Study On The Side-chain Alkylation Of Toluene To Styrene Over Modified CsX Molecular Sieve

As an important monomer in the chemical industry,styrene is mainly applied to the production of synthetic plastics and rubber,and is also widely used in the manufacturing industries of household appliances,pharmaceuticals and dyes.The industrial production of styrene is divided into two steps.Ethylbenzene is first obtained by the alkylation of benzene and ethylene,and then styrene is produced by catalytic dehydrogenation of ethylbenzene.Although the technology is mature,the production process of ethylbenzene consumes too much energy,resulting in high production cost of styrene.Styrene and ethylbenzene can be produced by side-chain alkylation of toluene with methanol belonging to coal chemical product over base catalyst,which may get rid of the dependence of styrene production on petroleum industry.The reaction materials toluene and methanol and the by-products of side-chain alkylation are easy to obtain and separate,respectively.Moreover,the energy consumption of the process is low.Therefore,side-chain alkylation may be a potential route for styrene production.The process is simple and easy to operate.The development of catalyst system for the side-chain akylation has been an important direction of this subject.In this paper,the possibility and degree of side-chain alkylation of toluene with methanol were calculated thermodynamically.In addition,the thermodynamic data of toluene-formaldehyde reaction system were analyzed according to the accepted reaction mechanism.The thermodynamic analysis showed that side-chain alkylation of toluene with formaldehyde could be carried out spontaneously to a high degree,and the higher the temperature was,the more adverse the side chain alkylation was.Methanol dehydrogenation occurs spontaneously at high temperature,but the excessive decomposition of methanol is more likely to occur than the formation of formaldehyde.Formaldehyde is easy to decompose,so the utilization rate of methanol should be improved,and the catalyst system that can promote methanol anaerobic dehydrogenation to formaldehyde should be considered.According to the selective catalytic characteristics of the molecular sieve,the carrier was determined to be Na X molecular sieve.The modified Cs X molecular sieve catalyst was prepared by adding the dehydrogenation agent sodium carbonate and iron oxide in the main way of the alkali metal Cs modification.The suitable catalyst preparation conditions were obtained by adjusting the concentration of the impregnated liquid and the times of exchange.The catalytic performance of Cs X molecular sieve under different reaction conditions such as temperature,WHSV,carrier gas and the toluene/methanol molar ratio was investigated.The selectivity of styrene can reach more than 60%by adjusting the reaction conditions in the presence of Cs X molecular sieve.The effect of sodium carbonate loading on activity of Cs X was investigated.The Cs X and Na X loaded by sodium carbonate were used in combination to investigate the effect of the combination on side-chain alkylation activity.Sodium carbonate can promote side-chain alkylation activity of Cs X.The catalytic activity of Cs X molecular sieve can be improved by sodium carbonate loading.The selectivity of styrene is improved in combination of Cs X and Na X loaded by sodium carbonate.The effect of preparation conditions on catalytic activity was investigated with Fe as additive.The effect of mass ratio of catalyst on catalystic activity was investigated in combination of Cs X and Na X modified with Fe and sodium carbonate.FT-IR,NH₃-TPD,CO₂-TPD and other characterization methods were used to measure the changes of the structure and acid-base properties of the catalyst before and after modification.In contrast,the side chain alkylation activity of Cs X molecular sieve was greatly decreased by the addition of Fe additive.The acid-base characterization showed that Fe modification reduced the alkalinity of molecular sieve.The addition of Fe additive was not conducive to the occurrence of side chain alkylation,while the acid-base properties of Cs X molecular sieve and sodium carbonate modified Cs X molecular sieve were suitable for side-chain alkylation of toluene with methanol.The acid-base matching relationship of the Cs X before and after modification with sodium carbonate are more reasonable for side-chain alkylation.In addition,a formaldehyde-generating device was designed to explore side-chain alkylation reaction between toluene and formaldehyde.The preparation of styrene by reaction of toluene with formaldehyde catalyzed by Cs X molecular sieve was investigated.The results show that it is not easy to prepare styrene by side-chain alkylation of toluene with formaldehyde instead of toluene with methanol.