Ga/H-ZSM-5催化剂中Ga2O22+上丙烷C-H键的活化及相应反应机理

doi:10.1016/S1872-2067(24)60065-3

摘要/Abstract

摘要：

丙烯是合成多种化学品的关键原料, 丙烷脱氢(PDH)反应是一种将丙烷转化为丙烯的重要工业过程. 传统的铂和铬基催化剂具有较高的PDH反应活性, 但存在成本高、环境不友好等问题. 因此, 寻找同时具有高活性和高选择性的新型催化剂成为当前研究的重点. H-ZSM-5分子筛负载镓(Ga/H-ZSM-5)作为一种新型催化剂, 在PDH反应中表现出良好的应用前景. 本文研究了Ga/H-ZSM-5催化剂的PDH催化性能及其反应机理, 揭示了不同镓物种在催化过程中的作用, 为设计高效稳定的PDH催化剂提供了重要的理论依据和实验支持.

本文主要研究了不同H-ZSM-5载体上镓物种的形态和分布对PDH催化性能的影响, 以及相应活性镓物种上丙烷脱氢的反应机理. 首先通过原位红外光谱(FTIR)和O₂的脉冲滴定反应, 结合PDH的反应速率结果, 证明了Ga₂O₂²⁺物种是PDH反应的主要活性位点. 在不同镓/铝比条件下, Ga₂O₂²⁺在B酸位(BAS)上占比不同. 当镓/铝比低于0.3时, Ga₂O₂²⁺/BAS比率随镓负载量增加而增加, 并在镓/铝比超过0.4后达到平台期. 同时, Ga₂O₂²⁺物种的浓度对丙烯生成速率(STY)有着决定性影响. 使用了五种来源不同、镓/铝比接近的H-ZSM-5载体, 通过原位FTIR光谱研究了负载镓后的样品中在H₂高温还原下形成的GaH物种, 并将其与PDH活性相关联. 研究结果表明, 在纯氢的气氛中, 不同载体上的Ga/H-ZSM-5样品形成的GaH物种在2035和2050 cm^-1附近有明显的红外吸收峰, 其中只有高波数的GaH峰(GaH_HW)的峰面积与PDH活性紧密关联, 而低波数的GaH峰(GaH_LW)对PDH活性没有明显贡献, 这些吸收峰分别对应于两种不同配位环境中的GaH物种. 结果显示, 在丙烷气氛中, GaH_LW迅速减小, 而GaH_HW保持稳定, 这表明GaH_HW物种在PDH反应中起到更重要的作用. 进一步采用H₂-D₂同位素交换实验研究了丙烷与GaH物种的相互作用机制, 从而提出PDH反应在活性Ga₂O₂²⁺物种上的反应机理: 丙烷首先在GaH上被活化生成Ga-C₃H₇和H₂, 该步骤也是反应的决速步(RDS); 然后C₃H₇发生β-消除得到丙烯, Ga物种恢复为GaH. 在GaH_LW对应的Ga物种上不能发生第二步反应, 因此不具有PDH活性. 由此可见, 在Ga/H-ZSM-5催化剂上, 只有部分对应于GaH_HW的Ga₂O₂²⁺物种是PDH反应的反应活性位点, 并且PDH反应机理需要经过一个两步的烷基化机理. 上述发现为理解Ga/H-ZSM-5催化剂在PDH反应中的作用机制提供了重要的实验依据和理论支持.

未来, PDH反应领域的研究将继续致力于开发高效、低成本的催化剂, 以及深入理解催化剂的作用机制. 本文揭示了Ga₂O₂²⁺物种在PDH反应中的关键作用, 为设计更高效的Ga基催化剂提供了新的思路和方法. 这有助于PDH催化剂的发展, 并有助于实现工业过程的绿色化和高效化.

关键词: 丙烷脱氢, Ga₂O₂²⁺, C-H键的活化, 镓物种

Abstract:

Propane dehydrogenation (PDH) on Ga/H-ZSM-5 catalysts is a promising reaction for propylene production, while the detail mechanism remains debatable. Ga₂O₂²⁺ stabilized by framework Al pairs have been identified as the most active species in Ga/H-ZSM-5 for PDH in our recent work. Here we demonstrate a strong correlation between the PDH activity and a fraction of Ga₂O₂²⁺ species corresponding to the infrared GaH band of higher wavenumber (GaH_HW) in reduced Ga/H-ZSM-5, instead of the overall Ga₂O₂²⁺ species, by employing five H-ZSM-5 supports sourced differently with comparable Si/Al ratio. This disparity in Ga₂O₂²⁺ species stems from their differing capacity in completing the catalytic cycle. Spectroscopic results suggest that PDH proceeds via a two-step mechanism: (1) C-H bond activation of propane on H-Ga₂O₂²⁺ species (rate determining step); (2) β-hydride elimination of adsorbed propyl group, which only occurs on active Ga₂O₂²⁺ species corresponding to GaH_HW.

Key words: Propane dehydrogenation, Ga₂O₂²⁺, Activation of C-H bond, Ga species

赵兆麒, 钟云竹, 常晓侠, 徐冰君. Ga/H-ZSM-5催化剂中Ga₂O₂²⁺上丙烷C-H键的活化及相应反应机理[J]. 催化学报, 2024, 64: 32-43.

Zhaoqi Zhao, Yunzhu Zhong, Xiaoxia Chang, Bingjun Xu. C-H bond activation of propane on Ga₂O₂²⁺ in Ga/H-ZSM-5 and its mechanistic implications[J]. Chinese Journal of Catalysis, 2024, 64: 32-43.

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链接本文: https://www.cjcatal.com/CN/10.1016/S1872-2067(24)60065-3

https://www.cjcatal.com/CN/Y2024/V64/I9/32

图/表 12

Table 1 The physicochemical characterization of five zeolite samples.

Sample	Si/Al ratio ^a	BAS density^b (μmol g^-1)	Micropore volume^c (mL g^-1)	Al pairs density^d(%)	Ga/Al ratio ^e	BAS_res^f (μmol g^-1)
A	10.2	1522	0.13	37.9	0.25	1269
B	11.2	1424	0.11	22.6	0.25	1382
C	9.0	1675	0.14	33.9	0.27	988
D	9.6	1212	0.12	24.8	0.24	825
E	11.4	1235	0.13	16.8	0.24	870

Fig. 1. (a) The Ga2O22+/BAS ratio as a function of the Ga/Al ratio on Sample A. (b) STY of C3H6 normalized to the Ga2O22+ density determined with pulse titration as a function of the Ga/Al ratio.

Fig. 2. (a) In-situ FTIR spectra of GaH formed in a pure H2 atmosphere in different Ga/H-ZSM-5 samples. The background spectrum was collected in the spectral cell with a dehydrated sample pellet at 500 °C under vacuum. Spectra presented have been background corrected. (b) Normalized GaH area determined in infrared spectra and PDH rates between different types of zeolites. Reaction conditions: 500 °C; C3H8 partial pressure, 5.07 kPa with balancing N2. The propane conversions are below 3.5% in the rate measurements.

Fig. 3. (a) In-situ FTIR spectra of GaH in Sample A when treated with C3H8 (5.07 kPa, N2 as the carrier gas). Background spectrum was collected in the transmission cell on a dehydrated sample pellet at 500 °C under vacuum. (b) Normalized GaHHW area determined in infrared spectra and PDH rates on the five zeolite samples investigated.

Fig. 4. (a) Peak fitting on the overlapping GaH peaks of sample A before and after oxidation. The spectra presented have been background corrected. (b) STY of propylene normalized to the amount of framework Al atoms vs. time-on-stream. Reaction conditions: 500 °C; C3H8 partial pressure, 5.07 kPa with balancing N2. Propane conversions are below 2.0% in the rate measurements.

Scheme 1. Schematic hypothesis of the transformation of Ga species under redox treatment on Ga/H-ZSM-5.

Fig. 5. In-situ FTIR spectra of the GaH species on Sample A under varied H2 partial pressure with peak fitting analysis: 5% (a); 10% (b); 100% (c) (N2 as the balance gas). Black dots represent the experimental data, thick black trace represents the overall fitting, and red lines represent the deconvoluted peaks. The raw spectra presented have been background corrected. (d) Normalized GaHLW and GaHHW peak areas under different H2 partial pressures.

Fig. 6. (a) In-situ FTIR spectra of GaH in Ga/H-ZSM-5 (sample A) during H2 treatment for 2, 6, 10, 14, 20, 40 min. (b) In-situ FTIR spectra of GaH during evacuation after H2 reduction and evacuating H2 for 2, 4, 6, 10, 18, 42 min. Background spectra were collected in the spectral cell with a dehydrated sample pellet at 500 °C under vacuum.

Scheme 2. Proposed PDH mechanism on D2-pretreated Ga2O22+ corresponding to GaHHW and GaHLW respectively.

Fig. 7. (a) In-situ FTIR spectra of GaH in the reduced Sample A (Ga/Al = 0.24) (i) being treated with D2 for ~20 min; (ii) after C3H8 treatment under PDH conditions. Background spectra were collected in the spectral cell with a dehydrated sample pellet under vacuum at 500 °C. (b) In-situ FTIR spectra of GaD in Sample A (Ga/Al = 0.24) (i) after H2-D2 exchange under D2 atmosphere; (ii) after C3H8 treatment followed by evacuation. Background spectra were collected in the spectral cell with a reduced sample pellet at 500 °C under H2 atmosphere. The spectra in (ii) and (iii) have been background corrected.

Scheme 3. Schematic of proposed mechanism for the PDH on Ga2O22+ species.

Fig. 8. (a) In-situ FTIR spectra collected on H2 treated Ga/H-MFI (black), Ga/H-BEA (blue), and Ga/H-MOR (green) at 500 °C. Background spectra were collected at the same conditions but without the H2 treatment. The spectra presented have been background corrected. (b) Space-time-yield (STY) of C3H6, CH4 and C2H4 normalized to the mass of the catalysts and corresponding selectivity to C3H6 on Ga/H-MFI, Ga/H-BEA and Ga/H-MOR. Reaction conditions: 500 °C; C3H8 partial pressure, 5.07 kPa with balancing N2. The propane conversions are below 3.5% in the rate measurements.

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Ga/H-ZSM-5催化剂中Ga₂O₂²⁺上丙烷C-H键的活化及相应反应机理

C-H bond activation of propane on Ga₂O₂²⁺ in Ga/H-ZSM-5 and its mechanistic implications

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