Proton conductivity of ordered mesoporous materials containing aluminium

In order to check the influence of Brønsted acidic Al sites in the walls of mesoporous materials on their proton conductivity, we introduced aluminium into the mesoporous SiO2 framework Si-MCM-41, which possesses hexagonally ordered channels, by using different Al sources, i.e. sodium aluminate, aluminium sulphate and aluminium isopropoxide. The successful synthesis of ordered mesoporous material with incorporated Al species was proven by X-ray diffraction, energy-dispersive X-ray spectroscopy, nitrogen adsorption, scanning electron microscopy and solid state 27Al MAS NMR. By realizing Si/Al ratios of 8–40 it was found that the proton conductivity of the mesoporous aluminium silicates measured by impedance spectroscopy significantly increases with the Al content, reaching a conductivity of 3 × 10−3 S cm−1 at 140 °C. The increase of the proton conductivities compared to Al-free Si-MCM-41 results from (i) an improved hydrophilicity enhancing the water storage capability, (ii) a decreased particle size from the micrometer to the nanometer scale (50–100 nm) and (iii) the existing Brønsted acidity in the mesoporous Al-MCM-41. The Al source NaAlO2 gives clearly the best results because the entire Al incorporated within the framework is tetragonally coordinated, while for samples prepared with Al2(SO4)3 or AIP also octahedral coordination of oxygen around the Al centers is observed by 27Al MAS NMR.