Rankin Andrew G M, Webb Paul B, Dawson Daniel M, Viger-Gravel Jasmine, Walder Brennan J, Emsley Lyndon, Ashbrook Sharon E
School of Chemistry, EaStCHEM and Centre of Magnetic Resonance, University of St Andrews, St. Andrews KY16 9ST, United Kingdom.
Sasol UK Ltd., Purdie Building, North Haugh, St. Andrews KY16 9ST, United Kingdom.
J Phys Chem C Nanomater Interfaces. 2017 Oct 19;121(41):22977-22984. doi: 10.1021/acs.jpcc.7b08137. Epub 2017 Sep 27.
Isotopic enrichment of Si and DNP-enhanced NMR spectroscopy are combined to determine the detailed surface structure of a silicated alumina catalyst. The significant sensitivity enhancement provided by DNP is vital to the acquisition of multinuclear and multidimensional experiments that provide information on the atomic-level structure of the species present at the surface. Isotopic enrichment not only facilitates spectral acquisition, particularly given the low (1.5 wt %) Si loading, but also enables spectra with higher resolution than those acquired using DNP to be obtained. The unexpected similarity of conventional, CP, and DNP NMR spectra is attributed to the presence of adventitious surface water that forms a sufficiently dense H network at the silica surface so as to mediate efficient polarization transfer to all Si species regardless of their chemical nature. Spectra reveal the presence of Si-O-Si linkages at the surface (identified as Q(3Al)-Q(3Al)) and confirm that the anchoring of the surface overlayer with the alumina occurs through Al and Al species only. This suggests the presence of Q/Q Si at the surface affects the neighboring Al species, modifying the surface structure and making it less likely Al environments are in close spatial proximity. In contrast, Q/Q species, bonded to the surface by fewer covalent bonds, have less of an effect on the surface, and more Al species are consequently found nearby. The combination of isotropic enrichment and DNP provides a definitive and fully quantitative description of the Si-modified alumina surface, and we demonstrate that almost one-third of the silicon at the surface is connected to another Si species, even at the low level of coverage used, lowering the propensity for the formation of Brønsted acid sites. This suggests that a variation in the synthetic procedure might be required to obtain a more even coverage for optimum performance. The work here will allow for more rigorous future investigations of structure-function relationships in these complex materials.
将硅的同位素富集与动态核极化（DNP）增强核磁共振光谱相结合，以确定硅铝酸盐催化剂的详细表面结构。DNP提供的显著灵敏度增强对于获取多核和多维实验至关重要，这些实验可提供有关表面存在物种的原子级结构的信息。同位素富集不仅有助于光谱采集，特别是考虑到低（1.5 wt%）的硅负载量，而且还能获得比使用DNP采集的光谱具有更高分辨率的光谱。传统NMR光谱、交叉极化（CP）NMR光谱和DNP NMR光谱出人意料的相似性归因于表面存在的吸附水，这些水在二氧化硅表面形成了足够致密的氢网络，从而介导了向所有硅物种的有效极化转移，而不管它们的化学性质如何。光谱揭示了表面存在Si-O-Si键（鉴定为Q(3Al)-Q(3Al)），并证实表面覆盖层与氧化铝的锚定仅通过铝和铝物种发生。这表明表面Q/Q硅的存在会影响相邻的铝物种，改变表面结构，使得铝环境在空间上紧密相邻的可能性降低。相比之下，通过较少共价键与表面结合的Q/Q物种对表面的影响较小，因此在附近发现更多的铝物种。各向同性富集和DNP的结合提供了对硅改性氧化铝表面的明确且完全定量的描述，并且我们证明即使在低覆盖度下，表面几乎三分之一的硅与另一个硅物种相连，这降低了布朗斯台德酸位点形成的倾向。这表明可能需要改变合成程序以获得更均匀的覆盖度以实现最佳性能。本文的工作将为未来更严格地研究这些复杂材料中的结构-功能关系提供支持。
