Spectroscopy under catalytic reaction conditions is a valuable approach for identifying active sites and surface intermediates. Depending on the catalytic transformations under investigation, the collection of high quality spectra can be a challenge because of demanding conditions. In addition, interpretation of the data may pose a difficult problem and requires the application of more than one method as well as an array of reference measurements to create individual species and follow their transformations. This investigation focused on the identification and the reactivity of hydrocarbon surface species on solid acids.<br/>Reactions of hydrocarbon fragments on solid acids play an important role in many catalytic transformations, including hydrocarbon cracking, alkylation, and isomerization and alcohol dehydrations including methanol-to-olefins conversion. Here, dehydration, oligomerization, elimination and hydride transfer steps were monitored by in situ UV-vis and in situ IR spectroscopy with simultaneous analysis of evolving products by mass spectrometry and gas chromatography. Zeolites with different framework structure (MFI, MOR, and BEA) and Si/Al ratios and other solid acids such as alumina and sulfated zirconia were compared. Surface species were created from alcohols and olefins with short carbon backbones (up to C6) and transformed through heating and reaction with other hydrocarbon species or bases.<br/>Hydrocarbon species on solid acid surfaces include π-complexes, alkoxides and ion pairs. Both, the acidity of the solid and the basicity of the hydrocarbon determine whether the proton transfer is complete. The recorded UV-vis and IR spectra showed that olefin adsorption or alcohol dehydration both lead to species that are quickly dehydrogenated to allylic cations. Acid site density and gas phase olefin concentration promoted the formation of such species. Hydride transfer reactions resulted in further dehydrogenation of surface species with concomitant formation of saturated gas phase products. Highly conjugated allylic surface species cyclized. The UV-vis spectra of the allylic species, which form ion pairs, were found to be more influenced by the constitution of the hydrocarbon backbone than by the structure of the surrounding zeolite framework. High temperatures (above ≈ 250 °C) led to the formation of six-membered rings (benzenium or naphthenium ions). The results provide insight as to how to prevent coking during acid-catalyzed transformations.

Carbocation chemistry on solid acid catalysts observed by in situ spectroscopy

256th National Meeting (2018)

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