The transition of chemical reactions from batch mode to continuous mode using a microreactor has been a much-studied topic recently. The advantages and possibilities created by this technology have been demonstrated for various organic reactions. The high surface to volume ratio, temperature control, and fast mass transfer make this technology especially interesting for heterogeneous catalysis.
A notable example of a reaction that benefits from microreactor technology is the hydrogenation, particularly those that are exothermic. Several examples of exothermic hydrogenations performed in microreactors have been demonstrated in the past and they can be classified into two broad groups. The first group, and probably the most popular, has the catalyst immobilized inside of the microreactor before the reaction. This immobilization can be done in various ways using standard methods: the catalyst might be immobilized in the reactor before assembly, use of devices made by thin-film methods, introduction of a catalyst bound onto a high-surface area material, or by grafting an organometallic-based catalyst onto the surface of the microreactor. The second group is limited to microreactors with larger channel size since the catalyst is pumped as a suspension through the channels.
This communication presents a third method for performing heterogeneous catalysis in microreactors, in which the liquid catalyst precursor is pumped continuously into the microreactor throughout the course of the reaction, as opposed to a reactant flowing over a fixed bed of a catalyst. Several benefits emerge by this continuous addition method: the ratio of catalyst to substrate is controllable during the run by adapting the flow rate; once a stabile system is obtained, the activity of the catalyst remains the same during the whole run. This in-situ production and immediate use of a catalyst will insure that the catalyst is in its most reactive form during the complete experiment. Furthermore, the preparation of the microreactor is, in this case, as simple as rinsing with the proper reagents. An example that demonstrates the benefits of the in-situ catalyst formation and reaction control is the use of a PdCl2 solution as a precursor to form Pd(0) particles as hydrogenation catalyst.