Highly active and robust Ni-MoS2 supported on mesoporous carbon: A nanocatalyst for hydrodeoxygenation reactions

Article abstract of DOI:10.1039/c9ra02143d

NiMoS₂ nanoparticles supported on carbon, synthesized by a microemulsion method were used as a nanocatalyst for hydrodeoxygenation (HDO) of a lignin model compound-guaiacol. Two types of carbon supports-mesoporous carbon (CMK-3) and activated carbon (AC) with a predominantly microporous structure, were studied to investigate the role of porosity and nature of the porous structure in catalyst activity. The activity of NiMoS₂/AC resulted in the complete guaiacol conversion at 13 h of reaction time to produce phenol (31.5 mol%) and cyclohexane (35.7 mol%) as the two main products. Contrastingly, NiMoS₂/CMK-3 needed a much lesser reaction time (6 h) to attain a similar conversion of guaiacol but gave different selectivities of phenol (25 mol%) and cyclohexane (55.5 mol%). Increased cyclohexane production with NiMoS₂/CMK-3 implied better deoxygenation of MoS₂ and enhanced hydrogenation capacity of Ni since phenol is a partially deoxygenated product of guaiacol while cyclohexane is a completely deoxygenated and hydrogenated product. The superior catalytic activity and deoxygenating behavior of NiMoS₂/CMK-3 catalysts could be attributed to the organized mesoporosity of the CMK-3 support in relation to the improved active phase distribution and access to active sites that facilitate the conversion of the reaction's product. Recyclability study implied NiMoS₂/CMK-3 was more stable without significant changes in the catalytic activity even after three reaction cycles.

Full text of DOI:10.1039/c9ra02143d

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Highly active and robust Ni–MoS supported on
2
mesoporous carbon: a nanocatalyst for
Cite this: RSC Adv., 2019, 9, 17194
hydrodeoxygenation reactions†
ab
c
d
a
Swathi Mukundan,
and Jorge Beltramini
Md A. Wahab, Luqman Atanda,
*
Muxina Konarova
de
2
NiMoS nanoparticles supported on carbon, synthesized by a microemulsion method were used as
a nanocatalyst for hydrodeoxygenation (HDO) of a lignin model compound – guaiacol. Two types of
carbon supports – mesoporous carbon (CMK-3) and activated carbon (AC) with a predominantly
microporous structure, were studied to investigate the role of porosity and nature of the porous
structure in catalyst activity. The activity of NiMoS
2
/AC resulted in the complete guaiacol conversion at
13 h of reaction time to produce phenol (31.5 mol%) and cyclohexane (35.7 mol%) as the two main
2
products. Contrastingly, NiMoS /CMK-3 needed a much lesser reaction time (6 h) to attain a similar
conversion of guaiacol but gave different selectivities of phenol (25 mol%) and cyclohexane (55.5 mol%).
Increased cyclohexane production with NiMoS /CMK-3 implied better deoxygenation of MoS and
2
2
enhanced hydrogenation capacity of Ni since phenol is a partially deoxygenated product of guaiacol
while cyclohexane is a completely deoxygenated and hydrogenated product. The superior catalytic
Received 20th March 2019
Accepted 9th May 2019
activity and deoxygenating behavior of NiMoS
mesoporosity of the CMK-3 support in relation to the improved active phase distribution and access to
active sites that facilitate the conversion of the reaction's product. Recyclability study implied NiMoS
2
/CMK-3 catalysts could be attributed to the organized
DOI: 10.1039/c9ra02143d
2
/
rsc.li/rsc-advances
CMK-3 was more stable without significant changes in the catalytic activity even after three reaction cycles.
chemical feedstock via catalytic biorening technologies will
serve as a renewable energy production. Among the various
Introduction
5,6
Lignin is the most abundant renewable aromatic resource in studies that have received increasing attention in recent years is
nature, however, it has been underutilized as a feedstock for the catalytic upgrade of lignin via hydrodeoxygenation (HDO),
1,2
7
biofuel production.
Meanwhile, lignin is currently used sometimes referred to as catalytic hydrotreatment. Catalytic
3,4
mostly as plasticizers or low-grade fuel by burning. With the hydrodeoxygenation (HDO) of lignin using molecular hydrogen
current depletion of non-renewable fossil fuels and the associ- is considered to be more advantageous than other conversion
ated environmental issues of pollution, production of chem- processes since it produces oil with a small amount of oxygen
icals and fuels from lignin can be expected to gain more interest that can be easily upgraded to hydrocarbon fuels. Alternatively,
in the foreseeable future. Owing to the vast availability of lignin HDO can be employed in the downstream processing of inter-
naturally in biomass or in the waste stream of pulp and paper mediate bio-oil obtained by pyrolysis or liquefaction of lignin in
industry, its transformation to useful products i.e. liquid fuel or order to improve the oil energy density by removal of the
oxygenated groups. During the HDO process of lignin, liquid
oxygenates such as guaiacol, phenol, catechol, etc. are gener-
ated. Product selectivity can be tailored based on the choice of
catalyst, hence crucial to improving yields of targeted valuable
chemicals is catalyst design.
a
Nanomaterials Center-AIBN, School of Chemical Engineering, The University of
Queensland, Brisbane, QLD-4072, Australia
8
b
Department of Applied Chemistry, Cochin University of Science and Technology,
Kochi, Kerala-682022, India
c
A remarkable effort has been dedicated to exploring the ideal
hydrotreating catalysts for the HDO process in the quest of
Centre for Tropical Crops and Biocommodities, Queensland University of Technology, producing high-quality fuels. Among the studied catalysts,
School of Chemistry, Physics and Mechanical Engineering, Faculty of Engineering,
Queensland University of Technology, Brisbane, QLD-4000, Australia
d
Brisbane, QLD-4000, Australia. E-mail: jorge.beltramini@qut.edu.au
e
carbon is considered an excellent support for hydroprocessing
IROAST, Department of Chemistry, Faculty of Advanced Science and Technology,
reactions due to its high surface area, amphoteric nature and
Kumamoto University, Kumamoto 860-8555, Japan
9–12
less tendency to generate coke.
For example, Ru–MnO sup-
x
†
Electronic supplementary information (ESI) available: The details containing
ported on carbon-black selectively catalyzed the conversion of
the detailed physical and chemical characterisation of carbon supports and
as-synthesised supported metal catalysts. See DOI: 10.1039/c9ra02143d
13
guaiacol to cyclohexanol and methanol. The nature of support
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is also critical to the reaction pathway of the HDO reaction. A dodecane (99.0% purity), guaiacol (99.0% purity), catechol
study of guaiacol conversion using CoMo catalyst supported on (99.0% purity), anisole (99.7% purity), cyclohexanol (99.0%
carbon and g-Al O showed that the demethoxylation reaction purity), cyclohexane (99.5% purity), cyclohexene (analytical
2
3
pathway that produces higher phenol/catechol ratio and less grade), veratrole (99.0% purity), cresol (99.0% purity), pluronic
coke formation was favoured on CoMo/C as compared to CoMo/ P-123, activated carbon (norit SX ultra) were all purchased from
1
4
g-Al
2
1
O
3
,
and found to be consistent with the report of Centeno Sigma-Aldrich.
5
et al. who observed that the rate of phenol formation is faster
on CoMo supported on carbon. Among the widely used carbon
support is activated carbon, nonetheless, its characteristic
15
Catalyst preparation
micropores can be easily blocked when using large reactant The highly ordered mesoporous carbon (CMK-3) support with
2
ꢀ1
molecules which oen leads to catalyst deactivation as was a high surface area of 1258 m g and uniform pore size of
observed in our previous study of guaiacol conversion using 3.85 nm was synthesised by using mesoporous silica (SBA-15) as
16
17,22
MoS
2
/C catalysts. In this context, the inuence of surface the hard templating agent and sucrose as a carbon source.
At
porosity of carbon support on the activity of the catalyst for rst, SBA-15 was prepared by dissolving 2 g of pluronic P-123
ꢁ
HDO reaction has yet to be explored. Mesoporous carbon is surfactant in 60 mL of 2 M HCl at 38 C under stirring. Then,
proven to be superior catalytic support because of its organised 4.2 g of the silica source, TEOS, was added dropwise to the
and controllable pore geometry that enables excellent distri- above solution under vigorous stirring. Aer stirring for 6–
ꢁ
bution of active species, thus enhancing catalytic activity and 8 min, the mixture was kept at 38 C for 24 h without stirring.
17
stability. Moreover, the wide pores of mesoporous carbon are Then, the solution was transferred into an autoclave and heated
ꢁ
benecial for the diffusion of reactants easily to reach the active for 48 h at 100 C, followed by cooling, ltration, washing with
1
8
ꢁ
sites of the catalysts. For example, improved cycling perfor- excess water and the collected sample was dried at 100
C
ꢁ
mance of MoS
2
/CMK-3 nanocomposites as an electrocatalytic overnight. Finally, the dried sample was calcined at 550 C for
material was attributed to the large pores of CMK-3.
Among the several methods that have been employed for the SBA-15 that possesses a high surface area of 719 m g and
synthesis of metal nanoparticles, incipient wetness impregna- pore size of 9.15 nm.
19
6 h in a muffle furnace to obtain a highly organized mesoporous
2
ꢀ1
tion synthesis route is a widely used technique for preparing
CMK-3 was prepared by impregnating the pore of SBA-15,
supported catalysts. This synthesis method adopts the concept which now serves as the silica template, with 1.25 g of sucrose
of distribution of metal precursor over the support, followed by dissolved in H SO solution (0.14 g of H SO in 5 mL deionized
2
4
2
4
calcination. With this technique, nanosized metal particles H O). Then the mixture was stirred for 6 h, followed by heating
2
ꢁ
ꢁ
could be obtained but difficult to achieve both the homoge- at 100 C for 6 h and 160 C for 6 h. This impregnation proce-
neous and narrow particle size distribution of metal species in dure was repeated with 0.8 g sucrose in 0.09 g H SO in 5 mL
2
4
2
the carbon supports. Moreover, the aggregation of metal parti- deionized H O. The same heating protocol was used as
cles at the pore entrance and the loosely bound metal particles mentioned before. Aer heating the sample, the carbon–silica
ꢁ
can leach out during the catalyst preparation step such as nanocomposite sample was carbonised at 850 C for 5 h under
washing with a solvent. One of the synthesis technique to a ow of N gas. Finally, the SBA-15 silica template from as-
2
overcome this problem is microemulsion (ME), which has been prepared carbon–silica nanocomposite was removed by
utilized as a good strategy to synthesize nanoscale metal parti- washing with 5 wt% hydrouoric acid (HF) solution at room
20
cles with narrow particle size distribution. High dispersion of temperature, leaving behind mesoporous carbon material
2
ꢀ1
metals over the support, less synthesis time and in situ reduc- (CMK-3) with a high surface area of 1258 m g and a pore size
ꢁ
tion of metals during synthesis are some other benets asso- of 3.85 nm. The CMK-3 was dried at 200 C overnight and used
21
ciated with the microemulsion technique. Therefore, in this thereaer as a catalyst support.
article, we utilise the ME method for the synthesis of NiMoS2
Supported NiMoS catalysts were prepared by W/O (water in
nanocatalyst supported on two carbon supports with different oil) microemulsion synthesis method as reported previously.
2
16,23
surface porosity and pore structures. The catalytic activity of Typically, 5.0 g of non-ionic brij-30 surfactant was dissolved in
these nanomaterials was tested for the conversion of guaiacol, 100 mL of cyclohexane to form an oil phase. Then 5 mL of
a lignin model compound. The selectivity of liquid products ammonium sulphide solution was added to the oil phase and
and the reaction mechanism were inuenced by the type of stirred for 1 h. Again, 2 mL aqueous solution of ammonium
carbon support used.
molybdate hexahydrate was slowly added to the above solution,
followed by 1 mL aqueous solution of nickel nitrate tetrahydrate
and allowed to stir for 4 h to form a black microemulsion. The
NiMoS nanoparticles thus formed were supported over carbon
2
supports (AC or CMK-3) by adding to the above microemulsion
Experimental section
Materials
Nickel(II) nitrate hexahydrate (Ni(NO
3
)
2
$6H
2
O) (>98.5% purity), an already dispersed carbon material in 50 mL of deionized
ammonium molybdate tetrahydrate ((NH ) Mo O $4H O) water, then stirred for an hour. Cyclohexane was subjected to
4
6
7
24
2
(
81.0–83.0% purity MoO basis), Brij 30, ammonium sulphide slow evaporation at room temperature followed by heat treat-
3
ꢁ
solution (20.0% in H
2
O), tetraethyl orthosilicate (TEOS) (98.0% ment at 550 C for 4 h under a ow of N
2
. The prepared catalysts
purity), sucrose (99.5% purity), sulphuric acid (99.9% purity), are labelled as NiMoS
2
/AC and NiMoS /CMK-3. The
2
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ꢁ
concentrations of molybdenum and nickel were calculated to be temperature ramped to 300 C and 450 rpm (H
2
: guaiacol
1
2 wt% and 3 wt%, respectively. MoS /AC and MoS /CMK-3 initial mole ratio ¼ 11 : 1). The reactions were conducted until
2
2
were also synthesised by the same procedure as mentioned the complete conversion of guaiacol. The liquid and gas prod-
above without the addition of the nickel precursor.
ucts were collected every hour and analysed. At the end of the
reaction, the catalyst was retrieved by ltration, washed with
ꢁ
ethanol and dried overnight at 50 C. For the recyclability tests,
Catalyst characterization
the catalysts were reduced before the next reaction cycle. Each
experimental run for the guaiacol HDO reaction was conducted
in triplicate and the results agreed within ꢃ4%.
X-ray diffraction (XRD) patterns of the used samples for this
study were recorded using Rigaku Miniex with mono-
ꢁ
chromatic Co Ka radiation (30 kV, 15 mA) for the range of 10 #
ꢁ
ꢁ
2
q # 90 (step size of 0.1 ) whereas Bruker radiation D8
advanced diffractometer with Cu Ka radiation was used for the Product analysis
ꢁ
ꢁ
range of 1 # 2q # 5 . Nitrogen adsorption–desorption
The liquid products retrieved on an hourly basis during the
reaction were collected in 2 mL Eppendorf tube and centrifuged
at 13 000 rpm for 3 minutes. 0.2 mL of the supernatant samples
were injected manually into a Shimadzu GC-17A gas chroma-
tography unit. The GC was equipped with a ame ionisation
detector (FID). CP-Sil 5 CB capillary column (30 m ꢂ 0.25 mm ꢂ
ꢁ
isotherms were obtained at ꢀ196 C using a Micrometrics
ꢁ
Tristar II 3020 system. The catalysts were degassed at 200 C
overnight on a vacuum line, prior to analysis. The specic
surface area was calculated by Brunauer–Emmett–Teller (BET)
method and pore size distribution curve using the BJH model
on the desorption isotherm curve. The chemical structure of the
catalysts was analysed by Raman spectroscopy using Renishaw
inVia Raman Microscope equipped with a Leica DM LM
Microscope using a 514 nm HPNIR diode laser as an excitation
source and 0.1 mW laser power. The Raman emission was
collected by 50ꢂ objective lens in a backscattering geometry in
0.39 m) was employed to analyse the reaction products. The
standards for guaiacol and other products were prepared in
ethyl acetate. The injector and detector temperatures were set at
ꢁ ꢁ
2
00 C and 230 C, respectively. The column temperature was
ꢁ
ꢁ
ꢀ1
held at 50 C for 5 min, followed by ramping at 15 C min to
ꢁ
ꢁ
ꢁ
ꢀ1
2
2
50 C. Then the temperature was ramped at 5 C min to
ꢀ
1
ꢀ1
the region of 100–2000 cm at 4 cm resolution and acqui-
sition time of 1.0 s. Transmission Electron Microscopy (TEM)
JOEL 2100 microscope operated at 200 kV, tted with energy
dispersive X-ray (EDS) detector was used for morphology char-
acterisation. Before TEM imaging, the samples were prepared
by ultrasonically dispersing the particles in ethanol. The solu-
tion was settled on carbon-coated Cu grids to prepare the TEM
samples. Inductively Coupled Plasma (ICP) analysis in a Varian
Vista Pro ICPOES instrument was used for bulk elemental
analysis. The samples were digested using a milestone Ethos 1
microwave digester. CHON–S analyser (FLASH EA 1112 series,
Thermo Electron Corporation) was used to quantify sulphur. X-
ray photoelectron spectra (XPS) was acquired using a Kratos
Axis ULTRA X-ray photoelectron spectrometer equipped with
a 165 mm hemispherical electron energy analyser using
a monochromatic Al Ka (1486.6 eV) X-ray source at 150 W (15
kV, 10 mA). A survey wide scan was collected at analyser pass
energy of 160 eV and multiplex (narrow) high resolution scans
at 20 eV. Base chamber pressure in the analysis chamber was 1.0
80 C. A volume of 0.25 mL liquid product was injected in a split
mode. A mass spectrometer (Shimadzu GCMS-QP2010 Ultra
instrument with Rxi-5ms 30 m, 0.25 ꢂ 1 mm column) was used
to determine unknown liquid products. Conversion (% C) of
guaiacol and product yield (% Y) were calculated in mol% as
follows:
ꢀ
ꢁ
Number of moles of guaiacol in product
Initial moles of guaiacol in feed
%
C ¼ 1 ꢀ
ꢂ 100
ꢀ
ꢁ
Number of moles of the product
Initial moles of guaiacol in feed
%Y ¼
ꢂ 100
Results and discussion
Hydrodeoxygenation of guaiacol with NiMoS
2 2
/AC and NiMoS /
CMK-3 catalysts
ꢀ
9
ꢀ8
ꢂ 10 torr and it increased to 1.0 ꢂ 10 torr during sample
analysis. The binding energies were referenced to the C 1s peak
of adventitious carbon at 284.6 eV to account for the charging
effects. Data analysis was done using Casa-XPS (v 2.3.12)
employing a Shirley-background subtraction prior to tting the
spectra using Gaussian–Lorentzian curves.
High interest in utilizing carbon materials as catalyst support
has tremendously increased due to its amphoteric nature,
remarkable exibility and stability. In order to be used in this

eld, carbon supports should not only have the high surface
area but also a highly organized pore structure, adequate pore
volume and uniform pore size that can easily accommodate the
active species within the pores of carbon supports during the
incorporation or functionalization process. Therefore, this
Guaiacol conversion tests
Guaiacol conversion tests were carried out in a 300 mL Parr study has considered two types of carbon supports: (i) micro-
batch reactor. Before the reaction, 10 wt% of the catalyst was porous carbon – activated carbon (AC) which was used as
ꢁ
reduced under the ow of H at 450 C for 3 h. Then the reduced received without modication (Fig. S1a and c†) and (ii) meso-
2
catalyst was transferred into the reactor vessel containing porous carbon – CMK-3 that was prepared via the mesoporous
guaiacol and dodecane (solvent). The vessel was sealed, ushed SBA-15 silica hard templating method. The detailed character-
with argon, pressurised to 50 bars with H
2
gas and the isation of the activated carbon (AC) and mesoporous carbon
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4
6
(
CMK-3) supports along with the supported metal catalysts are et al. reported both catechol and methyl catechol to be the
17,24–41
discussed in ESI.†
major products with CoMoS/g-Al O catalyst. However, inter-
2 3
The catalytic activity at the complete conversion of guaiacol estingly with CoMoS/ZrO2 catalyst, demethoxylation was the
with MoS supported on AC and CMK-3 is depicted in Fig. 1. major pathway producing phenol directly. Irrespective of the
2
Phenol was the major product of the reaction and was an slightly acidic nature of ZrO
2
support, the promoter effect of Co
intermediate for the formation of hydrogenated products such on ZrO created a unique active site which promotes direct
2
46
as cyclohexanol, cyclohexene and cyclohexane. The other demethoxylation of guaiacol. Hence, it is already concluded
products evolved during the reaction were anisole, catechol, that acidity is not the only reason that favours catechol forma-
47
2
veratrole, benzene, cresol and methanol. Almost all the prod- tion. Similarly, Ruiz et al. examined the activity of MoS sup-
ucts were partially/completely deoxygenated, except catechol ported on four different activated carbons with different
and veratrole. However, these products are observed in minor textural and chemical properties. Regardless of the neutral
7
quantities. H , CO and CH were among the gaseous products character of the carbon support, the authors still observed the
2
4
observed. It is known that guaiacol produces phenol by two formation of catechol. Considering the phenol/catechol ratio,
pathways as shown in Scheme 1; namely (i) demethoxylation the value was constant (ca. 0.3) with all the supported catalysts
(
DMO) – direct production of phenol from guaiacol by cleaving irrespective of textural and chemical properties differences
7,42,43
47
the C–OCH
3
bond, producing methanol as a by-product
between the four supports. Similarly, with our catalysts, DME
and (ii) demethylation (DME) – where the guaiacol is rst con- and DMO of guaiacol to phenol, both were observed on MoS
2
/
verted to catechol by cleaving O–CH bond. Thus, the formation AC. However, DME did not take place when MoS /CMK-3 was
3
2
of catechol was also accompanied by the cogeneration of CH4. used but DMO was the only route for phenol formation. The
The formation of catechol increases the coke formation by conversion of guaiacol using MoS /AC catalyst as function of
2
44
condensation reactions. The acidity of the catalyst support was time, as well as the reaction mechanism was discussed in our
45
16
reported to be the key reason for catechol formation. Bui previous work. Apart from the non-formation of catechol with
MoS /CMK-3 catalyst, we did not observe considerable differ-
2
ence in the selectivity of products at complete conversion as can
be seen from Fig. 1.
In order to increase the deoxygenation tendency of the
catalyst, we introduced Ni as a promoter for the active MoS2
species in this study. Among metal catalysts, nickel is an inex-
pensive and available transition metal which has been
48
employed traditionally for hydrogenation reactions. Applica-
tion of Ni-based catalysts for HDO reactions primarily produce
hydrogenated products where the order of deoxygenation is
4
9
low. For instance, with La1ꢀxCe
x
NiO
3
(x ¼ 0 to 0.9) catalyst,
hydrogenation of guaiacol ring was primarily the observed
pathway that produced cyclohexanol and methoxy cyclo-
hexanol.
50
Similarly, cyclohexane, cyclohexanone and 1-
methylcyclohexane-1,2-diol were the major products observed
51
2 2 2 3 2
by Bykova et al. with CeO , ZrO , Al O and SiO supported Ni
catalysts for HDO of guaiacol. The formation of the hydroge-
nated products was also accompanied by the consumption of
a signicant amount of hydrogen, which is expensive. However,
when Ni was used as a promoter for MoS catalyst, Ni–Mo–S
2
species was formed which created a synergistic effect between
Ni and Mo that promoted the deoxygenating tendency of the
52
catalyst.
Hence in this work, NiMoS
2
/CMK-3 catalyst was tested for
the conversion of guaiacol, and the activity was compared with
that of AC supported catalyst. Complete conversion of guaiacol
was achieved aer 6 h reaction time with NiMoS
2
/CMK-3, in
contrast, 13 h was needed for the case of using NiMoS /AC
2
(Fig. 2). The fast conversion of guaiacol with CMK-3 supported
catalysts can be ascribed to the presence of more surface area
available for the reaction when compared to NiMoS /AC (as
2
observed from BET analysis result in Table S1†).
The conversion of guaiacol with time and the selectivity of
liquid products are given in Fig. 2. At 1 h reaction time, 27% of
Fig. 1 Comparison of the selectivities of liquid products (mol%) by (a)
MoS /AC and MoS /CMK-3 catalysts (b) NiMoS /AC and NiMoS
CMK-3 catalysts for the conversion of guaiacol after complete
conversion.
2
2
2
2
/
2
guaiacol was converted with NiMoS /AC, where the liquid
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2
Scheme 1 Reaction network of hydrodeoxygenation of guaiacol by carbon supported NiMoS catalyst.
product selectivity was 44% of phenol and 19% of catechol. cyclohexene to cyclohexane also occurred (4.6%) within 1 h
There is also a co-generation of 11% methanol formed as result reaction time. On the other hand, 35% guaiacol was converted
of the direct deoxygenation of guaiacol to phenol. Methane at 1 h with NiMoS /CMK-3 catalyst. Contrastingly, the main
2
which is coproduced during the formation of catechol was also products produced were phenol (25%), cyclohexanol (26%) and
observed in the gaseous product. The hydrogenation of phenol 20% cyclohexane respectively. On the other hand, it was also
to cyclohexanol was observed from the start of the reaction observed that 10% cyclohexene and 10% methanol were
(7%). The formation of cyclohexene from cyclohexanol occurs produced within 1 h reaction time. There was no trace of cate-
through the production of cyclohexanone which is an inter- chol detected by GC-MS. On NiMoS /C catalyst, increasing
2
mediate compound. As cyclohexanone was not detected in the reaction time, the selectivity towards phenol constantly
reaction product, indicates a rapid conversion of cyclohexanone increased up to 4 h where it reached a 52% maximum, followed
to cyclohexene (10.1%). The further hydrogenation of by a constant drop till the end of the reaction. The decrease in
phenol selectivity was accompanied by an increase in cyclo-
hexane and cyclohexene selectivity, which concludes that
phenol was converted to cyclohexane. The catechol selectivity
decreased sharply aer 1 h, and the nal product collected at
the complete conversion of guaiacol showed no trace of cate-
chol. The other products such as cresol, anisole, veratrole,
benzene were produced at the onset of reaction but there was no
increase in selectivity aer 1 h of reaction. We can also infer
from this reaction pattern that during the rst hour, the kinetics
of the reaction was fast for all deoxygenation, hydrogenation
and methylation reactions that were taking place. Aer 1 h, the
methylation reaction was suppressed. Looking into the reaction
mechanism with NiMoS /CMK-3, there was no trace of catechol
2
from the onset of reaction. However, methane was still detected
in the gas product analysis, assuming that might be produced
through decomposition of methanol.
The organized mesopore structure of CMK-3 can facilitate
the better distribution of active species (Ni and MoS ) that
2
enhances the contact time of guaiacol with the catalyst active
phases. On the other hand, on NiMoS /AC, larger metal species
2
are deposited on the micropores of activated carbon reducing
the active surface area of the catalyst that can be noticed from
Table S1.† Hence, we may conclude that the active metal species
are deposited on the surface of the AC support, wherein inter-
action of the reactants with active species occurs mostly at the
surface level. The channel-like pores of the CMK-3 act as
a nanoreactor containing the active catalyst species. Thus,
guaiacol has easy accessibility to get in contact with the active
Fig. 2 Conversion (mol%) of guaiacol and selectivities of liquid
products produced (mol%) with respect to increase in reaction time
34
with (a) NiMoS
2 2
/AC and (b) NiMoS /CMK-3 catalysts.
metal sites, as illustrated in Fig. 3. Ferrari et al. concluded that
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conversion of guaiacol by NiMoS
2 2
/AC and NiMoS /CMK-3
catalysts for three cycles is depicted in Fig. 4.
During the recyclability experiment, there was a drastic drop
in the catalytic activity with NiMoS /AC catalyst. 93.2% guaiacol
2
was converted during the second cycle which was further
reduced to 88.6% during the third cycle in 13 h of reaction time.
Further discussing on the selectivity of products, the selectivity
towards phenol decreased from 31.4% to 25.8% respectively
aer the third cycle which is an overall 18% decrease in selec-
tivity. Similarly, the cyclohexane selectivity decreased from
35.7% to 28%. On the other hand, the methylated product
2
Fig. 3 Figure illustrating the Ni and MoS active phase distribution over
(
a) activated carbon (b) CMK-3 supports.
increased from 3.9% to 4.9% by third cycle. We have already
reported that the increase in methylated products was mainly
due to the formation of MoO
the dispersion of catalytic metal species and the accessibility of MoS /AC catalyst recyclability test. It can also be supported by
active sites are the crucial steps in guaiacol conversion. This the decrease in the S : Mo ratio in bulk (2.05 : 1 to 1.95 : 1) and
x
species which was also noticed by
16
2
53
observation was strongly supported by Ghampson et al. who surface (1.92 : 1 to 1.79 : 1) as estimated by ICP and XPS anal-
have used four different activated carbons as support for Mo
catalyst.
2
N
yses (Table 1). There was also a decrease in catalyst surface area
from 153 m g to 102 m g aer third cycle accounting for
2
ꢀ1
2
ꢀ1
While testing these catalysts for the conversion of guaiacol, a 33% loss in active surface area. From the current result, we
they noticed that the support with high mesopore content can infer that one of the main reason for the catalyst deactiva-
54
possessed the highly exposed active sites for reaction. Lee et al.
tion could be the loss of catalyst surface area available for
proposed that CoMoS/nanoporous carbon exhibit higher catalytic reaction. Zanuttini et al. also described rapid deacti-
activity for HDS reaction when compared to alumina and acti- vation of Pt/zeolite, for the conversion of m-cresol due to the
56
vated carbon supported catalyst. The high surface area and the micropore blockage of the support. The coke formation in the
organized pore geometry are the main reasons for the enhanced catalyst was also calculated using CHNSO analysis and the
activity. They noted similar observation while evaluating the amount of coke formed aer 3 cycles was found to be 1.1%.
activity of Pt/mesoporous carbon for methanol oxidation. The Generally, carbon supported catalyst were reported to produce
superior activity is attributed to the high dispersion of Pt less coke when compared to acidic catalysts such as alumina,
2
species in the pores, the orientation of nanochannels and easy ZrO etc. which was attributed to the neutral/less acidic nature
55
57,58
accessibility of the pores by the reactant.
of carbon.
Regarding the activity of NiMoS
2
/CMK-3, the complete
guaiacol conversion was achieved within 6 h even aer 3 reac-
tion cycles. Considering the selectivity, the selectivity towards
phenol decreases from 25.4% aer rst cycle to 23.9% aer
third cycle, which is a 5.9% selectivity difference. It is note-
worthy to mention that the selectivity towards cyclohexane was
not affected even aer the third cycle.
2
Catalyst recyclability – NiMoS /CMK-3
The catalyst aer being used for the reaction was recovered by
ꢁ

ltration, washed with ethanol and dried overnight at 50 C.
Aerwards, was reduced before each recyclability test. The
reactions were conducted for 13 h and 6 h with NiMoS /AC and
NiMoS /CMK-3 catalysts, respectively. These are the times that
2
The characterisation of spent catalyst in Table 1 shows
interesting results. A similar decrease in S : Mo ratio was
2
the catalysts attained complete conversion at the rst catalytic
run. The selectivity of liquid products (mol%) and the
2
noticed with NiMoS /CMK-3 also as previously noticed with
2
Table 1 Surface and bulk sulphur composition, S : Mo ratio, BET surface area and pore volume of spent NiMoS /C catalyst after cycle 1, 2 and 3
Amount of
sulphur (at%)
S/Mo (at. ratio)
BET surface area
a
b
a
b
2
ꢀ1
3
ꢀ1
b
Catalyst
Surface
Bulk
Surface
Bulk
(m g
)
Pore volume (cm g
)
Coke deposition (%)
NiMoS
NiMoS
NiMoS
NiMoS
NiMoS
NiMoS
NiMoS
NiMoS
2
2
2
2
2
2
2
2
/AC – fresh
/AC – cycle-1
/AC – cycle-2
/AC – cycle-3
/CMK-3 – fresh
/CMK-3 – cycle-1
/CMK-3 – cycle-2
/CMK-3 – cycle-3
9.8
9.9
9.7
8.7
9.1
8.9
8.9
8.7
3.2
3.0
2.9
2.7
3.3
3.2
3.0
2.8
1.92 : 1
1.88 : 1
1.82 : 1
1.79 : 1
1.92 : 1
1.90 : 1
1.87 : 1
1.85 : 1
2.05 : 1
2.02 : 1
1.99 : 1
1.95 : 1
1.98 : 1
1.97 : 1
1.97 : 1
1.92 : 1
153
135
119
102
453
448
441
437
0.21
0.19
0.17
0.17
0.33
0.27
0.24
0.21
0
0.6
1.0
1.1
0
0.3
0.6
0.7
a
b
Analyzed by XPS. Analyzed by CHNS elemental analyzer.
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NiMoS
2
/AC. This data supports corelate with an increase in (6.3%) were similar for NiMoS
2
/AC (14.4% – cyclohexene, 5.3% –
methylated product from 2.4% – cycle 1 to 3.9% – cycle 3. cyclohexanol). Hence, we can conclude that promotion of MoS2
Surprisingly the mesopore surface area decrease aer third by Ni, increases the hydrogenation tendency. On the other
cycle was only 3.5%. It is to be noted that the pore volume hand, a similar trend was not observed with CMK-3 supported
3
ꢀ1
3
ꢀ1
decreased from 0.33 cm g to 0.21 cm g ; with an amount of catalyst. The selectivity of cyclohexene was 10.6% for MoS
coke deposited of 0.7%. Hence, we can conclude that the coke CMK-3 which reduced to 1.8% on NiMoS /CMK-3 but accom-
formed were deposited inside the pores which reduced the pore panied by increased cyclohexane (50%) which is a product of
volume of the catalyst, however coke did not block the meso- intense hydrogenation. Thus, the promotion of Ni to MoS
2
/
2
2
/
pore of the catalyst and hence only 3.5% of the surface area was CMK-3 increases the hydrogenation as well as the deoxygen-
decreased. Hence, the remarkable activity of the NiMoS /CMK-3 ation of guaiacol to a greater extent. The kinetic diameter of
2
59
catalyst could then be attributed to the mesoporosity of the guaiacol was calculated to be 0.0668 nm. Since most of the
CMK-3 support.
pores of activated carbon falls in the micropore region, the
diffusion of guaiacol into the pores of activated carbon was
limited. Hence, the conversion of guaiacol mostly occurred at
the surface level. On the other hand, the high surface area of
Discussion
The inuence of the difference in carbon surface porosity was CMK-3 along with its mesoporosity (3.85 nm pore size) facili-
not in an appreciable amount for the conversion of guaiacol tates better dispersion of the active sites both within and
with MoS
2
/C. Notably, when Ni was introduced as a promotor outside of the pore channels of the support. As a result, there is
the selectivity of products changed signicantly. For example, an improved contact of the reactant with the catalytic active
the use of NiMoS /AC produced phenol (31.5%) and cyclo- sites, thus increasing the rate of the reaction and hence, the
2
hexane (35.7%) as the major products compared to the MoS /AC complete conversion of guaiacol was achieved in 6 h with
2
where phenol (62.2%) was produced to MoS
followed by cyclohexane (7.6%) (Fig. 1).
2
/AC predominantly NiMoS
2
/CMK-3 which was much quicker than the 13 h
/AC to achieve a similar conversion. This
required for NiMoS
2
The main inuence of Ni to MoS
2
was the increase in deox- fact is supported by XPS data (Table S2†) that shows that Ni
ygenation and hydrogenation activity. So, the deoxygenation of species are predominantly deposited inside the pores of CMK-3
guaiacol and the hydrogenation of phenol occurred simulta- (surfaceNi/Mo ¼ 0.16) whereas, for AC (surfaceNi/Mo ¼ 0.31), Ni
neously which increased the conversion of phenol and selec- was deposited mostly on the catalyst surface.
tivity of cyclohexane. Also, comparing the other hydrogenated
compounds such as cyclohexene (10.6%) and cyclohexanol
Conclusions
In summary, the microemulsion technique has been used for
the successful synthesis of NiMoS nanocatalyst supported on
2
two different carbon materials containing different surface
porosity. The catalysts were tested for HDO of guaiacol. Phenol
and cyclohexane were produced as the major products. The
selectivity of products was inuenced by the pore geometry of
the carbon supports which is ascribed to the distribution of Ni
and MoS active species. The Ni–Mo–S phase, along with the
2
easy accessibility to the active phases by guaiacol in the NiMoS2/
CMK-3 catalysts exhibited superior deoxygenation and hydro-
genation activity than the activated carbon supported catalyst.
Mesoporous carbon is a perfect choice to be used as support for
hydrodeoxygenation reaction for their mesoporosity and pore
geometry. Hence, the use of the high surface area, mesoporous
materials as support for HDO reactions are highly
recommended.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
We gratefully acknowledge nancial support from the Nano-
materials Center (NANOMAC), The University of Queensland.
Fig. 4 The selectivities of liquid products (mol%) and the conversion of
guaiacol by NiMoS
2
/AC and NiMoS
2
/CMK-3 catalysts for three cycles. Sincere thanks to Dr Miaoqiang Lyu, Department of chemical
17200 | RSC Adv., 2019, 9, 17194–17202
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