Multi-shelled Al2O3 coated CaO microspheres for CO2 capture

Emissions of carbon dioxide (CO₂) is considered as the main reason for global warming and ocean acidification and hence many technologies for CO₂ capture are currently emerging. Limestone-derived CaO possesses a high CO₂ uptake capacity (≈0.78 g CO₂/g of CaO) and exhibits fast kinetics of the CO₂ capture and release. Nevertheless, the poor cyclic stability stems from high sintering temperatures, 600–700 °C for CO₂ capture and ≥ 900 °C for sorbent regeneration causes irreversible detrimental changes in their textural properties. Incorporation of stabilizers such as Al₂O₃ though helps to improve the cyclic stability of CaO, the quantity of such stabilizers should be minimized to retain a high CO₂ uptake capacity. Mass transport limitation is yet another issue and for optimal performance, the ideal grain/particle size of CaO should be <100 nm.

Researchers at Department of Mechanical and Process Engineering and Department of Chemistry and Applied Biosciences, ETH Zürich, Switzerland have developed Al₂O₃ coated CaO microspheres for CO₂ capture. In their design approach, porous hollow spherical microstructures composed of nanostructured CaO served as the CO₂ sorbent. The voids in CaO microspheres enhance the surface-to-volume ratio, decrease the mass transport length for CO₂ and act as a buffer to accommodate large volume changes originated from the difference in molar volumes of CaCO₃ (36.9 cm³/mol) and CaO (16.7 cm³/mol). To increase the sintering resistance, the CaO microspheres are coated with a thin layer of Al₂O₃ (< 3 nm) by atomic layer deposition (ALD). The structural design is schematically represented in Fig. 1.

Fig. 1 Structural design of Al₂O₃ coated CaO microspheres for CO₂ capture

6.10 g of glucose and 4 g of Ca(NO₃)₂.4H₂O were dissolved in 15 ml of deionized (DI) water. Then, varying concentrations of urea (0 M, 2 M, 3 M and 6 M) dissolved in 3 ml of DI water was added to it. This reaction mixture in a glass vial was transferred to a 45 ml PTFE-lined stainless steel autoclave and subjected to hydrothermally treatment  at 170 °C for 24 h. The resultant black powder was filtered, thoroughly washed with DI water and ethanol, dried overnight at 80 °C and calcined at 800 °C for 1 h. The CaO sorbents prepared using 0 M, 2 M, 3 M and 6 M urea were denoted as Ca-0M, Ca-2M, Ca-3M and Ca-6M, respectively.

Atomic layer deposition (ALD) was employed for the coat conformal deposition of the Al₂O₃ over CaO. The CaO sorbent sample was alternatively exposed to pulse injections of trimethylaluminum (TMA) and DI water at 300 °C in which the pulse and purge times were set as 1 s and 10 s, respectively. Nitrogen served as purge as well as carrier gas. The deposition process was carried out for 10, 20 and 30 cycles to vary the thickness of the Al₂O₃ coating as 0.9, 1.8, and 2.7 nm, respectively. The Al₂O₃ coated CaO sorbents were denoted as Ca-xM-Al(10), Ca-xM-Al(20), and Ca-xM-Al(30), respectively where xM refers to the molarity of the urea and the number in the parenthesis represent the number of cycles employed for Al₂O₃ coating.

During hydrothermal treatment at 170 °C for 24 h, hydrolysis of urea increase the pH of the reaction mixture, resulting in the precipitation of CaCO₃. Glucose enables development of an interconnected network while Ca(NO₃)₂ increases the diameter of carbonaceous microspheres. The hydrolysis of urea enables a homogenous distribution of Ca within the carbonaceous spheres; the higher the concentration of urea, the greater is the level of incorporation of Ca, which helps to inhibit the oxidative decomposition of the inner core and increase the decomposition temperature. The mechanism involves simultaneous occurrence of condensation, polymerization, and carbonization of glucose with the binding of Ca²⁺ ions to the template surface and precipitation of CaCO₃ nanoparticles, resulting in a homogeneous distribution of Ca compounds within the carbonaceous matrix. Calcination at 800 °C for 1 h leads to the formation of a hollow, multi-shell structure (Fig. 2).

Fig. 2 Hydrothermal treatment of an aqueous solution of glucose, urea, and the Ca precursor after calcination results in multi-shelled hollow microspheres.

The CO₂ uptake performance of the sorbents assessed by TGA reveals that after 10 cycles the synthesized CaO sorbents outperforms limestone-derived CaO by several folds. Nevertheless, all of them experience deactivation (18.7%, 19.9%, and 31.5% decrease in capacity for Ca-2M, Ca-3M, and Ca-6M, respectively, over 10 cycles) due to the formation of smaller CaO microspheres with a reduced central void volume in the absence of a structural stabilizer. The cyclic stability of Al₂O₃ coated CO­₂ sorbents is significantly improved;  the capacity retention of the sorbents is increased to 87.1%, 92.1%, and 92.4% for 0.9, 1.8, and 2.7 nm thick Al₂O₃ coated CO­₂ sorbents, over 10 cycles of operation. After 30 cycles of operation, 0.9 nm thick Al₂O₃ coated CO­₂ sorbent  exhibits a 80.5% capacity retention, which exceeds performance of benchmark limestone-derived CaO by ≈500%. FIB cross-sections of the CO₂ sorbent confirm that the hollow, spherical structure is largely preserved after 30 cycles of operation (Fig. 3).

Fig. 3 (a) CO₂ uptake performance of uncoated and Al₂O₃ coated CaO sorbents; (b, c) FIB cross-sections of Al₂O₃ coated CaO sorbent: (b) calcined state; and (c) carbonated state after exposed for 30 cycles of calcination and carbonation.

The improved performance of the synthesized sorbents is due to the ability of (i) central void to accommodate the volumetric changes during cyclic operation; (ii) porous shells to favour transport of CO₂; (iii) shell-comprising nanoparticles (~100 nm) that ensure occurrence of carbonation reaction in the kinetically controlled regime; and (iv) homogeneous coating of Al₂O₃ that increases the thermal stability and enables long-term utilization of CaO-based CO₂ sorbents.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer to Andac Armutlulu et al., Adv. Mater. 2017, 1702896, DOI: 10.1002/adma.201702896

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