In situ Raman spectroscopy to monitor the hydrolysis of acetal in microreactors

In order to obtain a better understanding of the physical and chemical processes within micromixers or microreactors and to optimize these devices it is necessary to monitor the concentrations within the microchannels. To get chemical information, laser Raman spectroscopy can be used. This method is very selective for individual chemical compounds, allows a spatial resolution of 10 μm within fluids and a quantitative analysis. We examined the hydrolysis of the acetal 2,2-dimethoxypropane to acetone and methanol in the presence of hydrogen ions as catalyst. This reaction can be used to characterize micromixers. The aim of this work is the in situ monitoring of concentrations of acetal and its products, acetone and methanol, during the hydrolysis of acetal within a T-shaped micromixer with a channel width of 0.4 mm and a channel depth of 0.2 mm. In these experiments a continuous-wave argon ion laser was used as an excitation source. The laser radiation was coupled into a microscope and into the micromixer covered with a quartz plate. A special microscope objective was used. It allows the correction of optical aberrations resulting from quartz plates up to a thickness of 2 mm. Concentration profiles of acetal, methanol, and acetone were measured across the width of the microchannel.