Low-Temperature Catalytic Hydrogenolysis of Guaiacol to Phenol over Al-Doped SBA-15 Supported Ni Catalysts

Article abstract of DOI:10.1002/cctc.202000712

Selective hydrogenolysis of aromatic carbon-oxygen (C_aryl?O) bonds is a key strategy for the generation of aromatic chemicals from lignin. However, this process is usually operated at high temperatures and pressures over hydrogenation catalysts, resulting in a low selectivity for aromatics and an extra consumption of hydrogen. Here, a series of Al-doped SBA-15 mesoporous materials with different Si/Al molar ratios (Al-SBA-15) were prepared via a post-synthesis method using NaAlO₂ as the Al source, and then Al-SBA-15 supported Ni catalysts (Ni/Al-SBA-15) were prepared by a deposition-precipitation method using urea as the hydrolysis reagent. The prepared supports and catalysts were extensively characterized using various techniques such as XRD, N₂ adsorption/desorption, TEM, ²⁷Al NMR, NH₃-TPD, XPS, H₂-TPR, and pyridine-FT-IR, and the catalysts were evaluated in the hydrogenolysis of the C_aryl?O bond in guaiacol and lignin derived compounds under mild conditions. The effects of the Si/Al ratio in catalyst and reaction parameters on guaiacol conversion and product distribution were investigated in detail, associated with solvent effect. The incorporation of Al into the framework of SBA-15 can improve the Lewis acidity and the dispersion of the supported Ni particles and yet modulate the metal-support interactions, which are propitious to the hydrogenolysis of the C_aryl?O bond in guaiacol. The catalyst Ni/Al-SBA-15 with a Si/Al molar ratio of 10 shows the best performance with a guaiacol conversion of 87.4 % and a phenol selectivity of 76.9 % under the mild conditions conducted, because of its proper acidity, suitable metal-support interactions, and high dispersion of the active species. The present study would stimulate research and development in multi-functional catalysts for the generation of valuable chemicals from biomass.

Full text of DOI:10.1002/cctc.202000712

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Low-Temperature Catalytic Hydrogenolysis of Guaiacol to Phenol
over Al-Doped SBA-15 Supported Ni Catalysts
Authors: Rui Ma, Qiuyue Wang, Yufang Chen, Guanheng Yang, Ping
Deng, Xinqing Lu, Yanghe Fu, and Weidong Zhu
This manuscript has been accepted after peer review and appears as an
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To be cited as: ChemCatChem 10.1002/cctc.202000712
Link to VoR: https://doi.org/10.1002/cctc.202000712
01/2020

10.1002/cctc.202000712
ChemCatChem
FULL PAPER
Low-Temperature Catalytic Hydrogenolysis of Guaiacol to Phenol
over Al-Doped SBA-15 Supported Ni Catalysts
Qiuyue Wang, Yufang Chen, Guanheng Yang, Ping Deng, Xinqing Lu, Rui Ma,* Yanghe Fu, Weidong
Zhu*[a]
[a]
Q. Y. Wang, Y. F. Chen, G. H. Yang, P. Deng, Dr. X. Q. Lu, Dr. R. Ma, Dr. Y. H. Fu, Prof. W. D. Zhu
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry
Zhejiang Normal University
Jinhua 321004, People’s Republic of China
E-mails: mr@zjnu.cn (R. Ma), weidongzhu@zjnu.cn (W. D. Zhu)
Abstract: Selective hydrogenolysis of aromatic carbon-oxygen (Caryl
–
oxygenates, and the cleavage of the Caryl-O bonds in lignin
currently represents one of the grand challenges due to the high
Caryl-O bond energy (218-314 kJ/mol).3 Hydrogenolysis is
regarded as an effectively upgrading strategy to the cleavage of
the Caryl-O bonds in lignin and lignin-derived oils, rendering them
closer in value to conventional fossil fuels or upgrading them to
high valued aromatics.
As the complex composition of raw lignin-derived phenolic
compounds, model compounds that have similar properties, such
as eugenol, syringol, and guaiacol, are chosen for the initial
screening of catalysts for the following upgrading of lignin-derived
phenolic compounds.4 Among which, guaiacol is regarded as a
model compound representing phenols derived from lignin, as
there exists both −OH and −OCH3 groups in guaiacol.5-10
Hydrogenolysis of guaiacol involves the direct removal of oxygen
from bio-oil via Caryl–O bond cleavage while it is challenging, since
high yields of aromatic hydrocarbons can only be achieved by
selectively cleaving the strong Caryl–O bond without
hydrogenating the aromatic ring.
Reports have shown that the catalyst property is a critical
factor regulating the hydrogenolysis route, as shown in Table S1.
Supported noble metals, such as Pd, Ru, Au, Rh, and Re, as well
as base metals, such as Co, Ni, Fe, and their heterometallic alloys,
are found to possess a high hydrogenolysis and hydrogenation
activity, but some undesirable reactions, including the
hydrogenation of benzene ring, C-C bond hydrogenolysis as well
as methanation, also prevail on them, resulting in a low aromatics
yield and a high H2 consumption.5-10 Hydrodeoxygenation using H
donors (e. g. alcohols, formic acid and water) has been
considered as a promising method to improve the selectivity
towards aromatic hydrocarbons over supported noble metals.5g, h
Mo-based hydrotreating catalysts, e.g., CoMo sulfide, NiMo oxide,
MoO3, and Mo2C catalysts, have been investigated for the
hydrogenolysis of guaiacol, but demand high H2 pressures and
high temperatures. Besides, sulfur-containing reagents are
required to add to the feedstock to keep sulfide catalysts from
deactivation, consequently causing the sulfur contaminated
products.7 Recently, supported Ni-based catalysts are recognized
as efficient and low cost ones in the hydrogenolysis of guaiacol
and a range of materials have been reported as catalyst supports,
including γ-Al2O3, mesoporous SiO2, ZrO2, zeolites, TiO2, and
activated carbons.8-10 Zhou et al.9 reported that guaiacol could be
converted into cyclohexanol over Co-promoted Ni/γ-Al2O3
catalysts at 200 oC and a H2 pressure of 5 MPa and some proper
acidity and interactions between metal species and support would
O) bonds is a key strategy for the generation of aromatic chemicals
from lignin. However, this process is usually operated at high
temperatures and pressures over hydrogenation catalysts, resulting
in a low selectivity for aromatics and an extra consumption of
hydrogen. Here, a series of Al-doped SBA-15 mesoporous materials
with different Si/Al molar ratios (Al-SBA-15) were prepared via a post-
synthesis method using NaAlO2 as the Al source, and then Al-SBA-
15 supported Ni catalysts (Ni/Al-SBA-15) were prepared by a
deposition-precipitation method using urea as the hydrolysis reagent.
The prepared supports and catalysts were extensively characterized
using various techniques such as XRD, N2 adsorption/desorption,
TEM, 27Al NMR, NH3-TPD, XPS, H2-TPR, and pyridine-FT-IR, and the
catalysts were evaluated in the hydrogenolysis of the Caryl-O bond in
guaiacol and lignin derived compounds under mild conditions. The
effects of the Si/Al ratio in catalyst and reaction parameters on
guaiacol conversion and product distribution were investigated in
detail, associated with solvent effect. The incorporation of Al into the
framework of SBA-15 can improve the Lewis acidity and the
dispersion of the supported Ni particles and yet modulate the metal–
support interactions, which are propitious to the hydrogenolysis of the
Caryl-O bond in guaiacol. The catalyst Ni/Al-SBA-15 with a Si/Al molar
ratio of 10 shows the best performance with a guaiacol conversion of
87.4% and a phenol selectivity of 76.9% under the mild conditions
conducted, because of its proper acidity, suitable metal-support
interactions, and high dispersion of the active species. The present
study would stimulate research and development in multi-functional
catalysts for the generation of valuable chemicals from biomass.
Introduction
Lignin is constructed from methoxylated phenylpropanoid
subunits bridged by numerous ether linkages (C-O-C) as well as
hydroxyl (–OH) and methoxyl (–OMe) side groups.1 The utilization
of lignin as a sustainable resource for the production of fuels and
chemicals has garnered much attention in the last decade. Lignin-
derived oils, which contain aryl ether fragments (Caryl-O bonds),
are produced from lignin by thermal and catalytic processes of
polymer fragmentation.2 However, the wide application of lignin-
derived oils is limited due to several drawbacks such as low
energy density and thermal stability, immiscibility with
hydrocarbons and wide variation in boiling points, which are
mainly caused by the high content of Caryl-O bonds.3 Unfortunately,
these molecules are much more refractory than sugar-derived
1
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10.1002/cctc.202000712
ChemCatChem
FULL PAPER
be helpful for the hydrogenolysis of guaiacol. Broglia et al.10
investigated the effects of the support surface properties of
Ni/Al2O3-SiO2 on the hydrogenolysis of guaiacol into phenol and
found that both guaiacol conversion and product distribution were
markedly affected by the morphological and surface features of
the support, and the high conversion of guaiacol was achieved up
to 84% at a H2 pressure of 5 MPa and 300 oC. As a general trend,
the appropriate acidity of catalysts is favorable for enhancing their
performance in the hydrogenolysis of guaiacol.5,6 In addition, the
strong metal–support interactions could modulate the stacked
morphology and degree of active species to produce more active
centers with a high activity. Unfortunately, recent research has
been mainly focused on pure oxides as supports and the
hydrogenolysis has been mainly carried out at relatively harsh
conditions like reaction temperatures higher than 200 °C and H2
pressures higher than 5 MPa, resulting in the high consumption
of H2 and full aromatic-ring saturation. Thus, it is imperative to
develop Ni-based catalysts with high catalytic activities for the
hydrogenolysis of guaiacol that could be achieved at relatively low
temperatures and H2 pressures.
leading to an increase in the lattice parameters and the formation
of Al-O-Si structure to some extent. In all the catalysts, the weak
diffraction peaks of metallic Ni are observed at 44.3o, 51.9°, and
76.4°, respectively, reflecting the high dispersion of Ni in the
reduced catalysts. Additionally, the peaks at 2θ = 35.2o and 61.2o,
ascribed to NiO species, respectively, are also observed in the
XRD pattern, indicating that both metal Ni and NiO species
appear on the surface of the support. It can be seen that the
intensity of the diffraction peaks of metallic Ni decreases with
increasing the incorporating amount of Al into SBA-15, suggesting
that the incorporation of Al would lead to the better dispersion of
metallic Ni species on the support. A similar behavior was also
observed on Al-modified mesoporous SiO2 supported NiMo
catalysts.13 Consistently, the diffraction peaks that are
characteristic for Al2O3 are not detected in the current samples.
In this work, we report high selectivity and stability Ni
supported catalysts with a high H2-utilization efficiency for the
hydrogenolysis of guaiacol to phenol at mild conditions. A series
of Al-doped SBA-15 materials with various Si/Al ratios (Al-SBA-
15) were prepared via a post-synthesis method with sodium
aluminate (NaAlO2) as the Al source for effectively incorporating
tetrahedral aluminum species into pure silica SBA-15 in a basic
condition, and the prepared Al-SBA-15 materials were used as
supports to synthesize Ni/Al-SBA-15 catalysts via a deposition-
precipitation method, and these prepared supports and catalysts
were then characterized by different techniques. It is found that
the incorporation of Al species into SBA-15 framework can
increase the Lewis acidity and yet enhance the metal–support
interactions to improve the catalytic activity. The synergistic
effects of the incorporated Al species and Ni species on the
hydrogenolysis were elucidated. Further, the effects of the
reaction parameters, such as type of solvent, temperature, and
initial H2 pressure, on guaiacol conversion and product
distribution were investigated in detail.
Results and Discussion
Figure 1. (a) Small-angle and wide-angle XRD patterns of the
reduced catalysts, (b) TEM image and particle size distribution of
Ni/Al-SBA-15(10), (c) TEM image and particle size distribution of
Ni/Al-SBA-15(30), and (d) TEM image and particle size
distribution of Ni/Al-SBA-15(50).
Characterization of Supports and Catalysts. The typical small-
angle and wide-angle XRD patterns of the reduced Ni/SBA-15
and Ni/Al-SBA-15 with different Si/Al ratios are shown in Figure 1-
a1-2. Three strong diffraction peaks are observed with 2θ at
around 0.85°, 1.42°, and 1.65°, respectively, indexed as (100),
(110), and (200) diffractions of SBA-15, reflecting the presence of
The TEM images and size distributions of Ni species
(including metallic nickel and nickel oxides) supported on Al-SBA-
15 are shown in Figure 1b-d. For all the samples, the
characteristic mesostructure of Al-SBA-15 is visible, and the
particles of Ni species can be clearly differentiated ranging from
2.0 to 10.0 nm. According to the surface area weighted average
diameter, the Ni particle sizes of the samples are further
calculated as follows:
a
typical well-ordered two-dimensional hexagonal P6mm
mesostructured phase.11 The intensity of (110) and (200)
diffraction peaks increases while the (100) diffraction peak keeps
constant with a slightly shift toward the low angle side with
decreasing the Si/Al molar ratio from 50 to 10, demonstrating that
the structural order is enhanced due to the incorporation of Al into
the framework of SBA-15, in correspondence with the observation
by Chang and Lin.12 The shift corresponds to an increased length
of the unit cell α0 from 11.11 to 12.10 nm, calculated from the
ds= ∑ nid3i /∑ nid2i
(1)
formula α0=2d100√3 as shown in Table S2. The shifting of the
Bragg angle to a slightly low value indicates that some Al3+
species (rAl3+=0.054 nm) enter the lattice of SiO2 (rSi4+=0.040 nm),
where ni is the number of the counted particles of Ni species with
diameter di, in which the total number of the counted particles of
Ni species was more than 80.
2
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10.1002/cctc.202000712
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The average Ni particle size is calculated to be 7.6 nm in
oxygen bridges, the so-called ‘‘bridging hydroxyl groups”, are
helpful for improving the total acidity and yet creating the acidic
sites.13,14
diameter in Ni/Al-SBA-15(50). Additionally, the average particle
size of Ni species in the catalysts remarkably decreases from 7.6
to 3.8 nm with decreasing the Si/Al molar ratio in Al-SBA-15 from
50 to 10, confirming that the incorporation of Al would improve the
dispersion of Ni species on the support further, which is in good
accordance with the XRD results presented in Figure 1a. Besides,
with increasing the incorporated Al amount in SBA-15, the catalyst
possesses a much narrower size distribution of the particles of Ni
species on the support, implying that the interactions between Ni
species and support become stronger.
The results on the N2 adsorption−desorption isotherms and
the corresponding pore size distributions of the supports and
catalysts are presented in Figure S1 and Table S2. The hysteretic
capillary condensation is observed in a relative pressure range
from 0.66 to 0.9, suggesting the presence of characteristic
mesopores in all the samples investigated. Additionally, Ni/SBA-
15 has a narrow pore size distribution with the most probable pore
diameter centered at 8.3 nm, nearly identical to that of SBA-15,
reflecting that the incorporated Ni species onto the support do not
markedly affect the mesostructure of SBA-15.
The H2-TPR study was carried out to investigate the
reducibility of Ni oxides on the support, and these results are
shown in Figure 2b. The H2-TPR profile of the reference sample
NiO shows a sharp H2 consumption peak at ca. 340 °C, ascribed
to the reduction of crystalline NiO. In contrast, there is a broad
o
reduction peak in a temperature range of 400-600 C in the H2-
TPR profile of calcined Ni/SBA-15, suggesting the presence of the
reducible Ni oxides that strongly interact with SBA-15.
Interestingly, when Al-SBA-15 materials with different Si/Al ratios
are used as the supports, there are the significant shifts of the H2
consumption peaks in the H2-TPR profiles of calcined Ni/Al-SBA-
15 catalysts to higher temperature ranges, indicating that the Ni
oxide species are more stable, compared to those in Ni/SBA-15.
Besides, an up-shift of the reduction peak in terms of temperature
is observed, when the Si/Al molar ratio in Al-SBA-15 is decreased,
suggesting that the interactions between Ni oxide species and
support would be enhanced because of more surface-attached
hydroxyl groups and Al-O-Ni bonds available with increasing the
incorporated amount of Al into SBA-15. These pronounced
interactions between Ni oxide species and support, consequently,
lead to an increase in the dispersion of Ni species in the catalysts,
which could help to inhibit the thermal transmigration and sintering
of the metal species during calcination and reduction. A similar
observation is reported in Al-modified mesoporous SiO2
supported Ni catalysts.12,13 Therefore, the effects of the
incorporated Al in SBA-15 on the dispersion of Ni species on the
support would inevitably contribute to the improvement in the
catalytic activity.
The number and strength of the acid sites on SBA-15 and
Al-SBA-15 supported Ni catalysts were measured by NH3-TPD.
The acid strength is classified to be weak (100–250 oC), medium
o
o
(250–400 C), and strong (>400 C) in terms of the desorption
temperature of NH3.15 As shown in Figure 2c, only the weak acid
sites are detected in Ni/SBA-15 at ca. 180 oC and 140 oC,
respectively, while Ni/Al-SBA-15 catalysts show similar profiles
with a main broad asymmetric NH3 desorption peak at ca. 280 oC
o
and an additional wide, small shoulder centered at ca. 190 C,
corresponding to medium and weak acid sites, respectively,
suggesting that the incorporation of Al species can improve the
acid strength of the catalyst. The medium acid sites,
corresponding to the desorption peak at ca. 280 °C, supply the
active sites for the cleavage reaction of ether bonds in guaiacol
and lignin model compounds.16 Moreover, the total acid numbers
of the catalysts can be calculated by the desorbed amounts of
NH3, and these results are summarized in Table S2. A substantial
improvement in the acid number of Ni/Al-SBA-15 is clearly
observed, which is enhanced with increasing the incorporated Al
amount into SBA-15.
Although it has been reported that the total number and
strength of acid sites on various catalysts are closely related to
their catalytic activities for the hydrogenolysis and/or
hydrogenation of guaiacol, the contribution of separate acid sites,
such as Brønsted and Lewis acid sites, to the ether bond cleavage
reaction activities of guaiacol has not been well-characterized
until now.3,10 Therefore, the pyridine adsorption IR measurements
were performed to further determine the Brønsted and Lewis
acidities of all the reduced catalysts, and these results are shown
in Figure 2d. The peaks at 1454 and 1623 cm-1 are assigned to
Figure 2 27Al MAS NMR spectra (a) of the reduced catalysts, H2-
TPR profiles (b) of NiO and the calcined catalysts, NH3-TPD
profiles (c) and pyridine adsorption FT-IR spectra (d) of the
reduced catalysts.
27Al-NMR spectroscopy was performed to analyze the
existent states and distribution of Al species in SBA-15. In general,
the chemical shifts at 0 and 53 ppm are assigned to octahedrally
and tetrahedrally coordinated Al, respectively.12,13 As shown in
Figure 2a, only one sharp peak at 53 ppm is observed, assigned
to a tetrahedrally coordinated Al framework in Al-SBA-15. This
indicates that all the Al atoms are located in a tetrahedral
environment and covalently bound to Si atoms via oxygen bridges,
confirming that Al species are successfully incorporated into the
framework of SBA-15 via the post-synthesis method. Moreover,
the tetrahedrally coordinated Al species, linking Si atoms via
3
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10.1002/cctc.202000712
ChemCatChem
FULL PAPER
pyridine adsorbed onto the Lewis acid sites, suggesting that the
coordinatively unsaturated Al3+ sites are present on the surface of
SBA-15.17 The Brønsted acid sites are not observed on Ni/Al-
SBA-15, due to the absence of the adsorption peaks at 1545 and
1640 cm-1, respectively.18 These results are different from those
in the literature,18 in which both Brønsted and Lewis acids are
present on the surface of Al-doped SBA-15 prepared using
aluminum isopropoxide or aluminum nitrate as the Al source. In
the current case, the absence of the Brønsted acid sites in the
catalysts is due to the compensated Na+ for the negative charges
of the framework of Al-SBA-15.
Scheme 1. Possible reaction network for guaiacol conversion.
Catalytic Conversion of Guaiacol. Herein the primary products
of guaiacol conversion over the Ni-based catalysts were
determined at 120 oC in aqueous phase. The guaiacol conversion
and product distribution are presented in Figure 4. Phenol and
catechol are the primary aromatic products, and cyclohexanol, 2-
methoxy cyclohexanol, and cyclohexanone as hydrogenation
products are the byproducts. Besides, methanol and a small
amount of methane were also detected. This product distribution
suggests that the selective hydrogenolysis of the Caryl-O bond
forms phenol, the hydrolysis of guaiacol produces catechol, the
hydrogenation of guaiacol and phenol forms 2-methoxy
cyclohexanol, cyclohexanone, and cyclohexanol, see the reaction
network shown in Scheme 1. The properties of the catalyst
support used significantly affect both guaiacol conversion and
product selectivity. The different Ni particle sizes and interactions
between Ni species and support in the catalysts revealed by the
TEM (Figure 1) and H2-TPR (Figure 2b) characterizations are
responsible for the difference in the catalytic activity, showing that
the better dispersion of the metallic Ni on the support, the higher
guaiacol conversion and phenol selectivity, and Ni/Al-SBA-15(10)
shows the best catalytic performance in terms of guaiacol
conversion and phenol selectivity. These results are in good
accordance with those observed by Zhou and Schutlser.9,21
Figure 3. Ni 2p XPS spectra (a), Al 2p XPS spectra (b) and the
Ni2+/Ni0 atomic ratio versus the Si/Al molar ratio in the reduced
catalysts, I: Ni/Al-SBA-15(50), II: Ni/Al-SBA-15(30), III: Ni/Al-SBA-
15(10), and IV: Ni/SBA-15.
The surface element compositions of Ni/SBA-15 and Ni/Al-
SBA-15 were analyzed by the in-situ XPS technique, and Ni 2p
and Al 2p XPS spectra and corresponding binding energies of the
catalysts are shown in Figure 3 and Table S3. The Ni0 2p3/2, Ni0
2p1/2, and their satellite core level binding energies appear at ca.
853.1, 870.1, 862.4, and 881.8 eV, respectively.19 The peaks at
857.5 and 874.4 eV are assigned to Ni2+ 2p3/2 and Ni2+ 2p1/2
,
suggesting that both metallic Ni and NiO are present on the
surface of support,19 in accordance with the analysis of the XRD
patterns (Figure 1a-2). Compared with a binding energy of 852.8
eV (Ni0 2p3/2) of unsupported metallic Ni, the positive shift of the
binding energy suggests the stronger interactions between Ni and
support available, see the results shown in Table S3. It should be
stressed that the different Al amounts incorporated into SBA-15
do not affect the Ni 2p binding energy shift, indicating that there is
no electronic effect of Al on Ni. The Ni2+/Ni0 atomic ratio in Ni/Al-
SBA-15 is increased, compared with that in Ni/SBA-15, indicating
that the incorporation of Al into SBA-15 may have an adverse
effect on the reduction of NiO species into metallic Ni, as shown
in Figure 3c. The H2-TPR results (Figure 2b) also confirm that the
reducibility of Ni/Al-SBA-15 decreases because of its reduction
peak at ca. 650 °C while that at 540 °C for Ni/SBA-15, which is in
good accordance with the XPS results. Deconvolution of the Al 2p
spectrum of Ni/Al-SBA-15 reveals one peak at a binding energy
of 74.4 eV, corresponding to the tetrahedral Al3+ state, implying
that the Al3+ is covalently bound to four Si atoms via oxygen
bridges.13,20
Figure 4. Guaiacol conversion and product distribution over (A)
Ni/SBA-15, (B) Ni/Al-SBA-15(50), (C) Ni/Al-SBA-15(30), and (D)
Ni/Al-SBA-15(10). Reaction conditions: guaiacol (3.00 g),
catalyst (0.20 g), H2O (100 mL), 120 oC, 3 h, initial H2 pressure
(0.1 MPa) and stirring at 700 rpm.
4
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The acid sites could act as an H· radical acceptor and
transferrer, which could greatly increase the reaction rate by the
adjustment of the H· transfer length in the hydrogenolysis of
guaiacol at low H2 pressures over RuNi/HZSM-5 catalysts.8 Here,
over Ni/SBA-15, the guaiacol conversion is only 42.5% and the
main product is methoxy cyclohexanol with a selectivity of 72.8%,
see Figure 4. When Al-SBA-15 materials are used as the supports,
a significant change in the product distribution is observed. Over
Ni/Al-SBA-15(50), the guaiacol conversion is increased up to
68.9%, meanwhile the main products are catechol and phenol
with selectivities of 43.2% and 24.2%, respectively. When the
incorporated amount of Al into SBA-15 reaches its maximum in
the current investigation, i.e. a Si/Al molar ratio of 10 in Al-SBA-
15, the highest conversion of guaiacol and the highest selectivity
for phenol, 87.4% and 76.9%, respectively, are achieved.
Combined with the results on the Lewis acid characterization
(Figure 2c and d), where a substantial improvement on the Lewis
acid number associated with a decrease in the Lewis acid
strength is observed with increasing the incorporated Al amount
into SBA-15, indicating that the strong acid sites would be
responsible for the catalytic reaction to form catechol while the
moderate Lewis acid sites would be favorable for the
hydrogenolysis of guaiacol to produce phenol. This observation is
in good accordance with Yu’s results,22 in which the higher
conversion of guaiacol could be obtained when the support Al-
SBA-15 with a lower Si/Al molar ratio was used.
Figure 5. Effects of reaction temperature on guaiacol conversion
and product distribution over Ni/Al-SBA-15(10). Reaction
conditions: guaiacol (3.00 g), Ni/Al-SBA-15(10) catalyst (0.20 g),
H2O (100 mL), 3 h, initial H2 pressure (0.1 MPa) and stirring at
700 rpm.
The effects of reaction temperature on the guaiacol
conversion and product distribution over Ni/Al-SBA-15 (10) are
presented in Figure 5. The guaiacol conversion is significantly
increased from 27.6 to 98.8% with increasing the reaction
temperature from 80 to 160 °C. On the other hand, the guaiacol
conversion is dramatically dropped to 4.7% as the reaction
o
temperature is increased up to 200 C. This is mainly due to the
Table 1. Effects of solvent on guaiacol conversion and product
distributiona
structure collapse of the catalyst under such harsh hydrothermal
conditions, as shown in Figure S2. In a temperature range of 80–
160 °C, an increase in the reaction temperature results in a
decrease in the selectivity for phenol from 80.4% to 42.6% while
an enhanced selectivity for cyclohexanol from 3.1% to 28.7%.
These results indicate that low temperatures are favorable for the
hydrogenolysis of guaiacol into phenol while high temperatures
are favorable for the aromatic-ring hydrogenation, in good
accordance with the observation by Zhou et al.9 and Nakagawa
et al.17 In summary, in the current study, the hydrogenolysis of
guaiacol over Ni/Al-SBA-15(10) has a conversion of 87.4% with a
phenol selectivity of 76.9% at 120 oC and an initial H2 pressure of
0.1 MPa, which can be considered as the best results in terms of
phenol yield over a non-noble metal based catalyst under the mild
conditions.
Entr
y
Conv.
(%)
Product Sel. (%)
Solvent
1
2
3
4
5
1
2
3
Water
Ethanol
n-Hexane
87.4
48.2
31.2
76.9 3.6
6.8
6.3
2.6
5.8
2.6
3.2 21.3 57.8 6.5
2.7 24.8 61.2 5.3
a Reaction conditions: guaiacol (3.00 g), solvent (100 mL), Ni/Al-
SBA-15 catalyst (0.20 g), initial hydrogen pressure (0.1 MPa),
120 °C, 3 h, and stirring at 700 rpm.
In this study, the effects of different solvents, such as water,
ethanol, and n-hexane, on the conversion of guaiacol over Ni/Al-
SBA-15(10) were investigated, and these results are shown in
Table 1. The maximum guaiacol conversion is 87.4% with a
phenol selectivity of 76.9% using water as the solvent. On the
other hand, the guaiacol conversion is only 48.2% or 31.2% using
ethanol or n-hexane as the solvent. This may be related to the
relatively weak polarity of ethanol and n-hexane in comparison
with that of water. These results on the effects of the used solvent
are in good agreement with those observed by Ma et al.7b and
Zhou et al.9 Differently, phenol, produced with a high selectivity of
76.9% in water as the solvent, is produced with a selectivity of
only 3.8% (2.6%) using ethanol (n-hexane) as the solvent.
Guaiacol is converted to 2-methoxy cyclohexanol and
cyclohexanol with selectivities of 21.3% and 57.8%, respectively,
using ethanol as the solvent, and when n-hexane is used as the
solvent, a higher total selectivity of 86.0% for 2-methoxy
cyclohexanol and cyclohexanol is obtained. Therefore, it seems
that the aromatic-ring hydrogenation would dominate the reaction
pathway, when either ethanol or n-hexane is used as solvent.
Figure 6. Effects of initial H2 pressure on guaiacol conversion and
product distribution over Ni/Al-SBA-15(10). Reaction conditions:
5
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10.1002/cctc.202000712
ChemCatChem
FULL PAPER
guaiacol (3.00 g), Ni/Al-SBA-15(10) catalyst (0.20 g), H2O (100
mL), 120 oC, 3 h, and stirring at 700 rpm.
selectivity for 2-methoxy-cyclohexanol increases with increasing
the initial H2 pressure, suggesting that excessive H2 would
contribute to the hydrogenation of the aromatic ring in phenol and
guaiacol. In addition, increasing initial H2 pressure would also
enhance the solubility of H2 into the reaction solution, promoting
the hydrogenation of the hydrodeoxygenation products.
The effects of initial H2 pressures varying from 0.1 to 1.0
MPa on the guaiacol conversion and phenol selectivity were
investigated at 120 °C over Ni/Al-SBA-15(10), see Figure 6. The
guaiacol conversion increases slightly from 84.4% to 92.3% while
the selectivity for phenol decreases dramatically from 76.9% to
2.1% with increasing the initial H2 pressure from 0.1 to 1.0 MPa.
On the other hand, the selectivity for cyclohexanol reaches its
maximum at an initial H2 pressure of 1.0 MPa. Moreover, the
Table 2. Products distribution for hydrogenolysis of different model compounds over Ni/Al-SBA-15(10)a
Entry
1
Substrate
Conv. (%)
94.3
Selectivity for the main products (%)
Others
3.3
83.6
75.7
3.4
9.2
4.5
2.4
5.2
4.6
Others
3.8
2
3
86.7
77.2
4.3
4.8
Others
3.2
65.8
62.1
17.3
26.6
6.2
2.2
2.7
4.6
Others
2.7
4
5
80.2
82.8
1.8
Others
3.7
82.7
4.9
6.3
2.4
Others
3.2
6
76.8
63.6
25.4
2.1
3.8
1.9
a Reaction conditions: substrate (3.00 g), H2O (100 mL), catalyst (0.20 g), initial hydrogen pressure (0.1 MPa), 120 °C, 3 h, and stirring
at 700 rpm.
On the basis of the understanding of the model reaction
based on guaiacol that contains methoxy group and phenolic
hydroxy group, it was nevertheless important to determine
whether other varieties of Caryl-O bonds can be cleaved over the
currently developed catalysts under the same reaction conditions.
Therefore, the hydrogenolysis reactions of other model
compounds over Ni/Al-SBA-15(10) were performed and the
corresponding results are summarized in Table 2. As expected,
1,3-dimethoxybenzene and 1,3,5-trimethoxybenzene are
converted into benzene with anisole as the intermediate, which is
principally converted to benzene with a selectivity of 83.6%. On
the other hand, 1,3,5-trimethoxybenzene has a lower conversion,
6
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10.1002/cctc.202000712
ChemCatChem
FULL PAPER
compared to 1,3-dimethoxybenzene and anisole, which can be
contributed to the enhanced Caryl-O bond energy caused by the
extra electron-donating group (-OCH3).23 For the conversion of
syringol, partially deoxygenated products such as phenol,
guaiacol, catechol, and methoxy cyclohexanol are identified by a
liquid product analysis, among which phenol is the main product
with a selectivity of 62.1% and guaiacol is as demethoxylation
(removal of -OCH3) product from the onset of the reaction with a
selectivity of 26.6% by the end of the reaction. In addition, the
hydrogenolysis reactions of 4-n-propylguaiacol and 4-n-
propylsyringol were also performed, and it is found that 4-n-
propylphenol is the main product via demethoxylation. Based on
the above results, it can be concluded that Ni/Al-SBA-15(10)
possesses some excellent catalytic properties for cleavaging the
Caryl-O bonds in lignin-derived compounds.
as the solvent helps the hydrogenolysis of guaiacol to form phenol
while the weak polarity ethanol or n-hexane as the solvent
enhances the hydrogenation of guaiacol to form 2-methoxy
cyclohexanol and cyclohexanol. Low reaction temperatures are
favorable for the hydrogenolysis of guaiacol into phenol while high
reaction temperatures are favorable for the aromatic-ring
hydrogenation. Furthermore, low initial H2 pressures are favorable
for the hydrogenolysis of guaiacol into phenol. In addition, the
developed catalyst also show some excellent performance in the
hydrogenolysis of other model compounds. The current work,
therefore, indicates that the Al-doped SBA-15 supported Ni
catalyst, showing a high activity, selectivity for phenol, and
stability under the mild reaction conditions, would be alternatives
of sulfide or noble-metal catalysts for the upgrading of lignin.
Table 3. Reusability of Ni/Al-SBA-15(10)a
Experimental Section
Chemicals. All of the used reagents were of analytic purity grade
and purchased from Sinopharm Chemical Reagent Co., Ltd.
Materials Preparation. SBA-15 was prepared following the
method reported in the literature.11 Typically, 8.00 g of the
surfactant P123 (EO20PO70EO20, average molecular weight =
5800) was added to 300 mL of 1.6 M HCl at 40 °C until completely
dissolved, and 18.00 g of tetraethyl orthosilicate (TEOS) was then
added. The mixture was further stirred at 40 °C for 24 h.
Afterwards it was charged into a Teflon-lined stainless autoclave
and heated at 100 °C for 48 h. After the resultant precipitate was
collected by filtration, the cake was washed with ethanol and
deionized water, and finally dried in an oven at 80 °C for 10 h and
calcined in static air at 550 oC for 6 h.
Product Sel. (%)
Entry
Conv. (%)
1
2
3
4
5
1
2
3
4
5
87.4
86.5
84.7
85.4
84.7
76.9
76.4
75.8
76.2
76.5
3.6
3.4
3.3
3.4
3.5
6.8
6.7
6.8
6.4
6.2
6.3
6.4
6.2
6.1
6.5
2.6
2.4
2.7
2.3
2.5
a
Reaction conditions: guaiacol (3.00 g), H2O (100 mL), catalyst
(0.20 g), initial hydrogen pressure (0.1 MPa), 120 °C, 3 h, and
stirring at 700 rpm.
In addition,
a five-run experiment of the selective
Al-SBA-15 supports with different molar ratios of Si/Al were
synthesized via a post-synthesis method using sodium aluminate
(NaAlO2) as Al source and deionized water as solvent. A typical
synthesis of Al-SBA-15 was performed as follows: a certain
amount of NaAlO2 was dissolved in 250 mL of deionized water
under magnetic stirring. Then 2.00 g of the above-synthesized
SBA-15 was added into the solution and the mixture was stirred
for 12 h at 40 C. All the synthesized Al-SBA-15 materials with
different Si/Al molar ratios were obtained after calcination in static
air at 550 oC for 6 h.
hydrogenolysis of guaiacol over Ni/Al-SBA-15 was carried out to
check the reusability (Table 3). The catalyst was filtered from the
reaction medium after each run and dried at 80 °C after washed
with ethanol. Then the used catalyst was reused in the next run
for the hydrogenolysis of guaiacol under the same reaction
conditions. Obviously, the recycling use of the catalyst for five
runs shows no obvious change in the catalytic activity and
selectivity, suggesting that the catalyst is stable enough during
the hydrogenolysis.
o
The supported Ni/Al-SBA-15 catalysts were prepared by a
deposition-precipitation method with urea as the hydrolysis agent,
according to the procedure reported in the literature.22 In a typical
synthesis, 2.47 g of Ni(NO3)2·6H2O was added to 250 mL of
deionized water, and the mixture was divided into two parts: 6.30
Conclusion
In summary, the selective hydrogenolysis of the Caryl–O
bond in guaiacol was symmetrically carried out over Al-doped
SBA-15 supported Ni catalysts under the relatively mild conditions. g of urea was added into the first part with 50 mL of the mixture,
The results show that the incorporation of Al into the framework
of SBA-15 can improve the Lewis acidity and the dispersion of the
supported Ni particles and yet modulate the metal–support
interactions, which are propitious to the hydrogenolysis of the
Caryl-O bond in guaiacol. When the incorporated amount of Al into
SBA-15 reaches its maximum in the current investigation, i.e. a
Si/Al molar ratio of 10 in Al-SBA-15, the highest conversion of
guaiacol and the highest selectivity for phenol, 87.4% and 76.9%,
respectively, are achieved. In addition, the type of the solvents
used in the reaction would significantly affect the guaiacol
conversion and phenol selectivity, and the strong polarity water
and 2.00 g of Al-SBA-15 and the second part of the mixture were
then together put into a flask at 90 °C, in which the pH was
adjusted to 2 with diluted HNO3. The solution with urea was then
added to the flask with Al-SBA-15 at a flow rate of 1 mL/min, after
which the suspension was magnetically stirred for 12 h. The
suspension was then cooled to room temperature, and the solid
was filtered and washed with distilled water three times. Finally,
the sample was dried at 120 °C for 24 h, calcined in flowing air at
550 °C for 6 h, and reduced in flowing H2 at 400 °C for 2 h. The
obtained sample was nominated as Ni/Al-SBA-15(x), in which x
represents the molar ratio of Si/Al in Al-SBA-15.
7
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10.1002/cctc.202000712
ChemCatChem
FULL PAPER
Characterization. The powder X-ray diffraction (XRD) patterns of
fresh catalyst, and the conversion and selectivity were calculated
based on the average of the five-times measurements and
relative error was within 1%.
the samples were obtained on
a D8 Advanced X-ray
diffractometer (Bruker) using with a Cu Kα radiation source (λ =
0.15406 nm) at a voltage of 40 kV and a current of 40 mA. The
morphological and structural details of the samples were
investigated using
a
FEI Talos-S transmission electron
Acknowledgements
microscope (TEM) with an accelerating voltage of 200 kV. The
textural properties of the samples were determined by N2
adsorption−desorption isotherms at -196 °C using a Micromeritics
ASAP 2020 instrument. The H2-TPR and NH3-TPD
measurements of the calcined samples were carried out on a
This work was financially supported by Zhejiang Provincial
Natural Science Foundation of China (LQ17B060001) and
National Natural Science Foundation of China (21706239).
Micromeritics AutoChem 2920 instrument with
a thermal
Keywords: Guaiacol • Selective hydrogenolysis • Phenol • Ni-
Based catalysts • SBA-15
conductivity detector. The solid-state NMR spectra of the samples
were recorded on a Bruker 400 MHZ Ultra-Shield spectrometer
with a resonance frequency of 104.3 MHz for 27Al detection. The
adsorption of pyridine was performed on a Thermo Fisher iS 50
FT-IR spectrometer equipped with a DTGS detector at an optical
resolution of 4 cm-1 by carrying out 64 scans. The surface analysis
of the reduced catalysts was conducted with an X-ray
photoelectron spectrometer (XPS) (ESCALAB 250, Thermo
Fisher) equipped with a monochromatized Al Kα X-ray source (hv
= 1486.6 eV) and a pass energy of 40 eV.
Catalytic Hydrogenolysis. The catalytic reaction was carried out
with a 300 mL batch reactor (Parr 4566, made of Hastelloy)
equipped with a temperature controller (Parr 4848). In a typical
run, 3.00 g of guaiacol, 0.20 g of catalyst, and 100 mL of deionized
water were loaded into the reactor. The reactor was then
evacuated and purged with high-purity nitrogen for five times and
finally pressurized to the desired pressure with hydrogen. The
sealed reactor was then heated to 120 oC with a stirring speed of
700 rpm. After the reaction, the reactor was quenched to ambient
temperature using piping water, and the liquid products were
collected using ethanol as solvent and analyzed by an Agilent
7890B gas chromatography (GC) with a flame ionization detector
and an Agilent 7890-5975 GC−mass spectroscopy (GC−MS).
Sym-trimethylbenzene was used as an internal standard to
calibrate the liquid product concentrations. Two HP-5 MS capillary
columns (30 m × 0.25 mm × 0.25 µm) were used in the two GCs
for the analysis. For the reusability test, after the reaction, the
catalyst was separated from the reaction medium, repeatedly
washed with ethanol twice, and then dried overnight in a vacuum
oven at 80 oC.
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Entry for the Table of Contents
The developed Al-doped SBA-15 supported Ni catalyst shows some excellent performance in the selective cleavage of the Caryl–O
bonds in guaiacol under mild conditions, because of its proper acidity, suitable metal-support interactions, and high dispersion of the
active species. The present study would stimulate research and development in multi-functional catalysts for the generation of valuable
chemicals from biomass.
10
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