Catalyst Design And Performance Study Based On Selective Oxidation Of N-methylmorpholin

Cellulose is a natural polymer material with abundant reserves,which has a promising application in many fields such as textile and paper making,and has a great impact on human production and life.However,cellulose is very difficult to dissolve due to its complex aggregation structure and hydrogen bonding in intra-as well as intermolecular,which restricts its application.Among many solvents,N-methylmorpholine-N-oxide（NMMO）is non-toxic and harmless,which is considered to be an excellent solvent for cellulose dissolution.However,conventional catalysts suffer from low catalytic efficiency and poor selectivity in the preparation of NMMO.Thus,a series of titanium-based and organic-inorganic hybrid catalysts were synthesized in this work and applied in the selective oxidation of N-methylmorpholine（NMM）using H₂O₂ as an oxidant.The catalytic activity and selectivity with different catalysts were investigated,and the catalytic reaction mechanism was finally discussed,providing a theoretical basis for the industrial preparation of NMMO.The main contents are as follows:（1）A series of ZnO-TiO₂ nanocomposites were prepared by using the sol-gel method and used as catalysts for NMMO synthesis.The effects of Zn O content and calcination temperature on the structure and the catalytic activity of the catalysts were systematically investigated.The introduction of Zn O promoted the generation of oxygen vacancies on the surface of Ti O₂,which increased the number of adsorption/catalytic active sites on the surface of the Zn O-Ti O₂ composite.The study showed that active metal peroxy species（Ti-OOH）formed on the surface of the catalyst during the reaction,which improved the selectivity.The Zn O-Ti O₂ composite with 20%Zn O content showed the best performance with a high conversion（95.5%）and the selectivity（96.5%）of NMMO at 35°C for 3 h.In addition,the catalyst showed good stability after 5 cycles.（2）Fe,Co,Ni metal-doped TiO₂ catalysts were prepared by ion exchange strategy.The effects of different metal species and amounts on the structure and performance of the catalyst were studied.The results showed that the interaction between doped metal species and Ti O₂ promoted the formation of surface oxygen vacancies of Ti O₂nanowires,while also promoted the production of more strongly oxidizing·OH radicals.Finally,the selective oxidation of NMM was accelerated by the formation of active metal peroxy species and·OH radicals on the catalyst surface during the reaction.At a Fe-doping content of 1.75%,the content of oxygen defects on the catalyst surface is the highest（16.9%）,leading to the best catalytic activity.The yield of NMMO reached97.5%,significantly improving the performance compared to pure Ti O₂.（3）Decatungstate organic-inorganic hybrid catalysts[CH₂COOHmim]₄W₁₀O₃₂,[（CH₂）₃COOHmim]₄W₁₀O₃₂and[Bmim]₄W₁₀O₃₂ have been prepared simply from their corresponding bisulfate-based acidic ionic liquids.The double catalytic active sites of hybrid material,including the W₁₀O₃₂^4–anion and-COOH group,realized the efficient oxidation of NMM.It was found that the organic cation enhanced the affinity between NMM and catalyst.Increasing the length of the cation side chain,which can further improve the performance.The results showed that[（CH₂）₃COOHmim]₄W₁₀O₃₂showed the best catalytic performance,and the conversion of NMM and the selectivity of NMMO reached 99.8%and 97.7%after reacting at 40℃for 80 min,respectively.（4）Phosphotungstate organic-inorganic hybrids[CH₂COOHmim]H₂PW₁₂O₄₀,[（CH₂）₃COOHmim]H₂PW₁₂O₄₀and[Bmim]H₂PW₁₂O₄₀ with dual catalytic oxidation active sites were designed and prepared from bisulfate ionic liquid.The results showed that these phosphotungstate hybrids had high catalytic activity,among which[（CH₂）₃COOHmim]H₂PW₁₂O₄₀ had the best catalytic activity,and the conversion of NMM and the selectivity of NMMO reached 98.7%and 98.8%after reacting at 35℃for 70 min,respectively.Mechanism studies found that the effect of the-COOH group and the length of the side chain on its catalytic performance showed the same trend as decatungstate hybrids.