Department of Chemistry, Oklahoma State University , Stillwater, Oklahoma 74078, United States.
J Am Chem Soc. 2017 Dec 27;139(51):18698-18704. doi: 10.1021/jacs.7b10940. Epub 2017 Dec 14.
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 Brönsted 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 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 Brönsted 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 Brönsted 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 Brönsted 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.
在 MFI 沸石结构家族的一员 HZSM-5 沸石中，对多个反应部位的直接观察与工业上重要沸石中只有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 Brønsted 酸位质子外，还有两种质子物种与烷烃烃类试剂在室温下发生氢氘交换，包括一种沸石部分，其 H 化学位移约为 12-15 ppm，这是首次报道。尽管约 13 ppm 的化学位移值与文献中计算预测的沸石中表面稳定的羟离子一致，但数据表明该信号不是来自羟离子，而是来自靠近 Brønsted 晶格位的晶格外铝醇物种上的羟基，即一小部分高度去屏蔽的酸位。双共振实验表明，这种物质与 Al 原子接近，类似于 Brønsted 酸位质子。这些位可以通过适当的后合成化学处理去除，得到一种对同位素 H/D 交换反应活性降低的催化剂。此外，文献中以前讨论过的其他晶格外铝羟基基团，其质子被认为是无反应性的，现在也首次显示出与烃类探针分子反应。二维交换 NMR 揭示了 Brønsted 位和这两种类型的晶格外 Al-OH 物种之间的直接质子交换，它还揭示了晶格外 Al-OH 物种与烷烃试剂之间意想不到的质子交换。
