Operando ATR-FTIR analysis of liquid-phase catalytic reactions: can heterogeneous catalysts be observed?

The applicability of a React-IR(TM) system, containing a diamond attenuated total reflection (ATR) crystal, in heterogeneous liquid-phase catalytic reactions was evaluated by analysis of the catalytic esterification of 1-octanol and hexanoic acid over a Nafion/silica catalyst in an open reflux configuration at atmospheric pressure. The reaction was performed in either cumene (at 427 K) or n-decane (at 447 K). The concentration profiles of the esterification reaction, as determined by this real-time in situ IR spectroscopic technique, are in qualitative and quantitative agreement with those determined by conventional off-line GC analysis. Interestingly, besides the bands assigned to the ester, alcohol, and acid, an additional strong and broad absorption band was observed at around 1100 cm(-1) in the spectra during the esterification reaction in cumene (at 427 K). It was assessed by variation of the reaction mixture that this band is a result of the reaction of silica with octanol, yielding Si-O-R functionalities. The relevance of this reaction for the kinetics of the studied catalytic esterification, is discussed. More importantly, the contribution of solid particles and leached species to the 1100 cm(-1) band is evaluated. Strong indications exist that solid catalyst particles are contributing to the spectra, implying that on-line analysis of intermediate species adsorbed on heterogeneous catalysts is in specific cases possible using the React-IR(TM) technique. (C) 2003 Elsevier B.V. All rights reserved.