Synthesis Of ZSM-5 Zeolite And Its Performance On Methanol To Gasoline Reaction

The shortage of petroleum resources in china and the high proportion of oil supplied from overseas,especially,oil dependency on import will increase to 60%by 2015,has become a serious threaten to china's energy security.Considering the relatively abundant reserves of coal resources in china,the development of new technology for coal chemical industry is an important choice to change national energy supply and improve the national energy security.Fischer Tropsch(F-T)process is a well-known process of traditional coal to liquids technology which can convert coal into heavy oil;Methanol to gasoline(MTG)technology,a new type of coal industry technology,can be used to directly synthesis gasoline from methanol,which is a powerful supplement to F-T coal-to-liquids technology.The development of MTG technology can be a meaningful method to effectively solve the domestic excess of methanol,and to revitalize the coal chemical industry chain.ZSM-5 zeolite is a typical catalyst in MTG reaction.The regular arrangement of ZSM-5 zeolite's micropores indeed gives a performance of the shape-selective catalysis.However,micropores of ZSM-5 also lead to heavily diffusion limitation,and rapidly cause catalyst deactivation because of the coke deposition blocking the reactants from accessing the active sites.Hence,the catalyst requires frequent regeneration during industrial production.The regeneration of the catalyst not only increases the cost but also reduce the catalyst stability.It is,therefore,very important to investigate the diffusion limitation of ZSM-5 to improve catalytic performance and catalyst lifetime.Many studies indicated that the crystal size and pore structure of zeolite are key factors influencing the diffusion limitation.In this paper,nano-sized ZSM-5 and hierarchical ZSM-5 zeolites were synthesized and used to replace micro-sized ZSM-5.Based on XRD,NH3-TPD,BET,TEM and XPS characterizations,the influence of crystallization temperature,crystallization time and the concentration of APTMS on crystal size,pore structure and acid properties of ZSM-5 was studied.The MTG performance of the catalysts were further tested in a fixed bed reactor.The relationship between diffusion path length,reaction zone area and acid properties of ZSM-5 and the catalyst activity,product selectivity and catalyst lifetime was obtained.The results provided significant guidance and supports to the development of novel industrial catalyst with excellent stability.The main conclusions are as follows:(1)The crystal size of ZSM-5 zeolite is controlled by adjusting the crystallization temperature.The crystal size of ZSM-5 increased with the increase of crystallization temperature.And the catalyst is rapidly deactivated by the deposition of coke due to the increased diffusion limitation.In addition,the amount of strong acid sites and the strength of acid site increased with the decrease of crystallization temperature,which increased the rate of coke formation.The catalyst lifetime of HZSM-5 crystallized under 110? can reach 450 h(WHSV=2.1 g.g-1.h-1).(2)Effect of crystallization time on the texture and acid property of ZSM-5 zeolite and its MTG performance is investigated.The amount of strong acid site and the strength of acid site of ZSM-5 zeolite increased with the increase of crystallization time.This is ascribed to the increased framework Al and also Si/Al on external surface.The initial activity of MTG reaction and the rate of coke formation increased with the increase of crystallization time.In addition,the crystal size of ZSM-5 zeolite increased with the increase of crystallization time,promoting the catalyst deactivation because of the increased diffusion limitation.In summary,the HZSM-5 obtained by 6 day of crystallization has the longest catalyst lifetime(60 h)on MTG performance(WHSV=9.48 g.g-1.h-1).(3)The effect of crystal size and morphology of ZSM-5 zeolite was investigated.Nano-sized ZSM-5 has high surface area and short diffusion path length,resulting long-term catalytic stability and high selectivity to aromatics and iso-paraffins.In addition,the large reaction zone of Nanosphere-HZSM-5 makes initial liquid hydrocarbon yield increase with the increase of the WHSV.In contrast,the small reaction zone of Con-HZSM-5 makes the liquid hydrocarbon yield decrease with the increase of the WHSV.(4)Hierarchical ZSM-5 zeolites were synthesized by using organosilane surfactant(APTMS),and the effect of concertration of APTMS on the process of mesopore formation was further investigated.The pore structure of hierarchical ZSM-5 with intercrystalline mesoporous and intracrystalline mesoporous can further decrease the coke deposition in microporous of ZSM-5.And the rate of coke formation decreased with the increase of the concentration of APTMS.This is ascribed to the decreased amount and strength of strong acid and the formation of moderate strong acid.Finally,due to its appropriate pore structure and acid properties,the catalytic lifetime of Meso-HZSM-5-5%on MTG reaction was 132 h(WHSV=9.48 g.g'1.h-1),which is considerably longer than nanosized ZSM-5 catalyst.