Polymer-immobilized, hybrid multi-catalyst architecture for enhanced electrochemical oxidation of glycerol

Article abstract of DOI:10.1039/c7cc05724e

The development of a hybrid, tri-catalytic architecture is demonstrated by immobilizing MWCNTs, TEMPO-modified linear poly(ethylenimine) and oxalate decarboxylase on an electrode to enable enhanced electrochemical oxidation of glycerol. This immobilized, hybrid catalytic motif results in a synergistic 3.3-fold enhancement of glycerol oxidation and collects up to 14 electrons per molecule of glycerol.

Full text of DOI:10.1039/c7cc05724e

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Reactivity differences of Sc
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Polymer‐Immobilized, Hybrid Multi‐Catalyst Architecture for
Enhanced Electrochemical Oxidation of Glycerol
Florika C. Macazo, David P. Hickey, Sofiene Abdellaoui, Matthew S. Sigman, and Shelley D.
Minteer*
Received 00th July 2017
Accepted 00th August 2017
DOI: 10.1039/x0xx00000x
www.rsc.org/
The development of
a
hybrid, tri‐catalytic architecture is from mesoxalate and oxalate that is process‐limiting in the oxidation
8
‐10
cascade.
We have previously employed a hybrid catalytic
demonstrated by immobilizing MWCNTs, TEMPO‐modified linear
poly(ethylenimine) and oxalate decarboxylase on an electrode to
enable enhanced electrochemical oxidation of glycerol. This
immobilized, hybrid catalytic motif results in a synergistic 3.3‐fold
enhancement of glycerol oxidation and collects up to 14 electrons
per molecule of glycerol.
approach that utilizes a simple organic catalyst, TEMPO, to
electrochemically mediate alcohol and aldehyde oxidation in the first
five steps of the glycerol oxidation pathway (Scheme 1).
1
1
Hybrid biofuel cells combine biological, small molecular, and/or
metallic catalysts to electrochemically oxidize high‐energy density
fuel sources.1 Complex biofuels constitute a unique and abundant
subset of such molecules that can be electrocatalytically oxidized to
‐3
4
, 5
generate electricity.
By utilizing multiple catalytic motifs, it is
possible to facilitate multiple oxidative steps for a single fuel
molecule.3 However, integration of additional catalytic modalities
becomes increasingly complicated by the disparate reaction
conditions required by each additional catalyst, as well as limitations
arising from slow substrate diffusion between adjacent catalytic
sites.7 In addition, the combination of homogeneous and
heterogeneous catalytic motifs often requires relatively high
concentrations of the homogeneous component (i.e. high enzyme or
small molecule catalyst concentration). Therefore, there remains a
pressing need to design cost‐effective molecular constructs that can
efficiently integrate multiple catalytic motifs without a deleterious
, 6, 7
Scheme. 1 Electrocatalytic oxidation cascade of glycerol by MWCNT/TEMPO‐
impact to the overall catalytic activity, while enabling deep LPEI/OxDc multi‐catalytic system. Oxidation reactions catalyzed by TEMPO‐
electrochemical oxidation of fuels.
LPEI are indicated by blue arrows (steps 1 to 5, 7, 9), while decarboxylation
In this context, glycerol has been established as a promising fuel reactions catalyzed by MWCNTs and OxDc are shown in black (step 6) and red
4
, 8
(step 8) arrows, respectively.
source due to its exceptionally high energy density ; however,
electrochemical conversion of glycerol to CO presents a unique
2
This catalyst has been reported to electrochemically oxidize
primary and secondary alcohols to aldehydes and ketones,
respectively, as well as aldehydes to carboxylic acids under aqueous
challenge due to the variety of reactions required, including primary
and secondary alcohol oxidations to the resultant carbonyl
compounds, aldehyde oxidation to a carboxylic acid and carbon‐
carbon bond cleavage processes. While the possible oxidative
pathways from glycerol to tartrate, and subsequently to mesoxalate,
1
2‐19
conditions.
oxidatively cleave CO
However, nitroxyl radical catalysts are not known to
2
. To cleave carbon dioxide from oxalate in the
later steps of the cascade, we previously employed an enzyme,
are numerous and may be ambiguous, it is often the cleavage of CO
2
oxalate decarboxylase (OxDc), in creating a hybrid catalytic system to
2
0
cleave CO
2
from oxalate in the later steps of the cascade. Yet, even
cleavage from
under optimum pH and temperature, the rate of CO
2
a. Department of Chemistry, University of Utah, 315 S 1400 E, Salt Lake City, UT
8
4112, USA. E‐mail: minteer@chem.utah.edu
mesoxalate by OxDc is slow when compared to TEMPO‐mediated
electrochemical oxidation of alcohols and aldehydes due to low
†
Electronic Supplementary Information (ESI) available: Experimental section,
catalytic cyclic voltammograms, titration curves, and GC spectra as noted in the text.
See DOI: 10.1039/x0xx00000x
20
promiscuity of the enzyme catalyst. Furthermore, the high bulk
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enzyme concentration required to compensate for catalytic OxDc may not be a conventional enzyme that is typically employed
deficiency of OxDc is both cost and space prohibitive on the electrode in bioelectrocatalysis, its utility was logical for our purpose.
surface. We hypothesized that this problem may be addressed by co‐ Specifically, we were enthusiastic about its high specific and catalytic
2
0, 24‐26
immobilizing three promiscuous catalytic motifs, a small organic activity,
catalyst such as TEMPO, an enzymatic catalyst and catalytically active to protein bioengineering.
carbon nanotubes, to target process‐limiting steps and selectively the decarboxylation of oxalate via cleavage of the C–C bond,
ease of expression and purification, and amenability
2
0, 22, 24
Moreover, OxDc should catalyze
enhance the overall rate of glycerol oxidation.
Herein, we demonstrate the rational incorporation of three converted to CO by TEMPO‐LPEI, as reported previously.
2
producing formic acid as an intermediate, which can efficiently be
2
0, 27
catalytic motifs onto a single electrode configuration to increase the
overall oxidation rate of glycerol to CO . Specifically, we employed a LPEI/OxDc system to electro‐oxidize mesoxalate, oxalate, and
recently developed TEMPO‐modified linear poly(ethylenimine) formate by carrying out constant potential amperometry with the
TEMPO‐LPEI) as a molecular scaffold to simultaneously immobilize potential held at 0.70 V (vs. SCE) for all systems. Fig. 1 shows
We evaluated the catalytic ability of the hybrid MWCNT/TEMPO‐
2
(
TEMPO, carboxylated multi‐walled carbon nanotubes (COOH‐ comparative amperometric titration curves for the immobilized
MWCNTs) and OxDc directly onto an electrode surface to catalyze MWCNT/TEMPO‐LPEI/OxDc systems when challenged with
alcohol oxidation, cleavage of CO
2
from mesoxalate, and hydrolytic increasing concentrations of mesoxalate, oxalate, and formate. The
CO cleavage of oxalate, respectively. Our strategy relies on the titration curves reveal distinct differences in the electrocatalytic
2
ability of the polymer hydrogel, TEMPO‐LPEI, to effectively activities of the hybrid system towards intermediates involved in the
immobilize OxDc without substantial detriment to its catalytic rate‐limiting steps of the glycerol oxidation cascade, generating
‐
2
‐
activity, as well as the ability of catalytically active MWCNTs to maximum current densities of ~0.5 ± 0.0 mA cm , ~2.0 ± 0.3 mA cm
facilitate the cleavage of carbon dioxide from mesoxalate. By
2
‐2
, and ~2.7 ± 0.4 mA cm for mesoxalate, oxalate, and formate,
confining them into single electrode configuration using respectively. Noticeably, the multi‐catalytic system exhibits the
a
‐
2
crosslinking chemistry as opposed to a typical solution‐based highest catalytic current densities (~2.7 ± 0.4 mA cm ) in the
technique, we eliminated the need for additional catalyst diffusion presence of formate (within the concentration range of 0 – 60 mM)
steps to the interrogating electrode. Consequently, this enables and may be attributed to the high activity of TEMPO‐LPEI towards
faster electron transfer rates between the substrates, catalysts and formate.
the electrode surface. The use of concentrations lower than 60 mM in the titration
2
7
Prior to hybridizing with an enzyme, we first tested the experiments is due to the insolubility of mesoxalate and oxalate at
electrocatalytic activity of immobilized TEMPO‐LPEI against glycerol, high concentrations (> 60 mM) even with constant stirring, as well as
glyceraldehyde and formate, which generated maximum current observed delamination of the films from the electrode surface at
‐
2
‐2
‐2
densities of 0.70 mA cm , 0.16 mA cm , and 2.86 mA cm , high substrate concentrations. We attribute the instability (i.e., film
respectively. This confirms previous findings that TEMPO‐LPEI is delamination) of the cross‐linked films to the rapid production of
capable of rapidly oxidizing several key intermediates of the glycerol bubbles at the electrode surface when the substrate concentration
2 2
oxidation cascade. Furthermore, TEMPO‐NH ‐based systems have is high (indicative of CO production), which significantly weakens the
been shown to catalyze the first five oxidation reactions of the cross‐linking of the polymer to the surface as the gas bubbles
glycerol oxidation cascade and produce mesoxalate as the become entrapped in the hydrogel matrix especially during swelling.
intermediate.2
0, 21
However, not all TEMPO derivatives are able to Of note, the TEMPO‐LPEI hydrogel polymer is known to swell to
catalyze the electrochemical oxidation of tartrate to mesoxalate, facilitate rapid diffusion of the substrates, which is one of the reasons
2
1
adding a further impediment to completing the cascade. Thus, we why this system was selected to act both as an oxidation catalyst and
2
7
initiated our study by examining the hybrid, multi‐catalytic system as an anchor onto an electrode surface.
for the electrochemical oxidation of tartrate. Our data demonstrates
that immobilized MWCNT/TEMPO‐LPEI/OxDc exhibits catalytic
activity towards tartrate, as evidenced by the increased oxidative
‐
2
current (~3.89 mA cm at 0.70 V vs. SCE) in the presence of tartrate
Fig. S1, ESI†). When compared to relevant controls, catalytic
(
oxidation of tartrate was not observed, as shown by the absence of
an oxidative electrocatalytic wave for the immobilized MWCNTs, as
well as lack of increase in the kinetic activity of OxDc in the presence
of tartrate (Fig. S1, ESI†). This implies that tartrate oxidation is not
contributing significantly to the process‐limiting steps of the
oxidation cascade and confirms the ability of the multi‐catalyst
system in mediating the first five oxidation steps of the glycerol
oxidation cascade.
In fabricating the hybrid MWCNT/TEMPO‐LPEI/enzyme modality,
we first attempted to cross‐link MWCNT/TEMPO‐LPEI with oxalate
oxidase (OxOx), an oxidoreductase enzyme that will catalyze the
2
0,
oxidation of oxalate. However, the enzyme had low specific activity
2
2, 23
(
data not shown) and notable limited overlap with the working Fig 1. Catalytic oxidation of mesoxalate (red curve), oxalate (green curve), and
formate (blue curve) by hybrid MWCNT/TEMPO‐LPEI/OxDc immobilized on a
carbon electrode surface. Amperometric titration curves obtained with Eapp
.70 V vs. SCE. Data points fitted into a Michaelis‐Menten model.
21, 23
pH range of TEMPO.
In light of this, we explored the possibility
=
of hybridizing and immobilizing a non‐redox enzyme, oxalate
decarboxylase (OxDc), with TEMPO‐LPEI, as this enzyme has been
demonstrated to exhibit higher substrate activity and catalytic
0
To confirm if the catalytic current densities observed for the hybrid
2
0
response when challenged with mesoxalate and oxalate. While
catalytic systems were in fact caused by the enzymatic activities
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towards mesoxalate, oxalate, and formate and not just due to the
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These results show that the catalytic current densities observed
high reactivity of the TEMPO‐LPEI catalyst, we carried out control when OxDc is deactivated (denatured) are due to the TEMPO‐LPEI
experiments by immobilizing denatured OxDc to deactivate or lower and MWCNTs acting on their corresponding substrates, but when
the enzyme activity, and compared the resulting activities against the OxDc is active and carries out the decarboxylation of oxalate, a
active OxDc. The oxidative current densities of both the active and synergistic enhancement in the electrocatalytic oxidation of glycerol
denatured forms of OxDc did not show significant difference in is enabled. As these results provide evidence of the ability of the
catalytic activity towards mesoxalate (Fig. 2) indicating that the hybrid system to catalytically transform mesoxalate, oxalate and
enzyme was not involved in the catalytic reaction of mesoxalate to formate – the last three intermediates contributing to the rate‐
glyoxylate. Since TEMPO‐LPEI is not known to catalyze the cleavage limiting steps of the cascade (Fig. 1), we sought to demonstrate the
of CO
2
from mesoxalate, this strongly suggests that MWCNTs were potential of this hybrid multi‐catalytic system in enhancing the rate
, and verify that the oxidation
catalytically‐active towards mesoxalate and catalyzed the of oxidation of glycerol to produce CO
2
decarboxylation of mesoxalate (Fig. 1, red curve and Fig. 2). cascade for glycerol can occur at ambient conditions. Therefore, we
Comparative cyclic voltammograms obtained for bare (Fig. S2, left, examined the ability of the hybrid system to electrochemically
ESI†) and MWCNT‐modified (Fig. S2, right, ESI†) electrodes also oxidize glycerol to CO , on the basis that the MWCNTs and OxDc will
2
confirm this observed catalytic activity of MWCNTs towards perform cleavage of the C–C bond of mesoxalate and oxalate,
mesoxalate, as evidenced by the occurrence of an oxidative wave respectively, and facilitate faster rate of electrochemical oxidation of
starting at ~ 0.6 V vs. SCE.
glycerol.
Fig. 3. Catalytic oxidation of oxalate by immobilized MWCNT/TEMPO‐
LPEI/OxDc. Titration curves obtained for the active (blue curves) and
denatured (black curves) forms of OxDc in the presence of oxalate.
Fig. 2. Catalytic oxidation of mesoxalate by immobilized MWCNT/TEMPO‐
LPEI/OxDc. Titration curves obtained for the active (blue curves) and
denatured (black curves) forms of OxDc in the presence of mesoxalate.
Employing similar experimental conditions as discussed above, we
In contrast, the active form of OxDc (Fig. 3, blue curve) performed a series of cyclic voltammetric experiments in the
demonstrated higher activity towards oxalate than the denatured presence of increasing concentrations of glycerol using the
form (Fig. 3, black curve), which implies that the oxidative current MWCNT/TEMPO‐LPEI/OxDc hybrid system (Fig. 4, blue curve) and
densities obtained were caused by enzyme activity and not just by MWCNT/TEMPO‐LPEI as a control ((Fig. 4, black curve) and catalytic
the catalytic activity of the TEMPO‐LPEI polymer.
current densities at 0.70 V (vs. SCE) were plotted against the
We also confirmed this result by performing an activity kinetic concentration of glycerol (Fig. 4). It is evident from Fig. 4 that a
assay for both the solution‐based (Fig. S3, black curve, ESI†) and significant 3.3‐fold enhancement in the peak oxidative current
immobilized OxDc (Fig. S3, blue curve, ESI†) against increasing density is obtained for the hybrid MWCNT/TEMPO‐LPEI/OxDc
concentrations of oxalate. Titration curves obtained for the solution‐ system in the presence of 1 M glycerol, generating a current density
‐
2
based OxDc and immobilized MWCNT/TEMPO‐LPEI/OxDc generated of ~1.3 ± 0.2 mA cm , as opposed to a current density of ~0.4 ± 0.1
‐
1
‐
‐2 generated by the MWCNT/TEMPO‐LPEI system (control).
V
,
max values of 0.366 ± 0.008 mol min and 0.066 ± 0.002 mol min mA cm
This data strongly suggests that the immobilized, hybrid
observed for the cross‐linked films can be attributed to diffusion MWCNT/TEMPO‐LPEI/OxDc multi‐catalytic system enables
enhanced electrochemical oxidation of glycerol to CO , as a result of
1
respectively (Fig. S3, ESI†). The decrease in enzyme activity
2
7
limitations caused by immobilization.
immobilized hybrid system still exhibited significant activity towards the combined catalytic activities of the enzyme, MWCNTs, and the
organic polymer towards the substrates involved in the process‐
findings, as well as verifying that OxDc is still active even when limiting steps of the glycerol oxidation cascade. We also confirmed
by performing a 24‐hour bulk electrolysis on
Nevertheless, the
2
‐
1
oxalate (0.066 ± 0.002 mol min ), thereby supporting the previous
immobilized on an electrode surface. Finally, the current densities the production of CO
2
were generally higher for both the active (Fig. S2, blue curve, ESI†) the immobilized MWCNT/TEMPO‐LPEI/OxDc in the presence of 50
and denatured OxDc (Fig. S2, black curve, ESI†) when formate was mM glycerol and used gas chromatography to detect the formation
the substrate, indicating that the TEMPO‐LPEI polymer effectively of CO
catalyzes the oxidation of formate, either in the presence or absence
of OxDc, consistent with the literature.
2
in the headspace (Fig. S4, blue curve, ESI†).
Consistent with our proposed scheme (Scheme 1), we observed
2
0, 27
that enhanced electrocatalytic activity of the hybrid system towards
glycerol is a consequence of the ability of the TEMPO‐LPEI polymer
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Polymer-immobilized MWCNT/TEMPO-LPEI/OxDc hybrid tri-catalytic motif enables
synergistic enhancement in the complete oxidation of glycerol.
a