The Mechanism of the Gas-Phase Esterification of Acetic Acid and Ethanol over MCM-41

Esterification reactions are conventionally carried out using homogeneous acids, such as sulphuric acid. Especially in view of catalyst separation, the use of heterogeneous catalysts is now heavily pursued. In the current work we have assessed the potential of MCM-41 as a solid esterification catalyst and tested this in esterification of acetic acid and ethanol as a model reaction. Whereas the mechanism for the homogeneously catalysed esterification is long known, this does not hold for the esterification over heterogeneous catalysts. Using transient and steady state experiments, temperature-programmed desorption (TPD), and isotopic labelling experiments, the esterification mechanism over MCM-41 (Si/Al=16) is studied. Gas-phase esterification of acetic acid with ethanol over MCM-41 (Si/Al=16) is shown to proceed via a Langmuir–Hinshelwood-type mechanism, involving a protonated acetic acid intermediate.