ALPO4-AL2O3 CATALYSTS WITH LOW-ALUMINA CONTENT .2. ACIDITY AND CATALYTIC ACTIVITY IN CYCLOHEXENE CONVERSION AND CUMENE CRACKING OF CATALYSTS OBTAINED WITH AQUEOUS AMMONIA

The influence of gamma-Al2O3 loading (5-25 wt.-%) and the calcination temperature of AlPO4-Al2O3 (APAL-A) catalysts on their surface acidity (number and strength distribution) was studied by means of the chemisorption (gas-chromatographically measured) of pyridine (PY) and 2,6-dimethylpyridine (DMPY) at temperatures in the 473-673 K range. In addition, cyclohexene skeletal isomerization (CSI) and cumene conversion (CC), were selected as dynamic methods for surface acid characterization. The AlPO4-Al2O3 catalysts were found to show greater number and strength of acid sites than each individual component. The addition of Al2O3 even in a small amount (5 wt.-%), could significantly improve the activity and catalytic activity of AlPO4 catalysts. Moreover, the catalytic activity of the APAl-A catalyst for both reactions varied with Al2O3 loading and, thus, it increased gradually as the Al2O3 content of the catalyst increased up to 15 wt.-%, after which it decreased. Good correlations between catalytic activity and the increase in acidic properties of the AlPO4 by Al2O3 loading (as determined by base chemisorption) have been obtained for both reactions. Qualitative DRIFT spectroscopy of probe molecules [pyridine (PY), 2,6-dimethylpyridine (DMPY) and hexamethyldisilazane (HMDS)] showed that P-OH and Al-OH groups disappeared on adsorption and that both Lewis and Bronsted acid sites existed on the surface of APAl-A catalysts. Furthermore, the involvement of strong acid sites in CSI and CC reactions has been investigated by selectively poisoning the acid sites with PY, DMPY and HMDS. The catalytic activity and selectivity of the APAl-A catalyst were strongly affected by this poisoning. Thus, Bronsted acidity was considered to be the common active site on APAl-A catalysts.