Highly Selective Preparation of Cyclohexanone Oxime from Liquid-Phase Catalytic Oxidation of Cyclohexylamine with Molecular Oxygen over Glucose-Modified TiO2 under Solvent-Free Conditions

Construction of Dual Active-Site NH2-MIL-125(Ti) for Efficient Selective Oxidation of Cyclohexylamine to Cyclohexanone Oxime

In this work, dual active-site Ti-incorporated metal-organic frameworks (MIL-125 and NH2-MIL-125) were synthesized by a simple solvothermal process and applied to prepare cyclohexanone oxime from cyclohexylamine oxidation. A low-temperature thermal calcination strategy was used for the modulation of surface properties while maintaining the crystal structure and morphology. The results demonstrated that novel bifunctional NH2-MIL-125@250 °C obtained from thermal calcination possessed a large surface area with both oxygen vacancies and surface hydroxyl-active sites, promoting the adsorption and activation of cyclohexylamine and oxygen molecules, respectively. Under the optimum conditions, the cyclohexylamine conversion was 44.3%, and the selectivity to cyclohexanone oxime was 83.0%. By comparison, the stability of MIL-125 and NH2-MIL-125 was investigated separately in cyclic tests, and the crystal structure and catalytic properties of NH2-MIL-125 have been shown to be more stable than those of MIL-125. Combined with density functional theory, it was further shown that NH2-MIL-125 displayed a higher adsorption and activation ability toward cyclohexylamine and oxygen than MIL-125 and had a more stable metal-organic ligand structure. Finally, a plausible reaction pathway for selective cyclohexylamine oxidation to cyclohexanone oxime was proposed. This work can give new insights into designing novel dual active-site catalysts for the efficient catalytic transformation of organic primary amines to corresponding oximes.

Engineered aldoxime dehydratase to enable the chemoenzymatic conversion of benzyl amines to aromatic nitriles

A chemoenzymatic strategy has been implemented to synthesize nitriles from benzyl amines under mild conditions. Aldoxime dehydratase (Oxd) plays a decisive role to convert aldoximes into corresponding nitriles. However, natural Oxds commonly exhibit extremely low catalytic capacity toward benzaldehyde oximes. Here, we engineered the OxdF1 from Pseudomonas putida F1 to enhance its catalytic efficiency toward benzaldehyde oximes by a semi-rational design strategy. The protein structure-based CAVER analysis indicates that M29, A147, F306, and L318 are located adjacent to the substrate tunnel entrance of OxdF1, which were responsible for the transportation of substrate into the active site. After two rounds of mutagenesis, the maximum activities of the mutants L318F and L318F/F306Y were 2.6 and 2.8 U/mg respectively, which were significantly higher than the wild OxdF1 of 0.7 U/mg. Meanwhile, the lipase type B from Candida antarctica was functionally expressed in Escherichia coli cells to selectively oxidize benzyl amines to aldoximes using urea-hydrogen peroxide adduct (UHP) as an oxidant in ethyl acetate. To merge the oxidation and dehydration reactions, a reductive extraction solution was added to remove the residue UHP, which is critical to eliminate its inhibition on the Oxd activity. Consequently, nine benzyl amines were efficiently converted into corresponding nitriles by the chemoenzymatic sequence.

Solvent-free liquid-phase selective catalytic oxidation of toluene to benzyl alcohol and benzaldehyde over CeO2–MnOx composite oxides

A novel CeO2–MnOx composite oxide was prepared by the improved sol–gel method. The synergistic catalysis of Mn3+/Mn2+ and Ce4+/Ce3+ was responsible for the good catalytic performance in the liquid phase solvent-free selective oxidation of toluene.