Electrocatalytic Hydrogenation of Guaiacol in Diverse Electrolytes Using a Stirred Slurry Reactor

Article abstract of DOI:10.1002/cssc.201902611

Electrocatalytic hydrogenation (ECH) of guaiacol was performed in a stirred slurry electrochemical reactor (SSER) using 5 wt % Pt/C catalyst in the cathode compartment. Different pairs of acid (H₂SO₄), neutral (NaCl), and alkaline (NaOH) catholyte–anolyte combinations separated by a Nafion 117 cation exchange membrane, were investigated by galvanostatic and potentiostatic electrolysis to probe the electrolyte and proton concentration effect on guaiacol conversion, product distribution, and Faradaic efficiency. The acid–acid and neutral–acid pairs were found to be the most effective. In the case of the neutral–acid pair, proton diffusion and migration through the membrane from the anolyte to the catholyte supplies the protons required for ECH. Typically, the two major hydrogenation products were cyclohexanol and 2-methoxycyclohexanol. However, ECH at constant cathode superficial current density (?182 mA cm^?2) and higher temperature (i.e., 60 °C) favored a pathway leading mainly to cyclohexanone. The guaiacol conversion routes were affected by temperature- and cathode potential-dependent surface coverage of adsorbed hydrogen radicals generated through electroreduction of protons.

Full text of DOI:10.1002/cssc.201902611

DOI: 10.1002/cssc.201902611
Full Papers
Electrocatalytic Hydrogenation of Guaiacol in Diverse
Electrolytes Using a Stirred Slurry Reactor
Yanuar Philip Wijaya,[a, b] Tobias Grossmann-Neuhaeusler,[c]
Robertus Dhimas Dhewangga Putra,[a, b] Kevin J. Smith,*[a] Chang Soo Kim,*[b, d] and
[a]
˝
Elod L. Gyenge*
Electrocatalytic hydrogenation (ECH) of guaiacol was per-
formed in a stirred slurry electrochemical reactor (SSER) using
5 wt% Pt/C catalyst in the cathode compartment. Different
pairs of acid (H2SO4), neutral (NaCl), and alkaline (NaOH) catho-
lyte–anolyte combinations separated by a Nafionꢀ 117 cation
exchange membrane, were investigated by galvanostatic and
potentiostatic electrolysis to probe the electrolyte and proton
concentration effect on guaiacol conversion, product distribu-
tion, and Faradaic efficiency. The acid–acid and neutral–acid
pairs were found to be the most effective. In the case of the
neutral–acid pair, proton diffusion and migration through the
membrane from the anolyte to the catholyte supplies the pro-
tons required for ECH. Typically, the two major hydrogenation
products were cyclohexanol and 2-methoxycyclohexanol. How-
ever, ECH at constant cathode superficial current density
(ꢂ182 mAcmꢂ2) and higher temperature (i.e., 608C) favored a
pathway leading mainly to cyclohexanone. The guaiacol con-
version routes were affected by temperature- and cathode po-
tential-dependent surface coverage of adsorbed hydrogen rad-
icals generated through electroreduction of protons.
Introduction
Lignin, as the most abundant renewable aromatic source, is
considered a promising feedstock for production of value-
added chemicals and polymeric materials.[1–3] The valorization
of lignin involves depolymerization and catalytic upgrading of
the resultant bio-oil, which otherwise has limited use owing to
its high content of oxygen (10–40 wt%) and water (15–
30 wt%), high viscosity, corrosiveness, and instability toward
polymerization.[4,5] The ideal upgrading process should aim for
energy densification and carbon retention, and to avoid cata-
lyst deactivation owing to oligomers/coke formation, which is
currently a main problem in the conventional (thermochemi-
cal) process.
erating pressure and temperature, easier control of product se-
lectivity and conversion by tuning the electrode potential or
the current density (potentiostatic or galvanostatic operation),
and lower waste and by-product generation.[6–8] Therefore, ECH
is expected to be more selective to the targeted products and
cleaner with less coke formation than the thermal process, re-
sulting in higher energy and carbon utilization efficiencies.[7,8]
In addition, ECH could also be integrated with renewable elec-
tricity to provide an overall greener process for utilizing biore-
sources.[9–12] A combination of fast pyrolysis and electrocatalytic
upgrading technologies using renewable electricity offers a
promising carbon retentive pathway for biomass to renewable
fuels and chemicals.[8,13]
An alternative upgrading process via electrochemical reduc-
tion, referred to as electrocatalytic hydrogenation (ECH), offers
important advantages, such as in situ generation of reactive
adsorbed hydrogen (via water or proton reduction), milder op-
There have been numerous studies on the upgrading of
lignin-derived compounds using electrocatalytic reduction pro-
cesses. In the electrocatalytic hydrogenolysis of lignin model
compounds, such as benzyl phenyl ether, using Raney nickel
electrodes in a solution of sodium chloride mixed with aque-
ous ethanol, Mahdavi et al.[7] reported that hydrogenolysis of
the ether bonds must be faster than hydrogenation of aromat-
ic rings for an efficient lignin reduction. Lignin degradation by
ECH seemed feasible since the efficiency of ether bond hydro-
genolysis could be optimized as a function of substrate con-
centration, current density, and temperature. In another study,
Saffron et al.[8] reported the use of ruthenium supported on ac-
tivated carbon cloth (Ru/ACC) as an efficient catalyst for ECH
of phenolic compounds, such as phenol, guaiacol, and syrin-
gol, under mild conditions (ꢀ808C and ambient pressure). The
guaiacol reduction was found to be more favorable under
acidic environment and higher temperature (ꢁ508C), with cy-
clohexanol and phenol as the major and intermediate prod-
[a] Y. P. Wijaya, R. D. Dhewangga Putra, Prof. K. J. Smith, Prof. E. L. Gyenge
Department of Chemical and Biological Engineering
The University of British Columbia
2360 East Mall, Vancouver, BC, V6T 1Z3 (Canada)
E-mail: kjs@mail.ubc.ca
elod.gyenge@ubc.ca
[b] Y. P. Wijaya, R. D. Dhewangga Putra, Dr. C. S. Kim
KIST-UBC On-site Laboratory, The University of British Columbia
2259 Lower Mall, Vancouver, BC, V6T 1Z4 (Canada)
E-mail: cskim@chbe.ubc.ca
[c] T. Grossmann-Neuhaeusler
Department of Chemistry, Technical University of Munich
Lichtenbergstrasse 4, 85748 Garching (Germany)
[d] Dr. C. S. Kim
Clean Energy Research Center, Korea Institute of Science and Technology
Seoul 02792 (Republic of Korea)
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ucts, respectively. Moreover, high carbon recovery (>80%) in
the liquid phase was obtained in the ECH of water-soluble bio-
oil, showing that ECH can stabilize bio-oil against polymeri-
zation.[8–10] More recently, a comparative study on ECH and
thermal catalytic hydrogenation (TCH) of phenol was reported
by Song et al.[11] using Pt/C, Rh/C, and Pd/C catalysts in differ-
ent acidic electrolytes (such as acetic acid, phosphoric acid,
and sulfuric acid) at a range of current inputs (ꢂ20 to ꢂ60 mA)
and temperatures (5 to 508C). This work demonstrated that
ECH and TCH were independent pathways as the electroreduc-
tion of protons to hydrogen gas did not contribute to the
rates of thermally catalyzed reactions. Moreover, the study of
reaction mechanisms also revealed that ECH proceeded via
Langmuir–Hinshelwood-type mechanism where adsorbed hy-
drogen radicals were produced on the catalyst metal particles
by electroreduction of protons, instead of dissociative chemi-
sorption of H2 as in TCH.[11,14]
Guaiacol is regarded as a representative monomer of bio-oil
owing to the prevalence of hydroxy and methoxy groups in
lignin derivatives. The plausible reaction pathways for ECH of
guaiacol is depicted in Scheme 1. Concerning the importance
of solvent effects (with broad pH range) in biomass conversion,
this work presents for the first time a comprehensive investiga-
tion of different catholyte–anolyte pairs of aqueous electro-
lytes in the ECH of guaiacol. Furthermore, insights are provided
regarding the synergistic effect of temperature and current/po-
tential on the reaction pathways by performing electrolysis ex-
periments at high cathode superficial current densities [e.g.,
above 100 mAcmꢂ2 (in absolute value)] to enhance the practi-
cal feasibility of the ECH process. The ECH of phenol, the latter
being an important intermediate, is also studied to better un-
derstand the overall reaction sequence and gain insights into
the ECH rates relative to guiacol.
In this study, ECH of lignin model compounds (guaiacol and
phenol) was performed using carbon-supported platinum (Pt/ Results and Discussion
C) catalyst in a stirred slurry electrochemical reactor (SSER). To
Electrochemical characterizations
date, the slurry reactor configuration for ECH is not widely ex-
plored. Most of the reported ECH studies used fixed bed elec-
trodes prepared either by electrodeposition or impregnation
methods. The SSER configuration is advantageous because of
effective liquid–solid mass transfer at the H2-evolving slurry
bed electrode and possibly higher active surface area. In an in-
dustrial application, the SSER could be implemented either as
a fluidized bed or as a moving bed electrocatalytic reactor. The
latter design allows for easier separation and regeneration of
the catalyst bed in case of poisoning. Carbon-supported plati-
num (5 wt%-Pt/C) is chosen as the catalyst because of excel-
lent catalytic activity for hydrogenation (at low catalyst load-
ing), and physico-chemical stability in acid media. In thermo-
chemical processes as well, Pt/C has been widely used for hy-
drogenation and/or hydrodeoxygenation reactions.
Cyclic voltammetry (CV) experiments were performed to evalu-
ate how the presence of organics (guaiacol and phenol)
through adsorption on Pt would affect the various potential re-
gions of the Pt voltammograms [hydrogen evolution (HER), hy-
drogen oxidation (HOR), oxygen evolution (OER), and oxygen
reduction reaction (ORR)] in different electrolytes of interest for
electrolysis.
Figure 1 shows the effect of guaiacol and phenol on the Pt
electrode cyclic voltammograms in acid, neutral, and alkaline
electrolytes. In the absence of organics, the HER, HOR, OER,
and ORR waves are clearly identified and labelled (Figure 1a).
Comparing Figure 1a and b, the presence of phenol in acid
and neutral electrolytes dramatically suppressed HER, HOR,
OER, and ORR. It is particularly relevant for the present work
Scheme 1. Plausible reaction pathways for electrocatalytic hydrogenation–hydrogenolysis of (a) guaiacol to (d) cyclohexanol and (c) cyclohexanone as target
products; (b) phenol, (e) 2-methoxycyclohexanone, and (f) 2-methoxycyclohexanol as the intermediates; and methanol as the byproduct.
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Figure 1. Effect of guaiacol and phenol on the Pt gauze cyclic voltammograms in different electrolytes: (a) and (c) acidic, neutral, and basic electrolytes with-
out any organics, (b) phenol-containing electrolytes, (d) guaiacol-containing electrolytes, (e) acidic electrolytes with the increasing guaiacol concentration,
(f) guaiacol-containing acidic electrolytes before and after the addition of dispersed catalyst (5 wt%-Pt/C). Concentration of all electrolytes: 0.2m, tempera-
ture: 258C.
the suppression of HER by 20 mm phenol, as shown by a more
than four times lower HER current density at around ꢂ0.6 V
(vs. Ag/AgCl). This was because of the strong adsorption of
phenol on the Pt surface. It was reported that phenol acts as a
scavenger of adsorbed hydrogen radicals,[11] thereby, decreas-
ing the HER current density at a given overpotential. In con-
trast, the presence of guaiacol at the same 20 mm concentra-
tion as phenol, suppressed only slightly those reactions (com-
pare Figure 1c and d), which suggests that the adsorption of
guaiacol on Pt is weaker (i.e., lower surface coverage) com-
pared to phenol. When the guaiacol concentration was in-
creased from 40 to 100 mm, the HER, HOR, OER, and ORR reac-
tions eventually faded (Figure 1e). The addition of the Pt/C cat-
alyst (i.e., catalyst slurry configuration) resulted in more nega-
tive cathodic currents in the H2 evolution region (Figure 1 f).
This demonstrates that the reduction of protons was enhanced
by the presence of the slurry catalyst. These CV results also
show that acidic electrolytes are more appropriate for guaiacol
reduction (owing to the least negative potentials for HER[15]).
For screening purposes, the acid (H2SO4, HClO4), neutral (NaCl,
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KCl), and base (NaOH, KOH) electrolytes with the same concen-
trations (0.2m) were used and the cyclic voltammograms show
no stark differences caused by the different anions in acid
media and likewise by the different cations in alkaline media.
The effect of different catholyte–anolyte pairs
The effects of different catholyte–anolyte pairs (i.e., acid–acid,
neutral–acid, and base–acid) on the guaiacol ECH were evalu-
ated
under
constant
superficial
current
density
(ꢂ109 mAcmꢂ2) and temperature (508C) conditions (selected
results presented in Table 2 and Figure 2). Guaiacol conversion
decreased in order of: acid–acid (38%)>neutral–acid (36%)>
acid–base (16%)>base–acid (0%). Faradaic efficiency [F.E., de-
fined in Equation (1.6) in the Supporting Information] de-
creased in order of: neutral–acid (94%)>acid–acid (82%)>
acid–base (27%)>base–acid (0%). The base–acid pair resulted
in poor performance (zero guaiacol conversion to hydrogena-
tion products), consistent with the results from potentiostatic
experiments (Table S1, entry 5). Instantaneous guaiacol depro-
tonation was observed as the catholyte color turned dark yel-
lowish (Figure S2). The protons from the anode compartment
generated also by water oxidation (OER) and transported
through the Nafion membrane were fast neutralized by the hy-
droxide ions at the cathode (produced also by water reduc-
tion), thereby, hampering the reduction of protons and ECH re-
actions, as confirmed by the constant high pH of catholyte
(pH=13.2). Under alkaline conditions, guaiacol is prone to de-
protonation, which hinders its adsorption onto the catalyst.[8]
Another possible explanation may also be a result of the insta-
bility of Pt/C in alkaline medium, which modifies the anchoring
sites of the Pt nanoparticles on the carbon surface, resulting in
detachment/loss of the metal.[18]
Thermodynamic analysis
Guaiacol conversion to cyclohexanol proceeds via two parallel
routes: (i) demethoxylation–ring saturation and (ii) ring satura-
tion–demethoxylation (Scheme 1).[8] The first route is marked
with the formation of phenol and cyclohexanone, whereas the
second route can be identified by the production of 2-methox-
ycyclohexanone and 2-methoxycyclohexanol. Both routes gen-
erate methanol as a by-product. The calculations of free
energy, enthalpy, and equilibrium reduction potentials for each
specific reaction at standard conditions are summarized in
Table 1. These data reveal that all the direct electrocatalytic re-
ductions of guaiacol are thermodynamically favorable and the
standard potentials are all above that of the HER. Kinetically,
however, as shown by the cyclic voltammograms (Figure 1)
there is no separate reduction wave for guaiacol. Hence, guaia-
col reduction can only occur in the potential domain of the
HER.
As shown by Table 1 and Scheme 1, the expected final prod-
uct of guaiacol ECH is cyclohexanol. Further hydrogenolysis of
cyclohexanol to cyclohexane (CꢂO cleavage), which involves
dehydration of cyclohexanol to cyclohexene (C6H12O!C6H10 +
H2O, DH0R = +25.6 kJmolꢂ1) and the subsequent hydrogena-
tion of cyclohexene to cyclohexane (C6H10 +2H+ +2eꢂ!C6H12,
DH0R =ꢂ117.1 kJmolꢂ1) is not feasible because of equilibrium
limitations, as the former reaction is inhibited by water as sol-
vent (i.e., aqueous electrolyte) and is endothermic in nature,
thus, much higher temperatures and/or overpotentials than
those employed in this study would be required for complete
deoxygenation via hydrogenolysis.[11,16]
The current was stable whenever acidic anolyte was used,
however in the acid-base pair the current dropped significantly
(Table 2, entry 4). Under this condition, it was not possible to
maintain the high cathode current density of ꢂ109 mAcmꢂ2
constant, hence implying that the alkaline anolyte was not fa-
vorable for ECH purposes since proton transport across the
membrane was limited and only a much lower current could
be used (Table 2, entry 4). Nonetheless, marginal guaiacol con-
Table 1. Standard Gibbs free energy, enthalpy, standard potential (at 298 K and 1 atm) and temperature coefficient of the standard potential for reactions
in ECH of guaiacol.[a]
Reaction
Equation[b]
DG0R
[kJmol-1]
DH0R
[kJmol-1]
DS0
[Jmolꢂ1 Kꢂ1
E0red
[VSHE
@E0T=@T
[mVKꢂ1
]
]
]
Route 1: Demethoxylation–Ring saturation
guaiacol to phenol
phenol to cyclohexanone
cyclohexanone to cyclohexanol
Route 2: Ring saturation–Demethoxylation
guaiacol to 2-methoxycyclohexanone
2-methoxycyclohexanone to 2-methoxycyclohexanol
2-methoxycyclohexanol to cyclohexanol
Cathode side reactions
C7H8O2 +2H+ +2eꢂ!C6H6O+CH4O
C6H6O+4H+ +4eꢂ!C6H10O
C6H10O+2H+ +2eꢂ!C6H12O
ꢂ73.07
ꢂ57.85
ꢂ21.94
ꢂ70.15
ꢂ120.94
ꢂ75.90
9.80
ꢂ211.71
ꢂ181.07
0.38
0.15
0.11
0.05
ꢂ0.55
ꢂ0.94
C7H8O2 +4H+ +4eꢂ!C7H12O2
C7H12O2 +2H+ +2eꢂ!C7H14O2
C7H14O2 +2H+ +2eꢂ!C6H12O+CH4O
ꢂ55.93
ꢂ34.87
ꢂ74.99
ꢂ118.26
ꢂ47.00
ꢂ209.16
ꢂ40.70
ꢂ89.73
0.15
0.18
0.39
ꢂ0.54
ꢂ0.21
ꢂ0.47
ꢂ101.73
HER
(a) 2H+ +2eꢂ!H2
0.00
474.28
0.00
571.66
0.00
326.77
0.00
ꢂ0.83
0.00
1.69
(b) 2H2O+2eꢂ!H2 +2OHꢂ
Anode reactions
ORR
(a) 2H2O!O2 +4H+ +4eꢂ
ꢂ474.28
ꢂ571.66
ꢂ326.77
1.23
0.40
ꢂ0.85
(b) 4OHꢂ!O2 +2H2O+4eꢂ
474.28
571.66
326.77
0.85
[a] DG0R =standard Gibbs free energy of the reaction; DHR0 =standard enthalpy of the reaction; DS0 =standard entropy of the reaction; Er0ed =standard re-
duction potential of the reaction; @E0T=@T =temperature coefficient of the standard equilibrium electrode potential. All the thermodynamic data were ob-
tained based on NIST and Joback method. [b] reaction (a) in acid media, (b) in neutral or alkaline media.
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Table 2. ECH of guaiacol in different catholyte--anolyte pairs under galvanostatic control.[a]
Entry
C
A
E
[V]
pH
(C)
X
[%]
S [%]
(1)
C.B.
[%]
F.E.
[%]
rx,0
(A)
(2)
(3)
(4)
(5)
(6)
4.23
1.32
0.00
[molsꢂ1 gꢂ1
]
1
2
3
4[b]
H2SO4
NaCl
NaOH
H2SO4
H2SO4
H2SO4
H2SO4
NaOH
ꢂ0.7
ꢂ2.4
ꢂ2.3
ꢂ0.2
0.66
0.54
0.64
0.70
6.01
37.88
36.36
0.00
19.55
28.00
0.00
30.44
16.56
0.00
10.79
26.79
0.00
25.46
18.55
0.00
9.52
8.78
0.00
9.46
93.07
94.44
96.22
95.33
82.02
93.99
0.00
1.67ꢂ10ꢂ4
1.58ꢂ10ꢂ4
0.00
8.18
13.21
0.87
16.39
5.96
29.65
6.93
20.29
27.71
27.33
8.64ꢂ10ꢂ5
[a] C=catholyte; A=anolyte; E=cathode potential (vs. Ag/AgCl); X=guaiacol conversion; S=normalized product selectivity (C mol%) for cyclohexanol
(1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), methanol (5), and phenol (6); C.B. =carbon balance; F.E.=Faradaic effi-
ꢂ1) by t=30 min. Conditions: T=508C, I=ꢂ0.3 A (j=ꢂ109 mAcmꢂ2), catalyst=5 wt% Pt/C (0.133 g), reac-
ꢂ1
ciency; rx,0 =initial reaction rates (mols
g
metal
tant/metal molar ratio (R/M)=315, reaction time=2 h, concentration of all electrolytes: 0.2m, pH was measured at the end of reaction. [b] I=ꢂ0.01 A (j=
ꢂ3.63 mAcmꢂ2).
Figure 2. Galvanostatic ECH of guaiacol in the two most effective catholyte–anolyte pairs: acid–acid and neutral–acid. Guaiacol conversion and product nor-
malized selectivity (C mol%) in (a) acid–acid and (b) neutral–acid. Catholyte pH and Faradaic efficiency in (c) acid–acid and (d) neutral–acid; at t=0 the pH is
determined by proton diffusion from the anolyte during the pre-electrolysis (mixing) period. Acid: H2SO4 0.2m, neutral: NaCl 0.2 m. Conditions:
j=ꢂ109 mAcmꢂ2, T=508C, t=4 h. Initial guaiacol concentration: 0.1 m. Catalyst: 5 wt% Pt/C (0.133 g). All the experiments were performed in the jacketed
H-cell. Other conditions are given in Table 2.
version (ꢂ16.4%) was achieved, suggesting the protons result- observed in the ECH of phenol (Table 3) and the ECH under
ing from acid dissociation on the cathode were also involved potentiostatic conditions (Table S1–S2, Figure S1), implying the
in the reaction. The highest F.E. was obtained with the neutral– consistent electrolyte effect on the overall ECH process. The
acid pair, indicating that the protons for ECH reactions were detailed results from ECH of guaiacol (and phenol) under po-
mostly supplied from the anode side. The proton flux from the tentiostatic control, including electrolyte screening and catho-
anode to the cathode through the membrane is related to the lyte–anolyte pair effects, are presented in the Supporting Infor-
applied current density. Thus, under galvanostatic operation a mation.
controlled flux of protons is assured. Similar trends were also
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Table 3. ECH of phenol in different catholyte–anolyte pairs under galvanostatic control.
Entry
C
A
E
[V]
pH
(C)
X
[%]
S [%]
(1)
C.B.
[%]
F.E.
[%]
rx,0
(A)
(2)
[molsꢂ1 gꢂ1
]
1
2
3
4[b]
H2SO4
NaCl
NaOH
H2SO4
H2SO4
H2SO4
H2SO4
NaOH
ꢂ0.7
ꢂ2.3
ꢂ2.0
ꢂ0.2
0.71
0.63
0.65
0.65
8.39
38.95
39.52
0.00
23.73
25.67
0.00
76.27
74.33
0.00
95.37
99.89
95.48
97.83
82.74
92.39
0.00
1.74ꢂ10ꢂ4
1.62ꢂ10ꢂ4
0.00
8.05
13.18
0.94
17.80
15.33
84.67
25.49
8.85ꢂ10ꢂ5
[a] C=catholyte; A=anolyte; E=cathode potential (vs. Ag/AgCl); X=phenol conversion; S=normalized product selectivity (C mol%) for cyclohexanol (1),
ꢂ1
cyclohexanone (2); C.B.=carbon balance; F.E.=Faradaic efficiency; rx,0 =initial conversion rates (mols
g
ꢂ1), after 30 min. Conditions: T=508C, I=ꢂ0.3
metal
A (j=ꢂ109 mAcmꢂ2), catalyst=5 wt% Pt/C (0.125 g), reactant/metal molar ratio (R/M)=332, reaction time=2 h, concentration of all electrolytes=0.2m,
pH was measured after the reaction. [b] I=ꢂ0.01 A.
Furthermore, the conversion and selectivity profiles over ex- to 70% in 4 h, but it was still higher than in the case of acid–
tended reaction time (4 h) show differences between galvano- acid pair.
static and potentiostatic ECH, respectively (compare Figure 2a
Similar general trends were also observed for phenol ECH
and b vs. S1b and c). The stark contrast can be seen especially (Figure 3). Phenol conversion, which proceeds in a series reac-
in the acid–acid pairs, owing to the large differences in the tion, was found to be higher than guaiacol in either of the
cathodic potential (ꢂ0.7 vs.ꢂ2.0 V) between the two types of electrolyte pairs (Figure 2 vs. Figure 3, see also Figure S1). At a
experiments. In galvanostatic experiments, owing to lower fixed current density (j=ꢂ109 mAcmꢂ2) and temperature
cathode potential of ꢂ0.7 V, higher selectivities of cyclohexa- (508C), the acid–acid pair gave lower cathode potential
none (24–32%) and 2-methoxycyclohexanone (22–25%) were (ꢂ0.7 V) than the neutral–acid pair (ꢂ2.3 V), thus cyclohexa-
obtained (Figure 2a). This could be attributed to the lower ad- none selectivity was higher than cyclohexanol (Figure 3a vs.
sorbed hydrogen (Hads) surface coverages, hence the full hydro- 3b). It is noteworthy that phenol conversion was always higher
genation reactions generating cyclohexanol and 2-methoxycy- than guaiacol under galvanostatic (Table 2 vs. Table 3) as well
clohexanol, respectively, were hampered. In contrast, under po- as potentiostatic conditions (Table S1 vs. Table S2). This applies
tentiostatic reduction at ꢂ2.0 V, the cyclohexanol and 2-me- also for the alkaline catholyte where a marginal phenol conver-
thoxycyclohexanol selectivities were higher (Figure 2a vs. Fig- sion (ꢁ10%) was still obtained (Table S2, entries 4 and 5). Re-
ure S1b). Thus, these results obtained at similar temperatures markably, phenol was fully converted to cyclohexanol either in
[i.e., 50 and 538C for the galvanostatic (Figure 2a) and poten- the acid–acid pair (only after 2 h) or the neutral–acid pair (after
tiostatic (Figure S1b) experiments, respectively] indicate that 4 h), clearly showing that phenol is more reactive (than guaia-
more negative cathodic potentials increase the Hads coverages, col) toward hydrogenation (Figure S1). This finding is also sup-
as also reported by Song et al. in the ECH of benzaldehyde ported by its scavenging effect on hydrogen radicals[11] and
over carbon-supported metals,[14] favoring complete hydroge- the strong suppression of the HER wave in the CV (Figure 1b).
nation (ring saturation). Note that, in addition to cathodic po- Obviously, the higher functionalities in the aromatic com-
tential, temperature also synergistically affected the product pounds render more difficulties in converting the substrates.
distribution, for instance as shown by the remarkably higher Recently, Garedew et al. reported the effects of functional
cyclohexanone selectivities under galvanostatic conditions (24– group types on the reactivity, suggesting that the more com-
32% in acid–acid, 5–32% in neutral–acid) as compared to plex the functional group, the lower the conversion.[17]
those obtained under potentiostatic conditions (2–6% in acid–
acid, 4–13% in neutral–acid).
Galvanostatic experiments in acidic electrolytes
The time-dependent profiles for the guaiacol ECH suggest
clear evidence that each route is a series reaction, as indicated The ECH of guaiacol was further investigated in the pair of
by maxima in the intermediates (Figure 2a and b). Comparing acid (H2SO4) electrolytes using a jacketed cell under galvano-
the product distribution for ECH of guaiacol in acid and neutral static control to evaluate the effects of acid concentration
catholytes (Table 2, entries 1 and 2 and Figure 2a and b), it is (0.2–1.0m), cathode superficial current density (ꢂ109 to
noteworthy that the reaction pathway was also affected by the ꢂ255 mAcmꢂ2 corresponding to current loads of ꢂ60 to
catholyte pH. The catholyte pH profiles were dramatically dif- ꢂ140 AgPtꢂ1) and temperature (30–608C).
ferent between the acid–acid and neutral–acid pairs (Figure 2c
Increasing the cathode superficial current density to
vs. 2d). In the former case, the pH was relatively constant (0.7– ꢂ182 mAcmꢂ2 requires increased catholyte acid concentra-
0.8), however in the latter case, the pH of NaCl (0.2m) initially tions (e.g., 1m) to sustain the F.E. at (or above) 40% after 5 h
dropped to 2.1 because of proton diffusion from the anolyte of electrolysis (Figure 4a). The positive effect of acid concentra-
during dissolution of guaiacol without current applied (at t= tion on guaiacol conversion and Faradaic efficiency was also
0). Once the electrolysis started, the pH increased over time observed under potentiostatic conditions (Figure S6). En-
and after 4 h, the final pH was 9.2 because of water reduction hanced guaiacol conversion and cyclohexanol selectivity was
to H2 and OHꢂ. The F.E. gradually decreased from near 100% obtained when the superficial current density was increased at
a constant acid concentration and temperature (Figure 4b). At
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Figure 3. Galvanostatic ECH of phenol in the two most effective catholyte--anolyte pairs: acid–acid and neutral–acid. Phenol conversion and product selectivi-
ty over time in (a) acid–acid and (b) neutral–acid. Time-dependent profiles of catholyte pH and Faradaic efficiency in (c) acid–acid and(d) neutral–acid. Acid:
H2SO4 (0.2m), neutral: NaCl (0.2m). Conditions: I=ꢂ0.3 A, T=508C, t=4 h. Initial phenol concentration: 0.1 m. Catalyst: 5 wt% Pt/C (0.125 g). All the experi-
ments were performed in the jacketed H-cell. Other conditions are given in Table 3.
the highest current density (ꢂ255 mAcmꢂ2), cyclohexanol was
the most dominant product (selectivity ꢁ38%) and the guaia-
col conversion was 77% after 5 h reaction (Figure 4b). Howev-
er, these improvements were obtained at the expense of F.E.
decrease (from 48% to 37% after 5 h), presumably because of
the enhanced HER. The optimal combination of current density
and acid concentration must provide sufficient Hads generated
by the Volmer step (H+ +eꢂ!Hads) to sustain high guaiacol
higher cyclohexanone selectivity (35%) and much lower 2-me-
thoxycyclohexanol selectivity (7.5%) compared to the results
obtained at lower temperatures (30–408C). The higher temper-
ature promoted the desorption of Hads, thus, lowering the sur-
face coverage and changing the reaction pathway. Moreover,
the increased temperature (DT=308C) decreased the cathode
potential by 0.1 V (in absolute value) at a constant current den-
sity, which also affected the Hads coverage. Both cyclohexanone
and cyclohexanol, the mixture of which is known as ketone-al-
cohol (KA) oil, represent important chemicals for the synthesis
of nylon precursors, such as caprolactam for nylon 6 and
adipic acid for nylon 66.[20,21] Overall, the galvanostatic experi-
ments demonstrated that the reactor can be efficiently operat-
ed at high superficial current density and catalyst mass activity
of ꢂ109 to ꢂ225 mAcmꢂ2 and ꢂ60 to ꢂ140 AgPtꢂ1, respective-
ly. Temperature control is evidently required to enable selectivi-
ty control as the current/potential and temperature could syn-
ergistically affect the Hads coverage, leading to the higher selec-
tivity for cyclohexanone. Without temperature control, the
product distribution from guaiacol ECH was dominated by cy-
clohexanol and 2-methoxycyclohexanol, regardless of the cath-
ode potential and electrolyte type (Figure S1 and Figure S8,
see also Table S3–S4).
conversion while minimizing either the Tafel (Hads +Hads!H2,(g)
)
or Heyrovsky steps (H+ +Hads +eꢂ!H2,(g)) leading to hydrogen
gas evolution. The undesirable shift toward HER as the organic
reactants being consumed was also reported elsewhere.[17]
Higher initial guaiacol concentration would then expectedly
result in higher F.E. because more organic substrates were
available (i.e., higher reactant surface coverage) and this was
confirmed either by galvanostatic or potentiostatic experi-
ments (Figure S3). As the conversion increased, F.E. decreased
because fewer catalytic active sites were occupied by the or-
ganics. This can be clearly seen in the guaiacol conversion and
Faradaic efficiency time-dependent profiles from the selected
experiments (Figure 4d–f). Surface coverage of both organic
molecules and atomic hydrogen is crucial to promote the ECH
over HER.
Interestingly, at higher temperature (608C), as shown by Fig-
ure 4c (see also Figure 4 f), the demethoxylation route was
more favored than the ring saturation as indicated by the
A graphical kinetic analysis revealed the ECH of guaiacol in
this experiment appears to best fit an apparent second order
reaction with an apparent activation energy of 23.7 kJmolꢂ1
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Figure 4. The effect of (a) acid concentration, (b) superficial cathode current density, and (c) temperature on the ECH of guaiacol using the catholyte–anolyte
pair of H2SO4 electrolytes after 5 h of electrolysis. Guaiacol conversion, product selectivity, and F.E. profiles as function of reaction time from the corresponding
experiments (d, e, f). Operating conditions: (a) j=ꢂ182 mAcmꢂ2 and T=508C, (b) [H2SO4]=0.5m and T=508C, (c) [H2SO4]=0.5m and j=ꢂ182 mAcmꢂ2. All
the experiments were performed in the jacketed H-cell using 5 wt% Pt/C (0.10 g) under galvanostatic control. Initial guaiacol concentration: 0.1 m.
(Figure S4). The activation energy is slightly lower than that re-
ported for ECH of phenol on Pt/C (28.9 kJmolꢂ1).[11] This differ-
ence is reasonable because the reaction conditions were also
different, the phenol ECH was performed in acetic acid (pH 5)
at ꢂ0.75 V vs. Ag/AgCl, whereas the guaiacol ECH was carried
out in 0.5m H2SO4 (pH 0.37) at around ꢂ0.65 V vs. Ag/AgCl cor-
The rate constant for guaiacol ECH (expressed in Lmolꢂ1 sꢂ1) in-
creased with temperature in the following order: 5.0ꢂ10ꢂ4
(308C) <6.7ꢂ10ꢂ4 (408C)<8.8ꢂ10ꢂ4 (508C)<1.2ꢂ10ꢂ3 (608C).
In contrast to guaiacol, phenol ECH was found to be an appar-
ent zero order reaction (with respect to phenol) at the operat-
ing conditions tested in this work (Figure S5) and also reported
elsewhere.[11] Phenol is highly reactive toward hydrogenation
responding to a superficial current density of ꢂ182 mAcmꢂ2
.
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(acts as the H scavenger), characterized by fast adsorption and
nearly constant surface coverage.
could also be a viable option at low loadings. In this work, the
molar ratio of guaiacol to Pt was typically 315, meaning that
only very small amount of Pt was used. In the acidic environ-
ment on the other hand, Pt/C has superior durability and can
be reused.
A plausible explanation for the apparent second-order reac-
tion of guaiacol is that adsorption of guaiacol on Pt/C could
be the rate determining step. The adsorption rate equation
may appear second-order with respect to the adsorbate sur-
face concentration as proposed originally by Blanchard
et al.[22,23] Unlike phenol, the guaiacol conversion rate is depen-
dent on the initial guaiacol concentration (Figure S3). ECH of
guaiacol may be interpreted as first- or second-order according
to marginal differences in the correlation coefficient and de-
pending on the operating conditions (i.e., initial reactant con-
centration, stirring rates, catalyst loading). For such a complex
catalytic reaction the apparent reaction order is not an intrinsic
reaction kinetic parameter. Thermal hydrodeoxygenation
(HDO) of guaiacol to phenol, catechol, and cyclopentanone
over Pt/C at much higher temperatures (275–3258C) was also
reported to be second-order with respect to guaiacol.[22] At ele-
vated temperatures (350–4008C) using external H2 (4 MPa),
benzene and phenol were the most dominant products on
carbon-supported metal catalysts such as Ru/C and Mo/C in a
continuous pack-bed reactor.[24] However, at 1608C under
1.5 MPa H2, demethoxylation and ring saturation products
(e.g., cyclohexanol, 2-methoxycyclohexanol, and methanol)
were obtained over Ru/C-MgO catalyst.[25] These results clearly
suggested that full deoxygenation of guaiacol to benzene pro-
ceeds more favorably at much higher temperatures (>2008C)
via a thermocatalytic process. This work, on the other hand,
demonstrates that effective ring saturation and partial deoxy-
genation of guaiacol to cyclohexanol can be achieved via a
mild electrocatalytic reduction process.
The reaction pathways and product distributions were af-
fected by synergistic interaction effects among the major varia-
bles, such as proton concentration, temperature, and cathode
potential (or superficial cathode current density), influencing
the adsorbed hydrogen (Hads) surface coverage, showing the
possibility for product selectivity control. This work demon-
strates that in a slurry reactor the ECH of lignin model com-
pounds can be performed at high superficial cathode current
densities and catalyst mass activities (e.g., ꢂ255 mAcmꢂ2 and
ꢂ1
ꢂ140 AgPt respectively), therefore, contributing to the devel-
,
opment of a mild process for catalytic conversion of lignin-de-
rived substrates. Other advantages of the SSER configuration
include: i) ease of catalyst bed preparation, eliminating the
need for catalyst deposition on an electrically conductive sub-
strate; ii) favorable multi-phase mass and heat transfer condi-
tions; and iii) scale-up feasibility as either fluidized bed or
moving bed electrocatalytic reactors. Potential limitations of
the SSER may be associated with catalyst erosion owing to in-
tense friction among the catalyst particles, possibility of non-
uniform polarization of the electrocatalyst bed caused by loss
of electric contact between the current feeder and particles
and need for separation and recovery of the catalyst from the
liquid product.
Experimental Section
Conclusions
Electrochemical cell: Stirred slurry configuration
Electrocatalytic hydrogenation (ECH) of guaiacol was investi-
gated under mild conditions (1 atm, 25–608C) using a mem-
brane-divided stirred slurry electrochemical reactor (SSER) con-
figuration with dispersed 5 wt% Pt/C catalyst in the cathode
compartment. Different pairs of catholytes and anolytes were
studied using either potentiostatic or galvanostatic control, to
determine the electrolyte effect on guaiacol conversion, prod-
uct distribution, and Faradaic efficiency. The most effective
pairs were the acid–acid and neutral–acid catholyte–anolyte
combinations, whereas in alkaline catholytes the rate of guaia-
col ECH was virtually zero due to deprotonation and extremely
low catalytic activity.
The experimental setup for ECH is displayed in Figure 5. Two H-
cells were employed; one without temperature control (i.e., non-
jacketed) and the another one with temperature control (jacketed
cell). The former was used for preliminary screening experiments
under potentiostatic control (results in the Supporting Informa-
tion), whereas the latter was used for experiments under galvano-
static control. The H-cells were equipped on the cathode side with
a Luggin probe for the reference electrode (filled with 3m KCl) and
a proton exchange membrane (Nafionꢀ117). A high-surface area Pt
wire cylindrical mesh was used as anode, whereas Pt gauze served
as cathode current feeder. The geometric Pt electrode areas were
similar (ca. 2.75 cm2). The inter-electrode gap was ca. 6 cm. Typical-
ly, guaiacol (1.0–1.3 g) or phenol (0.8–1.0 g) was dissolved in the
catholyte (80 mL for the non-jacketed cell or 100 mL for the jacket-
ed cell) to prepare the initial substrate concentration of 0.1 M. In
addition, the catalyst (5 wt% Pt/C, 0.10–0.13 g) was also dispersed
in the catholyte. The H-cell setup is well suited for easy testing of
different ECH reaction conditions, product distributions, and elec-
trode polarizations in the cathode compartment. However, owing
to the high inter-electrode gap and batch mode of operation with
static anolyte the H-cell configuration cannot be used to draw in-
dustrially relevant conclusions about the overall cell potential and
energy efficiency of the system. Note that the reduction currents
are expressed with negative sign throughout this paper: “higher”
current means “more negative” current.
Under galvanostatic control, the neutral (NaCl 0.2m)–acid
(H2SO4 0.2m) pair gave higher Faradaic efficiencies and cyclo-
hexanol selectivities compared to the acid–acid pair (e.g., F.E.
of 70% vs. 65% after 4 h electrolysis at 508C, and superficial
cathode current density of ꢂ109 mAcmꢂ2). The NaCl catholyte
pH (between 2.1 and 9.2 after 4 h) was determined by the
proton transport from the anolyte (diffusion and migration
across Nafion) combined with either proton electroreduction
on Pt/C or OHꢂ generation by water electroreduction at higher
pH. The use of neutral catholyte opens the possibility of em-
ploying non-precious metal catalysts even though Pt/C slurry
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Product analysis and catalyst characterization
Sample aliquots (1 mL) of catholyte were taken before reaction
and periodically over the course of the reaction, whereas the
sample aliquots of anolyte were only taken after each reaction to
check if there was any substrate crossover from cathode to anode.
All the samples were filtered to separate the catalyst and then ex-
tracted with n-butanol (2 mL). Any acid-containing sample typically
required saturation with inorganic salt (NaCl) and the mixture was
left overnight to ensure a complete phase separation. Qualitative
(identification) and quantitative analyses of the products were per-
formed by gas chromatography (GC, Agilent 7820A) with a mass
spectrometer (MSD, Agilent 5975). The organic solution was fil-
tered using a 0.45 mm syringe filter prior to each analysis. The sep-
aration of compounds was performed in an HP-INNOWax column
(30 mꢂ250 mmꢂ0.25 mm) with the injector temperature was set at
2508C with a split ratio of 50:1. The GC oven program started at
508C for 5 min, heated to 1508C at 58Cminꢂ1 and then to 2608C
at 158Cminꢂ1 before being held at 2608C for 5 min. The products
were quantitatively determined using calibration curves and re-
sponse factors of each compound with standard chemicals. The
calculations of F.E., conversion, selectivity, and carbon balance are
provided in the Supporting Information, along with the catalyst
characterization method and durability test results.
Figure 5. A sketch of electrochemical cell setup (H-cell) for ECH of guaiacol
and phenol: (1) cathode, (2) anode, (3) reference electrode (Ag/AgCl/3m
KCl), (4) fritted side compartment (Luggin probe) for reference electrode
filled with 3m KCl, (5) cation exchange membrane, and (6) catalyst slurry.
Cyclic voltammetry and electrocatalytic hydrogenation ex-
perimental protocols
Acknowledgements
CV tests were performed in three different electrolytes: acid
(HClO4, H2SO4), neutral (NaCl, KCl), and base (NaOH, KOH), at the
same concentration (0.2m) using Pt gauze as the working elec-
trode and the cylindrical Pt mesh as the counter electrode in the
H-cell (Figure 5). Initially, CV scans were run for 50 cycles with a
scan rate of 100 mVsꢂ1 to clean the Pt working electrode surface
and obtain stable voltammograms. Guaiacol or phenol was then
added to the electrolyte such that to obtain a concentration range
between 0.02 to 0.1 m. The CV tests were run for 50 cycles with a
scan rate of 100 mVsꢂ1. The potential window for the acid, neutral,
and base solution was ꢂ0.60 to 2.00 V, ꢂ1.50 to 2.00 V, and ꢂ1.50
to 1.25 V, respectively. Note that all the potentials herein are refer-
enced vs. Ag/AgCl/KCl saturated.
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 Prof. Colin Oloman and Dr. Amir M. Dehkhoda
for fruitful discussions and technical support. We also thank
Bradford Ross for TEM measurements at Bioimaging Facility (BIF),
UBC.
Keywords: electrocatalytic hydrogenation · electrochemical
reactor · guaiacol · lignin · stirred slurry
Electrocatalytic hydrogenation experiments were conducted in the
H-cell using two modes of operation: potentiostatic and galvano-
static. In case of the former, a potentiostat (CH Instruments Electro-
chemical Workstation, Model 1100 C) was used, whereas for the
latter a power supply (BK Precision 9110) was employed. For tem-
perature control, a jacketed H-cell (CANSCI Glass Products Ltd.)
was connected to a recirculating water bath (Fisher Scientific Iso-
temp 3016HS). Initially, the H-cell was filled with the catholyte and
anolyte, and the cell equipped with the cathode and anode was
polarized for 10 min without the addition of either the organic
substrate or the catalyst slurry. The organic substrate (e.g., guaia-
col) was then dissolved in the catholyte and stirred for 30 min
without the cell being polarized before adding the catalyst. After-
wards the catalyst was added to the catholyte and then ECH was
performed at either constant potential or constant current under
constant stirring (ca. 350 rpm) of the catholyte. Before and after ex-
periment, the pH of the electrolytes was measured using a pH
meter (Thermo Scientific Orion Star A2110). The electrolyte conduc-
tivity was also measured using a conductivity meter (Thermo Orion
105). At the end of the experiment, the catalyst was recovered by
vacuum filtration. The H-cell was washed thoroughly with a mix-
ture of water and acetone after each use, whereas the current
feeders were cleaned ultrasonically for 60 min in distilled water.
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Y. P. Wijaya, T. Grossmann-Neuhaeusler,
R. D. Dhewangga Putra, K. J. Smith,*
C. S. Kim,* E. L. Gyenge*
&& – &&
Electrocatalytic Hydrogenation of
Guaiacol in Diverse Electrolytes Using
a Stirred Slurry Reactor
Electrolyte matching: Electrocatalytic
hydrogenation of guaiacol on Pt/C is in-
vestigated under mild conditions using
a membrane-divided stirred slurry elec-
trochemical reactor configuration. Dif-
ferent pairs of catholytes and anolytes
are studied using either potentiostatic
or galvanostatic control, to determine
the electrolyte effect on guaiacol con-
version, product distribution, and Fara-
daic efficiency in synergy with the ap-
plied current density, electrode poten-
tial and temperature.
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