Promotional effect of Fe on performance of Ni/SiO2 for deoxygenation of methyl laurate as a model compound to hydrocarbons

Article abstract of DOI:10.1039/c4ra07932a

Ni/SiO₂, Fe/SiO₂ and bimetallic FeNi/SiO₂ catalysts with different Fe/Ni weight ratios were prepared by incipient-wetness impregnation method for the deoxygenation of methyl laurate to hydrocarbons. It was found that a suitable amount of Fe enhanced the activity of Ni/SiO₂ for the deoxygenation of methyl laurate, and FeNi(0.25)/SiO₂ with a Fe/Ni weight ratio of 0.25 showed the best activity. Moreover, the addition of Fe to Ni/SiO₂ significantly promoted the hydrodeoxygenation pathway to produce more C12 hydrocarbon and suppressed the activity for C-C hydrogenolysis. The effect of Fe on the performance of Ni/SiO₂ is ascribed the formation of the NiFe alloy particles, particularly with the Fe-enriched surface at low Fe content, and the existence of oxygen vacancies in Fe oxides. A mechanism is proposed to explain the promoting effect of Fe, which involves the synergism between iron sites with strong oxophilicity and nickel sites with high ability to activate hydrogen. Besides, the effect of reaction conditions and catalyst stability were also investigated.

Full text of DOI:10.1039/c4ra07932a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Promotional effect of Fe on performance of Ni/SiO2
for deoxygenation of methyl laurate as a model
compound to hydrocarbons†
Cite this: RSC Adv., 2014, 4, 46427
*
Xinbin Yu, Jixiang Chen and Tianyu Ren
Ni/SiO2, Fe/SiO2 and bimetallic FeNi/SiO2 catalysts with different Fe/Ni weight ratios were prepared by
incipient-wetness impregnation method for the deoxygenation of methyl laurate to hydrocarbons. It was
found that a suitable amount of Fe enhanced the activity of Ni/SiO2 for the deoxygenation of methyl
laurate, and FeNi(0.25)/SiO2 with a Fe/Ni weight ratio of 0.25 showed the best activity. Moreover, the
addition of Fe to Ni/SiO2 significantly promoted the hydrodeoxygenation pathway to produce more C12
hydrocarbon and suppressed the activity for C–C hydrogenolysis. The effect of Fe on the performance
of Ni/SiO2 is ascribed the formation of the NiFe alloy particles, particularly with the Fe-enriched surface
at low Fe content, and the existence of oxygen vacancies in Fe oxides. A mechanism is proposed to
explain the promoting effect of Fe, which involves the synergism between iron sites with strong
oxophilicity and nickel sites with high ability to activate hydrogen. Besides, the effect of reaction
conditions and catalyst stability were also investigated.
Received 1st August 2014
Accepted 10th September 2014
DOI: 10.1039/c4ra07932a
www.rsc.org/advances
necessary to add sulfur to the feedstock to keep sulde catalysts
in a highly active state, which negatively affects the downstream
processes.14,15 It is therefore desirable to nd alternative cata-
1 Introduction
The rapid consumption of limited reserves of fossil fuels and
the subsequent negative impacts on the environment compel
researchers worldwide to nd alternative energy resources. In
recent years, bio-fuel has been seen as a potential solution to
these problems due to its renewability and intrinsic carbon
neutrality.1 Because of the high energy density and easy pro-
cessing, vegetable oils are currently being widely used for
production of biodiesel through transesterication.2 However,
stemming from its high oxygen content, biodiesel suffers from
some detrimental problems in practice, such as poor stability,
low heating value and high freezing point.3,4 Therefore, catalytic
hydrodeoxygenation of vegetable oils has attracted great atten-
tion because it can remove oxygen atoms and hydrogenate the
unsaturated bonds producing diesel-like hydrocarbons (also
called green diesel).4–6
lysts for the hydrodeoxygenation of vegetable oils. Ni-based
catalysts prove to be highly active in deoxygenation of fatty acid
esters.4 However, they catalyze deoxygenation mainly through
decarbonylation leading to a lower carbon yield, contrary to the
concept of atom economy.16,17 Moreover, they possess a high
activity for C–C hydrogenolysis at moderate temperature, which
not only further decreases the carbon yield but also consumes a
large amount of hydrogen. Therefore, it is of interest to utilize
the advantages of nickel catalysts and while avoiding their
drawbacks.
Bimetallic catalysts have been widely applied in the petro-
leum industry. Incorporating a second metallic element could
markedly alter the activity and selectivity of the rst one due to
electronic and geometric effects.18–20 Thus, it is possible to
modify the reactivity of nickel through the addition of another
element. One of the potential candidates is iron. Iron catalysts
have been used in the hydrogenation of carboxylic acid to
aldehyde, hydrodeoxygenation of microalgal oil to green diesel,
hydrodeoxygenation of guaiacol and selective reductive cleavage
of inert aryl C–O bonds.21–24 Recently, it has been reported that
FeNi catalysts show special performance in the conversion of
furfural to methylfuran and the hydrodeoxygenation of furfuryl
alcohol, benzene alcohol and ethyl oenanthate.25,26 In addition,
iron species have redox properties as cerium and zirconium
species, and cerium and zirconium species have been effectively
used as supports or additives to Ni-based catalysts.27–30 The
synergistic effect between Ni and cerium and zirconium species
Hydrodeoxygenation catalysts are mainly based on noble
metals (Pd, Ru, etc.) and metal suldes (such as MoS2, NiMoS,
and CoMoS).7–10 Although noble metals show good perfor-
mance, their high price signicantly increases the operation
costs. When employing the metal suldes, the byproduct water
immensely suppresses their activity.11–13 Moreover, it is
Tianjin Key Laboratory of Applied Catalysis Science and Technology, Department of
Catalysis Science and Engineering, School of Chemical Engineering and Technology,
Tianjin University, Tianjin 300072, China. E-mail: jxchen@tju.edu.cn; Fax: +86 22
87894301; Tel: +86 22 27890865
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07932a
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46427–46436 | 46427

View Article Online
RSC Advances
Paper
accounts for the good performance, which might also occur mL minꢀ1) at 723 K for 1 h. Aer the temperature cooling to 303
between Ni and Fe. Since Fe has a higher oxygen affinity than K, H2 (50 mL) pulses were injected into the Ar stream until the
Ni,31 iron oxide species might be available during the reaction effluent area of consecutive pulses was constant.
and their oxygen vacancies can facilitate the reaction by binding
X-ray diffraction (XRD) patterns were obtained on a D8 Focus
and subsequently activating the oxygenates.32–34 Although FeNi powder diffractometer operated at 40 kV and 40 mA using Cu Ka
catalysts have been reported for the deoxygenation of bio- radiation (l ¼ 0.15406 nm). Crystallite size was calculated using
oil,16,25,26 explicit studies remain scarce about the effect of Fe on the Scherrer equation.
the performance of nickel-based catalyst for the deoxygenation
of fatty esters.
Transmission electron microscope (TEM) images were
obtained on a JEM-2100F transmission electron microscope.
In the present work, we prepared silica supported Ni, Fe and High angle annular dark eld transmission electron microscopy
FeNi catalysts with different Fe/Ni weight ratios and then (HAADF-STEM) images were acquired on Tecnai G2 F20 trans-
investigated the effect of iron on the catalyst performance in the mission electron microscope with an FEI Titan operated at 200
deoxygenation of methyl laurate as a model compound to kV. For TEM characterization, the power catalysts were ultra-
hydrocarbons. The effects of reaction conditions as well as sonically dispersed in ethanol and then deposited on copper
catalyst stability were also investigated. The result contributes grids covered with a holey carbon lm. Over 300 particles were
to understanding the origin of the promotional effects of Fe and involved to measure the metal particle size distribution. Average
P
P
the design of efficient catalysts for the production of green metal particle size was calculated by nidi/ ni (di: diameter of
diesel with high carbon yield.
particle i, ni: particles with di). The standard deviation of the
mean (sm) was calculated by35
X
di2 ꢀ nd2
2 Experimental
2.1 Catalyst preparation
sm
¼
n2
SiO2-supported FeNi catalysts with different Fe/Ni weight ratios
(0.0625, 0.125, 0.25, 0.5 and 1) were prepared by incipient-
wetness impregnation method, in which the content of nickel
was kept at 8 wt%. Typically, to prepare the catalyst with Fe/Ni
weight ratio of 0.25, 10 g SiO2 (Qingdao Haiyang Chemicals Co.,
Ltd., 150–300 mm) was impregnated with an aqueous solution
containing 1.608 g (3.980 mmol) Fe(NO3)3$9H2O and 4.404 g
(15.1 mmol) Ni(NO3)2$6H2O. Then, the sample was dried at
room temperature for 48 h and subsequently at 393 K for 12 h.
Aer that, the sample was calcined at 773 K for 4 h to produce
catalyst precursor. The catalyst was generated from the
precursor via reduction at 723 K for 1 h. For comparison, silica
supported nickel and iron (8 wt%) were prepared with the
procedures same to those of FeNi catalysts. The FeNi catalysts
are labeled as FeNi(x)/SiO2, where x indicates the Fe/Ni weight
ratio. The properties of the fresh and spent catalysts are
summarized in Table 1.
Electron spin resonance (ESR) spectrum of reduced sample
was collected by a Bruker ESP320 spectrometer at room
temperature. Bruker ESP320E soware and the special Bruker
program were used for data analysis. The spectrum was
acquired by multiple scans in order to achieve a satised signal
to noise ratio. Before the ESR analysis, the sample was reduced
at 723 K for 1 hour and then transferred to a tube in a glove box
and well-sealed. Then X-band (y ¼ 9.78 GHz) was recorded and
the magnetic eld was swept from 2000 Gauss to 4000 Gauss.
X-ray photoelectron spectroscopy (XPS) was performed on
PHI5000VersaProbe instrument with Al Ka radiation (1486.6 eV)
for surface elemental analysis.
Thermogravimetric analysis (TGA) was performed on Perki-
nelmer Pyris 1 TGA instrument with 10 mg sample at the air
ow rate of 100 mL minꢀ1 and temperature ramping rate of 10 K
minꢀ1 from room temperature to 1170 K.
2.2 Characterization
2.3 Activity test
N2 adsorption isotherms were obtained at 77 K using Quan- The reactivity of catalysts for the deoxygenation of methyl lau-
tachrom Quadra Sorb SI. The Brunauer–Emmett–Teller (BET) rate as a model compound was tested on a continuous ow
was used to determine the specic surface area (SBET). The pore stainless-steel xed-bed reactor (12 mm in inner diameter). In a
volume and average pore size was determined by the Barrett– typical run, 0.5 g catalyst was mixed with 4.0 g quartz sands with
Joyner–Halenda (BJH) method using the desorption branch of the same diameter and loaded on 1.0 g quartz sands (450–900
the isotherm at a relative pressure of 0.99.
mm). 2.0 g quartz sands (150–300 mm) were placed on the sample
H2 temperature-programmed reduction (H2-TPR) was to preheat the reactant. The precursor was reduced in situ by H2
carried out in a U-tube quartz reactor (4 mm in inner diameter). (>99.9%, 100 mL minꢀ1) with temperature increasing from
The sample (50 mg) was reduced in a 10% H2/N2 (60 mL minꢀ1
)
room temperature to 723 K at a rate of 5 K minꢀ1 and main-
stream at a ramping rate of 10 K minꢀ1 to 1273 K. The H2 taining at 723 K for 1 h. Methyl laurate was pumped into the
consumption was monitored continuously by
conductivity detector (TCD).
a
thermal reactor with weight hourly space velocity (WHSV) and H2/methyl
laurate molar ratio equaling 10 hꢀ1 and 25, respectively. The
H2 chemisorption was conducted on the same apparatus as liquid products were identied using gas chromatograph (GC)
H2-TPR. The catalyst precursor (100 mg) was in situ reduced at standards and an Agilent GC6890- MS5973N and quantitatively
723 K for 1 h in H2 (60 mL minꢀ1), and then ushed by Ar (60 analyzed on a SP-3420 GC equipped with a FID and a HP-5
46428 | RSC Adv., 2014, 4, 46427–46436
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
capillary column (30 m ꢁ 0.33 mm ꢁ 0.5 m). Tetrahydronaph-
thalene was used as internal standard for quantitative analysis
of liquid products. In addition, the reaction was examined to be
carried out with negligible mass- and heat-transfer limitations.
Methyl laurate conversion (X) and the selectivity to product i
(Si) were dened as follows:
n0 ꢀ n
X ¼
ꢁ 100%;
n0
ni
Si ¼
ꢁ 100%
n0 ꢀ n
where n0 and n refer to the moles of methyl laurate in the feed
and the product, respectively; ni denotes the moles of methyl
laurate converted to product i (e.g., n-undecane, n-dodecane,
and oxygenated intermediates).
3 Results and discussion
3.1 Catalyst characterization
3.1.1 Characterizations of precursor. Fig. 1 shows the XRD
patterns of the catalyst precursors. For the Ni/SiO2 precursor,
the peaks at 37.2ꢂ, 43.3ꢂ and 62.9ꢂ are ascribed to NiO (JCPDS
71-1179). No apparent peaks attributable to iron oxide were
detected for the Fe/SiO2 precursor. In the patterns of the FeNi/
SiO2 precursor, the intensity of peaks due to NiO gradually
decreased with increasing Fe content, indicating there was an
interaction between NiO and Fe oxides promoting the disper-
sion of NiO. This is further reected by the H2-TPR results.
Fig. 2 shows the H2-TPR proles of catalyst precursors. The
H2-TPR prole for Ni/SiO2 precursor exhibited a main reduction
peak at about 660 K with a shoulder peak at about 735 K, which
are ascribed to the reductions of NiO and Ni silicate species,
respectively.36,37 The H2-TPR prole of Fe/SiO2 precursor was
distinctively different from those of others. The rst peak at
530–780 K could be attributed to the reduction of hematite to
magnetite and the second peak above 1200 K could be ascribed
to the reduction of magnetite to zero valent iron.38–40 In contrast
to the Fe/SiO2 precursor, bimetallic FeNi/SiO2 precursors did
not display any peaks above 800 K, implying the simultaneous
reduction of iron oxide and nickel oxide.25,41 In other words, Ni
promoted the reducibility of Fe oxides, consistent with other
investigations.42 This is further conrmed by the increasing H2
consumption with the rising Fe content in the bimetallic
samples because Ni content is kept constant and Fe/SiO2 itself
cannot be completely reduced below 780 K. The promotion
effect is due to easy H2 dissociation on Ni, followed by spillover
of hydrogen to iron oxide.42 The interaction between Ni and Fe
leads to the formation of a NiFe alloy, which is substantiated by
the following XRD results of the reduced samples. In addition,
the intensities of peaks belonging to nickel silicate species
declined with the increase of Fe content, indicating that Fe
species could suppress the formation of nickel silicate species
possibly because certain amounts of Si–OH bonded with the Fe
species instead of the Ni ones.43
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46427–46436 | 46429

View Article Online
RSC Advances
Paper
Fig. 1 XRD patterns of catalyst precursors. (a) Fe/SiO2, (b) Ni/SiO2, and
bimetallic FeNi/SiO2 with Fe/Ni weight ratios of (c) 0.0625 : 1, (d)
0.125 : 1, (e) 0.25 : 1, (f) 0.5 : 1, and (g) 1 : 1.
Fig. 3 XRD patterns of reduced samples. (a) Fe/SiO2, (b) Ni/SiO2, and
bimetallic FeNi/SiO2 with Fe/Ni weight ratios of (c) 0.0625 : 1, (d)
0.125 : 1, (e) 0.25 : 1, (f) 0.5 : 1, and (g) 1 : 1.
3.1.2 Characterizations of catalysts. The XRD patterns of
reduced catalysts are presented in Fig. 3. For Ni/SiO2, two
diffraction peaks centered at 2q ¼ 44.5ꢂ and 51.8ꢂ correspond to
(111) and (200) reections of the fcc Ni metal phase (JCPDS 01-
070-1849). However, for bimetallic catalysts, the peak corre-
sponding to (111) gradually shied to low angle with the
increase of Fe content, in accordance with other reports.16,36,44
This is ascribed to the formation of NiFe alloy with fcc structure
whose peak is located at 2q ¼ 43.5ꢂ.45,46 The more the Fe atoms
existed in the alloy, the more obvious the shi was.16,47 No peaks
due to monometallic Fe were detected for Fe/SiO2, which is
ascribed to the high dispersion of Fe as well as the incomplete
reduction of Fe oxides at 723 K. The average diameters of Ni or
NiFe alloy crystallites were calculated using Scherrer equation
based on the (111) reections (Table 1). The crystallite size
decreased with increasing Fe content, indicating that Fe
promoted the dispersion of Ni. This is further veried by the
TEM results.
Fig. 4 shows the TEM images and metal particle size distri-
butions of some reduced catalysts. The average metal particle
sizes in Ni/SiO2 and Fe/SiO2 were 6.0 and 4.5 nm, respectively.
The average metal particle sizes in bimetallic FeNi/SiO2 were
smaller than that of Ni/SiO2. As indicated by standard deviation
(Table 1), the particle diameter distributions for all catalysts
were wide. However, the standard deviation of the mean (sm)
was very small, indicating that the observed distribution closely
approached the true population.35 Moreover, it is generally
acknowledged that the particles grow large with the increasing
metal loading, which was, however, not the case of the present
samples. In all, Fe promoted the dispersion of Ni as reected by
both XRD and TEM results.
From the thermodynamic viewpoint,48 it is expected that the
more stable FeNi clusters will have the nickel inside,49 since the
cohesive energy of bulk nickel is slightly greater than for bulk
iron (428 vs. 413 kJ molꢀ1),50 which have been reported by many
references.16,41,42,45,51 To further verify the formation of NiFe alloy
and the Fe and Ni concentration distribution in the bimetallic
catalysts, FeNi(0.5)/SiO2 was characterized by HRTEM and
STEM-EDS (see Fig. 5 and 6). For FeNi(0.5)/SiO2, the lattice
spacing of the fringes was 0.205 nm, which can be attributed to
the (111) plane of fcc NiFe alloy (Fig. 5). Elemental mapping
analysis for FeNi(0.5)/SiO2 sample based on HAADF-STEM-EDS
(Fig. 6(B)) displays a more uniform distribution of Fe than that
of Ni. The Fe and Ni distributions within an individual alloy
particle were indicated by STEM-EDS line-scanning spectra
(Fig. 6(D), (F) and (H)), which suggested that the Fe and Ni
Fig. 2 H2-TPR profiles of catalyst precursors. (a) Fe/SiO2, (b) Ni/SiO2,
and bimetallic FeNi/SiO2 with Fe/Ni weight ratios of (c) 0.0625 : 1, (d)
0.125 : 1, (e) 0.25 : 1, (f) 0.5 : 1, and (g) 1 : 1.
46430 | RSC Adv., 2014, 4, 46427–46436
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
Fig. 4 TEM images of reduced samples. (a) Ni/SiO2, bimetallic FeNi/SiO2 with Fe/Ni weight ratios of (b) 0.25 : 1, (c) 0.5 : 1, (d) 1 : 1 and (e) Fe/SiO2.
To obtain information about the catalyst surface, FeNi(0.5)/
SiO2 was also characterized by XPS. The surface Fe/Ni atomic
ratio determined from XPS spectrum was about 2.5, i.e., about
ve times the nominal bulk atomic Fe/Ni ratio (0.5). Thus, Fe
species were rich on the catalyst surface.
H2 uptakes of the reduced catalysts are displayed in Table 1.
H2 uptake of Ni/SiO2 was 12.5 mmol gꢀ1. Interestingly, although
the addition of Fe to Ni/SiO2 promoted the metal dispersion as
indicated by XRD and TEM results, it indeed reduced the H2
uptake. Moreover, with the increase in Fe content, H2 uptakes
drastically decreased and there was no apparent H2 chemi-
sorption for FeNi(x)/SiO2 (x > 0.125). That is, the addition of Fe
blocked the H2 adsorption on Ni sites. This is possibly due to
coverage of Ni sites with Fe species on the Fe-enriched surface,51
because Fe has a lower activity for H2 adsorption and activation.
Fig. 5 HRTEM image of FeNi/SiO2 with Fe/Ni weight ratio of 0.5 : 1.
Similar effects of Fe have been widely reported.44,47,52,53
The BET surface area and pore structure of reduced catalysts
are also listed in Table 1. The BET surface area decreased with
the increase of Fe content (x $ 0.125). Compared to the Ni/SiO2
sample, FeNi (0.125–0.5)/SiO2 had a larger surface area, which
distributions in the bimetallic particles varied in terms of the Fe
content. For the FeNi(0.25)/SiO2 sample, the M shape of the
line-scanning spectrum (0–15 nm) for Fe implied that Fe was
indicated that a suitable amount of Fe could improve the BET
enriched on the particle surface. However, with increasing Fe
surface of Ni/SiO2.
content (i.e., for the FeNi(0.5)/SiO2 and FeNi(1)/SiO2 samples),
both Ni and Fe tended to be homogenous distribution in the
single alloy particle. That is, Fe might be enriched on the
3.2 Catalytic reactivity
bimetallic particles at low Fe contents. This is also supported by
the following results, i.e., sharp decreases of the H2 chemi-
sorption (Table 1) and the C–C bond hydrogenolysis and
decarbonylation ability of Ni (Fig. 7) due to the addition of tiny
amount of Fe.
A control experiment lling the reactor tube with SiO2 quartz
sand was conducted under the same reaction conditions which
exhibited scarce conversion of methyl laurate. The main liquid
products in the deoxygenation of methyl laurate on mono-
metallic and bimetallic catalysts were n-undecane (C11), n-
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46427–46436 | 46431

View Article Online
RSC Advances
Paper
Fig. 6 STEM images and structural characterizations for FeNi/SiO2
with Fe/Ni weight ratios of (A), (E) 0.5 : 1, (C) 0.25 : 1 and (G) 1 : 1. (B):
EDS mapping of the square area in (A); (D), (F) and (H): EDS line-
scanning spectra for Fe–K (black) and Ni–K (red) along the red line in
(C), (E) and (G), respectively.
dodecane (C12), cracked hydrocarbons (C7–C10, including n-
heptane, n-octane, n-nonane and n-decane), and oxygenated
intermediates (dodecanol, dodecanal, dodecoic acid and lauryl
laurate). Among the oxygenated intermediates, dodecanol was
dominating (except the case of Fe/SiO2, in which both dodeca-
nol and lauryl laurate were major products). In addition, there
were very low amounts of isomerized hydrocarbons, alkenes
and cyclododecane. The reaction network is proposed in
Scheme 1 based on previous studies.54,55
3.2.1 Effect of Fe/Ni ratio. Fig. 7 displays the effect of the
Fe/Ni weight ratios on the catalyst reactivity. Fig. 7(a) indicated
that Fe/SiO2 had a lower activity for deoxygenation of methyl
laurate than Ni/SiO2. However, Fe presented a promoting effect
on the activity of Ni/SiO2. The FeNi/SiO2 catalysts with Fe/Ni
ratio #0.5 gave a higher conversion than Ni/SiO2. As the Fe
Fig. 7 Performance of deoxygenation of methyl laurate as a function
of temperature and Fe/Ni weight ratios. Other products include
dodecanal, dodecoic acid, cyclododecane and lauryl laurate. Reaction
conditions: 3 MPa H2, WHSV of 10 hꢀ1, H2/methyl laurate molar ratio
of 25.
46432 | RSC Adv., 2014, 4, 46427–46436
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
the eta-2-(C,O) species is more stable on NiFe alloy than on
monometallic Ni.25 From ESR spectra (Fig. S2†), we also found
the existence of Fe–O–Fe species with ferri-, ferro-, and/or
antiferromagnetic behavior on FeNi(0.25)/SiO2,56 in agreement
with the other observations.16,57–59 The oxygen vacancies on the
iron oxide can bind oxygen atom in the C]O group via the
donation of the lone pair of electrons and subsequently the
C]O bond is activated.21,32–34,60,61 The strong oxygen affinity of
Fe and the role of oxygen vacancies in Fe oxide can account for
the dominating HDO pathway on Fe/SiO2 because the activated
C]O group via O adsorption on Fe site can be readily hydro-
genated to dodecanol and subsequently dehydrated/hydroge-
nated to produce C12 hydrocarbon. For bimetallic FeNi/SiO2,
the oxygen in C]O group is more preferentially adsorbed on Fe
sites (either metallic Fe atoms or Fe oxides) than the Ni ones.
The Ni and Fe sites could be synergetic in promoting the
conversion of methyl laurate. In other words, H2 is easily acti-
vated and dissociated on Ni sites and spills over to the neigh-
bouring Fe sites that adsorb oxygen atom of C]O group of
methyl laurate, where the hydrogenation takes place leading to
the HDO product. The process is proposed in Scheme 2. As a
result, the synergism between Ni and Fe sites promotes not only
the conversion of methyl laurate but also the HDO pathway (i.e.,
formation of C12). Further, an interesting phenomenon is
noteworthy as shown in Fig. 7(b), i.e., the selectivity to dodec-
Scheme 1 Reaction network of deoxygenation of methyl laurate.
content increased, the conversion exhibited a slight volcano
curve and reached the maximum at the Fe/Ni ratio of 0.25. With
the Fe/Ni ratio increasing from 0 to 0.25, the conversion
increased from 53.7% to 65% at 573 K and from 78% to 87% at
593 K. However, the conversion decreased as the Fe/Ni ratio
exceeded 0.25. Clearly, suitable Fe contents were needed for
enhancing the activity of Ni/SiO2. Furthermore, the addition of
Fe to Ni/SiO2 remarkably changed the product distribution. As
depicted in the Fig. 7(b) and (c), the main hydrogenated prod-
ucts on Ni/SiO2 and Fe/SiO2 were C11 and C12 hydrocarbons,
respectively. That is, the deoxygenation of methyl laurate on Ni/
SiO2 and Fe/SiO2 mainly occurred via decarbonylation and HDO
pathways, respectively. The addition of Fe to Ni/SiO2 reduced
the selectivity to C11(SC11) but enhanced the selectivity to
C12(SC12). This is more obviously reected by the molar ratio
between C11 and C12 hydrocarbons (C11/C12 ratio), which
represents the selectivity between decarbonylation and HDO
pathways. As shown in (Fig. 7(c)), the C11/C12 ratio sharply
decreased as the Fe/Ni ratio increased and then almost did not
change when the Fe/Ni ratio was larger than 0.25. Besides, the
addition of Fe to Ni/SiO2 restrained the formation of cracked
products (see Fig. 7(b)). The larger the Fe/Ni ratio was, the less
the cracked products formed, which was more obvious at high
temperature. At 633 K, for instance, the selectivity to cracked
products (C7–C10) was 67% on Ni/SiO2, while it was only 2% on
FeNi(0.25)/SiO2. The reduction of the selectivity to cracked
products is very signicant in practice because of enhancing the
carbon yield as well as reducing H2 consumption. In short, Fe
made a signicant effect on the performance of Ni/SiO2. This is
mainly related to the formation of NiFe alloy.
anol (Sdodecanol
) increased with the Fe content. This is
undoubtedly attributable to the promoting role of Fe for the
HDO pathway since dodecanol is an intermediate in the HDO
pathway. The promotional effect on the HDO pathway due to
the increase in oxygen affinity of metal sites has also been found
in the metal phosphide and sulde catalysts.55,62 As indicated by
As indicated in Section 3.1.2, Fe was enriched on the surface
of the NiFe alloy particles, especially at low Fe contents, and also
restrained the H2 chemisorption. Moreover, Fe/SiO2 showed a
lower activity than Ni/SiO2 for deoxygenation of methyl laurate.
Surprisingly, the activity of Ni/SiO2 was enhanced when a suit-
able amount of Fe (Fe/Ni weight ratio #0.5) was added. We
suggest that this promoting effect results from the synergism
between Ni and Fe species.
It is known that Ni has higher H2 adsorption and activation
abilities but a weaker oxygen affinity than Fe.16,31 Based on the
DFT calculations, it has been found that the metal–oxygen
affinity follows the sequence of Fe > NiFe alloy > Ni,16 and that
Scheme 2 Possible reaction mechanism for the HDO pathway in the
deoxygenation of methyl laurate on FeNi/SiO2, (a) hydrogenolysis to
dodecoic acid; (b) reduction to dodecanal; (c) reduction to dodecanol;
(d) dehydration and then hydrogenation to n-C12H26; R denotes n-
C11H23
.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46427–46436 | 46433

View Article Online
RSC Advances
Paper
Fig. 7(a), to reach maximum synergism for the catalyst activity, hꢀ1, the conversion and the selectivity to C11 (SC11) decreased
the Fe/Ni ratio should be 0.25. When the Fe/Ni ratio exceeds from 98% to 43% and from 71% to 35%, respectively, while the
0.25, the excess Fe covers the Ni sites, leading to a decrease in selectivity to C12 (SC12) did not change. The C11/C12 ratio ten-
the hydrogen-activating ability and subsequently the activity,63 ded to decrease, indicating that HDO pathway was promoted by
such as the case for FeNi(1)/SiO2.
increasing WHSV. This is more obviously reected by the C11/
As mentioned above, the selectivity to cracked hydrocarbons C12* ratio (C12* includes C12 and dodecanol) as dodecanol is
was remarkably reduced by the addition of Fe to Ni/SiO2. This is an intermediate in the HDO pathway. In addition, the selectivity
also related to the formation of NiFe alloy and the Fe rich to dodecanol (Sdodecanol) increased from 0 to 30%. The increase
surface of NiFe alloy particles. It is well accepted that the C–C of WHSV corresponds to the decrease of the contact time of
hydrogenolysis requires consecutive Ni sites.64 However, the reactants on the active sites, which reduces the conversion of
formation of NiFe alloy with Fe enriched surface leads to the methyl laurate as well as intermediates (for instance, dodeca-
disruption of the adjacent Ni atoms, thus the C–C hydro- nol). The result also indicates that the dehydration of dodecanol
genolysis was suppressed. Similarly, Cu has also been found may be the rate-determining step during the deoxygenation of
decreasing the C–C hydrogenolysis activity of Ni due to geom- methyl laurate.
etry effect.20,65
The effect of H2 pressure on the performance of FeNi(0.25)/
3.2.2 Effect of reaction conditions. Fig. 7 also shows the SiO2 is shown in Fig. 9. With the pressure increasing from 2 to 5
effect of temperature on the performance of different catalysts. MPa, the conversion and the selectivity to C11 and C12 tended
For each catalyst, increasing temperature promoted the to slightly decrease, while the selectivity to dodecanol increased
conversion of methyl laurate due to the increased reaction rate. from 10% to 22%. The C11/C12 ratio increased with the rise of
As the temperature rose from 573 to 633 K, the total selectivity to pressure. This seems to indicate that the increase of H2 pressure
C11 and C12 rstly increased and then decreased for Ni/SiO2 favors the decarbonylation pathway. In fact, since dodecanol
and FeNi(x)/SiO2 (x ¼ 0.0625 and 0.125), while it always was an intermediate during the HDO pathway, the C11/C12*
increased for FeNi(x)/SiO2(x ¼ 0.25–1). At high temperature, the might be more reasonable to reect the selectivity between the
decreased selectivity to C11 and C12 is ascribed to the promoted decarbonylation and HDO pathways, and it gradually decreased
cracking. It is noteworthy that the effect of temperature on the with the increasing pressure. This suggests that high pressure
selectivity to cracked products is more pronounced for Ni/SiO2 favors the HDO pathway, which is probably due to the increased
in comparison with FeNi(x)/SiO2. This further conrms that the hydrogen concentration on the catalyst surface.
formation of NiFe alloy is vital for the performance of FeNi(x)/
3.2.3 Catalyst stability. Stability is an important property of
SiO2. In addition, Sdodecanol decreased with the rise of temper- catalyst. Fig. 10 shows the conversion, total selectivity to C11
ature because high temperature favored the dehydration of and C12 (SC11+C12) and C11/C12 ratio on Ni/SiO2 and FeNi(0.25)/
dodecanol. As dodecanol was detected as the main intermediate SiO2 as a function of time. For FeNi(0.25)/SiO2, the initial
on FeNi(x)/SiO2(x $ 0.125), its dehydration might be a rate- conversion was nearly 100% and decreased to 95% at the 61st
determining step during the deoxygenation of methyl laurate.
Since FeNi(0.25)/SiO2 exhibited good deoxygenation ning and decreased to 87% at the 61st hour. Clearly, both Ni/
performance, the effects of WHSV and H2 pressure on its SiO2 and FeNi(0.25)/SiO2 deactivated but they had similar
performance were investigated. stability during 61 hours. SC11+C12 on FeNi(0.25)/SiO2
hour. For Ni/SiO2, the conversion was about 94% at the begin-
a
Fig. 8 displays the effect of WHSV of methyl laurate on the maintained above 98% during 61 hours, much larger than that
performance of FeNi(0.25)/SiO2. As the WHSV rose from 3 to 20
Fig. 8 Deoxygenation of methyl laurate on FeNi/SiO2 with Fe/Ni Fig. 9 Deoxygenation of methyl laurate on FeNi/SiO2 with Fe/Ni
weight ratio of 0.25 : 1 as a function of WHSV. Reaction conditions: weight ratio of 0.25 : 1 as a function of H2 pressure. Reaction condi-
573 K, 3 MPa H2, H2/methyl laurate molar ratio of 25.
tions: 573 K, WHSV of 10 hꢀ1, H2/methyl laurate molar ratio of 25.
46434 | RSC Adv., 2014, 4, 46427–46436
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
RSC Advances
through HDO pathway. Among the catalysts tested, FeNi(0.25)/
SiO2 exhibited the best performance. We also investigated the
effect of reaction conditions on the catalysts performance. What
is worth mentioning here is that as the temperature increased,
the cracking and decarbonylation pathway were promoted more
remarkably on Ni/SiO2 than on FeNi/SiO2. This further indicates
that there is an essential difference in structure and perfor-
mance between Ni/SiO2 and FeNi/SiO2. On the whole, the
increase of temperature and the deceases of WHSV and H2
pressure enhanced the catalyst activity while promoted the
decarbonylation pathway rather than the HDO one. Under the
condition of 613 K, 3 MPa, WHSV of 10 hꢀ1 and H2/methyl
laurate molar ratio of 25, FeNi(0.25)/SiO2 had comparable
stability to Ni/SiO2 during 61 h. Aer reaction for 61 h,
FeNi(0.25)/SiO2 gave the conversion of 94% and the selectivity to
C11 and C12 of 98%, much higher than those on Ni/SiO2. Our
nding may provide important information for the rational
design of Ni-based catalysts for the production of green diesel
with high carbon yield.
Fig. 10 Conversion, selectivity to C11 and C12 and C11/C12 ratio on
Ni/SiO2 and FeNi/SiO2 with Fe/Ni weight ratio of 0.25 : 1 as the
function of time. Reaction conditions: 613 K, 3 MPa H2, WHSV of 10
h
ꢀ1, H2/methyl laurate molar ratio of 25.
Acknowledgements
The authors gratefully acknowledge the supports from the
National Natural Science Foundation of China (no. 21176177),
the Natural Science Foundation of Tianjin (no. 12JCYBJC132 00)
and the Program of Introducing Talents to the University
Disciplines (B06006).
(70–80%) on Ni/SiO2. This is due to the higher selectivity to
cracked products on Ni/SiO2 (see Fig. S2†). In addition, with the
extension of time, the C11/C12 ratio steadily increased from 1.8
to 7.8 on FeNi(0.25)/SiO2, while it uctuated between 40 and 51
on Ni/SiO2. On the whole, compared with Ni/SiO2, FeNi(0.25)/
SiO2 gave a higher activity and selectivity to target hydrocarbons
as well as similar stability.
To obtain the properties of catalyst aer reaction, the spent
catalysts were characterized. From the metal particle sizes
before and aer reaction for 61 h (see Table 1, Fig. S3 and S4†),
it was found that the sintering of metal particles on Ni/SiO2 was
more serious than that on FeNi(0.25)/SiO2. That is, the addition
of Fe inhibited the growth of Ni particles. TGA (Fig. S5†) showed
that a little more amount of coke formed on FeNi(0.25)/
SiO2(8.9%) than on Ni/SiO2(6.3%). We speculate that the
FeNi(0.25)/SiO2 deactivation is mainly attributed to coke, while
the Ni/SiO2 deactivation is also derived from the sintering of Ni
particles apart from coke. The BET surface areas of spent
catalysts were displayed in Table 1. Compared to that of the
fresh samples, spent catalysts had a much lower BET surface
area, which could mainly be ascribed to the formation of coke.
Notes and references
1 A. Corma, G. W. Huber, L. Sauvanaud and P. O'connor, J.
Catal., 2007, 247, 307–327.
¨
2 S. Lestari, P. Maki-Arvela, J. Beltramini, G. Q. Lu and
D. Y. Murzin, ChemSusChem, 2009, 2, 1109–1119.
3 E. Santillan-Jimenez, T. Morgan, J. Lacny, S. Mohapatra and
M. Crocker, Fuel, 2013, 103, 1010–1017.
4 H. Zuo, Q. Liu, T. Wang, L. Ma, Q. Zhang and Q. Zhang,
Energy Fuels, 2012, 26, 3747–3755.
5 K. Murata, Y. Liu, M. Inaba and I. Takahara, Energy Fuels,
2010, 24, 2404–2409.
¨
6 I. Simakova, O. Simakova, P. Maki-Arvela, A. Simakov,
M. Estrada and D. Y. Murzin, Appl. Catal., A, 2009, 355,
100–108.
7 L. He, C. Wu, H. Cheng, Y. Yu and F. Zhao, Catal. Sci.
Technol., 2012, 2, 1328–1331.
4 Conclusions
8 M. Chiappero, P. T. M. Do, S. Crossley, L. L. Lobban and
D. E. Resasco, Fuel, 2011, 90, 1155–1165.
9 B. Veriansyah, J. Y. Han, S. K. Kim, S. A. Hong, Y. J. Kim,
J. S. Lim, Y. W. Shu, S. G. Oh and J. Kim, Fuel, 2012, 94,
578–585.
In the present work, we provided an approach to improving
activity and tuning the production distribution by addition of Fe
to Ni/SiO2, and proposed that the NiFe alloy particles, especially
with Fe-enriched surface at low Fe loading and the oxygen
vacancy in FeOx contributed to the special performance of FeNi/ 10 E. Furimsky, Catal. Today, 2013, 217, 13–56.
SiO2. The geometric effect of Fe signicantly restrained the C–C 11 E. Laurent and B. Delmon, J. Catal., 1994, 146, 281–291.
hydrogenolysis activity of Ni/SiO2. On the surface of NiFe alloy 12 Y. Yoshimura, T. Sato, H. Shimada, N. Matsubayashi and
particles, the Ni and Fe species, which respectively possessed a
A. Nishijima, Appl. Catal., 1991, 73, 55–63.
high ability to activate hydrogen and a strong oxygen affinity, 13 O. Senol, T. R. Viljava and A. O. I. Krause, Catal. Today, 2005,
were synergetic in promoting the conversion of methyl laurate
106, 186–189.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 46427–46436 | 46435

View Article Online
RSC Advances
Paper
14 A. Y. Bunch, X. Wang and U. S. Ozkan, J. Mol. Catal. A: Chem., 38 X. Gao, J. Shen, Y. Hsia and Y. Chen, J. Chem. Soc., Faraday
2007, 270, 264–272. Trans., 1993, 89, 1079–1084.
15 D. Kubicka and J. Horacek, Appl. Catal., A, 2011, 394, 9–17. 39 X. Ge, M. Li and J. Shen, J. Solid State Chem., 2001, 161, 38–
ˇ
´ˇ
16 L. Nie, P. M. de Souza, F. B. Noronha, W. An, T. Sooknoi and
D. E. Resasco, J. Mol. Catal. A: Chem., 2014, 388, 47–55.
17 D. E. Resasco, J. Phys. Chem. Lett., 2011, 2, 2294–2295.
44.
40 E. E. Unmuth, L. H. Schwartz and J. B. Butt, J. Catal., 1980,
61, 242–255.
18 J. H. Sinfelt, Bimetallic catalysts: discoveries, concepts, and 41 W. Gao, C. Li, H. Chen, M. Wu, S. He, M. Wei, D. G. Evans
applications, Wiley, New York, 1983. and X. Duan, Green Chem., 2014, 16, 1560–1568.
19 S. Zafeiratos, S. Piccinin and D. Teschner, Catal. Sci. 42 T. Ishihara, K. Eguchi and H. Arai, Appl. Catal., 1987, 30,
Technol., 2012, 2, 1787–1801. 225–238.
20 S. A. Khromova, A. A. Smirnov, O. A. Bulavchenko, 43 C. R. F. Lund and J. A. Dumesic, J. Catal., 1981, 72, 21–30.
A. A. Saraev, V. V. Kaichev, S. I. Reshetnikov and 44 D. Li, M. Koike, L. Wang, Y. Nakagawa, Y. Xu and
V. A. Yakovlev, Appl. Catal., A, 2014, 470, 261–270.
21 E. Grootendorst, R. Pestman, R. Koster and V. Ponec, J. 45 L. Wang, D. Li, M. Koike, S. Koso, Y. Nakagawa, Y. Xu and
Catal., 1994, 148, 261–269. K. Tomishige, Appl. Catal., A, 2011, 392, 248–255.
22 K. Kandel, J. W. Anderegg, N. C. Nelson, U. Chaudhary and 46 K. Gheisari, S. Javadpour, J. T. Oh and M. Ghaffari, J. Alloys
I. I. Slowing, J. Catal., 2014, 314, 142–148. Compd., 2009, 472, 416–420.
K. Tomishige, ChemSusChem, 2014, 7, 510–522.
23 R. Olcese, M. Bettahar, D. Petitjean, B. Malaman, 47 D. Pandey and G. Deo, J. Mol. Catal. A: Chem., 2014, 382, 23.
F. Giovanella and A. Dufour, Appl. Catal., B, 2012, 115–116, 48 J. H. Sinfelt, Acc. Chem. Res., 1977, 10, 15–20.
63–73.
49 E. K. Parks, K. P. Kerns and S. J. Riley, Chem. Phys., 2000, 262,
151–167.
50 C. Kittel, Introduction to Solid State Physics, Wiley, New York,
5th edn, 1976.
51 T. Mizushima, K. Tohji, Y. Udagawa, M. Harada, M. Ishikawa
and A. Ueno, J. Catal., 1988, 112, 282–289.
24 Y. Ren, M. Yan, J. Wang, Z. C. Zhang and K. Yao, Angew.
Chem., Int. Ed., 2013, 52, 12674–12678.
25 S. Sitthisa, W. An and D. E. Resasco, J. Catal., 2011, 284, 90–
101.
26 S. Leng, X. Wang, X. He, L. Liu, Y. E. Liu, X. Zhong,
G. Zhuang and J. G. Wang, Catal. Commun., 2013, 41, 34–37. 52 H. Tanaka, R. Kaino, K. Okumura, T. Kizuka, Y. Nakagawa
27 T. Kimura, T. Miyazawa, J. Nishikawa, S. Kado, K. Okumura, and K. Tomishige, Appl. Catal., A, 2010, 378, 175–186.
T. Miyao, N. Shuichi, K. Kimio and K. Tomishige, Appl. 53 L. Guczi, Catal. Rev.: Sci. Eng., 1981, 23, 329–376.
Catal., B, 2006, 68, 160–170.
54 Y. Yang, J. Chen and H. Shi, Energy Fuels, 2013, 27, 3400–
28 B. X. Peng, X. G. Yuan, C. Zhao and J. A. Lercher, J. Am. Chem.
Soc., 2012, 134, 9400–9405.
3409.
55 J. Chen, H. Shi, L. Li and K. Li, Appl. Catal., B, 2014, 144, 870–
884.
˜´
29 V. M. Gonzalez-DelaCruz, J. P. Holgado, R. Perenıguez and
A. Caballero, J. Catal., 2008, 257, 307–314.
30 V. A. Yakovlev, S. A. Khromova, O. V. Sherstyuk,
V. O. Dundich, D. Yu. Ermakov, V. M. Novopashina,
56 P. Decyk, M. Trejda, M. Ziolek, J. Kujawa, K. Głaszczka,
M. Bettahar, S. Monteverdi and M. Mercy, J. Catal., 2003,
219, 146–155.
M. Yu. Lebedev, O. Bulavchenko and V. N. Parmon, Catal. 57 G. Connell and J. Dumesic, J. Catal., 1987, 105, 285–298.
Today, 2009, 144, 362–366.
31 T. B. Reed, Free energy of formation of binary compounds, MIT
press, 1971.
58 C. N. R. Rao, G. U. Kulkarni, K. R. Kannan and S. Chaturvedi,
J. Phys. Chem., 1992, 96, 7379–7385.
59 G. B. Raupp and W. N. Delgass, J. Catal., 1979, 58, 337–347.
32 R. Pestman, R. Koster, J. Pieterse and V. Ponec, J. Catal., 60 W. Rachmady, J. Catal., 2002, 208, 158–169.
1997, 168, 255–264.
61 W. Rachmady and M. A. Vannice, J. Catal., 2002, 209, 87–98.
33 R. Pestman, R. Koster, E. Boellaard, A. Van der Kraan and 62 C. Dupont, R. Lemeur, A. Daudin and P. Raybaud, J. Catal.,
V. Ponec, J. Catal., 1998, 174, 142–152.
34 T. Yokoyama and N. Yamagata, Appl. Catal., A, 2001, 221, 63 T. Marinelli, S. Nabuurs and V. Ponec, J. Catal., 1995, 151,
227–239. 431–438.
35 R. J. Matyi, L. H. Schwartz and J. B. Butt, Catal. Rev.: Sci. Eng., 64 J. H. Sinfelt, J. L. Carter and D. J. C. Yates, J. Catal., 1972, 24,
2011, 279, 276–286.
1987, 29, 41.
283–296.
36 J. Ashok and S. Kawi, ACS Catal., 2014, 4, 89–301.
37 B. Mile, D. Stirling, M. A. Zammitt, A. Lovell and M. Webb, J.
Catal., 1988, 114, 217–229.
65 I. Gandarias, J. Requies, P. L. Arias, U. Armbruster and
A. Martin, J. Catal., 2012, 290, 79–89.
46436 | RSC Adv., 2014, 4, 46427–46436
This journal is © The Royal Society of Chemistry 2014