THE ROLE OF SODIUM AND STRUCTURE ON THE CATALYTIC BEHAVIOR OF ALUMINA.2. IR SPECTROSCOPY

A series of alumina samples differing in structure (gamma, delta and a lpha) and in Na content, were studied by infrared spectroscopy. The sa me set of samples was also studied using 2-propanol dehydration as a p robe reaction and using temperature-programmed reaction of 2-propanol [S. Srinivasan, C.R. Narayanan, A. Biaglow, R. Gorte and A.K. Datye, A ppl. Catal. A, 000 (1995) 000]. Transitional aluminas possess well def ined hydroxyl bands whose nature and intensity is affected by alumina structure and impurity content. The most reactive aluminas exhibit a p rominent hydroxyl band at 3770 cm(-1) Another characteristic of the mo st reactive aluminas is the formation of a band at 1622 cm(-1) after p yridine adsorption and evacuation at room temperature. Addition of Na as well as thermal treatments that transform gamma- to delta-alumina h ave a similar effect on aluminas, namely, an attenuation of the promin ent 3770 cm(-1) hydroxyl band and the 1622 cm(-1) pyridine band. In bo th cases, the effect appears to be steric in nature, indicative of hin dered access of the probe molecule to the reactive sites. These spectr oscopy results correlate well with the trends observed in catalytic re activity.