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

The conversion of C2H4 in the absence and presence of H2 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 H2, these materials catalyze the conversion of C2H4 into C2H6 with measurable rates but show no sign of C2H4 dimerization. The C2H4 hydrogenation activity of these materials increases with the Si/Al ratio, implying that in this case, the strength of Bronsted acid sites (i.e., Si‒OH‒Al) is an important factor for the hydrogenation process. The adsorption of both C2H4 and H2 over the Bronsted acid sites was calculated to be exothermic without activating the H2 molecule, while the conversion of C2H4 into ethoxy species is slightly endothermic. Heterolytic dissociation of H2 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 Bronsted acid sites with extraframework Al species is indeed required ...