Comparative Influence of Surface Tungstate Species and Bulk Amorphous WO3 Particles on the Acidity and Catalytic Activity of Tungsten Oxide Supported on Silica

Abstract

Catalysts based on tungsten oxide supported on silica and containing up to 2.5 W atoms nm–2 were studied for the purpose of establishing quantitative relationships between the nature and abundance of surface species, acidity, and catalytic activity. Synthesis conditions that maximize the formation of amorphous WO3 were adopted to assess its influence on the overall acidity and activity. The results were compared to those of conventionally prepared catalysts. The catalysts’ structure was characterized by FTIR, Raman, and UV spectroscopies. The acidity of the solids was monitored by adsorption of 2,6-dimethylpyridine followed by FTIR, and their catalytic activity was tested for the reaction of propan-2-ol dehydration. A methodology for quantitation of W species (WOx surface species, amorphous or crystalline WO3 particles) detected following calcination at 673 or 773 K was developed. For both calcination temperatures, surface W species were first formed with initial W addition. Their abundance increased with increasing W content up to ca. 0.7 atom W nm–2 and levels off for higher W surface densities. The leveling off was concomitant with the detection and rapid development of amorphous or crystalline WO3 phase (catalysts calcined respectively at 673 or 773 K). The comparison of W speciation in both series indicated that amorphous WO3 phase is quantitatively transformed to crystalline WO3 on increasing the calcination temperature from 673 to 773 K. The observed development of Brønsted acidity of the solids was associated with W deposition. The abundance of these acid sites correlated directly with the catalytic activity. The study of structure–activity relationship showed that all W species were active. Modeling of the results permitted an estimation of the intrinsic activity of each W species. The values thus obtained suggest that amorphous WO3 particles exhibit the highest activity per surface W atom.