molecules Review Designing Microflowreactors for Photocatalysis Using Sonochemistry: A Systematic Review Article Swaraj Rashmi Pradhan 1,* , Ramón Fernando Colmenares-Quintero 2 and Juan Carlos Colmenares Quintero 1,* 1 Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland 2 Universidad Cooperativa de Colombia, Calle 50A No. 41–34 Medellín, Colombia; [email protected] * Correspondence: [email protected] (S.R.P.); [email protected] (J.C.C.Q.); Tel.: +48-22-343-3215 (J.C.C.Q.) Academic Editor: Gregory Chatel Received: 31 July 2019; Accepted: 8 September 2019; Published: 12 September 2019 Abstract: Use of sonication for designing and fabricating reactors, especially the deposition of catalysts inside a microreactor, is a modern approach. There are many reports that prove that a microreactor is a better setup compared with batch reactors for carrying out catalytic reactions. Microreactors have better energy efficiency, reaction rate, safety, a much finer degree of process control, better molecular diffusion, and heat-transfer properties compared with the conventional batch reactor. The use of microreactors for photocatalytic reactions is also being considered to be the appropriate reactor configuration because of its improved irradiation profile, better light penetration through the entire reactor depth, and higher spatial illumination homogeneity. Ultrasound has been used efficiently for the synthesis of materials, degradation of organic compounds, and fuel production, among other applications. The recent increase in energy demands, as well as the stringent environmental stress due to pollution, have resulted in the need to develop green chemistry-based processes to generate and remove contaminants in a more environmentally friendly and cost-effective manner. It is possible to carry out the synthesis and deposition of catalysts inside the reactor using the ultrasound-promoted method in the microfluidic system. In addition, the synergistic effect generated by photocatalysis and sonochemistry in a microreactor can be used for the production of different chemicals, which have high value in the pharmaceutical and chemical industries. The current review highlights the use of both photocatalysis and sonochemistry for developing microreactors and their applications. Keywords: ultrasound; flow microreactor; photocatalysis; water/air detoxification; organic synthesis; semiconductor 1. Introduction With the continuous and prosperous development of modern civilizations, environmental contamination has spread far and wide. Faced with this issue, humankind reached a consensus on the need for environmental treatment and remediation. Green chemistry is the implementation of twelve principles [1] (Figure1) that lowers the use or generation of hazardous substances in the design, manufacture, and application of chemical products [2]. Our society is increasingly demanding the innovation of newer approaches to be sustainable in order to preserve the environment. It is crucial for these approaches to be less dependent on self-depleting sources or sources that effuse green-house gases in use. Molecules 2019, 24, 3315; doi:10.3390/molecules24183315 www.mdpi.com/journal/molecules Molecules 2019, 24, 3315 2 of 22 Molecules 2019, 24, x FOR PEER REVIEW 2 of 23 Figure 1. TheThe twelvetwelve principlesprinciples ofof greengreen chemistry.chemistry. PhotocatalysisPhotocatalysis occupiesoccupies anan essentialessential placeplace inin thethe ecologicalecological equilibriumequilibrium andand isis aa goodgood exampleexample ofof greengreen chemistrychemistry [3[3]].. Photocatalysis activates activates reactions depending on thethe lightlight (clean(clean andand superabundantlysuperabundantly availableavailable fromfrom thethe Sun)Sun) asas anan energyenergy source.source. Therefore,Therefore, researchresearch onon thethe utilizationutilization ofof solarsolar energyenergy hashas continuedcontinued toto bebe anan importantimportant topictopic [ 2[2,4],4].. PhotocatalysisPhotocatalysis inin microreactorsmicroreactors isis attractingattracting thethe attentionattention ofof manymany researchersresearchers becausebecause ofof itsits greenergreener aspect.aspect. BecauseBecause ofof reducedreduced reagentreagent requirements,requirements, shortershorter reactionreaction time,time, lesseninglessening ofof by-products,by-products, andand minimizedminimized energyenergy consumption,consumption, microreactorsmicroreactors are are regarded regarded as as a a green green synthetic synthetic approach approach [5]. [5] Nowadays,. Nowadays, many many groups groups are are working working on synthesizingon synthesizing catalysts catalysts inside inside a microreactor. a microreactor. Among Among the catalysts, the catalysts, titania-based titania- catalystsbased catalysts are well-known are well- photocatalystsknown photocatalyst under UVs under (Ultraviolet) UV (Ultraviolet) light and light have and been ha identifiedve been identified as a form as of a technologyform of technology playing anplaying important an important role in solving role in many solving of themany problems of the problems in water purificationin water purification [6]. [6]. InIn thisthis reviewreview article,article, afterafter aa brief introduction, theoretical backgrounds of the flow flow microreactor microreactor,, ultrasound,ultrasound, andand theirtheir combinedcombined studiesstudies areare discussed.discussed. Ultrasound irradiation is acceptedaccepted asas anan environmentallyenvironmentally benignbenign techniquetechnique toto carrycarry outout chemicalchemical reactionsreactions[ [7]7].. TheThe applicationapplication ofof ultrasoundultrasound waveswaves hashas beenbeen consideredconsidered asas anan agreeableagreeable techniquetechnique inin chemistry.chemistry. EarlyEarly worksworks onon catalystcatalyst synthesissynthesis inin aa microreactormicroreactor usingusing ultrasoundultrasound andand theirtheir comparisoncomparison withwith conventionalconventional batchbatch experimentsexperiments andand futurefuture challengeschallenges areare reviewedreviewed in in this this article. article. AsAs illustratedillustrated in in Figure Figure2, the 2, scientificthe scientific community community is trying is totrying make ourto make planet our green planet by combining green by chemicalcombining engineering chemical (e.g.,engineering manufacturing (e.g., manufacturing microchannels microchannel by ultrasound)s by with ultrasound) material chemistry with material (e.g., photocatalysts).chemistry (e.g., photocatalyst The major purposes). The ofmajor this purpose review articleof this isreview to highlight article is the to challengeshighlight the ahead challenges of the designahead andof the development design and ofdevelopment (photo)catalytic of (photo)catalytic microfluidic reactors microfluidic using ultrasound.reactors using To ultrasound the best of our. To knowledge,the best of thisour isknowledge, the first technical this is reviewthe first in technical the field ofreview microflow in the reactors field of for microflow photocatalysis reactor usings for sonochemistry,photocatalysis whichusing sonochemistry is promising for, which the upcoming is promising studies for inthe this upcoming branch of studies science. in this branch of science. 2 Molecules 2019, 24, 3315 3 of 22 Molecules 2019, 24, x FOR PEER REVIEW 3 of 23 FigureFigure 2.2. Two branches,branches, trying trying to to produce produce greener greener chemistry chemistry.. 2.2. Theoretical Theoretical Background Background ThereThere are two two main main types typesof flow- ofreactor flow-reactor system that systempeople use that for synthetic people photochemistry use for synthetic— photochemistry—micromicro and macro flow and[8]. macroFor various flow [applications8]. For various, people applications, use the m peopleicro-synthesis use the micro-synthesistechnique in techniquedisciplines in of disciplines both engineering of both and engineering sciences. andDefinition sciences. of aDefinition microfluidic of segment a microfluidic in a microreactor segment in a microreactoris described isas described a minimum as aunit minimum having properties unit having that properties can be used that to can improve be used various to improve operati variousons operationsand reactions and reactions[5]. Microreactors [5]. Microreactors offer new opossibilitiesffer new possibilities of reactions. of reactions.Microreactors Microreactors have been haveproven been provento be tohighly be highly effective effective for catalytic for catalytic reactions reactions because because of their of theirindispens indispensableable advantages advantages,, such as such asuniform uniform illumination illumination without without light light attenuation, attenuation, large large surface surface-to-volume-to-volume ratio ratio and and,, consequently, consequently, attaining of a high heat and mass transfer rate, and the resultant satisfactory catalytic effect [9,10]. attaining of a high heat and mass transfer rate, and the resultant satisfactory catalytic effect [9,10]. Also, one can easily control the contact time, shape, and size of the interface between fluids in these Also, one can easily control the contact time, shape, and size of the interface between fluids in these systems [11,12]. The aforementioned attributes make microreactors ideal for highly exothermic and systems [11,12]. The aforementioned attributes make microreactors ideal for highly exothermic and fast reactions. fast reactions. Recently,