ATR-FTIR Study of the Decomposition of Acetic Anhydride on Fosfotungstic Wells-Dawson Heteropolyacid Using Concentration-Modulation Excitation Spectroscopy

ALEJO AGUIRRE; ADRIAN L. BONIVARDI; SILVANA R. MATKOVIC; LAURA E. BRIAND; SEBASTIAN E. COLLINS

TOPICS IN CATALYSIS

SPRINGER/PLENUM PUBLISHERS

Año: 2011 vol. 54 p. 229 - 235

1022-5528

The decomposition of acetic anhydride in liquid phase on a fosfotunstic Wells-Dawson heteropoly acid (HPA) was investigated by in situ ATR-FTIR spectroscopy. Transient and concentration-modulation excitation spectroscopy (MES) experiments in combination with phase-sensitive (PSD) detection were used to monitor the solid-liquid interface. The MES method is based on the periodic variation of a parameter of the reaction media such as, the reactant concentration. That periodic alteration causes varying infrared signals of the surface adsorbed species that are subsequently demodulated with the PSD methodology. Thus, the separation of the static signals from the changing ones is achieved, and species with different response can be observed in the spectra.Using MES-PSD coupled with ATR-FTIR, acetic anhydride was observed to decompose to acetic acid, acetate and acyl species, involving Brønsted acid sites of the HPA catalyst. The acyl species is a very unstable intermediate and it is the key intermediate in the Friedel-Craft acylation reactions. Moreover, the acetate groups are spectator species since remain strongly adsorbed on the catalyst surface and do not further react.