Process development for the continuous epoxidation of renewable terpenes using “Mesoscale” 3D-printed oscillatory baffled reactor

Abstract

A continuous process was developed for the epoxidation of (R)-(+)-limonene and α-pinene with an oxidant (H2O2) using a polytungstophosphate catalyst in a mesoscale Oscillatory Baffled Reactor (mesoOBR). Waste biomass derived (R)-(+)-limonene and α-pinene were used as an alternative to petrochemical-based epoxides. A selective process towards the epoxides was investigated by the screening of process parameters including temperature, oxidant molar ratio, sodium sulphate (Na2SO4) amount, acid (H2SO4) concentration and solvent type. The mass and heat transfer limitation associated with the exothermic and biphasic epoxidation reaction was overcome using new 3D-printed baffles in the mesoOBR platform. Screening the process parameters for the epoxidation of (R)-(+)-limonene revealed that a high H2O2 conversion (~95 %) and selectivity to the limonene-1,2-epoxide (100 %), was able to achieve in 15 minutes with a single-step addition of H2O2. The operating conditions included a 50 °C temperature in an organic solvent-free environment, with a (R)-(+)- limonene/H2O2/catalyst molar ratio of 4:1:0.005. To prevent the hydrolysis of the epoxide, the reaction mixture was saturated with Na2SO4 (5.7 g). An acid concentration of lower than 0.04 M was used and found to have a significant effect on the selectivity. Kinetic studies were performed to allow modelling of the reaction scheme. A kinetic investigation showed that the reaction was first-order in terms of (R)-(+)-limonene and catalyst concentration, and fractional order (~0.5) with respect to the H2O2 concentration. The activation energy for the formation of limonene-1,2-epoxide and limonene1,2-diol was determined to be ~36 and 79 kJ mol‒1 , respectively. The epoxidation of α-pinene with H2O2 was also performed using polytungstophosphate catalyst. The variables in the screening parameters were temperatures (30–70 °C), oxidant amount (100-200 mol%), acid concentrations (0.02-0.09 M) and solvent types (1,2- dichloroethane, toluene, p-cymene, and acetonitrile). Screening the process parameters revealed that a 100% selective epoxidation of α-pinene to α-pinene oxide was possible with negligible side-product formation within a short reaction time (~20 minutes), using process conditions of a 50 °C temperature in an organic solvent-free environment and a α-pinene/H2O2/catalyst molar ratio of 5:1:0.01. A kinetic investigation also showed that the reaction was first-order in terms of α-pinene and catalyst concentration, and fractional order (~0.5) with respect to the H2O2 concentration. The activation energy of ~35 kJ mol-1 was obtained for the epoxidation of αpinene, which was similar to ~36 kJ mol-1 for (R)-(+)-limonene. Novel 3D-printed orifice baffles were integrated with a mesoscale oscillatory baffled reactor for the continuous epoxidation of terpenes ((R)-(+)-limonene and α-pinene) with H2O2 in an organic solvent-free environment. The biphasic reaction is highly exothermic, usually requiring solvent, tight temperature control and effective multiphase mixing. The performance of the new 3D-printed single, tri- and multi-orifice baffles was compared to conventional helical and integral baffles. The performance investigated were the mixing intensity, induction period, multi steady state and heat removal capability. Passive isothermalisation was also investigated using mesoOBR in a heat pipe assembly. The tri- and multi-orifice baffles were able to overcome mixing limitation in continuous epoxidation and achieved a comparable rate of reaction to batch epoxidation at mixing condition of oscillatory Reynolds number (Reo) >850 and Reo >500, respectively. Both baffles exhibited rapid steady state attainment, shorter induction period at t = 1.5 residence time (τ) and better reproducibility with product variation of ~ 1.3%. Other mesoOBRs designs had induction times of 2.0 τ – 3.0 τ and product variations in the range of 1.6 – 2.1 %. The helically baffled mesoOBR designs demonstrated effective heat transfer capability, allowing the reaction to being operated isothermally with ±1 °C temperature variation in an organic solvent-free condition. Thisremoves the need of a solvent, thus reducing reaction volume by a 5-fold. The timescale for the reaction was reduced from ~ 8 hours in a conventional process to 30 minutes in the multi-orifice mesoOBR, a 16-fold reduction. Therefore, a better process has been developed for a continuous epoxidation of (R)-(+)- limonene and α-pinene with H2O2 using multi-orifice mesoOBRs, with a potential intensification factor of ~ 80.