Biomass as a renewable and abundantly available carbon source is a promising alternative to fossil resources for the production of chemicals and fuels. The development of biobased chemistry, along with catalyst design, has received much research attention over recent years. However, dedicated reactor concepts for the conversion of biomass and its derivatives are a relatively new research field. Continuous flow microreactors are a promising tool for process intensification, especially for reactions in multiphase systems. In this work, the potential of microreactors for the catalytic conversion of biomass derivatives to value-added chemicals and fuels is critically reviewed. Emphases are laid on the biphasic synthesis of furans from sugars, oxidation and hydrogenation of biomass derivatives. Microreactor processing has been shown capable of improving the efficiency of many biobased reactions, due to the transport intensification and a fine control over the process. Microreactors are expected to contribute in accelerating the technological development of biomass conversion and have a promising potential for industrial application in this area.

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Homogeneous Co/Mn/Br catalyzed aerobic oxidation of benzyl alcohol in acetic acid to benzaldehyde was performed in polytetrafluoroethylene microreactors operated under slug flow at temperatures up to 150 C and pressures up to 5 bar. Depending on the bubble velocity and length, a wetted or dewetted slug flow was observed, characterized typically by a complete or partially wetting liquid film around the bubble body. The latter flow suffered from a limited interfacial area for mass transfer. Experiments at temperatures up to ca. 90 C were under kinetic control given no product yield difference under wetted and dewetted slug flows and were used to establish a simplified kinetic expression (first order in benzyl alcohol and zero order in oxygen). This allows to develop a mass transfer model combined with an instantaneous reaction regime that well described the experimental results at higher temperatures where mass transfer was limiting in the dewetted slug flow.

Lignin is a promising feedstock for the replacement of conventional carbon sources for the production of chemicals and fuels. In this paper, results are reported for the depolymerization of various residual lignins in the absence of a catalyst by utilizing ozone. Reactions were performed in a microreactor setup ensuring high gas–liquid mass transfer rates, a low inventory of ozone, and straightforward scale-up possibilities. The ozonation is demonstrated using a representative model compound (vanillin) and various lignins (pyrolytic and organosolv) dissolved in methanol (2.5 wt %). Experiments were performed under ambient conditions, at gas–liquid flow ratios ranging from 30 to 90 and short residence times on the order of 12–24 s. Analyses of the products after methanol removal revealed the presence of (di)­carboxylic acids, methyl esters, and acetals. Extensive depolymerization was achieved (i.e., up to 30% for pyrolytic lignin and 70% for organosolv lignins). Furthermore, a two-step approach in which the ozonated lignin is further hydrotreated (350–400 °C, 100 bar H2, 4 h, Pd/C as catalyst) showed a substantial increase in depolymerization efficiency, yielding a 2.5-fold increased monomer yield in the product oil compared to a hydrotreatment step only.

Oxidation of 5-hydroxymethylfurfural (HMF) using air or pure oxygen was performed in polytetrafluoroethylene capillary microreactors under gas-liquid slug flow, with Co/Mn/Br as the homogeneous catalyst in the acetic acid solvent. The temperature was varied from 90 to 165 • C at a pressure of 1 or 5 bar. At atmospheric pressure conditions (and 90 • C), acetaldehyde was further added as a co-oxidant to accelerate the reaction. At 150 • C, 5 bar oxygen and a residence time of 2.73 min, an HMF conversion of 99.2% was obtained, with the yields of 2,5diformylfuran (DFF), 5-formylfurancarboxylic acid (FFCA) and 2,5-furandicarboxylic acid (FDCA) being 22.9%, 46.7%, and 23.8%, respectively. By operation under wetted slug flows and elevated partial oxygen pressures, mass transfer limitations and oxygen depletion in the microreactor could be eliminated. This allowed to run the microreactor under kinetically controlled conditions, where both the HMF consumption and DFF formation were found zero order in partial oxygen pressure and roughly first order in HMF. The total selectivity towards DFF/ FFCA/FDCA was ca. 40% at low partial oxygen pressures due to the dominant occurrence of side reactions. By using pure oxygen at 5 bar the total selectivity was improved to 60-94%. The space time yields of DFF and FFCA in the microreactor exceeded those obtained in conventional (semi-)batch reactors at slightly elevated temperatures and pressures, due to the superior mass transfer and higher initial HMF concentrations in the microreactor. For highly efficient FDCA synthesis, more dedicated microreactor operations are needed to tackle its precipitation.

The enzymatic esterification of oleic acid and 1-butanol to butyl oleate was performed in an aqueous–organic system in capillary microreactors with various inner diameters operated under slug flow. The free Rhizomucor miehei lipase in the aqueous phase was used as a catalyst and n-heptane as the organic solvent. A close to 100% yield of butyl oleate could be achieved in the microreactor made of polytetrafluoroethylene within 30 min residence time at 30 °C. The reaction rate is well described by the existing kinetic model based on a Ping Pong Bi Bi mechanism with competitive inhibition of 1-butanol. This model was extended to describe the effect of the interfacial area and aqueous-to-organic flow ratio in microreactors. By performing the reaction at low aqueous-to-organic flow ratios in hydrophilic microreactors (e.g., made of stainless steel), the enzyme turnover number could be enhanced significantly, making it promising for process intensification.

The quality of cocoa depends on both the origin of the cacao and the processing stages. The roasting process is critical because it develops the aroma and flavor, changing the beans' chemical composition significantly by chemical reactions induced by thermal energy. Aspects have been identified as the main differences between bulk cocoa and fine cocoa, the effect of time and temperature on the formation of the flavor and aroma, and the differences between conductive heating in an oven, convective with airflow, and steam flow. Thermal energy initially causes drying, then non-enzymatic browning chemical reactions (Maillard reaction, Strecker degradation, oxidation of lipids, and polyphenols), which produce volatile and non-volatile chemical compounds related to the flavor and aroma of cocoa roasted. This review identified that the effect of the heating rate on the physicochemical conversion of cocoa is still unknown, and the process has not been evaluated in inert atmospheres, which could drastically influence the avoidance of oxidation reactions. The effect of particle size on the performance of product quality is still unknown. A more in-depth explanation of energy, mass, and chemical kinetic transfer phenomena in roasting is needed to allow a deep understanding of the effect of process parameters. In order to achieve the above challenges, experimentation and modeling under kinetic control (small-scale) are proposed to allow the evaluation of the effects of the process parameters and the development of new roasting technologies in favor of product quality. Therefore, this work seeks to encourage scientists to work under a non-traditional scheme and generate new knowledge.