An important method used to characterize porous materials is measuring gas and vapor sorption. The uptake capacity is yielded directly by isotherms, which can be used to measure the suitability of microporous materials for a variety of separation and storage applications.
Examples of such applications include post-combustion CO2 capture from syngas, thermochemical energy storage using water adsorption and desorption, and energy gas storage and release.
Both the kinetics and equilibria of gas and vapor sorption can be measured using a variety of different techniques, including gravimetry, gas chromatography, and manometry. By analyzing sorption kinetics, it is possible to obtain information regarding the diffusion of sorbate molecules through the pore network.
Valuable thermodynamic data is provided by equilibrium isotherms. This article demonstrates how to use the gravimetric technique to obtain extremely accurate sorption isotherms and kinetics, under a variety of diverse conditions.
Results and Discussion
Figure 1 illustrates a contribution to an interlaboratory study recently carried out for high-pressure CO2 isotherms on a reference microporous zeolite. The study was organized by the US National Institute of Standards and Technology’s (NIST) Facility for Adsorbent Characterization and Testing (FACT Lab).
Figure 1: CO2 adsorption-desorption isotherms on a reference microporous zeolite at 293 K.
Improving the repeatability and reproducibility of high-pressure adsorption isotherms was one of the goals of the study. Three different XEMIS-001 instruments (A, B, C) were used to measure adsorption-desorption isotherms, with two different samples (S1, S2) and two repeats (R1, R2) – 40 mg was the approximate sample size. The fantastic repeatability across all 24 isotherms is noteworthy.
Figure 2 shows a set of adsorption isotherms for CH4 on alum shale at pressures up to 10 MPa and temperatures at 15 K intervals between 298 and 358 K. The experimental conditions closely mirrored those which naturally occur in shale beds.
Figure 2: CH4 adsorption isotherms on alum shale under geologically relevant conditions.
As a result of low micropore volumes of kerogen (the constituent which is primarily responsible for gas adsorption in shales) below 0.2 mmol g-1, uptake levels are very low. Complementary low-pressure gas adsorption measurements can be used to determine such volumes, including CO2 and CH4 at their respective boiling points of 195 K and 112 K.
Figure 3 denotes a set of water sorption isotherms on a metal-organic framework (MOF), MIL-160 (AI), which is used as a model material in a prototype thermochemical energy storage reactor.
Demonstrated in the results is a behavioral change in the material’s water sorption as a function of pressure and temperature. The working capacity of the material for this application is also indicated.
Figure 3: H2O adsorption-desorption isotherms on a 27 mg MOF sample.6
Data reproduced with permission of Wiley and Sons.
Water sorption and desorption kinetics were additionally assessed under cycling conditions for the prototype reactor. In terms of charge and discharge rates, the kinetics directly relate to the reactor’s performance.
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This information has been sourced, reviewed and adapted from materials provided by Hiden Isochema.
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