The Research On The Structure-activity Relationship Of ZSM-5 Zeolites In The Methanol To Propylene Reaction

The conversion of methanol to propylene?MTP?process is suitable for China because of its richment of coal and methanol resources as well as the realistic supply and demand relationship of downstream products such as polypropylene.This work studied the research on the structure-activity relationship of ZSM-5 zeolites in the methanol to propylene reaction in order to give useful information for the design and prepare efficient MTP catalyst with high propylene selectivity and propylene/ethylene?P/E?ratio as well as excellent catalyst lifetime..Firstly,the rules of MTP reaction over ZSM-5 zeolite were investigated.The product distributions of MTP reaction over ZSM-5 zeolites were studied using different SiO₂/Al₂O₃ ratios?30,80 and 250?,different reaction temperatures?350⁵00??and different methanol conversions?2⁹9%?.The results showed that the hydrocarbon pool mechanism?one of dual-cycle mechanisms?in MTP reaction played the dominant role at a low methanol conversion or reaction temperature,in which the propylene was derived from chain growth of ethylene.However,the formation aromatic species were inhibited at a high temperature,resulting in the formation of olefins from olefin cycle.In addition,the occurrence of diffusion problem has been found in the ZSM-5 zeolite,and the zeolite with high SiO₂/Al₂O₃ ratios reduced the formation of aromatic species and then the olefin cycle played the dominant role in the MTP reaction.Secondly,the research on the structure-activity relationship between the properties of ZSM-5 zeolites and catalytic performance in MTP reaction was conducted.The results indicated a high SiO₂/Al₂O₃ ratio leads to a high propylene selectivity and P/E ratio over ZSM-5 zeolites with different crystal sizes?150 nm³ ?m?.However,the single-route lifetime of zeolites firstly increased and then decreased with the increase of SiO₂/Al₂O₃ ratios,in which the zeolite with SiO₂/Al₂O₃ ratio of 400 showed the longest single-route lifetime.Moreover,the performance could be improved by reducing the crystal size of zeolites,in which the one with a small crystal size of 150 nm showed the highest propylene selectivity and P/E ratio and the longest single-route lifetime.Therefore,the SiO₂/Al₂O₃ ratios and different crystal sizes should be synchronized to promote the catalytic performance of zeolites.