Introduction of MOF composites for CO2 capture

Usually, the peculiarities of MOFs cannot be exploited to their full potential, due to certain limitations in stability, shaping and scalability. Considering these drawbacks, hybridizing MOFs with porous supports has been emerged as a promising solution to overcome the described restrictions. Recent advancement on MOF composites research has demonstrated that a rational combination of two components within a composite promotes positive synergistic effects and simultaneously moderates the drawbacks of the single materials.78

Several examples of MOF composites with graphene,79 graphene oxide,80 MCM-41,81 ionic liquids,82 clays83 and polymers84 have been developed and successfully applied to CO2 adsorption, aiming to synthesize a material for CO2 capture.

A remarkable example has been elaborated by Prof. Rao and coworkers, preparing graphene reinforced Mg- and Ni- MOF-74 (Figure 4). The composites show a surface area increase of 200–300 m2/g compared to the sole MOFs, which allows an increment of CO2 adsorption capacity of 3–5 % at 0.15 atm and 6–10 % at 1 atm. Furthermore, the composite materials displayed enhanced mechanical properties.79

Recently, new in-situ synthesized composite materials of graphene oxide (GO) with incorporated UiO-66 and UiO-66-NH2 have been reported, which exhibited an enhanced, high performance for CO2 capture. For these materials both physisorption and chemisorption of CO2 has been observed.80a

Other interesting results have been reported for a composite consisting of the mesoporous silicate MCM-41 as matrix and the copper based MOF HKUST-1 as filler. In this case, a 30 % increase in the CO2 adsorption capacity compared to pristine MCM-41 was demonstrated.81

In a comparative study the effects of combining ionic liquids (ILs) and MOFs were investigated, incorporating [BMIM][BF4] and [BMIM][PF6] into MIL-53(Al), HKUST-1, and ZIF-8.82a The ILs had a negative effect on the CO2 adsorption capacity of HKUST-1 since they bind at the unsaturated copper sites. A moderate CO2 capacity increase was registered using ZIF-8/[BMIM][PF6] composites. However, the best results were achieved using MIL-53 (Al) MOF in combination with ionic liquids. Cycling adsorption–desorption measurements showed that the [BMIM][MeSO4]/MIL-53(Al) composite material could be reused after regeneration with almost no decrease in the CO2 uptake capacity for at least six subsequent cycles. These results indicate that any substantial IL leaching can be excluded.82c

The in-situ synthesis of MOF using a solid matrix as template has been proved to be an effective technique for the synthesis of composites. An aminopropyl functionalized magnesium phyllosilicate (aminoclay: AC) [H2N(CH2)3]8Si8Mg6O16(OH)4 has been used to stabilize in-situ prepared HKUST-1 MOF nanoparticles.83 The HKUST-1-nanoparticles@AC composite showed a CO2 adsorption capacity at 1 bar that was 39 % higher than that of the bulk HKUST-1 MOF, indeed the Qst for the bulk MOF and the composite are 24 and 38 kJ mol−1, respectively. Although the AC does not adsorb any relevant amount of CO2, the stronger CO2 affinity of HKUST-1-nanoparticles @AC can be ascribed to the active sites formed at the MOF NPs-AC interface, which enable the interaction between the substrate and the quadrupolar CO2, reaching a total capacity comparable to the benchmark MOF-74.

A large number of MOF-polymer composites have been also reported in the past years, giving rise to materials with improved CO2 adsorption capacity. Nitrogen rich polymers such as polydopamine, polyethyleneimine, polyether-block-amide have been used in combination with Ni-pyrazolate, MIL-101 and MOF-808.84a

In-situ polymerization of styrene, using MOF-5 (Zn4O(BDC)3) as template, dramatically increased the moisture stability of the MOF, giving rise to a material with a higher CO2 adsorption capacity.84b

In general, due to the small size of the guest molecules, CO2 capture is only effective using microporous MOFs. A smart approach to overcome this restriction has been recently reported by Wu et al.84c The encapsulation of dye molecules within the pores of a mesoporous MOF CZJ-10 [Cu6L4(H2O)6] ⋅ 13DMF (H3L=4,4’,4’’-benzene-1,3,5-triyl-tricinnamic acid) has been carried out to increase the exploitation of vacant cavities with dynamic binding sites for CO2 adsorption. Three different composites were prepared with rhodamine B, Congo Red and melanin. Although the vacant pore spaces were remarkably reduced, the carbon dioxide adsorption capacity of these dye encapsulated hybrid materials could be remarkably increased thanks to the insertion of supplementary binding sites for CO2 within the cavities of CZJ-10.

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