Direct synthesis of methyl isobutyl ketone in gas-phase reaction over palladium-loaded hydroxyapatite

Several compositions of palladium-loaded calcium hydroxyapatite Pd(x)/CaHAp were prepared and characterized by XRD, IR, and UV–visible spectroscopy and temperature-programmed reduction (TPR). The acid–base properties of the samples were studied with the use of butan-2-ol conversion and IR spectroscopy coupled with lutidine adsorption/desorption. Calcium hydroxyapatite bears Lewis acid sites of moderate strength and a limited number of Brønsted acid sites, which produce butenes by dehydration. The selectivity of the dehydrogenation reaction (formation of methyl ethyl ketone) is very limited in the absence of oxygen but indicates the existence on the surface of phosphate of basic sites that are as essential as the acid sites in the direct synthesis of methyl isobutyl ketone (MIBK) from acetone and H2. Dispersion of Pd on the hydroxyapatite introduces redox properties and the bifunctionality needed in the hydrogenation of mesityl oxide (MO). Temperature-programmed reduction (TPR) of Pd(x)/CaHAp catalysts shows a sharp peak of H2 consumption between 268 and 276 K, depending on palladium loading, followed by a negative peak at about 338 K, which is associated with the decomposition of β-PdH. Hydrogen chemisorption measurements carried out at 353 K allowed the determination of the size of palladium particles, which varies between 8 and 28 nm. The Pd(x)/CaHAp catalysts showed good performance in the low-pressure one-step synthesis of MIBK. They are stable under the experimental conditions, and they exhibit a MIBK and MIBC selectivity that reaches 92% for a conversion of 22% and an optimal Pd loading of 2 wt%. Other parameters of the reaction (such as temperature of reaction, residence time, H2/acetone ratio) were studied. They all play an important role in acetone conversion and product distribution. However, the reaction temperature has the most significant influence, since a maximum of the global conversion was observed at 423 K.