Liquid-phase Oxidation Of Cyclohexane Research Articles

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

CeO2–TiO2 Nanoparticle-Grafted gC3N4 Sheets as an Efficient Catalyst for the Oxidation of Cyclohexane to KA oil

Here, we report a sustainable catalytic process operating on CeO2–TiO2 nanoparticle-grafted gC3N4 sheets (gC3N4/CeO2–TiO2) in the presence of a H2O2 green oxidant for the liquid phase oxidation of cyclohexane to produce KA oil (cyclohexanone + cyclohexanol). The process achieved enhanced cyclohexane conversion (62%) toward 95% KA oil (cyclohexanone 83% + cyclohexanol 12%), which is the highest ever reported to the best of our knowledge. The excellent catalytic behavior of gC3N4/CeO2–TiO2 in the present study due to its enhanced active sites, that is, Ce3+ and Ti3+, is facilitated by the redox reaction between CeO2–TiO2 and gC3N4 sheets. The high stability of gC3N4/CeO2–TiO2 catalyst in the liquid phase reaction study indicates the robust nature of this catalyst facilitated by strong interactions between CeO2–TiO2 species and electron-rich sites (N and π) of gC3N4 sheets. The overall study suggests that the gC3N4/CeO2–TiO2 material can be a promising catalyst for the sustainable production of KA oil from cyclohexane oxidation in the presence of the H2O2 oxidant.

Fabrication and Characterization of Cu Nanoparticles Dispersed on ZnAl-Layered Double Hydroxide Nanocatalysts for the Oxidation of Cyclohexane.

In the chemical industry, designing high-performance catalysts for the oxidation of cyclohexane into value-added products such as cyclohexanol and cyclohexanone (the combination is known as KA oil) is critical. The catalytic activity of copper nanoparticles supported on layered double hydroxide (LDH) for the liquid phase oxidation of cyclohexane was examined in this study. In this work, we have developed Cu nanoparticles supported on layered double hydroxide nanocatalysts, abbreviated as CuNPs@LDH, by the chemical reduction approach. Various physical methods were used to characterize the resulting material, including ICP-AES, XRD, FTIR, SEM, EDX, HRTEM, and BET surface area. The catalytic activity of copper nanoparticles supported on LDH was examined for the liquid phase oxidation of cyclohexane with tert-butyl hydroperoxide. CuNPs@LDH nanocatalysts with an excellent 52.3% conversion of cyclohexane with 97.2% selectivity of KA oil was obtained after 6 h at 353 K. The hot filtration test further indicated that CuNPs@LDH was a heterogeneous catalyst that could be recycled at least six times without suffering a substantial reduction in its catalytic activity.

Cover Feature: Liquid‐Phase Cyclohexene Oxidation with O2 over Spray‐Flame‐Synthesized La1−xSrxCoO3 Perovskite Nanoparticles (Chem. Eur. J. 68/2021)

The liquid-phase oxidation of cyclohexene was studied over a series of Sr-substituted La1−xSrxCoO3 catalysts of perovskite structure under mild reaction conditions with O2 as oxidant in an autoclave. The substitution of Sr into the perovskite structure led to the formation of catalytically active oxygen species. In-situ ATR IR measurements provided a comprehensive insight into the reaction mechanism under oxidation reaction conditions. More information can be found in the Full Paper by H. Wiggers, B. Peng et al. (DOI: 10.1002/chem.202103381).

Synthesis and characterization of novel transition metal complexes with L-Proline and their catalytic activity evaluation towards cyclohexane oxidation

Some of first row transition metals [M = Mn (II), Fe (II) and Co (II)] complexed with L-proline amino acid were synthesized by thermal heating and characterized by FT-IR, UV–visible spectroscopy, XRD, SEM and TEM. The characterization data showed that the metal ions were bonded through N- and O-donor atoms of the amino acid. The metals-L-proline complexes exist in mixed particle sizes of amorphous and agglomerated nature. The prepared complexes were investigated to catalyze the liquid phase oxidation of cyclohexane in the presence of TBHP as an oxidant. Iron-proline complex was found to be an excellent catalyst with high selectivity towards K/A oil under the optimized reaction conditions.

Mechanisms for the formation of ester compounds in the liquid-phase oxidation of cyclohexane

The channels of formation of ester compounds in the liquid-phase oxidation of cyclohexane are reviewed. At least 99.8% cyclohexyl esters of mono- and dicarboxylic acids are formed as a result of cyclohexanol acylation with mixed anhydrides including residues of all compounds with carboxyl groups present in the oxidized cyclohexane. The formation of mixed anhydrides is related to the fast reacylation of adipic anhydride with carboxylic acids. Alcoholysis of mixed anhydrides containing the formic acid residue with cyclohexanol leads mainly to cyclohexyl formate. The formation of e-caprolactone during cyclohexane oxidation is the result of the Baeyer–Villiger oxidation of cyclohexanone with peroxy acids through α-hydroxy-α-hydroperoxy ester. A decrease in the yield of e-caprolactone is reached under the conditions of decomposition or reduction of peroxy acids in the presence the compounds of variable valency metals or sulfur-containing reagents, respectively. The cobalt(II) and manganese(II) compounds stimulate the homolytic decomposition of α-hydroxy-α-hydroperoxy ester during which no lactone is formed. The cyclization of 5-hydroxypentanoic acid, which was formed by the oxidative transformations of cyclohexanone, leads to δ-valerolactone. The compounds of variable valency metals slightly affect the yield of δ-valerolactone. The mechanism of ester formation associated with the acylation of oxiranes formed by the radical chain oxidation of carbonyl compounds at the β-CH bonds was proposed.

Multilayer‐Functionalized Reduced Graphene Oxide Decorated with Gold Nanoparticles as a Designed Nanonanocatalyst for the Selective Oxidation of Cyclohexene by Molecular Oxygen in a Solvent‐Free System

The aerobic oxidation of cyclohexene is of great significance from the viewpoints of both fundamental and industry studies as it can transfer the petrochemical feedstock into valuable chemicals. In this research, gold nanoparticles were synthesized on the multi‐layer functionalized reduced graphene oxide. The surface of reduced graphene oxide (rGO) was modified with hydrophobic and hydrophilic layers to create the rGO with scattered hydrophilic positions. The gold nanoparticles were synthesized and immobilized simultaneously in small hydrophilic micro reactors in a mild condition. Characterization of synthesized nanocatalyst was confirmed with different techniques such as TEM, XRD, FT‐IR, and SEM. TEM images of synthesized catalyst show the gold nanoparticles have diameters less than 5 nm. Designed nanonanocatalyst was investigated for the selective liquid phase oxidation of cyclohexene with molecular oxygen in solvent free condition which after optimized conditions a maximum of 88% conversion and 91% selectivity was obtained.

Mechanisms of the formation of carboxylic acids and their anhydrides during the liquid-phase oxidation of cyclohexane

The channels of formation and transformation of bifunctional (C5, C6) and monofunctional acids (C1, C5, C6) and their anhydrides during the liquid-phase oxidation of cyclohexane are reviewed. Adipic acid and adipic anhydride are predominantly formed by the radical-chain oxidation of 2-hydroxycyclohexanone. Destructive transformations of 1-hydroxycyclohexyloxyl and cyclohexyloxyl (at a low conversion of cyclohexane) radicals, which are formed by the homolytic decomposition of 1-hydroperoxy-1-hydroxy- or 1-hydroxy-1-alkylperoxy-, and hydroperoxycyclohexane, respectively, afford 6-hydroxyhexanoic and caproic acids. 6-Oxohexanoic acid can be formed by the decomposition of 2-hydroperoxycyclohexanone via the nonradical and radical routes, as well as by the oxidation of 2-hydroxycyclohexanone with peroxide compounds. Valeric and 5-hydroxypentanoic acids are predominantly resulted from the destructive transformations of the 2-oxocyclohexyloxyl radical. The shortening of carbon chain is explained by decarboxylation, decarbonylation, and elimination of formic acid. The ring closure of 5-hydroxypentanoic acid to 8-valerolactone prevents its further oxidation. The schemes for the formation of 5-oxopentanoic and glutaric acids by the oxidation of 6-oxohexanoic acid at the C(5)-H bonds were proposed. The transacylation of the initially formed anhydrides with carboxylic acids leads to mixed anhydrides, and their alcoholysis by cyclohexanol affords all cyclohexyl esters of mono- and dicarboxylic acids.

Mesoporous Co3O4 as a New Catalyst for Allylic Oxidation of Cyclohexene

Mesoporous cobalt oxide was investigated for the liquid phase oxidation of cyclohexene using tertiobutylhydroperoxide (TBHP) as an oxidant. The results were compared with several series of supported cobalt catalysts to study the influence of the cobalt loading and solvents on the overall conversion and selectivity. Mesoporous cobalt was synthesized through the nanocasting route using siliceous SBA-15 mesoporous material as a hard template and cobalt nitrate as the cobalt oxide precursor. Supported cobalt oxide catalysts (Co/MxOy) were synthesized by the impregnation method using two loadings (1 and 5 wt.%) and Al2O3, TiO2, and ZrO2 as supports. Samples were characterised by means: elemental analysis, X-ray powder Diffraction (XRD), BET (surface area), UV-Vis DR Spectroscopy, and MET. The results obtained showed that the cobalt oxide retains the mesoporous structure of SBA-15, and in all Co/MxOy, crystalline Co3O4 and CoO phases are observed. The mesoporous cobalt oxide is more active than the supported cobalt catalysts in the allylic oxidation of cyclohexene, with a conversion of 78 % of cyclohexene and 43.3 % selectivity toward 2-cyclohexene-1-ol. The highest activity of mesoporous cobalt oxide could be ascribed to its largest surface area. Furthermore, Co3O4 has both Lewis and Brönsted acidic sites whereas Co/MxOy has only Lewis acidic sites, which could also explain its superior catalytic activity. Moreover, mesoporous cobalt oxide was more stable than supported cobalt catalysts. Therefore, this catalyst is promising for allylic oxidation of alkenes.

Fabrication of ultra‐small ruthenium nanoparticles on porous modified reduced graphene oxide and its application in solvent‐free oxidation of cyclohexene with molecular oxygen

In the present work, ruthenium nanoparticles were synthesized on functionalized reduced graphene oxide (rGO). First, the surface of rGO was modified with two para‐arsanilic acid and dithiooxamide promoters to create the rGO with scattered hydrophilic positions. The ruthenium nanoparticles were synthesized and immobilized simultaneously in small hydrophilic micro‐reactors under mild conditions. Characterization of the synthesized nanocatalyst was confirmed with different techniques, such as transmission electron microscopy (TEM), X‐ray diffraction, Fourier transform‐infrared and scanning electron microscopy. TEM images of the synthesized catalyst show the Ru nanoparticles have diameters less than 6 nm. The designed nanocatalyst was investigated for the selective liquid phase oxidation of cyclohexene with molecular oxygen in an autoclave under solvent‐free conditions, which obtained a maximum of 91% conversion and 90% selectivity under optimized conditions. The catalytic activity over rGO‐AO‐TO/Ru recycled catalyst remained at a satisfactory state after at least five runs.

Liquid-phase oxidation of cyclohexane. Cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone, mechanisms of formation and transformation

The mechanisms of formation and transformation of cyclohexyl hydroperoxide, cyclohexanol, and cyclohexanone in the liquid-phase oxidation of cyclohexane are reviewed. Cyclohexyl hydroperoxide is formed in chain propagation reactions and decomposes via the degenerate branching reactions under the action of alkoxyl, alkyl, and peroxyl radicals, as well as via nonradical channels to form alcohol and ketone. The radicals of two types, $${\rm{H}}{{\rm{O}}_2}^ \cdot $$ radical and α-hydroxycyclohexylperoxyl radical, are involved in chain propagation in the course of cyclohexanol oxidation. In the chain propagation reactions, $${\rm{H}}{{\rm{O}}_2}^ \cdot $$ radicals are more reactive than α-hydroxycyclohexylperoxyl radicals. The dynamic equilibrium was established in the dissociation of 1-hydroxycyclohexylperoxyl radical to cyclohexanone and radical $${\rm{H}}{{\rm{O}}_2}^ \cdot $$ . During oxidation of cyclohexane, cyclohexanone is formed from peroxyl radicals, as well as by the decomposition of cyclohexyl hydroperoxide and chain-radical oxidation of cyclohexanol. The oxidation of cyclohexanone in positions 2 and 6 affords 2-hydroperoxycyclohexanone, which predominantly decomposes to form 2-hydroxycyclohexanone. The oxidation of 2-hydroxy-cyclohexanone proceeds via the main channels of destruction of the carbon chain of cyclohexane leading to adipic acid and adipic anhydride. The oxidation products of C—H bonds of all types in alcohol and ketone are accumulated simultaneously with the oxidation products of C(1)—H bonds in cyclohexanol and of C(2)—H and C(6)—H bonds in cyclohexanone.

Support effect on the reactivity and stability of Au25(SR)18 and Au144(SR)60 nanoclusters in liquid phase cyclohexane oxidation

Abstract Using oxide supported atomically precise gold nanoclusters is an emerging field in heterogeneous catalysis. Such well-defined nanocatalysts represent a rather new model system enabling fundamental insights in catalytic reactions. In the present work, the stability of Au25 and Au144 clusters, supported either on TiO2 or SiO2, was examined upon thermal air pretreatment and, for the first time, upon liquid phase oxidation reaction. A pronounced influence of the support (TiO2 vs. SiO2) and cluster size (Au25 vs. Au144) was revealed by XAS, DRS and STEM. Upon pretreatment, Au144 was more stable which may be related to its specific cluster core structure and staple configuration. The catalytic properties in liquid phase cyclohexane oxidation were clearly size dependent, with Au144 yielding higher TOF values, particularly in the case of SiO2 supported catalysts. However, with respect to selectivity, TiO2 supported catalysts led to higher KA production than SiO2 supported ones. This can be explained by the different reaction pathways, as observed by in situ ATR. HERFD-XAS measurements of Au144/TiO2 catalysts revealed a pronounced cluster structure modification towards bulk gold during the reaction, in contrast to a high stability of Au144/SiO2. This study demonstrates the important role the support material has on the reactivity and stability of gold nanoclusters, which is key for their catalytic function.

HKUST-1 Supported on Zirconium Phosphate as an Efficient Catalyst for Solvent Free Oxidation of Cyclohexene: DFT Study

Layer by layer metal-organic framework (MOF) supported on zirconium phosphate (ZrP) was synthesized at very mild conditions and used for the liquid phase oxidation of cyclohexene in solvent free condition in the presence of molecular oxygen. The MOF-ZrP was characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), thermal gravimetric analyzer (TGA), Fourier-transform infrared spectrometer (FT-IR) and Brunauer-Emmett-Teller (BET) surface area analyzer. The characterization shows a smooth morphology of MOF-ZrP with good stability under 200 °C having surface area 285 m2/g. The catalytic activity of the MOF-ZrP revealed that increase of layers of MOF on ZrP enhances conversion, as well as selectivity of oxidation of cyclohexene. DFT studies were used to explore the structure and electron properties of HKUST-1 (Hong Kong University of Science and Technology), which is a clue for the catalytic behavior of the catalyst.

INTENSIFICATION OF THE CYCLOHEXANE LIQUID PHASE OXIDATION PROCESS

Liquid-phase oxidation of cyclohexane to cyclohexanol and cyclohexanone was studied in the absence of solvents under an air pressure of 0.5-5 MPa, in the temperature range 115-150 °C, catalyzed by N-hydroxyphthalimide (N-HPI). It was established for the first time that the use of N-HPI as a catalyst in place of the conventionally used metal salts of variable valence allowed a 2-3-fold increase in the conversion of the initial hydrocarbon and selectivity from 70-75 to 90%. The combined use of N-HPI with cobalt(II) acetate results in an additional increase in the conversion of cyclohexane by 30-40%, the selectivity of cyclohexanol and cyclohexanone formation to 94-97%, which seems to be due to the synergistic effect between the two components of the catalyst. The mechanism of catalytic oxidation of cyclohexane to cyclohexanol and cyclohexanone is discussed. It has been suggested that N-HPI plays a dual role in the oxidation of cyclohexane: it catalyzes the conversion of cyclohexane to cyclohexanol and cyclohexanone and, on the other hand, promotes the conversion of cyclohexanol to cyclohexanone, thereby substantially reducing the formation of adipic acid and its esters, by-products of the reaction, and increases selectivity of oxidation. This also explains the unusually high (1.3-1.5 : 1) ketone: alcohol ratio in the oxidation products of cyclohexane in the presence of N-HPI. The high selectivity of the formation of the desired products, the conversion of cyclohexane, the moderate temperature, the available catalyst, suggest that this method of oxidizing cyclohexane to cyclohexanol and cyclohexanone may be of interest for further practical use.

Liquid phase oxidation of cyclohexane over mesoporous cobalt silicates molecular sieves synthesized in strong acidic media by assembly of preformed CoS-1 precursors with triblock copolymer

Cobalt–incorporated mesoporous silica materials [MCoS(n), n = Si/Co = 20; 60] have been successfully prepared in strong acidic media by assembly of preformed CoS-1 precursors with triblock copolymer of the Pluronic type (P123) by a two steps procedure. They were characterized by various techniques including XRD, BET, FT-IR spectroscopy and diffuse reflectance UV–Vis (DRUV–Vis). The results show that the calcined MCoS materials contain Co(II) ions in tetrahedral environments under low and high Co content and the mesoporous walls contain the MFI structure building units. Liquid phase oxidation of cyclohexane using TBHP as oxidant was investigated on MCoS(n). The major products detected are cyclohexanone and cyclohexanol in all cases. The MCoS(n) catalysts are more active when water free TBHP was used and the conversion of cyclohexane increases when cyclohexane to TBHP molar ratio increases from 1:1 to 1:2, while the cyclohexanol selectivity decreases, which is ascribed to a further oxidation of cyclohexanol to cyclohexanone. The catalytic activity of MCoS(n) materials is enhanced as the cobalt content increases. It is worth noting that MCoS(20)is more active than Co-SBA15(20). The most active catalyst MCoS(20) acts as truly heterogeneous catalyst and was very stable after four runs.

Biosynthesized Au/TiO2@SBA-15 catalysts for selective oxidation of cyclohexane with O2

A variety of TiO2@SBA-15 supporters with various TiO2 loadings were synthesized using a facile sol-gel method. Gold (Au)-based catalysts were prepared with an environmentally benign and economical bioreduction method via Cacumen Platycladi (CP) leaf extract and immobilized on various TiO2@SBA-15 supporters with different TiO2 loadings. The as-prepared biosynthesized Au catalysts were applied in the liquid-phase cyclohexane oxidation. The results showed that the Au nanoparticles were well-dispersed on TiO2@SBA-15, and the Au existed as Au0. These biosynthesized Au catalysts are promising for cyclohexane oxidation, achieving a turnover frequency up to 3,426 h−1 with a 14.1% cyclohexane conversion rate.

Selective liquid phase oxidation of cyclohexene over magnetic Fe3O4/graphene oxide nanocomposite

Magnetic graphene based nanocomposite catalysts were used for the first time in cyclohexene oxidation reaction. Fe3O4 was found to be the active phase of Fe in the catalysts as evident from the XRD analysis of the samples. Presence of graphene as graphene oxide in the nanocomposites is confirmed from FTIR spectral analysis. The distribution of more or less spherical particles of Fe3O4 over the graphene sheets was evident from the TEM photographs. The activity of bare Fe3O4 increased drastically upon graphene incorporation. Maximum conversion of 92% was attained using tert-butyl hydroperoxide as the oxidant over the best nanocomposite catalyst in which Fe3O4 was supported over 2 wt% graphene oxide. Using H2O2, double bond oxidation was the major reaction (78.27% conversion) and 1,2-cyclohexane diol was the major product (87.89% selectivity) under the selected reaction conditions of 0.05 g of catalyst with 5 mL of acetonitrile solvent at 70 °C in the 6 h reaction between 2 mmol of cyclohexene and 10 mmol of oxidant. Catalysts recovery from the reaction mixture was very easy by the use of a magnet that in turn facilitated the effective reusability of the Fe3O4 /graphene nanocomposite. The reused catalyst was characterized using TEM and FTIR spectral analyses and it is found that the partial loss in activity can be a resultant of the oxidation of graphenic C=C and the formation of epoxy linkage and –OH groups in the sheets, which hinders the efficient electron migration during catalysis.

Gold nanoclusters prepared from an eighteenth century two-phases procedure supported on thiol-containing SBA-15 for liquid phase oxidation of cyclohexene with molecular oxygen

The present work describes the incorporation of finely divided and well-dispersed Au species into SBA-15 mesoporous silica functionalized with mercaptopropyl groups, and the catalytic activity of the resulting Au-materials in the liquid phase oxidation of cyclohexene with molecular oxygen at atmospheric pressure. The gold entities have been prepared through a two-phase system described in the eighteenth century involving a solution of gold in a mixture of nitric acid and ammonium chloride (aqua regia) and rosemary oil. Gold nanoclusters, which present a disordered structure and sizes below 3nm, and even single gold atoms which may be anchored/stabilized through sulphur groups are found immobilized on the mesoporous support, but no gold nanoparticles (with well-defined structure and usually over ∼5nm) are detected. Such observations have been carried out by Cs-corrected STEM-HAADF where the visualization of bright spots is attributable to these individual entities. These materials are active in the oxidation of cyclohexene, but their relative activity is governed by the synthesis conditions. Partial aggregation of the gold entities into nanoparticles has been observed during the reaction, which are mainly responsible for the catalytic activity. The allylic oxidation of the cyclohexene ring over the double bond epoxidation is strongly favored, and at high conversion the two dominants products are 2-cyclohexen-1-one and 2-cyclohexen-1-ol.

Cyclohexane oxidation with an O2–H2 mixture in the presence of a two-component Pt/C–heteropoly acid catalyst and ionic liquids

Approaches to increase the efficiency of Pt/C–heteropoly acid catalyst in a liquid-phase oxidation of cyclohexane using an O2–H2 mixture were studied. It was shown that small additives of ionic liquid (BMImBr, Bu4NBr, or Bu4NHSO4) significantly improve the catalytic effect of the Pt/C–H3PMo12O40–CH3CN system at 35°C, by slowing the rate of side reactions resulting in water formation, increasing the rate of oxygenate formation, and inhibiting their secondary oxidation reactions. The efficiency of H2 consumption increases from 2 to 18–25%, while the selectivity of cyclohexane conversion is 92–98%. The substitution of one or two Mo(VI) ions by V(V) in the structure of the heteropoly acid decreases these parameters. In the presence of Bu4NHSO4, a Pt/C catalyst can be used many times. During the reaction, the heteropoly acid present in the solution is in a reduced state under the action of the reaction medium and undergoes reversible redox transformations. The nature of the catalytic action of the studied system is explained from the viewpoint of the effect of ionic liquids on the properties of a Pt/C catalyst in activating O2, heteropoly molybdate chemistry, and the known mechanisms of the peroxide oxidation of hydrocarbons.

Study of cobalt molybdenum oxide supported on mesoporous silica for liquid phase cyclohexane oxidation

Liquid phase oxidation of cyclohexane, by molecular oxygen, has been carried out using cobalt molybdenum oxide (CoMoO4) catalyst supported on mesoporous silica supports (SBA-15, KIT-6 and FDU-12). For each support, the catalyst activity has been studied as a function of loading, pore size, and calcination temperature. The catalysts were characterized using surface area analysis (BET), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Among the three supported catalysts, the one on FDU-12 shows the highest activity. The catalysts with lower loading, lower calcination temperature and lower pore size of support show higher activity but with lower selectivity. All the studied catalysts gave about 7–8% conversion, with selectivity of 85% for cyclohexanone and cyclohexanol (KA oil), before deactivation took over. The deactivated catalysts could be fully regenerated by re-calcination in every case. With such regeneration between runs, the catalysts are shown to retain their activity and selectivity over multiple cycles. The kinetic model which was proposed for the unsupported catalyst has been shown to work well for the supported catalysts as well.