Microwave Synthesis of a Porous Metal-Organic Framework, Zinc Terephthalate MOF-5

Abstract

Organic-inorganic porous hybrid materials are currently of great interest and importance for the novel coordination structure, relatively facile preparation, special properties and potential practical applications. Recent studies have been focused on the design and synthesis of porous materials which possess a very low density, high surface area and specific functionality. Among these materials, metal organic frameworks (MOFs), originally coined and intensively studied by Yaghi et al. are crystalline and well-defined structures, consisting of metal ions or oxo-bridged metal clusters linked by organic carboxylates. In particular, a series of MOF-5s have open metal frameworks with high Langmuir surface areas (1000-4500 m/g) and thermal stability. These porous “organic” zeolites are mimicking the frame and properties of the zeolite, and thus many potential industrial applications arise from the promising properties of these high-surface materials. The structural architecture of MOFs can be extended to the real zeotype frameworks by Ferey et al. who showed the MTN type structure from chromium oxo cluster ion and terephthalate, very recently. In general, MOFs can be synthesized by a slow diffusion method diffusion of amine into a solution containing metal salt and organic carboxylic acid for weeks or a solvothermal reaction technique for hours. Recent literature, however, revealed that it is not easy to prepare MOF-5 with a high surface area by both methods. Resulting MOF nanocrystalline materials and space group Fm-3m like MOF-5 materials had Langmuir surface areas of 727 and 572 m/g, respectively, but these figures are far below the reported value of 2900 m/g by Yaghi. Furthermore, both groups noticed that their MOFs slowly tend to decompose in moisture and acid, thus this instability may limit their practical applications. On the other hand, a microwave irradiation method has been studied in the syntheses of not only organic molecules but also inorganic materials. More recently, microwaveassisted synthesis of inorganic materials requiring several days for their crystallization has attracted much attention. This technique provides synthesis methods of porous materials with saving the reaction time within a few minutes and offering phase selectivity and facile morphology control. Very recently, some of us recently reported that microwave technique can be well applied to the rapid synthesis as well as phase selective crystallization of porous hybrid solids including MIL-77, MIL-100 and MIL-101. Here we describe the first result on the microwave synthesis of MOF-5, Zn4(O)(BDC)3·guest molecules (BDC = 1,4benzenedicarboxylate) with a high surface area, as named MW MOF-5 (Microwave MOF-5). MW MOF-5 was synthesized in N,N'-diethylformamide (DEF) using microwave irradiation. Equimolar amount of H2BDC (1.57 mg, 0.60 × 10−2 mmol) and zinc nitrate tetrahydrate (1.50 mg, 0.60 × 10−2mmol) in a DEF (0.6 mL) were place in a 10 mL tube. The tube was sealed with a robber septum and placed in a microwave oven (Discover, CEM, maximum power of 300 W). The resulting mixture was heated at 95 C, held for 9 min and then cooled to room temperature. The colorless crystalline materials (2.10 mg, 27% yield) were obtained by centrifuging, washing with N,N'-dimethylformamide (DMF, 3 × 5 mL) and drying briefly in the air. The morphology of MW MOF-5 was studied by scanning electron microscopy (SEM) after gold deposition. The SEM image shows three dimensional cube-like microcrystals with the length of 2-4 mm as illustrated in Figure 1. Its morphology was consistent with the previous result which was obtained from slow diffusion with addition of H2O2, even though SEM image of MOF-5 produced by H2O2 is not clear.