H-ZSM-5分子筛硅铝比对其催化的甲醇转化丙烯选择性的影响

本研究合成了一系列硅铝比不同（SiO₂/Al₂O₃=50-4000）,但晶粒粒径相近的ZSM-5分子筛,并考察了硅铝比对甲醇转化反应丙烯选择性的影响。采用XRD、N₂吸附-脱附、NH₃-TPD和Py-FTIR方法对合成的HZSM-5分子筛进行物化性质表征。实验结果表明,随着硅铝比的增大,初始甲醇转化率降低,其中,硅铝比为50-1600的样品可以实现甲醇的完全转化。在甲醇完全转化的条件下,随着硅铝比的增大,丙烯选择性单调增加,从机理角度出发,揭示了甲醇转化制丙烯反应中,甲基化/裂化循环相较于甲基化/脱烷基化循环进行程度更大。此外,本研究提出了在甲醇完全转化条件下,保证最大丙烯选择性所需要的临界酸密度值（[AS]_S）,当甲醇进料量为0.162g/min时,该临界值为0.175μmol/m²。

Effect of silicon to aluminum ratio on the selectivity to propene in methanol conversion over H-ZSM-5 zeolites

A series of H-ZSM-5 zeolites with a silicon to aluminum ratio of 50–4000 but similar crystal size were synthesized and characterized by XRD, N₂ sorption, NH₃-TPD and Py-FTIR; the intrinsic effect of silicon to aluminum ratio on the selectivity to propene in the conversion of methanol to propene (MTP) was investigated. The results show that a complete conversion of methanol can be initially achieved over H-ZSM-5 with a silicon to aluminum ratio from 50 to 1600 and then the initial conversion of methanol decreases progressively with further increasing the silicon to aluminum ratio. Meanwhile, the selectivity to propene increases monotonically with an increase in the silicon to aluminum ratio of H-ZSM-5 for MTP with a complete methanol conversion, suggesting that a high Si/Al ratio for H-ZSM-5 may enhance the propagation of the alkene-based methylation/cracking cycle relative to the arene-based methylation/dealkylation cycle in MTP. A critical value of acid density, viz., [AS]_S, is required to achieve the maximum propene selectivity for MTP with a complete methanol conversion; this critical [AS]_S value is 0.175 μmol/m² for the H-ZSM-5 zeolite under current reaction conditions.