The ability to measure and predict molecular diffusion coefficients in multicomponent mixtures is not only of fundamental scientific interest but also of significant relevance in understanding how catalytic processes proceed. In the present work, the direct measurement of the molecular diffusion of H₂ and CO gas-phase species diffusing in n-alkane mixtures using pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) methods is reported. The work is of direct relevance to Fischer–Tropsch (FT) catalysis, with the measurements being made of the gas–wax system with the wax in both the bulk liquid state and when confined within a titania catalyst support, at temperatures and pressures typical of low-temperature FT synthesis. Molecular diffusion coefficients of H₂ and CO within wax-saturated porous titania in the range (1.00–2.43) × 10^–8 and (6.44–8.50) × 10^–9 m² s^–1, respectively, were measured in the temperature range of 140–240 and 200–240 °C for H₂ and CO, respectively, at a pressure of 40 bar. The wax mixture was typical of a wax produced during FT catalysis and had a molar average carbon number of 36. It is shown that the hydrogen diffusion coefficient within this wax mixture is consistent, to within experimental error, with the hydrogen diffusion coefficient measured in pure single-component n-hexatriacontane (n-C₃₆) wax; this result held with the waxes in the bulk liquid state and when confined within the porous titania. The tortuosity of the porous titania was also measured using PFG NMR and found to be 1.77; this value is independent of temperature. The ability of existing correlations to predict these experimentally determined data was then critically evaluated. Although the Wilke–Chang correlation was found to underestimate the molecular diffusion coefficients of both H₂ and CO diffusing in the wax in both the bulk state and when confined within the porous titania, parameterized correlations based on the rough hard sphere model, having accounted for the experimentally determined tortuosity factor, predicted the H₂ and CO diffusion within bulk and confined wax to within 3%.

中文翻译：

---

密闭在多孔二氧化钛催化剂载体中蜡混合物中H ₂和CO扩散系数的实验测定

测量和预测多组分混合物中分子扩散系数的能力不仅具有根本的科学意义，而且在理解催化过程的进行过程中也具有重要意义。在本工作中，已报道了使用脉冲场梯度（PFG）核磁共振（NMR）方法直接测量H ₂和CO气相物种在正构烷烃混合物中扩散的分子扩散。这项工作与费-托（FT）催化直接相关，测量是在蜡和蜡在散装液态和密闭于二氧化钛催化剂载体中的情况下，在典型的温度和压力下进行的。低温FT合成。H _2的分子扩散系数在140–240和200的温度范围内分别测量了蜡饱和的多孔二氧化钛中的（1.00–2.43）×10 ^–8和（6.44–8.50）×10 ^–9 m ² s ^–1范围内的CO和CO对于H ₂和CO，在40 bar的压力下分别为–240°C 。该蜡混合物是FT催化过程中产生的蜡的典型特征，其摩尔平均碳数为36。结果表明，该蜡混合物中的氢扩散系数与纯净氢扩散系数一致，在实验误差范围内。单组分正己三aco烷（n -C ₃₆）蜡；当蜡处于散装液态时，并被限制在多孔二氧化钛中时，这种结果得以保持。还使用PFG NMR测量了多孔二氧化钛的曲折度，为1.77；结果为1.77。该值与温度无关。然后严格评估现有相关性预测这些实验确定的数据的能力。尽管发现Wilke-Chang相关性低估了蜡的H ₂和CO扩散的分子扩散系数，无论是在本体状态还是在多孔二氧化钛中，但基于粗糙硬球模型的参数化相关性已经说明了通过实验确定的曲折系数，可以预测H ₂和CO在散装和承压蜡中的扩散在3％以内。