Accurate Measurement of the Photon Flux Received Inside Two

Accurate Measurement of the Photon Flux Received Inside Two Continuous Flow Microphotoreactors by Actinometry Tristan Aillet, Karine Loubiere, Odile Dechy-Cabaret, Laurent E. Prat

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Tristan Aillet, Karine Loubiere, Odile Dechy-Cabaret, Laurent E. Prat. Accurate Measurement of the Photon Flux Received Inside Two Continuous Flow Microphotoreactors by Actinometry. International Journal of Chemical Reactor Engineering, De Gruyter, 2014, vol. 12 (n° 1), pp. 1-13. 10.1515/ijcre- 2013-0121. hal-01153086

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To link to this article : DOI:10.1515/ijcre-2013-0121 URL : http://dx.doi.org/10.1515/ijcre-2013-0121

To cite this version : Aillet, Tristan and Loubiere, Karine and Dechy-Cabaret, Odile and Prat, Laurent E. Accurate Measurement of the Photon Flux Received Inside Two Continuous Flow Microphotoreactors by Actinometry. (2014) International Journal of Chemical Reactor Engineering, vol. 12 (n° 1). pp. 1-13. ISSN 1542-6580

Any correspondance concerning this service should be sent to the repository administrator: [email protected] Tristan Aillet, Karine Loubiere*, Odile Dechy-Cabaret, and Laurent Prat Accurate Measurement of the Photon Flux Received Inside Two Continuous Flow Microphotoreactors by Actinometry

Abstract: In this study, the photon flux received in two 1 Introduction continuous flow microphotoreactors was measured by actinometry (potassium ferrioxalate). The microphotoreactors Recently, microreactor technology has been successfully had two different geometries and were irradiated by either investigated in the organic chemistry field. The micro-scale a polychromatic or a monochromatic light source. A model offers several advantages, such as short molecular diffusion considering the partial absorption of photons through the distances, intensified heat and mass transfers, and small reactor depth and, if required, the polychromatic character amounts of reactants used. In the case of photochemistry, of the light source and the dependence of the actinometer microreactors also allow efficient light penetration even for properties on the wavelength were formulated to describe relatively concentrated solutions. For these reasons, micro- the variation of the actinometer conversion with the irra- reactors are increasingly being adopted for photochemical diation time. The photon flux received in the microphotor- reactions, including photocycloadditions, photooxygena- eactors could be thus accurately calculated as a function of tions, photoisomerizations and photocatalytic reactions the emitted wavelength. The same methodology was then (Coyle and Oelgemöller 2008; Oelgemoeller 2012; Knowles, applied to measure the photon flux received in a batch Elliott, and Booker-Milburn 2012). Although the benefits of immersion well photoreactor. The radiant power received microreactors for photoconversion are now accepted, there in each photoreactor was compared to that emitted by the is still a lack of objective criteria for understanding and lamp and major differences were found, thus confirming modelling the positive effect of the microspace, and thus the need for this kind of in situ measurement. Finally, for rigorously transposing photochemical synthesis from some guidelines based on a knowledge of the photon flux batch to intensified continuous flow reactors. This requires were proposed to compare various photoreactors. They a modelling approach in which hydrodynamic and radiative revealed in particular that the choice of the most efficient transfer phenomena are coupled via the reaction kinetics photoreactor depended on the criteria chosen to evaluate term. In addition, microreactors are widely used for studying the performances (i.e. productivity, Space Time Yield). reactions and acquiring the associated kinetic data (Mozharov et al. 2011). By analogy, we could imagine using photochemistry, microphotoreactor, chemical Keywords: microreactors to determine some intrinsic properties of a actinometry, photon flux photochemical reaction, such as the quantum yield. In all cases, for modelling, for data acquisition purposes, or for optimizing reaction performance, it is essential *Corresponding author: Karine Loubiere, Université de Toulouse, −1 INPT, ENSIACET, F-31432 Toulouse, France; CNRS, Laboratoire de to know the photon flux (einstein s ) actually received in the Génie Chimique (LGC UMR 5503), 4 allée Emile Monso, BP 84234, continuous flow microphotoreactor. Up to now, this photon F-31432 Toulouse, France, E-mail: [email protected] flux has not been accurately measured in such systems, and Tristan Aillet, Université de Toulouse, INPT, ENSIACET, F-31432 most studies have estimated this parameter either by a mod- Toulouse, France; CNRS, Laboratoire de Génie Chimique (LGC UMR elling approach (Aillet et al. 2013) or from the characteristics 5503), 4 allée Emile Monso, BP 84234, F-31432 Toulouse, France, E-mail: [email protected] of the light source (Sugimoto et al. 2009; Shvydkiv et al. 2011; Odile Dechy-Cabaret, Université de Toulouse, INPT, ENSIACET, Aida et al. 2012). For example, Sugimoto et al. (2009) carried F-31432 Toulouse, France; CNRS, Laboratoire de Chimie de out the Barton reaction of a steroidal substrate in various Coordination (LCC UPR 8241), 205 route de Narbonne, BP 44099, continuous microphotoreactors and compared the reaction F-31077 Toulouse, France, E-mail: [email protected] performances according to the light sources and the material Laurent Prat, Université de Toulouse, INPT, ENSIACET, F-31432 of the microreactor glass top. To do this, they measured the Toulouse, France; CNRS, Laboratoire de Génie Chimique (LGC UMR 5503), 4 allée Emile Monso, BP 84234, F-31432 Toulouse, France, characteristics of the light transmitted through each type of E-mail: [email protected] reactor top. Shvydkiv et al. (2011) compared the performances of a batch Rayonet reactor and various micro- actinometer conversion with the irradiation time and thus reactors. They used the data given by the lamp manufacturer to accurately calculate the photon flux received in the micro- and evaluated, for example, the lamp power per unit of photoreactor as a function of the wavelength emitted by the irradiated surface (W cm−2). In a recent paper (Aillet et al. lamp. The same methodology was then used to measure the 2013), we deduced the photon flux from a modelling photon flux received in a batch immersion well photoreactor. approach in which the polychromatic light source was Finally, the radiant power received in each photoreactor was assumed to be monochromatic, and hence pointed out the compared to that emitted by the lamp, and some guidelines important role of the photon flux when comparing different based on a knowledge of the photon flux were proposed to photoreactors. Thus, in all cases, the photon fluxes consid- compare various photoreactors. ered remain approximate in so far as they are not equal to the fluxes actually received in the microphotoreactor, which depend, of course, not only on the characteristics of the 2 Material and methods light source but also on the reactor exposition to the source, on the reflectance and transmittance of the reactor material. In classical photoreactors, the photon flux is com- 2.1 Microphotoreactors and batch monly determined by direct measurements using a radio- photoreactor meter. However, this method cannot be applied in microreactors since their dimensions are smaller than The first continuous flow microphotoreactor (A) used in this those of the sensor and also because, in most cases, the study (Figure 1a) had a design encountered in many different irradiated area remains difficult to determine accurately. studies (Aillet et al. 2013; Hook et al. 2005; Lainchbury et al. An efficient alternative is to use an actinometer, which 2008; Vaske et al. 2010), classically named “capillary tower”. involves a simple photochemical reaction with a known It was constructed by winding Fluorinated Ethylene quantum yield. Despite that this well-established method Propylene (FEP) tubing (508 μm inner diameter d, 1587.5 μm is commonly applied in photochemical reactors of large outer diameter, 4 m length) in a single pass around an immer- scale (see for example (Yang, Pehkonen, and Ray 2004; sion well made of Pyrex, which contained a high-pressure Hg Yang et al. 2005; Cornet, Marty, and Gros 1997; Zalazar lamp. The temperature was not measured directly inside the et al. 2005)), we have not found any works implementing microphotoreactor, only the stability of the temperature (8°C) actinometry in microphotoreactors. of the cooling water circulating in the double jacket was In this context, this article aims to show that the photon checked. Aluminium foil was also used to protect the supply flux actually received in a continuous flow microphotoreac- syringe and the inlet and outlet sections of the tubing from UV tor can be accurately and easily measured by actinometry. light, thus ensuring that the photochemical reaction took The actinometer chosen was potassium ferrioxalate, as it is a place only in the tubing section wound around the well. The standard one recommended by IUPAC (Kuhn, Braslavsky, solution to be irradiated was fed into the reactor tubing by a and Schmidt 2004; Parker 1953; Hatchard and Parker 1956; high-pressure syringe pump (neMESYS High-Pressure Lehóczki, Józsa, and Ősz 2013; Allmand and Young 1931; Module, Cetoni®) equipped with 20 mL syringes. Hook et al. 2005). The photon flux was measured in two The second continuous flow microphotoreactor (B) continuous flow microphotoreactors of different geometries consisted of an FEP tube (508 μm inner diameter d,1587.5 and irradiated by either a polychromatic light source (high- μm outer diameter, 2.65 m length) which was fixed in a pressure mercury lamp) or a monochromatic light source channel carved in a flat aluminium plate (Figure 1b). To (UV-LED array). Due to the reactor dimensions, some pre- maximize the space, in particular with respect to the surface cautions were required to correctly transpose this standard irradiated by the light source, the FEP tube was wound in a method from the macro- to the micro-scale. Firstly, because spiral geometry; the inlet of the tube was located at the part of the incident photon flux was inevitably transmitted centre of the spiral. The surface was then illuminated with a over the reactor thickness, a kinetic model was necessary to UV-LED array composed of 9 LEDS emitting at 365 nm. The account for this partial absorption of photons. In addition, as radiant lamp power could be changed by varying the elec- the measurements were, in some cases, performed under trical intensity Ia supplying the UV-LED array. The height h polychromatic irradiation without light filtering, the depen- between the UV-LED source and the irradiated surface (i.e. dence of the intrinsic actinometer properties (quantum yield, the FEP tube) was fixed at 10 cm in order to obtain uniform Napierian molar absorption coefficient) on the wavelength illumination at the reactor surface. was considered. Through this consistent analysis, it was Some actinometry measurements were also run in a possible to model the experimental variation of the conventional batch photoreactor (Figure 1c) having the Table 1 Geometrical characteristics of the photoreactors a Parameters Microphotoreactor Batch Coolant IN Coolant OUT photoreactor AB

Volume of 0.81 0.54 225

reactor Vr (mL) Optical path 0.0508 0.0508 0.62 Mixture OUT length l (cm) Length of 4 2.65 – tube L (m) Mixture IN Inner radius –– 2.50

RW (cm) Outer radius –– 3.12

RW þ l (cm) b Lamp type High-pressure UV-LED High-pressure UV-LED array Hg lamp array (quasi- Hg lamp (Polychromatic) monochromatic) (Polychromatic) Mixture OUT Operating mode Continuous Continuous Batch

Tλ ¼ 0; Tλ > ¼ 1 ð1Þ h 300 300 where Tλ is the material transmittance at the wavelength λ (0 Tλ 1). Note that, rigorously speaking, the com- plete transmittance spectrum of the Pyrex material should be considered. Nevertheless, as the radiant energy emitted by the mercury lamp near 300 nm is weak Mixture IN in comparison to the other radiations participating in the actinometry reaction, this assumption remains c reasonable. In the same way, the tubing material (i.e. FEP) was assumed to be completely transparent to UV radiation Coolant IN Coolant OUT above 230 nm (Aida et al. 2012; Allmand and Young 1931).

2.2 Light sources

Rw Two different light sources were used in this study. The lamp used in the microphotoreactor A and in the

Rw+l batch photoreactor was a mercury vapour discharge lamp (high-pressure Hg Ba/Sr lamp, 125 W, HPK Heraeus®). According to its spectral distribution, the polychromatic Figure 1 (a) Microphotoreactor A. (b) Microphotoreactor B. (c) Batch immersion well photoreactor behaviour of the lamp must be taken into account as no specific equipment for light filtering was used in the present experiments. Generally, manufacturers give the same immersion well and the same light source as the spectral distribution in terms of relative radiant exitance: ones used for the microphotoreactor A. The volume of the Me;λ S ;λ ¼ ð2Þ irradiated solution, Vr, was equal to 225 mL here. e Me;max The main geometrical characteristics of the photoreactors are detailed in Table 1. where the radiant exitance Me;λ represents the power For the two photoreactors irradiated by the mercury emitted from the lamp per unit of source surface area −2 λ lamp, radiations at wavelength below 300 nm were (in W m ) at a given wavelength . Me;max is the max- assumed to be completely absorbed by the Pyrex material imum value observed within the spectral emission leading to: domain of the lamp. Note that the subscripts “e” and “p” denote the physical quantities expressed in radio- monochromatic since the output energy is narrowly (6 10 metric units (i.e. in watts) and in photon units (i.e. in nm) distributed around the 365 nm wavelength. einstein s−1) respectively.

Multiplying eq. (2) by Me;max gives the radiant exitance of the lamp: 2.3 Actinometer M ;λ ¼ M ; S ;λ ð3Þ e e max e ðÞ3 ! ðÞþ½2 þ ð Þ 2FeC2O4 3 2Fe C2O4 3C2O4 2CO2 6 Then, we convert this value into photon quantities, i.e. einstein s−1. The actinometer used in this study was potassium ferrioxalate. It is based on the photodecomposition of the com- 1 3– ¼ ð Þ plex [Fe(C2O4)3] in water (eq. (6)). This actinometer is Mp;λ Me;maxSe;λ Δ 4 NA Eλ largely described in the literature (Kuhn, Braslavsky, and hc Schmidt 2004; Parker 1953; Hatchard and Parker 1956; where ΔEλ ¼ λ is the energy of a photon (J), h the − Ő Planck constant (6.6256 10 34 J s), c the speed of Lehóczki, Józsa, and sz 2013; Allmand and Young 1931). 8 −1 It absorbs in a broad spectral domain and the quantum light (2.9979 10 ms ), and NA the Avogadro constant − – (6.02 1023 mol 1). yield is relatively constant in this domain (300 450 nm). Lastly, the density function of the lamp, gλ,is Moreover, the quantum yield does not depend on the expressed for each wavelength as: temperature, the presence of oxygen in the mixture or reactant concentration (Parker 1953). Despite these well- Mp;λ λ Se;λ gλ ¼ P ¼ P ð5Þ established properties, the ferrioxalate actinometer suf- M ;λ λ S ;λ λi p i λi i e i fers from its low solubility in water and from the precipi- P tation of the ferrous oxalate produced when it is exposed where λi Mp;λi represents the summation of the power emitted at each wavelength. The values of the density to radiation. These characteristics were particularly dis- function for the mercury lamp are presented in Figure 2: advantageous when operating in a microphotoreactor: it they confirm that, for the mercury lamp, the most impor- was impossible to work under full absorption conditions. tant lines are at 365 nm, 436 nm and 546 nm. In addition, the precipitation of the ferrous oxalate made The second light source used (microphotoreactor B) was quantitative energy measurements difficult and, in the a UV-LED array made by the company Led Engineering extreme, could lead to clogging of the microphotoreactor. Development (Toulouse, France). It was built with 9 Nichia Lastly, some gaseous carbon dioxide could be produced, LED (models NCSU033B). The total output power was 2.925 thus generating bubbles in the microphotoreactor. W (325 mW per LED). According to the spectral distribution Consequently, to avoid significant gas generation, (Figure 2), the UV-LED source can be considered as maintain a homogeneous solution and prevent clogging,

0.3 0.035

0.25 0.03

0.025 0.2 g λ g λ 0.02 0.15 0.015 UV-LED array UV-LED array

Mercury lamp lamp Mercury 0.1 0.01 Materiall absorption domain Materiall absorption 0.05 0.005

0 0 250 300 350 400 450 500 550 600 λ (nm)

Figure 2 Density function gλ of the lamps. Black bars correspond to the lines of the mercury lamp and dashed line corresponds to the spectral distribution of the UV-LED array −1 it was imperative to operate at low conversions. In prac- at an initial concentration CA0 of 0.16 mol L , which tice, this implied working with short residence times (few corresponded to the maximum actinometer solubility seconds) and thus with high flow rates Q (see Table 3). observed. The conversions were always kept below 20%. For that, specific pumps and a high-pressure syringe The Napierian molar absorption coefficient of the ® (neMESYS High-Pressure Module, Cetoni ) were used to potassium ferrioxalate κλ was experimentally determined overcome large pressure drops (15–30 bars). Note that the according to wavelengths using a spectrometer (Ultraspec range of the flow rates involved in the microphotoreac- 1000 Pharmacia Biotech®), as shown in Figure 3. It can tors A and B was not the same; this was due to the fact be observed that κλ was relatively important from 300 nm that the residence times required for keeping the conver- and it progressively decreased from 300 nm to 490 nm. sions low were directly correlated to the radiant power of This means that, for a given concentration, the Napierian the lamps, which was significantly different in the two absorbance Ae (see next section) was strong around 300 microphotoreactors (see Section 5.1). nm, weak near 450 nm and zero above 490 nm. Table 2 reports the set of experimental conditions for Consequently, the different wavelengths emitted by the all the photoreactors. All the experiments were performed mercury lamp were not equally absorbed, thus justifying the use of the polychromatic model. Table 2 Operating conditions for implementing actinometry in the In Figure 3, it is also interesting to note that the two microphotoreactors and in the batch photoreactor (CA0 is the quantum yield of the ferrioxalate system varied little initial concentration of the ferrioxalate solution, X the actinometer with the wavelength (data extracted from Hatchard and conversion, τ the residence time in the microphotoreactor, t the Parker (1956)): it remained ranged between 1 and 1.2. irradiation time in the batch photoreactor)

Parameters Microphotoreactor Batch photoreactor 2.4 Protocol for implementing the AB actinometry method

−1 CA0 (mol L ) 0.16 0.16 0.16 −1 2.4.1 Potassium ferrioxalate preparation CH2SO4 (mol L ) 0.2 0.1 0.1 X (%) 10 < X < 20 <10 <10 τ or t (s) 1.125–25–60 30–180 In this study, potassium ferrioxalate was first prepared by Q (mL min−1)24–43 0.75–6 – mixing 3 volumes of potassium oxalate monohydrate −1 K2C2O4 H2O [#CAS: 6487-48-5] (1.5 mol L ) and 1 volume

Ferrioxalate absorption domain (× 102)

) 70 1.4 −1 cm

−1 60 1.2 (L mol λ 50 1

40 0.8

30 0.6 from Hatchard (extracted λ

20 0.4 (1956)) (−) and Parker Pyrex absorption Pyrex domain

10 0.2 Quantum yield φ

Napierian molar absorption coefficient κ absorption molar Napierian 0 0 250 300 350 400 450 500 550 600 Wavelength (nm)

Figure 3 Dependence of the actinometer properties on the wavelength (Napierian molar absorption coefficient, quantum yield) − 1 hυ of ferric chloride FeCl3 [#CAS: 7705-08-0] (1.5 mol L ). The A ! B ð8Þ potassium ferrioxalate precipitate was then filtered, recrys- tallized three times in water and dried at 40°C for 20 If the batch photoreactor can be assumed to be perfectly hours. Then, an appropriate mass of potassium ferrioxa- mixed, the consumption rate of the compound A in the late was dissolved in sulphuric acid solution to obtain a entire volume of the reactor can be expressed, for a −1 monochromatic source of wavelength λ, as: potassium ferrioxalate solution of CA0 ¼ 0.16 mol L (the concentrations in sulphuric acid are reported in Table 2). A dCA a ¼ ’λ < L > ð9Þ second protocol was tested, which consisted of directly dt p;λ preparing the potassium ferrioxalate solution (CA0 ¼ − where C is the mean concentration of the compound A 0.16 mol L 1) in sulphuric acid solution by adding the A in the reactor and ’λ is the quantum yield of the reaction appropriate amount of potassium oxalate and ferric chlor- at the wavelength λ (mol einstein−1). ide. Various actinometry experiments were performed < La > is the mean absorbed photon flux density by using either the first or the second protocol. No differences p;λ the compound A at the wavelength λ (expressed in ein- were observed in terms of conversion, thus demonstrating stein m−3 s−1). This parameter is obtained by averaging, that the two protocols were equivalent. over the entire volume of the reactor (V ), the absorbed Finally, note that the whole procedure (potassium fer- r local radiance La which, considering a collimated beam rioxalate preparation and analysis) was carried out under p;λ red light to avoid potassium ferrioxalate decomposition. of radiation, is written as Cassano et al. (1995): a ¼ κ ð Þ Lp;λ λCALp;λ 10 2.4.2 Ferrioxalate analysis 2 −1 κλ is the Napierian molar absorption coefficient (m mol ) λ Potassium ferrioxalate decomposition is commonly evalu- of compound A at the wavelength and Lp;λ is the photon λ −2 −1 ated by complexation of the ferrous ions produced with o- radiance at the wavelength (einstein m s ) which is phenanthroline, leading to a complex with a strong red determined from the radiative balance equation: colour. In our trials, for the photoreactors, 2 mL (V0)ofthe ÑðLp;λuÞ¼κλ CA Lp;λ ð11Þ samples was firstly diluted with 50 mL of deionized water where u is the directional unit vector of the light beam (V1 ¼ 52 mL). Then 2 mL (V2) of the diluted samples were direction. mixed with 5 mL of o-phenanthroline (Co-phenanthroline ¼ 1gL−1), 5 mL of buffer solution (prepared with 36 mL of Eq. (11) requires the introduction of the incoming w −1 photon radiance L at the surface surrounding the sulphuric acid 1 mol L , 60 mL of sodium acetate 1 mol p;λ L−1 and completed to 100 mL with water) and finally 5 mL absorbing volume, i.e. the incident photon radiance at the reactor wall. Note that, as we consider a collimated of water (V3 ¼ 17 mL). Samples were stirred for 30 min. w The absorbance of the samples was recorded at 510 nm. beam, this photon radiance Lp;λ can also be called the Then, conversion was calculated as follows: flux density of photons received at the reactor wall. Finally, considering a perfectly mixed batch photoreac- C 2þ 1 A510 V3 V1 X ¼ Fe ¼ ð7Þ tor, a single absorbing species (compound A) at the wave- CA0 CA0 k V2 V0 length of interest λ and a collimated beam of radiation with where A510 is the absorbance of the mixture, k is the slope a direction u perpendicular to the irradiated surface, the ¼ ðÞþ of the calibration curve A510 fCFe2 obtained by using absorbed photon flux density is expressed as follows: the standard solutions of Fe2 þ /o-phenanthroline pre- q ;λ pared according to the procedure described by Hatchard < a > ¼ p ð Þ Lp;λ fλ 12 and Parker (1956). In the present study, the slope of the Vr −1 calibration curve k was found to be 10,980 L mol . where fλ is the fraction of the light absorbed at the wavelength λ defined as:

A κλ C l 3 Modelling fλ ¼ 1 e e ¼ 1 e A ð13Þ

With Ae the Napierian absorbance and l the optical path 3.1 Case 1: monochromatic source length. In eq. (12), qp;λ is the photon flux received (einstein − Potassium ferrioxalate, like many actinometers, can be s 1) over the entire volume of reactor at the wavelength λ. described with the following reaction scheme: It depends on the incoming photon flux at the surface surrounding the absorbing volume (qp0;λÞ and on the We recall that eq. (15) is valid when the following condi- reactor material transmittance (Tλ), such as: tions are fulfilled (Aillet et al. 2013; Shvydkiv et al. 2010): (i) a perfectly mixed batch photoreactor, (ii) a single q ;λ ¼ q ;λTλ ð14Þ p p0 absorbing species, (iii) a monochromatic light source,

Note that, in this study, qp;λ and qp0;λ are equal. Indeed, (iv) a collimated beam of radiation. The first two condi- we assume the photoreactor material (i.e. Pyrex or FEP) tions are always satisfied when the actinometry method perfectly transparent to UV radiation above 300 nm. is implemented because the conversion is kept low (i.e. w The flux density of photons Lp;λ received at the reac- the distribution of the concentration remains close to the tor wall (at the wavelength λ) can be calculated from the one initially assumed to be homogeneous and the main photon flux qp;λ (see Table 3) (Roger and Villermaux absorbing species is the initial reactant). 1983). For that, a parallel plate reactor model with a However, eq. (15) also remains true if, instead of a thickness equal to the inner tube diameter (l ¼ d) and perfectly mixed batch photoreactor, we consider a con- illuminated from one side is considered for the two tinuous photoreactor that can be modelled as a plug flow microphotoreactors, whereas a perfect annular geometry reactor. In that case, the irradiation time t in eq. (15) is assumed for the batch photoreactor (Rw is the inner should be replaced by the residence time τ defined as: radius of the immersion well and Rwþl is the outer radius, V as illustrated in Figure 1c). Finally, note that this calcula- τ ¼ r ð17Þ tion assumes that the flux density of photons received on Q the reactor wall is homogeneous. We will later use eqs (15) and (17) to calculate the photon

flux qp;λ received in the microphotoreactor B irradiated by w Table 3 Calculations of the flux density of photons Lp;λ the UV-LED array (at 365 nm).

w −2 −1 Photoreactors l (m) Lp (einstein m s ) q d 3.2 Case 2: polychromatic source Continuous flow d p V microphotoreactors r 2 2 qp RWþl RW In this study, the microphotoreactor A and the batch Batch photoreactor RWþl Rw 2Vr Rw photoreactor were irradiated by the mercury vapour discharge lamp, which was polychromatic (Figure 2), and no radiation filters were used in our experiments. A practical operating condition is to work under full In such cases, eq. (15) is no longer valid. To overcome absorption so as to use the incident light optimally (i.e. this limitation, a more complex model should be consid- not to waste incident photons). This situation occurs ered. It consists in discretizing the range of wavelengths when the fraction of light absorbed fλ tends to 1. In this covered by the lamp emission into several elementary case, qp;λ is determined directly from the slope of the intervals Δλi in which the wavelength-dependent para- curve of the concentration of compound A versus time. meters (quantum yield ’λ, material transmittance Tλ, Nevertheless, as the optical path length l is short in the and Napierian molar absorption coefficient κλ) can be ¼ ¼ microphotoreactors (l d 508 µm) and the actin- averaged and considered constant (Zalazar et al. 2005). ometer solubility is low, full absorption conditions are As the lamp has emitted line, each line corresponds to a difficult to obtain. Consequently, partial absorption con- wavelength interval Δλi. ditions (fλ < 1) classically occur in the microphotoreactors Thus, an equation similar to eq. (9) exists for all and thus, the exponential term in eq. (12) must be taken discrete wavelength intervals Δλi and can be written by into account. introducing the density function gλ of the lamp (eq. (5)) Finally, the use of eqs (9), (12), (13) and (14) leads to as: the following integrated equation: X dX 1 qp;0 ¼ ½Tλ ’λ gλ fλ κλCA0l Δλi i i i i qp;λ 1 1 e dt CA0 Vr ’λ t ¼ C 0X þ ln ð15Þ ð Þ A κλC ðÞ1X l 18 Vr κλl 1 e A0 X hi 1 qp;0 κλ C ð1XÞl ¼ Tλ ’λ gλ ð1 e i A0 Þ Δλi i i i CA0 Vr where X is the actinometer conversion defined as:

where qp0 is the total incoming photon flux at the surface ¼ ð ÞðÞ CA CA0 1 X 16 surrounding the absorbing volume (the term “total” meaning for all discrete wavelength intervals Δλi emitted 0.35 by the lamp). ’ 0.3 Note that, in eq. (18), the term (Tλi λi gλi fλi ) is equal to zero: +10% – in the batch photoreactor and the microphotoreactor 0.25

A, for each Δλi below 300 nm as the Pyrex transmit- −10% 0.2 tance Tλ is considered equal to zero (eq. (1)), – in the microphotoreactor B, for each Δλ below 230 i 0.15 nm as the FEP transmittance Tλ is then considered Conversion (X) equal to zero, 0.1 – whatever the photoreactors, for each Δλi above 490 nm as the Napierian molar absorption coefficient of the 0.05 actinometer κλ is considered equal to zero (Figure 3). 0 0 0.5 1 1.5 2 2.5 The next step is to solve eq. (18) numerically and to Residence time (s) determine qp0 by minimizing the quadratic error R (Gauss–Newton algorithm) defined as: Figure 4 Actinometer conversion versus irradiation time in the ○ X hi microphotoreactor A. Experimental conversions ( ) and conversions 2 – R ¼ ðÞX i ðÞX i ð19Þ predicted by eq. (18) ( ) i model exp − seconds because the pressure drop involved was too The photon flux received (einstein s 1) over the entire high. The experimental data reported, which correspond volume of reactor at the wavelength λ, qp;λ, is calculated to several runs, are reproducible and show little disper- from the material transmittance Tλ and the density func- sion (less than 10 %) despite the multiple dilutions asso- tion gλ of the lamp, such as: ciated with the analytical method. As already observed in qp;λ ¼ Tλ gλ qp0 ð20Þ the literature (Coyle and Oelgemöller 2008; Oelgemoeller − 2012; Knowles, Elliott, and Booker-Milburn 2012; At last, the total photon flux received (einstein s 1) over Mozharov et al. 2011; Aillet et al. 2013; Sugimoto et al. the entire volume of reactor, qp, is deduced from: X 2009; Shvydkiv et al. 2011; Aida et al. 2012), the times q ¼ q ;λ ð21Þ required to reach a given conversion are very short in p Δλi p i comparison with those found in conventional photoreac- As in case 1 (monochromatic source), qp and qp0 are tors. This is mainly due to the fact that the number of equal in this study (Tλ > 300 ¼ 1 for Pyrex and Tλ > 230 ¼ 1 reacting species moles involved (CA0 Vr) compared to the for FEP). photon flux received is smaller in microphotoreactors, as Eq. (18) will thus be used for calculating the photon we showed in a previous paper (Aillet et al. 2013). flux qp received in the microphotoreactor A (in this case, t Figure 4 also shows good agreement between τ will be replaced by defined in eq. (17)) and in the batch the conversions predicted by the polychromatic model photoreactor, both being irradiated by the mercury lamp. (eq. (18)) and the experimental ones, thus confirming the relevancy of the assumptions made in the modelling.

From this modelling (eq. (18)), the photon flux qp 4 Results received in the microphotoreactor was found to be equal to:

q ¼ 26:2 106 einstein s1 ð22Þ 4.1 Microphotoreactor A p

4.1.1 Photon flux received

Figure 4 presents the experimental conversions in potas- 4.1.2 Polychromatic source model versus sium ferrioxalate versus irradiation times (i.e. residence monochromatic source model times) in the microphotoreactor A irradiated by the polychromatic mercury lamp. Note that no experimental mea- From the received photon flux qp calculated above (eq. (22)), surements were possible for residence times below 1.13 it is interesting to look for the photon flux qp;λ received at Table 4 Photon flux (qp;λ) received at each wavelength in the of 365 nm (i.e. the Napierian molar absorption coeffi- microphotoreactor A cient and the quantum yield at 365 nm are considered). −6 −1 −6 −1 λ (nm) qp;λ (10 einstein s ) λ (nm) qp;λ (10 einstein s ) – Secondly, this photon flux of 11:2 106 einstein s1 254 0.44 365 4.07 does not take account of the lamp wavelengths that 265 0.85 405 1.32 are not absorbed. This can have important conse- 280 0.43 436 3.69 quences. For example, consider a photochemical 297 0.51 546 6.34 synthesis carried out in the microphotoreactor A for 302 1.24 577 2.41 313 2.05 579 2.37 which the absorption domain of the absorbing spe- 334 0.48 cies differs from that of the actinometer species. In this case, this photon flux will not be valid. In an extreme case, let us imagine that, for this photoche- each wavelength λ emitted by the mercury lamp. For this mical reaction, the species absorbs weakly around purpose, the density function of the lamp gλ and the material 365 nm but strongly at 546 nm. It is clear that this transmittance Tλ should be introduced as shown in eq. (20). photon flux calculated at 365 nm by considering a Table 4 shows the associated values at each wave- monochromatic source will no longer be valid, as the length. Given the lamp density function, it is logically potassium ferrioxalate actinometer does not absorb observed that the main photon fluxes received correspond at 546 nm. to365nm,436nmand546nm.However,itcanbeseen that, despite the fact that the actinometer absorbs only in For these reasons, it is strongly recommended to use the the range between 300 nm and 490 nm, this modelling polychromatic modelling (eq. (18)) to calculate the makes it possible to determine the photon flux received at photon flux received qp in a photoreactor irradiated other wavelengths. The photon fluxes received for wave- with polychromatic light when no light filtering is used. lengths below 300 nm are reported in Table 4 but we For the continuous flow microphotoreactor A, the should keep in mind that these values will never be received photon flux given by eq. (22) should thus be received in the photoreactor, unless the vessel material is considered instead of the one given by eq. (23). changed for a transparent material such as quartz. Concerning the photon fluxes received for wavelengths above 490 nm, the accuracy of the values reported strongly depends on the validity of the assumption made on the 4.2 Microphotoreactor B material transmittance (Tλ > 490 ¼ 1Þ. It is interesting to discuss the consequences on a Figure 5a presents the conversions in potassium common hypothesis that would assimilate the mercury ferrioxalate versus irradiation times (i.e. residence lamp to a monochromatic source. In this framework, it is times) in the microphotoreactor B when the UV-LED logical, considering the absorption domain of the ferriox- array is supplied with different current intensities (ran- alate actinometer (Figure 3) and the emission spectrum of ging from 100 mA to 400 mA). Good agreement is also the lamp (Figure 2), to choose 365 nm as the main wave- observed here between the conversions predicted by the length taking part in the photoreaction. The photon flux monochromatic model (eqs (15) and (17)) and the experi- received qp is then calculated from eqs (15) and (17) by mental conversions. taking the quantum yield and the Napierian molar In Figure 5b, the photon flux received in the reactor absorption coefficient at the wavelength of 365 nm. The at λ ¼ 365 nm is plotted as a function of the current following value is obtained: intensity supplying the UV-LED array. The curve

qp ¼ f ðIaÞ is linear with a correlation coefficient > 0.99, q ¼ 11:2 106 einstein s1 ð23Þ p which is in agreement with the LED manufacturer’s data. When this is compared to the photon flux calculated by Finally, the maximum current intensity eq. (22) (i.e. polychromatic model), significant differences recommended by the LED manufacturer being 500 mA, are observed. Such findings are not surprising: the maximum photon flux that can be received in – Firstly, the photon flux obtained in eq. (23) does not the microphotoreactor B can be extrapolated from represent the real flux at 365 nm but is the result of Figure 5b: the contributions of all the wavelengths, which are q ¼ 3:82 107 einstein s1 ð24Þ assumed to be absorbed equally to the wavelength p 0.06 0.1 a

0.05 0.08

0.04 0.06 0.03 Ia = 100 mA 0.04 Ia = 200 mA Conversion (X) Conversion 0.02 Conversion (X) Ia = 300 mA 0.01 0.02 Ia = 400 mA

0 0 02040 0 50 100 150 Residence time (s) Residence time (s) (X 10−7) Figure 6 Actinometer conversion versus irradiation time in the 4.5 b batch photoreactor. Experimental conversions (○) and conversions 4 predicted by eq. (18) (–) 3.5 )

1 3 − Photon flux ( ;λ) received in the batch photoreactor at 2.5 y = 7.6346x Table 5 qp R ² = 0.9984 each wavelength emitted by the mercury lamp 2

(einstein s 1.5 − − − −

p 6 1 6 1 λ (nm) qp;λ (10 einstein s ) λ (nm) qp;λ (10 einstein s ) q 1 0.5 254 0.79 365 7.40 265 1.55 405 2.39 0 0 0.1 0.2 0.3 0.4 0.5 0.6 280 0.78 436 6.70 297 0.93 546 11.53 Ia(A) 302 2.24 577 4.38 Figure 5 (a) Actinometer conversion versus irradiation time in the 313 3.72 579 4.30 microphotoreactor B for different current intensities supplying the 334 0.87 UV-LED array. Experimental conversions (symbols) and conversions predicted by eqs (15)–(17) (straight lines). (b) Photon flux received versus the current intensity supplying to the UV-LED array Again, we can express the above value for eachwave- length emitted by the mercury lamp with the help of the emission density function of the lamp (eq. (20)). The 4.3 Batch photoreactor same conclusions can be drawn as from Table 5.

Figure 6 presents the experimental conversions in potassium ferrioxalate versus irradiation times in the batch photoreactor irradiated by the polychromatic mercury 5 Discussion lamp. It can be observed that the irradiation times required to reach a given conversion are significantly 5.1 Comparison between the power received higher than in the continuous microphotoreactors. As in the photoreactors and the radiant shown by Aillet et al. (2013), this is linked with the fact that the number of reacting species moles involved power emitted by the lamp

(CA0 Vr) compared to the photon flux received is larger in the batch photoreactor. The photon fluxes received in each photoreactor (eqs (22), Using the same methodology as for the microphotor- (24) and (25)) can be also expressed in radiometric units eactor A (i.e. the polychromatic model described by (i.e. in watts) according to: eq. (18)), the photon flux received in the batch photo- qe;λ ¼ qp;λ ΔEλ NA ð26Þ reactor irradiated by the mercury lamp was calculated: In the case of a polychromatic source (batch photoreactor 6 1 qp ¼ 47:6 10 einstein s ð25Þ and microphotoreactor A), the total radiant power is then obtained by summing the radiant power emitted at each Table 7 Photon flux (qp) received, absorbed photon flux density ( qp ) and photon flux density received at the reactor wall (Lw)ata wavelength: Vr p X wavelength of 365 nm in the different photoreactors ¼ ð Þ qe qe;λi 27 Δλi Parameters Microphotoreactor Batch In eq. (27), the photon fluxes at wavelengths below 300 photoreactora nm are not taken into account as the Pyrex transmittance Aa B is considered equal to zero. −6 −1 qp (10 einstein s ) 4.07 0.382 7.40 These values are reported in Table 6, together with qp (einstein m−3 s−1) 5.02 0.71 0.033 Vr w −3 b b c the lamp radiant power P given by the manufacturer, for Lp (10 einstein 2.55 0.36 0.17 comparison purposes. Clearly, whatever the photoreac- m−2 s−1) tor, the power actually received inside it is less than Notes: a The values correspond to the wavelength of 365 nm when the 11% of the lamp radiant power. b w light source is the polychromatic mercury lamp; Lp is calculated by assuming that the microphotoreactors can be described as parallel plate ¼ c w reactors (l d) (see Table 3); Lp is calculated by using annular Table 6 Total photon flux received (qe) and lamp radiant power (P) geometry to describe the batch photoreactor (see Table 3). for each photoreactor

Parameters Microphotoreactor Batch photoreactor photoreactors irradiated with the polychromatic mercury AB lamp, the values in Table 7 are calculated from the lamp density function at this wavelength of 365 nm. Rigorously Lamp radiant power 125 2.925 125 (manufacturer’s data) P (W) speaking, the following comparison is valid only for plug

Power received qe (W) 6.07 0.13 11.9 flow reactors or perfectly mixed photoreactors, that is to say when the mixing phenomena along the optical path length, i.e. in each cross-sectional area of the reactor In the case of the mercury lamp, this difference can be (plug-flow reactor), or in the entire volume of the reactor explained by the fact that the majority of the emission is (perfectly mixed photoreactor) are strong enough to in the infrared domain (that is why a cooling system ensure homogeneous concentration. Lastly, this compar- should be used in such systems). ison will assume that the fraction of light absorbed fλ is With the UV-LED array, this value is low in compar- identical in the three photoreactors. ison with the total radiant power emitted of 2.925 W, From the photon flux received qp at 365 nm for each meaning that the design of the microphotoreactor B is photoreactor, the absorbed photon flux density qp and Vr not optimal. In the future, it could be easily improved by w the photon flux density received Lp at the reactor wall increasing the tube length under the area irradiated by (Table 3) can be calculated: they are reported in Table 7. the UV-LED array, or by adding some reflectors. The comparison of the photoreactors depends on the Finally, these findings demonstrate that the photon criteria that we choose to evaluate their performances. flux actually received in the reactor must imperatively be If the productivity (moles of compound produced per measured as it can be very different from a rough estima- time unit) is selected as the main criterion, it is interest- tion based on the power emitted from the lamp, which ing to look at the photon flux received qp because it is does not take the reactor exposition, or the reflectance directly correlated with the productivity. Considering the and transmittance of the reactor material into account. small amount dN ¼ Vr CA0 dX generated in the small irra- Such measurements would also be a useful tool for opti- diation time interval dt, the productivity can be related to mizing the photoreactor design and exposition with the photon flux qp as follows: respect to the light source. dN X ¼ ¼ ; ðÞ¼λ ’λ λ λ Productivity qp 0 Δλ T i i g i f i F1 qp dt i ð Þ 5.2 Comparison of photoreactors 28 In this case, it can be observed that the batch photoreactor The objective of this last section is to compare the differ- was more efficient in this study (Table 7). Nevertheless, the ent photoreactors investigated in this study by using the microphotoreactor A could easily be improved by increas- measured photon fluxes received. To do this, we consider ing the tube length so as to receive more light. In the case the wavelength of 365 nm. In consequence, for the two of the microphotoreactor B, the radiant power of the w UV-LED array is still lower than that of the classic mercury value of the photon flux density at the reactor wall Lp lamp, even though some promising improvements in LED can be a drawback, in particular when the reagents are powers can be expected in the future. Moreover, from likely to decompose under excessive light or when there Table 6, it is clear that the design of the microphotoreactor are several absorbing species in the medium. The mixing B can be optimized (for example by increasing the tube along the light path length inside the reactor could be length) since only 4.5% of the radiant lamp power is insufficiently efficient to renew the area close to the received. irradiated surface and would thus cause excessive light The performance of each photoreactor can also be exposure of the absorbing species present. The potential assessed using the differential Space Time Yield (STY). consequences are a photodegradation of the products This parameter evaluates the small amount and/or a drastic decrease in the reaction rate since the dN ¼ Vr CA0 dX generated in the time interval dt and per exchange of reactants out of and into the area close to the unit of volume Vr is directly related to the photon flux irradiated surface would be limited. In these circum- received per unit volume, qp as: stances, the reactor comparison proposed above is no Vr longer valid and more advanced modelling becomes 1 dN q STY ¼ ¼ F p ð29Þ necessary to accurately predict reaction conversion and V dt 2 V r r selectivity. Such advanced modelling is based on the Based on this criterion, the two microphotoreactors are resolution, in two or three spatial dimensions, of the clearly the most efficient (eq. (30)), as commonly observed radiative transfer equation (eq. (11)) coupled with funda- in previous studies (Aillet et al. 2013; Sugimoto et al. 2009; mental conservation equations (mass, momentum and Shvydkiv et al. 2011; Aida et al. 2012; Shvydkiv et al. 2010): energy). The photon flux density received at the reactor w 8 wall Lp that is here measured then becomes essential > qp > STY Vr data as it constitutes one of the boundary conditions of > A ¼ A ¼ > 153 <> STYbatch qp the system of equations to be solved. Vr batch ð30Þ > q > p > STY Vr > B ¼ B ¼ :> 22 6 Conclusions STYbatch qp Vr batch This parameter has made the success of microreactors as The photon fluxes received in two continuous flow micro- such systems enable small amounts of products to be photoreactors were measured accurately and easily by obtained quickly, which is an undeniable advantage in actinometry. Whatever the photoreactor, significant differ- the R&D field as it improves the screening of new reac- ences between the photon flux actually received and the tion pathways. radiant power emitted by the lamp were highlighted, thus Finally, we can look at the photon flux density confirming the importance of such in situ measurements. w Some guidelines based on a knowledge of the photon flux received at the irradiated wall Lp (Table 7) in order to compare photoreactors. In this case, it is interesting to were also proposed to compare various photoreactors. In note that the microphotoreactor B and the batch photo- future studies, this photon flux will constitute a key data reactor are quite similar, whereas the microphotoreactor for sizing a microphotoreactor according to the photoche- w mical reaction under test and/or for transposing a photo- A exhibits a higher Lp . A direct comparison of the photoreactors based on this parameter is still difficult. chemical reaction from a conventional batch reactor to an Nevertheless, it must be kept in mind that too high a intensified continuous flow reactor.

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