A New Paradigm for Anion Trapping in High Capacity and Selectivity: Crystal-to-Crystal Transformation of Cationic Materials

Abstract

We describe a new methodology to the selective trapping of priority pollutants that occur inherently as oxo-anions (e.g., perchlorate, chromate, arsenate, pertechnetate, etc.) or organic anions (e.g., salicylate, pharmaceuticals, and their metabolites, which are often chlorinated into potentially more harmful compounds). The typical approach to trapping anions is exchange into cationic hosts such as resins or layered double hydroxides. Both capacity and selectivity are limited by the equilibrium of the process and moreover are often subject to interference, e.g. by carbonate that is always present in water from atmospheric CO(2). Our approach takes advantage of the metastability of our cationically charged materials to instead trap by recrystallization to a new structure. Exceptionally high adsorption capacities for permanganate and perrhenate--studied as models for pertechnetate--were found for a Ag(I)-based cationic extended framework. The exchange capacity reached 292 and 602 mg/g, respectively, over five times the exchange capacity compared to conventional layered double hydroxides. Our cationic material can also selectively trap these and other toxic oxo-anions when nontoxic anions (e.g., nitrate, carbonate) were present in an over 100-fold excess concentration.

A Hexameric Cationic Copper(II) Metallacrown as a Pertechnetate and Perrhenate Scavenger

“…[17] The lesser ability of 3'·5H 2 Ot ot rap MO 4 À is probablyr elatedt ot he efficient packing of NO 3 À in the crystal, as observed in the crystal structure of its parent 3'·5H 2 O( see above). In fact, the adsorption capacity at aC u:MO 4 À ratio of 1:1i sc omparable to the best results obtained with other cationic materials.…”

“…[17] Therefore, we also investigated the ReO 4 À exchange selectivity of 1 in the presence of nitratea nd sulfate anions for 1:ReO 4 À :competing anion molarr atios of 1:1:1a nd 1:1:50 (see Supporting Information). [17] Therefore, we also investigated the ReO 4 À exchange selectivity of 1 in the presence of nitratea nd sulfate anions for 1:ReO 4 À :competing anion molarr atios of 1:1:1a nd 1:1:50 (see Supporting Information).…”

“…Other promising materials based on extended cationic networks include metal-organic frameworks (MOFs) [12] in which guest anions are susceptible to exchange. [17] Finally,m olecular-trappingc omplexes with recognition sites for anions can bind ion pairs through cooperative interactions. [17] Finally,m olecular-trappingc omplexes with recognition sites for anions can bind ion pairs through cooperative interactions.…”

“…[13][14][15][16] Some of these materials have shown ah igh capacityt oa dsorb MnO 4 À and ReO 4 À . [17] Finally,m olecular-trappingc omplexes with recognition sites for anions can bind ion pairs through cooperative interactions. [18][19][20] As in the design of MOFs,o rganic ligandsm ust in- corporate recognition sites for both the cation, usually at ransition metal,a nd the anion to be exchanged.…”

Neutral N-donor ligand based flexible metal–organic frameworks

“…This provides a simplistic route for the preparation of cationic frameworks, in case where the counter-anion is left uncoordinated to the metal centre. [13][14][15][16][17][18][19] Broadly, dynamic…”