Microflow High-p,T Intensification of Vitamin D3 Synthesis Using an Ultraviolet Lamp

Article abstract of DOI:10.1021/acs.oprd.7b00318

Herewith a new process concept for synthesis is presented which combines both UV-photoirradiation and high-p,T intensification (photo-high-p,T) in continuous flow. The application of this procedure to Vitamin D₃ synthesis promotes thermal shifting of the equilibrium from the reaction intermediate to the product. This is enabled by microreactors which allow operation under harsh conditions such as the high temperature used here. This provides, to our best knowledge, a new kind of process combination (novel process window). As a result, in less than 1 min, 42% conversion of 7-dehydrocholesterol can be achieved giving a 17% yield and 40% selectivity of Vitamin D₃. This approach enhances productivity by up to 2 orders of magnitude compared with the current capillary based vitamin D₃ synthesis, because, under the microflow conditions, photochemistry can be performed at fairly high concentration and up to 20 times faster.

Full text of DOI:10.1021/acs.oprd.7b00318

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Full Paper
Micro-flow high-p,T intensification of
3
Vitamin D synthesis using a ultraviolet lamp
Marc Escribà-Gelonch, Timothy Noël, and Volker Hessel
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00318 • Publication Date (Web): 20 Dec 2017
Downloaded from http://pubs.acs.org on December 20, 2017
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Micro-flow high-p,T intensification of Vitamin D synthesis using a
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ultraviolet lamp
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Marc Escriba-Gelonch *, Timothy Noël , Volker Hessel *
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Micro Flow Chemistry and Process Technology, Department of Chemical Engineering and
Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands
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*correspondence authors: m.escriba.gelonch@tue.nl, v.hessel@tue.nl
Abstract
Herewith a new process concept for synthesis is presented which combines both UV-photo-
irradiation and high-p,T intensification (photo-high-p,T) in continous-flow. The application of
3
this procedure to Vitamin D synthesis gives the motivation of shifting thermally the
equilibrium from the reaction intermediate to the product. This is enabled by microreactors
which allow to operate under harsh conditions such as the high temperature used here. This
provides, to our best knowledge, a new kind of process combination (novel process window).
As a result, in less than one minute 42 % conversion of 7-dehydrocholesterol can be achieved
giving 17 % yield and 40 % selectivity of Vitamin D
3
. This approach enhances the productivity
synthesis,
up to 2 orders of magnitude compared with the current capillary based vitamin D
3
because under the micro-flow conditions, photochemistry can be performed at fairly high
concentration and up to 20 times faster.
3
Keywords: Process intensification, UV-intensification, continuous flow, vitamin D .
1. Introduction
1
Recently, FDA as regulatory authority pushed the manufacturers of pharmaceutical and food
2
industry (e.g. Novartis-MIT ) to promote use of continuous end-to-end processing. In a
research environment, the use of microreactors and flow chemistry is an essential part of this
goal, since they have proved the possibility to combine high pressure (p) and high
temperatures (T) in a high-p,T fashion. This tandem gives the chance to achieve process
3
intensification which derives in shorter reaction times. Such chemical intensification is part of
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,5
novel process windows (NPW).
Following NPW, in this paper a fully continuous process intensification for the synthesis of
Vitamin D (VD3) is proposed. Therefore, the coupling of NPW with photochemistry gives the
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concept of photo-high-p,T intensification. The complex chemical synthesis of VD3 starts with 7-
dehydrocholesterol (7DHC), commonly known as provitamin D, industrially produced from
cholesterol obtained from the skin of animals. The irradiation of 7DHC with UV-B light most
6
efficiently in the range of 275-300 nm (Scheme 1) gives a bond rearrangement resulting in an
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electrocyclic ring opening (Previtamin D), followed by an antarafacial [1,7] sigmatropic
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hydrogen rearrangement . Dauben et al. suggested that the VD3 productivity actually does
not depend on 7DHC excitation wavelength, but on P3’s. Yet, in the first step, the reversible
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ring-closure can give also the 7DHC isomer Lumisterol (L3) , but with low provability with high
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photon energy . In the second, the rearrangement of previtamin D (P3) can give VD3 as a
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result in an equilibrium shifted thermally . Unfortunately, the process has to be stopped after
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short irradiations because of the synthesis of tachysterol (T3) , which has more favourable
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quantum yield in this stage . In addition, the conformation of P3 can also influence the
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equilibrium towards T3 . Besides T3, long irradiations can give also toxisterols . Therefore,
transferring the process into continuous flow means short irradiation and high pressure in a
temperature dependent matter.
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Scheme 1: Chemical synthesis and most accepted mechanistic pathway for Vitamin D .
In the current production of VD3, a solution of 7DHC in diethyl ether is shortly photo-activated
up to 10 % conversion using a high-pressure or medium-pressure mercury lamp in a quartz
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made recirculation reactor , where the upper temperature limit is strictly limited for photo-
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reactions. Thompson stated that the use of higher temperatures (than ambient, >20 ⁰C),
shifts the equilibrium towards higher amounts of P3, i.e. higher 7DHC conversion. From an
initial solution at room temperature of 93:7 P3:VD3, after an equilibrium of just 30 min at 100
⁰C, the P3:VD3 isomeric proportion turned to 72:28. Vice versa, processing at temperatures
below 20 ⁰C can lead to the formation of side products (L3 and T3) and the share of the latter
increases with longer light exposure giving so that the net result is low reaction selectivity as
well. In an industrial point of view, severe safety constraints are given due to the high photo-
energy introduced and the highly reactive intermediates generated. The synthesis using the
photo-activation of 7DHC is thus kept separate from the following isomerization to VD3 at
medium temperature. Accordingly, the combined use of high light energy, pressure and
temperatures (photo-high-p,T) is not established and probably would be prohibited when
using conventional equipment. This new kind of NPW is, however, made possible using photo-
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microreactors and a major key is the much higher light penetration
due to the miniature
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characteristic dimension . Microreactors are known to allow safe operation under classical
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harmful process conditions
and to achieve with all their process benefits a good
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sustainability profile
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In this scenario, VD3 could be an ideal candidate, since it is known to be one of the most
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thermally stable vitamins.
In addition, operating under pressure has been demonstrated to
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an efficient way to increase the VD3 productivity at room temperature . Therefore, the
combination of irradiation, pressure and temperature in one photo-reactor is a new approach
to achieve control over the side product formation, while boosting conversion. The latter is
also promoted by the high light penetration, common in micro-flow reactors, which allows to
operate at high concentrations close to solubility limits. Thus, finally the approach presented
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might be a new means to increase the productivity of vitamin D . Moreover, the advantageous
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surface-to-volume ratio of micro-flow reactors together with using modern lamps allows to
increase both the photon intensity and energy efficiency.
To date, very few continuous flow studies related with the synthesis of VD3 have been
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reported and none applying photo-high-p,T and NPW principles.
Fuse et al.
described a
continuous process for the production of 1α,25-(OH) -vitamin D
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operating in two steps. A first
irradiation of a mixture of 7DHC analogues at 313-578 nm (high-pressure mercury lamp)
achieved a mixture of VD3 analogues, which was irradiated again in a second step with 360 nm
-1
light at 100 ⁰C. The flow rate was 0.3-0.6 mL.h of a solution of 0.03 M of 7DHC analogues
mixture at atmospheric pressure. The process gave 28 % yield of 1,25-(OH)
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-VD3 with a
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productivity of 1.9 mg/h and a residence time of 15 min. Other studies in UV-microflow
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chemistry has been described successfully related to UV-photo Claisen rearrangement, or for
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isomerization of cyclooctene derivatives, using also inline packed bed reactors and operating
at atmospheric pressure. Continuous-flow photochemistry with microreactors has been used
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in organic synthesis, material science, and water treatment . Nevertheless the need of UV-
light for VD3 synthesis makes the use of quartz essential.
Inspired by these examples, a photo-high-p,T setup was developed with the capability of
operating together, but during short times as demanded by VD3 chemistry, with high pressure,
temperature and irradiation conditions in a safe mode in one step, i.e. using the advantages
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common to micro process engineering . The setup allows to operate at reaction temperature
above a solvent’s boiling point (super heating), which is known to reduce the reaction time of
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purely thermal reaction by several orders of magnitude compared with batch synthesis . Due
to the dangerous properties of ether peroxides generated by diethyl ether at high
temperatures, in this study tert-butyl methyl ether (t-BME) is used to minimize as possible
these inconveniences because of steric hindrance. The formation of tert-butyl radicals from
due to thermal and UV-irradiation, are minimized due to the short residence times, and the
absence of interfering compounds and oxygen inside the micro-channels, which gives t-BME a
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supplementary UV-stability
.
2. Material and methods
In order to perform experiments in a small statistically relevant fashion, three reaction
variables were taken into account: temperature, concentration and irradiation time. For each
variable, at least three levels were considered, and each experiment was carried out at least
three times. The levels of each variable are described in the corresponding section.
For each experiment, a solution of the desired concentration of 7DHC (>90 % Cayman
Chemical Company) in t-BME (99 % Alfa Aesar) was pumped using an ISCO pump (100D series).
All chemicals were used as received. The pressure was monitored by the pressure sensor of
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the ISCO pump. IDEX PEEK Ora 0.55 mm (1/16”) ID tubing and the corresponding fittings were
used for the connections. Quartz (HOQ – 90 % transparency at 280 nm) coil 0.5 mm ID and 3.2
mm ED was used in the thermally controlled (photo-high-p,T) irradiation chamber (Scheme 2)
made of reflective aluminium in order to enhance internal reflections and therefore the
irradiation in all sides of the coil. A Phillips HOK 4/120 mercury lamp was used to give light
energy during the reaction, which was carried out at different temperatures above the boiling
point of the solvent (55 ⁰C). The spectrum of the lamp is provided in Figure 1. The HOK lamp
was placed in one side, 6 mm out of the reactor. The light reflection was previously evaluated
(Zemax® simulation) to ensure that all sections of the quartz coil got the same light energy. In
order to achieve such intensification, the system was pressurized at 34 bar (33.5 atm) using a
back pressure regulator IDEX BPR (PEEK). Once irradiated at high temperature and high
pressure at the same time, the temperature was fastly lowered, taking advantage of the high
heat transfer provided by the capillary reactors. The sampling was carried out on ice using
Supelco 7 ml Clear Vials, with screw cap and PTFE liner. All samples were stored at -20 ⁰C in
nitrogen atmosphere for 1 hour before HPLC analysis. The latter was carried out using HPLC
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(Shimadzu UFLC-XR) with a GraceSmart RP 18 5u column (150 mm x 4.6 mm), using cholesterol
(95 % Alfa Aesar) as internal standard (IS), 100 % acetonitrile (99 % Merck) as mobile phase
and UV-visible Shimadzu diode array detector (RID-10A). The whole absorption spectra is
shown in Figure 2. In these conditions all peaks were clearly separated. Retention time as well
as UV-Vis spectra were used for identification (Figure 3): for VD3 characteristic peaks at 213
nm and 264 nm were found (Figure 3a). For 7DHC the peaks were at 272 and 282 nm (Figure
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b). Standards of 7DHC and VD3 (Alfa Aesar 95 %) were used for quantification at 190 nm,
quantifying the side products together as the difference according to sampling.
Scheme 2: Design view of the photochemical and thermal micro-flow system
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Figure 1: Full spectrum of Phillips HOK 4/120 mercury lamp (provided by Phillips)
Figure 2: Absorption spectra of most of compounds involved.
a)
b)
Figure 3: Absorption spectra of VD3 (a) and 7DHC (b).
3. Results and discussion
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.1. Effect of temperature
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A set of temperatures above the boiling point of the solvent were checked between 80 and
260 ⁰C. Despite the fact that temperatures can easily affect vitamins, cholecalcipherol (VD3) is
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known for its thermal resistance . In addition, flow chemistry allows to work under anoxic
conditions, which enhances even more the resistance of these compounds since the oxidation
risk is avoided. Hence, in order to ensure the 7DHC stability under the harsh conditions in
continuous flow, a high-p,T test was previously done without using light energy. In this case,
the decrease of 7DHC and the existence of other peaks was evaluated. As shown in Figure 4,
losses of 7DHC are relevant only above 200 ⁰C. Also some important changes in the range
between 240-260 ⁰C were found. This could suggest a critical stability point for 7DHC in this
range. These differences are also more important with longer thermal residence times (TRT). In
order to ensure the operating temperature in the irradiation chamber, when needed, a dark
pre-heating step was also placed in the setup before the irradiation chamber (Scheme 3). The
irradiation time (IT) then only refers to one part of the TRT, which is longer. In this connection,
Table 1 shows the equivalence between TRT and IT in the range from 43 until 300 s. It is
important to note at this point that VD3 was not detected in any of these experiments. Thus
and as expected, photo-activation is strictly needed.
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Scheme 3: Correlation between irradiation times and thermal residence time. (BPR = back pressure regulator)
Irradiation time (s)
Thermal residence time (s)
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Table 1: Correlation between irradiation times and thermal residence time.
After the stability test, the reaction was carried out using the UV-lamp with 400 W power. As
this mainly refers to heating power and unavoidably heats up anything in close distance, this
was used also to maintain the temperature in the irradiation chamber. The additional heating
by the oven then guaranteed to achieve the temperature desired. The temperature was
controlled with three thermocouples located in the irradiation chamber. The energy supplied
by the UV-lamp accordingly reduces the energy demand of the oven, which results in lower
energy in the pre-heating (dark) part of the setup. Here, the desired temperature was reached
only in the last sections of the pre-heating, reducing the thermal stress of 7DHC.
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Figure 4: Decomposition of 7DHC at different temperatures and different residence times.
Figure 5 shows the evolution of the reaction at different temperatures with different
irradiation times using a 0.05 M solution of 7DHC in t-BME as starting material. In the light of
these results, apparently no relevant reaction is detected below 100 ⁰C. Yet, Figure 5 shows
that with the same irradiation time, a temperature effect is given, which is seen to be
synergistic together with the irradiation. Furthermore, the behavior of the reaction is different
above and below 200 ⁰C. Above this temperature and at almost stable 7DHC conversion, the
amount of VD3 decreases to the expense of increasingly formed side compounds, which
include P3, L3, T3, and also otherwise decomposed 7DHC. This phenomenon may be caused
both by VD3 decomposition or shift in the equilibrium of VD3 synthesis, enhancing the amount
of P3 and probably the amount of T3 and L3 as well. This is not observed below 200 ⁰C where,
despite lower conversions and lower yield, where more control over side products is given. Yet
note that side product formation is inherent to the reaction because of the intermediate P3.
The maximum conversions achieved are in the range of 40-80 %, depending on the irradiation
time, with a maximum yield of VD3 around 20-25 %. It should be stressed that such
conversions and yields are reached with really short times by using the photo-high-p,T micro-
flow setup. Below 160 ⁰C, the difference in the amount of side compounds between 2 and 5
min irradiation times can be explained because of the amount of P3. Yet, since P3 is the
reaction intermediate, a high increase of P3 is expected derived from photo-activation in the
first steps. Simultaneously to P3 synthesis, isomerization is also given, and therefore P3 is
balanced between synthesis and isomerization. According to Figure 5, the maximum of P3 is
around 2 min IT. Yet, after 5 min IT, the higher yield of VD3 in the same conditions confirms
that thermal isomerization is also carried out. The results of Figure 5 also suggest that at
temperatures above 200 ⁰C, both the kinetics of side compounds together with the
decomposition of 7DHC and VD3 give lower VD3 yields, making the reaction inefficient. The
latter can only be controlled by reducing IT (43 s), but assuming at the same time a reduction
in both conversion and yield. However, the unreacted and unaltered 7DHC and P3 can be
recycled after 43 s IT.
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a)
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Figure 5: Effect of the temperature on the 7DHC conversion and the VD3 yield operating at 34 bar with (a) 43 s (b)
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min and (c) 5 min irradiation time.
3.2. Effect of concentration
The limit solubility of 7DHC in t-BME was experimentally determined and found to be 0.22 M
84.5 mg/mL) at room temperature. Thus, the concentration used in the experiments was set
(
equal or lower this value. The solubility of all other isomers involved in the reaction is much
higher than the one obtained for 7DHC. Hence, for this variable three levels were fixed: low
(
0.05 M), medium (0.11 M) and high (0.22 M). The goal of this study is to unveil the influence
of the concentration of the initial 7DHC on the reaction yield. This could be correlated with the
effect of the concentration in the light transfer according to the Lambert Beer law (L-B).
Following L-B, the higher the concentration, the lower should be the transmittance of 7DHC
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solution. Nevertheless, a negligible effect of the concentration was found and the effect using
different concentrations was very similar. For these experiments, a 3.2 mm external diameter
and 0.5 mm internal diameter capillary was used. Therefore, as expected by the excellent light
penetration in the quartz made microcapillaries (transparency above 90 % at 280 nm according
to the supplier), there are even at high concentration enough photons for reaction at any place
within the capillary. Also the fact that we used reflective aluminium in the internal part of the
irradiation chamber, favoured internal reflections and therefore the efficient irradiation in all
sides.Thus conversion is almost invariant with concentration. Yet, all reactions involved
(including side ones) are intramolecular and thus hardly any dilution effect was to be expected.
This conclusion is important in terms of productivity, since allows operating with the highest
concentration possible. Following this, and taking into account (i) the checked concentrations
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1
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1
1
2
2
2
2
2
2
2
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2
2
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4
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4
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5
5
5
5
5
6
0
1
2
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5
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7
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9
0
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8
9
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7
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(
0.05 M, 0.01 M and 0.22 M), (ii) the mean of the VD3 yields for each case, and (iii) the
corresponding flow rate according to the length of the capillary tested for each IT (20, 7.12 and
.67 ml/h respectively) and (iv) a 90 % VD3 recovery in the purification step, the productivity
results in Table 2. Besides the highest productivity of the 43 s IT capillary, Table 2 shows that
2
7
[
DHC
M]
7DHC
[mg/h]
IT
[s]
Productivity
[g/month]
Tablets/year
[milions]
[mg/h]
69
[g/day]
1.7
[kg/year]
0.6
0
.05
.11
.22
384
43
120
300
43
50
23
40
18
5
1
37
32
0.8
0.3
5
1
9
0.2
7
0.1
0
845
01
152
70
3.6
109
51
1.3
88
41
12
175
81
23
3
120
300
43
1.7
0.6
113
20
0.5
15
0.2
0
1690
304
141
41
7.3
219
101
29
2.6
6
2
01
26
120
300
3.4
1.2
1.0
0.4
Table 2: Estimation of anual average VD3 productivity of a single capillary using photo-high-p,T setup.
operating with the described continuous photo-high-p,T setup, is possible to produce at least
in the order of tens of millions of tablets (considering 15 μg VD3/tablet) per year using photo-
high-p,T with just one coil as described in this paper. In addition, especially operating with the
shortest IT, 7DHC and P3 can be recycled, and therefore converted to VD3 (recycling in not
2
8
considered in table 2). Comparing these results with previous literature (Fuse et al., 1.9 mg/h
productivity with 15 min residence time), photo-high-p,T intensification of VD3 synthesis leads
to 40 times better productivity with the lowest concentration (0.05 M), and 178 times better
28
productivity with the highest concentration (0.22 M). Despite Fuse et al. referred to calcitriol
a more unstable derivative) instead of VD3, the productivity of VD3 with the configuration
(
described here is 1 and even 2 orders of magnitude better than the process reported before.
Also the residence time is 20 times shorter than comparable continuous setups.
Taking Table 2 as a reference and considering our process design, an outlook for the scaling up
of the vitamin synthesis is proposed in Figure 6. Quartz tubing are enlarged to mm-scale (by a
4
0
factor of 10 possibly) and favourably use gas-liquid segmented flow. The thin liquid layers
around the gas bubbles preserve the original characteristic dimensions of the micro-flow used
in this study (own results to be published), while the characteristic dimensions of the whole
segmented flow can can be in meso range giving already per capillary much enlarged
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throughput. Those tubing are still placed in parallel manner to further enlarge the throughput,
surrounded and illuminated from a slightly separated, to avoid direct contact, reflecting hot
aluminium surface which has semi-circular shape. This multitube configuration is suitable to be
numbered up as suggested in Figure 6, yet the degree of numbering up is probably
exaggerated (for reasons of being generic). For pharma-scale applications it might be enough
to have one lamp and possibly 10 parallel channels. In between, UV lamps are placed quartz-
jacketed in order to isolate them from overheating.
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Figure 6: Outlook of the scaling up procedure for medium scale production.
3
.3.Effect of irradiation time (IT)
4
1
Flow chemistry and NPW are known for its capability to reduce order-of-magnitude the
3
6
residence times by intensifying the reaction kinetics . Here, the residence time (irradiation
time) is arranged simply by changing the flow-rate of the solution through the reactor. Hence
three levels of this factor are considered: 43 s, 2 min and 5 min. Figure 7a shows a comparative
view of the effect of irradiation time on VD3 yield at different temperatures using in all cases
the same concentration (0.05 M). Apparently, the VD3 yield raises below 200 ⁰C and decreases
above this temperature. This is mainly because of (i) 7DHC and VD3 decomposition, and (ii)
because thermal equilibrium clearly allows side compounds above 200 ⁰C. Moreover, below
2
00 ⁰C the shorter the IT is, the lower is the yield. Nevertheless, the optimum conditions for P3
using 2 min IT, and therefore its conversion to mainly VD3, can explain why the VD3 yield at 2
min IT is higher than the yield at the longest residence time above 200 ⁰C. In the light of these
results, it can be concluded that photo-high-p,T micro-flow setup operates close to the stability
limits of 7DHC and VD3, and achieves the maximum intensification possible.
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The selectivity of the reaction has a decreasing tendency (Figure 7b). At low conversions (low
temperatures) high selectivity is found, while at medium and especially high temperature the
selectivity becomes very low because of the side compounds and decomposition. The latter is
especially relevant above 200 ⁰C. The low selectivity at 120 s reaction time is because of P3
peak, as discussed above.
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a)
b)
Figure 7: Effect of the irradiation time on the (a) VD3 yield and (b) in the reaction selectivity, operating at 34 bar
and 0.05 M 7DHC concentration.
3
.4.Statistical analysis: Principal Component Analysis (PCA)
In order to show the relevance of the reaction variables studied for VD3 yield quantification, a
Principal Component Analysis (PCA) was performed using The Unscrambler® software. With
this method the experimental variables with most variability can be identified and this
information can be used for process optimization. In this connection, the loadings plot (Figure
8
) shows the desired relation among studied variables and the subspace dimensions.
According to this plot, the irradiation time (IT in the plot) accounts for 72 % of the variability of
the VD3 yield, while the temperature only accounts for 28 %. This confirms that IT plays a
more relevant role ahead of T, and therefore, this is the key variable for the VD3 synthesis.
Statistically, this can be explained because the T profiles are essentially rather similar. In
addition, temperature shows up a synergistic effect, but in a secondary component. It is also
important to note that concentration is in the center of the axis. This confirms that
concentration slightly contributes to the variability of VD3 yield in the reaction using the
performed setup, so in capillary based photo-high-p,T.
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Figure 8: Effect of the reaction variables on the VD3 yield operating at 34 bar.
4
. Conclusions
Process intensification both by photochemistry and thermal intensification may open
additional options for photo-reactions, yet is difficult to be combined in one step using
conventional procedures and equipment because of safety risks. In this manuscript, this new
kind of novel process windows is presented and investigated. The photo-high-p,T micro-flow
setup used enables to operate at high pressure and high temperature, while allowing also for
irradiation in a safe manner.
The synthesis of Vitamin D was proven in photo-high-p,T conditions. Main result is a reduction
3
of the reaction time down to 43 s, which means several times an order of magnitude shorter
than the same process carried out in batch. Also previous comparable flow studies set the
residence time in around 15 min. A 42 % conversion of 7DHC is achieved, giving a 17 % yield of
3
vitamin D and 25 % of intermediates. This amounts to 40 % selectivity. In addition, working
under harsh conditions showed up thermal limitation of performing the reaction above 200 ⁰C.
Yet interestingly, the optimal working conditions were found close to that at 200 ⁰C. As a
promising result, because of the small size of the capillary, no decrease of conversion within
the range of concentration was found. The highest concentration used was close to the
solubility limit the solvent used. This demands for the use of new, tailored solvents which are
specialised for the needs of flow processing (see www.one-flow.org). As given here, solvents
would be need to identified which allow to work in the molar range, which then probably will
show the concentration dependence due to remarkable absorption losses. Nevertheless, the
annual productivity achieved with the current photo-high-p,T micro-flow technique (just one
micro-capillary) suffices for several tens of millions of tablets, and is between 1 and 2 orders of
magnitude higher than comparable continuous processes described in the literature.
Moreover, a statistical analysis was performed in order to evaluate the influence of the
3
variables (concentration, irradiation time and temperature) on the yield of vitamin D . As a
main and not surprising result, irradiation time is the key variable in this reaction. As another
main, truly new and insightful result, it was found that the temperature has also a synergistic
influence in the irradiation step when both are combined at the same time. The common view
is that the reaction is performed in two steps: photochemical and thermal. Nevertheless, these
steps are performed in conventional equipment separating the steps spatially and timewise.
Yet, this paper gives for the first time an approach of high T and irradiation at the same time,
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suggesting that temperature plays a synergistic effect with irradiation time in the reaction
kinetics. As evident from the discussion of the single plots and as quoted above, the study also
confirms that concentration does not have any role on the reaction performance.
Acknowledgements
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This project was funded by the Horizon 2020: Marie Skolodowska-Curie Individual Fellowship
awarded by Dr. Marc Escribà Gelonch under grant agreement number 659233.
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