Characterization of the acid properties of tungsten/zirconia catalysts using adsorption microcalorimetry and n-pentane isomerization activity

Ammonia adsorption microcalorimetry was conducted on various solid acid tun gsten/zirconia catalysts prepared by different techniques. The calorimetric data were compared to catalytic test results using n-pentane isomerization as a measure of acid activity. The results show that (1) the co-precipitat ion method of making the tungsten/zirconia catalyst produces a greater numb er of acidic sites than impregnating tungsten on hydrous zirconia, resultin g in a more active catalyst, and (2) the addition of small amounts of iron to the tungsten/zirconia catalyst increases the acid site strength as deter mined by ammonia adsorption and improves the paraffin isomerization activit y. The calorimetry data indicate that the acid site strength of the tungste n/zirconia materials is similar to or slightly higher than that found in ze olites or sulfated zirconia and is comparable to sulfuric acid. However, th e paraffin isomerization activity results suggest that the acid sites of th e tungsten/zirconia catalyst should be about four orders of magnitude more active than that of zeolite beta on the basis of turnover frequency. Our ex perimental results indicate a lack of correlation between the heat of ammon ia adsorption with catalytic activity. Comparisons of catalytic activity be tween materials based entirely on acid strength may not be valid, and kinet ic probes would be more appropriate. (C) 1999 Academic Press.