Efficient hydrodeoxygenation of lignin-derived phenols and dimeric ethers with synergistic [Bmim]PF6-Ru/SBA-15 catalysis under acid free conditions

Article abstract of DOI:10.1039/c8gc03775b

Selective catalytic hydrotreatment of lignin-derived phenols and dimeric ethers into alkanes is crucial for utilization of lignin and its fragments. Herein, we developed an efficient catalytic system with well-dispersed metal nanoparticles supported on SBA-15 synergistic with an ionic liquid. The catalytic system could catalyze the hydrodeoxygenation (HDO) of various monomeric and dimeric lignin-derived phenols into the corresponding alkanes under acid free conditions. The synergistic [Bmim]PF₆-Ru/SBA-15 (1-butyl-3-methylimidazolium hexafluorophosphate) catalysis exhibited the best HDO activity for lignin-derived phenols and dimeric ethers with >99.0% conversion and maximum >98.0% selectivity of the corresponding alkanes. The yield of cyclohexane from diphenyl ether was 97.7% with 100% conversion under 2 MPa H₂ at 130 °C for 6 h. The mechanism investigation confirmed that the Ru/SBA-15 catalyst and the anion of [Bmim]PF₆ played crucial roles in the hydrogenation process and deoxidization process, respectively. The catalytic system was reused six times for HDO of diphenyl ether to test its stability.

Full text of DOI:10.1039/c8gc03775b

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Efficient hydrodeoxygenation of lignin-derived
phenols and dimeric ethers with synergistic
[Bmim]PF6-Ru/SBA-15 catalysis under acid free
conditions†
Cite this: DOI: 10.1039/c8gc03775b
Shaoqi Yang,a,b Xingmei Lu,
*
a,b Haoyu Yao,a Jiayu Xin,a Junli Xu,a Ying Kang,a
a,b
Yongqing Yang,a Guangming Caia and Suojiang Zhang
*
Selective catalytic hydrotreatment of lignin-derived phenols and dimeric ethers into alkanes is crucial for
utilization of lignin and its fragments. Herein, we developed an efficient catalytic system with well-
dispersed metal nanoparticles supported on SBA-15 synergistic with an ionic liquid. The catalytic system
could catalyze the hydrodeoxygenation (HDO) of various monomeric and dimeric lignin-derived phenols
into the corresponding alkanes under acid free conditions. The synergistic [Bmim]PF6-Ru/SBA-15
(1-butyl-3-methylimidazolium hexafluorophosphate) catalysis exhibited the best HDO activity for lignin-
derived phenols and dimeric ethers with >99.0% conversion and maximum >98.0% selectivity of the
corresponding alkanes. The yield of cyclohexane from diphenyl ether was 97.7% with 100% conversion
under 2 MPa H2 at 130 °C for 6 h. The mechanism investigation confirmed that the Ru/SBA-15 catalyst
and the anion of [Bmim]PF6 played crucial roles in the hydrogenation process and deoxidization process,
respectively. The catalytic system was reused six times for HDO of diphenyl ether to test its stability.
Received 3rd December 2018,
Accepted 2nd January 2019
DOI: 10.1039/c8gc03775b
rsc.li/greenchem
H3PO4 in [Bmin]PF6,13 etc. However, some disadvantages
about the reported catalytic system still limit their further utili-
1. Introduction
Lignin is the most abundant and renewable aromatic resource zation for HDO of lignin, such as the hard separation of pro-
in nature, which accounts for about 15–30 wt% of biomass.1 ducts and recycling of catalysts, and harsh reaction conditions
Conversion of lignin to chemicals and fuels has been con- for heterogeneous systems.14 It should be mentioned that
sidered as one of the efficient ways for full utilization of this exploration of novel catalytic systems is also very important for
resource.2,3 Hydrodeoxygenation (HDO) of lignin is one of the a reaction because the catalysts always need the assistance of
important strategies, which is currently applied for chemical solvents in their catalyzing processes. Therefore, exploration of
depolymerization and catalytic conversion of lignin into the efficient catalytic systems for HDO of lignin-derived com-
corresponding lower molecular weight alkanes.4–8 Due to the pounds under milder conditions is still necessary.
complex chemical structures of lignin, most researchers still
The HDO strategy for lignin includes two-step conversion
focus their attention on employing lignin model compounds processes, which could produce less mixed products by the
as the feedstock in their studies.9,10 To date, a series of homo- incorporation of hydro-processing and conversion pro-
geneous or heterogeneous catalytic systems have been devel- cesses.15,16 However, most of these reactions for HDO of
oped for the HDO of lignin-derived dimeric ethers and lignin-derived compounds were conducted in water, alcohols
phenols into alkanes,11 including Rh nanoparticles (NPs), Pt and other organic solvents over various metal catalysts.17
NPs, Ru NPs in [Bmim]BF4 and [Bmim]NTf2,12 metal NPs, There have been relatively fewer reports about HDO of lignin
model compounds in ionic liquids (ILs) up to now.1 In recent
years, ILs as one kind of desirable solvent were not only
aBeijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green
Process and Engineering, State Key Laboratory of Multiphase Complex Systems,
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190,
PR China. E-mail: xmlu@ipe.ac.cn, sjzhang@ipe.ac.cn
applied to dissolve lignin materials but also used as
efficient catalysts for the conversion of biomass-derived
compounds.18–23 Meanwhile, ILs are widely considered as
green solvents for heterogeneous catalytic systems, and could
also ensure the uniform dispersion of metal NPs thus forming
a pseudo-homogeneous system.13,24 Yan et al. firstly reported a
bifunctional catalytic system based on metal NPs and Brønsted
bSino Danish College, University of Chinese Academy of Sciences, Beijing 100049,
PR China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc03775b
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acidic ILs, which could be used for efficient HDO of lignin- ([Bmim]BF4), etc., were purchased from Lanzhou Institute of
derived phenols into alkanes in a one-pot process.12 Recently, Chemical Physics with 99.9% purity. H2 was provided by
Chen et al. reported the pseudo-homogeneous system incor- Beijing Beiwen Gas Factory, and the purity was 99.999%.
poration of metal NPs into ILs for HDO of lignin-derived Activated carbon, SiO2, γ-Al2O3 and SBA-15 (pore size 6–11 nm)
model compounds.13,25 However, H3PO4 was obligatory for were obtained from Nanjing XFNANO Materials Tech Co., Ltd.
cleavage of C–O bonds in lignin-derived dimeric ether. Hence
2.2. Experiments
the acid catalytic action for the cracking of C–O is still the
main reported mechanism.13,26 Nevertheless the development
2.2.1. Catalyst preparation. All the catalysts of metal
of efficient catalytic systems under Brønsted acid free con- loading on SBA-15 were prepared with a simple impregnation
ditions is highly desirable for industrial valorization of lignin method. First, SBA-15 (2 g) was impregnated with an aqueous
resource because Brønsted acids always corrode the autoclave solution of RuCl3·xH2O (0.25 g dissolved in 20 ml water), fol-
during a reaction process.
lowed by intensive mixing with microwaves at 100 MW for
Herein, we described an efficient catalytic system, which 2 hours. Thereafter, the solvent was evaporated in an air dry
was formed with well-dispersed metal NPs supported on oven at 100 °C for 24 hours. A tube furnace was used for calci-
SBA-15 synergistic with an ionic liquid. This catalytic system nations of the dried catalysts with an argon flow at 500 °C for
showed efficient activity for HDO of lignin-derived phenols 5 hours. At last, reduction of the catalysts was conducted with
and dimeric ethers under acid free conditions. The metal cata- a hydrogen flow at 500 °C for 4 hours. The heating rates for
lysts were characterized by scanning electron microscopy calcinations of catalysts were 2 °C min−1. Other catalysts were
(SEM), transmission electron microscopy (TEM), X-Ray prepared with a similar method. Pd/SBA-15 was obtained from
Diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area SBA-15 impregnation with an ethanol solution of PdCl2
analysis and X-ray photoelectron spectroscopy (XPS). instead of aqueous solution. The loading amount of metals
Meanwhile, different solvents including alcohols of low mole- was 5 wt% in all catalysts.
cular weight and ILs, catalysts based on various metals, H2
2.2.2. Catalyst characterization. The morphology of cata-
pressure, reaction temperatures and times were employed to lysts was analyzed by Scanning Electron Microscopy (SEM).
explore the optimal reaction conditions. The possible reaction The catalysts were scanned in a Hitachi SU8020 SEM under
mechanisms for phenol and diphenyl ether were deduced accelerated electrons with 5 keV of energy. Transmission elec-
based on the GC-MS results. Various lignin-derived phenolic tron microscopy (TEM) images were obtained by using a TEM
and ether compounds were used as the substrates to explore (JEM 2100-F (UHR)) operated at 200 keV. The ethanol suspen-
the applicability of our catalytic system. Meanwhile, the cata- sion of catalysts was deposited on a copper grid coated with a
lytic system of [Bmim]PF6-Ru/SBA-15 was reused six times with carbon film for the TEM observation. X-ray diffraction (XRD)
diphenyl ether as the substrate.
patterns were conducted on an Smartlab(9) Rigaku diffract-
ometer under the conditions of Cu Kα radiation at 40 kV and
200 mA with 2θ ranging from 5°–90°. The specific surface area
and pore diameter of the catalysts were obtained by BET
surface area analysis in an ASAP 2020HD88 BET system from
Micromeritics Instrument (Shanghai) Ltd. The H2-TPR of the
2. Materials and methods
2.1. Materials
Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, 99.9%), palla- catalysts was measured by using a BELCAT-B-TT (BEL) instru-
dium chloride (PdCl2, 99.9%), rhodium chloride hydrate ment. Firstly, 0.05 g of sample was pretreated at 150 °C for 1 h
(RhCl3·3H2O, 99.9%), and hydrogen tetrachloroaurate hydrate with the flowing helium. After the quartz reactor was cooled
(99.8%) were purchased from J&K Scientific Ltd. Ruthenium down to 40 °C, the temperature of the system was raised up to
chloride hydrate (RuCl3·xH2O, 35.0–42.0% Ru basis), phenol 700 °C at the rate of 10 °C min−1 with a flow of a H2–Ar gas
(≥99.0%), tert-butyl methyl ether (GC, ≥99.9%), ammonium mixture (5 vol% H2). XPS data were obtained by using a
(para)tungstate hydrate ((NH4)10H2(W2O7)6·xH2O, 99.5%), Thermo Fisher Scientific ESCALAB 250Xi instrument, which
dodecane (GC, ≥99.0%), cyclohexane (GC, >99.5%), ethylcyclo- was operated at the pass energy of 20.0 eV for a single element
hexane (GC, >99.5%) and all of the substrates (GC, >98%) were and 100 eV for the whole survey.
purchased from Aladdin Chemistry Co., Ltd. Guaiacol (CP,
2.2.3. Conversion of lignin-derived compounds and
≥99.0%), diphenyl ether (99.5%), absolute methanol (CP), product analysis. The lignin model compounds (1 mmol), cata-
ethanol (CP), isopropanol (CP) and copper chloride (CuCl2, lysts (0.1 g), ionic liquid (2 g) and dodecane (1 mmol) were
99.0%) were purchased from Sinopharm Chemical Reagent added into an autoclave with a Teflon® cell. The reaction
Co., Ltd. 2-Phenoxyacetophenone (>98.0%) was bought from system was sealed and purged with hydrogen three times, and
TCI Co., Ltd. Ammonium perrhenate(VII) (≥99.0%) and 2,3- then pressurized with 2 MPa H2 at room temperature. The
dihydrobenzo[b]furan (98.0%) were bought from Alfa Aesar autoclave was heated and stirred at a given temperature and
Co., Ltd. All ILs, including 1-n-butyl-3-methylimidazolium time in a heating block with a stirring speed of 600 rpm. After
N-bis-(trifluoromethanesulfonyl) imidate ([Bmim]NTf2), 1-n- the reaction finished, methyl tert-butyl ether (MBTE, 8 ml) was
butyl-3-methylimidazolium hexafluoro-phosphate ([Bmim] used to extract the final products. The products were analyzed
PF6), and 1-n-butyl-3-methylimidazolium tetrafluoroborate by using gas chromatography mass spectrometry (GC-MS,
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Shimadzu GCMS-QP2020), and quantified with gas chromato- the morphology of all the catalysts, which can be seen in the
graphy equipment (GC-2014, Shimadzu). The GC system was ESI (Fig. S1†). There was no significant change in the mor-
equipped with a capillary column from Agilent (RTX-5®, 30 m phology of catalysts compared with their carrier.
× 0.25 mm × 0.25 μm) connecting a flame ionization detector
(FID) for quantification. N-Dodecane was used as the internal metals. The crystal types of Ru and Re were hexagonal struc-
standard in the experiments. ture, while others were face-centered cubic (fcc). From the XPS
The XRD data in Fig. S2† provide the crystal types of
2.2.4. Catalytic system recycling. After the reaction, 5 ml data (Fig. 1), it could be confirmed that the metals had two
methyl tert-butyl ether was used to wash the catalytic system valence states, most of them being the reduced state and a
(containing a metal catalyst and IL) three times. The catalytic small amount of the oxidized state. There is no doubt that the
system was further dried at 45 °C under high vacuum for reduced state metal is the active phase of the catalysts.13 The
10 hours. After that, the catalytic system was used for the next oxidized state of the metal was owing to the oxidation of the
run directly.
metal in air, which was beneficial for the stability of
catalysts.33
It has been reported that the size of metallic nanoparticles
has a significant influence on the activity and selectivity of cat-
alysts for a reaction.13,34,35 The smaller metal particles have
higher activity for metal catalysts in many cases.13 The mean
sizes of metals supported on carriers in this work were deter-
mined by TEM of randomly selected areas and based on 100
particles. Fig. 2 shows the representative TEM micrographs
with the corresponding metal particle size distribution. It con-
firmed that the metal particles were well dispersed on the
surface of their carriers. The mean diameter of metal particles
decreased in the order of Au > Pd ≫ Pt > Ir ∼ Rh > Ru. The
catalytic activity of catalysts was consistent with the rule of
Ru > Ir ∼ Rh > Pt ≫ Pd > Au. The low catalytic activity of
W/SBA-15, Re/SBA-15 and Cu/SBA-15 could be attributed to the
unformed metal crystals, which can be seen in the ESI
(Fig. S3†).
3. Results and discussion
3.1. Characterization of catalysts
The characteristics of the catalysts were measured by using a
series of methods. Table 1 presents the specific surface area,
pore volume and pore size for the supported Ru catalysts. The
surface area of the catalysts was determined through the
Brunauer–Emmett–Teller (BET) method. The average pore
volume and pore size were tested by the Barrett–Joyner–
Halenda (BJH) method. Normally a high surface area and pore
volume with an appropriate pore size could promote most of
the catalytic reactions.27–30 SBA-15 has been widely used as a
catalyst carrier due to its high surface area and good thermal
stability,31 and it contains well-ordered hexagonal mesoporous
silica structures.32 Herein, Ru/SBA-15 presented the most
efficient catalytic activity for HDO of lignin-derived phenolic
and ether compounds due to its high surface area and suitable
pore volume and pore size. In addition, SEM graphics showed
3.2. Conversion of lignin-derived compounds
Heterogeneous catalytic systems are highly desirable for indus-
trial application owing to their convenient separation of pro-
ducts after the reaction. In order to evaluate our catalytic
system, phenol and diphenyl ether were used as the raw
materials to verify the catalytic activity of the IL system firstly.
Different from the reported strategies both in the IL system
and organic system, this catalytic system avoided the addition
of the protic acid H3PO4.13,17,25 Phenol and diphenyl ether rea-
lized efficient catalytic hydrogenation under mild conditions.
Table 1 BET characteristics of metal catalysts
Sample
SBET (m2 g−1
)
Pore volume (cm3 g−1
)
Pore size (nm)
Ru/C
1110
406
218
134
0.67
1.00
1.08
0.64
3.43
10.27
22.12
19.17
Ru/SBA-15
Ru/SiO2
Ru/γ-Al2O3
Fig. 1 XPS spectra of the catalysts (a) Ru/SBA-15 3d and (b) Ir-SBA-15 4f.
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Fig. 2 TEM images of (a) Ru/SBA-15, (b) Ir/SBA-15, (c) Rh/SBA-15, and (d) Pt/SBA-15.
Various lignin-derived compounds were also applied as the of cyclohexane was 95.8%. Meanwhile, Ir/SBA-15 and Rh/
substrates to investigate this catalytic system. SBA-15 also showed good performance for HDO of phenol, the
3.2.1. Phenol. To access the HDO activity of the syn- conversion of phenol was 100%, and the yield of cyclohexane
thesized catalysts to lignin-derived compounds, phenol was was 81.8% and 76.2%, respectively. However, for W/SBA-15,
used as the raw material to explore the optimal reaction con- Cu/SBA-15 and Au/SBA-15, trace phenol was converted, and the
ditions. Table 2 shows the catalytic activity of various metal yields of cyclohexane were about 1.0% at 130 °C for 6 h
catalysts for the HDO of phenol at 130 °C with a reaction time (Table S1 in the ESI†). It should be noted that besides cyclo-
of 6 h. No reaction occurred without metal catalysts (Table 2, hexane, bicyclohexane was also detected as the by-product in
entries 1 and 2). Catalysts with various metal loadings on some experiments as mentioned in a few studies13,36 (Table 2,
SBA-15 demonstrated significant differences in activity. Ru/ entries 3, 5 and 8), which will be discussed in the following
SBA-15 showed the most excellent catalytic activity for HDO of section.
phenol compared with other metal loading catalysts (Table 2,
In addition, compared with different metal loadings on
Table S1†). The conversion of phenol was 100%, and the yield SBA-15, the catalyst carrier showed a weak effect on the cata-
lytic activity (Table 2, see entries 3 and 7–9). Among the
different Ru-based catalysts, Ru/C exhibited excellent catalytic
Table 2 Catalytic activity of different catalysts for the HDO of phenola
activity similar to SBA-15 for HDO of phenol owing to its high
specific surface area, while Ru/SBA-15 showed the best cata-
lytic activity for HDO of diphenyl ether compared with other
catalysts owing to the high surface area and suitable pore
volume and pore size of SBA-15.
In order to find the best catalytic system by combination
of solvents and metal catalysts, various ILs and organic sol-
vents were tested in this work. Table 3 presents the effects of
solvents on the HDO of phenol by using Ru/SBA-15 as the
catalyst at 130 °C for 6 h. [Bmim]PF6 showed obvious advan-
tages in the hydrogenated deoxidization process compared
with other ILs and organic solvents as well. There was no
cyclohexanol detected in the metal catalyst screening experi-
ments (see Table 2), while in the solvent screening experi-
ments, cyclohexanol was detected in most of the processes. It
confirmed that the solvent type is the major factor for the
deoxidization process from cyclohexanol to cyclohexene. It
should be noted that the catalysts could well disperse in ILs,
while in organic solvents, the catalysts were likely to aggre-
gate.37 Therefore, high catalytic efficiency could be obtained
by using the IL catalytic system (see Table 3, Table S2 in the
ESI†).
Yieldb (%)
Entry
Catalyst
Conversionb (%)
1a
1b
1c
1
2
3
4
5
6
7
8
None
SBA-15
0
0
0
0
0
0
0
0
2.1
0
4.8
0
0
3.5
0
Ru/SBA-15
Ir/SBA-15
Rh/SBA-15
Pt/SBA-15
Ru/C
Ru/SiO2
Ru/γ-Al2O3
Pt NPs
100
100
100
62.5
100
100
83.8
100
95.8
81.8
76.2
56.8
98.5
80.5
65.2
98.0
0
0
0
0
0
0
1.2
0
9
10c
2.0
a Reaction conditions: Phenol, 1 mmol; catalyst 0.1 g; [Bmim]PF6,
2.0 g; reaction temperature, 130 °C (referred to as the constant-temp-
erature air bath temperature); reaction time, 6 h; pressure of H2,
2 MPa. b The conversion and yields were determined by GC using
n-dodecane as an internal standard. c T = 130 °C, pressure of H2 = 5
MPa, 10 h, 0.1 g of H3PO4, 2 g of [Bmim]PF6, substrate/Pt = 100.13
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Table 3 The effect of different solvents on HDO of phenola
Yieldb (%)
Entry
Solvent
Conversionb (%)
1a
1b
1c
c
1
2
3
4
5
6
[Bmim]PF6c
[Bmim]BF4
100
100
100
92.2
100
100
95.8
2.1
1.5
7.4
1.5
0
2.1
0
0
0
0
95.8
55.8
83.1
98.3
74.5
c
[Bmim]NTf2c
[Emim]NTf2
Isopropanold
n-Heptaned
12.7
0
Fig. 4 Effect of hydrogen pressure on the HDO of phenol. Reaction
conditions: Ru@SBA-15, 0.1 g; phenol, 1 mmol; [Bmim]PF6, 2.0 g; reac-
tion time, 6 h; reaction temperature, 130 °C. The conversion and yield
were determined by GC with n-dodecane as a standard.
a Reaction conditions: Phenol, 1 mmol; Ru/SBA-15, 0.1 g. b The conver-
sion and yields were determined by GC using n-dodecane as an
internal standard. c IL, 2.0 g. d Solvent, 4.0 g; reaction temperature,
130 °C (referred to as the constant-temperature air bath temperature);
reaction time, 6 h; pressure of H2, 2 MPa.
(Fig. 3a) and different times at 130 °C (Fig. 3b). When the
temperature was 90 °C, the main product was cyclohexanol
with a yield of 81.1%. When the temperature reached up to
130 °C, the main product was cyclohexane with a yield of
95.8%. Thus the deoxidization process was strongly dependent
on the reaction temperature in this catalytic system. As can be
seen in Fig. 3b, the optimal reaction time was 6 h at 130 °C.
Meanwhile, the effect of hydrogen pressure on phenol HDO
was also studied (see Fig. 4). The yield of cyclohexane
increased from 91.0% to 95.8%, when the pressure increased
from 1 MPa to 2 MPa. However, when the hydrogen pressure
increased to 4 MPa, the yield of bicyclohexane was up to
18.5%, indicating that high hydrogen pressure promoted the
reaction of aldol condensation between cyclohexanone and
cyclohexanol (see Scheme 1). Similar to the reaction processes
of phenol HDO reported by Chen et al.,13 cyclohexanone could
be obtained by phenol hydrogenation with a metal catalyst in
IL. Then cyclohexanone was converted to cyclohexanol quickly,
and the subsequent deoxygenation yields cyclohexene. Finally,
cyclohexene hydrogenated to cyclohexane.
Previous research had confirmed that the catalytic activity
of catalysts for HDO of lignin-derived model compounds
decreases in the order of Ru ∼ Pt ∼ Rh ≫ Pd ∼ Re in most
cases.13,25 The surface catalytic mechanism for heterogeneous
catalytic reactions is still widely accepted by most of the
researchers. Thus the activity and selectivity of catalysts are
mainly ascribed to the active surface of metal particles loaded
on the catalyst carrier.13 A large specific surface area of the
catalyst carrier facilitates the dispersion of metal, which also
contributes to provide more surface active sites for the
metal.38,39 The catalyst BET characterization section has con-
firmed that the specific surface area of the catalysts decreases
in the order of activated C > SBA-15 > SiO2 > γ-Al2O3. This
trend was in line with the activity of catalysts based on ruthe-
nium metal (see Table 1).
The temperature and time are very important parameters
for a reaction. In most cases, the reaction temperature deter-
mines whether a reaction can occur, while the time determines
the degree of reaction.17,40 Fig. 3 shows the results of phenol
HDO over Ru/SBA-15 catalyst at different temperatures for 6 h
Fig. 3 Effect of reaction temperature (a) and reaction time (b) on the HDO of phenol. Reaction conditions: Ru/SBA-15, 0.1 g; phenol, 1 mmol;
[Bmim]PF6, 2.0 g; pressure of H2, 2 MPa; reaction time, 6 h for (a). Reaction temperature for (b) was 130 °C and other conditions were the same as
for (a). The conversion and yield were determined by GC with n-dodecane as a standard.
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(with the same cation with [Bmim]PF6, Table 3, entries 2 and
3). Based on the abovementioned experiment data (including
the data in Table 2, entries 1 and 2), we could confirm that the
reactions of phenol to cyclohexanone and cyclohexene to cyclo-
hexane were mainly catalyzed by a metal catalyst. In addition,
when cyclohexanone was used as a substrate, 80.9% of cyclo-
hexanone was not converted. Meanwhile 2-(1-cyclohexenyl)
cyclohexanone with a yield of 6.0% was detected after the reac-
tion (see Fig. S4b in the ESI†). This result could confirm that
the reaction of cyclohexanone further converting to cyclohexa-
nol was also catalyzed by the metal catalyst. Moreover the aldol
condensation reaction between cyclohexanone was promoted
by [Bmim]PF6. The possible reaction mechanism is presented
in Scheme 1.46,47 Furthermore, it should be noted that the pro-
ducts of cyclohexane and bicyclohexyl were not dissolved in
[Bmim]PF6, meanwhile ILs have a better solubility for hydro-
gen than many solvents, and these also facilitate the full
reaction.13,23,25
3.2.2. Diphenyl ether. Table 4 presents the results of diphe-
nyl ether HDO. Similar to the results of phenol HDO, Ru/
SBA-15 showed the most excellent catalytic performance com-
pared with other catalysts. The conversion of diphenyl ether
was 100% at 130 °C for 6 h, and the yields of cyclohexane and
bicyclohexane were 97.7% and 1.1%, respectively. Ru/C
Scheme 1 Reaction pathway of phenol in catalytic HDO treatment.
Two other reaction pathways might also occur during the showed relatively low catalytic activity for HDO of diphenyl
abovementioned reaction. Cyclohexyl phenol could be gener- ether compared with phenol, which could be attributed to the
ated through a cyclohexyl cation recombining with phenol by aggregation of the catalyst in IL. Previous studies have con-
the electrophilic aromatic substitution reaction (see firmed that a proton acid could facilitate the electrophilic aro-
Scheme 1).41–43 However, different from the previous work, matic substitution and aldol condensation reaction to produce
cyclohexyl phenol was not detected in this reaction system, bicyclohexane.13,36,41–44 In this work, there were very few by-
even when the reaction was stopped at 1 h or 2 h. The reason products for the diphenyl ether HDO process using Ru/SBA-15
could be that cyclohexanol needs to be activated with protons in [Bmim]PF6, because no proton acid was added in the
and then reacts with phenol in this reaction.13 Nevertheless, system.
there was no proton acid added in our catalytic system, while
In order to explore diphenyl ether HDO pathways, the reac-
the aldol condensation reaction to produce 2-cyclohexyl cyclo- tion was carried out at different times (0.5 h, 1 h, 4 h). The
hexanone could happen in our catalytic system (Scheme 1). A
small amount of cyclohexyl cyclohexanone and bicyclohexyl
Table 4 The effect of different catalysts on the HDO of diphenyl ethera
were observed. The reaction pathway for the acid catalytic
mechanism has been reported in the previous work.36,44,45 As
far as we know, the mechanistic study of this side reaction is
still unreported with ILs as the catalyst. The possible reaction
pathway of the by-product bicyclohexyl is depicted in
Scheme 1.
Yieldb (%)
In order to analyse the impact factors of metal catalysts and
ILs for this reaction pathway, control tests were conducted
with cyclohexanol and cyclohexanone as substrates respect-
ively, and no metal catalysts were added in these experiments.
There was no cyclohexyl cyclohexanone and bicyclohexyl
detected with cyclohexanol as the substrate, meanwhile 90.2%
yield of cyclohexene and a trace of cyclohexane were detected
(see Fig. S4a in the ESI†). The conversion of cyclohexanol was
100%. These results could confirm that the deoxidization
process is promoted by an IL. We also can further conclude
that the main factor for the deoxidization step is the anion of
[Bmim]PF6, because the main product is cyclohexanol when
the reaction was conducted in [Bmim]BF4 and [Bmim]NTf2
Entry
Catalyst
Conversionb (%)
2a
2b
1
2
3
4
5
6
7c
None
SBA-15
Ru/γ-Al2O3
Ru/C
Ru/SiO2
Ru/SBA-15
Ru NPs
0
2.1
30.6
100
100
100
100
0
1.9
27.0
83. 6
91.5
97.7
89.0
0
0
0
0
0
1.1
11.0
a Reaction conditions: Diphenyl ether, 1 mmol; catalyst 0.1 g; [Bmim]
PF6, 2.0 g; reaction temperature, 130 °C (referred to as the constant-
temperature air bath temperature); reaction time, 6 h; pressure of H2, 2
MPa. b The conversion and yields were determined by GC using
n-dodecane as an internal standard. c T = 130 °C, pressure of H2
5
MPa, 10 h, 0.1 g of H3PO4, 2 g of [Bmim]PF6, substrate/Pt = 100.13
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Table 5 HDO of lignin-derived compoundsa
Yields of the
major productsb
(%)
Entry
1
Substrate
Tb (°C)
Con.b (%)
>99.0
150
95.2
2
3
4
5
150
130
130
130
>99.0
>99.0
>99.0
>99.0
79.1
94.3
95.1
22.0
99.2
99.4
99.0
79.1
69.1
99.1
6
7
8
9
130
130
130
130
130
>99.0
>99.0
>99.0
>99.0
>99.0
63.2
62.0
51.0
19.1
Scheme 2 Reaction pathway of diphenyl ether in catalytic HDO
treatment.
intermediate products of the diphenyl ether HDO process were 10
detected by GC-MS (see Fig. S5 in the ESI†). The possible reac-
tion pathways of diphenyl ether are depicted in Scheme 2. It
was reported that the C–O bond in diphenyl ether could be
99.5
68.3
96.8
broken via either acid-catalysis to phenol (route 1)13,25 or
11
130
>99.0
metal-catalysis to phenol and benzene (route 3).14,26 There was
no acid added in this catalytic system, therefore the broken
C–O bond was catalyzed by metal catalysts in this work. As far
as we know, the reaction pathways of this work were different
from the reported pathways for the diphenyl ether HDO
process in the IL system (Scheme 2, route 2). Diphenyl ether
was converted to oxydicyclohexane first; further cleavage of the
C–O bond to cyclohexane and cyclohexanol occurred, and then
cyclohexanol was converted to cyclohexane and bicyclohexyl by
the HDO process which was similar to the conversion pathway
of phenol.
a Reaction conditions: Substrate 1 mmol, Ru/SBA-15, 0.1 g; [Bmim]PF6,
2.0 g; temperature, 130 °C (referred to as the constant-temperature air
bath temperature); pressure of H2, 2 MPa. b T = temperature, Con. =
conversion; the conversion and yields were determined by GC using
n-dodecane as an internal standard.
3.2.3. HDO of other lignin-derived compounds. In order to
explore the applicability of the catalytic system, other lignin-
A possible mechanism is proposed in Scheme 2 for the derived phenolic and ether compounds were tested under 2
diphenyl ether HDO process to cyclohexane and bicyclohexyl. MPa hydrogen pressure (Table 5). The results indicated that
It is well known that noble metals can activate hydrogen for the catalytic system had high catalytic activity for HDO of
most of the hydrogenation reactions.48–50 For our catalytic lignin-derived phenolic and ether compounds. High yields of
system, diphenyl ether was converted by direct hydrogenation alkenes could be obtained after the reaction with >99.0% con-
of aromatic rings to oxydicyclohexane, and then the cleavage version of substrates at 130 °C for 6 h (see entries 3–11).
of the C–O bond to produce cyclohexane and cyclohexanone However, when the phenolic hydroxyl group was substituted by
through the formation of a weak coordinative bond between the methoxy group, the reactivity was affected significantly.
the C4-O-C5 group and the Ru nanoparticles.46,47,51 After increasing the reaction temperature from 130 °C to
Furthermore the cyclohexanone converted to cyclohexane and 150 °C, the yields of cyclohexane increased from 61.0% to
bicyclohexanyl, which was similar to the pathway of phenol. 95.2% with guaiacol as the substrate, and the yields of ethyl-
Except for diphenyl ether, the abovementioned reaction cyclohexane increased from 29.0% to 79.1% with benzofuran as
mechanism was also suitable for lignin-derived ether com- the substrate, respectively. This phenomenon could be attribu-
pounds as described in the following section.
ted to the higher bond dissociation energy of C–O for methoxy
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tion pathway of diphenyl ether was different from the metal
NPs–IL catalytic system. Diphenyl ether was converted to oxydi-
cyclohexane firstly with further cleavage of the C–O bond to
cyclohexane and cyclohexanol. The research confirmed that
both Ru/SBA-15 and [Bmim]PF6 played important roles in the
catalytic HDO processes, respectively. Ru nanoparticles well
dispersed on SBA-15, which promoted the hydrogenation pro-
cesses from raw materials to the corresponding alkanes, while
[Bmim]PF6 catalyzed the deoxidization processes during
lignin-derived phenol and dimeric ether HDO reaction. The
anion of [Bmim]PF6 was the main factor for catalyzing deoxidi-
zation processes. Moreover, this catalytic system exhibited
high HDO activity for various lignin-derived phenols and
dimeric ethers with a similar catalytic mechanism to phenol
Fig. 5 Recycling of the catalytic system. Reaction conditions:
Ru@SBA-15, 0.1 g; diphenyl ether, 1 mmol; [Bmim]PF6, 2.0 g; reaction and diphenyl ether.
time, 6 h; reaction temperature, 130 °C. The conversion and yield were
determined by GC with n-dodecane as a standard.
Conflicts of interest
groups than phenolic hydroxyl groups.25,52 For the guaiacol,
when the reaction was conducted at 130 °C for 6 h, a certain
amount of methoxycyclohexane was detected after the reaction
(see Fig. S6 in the ESI†). While there was no methoxycyclo-
hexane detected after the experiment at 150 °C for 6 h, the
main product was cyclohexane with a yield of 95.2%. It further
verified that the mechanism of the guaiacol HDO process was
similar to that with the diphenyl ether.
There are no conflicts to declare.
Acknowledgements
This research was supported financially by the National
Natural Science Foundation of China [No. 21476234, 21878292
and 21606240], the K. C. Wong Education Foundation and the
Strategic Priority Research Program of the Chinese Academy of
Science [No. XDA21060300].
3.2.4. Recycling test. The stability of the catalytic system
for Ru/SBA-15 dispersed in [Bmim]PF6 was investigated by cat-
alyzing diphenyl ether to cyclohexane. The results of the re-
cycling test are shown in Fig. 5. After recycling four times,
there was no significant loss of activity for the catalytic system.
The yield of cyclohexane still reached up to 95.6% for the
fourth experiment with 100% conversion of diphenyl ether. It
could be attributed to the Ru NPs homodispersed on the
SBA-15 surface, which avoided the aggregation of metal nano-
particles. Thus there was no deactivation of the catalytic
system like the reported methods.13,17 However, after running
six times, the conversion of diphenyl ether and the yield of
cyclohexane had a certain decrease. The deactivation of the
catalytic system could be attributed to the aggregation of nano-
metal particles after falling off from SBA-15 (see Fig. S7†).
Meanwhile, the loss of catalyst and IL during the separation
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