Theoretical study of the formation of acetyl intermediates over a cluster model of ZSM-5 zeolite

CORREGIDOR, PABLO FERNANDO; OTTAVIANELLI, EMILCE; ACOSTA, DELICIA ESTER; DESTÉFANIS, HUGO ALBERTO

Quito

Congreso; Congreso de químicos teóricos de expresión latina: QUITEL 2014; 2014

Universidad San Francisco de Quito

The adsorption on Brønsted acid sites of typical acylating agents with formula CH3CO-G (G = -OH for acetic acid, -OCOCH3 for acetic anhydride, -Cl for acetyl chloride and -OCHCH2 for vinyl chloride), has been theoretical studied by applying QM calculations at DFT level on a 3T-cluster model of ZSM-5, one of the most important zeolite with many important uses in heterogeneous catalysis. We investigated how the adsorption energies and the energetics of the acyl intermediate formation from different acylating agents vary in the3T model. The geometries of the reactants, products, 3T-cluster, complexes and transition states have been optimized by using the B3LYP/6-311+G(d) level. The stationary points have been characterized by the vibrational frequency analysis.All the stationary points have been positively identified for minimum (number of imaginary frequencies, NIMAG=0) and transition states (NIMAG=1). Intrinsic reaction coordinates (IRC) were reconstructed by following the steepest descent path toward reactants at one side and products at the other side, starting form reoptimized transition structures at the AM1 level. The results indicate that the adsorption follows a concerted mechanism where the OH of the zeolite protonates the O-G group of the acylating agent.Simultaneously, the carboxylic Carbon interacts covalently with the Oxygen atom of the same OH group from the zeolite. Adsorption energies of the substrates were calculated by taking the difference in energy between the zeolite-substrate complex, the isolated zeolite and the isolated substrate. The calculated activation energies for the formation of the acylium-ion-like acetyl-zeolite intermediate of acetic acid, acetic anhydride, acetyl chloride and vinyl acetate were 57.5, 41.0, 36.9 and 41.5 kcal/mol, respectively. The acetyl-zeolite intermediate is likely the most important in acylation reactions carried out over zeolite catalysts, these results are in agreement with those informed in previous reports. Similarly, the acylium-ion-like acetyl-zeolite intermediate that these molecules generate is completely analogous to the carbenium-ion-like alkoxide intermediate found by dehydration of alcohols or protonation of small olefins over Brønsted acid sites.