水热处理对MCM-22催化剂酸性、孔结构及甲苯/甲醇烷基化性能的影响

采用XRD, NH3-TPD, IR和低温氮气吸附等方法研究了分别以纯水蒸气和质量分数为6％的氨水蒸气处理MCM-22分子筛催化剂后, 其酸性和孔结构的变化, 并以甲苯、甲醇烷基化为探针反应考察了催化剂的催化性能. 研究结果表明, 在两种不同介质中和处理温度不高于400 ℃条件下, 催化剂的总酸量变化不大, 强酸中心有所增加; 处理温度高于500 ℃后, 催化剂的总酸量明显下降, 强酸中心基本消失; 经水热处理后, MCM-22分子筛催化剂中形成了孔径不均匀的二次孔, 平均孔径增大. 随着处理温度的提高, 催化剂的活性降低, 对二甲苯和邻二甲苯的选择性上升. 经500 ℃纯水蒸气处理5 h的MCM-22催化剂, 具有适宜的酸强度和酸类型分布, 有利于甲苯甲醇烷基化反应的进行, 且催化剂维持了较高的催化活性并具有一定的对位选择性(甲苯转化率和对二甲苯选择性分别29.22％和42.16％).

Effect of Hydrothermal Treatment of MCM-22 Catalyst on the Acidity, Pore Structure and Alkylation Properties of Toluene with Methanol

The change of acidity and pore structure of MCM-22 zeolite catalysts treated by pure steam and ammonia steam(mass fraction, 6%) were studied by means of XRD, NH3-TPD, IR and N2 adsorption. And the catalytic performance of catalysts was investigated via alkylation of toluene with methanol as the probe reaction. The results show that the total acid amount changed indistinctively and strong acid sites increased relatively if the treated temperature was not higher than 400 ℃ even in two different treated media. However, the total acid amount decreased obviously and strong acid sites nearly disappeared as the treated temperature was beyond 500 ℃, and the second pore with different pore sizes formed and average pore diameter enlarged of MCM-22 zeolite catalysts after hydrothermal treatment. The catalytic activity declined as the selectivity of para-xylene and ortho-xylene improved with increasing treated temperature. The acid strength and acid distribution of MCM-22 catalyst treated by pure steam for 5 h were appropriate for alkylation of toluene with methanol, and the catalyst maintained relatively high catalytic activity and processed some para-xylene selectivity(the toluene conversion and para-xylene were 29.22% and 42.16%, respectively).