Outlook of using MOF for direct air capture

Although the use of MOFs for DAC remains a very promising methodology to reduce the impact of atmospheric CO2 on the global climate, its technical application is still very challenging and further research efforts are necessary. Several requirements have to be fulfilled in order to generate a material that can be conveniently used for CO2 capture. First of all, the selectivity towards CO2 and the adsorption capacity must be remarkable to ensure a significant reduction of the CO2 gas concentration in the atmosphere. Moreover, the material must be stable and has to be reused without any loss of performance for a theoretically infinite number of adsorption/desorption cycles. Besides, the amount of energy required to desorb the CO2 guest molecules from the MOF has to be as low as possible. Another issue that needs to be addressed is the low CO2 selectivity of several MOFs in the presence of moisture. Considering that practical CO2 adsorption occurs under humid conditions, the water stability and the CO2 affinity of the MOFs have to be improved.

Despite the increasing and impressive amount of research, no pilot-scale study on the application of MOFs for carbon dioxide capture exists yet.

An outstanding acceleration in the design of MOFs with suitable features for CO2 capture and their implementation in DAC is expected to be produced by computational screening.

Several classes of strong CO2-binding sites, which leverage the CO2/N2 selectivity of MOFs in wet flue gases, have been identified using data mining. The results are based on a screening database of over 300,000 MOFs.95 This work allowed to synthesized two water-stable MOFs showing carbon-capture capacity that outperform that of some commercial materials. Danaci et al. have developed an adsorbent screening tool to evaluate adsorbents for CO2 capture by considering process economics and other industrially relevant factors.96 Specifically, 25 adsorbents (1 activated carbon, 2 zeolites, 22 MOFs) were evaluated with regard to performance limitations – i. e. CO2 purity and recovery as well as costs. It was concluded that low nitrogen adsorption and adequate adsorption enthalpies are key to achieving good process performance and reducing costs. Between the analyzed substances, it was assessed that UTSA-16 is the best material for post-combustion capture of CO2 showing performance comparable to the benchmark (zeolite 13X) and moderate costs.

Considering only the total gravimetric CO2 uptake capacity at 0.15 and 1 bar at 298 K, Mg-MOF-74 is still the best performing MOF, adsorbing 5.9 and 8.1 mmol CO2 per gram respectively.97 Nevertheless, the selectivity of Mg-MOF-74 towards CO2 is still not satisfactory and, owing to the low relative concentration of CO2 in the atmosphere it is not applicable for direct air capture.

Considering both CO2 adsorption energetics and uptakes, the NbOFFIVE-1-Ni (which has not been assessed in ref. 96) shows the best uptake capacity for carbon dioxide at 400 ppm together with optimal regeneration energy (Figure 6).48 NbOFFIVE-1-Ni is an ideal material for carbon capture at very low CO2 concentrations due to its peculiar structure with contracted square-channels that are functionalized with proximal fluorine moieties. However, a considerable CO2 uptake reduction has been observed when the adsorption has been performed under 75 % relative humidity, which limits the applicability of this material to dry atmospheric conditions.

Diamine functionalized Mg2(dobpdc) materials appear therefore the best option to date for the realization of a MOF-based carbon dioxide capture process.858993 These materials in fact own a good CO2 uptake capacity together with a significant stability under water vapor and a general structural robustness. In contrast to the MOFs described previously, CO2 is adsorbed into diamine functionalized Mg2(dobpdc) chemically, which requires a higher energy expenditure to regenerate the sorbent. Further optimization of this MOF, aimed to increase both the uptake capacity and, most specifically, the heat of desorption, are still needed to deliver a material designed for DAC of CO2. Taking into account the moderate price of the reagents and the relatively easy and scalable synthetic process, functionalized Mg2(dobpdc) MOF can be seriously considered as a promising candidate for future technical processes.

Considering their novelty in comparison to the other adsorbent types, and their tunability, which paves the way for innumerable research opportunities, MOFs can definitely boost the future application of DAC.98 Several matters still need to be faced to realize a negative carbon technology based on MOFs, however two main priorities can be already identified: i) increase the CO2 affinity of MOFs in presence of water; ii) realize materials that are easily shaped and produced through scalable processes.

Leave a Comment