甲醇在活性Al_2O_3催化剂表面的吸附与脱水反应

Adsorption and Dehydration of Methanol on Al_2O_3 Catalyst

采用X射线衍射、红外光谱、程序升温脱附和微反等技术对一系列不同方法制备的活性Al2 O3 催化剂的结构和性能进行了表征 ,探讨了催化剂的结构与甲醇脱水反应性能之间的构效关系 ,提出了甲醇在活性Al2 O3 催化剂表面脱水的反应机理 .结果表明 ,活性Al2 O3 主要由γ Al2 O3 和无定形的Al2 O3 组成 ,其表面存在σⅠ 和σⅡ 两种L酸吸附位 .甲醇在Al2 O3 表面有两种吸附态 ,即分子吸附态和解离吸附态 ,其中甲醇的分子吸附为可逆吸附 ,而解离吸附态甲醇 (即甲氧基 )在催化剂表面解离成表面甲氧基和羟基 ,当反应温度较高时 ,表面甲氧基会进一步分解产生CO ,H2 以及少量的CO2 ,CH4和C等 .二甲醚的生成是分子吸附态甲醇与临近的解离吸附态甲醇相互作用的结果 .

X-ray diffraction, infrared spectroscopy, temperature-programmed desorption coupled with mass spectroscopy, and micro-reactor techniques were employed to study the relationship between the structure and the catalytic activity of the methanol dehydration catalyst of Al 2O 3 with high activity prepared by different methods. The reaction mechanism of methanol dehydration on the catalyst was also discussed. The results show that the primary crystal structure of the catalyst consists of γ-Al 2O 3 and amorphous Al 2O 3 with two types of Lewis acid sites, σ Ⅰ and σ Ⅱ, instead of Brnsted acid sites, existing on the catalyst surface, which play important roles in the methanol dehydration. There are two adsorption states of methanol on Lewis acid sites of the catalyst, \{i.e.,\} the molecular adsorption state and the dissociative adsorption state. The molecular adsorption of methanol on the catalyst is reversible, while the dissociative adsorption of methanol is irreversible. The dissociatively adsorbed methanol can further decompose to form CO, H 2, little CO 2, CH 4, and C. The reaction of the molecularly adsorbed methanol with the dissociatively adsorbed methanol leads to the formation of dimethyl ether.