**Shuang Wang, Jianfei Zhao \* and Kang Yang**

School of Mechatronic Engineering and Automation, Shanghai University, Baoshan District, Shanghai 200444, China<EMAIL_ADDRESS>(S.W<EMAIL_ADDRESS>(K.Y.)

**\*** Correspondence<EMAIL_ADDRESS>Tel.: +86-021-6613-0935

Received: 5 November 2019; Accepted: 12 December 2019; Published: 14 December 2019

**Abstract:** In this paper, a new sensorless control scheme with the injection of a high-frequency square-wave voltage of an interior permanent-magnet synchronous motor (IPMSM) at low- and zero-speed operation is proposed. Conventional schemes may face the problems of obvious current sampling noise and slow identification in the process of magnetic polarity detection at zero speed operation, and the effects of inverter voltage error on the rotor position estimation accuracy at low speed operation. Based on the principle analysis of d-axis magnetic circuit characteristics, a method for determining the direction of magnetic polarity of d-axis two-opposite DC voltage offset by uninterruptible square-wave injection is proposed, which is fast in convergence rate of magnetic polarity detection and more distinct. In addition, the strategy injects a two-opposite high-frequency square-wave voltage vectors other than the one voltage vector into the estimated synchronous reference frame (SRF), which can reduce the effects of inverter voltage error on the rotor position estimation accuracy. With this approach, low-pass filter (LPF) and band-pass filter (BPF), which are used to obtain the fundamental current component and high-frequency current response with rotor position information respectively in the conventional sensorless control, are removed to simplify the signal process for estimating the rotor position and further improve control bandwidth. Finally, the experimental results on an IPMSM drive platform indicate that the rotor position with good steady state and dynamic performance can be obtained accurately at low-and zero-speed operation with the sensorless control strategy.

**Keywords:** sensorless control; high frequency square-wave voltage; interior permanent-magnet synchronous motor (IPMSM); magnetic polarity detection; rotor position estimation


**3. Glyphosate residues in the atmosphere**

#### **3.1 General aspects**

The environmental pollution instigated by the use of plant protection products, commonly referred to as pesticides, is one of the most serious problems that facing the world due to their potential toxicity, high persistence, and slow degradation. Pesticide fate in the environment is characterized by a number of complex processes occurring in different environmental compartments, such as air, soils, and plants [105]. A wide variety of pesticides has been detected in different environmental media, including water bodies, soil, and the atmosphere. The extended use of pesticides containing persistent active ingredients can lead to raised concentrations due to the accumulation in the environment and long-term exposure to nontarget organisms.

Since the last decades, there has been an increasing global concern over the human health impacts attributed to the environmental pollution and specifically to air pollution. During applications, a noteworthy segment of applied pesticides ranged from 15 to 40% is dispersed in the atmosphere and can travel with longrange atmospheric transport [106]. Thus, the atmosphere has been considered as an important spread vector at local, regional, and global scales. It has been reported in the international literature that air pesticide contamination was observed both in urban and rural areas with concentration levels ranging from some picograms to several nanograms per cubic meter [107]. However, the contamination of air by pesticides is an aspect of atmospheric pollution that remains less documented than that of other environments.

Worry over the transport of pesticides in air started in the 1960s with the detection of persistent and volatile substances such as DDT, dieldrin, and aldrin far from their application sites. The first legislation to consider air as an exposure route was in the United States in 1971. Since then the issue of pesticides in air has been subject to sporadic regulatory concern, especially in Europe [108].

Milestone legislation in Europe concerning pesticides in the atmosphere occurred in 1996 with the Stockholm convention on persistent organic pollutants (POPs). This regulation covers all chemicals, including pesticides, and lays down principles to identify substances for which aerial transport may be noteworthy [108].

Long-range transport in air and water can result in the exposure of remote and particularly vulnerable ecosystems such as the Arctic [109, 110].

Pesticides enter into the atmosphere, and their residues can move away from the application sites resulting in accidental exposure for humans, animals, and plants, close or distant the treated sites. It is well recognized that the exposure and effect assessment of pesticides should not be constrained to the target area, and its close zone because this does not adequately cover possible hazards associated with their use.

The most common routes of pesticide entry into the atmosphere could be the drift during their application, volatilization from the soil, surface water or crop foliage, as well as wind erosion of deposited residues [111–113]. Once they enter in the atmosphere, pesticides are distributed between the gaseous and particulate phases depending on parameters such as:

In the atmosphere, pesticides are distributed between particle and vapor phases based on their vapor pressure, the ambient temperature, and the concentration of suspended particulate matter. Taking into account the low volatility of the majority of the most commonly used pesticides; it could be considered that they are often absorbed on the surface of atmospheric particles. In that way they may incur transformation processes resulting in the formation of secondary metabolites which could be even more hazardous than the parent released compounds [106]. Pesticides released into the atmosphere can settle to the ground, be broken down by sunlight and water, or dissipate into the surrounding air.

#### *3.1.1 Transfer processes of pesticides in the air*

During and after the application of a pesticide, a considerable portion of the amount applied may enter into the atmosphere through many different routes (the most important will be briefly discussed) and consequently may be transported over shorter and longer distance.

Through spray application of pesticides, a fraction of the spray would exist as pesticides in the gas phase and as small droplets or particles. The latter do not reach their target due to their extremely small size and cannot be captured by drift collectors. This fraction that exists in the gas phase and as aerosol should be taken into account along with drift.

Volatilization is defined as the transfer of pesticide residues into the gas phase after application. Volatilization from treated areas is a constant process and could be

**137**

*Glyphosate Residues in Soil and Air: An Integrated Review*

the main dissipative route for numerous pesticides [115]. Its extent is governed by the physical and chemical properties of the pesticide such as vapor pressure and Henry's law constant; the application parameters such as the droplet size and the water volume; and finally the climatic conditions during and after application [108, 116]. Volatilization may be swayed by relative humidity, the atmospheric pressure, and the wind velocity [117]. The compound's volatility with medium vapor pressure values is significantly influenced by environmental and application factors, whereas substances with high vapor pressure values present high volatilization which does not depend on other factors. It is broadly established in the literature that vapor pressure can be used to categorize pesticides with a very high or with no volatilization potential. Vapor pressure also rules the partitioning of a semi-volatile constituent between the gas and the airborne particle phases. According to Bidleman substances with a vapor pressure

Pa are mainly expected in the vapor phase, while those with

and 10−5

A significant amount of pesticides entering into the atmosphere for several days or weeks after pesticide application comprises volatilization from the soil and plant surfaces as well as wind erosion of soil particles containing sorbed pesticides [119, 120]. Many parameters such as the physicochemical properties of the pesticide (vapor pressure, solubility, adsorption coefficient, molecular mass, and chemical nature), the soil properties (water content, soil density, soil organic matter content, clay content/texture, soil pH), the weather conditions (air temperature, solar radiation, rain, air humidity, and wind), and the agricultural practices used (application date and rate and formulation type) may influence the

Volatilization from plants is considered up to three times higher than soil volatilization under similar meteorological conditions. The vapor pressure and Henry's law constant are the physicochemical characteristics of the compound that seem to be related with the degree of volatilization. Additionally, application methods and weather conditions may also play an important role in the volatilization process

The Focus Air group has deemed that vapor pressure is the most significant factor affecting volatilization and deemed that active ingredients applied to soil

enter in the air and for that reason require a risk assessment evaluation before

Pesticides existing in the aerial phase could be carried by wind and deposited accidentally in untreated areas by dry (gas and particle) and wet (rain and snow)

The atmosphere could be efficiently cleaned of suspended particulate matter to which pesticides might be sorbed by rainfall, and thus gas-phase pesticides can

High pesticide concentrations in the air could be considered seasonal and often associated with local use and thus occur during the spraying months [123]. The physical and chemical properties of each pesticide also play a significant role in determining if a pesticide converts airborne, whether it then exists primarily in the gaseous or particle phase, and how efficiently rainfall removes it from the atmosphere. The period of time that a pesticide is applied, its amount, and the cultivated area play also significant roles in whether a pesticide exists in the atmosphere and at

Pa solely exist in the particle-adsorbed phase [118].

Pa values partition between

Pa and active ingredients applied to

Pa have a high possibility to

*DOI: http://dx.doi.org/10.5772/intechopen.93066*

value higher than 10−2

these phases.

vapor pressure value lower 10−5

volatilization process [111].

from plants [121].

authorization [108].

which concentration [113].

deposition [122].

Pesticides with vapor pressure between 10−2

with vapor pressure values higher than 10−4

partition directly into a falling raindrop [122].

plants with vapor pressure values higher than 10−5

*Pests, Weeds and Diseases in Agricultural Crop and Animal Husbandry Production*

identify substances for which aerial transport may be noteworthy [108].

particularly vulnerable ecosystems such as the Arctic [109, 110].

depending on parameters such as:

• Vapor pressure

• Water solubility

• Henry's law constant

• Weather conditions [114]

• Octanol-air partition coefficient

• Total suspended particulate matter

*3.1.1 Transfer processes of pesticides in the air*

over shorter and longer distance.

account along with drift.

sunlight and water, or dissipate into the surrounding air.

Milestone legislation in Europe concerning pesticides in the atmosphere occurred in 1996 with the Stockholm convention on persistent organic pollutants (POPs). This regulation covers all chemicals, including pesticides, and lays down principles to

Long-range transport in air and water can result in the exposure of remote and

Pesticides enter into the atmosphere, and their residues can move away from the application sites resulting in accidental exposure for humans, animals, and plants, close or distant the treated sites. It is well recognized that the exposure and effect assessment of pesticides should not be constrained to the target area, and its close zone because this does not adequately cover possible hazards associated with their use. The most common routes of pesticide entry into the atmosphere could be the drift during their application, volatilization from the soil, surface water or crop foliage, as well as wind erosion of deposited residues [111–113]. Once they enter in the atmosphere, pesticides are distributed between the gaseous and particulate phases

In the atmosphere, pesticides are distributed between particle and vapor phases based on their vapor pressure, the ambient temperature, and the concentration of suspended particulate matter. Taking into account the low volatility of the majority of the most commonly used pesticides; it could be considered that they are often absorbed on the surface of atmospheric particles. In that way they may incur transformation processes resulting in the formation of secondary metabolites which could be even more hazardous than the parent released compounds [106]. Pesticides released into the atmosphere can settle to the ground, be broken down by

During and after the application of a pesticide, a considerable portion of the amount applied may enter into the atmosphere through many different routes (the most important will be briefly discussed) and consequently may be transported

Through spray application of pesticides, a fraction of the spray would exist as pesticides in the gas phase and as small droplets or particles. The latter do not reach their target due to their extremely small size and cannot be captured by drift collectors. This fraction that exists in the gas phase and as aerosol should be taken into

Volatilization is defined as the transfer of pesticide residues into the gas phase after application. Volatilization from treated areas is a constant process and could be

**136**

the main dissipative route for numerous pesticides [115]. Its extent is governed by the physical and chemical properties of the pesticide such as vapor pressure and Henry's law constant; the application parameters such as the droplet size and the water volume; and finally the climatic conditions during and after application [108, 116]. Volatilization may be swayed by relative humidity, the atmospheric pressure, and the wind velocity [117]. The compound's volatility with medium vapor pressure values is significantly influenced by environmental and application factors, whereas substances with high vapor pressure values present high volatilization which does not depend on other factors. It is broadly established in the literature that vapor pressure can be used to categorize pesticides with a very high or with no volatilization potential. Vapor pressure also rules the partitioning of a semi-volatile constituent between the gas and the airborne particle phases. According to Bidleman substances with a vapor pressure value higher than 10−2 Pa are mainly expected in the vapor phase, while those with vapor pressure value lower 10−5 Pa solely exist in the particle-adsorbed phase [118]. Pesticides with vapor pressure between 10−2 and 10−5 Pa values partition between these phases.

A significant amount of pesticides entering into the atmosphere for several days or weeks after pesticide application comprises volatilization from the soil and plant surfaces as well as wind erosion of soil particles containing sorbed pesticides [119, 120]. Many parameters such as the physicochemical properties of the pesticide (vapor pressure, solubility, adsorption coefficient, molecular mass, and chemical nature), the soil properties (water content, soil density, soil organic matter content, clay content/texture, soil pH), the weather conditions (air temperature, solar radiation, rain, air humidity, and wind), and the agricultural practices used (application date and rate and formulation type) may influence the volatilization process [111].

Volatilization from plants is considered up to three times higher than soil volatilization under similar meteorological conditions. The vapor pressure and Henry's law constant are the physicochemical characteristics of the compound that seem to be related with the degree of volatilization. Additionally, application methods and weather conditions may also play an important role in the volatilization process from plants [121].

The Focus Air group has deemed that vapor pressure is the most significant factor affecting volatilization and deemed that active ingredients applied to soil with vapor pressure values higher than 10−4 Pa and active ingredients applied to plants with vapor pressure values higher than 10−5 Pa have a high possibility to enter in the air and for that reason require a risk assessment evaluation before authorization [108].

Pesticides existing in the aerial phase could be carried by wind and deposited accidentally in untreated areas by dry (gas and particle) and wet (rain and snow) deposition [122].

The atmosphere could be efficiently cleaned of suspended particulate matter to which pesticides might be sorbed by rainfall, and thus gas-phase pesticides can partition directly into a falling raindrop [122].

High pesticide concentrations in the air could be considered seasonal and often associated with local use and thus occur during the spraying months [123]. The physical and chemical properties of each pesticide also play a significant role in determining if a pesticide converts airborne, whether it then exists primarily in the gaseous or particle phase, and how efficiently rainfall removes it from the atmosphere. The period of time that a pesticide is applied, its amount, and the cultivated area play also significant roles in whether a pesticide exists in the atmosphere and at which concentration [113].

#### *3.1.2 Glyphosate occurrence in the air*

Glyphosate (N-[phosphonomethyl] glycine), a broad-spectrum, nonselective, and post emergence herbicide, is the most widely used pesticide worldwide.

Although numerous laboratory and field studies have been carried out for the determination of glyphosate and AMPA in the aquatic environment, there are limited studies in field soils. Furthermore, atmospheric concentrations of glyphosate and AMPA are shabbily documented as very few studies have monitored them in the atmosphere [124].

The first report about the atmospheric concentrations of glyphosate and AMPA had been published in 1991 in order to present the results of a study that had been conducted in 1988 in northeastern Finland for measuring the workers' exposure to glyphosate when they used sprayers connected to brush saws. In that study glyphosate was determined from the breathing zone and from urine samples. Based on the results of this study and at the end of the spraying week, two air samples were found to have measurable levels of glyphosate at concentrations 2.8 and 15.7 μg m−3 . AMPA had not been detected in any of the air samples [125].

In 2002, Humphries et al. examined the atmospheric samples at three different sites in east-central Alberta. For the purposes of the study, air samples were collected before the application of glyphosate and after its application and for 24 h time period at regular intervals. Glyphosate was not detected in any of the collected air samples at levels above the method LOQ; however, it was detected in few particulate samples [126]. The nonexistence of glyphosate in the polyurethane foam indicates that glyphosate is not released as the vapor forms into the atmosphere but rather is carried by a particulate matter.

In 2004, glyphosate was examined in 59 atmospheric samples in Hauts-de-France Region in France, with a detection occurrence of 14% and a maximum concentration of 0.19 ng m−3 [124, 127].

Chang et al. reported that both glyphosate and AMPA had been detected in the ambient air of Iowa, Indiana, and Mississippi during two growing seasons of the years 2007 and 2008. Atmospheric concentrations of glyphosate reached 9.1 and 5.4 ngm−3 in Mississippi and Iowa agricultural areas, respectively; however atmospheric concentrations of AMPA touched 0.49 and 0.97 ngm−3 in Mississippi and Iowa, correspondingly. It had been concluded that the existence of glyphosate in air is due to spray drift or wind erosion as it is not a volatile compound whereas AMPA presence is due to wind erosion as it is a glyphosate degradation product and it is formed in soil [128]. The authors provided also measurements in rainwater and estimated that 97% of glyphosate existing in the atmosphere could be removed by weekly rainfall greater than 30 mm [129].

Morshed et al. determined the atmospheric concentrations of glyphosate in treated fields in Malaysia during spray applications by a mist blower [129]. The maximum concentration of 42.96 μgm−3 was measured for glyphosate, and additionally a first modeling attempt for the estimation of glyphosate emission to the atmosphere at regional level was done; however, there were no measurements to confirm the model output.

In 2014, and specifically from July to November, Sousa et al. performed a study in northeastern Brazil, in the municipality of Limoeiro do Norte-Ceará, in urban and rural areas, for the determination of the atmospheric concentrations of glyphosate. Glyphosate detected at concentrations ranged between 0.313 and 2.939 μg m3 in all collected atmospheric samples [130].

During the years 2015–2016, glyphosate and AMPA were searched in 142 air samples during a 2-year field campaign in France. Samples were taken from both nonagricultural and agricultural areas, while atmospheric concentrations of glyphosate

**139**

*Glyphosate Residues in Soil and Air: An Integrated Review*

were detected at an overall frequency of 7%. AMPA was not detected in any sample.

rural site of Cavaillon. As regards the temporal distribution of glyphosate, it had been pointed out that there was no reproducible detection pattern from 2015 to 2016 [125].

Generally, a few number of monitoring studies have been conducted for the determination of pesticide residues in atmospheric samples. These studies could not provide consistent results due to the variability in experimental conditions, the lack of consistency in sampling methodologies, the variation in collection time and duration, the analytes selected, the analytical methods used [131], as well as the method detection limits. Most of the studies have been performed at the national level, they are short-term as they lasted from 1 to 2 years, and for that reason, the overall conclusion on the long-term trends and the atmospheric movements of

Pesticides existing in the atmosphere are usually at very low concentrations, and thus appropriate sampling and techniques are necessary. The most common sampling techniques used for pesticides in the ambient air could be separated into two

Active samplers allow the pesticides existing in gaseous and particulate phases to be trapped by pumping air through a filter followed by a solid adsorbent. Thus, pesticides standing in the gas phase are stacked by the solid adsorbent, whereas

Pesticides present in the atmosphere could be sampled through low-volume or high-volume samplers. As pesticide residues in the atmosphere are at very low

For sampling of semi-volatile pesticides, the use of diffusion denuder systems, which consist of a series of coaxial glass tubes coated with an appropriate adsorbent

Passive air samplers are devices that collect pesticides from the air without the use of pump, and they are comprised of an accumulating intermediate which has a high retention capacity for the target analytes. Passive samplers are able to gather only the free gaseous phase pesticides, while the length of sampling range from few weeks to several months, considerably larger than the usual time required using the

In 1991, Jauhiainen et al. collected air samples for the determination of glyphosate from the breathing zone through a portable pump onto an absorption liquid [125]. The air samples collected were first evaporated to dryness and then dissolved

Chang et al. used high-volume active samplers for collecting air samples for the determination of glyphosate. The glass fiber filters used were baked at 550°C,

categories: the active and the passive or diffuse samplers [132].

pesticides in the particulate phase are maintained in the filter.

concentrations, high-volume samplers are usually used [121].

through which the air flows, is proposed [121].

with trifluoroethanol and trifluoroacetic anhydrite.

was measured for glyphosate in the

*DOI: http://dx.doi.org/10.5772/intechopen.93066*

The maximum concentration of 1.04 ng m−3

*3.1.3 Monitoring studies for pesticides in the air*

pesticides could not been reached [108].

*3.2.1 Sampling and extraction procedures*

**3.2 Determination of glyphosate**

*<IP_ADDRESS> Active sampling*

*<IP_ADDRESS> Passive sampling*

active ones [121].

*Pests, Weeds and Diseases in Agricultural Crop and Animal Husbandry Production*

Glyphosate (N-[phosphonomethyl] glycine), a broad-spectrum, nonselective,

Although numerous laboratory and field studies have been carried out for the determination of glyphosate and AMPA in the aquatic environment, there are limited studies in field soils. Furthermore, atmospheric concentrations of glyphosate and AMPA are shabbily documented as very few studies have monitored them

The first report about the atmospheric concentrations of glyphosate and AMPA had been published in 1991 in order to present the results of a study that had been conducted in 1988 in northeastern Finland for measuring the workers' exposure to glyphosate when they used sprayers connected to brush saws. In that study glyphosate was determined from the breathing zone and from urine samples. Based on the results of this study and at the end of the spraying week, two air samples were found to have measurable levels of glyphosate at concentrations 2.8 and 15.7 μg m−3

In 2002, Humphries et al. examined the atmospheric samples at three different sites in east-central Alberta. For the purposes of the study, air samples were collected before the application of glyphosate and after its application and for 24 h time period at regular intervals. Glyphosate was not detected in any of the collected air samples at levels above the method LOQ; however, it was detected in few particulate samples [126]. The nonexistence of glyphosate in the polyurethane foam indicates that glyphosate is not released as the vapor forms into the atmosphere but

In 2004, glyphosate was examined in 59 atmospheric samples in Hauts-de-France Region in France, with a detection occurrence of 14% and a maximum

Chang et al. reported that both glyphosate and AMPA had been detected in the ambient air of Iowa, Indiana, and Mississippi during two growing seasons of the years 2007 and 2008. Atmospheric concentrations of glyphosate reached 9.1

and Iowa, correspondingly. It had been concluded that the existence of glyphosate in air is due to spray drift or wind erosion as it is not a volatile compound whereas AMPA presence is due to wind erosion as it is a glyphosate degradation product and it is formed in soil [128]. The authors provided also measurements in rainwater and estimated that 97% of glyphosate existing in the atmosphere could be removed by

Morshed et al. determined the atmospheric concentrations of glyphosate in treated fields in Malaysia during spray applications by a mist blower [129]. The

tionally a first modeling attempt for the estimation of glyphosate emission to the atmosphere at regional level was done; however, there were no measurements to

During the years 2015–2016, glyphosate and AMPA were searched in 142 air samples during a 2-year field campaign in France. Samples were taken from both nonagricultural and agricultural areas, while atmospheric concentrations of glyphosate

In 2014, and specifically from July to November, Sousa et al. performed a study in northeastern Brazil, in the municipality of Limoeiro do Norte-Ceará, in urban and rural areas, for the determination of the atmospheric concentrations of glyphosate. Glyphosate detected at concentrations ranged between 0.313 and 2.939 μg m3

in Mississippi and Iowa agricultural areas, respectively; however

[124, 127].

atmospheric concentrations of AMPA touched 0.49 and 0.97 ngm−3

.

in Mississippi

was measured for glyphosate, and addi-

and post emergence herbicide, is the most widely used pesticide worldwide.

AMPA had not been detected in any of the air samples [125].

rather is carried by a particulate matter.

weekly rainfall greater than 30 mm [129].

maximum concentration of 42.96 μgm−3

in all collected atmospheric samples [130].

confirm the model output.

concentration of 0.19 ng m−3

and 5.4 ngm−3

*3.1.2 Glyphosate occurrence in the air*

in the atmosphere [124].

**138**

were detected at an overall frequency of 7%. AMPA was not detected in any sample. The maximum concentration of 1.04 ng m−3 was measured for glyphosate in the rural site of Cavaillon. As regards the temporal distribution of glyphosate, it had been pointed out that there was no reproducible detection pattern from 2015 to 2016 [125].


(2) Transparency

Long-term planning and operational decision-making are guided by the Colorado River Simulation System (CRSS), a river system model that requires explicit and transparent assumptions regarding water availability, water deliveries and allocation rules [108,109]. The 2010–2012 Colorado River Basin Study used CRSS to engage stakeholders and establish a common language to navigate trade-offs associated with intensifying scarcity and shortage risks.

The reliance on CRSS for stakeholder engagement and water planning involves both strengths and weaknesses. On the one hand, the federal agency managing the reservoirs in the basin note that, as a result of the modeling system, 'transparency facilitated stakeholders being on relatively equal ground, rather than [certain parties] having an advantage' [108]. At the same time, the barriers to entry are substantial, which involves a steep learning curve that has required the capacity for building for historically marginalized groups, including environmental and indigenous stakeholders.


**Author details**

Jules Rutebuka Rwanda Agriculture and Animal Resources Development Board (RAB), Kigali City, Rwanda

\*Address all correspondence to<EMAIL_ADDRESS>
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Erosion Control Success Stories and Challenges in the Context of Sustainable Landscape… DOI: http://dx.doi.org/10.5772/intechopen.96267*


In NU-1000 single-ion-based iron(III) species have been incorporated using solutionphase post-synthetic metalation with two different iron(III) precursors. The resulting NU-1000-Fe-NO3 and NU-1000-Fe-Cl frameworks show two crystallographically independent Fe sites (Fe1 resides in the c-pore and Fe2 in the hexagonal mesopore), coordinated to the bridging and terminal oxygens of the Zr6 node, with Fe−O distances in NU-1000- Fe-Cl being much longer than those of NU-1000-Fe-NO3 (Figure 5) [52]. Epoxidation of cyclohexene in vapour H2O2 with NU-1000-Fe-NO3 as catalysts initially yields cyclohexene epoxide derived from heterolytic activation of H2O2, which in turn hydrolyzes rapidly to *trans*-cyclohexanediol. Otherwise, NU-1000-Fe-Cl yields a mixture of products and by-products, derived from the radical oxidation products due to homolytic activation of H2O2 [53,54]. This behaviour is probably due to the difference in the metal−node distance between the frameworks, the active site rearranging differently.

**Figure 5.** (**a**) Crystal structures of NU-1000-Fe-NO3 (**b**) Structures of the inorganic Zr6 -nodes.

∼ ′ ′ One-step template-free synthesis of ultrathin (∼5 nm) mixed-valence {V16} clustersbased MOF nanosheets [Ni(4,4′ -bpy)2]2 [V7 IVV9 VO38Cl]·(4,4′ -bpy)·6H2O (NENU-MV-1) has been also reported. A large number of vanadium catalytically active sites in the NENU-MV-1 nanosheet allowed excellent cyclohexene oxidation under air exhibiting a conversion of 95%. Moreover, the nanometer scale of the catalyst increased the catalytic activity 2.7 times compared to the bulk crystal (0.25 mm) for olefin epoxidation. Excellent catalytic performances have been shown for different olefin substrates [55].

**Table 2.** Mixed Metal MOFs in Olefin Epoxidation.

a tBuOH = *tert*-butyl alcohol; TBHP = *tert*-butylhydroperoxide; IBA = isobutyraldehyde.

#### *2.3. Organic Linkers with Functional Catalytically Active Sites*

Functional groups such as amino, pyridyl, amide, sulfonic acid, etc. present in organic linkers serve as active sites for catalysis and strongly influence the intrinsic catalytic activity of the MOFs through inductive effects. In addition, organic linkers can be catalytically active when organic functional groups and/or functional molecular catalysts (e.g., metalloporphyrins, salen and related ligands, chiral molecules, Schiff-base complexes, etc.) are introduced by post-synthetic ways. Alternatively, the same functional molecular catalysts can also be used as building units to fabricate MOFs.

Molybdenum complexes have been widely applied as homogeneous catalysts for the epoxidation of alkenes by H2O2 and organic hydroperoxide, a complete conversion and selectivity being reported. To overcome the recoverability and reusability issues correlated to the use of homogeneous molybdenum catalysts, molybdenylacetylacetonate has been supported on TMU-16-NH2 [Zn2(NH2-BDC)2(4-bpdh)]·3DMF, an amine-functionalized two-fold interpenetrated MOF via dative and combined covalent and dative post-synthetic modification [57].

A high porous NU-1000 MOF has been post-modified with the chiral L-tartaric acid, by SALI (solvent-assisted ligand incorporation) to build a chiral Zr-based MOF [C-NU-1000] [58]. Moreover, another active catalytic site, molybdenyl acetylacetonate, MoO2(acac)2, was incorporated on chiral NU-1000 to explore catalytic performance in the asymmetric epoxidation of olefins (Figure 6a) [59]. When olefins approach by pro-Sor R-face to the catalytic active center, they interact with the OH group of the tartrate through H-bond which induces chirality generating two chiral intermediates. The [C-NU-1000-Mo] catalyst, used in the epoxidation of styrene and 1-decene, can discriminate the *S* configuration in epoxides (Figure 6b).

**Figure 6.** (**a**) The preparation of [C-NU-1000-Mo] catalyst with solvent-assisted ligand incorporation (SALI); (**b**) simplification of the chirality induction mechanism of [C-NU-1000-Mo] when olefins approach by pro-S- or R-face to the catalytic active center.

The free amine group available on UiO-66-NH2 has been post-synthetically modified with salicylaldehyde (SA) or thiophene-2-carbaldehyde (TC) to graft a Schiff base in which MoO2(acac)2 could be immobilized. Efficient olefin epoxidation catalysed by UiO-66- NH2-SA-Mo and UiO-66-NH2-TC-Mo has been described and no Mo active site leaching was detected [60]. In the same way, MoO(O2)2·2DMF was immobilized onto UiO-66(NH2) functionalized with salicylaldehyde (Sal) (UiO-66-sal-MoD), pyridine-2-aldehyde (PI) (UiO-66-PI-MoD) and 2-pyridine chloride (PC) (UiO-66-PC-MoD). All of them allowed a high dispersion of Mo catalyst, the large pores of MOFs guarantee adequate contact between the substrate and the catalytic active center, thus improving the efficiency of cyclic olefins epoxidation [61].

μ μ − Molybdenum(VI) oxide was deposited on the eight-connected Zr6(µ3-O)4(µ3-OH)4 (H2O)4(OH)4 nodes connected by 1,3,5,8-(*p*-benzoate) pyrene linkers (TBAPy4−) of the mesoporous NU-1000, via condensation phase through solvothermal deposition in MOF (SIM) [62]. The stable Mo-SIM system exhibits a high conversion for cyclohexene epoxidation without leaching of molybdenum catalyst compared to Mo supported on bulk zirconia (Mo-ZrO2), in which significant leaching of the catalytic species has been observed [60].

Molybdenum tricarbonyl complexes are known to be effective catalysts for the epoxidation of olefins. They form an oxomolybdenum (VI) species in the presence of *tert*-butyl hydroperoxide (TBHP) as an oxidant which acts as highly active catalytic sites for the epoxidation of olefin. M(CO)6 was deposited on UiO-66 and UiO-67 by chemical vapor deposition (CVD) treatment, UiO-66-Mo(CO)3, and UiO-67-Mo(CO)**3** heterogeneous catalysts being fabricated. Herein, the larger tetrahedral and octahedral cavities of UiO-67 enable more accessibility of cyclooctene to catalytically active sites showing higher catalytic activity for the cyclooctene conversion than UiO-66-M(CO)3 [33].

Several attempts have been made to immobilize oxovanadium(IV) complexes on different solid materials and create heterogeneous catalytic systems for the epoxidation of allylic alcohols [63–66]. A catalyst has been designed by immobilizing oxovanadium(IV) species on UiO-66(NH2) via post-synthetic modification and by using two different pathways. At first, the amino-functionalized UiO-66(NH2) was modified with salicylaldehyde to produce salicylideneimine modified UiO-66 (UiO-66-SI), subsequently [VO(acac)2] was reacted with UiO-66-SI to give UiO-66-SI/VO(acac). In another pathway, UiO-66(NH2) directly reacted with [VO(acac)2] to produce UiO-66-N/VO(acac)2 [67]. Excellent catalytic activity in the regioselective epoxidation of geraniol was obtained when UiO-66-SI/VO(acac) and UiO-66-N/VO(acac)2 systems were employed by using reaction times of 60 and 120 min, respectively. The proper pore size of and the high dispersion of the catalytic sites on UiO-66-SI/VO(acac) and UiO-66-N/VO(acac)2 guarantee good access of the substrate to the active sites.

Metallosalen-based crystalline porous materials have been realized for heterogeneous catalytic applications towards cyclopropanation, alkene epoxidation and hydrolytic kinetic resolution of epoxides with interesting enantioselectivities [5]. UiO-68-Me has been modified via post-synthetic exchange (PSE) with single- and mixed-M(salen) linker (M = Cu, Fe, Cr, V, Mn) to fabricate UiO-66(NH2) attractive species for heterogeneous asymmetric catalysis, useful to overcome the problem of metal leaching. It was found that the single-M(salen) chiral MOFs (R)-UiO-68-Mn and (R)-UiO-68-Fe catalyse the epoxidation of alkenes to epoxides with up to a 98% ee of epoxide and 97% ee, respectively. The different catalytic metal centers in the mixed-(M)salen species UiO-68-Mn-Cr gave consecutive reactions starting from the epoxidation of alkene followed by ring–opening reaction of epoxide to produce the desired amino alcohol in 80−85% yields with 80−99.5% ee. Catalytic activity and enantioselectivity of all chiral UiO-68 catalysts remain unchanged for 10 cycles [68].

Encapsulation of Cu- or Ni-salen species in NH2-MIL-101(Cr) through one-pot method gave a series of effective heterogeneous catalysts in the styrene oxidation under mild conditions. Specifically, the styrene conversion obtained using TBHP was 98.78%. The concentrated electronic density around Cu(II) in the Cu salen@NH2-MIL-101(Cr) catalyst promoted the formation of tBuOOCu(III)-salen enhancing the selectivity to epoxide [69].

A recent synthetic strategy resides in the incorporation of different functionality in one single framework to generate a multivariate MOF (MTV-MOFs). On this basis, a chiral MOF based on multiple metallosalen bridging ligands has been synthesised. Firstly, M(salen)-derived dicarboxylate ligands H2L5M [M = Cu, VO, CrCl, MnCl, Fe(OAc), and Co(OAc)] were synthesized by reactions of N,N′ -bis(3-*tert*-butyl-5-(carboxyl)salicylide (H4L5) and the corresponding metal salts in MeOH at room temperature. Secondly, the crystals of binary or ternary MTV-MOFs (CuV, CuMn, CuCr, CuFe, and CuCo) were obtained by heating a 1:1 or 1:1:1 mixture of H2L M with Zn(NO3)2·6H2O at 80 ◦C in DMF, [Zn4O(L5M,M′ )3] species is obtained. Both [Zn4O(L5Cu,Mn)3] and [Zn4O(L5Cu,Fe)3] showed efficient catalytic performances for asymmetric epoxidation of alkenes, affording up to 93% and 90% ee of the epoxides, respectively. Moreover, in the ternary heterogeneous catalyst [Zn4O(L5Cu,Mn,Co)3], the combination of Mn3+ and Co3+ promotes the epoxidation of alkene followed by enantioselective hydrolysis of epoxide to afford diols [70].

The achiral Zr-MOF, [PCN-224(Mn(Cl)], based on tetratopic ligand [manganese (chloride) tetrakis(4-carboxyphenyl)porphyrin [Mn(Cl)-TCPP], has been post-synthetically modified with tartrate anion, as a chiral auxiliary. The final chiral PCN-224-Mn(tart) contains

two active metal sites (Zr and Mn) as Lewis acid centers and the chiral tartrate counterion, as Brønsted acid sites (OH functional group) has been investigated as chiral nucleophile catalyst towards both asymmetric epoxidation and CO2 fixation. Asymmetric epoxidation of several aromatic and aliphatic olefins like styrene, *trans*-stilbene, 4-methylstyrene, αmethylstyrene, 1-phenyl-1-cyclohexene, 1-decene, and 1-octene has been tested by using PCN-224-Mn(tart) with aldehyde as co-catalyst, CH3CN, and O2. The all-reaction conversions were completed with an optimum range of epoxide selectivity 83–100% and high ee (84−100%). Several factors allow high enantioselectivity in the formation of the epoxide: the framework porosity, the active Mn center, a preferred face of the olefin (pro-S or -R face) close to produce the more stable configuration, and the noncovalent interactions between H atom of the olefinic double bond of the preferred face and chiral centers (Table 3) [71].

**Table 3.** MOFs with functionalized organic linker in olefin epoxidation.

a

#### **Table 3.** *Cont.*

sPhIO = 2-(tertbutylsulfonyl)iodosylbenzene; IBA = isobutyraldehyde; CHP = cumene hydroperoxide; TBHP = *tert*-butylhydroperoxide.

#### **3. Epoxidation with MOF-Based Composites**

One possible way to improve the chemical and mechanical stability of MOFs as potentially heterogeneous catalysts is their immobilization onto/into supports. In this contest, solid polymer, graphene, and inorganic particles [72] or inorganic polymers [73] are largely employed as supports.

To overcome the poor hydrostability of [Cu3-BTC2] [74], a porous dendrimer-like porous silica nanoparticles (DPSNs) has been utilized as a carrier to support Cu-BTC Nps. The nanocomposites DPSNs@Cu-BTC were prepared by growing Cu2O NPs in the centerradial porous channels of DPSNs. After that, Cu2O NPs were dissolved in the presence of acid, oxidant and 1,3,5-benzenetricarboxylic acid (H3BTC) [75]. The obtained Cu-BTC NPs have shown limited growth and a uniform distribution without agglomeration. The small size of Cu-BTC NPs (40 ± 25 nm) is useful in the aerobic epoxidation of various cyclic olefins achieving high catalytic activity without by-products. Good yield and selectivity were detected with inert terminal linear alkenes. Otherwise, epoxidation of styrene only achieved 65% of conversion due to the kinetic instability of styrene oxide (Table 4) [76].

The amphiphilic MIL-101-GH, a porous hierarchical material, has been explored as catalyst for the biphasic epoxidation reaction of 1-octene with H2O2. MIL-101-GH hydrogel was obtained by dispersing MIL-101 nanoparticles homogeneously in aqueous graphene oxide (GO) solutions. The TS-1 catalyst, commercially used in this biphasic reaction, was then introduced in MIL-101-GH. The resulting system, MIL-101-GH-TS-1, overcame the lower activity toward olefin epoxidation of TS-1, and the amphiphilic MIL-101-GH increased the contact areas of TS-1 with both H2O2 and 1-octene. The catalytic performance of MIL-101-GH-TS-1 has been much higher than that of single TS-1 and the 1,2-epoxyoctane was obtained without other by-products [77].

Polyoxometalate-based (POMs) heterogeneous catalysts are attractive species in the catalytic epoxidation of olefin. They have got great catalytic activity, selectivity, and easy separation but their leaching mainly due to the strong complexing capability of solvent and H2O2 oxidants, represents the major obstacle in the possible applications [78,79]. To overcome the stability issue of POMs, the polyoxomolybdic cobalt (CoPMA) and polyoxomolybdic acid (PMA) species were incorporated into UiO-bpy, a Zr-based MOFs, through self-assembly process under solvothermal condition [80]. CoPMA@UiO-bpy showed the highest catalytic activity for cyclooctene oxidation with H2O2 and also for the oxidation of styrene and 1-octene with O2 as oxidant and *tert*-butyl hydroperoxide (t-BuOOH) as initiator. This is due to the uniform distribution and better immobilization of POM clusters within the size-matched cages of Zr-MOFs owing to the presence of bipyridine groups in the UiO-bpy framework. It is noteworthy that CoPMA@UiO-bpy shows excellent recyclability and stability against the leaching of active POM species.

Composite material has been obtained by encapsulating H5-PMo10V2O40 polyoxometalates (POMs) and 1-octyl-3-methylimidazolium bromide, ionic liquids (ILs), in the mesoporous cages and large surface area of MIL-100 (Fe). The synergic effect of ILs, Lewis and Brønsted acid sites in both PMo10V2 species and MOF created a PMo10V2-ILs@MIL-100(Fe) hybrid with significant catalytic properties in cycloolefins epoxidation. Indeed,

the PMo10V2 was activated by the imidazolium cations originated from ILs and the incorporation on MIL-100(Fe) prevented the leaching of POMs [81]. This composite is easily regenerated for 12 cycles without loss catalytic performance [82]. γ

MIL-100(Fe) combined with the polyoxometalate (C16H36N)6K2[γ-SiW10O36] has been reported to catalyse epoxidation of 3Z,6Z,9Z-octadecatriene to the corresponding 6,7-epoxide with high site selectivity (82.35%). The conversion catalysed by POM/MIL-100(Fe) exhibits a greater performance when the MOF contains unsaturated Lewis acid iron ions [83]. The main product of this epoxidation is a sex pheromone of *E. obliqua* Prout and can be potentially used in pest insect control with environmental friendliness.

Two POMs-based MOFs, [Cu6(bip)12(PMoVI12O40)2(PMoVMoVI11O40O2)]·8H2O and [Co3 IICo2 III(H2bib)2(Hbib)2(PW9O34)2(H2O)6]·6H2O (H2bip = 1,3-bis(imidazolyl)propane; bib = 1,4-bis(imidazol)butane)), have been fabricated using a flexible N-containing bidentate ligands via hydrothermal condition. They have been employed in the catalytic processes for selective alkene epoxidation and recycled four times without loss of quality (Figure 7) [84].

**Figure 7.** (**a**) The 2D structure of [Cu6 (bip)12(PMoVI 12O40)2 (PMoVMoVI 11O40O2 )]·8H2O; (**b**) the coordination environment of the Cu(II) cations. Hydrogens and hydroxyls are omitted for clarity. Light-blue polyhedral correspond to the (PMo12) polyanion.

Metal nanoparticles can grow without agglomeration in a porous matrix to produce a stable and active heterogeneous catalyst. Pd NPs have been loaded on the pre-synthesized UiO-66-NH2 using a simple solution impregnation method and NaBH4 reduction. The amino groups in the linkers allow a strong interaction with Pd (II) ions which is essential to yielding well-dispersed Pd/UiO-66-NH2 catalyst. The experiments suggest that the best catalytic activity for styrene epoxidation has been found under Pd NPs loadings of 3.69 wt% [85].

A dually functionalized catalytic system for the tandem H2O2-generation/alkeneoxidation reaction has been realized. A microcrystal of UiO-66-NH2 has been used as a platform to encapsulate Au and Pd metal NPs and later Pd/Au@UiO-66-NH2 surfaces have been post-synthetically modified with a (sal)MoVI (sal = salicylaldimine) molecular epoxidation catalyst. The porosity of Pd@UiO-66-sal(Mo) allows H2 and O2 gases to come into contact with the encapsulated NPs to generate H2O2. The synergic effect of the generated H2O2 and (sal)MoVI in a MOF enhanced epoxide productivity reducing alkene hydrogenation side reaction. This study showed that (sal)Mo moieties in Pd@UiO-66-NH2

epoxidize cis-cyclooctene substrate faster, leading to the more effective usage of the H2O2 oxidant [86].

Systems composed of a magnetic uniform Fe3O4(PAA) microspheres core and of a copper-doped MOF shell demonstrated an easily catalyst recovery approach improving turnover number and turnover frequency. In addition, these magnetic core–shell heterogeneous catalysts improve both stability of the metal active site and dispersity of catalyst materials reducing the metal leaching. Two interesting magnetic core-shell copper-doped catalysts, Fe3O4@P4VP@ZIF-8 and Fe3O4/Cu3(BTC)2 have been prepared by combining the solvothermal method with layer-by-layer assembly. Initially, monodispersed PAAmodified Fe3O4 particles were synthesized by solvothermal methods [87]. In the case of Fe3O4/Cu3(BTC)2, Fe3O4 particles were alternately immersed in solutions containing Cu(CH3COO)2·H2O and H3BTC such that Cu3(BTC)2 nanocrystals grow layer-by-layer on the surface of PAA- modified Fe3O4 particles. This nanosized porous structure increases the contact between the Cu(II) active sites present in the Cu3(BTC)2 shell and the catalytic substrates [88]. In Fe3O4@P4VP@ZIF-8 catalyst, on the other hand, the Fe3O4(PAA) core has been coated with P4VP middle layer to adsorb a large number of Zn2+ for the growth of the ZIF-8 shell thickness on the surface of the core–shell Fe3O4(PAA)@P4VP. Then, the Zn2+ ions were partially substituted by Cu2+ ions in the ZIF-8 shell framework. The ions exchange allowed a well-dispersed copper active site in the resulting copper-doped ZIF-8 structure, avoiding their leaching [89].

Aerobic epoxidation of cyclic olefins (e.g., cyclohexene, norbornene) using both magnetic core–shell copper-doped Fe3O4@P4VP@ZIF-8 and Fe3O4/Cu3(BTC)2 as heterogeneous catalyst achieved high conversion and selectivity (99%) in the formation of the epoxide under mild reaction conditions. Epoxidation of styrene by using Fe3O4@P4VP@ZIF-8 as a catalyst has brought only 54% selectivity of the desired epoxide owing to the kinetic instability of styrene oxide and its oxidation into benzaldehyde [90].

A series of Zr-based core-shell MOF composites with mesoporous cores and microporous shells have been synthesized by solvothermal under kinetic control. PCN-222(Fe) crystals have been synthesized and used as seed crystals to grow the Zr-BPDC(UiO-67) crystals. Meso- and micro-porosity inside of PCN-222(Fe)@Zr-BPDC(UiO-67) drives the catalytic performances for olefin epoxidation reaction [91]. Indeed, the core MOF with Fe-porphyrin moieties represents the catalytic center, while the shell controls the selectivity of the substrate through tuneable pore size. This size-selective catalyst showed almost complete conversions for small olefins.

**Table 4.** MOF-based composites for epoxidation reaction.

**Table 4.** *Cont.*

a tBuOH = *tert*-butyl alcohol; TMA = trimethylacetaldehyde; IBA = isobutyraldehyde.

#### **4. CO2 Epoxide Cycloaddition to Cyclic Carbonates**

Cycloaddition reaction of CO2 with epoxides represents one of the most economically efficient approaches in the production of cyclic organic carbonates with relevant applications ranging from raw materials in the pharmaceuticals industry, polar aprotic solvents, electrolytes in lithium batteries, lubricants, precursors for polycarbonate materials, and other fine chemicals.

CO2, being a C1 feedstock has, in fact, a high potential from the chemical point of view [92]. CO2 can be employed in the highly atom-economical acid-catalysed epoxides cycloaddition to give cyclic organic carbonates, relevant species for industrial applications [93]. The cyclic carbonates (OCs) have been also used as intermediates for engineered polymers, as a lubricant (in 1987 Agip Petroli added dialkylcarbonates as lubricant in a formulation of semisynthetic gasoline engine oil components), and more recently found application in varnish production, green solvents or electrolytes in lithium-ion batteries.

The CO2 cycloaddition mechanism involves an acid catalyst (Lewis or Brønsted acid) that coordinates to the epoxide substrate activating it toward nucleophilic attack by the co-catalyst (e.g., typically a tetraalkylammonium halide). The resulting halo-alkoxide intermediate reacts with carbon dioxide to generate the cyclic carbonate and subsequently regeneration of both catalyst and co-catalyst [93].

The CO2 fixation reaction catalysed by homogeneous or heterogeneous catalysts has been extensively investigated, however some drawbacks remain. Differently from the homogeneous, heterogeneous catalysts (e.g., ionic liquid-supported solids [94–96], polymers [96,97], and porous organic frameworks [98]) have the advantages of easy separation and regeneration of the catalyst, but they often required rough conditions (high temperature, pressure, and time) due to a lack of accessible surface area for accelerating interactions of CO2 and reagents with active sites. Therefore, the high surface area, tunability, and CO2 sorption capacity of MOFs can be beneficial for improving the efficiency of the CO2 cycloaddition reaction. Lewis acid metal centers and Brønsted acid groups in MOFs can promote the activation of the epoxide ring, while the functional groups in the ligands can act as Lewis/basic sites improving not only the CO2 affinity inside the pore but also can fulfil the role of co-catalyst (Figure 8) [99].

**Figure 8.** Proposed mechanism for the cycloaddition of CO2 to epoxide with Lewis acid and base catalysts.

− The Hf-cluster-based NU-1000 (Hf-NU-1000) demonstrated excellent catalytic activity, greater than the Zr-cluster-based NU-1000 under the same mild reaction conditions [100]. Indeed, the presence of high density stronger acidic Brønsted sites, due to stronger M−O bonds, gave a complete and quantitative conversion of styrene oxide and propylene oxide to form cyclic carbonates. Moreover, high yields have been detected for the cycloaddition reaction of CO2 with industrially important epoxide divinylbenzene dioxide (DVBDO) [101].

≡ π ′ − − Large pores in the MOFs, easily functionalized by polar groups, can promote CO2 fixation in a short reaction time under ambient CO2 pressure and moderate temperature without the use of solvent. Within the mesoporous M-MOF-184 series (M = Co, Ni, Mg, Zn), Zn-MOF-184 achieved efficient catalysis performances to convert CO2 to cyclic carbonates under ambient conditions for several epoxy substrate, due to the presence of high concentration of accessibly acidic metals, basic 2-oxidobenzoate anion sites and to the high polarity induced by C≡C bonds and π systems from the phenyl rings in the linkers. Low conversion has been detected for larger epoxides due to limit diffusion into the MOF pores of reactants toward the active sites [102]. The hydrothermally synthesized flexible Zn-based {[Zn2(TBIB)2(HTCPB)2]·9DMF·19H2O}n, has been synthesized employing two types of large linkers 1,3,5-tri(1H-benzo[d]imidazol-1-yl)benzene (TBIB) and 1,3,5-tris(4′ carboxyphenyl-)benzene (H3TCPB). A porous structure with 1D channels was generated via noncovalent supramolecular interactions between the layers. The presence of free protonated carboxylic acid groups(−COOH), carbonyl groups (−C=O), and the presence of Lewis basic sites from the rich N-containing TBIB on the surface pores enhance the selectivity toward CO2. Moreover, the COOH group helps in catalysing the CO2 cycloaddition reaction efficiently through noncovalent interaction with the epoxide substrate, followed by ring-opening upon nucleophilic attack of co-catalyst [103].

Excellent conversions of epichlorohydrin and 2-vinyloxirane have been obtained using as heterogeneous catalyst [Zn4OL43]n based on the meta-substituted 2,2′ ,6,6′ -tetramethoxy-4,4′ -biphenyldicarboxyate ligand [39].

′ ′ ′ − Zeolitic imidazolate frameworks are known for their high CO2 solubility and capture ability [104], especially the chloro-functionalized ZIF-95 [105]. The CO2 cycloaddition to propylene oxide by using ZIF-95 and a quaternary ammonium salt as cocatalyst procured over 99% selectivity to the desired propylene carbonate product under moderate conditions [106]. Also the imidazolate-containing species Im-UiO66(Zr)MOF reacts with methyl iodine to produce (I−)MeIm-UiO-66 that demonstrate efficiency in the CO2 cycloaddition reaction toward a broad range of substrates, in this case without the addition of co-catalyst [107].

Conversely, imidazolium-based IL units were grafted and immobilized into UiO-67 via direct ligand functionalization that, considering the post synthetic approach, is a quantitative method. The obtained species show a high density of IL sites. UiO-67-IL converts epichlorohydrin substrate in 95% yield under co-catalyst and solvent-free conditions. The yield increases to 99% in a shorter time when TBAB was employed (TBAB = tetrabutylammonium bromide) [108] (Table 5).

UiO-66-NH2 pores were modified with ILs such as methylimidazolium bromide and methylbenzimidazolium bromide by coupling reactions, to generate ILA@U6N and ILB@U6N MOFs. The Lewis acid sites (for activation of the epoxide) and the IL functional sites (for epoxide ring-opening) efficiently catalyse the epichlorohydrin conversion under mild conditions [109].

A linear ionic polymer was inserted inside the MIL-101(Cr) via in situ polymerization to form polyILs@MIL-101(Cr) stable heterogeneous composites. This polyILs@MIL-101 is able to catalyse the CO2 cycloaddition reaction with various epoxides with good to excellent conversions, including terminal epoxides with both electron-withdrawing and electron-donating substituents without the need of co-catalyst [110].

A new multimodal catalytic system has been designed via two steps post-synthetic modification of the metal nodes in the NU-1000 framework. A tandem functionalization was performed starting from the incorporation of *ortho*-, *meta*-, and *para*-pyridinecarboxylic acids into the framework of NU-1000(M), then the pyridine moieties were alkylated with various haloalkanes (CH3I, C4H9I, C4H9Br, and C6H4F9I) to introduce co-catalyst moieties near to the inorganic node [111]. Among catalysts, NU-1000(Zr) functionalized with 4-PyCOOH and CH3I, i.e., SALI-4-Py-I-(Zr), showed the highest styrene carbonate yield without co-catalyst, the epoxy ring being activated upon coordination to Zr4+ center (Lewis acid site) and the halogen anion opening the epoxy ring by nucleophilic attack on the less sterically hindered carbon atom [111].

Two 3D metal-cyclam-based zirconium MOFs [Zr6(µ3-OH)8(OH)8(M-L)4] (where M = Cu(II) or Ni(II), L1 = 6,13-dicarboxy-1,4,8,11-tetraazacyclotetradecane) were prepared, namely VPI-100 (Cu) and VPI-100 (Ni) (VPI = Virginia Polytechnic Institute), respectively. A two-step solvothermal synthesis has been necessary to build the MOFs. Initially, a zirconium-oxo cluster was assembled, then cyclam was added. The presence of accessible Cu2+/Ni2+ metal active sites in the metallocyclams and of the coordinatively unsaturated Zr4+ sites in the equatorial plane of the Zr6 cluster in VPI-100 improved their catalytic activity toward CO2 cycloaddition to various organic epoxides [112].

Another strategy developed to increase the catalytic performances is based on the incorporation of an amine group in MOFs. Essentially, the amino group has the dual advantage of acting as an electron donor (Lewis base) toward CO2 and increasing the local concentration of CO2 near catalytic centres through a high CO2 adsorption [113,114].

The amine-functionalized NH2-MIL-101(Al) has been synthesized using a solvothermal or microwave method and its catalytic activity in the solvent-free cycloaddition of CO2 to styrene oxide achieved nearly total conversion and selectivity in 96% yield, with a TOF of 23.5 h−1 [115]. The coordinatively unsaturated aluminium centers present in the SBUs (Lewis acidic sites) bind the epoxides and activate them toward ring-opening, this step is immediately followed by the attack of the bulky bromide ions of TBAB. The pendant amino groups polarize the CO2 molecules, through the nucleophilic attack at the carbon atom, and facilitate CO2 insertion and cycloaddition (Figure 9). During the catalytic reaction, the micro and mesoporous of the framework facilitate the diffusion of substrates and reactants to enhance their interactions [116].

Recently, the acid-base pair UiO-66-NH2 has been used to synthesize bio-based fivemembered cyclic carbonate from vegetable oil methyl ester by CO2 fixation. At first, 95% of double bonds in the O-acetyl methyl ricinoleate starting material were converted to epoxide through an enzyme-catalyzed process. Then, the cycloaddition of epoxy fatty acid methyl esters was performed in the presence of UiO-66-NH2 as catalyst and TBAB as co-catalyst for CO2 fixation. At 120 ◦C under 3 MPa CO2 pressure for 12 h, the reaction conversion reached 94.4% [117] (Table 5).

A series of diamino-tagged zinc bipyrazolate MOFs have been investigated as heterogeneous catalyst in the reaction of CO2 with the epoxides epichlorohydrin and epibromohydrin to give the corresponding cyclic carbonates at 393 K and pCO2 5 bar under relatively mild conditions (solvent and co-catalyst-free) [118]. The presence of amino group in the MOFs pores increased the CO2 storage capacity as well as the catalytic performances. The epoxide has been activated through halogen-amine interaction which was observed in structure of the [epibromohydrin@Zn(3,3′ -(NH2)2BPZ)] adduct. The isomeric Lewis basic site (NH2) in Zn(3,5 NH2-Bpz) (64% yield) improves more than twice the catalytic transformation of epichlorohydrin compared to its mono(amino) parent Zn(BPZNH2) (32% yield) [118].

**Figure 9.** Proposed mechanism for the cycloaddition of styrene oxide and CO2 using tetrabutylammonium bromide.

Post-synthetic metalation of organic linkers is employed strategically to tailor the MOFs' properties. In Hf-Bipy-UiO-67, the 2,2-bipyridine-5,5-dicarboxylate ligand was grafted with Mn(OAc)2 and the resulting Hf-Bipy-UiO-67(Mn(OAc)2 showed that synergy of the binary Lewis acid function significantly enhances the CO2 uptake capacity and catalytic performance of the cycloaddition reaction under mild conditions [119].

Vanadium chlorides have been used to produce the post-metalated Zr-based MOF-VCl3 and MOF-VCl4, with biphenyl-4,4′ -dicarboxylic acid, and 2,2′ -bipyridine- 5,5′ -dicarboxylic acid, respectively, which provide Lewis basic sites. Their high catalytic activity in the CO2 cycloaddition to various organic epoxides was attributed to the accessible Cu2+/Ni2+ metal active sites in the metallocyclams and the presence of coordinatively unsaturated Zr4+ sites in the equatorial plane of the Zr6 cluster in VPI-100 MOFs [112].

′ UiO-type MOFs become susceptible to water and alkaline solution when the length of the carboxylic linker increase. A series of UiO-type MOF named ZSF, incorporating chiral metallosalen as linker has been produced. The chemically stable ZSF-1 MOF, synthesized by dissolving a mixture of ZrCl4, Cy-salen-Ni, and modulators (trifluoroacetic acid), showed excellent catalytic performance for the conversion of CO2 with epoxides into cyclic carbonates. The tetrahedral cages of ZSF-1 decorated with salen-Ni moieties entrap efficiently CO2 and activate the substrate. ZSF-1 catalyses efficiently the asymmetric cycloaddition of CO2 with styrene oxide giving 94% yield of the resulting cyclic carbonate [120]. With other epoxides, specifically epichlorohydrin, the catalytic activity of ZSF-1 increases until to 99% of conversion thanks to the presence of electron-withdrawing Cl group, which promotes the nucleophilic attack of Br− during the ring-opening process.

′ ′ ′ The chiral PCN-224-Mn(tart) (see Section 2.3) has been used in asymmetric CO2 cycloaddition to styrene epoxide, its derivative showing conversions of 96% and 87%, respectively. The missing-linker defects in the Zr cluster and in the Mn center are Lewis acids inducing catalytic ability into the framework for CO2 chemical fixation. In addition, the auxiliary chiral tartrate anions, and the co-catalyst (Bu4NBr) act as nucleophiles generating a chiral epoxide, semi-intermediate, starting from prochiral styrene substrate. The CO2 addition leads asymmetrically to cyclic carbonate with a high ee, and it is related to the interaction of the chiral centers and substrate pro R/S face. Moreover, catalytic reactions with PCN-224-Mn(tart) were performed at low energy and ambient pressure and temperature [71].

**Table 5.** MOF-based composites for cycloaddition reaction.

a TBAB = tetrabutylammonium bromide.

#### **5. Conclusions**

MOF-based catalysts are now a very promising class of compounds as they merge relevant characteristics of both homogeneous and heterogeneous catalysts. They can be easily modified by changing linkers substituents to increase affinity for reactants, or by growing the number of active catalytic sites.

In this review, we have explored the ability of MOFs, MOF nanocomposites and mixed metal species toward olefin epoxidation and carbon dioxide cycloaddition.

We have observed that the olefin conversion and the epoxide selectivity are strongly dependent on the metal nodes/clusters, Co and Cu species being the most efficient, in some cases as for the epoxidation of a-pinene by Co-MOF-150-2 a conversion and an epoxide selectivity close to 100% being found.

Mixed metal MOFs can be also successfully employed in styrene and cyclohexene epoxidation, the best results being obtained with Cu/Co, Mn/Cu, and Ni/V species.

Selected functional groups introduced in organic linkers can also act as catalytically active sites. Amino, pyridyl, amide and sulfonic acid groups, but also metalloporphyrins, vanadium and molybdenum acetylacetonate, tartaric acid, salen and analogous molecules can be inserted or deposited to obtain also greater selectivity. UiO-66, UiO67, and PCN-224, appropriately functionalized can induce a complete conversion and selectivity as in the case of the geraniol epoxidation.

MOF-based composites are often employed to increase the hydrostability of selected MOFs or to perform epoxidation also of specific substrates as norbornene or octadecatriene. Specifically, a porous dendrimer-like porous silica nanoparticles (DPSNs) used as a carrier to support Cu-BTC NPs overcame the poor hydrostability of [Cu3-BTC2] MOF achieving high catalytic activity without by-products under mild reaction conditions.

Finally, MOFs and MOF-based composites show a great efficiency toward CO2 cycloaddition to epoxides, conversion being generally in the range 70–100% and selectivity close to 100%. The use of chiral ligands and amine-functionalized ligands seems to be very promising. The CO2 binding mode can in fact open new strategies for activation of CO2 and its transformation.

However, the low reactivity and inert nature of CO2 make its incorporation and activation into organic substrates still a challenge. Currently, the heterogeneous MOFsbased catalysts, as well as the technical system, remain at the laboratory scale and that makes the costs of productions of these materials extremely pricey. It is desirable that the improvement of MOFs-based catalysts might lead to technically viable efficiencies to industrial production to allow their large-scale application, in the next future. This review clearly shows that MOFs are now perspective materials and valid candidates for catalytic epoxidation and CO2 cycloaddition reactions.

**Author Contributions:** Conceptualization, A.T. and C.P.; methodology, A.T. and C.P.; software, A.T. and C.P.; validation, A.T. and C.P.; data curation, A.T. and C.P.; writing—original draft preparation, A.T. and C.P.; writing—review and editing, A.T. and C.P.; funding acquisition, C.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by University of Camerino.

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


**2.1. The parity operator**

Let us consider the parity operator in (3+1)QED [17]

$$
\widehat{P} = i\gamma\_4 \widehat{\Lambda}\_{\mathbf{n}}.\tag{1}
$$

Here, *iγ*4 is the inversion operator and

$$
\widehat{\Lambda}\_{\mathbf{n}} = e^{-i\frac{\pi}{2}\Sigma\mathbf{n}} = -i\Sigma\mathbf{n}
$$

is the operator of rotation by *π* about an **n** axis perpendicular to the graphene plane. In standard representation,

$$
\Sigma = \begin{pmatrix} \sigma \ 0 \\ 0 \end{pmatrix}\_{\prime}
$$

where σ denotes Pauli matrices, and

$$
\gamma\_4 \equiv \mathcal{J} = \begin{pmatrix} I & 0 \\ 0 & -I \end{pmatrix}
$$

where *I* is the 2×2 unit matrix. It is clear that operator (1) is analogous to the parity operator *iγ*5*n* in QED, where *n* = γ**n**,

$$
\gamma = \begin{pmatrix} \begin{smallmatrix} 0 & \sigma \\ -\sigma & 0 \end{smallmatrix} \end{pmatrix}, \quad \gamma\_5 = \gamma\_1 \gamma\_2 \gamma\_3 \gamma\_4 = i \begin{pmatrix} \begin{smallmatrix} 0 \\ I & 0 \end{smallmatrix} \end{pmatrix}.
$$

The eigen functions of this operator describe electron polarization states [18].

Charge carrier states in graphene can be described in terms of helicity defined as the eigenvalue of the operator *h* = σ · **p**/(2|**p**|). The projection of pseudospin on the direction of quasimomentum *p* indicates the valley in the Brillouin zone where electrons or holes belong (*K* or *K* point in Fig. 2). Positive helicity corresponds to electrons and holes with wavevectors near the *K* and *K* points, respectively; negative helicity corresponds to electrons and holes with wavevectors near the *K* and *K* points, respectively [19].

Massless states with opposite helicities are decoupled [20]. In addition, charge carriers have chiral symmetry (helicity is conserved), and parity can be defined for both massless and massive carriers1. In other words, a higher symmetry of massless charge carriers implies the existence of an additional quantum number: helicity. Whereas parity distinguishes only between the valleys where carrier states belong (*λ* = +1 and −1 for states close to the *K* and *K* points, respectively), helicity differs between a particle (electron) and an antiparticle (hole). However, chiral symmetry is broken for massive charge carriers. So, helicity is not a good quantum number any longer. Carrier states in a planar heterostructure combining gapless and gapped graphene should be characterized by parity.

Recall that the Dirac equation describing massless carriers in graphene in terms of 4×4 matrices is derived by assuming that they are spinless and have two valley degrees of freedom [10]. When analysis is restricted to charge carriers in one valley, the Dirac equation can be reduced to a 2×2 matrix representation by Weyl's equation for a massless fermion analogous to neutrino in two Euclidean dimensions [22]. The carrier energy spectrum with a pseudospin splitting in a planar heterostructure combining gapless and gapped graphene cannot be correctly analyzed in the 2×2 representation. For similar reasons, the representation of the Dirac algebra in terms of 2×2 matrices is not sufficient for describing the chiral symmetry breaking in (2+1)QED [23].

Using the two-dimensional 4×4 Dirac equation to describe charge carriers in a graphene-based nanostructure, we can study pseudospin effects following an approach to narrow-gap semiconductor heterostructures based on the Dirac model [24]. This makes methods developed for solving problems in the spintronics of narrow-gap semiconductor heterostructures applicable to graphene-based ones [25–33].


*3.1. Characteristics of Participants*

A total of 1168 students completed the survey (1237 students were informed and the valid rate was 94.4%). The age of the participants was 17–25; the numbers of freshmen, sophomores, juniors, seniors and graduate students were 397, 328, 102, 68 and 273, respectively. Of the respondents, 34.93% were female and 65.07% were male. A total of 599 lived in an urban region and 569 lived in a rural region. The annual household incomes were above 0.8 million (0.17%), 0.4–0.8 million (1.71%), 0.2–0.4 million (11.39%) and below 0.2 million (86.73%). About 99.66% participants were healthy and 0.34% were exposed, and no suspicion or infected existed. The number of participants who lived in the low risk regions was 385 (32.96%); those in the middle risk regions and the high risk regions were 681 (58.30%) and 102 (8.73%), respectively. The demographic characteristics of participants is summarized in Table 1.

**Table 1.** Demographic characteristics of participants (*n* = 1168).