Copper(II) immobilized on silica extracted from foxtail millet husk: a heterogeneous catalyst for the oxidation of tertiary amines under ambient conditions

Abstract

Foxtail millet husk has been used as a new source of silica for preparing copper catalysts via a sol–gel technique. X-Ray diffraction shows that silica is amorphous in the ash and catalyst I while in catalyst II it crystallizes to one of its polymorphs, β-cristobalite. Scanning electron micrographs show the particles to be well dispersed in the case of ash, but irregular and agglomerated in catalyst I and II. Transmission electron microscopy confirmed that catalyst II is a nano-composite. X-Ray fluorescence detected that the ash contains 98 % silica. The basic unit, SiO4 tethedra present in the prepared materials was discovered using IR spectroscopy. ESR observed the presence of Cu(II) in both catalyst. X-Ray photoelectron spectroscopy asserted the presence of only Cu(II) in catalyst I and both Cu(0) and Cu(II) in catalyst II. The prepared materials were utilized as catalyst in the oxidation of some tertiary amines to N-oxides, with molecular oxygen as the oxidant in methanol, under ambient conditions of temperature and pressure. The reaction was monitored online. Catalyst I shows better reactivity than II towards the oxidation reaction. High performance liquid chromatography was used to analyse the oxidation products.