H3PW12O40-supported MCM-41 acid catalyst for the gas-phase synthesis of MTBE

Abstract

HPW (H3PW12O40) was impregnated onto the surface of fluorinated Si-MCM-41 with an excellent hydrothermal stability. The ordered hexagonal mesostructure of the Si-MCM-41 support is maintained after the impregnation of 50-wt% HPW. The HPW amount has a remarkable effect on the intensity of the main XRD reflection [100] peak of the Si-MCM-41 support. No visible crystalline diffraction peaks appear above 10° 2θ for these samples. This is assigned to the use of a well-ordered MCM-41 host in this work. A sharp mesoporous inflection is observed at relative pressures between p/po = 0.25 and 0.40 on the isotherms of these samples. The infrared framework vibration bands of Si-MCM-41 are at 1236, 1090, 965, 800, 564 and 465 cm 1, easily overlap with those of Keggin structure of HPW. It is observed that the intensity of the 800-cm-1 (W-O-W) band is almost proportional to the increase of HPW amount on MCM-41. Once Si-MCM-41 is loaded with HPW, Brønsted and Lewis acidities are revealed on the HPW/MCM-41 catalysts, with an ascending order in the surface acidity of Si-MCM-41 ≪ HPW(15%)/MCM-41 < HPW(30%)/MCM-41 < HPW(50%)/ MCM-41. For the gas-phase synthesis of MTBE, the HPW/MCM-41 catalysts show high activity at optimal temperatures, and high on-stream steady activity until 110 h at 100 °C, comparable to that of the Amberlyst-15 catalyst. But the temperature window to achieve an ideal conversion narrows gradually from 80-100 °C for Amberlyst-15, to 90-100 °C for HPW(30-50%)/MCM-41 catalysts, and to 100 °C for HPW(15%)/MCM-41.