| A small amount of mixed xylenol separated from coal tar has limited practical value,and it can be converted into phenol and cresol with more widely used chemical products by catalytic hydrodealkylation method,which is an effective method to realize comprehensive utilization of xylenol.Microporous zeolite has strong acid sites that can undergo dealkylation reactions,but the narrow pores limit the effective diffuseion of reactants and products,resulting in secondary cracking,which reduces product selectivity.Al-MCM-41molecular sieve has a large pore structure which is conducive to the diffusion of reactants and improve the selectivity of the product,but its weak acidity leads to low catalytic dealkylation reaction activity.Composite molecular sieves is synthesized from two or more kinds of molecular sieves,which combine the advantages of two or more kinds of molecular sieves to show special catalytic performance.In this study,HY/MCM-41 coated composite molecular sieve was successfully synthesized by two-step crystallization,and the reaction performance of the hydrodemethylation of xylenol was investigated after loading the active component TiO2.Finally,the process conditions were optimized.In this study,3,4-xylenol was used as a model compound,and the optimal catalyst support for HY was screened by comparing the demethylation performance of HZSM-5,HY,HMCM-41,Hβand HMOR molecular sieve supported TiO2catalyst.Furthermore,the effects of HY modified with different concentrations of titanium sulfate solution on its catalyst structure,acid properties and catalytic dealkylation performance were investigated.The results show that the effect of HY molecular sieves modified with different concentrations of titanium sulfate solution on the catalyst performance is related to the acidity of the catalyst and the dispersion of TiO2on the HY surface.As the concentration of the titanium sulfate solution increases,the amount of acid in the catalyst gradually increases.However,the excessive loading of TiO2causes the TiO2to agglomerate on the outer surface of the support and block the catalyst pores.Therefore,it is appropriate to determine the concentration of the modified HY titanium sulfate solution as 0.15 mol/L.At this time,the result of the catalyzed xylenol demethylation reaction was:the conversion of 3,4-xylenol,the selectivity to toluene,phenol and m-cresol were 57.12%,15.43%,60.97%and 23.6%.The silicon/aluminum source dissolved by HY zeolite under alkaline conditions was used as the"nutrient"for the synthesis of MCM-41,and the silicon/aluminum source and the the template agent were added to synthesize HY/MCM-41 coated composite molecular sieve with MCM-41 grown on the surface of HY zeolite by two-step crystallization.The synthesized HY/MCM-41 coated composite molecular sieve was characterized by XRD,SEM,N2adsorption-desorption and NH3-TPD methods.The results show that the HY/MCM-41 composite molecular sieve is a coated composite molecular sieve with micro/mesoporous structure and strong acidity.The optimal synthesis conditions are:the addition amount of HY is m（HY suspension）/m（system）=50%,the p H of the system is 11,the crystallization time is 48 h,and the crystallization temperature is 120℃.The effects of micro/mesoporous molecular sieve catalysts containing TiO2active components on the hydrodemethylation performance of 3,4-xylenol were investigated.Compared with TiO2/YM-m catalyst,TiO2/YM-c composite catalyst has a synergistic effect due to its coating structure,unique pore structure and acid properties,which makes it show good catalytic performance in the 3,4-xylenol hydrodemethylation reaction.Under the optimal conditions of a reaction temperature of 400℃,the reaction pressure of 2.5 MPa,the hydrogen to phenol molar ratio of 55,and the liquid hourly space velocity of 1 h-1,the conversion of 3,4-xylenol,the selectivity to phenol and m-cresol were 50.82%,62.17%and36.32%. |
