THEORETICAL STUDY OF THE ADSORPTION OF ACILATING AGENTS OVER ZSM-5 ZEOLITE USING THE 3T-DFT CLUSTER MODEL

CORREGIDOR, PABLO; OTTAVIANELLI, EMILCE; ACOSTA, DELICIA; DESTÉFANIS, HUGO

DF-Mexico

Congreso; XXII International Materials Research Congress IMCR 2013; 2013

Sociedad Mexicana de Materiales

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 Quantum Mechanical (QM) calculations at DFT level on a 3T cluster model of ZSM-5, one of the most important zeolite material 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 this zeolite model. The geometries of the reactants, products, 3T cluster, complexes and transition states have been optimized by using the B3LYP/6-311+G(d) method. 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 state (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 reaction corresponds to a concerted mechanism where the OH group of the zeolite protonates the O-G group and simultaneously, the carbonylic Carbon of the acylating agent bounds directly to the Oxygen atom of the same OH group. 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, acetyl 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 important intermediate in acylation reactions carried out over zeolite catalysts, these results are in agreement with those informed in previous reports [1]. Similarly, the acylium-ion-like, acetyl-zeolite intermediate that these molecules form is completely analogous to the carbenium-ion-like, alkoxide intermediates formed at Brønsted-acid sites by dehydration of alcohols or protonation of small olefins [2].