Selective hydrogenation of phenylacetylene to styrene on a Pd/TiO ₂ coating in a microreactor

The selective hydrogenation of phenylacetylene to styrene by hydrogen was studied in the 25-50°C temperature range in a continuous flow microchannel of 250 μm i.d. with a Pd supported on mesoporous TiO₂ catalyst. The inner wall of the microreactor was dip-coated with a prehydrolyzed titania precursor solution containing an EO_xPO_yEO_x amphiphilic triblock copolymer (EO = ethylene oxide, PO = propylene oxide, x = 106, y = 70) and suspended palladium nanoparticles with an average diameter of 4 nm. Solvent was withdrawn from the channel at a rate of 1 cm/s yielding a 120 nm continuous titania layer with a cubic mesostructure after drying and calcination. The organic copolymer was removed by calcination at 300°C under a residual pressure of 15 mbar. The coating porosity was 0.4, resulting in overall coating mass of 1.32 mg with a 1 wt.% Pd content. The liquid flow was varied between 1 and 10 μL/min and the hydrogen flow between 250 and 550 μL/min (STP). This corresponds to superficial velocities of 0.034-0.34 cm/s and 8-18 cm/s for liquid and gas, respectively. The maximum selectivity to styrene of 95 % was observed at a phenylacetylene conversion of 88%. The reaction rate in terms of TOF was found to be up to 2 s^-1.