With respect to the potential application of indium-modified zeolites for light alkane conversion to simple aromatic hydrocarbons and their oxidation to carboxylic acids, the pathways of isobutane transformation on the In/H-ZSM-5 zeolite have been investigated by a combination of solid-state NMR spectroscopy, FTIR spectroscopy, and density functional theory (DFT) calculations. It is found that isobutane undergoes activation on InO⁺ sites of In/H-ZSM-5 zeolites via the “alkyl” pathway to yield isobutylindium species, which precedes the formation of isobutene at 296 K. Analysis of isobutene transformation on In/H-ZSM-5 shows the formation of allyl-like intermediates, alkoxy species, and cyclopentenyl cations from the alkene. This implies that oligomerization and aromatization of isobutene formed from the alkane occurred with the involvement of both InO⁺ sites and Brønsted acid sites (BASs) at 473–573 K. At higher temperatures (T ≥ 623 K), C₂–C₄ surface carboxylic species have been found as the products of isobutane oxidation. It is inferred that the direct oxidation of isobutane molecules to carboxylate species does not occur, contrary to the earlier found cases with propane and n-butane transformation on In-modified zeolites. Additionally, DFT calculations have been used to analyze the localization of indium species in the ZSM-5 zeolite, the energies of isobutane and isobutene adsorption on differently located active sites (InO⁺ and BAS), and the mechanisms of C–H bond activation in the alkane.

中文翻译：

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改性 ZSM-5 沸石上的异丁烷活化和转化：来自固态 NMR、FTIR 光谱和 DFT 计算的见解


关于铟改性沸石在轻质烷烃转化为简单芳烃及其氧化为羧酸方面的潜在应用，通过固相结合研究了 In/H-ZSM-5 沸石上异丁烷转化的途径。状态核磁共振波谱、傅里叶变换红外光谱和密度泛函理论 (DFT) 计算。研究发现，异丁烷通过“烷基”途径在 In/H-ZSM-5 沸石的 InO ⁺位点上进行活化，产生异丁基铟物质，然后在 296 K 下形成异丁烯。 In/H- 上​​异丁烯转化的分析ZSM-5 显示从烯烃形成烯丙基类中间体、烷氧基物质和环戊烯基阳离子。这意味着在 473–573 K 温度下，由烷烃形成的异丁烯的低聚和芳构化在 InO ⁺位点和布朗斯台德酸位点 (BAS) 的参与下发生。在较高温度 ( T ≥ 623 K) 下，C ₂ –C ₄表面已发现羧基物质是异丁烷氧化的产物。据推测，异丁烷分子不会直接氧化成羧酸盐物质，这与早期发现的丙烷和正丁烷在 In 改性沸石上转化的情况相反。此外，DFT计算还用于分析ZSM-5沸石中铟物质的定位、异丁烷和异丁烯在不同位置的活性位点（InO ⁺和BAS）上的吸附能量，以及C-H键活化的机制。烷烃。