The effect of aqueous complexation and gibbsite surface sites on the decarboxylation rate of malonate

The effect of metal-malonate complexation on the decarboxylation rate of malonate was studied by comparing the decarboxylation rates observed for malonate-bearing solutions with and without aqueous metal at 55, 80, and 90°C. The presence of high concentrations of an aqueous metal significantly slows malonate decarboxylation and the effect appears to increase with increasing concentration of metal in solution. This suggests that aqueous complexation significantly slows malonate decarboxylation. The change in decarboxylation rate due to complexation correlates reasonably well with the percent of malonate present in solution as a metal-malonate complex, and a single correlation appears to hold for all of the metals investigated. Malonate decarboxylation rates were also measured in the presence of powdered gibbsite. Due to aqueous Al-malonate complexation, the fluid in the presence of gibbsite contained elevated Al concentrations. However, comparison of malonate decarboxylation in solutions with gibbsite to the decarboxylation rates measured in gibbsite-free, aqueous Al-bearing solutions allows for isolation of the effect of the mineral surface on the decarboxylation rate. The results indicate that the effect of aluminum hydroxide mineral surface sites on malonate decarboxylation is negligible and that the primary controls on decarboxylation, in the absence of other mineral surface catalysts, are pH, temperature, and the extent of aqueous metalmalonate complexation.