(582q) Microreactor Techniques for Analysis of Complex Reactions

Saurabh Maduskar, Paul J. Dauenhauer

Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. SE, 484 Amundson Hall, Minneapolis, MN 55455, USA.

Limitations of analytical systems is the major hurdle in understanding complex chemical reactions such as biomass pyrolysis. Cellulose is a major constituent of biomass and the most abundant biopolymer in the world. During high temperature pyrolysis, it transforms into a liquid intermediate before depolymerizing and volatilizing into condensable bio-oil products for upgrading into fuels and chemicals.^[1] The underlying pyrolysis chemistry has been studied for decades, and numerous conflicting mechanisms and kinetics models have been proposed. Lack of the experimental verification of such kinetic models is primarily due to the complexity of the analytical problem. Limitations of conventional analytical systems to measure kinetics of cellulose pyrolysis were identified in terms of five experimental criteria, such as sample length scale, temperature ramp during heating and cooling, sweeping gas flowrate, temperature measurement and control, and product quantification.

To overcome these challenges, Pulse-Heated Analysis of Solid Reactions (PHASR), an experimental microreactor system was developed. The PHASR reactor rapidly heats and cools (~10⁶⁰C/min) thin film solid samples (10-50 µm thick) at millisecond time scales to control reaction progression, allowing for quantification of complex mixture of vapor, gas, and intermediate products as a function of reaction time.^[3] Further, quantification of each carbon-containing analyte in such complex mixtures by existing methods (flame ionization detection) requires extensive identification and calibration. An integrated microreactor system called the Quantitative Carbon Detector (QCD) for use with current gas chromatography techniques for calibration-free quantitation of analyte mixtures was developed. ^[5] Combined heating, catalytic combustion, methanation and gas co-reactant mixing within a single modular reactor fully converts all analytes to methane (>99.9%) within a thermodynamic operable regime. Residence time distribution of the QCD reveals negligible loss in chromatographic resolution consistent with fine separation of complex mixtures including cellulose pyrolysis products.

PHASR/GC-QCD reactor system was used to measure millisecond-resolved evolution of cellulose and its volatile fragmentation products over a wide range of temperature (400-525 ⁰C). The results obtained clearly demonstrates the effect of experimental conditions such as sample length scale on product distribution. Energetics of glycosidic bond cleavage was measured by analyzing cyclodextrin, a demonstrated surrogate of cellulose for pyrolysis reactions.^[1,4] Combining the rates of glycosidic bond cleavage with that of product formations, apparent kinetics of product formation was evaluated. Using these next-generation analytical techniques, mechanisms of complex chemical reactions such as pyrolysis of biomass can be elucidated. (1) Mettler, M. S.; Mushrif, S. H.; Paulsen, A. D.; Javadekar, A. D.; Vlachos, D. G.; Dauenhauer, P. J. Energy Environ. Sci. 2012, 5, 5414.

(2) Mettler, M. S.; Vlachos, D. G.; Dauenhauer, P. J. Energy Environ. Sci. 2012, 5, 7797.

(3) Krumm, C.; Pfaendtner, J.; Dauenhauer, P. J. Chem. Mater. 2016.

(4) Zhu, C.; Krumm, C.; Facas, G.; Neurock, M.; Dauenhauer, P.J. Reaction Chemistry & Engineering 2017.

(5) S. Maduskar, A.R. Teixeira, A.D. Paulsen, C. Krumm, T.J. Mountziaris, W. Fan, P.J. Dauenhauer, Lab on a Chip, 2015, 15, 440-447.