To demonstrate that monolayer coverage of a complex can be estimated from the footprint area of the complex and surface area of the support, various loadings of two dinuclear copper complexes, bis(heptane-2,4,6-trionato)dicopper(II) [Cu 2 (daa) 2 ] and bis(1,5-diphenyl-1,3,5pentanetrionato)dicopper(II) [Cu 2 (dba) 2 ], were supported on Cab-O-Sil by batch impregnation. The supported samples were characterized by elemental analysis, powder XRD, DRIFTS, EPR spectroscopy, and TGA. Elemental analysis and TGA data confirm that the Cu 2 (daa) 2 complex loses one triketonate ligand upon adsorption onto silica from THF solution and therefore is not suitable for estimating monolayer coverage. By contrast, the Cu 2 (dba) 2 complex was adsorbed on the silica surface intact from CH 2 Cl 2 . PXRD and DRIFTS confirmed the partial loading of the second Cu 2 (dba) 2 layer on the Cab-O-Sil surface for samples containing ≥ 2.64 wt% copper. This agrees with a predicted loading based on the surface area of the support and footprint of the complex which was determined by X-ray crystallography of the dipyridine adduct. This is the first dicopper bis triketonate complex in which the pyridines are cis.

Models of silica supported cobalt catalysts were formed by decorating the surface of Cab-O-Sil with the dinuclear cobalt complex, bis(1,5-diphenyl-1,3,5-pentanetrionato) dicobalt(II), [Co 2 (dba) 2 ]. To confirm the formation of a monolayer and multilayer of the complex on Cab-O-Sil, various loadings of [Co 2 (dba) 2 ] were combined with Cab-O-Sil by batch impregnation. The supported samples were characterized by elemental analysis, DRIFTS, and TGA to show that the Co 2 (dba) 2 complex was adsorbed on the silica surface intact and DRIFTS confirmed the loading of the Co 2 (dba) 2 as a monolayer on the Cab-O-Sil up to a loading < 2.09 wt% Co. This experimental monolayer loading confirmed the predicted loading (2.06 wt% Co metal) based on the surface area of the support, 200 m 2 /g, and the footprint area of the complex (1.866 nm 2). When this precatalyst was decomposed in air and reduced in hydrogen, the morphology of the resulting particles depended upon the initial disposition of the metal complex on the support. Monolayer films of the metal complex decomposed into metal or metal oxide particles which are too small to give XRD reflections (2.09 wt% Co); whereas, catalyst derived from a multilayer film (4.1 wt% Co) showed large metal particles developing a very sharp XRD reflection from the Co(111) index. Both catalysts were active for the Fischer-Trøpsch reaction at 350 o C with the catalyst derived from the multilayers of complex showing an activity, as characterized by first order rates constants, about 33 times that of the catalyst derived from a monolayer of the complex:71.4 vs 2.16 (mol Co-hour)-1. The selectivity to liquid hydrocarbons showed a remarkably narrow distribution of molecular weights, comprised of mainly iso-and cycloalkanes having 6-8 carbons.

Silica-supported model copper catalysts were prepared by supporting bis(1,5-diphenyl-1,3,5-pentanetrionato)dicopper(II), Cu 2 (dba) 2 , on Cab-O-Sil by a batch impregnation technique. This metal complex showed a strong affinity for the silica support, developing monolayer coverages near the value predicted from a consideration of the size and shape of the planar metal complex (2.6 wt % Cu). The supported catalysts were subsequently activated by decomposing the organic ligands at 400 °C in air followed by reduction with 2% H 2 /He at 250 °C. One sample was prepared having a loading of 3.70 wt % Cu 2 (dba) 2 /silica catalyst, and it was examined for the methanol synthesis reaction under the following conditions: 250 °C with an equimolar gas mixture of CO and H 2 in a high-pressure batch reactor. Kinetic data over the model catalyst were fit to a rate equation, second order in the limiting reactant (H 2 ), with a pseudo -second-order rate constant k 2 [CO] o [H 2 ] o = 0.0957 [h-g total Cu] −1 . A control experiment using a commercial catalyst, Cu/ZnO/Al 2 O 3 with a copper loading of 41.20 wt %, showed a value of k 2 [CO] o [H 2 ] o = 0.793 [h-g total Cu] −1 . A fresh sample of Cu 2 (dba) 2 /silica was examined for methanol decomposition reaction at 220 °C. The model catalyst shows a methanol decomposition first-order rate constant greater than that of the commercial Cu/ZnO/Al 2 O 3 catalyst: 1.59 × 10 –1 [min-g total Cu] −1 versus 9.6 × 10 –3 [min-g total Cu] −1 . X-ray diffraction analyzes confirm the presence of CuO particles in both catalysts after calcinations. Copper metal particles were found in both catalysts (fractional Cu dispersions were 0.11 and 0.16 on commercial and model catalysts, respectively) after the reduced catalysts were used in both the methanol synthesis and decomposition reactions. Using the values of copper dispersion found in these samples, we recalculated the rate constants for the two reactions per unit surface copper. These refined rate constants showed the same trends as those reported per total amount of Cu. One role of the promoter(s) in the commercial catalyst is the inhibition of the methanol decomposition reaction, thus allowing higher MeOH synthesis reaction rates in those regimes not controlled by thermodynamics.

No abstract