Capturing CO2 using metal-organic framework (MOF)

The steady increase in concentration of CO₂ in the atmosphere (from 310 ppm to > 380 ppm during the past five decades) and its continuous increasing trend until this moment, is really a matter of concern. Power plants contribute to ~ 60% of the total CO₂ emission worldwide. Hence, development of effective CO₂ capture systems that could selectively remove CO₂ from the exhaust gas is warranted. Porous metal-organic frameworks (MOFs) are promising for CO₂ capture. Nevertheless, development of MOFs for CO₂ capture directly from the exhaust gas of power plants is indeed challenging.

Researchers at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, University of Science and Technology of China, Hefei National Laboratory for Physical Sciences at the Microscale and Wenzhou University, China have designed and synthesized a Cu(II)-MOF (FJI-H14) with high density of open metal sites (OMS) and Lewis basic sites (LBS) in which both OMS and LBS interact synergistically with CO₂ and help to capture it.

A mixture of 2,5-di(1H-1,2,4-triazol-1-yl)terephthalic acid (H₂BTTA) (0.05 mM) and Cu(NO₃)₂·3H₂O (0.05 mM) in H₂O (4 ml) in a sealed Teflon vial under hydrothermal conditions at 120 °C for 3 days has lead to the formation of rod-shaped blue crystals of FJI-H14 ([Cu(BTTA)H₂O]_n·6_nH₂O) with 73% yield based on the organic ligand H₂BTTA (Fig. 1).

Fig. 1 Structural illustration of FJI-H14: (a) ligand H₂BTTA; (b) co-ordination environment of Cu(II) ions with BTTA; (c) one-dimensional nano-porous channels; and (d) topology of MOF (Cu atom, cyan; C atom, gray; O atom, red; N atom, blue; H atom, white)

The FJI-H14 is stable in boiling water as well as in acidic and basic environments (pH: 2 to 12) at temperatures as high as 373 K. It is also thermally stable up to 230 °C. The Brunauer–Emmett–Teller (BET) specific surface area of FJIH14 is 904 m²/g and its Langmuir-specific surface area is 1004 m²/g. The total pore volume of FJIH14 estimated from CO₂ isotherm is 0.45 cm³/g. The high porosity and high concentration of open active sites in the framework has lead to an increase in the extent of CO₂ uptake up to 279 cm³/g (Fig. 2(a)). The strong absorption bands at 2,340 cm⁻¹ and 2,328 cm⁻¹ in the IR spectra indicate that the CO₂ molecules tend to stack around the open Cu(II) sites, which is also in line with the theoretical calculations. Besides high adsorption capacity, reusability is an important property for any adsorbent. FJI-H14 maintains 100% adsorption capacity even after five cycles of adsorption, suggesting its suitability as a reusable adsorbent for CO₂ capture (Fig. 2(b)).

Since the flue gas from power plants contains a large amount of N₂ (73–77 %) than CO₂ (15–16 %), CO₂/N₂ selectivity is a crucial parameter in CO₂ capture applications. The CO₂/N₂ selectivity FJI-H14 (for the 15/85 CO₂/N₂ mixture at 298 K and at 1 atm) is 51. The high selectivity for  adsorption of CO₂ over N₂ suggests that the densely populated open active sites in the framework have a positive effect on CO₂ adsorption. The relatively narrow pores in FJIH14 could have easily blocked the relatively large N₂ molecules thus favouring selectivity for CO₂ (Figs. 2(c) and 2(d)). FJI-H14 is also capable of catalyzing chemical transformation of CO₂ into value-added chemicals, such as dimethyl carbonate, cyclic carbonates, N,N’-disubstituted ureas or formic acid.

Fig. 2 Experimental CO₂ adsorption by FJI-H14: (a) CO₂ adsorption isotherm for FJI-H14 at 195 K; (b) Cycles of CO₂ adsorption for FJI-H14 at 298 K; (c) N₂ and CO₂ adsorption isotherms for FJI-H14 at 298 K; and (d) CO₂/N₂ selectivity for 15/85 CO₂/N₂ mixture at 298 K.

FJI-H14 possesses the characteristics of an ideal MOF in terms of high CO₂ uptake at ambient conditions, excellent chemical and thermal stabilities, selectivity for CO₂ over N₂, reusability, direct and smooth conversion of CO₂ into corresponding cyclic carbonates and ease of preparation at large scale.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Liang et al., Carbon dioxide capture and conversion by an acid-base resistant metal-organic framework, Nature Communications, 8 (2017) 1233, DOI: 10.1038/s41467-017-01166-3