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Electrocatalytic Hydrogenation and Hydrogenolysis of Furfural and the Impact of Homogeneous Side Reactions of Furanic Compounds in Acidic Electrolytes Sungyup Jung, and Elizabeth J Biddinger ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01314 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 9, 2016

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ACS Sustainable Chemistry & Engineering

Electrocatalytic Hydrogenation and Hydrogenolysis of Furfural and the Impact of Homogeneous Side Reactions of Furanic Compounds in Acidic Electrolytes

Sungyup Jung and Elizabeth J. Biddinger* Department of Chemical Engineering The City College of New York, CUNY, 140th Street and Convent Avenue, New York, NY 10031, USA *Corresponding author: [email protected] Tel: +1) 212-650-6323

Keywords:

furfural,

furfuryl

alcohol,

2-methyl

hydrogenation, hydrogenolysis and copper electrocatalyst

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ACS Paragon Plus Environment

furan,

electrocatalytic


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Abstract The electrochemical hydrogenation and hydrogenolysis (ECH) of furfural (FF) on a copper electrocatalyst has been investigated to produce biofuels and fine chemicals in an H-type batch reactor at room temperature. We report a systematic study of ECH of FF to gain a better understanding of the relationships between products and reaction conditions: current density, electrolyte and co-solvent ratio in acidic solutions. The acidity of electrolytes had the most significant impact on the product distribution. Mildly acidic electrolytes mainly produced furfuryl alcohol (FA), while strongly acidic electrolytes produced both 2-methyl furan (MF) and FA. Also, the yield of products depended on the current density and reaction time when equivalent charge was transferred to the reaction. However, the mole balance accounting for FF, MF and FA was not higher than 70% in any reaction condition when the theoretical amount of electrons for complete MF production from FF (e-/FF = 4) was transferred to the system. The investigation of non-electrochemical homogeneous side reactions suggested that the low mole balance in a mildly acidic electrolyte may be from the charge transfer promoted side reactions on the copper electrode. On the other hand, it was shown that the low mole balance in strongly acidic electrolytes was due to homogeneous side reactions.

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Introduction Current developments in renewable and sustainable energies helps to diminish the demand for fossil fuels, however, organic carbon resources also have to be supplied in order to totally replace fossil fuels. In this regard, biomass is considered a promising sustainable resource for organic fuels and carbon-based materials1-3. Furfural (FF) is a C5 platform chemical, which is produced by the hydrolysis and dehydration of xylan contained in lignocellulose4,5. It offers a rich source of derivatives that can be used as industrial chemicals and biofuel components such as furfuryl alcohol (FA) and 2methyl furan (MF). FA can be used in corrosion resistant coatings, adhesives, binders and polymer concretes6,7. MF can be utilized in perfumes, pharmaceutical intermediates and for the synthesis of pesticides8, as well as can be considered as a biofuel itself or a fuel intermediate7,912

. Traditional chemical hydrogenation and hydrogenolysis (CH) requires a large quantity of

externally supplied hydrogen gas, high temperature and high pressure. In contrast, electrochemical hydrogenation and hydrogenolysis (ECH) does not require external hydrogen gas and offers a sustainable way to produce fine chemicals and biofuels at room temperature and pressure when paired with renewable (and frequently intermittent) electricity sources. From the absence of external hydrogen gas feedstock, biomass-derived chemicals can be produced in remote locations where hydrogen is not readily available. Another advantage of electrochemical reactions is that the reaction on the electrocatalyst can be controlled by current or potential changes13. The main difference between CH and ECH is the source of hydrogen supplied to a system. For ECH, chemisorbed hydrogen, (H)M, must be on the surface of a catalyst, M, for

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surface-mediated hydrogenation. The Menard and the Navarro groups suggested the mechanisms of ECH for unsaturated organic compounds14-18, and modified reactions for the specific case of ECH of FF are shown below. In acidic electrolytes, the chemisorbed hydrogen is formed according to reaction 1, and is formed according to reaction 2 in alkaline electrolytes. In contrast, CH requires H2 gas to obtain chemisorbed hydrogen (reaction 3). H3O+ + e- + M  (H)M + H2O

(Reaction 1)

H2O + e- + M  (H)M + OH-

(Reaction 2)

H2(g) + 2M  2(H)M

(Reaction 3)

The concurrent reaction on the catalyst or electrocatalyst is the adsorption of FF, denoted as (F=O), onto an active site A (reaction 4); followed by the hydrogenation of FF to FA (FHOH) (reaction 5); and then desorption of the FA (FH-OH) from the surface (reaction 6). Further hydrogenolysis of FA can occur to produce MF (F-H2) (reaction 7), followed by desorption of MF from the surface (reaction 8). (F=O) + A  (F=O)A

(Reaction 4)

(F=O)A + 2(H)M  (FH-OH)A + 2M

(Reaction 5)

(FH-OH)A  FH-OH + A

(Reaction 6)

(FH-OH)A + 2(H)M  (F-H2)A + H2O + 2M

(Reaction 7)

(F-H2)A  F-H2 + A

(Reaction 8)

In spite of these key advantages, ECH of FF has not been broadly studied. In the initial studies of ECH of FF, the primary product was FA. The Belgsir group investigated electrocatalytic reduction and oxidation of FF with Au, Pt, Cu, Ni and Pb19,20. Their primary product was FA, but they also produced MF, 1,5-pentanediol and hydroxyfuroin20. Chu, et al. and Wang, et al. used titanium dioxide-based electrodes to produce FA21,22. Among reports for

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ECH of FF to FA, the highest selectivity of FA (99%) was obtained by Zhao, et al. on Pt supported on activated carbon fibers23. Recent studies reported that MF can be produced with strongly acidic solutions and/or high potentials. The Huber group reported four furanic compounds, including MF and FA on Pt/C and Pd/C electrodes with a continuous electrocatalytic membrane reactor. In their research, product selectivity depended on applied potentials at low conversion of FF (≤ 6%)24. The Saffron group reported FA and MF production in acidic electrolytes with a sacrificial anode25. Among the studies for ECH of FF to MF, Nilges, et al. reported the highest selectivity of MF (c.a. 80%) on Cu in 0.5 M H2SO426. To summarize the studies for ECH of FF, research groups have utilized extremely different conditions and obtained a variety of results, making understanding of the ECH of FF reaction difficult. pH’s ranging from 0 to 13 and many electrocatalysts have been used, resulting in a wide variety of reaction results. Two broad observations made from the results are as follows: (1) high concentration of FF restricted undesirable H2 generation and, (2) MF generally appeared as a product only in strongly acidic electrolytes (pH ≤ 1). In this paper, we performed a systematic study of ECH of FF on a Cu electrode to gain a better understanding of the relationship between reaction efficiency (yield, selectivity, conversion and mole balance) and reaction conditions (current density, acidity and co-solvent ratio). This study was performed as a function of equivalent charge transfer (normalized reaction time) to observe the importance of reaction time. A Cu electrode was used as the electrocatalyst because both MF and FA have been observed using it and there is a possibility that reaction selectivity can be tuned by reaction conditions on a Cu electrode. Moreover, Cu has a low tendency for hydrogen evolution which competes with ECH of FF in aqueous solutions27. This

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means that electron consumption from hydrogen evolution can be suppressed compared to more noble metals, resulting in the possibility of higher faradaic efficiencies for ECH of FF. In addition, we investigated the low mole balance from ECH of FF in acidic electrolytes by studying non-electrochemical homogeneous reactions as a function of reaction time.

Experimental Materials Furfural (FF; 99%, Sigma-Aldrich), furfuryl alcohol (FA; 98%, Acros Organics) and 2methyl furan (MF; 99%, 0.024% BHT contained as stabilizer, Aldrich) were used as received. Ammonium chloride (NH4Cl, 99.998%, Aldrich), sulfuric acid (H2SO4, trace metal grade, 93 – 98%, Fisher Scientific) and hydrochloric acid (HCl, certified ACS plus, 37.5% diluted in aqueous solution, Fisher Scientific) were used as supporting electrolytes with DI water (resistance ≥ 18.2 MΩ) and acetonitrile (optima grade, Fisher Scientific) co-solvents (20, 50 and 80% acetonitrile by volume). Cu foil (99.999%, 0.1 mm thickness, Alfa Aesar) and Cu wire (99.999%, d = 0.5 mm, Alfa Aesar) were used for making electrode flags (2 cm x 1.5 cm). The in-house made working electrodes were used after pretreatment as follows: 10 min sonication in acetone (optima grade, Fisher Scientific) and washing with DI water, followed by immersion in 10 v/v% HCl for 1 min, then DI water washing again. New Cu working electrodes were used for each experiment. The counter electrode was a Pt gauze flag (2.5 cm x 2.5 cm), composed of Pt wire (99.95%, d = 0.5 mm, Alfa Aesar) and Pt gauze (99.9%, 0.1 mm thickness and 52 mesh woven, Alfa Aesar). A silver/silver chloride (Ag/AgCl) electrode (MR-5275, BASi) in 3 M sodium chloride (NaCl) was used as the reference electrode.

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Electrochemical cell setup A divided H-type cell was used for ECH of FF with a cation exchange membrane (Nafion 117, Aldrich). 30 ml of 100 mM FF in acetonitrile and aqueous acidic electrolytes (0.2 M NH4Cl, 0.1 M HCl, 0.1 M H2SO4, 0.5 M HCl and 0.5 M H2SO4) were used as catholytes. Furfural-free 30 ml acidic electrolytes with 20% acetonitrile (by volume) were used as anolytes. A Liebig condenser with chiller (8oC) was connected from the catholyte to a solvent trap. The solvent trap was filled with 60 ml acetonitrile and held in a -10°C to -20°C bath to capture any evaporated MF during ECH of FF. The catholyte was purged for a minimum of 15 min with nitrogen (35 40 ml/min) prior to ECH of FF being performed galvanostatically, with the nitrogen gas continuously flowing in the system until the end of the experiment. Three current densities (j = 5, -10 or -15 mA/cm2) were utilized. Magnetic stirring (700 rpm) was utilized for constant mixing. The total reaction time was 10.74 hr, 5.37 hr and 3.55 hr at j = -5, -10 and -15 mA/cm2, respectively, with the calculation further described below. Each experiment was run at least three times, with error bars shown as ± 1 standard deviation. All reaction experiments were run at ambient conditions.

Non-electrochemical homogeneous reaction In order to investigate the side reactions in acidic electrolytes, the concentration change of FF, FA and MF solutions was monitored without electron transfer. 50 mM FF, 25 mM FA and 25 mM MF were added to each acidic electrolyte (25 ml and 20-vol% acetonitrile) in a capped glass vial. The resulting concentration was analyzed at 0, 0.5, 1.5, 3 and 5.37 hr after mixing. In order to monitor the effects of Nafion acidity in this system, homogeneous reaction tests were

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also performed in the presence of Nafion with mildly acidic electrolyte and non-electrolyte solutions. Each test was run at least three times, and error bars shown are ± 1 standard deviation.

Analysis For analysis, a 0.25 ml aliquot of the catholyte or the homogeneous reaction solution was sampled, and then mixed with 2 ml chloroform (HPLC grade, Fisher Scientific) and 50 mg sodium chloride (NaCl; Certified ACS grade, Fisher Scientific) to separate the aqueous electrolyte layer and the organic solvent layer. The organic layer was extracted from the mixture, and it was further dried, using sodium sulfate (Na2SO4). The solvent trap was also analyzed by taking a 0.5 ml aliquot from the solvent trap and diluting 10 times with acetonitrile. Products were analyzed on a gas chromatography mass spectrometer (GC-MS; QP-2010 Ultra, Shimadzu) using a ZB-Waxplus (ℓ = 30 m, d = 0.25 mm, thickness = 0.25 μm, Phenomenex). Qualitative peak identification was conducted by comparison of NIST library and collected MS data by matching peak positions with FF, FA and MF standard solution. Quantitative results were calculated with standard calibration curve of FF, FA and MF solution.

Calculations The number of electrons transferred was calculated based on Faraday’s law of electrochemistry: Q = znF

(eq. 1)

Here, Q is the total charge passed in the circuit to convert n moles of a reactant to products by a reaction including z electrons per molecules of the reactant, and F is Faraday constant, F = 96485.4 C/mol. Assuming 100% Faradaic efficiency of electron transfer and 100%

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conversion of FF to MF (z = 4), total reaction time (3.55 to 10.74 hr) depended on current densities. The conversion, yield, selectivity, and mole balance were calculated based on the following equations: Conversion of FF = ([FF]i – [FF]t)/[FF]i * 100

(eq. 2)

Yield of FA or MF = ([FA]t or [MF]t)/[FF]i * 100

(eq. 3)

Selectivity of FA or MF = ([FA]t or [MF]t)/([FF]i – [FF]t) * 100

(eq. 4)

Mole balance = ([FF]t + [FA]t + [MF]t)/[FF]i * 100

(eq. 5)

Molar concentration of FF, FA and MF was used for the calculations. Subscripts i and t indicate initial concentration and concentration at sampling time, respectively. Faradaic efficiency was determined starting with equation 6 for the case of 100% FE. znF = it * FE

(eq. 6)

When the Faradaic efficiency, FE, is 1 (or 100%), all the electrons are used for ECH of FF, while lower Faradic efficiency means that electrons may go into side reactions, which allows a relation between the mass balance and Faraday’s Law to be written. Because the detected products MF and FA have different number of electrons transferred per FF converted, the relationship between Faradaic efficiency and the resulting products is: Faradaic efficiency = (4*[MF]t + 2*[FA]t + 2*([FF]conv – [FA]t – [MF]t))/(4*[FF]i) * 100 (eq. 7) In this equation it is assumed that the any reacted FF ([FF]conv) involved 2 electrons being transferred initially, before any further reaction (to MF or side products) occurred.

Results and Discussion

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Based upon the literature, it is known that ECH reactions significantly depend on the type of electrodes, current density, and solution properties including solvent used, pH, and electrolyte. How these parameters impact ECH is less clear. In order to gain a better understanding of the variability, the effects of electrolyte and current density on the ECH of FF were studied here. Also, the effects of co-solvent (acetonitrile/water) ratio and current density were investigated. Moreover, non-electrochemical homogeneous reactions were studied to investigate undetectable reactions during ECH of FF.

Effects of electrolyte and current density on ECH of FF To compare the effects of current density in different pH, three acidic electrolytes, 0.2 M NH4Cl (pH 3.4), 0.1 M H2SO4 (pH 1.1) and 0.5 M H2SO4 (pH 0) were used in 20-vol% acetonitrile and 80-vol% water co-solvent. The current densities (j) used were -5, -10 and -15 mA/cm2. Figure 1 shows the normalized concentration change of FF (CFF/C0), yield of FA (YFA) and MF (YMF) as a function of equivalent charge transfer (e-/FF) with 20-vol% acetonitrile solutions. Here, CFF and C0 indicate the concentration of FF at sampling time and initial concentration of FF, respectively. In figure 1 (a – c), the x-axis is the ratio of the number of electrons transferred to the initial number of furfural molecules (100 mM FF in 30 ml solution). This convention was used because the electron transfer rate at j = -15 mA/cm2 is three times faster than that at j = -5 mA/cm2, meaning the total reaction time is a function of the current density (10.74 hr for -5 mA/cm2 and to 3.55 hr for -15 mA/cm2, for example). This is essentially a normalized reaction time, and the total reaction time is based on e-/FF = 4 because FF

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theoretically requires four electrons to produce MF. The overall reaction of ECH of FF to both FA and MF in acidic electrolytes was shown in scheme 1. The acidity of electrolytes has significant influence on the product distribution, while current density impacts the product yield. In 0.2 M NH4Cl (figure 1-a), the main product was FA with 23.4% yield at j = -10 mA/cm2 when e-/FF = 4. The yield of FA increased from 15.0% to 23.4% as current density increased (-5 to -10 mA/cm2), but the total conversion of FF was higher than 93% regardless of current density. This indicates that higher current density produced more FA, and slower reaction led to the low product yield with same conversion in 0.2 M NH4Cl. The consumption of FF and production of FA was high until e-/FF was near 1, with nearly stable concentrations being obtained when e-/FF ≥ 2 in 0.2 M NH4Cl. This is likely because FF requires two electrons to produce FA, and further reaction to MF was negligibly shown. The observation of the product being FA in 0.2 M NH4Cl is consistent with the result of Li, et al., who used a water/methanol co-solvent with a sacrificial anode25. As seen in the figure 1-b and 1-c, both FA and MF were produced in sulfuric acidcontaining electrolytes. In 0.1 M H2SO4 (figure 1-b), at both j = -5 and -10 mA/cm2, the product distribution was similar with both FA and MF having yields between 11 and 14%. When e-/FF ≥ 2, the yield of MF and FA in 0.1 M H2SO4 stabilized, and the reaction rate of FF rapidly decreased. Also, the final conversion of FF in 0.1 M H2SO4 was less than 76%, which means that some of electrons supplied to this system were not used for FF hydrogenation. The meaning of e/FF = 4 is the theoretical amount of electrons for 100% conversion of FF into MF. In this regard, 76% conversion of reactant indirectly proves the formation of in situ side reactions that consume electrons.

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Figure 1-c shows that MF was the primary product in 0.5 M H2SO4. The yield of MF and FA increased as current density increased similar to 0.2 M NH4Cl. When e-/FF ≥ 2, the yield of MF steadily increased as conversion of FF increased at high current densities (-10 and -15 mA/cm2), however the yield of MF remained constant as conversion of FF increased at j = -5 mA/cm2. This indicates that ECH of FF produced FA and MF, but at longer reaction durations other species were able to form beyond FA and MF that accounted for the further consumption of FF. Possible side reactions, resulting in a low conversion in 0.1 M H2SO4, are hydrogen generation via Heyrovsky (reaction 9) or/and Tafel (reaction 10) reactions. The hydrogenation and hydrogenolysis steps (reactions 5 and 7) compete with electrochemical and chemical hydrogen desorption on a catalyst surface (reactions 9 and 10) to generate hydrogen gas14,17. H3O+ + (H)M + e-  M + H2 + H2O

(Reaction 9)

2(H)M  2M + H2

(Reaction 10)

Faradaic efficiency is another way to examine the balance between the desired ECH reactions and any side reactions. For example, at j = -10 mA/cm2, the Faradaic efficiency was 40.6% in 0.1M H2SO4 and 56.8% in 0.5M H2SO4, indicating significant losses are occurring in this system due to electron-transfer side reactions such as hydrogen generation. Table S1 lists the Faradaic efficiencies for the reactions examined. In order to clarify the effects of acidity, 0.1 M HCl (figure S1 and S3) and 0.5 M HCl (figure S2 and S3) in 20-vol% acetonitrile were used as a comparison to 0.1 M and 0.5 M H2SO4. This allowed verifying the impact of pH, rather the impact of anion change between NH4Cl and H2SO4. In the ECH of FF with HCl electrolyte, it was shown that product distribution mainly depended on acidity of solution, not electrolyte anions. Similar to the results in sulfuric acid, MF

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was the primary product in 0.5 M HCl, and the highest current density (j = -15 mA/cm2) led to highest yield of MF (29.6%) and FA (5.9%) compared to the lowest current density (YMF: 15.6, YFA: 1.9% at j = -5 mA/cm2) when e-/FF = 4. Both MF and FA were produced in 0.1 M HCl with low YMF/YFA ratio (< 1.6). The conversion of FF, selectivity of products and mole balance at j = -10 mA/cm2 when e-/FF = 4 with 20-vol% acetonitrile in the different electrolytes are summarized in Figure 1-d. It can be seen that the highest selectivity of FA (25.1%) was obtained in 0.2 M NH4Cl, and that of MF (31.3%) was obtained in 0.5 M H2SO4. As depicted in scheme 1, conversion of FF to FA requires 2 electrons and 2 protons, and conversion from FF to MF requires 4 electrons and 4 protons. Low pH can provide a higher driving force for H-adsorption on the surface of electrode, which can lead to formation of more hydrogenated products, MF in this study. In addition to pH effects alone, the potentials the reactions were operated at can impact the resulting products. Favorability of electrochemical reactions is potential dependent. The experiments performed in this study were run at constant current conditions, allowing for variable potential. More negative reduction potentials were observed for primary FA production in the 0.2M NH4Cl when compared to the other electrolytes at the same current densities. Figure S4 shows the reduction potential ranges for the electrolytes became more negative going from 0.5 M H2SO4 (- 0.6 to - 0.8 V vs. RHE) to 0.1 M H2SO4 (- 0.8 to - 0.9V vs. RHE), to 0.2 M NH4Cl (- 1.1 to - 1.8 V vs. RHE) at constant current density (j = - 10 mA/cm2). The more negative reduction potential for the NH4Cl solution likely led to additional side reactions occurring electrochemically. Further studies as a function of potential would be needed to draw clearer conclusions between product distribution, potential, and pH which is outside the scope of this particular study.

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The mole balance indicates a conservation of materials by the calculation of products and reactants accumulated in the reaction system. In this study, the mole balance accounts for the FA and MF produced, and the FF unreacted. With 20-vol% acetonitrile, the highest mole balance was 58.8% in 0.1 M H2SO4 at j = -5 mA/cm2 at the end of reaction (e-/FF = 4). The lowest mole balance was 21.1% in 0.2 M NH4Cl at j = -5 mA/cm2. When MF was produced equally or higher than FA (0.1 and 0.5 M H2SO4), higher mole balance was obtained than when the primary product was FA (0.2 M NH4Cl). This indicates that FA may be more involved in side reactions. FA is accumulated only in the acidic catholyte cell due to its low vapor pressure (0.609 mm Hg at 25oC)28, but MF was obtained in the both catholyte and solvent trap due to its high vapor pressure (174 mm Hg at 25oC)29. Products remaining in the catholyte may lead to further side reactions in the presence of acidic electrolytes. In order to support this argument, nonelectrochemical homogeneous reactions will be addressed later in the article. Before nonelectrochemical homogeneous reactions, the effects of acetonitrile co-solvent ratio and current density will be shown.

Effects of co-solvent ratio and current density on ECH of FF Acetonitrile and water co-solvent were used in ECH of FF for enhancing the solubility of hydrophobic FF and to investigate the effects of aprotic polar solvents on biomass conversion. According to the reports related to biomass conversion and hydrogenation, furanic compounds can be degraded or/and form large molecular weight side products in aqueous or/and acidic solutions30. These side products are known as humins31, and are generally difficult to detect and analyze utilizing many traditional analytical tools such as GC/MS or HPLC. The presence of polar aprotic organic co-solvents has tended to hinder side reactions and degradation of biomass

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compounds30,32-35. In the observations above using 20-vol% acetonitrile, the mole balance of FF, FA and MF after ECH in the three acidic electrolytes was less than 58.8%. In this regard, effects of acetonitrile ratio in the catholyte were studied to determine if increasing the aprotic proportion would hinder side reactions or/and degradation of furanic compounds in the system. Figure 2 shows the effects of acetonitrile ratio in 0.5 M H2SO4 with j = -10 mA/cm2 at e/FF = 4. The mole balance increased from 48.8% in 20-vol% acetonitrile to 63.0% in 50-vol% acetonitrile. Further increasing the acetonitrile fraction to 80-vol% did not improve the mole balance more. Interestingly, the acetonitrile composition also had an impact on the FF conversion, dropping from 86.6% in 20-vol% acetonitrile to 57.6% in 80-vol% acetonitrile. Products selectivity increased from 31.3% (MF) and 9.6% (FA) in 20-vol% acetonitrile to 36.4% (MF) and 10.6% (FA) in 50-vol% acetonitrile. Again, raising the acetonitrile fraction to 80-vol% did not improve the selectivity of MF and FA. The effects of 50-vol% acetonitrile in 0.2 M NH4Cl at j = -10 mA/cm2 is shown in figure S5. Both product yield of FA and mole balance decreased from 20-vol% to 50-vol% acetonitrile unlike 0.5 M H2SO4. Detailed product yield and conversion of FF in different acetonitrile ratio are shown in figure S5 – S8 as a function of equivalent charge transfer. This study shows that the combination of acetonitrile ratio and electrolyte impacted reaction performance, not just one factor above. In addition, current density apparently increased the yield and selectivity of products within the same solution as shown in figure 1 and figure 3 (20-vol% acetonitrile). Effects of current density along with acetonitrile ratios were shown in figure S8 to expand this argument in every acetonitrile ratio. Figure 3 and S8, in common, show that higher current density increased products selectivity and mole balance in all the acetonitrile ratios (20 – 80vol%).

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The above results show that the mole balance steadily decreased as reaction time increased. From another report, it was also shown that the mole balance did not close after ECH of FF25. To further investigate this trend, yield of FA and MF products, conversion of FF and mole balance was plotted together as a function of FF conversion (figure 4). This allows a direct comparison between the molecules converted and the desired products, MF and FA. The results for 0.2 M NH4Cl (figure 4-a) and in 0.5 M H2SO4 (figure 4-b) at j = -10 mA/cm2 are shown. Trend lines using regression analysis (95% confidence) of mole balance (FF + FA + MF), yield of FA and MF are plotted. Mole balance and products yield at all acetonitrile ratios were combined together to plot one line, since their values were not remarkably different with the acetonitrile ratios as a function of conversion. A high R2 (≥ 0.91) in both figures show that plotting in one line is reasonable, regardless of acetonitrile ratio. The two graphs in figure 4 demonstrate, in common, that undetected side products were obtained as a function of XFF, because the magnitude of the slope for the mole balance change is higher than the slope of MF and FA yield in both 0.2 M NH4Cl and 0.5 M H2SO4. However, the rate of mole balance decreasing was faster in 0.2 M NH4Cl than in 0.5 M H2SO4. This may be because FA, present only in the catholyte, could be more involved in side reactions than MF, present in the both solvent trap and catholyte. In 0.2 M NH4Cl, the mole balance was 80.0% at XFF = 31.4%, but decreased to 31.2% at XFF = 93.3% in 20-vol% acetonitrile (figure 4-a). For the same points, yield of FA was 10.8% at XFF = 31.4%, and 23.4% at XFF = 93.3% in 20-vol% acetonitrile. MF was negligibly produced in 0.2 M NH4Cl. The same trend was observed in 0.5 M H2SO4. The mole balance was 89.0% at XFF = 17.9%, but decreased to 48.8% at XFF = 86.6% in 20-vol% acetonitrile (figure 4-b). For the same

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points, YMF increased from 4.7 to 27.1%, and YFA increased from 2.1 to 8.3% at the same conversion in 0.5 M H2SO4 Although the yield of FA and MF increased proportionally to the conversion of FF (figure 3), the selectivity of both FA and MF remained constant or slightly decreased with conversion (figure S9 and S10). This comparison of yield to selectivity indicates that the rate of FA, MF and side products formation was relatively constant with the rate of side products formation slightly increasing as conversion increased.

Non-electrochemical homogeneous reaction From the above experiments for the ECH of FF, both MF and FA were produced in strongly acidic electrolytes, while FA was the primary product in 0.2 M NH4Cl. However, the highest mole balance was less than 66.3% at e-/FF = 4 (figure S8). It was revealed that the mole balance was mainly current density dependent with a lesser effect from solvent ratio. For the equivalent charge transfer (e-/FF = 4) at three current densities (-5, -10 and -15 mA/cm2), different reaction times were used for ECH of FF (10.74, 5.37 and 3.55 hr). This means that lower current densities had longer reaction time. Therefore, it is required to further investigate why these low mole balances were obtained during the ECH of FF in acidic electrolytes. The underlying phenomena for this were studied in non-electrochemical homogeneous reactions. To produce MF from ECH of FF, strongly acidic electrolytes were required in both this study and studies reported in the literature25,26. However, these acidic electrolytes may cause polymerization or oligomerization among furanic compounds 6,36,37. It has been reported that FA can be polymerized with condensation reactions, resulting in polyfurfuryl alcohol (PFA) resin

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using acidic catalysts6,36. Also, homogeneous trimerization of MF in the presence of water and sulfuric acid at 60oC has been shown37. In order to quantitatively analyze the solution-based homogeneous side reactions that might lead to low mole balance in ECH of FF in electrolyte solutions, solutions of 50 mM FF, 25 mM FA and 25 mM MF in acidic electrolytes using 20-vol% acetonitrile were analyzed. The solutions were analyzed for 5.37 hr. This total reaction time was based on the time for e-/FF = 4 at j = -10 mA/cm2. Since Nafion is a highly acidic itself38 and could serve as an acid catalyst during ECH of FF, homogeneous tests with Nafion were also run in electrolyte-free and 0.2M NH4Cl solutions. Figure 5 shows that the MF concentration significantly decreased in the presence of 0.1 and 0.5 M H2SO4 with time. Also, large concentration drops for FF and FA were shown in 0.5 M H2SO4. However, there was no significant concentration drop for furanic compounds in 0.2 M NH4Cl. The presence of Nafion in electrolyte-free and 0.2 M NH4Cl solutions also did not show significant concentration change for furanic compounds. In 0.5 M H2SO4, FF concentration decreased by 10.2% after 5.37 hr, while it was constant in the other three solutions (figure 4-a). A 52.6% drop in FA concentration was observed in 0.5 M H2SO4, while losses were less than 10% in other electrolytes (figure 5-b). Such a large drop of FF in 0.5 M H2SO4 may suggest the inclusion of FF in the condensation polymerization of FA as the head or tail monomer of the FA polymer. It is also possible that FA could be polymerized without any charge transfer in 0.5 M H2SO4. The MF concentration dramatically decreased in both 0.5 M H2SO4 (64.3%) and 0.1 M H2SO4 (25.6%) as shown in figure 5-c. These significant MF concentration decreases showed indirect evidence of side reactions: polymerization/oligomerization or/and other unknown reactions including humin production. Li, et al. suggested the possible reason of low mole

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balance in ECH of FF might be from evaporation of volatile MF25, but it is unlikely to happen in our system because the use of a solvent trap to capture volatile MF. The disparity between lack of furanic compound loss in the homogeneous study in 0.2 M NH4Cl and the low mole balance electrochemically in this solution suggests charge transfer promoted side reactions occur. The mole balance from the homogeneous reaction was 97.7% while with -10 mA/cm2 current flow the mole balance was 31.1% at the same reaction time. From figure 4-a, it was also observed that the mole balance rapidly decreased as the cumulative charge transfer increased. Promotion of FA polymerization in the presence of current flow may be a cause of this. The homogeneous reaction in 0.5 M H2SO4 showed remarkable concentration drops of furanic compounds with time. The mole balance after homogeneous reaction was 65.6%, which suggests acid promoted homogeneous side reactions in the absence of current flow. With the current flow (j = -10 mA/cm2), the mole balance after ECH of FF was 48.8% at the same time, which was higher than 0.2 M NH4Cl that did not show homogeneous side reaction. These observations suggest that rapid evaporation of MF from the catholyte into the solvent trap lead to less side reactions of MF. This is because MF captured in an acetonitrile solvent trap was not exposed to a sulfuric acid for extended period of time, preventing extensive side reactions with it. In contrast, FA produced accumulates in the catholyte giving more opportunity for the interaction with acidic electrolytes. This indirectly provides the evidence for higher mole balance observed in 0.5 M H2SO4 than in 0.2 M NH4Cl, even though the concentration of FA and MF decreased more due to homogeneous reactions in 0.5 M H2SO4 than 0.2 M NH4Cl.

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Further evidence of polymeric side products being formed could visually be confirmed by the presence of solidified products, visible to the naked eye, on Cu electrodes after reactions. Also, these solids were observed suspended in the solution during the reactions. Surface analysis of the Cu electrodes after ECH of FF was performed using XPS (X-ray photoelectron spectroscopy) (figure S11). A dark carbon film covered the Cu electrode with a thickness greater than the 5-10 nm analysis depth of XPS on the Cu electrode used in 0.5 M H2SO4 (20-vol% acetonitrile) at a current density of - 5 mA/cm2. Only a small signal from Cu was observed on the Cu electrode after use in 0.2 M NH4Cl, indicating that it also had a carbon film build up. The likely source of the carbonaceous film on the used Cu electrodes is long-chain carbon based materials such as polyfurfuryl alcohol or humins.

Conclusion ECH of FF on Cu has been studied to gain a better understanding between products and reaction conditions as a function of equivalent charge transfer (or normalized reaction time). Both MF and FA have been obtained in strongly acidic electrolytes (0.1 and 0.5 M H2SO4), while FA was the primary product in a mildly acidic electrolyte (0.2 M NH4Cl). Product yield and selectivity increased as a current density increased when equivalent charge was applied to the reaction (e-/FF = 4), but did not influence the product distribution. These observations indicate that pH is a major factor that dictates product selectivity, while faster charge transfer and shorter reaction time can further improve products yield and selectivity. Although changing the acetonitrile ratio improved the products yield and selectivity in some cases, a consistent correlation between products and acetonitrile ratios was not observed. After ECH of FF, the mole balance was less than 70% in every reaction. In order to reveal the reason of low balance from both electrolytes, non-electrochemical homogeneous

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reactions were studied. From non-electrochemical homogeneous reactions, it was suggested that the exposure of FF, FA and MF to sulfuric acidic-containing electrolytes may promote homogeneous side reactions with products that were not detected by GC/MS. It was also shown that charge transfer promoted homogeneous side reactions in 0.2 M NH4Cl likely occur. This explains why higher current densities resulted in more FA and MF because of the reduced duration of total reaction. Solutions and experimental conditions need to be carefully selected for ECH of FF. Fast hydrogenolysis for MF production in acidic electrolytes and rapid separation of MF from the solution will improve the yield of MF, decreasing side products formation.

Acknolwedgments The City College of New York Grove Foundation is gratefully acknowledged by the authors for its financial support.

Supporting Information Faradaic efficiency (table S1), ECH of FF in HCl (figure S1 – S3), potential profiles in 20-vol% acetonitrile (figure S4), effects of acetonitrile ratio (figure S5 – S8), selectivity plot (figure S9 and S10) and XPS Cu 2p spectra (figure S11) are included.

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List of Figures and Schemes Scheme 1. Pathway of ECH of FF to FA and MF

Figure 1. ECH of FF on Cu cathode in 20-vol% acetonitrile: (a) 0.2 M NH4Cl, (b) 0.1 M H2SO4 and (c) 0.5 M H2SO4. Numbers indicated in parentheses of legends (a – c) are current densities (mA/cm2). (d) Selectivity of products, conversion of FF and mole balance at e-/FF = 4 and j = -10 mA/cm2, calculated from (a) – (c). Figure 2. ECH of FF in 0.5 M H2SO4 at e-/FF = 4: effects of acetonitrile ratio at j = -10 mA/cm2. Figure 3. ECH of FF in 0.5 M H2SO4 at e-/FF = 4: effects of current density in 20-vol% acetonitrile.

Figure 4. Mole balance and yield of products as a function of conversion. Results of (a) 0.2 M NH4Cl and (b) 0.5 M H2SO4 at j = -10 mA/cm2. Trend lines of mole balance (MB) and yields combine all the acetonitrile ratios together in data set.

Figure 5. Concentration changes of (a) FF, (b) FA and (c) MF in the absence of charge transfer in 20-vol% acetonitrile. Initial concentration was 50 mM FF, 25 mM FA and 25 mM MF.

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Table of Contents Graphical Abstract Title: Electrocatalytic Hydrogenation and Hydrogenolysis of Furfural and the Impact of Homogeneous Side Reactions of Furanic Compounds in Acidic Electrolytes

Author name: Sungyup Jung and Elizabeth J. Biddinger

Electrochemical hydrogenation of biomass-derived furfural to fuel and fine chemical intermediates is highly dependent upon reaction conditions

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Scheme 1. Pathway of ECH of FF to FA and MF 170x46mm (300 x 300 DPI)



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Figure 1. ECH of FF on Cu cathode in 20-vol% acetonitrile: (a) 0.2 M NH4Cl, (b) 0.1 M H2SO4 and (c) 0.5 M H2SO4. Numbers indicated in parentheses of legends (a – c) are current densities (mA/cm2). (d) Selectivity of products, conversion of FF and mole balance at e-/FF = 4 and j = -10 mA/cm2, calculated from (a) – (c). 249x179mm (300 x 300 DPI)


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Figure 2. ECH of FF in 0.5 M H2SO4 at e-/FF = 4: effects of acetonitrile ratio at j = -10 mA/cm2. 238x162mm (300 x 300 DPI)


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Figure 3. ECH of FF in 0.5 M H2SO4 at e-/FF = 4: effects of current density in 20-vol% acetonitrile. 238x162mm (300 x 300 DPI)



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Figure 4. Mole balance and yield of products as a function of conversion. Results of (a) 0.2 M NH4Cl and (b) 0.5 M H2SO4 at j = -10 mA/cm2. Trend lines of mole balance (MB) and yields combine all the acetonitrile ratios together in data set. 238x164mm (300 x 300 DPI)


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Figure 4. Mole balance and yield of products as a function of conversion. Results of (a) 0.2 M NH4Cl and (b) 0.5 M H2SO4 at j = -10 mA/cm2. Trend lines of mole balance (MB) and yields combine all the acetonitrile ratios together in data set. 243x162mm (300 x 300 DPI)



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Figure 5. Concentration changes of (a) FF, (b) FA and (c) MF in the absence of charge transfer in 20-vol% acetonitrile. Initial concentration was 50 mM FF, 25 mM FA and 25 mM MF. 237x172mm (300 x 300 DPI)


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Graphical Abstract 228x132mm (300 x 300 DPI)


Electrocatalytic Hydrogenation and ... - ACS Publications

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