Xiaotong H.
Chadderdon
a,
David J.
Chadderdon
a,
Toni
Pfennig
ab,
Brent H.
Shanks
ab and
Wenzhen
Li
*ac
aDepartment of Chemical and Biological Engineering, Iowa State University, Ames 50011, USA. E-mail: wzli@iastate.edu
bNSF Engineering Research Center for Biorenewable Chemicals (CBiRC), Iowa State University, Ames 50011, USA
cUS Department of Energy Ames Laboratory, Ames 50011, USA
First published on 23rd October 2019
Electrochemical conversion of biomass-derived compounds is a promising route for sustainable chemical production. Herein, we report unprecedentedly high efficiency for conversion of 5-(hydroxymethyl)furfural (HMF) to biobased monomers by pairing HMF reduction and oxidation half-reactions in one electrochemical cell. Electrocatalytic hydrogenation of HMF to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved under mild conditions using carbon-supported Ag nanoparticles (Ag/C) as the cathode catalyst. The competition between Ag-catalyzed HMF hydrogenation to BHMF and undesired HMF hydrodimerization and hydrogen evolution reactions was sensitive to cathode potential. Also, the carbon support material in Ag/C was active for HMF reduction at strongly cathodic potentials, leading to additional hydrodimerization and low BHMF selectivity. Accordingly, precise control of the cathode potential was implemented to achieve high BHMF selectivity and efficiency. In contrast, the selectivity of HMF oxidation facilitated by a homogeneous electrocatalyst, 4-acetamido-TEMPO (ACT, TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl), together with an inexpensive carbon felt electrode, was insensitive to anode potential. Thus, it was feasible to conduct HMF hydrogenation to BHMF and oxidation to 2,5-furandicarboxylic acid (FDCA) in a single divided cell operated under cathode potential control. Electrocatalytic HMF conversion in the paired cell achieved high yields of BHMF and FDCA (85% and 98%, respectively) and a combined electron efficiency of 187%, corresponding to a nearly two-fold enhancement compared to the unpaired cells.
Electrocatalytic hydrogenation (ECH) of biomass-derived molecules has received significant attention in recent years.10–16 ECH is analogous to thermal catalytic hydrogenation, with the key difference that surface-adsorbed hydrogen atoms are generated electrochemically from water or proton reduction, rather than from H2 dissociation.17 In this way, the large kinetic barriers for H2 dissociation are avoided, therefore allowing ECH to proceed at mild temperatures and pressures and without the need for conventional hydrogenation catalysts (e.g., Pt, Pd, Ni). However, it can be challenging to obtain high selectivity and efficiency for the desired transformation because multiple reaction pathways and competing reactions may exist.14 The electrocatalytic hydrogen evolution reaction (HER) is operable on metal electrodes at cathodic potentials, and consumes adsorbed hydrogen atoms in competition with ECH. Additionally, carbonyl-containing substrates may undergo direct electroreduction and hydrodimerization reactions.18
Electrochemical half-reactions occur in pairs, yet many commercialized electrochemical processes only utilize one of the two electrodes for generation of desired products.19 For example, electrochemical reductions are typically paired with water oxidation as a benign counter reaction. However, water oxidation has sluggish kinetics and the oxygen gas produced is not valuable.20 Therefore, substantial gains in terms of economic feasibility and energy efficiency can be made by pairing two half-reactions that generate desired products in a single electrochemical cell.21–23 In this way, the theoretical maximum electron efficiency is 200%, two times greater than in conventional unpaired cells. Moreover, the paired electrolysis approach may lower capital and operation costs by reducing the number of reactors or processing steps required. Nevertheless, there are challenges arising from mismatched optimal current densities for the two half-reactions, chemical incompatibilities, and crossover issues.24
Utilizing renewable feedstocks is a pillar of green chemistry.25 One promising renewable molecule is 5-(hydroxymethyl)furfural (HMF), which is accessible from biomass through the dehydration of fructose or glucose, and is an important platform chemical that can be diversified into a variety of value-added chemicals and fuels.26–28 Electrocatalytic conversion has been recognized as a promising method for either HMF hydrogenation or oxidation to biobased monomers (Scheme 1).29 Specifically, the selective hydrogenation of HMF generates 2,5-bis(hydroxymethyl)furan (BHMF), which is an important precursor for production of polyesters and polyurethane foams.30 Selective oxidation of HMF generates 2,5-furandicarboxylic acid (FDCA). FDCA is a feedstock for production of polyethylene 2,5-furandicarboxylate (PEF), a biobased alternative to petroleum-derived polyethylene terephthalate.31
It was shown in two separate reports that electrocatalytic hydrogenation of HMF to BHMF and redox-mediated oxidation of HMF to FDCA can both be conducted in mildly basic electrolytes (i.e. pH 9.2) with good selectivity.13,32 Such conditions are well-suited for electrochemical conversion of biomass-derived chemicals including HMF, as those chemicals are typically unstable in very acidic or basic environments. Moreover, there is an opportunity to integrate the two HMF half-reactions in a paired electrochemical cell without additional issues related to electrolyte or HMF crossover. The overall cell reaction consumes only HMF, water, and electricity, and generate no waste products. Despite these benefits, there are only a few reports of paired electrocatalytic conversion of HMF in literature.33,34
Herein, we demonstrate the paired electrocatalytic conversion of HMF to BHMF and FDCA. A self-synthesized Ag/C catalyst facilitated HMF hydrogenation to BHMF with enhanced faradaic efficiency compared to a polycrystalline Ag electrode. We elucidated the contribution of the carbon black (CB) support material in Ag/C to the observed product selectivity; undesired HMF hydrodimerization occurred on CB at cathode potentials more negative than −1.2 V vs. Ag/AgCl. For Ag/C, operating at potentials more positive than −1.4 V vs. Ag/AgCl alleviated the impact of HMF reduction on CB; the optimal BHMF selectivity and faradaic efficiency was obtained at −1.3 V vs. Ag/AgCl. For the anode reaction, a homogeneous electrocatalyst, 4-acetamido-TEMPO (ACT, TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl), enabled indirect electrochemical HMF oxidation to FDCA with nearly 100% faradaic efficiency. The ACT-mediated HMF oxidation was conducted using an inexpensive carbon felt anode and mild electrolyte conditions (pH 9.2 buffer, same as catholyte). Electrocatalytic HMF conversion in a paired electrochemical cell achieved a combined electron efficiency to BHMF and FDCA of 187%, which corresponds to a nearly two-fold enhancement compared to the unpaired cells. The individual yields for BHMF and FDCA (85% and 98%, respectively) were similar to those in unpaired cells, indicating that the two half-reactions were compatible and proceeded without major complications or adverse effects. This approach satisfies many principles of green chemistry and demonstrates the feasibility of paired electrosynthesis for biomass conversion.
Cyclic voltammetry revealed that Ag/C, Ag-pc, and CB electrodes were active for HMF reduction (Fig. 2). The onset potentials for HMF reduction were −1.03 V, −1.05 V, and −1.21 V for Ag/C, Ag-pc, and CB, respectively, as defined herein as the potential at which background-corrected current density reached −0.1 mA cm−2. This suggests that the CB support material present in Ag/C may participate in HMF reduction at potentials more negative than −1.21 V. The peak currents densities for Ag/C and Ag-pc (−6.8 and −7.0 mA cm−2, respectively) were approximately two-fold higher than for CB (−3.3 mA cm−2). Koutecký–Levich analysis of the reduction waves indicated that the electron transfer number (n) with respect to HMF was approximately two for Ag-pc and Ag/C and one for CB (ESI, Fig. S10†). This indicates that different HMF reduction mechanisms may be operable for Ag-based electrodes than for CB electrodes. Fig. 2 also shows voltammograms measured in electrolytes without HMF, for which the reduction waves can be assigned to the hydrogen evolution reaction (HER). The HER current was negligible at potentials close to the onset of HMF reduction, but increased substantially at potentials more negative than about −1.4 V. This corresponds to the potential range at which a second reduction wave initiated for electrolytes containing HMF, suggesting that HMF reduction and HER may proceed concurrently at very negative potentials. The second reduction wave was less notable for CB, reflecting the lower HER activity for CB compared to Ag-based electrodes.
HMF reduction was studied using potential-controlled electrolysis over the range of −1.15 V to −1.5 V. Electrodes were prepared by drop-casting an ink dispersion of Ag/C onto a carbon paper substrate. The amount of Ag/C loaded onto the carbon paper was chosen to give similar Ag ECSA as the Ag-pc electrode, as determined by PbUPD stripping voltammetry (details in section 4.8). HMF reduction products for Ag/C and Ag-pc were evaluated in terms of selectivity and faradaic efficiency. The main HMF reduction products were BHMF from hydrogenation and BHH (5,5′-bis(hydroxymethyl)hydrofuroin) from hydrodimerization (Scheme 2). The product distribution was highly dependent on the cathode potential (Fig. 3). BHMF selectivity increased and BHH decreased with more negative potentials over the range of −1.15 V to −1.3 V. BHMF selectivity was notably higher for Ag/C than Ag-pc over this potential range; however, it decreased at more negative potentials for Ag/C, corresponding to increased formation of BHH and unidentified products. In contrast, BHMF selectivity continued to rise with more negative potentials for Ag-pc. Although, HER was active at those potentials and the faradaic efficiency for BHMF was lower (e.g. 73.7% at −1.5 V for Ag-pc). As a result, the optimal BHMF generation in terms of faradaic efficiency was obtained using Ag/C at −1.3 V, for which BHMF efficiency reached 96.2%. Stability tests were conducted by reusing one Ag/C electrode for four consecutive trials. HMF conversion rate and BHMF efficiency decreased slightly after the first trial but remained stable for the remaining trials (ESI, Fig. S11†). The initial drop in performance may be due to Ag particle agglomeration; larger Ag NPs were observed by TEM following the stability test (ESI, Fig. S12†). The ICP-AES analysis of the electrolyte solution showed that Ag leaching was negligible (i.e. less than detection limit of 0.6 ppb Ag in electrolyte) after two hours of HMF reduction.
HMF reduction was conducted using CB-modified carbon paper electrodes to decouple the contributions of Ag NPs and the CB support material to the observed product selectivities for Ag/C. HMF reduction products were detected at −1.2 V for CB electrodes, however HMF conversion was insufficient (i.e. <1%) for quantitative product analysis. Fig. 4 shows that HMF conversion rates were enhanced at more negative potentials and reached 50% at −1.5 V, which is comparable to the values for Ag/C and Ag-pc (i.e., 49% and 52%, respectively at −1.5 V). In sharp contrast to Ag-based electrodes, BHH was the major detected product for CB electrodes, and very little BHMF was generated (i.e. <3% selectivity). Also, HER was a minor contribution to the total charge passed (i.e. ∼2% faradaic efficiency at −1.5 V), reflecting the poor HER activity of CB.
The combined selectivities for BHH and BHMF were low (i.e. 32–35%), indicating that other products were generated from HMF reduction with CB electrodes. We detected several unknown products in the HPLC chromatographs and 1H NMR spectra, which were not identified or quantified in this work. As previously discussed, the Koutecký–Levich analysis indicated that HMF reduction on CB electrodes proceeds mainly by a one-electron transfer process. Therefore, we hypothesize that the unidentified species are dimer or oligomer byproducts of HMF hydrodimerization reactions, which consume one electron per HMF, rather than products of hydrogenation or hydrogenolysis reactions as suggested in an earlier report.37 These results show that CB electrodes were active for HMF reduction and generated substantial amounts of BHH and unidentified products starting around −1.3 V. On this basis, we attribute the lower BHMF and total selectivities for Ag/C compared to Ag-pc at −1.4 V and −1.5 V (Fig. 3) to the activity of the CB support material present in Ag/C.
Regarding the mechanism of HMF hydrogenation to BHMF in basic or neutral media, there is uncertainty whether the hydrogen source is water or surface adsorbed hydrogen (H*),11,13,29,37 as depicted in eqn (1) and (2), respectively.
HMF + 2H2O + 2e− → BHMF + 2OH− | (1) |
HMF + 2H* → BHMF | (2) |
The latter pathway is known as electrocatalytic hydrogenation (ECH).17 The electrochemical formation of H* (i.e. Volmer step) occurs by water reduction in basic or neutral media (eqn (3)),38 in which * designates a surface site.
H2O + * + e− → H* + OH− | (3) |
ECH reactions involve strong interactions between the electrode surface and reactants, and are therefore highly dependent on the nature of the electrode material.39 Kwon et al. hypothesized that HMF hydrogenation to BHMF in neutral media occurs directly by water molecules (eqn (1)) on the basis of the nearly identical onset potentials observed for a wide range of metallic electrodes.37 On the other hand, Roylance et al. studied HMF reduction on Ag-based electrodes in basic media (i.e. pH 9.2) and reported that H* was likely involved in BHMF formation within the potential region where HER, and therefore H* formation, were possible.13 Our viewpoint is that ECH was the major BHMF generation pathway for Ag-based electrodes under the conditions reported herein. This is self-consistent with the low BHMF selectivity we report for CB electrodes (Fig. 4), as carbon is known to have very weak H* adsorption,40 and is therefore not expected to facilitate ECH reactions. However, it should be noted that BHMF was a major product for Ag/C and Ag-pc electrodes even within the potential region where HER current was negligible (i.e. −1.15 V ≥ E ≥ −1.3 V). A recent density functional theory (DFT) study of Ag cathodes indicated that the lowest energy pathway for HER via water reduction is the Volmer–Heyrovsky sequence, and the Heyrovsky step is rate-limiting.41 Therefore, it is very plausible that HMF hydrogenation by H* (i.e. ECH) can proceed within a potential region where the Volmer step is facile but HER is kinetically-limited by the Heyrovsky step.
We found that the selectivity of HMF reduction to BHMF or BHH was dependent on the cathode potential for Ag-based electrodes; BHMF selectivity increased at more negative potentials down to −1.3 V for Ag/C and −1.5 V for Ag-pc (Fig. 3). The electrochemical Volmer step (eqn (3)) is accelerated at more negative potentials, so one explanation for the selectivity trend is that BHMF formation was promoted by higher H* availability. Additional experiments were performed with different initial HMF concentrations to gain more insight regarding BHMF and BHH selectivity. Ag-pc electrodes were used instead of Ag/C to avoid contributions of the CB support material to the observed product selectivity. Fig. 5 shows that BHMF selectivity at −1.2 V decreased from 81% to 41% when the HMF concentration was increased from 5.0 mM to 50 mM. It is likely that higher concentrations led to increased surface coverage of HMF, and that BHMF formation via ECH was less favorable due to insufficient availability of H* relative to adsorbed HMF. Accordingly, another reasonable explanation for the higher BHMF selectivity observed at more negative potentials (Fig. 3) would be that higher degrees of HMF conversion were achieved at those potentials, and therefore lower bulk HMF concentrations were present. However, we conducted an extended electrolysis at −1.2 V and found that BHMF selectivity (56%) was still notably lower than at −1.3 V (i.e. 85%), even though similar HMF conversion was reached (ESI, Table S1†).
As a heterogeneous process, electrochemical HMF reduction may be subject to external mass-transport limitations. In this way, the HMF concentration at the electrode surface may be significantly lower than in the bulk electrolyte, which could impact the selectivity of HMF reduction. Quasi-steady-state current densities were measured over a wide potential range using conditions (i.e., reactor geometry, electrode size, stirring rate) identical to electrolysis experiments. Fig. 6 shows the logarithm of current density versus potential (log(j) vs. E) for Ag/C and Ag-pc. For both electrodes, Tafel-like behaviour (i.e. linear log(j) vs. E relationship) was observed within the potential range of approximately −1.05 to −1.15 V, indicating that HMF reduction rate was limited by charge-transfer kinetics.42 Within this potential region, HMF reduction currents were 2–3 times higher on Ag/C than Ag-pc, even though the Ag ECSA values were very similar (i.e. 2.52 cm2 and 2.51 cm2 for Ag/C and Ag-pc, respectively). This suggests that Ag/C may have intrinsically higher catalytic activity for HMF reduction than bulk, polycrystalline Ag; however, definitive elucidation of nanoscale or particle size effects is beyond the scope of this work. At potentials more negative than about −1.15 V, the plots of log(j) vs. E (Fig. 6) for both electrodes deviated from Tafel-like behavior, indicative of mass-transport control. Almost completely mass-transport-limited behavior (i.e. a current “plateau”) was observed from about −1.30 V to −1.50 V, which corresponds to the optimal potential range for BHMF formation (Fig. 3). We previously showed that BHMF was favored at low HMF concentrations (Fig. 5), so we hypothesize that the higher BHMF selectivity observed at more negative potentials was at least partially derived from lower local HMF concentrations at the electrode surface resulting from mass-transport limitations. In the same way, the enhanced BHMF selectivity for Ag/C compared to Ag-pc at mild potentials (i.e. −1.15 V and −1.2 V, Fig. 3) may be due to the higher current densities for Ag/C compared to Ag-pc, which would result in more significant mass-transport limitations and lower local HMF concentrations.
An alternative approach is to use a homogeneous electrocatalyst to facilitate indirect electrochemical HMF oxidation.32 Organic nitroxyl radical catalysts, such as TEMPO and its derivatives, are widely used for selective oxidation of alcohols.44,45 In particular, 4-acetamido-TEMPO (ACT) has been identified as a very promising homogeneous electrocatalyst for alcohol oxidation, owing to its superior activity and lower cost compared to other TEMPO derivitatives.46Fig. 7a shows the cyclic voltammogram for ACT with a glassy carbon electrode in borate buffer electrolyte (pH 9.2). ACT exhibited reversible one-electron oxidation/reduction waves; ACT was oxidized to an oxoammonium cation (ACT+) on the anodic sweep and subsequently reduced back to a nitroxyl radical on the cathodic sweep. After addition of HMF to the ACT-containing electrolyte, the anodic current was increased. This is attributed to the regeneration of ACT following the reaction between HMF and ACT+ (Fig. 7b). The regeneration of ACT may occur either by the reoxidation of the ACT hydroxylamine (ACTH) or by the comproportionation of ACT+ and ACTH.47 The cathodic wave disappeared in the presence of HMF because ACT+ was consumed during the HMF oxidation reaction, and therefore not present to be electrochemically reduced. No HMF oxidation current was observed in electrolyte without ACT, demonstrating that non-mediated HMF oxidation was not operable under these conditions.
ACT-mediated HMF oxidation was conducted in an H-type cell using a carbon felt anode. In this system, HMF is oxidized through the non-electrochemical reaction with ACT+ in solution; therefore, HMF product selectivity is not directly dependent on the anode potential. Fig. 8a shows that HMF oxidation can proceed by two pathways, both leading to FDCA. Accordingly, the selectivity of ACT-mediated HMF oxidation to FDCA is mainly determined by the overall extent of reaction. This was demonstrated by performing the reaction at three different anode potentials while controlling the total amount of charge passed for each experiment to obtain the same extent of reaction (i.e. ∼50%). In this way, HMF oxidation product distribution was largely unaffected as anode potential was varied between 0.7 V and 0.9 V (Fig. 8b). High HMF conversion and selectivity to FDCA was achieved after ACT-mediated HMF oxidation at 0.7 V was run to completion (i.e. 72.2 C of charge passed), as shown in Fig. 8c. The apparent reaction sequence was HMF → 2,5-diformylfuran (DFF) → 5-formyl-2-furoic acid (FFCA) → FDCA. The final yield and faradaic efficiency for FDCA were about 97% and 98%, respectively.
Cathode: HMF + 2H2O + 2e− → BHMF + 2OH− | (4) |
Anode: HMF + 6OH− → FDCA + 6e− + 4H2O | (5) |
Overall: 4HMF + 2H2O → 3BHMF + FDCA | (6) |
Fig. 9a shows that the individual yields for BHMF and FDCA reached 85% and 98%, respectively, after 72.2 C of charge was passed in the paired cell. The yields were very similar to those achieved in separate (i.e. unpaired) cells, which are also shown in Fig. 9a. These results show that the two HMF half-reactions were compatible and proceeded without severe complications or adverse effects when paired in a single cell. A key feature of the paired electrolysis is that each transferred electron participates in the generation of two desired products (i.e. BHMF and FDCA). Accordingly, the combined electron efficiency to BHMF and FDCA was 187% for the paired cell, a nearly two-fold enhancement compared to the unpaired cells (Fig. 9b). This is one of the first demonstrations of paired HMF electrolysis and to the best of our knowledge is the highest reported combined electron efficiency for HMF conversion (ESI, Table S2†).
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Linear sweep voltammograms for HMF reduction were collected at various electrode rotation rates (ω, RPM) using a modulated speed rotator (Pine Research Instrumentation) for Koutecký–Levich analysis. Koutecký–Levich plots (1/j versus ω−0.5) were constructed using the background-corrected current density values at −1.5 V. The electron transfer number for HMF reduction, n, was extracted from the slope of the Koutecký–Levich plot, defined by eqn (8):
Slope = (0.201nFD2/3υ−1/6C)−1 | (8) |
ACT-mediated HMF oxidation was performed in an H-type cell with a similar configuration. The anode electrode was carbon felt with dimensions of 1.5 cm by 1.0 cm exposed to the electrolyte. Ag-pc served as the counter electrode to facilitate the hydrogen evolution reaction. Carbon felt electrodes were washed with acetonitrile to remove organic residues and then rinsed with deionized water before each use. Unless noted otherwise, the anode electrolyte volume was 12.5 mL and contained 10.0 mM HMF and 1.0 mM ACT. The anode electrolyte was purged with argon gas (99.999%, Airgas, Inc.) throughout the reaction, and was stirred with a small PTFE-coated stir bar at 500 rpm.
The paired HMF electrolysis was performed in an H-type cell that combined the cathode configuration previously described for HMF hydrogenation and the anode configuration described for HMF oxidation. The paired cell was operated by controlling the cathode potential to be −1.3 V.
![]() | (9) |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9gc02264c |
This journal is © The Royal Society of Chemistry 2019 |