Solar Photooxygenations for the Manufacturing of Fine Chemicals—Technologies and Applications

molecules Review Solar Photooxygenations for the Manufacturing of Fine Chemicals—Technologies and Applications Jayson S. Wau 1, Mark J. Robertson 1 and Michael Oelgemöller 1,2,* 1 College of Science and Engineering, James Cook University, Townsville, QLD 4811, Australia; [email protected] (J.S.W.); [email protected] (M.J.R.) 2 Department of Organic and Macromolecular Chemistry, Ghent University, Krijgslaan 281 S4, 9000 Gent, Belgium * Correspondence: [email protected]; Tel.: +61-7-4781-4543 1 Abstract: Photooxygenation reactions involving singlet oxygen ( O2) are utilized industrially as a mild and sustainable access to oxygenated products. Due to the usage of organic dyes as photo- sensitizers, these transformations can be successfully conducted using natural sunlight. Modern solar chemical reactors enable outdoor operations on the demonstration (multigram) to technical (multikilogram) scales and have subsequently been employed for the manufacturing of fine chemicals such as fragrances or biologically active compounds. This review will highlight examples of solar photooxygenations for the manufacturing of industrially relevant target compounds and will discuss current challenges and opportunities of this sustainable methodology. Keywords: solar chemistry; photooxygenations; solar reactor; chemical manufacturing Citation: Wau, J.S.; Robertson, M.J.; Oelgemöller, M. Solar Photooxygenations for the 1. Introduction Manufacturing of Fine Photochemistry has recently seen a remarkable renaissance in synthetic organic chem- Chemicals—Technologies and istry [1–4]. This trend was sparked by the development of new photochemical transfor- Applications. Molecules 2021, 26, mations [5–8] as well as advances in photoreactor and light technologies [9–13]. However, 1685. https://doi.org/10.3390/ the large-scale realization of photochemical manufacturing processes is hindered by their molecules26061685 quantum yields and the significant installation and maintenance costs of artificial light sources [14,15]. As a result, industrial photochemistry is restricted to low-volume but Academic Editor: Axel G. Griesbeck high-value fine chemicals (such as fragrances, flavors or vitamins), or bulk but low-value chemicals (such as haloalkanes, oximes, sulfonyl chlorides and sulfonic acids) [14–19]. Received: 18 February 2021 Natural sunlight is regarded as an alternative and sustainable energy source for a range Accepted: 15 March 2021 of photochemical transformations [20–23]. Up until the early 20th century, photochemical Published: 17 March 2021 experiments were routinely performed by placing sealed flasks and tubes in direct sunlight [24–27]. Although this ‘flask in the sun’ approach is still followed today [28,29], the Publisher’s Note: MDPI stays neutral small scales and prolonged exposure times result in very low productivities of the desired with regard to jurisdictional claims in published maps and institutional affil- products. Recently, sunlight collectors and concentrators that were initially developed iations. for energy generation [30–33] have been modified for solar photochemical experimenta- tion [34]. Technical-scale solar degradation and detoxification processes have subsequently been successfully realized using these technologies [35–38]. In contrast, however, preparative solar chemistry for the manufacturing of chemicals is still comparably rare [39–47]. The main hurdles for a widespread implementation of solar synthesis are the discontinu- Copyright: © 2021 by the authors. ous availability and the low UV content (<400 nm) of just 3–5% of natural sunlight [48]. Licensee MDPI, Basel, Switzerland. Sunlight also consists largely of direct and diffuse radiation and their ratios vary with This article is an open access article distributed under the terms and the location and weather [49]. Solar manufacturing may thus be ideally performed in the conditions of the Creative Commons visible range (400–700 nm: 42–43% of solar spectrum [48]) for small annual production Attribution (CC BY) license (https:// targets (<100 tons/year) that do not require continuous, year-round operation under opti- 1 creativecommons.org/licenses/by/ mal solar light conditions. Photooxygenations with singlet oxygen ( O2) combine catalytic 4.0/). Molecules 2021, 26, 1685. https://doi.org/10.3390/molecules26061685 https://www.mdpi.com/journal/molecules Molecules 2021, 26, x FOR PEER REVIEW 2 of 28 Molecules 2021, 26, 1685 2 of 27 combine catalytic amounts of a dye sensitizer with visible light and yield a variety of ox- ygenatedamounts products of a dye [50–52]. sensitizer Phot withooxygenations visible light are and also yield performe a varietyd industrially of oxygenated to produce prod- low-volumeucts [50–52]. fine Photooxygenations chemicals such as are fragrances, also performed flavors industrially or pharmaceuticals to produce [53–55]. low-volume These featuresfine chemicals make photooxygenations such as fragrances, flavorsideal candid or pharmaceuticalsates for solar chemical [53–55]. Thesemanufacturing features make processes.photooxygenations ideal candidates for solar chemical manufacturing processes. 2.2. Solar Solar Reactors Reactors for for Synthetic Synthetic Applications Applications SolarSolar reactors reactors are are classified classified based based on on their their sun sun concentration factor factor (CF) (CF) and and range fromfrom non- non- to to highly highly concentrating concentrating systems. systems. Non- Non- and and low low concentrating concentrating reactors reactors are are char- char- acterizedacterized by by large large apertures apertures that that enable enable them them to to harvest harvest direct direct and and diffuse diffuse radiation. The The simplestsimplest non-concentrating non-concentrating devices areare flatbedflatbed reactors reactors with with large large surface surface areas, areas, thin thin bodies, bodies,and and operation operation volumes volumes of up of to up 30 to L (Figure30 L (Figure1a) [ 56 1a)]. These [56]. fixedThese reactors fixed reactors are best are tilted best to tiltedthe latitude to the latitude of the location of the location for optimal for optimal harvest harvest of sunlight. of sunlight. Improved Improved models models operate op- in eratecirculation in circulation mode with mode an externalwith an heatexternal exchanger. heat exchanger. The compound The compound parabolic collectorparabolic (CPC) col- lectoruses a(CPC) ‘round uses W’-shaped a ‘round polished W’-shaped aluminum polished reflector aluminum that reflector is tilted tothat the is localtilted latitude to the localand directslatitude all and available directs radiationall available onto radiation a central onto received a central tube received(Figure1 b)tube [57 (Figure,58] . These 1b) [57,58].advanced These systems advanced have systems a concentration have a concentr factoration of one factor sun of (or one slightly sun (or above) slightly and above) most andcommonly most commonly operate in operate circulating in circulating batch mode batch on mode volumes on volumes of up to 100of up L. to Several 100 L. reactorsSeveral reactorscan also can be connectedalso be connected in series in to series form to larger form units. larger Parabolic units. Parabolic trough reactorstrough reactors utilize large uti- lizereflectors large reflectors that focus that only focus direct only sunlight direct sunlight onto a reaction onto a reaction tube in their tube focalin their line focal and line can andreach can concentration reach concentration factors offactors 20–150 of suns20–150 [32 su].ns More [32]. modern More modern reactors reactors such as such the former as the formerPROPHIS PROPHIS loop (PaRabolic loop (PaRabolic trough-facility trough-facility for Organic for PHotochemicalOrganic PHotochemical synthesis, synthesis, Figure1c) Figureoperate 1c) on operate volumes on up volumes to 150 Lup and to 150 track L theand movement track the movement of the sun, of either the sun, horizontally either hor- or izontallythree-dimensionally or three-dimensionally (horizontally (horizontally and vertically) and [ 59vertically)]. [59]. (a) (b) (c) FigureFigure 1. 1. ((aa)) Circulating Circulating flatbed flatbed reactor reactor with with refl reflectiveective back back and and external external cooling cooling at at James James Cook Cook University University in in Townsville, Townsville, 2 Australia.Australia. ( (bb)) CPC CPC reactor reactor with with a a 2 2 m m2 apertureaperture at at James James Cook Cook University University in in Townsville, Townsville, Australia. ( (cc)) Former Former PROPHIS PROPHIS looploop at at the the German German Aerospace Aerospace Center Center (DLR) (DLR) in Cologne-Porz, Cologne-Porz, Germany (modified (modified from [[60]).60]). WhileWhile solar solar dish dish concentrators concentrators can can provide provide very very high high concentration concentration factors factors of of up up to to 50005000 suns, suns, these these sophisticated sophisticated devices devices are are rare rarelyly used used for for preparative preparative solar solar photochemical photochemical applications.applications. Since thethe receiver receiver in in the the focal foca pointl point easily easily reaches reaches extreme extreme temperature temperature ranges, ranges,dish reactors dish reactors are instead are advantageousinstead advantageo for thermochemicalus for thermochemical processes processes [33]. Likewise, [33]. Like- solar wise,furnaces solar can furnaces reach extreme can reach concentration

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