Ga speciation in Ga/H-ZSM-5 by in-situ transmission FTIR spectroscopy
Ga speciation in Ga/H-ZSM-5 by in-situ transmission FTIR spectroscopy
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DOI：
10.1016/j.jcat.2020.11.004
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发表时间：
2021
影响因子：
7.3
通讯作者：
Yong Yuan;Casper Brady;Leelavathi Annamalai;R. Lobo;Bingjun Xu
中科院分区：
文献类型：
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作者：
Yong Yuan;Casper Brady;Leelavathi Annamalai;R. Lobo;Bingjun Xu
Ga/H-ZSM-5催化剂是烷烃非氧化脱氢和脱氢芳构化反应的理想催化剂。然而，Ga的形态反应条件下仍然存在争议。本文采用原位傅里叶变换红外光谱（FTIR）技术，系统研究了三种Si/Al比（15，28和39）和宽范围Ga/Al比（0-1.7）的Ga/H-ZSM-5中Ga的形态。用吡啶进行的定量FTIR光谱表明，在Ga/BAS比高达0.7时，一个Ga原子大致取代了一个布朗斯台德酸中心（BAS），然而，即使使用过量的Ga，也只能交换H-ZSM-5上高达~80%的BAS。在15的低Si/Al比下，在550 °C下还原的Ga/H-ZSM-5上的GaHX带的强度在低Ga负载量（Ga/Al < 0.13）下几乎线性地增加，然后趋于平稳。相比之下，在具有39的高Si/Al比的Ga/H-ZSM-5上没有观察到可检测的GaHX带，其中Ga/Al比高达1.3。GaH_x谱带与Si/Al比和Ga/Al比的关系表明，Ga的形态随分子筛中BAS密度的变化而变化。我们假设成对的BAS位点优先与Ga+交换，导致Ga+−H+对位点的形成，而孤立的BAS交换形成孤立的Ga+物种。使用水作为探针分子，我们发现孤立的Ga+和Ga+−H+对位点具有不同的性质，即，前者在150 °C下容易被水氧化形成GaOOH物种，而后者在相同条件下无活性。这些结果为还原Ga/H-ZSM-5上存在两种类型的Ga物种提供了直接的实验证据，突出了它们在烷烃脱氢反应中具有不同催化活性的可能性。
H-ZSM-5 supported Ga (Ga/H-ZSM-5) has long been recognized as a promising catalyst for nonoxidative dehydrogenation and dehydroaromatization of alkanes. However, Ga speciation under reaction conditions remains controversial. In this work, in-situ transmission Fourier Transform infrared (FTIR) spectroscopy is employed to systematically investigate Ga speciation in Ga/H-ZSM-5 with three Si/Al ratios (15, 28 and 39) and a wide range of Ga/Al ratios (0–1.7). Quantitative FTIR spectroscopy with pyridine reveals that one Ga atom roughly replaces one Brønsted acid site (BAS) at Ga/BAS ratio up to 0.7, however, only up to ~80% of the BAS on the H-ZSM-5 can be exchanged even with excess amounts of Ga. At a low Si/Al ratio of 15, the intensity of GaHxbands on reduced Ga/H-ZSM-5 at 550 °C increases almost linearly at low Ga loadings (Ga/Al < 0.13), and then levels off. In contrast, no detectable GaHxbands are observed on Ga/H-ZSM-5 with a high Si/Al ratio of 39, with Ga/Al ratios up to 1.3. The dependence of GaHxbands on both the Si/Al ratios and the Ga/Al ratios shows that Ga speciation varies with BAS density in the zeolite. We hypothesize that paired BAS sites are preferentially exchanged with Ga+, leading to the formation of Ga+−H+pair sites, while the exchange of isolated BAS form isolated Ga+species. Using water as a probe molecule, we show that isolated Ga+and Ga+−H+pair sites have distinct properties, i.e., the former can be easily oxidized by water at 150 °C to form GaOOH species, while the latter is inactive under the same conditions. These results provide direct experimental evidence for the existence of two types of Ga species on reduced Ga/H-ZSM-5, highlighting the possibility that they have different catalytic activities in alkane dehydrogenation reactions.