Effect of water on silica-supported phosphotungstic acid catalysts for 1-butene double bond shift and alkane skeletal isomerization

The thermal stability, acidity, catalytic activity and regenerability of silica-supported phosphotungstic acid were evaluated in reference to the bulk solid acid. Samples were prepared with 5, 10, and 25 wt.% heteropolyacid (HPA) on fumed silica to examine the effect of loading on the properties of supported acid catalysts. X-ray absorption spectroscopy indicated that the supported heteropolyacids did not completely decompose when heated to 573 K. The catalytic activity of both bulk and supported HPAs for double bond shift and skeletal isomerization of light hydrocarbons decreased with increasing pretreatment temperatures, which suggests that the level of hydration is critical for acid catalysis. After pretreatment at 573 K, a well dispersed sample (5 wt.% HPA) catalyzed I-butene double bond isomerization, a reaction that requires weak acid sites, whereas the sample was inactive for pentane skeletal isomerization, a reaction that requires strong acid sites. Evidently, the well dispersed HPAs did not have sufficient acidity to catalyze pentane isomerization since higher loaded samples were active for the reaction under identical conditions. Ammonia sorption microcalorimetry results correlated well with reactivity, revealing that 5 wt.% HPA on silica had the lowest heat of ammonia sorption, while a 25 wt.% sample had initial heats near that of bulk heteropolyacid (150 kJ mol(-1)). Treatment of the catalysts with water at mild conditions recovered activity for the bulk and supported heteropolyacids for the 1-butene and pentane isomerization reactions whereas treatment in air or N2O was ineffective. These findings suggest that water treatment regenerates acid sites by rehydrating partially decomposed Keggin units. Results from in situ W-VIS spectroscopy of the catalyst samples support the above conclusion. (C) 2000 Elsevier Science B.V. All rights reserved.