Acid Amount Control Of ZSM-5 Zeolite And Its Influence In Methanol Aromatization

This paper aims to investigate the effect of acid amount on the methanol aromatization(MTA)performance of ZSM-5 zeolite.Through in-situ synthesis,post-treatment and metal modification,the effective regulation path of acid amount of ZSM-5 zeolite was explored.Combined with the results of spectral characterization analysis,MTA evaluation experiment and literature research on MTA reaction mechanism,the influence of acid amount change on the steps of MTA reaction process was analyzed.Four ZSM-5 zeolites with Si/Al ratios of 9,13,20 and 58 were synthesized by static method.The characterization and evaluation results show that the different Si/Al ratio will cause a large difference in the acid amount of the prepared molecular sieve.Among them,the Si/Al ratio of 13 molecular sieve has the lowest acid amount,the highest aromatic yield and the highest selectivity of high carbon aromatics.It indicates that a small amount of acid can ensure the aromatization process,but too low acid amount will inhibit the dealkylation of aromatics,thereby increasing the selectivity of high-carbon aromatics.ZSM-5 zeolite was treated with 0.1～0.3 mol/L oxalic acid aqueous solution.The characterization and MTA evaluation results of the obtained samples showed that the acid treatment mainly removed the non-framework aluminum on the zeolite,resulting in a decrease in the amount of weak acid,but had little effect on the amount of strong acid.This change enhanced the inhibitory effect on the dealkylation of aromatic hydrocarbons,resulting in a large change in the carbon number distribution of aromatic hydrocarbons.Zinc modification of ZSM-5 zeolites(Zn content 0～2.0wt%)was studied by ion exchange method.The characterization results of the obtained samples showed that zinc species would combine with B acid sites,reduce the amount of B acid,and produce new L acid sites,increasing the amount of L acid.MTA evaluation results show that the addition of Zn significantly improves the selectivity of aromatic hydrocarbons,which may be mainly due to the change of dehydrogenation performance of zinc species,which inhibits the transfer of hydrogen to olefins to produce alkanes.Among them,the Zn/ZSM-5 zeolite prepared with Si/Al ratio of 13 and Zn loading of 1.6 wt%has the best catalytic performance,and the aromatic yield can reach 27.5%.However,with the further increase of zinc loading,the large reduction of strong acid content inhibited the aromatic dealkylation reaction,which made the catalyst easy to coke and deactivate,and the stability decreased.Based on 1.6 wt%Zn/ZSM-5 zeolite,the effect of methanol space velocity(3～7 h-1)on the catalytic performance was investigated.The results showed that the aromatics yield and catalyst life decreased with the increase of methanol mass space velocity.The highest aromatic yield of 28.1%can be obtained under the condition of mass space velocity of 3 h-1.The reason may be that the suitable residence time of the material under the space velocity can make more primary product olefins fully,and is conducive to the dealkylation reaction,thus increasing the selectivity of low carbon aromatics.