Fourier-transform pulsed-field-gradient 1H nuclear magnetic resonance investigation of the diffusion of light n-alkanes in zeolite ZSM-5
Fourier-transform pulsed-field-gradient 1H nuclear magnetic resonance investigation of the diffusion of light n-alkanes in zeolite ZSM-5
复制标题
轻质正构烷烃在 ZSM-5 沸石中扩散的傅里叶变换脉冲场梯度 1H 核磁共振研究
DOI：
10.1039/ft9918701935
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发表时间：
1991
期刊：
影响因子：
--
通讯作者：
J. Kärger
中科院分区：
文献类型：
--
作者：
K. P. Datema;C. D. Ouden;Wessel D. Ylstra;H. Kuipers;M. Post;J. Kärger
本文用傅里叶变换脉冲场梯度核磁共振技术研究了几种正构烷烃（甲烷、正丁烷和正戊烷）在ZSM-5沸石中的扩散。NMR研究的目的是比较分子动力学计算和吸收测量，使用相同的系统所获得的结果。甲烷清楚地表现出双指数自旋回波衰减。这表明两种类型的扩散：晶内扩散和长程扩散，长程扩散是晶间和晶内扩散的组合。在小晶体的情况下，甲烷扩散到晶体之间的大孔中占主导地位的衰变。然而，正如预期的那样，对于较大的晶体，晶内（或晶内）扩散的贡献显着增加。从这些曲线中，测定了甲烷在ZSM-5中的晶内扩散系数（25 °C时为3.8 × 10-9 m ~ 2·s ~（-1））。NMR甲烷扩散数据与分子动力学计算得到的值吻合得很好。随后对正丁烷和正戊烷在ZSM-5中扩散的NMR测量表明，随着烃的链长增加，扩散急剧下降（在25 °C和20 kPa负载下，分别为11 × 10-11和4.4 × 10-11 m2 s-1）。为了允许与通过其他技术独立获得的扩散率进行比较，确定了ZSM-5中正戊烷的NMR自扩散系数的浓度依赖性，并发现随着山梨酸盐浓度的增加而降低。此外，从扩散速率的温度依赖性的活化能为正丁烷和正戊烷在ZSM-5中的扩散已被确定（8.4和12.6 kJ mol-1，在20 kPa，分别）。还将轻质正构烷烃在ZSM-5中扩散的PFG NMR和MD结果与来自文献的相关扩散数据（使用其他技术获得，即吸收方法、ZLC、MT）进行了比较。微观自扩散（从PFG NMR和MD）系统地不同约。两个数量级的慢得多，宏观扩散观察到的吸收，ZLC和MT方法。另一方面，正丁烷的自扩散率与PFG NMR和相同的系统使用频率响应法测量的传输扩散率之间有令人满意的协议。
Fourier-transform pulsed-field-gradient NMR measurements have been used to analyse the diffusion of some n-alkanes (methane, n-butane and n-pentane) in zeolite ZSM-5. The intention of the NMR study was to compare results obtained by molecular dynamics calculations and by uptake measurements, using the same systems. Methane clearly exhibits a bi-exponential spin-echo attenuation. This indicates two types of diffusion: intracrystalline diffusion and long-range diffusion, which is a combination of inter- and intra-crystalline diffusion. In the case of small crystals the diffusion of methane into the macropores between the crystals dominates the decay. However, as expected, for larger crystals, the contribution of intracrystalline (or micropore) diffusion increases significantly. From the curves, the coefficient of intracrystalline diffusion of methane in ZSM-5 has been determined (3.8 × 10–9 m2 s–1 at 25 °C). The NMR methane diffusion data are in good agreement with values obtained by molecular dynamics calculations. Subsequent NMR measurements of n-butane and n-pentane diffusion in ZSM-5 indicate that the diffusion decreases sharply with increasing chain length of the hydrocarbons (11 × 10–11 and 4.4 × 10–11 m2 s–1, respectively, at 25 °C and 20 kPa loading). To allow for a comparison with the diffusivities obtained independently by other techniques, the concentration dependence of the NMR self-diffusion coefficient of n-pentane in ZSM-5 was determined and was found to decrease with increasing sorbate concentration. In addition, from the temperature dependence of the diffusion rates the activation energies for the n-butane and n-pentane diffusion in ZSM-5 have been determined (8.4 and 12.6 kJ mol–1 at 20 kPa, respectively). The PFG NMR and MD results for the diffusion of light n-alkanes in ZSM-5 have also been compared with relevant diffusion data from the literature (obtained using other techniques, i.e. uptake methods, ZLC, MT). The microscopic self-diffusivity (from PFG NMR and MD) differs systematically by ca. two orders of magnitude from the much slower, macroscopic diffusion observed by uptake, ZLC and MT methods. On the other hand, there is satisfactory agreement between the self-diffusivity of n-butane obtained with PFG NMR and the transport diffusivity of the same system measured using the frequency response method.