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 diffract ion (XRD), CO chemisorption and high resolution transmission electron micro scopy (HRTEM). The gas-phase hydrogenation of phenol was employed as a mode l reaction to probe the dependence of catalytic activity/selectivity on cha nges in Pd particle size and surface acid-base properties. The catalyst pre pared from the acetate precursor exhibited the greatest Pd dispersion when compared with the chloride and amine precursors. The surface mobility of th e metal chloride resulted in larger Pd particles from the Cl-containing pre cursors while the presence of residual Cl in the activated catalysts lowere d the hydrogenation rate and was responsible for a decline in activity with time-on-stream. The effects of varying such process variables as temperatu re, hydrogen/phenol mol ratio and inlet molar phenol feed rate are presente d and discussed while the question of structure sensitivity is addressed. T he reaction exhibited a negative dependence on phenol partial pressure up t o 503 K but a positive dependence was evident at higher temperatures. The o rder of the reaction with respect to hydrogen remained positive and was clo se to unity at 563 K; an apparent activation energy of 63 kJ mol(-1) was re corded. The effect of doping the support with calcium and fluoride has show n that modifications to the acid-base properties of magnesia can be used to control catalytic activity/selectivity.