Water-assisted selective hydrodeoxygenation of phenol to benzene over the Ru composite catalyst in the biphasic process

Article abstract of DOI:10.1039/c8gc04017f

Aromatic hydrocarbons as a liquid fuel have unique combustion properties (high volume energy density, etc.). Thus preserving the aromatic rings while selectively cleaving the C-O bonds in the hydrodeoxygenation of lignin derived substituted phenols without additional consumption of H₂ is of crucial importance. In this regard, the composite materials, which are used as the catalyst support, were prepared through the hybridization of Nb₂O₅ (niobium oxide) with MC (micro-mesoporous carbon). Ru NPs (nanoparticles) were loaded on the Nb₂O₅-MC composite supports by the incipient wetness impregnation method. And then the as-synthesized catalysts were characterized by various techniques, including XRD, SEM, TEM, BET, NH₃-TPD, XPS, etc. Under mild conditions (200-250 °C and 2.0-10 bar H₂), the Ru/Nb₂O₅-MC catalyst was proved to be highly effective for the hydrodeoxygenation (HDO) of phenol. With regard to achieving a high selectivity to benzene, the biphasic catalytic process in the decalin/water mixed solvent was superior to the monophasic processes in either the decalin or water solvent under the same conditions. Water acted as a co-solvent that prevented the occurrence of side reactions and promoted the catalytic C-O bond scission of phenol. The synergistic effect of the biphasic solvents (decalin and water) and the Ru/Nb₂O₅-MC composite catalysts, which might stabilize the emulsions and decrease the activation energy of HDO, was investigated. Meanwhile, other probe reactions were conducted to elucidate the mechanism of the HDO of phenol. The application of the efficient biphasic catalytic process may provide a promising approach for improving lignin valorization.

Full text of DOI:10.1039/c8gc04017f

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CRITICAL REVIEW
G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
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DOI: 10.1039/C8GC04017F
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Water-assisted selective hydrodeoxygenation of phenol to
benzene over Ru composite catalyst in biphasic process†
Cheng Zhang,a,c Chuhua Jia,c Yang Cao,a Yao Yao,c Shaoqu Xie,c Shicheng Zhang,a,b* and Hongfei Linc*
Received 00th January 20xx,
Accepted 00th January 20xx
Aromatic hydrocarbons as a liquid fuel have unique combustion properties (high volume energy density, etc.). Thus
preserving the aromatic rings while selectively cleaving the C–O bonds in the hydrodeoxygenation of lignin derived
substituted phenols without additional consumption of H2 is of crucial importance. In this regard, the hybridization of
niobium oxide with MC (micro-mesoporous carbon) as the support was prepared by the incipient wetness impregnation
method and characterized by various techniques, including XRD, SEM, TEM, BET, NH3-TPD and XPS, etc. Under the mild
conditions (200-250 oC and 2.0-10 bar H2), the Ru/Nb2O5-MC catalyst was proved to be highly effective for the
hydrodeoxygenation (HDO) of phenol. With regard to achieving a high selectivity to benzene, the biphasic catalytic process
in the decalin/water mixed solvent was superior to the monophasic processes in either the decalin or water solvent under
the same conditions. Water acted as a co-solvent that prevented the occurrence of side reactions and promoted the catalytic
C–O bond scission of phenol. The synergistic effect of the biphasic solvents (decalin and water) and the Ru/Nb2O5-MC
composite catalysts, which might stabilize the emulsions and decrease the activation energy of HDO, was investigated.
Meanwhile, other probe reactions were conducted to elucidate the mechanism of the HDO of phenol. The application of the
efficient biphasic catalytic process may provide a promising approach for improving lignin valorization.
DOI: 10.1039/x0xx00000x
www.rsc.org/
compounds. HDO of phenol has been reported to proceed via two
generalized pathways, DDO and HYD.10–12 One pathway proceeds
through direct deoxygenation (DDO) in which the C–O bond in
1. Introduction
Thermochemical processes, such as pyrolysis, are the prevailing
methods of converting lignocellulosic biomass, which is the
abundant renewable carbon resources, to biofuels and chemicals. In
such a process, raw biomass is directly converted to crude bio-oils,
which, unfortunately, have a high oxygen content (30-50 wt.%) as
well as some significant disadvantages, such as high acid content,
viscosity, corrosion, chemical instability and low heating value.1–3
Thus, bio-oils require upgrading to fungible fuels or high value
chemicals through partial or complete removal of oxygen. One
attractive upgrading option is hydro-treating, i.e. catalytic
hydrodeoxygenation (HDO).4,5 The complex chemical structure of
lignocellulosic biomass leads to hundreds of oxygenated bio-oil
compounds, among which phenol and substituted phenols derived
from lignin are the precursors of synthesizing aromatic compounds
such as benzene, toluene, and xylene (BTX).6–8
phenol is directly cleaved to form benzene. The DDO utilizes one
hydrogen equivalent for reduction of one phenol molecule. Another
pathway is catalytic hydrogenation (HYD) of phenol to cyclohexane,
which consumes four hydrogen equivalents for deoxygenation.
Although the conversion of phenol into cyclohexane through deep
HDO reaction is technically feasible, it carries the penalty of
additional H2 consumption. Indeed, it is highly challenging to
selectively cleave the strong aryl ether C–O bond in phenol (414
kJ·mol-1) while preserving the aromatic rings.13 Severe process
conditions are typically required for HDO of phenolic compounds to
synthesize aromatic products in monophasic solvent at high reaction
temperatures (∼400 oC), which facilitates phenolic C–O bond
cleavage, and with high hydrogen pressures (∼30 MPa), which
alleviates catalyst deactivation by reducing coke formation.2,14
Moreover, there is
a well-known competition between the
Thus, phenol is often chosen as a model compound of lignin-
derived phenolics because it contains the Caromatic–OH,9 one of the
major oxygen-containing functional groups found in lignin-derived
hydrogenolysis of C–O bonds and the hydrogenation of the aromatic
rings during the HDO of phenol.15–17
The full realization of the bio-refinery concept hinges on the
development of highly active and selective catalysts.18 Therefore,
design of new functional catalysts with unique selectivity is the key
to overcome the barrier. Several alternatives to traditional catalysts,
including noble metal catalysts,2,5,11,19,20 metal phosphides,21,22
carbides,21,23 and nitrides,21,24 have been widely recognized, showing
differences in removing oxygen from phenolic model compounds.
Recently, niobium and titanium-based catalysts have shown great
promise in promoting the cleavage of C–O bonds.9,10,25,26 Moreover,
a. Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3),
Department of Environmental Science and Engineering, Fudan University,
Shanghai 200438, China. E-mail: zhangsc@fudan.edu.cn
b. Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, China
c. The Gene and Linda Voiland School of Chemical Engineering and Bioengineering,
Washington State University, Pullman, WA 99164, USA. E-mail: hongfei.lin@wsu.edu
†Electronic Supplementary Information (ESI) available: BET for 5Nb2O5-5MC support
and Ru/5Nb2O5-5MC catalyst, XPS spectra in the Nb 3d and colour photograph of
the emulsion, additional figures and tables. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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Ru-based catalysts with particle sizes of ∼2 nm showed the high Pluronic P123 (Mw = 5800, EO20PO70EO20), Pluronic F127 (Mw = 12600,
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activity and DDO selectivity.10,26,27 It is reported that the effect of EO106PO70EO106), niobium chloride (99%), rutDheOnI:iu10m.10a3c9e/tCyl8aGcCet0o4n0a17teF
ruthenium metal and acid supports under working conditions is (97%), and fructose were purchased from Sigma-Aldrich. All these
crucial for manipulating the competition steps of DDO and HYD. As H chemicals were used without further purification.
concentration decreases (or low hydrogen pressure) at high
2.2 Preparation of catalyst
temperatures, a disparity in the strength of the adsorbed H species,
The mesoporous materials were prepared by using P123 and F127 as
which are nearby the benzene rings and oxygen atoms, leads to
the composite template, fructose as the carbon source and niobium
hydrogenolysis of the C–O bond of aryl ethers.28–30 The phenolic OH
chloride as the niobium precursor via a hydrothermal method.
is replaced with H to form benzene by a direct deoxygenation (DDO)
Briefly, 0.60 g of P123 and 1.80 g of F127 were dissolved in 40 mL of
pathway.10,26 In addition, phenol can also be dehydroxylated to
water, followed by mixing with 1.20 g of niobium chloride and
benzene through hydrogenation of phenol to cyclohexanol,
different mass of fructose to get the desired MC/Nb2O5 ratios. For
dehydration of cyclohexanol to cyclohexene, and dehydrogenation
instance, when the mass ratio of MC to Nb2O5 was 5:5, it was
of cyclohexene to benzene.31,32
denoted by 5Nb2O5-5MC. After stirring for 2 h at room temperature,
On the other hand, the approaches of process intensification such
the solution was transferred into high-pressure reactors and
as in-situ separation are also promising to increase the selectivity.
Many researchers have attempted to couple biphasic
o
crystallized at 180 C in the oven for 24 h. Then the solution was
filtered and the filter cake was washed with deionized water until
reaction/separation system for high conversion and selectivity. In
reaching the neutral pH value and then was dried at 80 oC overnight.
biphasic catalysis, Pickering emulsions stabilized by solid catalyst
The as-prepared samples were calcined at 700 oC for 3 h under
particles provide a platform for catalyzing interfacial reactions,
nitrogen atmosphere to obtain the mesoporous niobium oxide-
increasing the contact surface area, and separating the product from
carbon supports finally.
All supported Ru catalysts were prepared by the incipient wetness
the system efficiently.33–35 Besides, the wettability of the particles
plays a significant role in the emulsion states (water in oil or oil in
impregnation method. The composite supports were impregnated
water) and the catalytic performance.36–38 In such a biphasic system,
with a Ru(acac)3 solution containing a Ru loading of 1 wt.%. The
organic solvent extracts hydrophobic intermediates and products
impregnated samples were then kept overnight for 24 h at room
and thus significantly impacts on the product distribution and
temperature followed with drying at 110 oC for 12 h. Finally, the
selectivity. For instance, HDO of lignin-derived model compounds
obtained solid was reduced in a H2 flow at 200 oC for 3 h prior to use
with a heating ramp of 5 oC·min−1.
(vanillin, guaiacol, eugenol, catechol, etc.) in the biphasic system
achieved higher yields of hydrocarbon products than that in the
The Ru content in these catalysts was obtained by inductively
monophasic system.33,39–41 Recently, our group has developed a
coupled plasma atomic emission spectroscopy (ICP-MS). The Ru wt.%
of the catalysts was 1±0.3%.
novel biphasic tandem catalytic process (biTCP), in which the
hydrophilic and hydrophobic catalysts were partitioned into the
aqueous and organic phases, respectively, for the conversion of 2.3 Characterization of catalyst
terpenoids to cycloalkanes.42,43 We further demonstrated that that
highly hydrophilic carbohydrates (i.e., fructose) can be selectively
converted into value-added hydrophobic furan derivative chemicals
in the “one-pot” biphasic tandem catalytic process.44 In our previous
X-ray powder diffraction (XRD) patterns were obtained on a Bruker-
AXS D2 PHASER powder X-ray diffractometer using Co-Kα radiation
source (λ= Å). Measurements were carried out between 10° and 90°
at 1°·min-1 and a step size of 0.02° in 2θ.
research, we observed that both the selectivity and the kinetic rate
The N2 adsorption isotherms of the CNs were measured at -196 oC
in the biphasic tandem catalytic process were significantly higher
using a Quantachrome Autosorb iQ2 instrument. The surface area
than those in the monophasic processes. Therefore, we conceive
(SBET) was calculated from the N2 isotherms by the fitting of
that biphasic catalytic process is a generalized “one-pot” synthesis
Brunauer-Emmett-Teller (BET) method. The total pore volume (VT)
approach to produce hydrophobic biofuels or bio-based chemicals
was determined from the amount of N2 adsorbed at a relative
pressure (P/P0) of ∼0.99. The pore size distributions (PSDs) were
obtained by Density Functional Theory (DFT) fitting of N2 adsorption
from relatively hydrophilic biomass feedstock.
In this paper, we are applying the similar approach to develop a
biphasic catalytic process for selectively converting phenol to
data. The micropore surface area (Smic) was obtained from that pores
benzene. Here we report our findings on a “one-pot” process of
less than 2 nm in DFT fitting, and microporosity was indicated by the
phenol conversion over the Ru/Nb2O5-MC catalysts to produce
Smic/SDFT
.
benzene in the biphasic water-decalin solvent. The catalysts were
rationally designed with a synergistic effect between the Ru, Nb2O5
and MC species that result in the high selectivity to benzene from the
conversion of phenol in the biphasic system.
X-ray photoelectron spectroscopy (XPS) analyses were performed
using a Multilab2000 X-ray photoelectron spectrometer with Mg−Kα
as the photon source. The binding energies (BEs) reference was taken
by using the C 1s peak at 284.6 eV.
The surface properties of the catalysts were investigated by
temperature-programmed desorption of ammonia (NH3-TPD). The
samples (50 mg) were reduced at 573 K for 1 h under a flow of H2 of
60 mL/min and then purged in He flow for 30 min. After reduction,
the sample was cooled to 373 K, and the feed composition was
switched to a mixture containing 20% NH3 in He (30 cm3·min−1) for
2. Experimental
2.1 Materials
Phenol (> 98%), decalin (> 99%) were purchased from Tokyo
Chemical Industry Co. (TCI). Dodecane (≥ 99%), guaiacol (> 98%),
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30 min. The physisorbed ammonia was flushed out with He flow for desorption isotherms are shown in Figure 1A. For MC, 3Nb2O5-5MC,
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DOI: 10.1039/C8GC04017F
1 h. Then, the catalyst was heated at 10 K/min under He to 873 K. 5Nb2O5-5MC and 8Nb2O5-5MC, the relatively broad knee in the low-
The reactor effluent was continuously monitored by mass pressure range suggests the existence of a significant amount of
spectrometry.
micropores. Moreover, the Nb2O5 support exhibits a type IV
The crystal size and morphology of the Nb2O5-MC support and the isotherm, the typical characteristic of mesoporous materials
location and size range of the Ru particles were investigated using according to the IUPAC classification.12 Figure 1B exhibits the pore
scanning transmission electron microscopy (STEM), high-resolution distribution calculated by the density functional theory (DFT) model.
transmission electron microscopy (HR-TEM) and energy dispersive X- In addition, the porous textural details of these composite materials
ray spectrocopy (EDS) analyses. The imaging was performed using a are summarized in Table 1. Moreover, it can be observed that the
FEI Tecnai F20 field-emission gun with an accelerating voltage of 200 specific surface areas of these five supports by both the BET
kV. A symmetrical multibeam illumination was used with a Gatan measurement and the DFT calculation decrease as the content of MC
Ultrascan CCD camera, a 30 mrd camera length, and a 1 nm probe decreases gradually. Especially, there are no micropores but only
size. Data processing and analysis were carried out using the Gatan mesoporous ones in the Nb2O5 support, which is consistent with the
Digital Micrograph software. Catalyst powder samples were pore distribution of Nb2O5 in Figure 1B and Table 1. Moreover, the
dispersed on lacy carbon copper grids.
BET surface area and the total pore volume changed slightly when Ru
The water contact angles were measured at ambient temperature was loaded onto the 5Nb2O5-5MC support, suggesting that Ru
on a self-supporting pressed sample disc by a contact angle meter nanoparticles (NPs) do not block or alter the pores of the support
(Dataphysics, OCA 20). Once a drop of water was deposited on the (Figure S1). Note that the pore size of the 5Nb2O5-5MC support is
surface of the sample disc, the contact angle was determined from near ∼7.0 nm (Table 1).
pictures immediately captured using
a charge-coupled-device
camera. The water droplets were introduced using a micro syringe,
and images were captured to measure the angle of the liquid solid
interface; each sample was recorded at three different points.
2.4 Hydrodeoxygenation of phenol
The HDO reactions were carried out in a 45 mL Parr reactor equipped
with a magnetic stirrer. In a typical experiment, phenol (0.05 g) and
catalyst (0.05 g) were dispersed in a 20 mL mixture of deionized
water and decalin. After purging the reactor with hydrogen 6 times,
10 bar of hydrogen was maintained at ambient temperature while
stirring at 700 rpm, and then the reactor was heated up to 200 oC. At
the end of the reaction period, the reactor was taken apart and
cooled to room temperature quickly. To break the emulsion before
analysis, the Nano hybrid particles were separated in two steps. In
the first one, a common paper filter was used. This coarse paper filter
(8-micron pore) trapped a large faction of the solid particles, which
quickly agglomerate over the surface of the filter. In the second step,
the two clear liquid phases obtained after filtration were separated
and centrifuged at 10000 rpm for 5 min respectively. Then the
sample of each phase were analysed by gas chromatography, GC-FID
and GC-MS.
Fig. 1 The nitrogen adsorption-desorption isotherms of five supports
(A). The pore size distribution calculated from the adsorption
isotherms using DFT method (B)
Table 1 Physical properties of MC, 3Nb2O5-5MC, 5Nb2O5-5MC,
8Nb2O5-5MC, Nb2O5 and Ru/5Nb2O5-5MC
aSBET
bSmicro
cVtatal
dDp
(nm)
Catalysts
(m2·g-1)
(m2·g-1)
(cm3·g-1)
MC
523.67
347.59
285.42
287.49
31.48
492.77
184.98
155.18
148.09
0.00
0.3356
0.6476
0.5314
0.3577
0.0966
0.5364
2.56
7.45
7.45
4.98
12.27
7.37
3Nb2O5-5MC
5Nb2O5-5MC
8Nb2O5-5MC
Nb2O5
The following definitions were used to quantitate experimental
datum:
Conversion (%)
= [moles of carbon in reactant consumed / moles of carbon in
reactant fed] × 100
Selectivity (%)
Ru/5Nb2O5-5MC
290.97
136.48
= [moles of carbon in the product / sum of moles of carbon in all
products] × 100
Yield (%)
a
Specific surface area calculated by Brunauer-Emmett-Teller (BET)
equation.
b Specific surface area of micro pores calculated by DFT method.
c The total pore volume was determined from the amount of nitrogen
adsorbed at a relative pressure of 0.99.
= Conversion (%) × Selectivity (%) ÷ 100
d Calculated from the maximum in the BJH pore size distribution.
3. Results and discussion
3.1 Characterization of the catalysts
3.1.2 XRD. The XRD patterns of fresh catalysts obtained using
incipient wetness impregnation method are shown in Figure 2. When
3.1.1 BET. The porous texture of these samples was characterized
by nitrogen adsorption–desorption at 77 K. The N2 adsorption–
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o
the calcination temperature was higher than 650 oC, the acidity.47,48 The NH3 desorption peaks below 250 C correspond to
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orthorhombic Nb2O5 phase was formed which is evidenced by the partially ionic NH4+ that are assigned to the weak Brønsted acid sites
DOI: 10.1039/C8GC04017F
reflection peaks indexed as (001), (180), (181), (002), (380), (212), (BAS), while the NH3 desorption peaks in the temperature range of
o
(202), (2160), (1181), and (382), respectively.45,46 Moreover, Figure 2 250 to 500 C are assigned to the strong Brønsted acid sites or the
also showed no X-ray diffraction peaks attributed to the bulk metallic coordinated NH3 bounded Lewis acid sites (LAS), both of which are
Ru phase probably due to the ultra-finely dispersed small Ru NPs, considered as the strong acid sites. The areas of the NH3 desorption
which was consistent with the HRTEM (Figure 3).
peaks which are assigned to the strong acid sites, the weak acid sites
and the total acid sites, respectively, are all normalized to the surface
area as shown in Figure 4. The NH3-TPD curves (Figure S2) of all the
tested catalyst samples exhibit the similar broad peaks at the
temperatures lower than 300 oC, indicating the presence of plentiful
weak acid sites on all of the catalysts. Interestingly, compared to the
single Nb2O5 support, the normalized concentrations of both the
weak and the total acid sites on the hybrid Nb2O5-MC supports
increased substantially, which may explain the better performance
of the Ru/Nb2O5-MC catalysts.
Fig. 2 Powder XRD patterns of the fresh Ru/Nb2O5, Ru/Nb2O5-MC and
Ru/MC catalysts (A). Zoom on Ru (101) and Ru (002) diffraction peak
zone from 40 to 50 degree (B).
3.1.3 SEM and TEM. The morphology and microstructures of the
Ru/5Nb2O5-5MC catalyst was characterized by the scanning electron
microscopy (SEM) and transmission electron microscopy (TEM). The
SEM images (Figure 3A and 3B) showed the 3D frameworks
constituted by the particles with randomly opened micro-
mesopores, indicating that the pore structures were consistent with
the ones observed in the BET results (Figure 1 and Table 1). A typical
TEM image, as presented in Figure 3C, indicates that the Ru NPs with
a diameter of ∼2.5 nm were well distributed on the 5Nb2O5-5MC
support. Notably, the well-defined crystal lattice fringes are revealed
by high-resolution TEM (HRTEM) (Figure 3D), demonstrating
apparently distinct morphology profile from the pristine Ru/5Nb2O5-
5MC hybrid support. Moreover, the Ru NPs have interplanar spacings
of 0.205 and 0.234ꢀnm, ascribed to the (101) and (100) crystal planes
of Ru, respectively (Figure 3D). Owing to the limitation of resolution,
the bright dots of Ru cannot be observed in the HAADF-STEM image
of Ru/5Nb2O5-5MC, but the EDS mapping proves the existence of
highly dispersed Ru species on the 5Nb2O5-5MC hybrid support
(Figure 3E). The element mapping results further reveal the uniform
distribution of Nb, C, and O, confirming the successful hybridization
of Nb2O5 with MC in the support.
3.1.4 NH3-TPD. The NH3-TPD measurements were used to
evaluate the strength and distribution of the acid sites on the
catalysts. The NH3 desorption profiles are depicted in Figure S2 and
the amount of desorbed NH3 is compiled in Figure 4. In order to
facilitate the analysis, the acid sites are further classified according
to their strength. This classification is based on the NH3 desorption
temperature. The acidity distribution was determined by fitting the
TPD curves with a Gaussian function and calculating the area
corresponding to weak (T < 250 oC) and strong (250 oC < T < 500 oC)
Fig. 4 Acid concentration and the ratio of weak to strong acid of
Ru/3Nb2O5-5MC, Ru/5Nb2O5-5MC, Ru/8Nb2O5-5MC and Ru/Nb2O5
catalysts.
3.1.5 Wettability. To investigate the wettability of Ru/Nb2O5-MC
catalysts, the contact angles of water on the surface of the catalysts
were measured and the results are shown in Figure S3. The Ru/MC
catalyst presents a maximum contact angle of 106°, indicating a very
hydrophobic character, while the Ru/Nb2O5 catalyst exhibits its good
hydrophilicity with near-zero level in contact angle. For the
Ru/Nb2O5-MC catalysts, as the content of MC decreased, the contact
angle gradually decreased, indicating that the surface of the Nb2O5-
MC composite support became more hydrophilic. In water-oil
biphasic solvents, the wettability of the catalyst supports played a
key role in forming Pickering emulsions,49 with which the desirable
catalytic efficiency could be achieved in the water-in-oil system. As
expected, the difference in the wettability causes different activity
and selectivity of the Ru/Nb2O5-MC catalysts. It is found that in
aqueous phase reactions, hydrophilic catalysts are more active for
the HDO of phenol, because phenol is more soluble in water relative
to in decalin. However, in the water-oil biphasic system, the
heterogeneity of the catalyst supports with the hybrid hydrophobic
MC and hydrophilic Nb2O5 domains may compartment the catalyst
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at the water-oil interface, and thus stabilize the Pickering emulsions. composite catalyst, which, therefore, could affectVitehweArtriceleaOctniloinne
DOI: 10.1039/C8GC04017F
Moreover, the reactants and intermediates with different selectivity.
hydrophobicity may preferably adsorb on the specific domains of the
Fig. 3 Representative SEM images (A, B) and TEM images (C, D) of the Ru/5Nb2O5-5MC catalyst. Inset (C) the corresponding Ru nanoparticle
size distribution histogram. HAADF-STEM image and corresponding EDS elemental mapping images of this hybrid catalyst (E).
3.1.6 XPS. The X-ray photoelectron spectroscopy characterization
of Ru in these composite catalysts is shown in Figure 5. The curve
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fitting analysis was done in order to resolve multiple Ru oxidative hydrogenolysis of C–O bonds more difficult than hydrogenation of
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states and to quantify the chemical changes at the growing interface. arenes, as it typically requires severe condiDtiOonI:s10th.1a0t39le/Ca8dGtCo0a40lo17wF
As seen in Figure 5A, with the C 1s-Ru 3d core level spectra, three selectivity to the desired end product, benzene.56 As such, there exist
different ruthenium chemical states were resolved. The resolved Ru two competing reactions, hydrogenolysis of C–O bonds and
3d3/2 peaks (I, II and III)at binding energy of ∼284.3, ∼285.2 and hydrogenation of the aromatic rings, during phenol HDO. While there
∼286.2 eV are originated from the un-reacted/metallic Ru0, reacted are ample researches on new catalytic materials to improve the
Rux+ and reacted Ruy+ states respectively.50–52 The ratio of Ru0 / (Ru0 selectivity to benzene from the HDO of phenol in a single solvent
+ Rux+ Ruy+) are approximately 27.8%, 39.6%, 40.7%, 36.1%, 39.9% phase, to the best of our knowledge, there is lack of investigations of
and 38.0% for the fresh Ru/MC, Ru/3Nb2O5-5MC, Ru/5Nb2O5-5MC, new catalytic materials in a biphasic solvent system. Herein, we
Ru/8Nb2O5-5MC, Ru/Nb2O5 and the spent Ru/5Nb2O5-5MC catalysts, design the Ru catalysts on the Nb2O5-MC (micro/meso-porous
respectively (see Figure 5A and Table S1). The results indicate that carbon) composite supports for the HDO of phenol in the water-
there are more metallic Ru nanoparticles and/or nanoclusters on the decalin biphasic solvent system. Metal oxide nanoparticles have
surface of 5Nb2O5-5MC, which is in agreement with the TEM previously been used to stabilize oil-in-water emulsions because
observation (Figure 3C and D). Moreover, in the cases of the other their hydrophilicity preferentially orients them toward the aqueous
supports, there are more Rux+ or Ruy+ ions to interact with Nb2O5 to phase at the interface.57 In contrast, hydrophobic MC can also
form Ru-O-Nb bonds, and thus a higher percentage of ruthenium stabilize emulsions, but of the water-in-oil variety. Therefore, by
oxides would be observed (Table S2 and Figure S4). On the other tuning the compositions of the hybrid Nb2O5-MC composites, we
hand, the O 1s spectra of these Ru catalysts are significantly different could modify the hydrophilic-hydrophobic balance and assemble
(Figure 5B). The peaks 531.2 eV in binding energy can be assigned to water-in-oil or oil-in-water emulsions reproducibly.
lattice oxygen (O2-, Oβ), while those at 533.1 eV are the characteristic
of oxygen vacancies (O/OH, Oα). It is common to evaluate the
concentration of oxygen vacancies using the Oα / (Oα+Oβ) ratio.51,53,54
The highest ratio is 63.0% for Ru/5Nb2O5-5MC among these catalysts
(Table S3), which suggests that the concentration of oxygen defects
on the surface of Ru/5Nb2O5-5MC is higher than those on the other
catalysts. The Ru oxide peak (O2-, Oβ) implies that surface oxides are
presented on the Ru nanoparticles. Thus, for the as-received Ru
catalysts without any reduction pretreatment, the Ru oxides are
present in a significant amount.
Fig. 6 Selectivity to benzene in HDO of phenol under the biphasic
(decalin and water) and monophasic (decalin or water) systems.
o
Conditions: phenol (0.05 g), Ru/5Nb2O5-5MC (0.05 g), 250 C, 2 bar
of hydrogen, 4 h.
We demonstrated that the Nb2O5-MC composite supported Ru
catalyst was active and selective towards to cleaving the C–O bonds
in phenol at moderate temperatures with pressurized H2 in
monophasic and biphasic systems. As shown in Figure 6, the
Fig. 5 XPS spectra in the C 1s-Ru 3d (A) and O 1s (B) regions for the
selectivity to benzene was ∼80% in the biphasic system (decalin and
fresh and spent (*) catalysts.
water), while ∼65% in the monophasic systems (decalin or water),
indicating that benzene was prone to be generated over the
Ru/5Nb2O5-5MC catalyst under a low hydrogen pressure (2 bar) and
at a relatively high temperature (ca. 250 oC). Significantly, the polar
and nonpolar solvents, water and decalin, played an important role
in determining the product distribution. Besides benzene,
cyclohexane was another major deoxygenated product when decalin
was used as the sole solvent. In contrast, in the presence of a polar
solvent, whether in the monophasic water system or in the biphasic
3.2 Catalytic activity
3.2.1 Influence of catalyst support and biphasic solvent system.
Catalytic cleavage of aromatic C–O bonds via hydrodeoxygenation
(HDO) is known to be challenging, requiring harsh conditions such as
high temperatures, high H2 pressures, strong acids, etc.55 Indeed, the
phenolic C–O bond energy is large (468 kJ/mol) and makes the direct
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water-decalin system, benzene is the only deoxygenated product. In the catalyst supports without Ru (Table S4). It is worthwhile to notice
View Article Online
all three solvent systems, the oxygenated by-products, cyclohexanol that the hydrophilic-hydrophobic balance (tDhOeI:p1r0o.p10o3r9ti/oCn8GoCf 0N4b021O7F5
and cyclohexanone, were presented. Benzene might be formed from and MC)of the Ru catalysts in the biphasic system plays a key role for
the DDO of phenol by cleaving the Csp2−O bond via hydrogenolysis, tuning the selectivity to benzene from HDO of phenol. Here, the
while an alternative pathway is through dehydration of cyclohexanol hydrophilic Nb2O5 species hold dual functions: the strong affinity to
followed with dehydrogenation of cyclohexene, which may be phenol, as well as the distinct capability of reducing the
promoted by Brønsted acids. For the Ru/5Nb2O5-5MC catalyst, the disassociation energy for C–O bonds.9 Moreover, the Ru
oxophilic sites of niobium, represented by incompletely coordinated nanoparticles were well dispersed on MC which possessed a
Nb5+ cations (Lewis acid sites) near the perimeter of the Ru metal hydrophobic character and high specific surface area (Table 1). The
particles, could be converted to Brønsted acid sites as exposed to amphiphilic nature of the Nb2O5-MC composites makes these
water. Furthermore, the Nb2O5-MC composites have a much higher catalysts to be preferentially compartmented at the water/oil
acidity than Nb2O5 (Figure 4). It is very interesting that the presence interface instead of dispersing in either bulk phase, which could
of water may suppress the hydrogenation of benzene to cyclohexane stabilize the emulsion and thus enhance the mass transfer of
in the biphasic solvent system.
molecules between the different phases (Figure S5). In addition, the
biphasic catalytic process can facilitate the separation of
hydrophobic molecules (such as benzene) from the aqueous phase,
and thus convert those molecules selectively in only one of the liquid
phases (i.e., phase-selectivity), simply based on differences in
solubility.58 The separation of the compounds into the decalin and
water solvent layers with different polarities alleviates the fragment
repolymerization. Therefore, high benzene yield can be obtained
without the formation of chars. Therefore, the superior performance
of the Ru/5Nb2O5-5MC catalyst for the HDO of phenol originates
from a combination of the strong affinity to phenol, the reduction of
the disassociation energy of the Caromatic–OH bonds, the biphasic
catalysis and the synergistic effect between Ru and Nb2O5-MC
composite support, leading to the high phenol conversion and
benzene selectivity.
3.2.2 Influence of the ratio of decalin/water in the biphasic
catalytic process. To unravel the nature behind the high catalytic
activity of the Ru/5Nb2O5-5MC catalyst for the HDO of phenol in the
biphasic catalytic process, further studies were carried out to
investigate the effect of the ratio of organic solvent to water. The as-
formed Pickering emulsion (Figure S6) provides a high interfacial
surface area where the catalytic reactions take place. Meanwhile,
the Pickering emulsion enhances local hydrogen concentration near
the interface because hydrogen has higher solubility in organic phase
than in water.49,59 In the Pickering emulsion stabilized by Ru/5Nb2O5-
5MC, the oil/water ratio is a crucial factor in determining the
properties and states of emulsion droplets, which would further
affect the catalytic performance of the emulsion. Therefore, it is
reasonable that there exist a proper decalin/water ratio to obtain the
highest yield of benzene, as shown in Table 2. The conversion of
phenol occurred in either hydrophobic decalin or hydrophilic water,
but in different manners. For instance, the HDO of phenol carried out
in monophasic decalin did not produce benzene, while produced
∼7.1% of benzene in monophasic water. However, the selectivity to
benzene increased substantially in the decalin/water biphasic
solvents and reached a maximum of 30.2% when the decalin/water
volume ratio was 15:5. We ascribe the improvements to the “solvent
cage effect”,60,61 which is that the phenol molecules are hemmed
into a cage of solvent by the catalyst at the oil-water interface of the
emulsion droplets. Since phenol is more soluble in water than in
decalin, phenol was more concentrated in water-in-decalin (W/O)
emulsion (Figure S6). These molecules in the solvent cage will collide
with each other more frequently than the molecules in the bulk
solvent. Accordingly, the HDO of phenol would be accelerated in the
Fig.
7 Conversion, selectivity to benzene, cyclohexanol and
cyclohexanone for the HDO of phenol at 200 oC and 6 bar H2 for 4h
over Ru/Nb2O5-MC catalyst with different ratio of Nb2O5 to MC
supports.
The HDO activity of supported Ru catalysts on the Nb2O5, MC, and
Nb2O5−MC composite supports were further examined at the
different ratios of MC to Nb2O5 (ranging from 5:0 to 0:5) in the
biphasic solvent system at a relatively low temperature (ca. 250 oC).
As shown in Figure 7, the conversions of phenol over various catalysts
are in the order of: Ru/8Nb2O5-5MC > Ru/Nb2O5 > Ru/MC >
Ru/5Nb2O5-5MC > Ru/3Nb2O5-5MC. The conversions over the Ru on
Nb2O5-MC composite support catalysts, except Ru/8Nb2O5-5MC, are
lower than those over the Ru on Nb2O5 or MC single support
catalysts. On the other hand, the selectivity to benzene over the Ru
on Nb2O5-MC composite catalysts was significantly improved as the
yield of benzene over the Ru catalyst on the single support (MC or
Nb2O5) was negligible in the biphasic system. Especially, the
selectivity to benzene over Ru supported on 3Nb2O5-5MC and
5Nb2O5-5MC composites were approximately five to six times higher
compared to those over Ru/8Nb2O5-5MC with high content of Nb2O5.
The reactions over the Ru catalysts with only the single support were
more prone to produce cyclohexanol and cyclohexanone through the
HYD pathway. However, the HDO of phenol did not occur over only
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W/O emulsions. Further decreasing the ratio of decalin to water led the Ru/Nb2O5-MC catalyst. The hydrolyzed protons frVoiemw wArtaictleerOnalninde
DOI: 10.1039/C8GC04017F
to a decrease in the selectivity of benzene, possibly due to the the pressurized H2 gas may be involved in the removal of phenolic
enlargement of the size of the emulsion droplets. Note that water is hydroxyl groups from the phenol molecules,62 which improves the
essential for the phenol HDO reaction to promote the production removal of oxygen functionalities during the HDO reactions.
yield of benzene. Water can donate a proton to Lewis acid sites on
Table 2 The effect of volumetric ratio between decalin and water for the HDO of phenol over Ru/5Nb2O5-5MC catalyst
Selectivity (C mol. %)
Decalin/water
(mL/mL)
Conversion
(wt. %)
Entry
1
2
3
4
5
20/0
15/5
10/10
5/15
0/20
100.0
100.0
99.8
0
100.0
0
0
0
30.2
24.7
20.0
7.1
64.5
68.2
64.7
83.0
5.3
4.8
3.0
3.0
2.3
99.9
12.3
7.0
99.9
o
Reaction conditions: 10 bar of hydrogen, 200 C, 4 h, Ru/5Nb2O5-5MC catalyst.
3.2.3 Influence of hydrogen pressure. The effect of hydrogen reached a maximum value of 30.2% at 200 oC when the H2 pressure
pressure on the catalytic activity of Ru/5Nb2O5-5MC for the HDO of was kept at 10 bar. Further increasing the reaction temperature led
phenol is significant, as shown in Figure 8. Without H2, phenol was to a sharp increase of the cyclohexanone yield, while, at the same
not reacted at all. However, the Ru/5Nb2O5-5MC catalyst exhibited time, the yields of benzene and cyclohexanol decreased to a
good catalytic activities for the HDO of phenol under the H2 negligible level, which indicated the existence of the reversible
atmosphere at a mild temperature (200 oC). With the increase of the reaction between cyclohexanone and cyclohexanol in this process.
H2 pressure from 2 to 10 bar, the conversion of phenol increased and Higher reaction temperatures were favorable for the production of
reached 100% and correspondingly, the selectivity to benzene cyclohexanone. Interestingly, at the high temperatures range (> 200
increased from ∼10% to ∼27.7%, when the H2 pressure was greater oC), increasing the temperature led to a decrease in the conversion
than 10 bar. The yield of cyclohexanone is peaked at about 2 bar of of phenol since the HDO of phenol is an exothermic reaction, which
H2, and meanwhile the yield of cyclohexanol increases gradually with is thermodynamically unfavorable at high temperatures.
increasing the H2 pressure. The results imply that cyclohexanone is
an intermediate product from the conversion of phenol and is
subsequently hydrogenated to form cyclohexanol. The saturation of
benzene via hydrogenation to produce cyclohexane started as the H2
pressure was over 10 bar and was completed at the H2 pressure of 15
bar. On the other hand, however, the yield of cyclohexanol reached
a maximum under 15 bar of H2 and was kept unchanged with further
increasing the H2 pressure. Comparable results have been reported
in the literature concerning on the effect of hydrogen pressure.10,29,63
These results suggested that the DDO pathway of converting phenol
to benzene is preferred at low H2 pressures while a higher H2 pressure
favors the generation of cyclohexane and cyclohexanol. Therefore,
to enhance the yield of aromatics and reduce the consumption of H2
in the HDO reaction, the H2 pressure should be optimized.
3.2.4 Influence of reaction temperature. Figure 9 shows that the
conversion of phenol and the selectivity to various products change
significantly in the biphasic catalyst process with the increase of
reaction temperature. When the reaction was carried out at a lower
temperature range (160-200 oC), it was found that the highly efficient
HDO of phenol was achieved over the Ru/5Nb2O5-5MC catalyst with
a full conversion of phenol, and cyclohexanol was the main product
Fig. 8 Effect of the H2 pressure. Reaction conditions: phenol (0.05 g),
Ru/5Nb2O5-5MC (0.05 g), decalin/water = 10 mL/10 mL, 200 oC, 4 h.
and only
a small amount of benzene, cyclohexane and
cyclohexanone was formed. Especially, the benzene selectivity
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decreased gradually, while the yield of cyclohexanol trended
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oppositely to that of cyclohexanone. TDheOI:r1e0s.1u0l3ts9/Cim8GpCly040th17aFt
cyclohexanone is an intermediate product and can be converted to
cyclohexanol over time. Interestingly, further prolonging the
reaction time led to a decrease of the yield in cyclohexanol and an
increase of the yield in benzene, which suggests the alternative
pathway of producing benzene from cyclohexanol via dehydration
and dehydrogenation (Table 3 and Scheme 1, route 1).12
Fig. 9 Effect of reaction temperature on the catalytic activity of
Ru/Nb2O5-MC for the HDO of phenol. Reaction conditions: phenol
(0.05 g), Ru/5Nb2O5-5MC (0.05 g), 10 bar of hydrogen, decalin/water
= 15 mL/5 mL, 4 h.
Fig. 11 Recycle of the catalyst. Reaction conditions: phenol (0.05 g),
Ru/5Nb2O5-5MC (0.05 g), decalin/water = 15 mL/5 mL, 10 bar of
hydrogen, 200 oC, 4 h.
3.2.6 Recyclability of the catalysts. Catalyst stability and
recyclability is important to evaluate heterogeneous catalysts. We
tested the catalytic activity of the Ru/5Nb2O5-5MC catalyst in a
typical biphasic catalytic process with successive runs and the results
of conversions and yields are listed in Figure 11. As a result, the
catalyst was successfully recycled for five times and it maintained a
good activity, with only a slight decrease in the product yield.
Fig. 10 Product distributions for HDO of phenol in biphasic system Interestingly, the catalyst can be partitioned into one phase so as to
over Ru/Nb2O5-MC catalyst versus the time. Reaction conditions: achieve the catalyst recycling via a simple phase separation without
phenol (0.05 g), Ru/5Nb2O5-5MC (0.05 g), 10 bar of hydrogen, any loss in activity and selectivity.65 Moreover, The XPS analysis of
decalin/water = 15 mL/5 mL, 200 oC.
the spent catalyst showed that Rux+ and Ruy+ in the catalyst were
further reduced to Ru0 particles under the reductive reaction
conditions (Figure 5).
3.2.7 Mechanism. Two different reaction pathways have been
typically proposed in the literature for the HDO of phenol: (i) the
direct deoxygenation (DDO) that involves the cleavage of the
C(sp2)−O bond by hydrogenolysis and (ii) sequential hydrogenation
(HYD) of the aromatic ring on metal sites followed by dehydration
of cyclohexanol on acid sites.12,64,66 In order to clarify the reaction
pathways of the HDO of phenol (especially, how the C−O bond was
cleaved) over the Ru/5Nb2O5-5MC catalyst in the biphasic catalytic
process, the probe reactions were performed using the possible
intermediates or products, such as benzene, cyclohexanol and
cyclohexanone, as the probe reactants (Table 3). We found that
3.2.5 Influence of reaction time. To further clarify the intrinsic
reaction pathway, the kinetics behavior of the phenol over
Ru/5Nb2O5-5MC was run for up to 6 hours in the water/decalin
biphasic solvent (Figure 10). Along with extending the reaction time,
the conversion of phenol kept increasing and finally reached the
plateau at full conversion. The selectivity to benzene increased from
∼10.1% at 0.5 h to ∼30% at the full conversion of phenol, indicating
that phenol was initially deoxygenated to benzene rapidly through
the cleavage of Caromatic−OH bond, implying that it is a fast step in the
HDO of phenol possibly owing to the significant Caromatic−OH bond
activation on NbOx species.64 Meanwhile, the yield of cyclohexanone
was peaked after about 0.5 h of the reaction time and then
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benzene was not liable to be converted to cyclohexane even under
a
View Article Online
DOI: 10.1039/C8GC04017F
Table 3 HDO of the intermediate products over Ru/Nb2O5-MC catalyst in biTCP.
Selectivity (C mol. %)
Conversion
Entry
Feed materials
(wt. %)
1
2
3
13.10
7.13
100.0
84.4
4.8
0.0
0.0
0.0
15.6
1.1
97.16
94.1
Conditions: 200 oC, 10 bar of hydrogen, 4 h, 700 rpm.
overall reaction network of the HDO of phenol in the biphasic
catalytic process, as shown in Scheme 1. The upper pathway (route
1) proceeds first through the catalytic hydrogenation (HYD) of phenol
to form an unstable cyclohexene intermediate, followed by two
potential pathways: (i) dehydrogenation to form benzene, or (ii) a
hydrogenation step to cyclohexane. The lower pathway (route 2) is
initiated by direct deoxygenation (DDO) of the C−O bond to produce
benzene which then directly converted to cyclohexane slightly.
4. Conclusions
In summary, we have demonstrated an efficient catalytic process of
converting phenol into benzene over the Ru/Nb2O5-MC catalysts in
the biphasic decalin/water solvent systems. The HDO of phenol was
promoted by a synergistic effect between the Ru nanoparticles
catalyst and the Nb2O5-MC composite support, in which Ru catalyzes
hydrogenation/dehydrogenation reactions while the Nb2O5-MC
composite acidic sites adsorb the oxygenated compounds, activate
the Caromatic−OH bonds, and promote dehydration reactions. The
tunable wettability of the Nb2O5-MC composite support was the key
parameter for the formation of emulsions in the biphasic system. The
presence of water in the biphasic catalytic process limits the
saturation of benzene by hydrogenation and thus improves the
selectivity towards benzene. The process conditions, including
reaction temperature, hydrogen partial pressure, and reaction time,
significantly affected the conversion of phenol and the selectivity to
benzene. In general, higher temperatures, lower hydrogen partial
pressures, and longer reaction time promoted the yield of benzene
from phenol in the biphasic catalytic process. Under the optimum
conditions (250 oC, 2 bar H2), a full conversion of phenol and a high
benzene selectivity of 80% were achieved over the Ru/5Nb2O5-5MC
catalyst in the biphasic catalytic process. The probe reaction studies
suggest that the formation of benzene is achieved mainly through
the direct deoxygenation route. Overall, this biphasic catalytic
process is excellent for phenol conversion, yielding a considerable
amount of benzene under mild conditions with the low hydrogen
consumption. The biphasic catalytic approach demonstrated in this
research is expected to be further extended to transform lignin-
derived phenolic compounds into aromatic compounds.
Scheme 1 Proposed reaction pathway of HDO of phenol in the biTCP
over the Ru/Nb2O5-MC catalyst.
relatively high H2 pressure (10 bar) as only ∼13.1% of benzene was
saturated by hydrogenation at 200 oC. This observation is in line with
the literature report that hydrogenation of benzene to cyclohexene
is thermodynamically unfavorable.67 Moreover, the conversion of
cyclohexanol was conducted to assess the relative concentration of
surface Brønsted acid sites68 on the supported Ru catalysts during
the reaction. Table 3 shows that only a small amount of cyclohexanol
was converted to cyclohexane and benzene through hydrogenation
and dehydrogenation, respectively. In detail, cyclohexanol was
dehydrated to form cyclohexene, and then underwent
a
hydrogenation step to produce cyclohexane, which is the dominant
route, as the yield of cyclohexane from cyclohexanol was as high as
∼84.4%. Contrarily, benzene was produced in a great disparity from
the conversion of cyclohexanol and phenol, respectively, suggesting
that in a typical biphasic catalytic process, benzene is produced
primarily from the direct conversion of phenol via hydrogenolysis.
Using cyclohexanone as feedstock, a large number of cyclohexanol
and a small amount of cyclohexane are produced. These products
could be formed by the following reaction pathways: the
hydrogenation of cyclohexanone to cyclohexanol and dehydration of
cyclohexanol to cyclohexene followed by
a
subsequent
hydrogenation step to produce cyclohexane. Herein, we propose an
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19 Y. K. Lugo-José, J. R. Monnier and C. T. Williams,VAiepwpAlr.tiCclaetOanll.inAe
Conflicts of interest
There are no conflicts to declare.
DOI: 10.1039/C8GC04017F
Gen., 2014, 469, 410–418.
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Acknowledgements
This material is based on work supported by the National
Science Foundation under grant no. CBET 1700482 (USA) and
the National Natural Science Foundation of China (No.
21577025), the National Key Technology Support Program (No.
2015BAD15B06), the National Key Research and Development
Program of China (No. 2017YFC0212205), and the International
Cooperation Project of Shanghai Municipal Science and
Technology Commission (No. 18230710700).
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