Synthesis And Application Of The Core-shell Particles With ZIF-8 Shells

Emerging as one of the most common structures of composite materials, core-shell structured architecture usually consists of an inner guest core encapsulated inside another of a different shell. Due to the shell coating, the core-shell architecture shows distinctive properties of different materials employed together, so it has attracted increasing interest for use in a variety of applications. The key in core-shell architecture synthesis is to choose the shell material, as it provides the possibility to manipulate the final properties of the core-shell architecture. On the other hand, ZIF-8 material, one of the most representative ZIF materials with a zeolite sod topology, has extensively been applied in catalysis, gas storage and gas separation fields due to its uniform pore structure, large surface area and pore volume, high chemical and thermal stability. Thus, we have designed and synthesized a series of ZIF-8 coated core-shell particles with unique properties and explored their use in proper fields. The main contents is as follows.(1) We prepared a magnetic Fe3O4 core-ZIF-8 shell core-shell microsphere by surface modification method, investigated the effects of some synthetic factors such as temperature, time, solid to liquid ratio and synthesis cycles on the ZIF-8 shell. With the help of external magnetic field, the magnetic Fe3O4@ZIF-8 core-shell microspheres were successfully immobilized on the inner wall of the stainless steel capillary to form a micro-reactor for Knoevenagel reaction. Results showed that the micro-reactor exhibited high activity for the reaction. Importantly, the loading and unloading of the magnetic core-shell catalyst could be easily achieved by the introduction and removal of the external magnetic field.(2) A layer of ZIF-8 shell was coated on the surface of Pd-loaded colloidal carbon sphere (CCS) to form the Pd/CCS@ZIF-8 core-shell microsphere. The CCS was first prepared by using glucose precursor under hydrothermal conditions, then Pd nanoparticles were successfully loaded onto the surface of CCS via self-reduction method. Then taking advantage of the abundant surface groups and inherently high surface negative charge density of the Pd/CCS, a layer of continuous ZIF-8 shell was directly grown on the surface of Pd/CCS without any other treatment. When employed as the catalyst for the liquid-phase hydrogenation of 1-hexene, the obtained Pd/CCS@ZIF-8 core-shell catalyst demonstrated excellent aggregation-resistant and leach-proof properties for the supported Pd nanoparticles which was due to the immobilization and protection of the ZIF-8 shell. Thus the core-shell catalyst could be reused for 6 times with negligible loss in catalytic activity.(3) Pd/MSS@ZIF-8 core-shell microsphere with a continuous and dense ZIF-8 shell was synthesized by application of a combination of surface modification method and two-stage varying temperature synthesis method. The surface modification procedure improved the affinity and adhesion between Pd/MSS microsphere and ZIF-8 shell and a two-stage varying temperature synthesis route effectively reduced the defects in the shell. In the liquid-phase hydrogenation test, the synthesized Pd/MSS@ZIF-8 core-shell catalyst exhibited size-selectivity, poisoning-resistant, Pd leaching-proof properties and good reusability.(4) We designed and synthesized a novel sandwich structured Fe3O4@Pd/SiO2@ZIF-8 core/shell/shell microsphere. The obtained Fe3O4@Pd/SiO2@ZIF-8 core/shell/shell microsphere gathered the superparamagnetic behavior of Fe3O4 core, catalytic activity of supported Pd nanoparticles and the protective and size-selectivity properties of the ZIF-8 shell. Therefore, when employed in the liquid-phase hydrogenation reactions, the Fe3O4@Pd/SiO2@ZIF-8 core/shell/shell catalyst exhibited several functions including magnetic recovery, size selectivity, poisoning resistance, Pd aggregation and leaching resistance, and good reusability.(5) Considering that all the supports used in the above experiments were spherical, we explored the possibility of growing ZIF-8 shell over the non-spherical support, ZSM-5, by using the surface modification method and synthesized a Pd/ZSM-5@ZIF-8 core-shell catalyst. Uniform-sized and monodispersed ZSM-5 particles with benzenoid morphology were prepared from organic templates-free gel by using seed-induced method. After Pd loading and treatment by PDDA/PSS solution, the ZSM-5 particles were coated by ZIF-8 shells to form the Pd/ZSM-5@ZIF-8 core-shell catalyst with size-selectivity property and good reusability.