Hydrotalcite-based catalysts for the synthesis of Methyl Isobutyl Ketone

Abstract

The investigations for more environmentally benign solid base catalysts have increased significantly for the production of bulk as well as fine chemicals due to the demands for cleaner processes and more stringent legislation. An interesting candidate for industrial applications in the production of both types of chemicals is activated hydrotalcite (HT), which shows promising results in liquid-phase reactions already under ambient conditions. The structure of HT, Mg6Al2(OH)16CO3. 4H2O, closely resembles that of brucite (Mg(OH)2) and consists of stacks of positively charged sheets of edge-shared octahedra. Charge-balancing anions, typically CO32-, and water molecules are situated in the interlayer between the cation layers. After activation, the carbonate ions have been replaced by OH- in the interlayer. These Brønsted base sites exhibit high activity in liquid-phase aldol-type condensations.

The research involved focused on the liquid-phase synthesis of methyl isobutyl ketone (MIBK) starting from acetone and H2. MIBK is an industrially important chemical, mainly used as a coating solvent. To obtain a catalyst system that converts acetone and H2 into MIBK in a single-step, activated HT for catalyzing the condensation and dehydration reaction was combined with supported Pd catalysts for hydrogenation.

The main challenges in the research focused in particularly on obtaining more fundamental insights and control of the nature and the accessibility of the Brønsted base sites in activated hydrotalcites, eventually leading to more efficient catalysts. Besides thorough characterization of the materials with for instance TEM, N2 physisorption, ICP, XRD and IR, the nature and accessibility of the active sites were assessed by means of probing with CO2 (calorimetry) and CDCl3 (DRIFTS). Accordingly, the large differences in catalytic properties observed in liquid-phase aldol-type condensations between differently treated activated HTs could be explained. Not only were we able to understand the differences in performance of activated HT, implementation of the findings resulted in hydrotalcite catalysts exhibiting unsurpassed activity in the self-condensation of acetone and condensation of citral with acetone. HT could be deposited on carbon nanofibers with extremely small platelet sizes, not reported earlier, exhibiting catalytic activity over four times that of unsupported HT. Furthermore, a novel bi-functional catalyst was obtained by deposition of Pd and HT on the same support (carbon nanofibers). Due to the well accessible pore system of the carbon nanofibers support bodies, a high number of active sites could be obtained, which is important to improve the activity, hence the efficiency of the catalyst. This catalyst system was indeed by far the most efficient catalyst for the liquid-phase synthesis of MIBK from acetone and H2 under mild conditions compared to Pd supported on activated hydrotalcites bi-functional catalysts and compared to physical mixtures of activated hydrotalcites and supported Pd catalysts.