Supercritical adsorption in micro- and meso-porous carbons and its utilisation for textural characterisation

Abstract

Understanding supercritical gas adsorption in porous carbons requires consistency between experimental measurements at representative conditions and theoretical adsorption models that correctly account for the solid's textural properties. We have measured unary CO 2 and CH 4 adsorption isotherms on a commercial mesoporous carbon up to 25 MPa at 40 • C, 60 • C and 80 • C. The experimental data are successfully described using a model based on the lattice Density Functional Theory (DFT) that has been newly developed for cylindrical pores and used alongside Ar (87K) physisorption to extract the representative pore sizes of the adsorbent. The agreement between model and experiments also includes important thermodynamic parameters, such as Henry constants and the isosteric heat of adsorption. The general applicability of our integrated workflow is validated by extending the analysis to a comprehensive literature data set on a microporous activated carbon. This comparison reveals the distinct pore-filling behaviour in micro-and mesopores at supercritical conditions, and highlights the limitations associated with using slit-pore models for the characterisation of porous carbons with significant amounts of mesoporosity. The lattice DFT represents a departure from simple adsorption models, such as the Langmuir equation, which cannot capture pore size dependent adsorption behaviour, and a practical alternative to molecular simulations, which are computationally expensive to implement.

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

“…1 for the two CO 2 isotherms, this choice becomes rather arbitrary for measurements carried out both slightly below and above the critical point, because the density shows a strong dependency on pressure. This uncertainty can lead to significant variations in the obtained textural parameters [29] and challenges the interpretation of adsorption isotherms measured at supercritical conditions [1]. The second important implication is that the bulk adsorptive can reach liquid-like densities, comparable to the adsorbed phase, thereby affecting material balance calculations for adsorption systems, as discussed in the next section.…”

“…While the focus here has been on comparing two gases on an adsorbent largely dominated by microporosity (approx. 75% of the pore volume [1]), we will expand the evaluation in this study by considering three classes of materials characterised by distinct fractions of mesoporosity.…”

“…Supercritical adsorption experiments reported in the literature on both crystalline and ordered porous materials provide direct evidence of adsorption behaviour, which can be ascribed to a characteristic distribution of micro-and meso-pores. Some of the traditional adsorbent morphologies that have been investigated include: zeolite crystals, where adsorption is mainly within the micropores [29]; mesoporous silicas, such as Controlled Pore Glass (CPG) [33,41], SBA-15 [9,40] and MCM-41 [31]; and mesoporous carbons [1,36]. These ordered porous solids can be considered as the building blocks for more complex porous structures, including hierarchical adsorbents [37] or disordered materials (e.g.…”

Linking multi-scale 3D microstructure to potential enhanced natural gas recovery and subsurface CO2storage for Bowland shale, UK

“…(1) High-Pressure Adsorption CO 2 and CH 4 adsorption measurements were made BOR and BOL at 7 MPa and 80°C (reservoir conditions). The measurements have been carried out on powdered samples (2g) in a Rubotherm Magnetic Suspension Balance following the procedure described in [27]. Samples were dried at 120°C under vacuum for at least 12 hours in-situ prior to the measurements.…”

Mechanically Stable Monolithic Porous Boron Nitride with High Volumetric Adsorption Capacity

“…Methane adsorption isotherms: High-pressure CH4 adsorption isotherms were carried out at 298 K in the pressure range of 0-70 bar using the equipment and method reported before. [55] In brief, CH4 uptake was measured gravimetrically using a Rubotherm Magnetic Suspension Balance (MSB) with an equilibration time of at least 90 min for each pressure point. The samples were initially degassed in a vacuum oven overnight at 393 K and approximately 0.2 mbar pressure.…”