Preparation,Characterization And Applications Of Novel Methanol Conversion Catalysts

Light olefins such as ethylene and propylene,which are the most important intermediates for making fine chemicals and petrochemicals,play a very pivotal role in the modern chemical industry.Nowadays they are mainly obtained from naphtha steamcracking and crude oil catalytic cracking processes.The fast depleting crude oil reservoirs and ever-increasing global demands for light olefins call for alternative and sustainable routes to producing light olefins.Particularly,China's special energy structure which is meager in oil,little in gas,and rich in coal makes the development of coal based light olefin production technologies one of the key strategies in ensuring national energy security and chemical feedstock supplies.Among various non-crude oil based routes to producing light olefins developed so far,methanol-to-hydrocarbons(MTH)is considered as the most promising one,because methanol can be conveniently made from syngas derived from different feedstocks such as coal,biomass,natural gas or even solid wastes.It has been known that,over different acidic zeolites or zeotype catalysts,different MTH products can be obtained via different reaction pathways,such as methanol to olefins(MTO)over SAPO-34 and methanol to propylene(MTP)over ZSM-5.In 2010,the first SAPO-34 based dimethyl ether or methanol to olefins(DMTO)unit was successfully erected by the research group of Zhongmin Liu in China.Despite the great success,the SAPO-34 based MTO catalysts prepared by the semi-synthesis technique have two drawbacks: one is that the active sites in SAPO-34 are covered by the binder and matrix,leading to the poorer accessibility of the active sites to the reactants;and the other is that SAPO-34 only has micropores,giving rise to the diffusion limitation of the reactants and products.Therefore,exploring new preparation methods to overcome the drawbacks of the semi-synthesis technique and thereby developing novel methanol conversion catalysts have been the focus of catalysis science and technology research.In this thesis,three novel MTH catalysts are designed and synthesized,and their physicochemical properties and catalytic performance are extensively characterized and assessed.The main achievements are summarized as follows:1.The first part of this thesis is to explore the possibility of synthesizing SAPO-34@kaolin composites by using kaolin microspheres(KMS)as the whole aluminum and silicon sources.The characterization results show that SAPO-34@kaolin composites(SAPO-34@KMS)can be synthesized by using the calcined KMS as the whole aluminum and silicon sources,but when used as the MTO catalysts they show short lifetime(10 min)and poorer selectivity(only 74%)to ethylene and propylene.Further analyses show that this is because: 1)the contents of SAPO-34 in the catalysts are too low(only 11.2%)to offer enough active sites for methanol conversion;and 2)SAPO-34 in the composites has large size(about 2 ?m × 1 ?m × 1 ?m)and only micropores and therefore imposes the diffusion limitation for the reactants and products.2.To solve the above problems,the second part of this thesis presents a novel method to in situ synthesize meso-microporous SAPO-34 in calcined KMS via the bottom-up rearrangement and reorganization of the active silicon and aluminum species in calcined KMS and the external phosphorous source added into the synthesis system.The synthesis is co-templated by cetyltrimethylammonium bromide(CTAB)and 3-(trimethoxysilyl)propyl dimethyl octadecyl ammonium chloride(TPOAC).It is shown that CTAB functions as an in-situ growth promoter to increase the SAPO-34 content in the resulting composites,TPOAC serves as a mesoporogen to generate mesopores in the SAPO-34 phase of the SAPO-34@KMS,and both CTAB and TPOAC play a role of crystal growth inhibitor,leading to the successful synthesis of SAPO-34@KMS with high SAPO-34 content(33.7%),small SAPO-34 crystal size(1 ?m × 0.2 ?m × 0.2 ?m)and the hierarchical micro-mesoporous structure of SAPO-34.The obtained composite exhibits outstanding MTO performance,with the selectivity to ethylene and propylene being increasing by 20% and the catalyst lifetime being remarkably prolonged from 50 to 200 min as compared with the semi-synthesized catalyst.3.The third part of this thesis describes a novel ZSM-5 seed assisted approach to in situ synthesize SAPO-34/ZSM-5@kaolin composites(SAPO-34/ZSM-5@KMS)with the approximately same total content of molecular sieves but with different mass ratios of ZSM-5 to SAPO-34 with the aim of tuning the product distribution of the MTO reaction.It is shown that in the resulting composites ZSM-5 and SAPO-34 are connected by chemical bonds and the mass ratios of SAPO-34 to ZSM-5 can be adjusted between 0.6-10 by changing the calcination temperature of KMS and the m(CTAB)/m(KMS)value of the initial synthesis gel.Importantly,we demonstrate that changing the mass ratio of SAPO-34 and ZSM-5 in the composites can manipulate the reaction pathway of methanol conversion,e.g.,decreasing the mass ratio of SAPO-34 and ZSM-5 in the composites increases the propagation of the olefin-based cycle.This points a new way to tune the product distribution of the MTO reaction.