Wet oxidation of maleic acid by copper(ii) Schiff base catalysts prepared using cashew nut shell liquid templates

Abstract

Novel heterogeneous copper(II) Schiff base catalysts have been successfully prepared using a Cashew Nut Shell Liquid (CNSL) templating agent. The preparation of catalyst supports was via a one-pot route and afforded aminopropyl-functionalized Micelle Templated Silica (MTS-AMP). The MTS-AMP support was then condensed with pyridine-2-carboxyaldehyde, 2-hydroxybenzaldehyde, 2-hydroxynaphthaldehyde to produce the corresponding Schiff bases. In each case, the Schiff base was complexed with copper(II) acetate solution at room temperature. The supports and heterogeneous catalysts were porous with pore diameters of up to 25 nm and grain sizes of up to 1.0 μm as revealed by Nitrogen Physisorption Study and Scanning Electron Microscopy (SEM) respectively. The maximum copper loading was about 3% w/w for heterogeneous catalysts supported on CNSL based supports whereas the maximum loading of about 2% w/w was obtained for copper catalysts prepared using commercially available dodecylamine and hexadecylamine templates. The catalysts were tested on the oxidation of maleic acid at room temperature using H2O2 as an oxidant and the catalysts had an excellent catalytic efficiency with a yield of up to 90% and turn over number of about 1000 in ten minutes. The performance of catalysts depended on the type of ligands, the template used to prepare the catalyst support and on the method of catalyst preparation. Catalysts prepared using CNSL templates were more efficient than those prepared using the commercially available templates. The catalysts prepared by a stepwise approach were robust and gave good results while those prepared by an imprinting approach leached out after one reaction cycle.