A molecular simulation study of commensurate-incommensurate adsorption of n-alkanes in cobalt formate frameworks

The channels of the cobalt formate frameworks consist of one-dimensional channels that have a zig-zag configuration. Propane (C3) has a length that commensurates with the channel segment length; longer n-alkanes such as n-butane (nC4), n-pentane (nC5) and n-hexane (nC6) have conformations that straddle two channel segments. Configurational-bias Monte Carlo (CBMC) simulations show that the adsorption strength of C3 is higher than that of n-butane (nC4) and n-pentane (nC5); this unusual hierarchy is a direct consequence of the commensurate-incommensurate adsorption. CBMC simulations also reveal the possibility of separating C3-nC6, C3-nC4, nC4-nC6 and nC4-nC5 liquid mixtures for which the adsorbed phase contains predominantly the shorter alkane. Molecular dynamics simulations show that the hierarchy of self-diffusivities is non-monotonic and is the mirror image of the hierarchy of adsorption strengths.