Characterization and performance of the bifunctional platinum-loaded calcium-hydroxyapatite in the one-step synthesis of methyl isobutyl ketone

Abstract

Calcium-hydroxyapatite catalysts loaded with different amounts of platinum Pt(x)/CaHAp were synthesized and characterized by N2-adsorption, X-ray diffraction, transmission electron microscopy, Fourier transform infrared, ultraviolet–vis–NIR spectroscopy and temperature programmed reduction. It was found that the loaded platinum was mainly exchanged and dispersed on the apatite surface, forming particles with an average size of 2nm. The exchanged platinum, as shown by XRD, did not affect the apatite structure or its crystallinity. The specific surface area of the bare CaHAp as well as the mean pore diameter, diminished by increasing the Pt loading, probably because the pores were partially blocked. The Pt(x)/CaHAp samples calcined at 500°C contained, beside the exchanged platinum, Pt and PtO. H2-TPR results showed that the platinum occupied on the apatite at least two different sites.The acid–base and redox properties of the catalysts were studied using, as the probe reaction, butan-2-ol conversion. The impregnation of CaHAp with Pt decreased its natural basic properties and enhanced markedly the dehydrogenation of butan-2-ol into methyl ethyl ketone (MEK). In the presence of air, the production of MEK increased abruptly with the temperature. The reaction started at 30°C, passed through a maximum (90% of conversion) at 60°C and then it decreased rapidly. This important activity at low temperatures was attributed to Pt and Pt2+ species associated with the basic Ca2+–O2− groups of the apatite.The synthesized bifunctional catalysts were also investigated in the acetone conversion, in one step, into methyl isobutyl ketone (MIBK). The optimal performance was achieved with the sample loaded with 0.5wt.% Pt. It displayed at 150°C and stationary state a MIBK yield of 23% with a selectivity of 74%. Moreover, all the Pt(x)/CaHAp catalysts showed an acceptable stability over time on stream with no production of heavy compounds.