Takaba Hiromitsu, Yamamoto Atsushi, Hayamizu Kikuko, Nakao Shin-ichi
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-8685, Japan.
J Phys Chem B. 2005 Jul 28;109(29):13871-6. doi: 10.1021/jp044966z.
1H pulse field-gradient (PFG) spin-echo NMR was performed to measure the diffusivity of methane in a polycrystalline MFI-type silicalite membrane. Measured diffusivities decreased with an increase in the diffusion distance and converged to the constant value. This result suggests the presence of a transport barrier in the membrane. The long-time diffusivity in the membrane was 3.7 x 10(-9) m2/s, which was a factor of 3 smaller than reported values in a single crystal. The distance between the transport barriers was estimated to be much larger than 6 mum from the relationship of diffusivity with displacement. It should be noted that the estimated distances were larger than the smallest dimension of the crystals appearing in the membrane surface. Gas permeation and pervaporation tests were carried out on the same sample for which NMR measurements were taken. The estimated methane flux using measured long-time diffusivity by the permeation theory overestimated the experimental value, although it is closer to the experimental value than the value estimated using the short-time diffusivity. These results mean that the methane diffusivity in a silicalite membrane is much smaller than that in a single crystal.
采用1H脉冲场梯度（PFG）自旋回波核磁共振技术测量了甲烷在多晶MFI型硅铝酸盐膜中的扩散系数。测量得到的扩散系数随扩散距离的增加而降低，并收敛于恒定值。该结果表明膜中存在传输屏障。膜中的长时间扩散系数为3.7×10^(-9) m²/s，比单晶中的报道值小3倍。根据扩散系数与位移的关系，估计传输屏障之间的距离远大于6μm。需要注意的是，估计的距离大于膜表面出现的晶体的最小尺寸。对进行了核磁共振测量的同一样品进行了气体渗透和渗透蒸发测试。根据渗透理论，使用测得的长时间扩散系数估算的甲烷通量高估了实验值，尽管它比使用短时间扩散系数估算的值更接近实验值。这些结果意味着硅铝酸盐膜中甲烷的扩散系数远小于单晶中的扩散系数。
