Electrocatalytic Hydrogenation of Oxygenated Compounds in

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Electrocatalytic Hydrogenation of Oxygenated Compounds in Aqueous Phase Udishnu Sanyal, Juan A. Lopez-Ruiz, Asanga Padmaperuma, Jamie Holladay, and Oliver Y. Gutiérrez Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.8b00236 • Publication Date (Web): 24 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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Organic Process Research & Development

Electrocatalytic Hydrogenation of Oxygenated Compounds in Aqueous Phase Udishnu Sanyal, Juan Lopez-Ruiz, Asanga B. Padmaperuma, Jamie Holladay, Oliver Y. Gutiérrez* Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States Corresponding author: [email protected] Tel. +1 509 3717649

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Organic Process Research & Development 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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Abstract Electrocatalytic hydrogenation is a fascinating strategy to hydrogenate biogenic compounds at ambient conditions by replacing the thermal and H2 inputs by cathodic potential. This work compares the performance of this approach (in aqueous phase at room temperature) for a variety of model oxygenated compounds reacted over a series of metals. The target functionalities were carbonyl groups, aromatic rings, and ether bonds. All metals explored (Pt, Rh, Pd, Cu) are active for the reduction of carbonyl compounds to alcohols. For instance, conversion rates of benzaldehyde increased as a function of metal as: Pt < Rh < Pd (Cu was tested at different conditions). In contrast, only Rh and Pt were found active for hydrogenation of aromatic rings (Rh was more active than Pt). Comparing among the target functionalities, carbonyl groups are more reactive than aromatic rings and ether bonds in phenolic compounds and di-aryl ethers on all explore metals. This carbonyl reactivity, however, is enabled by the aromaticity of the molecule. Hence, the reactivity trend of the examined molecules is: butyraldehyde < furfural < acetophenone < benzaldehyde. For phenolic compounds, phenol is more reactive than cresol and methoxy phenol. Thus, the presence of substituting groups at the functionality being converted (either carbonyl or aromatic ring) decreases conversion rates. Ether bonds are cleaved under electrocatalytic conditions, which opens two main pathways for the conversion of aryl ethers: hydrogenation of the aromatic ring and hydrogenolysis of the ether bonds, whereas hydrolysis occurs as a minor pathway. Electrocatalytic hydrogenation competes with the H2 evolution reaction at the conditions of the tests, therefore Faradaic efficiency (fraction of current utilized in hydrogenation) and hydrogenation rates are correlated. That is, within the potential range explored, increasing hydrogenation rates lead to higher Faradaic efficiencies. The slope of this correlation, however, depends on the potential and on the functionality being hydrogenated. Keywords: Hydrogenation, electrocatalysis, metal catalysis, aqueous phase hydrogenation, biomass conversion.

Introduction Renewable and underutilized hydrocarbon resources are the main alternative to fossil-based technologies for sustaining, at least partially, our fuel and chemical industry. Just converting waste biomass via bio-oil to fuels and chemicals has the potential to displace up to 20 % of



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fossil oil consumed in the world.1 The main challenges towards the utilization of these abundant resources is their heterogeneity, geographic dispersion and their generation rates, which are incompatible to those of petroleum and chemical manufacturing. Thus, the scale of the chemical manufacturing technology has to shrink (compared to typical industrial practice) and the severity (pressure and temperature) of the process needs to decrease, and to be compatible with large concentrations of oxygenated compounds and water in the feed. However, the losses in rates expected with the shift of chemical technologies to milder reactions must be avoided to come up with viable novel processes.1 Thus, disruptive technologies are needed to enable lowtemperature, low-scale, oxygen-compatible technology. In this context, the approach of combining electrochemistry with heterogeneous catalysis can contribute to achieving low temperature conversions and developing modular processes responding to the grand challenge for scientists of increasing the diversity of resources for production of chemicals and energy.2 Hydrogenation is a mandatory step in the conversion of biogenic feeds to fuels and many valuable chemicals.3-6 Performing this step at mild conditions in the presence of an external bias, i.e., electrochemical hydrogenation is more relevant than ever before because of the increasing availability of renewable electrical energy, and the significant potential energy and cost savings of replacing thermal and H2 inputs with cathodic potential.1 However, this approach has been explored mostly for preparative purposes and relatively few investigations have been directed to implement low temperature conversion of biogenic feedstocks in aqueous phase.7-14 Several aspects of the electrocatalytic hydrogenation has been explored to different extents such as solvent effects,7,

15

metal dispersion,16-19 and cathodic potentials.14,

20-21

Due to the

increasing interest in the topic, recent years have seen a significant increase in reports focusing on the mechanistic details of organic electrocatalytic hydrogenation and on scaling up the process.12, 22-25 However, it is challenging to make comparisons across different reports because basic details are often not reported such as, applied potential, area of metal exposed, intrinsic turnovers, preconditioning of the material, etc. Although general trends emerge from literature, metal activity, reactivity of the molecules, effects of reaction parameter often can be only quantitatively discussed. In this work, we report the electrocatalytic hydrogenation in aqueous phase of a series of model compounds on selected metals (carbon-supported Pd, Rh, and Pt) focusing on the reactivity of carbonyl groups, aromatic rings and ether bonds because they are some of the most common functionalities in biomass-derived molecules. We compare the performance of the reactions in terms of electrocatalytic hydrogenation rates and efficiencies (comparing hydrogenation of organic compounds with the H2 evolution reaction) observed at identical


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conditions. We revisit findings reported before, report new observations, and discuss the intrinsic activity of the explored metals, the reaction mechanisms and their consequences for catalytic rates.

Scheme 1. The three main processes occurring during electrocatalytic hydrogenation: proton reduction leading to hydrogen adsorbed on the metal surface (Volmer step); electrocatalytic hydrogenation that can occur upon the reaction of the organic compound with a proton-electron pair or with an adsorbed hydrogen; H2 evolution via Heyrovsky or Tafel steps. In this scheme M denotes an active site at the metal surface and the different elementary steps are grouped as the step for production of adsorbed H (Volmer step), electrocatalytic hydrogenation and H2 evolution. As a preliminary note, we have to emphasize that in most of the systems explored and reported in literature, electrocatalytic hydrogenation is accompanied by the H2 evolution reaction (HER) because it is difficult to hinder the interaction of protons with charged surfaces without affecting hydrogenation. HER occurs readily and becomes the prevalent side reaction competing with hydrogenation for reduction equivalents. The mechanisms of both reactions are closely related as shown in Scheme 1. Upon proton reduction according to the Volmer step, the produced adsorbed hydrogen has two choices, either react toward HER or toward hydrogenation if an adsorbed reactive organic compound is present. HER can occur via reaction between neutral species or via an electrochemical elementary step (Tafel and Heyrovsky steps, respectively). It is possible that the hydrogenation of the organic compound occurs as well via addition of neutral adsorbed hydrogen or via proton coupled electron transfer (PCET). The nature of



electrochemical organic reduction might depend on many reaction parameters but it is clear that stepwise electron addition is enable at relatively high cathodic potentials leading to radical (outer sphere) products.24 In this work we report the electrocatalytic hydrogenation of phenol on Pt/C and Pd/C and compared the observations with previous results obtained for phenol and aryl ether conversion on Rh/C.14 We compare the catalytic vector of phenol with that of benzaldehyde on the same metals25 and report the reactivity of diverse carbonyl compounds on Rh/C (i.e., benzaldehyde, acetophenone and furfural). Then, we give an overview of the hydrogenation rates and Faradaic efficiencies of all compounds that we report in this paper and those reported in previous contributions,14,

25

all measured at the same conditions. Finally, we illustrate the continuous

electrocatalytic hydrogenation of the same carbonyl compounds at conditions that deviate from those of ideal batch operation.

Experimental Chemicals The chemicals used in the experiments were obtained from Sigma Aldrich: phenol (≥99.0%), methylphenol (≥99.0%), methoxyphenol (≥99.0%), diphenyl ether (≥99.0%), p-tolyl ether (≥99.0%), benzyl phenyl ether (≥99.0%), acetophenone (≥99.0%), furfural (≥99.0%), butyraldehyde (≥99.0%), and benzaldehyde (≥99.0%). An acetate buffer (pH 4.6; 3M sodium acetate and acetic acid) was used as electrolyte in the anode and cathode compartments. Catalysts and Characterization The powder catalysts, metals on activated carbon, reported in this work were purchased from Sigma Aldrich. The physicochemical characterization: textural properties (surface area, pore volume), area of exposed metal, metal content, and metal particle size as well as the characterization methods are reported in Refs.14, 25 The concentration of exposed metal (Table S2), as determined by hydrogen chemisorption, was used to calculate the TOF as described in the supporting information. Two catalysts were prepared by depositing Pd and Cu directly on carbon felts. They were prepared by impregnating the graphitic carbon felt (Alfa-Aesar #43200) with solutions of palladium (II) acetate (Pd(O₂CCH₃)₂, Sigma-Aldrich, 98%) or copper acetate (Cu(O2CCH3)2, Sigma-Aldrich, 98%) dissolved in tetrahydrofuran (Sigma-Aldrich, anhydrous, ≥99.9%). After


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impregnation, the felt was dried for 4 h at room temperature and then treated at 483 K in air for 1 hour and at 783 K for an additional hour. The metal loadings in the final felt were 4 wt. %. The materials were reduced in situ at constant current as described in the following section. Electrocatalytic hydrogenation Electrocatalytic hydrogenation was performed in a two-compartment H-cell described in refs.14, 25

Nafion 117 (Ion Power, Inc.) was used to separate cathodic and anodic compartments. Prior

to use, the Nafion membrane was treated in a H2O2 solution (3 vol. %) and in sulfuric acid (2 M). A piece of carbon felt (Alfa Aesar >99.0%, 3.2 mm thickness) attached to a graphite rod (Sigma Aldrich, 99.99%) was used as working electrode in the cathode compartment, whereas a Pt mesh was used as counter electrode in the anodic compartment. Ag/AgCl electrode with a double junction protection was used as reference electrode. A volume of 60 mL of the buffer solution was used in the cathode compartment. The powder metal/carbon (10 mg) catalyst was infiltrated in the carbon felt in each experiment. Before adding the reactant, the catalyst was polarized for 30 min at -40 mA. All experiments were performed at fixed potential and atmospheric pressure while a flow of N2 was kept through the reactant solution and the temperature was controlled with a circulator (Julabo F25-ED). The anode compartment was filled with 60 mL of the same buffer solution used in the cathode compartment. All electrochemical procedures were performed with an electrochemical workstation VSP-300, Bio Logic. The presence of internal mass transfer limitations was assessed in Ref.,25 where the effectiveness factor and Weisz modulus indicated that the measured kinetics were not limited by internal mass transport. Some experiments were performed on Pd and Cu deposited directly on carbon felts using the continuous flow reactor described in detail elsewhere.26 Two Cole-Parmer gear pumps were used to feed anolyte and catholyte at flow rates of 2 ml min-1. An AMETEK® VersaSTAT 4 Potentiostat Galvanostat was used to operate the cell. The volume of the gas evolved was monitored with a bubble gas flow meter to the glass vial. Prior to the tests the catalyst was reduced in situ at constant current of -220 mA (potential: -1.4V vs Ag/AgCl) for 2h using a catholyte composed of methanol:H2O (50:50 wt.) with 5 wt.% acetic acid. After reaction, Pd remained in metallic state as shown in Figure S3a. Cu oxidized rapidly as shown in Figure S3b. The Pourbaix diagram shown in Figure 3c, however, shows that Cu remains in metallic state at the conditions of the experiments as long as the potential remains more negative than -0.35 VAg/AgCl at the pH of the experiment, i.e., at pH 2.5.



The experiments were performed at room temperature, at constant cell potential. Steady state operation was reached after 15 minutes of changing the reaction conditions (feed composition and full cell potential). The concentration of the catholyte in all experiments was 20 mM substrate and 40 mM diglyme (internal standard) in a mixture of IPA:H2O (50:50 wt.) and 5 wt.% acetic acid (pH 3.1). The anolyte was 1 M KOH in MeOH:H2O (10:90 wt. %). Catalyst deactivation was not observed under the reaction conditions explored in this study. Product analysis The reactions in the batch, H-cell were monitored by withdrawing aliquots of 1 mL from the cathode compartment and extracting the compounds from the liquid phase with 3 mL of ethyl acetate (Sigma-Aldrich, ≥99.9%, HPLC). The organic phase was further dried over Na2SO4, and was analyzed by gas chromatography coupled with mass spectrometry (Shimadzu GCMSQP2010, with a plot Q capillary column, 30m x 250 µm, and a thermal conductivity detector). The amounts of H2 evolved during the experiments were determined by volumetric measurements or by analysis of the gas phase with a mass spectrometer. Conversion is determined as the change of concentration of reactant per unit time and Faradaic efficiency (FE) is defined as the fraction of the current utilized for hydrogenation of organic compounds compared to the total current passed during the reaction. Turnover frequencies (TOF) were calculated by normalizing the conversion rates of the organic compounds by the concentration of exposed metal atoms. Intrinsic H addition rates (TOFH-add) were determined by multiplying the TOF of molecular conversion by the number of electrons consumed for the first reaction step (e.g., two and four for the hydrogenation of carbonyl groups in benzaldehyde and aromatic ring in phenol, respectively). The reaction products obtained in the continuous flow reactor were analyzed by highperformance liquid chromatography using a Waters 2414 refractive index detector. A Bio-Rad Aminex HPX-87H ion exclusion column (300mm × 7.8mm) was used for anolyte separation. Sulfuric acid (0.005M) was used as eluent at a flow rate of 0.55 ml min-1.

Results and discussion Catalyst characterization.


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All powder catalysts (metals supported on carbon) contained 5 wt. % of the metal and their characterization has been reported in Refs.14,

25

The catalysts on the felt had 4 wt. % of the

metal and its characterization has been reported before.26-27 Conversion of phenol on Pt, Rh, and Pd. Phenol is the simplest phenolic compound and is thus utilized as a key model compound for lignin-derived feedstocks and to obtain activity-structure correlations.28 Thus, electrocatalytic hydrogenation of phenols has been well documented in literature. Table S1 in the supporting information shows and scheme with the series of events that lead to hydrogenation, emphasizing key elements of the process and a qualitative description of some of the common findings in literature. The electrocatalytic conversion of phenol has been explored mainly on Ptgroup metals and the studies have shown that at room temperature and at pH 4.6, cyclohexanone is the primary product and cyclohexanol is the secondary and final product.7, 12, 14, 29-31

The concentration profiles that we found for the conversion of phenol on Pt/C (shown in

Figure 1A) agree well with this description as we also found for the conversion on Rh/C.14 Mechanistically, the hydrogenation of the phenolic ring has been reported to occur via transfer of neutral hydrogen (H) adsorbed at the surface, i.e., it is not an electrochemical step.32-33 In support of this claim, we found that the activation energy of phenol hydrogenation on Pt/C is identical if it is driven by H2 or by cathodic potential as shown in Figure 1B that reports an activation energy of 34 kJ mol-1. This value is very similar to that reported before (30 kJ mol-1) for electrocatalytic phenol hydrogenation on Pt at conditions with large potential drops.31 Thus, the effective cathodic potential seems to control the rate at which adsorbed hydrogen is made available rather than the energy barrier the reaction or the nature of the elementary steps.



Figure 1. A: typical concentration profiles of phenol undergoing hydrogenation on Pt/C at -0.7 V vs Ag/AgCl (empty symbols) or with 1 bar H2 (filled symbols) with initial concentration of 20 mM phenol in acetic buffer with pH 4.6. The symbols represent the concentration of phenol (circles), cyclohexanone (squares) and cyclohexanol (triangles). B: reaction rates for phenol hydrogenation obtained at varying temperatures using cathodic potential at -0.7 V vs Ag/AgCl (empty symbols) or 1 bar H2 (filled symbols) flowing through the system. Rh/C and Pt/C are active for phenol hydrogenation with rates linearly increasing with negative potential as shown in Figure 2A. Pd/C, in contrast, is relatively inactive for the reaction in agreement with previous attempts.7, 31 Rh is more active than Pt and the FE follows the same trend, i.e, the FE for phenol (and cyclohexanone) hydrogenation is the highest on Rh/C followed by Pt/C, whereas the FE on Pd/C is close to null (Figure 2B). Note that the FE on Rh and Pt do not vary to a large extent in the potential window explored. It also must be emphasized that Pd/C generated a current of, e.g., -119 mA at -0.7 V vs Ag/AgCl but all of it is utilized for HER. For comparison, the observed currents were -121 mA and -140 mA on Pt/C and Rh/C, respectively, at the same conditions.


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Figure 2. A: Hydrogenation rates of phenol on Rh/C (blue circles), Pt/C (green triangles), and Pd/C (red squares) at varying potentials and pH 4.6 in acetic buffer and room temperature. B: corresponding Faradaic efficiencies. The data of the reaction on Rh/C was adapted from a previous report.14 Adapted with permission from reference.14 Copyright 2016 Elsevier.

Conversion of benzaldehyde on Pt, Rh, and Pd. Benzaldehyde, in contrast to phenol, undergoes hydrogenation of the carbonyl group leading to benzyl alcohol without ring hydrogenation or any other step at mild pH.25 Reference experiments applying benzyl alcohol as starting reactant showed that it is indeed inactive at the explored conditions. The trend of activity for carbonyl hydrogenation also deeply contrasts that of ring hydrogenation. Figure 3A shows that Pt/C and Rh/C have similar activity in the explored potential range. Pd/C exhibits similar activity at -0.7 vs Ag/AgCl or less negative potentials but it becomes the most active catalyst at more negative potentials. In all cases, the rates increase



with increasingly negative potentials. The most outstanding feature of the catalytic vector of Pd/C is that it exhibits near full selectivity toward hydrogenation as shown in Figure 3B (FE above 95 %). Although the FE on Rh/C remains invariant, Pt/C shows decreasing FE as the cathodic potential becomes more negative.

Figure 3. A: Hydrogenation rates of benzaldehyde on Rh/C (blue circles), Pt/C (green triangles), and Pd/C (red squares) at varying potentials and pH 4.6 in acetic buffer and room temperature. B: corresponding Faradaic efficiencies. The data has been adapted from Ref.25 Adapted with permission from reference.25 Copyright 2018 Elsevier. We attribute the contrasting reactivity of the aromatic rings in phenol and benzaldehyde to different adsorption strength induced by the methylene bridge. For instance, phenol and benzaldehyde interact more weakly with metal surfaces than benzene due to the steric hindrance that the hydroxyl and aldehyde groups pose to the ring distortion needed to form the coplanar η6 adsorbed states.34 Gas-phase adsorption energies of phenol and benzaldehyde are 10 kJmol-1 and 20 kJmol-1, respectively, lower than that of benzene.35 In solution, the


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adsorption energy of polar molecules decrease further,36 i.e., by up to 25 %.37 For benzaldehyde, there has been claimed a weakened interaction with the metal due to repulsion of the aromatic ring from the metal surface.38 On the other hand, it is surprising that the sole presence of the hydroxymethyl group in benzyl alcohol hinders the η6 adsorption needed for the consecutive hydrogenation of the aromatic ring. The interaction strength of the organic compound with the metal seems to have a key role in electrocatalytic hydrogenation. However, more work is needed to understand this specific low-temperature reactivity of organic compounds. Comparing across organic compounds, it is clear that the hydrogenation of the carbonyl group in benzaldehyde is faster than the hydrogenation of the aromatic ring in phenol, e.g., ~1800 h-1 (benzaldehyde hydrogenation on Pt/C, Pd/C, and Rh/C) and 280 h-1, or 450 h-1 (phenol hydrogenation on Pt/C and Rh/C, respectively) at -0.7 V vs Ag/AgCl. The corresponding intrinsic H addition rates (TOFH-add) are ~3600 h-1 (benzaldehyde hydrogenation proceeds upon the addition of two electrons) and 1120 h-1, or 1800 h-1 (phenol hydrogenation proceeds upon the addition of four electrons). This difference is due to the intrinsically higher reactivity of the carbonyl group compared to aromatic rings. The reactivity of the former, associated to a high polarization of the C=O bond has led to the postulation of PCET mechanisms for the reduction on benzaldehyde taken as basis the numerous differences in hydrogenation (activation barriers, reaction orders and activation energies) when it is driven by cathodic potential or by dissociated H2.25, 39 Conversion of carbonyl compounds and ethers on Rh/C. In order to explore the chemistry of carbonyl hydrogenation, we compared the electrocatalytic hydrogenation of benzaldehyde (aromatic aldehyde) with acetophenone (aromatic ketone), and furfural (pseudo aromatic aldehyde). In all cases, the only products were the corresponding alcohols:

benzyl

alcohol,

1-phenylethanol

and

furfuryl

alcohol,

(from

benzaldehyde,

acetophenone, and furfural respectively). In all cases, the conversion rates increase linearly with increasingly negative cathodic potential (Figure 4A), whereas the FE does not vary to a large extent in the potential window explored (Figure 4B). These trend indicates that the reactivity decreases with substitution, i.e., the rates of acetophenone conversion are lower than those of benzaldehyde in a similar way as it was observed for substituted phenols compared to phenol on the same Rh/C catalyst.14 Aromaticity, on the other hand, also seems to play a role in reactivity as furfural, a pseudo aromatic compound, exhibits lower reactivity than benzaldehyde and acetophenone while butyraldehyde (aliphatic aldehyde) was unreactive at the same



conditions. Finally, the FE follows the same trend as the conversion rates, i.e., it decreases from ~60 % for benzaldehyde to ~40 % for acetophenone and to ~20 % for furfural (Figures 4A and 4B). Figure 4 also shows that the hydrogenation of furfural on Pt/C leads to lower rates and FE than on Rh/C although the main trends persist, i.e., increasing rates and slightly decreasing FE with cathodic potential.

Figure 4. A: Hydrogenation rates of benzaldehyde, acetophenone and furfural on Rh/C at varying cathodic potentials, pH 4.6 and room temperature. The corresponding reactions are shown on the right. B: Corresponding Faradaic efficiencies. The symbols represent the data obtained using: benzaldehyde (blue circles), acetophenone (green triangles), and furfural (red squares). As a comparison, the results of furfural hydrogenation on Pt/C are also shown (filled red squares). The data of benzaldehyde electrocatalytic hydrogenation was taken from Ref.25 Adapted with permission from reference.25 Copyright 2018 Elsevier. Among the explored catalysts, Rh/C showed high activity for both phenol hydrogenation and carbonyl hydrogenation maintaining moderately high FE. Therefore, it has been selected to perform specific studies with a wider variety of compounds. In a previous study,14 we show that Rh is also active for the electrocatalytic reduction of aryl ethers, which is a family of compounds that follow at least two parallel conversion routes: hydrogenation of aromatic rings and C-O


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bond cleavage of the ether bond. This is shown for two model compounds in Scheme 2. In the hydrogenation routes, the aromatic rings are sequentially hydrogenated and if a carbonyl group forms leading to ketones, they are hydrogenated as well yielding aliphatic cyclic alcohols (analogously to the network for phenol hydrogenation). In the hydrogenolysis routes, the C-O bond scission leads to phenolic and aromatic compounds that then undergo hydrogenation. The reaction conditions can be optimized towards C-O bond hydrogenolysis.8 However, the selectivity to these two pathways depends on the structure of the ether, i.e., the strength of the ether bond seems to be determining. In the case of methoxyphenol, for instance, the selectivity to hydrogenolysis is 10 % and the strength of the C-O bond that undergoes scission is 415 kJ mol-1. In case of benzyl phenyl ether, the relatively weak α-C-O bond (218 kJ mol-1) allows for a hydrogenolysis selectivity of 60 %. It is important to note that the hydrogenolytic bond cleavage allows for the production of hydrocarbons. Thus, the electrocatalytic reduction of biogenic feeds has the potential of yielding hydrocarbons depending on the molecular structure. There is a second C-O bond cleavage mechanism identified in the studies, i.e., hydrolysis, which cleaves the bond inserting water. This mechanism, however, is minor with selectivity below 4 %. It worth noting that in the original report, benzyl alcohol, one of the hydrolysis products (from benzyl phenyl ether), was suggested to hydrogenate to cyclohexanemethanol. However, studies on hydrogenation of benzaldehyde and benzyl alcohol (see below) have shown that the latter is not hydrogenated

under

the

conditions

cyclohexanemethanol observed in Ref.

of 14

our

studies.

Thus,

the minor

amounts

of

are concluded to be produced by hydrolysis of a

partially hydrogenated ether as described in Scheme 2.

Scheme 2. Reaction networks of the electrocatalytic conversion of methoxyphenol (A) and benzyl phenyl ether (B) on Rh/C at room temperature and cathodic potentials more positive than -1 V vs Ag/AgCl. The networks and data were adapted From Ref.14 The elements in red



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(hydrolysis of the hydrogenated intermediate) is a corrections proposed to that reported network. Adapted with permission from reference.14 Copyright 2016 Elsevier. To put the reactivity of ethers in perspective, Table 1 compares the results of hydrogenation of the most reactive molecule found so far, benzaldehyde, with phenol and ethers. At conditions, where similar currents are drawn by Rh/C (around -100 mA), the conversion of 4methoxyphenol and the associated FE (138 h-1, 35 %) are just half those observed for phenol conversion (296 h-1, 68 %). The rates and efficiencies decrease further for diphenyl ether and ptolyl ether (60 h-1, 43 % and 43 h-1, 18 %). Altogether, it seems that the hydrogenation rates of the aromatic ring decrease with increasing the complexity of the molecule but without affecting the rates at which protons are reduced. The case of hydrogenation of benzyl phenyl ether is different because it reacts mostly via hydrogenolysis (selectivity of 60%), whereas the main contributions for the conversion of the other ethers come from hydrogenation of the ring. Thus, hydrogenation of benzyl phenyl exhibits the highest conversion rates and FE among the explored di-aryl ethers. Also note that more negative applied potentials are needed to draw ~100 mA in case of di-aryl ethers than for the other model compounds but we attribute this, at least partially, to the presence of i-propanol in the mixture (needed to achieve the dissolution of the reactant). Based in the rates of H addition (Table 1), all the discussed differences in reactivity decrease but the trends remain invariant.

Table 1. Currents, TOF, intrinsic H addition rates, and Faradaic efficiencies observed at fixed potentials during the electrocatalytic hydrogenation of selected aromatic compounds on Rh/C. All reactions were performed at room temperature, atmospheric pressure, pH 4.6 in acetate buffer or in a mixture of acetate buffer and i-propanol (for the di-aryl ethers). Data taken from Refs.14, 25 Benzaldehy Phenol2 de1

4Methoxyphenol2

Diphenyl ether3

p-Tolyl ether3

Benzyl phenyl ether3

Potential (V vs Ag/AgCl)

-0.7

-0.6

-0.6

-0.9

-0.9

-0.9

Current (mA)

-125

-100

-95

-100

-100

-100

Initial TOF (h-1)

1419

296

138

60

43

88

TOFH-add (h-1)

2838

1184

524

264

189

317

FE (%)

59

68

35

25

18

36


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Reaction conditions:

1)

1.210-3 moles of benzaldehyde; 10 mg of 5wt% Rh/C catalyst.

moles of phenol; 20 mg of 5wt% Rh/C catalyst.

3)

2)

9.610-4

510-4 moles of diphenyl ether, p-Tolyl ether

and benzyl phenyl ether; 50 mg of 5wt% Rh/C

General comparison of rates and efficiencies. Figure 5 shows a complete overview of the reactivity of several model compounds (conversion rates and associated Faradic efficiencies observed in the batch cell) grouped by families, over Rh, Pt, and Pd. It is interesting that TOFs and FE spam in such wide ranges strongly depending on the reactant-metal combination. It is clear that aromatic carbonyl groups (benzaldehyde, acetophenone) are more reactive than phenolic compounds and also convert on all metals tested. Phenolic compounds, in contrast, are not reactive on Pd. Butyraldehyde also exhibits very low reactivity on Rh. Aryl ethers are, in general, the least reactive molecules. By grouping the data of different model compounds in families, correlations between FE and conversion rates arise. That is, for a given functionality being hydrogenated at the same potential, the faster the hydrogenation occurs, the higher the FE is. This correlation likely results from the competitive nature of hydrogenation and HER highlighting the importance of the interaction of the organic with the metal surface. This correlation, however, does not necessarily imply that the best candidates for electrocatalytic hydrogenation are metals with high overpotential for HER because adsorbed hydrogen is anyways needed for the reaction. On the other hand, the metal should still be able to adsorb the organic compound and to add hydrogen to it, which is a slow process on metals with high HER overpotential. Finally, at the very negative potentials accessed on base-metals, radical (outer sphere) mechanisms operate, which are difficult to control and do not guarantee that H2 evolution can be controlled.



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Figure 5. Comparison of the reactivity (Faradaic efficiency and intrinsic conversion rates) of different compounds undergoing hydrogenation on Rh, Pt, and Pd (we also present the data of benzaldehyde converted on Ni). The labels denote the model compounds and the metal used for the reaction. The symbols represent: phenolic compounds (blue circles), aryl ethers (green triangles),

and

carbonyl

compounds

(orange

squares).The

abbreviations

are:

APO

(acetophenone), BZH (benzaldehyde), BPE (benzyl phenyl ether), BTH (butyraldehyde), BPE (benzyl phenyl ether), BZH (benzaldehyde), DPE (diphenyl ether), FRL (furfural), MePOH (methylphenol), MXPOH (methoxyphenol),POH (phenol). Most of the data was obtained under 0.8 V vs Ag/AgCl of applied potential, in acetate buffer solution, room temperature and pH 4.6. Experiments using BPE, DPE, and PTE were performed in water:isopropanol mixtures under -1 V vs Ag/AgCl of applied potential. Data of the conversion of BPE, DPE, PTE, MePOH, MXPOH, and POH on Rh was taken from Ref.14 Adapted with permission from reference.14 Copyright 2016 Elsevier. Data of the conversion of BZH on Pd, Rh, Pt, and Ni was taken from Ref.25 Adapted with permission from reference.25 Copyright 2018 Elsevier. Toward continuous electrochemical hydrogenation. We are expanding our research towards designing continuous processes and usage of base metals as catalysts. Therefore, the electrocatalytic hydrogenation of a series of carbonyl model compounds was performed in a flow mode on Pd and Cu deposited directly on felts. For the series of carbonyl compounds that were tested, benzaldehyde was the most reactive molecule followed by furfural and acetophenone with similar reactivity (Figure 6). This trend is the same on both Pd/Cfelt and Cu/Cfelt and in all cases the only products obtained were the corresponding


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aromatic alcohols produced upon hydrogenation of the carbonyl group. Cu has lower activity and lower efficiencies than Pd but still in the same range. The results in Figures 6A and 6B clearly show that electrocatalytic hydrogenation can be successfully translated from a batch cell to a continuous flow reactor and that a base metal such as Cu is active and stable under electrochemical conditions. Another example is Ni/C which is compared in Figure 5 with Pt-group metals for benzaldehyde hydrogenation. Although the conversion rates and FE on Cu (Figure 6B) and Ni (Figure 5) are lower than on Pt-group metals, the difference in activity, is not as large as expected from the performance of the two groups of metals for gas-phase hydrogenation.40 This can be a determining factor for scaling up as the catalyst cost may be minimize with a base metal without proportional losses in activity. The FE, on the other hand, is higher on Pt-group metals than on Cu or Ni, which is unexpected considering the high overpotential for H2 evolution of the base metals. Another feature that must be noted about the results in Figure 6 is that, at a given current, the FE is proportional to conversion rates and different molecules fall on the same correlation. This is well in line with the trends observed in Figure 5. This, together with the higher FE of Pt-groups metals for carbonyl reduction than Cu and Ni, suggests that the descriptor for electrocatalytic hydrogenation will not be the overpotential for H2 evolution but the adsorption energy of the organic compounds with the catalyst surface. It is interesting to note that in the experiments shown in Figure 6A, the hydrogenation of benzaldehyde on Pd was not as selective as that reported in Figures 3B and 5. A parallel study shows that this difference is due to the presence in the reaction media of a co-solvent, ipropanol, which impacts the interaction of hydronium with the metal surface.26 At present we speculate that similar effects lead to the similar reactivity of furfural and acetophenone (the former is slightly more reactive) in the presence of the alcohol, whereas in pure buffer acetophenone is more reactive than furfural.



Figure 6. A: Rates and Faradaic efficiencies observed during electrocatalytic hydrogenation on Pd deposited on a carbon felt. B: Rates and Faradaic efficiencies observed during electrocatalytic hydrogenation on Cu deposited on a carbon felt. The model molecules were: benzaldehyde (blue circles), furfural (red squares) and acetophenone (green triangles). All the reactions were performed in a continuous flow reactor at room temperature and the currents indicated in the figure (the dashed lines are to guide the eye). The concentration of the reactants was 20 mM in mixtures of isopropyl alcohol and water (50:50 wt.) containing 5 wt. % acetic acid. Conclusions The present work demonstrates that one of the advantages of electrocatalytic hydrogenation is the usage of cathodic potential as an additional parameter for controlling reaction rates. It controls both activity and selectivity of hydrogenation, which can be translated from batch to continuous flow operation. The prevalent side reaction, H2 evolution, seems to be unavoidable when using base and Pt-group metals although it can be controlled by choosing appropriate reactant-metal combinations.


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In ideal conditions, i.e., pure electrolyte, increasing cathodic potentials increase the reaction rates however, the trend in Faradic efficiency depends on the metal. The presence of substituting groups next to the functionality undergoing hydrogenation decreases conversion rates. Overall, the dependences of rates and efficiencies on cathodic potential strongly depend on the molecular motifs being converted. Among the explored compounds, the general reactivity trend is: diaryl ethers < aryl ethers < phenol < aromatic carbonyl compounds. Thus, both conversion and Faradaic efficiency decrease with increasing the complexity of the molecule. Among the reactant-metal combinations explored in ideal conditions, Pt catalyze ring and carbonyl hydrogenation albeit with the lowest Faradaic efficiencies, which is tentatively attributed to its high activity for H2 evolution. Rh seems to offer a good compromise as it converts aromatic rings, carbonyl groups and ether bonds all with relatively good efficiency. Pd shows unique behavior as it hydrogenates benzaldehyde at faster rates than those of H2 evolution, suppressing this parallel reaction. In contrast, it is inactive for hydrogenation of phenolic compounds. Results obtained in a continuous flow reactor show better performance for Pd than for Cu. The differences, however, are not as large as expected. The presence of a co-solvent in flow mode suppresses the outstanding efficiency showed by Pd in ideal conditions. Thus, Pd-based systems must be studied in more depth to understand the requirements for efficient hydrogenation of multiple oxygenated functionalities. The effect of co-solvent is another key issue that has to be explored carefully because it strongly influence the structure-activity correlations.

Acknowledgements The authors would like to thank Dr. Yang Song, Prof. Johannes Lercher, and the group of Prof. Hubert A. Gasteiger at the Technische Universität München for advice and support. The authors are grateful to Donald M. Camaioni, Nirala Singh, and Juan Herranz for fruitful discussions. The research described in this paper is part of the Chemical Transformation Initiative at Pacific Northwest National Laboratory (PNNL), conducted under the Laboratory Directed Research and Development Program at PNNL, a multi-program national laboratory operated by Battelle for the U.S. Department of Energy.

Supporting Information



Page 22 of 24

Survey of investigations of the electrocatalytic hydrogenation of phenolic compounds, description and examples

of

the calculations

of

turnover frequencies

(TOF),

and

characterization of catalysts prepared on carbon felts.

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Electrocatalytic Hydrogenation of Oxygenated Compounds in

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