MnCl2-H4SiW12O40/SiO2催化氧化二甲醚制取甲缩醛

 采用浸渍法制备了H4SiW12O40/SiO2杂多酸催化剂,分别使用MnCl2, SnCl4和CuCl2对其进行修饰,并在常压连续流动固定床反应器中考察了催化剂对二甲醚选择氧化制取甲缩醛的催化活性. 结果表明, MnCl2修饰的H4SiW12O40/SiO2催化剂的活性和甲缩醛选择性均高于SnCl4和CuCl2修饰的催化剂. 进一步考察了不同MnCl2含量及反应温度对反应的影响. 在MnCl2含量为5%, 反应温度为593 K时, MnCl2-H4SiW12O40/SiO2催化剂的活性和甲缩醛选择性最佳,二甲醚转化率为8.6%, 甲缩醛选择性为36.3%. X射线衍射结果显示, MnCl2与H4SiW12O40相互作用并均匀地分散在载体上. 用红外光谱研究了MnCl2-H4SiW12O40/SiO2催化剂的结构,发现改性后的催化剂基本保持了杂多酸的Keggin结构. NH3程序升温脱附结果显示, MnCl2的加入较明显地降低了催化剂的酸强度和酸中心数目.

Catalytic Oxidation of Dimethyl Ether to Dimethoxymethane over MnCl2-H4SiW12O40/SiO2 Catalyst

The H_4SiW_(12)O_(40)/SiO_2 heteropolyacid catalyst for the catalytic oxidation of dimethyl ether to dimethoxymethane was prepared by the impregnation method.MnCl_2,SnCl_4,and CuCl_2 were used to modify the catalyst to improve its activity and selectivity.The catalytic oxidation reaction was carried out in a continuous flow fixed-bed reactor.H_4SiW_(12)O_(40)/SiO_2 is active for the dimethyl ether oxidation,but the selectivity for dimethoxymethane is as low as 4.8%.Modification of H_4SiW_(12)O_(40)/SiO_2 with 5% MnCl_2 significantly improves the dimethoxymethane selectivity up to 27.9% at 633 K,while SnCl_4-and CuCl_2-modified catalysts give dimethoxymethane selectivities of 16.4% and 0.4%,respectively,under the same conditions.The effects of the MnCl_2 content(2%-20%) and reaction temperature(573-633 K) on the reaction were investigated.A dimethyl ether conversion of 8.6% and a dimethoxymethane selectivity of 36.3% were obtained under the optimum conditions of 593 K and 5% MnCl_2 content.X-ray diffraction patterns of the catalysts show that MnCl_2 and H_4SiW_(12)O_(40) interact and are dispersed uniformly on the support.Infrared spectra demonstrate that the Keggin structure of H_4SiW_(12)O_(40) remains almost unchanged over the modified catalyst.Ammonia temperature-programmed desorption profiles indicate that MnCl_2 modification reduces the acidity of H_4SiW_(12)O_(40)/SiO_2 by decreasing the acid center numbers.