Key role of temperature monitoring in interpretation of microwave effect on transesterification and esterification reactions for biodiesel production

•Temperature monitoring is more accurate using an optical fiber than an infrared sensor.•Transesterification is not accelerated under microwave irradiation.•Esterification with methanol is not accelerated under microwave irradiation.•Main cause of “the microwave effect” is temperature underestimation.•Temperature underestimation may be associated with superheating of methanol. Microwave effects have been quantified, comparing activation energies and pre-exponential factors to those obtained in a conventionally-heated reactor for biodiesel production from waste cooking oils via transesterification and esterification reactions. Several publications report an enhancement of biodiesel production using microwaves, however recent reviews highlight poor temperature measurements in microwave reactors give misleading reaction performances. Operating conditions have therefore been carefully chosen to investigate non-thermal microwave effects alone. Temperature is monitored by an optical fiber sensor, which is more accurate than infrared sensors. For the transesterification reaction, the activation energy is 37.1kJ/mol (20.1–54.2kJ/mol) in the microwave-heated reactor compared with 31.6kJ/mol (14.6–48.7kJ/mol) in the conventionally-heated reactor. For the esterification reaction, the activation energy is 45.4kJ/mol (31.8–58.9kJ/mol) for the microwave-heated reactor compared with 56.1kJ/mol (55.7–56.4kJ/mol) for conventionally-heated reactor. The results confirm the absence of non-thermal microwave effects for homogenous-catalyzed reactions.