Initial O2 Pressure Research Articles

Production of Diethyl Maleate via Oxidative Depolymerization of Organosolv Lignin from Wheat Stalk over the Cooperative Acidic Ionic Liquid Pair

Lignin, the second largest component of biomass, is considered as an important alternative source of fossil reserves for the production of fuels and chemicals. Here, we developed a novel method to oxidatively degrade organosolv lignin into value-added four-carbon esters, particularly diethyl maleate (DEM), with the cooperative catalyst consisting of 1-(3-sulfobutyl) triethylammonium hydrogen sulfate ([BSTEA]HSO4) and 1-butyl-3-methylimidazolium ferric chloride ([BMIM]Fe2Cl7). Under optimized conditions (1.00 MPa initial O2 pressure, 160 °C, 5 h), the lignin aromatic ring was effectively cleaved by oxidation to form DEM with a yield of 15.85% and a selectivity of 44.25% in the presence of the synergistic catalyst of [BMIM]Fe2Cl7-[BSMIM]HSO4 (1/3, mol/mol). The structure and composition analysis of lignin residues and liquid products confirmed that the aromatic units in lignin were effectively and selectively oxidized. Furthermore, the catalytic oxidation of lignin model compounds was explored for obtaining a possible reaction pathway of oxidative cleavage of lignin aromatic units to DEM. This study provides a promising alternative method for the production of traditional petroleum-based chemicals.

Catalytic Oxidation of Glycerol over Pt Supported on MOF-Derived Carbon Nanosheets.

A series of nitrogen-doped porous carbon nanosheets (NPCNs) doped with transition-metal-supported Pt catalysts were prepared by colloidal deposition and evaluated for the selective oxidation of glycerol to glyceric acid (GLYA) under nonalkaline conditions. The transition metal contained in the catalyst was found to affect its performance and selectivity for GLYA, with the Pt/Zr@NPCN catalyst showing the highest catalytic activity and selectivity. These materials were characterized using Brunauer-Emmett-Teller surface area analysis, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and CO2 temperature-programmed desorption. The results showed that the small size of the Pt nanoparticles, the interaction between the Pt nanoparticles and the support, and the unique textural properties of the catalyst all promoted glycerol conversion and GLYA selectivity. A Zr concentration of 1.5 wt % and a support preparation temperature of 800 °C were found to provide a catalyst with the optimal performance that exhibited a glycerol conversion and selectivity for GLYA of 68.62 and 77.29%, respectively, at an initial O2 pressure of 10 bar and 60 °C after 6 h. Even after being recycled five times, this material provided a GLYA selectivity of approximately 75%, although the glycerol conversion decreased from 68 to 50%. The insights may provide new suggestions on the design of efficient support for the selective oxidation of polyols.

Facile conversion of glycerol to 1,3-dihydroxyacetone by using mesoporous CuO–SnO2 composite oxide supported Au catalysts

Selective catalytic oxidation of polyols, e.g., the selective catalytic oxidation of the secondary –OH bond in glycerol, remains a considerable challenge. In this study, a series of mesoporous CuO–SnO2 composite oxides were prepared by a hard-template method and used to support Au catalysts for the selective oxidation of glycerol to 1,3-dihydroxyacetone (DHA) under base-free conditions. Catalysts with different Cu:Sn molar ratios gave different catalytic performances. A high conversion of glycerol (100%) and selectivity for DHA (94.7%) were obtained in 2 h at 80 °C and PO2 = 1 MPa over the Au/CuO–SnO2-3:1 catalyst. Further investigation indicated that the high catalytic activity of Au/CuO–SnO2-3:1 is related to the small size and high dispersion of Au nanoparticles (NPs), the interactions between the Au NPs and the support, the synergistic effect between CuO and SnO2, and the amount of surface lattice oxygen species. Various reaction parameters, namely the glycerol:Au molar ratio, the reaction temperature, the initial O2 pressure, the reaction time, and the support calcination temperature were studied. Although the conversion rate by the catalyst decreased after four cycles, the selectivity remained above 86%. Density functional theory calculations showed that the synergy between CuO and SnO2 improves the catalytic activity in glycerol oxidation to DHA. The results show that mesoporous composite oxide supports have a wide range of potential applications in the selective oxidation of glycerol to other high-value-added products.

Lignin-first biorefinery of corn stalk via zirconium(IV) chloride/sodium hydroxide-catalyzed aerobic oxidation to produce phenolic carbonyls

Lignin-first biorefinery of corn stalk via ZrCl4/NaOH-catalyzed aerobic oxidation for phenolic carbonyls production was reported. Under the co-catalysis of ZrCl4 and NaOH, lignin in corn stalk was oxidized into phenolic aldehydes (p-hydroxybenzaldehyde, vanillin, and syringaldehyde), ketones (p-hydroxyacetophenone, acetovanillone, and acetosyringone), acids (p-hydroxybenzoic acid and vanillic acid), and other derivatives. Reaction conditions, including time, temperature, ZrCl4 dosage, NaOH dosage, MeCN/H2O ratio, and initial O2 pressure were comprehensively screened, and the optimal lignin-derived monomer yields of 13.2 wt% was obtained. Among these aromatic compounds, phenolic aldehydes were the main products, and the overall selectivity of phenolic carbonyls was as high as 93%. Cellulose-rich residues after lignin-first oxidation were further characterized by thermogravimetry and analytical pyrolysis with corn stalk as the control, proving the good fragmentation and dissolution of lignin streams. In general, ZrCl4/NaOH-catalyzed lignin-first oxidation provided a novel approach for lignin valorization, and achieved the highest reported phenolic carbonyls selectivity.

1-Ethyl-3-methylimidazolium acetate ionic liquid as simple and efficient catalytic system for the oxidative depolymerization of alkali lignin

The oxidative depolymerization of alkali lignin (AL) in 1-ethyl-3-methylimidazolium acetate ([C2C1im]OAc) system without additional catalyst was investigated under mild conditions (initial O2 pressure of 1.5 MPa, 80 °C–100 °C). Compared with other ionic liquids (ILs), the cooperation of imidazolium cation and acetate anion successfully enhanced AL conversion. Among the investigated imidazolium acetate ILs with ethyl- to octyl-side chains, [C2C1im]OAc presented the best catalytic capacity for AL oxidative depolymerization. Adding an appropriate amount of water to [C2C1im]OAc can further improve the reaction efficiency. In the [C2C1im]OAc system with the addition of 0.10–0.25 mL of water, approximately 77 wt% AL was depolymerized into small molecule soluble products at 100 °C for 2 h. The extracted oil was composed mainly of phenolic derived compounds. With the use of the [C2C1im]OAc-based system, the specific inter-unit linkages of lignin were broken down, and residual lignin with low molecular weight and narrow polydispersity index (1.88–1.96) was obtained. Compared with that in AL conversion with fresh [C2C1im]OAc, only a minimal decrease (~3.2%) was observed with the recovered IL until the fifth cycle. These findings revealed that [C2C1im]OAc–based system is a simple and efficient catalytic system for lignin oxidative depolymerization.

Catalytic O2 oxidation of lignite to carboxylic acids with iron-based catalysts in acidic aqueous solutions

Carboxylic acids (CAs) are important industrial materials. The properties of lignite determine that it has great potential to produce CAs by oxidation. In this work, an environmentally benign catalyst, Fe2(SO4)3, was used to oxidize lignite by O2 in acidic aqueous solutions to produce CAs. The effects of Fe2(SO4)3 concentration, H2SO4 concentration, reaction temperature, reaction time and initial O2 pressure on lignite conversion and CAs yield were studied. We have found that the catalytic system can effectively promote the oxidation of lignite. H2SO4 could adjust the activity of Fe2(SO4)3, which influences the yields of formic acid and benzene carboxylic acids (BCAs) greatly. A CAs yield of 48.3 wt% was obtained, including 21.6 wt% formic acid, 12.7 wt% acetic acid, 0.9 wt% succinic acid, 4.2 wt% oxalic acid and 8.9 wt% BCAs. The catalytic system can be reused after the extraction of products. Moreover, FeS2 is readily available as the main component of pyrite that is an inorganic mineral of coal. Thus, FeS2 was also studied for the oxidation of lignite. The catalytic system of FeS2/H2SO4 achieves the same CAs yield as Fe2(SO4)3/H2SO4 with the same amounts of iron and sulfur. In the oxidation conditions, iron and sulfur of FeS2 were converted to Fe3+ and SO42−, respectively. The catalysts developed in this work are non-corrosive, easy to obtain and environmentally friendly.

Formic acid and acetic acid production from corn cob by catalytic oxidation using O2

Direct conversion of raw biomass to fine chemicals is of significant conceptual and practical interest. Up to now, the direct conversion of raw biomass to fine chemicals is still a big challenge. The oxidation of biomass to formic acid (FA) and acetic acid (AA) is a promising industrial process in the future. In this work, we reported that a binary catalyst system of Keggin-type heteropoly acid H5PV2Mo10O40+H2SO4 was efficient to directly convert corn cob, a highly abundant renewable agricultural waste, to FA and AA using O2 as the oxidant in aqueous solutions. The effects of catalyst amount, H2SO4 concentration, reaction temperature, reaction time and initial O2 pressure on the conversions of corn cob were investigated. Under optimized reaction conditions, corn cob was transformed with a FA yield of 42.5% and an AA yield of 9.1% (based on carbon) after 30min of reaction at 170°C. In the oxidation, the pH of the aqueous solution plays a key role. The influences of pH on solid degradation, reaction pathway and oxidation of the components of corn cob were discussed. Hemicellulose is oxidized rapidly followed by partly oxidation of lignin, and cellulose is the most difficult converted part. Products FA and AA mainly come from polysaccharides (hemicellulose and cellulose) and pH has a major influence on cellulose. The recycling of the catalyst system is demonstrated and shows good reusability.

Bio-gel template synthesis of CoFe2O4 nano-catalysts and application in aerobic oxidation of cyclohexane

Magnetic cobalt ferrite (CoFe2O4) particles were synthesized via a solvothermal pathway by using the gel of natural lotus roots as bio-gel template. Samples with porous structure and of large specific area (∼95.6 m2/g) were prepared, which were assembled by CoFe2O4 particles of uniform sizes (ca. 15.0 nm). The annealed CoFe2O4 nanoparticles were proven to be highly efficient catalysts for aerobic oxidation of cyclohexane. Factors such as oxygen pressure, temperature, reaction time and self-catalysis by adding cyclohexanone have been studied in view of improving the conversion of cyclohexane as well as the selectivity of KA oil (cyclohexanone and cyclohexanol). Optimum parameters were found at an initial O2 pressure of 1.6 MPa when the reaction was conducted for 6.0 h at the temperature of 150 °C. The maximum conversion of cyclohexane oxidation reaches 16.43%, and the selectivity 90.3% for KA oil was obtained. This research provides a facile way of developing green catalysts in the aerobic oxidation of cyclohexane.

Catalytic oxidation of lignite to carboxylic acids by molecular oxygen in an aqueous FeCl3 solution

Carboxylic acids (CAs) are important chemicals widely used in industrial manufacturing. They can be produced with high yields from O2 oxidation of lignite and the process is regarded promising in the future. However, current methods for the process consume a large amount of mineral alkali and acids, or involve catalysts that are harmful to environment and human beings, and require high temperatures. In this work, we have screened various catalysts and found an iron-based catalyst (FeCl3, Fe2(SO4)3) to be beneficial for the process. The products include formic acid, acetic acid, oxalic acid, succinic acid and benzene carboxylic acids (BCAs, including 12 types). The effects of FeCl3 concentration, H2SO4 concentration, reaction temperature, initial O2 pressure and reaction time on lignite conversion and CAs yield are studied. It is found that H2SO4 can change the catalytic activity of FeCl3 resulting in a synergistic effect for CAs production. In the oxidation process, Fe3+ is reduced to Fe2+ by lignite while O2 reoxidizes Fe2+ to Fe3+ to form a redox cycle. The catalyst can be reused without significant decline in activity. The method developed in this work requires no alkali and consumes little H2SO4, and is environmentally benign.

Catalytic Oxidation of Lignite to Carboxylic Acids in Aqueous H5PV2Mo10O40/H2SO4 Solution with Molecular Oxygen

Carboxylic acids (CAs) are considered as an important group of chemicals and are widely used in the chemical industry. The production of CAs via oxidation of lignite is a promising industrial process. In the present work, we found that H5PV2Mo10O40 (HPA) was beneficial to catalytic oxidation of lignite by O2 in aqueous H2SO4 solutions to produce CAs. The effects of HPA concentration, H2SO4 concentration, reaction temperature, initial O2 pressure, and reaction time on lignite conversion and CA yield were studied. The catalytic oxidation of lignite in aqueous HPA/H2SO4 solutions with O2 can yield 56.9 wt % CAs, including 33.5 wt % formic acid, 14.4 wt % acetic acid, 1.5 wt % succinic acid, 1.1 wt % oxalic acid, and 6.4 wt % benzene carboxylic acids (BCAs, including 12 types) at 170 °C and 3 MPa for 60 min. In this catalyst system, the existence of H2SO4 can change the catalytic activity of HPA, and the synergistic effect of HPA and H2SO4 can significantly promote the production of CAs. VV can oxidize lignit...

Solvent-free selective oxidation of cyclohexane with molecular oxygen over manganese oxides: Effect of the calcination temperature

The effects of calcination temperature on the physicochemical properties of manganese oxide catalysts prepared by a precipitation method were assessed by X-ray diffraction, N2 adsorption-desorption, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, O2 temperature-programmed desorption, and thermogravimetry-differential analysis. The catalytic performance of each of these materials during the selective oxidation of cyclohexane with oxygen in a solvent-free system was subsequently examined. It was found that the MnOx-500 catalyst, calcined at 500 °C, consisted of a Mn2O3 phase in addition to Mn5O8 and Mn3O4 phases and possessed a low surface area. Unlike MnOx-500, the MnOx-400 catalyst prepared at 400 °C was composed solely of Mn3O4 and Mn5O8 and had a higher surface area. The pronounced catalytic activity of this latter material for the oxidation of cyclohexene was determined to result from numerous factors, including a higher concentration of surface adsorbed oxygen, greater quantities of the surface Mn4+ ions that promote oxygen mobility and the extent of O2 adsorption and reducibility on the catalyst. The effects of various reaction conditions on the activity of the MnOx-400 during the oxidation of cyclohexane were also evaluated, such as the reaction temperature, reaction time, and initial oxygen pressure. Following a 4 h reaction at an initial O2 pressure of 0.5 MPa and 140 °C, an 8.0% cyclohexane conversion and 5.0% yield of cyclohexanol and cyclohexanone were achieved over the MnOx-400 catalyst. In contrast, employing MnOx-500 resulted in a 6.1% conversion of cyclohexane and 75% selectivity for cyclohexanol and cyclohexanone. After being recycled through 10 replicate uses, the catalytic activity of the MnOx-400 catalyst was unchanged, demonstrating its good stability.

Production of Benzene Poly(carboxylic acid)s and Small-Molecule Fatty Acids from Lignite by Catalytic Oxidation in NaVO3/H2SO4 Aqueous Solution with Molecular Oxygen

Carboxylic acids are widely used in industry and are considered an important type of chemicals. The production of carboxylic acids through oxidation of lignite is very promising. The traditional alkali−oxygen oxidation of lignite can produce high yields of carboxylic acids, but the process consumes a great deal of alkali and acids and the high reaction temperature increases the energy consumption. In the present work, we found that carboxylic acids, including small-molecule fatty acids and benzene poly(carboxylic acid)s, could be obtained by catalytic oxidation of lignite in NaVO3/H2SO4 aqueous solution with molecular O2. The effects of NaVO3/coal mass ratio, H2SO4 content, reaction temperature, initial O2 pressure and reaction time on the conversion of lignite and yield of carboxylic acids were investigated. In the process of reaction, lignite is first converted into water-soluble intermediates, which are then converted into carboxylic acids. The second step is the rate controlling step. It has been foun...

Depolymerization of lignin by catalytic oxidation with aqueous polyoxometalates

In this work a method for the depolymerization of lignin by catalytic oxidation was studied. Using H5PMo10V2O40 as the catalyst, various lignin spices including pyrolytic lignin, hydrolytic lignin, alkali lignin, sodium lignosulfonate and calcium lignosulfonate were tested for the oxidative degradation. The influence of reaction conditions, i.e. reaction temperature, solvent, reaction time, initial O2 pressure and catalyst concentration were investigated. Under optimized conditions, the highest yield of 65.21% of the oxidative degradation products was achieved from pyrolytic lignin, and the main products were aromatic compounds and dimethyl succinate. Moreover, the H5PMo10V2O40 catalyst was recycled for five times without significant deactivation.