Electrochemical Valorization of Furfural to Maleic Acid - ACS

Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Electrochemical Valorization of Furfural to Maleic Acid Stephen R. Kubota and Kyoung-Shin Choi* Department of Chemistry, University of Wisconsin−Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States

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S Supporting Information *

ABSTRACT: Maleic acid (MA) is a platform chemical used for various industrial processes. In this study, a new electrochemical oxidation method that can convert furfural, which is one of the most important C5 platform chemicals derived from cellulosic biomass, to MA is reported. This method can produce MA in aqueous media under ambient temperature and pressure without the use of oxidizing agents. The use of acidic media, which promotes opening of the furan ring during oxidation, was critical for the formation of MA. PbO2, MnO2, and Pt, which are stable in acidic solutions under strongly oxidizing potentials, were investigated as anodes for furfural oxidation. The results showed that PbO2 is the only catalyst that can convert furfural to MA as the major product. It was also revealed that the electrochemical conversion of furfural to MA proceeds with 2-furanol as an intermediate. KEYWORDS: Furfural, Maleic acid, Lead oxide, Electrochemical oxidation, Electrocatalysis, Biomass conversion

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INTRODUCTION Lignocellulosic biomass provides a great abundance of renewable, carbon-based compounds that can be converted into alternatives to fossil fuel-derived fuels and chemicals.1−5 For example, furfural is a C5 compound that is currently being produced on an industrial scale from biomass sources and has been identified as a platform molecule to synthesize a multitude of value-added products.1−3 Maleic acid (MA) is one such compound that can be made from furfural and is used in a variety of processes and applications including the manufacture of lubricant additives, resins, surface coatings, plasticizers, and pharmaceuticals.6,7 Currently, MA is industrially produced by the hydrolysis of maleic anhydride which is made by the oxidation of fossil-fuel derived chemicals.7 Due to the value of MA as a chemical intermediate for a variety of important industrial processes, strategies to replace fossil-fuel derived starting compounds with sustainable alternatives are very desirable. Previously reported strategies to convert furfural to MA include the use of gas-phase reactions,8,9 high pressures of O2,10−12 or chemical oxidants.13−15 In this study, we report a new electrochemical oxidation process for the conversion of furfural to MA, which provides the benefits of being able to perform the oxidation in aqueous solutions under ambient temperatures and pressures. Because the oxidation is driven by an applied electrochemical potential, the need for a stoichiometric amount of chemical oxidants is removed.16−19 Additionally, the electrons generated from the oxidation of furfural can be used for valuable, controllable reduction reactions, such as the reduction of water to H2.16 The considerable and continuing decrease in the cost of electricity provided by renewable energy sources further encourages the © XXXX American Chemical Society

development of electrochemical processes for biomass conversion.20 The conversion of furfural to MA requires an eight electron oxidation and opening of the furan ring (Figure 1a). A simpler

Figure 1. Balanced equations for the oxidation of furfural to (a) MA and (b) furoic acid.

and more straightforward oxidation of furfural is a two electron oxidation that does not involve ring opening, which results in the formation of furoic acid (Figure 1b). Electrochemical oxidation of furfural to furoic acid has been observed in basic aqueous conditions.21−23 Since opening of the furan ring has been reported to be facilitated in acidic conditions,24−27 we aimed to investigate electrochemical oxidation of furfural in acidic media to promote the formation of MA. Since not many materials are stable in acidic media under highly oxidizing potentials, the materials that can be used as an anode are very limited. Furthermore, since water oxidation can Received: June 8, 2018 Revised: June 20, 2018

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DOI: 10.1021/acssuschemeng.8b02698 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

Figure 2. LSVs of (a) a PbO2 anode, (b) a MnO2 anode, and (c) a Pt anode obtained in a pH 1 H2SO4 solution without (black) and with (red) 10 mM furfural at a scan rate of 5 mV s−1.

Pt working electrode (WE) along with a Ag/AgCl (4 M KCl) reference electrode (RE), and the cathode compartment contained 15 mL of a pH 1 H2SO4 solution with a submersed Pt counter electrode (CE). The ideal half-reaction at each electrode and overall reaction are summarized below (eqs 1−3).

be a major competing reaction with the oxidation of furfural in aqueous solutions, materials that are known to be highly catalytic for water oxidation may not be favorable for achieving a high Faradaic efficiency (FE) for furfural oxidation. Based on these considerations, we selected PbO2 as the most promising candidate; PbO2 is stable in acidic media under strongly oxidizing potentials28 while not being catalytic for water oxidation. In this study, we investigated the performance of a PbO2 anode for electrochemical oxidation of furfural in comparison with MnO2 and Pt anodes that are also stable in acidic media under strongly oxidizing potentials.28

Anode reaction: Furfural + 4H 2O → MA + CO2 + 8H+ + 8e− Cathode reaction: 8H+ + 8e− → 4H 2

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(1) (2)

Overall reaction: Furfural + 4H 2O → MA + CO2 + 4H 2

RESULTS AND DISCUSSION The PbO2 and MnO2 electrodes used in this study were prepared by electrodeposition while the Pt electrode was prepared by sputter coating. The synthesis and characterization details can be found in the Supporting Information (Figures S1−3). The electrocatalytic activity of PbO2 for furfural oxidation was first examined by linear sweep voltammetry (LSV) in a pH 1 H2SO4 solution with and without 10 mM furfural (Figure 2a). The anodic current onset at around 1.85 V vs RHE observed in the solution that does not contain furfural is due to water oxidation. When furfural is present, an earlier onset at around 1.60 V was observed for furfural oxidation, indicating that furfural oxidation is favorable over water oxidation on PbO2. We also obtained LSVs under the same conditions using MnO2 and Pt as the anode (Figure 2b,c). The MnO2 anode also displayed an earlier onset when furfural was added. In fact, the onset potential of MnO2 for furfural oxidation was more negative than that of PbO2. However, since MnO2 is much more catalytic for water oxidation than PbO2, its onset potential for water oxidation was also more negative than that of PbO2. As a result, MnO2 has a narrower potential window between furfural and water oxidation onsets. This means that while PbO2 may require a more positive applied potential to oxidize furfural, PbO2 may have a higher chance to oxidize furfural without inducing substantial water oxidation, thus achieving a higher FE for furfural oxidation. Unlike PbO2 and MnO2, Pt showed a delayed onset and decreased current density when furfural was added to solution. This behavior indicates that furfural oxidation is less favored over water oxidation on Pt. To determine the oxidation products of furfural oxidation, constant potential oxidation of furfural was performed in a divided cell where the anode compartment was separated from the cathode compartment by a fine glass frit. The anode compartment contained 15 mL of a pH 1 H2SO4 solution containing 10 mM furfural with a submersed PbO2, MnO2, or

(3)

The conversion of furfural and the yield of furfural oxidation products were monitored during the oxidation reaction by high-performance liquid chromatography (HPLC). The furfural conversion, yields of oxidation products, and FE for the oxidation products were calculated from eqs 4−6, and the results obtained using the PbO2, MnO2, and Pt anode are summarized in Figure 3 and Tables S1−S3. Furfural conversion (%) mol of furfural consumed = × 100% mol of initial furfural Product yield (%) =

mol of product formed × 100% mol of initial furfural

Faradaic efficiency (FE)(%) charge used for product formation = × 100% total charge passed

(4)

(5)

(6)

The stoichiometric charge required to convert furfural in 15 mL of a 10 mM furfural solution to MA is 115.8 C. Figure 3a shows the furfural conversion and product yields by PbO2 monitored while passing 250 C at 2.0 V vs RHE. The result shows that PbO2 can convert furfural to MA using 2-furanol as an intermediate product. The conversion of furfural to 2furanol is a four electron oxidation process that involves the loss of one carbon from the furfural in the form of CO2 (Figure 4a). The conversion of 2-furanol to MA requires an additional four electron oxidation with opening of the furan ring (Figure 4b). The substantial accumulation of 2-furanol before the conversion of 2-furanol to MA suggests that the rate for the oxidation of furfural to 2-furanol are faster than the rate for the oxidation of 2-furanol to MA. We note that furoic acid, which can be obtained by a twoelectron oxidation of the aldehyde group to a carboxylic acid B


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ACS Sustainable Chemistry & Engineering

Figure 4. Balanced equations for the oxidation of (a) furfural to 2furanol and (b) 2-furanol to MA.

constant potential oxidation of furoic acid at 2.0 V vs RHE yielded both 2-furanol and MA along with other unidentifiable products (Table S4). These results suggest that although furoic acid was not detected during furfural oxidation, the possibility that furoic acid may be produced and then immediately further oxidized on the PbO2 anode cannot be excluded. At 250 C charge passed, a yield of 65.1% MA was achieved for PbO2 with 20.2% 2-furanol still remaining in solution (Figure 3a and Table S1). Passing additional charge resulted in a decrease of 2-furanol but not a corresponding increase in MA although no other product was detected. We note that according to the LSV shown in Figure 2a, water oxidation is also possible on PbO2 at 2.0 V vs RHE. (Furfural oxidation at a potential more negative than 2.0 V vs RHE resulted in a very low current density, making it impossible to practically complete the reaction.) Therefore, although furfural oxidation is dominant in the beginning of the reaction, during the last stages of oxidizing 2-furanol, when not much 2-furanol is available in solution, water oxidation becomes a dominant reaction. We suspect that the oxidation of 2-furanol carried out under dominant water oxidation may have resulted in the formation of an undesirable side product that is not detectable by HPLC (i.e., humins). Future work can aim to optimize the synthesis of the PbO2 anode by either improving the morphology, such as increasing the surface area, or tuning the composition, such as incorporating dopants, so that furfural oxidation can be achieved more efficiently at potentials that are more negative than the onset potential for water oxidation. Furfural oxidation on MnO2 was conducted at 1.8 V vs RHE instead of 2.0 V vs RHE because the current density generated by MnO2 at 1.8 V vs RHE was comparable to that generated by PbO2 at 2.0 V vs RHE, as can be predicted by their LSVs (Figure 2a,b). Furthermore, at 2.0 V vs RHE furfural oxidation was significantly suppressed on MnO2 due to dominant water oxidation. Even at 1.8 V vs RHE, MnO2 showed water oxidation as a considerable competing reaction, and 23.7% furfural remained unconverted after passing 100 C (Figure 3b). For comparison, nearly 100% conversion of furfural was achieved at 100 C with PbO2 (Figure 3a). The yield of MA achieved by MnO2 at 250 C was only 18.1% with 29.0% 2furanol remaining. Unlike with PbO2, the formation and then disappearance of a small amount of furoic acid were observed. This shows that on MnO2 furoic acid definitively serves as an intermediate that is consumed for further reactions. We note that the sum of the product yields (yields of MA, 2-furanol, and furoic acid) is significantly lower than that of the furfural conversion. For example, this discrepancy, referred to as “furfural missing” in Table S2, was 51.2% at 250 C. We postulate that this is likely due to undesirable polymerization reactions, resulting in the formation of humins that cannot be detected. As previously mentioned, we postulate that

Figure 3. Conversion of furfural (%) and yield (%) of oxidation products obtained using (a) PbO2 anodes at 2.0 V vs RHE, (b) MnO2 anodes at 1.8 V vs RHE, and (c) Pt anodes at 2.2 V vs RHE during the course of electrochemical oxidation of furfural in a 15 mL pH 1 H2SO4 solution containing 10 mM furfural over various amounts of charge passed.

group (Figure 1b), was not detected at all. However, it may be possible that furoic acid is an extremely short-lived intermediate that can be rapidly oxidized to the next intermediate. Therefore, we investigated the oxidaiton of fuoric acid using a pH 1 H2SO4 solution containing 10 mM furoic acid. Indeed, when a LSV of a PbO2 anode was performed in this solution, the current density was larger than that of the LSV obtained with 10 mM furfural in the potential region where water oxidation is not dominant (Figure S4). This indicates that the oxidation of furoic acid occurs more readily than the oxidation of furfural on PbO2. Aditionally, C


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concurrent oxidation of furfural or furfural intermediates with water oxidation may increase the formation of humins. Therefore, on MnO2, where water oxidation is dominant at all times, more humins can be formed, resulting in considerable furfural missing. Nonselective furfural oxidation and significant furfural missing makes MnO2 an undesirable electrocatalyst for furfural oxidation in acidic solutions. Finally, furfural oxidation on Pt was conducted at 2.2 V vs RHE. Furfural oxidation at a potential more negative than 2.2 V vs RHE did not generate a sufficient current density to complete the reaction as can be predicted by its LSV (Figure 2c). Pt showed the worst furfural oxidation ability of the three materials examined (Figure 3c). The majority of all furfural converted resulted in the formation of undetectable humins, resulting in 88.7% missing furfural at 250 C. The yields for MA and 2-furanol were only 5.2% and 6.0%, respectively, at 250 C (Table S3). We additionally investigated the performance of PbO2 for furfural oxidation in pH 7 and pH 13 solutions to examine how critical the acidic pH is for the conversion of furfural to MA on PbO2. LSVs in pH 7 and 14 solutions showed little difference between LSVs obtained with and without furfural, indicating that furfural oxidation is not favored over water oxidation on PbO2 in neutral and basic solutions (Figure S5). Constant potential oxidation of furfural in these solutions showed that the furfural conversion at 100 C was only about 10−20%, and the majority of the current was used for water oxidation (Table S5). Only negligible amounts (yield < 2%) of 2-furanol, furoic acid, and MA were produced in a pH 7 solution at 100 C while furoic acid was obtained as the dominant product with a yield of 4.1% in a pH 13 solution at 100 C. This result confirmed that the efficient and selective electrochemical conversion of furfural to MA is possible by the combination of PbO2 and an acidic condition. In summary, we developed a new electrochemical method to convert furfural to MA. In order to simultaneously achieve oxidation of furfural and opening of the furan ring to form MA, the use of acidic solutions was critical. Therefore, PbO2, MnO2, and Pt were explored as acid-stable anodes for furfural oxidation. PbO2 displayed the best performance with a MA yield of 65.1%. 2-Furanol was identified as an intermediate. It is expected that optimizations of PbO2 to enhance its current density for furfural oxidation at potentials below the onset potential for water oxidation will further increase the selectivity for MA production. MnO2 and Pt were found to be unsuitable as anodes for furfural oxidation because dominant water oxidation on these electrodes not only made furfural oxidation inefficient but also facilitated the conversion of furfural to undesirable and undetectable furfural products (i.e., humins).

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Kyoung-Shin Choi: 0000-0003-1945-8794 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by University of Wisconsin− Madison.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02698. Materials and methods, SEM, XRD, and additional LSVs

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REFERENCES

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of PbO2, and electrolysis data (PDF)

AUTHOR INFORMATION

Corresponding Author

*K.-S. Choi. E-mail: [email protected]. D


Letter

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Electrochemical Valorization of Furfural to Maleic Acid - ACS

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