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

A. AGUIRRE; A. BONIVARDI; S. MATKOVIC; L. BRIAND; S. COLLINS

TOPICS IN CATALYSIS

SPRINGER/PLENUM PUBLISHERS

Lugar: New York; Año: 2011 vol. 54 p. 229 - 235

1022-5528

The decomposition of acetic anhydride inliquid phase on a fosfotungstic Wells–Dawson heteropolyacid (HPA) was investigated by in situ ATR-FTIR spectroscopy.Transient and concentration-modulation excitationspectroscopy (MES) experiments in combination withphase-sensitive detection (PSD) were used to monitor thesolid–liquid interface. The MES method is based on theperiodic variation of a parameter of the reaction mediasuch as, the reactant concentration. That periodic alterationcauses varying infrared signals of the surface adsorbedspecies that are subsequently demodulated with the PSDmethodology. Thus, the separation of the static signalsfrom the changing ones is achieved, and species with differentresponse can be observed in the spectra. Using MESPSDcoupled with ATR-FTIR, acetic anhydride wasobserved to decompose to acetic acid, acetate and acyl[CH3C(O)?] species, involving Brønsted acid sites of theHPA catalyst. The CH3C(O)? is a very unstable intermediatespecies and it is the key intermediate in the Friedel–Crafts acylation reactions. Moreover, the acetate groups arespectator species since remain strongly adsorbed on thecatalyst surface and do not further react.