Carbon incorporated MOF for CO2 capture

https://doi.org/10.3390/en15093473

“Recently, the incorporation of carbon-based compounds to MOFs to produce MOF-carbon composites has been intensively studied. MOF-carbon composites can be considered as a new material that combines the advantages of MOFs, such as well-defined structure and texture properties, high porosity, etc., and properties of carbon-based materials, such as high mechanical and elastic strength, chemical and thermal stability, low weight, low toxicity, and low costs. Such new functionality could be obtained after synergetic interaction between the MOF matrix and the carbon matrix due to changes in chemical composition, structure, and porosity [49]. Functionalities of the C-containing MOF-composites including structure change, enhanced stabilities, and template effects are vital to meet the prerequisites in practical occasions and enrich the prospective contents of MOFs [50]. According to the literature, the application of some adsorbents with high surface areas, such as graphite, graphene, graphene oxide (GO), and carbon nanotubes, significantly improves the gas adsorption performances of the MOF-composites by optimizing the texture and morphology of the solid to create an additional microporosity and interstitial porosity [11,50,51,52,53,54,55,56,57,58,59,60,61]. This promotes the gas transport to the adsorption sites and increases total accessible pore volumes. Recent studies report that due to the formation of secondary micro- and mesoporosities in the nanostructure of composite materials, carbon-doped MOF-composites are able to increase gas adsorption capacity and separation efficiency compared to their parent material [52,53]. For instance, Szcznesniak and Choma reported that graphene containing CuBTC composites are potential materials for large-scale selective CO₂ capture at ambient conditions. They found that most graphene-containing composites show lower adsorption capacities; nevertheless, they exhibit an enhanced CO₂/N₂ selectivity in comparison to bare samples [54]. It has been reported that GO, as well as other materials, can act as a substrate or template in the formation of MOF-composites; for example, oxygen-containing functional groups of GO can act as seed sites for crystallization and HKUST-1 nanocrystal dispersion. Studies also indicate that GO affects the growth of MOF nanocrystals. The optimal addition of GO allows for the obtainment of a composite with a larger specific surface area, as well as to increase the CO₂ adsorption capacity compared to pristine HKUST-1 [44]. Shen et al. reported that UTSA-16-GO composites have improved thermal stability, and that CO₂/CH₄ selectivity is three times higher compared with UTSA-16 alone [30]. As with GO, carboxylate groups of CNTs are reported to act as nucleation sites to support continuous growth of HKUST-1 [52]. CNT-containing composites also have higher CO₂ adsorption capacities in comparison to pristine MOF, e.g., the CO₂ uptake at 298 K, 18 bar for CNT@Cu₃(BTC)₂ reach 595 mg/g, which is more than that of pristine MOF (295 mg/g) at the same conditions [57]. So far, MOF-carbon composites containing graphene [45], graphene oxide (GO) [44,45,48,54,56,58], graphene aerogel [23], and carbon nanotubes (CNT) [55,57] have been intensively studied, but less attention has been paid to biochar as a potential carbon-containing MOF-composite. Yet, several definitions of biochar [51] have been proposed, but according to IBI [60], biochar is the solid material derived from various biomass feedstocks under oxygen-limited thermal conversion processes. Biochar is characterized by a porous structure, abundant functional groups, various inorganic nutrients, and high carbon stability. Therefore, biochar can be used for a variety of purposes, including immobilization of pollutants, flue gas purification, and in situ carbon storage [61]. It is quite similar to activated carbon, which is also produced by pyrolysis, with a characteristic medium or large surface areas. However, unlike activated carbon, biochar is usually not activated or processed. In addition, biochar contains a noncarbonated fraction that can interact with other substances. In particular, the amount of O-containing carboxyl groups, hydroxyl groups, and phenolic surface functional groups in biochar could effectively be able to co-ordinate with the metallic centers of MOFs, leading to the growth of MOF crystals onto biochar [51,52]. The biochar component in the MOF-biochar composite could enhance the mechanical strength of the composite as well as serve as secondary adsorption sites.”