Valorization of biomass-derived substrates via electrocatalytic hydrogenation-hydrogenolysis (ECH) is an attractive approach for selective production of organic chemicals. The electrocatalytic activity is strongly dependent on the surface coverage of adsorbed hydrogen radicals, which is a complex function of the catalytically active surface sites, electrolyte (pH and composition) and electrode potential. The performance of carbon-supported catalysts (Pt/C, Ru/C, Pd/C) was explored in the ECH of phenol and guaiacol in a stirred slurry electrochemical reactor where the cathode and anode compartments were separated by a Nafion® 117 membrane. Acid (H2SO4) and neutral (NaCl) catholytes were used. Pt/C showed superior activity in the acid-acid electrolyte pair, while the activity of Ru/C and Pd/C were significantly improved in the neutral-acid catholyte-anolyte pairs. By pairing neutral catholyte and acid anolyte, the anodic protons transported through the membrane can be effectively utilized for ECH reactions. In terms of reaction pathways for guaiacol ECH, ring saturation leading to 2-methoxycyclohexanol was generally the dominant pathway. However, for Pt/C in either 0.2 or 0.5 M NaCl catholyte paired with 0.5 M H2SO4 anolyte the demethoxylation–ring saturation pathway producing cyclohexanol and cyclohexanone was equally competitive at a constant superficial current density of -109 mA cm−2 and 50 0C. Efficient reductive upgrading of lignin model compounds can be achieved under mild conditions via electrocatalysis in the slurry reactor by exploiting synergistic effects between the catalyst and electrolyte.
Electrocatalytic hydrogenation-hydrogenolysis (ECH) is a promising approach for synthesis of bio-based chemicals [1,2,3]. The advantages of this process over the classic thermocatalytic routes are mainly attributed to the mild operating conditions using aqueous electrolytes with in situ H2 generation and the feasibility of product selectivity control by synergistic interactions between the electrode potential, current density, and temperature. As the cost of renewable electricity (e.g., wind and solar) becomes more economical, the electrochemical pathway could have a significant impact on the overall biomass conversion process and electricity storage in a chemical form [4, 5].
ECH has been studied in many applications for the reductive upgrading of biomass-derived substrates. Using carbohydrate derivatives, for instance, ECH of furfural has been demonstrated to produce furfuryl alcohol and 2-methylfuran using palladium and copper electrocatalysts [6, 7]. ECH of levulinic acid, the acid-catalyzed hydrolysis product of carbohydrates, has also been reported using a lead electrode to synthesize valeric acid and gamma valerolactone . As representative of lignin derivatives, phenolic compounds (e.g., phenol, guaiacol, syringol) can be electrocatalytically hydrogenated to a variety of products using ruthenium supported on activated carbon cloth (Ru/ACC) [9, 10]. Recently, it was shown that benzaldehyde can be selectively reduced to benzyl alcohol using carbon-supported metals incorporated on a carbon felt as the working electrode . Overall, the synthesis of bio-based chemicals via electrocatalytic reduction has been shown to be feasible under mild conditions without external H2 supply.
Most of the ECH studies were conducted in acidic media requiring catalysts which are stable and active under these conditions. The electrolyte pH plays an important role in the reaction affecting the catalyst stability, activity, and product distribution. Ruthenium has been reported as one of the least active metals toward H2 evolution reaction in acid media with increased ECH activity when supported on activated carbon [10, 12]. The activity of Ru/C was, however, inhibited by the presence of H2SO4 due to sulfate poisoning, similar to the sulfur poisoning in case of the liquid-phase hydrogenation of levulinic acid to gamma-valerolactone . On the other hand, ruthenium-based catalysts were recently reported to be more active than platinum toward H2 evolution reaction (HER) in alkaline media [14, 15]. In this regard, the choice of electrolyte and electrocatalyst are pivotal for the electrocatalytic process development.
Herein, a mild electrosynthesis of valuable chemicals from phenolic monomers (e.g., phenol and guaiacol) is presented using a stirred slurry electrochemical reactor (SSER). Cyclohexanol and cyclohexanone are among the main target products, which represent important chemicals for the industrial synthesis of Nylon polymers. The advantages of SSER compared to the fixed bed electrode configuration are simpler catalyst preparation, recovery and recycling, higher utilization efficiency of the catalyst bed, favorable liquid–solid mass and heat transfer and feasible operation at industrially relevant superficial current densities (i.e., above 100 mA cm−2 (in absolute value)). In the present work, following up on our previous study regarding the electrolyte effect on ECH , the catalytic activities of Pt/C, Ru/C, and Pd/C are comparatively studied for the ECH of phenol and guaiacol using acid-acid and neutral-acid catholyte-anolyte pairs of different concentrations.
The electrolysis experiments were conducted in a jacketed H-cell (CANSCI Glass Products Ltd.) with a Luggin probe (filled with 3 M KCl) for the reference electrode (Ag/AgCl) on the cathode side and a proton exchange membrane (Nafion® 117) between the chambers (Fig. 1). A cylindrical Pt wire was used as anode while Pt gauze (2.5 cm × 1.1 cm) served as cathode current feeder. A power supply (BK Precision 9110) was used for constant current operation (galvanostatic) in the H-cell connected to a recirculating water bath (Fisher Scientific Isotemp 3016 HS) for temperature control.
Initially, the H-cell was filled with catholyte (100 mL) and anolyte (40 mL) solutions. The catholyte was either H2SO4 (0.2 M) or NaCl (0.2 or 0.5 M) paired with acidic anolyte (H2SO4 0.2 or 0.5 M). The organic substrate (phenol or guaiacol) was then dissolved in the catholyte to obtain the initial reactant concentration of ca. 100 mM and stirred for 30 min before adding the supported catalyst powder (used as received without any treatment). A pre-electrolysis (initial cell polarization) was done for 5 min at − 0.5 A to activate the catalyst and ensure stable operation. ECH was then performed under galvanostatic control at the desired superficial current density and temperature with constant stirring (ca. 240 rpm with the magnetic stirrer bar length of 3.6 cm) of the catholyte with the slurry catalyst for 4 h. The stirring rate was determined experimentally by observing the dispersion of the catalyst at different rates. Low stirring rates caused poor dispersion whereas too high stirring rates created undesired vortex and random mixing patterns. Thus, for the cell size used in the present work 240 rpm was considered optimal.
Sample aliquots (1 mL) of catholyte were taken before reaction and periodically during the reaction using a syringe filter (to collect only the liquid part from the slurry while the retained solid portion was returned to the reactor to minimize catalyst loss). The anolyte sample aliquots were only taken after 4 h reaction time to check the possibility of diffusive loss from cathode to anode. The pH of electrolytes was measured using a pH meter (Thermo Scientific Orion Star A2110). The H-cell was subsequently washed, while the electrodes were cleaned ultrasonically for 30–60 min in distilled water.
Prior to the analysis, the samples were extracted with n-butanol (2 mL) and left overnight to ensure a complete phase separation. Product analyses were performed using gas chromatography (GC, Agilent 7820 A) with a mass selective detector (MSD, Agilent 5975). The organic solution was filtered using a 0.45 μm syringe filter before each analysis. The GC unit employed an HP-INNOWax column (30 m × 250 μm × 0.25 μm) and the injector temperature was set to 250 °C with a split ratio of 50:1. The oven program started at 40 °C for 5 min, heated to 150 °C at 5 °C/min and then to 260 °C at 15 °C/min, and the temperature was held for 5 min. The quantitative analysis of the products was performed based on calibration curves and response factors of each compound with standard chemicals. Parameter calculations, such as conversion, yield, selectivity, reaction rate, carbon balance, and Faradaic efficiency as well as materials and catalyst characterization data are provided in the Supporting Information (SI). In order to comparably evaluate the catalyst performance, reaction rate was used as the descriptor; and is defined as the mole of reactant (i.e. phenol or guaiacol) converted per hour (determined after t = 2 h) normalized by the mole of dispersed metal (Pt, Ru, or Pd).
Results and discussion
Synergy between electrocatalyst and electrolyte in the ECH
In the SSER configuration with carbon supported metal catalysts, an effective ECH requires at least three conditions: (i) availability of electrons on the dispersed catalyst particles which implies good electric contact among the particles and current feeder, (ii) efficient reduction of protons on the catalyst particles to generate adsorbed hydrogen (Hads) on the catalyst surface, and (iii) contact between reactant (organic) molecules and catalyst particles in the electric field. Our previous observations indicated that if one of the above-mentioned conditions was not met, the ECH could not proceed and virtually no hydrogenation products were identified . The presence of dispersed metal catalyst on the carbon support is essential since electrolysis of guaiacol using only activated charcoal resulted in no products (Table S2, Figure S2), suggesting that the reaction pathway involves Hads formed on the metal sites.
The synergy between electrocatalyst and electrolyte aims to maximize the Faradaic efficiency (F.E.), which implies that the electrons are predominantly used for the ECH of organics over the hydrogen evolution reaction (HER). Concerning the electrolyte effect, it is noteworthy that the proton/water reduction and hydrogen evolution reactions proceed differently in acid, neutral, or base solutions (Table 1). The first step in the electrocatalytic H2 evolution is the formation of adsorbed hydrogen radicals (Volmer steps: R1 and R4), which play an important role in ECH of the organics (see the elementary reactions in Table S3–S4). Hydrogen gas can be generated either via the electrocatalytic Heyrovsky step (R2 and R5), or via the thermocatalytic recombination of Hads, referred to as Tafel step (R3 or R6). In alkaline electrolytes, the HER kinetics is generally more sluggish than in acidic media due to an additional water dissociation step and the catalyst stability is poorer as well . Different catalyst and electrolyte properties would then expectedly determine the ECH efficiency. Experimental results on the electrocatalyst-electrolyte synergy are presented in the following sections.
Catalyst performance in acid-acid and neutral-acid pairs
The effects of different metal catalysts were investigated under the same operating conditions (i.e. j = − 109 mA cm−2, T = 50 °C, and t = 4 h) with the same catalyst amount (0.5 g) for ECH of phenol and guaiacol. In all cases, Pt/C showed superior activity than Ru/C and Pd/C, either in the acid-acid or neutral-acid pairs, possibly because of its higher surface area and metal dispersion (see Figure S1 and Table S1). In the ECH of phenol under acidic conditions (pH = 0.7–0.8), the reaction rate (in mmol per mmol dispersed metal h−1) decreases as follows: Pt/C (64) > Pd/C (33) > Ru/C (16). In the acid-acid pairs, the highest conversion (72%) and F.E. (90%) were obtained with Pt/C (Fig. 2a). With neutral-acid pairs, significant improvements were noticed in the activity of Ru/C and Pd/C (Fig. 2b), resulting in higher rates [Pd/C (52), Ru/C (32)] and phenol conversions (70–75%), cyclohexanol selectivities (70–100%) and F.E. (> 96%). Under these conditions, the catholyte pH increased from approximately 2 to 10 because of water reduction to hydroxide ions (beside H2), suggesting that the anodic protons transported through the cation exchange membrane were also effectively reduced on the catalyst surface.
Similar trends were observed in the ECH of guaiacol. In the acid-acid pair, the reaction rate decreases as Pt/C (54) > Pd/C (7) ≈ Ru/C (6). The best performing catalyst, Pt/C, resulted in higher guaiacol conversion (60%) and F.E. (69%) after 4 h reaction (Fig. 3a). Interestingly, in the neutral-acid pair, the activity of Pt/C dropped while Ru/C and Pd/C showed increased activities. The lower Pt/C activity in ECH of guaiacol under high pH (> 9) conditions had been reported previously , which could be attributed to the hampered guaiacol adsorption on the catalyst caused by the deprotonation  as well as the loss of Pt nanoparticles due to modification of the anchoring sites of the particles on the support . Ru/C showed the most dramatic increase in activity, resulting in over 5 times faster reaction rates (32 mmol h−1 vs. 6 mmol h−1, per mmol dispersed metal) with nearly 8 times higher conversion (48% vs. 6%) and 7 times higher F.E. (61% vs. 9%) compared to those obtained in the acid-acid pair. The higher activity of Ru-based catalysts than Pt toward HER was ascribed to the atomically dispersed Ru within the carbon matrix lowering the kinetic barrier for water dissociation . Meanwhile, Pd/C showed moderate activity in the guaiacol ECH, either in acid-acid or neutral-acid pair, which might be attributed to the instability (degradation and agglomeration) of its nanoparticles under acidic or alkaline conditions .
As for the product distribution, Ru/C favored full hydrogenation of the benzene ring producing mainly cyclohexanol (in the phenol ECH) and 2-methoxycyclohexanol (in the guaiacol ECH) (Figs. 2a, b and 3a, b). Carbon- and oxide-supported Ru were also recognized as efficient catalysts in the aqueous-phase hydrogenation of carbonyl compounds into the corresponding alcohols . Similarly, Pd/C also preferred ring saturation of guaiacol as shown by the dominant selectivity of 2-methoxycyclohexanol. This trend was also observed in the thermocatalytic hydrogenation of guaiacol over Pd/C at 200–260 °C, resulting in 2-methoxycyclohexanol as the major products (with only a small amount of cyclohexanol) . Generally, the hydrogenolysis of the C–O bond from guaiacol was difficult in case Pd/C as shown by the lowest cyclohexanol selectivity (Fig. 3a, b).
Having proven the good performance of Ru/C at high pH (i.e., prevalent in the neutral-acid electrolyte combination), increasing the Ru/C amount in the slurry (from 0.15 g to 1 g) generated a remarkable increase of conversion (from 13 to 73%) and F.E. (from 17 to 76%) after 4 h reaction time (Fig. 4a). This trend showed an obvious difference in terms of Ru/C activity compared to Pt/C under alkaline conditions (see SI, Figure S3b). In contrast to Ru/C, however, Pd/C did not show good activity in the guaiacol ECH even at high catalyst concentrations (Fig. 4b). In all cases with the neutral-acid pairs, the higher catalyst loading accelerated the catholyte pH increases, indicating that the protons were consumed faster thereby favoring the water reduction to hydrogen and hydroxide ions (Figs. 4 and S3b).
A secondary effect of the higher catalyst loading experiments was the lower carbon recovery due to adsorption of the organics on the supported catalyst and mainly on the activated charcoal support (see Table S6 in SI). Carbon balance analysis was further conducted in a series of blank experiments where the organic compounds in solution were mixed with the supported catalyst with no current applied. The results and discussion are provided in the SI, showing that adsorption of organics was mainly responsible for the decreasing carbon balance at the higher catalyst loading. However, note that all conversion data reported in this study were calculated by excluding the contributions from carbon losses to avoid overestimations. Thus, all the values purely represent the conversion from reaction.
The plausible reaction pathways for the ECH of guaiacol and phenol together with the activity order of the catalysts based on the reaction rates are illustrated in Fig. 5. Under similar conditions, phenol was converted faster than guaiacol regardless of the catalyst, showing its high reactivity in ECH. While phenol ECH proceeds in a series reaction to cyclohexanone and cyclohexanol, guaiacol ECH occurs in a parallel pathway involving demethoxylation and benzene ring saturation steps. In a recent publication, the phenol ECH has been reported to be a zeroth order reaction  as opposed to the guaiacol, which may be first or second order reaction depending on the operating conditions . Therefore, the phenol reaction rate is independent of the initial reactant concentration with hydrogenation of the adsorbed phenol being the rate determining step (RDS) , while the guaiacol ECH rate is dependent on the initial concentration in which guaiacol competitive adsorption with Hads was the likely RDS . Due to these mechanistic differences, different catalytic performances are expected.
The effect of electrolyte concentration
As the neutral-acid catholyte-anolyte pair was found to be advantageous for ECH , it is worthwhile to further investigate the impact of different electrolyte concentrations. Here, the ECH of guaiacol was performed using the combinations of neutral catholyte (NaCl) and acidic anolyte (H2SO4) with two different concentrations (0.2 M or 0.5 M) at a constant superficial current density (-109 mA cm−2) and temperature (50 °C). The same catalyst (Pt/C, Ru/C or Pd/C) loading was used (0.2 g, corresponding to a catalyst concentration of 13 wt.%) for all the experiments. Conversion, selectivity, F.E., and catholyte pH were monitored over the course of the reaction to better understand the influence of pH changes on the catalyst performance as well as the product distribution.
In case of Pt/C, the guaiacol conversion and F.E. increased with the anolyte proton concentration (Fig. 6a, b), confirming that Pt/C works more effectively at the lower pH conditions (1.8) where higher guaiacol conversion (65%) is achieved in the NaCl (0.2 M) catholyte paired with H2SO4 (0.5 M) anolyte. Product distributions were affected by the pH as shown by the time-dependent profiles (Fig. 6). At high pH (> 9), direct ring saturation route was predominant with the highest selectivity to 2-methoxycyclohexanol (62–72%) obtained at moderate guaiacol conversion (39–47%) (Fig. 6a, c). Meanwhile at lower pH (< 2), due to higher flux of proton across the membrane from the higher (0.5 M) H2SO4 concentration anolyte to the catholyte, the demethoxylation step gained significance as indicated by the fairly similar selectivity for cyclohexanol, cyclohexanone and 2-methoxycyclohexanol (Fig. 6b, d). The latter trends were also evident when low Pt/C loading was used in the acid-acid and neutral-acid pairs (Figure S5–S6, Table S5). Operation at higher catholyte NaCl concentration (i.e. 0.5 M vs. 0.2 M) lowered both the guaiacol conversion and F.E. especially after 2 h, but enhanced the selectivity toward cyclohexanol (i.e., demethoxylation – ring saturation pathway) particularly in case of 0.5 M NaCl – 0.5 M H2SO4 catholyte–anolyte pair (Fig. 6c, d). It is proposed that the interaction between surface pH and specific adsorption of Cl− on Pt could have altered the reaction pathways enhancing the competitiveness of demethoxylation – ring saturation.
In contrast, both Ru/C and Pd/C favored only the ring saturation pathway with 2-methoxycyclohexanol as the main product (selectivity ≥ 60%) under all the explored conditions (Figs. 7 and 8). Thus, catholyte pH changes did not dramatically affect the product distribution in the guaiacol ECH over Ru/C or Pd/C (unlike in the Pt/C cases). Furthermore, both the guaiacol conversion and F.E. were lower on Ru/C and Pd/C compared with Pt/C, with somewhat better performance under alkaline conditions for the former. The poor activity of Ru/C in acidic conditions was also found in the ECH of guaiacol using different acid electrolytes (0.2 M HClO4, 0.2 M HCl) whereby virtually no products were identified even after 12 h reaction (results not shown). Ru atoms are prone to dissolution in acidic media owing to corrosion and changes of the Ru oxidation state . The highest guaiacol conversions with Ru/C (34%) and Pd/C (28%) were obtained using the pair of NaCl (0.5 M) catholyte and H2SO4 (0.2 M) anolyte which showed the highest pH after 4 h reaction. Comparing the catholyte pH profiles, it is clear that with higher anolyte acid concentration (0.5 M) the catholyte pH remained low regardless of the catalyst, (Figs. 6f, 7f, and 8f). Moreover, the profiles in the first (Figs. 6e, 7e, and 8e), third (Figs. 6g, 7g, and 8g), and fourth pairs (Figs. 6h, 7h, and 8h) reveal the fastest pH increases over Pt/C, followed by Ru/C and Pd/C (see the overlay profiles in Figure S10). This shows that Pt/C catalyzes water reduction to hydroxide ions faster than Ru/C and Pd/C. In neutral electrolyte, this reaction is important as the first step in the electrocatalytic reaction that forms Hads (Table 1, R4). The faster Hads formation is desirable to promote the ECH rates over HER as long as the organics are present.
The overall results of the guaiacol galvanostatic ECH using different concentrations of neutral-acid electrolyte pairs with Pt/C, Ru/C, and Pd/C are also summarized in Table 2. In general, it can be stated that the measured cathode potentials under galvanostatic conditions for the three investigated catalyst beds were fairly similar for identical electrolyte compositions (Table 2). Therefore, the reaction rate results are reflective of the specific performance for the investigated catalyst beds. It is noted, however, that the measured cathode potentials cannot be taken as the true values on the catalyst surface due to two effects: (i) ohmic potential losses in the catholyte due to H2 gas evolving slurry of dispersed catalyst particles, and (ii) non-uniform potential and current distribution in the slurry bed. In all cases, higher catholyte NaCl concentration (0.5 M) resulted in lower (less negative) measured cathode potentials at constant operating current density because of lower ohmic resistance (Table 2).
In terms of the reaction rates, Pt/C clearly showed superior activity (around 6–7 times higher) than Ru/C and Pd/C (Table 2). The higher anolyte concentration positively affected the guaiacol conversion and F.E. over Pt/C, but it had a negative effect in case of Ru/C and Pd/C, implying that Pt/C works better for ECH of guaiacol at lower pH, while Ru/C (or Pd/C) is more effective at higher pH conditions.
Electrocatalytic reduction of lignin model compounds (i.e., phenol and guaiacol) has been investigated in a stirred slurry electrochemical reactor (SSER) under mild conditions (1 atm, 50 °C). Three different catalysts (Pt/C, Ru/C, Pd/C, with the same metal content of 5 wt.%) were tested in acid-acid and neutral-acid catholyte-anolyte pairs at constant superficial current density (− 109 mA cm−2). All the catalysts displayed good hydrogenation activity for phenol, where the activity decreased in the order of Pt/C > Pd/C > Ru/C, either in the acid-acid or neutral-acid pairs. In the ECH of guaiacol, Pt/C was the most active in the acidic electrolyte pair at low pH (< 0.8), however the activity of Ru/C improved in the neutral-acid pair when the catholyte pH increased due to the formation of hydroxide ions via water reduction. Such improvements were observed for either phenol or guaiacol ECH, implying that high pH (9–11) conditions were more favorable for Ru/C. The catalyst activity in the ECH of guaiacol decreased as follows Pt/C > Ru/C > Pd/C.
In terms of reaction pathways for guaiacol ECH, ring saturation leading to 2-methoxycyclohexanol was the dominant pathway, with the exception of Pt/C operated in either 0.2 or 0.5 M NaCl catholyte and 0.5 M H2SO4 anolyte. In the latter case demethoxylation–ring saturation producing cyclohexanol and cyclohexanone was equally competitive. Further fundamental studies are required to better understand these effects and their impact on the ECH pathways. Theoretical studies and molecular simulations may provide insights on how different metal surfaces affect the ECH mechanisms as well as the HER catalysis. In situ microscopic, structural, compositional characterizations could help identify the changes on the catalyst surface during the ECH and HER reactions .
Pairing neutral (NaCl) catholyte with acidic (H2SO4) anolyte has been shown to improve the Faradaic efficiency with the possibility of pH and selectivity control, which reveals the electrocatalyst-electrolyte synergy and opens new opportunities for process design. With respect to the reactor design, the SSER could be implemented in a large-scale application as either a fluidized bed or moving bed electrocatalytic reactor. This work focused on the comparative investigation of the fresh carbon-supported metal catalyst performance in different electrolyte pairs. Reusability tests of Pt/C have been performed in our previous work using acid (H2SO4) and neutral (NaCl) catholytes. The spent Pt/C catalysts remained active after simple recovery steps (filtration and oven drying), despite some morphological changes . Stability and characterization experiments for Ru/C and Pd/C are subject of further research.
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This work is supported by a collaborative project between the University of British Columbia and Korea Institute of Science and Technology (KIST) on-site laboratory. The support of NSERC through the Discovery Grant (for EG) is gratefully acknowledged. The authors thank Priyanthika Adinamozhi, Daichi Hirata, and Robertus D. D. Putra for the technical supports.
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Wijaya, Y.P., Smith, K.J., Kim, C.S. et al. Synergistic effects between electrocatalyst and electrolyte in the electrocatalytic reduction of lignin model compounds in a stirred slurry reactor. J Appl Electrochem 51, 51–63 (2021). https://doi.org/10.1007/s10800-020-01429-w
- Biomass conversion
- Electrocatalytic hydrogenation