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Review

Recent Advances in Greener and Energy Efficient Alkene Epoxidation Processes

by
Misbahu Ladan Mohammed
1 and
Basudeb Saha
2,*
1
Department of Pure and Applied Chemistry, Usmanu Danfodiyo University, Sokoto 840004, Nigeria
2
Engineering Department, Lancaster University, Lancaster LA1 4YW, UK
*
Author to whom correspondence should be addressed.
Energies 2022, 15(8), 2858; https://doi.org/10.3390/en15082858
Submission received: 27 January 2022 / Revised: 29 March 2022 / Accepted: 11 April 2022 / Published: 13 April 2022
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Abstract

:
The chemical industry is considered to be one of the largest consumers of energy in the manufacturing sector. As the cost of energy is rising rapidly, coupled with the increasingly stringent standards for the release of harmful chemicals and gases into the environment, more attention is now focused on developing energy efficient chemical processes that could significantly reduce both operational costs and greenhouse gas emissions. Alkene epoxidation is an important chemical process as the resultant epoxides are highly reactive compounds that are used as platform chemicals for the production of commercially important products for flavours, fragrances, paints and pharmaceuticals. A number of epoxidation methods have been developed over the past decade with the ultimate aim of minimising waste generation and energy consumption. In this review paper, some of the recent advances in epoxides synthesis using energy efficient processes are discussed. The epoxidation methods may provide sustainability in terms of environmental impact and energy consumption.

Graphical Abstract

1. Introduction

The rate of energy consumption, either for electricity, steam or heating purposes, has increased rapidly during the past decades due to rapid global developments in the transportation, industrial, agricultural, residential, commercial and other related sectors [1]. Fossil fuels including coal, oil, and natural gas are currently the largest sources of energy worldwide. However, the use of carbon-based fossil fuels has contributed significantly to an increase in the concentration of CO2 and related greenhouse gases in the atmosphere, with negative impacts on the environment [2,3,4]. Consequently, highly industrialised nations popularly known as the G8 have promulgated measures aimed at cutting greenhouse gas emissions associated with energy exploration and utilisation to prevent further deterioration of the environment [5,6]. In this respect, miniaturisation of chemical processes through intensification and integration of multiple operations plays a significant role by providing means of reducing energy consumption and waste generation [7,8,9,10].
Epoxides, also known as oxiranes, are mainly products of alkene epoxidation. They are key raw materials or intermediates in organic synthesis, particularly for the functionalisation of substrates and the production of a wide variety of chemicals such as pharmaceuticals, plastics, paints and adhesives [11,12,13,14,15]. The highest consumption of ethylene oxide (C2H4O) worldwide is in the production of ethylene glycol (C2H6O2), which is a building block in the production of pharmaceuticals, textiles, automobiles and detergents [16]. Similarly, propylene oxide (C3H6O) has found applications in the production of cosmetics, drugs, and plasticisers, and in the manufacture of unsaturated polyester resins used in the textile and construction industries [17]. Furthermore, epichlorohydrin (C3H5ClO) is an important epoxide that is widely employed in the manufacture of epoxy resins, glycerols, plastics and elastomers [18]. Epoxidation of limonene and α-pinene yields the corresponding 1,2-limonene oxide and α-pinene oxide, which are both vital intermediates for the production of fragrances, perfumes, food additives and pharmaceuticals [19].
The conventional epoxidation methods in the fine chemical industry employ in situ generated peracids such as peracetic (C2H4O3) or performic acid (CH2O3) as an oxidising reagent in liquid phase batch reactions in the presence of a mineral acid as a catalyst [20,21]. However, the employment of peracid for alkene epoxidation is not an environmentally benign method as an equivalent amount of acid waste is produced. In addition, there are safety issues associated with the handling and storage of peracid [21]. Molecular oxygen is probably the most suitable oxidant for alkene epoxidation in terms of environmental and economic considerations due to its high oxygen content and its ability to produce water as the only by-product. However, one of the major limitations of epoxidation with molecular oxygen is low product selectivity [22]. Similarly, hydrogen peroxide (H2O2) is another eco-friendly reagent for epoxidation because water is its only waste product [23]. As in the case with molecular oxygen, epoxidation with H2O2 can result in poor product selectivity [24], although researchers in recent years have developed catalytic systems that can activate the oxidant to yield up to 98% epoxide selectivity [25]. By comparison, there are limited applications of hypochlorites as oxygen sources for epoxidation due to their serious health and environmental hazards; these include the release of toxic gases such as chlorine when acidified or heated, and their reaction with ammonia or with substances that generate ammonia to yield chloramines, which are also toxic and have the explosive potential [26,27]. Alkyl hydroperoxides including tert-butyl hydroperoxide (C4H10O2) (TBHP), cumene hydroperoxide (C9H12O2) and ethylbenzyl hydroperoxide (C9H12O2) are commonly used as oxygen sources in epoxidation reactions since the reagents are readily available and inexpensive [28,29,30]. However, tert-butyl hydroperoxide (TBHP) has received considerable attention in recent times as an oxidant of choice for the reaction due to its numerous advantages including high thermal conductivity, good solubility in polar solvents, and neutral pH [31,32,33]. In addition, the oxidant is atom efficient since it yields tert-butanol, a major industrial feedstock, as a by-product [34,35,36].
There are numerous examples of epoxidation reactions carried out with a broad range of heterogeneous catalysts based on either transition metals or main group elements. Some of the catalysts tend to be more active on certain alkene substrates than others and the selectivity of a particular catalyst for epoxidation depends on the properties of the active component, the combination of ligands and the oxidant used. For instance, a number of heterogeneous catalysts have been developed for epoxidation reaction by immobilisation of catalytically active metal species on organic or inorganic materials, such as alumina [37,38], silica [39,40], zeolites [41,42], polymers [43], ion-exchange resins [44,45] and metal organic frameworks [46]. However, polymers have gained more attention as a suitable support for transition metal catalysts as they are inert, non-toxic, insoluble and often recyclable [11,12,13,31,34,47].
The main purpose of this review paper is to highlight some of the techniques being developed to achieve intensification of the alkene epoxidation process by reducing the volume of hazardous material and replacing large, energy-intensive equipment and stages with those that are smaller, less expensive and energy efficient.

2. Important Reactions of Epoxides

Epoxides are highly reactive cyclic ethers having a three-membered ring structure with an oxygen atom connected to two neighbouring carbon atoms by single bonds. The high reactivity of epoxides is due to their polarity and the strain of the three-membered ring, which weakens the carbon-oxygen (C-O) bond [48]. As shown in Scheme 1, epoxides can readily undergo ring-opening reactions and form a variety of products that can serve as a starting material or intermediate in the production of commercially important products for flavours, adhesives, fragrances, paints and pharmaceuticals [14,48,49]. The reactions involving epoxides are usually catalysed by many acidic and basic substances, and by acceptor-donor compounds [50,51].
The reaction of epoxides and CO2 yields carbonates and polycarbonates as two major valuable products [52]. However, the selectivity toward either of the products depends on the reaction conditions and the type of catalysts used [53,54]. A number of transition metal-based catalysts have shown good activity in the solvent-free synthesis of propylene carbonate by cycloaddition reaction of propylene oxide and CO2 under mild reaction conditions [55,56].
Epoxides can be cleaved in hydrolysis by either aqueous base (NaOH/H2O) or acid (H2SO4/H2O) to form vicinal diol in the SN2 or SN1 mechanism [57,58]. In the presence of an aqueous base, the nucleophile (H2O) attacks the least substituted carbon atom based on an SN2-like reaction leading to ring-opening of the epoxide, whereas in the aqueous acid solution, the more substituted carbon is the site of nucleophilic attack according to the SN1 displacement mechanism [59]. Similarly, epoxides undergo cleavage of the ether bond in the presence of anhydrous acids including HCl, HI, HBr to form halohydrin in the SN1- or SN2-like reactions, depending on whether the carbon attacked by the halogen anion is a primary, secondary or tertiary carbon [60,61].
Epoxides are capable of undergoing intramolecular rearrangement often initiated by Lewis or Brꝋnsted acids to form aldehydes or ketones [62,63,64]. Ring-opening of epoxide with Ketene Silyl Acetals is regarded as the best approach for the preparation of an important class of extensive biologically active compounds, γ-lactones [65,66,67,68]. Furthermore, the reduction of epoxide by lithium aluminium hydride (LiAlH4) occurs at the least sterically hindered side of the epoxide to give the corresponding alcohol [69,70]. The nucleophilic addition reaction of epoxides with carboxylic acids such as pivalic acid (C5H10O2) results in acidolysis ring-opening to form hydroxyalkyl esters [71]. However, when epichlorohydrin (C3H5ClO) is used as the epoxy compound, the hydroxyalkyl ester formed can further undergo the ring-closure reaction with alkali by dehydrochlorination to yield vital intermediates for the coating and polymer industries, such as glycidyl methacrylate [72,73] and glycidyl neodecanoate [74,75]. The reaction schemes of the notable reactions of epoxides and their products are presented in Scheme 1.

3. Energy Efficient Alkene Epoxidation Processes

The search for an energy efficient method for the preparation of epoxide has been an active research area in recent years due to the significance of the compound, not only as a synthetic end product but as a versatile intermediate in organic synthesis. To date, several methods of epoxide synthesis have been developed and most of these methods employ alkene as a starting material, which is subsequently oxidised by a suitable reagent to yield the desired product. However, some alternative methods that utilise aldehydes and imines for the production of epoxides have also been reported.

3.1. Continuous Flow Epoxidation Process

The development of a continuous flow approach to synthetic organic chemistry has received considerable interest over the last few decades [76,77,78]. The inherently closed nature of continuous flow reactors enhances the reaction safety due to proper containment of harmful or toxic reagents, and allows rapid optimisation of reaction conditions from a small quantity of reactants under different reaction conditions with minimal energy consumption. The FlowSyn reactor (supplied by Uniqsis Ltd.) is one of the energy efficient continuous flow reactors currently employed for laboratory-scale synthesis of various organic products [79]. We earlier employed the FlowSyn reactor for continuous epoxidation of 1-hexene (C6H12) and 4-vinyl-1-cyclohexene (C8H12) using tert-butyl hydroperoxide (TBHP) and a polystyrene 2-(aminomethyl)pyridine supported molybdenum(VI) complex (Ps.AMP.Mo) as a catalyst (Scheme 2) [12,31]. The catalyst was prepared by immobilising the molybdenum metal species derived from molybdenyl acetylacetonate (MoO2(acac)2) on an already synthesised cross-linked polystyrene-based resin code-named polystyrene 2-aminomethyl(pyridine) (Ps.AMP), as shown in Scheme 3, Scheme 4 and Scheme 5.
Continuous flow epoxidation has enabled rapid evaluation of the catalytic performance of Ps.AMP.Mo from a small quantity of reactants under different reaction conditions. For instance, the catalyst has demonstrated high catalytic performance in the continuous epoxidation of 4-vinyl-1-cyclohexene (C8H12) as compared to 1-hexene (C6H12). An experiment carried out at a feed molar ratio of 4-vinyl-1-cyclohexene (C8H12) to TBHP of 5:1, reaction temperature of 353 K and feed flow rate of 0.1 mL/min resulted in ~95% conversion of TBHP and ~82% yield of 4-vinyl-1-cyclohexane 1,2-epoxide (C8H12O) at steady state. However, continuous epoxidation of 1-hexene (C6H12) at similar conditions gave ~79% conversion of TBHP and ~64% yield of 1,2-epoxyhexane. The continuous epoxidation in a FlowSyn reactor has shown considerable time savings, high reproducibility and selectivity, along with remarkable improvements in catalyst stability compared with the reactions carried out in a batch reactor [13,80]. The schematic of the experimental set-up for the continuous epoxidation process in the FlowSyn reactor is shown in Figure 1.

3.2. Epoxidation in a Reactive Distillation Column (RDC)

Reactive distillation (RD) is a unit operation that combines chemical reaction and distillation in the same vessel in a single step. By conducting chemical reaction and product separation simultaneously, equilibrium can be shifted to achieve almost complete conversion of the feedstock [81,82,83,84,85,86,87]. Thus, RD technology has many key advantages, such as reduced investment and operating costs, and significant energy savings, as it can simplify complex separation and purification processes [88]. Saha and co-workers [81,82,83,84,85,86,87] designed an RDC for the continuous epoxidation process using molybdenum-based complexes as catalysts for epoxidation of alkenes/terpenes using tert-butyl hydroperoxide (TBHP) as an oxidant. The RDC consists of three distinct parts: a catalytic section containing the catalyst, packed in a well-structured “rolled belt” type catalyst packing. The catalytic section is enclosed between a non-reactive enriching section and a non-reactive stripping section, both packed with ceramic Raschig rings, as shown in Figure 2.
The epoxidation process developed by Saha and co-workers [81,82,83,84,85,86,87] may be termed as atom efficient since it forms tert-butanol, which is a valuable industrial feedstock as a co-product of the reaction. In addition, the process achieved nearly 100% conversion of cyclohexene [81,82,83,84,85,86,87] and 4-vinyl-1-cyclohexene (C8H12) [31] to cyclohexene oxide (C6H10O) and 4-vinyl-1-cyclohexane, 1-2-epoxide (C8H12O), respectively, under mild reaction conditions.
The conventional method for the production of epichlorohydrin (C3H5ClO) uses energy-intensive multistep processes involving the addition of chlorine to propene (C3H6) at high temperatures to give allyl chloride (C3H5Cl) and, subsequently, reacting the product with hypochlorous acid (HOCl) to form two dichlorohydrins isomers. Finally, dichlorohydrins are reacted with sodium hydroxide (NaOH) to yield epichlorohydrin (C3H5ClO) [89,90]. In addition to the high energy requirements, this method has major drawbacks, including the application of hazardous chlorine, low atom efficiency of the chlorine and a large number of by-products for disposal [91]. However, a more energy efficient and sustainable process for the production of epichlorohydrin (C3H5ClO) can be achieved by utilising glycerol (C3H8O3) based on two reaction steps: catalytic chlorination of liquid glycerol (C3H8O3) with gaseous hydrochloric acid (HCl), and then, reacting the dichlorinated compounds formed with an inorganic base in an RDC to form the product (Scheme 6) [92,93]. Although the stoichiometric bases used in the second step of the reaction may be soluble in the reaction mixture, thus increasing the difficulty in product isolation and purification, this problem can be surmounted when a heterogeneous catalyst is applied in the first step, and can be easily recovered from the reaction by filtration [94].

3.3. Microwave-Assisted Epoxidation

The microwave offers energy-efficient heating by the interaction of the generated electromagnetic radiations with the molecules (polar), thereby generating heat for the reaction without direct contact with the reaction mixture. The microwave provides substantial energy savings due to rapid and uniform heating during the chemical reaction in contrast to conventional heating by the conduction and convection mechanisms [95,96,97].
The epoxidation of oleic acid (C18H34O2) by peracetic acid carried out using microwave heating has recorded significant energy savings by enhancing the perhydrolysis step in the aqueous phase (Scheme 7) and reducing the reaction time by up to 50% compared with the reaction carried out using conventional heating with a heat exchanger [98]. The combination of microwave-induced heating and catalytic monolithic reactors has recorded higher energy efficiencies in the ethylene (C2H4) epoxidation to ethylene oxide (C2H4O) due to selective heating of the catalyst by the microwave compared to conventional electrically induced heating [99]. It was observed that the catalyst was heated rapidly under microwave irradiation, while the gas stream remains at a lower temperature. Thus, the gas temperature in the case of microwave heating was ~150 °C, about 70 °C lower in comparison with conventional heating conditions for conversion of 9% [99]. Epoxidation of α-pinene with molecular oxygen yields three products (Scheme 8) and the selectivity to the epoxide depends on the catalyst used and operating conditions [100].
Cobalt(II) acetate tetrahydrate (C4H6CoO4.4H2O) catalyst supported on Mordenite (zeolite), prepared by impregnation with microwave heating, resulted in up to 92.7 mol% conversion of α-pinene in 4 h at 90 °C in the epoxidation with air, which is about 1.5 times higher than the conversion obtained using the catalyst prepared by conventional impregnation heating [101]. The higher activity and selectivity recorded by the catalysts was attributed to the ability of the microwave heating to produce and uniformly disperse the CoOx nanoparticles in the mordenite support [101]. A comparison of microwave-assisted epoxidation of α-pinene and styrene with air and the traditional heating method under atmospheric pressure showed that using a microwave heating method may yield an excellent efficiency of epoxidation. For instance, when using microwaves to heat the epoxidation reaction, the conversion of α-pinene and styrene reaches 85.3% and 86.5%, respectively. However, under the same conditions, the reaction conducted using the traditional heating method shows that the substrates were substantially unreacted, resulting in only 5.2% and 3.4% conversion of α-pinene and styrene, respectively [102].

3.4. Epoxidation in Microreactors

Microreactors are chemical reactors having extremely small dimensions and made from silicon, polymers, glass, metal and other materials, employing a variety of fabrication techniques often requiring special equipment and skills to fit the intended reaction [103,104]. The small channel sizes provide a high surface area to volume ratio, which results in uniform heating across the reaction site, in addition to efficient heat and mass transfer characteristics [105,106]. Microreactors have helped to minimise reagent and energy consumption in chemical synthesis due to their small dimensions, and allow safe handling of hazardous or highly exothermic reactions [107,108].
A microreactor coated with a catalytic TiO2 layer has achieved higher oxirane selectivity (92%) in the epoxidation of methyl oleate (C19H36O2) with H2O2 as oxidant and recorded a higher reaction rate of epoxide production (23 times) compared with the reaction carried out in a conventional batch reactor with an average yield of 75% (Scheme 9) [109]. The higher epoxide selectivity obtained in the microreactor was attributed to the efficient mixing of reactants, high surface to volume ratio and accurate control of the substrate to oxidant ratio [109]. Moreover, a microfluidic device that generates dielectric barrier discharge (DBD) plasma at the gas–liquid interface has efficiently diffused the reactive oxygen species (ROS) to the liquid phase via microbubbles in trans-stilbene (C14H12) epoxidation to a trans-stilbene epoxide (C14H12O) [106]. The highest epoxide yield of ~94% was obtained at the optimum operating conditions of short bubble–liquid contact times (~2 s), with frequent exposure to freshly generated microbubbles containing reactive oxygen species by continuous liquid recirculation [106].
Monolithic microreactors are continuous unitary structures consisting of parallel channels, with the catalyst either incorporated into a thin layer of a porous oxide deposited on the channel wall (acting as support for the catalyst) or into the wall of the reactor itself [110,111]. A catalytic monolith reactor offers high catalytic efficiency in the chemical reaction due to high concentrations of active sites per unit volume, and provides better heat and mass transfer, and very low pressure drop along the channels [112,113,114]. A flow reactor used for the continuous epoxidation of styrene (C8H8) with TBHP as an oxidant in the presence of a monolith catalyst resulted in good conversion (55%) and excellent selectivity (74%) towards styrene oxide (C8H8O) within 50 min residence time, and allowed easy separation of the catalyst and recyclability of more than seven cycles without a significant loss in catalytic activity [115]. A comparison of a monolith reactor and a Confined Taylor Flow (CTF) reactor in the heterogeneous catalytic epoxidation of propene (C3H6) to propene oxide (C3H6O) with hydrogen peroxide revealed that the production rate of propene oxide (C3H6O) was higher in the monolith reactor in the entire range of operating pressures due to its larger catalyst coating area, larger mass-transfer surface area and more frequent recycling of liquid flow [116].

3.5. Sonochemical Synthesis of Epoxides

Sonochemical synthesis (sonochemistry) involves chemical reactions between molecules due to the application of powerful ultrasound radiation usually in the range of 20 kHz–10 MHz [117]. The chemical reaction is derived from a physical phenomenon referred to as acoustic cavitation, which is responsible for the formation, growth, and collapse of tiny acoustic bubbles inside a liquid, thereby inducing high temperatures (up to 5000 K) and very high pressures (up to 1000 atm) inside such cavities, in addition to a shock wave at the interface and bulk liquids leading to enhanced heat and mass transfer, reduced reaction times and lower energy consumption [118,119,120,121].
A catalytic system consisting of a combination of mild mixing resulting from high-frequency ultrasonic irradiation (800 kHz) with precise temperature regulation of the double jacketed sonochemical reactor has achieved a higher yield of cyclooctene oxide (C8H14O) (96%) and selectivity (98%) within 30 min in the epoxidation of 1-octene mediated by H2O2 and H2WO4 compared with silent conditions (Scheme 10) [122]. Similarly, ultrasonic-assisted limonene epoxidation using in situ generated dimethyl dioxirane, C3H6O2 (DMDO), as the oxidising agent, achieved 100% yield of the limonene dioxide product within 4.5 min, compared to 97% yield obtained in the reaction conducted using conventional agitation with a magnetic stirrer after 1.5 h [123]. A proposed two-step mechanism for the reaction is presented in Scheme 11. Furthermore, epoxidation of α-pinene (C10H16) to α-pinene oxide (C10H16O) under ultrasound conditions by DMDO yielded 100% after 4 min, and took 60 min to achieve a similar yield using the traditional method [123].
Chavan and co-workers [124] observed a substantial reduction in the reaction time for soybean oil epoxidation using the ultrasonic horn in the presence of tetra-n-butyl ammonium bromide as a phase transfer catalyst for similar levels of conversion compared with the conventional stirring approach. It was found that the relative percentage conversion to epoxide using the conventional method was about 87% in 10 h, whereas the reaction carried out in an ultrasound horn under optimised conditions (20 kHz with pulse of 5 s ON and 5 s OFF) achieved almost 83% in 4 h of reaction time.

4. Conclusions

It can be deduced from the foregoing that the technologies used for the synthesis of epoxides are continuously being improved with a view to reducing the environmental impact, energy consumption and overall costs. The recent advances in this respect are geared towards the process intensification approaches, by utilising equipment and processes that are smaller, safer, less expensive and more energy efficient. Thus, industrial application of the energy efficient epoxidation techniques may significantly reduce the cost associated with energy consumption, waste generation and product purification in epoxide production. For instance, the continuous flow epoxidation we previously carried out in a FlowSyn reactor shows substantial benefits, which include fast heat and mass transfer, short setup and reaction times, the flexibility of scaling-up reactions, complete non-attended operation and remarkable catalyst stability compared with the experiments carried out in a classical batch reactor under similar reaction conditions.
Although epoxidation processes described in this review have been successfully carried out at the laboratory scale, the development of scaled-up processes is recommended first of all to move the chemistry on from small-scale laboratory reactions to large-scale industrial production. The energy-efficient epoxidation techniques, including continuous flow chemistry and reactive distillation, in addition to microwave, microreactor and sonochemical syntheses, can significantly reduce the cost associated with energy consumption, waste generation and product purification in epoxide production.

Author Contributions

M.L.M.: Conceptualization, methodology, writing—original draft preparation, formal analysis; B.S.: Conceptualization, methodology, writing—review and editing, formal analysis. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

AMP2-aminomethyl pyridine
CTFConfined Taylor Flow
DBDDielectric Barrier Discharge
DMDODimethyl dioxirane
DVBdivinylbenzene
MoO2(acac)2Molybdenyl acetylacetonate
Ps.AMPPolystyrene 2-(aminomethyl) pyridine
Ps.AMP.MoPolystyrene 2-(aminomethyl) pyridine supported Mo(VI) complex
RDCReactive Distillation Column
ROSReactive Oxygen Species
TBHPtert-butyl hydroperoxide
VBCVinylbenzyl chloride

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Energies 15 02858 sch001 550
Scheme 1. Schemes of some important reactions of epoxide.
Scheme 1. Schemes of some important reactions of epoxide.
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Energies 15 02858 sch002 550
Scheme 2. Reaction schemes for epoxidation of (a) 1-hexene; (b) 4-vinyl-1-cyclohexene with TBHP catalysed by Ps.AMP.Mo complex.
Scheme 2. Reaction schemes for epoxidation of (a) 1-hexene; (b) 4-vinyl-1-cyclohexene with TBHP catalysed by Ps.AMP.Mo complex.
Energies 15 02858 sch002
Energies 15 02858 sch003 550
Scheme 3. Synthesis of poly (divinylbenzene-co-vinylbenzyl chloride-co-styrene) (Ps.VBC) resin.
Scheme 3. Synthesis of poly (divinylbenzene-co-vinylbenzyl chloride-co-styrene) (Ps.VBC) resin.
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Scheme 4. Synthesis of polystyrene 2-(aminomethyl)pyridine (Ps.AMP) beads.
Scheme 4. Synthesis of polystyrene 2-(aminomethyl)pyridine (Ps.AMP) beads.
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Scheme 5. Loading of Mo(VI) into polystyrene 2-(aminomethyl)pyridine (Ps.AMP) beads to produce polystyrene 2-(aminomethyl)pyridine supported Mo(VI) (Ps.AMP.Mo) complex.
Scheme 5. Loading of Mo(VI) into polystyrene 2-(aminomethyl)pyridine (Ps.AMP) beads to produce polystyrene 2-(aminomethyl)pyridine supported Mo(VI) (Ps.AMP.Mo) complex.
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Figure 1. Schematic representation of continuous epoxidation of 4-vinyl-1-cyclohexene with TBHP using a FlowSyn reactor.
Figure 1. Schematic representation of continuous epoxidation of 4-vinyl-1-cyclohexene with TBHP using a FlowSyn reactor.
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Figure 2. Schematic of an RDC set-up.
Figure 2. Schematic of an RDC set-up.
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Scheme 6. Reaction scheme for the conversion of glycerol into epichlorohydrin [93].
Scheme 6. Reaction scheme for the conversion of glycerol into epichlorohydrin [93].
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Scheme 7. Epoxidation of oleic acid using acetic acid by the Prilezhaev method [98].
Scheme 7. Epoxidation of oleic acid using acetic acid by the Prilezhaev method [98].
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Scheme 8. Possible products of α-pinene epoxidation with air.
Scheme 8. Possible products of α-pinene epoxidation with air.
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Scheme 9. Epoxidation of methyl oleate with H2O2.
Scheme 9. Epoxidation of methyl oleate with H2O2.
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Scheme 10. Epoxidation of 1-octene with H2O2.
Scheme 10. Epoxidation of 1-octene with H2O2.
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Scheme 11. Epoxidation mechanism of limonene to limonene dioxide by dimethyl dioxirane (DMDO) [123].
Scheme 11. Epoxidation mechanism of limonene to limonene dioxide by dimethyl dioxirane (DMDO) [123].
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Mohammed, M.L.; Saha, B. Recent Advances in Greener and Energy Efficient Alkene Epoxidation Processes. Energies 2022, 15, 2858. https://doi.org/10.3390/en15082858

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Mohammed ML, Saha B. Recent Advances in Greener and Energy Efficient Alkene Epoxidation Processes. Energies. 2022; 15(8):2858. https://doi.org/10.3390/en15082858

Chicago/Turabian Style

Mohammed, Misbahu Ladan, and Basudeb Saha. 2022. "Recent Advances in Greener and Energy Efficient Alkene Epoxidation Processes" Energies 15, no. 8: 2858. https://doi.org/10.3390/en15082858

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