Graphite encapsulated molybdenum carbide core/shell nanocomposite for highly selective conversion of guaiacol to phenolic compounds in methanol

Article abstract of DOI:10.1016/j.apcata.2016.09.016

Graphite encapsulated molybdenum carbides (Mo₂C@C) were synthesized via the hydrothermal carbonization of a solution of glucose and ammonium molybdate followed by temperature programmed reduction. Characterization and structural analyses revealed that the synthesized Mo₂C@C nanoparticles had a molybdenum core/carbon shell structure with a particle size ranging from 50 nm to 100 nm and a core size range of 5–45 nm. The catalytic performance of the graphite encapsulated molybdenum carbides was evaluated on conversion of guaiacol to phenolic compounds in methanol. At 340 °C under 2.8 MPa hydrogen pressure, a 76.3% guaiacol conversion was obtained with selectivities of 68.6% for phenol and 93.5% for phenolic compounds. Thus, Mo₂C@C showed high selectivity for phenolic compounds in methanol.

Full text of DOI:10.1016/j.apcata.2016.09.016

Accepted Manuscript
Title: Graphite encapsulated molybdenum carbide core/shell
nanocomposite for highly selective conversion of guaiacol to
phenolic compounds in methanol
Author: Rui Li Abolghasem Shahbazi Lijun Wang Bo Zhang
Albert M. Hung David C. Dayton
PII:
S0926-860X(16)30480-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.09.016
APCATA 16012
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
19-7-2016
23-9-2016
26-9-2016
Please cite this article as: Rui Li, Abolghasem Shahbazi, Lijun Wang, Bo Zhang, Albert
M.Hung, David C.Dayton, Graphite encapsulated molybdenum carbide core/shell
nanocomposite for highly selective conversion of guaiacol to phenolic compounds in
methanol, Applied Catalysis A, General http://dx.doi.org/10.1016/j.apcata.2016.09.016
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Graphite encapsulated molybdenum carbide core/shell
2 nanocomposite for highly selective conversion of guaiacol
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to phenolic compounds in methanol
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Rui Lia, Abolghasem Shahbazib*, Lijun Wangb, Bo Zhangb, Albert M. Hunga, David C.
Daytonc
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a Joint School of Nanoscience and Nanoengineering, North Carolina A & T State
University, Greensboro, NC 27411, USA
b Biological Engineering Program, Department of Natural Resources and Environmental
Design, North Carolina A & T State University, 1601 East Market Street, Greensboro,
NC 27411
c Energy Technology Division, RTI International, 3040 East Cornwallis Road, Research
Triangle Park, Durham, NC 27709
Graphical abstract
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Highlights
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1. The molybdenum core/graphite shell nanoparticles were synthesized by combining hydrothermal
carbonization and temperature programmed reduction.
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2. The mechanism for the material formation is reported.
3. Highly selective conversion of guaiacol to phenolic compounds is achieved in alcohol.
4. Its reaction pathway is proposed.
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5. Both demethoxylation (DMO) pathway and consecutive demethylation (DME) followed by a
dehydroxylation proceed concurrently to form phenol.
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Abstract
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Graphite encapsulated molybdenum carbides (Mo2C@C) were synthesized via the
hydrothermal carbonization of a solution of glucose and ammonium molybdate followed
by temperature programmed reduction. Characterization and structural analyses revealed
that the synthesized Mo2C@C nanoparticles had a molybdenum core/carbon shell
structure with a particle size ranging from 50 nm to 100 nm and a core size range of 5-45
nm. The catalytic performance of the graphite encapsulated molybdenum carbides was
evaluated on conversion of guaiacol to phenolic compounds in methanol. At 340°C under
2.8 MPa hydrogen pressure, a 76.3% guaiacol conversion was obtained with selectivities
of 68.6% for phenol and 93.5% for phenolic compounds. Thus, Mo2C@C showed high
selectivity for phenolic compounds in methanol.
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Keywords: Encapsulation; Molybdenum carbides; Nanoparticles; Hydrothermal
carbonization; Phenolic compounds
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1. Introduction
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Bio-oil produced from the pyrolysis of lignocellulosic biomass consists of a significant
amount of phenolic compounds that were mainly derived from lignin [1]. The presence of
these compounds with high oxygen contents leads to undesired properties for the bio-oil,
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such as low heating value, thermal instability, high acidity, high viscosity, and incomplete
volatility. Therefore, hydrodeoxygenation is often considered as a competitive route for
upgrading bio-oil to transportation fuels or desired fine chemicals [2-5]. Guaiacol is one of
the major phenolic compounds in pyrolytic bio-oil. Guaiacol has the greater propensity for
coking than other phenolic compounds [6], and the complete deoxygenation is challenging
due to its oxygenated hydroxyl and methoxy functional groups. Guaiacol was thus chosen
as a model compound for the current study on the catalytic conversion of bio-oil
components to desired chemicals.
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The most widely studied catalysts for guaiacol hydrogenation are noble metal catalysts
(such as supported Ru [7], Re [8], Rh [9, 10], Pd [7, 10], and Pt [11-14]) and molybdenum-
based sulfide catalysts (such as sulfided CoMo [15-17] and NiMo [9, 18]), and the reaction
schemes proposed in literature are summarized in Fig. 1. Noble metals are commonly
presented as highly active hydrogenation catalysts. Noble metal catalysts could fully
hydrogenate the guaiacol benzene ring, and then demethoxylate and dehydoxylate
oxygenates to form saturated deoxygenated hydrocarbons, e.g. cyclohexanol and
cyclohexane. Hydrogenation of guaiacol over Mo-based sulfide catalysts starts with
demethylation and demethoxylation, which is followed by the benzene ring saturation to
form major products such as catechol, phenol, benzene, and cyclohexane. Both noble
metals and sulfide catalysts have their own shortcomings. For example, the use of noble
metal produces completely saturated hydrocarbons, resulting in undesired H2 consumption;
while sulfide catalysts can be deactivated easily without sulfur addition [19].
Hydrogenation catalysts for phenolic compounds should have high activities towards
selected products without causing saturation of the double bonds. Recently, molybdenum
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carbide catalysts showed promising performances in many reactions involving hydrogen,
including isomerization [20], hydrogenation [21-23], hydrodesulfurization [24-26],
hydrodenitrogenation [27], and deoxygenation [28, 29]. Simultaneously, they exhibited
good activity and selectivity in C–O/C=O bond cleavage [28, 30, 31], leading to dominant
products with unsaturated C=C bonds rather than alkanes [32-34]. Therefore, molybdenum
carbide was chosen as an efficient catalyst for the selective conversion of guaiacol to
phenolic compounds.
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Figure 1. Reaction pathways for guaiacol hydrogenation over noble metal catalysts and
Mo-based sulfide catalysts (according to ref. 3-14).
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The conventional method for the synthesis of molybdenum carbides is temperature
programmed reduction (TPRe) [35, 36]. The TPRe method involves gas-solid reactions
between oxide or nitride precursors and a mixture of hydrogen and carbon-containing gas
such as CH4 [36-38], C2H4 [39], C2H6 [40], C3H8 [41, 42], C4H10 [43], etc. Unfortunately,
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high temperature during the TPRe process favors particle agglomeration, which makes it
impossible to obtain a catalyst with nanoscale particles capable of potentially higher
activities. In order to avoid the agglomeration of catalyst particles, one strategy is to
prepare core/shell structure catalysts by using the hydrothermal carbonization (HTC)
method. Hydrothermal carbonization has been employed successfully to synthesize
carbonaceous spheres with controlled sizes from a solution of saccharides [44] and
encapsulate metal nanoparticles in carbon nanospheres [45-49]. Through this method,
homogeneously dispersed metal nanoparticles are encapsulated and isolated largely by
permeable carbonaceous matrices that are generated in-situ [50].
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Lignin is the only relevant renewable feedstock for aromatic compounds [51], but with
low selectivity for specific compound [52]. In this study, carbon encapsulated molybdenum
carbides were synthesized via a one-step hydrothermal carbonization followed by TPRe,
and the catalyst activity was tested on conversion of guaiacol to phenolic compounds in
methanol. The possible mechanism for the fabrication of this core/shell nanoparticle is
proposed. The effects of temperature, pressure of hydrogen, and solvents on phenolic
compounds production were investigated to elucidate the reaction pathway.
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2. Experimental
2.1 Catalyst synthesis
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Carbon encapsulated molybdenum carbides were prepared via hydrothermal
carbonization of a solution of glucose and ammonium molybdate followed by temperature
programmed reduction. A total of 0.0015 mol ammonium molybdate tetrahydrate
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((NH4)6Mo7O24·4H2O, 99%, Sigma-Aldrich) and 0.132 mol glucose (96%, Sigma-Aldrich)
were dissolved in 600 mL deionized water. The solution was then treated hydrothermally
in an autoclave at 180oC for 8 h. The resultant precipitate was washed thoroughly with
deionized water and ethanol. The solid product was dried at 80oC. This hydrothermal
carbonization derived compound was denoted as MoO2@C, in which MoO2 represents
molybdenum oxide including molybdenum dioxide and molybdenum trioxide. The dried
MoO2@C was further carbonized in 20 vol% CH4 and 80 vol% H2 at a flow rate of 50
o
mL/min. The carburization temperature was raised to 300oC at a heating rate of 5 C/min
o
and then ramped to 900oC at a heating rate of 1 C/min and kept at 900oC for 2 h. The
resulting material was then cooled down to room temperature in argon and subsequently
passivated under 1 vol% O2 and 99 vol% N2. The synthesized sample from the
carbonization process was denoted as Mo2C@C.
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2.2 Catalyst characterization
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X-ray powder diffraction (XRD) patterns were obtained using an Agilent Gemini X-
ray Diffraction System (Santa Clara, CA, USA) operated at 40 kV and 40 mA with Cu-Kα
radiation. The morphology was examined on a JEOL JSM-7600F scanning electron
microscope (SEM) (Tokyo, Japan) operated at 2 kV with energy-dispersive X-ray
spectroscopy (EDX). Prior to the imaging, the sample was sputter-coated with 5 nm of
gold-palladium. High-resolution transmission electron microscopy (HRTEM) was
performed with a JEOL 2010F operated at 200 kV (Tokyo, Japan). Fourier Transform
Infrared (FT-IR) spectra were collected on a Varian 670 FT-IR spectrometer (Santa Clara,
CA, USA) equipped with a single-bounce diamond attenuated total reflectance accessory.
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The amount of Mo in catalysts was measured using a Varian 710-ES inductively coupled
plasma optical emission spectrometer (ICP-OES) (Santa Clara, CA, USA). The content of
carbon and hydrogen were determined using a Perkin-Elmer 2400 CHN/S analyzer
(Waltham, MA, USA). Thermogravimetric analyses (TGA) were performed using a TA
SDT Q600 thermogravimetric analyzer (New Castle, DE, USA), in which samples were
heated to 900ºC at a heating rate 10 ºC/min in nitrogen. Surface area and porosity were
analyzed by measuring physical nitrogen adsorption using a Micromeritics ASAP 2020
(Norcross, GA, USA).
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2.3 Catalytic tests
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The catalytic performance of Mo2C@C was evaluated in a 75 ml Parr stainless steel
batch autoclave (St, Moline, IL, USA). A mixture of 0.2 mL guaiacol (98%, Sigma-
Aldrich), 10 mL solvent methanol, 0.05 mL hexadecane (99%, Sigma-Aldrich) as the
internal standard, and 0.1 g catalyst was loaded into the reactor vessel. The vessel was
evacuated by pressurization-depressurization with nitrogen to purge out any air and then
charged with hydrogen to a certain pressure. A higher initial hydrogen pressure of 2.8 MPa
was used to ensure excess hydrogen supplement. The hydrotreating process proceeded at a
given temperature for 4 h. Table S1 gives the experimental conditions. After the
hydrotreatment, the liquid samples were analyzed by using an Agilent 7890A gas
chromatograph (Santa Clara, CA, USA) equipped with a 5975C mass spectrometry (GC-
MS), a flame ionization detector (FID), and a DB-1ms capillary column. Helium gas was
used as carrier gas with a split ratio of 50:1. The GC oven was held at 35°C for 5 min, then
ramped to 200°C at 5 °C/min.
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The conversion was determined by averaging the data measured twice. The conversion
efficiency and selectivity of guaiacol during hydrotreating were defined as:
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(1)
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(2)
where “N” represents the moles of the reactants or products.
3. Results and discussion
3.1 Characterization of carbon encapsulated molybdenum oxide
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Carbon encapsulated molybdenum oxide (MoO2@C) was synthesized via
hydrothermal carbonization of a solution of glucose and ammonium molybdate followed
by drying at 80oC. XRD spectra showed that this synthesis method formed a poorly
crystalline phase (Fig. 2a), which was assigned to hexagonal MoO2 (PDF 50-0739) with
2θ peaks at 36.5° (1 0 0), 53.8° (1 0 2), 65.7° (1 1 0), and 41.4° (1 0 1). The carbonaceous
phase in these samples was amorphous as no graphitic carbon peaks were observed. EDX
analysis confirmed the exclusive existence of Mo, C, and O in the as-synthesized MoO2@C.
The Mo element was evenly distributed in MoO2@C (supplementary information Fig.S1).
The elemental analysis shows that the contents of C and Mo were 51.0 and 15.9 wt%,
respectively. The BET surface area and total pore volume of the catalyst were 37.6 m2/g
and 0.25 cm3/g, respectively. Morphological images of MoO2@C exhibited a porous
structure composed of numerous attached carbonaceous spheres with sizes ranging from
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50 to 100 nm (Fig. 3a-3b). In Fig. 4a, the HRTEM shows that the catalyst had a core-shell
structure with the molybdenum oxide encapsulated inside carbonaceous spheres during the
hydrothermal carbonization treatment. Molybdenum oxide cores had a size distribution
between 5 and 56 nm and were dispersed homogeneously in a carbonaceous matrix. The
lattice spacing of 0.25 and 0.17 nm (Fig. 4b) could be assigned to the (1 0 0) and (1 0 2)
planes of MoO2. Fig. 5a shows the surface functional groups on the carbonaceous shells
identified by FT-IR spectroscopy. The bands around 1615 cm-1 were attributed to in-plane
C=C stretching vibration in an aromatic ring, and the band at 767 cm-1 was for aromatic
C–H out-of-plane bending vibrations [53]. These characteristic absorptions of aromatic
rings confirmed the aromatization of glucose during the hydrothermal treatment. The band
at 1698 cm-1 was attributed to C=O vibration of aldehydes, ketones, carboxylic acids, or
esters [48]. The broad band centered around 3400 cm-1 corresponded to O–H stretching
vibration and 1292 cm-1 was from C–OH bending vibration, which implied the existence
of hydroxyl groups. The bands at 2975 and 2934 cm−1 were stretching vibrations of
aliphatic C–H, which showed that the carbonaceous shell also possessed an aliphatic
structure. In summary, the dehydration and subsequent aromatization of glucose during
hydrothermal treatment resulted in carbonaceous hydrophilic shells with abundant –OH
and C=O groups.
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3.2 Characterization of graphite encapsulated molybdenum carbide
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Graphite encapsulated molybdenum carbide (Mo2C@C) was synthesized through the
process of the temperature programmed reduction at 900oC under 20% CH4/80% H2. TPRe
treatment resulted in the formation of hexagonal β-Mo2C (PDF 35-0787) with 2θ peaks at
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~39.4° (1 0 1), 38.0° (0 0 2), 52.1° (1 0 2), 34.4° (1 0 0), 61.7° (1 1 0), 69.6° (1 0 3), and
74.7° (1 1 2) (Fig. 2b). Besides the peaks of β-Mo2C, the diffraction peaks at 26.1° (0 0 2),
42.7° (1 0 0), and 44.4° (1 0 1) confirmed the formation of graphite (PDF 41-1487). Clair
et al. [54] and Hanif et al. [40] also observed a similar phenomenon that graphite was
formed from methane at a temperature higher than 700oC. The TPRe treatment did not
significantly change the morphology of Mo2C@C and the size of carbonaceous shells for
Mo2C@C that was still in the range of 50-100 nm as shown in Fig. 3d. EDX analysis
showed that Mo and C were evenly distributed in the sample (supplementary information
Fig.S2). The HRTEM images indicated that the size of the molybdenum carbide cores was
between 32 and 45 nm in Fig. 4c and between 5-15 nm in Fig. 4d, and the carbonaceous
matrix effectively inhibited the agglomeration of the molybdenum nanoparticles. The
lattice spacings of 0.23, 0.24, and 0.26 nm could be assigned to the (1 0 1), (0 0 2), and (1
0 0) planes of Mo2C, respectively, while the lattice spacing of 0.34 nm could be assigned
to the (0 0 2) plane of graphite. A multilayer graphite was found to be formed on the surface
of molybdenum carbide particles in Fig. 4d. The BET surface area and the total pore
volume of the synthesized material increased significantly from 37.6 m2/g to 177.0 m2/g
and 0.25 cm3/g to 0.34 cm3/g, respectively, while the carbon content increased from 51.0
wt% to 55.7 wt% and the oxygen content decreased from 28.3 wt% to 12.7 wt% in Table
1. This core-shell structured porous material with large pore sizes acts as a physical barrier,
but still allow reactants reach reactive sites and products flow freely through the pores,
which is in agreement with the results reported in the literature [50]. FT-IR spectra of
Mo2C@C indicated that most of functional groups on the carbon surface of MoO2@C were
removed during the heat treatment. The hydroxyl bands at 3400 cm-1 and 1292 cm-1 were
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not observed, indicating that dehydration occurred during carbonization. The
disappearance of C=O at 1698 cm-1 was associated with the decomposition of carboxylic
acids. The disappearance of the peak at 767 cm-1 and 1615 cm-1 was due to internal
structural reorganizations of aromatic structures and the further carbonization of the shell
during the heat treatment. Therefore, almost all the oxygen-containing functional groups
on the surface were removed at 900°C. This process may also have resulted in the
development of mesopores in the carbonaceous matrix of Mo2C@C as evidenced by the
significant increase in BET surface area. Newly developed pores would increase not only
the surface area but also the penetration of reactive species for catalytic reactions.
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Figure 2. XRD patterns of (a) MoO2@C, and (b) Mo2C@C.
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Figure 3. SEM images of (a, b) MoO2@C, and (c, d) Mo2C@C.
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Figure 4. HRTEM images of (a, b) MoO2@C, and (c, d) Mo2C@C.
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Figure 5. FT-IR spectra of (a) MoO2@C, and (b) Mo2C@C.
Table 1. Textural parameters of materials (on dry basis).
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BETc
Pore Diameterd Pore Volumee
Sample
Ca (wt%) Ha (wt%) Mob (wt%)
(m² g-1)
(nm)
(cm3 g-1)
MoO2@C
Mo2C@C
51.0
55.7
4.8
4.7
15.9
26.9
37.6
40.1
31.4
0.25
0.34
177.0
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a Measured with the Perkin-Elmer 2400 CHN/S analyzer.
b Measured with ICP
c Brunauer-Emmet-Teller (BET) surface area.
d Pore diameter calculated by the Barrett-Joyner-Halenda (BJH) method using adsorption branches.
e Total pore volume calculated from the amount of nitrogen adsorbed at a relative pressure of 0.995.
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3.3 Mechanism for the formation of carbon encapsulated molybdenum nanoparticle
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The mechanism of formation of carbon nanospheres prepared under hydrothermal
conditions in the temperature range of 160-200 °C has been well summarized in studies
[55-58]. However, the mechanism for the formation of metal oxide within carbonaceous
spheres by one pot method is still under investigation. The interplay between the metal
oxide and carbonaceous spheres and the precipitation kinetics of both phases are crucial to
carbon encapsulated metal oxide formation [47]. Fig. 6 is a schematic illustration
explaining the possible formation process of molybdenum nanoparticles encapsulated in
carbon by in situ hydrothermal reduction of molybdenum ions with glucose based on our
characterization results. At the initial stage, glucose is hydrolyzed to small carbonaceous
colloids. Meanwhile, molybdenum is reduced from the oxidation state Mo6+ in
(NH4)6Mo7O24·4H2O to Mo4+ in MoO2 due to the ability of glucose degradation products
to act as reductants [59]. The molybdenum nanoparticles combine with carbonaceous
colloids through Coulombic interactions until condensing from water [45]. The further
growth of carbon spheres via intermolecular dehydration of the surface functional groups
follows the LaMer model [60]. Because homogeneous nucleation requires higher
activation energy than heterogeneous nucleation [61], pure carbon nanospheres form at
much lower rate than carbon encapsulated molybdenum oxide nanoparticles. All carbon
encapsulated molybdenum oxide nanoparticles and pure carbon nanospheres interconnect
randomly in three dimensions to form a hydrophilic microcomposite. During TPRe,
molybdenum oxide reacts with methane and hydrogen to form hexagonal molybdenum
carbide with a preserved structure, while graphite is formed on the surface of molybdenum
carbide particles from methane.
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Figure 6. Schematic illustration for possible formation process for carbon encapsulated
molybdenum particles.
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(a) An aqueous solution of glucose and ammonium molybdate. (b) Molybdenum oxide (green dots)
particles nucleate during HTC. (c) Molybdenum oxides (green core) are encapsulated in carbon shell (black
shell) and carbonaceous colloids (black dots) formed. (d and e) The carbon encapsulated molybdenum
oxides and carbon nanospheres randomly interconnect together to form a microcomposite. (f) Molybdenum
carbides were synthesized by TPRe.
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3.4 Catalytic performance of carbon encapsulated molybdenum carbide
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The product profiles can be found in Table S2 of the supporting information. For
comparison, the guaiacol conversion efficiencies in the absence of the catalyst at 340°C
were 28.4% and 13.6% for the experiments with and without 2.8 MPa H2, respectively. Fig.
7 shows the product yields obtained at reaction temperatures of 300°C, 320°C, and 340°C
under a H2 pressure of 2.8 MPa for 4 h. The increase of temperature from 300°C to 340°C
increased the conversion efficiency of guaiacol from 32.5% to 76.3%, and the selectivities
of phenol and phenolic compounds from 66.1% to 68.6% and 81.2% to 93.5%, respectively.
The selectivity of catechol that is a reaction intermediate decreased with increasing
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temperature, which means that more catechol was converted to phenol at a higher
temperature. After the reaction, H2, CH4, and CO2 were detected in the gas phase.
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Figure 7. Conversion of guaiacol, and the selectivity to phenol, cresols, xylenols,
veratrole, and catechol over Mo2C@C at temperatures of 300°C, 320°C, and 340°C.
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As shown in Fig. 8, when the initial H2 pressure was increased from 0.0 to 2.8 MPa,
the conversion efficiency of guaiacol increased from 50.9% to 76.3% and the phenolic
selectivity increased from 67.0 to 93.5%. In the absence of initial H2, methanol also served
as a hydrogen donor. However, the selectivity towards phenol without H2 was only half of
that of the process with H2. High hydrogen pressures increased the selectivity for phenolic
compounds. Compared with the similar research [34], under the same reaction condition
(in methanol, at 340ºC, under 0 MPa H2, for 4 h), the main products from guaiacol
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conversion between two studies were the same. But the selectivity in this study is higher
than that in the previous study.
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Figure 8. Conversion of guaiacol, and the selectivity to phenol, cresols, xylenols,
veratrole, and catechol over Mo2C@C at an initial H2 pressure of 0, 1.4, and 2.8 MPa.
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The reusability of the catalyst was evaluated at 340oC for 4 h in the methanol solvent
for three cycles. A standard reaction was first performed on a freshly prepared Mo2C@C
catalyst. After each cycle, the spent catalyst was retrieved, dried at 105ºC overnight, and
reused directly in the next run without reactivation. As shown in Fig. 9, the conversion of
guaiacol decreased slightly from 76.3% for the first cycle to 62.9% for the third cycle,
while the selectivity towards phenol decreased from 68.6% to 64.1% and phenolic
compounds decreased from 93.5% to 90.8%.
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Figure 9. Conversion of guaiacol, and the selectivity to phenol, cresols, xylenols,
veratrole, and catechol over recycled Mo2C@C.
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3.5 Reaction pathway for phenolic compounds production over Mo2C@C
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The product profiles showed that the selectivities towards phenol, cresols (o-Cresol,
and m-Cresol), and xylenols (2, 3-dimethylphenol and 2, 6-dimethylphenol) were high.
Trace amounts of veratrole, catechol, and anisole were also measured, but other
transalkylation products were not present due to their selectivities less than 1%. Less than
6% selectivity of veratrole suggested that transalkylation was insignificant, which is in
agreement with the results reported in the literature [11, 62]. Neither completely
deoxygenated products (such as benzene and toluene) nor ring hydrogenation products
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(such as cyclohexane, cyclohexene, cyclohexanone, etc.) were detected. High selectivity
for phenol formation implied that the removal of the methoxy group was more favorable
than the removal of the hydroxyl group of guaiacol in the alcohol solution. As no benzene
was observed, the removal of the hydroxyl group to form benzene is more difficult and is
not favorable when guaiacol is still present [33]. Based on the experimental results,
possible reaction pathways for phenolic compounds production over Mo2C@C are
summarized in Fig. 10. Methyl substituents of cresols and xylenols were mainly formed
through methyl-substitution from solvent rather than through transalkylation of guaiacol
[63]. The comparative test was carried under the same condition in ethanol, and the main
products were phenol, ethylphenols, and diethylphenols. This test proved that alkyl groups
on cresols and xylenols were mainly from solvent via alky-substitution reactions. It is
widely accepted that the selectivity of carbon supported catalysts is high, though their
hydrogenation activities are lower than that of catalysts on acidic supports [15, 64].
Therefore, both the active metal Mo and the nature of the carbon support affected catalytic
activities and determined the reaction pathway.
21

341
342
343
Figure 10. Proposed reaction pathway for guaiacol conversion over Mo2C@C catalyst in
methanol.
344
345
4. Conclusions
346
347
348
349
The fabrication of highly dispersed molybdenum oxide nanoparticles embedded in
carbon nanospheres was achieved by one-pot hydrothermal carbonization of an
inexpensive precursor solution at a mild temperature. After TPRe, graphite encapsulated
molybdenum carbide Mo2C@C was formed, which was composed of aggregates of
22

350
351
molybdenum core and carbon shell nanoparticles. The sizes of Mo2C@C particles and
molybdenum cores were 50-100 nm and 5-45 nm, respectively.
352
353
354
355
356
357
358
359
360
361
Mo2C@C catalyst showed high selectivity to phenolic compounds in an alcohol
solution, which exemplified the confining effect of the carbonaceous matrix to inhibit the
aggregation of molybdenum nanoparticles during both the activation and reaction
processes at a high temperature. The highest conversion efficiency and highest selectivities
towards phenol and phenolics were obtained at 340°C under 2.8 MPa hydrogen pressure.
Both demethoxylation and consecutive demethylation followed by a dehydroxylation
proceeded concurrently to form phenol. Substitution of methyl or ethyl groups in phenol
were from solvent methanol or ethanol. Moreover, no significant loss of catalytic activity
even after three recycle times demonstrated that the core-shell structure of catalysts
effectively offered the resistance to deactivation and sintering.
362
363 Acknowledgments
364
365
366
367
368
369
370
This work was supported by US-Department of Energy, Renewable Energy Center
[award number: DE-EE0003138] and NSF-CREST Center for Bioenergy [Award No.
HRD-1242152]. The HRTEM assistance of Dr. Yang Liu, in Analytical Instrumentation
Facility at North Carolina State University supported by the State of North Carolina and
the National Science Foundation, is gratefully acknowledged. The authors also thank Mr.
Bryce M. Holmes and Mr. John R. Carver of Department of Natural Resources and
Environmental Design, North Carolina A & T State University for using GC apparatus.
371
23

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