1H and 13C nuclear magnetic resonance investigations of the Cu/Zn/Al oxide methanol-synthesis catalyst

Abstract

¹ H Nuclear magnetic resonance measurements are reported for the methanol-synthesis heterogeneous catalyst system based on Cu / Zn / Al-oxides. Spectra of the hydrogen-reduced catalyst at room temperature show two peaks, one of which is narrow and shifted to high frequency by 85 ppm. This is assigned to hydrogen dissociatively adsorbed onto the copper metal surface, the interaction of the protons with the metal conduction electrons giving a Knight shift. The shift is independent of hydrogen pressure up to 50 Torr, ¶ and the isotherm obtained from the intensity of the Knight-shifted peak as a function of hydrogen pressure is consistent with the assignment. The amount of copper metal surface available for hydrogen chemisorption, as detected by ¹H n.m.r. spectroscopy, is very vairbale.

The second peak in the ¹H n.m.r. spectra of the reduced catalyst occurs close to the normal resonance position of protons in diamagnetic systems. Partially T₁-relaxed ¹H spectra show this peak to be a composite, with up to three components distinguishable on the basis of thier spin–lattice relaxation and linewidth characteristics. Measurements of ¹H spin–lattice relaxation behaviour are presented for samples of zinc oxide, the spinel catalyst precursor and a copper-free catalyst sample to assist in assignment of lines in the spectra of reduced catalyst samples. The data are interpreted in terms of four proton populations, these being hydrogens associated with the oxide phase (probably ZnH species), hydroxyl hydrogens on the zinc oxide surface, hydrogens associated with the alumina-containing oxide phase and the hydrogen atoms chemisorbed on copper metal surfaces. Comparisons of the n.m.r. properties for the reduced catalyst samples with those for the non-copper-containing samples suggest that the copper component in the reduced catalyst has the effect of substantially increasing the population assigned as ZnH species. In addition it substantially lowers their spin–lattice relaxation time and that of the alumina-associated proton population. The values of these relaxation times are suggestive of an electronic mechanism, being too short for typical neclear dipole–dipole and similar mechanisms. The peaks assigned to ZnH and CuH species are shown to derive directly from adsorbed hydrogen. Measurements of the spin–lattice realxation of the total proton magnetisation for the reduced catalyst in the presence of hydrogen gas demonstrate that the relaxation of the component populations are coupled via exchange through surface or spin diffusion.

Preliminary measurements are reported of ¹³C spectra for reduced cataylst samples reacted with ¹³C-enriched methanol, CO, CO₂ and CO–H₂ and CO₂–H₂ mixtures. All samples show evidence of a carbon species which has no detectable dipolar coupling to protons. The sample exposed to ¹³CO shows, in addition, a peak at ca. 350 ppm which is assigned to a Knight-shifted ¹³C resonance from ¹³CO bound to copper metal. In the presence of an excess of hydrogen this peak is absent suggesting displacement of the CO by hydrogen.