Coke assessment in spectroscopic and chromatographic studies of zeolite spent catalysts in ethylene oligomerisation

Over time, all catalysts lose their efficiency and selectivity for the reaction. The primary cause for zeolite deactivation in hydrocarbon reactions is the formation of coke deposits. The coke nature and location interdependence with the catalysts' structural, textural, and acidic characteristics were studied. For that, an ethylene oligomerisation as a model reaction was applied, with ethylene being a crucial intermediate in numerous sustainable processes. The parent ZSM-5 zeolite and its hierarchised analogues of different porous and acidic characteristics were chosen for study. Thermogravimetric and chromatographic analysis of coke deposited on spent catalysts (after 16 h of ethylene oligomerisation), alongside advanced FT-IR and UV–vis spectroscopic studies of temperature-programmed oxidation (TPO) of coke coupled with mass spectrometry, were conducted. As the lower V_micro value corresponded to a reduced coke content, it is concluded that coke was predominantly formed in micropores, and the introduction of mesoporosity restricted its formation. In addition, spectroscopic investigations revealed that the high concentration of acid centres in micropores for parent and mildly modified samples favoured the formation of complex aromatic compounds. GC-MS analysis confirmed the presence of the aliphatic long-chain hydrocarbons, alkyl-substituted benzenes, naphthalenes, phenanthrenes, and pyrenic species. While naphthalenes were formed over microporous and mildly modified samples, only minor content was found for severely treated catalysts. Also, the aliphatic long-chain hydrocarbons and monoaromatics required lower temperatures for oxidation than polyaromatic species. These findings highlight that strong acid sites are responsible for forming polyaromatic compounds such as phenanthrene or pyrene-like species.