Cyclohexene Oxide Research Articles

Evaluating Heterodinuclear Mg(II)M(II) (M = Mn, Fe, Ni, Cu, and Zn) Catalysts for the Chemical Recycling of Poly(cyclohexene carbonate).

Polymer chemical recycling to monomers (CRM) is important to help achieve a circular plastic economy, but the "rules" governing catalyst design for such processes remain unclear. Here, carbon dioxide-derived polycarbonates undergo CRM to produce epoxides and carbon dioxide. A series of dinuclear catalysts, Mg(II)M(II) where M(II) = Mg, Mn, Fe, Co, Ni, Cu, and Zn, are compared for poly(cyclohexene carbonate) depolymerizations. The recycling is conducted in the solid state, at 140 °C monitored using thermal gravimetric analyses, or performed at larger-scale using laboratory glassware. The most active catalysts are, in order of decreasing rate, Mg(II)Co(II), Mg(II)Ni(II), and Mg(II)Zn(II), with the highest activity reaching 8100 h-1 and with >99% selectivity for cyclohexene oxide. Both the activity and selectivity values are the highest yet reported in this field, and the catalysts operate at low loadings and moderate temperatures (from 1:300 to 1:5000, 140 °C). For the best heterodinuclear catalysts, the depolymerization kinetics and activation barriers are determined. The rates in both reverse depolymerization and forward CHO/CO2 polymerization catalysis show broadly similar trends, but the processes feature different intermediates; forward polymerization depends upon a metal-carbonate intermediate, while reverse depolymerization depends upon a metal-alkoxide intermediate. These dinuclear catalysts are attractive for the chemical recycling of carbon dioxide-derived plastics and should be prioritized for recycling of other oxygenated polymers and copolymers, including polyesters and polyethers. This work provides insights into the factors controlling depolymerization catalysis and steers future recycling catalyst design toward exploitation of lightweight and abundant s-block metals, such as Mg(II).

Fabrication of single Co sites in graphitic carbon nitride via the ion exchange to boost aerobic cyclohexane oxidation

Cyclohexane oxidation is an industrial process for manufacturing cyclohexanol, cyclohexanone and adipic acid, while it remains a huge challenge to develop highly efficient catalysts for intensifying this process. Herein, a bottom-up synthesis approach, coupling the one-pot and ion exchange strategy, is developed to fabricate atomically dispersed Co on graphitic carbon nitride (g-C3N4) for the aerobic cyclohexane oxidation. The loading of the single-atom Co on Co/g-C3N4 can be facilely tuned by adjusting the ion exchange extent of Na + on Na/g-C3N4 precursor. Comprehensive characterizations demonstrate the atomical location of Co in sixfold cavities of g-C3N4, accompanied by the electron interaction between Co atoms and g-C3N4. The resulting Co/g–C3N4–1 catalyst possesses the superior oxidation performance with the 28.8 % cyclohexane conversion and 93.0 % overall selectivity toward desired products, and it also exhibits an acceptable universality to the catalytic oxidation of various hydrocarbons. The mechanism study revealed that besides the dissociation of O2 molecules on single-atom Co sites, the electron-rich g-C3N4 surface stemming from atomic Co doping facilitated the cyclohexane adsorption, which synergically boosted the subsequent oxidation reaction following the surface catalytic mechanism. We anticipate that such a concept of fabricating atomically dispersed metals via the ion exchange strategy provides a novel insight and feasible approach for the design of highly efficient single-atom catalysts for the cyclohexane oxidation and beyond.

Autoignition of cyclohexane at low-to-intermediate temperatures: Rapid compression machine experiments and improved comprehensive chemical kinetic model

Cycloalkanes are an important class of hydrocarbons found in conventional and alternative transportation fuels. Being one of the simplest practical cycloalkanes, cyclohexane is often used as a surrogate in studies to understand the oxidation, ignition, and sooting characteristics of cycloalkanes more broadly. Although cyclohexane has been extensively studied, very limited datasets for cyclohexane oxidation at low-to-intermediate temperatures are available in the literature. Moreover, the literature autoignition data from rapid compression machines (RCMs) did not provide information on volume histories or experimental heat transfer characteristics which can significantly impact model predictions, thereby hindering the detailed validation of chemical kinetic models. It has also been observed that the cyclohexane kinetic models developed so far are not able to predict the literature data across a wide range of operating and mixture conditions. Hence, further studies are recommended to understand and improve model predictions for cyclohexane oxidation. In this study, RCM experiments were conducted using cyclohexane/air mixtures at varying compressed pressures (PC =15, 20, and 30 bar), equivalence ratios (ϕ=0.5, 1.0, and 2.0), and compressed temperatures (TC=670–960 K). The current RCM results of cyclohexane were found to complement well with the literature data, with the first-stage IDTs at high pressures and volume histories at all test conditions being first reported for better development and validation of chemical kinetic models. In addition, a detailed chemical kinetic model for the cyclohexane oxidation has been developed and validated using both newly acquired experimental IDTs in this study and literature experimental studies (IDTs in RCMs and STs, species profiles in jet-stirred reactors, and laminar burning velocities). Unlike literature models, the proposed model was observed to provide an overall better prediction for experimental results over a wide range of conditions. Chemical kinetic analysis of the developed model was further conducted to identify the reaction pathways controlling the autoignition of cyclohexane.

Selective oxidation of cyclohexane over nanostructured manganese tungstate: Contribution of Mn and W related sites

Nanostructured MnWO4 catalysts prepared with different initial pH exhibited systematic variation in the activity in the selective oxidation of cyclohexane. 2-propanol TPD showed that the ratio of Mn to W related sites increased with the initial synthesis pH. W sites were found to be mainly active in the formation of CHHP and dehydrogenation of cyclohexanol, while Mn sites appeared to be more active in decomposition of CHHP and deep oxidation, giving a significant selectivity to adipic acid. The high yield of KA oil at intermediate synthesis pH suggests an optimum balance of Mn and W surface sites.

The Deposition of Isolated Fe(3+) Species in Mesoporous Silicon for Oxidation of Cyclohexane

AbstractThe balance of activity and selectivity in liquid alkane oxidation is challenging for the design of supported Fe catalysts. When Fe species leach to the solvent, uncontrolled free radical chain reactions happen. Herein, isolated Fe(3+) species were constructed by forming a strong Fe−O−Si bond for the selective oxidation of cyclohexane to cyclohexanone by H2O2. Compared to the supported FeOx clusters, the strong Fe−O−Si bond between isolated Fe(3+) species and SiO2 prevents the leaching of Fe in strong oxidation (H2O2) reaction conditions and dominates the non‐free radical mechanism. The turnover frequency over 10FeOx/SBA‐15 reached 15.2 h−1, higher than for reported Fe‐based catalysts. High selectivity of cyclohexanone is maintained at different conversions. Moreover, the (SiO)xFe3+(OH)3‐x−OOH active species were detected by Raman and FTIR and are generated from the oxidation of isolated Fe species. The strong Fe−O−Si bond and non‐free‐radical mechanism by (SiO)xFe3+(OH)3‐x−OOH active species induce high activity and selectivity for the oxidation of many other alkanes.

SiO2 supported Co3O4 catalyst for selective oxidation of cyclohexene with molecular oxygen

SiO2 supported Co3O4 catalyst for selective oxidation of cyclohexene with molecular oxygen

Metal-catalyzed copolymerizations of epoxides and carbon disulfide for high-refractive index low absorbance adhesives and plastics.

High-refractive index plastics are useful materials due to their optical properties, ease of processing, and low-costs compared to their inorganic counterparts. Catalytic carbon disulfide (CS2) copolymerization with epoxides is one method for producing low-cost high refractive index polymers. The reaction is accompanied by an oxygen-sulfur exchange reaction which produces irregular microstructures in the repeating units. In this study, metal salen catalysts were investigated with different metal centers (Al, Cr, Co) and salen ligand electronics, sterics, backbones, and co-catalyst in the copolymerization of CS2 with propylene oxide (PO) and cyclohexene oxide (CHO). The results reveal the essential nature of Cr metal centers on reactivity and the backbone geometry on monomer selectivity. There were no significant impacts on the O-S exchange reaction when ligand design changed, however PO and CHO/CS2 copolymers yield different monothiocarbonate microstructures. Additionally, the effects of microstructure on optical and thermal properties were investigated using spectroscopic ellipsometry and calorimetry, respectively. The CHO system produced high T g plastics (93°C) with high refractive indexes (n up to 1.64), modest absorbance (κ < 0.020), and Abbe numbers of 32.2 while PO yielded low T g adhesives (T g = 9°C) with high refractive indexes (n up to 1.73), low absorbance (κ < 0.005), and low Abbe numbers (V D = 19.1).

An Overview of Heterogeneous Transition Metal‐Based Catalysts for Cyclohexene Epoxidation Reaction

AbstractCyclohexane epoxide, which contains highly active epoxy groups, plays a crucial role as an intermediate in the preparation of fine chemicals. However, controlling the epoxidation pathway of cyclohexene is challenging due to issues such as the allylic oxidation of cyclohexene and the ring opening of cyclohexane epoxide during the cyclohexene epoxidation process to form cyclohexane oxide. This review focuses on the structure‐activity relationships and synthesis processes of various heterogeneous transition metal‐based catalysts used in cyclohexene epoxidation reactions, including molybdenum(Mo)‐based, tungsten(W)‐based, vanadium(V)‐based, titanium(Ti)‐based, cobalt(Co)‐based, and other catalysts. Initially, the mechanism of cyclohexene epoxidation by transition metal‐based catalysts is examined from the perspective of catalytic active centers. Subsequently, the current research of cyclohexene epoxidation catalysts is summarized based on the perspective of catalyst support. Additionally, the differences between alkyl hydroperoxide, hydrogen peroxide (H2O2), and oxygen (O2) as oxidants are analyzed. Finally, the main factors influencing catalytic performance are summarized, and reasonable suggestions for catalyst design are proposed. This work provides scientific support for the advancement of the olefin epoxidation industry.

Phenylalanine-Based Co2+ and Cd2+ 1D Coordination Polymers: Structural Properties and Catalytic Application for Solvent-Free Aerobic Oxidation of Cycloalkene.

Two 1D coordination polymers (CPs) with general formula [M(L)(H2O)(AcO)]n, (M = Co (1) or Cd (2), AcO = acetate anion and L denotes l-phenylalanine based ligand), were synthesized and fully characterized by various spectroscopies (UV-vis, FTIR, and NMR), thermal techniques, magnetic measurements (for 1), and single-crystal and powder X-ray diffraction studies. They can be described as "ribbon-like" 1D polymers constructed through a zigzag arrangement. The polymeric structure is developed due to the coordination mode adopted by the amino acid ligand, classified as μ3-N1O1:O1:O2, which simultaneously links three metal centers. This moiety also plays an important role as a magnetic coupler between metal centers in the cobalt system, which shows a weak antiferromagnetic interaction. Both CPs have also been used in the catalytic oxidation of cyclohexene with molecular oxygen (O2) as an oxidant. Under mild conditions, both compounds demonstrated remarkable catalytic activity, with the cobalt system being more efficient than the cadmium analogue (conversion: 73 and 58% and selectivity for the major product, 2-cyclohexanone: 63 and 55%, for 1 and 2, respectively). Leaching experiments and the results obtained using a radical quencher are consistent with a radical-mediated mechanism for the Co compound. The presence of the superoxide radical was also confirmed using EPR spectroscopy and DMPO as a spin trap, which was further validated by DFT calculations. The activity observed for the Cd analogue is attributed to the organic scaffold assisted by the templating effect of the metal ion.

Highly efficient one-pot aerobic oxidation of cyclohexane to adipic acid using task-specific metal-based ionic liquid

Adipic acid is a significant building block of polymers and is manufactured on a massive scale, primarily to produce nylon. However, the current route depends heavily on using corrosive HNO3 while releasing N2O as a potent greenhouse gas. Herein, we developed an efficient strategy for the highly selective aerobic oxidation of cyclohexane into adipic acid using metal-based ionic liquid as a catalyst. Particularly, the optimised catalyst 1-carboxymethyl-3-methylimidazolium chloride cobalt exhibited a high selectivity towards adipic acid (73.9%). It was discovered that the electron-donating ability of anions and the electrostatic between cations and anions could influence the charge density of Co(II) by anion coordination and then influence the oxidation performance. Meanwhile, the hydrogen bond interaction between the hydrogen-bond donor groups of the IL and cyclohexanone, as well as between the hydrogen-bond donor groups of the IL and the carboxylic radicals could promote the formation of adipic acid. This simple, efficient and green protocol has great potential for industrial applications in adipic acid production.

Cyclo-addition of carbon dioxide to epoxides employing hierarchical silicoaluminophosphates molecular sieves

Hierarchical silicoaluminophosphates comprising dual micro/meso pore structures have been explored, and proven to be effective, robust, and recyclable materials for the production of cyclic carbonates in the absence of co-catalyst. Upon activation at the temperature range of 80–100 °C, treated SAPOs have been examined as catalysts for the reaction. The crystalline structures of the hierarchical SAPO were checked by P-XRD. FE-SEM images confirmed the morphology and TEM measurement showed the periodicity of the framework. The BET analysis revealed the surface modification. In the cycloaddition of CO2 to epoxide (cyclohexene oxide, and styrene oxide), the hierarchical silicoaluminophosphate materials were explored where the activity of post-synthetic treated materials as catalyst was discovered to be superior. 100 % and 93 % reactant conversion were found for treated SAPO-34 and SAPO-5 catalyst, respectively.

Stereoselective Copolymerization of Ring-Strained Pentacyclic Anhydride with Cyclohexene Oxide: Cocatalyst-to-Catalyst Ratio Dependence of Polymerization Mechanism

Stereoselective Copolymerization of Ring-Strained Pentacyclic Anhydride with Cyclohexene Oxide: Cocatalyst-to-Catalyst Ratio Dependence of Polymerization Mechanism

Multinuclear Zinc-Magnesium Hydroxide Carboxylates: A Predesigned Model System for Copolymerization of CO2 with Epoxides.

Among numerous catalysts in the ring-opening copolymerization of epoxides with carbon dioxide (CO2), zinc dicarboxylate complexes are the most common type, and in the family of metal-based homogeneous catalysts, zinc and magnesium complexes have attracted widespread attention. We report on the synthesis and structural characterization of a zinc-magnesium benzoate framework templated by the central hydroxide anion with μ3-κ2:κ2:κ2 coordination mode, [ZnMg2(μ3-OH)(O2CPh)5]n (n = 1 or 2). The resulting heterometallic system forms stable Lewis acid-base adducts with tetrahydrofuran (THF) and cyclohexene oxide (CHO), which crystallize as the hexanuclear zinc-magnesium hydroxide carboxylate cluster [ZnMg2(μ3-OH)(O2CPh)5(L)2]2 (L = THF or CHO). Their X-ray crystal structure analysis revealed that the Zn center prefers 4-fold coordination and the Mg centers demonstrated the ability to accommodate higher coordination numbers, and as a result, the heterocyclic molecules are exclusively bonded to 6-fold Mg atoms. The heteronuclear carboxylate aggregates appeared active in the copolymerization reaction at elevated temperatures to produce an alternating poly(cyclohexene carbonate).

Efficient cycloaddition of carbon dioxide with epoxide to produce five-membered cyclic carbonates by amine-functionalized silicoaluminophosphates molecular sieves

The synthesis of functionalized microporous materials as very effective catalysts is a significant and rapidly increasing research topic with the goal for providing more efficient energy alternatives and contributing to the reduction of atmospheric CO2 levels. Carbon dioxide is a greenhouse gas that traps heat in the atmosphere, contributing to global warming and climate change. SAPO-5 and SAPO-34 molecular sieves were post-synthetically –NH2 functionalized, and the existence of –NH2 functionalities in the SAPO structure wall was demonstrated by FT-IR, CHN analysis. Powder XRD confirmed the crystalline structures of the modified SAPO-34 materials with amine functionalities. FE-SEM examined the morphology of the materials. The functionalized silicoaluminophosphate materials were investigated in the cyclo-addition of CO2 to epoxide (cyclohexene oxide and styrene oxide), where the catalytic performance of post-synthetically functionalized materials as catalyst were found to be superior. The amine group of the silicoaluminophosphates materials were essential for the activation of CO2. For treated SAPO-34, reactant conversion in cyclohexene oxide and styrene oxide was reported to be 94% and 93%, respectively. Because of the high recyclability of these materials, the catalysts are found to be both cost-effective and environmentally benign.

Liquid Phase Oxidation of Cyclohexene to Cyclohexenone over M (M = Co, Mn, Cr, Cu)/UiO-66-Sal Catalysts

Liquid Phase Oxidation of Cyclohexene to Cyclohexenone over M (M = Co, Mn, Cr, Cu)/UiO-66-Sal Catalysts

Catalytic cyclohexane oxidation to KA oil using novel hydrosoluble copper(I) complexes bearing aminophosphine ligands: Water as a selectivity promoter

Reacting the alkylated 1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane (PTA), namely (PTA-CH2-C6H4-p-COOH)Br (1) or (PTA-CH2-C6H3-p-OH-m-CHO)Cl (2), with CuBr, CuI or [Cu(CH3CN)4]BF4, under mild conditions using different stoichiometric ratios resulted in the formation of the new water soluble copper complexes [CuBr2(PTA-CH2-C6H4-p-COOH)2]X (X = Br {3a}, I {3b} or BF4 {3c}), [CuBr(PTA-CH2-C6H4-p-COOH)3]Br3 (4) and [CuCl2(PTA-CH2-C6H3-p-OH-m-CHO)2]X (X = I {5a} or Cl {5b}). The compounds were characterized using multinuclear NMR, FTIR, ESI-MS and elemental analysis. The molecular structure of the neutral form of 3 was established by single crystal X-ray diffraction. In a homogeneous aqueous medium, the catalytic activity of the hydrophilic complexes was assessed for the cyclohexane oxidation to the KA oil (a mixture of cyclohexanone and cyclohexanol). The selectivity towards cyclohexanone is controlled by water, increasing with the H2O amount, and becoming higher than that of cyclohexanol for the 25–75% range of water in the solvent composition. Complex 5a exhibited the highest catalytic activity, achieving up to 21% conversion after 4 h at 50 °C using 0.5 mol% of the catalyst. Utilizing a similar loading of 5a, under microwave irradiation, afforded 18% of yield after 1.5 h at 70 °C.

The synthesis of multifunctional cellulose graft alternating copolymers of 3,4-dihydrocoumarin and epoxides in DBU/DMSO/CO2 solvent system

Cellulose graft copolymers having well-defined structures could incorporate the characteristics of both the cellulose skeleton and side chains, providing a new method for the preparation functionalised cellulose derivatives. Herein, a series of multifunctional cellulose grafted, alternating 3,4-dihydrocoumarin (DHC) and epoxide (EPO) copolymers (cell-g-P(DHC-alt-EPO)) were prepared in a metal-free DBU/DMSO/CO2 solvent system without adding additional catalyst. Four examples of cell-g-P(DHC-alt-EPO) with tunable thermal and optical properties were synthesized by copolymerization of DHC with styrene oxide (SO), propylene oxide (PO), cyclohexene oxide (CHO) or furfuryl glycidyl ether (FGE) onto cellulose. The nonconjugated cell-g-P(DHC-alt-EPO) showed UV absorption properties with the maximum absorption peak at 282 nm and 295 nm and photoluminescence performance. A clustering-triggered emission mechanism was confirmed and consistent with DFT theoretical calculations. In DMSO solution, the copolymer (DHCSO5) with DP of 11.64 showed ACQ behaviour as the concentration increased. In addition, DHCSO5 had good antioxidant capacity with an instantaneous radical scavenging activity of 2,2-diphenyl-1-picrylhydrazine (DPPH) up to 65 % at a concentration of 40 mg/ ml and increased to 100 % after 30 min. Thus, the multifunctional cell-g-P(DHC-alt-EPO) materials had a variety of potential applications in the fields of fluorescent printing, bio-imaging, UV- shielding and antioxidants.

Performance of cross-linked enzyme crystals of engineered halohydrin dehalogenase HheG in different chemical reactor systems.

Halohydrin dehalogenase HheG is an industrially interesting biocatalyst for the preparation of different β-substituted alcohols starting from bulky internal epoxides. We previously demonstrated that the immobilization of different HheG variants in the form of cross-linked enzyme crystals (CLECs)yielded stable and reusable enzyme immobilizes with increased resistance regarding temperature, pH, and the presence of organic solvents. Now, to further establish their preparative applicability, HheG D114C CLECs cross-linked with bis-maleimidoethane have been successfully produced on a larger scale using a stirred crystallization approach, and their application in different chemical reactor types (stirred tank reactor, fluidized bed reactor, and packed bed reactor) was systematically studied and compared for the ring opening of cyclohexene oxide with azide. This revealed the highest obtained space-time yield of 23.9 kgproduct gCLEC -1 h-1 Lreactor volume -1 along with the highest achieved product enantiomeric excess [64%] for application in a packed-bed reactor. Additionally, lyophilization of those CLECs yielded a storage-stable HheG preparation that still retained 67% of initial activity (after lyophilization) after 6 months of storage at room temperature.

Turning on Asymmetric Catalysis of Achiral Metal-Organic Frameworks by Imparting Chiral Microenvironment.

The development of heterogeneous asymmetric catalysts has attracted increasing interest in synthetic chemistry but mostly relies on the immobilization of homogeneous chiral catalysts. Herein, a series of chiral metal-organic frameworks (MOFs) have been fabricated by anchoring similar chiral hydroxylated molecules (catalytically inactive) with different lengths onto Zr-oxo clusters in achiral PCN-222(Cu). The resulting chiral MOFs exhibit regulated enantioselectivity up to 83 % ee in the asymmetric ring-opening of cyclohexene oxide. The chiral molecules furnished onto the catalytic Lewis sites in the MOF create multilevel microenvironment, including the hydrogen interaction between the substrate and the chiral -OH group, the steric hindrance endowed by the benzene ring on the chiral molecules, and the proximity between the catalytic sites and chiral molecules confined in the MOF pores, which play crucial roles and synergistically promote chiral catalysis. This work nicely achieves heterogeneous enantioselective catalysis by chiral microenvironment modulation around Lewis acid sites.

Turning on Asymmetric Catalysis of Achiral Metal‐Organic Frameworks by Imparting Chiral Microenvironment

AbstractThe development of heterogeneous asymmetric catalysts has attracted increasing interest in synthetic chemistry but mostly relies on the immobilization of homogeneous chiral catalysts. Herein, a series of chiral metal–organic frameworks (MOFs) have been fabricated by anchoring similar chiral hydroxylated molecules (catalytically inactive) with different lengths onto Zr‐oxo clusters in achiral PCN‐222(Cu). The resulting chiral MOFs exhibit regulated enantioselectivity up to 83 % ee in the asymmetric ring‐opening of cyclohexene oxide. The chiral molecules furnished onto the catalytic Lewis sites in the MOF create multilevel microenvironment, including the hydrogen interaction between the substrate and the chiral −OH group, the steric hindrance endowed by the benzene ring on the chiral molecules, and the proximity between the catalytic sites and chiral molecules confined in the MOF pores, which play crucial roles and synergistically promote chiral catalysis. This work nicely achieves heterogeneous enantioselective catalysis by chiral microenvironment modulation around Lewis acid sites.