Identification of carbonaceous deposits formed on H-mordenite during alkane isomerization

To identify hydrocarbon surface species accumulating during alkane isomerization, a strategy involving in situ UV–vis and FTIR spectroscopies, reaction of formed species with various bases, adsorption of reference compounds, and extraction of spent catalysts was applied. During conversion of n-butane or n-pentane on H-mordenite at reaction temperatures below ≈550 K, species characterized by an intense absorption band at a wavelength of 292–296 nm were observed by in situ diffuse reflectance UV–vis spectroscopy. Species with a comparable electronic signature formed after adsorption of 1-butene, 1-pentene, or 1-hexene, indicating that olefins are intermediates in the formation of carbonaceous deposits from alkanes. The chromophore was largely invariant to the carbon chain length and the species were identified as alkyl-substituted cyclopentenyl cations. Carbonaceous deposits formed at temperatures higher than ≈550 K consisted of methyl-substituted naphthalenes, anthracenes, and tetracenes; these species also existed as stable cations on the zeolite during catalysis, producing a broad absorption at 350–500 nm. The polycyclic aromatic species were neutralized by water vapor, whereas the alkyl-substituted cyclopentenyl cations required a stronger base, such as ammonia.