Mechanism and Kinetics of Propane and <i>n</i>-Butane Dehydrogenation over Isolated and Nested ≡SiOZn–OH Sites Grafted onto Silanol Nests of Dealuminated Beta Zeolite
Yanfei Zhang, Liang Qi, Danna Nozik, Chaochao Dun, Jeffrey J. Urban, Alexis T. Bell
Abstract
Zn Lewis acid centers were grafted onto the silanol nest created by dealumination of H-BEA zeolite (DeAlBEA). The resulting material was characterized and investigated for propane dehydrogenation to propene and n -butane dehydrogenation to 1,3-butadiene (1,3-BD). For Zn/Al molar ratios (Al is the molar amount in H-BEA) below 0.12, Zn sites are present as isolated (≡SiOZn–OH) species, but for Zn/Al ratios between 0.12 and 0.60, the ≡SiOZn–OH species form nests in which enhanced electron transfer between Zn and O atoms of the neighboring ≡SiOZn–OH group and H-bonding interaction between adjacent Zn–OH groups occur. The turnover frequency (TOF) for both propane and n -butane dehydrogenation is virtually identical for Zn-DeAlBEA for Zn/Al < 0.12 and then increases almost linearly with increasing Zn/Al ratio from 0.12 to 0.36, indicating the superior activity of Zn atoms in ≡SiOZn–OH nests. In the case of 1-butene dehydrogenation, identical activity is observed for both isolated and nested ≡SiOZn–OH sites. The kinetics of these three reactions was investigated to clarify the difference in activity. The rate coefficient for the forward reaction (dehydrogenation) was found to be 173 mol propene/(mol Zn sites·bar·h) at 773 K over ≡SiOZn–OH nests, and that for the forward n -butane dehydrogenation was found to be 1193 mol butene/(mol Zn sites·bar·h) at 823 K, a value that is significantly higher than those for most other supported non-noble metal catalysts. Regeneration experiments for propane and n -butane dehydrogenation over 0.60Zn-DeAlBEA suggest a good stability of Zn atom in ≡SiOZn–OH nests.