Propane dehydrogenation catalyzed by single Lewis acid site in Sn-Beta zeolite

The gap between supply and demand of propylene has become more and more evident, because of a large consumption of the downstream products derived from propylene. Propane dehydrogenation (PDH) constitutes an important alternative for the production of propylene, and thus considerable attention has been paid to the development of eco-friendly and cost-efficient catalysts for this process. Herein, we discover that the Sn-Beta zeolite with Lewis acid sites can activate the C-H bond, and exhibits high catalytic performance in the PDH. XRD, STEM, and XPS characterizations confirm that Sn species are incorporated into the zeolite framework, and H-2-TPR suggests that there is a strong interaction between Sn species and zeolite framework. It is found that the Lewis acid is the active site for dehydrogenation reaction, and the Bronsted acid is responsible for cracking reaction. The dehydrogenation rate/cracking rate is positively proportional to the L/B ratio, and a high L/B ratio is beneficial for the propane dehydrogenation reaction. The Na-Sn-Beta-30 catalyst possessing the highest amount of Lewis acid but the lowest Bronsted/Lewis ratio, exhibits the best performance in the PDH, which delivers propane conversion of 40% and propylene selectivity of 92%. Most importantly, these Sn-Beta zeolites are extremely stable without any detectable deactivation under the harsh reaction condition for 72 h. Density functional theory calculations reveal that both Sn and adjacent O atom or OH group cooperatively act as the active sites. The PDH occurs through the direct reaction mechanism in which hydrogen molecule is produced by the direct coupling of H atom of primary C3H7 motif with the Bronsted proton in closed sites or the proton of water in open sites. It seems that open sites are more reactive than the closed ones, and the intrinsic enthalpy barriers are calculated to be 242 - 301 kJ/mol depending on the hydroxylation extents. These efficient Sn-Beta zeolites could provide a new possibility for the development of a new generation of PDH catalysts with a high stability for the production of propylene. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

The gap between supply and demand for propylene is increasing, and propane dehydrogenation (PDH) with Lewis acid sites has been identified as an effective alternative for propylene production. The Sn-Beta zeolite catalyst showed high catalytic performance in PDH, with a positive correlation between dehydrogenation rate/cracking rate and the Lewis/Bronsted ratio. The Na-Sn-Beta-30 catalyst exhibited the best performance in PDH, delivering high propane conversion and propylene selectivity. The Sn-Beta zeolites are extremely stable and have potential for development as a new generation of PDH catalysts for propylene production.