Compensation effect in isopropanol dehydration over heteropoly acid catalysts at a gas–solid interface

Kinetics of isopropanol dehydration at a gas–solid interface over a range of bulk and supported Brønsted acid catalysts based on H3PW12O40 (HPW) Keggin-type heteropoly acid was studied in a continuous flow fixed-bed reactor in the temperature range of 55–135 °C and isopropanol partial pressure of 0.94–5.52 kPa. The bulk catalysts included HPW itself and its Cs acid salts CsnH3−nPW12O40 (CsnH3−nPW). The supported catalysts comprised of 15 wt.% HPW supported on SiO2, TiO2, ZrO2 and Nb2O5. Under the conditions studied, the reaction was found to be of zero order in isopropanol. The turnover rate decreased in the order HPW > Cs2.5H0.5PW ≈ Cs2HPW > HPW/SiO2 > HPW/TiO2 > HPW/Nb2O5 ≈ HPW/ZrO2, which is in line with the acid strength of these catalysts. The true activation energies E and pre-exponential factors A obtained from zero-order kinetics, were found to exhibit a compensation linear relationship ln A = mE + c. Moreover, the bulk and supported catalysts exhibited different compensation plots. This is suggested to be due to the different chemical structure of Brønsted acid sites in these catalysts, resulting in differing reaction mechanisms. The bulk catalysts possess strong surface H+ sites located on peripheral (bridging) oxygen atoms in the Keggin unit, whereas supported HPW catalysts have weaker H+ sites probably located on oxygen atoms of support. Consequently, on the bulk catalysts, isopropanol dehydration is suggested to occur via E1 elimination pathway, whereas on the supported catalysts it might also involve a contribution of E2 elimination pathway.

Activation energies and pre-exponential factors exhibit compensation relationship, which is different for bulk and supported HPA catalysts. This is rationalised mechanistically via E1 and E2 elimination pathways.Figure optionsDownload high-quality image (62 K)Download as PowerPoint slideHighlights
► Turnover reaction rates correlate with the acid strength of HPA catalysts.
► Activation energies and pre-exponential factors exhibit compensation relationship.
► Bulk and supported HPA catalysts show different compensation plots.
► This is rationalised mechanistically via E1 and E2 elimination pathways.