Study On Conversion Of Methanol To Organic Chemicals Over Zeolite Catalysts

Being an important product from C1 chemical industry, methanol is not only clean fuel but also an important chemical feedstock. The studies on the application of methanol in green processes to synthesize various derived products has been the focus of C1 chemistry research. At present, dimethyl ether(DME) can be synthesized by dehydration of methanol, which is the primary method for DME production. On the other hand, the catalytic system have some defects such as local overheating in catalysts bed, too much olefin by-products and preference of coke. Toluene alkylation with methanol is an new method to produce para-xylene(PX) by far. The present work are focused on how to increase the para-selectivity. It is of great value to develop chemical processes to synthesize DME and PX based on zeolite catalysts, which will enhance integrated utilization of methanol and development of C1 chemical industry.This paper aims to develop a green chemical process to synthesize DME and PX. various factors have been investigated, including synthesis of zeolites, structure characters of zeolite catalysts and reaction technique. The primary results have been listed as follows:1. Influence of zeolite topology structure, acidity and reaction technique have been investigated in methanol dehydration reaction. The results have shown that ZSM-22, ZSM-23 and FER zeolites have suitable channel structure restricting the formation of hydrocarbon pool intermediate, which helps to increase DME selectivity. Bronsted and Lewis acidity sites are both active to catalyze methanol dehydration. Meanwhile, olefin and other hydrocarbons are formed primarily on strong acidity sites of zeolites. The experiment has shown an optimized reaction technique using HZSM-23 as catalyst: 0.5g catalyst, feed WHSV 5h^-1, reaction temperature 300℃, pressure 0.1MPa and corresponding methanol conversion is 95.1%, DME selectivity is 95.5%. Finally, a possible mechanism for methanol dehydration on zeolite catalysts was put forward based on literature and above analysis. 2. Influence of zeolite topology structure, acidity, crystal size and reaction technique have been investigated in toluene alkylation with methanol, following with modification on zeolites with boron acid. The results have shown that ZSM-5 is fit to catalyze this reaction for its suitable channel structure and adjustable acidity. Toluene disproportionation and PX isomerization can be inhibited when increasing SiO₂/Al₂O₃ ratios and modifying zeolites with boron, which can be attributed to the reduction of strong acidity. Meanwhile, Enhancement of zeolitic diffusion restriction can be realized by adjustment of crystal size, so that a higher PX selectivity can be achieved.3. ZSM-23 zeolite has been synthesized successfully with isopropylamine(iPA) as template, instead of expensive traditional templates, such as pyrrolidine, isopropanolamine, Diquat-7, Diquat-8 and Diquat-12. The results have shown that ZSM-23 zeolite with pure crystalline and high crystallinity can be synthesized when SiO₂/Al₂O₃ ratio remained between 60 and 150. Template concentration, gel alkalinity and water content are critical factors leading to crystalline transition and morphology transition of products. ZSM-23 zeolite with nano size has been synthesized steadily at optimized experimental conditions.