Spontaneous Liquid-Gas Imbibition On Carbon Molecular Sieves

The porous material is a kind of functional materials with superior performance. It has a large number of pores. Thus, exploring the pore structure of the porous material is an important content of porous material's characterization. Gas adsorption at low temperature has been widely used in characterizing the properties of the pore structure. However, this method can't be used to characterize the ultramicropore (<0.7nm) because of diffusional problems of gas molecules. Hence, the accurate and simple methods for measurements of ulramicropores are highly desirable. Carbon molecular sieves (CMSs) are porous carbons with clearly distinguishable macropores and very sharp ultramicropore size distributions (size < 0.7 nm). The ultramicropore size of molecular size from either the pore mouths or the pore itself creating a barrier for molecule diffusion. The barrier can be the rate-limiting factor in the transport of larger molecules. Hence, CMSs have significant kinetic selectivity of gas mixture with different molecular sizes as a result of differences in the adsorption rates.In this work, spontaneous liquid-gas imbibition has been applied to the assessment of the structural characteristics of micropores in CMSs. The pseudo-second-order (PSO) kinetic model and linear driving force (LDF) model were employed to interpret the kinetics of spontaneous imbibition to assess the average size and size distribution of ultramicropore mouths in CMSs, and an assessment for the O2/N2 separation performance of CMSs by pressure swing adsorption (PSA) has been established. Both the theory analysis and expemental results demonstrated that the gas recovery on CMSs is mainly controlled by the surface adsorption of the water, the diffusion of the gas along the ultramicropores or through the barrier at the ultramicropore mouths. The gas recovery takes place in the ultramicropores with narrower pore mouths prior to the ultramicropore with wider pore mouths. Moreover, an assessment method for the O2/N2 separation ability of CMSs in PSA was established by introducing an O2/N2 selectivity coefficient K. When the O2/N2 selectivity coefficient K is equal to zero, the CMS owns the most appropriate size uniformity of ultramicropore mouths to adsorb O2 over N2; and with K gradually deviating zero, the O2/N2 separation ability reduces. The spontaneous liquid-gas imbibition method is a simple and useful tool to assess the ultramicropore mouths and the O2/N2 separation ability of CMSs in PSA.