Mild electrocatalytic hydrogenation and hydrodeoxygenation of bio-oil derived phenolic compounds using ruthenium supported on activated carbon cloth

Article abstract of DOI:10.1039/c2gc35552c

Electrocatalytic hydrogenation (ECH) is an option for stabilizing or upgrading bio-oil that employs mild conditions (≤80 °C and ambient pressure) compared to hydrotreatment. In this study, phenol, guaiacol (2-methoxyphenol), and syringol (2,6-dimethoxyphenol) were chosen as model lignin-like substrates because of their abundance in bio-oil and their high resistance to hydrogenation relative to the carbonyl compounds. Cathodic reduction was catalyzed by ruthenium supported on activated carbon cloth (Ru/ACC), a novel electrocatalyst. Incipient wetness impregnation and cation exchange methods were employed to prepare the electrocatalyst using three different ruthenium precursors. Scanning electron microscopy revealed that ruthenium nanoparticles within the range of 10 to 20 nm were deposited on the support by both catalyst synthesis methods. Catalysts prepared by cation exchange were more active than those prepared using incipient wetness impregnation, presumably because of support surface functionalization by the oxidation pretreatment. When using incipient wetness impregnation, catalysts synthesized with precursor hexaammineruthenium(iii) chloride showed the best activity and electrochemical efficiency, followed by catalysts prepared with ruthenium(iii) chloride and ruthenium(iii) nitrosyl nitrate. The Ru/ACC electrocatalyst reduced guaiacol, phenol and syringol with similar electrochemical efficiencies, but temperature was an important variable; the electrochemical efficiency for guaiacol reduction increased from 8% at 25 °C to 17% at 50 °C, but then dropped back to 10% at 80 °C. Solution pH also affected catalyst activity and product selectivity, with acidic conditions favoring guaiacol conversion, electrochemical efficiency and cyclohexanol selectivity.

Full text of DOI:10.1039/c2gc35552c

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PAPER
Mild electrocatalytic hydrogenation and hydrodeoxygenation of bio-oil
derived phenolic compounds using ruthenium supported on activated
carbon cloth
a,b
a
c
c
b
Zhenglong Li, Mahlet Garedew, Chun Ho Lam, James E. Jackson, Dennis J. Miller and
a,b,d
Christopher M. Saffron*
Received 12th April 2012, Accepted 26th June 2012
DOI: 10.1039/c2gc35552c
Electrocatalytic hydrogenation (ECH) is an option for stabilizing or upgrading bio-oil that employs mild
conditions (≤80 °C and ambient pressure) compared to hydrotreatment. In this study, phenol, guaiacol
(2-methoxyphenol), and syringol (2,6-dimethoxyphenol) were chosen as model lignin-like substrates
because of their abundance in bio-oil and their high resistance to hydrogenation relative to the carbonyl
compounds. Cathodic reduction was catalyzed by ruthenium supported on activated carbon cloth
(Ru/ACC), a novel electrocatalyst. Incipient wetness impregnation and cation exchange methods were
employed to prepare the electrocatalyst using three different ruthenium precursors. Scanning electron
microscopy revealed that ruthenium nanoparticles within the range of 10 to 20 nm were deposited on the
support by both catalyst synthesis methods. Catalysts prepared by cation exchange were more active than
those prepared using incipient wetness impregnation, presumably because of support surface
functionalization by the oxidation pretreatment. When using incipient wetness impregnation, catalysts
synthesized with precursor hexaammineruthenium(III) chloride showed the best activity and
electrochemical efficiency, followed by catalysts prepared with ruthenium(III) chloride and ruthenium(III)
nitrosyl nitrate. The Ru/ACC electrocatalyst reduced guaiacol, phenol and syringol with similar
electrochemical efficiencies, but temperature was an important variable; the electrochemical efficiency for
guaiacol reduction increased from 8% at 25 °C to 17% at 50 °C, but then dropped back to 10% at 80 °C.
Solution pH also affected catalyst activity and product selectivity, with acidic conditions favoring guaiacol
conversion, electrochemical efficiency and cyclohexanol selectivity.
3
,4
Introduction
phenols and sugars to saturated alcohols and polyols.
However, such hydrogenation is typically run at temperatures
3,5,6
Bio-oil, the liquid product from fast pyrolysis of biomass, is a
promising sustainable material for transportation fuel production.
However, the raw bio-oil is not ready for end use because of its
tendency to polymerize and to corrode metal containers and
pipes. This reactivity is largely due to the presence of unsatu-
rated oxygenates such as aldehydes, ketones, carboxylic acids
over 100 °C,
high enough to cause accelerated bio-oil
7
polymerization. We propose instead electrocatalytic hydrogen-
ation (ECH) which operates at less than 80 °C and ambient
pressure; these mild conditions minimize both polymerization
and catalyst deactivation by coke formation. During ECH,
reduction of protons from solution forms atomic hydrogen
in situ on the catalytic electrode surface where it is needed to
hydrogenate organic substrates. This strategy simplifies proces-
sing and avoids the need for externally supplied, fossil-based
hydrogen gas and associated handling equipment. Ideally, the
needed electricity would come from carbon-free sources such as
solar, wind, or even nuclear power. Electrocatalytic hydrogen-
ation thus represents a green, carbon-retentive pathway for stabil-
ization (and potentially further upgrading) of biomass-derived
bio-oil to produce fuels and chemicals.
1,2
and phenols.
Thus, for long-term storage and any further
upgrading, bio-oil must first be stabilized in order to minimize
downstream coke formation, catalyst deactivation and carbon
loss to the gas phase.
Catalytic hydrogenation has been shown to be a good method
for bio-oil stabilization, converting most aldehydes, ketones,
3
a
Department of Biosystems & Agricultural Engineering, Michigan State
University, East Lansing, MI 48824, USA. E-mail: saffronc@egr.msu.edu
Department of Chemical Engineering & Materials Science, Michigan
Lignin-derived phenolic compounds account for 25–30% of
b
8,9
the raw bio-oil, but have a lower oxygen content (22–30 vs.
State University, USA
10
c
33–40 wt% for raw bio-oil, dry basis). Compared to other bio-
Department of Chemistry, Michigan State University, USA
Department of Forestry, Michigan State University, USA
d
oil components such as aldehydes and ketones, phenolics are
2540 | Green Chem., 2012, 14, 2540–2549
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Table 1 BET surface area, micropore area and micropore volume of the blank ACC and the catalysts
BET surface area
Micropore area
Micropore volume
2
−1
2
−1
3
−1
Sample name
Preparation method
Precursor
(m g
)
(m g
)
(cm g
)
Blank ACC
DI water wash H reduction at 220 °C and 500 psi
2
—
1010
904
800
879
341
685
599
551
612
139
0.30
0.27
0.26
0.28
0.06
3
3
3
6
-IW-Cl
Incipient wetness
Incipient wetness
Incipient wetness
Cation exchange
RuCl
Ru(NH
Ru(NO)(NO
Ru(NH Cl
3
3
-IW-NH3
-IW-NO
.6-CE-NH3
3 6 3
) Cl
3 3
)
3
)
6
1
1
more resistant to hydrogenation. Therefore, successful ECH of
phenol is a significant step towards its application for raw bio-oil
stabilization.
at 220 °C and 500 psi. The blank ACC has a surface area of
2
−1
1010 m g , very similar to the value reported in the litera-
3
2
12
ture. Compared with other supports used for cathodes, this
kind of support has much larger surface area. The majority of
12–17
Previous research
on ECH of phenolic compounds has
shown that large cathode surface areas are usually necessary to
achieve high reaction rates. Electrodes made with pressed
metallic powder particles have been successfully used,
their mechanical strength is weak without addition of a binding
material and much surface area is lost in the powder pressing
step. To overcome the disadvantages of pressed electrodes,
Menard and co-workers developed a new electrode by entrap-
ping catalytic powders into reticulated vitreous carbon
This is an effective system to test different classi-
cal catalytic hydrogenation catalysts using electrochemistry.
3
−1
the pores are micropores and the volume is 0.30 cm g
(Table 1). Upon loading with different ruthenium precursors, the
supports’ micropore volumes decreased, suggesting that some of
the micropores were blocked by ruthenium. As shown in
Table 1, catalyst 6.6-CE-NH3,† prepared by cation exchange,
has a very different surface area and micropore volume than the
18,19
but
20
−
3
other catalysts because the support was pretreated in 1 mol dm
33
boiling nitric acid for 24 h. According to Pradhan and Sandle,
12,21–23
(RVC).
oxidation with nitric acid generates surface oxygen moieties at
12,24
the entrance of the pores, restricting access of N molecules to
2
However, the catalyst powders are mobile, so the amount of
active catalysts involved in the actual hydrogenation is difficult
to measure and the mechanically fragile electrode may be
destroyed by strong stirring.
the micropores. Thus this type of catalyst shows reduced surface
area and micropore volume.
Fig. 1 shows a SEM image of the blank ACC FM100. With
34
its carbon fibers knitted together, the conductivity is very high,
To develop a well-defined catalytic cathode, we immobilized
ruthenium onto activated carbon cloth (ACC) using incipient
wetness impregnation and cation exchange methods. As a cata-
lyst support, the advantages of ACC include high rates of
making it a good electrocatalyst support. Because it is mono-
lithic, filtration or decantation is not needed after the reaction.
This flexible support can conveniently be cut and shaped to fit
into different kinds of reactors.
2
−1 25
adsorption, high surface area (1000 to 2100 m g
tial for easy in situ regeneration. Ruthenium supported on
carbon has been shown to be an efficient catalyst for classical
)
and poten-
SEM images of the catalysts (IW series) prepared by incipient
wetness impregnation with precursor Ru(NH ) Cl are shown in
2
6
3
6
3
Fig. 2(a) and (b). As seen in Fig. 2(a), most of the ruthenium
was distributed into white spots larger than 100 nm. Fig. 2(b)
shows that the white spots were made of accumulated ruthenium
particles. For the cation exchange (CE series) preparations, the
27
chemical hydrogenation of various organics such as phenols,
28,29
30
31
organic acids,
amino acids and bio-oil itself.
In this investigation, ECH of guaiacol was assessed in terms
of reactivity, product selectivity and electrochemical efficiency
HNO pre-oxidation of the ACC fiber surface forms oxygenated
3
(E.E.) as functions of catalyst parameters (e.g. preparation
functional groups such as carboxylic acid, lactone, phenolic
33
methods, precursor types, ruthenium content, type of support)
and reaction conditions (temperature and pH). In order to
demonstrate chemical reduction on a range of phenolic com-
pounds, the ECH of phenol and syringol were also investigated
using the same conditions as for guaiacol ECH.
hydroxyl and quinone groups. When loading the ACC in the
ruthenium salt solution, metal cations ion pair with the surface
anionic sites. After catalyst reduction under hydrogen, white
spots are observed on the ACC fiber (Fig. 2(c)). Magnification of
these white spots reveals nanoparticles in the 10–20 nm range.
Compared with the IW preparation, these nanoparticles tend to
be isolated from each other as opposed to forming a continuous
sheet (Fig. 2(d)).
Results and discussion
Characterization of catalysts
†
Catalyst codes are used to refer to each type of catalyst and catalyst
As mentioned in the introduction, a high surface area cathode is
necessary for the efficient reduction of guaiacol. The catalytic
cathodes’ BET surface area, micropore area and micropore
volume were analyzed (Table 1). In order to assess the under-
lying support properties of the catalyst supports, ACC was pre-
treated in the same manner (designated the “blank ACC”) as the
catalyst, by washing with DI water and reducing with hydrogen
preparation method. In 6.6-CE-NH3, the number “6.6” is the ruthenium
weight percent, “CE” is the method cation exchange and “NH3” means
the precursor is Ru(NH ) Cl . For incipient wetness impregnation, “IW”
is used to indicate the method. Designations for the other two precur-
sors, RuCl and Ru(NO)(NO , are “Cl” and “NO” respectively. For the
catalysts (3 wt% Ru) prepared on HCl-washed ACC with RuCl as pre-
3
6
3
3
3 3
)
3
cursor, the code 3-IW-Cl-AW was used, where “AW” stands for acid
washed.
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Mechanism of guaiacol electrocatalytic hydrogenation with
Ru/ACC
include: formation of adsorbed atomic hydrogen (eqn (1)),
adsorption of guaiacol (eqn (2)), reactions between adsorbed
guaiacol and adsorbed hydrogen (eqn (3)), demethoxylation of
guaiacol (eqn (4)), hydrogenation of phenol to cyclohexanone
3
5
According to Sergeev and Hartwig, a detailed reaction mech-
anism remains unclear, even for an aryl ether hydrogenolysis
catalyzed by a well-defined nickel complex. For the present
guaiacol cleavage, effected by heterogeneous Ru/ACC catalysts,
(
eqn (5)) and cyclohexanol (eqn (6)), and desorption of the pro-
28,36
ducts (eqns (7) and (8)).
2
1
we follow the lead of Dabo et al. and simply outline the trans-
formation in a manner analogous to that of pi-bond hydrogen-
ation (Scheme 1). Reactions involved in ECH of guaiacol
þ
ꢀ
H3O þ e þ M ! MðHÞ þ H2O
ð1Þ
ð2Þ
Scheme 1 Schematic representation of the electrocatalytic hydrogen-
ation mechanism with Ru/ACC, where YvZ is the unsaturated organic
compound, YH–ZH is the hydrogenated product.
Fig. 1 SEM image of the blank ACC FM100. Scale bar: 500 μm.
3 6 3
Fig. 2 (a) and (b) SEM images of 3-IW-NH3 prepared with incipient wetness impregnation using Ru(NH ) Cl as precursor; (c) and (d) SEM
images of 5-CE-NH3 prepared with cation exchange method using Ru(NH
d) 100 nm.
3
)
6
Cl
3
as precursor. Scale bar: (a) 100 nm; (b) 100 nm; (c) 1 μm;
(
2542 | Green Chem., 2012, 14, 2540–2549
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differently in chemical catalytic hydrogenation vs. ECH. For
ð3Þ
ECH, atomic hydrogen directly forms on the cathode surface
(eqn (1)), so reduction can occur at ambient pressure. For cataly-
tic hydrogenation, H gas must dissolve first, but it is poorly
2
soluble in water, so high pressure is usually needed for aqueous
reactions. However, under ambient pressure, the H2 concen-
trations in solution and on the catalyst surface would be low.
Furthermore, the temperatures used here are much lower than the
11
1
25 °C reported by Vispute as the lowest temperature where
ð4Þ
ð5Þ
guaiacol starts to be chemically reduced.
ECH of guaiacol using different catalysts: preparation methods
and precursor effects
ECH of guaiacol was studied with several different catalysts pre-
pared using two methods (incipient wetness impregnation and
cation exchange) with different precursors. Various conditions
were employed, including different reaction temperatures, elec-
trolytes and currents. The results were summarized in Table 2.
The effects of these conditions on guaiacol conversion, electro-
chemical efficiency and product selectivities are discussed in
detail in the following text.
ð6Þ
ð7Þ
ð8Þ
Products detected using GC/MS include cyclohexanol, cis-
2
-methoxycyclohexanol, trans-2-methoxycyclohexanol and
phenol (Table 2, Fig. 3). However, phenol only appeared in mea-
surable quantities with 3-IW-NH3, 5-IW-NH3 and 3-IW-NO as
cathodes, indicating that most of the catalysts efficiently hydro-
genated phenol. The reaction pathway for ECH of guaiacol to
the major products is shown in Scheme 2. While demethylation
is one of the major reactions in traditional catalytic deoxygena-
Here, M is the metal active site for hydrogen or organic com-
pounds, M(H) is the chemisorbed atomic hydrogen. Besides the
reactions listed above, hydrogen desorption via the Tafel
eqn (9)) or Heyrovsky (eqn (10)) processes competes with the
desired hydrogenation (eqn (3)–(6)) decreasing its electrochemi-
(
39
tion of guaiacol, it is not observed during ECH; demethoxyla-
tion dominates instead. Thus ECH of bio-oil retains more carbon
in the liquid products than conventional upgrading.
3
7,38
cal efficiency (E.E.).
As a practical matter, for bio-oil
upgrading, this byproduct hydrogen could potentially be cap-
tured and used as a valuable reducing agent for further
hydrotreatment.
Deoxygenation of oxygenated compounds is one ultimate
goal for bio-oil upgrading. Though deoxygenation is difficult at
such mild conditions, partial deoxygenation of guaiacol was
observed, resulting in two products, cyclohexanol and phenol.
Cyclohexanol is a major product while phenol is only an inter-
mediate. Among the catalysts studied, 5-IW-NO and 3-IW-Cl
gave the greatest selectivity to cyclohexanol although their
guaiacol conversion activities were relatively low (Fig. 4).
When we compared the catalysts with the same nominal ruthe-
nium content (3 wt%), 3-CE-NH3 resulted in slightly higher
guaiacol conversion than 3-IW-NH3 (Fig. 4) even though the
metal dispersions (measured from hydrogen chemisorption) are
very similar. The pretreatment in the cation exchange method
functionalizes the support surface by increasing acidic groups,
MðHÞ þ MðHÞ ! H2 þ 2M
ð9Þ
þ
ꢀ
MðHÞ þ H þ e ! H2 þ M
ð10Þ
Control experiments
The first control experiment used only blank ACC as cathode
material; at 80 °C and ambient pressure, no hydrogenation of
guaiacol was observed. For this control experiment, there were
no metal active sites for atomic hydrogen, and no catalyst for the
hydrogenation reaction. Therefore, ACC alone cannot hydro-
genate guaiacol.
40
which likely enhances guaiacol adsorption and thus, electro-
catalytic activity.
The second control experiment used 3-IW-NH3 catalyst at
0 °C and ambient pressure, with no current passed. No guaiacol
was converted, showing that hydrogen is essential for
hydrogenation.
The third control experiment was performed on 3-IW-NH3,
with no current passed through the electrochemical cell.
Ru/ACC catalysts prepared from different precursors using
incipient wetness impregnation showed different activities
toward ECH of guaiacol. Ru(NH ) Cl exhibited the highest
8
3
6
3
activity among the three precursors, followed by RuCl3 and
Ru(NO)(NO ) . The presence of residual nitrogen on the catalyst
3
3
and an uneven ruthenium distribution could be responsible for
the reduced performance of the catalyst prepared with Ru(NO)-
However, H gas was supplied by bubbling through the solution
2
41
at 80 °C and ambient pressure. No conversion of guaiacol was
observed either. Catalyst surface-bound atomic hydrogen forms
(NO ) . This notion is supported by the blocky accumulations
3 3
42
of ruthenium seen in Fig. 5. Similarly, Nurunnabi et al.
Green Chem., 2012, 14, 2540–2549 | 2543
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Table 2 Guaiacol conversion, electrochemical efficiency and product selectivities for ECH of guaiacol at various conditions
Product selectivity (%)
Temp.
(°C)
Electrolyte
Current
(mA)
Conversion
(%)
Cis-2-
MCH
Trans-2-
MCH
0.2 (mol dm−
3
)
E.E. (%)
CH
P
Catalysts
1
2
3
4
5
6
7
8
9
1
3-IW-Cl
80
80
80
80
80
80
80
25
50
25
50
80
80
80
80
80
80
80
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
NaCl
NaOH
NaOH
HCl
HCl
HCl
HCl
100
100
100
100
100
100
100
100
100
100
100
100
100
100
40
52 ± 0.4
69 ± 1.3
48 ± 8.7
38 ± 0.5
53 ± 1.6
52 ± 6.2
75 ± 3.9
13 ± 0.4
36 ± 12
51 ± 0.8
60 ± 13
64 ± 8.8
62 ± 2.4
45 ± 2.3
71 ± 3.3
69 ± 0.0
65 ± 4.2
60 ± <0.01
10 ± 2.1
31 ± 2.9
19 ± 4.3
9.8 ± 0.5
19 ± 0.4
19 ± 3.5
30 ± 4.3
7.7 ± 1.8
17 ± 1.6
19 ± 6.6
31 ± 4.9
20 ± 2.2
28 ± 1.0
20 ± 1.2
27 ± 0.1
28 ± 3.9
23 ± 0.6
22 ± <0.01
65 ± 7.1
58 ± 3.1
59 ± 11
31 ± 4.8
25 ± 3.2
29 ± 3.1
36 ± 0.0
22 ± 0.4
37 ± 5.0
36 ± 2.3
62 ± 6.5
56 ± 1.4
59 ± 9.3
47 ± 2.8
42 ± 1.3
38 ± 0.5
36 ± 2.3
28 ± 1.2
30 ± 0.4
38 ± 0.8
37 ± <0.01
4.4 ± 2.3
12 ± 0.3
9.8 ± 6.4
12 ± 1.0
5.2 ± 0.8
14 ± 2.7
11 ± 2.0
12 ± 6.9
14 ± 0.0
16 ± 0.9
15 ± 2.2
17 ± 2.0
13 ± 0.6
11 ± 1.9
11 ± 0.7
13 ± 0.1
14 ± 0.5
14 ± <0.01
—
3.7 ± 0.4
2.4 ± 2.8
5.9 ± 0.1
—
—
—
—
—
—
—
—
—
6.6 ± 0.4
—
—
3-IW-NH3
5-IW-NH3
3-IW-NO
47 ± 0.9
72 ± 1.3
48 ± 2.3
53 ± 1.4
26 ± 0.3
30 ± 1.4
25 ± 8.4
38 ± 0.5
42 ± 3.3
48 ± 0.2
46 ± 3.8
61 ± 1.9
57 ± 0.5
45 ± 1.3
49 ± <0.01
5-IW-NO
1-CE-NH3
3-CE-NH3
3-IW-Cl
3-IW-Cl
0
1.5-CE-NH3
1.5-CE-NH3
3-CE-NH3
3-CE-NH3
3-IW-NH3
3-IW-NH3
3-IW-NH3
3-IW-NH3
3-IW-NH3
1
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
70
130
160
2.1 ± 0.0
—
CH: cyclohexanol; Cis-2-MCH: Cis-2-methoxycyclohexanol; Trans-2-MCH: Trans-2-methoxycyclohexanol; P: phenol.
Fig. 3 Product selectivities for ECH of guaiacol at 80 °C and ambient
pressure using different catalysts with 0.2 mol dm HCl as catholyte.
Fig. 4 Guaiacol conversion and electrochemical efficiency for ECH of
−3
guaiacol at 80 °C and ambient pressure using different catalysts with
3
0
.2 mol dm− HCl as catholyte.
Scheme 2 Reaction pathway for ECH of guaiacol to the major pro-
ducts; 2-methoxycyclohexanol includes both cis and trans isomers.
showed low CO conversion during Fischer-Tropsch synthesis
43
with catalysts prepared from Ru(NO)(NO ) . Diaz et al. also
3
3
reported that Pd/C prepared with a nitrate precursor (Pd(NO ) )
3
2
showed a lower overall hydrogenation activity.
Fig. 5 Ruthenium accumulation for the catalyst 3-IW-NO prepared
The electrochemical efficiencies of guaiacol ECH at 80 °C
3 3
with incipient wetness impregnation using Ru(NO)(NO ) as precursor.
using different catalysts were compared as summarized in Fig. 4.
Scale bar: (a) 10 μm.
2544 | Green Chem., 2012, 14, 2540–2549
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Both 3-IW-NH3 and 3-CE-NH3 catalysts show >30% electro-
methoxycyclohexanol isomer was expected to be more stable,
the cis isomer was always the major product at these studied
temperatures. Presumably, during hydrogenation, the aromatic
ring lies down on the ruthenium active surface and atomic hydro-
gens are added to the one face, thus forming cis-2-methoxyclo-
hexanol. This phenomenon was also observed when using
catalyst prepared by the cation exchange method (Fig. 7).
chemical efficiency, higher than the 26.3% maximum reported
1
2
E.E. for phenol ECH, obtained using a Pd catalyst. One poss-
ible reason for these high E.E. values is that the present immobil-
ization schemes achieve more intimate metal-support electrical
contact.
48
Solladié-Cavallo et al. showed a similar result for hydrogen-
Temperature effect
ation of substituted phenols over Ru/Al O .
2
3
The effect of temperature on guaiacol ECH was studied with
0
.2 mol dm− HCl as catholyte for 2 h under a constant current
3
Catalyst support effect
of 100 mA. Three different temperatures, 25, 50 and 80 °C, were
studied, all much lower than those used in classical catalytic con-
version of phenolic compounds.
The E.E. is a function of the competition between electrocata-
lytic hydrogenation and hydrogen desorption. Raising the temp-
erature from 25 °C to 50 °C increased E.E. (Fig. 6) from 8% to
Analysis by EDX reveals that there are other elements in the orig-
inal ACC besides C and O, including Al, Zn and S (Table 3).
Some elements, especially S, may have strong chemisorption
with ruthenium, blocking active sites and moderating the hydro-
4
4,45
49
genation reaction. Thus catalyst performance was assessed
based on original and demineralized ACC. ACC was deminera-
lized by washing in boiling hydrochloric acid for 3 days, resulting
in the decrease of the ash content from 5 wt% to around 0.1 wt%.
Catalysts made from original and demineralized ACC supports
were compared during ECH of guaiacol using 0.2 mol dm− HCl
as catholyte at 25, 50 and 80 °C (Fig. 8). The trivial differences
found indicate that the extra elements in the original ACC do not
interfere with the Ru catalytic activity.
1
7% but further heating to 80 °C dropped it back to 10%. This
indicates that electrocatalytic hydrogenation was favored from
5 °C to 50 °C, while hydrogen desorption rates accelerated as
2
temperature increased from 50 °C to 80 °C. Similar E.E. changes
on the ECH of indigo to leuco indigo were observed by Roessler
3
46
et al., who found that E.E. rose as temperature was ramped up
from 30 °C to 60–80 °C, but then dropped off above this range.
2
1
However, Dabo et al. found continuous E.E. improvement
from 25 °C to 75 °C during ECH of 4-phenoxyphenol to phenol
−3
over 5% Pd/C in 1 mol dm NaOH. Amouzegar and Sava-
4
7
pH effect
dogo also saw continuous E.E. increases from 5 °C to 40 °C to
0 °C during ECH of phenol to cyclohexanol on platinum dis-
persed on graphite particles; the E.E. increase from 40 °C to
6
All experiments discussed above were carried out in 0.2 mol dm−3
HCl solution. To probe pH effects, the performance of these
6
0 °C, however, was much smaller than that from 5 °C to 40 °C.
Cation exchange prepared catalyst shows good activity toward
guaiacol conversion at 25 °C, very similar to that at 50 °C
Fig. 7). However, guaiacol conversion for incipient wetness
(
impregnation prepared catalyst decreased dramatically from 36%
to 13% (Fig. 6). Again, catalysts prepared by cation exchange
show better performance than those prepared by incipient
wetness impregnation.
The effects of temperature on the product selectivities using
3
-IW-Cl are shown in Fig. 6. At 80 °C, cyclohexanol was the
dominant product, while cis-2-methoxycyclohexanol became the
largest one at 25 °C and 50 °C. Though the trans-2-
Fig. 7 Product selectivities, guaiacol conversion and E.E. for ECH of
guaiacol at different temperatures using 1.5-CE-NH3.
Table 3 Energy dispersive X-ray analyses of the original ACC and
washed ACC
Weight (%)
Element
Original ACC
Washed ACC
C
O
Cl
Al
S
80.9 ± 0.48
12.1 ± 0.03
—
2.6 ± 0.16
0.3 ± 0.06
4.1 ± 0.29
92.0 ± 0.05
7.3 ± 0.05
0.8 ± 0.01
—
—
—
Fig. 6 Product selectivities, guaiacol conversion and E.E. for ECH of
guaiacol at different temperatures using 3-IW-Cl as cathode. Bars refer
to product selectivities. G: guaiacol.
Zn
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−
3
ECH at 80 °C and ambient pressure with 0.2 mol dm−3 HCl as
catholyte. Because it was difficult to measure the actual effective
electrode surface area, current is directly used without calculat-
ing current density. As shown in Fig. 10, guaiacol conversion
and electrochemical efficiency are invariant when current is in
the range of 40 mA to 100 mA and above 100 mA, both
decrease slightly. The current density effect on guaiacol conver-
sion and E.E. is not very obvious for ECH of guaiacol using
Ru/ACC as cathode because the surface area of the cathode is
relatively high and the absolute change of the current density is
very small. Additionally, product selectivities are only slightly
affected. Selectivity for cyclohexanol formation is higher for the
current range from 40 mA to 100 mA than 130 mA and
160 mA.
catalysts in neutral (0.2 mol dm NaCl as catholyte) and basic
−3
solution (0.2 mol dm NaOH as catholyte) was examined here.
All other conditions were the same as the runs with 0.2 mol dm−
HCl as electrolyte. When using 3-CE-NH3 and 3-IW-NH3 as
catalysts, guaiacol conversion was favored under acidic con-
ditions (Fig. 9). Guaiacol adsorption onto catalyst sites (eqn (2))
is a key step in the hydrogenation reaction. The ionization state
of guaiacol varies with solution pH, potentially affecting its
adsorption characteristics. In acidic and neutral conditions,
3
50
guaiacol (pK = 9.9) is mainly a neutral molecule, while depro-
a
tonation of guaiacol to form phenolate anion occurs under alka-
line conditions. As ionic forms prefer to remain in the polar
solution and the carbon support preferentially adsorbs neutral
species, adsorption onto the support is reduced. Furthermore, the
resonance delocalization in the phenolate form shifts the thermo-
12
dynamics and makes the ECH of guaiacol more difficult. Thus,
greater guaiacol conversion was observed in acidic than in alka-
line solution. A similar effect on E.E. was observed, showing
higher E.E. under acidic conditions. Likewise, selectivity to
cyclohexanol was higher under acidic conditions, indicating a
higher level of demethoxylation of guaiacol (Fig. 9).
ECH of other phenolic compounds: phenol and syringol
In addition to guaiacol, both phenol and syringol served as
lignin-derived model compounds. ECH of these two model com-
−3
pounds was carried out in 0.2 mol dm HCl catholyte solution
at 80 °C and ambient pressure. ECH of phenol generates two
products, cyclohexanol and cyclohexanone (Table 4). Cyclohexa-
nol is the major product, consistent with separate observations of
rapid, efficient reduction of cyclohexanone to cyclohexanol. Five
major products are obtained from ECH of syringol: cyclohexa-
nol, cis-2-methoxycyclohexanol, trans-2-methoxycyclohexanol,
Current density effect
Current density effects on guaiacol conversion, electrochemical
efficiency and product selectivities were studied for guaiacol
2
-methoxycyclohexanone and guaiacol (Table 4); these products
are similar to those obtained from ECH of guaiacol and point to
demethoxylation as the first step from syringol.
Equal amounts of charge were passed for ECH of phenol and
syringol, but ECH of phenol requires fewer electrons than ECH
of syringol, resulting in higher conversion of phenol. The
π-system’s electron density increases as the methoxylation
degree increases from phenol to syringol, so syringol should be
more difficult to reduce. However, similar E.E. values were
obtained for ECH of both model compounds.
Catalyst stability
Fig. 8 ECH of guaiacol using catalysts prepared on HCl washed ACC
Catalyst deactivation is an important concern during catalytic
bio-oil upgrading. To evaluate the catalyst stability during ECH
−3
(3-IW-Cl-AW) and original ACC (3-IW-Cl) with 0.2 mol dm HCl as
electrolyte at different temperatures.
Fig. 9 ECH of guaiacol using catalysts 3-CE-NH3 and 3-IW-NH3
under different pH conditions at 80 °C and ambient pressure. Bars refer
to product selectivities.
Fig. 10 ECH of guaiacol using catalyst 3-IW-NH3 under different cur-
rents at 80 °C and ambient pressure. Bars refer to product selectivities.
2546 | Green Chem., 2012, 14, 2540–2549
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Table 4 ECH of phenol and syringol using 1.5-CE-NH3 at 80 °C and ambient pressure with 0.2 mol dm− HCl as catholyte
3
Product selectivity (%)
Conversion E.E.
Cis-2-
Trans-2-
2-
Reactants (%) (%) Cyclohexanol Cyclohexanone methoxycyclohexanol methoxycyclohexanol methoxycyclohexanone Guaiacol
Phenol
Syringol 58
89
29
29
99
35
1
0
—
27
—
9
—
13
—
16
Experimental
Reagents and materials
Guaiacol (98+%) and 2-methoxycyclohexanol (99%) were
obtained from Alfa Aesar. 2,6-Dimethoxyphenol (syringol)
(
(
99%) was purchased from Sigma-Aldrich and cyclohexanol
99+%) was from Mallinckrodt. All of these compounds were
used as received without further purification. Phenol, from
Mallinckrodt, was purified by sublimation to remove moisture.
Zorflex® ACC FM100 was obtained from the Calgon Carbon
Co. Ruthenium(III) chloride (RuCl ) hydrate (99.9% PGM
3
basis, Ru 38% min), hexaammineruthenium(III) chloride
(
(
Ru(NH ) Cl , Ru 32.1% min) and ruthenium(III) nitrosyl nitrate
3 6 3
Ru(NO)(NO ) , Ru 31.3% min) were bought from Alfa Aesar.
3 3
Fig. 11 Time course of guaiacol conversion for the initial reaction, the
first and the second reuse of the catalyst 3-IW-NH3 using 0.2 mol dm−
HCl as catholyte at 80 °C and ambient pressure.
3
Catalyst preparation
of phenolic compounds, the catalysts were reused two times.
After each use of the catalytic cathode, it was washed overnight
using DI water, followed by drying under vacuum in the dessica-
tor. At the beginning of the next experiment, pre-electrolysis was
carried out at 80 mA for 10 min. As shown in Fig. 11, guaiacol
conversion did not change significantly with each reuse of the cat-
alyst. Also, ICP-OES analysis of the solution showed that there
was no significant ruthenium leaching into the solution. This
observed stability of the Ru/ACC catalytic cathode is likely due to
the mild conditions required for effective ECH of guaiacol.
Zorflex® ACC FM100 was chosen as the support for the ruthe-
nium catalysts due to good conductivity and high surface area.
For most of the experiments, ACC was washed in DI water; this
material was labeled “original ACC.” According to energy dis-
persive X-ray spectroscopy (EDX) analysis, there were some
mineral impurities in the original cloth (Table 3). To probe the
effects of the catalyst support, “washed ACC” was obtained by
washing ACC in boiling concentrated HCl solution for three
days, thoroughly rinsing with DI water to remove residual HCl,
and then drying in the oven at 105 °C. EDX of this material only
showed C, O and small amounts of Cl, presumably a residue
from the HCl washing, as there was no Cl in the original ACC.
Two methods were used for the catalyst preparation: incipient
wetness impregnation and cation exchange. For incipient
wetness impregnation, three precursors were used, including
Ru(NH ) Cl , Ru(NO)(NO ) and RuCl . For Ru(NH ) Cl and
Conclusions
This work shows that Ru/ACC is an efficient catalyst for electro-
catalytic hydrogenation and partial hydrodeoxygenation of phe-
nolic compounds under mild conditions compared to other
catalytic reductions, including other ECH schemes. Catalyst
comparisons demonstrated that Ru/ACC catalysts prepared via
the cation exchange method show much better activity than
those prepared by incipient wetness impregnation. When using
incipient wetness impregnation, Ru(NH ) Cl showed the
3
6
3
3 3
3
3 6
3
Ru(NO)(NO ) , two ruthenium loadings (nominal 3 wt% and
3
3
5 wt%) were prepared, while only 3 wt% (nominal) ruthenium
content was used for RuCl because of its relatively low solubi-
3
lity in water. A piece of ACC (1.3 cm × 3.0 cm) was first soaked
in ruthenium precursor solution to saturate the ACC pores.
Kimwipes® were then used to remove the excess solution after
the ACC was soaked. The wet ACC was dried under room con-
ditions, then vacuum dried at room temperature, and finally
3
6
3
highest activity among the three precursors studied. Within the
range 25–80 °C, higher temperature was found to favor guaiacol
conversion and deoxygenation. Lower pH resulted in higher
guaiacol conversion and electrochemical efficiencies. Further-
more, phenol and syringol can be hydrogenated using Ru/ACC
catalysts, showing electrochemical efficiencies similar to those
found for guaiacol. Based on the results from this investigation,
electrocatalytic hydrogenation with Ru/ACC is a potential strat-
egy for ambient pressure hydrogenation of phenolic compounds
at low temperatures, and it may offer significant advantages for
future bio-oil stabilization and upgrading.
reduced with H in a Parr pressure reactor (model 452HC) at
2
500 psi and 220 °C for more than 12 h. During the reduction,
hydrogen was supplied continuously and the reactor was purged
occasionally to remove the product gases.
Ru(NH ) Cl was used as the precursor for cation exchange
3
6
3
preparation and five ruthenium loadings (nominal 1 wt%, 1.5 wt%,
3 wt%, 5 wt% and 6.6 wt%) were prepared. A piece of ACC was
first pre-oxidized using boiling 1 mol dm− HNO solution for
3
3
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3
3
2
4 h. The pretreated ACC was then washed thoroughly with
pre-electrolysis (80 mA) was applied to activate the ruthenium
catalyst. After the ruthenium was reduced, 1 mL 620 mmol dm−
guaiacol solution in isopropanol was added to the cathode
chamber to make an initial concentration of guaiacol equal
to 20 mmol dm . For all the experiments, ECH was performed
for 2 h.
3
DI water to remove the residual HNO and dried under vacuum
at room temperature. After drying, it was soaked in Ru(NH ) Cl
3
solution with 1 mol dm ammonia overnight to exchange
Ru(NH3)6 onto the cloth. The ACC was removed, washed
carefully with DI water, and then dried under vacuum at room
3
3
6
−3
3+
51
−3
temperature, and reduced as above with H at 500 psi and 220 °C.
2
Product analysis
Catalyst characterization
Chemical analysis proceeded by withdrawing 1 mL sample
aliquots at discrete time intervals from the cathode and the anode
chambers. The samples were further saturated with NaCl,
acidified to pH = 1 and then 1 mL chloroform was added to
extract the organics. After the experiments, the Ru/ACC cata-
lyst was immersed in 5 mL chloroform and the solution was
sonicated for 15 min to desorb any adsorbed organics into the
solution. The solution was then filtered using a 0.22 μm syringe
filter for GC/MS analysis.
Ruthenium contents of the catalysts were measured on a Varian
7
10-ES inductively coupled plasma optical emission spec-
trometer (ICP-OES). The catalysts were digested using aqua
regia in a boiling water bath for 4 h, filtered and diluted with DI
water. The standards prepared with RuCl were used to quantify
the ruthenium content over a concentration range of 0.08 ppm to
12
3
5
0 ppm.
Scanning electron microscopy (SEM) on a JEOL JSM-7500F
and a JEOL 6400V were used to image the catalyst support and
the morphology of ruthenium particles on the support. The cata-
lysts were mounted onto the aluminum stubs with carbon paste
and then dried under vacuum overnight. Secondary electron
imaging was used to obtain the images. Surface chemical com-
position was characterized by EDX coupled with the JEOL
All the samples were analyzed on a Shimadzu QP-5050A
GC/MS. The GC used a Restek Rtx-1701 capillary column,
60 m × 0.25 mm with a 0.25 μm film thickness, a 1.0 ml min−
helium carrier gas flow rate, and a split ratio of 1 : 40. The injec-
tor temperature was set at 270 °C. The GC oven program started
at 40 °C for 1 min, and then heated at 15 °C min− to 260 °C.
Mass spectrometry was operated in the electron ionization (EI)
mode at an ionization energy of 80 eV, a m/z ranging from 28 to
400, and a sampling interval of 0.34 s. Species associated with
each chromatographic peak were identified by comparing their
observed mass spectrum with the NIST library and then
confirmed by injection of authentic samples. External standards
were also used to identify compounds and quantify the peaks.
1
1
6
400V SEM.
Brunauer–Emmett–Teller (BET) surface area, micropore area
and micropore volume of the samples were measured on a
Micromeritics® ASAP 2010 system using a static volumetric
adsorption and desorption method. Nitrogen was used as the
adsorptive gas and the measurement was made at 77 K. Nitrogen
pressure was increased until 99% of the nitrogen saturation
pressure was reached. The total surface area of the sample was
calculated using the BET method from the adsorption isotherm
from 0.06 to 0.20 relative pressures. The micropore volume was
calculated from the desorption isotherm using the BJH (Barrett,
Joyner and Hallender) method.
Calculations
The conversion, selectivity and electrochemical efficiency were
calculated according to the following equations:
5
2
A method devised by NREL was used to analyze the ash
content in the original ACC and in the HCl washed ACC.
Samples of 0.5 to 2.0 g were weighed to the nearest 0.1 mg and
transferred to dried crucibles. A muffle furnace set to 575 ±
Conversion ¼ ðmoles reactant consumed=initial moles reactantÞ
ꢁ
100
2
5 °C was used to ash the samples to constant weights. After
ð11Þ
Selectivity ¼ ðmoles desired product=total moles productsÞ
100:
cooling in a desiccator, the residue was weighed to determine the
ash content.
ꢁ
Experimental setup
ð12Þ
Methanol was not accounted for the selectivity calculation.
Electrochemical hydrogenation was carried out in a two-chamber
53
glass H-cell, separated with a DuPont® Nafion-117 membrane.
−3
−3
E:E: ¼ ðElectrons used to generate products=Total electrons passedÞ
ð13Þ
Catholyte (30 mL) consisted of 0.2 mol dm HCl, 0.2 mol dm
−3
NaCl or 0.2 mol dm NaOH, depending on the experiments.
Ru/ACC prepared as described above was used as the working
electrode (cathode). Anolyte (30 mL) consisted of 0.2 M
phosphate buffer (pH = 7), and a Pt wire was used as the counter
electrode (anode). The whole cell was placed in a water bath for
experiments at controlled temperatures such as 50 °C and 80 °C.
ECH was carried out under galvanostatic control (100 mA) with
a dual channel potentiostat from Lambda (Model: LPD 422A
FM). Before the electrochemical hydrogenation, 10 minutes of
Acknowledgements
The authors would like to thank the Michigan State University
Foundation and AgBioResearch for financial support. Assistance
provided by Dr. Ambareesh Murkute, Dr. Lars Peereboom and
Mr. Xianfeng Ma with BET analysis is warmly acknowledged.
2548 | Green Chem., 2012, 14, 2540–2549
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