Intrinsic role of pH in altering catalyst properties of NiMoP over alumino-silicate for the vapour phase hydrodeoxygenation of methyl heptanoate

Article abstract of DOI:10.1039/C8NJ05145C

Monometallic and bimetallic Ni₂P, MoP, and NiMoP active species were successfully impregnated on thermally stable, high surface area mesoporous alumino-silicate with an Si/Al ratio of 10 at room temperature via a facile wet impregnation method under both acidic and basic conditions using HCl and NH₄OH as pH regulators, respectively. Furthermore, the intrinsic role of pH in altering the physicochemical properties of the catalysts was comprehensively evaluated. The catalysts were tested in a high-pressure stainless steel fixed bed reactor at different temperatures ranging from 275-350 °C, under 10-40 bar hydrogen pressure for the hydrodeoxygenation (HDO) of methyl heptanoate. The reaction pathway and product distribution of methyl heptanoate were manifested at different temperatures and pressures. The HDO activity and synergistic factor were found to be remarkably higher for the NiMoP/MAS (10)-A catalyst than the NiMoP/MAS (10)-B catalyst and its monometallic counterparts. This investigation proves that the NiMoP/MAS (10)-A catalyst is a promising catalyst for green fuel production from non-edible oils through hydrodeoxygenation. It was also unequivocally confirmed that the catalytic process does not suffer from any mass transfer resistance; thus, making the scaling up of the reaction more feasible.

Full text of DOI:10.1039/C8NJ05145C

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C8NJ05145C
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Intrinsic Role of pH in altering Catalyst Properties of NiMoP over
Alumino-Silicate for the Vapour phase Hydrodeoxygenation of
Methyl Heptanoate
Received 00th January 20xx,
Accepted 00th January 20xx
Arumugam Ramesh, Perumal Tamizhdurai, Krishnan Suthagar, kandhasamy Sureshkumar, Gubert
Sonia Theres, Kannan Shanthi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Monometallic and bimetallic Ni2P, MoP, and NiMoP active species were successfully impregnated on thermally
stable, high surface area mesoporous alumino-silicate with Si/Al 10 ratio, at room temperature by a facile wet
impregnation method under both acidic and basic medium, using HCl and NH4OH as pH regulators. The intrinsic role of pH
in altering the physicochemical properties of the catalysts was comprehensively evaluated. The catalysts were tested in a
high-pressure stainless steel fixed fed reactor, at different temperatures ranging from 275-350oC, under 10-40 bar
hydrogen pressure for hydrodeoxygenation (HDO) of methyl heptanoate. The reaction pathway and product distribution
of methyl heptanoate is manifested at different temperature and pressure. HDO activity and synergetic factor were found
to be remarkably higher for NiMoP/MAS (10)-A catalyst than NiMoP/MAS (10)-B catalyst or its monometallic counterparts.
This investigation proves that NiMoP/MAS (10)-A catalyst can potentially be a promising catalyst for green fuel production
from non-edible oils through hydrodeoxygenation. It was also unequivocally confirmed that the catalytic process did not
suffer from any mass transfer resistance thus making the scale-up process of the reaction more feasible.
Introduction
Conventional catalytic hydrotreatment process is one
of the most adopted processes to remove the oxygen content
The rapid depletion of conventional fossil fuels
from naturally available bio-oils [3, 4]. Numerous reports have
been published for hydrodeoxygenation (HDO) of non-edible
oils [5-6]. Methyl heptanoate, a simple ester of triglycerides
present in all the non-edible oils is essentially subjected to
hydrocarbon fuel upgradation via catalytic transformation; this
is an emerging field and the development of an efficient
leading to inevitable fossil fuel shortage and degradation of
the ecosystem owing to the constant excessive use of non-
renewable fossil fuels are the two pivotal reasons, that has
made exploring of alternative resources an urgent need of the
hour. Use of non-edible oils obtained from biomass to produce
fuel precursors and fuel components is one such alternative
resource. However, the most significant step towards the
generation of a greener and a more secured energy for future
is the development of an efficient catalyst paving the way for
an efficient process. Such development could be an antidote to
the challenges faced by the industries associated with fuel
production [1]. The high oxygen content present in bio-oil limits
its direct usage as a fuel, since it possesses deleterious
properties such as poor heating value, thermal instability, non-
volatility, high viscosity, and immiscibility with fossil fuels;
hence bio-oil up gradation process becomes essential and
inevitable in order to upgrade its quality as a fuel[2].
catalyst for the HDO pathway is being vehemently explored [7]
.
Hydrotreating catalysts owning transition metal
compounds have shown excellent selectivity and conversion
towards the removal of nitrogen and sulfur atoms from crude
oil [8, 9]. Particularly, γ-alumina supported Ni-Mo catalyst has
been extensively used as a hydrotreating catalyst in oil-
refineries [10]. Even though gas phase preparation of Ni-Mo
sulfide catalyst is highly active, they are found to be
thermodynamically less stable and the unreacted sulfur
molecules existing at high temperature poses a great threat to
the eco-system [11]. In order to overcome these drawbacks,
metal catalysts have been recently prepared in the phosphide
form, which not only is more stable and highly active but also
retains its structure after prolonged use [12]. The superior
textural properties, thermal stability and acidity possessed by
the metal phosphide catalysts have been found responsible for
significant enhancement of activity of the catalysts [13]. It is
reported that transitions metal phosphides Mo/P, WP, Fe2P,
Department of Chemistry, Anna University, Chennai-600025, Tamil Nadu, India
* Corresponding Author: K. Shanthi (shanthiramesh@annauniv.edu.in)
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Co2P, and Ni2P showed excellent catalytic activity towards HDO
of lignin model compounds [14-15]. Ni as a promoter for Mo
based catalyst and introduction of phosphorous to Ni
Experimental
Materials
View Article Online
DOI: 10.1039/C8NJ05145C
Tetraethylorthosilicate (TEOS), methyl heptanoate,
cetyltrimethylammonium bromide (CTAB), nickel nitrate
hexahydrate, ammonium heptamolybdate, ammonium
dihydrogen phosphate, aluminium nitrate were obtained from
Merck. m-Xylene, and hydrochloric acid (35-38%) were
purchased from S.D fine chemicals. Polyethylene glycol Mw-
4000 (PEG-4000) was purchased from molychem. Solvents
such as methanol, acetone, ethanol and other chemicals were
used as such without any further purification.
promoted catalysts; both have been reported to play a
[15]
significant role in enhancing the efficacy of the catalyst
.
Ni2P has been reported to be an active component for
hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and
hydrodechlorination types of reactions with superior catalytic
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activity [16-17]
.
Even though Al2O3, ZrO2 and TiO2 supported Ni-Mo
catalysts have been used for HDO of esters, phenolics, cyclic
ketones and aldehydes [17-18], there is still demand for
development of new supports with improved textural
property, morphology and thermal stability [19]. Mesoporous
silica has been widely used as support for Ni-Mo catalysts with
improved textural properties, homogeneously dispersed active
species and increased acidity [20]. Various mesoporous silicates
such as MCM-41, HMS, SBA-15 and KIT-6 have been widely
used as supports in hydrotreating catalysts [8, 21]. However, the
preparation of alumino-silicates with low Si/Al ratio is tedious
due to the difficulty in incorporating aluminium into the
mesoporous silica framework by employing one-pot
Synthesis of mesoporous alumino-silicate (MAS)
Mesoporous alumino-silicate (MAS) was synthesized
using PEG-4000 as particle size regulator and CTAB as structure
directing agent at room temperature (RT) based on a
procedure reported by Zhai et al [21]. After homogenizing the
surfactant solution mixture, calculated amount of TEOS and Al
(NO3)3 with Si/Al=10 ratio were slowly added to the above
reaction mixture, maintained at pH 9.0 by using 25% NH4OH
solution. Then, the reaction mixture was continuously stirred
for 2 days at room temperature. The obtained solid product
was filtered, washed well with doubly distilled water (DD) and
dried at 80oC for 24 h. The dried material was calcined at
550oC for 6 h. The sample was denoted as MAS (10).
hydrothermal method [22-23]
.
The preparation of Ni2P, MoP, and NiMoP based
catalysts have been reported by wet impregnation,
temperature programmed reduction, and sequential pore-
[10, 24-25]
filling impregnation methods
.
The catalysts
synthesized by these reported methods when scaled up from
pilot to industrial reaction conditions, suffered from a major
drawback of structural instability caused by the conversion of
the active metals into their respective oxide forms in the long
run. In order to overcome this flaw, the intrinsic role of pH in
the catalysts synthesis thereby affecting the physicochemical
properties and the catalytic activity of the catalysts has been
comprehensively evaluated and it was observed that under
optimized pH values the synthesized catalysts possessed
improved textural properties and excellent structural stability
Catalysts preparation
Ni2P, MoP and NiMoP/MAS (10) catalysts were
synthesized by wet impregnation method on the basis of
previously reported literature by employing acidic as well as
basic medium [24]. Nickel nitrate, ammonium heptamolybdate
and ammonium dihydrogen phosphate were used as nickel,
molybdenum, and phosphorus precursors respectively. Metal
precursor solution was prepared using the following ratios
Ni/P = 0.3, P/Mo = 0.06 and Ni/Mo = 0.2 dissolved in DD water
for NiP, MoP and NiMoP active-metal composites respectively.
The prepared precursor solution was added to the already
synthesized MAS (10) support under acidic (35% of HCl) and
basic (25% of aqueous ammonia) medium and stirred using a
stirrer. Ni-MoP catalyst is pale green. Ni2P and MoP are in
white colour in both acidic and basic medium. All the prepared
materials were washed with water, dried in vacuum followed
by temperature programmed reduction up to 650oC under H2
atmosphere for 6 h. The catalysts were designated as
Ni2P/MAS (10)-A, MoP/MAS (10)-A, MoP/MAS (10)-B,
NiMoP/MAS (10)-A and NiMoP/MAS (10)-B wherein A & B
denote the acidic and basic condition adopted in the synthesis
respectively.
[18]
.
In this present work, two vital aspects pertaining to
the catalytic hydrotreating of non-edible oil based model
compound that was focused upon were, synthesizing
structurally stable material by employing a simple preparation
method and seeking an efficient and energy economical
catalyst for low-temperature HDO reaction. Along the same
lines, MAS (10) with high specific surface area was synthesized
by a simple sol-gel method at room temperature. The effective
formation of Ni2P, MoP, NiMoP catalysts over the MAS (10)
support was controlled by acidic and basic medium using HCl
and NH4OH respectively. The effect of pH on the
physiochemical properties and the catalytic activity of the
catalysts, synergistic factor in bimetallic catalysts, the product
distribution and the reaction pathways in hydrodeoxygenation
were all comprehensively evaluated. Each catalyst was tested
under long term experimental conditions incorporated in the
high pressure fixed bed vapour phase reactor; under optimized
reaction parameters and the structural-activity relationship of
the catalysts were also evaluated.
Materials characterization
All the synthesized catalysts were characterized by
various physicochemical techniques. The diffraction pattern of
the samples was recorded using BRUCKER D8 diffractometer in
the range of 0.5-5o and 10-80o 2θ values with step scan rate of
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0.2 deg/min using Cu Kλ as the radiation source (λ=1.548 Å). N2 Where, Fo = Flow rate of methyl heptanoate (mol g-1 s−1), Y =
sorption isotherms were obtained using QUADRASORB SI methyl heptanoate conversion (%) and w = weight of the
automatic analyzer at liquid N2 temperature. H2-Temperature catalyst (g).
View Article Online
DOI: 10.1039/C8NJ05145C
Programmed Reduction (H2-TPR) and NH3-Temperature The Turnover frequency (TOF) was calculated using the
Programmed Desorption (NH3-TPD) studies were carried out following equation
with QUADRASORB SI. NH3-TPD/H2-TPR by using 5%H2/95%Ar
TOF (s-1) = r (mol g-1 s-1) / quantity of sites (μmol g-1) (4)
and 10%NH3/90%He gas mixtures respectively. Before the Where, the quantity of active sites is calculated from H2
o
TPD/TPR analysis, 0.05 g of sample was pre-treated at 400 C chemisorption (μmol g-1).
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for 2 h under helium flow (80 mL min-1). DR UV-Vis spectra of
the samples were recorded (200-800 nm) using Shimadzu UV-
2450 spectrophotometer equipped with diffuse reflection
Results and discussion
attachment and BaSO4 was used as a reference. Raman
XRD analysis
spectra were recorded using EZ-Raman 785 series, Diode Laser
In low angle XRD patterns of all synthesized materials
power 60 mW. FTIR spectra were recorded using PerkinElmer
are displayed in Figure 1A. In case of Ni2P/MAS (10) catalysts a
Spectrometer (FT-IR C101375). The prepared sample was
sharp intense diffraction peak at 2θ 2.42o and a less prominent
mixed with KBr and compressed to form a thin pellet for FT-IR
peak at 3.46o corresponding to the 100 and 110 reflection
analysis.
planes were evidently observed, indicating long-range ordered
mesoporous framework with hexagonal symmetry in the
synthesized MAS (10). Absence of other planes characteristic
Catalytic activity and reactor set-up
to that of MAS were not observed which may be due to the
high loading of aluminium content on the silica frame work [39]
.
All the prepared Ni2P/MAS (10)-A, MoP/MAS (10)-A,
MoP/MAS (10)-B, NiMoP/MAS (10)-A, and NiMoP/MAS (10)-B
catalysts were screened for HDO of methyl heptanoate by
tuning the various reaction parameters such as temperature
(250 to 375oC), pressure (10 to 40 bar) and WHSV (1.7- 8.5 h-1).
The product was collected for every one hour interval and
analyzed in a gas chromatograph (Shimadzu-GC-17A with DB-5
column). Prior to each run, all the catalysts were activated
under nitrogen atmosphere at 400oC followed by passing ultra-
high pure hydrogen gas for 4h under 4 bar pressure.
In case of monometallic MoP/MAS (10)-A and MoP/MAS (10)-
B catalysts, the diffraction peak corresponding to the 100
plane of MAS was observed at 2.39o whereas in the bimetallic
NiMoP/MAS (10) A and NiMoP/MAS (10) B catalysts the peak
due to 100 plane of MAS was shifted to 2θ value of 2.42o. The
mesoporous nature of MAS was found to be retained and not
distorted as evident as seen from HR-TEM images (Figure 6).
Figure 1B depicts the high angle XRD pattern of all the
prepared catalysts. XRD pattern of Ni2P/MAS (10) showed a
broad peak at 2θ value 23o corresponding to the amorphous
nature of silica-alumina framework of MAS support. Absence
of any peak characteristic to the Ni2P phase in the diffraction
pattern of Ni2P/MAS (10) catalyst, implies that the phase may
be well dispersed over MAS (10) support [26-27]. The XRD
pattern of MoP/MAS (10) (JCPDS#24-0771) catalysts prepared
under both acidic and basic medium showed a common set of
peaks at 2θ = 26.89o, 32.57o, 44.11o, 55.7o, 62.8o, 67.49o and
73.9o corresponding to the respective planes (001), (100),
(101), (110), (111), (102) and (201) of MoP [28]. In addition,
peaks characteristic to the MoO3 phase were also observed at
2θ = 11.1o, 30.64o, 36.05o, 39.03o, 48.32o, 51.09o and 62.88o
[29]. There was no significant change observed in the peak
position and peak intensity of MoP based on the medium of
synthesis i.e. both MoP/MAS (10) A & MoP/MAS (10) B
exhibited almost similar diffraction pattern. For bimetallic
NiMoP/MAS (10) (JCPDS#30-0862) catalysts were prepared
under both acidic and basic medium, a common set of, sharp
crystalline peaks corresponding to the MoP phase formation
over the MAS (10) support were observed. In addition to these
peaks, a peak at 2θ = 27.5o, due to the formation of crystalline
Ni2P phase, was also observed for the catalyst prepared under
Vapour phase HDO of methyl heptanoate was carried
out in a high throughput reactor setup; Stainless steel high
pressure fixed bed flow reactor (100 bar) fabricated by Amar
Equipment (India) Pvt. Ltd, Mumbai, India. A schematic
diagram of the high pressure fixed bed flow reactor set-up is
shown in Figure S1. In this work, high purity hydrogen and
nitrogen gas and methyl heptanoate dissolved in m-xylene
were used as a feed. Both temperature and pressure were
monitored by the pneumatic control panel. The pelletized
catalyst was packed with glass beads (3.5 mm diameter) inside
the bed (15 mm inner diameter and 380 mm in length) which
was in turn densely packed with silica wool to maintain a
uniform temperature throughout the bed. The gas feed (N2,
H2, and O2) along with vaporized methyl heptanoate was
introduced as a single feed to the reactor tube with the aid of
high pressure pump.
The % of methyl heptanoate (MH) conversion (Y) and
selectivity (Si) were calculated using the following equations
Y= (mole of MH) in-(mole of MH)out/(mole of MH)in × 100% (1)
Si = (mole of Product i)/(sum of moles all products) × 100 % (2)
acidic medium [30]
.
The specific reaction rate (SRR, r) was calculated by using the
formula
w/(Fo)=∫dXA/r=1/r∫dXA=XA/r
(3)
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Table 1 Textural properties of catalysts, Acid site distribution, H2 consumption, edge energies: Ni2P/MAS (10)-A, MoPV/MiewAASrtic(l1e0O)n-Alin,e
MoP/MAS (10)-B, NiMoP/MAS (10)-A, and NiMoP/MAS (10)-B catalysts
DOI: 10.1039/C8NJ05145C
aS
(m2 g-1)
c d
BJH
(nm)
dReduction
Temperature
(oC)
dVolume of H2
consumption
(μmol g-1)
eAcidity
fPhase
formed Energy
gEdge
bV
(cm3g-1)
0.91
BET
Catalysts entry
(mmol NH3 g-1 cat)
t
(Eg) eV
WA
SA
TA
MAS (10)
855
9.0
8.9
8.5
8.3
8.9
-
-
-
-
-
-
-
Ni2P/MAS (10)-A
MoP/MAS (10)-A
MoP/MAS (10)-B
NiMoP/MAS(10)-A
825
0.90
0.79
0.77
0.71
237,337, 377
547,726, 850
540,675, 789
70
0.42
0.90
0.55
0.11
0.01
0.01
0.01
1.02
0.43
0.91
0.56
1.13
Ni2P
MoP
MoP
-
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662
155
135
180
3.45
3.54
3.80
640
580
449,540,675,
789
552,772,850
Ni2P,
NiMoP
Ni(OH)
2,MoP
NiMoP/MAS(10)-B
510
0.68
8.6
151
0.5
0.13
0.63
3.73
a,b,c obtained from BET isotherm, d & e obtained from H2-TPR and NH3-TPD, f DRS-Uv-Vis Spectrum, FT-IR and Raman, g obtained from DRS-
Uv-Vis Spectrum, (BJH) Barrett-Joyner-Halenda Analysis, WA= Weak acid, TA = total acidity and SA= Strong acid.
This peak was observed to be less prominent in case of MoP/MAS (10). Compared to the monometallic catalysts, the
NiMoP/MAS (10) B catalyst, which is due to the more favoured bimetallic catalysts exhibited reduced pore characteristic
precipitation reaction of Ni2+ as Ni(OH)2 under basic pH values; this can be attributed to the greater degree of pore
conditions, thus hampering the formation of Ni2P in blockage and surface area coverage caused by the bimetallic
NiMoP/MAS (10) B catalyst. No additional peaks corresponding species (NiMoP) as compared to their monometallic
to Ni or Mo oxides could be identified; this observation is in counterparts (Ni2P and MoP), on the support (Table-1) [20]. It is
accordance with the literature report [27]
.
inferred from Table-1 that monometallic (MoP/MAS 10) and
In the monometallic catalyst, Ni2P is 3% and it has bimetallic (NiMoP/MAS 10) catalysts prepared under acidic
only Ni species whose concentration is relatively very low medium showed beneficial textural properties than the one
compared to the total metallic species present in other prepared under basic medium [18]
catalysts (MoP and Ni-MoP). Hence, in the Ni2P/MAS (10)-A
catalyst, Ni is highly dispersed over the support MAS (10).
There is no crystalline plane appeared in Ni2P/MAS (10)
catalyst when compared to 12% of MoP/MAS (10) and
NiMoP/MAS (10) catalysts as seen in Figure 1B.
.
Figure 2 N2-adsorption desorption isotherms (A) and Pore size
distribution curves (B) of catalysts
DR Uv Vis spectroscopy
DR UV–vis spectra of all the catalysts are shown in
Figure 3. In case of Ni2P/MAS (10) A, absorbance bands around
240–370 nm and a broad band in the range of 550–800 nm are
Figure 1 XRD pattern of catalysts (A) low angle (B) high angle
[31]
due to the octahedral framework of Ni2+ in Ni2P phase
.
MoP/MAS (10) catalysts prepared under acidic and basic
medium displayed absorbance peaks around 220, 230-285 and
N2 Sorption study
BET isotherms and pore size distribution curves of
Ni2P, MoP, and NiMoP supported MAS (10) catalysts are
shown in Figure 2 (A) and (B) respectively. All the prepared
catalysts displayed type-IV isotherm (H1 type hysteresis loop)
with a sharp nitrogen uptake in P/Po range of 0.5 -0.9 P/Po
confirming the mesoporous nature of the catalysts [21]. In case
of monometallic catalysts, Ni2P/MAS (10) exhibits greater
surface area, pore volume and pore diameter as compared to
285-350 nm which can be attributed to the Td and Oh
[20,
environment of Mo respectively
32]. In case of the
absorbance peaks observed in common, in the range of 285-
359 nm, for both acidic and basic medium synthesized
MoP/MAS (10) catalyst, a shift in the absorbance peaks
towards higher wavelength was noted in case of MoP/MAS
(10) A, indicating greater number of Oh coordinated species in
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the MoP/MAS (10) A as compared to the MoP/MAS (10) B total acidity values measured on bimetallic NiMoP/MAS (10)-A,
View Article Online
DOI: 10.1039/C8NJ05145C
-1
catalyst. In case of bimetallic catalysts, NiMoP/MAS (10) all the was found to be the highest (1.13 mmol g ). This can be
absorbance bands corresponding to Td and Oh environment of attributed to the creation of large amount of H+ ions and H
Mo were evident at 220, 230-285 and 285-350 nm atoms on the catalyst surface by the Ni2P species [20]. The
respectively. A similar trend, as observed in case of surface hydroxyl groups on the support, stabilizes these
monometallic MoP/MAS (10) catalysts, was more evidently hydrogen species which in turn is responsible for high acidity
observed in case of bimetallic NiMoP/MAS (10); a shift in the value. The order of acidity follows the trend; NiMoP/MAS (10)-
absorbance bands towards the higher wavelength was A > MoP/MAS (10)-A > NiMoP/MAS (10)-B> MoP/MAS (10)-B >
observed in case of NiMoP/MAS (10) A as compared to Ni2P/MAS (10)-A catalysts.
10
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NiMoP/MAS (10) B, indicating more number of octahedrally
bonded species in the NiMoP/MAS (10) A than the
NiMoP/MAS (10)
B
catalyst. Other than the bands
characteristic to the Mo species, both acidic and basic medium
synthesized NiMoP/MAS (10) catalysts exhibited absorption
bands at 600 and 750 nm, characteristic to the octahedral
species of Ni2+.
Moreover, it is an established fact that an increase in
Al content in the alumino-silicates increases the edge energy
(Eg) values in terms of the blue shift of molybdenum
absorption edge as reported in literature [33]. The edge energy
value for the bimetallic NiMoP/MAS (10)-A catalyst was
calculated to be 3.80 eV which was found to be higher than
the edge energy value of NiMoP/MAS (10)-B (3.73 eV) (Table
1); this clearly implies and once again emphasizes that a large
number of octahedral coordinated molybdenum phosphate
species was formed in case of NiMoP/MAS (10) A as compared
Figure 4 NH3-Temperature Programmed Desorption profiles of
catalysts
to NiMoP/MAS (10) B catalyst [39]
.
H2-TPR analysis
In order to ascertain the fact, that the conditions
employed in the method of preparation, the metal precursors
used and the type of support can alter the reduction
temperature of Ni and Mo active metal species [36], H2-TPR
study was carried out for all the synthesized catalysts. The H2-
TPR profiles of Ni2P/MAS (10)-A, MoP/MAS (10)-A, MoP/MAS
(10)-B, NiMoP/MAS (10)-A, and NiMoP/MAS (10)-B catalysts
are displayed in Figure 5. The as-synthesized catalysts were
subjected to TPR using H2 gas. The H2-TPR profiles were
recorded up to 650oC at the rate of 1oC min-1. In case of
Ni2P/MAS (10) catalyst, two types of reduction peaks were
displayed at 337 and 377oC corresponding to the reduction of
Ni2+ in the Ni2P phase. The preparation conditions employed in
the formation of Ni2P phase facilitates the reduction of Ni at
lower temperature (377oC) as compared to the previously
reported data wherein the reduction of supported Ni2+ species
has been recorded at higher temperature [37]. NiMoP/MAS (10)
A and B showed three types of reduction peaks at 540, 740
and 820oC corresponding to the reduction of different Mo
species. Reduction peak observed at 540oC is due to the
reduction of Mo6+ to Mo4+ in the form of octahedral
polymolybdates, whereas reduction peak observed at 740oC is
ascribed to Ni2+ to Ni0 in the bimetallic nature of active metals
[38, 52]. Further the reduction peak at 820oC is ascribed to the
Figure
3
DR
Uv-Vis
spectra
of
catalysts
NH3-TPD analysis
Temperature programmed desorption profiles of all
the catalysts are shown in Figure 4. The acid sites of the solid
samples were categorized as weak (100 - 250oC), medium (250
- 450oC) and strong acid sites (above 450oC) based on the
temperature programmed desorption of NH3 at various
temperatures [34]. Table 1 summarizes the total acidity of all
the catalysts prepared under both acidic and basic medium.
The NH3-TPD profiles of monometallic MoP/MAS (10) and
bimetallic NiMoP/MAS (10) catalysts were greatly influenced
by the medium of preparation (acidic or basic) and
correspondingly exhibited strong and weak acid sites. The total
acidity values of all the catalysts were measured to be in the
range of 0.43 to 1.13 mmol g-1. Among all the catalysts, the
reduction of tetrahedrally coordinated of Mo6+ to Mo0 [38, 52]
.
For the catalyst NiMoP/MAS (10)-A prepared under
acidic medium, a lower temperature shoulder peak at 449oC,
arising due to the reduction of the Ni2P phase was evident;
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however, such a low-temperature reduction peak was not
observed in the NiMoP/MAS (10)-B catalyst. This evidently determining the structural informationDoOfI:t1h0e.10m39a/Cte8NriJa0l.51T4h5Ce
proves that the enhanced reduction capacity of NiMoP/MAS Raman spectra for Ni2P/MAS (10)-A, MoP/MAS (10)-A,
(10)-A catalyst has increased the performance of hydrotreating MoP/MAS (10)-B, NiMoP/MAS (10)-A, and NiMoP/MAS (10)-B
reaction when compared to the NiMoP/MAS (10)-B catalyst.
The hydrogen consumptions values calculated by H2- two Raman bands at 855 and 416 cm-1 were observed, which
TPR profiles are tabulated in Table 1. The hydrogen reveals the formation of Ni2P phase over MAS (10) support [12]
consumption value of NiMoP/MAS (10)-A (180 μmol g-1) is In case of MoP/MAS (10)-A catalyst, six Raman bands were
greater than Ni2P/MAS (10) (70 μmol g-1) catalyst. The observed at 121, 285, 418, 666, 817, and 990 cm−1
activities of catalysts were direct by correlated with the corresponding to the crystalline MoP phase, among these the
ARTICLE
Raman spectroscopy is an importaVnietw Arttiocloe Ol nlinine
1
2
3
4
5
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8
are presented in Figure S3. In case of Ni2P/MAS (10) A catalyst,
.
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hydrogen consumption values.
bands observed at 285, 817 and 990 cm−1 can be attributed to
the polymeric form of molybdate [41]. The set of Raman bands
observed in case of the monometallic catalysts were sharp,
indicating crystalline nature; whereas the bands were broad
indicating amorphous nature in case of bimetallic catalysts.
For the bimetallic catalysts, there are six Raman bands
observed at 269, 427, 653, 821, 989, and 1087 cm-1 that can be
ascribed to the amorphous nature of NiMoP phase, among
these the peaks observed at 269, 821, and 989 cm-1 reveals the
formation of Ni-Mo-P-O bond. As compared to monometallic
MoP/MAS(10), peak at 1087 cm-1 due to νas (P-O) of PO4 group
[42]
was much more evident in the bimetallic catalyst
and the
broad peak at 989 cm−1 ascribed to νas (Mo = O) of octahedral
coordinated polymeric molybdate was observed both in case
of monometallic MoP/MAS (10) as well as bimetallic catalyst.
Other common bands observed in both monometallic
MoP/MAS (10) and the bimetallic catalysts included the strong
band at 817 cm−1 attributed to νas (Mo-O-Mo) of polymeric
molybdate and another weak band at 427 cm−1 ascribed to the
vibrational mode of the P-O-Mo bond [39]. In comparison to all
the catalysts, NiMoP catalyst synthesized under basic medium
possessed the most amorphous nature which can be inferred
from the broad Raman bands. Moreover, the band at 805 cm-1
observed in NiMoP/MAS (10)-B catalyst, corresponds to the Ni
(OH)2 phase. Both, its amorphous nature and the precipitation
of Ni2+ as Ni (OH)2 were also hinted from the XRD results of
NiMoP/MAS (10)B as discussed in XRD.
Figure 5 H2-Temperature Programmed Reduction Profiles of
catalysts
FT-IR Spectroscopy
The FT-IR spectra of all the synthesized catalysts are
displayed in Figure S2. The peaks pertaining to that of MAS
(10) support were seen in common in case of all the
synthesized catalysts. The peaks observed at 1058 and 802 cm-
1
can be unambiguously ascribed to the asymmetric and
symmetric stretching vibrations respectively of the Si-O-Al
bond in the support. The absorbance band observed at 3500
cm-1 corresponds to the -OH group of the MAS (10) support
[26]. The absorbance band observed at 1050 cm-1 is due to the
formation of P=O and P-O-Ni bond in all the prepared catalysts
which is in accordance with the previously reported literature
[39]. The stretching vibration mode of Ni-P bond appeared
around 900-1000 cm-1 and the bending mode vibration was
observed at 481 cm-1 in case of Ni2P/MAS (10) and
NiMoP/MAS(10) [28]. The broad band at 1050 cm-1 in Ni2P/MAS
(10)-A catalyst is ascribed to strong asymmetric vibration of Si-
O-P. The band observed at 953 cm-1 is a clear indication of P-Ni
bond in NiMoP/MAS (10)-A catalyst and the absorption band
at 965 cm−1 corresponds to Mo-O-P bond [37, 40]. The results
confirm that the bands characteristic to NiP and MoP were
evidently observed in the MAS (10) supported bimetallic
NiMoP catalysts prepared under acidic medium. However,
these characteristic bands were weak and less intense in
MoP/MAS (10)-B and NiMoP/MAS (10)-B catalysts.
SEM and HR-TEM studies
SEM images of NiMoP/MAS (10)-A catalyst is shown
in Figure 6 A-B. The images of the NiMoP/MAS (10)-A catalyst
seems like an encrusted surface with spherical morphology of
the support. Even after deposition of NiMoP species over the
MAS (10) support, the spherical morphology of the support
has been retained. HR-TEM images of NiMoP/MAS (10)-A
catalyst is shown in Figure 6 C-D. The catalyst showed ordered
mesoporous nature of the support and the observed dark
patches confirms the dispersion of NiMoP species over the
support.Both SEM and HR-TEM images reveal spherical
morphology, ordered porous nature and homogeneous
dispersion of NiMoP/MAS species over the MAS (10) support.
It must also be noted that the low pH conditions (acidic
medium) deployed in the synthesis of NiMoP/MAS (10)-A
catalyst, has not destructively affected the morphology of
either the support or the catalytic active species; however its
Raman analysis
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crowding of the reactant molecule on the catalyst surface,
constructive benefits are evidently reflected in its catalytic
activity as discussed further.
View Article Online
which results in decreased reactant contact time over the
DOI: 10.1039/C8NJ05145C
catalyst. Once again, NiMoP/MAS (10)-A catalyst prepared
under acidic medium exhibited maximum methyl heptanoate
conversion as compared to NiMoP/MAS (10)-B and all the
other synthesized catalysts. Based on this observation, 30 bar
H2 pressure was chosen as the optimum pressure for further
studies.
10
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Figure 6 SEM (A & B) and HR-TEM images (C & D) of
NiMoP/MAS (10)-A catalyst
Figure 7 (A) Effect of reaction temperature on HDO of Methyl
heptanoate at 30 bar H2 pressure, (B) Effect of reaction H2
pressure on HDO of Methyl heptanoate at 350oC. (Reaction
conditions; Methyl heptanoate 7 % in m-xylene, WHSV= 3.42h-
Catalytic activity
1
,
H2
flow
rate
=
1200
ml/h)
Effect of reaction temperature
The Hydrodeoxygenation activity of Ni2P/MAS (10)-A,
MoP/MAS (10)-A, MoP/MAS (9)-B, NiMoP/MAS (10)-A, and
NiMoP/MAS (10)-B catalysts were measured in the
temperatures ranging from 250 to 375oC under 30 bar H2
pressure and the results are shown in Figure 7 A. At 250 oC the
conversion of methyl heptanoate over the catalysts Ni2P/MAS
(10)-A, MoP/MAS (10)-A, MoP/MAS (10)-B, NiMoP/MAS (10)-
A, and NiMoP/MAS (10)-B was calculated to be 50, 60, 59, 72,
and 65 % respectively and further increase in the temperature
HDO activity of the catalysts
Catalyst A detailed comparison of the NiMoP/MAS (10)-A was
made at 275oC and 350oC under 30 bar H2 pressure with all the
other catalysts in order to understand the HDO efficiency;
these results are displayed in Figure S4 A & B; the SRR and TOF
values are tabulated in Table 2. The possible route for
upgradation of methyl heptanoate takes place either by
decarboxylation to C6 alkane or HDO to C7 alkane. At 275oC,
most of the reactant molecule prefers to undergo HDO to C7
hydrocarbon (50% selectivity) rather than decarboxylation to
C6 hydrocarbon (20% selectivity) on the Ni2P/MAS (10)
catalyst, since the Ni2+ sites are associated with superior
hydrogenation ability which has been evidently proved for
fatty acid methyl ester (FAME) conversion on Ni2P/SBA-15
catalyst [43]. At 275oC, similar HDO pathway was followed by
both MoP/MAS (10)-A & MoP/MAS (10)-B catalysts but with
higher conversion (62 & 58%) and selectivity towards C7
hydrocarbon (58 and 55%) compared to Ni2P/MAS (10)
catalysts. It was also observed that HDO to C7 hydrocarbon is
more favourable over MoP/MAS (10)-A than over MoP/MAS
(10)-B catalyst. The catalytic activity not only depends on the
textual properties of the catalysts but also on various
parameters involved such as hydrogen consumption (H2-TPR),
metal - support interaction (H2-TPR profiles), acidity (NH3-TPD),
surface coverage of Mo-Ni active metals (BET analysis) and
chemical environment of the catalysts (Raman and DRS-Uv-
Visible).
o
up to 350 C led to further increase in methyl heptanoate
o
conversion. At 350 C, the methyl heptanoate conversion over
the catalysts was evaluated to be 57, 77, 76, 99, and 80 %
o
respectively. Further increase in temperature beyond 350 C,
there was a slight decrease in the conversion which may be
probably due to coke formation on the surface of the catalysts
as shown in Figure 7 (A). NiMoP/MAS (10)-A catalyst prepared
under acidic medium showed maximum conversion towards
methyl heptanoate (>99%) as compared to NiMoP/MAS (10)-B
o
and all the other synthesized catalysts. Hence, 350 C was
chosen as the optimum temperature for further studies.
Effect of reaction H2 pressure
Effect The effect of pressure on hydrodeoxygenation
of methyl heptanoate was studied on all the catalysts and the
results are shown in Figure 7 B. Initially, the reaction was
carried out at 350 oC under 10 bar H2 pressure. The conversion
of methyl heptanoate on Ni2P/MAS (10)-A, MoP/MAS (10)-A,
MoP/MAS (10)-B, NiMoP/MAS (10)-A, and NiMoP/MAS (10)-B
catalysts were calculated to be 45, 71, 60, 83 and 66.8 %
respectively. Upon increasing the pressure up to 30 bar,
maximum conversion was recorded for all the catalysts. On
further increasing the H2 pressure (40 bar), the conversion
slightly decreased for all the catalysts, probably due to
Table 2 Comparative account of catalytic activity, Specific
Reaction Rate, TOF, Metal Dispersion, No of H2 active sites for
HDO of methyl heptanoate on Ni2P/MAS (10)-A, MoP/MAS
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(10)-A, MoP/MAS (10)-B, NiMoP/MAS (10)-A, and NiMoP/MAS [43]. Then hydrogenation of 1-heptene leads to n-heptane (C7
View Article Online
(10)-B catalysts
DOI: 10.1039/C8NJ05145C
hydrocarbon) (Figure S5). In the second route, hydrolysis of
methyl heptanoate takes place to form heptanoic acid and
aH2
SRR x 10-4
TOF s-1
methanol followed by decarboxylation to produce C6
hydrocarbon. Among the routes, the first route was favoured
at 275oC with less amount of formation of CO2 leading to the
formation of C7 hydrocarbon predominantly on all the
catalysts. The second route occurred at 350oC by predominant
decarboxylation of methyl heptanoate to form C6
hydrocarbon. This has been reported to be the preferred
reaction at high temperature [44]. The temperature of the
reaction seemingly decides the pathway of the reaction under
the assayed reaction conditions.
active
site ×
mol g−1 s−1
Catalysts
SF
275
350
275
350
10-6 g-1)
(oC)
(oC)
(oC)
(oC)
Ni2P/MAS(10)A
20.8
3.8
4.5
1.8
2.1
-
10
11
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44
45
46
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50
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52
53
54
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57
58
59
60
MoP/MAS (10)-A
MoP/MAS (10)-B
12.7
11.2
4.3
4.0
5.5
5.2
3.3
3.1
4.3
4.0
-
-
NiMoP/MAS (10) A
NiMoP/MAS (10)-B
8.50
9.00
5.2
4.4
6.9
6.1
6.1
4.9
8.1
6.7
3.8
3.2
aH2 chemisorption Studies SF = synergetic factor, and Reaction
conditions: 7% Methyl heptanoate, 30 bar H2 pressure, reaction time: 5
h; catalyst pellet size 0.5-1 mm, catalysts weight 2 g, WHSV 3.4 h-1and
Catalytic activity
H2
flow
rate
=
1200
ml/h.
At 275oC temperature, both bimetallic catalysts
The catalytic activity of Ni2P/MAS (10)-A, MoP/MAS
(10)-A, MoP/MAS (10)-B, NiMoP/MAS (10)-A, and NiMoP/MAS
(10)-B catalysts were calculated at 350oC temperature under
30 bar H2 pressure and the results are shown in Figure S6;
specific reaction rate (SRR), TOF values, conversion and
selectivity of HDO reaction towards methyl heptanoate are
included in Table 2. The monometallic Ni2P/MAS (10) catalyst
exhibited least conversion of about 65% and SRR and TOF
values of 4.5 × 10−4 mol g−1 s−1 and 2.1 s-1 respectively, which
may be due to the undesirable hydrogenation ability of the
monometallic NiP phase on the support. It is clear that the
reactant molecule on NiP phase prefers hydrogenation instead
of C-O cleavage.
NiMoP/MAS (10)-A and NiMoP/MAS (10)-B displayed
significant improvement in conversion (75 and 64%) and
selectivity to C7 hydrocarbon (70 and 60%) due to the
presence of promoter Ni2P phase. In order to understand the
Ni2P promotion, the synergetic factor for the bimetallic
catalysts was calculated and is presented in Table (2). Large
value of acidity and hydrogen spill over are responsible for the
high synergetic factor of the NiMoP/MAS (10)-A as evidenced
from the TPD/TPR results. The high hydrogen consumption
values of pre-reduced Mo species on NiMoP/MAS (10)-A
catalyst supports this positive synergism (Table 2). The H2
consumption values for various catalysts are shown in Table 1.
This may be attributed due to the activation and stabilization
of MoP phase by the promoter phase Ni2P.
The monometallic MoP/MAS (10)-A catalyst exhibited
moderate conversion (80%), SRR (5.5 × 10−4 mol g−1 s−1) and
TOF (4.3 s-1) values. Among all the catalysts, NiMoP/MAS (10)-
A catalyst showed maximum conversion (100 %) with C6-C7
hydrocarbon selectivity. SRR and TOF values for the
NiMoP/MAS (10)-A catalyst was calculated to be 6.9 × 10−4 mol
g−1 s−1 and 8.1 s-1. The maximum HDO rate of the catalyst can
be correlated with high hydrogen uptake value (180 mmol g-1),
increased total acidity (1.13 mmol g-1) and fine dispersion of
the active metal compounds over the support as evidenced
from the NH3-TPD/H2-TPR and XRD results. Moreover, NiP
phase present in the bimetallic NiMoP/MAS (10)-A catalyst
It seems to be clear that high temperature (at 350oC)
favours C6 hydrocarbon formation which inclines to follow the
decarboxylation process rather than the HDO pathway on all
the catalysts. Compared to the monometallic catalysts,
extensive C6 hydrocarbon formation is observed on bimetallic
catalysts as shown in Figure S4 B and Table 2. Hence it is clear
that, decarboxylation process becomes more favourable when
both Ni2P and MoP phases coexists rather than over
monometallic phases, Ni2P or MoP. Further it can be
understood that HDO and decarboxylation of methyl
heptanoate are dependent on both temperature and type of
metallic phase. Decarboxylation and hydrodeoxygenation
pathways are simultaneously taking place on all the catalysts
at all the temperatures. Hence, by adjusting the reaction
temperature and metallic phase, hydrodeoxygenation versus
decarboxylation of methyl heptanoate can be controlled.
potentially enhances the hydrogen spill over on the surface,
[23]
which minimizes coke formation on the catalyst surface
.
The activity of the catalysts follows the trend Ni-MoP/MAS
(10)-A>Ni-MoP/MAS (10)-B>MoP/MAS (10)-A>MoP/MAS (10)-
B>Ni2P/MAS (10)-A. The superior HDO activity of NiMoP/MAS
(10)-A catalyst is strongly related to the acidic method of
preparation and promotional sites offered by the Ni2P phase.
Moreover, it is evident from Table 2 that the value of
synergetic effect of the NiMoP/MAS (10)-A catalyst is more
positive than NiMoP/MAS (10)-B. This directly explains why
the production of spillover hydrogen is higher in the former
catalyst than the latter at low temperature. Hence, the formed
Ni2P phase can split the hydrogen which can be adsorbed over
Plausible reaction path ways
The reaction pathways conceivably followed by all the
catalysts are shown in Figure S5. HDO of methyl heptanoate
takes place via two major routes namely hydrodeoxygenation
and decarboxylation. In the first route, HDO of methyl
heptanoate occurs by the formation of 1-heptanol and
methanol, followed by E2 elimination to produce 1-heptene
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View Article Online
DOI: 10.1039/C8NJ05145C
Table 3 Critical evaluation of the catalytic potency of various catalysts for vapour phase HDO reaction
S.
N
1
Catalysts
Reactant
Conversion (%)
Reaction
condition
TOF Paraffin’ Ref
s-1
s
NiMoP/MAS
(10)-A
Methyl Heptanoate
100% Yield 92 wt% 3MPa,WHSV=3.4h-1, 8.1
350 oC
C6-C7
This
work
[47]
[47]
[48]
2
3
4
Ni2P/SAPO-11
Ni3P-i12P5/Al2O3 Methyl Laurate
NiMo/Al2O3 Sun flower
Methyl Laurate
99 wt%
99 wt%
82-90 wt%
3MPa, 300-340oC
3.0MPa, 300-340oC
8MPa,LHSV=1h-1
360-380°C
5MPa,340-
380°C,LHSV=2.4 h-1
0.68 C11-C12
0.28 C11-C12
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
-
-
-
-
C15-C20
C15-C20
C8-C29
5
6
7
NiW/Al2O3-SiO2 Soybean
NiMo/Al2O3
Hydrotreating
catalyst
85-95 wt%
[49]
[50]
[51]
Waste
cooking oil
Palm
72.62%,
71wt%,
Yield:11.93wt%
Yield: 330-398°C,
8.3MPa,LHSV=1 h-1
300-320 ºC,1 atm
NiMo/zeolite
C15-C20
the MoP to activate the catalytic centres which are responsible surface of the catalyst which results in the reactant molecules
for enhanced hydrogenation and effective HDO ability of being detached from the catalysts bed as unreacted
NiMoP/MAS (10)-A catalyst.
molecules. Hence, this study confirms the optimum WHSV to
Based on the available literature, a comparative study be 3.4 h-1 for maximum conversion and selectivity under
was done, in terms of the catalyst and the model compound optimized reaction conditions.
used, reaction conditions employed, the hydrocarbon formed,
the % conversion, yield and TOF for the reaction, in order to
ascertain the competency of the catalyst synthesized in this
work with respect to the already published work for vapour
phase HDO reactions. The available data are tabulated and
presented in Table 3. Fascinatingly, it was discerned that the
catalyst identified in the present investigation was not only
competent but also excelled the other catalysts in almost all
parameters. Another striking characteristic that was brought
Figure 8 (A) Effect of WHSV on HDO of methyl heptanoate and
(B) Time on stream for HDO of methyl heptanoate at 350oC
to light is that the catalyst presented in this investigation
under
30
bar
H2
pressure.
generated the maximum yield of hydrocarbon at lower
pressure as compared to the rest, which means it is a low
energy consumption process. The superior activity of the
synthesized catalyst originate from the combinatorial factors
viz., ordered mesoporous support with high specific surface
area and optimum acidity originated from the alumino silicate
framework; the promotional effect of Ni2P and combination
with MoP phase recorded the synergistic benefit. The
optimum pH is to be maintained during the catalyst synthesis
is a key parameter to tune the physicochemical characteristics
of the catalyst and achieve high activity.
Effect of time-on-stream was evaluated up to 15 h on
highly active NiMoP/MAS (10)-A catalyst for conversion and
C6-C7 hydrocarbon selectivity of methyl heptanoate at
optimized temperature (350oC) and pressure (30 bar) and the
results are displayed in Figure 8 B. After 1 h, it was observed
that 60% of methyl heptanoate was converted with 78%
selectivity of C6-C7 hydrocarbon. The conversion and HDO
activity of the catalyst increased with increase in time and
reached maximum activity at 5h, beyond which it remained
constant. The C6-C7 hydrocarbon selectivity of the catalyst was
calculated to be 99%. It is obvious that the steady state
condition has been maintained after 5h in the 15h experiment.
The experiment could not be carried out beyond 15h due to
the limitation of the reactor capacity volume.
Effect of WHSV and Time on Stream
Effect of WHSV was studied using the best
NiMoP/MAS (10)-A catalyst for conversion and hydrocarbon
selectivity of methyl heptanoate at optimized temperature
(350oC) and pressure (30 bar H2 pressure). The change in
conversion and selectivity was investigated in the range of 1.7-
8.5 h-1 and results are presented in Figure 8 A. Maximum
conversion and hydrocarbon selectivity (C6-C7) were measured
to be 100% and 98% respectively at 3.4 h-1. Beyond the space
velocity a 3.4 h-1, the conversion and selectivity decreased due
to less residence time of the reactant molecules over the
It is reusable. The used catalyst can be regenerated
by heating at 400oC for 6h under inert atmosphere to remove
the surface adsorbed carbon residue and followed by
reduction with high pure hydrogen to from the active metal
species for the HDO reaction.
pH role in catalysts
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In this work, pH plays a vital role in which it can alter which once again showed no change in the TOF values, despite
View Article Online
DOI: 10.1039/C8NJ05145C
the properties of the catalysts. Catalysts are prepared in both the changes made in the size of the catalyst bed. This is
acidic and basic medium by altering the pH. By altering pH, apparently due to the absence of mass transfer resistance.
intrinsic and extrinsic properties of catalyst such as surface Thus since the kinetics of the reaction is not altered by the
composition, surface acidity, reducibility, chemisorption, variables, it can be unequivocally confirmed that the catalytic
surface coordination and surface morphology are significantly process carried out, does not suffer from any mass transfer
modified. In this investigation, the catalyst were prepared limitations.
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using active metals like nickel, molybdenum and phosphate
with the ratios of Ni/P = 0.3, Mo/P = 0.06 and Ni/Mo = 0.2 by
impregnation method. These catalysts were examined by
different physico-chemical characterization techniques and the
results revealed that at low pH (acidic medium), nickel species
are highly dispersed over support and form Ni2P species in Ni-
Conclusion
The significant features of the report include; one pot
room temperature synthesis of thermally stable, high surface
area mesoporous Alumino-Silicate (Si/Al=10) which served as
MoP/MAS (10) catalysts. (H2-TPR profile (Table 1)). Nickel act an excellent support facilitating homogeneous dispersion of
the active metal species (Ni2P, MoP and NiMoP) and retaining
its superior pore characteristic properties even after metal
dispersion both on the external surface and inside of the
mesoporous framework. Comprehensive evaluation of the
physicochemical characterization results of all the synthesized
catalysts showcase superior properties for NiMoP/MAS (10)-A
such as homogeneous dispersion (TEM), pre-reducibility (H2-
TPR), high hydrogen consumption (H2-TPR, 180 mmol/g) and
enhanced surface acidity (NH3-TPD, 1.13 mmol/g), which
evidently justifies the intrinsic role of pH in altering the
physicochemical properties of the synthesized materials.
Among all the catalysts tested for vapour phase hydrotreating
as a promoter by hydrogen spillover effect, which play an
important role in HDO activity. In the catalysts synthesized
under acidic medium, the H2-TPR and DR UV-Vis spectra
indicated there is a strong metal support interaction (Figure 3
& 5 and Table 1). However, at high pH (basic medium), nickel
species tend to precipitate as nickel hydroxide Ni (OH)2,
thereby showing low the HDO activity.
Mass Transfer Considerations
Major steps involved in the catalytic reaction system
are (a) diffusion of hydrogen gas feed into the liquid feed (b)
adsorption of reactant from the bulk feed onto the catalyst of methyl heptanoate, NiMoP/MAS (10)-A has been the most
active catalyst yielding the C7 hydrocarbon predominantly
through the HDO pathway, with a selectivity and conversion of
66 and 62% respectively at a low temperature of 275oC which
demonstrates the high oxygen removal efficiency of the
NiMoP/MAS (10)-A catalyst at 275oC with high synergism. Both
internal as well as external mass transfer resistance was
unequivocally ascertained. Thus NiMoP/MAS (10)-A has been
suggested as a commendable catalyst for green fuel
production from non-edible oils through hydrodeoxygenation
under the experimental conditions.
surface (c) chemical reaction at the active sites of the catalyst
(d) product desorption and diffusion from the catalyst surface.
The slowest step determines the overall rate of the reaction. In
order to rule out mass transfer limitations in a catalytic
process, a deeper investigation has to be done to prove that
the diffusion controlled steps do not alter the kinetics of the
reaction. The rate of a reaction is projected from the TOF
values. Thus in order to prove that both internal as well as
external resistance to mass transfer are absent, two sets of
experiments were carried out, based on the reports published
by Ancheyta et al. [45]
.
Acknowledgments
The authors are grateful for the financial support
provided by Defence Research and Development Organisation
In the first set of experiments the temperature
(350oC), pressure (30 bar) and WHSV (3.4 h-1) parameters were
kept constant. However, the feed flow rate and the amount of
catalyst were varied from 0.113 to 0.227 ml min-1 and 2 to 4 g,
respectively. The corresponding results have been tabulated in
Table S1. Comprehension of these results show that, despite
changes in the flow rate of the feed no significant change was
observed in the TOF values, which proves that external mass
(DRDO),
New
Delhi,
the
project
sanction
no.
EPIR/ER/1203047/M/01/1535, dt. 16. 05. 2014. We are
thankful to DRDO, University Grants Commission (UGC), and
Department of Science and Technology for providing adequate
instrumentation facility in the Department of Chemistry, Anna
University, Chennai, Tamil Nadu, India.
transfer resistance was absent [46]
.
In another set of experiments, to show that the
kinetics of the reaction is not affected by the internal mass
transfer resistance, temperature (350oC), pressure (30 bar),
feed flow rate, catalyst amount (2g) and WHSV (3.4 h-1) were
kept constants. The catalysts were finely powdered and
packed into two different pellet sizes (0.5-1 mm and 3-4 mm).
The corresponding results have been included in Table S1,
Conflict of Interests
The authors declare that there is no conflict of
interests regarding the publication of this paper.
Notes and References
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