Zn/H-ZSM-5 催化剂中的锌形态和丙烷脱氢,ACS Catalysis

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Zn/H-ZSM-5 催化剂中的锌形态和丙烷脱氢
ACS Catalysis ( IF 11.3 ) Pub Date : 2023-03-28 , DOI: 10.1021/acscatal.2c05898
Yong Yuan ₁ , Raul F. Lobo ₁

Affiliation

Zn/H-ZSM-5 催化剂经常被研究用于丙烷脱氢 (PDH)；然而，由于系统的复杂性，活性位点仍未解决。我们采用原位 FTIR 光谱和动力学方法将 Zn/H-ZSM-5 中的 Zn 形态和 PDH 活性与两个 Si/Al 比（15 和 39）和一系列 Zn/Al 比（0-1.7）相关联. 锌的增量添加表明 Zn ²⁺位点优先在 H-ZSM-5 上形成，而不是成对的 Al 位点的一部分，然后是 [Zn-O-Zn] ²⁺和 [ZnOH] ⁺位点，然后是 ZnO _x簇。H-ZSM-5 中的[Zn-OH] ⁺和 [Zn-O-Zn] ^{2+位点对于 PDH 比分离的 Zn}²⁺更具活性和选择性。[锌-OH]⁺物种在流中随时间升华，导致催化剂失活，而 [Zn-O-Zn] ²⁺物种即使在高温还原（750 °C 60 分钟）后也是稳定的。三个不同的 Zn 位点（[Zn-O-Zn] ²⁺、Zn ²⁺和 [ZnOH] ⁺）显示出相似的丙烷反应级数（接近 1）和 H ₂反应级数（接近 0）。结合丙烷在 550 °C 下流过催化剂时氢化锌的缺乏，得出丙烷吸附和解离是一个决速步骤和 H ₂脱附速度快。这项工作表明，制备具有丰富铝对的 H-ZSM-5 可能是形成用于丙烷脱氢的稳定和选择性 Zn/H-ZSM-5 催化剂的策略。还强调的是，检查金属/Al 比率和沸石的 Al 分布的影响对于识别金属-沸石系统中的金属阳离子至关重要。

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Zinc Speciation and Propane Dehydrogenation in Zn/H-ZSM-5 Catalysts

Zn/H-ZSM-5 catalysts have been frequently investigated for propane dehydrogenation (PDH); however, the active site remains unresolved due to the complexity of the system. We employed in situ FTIR spectroscopy and a kinetics method to correlate the Zn speciation and PDH activity in Zn/H-ZSM-5 with two Si/Al ratios (15 and 39) and a range of Zn/Al ratios (0–1.7). Incremental additions of zinc show that Zn²⁺ sites are preferentially formed on H-ZSM-5 over a fraction of paired Al sites followed by [Zn-O-Zn]²⁺ and [ZnOH]⁺ sites and then ZnO_x clusters. The [Zn-OH]⁺ and [Zn-O-Zn]²⁺ sites in H-ZSM-5 are more active and selective than isolated Zn²⁺ for PDH. [Zn-OH]⁺ species sublimate over time on stream, leading to catalyst deactivation, while [Zn-O-Zn]²⁺ species are stable even after high-temperature reduction (750 °C for 60 min). Three distinct Zn sites ([Zn-O-Zn]²⁺, Zn²⁺, and [ZnOH]⁺) show a similar propane reaction order (close to 1) and H₂ reaction order (close to 0). Combined with the lack of Zn hydride when propane flows over the catalyst at 550 °C, it is concluded that propane adsorption and dissociation is a rate-determining step and H₂ desorption is fast. This work indicates that the preparation of H-ZSM-5 with abundant Al pairs may be a strategy to form stable and selective Zn/H-ZSM-5 catalysts for propane dehydrogenation. It is also highlighted that examining the effect of both metal/Al ratios and Al distribution of the zeolite is crucial in identifying the metal cations in metal–zeolite systems.

更新日期：2023-03-28

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