Scale-up Study of Benzoic Acid Alkylation in Flow: From Microflow Capillary Reactor to a Milliflow Reactor

The esterification reaction is one of the most important reaction in organic chemistry, and the development of scalable continuous flow conditions is of high interest. Here we describe a scaling-up study of benzoic acid alkylation by MeI in the presence of superbases from a capillary reactor (282 mg·h–1 at 75 °C) to a low flow reactor (LFR, 6.82 g·h–1 at 80 °C). First, the optimization of the base demonstrated that 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) was a good compromise in terms of basicity and price. A comparison between batch and LFR reaction showed similar kinetic rates. We also observed a maximum reactivity for a flow rate of 3 mL·min–1 which corresponds to an optimum mixing of the entering fluids. A colorimetric study permits to show a visual change in the flow regime (dispersed to stratified) by varying the flow rate with a maximum conversion obtained at 3 mL·min–1 corresponding to a 105 s residence time, demonstrating that the studied reaction is sensitive to mixing conditions and that LFR presents some mixing limitations for low flow rates. After thermal optimization (80 °C), the reaction has been improved to a productivity of 6.82 g/h at the lab scale using DBU as the base and Eyring/Arrhenius plots permitted to extract activation energies.