Wireless μLED packed beds for scalable continuous multiphasic photochemistry†

Abstract

Photochemical and photocatalytic reactions are a powerful emerging tool in the green synthesis of organic molecules. In contrast to thermochemical reactions, they promise greater energy efficiency, milder reaction conditions, and a decrease in the number of synthesis steps. Unfortunately, conventional batch photochemical systems are not inherently scalable, making translation to industrial applications challenging. Fundamentally, this is most clearly attributed to the penetration depth of light, as constrained by the Beer–Lambert relationship: as the size of the reactor is increased, the depth of light penetration into liquid medium decreases exponentially. Small-diameter plug flow reactors with external illumination have recently been employed industrially to 1) transition photochemistry from batch to continuous flow, and 2) overcome light penetration challenges by employing millimeter-scale optical paths; however these often present with substantial pressure drops and scalability challenges. In this work, a fixed bed reactor is packed with wireless μLEDs (μLED-PBR) and engineered to scale the oxidation of α-terpinene using a homogeneous rose-bengal photosensitizer. Utilizing μLEDs as packing allows for internal volumetrically scalable illumination from 250 or 500 μLEDs. Not only is the μLED packing efficient at delivering photons, but it also statically induces turbulence and mixing of the biphasic streams within the reactor. Unlike tubular plug flow reactors, the μLED-PBR design is volumetrically scalable. During operation, a co-current trickle flow regime was established with a 29 μm liquid film flowing over the μLEDs. In stark contrast to those typical in small channel tubular flow reactors, the packed bed experienced negligible hydrodynamic pressure drop penalties. The photochemical space time yield of the reactor normalized to the power consumption for the μLED-PBR was three orders of magnitude greater than other externally illuminated thin film flow reactors for the same chemistry: 1411 mmol W⁻¹ per day compared to 1.34 mmol W⁻¹ per day.