Synthesis Of SAPO-18 Molecular Sieve Modification, And Catalytic Properties

With the world economy becoming more interconnected and dependent, finding new sources of energy and industrial valuable products has become very critical to remain competitive in meeting market demand. With energy demand far outstriping energy supply this focus seems more relevant th an ever. In recent years the methanol to olefins (MTO) process has received wide attention as it provides an indirect route in reaching that goal. The catalysts as well as the corresponding technology are the key points for the MTO process. Up to now, ZSM-5、SAPO-44、SAPO-47and SAPO-34molecular sieves have received many attentions due to their high activity for methanol conversion and high selectivity toward ethylene and propylene. In catalytic reactions these microporous materials particularly silicoaluminophosphate oxides (SAPO) give a narrow range of product distribution with a high selectivity towards light olefins, however, diffusion of larger molecules through their narrow pores is very restricted. Their unique selectivity ability has spurred considerable interest in the research community in finding better ways to making them more efficient in performance and overcoming rapid coke formation brought about by larger molecules formed inside the pores. In this dissertation, a novel silicoaluminophosphate zeolite with AEI structure (SAPO-18) was synthesized and modified for being used in the reaction of dimethyl ether to olefins. SAPO-18and modified SAPO-18have been characterized by XRD, NH3-TPD techniques and so on. The catalyfic performance of SAPO-18and modified SAPO-18for dimethyl ether to olefins (abbr. DTO) were studied.Firstly, in this paper, SiO2/Al2O3ratio, crystallization time, metal atoms, so on that have important influences on the structure of SAPO-18molecular sieve have been investigated. The results show that the best SiO2/Al2O3ratio is0.6and the optimum crystallization time was four days. Also, these catalysts show higher activity and selectivity to light olefins (ethylene and propylene) than others.Secondly, the effect of the incorporating of the metal atoms SAPO-18in molecular sieve structure and acidity was studied. The molecular sieves after incorporating metals into the framework have been found exhibiting the acid distribution and high selectivity to light olefins in DTO reaction. The results indicate that the alkali metal oxide SAPO-18reduced in acidity. Mg-SAPO-18and Ca-SAPO-18show a better catalytic performance in DTO reaction. Ba-SAPO-18increased the coke rate in DTO reaction, which extended the lifetime of the molecular sieve. Similarly, the SAPO-18modified by rare earth metal(La、Ce) and Ni metal ion reduced the acidity of catalyst and therefore increased the selectivity to olefins.