Effect Of Mesoporous Structure On Methanol To Aromatics Performance Over Nano-Zn/ZSM-5 Catalysts

Aromatics are fundamental raw materials for chemical industry,and are mainly produced by catalytic reforming and cracking using crude oil.With the decrease of oil resources and the fluctuation of crude oil prices,it is of great significance to use new materials to produce aromatics with new routes.Among the alternative processes,aromatics synthesis from coal by methanol?MTA?is a promising technology.ZSM-5 zeolite has well-defined microporous channels,high surface area,strong acidity,high hydrothermal stability,and good aromatization performance.However,the surface acidity of ZSM-5 is complex,side reactions are numerous,the selectivity of aromatics is low and microporous diffusion restrictions are easy to cause coke deactivation.Reducing crystal size and introducing mesoporous structure can effectively shorten reaction path,promote product diffusion and catalytic stability.The impregnation of zinc metal species on zeolite could increase the amount of Lewis acid,which is conducive to dehydrogenation reaction,thus significantly improve the aromatics selectivity of MTA reaction.In this paper,three structural factors of ZSM-5 including crystal size,mesoporous and additives were discussed and the effect of their coupling on the performance of MTA reaction were investigated in order to find out the best catalytic effect after the combination of different structural factors and provide the preparation direction of high-performance aromatization catalysts.In order to solve the problem of non-uniform desilication and pore formation caused by the non-uniform distribution of nano ZSM-5 bulk aluminum,uniform mesopores were introduced in its bulk and surface acidity was optimized by the successive treatment of hydrochloric acid,oxalic acid and mixed alkali.The effect of mesoporous construction on the catalytic performance was preliminarily discussed.The main conclusions are as follows:?1?It was found that after alkali treatment,the external surface area of micron ZSM-5 increased from 48 to 179 m²/g and the amount of acid decreased from 1.21 to 0.91 mmol/g.However,for the nano ZSM-5 with short diffusion channel,it was difficult to improve pore diffusion further after alkali treatment,resulted in its external surface area decreased 106 m²/g.But the amount of acid was maintained at 0.74 mmol/g.The micron surface area reduced due to part of the pore structure blockage after loading Zn.But the degradation degree of mesoporous ZSM-5 loaded with Zn was smaller than that of parent loaded with Zn.?2?The results showed that nano ZSM-5 had short diffusion channel,low acid amount and high catalytic lifetime,but the lower selectivity of aromatics than micron ZSM-5.After alkali treatment,although the external surface area was smaller than that of micron mesoporous ZSM-5,its catalytic lifetime was significantly higher due to nano ZSM-5 with short pore channels.It showed that mesoporous connectivity of micron mesoporous ZSM-5 needs to be improved.Because the acid amount of mesoporous nano-ZSM-5 was basically unchanged and the external surface area was slightly reduced,the selectivity of aromatics maintained well.After loading Zn,although the catalytic lifetime was significantly reduced,the catalytic stability was still high.The catalytic lifetime was up to 84 h when space velocity of 8 h^-1,and the aromatics selectivity was up to 80.82%at space velocity of 2 h^-1.Considering comprehensively,nano-ZSM-5has short pore length and it can maintain good acidity after alkali treatment.The mesoporous nano-ZSM-5 exhibits high catalytic stability and aromatics selectivity after adding Zn.?3?It was found that after TPA⁺was introduced into NaOH solution,the degree of hollowness was reduced based on the protection of TPA⁺on the silicon around the adsorption site.And the amount of acid significantly increased from0.48 to 0.60 mmol/g after desilication.When treated with hydrochloric acid followed by alkali,some of the framework aluminium was removed and the aluminium gradient was reduced,but some non-skeleton aluminium will be formed,which inhibit the subsequent alkali desilication and reduce acid amount.When the sample treated with hydrochloric acid was treated with oxalic acid,the extra-framework aluminum on the external surface was eliminated selectively,so the mixed alkali treatment could promote the formation of uniform small mesoporous,and the amount of acid increase to 0.51 mmol/g.?4?The results showed that the hollow structure formed by NaOH solution treatment was conducive to product diffusion.And the catalytic lifetime increased from 100 h to 130 h,but the selectivity of aromatics decreased to44.9%.When TPA⁺was introduced into the NaOH solution,the strong acid amount increased and the catalytic lifetime decreased slightly,but the selectivity of aromatics increased to 48.0%.When hydrochloric acid pretreatment before mixed alkali treatment,although there was no obvious mesoporous,the catalytic lifetiem increased to 150 h due to a decrease in total acid amount.With the increase of oxalic acid treatment,the amount of acid increased and the surface extra-framework aluminium decreased,resulting in the catalytic lifetime increased to 155 h,and the selectivity of aromatics increased to 51.1%.After adding Zn,the selectivity of aromatics increased to 84.5%,and the selectivity of BTX was 50.9%at space velocity of 2 h^-1.