热扩散法制备MoO3/SiO2催化草酸二甲酯和苯酚酯交换反应

摘要

采用热扩散法(TS)和等体积浸渍法制备了MoO3/SiO2催化剂用于草酸二甲酯和苯酚酯交换反应.结果表明,热扩散法制备的10%MoO3/SiO2-TS催化剂较等体积浸渍法制备的10%MoO3/SiO2-C催化剂具有更好的催化性能.运用X射线衍射、Raman光谱、X射线光电子能谱分析、吡啶吸附红外光谱、NH3程序升温脱附等手段对催化剂进行了表征,发现虽然两种方法制备的催化剂都只有弱Lewis酸中心,钼均以氧化钼单体形式存在,未形成解离和聚合,但是10%MoO3/SiO2-TS催化剂较10%MoO3/SiO2-C催化剂表面钼含量更高且MoO3分散得更好.在苯酚用量为0.2 mol,10%MoO3/SiO2-TS催化剂用量为1.2 g,反应温度为180°C,草酸二甲酯与苯酚的摩尔比为2,反应时间为4 h的优化条件下,苯酚转化率可达70.9%,甲基苯基草酸酯和草酸二苯酯的收率分别达63.1%和7.7%.%MoO3/SiO2 catalysts for the transesterification of dimethyl oxalate (DMO) with phenol were pre-pared by both the thermal spreading (TS) and incipient wetness impregnation methods. The results showed that the 10%MoO3/SiO2 catalyst prepared by TS (10%MoO3/SiO2-TS) exhibited higher catalytic performance compared with the 10%MoO3/SiO2 catalyst prepared by incipient wetness impregnation (10%MoO3/SiO2-C). The catalysts were characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, pyridine-IR spectroscopy, and NH3 temperature-programmed desorption. These analyses indicated that weak Lewis acid sites were formed on the catalyst surfaces and that the Mo species were present as monomeric MoO3 rather than as isolated molybdenum oxide or polymolybdate species on both catalysts, although the 10%MoO3/SiO2-TS exhibited better dispersion of MoO3 and a higher surface Mo content than the 10%MoO3/SiO2-C. Under the optimal transesterification reaction conditions (1.2 g 10%MoO3/SiO2-TS, T = 180 °C, n(DMO)(phenol) = 2, t = 4 h), the conversion of phenol was 70.9%, and the yields of methyl phe-nyl oxalate and diphenyl oxalate were 63.1% and 7.7%, respectively.