Anion Separation with Metal–Organic Frameworks

Abstract

The application of metal-organic frameworks (MOFs) to anion separations with a special emphasis on anion selectivity is reviewed. The coordination frameworks are classified on the basis of the main interactions to the included anion, from weak and non-specific van der Waals forces to more specific interactions such as coordination to Lewis acid metal centers or hydrogen bonding. The importance of anion solvation phenomena to the observed anion selectivities is highlighted, and strategies for reversing the Hofmeister bias that favors large, less hydrophilic anions, and for obtaining peak

Pre-organisation or a hydrogen bonding mismatch: silver(I) diamide ligand coordination polymers versus discrete metallo-macrocyclic assemblies

“…Strategies to selectively complex anions require the synthesis of hosts utilising one or more of a toolbox of interactions, 11 including hydrogen bonding donor groups, 10,12 π-acidic heteroarene scaffolds, [14][15][16][17] cationic groups for electrostatic interactions 11,18 and Lewis acidic moieties. 11 The investigation of metallo-supramolecular assemblies 5,[19][20][21][22] and coordination polymers [23][24][25] as anion receptors and for anion separation or sequestration has also received attention. In the context of developing such systems for anion inclusion, the use of a metallo-supramolecular species to bind anions can provide a number of advantages over neutral organic receptors.…”

A theoretical study of the neutral and the double-charged cation of cyclo[8]pyrrole and its interaction with inorganic anions

“…The main conclusion was that 1 is able to extract the sulfate anion, SO 4 2-, in the presence of a large excess of the nitrate anion, NO 3 -, from a water solution overcoming the Hofmeister bias [5,6].…”

Selective anion exchange with nanogated isoreticular positive metal-organic frameworks

“…M etal-organic frameworks (MOFs) have been recognized as an excellent platform for host-guest chemistry, which includes gas storage [1][2][3][4][5] , ion-exchange [6][7][8][9][10] , separation [11][12][13][14][15][16] , and so on [17][18][19][20][21][22][23] . The pore size of MOFs varies from a few angstroms to a few nanometres, making them ideal for incorporating species ranging from small gas molecules [24][25][26] and organic molecules 27,28 to large inorganic and organic species [29][30][31][32] .…”

“…Although ion-exchange of small or bulky guest species into charged frameworks has been observed [6][7][8][9][10]31 , most of the studies were focused on the capture of specific metal ions 6,7 or the enhancement of gas-sorption capability 8 . There are also examples of liquid chromatography for organic dyes using Zn MOFs with neutral framework [33][34][35] , which are less likely to involve ion-exchange.…”

“…Among reported MOFs, there are comparatively few permanently porous MOFs with positive framework (P-MOFs) 6,36,37 , whose rarity is also mirrored in other types of porous materials, including zeolites [38][39][40][41][42][43][44][45] . In addition, known P-MOFs often have low stability or small pore size that only permits the exchange of small inorganic anions 7,10 . Therefore, the construction of P-MOFs with tunable open architecture that will allow for the exchange and separation of bulky anions is highly desirable.…”