Measurement and interpretation of unary supercritical gas adsorption isotherms in micro-mesoporous solids

Abstract

Gas adsorption at high pressures in porous solids is commonly quantified in terms of the excess amount adsorbed. Despite the wide spectrum of adsorbent morphologies available, the analysis of excess adsorption isotherms has mostly focused on microporous materials and the role of mesoporosity remains largely unexplored. Here, we present supercritical CO2 adsorption isotherms measured at T=308 K in the pressure range p=0.02{-}21 MPa on three adsorbents with distinct fractions of microporosity, phi_2, namely a microporous metal-organic framework (phi_2=70%), a micro-mesoporous zeolite (phi_2=38%) and a mesoporous carbon (phi_2<0.1%). The results are compared systematically in terms of excess and net adsorption relative to two distinct reference states–the space filled with gas in the presence/absence of adsorbent–that are defined from two separate experiments using helium as the probing gas. We discuss the inherent difficulties in extracting from the supercritical adsorption isotherms quantitative information on the properties of the adsorbed phase (its density or volume), because of the nonuniform distribution of the latter within and across the different classes of pore sizes. Yet, the data clearly reveal pore-size dependent adsorption behaviour, which can be used to identify characteristic types of isotherm and to complement the information obtained using the more traditional textural analysis by physisorption.