MODULATED TRANSIENT EXPERIMENTS BY ATR-FTIR FOR THE IDENTIFICATION OF INTERMEDITES IN SOLID-LIQUID INTERPHASE

S. COLLINS, L. BRIAND, A. BONIVARDI

Sao Pedro, Brasil

Simposio; Fifth San Luis Symposium on Surfaces, Interfaces and Catalysis; 2010

Pan-American Advanced Studies Institute

Attenuated total reflection (ATR) infrared (IR) spectroscopy is a powerful tool for the investigation of reaction pathways in solid(catalyst)/liquid(reactive) systems, because it provides the detection of species adsorbed on a catalyst under reaction conditions. However, the presence of strongly absorbent solvents, spectator species and/or the catalyst itself can make very difficult or preclude the identification and tracking of the true active species, that is, the intermediates. Then, transient experiments are widely applied in the analysis of reaction intermediates by perturbing a catalytic system working under steady state conditions (ss). Forexample, modulation excitation spectroscopy (MES) with phase sensitive detection (PSD) have proved to be an adequate methodology to separate a weak response from a strong background signal [Rev. Sci. Instrum. 72(2001)3782]. MES-PSD is based on the disturbance of a ss system by the periodical variation of an external parameter such as the temperature, the pressure, or the concentration of a reactant. Therefore, all the species in the system that are affected by this parameter will also change periodically with the same frequency of the perturbation, but with a phase lag. Thus, MES-PSD applied to in situ ATR-IR experiments can allow the sensitive and selective detection of species directly involved in a reaction at the liquid/solid interface, and the monitoring of their dynamic behaviors. In this work we studied the adsorption and decomposition of acetic anhydride in liquid phase on phosphotunstic Wells-Dawson heteropoly acid (HPA) using MES-PSD coupled with ATR-IR. The acyl [CH3C(O)+] intermediate species bonded to HPA was identified: vas(COO) = 1650 cm-1, vs(COO) = 1322 cm-1, d(CH) = 1455 cm-1. This very unstable species is produced by the splitting of acetic anhydride molecule, and it is the key intermediate in the Friedel-Craft acylation reaction ofisobutylbencene towards p-isobutylacetophenone, which is a step of the ibuprofen synthesis. The by-products of the reaction are acetic acid and acetate.