Hydrogenation of lignin-derived phenolic compounds over step by step precipitated Ni/SiO2

Article abstract of DOI:10.1039/c5ra23041a

The harsh reaction conditions for the valorization of lignin-derived phenolic compounds considerably limit the efficient utilization of the lignin derivatives. Here, we put forward a high efficient and selective hydrogenation process for phenolic compounds at a mild condition over step by step precipitated Ni/SiO₂ catalyst. The properties of the Ni/SiO₂ catalysts by different preparation methods were detailedly compared using various characterization measurements. Catalytic activity of the catalysts was tested by the hydrogenation of guaiacol, and the results showed that guaiacol could be completely converted into cyclohexanol with 99.9% selectivity at 120 °C, 2 MPa H₂ atmosphere for 2 h. Other typical lignin-derived phenolic compounds also had excellent hydrogenation performance and great energy efficiency. Catalyst characterization results demonstrated that the high catalytic activity of the step by step precipitated Ni/SiO₂ was mainly ascribed to its polyporous spherical structure, which led to the large specific surface area and high nickel dispersion. The appropriate acidity of the catalyst also promoted the catalytic performance significantly. Furthermore, the catalyst exhibited an excellent recyclability, where no significant loss of the catalytic activity was showed out after 3 runs.

Full text of DOI:10.1039/c5ra23041a

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DOI: 10.1039/C5RA23041A
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Hydrogenation of Lignin-derived Phenolic Compounds over Step
by Step Precipitated Ni/SiO2
Received 00th January 20xx,
Accepted 00th January 20xx
Riyang Shu,a,b Qi Zhang,*a Ying Xu,*a Jinxing Long,a Longlong Ma,a Tiejun Wang,a Pengru Chena,b
and Qingyun Wua
DOI: 10.1039/x0xx00000x
The harsh reaction conditions for the valorization of lignin-derived phenolic compounds considerably limited the efficient
utilization of the lignin derivatives. Here, we put forward a high efficient and selective hydrogenation process for phenolic
compounds at a mild condition over step by step precipitated Ni/SiO2 catalyst. The properties of the Ni/SiO2 catalysts by
different preparation methods were detailedly compared using various characterization measurements. Catalytic activity
of the catalysts was tested by the hydrogenation of guaiacol, and the results showed that guaiacol could be completely
converted into cyclohexanol with 99.9% selectivity at 120 °C, 2 MPa H2 atmosphere for 2 h. Other typical lignin-derived
phenolic compounds also had excellent hydrogenation performance and great energy efficiency. Catalyst characterization
results demonstrated that the high catalytic activity of the step by step precipitated Ni/SiO2 was mainly ascribed to its
polyporous spherical structure, which led to the large specific surface area and high nickel dispersion. The appropriate
acidity of the catalyst also promoted the catalytic performance significantly. Furthermore, the catalyst exhibited an
excellent recyclability, where no significant loss of the catalytic activity was showed out after 3 runs.
www.rsc.org/
lignin derivatives.12 Wherein, the CAR-OCH3 is easy to cleave by
hydrogenolysis,13,14 while the removement of the oxygen in
CAR-OH is much harder. The hydrodeoxygenation (HDO)
1. Introduction
With the depletion of petroleum deposition and the increasing
environmental problems, lignin, the byproduct of the biomass
refinery, is attracting the global attention as an important
renewable feedstock for the production of fuels, chemicals and
energy.1 Due to its 4-propenyl-phenols units interlinked
through C-O-C ether covalent bonds,2 lignin can be broken
down into phenolic monomers, dimers and oligomers by
means of catalytic and thermochemical processes.3-6 These
phenolic compounds are potential to be high grade fuels and
chemicals. However, owing to the high oxygen content,
corrosivity and immiscibility,7 they cannot be directly used as
biofuels. The unsaturated aromatic compounds have a poor
chemical stability and are flexible to repolymerize, which will
result in significant char formation.8,9 Therefore, the
researches for lignin depolymerization products conversion to
stable and high-value compounds have recently drawn
unprecedented attention.
method was usually used for the upgrade of guaiacol.15-17
Sulfided CoMo and NiMo are the most common catalysts in
the early time.18-20 While, the continuous addition of sulfur is
required in the reactant stream to maintain the catalyst
activity. Catalyst deactivation also occurs easily for the high
coke formation, hydrothermal instability, and catalyst sintering.
The noble metals receive widespread attention due to their
scarce characteristic of the aromatic ring hydrogenation and
anti-carbon deposition. For example, Lin’s study21
demonstrated that 100% conversion of guaiacol could be
obtained over Rh catalysts at 300 °C. The research group of
Kou22-24 also had done lots of interesting work in this issue.
They found that guaiacol could be completely converted into
cycloalkane with Pd/C and H3PO4 catalysts at 250 °C. However,
the high cost for these catalysts is inevitable, and the reaction
conditions have yet to be much milder.
The HDO process involves the complete deoxygenation, and
hydrocarbons such as benzene and cyclohexane were the
desired products. The acid centres are needed to catalyze the
deoxygenation and the high temperature is necessary for the
high reaction activation energy.24,25 But, the use of a high
temperature process is not suitable for the hydrogenation of
real lignin derived products because of the simultaneous
existence of phenolic monomers and oligomers.9 They contain
Guaiacol, as one of the most abundant products from the
lignin depolymerization processes, is a popular model
compound for phenolic monomer valorization study.10,11 It
contains three kinds of C-O bonds for CAR-OH, CAR-OCH3 and
CARO-CH3, which are typically and frequently found in various
a
large amount of unsaturated structures and can
repolymerize at elevated temperatures to produce the
undesired char. Remarkably, the selective hydrogenation
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reaction is much easier due to its low reaction activation treatment procedure as above. For convenience, the step by
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energy23, which can realize even over the non-noble metal step precipitated Ni/SiO2, co-precipDitOatI:e1d0.10N39i//CS5iORA2230a4n1dA
catalyst.14,26 The platform chemical compounds or high-value supported Ni/SiO2 catalysts were denoted as SS-Ni/SiO2, CP-
biofuel precursors, such as cyclohexanol, are prone to obtain Ni/SiO2 and SP-Ni/SiO2, respectively.
at low temperature likewise. Moreover, the hydrogenation is
very fast, in which the rate is several orders of magnitude 2.3. Catalyst characterization
higher than the deoxygenation reaction.24,25 Therefore, the
The surface area and pore size distribution of the catalysts
selective hydrogenation process is potential, energy-saving
were carried out over a Quantachrome chemical adsorption
and easy to realize.
instrument. In a typical process, the catalysts were degassed at
280 °C for 10 h prior to measurement, and the surface area
In this work, a mild and high selective process for the
hydrogenation of the lignin-derived phenolic compounds was
was calculated by the Brunauer Emmett Teller (BET) method.
proposed in the presence of different Ni/SiO2 catalysts. Results
Powder X-ray diffraction (XRD) patterns of the catalysts were
showed that the step by step precipitated Ni/SiO2 had a great
measured using X-ray diffraction radiation (X Pert Pro MPD
with Cu Kα (k=0.154) radiation, Philip, the scanning angle (2θ)
performance on the hydrogenation of guaiacol and other
typical phenolic compounds. Detailed characterization
ranged from 10° to 80°). X-ray fluorescence (XRF) was
demonstrated that the structure of the catalysts influenced
conducted on the PHI-5300/ESCA instrument. Scanning
the catalytic activity significantly. This novel catalysis process is
electron microscope (SEM) images of the catalysts were
a quite promising technique for the upgrade of lignin-derived
obtained on a Hitach S-4800 instrument (10 kV). Transmission
phenolic compounds, producing the platform chemical
electron microscopy (TEM) and energy dispersive
spectrometer (EDS) investigations were measured on a Jeol
compounds or high-value biofuel precursors.
TEM-100CX instrument at 200 kV accelerating voltage.
Hydrogen temperature programmed reduction (H2-TPR) and
desorption (H2-TPD), as well as ammonia temperature
2. Experimental
2.1. Materials
programmed desorption (NH3-TPD) investigations were
conducted in a same U-tube quartz reactor with a thermal
conductivity detector (TCD). The Ni metal leaching of the
catalyst in the decalin was determined by inductively coupled
plasma-atomic emission spectrometry (ICP-AES, OPTIMA 8000).
Fourier Translation Infrared Spectroscopy (FT-IR) was obtained
on a Nicolet iS50 FT-IR spectrometer using KBr pelleting
method.
Tetraethylorthosilicate (TEOS) and cetylhexadecyl trimethyl
ammonium bromide (CTAB) were purchased from Da Mao
chemical reagent factory. Ni(NO3)3 was provided by Tianjin Fu
Chen chemical reagent factory. Poly-vinylpyrrolidone (PVP)
was purchased from Shanghai Yuan Ju biotechnology Co., Ltd.
Guaiacol was purchased from Sinopharm Chemical Reagent
Co., Ltd. Other reagents were supplied by Jinhuada Chemical
Reagent Co., Ltd. All reagents were analytic grade and were
used without further purification.
2.4. Hydrogenation of lignin-derived phenolic compounds
All the hydrogenation reactions of lignin-derived phenolic
compounds were carried out in a 50 ml stainless autoclave
equipped with an electromagnetic driven stirrer. Typically, 0.1
g catalyst, 0.2 g phenolic compound and 20 ml decalin were
added into the autoclave. After purging with hydrogen three
times, the reactor was pressurized to 2 MPa at room
temperature and heated to 120 °C for 2 h under vigorous
stirring of 400 rpm. Repeated activity measurements were
made and the error bars were determined and added to the
reaction data.
The components of liquid products were identified by gas
chromatography/mass spectrometry (GC-MS, Agilent 5890)
equipped with a column of HP-INNOWAX, 30 m × 0.25 mm ×
0.25 um. The oven temperature was programmed as 60 °C
hold 2 min, and then ramped up to 260 °C with 10 °C min-1 and
hold for another 10 min. The injector was kept at 280 °C in
split mode (5:1) with helium as the carrier gas. Quantitative
analysis of the products was determined by a SHIMADZU GC
2014C with a flame ionization detector (FID) and a HP-
INNOWAX column. The oven temperature program was the
same as the GC-MS analysis. The conversion of phenolic model
compounds and the product selectivity were calculated based
on the following equations:
2.2. Catalyst preparation
The step by step precipitated Ni/SiO2 catalyst was prepared by
a modified method.27 In brief, 7.0 g Ni(NO3)3 and 1.0 g poly-
vinylpyrrolidone (PVP) was added to a 700 ml ethanol-water
(4:3) solution. After the solution was stirred for 5 min, 120 ml
.
NH3 H2O (25 wt%) was injected dropwise under strong
agitation. Then, 6.0 g hexadecyl trimethyl ammonium bromide
(CTAB) was added, and the suspension was stirred for another
10 min. Subsequently, 20 ml tetraethylorthosilicate (TEOS) was
injected into the suspension dropwise. Stirring vigorously for
72 h, the precipitation was filtrated, washed three times with
distilled water and ethanol, and dried at 60 °C in air for 12 h.
Then the catalyst was calcined at 500 °C for 6 h. After
reduction in a slow H2 flow at 550 °C for 6 h, the step by step
precipitated Ni/SiO2 catalyst sample was obtained. The
method for synthesizing the co-precipitated Ni/SiO2 catalyst28
was similar with that for the step by step precipitated Ni/SiO2
catalyst, and only the addition order of reagents was changed,
.
in which NH3 H2O was added after the injection of TEOS. The
supported Ni/SiO2 catalyst was prepared by wet impregnation
method.29 The SiO2 support was obtained by the hydrolysis of
.
TEOS (20 ml) using 25 wt% NH3 H2O (120 ml) with the same
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Conversion (%) = ( molesin – molesout ) / molesin × 100%
Selectivity (%) = moles (product)i /Σmoles (product) × 100%
View Article Online
DOI: 10.1039/C5RA23041A
3. Results and discussion
3.1. Catalyst characterization
N2 adsorption/desorption isotherms of different Ni/SiO2
catalysts and SiO2 support were displayed in Fig. S1. And their
corresponding pore size distributions were also exhibited.
Table 1 summarized the textural parameters of these catalysts.
The preparation methods had a significant effect on the BET
surface area and the pore property. The support had a surface
area of 92.08 m2 g-1 and a total pore volume of 0.56 cm3 g-1.
However, when it was impregnated with nickel, the values of
obtained SP-Ni/SiO2 decreased to 67.63 m2 g-1 and 0.47 cm3 g-1
respectively. Meanwhile, the pore diameter did not change.
The pore closure by the active phase precursor30,31 were
considered to be responsible for it. As for the CP-Ni/SiO2, a
declination of the pore volume was showed with the increase
of both the surface area and the pore diameter, compared to
the single support. The reason was considered to be that the
Ni species were inlaid inside the SiO2 with bad closure of the
pore. Moreover, it should be noted that both the surface area
and the total pore volume of the SS-Ni/SiO2 were greatly
increased to 450.75 m2 g-1 and 1.06 cm3 g-1, while the pore
diameter decreased to 3.04 nm. This was due to the high
mesoporous nature of the silica phase.32 The large surface area
suggested its potential application in catalytic reaction.
Fig. 1 XRD patterns of the different Ni/SiO2 catalysts.
was considered to result from that the Ni particles were quite
tiny and well dispersed in the interior of the SiO2 phase, based
on the preparation method.
In order to confirm the high dispersion of Ni particles, the
SEM and TEM measurements were used to observe the
morphology of the different Ni/SiO2 catalysts. As showed in
the SEM image of SS-Ni/SiO2 sample (Fig. 2a), the spherical
composite was uniformly obtained with particle size of 200-
400 nm, which was much larger compared with other reported
Ni/SiO2 catalysts.35,36 They were exhibiting a relative
The XRD patterns of the different Ni/SiO2 catalysts were
exhibited in Fig. 1. Wherein, the patterns of CP-Ni/SiO2 and SP-
Ni/SiO2 samples were very similar. The diffraction peaks at 2
ꢀ
values of 44.3°, 51.4°, and 76.1° were showed, which were
respective related to the crystal planes of (111), (200), and
(220) of metallic nickel phase.33 And the peaks of SP-Ni/SiO2
sample were stronger, indicating the greater crystallinity and
bigger crystal grain size. However, the SS-Ni/SiO2 sample was
quite different. There was only a broad peak at about 22.3°
that ascribed to the SiO2 phase, and no diffraction peaks
belonging to Ni phase could be detected. Obviously, the SiO2
showed no crystallinity demonstrated that it existed as
amorphous phase.32,34 While, the disappearance of Ni species
Table 1 Surface properties of the different Ni/SiO2 catalysts.
Catalystsa
Surface
areab
Total pore
volumec
(cm3 g-1)
1.06
Pore
diameterc
(nm)
(m2 g-1)
450.75
SS-Ni/SiO2
CP-Ni/SiO2
3.04
238.93
67.63
92.08
0.28
0.47
0.56
3.41
3.82
3.82
SP-Ni/SiO2
SiO2
a
SS, step by step precipitated; CP, co-precipitated; SP,
supported.
Fig. 2 (a) SEM and TEM images of SS-Ni/SiO2, (b) STEM and EDS
images of SS-Ni/SiO2, (c) SEM and TEM images of CP-Ni/SiO2, (d)
SEM and TEM images of SP-Ni/SiO2
b MultiPoint Brunauer, Emmett &Teller (BET) method.
c Barrett, Joyner & Halenda (BJH) method.
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the aggregation of the Ni particles was obvious in the SP-
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DOI: 10.1039/C5RA23041A
Ni/SiO2 and CP-Ni/SiO2, but not in the SS-Ni/SiO2, which
indicated that the Ni dispersion of SS-Ni/SiO2 was much better
than the others.
The H2-TPR profiles of the different Ni/SiO2 catalysts were
given in Fig. 3, in which a low-temperature peak centered at
445 °C and a high-temperature peak around 840 °C were
observed. The reduction peak at low temperature was
attributed to the reduction of NiO less interacted with the
support, while the one at high temperature corresponded to
the reduction of NiO under strong interaction with the
support.37 The SP-Ni/SiO2 only exhibited a stronger low-
temperature peak and the CP-Ni/SiO2 showed both the low-
temperature and high-temperature peaks. This difference was
resulted from the preparation methods, which made the CP-
Ni/SiO2 have a strong interaction with the support.38
Particularly, the SS-Ni/SiO2 catalyst only had a broad reduction
band between 600 °C and 880 °C. This was probably due to the
Fig. 3 H2-TPR profiles of the different Ni/SiO2 catalysts
homogeneous smooth external surface with porosity. The formation of the nickel silicate species,34 which were strongly
STEM images (Fig. 2b) exhibited that the small Ni particles with interacted with the support and hard to reduce.
nanoscale were highly dispersed not only on the surface, but
To make an accurate comparison of nickel dispersion, H2-
also in the interior of catalysts. This special structure was TPD measurements for reduced different Ni/SiO2 catalysts
affected by the preparation procedure significantly. The tiny Ni were carried out as represented in Fig. S2. Generally, there
particles were firstly precipitated and suspended in the were two hydrogen desorption regions: the low temperature
solution. Afterward, the SiO2 precipitated and encapsulated at 250 °C was attributed to hydrogen desorbed from the active
many Ni particles, then the large composite spheres formed. metal species, and the high temperature at 550 °C was related
Some Ni particles also attached to the surface of catalysts to hydrogen desorbed from subsurface layers or to spillover
spontaneously. The EDS images (Fig. 2b) demonstrated the Ni hydrogen.39,40 As observed, the SP-Ni/SiO2 had an obvious low
particles were quite small and the size was nanoscale, which temperature peak, while, the SS-Ni/SiO2 and CP-Ni/SiO2 had an
verified the high dispersion of the Ni phase in the SS-Ni/SiO2 high temperature peak. The peak intensity of SS-Ni/SiO2 was
sample. These phenomena explained well the large BET much higher than that of SP-Ni/SiO2 and CP-Ni/SiO2, which
surface area (Table 1) and the disappearance of the Ni species indicated the high Ni dispersion of SS-Ni/SiO2. The high peak
diffraction peaks in the XRD pattern (Fig. 1). However, the intensity also demonstrated that the hydrogen desorption
other two Ni/SiO2 samples were quite different. As for CP- from subsurface layers or hydrogen spillover in SS-Ni/SiO2
Ni/SiO2 (Fig. 2c), the Ni and SiO2 were coprecipitated, and the sample occurred more seriously.39,40 The quantitative analysis
interconnected structure was clustered together, where the of hydrogen uptake by exposed nickel species of the catalysts
size was much smaller than SS-Ni/SiO2. Moreover, the SP- was conducted based on the H2-TPD peaks area. As showed in
Ni/SiO2 sample showed that the Ni species were well Table 2, although the SS-Ni/SiO2 catalyst had the lowest Ni
supported on the SiO2 surface (Fig. 2d) and their average size loading (average 14.2%), its H2 uptake was the most, with
was much larger than that of CP-Ni/SiO2. It explained well its 360.2 umol/g. The nickel dispersion was also calculated out.
stronger diffraction peaks in XRD patterns (Fig. 1). Remarkably, The SS-Ni/SiO2 catalyst exhibited 29.8% nickel dispersion,
which was higher than that of CP-Ni/SiO2 and SP-Ni/SiO2. This
was also in well accordance with the XRD patterns (Fig. 1) and
Table 2 The dispersion of the different reduced Ni/SiO2
STEM, EDS results (Fig. 2).
catalysts.
The NH3-TPD experiments were also carried out and
Catalystsa
Ni
(wt%)
XRF
loading H2
uptakeb
(umol/g)
Nickel
dispersion
(%)
depicted in Fig. 4. The CP-Ni/SiO2 and SP-Ni/SiO2 exhibited only
one weak low-temperature desorption peak, which indicated
that they had only weak acid sites. However, two desorption
peaks arising from NH3 could be detected in the SS-Ni/SiO2
profile. According to the desorption temperature and the
corresponding classification of acid sites,34,41 the SS-Ni/SiO2
catalyst had not only weak, but also strong acid sites with
desorption temperature around 175 °C and 400 °C,
respectively. It demonstrated that the acid sites of the SS-
Ni/SiO2 were much stronger than that of the CP-Ni/SiO2 and
SP-Ni/SiO2 catalysts, although these catalysts with SiO2 support
were much weaker than those with acid supports, such as
EDS
20% SS-
Ni/SiO2
20% CP-
Ni/SiO2
20% SP-
15.85 12.56 360.2
17.71 14.32 150.1
18.13 15.74 137.1
29.8
11.0
9.5
Ni/SiO2
a
SS, step by step precipitated; CP, co-precipitated; SP,
supported.
b Calculated from peak area of H2-TPD profiles.
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especially on the existence of a large amountVieowf ArhtiyclderOonxlinyel
group.46 And the peak intensity of this group in the SS-Ni/SiO2
DOI: 10.1039/C5RA23041A
catalyst was much higher than that in the CP-Ni/SiO2 and SP-
Ni/SiO2 catalysts.
3.2. Adsorption ability of guaiacol
The catalysts adsorption ability of the substrate was first
tested. The adsorption curve of guaiacol on different Ni/SiO2
catalysts (Fig. 5) showed the SS-Ni/SiO2 catalyst had much
stronger adsorption ability than the others. Wherein, guaiacol
had been substantially adsorbed to the surface of SS-Ni/SiO2
catalyst in the initial 20 min, and reached to the saturated
state with the quantity of about 0.9 mmol/g. While, as for the
CP-Ni/SiO2 and SP-Ni/SiO2, their adsorption ability was much
weaker, and the quantity was only about 0.2 mmol/g. This
difference was probably due to the specific surface area of
catalysts (Table 1), as well as the amount of hydroxyl groups
over the catalysts (Fig. S3), which had strong absorption to the
aromatic hydroxyl groups of guaiacol.47,48
Fig. 4 NH3-TPD profiles of the different Ni/SiO2 catalysts.
Ni/HZSM-525, Ni/Al2O334. And obviously, the amount of
desorbed NH3 represented by the area of desorption peaks
also indicated that the SS-Ni/SiO2 catalyst had much more acid
sites than the CP-Ni/SiO2 and SP-Ni/SiO2. This was probably
3.3. Catalytic hydrogenation of guaiacol
resulted from the unreduced nickel silicate, which could form The catalytic activity of the different Ni/SiO2 catalysts on the
the [Ni(NH3)5]2+ ions.42-44 The explanation correlated well with hydrogenation of guaiacol was investigated (Table 3). In order
the strong interaction between nickel and silica in the SS- to calculate the turnover frequency (TOF) values, the H2-TPD
Ni/SiO2 catalyst, which was testified bythe H2-TPR profile in Fig. profiles of different SS-Ni/SiO2 catalysts were extra measured
3.
(Fig. S4) and the Ni dispersions of all the samples were given
The FT-IR analysis of the different Ni/SiO2 catalysts was out (Table S1). As a blank control, the single support SiO2 had
showed in Fig. S3. The large band in the range 3500-3400 cm-1 not any effect on the hydrogenation reaction at 120 °C.
corresponded to the molecular water or strongly H-bonded However, over the SS-Ni/SiO2 catalyst, the guaiacol could be
SiOH group. The band 1620 cm-1 was attributed to the converted into cyclohexanol with 99.4% conversion and 5.26 h-
1
vibrations of HOH bond deformation, 1100 cm-1 corresponding
TOF value, performing much better than those over the SP-
to the stretching vibration of Si-O-Si bond, 800 cm-1 Ni/SiO2 (5.8% of conversion) and the CP-Ni/SiO2 (3.4% of
corresponding to tetraedric SiO4, 450 cm-1 attributed to the conversion). CH3OH was detected, deriving from the efficient
vibrations of Si-O bond.45,46 The FT-IR profiles of different hydrogenolysis of the -OCH3 side chain of guaiacol, which was
Ni/SiO2 catalysts exhibited the similar peak types, while, the promoted by the synergistic effect of acid sites (Fig. 4) and Ni
distribution of groups was great different. Wherein, the SS- particles.25 While, this work focused on the selective
Ni/SiO2 catalyst had a similar property with the sol-gel catalyst, hydrogenation products, therefore the CH3OH yield was not
presented in the following discussion.
As for the guaiacol conversion into cyclohexanol, Escalona et
al. had reported their study at the reaction temperature of
300 °C, with the cyclohexanol selectivity of 60%.11 Also,
Guvenatam et al. had successfully realized the reaction at the
temperature of 200 °C over noble metal catalyst, but the
conversion of guaiacol only reached 80%, with the
cyclohexanol selectivity of 70%.12 Comparatively, the reaction
condition in this process was milder and the performance was
more excellent. According to the detailed characterization of
the catalysts above, the high catalytic activity of SS-Ni/SiO2 was
mainly ascribed to its polyporous spherical structure (Fig. 2),
which led to the large specific surface area (Table 1) and high
nickel dispersion (Table 2). The appropriate acidity of the
catalyst (Fig. 4), as well as the strong guaiacol adsorption
ability (Fig. 5), also promoted the catalytic performance
significantly. Moreover, the silica phase was polyporous with a
Fig. 5 Adsorption curve of guaiacol on the different Ni/SiO2
catalysts.
huge amount of 3.04 nm mesopores (Table 1), which allowed
guaiacol molecules (molecular diameter 0.67 nm49) to pass
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Table 3 The effects of different catalysts on the guaiacol
hydrogenation.a
View Article Online
DOI: 10.1039/C5RA23041A
Catalystb
Temperature Conversionc TOF (h-1)
(°C)
(%)
SiO2
120
0
0
20% CP-Ni/SiO2 120
20% SP-Ni/SiO2 120
20% SS-Ni/SiO2 120
15% SS-Ni/SiO2 120
10% SS-Ni/SiO2 120
3.4
0.49
0.96
5.26
5.98
6.01
5.46
5.8
99.4
89.2
68.4
39.4
5% SS-Ni/SiO2 120
a
Reaction condition: 20 ml decalin solvent, 0.2 g guaiacol,
0.1 g catalyst, 3 MPa H2, 3 h.
CP, co-precipitated; SP, supported; SS, step by step
precipitated.
The hydrogenation product was cyclohexanol and the
selectivity was 99.9%.
b
c
Fig. 6 Effects of different (a) reaction temperature, (b) reaction
time, (c) hydrogen pressure and (d) catalyst dosage on guaiacol
hydrogenation.a
through and interact with active sites for hydrogenolysis and
hydrogenation. Table 3 also showed that the Ni content of the
catalyst was a crucial factor for guaiacol hydrogenation. The
higher Ni content meant more hydrogenation active sites and
more acid sites (the unreducednickel silicate). Therefore, with
the increase of Ni content, the guaiacol conversion was
gradually increased, and almost 100% conversion could be
achieved over 20% SS-Ni/SiO2.
a
Reaction condition: (a) 0.2 g guaiacol, 0.1 g 20% SS-Ni/SiO2, 20
ml decalin, 3 MPa H2, 3 h; (b) 0.2 g guaiacol, 0.1 g 20% SS-Ni/SiO2,
20 ml decalin, 3 MPa H2, 120 °C; (c) 0.2 g guaiacol, 0.1 g 20% SS-
Ni/SiO2, 20 ml decalin, 120 °C, 2 h; (d) 0.2 g guaiacol, 20 ml
decalin, 2 MPa H2, 120 °C, 2 h.
With the 20% SS-Ni/SiO2 catalyst, the influences of reaction
temperature, reaction time, hydrogen pressure and catalyst
dosage had also been investigated (Fig. 6). Fig. 6a showed that
reaction temperature remarkably affected the guaiacol
hydrogenation process. The conversion of guaiacol rised up
gradually, with the increase of the reaction temperature. For
example, 63.7% guaiacol was hydrogenated at 100 °C, however,
when it reached to 120 °C, the guaiacol was completely
converted into cyclohexanol with 99.9% selectivity. This was a
mild temperature condition for the guaiacol hydrogenation
process in comparison with the reported literatures.11-14,25 The
reaction time also had a pronounced effect on the conversion of
guaiacol (Fig. 6b). In the investigated time range of 0.75 to 3.0 h,
the conversion of guaiacol gradually increased from 33.1% to
99.4%. Remarkably, the guaiacol conversion suffered a sharply
increase from 49.1% to 98.9% when the reaction time was
prolonged from 1.5 to 2.0 h. The induction period of the
heterogeneous catalyst were considered to be the main reasons
for this significant increase.50 This hydrogenation process was H2
consumed, therefore, the increase of H2 pressure favored the
guaiacol conversion. When the pushed H2 pressure was more
than 2 MPa, the increase of the performance was insignificant
because almost complete conversion of the feedstock (100%)
was achieved (Fig. 6c). And thus, 2 MPa hydrogen pressure was
obviously the most suitable for this hydrogenation process. Fig.
6d showed the influence of catalyst dosages. Obviously, the
augment of catalyst dosages meant more catalytic activity
centers, resulting in higher guaiacol conversion. For instance,
79.9% conversion was shown up over 0.06 g SS-Ni/SiO2
catalyst,while nearly 100% conversion was obtained when the
catalyst dosage increased to 0.1 g.
3.4. Hydrogenation of other typical phenolic compounds
In an extension to guaiacol, a series of other typical lignin-
derived phenolic compounds with various structures were
investigated under the optimized condition (120 °C, 2 MPa H2
for 2 h) over the SS-Ni/SiO2 catalyst. Table 4 listed the
hydrogenation results of seven kinds of lignin-derived phenolic
compounds in detail. As the representative phenolic
monomers, phenol, p-cresol and benzyl alcohol were all
completely converted into the corresponding saturated
alcohol with the selectivity up to 99.9% through the aromatic
ring hydrogenation (Table 4, Entry 1-3). The catalyst also
performed well on the hydrogenation of 2,4-dimethyl-phenol,
with 2,4-dimethylcyclohexanol as the major product and a
small amount of saturated alkane obtained (Table 4, Entry 4).
Unexpectedly, 5-eugenol and vanillin could not achieve the
hydrogenation of the aromatic ring, and that of the carbon
double bond and aldehyde group occurred instead. 2-
methoxy-4-propyl-phenol and 2-methoxy-4-methyl-phenol
were their major products, respectively (Table 4, Entry 5, 6). It
indicated that the species and position of the substituents in
aromatic ring affected the chemical property greatly. And the
carbon double bond and aldehyde group were more active
than the aromatic ring during this process, which accorded
well with the results of our previous study.51 Moreover, the
diphenyl ether, as the example of dimeric phenolic compounds,
was mostly converted into hydrogenation product rather than
6 | J. Name., 2012, 00, 1-3
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Journal Name
Table 4 Hydrogenation of other lignin-derived phenolic compounds over the SS-Ni/SiO2 catalyst.a
View Article Online
DOI: 10.1039/C5RA23041A
Entry
Substrates
Conversion(%)
Selectivity(%)
H
O
O
H
1
99.9
99.9
OH
OH
2
97.8
99.9
99.9
OH
OH
OH
3
4
98.5
96.4
H
O
HO
23.6
13.3
63.0
H
O
H
O
H
O
5
6
99.9
99.8
O
O
53.8
44.4
HO
HO
HO
H
O
O
O
O
O
89.0
26.2
4.1
3.4
O
H
O
O
7
95.8
25.4
46.8
a Condition: 0.2 g phenolic compounds, 0.1 g 20% SS-Ni/SiO2 catalyst, 20 ml decalin, 2 MPa H2, 120 °C, 2 h.
cyclohexane, which was much different from the previous responsible for it. Moreover, the TEM, STEM and EDS images
study of Zhang52 (Table 4, Entry 7). The much weaker acidity of of the catalysts before and after reaction (Fig. S7) were
the SS-Ni/SiO2 compared with Ru/HZSM-5, was considered to measured, where no significant change on the Ni particle size
be the main reason for this difference.
and morphology was exhibited after treatment as well. The FT-
In summary, the typical lignin-derived phenolic compounds IR spectra analysis of the fresh and recovered SS-Ni/SiO2
could be effectively converted into saturated alcohol with high catalyst were also compared. The characteristic infrared
yields, which suggested that the catalytic hydrogenation absorption peaks of repolymerized products could be found in
reaction routine of the phenolic compounds was highly atom the recovered catalyst. For example, the stretching of -OCH3
economic and energy efficient, and the SS-Ni/SiO2 catalyst was and -CH2 bonds (2952 and 2845 cm-1), and the vibration of
potential to apply widely for the upgrade of diverse phenolic aromatic ring (1606, 1514, 1462 and 843 cm-1)53,54 could be
compounds.
observed clearly (Fig. S8). Therefore, it was considered that the
formation of char and tar, which were caused by the
repolymerization of feedstocks,54,55 was the main reason for
the slight activity decrease. The FT-IR spectra also exhibited
that the peaks at 3400, 1630 and 675 cm-1 were enhanced,
indicating the H2O formation. And the interaction between the
catalyst and the formed water was also responsible for the
activity declination. To prove this conclusion, we calcined the
recovered catalyst after 4 runs at 500 °C under air atmosphere
and then reduced at 550 °C for 6 h at the flow of hydrogen.
Remarkably, the conversion had gone up back to 100% at the
next run. It indicated that the catalysts could be refreshed
through simple methods, which was a prominent advantage of
the catalyst for the further industrial application. Overall, the
SS-Ni/SiO2 catalyst verified its excellent recyclability clearly,
further suggesting the good stability of catalyst structure.
3.5. Catalyst recycle
The recyclability of the SS-Ni/SiO2 catalyst was also
investigated carefully. As shown in Fig. S5, the catalyst
exhibited an excellent recyclability for the first 3 runs, where
no significant decrease of the activity loss was detected (more
than 99.4%). However, in the fourth run, the conversion of
guaiacol was slight decreased to 92.1%. The SEM images of the
fresh and recovered SS-Ni/SiO2 (Fig. S6) demonstrated that
there was no significant change on the particle size and
morphology. However, a slight oily substance and char-liked
material were observed coating on the recovered catalyst
surface. The corresponding EDS results showed the carbon
increased remarkably and the Ni loading amount nearly did
not change. This was verified by the ICP-AES analysis result,
which showed no obvious Ni leaching in the solvent. The
strong interaction between Ni species and SiO2, which was
confirmed by the H2-TPR profiles (Fig. 2), was considered to be
4. Conclusion
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ARTICLE
Journal Name
An efficient and promising strategy for lignin-derived phenolic 14 J. X. Long, R. Y. Shu, Q. Y. Wu, Z. Q. Yuan, T. JV.iWewaAnrtigcl,eYO.nXlinue,
DOI: 10.1039/C5RA23041A
compounds conversion to the platform chemical compounds
and the low-cost high-grade biofuel was proposed via the high
X. H. Zhang, Q. Zhang and L. L. Ma, Energy Convers.
Manage., 2015, 105, 570-577.
selective catalytic hydrogenation process. The results showed 15 N. Yan, Y. Yuan, R. Dykeman, Y. Kou and P. J. Dyson,
that guaiacol could be completely converted to cyclohexanol Angew. Chem. Int. Ed., 2010, 49, 5549-5553.
with 99.9% selectivity over the step by step precipitated 16 D. D. Laskar, M. P. Tucker, X. W. Chen, G. Helms and B.
Ni/SiO2 catalyst at a mild condition of 120 °C, 2 MPa H2 Yang, Green Chem., 2014, 16, 897-910.
atmosphere for 2 h. Detailed investigation on the catalyst 17 W.Y. Wang, Z.Q. Qiao, K. Zhang, P.L. Liu, Y.Q. Yang, K. Wu,
indicated that the polyporous spherical structure were mainly RSC Adv., 2014, , 37288-37295.
responsible for the high catalytic activity, which led to the 18 J. B. Bredenberg, M. Huuska and P. Toropainen, J. Catal.,
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derived phenolic compounds at this mild condition. Moreover, 21 Y. C. Lin, C. L. Li, H. P. Wan, H. T. Lee and C. F. Liu, Energy
the catalyst showed an excellent recyclability, where no
Fuels, 2011, 25, 890-896.
significant loss of the catalytic activity was exhibited after 3 22 C. Zhao, H. Z. Wang, N. Yan, C. X. Xiao, X. D. Mu, P. J.
runs. And thus, this novel process is a beneficial reference for
Dyson and Y. Kou, J. Catal., 2007, 250, 33-40.
future utilization of the lignin-derived phenolic compounds 23 C. Zhao, Y. Kou, A. A. Lemonidou, X. B. Li and J. A. Lercher,
4
and the renewable lignin.
Angew. Chem. Int. Ed., 2009, 48, 3987-3990.
24 C. Zhao, J. Y. He, A. A. Lemonidou, X. B. Li and J. A.
Lercher, J. Catal., 2011, 280, 8-16.
25 W. J. Song, Y. S. Liu, E. Baráth, C. Zhao and J. A. Lercher,
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26 C. Zhao, S. Kasakov, J. Y. He and J. A. Lercher, J. Catal.,
2012, 296, 12-23.
27 C. F. Wu and P. T. Williams, Appl. Catal. B: Environ., 2011,
102, 251-259.
Acknowledgements
The authors gratefully acknowledge the financial support of
the National Key Technology R&D Program, (NO.
2014BAD02B01) and the Natural Science Foundation of China
(NO. 51476178).
28 C. F. Wu, Z. C. Wang, J. Huang, P. T. Williams, Fuel, 2013,
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29 A. D. Consuegra, P. M. Patterson and M. A. Keane, Appl.
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30 P. H. Blanco, C. F. Wu, J. A. Onwudili and P. T. Williams,
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31 A. Saadi, R. Merabiti, Z. Rassoul and M. M. Bettahar, J.
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32 Y. B. Hu, K. Tao, C. Z. Wu, C. Zhou, H. F. Yin and S. H. Zhou,
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Graphical abstract
An efficient and mild process for the hydrogenation of phenolic compounds over step by step
precipitated Ni/SiO2 catalyst was presented.