Application of Radiofrequency Heating for Process Intensification in Pd Catalysed Hydrogenation Over Composite Magnetic Microparticles

Application of radiofrequency heating for process intensification in Pd catalysed hydrogenation over composite magnetic microparticles

Thomas K. Houlding, Pengzhao Gao, Evgeny V. Rebrov

School of Chemistry & Chemical Engineering, Queen's University Belfast, Stranmillis Road, BT9 5AG, Belfast, UK

Recently, much research in process intensification has been focussed on the development multi-functional reactors, improving processes by system integration [1]. The use of NiZn ferrite nanoparticles embedded in a mesoporous microparticles for radiofrequency (RF) heating, magnetic separation and enhancement of mass transfer in the laminar flow within a continuous flow system provides a novel process intensified platform for system integration. The objective of this study was to explore novel Pd-based supported catalysts for selective and economically viable fine chemical synthesis.

The composite titania microparticles were prepared by sol-gel method from titania precursor in the presence of a structure directing agent and NiZn ferrite nanoparticles (grain size 15-25 nm) in the sol following earlier developed methodology [2,3].  After calcination at 300°C, the porous network was filled with a Pd precursor by incipient wetness impregnation. Then the microparticles were dried, calcined and reduced in hydrogen flow.

The heating rate of composite microparticles in the AC field (100-400 kHz, current 20-100 A) was determined of powders and slurries containing the desired amount of solid material. Induction coils with either 4, 5 or 6 turns were used as applicators to generate RF fields with different frequencies.  During operation the induction coil was cooled from inside by circulating water at 10^oC. The heating rates were studied as a function of ferrite composition (Ni and Zn content), the total loading of ferrite onto titinia microparticles as well as of the field strength and frequency. The temperature was measured with a fibre optic probe. The highest heating rate was observed over microparticles containing NiZn ferrite nanoparticles with low coercivity, with average powder heating rates of up to 40 K/min. Specific absorption rates (SAR) of up to 10 W/g were observed for composite microparticles containing magnetite nanoparticles.

Figure 1. Specific absorption rate of SiO₂@magnetite microparticles as a function of current and coil type

The catalytic activity of composite catalysts in hydrogenation reactions was determined in a reactor made of PEEK at a total H₂ pressure of 3 bar. Reference experiments were carried out using conventional heating with a thermostat. During experiments under RF-heating, the liquid temperature remains considerably below the catalyst temperature, which was estimated from the Arrhenius plot for the reaction rate of several selected transformations.

References

[1] Hessel V. Novel Process Windows – Gate to Maximizing Process Intensification via Flow Chemistry. Chem. Eng. Technol. 2009;32:1655-1681.

[2] Rebrov EV, Berenguer-Murcia A, Skelton HE, Johnson BFG, Wheatley AEH, Schouten JC. Capillary microreactors wall-coated with mesoporous titania thin film catalyst supports. Lab Chip. 2009;9:503-506.

[3] Protasova LN, Rebrov EV, Skelton HE, Wheatley AEH, Schouten JC. A kinetic study of the liquid-phase hydrogenation of citral on Au/TiO2 and Pt–Sn/TiO2 thin films in capillary microreactors. Appl. Catal. A. 2011;399:12-21.