Radical Species Detection and Their Nature Evolution with Catalyst Deactivation in the Ethanol-to-Hydrocarbon Reaction over HZSM-5 Zeolite

HZSM-5 (Si/Al ratio = 16) zeolite was found to be a very stable and efficient catalyst for ethanol transformation into hydrocarbons at 350 degrees C and 30 bar total pressure. Deactivation, in our operating conditions, was only observed after 16 h on stream, and for ethylene transformation into higher hydrocarbons only. Carbon deposit evolution with time-on-stream (TOS) was fully characterized using IR spectroscopy, GC-MS (after CH2Cl2 extraction and HF solubilization), and electron paramagnetic resonance (EPR) techniques. The carbon content was very high from the reaction beginning, leading to great losses, of microporosity and acidity. Nevertheless, C3+ hydrocarbons yield remained high even after 30 h on stream. EPR analysis allowed us to show. the existence of free radical species among the species adsorbed, from the reaction beginning. A decay period of the number of radical species, as well as a change in their chemical nature coincides with the moment of deactivation of the catalyst, leading to a decrease in the formation of C3+ hydrocarbons. The existence of reactive radical species could explain the high catalytic performances of the catalyst at 30 h TOS, considering the losses in acidity and microporosity. The apparent correlation between the formation of C3+ hydrocarbons and the existence of active radical species could indicate the existence of radical reactions, which should occur at pore mouth. The correlation between the analytical and the catalytic results should be instructive to a better understanding of the deactivation as well as ethanol's transformation reaction mechanism.