Scale up study of capillary microreactors in solvent-free semihydrogenation of 2‐methyl‐3‐butyn‐2‐ol

Article abstract of DOI:10.1016/j.cattod.2016.03.028

A 2.5 wt.% Pd/ZnO catalytic coating has been deposited onto the inner wall of capillary reactors with a diameter of 0.53 and 1.6 mm. The coatings were characterised by XRD, SEM, TEM and elemental analysis. The performance of catalytic reactors was studied in solvent-free hydrogenation of 2-methyl-3-butyn-2-ol. No mass transfer limitations was observed in the reactor with a diameter of 0.53 mm up to a catalyst loading of 1.0 kg_(Pd) m^?3. The activity and selectivity of the catalysts has been studied in a batch reactor to develop a kinetic model. The kinetic model was combined with the reactor model to describe the obtained data in a wide range of reaction conditions. The model was applied to calculate the range of reaction conditions to reach a production rate of liquid product of 10–50 kg a day in a single catalytic capillary reactor.

Full text of DOI:10.1016/j.cattod.2016.03.028

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Scale up study of capillary microreactors in solvent-free
semihydrogenation of 2-methyl-3-butyn-2-ol
Nikolay Cherkasova, Ma ’moun Al-Rawashdehb, Alex O. Ibhadonc, Evgeny V. Rebrova,d,∗
a School of Engineering, University of Warwick, Library Road Coventry, CV4 7AL, United Kingdom
b TreeScaling BV, Helix Building, P.O. Box 513, 5600 MB Eindhoven, Netherlands
c Catalysis and Reactor Engineering Research Group, Department of Chemistry and School of Biological, Biomedical and Environmental Sciences, University
of Hull, Cottingham Road, Hull HU6 7RX, United Kingdom
d Department of Biotechnology and Chemistry, Tver State Technical University, Tver, 170026, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
A 2.5 wt.% Pd/ZnO catalytic coating has been deposited onto the inner wall of capillary reactors with a
diameter of 0.53 and 1.6 mm. The coatings were characterised by XRD, SEM, TEM and elemental analysis.
The performance of catalytic reactors was studied in solvent-free hydrogenation of 2-methyl-3-butyn-2-
ol. No mass transfer limitations was observed in the reactor with a diameter of 0.53 mm up to a catalyst
loading of 1.0 kg(Pd) m−3. The activity and selectivity of the catalysts has been studied in a batch reactor
to develop a kinetic model. The kinetic model was combined with the reactor model to describe the
obtained data in a wide range of reaction conditions. The model was applied to calculate the range of
reaction conditions to reach a production rate of liquid product of 10–50 kg a day in a single catalytic
capillary reactor.
Received 13 December 2015
Received in revised form 18 March 2016
Accepted 20 March 2016
Available online xxx
Keywords:
Semihydrogenation
Catalytic coatings
Microreactor
Palladium
© 2016 Elsevier B.V. All rights reserved.
Alkynol
Acetylene
1. Introduction
results in substantial process intensification [6–8]. An important
societal advantage of safer microreactor processes is the synthesis
Process safety and small environmental footprint are one of
the main challenges of chemical industry, and microreactors seem
to be one of the most promising technologies. Small dimensions
of microreactors provide large surface to volume ratio, orders of
magnitude higher than that of conventional reactors, and result in
high heat and mass transfer rates [1–4]. Quick mass transfer opens
new ways for improved product selectivity by avoiding mass and
heat transfer limitations which eliminate hot spots that generally
lead to over-reaction, catalyst deactivation and rise safety con-
cerns. Another important benefit brought about by microreactors
lies with the ability to use high reaction temperature and pressure
without compromising on the process safety. Even in the case of
catastrophic reactor malfunction only very small amount of harm-
ful substances will be released due to the low liquid hold up [5].
Because of intrinsic safety and excellent reaction control, microre-
actors can be utilised under reaction conditions that are prohibited
for large-scale reactors. This opens new operational regimes which
on demand in smaller scale at the place where products are needed,
the approach which decreases concentration of industrial units and
reduces transportation costs [7].
Microreactor technology is successfully used for a number of
processes both in academia and in industry [9–12]. The advantages
of microreactors have been clearly demonstrated for non-catalytic
gas-liquid and liquid–liquid reactions that require precise con-
trol over reaction conditions, or involve dangerous chemicals
such as corrosive fluorine or explosive diazo-compounds [13–16].
However, considering that the vast majority of modern chemical
processes utilise heterogeneous catalysts to replace stoichiometric
reactions due to obvious economic and environmental advantages,
the application of catalytic coatings in microreactors is dispropor-
tionately scarcely studied [17]. A few gas-phase heterogeneously
catalysed reactions have been studied in various reactions such as
Fisher-Tropsch synthesis, CO2 hydrogenation, water gas shift [18],
preferential CO oxidation [19], and complete oxidation of organic
compounds [20] demonstrating a 2-5 fold increase in reaction rates
compared to conventional reactors [21–24].
However, the class of heterogeneously-catalysed multiphase
(liquid-liquid or gas-liquid) reactions received limited attention
because of the difficulty to precisely control catalyst-substrate
∗
Corresponding author at: School of Engineering, University of Warwick, Library
Road Coventry, CV4 7AL, United Kingdom.
E-mail address: e.rebrov@warwick.ac.uk (E.V. Rebrov).
http://dx.doi.org/10.1016/j.cattod.2016.03.028
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: N. Cherkasov, et al., Scale up study of capillary microreactors in solvent-free semihydrogenation of
2-methyl-3-butyn-2-ol, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.028

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interactions. The reactions can be performed either using a (i)
catalyst particles in a micro fixed bed configuration [2,25], (ii)
magnetically-supported catalysts held at the reactor walls with
magnetic field [26–29], (iii) introducing a slurry of solid catalyst
particles into the reactant flow [30], (iv) catalytic wall coatings
obtained via hydrothermal synthesis [31,32] or methods derived
from dipcoating, spincoating or washcoating [33–39]. Micro fixed
bed reactors with a typical pellet size below 100 ␮m seem to offer
a simple way of catalyst introduction and they are suitable for any
catalyst. However, catalytic beds have a number of problems such
as bed densification with time resulting in considerable pressure
build-up which requires catalyst dilution with a hard inert mate-
rial. Also, the removal of heat from the reaction zone to the external
walls could create considerable radial gradients in the reactor in a
highly exothermic reaction. Finally, liquid channelling may result in
very wide residence time distribution and therefore poor product
selectivity in consecutive reactions [40–42]. However, for a limited
class of reactions that have no side products, packed-bed reac-
tors can successfully be applied [5,40]. Magnetically-recoverable
catalysts offer a convenient way of catalyst introduction and sep-
aration, but their benefits are counterbalanced by more complex
reactor design required [43–45]. A slurry of solid catalyst particles
can be introduced into a liquid stream and provides good selectiv-
ity towards intermediate products which is comparable to that of
an ideal stirred-tank reactor [30]. However, this approach requires
an additional expensive step to separate catalysts from the reaction
products. Also the reactant to catalyst ratio is much lower compared
to other reactor types in order to provide good catalyst distribution
and high conversion. The wall coated capillary reactors combine
simplicity of operation and high throughput with excellent reaction
control which lead to high selectivity towards intermediate prod-
ucts [33,34,46]. Their synthesis methods have been considerably
improved over the last decade and very stable catalytic coatings
were obtained for a range of industrial applications following a
recently developed synthesis method [33].
There exists a limited number of publications describing the
upscaling of catalytic capillary microreactors [47,48]. While the
numbering up approach looks very promising to increase the
throughput, it might require 102–105 parallel channels to produce
1–100 ton of product per year. The exact number of microchannels
depends on the maximum allowed pressure drop over the reactor
and the channel length which is often limited by available microfab-
rication methods [49–52]. Therefore the numbering up approach
needs to be combined with scaling up of the reactor dimensions to
bring the throughput to the industrial scale while avoiding mass or
heat transfer limitations.
The aim of the work was to assess feasibility of industrially-
relevant semihydrogenation in wall-coated capillary microreactors
and to study the feasibility of reactor upscaling to reach the desired
production capacity. In this study, selective hydrogenation (semi-
hydrogenation) of 2-methyl-3-butyn-2-ol (MBY) over a Pd/ZnO
catalyst was chosen as a model reaction representing a wide class of
important gas-liquid reactions catalysed by a supported heteroge-
neous catalyst. The replacement of traditional batch reactors with
slurry catalysts by catalytic capillary microreactors is expected to
provide (i) substantial safety benefits due to minimisation of explo-
sive hydrogen utilisation and (ii) decreased labour costs due to
quick process optimisation.
length of 1 m, and (ii) glass tubes millireactors with an internal
diameter of 1.6 mm, and a length of 0.2 m. The reactors are further
referred to as r0.53-xx, where 0.53 is the internal diameter in mm
and xx is the total Pd loading in kg m−3(reactor). A 2.5 wt.% Pd/ZnO
catalytic coating was deposited onto the inner reactor walls fol-
lowing the previously reported method [33]. The sol containing
catalyst precursors was introduced into the capillary reactors and
the solvent was evaporated by moving the capillary into the oven at
200 ◦C at the displacement speed of 0.3 mm s−1. The coating thick-
ness was varied from 2 to 65 ␮m adjusting the number of coatings
cycles and the concentration of precursors in the initial sol. The sur-
factants were removed by heating the coated reactor in a vacuum
oven (1 mbar residual pressure) at 200 ◦C overnight. After cooling
to room temperature, the reactors were consecutively flushed with
water and acetone (1 mL min−1, 10 min each) followed by drying in
air.
Several coated reactors were cut and glued on a microscope
stubs with epoxy. Then several (typically 6-9) cross-sections from
every reactor were studied on a TM-1000 Hitachi scanning electron
microscope (SEM) in a high-pressure mode to minimise charg-
ing without deposition of an additional conductive coating. The
transmission electron microscopy (TEM) images of the coatings
were obtained using a Jeol 2010 transmission electron microscope
equipped with an energy-dispersive X-ray spectrometer (EDX,
Oxford Instruments). The study was performed from 5-8 differ-
ent regions to obtain representative data. In this study, the coating
from the reactors was removed, dispersed in ethanol under son-
ication and a few droplets of the dispersion were dropped on a
carbon-coated copper grid.
Powder X-ray diffraction (XRD) measurements were performed
using an Empyrean X-ray diffractometer equipped with monochro-
matic K␣-Cu X-ray source and a PIXcel linear detector. The scanning
was performed in a stepwise mode within a 2␪ range of 20–85◦ with
a step length of 0.0390◦ 2␪, and a step time of 60 min studying the
coating removed from the capillaries on a zero background sample
holder.
After the reaction, the total catalyst loading in the capillaries was
determined dissolving the coating by withdrawing 1 mL of aqua
regia (1:3 volume mixture of concentrated HCl and HNO3, Sigma-
Aldrich) at 0.4 mL min−1 through the capillaries. The resulting
solution was collected in a 5 mL volumetric flask and diluted with
deionised water. The solutions were studied using a PerkinElmer
Optima 5300DV emission inductively coupled plasma spectrome-
ter.
Turn-over frequency has been calculated as a ratio of the ini-
tial reaction rate (in mol gcatalyst−1 s−1) and the number of surface
Pd sites (in mol gcatalyst−1). The latter was calculated via dispersion
using the empirical correlation D = 0.9/dPd(nm) with the average Pd
nanoparticle diameter determined by TEM [53].
2.2. Capillary reactor testing
The experimental setup consists of a continuous flow syringe
pump fed with MBY (Sigma-Aldrich, >98 wt.%) without solvent,
mass flow controllers and a pressure transducer connected via an
X-joint to the capillary reactor (Fig. 1). The reactor was placed into
a thermostatic water bath at the temperature of 70.0 0.4 ◦C. The
reactor outlet was connected to an automatic sample collector via
an additional silica capillary (i.d. 250 ␮m) that provided a pres-
sure drop of 0.20 0.03 bar. The liquid samples were diluted 100:1
with hexane and analysed using a Varian 430 gas chromatograph
equipped with a 30 m Stabiwax capillary column and a flame ioni-
sation detector.
2. Experimental
2.1. Reactor preparation and characterisation
Two groups of capillary reactors were used: (i) fused silica cap-
illary microreactors with an internal diameter of 0.53 mm and a
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2.4. Batch reactor testing
3
Hydrogenation in a batch reactor at atmospheric pressure was
performed in a 10 mL two-neck round bottom flask equipped with a
water-cooled condenser, and a septum for gas introduction and liq-
uid withdrawal. The 2 wt.% Pd/ZnO reference catalyst (30.0 mg) was
added to 4.0 mL of MBY without any additional solvent. The reac-
tion mixture was stirred using a magnetic stirrer. The stirring rates
were in the range of 600–1200 rpm. The reaction temperature was
controlled with a water bath at 70.0 0.1 ◦C. The gases were intro-
duced to the bottom of the reactor via a 250 ␮m id silica capillary
and the flow rates were controlled by mass-flow controllers. The
reactor was flushed with a flow of 20 mL min−1 of nitrogen (99.999
vol.%, BOC) for 10 min to remove air, then hydrogen was quickly
introduced at a flow of 50 mL min−1 for 1 min to displace nitro-
gen. Afterwards, the reaction was performed feeding a hydrogen
flow of 15 mL min−1 through the reaction mixture and withdraw-
ing 20 ␮L of the reaction mixture regularly for off-line analysis. The
absence of mass transfer limitations was confirmed by studying the
reaction rate at various stirring speeds and catalyst amounts. The
liquid samples were diluted 100:1 with heptane and analysed using
a Shimadzu 2010 gas chromatograph equipped with a Stabiwax
capillary column and a flame ionisation detector.
Fig. 1. Scheme of the capillary reactor setup.
2.3. Reference catalyst synthesis
A reference Pd/ZnO powder catalyst was used for kinetic study
in a batch reactor. The reference catalyst was synthesised by a
conventional polyol method [54]. A powder of ZnO, 2 g, particle
sizes <20 ␮m (Alfa Aesar), was added into a solution of palla-
dium acetate (>99 wt.%, Sigma-Aldrich) in 50 mL ethylene glycol
(>99 wt.%, Sigma-Aldrich). The amount of palladium precursor was
adjusted to obtain a 2 wt.% Pd/ZnO catalyst because this Pd content
provided Pd nanoparticles of the same dimensions as the 2.5 wt.%
Pd/ZnO obtained in capillary reactor coatings. The dissolved air was
displaced with a nitrogen flow of 100 mL min−1 followed by reflux
heating for 2 h. Then, the solution was centrifuged, washed with
water (2 × 30 mL), acetone (2 × 30 mL) and dried in nitrogen. The
catalyst was characterised by N2 adsorption and TEM. The complete
reduction of Pd was confirmed by elemental analysis data.
3. Results and discussion
3.1. Characterisation of the capillary reactors
Fig. 2 shows representative SEM images of catalytic coatings
in the r0.53 reactors. No continuous ZnO layer was obtained
at the catalyst loading corresponding to the Pd content below
0.7 kg(Pd) mr−3. These coatings consisted of separate particles of
Pd/ZnO deposited onto the reactor walls (Fig. 2a, b). Statistical anal-
ysis of the coating in 5 capillary reactors with the Pd loading below
0.7 kg(Pd) mr−3 shows that the dimensions of catalytic particles are
independent of total Pd content and are about 2.3 2.0 ␮m (aver-
Fig. 2. Cross sectional SEM photographs of the 0.53 mm i.d. capillary reactors wall-coated with Pd/ZnO, total Pd content is (a) 0.24, (b) 0.64, (c, d) 2.35 kg (Pd) m−3(reactor).
Please cite this article in press as: N. Cherkasov, et al., Scale up study of capillary microreactors in solvent-free semihydrogenation of
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Fig. 3. Cross sectional SEM photographs of the 1.6 mm i.d. capillary reactors wall-coated with Pd/ZnO, total Pd content is (a) 0.17, (b) 1.03, (c) 1.23, (d) 1.76 kg (Pd) m−3(reactor).
age value standard deviation). Likely, the formation of individual
particles was caused by capillary forces of the evaporating solvent.
At a higher catalyst content, however, the capillary was coated with
a rather uniform catalytic layer with a thickness of 11.9 6.0 ␮m.
The coating consisted of ZnO crystallite platelets which formed cav-
ities with a characteristic size in the micrometer range (Fig. 2c,
d).
SEM study of various r1.6 reactors showed that the coating was
not uniform at the catalyst loading below 0.7 kg(Pd) mr−3. The
coating consisted of individual ZnO particles with a mean size of
6.7 5.2 ␮m. At a higher catalyst content, a uniform coating was
obtained onto the reactor walls with the thickness increasing from
36 22 to 65 30 ␮m for the r1.6–1.0 and r1.6–1.8 reactors, respec-
tively. A Pd loading in all the capillary reactors of 2.53 0.37 wt.%
was determined by EDX analysis. This value is in a good agreement
with the nominal Pd loading of 2.5 wt.%. This demonstrates that no
Pd was lost during the coating and washing process (Fig. 3).
The powder XRD pattern of the Pd/ZnO catalyst (Fig. 4) showed
that wurtzite was the only ZnO phase in the coatings. Its crystallite
size was estimated using Scherrer’s formula to be about 30 nm. This
is a typical size obtained in the developed sol–gel method after a
calcination step at 200 ◦C as low calcination temperature prevents
sintering of ZnO nanoparticles. A small Pd peak is present at 40◦ 2␪.
Fig. 5 shows a characteristic TEM image of a coating detached
from the r1.6–1.0 reactor. It can been seen in Fig. 5 that the Pd
particles with a mean size of 3.0 1.0 nm are evenly distributed
through the ZnO framework. These particles are formed by several
smaller nanoparticles about 1.3 nm in diameter. The morphology
and dimensions of Pd nanoparticles were the same for r0.53 and
r1.6 capillary reactors, i.e. change in the total Pd content in the
reactors was caused only by the increase in the Pd/ZnO coating
thickness.
Fig. 4. Powder X-ray diffraction pattern of the Pd/ZnO catalytic coating extracted
Fig. 5. A representative TEM microphotograph of the Pd/ZnO coating removed from
from a millireactor.
the r1.6–1.0 reactor.
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5
Fig. 8. Scheme of 2-methyl-3-butyn-2-ol hydrogenation reactions.
tant flow rate. The zero-order reaction kinetics is described by Eq.
(1):
CMBY
=
CM0 BY (1 − k0ꢀ ꢀres
)
(1),
V
where ␶
is the liquid residence time (ꢀres
=
cat εL), Vcat is the
res
F
catalyst volume, εL is the liquid hold-up in the rVeactor, and Fv is
the volumetric liquid flow rate, and k0ꢀ is the pseudo zero-order
reaction rate constant. Eq. (1) can be rearranged in terms of MBY
conversion (XMBY ):
Fig. 6. MBY hydrogenation over a Pd/ZnO catalytic coating in capillary reactors (᭿,ꢀ)
0.53 mm i.d. (r0.53) and (ꢁ) 1.6 mm i.d. (r1.6). Liquid flow rate: (᭿,ꢁ) 10 ␮L min−1
or (ꢀ) 40 ␮L min−1, reaction temperature: 70C, H2 flow rate: 10 mL min−1 (STP),
absolute reaction pressure 1.2 bar.
XMBY
=
k0ꢀ ꢀres
(2)
Fig. 7 shows the change in reactant concentration as a function of
3.2. Semihydrogenation in capillary reactors
residence time. It can be seen that no mass transfer limitations were
−3
observed for the r0.53 up to the catalytic loading of 1 kg(Pd) mr
.
Two series of capillary reactors, r0.53 and r1.6, were studied in
solvent-free MBY hydrogenation. The conditions were selected to
ensure that the conversion of MBY was below 80%, which allowed
for determination of apparent reaction rates directly from MBY con-
version as the reaction rate follows a pseudo-zero order kinetics
[55,56]. The alkene selectivity above 97.5% was observed for all the
reactors studied except for the r1.6–1.8 reactor. These data show
that there were no internal diffusion limitations of MBY or liquid
channelling that usually substantially decrease selectivity.
It can be seen in Fig. 6, the MBY conversion in r0.53 reactors
with different Pd loadings was linearly proportional to the cat-
alyst loading up to a Pd content of 0.75 kg m−3 at a liquid flow
rate of 10 ␮L min−1. This confirms the absence of mass transfer
limitations. At a higher Pd loading of 2.4 kg m−3, the full MBY con-
version was observed therefore the flow rates was increased to
40 ␮L min−1 to get kinetic data. At the higher flow rate, the MBY
conversion increased linearly with catalyst loading up to a Pd con-
tent of 0.7 kg m−3. At a higher Pd loading of 2.4 kg m−3 the reaction
rate was affected by the onset of mass-transfer limitations. For the
r1.6 reactors, similarly, the MBY conversion was proportional to the
Pd loading in the whole range of Pd loadings studied.
For the r1.6 reactors, the observed rates were lower than that in the
r0.53 reactors suggesting the presence of mass transfer limitations.
3.3. Semihydrogenation in a batch reactor
The MBY semihydrogenation was performed over a 2 wt.%
Pd/ZnO catalyst in a batch reactor. The reaction rate was inde-
pendent on the stirring speed above 900 rpm confirming the
absence of mass transfer limitations. The catalyst was non-porous
(Vpore < 50 ␮L g−1) with a specific surface area of 4.1 m2 g−1. The
Pd nanoparticles were 3.2 1.1 nm according to TEM data (not
shown). The Pd dispersion in the model catalyst was the same as
that in the catalytic coatings which allowed direct data comparison
between the model catalyst and catalytic coatings.
The reaction kinetics was modelled to explain the difference in
the performance between r0.53 and r 1.6 reactors. The MBY hydro-
genation reaction scheme (Fig. 8) consists of three main reactions:
(i) MBY hydrogenation to MBE followed by (ii) MBE hydrogenation
to MBA and (iii) a direct hydrogenation step of MBY to MBA.
The concentration profiles of the reactant and products in the
MBY hydrogenation over the 2 wt.% Pd/ZnO catalyst in the batch
reactor are shown in Fig. 9. The concentration profiles were mod-
elled using the Langmuir-Hinshelwood kinetics with competitive
adsorption of organic species and dissociated hydrogen on the cat-
alyst surface with the rate equation 3-5 [55,56]. Considering vapour
The reactant conversion in a zero-order catalytic reaction is pro-
portional to the amount of catalyst in the reactor, the fraction of the
total reactor volume occupied by the liquid (which is determined
by the liquid hold up) and inversely proportional to the liquid reac-
Fig. 7. MBY hydrogenation as a function of normalised catalyst mass over a Pd/ZnO
catalytic coating in capillary reactors (᭿,ꢀ) 0.53 mm i.d. (r0.53) and (ꢁ) 1.6 mm i.d.
(r1.6). Liquid flow rate: (᭿,ꢁ) 10 ␮L min−1 or (ꢀ) 40 ␮L min−1, reaction temperature:
70C, H2 flow rate: 10 mL min−1 (STP), absolute reaction pressure 1.2 bar.
Fig. 9. Concentration profiles of solvent-free MBY hydrogenation in a batch reactor
on the 2.% Pd/ZnO reference catalyst at 70C and ambient H2 pressure. Curves present
results of kinetic modelling.
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Table 1
Kinetic parameters for MBY hydrogenation obtained by regression analysis of experimental data.
Parameter
Unit
value
68% c.i.
95% c. i.
∗
k1∗
mol L−1 s−1 g−1Pd bar−1
2.71 × 101
4.37 × 10−1
1.73 × 10−1
9.38 × 10−2
1.71 × 10−2
(2.68–2.79) × 101
(2.59–2.90) × 101
(1.88–9.82 × 10−1
(0.00–4.02) × 10−1
(6.25–14.3) × 10−2
(1.08–2.71) × 10−2
k2∗
mol L−1 s−1 g−1Pd bar−1
(3.74 − 6.33) × 10−1
(0.23 − 2.62) × 10−1
(8.68 − 11.2) × 10−2
(1.56 − 2.10) × 10−2
k3
mol L−1 s−1 g−1Pd bar−2
–
–
Q1
Q2
c.i. = confidence interval.
pressure of MBY of 0.27 bar at the reaction temperature, hydrogen
pressure (PH2) in the system was taken as 0.73 bar.
k1∗CMBY pH2
r1
r2
r3
=
=
=
(3)
(4)
(5)
2
2
(Q1CMBE + CMBY + Q2CMBA
)
)
)
k2∗CMBEpH2
(Q1CMBE + CMBY + Q2CMBA
k3∗CMBY p2H2
2
(Q1CMBE + CMBY + Q2CMBA
where CMBY , CMBE and CMBA are the concentrations of the organic
species, k∗1-k∗ are the apparent rate constants of the correspond-
ing reaction steps, Q1 = KMBE/KMBY , Q2 = KMBA/KMBY are the ratios of
adsorption constants.
3
Fig. 10. Parity plot for MBY conversion in the microreactors for capillary 0.53 mm
i.d. microreactors (r0.53) at liquid flow rates of (᭿) 10 ␮L min−1 and (ꢀ) 40 ␮L min−1
,
and in 1.6 mm i.d. millireactors (r1.6) at liquid flow rate of (ꢁ) 10 ␮L min−1. Dashed
line corresponds to perfect agreement. The numbers in the parentheses refer to the
Pd loading in kg m−3 in the corresponding reactors.
The rate equations (Eqs. (3–5)) were solved numerically in the
Matlab software using a Runge-Kutta method optimised for stiff
systems of ordinary differential equations (ode23tb solver). The
regression analysis was performed using a non-linear weighted
least squares routine using Levenberg-Marquardt with the statisti-
cal weights reciprocal to the experimental uncertainties. The error
analysis of the kinetic parameters was performed using a Monte-
Carlo method analysing results on 2000 data sets with the initial
data normally distributed around the experimental results [57].
The obtained constants are listed in Table 1. The values of k2* and
The Weisz-Prater criterion was calculated to estimate internal mass
transfer limitations of hydrogen and organic species (Eq. (6)):
ꢁ
hc2
Cs Deff
NW−P
=
(6)
*
*
where ꢁ is the apparent reaction rate, hc is the coating thickness,
Cs is the concentration of the diffusing species with the effective
diffusivity of Deff . Diffusivities of dissolved hydrogen and organic
species of 7.0 × 10−9 m2 s−1 and 2.0 × 10−9 m2 s−1 were estimated
according to Vannice [63]. A value of NW-P = 0.5 for hydrogen in the
coating with a thickness of 6 ␮m, indicates the presence of internal
diffusion limitations. For organic species, however, diffusion limita-
tions were highly unlikely because their concentrations, 4 orders of
magnitude higher than that of hydrogen, resulted in NW-P numbers
below 10−3. Still, at the high MBY conversion and for thick catalytic
coating observed in the r1.6k1.8 reactor, NW-P reached values ∼1
suggesting pore limitations of MBE. Indeed, low MBE selectivity of
91% for the r1.6–1.8 reactor indicates that pore diffusion limitations
were observed for both organic and hydrogen species. The observed
linearity with Pd content in Fig. 7 is likely caused by comparable
intrinsic reaction and mass transfer rates under the studied condi-
tions. Furthermore, pore diffusion limitations cannot be removed
at a higher hydrogen pressure because the increase in the concen-
tration of dissolved hydrogen will be compensated by the increased
reaction rates resulting in the same Weisz-Prater numbers.
The pore limitations agree with the modelling performed by
Warnier, who showed that pore diffusion limitations are observed
faster than external transfer in a gas-liquid slug flow [64]. Con-
sidering the boundary value for the Weisz-Prater number of 0.3,
the thickness of the catalytic layer where no hydrogen diffusion
limitations are expected is 5 ␮m. This thickness is in excellent
agreement with the experimental results, which show that mass
transfer was observed for all the reactors with the thicker catalytic
coating (Fig. 6).
k3 are more than 100 times lower than k1 value which explains
*
the very high MBE selectivity observed in Fig. 9. The k3 rate con-
stant has wide confidence intervals and demonstrates that the
experimental data can be accurately described considering only
two consecutive reaction stages. This result is in line with the data
reported for Pd and Pd-Bi catalysts [55,56,58]. The KMBE/KMBY and
KMBA/KMBY ratios are below 0.1. This result agrees with previously
published data reporting preferential adsorption of alkynes on Pd
catalysts which is another factor contributing to the high alkene
selectivity [59,60]. TOF calculated using TEM data was 93 s−1, which
agrees with the TOF of about 60 s−1 obtained for 0.2% Pd/ZnO cat-
alyst under similar conditions [61].
3.4. Semihydrogenation in a capillary reactor
The kinetic parameters were applied to the capillary reactors
and the corresponding parity plot is shown in Fig. 10. There is a
good agreement between the experimental data and model pre-
dictions for the r0.53 reactors, which confirms the absence of mass
transfer limitations. The deviations in the area of low MBY conver-
sions are likely caused by uncertainties in elemental analysis and
variations in the dispersion of Pd nanoparticles. The experimental
data obtained in the r0.53–2.4 reactor demonstrated substantial
deviation from predicted values due to the presence of substantial
mass transfer limitations in this reactor.
For the r1.6 reactors, the picture was completely different as
all the points show reaction rates much lower than that expected
for the modelled conditions, i.e. without mass transfer limitations.
Please cite this article in press as: N. Cherkasov, et al., Scale up study of capillary microreactors in solvent-free semihydrogenation of
2-methyl-3-butyn-2-ol, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2016.03.028

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Two series of capillary reactors with the internal diameters
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