Dual-functionalized ZIF-۸ with increased active metal sites as efficient catalyst for chemical hydrogen generation

Metal-organic frameworks (MOFs) constructed by metal ions/clusters and organic linkers have been considered as an exquisite category of porous materials with prominent and significant catalytic capabilities. Also, zeolitic imidazolate frameworks (ZIFs) are a class of MOFs that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions (e.g. Fe, Co, Zn) connected by imidazolate linkers. Unfortunately, the catalytic active sites in many MOFs are not accessible. Two approaches have been applied for dominance on major disadvantages of MOFs and their application as a highly versatile platform for catalysis: ۱) Pore engineering to achieve suitable catalytic active MOFs and modulating their catalytic comportments. ۲) Producing coordinatively unsaturated metal active sites by removal of the coordinated solvent molecules and/or controlled thermal deligandation processes to create the Lewis acid centers. In this study, the introduction of copper (II) to ZIF-۸ structure, referred to as Cu/Zn-ZIF-۸, was successfully achieved through de novo synthesis. Subsequently, post-modification of framework was employed to obtain quasi-Cu/Zn-ZIF-۸ (Q-Cu/Zn-ZIF-۸) containing high density of dual active metal sites as Lewis acid centers with maintaining the structural and thermal stability of the parent ZIF-۸. For this purpose, the synthesized Cu/Zn-ZIF-۸ was subjected to ۱۸۰°C for ۵ min under an air atmosphere to remove a fraction of linkers. These compounds were employed for hydrogen generation by catalytic hydrolysis of NaBH۴. Among them, the Q-Cu/Zn-ZIF-۸ shows the highest turnover frequency at room temperature in the absence of any accelerator such as base. The unsaturated dual copper and zinc active sites with simultaneous presence of two types of micro- and meso-pores in Q-Cu/Zn-ZIF-۸ are responsible for this excellent catalytic performance. The different efficient factors such as various amounts of catalyst, concentration of NaBH۴ and reaction time as well as kinetics of hydrogen generation rate including activation energy and thermodynamic parameters of enthalpy and entropy were also investigated.

کلیدواژه ها:

Dual active metal sites ، Post modification ، De novo synthesis ، ZIF-۸