High-Connectivity Approach to a Hydrolytically Stable Metal–Organic Framework for CO₂ Capture from Flue Gas

Abstract

T here have been significant advances in the design and development of porous coordination polymers (PCPs), or metal−organic frameworks (MOFs), over the past three decades, since the initial design philosophy was outlined by Hoskins and Robson. 1 MOFs are of interest due to their high porosity with potential to be used in a wide range of applications including gas capture and storage, separations and catalysis. 2−4 MOFs are often compared with zeolites as both classes of materials contain accessible internal void space; however, the ability to chemically tune the ligands of MOFs allows for the inclusion of different functionalities. One of the advantages that zeolites hold over MOFs is their stability to a wider range of conditions, with MOFs being thermally stable but hydrothermally less stable. This restricts MOFs from being applied to processes such as postcombustion CO 2 capture technologies as the flue gas contains 6−7% water vapor. 5,6 Thus, it is crucial that if MOFs are to be used for industrial applications, the problem with stability needs to be addressed and overcome, in this case for example via synthetic design. True hydrolytic stability is still relatively rare, typically arising from inert metal clusters. 7 Work by Gelfand and Shimizu described a systematic method for parametrizing the hydrolytic stability of MOFs by grading the kinetic and thermodynamic water stability, types of water exposure, methods of probing water stability and degrees of water stability. 8 This analysis resulted in a grading system that compares the various levels of hydrolytic stability, using a number from 1 to 6 (1 = near ambient conditions, 6 = boiling water) to denote harshness of exposure and a letter from A to D (A = retention of crystallinity and porosity, D = loss of crystallinity and porosity) to denote proof of stability. The authors note that these definitions should be used as guidelines for discussion and not quantitative metrics.There have been a few examples of MOFs such as HKUST-1, 9 UiO-66, 10 MIL-100, 11 and MIL-101 12,13 that have been shown to not only be relatively stable in humid conditions but to even have enhanced CO 2 sorption in humid environments. 14−17 HKUST-1 was one of the frameworks listed as a case study by Shimizu, which when placed in water at 50 °C for 24 h showed a change in the PXRD pattern and decrease in internal surface area from 1340 to 647 m 2 /g. 18 This behavior resulted in a classification of 4B stability, however when HKUST-1 was exposed to humidity it was found to degrade into a nonporous phase, 19 which changes its classification to a 3D rating. Another material studied was the ZIF-8 network, which showed 6C stability when placed in boiling water for 1 week. 20 A comprehensive review by Li et al. covered

Improving MOF stability: approaches and applications

“…A hydrostable and highly connected cadmium(II)based MOF was produced using a triaminepentacarboxylic acid ligand. 165 Flue gas simulation (3.9% H 2 O:14.4% CO 2 :81.7% N 2 ) experiments showed that this MOF preserved its CO 2 adsorption performance even aer three consecutive cycles, indicating its outstanding water stability. Stable MOF-based composites showed similar results.…”

Understanding the Effect of Water on CO2 Adsorption

“…This includes the occurrence of well-protected metals with high coordination numbers, such as Zr-based UiO-66 339 and MIL-140, 340 numerous porous coordination networks (PCNs), 341−343 Hfbased DUTs, 344 phosphonate-based MOFs, 345−347 and others. 348 Catenation, i.e., interpenetration of two or more identical and independent frameworks, is another steric factor that improves water stability by locking labile ligands in place. 349−352 The higher water-stability of Zn(BDC)(BPY) 0.5 or MOF-508 (BDY = 4,4′-bipyridine) with catenation, compared to Zn(BDC)(DABCO) 0.5 or DMOF (DABCO: 1,4-diazabicyclo[2.2.2]octane) without catenation, offers an illustrative example.…”

“…Using water-repellent fluorinated ligands blocked water adsorption altogether. , Incorporation of polymers can also increase the MOF hydrophobicity and thus their hydrolytic stability. , In addition to hydrophobicity, steric factors may also reduce the rate of hydrolysis. This includes the occurrence of well-protected metals with high coordination numbers, such as Zr-based UiO-66 and MIL-140, numerous porous coordination networks (PCNs), − Hf-based DUTs, phosphonate-based MOFs, − and others . Catenation, i.e., interpenetration of two or more identical and independent frameworks, is another steric factor that improves water stability by locking labile ligands in place. − The higher water-stability of Zn­(BDC)­(BPY) 0.5 or MOF-508 (BDY = 4,4′-bipyridine) with catenation, compared to Zn­(BDC)­(DABCO) 0.5 or DMOF (DABCO: 1,4-diazabicyclo[2.2.2]­octane) without catenation, offers an illustrative example .…”

Kinetically Controlled Reticular Assembly of a Chemically Stable Mesoporous Ni(II)-Pyrazolate Metal–Organic Framework

“…As a family of porous crystalline materials assembled by organic and inorganic building units, metal–organic frameworks (MOFs) have achieved tremendous advances in chemistry and material science due to their structural and functional diversity. − Essentially, the modular nature of MOFs endows them with tailorable composition, adjustable architecture, and tunable property. − These intrinsic merits enable MOFs to perform superiorly over traditional porous materials in various applications, such as gas storage and separation, , chemical sensing, − and heterogeneous catalysis. , Although plenty of MOFs have been synthesized and utilized, the inherent framework instability and small pore size have limited their further involvement in many critical applications under harsh conditions. − …”