A series of zinc oxide and copper(0)
colloidal nanocatalysts, produced
by a one-pot synthesis, are shown to catalyze the hydrogenation of
carbon dioxide to methanol. The catalysts are produced by the reaction
between diethyl zinc and bis(carboxylato/phosphinato)copper(II) precursors.
The reaction leads to the formation of a precatalyst solution, characterized
using various spectroscopic (NMR, UV–vis spectroscopy) and
X-ray diffraction/absorption (powder XRD, EXAFS, XANES) techniques.
The combined characterization methods indicate that the precatalyst
solution contains copper(0) nanoparticles and a mixture of diethyl
zinc and an ethyl zinc stearate cluster compound [Et4Zn5(stearate)6]. The catalysts are applied, at 523
K with a 50 bar total pressure of a 3:1 mixture of H2/CO2, in the solution phase, quasi-homogeneous, hydrogenation
of carbon dioxide, and they show high activities (>55 mmol/gZnOCu/h of methanol). The postreaction catalyst solution is
characterized
using a range of spectroscopies, X-ray diffraction techniques, and
transmission electron microscopy (TEM). These analyses show the formation
of a mixture of zinc oxide nanoparticles, of size 2–7 nm and
small copper nanoparticles. The catalyst composition can be easily
adjusted, and the influence of the relative loadings of ZnO/Cu, the
precursor complexes and the total catalyst concentration on the catalytic
activity are all investigated. The optimum system, comprising a 55:45
loading of ZnO/Cu, shows equivalent activity to a commercial, activated
methanol synthesis catalyst. These findings indicate that using diethyl
zinc to reduce copper precursors in situ leads to catalysts with excellent
activities for the production of methanol from carbon dioxide.