Room-Temperature Ethene Hydrogenation Activity of Transition-Metal-Free HY Zeolites

The conversion of C 2 H 4 in the absence and presence of H 2 was examined over dealuminated HY30, HY15, and HY2.6 zeolites with different Si/Al ratios under ambient conditions (1 atm, 25 °C). Only in the presence of H 2 did these materials catalyze the conversion of C 2 H 4 into C 2 H 6 with measurable rates and no sign of C 2 H 4 dimerization. The C 2 H 4 hydrogenation activity of these materials increases with the Si/Al ratio, implying that in this case, the strength of Brønsted acid sites (i.e., Si–OH–Al) is an important factor for the hydrogenation process. The adsorption of both C 2 H 4 and H 2 over Brønsted acid sites was calculated to be exothermic without activating the H 2 molecule, while the conversion of C 2 H 4 into ethoxy species is slightly endothermic. Heterolytic dissociation of H 2 is strongly unfavorable over Al/O or Si/O sites of the zeolite framework, but it is exothermic over the extraframework Al species, indicating that cooperation of Brønsted acid sites with extraframework Al species is indeed required for transition-metal-free zeolites to exhibit hydrogenation properties.