Evaluating the Robustness of Metal–Organic Frameworks for Synthetic Chemistry

Abstract

Metal–organic frameworks (MOFs) are emerging as sustainable reagents and catalysts with promising applications in synthetic chemistry. Although the hydrothermal stabilities of MOFs have been well studied, their robustness toward various reagents, including acids, bases, nucleophiles, electrophiles, oxidants, and reductants, remains poorly characterized. As such, heterogeneous platforms for promising catalysts are generally identified on an ad hoc basis and have largely been limited to carboxylate frameworks to date. To address these limitations, here we systematically characterize the robustness of 17 representative carboxylate, salicylate, and azolate MOFs toward 30 conditions representing the scope of synthetic organic chemistry. Specifically, analysis of the full width at half-maximum of powder X-ray diffraction patterns, as well as infrared spectroscopy, 77 K N2 adsorption measurements, and scanning electron microscopy in select cases are employed to appraise framework degradation and dissolution under a range of representative conditions. Our studies demonstrate that azolate MOFs, such as Fe2(bdp)3 (bdp2– = 4,4′-(1,4-phenylene)­bis­(pyrazolate)), generally possess excellent chemical stabilities under myriad conditions. In addition, we find that carboxylate and salicylate frameworks possess complementary stabilities, with carboxylate MOFs possessing superior robustness toward acids, electrophiles, and oxidants, and salicylate MOFs demonstrating improved robustness toward bases, nucleophiles, and reductants. The guidelines provided herein should facilitate the rational design of robust frameworks for applications in synthetic chemistry and guide the development of new strategies for the postsynthetic modification of MOFs as well.

Using Machine Learning and Data Mining to Leverage Community Knowledge for the Engineering of Stable Metal–Organic Frameworks

“…4,[7][8][9][10][11][12] Despite challenges in MOF synthesis [13][14] and post-synthetic modification 15 , intense synthesis effort has led to rapid growth 16 in the number of reports of experimentally realized MOFs. MOFs with varying pore size, metals, and linker chemistry have demonstrated desirable properties for storage [17][18] , separations 7,19 , sensing [20][21] , conductivity [22][23][24] , and catalysis [25][26][27][28][29] . Most MOFs are formed with solvent present in their pores that must be removed for such applications.…”

“…Metal–organic frameworks (MOFs) have the potential to revolutionize catalysis and functional materials design due to their reticular nature and well-defined, isolated metal sites. , To meet numerous design criteria, an exhaustive search of MOFs must be carried out for each potential application, beyond the tens of MOFs that an experimentalist could synthesize. − The modularity of inorganic secondary building units (SBUs) and linkers that are the building blocks of MOFs have motivated the search for new materials via virtual high throughput screening (VHTS). ,− Despite challenges in MOF synthesis , and postsynthetic modification, intense synthesis effort has led to rapid growth in the number of reports of experimentally realized MOFs. MOFs with varying pore size, metals, and linker chemistry have demonstrated desirable properties for storage, , separations, , sensing, , conductivity, − and catalysis. − Most MOFs are formed with solvent present in their pores that must be removed for such applications. Although new methods for solvent removal have been developed to activate these materials, , many crystalline MOFs collapse upon activation, ,, rendering them unusable …”

Carbon Dioxide Capture at Nucleophilic Hydroxide Sites in Oxidation-Resistant Cyclodextrin-Based Metal-Organic Frameworks

“…35,36 In contrast to the wide study of amine-functionalized porous solids for CO2 capture, hydroxide-and alkoxide-functionalized porous solids remain remarkably understudied. 32 This may be due in part to the generally poor stability of traditional solid supports such as mesoporous silicas 37 and MOFs 38,39 towards OH − . Nonetheless, previous studies have shown that robust azolate MOFs bearing terminal M−OH (M = Zn, Ni, Co) sites can undergo selective and reversible reaction with CO2 to form metal-bound bicarbonates, akin to the enzyme carbonic anhydrase.…”

Ultrastable High-Connected Chromium Metal–Organic Frameworks

“…After nearly two decades since the first report of Cr-MOF, there are still few Cr-MOFs. Furthermore, known low-connected Cr-MOFs have limited stability, especially in basic solution (stable only at pH ≤ 12). ,, …”

“…Furthermore, known low-connected Cr-MOFs have limited stability, especially in basic solution (stable only at pH ≤ 12). 29,31,32 To make new Cr-MOFs with superior stability, going beyond currently known 6-connected Cr 3 O frameworks is a worthy pursuit. High-connected Cr-MOFs are expected to be difficult to crystallize, given what we know about lowconnected Cr-MOFs.…”