Phenol hydrogenation over palladium supported on magnesia: Relationship between catalyst structure and performance

A series of magnesia-supported palladium catalysts (Pd loading in the range 0.5-7.0 wt.%) has been prepared by impregnation from aqueous solutions of PdCl2, Pd(NH3)(4)Cl-2 and Pd(CH3COO)(2) and characterised by X-ray diffraction (XRD), CO chemisorption and high resolution transmission electron microscopy (HRTEM). The gas-phase hydrogenation of phenol was employed as a model reaction to probe the dependence of catalytic activity/selectivity on changes in Pd particle size and surface acid-base properties. The catalyst prepared from the acetate precursor exhibited the greatest Pd dispersion when compared with the chloride and amine precursors. The surface mobility of the metal chloride resulted in larger Pd particles from the Cl-containing precursors while the presence of residual Cl in the activated catalysts lowered the hydrogenation rate and was responsible for a decline in activity with time-on-stream. The effects of varying such process variables as temperature, hydrogen/phenol mol ratio and inlet molar phenol feed rate are presented and discussed while the question of structure sensitivity is addressed. The reaction exhibited a negative dependence on phenol partial pressure up to 503 K but a positive dependence was evident at higher temperatures. The order of the reaction with respect to hydrogen remained positive and was close to unity at 563 K; an apparent activation energy of 63 kJ mol(-1) was recorded. The effect of doping the support with calcium and fluoride has shown that modifications to the acid-base properties of magnesia can be used to control catalytic activity/selectivity.