Theoretical adsorption enthalpies of alkyl chlorides in acidic zeolite catalysts

The adsorption enthalpies, Delta H-ad, of a series of alkyl chlorides in acidic (H-FAU) zeolite have been investigated using density functional theory (DFT) and molecular mechanics methods. Two models were employed to simulate the zeolite cluster, each containing one Bronsted acid site. The first model, corresponding to the formula Al(OH)(2)(OHp)(SiOH2)(11), forms a 12-membered ring (12T) constructed from 11 Si tetrahedral atoms and one Al atom in the windows of the twelve O atoms and has been fully optimized at the B3LYP/6-31+G(d,p) level of theory. The second model is represented by the more realistic cluster of 84 tetrahedral units (84T) and has been examined with the help of the ONIOM2 approach using the two layered schemes B3LYP/6-31+G(d,p):UFF and M06-2X/6-31+G(d,p):UFF in two series of calculations. The latter series using the newly developed M06-2X functional by Truhlar and coworkers (Zhao et al., J Chem Phys 123:161103, 2005; Zhao and Truhlar, Theor Chem Acc 120:215, 2008), is expected to make a more reliable and realistic estimation of the effect of the dispersion forces which are quite important in this type of systems. The calculated structural variations and adsorption enthalpies of the van der Waals 1:1 adsorption complex assumingly formed, are shown to exhibit an impressive dependence both on the size of the zeolite cluster model and on the theoretical method employed, particularly the M06-2X functional. Indeed, the M06-2X calculations bring out most clearly the important role and the significant contribution of the dispersion forces to the adsorption enthalpies of a series of the alkyl chlorides on acidic zeolite leading to Delta H-ad values higher by more than 30 % compared to the B3LYP results. Finally, the present calculations reconfirm the dependence of the interaction energy on the size and the branching of the alkyl radical.