Monitoring of chemical reactions within microreactors using an inverted Raman microscopic spectrometer

Abstract

An inverted Raman microscope spectrometer has been used to profile the spatial evolution of reactant and product concentrations for a chemical reaction within a microreactor operating under hydrodynamic flow control. The Raman spectrometer was equipped with a laser source at wavelength of 780 nm, confocal optics, a holographic transmission grating, and a charge-coupled device (CCD) detector. The microreactor consisted of a T-shaped channel network etched within a 0.5 mm thick glass bottom plate that was thermally bonded to a 0.5 mm thick glass top plate. The ends of the channel network were connected to reagent reservoirs that were linked to a syringe pump for driving the solutions by hydrodynamic pumping within the channels. The microchannels were 221 micro m wide and 73 micro m deep. The synthesis of ethyl acetate from ethanol and acetic acid was investigated as a model system within the microreactor as Raman scattering bands for each reactant and product species were clearly resolved. Raman spectral intensities of each band were proportional to concentration for each species and hence all concentrations could be quantitatively measured after calibration. By scanning specific Raman bands within a selected area in the microchannel network at given steps in the X-Y plane, spatially resolved concentration profiles were obtained under steady-state flow conditions. Under the flow conditions used, different positions within the concentration profile correspond to different times after contact and mixing of the reagents, thereby enabling one to observe the time dependence of the product formation. Raman microscopy provides a useful complementary technique to UV/VIS absorbance and fluorescence methods for the in situ monitoring and analysis of chemical reaction species having their lowest S(0)-S(1) absorption bands too far in the UV to be of use, due to their probable overlap with the bands from other reactant, product and solvent molecules.

Coherent anti-Stokes Raman scattering microscopy for quantitative characterization of mixing and flow in microfluidics

“…Building on this success, microfluidic devices with integrated Raman microscopy have shown they can monitor thermal and chemical gradients, product formation, and conformational changes [4][5][6][7][8][9]. Within such devices, optical resolution of molecular kinetics is limited more by the optical sectioning than by the stability of the conditions within the microfluidic device.…”

Unconventional detection methods for microfluidic devices

“…When compared with macroscale reactors, the time needed to extract the necessary information was reduced from 48 h to just 10 min. For the same purpose, Fletcher et al [23] also developed a simple t-shaped microreactor on Pyrex glass. They profiled the spatial and temporal evolution of reactant and product concentrations for a chemical reaction under hydrodynamic flow control.…”

Simultaneous in situ characterisation of bubble dynamics and a spatially resolved concentration profile: a combined Mach–Zehnder holography and confocal Raman-spectroscopy sensor system

“…Within the last decades, the method has been further developed for in situ process analysis and reaction monitoring in research, pharmaceutical operations and the chemical industry (Fletcher et al, 2003;Kessler et al, 2016;Lewis, 2001;Vankeirsbilck et al, 2015;Rantanen, 2007). At the Institute for Micro Process Engineering, previous work was done on the development of an in situ laser Raman system to examine mixing processes (Rinke et al, 2011) and the kinetics of the oxidation of cyclohexane (Fräulin et al, 2013(Fräulin et al, , 2014.…”