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

Abstract

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, [Al3 (OH)3 (HTCS)2 ] (AlTCS-1) and [Al5 O2 (OH)3 (TCS)2 (H2 O)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 24 h. The BET specific surface areas of AlTCS-1 and -2 are 11 and 1506 m2 g-1 , respectively. AlTCS-2 takes up 51 cm3 (STP) g-1 CO2 and 15 cm3 (STP) g-1 CH4 at 298 K and 1 bar, which is relatively high among MOF materials. AlTCS-1 takes up 30 cm3 g-1 CO2 and 4.2 cm3 g-1 CH4 at 298 K and 1 bar. 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 50 μmol g-1 h-1 for the first 10 h in a mixture of water and triethanolamine.