Caihong Yang

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

The H4SiW12O40/SiO2 heteropolyacid catalyst for the catalytic oxidation of dimethyl ether to dimethoxymethane was prepared by the impregnation method. MnCl2, SnCl4, and CuCl2 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. H4SiW12O40/SiO2 is active for the oxidation of dimethyl ether, but the selectivity for dimethoxymethane is as low as 4.8%. Modification of H4SiW12O40/SiO2 with 5% MnCl2 significantly improves the dimethoxymethane selectivity up to 27.9% at 633 K, while SnCl4- and CuCl2-modified catalysts give dimethoxymethane selectivities of 16.4% and 0.4%, respectively, under the same conditions. The effects of the MnCl2 content (2%–20%) and the 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% MnCl2 content. X-ray diffraction patterns of the catalysts show that MnCl2 and H4SiW12O40 interact and are dispersed uniformly on the support. Infrared spectra demonstrate that the Keggin structure of H4SiW12O40 remains almost unchanged over the modified catalyst. Temperature-programmed desorption profiles of ammonia indicate that MnCl2 modification reduces the acidity of H4SiW12O40/SiO2 by decreasing the acid center numbers.

Catalytic Oxidation of Dimethyl Ether to Dimethoxymethane over Cs Modified H3PW12O40/SiO2 Catalysts

Abstract The attractive utilization route for one-step catalytic oxidation of dimethyl ether to dimethoxymethane was successfully carried out over the H3PW12O40(40%)/SiO2 catalyst, modified by Cs, K, Ni, and V. The Cs modification of H3PW12O40(40%)/SiO2 gave the most promising result of 20% dimethyl ether conversion and 34.8% dimethoxymethane selectivity. Dimethoxymethane could be synthe-sized via methoxy groups decomposed from dimethyl ether through the synergistic effect between the acid sites and the redox sites of Cs modified H3PW12O40(40%)/SiO2.

Effect of different Mn salt precursors on Mn-H4SiW12O40/SiO2 used for dimethoxymethane synthesis from dimethyl ether oxidation

Abstract Mn-H4SiW12O40/SiO2 heteropolyacid catalysts were prepared by impregnation method from different Mn salt precursors, such as MnSO4, Mn(NO3)2, MnCl2 and Mn(CH3COO)2. The catalytic oxidation reaction of dimethyl ether (DME) to dimethoxymethane (DMM) was carried out in a continuous flow type fixed-bed reactor with a ratio of NDME/NO2 = 1:1. It is found that the sequence of catalytic activity for DMM synthesis is MnCl2-H4SiW12O40/SiO2>Mn(NO3)2-H4SiW12O40/SiO2>MnSO4-H4SiW12O40/SiO2>Mn(AC)2-H4SiW12O40/SiO2. The effects of reaction temperature (573 −633 K) on the catalysts were also investigated. With the increase of reaction temperature, the DME oxidation reaction is more exquisite over MnSO4-H4SiW12O40/SiO2 catalyst. 42.4% of DME conversion and 0.9% of DMM selectivity have also been obtained at 613 K. However, MnCl2 modified H4SiW12O40/SiO2 catalyst obtains higher DMM selectivity (37.5%, at 593 K) than other three catalysts at mild reaction conditions. H2-TPR profiles show that MnSO4 modification demonstrates stronger oxidative performance at high temperature than other catalysts, while MnCl2-H4SiW12O40/SiO2 catalyst exhibits better oxidative performance at low temperature. XRD patterns of the catalysts show that the diffraction peaks are strong and MnO2 diffraction peak is also found over the MnCl2 modified catalyst.