Summary of MOF-PEI adsorbents for CO2 capture

https://doi.org/10.1016/j.ijggc.2020.103005

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MOFs are a new class of versatile crystalline materials comprising organic ligand – attached metallic ions or clusters arranged in an open network of extended porosity. CO₂ adsorption performance in terms of capacity, selectivity, and kinetics in MOFs can be tuned on demand by controlling the surface chemical potential as well as pore size and shape through simple organic ligand variations (Adil et al., 2017; Anastasiou et al., 2018; Li et al., 2009; Pokhrel et al., 2018b; Sabouni et al., 2014; Samanta et al., 2011; Sumida et al., 2011). In this section, an overview of literature related to PEI – functionalized MOF CO₂ adsorbent systems is presented.

Enhancement of the dynamic CO₂ capacity and selectivity of UiO-66 after PEI functionalization was observed by Xian et al. (Xian et al., 2015a). For 30 wt% PEI – loaded UiO-66, uptakes of 1.65 and 2.41 mmol g⁻¹ were acquired in dry and 55 % relative humidity conditions, respectively, at 65 °C, combined with high selectivity values of 111 and 251 in dry and humid conditions, respectively. Upon cyclic adsorption-desorption operation, the capability of the developed adsorbents was maintained to a good extent. An improved CO₂/N₂ selectivity of 48 and an uptake of 3.2 mmol g⁻¹ were reported at 298 K and 1 bar by Zhu et al. (Zhu et al., 2019b) for 41 wt% BPEI (M_w = 600) – functionalized UiO-66-NH₂ developed by Schiff base reaction in the presence of glutaraldehyde. For comparison, UiO-66-NH₂ exhibited a selectivity of 25 and a capacity of 2.7 mmol g⁻¹. In addition, the functionalized adsorbent exhibited moisture resistance, cyclic stability, and a moderate heat of adsorption (68 kJ mol⁻¹ CO₂).

Darunte et al. (Darunte et al., 2016) carried out functionalization of MIL-101 (Cr) using BPEI with M_w of 800. Gravimetric CO₂ adsorption studies at 25 °C revealed a capacity of 1.35 mmol g⁻¹ for BPEI loading of 1.75 mmol g⁻¹ MOF. Moreover, an amine efficiency of 11 % was acquired for the adsorbent with BPEI content of 0.97 mmol g⁻¹ MOF. Cyclic studies indicated a rather minor loss of uptake for the adsorbent with 1.06 mmol PEI g⁻¹ MOF, while the system with PEI loading of 1.1 mmol g⁻¹ of MOF exhibited also enhanced kinetics. An adsorbent based on 70 wt% PEI (M_w = 800) – impregnated MIL-101 (Cr) was reported by Mutyala et al. (Mutyala et al., 2019) exhibiting a capacity of 3.81 mmol g⁻¹ at 75 °C and 1 bar that was raised to 4.4 mmol g⁻¹ in the presence of moisture due to ammonium bicarbonate generation. The capacity of neat MIL-101 (Cr) was 0.8 mmol g⁻¹. This adsorbent system also exhibited appreciable stability under adsorption-desorption cycles, while application of the Avrami model supported the major involvement of chemisorption. At 25 °C and 1 bar, Gaikwad et al. (Gaikwad et al., 2019) recorded a CO₂ uptake of 3 mmol g⁻¹ for 20 wt% BPEI (M_w = 800) – impregnated bimetallic MIL-101 (Cr, Mg), whereas the N₂ adsorption was decreased. About 35 % reduction in adsorption performance was observed after exposure in combined humid and acidic (NO_x and SO_x) environment. PEI – functionalized MIL-101 was developed by Lin et al. (Lin et al., 2013) exhibiting significant uptake and selectivity even at low pressure. Specifically, selectivity values of 770 and 1200 were obtained at 25 and 50 °C, respectively, for a CO₂/N₂ mixture at 150 mbar, while the respective capacities were 4.2 and 3.4 mmol g⁻¹ for 100 wt% LPEI (M_w = 300) – loaded MIL-101. The observed adsorption kinetics was also enhanced.

Zeolitic imidazolate frameworks (ZIFs) is a subcategory of MOFs consisting of metal cluster – linked imidazole ligands that are topologically analogous to zeolites. Xian et al. (Xian et al., 2015b) highlighted the positive impact of PEI loading on the CO₂ adsorption of ZIF-8. Indeed, the adsorption capacity of ZIF-8 was increased from 0.126 to 1.61 mmol g⁻¹ at 65 °C and 1 atm after PEI functionalization (45 wt% of PEI), which was further increased in the influence of moisture. Also, the CO₂/N₂ selectivity of this adsorbent was increased to 62 from 1.6. This system was thermally stable up to 177 °C. Huang et al. (Huang and Feng, 2018) studied hybrid adsorbents comprising ZIF-8 and PEI – loaded GO for CO₂ capture. Adsorption studies conducted at 273 K and 1 atm revealed a 6-fold enhancement in capacity (181.04 cm³/g) as compared to neat GO (26.04 cm³/g) and ZIF-8 (30.79 cm³/g), which was attributed to the combinatorial effect of PEI functionality and the effective ZIF-8 proliferation in the GO interlayers. A CO₂/N₂ selectivity value of 184 was reported for this hybrid adsorbent due to the occupancy of amine moieties. Pokhrel et al. (Pokhrel et al., 2018a) studied PEI (M_w = 800) – loaded ZIF-8 obtaining a capture capacity of ca. 0.92 mmol g⁻¹ at 1 bar and 30 °C in dry CO₂, while ZIF-8/GO with various modes of functionalizations of the MOF and the GO surfaces were also prepared and studied under dry and humid conditions. Upon impregnation of 40 wt% PEI (M_w = 600) into thermally activated (at 450 °C) Zn/Co ZIF, Cheng et al. (Cheng et al., 2019) achieved a CO₂ capture capacity of 1.82 mmol g⁻¹ at 298 K and 1 bar, whereas the capacity of unmodified Zn/Co ZIF was only 0.70 mmol g⁻¹. Until 9 repeated cycles of adsorption/desorption, the studied adsorbent exhibited an appreciable reversibility as well. The above studies reveal the potential of PEI functionalization to enhance the performance of MOF CO₂ sorbents, specifically, the capture capacity and selectivity of MIL-101, UiO-66-NH₂, and ZIF-8.

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