Hydrodeoxygenation of lignin model compounds over a copper chromite catalyst

Article abstract of DOI:10.1016/j.apcata.2012.09.047

The hydrodeoxygenation of benzyl alcohol, phenol, anisole, o-cresol, catechol, guaiacol, and vanillyl alcohol were carried out from 150 to 275 °C at 50 bar H₂ with a CuCr₂O₄·CuO catalyst in a decalin solvent. The hydroxymethyl group of benzyl alcohol was found to be highly reactive towards hydrogenolysis to form toluene. Demethoxylation of anisole to form benzene was found to be the primary reaction pathway in contrast to demethylation and transalkylation reactions, which are more prevalent for conventional hydrotreating catalysts. The hydroxyl group of phenol strongly activated the aromatic ring towards hydrogenation forming cyclohexanol which was subsequently dehydrated and hydrogenated to form cyclohexane. Reaction networks of increasing complexity were devised for the major functional groups and integrated to describe the most complex molecule studied, vanillyl alcohol.

Full text of DOI:10.1016/j.apcata.2012.09.047

Applied Catalysis A: General 447–448 (2012) 144–150
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Hydrodeoxygenation of lignin model compounds over a copper
chromite catalyst
Keenan L. Deutsch, Brent H. Shanks∗
Department of Chemical & Biological Engineering, Iowa State University, Ames, IA 50011, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 April 2012
Received in revised form 5 September 2012
Accepted 13 September 2012
Available online 8 October 2012
The hydrodeoxygenation of benzyl alcohol, phenol, anisole, o-cresol, catechol, guaiacol, and vanillyl alco-
hol were carried out from 150 to 275 ◦C at 50 bar H2 with a CuCr2O4·CuO catalyst in a decalin solvent.
The hydroxymethyl group of benzyl alcohol was found to be highly reactive towards hydrogenolysis
to form toluene. Demethoxylation of anisole to form benzene was found to be the primary reaction
pathway in contrast to demethylation and transalkylation reactions, which are more prevalent for con-
ventional hydrotreating catalysts. The hydroxyl group of phenol strongly activated the aromatic ring
towards hydrogenation forming cyclohexanol which was subsequently dehydrated and hydrogenated
to form cyclohexane. Reaction networks of increasing complexity were devised for the major functional
groups and integrated to describe the most complex molecule studied, vanillyl alcohol.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Hydrogenolysis
Hydrodeoxygenation
Copper catalyst
Lignin
Bio-oil upgrading
Copper chromite
1. Introduction
liquid product from fast pyrolysis [6]. The lignin-derived compo-
nents of biomass are commonly used as model compounds for
The utilization of biomass to produce fuels and chemicals is a
topic of increasing importance as petroleum prices rise and reserves
diminish. Numerous technologies are under investigation to utilize
the various components of biomass: cellulose, hemicellulose and
lignin. A key challenge in the chemical and thermal conversion of
biomass is the reduction of oxygen content to produce fuels and
chemicals that are compatible with conventional petroleum-based
products. Hydrodeoxygenation (HDO) is a promising upgrading
technology that has gained considerable attention in recent years
due to the high oxygen content of biomass derived feedstocks,
especially in the context of pyrolysis oil upgrading [1–5]. Of partic-
ular importance is the minimization of hydrogen consumption by
developing catalysts that can perform HDO while minimizing the
hydrogenation of unsaturated phenolic and furanic species that are
common in biorenewable feedstocks.
HDO because they possess the aromaticity that is important to
maintain to minimize hydrogen consumption [2]. Furthermore,
phenolic compounds contain several oxygen functionalities includ-
ing hydroxyl groups bound to aromatic and aliphatic carbons and
methoxy groups that all have different HDO reaction pathways and
susceptibilities [7].
Numerous HDO studies have investigated the use of conven-
tional hydrotreating catalysts consisting of sulphided NiMo–Al2O3
or CoMo–Al2O3 for bio-oil upgrading [8–32]. Both catalysts have
shown to be effective at HDO, where NiMo–Al2O3 favors hydro-
genated products and CoMo–Al2O3 favors aromatic products.
However, there are several concerns with using sulphided cata-
lysts in HDO such as the loss of the sulphided phase over the
course of processing and the introduction of sulphur into the prod-
uct stream [11,13,15,21,30,33,34]. Some researchers have proposed
co-feeding H2S to maintain the sulphided phase; however, H2S
competitively adsorbs with oxygenated compounds and dispropor-
tionally decreases the selectivity towards hydrogenolysis, which
lowers the production of aromatics. Coking has also been shown
to be a significant issue with the conventional hydrotreating cat-
alysts, which has been attributed to the acidity of the support
[11,16,17,27,35,36].
Conventional hydrotreating processes have focused primarily
on hydrodesulphurization (HDS) and hydrodenitrogenation (HDN)
because oxygen content in crude oil is very low and does not
pose the same environment issues as burning S- or N-containing
fuels [1]. Many studies of HDO have focused on the phenolic com-
ponents of bio-oil which represent a considerable portion of the
Alternatives to the sulphided molybdenum hydrotreating cata-
lysts have been investigated in more recent years [33–46]. Noble
metals have shown high activities in HDO; however, aromatic sat-
uration is observed unless high temperatures and atmospheric
∗
Corresponding author. Tel.: +1 515 294 1895; fax: +1 515 294 1269.
E-mail address: bshanks@iastate.edu (B.H. Shanks).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.09.047

K.L. Deutsch, B.H. Shanks / Applied Catalysis A: General 447–448 (2012) 144–150
145
standard of ethanol and identified and quantified using an Agilent
GC–MS/FID equipped with a HP-5MS column. The HP-5MS col-
umn was unable to separate benzene and cyclohexane; therefore,
these two compounds were quantified by m/z values of 78 and 84
respectively.
The catalysts were recovered after reaction and dried for 24 h at
100 ◦C prior to analysis of coking with a Perkin Elmer Simultaneous
Thermal Analyzer (STA 6000). This analysis consisted of 5–15 mg
of dried catalyst undergoing a catalyst oxidation step followed by
a coke oxidation step under 20 mL/min of air flow. A temperature
ramp from 50 to 250 ◦C at 10 ◦C/min followed by a 120 min dwell
was used to completely dry and oxidize the bulk of the copper cata-
lysts, which can form passivated oxide layers on the outside of the
catalyst upon exposure to an air atmosphere. Following this step
was a ramp from 250 to 700 ◦C at 5 ◦C/min in which the observed
weight loss was attributed to the oxidation of coke. All mass bal-
ances could be closed within 95% of the total carbon fed to the
system.
Fig. 1. Primary reactants used for HDO (a) benzyl alcohol, (b) phenol, (c) anisole, (d)
o-cresol, (e) catechol, (f) guaiacol, (g) 2-methoxy-4-methylphenol and (h) vanillyl
alcohol.
pressures are used to favor aromatic over saturated products.
Considering the challenges associated with pyrolysis oil and the
desired scale of the technologies, a base metal catalyst may be an
appropriate alternative to conventional hydrotreating catalysts and
noble metal catalysts [5]. Copper mixed metal oxide catalysts are
interesting materials for HDO because they tend to have weak acid-
ity, high hydrogenolysis activity, and low hydrogenation tendency
for furanic compounds [47–51].
An important aspect of catalyst research is developing an under-
standing of the reaction network in complex systems such as
the HDO of diverse mono-functional and multi-functional phe-
nolic species. The current work investigated the HDO of benzyl
alcohol, phenol, anisole, o-cresol, catechol, guaiacol, 2-methoxy-4-
methylphenol, and vanillyl alcohol with a copper chromite catalyst
from 150 to 275 ◦C at 50 bar H2 in a decalin solvent. The primary
reactants used are shown in Fig. 1. Benzene, toluene, cyclohexanol,
and methanol were also subjected to HDO conditions to understand
the stability of the primary products at high conversions. A reac-
tion network for copper was developed which can be contrasted to
those of conventional hydrotreating catalysts (S–CoMo–Al2O3 and
S–NiMo–Al2O3) and noble metal catalysts [8,42].
3. Results
3.1. HDO of mono-functional phenolics
The HDO of phenol was performed at 200–250 ◦C and the
resulting reaction profiles can be seen in Fig. 2. The reaction
of phenol resulted in the hydrogenation to cyclohexanol as the
major product. Minor products included benzene, cyclohexene
and cyclohexane. Benzene was not observed at 200 ◦C, whereas
it was observed in a 1:10 and 1:20 ratio to cyclohexane at 225
and 250 ◦C, respectively. Cyclohexene was found at a maxima
concentration of less than 1 mM during each run. Cyclohexanol
was also reacted under HDO conditions between 225 and 275 ◦C
and the evolution of cyclohexene and cyclohexane showed the
same trends as observed in the HDO of phenol. Benzene was
detected at a concentration of ∼0.3 mM by the end of the 275 ◦C
run but was not detected at any other sampling point of the reac-
tions.
The product distribution of the HDO of anisole can be seen in
Fig. 3. Anisole was considerably less reactive than phenol and after
20 h of reaction had conversions of 20, 52, and 88 mol% at 225, 250,
and 275 ◦C, respectively. The major products included benzene,
cyclohexane, cyclohexanol and methoxycyclohexane. Methanol
was not observed by GC. The selectivity towards benzene decreased
from 46% to 30 mol% with increasing the reaction temperature from
225 to 275 ◦C.
As seen in Fig. 4, the reactivity of benzene was examined under
the same HDO conditions. These data demonstrated that the hydro-
genation of benzene contributed to the loss of selectivity towards
benzene under these conditions. Methanol was also subjected to
the same HDO conditions and was observed only in trace amounts
at any sampling period. This result was likely caused by the vapor
pressure exceeding the reaction pressure as methanol approached
and became supercritical at 240 ◦C.
The HDO of benzyl alcohol occurred the most readily of the
compounds studied. The reaction profiles, shown in Fig. 5, exhibit
100 mol% conversion after 20 h at 150 ◦C. Further increasing the
reaction temperature to 175 and 200 ◦C achieved complete con-
version in 5 and 2 h, respectively. In separate experiments, the
hydrogenation of toluene to methylcyclohexane was found to
reach 8 mol% conversion after 20 h at 225 ◦C. Therefore, the hydro-
genation of toluene to methylcyclohexane was likely negligible at
150–175 ◦C and may have occurred to a very minor extent at 200 ◦C.
The hydrogenation of toluene occurred at approximately 60% of the
rate of benzene hydrogenation.
2. Materials and methods
The following compounds were used in reaction studies or to
calibrate the mass spectrometer and flame ionization detector of
the gas chromatograph (GC–MS/FID): phenol (Sigma 99%), anisole
(Acros 99%), benzyl alcohol (Fisher), guaiacol (Acros 99+%), catechol
(Acros 99+%), o-cresol (Acros 99%), 2-methoxy-4-methylphenol
(Acros 99%), vanillyl alcohol (Acros 99%), benzene (EMD Chem),
toluene (Fisher ACS), cyclohexane (Fisher ACS), cyclohexanol
(Fisher reagent grade), methanol (Fisher ACS), xylenes (Fisher ACS),
methylcyclohexane (Acros 99%) and decalin (Acros 98%). The cop-
per chromite catalyst (Acros) was used as received. The catalyst
contained a stoichiometric amount of CuCr2O4 and CuO. The as
received surface area was 10.3 m2/g. The physical and chemical
properties of this reduced catalyst have been previously reported
[51].
Reactions were carried out in 75 mL Parr autoclave reactors
with 50 mL of reaction solution at 100 mM reactant concentra-
tion, 100 mg catalyst, 50 bar H2 (UHP/Zero Linweld), 500 rpm (this
stirring rate was found to be outside the external mass trans-
fer limited region for the reaction through examining a range
of stirring rates below 500 rpm), sampling at 0, 2, 5, 9, and
20 h, and temperatures ranging from 150 to 275 ◦C depending
on the reactant. Liquid samples were prepared with an external

146
K.L. Deutsch, B.H. Shanks / Applied Catalysis A: General 447–448 (2012) 144–150
100
80
60
40
20
0
100
80
60
40
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0
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80
60
40
20
0
0
5
10
t [Hr]
15
20
0
5
10
t [Hr]
15
20
0
5
10
t [Hr]
15
20
(c)
(a)
(b)
Fig. 2. HDO of phenol at 50 bar H2 with (᭹) phenol, (ꢀ) cyclohexanol, (ꢀ) cyclohexane: (a) 200 ◦C, (b) 225 ◦C and (c) 250 ◦C.
30
25
20
15
10
5
30
25
20
15
10
5
30
25
20
15
10
5
0
0
0
0
5
10
Hr
15
20
0
5
10
t [
15
20
0
5
10
t [
15
20
(a)
(b)
(c)
]
]
]
t [
Hr
Hr
Fig. 3. HDO of anisole at 50 bar H2 with product formation: (ꢁ) benzene, (ꢀ) cyclohexane, (ꢀ) methoxycyclohexane and (ꢀ) cyclohexanol: (a) 225 ◦C, (b) 250 ◦C and (c)
275 ◦C.
120
100
80
60
40
20
0
120
100
80
60
40
20
0
120
100
80
60
40
20
0
0
5
10
15
20
0
5
10
15
20
0
5
10
15
20
(a)
(b)
(c)
t [Hr]
t [Hr]
t [Hr]
Fig. 4. HDO of benzene at 50 bar H2 with (ꢁ) benzene and (ꢀ) cyclohexane: (a) 225 ◦C, (b) 250 ◦C and (c) 275 ◦C.
3.2. HDO of bi-functional phenolics
form 2-methylcyclohexanol was the primary reaction with sub-
sequent dehydration and hydrogenation form methylcyclohexane.
Small amounts of 2-methylcyclohexanone, methylcyclohexene
and toluene were also observed. The ratio of methycyclohex-
anone:methylcyclohexanol increased from 0.02 to 0.07 with
increasing reaction temperatures. The amount of toluene also
increased with temperature and had a maximum concentration of
∼1 mM after 20 h.
The presence of an additional functional group in the bi-
functional phenolics increased the complexity of the reaction
products; however, the same general trends were observed com-
pared to the mono-functional phenolics. The HDO of o-cresol
was very similar to phenol, however, o-cresol was less reac-
tive. As seen in Fig. 6, hydrogenation of the aromatic ring to
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
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0
0
5
10
t [Hr]
15
20
0
5
10
t [Hr]
15
20
0
5
10
t [Hr]
15
20
(a)
(b)
(c)
Fig. 5. HDO of benzyl alcohol at 50 bar H2 with (᭹) benzyl alcohol, (ꢂ) toluene and (ꢁ) methylcyclohexane: (a) 150 ◦C, (b) 175 ◦C and (c) 200 ◦C.

K.L. Deutsch, B.H. Shanks / Applied Catalysis A: General 447–448 (2012) 144–150
147
100
80
60
40
20
0
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80
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0
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80
60
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0
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10
t [Hr
15
20
0
5
10
t [Hr
15
20
0
5
10
t [Hr
15
20
(a)
(b)
(c)
]
]
]
Fig. 6. HDO of o-cresol at 50 bar H2 with (᭹) o-cresol, (ꢀ) 2-methylcyclohexanol and (ꢁ) methylcyclohexane: (a) 225 ◦C, (b) 250 ◦C and (c) 275 ◦C.
100
80
60
40
20
0
100
80
60
40
20
0
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80
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0
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5
10
t [Hr
15
20
0
5
10
t [Hr
15
20
0
5
10
t [Hr
15
20
(a)
(b)
(c)
]
]
]
Fig. 7. HDO of guaiacol at 50 bar H2 with (ꢂ) guaiacol, (ꢀ) cyclohexanol, (♦) 1-methyl-1,2-cyclohexanediol and (ꢀ) cyclohexane: (a) 225 ◦C, (b) 250 ◦C and (c) 275 ◦C.
The product distribution of guaiacol was the most complex of
any of the phenolic compounds. The main products were cyclo-
hexanol, 1-methyl-1,2-cyclohexanediol and cyclohexane as shown
in Fig. 7. Minor products included benzene, phenol, anisole, 1,2-
dimethoxybenzene, 2-methylcyclohexanol, methoxycyclohexane
and cyclohexanone. While the highest concentrations of any of
the minor products did not exceed 3 mM, the sum of these prod-
ucts did account for a significant amount of the overall carbon
balance.
Catechol was not soluble in the decalin solvent at room tempera-
ture and therefore it was not detected in the liquid samples. The first
sampling at t = 0 was skipped to minimize the loss of undissolved
reactant. When catechol was reacted at 225 ◦C, only 2 mol% conver-
sion was observed based on product formation. Higher conversions
of 12 and 70 mol% were observed at 250 and 275 ◦C, respectively,
most likely due to the solvation of catechol enabling its subsequent
reaction (Fig. 8). The two major products were cyclohexanol and
hydroxymethylcyclopentane. Minor products with concentrations
of up to 2 mM included cyclohexene, cyclohexane and cyclohex-
anone.
3.3. HDO of tri-functional phenolics
Vanillyl alcohol suffered from the same solubility issues as
catechol; however, it was found that the hydroxymethyl group
was so reactive at 225–275 ◦C, that 2-methoxy-4-methylphenol
was immediately formed. Therefore, the HDO of vanillyl alcohol
was no different than starting with 2-methoxy-4-methylphenol
and the reaction distribution of the latter is shown in Fig. 9. The
product distribution of 2-methoxy-4-methylphenol was very sim-
ilar to guaiacol except with a 4-methyl substitution. The major
product was 4-methylcyclohexanol, which led to the subsequent
production of methylcyclohexene and methylcyclohexane. The
minor products of less than 3 mM concentrations included toluene,
4-methylcyclohexanone, 3-methylanisole, 3,4-dimethoxytoluene,
and 2,4-dimethylcyclohexanol.
60
50
40
30
20
10
0
60
50
40
30
20
10
0
0
5
10
t [Hr
15
20
0
5
10
t [Hr
15
20
(a)
(b)
]
]
Fig. 8. HDO of catechol at 50 bar H2 with (ꢀ) cyclohexanol and (ꢁ) hydroxymethylcyclopentane: (a) 250 ◦C and (b) 275 ◦C.

148
K.L. Deutsch, B.H. Shanks / Applied Catalysis A: General 447–448 (2012) 144–150
100
80
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0
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t [Hr
15
20
0
5
10
t [Hr
15
20
(a)
(b)
(c)
t [Hr
]
]
]
Fig. 9. HDO of 2-methoxy-4-methylphenol at 50 bar H2 with (ꢃ) 2-methoxy-4-methylphenol, (ꢀ) 4-methylcyclohexanol, (♦) methylcyclohexane and (ꢀ) cyclohexane: (a)
225 ◦C, (b) 250 ◦C and (c) 275 ◦C.
3.4. Coking studies
by two adsorption modes of the conventional hydrotreating cat-
alysts including a ␴-bond adsorption of the hydroxyl group which
leads to direct hydrogenolysis and a ␲-bond adsorption which
results in ring hydrogenation [30,31,52]. Supported Ni catalysts
were also seen to favor the production of cyclohexane; however, at
atmospheric pressures and higher temperatures the formation of
benzene became the major product [44,45]. High temperatures also
favored the formation of cyclohexanone. A similar trend has been
observed with noble metal catalysts [41–45]. Therefore, the reac-
tion path of this hydrogenolysis step can become more ambiguous
at higher temperatures and lower pressures due to the equilibrium
between benzene and cyclohexane. A significant shift in selectiv-
ity from cyclohexane to benzene at higher temperatures was not
observed in this work due to the high hydrogen pressure. While
it was not unexpected to see significant hydrogenation of the aro-
matic ring at high pressure, the relatively low selectivity to direct
deoxygenation was unexpected because Cu usually has lower affin-
ity towards hydrogenation than many noble metals or Co.
The amountof coke on the catalystsrangedfrom 0.26 to 1.44 wt%
of the total reactant. Most reactants showed weight loss upon oxi-
dation between 220 and 400 ◦C that accounted for 0.5–0.75 wt% of
the initial reactant. The reactant with the highest coking was cat-
echol, whereas benzyl alcohol exhibited the lowest degree of coke
deposition. Of the secondary products, methanol gave the highest
amount of coke corresponding to 3.2 wt% of the total reactant in
the reaction. All catalysts showed a higher degree of coking with
increased reaction temperature. It is important to note that this
analysis may have underestimated the amount of coke by convo-
luting the weight loss upon coke oxidation with the weight gained
by catalyst bulk oxidation. However, the relative amounts of coke
for the compounds were similar.
4. Discussion
As shown in Fig. 11, the reaction network for anisole
could include three main pathways: direct deoxygenation via
demethoxylation, hydrogenation and demethylation. The direct
deoxygenation pathway to benzene appeared to be the most preva-
lent pathway with benzene selectivities of 30–46% after 20 h at
conversions of 20–88%. This high selectivity could not be explained
by a pathway involving phenol considering that phenol yielded pri-
marily cyclohexane. The amount of product that formed through
this pathway was higher than indicated by the benzene selectivity
because some of the benzene was subsequently hydrogenated to
cyclohexane. The second most significant pathway was the hydro-
genation of anisole to methoxycyclohexane. Similar to phenol,
the methoxy group activated the aromatic ring for hydrogena-
tion, although to a lesser extent. Methoxycyclohexane reacted
further through either demethoxylation to form cyclohexane or
demethylation to form cyclohexanol. Assuming similar amounts of
demethoxylation of anisole and methoxycyclohexane, cyclohexane
Most HDO catalysts possess two active sites including a metal
center that dissociates hydrogen to catalyze hydrogenation or
hydrogenolysis and an acid group that performs dehydration and
transalkylation. The behavior and balance of these two sites are
what dictate the overall selectivity in the HDO of phenolic com-
pounds. In this contribution, Cu0 constitutes the hydrogenation and
hydrogenolysis component and Cr2O3 is a weakly acidic oxide that
catalyzes the dehydration and transalkylation.
In the HDO of phenol, there are two generally proposed
reaction pathways for deoxygenation: direct deoxygenation via
hydrogenolysis of the Caromatic OH bond to form benzene or
a hydrogenation route in which cyclohexane is formed by the
hydrogenation of phenol to cyclohexanol, dehydration to cyclohex-
ene, and subsequent hydrogenation. These pathways are shown
in Fig. 10. Copper chromite between 200 and 250 ◦C and at
high pressures strongly favors the saturation of the aromatic
ring which eventually yields cyclohexane. The increased tendency
towards hydrogenation can be attributed to the strongly electron-
donating resonance effect of the hydroxyl group [52]. Conventional
sulphided CoMo–Al2O3 catalysts generally favor benzene forma-
tion over hydrogenated products while the sulphided NiMo–Al2O3
favors cyclohexane [8,20,22,24–28,30]. This has been explained
Fig. 10. Reaction network for phenol HDO with CuCr2O4·CuO at 200–250 ◦C and
Fig. 11. Reaction network for anisole HDO with CuCr2O4·CuO at 200–250 ◦C and
50 bar H2.
50 bar H2.

K.L. Deutsch, B.H. Shanks / Applied Catalysis A: General 447–448 (2012) 144–150
149
was also produced through this reaction pathway. The least preva-
lent of the possible reaction pathways involved the demethylation
to form phenol and subsequent reaction to cyclohexanol and cyclo-
hexane. A majority of the cyclohexanol observed was produced
via phenol with the remaining fraction from methoxycyclohexane.
The cyclohexane observed during the HDO of anisole was likely
produced in similar quantities from benzene, methoxycyclohex-
ane, and cyclohexanol based on the relative reactivities of each
group and the product concentrations observed during the reaction.
The significance of the demethoxylation pathway with the copper
chromite catalyst may have been due to copper demonstrating a
unique selectivity towards this reaction. Another possibility was
that the low acidity of this catalyst allowed the methoxy group to
survive long enough to be reacted as shown in Fig. 11, whereas other
common HDO catalysts have higher acidity and rapidly demethy-
late or transalkylate the methoxy group.
The relatively low amount of demethylation and absence of
transalkylation were in stark contrast with results found using
conventional hydrotreating catalysts or noble metal catalysts sup-
ported on alumina or zeolites. Reaction networks from these
catalysts would generally feature the pathway of anisole demethyl-
ation to phenol with the HDO occurring predominately via phenol
with the absence of a demethoxylation as a significant deoxygen-
ation pathway. The strong acidity of the support usually accounts
for high activity towards the demethylation and transalkylation of
anisole; however, this acidity has also been attributed to coke for-
mation, which is a major form of deactivation for these catalysts
[16,27,35]. Furthermore, the demethylation and transalkylation
activity of these catalysts routinely exceeded the deoxygenation
activity of phenol and cresols, which may exacerbate the issue with
coking. In contrast, weakly acidic Cr2O3 was likely the only signif-
icant acidic group for the reduced CuCr2O4·CuO catalyst [3]. The
weak acidity of the reduced copper chromite catalyst was likely
the reason for low demethylation and transalkylation rates.
The HDO of the bifunctional phenolic compounds used in the
study generally yielded the same qualitative results as would occur
by superimposing the two individual functionalities. For example,
the reaction network of o-cresol was analogous to phenol with o-
cresol mainly reacting through hydrogenation of the aromatic ring
followed by dehydration to methylcyclohexane. A minor difference
was that o-cresol was less reactive than phenol which has been
ascribed to steric and electronic effects in literature [44]. This effect
was also observed when comparing the relative hydrogenation
rates of benzene and toluene in which the methyl group lowered
the hydrogenation activity.
Fig. 12. Reaction network for guaiacol HDO with CuCr2O4·CuO at 200–250 ◦C and
50 bar H2.
immediately following biphenate formation [31,32,35]. A similar
methyl scavenging phenomenon has been observed with the thiol
groups of methoxythiophenols and 1,2-dimethoxybenzene and has
also been observed with guaiacol in previous literature [31,32].
Since the transalkylation to the methoxy carbon was not observed
with anisole, the addition of the methyl group likely occurred at
the carbon of the hydroxyl group in guaiacol. The tertiary alco-
hol, 1-methyl-1,2-cyclohexanol, was dehydrated first considering
the high reactivity of tertiary alcohols and this would explain
the exclusive presence of only 2-methylcyclohexanol and not 1-
methylcyclohexanol or a mixture of the two in the reaction product.
The cyclohexanone present from guaiacol HDO was likely a prod-
uct of the dehydrogenation of cyclohexanol because its presence
was only observed at high concentrations of cyclohexanol. It was
also observed as a product in the pure cyclohexanol runs and its
presence increased with increasing reaction temperature. There-
fore, cyclohexanone could be considered a sink for cyclohexanol
during the course of reaction caused by the equilibrium between
hydrogenation and dehydrogenation and was excluded from the
reaction network. One benefit of the low demethylation activity
for the copper catalyst was the circumvention of a pathway involv-
ing catechol or 4-methylcatechol, because these compounds have
been attributed to high coke formation [35]. In coking studies, it
was found that catechol led to comparable amounts of coke to that
from anisole and phenol.
Catechol also underwent a HDO pathway very similar to
phenol in that the aromatic ring was hydrogenated followed
by subsequent dehydration and hydrogenation steps to form
cyclohexane. An additional reaction that occurred with the 1,2-
cyclohexanediol was attributed to a pinacol rearrangement in
which 1,2-cyclohexanediol was converted into cyclohexanone,
formylcyclopentane and cyclohexadiene [53]. Formylcyclopentane
was subsequently hydrogenated to form the observed hydrox-
ymethylcyclopentane.
The HDO of vanillyl alcohol and 2-methoxy-4-methylphenol
were identical due to the labile hydroxymethyl group, which was
also demonstrated in the HDO of benzyl alcohol. The HDO of 2-
methoxy-4-methylphenol and vanillyl alcohol followed the same
reaction network as guaiacol but with a 4-methyl substitution. Sim-
ilarly to o-cresol, the methyl substituent lowered the reactivity of
2-methoxy-4-methylphenol relative to guaiacol.
In a similar fashion, the HDO of guaiacol showed the same
qualitative trends as for phenol and anisole but with a couple
points of distinction. As shown in Fig. 12, phenol and anisole path-
ways are integral parts of the observed reaction network. The
primary product was cyclohexanol that was likely formed by the
demethoxylation of guaiacol to phenol, which rapidly reduces to
cyclohexanol. Two alkylation reactions were also observed that
included the addition of a methyl group to the hydroxyl group
to form 1,2-dimethoxybenzene and the addition of the methyl
group to the aromatic ring to form 1-methyl-1,2-cyclohexanediol.
The former reaction is likely due to an adsorption mode prior
to biphenate formation and the latter due to a transalkylation
5. Conclusion
The hydrodeoxygenation of several mono-function, bi-
functional and tri-functional phenolics was carried out with a
CuCr2O4·CuO catalyst at 150–275 ◦C and 50 bar H2. Reaction
networks were devised for increasingly complicated phenolic
systems that were able to describe the HDO of vanillyl alcohol.
Hydroxymethyl groups of benzyl alcohol and vanillyl alcohol were
found to be readily reduced to form toluene and 2-methoxy-4-
methylphenol, respectively, at as low as 150 ◦C. Methoxy groups
were demethoxylated by the copper catalyst to retain the aromatic

150
K.L. Deutsch, B.H. Shanks / Applied Catalysis A: General 447–448 (2012) 144–150
functionality of anisole, guaiacol, 2-methoxy-4-methylphenol, and
vanillyl alcohol, which has not been commonly observed with
other HDO catalysts. Minimal demethylation and transalkylation
were observed in the reactions with CuCr2O4·CuO likely due to
the relatively low acidity of the support compared to alumina
and zeolites. Phenolic and methoxy groups were also found to
activate the aromatic ring towards hydrogenation. The deoxy-
genation of phenols occurred via hydrogenation, dehydration and
hydrogenation to form cyclohexane derivatives. The dehydration
of the cyclohexanol derivatives was rate limiting step of this
HDO process. The use of higher reaction temperatures and lower
pressures may increase the formation of aromatics from phenols
by favoring dehydrogenation. Overall, it was found that copper
catalysts exhibit unique reaction selectivities and networks for the
HDO of aromatic hydroxyl and methoxy groups when compared
to conventional sulphided catalysts and supported noble metal
catalysts.
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