“Ni-4PyC exhibits exceptional stability and recyclability properties, which are critical attributes of solid sorbents when used in industrial PSA systems. A sample of 1 was exposed to steam for 160 hours, and the powder x-ray diffraction (PXRD) pattern remains essentially unchanged as shown in Fig. 5A (blue and magenta patterns). In another key experiment, 1 was maintained under a constant stream of humidified CO2 [~30% relative humidity (RH)] for over 48 hours. Figure 5B reveals that the CO2 adsorption isotherms are the same before and after this treatment. Also, a water vapor isotherm and a post-adsorption PXRD further support 1’s hydrolytic stability (fig. S25). Even harsher humidity treatments did not seem to reduce the CO2 capacity of 1 (figs. S26 and S27). The exceptional hydrolytic stability of 1 is accompanied by an excellent hydrostatic stability (52). 1 was found to retain its original porous structure after being subjected to 70-bar pressure for 24 hours as shown by the PXRD patterns given in Fig. 5A (blue and green patterns). In terms of shelf life, we found that 1 retains >90% of its porosity even after exposure to ambient air for over 6 months (fig. S29).”
(A) PXRDs showing the hydrolytic, hydrostatic stabilities and the homogeneity of the milligram- and gram-scale syntheses of 1. (B) CO2 adsorption isotherms of 1 at 273 K for as made and following exposure to humid (30% RH) CO2 for 48 hours (filled circles, adsorption; open circles, desorption). (C) TGA cycling data on 1 carried out at 308 K. Blue, CO2 flow; red, N2 flow. DSC, differential scanning calorimetry. (D) Diffusion coefficient (Dc) as a function of CO2 loading from eight loadings at 273 K for both the powder and pelletized forms of 1. Average diffusion coefficients for the powder and the pellet are 3.03 × 10−9 and 1.66 × 10−9 m2 s−1, respectively.”
“
1 has smooth adsorption-desorption characteristics as observed from thermogravimetric analysis (TGA) cycling experiments, the results of which are shown in
Fig. 5C. In these experiments, the CO
2 uptake is cycled up to ~6.5% by weight at 35°C. This easy removal of CO
2 is attributable to the moderate interactions of CO
2 with the framework (HOA, 25 to 30 kJ/mol, optimal for a pressure swing) (
41). These cycling experiments produce the same results if He is used as a sweep gas.”
“When used as a solid sorbent in a PSA system, the small pores of an ultra-microporous MOF may severely restrict the adsorption and desorption times under practical operating conditions. This necessitates an investigation of the kinetics associated with the diffusion of CO
2 within the pores of
1. A high-resolution rate of adsorption measurement was carried out using the ASAP 2020 HD instrument at 273 K in the pressure range of 0 to 1 bar, and eight different pressure points were used to determine the diffusion coefficients by fitting them against a spherical pore model (fig. S31) (
53). From this single-component adsorption, an average diffusion coefficient of 3.03 × 10
−9 m
2 s
−1 for CO
2 was obtained (
Fig. 5D). Meanwhile, the simulations gave a diffusion coefficient of 3.73 × 10
−9 m
2 s
−1 at 298 K under the flue gas compositions (fig. S31). This diffusivity is comparable to those observed in some of the microporous MOFs: ZIF-8, 8 × 10
−10; MIL-53(Cr), ~5 × 10
−8; MOF-5, 1.17 × 10
−9; and MOF-177, 2.3 × 10
−9 m
2 s
−1 at 298 K (
54–
58). The diffusion coefficient of
1 is also two orders of magnitude higher than that of zeolite 13X, which is currently used in PSA scrubbers for natural gas purification (
56,
58). When the CO
2 self-diffusion coefficient was measured using a pelletized form of
1, only a little drop in its value was observed (1.66 × 10
−9 m
2 s
−1,
Fig. 5D).”
