are usually inversely related [10]. Meanwhile, zeolites are known as shape-selective catalysts due to the presence of micropores with a pore diameter close to the molecular diameter of the products. Hence, in order to obtain high PX selectivity, it is important to reduce the acid sites, decrease the external surface area or modify the pore-openings by post synthesis modification. A number of modification methods are reported to improve para-selectivity, including silanation [24,25], precooking [26] and metal/non-metal oxide impregnation [27,28]. Among them, the impregnation of phosphate species on ZSM-5 has been shown to be convenient and effective to achieve a high PX selectivity [3,29,30]. The earlier study by Kaeding and coworkers [27,31] showed that the oxides of phosphorus species in zeolite pores imparted a diffusion barrier in the micropores exhibiting an increase in the para selectivity. Védrine et al. [32] found that phosphorus species passivated acid sites primarily at the entrance of the channels of the zeolite and the strong acid sites remained unmodified. They concluded that the high selectivity for para isomers of xylene (X) were assumed to be related to the narrowing of the pore size and the zeolite framework phosphorus species, rather than to the modification of acid sites' strength. Janardhan and coworkers [30] reported that the introduction of phosphorous into ZSM-5 zeolite generated new acid sites with pore narrowing, which were formed by the interaction of phosphates with bridging hydroxyl groups of ZSM-5 zeolites in the micropores. The active sites generated monolayer islands over zeolite surface and narrowed the pore opening, leading to a high selectivity of PX for alkylation and disproportionation reactions. Recently, the non-petroleum route to produce PX through MTA reaction has been increasingly investigated. Miyake et al. [33] reported a Zn/ZSM-5@silicalite-1 core-shell zeolite catalyst which showed a high xylene yield and PX selectivity, and the yield of PX could reach 40.7%. Wei et al. [34] designed a Zn–P–Si–ZSM-5 catalyst which showed increased selectivity PX (89.6%) in X for MTA reaction. The catalyst was impregnated with a Zn and P over the ZSM-5 sample, and then SiO2 was loaded by chemical liquid phase deposition (CLD) modification. Zhu et al. [2] prepared Mg–Zn–Si-HZSM-5 catalyst by using the CLD method with polyphenylmethylsiloxane and then introducing Zn and Mg through vacuum impregnation. The prepared catalyst showed a high selectivity of PX in X (98.9%) and a relatively long lifetime for the MTA.

In this work, the highly shape-selective P/HZSM-5 catalysts for the MTA reaction were prepared by incipient wetness impregnation with H3PO4 as a source of phosphorus and the different amounts of loaded phosphorus were obtained by changing the concentration of H3PO4 solution. Then, the obtained samples were investigated by X-ray powder diffraction (XRD), infrared spectra for pyridine adsorption (Py-IR), N2 adsorption–desorption, temperature programmed desorption of NH3 (NH3-TPD), solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) and thermogravimetric analysis (TG). Due to the fine modification of pore size and acidic properties, the selectivity of PX in X and the selectivity for xylene in aromatics over P/HZSM-5 catalyst both increased.

#### **2. Results and Discussion**

#### *2.1. Catalyst Characterization*

As shown in Figure 1, all the zeolite catalysts exhibited the similar peaks, which were the typical MFI structure. (JCPDS no. 73–1138) No new diffraction peak could be observed after the introduction of P species, even at the loadings of 5 wt.% P. The results indicated that the introduction of P did not destroy the structure of ZSM-5 framework severely and P was highly dispersed in ZSM-5 zeolite. Also, the relative crystallinities of prepared catalysts as shown in Table 1 decreased evidently. The crystallinity of the ZSM-5 zeolite was reduced to 76.4% after the loading of 5 wt.% P, which was most likely due to the framework defects caused by a certain degree of dealumination of the framework [2], as illustrated in the following 27Al MAS NMR results (Figure 6A). The surface area and pore volume of HZSM-5 and P/HZSM-5 samples calculated by nitrogen adsorption–desorption were shown in Table 1. The BET surface area, external surface area and micropore volume all obviously decreased with the increase in P doping concentration, while the mesopore volume was almost constant, which could be ascribed to the fact that the loadings of P species blocked or destroyed some micropores in HZSM-5

during the process of phosphorus modification [35]. This indicated that the phosphorus species were not only doped on the external surface but also in the ZSM-5 channels. Compared with the total surface area, the external surface area decreased more obviously. The external surface area had decreased by more than 57% with 5 wt.% P loading.

**Figure 1.** XRD patterns of the different catalysts: (**a**) HZSM-5, (**b**) 0.5% P/HZSM-5, (**c**) 1% P/HZSM-5, (**d**) 2% P/HZSM-5, (**e**) 3% P/HZSM-5 and (**f**) 5% P/HZSM-5.

Note: *S*BET, BET surface area; *S*e, external surface area, calculated by the t-plot method; *V*mes, mesopore volume; *V*mic, micropore volume, obtained by the t-plot method; the relative crystallinity is compared with the parent HZSM-5 having the strongest diffraction intensity.

The incorporation of phosphorus obviously influenced the porous structure of HZSM-5, as illustrated in Figure 2. The loading of P made the pore size distribution of HZSM-5 a little narrow and the pore size become remarkably smaller. The shrinkage of pore size in P modified HZSM-5 zeolites may be attributed to the formation of hydroxy-bridged P species in ZSM-5 zeolites or the accumulation of P species in the channels of ZSM-5 zeolites, which may impart a diffusion barrier and influence product diffusion in the micropores resulting in an increase in shape selectivity [30]. In order to reveal the probable relationship between the pore size and the catalytic properties, the adsorption isotherms of PX and MX on HZSM-5 and 5% P/HZSM-5 are shown in Figure 3. The adsorption results verified that m-xylene was more diffusion-limited than p-xylene on P modified HZSM-5 catalyst. This indicated that the introduction of P was beneficial to enhance the competitive adsorption of p-xylene because of pore narrowing. A similar phenomenon was found when SiO2 was loaded on the surface of the ZSM-5 catalyst. Compared with the unmodified ZSM-5 catalyst, the Si-modified catalyst enhanced the diffusion barrier in the mouth or channel of the zeolite and significantly inhibited the adsorption and diffusion of m-xylene [6].

**Figure 2.** Pore size distribution of parent and P modified HZSM-5 zeolites: (**a**) HZSM-5, (**b**) 0.5% P/HZSM-5, (**c**) 1% P/HZSM-5, (**d**) 2% P/HZSM-5, (**e**) 3% P/HZSM-5 and (**f**) 5% P/HZSM-5.

**Figure 3.** Adsorption isotherms of para-xylene (PX) (**A**) and meta-xylene (MX) (**B**) on HZSM-5 (**a**) and 5% P/HZSM-5 (**b**).

The total acid quantity and strength of acid sites in parent and P-modified HZSM-5 zeolites were obtained by NH3-TPD, as shown in Figure 4. The HZSM-5 zeolite exhibited a typical NH3- TPD profile with two characteristic peaks at about 190 and 380 ◦C, representing the ammonia adsorbed on the weak acid sites and strong acid sites, respectively [36]. After the doping with P, the evident changes in the peak temperature and area of the strong acid sites were observed. The peak temperature of the strong acid sites shifted towards a low temperature and the peak area decreased obviously. The more P doping was used, the smaller the peak area became, indicating that the decrease in the amount and strength of strong acid sites was due to the doping with P. The low-temperature peak also slightly shifted toward a low temperature, demonstrating that the loading of P also reduced weak acid strength. The difference was that the density of weak acid sites in low temperature was less affected with the increase in P loadings (as listed in Table 2).

Chemisorption of pyridine on HZSM-5 and P-modified HZSM-5 zeolites also illustrated the changes in zeolite acidity. As shown in Figure 5, all catalysts showed absorption peaks at about 1545 and 1450 cm−1, which were ascribed to pyridine chemisorbed on Brönsted (B) acid sites and pyridine adsorbed on Lewis (L) acid sites, respectively [37,38]. The incorporation of phosphorus led to a dramatic decline in the amount of both Brönsted and Lewis acidic sites. When small amounts of P were added (≤1 wt.%), the acid amount of the Brönsted acid sites showed a sharp decrease compared to that of the Lewis acid sites (as listed in Table 2). Whereas, further increasing phosphorus content from 1 to 5 wt.% resulted in an evident decrease in the acid amount of the Lewis acid sites and a slight decrease in the amount of the Brönsted acid sites. In summary, the loading of phosphorus reduced both the acid strength and amount of HZSM-5 zeolite, especially strong acid sites.

**Figure 4.** NH3-TPD profiles of parent and P-modified HZSM-5 zeolites: (**a**) HZSM-5, (**b**) 0.5% P/HZSM-5, (**c**) 1% P/HZSM-5, (**d**) 2% P/HZSM-5, (**e**) 3% P/HZSM-5 and (**f**) 5% P/HZSM-5.

**Table 2.** Acidity characterization of HZSM-5 and P/HZSM-5 zeolites.

a Determined by NH3-TPD. b Obtained by Py-IR. L/B, the ratio of the amount of Lewis acidic sites to that of Brönsted acidic sites.

**Figure 5.** FT-IR spectra of pyridine adsorption on parent and P-modified HZSM-5 zeolites: (**a**) HZSM-5, (**b**) 0.5% P/HZSM-5, (**c**) 1% P/HZSM-5, (**d**) 2% P/HZSM-5, (**e**) 3% P/HZSM-5 and (**f**) 5% P/HZSM-5.

27Al MAS NMR has been extensively used as an effective technique to identify states of the Al atoms in the zeolite. Figure 6A showed 27Al MAS NMR spectra of HZSM-5 and P/HZSM-5 samples. For the parent HZSM-5 zeolite, a characteristic peak of typical Al species around 53 ppm was detected that could be ascribed to a tetrahedral framework aluminum [32]. A relatively weak signal appeared at 0 ppm which could be assigned to octahedral aluminum species, revealing that there existed a small number of extra-framework aluminum species in the HZSM-5 zeolite. With increasing impregnation degree, new peaks at about −14 and 40 ppm appeared. The signal at −14 ppm was related to octahedral aluminum interacting with a P atom [39]. The peak around 40 ppm was generally assigned to a tetrahedral framework aluminum or extra-framework aluminum species in a distorted environment [40]. With the increase in P loadings, the intensity of signals at −14 and 40 ppm increased while that at 53 ppm decreased, illustrating that the dealumination of the HZSM-5 zeolite framework occurred as a result of the phosphate impregnation, consistent with the XRD, micropore volume and acidity measurement results.

**Figure 6.** 27Al MAS NMR spectra (**A**) and 31P MAS NMR spectra (**B**) of parent and P-modified HZSM-5 zeolites: (**a**) HZSM-5, (**b**) 0.5% P/HZSM-5, (**c**) 1% P/HZSM-5, (**d**) 2% P/HZSM-5, (**e**) 3% P/HZSM-5 and (**f**) 5% P/HZSM-5.

The 31P MAS NMR spectra of P/HZSM-5 zeolites were shown in Figure 6B. The peak at 0 ppm was ascribed to excess P species which did not interact with framework aluminum. The signal around−6 ppm was assigned to P species in pyrophosphoric acid or pyrophosphates, whereas the −12 ppm signal was assigned to P species for intermediate groups in short-chain polyphosphates or pyrophosphates. The signals from −18 to −40 ppm were mainly due to extra-framework aluminophosphate and highly condensed polyphosphate complexes. Another signal appearing at −46 ppm could be assigned to intermediate P4O10 groups [32,41]. As can be seen in Figure 6B, the signals at 0 and −12 ppm increased slightly with the increase in P content, whereas the peaks from −18 to −40 ppm decreased evidently. This indicated that the phosphorus content had a remarkable effect on the existent state of the phosphorus species, which may have an influence on the catalytic performance.

#### *2.2. Influence of Phosphorus on the Catalytic Performance*

The effect of phosphorus on the catalytic performance was illustrated in Figure 7 and Table 3. The introduction of phosphorus exhibited a significant impact on the catalytic stability, methanol conversion, and product selectivity. Compared with the unmodified HZSM-5, a small amount of phosphorus (≤1 wt.%) could enhance the catalyst stability at a certain degree. Further increasing the P content, the catalyst stability and methanol conversion both decreased evidently. Equilibrium mixtures of xylene isomers generally contain only 23%–24% PX. As can be seen in Figure 7B, the PX selectivity in xylene was 23.8% on unmodified HZSM-5 zeolite. The introduction of phosphorus to HZSM-5 zeolite appreciably increased the selectivity for PX in xylene in the MTA reaction. The selectivity of PX in xylene increased with the increase in P content and it reached nearly to 90% when P content was

5 wt.%. Meanwhile, the selectivity for xylene in aromatics was enhanced with increasing the P content. The further detailed results were listed in Table 3. The modified catalysts also greatly improved the selectivity of light olefins (C2 =~C5 =) and decreased the generation of aromatics and alkanes (C1 −~C4 −).

**Figure 7.** Methanol conversion with the time on stream (TOS) (**A**) and product selectivity at a TOS of 12.5 h (**B**) over parent and P-modified HZSM-5 zeolites: (**a**) HZSM-5, (**b**) 0.5% P/HZSM-5, (**c**) 1% P/HZSM-5, (**d**) 2% P/HZSM-5, (**e**) 3% P/HZSM-5 and (**f**) 5% P/HZSM-5.

**Table 3.** Products distribution of the methanol to aromatics (MTA) reaction over different catalysts.

a Others, H2 and COX; b the yield of PX is calculated through methanol conversion × aromatics selectivity × xylene selectivity in aromatics × PX selectivity in X; Conv.MeOH, methanol conversion. The data were obtained at 12.5 h.

Generally, in the MTA reaction, methanol is dehydrated to form dimethyl ether with the action of protons, which can be converted to light olefins by further dehydration and methylation. The light olefins are further developed into C5+ non-aromatic hydrocarbons by polymerization and cyclization. Then the aromatics and alkanes are generated by hydrogen transfer and dehydro–cyclization reactions under the action of Brönsted acid sites [14,42]. The results of acidity measurement showed that the amount of the Brönsted acid sites decreased obviously after the doping of P, which was consist with the changes in the selectivity of aromatics and alkanes. In addition, because phosphorus can enter the channels of catalyst and cover the strong acid sites of the inner surface, the amount and strength of strong acid sites decreased significantly with P doping, resulting in the poor methanol conversion [43].

The spent catalysts were studied using thermogravimetric analysis. The weight loss of the samples on the range of 300–750 ◦C revealed the decomposition of coke deposition [9]. As shown in Figure 8, the amounts of coke deposition in the used P/HZSM-5 catalysts decreased to a certain extent. Especially, increasing P content to more than 1% resulted in an obvious decrease in coke amounts. As can be seen from the catalytic performance (Figure 7 and Table 3), when P content was above 1%, the catalyst stability and methanol conversion both decreased evidently, whereas the selectivity of PX in X improved greatly. The phenomenon may be due to the fact that high level loadings of P blocked more pore volume and external surface area, which reduced the resistance of coke deposition of the catalyst. The narrowed pore size lead by the introduction of P limited the diffusion of products and made the micropore entrance block quickly, resulting in a fast coke deposition rate and deactivation of the catalyst.

**Figure 8.** Thermogravimetric analysis (TG) curves of the spent HZSM-5 and P/HZSM-5 catalysts.

Meanwhile, the introduction of P reduced external surface area and narrowed the pore size, leading to an increase in the selectivity of xylene and PX. It is reported that the narrowing of the pore openings and the poisoning of external acid sites can enhance the PX selectivity in X isomers in MTA reaction [2]. Firstly, the narrowed pore size made the generation of PX easier than that of other X isomers and C9+ aromatics, and the small amount of other X isomers and C9+ aromatics formed was also difficult to diffuse out the zeolite channel. Moreover, the isomerization reaction of PX was inhibited after PX diffusing to the external surface which was poisoned by P species [42]. Therefore, the P/HZSM-5 was highly shape-selective for PX in the MTA reaction. However, the PX yield did not increase significantly as expected because of the decrease in the selectivity of aromatics and methanol conversion. The previous work found that the introduction of Zn species could generate the Zn-Lewis acid sites, which was beneficial to the formation of aromatics through the dehydration and cyclization reactions of light olefins [14]. Therefore, it is suggested that the catalyst with a high aromatics selectivity and high PX selectivity could be obtained by loading Zn over 5% P/HZSM-5.

#### **3. Materials and Methods**

#### *3.1. Catalyst Preparation*

To obtain the sample with uniform crystal size, ZSM-5 zeolite was synthesized by seed-induction synthesis. The silicalite-1 seed was synthesized by modifying the procedures reported in reference [44]. The obtained colloidal silicalite-1 was directly used as a seed to prepare ZSM-5 zeolites. ZSM-5 samples were synthesized with the molar composition: SiO2:0.011Al2O3:0.02Na2O:0.15TPAOH:30H2O prepared from silica sol (30 wt.% SiO2), sodium aluminate (NaAlO2, Al2O3 ≥ 41 wt.%), tetrapropylammonium hydroxide (TPAOH, 48.67 wt.% in aqueous solution). The synthesis mixture was conducted at 170 ◦C in a teflon-lined, stainless-steel autoclave under rotation (20 r/min). The obtained samples were recovered by centrifugation and repeatedly re-suspended in distilled water to remove physically attached templates. The recovered samples were dried at 100 ◦C overnight and calcined at 560 ◦C for 10 h in air, repeatedly ion-exchanging Na+ with aqueous NH4NO3 solution (1 M, m(liquid)/m(solid) = 40) at 80 ◦C for 6 h, and subsequently calcined at 560 ◦C for 6 h.

Incipient wetness impregnation was used to incorporate Phosphorous species into HZSM-5 and the different phosphorous content was obtained by changing the concentration of H3PO4 solution, then dried at 100 ◦C overnight and calcined at 560 ◦C for 6 h under ambient condition.

#### *3.2. Catalyst Characterization*

X-ray powder diffraction (XRD) patterns were collected on a Rigaku MiniFlex II desktop X-ray diffractometer (Rigaku, Tokyo, Japan) with monochromated Cu *K*α radiation (30 kV and 15 mA). By assuming that the zeolite sample had the largest peak area in the range of 2θ from 22◦ to 25◦ (here, it was the parent HZSM-5), as a reference had a crystallinity of 100%, the relative crystallinity of the P/HZSM-5 catalyst was then estimated by comparing its total peak area in this 2θ range with that of the parent HZSM-5.

Nitrogen adsorption/desorption isotherms were measured at –196 ◦C on a BELSORP-max gas adsorption analyzer (MicrotracBEL, Osaka, Japan). The adsorption isotherms of p-xylene and m-xylene on the zeolites were measured at 25 ◦C on the BELSORP-max instrument. The measurement process and result analysis were similar to those described in our previous work [14]. Similarly, temperature-programmed desorption of NH3 (AutoChem II 2920 chemisorption analyzer, Micromeritics, Atlanta, GA, USA) (NH3-TPD, to obtain the amount and strength of acid sites), and infrared spectra for pyridine adsorption (Tensor 27 FT-IR spectrometer, Bruker, Karlsruhe, Germany) (Py-IR, to get the acidity of Brönsted and Lewis acidic sites [45]), were performed by following similar procedures with the same apparatus as reported previously [12,14].

Solid-state NMR investigations were performed on Bruker Avance III 600 spectrometer (Bruker, Karlsruhe, Germany). 31P MAS NMR was recorded with 85% phosphoric acid as the reference at 161.9 MHz with a pulse length of 3 μs (60◦) and pulse interval of 60 s. 27Al MAS NMR was carried out with 1.0 M aqueous solution of Al(NO3)3 as the reference at 104.3 MHz, with the pulse of 1.7 μs, and a pulse interval of 60 s. All the data were acquired at a spinning speed of 8 kHz.

Coke deposition after MTA reactions was performed by TG on a Rigaku Thermo plus Evo TG 8120 instrument (Rigaku, Tokyo, Japan). The spent catalysts were combusted from room temperature to 800 ◦C with a heating rate of 10 ◦C min−1 in air.

#### *3.3. Catalyst Evaluation*

The MTA reaction was performed in a fixed-bed micro reactor (Pengxiang, Tianjin, China). The reaction was carried out with methanol WHSV of 3.2 h−1 at 390 ◦C and 0.5 MPa. The zeolite catalysts were crushed and sieved to 20–40 mesh and pretreated at reaction temperature for 8 h in a nitrogen flow. The gas and liquid products were separated with a cold trap and analyzed by gas chromatographs (7890A, Agilent, Palo alto, CA, USA). The specific parameters of GC were the same values as described previously [14].

#### **4. Conclusions**

The phosphorous-modified HZSM-5 zeolites were prepared by incipient wetness impregnation. The effect of phosphorous on the structure of HZSM-5 and its relationship with the catalytic performance of P/H-ZSM-5 catalysts in MTA was studied. The results indicated that the introduction of P could reduce micropore volume and external surface area, narrow the pore size and decrease the amount and strength of strong acid sites. The pore size and acid properties of zeolite were finely modified by introducing phosphorous, which resulted in the P/HZSM-5 catalyst enhancing the PX selectivity in X isomers for MTA reaction. The PX selectivity in X and the selectivity for xylene in aromatics both improved significantly with the increase in P content. However, excessive P (>1%) also resulted in the decrease in aromatic selectivity and methanol conversion. The PX selectivity (in xylene) could reach nearly to 90% when P content was 5 wt.%, while the selectivity of xylene in aromatics was enhanced from 41.3% to 60.2%. Owing to the decrease in the selectivity of aromatics and methanol conversion, the yield of PX needs to be further improved. It is suggested that the catalyst with a high PX selectivity in X and high PX yield could be obtained by loading Zn over 5% P/HZSM-5 which can generate the Zn-Lewis acid sites to enhance the formation of aromatics through the dehydration and cyclization reactions of light olefins.

**Author Contributions:** Conceptualization, X.N., M.D. and W.F.; Data curation, K.W. and Y.-e.D.; Formal analysis, X.N. and K.W.; Funding acquisition, X.N., Y.-e.D. and Y.C.; Investigation, Y.B.; Methodology, X.N.; Writing—original draft, X.N.; Writing—review and editing, X.N., K.W. and Y.C.. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This work was financially supported by the Doctor Research Funds of Jinzhong University, Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi (No. 2019L0881), the Applied Basic Research Project of Shanxi (No. 201901D111303), Shanxi "1331 Project" Key Innovative Research Team (PY201817), Jinzhong University "1331 Project" Key Innovative Research Team (jzxycxtd2017004 and jzxycxtd2019005) and the Research Project of Science and Technology of the Henan Province<PHONE_NUMBER>65).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## **Pt Modified Heterogeneous Catalysts Combined with Ozonation for the Removal of Diclofenac from Aqueous Solutions and the Fate of by-Products**

**Soudabeh Saeid 1, Matilda Kråkström 2, Pasi Tolvanen 1, Narendra Kumar 1,\*, Kari Eränen 1, Jyri-Pekka Mikkola 1,3, Leif Kronberg 2, Patrik Eklund 2, Atte Aho 1, Heikki Palonen 4, Markus Perula 5, Andrey Shchukarev 3 and Tapio Salmi 1,\***

Received: 7 February 2020; Accepted: 9 March 2020; Published: 12 March 2020

**Abstract:** The degradation of the pharmaceutical compound diclofenac in an aqueous solution was studied with an advanced oxidation method, catalytic ozonation. Diclofenac was destroyed in a few minutes by ozonation but several long-lasting degradation by-products were formed. For this reason, the combination of heterogeneous catalysts and ozonation was applied to eliminate them completely. The kinetics of the diclofenac degradation and the formation of by-products were thoroughly investigated. Loading of Pt on the catalysts resulted in an improvement of the activity. The Mesoporous Molecular Sieves (MCM) were one of the promising catalysts for the degradation of organic pollutants. In this study, six heterogeneous catalysts were screened, primarily MCM-22-100 catalysts with different Pt concentrations loaded via the evaporation-impregnation (EIM) method, and they were applied on the degradation of diclofenac. It was found that the presence of Pt improved the degradation of diclofenac and gave lower concentrations of by-products. The 2 wt % Pt-H-MCM-22-100-EIM demonstrated the highest degradation rate compared to the proton form, 1% or 5 wt % Pt concentration, i.e., an optimum was found in between. Pt-H-Y-12-IE and Pt-γ-Al2O3 (UOP)-IMP catalysts were applied and compared with the MCM-22 structure. Upon use of both of these catalysts, an improvement in the degradation of diclofenac and by-products was observed, and the 2 wt % Pt-H-MCM-22-100-EIM illustrated the maximum activity. All important characterization methods were applied to understand the behavior of the catalysts (X-ray powder diffraction, transmission electron microscopy, nitrogen physisorption, scanning electron microscopy, energy dispersive X-ray micro-analyses, pyridine adsorption-desorption with FTIR spectroscopy, X-ray photoelectron spectroscopy). Finally, leaching of Pt and Al were analyzed by inductively coupled optical emission spectrometry.

**Keywords:** diclofenac; catalytic ozonation; heterogeneous catalyst; catalyst characterization; advanced oxidation processes

#### **1. Introduction**

The appearance of pharmaceutical compounds in the aquatic environment, particularly in the surface waters, has become a global concern because various pharmaceuticals are frequently detected in wastewaters, surface waters as well as ground waters around the world. In rivers and surface waters, pharmaceuticals are sometimes observed at very high concentrations, even up to mg per liter scales [1–3]. This is mainly due to the residuals of veterinary and medical pharmaceuticals discharged to surface waters from the excreta of human and animals, the sewage effluent of water treatment processes, as well as hospital and domestic wastewaters [4]. Owing to the typically strong biological activity of the pharmaceuticals, they are harmful for marine animals, micro-organisms and potentially human beings as well [5,6]. Most of these compounds are persistent to the treatment applied in conventional wastewater treatment plants [7].

Diclofenac (2-(2-(2,6-dichlorophenylamino) phenyl) acetic acid), or DCF, is a non-steroidal anti-inflammatory drug (NSAID). The molecular structure of DCF is displayed in Figure 1. DCF can be used as oral tablets or as a topical gel for the prescription of osteoarthritis, rheumatoid arthritis, migraine headache, and menstrual cramps [8,9]. DCF has been identified in numerous freshwaters worldwide, and more than 100 publications have announced its occurrence in surface waters and ground waters. This medicine is analyzed in freshwaters at Eco toxicological levels [10]. As an example, DCF was reported by the Ministry of Health of Malaysia (2010) as one the most generally used domestic drug, and it was detected in the three largest rivers Lui, Gombak, and Selangor at concentrations of 2.76, 4.84 ng/L, and 4.30 ng/L, respectively [4]. In Pakistan, DCF and five of its transformation by-products have been detected in the Malir and Lyari River, as well as in surface and effluent waters of Karachi. The highest measured concentrations of DCF, 4 -hydroxydiclofenac, and 5-hydroxydiclofenac were 85,000 ng/L, 18,000 ng/L, and 3000 ng/L, respectively [11]. The occurrence of DCF has been studied in waste waters of Colombian rivers too. Concentrations corresponding to 400 ng/L at Bogotá and 256 ng/L at Medellin wastewater influent have been detected. Even after the water treatment plants (in the effluents), the concentrations were 340 ng/L and 170 ng/L [2]. In Europe, too, the concentrations of DCF are alarming. For example, in the surface waters of Milan it was detected at up to 100 ng/L [6]. In the investigation of the concentrations of pharmaceuticals along a 32 km expansion of a highly wastewater contaminated watercourse (rivers and lakes) in Eastern Finland, the DCF load varied from 6000 mg/day to 10,000 mg/day depending on the seasons [12].

**Figure 1.** Structural formula of diclofenac (DCF).

DCF is reported to have potentially adverse effects on aquatic organisms such as that it has an influence on the kidney and gill integrity in the brown trout a salmonid kind and more over it have potential of bioaccumulation via food chain [13–15]. Stickleback fish which was exposed to sewage effluents containing DCF at low concentrations (μg/L) were affected by kidney alterations [16].

Usually, DCF is not entirely degraded via wastewater treatment methods. It is in fact poorly biodegradable throughout the entire wastewater treatment plants [8,17]. Consequently, it remains in effluents and pollutes surface and ground waters which are the principal sources of drinking water. Indeed, frequent detection of DCF in drinking water sources prove this evidence [18,19]. DCF can

potentially interact with organic and inorganic contaminants in the environment, especially in the wastewater treatment processes, for instance with organic pollutants and metals, which may drive to the production of other harmful pollutants [20].

Various studies have focused on how to decrease the toxicity and improve the degradation of emerging pharmaceutical pollutants. Advanced oxidation processes (AOPs) are one of the most promising technologies for the removal of organic macro-pollutants. They could provide practical technologies to the degradation of non-biodegradable pharmaceuticals in contaminated wastewater as a replacement to the biological treatment [21–23]. Some methods have been proposed for the removal of DCF from wastewater using adsorption [24], photocatalytic degradation [25], the use of microorganisms [26], and sludge treatment [8]. Still, most of the treatments proposed have issues such as long-term procedure, low effectiveness, adverse environmental impact, high cost, or they have been proposed without analyzing the by-products at all [24]. A promising AOP method is ozonation, because ozone has the ability to attack aromatic molecules and unsaturated bonds, and it is a well-known method for the removal of pharmaceuticals [27,28]. There is, however, a risk concerning the formation of toxic by-products appearing during ozone treatment [29]. While 99% of DCF can be removed 99% by ozonation, only 24% of it was mineralized and the toxicity was only slightly decreased [30]. Combination of heterogeneous catalysis and ozonation improves the removal of organic micro pollutants by the transformation of ozone into high reactive species or by direct reactions on the surface of catalysts [31]. Aguinaco et al. investigated the elimination of DCF from the water via photocatalytic ozonation, where TiO2 was employed as a catalyst. Their study revealed that DCF was removed after 6 min, and the total amount of organic compounds were removed to 60% after 60 min of reaction [32]. Loading Pt on supported catalysts increases their activity during the ozonation process in the elimination of organic pollutants such as Pt-supported alumina in the removal of paracetamol and Pt-Ce/BEA catalyst in toluene oxidation [33,34].

This work investigates the efficiency of catalytic ozonation on the removal of DCF and the evolution of by-products. Pt-modified catalysts were synthetized, several types of characterizations were applied to understand the catalytic behavior. The effect of the Pt loading on the catalysts, the catalyst concentration and catalyst structures in the transformation of DCF and its degradation by-products were studied, too.

#### **2. Results and Discussion**

#### *2.1. Results Physico-Chemical Characterization of Pt-Modified Catalysts*

#### 2.1.1. X-Ray Powder Diffraction (XRD)

X-ray powder diffraction was used to determine the phase purities and structures of H-MCM-22-100, 1 wt % Pt-MCM-22-100-EIM, 2 wt % Pt-MCM-22-100-EIM, 5 wt % Pt-MCM-22-100-EIM, Pt-H-Y-12-IE and Pt-γ-Al2O3 (UOP)-IMP catalysts. The measured XRD patterns of the MCM-22-100 samples are shown in Figure 2a. (2θ: 0–40) and 2b. (2θ: 40–100). The reference pattern fitted to the data is a zeolite structure with the MWW framework [35]. The 2% and 5 wt % Pt samples were phase pure. The detection limit for the impurities is around 1 vol.% when the reference pattern fits the data well, like in this case. The 1 wt % Pt sample has a hint of Si phase visible. The sample without Pt has extra peaks possibly originating from a different type of a framework. Estimating from the relative peak intensity, amount of the impurity phase was about 10–20 vol.%. The results are collected in Table 1. The lattice parameters given in the table were obtained from the Rietveld refinements and the crystal sizes were estimated from the FWHMs of the (102) peaks using the Scherrer equation after correcting the FWHMs for the instrument resolution. The agreement between the fits and the measured XRD data is good in the MCM-22-100 sample series. Thus, the phase identification and the lattice parameters can be reported with a high degree of certainty. The impurities in the H-MCM-22-100 sample could have two different sources: one is a large unit cell structure visible as peaks at the low angles and the other

is a small unit cell structure visible as a shoulder in the peak at 26◦ and as a separate peak at 49.5◦. The latter peaks do not match with Al, Si or Al2O3.

The measured XRD pattern of the Pt-H-Y-12-IE sample is shown in Figure 2c. The reference pattern fitted to the data is a zeolite structure with the Faujasite (FAU) framework [36]. In addition to the FAU framework, the Pt-H-Y-12-IE sample shows a Pt phase. A fcc structure was used to fit the Pt phase. The fit results are collected in Table 1. The lattice parameters given in the table were obtained from the Rietveld refinements and the crystal sizes were estimated from the FWHMs of the (331) peak for FAU and of the (111) peak for Pt phase using the Scherrer equation after correcting the FWHMs for the instrument resolution.

The measured XRD pattern of the Pt-γ-Al2O3 (UOP)-IMP sample is displayed in Figure 2d. The pattern shows broad smooth features which contain multiple peaks smeared by the Scherrer broadening into a single large peak. The data were Rietveld refined using a tetragonal unit cell [37]. The fit results are collected in Table 1. The lattice parameters and the crystal sizes given in the table for the Pt-γ-Al2O3 (UOP)-IMP sample were obtained from the Rietveld refinements.

The majority phase in all the four types of MCM-22 zeolite catalysts were MWW. There were some impurities present in the catalysts, which were present as SiO2 domains in the synthesized MCM-22 zeolite. The presence of SiO2 phase in the MCM-22 catalysts were attributed to the unreacted source of silica. It is noteworthy to mention that some of the peaks appearing, which were not identified as SiO2, Al2O3 can be attributed to the PtO2 oxide phase. The three Pt-modified catalysts 1 wt % Pt-MCM-22-100-EIM, 2 wt % Pt-MCM-22-100-EIM and 5 wt % Pt-MCM-22-100-EIM, peaks present as PtO2 should be observed. However, due to small amounts of Pt in 1 wt % Pt-MCM-22-100-EIM and 2 wt % Pt-MCM-22-100-EIM, PtO2 phase was not visible. Further explanation for the absence of the PtO2 phase could be nano size of PtO2 < 3 nm which are not detected by an X-ray powder diffractometer.

The agreement between the fits and the measured XRD data is reasonable in both Pt-H-Y-12-IE and Pt-γ-Al2O3(UOP)-IMP samples. Resolving a small fraction of Pt particles from the γ-Al2O3 phase is very challenging because all the Pt peaks, except for a single peak at 80◦, coincide with the γ -Al2O3 peaks. To detected small amounts of Pt in γ-Al2O3 would need to be compared the Pt-γ-Al2O3(UOP)-IMP sample measurement to a clean reference sample of γ-Al2O3.

**Table 1.** The Rietveld refinement results for the H-MCM-22-100, 1 wt % Pt-MCM-22-100-EIM, 2 wt % Pt-MCM-22-100-EIM, 5 wt % Pt-MCM-22-100-EIM Pt-H-Y-12-IE and Pt-γ-Al2O3 (UOP)-IMP catalysts.

**Figure 2.** X-ray powder diffraction patterns of H-MCM-22-100, 1 wt % Pt-MCM-22-100-EIM, 2 wt % Pt-MCM-22-100-EIM, 5 wt % Pt-MCM-22-100-EIM (**a**,**b**), Pt-H-Y-12-IE (**c**) and Pt-γ-Al2O3 (UOP)-IMP (**d**) catalysts.

#### 2.1.2. Transmission Electron Microscopy (TEM)

The TEM images of (a) H-MCM-22-100, (b) 1 wt % Pt-MCM-22-100-EIM, (c) 2 wt % Pt-MCM-22-100-EIM, (d) 5 wt % Pt-MCM-22-100-EIM, (e) Pt-H-Y-12-IE and (f) Pt-γ-Al2O3 (UOP)-IMP catalysts and Pt particle size distributions are illustrated in histograms displayed in Figure 3. Figure 3a shows the structure of H-MCM-22-100 by TEM images, the lengths of the channels can be measured from these images between 1.12–1.63 nm.

The TEM images exhibited the largest Pt particles for the Pt-H-Y-IE catalyst (30.6 nm) and the second largest Pt particles for 1 wt % Pt-MCM-22-100-EIM (24.9 nm). The smallest average Pt- particles were obtained for 5 wt % Pt-MCM-22-100-EIM (2.7 nm) catalyst. The explanation for the very small nanoparticles of Pt in 5 wt % Pt-MCM-22-100-EIM compared to 1 wt % Pt-MCM-22-100-EIM can be attributed to a very high dispersion of Pt on the H-MCM-22-100 catalyst. Furthermore, high dispersion of Pt nanoparticles in H-MCM-22-100 can also be attributed to the presence of Pt nanoparticles only at the size of 5 nm (Figure 3d). Additionally, the description of variations in the average Pt particles is attributed to the methods of Pt introduction in the support materials H-MCM-22, H-Y and Al2O3 (Table 2). Furthermore, the structures of the supports H-MCM-22, H-Y and Al2O3, surface area, Brønsted and Lewis acid sites of these catalytic materials can also influence the formation of Pt particles, size and dispersion.

**Figure 3.** *Cont.*

**Figure 3.** Transmission electron microscope (TEM) images and Pt particle size distribution histograms of (**a**) H-MCM-22-100, (**b**) 1 wt % Pt-MCM-22-100-EIM, (**c**) 2% nwt Pt-MCM-22-100-EIM, (**d**) 5% nwt Pt-MCM-22-100-EIM, (**e**) Pt-H-Y-12-IE, and (**f**) Pt-γ-Al2O3 (UOP)-IMP.

**Table 2.** Pt particle size average of 1 wt % Pt-MCM-22-100, 2 wt % Pt-MCM-22-100, 5 wt % Pt-MCM-22-100-EIM, Pt-H-Y-12-IE and Pt-γ-Al2O3 (UOP)-IMP catalysts.

#### 2.1.3. Nitrogen Physisorption

The catalysts were characterized by nitrogen physisorption for the measurement of the specific surface areas and the specific pore volumes. The surface areas were calculated the aid of the Brunauer-Emmett-Teller (BET) theory. The results from nitrogen physisorption analysis are presented in Table 3.

**Table 3.** Specific surface area and pore volume of the catalysts used in the ozonation experiments.

The highest specific surface areas and pore volumes were obtained for the Pt-H-Y-12-IE catalyst (835 m2/g). For the spent Pt-H-Y-12-IE catalyst a decrease of the surface area and pore volume was observed. This decrease in the surface area and pore volume is attributed to the coke formation in the pores of the Pt-H-Y-IE zeolite catalyst. All the spent catalysts studied in the reaction showed a decrease in the surface area and the pore volume. However, the extent to which the decrease in surface area and pore volume took place, was dependent on the structure of zeolite, i.e., H-MCM-22, H-Y and Al2O3. The Pt-γ-Al2O3 (UOP)-IMP spent catalyst showed the highest decrease in the surface area, followed by H-MCM-22 spent zeolite catalyst according to Table 3. The Pt-modified H-MCM-22 and H-Y-12 zeolite catalysts exhibited lower surface areas than the Pt-modified H-MCM-22, H-Y-12 fresh catalysts. The reason for such a decrease in the surface area for Pt-γ-Al2O3 (UOP)-EIM (189 m2/g) is probably due to the coke formation in the pores of the catalysts. The Pt-modified MCM-22-100-EIM catalysts exhibited a smaller decrease in the surface area (Table 3) for the spent catalysts. A plausible explanation for the lower decrease in the surface area for 1 wt % Pt-, 2 wt % Pt-, 5 wt % Pt-H-MCM-22-100-EIM can be attributed to the presence of Pt- in the MCM-22 zeolite catalysts, which has a coke inhibiting property. Similarly, decrease in the surface area for the Pt-H-Y-IE catalyst (767 m2/g) spent catalyst was much lower than the fresh Pt-H-Y-IE (835 m2/g) catalyst.

#### 2.1.4. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Microanalyses

Scanning electron microscopy (SEM) was applied for the measurement of the crystal size, shape and distributions of the synthesized catalytic materials. Furthermore, the crystal size distributions for the H-MCM-22, Pt-modified H-MCM-22-100-EIM, H-Y-IE and γ-Al2O3 (UOP)-IMP catalysts were measured as (100–800 nm) and (300–800 nm), respectively (Figure 4). The differences in the crystal size distributions for Pt-modified H-MCM-22-100-EIM and Pt-modified H-Y-IE are attributed to the variations in the shapes and structures of the studied H-MCM-22 (MWW) and H-Y (FAU) zeolites. It is noteworthy to mention that modifications of pristine H-MCM-22 and H-Y zeolite catalysts with Pt- did not influence the parent structures (Figure 4). The largest average crystal size (396 nm) was measured for 5 wt % Pt-H-MCM-22-100-EIM catalyst among the 1 wt % Pt-, 2 wt % Pt-, 5 wt % Pt- catalysts. The second largest crystals were measured for H-MCM-22 (386 nm) catalyst. The Pt-H-Y-IE zeolite catalyst exhibited the average crystal size (493 nm). The crystal size distribution of the (a) H-MCM-22-100, (b) 1 wt % Pt-MCM-22-100-EIM, (c) 2 wt % Pt-MCM-22-100-EIM, (d) 5 wt % Pt-MCM-22-100-EIM, (e) Pt-H-Y-12-IE and (f) Pt-γ-Al2O3 (UOP)-IMP catalysts are presented in Figure 4a–f.

**Figure 4.** *Cont.*

**Figure 4.** SEM images and crystal size distribution histograms of (**a**) H-MCM-22-100, (**b**) 1 wt % Pt-MCM-22-100-EIM, (**c**) 2 wt % Pt-MCM-22-100-EIM, (**d**) 5 wt % Pt-MCM-22-100-EIM, (**e**) Pt-H-Y-12-IE and (**f**) Pt-γ-Al2O3 (UOP)-IMP catalysts.

The amounts of Pt (wt %) in the Pt-modified H-MCM-22-100-EIM, H-Y-12 and γ-Al2O3 catalysts were determined using energy dispersive X-ray micro-analysis. The smallest amount of Pt (0.63 wt %) (Table 4) was measured for 1 wt % Pt-H-MCM-22-100-EIM catalyst and the largest amount of Pt (7.13 wt %) was measured for Pt-γ-Al2O3 catalyst. The differences in the loadings of Pt in H-MCM-22-100, H-Y-12 and γ -Al2O3 catalysts were attributed to the methods of the catalyst synthesis, the support structure and the SiO2/Al2O3 ratio of the zeolites.

**Table 4.** Average crystal size and Pt concentration of H-MCM-22-100, 1 wt % Pt-MCM-22-100, 2 wt % Pt-MCM-22-100, 5 wt % Pt-MCM-22-100-EIM, Pt-H-Y-12-IE and Pt-γ-Al2O3 (UOP)-IMP catalysts.

The amount of *Brønsted* and *Lewis* acid sites of the pristine H-MCM-22-100 and Pt-modified H-MCM-22-100-EIM, H-Y-12-IE and γ-Al2O3 catalysts were determined by FTIR spectroscopy using pyridine adsorption/desorption. Pyridine was desorbed at 250, 350, and 450 ◦C, and these temperatures indicate the presence of weak, medium and strong acid sites (Table 5).

The largest amount of *Brønsted* acid sites was obtained for Pt-H-Y-12-IE (145 μmol/g), the zeolite catalyst with lowest SiO2/Al2O3 ratio 12, the second largest amount of *Brønsted* acid site was obtained for the 1 wt % Pt-MCM-22-100-EIM catalyst (111 μmol/g), the lower amount of *Brønsted* acid cites is attributed to the high SiO2/Al2O3 ratio 100. It is noteworthy to mention that the H-MCM-22 zeolite catalyst exhibited a much lower amount of *Brønsted* acid sites (33 μmol/g). The Pt-modified H-MCM-22 i.e., 1 wt % Pt-MCM-22-100-EIM, 2 wt % Pt-MCM-22-100-EIM and 5 wt % Pt-MCM-22-100-EIM catalysts, exhibited an increase of the *Brønsted* acid sites, the enhancement is attributed to the presence of Pt in the structure of MCM-22. The Pt-γ-Al2O3 (UOP)-IMP catalyst exhibited the lowest amount of *Brønsted* acid sites, which is attributed to the lower amount of acid sites in γ-Al2O3.

**Table 5.** *Brønsted* and *Lewis* acidities of proton and Pt modified zeolites.

2.1.6. Characterization of the Oxidation States of Pt in Pt-Modified H-MCM-22-100-EIM, Pt-H-Y-12-EIM and Pt-γ-Al2O3-EIM Catalysts by X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) was used to analyze the oxidation states of Pt (PtO, PtO2 and Pt0) catalysts in 1 wt % Pt-H-MCM-22-100-EIM, 2 wt % Pt-H-MCM-22-100-EIM, 5 wt % Pt-H-MCM-22-100-EIM, Pt-H-Y-12-EIM and Pt-γ-Al2O3 (UOP)-EIM catalysts. The presence of PtO, PtO2 and Pt0 were observed in all the above-mentioned Pt-modified catalysts (Figure 5). The binding energies (eV) of the Pt0 – 71 (eV), PtO – 72 (eV), PtO2 – 74 (eV) were measured for all the studied Pt-modified catalysts. It was inferred from these binding energies that the surface of Pt modified catalytic materials consisted of oxidation states of Pt0, PtO and PtO2, respectively. Similar binding energies for Pt0, PtO, PtO2 have been previously reported by J. Z. Shyu et al. [38] and R. Bouwman et al. [39].

**Figure 5.** *Cont.*

**Figure 5.** *Cont.*

**Figure 5.** XPS spectra of (**a**) H-MCM-22-100, (**b**) 1 wt % Pt-MCM-22-100, (**c**) 2 wt % Pt-MCM-22-100, (**d**) 5 wt % Pt-MCM-22-100, (**e**) Pt-H-Y-12-IE and (**f**) Pt-γ-Al2O3 (UOP)-IMP catalysts.

#### *2.2. Non-Catalytic and Catalytic Ozonation of Diclofenac*

#### 2.2.1. Influence of Pt Modified Catalysts in Degradation of Diclofenac

The degradation of DCF was investigated by non-catalytic as well as catalytic experiments using ozone as the oxidant. Initially, the catalysts H-MCM-22-100 with different Pt-modified concentrations were used (Figure 6). The kinetic experiments revealed that the degradation rate of DCF was higher in the presence of heterogeneous catalysts compared to non-catalytic experiments. The 2 wt % Pt-MCM-22-100-EIM showed the highest decomposition rate, 95% of DCF had transformed at 4 min, whereas there was still 65% of DCF was left when no catalyst was used. Moreover, the 2 wt % Pt-MCM-22-100-EIM demonstrated a higher degradation efficiency compared to 5 wt % Pt-MCM-22-100-EIM. The excessive loading of metals on the support could decrease the amount of active sites, thus weakening the interaction among the metal and the support [40].

**Figure 6.** DCF degradation by ozonation and H-MCM-22-100, 1 wt % Pt-MCM-22-100-EIM, 2 wt % Pt-MCM-22-100-EIM and 5 wt % Pt-MCM-22-100-EIM catalysts. CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L, C catalysts = 0.5 g/L.

In addition, ozonation of DCF was investigated with 0, 0.25, 0.50 and 1.00 g of 2 wt % Pt-MCM-22-100-EIM catalyst. The results reveal that using 0.25 g of 2 wt % Pt-MCM-22-100-EIM catalyst increased the degradation rate compared to non-catalytic experiments. Moreover, 0.5 g and 1 g of 2 wt % Pt-MCM-22-100-EIM catalyst showed a maximum degradation rate of DCF. After 4 min of ozonation, 65% of DCF was still left in case of non-catalytic experiments whereas in case of catalytic reactions, 35% remained when 0.25 g catalyst was added, 8.3% when 0.5 g was added and 13% when 1 g of 2 wt % Pt-MCM-22-100-EIM catalyst was added (Figure 7a). Moreover, the normalized figure (Figure 7b) of these three amounts of catalyst illustrates an overlap of the kinetic curves revealing that the degradation rate is not dependent on the catalyst amount, which suggests the absence of gas-liquid mass transfer limitations.

**Figure 7.** (**a**) DCF degradation by ozonation without, and with 0.25, 0.50 and 1.00 g of 2 wt % Pt-MCM-22–100 catalyst, (**b**) Normalized. CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L.

A comprehensive study on the ozonation of DCF using different Pt-modified catalyst structures was conducted to evaluate its effect on the degradation of DCF. Two catalysts, Pt-H-Y-IE and Pt-γ-Al2O3 (UOP)-IMP were used and compared with the 2 wt % Pt-MCM-22-100-EIM. Figure 8 reveals that the transformation of DCF was relatively similar for these three catalysts.

**Figure 8.** DCF degradation by ozonation and 2 wt % Pt-MCM-22-100-EIM, Pt-γ-Al2O3 (UOP)-IMP and Pt-H-Y-12-IE catalysts. CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L, C catalysts = 0.5 g/L.

#### 2.2.2. Quantification of Ozonation Transformation-Products

According to the literature, the advanced oxidation process of DCF leads to the formation of one or more hydroxylated products [30,41]. Two of the hydroxylated products (4'-OH-DCF and 5-OH-DCF) were purchased (Figure 9) and an MRM method was developed to quantify them in order to investigate the effect of heterogeneous catalysis on the formation and transformation of hydroxylated DCF products. The transition used to quantify both 4'-OH-DCF and 5-OH-DCF was the same (m/z 310 -> 266), but the isomers were distinguished by their different retention times. The qualitative transitions were different for the isomers. The presence of the hydroxylated internal standard of the 4'-OH-DCF isomer further helped with distinguishing between the isomers. Chen et al. investigated the mineralization of DCF by ozonation combined with Fe-MCM-41 and Fe-MCM-48 catalysts. Their studies illustrated that mineralization of DCF using these catalysts is higher compared to non-catalytic ozonation [42,43]. For this reason, the MCM-structured catalyst was applied in our experiments to study the degradation of by-products and the performance of the MCM catalyst was compared by other structures.

**Figure 9.** DCF and its major hydroxylation products.

#### 2.2.3. 5-Hydroxy Diclofenac

Figure 10 illustrates the growth of one of the by-products, 5-OH-DCF, which emerges during the ozonation of DCF. In these experiments, using a heterogeneous catalyst resulted in a lower concentration of 5-OH-DCF compared to non-catalytic experiments. The lowest concentration of 5-OH-DCF was observed for the 2 wt % Pt-MCM-22-100-EIM catalyst. The presence of the heterogeneous catalysts affects the formation of 5-OH-DCF. When only the H-MCM-22-100 catalyst was used, the concentration of 5-OH-DCF increased more rapidly than in the metal-catalyzed experiments. The presence of Pt-modified catalyst resulted in a decrease of the maximum concentration of 5-OH-DCF.

**Figure 10.** 5-OH-DCF concentration during the decomposition of DCF by catalytic and non-catalytic ozonation. CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L, C catalysts = 0.5 g/L.

Three different catalyst amounts 0.25, 0.5, 1 g of 2 wt % Pt-MCM-22-100-EIM were studied (Figure 11). The formation of 5-OH-DCF decreased when the higher amount of catalyst was employed. As 1 g of catalyst was used, 5-OH-DCF could not be detected at all, which implies that the catalyst inhibits the formation of 5-OH-DCF.

**Figure 11.** 5-OH-DCF concentration during the ozonation of DCF using different amount of 2 wt % Pt-MCM-22-100-EIM catalyst (0.25, 0.50 and 1.00 g/L). CDCF = 30 mg/L, gas flow rate = 110 mL/min, T= 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L.

The transformation of 5-OH-DCF with three different catalyst structures in the ozonation process of DCF are illustrated in Figure 12. The decrease of 5-OH-DCF was strongly influenced by different kinds of Pt modified catalysts, especially with 2 wt % Pt-MCM-22-100-EIM and the use of this catalyst resulted in the lowest amount of 5-OH-DCF. Moreover, the concentration of 5-OH-DCF started to decline after 4 min in the presence of the Pt-H-Y-12-IE catalyst.

**Figure 12.** 5-OH-DCF concentration during the decomposition of DCF by catalytic and non-catalytic ozonation. CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L, C catalysts = 0.5 g/L.

#### 2.2.4. 4'-Hydroxy Diclofenac

The other main by-product formed was 4'-OH-DCF (Figure 9). During non-catalytic experiments, the concentration of 4'-OH-DCF increased to a maximum in 4 minutes but remained stable until the end of the experiment (Figure 13). As MCM-22-100 was used, the maximum concentration of 4'-OH-DCF was lower than in the non-catalytic experiment, and the concentration of 4'-OH-DCF started to decrease towards the end of the experiment. The lowest final concentration was observed as 2 wt % Pt-MCM-22-100 was used.

**Figure 13.** 4´OH-DCF concentration during the decomposition of DCF by catalytic and non-catalytic ozonation. CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L, C catalysts = 0.5 g/L.

As mentioned before, three different catalyst amounts (0.25, 0.5, 1 g) of 2 wt % Pt-MCM-22-100-EIM were studied in DCF elimination (Figure 14). The maximum concentration of 4'-OH-DCF which was formed decreased as the amount of catalyst increased. The concentration of 4'-OH-DCF after 10 min of ozonation was 0.06 mg/L only when 1 g of 2 wt % Pt-MCM-22-100-EIM was used, compared to 2.63 mg/L when no catalyst was present. This implies that the catalyst inhibits the formation of 4'-OH-DCF and/or enhances the transformation of it to the low molecular by-products.

**Figure 14.** 4'OH-DCF concentration during the ozonation of DCF using different amount of 2 wt % Pt-MCM-22-100 catalyst (0.25, 0.50 and 1.00 g). CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L.

The effects of the different structures of Pt-modified catalysts combined with the ozonation on the transformation of 4'-OH-DCF are displayed in Figure 15. The amount of 4'-OH-DCF increased to around 1 mg/L at 2 min; later it declined to zero at 10 min in catalytic ozonation. All these three catalysts turned to be efficient upon transformation of 4'-OH-DCF compared to the non-catalytic process.

**Figure 15.** 4'OH-DCF concentration during the decomposition of DCF by catalytic and non-catalytic ozonation. CDCF = 30 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L, C catalysts = 0.5 g/L.

The mechanisms that could enhance the degradation of DCF and by-products in the presence of heterogeneous catalysts were as follows: the first pathway is the ozone adsorption on the catalyst surface, following the generation of more active radical species which react with DCF and by-products; the second pathway is that the DCF and/or by-products adsorb on the catalyst surface resulting in a reaction by dissolved ozone; and the third pathway is that the ozone, DCF and its by-products adsorb on the catalyst surface and react by direct or indirect reaction on the surface [44].

2.2.5. Leaching of Pt and Al in the Reaction Media During the Catalytic Ozonation of Diclofenac

The Al and Pt concentrations were analyzed from the end samples of the catalytic ozonation of DCF (Figure 16). The results from this experiments revealed that 2 wt % Pt-MCM-22-100-EIM and 5 wt % Pt-MCM-22-100-EIM catalysts have the highest leaching of Pt about 15 to 18%, also MCM-22-100 catalysts showed a small amount of Al leaching nearby 5%. Moreover, the Pt-γ-Al2O3-IMP and Pt-H-Y-12-IE catalysts exhibited a small amount of Pt and Al less than 5% in these experiments. Plausible explanations for the Pt and Al leaching during the catalytic ozonation of diclofenac can be attributed to the contents of Pt in the Pt-H-MCM-22-100-EIM, Pt-H-Y-12-EIM and Pt-γ-Al2O3 catalysts. Furthermore, the extent of leaching of Pt can be also attributed to the structure and acidic properties of

H-MCM-22-100, H-Y-12 and Al2O3. In general, the leaching of the metals during ozonation for the applied catalysts was quite low.

**Figure 16.** Leaching of aluminum and Platinum (%) in the catalytic ozonation of DCF at 20 ◦C.

#### **3. Materials and Methods**

#### *3.1. Chemicals*

DCF (2-[(2,6-dichlorophenyl)amino]- phenylacetic acid, C14H11Cl2NO2, MW: 296.148 g/mol, CAS number: 15307-86-5, >98% purity) was purchased from Sigma-Aldrich, St. Louis, MO, USA). The water used in the LC-MS analysis was purified using a Millipore Simplicity 185 system (Millipore S.A.S., Molsheim, France). The acetonitrile used in the LC-MS analysis was of LC-MS grade obtained from Fischer scientificTM. Finland, ammonium formate was obtained from Fischer Scientific. Ozone was produced in situ using Nano ozonator. The catalysts used were synthesized by the methods described below.

#### *3.2. Preparation of Pt-Modified H-MCM-22-100, H-Y-12 and* γ*-Al2O3 Catalysts*

Six different heterogeneous catalysts H-MCM-22-100, 1 wt % Pt-MCM-22-100-EIM, 2 wt % Pt-MCM-22-100-EIM, 5 wt % Pt-MCM-22-100-EIM, Pt-H-Y-12-IE and Pt-γ-Al2O3 (UOP)-IMP were synthesized. The Na-MCM-22-100 was synthesized in the laboratory [45]. The Pt-MCM-22-100 modified catalysts were prepared by the evaporation impregnation (EIM) method. The modification of the MCM-22-100 and Pt(NO3)2 solution was carried out in a rotator evaporator at 60 ◦C for 24 h. After the modification, the Pt-MCM-22-100 catalysts were dried at 100 ◦C in oven and calcined at 450 ◦C for 240 min in muffle oven [46]. In addition, the Pt-γ-Al2O3 (UOP)-IMP likewise was prepared by EIM method. Pt-H-Y-12-IE was prepared by solution ion exchange (IE) method, which was carried out in a beaker using an aqueous solution of Pt(NO3)2 and H-Y-12 for 24h. The catalyst was filtered and washed with two liters of distilled water and the Pt-H-Y-12-IE catalyst was dried in an oven at 100 ◦C. Later, the Pt-H-Y-12-IE catalyst was calcined at 450 ◦C in a muffle oven for 4.5 h.

#### *3.3. Catalytic Physico-Chemical Characterizations*

The crystallinity and structural properties of the catalysts were characterized using a powder X-ray diffraction (XRD) with PANalytical Empyrean diffractometer in Bragg-Brentano mode. The incident X-ray beam was collimated with parallel beam optics consisting of a 1/2 divergence slit, a 10 mm mask, a 0.04 rad Soller, a Göbel mirror and 1 antiscatter slit. On the diffracted side, a 7.5 mm antiscatter slit, a 0.04 rad Soller and a PIXcel3D detector array by 255 × 255 pixels were applied. The Göbel mirror monochromatizes the beam into Cu-Kα1 and Cu-Kα2 lines giving an average wavelength of λ = 1.542 Å. The powder samples were determined on a siliceous zero background sample holder from 5◦ to 100◦ including a step size of 0.026 and counting for 120 s per step. The XRD data was fitted via the Rietveld refinement using the MAUD software [47]. The Pt particle size distributions and structural properties were measured from images obtained with JEM-1400 Plus transmission electron microscope (TEM, model JEM 1400 plus: Jeol Ltd., Tokyo, Japan) by a voltage of 120 kV. The specific surface area and pore volume were measured by nitrogen physisorption using the Sorptomatic 1900 apparatus Carlo Erba Instrument. The fresh catalysts outgassed for 3 h at 150 ◦C while the catalyst in the ozonation process was outgassed for 3 h at 100 ◦C. The specific surface areas and pore distribution were interpreted by using the BET: nitrogen physisorption (Carlo Erba Sorptomatic 1900-Fisons Instruments, Milan, Italy) and Dubinin equations. The morphology and crystal size distribution catalysts were studied by a scanning electron microscope Zeiss Leo 1530 Gemini equipped by ThermoNORAN vantage X-ray detector. Energy-dispersive X-ray analysis (EDXA: Zeiss Leo Gemini 1530, oberkochen, Germany) was carried out with the same instrument to analyze the Pt and Al contents of the catalysts.

The amount of *Brønsted* and *Lewis* acid sites in the catalysts were determined by FTIR (ATI Mattson Infinity series, Madison, U.S.A) using pyridine as a probe molecules. Molar extinction coefficients from Emeis were used to quantify *Brønsted* and *Lewis* acid sites at bands 1545 cm−1 and 1455 cm−1 through pyridine as probe molecule.

X-ray photoelectron spectroscopy (XPS) was used to investigate the oxidation state of Pt. A Kratos Axis Ultra DLD electron spectrometer with monochromated Al K source was operated at 150 W. Analyser pass energy of 160 eV for acquiring wide spectra and a pass energy of 20 eV for individual photoelectron lines were used. The surface potential was stabilized with the spectrometer charge neutralization system and the binding energy (BE) scale was referenced to the C 1s line of aliphatic carbon, set at 285.0 eV. Processing of the spectra was accomplished with the Kratos software. The powder samples were gently hand-pressed into a pellet directly on a sample holder using clean Ni spatula. Energy dispersive X-ray microanalysis was used to determine the chemical compositions of fresh catalytic materials.

The content of metals in the spent catalysts after catalytic ozonation process was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES: PerkinElmer precisely. Selton, USA), using an Optima 5300 DV Perkin Elmer instrument to investigate the potential leaching of Pt and Al.

#### *3.4. Kinetic Experiments*

Ozonation experiments were conducted in the presence and absence of heterogeneous catalysts in a double jacket glass reactor having a 1000 mL capacity. The reactor vessel was covered by aluminum foil to prevent photo-degradation. The reactor was connected to an ozone generator (Absolute Ozone, Nano model, Canada), and the gas was continuously dispersed to the liquid by a 7 μm disperser placed on the bottom of the reactor vessel. The kinetic experiments were operating in two phases (gas-liquid) when no catalyst was used, or in three phases (gas, liquid and solid) when a catalyst was used. The gas flow rate in these experiments was 110 mL/min (mixed oxygen 108.5 mL/min and nitrogen 1.5 mL/min) which produced 21 mg/L of ozone in gas phase (determined by iodine volumetric titration). In the liquid phase, 0.03 g of DCF was dissolved in 1000 mL of de-ionized water to provide 30 mg/L of DCF solution. The solid catalyst was, immobilized in a SpinchemTM rotating bed reactor using a 200 μm mesh that allowed the trapping of the catalyst (150–500 μm catalyst particle sizes) inside the stirrer. The stirring rate was 900 rpm to provide a high mass transfer rate between the three phases. Due to a rapid reaction of DCF by ozone, the total reaction time was 10 min and samples were collected at 0, 0.5, 1, 1.5, 2, 3, 4 and 10 min, respectively. The dissolved ozone concentration was 0.44 mg/L, determined by the indigo method. Because this reaction is so rapid compared to ibuprofen degradation described in our earlier study (2018) [46] a much smaller gas flow (110 mL/min) was used and therefore dissolved ozone concentration was only 0.44 mg/L. Even if 30 mg/L of DCF is much higher than ordinarily

detected in the aquatic environments using a concentration of 30 mg/L enables to identify and classify the transformed by-products at low concentration level (Figure 17).

**Figure 17.** Schematic view of the semi-batch reactor system for the ozonation of DCF.

#### *3.5. Quantification of DCF*

DCF was quantified by using the LC-UV method. The chromatographic separation was performed using an Agilent 1100 binary pump equipped with a vacuum degasser, an autosampler, a thermostated column oven set to 30 ◦C, a variable wavelength detector, and a Waters Atlantis T3 C18 column 2.1 × 100 mm, 3 μm) with a pre-column made from the same material. The eluents were 0.1% formic acid in water (A) and 0.1% formic acid in acetonitrile (B). Initially the composition was kept at 30% (B) for 2 min, then the composition increased linearly to 95% (B) over 8 min. The eluent composition was held at 95% (B) for 1 min before being returned to the initial conditions over the next 0.1 min and given 8.9 min for equilibration. The gas flow rate was 0.3 mL/min and the injection volume was 30 μL. The detector was set to 254 nm. For quantification, a seven-point calibration curve was prepared in water by diluting the stock solution. The ozonated samples were injected without sample preparation.

#### *3.6. Quantification of 4'-OH-DCF and 5-OH-DCF*

For LC-MS/MS analysis of 4'-OH-DCF and 5-OH-DCF, an Agilent 6460 triple quadrupole mass spectrometer equipped with an Agilent Jet Spray electrospray ionization (ESI source was used in full scan and MSn scan modes. Manufacturer: Agilent Technologies, USA) source was used in the multiple reaction monitoring (MRM) mode. Nitrogen was used as the drying, sheath, nebulizer, and collision gas. The drying gas and sheath gas were held at 6 and 12 L/min respectively and the gas flow and sheath gas flow were heated to 180 and 400 ◦C, respectively. The nebulizer pressure was set to 20 psi. A capillary voltage of 1500 V and a nozzle voltage of 500 V were applied. The cell acceleration voltage was 3 V. The fragmentor voltage and collision energy were optimized for the compounds individually using the Mass Hunter Optimizer software (Table 6). The chromatographic separation was performed using an Agilent 1290 binary pump equipped with a vacuum degasser, an autosampler, a thermostated column oven set to 30 ◦C, and a Waters xbrigde C18 column (2.1 × 50 mm, 3 μm) with a pre-column made from the same material. The eluents used were 10 mM ammonium formate in water (A) and acetonitrile (B). Initially the composition was held at 10% (B) for 0.5 min, the composition increased linearly to 30% (B) over 2.5 min, followed by a linear increase to 95% (B) over 1 min. The eluent

composition was held at 95% (B) for 1 min before being returned to the initial conditions over the next 0.1 min and given 1.4 min for equilibration. The flow rate was 0.4 mL/min and the injection volume was 10 μL. The internal standard method was used for the quantification. The quantification was performed using a ten-point calibration curve prepared in water. An internal standard (100 ng/mL) was added to the calibration samples. The ozonated samples were prepared by adding 50 μL of internal standard solution (1000 ng/mL) to 450 μL of the sample.

**Table 6.** Mass spectrometric parameters for DCF transformation products.

#### **4. Conclusions**

Pt-modified H-MCM-22-100, H-Y-12 and γ-Al2O3 catalysts were synthesized using evaporation impregnation method. The physico-chemical characterization of the catalytic materials was carried out using several techniques such as XRD, SEM, EDX, TEM, XPS and N2-physisorption. Pt-modified H-MCM-22-100, H-Y-12 and Al2O3 catalysts were observed to be active in the degradation of diclofenac. Furthermore, by-products of diclofenac such as 5-hydroxydiclofenac, 4-hydroxydiclofenac were removed from the aqueous phase using 1 wt % Pt-H-MCM-22-100-EIM, 2 wt % Pt-H-MCM-22-100-EIM, 5 wt % Pt-H-MCM-22-100-EIM, Pt-γ-Al2O3 (UOP)-IMP and Pt-H-Y-12-EIM and catalysts. The main selected factors for this work were the concentration of Pt on MCM-22-100 catalyst, concentration of catalysts in ozonation process and different types of Pt-modified catalysts for degradation of DCF. The highest degradation rate of DCF, side products 5-hydroxy diclofenac and 4-hydroxydeclofenac was obtained using 2 wt % Pt-MCM-22-100-EIM catalyst. It was observed that 0.5 g/L and 1 g/L amount of 2 wt % Pt-MCM-22-100-EIM catalyst gives a similar degradation efficiency of DCF and ozonation by-products. Moreover, most of applied catalysts were exhibited low decrease in the surface area and pore volume, which it was expressed that these kinds of catalysts could be used again.

**Author Contributions:** S.S. catalytic and non-catalytic ozonation experiments, preparation and characterization of catalysts, writing. M.K. analysis and quantification of DCF and by-product. senior scientists and supervisors with the following competences: P.T.: ozonation technology, N.K.: catalyst specialist, K.E.: reactor design, T.S.: chemical kinetics and experimental planning, J.-P.M.: stirrer expert, L.K.: analysis of organic components in aqueous environment, P.E.: organic reaction technology, H.P.: XRD expert, M.P.: TEM expert, A.A.: FTIR expert, A.S.: XPS expert. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded Centret for internationell mobilitet och internationellt samarbete (CIMO), 4.2.2016. Finnish fondation for technology promotion (TES), 2016. Finnish fondation for technology promotion (TES), 2017.Svenska litteratursällskapet i Finland (SLS), 21.11.2016. graduate school of Åbo Ackademi: Graduate School in Chemical Engineering (GSCE) for 2018 and 2019.

**Acknowledgments:** This work is part of the activities of the Johan Gadolin Process Chemistry Centre (PCC) at the Åbo Akademi University. The foundation support from Graduate School in Chemical Engineering (GSCE) is gratefully acknowledged (by Soudabeh Saeid). Spinchem AB is acknowledged for providing the Rotating Bed Reactor.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

#### *Article*

## **Catalytic Properties of Microporous Zeolite Catalysts in Synthesis of Isosorbide from Sorbitol by Dehydration**

### **Sangmin Jeong 1, Ki-Joon Jeon 1, Young-Kwon Park 2, Byung-Joo Kim 3, Kyong-Hwan Chung 4 and Sang-Chul Jung 4,\***

Received: 27 December 2019; Accepted: 21 January 2020; Published: 23 January 2020

**Abstract:** As bisphenol A has been found to cause hormonal disturbances, the natural biomaterial isosorbide is emerging as a substitute. In this study, a method for isosorbide synthesis from sorbitol was proposed by dehydration under high temperature and high pressure reaction. Microporous zeolites and Amberlyst 35 solid acids with various acid strengths and pore characteristics were applied as catalysts. In the synthesis of isosorbide from sorbitol, the acidity of the catalyst was the main factor. MOR and MFI zeolite catalysts with high acid strength and small pore size showed low conversion of sorbitol and low yield of isosorbide. On the other hand, the conversion of sorbitol was high in BEA zeolite with moderate acid strength. Amberlyst 35 solid acid catalysts showed a relatively high conversion of sorbitol, but low yield of isosorbide. The Amberlyst 35 solid acid catalyst without micropores did not show any inhibitory effects on the production of by-products. However, in the BEA zeolite catalyst, which has a relatively large pore structure compared with the MOR and MFI zeolites, the formation of by-products was suppressed in the pores, thereby improving the yield of isosorbide.

**Keywords:** isosorbide; solid acid catalyst; sorbitol; dehydration; bisphenol A

#### **1. Introduction**

Bisphenol A (4,4'-isopropylidenediphenol, BPA) is a monomer synthesized by the condensation of acetone with two molecules of phenol. BPA is a typical female hormone disruptor and is a raw material of epoxy resin or polycarbonate. BPA has also been used as a coating material to prevent cans from corrosion, or as a plastic additive to increase heat resistance and durability. Polycarbonate has high heat resistance and transparency and is widely used for household goods such as water bottles, baby bottles, food storage containers, and industrial products such as CDs. In addition, BPA is used in a number of products used in daily life such as disposable paper cups and thermal paper receipts. However, several studies have raised the risks of BPA including endocrine disruption, metabolic disorders, hypertension, and premature maturation [1,2]. When food is stored in packaging using BPA, a report suggested that BPA may elute and affect people through food, limiting the use of BPA [3]. In particular, infants were considered to be sensitive to BPA exposure, and their use was completely prohibited [4–9].

Due to the potential risks of BPA, various attempts have been made to replace BPA as regulations progress. The first attempts were made to utilize the same bisphenol-based materials, bisphenol E (BPE), bisphenol S (BPS), and bisphenol F (BPF) [10]. The study on amphibians has shown that BPA inhibits the activity of γ-secretase in early amphibian embryos. However, this phenomenon did not occur when treated with the same concentration of BPE and BPF [11]. Although BPA requires two methyl groups to inhibit γ-secretase activity, it was concluded that BPE and BPF did not have teratogenic effects such as BPA because there was only one methyl group in each. BPS and BPF have almost the same physical properties as BPA, and have been used in many products such as can coatings, thermal paper receipts, and polycarbonates [12].

Indeed, the dangers of BPA have been emphasized and as an alternative, BPA-free products have begun to be used. However, BPA, BPF, and BPS have similar chemical structures to BPA, and these alternatives have also been shown to cause endocrine disruption [13]. Papers comparing BPF and BPA reported the same anti-androgen activity in proportion to the concentration of both substances in vitro [14–17]. The same risks as BPA were seen in BPS as well as in BPF. Recently published studies have reported a 180–240% increase in neuronal development near the hypothalamus of zebra fish, even at very low levels (0.0068 μM) of BPA and BPS, suggesting a link between BPA and BPS.

BPA, phthalates, and nonylphenols, which are the major environmental hormone obstacles, are the main raw materials of plastics and detergents and have a high exposure in daily life [18]. Recognition of the dangers of environmental hormonal barriers is driving global regulations on these substances and replacing existing products with products that use less dangerous ingredients. However, since there are no clear alternatives right now, these environmental hormonal obstacles are still being used. Research continues to raise concerns about alternatives, as some alternatives have been shown to present similar risks to existing environmental hormone barriers [19].

As BPA substitutes using synthetic compounds still show endocrine disrupting effects, efforts have been made to find alternatives in natural products. Sorbitol is a major source of biomass as a useful chemical [20–24] and is considered one of the 10 most important sources of biomass [25]. Generally, sorbitol is produced through hydrogenation of glucose derived from starch. In the last decade, much effort has been made to produce sorbitol from cellulose through hydrolysis hydrogenation. Much research has expanded the potential of sorbitol as a bio-based feedstock and is being produced in high yield from cellulose [26–36].

Isosorbide is a 100% natural biomaterial made from corn. Isosorbide (1,4:3,6-dianhydro-D-sorbitol) is produced through the dehydration of the double molecule of sorbitol. Starch is extracted from corn and is made by glucose, sorbitol, etc. Isosorbide is produced by enzymatic glycosylation of starch from corn, followed by hydrogenation, followed by dehydration. Isosorbide is widely used in a variety of industries such as the pharmaceutical industry due to its high stability, two symmetric OH groups, and its unique properties [37,38]. Another important application of isosorbide is that it is used as a plastic monomer [39,40]. Poly(ethylene-co-isosorbide) terephthalate is a bio-based alternative to polyethylene terephthalate (PET) and has a higher glass transition temperature than PET [41]. Isosorbide is also attracting attention as a highly functional material that can replace BPA in polycarbonate and epoxy resin production [42].

Plastics made of isosorbide as the raw material have distinguished advantages such as superior transparency and surface hardness as well as eco-friendly properties such as biodegradability and nontoxicity, compared to plastics made of petrochemicals. It is expected to be widely used for exterior materials of electronic devices such as mobile devices and TVs, liquid crystal films of smart phones, automobile dashboards, food containers, and eco-friendly building materials. Isosorbide, which can be synthesized from plants such as corn, is attracting attention as an environmentally friendly alternative due to its high biodegradability and excellent heat resistance [43].

Zeolites have well-defined structures that are microporous crystalline solids. Generally, they are composed of silicon, aluminum, and oxygen in their framework and cations. They have been applied in many fields of catalysis, generating intense interest in these materials in industrial and academic laboratories. They present appreciable acid activity with shape-selective features as catalysts [44,45]. The FAU zeolite consists of 12-oxygen rings, with pores with a of diameter 7.4 Å in a tetrahedral configuration, with large pores called supercages at the intersection. The MFI zeolite has an unusual structure in which straight pores with 5.4 × 5.6 Å cross sections and zig-zag pores with 5.1 × 5.5 Å intersect each other. The pores of the MWW zeolite consist of two independent 10-oxygen rings. The two-dimensional 10-oxygen ring is sinusoidal and the other is composed of a 10-oxygen ring and 12-oxygen ring. The pores of the BEA zeolite consisted of 12-oxygen rings, and the pore size parallel to the [001] plane was small, but the pore size of the [100] plane was large. The pores were curved like sinusoids. The MOR zeolite has a straight main hole consisting of 12-oxygen rings and pores consisting of 8-oxygen rings there between.

In this study, we propose a method for isosorbide synthesis that is drawing attention as an alternative to BPA. Sorbitol is used as a raw material to investigate the reaction characteristics of isosorbide synthesis and to find the optimum catalyst. Various zeolites were introduced as a solid acid catalyst. The dehydration reaction of sorbitol, according to the physicochemical properties of the catalyst, and the synthesis process of isosorbide are discussed.

#### **2. Results and Discussion**

#### *2.1. Characteristics of the Catalysts*

The X-ray diffraction pattern of the zeolite catalyst used in the reaction is shown in Figure 1. The characteristic diffraction peaks of the synthesized zeolites agreed well with the position and size ratio of the characteristic diffraction peaks presented in the literature [46]. SEM images of the zeolites are shown in Figure 2. The particles of the BEA, FAU, MFI zeolite were small regular cubes. The crystal size of the MOR zeolite was rod-shaped, about 1.5 μm. The surface of the MWW zeolite was lumpy in shape, very tangled. The particle size of BEA zeolite was very small, 0.2 μm or less. The FAU and MFI zeolites were uniform in size, approximately 0.5 μm. Figure 3 shows the nitrogen adsorption isotherms of the zeolite catalysts. Zeolites used in experiments such as the MFI zeolite are zeolites with microporous development and have a typical Langmuir adsorption isotherm pattern due to the large number of micropores. On the other hand, the relatively large pore size and the wide surface area of the BEA zeolite increased the adsorption amount at a relative pressure of 0.2~0.7. The reason for the large adsorption amount in the BEA and FAU zeolites is that the pore size of the zeolite is large and there is a lot of empty space such as a supercage in the pore.

**Figure 1.** XRD patterns of the MWW, BEA, MOR, FAU, and MFI zeolites.

**Figure 2.** SEM images of the MWW, BEA, MOR, FAU, and MFI zeolites.

**Figure 3.** NH3-temperature programmed desorption (TPD) profiles of the various zeolites.

The BET surface area and pore structure parameters obtained from nitrogen adsorption isotherms are summarized in Table 1. Unlike other zeolites such as MOR, the MFI zeolite has a pore size of 10-membered oxygen rings, and thus the pore size is smaller than that of other zeolites. MOR zeolites have a straight pore structure, whereas MFI zeolites are bent in a zigzag shape. The BET surface area was larger in the order of BEA > FAU > MOR > MFI zeolite.

**Table 1.** Physical properties of the zeolite catalysts used in this work.

1 determined from the t-plot method.

The NH3-TPD results are shown in Figure 4 to compare the acidity of zeolites from the ammonia desorption curve. Only one bud appeared in the TPD curve of the FAU zeolite shown on the left side of the figure. On the other hand, the TPD curves of other zeolites can be divided into two desorption buds around 200 ◦C and 300–500 ◦C. It is known that buds appearing at low temperatures are due to the desorption of physisorbed ammonia and buds appearing at high temperatures due to the desorption of ammonia adsorbed at strong acid sites [47]. The lower temperature and smaller size of the second desorption bud of the MWW and BEA zeolite means weak and low acidity. Acid strength can be estimated by the maximum peak temperature (*T*max) of the desorption peak associated with the activation energy for ammonia desorption [48]. The acid strength of the MWW zeolite was similar to that of the MFI zeolite. The order of acid strength was FAU BEA < MWW MFI < MOR. MOR zeolites had the highest acidity compared to other zeolites, with the highest peak temperature of the second desorption bud at 480 ◦C. The MFI zeolite had the largest number of scattering points as the second detachable bud was the largest.

**Figure 4.** NH3-TPD profiles from the MWW, BEA, MOR, FAU and MFI zeolites.

Figure 5 shows the adsorption behavior of *o*-xylene on the MWW, BEA, MOR, FAU, and MFI zeolites. The behavior shows the adsorption of *o*-xylene inside the pores of the zeolite. *o*-xylene was rapidly adsorbed on BEA and FAU with a large pore space. The *o*-xylene adsorption levels at MOR and MWW were lower than for the BEA and FAU zeolite catalysts. Adsorption of *o*-xylene on the MFI zeolite with narrow pore entrance and curved pore structure was slower than that of other zeolites. This suggests that steric hindrance due to various pore structures of zeolites can affect the reaction.

**Figure 5.** Adsorption of *o*-xylene on the MWW, BEA, MOR, FAU, and MFI zeolites.

#### *2.2. Reaction Characteristics of Isosorbide Synthesis on the Catalysts*

As shown in Scheme 1, sorbitol is dehydrated to either 1,4-sorbitan, 1,5-sorbitan, or 2,5-sorbitan. 1,4-sorbitan reacts further to isosorbide, while 1,5- and 2,5-sorbitan did not exhibit in the high-performance liquid chromatography (HPLC) chromatogram. Analytical peaks of the products analyzed by HPLC are shown in Figure 6. Detection peaks for the raw and reactant sorbitol, the intermediate product 1,4-sorbitan, and the desired product isosorbide were observed. Figure 7

shows the conversion of sorbitol and the yield of isosorbide according to the reaction temperature in the BEA zeolite catalyst. At the reaction temperature in the range of 160 ◦C to 220 ◦C, the internal pressure of the autoclave ranged from 7 bar to 10 bar. The conversion of sorbitol and the yield of isosorbide were the highest at the reaction temperature of 200 ◦C and decreased with increasing reaction temperature. If the reaction temperature is higher than 200 ◦C, it appears that the catalyst deactivation occurs by carbon deposition.

**Scheme 1.** Reaction scheme of isosorbide synthesis from sorbitol dehydration.

**Figure 6.** High-performance liquid chromatography (HPLC) chromatogram of products.

**Figure 7.** Conversion of sorbitol and yield of isosorbide by dehydration reaction over the BEA zeolite catalyst.

Figure 8 shows the change of sorbitol conversion and isosorbide yield in the BEA and FAU zeolites and Amberlyst 35 solid acid catalyst. The conversion of sorbitol and isosorbide yield in the MFI zeolite with high acid strength, but small pore size was lower than that of the BEA zeolite or Amberlyst 35. In comparison, the BEA zeolite showed the highest sorbitol conversion and isosorbide yield among the applied catalysts. The conversion of sorbitol with a relatively large molecular size was higher in the BEA zeolite with a larger pore size than the MFI zeolite with a smaller pore size and higher reactivity at medium acid strength than strong acid strength. Amberlyst 35 catalysts had relatively high conversions, but slightly lower yields of isosorbide. It is suggested that this was caused by not showing the effect of inhibiting the formation of by-products by the pores in the conversion reaction. Table 2 summarizes the sorbitol conversion and isosorbide yields investigated by applying various acid catalysts. As can be seen from this result, the BEA zeolite showed the best reactivity in this synthesis reaction. If the acid strength of the catalyst was too strong, the conversion of sorbitol and the yield of isosorbide were very low. The conversion of sorbitol was high in the catalyst with a medium acid strength. MOR and MFI zeolite catalysts with high acid strength and small pore size showed a very low conversion of sorbitol and yield of isosorbide. The conversion of sorbitol was the highest in the BEA zeolite catalyst and the yield of isosorbide was higher than that of other catalysts. 1,4-sorbitan was mainly obtained as a by-product. It was determined that the yield was increased because the production of by-products during the reaction was suppressed by the steric effect of the pore structure of the BEA zeolite.

**Figure 8.** Conversion of sorbitol and isosorbide yield as a function of process time on the (**a**) BEA zeolite, (**b**) Amberlyst 35, and (**c**) FAU zeolite.

**(h)**

**Table 2.** Synthesis of isosorbide by dehydration of sorbitol on various catalysts after 5 h of process time at 200 ◦C.

#### **3. Experimental**

#### *3.1. Materials and Catalysts*

D-sorbitol (99%, Sigma-Aldrich, St. Louis, MO, USA) was used as a reactant, and mixed with a catalyst for the dehydration reaction in an autoclave reactor. The catalysts were zeolite acid catalysts with micropores and an Amberlyst 35 (Sigma-Aldrich, St. Louis, MO, USA) solid acid catalyst. The zeolites were MOR, MFI, FAU, BEA, and MWW zeolite catalysts with different pore sizes and different acidity. MOR zeolites (Si/Al = 10)) with a Si/Al molar ratio of 10 was purchased from Tosoh. To convert to the H-type MOR zeolite, Na+ in MOR zeolite was ion-exchanged with a solution of 0.5 M ammonium nitrate (>99 wt%, Duksan, Seoul, Korea) at 60 ◦C and calcined at 550 ◦C for 6 h to form H-type MOR zeolite.

The MFI zeolite was synthesized from the liquor containing a mixture of colloidal silica (Ludox, 40 wt% SiO2, Sigma-Aldrich, St. Louis, MO, USA), aluminum hydroxide (64 wt%, Sigma-Aldrich, St. Louis, MO, USA), potassium hydroxide (80 wt%, Duksan, Seoul, Korea), and secondary distilled water. After aging for 12 hours, the mixture was heated at 190 ◦C for two days in a high pressure reactor. The Si/Al molar ratio of the MFI zeolite synthesized by the concentration of the synthetic mother liquor was 25. The MWW (Si/Al = 13) zeolite was prepared according to the methodology reported elsewhere [49]. The ion exchange process for converting the H-type MFI zeolite was applied in the same manner as in the preparation of the MOR zeolite. BEA and FAU zeolites were prepared by ion-exchanging Na-BEA (Si/Al = 13, Zeolyst Co., Kansas City, USA) and Na-FAU (Si/Al = 5, Zeolyst Co., Kansas City, USA) in the same manner as above. BEA and FAU zeolites were also washed, dried, and calcined. The zeolite used in the experiment was named MOR, MFI, FAU, BEA, and MWW according to the International Zeolite Association (IZA) code name, and the Si/Al molar ratio was written in parentheses after the name. SBA-15 was synthesized according to the methods presented in the literature [50].

#### *3.2. Preparation of Isosorbide from Sorbitol*

Isosorbide was synthesized by the dehydration reaction at high temperature and high pressure. As a reactant, 0.5 g of D-sorbitol (99%, Sigma-Aldrich, St. Louis, MO, USA) was dissolved in 20 mL of distilled water, followed by reaction with 0.1 g of a catalyst. The reactor used a batch stainless autoclave capable of temperature maintenance and magnetic stirring. The reactant and the catalyst were added to the reactor and stirred at 500 rpm. The dehydration reaction was carried out in the temperature range of 180 ◦C to 220 ◦C. The composition of the product produced by the reaction was analyzed for composition using high-performance liquid chromatography (HPLC; Shimadzu, LC-20A, Kyoto, Japan) equipped with a refractive index detector and a REZEX RCM-monosaccharide column.

#### *3.3. Characterization of the Catalysts and Products*

The X-ray diffraction (XRD) pattern of the zeolite was irradiated with an X-ray diffractometer (D/MAX Ultima III, Rigaku, Tokyo, Japan). CuKα X-rays passed through the Ni-filter at 40 kV and 40 mA conditions were injected at a rate of 2◦/min over a range of 5–50◦. The particle size and shape of the zeolite was observed by scanning electron microscope (SEM; Hitachi, S-4700, Tokyo, Japan). The Si/Al molar ratio was calculated by measuring the silicon and aluminum content with energy-dispersive X-ray spectroscopy (EDX; Horiba, EX-250, Tokyo, Japan) mounted on the SEM. Nitrogen adsorption isotherms were measured by a nano-porosity analyzer (Nano Porosity Analyzer, Mirae SI Co., nanoPOROSITY-XQ, Gwangju, Korea). After exhausting at 150 ◦C for 2 h, nitrogen was adsorbed at −196 ◦C. Surface area was calculated using the Brunauer–Emmett–Teller (BET) equation.

In order to investigate the acidity of the zeolite catalyst, the ammonia elevated temperature desorption (NH3-TPD) curve was drawn using a temperature desorption test apparatus (Bel Japan, BELCAT, Osaka, Japan). The catalyst was evacuated at 550 ◦C for 1 h in a helium stream and then cooled to 150 ◦C. Ammonia gas was sent in pulses to the catalyst and adsorbed until saturated. In order to remove the physically adsorbed ammonia, it was exhausted while flowing a helium stream for 1 h, and the ammonia desorbed while analyzing the temperature. The temperature was raised to 600 ◦C at a rate of 10 ◦C/min. The amount of the acid point of the zeolite catalyst was calculated by deconvolution of the measured TPD curve to obtain the amount of weak and strong acid points.

#### **4. Conclusions**

Sorbitol was used as a raw material to prepare isosorbide by high temperature and high pressure reaction. Sorbitol conversion was higher for the BEA zeolites and Amberlyst 35 solid acid catalysts with moderate acid strengths than for the acidic MOR or MFI zeolites. The yield of isosorbide was not high in the MOR or MFI zeolite catalysts with a small pore size, but relatively high in the BEA zeolite catalyst. It was shown that the relatively large molecular size of sorbitol was enhanced by the inhibitory effect of the byproduct formation in the BEA zeolite catalysts with pores of similar size to the reactant and product molecular sizes than the zeolite catalysts with small pore sizes. The Amberlyst 35 solid acid had a relatively high conversion of sorbitol, but not a high yield of isosorbide. This was caused by not showing the effect of inhibiting the formation of by-products by pores in the conversion reaction. In contrast, the BEA zeolite was found to increase the yield because it suppresses the formation of by-products by the pore structure of the catalyst.

**Author Contributions:** S.J., K.-H.C., and S.-C.J. designed the experiments. S.J. and K.-H.C. performed the experiments and wrote the original drafted paper. K.-J.J. interpreted the data. Y.-K.P. and B.-J.K. contributed to the analysis. S.-C.J. supervised the experiments and paper. All authors discussed the results and contributed to the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** We do not have Funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*

### **Advanced Oxidation Process for Degradation of Carbamazepine from Aqueous Solution: Influence of Metal Modified Microporous, Mesoporous Catalysts on the Ozonation Process**

**Soudabeh Saeid 1, Matilda Kråkström 2, Pasi Tolvanen 1, Narendra Kumar 1,\*, Kari Eränen 1, Jyri-Pekka Mikkola 1,3, Leif Kronberg 2, Patrik Eklund 2, Markus Peurla 4, Atte Aho 1, Andrey Shchukarev 3 and Tapio Salmi 1,\***

Received: 22 November 2019; Accepted: 30 December 2019; Published: 7 January 2020

**Abstract:** Carbamazepine (CBZ), a widely used pharmaceutical compound, is one of the most detected drugs in surface waters. The purpose of this work was to identify an active and durable catalyst, which, in combination with an ozonation process, could be used to remove CBZ and its degradation products. It was found that the CBZ was completely transformed after ozonation within the first minutes of the treatment. However, the resulting degradation products, 1-(2-benzaldehyde)- 4-hydro-(1H,3H)-quinazoline-2-one (BQM) and 1-(2-benzaldehyde)-(1H,3H)-quinazoline-2,4-dione (BQD), were more resistant during the ozonation process. The formation and degradation of these products were studied in more detail and a thorough catalytic screening was conducted to reveal the reaction kinetics of both the CBZ and its degradation products. The work was performed by non-catalytic ozonation and with six different heterogeneous catalysts (Pt-MCM-41-IS, Ru-MCM-41-IS, Pd-H-Y-12-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM) operating at two temperatures 20 ◦C and 50 ◦C. The influence of temperature on degradation kinetics of CBZ, BQM and BQD was studied. The results exhibited a notable difference in the catalytic behavior by varying temperature. The higher reactor temperature (50 ◦C) showed a higher activity of the catalysts but a lower concentration of dissolved ozone. Most of the catalysts exhibited higher removal rate for BQM and BQD compared to non-catalytic experiments in both temperatures. The Pd-H-Y-12-EIM catalyst illustrated a higher degradation rate of by-products at 50 ◦C compared to other catalysts.

**Keywords:** carbamazepine; ozone; zeolites; catalysts synthesis and characterization; catalytic ozonation

#### **1. Introduction**

The frequent appearance of pharmaceuticals as high as mg/L levels in the in ground water and surface waters has been reported in several studies during the last decades [1]. A notable attention has

been attracted due to their potential hazard effects on the aquatic ecosystem and human health [2,3]. An emerging concern exists of what exactly happens with the pharmaceutical contaminants in the aquatic environment. The origin of these compounds comes from the fact that they have been employed in several domains of the human activity, for example, human medication, veterinary purposes and aquaculture, as well as large concentrations of them leaking into the sea from pharmaceutical industries. Conventional wastewater treatment is not able to efficiently eliminate most of these pharmaceuticals from wastewaters, not to mention their degradation products [4,5].

Carbamazepine (5H-dibenzo (b,f) azepine-5-carboxamide, shortly CBZ) is a psychiatric pharmaceutical used for treatment of epilepsy and trigeminal neuralgia a chronic pain disorder [6]. CBZ was displayed in a list of essential medicine requirements for a basic health-care in the World Health Organization (WHO) Model List of Essential Medicines (March 2017) in the group of anticonvulsant and antiepileptic pharmaceuticals [7]. Only 28% of CBZ is metabolized by the human body, while the rest is excreted during urination. Before that, it is enzymatically transformed to 10, 11-epoxy-10, 11-dihydro-CBM, and from the hydrolysis of these intermediates, CBZ-diOH is produced. For this reason, CBZ-diOH was one the most abundant CBZ compounds detected in water [8]. CBZ is one of the commonly detected compounds in the effluents. It is resistant against biodegradation, and sometimes the concentration in the effluent has been larger than influent, perhaps due to the conjugates originating from the parent compounds, i.e., hydrolysis through biological treatment [9]. The CBZ and metabolite removal efficiency was −11–18% toward human waste treatment plants, −40% to 25% in the sewage treatment plants and −100–56% in hospital wastewater treatment plants. The concentration of CBZ in the influent and effluent samples were abnormal, this is due to the fact of the low removal rate of CBZ [8].

CBZ has been detected as good as everywhere in the Baltic Sea, in Northern Europe. However, owing to the long turnover time (a very long half-life time in the Baltic Sea, exceeding 3.5 years) and a low elimination rate of CBZ, the wastewater treatment plants exhibit negative removal efficiencies (for instance in Finland with a −41% removal efficiency in wastewater treatment plants). Consequently, it is estimated that more than 55 tons of CBZ has accumulated in the Baltic Sea [10]. CBZ has been detected in the surface water of Milan up to 100 ng/L concentration and it is most frequent drug found in Serbian rivers [11,12]. The research of German and Portuguese surface waters revealed that CBZ and its metabolites were detected to 5000 ng/L in the surface waters, which they were barely decomposed during the wastewater treatment [13].

Significant concentrations of CBZ and five of its metabolites have been detected in urban groundwater in the Spanish rivers Poble Sec and Besòs. Moreover, at the delta of Besòs restored via a river polluted through treated effluent from several treatment plants, CBZ and two of CBZ hydroxyl metabolites were not eliminated following those degrading conditions [14]. Moreover, CBZ was detected in Germany, in the form of the human metabolites, and transformation by-products of CBZ in the several groundwater springs, surface waters and treated wastewater. The genotoxicity of these compounds determined by the so-called silico method (distributed structure-searchable toxicity). These research results revealed that these contaminants have a potential hazard to mammalians, including human beings [15].

The presence of CBZ in the environment poses a potential risk to the aquatic life and human health. This has led to increasing attention to the elimination of CBZ compound through the wastewater treatment plants before discharge into the water basin [16–18]. Several methods have been proposed for the removal of CBZ from wastewaters, for instance, an adsorptive method utilizing biosynthesized hematite nanoparticles. This study did not eliminate the pollutants but instead they were accumulated and pollutants were transferred from aqueous solution to the adsorbent phase [19]. Some another studies concerning an activated sludge system consisting of a membrane bioreactor, the removal was rather low, around 35% due to of recalcitrance [20], or gamma irradiation coupled with microbiological treatment. With this technology, almost 99% of CBZ could be removed, as well as decreasing the TOC by 80%. However the treatment duration was 250 h, which is a quite long period [21]. Advanced oxidation

processes (AOP) have as well been applied for degradation of CBZ such as, radiation-induced activation of peroxymonosulfate, which was reported by Wang et al. [16]. In this investigation, nine intermediate products were detected, and the scientists observed that the limiting step for CBZ mineralization in the system was the decomposition of CBZ intermediates. Low mineralization and hazard intermediates were the biggest obstacles with most of the CBZ treatment trials in the study.

Ozonation has been proposed and applied as a method for removing pharmaceuticals from hospital wastewaters, both in a laboratory and pilot scale. In the laboratory scale, the removal has been successful [22]. Besides, ozonation as an additional treatment step in wastewater treatment has become generally acknowledge for eliminating pharmaceuticals and other micro-pollutants [23]. To improve the ozonation process, more research is needed to study the formation of hydroxyl radicals and the formation of dangerous degradation products of ozonation [9]. Ozonation of CBZ gives some resistant by-products due to the ozone reaction by the olefin group in CBZ, producing an ozonide, which divides the double bond [24]. The major identified by-products from ozonation of CBZ were BQM and BQD [25]. Some methods have been proposed to improve the ozonation of CBZ due to reducing the by-product such as combination by soil aquifer treatment column that enhances degradation of BQD yet the other by-products were more resistance up to 5–6 days [26].

Catalytic ozonation is recommended as an AOP for wastewater treatment, due to enhancing the removal efficiency and its potential to mineralize the organic compounds created. Catalysts improve the decomposition of ozone in the water to produce extremely active hydroxyl radicals [27,28]. Several studies have revealed that coupling ozonation to heterogeneous catalysis is very efficient in oxidizing pharmaceuticals from wastewater. For instance, ozonation combined with a granular activated carbon (GAC) filter can dramatically diminish the CBZ content and its human metabolites [15,29]. Zeolite catalysts exhibited an improvement in the ozonation of the pharmaceutical compound paracetamol [30] and ibuprofen [31]. Moreover, loading of the metals on zeolite catalysts increases the catalytic ozonation efficiency [32].

In the current work, non-catalytic and catalytic ozonation was employed to study the removal of CBZ and its by-products in aqueous solutions. The task is challenging because the degradation rates of the by-products are relatively low. A high concentration of CBZ (30 mg/L) was selected to study the kinetics of the by-products of the degradation. For this purpose, the main products were isolated, and their concentration was monitored with high performance liquid chromatography (HPLC). In order to optimize the degradation of the by-products, the catalysts Pt-MCM-41-IS, Ru-MCM-41-IS, Pd-H-Y-12-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM were used, at two different reaction temperatures. The catalysts were characterized with revealed methods to explain their activities in the experiments.

#### **2. Results and Discussion**

#### *2.1. Physico-Chemical Characterization Results: Catalyst Structure and Surface Properties*

#### 2.1.1. Transmission Electron Microscopy

The TEM images of (a) Pt-MCM-41-IS, (b) Ru-MCM-41-IS, (c) Pd-H-Y-12-EIM, (d) Pt-H-Y-12-EIM, (e) Pd-H-Beta-300-EIM and (f) Cu-MCM-41-A-EIM catalysts and Pt, Ru, Pd and Cu particle size distributions presented in the form of histograms, are given in Figure 1a–f. The average metal particle size and the distributions were measured and counted from the TEM images, which are displayed in Table 1.

**Figure 1.** (**a**) TEM image and Pt particle size distribution histogram of Pt-MCM-41-IS, catalyst. (**b**) TEM image and Ru particle size distribution histogram of Ru-MCM-41-IS catalyst. (**c**) TEM image and Pd particle size distribution histogram of Pd-H-Y-12-EIM catalyst. (**d**) TEM image and Pt particle size distribution histogram of Pt-H-Y-12-EIM catalyst. (**e**) TEM image and Pd particle size distribution histogram of Pd-H-Beta-300-EIM catalyst. (**f**) TEM image and Cu particle size distribution histogram of Cu-MCM-41-A-EIM catalyst.

The smallest average Pt particle sizes were measured for Pt-H-Y-12-EIM (5.72 nm), whereas the second smallest average Pd particle was measured for Pd-H-Beta-300-EIM (6.11 nm). The largest average particle size was observed for Pt-MCM-41-IS (13.395 nm). The mesoporous material Ru-MCM-41-IS catalyst synthesized also exhibited large average particles (13.29 nm).

It was concluded from the results of averages particle sizes (Table 1), that the method of introducing Pt-, Ru- into the mesoporous MCM-41 influenced the sizes of the particles. Hence, an in-situ method of introducing Pt- and Ru- resulted in large average particles sizes, once again confirming the significance of method of metal introduction in the MCM-41, mesoporous material.

For Cu-MCM-41-A-EIM, the mesoporous catalyst synthesized using the evaporation impregnation method, resulted in the average Cu particle size of 7.82 nm. Therefore, an introduction of metals (Pt-, Pdand Cu-) using the evaporation impregnation method resulted in average smaller particle sizes.

#### 2.1.2. Nitrogen Physisorption

The specific surface areas and specific pore volumes of fresh and spent Pt-, Ru and the Cu-MCM-41 mesoporous catalyst and Pd- and Pt-H-Y-12 catalysts were measured using nitrogen physisorption and results are collected in Table 2. The lowest specific surface area was obtained for the fresh Pt-MCM-41-IS (429 m2/g) catalyst and this value increased after the catalyst had been used. The highest specific surface areas were determined for Pt-H-Y-12-EIM (857 m2/g) and Pd-H-Beta-300-EIM (808 m2/g).

**Table 2.** Specific surface area and pore volume of Pt-MCM-41-IS, Ru-MCM-41-IS, Pd-H-Y-12-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM catalysts applied in the experiments.

#### 2.1.3. Scanning Electron Microscopy

The morphology, size and shape of the catalysts crystals were analyzed by scanning electron microscopy (SEM). The scanning electron micrographs of the Pt-MCM-41-IS, Ru-MCM-41-IS, Pd-H-Y-12-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM catalysts studied in the removal of CBZ from aqueous solutions exhibited a crystal morphology similar to that of H-MCM-41, H-Y-12 and H-Beta-300 catalyst, prior to their modifications with Pt-, Ru-, Pd- and Cumetals. The crystal size distribution of the (a) Pt-MCM-41-IS, (b) Ru-MCM-41-IS, (c) Pd-H-Y-12-EIM, (d) Pt-H-Y-12-EIM, (e) Pd-H-Beta-300-EIM and (f) Cu-MCM-41-A-EIM in the form of histograms are displayed in Figure 2a–f. The average size of the catalyst crystals is presented in the Table 3. The largest average crystal size (272.45 nm) was obtained for the Pd-H-Beta-300-EIM synthesized

using evaporation impregnation. The second largest average crystal size measured for Pd-H-Y-12-EIM (257.12 nm) synthesized by using evaporation impregnation. The third largest average crystal size was measured for Pt-MCM-41-IS (219.23), prepared by the in-situ method. The explanation for the variations in the crystal sizes for synthesized catalysts is the structures of the H-Beta-300, H-Y-12 and MCM-41 mesoporous materials. Furthermore, the methods of the Pt-, Ru-, Pd- and Cu- introduction and the synthesis conditions can influence the average crystal size. The smallest crystal size (88.23 nm) was obtained for Cu-MCM-41-A-EIM, the catalyst synthesized using the evaporation impregnation method.

**Figure 2.** *Cont.*

**Figure 2.** SEM image and crystal size distribution histogram of (**a**) Pt-MCM-41-IS. (**b**) Ru-MCM-41-IS catalyst. (**c**) Pd-H-Y-12-EIM. (**d**) Pt-H-Y-12-EIM. (**e**) Pd-H-Beta-300-EIM and (**f**) Cu-MCM-41-A-EIM catalysts.

#### 2.1.4. Energy Dispersive X-ray Microanalysis

The Pt, Ru, Pd and Cu contents inside the Pt-MCM-41-IS, Ru-MCM-41-IS, Pd-H-Y-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM catalysts are given in Table 3. The highest amount of Pt was found for the Pt-MCM-41-IS catalyst. The largest amount of metal content (Pt) 7.12 wt% was measured for Pt-MCM-41-IS catalyst synthesized using the in situ method (Table 3). The Pd-H-Y-12-EIM catalyst synthesized using the evaporation method exhibited second highest metal (Pd) content 5.34 wt% Cu metal content (Table 3).

#### 2.1.5. Pyridine Adsorption-Desorption with FTIR Spectroscopy

The concentration of Brønsted at 1545 cm−1 and Lewis at 1450 cm−1 acid sites were measured with FTIR using pyridine as a probe molecule. At 250–350 ◦C, pyridine desorption demonstrate weak, medium and strong sites, whereas at 350–450 ◦C it exhibited medium and strong sites and finally at 450 ◦C, strong sites prevailed. The acidity of the Pt-MCM-41-IS, Ru-MCM-41-IS, Pd-H-Y-12-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM catalysts analyzed via FTIR is listed in Table 4. Following that, the data of the determination were converted into concentrations using the extinction coefficients of Emeis [33]. It was noticed that Pd-H-Y-EIM showed the presence of largest Brønsted acidity (237 μmol/g) at 250 ◦C compared to other catalyst. The catalysts Cu-MCM-41-EIM and

Pd-H-Y-12-EIM catalysts had higher Lewis acidities (56 μmol/g and 52 μmol/g) at 250 ◦C, compared to the other catalysts.

2.1.6. Characterization by X-ray Photoelectron Spectroscopy

The surface composition and oxidation state of metals were determined by the X-ray photoelectron spectroscopy (XPS) method. The XPS analysis indicated the oxidation states of Pt◦ and PtO on the Pt-MCM-41-IS catalyst (Figure 3a) Similarly oxidation state of Pt in the Pt-H-Y-12-EIM was measured as Pt, PtO2 (Figure 3d). The oxidation states of Ru in Ru-MCM-41-IS was determined as Ru◦ and RuO2 (Figure 3b). The oxidation states of Pd in Pd-H-Y-12-EIM and Pd-H-Beta-300-EIM were measured to be Pd◦ (Figure 3c,e). The oxidation state of Cu in Cu-MCM-41-A-EIM was measured to be Cu◦ and CuO (Figure 3f).

**Figure 3.** *Cont.*

**Figure 3.** *Cont.*

**Figure 3.** X-ray photoelectron spectroscopy (XPS) spectra of (**a**) Pt-MCM-41-IS, (**b**) Ru-MCM-41-IS, (**c**) Pd-H-Y-12-EIM, (**d**) Pt-H-Y-12-EIM, (**e**) Pd-H-Beta-300-EIM and (**f**) Cu-MCM-41-A-EIM catalysts.

#### *2.2. Non-Catalytic and Catalytic Ozonation of Carbamazepine*

2.2.1. Influence of Different Catalysts in the Degradation of Carbamazepine and its By-Products in the Presence of Ozone

The influence of different temperatures (5 ◦C, 20 ◦C and 50 ◦C) on the ozonation of CBZ on the reaction rate was evaluated the results are displayed in Figure 4a. These experiments illustrate that the transformation rate of CBZ was rapid and approximately equal to each other, at these three temperatures. After 5 min of ozonation, all the CBZ was transformed. Somathilake et al. obtained similar results from the ozonation of CBZ [34]. The degradation kinetics of CBZ by 0.5 g/L of Pd-H-Beta-300-EIM was studied in the absence and presence of ozone (Figure 4b). This experiment revealed that the CBZ concentration did not change in the absence of ozone, which indicates that the catalyst does not absorb CBZ and it is not transformed without the presence of ozone.

**Figure 4.** (**a**) CBZ degradation by ozonation at different temperatures (5 ◦C, 10 ◦C, 20 ◦C and 50 ◦C). (**b**) CBZ degradation by catalysis in the presence and absence of ozone at temperature 20 ◦C (stirring speed = 900 rpm, CO3,g = 21 mg/L, CCBZ = 35 mg/L, gas flow rate = 110 mL/min, Ccatalysts = 0.5 g/L).

An identical series of catalytic ozonation experiments were conducted at 20 ◦C in order to evaluate the effect of different heterogeneous catalysts on the degradation rate of CBZ. Figure 5a shows that Cu-MCM-41-A-EIM and Ru-MCM-41-IS showed higher degradation rates compared to other catalysts the conversion of CBZ was complete already after two minutes for the Cu-catalyst and the Ru-catalyst was almost as active.

**Figure 5.** CBZ degradation by ozonation and ozonation combined with catalysis. CCBZ = 35 mg/L, gas flow rate = 110 mL/min, stirring speed = 900 rpm, CO3, g = 21 mg/L, C catalysts = 0.5 g/L. (**a**) T = 20 ◦C and (**b**) T = 50 ◦C.

A similar series of catalytic ozonation experiments were carried out at 50 ◦C, to evaluate the effect of increased temperature on the catalyst activity while ozonating CBZ (Figure 5b). As revealed in our previous work, the dissolved ozone concentration decreases at higher temperature but, on the other hand, the activity of catalysts increases at higher temperature [31,35]. Here, the Pd-H-Y-12-EIM and Pd-H-Beta-300-EIM illustrated the highest decomposition rate compared to other catalysts and the non-catalytic experiment. Due to the rapid reaction rate, it is hard to evaluate the temperature effect on decomposition rate of CBZ.

#### 2.2.2. Quantification of Catalytic and Non-Catalytic Ozonation Products

Han et al. studied [36] cytotoxicity and genotoxicity of the removal CBZ through chlorination, chloro-amination and ozonation processes. The CBZ solely induces chromosomal damage, and on the other hand, the genotoxicity of the CBZ residues after each treatment was found to be higher. Moreover, it was demonstrated that the most efficient way for the degradation of CBZ was by ozonation among the other treatments. However, the by-products formed by ozonation exhibited the highest cytotoxicity and an elevated genotoxicity. The authors suspected that the by-products BQM and BQD might cause chromosomal damage effects.

The main ozonation pathway of CBZ is a rapid, almost instantaneous reaction leading to BQM (Figure 6). BQM is transformed further slowly to BQD [37]. In order to investigate the effect of heterogeneous catalysts on the formation and transformation of BQM and BQD, the products were isolated from the kinetic experiments and a quantification method was developed.

**Figure 6.** CBZ and its main ozonation products.

The temperature effect on the formation of by-products was also studied for the non-catalytic ozonation using the same temperatures as above. The influence of the reactor temperature on the ozonation of CBZ into BQM and BQD is illustrated in Figure 7. It can be noticed that at higher temperature (50 ◦C), the concentration of BQM remained at a high level and after 4 h it had only decreased to 19.5 mg/L from the highest value of 28 mg/mL. At lower temperatures (20 ◦C and 5 ◦C), the reaction proceeded more rapidly, and almost all of BQM was removed or transformed after four hours. The concentration of the secondary by-product BQD increased simultaneously to 20–24 mg/L when performing the experiments at low temperature (5–20 ◦C), but it remained low at the higher temperature (50 ◦C), due to a slow transformation of BQM, BQD formed slower at higher temperatures. If the experiment at 50 ◦C had been continued, the concentration of BQD might have been the same as for the experiments at lower temperatures. It can be concluded that by lowering the temperature due to higher dissolved ozone concentration, CBZ rapidly converts to BQM, and later on it transforms to BQD faster than at the higher temperature (50 ◦C).

**Figure 7.** (**a**) BQM and (**b**) BQD concentration during the ozonation of CBZ. CCBZ = 35 mg/L, gas flow rate = 110 mL/min, T = 5 ◦C, 10 ◦C, 20 ◦C and 50 ◦C, stirring speed = 900 rpm, CO3, g = 21 mg/L.

Thereafter, several solid catalysts, which were synthesized and characterized by our group, were employed to reveal the catalytic effect on the CBZ. The formation kinetic of BQM and BQD are displayed in Figures 8 and 9. It was observed that during the first minutes of experiments, the BQM

concentration increased to a maximum 25.7 mg/L (Cu-MCM-41-A-EIM), later on it decreased in 2 h to a minimum of 4 mg/L. For non-catalytic ozonation at 20 ◦C, the BQM concentration increased to 25.7 mg/L but after 2 h it decreased to 4.8 mg/L. Pt-H-Y-12-EIM and Ru-MCM-41-IS catalysts showed a slightly higher transformation rate of BQM compared to the corresponding non-catalytic experiment. As revealed by Figure 9, BQD was slightly formed within 2 h. The maximum concentration of BQD was 20.2 mg/L for non-catalytic experiment. The catalyst Pd-H-Y-12-EIM showed the lowest BQD concentration (1.7 mg/L) also Pd-H-Beta-300 showed lower formation of BQD (11.9 mg/L). Three mechanisms could occur in these experiments, (1) ozone adsorption on the catalyst surface and generation of active radical species that react with BQM and BQD; (2) BQM and BQD sorption on the catalyst surface followed via the reaction by dissolved ozone in the water and (3) sorption of all three species ozone, BQM and BQD on the catalyst surface followed by a direct or indirect reaction [38].

**Figure 8.** BQM concentration during the catalytic and non-catalytic ozonation of CBZ. CCBZ = 35 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3,g = 21 mg/L, Ccatalysts = 0.5 g/L.

**Figure 9.** BQD concentration during the catalytic and non-catalytic ozonation of CBZ. CCBZ = 35 mg/L, gas flow rate = 110 mL/min, T = 20 ◦C, stirring speed = 900 rpm, CO3,g = 21 mg/L, Ccatalysts = 0.5 g/L.

The catalytic experiments were carried out under several temperatures. Figure 10 shows that in the first minutes, 27.7 mg/L of BQM was formed (similar to non-catalytic ozonation), but later on, the BQM concentration slightly decreased to 9 mg/L when using the Pd-H-Y-12-EIM catalyst. For non-catalytic ozonation at 50 ◦C, the BQM concentration increased to 27.7 mg/L, after which it decreased to 21.4 after 2 h, and after 4 h it had decreased to 19 mg/L. It can be noticed from results that the activity of catalyst increased with higher temperature, since dissolved ozone concentration is

very low at 50 ◦C (approximately five times lower than at 20 ◦C). For example, after 2 h of ozonation combined with Pd-H-Y-12-EIM catalyst, the BQM and BQD concentration were 11.2 mg/L and 0.9 at 50 ◦C, which were lower compared to 20 ◦C (BQM concentration was 12.8 mg/L and BQD concentration was 1.7 mg/L) with the same catalyst (Figure 11). One explanation for the increased catalytic efficiency at higher temperatures could be due to that instead of ozonation, catalytic oxidation is observed, where dissolved oxygen is adsorbed on metal oxides and the further generate active atomic oxygen species as well as lattice oxygen atoms, which are present on the metal oxides of catalysts [39].

**Figure 10.** BQM concentration during the catalytic and non-catalytic ozonation of CBZ. CCBZ = 35 mg/L, gas flow rate = 110 mL/min, T = 50 ◦C, stirring speed = 900 rpm, CO3,g = 21 mg/L, Ccatalysts = 0.5 g/L.

**Figure 11.** BQD concentration during the catalytic and non-catalytic ozonation of CBZ. CCBZ = 35 mg/L, gas flow rate = 110 mL/min, T = 50 ◦C, stirring speed = 900 rpm, CO3,g = 21 mg/L, Ccatalysts = 0.5 g/L.

These results were in line with a study performed by Rosal et al. who investigated catalytic ozonation of CBZ. Their ozonation results revealed that utilizing a titanium dioxide catalyst combined with ozonation provides a significant benefit where the ozone decomposition as well as ozonation reactions were improved, as well as an increase of the formation of hydroxyl radicals and rate of mineralization compared to non-catalytic ozonation [40].

A more detailed investigation of the fate of BQD was conducted using isolated BQD without CBZ and other by-products. Pure BQD was obtained by isolation from an ozonation experiment. BQD was dissolved in acetonitrile and deionized water. Non-catalytic and catalytic ozonation was carried out with this solution. Figure 12 demonstrates that after 4 h of non-catalytic ozonation only 6% of BQD remained and after 4 h catalytic ozonation only 3% of BQD, catalytic ozonation slightly increased transformation rate.

**Figure 12.** BQD concentration during the catalytic and non-catalytic ozonation of BQD. CBQD = 6 mg/L, CAcetonitrile = 6 mg/L, gas flow rate = 110 mL/min, T = 50 ◦C, stirring speed = 900 rpm, CO3,g = 21 mg/L, Ccatalysts = 0.5 g/L.

2.2.3. Leaching of Modified Metals and Aluminum from Catalysts

Inductively coupled optical emission spectrometry (ICP-OES) Optima 5300 DV Perkin Elmer instrument was applied to determine leaching of catalytic metals during ozonation at 20 ◦C. The aluminum and modified metal were considered in these experiments. Cu-MCM-41A-EIM catalyst showed high leaching of Cu 58.9% and Pd-H-Beta-300-EIM relatively high leaching of Pd and aluminum 14.2% and 20.26% respectively. On the contrary, Pt-H-Y-12-EIM, Ru-MCM-41-IS, Pt-MCM-41-IS showed no leaching for modified metal. Figure 13 illustrates the catalyst leaching during 2 h ozonation.

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**Figure 13.** Leaching of modified metal and aluminum (%) in the catalytic ozonation of CBZ at 20 ◦C.

#### **3. Materials and Methods**

#### *3.1. Chemicals*

Carbamazepine (CBZ; C15H12N2O, MW: 236.269 g/mol, CAS number: 298-46-4) was purchased from Sigma Life Science. The CBZ was in crystal powder form, therefore it was first dissolved in methanol (H3COH, MW: 32.04 g/mol, CAS number: 67-56-1), which was obtained from VWR (Briare, France). The water used in the LC-MS analysis was purified using a Millipore Simplicity 185 system (Millipore S.A.S., Molsheim, France). The acetonitrile used in the LC-MS analysis was of LC-MS grade

and was obtained from Fisher scientific and formic acid was obtained from Sigma-Aldrich. BQM and BQD were prepared from ozonation following to the procedure Kråkström et al. Stock solutions of BQM and BQD were prepared in acetonitrile.

#### *3.2. Catalyst Preparation*

The following solid catalysts were prepared in our laboratory: Pt-MCM-41-IS, Ru-MCM-41-IS, Pd-H-Y-12-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM. The Pt and Ru modification of Pt-MCM-41-IS and Ru-MCM-41-IS was carried out using the in-situ (IS) synthesis preparation method [41]. The modification methods for Pd, Pt and Cu used for the synthesis of Pd-H-Y-12-EIM, Pt-H-Y-12-EIM, Pd-H-Beta-300-EIM and Cu-MCM-41-A-EIM catalysts were evaporation impregnation (EIM) [31].

For the synthesis of Pd-H-Y-12-EIM, an aqueous solution of Pd(NO3)2 and H-Y-12 was placed in a Rotavapor for 24 h, until the aqueous phase was evaporated at 60 ◦C. The Pd-H-Y-12-EIM catalyst was transferred from the flask and dried in an oven overnight at 100 ◦C. Finally, it was calcined in a muffle oven at 450 ◦C for three hours.

#### *3.3. Physico-Chemical Characterization of Catalyst*

The electron micrographs, metal particle size and structural properties of the catalysts were investigated by transmission electron microscopy (TEM, model JEM 1400 plus: Jeol Ltd., Tokyo, Japan) using a transmission electron microscope (model JEM 1400 Plus), using 120 kV accelerating voltage and a resolution of 0.38 nm provided via OSIS Quemesa 11 Mpix digital camera (rephrase/split). The specific surface areas and pore volumes of the catalysts were determined by nitrogen physisorption (Carlo Erba Sorptomatic 1900, Fisons Instruments, Milan, Italy) and interpreted with Dubinin and B.E.T. (Brunauer–Emmett–Teller) equations. The fresh catalysts were outgassed at 150 ◦C, whereas the spent catalysts were outgassed at 100 ◦C for 3 h before each and every nitrogen adsorption measurement. Scanning electron microscopy coupled to energy disperse X-ray analysis (SEM/EDXA) was used to investigate morphology. Crystallite size and metal contents of catalysts was analyzed by SEM (Zeiss Leo Gemini 1530, Oberkochen, Germany) and energy dispersive X-ray micro-analysis. The catalyst acidities were measured by Fourier transform infrared spectroscopy (FTIR) spectroscopy (ATI Mattson Infinity series, Madison, MI, USA) by employing pyridine as the probe molecule. The X-ray photoelectron spectroscopy (XPS, Kratos Analytical, UK) analysis was performed using an X-ray spectrometer (lens mode hybrid resolution by mono aluminum). The specific elemental analysis was done using 20 eV pass energy.

#### *3.4. Kinetic Experiments in a Semi-Batch Reactor*

The ozonation experiments were conducted in a double jacket isothermal glass reactor operating in semibatch mode. In order to immobilize the solid catalyst used in these experiments, a Spinchem TM rotating bed reactor (RBR) was used, typically operating at 900 rpm to ensure vigorous mixing of the liquid and gas phases and to maximize the mass transfer between the gas–liquid interface and the solid catalyst surface. An ozone generator (Absolute Ozone, Nano model, Edmonton, AB, Canada) was used to provide ozone, using oxygen (108.5 mL/min) and nitrogen (1.5 mL/min) as the gas flow rate to the generator was, so the total gas flow was 110 mL/min. The feed gas was super-dried (dew point −60 ◦C). The ozonator produced a concentration of approximately 21 mg/L of ozone in the gas phase. The ozone concentration was determined by iodine volumetric titration [42]. In order to purge the gas constantly through the reactor vessel, a 7 μm disperser was used at the bottom of the reactor. The dissolved ozone concentration was determined to 1.668 mg/L at 5 ◦C, 0.441 mg/L at 20 ◦C and 0.0921 mg/L at 50 ◦C by the indigo method. The water solubility of CBZ is rather low, only 17.7 mg/L, while it is fully soluble in methanol. Therefore, the stock solution was prepared by dissolving 0.35 g CBZ in 100 mL of methanol. Thereafter, 10 mL of the stock solution was added to 1 L deionized water in the glass reactor in the beginning of each experiment. The initial concentration of CBZ was thus 35 mg/L, which is higher than

the concentration typically detected in surface waters. However, a high initial concentration allows us to detect and identify by-products of very low concentrations. In this study, the reaction time was varied from 180 to 240 min. The reactor system used in the experiments is displayed in Figure 14.

**Figure 14.** Schematic view of the semi-batch reactor system for degradation of CBZ.

#### *3.5. Quantification of CBZ, BQM and BQD*