Preparation Of Phosphomolybdate Heteropolyacid Catalyst And Its Application In Selective Oxidation Of Methacrolein

Methyl methacrylate（MMA）is an important chemical monomer which is mainly used to produce,for example,polymethyl methacrylate or chemical coatings.The current MMA production process mainly includes acetone cyanohydrin（ACH）,isobutylene oxidation and ethylene carbonylation,with the ACH method no longer meeting the requirements of clean production due to serious environmental pollution.Isobutylene oxidation and ethylene carbonylation,with green and clean production processes,are the main development directions for future MMA production in China.Whether it is isobutylene oxidation or ethylene carbonylation,the selective oxidation of methacrolein（MAL）to methacrylic acid（MAA）is one of the key steps.The selective oxidation of methacrylaldehyde（MAL）to methacrylic acid（MAA）is one of the key steps in either the isobutylene oxidation or the ethylene carbonylation process.The catalyst used for this reaction is a Keggin-type phosphomolybdenum heteropolymer,whose structure,acidity and oxidation properties can be modulated by changing the type and number of ligand or antibodies ions,but the effect of antibodies ions and ligands on the structure and active sites of the catalyst is not known.This thesis focuses on the effect of changes in the content of the counteracting ions Co²⁺and K⁺and the ligand atom vanadium on the performance of the catalyst.1.Firstly,heteropolymerate catalysts containing different Co²⁺contents,Cs（NH₄）_0.2Co_nH_1.8-2nPMo₁₂O₄₀（n=0.0,0.05,0.1,0.2,0.5,0.8）,were synthesized in this paper.Co²⁺was found to significantly improve the selective oxidation of MAL by heteropolymeric catalysts.The MAL oxidation activity is better when the heteropolymerate catalyst is doped with Co²⁺.Lower levels（n<0.5）of Co²⁺do not affect the Keggin structure of the catalyst after roasting and are beneficial for increasing the specific surface area of the catalyst;higher levels of Co²⁺（n≥0.5）reduce the stability of the Keggin structure.The doping of the catalyst with Co²⁺directly reduce with the secondary structure of the catalyst and the electrons in the heteropolymeric anion are attracted and deflected.The doping of Co²⁺improves the acid strength of the heteropolymeric catalyst,while the doping of Co²⁺improves the reducibility of the catalyst.When the amount of Co²⁺added is n=0.1 and a reaction temperature of 310℃,Cs（NH₄）_0.2Co_0.1H_1.6PMo₁₂O₄₀ on MAA had the highest yield of 66.67%,at which point the conversion of MAL was 77.85%and the selectivity of MAA was 85.64%.2.Based on the introduction of alkali metal K⁺in the third chapter,CsK_x（NH₄）_0.2Co_0.1H_1.6-xPMo₁₂O₄₀（x=0.0,0.2,0.5,1.0,1.5）series catalysts were synthesized.Within a certain range（x<1.5）,the introduction of K⁺can improve the conversion and selectivity of MAL on the catalyst.The introduction of K⁺will not change the cubic crystal phase structure of the catalyst,but can increase the specific surface area of the catalyst and form a uniform mesoporous channel structure,thereby providing more active sites for the MAL reaction.The introduction of K⁺to the heteropolymerate catalyst no longer contains weak acid sites that are detrimental to the MAL oxidation reaction,but at the same time reduces the amount of acid available to the catalyst.When the amount of K⁺added is x=0.5 and the reaction temperature is 310℃,the highest yield of MAA on CsK_0.5（NH₄）_0.2Co_0.1H_1.1PMo₁₂O₄₀ is 73.8%,and the conversion rate of MAL is 83.9%.The selectivity of MAA is 88.0%.3.The Cs（NH₄）_0.2Co_0.1K_0.5H_y+1.1PMo_12-yV_yO₄₀（y=0.0,0.5,1.0,1.5,2.5）series catalysts were synthesized by introducing the ligand atom vanadium over Co²⁺and K⁺doped heteropolymerate catalysts.The introduction of vanadium atoms into the primary structure of heteropolymeric catalysts reduces the symmetry of the Keggin structure and increases the insertion capacity of lattice oxygen.After catalyst roasting the V-O active species migrated from the primary to the secondary structure of the catalyst and had a higher activity compared to the Mo-O species.When vanadium was added at y=1.0,the catalyst had a maximum yield of 78.2%,at which point the conversion was 83.4%for MAL and92.8%for MAA.