jé et al., 2017).
In this work, monolithic CNT supported Pd catalyst is synthesized by one-pot process via combined palladium nitrate, CNT and alginate solution. During the synthesis process, the alginate played as biopolymer template to form alginate gel in the presence of CaCl2 solution with a homogeneous size and macroscopic shape. The obtained monolithic Ca-Pd@CNT beads with different Ca promoter concentration were employed as catalyst for the hydrogenation of CAL to HCAL. The monolithic structural carbon-based catalysts could be easily separated from the liquid reactant and thus facilitates the catalysts recycling. The structural and electronic properties of such monolithic Ca-Pd@CNT catalyst are analyzed by different characterization techniques to understand the structure-activity relationship in the presence of Ca promoter.
The commercial multi-walled carbon nanotubes (CNT) were pre-treated by hydrochloric acid (6 mol/L) at room temperature for 12 h in order to remove residual metal impurities. CaCl2, alginate and hydrochloric acid were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). Pd (NO3)2·2H2O was purchased from Alfa Aesar. All chemical agents were used as received without further purification.
Macroscopic shaped CNT beads (Liu et al., 2012; Ba et al., 2016):1 g of CNT was dispersed in 50 mL distilled water and treated by ultrasonic (400 W) for 60 min at room temperature. Then the mixture was heated to 50°C and maintained at this temperature for 10 min before a certain amount of alginate (1 wt.% with respect to the H2O) was slowly added to the above mixture. The suspension was kept stirring under vigorous speed (500 rpm) at 50°C for another 50 min in order to obtain a homogeneous mixture. To form the gel beads, the mixture was added dropwise into an aqueous CaCl2 (3 wt.%) solution at room temperature. The gel beads were left into CaCl2 aqueous overnight, and then the gel beads were separated and washed with distilled water to remove excess sodium and calcium ions. The gel beads were dried under vacuum overnight and calcined at 350°C under argon for 2 h with the ramp of 2°C/min.
Ca-Pd@CNTHCl−x (x = 0, 2, 4) catalysts: Macroscopic shaped catalysts with 1 wt.% Pd loadings was synthesized by an in-situ process which was similar with aforementioned preparation method shown in Figure 1. Typically, the Pd(NO3)2·2H2O was added into the CNT-containing suspension accompanied with the alginate. The gel beads were dried under vacuum and then calcined at 350°C under argon for 2 h with the ramp of 2°C/min to obtain the Ca-Pd@CNT sample. The as-synthesized Ca-Pd@CNT sample was reduced under pure H2 at 400°C for 2 h. Furthermore, to investigate the influence of Ca promoter on the catalytic performance, the Ca-Pd@CNT sample was treated in dilute hydrochloric acid (1 mol/L) at room temperature for 2 and 4 h, respectively. HCl washed Ca-Pd@CNT sample were denoted as Ca-Pd@CNTHCl−2h and Ca-Pd@CNTHCl−4h, respectively.
Figure 1. Schematic illustration of in-situ gel methodology for the synthesis of macroscopic shaped of Ca-Pd@CNT beads with Ca promoter.
Pd/AC, Pd/CNT and macroscopic shaped Ca-Pd/CNT catalyst: A certain amount of Pd(NO3)2 solution was added into the active carbon (AC), powdered CNT and as-prepared CNT beads by wetness impregnation method. The samples were also dried under vacuum and calcined at 350°C under argon for 2 h with the ramp of 2°C/min, and then reduced under pure H2 at 400°C for 2 h before the CAL chemoselsctive hydrogenation reaction.
The morphology and compositions measurements were performed using a scanning electron microscopy (SEM) instrument (JSM-7800F, Japan), elemental mapping was operated on JSM-7800F with probe corrector at 15 kV. The textural properties of the samples were analyzed by N2 adsorption-desorption measurements using a Micromeritics ASAP 2020 plus sorptometer operated at −196°C. The samples were degassed under vacuum at 180°C for 6 h before the analysis in order to desorb moisture and adsorbed volatile species on the sample surface. The specific surface area was calculated according to Brunauer-Emmett-Teller (BET) method. Pore volume and average pore diameter were determined from using desorption branch by Barret-Joyner-Halenda (BJH) method. Inductively coupled plasma optical emission spectrometer (ICP-OES) was used to measure Pd and Ca element concentration by an IRIS Intrepid II XSP instrument. The particle size was obtained through using Transmission electron microscopy (TEM) technology which was performed on Titan Themis ETEM G3 (Thermofisher) with image corrector operating at 300 kV. High angle annular dark filed scanning TEM (HAADF-STEM) and elemental mapping were operated on Hitachi HF5000 with probe corrector at 200 kV. A small amount catalyst was crushed and ultrasonicated in ethanol, and then a drop was deposited onto a copper grid covered with a holey carbon membrane for analysis. X-ray photoelectron on spectroscopy (XPS) were recorded on a Thermofisher ESCALAB 250Xi spectrometer equipped with monochromatic Al KC (hυ = 1486.6 eV, 15 kV, 10.8 mA). The binding energies were calibrated by carbon deposit C1s with Eb = 284.6 eV.
The liquid phase hydrogenation of cinnamaldehyde was performed in an autoclave reactor with pressure control system. The reaction process as follows: cinnamaldehyde was diluted in 1,4-dioxane to obtain a 0.45 mol/L reaction solution, and o-xylene was used as internal standard. Typically, 30 mg of catalyst was added into 5 mL reaction solution. The autoclave was then purged with hydrogen flow with four times to replace the air inside the autoclave and maintained the total pressure at 1 MPa at room temperature. Then, the autoclave was heated to 80°C in a water bath with a magnetic stirrer at a fixed rate (c.a. 800 rpm) to carry out the hydrogenation reaction. After the reaction, the reaction mixture was analyzed using gas chromatograph (GC-FID) equipped with an HP-5 column.
The macroscopic shaped Ca-Pd@CNT catalyst was synthesized by an ionic induced crosslinking method. A series of Ca-Pd@CNT beads catalysts with controlled macroscopic shaping was prepared with around 0.9 wt.% Pd loading which was confirmed through ICP-OES analysis (Table 1). As shown in Figure 2, the uniform spherical beads with homogeneous diameter around 1.5 mm could be formed successfully through the in-situ gel process. Compared to the initial CNT powder, the apparent volume of the shaped catalyst is smaller due to the shrinkage during the drying process (the inset digital photo of Figure 2a). The energy-dispersive X-ray (EDX) elemental mapping images of Ca-Pd@CNT beads (Figures 2c–g) showed that the Pd and Ca are uniformly dispersed on the Ca-Pd@CNT beads, and only few of them are aggregated in the macroscale. The element composition Ca-Pd@CNT beads derived from EDS spectra is 0.45 at% (Ca) and 0.09 at% (Pd) in atomic percentage, which is a little lower compared to the actual mass loading from the ICP-OES results (vide infra). The medium and high-resolution SEM images clearly show that CNT in the composites entangled highly to form a large porosity dense network and the microscopic filamentous structure of CNT can be seen at high magnification. The above results indicated that highly dispersed particle could be obtained on the surface of CNT with a special macroscale structure through a simple preparation method.
Table 1. Specific surface area from N2 adsorption-desorption isotherms, exactly Pd, Na, and Ca mass loading and catalytic performance in selective hydrogenation of CALa.
Figure 2. (a) Optical photos showing macroscopic shaped Ca-Pd@CNT catalyst. The inset image presents the volume of hybrid catalyst (1) and the compared initial CNT sample (2). (b) Low magnification SEM micrograph showing the gross morphology of the sphere structured catalyst. (c–g) SEM image and elemental mapping of Ca-Pd@CNT catalyst. (h,i) SEM images of Ca-Pd@CNT catalysts show the opened structure of as-synthesized beads.
TEM image of as-synthesized Ca-Pd@CNT beads reveals that the Pd NPs anchored on the surface of CNT with relatively homogeneous size (Figure 3a). The particle size distribution of Ca-Pd@CNT beads is centered at around 4.7 nm, while the comparative sample, Pd NPs supported on powdered CNT (Pd/CNT), displays an average particle size of 5.5 nm with a broaden size distribution (Figure 3). Such relative homogeneous Pd particle size on the Ca-Pd@CNT beads could be directly attribute to the strong interaction between the metal NPs with Ca species as well as the graphitic layers of support surface.
Figure 3. TEM images and Pd particle size distributions of (a,b) Ca-Pd@CNT beads and (c,d) Pd/CNT.
The chemoselective hydrogen