New mechanistic insights into the role of water in the dehydration of ethanol into ethylene over ZSM-5 catalysts at low temperature

Abstract

The low-temperature dehydration of bioethanol-to-ethylene is of great interest to reduce energy consumption and achieve high product purities in the biorefinery and olefin industry. Thermokinetic constraints, however, lead to low ethylene selectivity at low temperature. In this work, we integrate a new approach that combines a hierarchical acid H-form ZSM-5 (HZSM-5) with systematic catalytic testing to study how the physicochemical modification of the surface and intermediate catalytic species affect the ethanol-to-ethylene route at 225 °C. Four HZSM-5 zeolites were treated with OH species under basic conditions (OH⁻) or solely with H₂O. Kinetic evidence coupled to ²⁷Al-nuclear magnetic resonance, NH₃-temperature-programmed desorption and N₂ adsorption, as well as density-functional theory calculations, correlate ethylene selectivity with the appearance of new extra-framework Al(V) and Al(VI) species, acting as Lewis acid-sites. The adopted approach allows us to experimentally unveil the cooperative effect between Brønsted- and Lewis-acid sites that seem to play a key role in ethylene formation from ethanol at low-temperature via (i) a primary route via ethanol dimerization on neighboring Brønsted-acid sites to diethylether, which subsequently cracked on Lewis-acid sites to ethylene; (ii) a secondary route via the direct ethanol dehydration on Brønsted-acid sites. Theoretical calculations support the proposed catalytic cycle. These new insights shed light on the mechanism of ethanol-to-ethylene at low temperature, and on how the precise control over the strength of acid-sites and their population in HZSM-5 affects catalysis. This work progresses towards more active and stable catalysts, advancing into more mature low-temperature technologies for the dehydration of bioethanol into sustainable ethylene.