Synthesis and characterization of Al@MOF materials

The potential impregnation of Al(0) nanoparticles in the pores of three different metal-organic frameworks (MOFs), MIL-53-Al, HKUST-1, and UiO-67, was investigated through the suspension of the MOFs in AlH3·NMe2Et (1), followed by filtration, toluene wash, and heating to 150 °C under vacuum. Calculations based on the ratios of molecular and pore volumes provided idealized, benchmark impregnation capacities. Three successive impregnation cycles were performed to provide maximum incorporation of Al in the pores, and the materials were characterized after each impregnation cycle by ICP elemental analysis, BET surface area, and pore volume measurements. For MIL-53-Al, about half of the calculated amount of Al was incorporated into the MIL-53 pore structure, and PXRD data indicated a loss of crystallinity after the third incorporation cycle. Little Al incorporation was observed with HKUST-1, and the large decrease in surface area and pore volume, without significant change in the PXRD pattern, is attributed to pore blockage. Reaction of a large excess of 1 with UiO-67 was highly exothermic and evolved gas, likely from reaction with the μ3-OH groups in the UiO-67 structure. The resulting material was amorphous apart from metallic Al(0) crystals approximately 30 nm in size and larger than the UiO-67 pores, as determined by PXRD and 27Al MAS NMR spectroscopy. This material exhibited no apparent reaction with air or water and exposure to air gave little change in the 27Al MAS NMR spectrum. The Al(0) crystals thus appear to be protected from oxidation, presumably by the remaining UiO-67 framework.