Catalysis by solid acids has emerged from physical and conceptual transfer from the solution state to the surface. First solid acid catalysts included “mounted acids”, that is, supports soaked with mineral acids. Liquid superacids were followed by solid superacids. Concepts such as those of general and specific acid catalysis were shown to apply to soluble acids and to acid resins. Revisiting the analogies between catalysis in different phases, we investigate spectra and chemistry of hydrocarbons in three different environments by experiment (liquid phase, zeolite pore) and by theory (gas phase). Our species of interest are unsaturated carbocations and their conjugate bases (polyenes) because they play key roles as precursors of coke and as surface intermediates in methanol-to-hydrocarbons chemistry.<br/>We have collected UV–vis and IR spectra of various dienes, trienes, and the associated cations formed through protonation, in liquid phase or adsorbed on zeolites of various framework types. Spectral interpretation is refined by comparing with periodic DFT calculations of vibrational spectra using CP2K.<br/>The results, together with literature data, indicate far-reaching similarities among spectra of the same species recorded in different environments. This minimal influence of the surroundings on spectral signature applies for acyclic and cyclic dienes and trienes and their corresponding cations. The analogies between spectra in different environments can be exploited to establish correlations between molecular constitution and the position of characteristic electronic and vibrational absorptions.<br/>These correlations can be employed to identify key intermediates in hydrocarbon transformations and their reactions on zeolites such as cracking or cyclization. In situ IR and UV–vis spectra show that protonation and subsequent cyclization of 2,6-dimethylocta-2,4,6-triene lead to cyclopentenyl cations with different substitution patterns at the allyl system depending on the zeolite environment (MOR, MFI, TON), whereby large pores allow for the same selectivity as observed in liquid acids. The cyclopentenyl cations can be cracked at elevated temperatures (&gt; 200 °C), and the shrinkage (number of carbon atoms lost) can be measured by UV–vis spectroscopy and matched with the size of volatile product. These examples demonstrate that spectroscopic correlations make it possible to monitor elementary reaction steps on surfaces with great precision.

Analogies in acid catalysis: Unsaturated carbenium ions in liquid, solid, and gas phase environments

ACS Spring 2022

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