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

Abstract

Methylene blue was efficiently immobilized on silica micro- and nanoparticles by electrostatic interactions and the performances of the heterogenized photocatalysts were compared against the homogeneous conditions using the photooxidation of citronellol as a model reaction under red light, in a batch and a continuous flow photochemical reactor. In batch, the heterogeneous photocatalyst outperforms the homogeneous one, presumably due to kinetic and stability effects. The two catalytic systems are also compared in a flow reactor displaying improved mass transfer properties. We demonstrate that this results in a dramatic enhancement in photocatalyst stability, reactivity and productivity. This study highlights the importance of photocatalyst stability under homogeneous versus heterogenized conditions and in batch versus flow photochemistry.

Applications of red light photoredox catalysis in organic synthesis

“…However, after these initial works, though vast MB-catalyzed reactions were reported under the irradiation of blue or white light, 1b,g no red-light-mediated reactions were introduced until 2018. [12][13][14] Hecht and co-workers showed the quantitative isomerization of Z-azobenzene 11a to E-azobenzene 11b in the presence of MB and red light (Scheme 1c). 12 Later, the groups of Watanabe and Hosoya presented a rapid uncaging of alcohols or carboxylic acids of 3-acyl-2-methoxyindolizines 12 or 14 via red-light-induced MB-catalyzed photooxidation (Scheme 1d).…”

“…Recently, the Amara group reported the red light-induced photooxidation of citronellol 16 in the presence of MB or MB@SiO 2 in both batch and continuous flow photochemical reactors, indicating the solid compatibility of red light catalysis with flow chemistry (Scheme 1e). 14 Fig. 5 PCs involved in direct red light photoredox catalysis.…”

Modelling approaches to predict light absorption in gas-liquid flow photosensitized oxidations

“…[5] During the last two decades, singlet oxygen oxidation has become a common benchmark reaction in flow photochemistry. [6][7][8].…”

“…The photooxidations were performed in single [9][10][11], gas-liquid, and multi-phase flow (e.g. gasliquid-solid) [8,[12][13][14]. The optimization of operating conditions was studied for various gas-liquid flow patterns such as annular [10,[15][16][17], Taylor [18][19][20], bubbly flow [21][22][23] and more recently, aerosols [24,25].…”

Self-Sensitized Photooxidation of Naphthols to Naphthoquinones and the Use of Naphthoquinones as Visible Light Photocatalysts in Batch and Continuous Flow Reactors

“…Visible light photochemistry is particularly appealing from a green chemistry standpoint, as it enables a large variety of chemical transformations to operate under very mild conditions. − Because most organic compounds do not absorb visible light, the main method of driving such processes is to use a photocatalyst (also called a “photosensitizer”) to transfer electrons or energy to a given substrate upon excitation by one or multiple photons. − These processes can be mediated by either organic or organometallic photocatalysts, such as rose bengal, − porphyrins, methylene blue (MB), − or ruthenium trisbipyridine salts, − to name a few. Such photocatalysts and their derivatives have been widely used in organic synthesis, particularly in the context of singlet oxygen ( 1 O 2 ) mediated photooxidations (in this context the term “photosensitizer” is more appropriate) .…”