MOFs As Adsorbents for Gas Separations: A Comparative Study

Separation processes and especially the separation of gases form a major part of both the capital expenditure and the operating expenditure on many chemical plants. Gaseous mixtures are generally separated either by cryogenic distillation or solvent extraction, both of which are highly energy intensive and costly. A viable alternative to these is a pressure swing adsorption (PSA) process, but this requires an adsorbent which is capable of separating the gases efficiently.

Many different types of porous materials are being considered for use as adsorbents in such a process including activated carbons, mesoporous silicas, zeolites and metal-organic frameworks (MOFs) and indeed some of these are already being used for specific separations¹. The purpose of this study is firstly to identify which are the best parameters for comparing new materials and deciding which ones are most likely to be of interest for a specific separation, and secondly to see whether MOFs can compete with the more established materials such as activated carbons and zeolites.

Ideally the most relevant data for comparing adsorbents for a PSA process would be breakthrough curves from column experiments, either at the lab scale or in pilot plants. However these studies often require relatively large amounts of adsorbent and are not feasible for materials which are at the discovery stage, as many new MOFs are. Alternatively it is possible to measure static co-adsorption isotherms using about a gram of material, but it can be difficult to get reliable information from these experiments and so results are not widely available in the literature.

The data which is most readily available for new MOFs is the pure component isotherms measured at room temperature, given in terms of amount of gas adsorbed for a given mass of adsorbent. This is not directly relevant for a PSA process, however from this it is possible to extract a number of parameters which can then be used to compare the materials.

We have undertaken a study where we have measured pure gas isotherms for CO₂, CO and CH₄ on a number of different MOFs as well as a two reference materials, a zeolite and an activated carbon, and compared them based on the following criteria:

-          Maximum uptake in terms of amount adsorbed per gram of material (most readily available data)

-          Maximum uptake as a volume of gas adsorbed per volume of adsorbent (more relevant for a PSA process)

-          Working capacity, i.e. the difference between the amounts adsorbed at two operating pressures

-          Separation factors based on prediction models (IAST and VSM)

In addition to this, we have also measured the adsorption enthalpies for these systems using a Tian-Calvet type microcalorimeter coupled with a homemade gas dosing device. These measurements provide information as to the strength of the host-gas interaction and therefore how easy it is likely to be to regenerate the adsorbent. This data is also interesting for a PSA process because it gives an indication of the amount of heat released during adsorption, which will either have to be removed or which will significantly heat the column, if it is run as an adiabatic process.

Finally we have done a number of experiments to study the effect of water on the adsorption of gases in these materials, looking at both the stability of the material at different relative humidities and the competitive adsorption of CO₂ and water.

This study has shown that different materials stand out as good candidates depending on what criteria you use to compare them and that some materials which appear at first glance to be ideally suited for a particular separation in fact present some major drawbacks when you look at the complete picture. In addition, although in many cases the MOFs studied failed to compete with zeolites or activated carbons, some processes were identified in which MOFs could significantly outperform the reference materials.

This work is part of the European project FP7 MACADEMIA “MOFs as Adsorbents and Catalysts: Design and Engineering of Materials for Industrial Applications” (CP-IP-228862-2). Our thanks go to the laboratories of the Institut Lavoisier in Versailles, the Christian Albrechts Universität in Kiel and the Korean Research Institute for Chemical Technologies for the samples provided.

References

[1] S. P. Reynolds, A. D. Ebner, J. A. Ritter, Industrial and Engineering Chemistry Research. 45, 3256-3264, 2006.