A team led by Ulf Hanefeld at Delft University of Technology, the Netherlands, made a series mesoporous silicates containing different ratios of aluminium and zirconium metals. They found that in Brønsted acid catalysed reactions, the catalytic activity of the mixed metal silicates was higher than that of the aluminium-only silicate, which contains only Brønsted acid sites. Hanefeld claims that because the overall metal concentration is the same in all the catalysts and the overall acidity is almost identical to the aluminium-only silicate, this is proof of synergy between the aluminium-induced Brønsted acid sites and the zirconium-induced Lewis acid sites.
When Hanefeld tested the silicates in a Lewis acid catalysed reaction, he found that the activity increased with increasing zirconium concentration and was independent of the aluminium concentration. Hanefeld says that this demonstrates that synergy between the Brønsted and Lewis acid sites does not exist in Lewis acid catalysed reactions.
'For a long time there has been speculation about synergy between the different types of catalysts,' says Hanefeld. 'Now that we know that synergy exists and under which conditions, we can make use of it. Hopefully this will help to develop better catalysts, enabling a cleaner and greener chemistry.'
Original article: Selvedin Telalovi, Jeck Fei Ng, Rajamanickam Maheswari, Anand Ramanathan, Gaik Khuan Chuah and Ulf Hanefeld; "Synergy between Brønsted acid sites and Lewis acid sites"; Chem. Commun. 2008.