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

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.