金属氧化物改性的HZSM-5上甲苯与甲醇的烷基化反应

 考察了La2O3,MgO以及La2O3-MgO复合改性的HZSM-5催化剂的孔结构、表面酸性和吸附性能,以及它们在甲苯与甲醇烷基化反应中的催化性能. 未经改性的HZSM-5上甲苯甲基化反应产物组成为热力学平衡组成,而改性后的催化剂上目标产物对二甲苯选择性提高,但反应活性下降. La2O3改性使HZSM-5孔径缩小,孔道变窄,强酸和弱酸酸量均降低,目标产物选择性明显提高; MgO主要分布在沸石外表面和孔口,因而MgO改性的HZSM-5孔口尺寸稍有缩小,另外强酸酸量减少,弱酸酸量略有上升,对二甲苯选择性略有提高; 而La2O3-MgO复合改性的催化剂上对二甲苯选择性显著提高,达到93%. 结果表明,反应的对位选择性是孔径和表面酸性同时调变的结果,孔径效应比酸性分布对催化剂的对位选择性影响更大.

Methylation of Toluene with Methanol over Metal Oxide-Modified HZSM-5 Catalysts

A series of HZSM-5 zeolite catalysts modified with MgO, La_2O_3 and La_2O_3-MgO were prepared by the impregnation method and characterized by XRD, N_2-adsorption, NH_3-TPD, pyridine-adsorption IR spectroscopy and model reaction of 1,3,5-trimethylbenzene. Methylation of toluene with methanol to p-xylene was carried out over these catalysts. According to the research, MgO is mainly deposited on the external surface of HZSM-5, which results in the inactivation of external surface acid sites of MgO/HZSM-5, and the MgO/ HZSM-5 catalysts show only a little higher p-xylene selectivity than HZSM-5. However, highly dispersed La_2O_3 on La_2O_3/HZSM-5 covers the strong acid sites and narrows the pore channels of HZSM-5, which leads to a better p-xylene selectivity. The p-xylene selectivity of the La_2O_3-MgO/HZSM-5 catalyst is significantly improved to 93%, and the conversion of toluene is lowered a little to 20%, owing to the dual effect of La_2O_3 and MgO. The results show that the reduction of pore dimension of the HZSM-5 catalysts has a greater effect on the p-xylene selectivity enhancement than the suppression of strong acid sites and inactivation of external surface acid sites.