Electrocatalytic Conversion of Furanic Compounds - ACS Catalysis

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Electrocatalytic Conversion of Furanic Compounds Youngkook Kwon, Klaas Jan P Schouten, Jan Cornelis van der Waal, Ed de Jong, and Marc T.M. Koper ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01861 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 17, 2016

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ACS Catalysis

Electrocatalytic Conversion of Furanic Compounds Youngkook Kwon1,2, Klaas Jan P. Schouten1,3, Jan C. van der Waal3, Ed de Jong3, and Marc T.M. Koper1* 1. Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands 2. Carbon Resources Institute, Korea Research Institute of Chemical Technology, Daejeon 34114, Republic of Korea 3. Avantium Chemicals, Zekeringstraat 29, 1014 BV Amsterdam, The Netherlands

ABSTRACT The electrocatalytic conversion of furanic compounds i.e. mainly furfural and 5hydroxymethylfurfural, has recently emerged as a potentially scalable technology for both oxidation and hydrogenation processes because of its highly valuable products. However, its practical application in industry is currently limited by low catalytic activity and product selectivity. Thus, a better understanding of the catalytic reactions as well as a strategy for the catalyst design can bring solutions for a complete and selective conversion into desired products. In this perspective, we review the status and challenges of electrocatalytic oxidation and hydrogenation of furanic compounds including thermodynamics, voltammetric studies, and bulk electrolysis with important reaction parameters (i.e. catalyst, electrolyte, temperature etc.) and reaction mechanisms. In addition, we introduce ways of energy efficient electrocatalytic furanic synthesis by combining yields of anodic and cathodic reactions in a paired reactor or a reactor powered by a renewable energy source (i.e. solar energy). Current challenges and future opportunities are also discussed, aiming at the industrial applications. Keywords: furfural · hydroxymethylfurfural · electrocatalysis · oxidation · hydrogenation

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∗

To whom all correspondence should be addressed. E-mail: [email protected]

INTRODUCTION The production of chemicals, fuels, and solvents directly from biomass is of emerging interest to reduce the dependence on petroleum-based resources in light of increasing environmental, economic, and political challenges.1,2 Furfural (C5H4O2) and 5hydroxymethylfurfural (HMF, C6H6O3), representative furanic compounds generated by acid-catalyzed dehydration of pentose (C5) and hexose (C6) sugars, respectively, have potential as alternative commodity chemicals to fossil-fuel-based platform chemicals through oxidation, dehydration, and hydrogenation processes of functionalities (-C=O on furfural, -OH and –C=O on HMF) attached to their furan ring. On industrial scale, furfural has been produced ca. 2.5x105 ton/yr by hydrolysis and dehydration of agricultural by-products3,4 or by the cyclo-dehydration of xylose on acid catalysts4-7. These conventional processes can be improved by adding metal chloride salts to an acid solution, which promotes the formation of 1,2-enediol of xylose, thus favoring the subsequent acid catalysed dehydration to furfural.8 Recent advances in furfural synthesis have been reported in biphasic reactors with microwave heating (water/MIBK, 85% furfural yield estimated)6, H-mordenite (water/toluene, 98% furfural yield at 98% conversion of xylose)9 and one-pot conversion with H-USY catalyst (water, 12% furfural from hemicellulose)10. HMF has been known for over 100 years as one of the most promising platform chemicals11 and many researchers have demonstrated the attractive conversion of fructose to HMF in a monophasic systems i.e. water, organic solvents (methanol, DMSO)11,12, in ionic liquids13-16, and in a biphasic systems (i.e. water/MIBK)17 by using acid catalysts (i.e. HCl, H2SO4)18,19, solid acids (i.e. zeolites17, ion exchange resins20), or salts (i.e. LaCl3, CrCl2)21,22. However, at present, the industrial scale production of HMF is not yet established and therefore HMF is still very

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costly. Avantium, a Clean Tech Top 100 company, has announced the intention to scaleup the production of HMF and the HMF derivative methoxymethylfurfural (MMF) in the near future in a joint venture with BASF. Both HMF and MMF can be subsequently converted into 2,5-furandicarboxylic acid (FDCA)23,24, which is a bulk chemical to make a renewable polymer with properties superior to those of the PET polyesters.11,2527

Furfural, as illustrated in Scheme 1a, is the starting material for furfuryl alcohol and furoic acid, which are intermediates in pharmaceutical and polymer industry,25-27 and for 2-methylfuran (MF), a promising biofuel for ignition engines.28,29 Other furfural derivatives such furfuryl ethyl ether also show great promise as fuel additive.30 HMF, as shown in Scheme 1b, oxidation mainly targets FDCA and HMF hydrogenation targets specifically two chemicals; 2,5-dihydroxymethyl-tetrahydrofuran (DHMTHF) and 2,5dimethylfuran (DMF). DHMTHF finds applications as a solvent and a building block in polymer synthesis and DMF is a potential transportation fuel with a higher energy density (by 40%) and a higher boiling point (by 20 K) than ethanol, and a better miscibility with apolar petro-based fuels.31-33 Dumesic and co-workers33 introduced a process of HMF conversion into DMF over a CuRu catalyst and Schüth and coworkers2 reported 100% conversion of HMF within 10 min and 98% yield to DMF after 2 h over a PtCo catalyst. DMF is also an interesting diene in Diels-Alder reactions towards substituted benzenes such as terephthalic acid.11 These catalytic processes retain a reasonable proportion of the original chemical complexity of furfural and HMF, which is different from bioethanol fermented from sugars, and have the potential to generate high-tonnage chemicals.34

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(a) O O OH

- 2H + H2O

furoic acid (FA)

O

O

+ 2H

O Furfural C5H4O2

furfuryl alcohol (FAL)

- H2O

+ 2H - H2O

O

Hemicellulose (xylans)

OH

tetrahydrofurfuryl alcohol (THFA)

+ 2H - H2O

+ 4H

Xylose C5H10O5

O

+ 4H

OH

O

+ 4H

2-methylfuran (2-MF)

tetrahydromethylfuran (THMF)

(b) O O

O

- 2H

O HO

2,5-diformyl furan (DFF)

O

+ 2H

O

HO

5-HMF C6H6O3

O

Cellulose

2,5-dimethylfuran (DMF)

O

O HO

+ 4H - 2H2O

+ 2H

O

O

O

- H 2O

5-methylfurfural (5-MF)

- 2H

- 2H + H2O

+ 4H

O

O

5-formyl-furan carboxylic acid (FFCA)

OH

2,5-dihydroxymethyltetrahydrofuran (DHMTHF)

- H 2O

- 2H + H2O

O HO

O HO

- H2O

Fructose (Glucose) C6H12O6

O

+ 4H

2,5-dihydroxymethylfuran (DHMF)

+ 2H

- 2H + H2O

OH

O OH

2,5-furan dicarboxylic acid (FDCA)

HO

O OH

5-hydroxymethyl-2-furan carboxylic acid (HMFCA)

2,5-dimethyl-2,3-dihydrofuran (DMDHF)

+ 2H

O

2,5-dimethyltetrahydrofuran (DMTHF)

Scheme 1. Possible catalytic oxidation (blue) and hydrogenation (red) pathways of (a) furfural and (b) HMF.

In addition to the conventional catalytic conversion processes of furanic compounds operated at high temperature and pressure, electrocatalytic oxidation and reduction (redox) of furanic compounds is currently a topic of significant interest as a promising future green methodology, on the premise the electricity is generated from sustainable origins such as solar, wind, geothermal etc. There are several potential advantages of electrocatalytic furanic synthesis over other conventional methods in biomass conversion35-38:

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1) H2O is the proton source instead of high cost H2 gas, 2) low operating temperatures and pressures, 3) precise control of the reaction rate and product selectivity by applying carefully chosen potential or current, 4) the reactivity of the molecules of interest can be studied easily by simple and quick voltammetric techniques i.e. cyclic voltammetry (CV), 5) oxidation and hydrogenation products can be produced simultaneously in a continuous membrane reactor (i.e. paired electrolysis, Figure 1).

For instance, combining yields of anodic and cathodic reactions of furanic compounds in a reactor powered by a renewable energy source (i.e. solar, wind energy) would have the potential to double the energy efficiency in comparison with having a water splitting reaction as a counter reaction, which generates only a single useful product (i.e. hydrogen) at cathode. The design of the paired electrolysis cell is close to that of polymer electrolyte membrane (PEM) fuel cells and water electrolyzers, the developments of which can be used for a better design and eventually lower the costs of the reactor.

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Figure 1. Continuous electrocatalytic membrane reactor schematic for the electrocatalytic furfural/HMF oxidation at anode and hydrogenation at cathode powered by a renewable energy source.

The electrocatalytic conversion of furanic compounds is at an initial but growing stage and a better understanding of the catalytic reactions as well as a strategy for the catalyst design can bring solutions for a complete and selective conversion into desired products. In this perspective, the status and challenges of electrocatalytic oxidation and hydrogenation of furanic compounds will be introduced. Specifically, the thermodynamics of the conversion of furanic compounds will be provided together with their corresponding electrochemical equilibrium potentials, followed by a review of fundamental voltammetric studies on redox reactions of furanic compounds in a half or a paired electrolysis cell. This review will discuss important reaction parameters (i.e. catalyst, electrolyte, temperature etc.) and the reaction mechanisms. Current challenges and future research direction will also be discussed, aiming at the industrial application.

THERMODYNAMICS A better understanding of the thermodynamics of the oxidation and reduction of furanic compounds will help steer research directions towards specific biofuels and/or chemical building blocks. Verevkin et al.39 reported the Gibbs free energies of the product formation from HMF obtained by ab initio calculation, in good agreement with the thermochemical experimental results. Gibbs free energies (kJ mol-1) were calculated for the conversion of the furanic compounds in Figure 2. They can be converted into a standard equilibrium potential E0 (vs. NHE) by:

∆Go = - nFEo

6


(1)

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where ∆Go = Gibbs free energy of the reduction reaction (kJ mol-1); n = number of electrons; F = charge on one mole of electrons, 96,485 C mol-1; Eo = standard redox potential of the corresponding redox couple vs. the Normal Hydrogen Electrode (NHE). We distinguish between H addition/subtraction and O addition/subtraction reactions (as all reactions in Figure 2 are either H addition/subtraction or O addition/subtractions) and write the corresponding half-cell reactions. For hydrogen addition/subtraction, A + H2 ↔ AH2 (∆GHadd = −∆GHsub)

(2)

the corresponding half-cell reaction is A + 2 H+ + 2 e- ↔ AH2

(3)

with standard redox potential

EoA,2H+/AH2 = −∆GHadd / 2F

(vs. NHE)

(4)

For oxygen addition/subtraction, A-H + 1/2 O2 ↔ A-OH (∆GOadd = −∆GOsub)

(5)

we rewrite the reaction as a hydrogenolysis reaction A-H + H2O ↔ A-OH + H2

(∆GOadd + ∆GH2Osplit)

(6)

where ∆GH2Osplit is the ∆G of the water splitting reaction H2O → H2 + 1/2 O2, i.e. 237.2 kJ/mol, so that the corresponding half-cell reaction is A-H + H2O ↔ A-OH + 2 H+ + 2 e-

(7)

with standard redox potential

EoA-H,H2O/A-OH,2H+ = (∆GOadd + ∆GH2Osplit) / 2F

(vs. NHE)

(8)

Based on Eqn. (1~8) and the Gibbs free energies reported by Verevkin et al., the standard redox potentials for HMF conversion have been calculated and summarized in Table 1. We note that in the Gibbs free energy calculations, the solvation effects on the 7


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molecules are ignored because Verevkin et al. reported the ∆G’s in ref. 39 as gas-phase values. However, the solvation contributions roughly cancel (i.e. A + 2 H+ + 2 e- ↔ AH2) since solvation energy is presumably more or less the same for A and AH2. Hence we expect the values in Table 1 to be reasonably accurate indicators of the oxidative and reductive power of the corresponding redox couples. O HO

O

H2 ∆rGo1

O HO

OH

5-HMF

O

2 H2, -2 H2O ∆rGo2

DHMF

O

2 H2 ∆rGo3

DMTHF

DMF

O O HO

0.5 O2, -H2O ∆rGo4

O

O O

O

5-HMF

0.5 O2 ∆rGo5

DFF

O

O O

HO

O

0.5 O2 ∆rGo6

O HO

OH FDCA

FFCA

Figure 2. Reaction scheme for the HMF conversion. Hydrogenation reactions to 2,5dihydroxymethylfuran (DHMF), 2,5-dimethylfuran (DMF), and 2,5-dimethyltetrahydrofuran (DMTHF) and oxidation reactions to 2,5-diformylfuran (DFF), 2-formyl-5-carboxyfuran (FFCA), and 2,5-dicarboxyfuran (FDCA).39

Table 1. Gibbs free energy for the oxidation (in kJ mol-1) and hydrogenation of HMF39 and calculated standard cell potentials (V vs. NHE) Hydrogenation

Half-Cell Reaction

ΔrGo1add

-23.0

E

1

0.12

HMF + 2 H+ + 2 e- ↔ DHMF

ΔrGo2 add

-206.7

Eo2

1.76

DHMF + 4 H+ + 4 e- ↔ DMF + 2 H2O

ΔrGo3 add

-59.8

Eo3

0.15

DMF + 4 H+ + 4 e- ↔ DMTHF

o

Oxidation

Half-Cell Reaction

ΔrGo4 add

-249.4

Eo4

-1.29

HMF ↔ DFF + 2 H+ + 2 e-

ΔrGo5 add

-315.7

Eo5

0.41

DFF + H2O ↔ FFCA + 2 H+ + 2 e-

ΔrGo6 add

-320.6

Eo6

0.43

FFCA + H2O ↔ FDCA + 2 H+ + 2 e-

Huber and co-workers35 reported Gibbs free energy and standard cell potentials for the electrocatalytic furfural hydrogenation as presented in Table 2. Especially, FAL hydrogenation to 2-MF (∆rGo3) and THFA hydrogenation to THMF (∆rGo5) involve H2O so that standard redox potentials are recalculated based on hydrogenolysis reaction in Eqn. (6~8). The reaction scheme is illustrated in Figure 3. The hydrogenolysis of the 8


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ACS Catalysis

hydroxyl in FAL to form 2-MF is expected to be favourable compared to the hydrogenation of the furan ring to form THFA. In addition, hydrogenation of the nonconjugated hydroxyl group in THFA is more favourable than hydrogenating the furan ring in 2-MF to form THMF. O

H2 ∆rGo1

O Furfural

O OH

O

2 H2 ∆rGo2

OH THFA

FAL H2, - H2O

∆rGo3

O

H2, - H2O

∆rGo5

O

2 H2 ∆rGo4

2-MF

THMF

Figure 3. Reaction scheme for the hydrogenation of furfural.35

Table 2. Gibbs free energy (in kJ mol-1) and standard cell potentials (V vs. NHE) for the hydrogenation of furfural35 Hydrogenation

Half-Cell Reaction

ΔrGo1add

-35.95

E

1

0.19

Furfural + 2 H+ + 2 e- ↔ FAL

ΔrGo2 add

-80.97

Eo2

0.21

FAL + 4 H+ + 4 e- ↔ THFA

ΔrGo3 add

-25.27

Eo3

1.36

FAL + 2 H+ + 2 e- ↔ 2-MF + H2O

ΔrGo4 add

104.78

Eo4

-0.27

2-MF + 4 H+ + 4 e- ↔ THMF

ΔrGo5 add

160.48

Eo5

0.40

THFA + 2 H+ + 2 e- ↔ THMF + H2O

o

REDOX REACTIONS OF FURFURAL AND 5-HMF Hydrogenation of Furfural and HMF We have reported the electrocatalytic hydrogenation of HMF on a large number of pure solid metal electrodes in neutral (0.1 M Na2SO4, Figure 4) and acidic (0.5 M H2SO4, Figure 5) solutions by correlating voltammetry with on-line HPLC product analysis.40,41 These works provided ample insights into active catalysts in combination with

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electrolyte pH, leading to some key understandings. The onset potential for HER discussed here is defined at the potential where the cathodic current is -0.5 mA cm-2 and the onset potential for product formation is defined as the least negative potential for which a product is detected. Based on the observed experimental results in Figure 4,40 three groups of catalysts are distinguished in neutral condition: (1) metals mainly forming DHMF in Figure 4a and 4d (Fe, Ni, Ag, Zn, Cd and In), (2) metals forming DHMF and hydrogenolysis products depending on the applied potential in Figure 4b and 4e (Pd, Al, Bi, and Pb), and (3) metals forming mainly hydrogenolysis products in Figure 4c and 4f (Co, Au, Cu, Sn, and Sb). For 50 mM HMF in 0.1 M Na2SO4, all catalysts exhibit a very similar onset potential (-0.5±0.2 VRHE, also see Figure 6), attributed to an activation energy that is not strongly influenced by the nature of the unmodified catalysts. However, the nature of the catalysts influences significantly the final reaction product. Ag shows the highest activity among all metals towards DHMF formation (up to 13.1 mM cm-2 with a high selectivity of >85%), which is consistent with recent work reported by Choi and co-workers36.

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0

0

Pd Al

2

-1.2

-0.8

-0.4

0.0

0

-1.2

-0.8

-0.4

0.0

-1.6 0

-10

-20

Bi Pb

0 -1.5

-1.0

-0.5

-0.4

0.0

(f)

0.0

-10

-20

Sn Sb

6 3 0 -2.0

E / V (vs RHE)

-1.5

-1.0

-0.5

-2

Bi, DHMF Bi, others Pb, DHMF Pb, others

9

C products (mM cm )

-30

-2

-2

C products (mM cm )

-30 Zn, DHMF Zn, others Cd, DHMF Cd, others In, DHMF In, others

-0.8

-2

(e) j / mA cm

Zn Cd In

-2.0

-1.2

E / V (vs RHE)

-2

j / mA cm

-2

-10

5

1

E / V (vs RHE)

(d)

10

2

0

-1.6

E / V (vs RHE)

-20

Co, DHMF Co, others Au, DHMF Au, others Cu, DHMF Cu, others

-2

1

0

-1.6

Co Au Cu

-20

C products (mM cm )

0

-10

-30 3 Pd, DHMF Pd, others Al, DHMF Al, others

-2

5

(c)

-2

-20

C products (mM cm )

-2

C products (mM cm )

-10

-30 3 Fe, DHMF Fe, others Ni, DHMF Ni, others Ag, DHMF Ag, others

10

0

j / mA cm

j / mA cm

-2

j / mA cm

Fe Ni Ag

-20

-30 15

j / mA cm

0

(b)

-2

(a)

-10

C products (mM cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.0

3

Sn, DHMF Sn, others Sb, DHMF Sb, others

2

1

0 -2.0

E / V (vs RHE)

-1.5

-1.0

-0.5

0.0

E / V (vs RHE)

Figure 4. Electrocatalytic HMF (50 mM) reduction on metal catalysts in 0.1 M Na2SO4. Upper panels: current-density profiles (upper panels) with (solid line) and without (dashed line) HMF in the solution during linear sweep voltammetry with a scan rate of 1 mVs-1. Lower panels: concentration profiles of the corresponding reaction products, that is, DHMF and others, as a function of potential.40

In acidic solution (0.5 M H2SO4), three soluble products from HMF hydrogenation

are

distinguished:

dihydroxymethyltetrahydro-furan

2,5-dihydroxymethylfuran

(DHMTHF),

and

(DHMF),

2,5-

2,5-dimethyl-2,3-dihydrofuran

(DMDHF)41, as shown in Figure 5. Based on the dominant reaction products, the metal catalysts are divided into three groups: (1) metals mainly forming DHMF in Figure 5a and 5d (Fe, Ni, Cu, and Pb), (2) metals forming DHMF and DMDHF depending on the 11


ACS Catalysis

applied potentials in Figure 5b and 5e (Co, Ag, Au, Cd, Sb, and Bi), and (3) metals forming mainly DMDHF in Figure 5c and 5f (Pd, Pt, Al, Zn, In, and Sb). Among the pure metals, Ni and Sb are the most active catalysts for DHMF (0.95 mM cm-2 at ca. 0.35 VRHE and -20 mA cm-2) and DMDHF (0.7 mM cm-2 at -0.6 VRHE and -5 mA cm-2), respectively.41 0

Fe, DHMTHF Ni, DHMTHF Cu, DHMTHF

0.0 -0.3

0.0 Co, DHMTHF Ag, DHMTHF Au, DHMTHF

0.1

0.0 -0.9

0.0

-0.3

Pb -20

Pb, DHMF Pb, DMDHF

1.0

0.5

-2

0.0

C products / mM cm

-2

-30 1.5

C / mM cm

Pb, DHMTHF

0.0 -0.5

E / V (vs RHE)

Pd, DHMTHF Pt, DHMTHF Al, DHMTHF

0.1

0.0

0.0

-0.6

-0.3

0.0

j / mA cm

E / V (vs RHE) 0

(e)

-10

Cd Sn Bi

-20

-30 0.8

Cd, DHMF Cd, DMDHF Sn, DHMF Sn, DMDHF Bi, DHMF Bi, DMDHF

0.4

0.0 Cd, DHMTHF Sn, DHMTHF Bi, DHMTHF

0.2

0.1

0.0 -1.0

0.0

-0.9

j / mA cm

-10

-1.5

0.2

0.0

-2

j / mA cm

-2

j / mA cm

-2

C products / mM cm

-0.6

Pd, DHMF Pd, DMDHF Pt, DHMF Pt, DMDHF Al, DHMF Al, DMDHF

0.4

(f)

-2

0

(d)

0.1

Pd Pt Al

-40

E / V (vs RHE)

E / V (vs RHE) 0

-20

-2

j / mA cm

-2

0.2

C products / mM cm

0.4

-2

-0.6

(c)

-60 Co, DHMF Co, DMDHF Ag, DHMF Ag, DMDHF Au, DHMF Au, DMDHF

C products / mM cm

0.1

-60 0.6

-2

0.0

Co Ag Au

-40

C / mM cm

0.4

-20

-2

0.8

-0.9

-2

0

(b)

-2 Fe, DHMF Fe, DMDHF Ni, DHMF Ni, DMDHF Cu, DHMF Cu, DMDHF

-2

-60 1.2

C products / mM cm

Fe Ni Cu

-40

-2

-20

C / mM cm

C / mM cm

-2

C products / mM cm

-2

j / mA cm

-2

(a)

C / mM cm

0

C / mM cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-1.2

-0.8

-0.4

0.0

E / V (vs RHE)

-10

Zn In Sb

-20

-30 0.9

Zn, DHMF Zn, DMDHF In, DHMF In, DMDHF Sb, DHMF Sb, DMDHF

0.6

0.3

0.0 Zn, DHMTHF In, DHMTHF Sb, DHMTHF

0.1

0.0 -1.2

-0.8

-0.4

0.0

E / V (vs RHE)

Figure 5. Electrocatalytic HMF (50 mm) reduction on metal catalysts in 0.5 M H2SO4. Upper 12


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panels: current-density profiles (upper panels) with (solid line) and without (dashed line) HMF in the solution during linear sweep voltammetry with a scan rate of 1 mVs-1. Lower panels: concentration profiles of the corresponding reaction products, that is, DHMF, DMDHF, and DHMTHF, as a function of potential.41

Based on the observed trends in Figure 4 and 5, the onset potentials for HER and product formation from HMF hydrogenation are compared in Figure 6.41 In general, the onset potentials for HER and HMF hydrogenation in acidic solution are shifted positively compared to neutral solution and their differences in acid ( 95% selectivity and up to 95% yield, depending on the flow rate. When both reactions were paired, lower yields were obtained, in particular for the oxidation.

Discussion on the Electro-Oxidation of Furanics Based on the literature presented here, it is clear that the electrochemical oxidation of HMF can selectively yield DFF, or can proceed further to the fully oxidized FDCA. The reaction pathway seems to be strongly dependent on the electrocatalyst. The oxidation pathways of DHMF and HMF are shown in Figure 11, including the most suitable metal for each pathway, as reported in literature. For all experiments resulting in DFF, Pt electrodes are used, suggesting that Pt is a suitable catalyst for oxidizing hydroxymethyl groups to aldehydes. Au is a suitable electrocatalyst for the oxidation of aldehyde groups to carboxylic acid groups, as shown by Chadderdon et al.56 Pd and Ni can catalyze both the oxidation of hydroxymethyl groups and aldehyde groups, since the use

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of both metals yields significant amounts of FDCA55,56. To catalyze both oxidations, it is probably important that the required potentials for the oxidation of the hydroxymethyl group and for the oxidation of the aldehyde group are similar. This is indeed the case for Pd, as can be seen in Figure 11. Not only the electrode, but also the electrolyte and electrolyte pH have a strong influence on the reaction. Best results in terms of full oxidation to FDCA are obtained at pH > 10. O O Pt [52-54] Pd [56]

Pd [56]

O

HO

OH

HO

O

O

Ni [55]

O HO

OH

PdAu [56]

Pt [53] DHMF

O

O O

O

O HO

O DFF

5-HMF

FFCA

O

Au [56]

FDCA

Pd [56]

O OH

HO HMFCA

Figure 11. Oxidation pathways of di-substituted furanics in alkaline media, indicating the most suitable catalyst for each pathway published in literature.

PHOTO-ASSISTED ELECTROCATALYSIS OF FURANICS The conversion of HMF with high yield and FE using a photo-electrochemical cell (PEC) with a significantly reduced electrical energy input has been demonstrated by Choi and coworkers.36,59 A PEC with HMF oxidation as the anode reaction was constructed in which an n-type nanoporous BiVO4 electrode was used as a photoanode that absorbs photons to generate and separate electron-hole pairs as illustrated in Figure 12. After separation, electrons were transferred to the Pt counter electrode to reduce water to H2, whereas the holes that reach the surface of BiVO4 are used for TEMPOmediated HMF oxidation. A 0.5 M borate buffer solution (pH 9.2) was used as the electrolyte with 7.5 mM TEMPO and 5.0 mM HMF added only to the anolyte, which was separated from the catholyte by fritted glass. The TEMPO-mediated photo-

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oxidation of HMF was carried out at 1.04 V vs. RHE under AM 1.5 G illumination (100 mW cm-2) and the final yield of FDCA at 40 C was ≥99% as shown in Figure 13.

Figure 12. Schematic comparison of the photo-electrochemical and electrochemical cells. (a) Photo-electrochemical TEMPO-mediated HMF oxidation. (b) Electrochemical TEMPOmediated HMF oxidation. CB, conduction band; EF, Fermi energy.59

Figure 13. Photo-electrochemical TEMPO-mediated HMF oxidation. (a) Linear sweep voltammograms (LSVs) of a BiVO4 photoanode obtained under AM 1.5 G illumination (100 mW cm-2) (orange line) and a Au electrode in the dark (black line) in a 0.5 M borate buffer

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solution (pH 9.2) containing 5 mM HMF and 7.5 mM TEMPO. Scan rate, 10 mV s-1. (b) Conversion and yield (%) changes of HMF and its oxidation products during TEMPO-mediated photo-oxidation of HMF at 1.04 V versus RHE in a 0.5 M borate buffer solution containing 5 mM HMF and 7.5 mM TEMPO under AM 1.5 G illumination (100 mW cm-2). The result shows that the TEMPO-mediated photo-oxidation of HMF can be achieved with a high yield and FE and significantly reduce the applied potential necessary for TEMPO oxidation.59

Using an identical photo-assisted setup, Roylance et al.36 demonstrated the hydrogenation of HMF to DHMF on a Ag catalyst with high yield and FE. LSVs were compared for a two-electrode cell, composed of a Aggd (galvanic displacement of a Cu foil by Ag) cathode and a Pt anode and for a photo-electrochemical cell composed of a Aggd cathode and a BiVO4 photo-anode under AM 1.5 G illumination. The electrolyte was a 0.5 M borate buffer solution (pH 9.2) containing 0.02 M HMF. Results are shown in Figure 14c. In order to achieve a current density of 1 mA cm-2 in the electrochemical cell, application of 2.76 V between a Aggd cathode and a Pt anode was necessary. However, when a photo-electrochemical cell is used, 1 mA cm-2 was achieved at a potential of 0.92 V between the BiVO4 photo-anode and Aggd cathode, saving about 2 V. The reduction products, FE, and yield for photo-electrochemical reduction of HMF were analyzed after performing photo-electrochemical reduction at 1.0 V between the BiVO4 photoanode and the Aggd cathode (a two-electrode cell). The photoelectrochemical reduction results show that DHMF was produced with a FE of 94.1 % and a yield of 94.8 %.

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Figure 14. Photo-electrochemical vs. electrochemical HMF reduction. Schematic diagrams comparing the external bias necessary to achieve HMF reduction for (a) photo-electrochemical cell and (b) electrochemical cell. (c) LSVs obtained from photo-electrochemical cell (red) and electrochemical cell (black) for HMF reduction using a two-electrode setup in 0.5 M borate buffer (pH 9.2) containing 0.02 M HMF (scan rate: 5 mV s-1, illumination for photoelectrochemical cell: AM 1.5 G, 100 mW cm-2). A two-electrode cell was used for both cases.36

These studies strongly evidence that electrocatalysis of furanics can be carried out at a significantly decreased external bias with support of renewable energy sources (i.e. solar, wind etc). When better photo-electrodes (photoanode and/or photocathode) are developed with a smaller band gap and the cell design is improved to minimize the IR drop, photo-assisted paired electrolysis of furanics can be achieved with no external energy input.36,38 Furthermore, these works demonstrate the potential to double the energy efficiency by combining yields of anodic and cathodic reactions of furanic compounds in a paired-reactor.

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CONCLUSIONS AND PERSPECTIVE The increasing generation of renewable energy from sun and wind, which means effectively an increasing direct production of electricity, has renewed the interest in electrochemistry both as a production process and as a method for electricity storage. It is of crucial importance for the transfer to renewable energy that new methods and processes are found that are capable of dealing with the inherent problem of the strong fluctuations in the production volumes of renewable energy. Electrosynthesis is a promising method to convert large amounts of surplus electricity into chemicals. In this way, large amounts of electricity can be converted and stored as chemical energy. Moreover, the capacity is not limited to its size and energy density, as is the case with batteries. Electrochemistry and electrocatalysis have been known for many years, but so far large-scale synthetic applications have been limited to a few areas such as the production of halogens and metals. There have also been a few larger scale applications for organic electrosynthesis such as the Kolbe reaction (oxidative decarboxylation resulting in dimer formation) and the electrohydrodimerization of acrylonitrile.60 While electrosynthesis has been widely studied, new commercial applications have been very limited in the last decades. One of the reasons is that electrosynthesis requires a multidisciplinary approach: for successful implementation, one needs a combination of organic chemistry, electrochemistry, catalysis, electrochemical engineering, and process design and economics. Moreover, focus is often on high volume products, which usually have a low added value. Given that electrochemical processes have to be scaled out due to maximum cell dimensions, they will have a relatively high capital expenditure (CAPEX) requirement, and thus lack the economy-of-scale metrics to 30


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compete efficiently. It is therefore more likely that electrosynthesis will be competitive in the production of high added value products.

Current Challenges One of the main challenges is to identify an organic synthesis where an electrocatalytic step has a clear advantage compared to conventional homogeneous or heterogeneous catalysis. Replacing an existing process by an electrochemical process is often not beneficial; when hydrogen or oxygen can be used instead of electrochemistry this is often more economical, in particular for substrates and products with a low molecular weight. Such reactions require a lot of electricity per ton of substrate, and with current electricity prices they cannot compete with oxidation or reduction using oxygen or hydrogen gas.61 In our opinion, the opportunities for electrochemistry in the near future are processes that currently are producing a lot of (toxic) waste, which could be avoided using electrons as direct oxidators/reductors, processes that are currently not selective, or processes where the number of process steps can be reduced by using electrochemistry. Such processes would not only be environmentally friendlier because a green energy source can been used, but also because the environmental footprint in terms of side products and waste can be reduced. In this perspective we have described the electrochemical oxidation and reduction of HMF and related compounds. Since the prices for chemicals are usually determined in dollars/ton, it is beneficial to keep as much of the oxygen functionalities in the molecule as possible because this conserves the molecular mass of the starting biomass. On the other hand, since most processes are currently fossil based and, therefore, are focused on hydrocarbon building blocks with low oxygen functionalities there is a large market for hydrogenated biomass “drop-in” products with known

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properties and applications for which an established market exist. However, in the long term, the conversion of biomass into highly oxidized products will be more economic. The modular nature of electrochemistry is also a challenge from a capital-cost perspective. Therefore, once a suitable electrochemical conversion has been identified with high potential, one has to focus on an easy reactor design which can easily be mass produced at lower costs. The fact that electrochemistry can often be performed at room temperature and atmospheric pressure should enable such a reactor design. In addition, product separation is in all chemical processes an energy demanding step, certainly with water as substrate/product or solvent. It is therefore of utmost importance that also in electrochemistry high substrate/product concentrations are achieved. Electrochemistry of furanics might show additional benefits because research suggests that the presence of furanics inhibits steel corrosion, which will increase the lifetime of the electrocatalytic reactor, and thereby reduce the capital costs.62,63 Recent patents on the electrochemical conversion of furanics also indicate commercial interest.64-66

Future Opportunities One of the opportunities for electrochemistry is to integrate it within a biorefinery. The modular nature of electrochemical reactors allows fitting a variety of production scales. Integrating electrochemistry in a biorefinery can give advantages such as heat integration as well as waste reduction. Moreover, since various product streams are present in a biorefinery, this opens up new opportunities for combining an oxidation with a reduction step in a single electrochemical reactor, thereby increasing the overall energy efficiency. From the perspective of a biorefinery approach, electrochemistry on furans shows a good promise. The oxidized product FDCA has very interesting characteristics

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because it is one of the building blocks to make a new polyester, polyethylene furanoate (PEF). The barrier properties of PEF are much better than those of other polyesters used in the food packaging industry, which enables smaller and stronger bottles with a smaller footprint and opens up new packaging applications.67-72 The company Avantium has developed a new process to produce FDCA, and is currently starting to commercialize FDCA as well as PEF with its partners. Reduction of furans such as furfural and HMF results in the formation of another interesting class of compounds such as DMF (a potential fuel as well as chemical intermediate), furfuryl alcohol (used in foundry resins) and methylTHF (more benign solvent).

ACKNOWLEDGEMENTS This research has been partly performed within the framework of the CatchBio program. The authors gratefully acknowledge the support of the Smart Mix Program of the Netherlands Ministry of Economic Affairs and the Netherlands Ministry of Education, Culture and Science. This research has been partly performed as a project KK1601-A00 supported by the Korea Research Institute of Chemical Technology (KRICT).

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