HZSM-5型分子筛硅铝比对一步法合成二甲醚的影响

Direct DME synthesis catalyst contains methanol synthesis catalyst and solid acid catalyst for methanol dehydration. The general solid acid catalyst is γ-Al2O3, zeolite, or SiO2-Al2O3. And the optimal reaction temperature of HZSM-5 zeolite matches the methanol synthesis catalyst best. The key effect on the direct DME synthesis is due to the acidity of the HZSM-5. However the acidity of the HZSM-5 is up to the Si/Al ratio on the bone of the zoelite. In this paper, several kinds of HZSM-5 with different Si/Al ratios are used to investigate their effects on the direct DME synthesis. The acidity of the zeolite is tested by NH3-TPD. The HZSM-5 zeolites with different Si/Al ratios are mechanically mixed with the methanol synthesis catalyst respectively. The bi-functional catalyst is tested on the micro-reactor with an online tail gas analysis system. The feed gas composition is H2∶CO∶N2∶CO2∶CH4＝60∶30∶3∶4∶3. The reaction conditions are 493 K～553 K, 2 MPa～4 MPa, and 1 000 h-1～4 000 h-1. With the decrease of the Si/Al ratio, the acidity of the HZSM-5 increases and the CO single pass conversion increases. The selectivity order of CO to DME with different Si/Al ratio is sDME (cat38)> sDME (cat50)>sDME (cat25). The highest space time yield (STY) is 0.38 gDME/gcat·h－１ at the reaction condition of 553 K, 3 MPa and 4 000 h-1 on the catalyst with Si/Al ratio 25, which is the best Si/Al ratio among the the reaction condition range. From the NH3-TPD spectrum, the smaller the Si/Al value is, the stronger the acidity on the strong acid site (Brönsted acid) is. The dehydration process happens on the B acid site of the zeolite. With small Si/Al ratio, the results of methanol dehydration on the zeolite will be better. It can enhance the CO hydrogenation. So the CO single pass conversion increase with the decrease of the Si/Al ratio. If the Si/Al ratio is too small, the acidity on the B acid site will become too strong and the DME formed on the zeolite will be dehydrated further to form hydrocarbons such as C2H2 and C2H4. According to our study, the Si/Al ratio is 38 and 25, the selectivity to DME and the STY of DME can reach their best value respectively.