Morphologically Cross-Shaped Ru/HZSM-5 Catalyzes Tandem Hydrogenolysis of Guaiacol to Benzene in Water

Article abstract of DOI:10.1002/cctc.201701398

Hydrogenolysis of C?O bonds is an important tool for synthesis of valuable fuels and chemicals from biomass. In this contribution, we report that morphologically cross-shaped HZSM-5-loaded Ru nanoparticles have demonstrated high activity in the selective hydrogenolysis of guaiacol to benzene in water with 97 % yield and a rate of 7.8 g g^?1 h^?1 accompanied with high durability. N₂ sorption analysis showed that Ru/HZSM-5 (cross-shaped) had a large mesoporous surface area and pore volume for loading small and uniform Ru nanoparticles, as confirmed by TEM images. The stronger interaction of Ru and cross-shaped HZSM-5 was simultaneously confirmed by a higher hydrogen reduction temperature of RuO₂ on calcined Ru/HZSM-5, a blueshift of Ru^δ+-(CO)_n, Ru^δ+-(CO), and Ru⁰-(CO) species in the IR spectra of adsorbed CO, and a higher Ru 3d_5/2 binding energy in X-ray photoelectron spectroscopy measurements. The reaction constant in guaiacol hydrogenolysis to phenol over cross-shaped Ru/HZSM-5 (0.051 min^?1) was 3–4 times higher than that on spherical and cuboid Ru/HZSM-5 (0.012–0.029 min^?1) at identical conditions, attributed to the remarkable hydrogenolysis catalytic capability of Ru nanoparticles on cross-shaped HZSM-5. In addition, adsorption of guaiacol and hydrogen was more substantial on cross-shaped Ru/HZSM-5, as evidenced by of IR and mass spectroscopy, respectively. The higher adsorption of guaiacol is attributed to the abundant Lewis acid sites on cross-shaped Ru/HZSM-5, as the Al?OH enriched Lewis acid sites favor the adsorption of oxygen-containing guaiacol. The higher rate constant in the primary step, together with the adsorbed high concentrated reactant and hydrogen (with nearly first-order reaction kinetics) on cross-shaped Ru/HZSM-5, facilitates the overall tandem reaction, leading to an excellent hydrogenolysis catalyst working at hydrothermal conditions for biomass conversion.

Full text of DOI:10.1002/cctc.201701398

Accepted Article
Title: Morphologically Cross-shaped Ru/HZSM-5 Catalyzed Tandem
Hydrogenolysis of Guaiacol to Benzene in Water
Authors: Chen Zhao, Zhaoxia Zheng, and Zhicheng Luo
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10.1002/cctc.201701398
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Morphologically Cross-shaped Ru/HZSM-5 Catalyzed Tandem
Hydrogenolysis of Guaiacol to Benzene in Water
Zhaoxia Zheng [a], Zhicheng Luo [a], Chen Zhao *[a]
Abstract: Hydrogenolysis of C-O bonds is an important tool for
synthesis of valuable fuels and chemicals from biomass. In this
contribution, we report that morphologically cross-shaped HZSM-5
loaded Ru nanoparticles have demonstrated a high activity in
selective hydrogenolysis of guaiacol to benzene in water with a 97%
yield and a rate of 7.8 g·g-1·h -1 accompanied with high durability. The
N2 sorption showed that Ru/HZSM-5 (cross-shaped) had the large
mesoporous surface area and volume for loading small and uniform
Ru nanoparticles, as confirmed by TEM images. The stronger
interaction of Ru and cross-shaped HZSM-5 was simultaneously
confirmed by a higher hydrogen-reduction temperature of RuO2 on
calcined Ru/HZSM-5, a blue shift of Ruδ+-(CO)n, Ruδ+-(CO), and Ru0-
(CO) species in IR spectra of adsorbed CO, and a higher Ru 3d5/2
binding energy in X-ray photoelectron spectroscopy measurements.
The reaction constant in guaiacol hydrogenolysis to phenol over
cross-shaped Ru/HZSM-5 (0.051 min-1) was 3-4 times higher than
that on spherical and cuboid Ru/HZSM-5 (0.012-0.029 min-1) at
identical conditions, attributed to the remarkable hydrogenolysis
catalytic capability of Ru nanoparticles on cross-shaped HZSM-5. In
addition, adsorption of guaiacol and hydrogen was more substantial
on cross-shaped Ru/HZSM-5, as evidenced by respective detectors
of Infrared and mass spectroscopy. The higher adsorption of
guaiacol is attributed to the abundant Lewis acid sites (LAS) on
cross-shaped Ru/HZSM-5, as the Al-OH enriched LAS favor for the
adsorption of oxygen-containing guaiacol. The higher rate-constant
in the primary step, together with the adsorbed high concentrated
reactant and hydrogen (with nearly first reaction orders) on cross-
shaped Ru/HZSM-5 facilitates the tandem rates in such reaction to
some extent, leading to an excellent hydrogenolysis catalyst working
at hydrothermal conditions for biomass conversion.
intermediate. Selective HDO of guaiacol to benzene is quite
challenging,[3] as the reactant as well as intermediates and
products tends to be hydrogenated to saturated compounds in
the hydrogen atmosphere.
In the gas phase, sulfide and non-sulfide catalysts were
used for guaiacol HDO to aromatic hydrocarbons. Sulfide
catalysts such as NiMoS[4] and CoMoS[5] can catalyze aromatic
hydrocarbons production with high selectivity, but they are
susceptible to deactivate in presence of water. In addition, non-
sulfide catalysts including metal catalysts such as Pt,[6] Ni, [7] Fe,
[8]
Mo, [9] can be used to catalyze the selective HDO route in the
gas phase. Especially with Mo/C[9] or PdFe/C, [10] the selectivity
of aromatic hydrocarbon can be up to 80%. In addition, MoN2, [11]
MoO3-NiO[12] and NiP/SiO2
guaiacol.
can also catalyze arenes from
[13]
Water is wildly present in the biomass and in the HDO
product, and thus, the role of water as a suitable solvent should
be addressed for guaiacol HDO. In aqueous hydrothermal
conditions, our group have developed Ru/sulfate zirconia[14] and
Ru/HZSM-5 catalysts[15] for selectively HDO of phenolic
monomers and phenolic dimers to arenes. We also found that in
the presence of hydrogen, highly selective hydrogenolysis
without hydrogenation depends on the concentration and
intensity of H· adsorption during the reaction. Thus, the high
temperature and low hydrogen pressure environment is
beneficial to remove substantial H· species on the surface. In
that case, it not only reduces the surface H· concentration, but
also changes the spatial H· distribution on the surface. In
addition, Fu et al[16] also reported a similar Ru-WOx/SiAl catalyst
system to convert of phenols and phenyl ethers to arenes in
high-temperature water.
It is well recognized that hydrogenolysis is catalyzed by the
metal site, but the capability of metal would be dramatically
influenced by the support. Herein, we reported that
morphologically cross-shaped HZSM-5 loaded Ru nanoparticles
showed a high activity as well as high stability in selective
hydrogenolysis of guaiacol to benzene in a tandem step via
phenol intermediate in presence of high temperature (240 °C)
and a low hydrogen pressure (2 bar) in the water phase.
Extensive ex situ characterization and adsorption measurements
were performed in order to explain the best hydrogenolysis
performance on cross-shaped Ru/HZSM-5.
Introduction
Guaiacol is a representative phenolic compound derived
from lignin depolymerization,[1] anchoring about two oxygen-
containing functional groups: phenolic hydroxyl and phenolic
methoxy groups. Catalytic hydrodeoxygenation (HDO) is a
useful upgrading approach to convert phenolic mixture to fuels
or chemicals.[2] Compared to the fully hydrodeoxygenated
products of alkanes, aromatic hydrocarbons are shown to be
more attractive with higher energy density as important chemical
Results and Discussion
[a]
Zhaoxia Zheng，Zhicheng Luo，Prof. Dr. Chen Zhao
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, Department of Chemistry, East China Normal University,
Shanghai 200062, China
Comparison of three Ru/HZSM-5 samples with different
morphologies
E-mail: czhao@chem.ecnu.edu.cn
The Ru based catalysts have demonstrated relatively high
selectivity in hydrogenolysis of C-O bonds of bio-molecules such
as phenols, glycerol, cellulose, and lignin. Here in the present
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cctc.201701398
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work, three HZSM-5 samples with different morphologies (cross-
shaped, spherical, and cuboid) with comparable Si/Al2 ratios of
80 to 120 were synthesized for loading Ru nanoparticles in the
investigation of guaiacol hydrogenolysis to benzene at the
hydrothermal conditions. Three catalysts are annotated as
Subsequently, the particle information of Ru on three
samples was analyzed by XRD patterns and TEM images. The
diffraction peaks of 38.4°, 42.2° and 44.0° in XRD patterns were
assigned to hexagonal close-packed (hcp)-structured Ru (100),
Ru (002) and Ru (101) facets, respectively (Figure 2). It was
obviously observed that Ru/HZSM-5(120)-3 had a mixture of
hexagonal close-packed Ru (100), Ru (002) and Ru (101) facets,
while Ru/HZSM-5(80)-2 contained a Ru (101) phase, and no Ru
diffraction peaks was shown for Ru/HZSM-5(100)-1. Based on
the Scherrer equation for XRD patterns, this indicates that the
Ru nanoparticle sizes on three samples would follow the
increasing order, Ru/HZSM-5(100)-1 < Ru/HZSM-5(80)-2 <
Ru/HZSM-5(120)-3. In line with this result, TEM images showed
the average Ru particles on Ru/HZSM-5(100)-1, Ru/HZSM-
5(80)-2, and Ru/HZSM-5(120)-3 were 4.0±0.9, 4.5±0.8, 5.0±0.7
nm (Figure S1, Table 1). It can be also inferred that the small
and uniform Ru nanoparticles on the cross-shaped HZSM-5
maybe resulted from the high mesoporous surface area and
pore volume of the support.
Ru/HZSM-5(100)-1
(cross-shaped),
Ru/HZSM-5(80)-2
(spherical), and Ru/HZSM-5(120)-3 (cuboid), respectively. The
Ru nanoparticles are incorporated by the incipient wetness
impregnation method, with comparable loadings of ca. 5 wt% as
detected by inductively coupled plasma (ICP) analysis. The
morphologies of three samples were measured by scanning
electron microscope (SEM), as shown in Figure 1. HZSM-
5(100)-1 comprised of numerous cross-shaped crystals with
uniform size of ca. 1-2 μm, while HZSM-5(80)-2 was consisted
by 3-4 nm spherical small particles to construct into separate
large crystals (7-8 μm). In comparison, HZSM-5(120)-3 was
made up by regular cuboid-shaped particles with average sizes
of 1-2 μm. The morphological disparity may dramatically
influence the properties of supports such as surfaces, pore
structures, acidity, and in addition to the properties of metals.
The summarized data for physicochemical and acid
properties of three HZSM-5 and Ru/HZSM-5 catalysts are
compiled in Table S1 and Table 1. The N2 adsorption/desorption
data demonstrated that the micropore structures of three
Ru/HZSM-5 samples were quite similar, with Smicro of 254-283
m2·g -1 and Vmicro of 0.13-0.14 cm3·g -1. However, it was noted that
their mesopores structures were different. The Smeso surface
areas were 132, 61, and 29 m2·g -1 for Ru/HZSM-5(100)-1,
Ru/HZSM-5(80)-2, Ru/HZSM-5(120)-3, respectively. In an
accordance, the Smeso volumes were respectively decreased
from 0.49, 0.16, to 0.07 cm3·g -1.
The acidic sites of three Ru/HZSM-5 samples were
detected by IR spectra of adsorbed pyridine. Shown from the IR
results, the Brönsted acid sites of three samples were very close
at the range of 0.213-0.268 mmol·g-1. Whereas Lewis acid site
of cross-shaped Ru/HZSM-5(100)-1 (0.030 mmol·g-1) was two or
three times higher than that on Ru/HZSM-5(80)-2 and
Ru/HZSM-5(120)-3 (0.010-0.014 mmol·g-1). The abundant Lewis
acid sites on HZSM-5(100)-1 may greatly influence the
adsorption of guaiacol reactant, and then dramatically affect the
corresponding hydrogenolysis rate on the assumption of first-
order reaction towards guaiacol concentration.
(a)
(c)
(e)
(b)
(d)
(f)
2μm
2μm
2μm
5μm
5μm
5μm
Figure 1. SEM images of (a-b) HZSM-5(100)-1, (c-d) HZSM-5(80)-2, and (e-f)
HZSM-5(120)-3.
Table 1. The characterized physicochemical and acidic properties of three Ru/HZSM-5 samples.
Acid conc. (Py-IR)
a
b
b
b
b
c
Si/Al2
Smeso
Smicro
Vmeso
Vmicro
DRu
(mmol∙g-1)
Ru/HZSM-5
(mol∙mol-1)
(m2∙g-1)
(m2∙g-1)
(cm3∙g-1)
(cm3∙g-1)
(nm)
BAS
LAS
1
2
3
100
80
132
61
256
254
283
0.49
0.16
0.07
0.13
0.13
0.14
4.0±0.9
4.5±0.8
5.0±0.7
0.251
0.213
0.268
0.030
0.010
0.014
120
29
a Detected by ICP.
b Determined by N2 sorption.
c Calculated by TEM images.
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10.1002/cctc.201701398
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from the strongest Ru and HZSM-5 interaction. This is in an
agreement with the results from IR spectra of adsorbed CO and
TPR profile of adsorbed H2.
Ru(100).Ru(002) Ru(101)

Ru/HZSM-5(120)-3

.
(a)
128 C

Ru/HZSM-5(80)-2
Ru/HZSM-5(100)-1
Ru/HZSM-5(120)-3
Ru/HZSM-5(80)-2
Ru/HZSM-5(100)-1
140 C
156 C
10
20
30
40
50
60
70
80
2 (degree)
Figure 2. XRD patterns of Ru/HZSM-5(100)-1, Ru/HZSM-5(80)-2, and
Ru/HZSM-5(120)-3.
0
100 200 300 400 500 600 700 800
Temperature (C)
In a next step, the interaction of Ru and HZSM-5 support
was investigated by temperature programmed reduction of
hydrogen (TPR-H2), IR spectra of adsorbed CO (IR-CO), and X-
ray photoelectron spectroscopy (XPS) measurements. As shown
from the TPR-H2 profile in Figure 3a, the strong reduction peaks
at 120-190 °C were assigned to reduction of the RuO2 species
on the surface. The maximum peaks in reduction temperatures
of RuO2 in Ru/HZSM-5(100)-1, Ru/HZSM-5(80)-2 and
Ru/HZSM-5(120)-3 were located at 156, 140, and 128 °C,
respectively. The highest reduction temperature for RuO2 (156°C)
on cross-shaped HZSM-5-1 support implies the strongest
interaction of metal and support.
The IR spectra of adsorbed CO (Figure 3b) showed three
groups of adsorbed CO bands on Ru nanoparticles: HF1 (high-
frequency 1) at 2156-2133 cm-1, HF2 (high-frequency 2) at 2100-
2060 cm-1, and LF (low-frequency) bands at about 2040 ± 40
cm-1, which were attributed to Ruδ+-(CO)n, Ruδ+-(CO), and Ru0-
(CO) species, respectively. [17] The main bands of Ruδ+-(CO)n of
Ru/HZSM-5(100)-1, Ru/HZSM-5(80)-2, and Ru/HZSM-5(120)-3
were located at 2135, 2130 and 2128 cm-1, respectively.
Obviously, there showed a blue shift from Ru/HZSM-5(120)-3 to
Ru/HZSM-5(100)-1 for Ruδ+-(CO)n species. In a same trend, the
bands assigned to Ruδ+-(CO) and Ru0-(CO) also showed the
similar shift. In general, a blue shift in IR spectra indicates
relatively high transition energy of adsorbed CO species on Ru
surface. The relatively high transition energy of Ruδ+-(CO)n,
Ruδ+-(CO), and Ru0-(CO) species can be resulted from the
strong interaction of Ru and HZSM-5 support, as confirmed by
the results from TPR-H2 profile. The strong interaction may be
due to the electron transfer from Ru to HZSM-5(100)-1 support,
and consequently, an electron-deficient Ru metal center is
formed.
Ru+(CO)
(b)
Ru0CO
2071 cm-1
Ru+(CO)n
2128 cm-1
2002 cm-1
Ru/HZSM-5(120)-3
2074 cm-1
2005 cm-1
2130 cm-1
2135 cm-1
Ru/HZSM-5(80)-2
2080 cm-1
2020 cm-1
Ru/HZSM-5(100)-1
2250 2200 2150 2100 2050 2000 1950 1900
Wavenumber (cm-1)
C1s
Ru 3d5/2
(c)
Ru0
280.2 eV
Ru/HZSM-5(120)-3
280.5 eV
280.8 eV
Ru/HZSM-5(80)-2
Ru/HZSM-5(100)-1
276
278
280
282
284
286
288
Binding energy (eV)
Figure 3. (a) TPR-H2 profile, (b) IR spectra of adsorbed CO, and (c) Ru 3d5/2
XPS spectra of three Ru/HZSM-5 samples.
Evaluation of the catalytic performances for tandem
hydrogenolysis of guaiacol to benzene over three
Ru/HZSM-5 samples
XPS was used to illuminate the structural and chemical
state of Ru on the HZSM-5 surface. As shown in Figure 3c, the
main peak at 284.8 eV was assigned to the strong signal from
carbon. The binding energy (BE) for Ru 3d5/2 on Ru/HZSM-
5(100)-1, Ru/HZSM-5(80)-2, and Ru/HZSM-5(120)-3 was 280.8,
280.5, and 280.2 eV, respectively. It is demonstrated that the
shift of three samples to higher binding energy of Ru0 is due to
different metallic Ru(0) states. The highest binding energy of Ru
3d5/2 demonstrates a highest electronegativity on Ru resulted
The hydrogenolysis of guaiacol was evaluated on three
Ru/HZSM-5 catalysts in water with a fixed guaiacol and catalyst
ratio of 20 g·g-1. The conditions with a high temperature and a
low pressure (240 °C and 2 bar H2) was adopted, since the
adsorbed few H· species nearby the oxygen atom of guaiacol is
more strongly adsorbed under such conditions and thus
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preferred to be retained on the Ru surface.[18] Therefore,
selective hydrogenolysis of the CH3O-Ar bond is successfully
conducted at such conditions.
Ru/HZSM-5(100)-1:
Ru/HZSM-5(80)-2:
Ru/HZSM-5(120)-3:
Conv.
Conv.
Conv.
Phenol
Phenol
Phenol
Benzene
Benzene
Benzene
35
(a)
30
25
20
15
10
5
The plotted kinetics curves for guaiacol hydrogenolysis over
three Ru/HZSM-5 samples were displayed were in Figure 4a
and Table S2. After a reaction time of 80 min., the conversion
values attained 33%, 20% and 8%, respectively, on Ru/HZSM-
5(100)-1, Ru/HZSM-5(80)-2 and Ru/HZSM-5(120)-3. The gained
initial rates were 63, 41, 14 mmol·g-1·h -1, correspondingly. The
carbon balance in the liquid phase exceeded 93% in the liquid
phase. In addition, it was noted that primary product was phenol
with the high selectivity of guaiacol to phenol over three
Ru/HZSM-5 samples, which was originated from Ar-OCH3 bond
(bond dissociation energy: 409-421 kJ/mol) cleavage of guaiacol.
The yields of benzene were then attained at 8%, 6%, and 0.2%
at 80 min..
Apart from the primary phenol and its hydrogenated
products cyclohexanone and cyclohexanol from guaiacol
conversion, other products including benzene, anisole, 2-
methoxycyclohexanol, and methanol were also detected with
Ru/HZSM-5(100)-1 (Figure 4b). Based on this information, it can
be concluded that the main pathway for guaiacol conversion
proceeds with in tandem hydrogenolysis to phenol and methanol,
and subsequently phenol is cleaved into benzene and water.
The phenol intermediate can be also hydrogenated to
cyclohexanone and cyclohexanol in a minor part (Scheme 1). In
addition, Guaiacol can be also hydrogenolyzed to anisole or
hydrogenated to 2-methoxycyclohexanol as by-products. Thus,
the crucial factor in achieving the high selectivity of benzene is
to enhance the tandem hydrogenolysis capability on Ru center.
It has been demonstrated that the reaction orders for
hydrogen and guaiacol concentrations were 0.74 and 0.97,
respectively.[15] Since the hydrogen pressure before and after
the reaction was almost unchanged, the parameter for the
hydrogen pressure can be considered to be a constant. The
reaction order for the guaiacol concentration (0.97) approached
1, and therefore, the reaction steps and individual reaction
equation from guaiacol hydrogenolysis to phenol, and then
hydrogenolysis to benzene can be drawn in Figure 5a and
Figure 5b.
0
0
20
40
60
80
Time (min.)
(b)
Conv.
Phenol
Benzene
30
20
10
0
2
Methanol
Cyclohexanol
Cyclohexanone
Anisole
2-methoxycyclohexanol
1
0
0
20
40
60
80
Time (min.)
Figure 4. (a) Kinetics of hydrogenolysis of guaiacol over three Ru/HZSM-5
catalysts. (b) The product distributions on guaiacol hydrogenolysis over
Ru/HZSM-5(100)-1 as a function of time. General conditions: guaiacol (1.0 g),
Ru/HZSM-5 catalyst (0.05 g), H2O (100 mL), 240 °C, 2 bar H2 and 6 bar N2,
stirring at 600 rpm.
(a)
k1
k2
(b)
Figure 5. (a) A first-order reaction on tandem hydrogenolysis of guaiacol to
benzene. (b) Individual reaction equations for the hydrogenolysis of guaiacol
to benzene. General conditions: guaiacol (1.0 g), Ru/HZSM-5 (0.05 g), H2O
(100 mL), 240 °C, 2 bar H2 and 6 bar N2, stirring at 600 rpm.
Scheme 1. The proposed plausible routes on guaiacol hydrogenolysis over
Ru/HZSM-5 in presence of hydrogen.
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The Matlab software is used to simulate the kinetics curve
for the tandem hydrogenolysis of guaiacol to benzene via phenol
intermediate. As shown in Table 2, the curves simulated by
Matlab software are fitted well with the kinetics curves over three
Ru/HZSM-5 catalysts. The fitted reaction constants for the
individual steps are listed in Table 2.
range. In addition, the apparent activation energy and the true
activation energy shows the following relationship:
=
+
(1-θA)·ΔHA. Since the reaction pressure is low (2 bar) under the
current catalytic system, the value of θA is approximately 0. The
formula can be simplified as
=
+ ΔHA. When guaiacol
is preferentially adsorbed on Ru, the adsorption energy (ΔHA) is
-237 kJ·mol-1 at the corrected zero-point energy.[19] Therefore,
the true activation energy of hydrogenolysis of guaiacol at
selected conditions is calculated to be close to 253.6 kJ·mol-1.
With the Ru/HZSM-5-1 sample, the initial rate constant for
hydrogenolysis of guaiacol to phenol (k1 = 0.0051 min-1) was
much higher than the second step for hydrogenolysis of phenol
to benzene (k2 = 0.0034 min-1). In comparison to other two
Ru/HZSM-5 catalysts, the gained rates for the first step for
converting of guaiacol to phenol were 0.0051, 0.0029, and
0.0012 min-1, respectively. This implies the first hydrogenolysis
step rate on Ru/HZSM-5(100)-1 was two or three times higher
than that on other two Ru samples. The rate constants for the
second step of forming benzene from phenol were quite similar
at 0.0034, 0.0033, and 0.0030 min-1. In general, the higher rate
constant of k2 than k1 indicates that the initial step for
hydrogenolysis of guaiacol is the rate-limiting step on Ru/HZSM-
5 catalysts. The higher rate-constant in the two steps of guaiacol
hydrogenolysis on cross-shaped Ru/HZSM-5(100)-1 may be
attributed to the remarkable capability of supported Ru
nanoparticles.
Understanding the high activity of cross-shaped
Ru/HZSM-5(100)-1 for tandem hydrogenolysis of
guaiacol to benzene
Afterwards, we try to explore the reason for the high activity
of cross-shaped Ru/HZSM-5(100)-1 for hydrogenolysis of Ar-
OCH3 bonds of guaiacol. The rate for guaiacol hydrogenolysis
follows the equation I:
(Equation I)
The rate constant k stands for the hydrogenolysis capability
on the active site of Ru center. In the part of understanding the
individual steps in guaiacol to benzene, it has been
demonstrated that the calculated rate constant (0.0051 min-1) on
cross-shaped Ru/HZSM-5(100)-1 for the primary step was two
or three times higher than that on other two catalysts (0.0029
and 0.0012 min-1). The higher capability for C-O bond cleavage
on supported Ru nanoparticles of cross-shaped HZSM-5(100)-1
maybe resulted from the smaller and more uniform particles on
the high mesoporous surface areas of HZSM-5(100)-1, as well
as strong Ru and HZSM-5 interaction. The smaller size of Ru
nanoparticles is verified by TEM images and XRD patterns, and
the stronger interaction of Ru and the cross-shaped HZSM-5
support is confirmed by a higher reduction temperature in TPR-
H2 profile, a blue shift in IR spectra of absorbed CO, a stronger
electronegativity of Ru.
We used the method of IR spectra of absorbed guaiacol in
the gas phase in vacuum to reflect the adsorption of guaiacol
over three Ru/HZSM-5 samples in the aqueous phase at Figure
6a. Three main bands at 1500-1620 cm-1 were assigned to (C-
Cring) vibrations.[20] The ratios of integrated areas of guaiacol
adsorption intensities at 1508, 1597 and 1619 cm-1 were
2.02:1.56:1 for three Ru/HZSM-5 samples sequentially, showing
that the guaiacol adsorption capacity is substantially abundant
on Ru/HZSM-5(100)-1. The much higher guaiacol adsorption is
attributed to the Al-OH enriched Lewis acid sites on the support,
Table 2. Fitted curves and rate constants (k) for hydrogenolysis of guaiacol to
benzene over three Ru/HZSM-5 simulated by Matlab software.
Ru/HZSM-5(120)-3
Ru/HZSM-5(100)-1
Ru/HZSM-5(80)-2
k1 (min-1)
k2 (min-1)
Catalyst
guaiacol to phenol phenol to benzene
Ru/HZSM-5(100)-1
Ru/HZSM-5(80)-2
Ru/HZSM-5(120)-3
0.0051
0.0029
0.0012
0.0034
0.0033
0.0030
Later, the temperature dependence at the range from 180 to
220 °C towards the tandem hydrogenolysis of guaiacol was
explored over Ru/HZSM-5(100)-1 in presence of 2 bar H2
(Figure S3). With a temperature increase from 180 to 240 °C,
the conversion of guaiacol gradually increased from 21%, to
28%, 30%, and 33% at 80 min., suggesting low temperature
dependence towards the rate increase.
The Arrhenius plot is thus acquired based the
hydrogenolysis rate of the reaction at different temperatures
(Figure S4). On the basis of Arrhenius's law (lnk = lnA - Ea/RT),
we calculated the apparent activation energy (Ea) of this reaction
to be 16.6 kJ·mol-1. The small value of Ea indicates the
hydrogenolysis rates with a low hydrogen pressure (2 bar) show
a low temperature dependence at the selected temperature
as this electron-positive Al center can favorably adsorb the
[21]
electron-negative Ar-OCH3 and Ph-OH groups.
The higher
Lewis acids on cross-shaped HZSM-5(100)-1 are also
demonstrated by the IR spectra of absorbed pyridine as shown
in Figure S2. To prove this point, we also tested the adsorption
of guaiacol in the gas phase (Figure 6a) on the pure γ-Al2O3,
and the result revealed that the adsorption intensity of guaiacol
in IR spectrum was highly strong at 1508, 1597 and 1619 cm-1
with the Lewis Al center. This again demonstrates the higher
guaiacol on cross-shaped HZSM-5(100)-1 is due to its abundant
Lewis acid sites.
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1508 cm-1
(a)
(a)
Guaiacol-adsorption
Methanol
Others
Phenol
100
80
60
40
20
1597 cm-1
Al2O3
Benzene
1619 cm-1
Ru/HZSM-5(120)-3
Ru/HZSM-5(80)-2
Ru/HZSM-5(100)-1
1800 1750 1700 1650 1600 1550 1500 1450
0
Wavenumber (cm-1)
Ru/HZSM-5(120)-3
Ru/HZSM-5(100)-1 Ru/HZSM-5(80)-2
80
Conversion
Phenol
Benzene
(b)
H2-adsorption
(b)
Methanol
Cyclohexanone
Cyclohexanol
Anisole
Ru/HZSM-5(120)-3
60
40
20
0
Ru/HZSM-5(80)-2
Ru/HZSM-5(100)-1
0
50
100
150
200
250
300
Run 1
Run 2
Run 3
Run 4
Temperature (C)
Figure 6. (a) The IR spectra of absorbed guaiacol, and (b) TPD/H2-MS profiles
(c)
of three Ru/HZSM-5 samples.
100
80
60
40
20
0
In addition, the adsorption of another reactant (hydrogen) is
monitored by TPD/H2-MS technique on three Ru/HZSM-5
samples (Figure 6b). The maximum desorption temperatures for
adsorbed hydrogen located at around 100-110 °C. The ratios of
adsorbed H2 capacity of were 1.29:1.12:1 for three Ru/HZSM-5
samples. Basically, it is recognized that H2 is adsorbed on the
Ru nanoparticles, suggesting more accessible Ru nanoparticles
on cross-shaped HZSM-5(100)-1. Therefore, based on the
equation of r = k [guaiacol]adsa[H2]adsb, the higher adsorbed
reactants of guaiacol and hydrogen on the surface together with
the high hydrogenolysis capability on cross-shaped Ru/HZSM-
5(100)-1 (with a was 0.97 and b was 0.74) construct a promising
hydrogenolysis catalyst.
Phenol 4-Methyl-guaiacol
Syringol
Figure 7. (a) The product distributions on conversion of guaiacol to benzene
on three Ru/HZSM-5 samples, others refer to the products of cyclohexanone
and cyclohexanol. Reaction conditions: guaiacol (0.2 g), Ru/HZSM-5 (0.2 g),
H2O (100 mL), 240 °C, 2 bar H2 and 6 bar N2, 1 h, stirring at 600 rpm. (b)
Recycling test of the hydrodeoxygenation of guaiacol over Ru/HZSM-5(100)-1.
Reaction conditions: guaiacol (1.0 g), Ru/HZSM-5(100)-1 (0.2 g), H2O (100
mL), 240 °C, 2 bar H2 and 6 bar N2, 1 h, stirring at 600 rpm. (c) Selective
hydrodeoxygenation of lignin-derived phenolic monomers (phenol, 4-
methylguaiacol, and syringol) to aromatic hydrocarbons over Ru/HZSM-
5(100)-1. Reaction conditions: reactant (0.2 g), Ru/HZSM-5(100)-1 (0.2 g),
H2O (100 mL), 240 °C, 2 bar H2 and 6 bar N2, stirring at 600 rpm, reaction time:
1 h for phenol, 4 h for 4-methyl-guaiacol and syringol.
Catalytic and recycling tests
To test the maximum benzene production capability from
guaiacol, three Ru/HZSM-5 catalysts were compared at the
identical conditions (240 °C, 2 bar H2) for 1 h. The cross-shaped
Ru/HZSM-5(100)-1 attained nearly quantitative benzene
formation from guaiacol with a 97% yield of benzene and a 3%
yield of methanol (Figure 7a and Table S3), while the yields of
benzene only attained around 20% for Ru/HZSM-5(80)-2 and
Ru/HZSM-5(120)-3 sample. These data again demonstrate the
super high hydrogenolysis activity of Ru nanoparticles on cross-
shaped HZSM-5.
The durability of the best Ru/HZSM-5(100)-1 catalyst was
tested with four reaction cycles under this selected reaction
condition. After each reaction, the catalyst was filtered and then
dried in vacuum for the next use. As shown in Figure 7b and
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Synthesis of Cross-shaped HZSM-5
Table S4, the conversion of guaiacol within 1 h was from 66%
(fresh catalyst) to 63% (second cycle) and then to 62% (in the
third cycle) and 60% (in the fourth cycle), whereas the
conversion of the main products of phenol and benzene was
essentially maintained at a level of 40% and 20% in each cycle.
These results indicate that the catalyst is relatively stable under
this harsh hydrothermal condition. In addition, the reactant was
extended to diverse lignin derived monomers (phenol, 4-
methylguaiacol, and syringol), and they were quantitatively
converted to arenes at similar conditions (Fig. 7c). This suggests
that the developed catalytic system is versatile for arenes
production from hydrodeoxygenation of lignin pyrolysis oil.
Cross-shaped HZSM-5 zeolites were synthesized with the assistant
of active seeds[22]. Firstly, tetra-ethyl orthosilicate (TEOS) was dropped
into the solution including water and tetrapropylammonium hydroxide
(TPAOH, 25 wt%). After mixing at 353 K for 2 h, the gel (1.0 TEOS : 0.15
TPAOH : 14 H2O) was introduced into a Teflon-lined steel autoclave,
aging at 393 K for 3 h.
Piperidine (PI) was used as structure directing agent to synthesize
HZSM-5 zeolite Sodium hydroxide and aluminum sulfate were first
dissolved in the water. Then piperidine was added in the above solution.
Next, silica sol and active seeding gel were dropped into the mixed
solution and further stirred for 30 minutes, forming a gel composition of
1.0 SiO2 : 0.01 Al2O3 : 0.2 PI : 0.05 Na2O : 25 H2O. SiO2 in the active
seeding gel accounted for 1 % of the whole SiO2 in gel. The gel was
crystallized in a Teflonlined steel autoclave at 443 K for 72 h. The ZSM-5
product was collected by filtration followed by washing with distilled water
several times, dried at 373 K, and then calcined in air at 823 K for 6 h.
The ZSM-5 was brought into ammonium form. After filtration, washing
and drying at 373 K, the ammonium ion-exchanged zeolite was
subsequently calcined at 823 K for 6 h to give HZSM-5.
Conclusions
The hydrogenolysis of cellulose, lignin, and lipids is wildly
used in biomass conversion to produce fuels and chemicals.
Here, we reported that Ru nanoparticles supported on cross-
shaped HZSM-5 catalyzed a selective benzene production with
a 97% yield from the hydrogenolysis of guaiacol at hydrothermal
conditions in presence of a high temperature and a low
hydrogen pressure (240 ⁰C, 2 bar H2), with sequential rate
constants of 0.0051 and 0.0034 min-1 for steps of guaiacol to
phenol and phenol to benzene in a tandem reaction.
The characterization results show that the large
mesoporous surface area and the mesopore pore volume of the
cross-shaped HZSM-5 support was loaded with small and
uniform Ru nanoparticles. The strong interaction between Ru
and cross-shaped HZSM-5 support (as confirmed by TPR-H2)
leads to an electron-deficient Ru (as shown in IR spectra of
adsorbed CO) and a strong electronegative Ru (as measured by
XPS). Resultantly, cross-shaped Ru/HZSM-5 led to the overall
highest rate constant in hydrogenolysis of guaiacol to phenol. In
addition, the higher adsorbed reactants of guaiacol and
hydrogen on the surface of cross-shape HZSM-5 together with
the high hydrogenolysis capability on cross-shaped Ru/HZSM-5
construct a promising hydrogenolysis catalyst, offering high
Synthesis of Spherical HZSM-5
For spherical HZSM-5 synthesis, AIP was dissolved in a mixture of
TPAOH and distilled water with stirring at ambient temperature for 2 h.
Then TEOS was added drop-wise into the above solution, with a molar
composition of 1 SiO2 : 80 Al2O3 : 15 H2O : 0.25 TPAOH, stirring for 4 h.
The solution was transferred into a Teflon-lined stainless steel autoclave
o
and crystallization in an oven lasted for 72 h at 170 C. After cooling, the
solid product was washed to neutrality, and sequentially centrifugated to
remove supernatant liquid, and finally collected after drying at 110 oC
overnight. Prior to use, it was calcined in flowing air (flowing rate: 100
mL∙min-1) at 550 oC for 6 h.
Synthesis of Cuboid HZSM-5
Cuboid HZSM-5 zeolites were also synthesized with the assistant of
active seeds[23]
.
The colloidal silica was added into the
tetrapropylammonium hydroxide solution at room temperature, stirring for
4 h. Then the mixture (SiO2 : 0.35 TPAOH : 19.6 H2O) was transferred
into the Teflon-lined stainlesssteel autoclave and hydrothermally treated
at 353 K for 72 h. The obtained seeding gel was directly used for the
synthesis of parent HZSM-5.
potentials for used in selective hydrogenolysis of diverse Caliphatic
O and Caromatic-O bonds in biomass at hydrothermal conditions.
-
The Al2(SO4)3 was dissolved in the sulfuric acid solution as the
aluminum source. Then the solution was added into the water glass (SiO2,
27.10 wt%; Na2O, 8.39 wt%), after that the template tetraethylammonium
bromide and seeding gel (the active seeding gel accounted for 0.5 % of
the whole SiO2 in gel)was added to obtain amixture with amolar
composition of SiO2 : 1/120 Al2O3 : 0.05 Na2O : 0.05 TEABr : 30 H2O.
After hydrothermal synthesis at 458 K for 24 h, the resultant solid product
was recovered by filtration, being washed with deionized water for
several times and dried at 373 K overnight. The template was removed
by the calcination at a ramp of 2 K min−1 to 823 K for 6 h in air
atmosphere. The obtained zeolite was ion-exchanged with NH4Cl twice,
and the final product was denoted as HZSM-5.
Experimental Section
Materials
All chemicals were used without further purification: RuCl3∙3H2O
(J&K, ≥ 59.5 wt%), tetra-propylammonium hydroxide (TPAOH, Sinopec
Co., Ltd., 25 wt%), tetra-ethylorthosilicate (TEOS, Sinopec Co., Ltd.,
containing 28.4 wt% SiO2), tetraethylammonium bromide (TEABr, Alfa,
98 wt%), colloidal silica (Sigma-Aldrich, 30 wt%), piperidine (Aladdin,
≥99.5% analytical standard), aluminum sulfate octadecahydrate (Aladdin,
98.0%–102.0% ACS reagent), sodium hydroxide (Alfa Aesar, 98.0%
flake). Guaiacol (Sinopharm, > 98% GC assay), ethyl acetate (Sinopharm,
AR), phenol (Sinopharm, AR), 4-methylguaiacol (J&K, > 99% GC assay),
2,6-dimethoxy-4-methyl-phenol (Alfa Aesar, > 97% GC assay). Air, H2,
and N2 gases (99.999 vol.%) were supplied by Shanghai Pujiang
Specialty Gases Co., Ltd.
Synthesis of supported 5 wt% Ru/HZSM-5
The supported 5 wt. % Ru catalysts were prepared by incipient
wetness impregnation. Firstly a solution of RuCl3∙3H2O (6.35 mL, conc:
0.040 g/L) was added drop-wise to an aqueous suspension (150 mL H2O,
2.0 g support) with stirring at room temperature overnight. Afterwards,
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10.1002/cctc.201701398
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the as-formed catalysts were filtered, dried at 80 oC overnight. Finally, it
was calcined in flowing N2 (flowing rate: 100 mL/min) at 350 oC for 4 h,
and reduced in flowing hydrogen (flowing rate: 100 mL∙min-1) at 350 oC
for 4 h.
The recycling tests for guaiacol conversion with Ru/HZSM-5: the
Ru/HZSM-5 catalyst was examined in four consecutive catalytic runs for
guaiacol HDO by employing a batch of Ru/HZSM-5 catalyst at 240 oC
and 2 bar H2 for 1 h. After each reaction, the used Ru/HZSM-5 catalysts
were filtered and then dried at 60 oC in vacuum before next catalytic runs.
Catalyst characterization
The amounts of Si, Al, and Ru were quantified by inductively coupled
plasma (ICP) atomic emission spectroscopy on a Thermo IRIS Intrepid II
XSP. The crystal morphology and size were determined by scanning
Acknowledgements
This research was supported by the Key Research and
Development Program of the Ministry of Science and
Technology (Grant No. 2016YFB0701100), the Recruitment
Program of Global Young Experts in China, National Natural
Science Foundation of China (Grant No. 21573075), the
Shanghai Pujiang Program (Grant No. PJ1403500), and the
Foundation of Key Laboratory of Low-Carbon Conversion
Science & Engineering, Shanghai Advanced Research Institute,
Chinese Academy of Sciences.
electron microscopy (SEM) on
a Hitachi S-4800 microscope and
transmission electron microscopy (TEM) on
microscope at an accelerating voltage of 200 kV.
a
JEOL JEM-2100
using
Nitrogen sorption isotherms were obtained at 77
K
a
BELSORP-MAX instrument after degassing the samples for 10 h under
vacuum at 573 K. The surface areas were calculated by the Brunauer-
Emmett-Teller (BET) method. X-ray diffraction (XRD) patterns were
obtained with a Rigaku Ultima IV X-ray diffractometer (35 kV and 25 mA)
using Cu Kα radiation (λ = 1.5405 Å) over a 2θ ranging from 10o to 70o at
a scanning speed of 60o min−1. The TPR-H2 profile was recorded at a
heating rate of 5 K min−1 from 293 to 1073 K by using Micromeritics
tp5080 apparatus equipped with a thermal conductor detector (TCD).
The IR spectra of the adsorbed CO (IR-CO) were recorded with a
Nicolet NEXUS 670 FTIR spectrometer equipped with an in situ IR cell.
The samples were reduced in H2 at 623 K for 1 h and then adsorbed CO
at room temperature. The IR spectra of the adsorbed pyridine (IR-Py)
were measured on a Nicolet Fourier transform infrared spectrometer
(NEXUS 670) equipped with an in situ IR cell. The samples were
activated in a vacuum at 823 K for 1 h before equilibration with pyridine
at 423 K, and then evacuated at 423 K for 1 h.
Keywords: Selective hydrogenolysis • arenes • lignin • HZSM-5
morphology • Aqueous phase reaction
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Entry for the Table of Contents
FULL PAPER
Morphologically cross-shaped HZSM-
k2
Zhaoxia Zheng, Zhicheng Luo, Chen
Zhao*
k1
5
loaded Ru nanoparticles have
Ru/HZSM-5 k1(min-1
Cross-shaped 0.0051
)
k2(min-1
0.0034
0.0032
0.0030
)
demonstrated high activity (rate: 7.8
g·g-1·h -1) in selective hydrogenolysis
of guaiacol to benzene in water with a
nearly quantitative yield accompanied
with high durability.
Page No. – Page No.
Spherical
Cuboid
0.0029
0.0012
Ru/HZSM-5
Ru/HZSM-5
Morphologically Cross-shaped
Ru/HZSM-5 Catalyzed Tandem
Hydrogenolysis of Guaiacol to
Benzene in Water
Cross-shaped
Cuboid
Cuboid
Spherical
Spherical
Cross-shaped
Cross-shaped
1700 1650 1600 1550 1500 1450
Wavenumber (cm-1
0
50
100
150
200
Temperature (C)
)
Spherical
Cuboid
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