INSITU C-13 SOLID-STATE NMR-STUDY OF THE CU/ZNO/AL2O3 METHANOL SYNTHESIS CATALYST

The reactions of methanol, formic acid, paraformaldehyde, CO2, and CO were studied on Cu/ZnO/Al2O3, Cu/Al2O3, ZnO/Al2O3, ZnO, and Al2O3 by in situ C-13 solid-state NMR with magic-angle spinning (MAS). These studies relate to the chemistry of methanol synthesis using the standard Cu/ZnO/Al2O3 catalyst, which produces essentially all of the world's methanol supply. Methanol adsorbs on the alumina phase at 298 K to form a surface-bound terminal methoxy, but it forms a bridging methoxy on the ZnO phase. On catalysts containing Cu, these methoxy groups were oxidized to a surface-bound symmetrical formate at ca. 373 K. Formate is mobile on the surface at 493 K. Heating to ca. 523 K converted formate to carbonate or bicarbonate, but again only on catalysts containing Cu. These carbonates partially decomposed at 523 K to free CO2. Paraformaldehyde underwent a Cannizzaro-type disproportionation on all catalysts studied to form formate and methoxy groups. This observation is consistent with a transient intermediate role for formaldehyde. Evidence of a surface-bound dioxymethylene was seen on ZnO/Al2O3 during this reaction. CO2 readily adsorbs on Cu/ZnO/Al2O3 to form carbonate at 298 K. CO adsorbs to form a small amount of carbonate and a weakly bound surface carbonyl. Detailed measurements of relaxation rates and the principal components of the C-13 chemical shift tensors are reported for all catalyst/adsorbate systems. All adsorbates showed biexponential C-13 T1 behavior on Cu/ZnO/Al2O3 only. Our interpretation of this result is that small paramagnetic clusters of Cu are dispersed in an oxide phase. The results of this study are interpreted in terms of several mechanistic controversies.