Modular microreactor with integrated reflection element for online reaction monitoring using infrared spectroscopy

We report on the fabrication of an internal reflection element (IRE) combined with a modular polymer microfluidic chip that can be used for attenuated total reflection (ATR) infrared spectroscopy. The IRE is fabricated from a silicon wafer. Two different polymers are used for the fabrication of the two types of modular microfluidic chips, namely polydimethylsiloxane (PDMS) and cyclic olefin copolymer (COC). The microfluidic chip is modular in the sense that several layers of mixing channels, using the herringbone mixer principle, and reactions chambers, can be stacked to facilitate the study of the desired reaction. A model Paal-Knorr reaction is carried out to prove that the chip works as intended. Furthermore, we highlight the strength of IR spectroscopy as a tool for reaction monitoring by identifying the peaks and showing the different reaction orders at the different steps of the Paal-Knorr reaction. The reduction of the aldehyde groups indicates a (pseudo) first order reaction whereas the vibrational modes associated with the ring formation indicate a zero order reaction. This zero order reaction can be explained with literature, where it is suggested that water acts as a catalyst during the dehydration step, which is the final step in the pyrrole ring formation.