JOURNAL OF CATALYSIS 127, 75-85 (1991) Synthesis of Magnesium Oxide by the Sol-Gel Method: Effect of the pH on the Surface Hydroxylation T. LOPEZ, I. GARCIA-CRUZ, AND R. GOMEZ Universidad Autonoma Metropolitana-Ixtapalapa, Departamento de Quimica, A, P. 55-534, Ixtapalapa, Mexico D. F. 09340 Received August 23, 1989; revised February 20, 1990 Magnesium oxide was prepared by the sol-gel method. The method involves the hydrolysis of magnesium alkoxide in the presence of acid or basic catalysts followed by a polycondensation reaction. The synthesized solids were characterized by IR spectroscopy, X-ray diffraction, DTA, TGA, and electron microscopy. It is shown that the hydrolysis process depends on the catalyst's pH (HCI > CH3COOH > H2C204 > H20 > NH4OH). This effect can be explained by the reaction mechanism. The reaction is controlled by the hydrolysis at acid pH and by the polycondensation at a basic one. Solids prepared by this method are chemically homogeneous, very pure, and specifically hydroxylated. They can be widely used in the study of magnesium support effects in different catalytic reactions, c~ 1991AcademicPress. Inc. INTRODUCTION Earlier studies have shown that metal cat- alysts supported on MgO behave quite dif- ferently from the ones supported on silica or alumina. The magnesia support effect is observed in catalytic activity, in selectivity, and in the formation of bimetallic particles. For benzene hydrogenation (I) and methyl- cyclopentane hydrogenolysis (2) on sup- ported Ru catalysts, a high turnover value and a considerably smaller deactivation rate are reported for the Ru/MgO catalysts com- pared with Ru catalysts supported on SLOE, A1EOa, or TiO2. The MgO effect is also ob- served in the formation of bimetallic parti- cles of Pt-Ru/MgO catalysts which are un- usually enriched by Ru on their surface (3). Similar support effects have also been ob- served in Ru-Cu/MgO catalysts (4). The particular behavior of this support is not completely understood. However, some reasons have been suggested: (i) magnesia stabilizes the metal in uncommon oxidation states (5, 6); (ii) its morphology and crystal- line structure are easily modified (7); (iii) the occurrence of electronic transference between metal and support causes varia- tions in the catalytic properties (8); and (iv) the acid strength of the hydroxyl groups found in the surface of the support is directly related to the catalytic activity (9, 10). It is evident that characterization of the MgO becomes necessary for its use and for inter- pretation of its effects when it is employed as a metal support. MgO is obtained by thermal treatment of Mg(OH) 2 and MgCO 3. The oxide morphol- ogy depends on the preparation technique (calcination rate and temperature), but there is little information on the formation mecha- nism, especially on oxide hydroxylation. In this work the method of synthesis known as sol-gel (11), which consists of hydrolysis and condensation of a magne- sium alkoxide, is used. This method permits us to have different hydroxylated MgO pre- cursors due to variations of pH in the reac- tive medium. The homogeneity of a gel depends on the following parameters: the solubility of reac- tives in solvent, the sequence of addition of reactives, the temperature, and the pH of reaction. The mechanism for obtaining a gel from a sol is probably a nucleophilic substitution. 75 0021-9517/91 $3.00 Copyright © 1991 by Academic Press, Inc. All rightsof reproductionin any formreserved.