Highly effective continuous-flow monolithic silica microreactors for acid catalyzed processes

Article abstract of DOI:10.1016/j.apcata.2014.10.047

This work reports the performance of a new monolithic silica microreactor, activated with sulphonic acid groups in the continuous-flow synthesis of n-butyl acetate and n-butyl lactate. A reactive core of the reactor was made of a single silica rod with bi-continuous structure, containing flow-through pores with diameter in the range of 20-50 μm and mesopores of ca. 20 nm localized in silica struts. This structure resulted in low pressure drop, even at flow rates large enough to eliminate external mass transfer effect on the reaction kinetics. The microreactor functionalized with 0.65mmol/g of-SO₃H groups showed high activity and productivity in both esterification reactions carried out in temperatures up to 140 °C. Structural and catalytic stability of the microreactor was confirmed to demonstrate its process viability.

Full text of DOI:10.1016/j.apcata.2014.10.047

Applied Catalysis A: General 489 (2015) 203–208
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Highly effective continuous-flow monolithic silica microreactors for
acid catalyzed processes
A. Koreniuka, K. Maresza, K. Odrozekb, A.B. Jarze˛bskia,b, J. Mrowiec-Białon´ a,b,∗
a Institute of Chemical Engineering, Polish Academy of Sciences, Bałtycka 5, 44-100 Gliwice, Poland
b Department of Chemical Engineering and Process Design, Silesian University of Technology, Ks. M. Strzody 7, 44-100 Gliwice, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 August 2014
Received in revised form 21 October 2014
Accepted 23 October 2014
Available online 1 November 2014
This work reports the performance of a new monolithic silica microreactor, activated with sulphonic
acid groups in the continuous-flow synthesis of n-butyl acetate and n-butyl lactate. A reactive core of the
reactor was made of a single silica rod with bi-continuous structure, containing flow-through pores with
diameter in the range of 20–50 ␮m and mesopores of ca. 20 nm localized in silica struts. This structure
resulted in low pressure drop, even at flow rates large enough to eliminate external mass transfer effect
on the reaction kinetics. The microreactor functionalized with 0.65 mmol/g of SO3H groups showed
high activity and productivity in both esterification reactions carried out in temperatures up to 140 ◦C.
Structural and catalytic stability of the microreactor was confirmed to demonstrate its process viability.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Microreactors
Silica monoliths
Acid catalysis
Esterification
Hierarchical materials
1. Introduction
Silica-based continuous-flow monolithic microreactors, first
proposed by the Montpellier group, appeared to be extremely
Synthesis of most fine chemicals is typically carried out in a
liquid phase and batch operation. Their replacement by contin-
uous processes using microreactor-based technologies can make
them more effective, safer and to give products with stable proper-
ties. Catalysts beads, currently applied as column reactor packings
in fine chemicals’ synthesis, are usually of 50–100 ␮m diame-
ter. Therefore, their application often leads to serious operational
problems; excessive backpressure results in a maldistribution of
fluids, and thus formation of stagnant zones and hot spots, which
in addition to a fairly long diffusion path, leads to lower yields,
poorer selectivity and reduced catalyst life [1]. Continuous-flow
monolithic polymeric microreactors show important advantages:
defined system of flow-through pores and facile modification of the
surface using ample arsenals of chemical tools [2,3]. But their ther-
mal sensitivity and propensity to swell are serious drawbacks [4].
Typical ceramic (e.g. cordierite) monoliths, which feature bunch
of channels of 1–5 mm sizes, connected with smaller ones of ca.
0.1–1.0 mm, are aimed at gas-phase reactions [5].
promising for the cost-effective production of fine chemicals [6].
This stems from a unique, bi-continuous structure of the monoliths
in which flow-through macropores are connected to an extensive
network of meso- and micro-pores present in the silica skeleton
[6–8]. In effect, pressure drops are fairly small, even at considerable
throughputs, whereas easily accessible large surface area offers
very large concentration of active sites per unit volume. Initially,
such microreactor was made of a single monolith–MonoSil, the
surface of which was functionalized with NH2 or HSO3 groups
[6]. Control of its meso- and macro-porosity was obtained by com-
bining phase separation method, elaborated by Nakanishi [9–11],
with pseudomorphic synthesis [12,13], to obtain an ordered meso-
porosity of the silica skeleton. But more importantly perhaps, the
productivity of MonoSil microreactors in Knovenagel reaction and
acid transesterification was shown to be 13 and 18 times larger
than in the corresponding batch processes.
The original Nakanishi method was later modified [14,15] to
give silica monoliths with notably larger macropores (30–50 ␮m)
and more isotropic structure. Their pore structure was not
destroyed after immobilization of ionic liquids and the catalysts
obtained from the crushed monoliths were very active, selective
and stable in the Baeyer–Villiger reaction and also in aerobic oxi-
dation of primary alcohols [16–18].
∗
Corresponding author at: Polish Academy of Sciences, Institute of Chemical Engi-
neering, Bałtycka 5, 44-100 Gliwice, Poland. Tel.: +48 32 2310811;
fax: +48 32 2310318.
The up to date methods of silica modification [19] offer
huge potentials to develop microreactors with the structure
E-mail address: jmrowiec@polsl.pl (J. Mrowiec-Białon´ ).
http://dx.doi.org/10.1016/j.apcata.2014.10.047
0926-860X/© 2014 Elsevier B.V. All rights reserved.

204
A. Koreniuk et al. / Applied Catalysis A: General 489 (2015) 203–208
purposely designed to meet specific catalytic and process engi-
neering demands. Most recently these monoliths were successfully
converted into a miniaturized multichannel enzymatic reactor
[20] which could operate at flow rates up to about 20 cm3/min
at backpressure not exceeding 2.5 bar. Worth noting, the sucrose
hydrolysis catalysed by invertase appeared to proceed in this
microreactor with maximum rate over 1000 times faster than in
the MCF-based slurry system, and the enzyme confined in meso-
pores showed notably larger affinity to substrate than the native
one, a clear signature of hyperactivity effect.
For this reason we deemed it important to test the potentials
of continuous-flow microreactors made of similar silica monoliths
with ultra large (20–50 ␮m) macropores, but activated with sul-
phonic acid groups, in the esterification of acetic acid and lactic acid
with n-butanol. While considered here as model reactions [21–25],
they are both of a major practical interest. The n-butyl acetate is
commonly used as a solvent in the manufacturing of lacquer, artifi-
cial perfume, photographic films, plastics and safety glass, and also
as synthetic fruit flavouring in foods [26]. Typically, the esterifica-
tion of lactic acid is applied to recover and purify the acid obtained
by bacterial fermentation of carbohydrates [27,28]. But as all previ-
ous experiments with the reactors of this type were carried out in
relatively benign conditions [7,20], we considered it also important
to test whether they can effectively operate at elevated tempera-
ture.
Fig. 1. Scheme of microreactor setup.
(Mettler Toledo STAR 850). The weight loss and thermal effects
during heating with ramp of 10 deg/min and air flow of 60 cm3/min
were recorded in the range of 25–800 ◦C. Additionally, drop pres-
sure was measured in the continuous reaction conditions using
pressure controller (UNIK 5000, Ex-Calibra).
2.3. Catalytic measurements
The microreactor was tested in esterification of acetic acid and
lactic acid with n-butanol (pure p.a, Chempur, Poland). The reactor
setup is shown schematically in Fig. 1. The substrates solution was
stored in ice bath to prevent any further reaction.
2. Experimental part
The experiments with acetic acid were performed for the molar
ratio of substrates 1:1, at 75 ◦C using flow rates of 0.03, 0.06
and 0.09 cm3/min. Esterification of lactic acid was carried out
at 120 ◦C and 140 ◦C (measured with accuracy of 0.1 ◦C) with
flow rates of 0.03–0.4 cm3/min. The molar ratio of lactic acid
to n-butanol was 1:12, 1:6 and 1:1. The reaction progress was
evaluated from the amount of acetic/lactic acid in the mixture.
Acid concentration at the inlet and outlet of the microreactor
was measured by titration method and additionally confirmed
by gas chromatography (Agilent 7890 A, FID detector, HP-5 col-
umn).
2.1. Synthesis of silica monoliths
Silica rods of 4.5 mm diameter were synthesized using Nakan-
ishi method [9–11] with the modifications described in [15,16]. In
brief, 0.91 g of polyethylene glycol (PEG 35000) was dissolved in
10.5 cm3 of 1 M HNO3, next 7.6 cm3 of TEOS was added slowly,
followed by addition 0.4 g of cetyltrimethylammonium bromide
(CTAB). The solution was mixed at room temperature for 1 h and
then polypropylene tubes (5.7 mm i.d.) were filled with the sol.
After gelation at 40 ◦C (12 h) they were aged at the same tempera-
ture for seven days. Then the samples were treated in 1 M ammonia
aqueous solution at 90 ◦C for 9 h and after washing with water the
monoliths were dried at 40 ◦C for three days and finally calcined at
550 ◦C for 5 h. During the processing shrinkage of about 20% of the
size of mold was observed.
Single rods (1, 2 and 4 cm long) were embedded into a heat-
shrinkable PTFE tubes (DSG-Canusa), equipped with connectors
to obtain microreactors suitable for a continuous-flow opera-
tion. Then these microreactors were functionalized under flow
(48 h, 60 ◦C) with arenesulphonic acid groups using solutions of 2-
(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS; 50 wt%
solution in CH2Cl2) dissolved in anhydrous ethanol (99.6%) to pre-
vent uncontrolled hydrolysis.
Productivity of the microreactor and residence time was calcu-
lated from Eq. (1) and (2) [29]:
VT
P = C0 × Conv ×
(1)
(2)
ꢀ
VT × m
ꢀ =
F
where C0 is initial concentration of substrate [mmol/cm3], Conv is
the conversion coefficient, VT is the total pore volume [cm3/g], ꢀ is
the residence time [min], m is the mass of monolith [g], F is the flow
rate [cm3/min]. The esterification of acetic acid was also carried
out in batch reactor using round bottom flask (75 cm3) equipped
with a heating jacket and condenser. The reaction was performed
under agitation at 1000 rpm using the same temperature and molar
ratio of substrates as in the continuous process. The functionalized
monolith was crushed and particles of ca. 50 ␮m were applied as
catalyst at concentration of 0.74 wt%.
2.2. Characterization of materials
Macropore structure of the monoliths thus obtained was inves-
tigated by mercury porosimetry (Quantachrome, PoreMaster 60),
and by scanning electron microscopy (SEM, TM 30000 Hitachi).
Low temperature nitrogen sorption (ASAP Micromeritics 2010)
was applied to evaluate specific surface area (SBET), mesopore
volume and mesopore size distribution using desorption branch
of isotherm and BJH method. Before analysis the samples were
degassed for 24 h at 200 ◦C. The incorporation of functional groups
was confirmed by FT-IR analysis (samples were prepared by KBr
technique). Thermal properties and an amount of incorporated
active groups were determined by thermogravimetric method
3. Results and discussion
Crack free silica rods displayed in Fig. 2 featured three types of
pores detected by mercury porosimetry, nitrogen adsorption and
scanning electron microscopy: (i) small mesopores with diame-
ters ca. 3 nm and larger ones with maximum at 20 nm, originating
from the presence of CTAB micelles, applied as soft pore tem-
plates, and a hydrothermal treatment of monoliths in ammonia
solution, and (ii) ultra large macropores (flow-through channels)
with diameters in the range of 20–50 ␮m (Figs. 3 and 4) obtained by

A. Koreniuk et al. / Applied Catalysis A: General 489 (2015) 203–208
205
Fig. 2. Silica monoliths and SEM image of their structure.
Table 1
Texture parameters of monoliths.
Description
SBET (m2/g)
Vmes (cm3/g)
Vmac (cm3/g)
Vtotal (cm3/g)
dmes (nm)
dmac (nm)
Monolith (M)
Functionalized monolith (MF)
Monolith after reaction (MFR)
328
245
275
1.15
0.91
0.91
2.45
2.45
2.44
3.5
3.25
3.25
3.5/20
20
20
20–50
20–50
20–50
PEG-induced phase separation. The bi-continuous structure of
voids (macropores) and silica struts is well seen in the SEM
image displayed in Fig. 2. Separated spherical voids of ca. 1 ␮m
could also be observed in the silica skeleton, yet they were
hardly detected by mercury porosimetry owing to their inacces-
sibility (Fig. 4). We believe that they originate from small PEG
droplets. A pristine monolith (M) exhibited surface area of about
328 cm2/g and mesopore volume of 1.15 cm3/g (Table 1). After
its functionalization with arenesulphonic groups (MF) smaller
mesopores disappeared and this can be explained by a prefer-
ential deposition of functional groups’ precursor in that space
[30]. Clearly, it resulted in a decrease of specific surface area
(ca. 17%). The monoliths thus obtained appeared to possess
larger meso- and macropores compared to those reported before
[6–8].
800
A
M
MF
MFR
600
The presence of arenesulphonic groups on the monoliths’ sur-
face was investigated by FT-IR spectroscopy to obtain the spectra
shown in Fig. 5. In all materials the asymmetric and symmetric
400
200
0
stretching vibrations of the Si
O Si framework occur in the range
of 1000–1250 cm−1 and also 800 and 470 cm−1. Weak absorption
peak at around 960 cm−1 is attributed to stretching vibration of
Si OH group. Broad band in the range of 3000–3700 cm−1 corre-
sponds to asymmetric stretching of hydrogen bonded OH groups.
In the arenesulphonic-functionalized samples typical absorption
bands from aromatic ring are observed at 700 cm−1, 1403 cm−1
0.0
0.2
0.4
0.6
0.8
1.0
Relative preasure p/po
0.12
B
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
M
MF
0.10
0.08
0.06
0.04
0.02
0.00
MFR
1
10
100
0.01
0.1
1
10
100
1000
Pore diameter [nm]
Pore diameter [µm]
Fig. 3. Nitrogen adsorption/desorption isotherms (A) and pore size distributions (B)
Fig. 4. Cumulative pore size distribution in a pristine monolith obtained by mercury
of pristine monolith (M), after its activation (MF) and after 3-day operation (MFR).
porosimetry.

206
A. Koreniuk et al. / Applied Catalysis A: General 489 (2015) 203–208
0.8
0.6
0.4
0.2
0.0
M
MF
MFR
0
3
6
9
12
15
18
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber [cm-1]
Flow rate [cm3/min]
Fig. 7. Drop pressure vs. flow rate for 4 cm microreactor.
Fig. 5. FT-IR spectra of M, MF and MFR samples (the spectra were shifted for clarity).
60
50
40
30
20
10
0
and 1498 cm−1. Absorption in the range of 2850–3000 cm−1 and
the peak at 1457 cm−1 can be assigned to asymmetric stretching
and bending vibration of C H in methylene group connecting the
aromatic ring. The S O stretching vibrations of sulphonic groups
normally occur in the range of 1000–1200 cm−1. In the case of
silica-based materials those vibrations overlap with Si
O Si bands.
The amount of functional groups attached to the monoliths’ sur-
face and also their thermal stability were determined by thermal
analysis. TG curves of the samples activated with different amounts
of acidic groups are shown in Fig. 6A. Two steps of weight loss are
clearly visible on the TG curve, especially from the sample with the
100
A
0.15 mmol/g
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Concentration of acidic groups [mmol/g]
95
0.25 mmol/g
Fig. 8. Conversion of acetic acid vs. concentration of acidic groups for different flow
rates: (ꢀ) 0.03 cm3/min; (᭹) 0.06 cm3/min and (ꢁ) 0.09 cm3/min.
90
85
largest content of functional groups. The sharp weight loss below
120 ◦C is assigned to the removal of physisorbed water, whereas the
loss between 350 and 600 ◦C, corresponds to the decomposition of
organic groups. Desorption of water and decomposition of organics
appeared to be portrayed by two thermal effects; the endothermic
peak with maximum at 100 ◦C and the exothermic (550 ◦C), are well
seen on SDTA curve (Fig. 6B). The amount of incorporated groups
was calculated from mass loss in the range of 350–600 ◦C and the
values obtained were 0.15, 0.25 and 0.65 mmol/g. The largest value
is consistent with the reported dependence between the maximum
number of organic group, which could effectively be bound, and
the concentration of hydroxyl groups present on the silica surface
[31,32].
For the continuous-flow microreactors the relationship
between pressure drop and the flow rate through the monoliths
is of practical significance. As can be inferred from Fig. 7, which
shows the effect of liquid flow rate on pressure drop in the 4-cm
reactor, the pressure drop in this microreactor was significantly
smaller than that recorded before [7], due to the presence of much
larger flow-through channels. This eliminates the need for the use
of high-pressure metering pumps, typically applied in columns
packed with fine beads, with a positive effect on process economy.
As indicated, catalytic properties of the microreactor were
investigatedin the esterificationof acetic acid and lactic acid with n-
butanol. First, the relationship between the amount of acidic groups
and conversion was checked in esterification of acetic acid, at
80
75
0.65 mmol/g
0
100 200 300 400 500 600 700 800
Temperature [oC]
0.000
-0.005
-0.010
-0.015
-0.020
-0.025
4
B
TG
2
SDTA
0
-2
-4
0
100 200 300 400 500 600 700 800
Temperature [oC]
Fig. 6. TG curves of samples with different amount of incorporated acidic groups
(A) and DTG and SDTA curves recorded for MF with 0.65 mmol/g of active groups
(B).

A. Koreniuk et al. / Applied Catalysis A: General 489 (2015) 203–208
207
60
30
25
20
15
10
5
50
40
30
0
100 200 300 400 500 600 700 800
Time [min]
Fig. 11. Catalytic stability of microreactor in esterification of acetic acid with n-
butanol (reaction conditions: 75 ◦C, molar ratio 1:1, flow rate 0.03 cm3/min).
0
1 cm
2 cm
4 cm
1.4
100
Fig. 9. Conversion of acetic acid in microreactors of different length for the residence
time equal 155 s.
1.2
80
1.0
60
60
40
20
0
0.8
0.6
0.4
0.2
0.0
A
50
40
30
20
10
0
0.0
0.1
0.2
0.3
0.4
0.5
Flow rate [cm3/min]
Fig. 12. Conversion of lactic acid and productivity of ester vs. flow rate in reactions
carried out at (ꢀ) 140 ◦C and (᭹) 120 ◦C.
0.02
0.04
0.06
0.08
0.10
Fig. 10 shows the relationship of ester productivity and con-
version of acetic acid against the flow rate for microreactors of
different length (1–4 cm). As can be seen, the highest productivity
(1.78 mmol/g min) was achieved in the shortest one (1 cm) and for
the largest value of flow rate (0.09 cm3/min), and this is in accord
with expectations. An opposite relationship was observed for the
case of conversion; the highest value, of ca. 53%, was received for
4 cm reactor and the lowest value of flow rate.
The performance of the monolithic and batch reactors was com-
pared under identical conditions: the catalysts of 0.1444 g load, had
either the form of monolith or powder, reaction time was 6 h and
the volume of substrates was 21.6 cm3. The conversion of acid was
found to be ca. 42.5% and the productivity of ester reached the value
of 1.2 mmol/g min in the both cases. This confirms that diffusion
had no effect on the rate of reaction, unlike observed before for
Diels–Alder reaction study, in which productivity in batch reac-
tor was only half of that determined for the flow system [29]. But
not less importantly, good catalytic stability during 800 min-long
process was observed (Fig. 11).
Flow rate [cm3/min]
2.0
B
1.6
1.2
0.8
0.4
0.0
0.02
0.04
0.06
0.08
0.10
Flow rate [cm3/min]
To check the possibility of application these microreactors under
more severe conditions, the esterification of lactic acid with n-
butanol was performed. The results shown in Figs. 12 and 13
demonstrate almost full (99%) conversion of the acid when the
process was carried out at 140 ◦C, the molar ratio of lactic acid to
n-butanol equalled to 1:12 and the flow rate was in the range of
0.03–0.15 cm3/min. This conversion is similar to that reported for
batch process carried out over TiO2–Al2O3 catalyst under the same
conditions [24]. However, the productivity obtained in the mono-
lithic microreactor under study (for flow rate of 0.15 cm3/min)
reached the value of 0.874 mmol/g min, i.e. four times more than
reported. The increase in the flow rate to 0.3 cm3/min decreased
the conversion to 75%, but productivity increased by ca. 50%, to
Fig. 10. Conversion of acetic acid (A) and productivity of ester (B) vs. flow rate in
microreactors 1 cm (᭹), 2 cm (ꢁ) and 4 cm (ꢀ) long.
different flow rates, to find a linear dependence (Fig. 8). Therefore,
the bulk of experiments were performed for the monoliths with the
highest concentration of acid groups, i.e. 0.65 mmol/g.
In order to check and eliminate the effect of external mass
transfer on reaction kinetics, the reactors of different lengths were
compared for the same residence time (155 s) to obtain similar val-
ues of conversion (ca. 25%, cf. Fig. 9); a clear proof of the lack of
those limitations.

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A. Koreniuk et al. / Applied Catalysis A: General 489 (2015) 203–208
nanometric (Fig. 3) and micrometric scale (Fig. 15) and no change
in the acidic groups content was detected by IR and thermogravi-
metric analysis after that period (data not shown).
100
80
60
40
20
0
4. Conclusions
A facile procedure to prepare an effective continuous-flow
monolithic silica microreactor functionalized with arenesulphonic
acid groups was proposed. Due to the presence of flow-through
pores of 20–50 ␮m sizes these reactors can operate at low pressure
drops which eliminate the need for the use of high pressure pumps.
The relatively large specific surface area, ca. 250 m2/g, exhibited
by mesopores of ca. 20 nm present in silica skeleton allowed to
attaching acidic groups in amount of 0.65 mmol/g. The microrector
demonstrated very high activity and productivity and also good
stability of catalytic and structural properties in the continuous
esterification of acetic and lactic acid with n-butanol, even carried
out at the temperature as high as 140 ◦C.
0
60
120
180
240
300
360
Time [min]
Fig. 13. Conversion of lactic acid vs. time at different reaction conditions: (᭹) 140 ◦C,
1:12; (ꢀ) 120 ◦C, 1:12; (ꢂ) 120 ◦C, 1:6; (ꢁ) 140 ◦C, 1:1.
Acknowledgements
100
The support of the National Science Center for this project under
the project UMO-2011/01/B/ST8/03855 is gratefully acknowl-
edged.
90
80
70
60
turn off/on
turn off/on
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Fig. 15. SEM image of MFR monolith after esterification of lactic acid (reaction
conditions: 140 ◦C, molar ratio 1:12, flow rate 0.15 cm3/min).
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