Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM-5
Direct Detection of Multiple Acidic Proton Sites in Zeolite HZSM-5
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DOI：
10.1021/jacs.7b10940
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发表时间：
2017-12-27
影响因子：
15
通讯作者：
White, Jeffery L.
中科院分区：
文献类型：
--
作者：
Chen, Kuizhi;Abdolrhamani, Maryam;White, Jeffery L.
在沸石HZSM-5（沸石结构的MFI家族的成员）中直接观察到多个反应性位点与在工业上重要的沸石中仅一种类型的活性质子物种的传统观点相矛盾。除了众所周知的布朗斯台德酸位点质子，其他两个质子物种进行室温氢氘交换与烷烃试剂，包括一个沸石部分，其特征在于由一个广泛的H-1化学位移约。12-15 ppm，这是首次报道。虽然CA。13 ppm化学位移值与来自文献的关于沸石中表面稳定的氢钥离子的计算预测一致，数据表明信号不是由氢钥物质产生，而是由靠近布朗斯台德晶格位点的晶格外铝醇物质上的羟基产生，即，一小群高度去屏蔽的酸性位点。双共振实验表明，该物种是接近铝原子，类似于布朗斯台德酸位质子。这些位点可以通过适当的合成后化学处理去除，从而产生对于同位素H/D交换反应具有降低的活性的催化剂。此外，先前在文献中讨论的其他晶格外铝羟基基团，但其质子被认为是不反应的，也首次显示与烃探针分子反应。二维交换NMR揭示了Bronsted位点与这两种类型的晶格外Al OH物种之间的直接质子交换，并且还揭示了晶格外Al OH物种与烷烃试剂之间的意外质子交换。
Direct observation of multiple reactive sites in the zeolite HZSM-5, a member of the MFI family of zeolite structures, contradicts the traditional view of only one type of active protonic species in industrially important zeolites. In addition to the well-known Bronsted acid site proton, two other protonic species undergo room-temperature hydrogen deuterium exchange with an alkane hydrocarbon reagent, including one zeolite moiety characterized by a broad H-1 chemical shift at ca. 12-15 ppm that is reported here for the first time. Although the ca. 13 ppm chemical shift value is consistent with computational predictions from the literature for a surface-stabilized hydroxonium ion in a zeolite, data suggest that the signal does not arise from hydroxonium species but rather from hydroxyls on extra-lattice aluminol species proximate to Bronsted lattice sites, i.e., a small population of highly deshielded acid sites. Double-resonance experiments show that this species is proximate to Al atoms, similar to the Bronsted acid site proton. These sites can be removed by appropriate postsynthesis chemical treatment, yielding a catalyst with reduced activity for isotopic H/D exchange reactions. Additionally, other extra-lattice aluminum hydroxyl groups previously discussed in the literature but whose protons were considered unreactive are also shown for the first time to react with hydrocarbon probe molecules. Two-dimensional exchange NMR reveals direct proton exchange between the Bronsted site and these two types of extra-lattice Al OH species, and it also reveals unexpected proton exchange between extra-lattice Al OH species and an alkane reagent.