Abstract
Silica-supported model copper catalysts were prepared by supporting bis(1,5-diphenyl-1,3,5-pentanetrionato)dicopper(II), Cu2(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% H2/He at 250 °C. One sample was prepared having a loading of 3.70 wt % Cu2(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 H2 in a high-pressure batch reactor. Kinetic data over the model catalyst were fit to a rate equation, second order in the limiting reactant (H2), with a pseudo-second-order rate constant k 2[CO]o[H2]o = 0.0957 [h-g total Cu]-1. A control experiment using a commercial catalyst, Cu/ZnO/Al2O3 with a copper loading of 41.20 wt %, showed a value of k 2[CO]o[H2]o = 0.793 [h-g total Cu]-1. A fresh sample of Cu2(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/Al2O3catalyst: 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.