Preparation And Growth Mechanism Of Metal Organic Frameworks From Copper-based Nanomaterials

As new kinds of porous materials, metal-organic frameworks (MOFs) have attracted huge attention due to their high surface areas, tunable hole sizes and shapes, various chemical modified structures and diverse functionalities. MOFs are promising for gas adsorption and storage, gas separation, chemical sensors, catalysts and drug delivers, etc. Particularly, MOF films prepared on silicon, porous alumina or polymer substrates not only present an efficient and economical way to adapt to the industrial production, but also expand their applications. But so far, the researches about MOF films are still in the development and there are many challenges for the preparation of continuous MOF films, such as poor compactness, high energy consumption, long reaction time and toxicity of organic solvents, which restricted the performance improvement and the possibility of mass production. Besides, since most MOFs are generated and segregated from solutions, the nucleation and crystallization process must have significant influence on the crystal morphology, size and the quality of thin films. However, there are few studies about this process. A better understanding of the crystallization mechanism will undoubtedly help to control the crystal morphology for desired applications. The researches on the effects of additives in the preparation process of MOFs crystals and thin films are still at an early stage, so it is desirable to explore their functions.To solve the problems above, we used positive charged copper-based nanomaterials to prepare MOFs crystals and films and studied the growth mechanism. The main innovative results are as follows:1. Tubular MOP-18 crystals have been successfully prepared by cooling-induced crystallization from copper hydroxide nanostrands and 5-dodecyloxybenzene-1,3-dicarboxylic acid (5-OC12H25-mBDCH2) N,N-dimethylformamide (DMF)-ammonia solution. The dependence of the crystal morphologies on the cooling rate was investigated in detail. A slower cooling rate results in larger and longer MOP-18 tubes. The ammonia in the DMF solution plays a crucial role in the formation of MOP-18 tubes. It deprotonates the acid ligand to trigger the nucleation of MOP-18 and leads the growth of tubular crystals. A "diffusion driven instability" growth mechanism is proposed.2. Using copper hydroxide nanostrands films as copper source, we have prepared continuous MOP-18 membranes through solution reaction at room temperature for the first time. The optimized mole ratio of ammonia and the organic ligand 5-OC12H25-mBDCH2 is 1:2 and the optimum ligand concentration is around 22 mmol/L.3. Rapid preparation of HKUST-1 powders at room temperature has been achieved by using copper oxide nanoparticles as copper source and benzenetricarboxylic acid (H3BTC) water/ethanol mixture as solvent (vol. ethanol:water=1:1) through stirring.4. We prepared well intergrown HKUST-1 membranes at room temperature by immersing copper oxide nanoparticles films into H3BTC water/ethanol mixture where sodium formate was added. The type of solvent was studied and we found water and ethanol mixture was the optimal, compared to DMF and water. Sodium formate as an additive not only deprotonates the acid ligand and completes the reaction to form pure HKUST-1 membranes, but also improves the intergrowth among crystals to make the membranes more dense. More sodium formate, less reaction time. The optimized H3BTC concentration is 5-10 mmol/L. The ideal separation factors of the HKUST-1 membrane prepared under optimized conditions for H2/CH4, H2/N2 and H2/CO2 are 2.90,3.61 and 4.24, respectively, which approaches the corresponding Knudsen selectivity.