MOFs as chemisorbents for CO2 capture

https://doi.org/10.1002/cplu.202000072

“The current literature shows that despite the huge efforts to design and synthetize a material with high CO₂ capacity that can be easily regenerated, only little progress has been achieved. Indeed, the MOF-74 family remains one of the benchmark materials for CO₂ capture, even if some issues, like a reduced performance upon contact with moisture, have not been solved yet. Another limitation of MOF-74 is the relatively narrow dimension of the channels (∼11 Å diameter), which prevents a successful functionalization of the MOF with amines. The group of Prof. Long, aiming to unlock new potential ranges of capabilities, suggested employing MOFs owning larger cavities and openings, enabling more efficient gas diffusion and facilitating the material functionalization. As a consequence, they realized the synthesis of expanded analogues of the MOF-74 structure via a linker extension.85 Using 4,4’-dioxido-3,3’-biphenyldicarboxylate a MOF analogue to MOF-74 presenting 18.4-Å-wide channels has been synthesized and named M(dobpdc). These materials were subsequently functionalized with N,N’-dimethylethylenediamine (mmen) (Figure 5). When comparing Mg-MOF-74 and Mg₂-(dobpdc) a similar CO₂ affinity and capacity were measured. However, the alkylamine-MOF mmen-Mg₂-(dobpdc) exhibited a tremendously higher affinity for CO₂ at extraordinary low pressures. At 25 °C and 0.39 mbar, meaning close to the current CO₂ partial pressure on Earth’s atmosphere, the alkylamine-functionalized MOF has a CO₂ uptake capacity 15 times higher than Mg₂-(dobpdc). Diamine functionalized Mg2-(dobpdc) can behave as a ‘phase-change’ carbon dioxide sorbent, namely this functionalized MOF shows singular step-shaped CO₂ adsorption isotherms. The adsorption step shifts notably with temperature. A rational selection of the M²⁺ ion and appended diamine allows to precisely tune this step pressure. 1°,2°-alkylethylenediamines, such as N-ethylethylenediamine and N-(iso-propyl)ethylenediamine, display optimal performances for carbon capture from low concentration gas streams, as expected taking into account the step-shaped CO₂ adsorption isotherm at partial pressures ≤1 mbar and 40 °C and the negligible diamine loss. According to the experimental data, secondary, secondary (2°,2°) diamines functionalized materials are less promising candidates for future applications.86”

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A combination of computational and spectroscopic studies on the CO₂ adsorption of amine functionalized Mg₂-(dobpdc) MOFs showed that either the ammonium carbamate or carbamic acid formation between CO₂ and the amine is highly energetically favored compared to physisorptive adsorption.87 Furthermore, the CO₂ adsorption capacity retained both after exposure to humidity and after adsorption/desorption cycles at realistic dilute CO₂ concentrations.88

Spectroscopic and diffractometric measurements together with computational studies elucidated a singular mechanism in which, exceeding a metal-dependent breakthrough pressure, CO₂ molecules insert into metal-amine bonds, originating well-ordered chains of ammonium carbamate as a consequence of the amines rearrangement. Thus, outstanding CO₂ separation capacities are accomplished with slight temperature swings. Moreover, the energy of regeneration is substantially lower compared to the state-of-the-art aqueous amine solutions. The combination of DFT and NMR studies provided with additional insight into the chemisorption mechanism, thereby the formation of carbamate or carbamic acid formation can be identified and distinguished.89

Newly, analyzing together computational and structural studies, gas adsorption data and solid-state NMR spectra it was possible to reveal five different mechanisms in carbon dioxide adsorption in alcoholamine-and alkoxyalkylamine-functionalized MOFs.90

Yaghi et al. pointed out that primary alkylamine functionalized IRMOF-74-III adsorbs CO₂ as ammonium carbate under dry conditions and as carbamic acid when water vapor is present.91

To promote the activity of these amino functionalized MOFs it was proposed to use diamines bearing large alkyl groups.

Diamines with higher molecular weights have a positive effect on the thermal stability of the hybrid diamine-MOF towards diamine leaching in the presence of water vapor. Nevertheless, large amines, due to steric interactions, limit the formation of uniform ammonium carbamate chains in the case of Mg₂(dobpdc) and have a negative impact on CO₂ adsorption mechanism. Indeed, two-step CO₂ adsorption/desorption profiles are registered. Furthermore, increased H₂O co-adsorption in the presence of humidity is observed. MOFs with either a more uniformly hexagonal pore structure or longer organic linkers seem to do not suffer from these drawbacks.92 Nonetheless, larger diamines reduce the surface areas of the MOF, decreasing the gravimetric CO₂ uptake capacities.

In a following article, Mg₂(dobpdc) was functionalized with the cyclic diamine 2-(aminomethyl)piperidine. For this material, it was observed that hydrogen-bonding interactions between water and the carbamate chains enhance the CO₂ capture. Seeing the outstanding oxidative and thermal stability of 2-(aminomethyl)piperidine–Mg₂(dobpdc), its outstanding CO₂ uptake capacity, and its remarkable CO₂ capture rate from a simulated natural gas flue emission stream, this MOF is certainly one of the best candidate to date for this highly significant technology.93

Other MOFs with sufficiently large pores can accommodate amines. As an example, the CO₂ capture performance of MIL-101(Cr) was increased by alkylamine functionalization. The loading grade of the MIL-101(Cr) was demonstrated to be related to the polarity of the solvent used for the tethering process. Less polar solvents, or even better nonpolar, are more suitable for the encapsulation of an alkylamine into MOFs, promoting the CO₂ uptakes.94

Hence, amine functionalized MOFs, which allow chemisorption of CO₂, appear as superior materials for practical applications.

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