Synthesis, Characterization Of ZSM‐5 Zeolite And Studies On Its Catalytic Performance For The Reaction Of MTO

Light olefins(ethylene,propylene and butylene)are very important basic feedstocks of petrochemical industry. Olefins are traditionally produced from petroleum feedstock by steam and catalytic cracking processes. With the decrease of oil resources and the rise in oil prices, it is a trend to develop non-petroleum based technologies to produce olefins. Methanol to olefins (MTO) and methanol to propylene (MTP) processes are the strategy choices of Chinese petrochemical industry and have great potential in development and application.ZSM-5 zeolite was synthesized successfully using silica gel microspheres and silicon sol as double silicon source in this paper. As the proportion of silicon sol in the silicon source increased, the Si/Al ratio of obtained ZSM-5 zeolite increased. Results indicated that the utilization ratio of silicon sol was higher than silica gel microspheres in the synthesis of ZSM-5 zeolite.It was found that effects vary greatly according to different additives on the synthesis of ZSM-5 zeolite. Crystal morphologies were strongly influenced by the addition of NaF and many aggregates made up of sheets with the thickness of 50nm were produced. KCl and NH4F restrained the formation of ZSM-5 zeolite. A small amount of NaCl could decrease the crystal size of ZSM-5 zeolite, while the size increased when excessive NaCl was added. N-Methyl pyrrolidone and glycerol were beneficial to promote the crystal size of ZSM-5 zeolite and ZSM-5 zeolite with crystal size of 55μm was obtained when glycerol was used as additive.Catalytic performances of commercial ZSM-5 zeolite with micro size and nano size for the MTO reaction indicated that there were few strong acid sites for conventional Na-ZSM-5 and its reactivity for MTO reaction was very low. Microsize and nanosized H-ZSM-5 both showed high activity toward MTO reaction, while their selectivities to olefins were very low. Si/Al ratios of nanosized H-ZSM-5 could be enhanced greatly and the external surface areas were also increased by ammonium hexafluorosilicate modification, so the selectivity toward olefins was also increased greatly.Moreover, one type of Na-ZSM-5 with large partical size and low Na content was synthesized in this paper, it possessed lots of strong acid sites. This new type of Na-ZSM-5 exhibited high MTO activity without ammonium exchange. As the Si/Al ratio of Na-ZSM-5 increased, the acidity decreased and the selectivity to olefins increased. After ammonium exchange, the Na content decreased notably and the acidity was strengthened, the ethylene selectivity increased and the selectivity to propylene and butylene both decreased.Selectivity to olefins was increased by phosphorus modification and ethyl orthosilicate deposition modification, the selectivity to propylene was increased to 44.2% by using phosphorus modified Na-ZSM-5 as catalyst.In conclusions: strong acid sites are the active centers to catalyse MTO reaction, ZSM-5 zeolite with the proper amount of strong acid sites and large external specific surface area is beneficial for the MTO reaction.