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 μm thick compact layer of Pd90Au10 (10 wt.% Au), a 5 μm thick compact layer of PdGa (50 at.% Ga) and a thin film of Pd90Au10 deposited on a discontinuous V2O5 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 °C and a maximum phenol formation rate of 1.67 × 10−4 mol h−1 m−2 at 200 °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.

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. Selectivity reached up to 67% with a Pd90Au10 layer. The PdGa layer showed promising catalytic performance in hydrogen co-feed mode but suffered from a low hydrogen permeance in membrane mode.Figure optionsDownload high-quality image (48 K)Download as PowerPoint slideResearch highlights▶ Distributed supply of O2 via second membrane gave improved phenol selectivity compared to co-feed mode. ▶ PdAu and PdGa showed improved activity and selectivity for benzene hydroxylation in the gas phase with in situ-H2O2 compared to Pd and PdCu. ▶ Poor utilization of H2 and low reactivity of benzene render the system unattractive.