Pd/MgO: Catalyst characterization and phenol hydrogenation activity

The gas-phase hydrogenation of phenol (423 less than or equal to T less tha n or equal to 573 K) has been studied over a 1% w/w Pd/MgO catalyst prepare d by impregnation of MgO with (NH4)(2)PdCl6. The catalyst precursor was act ivated by precalcination in air at 473 K followed by reduction in hydrogen at 573 K. Temperature-programmed reduction/desorption (monitoring H-2 consu mption and NH3, H2O, CO, and CO2 release) has revealed the presence of ammo nium carbonate and/or ammonium hydrogen carbonate an the active surface in addition to a metallic palladium component. Whereas the latter was not dete ctable by X-ray diffraction due to the high metal dispersion, transmission electron microscopy revealed that the mean palladium particle diameter is 1 .3 +/- 0.2 nm, which corresponds to a palladium dispersion of D-Pd = 71%. B esides conventional and high-resolution transmission electron microscopy, s elected area electron diffraction provides some insight into the fine struc ture of the palladium crystallites. Impregnation followed by calcination is shown to transform MgO to Mg(OH)(2) while the additional reduction step ge nerates a surface phase that is composed of bath needle-like Periclase MgO and Mg(OH)(2). X-ray photoelectron spectrometric analyses of the activated catalyst has established the presence of zero-valent palladium which appear s to be electron rich as a result of metal-support interaction; a degree of palladium charging is also evident as well as residual surface chlorine. T he effects on fractional phenol conversion and reaction selectivity of vary ing such process variables as reaction time, temperature, and phenol molar feed rate are considered and the possibility of thermodynamic limitations i s addressed, Hydrogenation was observed to proceed in a stepwise fashion wi th cyclohexanone as the partially hydrogenated product and cyclohexanol as the fully hydrogenated product. The catalyst delivered a 96% selectivity wi th respect to cyclohexanone production at 423 K but the cyclohexanone yield decreased at higher temperatures as conversion declined and cyclohexanol w as increasingly preferred. Conversion and selectivity were both stable with prolonged catalyst use, i.e., time on stream in excess of 55 h, (C) 2000 A cademic Press.