Wet flue gas CO 2 capture and utilization using one-dimensional metal-organic chains.

Herein, we describe the use of an ultramicroporous metal-organic framework (MOF) with a composition of [Ni 3 (pzdc) 2 (ade) 2 (H 2 O) 1.5 ]·(H 2 O) 1.3 (pzdc: 3,5-pyrazole dicarboxylic acid; ade: adenine), for the selective capture of carbon dioxide (CO 2 ) from wet flue gas followed by its conversion to value-added products. This MOF is comprised of one-dimensional Ni(II)-pyrazole dicarboxylate-adenine chains; through pi-pi stacking and H-bonding interactions, these one-dimensional chains stack into a three-dimensional supramolecular structure with a one-dimensional pore network. Upon heating, our MOF undergoes a color change from light blue to lavender, indicating a change in the coordination geometry of Ni(II). Variable temperature ultraviolet-visible (UV/vis) spectroscopy data revealed a blue shift of the d-d transitions, suggesting a change in the Ni-coordination geometry from octahedral to a mixture of square planar and square pyramidal. The removal of the water molecules coordinated to Ni(II) leads to the generation of a MOF with open Ni(II) sites. Nitrogen isotherms collected at 77 K and 1 bar revealed that this MOF is microporous with a pore volume of 0.130 cm 3 g -1 . Carbon dioxide isotherms show a step in the uptake at low pressure, after which the CO 2 uptake is saturated. The step in the CO 2 uptake is likely attributable to the rearrangement of the three-dimensional supramolecular structure to accommodate CO 2 within its pores. The affinity of this MOF for CO 2 is 35.5 kJ mol -1 at low loadings, and it increases to 41.9 kJ mol -1 at high loadings. While our MOF is porous to CO 2 and water (H 2 O) at 298 K, it is not porous to N 2 , and the CO 2 /N 2 selectivity increases from 28.5 to 31.5 as a function of pressure. Breakthrough experiments reveal that this MOF can capture CO 2 from dry and wet flue gas with uptake capacities of 1.48 ± 0.01 and 1.14 ± 0.06 mmol g -1 , respectively. The MOF can be regenerated and reused at least three times, demonstrating consistent CO 2 uptake capacities. Upon understanding the sorption behavior of this MOF, catalysis experiments show that the MOF is catalytically active in the fixation of CO 2 into an epoxide ring for the formation of a cyclic carbonate. The turnover frequency for this reaction is 21.95 ± 0.03 h -1 . The MOF showed no catalytic deterioration after two cycles and maintained comparable catalytic activity when dry and wet CO 2 /N 2 mixtures were used. This highlights that both N 2 and H 2 O do not dramatically affect the catalytic activity of our MOF toward CO 2 fixation.