(504g) Understanding the Role of Promoters for Propane Dehydrogenation Catalysts

Zhao, Z. J. - Presenter, Tianjin University
Gong, J., Tianjin University
This talk focuses on the combined theoretical and experimental study on two types of promoters, metals and metal oxides, which can enhance the propylene selectivity and anti-coking ability for propane dehydrogenation (PDH) catalyst. PDH is one of the commercial propylene production technologies, prevailing with Cr-oxide- or Pt-based catalysts. Compared with Cr-oxide, Pt-based catalyst is more environmentally friendly and exhibits better activity with lower deactivation rate. However, both of them suffer loss of activity during the reaction, mainly due to the deposition of coke on catalyst surfaces. The deposition of coke can be inhibited by addition of different types of promoters. For Pt-based catalyst, two types of promoters were considered. The first type, late or post transition metals, such as Cu [1], Ga [2], can directly suppress propylene adsorption, thus, further diminishes the possibility of coke formation initiated from propylene deep dehydrogenation and C-C bond cleavage. A systematic density functional theory study has been performed which shows the importance of the alloy d band center on the determination of catalytic activity and selectivity. Unlike metal promoters, the present of oxide, TiO2 [3] and CeO2 [2], accelerates the removal of formed coke, by utilizing the redox property of the oxide to oxidize coke. Partially reduced TiOx (x<2) transfers electrons to Pt atoms, which reduces the adsorption strength of adsorbates. Thus, it facilitates the migration of coke precursors from the metal surface to the support where coke can be further removed from catalyst surface. The existence of Pt can, in turn, further enhances the redox property of the oxide, where more lattice O can be used in CeO2 to oxidize coke, enhancing the anti-coke ability of the catalyst [2]. A screening study indicates that PtIn alloy balances the PDH activity and selectivity which could be potentially an optimal catalyst for Pt alloy PDH catalyst.[4] In addition, the promoter effect on the oxide catalysts have also been considered, by two case studies with ZnO and VOx. For ZnO, Pt can serve as a promoter [5]. DFT calculations show that the trace amount of Pt (0.1 wt %) accelerates H2 desorption to prevent the reduction of ZnO thus superior stability is achieved, which not only minimizes Pt usage but also improves catalytic performance. For VOx, we found that the PDH activity and stability is closely related to the valance state of V[6] and surface OH group could stabilize the PDH catalyst [7]. In addition, the existence of Mg enhances the dispersion of VOx, and reduces the amount of V2O5 which is more active for coke deposition [8].

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[3] Jiang, F.; Zeng, L.; Li, S.; Liu, G.; Wang, S.; Gong, J. ACS Catal. 2015, 5, 438–447.

[4] Zha. S.; Sun G.; Wu, T.; Zhao, J.; Zhao, Z.-J.; Gong, J. Chem. Sci. 2018, DOI: 10.1039/c8sc00802g

[5] Liu, G.; Zeng, L.; Zhao, Z.-J.; Tian, H.; Wu, T.; Gong, J. ACS Catal. 2016, 6, 2158–2162.

[6] Liu, G.; Zhao, Z.-J.; Wu, T.; Zeng, L.; Gong, J. ACS Catal. 2016, 6, 5207–5214.

[7] Wu, T.; Liu, G.; Zeng, L.; Sun, G.; Chen, S.; Mu, R.; Gbonfoun, S.; Zhao, Z.-J.; Gong, J. AIChE J. 2017, 63, 4911-4919

[8] Zhao, Z.-J.; Wu, T; Xiong, C.; Sun, G.; Mu, R.; Zeng, L.; Gong, J. Angew. Chem. Int. Ed. 2018, DOI: 10.1002/anie.201800123