(83n) Selective Hydrogenation of Phenylacetylene to Styrene on a Pd/TiO2 Coating in a Microreactor

The synthesis of a large number of fine chemicals, particularly in the field of flavor and fragrance chemistry and pharmaceuticals, involves the selective hydrogenation of organic compounds as a critical step. Those multiphase reactions can be performed in microchannels with a catalytic coating deposited on the channel wall. Usually, the geometric surface of the microchannels is not sufficient for performing catalytic reactions. The efficient use of microstructured catalytic reactors requires a shaping of the catalyst by deposition of thin catalytic coatings at the walls of the reactor channels. Recently, inorganic mesoporous thin films have attracted considerable attention because of their large surface areas and narrow pore size distributions, which make them attractive candidates for catalyst supports [1]. Recent studies have shown that colloidal nanoparticles can be excellent hydrogenation catalysts when they are activated on mesoporous supports.

In this study, colloidal palladium nanoparticles of 2.4 nm in diameter suspended in ethanol (99.99 wt.%) were synthesized following a well established procedure [2]. Nanoparticle-doped mesoporous titania thin films were prepared using a precursor solution with the following composition: 1 TiO₂ (Titanium tetrabutoxide (TTB)): 0.005 Pluronic F127: 40 EtOH: 1.3 H₂O: 0.13 HNO₃. The incorporation of the Pd nanoparticles was done by using the ethanol-suspended nanoparticles solution to obtain 1 wt.% Pd loading in mesoporous titania. The solution was then prepared by adding the appropriate amounts of F127, water, and TEOS, in that respective order. The bottle was then capped and the mixture stirred at room temperature for 2 hours.

Prior to dip-coating, the internal surface of a fused silica capillary with an internal diameter of 250 mL and a length of 9 m was activated in a flow of 1 M NaOH aqueous solution (0.03 mL/min) for 15 minutes in order to enhance the adhesion of the coating to the channel wall. Then, the capillary was flushed for 15 minutes (0.03 mL/min) with demiwater and the titania sol, respectively. Finally, the titania sol was withdrawn from the capillary at a rate of 1 cm/s. The capillary was dried and calcined in an oven at a residual pressure of 15 mbar. The heating rate from 25 to 300 ^oC was 1 ^oC/min with a 1-hour dwelling time every 25 K below 200 ^oC, a 2-hour dwelling time at 225, 250 and 275 ^oC and a 4-hour dwelling time at 300 ^oC. A similar coating was deposited on 1 cm x 1cm flat silicon substrates to perform characterization by LA XRD, TEM, ethanol adsorption-desorption isotherms. The coating thickness was 120 nm, and the coating porosity was 0.4, resulting in overall coating mass of 1.32 mg with 1 wt.% Pd content.

Hydrogenation of phenylacetylene was studied in the 25-50 ^oC temperature range in the microchannel with the Pd/TiO₂ coating. A mixture of 10 vol.% phenylacetylene in methanol and hydrogen (99.9 vol.%) were mixed in a T-mixer prior to the microchannel. The liquid flow was varied between 1 and 10 mL/min and the hydrogen flow between 250 and 550 mL/min (STP) corresponds to superficial velocities of 0.034 - 0.34 cm/s and 8 - 18 cm/s for liquid and gas, respectively. Samples were collected at the outlet of the capillary, diluted by methanol and analyzed on a Varian CP-3800 gas chromatograph equipped with a CP-Sil 5CB capillary column and a FID detector.

Full conversion of phenylacetylene was observed at liquid flow rates below 3 mL/min in the whole range of temperatures studied. Ethylbenzene was the main product at those conditions. As the flow rate increases, the conversion decreases with simultaneous increase of selectivity to styrene (Figure 1). The maximum selectivity to styrene of 95% was observed at the highest liquid flow rate of 6 mL/min. The reaction rate in terms of TOF was found to be up to 2 s^-1.

Figure 1.   Phenylacetylene conversion and selectivity to styrene on a Pd/TiO₂ coating as a function of the liquid flow rate in the microchannel. The hydrogen flow rate was kept constant at 0.55 ml/min (STP). The liquid consists of a mixture of 10 vol.% of phenylacetylene in methanol.
Acknowledgements

The financial support of the Netherlands Organization for Scientific Research (NWO) and British Council (British Council-NWO Partnership Programme in Science, projects PPS 888 and 894) is gratefully acknowledged.

References

[1] T.S. Glazneva, E.V. Rebrov, J.C. Schouten, E.A. Paukshtis, and Z.R. Ismagilov, Thin Solid Films, 515 (2007)6391.

[2] S.Dominguez-Dominguez, A. Berenguer-Murcia, D. Cazorla-Amoros, A. Linares-Solano, J. Catal., 243 (2006) 74.