Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge, UK CB2 3RA.
Phys Chem Chem Phys. 2010 Mar 20;12(11):2619-24. doi: 10.1039/b921210h. Epub 2010 Jan 27.
This paper presents the application of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) to observe surface diffusion of 1-octene in porous 1 wt% Pd/theta-Al(2)O(3) catalyst trilobes. We demonstrate for the first time the ability to identify diffusion on the pore surfaces unambiguously at ambient conditions in saturated porous media; this technique is applicable to microporous and mesoporous materials in general. At very short observation times, two distinct diffusion regimes are present. These are associated with the bulk pore and pore surface diffusion of 1-octene; using the model proposed by Kärger for two site exchange we determined the diffusion coefficients of these regimes to be 1.3 x 10(-9) and 1.7 x 10(-11) m(2) s(-1), respectively, and the mean residence time of a molecule on the pore surface to be 150 ms. Treatment of the catalyst trilobes with a silane surface coating is seen to influence the surface such that a surface diffusion coefficient is no longer observed, supporting the interpretation that the molecular dynamics of surface diffusing species are influenced strongly by their interaction with hydroxyl groups on the alumina surface. This technique will enable further study and improved understanding of molecular transport in porous catalysts used in liquid-phase, heterogeneous catalytic processes.
本文介绍了脉冲梯度场（PFG）核磁共振（NMR）在观察 1-辛烯在多孔 1wt%Pd/θ-Al(2)O(3)催化剂三叶瓣中的表面扩散中的应用。我们首次证明了在环境条件下在饱和多孔介质中能够明确识别表面扩散的能力；该技术通常适用于微孔和介孔材料。在非常短的观测时间内，存在两种不同的扩散模式。这些与 1-辛烯的体相孔和孔表面扩散有关；使用 Kärger 提出的双位交换模型，我们确定了这些模式的扩散系数分别为 1.3×10(-9)和 1.7×10(-11) m(2) s(-1)，以及分子在孔表面上的平均停留时间为 150ms。用硅烷表面涂层处理三叶瓣会影响表面，从而不再观察到表面扩散系数，这支持了这样的解释，即表面扩散物种的分子动力学受到与其在氧化铝表面上的羟基相互作用的强烈影响。该技术将能够进一步研究和提高对用于液相反相催化过程的多孔催化剂中分子输运的理解。
