Hydrogenation of 2,2-dimethylol-1-butanal and 2,2-dimethylol-1-propanal to trimethylolpropane and trimethylolethane over a supported nickel catalyst

The hydrogenation kinetics of 2,2-dimethylol-1-butanal (TMP-aldol) and 2,2-dimethylol-1-propanal (TME-aldol) over a supported nickel catalyst were determined with experiments carried out in a batchwise operating autoclave at 50-90°C and 40-80 bar hydrogen. Water was used as the solvent. TMP- and TME-aldol were hydrogenated with 100% selectivity to the corresponding triols. The effects of the catalyst activation procedure and the formaldehyde concentration on the hydrogenation kinetics were studied with thermogravimetry, X-ray photoelectron spectroscopy, and hydrogenation experiments. Catalyst reduction at a high temperature (400°C) under hydrogen flow was favorable because of a more effective reduction of nickel oxides. Formaldehyde had a considerable retarding effect on the aldol hydrogenation: the hydrogenation rate was low until all of the formaldehyde was hydrogenated to methanol. The hydrogenation rate of TME-aldol was found to be significantly lower than that of TMP-aldol at low temperatures and pressures (60°C and 40 bar), whereas equally high rates for both aldol molecules were observed at the highest temperature and pressure studied. A kinetic model including the inhibitory effect of formaldehyde as well as real hydrogen solubility data was proposed for the aldol hydrogenation. The model is comprised of adsorption, desorption, and surface reaction steps. The rate equations based on the model were able to describe the experimentally recorded hydrogenation kinetics of TMP- and TME-aldol.