Surface Reactions of Acetone on Al 2 O 3 , TiO 2 , ZrO 2 , and CeO 2 : IR Spectroscopic Assessment of Impacts of the Surface Acid-Base Properties M. I. Zaki,* M. A. Hasan, and L. Pasupulety Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969 Safat, 13060 Kuwait Received July 11, 2000. In Final Form: November 7, 2000 Adsorption and surface reactions of acetone vapor were observed on the title oxides at room and higher temperatures (up to 400 °C), using in situ infrared spectroscopy. The results were correlated with results of infrared spectroscopy of adsorbed pyridine, to assess impacts of the surface acid-base properties. It was found that the availability of Lewis acid sites is essential for anchoring acetone molecules to the surface. Coexisting Lewis base sites catalyze condensation of the acetone molecules into mesityl oxide surface species, via formation and subsequent decomposition of enolate and diacetone alcohol species. When intimately coupled, the Lewis acid and base sites generate pair sites of particularly strong adsorption capacity toward condensation products thus formed. Consequently, surface active sites are blocked and adsorptive and catalytic interactions of acetone are largely suppressed. Introduction Surface chemistry of acetone has been the focus of attention of a number of recent research endeavors. 1-4 The thrust for this research interest has been the knowledge that catalytic hydrogenation of acetone is a versatile synthetic route to fine chemicals. 5-7 Among the products encountered are the industrially important 4-hydroxy-4-methylpentan-2-one, methyl isobutyl ketone (MIBK), diacetone alcohol (DAA), and mesityl oxide (MSO). 6,7 Hydrogenation of acetone has been found to occur on metal-oxide-supported Ni, Co, and Fe metal catalysts 5-7 at low temperatures (150-250 °C), near atmospheric pressure. Studies conducted to understand the surface chemistry of acetone were based largely on in situ IR probing of adsorption modes and species of acetone on metal oxide surfaces at low 1,2 and high 3,4 temperatures. The results obtained may lead to the following conclusions. First, acetone molecules are irreversibly adsorbed via coor- dination to Lewis acid sites ((CH 3 ) 2 CdOfM n+ ). Second, the acetone ligands may be activated for R-hydrogen abstraction and consequent formation of anionic enolate- type ions (CH 2 (CH 3 )C-O - fM n+ ), provided that the coor- dination site is strongly acidic and has a basic site (surface -OH - or -O 2- site) in close proximity. Third, an aldol- condensation-type of surface reaction may then occur, converting the enolate species into DAA ((CH 3 ) 2 C(OH)- CH 2 (CH 3 )CdOfM n+ ) and further to MSO ((CH 3 ) 2 Cd CH-(CH 3 )CdOfM n+ ) species. Fourth, occurrence of aldol condensation implies, according to the reaction mechanism published elsewhere, 8 the availability on the surface of acid-base site-pairs functioning in a concerted fashion. Last of all, acetone adsorbed species are con- verted at high temperatures (>200 °C) into acetate surface species. It is obvious from these conclusions that surface reactions of acetone are critically controlled by the acid-base properties of the surface. In an attempt to correlate catalysis-induced changes in the chemical composition of the acetone gas phase with the type of adsorbed acetone species, Fouad et al. 3,9 suggested that strong adsorption of the primary (coordinated) species and/or condensation (polymerized) products of acetone blocks the active sites for further adsorptive and catalytic interaction. This suggestion was based on observed decomposition reactions of methyl- butynol (giving “acetone + acetylene”) that slow markedly on surfaces (Y 3+ -doped MgO) exposing strong acid-base site-pairs, known to facilitate strong adsorption and subsequent condensation of the acetone thus produced. These authors 3,9 stressed that such aldol-condensation- type reactions require not only basic sites at which C-H bond activation takes place 8 but also coexisting Lewis acid sites to stabilize the reaction intermediates. This was verified earlier by blocking the Lewis acid sites via adsorption of pyridine. 10 Acetone produced via 2-propanol dehydrogenation on group IVB metal oxides was found to convert to isobutene and CH 4 gas-phase products at 300-400 °C, without any sign of formation of acetone condensation products on the surface. 11,12 Whether that was due to involvement of Lewis acid sites in coordinating isopropoxide species (or the simultaneously formed acetate species), leading to the absence on the test surfaces of cooperatively functioning acid-base site-pairs, is a question that could find no definitive answer by the results then com- municated. 11,12 As a matter of fact, dehydrogenation of * Corresponding author. E-mail: zaki@kuc01.kuniv.edu.kw. Fax: (0965)4846946. (1) Allian, M.; Borello, E.; Ugliengo, P.; Spano `, G.; Garrone, E. Langmuir 1995, 11, 4811. (2) Panov, A.; Fripiat, J. J. Langmuir 1998, 14, 3788. (3) Fouad, N. E.; Thomasson, P.; Kno ¨zinger, H. Appl. Catal., A 2000, 196, 125. (4) Zaki, M. I.; Hasan, M. A.; Al-Sagheer, L.; Pasupulety, L. Langmuir 2000, 16, 460. (5) Gandia, L.; Montes, M. Appl. Catal., A 1993, 101, L1. (6) Narayanan, S.; Unnikrishnan, R. Appl. Catal., A 1996, 145, 231. (7) Narayanan, S.; Unnikrishnan, R. J. Chem. Soc., Faraday Trans. 1998, 94, 1123. (8) Iglesia, E.; Barton, D. G.; Biscardi, J. A.; Gines, M. J. L.; Soled, S. L. Catal. Today 1997, 38, 339. (9) Fouad, N. E.; Thomasson, P.; Kno ¨zinger, H. Appl. Catal., A 2000, 194-195, 213. (10) Tanabe, K.; Saito, K. J. Catal. 1974, 35, 247. (11) Zaki, M. I.; Hussein, G. A. M.; El-Ammawy, H. A.; Mansour, S. A. A.; Poltz, J.; Kno ¨zinger, H. J. Mol. Catal. 1990, 57, 367. (12) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1723. 768 Langmuir 2001, 17, 768-774 10.1021/la000976p CCC: $20.00 © 2001 American Chemical Society Published on Web 01/03/2001