Stability Study of Important Metal Organic Frameworks (MOFs) and a Review on their Gas Adsorption Properties

Abstract

Metal Organic Frameworks (or, MOFs) have shown tremendous potential in adsorptive separation applications and gas storage owing to some of their extraordinary features in terms of specific surface area, pore volume, low to moderate heat of adsorption and fairly uniform pore size distribution. But, the success or failure of any adsorbent material largely depends on their stability in varying experimental conditions. In this work, we have highlighted the synthesis of 3 most versatile MOFs reported till date viz. Cu-BTC (or, HKUST-1), Cr-BDC (or, MIL-101) and Zn-BDC (or, MOF-5). Each of these MOFs after their successful synthesis and characterization were exposed to a regulated environmental conditionto study the effect of moisture sensitivity. Such a study is particularly important since any real time experiment with MOF is bound to come to terms with varying degree of moisture or water vapor, especially when exposed for longer duration. After detailed experimentation we concluded that a controlled exposure to ambient conditions didn’t have a severe effect on MOF’s thermal stability. Cr-BDC was found to be taking up more moisture during the course of time as compared to Cu-BTC and Zn-BDC. The degree of crystallinity appeared to be reduced over the time interval and surface morphology too gets affected.
Moreover, we have carried out a comprehensive review of 3 very important industrially and environmentally important gases viz. H2, CO and CO2 on these three MOF matrices. The reason behind choosing theses gases stems out from the fact that H2 is projected as a future fuel which may very well replace the conventional fossil fuels, both CO2 and CO are the most important green house gases and their emission needs to be effectivelyarrested, mixture of these gases are emitted from various sources e.g. steam reforming of naphtha, partial oxidation of hydrocarbons, metallurgical plants etc. Apart from these facts, physical properties of each of them are quite different. H2 is a non-polar gas whereas CO has a permanent dipole moment and CO2 has a quadrupole moment. Studying the effects of these physical properties could be interesting from a fundamental point of view to understand the adsorption phenomenon. The retrieved experimental data from literature was model fit using standard isotherm models viz. Langmuir, Freundlich, Freundlich-Langmuir, Dual Site Langmuir (DSL) and Virial models. Additionally, a comparative study between simulation data (available in literature) and experimental data (at same conditions)was carried out for a proper validation. CO was selected on the basis of its polarity and CH4 was chosen since it is non-polar. The adsorbent for the study was Cu-BTC.
Our findings are summarized as:
(I) All the isotherm models are not equally efficient in predicting the adsorption behavior in low and high pressure regime. Freundlich-Langmuir model is seen to be the best in explaining the adsorption behavior irrespective of the type of probe or adsorbent surface.
(II) The experimental H2 adsorption data as reported by various researchers varied considerably from lab to lab and H2 adsorption on none of the adsorbents studied in this work satisfies the Department of Energy (DoE) target of 6.5 wt%.
(III) Cr-BDC (or, MIL-101) showed the highest affinity for CO2. This uptake of CO2 is the highest reported till date.
(IV) Although experimental data on CO adsorption on any MOF material is scarce, but still within our review, we have found Cr-BDC to have the highest loading of CO. The higher loading can be attributed to very high surface area (ca. 3000 m2 g-1) for Cr-BDC amongst the studied MOFs.
(V) The comparison of simulation with experimental data of CO and CH4 on Cu-BTC has shown that for polar molecule e.g. CO, simulation data under predicts the experimental data whereas in the higher loading region simulation data over predicts. This is less marked for non-polar gas like CH4. It is worth mentioning that even though there are variations in simulation result predictions with experimental data but still Grand Canonical Monte Carlo (GCMC) simulation is a strong method in predicting experimental excess adsorption data particularly when total pore volume information and single crystal XRD data is available.