Synthesis Of Single-crystalline Hierarchical SSZ-13 Zeolites With Three-Dimensional Ordered Micro-spherical Array Structure And Catalytic Performance Of Methanol To Olefins

Methanol to olefins(MTO)is an important route for the use of coal resources to produce low-carbon olefins.SSZ-13 zeolite exhibits excellent catalytic performance in MTO reaction due to its special cage structure and good thermal stability.However,the narrow eight-membered pore-opening of the SSZ-13 zeolite leads to diffusion limitation,which causes the deposition of carbonaceous material in the pores,resulting in rapid deactivation of the catalyst.Shortening the diffusion length of the catalyst is an effective strategy for enhancing the mass transport in the catalyst.It can be achieved by preparing zeolite nanocrystals or introducing mesopores into the catalyst to form a hierarchical pore structure.In addition to the crystal size/diffusion length,the acidity of zeolites is also closely related to the catalytic performance.However,in practical synthesis,the crystal size and the acidity of zeolite catalysts are often difficult to be individually modulated,so it is difficult to study the influence of one factor on the catalytic performance without excluding the influence of the other.Therefore,this thesis proposes a synthesis of zeolite catalysts with precisely tunable three-dimensional ordered microsphere array structure,which allows separately modulating the size of the microspheres and the silica-alumina ratio of catalysts for the purpose of decoupling the effects of the crystal size and acidity on catalyst performance.The synthesis of the three-dimensional ordered microsphere array zeolite is based on three-dimensionally ordered porous carbon as an inert hard template,which is realized by steam-assisted crystallization(SAC)and multiple cycles of hydrothermal growth(MHT).The three-dimensionally ordered hierarchical pore structure with microspheres of 30～400 nm can be successfully obtained by both methods,and the silicon-aluminum ratio can also be controlled independently.Compared with the multi-cycle method,the catalyst prepared by steam-assisted method exhibits a more complete micro-spherical array structure and appears as cubic single crystals of several microns in macroscopic.The catalytic test results show that the particle size of the zeolite is the main factor that affects the life of the catalyst for methanol to olefins(MTO).When the ratio of silicon to aluminum is relatively low(Si/Al=10),the particle size greatly affects the catalytic performance.The catalyst life increases with the decrease of crystal domain size of SSZ-13 zeolite.The selectivity for ethene plus propylene increases with decrease of the crystal domain size.In the case of higher silicon to aluminum ratio(Si/Al=25),the catalyst life can be prolonged by reducing the grain size although the degree of the influence is much smaller than that in the case of lower Si/Al ratio.The results show that the 32 nm_Si/Al=10 zeolite molecular sieve exhibits the longest catalytic life(360 min)and high olefin selection(86.05%).