Syntheses of Exceptionally Stable Aluminum(III) Metal-Organic Frameworks: How to Grow High-Quality, Large, Single Crystals

The difficulty of obtaining large single crystals of aluminum carboxylate metal-organic frameworks (MOFs) for structure determinations has limited the development of these water and thermally stable MOFs. Herein, how large single crystals of known MIL-53(Al) and the first two tetrahedral ligand-based, visible-light-absorbing 3D Al-MOFs, [Al-3(OH)(3)(HTCS)(2)] (AlTCS-1) and [Al5O2(OH)(3)(TCS)(2)(H2O)(2)] (AlTCS-2; TCS=tetrakis(4-oxycarbonylphenyl)silane), are obtained in the presence of hydrofluoric or formic acid for conventional single-crystal diffraction measurements is presented. The technique of obtaining those single crystals has potential to be a general method for obtaining large and good-quality single crystals of Al-MOFs. AlTCS-1 and -2 are stable over a wide pH range (1-11), and AlTCS-1 is even stable in aqua regia solution for at least 24h. The BET specific surface areas of AlTCS-1 and -2 are 11 and 1506m(2)g(-1), respectively. AlTCS-2 takes up 51cm(3)(STP)g(-1) CO2 and 15cm(3)(STP)g(-1) CH4 at 298K and 1bar, which is relatively high among MOF materials. AlTCS-1 takes up 30cm(3)g(-1) CO2 and 4.2cm(3)g(-1) CH4 at 298K and 1bar. The rapid and stable photocurrent responses of AlTCS-1 and -2 under UV and visible-light illumination are observed. Moreover, AlTCS-1 photocatalyzes the water-splitting reaction under visible light with an average hydrogen evolution efficiency of 50molg(-1)h(-1) for the first 10h in a mixture of water and triethanolamine.