Laponite-derived porous clay heterostructures: III. The effect of alumination

Abstract

Al-containing porous clay heterostructures (PCH) were synthesised from Laponite by post-synthesis alumination using aluminium isopropoxide and aluminium nitrate as Al sources (Si/Al=30 and 10). The solids were characterized with 27Al, 29Si MAS NMR, FTIR, nitrogen adsorption/desorption at −196°C, NH3 TPD-MS, pyridine adsorption monitored by FTIR and catalytic decomposition of ethanol. Alumination resulted in substitution of Al for Si in the mesoporous silica component of PCH and formation of some amount of extra lattice Al species. A more uniform distribution of Al sites was obtained with the use of the inorganic Al source. Alumination caused a decrease of textural parameters, more pronounced upon impregnation with Al-isopropoxide. The parent PCH-L possessed little capacity for sorption of basic molecules. Alumination enhanced the samples acidity. Pyridine adsorption showed that the acidity, especially at elevated temperatures, was primarily determined by the presence of Lewis acid sites. Brønsted acid centers were less abundant and rather weak. Increase of the Al loading resulted in a higher number of Brønsted and Lewis acid sites. For a similar level of doping, more acid sites of both types were created with the use of Al nitrate. Significant concentration of pyridine attached via hydrogen bonding to weakly acidic hydroxyl groups was observed on all samples. The study of ethanol decomposition showed that catalytically meaningful acidic properties were associated with the presence of Brønsted and Lewis acid sites generated upon alumination. Weakly acidic hydroxyl groups did not participate in the reaction.