Chemical Physics ELSEVIER Chemical Physics 208 (1996) 331-340 Cationic, structural, and compositional effects on the surface structure of zeolitic aluminosilicate catalysts Jumras Limtrakul *, Duangkamol Tantanak Laboratory fi~r Computational and Applied Chemistry, Chemistry Department, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand Received 28 January 1996 Abstract The cationic, structural, and compositional influences on the structure and bonding of zeolitic aluminosilicates have been investigated with the density functional theory (DFT) method including local (VWN) and non-local spin density functionals (BLYP). Full optimization of structures has been carried out at the 6-3 I G */VWN and 6-3 I G */BLYP levels of theory for the different types of the ---Si-OH-AI~- unit in the secondary building unit of the zeolite cluster models [(OH)sHyAlxSi8_xOt2 ]Cx-y)- (x, y = 0, 1, 2, 4) and the silica model (OH)sSisOj2. Changes in the environment of the silicon and aluminium framework atoms with a given Si/AI ratio generate new different acid sites. The validity of Loewenstein's =-AI-O-AI~- avoidance rule is confirmed but Dempsey's =-AI-O-Si-O-AI-= avoidance rule does not hold with double four-membered ring aluminosilicate (D4R). The proton affinity (PA) of the silica model (OH)sSi6Oi2 at BLYP/6-31G * are evaluated to be 1403 _+ 15 kJ/mol which is in good agreement with the experimentally observed value of 1390 + 25 kJ/mol. Proton affinities of Bronsted hydroxyl groups in H forms of zeolites associated with different Si/AI ratios indicate that the higher the ratio, the less the proton is constrained which results in a stronger acid strength. Cations are found to have profound effects on the structure and bonding of zeolite clusters. The H ion has a strong perturbation on the important parameters i.e. Si-O, AI-O, OH bond lengths while the Li(I) has a modest effect. The PA of the BrOnsted OH groups in the zeolitic frameworks interacting with their cations are 1272.4 and 1279.3 kJ/mol for H2(OH)sAI2Si6OI2 and LiH(OH)sAI2Si6OI2, respectively, which correspond with decreasing bond lengths of the OH groups. These results indicate that the acid strength of the OH groups within the zeolitic framework is also determined by the presence of cations, in addition to compositional and structural effects. 1. Introduction Zeolites are porous frameworks of crystalline alu- minosilicates with a number of interesting physical and chemical properties. The hydroxyl groups in zeolites are of primary significance for the catalytic properties and have led to numerous industrially * Corresponding author. important applications, such as adsorbents and cata- lysts [1-10]. The performance of zeolitic catalysts depends on the framework composition (the Si/A! ratio) and the framework topology (the environment of the Si and AI atoms). The widely utilized primary building unit [11-15] H 3SiOHAIH 3 is too limited to investigate the effects mentioned above. The two-site binding model of 0301-0104/96/$15.00 Copyright © 1996 Elsevier Science B.V. All fights reserved. PH S0301-0104(96)00089-4