HZSM-5型分子筛硅铝比对一步法合成二甲醚的影响

以Cu/Zn/Al(摩尔比为6∶3∶1)甲醇合成催化剂与HZSM-5型分子筛混合，制备了一步法二甲醚合成催化剂。通过改用三种不同Si/Al摩尔比（摩尔比为25、38和50）的HZSM-5型分子筛，考察了催化剂中脱水组分（HZSM-5分子筛）的酸性对二甲醚合成的影响。结果表明，随着催化剂Si/Al摩尔比的降低，分子筛的酸性增强，使得CO单程转化率提高。当催化剂Si/Al＝38时，CO对二甲醚的选择性最高，可达到68.13％，其次是催化剂Si/Al＝50，选择性最差的是Si/Al＝25的催化剂。在553 K、 3 MPa和4 000 h－1的条件下，Si/Al＝25和Si/Al＝38的催化剂CO单程转化率和DME的选择性接近一致。在此条件下，两者的时空产率达到试验的最大值，分别为0.38 gDME/(gcat·h)和0.36 gDME/(gcat·h)，在试验范围内，一步法合成二甲醚催化剂最佳的Si/Al摩尔比为25。

Study of the effects of Si/Al ratio of HZSM-5 on the direct synthesis of DME

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.