Quantitative determination of acid sites on silica–alumina

The density of acid sites on amorphous silica alumina has been determined using a combination of gravimetric and spectroscopic analysis using pyridine and 2,4-lutidine and 2,6-lutidine for surfaces treated at different calcination temperatures. An attempt was made to correlate the number of acid sites determined by the different base molecules with the activity for liquid phase reaction of acetone to produce diacetone alcohol at 298 K. The best correlation was obtained with the number of Lewis acid sites which were able to retain pyridine after evacuation at 473 K. The lutidines underestimated the number of Lewis sites on the oxide surface. In particular, 2-6-lutidine was only able to detect Lewis acid sites in geometries at edges and other geometric imperfections on the solid and these were mainly generated after low temperature (573 K) calcination treatment.

Figure optionsDownload high-quality image (72 K)Download as PowerPoint slideResearch highlights▶ Quantification of both Lewis and Brønsted acid sites by combined FTIR and gravimetric measurement. ▶ Three nitrogen bases describe the distribution of sites of different acid strength and accessibility. ▶ Correlation between site density and activity for acetone condensation show the latter to be catalysed by Lewis acid sites.