(679d) In Situ Spectroscopic Analysis of Reactive Hydrocarbon Surface Species and Their Transformations

The formation of long-lived hydrocarbon surface species during solid acid-catalyzed reactions is not uncommon. Such species may be the precursors of persistent carbonaceous deposits that lead to deactivation of the catalyst as for example in fluid catalytic cracking, or they may be active intermediates as in the hydrocarbon pool-mediated conversion of methanol to olefins. The methods for characterization of hydrocarbon surface species can be subdivided into post-reaction and in situ analyses. Hydrofluoric acid digestion of the solid followed by GC-MS analysis [1] and temperature-programmed oxidation have long been important for identification, classification, and quantification of carbonaceous deposits. Molecular spectroscopies including UV–vis, IR, NMR, and EPR have been applied post-reaction or after quenching, and to some extent also during catalytic conversion [2-6]. However, a detailed spectroscopic characterization of the molecular species that are precursors to hydrocarbon pool species or carbonaceous deposits, or are key components of the hydrocarbon pool, is lacking, in particular by methods that are applicable under reaction conditions. In this work, we record spectroscopic signatures and monitor transformations of hydrocarbon species in situ, with the goal of acquiring the mechanistic understanding that will ultimately let us steer the surface chemistry towards active intermediates and away from coke.

A library of UV–vis and IR spectra of hydrocarbon surface species was created, and the assignments of the absorption bands were corroborated by collecting spectra of reference compounds and by comparing with periodic DFT calculations of vibrational spectra using CP2K. Various zeolites with MFI, BEA and MOR frameworks and a range of Si/Al ratios were explored as both hosts and catalysts. Alcohols, acyclic and cyclic conjugated polyenes, and aromatic compounds served as reactants and reference compounds. Diffuse reflectance UV–vis and diffuse reflectance FTIR spectra were recorded from subambient temperature to 500 °C using Harrick Scientific optical accessories and reaction chambers. Several short periodic DFT molecular dynamics simulations of various host-guest systems were performed at various temperatures to generate ensembles of host-guest configurations, which were then subjected to normal mode analysis to obtain vibrational frequencies and IR intensities. Ensemble-averaged spectra were computed for various species in MFI and MOR zeolites to assist in making spectroscopic assignments, and were quantitatively compared with experimental spectra to extract mole fractions of relevant linear and cyclic protonated guest species.

UV–vis and IR spectroscopies proved to be excellently suited to discriminate between surface species, specifically between neutral hydrocarbons and charged carbenium ions as well as between acyclic and cyclic types. UV–vis spectra afforded the ability to count the total number of carbons in the cyclic species. Spectra were found to be sensitive to the substitution pattern of the enylic cations, with the ability to distinguish between 1,3 and 1,2,3-alkyl substitution of the allylic system of cyclopentenyl cations; the electronic absorption of the former was shifted to longer wavelengths. Moreover, in situ experiments performed to observe the formation of cyclopentenyl cations from acyclic precursors provided evidence that the allylic substitution pattern is controlled by the shape selectivity of the zeolite framework topology. This substitution pattern also produces a characteristic shift in the vibrational stretching frequency of the allylic group, ν_asym(C=C‑C⁺).

The individual reaction steps during the formation and transformation of hydrocarbon species were tracked with the aid of the spectra library, extracted correlations between band position and molecular structure, and online gas phase analysis. Reaction sequences were found to depend on the starting point and the temperature, among other parameters. Hydride transfer leads to the release of volatile saturated hydrocarbons, while surface species become increasingly unsaturated. Cyclization of suitable polyenylic cations to five-membered rings begins at temperatures of about 100 °C. These five-membered rings are generally present and relatively stable in the monoenylic cation form in H‑form zeolites. At temperatures between 200–350 °C, these alkyl-substituted cyclopentenyl cations rearrange and cleave, typically producing short olefins (ethene and propene) and smaller cyclopentenyl cations.

In conclusion, the combination of UV–vis spectroscopy, IR spectroscopy and periodic DFT constitutes a powerful approach to identify reactive surface hydrocarbon species and intermediates. The established spectroscopic signature and trend are the foundation for future in situ analyses of important acid-catalyzed transformation.

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