Effect of precursor on the catalytic properties of Ni2P/SiO2 in methyl palmitate hydrodeoxygenation

Article abstract of DOI:10.1039/c6ra01171c

The effect of phosphorus precursor on the physicochemical and catalytic properties of silica-supported nickel phosphide catalysts in the hydrodeoxygenation (HDO) of aliphatic model compound methyl palmitate (C15H31COOCH3) has been considered. Nickel aceta

Full text of DOI:10.1039/c6ra01171c

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V. Deliy, P. V. Aleksandrov, E. Yu. Gerasimov, V. Pakharukova, E. G. Kodenev, A. B. Ayupov, A. S. Andreev,
O. B. Lapina and G. A. Bukhtiyarova, RSC Adv., 2016, DOI: 10.1039/C6RA01171C.
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Effect of precursor on the catalytic properties of Ni2P/SiO2 in
methyl palmitate hydrodeoxygenation†
Ivan V. Shamanaev,abc Irina V. Deliy,abc* Pavel V. Aleksandrov,ac Evgeny Yu. Gerasimov,abc
Vera P. Pakharukovaabc, Evgeny G. Kodenev,a Artem B. Ayupov,ab Andrey S. Andreev,a
Olga B. Lapina,a and Galina A. Bukhtiyarovaa
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The effect of phosphorus precursor on physicochemical and catalytic properties of silica-supported nickel phosphide
catalysts in hydrodeoxygenation (HDO) of aliphatic model compound – methyl palmitate (C15H31COOCH3) has been
considered. Nickel acetate (Ni(OAc)2) and diammonium hydrogen phosphate ((NH4)2HPO4) (phosphate precursor) or nickel
hydroxide (Ni(OH)2) and phosphorous acid (H3PO3) (phosphite precursor) have been used for the catalyst preparation by
incipient wetness impregnation of SiO2 with aqueous solution of the precursors followed by temperature-programmed
reduction in hydrogen flow. Chemical analysis (ICP-AES), H2-TPR, NH3-TPD, NMR, XRD, and TEM have been employed for
the characterization of the catalysts. The optimal reduction parameters of the silica-supported nickel phoshide catalysts
have been found for the phosphate and phosphite precursors in terms of their activity in methyl palmitate HDO. The
Ni2P/SiO2 catalysts prepared by phosphite method have shown the higher catalytic activity in comparison with the
Ni2P/SiO2 catalysts prepared by phosphate method. The activity has been shown to depend on the catalyst handling after
reduction: in situ reduced catalysts demonstrate the higher conversion of methyl palmitate that the samples exposed to
the reduction, passivation and re-reduction steps.
hydrodeoxygenation (HDO) – catalytic upgrading by H2 (at high
temperature 250-350 °С and high pressure 3.0-5.0 MPa).6–8
HDO of triglycerides or fatty acids results in high quality,
1. Introduction
The motives for the development and usage of alternative fuel
sources are growing due to limited petroleum resources,
unstable economic and political situations, unreliable supply of
energy, global warming, green-house gas emissions, and
environmental health hazards associated with the emissions
from fossil fuels.1–3
oxygen-free fuel compatible with conventional petrodiesel.9
The latest studies have shown that HDO of Cn fatty acid
based compounds (i.e. containing fatty acid residues with n
carbon atoms) involves different reaction routes,
decarbonylation/decarboxylation leading to Cn-1 hydrocarbons
and direct hydrodeoxygenation leading to Cn hydrocarbons. In
case of decarbonylation/decarboxylation reactions oxygen is
removed as CO or CO2, and H2O forms in case of direct
Biomass is a promising type of renewable sources to
replace fossil fuels in future. However, the majority of the bio-
feedstocks are edible, and this can lead to the food vs fuel
problem.4,5 Nevertheless, this problem can be overcome by
hydrodeoxygenation.
The
selectivity
towards
decarbonylation/decarboxylation and hydrodeoxygenation
depends on the reaction conditions and type of the catalyst
employed.10–13
using
second-generation
biofuels
produced
from
lignocellulosic biomass, waste fat from the food industry, non-
edible vegetable oils, tall oils, and animal fats.6,7
Sulfided NiMo/Al2O3 and CoMo/Al2O3 catalysts are widely
investigated14–17 and already used in commercial HDO
processes: NExBTL® Process (Neste Oil), EcofiningTM Process
for Green Diesel Production (UOP/Eni).3 However, these
systems display low stability in HDO reactions without addition
of sulfidation agent to the feed.18 Therefore, alternative
catalysts for HDO of fatty acid based feedstocks must be
developed.
Today the most feasible and versatile bio-feedstocks
include triglyceride-containing feedstocks such as palm oil,
rapeseed oil, and waste fats from the food industry.8 These
raw materials can be converted into hydrocarbons by
a.Boreskov Institute of Catalysis, pr. Lavrentieva 5, Novosibirsk, 630090, Russia.
b.Novosibirsk State University, Pirogova st. 2, Novosibirsk, 630090, Russia.
c. Research and Educational Center for Energy Efficient Catalysis, Novosibirsk
National Research University, Pirogova st. 2, Novosibirsk, 630090, Russia.
* E-mail: delij@catalysis.ru (I.V. Deliy); Fax: +7 3833308056; Tel: +7 3833269410
†Electronic Supplementary Information (ESI) available: FID relative sensitivities,
Weisz-Prater calculations, H2-TPR results, C16/C15 hydrocarbons molar ratio,
Conversions of methyl palmitate and oxygen-containing compounds. See
DOI: 10.1039/x0xx00000x
New non-sulfided HDO catalysts have received much
attention in the past decade. Active components of such
catalysts include transition metals,19–21 noble metals,22–24
transition metal carbides,25–27 nitrides,28 and phosphides.29–32
In recent years there has been growing interest in transition
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metal phosphide catalysts for HDO reactions,33–35 and Ni2P has for the catalyst preparation. Methyl palmitate (C15H31COOCH3,
been found to be one of the most active phases in HDO of «Sigma-Aldrich», ≥ 97%) was used as the reactant and as the
oxygen-containing
compounds
such
as
2- standard to calibrate the gas chromatograph. Palmitic acid
(C15H31COOH, «Acros Organics», ≥ 98%) and ASTM® D5307
methyltetrahydrofuran, methyl laurate and guaiacol.33,36,37
A widely used technique to prepare Ni2P-based catalysts is Crude Oil Internal Standard (n-tetradecane, n-pentadecane, n-
the high temperature reduction in hydrogen flow in the hexadecane, n-heptadecane) were used as the standards to
separate reactor (ex situ reduction) with subsequent calibrate the gas chromatograph. n-Dodecane (C12H26, «Acros
passivation to prevent oxidation of Ni2P nanoparticles in air Organics», ≥ 99%) was used as a solvent.
during the transfer to a catalytic reactor.33–35,38 Passivated
2.2. Catalyst synthesis
samples are usually re-reduced in catalytic reactor before
experiments. There is a number of works where the reduction The catalysts were prepared by the incipient wetness
has been performed directly in catalytic reactor (in situ impregnation of SiO2 with aqueous solutions of Ni and P
reduction).30,39 Therefore, it is essential to compare these precursors. The initial Ni/P molar ratio was 1/2 for all catalysts.
techniques to elucidate the effect of passivation and re- The catalysts were labelled as NiP_A and NiP_I with respect to
reduction steps on the catalytic properties of Ni2P/SiO2 the phosphorus-containing precursor used: phosphate (NiP_A)
catalysts.
and phosphite (NiP_I) (Ni content ~2.5 wt%). The catalysts
Water solutions of Ni2+ salts (nitrate, acetate) and with 5 wt% of Ni loading were labelled as NiP_Ah and NiP_Ih (h
phosphates (NH4H2PO4 and (NH4)2HPO4) are widely used as Ni meaning the high amount of Ni loading).
and P precursors.29–31,33–36,39,40 Using phosphorus precursors
Phosphate method (NiP_A and NiP_Ah). To prepare
with lower oxidation state of P (H3PO3, NaH2PO2) allows one to precursors according to this technique 0.693±0.001 g (or
reduce temperatures of formation and particle sizes of 2.772±0.001 g) of (NH4)2HPO4 was diluted in distilled water.
Ni2P.41,42 Bui et al. investigated the effect of precursor Then 0.635±0.001 g (or 2.540±0.001 g) of Ni(OAc)2·4H2O was
(phosphate or phosphite) on catalytic properties of Ni2P/SiO2, added at stirring. A yellow-green precipitate was formed.
CoP/SiO2, FeP/SiO2, MoP/SiO2, WP/SiO2 in HDO of 2- Subsequently 1.5 mL (or 3 mL) of concentrated HNO3 was
methyltetrahydrofuran.33 The comparison within each pair of added to dissolve the precipitate. 5 g of SiO2 support was
catalysts shows that the TOF values of CoP and MoP prepared impregnated by the obtained green solution. The precursors
by the phosphite method are higher than of those prepared by were air-dried at room temperature overnight and then dried
the phosphate method. Ni2P, WP, and FeP catalysts prepared at 110 °C in air for 3 h. Dried oxide precursors were calcined at
by the phosphate method demonstrate higher activity than 500 °C in air for 3 h and subsequently converted into desired
phosphite analogues. Up to now the catalysts prepared by phosphide by means of temperature-programmed reduction
phosphate and phosphite methods have not been directly (TPR) in H2 flow (100 mL/(min·g)) using the following
compared in HDO of vegetable oil or model compounds (fatty temperature program: heating to 370 °C at a heating rate of
acids, fatty acid esters).
3 °C/min and then to 550, 600 or 650 °C at a heating rate of
The aim of the present work is to investigate the effect of 1 °C/min and keeping at the reduction temperature for 1 h.
phosphorus precursor on physicochemical and catalytic After reduction the samples were passivated at room
properties of silica-supported nickel phosphide catalysts in temperature for 2 h in 1 vol% O2/He flow (40 mL/(min·g))
HDO of a vegetable oil model compound – methyl palmitate. before taking out into air.
The optimal reduction conditions have been determined for
Phosphite method (NiP_I and NiP_Ih). To prepare
the phosphate and phosphite precursor in terms of the activity precursors according to this technique 0.243±0.001 g (or
of Ni2P/SiO2 catalysts in methyl palmitate HDO. The effect of 0.972±0.001 g) of Ni(OH)2 and 0.430±0.001 g (or
passivation and re-reduction steps on catalytic properties of 1.720±0.001 g) of H3PO3 were diluted in distilled water at
Ni2P/SiO2 catalysts has been studied.
stirring. 5 g of SiO2 support was impregnated by the obtained
green solution. The precursors were dried at room
temperature in air overnight and then dried at 80 °C in air for
24 h. Dried precursors were reduced directly without
calcination by means of temperature-programmed reduction
(TPR) in H2 flow (100 mL/(min·g)) using the following
temperature program: heating to 500, 550 or 600 °C at a
heating rate of 1 °C/min and keeping at the reduction
temperature for 1 h. The passivation was carried out after
reduction as in case of NiP_A and NiP_Ah samples.
2. Experimental
2.1. Materials
Commercial spherical silica «KSKG» (BET surface area –
300 m2/g, pore volume – 0.80 cm3/g, average pore diameter –
11 nm) was purchased from «KhromAnalit» Ltd. (Russia) and
has been used as a support. Prior to use, the silica was dried at
110 °С for 7 h and calcined at 500 °C for 4 h, after that it was
crushed and sieved to 0.25-0.50 mm.
The NiP_Ah and NiP_Ih samples with higher concentration
were prepared to verify the features of precursor influence on
formation mechanism and physico-chemical properties of the
low-concentrated catalysts.
Ni(OAc)2·4H2O («Reachim», ≥98%), Ni(OH)2 («Acros
Organics», ≥ 98%), (NH4)2HPO4 («Alfa Aesar», Technical Grade),
HNO3 («Reachim», ~ 70%) and H3PO3 («Sigma-Aldrich», ≥ 97%)
were obtained from the commercial suppliers and were used
POx/SiO2 reference sample. 5 g of SiO2 support was
impregnated by aqueous solution of (NH4)2HPO4 followed by
2 | J. Name., 2012, 00, 1-3
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the subsequent drying in air at room temperature overnight, without oxygen access. Prior to NMR experiments, the
at 110 °C for 3 h and calcination at 500 °C for 3 h to obtain catalysts were transferred from ampoule to NMR rotor in a
oxide precursor. The POx/SiO2 (6 wt% of P) sample was used in glovebox under argon atmosphere. This technique helps to
H2-TPR experiments to determine the temperature range of minimize possible oxidation during NMR spectra acquisition.
phosphate group reduction.
Transmission electron microscopy (TEM). The samples
NiO/SiO2 reference sample. 5 g of SiO2 support was were studied using transmission electron microscopy (TEM)
impregnated by aqueous solution of Ni(OAc)2·4H2O followed with a JEM-2010 transmission electron microscope («JEOL»,
by the subsequent drying in air at room temperature Japan) with accelerating voltage of 200 kV and resolution of
overnight, at 110 °C for 3 h and calcination at 500 °C for 3 h to 0.14 nm. The catalyst samples were applied to copper gauze
obtain oxide precursor. The NiO/SiO2 (5 wt% of Ni) sample was by dispersing an ethanol suspension of the sample with an
used in H2-TPR experiments to determine the reduction ultrasonic disperser. To obtain statistical information, the
temperature of NiO.
structural parameters of ca. 250 particles were measured.
Analysis of the local elemental composition was carried out
with an energy-dispersive EDX spectrometer equipped with a
2.3. Catalyst characterization
Elemental analysis of the catalysts was performed using Si(Li) detector (energy resolution – 130 eV).
inductively coupled plasma atomic emission spectroscopy (ICP-
AES) on Optima 4300 DV («Perkin Elmer», France).
Powder X-ray diffraction analysis (XRD). The crystalline
structure of the catalysts was analysed by powder X-ray
N2 physisorption. The textural properties of the support diffraction. XRD patterns were obtained with an XTRA
were determined using nitrogen physisorption with an ASAP diffractometer (Switzerland) using CuKα radiation (wavelength
2400 instrument («Micromeritics», USA).
λ = 1.5418 Å). The measurements were carried out in a range
H2 temperature-programmed reduction (H2-TPR) of the of 2θ from 5 to 70° with scanning step of 0.05° and an
catalyst precursors were carried out in U-shape quartz reactor accumulation time of 3 s per point.
in 10 vol% H2/Ar flow (60 mL/(min·g)) at a heating rate of
2.4. Catalytic experiment
10 °C/min. The H2 consumption was determined by a thermo-
conductivity detector.
HDO tests of methyl palmitate were carried out in a trickle-bed
The acidic strength of the samples was analyzed by reactor. 0.5-2.1 mL of the catalyst precursor was diluted with
temperature-programmed desorption of ammonia (NH3-TPD) SiC (0.1-0.2 mm) to a total volume of 4.6 mL and loaded into
using an Autosorb-1 apparatus (Quantachrome Instruments) in the reactor. Before the experiments, the catalyst was reduced
the temperature range from 100 to 500 oC. Prior to adsorption in situ in H2 flow of 100 mL/(min·g) at the pressure of 0.1 MPa.
of ammonia 0.25 g of each samples was reduced at 600 oC for The NiP_A samples were heated to 370 °C at a heating rate of
1 h in a H2 flow (25 ml/min) and then cooled to 120 oC. 3 °C/min and then to 550, 600 or 650 °C at a heating rate of
Subsequently the sample was saturated with NH3 for 30 min. 1 °C/min and kept at the reduction temperature for 1 h. The
The physically adsorbed ammonia was desorbed from the NiP_I samples were heated to 500, 550 or 600 °C at a heating
sample with He flow (25 ml/min) at 120 °C for 30 min. rate of 1 °C/min and kept at the reduction temperature for 1 h.
Desorption of the chemically adsorbed part of ammonia was The samples after ex situ reduction and subsequent
started by increasing the temperature from 120 °C up to passivation were re-reduced in catalytic reactor immediately
500 °C at a heating rate of 10 °C/min. The desorbed NH3 was before the experiment in accordance with the common
detected by a TCD.
procedure: at 450 °C and 0.1 MPa for 2 h.33–36,38
NMR experiments were carried out using a Bruker Avance-
After reduction (or re-reduction) of the catalyst the reactor
400 spectrometer at the resonance frequency of 161.923 MHz was cooled to the reaction temperature and the liquid feed
for 31P. Magic Angle Spinning (MAS) spectra were measured was supplied into the reactor by means of a high-pressure
using a Bruker MAS NMR probe with 4-mm (outer diameter) pump («Gilson 305», France). 10 wt% of methyl palmitate in n-
ZrO2 rotors at spinning frequency of 15 kHz. The 31P spectra dodecane was employed as a reaction mixture, H2/methyl
were obtained using Hahn-echo sequence using a train of π/2- palmitate molar ratio was 95±2 in all experiments. Contact
τ-π pulses, the duration of π/2 was 2.4 μs. The interpulse delay time was varied by changing the flow rate and the catalyst
τ was chosen with respect of rotation frequency. The sequence volume in the range of WHSV = 10-100 h−1. The experiments
repetition delay was 100 ms due to very short 31P relaxation were performed at the temperatures of 290 °C and 270 °C and
time in Ni-P intermetallic compounds.43 Due to very large total pressure of 3.0 MPa. The reactor was stabilized for 2 h
spectra width all spectra were collected by frequency stepped and then the liquid effluent was collected every hour.
technique with a step of 600 ppm (with respect to 31P NMR
frequency, ~ 100 kHz) covering ~5000-6000 ppm spectral chromatograph («Agilent» N6890, USA) equipped with a
width, the details can be found elsewhere.44 The spectra were capillary column (HP-1MS, 30 m × 0.32 mm × 1
m) and a FID
The liquid reaction products were analysed by gas
ꢀ
collected at room temperature and referenced to 85% water detector. Relative sensitivities were used to calculate the
solution of orthophosphoric acid (for 31P experiments). The concentrations
accuracy of the chemical shift was ±0.5 ppm. The number of hexadecane-1-ol,
of palmityl palmitate,
hexadecane-2-ol,
hexadecanal,
hexadecenes,
transients was 16k in each frequency point. To avoid catalyst pentadecenes, methane and methanol (Table S1 of the
oxidation by air, all samples were sealed in glass ampoules Supporting Information).45 The reaction products were
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identified using a GC/MS technique (Agilent Technologies 7000 The chemical compositions of the catalysts after ex situ
GC/MS Triple QQQ GC System 7890A, USA) with a quartz reduction at 600 °C for 1 h are listed in Table 1. In order to
capillary column (HP-5MS, 30 m × 0.32 mm × 0.25 ꢀm). The prepare the Ni2P phase the impregnating solutions with the
total oxygen content in the reaction mixture was determined Ni/P molar ratio equal to 1/2 was used in accordance with the
with a CHNSO elemental analyser Vario EL Cube («Elementar literature data.36 An increase of the Ni/P ratio in the samples
Analysensysteme» GmbH, Germany).
after reduction at 600 °C is likely to be due to the formation of
Gaseous products were analysed online with a gas volatile phosphorus compounds during reduction (e.g. P4,
chromatograph («Chromos 1000», Russia) equipped with a PH3).40 NiP_I samples contain higher amount of P than NiP_A
column packed with 80/100 mesh HayeSep® («Sigma-Aldrich») samples (Table 1).
and a FID detector. The concentrations of CO and CO2 were
The textural parameters of the reduced catalysts were
analysed in the form of methane after methanation over obtained from the N2 adsorption-desorption isotherms at -
reduced Pd catalyst at 340 °C.
196 °C (Table 1). The specific surface area and pore volume of
The methyl palmitate conversion (
ꢀꢁꢂ) and oxygen NiP_A and NiP_I samples (Table 3) decrease slightly with the
conversion (ꢀꢃ) were calculated as common: active component loading in comparing with SiO2 support
(262-266 m2/g versus 300 m2/g of SiO2, and 0.67 cm3/g versus
0.79 cm3/g of SiO2). While the average pore diameter is
ꢄꢅꢆꢇꢈꢉꢄꢅꢆꢊꢋꢌ
ꢀꢁꢂ
=
∙ 100%
(1)
ꢄꢅꢆꢇꢈ
where ꢍꢁꢂꢎꢏ is the initial concentration of methyl palmitate virtually the same (10.5 nm in contrast to 10.6 nm of SiO2), the
(mol/L), ꢍꢁꢂꢐꢑꢒ is the concentration of methyl palmitate in the specific surface area of NiP_A and NiP_I the samples after HDO
reactor effluent (mol/L).
were changed slightly (Table 2).
Fig. 1 shows XRD patterns of the NiP_Ah, NiP_Ih, NiP_A and
NiP_I samples. All XRD curves contain the broad diffraction line
ꢄꢓꢇꢈꢉꢄꢓꢊꢋꢌ
ꢀꢃ
=
∙ 100%
(2)
ꢄꢓꢇꢈ
where ꢍꢃꢎꢏ is the initial concentration of oxygen in the at 2
θ ~ 15-30° which is typical of the amorphous SiO2. The
reaction mixture (mol/L), ꢍꢃꢐꢑꢒ is the concentration of oxygen NiP_Ah, NiP_Ih and NiP_A samples show the characteristic
in the reactor effluent (mol/L).
peaks of Ni2P phase:
2
θ
(111) = 40.7°,
2
2
θ
θ
(201) = 44.6°,
(002) = 54.4°,
It should be noted that conversion of methyl palmitate
slightly decreased during the 4-6 hours of experiments
2
2
θ
θ
(210) = 47.3°,
2θ (300) = 54.1°,
(211) = 55.0° (JCPDS#03-0953).46 The NiP_I sample does not
afterwards remained unchanged up to 12 hours. When the show any diffraction peaks after reduction at 600 °C (Fig. 1).
WHSV was changed the steady-state concentrations of methyl This fact could be attributed to the formation of small particles
palmitate and products of HDO were reached after 4-6 h.
that are not detectable by XRD. Average crystallite sizes of Ni2P
To estimate selectivity towards HDO routes we used particles (DXRD) were estimated using the Scherrer equation
⁄
ꢍꢔꢕ ꢍꢔꢖ molar ratio, which was calculated as follows:
and the full-width at half-maximum of the (111) reflection
(Table 1). For the NiP_A DXRD was 6.0 nm, but for the NiP_Ah
DXRD proved to be 10.0 nm. For NiP_Ih DXRD was 6.0 nm.
ꢄꢚꢗꢘ ꢜꢝꢞꢄꢚ
ꢄꢗꢘ
ꢄꢗꢙ
ꢛ
ꢛ
ꢗꢘ ꢜꢟ
=
(3)
ꢄꢚꢗꢙ ꢜꢟꢞꢄꢚ
ꢛ
ꢛ
ꢗꢙ ꢜꢠ
Table 1 Physicochemical characteristics of the ex situ reduced catalysts
where ꢍꢄ
is the concentration of n-hexadecane in the
ꢗꢘꢡꢜꢝ
Sample
Ni,
P,
Tred*,
Ni/P
XRD
DXRD
,
DTEM
,
ABET,
m2/g
reactor effluent (mol/L), ꢍꢄ
is the concentration of
ꢗꢘꢡꢜꢟ
wt% wt%
°C
molar phase
ratio
nm
nm
hexadecenes in the reactor effluent (mol/L), ꢍꢄ
is the
ꢗꢙꢡꢜꢟ
NiP_A
NiP_I
2.2
2.4
6.3
5.0
1.2
1.3
3.8
5.0
600
600
600
600
1/1.0
1/1.1
1/1.1
1/1.9
Ni2P
n/d
6.0
-
3.6
1.8
8.9
4.6
262
266
161
180
concentration of n-pentadecane in the reactor effluent
(mol/L), ꢍꢄ is the concentration of pentadecenes in the
ꢗꢙꢡꢜꢠ
NiP_Ah
NiP_Ih
Ni2P
Ni2P
10.0
6.0
reactor effluent (mol/L).
The catalytic activity (
* Tred – reduction temperature
ꢢꢡꢣꢃ) of the samples in methyl
Table 2 The catalysts characteristics after HDO of methyl palmitate
palmitate HDO was evaluated as follows:
ꢤꢓ∙ꢥ
ꢪꢐꢫ
Sample
Tred*,
°C
Ni/P molar ratio
after HDO (EDX)
1/0.8
DTEM after
HDO, nm
3.4
ABET
,
m2/g
257
254
253
262
245
249
243
ꢢꢡꢣꢃ
=
,
(4)
ꢦꢧꢨꢌ∙ꢄꢩꢇ ꢬ∙ꢭ
where ꢀꢃ is the conversion of oxygen-containing compounds
(%), is the molar rate of methyl palmitate fed into the
NiP_A
NiP_A
550
600
650
600
500
550
600
ꢮ
1/0.6
3.2
NiP_A
1/0.5
3.3
reactor (mol/h), ꢯꢰꢱꢒ is the catalyst weight (g) and ꢍꢲꢎ is the
nickel content in the catalyst (wt%). This approach could be
used in differential conditions where the conversions are
below 20%.
NiP_A (ex situ)
NiP_I
1/0.6
2.9
1/1.6
2.5
NiP_I
1/0.7
3.2
NiP_I
1/0.7
2.2
* Tred – reduction temperature
3. Results and discussion
3.1. Catalysts characterization
4 | J. Name., 2012, 00, 1-3
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Summarising the results of XRD and TEM data we can
conclude that the nature of the precursor affects the sizes of
the phosphide particles: using H3PO3 as P precursor instead of
(NH4)2HPO4 favours the formation of smaller Ni2P particles.
H2-TPR experiments were carried out to reveal possible
reasons for this effect and to estimate the temperatures of the
reduction processes. Fig. 3 shows H2 consumption during
reduction of the NiP_Ah and NiP_Ih precursors and during
reduction of the reference samples POx/SiO2 and NiO/SiO2.
Phosphates on the surface of POx/SiO2 sample are
gradually reduced starting at 600 °C, and then H2 consumption
accelerates as the temperature increases. Reduction of
NiO/SiO2 starts at 370 °C and the H2-TPR curve has two
maximums at about 415 °C and 550 °C (Fig. 3). The first
maximum is attributed to the reduction of NiO nanoparticles
to metallic Ni.47,48 The second maximum is explained by the
reduction of the Ni silicate to the metallic Ni and SiO2.49
5 wt% Ni
NiP_Ah
NiP_Ih
2.5 wt% Ni
NiP_A
NiP_I
JCPDS#03-0953 Ni2P
15 20 25 30 35 40 45 50 55 60 65
θ (°)
2
Fig. 1 XRD patterns of Ni phosphide catalysts reduced ex situ at 600 °C
H2-TPR curves of the NiP_Ah and NiP_Ih precursors are
completely different. H2 consumption by NiP_Ah sample starts
at about 600 °C and the reduction rate steadily increases
(Fig. 3). The NiP_Ah reduction curve resembles the reduction
curve of POx/SiO2, but H2 consumption is much higher.
Figure 2 shows the TEM images and the particle size
distributions of NiP_Ah (Fig. 2a), NiP_A (Fig. 2b), NiP_Ih
(Fig. 2c) and NiP_I (Fig. 2d) catalysts reduced at 600 °C, the
mean sizes of Ni2P particles on the surface of NiP_Ah, NiP_Ih,
NiP_A and NiP_I samples (DTEM) are listed in Table 1. NiP_A
sample contains Ni2P nanoparticles with the mean particle size
(DTEM) of 3.6 nm, DTEM of NiP_I samples is twice as low, 1.8 nm.
It is seen that particle size distribution of NiP_A sample is
Two different mechanisms of Ni phosphide formation from
phosphate precursor have been considered in the literature.49–
51
The first one proposes the intermediate formation of
metallic Ni during H2-TPR of SiO2 supported nickel phosphide
precursor followed by formation of Ni12P5 and Ni2P.49 Using 31
broader (~ 1.5-7.5 nm) than that of NiP_I sample (~ 1.0-
P
3.0 nm). The mean particle size increases with a growing Ni
content, and the NiP_Ah sample contains bigger particles with
a wider particle size distribution in comparison with NiP_Ih
one (Table 1, Fig. 2).
MAS NMR spectroscopy Stinner et al.50 suggested that the
reduced metallic Ni catalyses the reduction of the phosphate
50
50
(b) NiP_A
(a) NiP_Ah
40
40
30
20
10
0
30
20
10
0
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Size (nm)
Size (nm)
50
40
30
20
10
0
50
40
30
20
10
0
(d) NiP_I
(c) NiP_Ih
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Size (nm)
Size (nm)
Fig. 2 TEM images of NiP_Ah (a), NiP_A (b), NiP_Ih (c) and NiP_I (d) samples after reduction at 600 °C
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POx
POx/SiO2
NiP_I
NiP_A
SiO2
NiO+POx
NiP_Ah
NiP_Ih
POx
ꢂ
ꢁ
H3PO3 + H2O
ꢀ
H3PO4 + H2
ꢁ
ꢂ
4H3PO3 ꢀ 3H3PO4 + PH3
NiO
2PH3 ꢀ P2 + 3H2
Ni
NiSiO3
NiO/SiO2
100
200
300
400
500
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
2
3
4
5
6
7
8
9
0
1
Temperature (oC)
Temperature (°C)
Fig. 3 H2-TPR curves of POx/SiO2, NiP_Ah and NiP_Ih precursors and NiO/SiO2
Fig. 4 NH3-TPD profiles of NiP_A and NiP_I samples and SiO2 support
to form reactive P species (such as P2, P4 and PxHy).
The suggested sequence of the NiP_Ih precursor
Subsequently the P species react with Ni atoms to form the transformation is rather speculative. However, the main idea is
Ni12P5 and Ni2P phases. The other reduction mechanism does that nickel phosphide is formed at lower temperatures when
not consider the formation of metallic Ni nanoparticles. The H3PO3 is used as P precursor instead of (NH4)2HPO4.
appearance of α-Ni2P2O7 pyrophosphate has been observed at Apparently, the lower temperature of phosphide formation
the temperatures of 300-500 °C during reduction using in situ prevents sintering and led to smaller particles in the case of
XRD, XAS and 31P MAS NMR.51 According to this study, Ni(PO3)2 NiP_I catalysts. Besides, phosphite method (NiP_I) did not
and Ni2P phases are formed at the temperature of 550 °C, contain the high-temperature calcination step, while
while only Ni2P phase was detected after reduction at 650 °C. phosphate method includes the calcinations step of precursors
The Ni/P ratio is suggested to be the main reason of this in order to decompose nitrogen-containing salts. So, NiP_A
discrepancy: a high initial Ni/P molar ratio (Ni/P = 1/1.25) samples are subjected to two high-temperature procedures
favours the intermediate formation of metallic Ni.51
NiP_A and NiP_Ah catalysts were prepared using the case of NiP_I samples.
(calcination and reduction) instead of only reduction step in
solutions with lower Ni/P molar ratio equal to 1/2. There are
The composition and sizes of the phosphide particles in the
no H2-TPR peaks corresponding to metallic Ni formation in catalysts after reaction are presented in Table 2. After HDO of
NiP_Ah sample. Therefore, the reduction of NiP_Ah precursors methyl palmitate over NiP_A and NiP_I samples the Ni/P molar
is likely to occur without formation of metallic nickel ratio changes and P content gets lower in comparison with the
nanoparticles and goes through the second mechanism.51
samples before HDO (Table 1). This observation can be
The TPR curve of the NiP_Ih precursor contains two attributed to the leaching of the phosphorus excess during
downward-directed peaks at 350 °C and 550 °C, which can be HDO. The particle sizes (DTEM) do not differ significantly from
explained by the H2 evolution in the course of the precursor that obtained after ex situ reduction to discuss the change in
heating. H2 consumption is observed only after heating the particle sizes as a consequence of reaction.
sample to a temperature of 600 °C. Several mechanisms can
Fig. 4 shows the NH3-TPD profiles of NiP_A and NiP_I
result in the additional formation of hydrogen and downward- samples and SiO2 support. The entire samples show only peak
directed peaks appearance. H3PO3 and its salts are known to corresponding to weak acidic sites with Tmax at 231 oC.54 The
decompose into H3PO4 (or phosphates) and PH3 at the total quantities of acid sites estimated by integration of NH3
temperatures of 250-275 °C through the disproportionation desorption peaks are summarized in Table 3. Total acidity is
reaction:52,53
4H3PO3 → 3H3PO4 + PH3
increased to 110 μmol/g (+31%) for NiP_A sample and to
139 μmol/g (+65%) for NiP_I sample in comparing with the
PH3 appears to form H2 during Ni2+ reduction. Therefore, acidity of silica support.
downward-directed peaks are observed. H2O seems to be
Table 3 The characteristics of the NiP_A and NiP_I samples and SiO2 support
formed at higher temperatures due to phosphates and
phosphites reduction catalysed by nickel phosphide. H2O can
also react with H3PO3 or phosphite to form additional amount
of H2:
NH3-TPD
BET
ABET, m2/g
Vpore,
Sample
Tred*, oC
Quantity,
Tmax, oC
μmol/g
84
cm3/g
0.79
0.67
0.70
H3PO3 + H2O → H3PO4 + H2
SiO2
NiP_A
NiP_I
-
231
231
231
300
262
266
600
600
110
H3PO3 and phosphites are completely transformed to
phosphates or phosphide at 600 °C, and the consumption of
hydrogen at a temperature above 600 °C can be ascribed to
the reduction of phosphate residues.
139
* Tred – reduction temperature
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Fig. 5 displays Echo 31P 15 kHz MAS NMR spectra of NiP_Ih
catalysts reduced at different final temperatures from 500 °C
to 600 °C. First of all, two main regions of the spectra can be
highlighted. First is around 0 ppm corresponding to different
(3-n)-
oxide phosphorus species, i.e. phosphoric acid HnPO4
at
~ 0 ppm, terminal P2O74- at ~ -7 ppm, and internal (PO3 )n at ~ -
22 ppm phosphate groups in phosphoric acid oligomers.55–57
Moreover, an additional signal at ~ -35 ppm can correspond to
silicon hydrogen tripolyphosphate SiHP3O10, or to silicon
hydrogen phosphate monohydrate (Si(HPO4)2·H2O since the
position is not exact due to rather large full width at a half
maximum (FWHM) of observed lines.55
-
The second part of spectra in Fig. 5 is downfield broad line
with maximum at ~ 1500 ppm for 500 °C to 600 °C reduction
temperatures. The line is not symmetric, the main reason for
that is superposition of several signals. However, the NMR line
shape effects cannot be excluded. The maximum at
~ 1500 ppm most likely corresponds to Ni2P phase, and due to
large FWHM the second phosphorus center in this phase
cannot be detected. Nevertheless, the presence of other Ni-P
Fig. 5 Spin-echo 31P 15 kHz MAS NMR spectra of NiP_Ih catalysts reduced at different
final temperatures from 500°C to 600°C (top-down)
ꢗ
ꢟ
ꢗ
꣗꣘ꢨꢸ
꣎
ꣅꢁꢂ∣ꢣꢐ꣍ꣃꢰꢱꢏꣃ = 4.4 ∙ 10ꢉꢔꢖ
꣔꣓꣐ꢊ꣑ꢻꢧꢨꢈꢻ꣕ꢘ ꣖ ꣐ꢊ꣑ꢻꢧꢨꢈꢻ꣙
(8)
꣘ꢨꢸ
꣒꣐ꢊ꣑ꢻꢧꢨꢈꢻ
꣓ꢅꢆ
꣗
ꢅꢆ
Parameters used for the calculation are listed in Table S2 of
the Supporting Information. Estimated Weisz-Prater numbers
for hydrogen and methyl palmitate were 0.02 and 0.08
respectively, indicating the negligible internal mass transfer
effects on reaction rate.
50,58
intermetallic compounds as Ni12P5, Ni3P, NiP, and Ni5P4
could not be avoided on the bases of NMR data since (i) their
chemical shifts lie in the same region; (ii) large FWHM does
provide well separation of lines. The large FWHM can be result
of either small particle or inhomogeneous character
(defectiveness) of Ni-P intermetallic compounds. The amount
of oxide phosphorus groups is decreased with the increase of
reduction temperature being equal to 16, 12, and 8 at% in
NiP_Ih samples reduced at 500, 550, and 600 °C, respectively.
3.2.2. Re-reduction effect (in situ vs ex situ reduction)
In order to elucidate the effect of passivation on the catalytic
properties of phosphide-based catalysts the testing of the
same NiP_A precursors in methyl palmitate HDO was
performed after ex situ and in situ reduction. The ex situ
sample was passivated as described in experimental section,
then it was transferred to the catalytic reactor and re-reduced
at 450 °C for 2 h (the most frequently used re-reduction
conditions in literature).33–35,38 The in situ reduction means the
reduction of catalysts in the catalytic reactor immediately prior
to testing. In both cases the reduction was performed at
600 °C for 1 h.
3.2. Catalytic properties
3.2.1. Mass-transfer process in methyl palmitate HDO
In order to evaluate the effect of internal diffusion on catalytic
HDO of methyl palmitate the Weisz-Prater criterion was
used.59 To neglect diffusional limitations the Weisz-Prater
number estimated by the equation (5) should be less than 0.3:
ꢵꢅ.ꢶ.∙ꢷꢟ
ꢸꢨꢹꢌꢇꢧꢺꢻꢼ ꢾ 0.3
(5)
Fig. 6 shows the catalytic activities of NiP_A samples
reduced in situ and ex situ. The ex situ reduction at 600 °C
results in a lower activity of the NiP_A sample compared to in
ꢳꢴꢉꢂ
=
ꢢ
ꢄꢼ∙ꢣꢻꢽꢽ
where
ꢁ.ꢿ. is the observed reaction rate;
ꣀꣁꢱꣂꢒꢎꢰꢫꣃ꣄ is the catalyst particles radius;
ꢍ꣄ is the reactant surface concentration;
0,5
0,4
ꣅꣃ꣆꣆ is the effective diffusion coefficient estimated as follows:
꣈ꢔꢉ꣉꣊ꢟ
ꣅꣃ꣆꣆ = ꣅ꣇
(6)
NiP_A
NiP_A
ꢔꢞꢂ꣉
(ex situ)
0,3
0,2
0,1
0,0
where ꣅ꣇ is the bulk diffusivity;
is the ratio of the radius of the diffusing molecule to the
꣋
catalyst pore radius;
꣌ = 16.3 is the fitting parameter for used silica.59
Equations of Wilke and Chang were used for calculation of
the diffusion coefficients of hydrogen (ꣅꢡ ∣ꢣꢐ꣍ꣃꢰꢱꢏꣃ) and methyl
palmitate (ꣅꢁꢂ∣ꢣꢐ꣍ꣃꢰꢱꢏꣃ) in n-dodecane:
ꢟ
꣎
ꢤ∙ꢁ
꣐ꢊ꣑ꢻꢧꢨꢈꢻ
꣏
ꣅꢡ
ꢟ∣ꢣꢐ꣍ꣃꢰꢱꢏꣃ
= 1.1728 ∙ 10ꢉꢔꢕ
(7)
꣒꣐ꢊ꣑ꢻꢧꢨꢈꢻ∙꣓ꢛꢠ.ꢘ
Fig. 6 Activity of NiP_A samples reduced ex situ at 600 °C and in situ at 600 °C.
WHSV = 48 h-1, reaction temperature = 290 °C, reaction pressure = 3.0 MPa
ꢟ
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situ reduction of this sample at the same conditions. Ni/P same reaction conditions: WHSV = 48 h−1, 290 °C, 3.0 MPa. The
molar ratios of NiP_A (ex situ) and NiP_A (in situ) samples after conversion goes through a maximum in the case of NiP_A
methyl palmitate HDO were quite similar (Table 2). The catalysts, the temperature of 600 °C proved to be the optimal
particle sizes (DTEM) were 3.2 and 2.9 nm for in situ and ex situ reduction temperature for the Ni2P/SiO2 catalysts prepared
samples correspondingly. XRD analysis confirmed the Ni2P from the phosphate precursor (NiP_A). The activity of NiP_I
phase formation for both techniques. Thus the ex situ and in catalyst decreased with an increasing of reduction
situ samples were quite similar according to their bulk temperature from 500 to 600 °C, displaying completely
physicochemical properties. The difference in the catalytic different behavior of NiP_I samples. It should be noted that
properties can be explained by the change of the surface NiP_I samples exhibit higher activity than NiP_A samples.
properties of Ni2P particles induced by the contact with oxygen
during passivation.
The catalytic HDO of fatty acid esters is considered to occur
through a complex reaction network which involves hydrolysis,
Taking into account the difference in catalytic properties transesterification,
the recovery of the initial state of Ni2P surface seems to be decarbonylation, hydrogenation, hydrogenolysis, dehydration,
impossible by the re-reduction of the sample at 450
C. The dehydrogenation.14,15,20–22,32,60,61 Some of this stages proceed
esterification,
decarboxylation,
°
similar behaviour of H2-TPR patterns of the initial NiP_A with participation of acid centers, while others require
precursor and the same sample after reduction and presence of hydrogenation function.
passivation was observed (Fig. S1 of the Supporting
Information), confirming the necessity of higher temperature catalysts should be explained taking into account the overall
during the re-reduction procedure. scheme of methyl palmitate transformations and the nature of
Therefore, in order to compare the NiP_A and NiP_I centers responsible for the different stages of the reaction.
catalysts correctly and eliminate the influence of the The main products of methyl palmitate HDO are
The difference in catalytic properties of NiP_A and NiP_I
passivation, storage and re-reduction conditions on catalytic completely deoxygenated hydrocarbons, namely n-
properties in situ reduction technique was used in further pentadecane and n-hexadecane (Fig. 8 and Fig. 9). Minor
experiments.
quantities of intermediate products were also detected:
hexadecane-1-ol, hexadecane-2-ol, palmitic acid, palmityl
palmitate, hexadecanal, pentadecenes and hexadecenes (Fig. 8
and Fig. 9). C16/C15 ratio did not significantly depend on
reduction temperature for NiP_A and NiP_I samples and was
about 0.45-0.55 (Fig. S2 of the Supporting Information).
The main gas phase products over NiP_A and NiP_I
samples were CH4 and CO (Fig. 10), some amount of CO2 was
also detected in case of NiP_I catalysts pointing out the
possibility of palmitic acid transformation through the
decarboxylation route. The methanol formation was confirmed
by gas phase analysis over both catalytic systems, but the
quantitative data is missing because of the ability of methanol
to dissolve in water, one of the products of methyl palmitate
HDO.
3.2.3. Effect of precursor on catalytic properties of NiP_A and
NiP_I catalysts
The catalytic properties of Ni2P/SiO2 catalysts (NiP_A and NiP_I
samples) were investigated in methyl palmitate HDO in a
trickle-bed reactor at hydrogen pressure of 3.0 MPa and
temperatures of 290 °C. Taking into account the significant
differences in H2-TPR curves of the NiP_Ah and NiP_Ih
precursors the effect of reduction temperature on the catalytic
properties of the NiP_A and NiP_I catalysts was studied.
During the investigation the detailed analysis of the
intermediates, liquid and gaseous products of methyl
palmitate HDO was performed.
Fig. 7 shows the dependence of XMP on the reduction
temperature of NiP_A and NiP_I catalysts obtained at the
The tentative reaction network can be suggested taking
into account the liquid product distributions, gas phase
analysis and the literature data about HDO of fatty acid esters
(Scheme 1).
80
70
60
50
40
30
20
10
0
The different pathways can be considered for the methyl
palmitate conversion into intermediate products: palmitic acid
and hexadecanal. Both metallic and acid-base properties play
an important role in the transformation of aliphatic ester into
acid, which can proceed through hydrogenolysis of C-O bond
of ester group over metallic sites or through acid-catalysed
hydrolysis.36 Hydrogenolysis leads to CH4 formation, while
hydrolysis is accompanied by the appearance of methanol in
the reaction products.62
The direct transformation of methyl palmitate by
hydrogenolysis of the C-O σ-bond of the carboxylic group leads
to hexadecanal and methanol. Hexadecanal can be also
obtained as a result of palmitic acid hydrogenation, producing
water as a side product. Hexadecanal was proposed to be
hydrogenated to hexadecane-1-ol or eliminate CO with the
NiP_I
NiP_A
Fig. 7 XMP dependence on the reduction temperatures of NiP_A and NiP_I samples.
WHSV = 48 h-1, reaction temperature = 290 °C, reaction pressure = 3.0 MPa
8 | J. Name., 2012, 00, 1-3
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100
1,6x10-4
(a)
80
n-C15H32
60
40
20
XMP=21.7%
1,2x10-4
XMP=19.7%
n-C16H34
CO2
CH4
CO
0
0
8,0x10-5
10 20 30 40 50 60 70 80 90 100
XMP (%)
2,0
C16H33OH-1
4,0x10-5
(b)
C15H31COOH
1,5
1,0
0,5
0,0
C15H31COOC16H33
C16H33OH-2
0,0
C15H31CHO
C15H30
NiP_A
NiP_I
C16H32
0
10 20 30 40 50 60 70 80 90 100
Fig. 10 Gas phase products of methyl palmitate HDO over NiP_A and NiP_I samples at
the temperature of 290 °C, hydrogen pressure 3.0 MPa, WHSV = 48 h-1
XMP (%)
Fig. 8 Products distribution as a function of methyl palmitate conversion for NiP_A
sample, reduced at 600 °C. (a) – the main products: n-C15H32 – n-pentadecane, n-
C16H34 – n-hexadecane, (b) – the minor products: C16H33OH-1 – hexadecane-1-ol,
C15H31COOH – palmitic acid, C15H31COOC16H33 – palmityl palmitate, C16H33OH-2 –
be attributed to the excess of phosphorus in catalyst
precursors (Ni/P = 1/2) and the surplus P unreduced species
(HnPO4(3−n)−, P2O7 and (PO3 )n) after reduction. It is also
proposed that the most of –P-OH groups belong to silica-
bounded hydrogen phosphate ((SiO)2P=O(OH)), and a very
small fraction of –P-OH groups can belong to Ni2P phase.63 The
amount of acid sites was shown to increase as the P content in
the catalysts increases.63,65
4−
−
hexadecane-2-ol, C15H31CHO
–
hexadecanal, C15H30
–
pentadecenes, C16H32
–
hexadecenes.
WHSV = 10-100 h-1,
reaction
temperature = 290 °C,
reaction
pressure = 3.0 MPa
100
(a)
80
60
40
20
0
n-C15H32
n-C16H34
According to DRIFT and NH3-TPD studies Chen et al.
suggest a surface model for Ni2P/SiO2 that involves metal (Niδ+)
and acid (Lewis Niδ+ and Brønsted –P-OH) active sites.67
The acid-catalysed reactions (esterification, hydrolysis,
dehydration) seem to proceed with the participation of the
Brønsted acid sites of the catalyst, namely acid –P-OH surface
groups.36 It has been shown that –P-OH groups are strong
enough to donate H to N-bases compared to –Si-OH groups.63
Therefore, SiO2 can be considered to be inert since –Si-OH
groups acid groups are too weak.68
0
10 20 30 40 50 60 70 80 90 100
XMP (%)
2,0
1,5
1,0
0,5
0,0
C16H33OH-1
(b)
C15H31COOH
C15H31COOC16H33
C16H33OH-2
C15H31CHO
C15H30
C16H32
The hydrogenolysis and hydrogenation reactions take place
at the metal Niδ+ sites and consume the adsorbed H atoms.
The main features of all the experiments regarding the
reaction conditions or catalysts employed have been the minor
difference in conversions of methyl palmitate and overall
oxygen-containing compounds as well as the negligible
amounts of oxygen-containing intermediates (Fig. S3 of the
Supporting Information). The latter observation indicates that
0
10 20 30 40 50 60 70 80 90 100
XMP (%)
Fig. 9 Products distribution as a function of methyl palmitate conversion for NiP_I
sample, reduced at 600°C. (a) – the main products: n-C15H32 – n-pentadecane, n-C16H34
– n-hexadecane, (b) – the minor products: C16H33OH-1 – hexadecane-1-ol, C15H31COOH
– palmitic acid, C15H31COOC16H33 – palmityl palmitate, C16H33OH-2 – hexadecane-2-ol,
C15H31CHO – hexadecanal, C15H30 – pentadecenes, C16H32 – hexadecenes. WHSV = 10-
100 h-1, reaction temperature = 290 °C, reaction pressure = 3.0 MPa
formation of pentadecane.35,36 On the other hand, once methyl palmitate is converted the produced intermediate
hexadecane-1-ol can be dehydrated to hexadecenes, which are oxygen-containing compounds are immediately transformed
subsequently hydrogenated to n-hexadecane. Hexadecenes and therefore the conversion of methyl palmitate is the rate
can react with water to form hexadecane-2-ol that was limiting stage of overall HDO reaction over silica-supported
observed among the reaction products in negligible amount.
Palmityl palmitate was also observed among the reaction
Ni2P catalysts.
The methyl palmitate can be converted to the intermediate
products over both catalysts being the results of the oxygenated products through acid-catalysed hydrolysis or
esterification reaction of palmitic acid (or the metal-catalysed hydrogenolysis of C-O bonds. So, in order to
transesterification reaction of methyl palmitate) with explain the difference in catalytic properties of NiP_A and
hexadecane-1-ol (or hexadecane-2-ol – further HDO product).
NiP_I catalysts in the methyl palmitate conversion, acid and
It has been shown that Ni2P/SiO2 catalysts have Brønsted metal surface active sites should be taken into account. In
and Lewis acid sites on their surface.63-67 Brønsted acidity is turn, the precursor and reduction temperatures may influence
attributed to –P-OH groups while Lewis acidity is attributed to the amount and the ratio of different sites on the surface of
coordinative unsaturated Niδ+ sites.
Ni2P/SiO2.
NiP_A and Ni_I samples after reduction at different
The –P-OH band is observed in DRIFT spectra even after in
situ reduction at 550 °C of Ni2P/SiO2 catalysts.63 This fact can temperatures contain different amounts of P (Table 2), which
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Scheme 1 Tentative reaction network of methyl palmitate HDO
is the source for the formation of –P-OH surface groups. The corresponding decrease of methyl palmitate hydrolysis
higher activity of NiP_I samples can be explained by the higher reaction rate. The formation of Ni2P particles in the case of
amount of –P-OH surface groups which promote the hydrolysis phosphite precursor occurs at a lower temperature in
of methyl palmitate. The higher activity of NiP_I samples in accordance with H2-TPR data.
acid-catalysed reactions is confirmed by the formation of
palmityl palmitate, a side product (Fig. 8 and Fig. 9) of
4. Conclusions
esterification reaction (Scheme 1). The above reasoning
coincides with the higher acidity of NiP_I sample defined by
NH3-TPD technique (Table 3).
The synthesis of silica-supported Ni2P nanoparticles starting
from phosphate ((NH4)2HPO4) or phosphite (H3PO3) precursors
have been investigated using chemical analysis, H2-TPR, XRD
and TEM methods. The different reduction mechanisms of
NiP_A and NiP_I precursors have been proposed as a result of
H2-TPR data analysis. NiP_I precursor can be reduced at lower
temperatures due to nickel phosphite and H3PO3
decomposition leading to the reactive PH3 formation. On the
other hand, higher temperatures are needed to reduce
phosphate precursor to Ni2P. In line with this reasoning the
results of XRD and TEM data have revealed the formation of
smaller Ni2P particles in case of using H3PO3 as P precursor
instead of (NH4)2HPO4.
The conversion of methyl palmitate over NiP_A samples is
shown to go through the maximum with increasing of
reduction temperature, 600 °C being the optimal reduction
temperature. According to TEM analysis, the particle sizes in
NiP_A samples after HDO do not depend on reduction
temperature (Table 2). However, the reduction temperature
increase leads to a decrease in P content (Table 2). Reduction
at 650 °C gives stoichiometric Ni/P molar ratio in NiP_A
sample, but there is some excess of P for the NiP_A catalysts
reduced at 600 and 550 °C. Probably, the reduction
temperature of 600 °C is optimal because it allows us to form
more Ni2P phase (metal sites, responsible for hydrogenolysis
reaction) than at 550 °C and to save some additional amount
of P (acid sites, responsible for acid-catalysed reaction). Thus,
the optimal combination of metal and acid sites in the NiP_A
sample after reduction at 600 °C could provide the maximum
activity in methyl palmitate transformation through
hydrogenolysis and hydrolysis reaction.
The passivation and re-reduction steps showed
a
detrimental effect on catalytic properties and in situ reduction
of precursors should be used to obtain a reliable data about
properties of Ni phosphide catalysts.
Precursor nature of Ni phosphide catalysts influences their
activity in HDO of methyl palmitate, using H3PO3 as
phosphorus precursor allows us to obtain more active catalyst.
The activity of the catalysts in methyl palmitate HDO depends
not only on the precursor nature but also on the reduction
temperature of catalyst precursor. The optimal reduction
temperature of NiP_A precursors in terms of methyl palmitate
HDO activity is 600 °C, while the NiP_I catalyst shows higher
activity after reduction at 500 °C. The common features of all
the experiments are the coincidence of methyl palmitate and
The activity of NiP_I catalyst decreases with an increase of
reduction temperature from 500 to 600 °C, whereas the Ni/P
ratio is increased (from 1/1.6 to 1/0.7) and content of
phosphate groups is decreased (from 16 to 8 at%) in this range
of temperature. The decrease of the methyl palmitate
conversion with the reduction temperature can be explained
by the decrease of surface –P-OH groups and the
10 | J. Name., 2012, 00, 1-3
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overall oxygen conversion as well as negligible quantities of 17 J. Horáček, Z. Tišler, V. Rubáš and D. Kubička, Fuel, 2014, 121,
oxygen-containing intermediates. So, the methyl palmitate 57.
conversion is found to be a limiting step of overall HDO 18 D. Kubička and J. Horáček, Appl. Catal. A Gen., 2011, 394, 9.
reaction. The methyl palmitate can be converted to the 19 C. Ochoa-Hernández, Y. Yang, P. Pizarro, V. A. de la Peña
intermediate oxygenated products through acid-catalysed
hydrolysis or metal-catalysed hydrogenolysis of C-O bonds.
O’Shea, J. M. Coronado and D. P. Serrano, Catal. Today, 2013,
210, 81.
The higher amount of P in the NiP_I samples favours higher 20 R. G. Kukushkin, O. A. Bulavchenko, V. V. Kaichev and V. A.
amount of acid –P-OH groups and is likely to provide the Yakovlev, Appl. Catal. B Environ., 2015, 163, 531.
higher activity in the acid-catalysed reaction of methyl 21 N. Chen, S. Gong and E. W. Qian, Appl. Catal. B Environ., 2015,
palmitate hydrolysis to palmitic acid.
174-175, 253.
22 N. Chen, S. Gong, H. Shirai, T. Watanabe and E. W. Qian, Appl.
Catal. A Gen., 2013, 466, 105.
Acknowledgements
23 T. M. Yeh, R. L. Hockstad, S. Linic and P. E. Savage, Fuel, 2015,
156, 219.
This work was supported by Russian Academy of Sciences and
Federal Agency of Scientific Organizations (project V. 45.3.2).
The authors are grateful to Dr. V.A. Rogov for catalysts
characterization by H2-TPR, and to V.A. Utkin and
Dr. M.V. Shashkov for identification of the reaction products
using GC/MS technique.
24 M. Chiappero, P. T. M. Do, S. Crossley, L. L. Lobban and D. E.
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Graphical abstract
Effect of precursor on the catalytic properties of Ni2P/SiO2 in methyl palmitate
hydrodeoxygenation
Ivan V. Shamanaev,abc Irina V. Deliy,abc* Pavel V. Aleksandrov,ac Evgeny Yu. Gerasimov,abc Vera P. Pakharukovaabc, Evgeny G.
Kodenev,a Artem B. Ayupov,ab Andrey S. Andreev,a Olga B. Lapina,a and Galina A. Bukhtiyarovaa
a.Boreskov Institute of Catalysis, pr. Lavrentieva 5, Novosibirsk, 630090, Russia.
b.Novosibirsk State University, Pirogova st. 2, Novosibirsk, 630090, Russia.
c. Research and Educational Center for Energy Efficient Catalysis, Novosibirsk National Research University, Pirogova st. 2, Novosibirsk, 630090, Russia.
E-mail: delij@catalysis.ru
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