Active-site nature and reaction mechanism of supported chromium oxide catalysts in non-oxidative propane dehydrogenation: valence speciation, dynamic evolution, and the desorption/regeneration bottleneck

暂时无法连接题库，当前显示已保存的题面。最后核验： 2026-10-22

Chromium oxide supported on silica or alumina is, alongside Pt-Sn (Oleflex), one of the two commercially dominant catalyst systems for non-oxidative propane dehydrogenation (PDH) to propylene, yet after three decades the identity of the catalytically active chromium species and the operative reaction mechanism remain unsettled. A 2021 density-functional-theory (DFT) cluster study of isolated monomeric $Cr^{3+}$, $Cr^{5+}$, and $Cr^{6+}$ sites anchored to silica found $Cr^{3+}$ most favorable and computed that hydrogen recombination and $H_2$ release, not C–H activation, are the highest-barrier regeneration steps for the higher valences — but on idealized single-site models that leave open how real, multivalent, dynamically evolving catalysts work. The open core is a validated quantitative assignment of the active Cr species and mechanism for supported CrOx under working PDH conditions — spanning preparation/loading-dependent speciation, reaction–regeneration dynamics, oxygen- versus metal-centered activation, and the role of product desorption and site regeneration in deactivation and catalyst design.

Background

Non-oxidative propane dehydrogenation (PDH; $C_3H_8 \rightarrow C_3H_6 + H_2$) is one of the main on-purpose routes to propylene, a building block of the chemical industry. Two catalyst families dominate commercial practice: Pt-based systems (Oleflex) and supported chromium oxide on alumina (CATOFIN), the latter run in cyclic operation because the catalyst deactivates within minutes through coking and must be regenerated by burning off the coke in air. Understanding PDH catalysis on chromium oxide requires tracking chromium in several coexisting oxidation states and nuclearities: isolated monochromate species ($Cr^{6+}$, often written as chromate anchors $=Cr(=O)_2$), polychromates, ESR (electron spin resonance, the magnetic-resonance technique for paramagnetic centers)-detectable $Cr^{5+}$, and $Cr^{3+}$ in disordered or crystalline $\alpha$-$Cr_2O_3$-like domains; under the reducing reaction atmosphere and the oxidizing regeneration cycle these populations interconvert. Which of these species actually activates the C–H bond of propane has been debated since at least the early 1990s: spectroscopic studies of chromia/silica and chromia/alumina (Derossi et al., 1994) and ESR work on silica-supported chromia (Zou et al., 1999) produced divided assignments among $Cr^{3+}$, $Cr^{5+}$, and $Cr^{6+}$ species, and operando (under-working-conditions) UV–vis spectroscopy at pilot scale (Sattler et al., 2013) showed Cr speciation changing as the catalyst deactivates, without settling which species is causal. Against this background, Sun et al. (Molecular Catalysis 505, 2021, 111520) performed a systematic density-functional-theory (DFT, quantum-chemical computation of energies and barriers from the electron density) comparison — cluster models of a single isolated Cr atom anchored to a silica cutout, geometry and transition states at the B3LYP/6-311G** level (B3LYP, a widely used exchange–correlation functional; LANL2DZ effective-core potential for Cr), free energies at 873 K in the rigid-rotor harmonic-oscillator approximation (the standard treatment combining molecular translation/rotation with harmonic vibrations to convert electronic energies into finite-temperature free energies). They found: (i) the $Cr^{3+}$ model has the lowest activation and active-site-regeneration barriers of the three valences; (ii) first C–H activation barriers at terminal ($Cr{=}O$), bridge ($Cr{-}O{-}Si$), and hydroxyl ($Cr{-}OH$) oxygens are similar, hydroxyl oxygen being at least as reactive as terminal oxygen; (iii) the metal center itself can abstract H (barrier 0.49 eV in the $Cr^{3+}$ model), so activation is not exclusively oxygen-mediated; (iv) radical-like gas-phase $C_3H_7$ routes are favored over fully dissociative adsorption; and (v) for the higher valences $Cr^{5+}$/$Cr^{6+}$, regeneration by H recombination and $H_2$ desorption is the highest-barrier step (about 2.3–2.5 eV), far above the C–H activation barriers, while $Cr^{3+}$ regeneration is computed at about 1 eV or less. The study is explicitly a survey on idealized single-site models; the authors' own framing is that the active site and mechanism 'are still remaining unresolved due to the complicated valence of chromium and co-exists of several active sites'. The open question is therefore how this cluster-model picture maps onto real catalysts: which species actually operates under reaction conditions, how Cr valence populations evolve and depend on preparation, and whether the desorption/regeneration bottleneck governs the deactivation that forces industrial cyclic operation.

Problem Statement

For supported chromium oxide catalysts (with silica and alumina as the representative model and industrial supports) operating under non-oxidative propane dehydrogenation conditions, establish the nature of the active site and the reaction mechanism: (1) identify which chromium valence, coordination, and nuclearity species (isolated monochromate/dichromate anchors, $Cr^{5+}$ centers, $Cr^{3+}$ disordered or $\alpha$-$Cr_2O_3$-like domains, hydroxylated Cr–OH species) carries propane C–H activation, and quantify how the population of these species depends on chromium loading, preparation route, and calcination/reaction atmosphere; (2) determine how these populations dynamically interconvert under working conditions and over reaction–regeneration cycles (reduction of $Cr^{6+}$ toward $Cr^{3+}$, reoxidation during coke burn-off) and which transient states are catalytically causal versus spectator species; (3) establish whether C–H activation on the operative sites proceeds via oxygen-mediated abstraction (terminal, bridge, or hydroxyl oxygen), direct metal-centered abstraction at Cr, or both, and whether the propyl intermediate is radical-like or surface-bound; and (4) determine which elementary step — first or second C–H activation, H recombination and $H_2$ release, propylene desorption, or active-site regeneration — is rate- and stability-limiting on the operative sites, and thereby what governs deactivation and lifetime. The active-site and mechanism assignment for supported chromium oxide under working PDH conditions must be established beyond the idealized isolated single-site cluster models and quantitatively consistent with the available experimental evidence. Design guidance derived from the assignment — controlling the site population to improve activity, selectivity, and reaction–regeneration stability — echoes the source's aim to 'supply the doable optimization methods for further improvements' and is a desirable downstream application, not a completion requirement.

References

1. XiaoYing Sun, JiaHui Xue, Yu Ren, XinYu Li, LiJing Zhou, Bo Li, Zhen Zhao. Revealing nature of active site and reaction mechanism of supported chromium oxide catalyst in propane direct dehydrogenation. Molecular Catalysis 505 (2021) 111520. DOI: 10.1016/j.mcat.2021.111520. https://doi.org/10.1016/j.mcat.2021.111520
2. S. Derossi, G. Ferraris, S. Fremiotti, E. Garrone, G. Ghiotti, M.C. Campa, V. Indovina. Propane Dehydrogenation on Chromia/Silica and Chromia/Alumina Catalysts. Journal of Catalysis 148 (1994) 36–46. DOI: 10.1006/jcat.1994.1183. https://doi.org/10.1006/jcat.1994.1183
3. Hu Zou, Yuwang Han, Weijie Ji, Jianyi Shen. ESR Study on the Nature of Active Site of Silica-supported Chromia in Propane Dehydrogenation Reaction. Chemistry Letters 28 (1999) 761–762. DOI: 10.1246/cl.1999.761. https://doi.org/10.1246/cl.1999.761
4. J. J. H. B. Sattler, I. D. González-Jiménez, A. M. Mens, M. Arias, T. Visser, B. M. Weckhuysen. Operando UV-Vis spectroscopy of a catalytic solid in a pilot-scale reactor: deactivation of a CrOx/Al2O3 propane dehydrogenation catalyst. Chemical Communications 49 (2013) 1518. DOI: 10.1039/c2cc38978a. https://doi.org/10.1039/c2cc38978a
5. Jason P. Malizia, Stephen Kristy, Rong Xing, James Pittman, Shane Palmer, Shengguang Wang, Mingyong Sun, Rebecca Fushimi. Pulse Response Operando Spectroscopy for Resolving Transient Dynamics in the CrOx/Al2O3 Propane Dehydrogenation Catalyst. ACS Catalysis 15 (2025) 7496–7504. DOI: 10.1021/acscatal.5c00593. https://doi.org/10.1021/acscatal.5c00593
6. Hang Zhou, Guodong Qi, Yueying Chu, Liang Wang, Yu Hui, Zhangyi Yao, Feng Ryan Wang, Shengqi Chu, Yucai Qin, Lijuan Song, Lu Liu, Xuedi Qin, Jun Xu, Feng Deng, Feng-Shou Xiao. Ultralow Content of Chromium Species in Zeolite for Efficient Catalytic Propane Dehydrogenation. Journal of the American Chemical Society 147 (2025) 32432–32439. DOI: 10.1021/jacs.5c04531. https://doi.org/10.1021/jacs.5c04531
7. Zhong-Pan Hu, Gangqiang Qin, Jingfeng Han, Yijun Zheng, Zhen Liu, Yong Jiang, Xiaozhi Su, Te Ji, Min Li, Zhong-Yong Yuan, Jianping Xiao, Svetlana Mintova, Yingxu Wei, Zhongmin Liu. Synergy principle of single active centers and microenvironment for Cr-MFI-catalyzed alkane dehydrogenation. National Science Review 12 (2025) nwaf405. DOI: 10.1093/nsr/nwaf405. https://doi.org/10.1093/nsr/nwaf405
8. Yupeng Du, Yanxiao Wang, Chengtao Zhang, Rongzhao Li, Bo Wang, Shuo Li, Chaohe Yang. Theoretical and experimental investigations into light alkane dehydrogenation over chromium-containing catalyst. Fuel 320 (2022) 123893. DOI: 10.1016/j.fuel.2022.123893. https://doi.org/10.1016/j.fuel.2022.123893
9. Rongzhao Li, Chengtao Zhang, Shuo Li, Yupeng Du. Experimental and theoretical investigations into propane dehydrogenation over MIL-101(Cr/Al) derived catalysts. Molecular Catalysis 535 (2023) 112894. DOI: 10.1016/j.mcat.2022.112894. https://doi.org/10.1016/j.mcat.2022.112894
10. Drejc Kopač, Blaž Likozar, Matej Huš. Metal-doped Cr2O3 as a catalyst for non-oxidative propane and butane dehydrogenation: A multiscale kinetic study. Applied Surface Science 575 (2022) 151653. DOI: 10.1016/j.apsusc.2021.151653. https://doi.org/10.1016/j.apsusc.2021.151653
11. Dingding Jin, Haoxiang Xu, Jiqin Zhu, Daojian Cheng. Activation of Cr2O3 for propane dehydrogenation by doping with Pt single-atom promotor. Molecular Catalysis 551 (2023) 113624. DOI: 10.1016/j.mcat.2023.113624. https://doi.org/10.1016/j.mcat.2023.113624
12. Xue Cai, Zitong Zhang, Lili Cai, Xiaoyan Tian, Wenling Chu, Weishen Yang. Effect of Calcination Atmosphere on the Structure and Catalytic Behavior of Cr2O3/Al2O3 Catalysts for Dehydrogenation of Propane. Industrial & Engineering Chemistry Research 61 (2022) 16479–16488. DOI: 10.1021/acs.iecr.2c03031. https://doi.org/10.1021/acs.iecr.2c03031
13. L. Lara-Moreno, P. Sánchez-López, B. Pawelec, R. M. Navarro Yerga, S. A. Gomez, S. Fuentes Moyado, T. A. Zepeda. CO2-oxidative Dehydrogenation of Light Alkanes Catalyzed by Cr Catalysts Supported on Different Mesoporous Silica Structures. Topics in Catalysis 68 (2025) 1682–1700. DOI: 10.1007/s11244-024-02046-z. https://doi.org/10.1007/s11244-024-02046-z
14. Mufan Niu, Baojian Shen. Research progress on Cr-based catalysts for the CO2-assisted catalytic oxidative dehydrogenation of light alkanes to light olefins. Journal of Fuel Chemistry and Technology 53 (2025) 1283–1299. DOI: 10.1016/s1872-5813(25)60551-2. https://doi.org/10.1016/s1872-5813(25)60551-2