Modification of the catalytic properties of a Pd membrane catalyst for direct hydroxylation of benzene to phenol in a double-membrane reactor by sputtering of different catalyst systems

The surface of hydrogen-permeable PdCu membranes acting as a catalyst for direct hydroxylation of benzene to phenol in the gas phase in a novel catalytic double-membrane reactor was modified by sputtering on it different catalytic layers with the aim to increase the formation rate and selectivity to phenol. Three different systems are described: a 1 mu m thick compact layer of Pd(90)Au(10) (10 wt.% Au), a 5 mu m thick compact layer of PdGa (50 at.% Ga) and a thin film of Pd(90)Au(10) deposited on a discontinuous V(2)O(5) layer. The different systems were characterized by SEM, EDX, and mainly in terms of their catalytic properties for benzene hydroxylation. The formation rate and the selectivity to phenol could be increased substantially through the catalytic modification. With a maximum phenol selectivity of 67% at 150 degrees C and a maximum phenol formation rate of 1.67 x 10(-4) mol h(-1) m(-2) at 200 degrees C, PdAu reached the best performance in double-membrane operation mode. PdGa showed even more promising results compared to PdAu in kinetic experiments in co-feed operation mode, but suffers from the very low hydrogen permeability of PdGa which stands against its use as a continuous layer in the catalytic membrane reactor. (C) 2010 Elsevier B.V. All rights reserved.