Effects of the Si–O–Si Network Structure on the Permeation Properties of Silylated Ionic Liquid-Derived Membranes

We previously reported the permselectivities, microstructure, and permeation mechanisms of ionic silsesquioxane-based membranes, which are a class of chemically stabilized ionic liquid (IL) membranes, prepared from ionic trialkoxysilanes (i.e., silylated ILs (T-type)) via the sol–gel method. These membranes comprise dense IL regions and Si–O–Si network-derived micropores, and their permeation characteristics depended on the two permeation pathways. Establishing a method for control the permeation characteristics of silylated IL-derived membranes is important for application expansion. Therefore, in this study, an ionic dialkoxysilane (i.e., silylated IL (D-type))-derived membrane was developed, and the effects of the Si–O–Si network structure on its permeation characteristics are discussed. Attenuated total reflectance infrared spectroscopy, N2 adsorption, and nanopermporometry characterizations revealed that the structure of the newly developed membrane was consistent with the Si–O–Si linear structures and Si–O–Si rings, but not the Si–O–Si network-derived micropores. Both membranes showed selective methanol permeation against H2 at temperatures up to 473 K, but the calculation of the activation energy for methanol permeation clearly suggested that the IL-like properties of ionic dialkoxysilane-based membranes were better than those of the ionic silsesquioxane-based membranes with respect to methanol permeation.