Incorporation of a poly (ionic liquid) into UiO-66 and UiO-67 for anion exchange applications

Abstract

Metal Organic Frameworks (MOFs) are a new generation of porous materials which have received huge attention recently because of their ultra-high surface areas and very organized pores. To further exploit the advantages of MOFs and broaden their applications, other types of materials such as polymers are combined with MOFs to create composites with desired properties. One of the recent progress in this area is the development of an in situ polymerization concept to thread polymers within the nanospaces of MOFs. To functionalize the polymer~MOF composite for anion exchange, a post-synthetic amination step is added as demonstrated in the case of polyvinylbenzyltrimethyl ammonium chloride threaded in ZIF-8. However, this step requiring high pH is unfavorable to MOFs that are unstable in alkaline medium. In this thesis, I have addressed the issue by applying an alternative facile approach to prepare an anion-exchange polymer~MOF composite. An ionic liquid monomer, 3-methyl-1-vinylimidazolium iodide (3MVIM-I), consisting of an anion-exchange function is first introduced into Zr-based MOFs (UiO-66 and UiO-67) and then in situ polymerized. The preparation of the poly (ionic liquid) (PIL) threaded within UiO-66 and UiO-67 occurs without further need of post-synthetic amination step. Physical characterization techniques including FTIR, 1H-NMR, N2 sorption, EDS, PXRD, TGA and DSC all confirm successful incorporation of the P3MVIM-I in the nanocages of the UiO MOFs. The highly porous structures of UiO-66 and UiO-67 are retained after the in situ polymerization. The BET surface areas of P3MVIM-I~UiO-66 and P3MVIM-I~UiO-67 are 889 m2g-1 and 1581 m2g-1, respectively. In the ion exchange performance tests, both P3MVIM-I~UiO-66 and P3MVIM-I~UiO-67 demonstrate excellent ion selectivity over the pristine UiO-66 and UiO-67. More than 95 % of cationic Rhodamine B (Rh-B) solutions are rejected in both aqueous and organic media by P3MVIM-I~UiO-66 while over 90 % of Acid blue 9 are exchanged under the same condition. The commercial resin rejects ~65 % of Rh-B while the pristine UiO-66 shows no ion selectivity. P3MVIM-I~UiO-67 also demonstrates outstanding ion-selectivity. More than 90 % of AuCl4- is exchanged with I- in non-aqueous solvent (DMSO) within 30 minutes while 100 % rejection of K+ and Na+ is observed. The ion selectivity of ionic dyes by P3MVIM-I~UiO-67 follows the same trend as P3MVIM-I~UiO-66. Excellent exchange of AB9 with more than 95 % rejection of Rh-B by P3MVIM-I~UiO-67 is observed in ethanolic solution of mixed AB9 and Rh-B. Faster kinetics of ion exchange by P3MVIM-I~UiO-66 and P3MVIM-I~UiO-67 is observed in aqueous environment than in non-aqueous solution. In summary, the introduction of an anion exchange polymer by in situ polymerization of a directly inserted ionic liquid through UiO-66 and UiO-67 is successful. The ion exchange performance in aqueous and non-aqueous media is more outstanding than their pristine forms or the commercial available Amberlyst A26 ion-exchange resin.