Acid catalysis places a central role in the conversion of biomass- or petroleum-derived feedstocks to chemicals and fuels; for example, isomerization, dehydration, cracking, and alkylation all proceed on acid sites. These transformations involve organic surface species, which take various forms, including π-complexes, alkoxides, or ion pairs. In some cases, for example, in methanol-to-olefins conversion, long-lived organic surface moieties function as part of the catalytically active site (“hydrocarbon pool”). Often, organic surface species grow and form very stable carbonaceous deposits (“coke”) that block active sites and pores and thus deactivate the catalyst. Understanding surface reaction pathways is thus of paramount importance for controlling selectivity and avoiding coke formation. While mechanisms have been proposed in the literature, direct evidence for postulated intermediates and their involvement is often lacking.
In recent years, we have advanced the interpretation of IR and UV-vis spectra of observable surface species on solid acids through a variety of measures, including use of model reactants and well-defined catalysts such as zeolites, specific experiments to discriminate neutral and charged surface species, analogies with liquid phase chemistry, and insight from theory. The lecture introduces these methodologies and the resulting correlations. It is demonstrated how the acquired knowledge of spectra and surface reaction chemistry can be applied to assess proposed catalytic cycles.