Supported Transition Metal Oxides as Solid Base Catalysts

The initial objective of the work studied here was to investigate the effects of isomorphously-substituted transition metals in lattice sites in Layered Double Hydroxides (LDHs) in terms of basicity and catalytic activity in base-catalysed reactions and oxidation reactions. Experiments in which copper(II), nickel(II), cobalt(II), zinc(II) and iron(III) were substituted in the lattice of synthetic hydrotalcites showed that, on calcination, the resultant mixed oxides arising from hydrotalcites containing copper(II) exhibited the greatest enhancement of catalytic activity. Test reactions were the conversion of 4-methylpentan-2-ol to methylisobutylketone, the oxidation of benzyl alcohol and the Henry reaction between benzaldehyde and nitromethane. Results showed conclusively that copper(II) imparted base catalytic activity to the mixed metal oxides formed on calcination of the LDHs. This was confirmed through an extension to the study in which the copper(II) content in the LDHs was varied. It was shown that there is an optimum copper(II) loading, above which additional copper(II) is not effectively incorporated in the LDH lattice.
Other supports for copper(II) as an oxide were investigated, on the basis that activity was linked to effective dispersion of copper(II) oxide. A series of copper(II) dispersions on zirconia, on silica and on magnesium oxide were prepared and tested. The zirconia-supported catalysts were the most active so work was concentrated on these. Two methods for preparing copper(II) oxide/zirconia were investigated, using a solid state reaction and using a sol-gel process. Catalysts were prepared with molar copper contents of from 2 to 60 mol%. It was possible to see from powder X-ray diffraction that copper(II) was dispersed in the zirconia matrix at levels up to about 20% depending on the synthetic method. And it was shown that base-catalytic activity of these materials correlated with the amount of fully incorporated copper(II). Activities of these were generally higher than the mixed metal oxides made from the LDHs.
For comparison, another, different, approach to incorporating basicity in oxide supports was examined, in which alkylamino groups were tethered to silica supports, specifically a mesoporous molecular sieve form of silica, SBA-15. The activity of these materials were compared with those of the copper(II) based catalysts in the Henry reaction between nitromethane and benzaldehyde. An advantage of these catalysts was exploited by coordinating palladium(II) to the amine groups at different levels to impart oxidation catalytic activity. The resultant materials, in which only part of the amine functionality was coordinated by palladium(II), were tested as bifunctional catalysts, for combined oxidation and base catalytic activity, in the two step reaction between benzyl alcohol and nitromethane, which proceeds via an oxidation step to benzaldehyde followed by a base-catalysed step.
The overall conclusion was that copper(II) is effective at imparting base catalytic activity to a range of support oxides, and that it does not have significant activity towards oxidation reactions., contrary to some claims in the literature. Oxidation
activity can be obtained alongside base activity by preparing bifunctional catalysts but only through a different approach.