Electrochemically responsive heterogeneous catalysis for controlling reaction kinetics

Article abstract of DOI:10.1021/ja512224g

We report a method to control reaction kinetics using electrochemically responsive heterogeneous catalysis (ERHC). An ERHC system should possess a hybrid structure composed of an electron-conducting porous framework coated with redox-switchable catalysts. In contrast to other types of responsive catalysis, ERHC combines all the following desired characteristics for a catalysis control strategy: continuous variation of reaction rates as a function of the magnitude of external stimulus, easy integration into fixed-bed flow reactors, and precise spatial and temporal control of the catalyst activity. Herein we first demonstrate a facile approach to fabricating a model ERHC system that consists of carbon microfibers with conformal redox polymer coating. Second, using a Michael reaction whose kinetics depends on the redox state of the redox polymer catalyst, we show that use of different electrochemical potentials permits continuous adjustment of the reaction rates. The dependence of the reaction rate on the electrochemical potential generally agrees with the Nernstian prediction, with minor discrepancies due to the multilayer nature of the polymer film. Additionally, we show that the ERHC system can be employed to manipulate the shape of the reactant concentration-time profile in a batch reactor through applying customized potential-time programs. Furthermore, we perform COMSOL simulation for an ERHC-integrated flow reactor, demonstrating highly flexible manipulation of reactant concentrations as a function of both location and time. (Chemical Equation Presented).

Full text of DOI:10.1021/ja512224g

Article
pubs.acs.org/JACS
Electrochemically Responsive Heterogeneous Catalysis for
Controlling Reaction Kinetics
Xianwen Mao,† Wenda Tian,† Jie Wu,‡ Gregory C. Rutledge,*,† and T. Alan Hatton*,†
‡
†Department of Chemical Engineering and Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: We report a method to control reaction kinetics
using electrochemically responsive heterogeneous catalysis
(ERHC). An ERHC system should possess a hybrid structure
composed of an electron-conducting porous framework coated with
redox-switchable catalysts. In contrast to other types of responsive
catalysis, ERHC combines all the following desired characteristics
for a catalysis control strategy: continuous variation of reaction rates
as a function of the magnitude of external stimulus, easy integration
into fixed-bed flow reactors, and precise spatial and temporal
control of the catalyst activity. Herein we first demonstrate a facile
approach to fabricating a model ERHC system that consists of
carbon microfibers with conformal redox polymer coating. Second,
using a Michael reaction whose kinetics depends on the redox state
of the redox polymer catalyst, we show that use of different electrochemical potentials permits continuous adjustment of the
reaction rates. The dependence of the reaction rate on the electrochemical potential generally agrees with the Nernstian
prediction, with minor discrepancies due to the multilayer nature of the polymer film. Additionally, we show that the ERHC
system can be employed to manipulate the shape of the reactant concentration−time profile in a batch reactor through applying
customized potential−time programs. Furthermore, we perform COMSOL simulation for an ERHC-integrated flow reactor,
demonstrating highly flexible manipulation of reactant concentrations as a function of both location and time.
Herein we report a method to control reaction kinetics using
electrochemically responsive heterogeneous catalysis (ERHC)
(Figure 1). Our ERHC system possesses a hybrid structure
consisting of two key components (Figure 1a): (i) an electron-
conducting framework (e.g., interconnected conductive fibers)
and (ii) conformal coating of this framework with redox-
switchable catalysts whose activities can vary markedly with
changes in redox states. One unique advantage of an ERHC
system is that the electrochemical stimulus (i.e., potential) can
be used to manipulate continuously the number of activated
catalytic sites according to the Nernst equation.11 Figure 1b
illustrates a simple example case whereby the catalytic site is
activated when oxidized, and deactivated when reduced. When
the applied potential (E) on the conductive framework is much
lower than the formal potential (E0) of the redox-switchable
catalyst, most catalytic sites are deactivated and thus the
reaction is slow. When E ≫ E0, most catalytic sites are activated
and thus the reaction is fast. In “on/off” bimodal catalysis (e.g.,
several responsive gel-based catalysts6−10), variations of
external signals usually make reactions either fast or slow
(Figure 1c, purple line). In contrast, in an ERHC system, the
electrochemical potential can be employed to modulate the
INTRODUCTION
■
The ability to exert control over an entity or a process is
arguably the ultimate demonstration of our understanding of
that process and enables the full exploitation of its potential.
Stimuli-responsive systems offer advanced control methods
because external signals can be exploited elegantly to regulate
material properties.1−5 Chemists have recently begun to
incorporate control elements into catalyst design in response
to an increasing interest in responsive catalytic systems. Such
systems enable new strategies for the modulation of reaction
kinetics using various chemico-physical stimuli.6−10 The key to
achieving stimuli-controlled catalysis is the development of a
system in which the concentration or accessibility of the
catalytic site in reaction media can be adjusted in response to
external signals, such as temperature,6−9 pH,8 or solvent
composition.10 In most cases, the catalyst carrier (usually a soft
material such as a polymeric gel) undergoes morphological
and/or architectural changes upon exposure to an external
stimulus that result in variations in the catalyst concentration
and/or accessibility. For example, He et al.6 used a
thermoresponsive gel to move the catalyst into or out of the
reaction medium, thus turning the reaction on or off at will. In
another case, Wang et al.8 used temperature or pH to de-swell a
hydrogel, thereby concentrating the catalyst within the gel
matrix and thus accelerating the reaction rate.
Received: November 29, 2014
© XXXX American Chemical Society
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The ERHC concept is broadly applicable to a variety of
important reactions that can be catalyzed by redox-switchable
catalysts.23−28 This electrochemical modulation method may
also be employed to tune redox properties of the active sites in
enzymes, offering novel control strategies in enzyme
catalysis.29,30 Moreover, porous electrodes with tunable pore
size and surface area (e.g., electrospun carbon fiber webs)31,32
may be used as the conductive framework in an ERHC system,
providing possibilities to facilitate reactant diffusion and
improve catalysis performance.
RESULTS AND DISCUSSION
■
Fabrication and Characterization of a Model ERHC
System. The proof-of-concept ERHC system developed here
consists of a porous carbon fiber (CF) matrix with conformally
coated polyvinylferrocene (PVF). Ferrocene can function either
directly as a catalyst with a redox-controlled activity,28 or
indirectly as a redox-active ligand to adjust the reactivity of
metal complexes.26 The CF matrix serves as the electron-
conducting framework in this ERHC system. The PVF/CF
hybrid system was prepared by electrochemical oxidation-
induced deposition of PVF to the CF matrix (Figure 2a).33 For
details, see the Experimental Section. Application of a positive
electrochemical potential (0.8 V) to the CF matrix provides a
localized oxidative environment on the fiber surface. The
affinity of ferrocene for hydrophobic organic solvents (e.g.,
chloroform) is reduced upon oxidation.33,34 Therefore, PVF
Figure 1. The ERHC concept. (a) Conceptual drawing of an ERHC
system composed of interconnected conductive fibers with con-
formally coated with redox-switchable catalysts. (b) Schematic
illustration of electrochemical control over the number of active
sites and reaction rates. (c) Comparison between ERHC (green) and
“on/off” bimodal responsive catalysis (purple) for kinetic control.
reaction rate continuously and thus achieve intermediate rates
(Figure 1c, green data). Such flexible control over reaction rates
is highly desirable in chemical synthesis, especially for
mechanistic studies of reaction kinetics, selectivity control in
complex reaction networks, and safe heat removal in
exothermic reactions.12−15
More importantly, the electrochemical potential of an ERHC
system can be varied locally in real-time with high resolution,
allowing for precise spatial and temporal control of the
catalyst’s activity. Such exquisite control would benefit reaction
engineering tremendously, a main objective of which is to
adjust reactant distributions in reactors as functions of both
location and time.13,16−19 For controlled catalysis, chemical
(e.g., pH or solvent type) and thermal stimuli are commonly
adopted. However, although there are several ways to introduce
chemical species (e.g., dripping) and thermal energy (e.g.,
conventional or microwave heating) into a system, precise
control of chemical activity over location and time is difficult to
achieve (especially in a flow system) and is further hampered by
mass diffusion or heat dissipation processes.
Additionally, an ERHC system allows easy integration into
fixed-bed flow reactors, because, unlike soft materials-based
catalysts, activation/deactivation of the ERHC system does not
lead to significant changes in volume. Therefore, ERHC meets
a major goal of modern chemistry, that is, to combine the
advantages of heterogeneous catalysis and flow chemistry to
enhance the sustainability of chemical synthesis practices.12,20,21
However, many of the soft materials-based catalysts6−10
undergo significant morphological/structural changes (e.g.,
volumetric6,8−10 and sol−gel7 transitions) during the activa-
tion/deactivation process, and hence these systems cannot be
used easily in a fixed bed reactor that requires a fixed catalyst
volume and no catalyst leaching.21,22
Figure 2. Preparation of PVF/CF hybrids. (a) Schematics of
oxidation-induced conformal deposition of PVF onto CFs. (b) CVs
of a bare and a PVF-coated CF matrix. Scan rate: 50 mV/s. Electrolyte:
0.5 M NaClO4. (c) XPS spectra of a bare and a PVF-coated CF matrix.
(d) Ferrocene surface coverage versus deposition time. Insets:
corresponding SEM images of the specimens. Scale bar: 2 μm.
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initially soluble in chloroform becomes solvophobic and
subsequently precipitates onto the fiber when it is oxidized at
the fiber surface. The hydrodynamic diameter (dH) of the
polymer (molecular weight = 50 000 g/mol) in chloroform was
∼7.4 nm.33 The diameter of the carbon fiber was around 8 μm.
The presence of ferrocene moieties in an as-prepared hybrid
system was verified by cyclic voltammetry (CV) (Figure 2b).
The PVF-coated CF matrix prepared by a 10 min potentiostatic
deposition at 0.8 V showed pronounced anodic and cathodic
peaks at 0.41 and 0.22 V, respectively, characteristic of
ferrocene.35 The unmodified CF substrate showed no such
signals. Figure 2c shows the wide-range X-ray photoelectron
spectra (XPS) of the unmodified CFs and the PVF-coated CFs.
The spectrum of PVF-coated CFs possesses a C1S peak at 281
eV, an O1S peak at 532 eV, and two Fe2p peaks at 708 eV (2p 3/
2) and 721 eV (2p 1/2) due to the spin−orbital splitting of the
iron p orbital. The spectrum of the unmodified CFs does not
exhibit such Fe2p peaks. The XPS results, complementary to the
CV analysis, confirm the successful surface functionalization of
CFs by PVF.
The key factor to controlling the quality of the PVF coating
is the potentiostatic deposition time. Scanning electron
microscopy (SEM) images (Figure 2d insets) show a clear
morphological transition in the CF surfaces with different
deposition times. An unmodified CF exhibits a clean surface.
Deposition for 2 min led to non-uniform PVF aggregates that
only partially covered the fiber surface. With 10 and 20 min
deposition times, conformal, uniform coating around fibers with
complete surface coverage was achieved. However, a further
increase in deposition time to 30 min led to uneven coating and
cracking of the PVF film; we also observed that the initially
deposited film fell off the CF matrix in this latter case. This
poor coating quality might be due to the large thickness of the
polymer film and the low solubility of PVF+; both factors could
lead to mechanical instability of the deposited film.
Figure 3. Large-area SEM images of (a,b) unmodified CFs, (c,d) PVF-
coated CFs with 10 min deposition, and (e,f) PVF-coated CFs with 30
min deposition. Scale bar: 10 μm.
To evaluate quantitatively the deposition efficacy, we used
CV measurements to estimate the ferrocene surface coverage
(ΓFc, nmol/cm2) on the CFs (Figure 2d). For calculation
details, see the Experimental Section. ΓFc increased with
deposition time in the range of 0−20 min, but decreased
over the period from 20 to 30 min. This later decay indicates
loss of PVF, consistent with the SEM observation. Thus, an
intermediate deposition time (10−20 min) was thought to be
optimal since it consistently generated uniform conformal PVF
coatings. Large-area SEM images of unmodified CFs, and PVF-
coated CFs with 10 and 30 min deposition time are shown in
Figure 3.
Energy dispersive X-ray spectroscopic (EDS) elemental
mapping of Fe corroborated the assertion that the PVF-coated
CF substrate (10 min deposition) was completely covered by
the polymer (Figure 4a). In contrast, the EDS image of a bare
CF substrate exhibits negligible Fe signals (Figure 4b).
Additionally, the cross-sectional EDS (Fe and C) images of a
cryo-fractured sample indicates a clear core (C)−shell (Fe)
hybrid structure (Figure 4c).
Continuous Adjustment of Reaction Rates Using
ERHC. The model reaction chosen for demonstrating the use
of ERHC to control kinetics is the Michael addition of methyl
vinyl ketone (MVK) and ethyl-2-oxycyclopentane carboxylate
(E2OC) (Figure 5a). This class of reactions is important for
steroid synthesis and for forming carbon−carbon bonds in
many other organic compounds.36 Ferrocenium (Fc+) catalyzes
this reaction as a Lewis acid whereas ferrocene (Fc) has no
Figure 4. EDS elemental mapping. (a) Fe mapping and corresponding
SEM image of PVF-coated CFs (10 min deposition). (b) Fe mapping
and corresponding SEM image of unmodified CFs. (c) Fe mapping, C
mapping, and corresponding SEM image of the cross section of cryo-
fractured PVF-coated CFs (10 min deposition). Scale bar: 10 μm.
catalytic activity toward this reaction.28 This reaction is pseudo-
first order in MVK; such kinetic characteristics indicate that
E2OC reacts reversibly with the catalyst to form an adduct
which then reacts more slowly with MVK.28 The ERHC
concept may be broadly applicable to other types of Lewis acid-
catalyzed reactions of scientific and technological significance,
such as alkene alkylation, Friedel−Crafts reactions, aldol
reactions, and heterocyclic ring aperture.37 For heterogeneous
catalysis, it is convenient and conventional to express the
reaction rate as moles of reactants reacted per unit mass of the
catalyst.22 Thus, the rate law is written as −r (mol/(g catalyst·
min)) = −dm/dt = kappm, where m is the concentration of
MVK normalized to the catalyst mass (mol/g catalyst), and kapp
is the apparent first-order rate constant (min−1). Integration of
this differential equation results in ln m = −kappt + ln m0, where
m0 is the initial concentration of MVK. Figure 5b shows ln m
versus the reaction time, measured in the presence of a PVF/
CF system prepared by 10 min potentiostatic deposition. For
C
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is the standard redox potential for ferrocene, determined to be
0.370 0.025 V from five CV measurements, R is the ideal gas
constant, and T is the reaction temperature (298 K). Figure 5c
shows that kapp was generally consistent with kN, but exhibited a
less steep trend with decreasing potential than did kN.
Specifically, when the potential was at low values (0.0, 0.2,
and 0.3 V), kapp was larger than kN. One possible explanation
for the difference between kapp and kN is that the PVF coating
may have been a multilayer film whose redox composition did
not exhibit an ideal Nernstian dependence on potential; only a
redox monolayer can exhibit ideal Nernstian behavior.38 In a
multilayer film, each layer of ferrocene may experience a slightly
different potential. Thus, ferrocene molecules at the outermost
layer may still have been in the oxidized state even when the
electrode surface was at potentials much lower than E0; note
that the as-prepared PVF/CF system contained only Fc+. The
multilayer nature of the PVF coating was further elucidated by
using a redox polymer electrode (RPE) model to simulate the
instantaneous amperometric response of PVF during linear
potential sweeping (see SI Section S2).
Temporal Control in Batch Systems. To demonstrate
the ability of ERHC to exert temporal control over reaction
kinetics, we followed the concentration of MVK in a batch
reactor while using the electrochemical potential to modulate
the catalytic activity of the PVF/CF hybrid at different times.
Figure 6 shows the concentration of MVK in mol/L (CMVK) in
Figure 5. (a) Schematics of the Michael addition reaction of methyl
vinyl ketone and ethyl 2-oxycyclopentanecarboxylate. (b) ln m versus t
when 0.8 and 0.0 V were applied. Different symbols indicate three
independent measurements. (c) kapp (green) and kN (black) as a
function of potential. Error bars for kapp were obtained from four or
five measurements using different electrodes prepared under identical
conditions. Error bars for kN were from the standard deviation of the
E0 value for ferrocene.
details on the kinetic measurement, see the Experimental
Section. When a potential of 0.8 V (which provides 100%
conversion of Fc to Fc+35) was applied, m decayed markedly
with time. Linearity between ln m and t confirms the pseudo-
first order kinetics. When the potential was set at 0.0 V, m
remained almost unchanged. Three independent measurements
Figure 6. Concentration of MVK (purple circle) in a batch reactor as a
function of time in the presence of a PVF/CF hybrid (10 min
deposition), whose electrochemical potential was programmed to be
0.8 V (completely oxidized state) from 0 to 28 min, and 0.0 V
(completely reduced state) from 28 to 64 min, and 0.8 V (completely
oxidized state) again from 64 to 100 min. Error bars were obtained
from three measurements using different electrodes prepared under
identical conditions. The gray dash line shows the prediction from the
0.8 V
0.0 V
led to a kapp
= (6.0 0.5) × 10−3 min−1 and a kapp
=
(0.5 0.2) × 10−3 min−1, indicating that the PVF/CF catalyst
showed strong catalytic activity at 0.8 V and almost no activity
at 0.0 V. A potential of 0.8 V applied to a bare CF matrix did
not cause a concentration decay of MVK (Supporting
Information (SI) Figure S2). Chronoamperometry measure-
ments (SI Figure S3) show that the time scale for catalyst
activation (i.e., converting Fc to Fc+ electrochemically) was ∼5
s. Scaling analysis (SI Section S1) suggests that under our
experimental conditions the reactant mass transport time scale
was ∼0.1 s. Both these time scales were significantly shorter
than the reaction time scale (1/kapp0.8 V is ∼167 min); hence the
activation and transport processes had negligible influence on
0.8 V
0.0 V
batch system mass balance equations using the kapp
and kapp
values.
the reaction medium as a function of time, with the
electrochemical potential of the PVF/CF catalyst set to be at
0.8 V from 0 to 28 min (fully oxidized state), 0.0 V from 28 to
64 min (fully reduced state), and 0.8 V from 64 to 100 min.
Switching in situ between 0.8 and 0.0 V effectively turned the
reaction on and off, respectively. In Figure 6, CMVK decayed in
the first 28 min, remained almost unchanged for the next 36
min, and continued to decay afterward. Such a concentration−
time profile was consistent with the prediction (gray dash lines)
based upon the batch reactor mass balance equations using the
kapp0.0 V and kapp0.8 V values. This consistency also suggested that
the activation/deactivation of the catalyst was rapid, causing no
apparent discrepancy between the experimentally measured
profile and the prediction (which assumed that the activation or
deactivation process happened instantaneously). This rapid
the determination of kapp
.
To demonstrate the unique ability of an ERHC system to
control kinetics on demand and achieve multiple intermediate
reaction rates, we applied a series of different potentials
between 0.8 and 0.0 V to the PVF/CF system and measured
the corresponding reaction rates. As shown in Figure 5c (green
bar), we obtained intermediate kapp values that were higher than
0.0 V
kapp
and lower than kapp0.8 V. Figure 5c (black data) also
shows the rate constants predicted theoretically based upon the
Nernst equation: kN = kOXβ/(1 + β), with β = exp(F(E − E0)/
RT), where kOX is the rate constant when the catalyst is
completely oxidized (i.e., kapp0.8 V), F is the Faraday constant, E0
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Figure 7. (a−c) The CMVK−t relationships (b) in a batch reactor when two different potential−time profiles (a,c) were applied. The green circles or
red squares are experimentally determined concentrations. Error bars were obtained from three measurements using different electrodes prepared
0.4 V
under identical conditions. The shaded bands are the predictions from the batch system mass balance relationship using the kapp0.8 V, kapp0.6 V, kapp
,
kapp0.2 V, and kapp0.0 V values. (d,e) The first derivative (dCMVK/dt) (d) and second derivative (d2CMVK/dt2) (e) of the green line (theoretical) shown
in (b). (f,g) The first derivative (dCMVK/dt) (f) and second derivative (d2CMVK/dt2) (g) of the red line (theoretical) in (b).
catalyst activation/deactivation should be attributed to the fast
electron transfer kinetics of the ferrocene moiety.38
linear potential−time function. On the other hand, with the
“high−low” program, the CMVK−t relationship exhibited a
convex shape, with different decay constants (τP = 1/kappP, P is
the applied potential with values of 0.0, 0.2, 0.4, 0.6, and 0.8 V)
in each constant-potential segment. The first and second
derivatives in this case are shown in Figure 7f and 7g,
respectively. The distinction between the “low−high” case and
the “high−low” case can be also appreciated by comparing
Figure 7d with Figure 7f. In both cases, dCMVK/dt in each
individual segment was an increasing function of t. With the
“high−low” program, dCMVK/dt exhibited a general increasing
trend with time (indicated by the dashed arrow in Figure 7f).
However, with the “low−high” program, dCMVK/dt exhibited a
general decreasing trend with time (indicated by the dashed
arrow in Figure 7d). This general decreasing trend gave rise to
the apparently concave shape of the CMVK−t curve in Figure 7b.
Temporal and Spatial Control in Flow Systems
Demonstrated by COMSOL Simulation. A distinct
advantage of integrating ERHC into a flow reactor is the
flexible control over reactant concentrations as a function of
both location and time. We note that the spatial control in a
flow reactor may be unique to an ERHC system, due to its
heterogeneous nature combined with the fact that the
electrochemical stimulus can be applied locally with high
precision. To demonstrate the capability of ERHC to exert both
spatial and temporal control in a flow reactor, we performed a
COMSOL simulation on an ERHC-integrated packed-bed-like
tube reactor, composed of ∼40 000 stacked PVF/CF sheets
whose activity can be adjusted individually in real-time by
changes in their electrochemical potentials. The tube reactor
More interestingly, ERHC could be employed to create a
complicated shape of the reactant concentration−time profile
through applying a customized potential−time program. Figure
7 shows such an example. By applying two different potential−
time profiles, termed “low−high” (Figure 7a) and “high−low”
(Figure 7c) programs, we obtained two very different CMVK−t
curves (Figure 7b). The “low−high” program resulted in a slow
decay in CMVK in the first 60 min followed by a fast decay from
60 to 112 min, whereas the “high−low” program led to a fast
concentration decrease from 0 to 80 min and a slower decrease
from 80 to 112 min. The two shaded bands with asymptotic
color changes in Figure 7b were predictions from the batch
system mass balance equations with first-order kinetics using
0.0 V
the kapp0.8 V, kapp0.6 V, kapp0.4 V, kapp0.2 V, and kapp
values and
their standard deviations. The experimental data (circles and
squares in Figure 7b) were in good agreement with the
predictions.
It is also interesting to note that, when employing the “low−
high” program, the overall shape of the CMVK−t curve exhibited
a somewhat concave character, even though, in each segment
with a constant potential value, the CMVK−t relationship was
convex in nature. This convex character is clearly seen on
examination of the first and second derivatives of the calculated
CMVK−t curve: in each individual segment, the first derivative
(dCMVK/dt) was an increasing function of time (Figure 7d), and
the second derivative (d2CMVK/dt2) was positive (Figure 7e). In
fact, it would be possible to obtain a “real” concave curve by
using very small increments in potential, for example, using a
E
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model is illustrated schematically in Figure 8a. The reactor was
taken to be 10 m long and 1 m in diameter, with a catalyst mass
reactions. Results shown in Figure 9a−d indicate that the
ERHC control strategy could also be applied to these four
reactions using the PVF/CF hybrid. However, the kapp values
for reactions (e) and (f) were very low, and showed no
significant difference across different potentials. It was found
previously that, for iron(III) catalysis of the Michael reaction of
β-dicarbonyl compounds and enones, fast kinetics were
observed for reactions involving either cyclic ketoesters or
methyl vinyl ketone or both.39 Reactions (e) and (f) involve
neither ketoesters nor methyl vinyl ketone.
CONCLUSION
■
In this study, we have demonstrated a control strategy to
manipulate reaction kinetics through electrochemically respon-
sive heterogeneous catalysis (ERHC). We demonstrated a facile
electrochemical method with high controllability to fabricate a
proof-of-concept ERHC system that consisted of intercon-
nected carbon fibers with conformal PVF coating. The surface
functionalization efficiency could be varied systematically by
potentiostatic deposition time. This fabrication approach is very
versatile; other functional components, such as aniline, pyrrole,
carbon nanotubes and graphene oxides, could be electrochemi-
cally co-deposited with PVF to improve the catalysis perform-
ance.
Second, using a model first-order reaction whose kinetics
depends on the redox state of the PVF catalyst, we showed that
different electrochemical potentials could be used to vary the
reaction rates continuously. The dependence of the reaction rate
on the electrochemical potential was generally consistent with
the Nernstian prediction.
Figure 8. (a) Schematics of the ERHC-integrated flow reactor
employed in the COMSOL simulation. (b) CZ as a function of the
axial position within the reactor when a series of different potentials
were applied to all the catalyst sheets. (c) The applied potential
(green) and the corresponding CZ (black) as a function of position in
the z-direction. (d) The applied potential (green) and the
corresponding concentration Coutlet (black) as a function of time.
of 1700 kg and a packing density of 217 kg/m3. The inlet
concentration of MVK was fixed at 1 mol/g catalyst. For
simulation details, see SI Section S3. Figure 8b shows the
concentration of MVK (CZ) versus position along the z-axis of
the reactor when a constant potential was applied to all the
catalyst sheets, for different such potentials. Clearly, with
increasing potential, CZ decreased more quickly. For instance,
when 0.0 V was applied, CZ decayed by half at z = 4.31 m,
whereas with an applied potential of 0.8 V, CZ decreased by half
at z = 0.91 m. Conceivably, more complicated concentration
profiles can be obtained by applying different potentials at
different positions within the reactor. Figure 8c shows such an
example: a step-like concentration profile (black line) was
obtained through application of a square-wave-like potential
profile (green line). To further demonstrate temporal control,
we arbitrarily changed the potentials of all the catalyst sheets
over time (Figure 8d, green line). Consequently, an interesting
Additionally, we showed that, in a batch system, the PVF/CF
ERHC system could be utilized to manipulate the shape of the
reactant concentration−time profile through applying custom-
ized potential−time programs. We also carried out COMSOL
simulation for an ERHC-integrated flow reactor, demonstrating
highly flexible control over reactant concentrations as a
function of both location and time. Such a high degree of
flexibility in spatial and temporal control is almost unattainable
using conventional control strategies.
In contrast to other types of stimuli-triggered catalysis,6−10
ERHC combines all the following advantageous features:
continuous variation of reaction rates, easy integration into
fixed-bed flow reactors, and precise spatial and temporal control
over the catalyst activity. This electrochemical modulation
method is broadly applicable to a variety of reactions amenable
to redox-switchable catalysis,23−28 and may potentially offer
new control strategies in enzyme catalysis.29,30 The conductive
framework in an ERHC system may accommodate porous
carbon electrodes with tunable morphologies and electronic
properties,40 allowing further optimization of transport proper-
ties, electron transfer kinetics, and catalysis performance. With
broad applicability and high design flexibility, ERHC could pave
the way to intriguing novel applications in controlled catalysis,
chemical synthesis and reaction engineering.
relationship between the outlet concentration of MVK (Coutlet
)
and time was predicted (Figure 8d, black line). In this case, an
initial condition with a uniform MVK concentration of 1 mol/g
catalyst throughout the reactor was applied. The results shown
in Figure 8c,d suggest that ERHC allows flexible spatial and
temporal manipulation of reactant concentrations; such a high
degree of flexibility is almost unattainable using conventional
control strategies.
Tests of Other Reactions. We measured kapp values for six
other Michael addition reactions using the PVF/CF catalysts
(10 min deposition); the results are summarized in Figure 9.
Reactions (a), (b), (c), and (d) were found to show much
higher kapp values under the completely oxidized state (i.e., 0.8
V) than at the completely reduced conditions (i.e., 0.0 V). We
EXPERIMENTAL SECTION
■
Chemicals and Materials. Polyvinylferrocene (molecular weight
= 50 000 g/mol) was obtained from Polysciences. Methyl vinyl ketone,
ethyl 2-oxycyclopentanecarboxylate, 2-acetylcyclopentanone, ethyl
acetoacetate, ethyl 2-ethylacetoacetate, trans-4-phenyl-3-buten-2-one,
sodium perchlorate, tetrabutylammonium perchlorate, and chloroform
were purchased form Sigma-Aldrich. Deuterated methanol was
purchased from Cambridge Isotope Laboratories. All reagents were
also tested one intermediate potential (0.3 V), which gave rise
0.0 V
to a rate between the kapp
and kapp0.8 V values for these four
F
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Figure 9. kapp values obtained at three different electrochemical potentials using the PVF/CF hybrids as the catalysts for six other Michael addition
reactions: (a) methyl vinyl ketone + 2-acetylcyclopentanone; (b) methyl vinyl ketone + ethyl acetoacetate; (c) methyl vinyl ketone + ethyl 2-
ethylacetoacetate; (d) trans-4-phenyl-3-buten-2-one + ethyl 2-oxycyclopentanecarboxylate; (e) trans-4-phenyl-3-buten-2-one + ethyl acetoacetate; (f)
trans-4-phenyl-3-buten-2-one + ethyl 2-ethylacetoacetate. Error bars were obtained from three to four measurements using different electrodes
prepared under identical conditions.
V2
used as received throughout the study, without further purification or
Γ =
[ia(V) − ic(V)] dV/(2AveN )
∫
s
A
chemical modification unless otherwise noted. A platinum wire
auxiliary electrode and an Ag/AgCl (3 M NaCl) reference electrode
were purchased from BASi.
V
(1)
1
where Γ is the ferrocene surface coverage, V1 and V2 are the cutoff
potentials in cyclic voltammetry, ia(V) and ic(V) are the instantaneous
anodic and cathodic currents as a function of potential, vs is the scan
rate, e is the elementary charge, NA is Avogadro’s number, and A is the
total surface area of the CF matrix (calculated by the mass of the CF
matrix multiplied by its specific surface area, determined by nitrogen
adsorption isotherms by means of the Brunauer−Emmett−Teller
method). Note that eq 1 does not consider the charging current
because it was negligible compared to the faradiac current of ferrocene
under our experimental conditions. Equation 1 is a universal
expression to calculate total charges; it applies to cyclic voltammo-
grams of any shape since it uses the integral area of the cyclic
voltammogram/scan rate to represent the sum of anodic and cathodic
voltammetric charges.41
Kinetic Measurements. Equimolar mixtures of reactants (1 mL of
E2OC and 0.58 mL of MVK) and the supporting electrolyte (68 mg of
sodium perchlorate) were added to 4 mL of methanol. The reaction
between E2OC and MVK was carried out in an electrochemical cell
with the PVF/CF catalyst as the working electrode, a platinum wire as
the counter electrode, and an Ag/AgCl electrode as the reference
electrode. The reaction mixture was magnetically stirred at a speed of
220 rpm and kept at 298 K using a water bath. The progress of the
reaction was followed by the time dependence of the vinyl proton
Instrumentation. Scanning electron microscopy (JEOL-6010LA)
was used to investigate the morphologies of the PVF/CF catalysts and
perform energy dispersive elemental mapping. X-ray photoelectron
spectra were recorded with a Kratos Axis Ultra instrument equipped
with a monochromatic Al Kα source operated at 150 W. Electro-
chemical experiments were performed on an AutoLab PGSTAT 30
potentiost at with GPES software. All potentials in this work are
1
referred to a Ag/AgCl (3 M NaCl) reference electrode. H NMR
analysis was performed in deuterated methanol with a Bruker 400. The
nitrogen adsorption/desorption measurements were performed with
ASAP2020, Micromeritics.
Fabrication of PVF/CF Hybrids. A carbon fiber matrix (Toray,
TGP-H-060) with a nominal surface area of 1 cm2 and a thickness of
200 μm was immersed in 5 mL of chloroform solution containing 0.1
M tetrabutylammonium perchlorate and 10 mg/mL PVF. An
electrochemical potential of 0.8 V versus Ag/AgCl was applied to
the carbon fiber matrix for a period of 2, 10, 20, and 30 min to induce
the PVF deposition process. The surface functionalization efficiency
(i.e., ferrocene surface coverage) was calculated from the cyclic
voltammograms according to the following equation:41
G
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Journal of the American Chemical Society
Article
NMR signal at 6.3 ppm.28 Aliquots of 0.1 mL were taken from the 4
mL reaction mixture and mixed with 0.7 mL of deuterated methanol
for NMR analysis. The typical sampling frequency was around 10−20
min. Similar procedures were adopted for other reactions shown in
Figure 9.
COMSOL Simulation. Simulations were carried out with the
COMSOL (Multiphysics Version 4.2a) software package. The plug
flow module was used under either transient or steady state conditions
with first-order reaction kinetics for MVK. The relationship between
the potential applied and the corresponding reaction rate constant was
determined experimentally (see Figure 5c). The total mass of the
catalyst (1700 kg), or the size of the reactor (10 m in length and 1 m
in diameter), was chosen such that when the catalyst was fully oxidized
(fixed at a potential of 0.8 V), the concentration of MVK decayed to
zero at the outlet. The mass of the catalyst and the volume of the
reactor were correlated from the density of the PVF/CF catalyst.
There were approximately 40 000 sheets in the tube reactor. Details
for the COMSOL simulation are shown in SI Section S3.
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ASSOCIATED CONTENT
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■
S
Scaling analysis of the reactant transport process, details of the
RPE simulation, details of the COMSOL simulation, and
supplementary figures. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*rutledge@mit.edu
*tahatton@mit.edu
Notes
(30) Abou Hamdan, A.; Dementin, S.; Liebgott, P. P.; Gutierrez-
Sanz, O.; Richaud, P.; De Lacey, A. L.; Roussett, M.; Bertrand, P.;
Cournac, L.; Leger, C. J. Am. Chem. Soc. 2012, 134, 8368−8371.
(31) Mao, X.; Simeon, F.; Rutledge, G. C.; Hatton, T. A. Adv. Mater.
2013, 25, 1309−1314.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge the MITEI Seed Fund Grant for financial
support.
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(32) Mao, X.; Hatton, T. A.; Rutledge, G. C. Curr. Org. Chem. 2013,
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