Preparation of 12-Tungstophosphoric Acid Embedded in a Silica Matrix and Its Effect on the Activity of 1-Propanol Dehydration.

Heteropolyacids (HPAs) such as 12-tungstophosphoric acid (HPW) encapsulated in silica and other matrices are widely used in heterogeneous catalysis because of their enhanced catalytic stability in liquid and gas-phase reactions. In this work, we systematically studied, for the first time, HPW embedded in a silica matrix using different alcohols (methanol, ethanol, 1-propanol, 1-butanol, and 1-octanol) during the sol-gel preparation method. The catalysts, x%HPW@SiO2-alcohol (x = 10, 20, 30, and 40 wt %) were characterized by several methods: energy dispersive X-ray fluorescence (EDXRF), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), 31P MAS NMR, N2 sorption at low temperature (-196 °C), and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). The HPW Keggin structure was maintained under all preparations. The elemental and leaching tests showed that 20%HPW@SiO2-butanol was one of the most promising materials for catalysis. It was found a direct correlation between the textural properties (e.g., Brunauer-Emmett-Teller (BET) specific surface area) and the size of the carbon chain used in the synthesis. The larger the alcohol chain, the greater the tendency to form mesopores, which in turn increase the stability of the entrapped HPW. SEM/EDS and XRD results indicated a good dispersion of HPW on the inner surface of silica. A high conversion of 1-propanol (92%) and selectivity to propene (98%) were obtained with the 20%HPW@SiO2-butanol catalyst at 400 °C, which has Brønsted acid sites and a density of around 0.20 H+ nm-2. These combined properties (BET surface area, mesoporous volume, Brønsted acid density, and dispersion) contributed to enhancing the activity of the catalyst for this reaction under our experimental conditions.