Precise oxygen scission of lignin derived aryl ethers to quantitatively produce aromatic hydrocarbons in water

Article abstract of DOI:10.1039/c5gc01790d

To produce aromatic hydrocarbons from biomass, a novel route is reported for one-pot selective hydrodeoxygenation of a lignin derived aryl ether mixture to C₆-C₉ aromatic hydrocarbons over Ru/sulfate zirconia in the aqueous phase. The cascade steps undergo the initial precise cleavage of the C_aliphatic-O bond of phenolic dimers or C_aromatic-OCH₃ of phenolic monomers, as well as the full blockage of benzene-ring and cyclic-alkene hydrogenation to realize nearly quantitative aromatic hydrocarbon formation at 240 °C in the presence of low pressurized H₂ (2-8 bar). With a Ru catalyst, the primary and competitive steps in hydrogenolysis of the C-O bond and hydrogenation of the benzene ring are shown to be sensitive to temperature and hydrogen pressure, which can subtly modify the concentration and spatial distribution of the surface adsorbed H. A high temperature and a low hydrogen pressure are found to be essential for hydrogenolysis, since the H species nearby the oxygen atom are more strongly adsorbed under such conditions and thus preferred to be retained on the Ru surface. Herein it is defined as the "atom-induced H sorption effect", probably resulting from the analogous hydrogen-bond force between the adsorbed H and the adjacent oxygen atom on the metal surface. Besides the initial step, another key point for aromatic hydrocarbon formation is to optimize the target route of cycloalkene dehydrogenation, but suppress the parallel hydrogenation pathway to saturated cycloalkanes. It is found that the produced phenol intermediate serves as a suitable H-acceptor for the selected catalytic system during the whole conversion, via fast consumption of the in situ produced H from cyclohexene dehydrogenation. Such an interesting phenomenon of internal hydrogen transfer not only promotes the dehydrogenation and hydrogenation equilibrium of cyclohexene but also lowers the in situ H concentration on the Ru surface, both of which would be beneficial for reaching a high benzene yield. This hypothesis for internal hydrogen transfer is further confirmed by separate experiments and density functional theory modelling results.

Full text of DOI:10.1039/c5gc01790d

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DOI: 10.1039/C5GC01790D
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ARTICLE
Precise Oxygen Scission of Lignin Derived Aryl Ethers to
Quantitatively Produce Aromatic Hydrocarbons in Water
Zhicheng Luo,a YimengWang,a Mingyuan He,a Chen Zhao*a
Received 00th xx 20xx,
Accepted 00th xx 20xx
DOI: 10.1039/x0xx00000x
To produce aromatic hydrocarbons from biomass, a novel route is reported for one-pot selective hydrodeoxygenation of
lignin derived aryl ether mixture to C6-C9 aromatic hydrocarbons over Ru/sulfate zirconia in aqueous phase. The cascade
steps undergo the initial precise cleavage of Caliphatic-O bond of phenolic dimers or Caromatic-OCH3 of phenolic monomers, as
well as the full blockage of benzene-ring and cyclic-alkene hydrogenation to realize nearly quantitative aromatic
hydrocarbons formation at 240 °C in presence of a low pressurized H2 (2 - 8 bar). With Ru catalyst, the primary step in
competition of hydrogenolysis of C-O bond as well as hydrogenation of benzene ring is shown to be sensitive to
temperatures and hydrogen pressures, which can subtly modify the concentrations and spatial distributions of the surface
adsorbed H∙. A high temperature and a low hydrogen pressure are found to be essential for hydrogenolysis, since the H∙
species nearby the oxygen atom are more strongly adsorbed at such conditions and thus preferred to be reserved on the
Ru surface. Herein it is defined as “atom-induced H sorption effect”, probably resulted from the analogous hydrogen-bond
force between adsorbed H∙ with the adjacent oxygen atom on the metal surface. Besides the initial step, another key point
for aromatic hydrocarbons formation is to optimize the target route of cycloalkene dehydrogenation, but suppressing the
parallel hydrogenation pathway to saturated cycloalkane. It is found that the produced phenol intermediate serves as the
proper H-acceptor at selected catalytic system during the whole conversion, via fast consuming the in-situ produced H∙
from cyclohexene dehydrogenation. Such interesting phenomenon of internal hydrogen transfer not only promotes the
dehydrogenation and hydrogenation equilibrium on cyclohexene but also lowers the in-situ H∙ concentration on the Ru
surface, both of which would be beneficial for reaching a high benzene yield. This hypothesis for internal hydrogen
transfer is further confirmed by separate experiments and density functional theory modelling results.
www.rsc.org/
the ether bonds in presence of H2 without hydrogenation of
benzene-ring. Parallel hydrotreating routes including partial/
Introduction
full hydrogenation of benzene-ring, hydrogenolysis of Caromatic
-
O bonds, and hydrogenolysis of Caliphatic-O bonds co-exist, and
therefore, it is quite arduous to select a specific selective route
to defunctionalize the oxygen-containing groups as well as
simultaneously protect the benzene-rings in liquid phase.
The efficient utilization of lignocellulose biomass has attracted
increasing attention.1 As the second abundant lignocellulosic
resource, lignin is
a three-dimensional highly branched
aromatic bio-polymer composed of various C-O linkages
connected with methoxylated phenyl-propane units. Through
depolymerization, lignin is converted to diverse aryl ether
fragments comprising of Caromatic-OCH3, Caromatic-O-Caromatic, and
Caromatic- O-Caliphatic units.2 These phenolic fragments are rich in
inherent aromatic structures with high energy-density, but the
technique for precise removal of oxygen in these bio-derived
molecules to produce aromatic hydrocarbons has been a
challenging issue.3
Previous researches are primarily aimed at cleaving of the
C-O bonds of lignin derived fragments in the liquid phase. The
homogeneous catalysts5 such as Ni5a, Ru5b, V5c complexes, as
well as unsupported nanoparticles Fe6 and RuNi7a, AuNi7b, are
capable in selective hydrogenolysis of lignin-derived aryl
ethers to a mixture of aromatics and phenolics/cycloalcohols
in solvents under mild conditions. Besides, the heterogeneous
Ni-based catalysts shows considerable activities in cleaving the
Caliphatic–O bond of aryl ethers to produce aromatics and
cycloalcohols in the liquid phase, as well.8 The various –OCH3
substituted phenolic ethers can also be quantitatively
hydrodeoxygenated to cyclic alkanes with the bifunctional
metal/acid catalysts in water.9 Phenols can be dehydroxylated
by combined Raney Ni and HBEA catalysts using 2-PrOH as H-
donor.10 In the non-polar solvent decane, bimetallic FeMoP
catalyst can selectively cleave C–O bonds of aryl ethers to
produce aromatics at a high temperature of 400 °C.11 Apart
Because of the high C-O bond dissociation energy of aryl
ethers (218-314 kJ∙mol-1)4, it is contradictory to break down
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from the reactions occurred in the liquid phase, MoO3 was time, the intermediate phenol was converted to
shown to be active for hydrodeoxygenation of phenolic cyclohexanone and cyclohexanol with a rapid rate over Ru/C,
monomers to high yields of aromatic hydrocarbons in the gas and by contrast, the other intermediate ethyl-benzene was
phase.12
quite stable without any reacting. Such great hydrogenation
rate disparity was further evidenced by the separate
experiment that co-introduction of phenol and ethyl-benzene
over Ru/C led to a high phenol hydrogenation rate of 96
mmol∙g-1∙h-1, and by contrast a slow rate for ethyl-benzene
hydrogenation (7.0 mmol∙g-1∙h-1) at identical conditions (Fig.
S3), probably attributed to the better solubility of phenol in
water as well as the stronger adsorption of phenolic –OH
groups than ethyl-benzene on the Ru site, as similar as the
case on the Ni surface.8b In addition, it can be also observed
that a small part of benzene (5% yield) was formed from PEB
over Ru/C in high-temperature water at 100 min.
Water is present ubiquitously in the biomass resource,
and is a green and excellent solvent for upgrading of biomass
derived oxygenates as well. However, in neat aqueous phase it
has not been reported that lignin derived aryl ethers can be
selectively converted to more valuable aromatic hydrocarbons
in a one-step process. Herein, in this contribution we report a
new approach to selectively cleave and deoxygenate the aryl
ether mixture to C6-C9 aromatic hydrocarbons with a multi-
functional Ru/sulfate zirconia (SZ) catalyst in aqueous phase
via cascade steps.
Results and discussion
Selective cleavage and hydrodeoxygenation of phenethoxy-
benzene (
Lignin depolymerization generates an abundant variety of
phenolic aryl ethers.3 Firstly, a representative
-O-4 model
β-O-4 model compound of lignin) in water.
β
compound (contributing to 45-62% C-O linkages in lignin),
phenethoxybenzene (PEB, 1H and 13C NMR spectra were
displayed in Fig. S1) is selected to investigate the proper route
for aromatics formation. In the blank test, PEB was inactive in
the high-temperature water. In the whole hydrodeoxygenation
process, metal centers may greatly influence the pathways and
rates of individual steps. To explore the effect of three metals
in water, Ru/C, Pt/C, and Pd/C (5 wt%) were comparatively
studied at 240 °C in presence of 8 bar H2 in aqueous phase (see
Fig. S2). The results (Table S1) showed that Ru/C and Pd/C
both catalyzed high hydrogenolysis rates on PEB (165 and 270
mmol∙g-1∙h-1, respectively), while Pt/C performed poorly with a
slow rate of 47 mmol∙g-1∙h-1. The parallel route of hydrogen-
ation of benzene ring was shown to be active on Pd/C site
(rate: 47 mmol∙g-1∙h-1), whereas it was almost inert with Ru/C
(3.3 mmol∙g-1∙h-1). Through the rate comparison of hydrogen-
olysis to hydrogenation, Ru/C is more capable than Pd/C for
selective hydrogenolysis (hydrogenolysis / hydrogenation ratio
= 55) of the C-O bond of PEB (see Table S1).
It should be addressed that the Ru based catalysts, e.g.
Ru/C, have already exhibited uniquely high selectivity in
13
14
hydrogenolysis of C-O bonds in polyols and cellulose. For
achieving a more selective route for hydrogenolysis of C-O
bond, Ru/C is thus selected for further investigation. Kinetics
of PEB conversion with Ru/C at identical conditions (see Fig. 1a)
revealed that at an initial time PEB was cleaved to C6 phenol
and C8 ethyl-benzene with nearly 100% selectivity. It is notable
that it does not produce any benzene-ring hydrogenation
products, as well as the cleaved products of C6 benzene and C8
phenyl ethanol (see Fig. 1a). This indicates that at optimized
conditions, Ru catalyzes the exclusive route for cleaving of the
Caliphatic-O bond of PEB, while fully blocking the possible
hydrogenation pathway on benzene-ring and the hydrogen-
olysis route on Caromatic-O bond (Scheme S1). As a function of
Figure 1. (a) Ru/C (0.10 g) and (b) Ru/SZ (0.10 g) catalyzed PEB
hydrodeoxygenation as a function of time. (c) Selective hydrogen-
ation of PEB to aromatic hydrocarbons over diverse supported Ru
catalysts (5 wt.%, 0.10 g) for 1 h. General conditions: PEB (1.0 g), H2O
(100 mL), 240 °C, 8 bar H2, stirring at 650 rpm.
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The further strategy for target C6 aromatic hydrocarbon demonstrated by TEM image (supporting information at Fig.
formation is designed as dehydration of as-received S4a). The high-resolution TEM image illuminated that Ru
cyclohexanol over acid sites, subsequently dehydrogenation of nanoparticles are of high crystallinity, composed by multi-
the formed cyclohexene to benzene with metal sites at a low crystalline particles with exposed major Ru (100) facets, as well
hydrogen pressure, via suppressing the by-product as some minor Ru (111) and Ru (110) facets (Fig. S4b).
cyclohexane formation (see Scheme 1). To achieve it, some Ru Determined from the CO chemisorption measurement, the Ru
catalysts supported on solid acids were tested for catalyzing dispersion was 21%. SEM images (Fig. S4c) illuminated that the
the cascade reactions (see Figs. 1b and 1c). With Lewis acids Ru/SZ sample exhibited granulated and inhomogeneous shape
supported Ru such as Ru/Al2O3 and Ru/ZrO2, the cyclohexanol with a crystal size of around 1-10 μm. The acid site was
intermediate was stable at 35-40%, implying that Lewis acid determined to be 0.1078 mmol∙g-1, as characterized by the
sites are not active for dehydration in the selected aqueous temperature programmed desorption of ammonia (Fig. S4d).
system.9b Compared to H-zeolites supported Ru/HY (5), These results indicate that Ru nanoparticles are well dispersed
Ru/HBeta (20), Ru/HZSM-5 (5), and Ru/HZSM-5 (38), the on the acidic SZ carrier, constructing to a bi- or multi-
Ru/sulfate zirconia (SZ) sample catalyzed a much higher yield functional catalyst.
(73%) of C6 benzene and C8 ethyl-benzene for 1 h. If extending
the reaction time to 100 min., 90% C6 and C8 aromatic
Controlling the primary step of hydrogenolysis and
hydrocarbons can be achieved on Ru/SZ. Based on these
hydrogenation of PEB over Ru/SZ in water.
results, it is implied that the addition of SZ may facilitate
cyclohexanol dehydration to cyclohexene, and the produced
The crucial competing routes of hydrogenolysis (at position a
cyclohexene is selectively dehydrogenated to the target
or b) and hydrogenation (partial and full) can be substantially
benzene at the selected condition with a high temperature and
influenced via the adsorbed H∙ on the Ru/SZ surface (scheme
a low H2 pressure (240 °C, 8 bar H2). The kinetics of PEB
S1). The surface adsorbed H∙ is determined by multi-factors.
conversion to aromatics over Ru/SZ (Fig. 1b) revealed that
The influence of H2 pressure (see Fig. 2a) demonstrated that
Ru/SZ is capable of catalyzing all cascade steps including C-O
hydrogenolysis played as the major role in presence of a
bond cleavage of PEB (188 mmol∙g-1∙h-1), phenol
relatively low H2 pressure (≤ 10 bar) at 240 °C, and PEB was
hydrogenation to cyclohexanol, cyclohexanol dehydration to
selectively cleaved at the Caliphatic-O position to produce C6
cyclohexene, as well as cyclohexene dehydrogenation to
phenol and C8 ethyl-benzene. When the H2 pressure was
benzene, and therefore, it leads to the highly selective
increased to 30 bar, the major reaction pathway was altered to
aromatics formation via the integrated steps.
full hydrogenation (Fig. S5a). It indicates that the relatively
The characterization of Ru/SZ feathered that Ru
nanoparticles (synthesized by liquid phase HCHO reduction)
were shown to be uniform at sizes of 2.0 ± 0.4 nm, as
high concentrations of activated H∙ (at high H2 pressure)
mainly contribute to the hydrogenation of PEB, and as well
accelerate the cleavage rate of Caromatic-O of PEB. Consequently,
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dominated as the major route (yield
> 95%), and
the high H2 pressure leads to a complicated and unselective
product distribution, and the desired aromatic hydrocarbons
such as ethyl-benzene and benzene were substantially
hydrogenated to cycloalkanes. In explaining the effect of
hydrogen pressures, it is assumed that at the low H2 pressures
such as 2 bar, the surface adsorption sites are probably not
fully occupied, and with the increasing H∙ incorporation by
introducing a higher-pressure H2, the H∙ concentrations around
the benzene-rings will be greatly accumulated to accelerate
the hydrogenation rates since that PEB is preferably adsorbed
in the planar mode with an adsorption energy of 262 kJ∙mol-1,
as determined from the DFT modelling result (Fig. S6).
hydrogenation occurred in a minor part (see Fig. S5b),
suggesting that a high temperature, e.g., 240 °C, benefits for
the cleavage of Caliphatic-O bond of PEB (95% selectivity). Due to
4
the much lower C-O bond dissociation energy barrier, the
Caliphatic-O bond rather than Caromatic-O bond of PEB is cleaved at
a higher temperature.
The change of temperatures may modify the status of
the surface adsorbed H∙ species. Concerning on the
concentrations of surface adsorbed H∙ species, it is commonly
recognized that such H∙ species will be preferably desorbed at
increasingly high temperatures, especially when the
adsorption of H∙ on the metal surface is exothermic with a
typical enthalpy value of -125 to -150 kJ∙mol-1.15 Additionally,
the higher temperature would lower the solubility of hydrogen
gas in the selected aqueous system.16 In order to track the
concentration of adsorbed H∙ on the surface of Ru/SZ at two
different reaction temperatures (120 °C and 240 °C) in
presence of 8 bar H2, temperature programmed desorption of
H∙ (from 80 °C to 350 °C) was performed on the used Ru/SZ
(after reaction with PEB at designated conditions) in a He flow
monitored by a mass spectrometry. In the blank test with the
fresh catalyst Ru/SZ, no signal was obtained either with
thermal conductivity detector or with mass spectroscopy
detector (Fig. 3a). The physisorbed H∙ was then removed by
heating at 80 °C in a He flow for 0.5 h, and only the
chemisorbed H∙ was reserved. With the increasing
temperatures, the two-state peaks appeared at 150 °C and
300 °C detected by temperature programmed desorption may
be correlated with the occupation of threefold Ru sites on the
hcp and fcc facets, respectively, as suggested by Feulner and
Menzel17. The results manifested that the surface adsorbed H∙
was more abundant at the reaction temperature of 120 °C
than that at 240 °C, as compared by the normalized areas of
the detected H∙ signals (Fig. 3a).
(a)
100
80
60
Hydrogenolysis
40
20
0
Hydrogenation
6
10 14 18 22 26 30
Hydrogen pressure / bar
(b)
100
80
60
40
20
0
Besides the evidence derived from the detected surface
H∙ species at two working temperatures tracked by
temperature programmed desorption, we design another
experiment, that is, phenol hydrogenation over Ru/SZ in
presence of 8 bar H2 with varying temperatures (Fig. 3b). The
results showed that the rate for phenol hydrogenation sharply
decreased from 285 to 66 mmol∙g-1∙h-1 when the temperature
was increased from 160 to 260 °C. The rate express for phenol
Hydrogenolysis
Hydrogenation
hydrogenation is r = k⋅
[phenolads]a∙[Hads]b. In principle, the
solubility of phenol in water is infinite above 65 °C, and the
reaction orders of phenol (a) and hydrogen (b) are positive.18
This implies that the increase of temperature would not
change the concentration of adsorbed phenol, and the rate
constant k should be increased. However, oppositely the
obtained overall rate was decreased. Based on the rate
equation, it is rationally inferred that the reduced surface
adsorbed H∙ species [Hads] at higher temperatures leads to the
lower hydrogenation rate on phenol, when the value (k×
[phenolads]a) is increased.
120 140 160 180 200 220 240 260
T / ° C
Figure 2. Influence of (a) hydrogen pressure and (b) temperature
towards hydrogenolysis and hydrogenation of PEB over Ru/SZ.
Reaction conditions: PEB (1.0 g), Ru/SZ (5 wt.%, 0.10 g), H2O (100 mL),
1 h, stirring at 650 rpm, (a) 240 °C, (b) 8 bar H2.
In addition to the hydrogen pressure, the temperature
effect manifested that at relative low temperatures (120 -
200 °C, 8 bar H2), hydrogenation as well as hydrogenolysis (at
positions a and b, Scheme S1) co-existed (see Fig. 2b). With the
increasing temperatures from 200 to 260 °C, hydrogenolysis
With these two reliable proofs, it can be firmly concluded
that the higher temperatures (at selected range) would lower
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the concentrations of surface adsorbed H∙. However, this rich orbital of 2p (O) (Fig. 3d). Such force induces a strong H∙
gained information cannot fully explain the changed ratios of adsorption strength on the Ru surface. In comparison, π or P-π
hydrogenolysis to hydrogenation with temperature variations conjugation makes the benzene rings highly stable, and thus,
as displayed at Fig. 2b, as the aromatic benzene rings of PEB weakens the available interaction of adsorbed H∙ with benzene
are adsorbed in a planar model, which suggests that in rings of PEB (Fig. 3d). Resultantly a high selectivity of
principle the hydrogenation of benzene ring is more favored hydrogenolysis of the C-O bond rather than hydrogenation of
even with few adsorbed H∙ species on the Ru surface. We may benzene-rings is attained with few adsorbed H∙ species.
argue that hydrogenation of benzene ring needs higher
Therefore, the competition of hydrogenolysis to
amounts of metal-activated H∙ than hydrogenolysis of C-O
hydrogenation depends on the concentration together with
bonds, but this argument also cannot intrinsically support the
the strength of adsorbed H∙ at working conditions. If the
observed experimental phenomenon when the aromatic rings
adsorbed H∙ is abundant, the rates of hydrogenation and
are planarly-adsorbed.
hydrogenolysis are both speeded up. When the H∙
The only probable reason is that the higher temperatures concentration is decreased, a great disparity on strength of
change the surface adsorbed H∙ species not only by the adsorbed H∙ nearby the oxygen atom and benzene rings of PEB
concentrations, but also by the spacial distributions. In another leads to the high selectivity to the hydrogenolysis route on C-O
word, the weaker adsorbed H∙ species on the Ru surface bond. With the variation of temperatures and H2 pressures,
surrounding the benzene rings will be fast desorbed as the the reaction pathways for PEB conversion over Ru/SZ are
temperature increases, while the H∙ species nearby the oxygen depicted at Scheme S2. The low temperatures and high H2
atoms are strongly adsorbed due to the induction by the pressures are beneficial for the partial and full hydrogenation,
oxygen atom of PEB, and thus are reserved on the Ru surface together with the hydrogenolysis route on Caromatic-O bond at
even with a rising temperature (Fig. 3c). We define such position a. However, the targeted ether cleavage route in the
phenomenon as “atom-induced H adsorption effect”.
whole designed hydrodeoxygenation process favors the
specific conditions of the relatively high temperature and low
H2 pressure.
In fact, the electron of 1s (H) prefers to shift to the empty
orbital of 4d (Ru) when H∙ is adsorbed on the Ru surface,15 and
this leads to form a strong analogous hydrogen-bond force
between the electron-deficient orbital of 1s (H) and electron-
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Controlling the critical step of cyclohexene dehydrogenation converted to cyclohexanol. The internal H-transfer between
over Ru/SZ in water.
cyclohexene dehydrogenation and phenol hydrogenation at
selected condition helps shifting the reaction route to
specifically produce the dehydrogenated product benzene.
In the overall PEB hydrodeoxygenation process to aromatic
hydrocarbons, the critical step is controlling the selectivity of
the individual step of cyclohexene dehydrogenation to
benzene, and meanwhile preventing the parallel step of
cyclohexene hydrogenation to cyclohexane. Calculated by HSC
software, the thermodynamic data for cyclohexene conversion
to benzene and cyclohexane at 240 °C are displayed at Figure 4.
It shows that in the dehydrogenation reaction for forming
benzene, ΔG equals to -10.4 kJ∙mol-1 within an endothermic
reaction (ΔH = 26.9 kJ∙mol-1), while the other direction for
forming cyclohexane is exothermic (ΔH = - 24.7 kJ∙mol-1) and
ΔG equals to -6.9 kJ∙mol-1. By comparing the thermodynamic
data, it is suggested that dehydrogenation of cyclohexene to
benzene is more preferred at 240 °C under the ideal conditions
when the reaction approaches to the thermodynamic
equilibrium.
240 oC, 8 bar H2
Ru/SZ
(1)
Sele.: 100%
240 oC, 8 bar H2
(2)
(3)
+
+
OH
OH
+
+
Ru/SZ
Sele.: 40%
Sele.: 90%
Sele.: 60%
Sele.: 10%
240 oC, 8 bar H2
Ru/SZ
240 oC, 8 bar N2
Ru/SZ
+
OH
+
Sele.:100%
Sele.: 0
OH
ꢀ H2O
ꢀ2 H2
∆ꢀꢁꢂꢃ °ꢄ ꢅ ꢐꢁꢂ. ꢑ ꢈꢉ ∙ ꢊꢋꢌꢍꢎ
∆ꢏꢁꢂꢃ °ꢄ ꢅ ꢐꢆ. ꢇ ꢈꢉ ∙ ꢊꢋꢌꢍꢎ
OH
H2
H transfer
Hꢀacceptor
Figure
5. The reaction of cyclohexene (1), and co-reaction of
∆ꢀꢁꢂꢃ °ꢄ ꢅ ꢁꢆ. ꢇ ꢈꢉ ∙ ꢊꢋꢌꢍꢎ
∆ꢏꢁꢂꢃ °ꢄ ꢅ ꢐꢎꢃ. ꢂ ꢈꢉ ∙ ꢊꢋꢌꢍꢎ
cyclohexene and cyclohexanol (2), co-reaction of cyclohexene and
phenol (3) over Ru/SZ in presence or absence of H2 in the aqueous
phase. Reaction conditions: cyclohexene (1.0 g) or mixture of
cyclohexanol (0.5 g) and cyclohexene (0.5 g) or mixture of phenol (0.5
g) and cyclohexene (0.5 g), H2O (100 mL), 240 °C, 8 bar H2 or N2, 1 h.
To explore the key step of cyclohexene dehydrogenation,
this individual step was separately investigated over Ru/SZ at
selected condition (240 °C, 8 bar H2, see Fig. 5). Unexpectedly,
cyclohexane but not benzene was the solo product from
cyclohexene conversion with 100% selectively with Ru/SZ in
the aqueous phase. Considering that in the overall PEB
conversion over Ru/SZ, phenol and cyclohexanol were the two
major intermediates during the conversion (see Fig. 1b), the
co-reactions of cyclohexene and cyclohexanol/phenol were
conducted afterwards. These results showed that co-reacted
with cyclohexanol and cyclohexene, the selectivity to benzene
was increased to 40%, and more surprisingly, the target
benzene was enhanced to 90% when co-reacting of phenol
and cyclohexene at 240 °C in presence of 8 bar H2. The high
selectivity of benzene is speculated to the reason that the
phenol intermediate serves as the H-acceptor, which can
digest the in-situ produced H2 from cyclohexene
dehydrogenation. The consumption of H2 not only promotes
the dehydrogenation and hydrogenation equilibrium on
cyclohexene but also lowers the in-situ H∙ concentration on
the Ru surface, both of which would be beneficial for reaching
a high benzene yield. This is further proven by the comparative
experiment that in presence of N2, co-reaction of cyclohexene
and phenol led to the formation of 100% selectivity of benzene
from cyclohexene dehydrogenation, and phenol was partially
In the theory of internal hydrogen transfer between
cyclohexene dehydrogenation and phenol hydrogenation, it is
assumed that phenol is more preferably adsorbed on the Ru
surface than cyclohexene. Under such condition, then the
adsorbed phenol can easily accept the in-situ generated H∙
from cyclohexene dehydrogenation to complete the internal
hydrogen transfer process. To verify this assumption, the
adsorption energies of five intermediates (ethyl-benzene,
phenol, cyclohexanol, cyclohexanone, and cyclohexene) from
PEB conversion were calculated by DFT on Ru (0001) surface
(see Table 1). The DFT modelling results demonstrated that on
the clean Ru (0001) surface, the adsorption energies followed
the order, Eads (phenol)
> Eads (ethyl-benzene) > Eads
(cyclohexanone) > Eads (cyclohexanol) > Eads (cyclohexene). If
calculated on the Ru (0001) surface with a water mono-layer,
the adsorption energies of these compounds were decreased
due to the introduced water interference, but the order trend
was not changed, that is, the adsorption energy of phenol (-
160 kJ∙mol-1) was still the highest among these compounds,
and was four times higher than that of cyclohexene (-26
kJ∙mol-1). These DFT results confirm our inference that the
adsorption of phenol is much stronger than other
intermediates, and therefore, the presence of phenol as H-
acceptor is the critical factor for manipulating the individual
step of cyclohexene dehydrogenation.
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Table 1. Adsorption energies of reaction intermediates on Ru
(0001) surface calculated by DFT.
condition of 240 °C in presence of 8 bar H2 for 1.5 h (see Fig.6
and Fig. S7).
Clean surface
(kJ∙mol-1)
With water
monolayer (kJ∙mol-1)
Ethyl-benzene
Phenol
-148
-172
-49
-118
-160
-32
Cyclohexanol
Cyclohexanone
Cyclohexene
-77
-68
-76
-26
Selective hydrodeoxygenation of lignin-derived diverse aryl
ethers to aromatic hydrocarbons in water.
Based on the obtained knowledge, the designed catalytic
system was applied to hydrodeoxygenate the lignin derived
representative phenolic dimmers including β-O-4, α-O-4, and
4-O-5 model compounds (see Fig.6). These aryl ether
compounds were selectively hydrodeoxygenated to C6, C7, and
C8 aromatic hydrocarbons with yields exceeding 78% over
Ru/SZ in water at 240 °C with 8 bar H2 for 1.5 h.
Figure 7. Selective hydrodeoxygenation of lignin derived phenolic
monomers or mixture to aromatic hydrocarbons over Ru/SZ in water.
Reaction condition: reactant (1.5
mmol), Ru/SZ (5 wt%, 0.20 g), H2O
(100 mL), 240 °C, 2 bar H2 and 6 bar N2, 2 h, stirring at 650 rpm.
Apart from the phenolic dimers, there are substantial
amounts of substituted phenolic monomers formed from
lignin degradation. They have two types of oxygen-containing
groups, that is, the phenolic –OH and –OCH3 groups. Through
lowering the hydrogen pressure to 2 bar (with additional 6 bar
N2), the conversion of anisole and guaiacol reached nearly 90%
yields of C6 benzene at 240 °C after selectively removing the
phenolic –OH and/or –OCH3 groups over Ru/SZ in aqueous
phase (see Fig.7). In the conversion of guaiacol, in addition to
83% yield of benzene, 6% yield of toluene was also formed due
to the recombination of CH3OH with the benzene ring. When
4-methyl-, 4-ethyl-, and 4-n-allyl- substituted guaiacols were
hydrodeoxygenated, the yields of aromatic hydrocarbons were
lowered to respective 85%, 79%, and 71% (see Fig.7),
indicating the rates for hydrogenation of aryl-benzenes to
saturated cycloalkanes were highly accelerated on Ru/SZ. This
was verified by the experimental data that the hydrogenation
yield on toluene (35%) was 7% lower than that on ethyl-
benzene (42%) at same reaction conditions (Table S2). In
addition, the C-C alkylation products with substituted phenol
Figure 6. Selective hydrodeoxygenation of lignin derived phenolic
dimers or mixture to aromatic hydrocarbons over Ru/SZ in water.
Reaction condition: each reactant (5 mmol), Ru/SZ (5 wt.%, 0.10 g),
H2O (100 mL), 240 °C, 8 bar H2, stirring at 650 rpm. In the conversion of
a phenolic dimer mixture, each compound (2.5
wt.%, 0.15 g) were used.
mmol) and Ru/SZ (5
The oxygen elimination route follows the similar
mechanism as PEB conversion (see Scheme 1), that is, the
initial selective cleavage of ether bond to produce phenol and
aromatics, followed by a dedicatedly designed route for
sequential hydrogenation of phenol, dehydration of
cyclohexanol, and dehydrogenation of cyclohexene to benzene.
It should be noted that the side reactions of hydrogenation of
ether reactants and aromatic hydrocarbons are well
suppressed. Even treating with a mixture of β-O-4, α-O-4, and
4-O-5 aryl ethers with an equal molar ratio, a high yield (86%)
of C6-C8 aromatics mixture was obtained from the one-pot
hydrodeoxygenation process with Ru/SZ at the optimized
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ARTICLE
and methanol were not observed. It was tested that alkylation phase, only a trace amount of CH4 was detected (see Fig. S8b).
of 4-methyl-, 4-ethyl-, and 4-n-propyl- substituted phenol with Furthermore, almost quantitative transformation of mixed ten
methanol did not occur at selected conditions in aqueous kinds of phenolic dimer and monomer ethers to aromatics (95%
phase (Table S3), suggesting that two-molecular C-C coupling yield, consisting of 40% C6, 29% C7, 16% C8, and 9.4% C9
reactions are difficult to be catalyzed with solid acids in hydrocarbons) in a one-step process (see Fig. 8) verifies that
aqueous phase albeit that these solid acids are effective in the such developed system exerts the selective hydro-
gas phase or solvent-less environment. The more complex deoxygenation capability for aromatics formation from a
substituted syringols were also quantitatively converted to variety of phenolic aryl ethers. It should be addressed that
aromatic hydrocarbons at selected condition. In principle, the such C6-C9 aromatic hydrocarbon products mirrored the
mechanisms for cleaving the Caromatic-OH and Caromatic-OCH3 of distribution of ether reactants in the mixed phenolic oil (see
phenolic monomers are different from that of phenolic dimers. Fig. 5), suggesting that no polymerization occurs during the
The presence of large amounts of aryl phenols and methanol integrated process and
a high feasibility to further
implies that the hydrogenolysis of the Caromatic-OCH3 bond but deoxygenate of lignin derived complex oil ingredients. The
not the CH3-O bond appears as the primary step for conversion novel and cascade hydrodeoxygenation reaction steps towards
of –OCH3 substituted phenols.
aryl ethers leads to a highly integrated atom-economic process
for aromatic hydrocarbons formation via a subtly-manipulated
reaction route.
24
ethyl acetate
OH
OH
OCH3
20
OH
OCH3
OH
OCH3
OCH3
H3CO
OCH3
Conclusions
16
12
8
1
OCH3
2
3
4
5
OH
OH
H3CO
OCH3
O
A novel route is established for one-pot hydrodeoxygenation
of lignin derived aryl ether mixture to C6-C9 aromatic
hydrocarbons over Ru/SZ in the aqueous phase. The multi-
functional Ru/SZ catalyzes the selective cleavage of Caliphatic-O
bond of phenolic dimers or Caromatic-OCH3 of phenolic
monomers, as well as the full blockage of benzene-rings
hydrogenation, finally reaching the cascade steps to realize
nearly quantitative aromatic hydrocarbons formation at 240 °C
in presence of a low pressure H2. In manipulating the
selectivity to aromatic hydrocarbons with Ru catalysts, the
primary step is to operate the competition step of hydrogen-
olysis and hydrogenation, which depends on the concentration
together with the strength of adsorbed H∙ at working
conditions. When the H∙ concentration is decreased at high
temperatures or low H2 pressures, a great disparity on
strength of adsorbed H∙ species that are nearby the benzene
rings and oxygen atoms (atom-induced H adsorption effect)
leads to the high selectivity to the hydrogenolysis route on the
C-O bond of aryl ethers.
O
O
6
8
9
10
7
2
10
1
3
4
5 6 7 8
9
4
0
24
ethyl acetate
20
16
12
8
1
1
2
3
4
5
6
7
3
5
7
4
CH3OH
2
6
4
0
0
4
8
12
16
20
In addition, the controlling of the individual target step of
cyclohexene dehydrogenation is successfully realized by an
internal hydrogen transfer between cyclohexene dehydrogen-
ation and phenol hydrogenation in aqueous phase over Ru/SZ
at selected conditions. The function of phenol (acting as H∙
acceptor) not only promotes the dehydrogenation and
hydrogenation equilibrium on cyclohexene but also lowers the
in-situ H∙ concentration on the Ru surface, both of which
would be positive for reaching a high yield of aromatic hydro-
carbons. Proper metal site (Ru) and solid acid (SZ), together
with a favourable low hydrogen pressure (2-8 bar) and a
relative high temperature (240 °C) are crucial factors for
Time / min
Figure 8. GC chromatography spectra of (a) a reactant mixture of
phenolic monomers and dimers, and (b) aromatic hydrocarbon
products after one-pot selective hydrodeoxygenation of (a) mixed aryl
ethers over Ru/SZ in aqueous phase. Reaction conditions: each
compound (0.3125 mmol), Ru/SZ (5 wt%, 0.5 g), H2O (100 mL), 240 °C,
2 bar H2 and 6 bar N2, 5 h, stirring at 650 rpm. After reaction, ethyl
acetate was used as solvent to extract the organic products.
Selective hydrodeoxygenation of an aqueous mixture of
seven kinds of phenolic monomer mixture (anisole, guaiacols,
and syringols) allowed to quantitatively convert it to aromatic precisely coordinating multi-steps to achieve target routes and
aromatic hydrocarbons. This established selective route
provides a new promising technique for directly producing a
green aromatics pool from lignin derived bio-oil.
hydrocarbons mixtures including C6 benzene (35% yield), C7
toluene (33% yield), C8 ethyl-benzene (9.5% yield), and C9
propyl-benzene (13% yield) at 240 °C in presence of a low
hydrogen pressure of 2 bar (see Fig. 7 and Fig. S8a). In the
aqueous phase, methanol was produced from the
hydrogenolysis of Caromatic–OCH3 group of phenolic ethers. The
carbon balance exceeded 95% in the liquid phase. In the gas
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DOI: 10.1039/C5GC01790D
ARTICLE
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Acknowledgements
This research was supported by the Recruitment Program of
Global Young Experts (Thousand Youth Talents Plan), National
Natural Science Foundation of China (Grant No: 21573075),
and Shanghai Pujiang ProgramPJ1403500.
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Table of Content:
- H2O
OH
Internal
hydrogen
transfer
- H2O
OH
H2
H transfer
H acceptor
Hydrogen bond
2 p
CꢀO bond
cleavage
eꢀ
H
4 d
Ru
Ru
ꢁ
Reactants
This journal is © The Royal Society of Chemistry 20xx