Thermal dehydrogenation of propane (PDH) derived from shale gas is an attractive source of propylene for manufacturing polypropylene, propylene oxide, acrylic acid, and acrylonitrile. Pt is the only metal used as a catalyst in industrial PDH processes because it exhibits high activity for C–H bond activation and low activity for the undesired C–C bond cleavage. However, monometallic Pt catalysts are plagued by unsatisfactory selectivity and low resistance to sintering. Because PDH is endergonic, temperatures above 773 K are required to overcome equilibrium limitations, but at the expense of coking and Pt sintering. Large Pt ensembles bring about detrimental structure-sensitive side reactions including cracking and deep dehydrogenation, leading to coke formation, whereas small Pt clusters favor high activity and selectivity. Therefore, there is strong interest in stabilizing very small Pt clusters and ultimately Pt atoms. In this study, we build on the idea that atomically dispersed Pt in clusters containing Zn might lead to improved new class of PDH catalysts. To this end, we synthesized ≡SiOZn(OH) nests by grating Zn into the silanol nest created by dealumination of zeolite Beta (DeAlBEA) and then used them to anchor Pt atoms. We report a catalyst xPt-yZn-DeAlBEA (x and y indicate atomic ratios). This material was characterized with complementary spectroscopic and microscopic methods (HAADF-STEM, 1H and 29Si NMR, K-edge Zn and Pt XANES and EXAFS to show that Pt is dispersed atomically as cations with an apparent oxidation state ~ +4. The as-prepared catalyst undergoes reduction at 823 K to form hexa-coordinated Pt atoms best characterized as a PtZn6 intermetallic complex anchored to the zeolite. This new catalyst exhibits a PDH activity that is 2- to 4.5-fold higher and a deactivation rate that is 18 times slower than that previously reported best PtZn catalysts, as well as a propylene selectivity of nearly 100%. In the presence of a low partial pressure of H2, the catalyst works with virtually no activity loss after 160 h on stream. The high resistance of the catalyst to sintering is confirmed by the absence of significant structural changes.

Propane dehydrogenation catalyzed by single Pt atoms in ZnOx nests in dealuminated zeolite beta

ACS Fall 2021

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