Core-shell Structure ZSM-5@S-1 Catalyzes The Alkylation Of Toluene And Methanol To P-xylene

P-xylene,abbreviated as PX,is an important basic organic chemical raw material for the polyester industry.The process of preparing para-xylene by high-selectivity alkylation of toluene and methanol is a cost-effective production route.However,the acid center on the outer surface of the common ZSM-5 catalyst will cause the isomerization of p-xylene,thereby reducing the selectivity.In this thesis,a core-shell structured catalyst is prepared by coating the ZSM-5 molecular sieve with a non-acidic Silicate-1 shell layer to improve the selectivity to p-xylene.The research includes the following two parts:Preparation of small-grain Pt@ZSM-5@S-1 core-shell structure:Pt was introduced into ZSM-5 with a grain size of 200 nm by in-situ synthesis.TEM showed that Pt particles were well dispersed.Using Pt@ZSM-5 as the core,the Silicate-1 shell was grown epitaxially.And the effect of different coating amounts of the catalyst was investigated.The catalytic tests showed that the increase of the coating amount significantly improved the selectivity of p-xylene and slightly reduced the conversion of toluene.Among them,Pt@ZSM-5@3S-1exhibited the best performance with P-xylene selectivity of 73.5%and the toluene conversion of 10.9%,;the presence of Pt metal effectively improves the stability and reduces the contents of ethylene and propylene in the gaseous products,which is considered as carbon precursor.The carbon deposit after the reaction for 30 h time on stream is 60.9%of that with Pt@ZSM-5.This core-shell preparation method effectively improves the selectivity and stability of the catalyst.Preparation of large-grain ZSM-5@S-1 core-shell structure:First,by adjusting the content of seed crystals in the synthesis system,molecular sieve catalysts with different grain sizes are obtained.As the grain size increases,the diffusion channels become longer,the conversion of toluene is reduced,and the selectivity of p-xylene is increased.It is preferable that the large-grain Z5-5 with a length of 20μm is used as a precursor for secondary epitaxial growth to prepare a large-grain core-shell structured catalyst.By optimizing the secondary coating conditions to adjust the coating amount,shell-to-core ratio equals 1was found to be the optimal amount;and the water-silicon ratio for the crystallization solution,H₂O/SiO₂=150was found to be optimal;the amount of CTAB introduced in the epitaxial shell was shown to be best at CTAB/SiO₂=0.01.