Surface reactivity and self-oscillating oxidation of butan-2-ol over palladium loaded AlPO4

Article abstract of DOI:10.1016/j.apcata.2015.07.004

Abstract Palladium loaded AlPO₄ catalysts were synthesised and characterised by XRD, IR, UV-vis spectroscopy, temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR). The Pd²⁺ ions of the dispersed palladium on the amorphous surface of AlPO₄ are in D4h symmetry. They can partially be reduced into Pd⁰ even in dynamic conditions by a mixture of butan-2-ol and oxygen. TPD/TPSR showed that the active sites are formed by the couple PdO/Pd. The presence of PdO on the surface favours the production of methyl ethyl ketone (MEK). The CO₂ formation was never observed below 100 °C. Moreover, desorption of the MEK takes place in two waves arising from two different types of sites. Only the reactivity of the first set of sites which are located around 80 °C was considered in the present study. As a matter of fact, the oxidation of butan-2-ol into MEK (<1.3%) at 74.2 °C describes a counter-clockwise hysteresis attributed to a bistability induced by the competition between the redox processes that the surface and the reagents undergo. This sensibility of the catalyst to the parameters governing the reaction resulted in an oscillating oxidation of butan-2-ol. Spontaneous oscillations of the MEK production (α_one) appear around 85 °C and stop before 100 °C. Oscillations of the temperature of the catalyst bed were also recorded but their amplitude never exceeds 7 °C. An oscillatory regime appears also for butan-2-ol pressures ranging from 0.61 to 1.65 kPa and for a total flow rate between 80 and 260 mL min^-1. The oscillating oxidation of butan-2-ol is accompanied by a significant enhancement of the methyl ethyl ketone production.