Study On Preparation And Performance Of H₃PW_12-nMo_nO₃₄ Heteropoly Acid Supported On Modified Zeolite

Polyoxometalates(POM)are metal oxygen clusters formed by dehydration and condensation of acid salts of transition metals.Polyacids are clear structure and and acidity,make it a good catalyst for esterification,alkylation and other reactions.However,due to the small specific surface area of crystalline polyacids and the large solubility in the polar system,the catalytic activity of polyacids in esterification and alkylation is low and catalyst recovery is difficult.On the one hand,the specific surface area of polyacids can be increased,and then the catalytic efficiency of polyacids can be improved;on the other hand,the solubility of polyacids after immobilization is low in the polar system,which can be recovered and reused by centrifugation,filtration and other methods.In this paper,SBA-15,worm-like and MCM-41 were modified by subsequent synthesis.The supported catalysts of phosphotungstic acid,phosphomolybdic acid and phosphotungsticmolybdium acid were prepared by impregnation.The structure,morphology and specific surface area of the catalyst were characterized by FT-IR and XRD.On this basis,the catalysts were used in the synthesis of tributyl citrate and the alkylation of tert-butyl phenol.The optimum experimental conditions and reusability of catalyst were investigated.The specific research contents are as follows:Firstly,the mesoporous molecular sieve SBA-15 modified by 3-aminopropyltriethoxysilane coupling agent was used as the carrier.A series of Keggin type phosphotungsticmolybdium acid H3PW12-nMonO34(n=0,6,12)with different composition were prepared as solid acid catalysts.Study on the catalytic synthesis of tributyl citrate with PWMo@SBA-15/Apts as catalyst.The effe-cts of the composition of polyacid,the amount of catalyst,the molar ratio of acid to alcohol,the reaction time and the reaction temperature on the esterification rate of tributyl citrate were investigated.The optimum conditions for the synthesis of tributyl citrate were obtained.The results showed that the sequence of catalytic activity was PW12@SBA-15/Apts>PW6Mo6@SBA-15/Apts>PMo12@SBA-15/Apts.Secondly,the molecular sieves worm like,MCM-41 and SBA-15 with different pore structure and specific surface area were modified by 3-aminopropyltriethoxysilane coupling agent and a series of amino modified mesoporous molecular sieves were obtained.And then,a series of supported phosphotungstic acid catalysts were prepared by impregnation.Taking the synthesis of tributyl citrate by acid as the research object the effects of different molecular sieve structure,catalyst dosage,reaction temperature and molar ratio of acid and alcohol on the esterification rate of tributyl citrate were investigated by orthogonal method,and the best experimental conditions of tributyl citrate were obtained.The results showed that the sequence of catalytic activity was PW12@SBA-15/Apts>PW12@worm-like/Apts>PW12@MCM-41/Apts.Finally,SBA-15 molecular sieves covalently modified by Apts,Papts and Atapts with different structure and alkyl chain length were used as carriers.And phosphotungstic acid with good catalytic activity was used as catalyst.A series of SBA-15 supported phosphotungstic acid catalysts were prepared by impregnation based on the electrostatic interaction between polyacid anions and protonated silane coupling agent.In the synthesis of tert-butyl phenol catalyzed by SBA-15 supported phosphotungstic acid catalyst:n(phenol):n(TERT butanol)=1:2.5,the reaction temperature is 145?,the highest conversion of phenol is 98.3%.The effects of silane coupling agents with different structures,the amount of phosphotungstic acid,the amount of catalyst,the reaction temperature and the molar ratio of acid and alcohol on the esterification rate of tributyl citrate were investigated by orthogonal method and the best experimental conditions of tributyl citrate were obtained.The results showed that the order of acid catalysis of three kinds of silane coupling agent modified zeolites was PW12@SB A-15/Apts>PW12@SB A-15/Papts>PW12@SBA-15/Atapts.The esterification rate of TBC synthesized by PW12@SBA-15/Apts is 97.36%,and the esterification rate can still reach 74.38%after 8 times of reuse.