WI ELSEVIER Chemical Physics 193 (1995)79-87 Chemical Physics Adsorption of methanol in zeolite, gallosilicate and SAPO catalysts Jumras Limtrakul Laboratory for Computationaland Applied Chemistry, ChemistryDepartment, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand Received 14 June 1994;in final form 8 December 1994 Abstract Methanol adsorption in zeolite, gallosilicate and silicoaluminophosphate (SAPO) catalysts has been investigated within the framework of ab initio molecular orbital calculations. Full optimization of all cluster models and their complexes has been carried out at the DZP/SCF level of theory. Physisorbed methanol and methoxonium cation complexed to the framework catalyst are found for SAPO catalysts, the latter complexes are observed only at high coverages, while only hydrogen-bonded physisorbed methanol complexes are obtained for gallosilicates and zeolitic catalysts. The conversion energy of the hydrogen-bonded physisorbed structure, H3Si-OH-AI(OH)2-O-PH3/[CH3OH]2 to the methoxonium structure, H3Si-O-AI(OH)2-O-PH3]/[CH3OH~'][CH3OH], is about 6.69 kcal/mol. Comparison with hydrogen halides and related complexes of methanol shows that protonated SAPO/methanol is a very strong acid. I. Introduction The Bronsted acidity of catalysts generated from surface hydroxyls within their framework is of prime importance and has been the subject of numerous theoretical and experimental studies [1-9]. Of partic- ular interest in this active research is the methanol conversion to hydrocarbon over zeolitic catalysts, which is the foundation of certain industrially impor- tant reactions, i.e. the methanol to gasoline (MTG) or the methanol to olefin (MTO) processes [10,11]. One of the important steps of these processes in- volves the methanol adsorption on the catalyst sur- face. The reaction mechanisms and the nature of interaction with a zeolitic proton have been the subject of controversy [12,13]. In this work, the interaction of methanol with zeolite, gallosilicate, silicoaluminophosphate (SAPO) catalysts and the protonation of CH3OH by catalytic protons are investigated by ab initio quantum chemi- cal calculations on the successful model of H3SiOH-X(OH)2YH 3 (X = AI, Ga and Y = Si, P) [14-18] with the aims of: (a) comparing the changes in structures and bonding of the different framework structures of catalysts and their complexes, (b) deter- mining the influence of the structure on the catalytic properties of zeolites, gallosilicates and SAPO, (c) investigating the important consequences of methanol adsorption at low and at high coverages, and (d) obtaining the reaction energies for methanol desorp- tion, cluster deprotonation, ion-molecule formation and the conversion energy of the hydrogen-bonded physisorbed methanol complexes to methoxonium cation complexes. 0301-0104/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0301-0104(94)00418-8