Ruthenium Catalysts on ZSM-5/MCM-41 Micro-Mesoporous Support for Hydrodeoxygenation of Guaiacol in the Presence of Water

Article abstract of DOI:10.1134/S1070427219080172

A ruthenium-containing catalyst on ZSM-5/MCM-41 micro-mesoporous aluminosilicate support was synthesized. The micro-mesoporous support and supported catalyst were characterized by low-temperature nitrogen desorption/adsorption, temperature-programmed ammonia desorption, transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction with hydrogen, and energy-dispersive X-ray fluorescence analysis. Ru/ZSM-5/MCM-41 has high specific surface area (392 m² g^?1) and high acidity (1087 μmol g^?1); the mean ruthenium particle size is 1.7 nm. The catalyst was studied in hydrodeoxygenation of a modelbio-oil compound, guaiacol, in the presence of water at a hydrogen pressure of 5 MPa and temperatures in the interval 130–290°C. The catalyst obtained exhibits high activity in guaiacol hydrodeoxygenation: The conversion was 90%, and the selectivity with respect to cycloalkanes was 64% in 3-h reaction performed at a hydrogen pressure of 5 MPa and a temperature of 230°C.

Full text of DOI:10.1134/S1070427219080172

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2019, Vol. 92, No. 8, pp. 1170−1178. © Pleiades Publishing, Ltd., 2019.
Russian Text © The Author(s), 2019, published in Zhurnal Prikladnoi Khimii, 2019, Vol. 92, No. 8, pp. 1079−1088.
ORGANIC SYNTHESIS
AND INDUSTRIAL ORGANIC CHEMISTRY
Ruthenium Catalysts on ZSM-5/MCM-41 Micro-Mesoporous
Support for Hydrodeoxygenation of Guaiacol
in the Presence of Water
E. A. Rolduginaa,*, A. P. Glotovb, A. L. Isakova, A. L. Maksimova,c,
V. A. Vinokurovb, and E. A. Karakhanova
a Chemistry Department, Moscow State University, Moscow, 119991 Russia
b Gubkin University, National University of Oil and Gas, Moscow, 119991 Russia
c Topchiev Institute of Petrochemical Synthesis, Moscow, 119991 Russia
*e-mail: rolduginakate@mail.ru
Received May 25, 2019; revised June 17, 2019; accepted June 17, 2019
Abstract—A ruthenium-containing catalyst on ZSM-5/MCM-41 micro-mesoporous aluminosilicate support
was synthesized. The micro-mesoporous support and supported catalyst were characterized by low-temperature
nitrogen desorption/adsorption, temperature-programmed ammonia desorption, transmission electron microscopy,
X-ray photoelectron spectroscopy, temperature-programmed reduction with hydrogen, and energy-dispersive
X-ray fluorescence analysis. Ru/ZSM-5/MCM-41 has high specific surface area (392 m2 g–1) and high acidity
(1087 μmol g–1); the mean ruthenium particle size is 1.7 nm. The catalyst was studied in hydrodeoxygenation of
a modelbio-oil compound, guaiacol, in the presence of water at a hydrogen pressure of 5 MPa and temperatures
in the interval 130–290°С. The catalyst obtained exhibits high activity in guaiacol hydrodeoxygenation: The
conversion was 90%, and the selectivity with respect to cycloalkanes was 64% in 3-h reaction performed at a
hydrogen pressure of 5 MPa and a temperature of 230°С.
Keywords: nanoparticles, guaiacol, micro-mesoporous materials, hydrodeoxygenation, ruthenium
DOI: 10.1134/S1070427219080172
Lignocellulosic biomass, a wood waste from of various structures. Complex chemical composition
and high content of water formed by biomass polymer
degradation and of oxygen are responsible for high
polarity, high acidity, and thermal instability ofbio-
oil [1]. The removal of oxygen and water frombio-oil
and its joint processing with hydrocarbon petroleum
products on traditional oil-refining equipment
require an additional stage of improving thebio-
oil quality. A promising technology for processing
biological raw materials is catalytic hydrotreating
nonfood industry, is a promising biological raw
material, because its processing supports the zero
balance of СО2 in the atmosphere and allows
production of biofuel with the characteristics similar
to those of the existing motor fuels [1]. The biomass
processing by the fast pyrolysis technology yields
so-calledbio-oil differing from other biofuels in the
convenience of storage and transportation and in the
complexity of the composition, which allows using it
not only as a transport fuel but also as feedstock for
fine chemical synthesis.bio-oil contains hundreds of
organic compounds formed by thermal degradation
of cellulose and lignin and belonging to different
classes, such as low-molecular-weight alcohols, acids,
ethers, esters, aldehydes, ketones, furans, and phenols
(hydrodeoxygenation,
HDO),
in
whichbio-oil
components undergo such catalytic transformations
as cracking, decarbonylation, decarboxylation,
deoxygenation, and hydrogenation [2].
One of the factors ensuring the hydrodeoxygena-
tion efficiency is the development of stable and ac-
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RUTHENIUM CATALYSTS ON ZSM-5/MCM-41 MICRO-MESOPOROUS SUPPORT
1171
tive catalysts. The traditional HDO catalyst contains
EXPERIMENTAL
an active phase, metal particles immobilized on a
solid support. Three types of catalysts forbio-oil hy-
drodeoxygenation are known: traditional sulfidized
hydrotreating catalysts NiMo/Al2O3 and CoMo/Al2O3
[3]; heterogeneous catalysts based on transition met-
als (Ni [4], Mo [5], W [6], Fe [7] Cu [8], etc.); and
catalysts based on noble metals (Rh, Ru, Pt, and Pd
[9, 10]). Catalysts based on noble metals, in particu-
lar, Ru-containing catalysts exhibiting high activity in
processes performed in the presence of water [11–14]
formed in large amounts inbio-oil HDO, are of inter-
est in this respect. The support is an important compo-
nent of hydrodeoxygenation catalysts; its textural and
acid characteristics significantly influence the reaction
product distribution and the deoxygenation efficiency.
Initially classical γ-Al2O3 was used as a catalyst sup-
port inbio-oil processing; however, in the presence of
water γ-Al2O3 transforms into boehmite AlO(OH),
which can cause blocking and deactivation of the cata-
lyst active sites [15]. The following supports are used
as alternatives to γ-Al2O3: zeolites [16], carbon sup-
ports [9], SiO2 [9], ZrO2 [10], CeO2, TiO2 [17, 18], and
their mixtures [9, 19]. Mesoporous inorganic supports
are of particular interest for hydrodeoxygenation pro-
cesses [11, 20–26]. It is assumed that their developed
structure reduces the diffusion hindrance and favors
enhancement of the catalytic activity, and the optimum
acidity level ensures efficient deoxygenation [27–29].
Synthesis of ZSM-5/MCM-41 micro-mesoporous
material. ZSM-5/MCM-41 (Si/Al = 16) was prepared
by the procedure similar to that described in [30]. The
following chemicals were used: cetyltrimethylammo-
nium bromide (CTAB, Sigma–Aldrich, 98%), sodium
aluminate (Sigma–Aldrich), pyrogenic silica (Sigma–
Aldrich, 99.99%), tetrapropylammonium hydroxide
(TPAОН, Sigma–Aldrich, 1 М aqueous solution), so-
dium hydroxide (Irea 2000, chemically pure grade), and
distilled water. The gel containing templates of ZSM-
5 and mesoporous material had the following molar
composition: 3.1Na2O : 31SiO2 : Al2O3 : 1.9CTAB :
0.23TPAОН : 854H2O.
Synthesis of Ru/ZSM-5/MCM-41 catalyst. A
typical procedure for preparing Ru/ZSM-5/MCM-41
includes impregnation of the support with an aqueous
solution of ruthenium chloride, taken in an amount
corresponding to the moisture capacity, followed by
solvent removal on a water bath in an air stream. To this
end, a calculated amount of the support was added to
a solution of the metal salt at 25°С. After adding the
impregnating solution to the support, the liquid layer
over it did not exceed 2 mm. The mixture was stirred for
10-min periods with 10-min breaks (total time 2 h) and
left overnight for impregnation. After that, the sample
was dried in an air stream at 60°С for 6 h, at 100°C
for 3 h, and finally at 250°С for 3 h and was reduced
in a hydrogen stream at 400°С for 3 h. Ruthenium(III)
chloride (Aurat, ruthenium weight fraction no less than
46.5%) was used as a metal source. When preparing the
metal salt solution, the RuCl3 amount was calculated so
as to ensure 1 wt % ruthenium content of the catalyst.
Oneofmodelcompoundsfrequentlyusedforstudying
the relationships ofbio-oil hydrodeoxygenation is
guaiacol, because it contains both hydroxyl (Csp2OH),
and methoxy (Csp2OCH3) fragments as the most typical
fragments of oxygen-containing phenolic components
ofbio-oil [1]. Because it is known thatbio-oil contains
a large amount of water released from plant cell walls
in the course of biomass pyrolysis and formed by
hydrotreating of thebio-oil itself, it is necessary to
evaluate the effect of water on the catalytic system.
Therefore, in studying guaiacol HDO, we chose water
as the second component of the reaction mixture.
Devices and methods. The nitrogen adsorption/
desorption isotherms were recorded at 77 K with a
Gemini VII 2390 device. Prior to measurements, the
samples were degassed at 350°С for 6 h. The specific
surface area was calculated by the BET method using
the adsorption data in the relative pressure range p/p0 =
0.04–0.20. The pore volume and pore-size distribution
were determined from the adsorption branch of the
isotherms using the Barrett–Joyner–Halenda model.
The micropore volume was determined using the
Horvath–Kawasoe model. The specific pore volume
was determined from the amount of adsorbed nitrogen
at the relative pressure p/p0 = 0.99.
Thus, our goal was to evaluate the activity of Ru-
containing catalysts supported on ZSM-5/MCM-41
micro-mesoporous material in hydrodeoxygenation of a
model compound ofbio-oil, guaiacol, in the presence of
water.
The acidity of the catalyst sample was determined by
temperature-programmed ammonia desorption (TPD-
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1172
ROLDUGINA et al.
NH3) with a MicromeriticsAutoChem HPchemisorption
analyzer. The sample was finely divided, and the 1–2-
mm fraction was taken, placed in a reactor, and kept
in a helium flow at 400°C for 30 min, after which the
temperature was decreased to 100°C, and the sample
was saturated with ammonia for 30 min. Physically
adsorbed ammonia was removed in a helium flow until
the baseline drift ceased. Then, the temperature was
raised to 750°C at a rate of 20 deg min–1. The amount of
ammonia desorbed in the course of ammonia TPD was
calculated using the AutoChem HP V2.04 program.
consisting of 0.25 g of guaiacol (Sigma–Aldrich,
≥98%) and 0.25 g of water. The autoclave was sealed
and filled with hydrogen to a pressure of 5 MPa. The
reaction was performed in the temperature interval
130–290°С for 3 h; the temperature was monitored
with a thermocouple. After the reaction completion, the
autoclave was cooled below the room temperature and
opened. The catalyst was separated from the solution
by centrifugation. For complete dissolution of organic
compounds and homogenization of the reaction mixture,
isopropanol (Irea 2000, analytically pure grade) was
added to the sample prior to chromatographic analysis.
The hydrogenation products were analyzed with a
Kristallyuks 4000 М chromatograph equipped with
a flame ionization detector and a Petrocol® DH 50.2
capillary column (50 m × 0.25 mm, stationary phase
polydimethylsiloxane). The carrier gas was helium (flow
split ratio 1 : 90). The liquid products were additionally
analyzed by gas chromatography–mass spectrometry
with a Finnigan MAT 95 XL device. Its chromatograph
was equipped with a Varian VF-5MS capillary column
(30 m × 0.25 mm × 0.25 μm); the carrier gas was helium
(1.5 cm3 min–1).
A transmission electron microscopic (TEM) study of
the catalyst was performed with a Jeol JEM-2100 device
(magnification from 50 to 1 500 000, image resolution
0.19 nm at 200 kV).
The ruthenium content was determined by energy-
dispersive X-ray fluorescence (EDX) analysis with a
Thermo Fisher Scientific ARLQuant’X analyzer in a
vacuum. The results were processed using the UniQuant
method without references. The samples were prepared
as follows: Powdered samples were pressed in pellets on
a boric acid support and covered with a Lavsan [material
based on poly(ethylene terephthalate)] film, which was
pressed to the cell with a special ring.
RESULTS AND DISCUSSION
X-ray photoelectron spectroscopic (XPS) studies
were performed with a Kratos Axis Ultra DLD device
using AlKα excitation X-ray radiation (1486.6 eV). The
photoelectron peaks were calibrated using the С1s line
(binding energy 284.8 eV). The transmission energy of
the energy analyzer was 160 eV for the total spectrum
and 40 eV for separate lines.
The textural characteristics of the Ru/ZSM-5/
MCM-41 catalyst were determined by low-temperature
nitrogen adsorption–desorption. Ru/ZSM-5/MCM-41
is characterized by the adsorption–desorption isotherm
of type IV with a hysteresis loop confirming the
mesoporous structure (Fig. 1) [30, 31].
Temperature-programmed reduction with hydrogen
(TPR-Н2) was performed with a Micromeritics Auto-
Chem HP2950 chemisorption analyzer. Prior to the ex-
periments, the sample was calcined in a muffle furnace
in an air stream at 550°С for 4 h to convert the ruthe-
nium to the oxide form. The test sample (~0.1 g) was
placed in a quartz reactor and kept for 1 h in an argon
flow at 400°C. After that, the temperature was decreased
to 60°С, the flow rate of the argon–hydrogen mixture
(8 vol % Н2) of 30 mL min–1 was set, and the tempera-
ture was raised to 400°С at a rate of 10 deg min–1.
The catalyst obtained has high specific surface area
and high specific pore volume: SBET = 392 m2 g–1, Vpore
=
0.41 cm3 g–1 (Table 1). After impregnation of the micro-
mesoporous support with the ruthenium salt, the specific
surface area decreased only slightly. Thus, application
of 1 wt % metal does not significantly affect the texture
characteristics of the catalyst.
The Ru/ZSM-5/MCM-41 catalyst was studied by
TPD-NH3 (Table 2). Depending on the temperature
ranges of the ammonia desorption, the acid sites
can be subdivided into weak (Т < 300°С), medium-
strength (Т = 300–500°С), and strong (T > 500°С).
The Ru/ZSM-5/MCM-41 sample exhibits high acidity
(1087 μmol g–1) and contains mainly medium-strength
acid sites (841 μmol g–1) with small amounts of weak
(90 μmol g–1) and strong (157 μmol g–1) acid sites.
Procedure of catalytic experiments. The catalytic
experiments on HDO of guaiacol at elevated hydrogen
pressure were performed with vigorous stirring in
a 10-mL steel autoclave equipped with a magnetic
stirrer. The autoclave was charged with the calculated
amount of the catalyst (25 mg) and a model mixture
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RUTHENIUM CATALYSTS ON ZSM-5/MCM-41 MICRO-MESOPOROUS SUPPORT
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Table 1. Textural characteristics of micro-mesoporous aluminosilicate and catalyst based on it
BET specific
surface area,
m2 g–1
Micropore
surface area,
m2 g–1
Pore
Micropore
volume,
cm3 g–1
Fraction of
Pore
diameter, Å
Sample
volume,
micropores
cm3 g–1
(Smicro/SBET
)
ZSM-5/MCM-41
425
392
77
74
0.43
0.41
39
42
0.02
0.06
0.18
0.19
Ru/ZSM-5/MCM-41
Table 2. TPD-NH3 data for Ru/ZSM-5/MCM-41
Acid sites, μmol g–1
Total amount of acid sites, μmol
Sample
g–1
weak
90
medium
strong
157
Ru/ZSM-5/MCM-41
841
1087
The thermal desorption curve has a pronounced maxi-
mum at approximately 320°С, corresponding to medi-
um-strength acid sites, and weaker peaks at approxi-
mately 160 and 550°С, corresponding to weak and
strong acid sites (Fig. 2).
TEM examination confirms that the ZSM-5/MCM-
41 support is a crystalline mesophase material with
lamellar structure [30]. The secondary mesoporous
structure is formed via formation of slitlike voids, caused
by deformation of the hexagonal framework of МСМ-
41 in the course of the zeolite crystallization. Ordered
pores with a size of approximately 30–40 Å are seen in
the electron micrographs. The porous structure of ZSM-
5 zeolite can be clearly observed at high resolution
of the images (Fig. 3b). According to the TEM data,
ruthenium particles are uniformly distributed over
the support surface. Their mean size is approximately
1.7 nm; coarser particles (up to 6 nm) are present in a
small amount (Figs. 3c. 3d). The appearance of coarser
particles may be due to agglomeration of nanosized
particles in the course of the catalyst reduction in a
hydrogen stream at elevated temperature [32].
Relative pressure p/p0
Fig. 1. Nitrogen adsorption/desorption isotherms for
Ru/ZSM-5/MCM-41 and ZSM-5/MCM-41.
The curve of the temperature-programmed reduction
of Ru/ZSM-5/MCM-41 sample with hydrogen has two
peaks at 106 and 253°С [22], with the first peak having
a shoulder at 130–160°С (Fig. 4). The first maximum
corresponds to the reduction of RuO2 on the external
surface of ZSM-5/MCM-41 [33, 34]. The broad
shoulder can be attributed to the reduction of ruthenium
oxides and oxychlorides tightly bound to the surface
of the micro-mesoporous support; it requires higher
temperatures [34]. The maximum at 253°С belongs
Temperature, °C
Fig. 2. Temperature-programmed ammonia desorption
spectrum of Ru/ZSM-5/MCM-41.
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ROLDUGINA et al.
(b)
(a)
200 nm
20 nm
(d)
(c)
50 nm
Number of particles
Fig. 3. TEM images of (a, b) ZSM-5/MCM-41 and (c) Ru/ZSM-5/MCM-41 and (d) ruthenium particle size distribution.
to the reduction of difficultly accessible ruthenium in
narrow pores of ZSM-5/MCM-41 [35].
reliably. However, the spectrum deconvolution revealed
the presence of two states of Ru, Ru3d5/2 and Ru3d3/2,
with the binding energies of 279.9 and 283.8 eV, which
corresponds to ruthenium metal [32, 36]. Also, there
AccordingtotheTPR-Н2 data(Table3),theruthenium
content of Ru/ZSM-5/MCM-41 is 1.0 wt %, which is
confirmed by elemental analysis. The most pronounced
hydrogen uptake is observed at 106°С (0.194 mmol g–1),
suggesting the formation of ruthenium nanoparticles
mainly on the surface of the micro-mesoporous material.
Figure 5 shows the high-resolution Ru3d fragment of
the XPS spectrum of Ru/ZSM-5/MCM-41. Because of
the overlap of the С1s (285 eV) and Ru3d3/2 lines, only
the Ru3d5/2 component (280.1 eV) could be identified
Table 3. TPR-H2 data for Ru/ZSM-5/MCM-41
Peak temperatures in the reduction
curves, °C
106/253
–1
Amount of adsorbed H2, μmol gcat
0.194/0.01
1.0
Temperature, °C
Ruthenium content,a wt %
Fig. 4. Curve of temperature-programmed reduction of
Ru/ZSM-5/MCM-41 with hydrogen.
a Calculated assuming complete reduction of RuO2.
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RUTHENIUM CATALYSTS ON ZSM-5/MCM-41 MICRO-MESOPOROUS SUPPORT
1175
are maxima in the spectra at the energies of 280.8 and
285.2 eV, corresponding to ruthenium in the oxidized
state [36].
The catalytic activity of the Ru/ZSM-5/MCM-41
sample obtained was evaluated in hydrodeoxygenation
of a modelbio-oil compound, guaiacol, at 5 MPa of
Н2 in the temperature interval 130–290°С for 3 h. The
guaiacol/water weight ratio equal to 1 was chosen to
make the model mixture composition similar to that
of the realbio-oil, in which the water content can vary
in the interval 15–40 wt % depending on the initial
biomass composition and pyrolysis conditions [1]. It
should be noted that, in HDO experiments, an aqueous
solution containing no more than 8 wt % substrate is
used as a model mixture [9, 16]. This study was aimed
at evaluating the behavior of the catalyst in the presence
of a large amount of one of phenolicbio-oil components,
guaiacol (50% aqueous solution).
Binding energy, eV
Fig. 5. Ru3d X-ray photoelectron spectrum of Ru/ZSM-5/
MCM-41.
We have studied the activity of Ru/ZSM-5/MCM-
41 in HDO of guaiacol in the temperature interval
130–290°С. At 130°С, the guaiacol conversion was
45% (Fig. 6); on heating to 150°С, the conversion
sharply increased to 98%. In the temperature interval
150–210°С, the conversion was close to 100%. As the
reaction temperature was increased further, the guaiacol
conversion gradually decreased (to 90% at 230°С).
At still higher temperatures, the conversion decreased
drastically: to 68% at 250°С, reaching a constant value
of ~50% at 270–290°С.
Temperature, °C
Fig. 6. Guaiacol conversion in the presence of Ru/ZSM-5/
MCM-41 catalyst as a function of temperature.
increased, and their content reached 66%. Products
of direct deoxygenation of guaiacol, namely, anisole,
phenol (O-benzenes, ~1%), benzene (2%), and
cyclohexene (2%), were also detected in the reaction
products. On further temperature elevation to 250°С,
the trend toward active formation of cycloalkanes (with
a decrease in the total concentration of O-cyclohexanes
and methoxycyclohexanol) and toward a gradual
increase in the selectivity with respect to benzenes
was preserved. At 250°С, the selectivity with respect
to cycloalkanes reached a maximum (78%); the other
major products were benzene (10%), phenol, and anisole
(О-benzenes, 6%). At this reaction temperature, small
amounts of heavier guaiacol HDO products, methyl- and
dimethylphenols (polymethylphenols, ≤2%) appeared
in the product mixture. Further temperature elevation
to 270°С led to a sharp decrease in the concentration
of cycloalkanes to 42%; the content of О-cyclohexanes
and methoxycyclohexanol did not exceed 2 and 3%,
respectively. The fractions of phenol and anisole
The reaction temperature significantly influences
the distribution of guaiacol HDO products (Fig. 7).
HDO of guaiacol at 130°С yielded products of
direct hydrogenation of the guaiacol aromatic ring:
methoxycyclohexanol (selectivity 62%) and hydroxy-
and methoxycyclohexanes (О-cyclohexanes, 36%).
Cyclohexane, and also phenol and anisole (O-benzenes,
~1%) were detected in trace amounts. The temperature
elevation from 150 to 190°С led to a decrease in the
methylcyclohexanol content from 55 to 41%, with
the amount of О-cyclohexanes remaining on the level
of ~40%; the selectivity with respect to cycloalkanes
(cyclohexane and trace amounts of methylcyclohexane
and methylcyclopentane) gradually increased from 3
to 19%. At 210°С, the reaction mixture composition
became considerably more complex. The content of
methylcyclohexanol decreased to 24%, and that of
hydroxy- and methoxycyclohexanes, to 6% (in total);
the selectivity with respect to cycloalkanes sharply
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ROLDUGINA et al.
Cycloalkanes
Benzene
Cyclohexene
Hydroxy-, methoxycylcohexane
(O-cyclohexanes)
Phenol, anisole (O-benzenes)
Methoxycyclohexanol
Polymethylphenols
Selectivity, %
Fig. 7. Product distribution in HDO of guaiacol in the presence of Ru/ZSM-5/MCM-41 catalyst at various temperatures (130–290°С).
Cycloalkanes: cyclohexane and trace amounts of methylcyclohexane and methylcyclopentane.
(O-benzenes), benzene, and cyclohexene increased to
29, 12, and 11%, respectively. Elevation of the reaction
temperature to 290°С did not lead to significant changes
in the reaction product distribution. The selectivity with
respect to cycloalkanes slightly increased (to 50%),
the content of hydroxy- and methoxycyclohexane
(O-cyclohexanes) was 1%, and methoxycyclohexanol
was not detected. The selectivity with respect to
О-benzenes and cyclohexene slightly decreased: from
29 to 23 and from 11 to 9%, respectively. The selectivity
with respect to benzene increased from 12 to 17%.
At the reaction temperatures of 270 and 290°С (as at
250°С), polymethylphenols were detected in trace
amounts (<2%).
With increasing temperature, the contribution of this
pathway increases: At 290°С, the selectivity with re-
spect to benzene is 19%. In the temperature interval
210–250°С, the reaction rates for the first pathway
increase, which is manifested in a sharp increase in
the selectivity with respect to cycloalkanes (to 78% at
250°С) with a decrease in the selectivity with respect
to hydroxy- and methoxycyclohexanes and to meth-
oxycyclohexanol to 2% (in total). At reaction tempera-
tures higher than 250°С, cycloalkanes were detected
in the reaction products (>40%), with O-cyclohexanes
and methoxycyclohexanol virtually disappearing. As
compared to lower temperatures, the products of the
reaction at 270°С contained considerably more phe-
nol and anisole (О-benzenes, 6% at 250°С and 29% at
270°С). The other major intermediates were benzene
and cyclohexene. Such product distribution indicates
that, when HDO is performed at elevated temperatures
(>250°С), the second pathway, direct guaiacol deoxy-
genation to benzene, followed by its hydrogenation,
prevails.
The analysis results suggest that HDO of guaiacol
under the conditions of our experiments in the pres-
ence of Ru/ZSM-5/MCM-41 occurs along two parallel
pathways [37]. At low temperatures (up 210°С), the
prevalent process is direct hydrogenation of the aro-
matic ring of guaiacol, followed by deoxygenation, in
accordance with the scheme guaiacol → methoxycy-
clohexanol → hydroxycyclohexane → methoxycy-
clohexane → cyclohexane → methylcyclohexane. At
210°С, a side process starts to occur: direct deoxy-
genation of guaiacol to phenol and anisole and then
to benzene, followed by hydrogenation of benzene to
cyclohexene and cyclohexane (guaiacol → phenol →
anisole → benzene → cyclohexene → cyclohexane).
The occurrence of HDO along two pathways is pos-
sible owing to bifunctional properties of the Ru/ZSM-5/
MCM-41 catalyst: Hydrogenation of the aromatic ring
occurs on metallic active sites of the catalyst [9, 16], and
deoxygenation, on acid sites of the micro-mesoporous
support [32, 37]. For the efficient deoxygenation, the
catalyst should have a large number of acid sites ac-
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RUTHENIUM CATALYSTS ON ZSM-5/MCM-41 MICRO-MESOPOROUS SUPPORT
1177
tive in the given temperature interval; however, strong
CONFLICT OF INTEREST
acid sites favor the formation of polymethylation by-
products, which act as coke precursors and decrease
the catalyst activity [32]. It should be noted that, in
HDO of guaiacol in the presence of Ru/ZSM-5/MCM-
41 catalyst exhibiting high acidity (1087 μmol g–1) but
containing only a small amount of strong acid sites
(Fig. 2), the selectivity with respect to polymethylphe-
nols does not exceed 2% even at elevated temperatures
(>250°C). The developed hierarchic structure of the
support, ZSM-5/MCM-41, ensures the maximal ac-
cess of reagents to active sites of the catalyst, which
is manifested in high parameters of guaiacol conver-
sion at moderate reaction temperatures. However, at
temperatures higher than 250°С, the guaiacol conver-
sion sharply decreases from >90 to 50%. This may be
associated with the catalyst deactivation at elevated
temperatures, caused by the formation of intermediate
products [anisole, phenol (O-benzenes), polymethyl-
phenols], which can be adsorbed on active sites of the
catalyst more strongly than guaiacol [38, 39].
A.L. Maksimov is the Editor-in-Chief of Zhurnal
Prikladnoi Khimii/Russian Journal of Applied Chemis-
try; the other coauthors declare that they have no con-
flict of interest.
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