Etude infrarouge des sites d'adsorption des alcools sur alumine

Abstract

The adsorption of different alcohols ((CF 3) 3 COH, (CF 3) 2 CHOH, CF 3 CH 2 OH, CH 3 OH) has been studied by gravimetry and infrared spectroscopy. Gravimetric results show that the irreversibly and reversibly adsorbed quantities increase with alcohols basicity. Infrared spectra of the irreversible species formed on alumina samples activated at 1120 K provide evidence for alcoholate species. Moreover additional species are present in the case of methanol and 2-propanol: they are coordinated to Lewis acid sites. The reversible species are mainly hydrogen bonded. Poisoning experiments with bases ((CH 3) 2 O and (CH 3) 3 CC ≡ N) lead us to conclude that the number of acid sites not involved in the irreversible adsorption of alcohols increases from (CH 3) 2 CHOH to (CF 3) 3 COH. Use of CD 3 OCD 2H shows that these unoccupied sites are strong Lewis acid ones. These results with those obtained from the coadsorption of two alcohols show the heterogeneity of alumina surface sites; basic O 2− - weak Lewis acid Al 3+ pair sites are involved in the adsorption of the different alcohols. Moreover the surface presents other sites such as O 2+ — strong Lewis acid Al 3+ pair sites or strong Lewis acid Al 3+ sites which are also involved in the adsorption of alcohols with high pk a.