Zeolitic imidazolate frameworks (ZIFs) and their derivatives: synthesis and energy related applications

Abstract

Zeolitic imidazolate frameworks (ZIFs) represent a new and special class of metal organic frameworks comprised of imidazolate linkers and metal ions, with structures similar to conventional aluminosilicate zeolites. Their intrinsic porous characteristics, abundant functionalities as well as exceptional thermal and chemical stabilities, have led to a wide range of potential applications for various ZIF materials. Explosive research activities ranging from synthesis approaches to attractive applications of ZIFs have emerged in this rapidly developing field in the past 5 years. Moreover, ZIF materials act as outstanding templates or precursors to produce porous carbons and related nanostructured functional materials based on their high surface area, controllable structures and rich metal/organic species in their scaffolds. In this thesis, the synthesis and applications of ZIFs and their related nanostructured functional derivatives are demonstrated with special emphases on the applications in energy storage and conversion areas. In brief, this thesis presents the following research findings: 1). A cost-effective and facile preparation method for the synthesis of zeolitic imidazolate framework-8 (ZIF-8) material has been developed. ZIF-8 can be obtained from stoichiometric precursors in aqueous ammonia solution without any other additives at room temperature. The structures, particle sizes and textural properties of the resulting ZIF-8 materials can be controlled by the concentration of aqueous ammonia. In addition, the formation of ZIF-8 can be remarkably affected by different types of anions. The anion effect capacity is revealed to be SO42- > CH3COO- > Cl- > Br- > NO3-, which follows the classic Hofmeister anion sequence. 2). Composites containing ZIF-8 and various contents of graphene oxide (GO) have been successfully prepared for the first time by using an in-situ method in aqueous ammonia solution. Different material characterization techniques confirm the formation of strong interactions between ZIF-8 and GO in the resulting composites. The crystal sizes and the textural properties of the synthesized composites can be modulated by control of the added amount of GO. Interestingly, the in-situ synthesized composites exhibit enhanced CO2 adsorption energy and significant CO2 storage capacity. A mechanism has been proposed accordingly to the strong interactions and the synergistic effect between ZIF-8 and GO. 3). Apart from the investigation of ZIFs themselves, exploration of ZIF derivatives has been carried out in this thesis. Homogeneously-dispersed ZnO nano-particles embedded within N-doped porous carbon matrix using ZIF-8 as a template and precursor have been successfully synthesized by a simple one-step water steam carbonization route. The as-synthesized ZnO/C-S-S and ZnO/C-S-L exhibit high CO2 uptake capacities, CO2 selectivity, and CO2 adsorption energy. The mechanism of enhanced CO2 adsorption energy has been proposed and discussed. Moreover, because of excellent adsorption and degradation abilities, these ZnO/C-S-S and ZnO/C-S-L composites are highly efficient for methylene blue (MB) removal from wastewater under visible-light irradiation. Kinetics studies of MB removal show that the adsorption process is dominated by a pseudo-second-order adsorption model. 4). In addition, ZIF-67 derivatives ranging from cobalt-embedded porous N-doped carbon/carbon nanotubes to hollow carbon nano-onions under different carbonization temperatures have been prepared. The carbonization temperature effects on the morphology and electrocatalytic properties of resultants are well studied. An optimum carbonization temperature at 800 oC for electrocatalytic performance was proposed within the range 600-2000 oC. Due to the hierarchical porous carbon structure, N-doping effect and the homogeneous cobalt dispersion, the as-synthesized sample Co@C-800 nanocomposite exhibits excellent catalytic activities for both Oxygen reduction reaction (ORR) and Oxygen evolution reaction (OER) with good stability. 5). Further development of the other ZIF-67 derivatives has been investigated. Homogeneously dispersed cobalt sulfide/N, S co-doped porous carbon nanocomposites and nickel promoted cobalt sulfides/N, S co-doped porous carbon have been successfully prepared by a simple and efficient method via the simultaneous carbonization and sulfurization using ZIF-67 and Ni-substituted ZIF-67 as a precursor and template, respectively. Due to the hierarchical porous carbon structure, N, S co-doping effect and the homogeneous nanoparticle dispersion, the as-synthesized nanocomposites exhibit excellent catalytic activities for both ORR and Oxygen evolution reaction OER with good stability.