Conversion of lignin model compounds under mild conditions in pseudo-homogeneous systems

Article abstract of DOI:10.1039/c5gc03121d

To produce basic chemicals from lignin, depolymerization and removal of oxygen from lignin through C-O cleavage and hydrodeoxygenation (HDO) are crucial steps. In this study, a novel, pseudo-homogeneous catalyst system, consisting of uniformly stabilized noble metal nanoparticles (NPs) in ionic liquids is developed for the selective reductive cleavage of C-O and HDO. Phenol and guaiacol as lignin monomer model compounds are investigated to gain an insight into the possible HDO pathway, meanwhile, dimeric model compounds such as diphenyl ether and benzyl phenyl ether are studied for the cleavage of C-O bonds between aromatic units. Four types of NPs including Pd, Pt, Rh and Ru were synthesized in situ and well distributed in ILs without aggregation. These catalytic systems displayed almost 100% conversion for various monomeric and dimeric lignin model compounds at 130°C and were recycled several times without loss of activity. The catalytic selectivity of metals for HDO/C-O cleavage normally decreases in the order of Pt > Rh ～ Ru ? Pd, which is similar to the order of NP size, Pd ? Pt > Rh ～ Ru. With a mean diameter of 5.6 nm, Pt NPs in [Bmim]PF₆ are identified as the best catalytic system for the transformation of lignin monomeric and dimeric model compounds with an almost 100% conversion and maximum 97% selectivity.

Full text of DOI:10.1039/c5gc03121d

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Conversion of Lignin Model Compounds at Mild Conditions in
Pseudo-Homogeneous Systems
Received 00th January 20xx,
Accepted 00th January 20xx
Lu Chen, Jiayu Xin, Lingli Ni, Huixian Dong, Dongxia Yan, Xingmei Lu and Suojiang Zhang*
To produce basic chemicals from lignin, depolymerization and removal of oxygen from lignin through C-O cleavage and
hydrodeoxygenation (HDO) are crucial steps. In this study, a novel catalyst system, pseudo-homogeneous catalyst system,
consisting of uniformly stabilized noble metal nanoparticles (NPs) in ionic liquids is developed for the selective reductive
cleavage of C-O and HDO. Phenol and guaiacol as lignin monomer model compounds are investigated to get an insight into
the possible HDO pathway, meanwhile, dimeric model compounds, such as diphenyl ether, benzyl phenyl ether, are
studied for the cleave C-O bonds between aromatic units. Four types of NPs including Pd, Pt, Rh, Ru were in-suit
synthesized and well-distributed in ILs without aggregation. These catalytic systems displayed almost 100% conversion for
various monomer and dimer lignin model compounds at 130 °C and were recycled several times without lossing activity.
The catalytic selectivity of metals for HDO/C-O cleavage normally decreases with the order of Pt > Rh ~ Ru >> Pd that is
similar with the order of NPs size Pd >> Pt > Rh ~ Ru. With mean diameter of 5.6 nm, Pt NPs in [Bmim]PF6 is identified as
the best catalytic system for the transformation of lignin monomers and dimers model compounds with almost 100 %
conversion and maximum 97 % selectivity.
DOI: 10.1039/x0xx00000x
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hydrodeoxygenation (HDO), leads to less mixed products.10
One of the potential product by lignin reductive
depolymerisation and complete HDO is cyclohexane, which is
the building block of adipic acid, caprolactam,11 and the
precursor of nylon as the necessary chemicals in our daily life.
Another potential product, bicyclohexyl, is used frequently as a
penetrant and high boiling solvent.12
There are still two challenges in the twoꢀstep lignin conversion
strategy: (1) developing an efficiency catalyst that
depolymerises lignin through cleavage of linkages to produce
small basic chemicals; (2) finding a suitable medium that
dissolves lignin and facilitates the interaction between the
dispersed lignin and catalyst.6 Due to the overꢀfunctionalized
structure,5 depolymerisation of lignin into small molecules is a
tough synthetic problem for a long time.13 Among all the
linkages of lignin, CꢀOꢀC bonds (account for twoꢀthirds to
threeꢀquarters of all the linkages) and CꢀC bonds should be
cleaved through depolymerisation.14 Although CꢀOꢀC bonds are
relative readily to be cleaved than CꢀC bonds, the aryl CꢀO
Introduction
Since currently over 95% of organic chemicals are derived
from petroleum,1 the depletion of petroleum may gradually lead
to energy and chemicals crises. Thus, to find new alternatives
for petroleum has become one of the most urgent tasks. Lignin,
as the second most abundant biomass polymer on the earth and
the only largeꢀvolume renewable aromatics feedstock as well,
accounts for 10 to 30 %2, 3 by dry weight of biomass while up
to 40 %4, 5 by energy and is considered as one of the most
promising bulk feedstocks to produce platform chemicals.
Nonetheless, most of them from the pulp and paper industry are
underutilized feedstocks, which are just limited to low value
fuels.6, 7 Furthermore, more and more lignin will be generated
as the bioꢀrefineries which is based on lignocellulosic biomass
emerging in a large scale.8, 9 Efficient conversion of lignin not
only makes contribution to the petroleum crisis and economic
viability, but also relieves the solid waste disposal problem.
Compared with lignin conversion by a single step, a twoꢀstep
conversion strategy, which consists of depolymerisation and
Table 1 Types and Frequencies of Inter-Subunit Linkages in Softwood and Hardwood
Lignins (number of linkages per 100 C9 units) 15-20
Bond dissociation
energy/ kJ/mol
Softwood Hardwood
Linkage
Lignin /%
Lignin /%
β-O-4
α-O-4
β-5
226–303
156–203
-
45-50
6-8
9-12
4-8
65
-
6
4-O-5
346
1.5
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bonds cleavage is still a significant barrier because of the
relative high bond dissociation energy with the highest 346
kJ/mol as shown in Table 1.21, 22 Typically, the hydrogenolysis
of aryl CꢀO bonds is operated at high temperature (>250 °C)
and high H2 pressure (>3 MPa) over heterogeneous catalysts.10,
Experimental
2.1 Materials
The ILs used in this study, including 1ꢀallylꢀ3ꢀmethylꢀ
imidazolium Nꢀbisꢀ (trifluoromethanesulfonyl) imidates
([Amim]NTf2), 1ꢀnꢀbutylꢀ3ꢀmethylimidazolium hexafluoroꢀ
phosphate ([Bmim]PF6), and 1ꢀnꢀbutylꢀ3ꢀmethylimidazolium
tetrafluoroborates ([Bmim]BF4) etc., were dried under vacuum
at 80 °C for 24 h before used. The purity of IL was proved by
the 1HꢀNMR (Figure S1ꢀS6). Other chemicals, including phenol
(CP), guaiacol (CP), diphenyl ether (CP), 2ꢀphenylethyl phenyl
ether, (>98.0 wt%), benzyl phenyl ether (>97.0 wt%), and
benzofuran (>99.0 wt%) were obtained from J&K or Sigmaꢀ
Aldrich and used without further purication. The dealkalized
lignin (65.85% C, 7.01% H, 2.43% N, 24.03% O and 0.62% S
by difference) was purchased from J&K. Chloroplatinic acid
23
After depolymerisation, upgrading of monomers and dimers
through hydrogenation or HDO at mild conditions to remove
the additional groups (e.g. hydroxyl, methoxyl) is another
remaining challenge.24 The conventional (sulfide based)
heterogeneous catalysts tend to be deactivated rapidly and are
harmful to the environment by employing sulfur.25 The recently
emerged bifunctional catalysts, coupled hydrogenation site with
dehydration site, showed good performance in HDO at high
temperature (>200 °C) and high H2 partial pressure (3 ꢀ20 MPa)
in aqueous phase.26 However water would inhibit the
deoxygenation when the oxygen is removed as the form of
H2O. Furthermore, water is an antiꢀsolvent of lignin which
impedes the dissolution.27, 28
.
hexahydrate (H2PtCl6 6H2O), ruthenium chloride hydrate
.
(RuCl3 3H2O), palladium chloride (PdCl2), rhodium chloride
.
hydrate (RhCl3 3H2O) were purchased from J&K with 99.9%
With the improvement based on the bifunctional catalysts, ionic
liquids (ILs) as the substitution of water are not only
environmentally benign but also serve as good catalysts for
hydrolysis of CꢀO bonds of lignocellulose.29ꢀ31 Meanwhile, ILs
are regarded as green solvents and are more effective in lignin
dissolution than other solvents even at room temperature.6, 32ꢀ35
Nevertheless, to date, examples of ligninꢀbased chemical
production in ILs remain scarce.36ꢀ38 On the other hand, the
noble metals are known to all as highly active catalysts in both
hydrogenation39, 40 and CꢀO activation.14, 41 Owing to structural
directionality (IL effect), selfꢀorganization and electrostatic
stabilization effect, nanoparticles (NPs) are stable with high
activity in ILs.42ꢀ44 The perfect combination of ILs and NPs was
originally applied in olefins hydrogenation as pseudoꢀ
homogeneous catalyst,42 and then it was widely used in
hydrogenation, oxidation and CꢀC coupling.45ꢀ48
purity. H2 was received from Beijing Beiwen Gas Factory, and
the purity was 99.999%.
2.2 Experimental
2.2.1 Organosolv Lignin Synthesis
The real wood sawdust was screen by 120 mesh sieve, and then
it was extracted by ethanol–water (1:1) for 3 h at 180 °C. After
filtration, water was added into the filtrate to precipitate
organosolv lignin. After filtration again, the solid was dried in
vacuo for overnight. The composition organosolv lignin was
characterized by Elemental analysis (65.68 % C, 6.48 % H,
3.94 % N, 23.09 % O and 0.76% S by differeence) and FTIR
(Figure S7), and the molecular weight distribution of the
obtained organosolv lignin was determined by gel permeation
chromatography (GPC) (Figure S8).
Herein we describe a pseudoꢀhomogeneous catalytic system
(NPs/ILꢀH3PO4) for the HDO and CꢀO bonds cleavage, and this
catalytic system combines the virtues of homoꢀ and
heterogeneous catalysts, namely this catalyst is able to access
the reactive sites in lignin and to be recycled easily after the
reaction as well. The NPs were synthesized by reducing the
metal salt inꢀsitu in ILs, followed by directly used in the
HDO/CꢀO bond cleavages reactions. These reactions were
mimicked by lignin dimers model compounds: 2ꢀphenylethyl
phenyl ether (βꢀOꢀ4), benzyl phenyl ether (αꢀOꢀ4), etc. Since
less research has been done in lignin conversion after
depolymerisation32, these studies about the conversion of
phenol and guaiacol as the lignin monomers model compounds
are also significant. It was marvellous that most of the lignin
model compounds were completely converted at the mild
reaction temperature 130 °C, while the traditional
heterogeneous catalysts only showed little activity. To find out
the impact of anions and the length of the alkyl chain of cations
of ILs on the activity and selectivity of reactions, a series of ILs
have been tested. Lastly, noble metal catalysts, Pd, Pt, Rh and
Ru, were screened for HDO/CꢀO bond cleavages and the most
active one was evaluated in the recycling test.
2.2.2 Nanoparticles Formation and Reactions
In a typical experiment, the solution of chlorate salts of noble
metal (0.01 mmol) with ionic liquid (2 g) and methanol (1 g)
was stirred at room temperature for 15 min. The volatiles were
then removed under reduced pressure (0.1 bar) at 75°C for 2 h
in a rotary evaporator. The lignin model compounds (1 mmol)
or Organosolv lignin (0.1 g), also called substrates, and H3PO4
(0.15 g) were added into a autoclave with a teflon cell. The
autoclave was sealed and purged with hydrogen for three times
before pressurized with 5 MPa H2 at room temperature. Then
the autoclave was placed in a heating block maintained at
130 °C for 10 h with a stirring of 800 rpm. The products were
analyzed by gas chromatographyꢀmass spectrometer (GCꢀMS).
GCꢀMS analyses were performed in an Agilent 6890N/5975B
GCꢀMS equipped with a HPꢀ5MS capillary column (25 m ×
0.25 mm, using He as carrier gas). Samples were tested with
split ratio of 30:1 and an injection volume of 0.4 ꢁL. The GC
oven was programmed to begin with 40 °C for 3 min, and then
raised with the rate of 10 °C/min until the temperature reached
270 °C and held at the final temperature for 2 minutes giving a
total run time of just over 28 min.
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2.2.3 Sample Preparation and NPs Characterization
indicated in the Figure S9ꢀS12 is one of evidences for the
relatively strong interaction between ILs and the surface of
metal NPs even after isolation.46 The scanning electron
microscopy with energy dispersive Xꢀray (SEMꢀEDX) (Figure
S13ꢀS15) data is consistent with XPS patterns showing the
existence of O and F as well.
The NPs were isolated by centrifugation (5000 rpm) for 15
minutes, and then washed with dichloromethane (3 x 5 mL) and
ethanol (3 x 5 mL). The final solution was ultraꢀsonicated for 1
h. The ethanol suspension of nanoparticles was deposited on a
copper grid coated by carbon film and then used in transmission
electron microscopy (TEM) test. The morphology and size of
isolated NPs were observed by TEM (JEM 2100ꢀF (UHR))
operated at 200 keV, and scanning electron microscope (SEM)
(SU8020) examination which was operated at 15 keV.
Pd(0)
Pt(0)
PtO2
(b) Pt
(a) Pd
PdO
Pd(0) loss feature
Raw data
Raw data
Background
Background
Before the Xꢀray photoelectron spectroscopy (XPS) and Xꢀray
diffraction (XRD) tests, the isolated NPs was dried under
reduced pressure for 8 h. XPS data were recorded on a
ESCALAB 250Xi operated at Pass Energy 20.0 eV for signal
element and 100.0 eV for whole survey, while the XRD
instrument was collecting data by Smartlab (9) on a 2D image
345
340
335
330
85
80
75
70
65
Binding energy [eV]
Binding energy/eV
Rh(0)
Rh2O3
Ru(0)
RuO2
(c) Rh
(d) Ru
C
plate were integrated between 20° and 90° 2
0.05° and time of 1 s per step.
θ with a step size of
Raw data
Raw data
Background
Background
Results and discussion
320
315
310
305
300
295
290
285
280
275
3.1 Characterization of NPs
Binding energy/eV
Binding energy/eV
3.1.1 Active Component of NPs
Figure 1 XPS of the prepared NPs with [Bmim]PF6 showing the 3d region with the fitting
results, (a) Pd NPs, (b) Pt NPs, (c) Rh NPs, (d) Ru NPs (The C 1s signal was a contribution
from the carbon tape which was unavoidable during the test)
From the XPS results, all the NPs have two valence states,
reduced state and small amount of oxidized state as shown in
Figure 1. The present of O suggests that NPs surface is
susceptible to oxidation in air, and the oxidized surface may
also enhance the stability of NPs.49 Undoubtedly, the active
phase of NPs is the reduced state metal. The presence of F
Figure 2 TEM Micrographs and histograms showing the size distribution of NPs prepared in [Bmim]PF6 before reaction, (a) Pd NPs, (b) Pt NPs, (c) Rh NPs, (d) Ru NPs
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Figure 3 TEM Micrographs and histograms showing the size distribution of NPs prepared in [Bmim]PF6 after reaction, (a) Pd NPs, (b) Pt NPs, (c) Rh NPs, (d) Ru NPs
3.1.2 Mean Diameter of NPs
(a) Pd
(b) Pt
The size of NPs has strong influence over the activity and
selectivity of catalysts.50ꢀ52 The mean sizes of NPs in this study
were determined by TEM in randomly selected areas and based
on at least 150 particles. Since these NPs are isolated and
exposed to the air, the oxidized layer may enlarge the particle
size. Nonetheless, the mean diameters of these isolated particles
have been determined by TEM and usually were quite close to
those obtained from the nanoparticles dispersed in the ILs as
others reported.46 Representative TEM micrographs with their
corresponding histograms of the obtained four types NPs before
and after reaction are presented in Figure 3, while the mean
diameter of each NPs is listed in Table 2. The mean diameter
decreases with the order of Pd >> Pt > Rh > Ru. The solubility
of the metal precursor influences the size of the metal NPs.53
The precursor of the Pd, PdCl2, is difficult to be dissolved in
ILs and the formed Pd NPs have the largest size. To obtain a
better understanding of the recycle ability of NPs, the spent
catalysts were characterized using TEM as well. Before the
reactions, the NPs are uniformly dispersion with less
aggregation, while these trend to aggregate. The differences in
the mean diameters of NPs between before and after reactions
are usually less than 1 nm, which indicates the high possibility
of recycling and reusing.
(c) Rh
(d) Ru
Figure 4 Theoretical (right) and experimental (left) electron diffraction rings, (a) Pd NPs,
(b) Pt NPs, (c) Rh NPs, (d) Ru NPs
Table 2 Mean diameter of noble metal NPs
Mean Diameter/nm
Pd
Pt
Rh
Ru
Before Reaction
After Reaction
34.1
35.5
5.6
6.7
3.4
3.8
2.4
2.8
3.1.3 Crystal Form Selected Area Electron Diffraction (SAED) and
XRD
results, the order of diameters is the same, Pd > Pt >Ru > Rh.
The difference could due to the limitation of Scherrer equation
in small size NPs less than 5 nm. Another reason is the small
amount of test samples that leads to the random error.
The crystal form of each metal is determined by selected area
electron diffraction (SAED) in Figure 4. The electron
diffraction rings demonstrated that the crystal forms of Pd and
Pt NPs are polycrystals with preferred crystal orientations,
while Ru and Rh NPs are polycrystals without preferred crystal
orientations.
3.2 Conversion of Lignin and Model Compounds
The cleavages of CꢀO bonds and HDO performance of pseudoꢀ
homogeneous catalytic systems are investigated through lignin
monomers and dimers model compounds listed in Scheme 1.
The XRD data in Figure 5 confirmed the crystalline types of
NPs. Pd, Pt and Rh NPs are faceꢀcentered cubic (fcc), while Ru
NPs are hexagonal structure. Although the estimated mean
sizes of these NPs from XRD in Table S3 are larger than TEM
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of cyclohexane slightly declines when the length of the alkyl
chain increased to six. However, the anions of ILs have great
impact on the reaction, as phenol almost cannot be converted in
[Bmim]BF4.
Ning Yan and coꢀworkers reported the HDO of phenols in the
Brønsted acidic ILs/NPs system.54 The conversion and
selectivity of phenol to cyclohexane were excellent but not
good at the same time, the best selectivity to cyclohexane (98%)
could only be obtained with a moderate conversion (48%). The
results of our catalysts have little improvement with almost full
conversion and 98% selectivity. It should be noted that the byꢀ
produce, bicyclohexane, was also found in our study as
mentioned in few studies.55
Ycalc
Ycalc
(111)
Yobs
Yobs
(111)
YobsꢀYcalc
Bragg position
YobsꢀYcalc
Bragg position
(200)
(200)
(a)
Pd
(b)
Pt
(220)
(220)
(311)
(222)
(311)(222)
30
40
50
60
2θ/o
70
80
90
30
40
50
60
2θ/o
70
80
90
Ycalc
Ycalc
Yobs
Yobs
(111)
(101)
YobsꢀYcalc
YobsꢀYcalc
Bragg position
Bragg position
(002)
(111)
(200)
(220)
(c)
Rh
(d)
Ru
(311)
(222)
(102)
(110) (103) (112)
(004)
30
40
50
60
2θ/o
70
80
90
30
40
50
60
2θ/o
70
80
90
The possible reaction pathways of phenol are depicted in
Scheme 2, route 1 is the reaction pathway from phenol to main
product cyclohexene while the route 2 and 3 are the possible
reaction mechanisms of intermediate products, cyclohexyl
phenol and cyclohexyl cyclohexanone. It is well known that
noble metals (e.g. Pd, Pt) can activate H2,56ꢀ58 so almost all the
hydrogenation steps are catalysed by noble metals. Moreover,
the deoxygenation steps are promoted by acid sites. Through
the Route 1, the main product, cyclohexane, is generated by full
HDO. The phenol is converted to cyclohexanone first, and
further hydrogenated to cyclohexanol, and then the subsequent
deoxygenation yields cyclohexene. Finally, cyclohexene reacts
with hydrogen to produce cyclohexane. The phenol is more apt
to be dissolved in ILs, while cyclohexane tends to be separated
from the ILs. This biꢀphase phenomenal would facilitate the
reactions.
Figure 5 X-ray diffraction pattern of the isolated NPs dispersed in [Bmim]PF6, (a) Pd NPs,
(b) Pt NPs, (c) Rh NPs, (d) Ru NPs. Experimental data (black), refined fitting line (blue
line) and difference between the experimental values and the fitting model (red line)
(a)
(c)
(e)
(b)
(d)
(f)
Scheme 1 Lignin monomers and dimers model compounds (a) phenol; (b) guaiacol; (c)
diphenyl ether (4-O-5); (d) Benzyl phenyl ether (α-O-4); (e) 2-phenylethyl phenyl ether
(β-O-4); (f) Benzofuran (β-5)
A very interesting side produce, bicyclohexyl, was found in our
experiments and has not been reported in ILs in previous papers.
The mechanism of bicyclohexyl formation over heterogeneous
catalysts has been studied in aqueous and solventꢀfree system.55
However, as far as we know, the mechanism study of this side
reaction is still lacking with IL as the solvent. The possible
reaction pathway of side produce bicyclohexyl is depicted in
the Scheme 2, route 2 and 3. Through the electrophilic aromatic
substitution of phenol (Route 2), cyclohexyl cation may
recombine with phenol and then leads to the formation of
cyclohexyl phenol.59ꢀ61 Cyclohexanol combines with proton,
then reacts with the orthoꢀC of phenol to produce cyclohexyl
phenol. Then the cyclohexyl phenol is converted to
bicyclohexyl by HDO which is similar to the conversion
pathway of phenol. And 2ꢀcyclohexyl cyclohexanone may be
possible produced via aldol condensation (Route 3) between
cyclohexanone and phenol.55, 62 Those reaction pathways may
happen, because cyclohexyl cation is also stable in ILs.63
However, both Route 2 and 3 are difficult to occur. As Route 1
is much faster than other routes, the majority of the reactant is
converted to cyclohexane, and the concentration of
cyclohexanone and cyclohexanol are low. Besides, the low
reactivity of cyclohexanone and cyclohexanol with proton also
made the reactions of Route 2 and 3 remain difficult.
100
80
60
40
20
0
100
80
60
40
20
0
Pd/IL1 Pt/IL1 Rh/IL1 Ru/IL1 Pt/IL[1] Ru/IL[1]Pt/C/IL1
(1)
(2)
Figure 6 Yield of products from HDO of phenol over Pd, Pt, Ru, Rh NPs prepared in
[Bmim]PF6 and the result in Reference54 (Left); Yield of different ILs over Pt (Right) (T:
130 °C, P: 5 MPa, 10 h; H3PO4 0.1 g, [Bmim]PF6 2g, Substrates/Pt=100)
A) Phenol
Figure 6 shows the yield and the corresponding product
distribution from phenol under 5 MPa at 130 °C for 10 h. The
main products of phenol are cyclohexane with small amount of
bicyclohexyl as a side produce. The yield of cyclohexane with
Pt NPs is the best among all the catalysts as shown in Figure 6
(Left). The catalytic activity of metal site for phenol
hydrodeoxygenation to cyclohexane decreases with the order of
Pt > Rh ~ Ru >> Pd. The effect of the length of the alkyl chain
in cation is not very obvious in Figure 6 (Right), only the yield
To verify these assumptions, the reaction was stopped at 3 h
(the full reaction time was 10 h), and small amount of
cyclohexyl phenol, cyclohexyl cyclohexanone and bicyclohexyl
were observed. At the end of 10 h reaction, only bicyclohexyl
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was detected. It is presumably that cyclohexyl phenol and monomeric species.64 However, these recombination reactions
cyclohexyl cyclohexanone were completely converted to also occurred in the absence of noble metal, so the present of
bicyclohexyl. These small amount intermediate products are the proton should be the main reason for the formation of the side
evidences for the Route 2 and 3. As a further proof, produce bicyclohexyl. It was reported that cyclohexanol would
cyclohexanol and cyclohexanone were used as substrates like to yield cyclohexene rather than cyclohexane when water
respectively at the same condition without noble metal. Both was used as the solvent.26 It is supposed that IL has better
final mixtures contained bicyclohexyl, and cyclohexyl solubility of H2 than water. Therefore, cyclohexene can be
cyclohexanone was detected as well when the substrate was hydrogenated to cyclohexane in spite of the absence of noble
cyclohexanone. Some papers attributed their formation to the metal.
Ptꢀcatalyzed recombination of (partially) hydrogenated
OH
OH
OH
OH
+ H+
-H2O
Slow
Electrophilic aromatic substitution
Route 2
H2
Fast
+ H2
+ H2
OH
O
OH
O
H2
+2
Route 1
+ H2
+ H+
-H2O
H2
+ H2
Hydrogenation
By-Product
Main Product
O
O
Route 3
+ H2
Slow
Aldol condensation
H3O+
OH
+
H+
OH
OH2
OH
Route 2
H
O+
H
H
H+
O+
O
O
+
OH
O
H3O+
H
H
H
H2O
Route 3
Scheme 2 Reaction pathway of phenol in the catalytic hydrogenation treatment
Figure 7 Yield of products from HDO of guaiacol over Pd, Pt, Ru, Rh NPs prepared in
[Bmim]PF6 and the result in Reference (Left); Yield of different ILs over Pt (Right) (T:
130 °C, P: 5 MPa,10 h; H3PO4 0.1 g, [Bmim]PF6 2g, Substrates/Pt=100)
B) Guaiacol
Compare with phenol, the ꢀOCH3 group has large impact in the
HDO process. Guaiacol is converted to cyclohexane with a
selectivity of 91% at full conversion when Ru as the catalyst.
Except Ru, the performances of rest noble metals are not good.
It is supposed that the electron donating group may impede the
HDO reaction through stabilizing the transition state of
carbocation. ꢀOCH3 group is a barrier to form cyclohexanol.
The detection of bicyclohexyl proved our proposed mechanism.
A possible mechanism is shown in Scheme 3 for the HDO of
guaiacol to cyclohexane and bicyclohexyl. After hydrogenation
100
80
60
40
20
0
25
20
15
10
5
0
Pd/IL1
(1)
Pt/IL1
Ru/IL1
Rh/IL1
(2)
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and ketone/enol isomerization, guaiacol may be converted into cleavage. Significant amounts of recombination dimeric
2ꢀmethoxycyclohexanone. Reacting with hydrogen, 2ꢀ compounds, bicyclohexane, were also observed as well. The
methoxycyclohexanone is turned into methoxycyclohexanol.
effect of noble metals, cations and anions of ILs are also similar
Then methoxycyclohexanol can be hydrolyzed to to the results of phenol. It is declared that the cleavage of CꢀO
cyclohexanediole, and afterwards cyclohexanediole would be can take place via either the acidꢀcatalyzed hydrolysis or NPsꢀ
dehydrated into cyclohexanone very readily. It is speculated catalyzed hydrogenolysis.64 However, the conversion of
that the methoxycyclohexanol reaction pathway is first through diphenyl ether was failed without phosphoric acid under 5 MPa
acidꢀcatalyzed hydrolysis to yield 1,2ꢀcyclohexanediol and H2 at 130 °C for 10 h. So the only route at this reaction
followed by the (acidꢀcatalyzed) dehydration of the conditions is that diphenyl ether is firstly hydrolyzed to phenol
intermediate product to produce cyclohexanone.26 The and benzene through the acid site. Then the afterward steps are
afterwards reaction pathway are similar to that of phenol, that is the same as the HDO of phenol, so the products distribution of
the cyclohexanone convert into bicyclohexyl through aldol diphenyl ether are the same with phenol. The only difference
condensation.
between phenol and diphenyl ether is the good performance of
Pd NPs, and this phenomena owning to the easier
hydrogenation of benzene than phenol.
C) Diphenyl ether (4-O-5)
The results of diphenyl ether are similar to the results of phenol,
since it decomposes to phenol and benzene through CꢀO
Scheme 3 Reaction pathway of guaiacol in the catalytic hydrogenation treatment
Scheme 4 Reaction pathway of diphenyl ether in the catalytic hydrogenation treatment
100
80
60
40
20
0
100
80
60
40
20
0
Pd/IL1
(1)
Pt/IL1
Ru/IL1
Rh/IL1
(2)
Figure 8 Yield of products from HDO of guaiacol over the Pd, Pt, Ru, Rh NPs prepared in
[Bmim]PF6 and the result in Reference (Left); Yield of different ILs over Pt (Right) (T:
130 °C, P: 5 MPa,10 h; H3PO4 0.1 g, [Bmim]PF6 2g, Substrates/Pt=100)
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Journal Name
less activity in the side reactions. Compare with cyclohexanol,
the 2ꢀcyclohexylethanol is more inactive when reaction with
proton via electrophilic aromatic substitution and aldol
condensation, so majority of 2ꢀcyclohexylethanol converts to
ethylcyclohexane.
D) 2-phenylethyl phenyl ether (β-O-4), Benzyl phenyl ether (α-O-
4) and Benzofuran (β-5)
(I) 2-phenylethyl phenyl ether
4
β
O
NPs/IL, H3PO4
The results of benzyl phenyl ether are similar to the results of 2ꢀ
phenylethyl phenyl ether conversion, and the Pt NPs shows the
best yield, while the Ru is the worst one due to the selectivity.
The yields of two main products are lower compared to other
metals. The highly reactive Ru NPs may be the reason for the
appearance of dominant side produce.
The catalytic activity of metal site for benzofuran is very
different from others reactants, which decreases with the order
of Pd > Rh ~ Pt >> Ru. The [Bmim]PF6 shows much better
performance in selectivity than the ILs with BF4 or NTf2 anions.
+
130 oC, H2: 5 MPa, 10 h
(1)
(2)
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
100
80
60
40
20
0
Pd/IL1
(1)
Pt/IL1
Ru/IL1
Rh/IL1
(2)
(II) Benzyl phenyl ether
α
O
4
NPs/IL, H3PO4
+
+
130 oC, H2: 5 MPa, 10 h
(3)
(1)
(2)
100
80
60
40
20
0
100
80
60
40
20
0
100
100
80
60
40
20
0
80
60
40
20
0
Figure 10 Comparsion between different ILs and NPs
3.2 The Effect of Metal, Anions and Cations of ILs
Converge all the results in one figure, the region with high
conversion and high selectivity is zoomed in to select the best
combination of IL and noble metal. The most versatile
combination is the Pt/[Bmim]PF6 which provided full
conversion for most model compounds with selectivity higher
than 90%.
The catalytic selectivity of metal for HDO/CꢀO cleavage
decreased with the order of Pt > Rh ~ Ru >> Pd in most cases.
The precise method of titrating the number of active sites of
NPs hasn’t been found yet.66 In most cases, it is assumed that
only the surface atoms of NPs are active in catalytic reaction.45
Thus the activity and selectivity are only attributed to the size
of the NPs and the type of unit cell of metal.
The mean sizes of NPs are estimated through TEM. The poor
performance of Pd NPs owing to their large particle sizes even
before reaction is clearly shown in Table 2 and Figure 10. The
better selectivity of Pt NPs than Rh and Ru NPs are
corresponding to the order of mean diameter Pt > Rh > Ru. The
smaller NPs with lager effective contact area with reactants are
more active in the catalytic reactions, while lead to more side
reactions with less selectivity. Comparing with the diameter
before the reaction, the slightly increased mean diameters of
NPs indicates the possibility of recycling.
Apart from particle size, the surface atom is determined by the
type of unit cell as well. The XRD data displays the type of unit
cell of NPs in Figure 5. The surface atoms of fcc are more than
these of hexagonal in the same size. So Pt and Rh NPs are more
active and less selective than Ru NPs when their sizes are the
Pd/IL1
Pt/IL1
Ru/IL1
(3)
Rh/IL1
(2)
(1)
(III) Benzofuran
NPs/IL, H3PO4
+
Others
β
130 oC, H2: 5 MPa, 10 h
O
5
(1)
(2)
100
80
60
40
20
0
100
80
60
40
20
0
Pd/IL1
(1)
Pt/IL1
Ru/IL1
Rh/IL1
(2)
Figure 9 Yield of products over the Pd, Pt, Ru, Rh NPs prepared in [Bmim]PF6 and the
result in Reference (Left); Yield of different ILs over Pt (Right) (T: 130 °C, P: 5 MPa,10 h;
H3PO4 0.1 g, [Bmim]PF6 2g, Substrates/Pt=100)
βꢀOꢀ4 bonds are the most abundant CꢀO bonds in lignin.6, 65
The HDO/CꢀO cleavage performed good to yield the
corresponding products, cyclohexane and ethylcyclohexane.
However, there are still side reactions to form bipolymer, such
as bicyclohexane and 1,2ꢀdicyclohexylethane. Theoretically,
one mole 2ꢀphenylethyl phenyl ether would yield one mole
cyclohexane and one mole ethylcyclohexane. Actually, the
amount of ethylcyclohexane is more than cyclohexane owing to
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same in theory. While the selectivity decreases with the order
of Pt > Rh ~ Ru, so it is concluded that the activity and
selectivity are strongly dependent on the particle size rather
than the type of unit cell.
100
80
100
80
Considering the conversion depends on the nature of the IL as
well, a series of anions or cations are used in the conversion of
model compounds. A small decrease in selectivity is caused by
the increase in the length of alkyl chain of the cations. When
the length of the alkyl chain is six, the yield of main product
decreased obviously. The degree of organization in
imidazolium based ILs strongly depends on the presence of a
hydrogen atom at the C2 position of the imidazolium ring.67, 68
With longer alkyl chain, the structure of ILs tends to have more
aggregations in microscale.67, 69ꢀ71 This nonꢀuniform and coꢀ
adjacent structure leads to the formation of instable NPs which
caused NPs agglomeration. Furthermore, a recent study has
state that the longer alkyl chain of cations resulted in smaller
NPs size.72, 73 Thus the shorter the alkyl chain leads to higher
the selectivity.
60
60
H3PO4
H3PO4
Solid Acid
Solid Acid
40
40
0
2
4
6
8
10
12
14
16
Mass concentration of acid/%
Figure 11 The effect of the amount and types of acid (T: 130 °C, P: 5 MPa, 10 h;
[Bmim]PF6 2g; Diphenyl ether /Pt=100)
organosolv lignin are more difficult to occur than the lignin
model compounds.
During the depolymerisation, selfꢀcondensation reactions of
the lignin fragmentation are forming new CꢀC bonds that may
be counteracting depolymerisation.82 In addition the crossꢀ
linking reactions are more prone to occur under acid conditions
or in the presence of radicals.83 Nevertheless, it was unusually
to find products at such low temperature.84
However, the anions of ILs have great impact on the reaction.
ILs with NTf
2
and PF
6
anions are found to be more effective
than ILs with BF
4
due to their different hydrophobic properties.
The hydrophilia of ILs may affect the reaction, since the NPs
are sensitive to water. The small amount of water may cause the
aggregation of NPs which would result in the deactivation of
catalysts. The gas solubility also strongly depends on the nature
of the anion while it has only a minor relation with the cation.74,
3.4 The Effect of Acid
In Figure 11, the conversion of diphenyl ether increases with
the amount of acid at first, after the amount of acid is up to 0.1
g, the conversion is kept at 100%. The selectivity hardly
changed with the acid amount. One of the drawbacks of this
catalyst system is the unꢀavoided side reactions, and the
potential environmental damage by the dispose of phosphoric
acid even though it can be recycled. It is recommended to use
solid acids to replace it. A parallel experiment is carried out at
the same condition to use Amberlyst 15(H) as the catalyst
instead of H3PO4. The trend of conversion and selectivity are
similar to H3PO4. At the same dosage, the conversion of solid
acid is lower than H3PO4. And the optimal acid dosage is 0.15 g
which was larger than H3PO4. The slight lower activity of solid
acid may be caused by the limited mass transfer. Nevertheless
solid acid is still promising in this system.
75
It is believed that the higher gas solubility would contribute
to the hydrogenation. Since more hydrogen dissolved in ILs
than in water or most solvents, the possibility of the contact
between hydrogen and the surface of NPs would increase and
the activation of hydrogen by the NPs could also be easier. The
solubility of hydrogen in various ILs with different anions are
decreased with the following order: [Bmim]NTf2 > [Bmim]PF6 >
[Bmim]BF4 > H2O.76ꢀ79 In addition, the NPs size increases with
the size of the anions.80 The polar precursors are preferentially
dissolved in polar domains, then the reduction and NPs growth
occurs in the polar domains as well.81 Since the volume of the
polar domains are controlled by the size of anions, the NPs size
decreases linearly with the molecular volume of the anion in the
order of [Bmim]BF4 > [Bmim]PF6 > [Bmim]NTf2. As a result,
the order of selectivity is [Bmim]BF4
> [Bmim]PF6 >
3.5 The Recycling Test
[Bmim]NTf2, which is the same with the experiment results
except the [Bmim]BF4. The reason of low conversion in
[Bmim]BF4 may be caused by the large NPs size, which is
inactivate in the hydrogenation.
The slightly increased in diameter after reaction indicates the
possibility of reusing the catalyst. After three times recycling,
the Pt/[Bmim]PF6 still can full converts the substrate without
losing any activity. The inactivation of catalyst may be caused
by the increase of the size of the NPs after each reaction. The
lost of NPs at each run may another main reason as well.
3.3 Conversion of Dealkalized Lignin and Organosolv Lignin
Using the best combination of Pt and [Bmim]PF6 for
dealkalized lignin and organosolv lignin reactions at the same
conditions as previous, the liquid products yield is only 5% and
3%, respectively. The selectivity of main product cyclohexane
is accounting for 68% and 49%, respectively. In the reductive
depolymerisation step, CꢀO bonds in lignin with aromatic
network polymer chains are more inactive than these in model
compounds. The depolymerisation of dealkalized lignin and
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Run 1
Run 2
Run 4
Run 3
(1)
(2)
Figure 12 Catalyst recycles of diphenyl ether (T: 130 °C, P: 5 MPa, 10 h; [Bmim]PF6 2g;
Diphenyl ether /Pt=100)
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Conclusions
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Pt/[Bmim]PF6 is an effective pseudoꢀhomogeneous catalytic 13.
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diphenyl ether was found to follow the order of Pt > Rh ~ Ru >>
15.
Pd. The difference in the selectivity may mainly ascribe to the
particle size as proven by TEM. The length of the alkyl chain in
16.
cation has slight impact on the selectivity, while the hydrophilic
[Bmim]BF4 could hardly convert the model compounds. The
recycling test is the evidence of the well robustness of the
catalyst which do not lose activity for at least 3 runs. The
conversions of dealkalized lignin and organosolv lignin showed
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practical process options for the production of cyclohexane
from lignin even though the conversion is quite low.
Nevertheless, a meaningful advantage of this process is that the
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Acknowledgements
This work was supported by International S&T Cooperation
Program of China (2014DFA61670), National Natural Science
Foundation of China (No. 21210006, 21576269, 21276260,
21406230) and External Cooperation Program of BIC, Chinese
Academy of Sciences (No. GJHZ201306).
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