In industrial synthetic chemistry laboratories, reactions are generally carried out using batch-mode methodologies, stepwise reactions, and purifications to generate a final product. Each step has an associated yield of both the reaction itself and of the final purification that is largely dependent on the procedure being used, and the scientist carrying out the procedure. Continuous-flow reactors are one way of streamlining the process. Furthermore, microwave-enhanced, or microwave-assisted, chemistry has been demonstrated to aid in many of these areas; however, scaling has been a traditional problem with this technique.
Continuous-flow reactors offer many benefits for industrial chemistry, including easier reaction scaling and often better control of reaction conditions. Using microwaves in place of a conventional heater can generate higher product yields and purities, better control of reaction conditions, higher reproducibility, and sometimes even new chemical reactions, which further expand the synthetic toolbox.
This work uses an RF-powered continuous-flow reactor, developed for the extraction of biomarkers from regolith, which can also be used for microwave-enhanced synthetic reaction chemistries. The RF power source consists of a custom-built, fixed-frequency (60 GHz), millimeter-wave generator and a coupling waveguide that focuses the power onto the flowing aqueous sample. During operation, a liquid solution is continuously pumped through the micro-reactor by an ISCO 260D syringe pump. Head pressure is applied to prevent boiling of the sample when exposed to RF radiation. The flow restrictor is connected to the sample tubing by a micro-volume reducing union to maintain constant head pressures in the sample line. The sample is loaded with a syringe connected to the sample line by a micro-volume T-connector and a custom-made Labsmith syringe adapter. Swagelok valves control the sample loading and the water flow through the micro-reactor’s sample handling system.
There is a large push for greener synthetic chemical processes, requiring higher yield processing, less solvent consumption, fewer purification steps that use up further resources, and lower power requirements, among other conditions. The RF-powered micro-reactor offers microwave-enhanced chemical reactions in a continuous-flow operation. Only a handful of continuous-flow microwave reactors have been made, and these are generally modified microwave reactors that deliver a large amount (often over 100 W of power) of microwave energy at 2.45 GHz to the flowing solutions. The RF-powered continuous-flow reactor instead uses a directed 60-GHz source of up to 1W. The key development here is that by directing the power adsorption into a small volume, far less power is required to observe the same reactivity profiles.
This work was done by Valerie Scott and Xenia Amashukeli of Caltech for NASA’s Jet Propulsion Laboratory. NPO-49602