Improving Continuous Flow Singlet Oxygen Photooxygenation Reactions with Functionalized Mesoporous Silica Nanoparticles

Abstract

Continuous flow photochemistry relying on photosensitizers faces two main challenges: 1) Photodegradation (bleaching) and 2) the downstream removal of the photosensitizer. Rose bengal (RB) is a common photosensitizer utilized for photooxygenation reactions with singlet oxygen (1O2), but is notoriously sensitive to photobleaching and difficult to remove from reactor effluents. The heterogenization of photosensitizers on mesoporous silica nanoparticles (MSNs) is arguably a viable option for such applications. Herein, we report on the use of RB covalently incorporated into MSNs (RB@MSNs) for photooxygenation reactions under continuous flow conditions. RB@MSNs enable the 1O2 photooxygenation of various organic substrates upon irradiation with 540 nm LEDs. A series of organic substrates were evaluated including methionine, α‐terpinene, 2‐furoic acid, triphenylphosphine, citronellol and cyclopentadiene. These results emphasize an improved resistance to photobleaching, and the possibility to use RB@MSNs as an easily recoverable catalyst, which could be removed from the reactor effluent either a) by centrifugation or b) by in‐line membrane filtration.

Flow Photochemistry: Shine Some Light on Those Tubes!

“…The mixing within the suspension segments promotes the reaction, similarly to the gas-liquid version. Besides applications in singlet oxygen chemistry [52], Gilmore and Seeberger recently exploited this method for a (decarboxylative) fluorination catalyzed by a carbon nitride catalyst [53]. The combination of continuous stirring and the use of dense ionic-liquid solvent were critical to maintaining the solid catalyst in suspension prior to injection into the reactor.…”

“…Heterogenization of homogeneous catalysts to prevent degradation and facilitate separation is also a viable alternative. Dreesen, Monbaliu, and Heinrichs demonstrated this concept by anchoring a photocatalyst to silica nanoparticles for photooxygenation reactions [52]. A very fine colloidal suspension was obtained and easily pumped through the reactor without recourse to a slurry-Taylor.…”

Performances of Homogeneous and Heterogenized Methylene Blue on Silica Under Red Light in Batch and Continuous Flow Photochemical Reactors

“…Important efforts have been made to intensify photochemical processes by means of flow chemistry and this technology is the most promising for larger-scale photochemical manufacturing. However, this approach remains limited to homogeneous or gas/liquid conditions and yet, processing heterogeneous solid/liquid and solid/liquid/gas reactions is still underdeveloped (Carofiglio et al, 2008;Woźnica et al, 2014;Amara et al, 2015;Mendoza et al, 2018;Pieber et al, 2018;Blanchard et al, 2020;Mendoza et al, 2020;Radjagobalou et al, 2020).…”

Efficient Photooxygenation Process of Biosourced α-Terpinene by Combining Controlled LED-Driven Flow Photochemistry and Rose Bengal-Anchored Polymer Colloids

“…Such a concept has been successfully applied in microreactors for catalytic hydrogenation applications [57][58][59] but to date, only two studies have been reported for photooxygenation, and involved either conjugated microporous polymers (CMPs) 56 or functionalized mesoporous silica particles. 60 The most relevant strategy for achieving good operability appears to be the implementation of submicronic materials covalently grafted to an efficient organic photosensitizer. Their smaller sizes mean that they are not prone to settle (in particular at the microreactor walls where the velocities are very low), unlike micrometric beads (such as the commercial Sensitox ® polystyrene particle).…”

“…As a continuous flow of bubbles (air or pure oxygen) remains the most preferable option to supply oxygen to the reaction medium, this leads to operation under gas–liquid slurry flows (Figure ). Such a concept has been successfully applied in microreactors for catalytic hydrogenation applications − but, to date, only two studies have been reported for photooxygenation, and involved either conjugated microporous polymers (CMPs) or functionalized mesoporous silica particles . The most relevant strategy for achieving good operability appears to be the implementation of submicronic materials covalently grafted to an efficient organic photosensitizer.…”