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（英文）

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

本研究合成了一系列硅铝比不同(SiO_2/Al_2O_3=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、N_2吸附-脱附、NH_3-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0.162 g/min时,该临界值为0.175μmol/m^2。

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( M TP) 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. M eanwhile,the selectivity to propene increases monotonically with an increase in the silicon to aluminum ratio of H-ZSM-5 for M TP 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 M TP. A critical value of acid density,viz.,[AS]S,is required to achieve the maximum propene selectivity for M TP with a complete methanol conversion; this critical [AS]Svalue is 0. 175 μmol/m2 for the H-ZSM-5 zeolite under current reaction conditions.