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

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.