Continuous flow Fischer esterifications harnessing vibrational-coupled thin film fluidics

Article abstract of DOI:10.1039/c4ra11777h

Rapid Fischer esterification reactions occur under solventless, continuous flow conditions in dynamic thin films. This methodology uses limited catalyst, require no additional heat input and occurs within the confinements of an inexpensive vortex fluidic device (VFD). The associated mechanoenergy is primarily delivered from two types of vibration, which are manifested in sharp increases in the yield of the reactions. These vibrations promote the existence of Faraday waves that alter the instantaneous shear rates of the reactants within the rotating tube. Tuning the rotational speed of the device allows harmonic vibrations to be utilized in the synthesis of alkyl-based esters within both a high and low contact angle NMR tube. This journal is

Full text of DOI:10.1039/c4ra11777h

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Continuous flow Fischer esterifications harnessing
vibrational-coupled thin film fluidics†‡
Cite this: RSC Adv., 2015, 5, 1655
a
b
a
*
Joshua Britton, Stuart B. Dalziel and Colin L. Raston
Rapid Fischer esterification reactions occur under solventless, continuous flow conditions in dynamic thin
films. This methodology uses limited catalyst, require no additional heat input and occurs within the
confinements of an inexpensive vortex fluidic device (VFD). The associated mechanoenergy is primarily
delivered from two types of vibration, which are manifested in sharp increases in the yield of the reactions.
These vibrations promote the existence of Faraday waves that alter the instantaneous shear rates of the
reactants within the rotating tube. Tuning the rotational speed of the device allows harmonic vibrations to
be utilized in the synthesis of alkyl-based esters within both a high and low contact angle NMR tube.
Received 4th October 2014
Accepted 24th November 2014
DOI: 10.1039/c4ra11777h
www.rsc.org/advances
acid esters as in biodiesel production. Butyl acetate alone is an
important component in coatings, having a relatively low
Introduction
vapour pressure, featuring as a solvent for polymerization
reactions and as a fragrance in cosmetics.9,10 Currently,
advancing the production of esters (especially for biodiesel)
focuses around microbial lipases and oleaginous microorgan-
isms,11,12 with other processes using ion exchange resins13 and
heterogeneous catalysts such as MgO14 and Na2CO3.15 While
these systems are highly effective, there is a number of
concerns, including the generation of a waste stream, catalyst
lifetime, the use of high temperatures and pressures, and rather
specic conditions required for biological catalysts. In addition,
Lewis acid catalysts, for example, tin-octoate, require removal
aer processing.9 Overall, developing more versatile and more
benign alternative reactions, using smaller amounts of catalyst
at standard temperature and pressure, with lower environ-
mental impact and capital outlay are warranted.16,17
Reactions of alcohols with carboxylic acids to form esters are
fundamentally important in organic synthesis. Typically, these
esterication reactions require an acid catalyst to promote the
electrophilicity of the carbonyl moiety,1a with heating improving
the kinetics.1b As chemistry has progressed, so has the diversity
and complexity of such catalysts, with the ability to generate
esters in excellent yields.2–6 Nevertheless, the inherent
complexity, associated expense of such processing and the
safety of large scale batch processing can be questionable, as
can the difficulties in scaling up. To this end we have developed
high yielding esterication reactions, where the chemistry is
simplied, using continuous ow processing and simple acid
catalysts within the connements of the recently developed
vortex uidic device (VFD).7 This thin lm microuidic platform
allows the continuous ow processing of uids up to 7 mL
minꢀ1 or 20 mL minꢀ1 for the 7.0 mm or 17.5 mm internal
diameter borosilicate tubes (commercially available sizes of
NMR tubes) respectively, Fig. 1. Scaling up is possible by using
parallel arrays of such inexpensive VFD, as in conventional
microuidic processing involving channels. In this context,
clogging is not an issue for the VFD, as it can be for other
microuidic processing platforms.8
Esterication reactions are widely used in industry,
including the synthesis of alkyl-acetate based esters and fatty
aSchool of Chemical and Physical Sciences, Flinders University, Bedford Park, SA 5042,
Australia
Fig. 1 Photograph of a vortex fluidic device (VFD) housing a 17.5 mm
internal diameter borosilicate glass NMR tube at a 45 degree angle, q,
relative to the horizontal position, along with a schematic of the
dropping regime in the VFD and the subsequent dynamic thin film. The
liquid is introduced into the system through the metal jet feeds, the
liquid then whirls up the rotating tube and exits through a Teflon
channel in the housing unit at the top of the device. Full schematics
and workings of the device have been reported previously.7
bDepartment of Applied Mathematics and Theoretical Physics, University of
Cambridge, CB3 0WB, UK
† This research was supported by Flinders University, the University of
Cambridge, the Government of South Australia and the Australian Research
Council.
‡ Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra11777h
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The VFD can operate under so call continuous ow mode
where jet feeds deliver liquid to the base of the rapidly rotating
tube leading to intense shearing and micromixing in the
resulting dynamic thin lm, with the lm thickness primarily
governed by the rotational speed, ow rate and tilt angle, q of
the tube. Controlling the delivery of a drop of liquid onto the
hemispherical bottom of the VFD tube sets the level of shearing
and when the liquid exits the hemisphere, a helical wave is
formed, which is likely to provide additional shearing due to the
viscous drag of the liquid against the rotating tube. The intense
shear that is created at the base of the tube is also present in the
thin lm, which arises from the Stewartson/Ekman layers.18
Also noteworthy, is that the intense shear can result in a change
of the surface tension of the thin lm, which can facilitate high
mass transfer of gases into and out of the liquid.19
The VFD is a versatile processing platform in materials
chemistry, for example, in exfoliating graphene and h-BN,20
coating algal cells with graphene and a magnetic polymer,21
controlling the pore size of mesoporous silica22 and decorating
graphene with palladium nano-particles.23 In the context of the
present study, the VFD is effective in enhancing the dimeriza-
tion of cyclopentadienes, the direct base catalysed synthesis of
2,4,6-triarylpyridines from aromatic aldehydes and ketones in a
single pass,7 the stereospecic synthesis of calixarenes derived
from resorcinol or pyrogallol and aldehydes,24 and room
temperature conversion of sunower oil to biodiesel,25 all under
continuous ow conditions. The work herein focuses on
furthering fundamental organic chemistry, with an aim of
understanding the remarkable outcome of the above reactions
carried out using the device.
Fig. 2 (a) Variation in the temperature (DT) of n-butyl acetate pro-
cessed in the VFD operating in the confined mode (2 mL of n-butyl
acetate, 17.5 mm internal diameter tube), for different rotational
speeds, at q 30ꢁ, 45ꢁ and 60ꢁ (data points in triplicate), with the
temperature measured using a thermal imaging camera (0.20 K
resolution) before and after three minutes of rotation. Temperature
change of n-butyl acetate was measured before and then immediately
after processing, rather than measuring the temperature of the tube
during processing, acknowledging that the tube is not IR transparent.
(b) and (c) Thermal images after three minutes processing for 4000
and 5250 rpm respectively, at q 45ꢁ, measuring the temperature of the
NMR tube itself. See experimental for further details.
glass tube will fall into what one may call a resonance, and this
will generate more heat via friction. Thus, at 5250 rpm, for a
17.5 mm internal diameter tube, the rotational speed generates
a harmonic vibration, dramatically affecting the increase in
frictional heat generated (as well as chemical reactivity, see
further on). As a control, the heat transfer was monitored for a
rotating tube devoid of any solvent (ESI‡), and while heat was
produced at both the upper and lower parts of the device, like in
Fig. 2, it remains essentially localised around the Teon
housing for prolonged periods. Thus, the vibration of the glass
tube against the Teon housing results in some localised
heating. However, when solvent is present in the tube at the
harmonic vibration, heat is dispersed throughout the glass
tube, as shown in Fig. 2C. This further highlights a strong uid
dynamical response at selected rotational speeds, and such
vibrational effects are clearly observed at a 5250 rpm rotational
speed at q 45, Fig. 2.
The continuous ow mode of the VFD was initially explored
for the synthesis of n-butyl acetate. For comparison, as a control
experiment, the batch processing reaction catalysed by sul-
phuric acid at room temperature was carried out, reaching
#47% conversion aer 3 hours of vigorous stirring at 1400 rpm.
To achieve higher conversion rates of n-butyl acetate, literature
reports require heating between 90–110 ꢁC for a number of
hours.26–29 These elevated temperature methods include the use
of complex and expensive catalysts such as sulfonic acid func-
tionalised MIL-101,26 silicotungstic acid on bentonite,27 expan-
sible graphite,28 tin oxide29 and calcium chloride coupled with
Results and discussion
The temperature change (DT) of the thin lm in the VFD
operating in conned mode operation was monitored for 2 mL
of n-butyl acetate, to ascertain any temperature change associ-
ated with varying the q and rotational speed, Fig. 2. The choice
of q dramatically affects the temperature of the thin lm, with a
distinct change from 5000 rpm to 5250 rpm at q of 45ꢁ, for the
device operating at ambient temperature (17.5 mm internal
diameter tube). Aeꢁr three minutes at these operating param-
eters DT was +7.73 C (IR thermal imaging), which is 3.33 ꢁC
greater than DT at 5000 rpm. This value is large in comparison
to DT ꢂ +1 ꢁC at a q 30ꢁ or 60ꢁ when stepping from 5000 to 5250
rpm, Fig. 2. In contrast, at q 30ꢁ the highest +DT corresponds to
the step from 5500 to 6000 rpm, regaining the linear increase in
temperature. For all angles and speeds >6000 rpm, the higher
the rotational speed, the greater +DT. The vibration of the glass
tube within the upper housing unit whilst rotating leads to the
generation of frictional heat, Fig. 2, which is transferred into the
thin lm through the rotating glass tube. At certain rotational
speeds there are more or larger vibrations that lead to increased
amounts of friction, thus leading to greater heat transfer into
the thin lm, especially at 5250 rpm where there is a sharp +DT.
The sharpness of this increase in temperature is due to a more
effective vibration of the rotating tube against the Teon
housing unit. At certain rotational speeds, the vibration of the
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energy intensive microwave radiation.30 Nevertheless, batch relationship between ratio of reactants (as a measure of relative
processing is still limited to diffusion control and the need to concentrations) and overall conversion. Other acid catalysts
address environmental impacts remains. Herein we present an were tested (ESI‡), establishing sulphuric acid being the most
ambient pressure and temperature, continuous ow process for efficient, with concentrated nitric, hydrochloric and phosphoric
the synthesis of n-butyl acetate, which harnesses mechanoe- acid dramatically reducing the yield.
nergy in the VFD, increasing the yield (and also for other alkyl
esters) by 26% in 3.25 minutes, which is the residence time of up to four times in an attempt to increase the conversion,
the reactants in the tube. noting that starting material was still present in the product
The collected reaction mixture was recycled through the VFD
Both the concentration of acid catalyst and the ow rate were mixture aer a single pass through the VFD (ESI‡). Interest-
systematically varied for q 30ꢁ, 45ꢁ and 60ꢁ, at rotational speed ingly, this recycling resulted in some evaporation and a
of 5250 rpm, corresponding to the largest +DT, Fig. 3. The decrease in the amount of product isolated. Clearly, an impor-
increase in conversion at q 45ꢁ to 73% was the most striking for tant processing parameter for the VFD is residence time, which
a single pass at 0.50 mL minꢀ1 using 10% acid catalyst. is controlled by the ow rate, which was set at 0.50 mL minꢀ1
Conversions approaching this level were also observed for aer establishing that it results in the highest ester formation at
higher ow rates and lower catalyst loading. Logically, the the tilt angles explored.
vibrations causing the change in temperature of the system are
Finer control of the reaction was studied by varying the
also responsible for the overall increase in yield at a 45ꢁ tilt rotational speed, noting that as the rotational speed increases,
angle. This is further manifested when taking into consider- the thickness of the dynamic thin lm in the VFD decreases, as
ation the relatively small DT at 30ꢁ and 60ꢁ tilt angles, and the established for the conned mode of operation,7 and presum-
overall lower yield of n-butyl acetate at these angles for a rota- ably this also prevails under the continuous ow mode. For
tional speed of 5250 rpm, where there are no apparent vibra- $4500 rpm there is a general decrease in yield, arising from a
tions. Even though this system requires no additional input of small amount of evaporation of the reactant components, as the
temperature, the increase in temperature of the glass, due to lm gets thinner, Fig. 4. However, at 5250 and 6000 rpm the
vibrational induced friction, has little impact on the increase in yield is dramatically higher, for a very narrow speed range (ꢃ10
yield. This is observed when analysing the longer residence rpm) for a 17.5 mm internal diameter tube supported by a single
times (slower ow rate), which almost always leads to a decrease bearing system, with the top of the tube contained within a
in yield. Clearly the small rise in temperature of the system is 20.51 mm guide, having a headroom of 0.51 mm. The sharpness
not a substantial driving force behind the reaction. This is of the peaks at 5250 and 6000 rpm indicate that a mechanical
further demonstrated for higher rotational speeds, Fig. 4. The resonance is responsible for the increase in yield, driving a
largest +DT is at 7000 rpm, Fig. 2, but this is not a vibrational Faraday wave like uid dynamic response.31 The VFD dramati-
effect. Heat is generated by the fast rotating bearings, but the cally amplies the synthesis of the ester when a mechanical
lack of a distinct heat change corresponds to some of the lowest response is present. This Faraday wave enhancement occurs at
yields.
the rotational speeds indicated above; for all other rotational
Given that a tilt angle of 45ꢁ and a 0.50 mL minꢀ1 ow rate speeds tested, there seems to be no additional mechanical
gave the highest yield, the reaction was further optimized with enhancement.
these parameters xed. A 1 : 1.5 ratio of n-butanol : acetic acid
In further testing the presence of macro-vibrations between
was used so that excess acid could be easily removed, and this the glass tube and the housing chamber, a double bearing VFD
was subsequently shown to give the highest yield (ESI‡), and system was constructed with the top of the tube now also sup-
thus the use of the VFD does not follow a simple linear ported by a bearing. This resulted in the peak at 5250 rpm
Fig. 3 Percent formation of n-butyl acetate in the VFD (17.5 mm internal diameter NMR tube) at room temperature, as a function of flow rate and
percent catalyst for q (a) 30ꢁ, (b) 45ꢁ and (c) 60ꢁ relative to the horizontal position, at 5250 rpm (data points in triplicate). The tilt angle q, Fig. 1, was
measured by a digital protractor with a resolution of 0.01ꢁ.
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by the lm on the walls of the rapidly rotating tube, altering the
instantaneous shear rates. Strengthening this theory further is
that for both the continuous ow synthesis and conned mode
heating of n-butyl acetate, Fig. 2 and 4 respectively, for a single
bearing system, the optimum rotational rate was 5250 rpm. The
similarity of optimum rotational speed provides further
evidence that this is a mechanical resonance of the system.
Further to this model, if there is too much vibration then the
thin lm breaks up changing the dynamics, resulting in
reduced shear stress, leading to a decrease in yield. This was
observed for an inadvertently corroded bearing, with high levels
of noise and vibration resulting in a 35% decrease in yield.
The conversion to n-butyl acetate was typically lower in the
7.0 mm VFD for the same ow rate, and this relates to the lower
surface area and thus the decrease in residency time of the thin
lm relative to a 17.5 mm tube. There is an expected optimum
radius associated with the VFD tubes, noting that the radius at
which the drop hits the bottom of the tube sets the maximum
amount of shear possible. However, the dominant effect is the
residency time of the uid in the hemisphere at the bottom of
Fig. 4 Variation in yield of n-butyl acetate in 7.0 mm and 17.5 mm
internal diameter tubes in the VFD with either single or double bear-
ings. Single bearing regions are highlighted as macro-vibrational
effects and double bearing effects are highlighted as micro-vibrational
regions, with q 45ꢁ for varying flow rates (data points in triplicate).
disappearing, but the peak at 6000 rpm remaining, with the the tube. There is conceivably an optimum between the impact
overall yield decreasing slightly. Thus, the 5250 rpm peak arises drop radii and residency time within the hemispherical part of
from macro-vibrations imparting additional mechanical energy the VFD tube. We also note that changing the ow rate did not
to the NMR tube. However, new peaks emerge 250 rpm apart, at disrupt the position of these peaks, further indicating a
6250 and 6500 rpm, when using the double bearing system, mechanical resonance driving a secondary uid dynamical
indicating that this is an ordered form of mechanical energy, response that can exist due to the rapid rotation of the VFD
and is consistent with a harmonic micro-vibrational effect. The tube.
7.0 mm internal diameter (external diameter, 10.0 mm) VFD
The optimised parameters for preparing n-butyl acetate were
processor was also used for comparison, at the same ow rate, used for preparing a range of esters, without further individual
which reveals a similar pattern, but with different rotational optimisation, so that a direct comparison could be drawn,
speeds producing sharp increases in yields, centred around Table 1. However, an additional operating parameter was
4510 and 6490 rpm. The VFD unit housing the smaller diameter introduced, namely varying the nature of the inner surface on
tube also gives the rotating tube a small head room within the the NMR tubes, with the internal surface of the 17.5 mm VFD
housing unit at the top of the device, and the peak at 4510 rpm tube being coated with dodecyl alkyl silane, thereby creating a
is indicative of a strong macro-vibration of the borosilicate tube hydrophobic surface, as opposed to a hydrophilic surface for
against the housing chamber. This vibrational model is also the as received NMR tubes (ESI‡). While some yields decreased
consistent with the thermal IR images, Fig. 2, which show a for the hydrophobic surface, the yield of n-butyl acetate
large amount of heat being generated at the upper bearing,
arising from macro vibrational induced friction of the NMR
Table 1 Yields of esters prepared using the VFD operating at q 45ꢁ,
tube against the Teon sleeve. The 7.0 mm tube also has a peak
5250 rpm, for a 17.5 mm internal diameter tube (borosilicate glass or its
at 6490 rpm, close to the 6500 rpm for the 17.5 mm tube.
dodecyl silane treated analogue), with an alcohol : acid ratio of 1 : 1.5,
for a flow rate of 0.50 mL minꢀ1, which corresponds to 3.25 min
When the vibration at 5250 rpm was removed for the
17.5 mm tube, the chemical enhancement was also removed.
This presumably arises from a change in the stability of the thin
lm, thereby revealing new peaks at 6250 and 6500 rpm.
However, at 6010 rpm with a ow rate of 0.50 mL minꢀ1 there is
a large decrease in yield, and thus the vibration here appears to
have a detrimental effect on the stability of the thin lm as
further described below, or that the window for the vibration is
small. There were no sharp increases in yield, but nevertheless
there is a vibrational effect on the thin lm at 6010 rpm, as for
all other studies using the 17.5 mm internal diameter NMR
tube.
residence time
Yield (%) for borosilicate Yield (%) for silane
Ester
glass surfaces
treated surfaces
n-Propyl acetate
n-Butyl acetate
n-Pentyl acetate
n-Hexyl acetate
40
73
87
89
82
77
55
57
79
33
36
28
29
80
59
98
70
73
62
90
49
2
n-Octyl acetate
n-Butyl propionate
n-Pentyl propionate
n-Hexyl propionate
n-Propyl propionate
i-Propyl propionate
s-Butyl acetate
The uid based mechanism on how vibrations cause large
increases in yields relates to the stability of the thin lm.
Vibrations within the tube are likely to excite Faraday waves
through a modulation of the large centrifugal force experienced
6
11
s-Butyl propionate
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increased to 80% and the yield of n-hexyl propionate and n-hexyl
Temperature measurements were carried out with a high
acetate increased by 33% and 9% respectively. The decrease in level of condence using a FLIR-T62101 thermal imaging
yields of the other esters presumably arises from the established camera with a resolution of 0.02 K. The camera was calibrated
lm at the bottom of the tube becoming unstable for a hydro- using radiation sources that are traceable to National Standards
phobic surface, leading to a decrease in sustained shear, and at the SP Technical Research Institute of Sweden, or to NIST,
thus a decrease in reactivity. Surprisingly n-hexyl alcohol rather National Institute of Standards and Technology (USA).
than the longer chain n-octyl alcohol benets the most from the Measurements were in triplicate, with the largest error 0.37 ꢁC,
hydrophobic coating, and the ability to change a number of and were determined from the difference in temperature of the
processing parameters suggests that esterication reactions can uid tens of seconds before the experiment (outside of the tube)
be further improved on an individual basis.
and immediately aer processing, thus allowing an accurate
measurement of the temperature of the thin lm.
Conclusions
Notes and references
We have identied vibrations in two different diameter VFD
units which transfer additional mechanoenergy into the thin
lm, which is manifested in an increase in yield by #26%, to
>70% for n-butyl acetate, for a single pass for which the resi-
dence time is 3.25 minutes. This is in contrast to 47% yield of
n-butyl acetate for a three hour room temperature batch reac-
tion. The increase in yield is without the need for any further
modication to the VFD or the surface of the glass tubes. It
must be noted that these vibrations are not conned to a single
system, with several systems all giving reproducible results at
the rotational speeds indicated. Even though the measurements
of yield were carried out offline, the reaction was quenched on
exit of the system, thus providing an accurate measurement of
conversion. Overall, the results highlight the versatility of the
inexpensive VFD beyond earlier studies, now with a clearer
understanding of the repertoire of operating parameters, which
can be translated to reducing the environmental impact of
synthesis due to the minimisation of energy requirements and
the just in time processing capability.
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