Selective production of glycolaldehyde via hydrothermal pyrolysis of glucose: Experiments and microkinetic modeling

Pyrolysis of glucose and glucose-based carbohydrates has been shown to produce a range of chemical species that can be used directly as fuels and chemicals or as feedstocks to further chemical transformations. It is known that process operating conditions such as temperature, heating rate, reactor configuration, moisture content, and pretreatment method can have a significant effect on the observed product distribution. Pyrolysis of carbohydrates in the presence of water at high concentrations can significantly alter the product spectrum and favor the production of specific products at unusually high yields. In this work, we show that pyrolysis of aqueous glucose solutions at high temperatures can result in highly selective production of glycolaldehyde, a C2 hydrocarbon with an aldehyde and hydroxyl functionality, toward direct applications in the food industry or as a chemical building block toward value-added products. A glucose pyrolysis model that was developed previously was expanded to capture the pyrolysis kinetics of glucose at hydrothermal conditions, accurately reproducing the observed product yields for a range of temperatures and feedstock compositions. Dominant reaction families were identified and interrogated to quantify the effect of hydrothermal conditions on the predicted kinetics. High yields of glycolaldehyde were achieved experimentally, with the maximum values observed at moderate temperatures and pure glucose feed. Elevated temperatures and increasing fructose concentrations negatively affected observed glycolaldehyde yields, resulting in increased production of undesired decomposition products in the C1-C3 range. The results of this study include preferred operating conditions toward maximizing the yield of glycolaldehyde as described by a predictive kinetic model that explicitly accounts for the major reactions comprising a complex network taking place and the effect of the operating conditions on their relative contributions to glucose conversion and product yields.