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

•Carbonaceous deposits formed on H-mordenite during alkane isomerization or olefin adsorption are identified.•A strategy involving in situ diffuse reflectance UV–vis and IR spectroscopies, and extraction of spent catalysts is presented.•At temperatures below ≈550 K, alkyl-substituted cyclopentenyl cations are formed.•At temperatures above ≈550 K, methyl-substituted naphthalenes, anthracenes, and tetracenes and their cations are formed.•Substituted cyclopentenyl cations can be neutralized by ammonia, and cations of substituted acenes can be neutralized by water.

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.

Graphical abstractCarbonaceous deposits on H-mordenite formed during alkane and alkene conversion are identified as methyl-substituted cyclopentenyl cations and neutral and cationic methyl-substituted acenes by a combination of spectroscopic and extraction experiments.Figure optionsDownload full-size imageDownload high-quality image (123 K)Download as PowerPoint slide