One-Pot Conversion of Lignin into Naphthenes Catalyzed by a Heterogeneous Rhenium Oxide-Modified Iridium Compound

Article abstract of DOI:10.1002/cssc.201903286

The direct transformation of lignin into fuels and chemicals remains a huge challenge because of the recalcitrant and complicated structure of lignin. In this study, rhenium oxide-modified iridium supported on SiO₂ (Ir-ReO_x/SiO₂) is employed for the one-pot conversion of various lignin model compounds and lignin feedstocks into naphthenes. Up to 100 percent yield of cyclohexane from model compounds and 44.3 percent yield of naphthenes from lignin feedstocks are achieved. 2 D HSQC NMR spectroscopy before and after the reaction confirms the activity of Ir-ReO_x/SiO₂ in the cleavage of the C?O bonds and hydrodeoxygenation of the depolymerized products. H₂ temperature-programmed reduction, temperature-programmed desorption of NH₃, IR spectroscopy of pyridine adsorption, X-ray photoelectron spectroscopy, X-ray absorption fine structure analysis, and control experiments reveal that a synergistic effect between Ir and ReO_x in Ir-ReO_x/SiO₂ plays a crucial role in the high performance; ReO_x is mainly responsible for the cleavage of C?O bonds, whereas Ir is responsible for hydrodeoxygenation and saturation of the benzene rings. This methodology opens up an energy-efficient route for the direct conversion of lignin into valuable naphthenes.

Full text of DOI:10.1002/cssc.201903286

Accepted Article
Title: Heterogeneous rhenium oxide modified iridium catalyzed one-pot
conversion of lignin into naphthenes
Authors: Changzhi Li, Bo Zhang, Xinxin Li, Xiaoli Pan, Jianwei Ji,
Yujing Ren, Hua Wang, Na Ji, and Qiying Liu
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10.1002/cssc.201903286
ChemSusChem
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Heterogeneous rhenium oxide modified iridium catalyzed one-
port conversion of lignin into naphthenes
Xinxin Li,[a][b]† Bo Zhang,[a]† Xiaoli Pan,[a] Jianwei Ji,[c] Yujing Ren,[a] Hua Wang,[a] Na Ji,*[b] Qiying Liu,[d][e]
Changzhi Li*[a][e]
Abstract: Direct transformation of lignin into fuels and chemicals
remains huge challenge due to the stubborn and complicated lignin
structure. Here rhenium oxide modified iridium supported by SiO2 (Ir-
ReOx/SiO2) was employed for one-pot conversion of various lignin
model compounds and lignin feedstocks into naphthenes. Up to 100%
yield of cyclohexane from model compounds and 44.3% yield of
naphthenes from lignin feedstocks were achieved. 2D HSQC NMR
analysis before and after reaction confirmed the clear activity of Ir-
ReOx/SiO2 in cleavage of the C-O bonds and hydrodeoxygenation of
the depolymerized products. H2-temperature-programed reduction
(H2-TPR), temperature-programmed desorption of NH3 (NH3-TPD), IR
spectroscopy of pyridine adsorption (pyridine-IR), X-ray photoelectron
spectroscopy (XPS) and X-ray absorption fine structure (XAFS)
characterization, as well as control experiments revealed that a
synergistic effect between Ir and ReOx in Ir-ReOx/SiO2 played a crucial
role for the high performance; of which, ReOx was mainly responsible
for the cleavage of C-O bonds whilst Ir was in charge of HDO reaction
and saturation of the benzene rings. This methodology opens an
energy-efficient route for directly converting lignin into valuable
naphthenes.
renewable energy resource for the production of transportation
fuels.[2] As a major component in lignocellulose, lignin is regarded
as a waste product in pulp industry, and is often treated to supply
heat through burning or discarded.[3] From the chemical structure
of lignin, one can expect that it is a unique candidate for the
production of fuels and a wide variety of aromatic chemicals.[4]
However, the inactive property and the complexity of lignin make
it notorious resistant to chemical transformation, and effective
dissembling of lignin represents a huge challenge. So far, great
efforts have been achieved to address this challenge. One of the
most important and popular depolymerization method to produce
fuels is catalytic hydrodeoxygenation (HDO). For example,
numerous research groups have employed lignin monomers
(phenols, guaiacols, syringols and their derivatives) to assess the
potential for the transformation of lignin to fuels.[5] Further,
phenolic bio-oil, produced by fast pyrolysis or catalytic
depolymerization of lignin, is tested for the production of
naphthenes via HDO reaction over different heterogeneous
catalysts.[6] From the perspectives of application, the
development of advanced catalytic approaches for feasible one-
pot conversion of lignin into naphthenes would be benefit for cost-
effective utilization of biomass and is highly desirable, as
indicated by pioneer works of Rinaldi,[7] Zhao,[8] and Wang[9]
groups.
Introduction
Rhenium (Re) possesses low electronegative and has wide
range of oxidation states, hence can be used as a versatile
catalyst in various reactions including biomass conversion.[10] It is
reported that homogeneous methyltrioxorhenium (MTO)[11] and
supported ReOx catalysts[12][13] have the ability to remove two
adjacent hydroxy groups of polyols to obtain alkenes via
deoxydehydration (DODH) reaction. In the area of lignin
conversion, Crestini[14], Kühn[15] and our own[16] groups show that
rhenium-based catalysts such as MTO, Re2O7 and particularly
ReOx/AC[16c,16d] can effective cleavage of lignin C-O bonds,
however, the products are the mixture of aromatic monomers and
oligomers with the latter ones as the major component when
meeting realistic lignin, making the subsequent utilization
challenging and increasing difficulty in energy-intensive
separation/purification process.
In comparison with the monometallic catalyst, the use of
bimetallic catalysts is a promising strategy to modify the catalyst
property, which would increase both the activity and stability of
the catalyst, and tailor the selectivity to a particular product.
Noting that platinum-group metals (PGMs) such as Pd, Pt and Ir,
including non-noble metal Ni bear intrinsic properties in hydrogen
activation, it offers the possibility to modify the surface properties
of rhenium-based catalysts and improve the capability in
hydrogen-involved reactions. Herein, we prepare a series of
bimetallic catalysts composed of rhenium oxide and transition
metals for the directly catalyzing both lignin model compounds
and lignins into naphthenes. A clear synergistic effect is observed
With reducing the consumption of petroleum and concerning on
the growth of environment issues, the conversion of renewable
biomass to liquid fuels has attracted increasing attention.[1]
Lignocellulose, the most abundant form of biomass, is a
[a]
X. Li, Associate Prof. B. Zhang, X. Pan, Y. Ren, H. Wang, Prof. C.
Li,
CAS Key Laboratory of Science and Technology on Applied
Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023, China
E-mail: licz@dicp.ac.cn, jina@tju.edu.cn
[b]
[c]
X. Li, Associate Prof. N. Ji
School of Environmental Science and Engineering, Key Laboratory
of Biomass-Derived Gas and Oil for Chinese Petrochemical
Industry, Tianjin University, Tianjin 300350, China.
Dr. J. Ji
Shaanxi Key Laboratory of Catalysis, School of Chemistry and
Environment Science, Shaanxi Sci-tech University, 1 Dongyihuan
Road, Hanzhong, 723001, China
[d]
[e]
[†]
Prof. Q. Liu
Guangzhou Institute of Energy Conversion, Chinese Academy of
Sciences, Guangzhou 510640, China
Prof. Q. Liu, Prof. C. Li,
Dalian National Laboratory for Clean Energy, 457 Zhongshan Road,
Dalian, 116023, China
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end
of the document.((Please delete this text if not appropriate))
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and Ir-ReOx/SiO2 exhibits extraordinarily high catalytic activity,
activity. For the latter two catalysts, incomplete hydrogenation
representing one-pot conversion of realistic lignin into naphthenes. products such as ethylbenzene (85% and 12% yields over Pd-
ReOx/SiO2 and Pt-ReOx/SiO2, respectively) and cyclohexanol
(7% and 8% yields over Pd-ReOx/SiO2 and Pt-ReOx/SiO2,
respectively) were formed as the major by-products. Hence, Ir
Results and Discussion
acting as the second metal exhibited better performance of HDO
than the other three transition metals.
Screening of the catalyst for the hydrodeoxygenation of
phenethoxybenzene
Bimetallic catalysts composed of Ir and various metallic oxides
such as WOx, MoOx, VOx, and MnOx were then prepared for the
conversion of phenethoxybenzene. The results in Figure 1 shows
that both the conversion (<50%) and the yield of naphthenes
(<38%) dramatically decreased in all cases, as compared to the
performance of Ir-ReOx/SiO2. Based on the above results, Ir-
ReOx/SiO2 exhibited remarkable improvement in catalytic activity
compared with monometallic and other bimetallic catalysts.
Encouraged by the above promising result, the effect of various
supports (Al2O3, MgO, CeO2, ZnO and ZrO2) on the performance
was then explored. Figure 1 shows that the catalysts Ir-ReOx/MgO,
Ir-ReOx/CeO2, Ir-ReOx/ZnO, and Ir-ReOx/ZrO2 afforded low
conversions (< 30%) with the yields of naphthenes less than 20%.
While Ir-ReOx/Al2O3 gave much higher conversion up to 92%, but
the yield of cyclohexane was only moderate (86%). Exceptionally,
unsaturated product ethylbenzene was detected in the yield of
39%. The different catalytic activity for ring saturation may be
attributed to the larger surface area of SiO2 compared to Al2O3
(Table S1), which favors the dispersion of Ir species.
Nevertheless, Ir-ReOx/SiO2 still showed the highest activity.
Optimization of the reaction conditions (Figures S2-S3)
suggested that the proper reaction temperature, hydrogen
A series of heterogeneous rhenium oxide modified metal
catalysts composed of different metal sites and supports are
prepared by wetness impregnation of metal precursors followed
by a thermal treatment under air. Because lignin structure is
notoriously complicated[3a], in our initial study, lignin model
compound phenethoxybenzene is employed to assess the activity
of various catalysts.
As shown in Figure 1, sole support SiO2 and monometallic
catalysts Ir/SiO2 and ReOx/SiO2 exhibited very poor activity in the
conversion of phenethoxybenzene, providing yields of
cyclohexane and ethyl-cyclohexane less than 5%. In comparison,
the physical mixture of Ir/SiO2 and ReOx/SiO2 lead to an obvious
increase in catalytic activity, yielding cyclohexane and ethyl-
cyclohexane at 15% and 1%, respectively, probably due that the
Re species mixed with noble metal Ir also can work as catalytic
center for hydrogenation and C-O bonds cleavage.[10c]
Interestingly, the bimetallic Ir-ReOx/SiO2 significantly speeded up
the transformation process and displayed 97% yield of
cyclohexane, 11% yield of methyl-cyclohexane, and 85% yield of
ethyl-cyclohexane with almost complete conversion, highlighting
the gratifying synergistic effects between Ir and ReOx.
The conversion of various transition metals of Ir, Pd, Pt and Ni
modified by rhenium oxide were compared. Figure 1 shows that
Ni-ReOx/SiO2 gave the lowest conversion and yields of the
products, and Pd-ReOx/SiO2, Pt-ReOx/SiO2 provided higher
o
pressure and time were 260 C, 2 MPa H2 and 10 h. Figure S2
also showed that the yield of ethyl-cyclohexane increased clearly
with the increasing of the reaction temperature and hydrogen
Figure 1. Catalytic performance of various catalysts. The reaction condition: phenethoxybenzene (0.5 mmol), the catalyst (30 mg), n-hexane (30 mL), T = 260 oC,
2MPa H2, t=10 h. Ir/SiO2 (0.2 wt%Ir), ReOx/SiO2(2 wt% Re), Ir-ReOx/SiO2 (0.2 wt% Ir, 2 wt% Re); M-ReOx /SiO2 (0.2 wt% M, 2 wt% Re, M = Ni, Pt, Pd, Ir); Ir-
MOx/SiO2 (0.2 wt% Ir, 2 wt% M, M = Mn, W, Mo, V, Re); Ir-ReOx/MOx (0.2 wt% Ir, 2 wt% Re, M = Ce, Zr, Zn, Mg, Al, Si).
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10.1002/cssc.201903286
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two methoxy groups (syringyl subunit), as illustrated by the fact
that the yield of cyclohexane improved to 90% yield of
cyclohexane under 4 MPa H2 from 28% yield of cyclohexane and
19% yield of guaiacol under 2 MPa H2 (entry 7). The above results
suggested that the demethoxylation ability is promoted by
hydrogen pressure, the activity sequence of the aromatic units
follows hydroxyphenyl>guaiacol>syringyl types order under the
same condition. Moreover, the substrate with additional C=C
bond in sidechain group also needs high H2 pressure to guarantee
the high yield of cycloalkanes (entry 5 vs entry 6). Based on the
above results, all monomers have been converted to
cycloalkanes under optimized condition, suggesting that Ir-
ReOx/SiO2 exhibited excellent activity for hydrodeoxygenation of
various monomers.
pressure. By contrast, the yield of ethylbenzene decreased under
severe conditions, indicating that for the conversion of
phenethoxybenzene, C-O bonds were cleaved first, followed by
saturation of aromatic rings to afford cycloalkanes, which is
supported by time course profile (Figure S3), which will be further
discussed in the following part.
Hydrodeoxygenation of various lignin model compounds
To generalize the protocol, Ir-ReOx/SiO2 was employed to
convert the mono-phenols that are major components in lignin
bio-oil.[6a] As indicated in Table 1, these substrates, belonging to
syringyl, guaiacol and coumaryl types of units, reached complete
conversion to afford cycloalkanes as the dominant products. From
entries 1, 2 and 7, one can find that higher H2 pressure is required
to guarantee the deoxygenation for saturation of the substrates
containing more methoxy groups. For instance, phenol was easily
converted to cyclohexane quantitatively under 2 MPa H2 at 260
oC within 10 h (Entry 1). Whereas guaiacol with one methoxy
group provided 37% yield of cyclohexane with incomplete
hydrogenation product such as 3% yield of cyclohexanol and 9%
phenol under the same condition (Entry 2). Higher H2 pressure of
3 MPa could increase cyclohexane yield to 79%. Similar tendency
was observed in the case of 2,6-dimethoxyphenol with
In native lignin, at least two-thirds of the total linkages between
the aromatic units are ether bonds such as β-O-4 (β-aryl ether),
4-O-5 (diaryl ether) and α-O-4 (α-aryl ether), and less amount of
linkages are C-C bonds (β-β (resinaol), β-5 (phenylcoumaran), 5-
5, and β-1 (spirodienone)).[3a] To elucidate the potential
application of Ir-ReOx/SiO2 in the direct conversion of lignin,
model compounds with different types of linkages were tested
over Ir-ReOx/SiO2. The β-O-4 and α-O-4 model compounds,
containing typical ether linkages in lignin, were selectively
converted to C6-C9 cycloalkanes with the selectivity higher than
55%; meanwhile, C13-C14 bicycloalkanes with the selectivity less
than 20% were quantified, which were probably generated
through randomly free-radical reactions (Entries 1-7).[6a,17]
Lercher group also found some secondary recombination
products through the ionic and radical mechanism pathways
when conversion of α-O-4 model compounds.[18] The comparison
experiments of β-O-4 model compounds with different number of
OCH3 groups (Entries 3-7) indicate that more OCH3 groups would
cause lower yield of cycloalkanes under the same condition, and
hence high hydrogen pressure is required to guarantee
completely removal of OCH3, which is in agreement with the
monomer conversion results (Table 1, entries 2, 3, and 7). It is
worth noting that β-O-4 model compound with γ-OH which
represents lignin typical realistic segment underwent both C-O
bonds cleavage and HDO to give 57% C6-C14 cycloalkanes (Entry
7). Furthermore, 4-O-5 linkage is mainly existence in lignin
structure as the unit of oligomer-oligomer couplings and is
regarded as the most stubborn ether bonds due to the high bond
dissociation energies of aryl-aryl ether bond reaching up to 82.5
kcal/mol.[19] It is interesting to find that in our catalytic systems 4-
O-5 model compound diphenyl ether can be efficiently broken
down, affording up to 98% selectivity of cyclohexane with
quantitative conversion (Entry 8). C-C linkages such as β-1, and
5-5 model compounds, existing in different plant species, were
successfully converted to C14, C12 and C6 cycloalkanes,
respectively via fully removal of oxygen-containing ketone and
hydroxy functional groups (Entries 9-10). On the whole, all these
model compounds containing the major linkages of lignin can be
effectively converted to naphthenes with different carbon chain
without producing any aromatics, indicating that Ir-ReOx/SiO2
exhibited superior ability for hydrodeoxygenation and cleavage of
C-O bonds. Furthermore, the recycling experiment associated
Table 1. Conversion of various lignin monomers into cycloalkanes catalyzed by
[a]
Ir-ReOx/SiO2
[a]Reaction condition: substrate (0.5 mmol), Ir-ReOx/SiO2 (2 wt% Re, 0.2 wt% Ir,
30 mg), T = 260 oC, P = 3 MPa H2; [b]The C-balance is provided relative to the
C6 ring products owing to demethoxylation; [c] 2 MPa H2; [d] 4 MPa H2.
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Table 2. hydrodeoxygenation of various lignin dimers catalyzed by Ir-
[a]
ReOx/SiO2
Scheme 1. Proposed reaction pathways of phenethoxybenzene conversion.
Scheme 2. Proposed reaction pathways of 2-(2-methoxyphenoxy)-1-
phenylehtanol conversion.
Characterization of the catalysts
To explore the role of each metal species in Ir-ReOx/SiO2 and
unveil the fundamental behind the high catalytic activity, several
characterizations of the catalyst were performed. First,
[a]Reaction condition: substrate (0.5 mmol), Ir-ReOx/SiO2 (2 wt% Re, 0.2 wt% Ir,
30 mg), T = 260 oC, P = 3 MPa H2; [b] The C-balance is provided relative to the
C6 ring products owing to demethoxylation; [c] 2 MPa H2; [d] 4 MPa H2.
with incomplete conversions for lignin model compounds was
carried out (Figure S4). The results showed that Ir-ReOx/SiO2 can
be recycled for seven times with a loss in activity, due to the
leaching of rhenium oxide species according to ICP-AES
characterization (2.02 wt% Re in fresh catalyst vs 1.44 wt% Re in
used
catalyst; meanwhile,
the change of
surface
composition/property of the catalyst might be another reason for
the loss in activity.
With the aim to explore the pathway of different substrates, time
course experiments were carried out. According to the results
shown
in
Figure
S3,
the
yields
of
(2-
(cyclohexyloxy)ethyl)benzene and (2-cyclohexylethoxy)benzene
were firstly increased, then decreased with prolonging reaction
time, indicating that these two dimers are probably the
intermediates. The evolutions of ethylbenzene and cylohexanol
also indicates that hydrogenation and cleavage of C-O bonds
occurred. Thus, it is logical to suppose that the reaction pathway
of β-O-4 without hydroxyl group proceed three major pathways
(Scheme 1), as aforementioned before. While the substrate with
hydroxy and methoxy groups may undergo different routes.
Figure
S5
shows
time-course
results
using
2-(2-
methoxyphenoxy)-1-phenylehtanol as the substrate (Table 2,
entry 5). The evolutions of the substrate as well as the products
indicate that 2-(2-methoxyphenoxy)-1-phenylehtanol proceeded
first hydrodeoxygenation to remove hydroxyl, followed by a
cascade C-O bonds cleavage-hydrogenation to obtain
cycloalkanes (Scheme 2).
Figure 2. (a) HAADF-STEM images of Ir-ReOx/SiO2, (b, c, d, e, f) STEM-EDX
elemental mapping for Si, Ir, Re, O element of Ir-ReOx/SiO2, (g) EDX-linear
scanning, (h) the fast Fourier transform (FFT) of Ir-ReOx/SiO2.
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Table 3. Conversion of phenethoxybenzene catalyzed by Ir-ReOx/SiO2.[a]
Entry
Catalyst
Ir (wt%)-Re(wt%)
0.1-2
Conv.
[%][b]
56
Yield [%][b]
1
40
97
100
96
55
10
2
3
5
4
28
0
1
2
3
4
5
6
1
11
15
15
2
0.2-2
100
100
100
80
85
83
80
13
1
0.4-2
0
0.2-5
0
0.2-1
34
7
0.2-0.5
23
0
[a] Reaction condition: substrate (0.5 mmol), Ir-ReOx/SiO2 (30 mg), T = 260 oC,
P = 2MPa H2, t = 10 h; [b] The conversion and yields were determined by GC-
FID with an internal standard method.
the XRD profile of Ir-ReOx /SiO2 (Figure S6) showed no obvious
peaks of Re and Ir species, indicating the high dispersion of the
metal species on SiO2 support. This fact was also confirmed by
the TEM and EDX images in Figure 2. Typical HAADF-STEM
images of Ir-ReOx/SiO2 sample together with the corresponding
histograms of supported nano-particles (NPs) size distribution
(PSD) (Fig. 2a) showed that the NPs are uniformly distributed with
mean size of 2.1±0.7 nm. STEM-EDX elemental mapping (Fig.
2b-2f) also demonstrates that Ir and Re are highly dispersed on
SiO2. The EDX-linear scanning of single particle (Fig. 2g) exhibits
that NPs are IrOx and ReOx in the form of even smaller particles,
as supported by XPS characterization. The lattice spacings (111)
and (200) of Ir with a characteristic acute angle of 55o were
identified based on the JCPDS card (No. 46-1044). The results
from HRTEM are well consistent with the structure analysis from
the fast Fourier transform (FFT) and EDX-linear scanning.
Hydrogen temperature-programmed reduction (H2-TPR)
characterization results of various catalysts are presented in
Figure 3A, which is used to reveal the redox behavior and the
interaction between Ir and ReOx. Monometallic ReOx/SiO2
displayed a sharp hydrogen consumption peak at 338 oC,
suggesting that ReOx alone on the SiO2 support is difficult to be
reduced at the reaction temperature of 260 oC, Re species thus is
mainly presented as a mixture of high valence of rhenium oxides.
The monometallic Ir/SiO2 has a weak signal at 204 oC, hence the
in-situ formation of Ir(0) species is likely in charge of
hydrogenation in the transformation process at 260 oC. When
fixing Re amount in Ir-ReOx/SiO2, H2 consumption peak shifted to
lower temperature with increasing of Ir amount, suggesting that
the second metal Ir was in favor of decrease the reduction
temperature, consequently, it was benefit for hydrogenation
reaction. All bimetallic catalysts with different ratio of Ir/Re has
lower reduction temperature than both monometallic catalysts
(ReOx/SiO2 and Ir/SiO2), providing a direct evidence of synergistic
interaction between Ir and Re species, which may have huge
effect on catalytic activity. To support the above viewpoint, Ir-
ReOx/SiO2 catalysts with different Ir/Re ratios were employed for
the conversion of phenethoxybenzene. The results in Table 3
showed that the yield of cyclohexane, methyl-cyclohexane, and
ethyl-cyclohenxane increased with an increasing of Ir loading
Figure 3. (A) H2-TPR profiles of monometallic and bimetallic catalysts; (B) NH3-
TPD profiles and (C) acid content of Ir-ReOx/SiO2.
amount when fixing Re content, and unsaturation product
ethylbenzene was eventually vanished (entries 1-3). Whilst the
yield of ethylbenzene was firstly increased, then decreased in the
case of fixing Ir content with increasing Re amount (entries 2, 4-
6). Taking H2-TPR characterization (Figure 3A) into consideration,
Ir(0) could play an important role for hydrogenation.
The acidic property of Ir-ReOx/SiO2 catalyst was investigated
by NH3-TPD and IR spectroscopy of pyridine adsorption. NH3-
TPD profiles in Figure 3C revealed that acid amount increases
with the increasing of Re content in the bimetallic catalyst. Whilst
variation of Ir loading amount has little effect on the acid density
according to the results of 0.1Ir-2Re and 0.4Ir-2Re. XPS
characterization showed that no Re(0) was formed after reaction,
and prolonging the reaction time did not change the composition
of Re species (ReVI, ReIV, ReII) greatly (Figure 5), hence Ir-
ReOx/SiO2 still possesses the acid sites, despite the acid density
decreased after reaction comparing with fresh catalyst (123 vs
169 umol/g) according to NH3-TPD characterization.Therefore,
the acidity should be generated from ReOx species, which would
induce acid-catalyzed C-O bond cleavage and facilitate Ir-
catalyzed hydrogenolysis reactions.[5a, 20] The acidic property of
the catalystwas further confirmed by FTIR spectroscopy of
pyridine adsorbed (Figure 4). The IR bands at 1450 cm-1 and
1540 cm-1 are assigned to Lewis acid and Brønsted acid sites,
Figure 4. FTIR spectra of adsorbed pyridine on Ir/SiO2, ReOx/SiO2, and Ir-
ReOx/SiO2.
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Scheme 3. Control experiments.
Figure 5. X-ray photoelectron spectra of Ir-ReOx/SiO2 before and after reaction.
(left) Ir 4f, (right) Re 4f.
respectively; and the band at 1490 cm-1 is associated with both
Lewis and Brønsted acid sites.[21] Ir/SiO2 showed a very weak
Lewis acid peak at 1440 cm-1 whilst Brønsted acid site was absent.
In comparison, ReOx/SiO2 showed both Lewis and Brønsted acid
sites with strong peaks. Interestingly, the IR spectrum of pyridine
adsorbed on Ir-ReOx/SiO2 was similar to that on ReOx/SiO2 in
peak position, modulating the ration of Lewis and Brønsted acid.
In order to investigate the role of each metal in the HDO reaction,
several control experiments were carried out. The conversion of
In order to identify the specific active species, XPS and
EXAFS measurements were performed. To avoid oxidation of Re
and Ir species, all samples were treated in glove box before XPS
and EXAFS measurement. The binding energy (BE) of the
surface Re 4f7/2 and Ir 4f2/7 orbitals for Ir-ReOx/SiO2 are
summarized in Figure 5. The Ir4f region gave one contribution on
the surface of fresh Ir-ReOx/SiO2, which was assigned to IrIV(Ir4f
7/2 = 62.4 eV, IrO2) specie, which were assigned according to
reported value.[22] After 1 h, a new Ir4f7/2 peak at 61.0 eV
presented as major proportion, which was assigned to Irδ+ on the
basis of reported reference.[23] Within 10 h, the peak of IrIV (IrO2)
species was completely disappeared, along with decreasing the
amount of Irδ+. In contrast, Ir0 specie arose, indicating that IrIV
species was completely reduced to lower valence ones, which
was in agreement with H2-TPR results (Figure 3A). The in-situ
generated Ir0 should be responsible for the hydrogenation
2-(2-methoxyphenoxy)-1-phenylethanol
catalyzed
by
Ir-
ReOx/SiO2 under H2 afforded complete saturated naphthenes
(Equation 1, Scheme 3). In the case of Ar atmosphere, 24% yield
of 2-phenylacetaldehyde, 48% yield of guaiacol and 17% yield of
1-methoxy-2-(styryloxy)benzene were obtained, indicating that C-
O bondcleavage also occurred. To ascertain the role of rhenium
oxides, the same substratecatalyzed by ReOx/SiO2 under both H2
and Ar atmosphere were conducted (Equations 3 and 4). It was
found that most of the C-O bonds between the aromatic units
were cleaved, suggesting that ReOx makes major contribution to
C-O bond cleavage. The results in Equations 4-5 also gave the
same conclusion, because Ir/SiO2 provided much higher dimer
product than ReOx/SiO2 under Ar atmosphere (30%vs 2%). It
should be noted that in Equation 3, three saturated products were
obtained with the yields less than 20% over ReOx/SiO2 under H2,
which were much poorer than that obtained in the presence of Ir-
ReOx/SiO2 (Equation 1). These results, in combination with the
results of equations 5 and 6 (8% yield of cyclohexane was
obtained catalyzed by Ir/SiO2 in the presence of H2), indicated that
Ir should play a major role in hydrogenation reaction, which is in
agreement with H2-TPR characterization result. Taking into
account these comparative conversion results and the above
characterizations, it can be concluded that in Ir-
ReOx/SiO2catalyzed
2-(2-methoxyphenoxy)-1-phenylethanol
transformation, a synergistic effect between Ir and ReOx play a
crucial role for the high performance, of which, ReOx was mainly
responsible for the cleavage of C-O bond whilst Ir was in charge
of HDO reaction and saruration of the benzene rings.
Figure 6. (A, B) The results of Ir L3-edge and Re L3-edge XANES spectra; (C,
D) relation between white line area and valence of Ir and Re.
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Table 4. Conversion of various lignins into nathphenes over Ir-ReOx/SiO2.[a]
Entry
Lignin
Mass/Carbon yield of monomers [%][b][c]
Sum mass/
carbon yield of
monomers
[%][b][c]
Mass yield
of residue
[%][d]
Mass/Carbon
Total mass/
carbon yield of
naphthenes[%][b][c]
yield of lignin
oil [%][b][c]
1
2
3
Organosolv
lignin
Enzymolysis
lignin
Alkaline
lignin
3.2/4.8
3.6/5.4
3.1/4.4
6.7/10.1
0.7/1.0
0.5/0.7
2.2/3.3
3.6/5.4
2.2/3.1
7.2/10.8
1.9/2.8
0.8/1.1
19.3/29.0
37.3
30.1
22.6
11.4/15.3
11.6/14.2
14.3/16.6
30.7/44.3
21.4/28.8
20.9/25.9
9.8/14.6
6.6/9.3
[a]Reaction condition: lignin (300 mg), Ir-ReOx/SiO2 (300 mg, 1 wt% Ir, 5 wt% Re), n-hexane = 50 mL, T = 260 oC, P = 4 MPa H2, t = 10 h; [b] The mass yields were
determined by GC-FID with an internal standard method; [c] Carbon yield (%) = ((mass of hydrocarbons×C% in element analysis+mass of monomers ×C% in
monomers) /mass of original lignin×C% in element analysis)×100%; Monomers: cyclohexane, methyl-cyclohexane, ethyl-cyclohexane, propyl-cyclohexane; [d]
Mass yield of residue (%) = (the weight of the residue/the weight of original lignin)×100%.
reaction so as to obtain cycloalkanes. In the region of Re4f7/2,
fresh Ir-ReOx/SiO2 displayed ReVII and ReVI peaks at BEs around
46.5, and 44.2 eV; after reaction 1 h, four peaks at BEs around
46.5, 44.2, 42.4, and 41.0 eV appeared, which were assigned to
ReVII, ReVI, ReIV, and ReII respectively, according to reported
references.[24] Further prolonging the reaction time did not change
the composition of Re species greatly, suggesting that formation
of metal Re by H2 reduction of ReOx is hard to occur under
reaction conditions, which is consistent with H2-TPR
characterization results (Figure 3A). Therefore, Ir0 could have a
major contribution in hydrogenation.
Figure 6 shows the Ir-L3 edge and Re-L3 edge XANES
spectra of Ir-ReOx/SiO2 before and after reaction. On the basis of
white peak area, after reaction the average valences of Ir and Re
were decreased from 4.0 to 1.6, and 6.8 to 4.3, respectively.
Hence, both XPS and EXAFS characterizations gave the same
fact that the metal species in Ir-ReOx/SiO2 have been reduced to
lower valence states after reaction. Because H2-TPR
characterization has suggested that the metal species in Ir-
ReOx/SiO2 is easier to be reduced than monometallic catalysts
Ir/SiO2 and ReOx/SiO2, the above XPS and EXAFS
characterizations further implied that there was an interaction
between ReOx and Ir species, which are in accordance with
Tomishige’s finding that the interaction between Re and Ir species
is presence at above 210 oC reduction temperature.[25]
Suchsynergistic effects would increase the metallic property and
facilitate the C-O bonds cleavage and HDO process. Based on
XPS, EXAFS, H2-TPR, NH3-TPD and Pyridine-IR results, Ir0,
which has intrinsic activity in hydrogen-involved reactions, is
mainly responsible for hydrogenation/hydrogenolysis reactions,
whilst acidic ReOx facilitates the cleavage of C-O bonds.
4), of which, the carbon yield of C7-C9 cyclohexanes
(cyclohexane, methyl-cyclohexane, ethyl-cyclohexane, propyl-
cyclohexane) were 29.0%, 14.6% and 9.3% respectively. The
relative lower yield from alkaline lignin was probably due to higher
content of interunit C-C bond in alkaline lignin, which is more
difficult to release monomers. Generally, two-step process, i.e.
pyrolysis (>350 oC) followed by HDO of the pyrolysis oil involves
in the conversion of lignin to napthenes.[6b] In the few cases for the
direct conversion of lignin into napthenes (Table S4, entries 1, 3,
4, 6), reaction temperature of 300 oC and hydrogen pressure of 7
MPa are required, to reach so far highest 45% yield of
naphthenes.[7] Several researchers also reported upgrading of
bio-oil under similar or milder conditions (entries 2 and 5), none of
these cases obtained clear better results compared to the current
developed catalytic system.
To elucidate the deoxygenation extent of the liquid products
and look insight into the structure evolution after reaction, 2D
HSQC NMR and GC-MS analysis of the lignin materials and the
liquid oils were conducted. GC-MS analysis showed that all
compounds were saturated hydrocarbons, and no oxygen
containing compound was detected (Figure S7). 2D HSQC NMR
characterization further confirms the excellent activity of Ir-
ReOx/SiO2 in direct dissembling/deoxygenation of lignin. As
shown in Figure S8, in aromatic region, three primary aromatic
building blocks Syringyl (S), guaiacyl (G), and p-hydroxyphenyl
(H) units appeared in three fresh lignins. In addition, noncanonical
subunits such as p-hydroxybenzoate (PB), p-coumarate (pCA)
and Ferulate (FA) moieties were detected in enzymolysis lignin.
Interestingly, the signals in aromatic regions almost disappeared
for all three corresponding oil products, revealing that the
aromatic rings have been saturated after reaction. As a result, the
signals in hydrocarbon region of the three liquid oils remarkably
increased in comparison with that of the corresponding lignins.
Moreover, the signals of β-O-4 (A), β-5 (B) and β-β (C), as well
as methoxyl groups in the sidechain region nearly completely
vanished for the oil products, illustrating that the major linkages of
β-O-4, β-5 and β-β were deconstructed, and methoxyl groups of
Syringyl (S) and guaiacol (G) units were removed. It should be
noted that industrial lignin (enzymolysis lignin) and alkaline lignin
are well known for their stubborn property that cannot be digested
by most of the chemical regents, which are typically a waste
stream in paper factory and other biorefinery processes. The
conversion of these two lignins revealed that Ir-ReOx/SiO2 is an
efficient catalyst for converting various lignins to hydrocarbons
Direct conversion of lignin and woody biomass into liquid
hydrocarbons
Encouraged by the promising conversion results of lignin
monomers and dimers, direct deconstruction of realistic lignins
and woody biomass to naphthenes was investigated. Three
different sources of lignin, namely, organosolv poplar-lignin,
enzymolysis lignin and commercial alkaline-lignin are employed
to assess the robustness of the catalytic system. As expected, all
three lignins can be deconstructed to naphthenes over Ir-
ReOx/SiO2 (Table 4 and Figure S7).The carbon yields of the
naphthenes were 44.3%, 28.8% and 25.9%, from organosolv
lignin, enzymolysis lignin and alkaline lignin, respectively (Table
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10.1002/cssc.201903286
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three times to remove air and oxygen, then charged with desired hydrogen
pressure at room temperature and stirred (800 rpm). The reaction was
heated to the desired reaction temperature. After reaction, the reaction
mixture was rapidly cooled to room temperature and filtered. The filtrate
was analyzed by GC-MS and quantified with GC-FID by an internal
standard method (standard: n-tridecane, HP-5 column, 30 m × 0.32 mm ×
0.25 µm). For recycling experiment, after reaction, the solution was filtrated,
then the catalyst was dried at 60 oC within 5 h for next use. In the reaction
with a physical mixture of Ir/SiO2 (0.2 wt% Ir) and ReOx/SiO2 (2 wt% Re),
the catalysts were sequentially added into the autoclave without premixing
of catalysts.
(Table S3 and Figure S7). To confirm the activity of the catalyst,
the conversion of poplar woody biomass was carried out, giving
3.9% mass yield of oily products (25.2% based on lignin content)
and 2.2% mass yield of monomers (14.2% based on lignin content,
Table S4 and Figure S9). Additionally, all monomers are the
cycloalkanes. Overall, the catalyst has good activity not only in
cleavage of the major C-O linkages of lignin, but also for
deoxygenation and saturation of both various lignins and raw
woody biomass to naphthenes.
Typical procedure for lignin and woody biomass depolymerization:
Organosolv poplar lignin was extracted by a previous method.[26] And
enzymolysis lignin was obtained from Shandong Longlive Bio-Technology
Co., Ltd, utilizing alkaline extraction method for corn stalk lignin.
Commercial alkaline lignin (CAS: 8068-05-1) was obtained in Jianglaishiye
Company. The composition of poplar wood is referenced in our previous
work.[27] Typically,300 mg of lignin,300 mg of catalyst, and 50 ml of n-
hexane were added in the parr autoclave (100 mL). The reaction was
performed at 260 oC with 4 MPa H2and 10 h for conversion of lignin. 1 g of
poplar wood, 1 g of catalyst, and 50 ml of n-hexane were added in the parr
autoclave (100 mL). The reaction was performed at 260 oC with 4 MPa
H2and 24 h for conversion of poplar wood. After reaction, the reaction
mixture was rapidly cooled to room temperature and filtered. The solid
residue was washed three times (using 10 ml n-hexane each time) and
dried in the oven (60 oC) for 12 h. The filtrate was analyzed by GC-MS,
and quantified by GC-FID using an internal standard (standard: n-
tridecane, HP-5 column, 30 m × 0.32 mm × 0.25 µm). Then the filtrate was
concentrated at room temperature overnight to obtain a liquid oil, weight.
The monomers (cyclohexane, methyl-cyclohexane, ethyl-cyclohexane,
propyl-cyclohexane) were evaporated with solvent in the concentrated
process due to their low boiling point, thus were not included in the oily
hydrocarbons.
Conclusions
We have described a promising catalytic system for the
production of naphthenes from lignin over transition metal-
modified rhenium oxide catalysts. Ir-ReOx/SiO2 exhibited
excellent activity on not only lignin model compounds but also
realistic lignin feedstocks. The exceptional activity of Ir-ReOx/SiO2
is mainly attributed to the synergistic effect between Ir and ReOx
species based on characterization of the catalysts and control
experiments. The acid sites generated from ReOx species
facilitates the C-O bonds cleavage, whilst in-situ generated Ir0 is
in charge of hydrogenation in the catalytic process. This facile
process could be a promising strategy for the direct conversion of
renewable lignin feedstocks into value-added naphthenes.
Experimental Section
Catalyst preparation: all the catalysts were synthesized by wetness
impregnation of metal precursors, and then followed by a thermal
treatment. First, ReOx/SiO2 was synthesized by impregnating the SiO2
(Qingdao Ocean Chemical Ltd., used after calcination at 700 oC for 1 h)
with an aqueous solution of NH4ReO4 (Alfa-Aesar). After drying at room
Acknowledgements
o
temperature for 12 h and 120 C in the oven, ReOx/SiO2 was calcined in
Support from the National Natural Science Foundation of China
air at 500 oC for 3 h. Second, the resulting ReOx/SiO2 was impregnated
with an aqueous solution of H2IrCl6 (Sigma-Aldrich). After drying at room
temperature for 12 h, the sample was further dried at 120 oC in the oven
for 12 h, and then was calcined in air at 500 oC for 3 h with a 10 oC min-1
temperature ramp to obtain Ir-ReOx/SiO2 catalyst. The same preparation
procedure was followed to synthesize other bimetallic catalysts such as Ir-
ReOx/SM (SM: Al2O3 was home-made, and used after calcination at 700
oC for 1 h; MgO was from Sinopharm Chemical Reagent. Co., Ltd., and
used after calcination at 700 oC for 1 h; CeO2 was from Sinopharm
Chemical Reagent. Co., Ltd. and used after calcination at 600 oC for 3 h;
ZnO and ZrO2 was from Tianjin Kemiou Chemical Reagent Co. Ltd., used
after calcination at 700 oC for 1 h) and ReOx-M/SiO2 (M= Pd, Pt, Ni) as well
as MOx-Ir/SiO2 (M = V, W, Mo and Mn). The metal precursors were
Pd(NO3)2 (Shanghai 3A Chemical Reagent. Co., Ltd.), H2PtCl6·6H2O
(Tianjin Fengchuan Chemical Reagent Co., Ltd.), and Ni(NO3)2 (Tianjin
Damao Chemical Reagent Factory), Mn(NO3)2, NH4VO3 as well as
(NH4)6H2W12O40·xH2O (Shanghai Aladdin Bio-Chem Technology Co., Ltd.),
and (NH4)6Mo7O24·4H2O (Tianjin Kaida Chemical Reagent Co., Ltd.),
respectively. The reported metal loadings are on the basis of the
theoretical amount of metals used from impregnation. All catalysts were
used in the power with granule sized of 40-60 mesh.
(21878288，21690080, 21690083), DNL Cooperation Fund CAS
(DNL180302), the Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB17020100), and National
Key
R&D
Program
of
China
(2016YFA0202801,
2018YFB1501402) is gratefully acknowledged.
Keywords: Lignin • Hydrodeoxygenation • Ir-ReOx/SiO2 •
Naphthenes
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Entry for the Table of Contents (Please choose one layout)
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X. Li,† B. Zhang,† X. Pan, J. Ji, Y. Ren,
H. Wang, N. Ji*, Q. Liu, C. Li*
Page No. – Page No.
Heterogeneous Rhenium oxide
modified Iridium catalyzed one-port
conversion of lignin into naphthenes
Text for Table of Contents
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