Interactions of amino acids with H-ZSM-5 zeolite: An embedded ONIOM study

The adsorption of glycine and L-alanine on the H-ZSM-5 zeolite has been studied at an embedded ONIOM (MP2/6-31G(d,p):UFF) level of theory. The most stable adsorption structure involves ion-pair interactions between the protonated amino acid and the anionic zeolite framework. The adsorption energy is computed to be -31.3 and -34.8 kcal/mol for glycine and L-alanine, respectively. Two hydrogen bonded complexes are identified: one is the cyclic double hydrogen bonded complex via the interactions of the carboxyl group and the zeolite acid site having adsorption energies of -25.4 and -30.0 kcal/mol for glycine and L-alanine, respectively. The second is the hydrogen bonded complex via the interactions of the hydroxyl group and the zeolite acid site having weak interaction energies, -20.7 and -23.9 kcal/mol for glycine and L-alanine, respectively. The zwitterion form is not found in the acidic H-ZSM-5 due to the acid-base reaction, but the glycine zwitterion is found to be stably adsorbed on the Na-ZSM-5 with the adsorption energy of -24.8 kcal/mol. In addition to the interactions with the Bronsted acid site of the zeolite, the interactions with the nearby oxygen framework and van der Waals interactions with the zeolite walls are also found to be important for stabilizing the adsorbed amino acids and zwitterion, suggesting the role of the zeolite framework as a "solid solvent molecule". (C) 2008 Elsevier B.V. All rights reserved.