Ir/Ni Photoredox Dual Catalysis with Heterogeneous Base Enabled by an Oscillatory Plug Flow Photoreactor

Article abstract of DOI:10.1021/acs.oprd.0c00150

Continuous flow reactor technology has a proven track record in enabling photochemical transformations. However, transfer of a photochemical batch process to a flow protocol often remains elusive, especially when solid reagents or catalysts are employed.

Full text of DOI:10.1021/acs.oprd.0c00150

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Communication
Ir/Ni Photoredox Dual Catalysis with Heterogeneous
Base Enabled by an Oscillatory Plug Flow Photoreactor
Wouter Debrouwer, Wim Kimpe, Ruben Dangreau, Kevin Huvaere,
Hannes P. L. Gemoets, Milad Mottaghi, Simon Kuhn, and Koen Van Aken
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.0c00150 • Publication Date (Web): 04 Jun 2020
Downloaded from pubs.acs.org on June 4, 2020
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Ir/Ni Photoredox Dual Catalysis with Heterogeneous
Base Enabled by an Oscillatory Plug Flow
Photoreactor
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Wouter Debrouwer,a* Wim Kimpe,a Ruben Dangreau,a Kevin Huvaere,a Hannes P.L. Gemoets,b
Milad Mottaghi,c Simon Kuhn,c Koen Van Aken ab
a EcoSynth, Industrielaan 12, 9800 Deinze, Belgium
b Creaflow, Industrielaan 12, 9800 Deinze, Belgium
c Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, 3001, Leuven,
Belgium
wdebrouwer@ecosynth.be
Date received: xx/xx/2020
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Abstract
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Continuous flow reactor technology has a proven track record in enabling photochemical
transformations. However, transfer of a photochemical batch process to a flow protocol often
remains elusive, especially when solid reagents or catalysts are employed. In this work, application
of an oscillatory plug flow photoreactor enabled a heterogeneous MacMillan-type C(sp²) – C(sp³)
cross-electrophile coupling. Combination of an oscillatory flow regime with static mixing
elements imparts exquisite control over mixing intensity and residence time distribution (RTD),
pinpointing a mindset shift concerning slurry handling in continuous flow reactors. The C(sp²) –
C(sp³) cross-electrophile coupling was successfully transferred from batch to flow, resulting in an
intensified slurry process with significantly reduced reaction time and increased productivity (0.87
g/h).
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Keywords (4-6): HANU reactor, plug flow, oscillatory flow, photoredox catalysis, dual catalysis,
slurry handling
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Introduction
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During the past 15 years, the synthetic organic chemist’s toolbox has expanded tremendously
by the elaboration, control and design of radical-driven transformations. Indeed, access to transient
one-electron species has been facilitated thermally,1 photochemically2-4 and electrochemically.5-7
In particular, the amount of reports on photoredox catalysis has grown exponentially since 2010,
corroborating widespread adoption in the community.2 Insights in the underlying chemistry and
the ability to access radicals by tuning catalyst photophysical properties have resulted in a myriad
of compelling organic transformations.2-4
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Though empowering those involved in small-scale organic synthesis, photoredox catalysis often
proves strenuous when scaling-up. Apart from the usual mass and energy transfer considerations,
photon transport to the bulk of the matrix plays a pivotal role.8 According to Beer-Lambert’s law,
light attenuation in an absorbing medium follows an exponential decrease.9 Enlarging reactor
dimensions is therefore a poor scale-up strategy, which explains exactly why photochemical
transformations strongly benefit from continuous flow reactor technology. The short path length
for light penetration (mm range), the high surface-to-volume ratio and the excellent process control
outcompete the batch approach.10-12 Preparative manufacture in flow relies on judicious scale-up
of reactor dimensions (i.e. “smart dimensioning”), scale-out or numbering-up.13-15 Still, continuous
flow reactors are often ill-performing when solids handling is concerned, a limiting or even
prohibitive factor when envisioning slurry transformations.11,16-18
It is the aim of the presented work to demonstrate that such issues can be countered by an
innovative plug flow reactor platform, capable of handling solids and particularly apt for
photochemistry. This was exemplified by transfer of a heterogeneous MacMillan-type cross-
electrophile C(sp2) – C(sp3) coupling from batch to continuous flow.19
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Results and Discussion
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1) Reactor design
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The result of a proprietary design, a flow reactor system that addresses the hurdles commonly
encountered upon scaling photochemical transformations was developed.8,10,12 This concept
consists of three major pillars combined: 1) Continuous processing in a linear plate reactor, 2)
utilizing an Oscillatory flow regime which is 3) combined with multiple STAtic mixing elements
in the process channel. These features combined constitute the very essence of the COSTA
technology (Figure 1), from which a photochemical reactor, the HANU reactor, was conceived.
Characteristics and dimensions of the lab-scale reactor are outlined in Table 1.
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Figure 1. Principles of the COSTA technology. The COSTA technology consists of 3 pillars: continuous processing in a
linear plate reactor by means of an oscillating flow regime and static mixing elements. The oscillating flow regime is
characterized by a net flow onto which a periodic flow is superimposed.20
Table 1. Lab-scale HANU HX 15 reactor characteristics.
Dimensions
530 x 60 x 45 mm
Reactor volume (irradiated)
Window
15 (14) mL
Borosilicate, Quartz (480 x 17 mm)
-20 °C to + 80°C
Temperature range
Pressure rating
10 barg
Static mixing elements
Reactor material
Other wetted parts
Irradiation sources
Cubic (2 x 2 x 2 mm)
Stainless steel 316L or Hastelloy C276
FEP, PTFE
Mercury lamps, excimers, LEDs
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The reactor is operated in conjunction with a pulsator, a device that superimposes a periodic
oscillation on the net flow generated by a metering pump.20-21 This oscillatory flow regime and
repeated split-and-recombine flow path ensure adequate mixing regardless of the net flow rate. In
other terms, reactor hydrodynamics are predominantly decoupled from the net flow rate. As a
consequence, pulsator amplitude and frequency are introduced as new process parameters
affecting hydrodynamics, residence time distribution (RTD), energy dissipation rate and related
transport phenomena, all of which may be optimized on a case-by-case basis. This was exemplified
by the Kappe group utilizing the same reactor to enable C–N coupling with a heterogeneous
catalyst.22 Depending on process requirements, reactor material, volume and static mixing
elements can be customized.
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Having determined a set of oscillatory flow parameters, the scale-up principle relies on widening
the process channel without affecting photon path length, mass and energy transfer, and pressure
drop. A “pilot-scale” reactor with 10-fold increased volume was constructed and validated, though
transfer of the C(sp2) – C(sp3) coupling under investigation here from lab-scale will be the subject
of future work.
2) RTD
As a key parameter in characterizing reactor performance, residence time distributions under
varying conditions were determined. Absorbance curves were recorded by means of tracer
injections (10 mM methylene blue in ethanol) and in-line UV-VIS spectrophotometry at reactor
outlet. Data thus obtained was transformed into E-curves and analyzed using open-open vessel
boundary conditions, after which sloppy tracer input correction was applied.23 Effect of pulsation
amplitude, frequency and flow rate were on RTD were evaluated (Figure 2). In view of a likely
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reaction time reduction from 6 h in the corresponding batch process, 30 minutes was tentatively
selected as starting point for the RTD experiments.
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Figure 2. E-curves as function of pulsation amplitude (x0, % of maximum pulsation amplitude, 100% corresponds to 7 mm
center-to-peak amplitude, top left graph), frequency (top right) and flow rate (bottom).
Higher pulsation amplitudes (Q = 0.5 mL/min, f = 3 Hz) lead to broader RTD, the result of more
axial dispersion. On the other hand, lowering pulsation frequency causes narrowing distributions,
as indeed each additional pulse provokes further axial dispersion. An increased flow rate is
favorable too, although this may be the result of reducing the total amount of pulses. In all cases
pulsed RTD curves are superior to the non-pulsed system, an observation that was confirmed
computationally (see SI). Further optimization by means of statistical methods (including design
of experiment, DOE) would benefit RTD outcome, but is beyond the scope of this work. Moreover,
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experimental conditions imparting optimal RTD are not necessarily ideal for the specific chemical
reactions under investigation. Slurry handling is pivotal in this work and, while best for RTD, very
low pulsation frequency is counterproductive for maintaining stable suspensions. In reality, finding
the optimal trade-off between RTD and mixing intensity seems best practice.22
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Computational Fluid Dynamic (CFD) simulations support the advantageous effect of pulsation
on RTD (see SI). In addition, by computing tracer transport over time, such simulations allow
comparison of vertical mixing time in absence and presence of pulsation. A drastic increase in
species transport normal to the flow direction is observed with pulsation. The time required for the
scalar species to be transported half the channel height is below 1 s when pulsation is applied,
whereas the tracer did not reach the center of the flow channel within the 7 s of simulation time
(Figure 3).
Figure 3. Concentration field in the reactor t =1.0 s after tracer injection for a flow rate of 1 ml/min. Left: no pulsation,
Right: pulsation frequency of 3 Hz and 33% amplitude.
3) Actinometry
In scientific literature on photoredox catalysis, authors often neglect to quantitatively describe
kinetic parameters and to unambiguously characterize irradiation sources and temperature control
measures. There is growing demand to accurately disseminate on reactor/lamp setup, distance,
emission spectrum and operational parameters.24 In particular, actinometric data facilitates
reproducibility and quantitative comparison between experimental results.25-26
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The photoredox catalyst used in MacMillan’s work, Ir[dF(CF3)ppy]2(dtbbpy)PF6, displays
photon absorption up to ca. 480 nm, with λmax at 380 nm.19,27 Use of blue LEDs with multiple
emission peaks at 470 nm, 430 nm and 390 nm is reported (34 W Kessil H150b).19,28 The choice
for 405 nm irradiation, available as standard 75 W LED source to mount on the reactor, is thus
well justified (irradiation source details are summarized in SI). In order to accurately measure
incident photon flux in the reactor, Loubière’s chemical actinometry protocol using potassium
ferrioxalate was employed.29,30 For simplicity, the monochromatic irradiation model was adopted
(LED emission spectrum half width Δλ = 15 nm). From ferrioxalate conversion as function of
irradiation time (Figure 4), the corresponding photon flux entering the process channel qn,p was
calculated as 2.96 x 10-5 mol photons/s.
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Actinometer conversion (%)
10%
9%
8%
7%
R² = 0.9646
6%
5%
4%
3%
2%
1%
0%
0
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irradiation time (s)
Figure 4. Actinometer conversion as function of irradiation time.
4) Slurry handling
In MacMillan’s seminal work, the heterogeneous bases K2CO3, Na2CO3 and LiOH were
evaluated and finally Na2CO3 (2 equiv.) was put forward as most effective for C(sp2) – C(sp3)
cross-electrophile coupling.19 It performs best with non-heteroaryl substrates whereas
heteroaromatic bromides are best combined with anhydrous LiOH. In case homogeneous
conditions are desired, 2,6-lutidine may replace the inorganic base. However, heterocycles require
at least 5 equiv. to reach yields similar to LiOH.31 Taking into account its higher cost (roughly one
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order of magnitude per mole) and inevitable post-processing removal issues, 2,6-lutidine’s
supposed advantages are undone and the choice for Na2CO3 becomes evident, especially during
process development and scale-up.
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Continuous flow reactor technology creates opportunities for many photochemical reactions, but
the heterogeneous nature of some creates new challenges.10 Upon translating MacMillan’s batch
protocol to flow, the particular difficulty is handling and conveying a Na2CO3 suspension in 1,2-
dimethoxyethane (DME). Typically, plug flow reactors are considered incompatible with solids as
they suffer from plugging and poor solids distribution, while conveying solids requires adequate
pump selection. The integrated HANU concept, on the contrary, anticipates to these issues and
performs exceptionally well in terms of slurry handling. Suspended solids do not sediment because
the oscillatory flow generates high local velocities that constantly resuspend particles and prevent
reactor fouling. Pulsation parameters may be adapted to optimize suspension stability.
Nevertheless, it must be noted that fouling and/or deposition are issues inherently associated with
slurry processing depending on the nature and characteristics of the particles and reactor material,
both in batch and flow. Control and knowledge of slurry physicochemical properties (e.g. particle
size distribution, contact angle, etc.) helps to prevent such issues, and this is the subject of ongoing
research.
MacMillan’s cross-electrophile coupling thus requires 2 equiv. Na2CO3 in DME, corresponding
to ca. 2.2 wt%.19 DME is not viscous (µ = 0.46 mPa.s at 20 °C) with a density of 0.86 kg/L (20
°C), as compared to 2.54 kg/L for Na2CO3.32 Stokes’ law does not accurately describe particle
sedimentation under conditions prevailing in the reactor, but a similar driving force for settling is
expected and observed accordingly (low solvent viscosity and Δρ = 1.68 kg/L). Preliminary
experiments demonstrated that suspension stability inside the reactor remained excellent due to
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the applied oscillations, but issues manifested in supply tubing to the pump. Therefore,
investigations continued with the 2.2 wt% Na2CO3 slurry in DME, now in recirculation mode using
a peristaltic pump and 0.8 mm ID FEP tubing at different flow rates (Table 2). High flow rates
(corresponding to relatively short irradiation times, entries 1-2) proved effective but lower flow
rates led to sedimentation of the larger particle fraction in the pump aspiration line. Reduction of
the Na2CO3 particle size by wet milling decreased the Sauter mean diameter from 122 µm to 14
µm (see SI for details). The resulting suspensions were maintained in the aspiration line even at
very low flow rates, ensuring accurate and precise dosing during reaction.
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Table 2. Impact of flow rate and Na2CO3 particle size on stability of a 2.2 wt% suspension in DME transferred through 0.8
mm inner diameter FEP tubing.
Entry
d32 (µm)a
122
122
122
14
Q (mL/min)b
τirr (min)c
2.8
Observations
Slurry conveyed
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14
Slurry conveyed
0.4
0.4
0.2
0.1
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Sedimentation in pump supply line
Slurry conveyed
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14
70
Slurry conveyed
14
140
Slurry conveyed
a Na2CO3 Sauter mean diameter; b flow rate; c corresponding irradiation time in the reactor
5) Dual Ni/Ir Photoredox cross-electrophile coupling
Having identified conditions that allow conveying the Na2CO3 slurry required for the C(sp2) –
C(sp3) cross-electrophile coupling, the chemical transformation itself was then evaluated. The aim
was to present a platform capable of reducing multiple-hour batch reactions to faster and
performant flow protocols. A stirred suspension was fed to the integrated photoreactor by means
of a peristaltic pump connected to a pulsator in series (Figure 5). The oscillating process stream
was exposed to 405 nm irradiation upon entering the reactor channels and left the system through
a dome-loaded back-pressure regulator (BPR) after the desired residence time.
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Figure 5. Schematic representation of experimental setup used for MacMillan’s light-driven cross-electrophile coupling.
The Na2CO3, base remains suspended in the DME reaction solvent by the oscillatory flow regime. The complementary 405
nm LED source covers the entire reactor surface window, thus creating a homogeneous photon flux inside the reaction
matrix.
In literature, temperature was maintained at 25 °C by means of convective cooling and no
elevated temperatures were assessed. Since the reactor applied here is equipped with an integrated
heat exchanger and process thermostat, influence of reaction temperature was readily investigated.
Preliminary experiments demonstrate that reaction temperature plays an important role, suggesting
the rate-determining step is not photoredox catalyst excitation, but likely a thermal step in the
nickel catalyst cycle or a transport phenomenon. The desired reaction proceeds faster at 50 °C
without compromising selectivity towards the desired coupled product. To further optimize the
thermal component of the catalytic cycle, Ni-loading was doubled to 1 mol% while 2 mol% dtbbpy
facilitated catalyst dissolution prior to reaction.33-34
We then engaged in a relevant reductive cross-coupling example between methyl 4-
bromobenzoate and 4-bromotetrahydropyran. Aiming to maximize productivity, an ambitious 30
minutes of irradiation resulted in complete conversion of the aryl bromide (Table 3). Yield was
determined both via 1H NMR and isolation of the coupled product, with each method resulting in
1
76%.19 As such, yields obtained via H NMR spectroscopy using internal standard were proven
accurate. Observed side products include solvent-coupled methyl benzoate and methyl benzoate.
Further reduction to 20 minutes irradiation furnished an almost identical outcome with
substantially increased productivity. Shorter irradiation times (< 20 min) did not lead to full
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conversion, suggesting the limits of the system were reached under the prevailing reaction
conditions (entries 3-4). Interestingly, further temperature increase to 65 °C improved conversion
though selectivity eroded. This was perceived by the unproportionally darker reactor effluent.35
Considering 50 °C as optimal reaction temperature, irradiation intensity was next halved (entry 6
vs. entry 2). This resulted in only a slight drop in conversion and yield, corroborating the stronger
temperature dependence observed earlier. As a final effort to further boost productivity, reactant
concentration was increased from 2.3 to 4.2 wt%. An irradiation time of 20 minutes was
insufficient but after 30 minutes conversion was complete, culminating in the production of 0.87
g/h of the desired coupled product.
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Table 3. Evaluation of irradiation time, substrate concentration, light intensity, and reaction temperature on conversion
and yield of the reductive cross-coupling between methyl 4-bromobenzoate and 4-bromotetrahydropyran, as carried out
under continuous flow regime.
τa
(min)
30
Cb
(wt%)
2.3
Lamp power
(%)
T
(°C)
50
50
50
50
65
50
50
50
Conversionc
Yieldd
(%)
76
Productivity
(g/h)
0.44
Entry
(%)
100
100
92
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6
7
100
20
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100
77
0.71
15
2.3
100
72
0.85
10
2.3
100
72
61
1.11
10
2.3
100
80
64
1.12
20
2.3
50
93
74
0.64
20
4.2
100
95
79
1.33
8
30
4.2
100
100
80
0.87
a
b
c
irradiation time; methyl 4-bromobenzoate concentration; conversion of methyl 4-bromobenzoate as determined by GC-FID
analyses using dodecane as internal standard; d yield of coupled product as determined by H NMR using mesitylene as internal
1
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There are many pitfalls associated with a direct comparison between different photoreactors or
setups. In particular, the incident photon flux is often different due to variations in lamp type,
manufacturer, emitted wavelength, lamp-reactor distance, reactor and light source design, etc.
Table 4 presents an overview of the results obtained for the identical transformation by MacMillan
(batch),19 Jensen (CSTR-cascade)36 and in this work. In MacMillan’s seminal methodology paper
0.50 mmol methyl 4-bromobenzoate was converted in 6 hours with 79% yield, producing an
average 0.014 g/h of desired product. A 34 W Kessil blue LED was employed. The space-time
yield (STY) was 2.9 g/L/h, though it must be noted that the paper’s aim was not productivity
optimization, but methodology development.19 Jensen’s cascade of 5 CSTRs was irradiated by
means of 3 identical 40 W Kessil A160 WE lamps focused onto the reactor window using lenses.
With this setup 0.077 g/h output was realized corresponding to a STY of 14.5 g/L/h.36 In this work,
finally, 0.87 g/h was produced in an oscillatory plug flow reactor irradiated with quasi-
monochromatic 405 nm LEDs. The STY thus achieved amounts to 62.1 g/L/h, a marked
improvement as compared to the state-of-the-art. The higher incident photon flux per reactor
volume (qn,p/Vr) and higher surface-to-volume ratio (airr/Vr) certainly contribute to this
enhancement, highlighting the importance of reactor design, irradiation source power and
integration in the setup. Finally, the photocatalytic space-time yield (PSTY) is a metric that takes
into account both reactor productivity and irradiation source electrical consumption.37 Though
similar to STY, this value also embodies the efficiency with which lamp input power is translated
into product.
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Table 4. Overview of reaction conditions published for Ir/Ni photoredox C(sp2) – C(sp3) cross-electrophile coupling of
methyl 4-bromobenzoate and 4-bromotetrahydropyran using Na2CO3.
Ca
(mM)
10
Vrb
(mL)
5
τc
(h)
6
T
(°C)
25
Pd
(W)
34i
Productivity
(g/h)
STYe
(g/L/h)
2.9
qn,p/Vrf
(mol/s/m³)
NR
airr/Vrg
(cm-1)
NR
PSTYh
Source
(g/L/h/W)
4.21 x 10-04
6.41 x 10-04
1.16 x 10-02
MacMillan19
Jensen36
0.014
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40
5.3
0.5
35
120j
75l
0.077
14.5
0.82k
1.3
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this work
20
14
0.5
50
0.870
62.1
2.12
4.4
a
b
methyl 4-bromobenzoate concentration; reaction volume; c irradiation time; d lamp input power; e space-time yield; f incident
photon flux (mol photons/s) per reactor volume; reactor irradiated surface-to-volume ratio; h photocatalytic space-time yield,
g
defined as the ratio between STY and the standardized lamp power, which is the lamp input power scaled to the power that would
irradiate a reactor of 1 L37; i Kessil blue 34 W LED; j 3 x Kessil A160WE 40 W LED; k 0.27488 mol/s/m³ was reported for one
lamp based on chemical actinometry (See SI of the reference).36 This value was multiplied by 3 since 3 lamps were used during
reaction.; l 405 nm LEDs; NR = not reported.
Conclusion
The HANU reactor was demonstrated as a continuous plug flow system that is particularly
suitable for slurry processing and facilitates transfer of heterogeneous (photochemical)
transformations from batch to continuous flow. Several residence time distribution experiments
were recorded, assessing the effect of pulsation parameters and flow rate. Incident photon flux was
quantified by means of ferrioxalate actinometry and the influence of Na2CO3 particle size on
suspension stability was evaluated. The oscillatory flow regime was essential in maintaining stable
reactant suspensions. A heterogeneous, milliliter-scale batch Ni/Ir photoredox dual catalyzed
transformation was successfully transferred to a plug flow photoreactor. Reaction time was
significantly reduced from 6 hours in batch mode to 20 minutes in continuous flow flow, offering
perspective for reconsidering the batch approach of a multitude of other (heterogeneous)
(photo)chemical reactions.
Experimental Section
A two-neck 100 mL round bottom flask was equipped with a magnetic stirring bar, photocatalyst
Ir[dF(CF3)ppy]2(dtbbpy)PF6 (79 mg, 0.07 mmol, 0.01 equiv.), methyl 4-bromo-benzoate (1.505 g,
7.00 mmol, 1.0 equiv.), neopentyl bromide (1.32 mL, 1.586 g, 10.50 mmol, 1.5 equiv.),
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tris(trimethylsilyl)silane (2.16 mL, 1.741 g, 7.00 mmol, 1.0 equiv.), 28.00 mL of a suspension of
6 wt% Na2CO3 in DME (24.731 g, 1.484 g Na2CO3, 14.00 mmol, 2.0 equiv.) and 28 mL DME
(23.894 g). The flask was sealed, purged with N2 and covered in aluminium foil. A separate 25
mL round bottom flask was charged with NiCl2·glyme (15 mg, 0.07 mmol, 0.01 equiv.) and 4,4’-
di-tert-butyl-2,2’-bypridine (38 mg, 0.14 mmol, 0.02 equiv.). The flask was sealed and purged
with N2. 14.00 mL DME (12.156 g) was added and the precatalyst solution was sonicated for 5
minutes, after which it was quantitatively transferred into the two-neck reactant flask. The solution
was degassed by sparging with N2 while stirring for 5 minutes. The stirred suspension was pumped
into the pulsator and reactor using a Vapourtec SF-10 peristaltic pump equipped with blue tube.
Thermostat temperature was set to 50 °C and 2 bar back-pressure was applied. A representative
sample was collected after 3 residence times and analyzed by GC-FID and 1H NMR spectroscopy.
Supporting information
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The Supporting Information is available free of charge on the ACS Publications website at DOI:
xxx.xxx.
The supporting information contains general experimental details, CFD details, irradiation
source characteristics, particle size distributions and compound characterization data.
Corresponding Author
*Wouter Debrouwer (wdebrouwer@ecosynth.be)
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
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ACKNOWLEDGMENT
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Financial support from Vlaio (Flanders Innovation and Entrepeneurship) is gratefully
acknowledged (HBC.2019.2081). We thank Bert Metten and Sophie Nols (Ajinomoto Bio-Pharma
Services) for wet milling of Na2CO3.
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ABBREVIATIONS
airr
irradiated reactor surface [cm²]
back-pressure regulator
concentration [wt %]
BPR
C
CFD
CSTR
DME
DOE
d32
computational fluid dynamic
continuous stirred tank reactor
1,2-dimethoxyethane
Design of Experiments
Sauter mean diameter [µm]
frequency [Hz]
f
FEP
ID
fluorinated ethylene propylene
inner diameter [mm]
LED
P
light-emitting diode
lamp power [W]
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PTFE
polytetrafluoroethylene
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Q
flow rate [mL/min]
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qn,p
photon flux [mol photons/s]
residence time distribution
temperature [°C]
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RTD
T
UV-VIS
ultraviolet – visible light
reactor volume [mL]
Vr
x0
µ
center-to-peak amplitude [% of maximum or mm]
dynamic viscosity [mPa.s]
density [kg/L]
ρ
τirr
irradiation time [min]
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