Liquid phase hydrogenation in a structured multichannel reactor Xiaolei Fan, Alexei A. Lapkin, Pawel K. Plucinski * Centre for Sustainable Chemical Technology, Department of Chemical Engineering, University of Bath, Bath BA2 7AY, UK 1. Introduction Advances in the field of process intensification have led to the development of micro-heat exchanger and compact heat exchan- ger and reactors. These consist of multichannel structures that may have individual channels with hydraulic diameters in the range of 0.05–5 mm [1–3]. In comparison with the conventional multi- phase reactors, these compact structures provide more intensive conditions for promoting catalytic reactions due to the significant reduction of the length scales across which mass and heat transfer should occur. For example, compared with the values reported for the traditional large-scale multiphase packed-bed reactors, more than a 100-fold increase in the gas/liquid mass transfer rate was measured for a microreactor with 0.625 mm reaction channel, which in return significantly accelerated the catalytic reactions [2]. In our previous studies, we demonstrated the use of a compact multifunctional reactor as an effective tool for performing partial oxidation of organic feedstocks. The reactor was integrated with a static mixer and a micro-heat exchanger which enable excellent mixing of reagents prior to entering the catalytic channel, and good heat transfer efficiency for safe running of reactions with significant heat effects. Selective oxidation of aromatic alcohols by molecular oxygen was demonstrated at up to 24 atm undiluted oxygen pressure, which was enabled by (i) highly efficient heat removal in an integral micro-heat exchangers and (ii) the absence of gas-phase volume in which an explosive mixture of hydrocarbon vapour and oxygen may be produced [3]. Continuous operation is another major advantage offered by micro/compact reactors. Adopting this reactor technology in chemistry as a potential solution to circumvent limitations of batch processes is often called ‘flow chemistry’. A prominent benefit of microreactor technology is high surface-to-volume ratio which enables a narrow temperature profile along the reactor and efficient cooling. This feature offers the opportunity to successfully execute highly exothermic/endothermic chemical transformations which are usually inhibited in larger batch reactors. Gross et al. [4] demonstrated the use of a flow reactor in the synthesis of NBI- 75043 (an anti-insomnia drug) to promote the key halogen–metal exchange step whose highly exothermic nature limited this total synthesis route in a large-scale batch. Small reaction volume also endows microreaction technology with the inherent safety, which allows us to perform hazardous reactions and reactions using toxic reagents, for instance, ring-expansion reaction involved diazo compound as a reactant [5]. In this study, we expanded the application scope of the compact reactor [3,6] to the three-phase catalytic hydrogenation (hydro- genation of benzaldehyde as a model reaction) and continuous tandem C–C Heck coupling with consecutive hydrogenation Catalysis Today 147S (2009) S313–S318 ARTICLE INFO Article history: Available online 5 August 2009 Keywords: Hydrogenation Tandem reaction Compact reactors Multifunctional reactors Flow chemistry ABSTRACT A compact, structured, multichannel reactor was tested for two exemplary reactions: the selective hydrogenation of aromatic aldehyde and the tandem C–C coupling–hydrogenation. In the case of hydrogenation reaction up to 50% yield with ca. 96% selectivity was attained in a single pass (single channel of 10 cm length of catalytic bed) at a liquid phase residence time of ca. 10 s, proving the effectiveness of the designed reactor. By controlling the point of injection of hydrogen into the reactor an increase in the yield of hydrogenation up to 73% was achieved using two channels in a consecutive mode. The designed compact reactor was also proven to be an excellent tool for kinetic studies: the kinetics of the three-phase hydrogenation was evaluated showing: (i) reaction limitation for applied reaction conditions, (ii) Langmuir–Hinshelwood mechanism of hydrogenation, and finally (iii) the dominating role of adsorption of reactant and absorption of hydrogen in the mechanism of hydrogenation at higher temperatures. Furthermore, the compact reactor was also successfully used for conducting sequential coupling Heck reaction with consecutive hydrogenation of double C–C bond. A stepwise conversion of the substrates to the final product was achieved with ca. 6 min residence time at relatively low temperature and pressure. In conclusion, the developed structured compact reactor was demonstrated as a promising alternative to replace conventional batch reactors and establish continuous synthesis of pharmaceutical intermediates and specialty chemicals. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +44 1225 386961; fax: +44 1225 385713. E-mail address: p.plucinski@bath.ac.uk (P.K. Plucinski). Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod 0920-5861/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2009.07.048