Functionalized metal-organic frameworks for the applications of ion-exchange and hydrogen storage

Abstract

As a new class of nanoporous materials, metal-organic frameworks (MOFs) exhibit high degree of porosity and ultra-high surface area. The architecture and functionality of MOFs are more diverse than that of other classes of porous materials. Apart from the high surface area, high porosity, and extensive multiplicity, MOF has combined the advantages of a ceramic matrix having a regular pore structure and those of a polymer with flexibility. Integrating functional groups into MOFs has been intensively studied to extend the applications of MOFs. Among the various approaches to functionalize MOFs, (i) inclusion of functional species and (ii) post synthesis modifications (PSMs) are the two that are general, effective and straightforward. In this thesis, these two strategies are adopted to functionalize MOFs. Basically, the study in this thesis can fall into two parts: (i) threading polyelectrolyte inside MOF for ion exchange study and (ii) covalently modifying MOFs via PSMs to mediate dehydrogenation from ammonia borane. The motivations behind these study and general conclusions are given as follows:

Commercialized ion-exchange resins have many shortcomings such as irregular pore structure in polymeric resin with ion-exchange sites inaccessible to ions. The key to address these issues is to expose ion-exchange sites and thus improve contacting efficiency with ionic guest species. Exploiting the advantages of the porous structure of MOFs, we thread non-cross-linked polyelectrolytes in MOFs in steps of monomer impregnation and in-situ polymerization. The polyelectrolyte threaded in MOF has a larger specific volume compared to its bulk state and possesses advantageous properties. The fixed charges of the polyelectrolyte are exposed for full interaction with solvated ions and solvent without the need of swelling or restructuring the porous framework. As two concrete examples, ZIF-8 and MIL-101 are used to cage anionic and cationic polyelectrolyte, respectively. Polyvinyl benzyl trimethylammonium hydroxide (PVBTAH) caged in ZIF-8 is synthesized in steps of impregnating the chloro-monomer, in-situ polymerization, amination, and alkaline ion-exchange. The synthesized PVBTAH~ZIF-8 material exhibits superior nitrate and gold cyanide anions exchange kinetics compared to conventional ion-exchange resin. Sodium poly(4-styrene sulfonate) threaded in MIL-101 (NaPSS~MIL-101) is synthesized directly with polymerization in situ inside MOF. NaPSS~MIL-101 exhibits high ion exchange capacity, superior exchange kinetics, high selectivity with co-ion rejection, reversibility, and durability.

Along the lines of the “polyelectrolyte threaded in MOF” concept, we fabricate a new class of MOF-based membrane where the polyelectrolyte is encalpuslated inside the caviteis of the MOF layer. The motivation behind this lies in the ordered pore structure and evenly charged nanospace of polyelectrolyte~MOFs materials. These features may offer ion selectivity based on charge and size. Herein, a MOF-based cation-exchange membrane, poly(sodium vinyl sulfonate-co-acrylic acid)~MIL-53 (poly(VS-co-AA)~MIL-53)/AAO membrane is synthesized. Various characterizations are conducted to investigate the structure and composition of poly(VS-co-AA)~MIL-53)/AAO. It is demonstrated that the structure features , including inherent porosity of MIL-53 and charged groups of poly(VS-co-AA), synergetically contribute to the size-selective caion-exchange properties.

As an effort to explore application of the postsynthesis functionalization strategies, different functionalities of MOFs are employed to improve hydrogen release from ammonia borane. Confinement and additive effects have been widely utilized to improve the hydrogen release from ammonia borane. However, there is rarely a case of evaluating both effects on ammonia borane thermolysis in one single system. We conceive that these two effects can be readily integrated into MOFs because various group can be attached on the open skeleton of MOFs via PSMs. It is observed that amino and amide groups on MIL-101(Cr) can interact with ammonia borane to enable hydrogen release at a reduced temperature with higher purity than pristine MIL-101(Cr). In addition, the effective amount of hydrogen released significantly increases. A series of characterizations and control experiments indicate that the improved hydrogen release is attributed to the collabrative roles of functional groups and nanoconfinement of the functionalized MIL-101(Cr).