Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, West Cambridge Site, Cambridge, CB3 0AS, UK.
Faraday Discuss. 2017 Oct 26;204:439-452. doi: 10.1039/c7fd00098g.
NMR relaxation has recently emerged as a novel and non-invasive tool for probing the surface dynamics of adsorbate molecules within liquid-saturated mesoporous catalysts. The elucidation of such dynamics is of particular relevance to the study and development of solvated green catalytic processes, such as the production of chemicals and fuels from bio-resources. In this paper we develop and implement a protocol using high field H NMR spin-lattice relaxation as a probe of the reorientational dynamics of liquids imbibed within mesoporous oxide materials. The observed relaxation of liquids within mesoporous materials is highly sensitive to the adsorbed surface layer, giving insight into tumbling behaviour of spin-bearing chemical environments at the pore surface. As a prototypical example of relevance to liquid-phase catalytic systems, we examine the mobility of liquid methanol within a range of common catalyst supports. In particular, through the calculation and comparison of a suitable interaction parameter, we assess and quantify changes to these surface dynamics upon replacing surface hydroxyl groups with hydrophobic alkyl chains. Our results indicate that the molecular tumbling of adsorbed methanol is enhanced upon surface passivation due to the suppression of surface-adsorbate hydrogen bonding interactions, and tends towards that of the unrestricted bulk liquid. A complex analysis in which we account for the influence of changing pore structure and surface chemistry upon passivation is discussed. The results presented highlight the use of NMR spin-lattice relaxation measurements as a non-invasive probe of molecular dynamics at surfaces of interest to liquid-phase heterogeneous catalysis.
NMR 弛豫最近已成为一种新颖的、非侵入性的工具，可用于探测液体饱和介孔催化剂中吸附分子的表面动力学。阐明这种动力学对于研究和开发溶剂化绿色催化过程（如从生物资源中生产化学品和燃料）具有特别重要的意义。在本文中，我们开发并实施了一种使用高场 H NMR 自旋晶格弛豫作为介孔氧化物材料中吸收液体的重取向动力学探针的方案。观察到的介孔材料中液体的弛豫对吸附表面层非常敏感，可深入了解孔表面处带有自旋的化学环境的翻滚行为。作为与液相催化体系相关的典型范例，我们研究了一系列常见催化剂载体中液体甲醇的迁移率。特别是，通过计算和比较合适的相互作用参数，我们评估并量化了用疏水性烷基链取代表面羟基基团对这些表面动力学的影响。我们的结果表明，由于抑制了表面-吸附物氢键相互作用，吸附甲醇的分子翻滚在表面钝化后得到增强，并且趋于无限制的体相液体。讨论了一种复杂的分析方法，其中我们考虑了孔结构和表面化学变化对钝化的影响。所提出的结果突出了 NMR 自旋晶格弛豫测量作为对液相多相催化中感兴趣的表面分子动力学的非侵入性探针的用途。
