Abstract

Using microstructured devices in chemical engineering provides several advantages over conventional, and mostly batch, reaction systems. Due to the decrease in length scale an increased surface-to-volume ratio is obtained, which is beneficial to achieve enhanced mass and heat transfer coefficients. However, there are also challenges associated with microchemical devices, especially the prevention of clogging in the presence of solid particles. 

To address this issue, we developed a Teflon stack microreactor with an integrated piezoelectric actuator for conducting chemical synthesis with solid products. The microreactors are demonstrated with palladium-catalyzed C–N cross-coupling reactions, which are prone to clogging microchannels by forming insoluble salts as by-products. To prevent microchannel clogging, the size of the precipitating particles must be minimized and their interactions with the walls reduced. This is achieved by fabrication the microreactor completely out of Teflon (PTFE) plates with an integrated piezoelectric element as acoustic actuator. Investigations of the ultrasonic waveform applied by the piezoelectric actuator reveal an optimal value of 50 kHz at a load power of 30 W. Operating the system at these conditions, the newly developed Teflon microreactor handles the insoluble solids formed and no clogging is observed. The investigated reactions reach full conversion in very short reactions times and high isolated yields are obtained (> 95% yield). In a second step, we apply this microreactor to the precipitation of hydroxyapatite to obtain crystals of desired size.