P. Kumar et al. / Applied Catalysis A: General 471 (2014) 28–38
29
Extensive works have been published in the past on deoxy-
genation of several variety of fatty acids such as dodecanoic acid
Nomenclature
[
[
14], oleic acid [15,16], linoleic acid [14,16,17], lauric acid [18], SA
16,19], C17-C20 and C22 fatty acids [20]. Several research articles
HDO
hydrodeoxygenation
HEPD
HEXD
NiAl
n-heptadecane
n-hexadecane
Ni/␥-Al O
have also been published on deoxygenation of esters of various fatty
acids such as methyl octanoate [21], methyl and ethyl heptanoate
[
heptanoate [24]. Various metals (Pd, Ni, Ru, Pt, Ir, Os, Mo, Co, Cu,
and Rh) supported on carbon and numerous metal oxides (CaO-
MgO, MgO-Al O , ZrO -CeO , Al O , ZrO , CeO , SiO , carbon, and
SBA-15) were employed as catalyst for deoxygenation of fatty acids
and their esters [17–19,26–28]. The Pd/C catalysts displayed very
good catalytic activity for deoxygenation of fatty acids and their
esters [26].
2
3
22,23], methyl heptanoate [24,25], methyl hexanoate, and methyl
OCTD
n-octadecane
OCTDL l-octadecanol
PEND
SA
n-pentadecane
stearic acid
2
3
2
2
2
3
2
2
2
5
1
1
2
1
1
NiAl
5 wt.% Ni/␥-Al O3
2
0NiAl 10 wt.% Ni/␥-Al O3
5NiAl 15 wt.% Ni/␥-Al O3
5NiAl 25 wt.% Ni/␥-Al O3
0NiSi 10 wt.% Ni/SiO2
2
2
2
However, only a few studies were reported on deoxygenation of
SA. Snare et al. first reported catalyst/support screening for deox-
ygenation of SA at 573 K and 6 bars of He pressure in a semi-batch
reactor and Pd/C was reported to be the best catalyst [26]. Lestari
et al. reported deoxygenation of SA over palladium catalysts sup-
ported on nanocomposite carbon Sibunit in a semi-batch reactor
in the temperature range of 543–573 K using 17 bars of helium
pressure and n-heptadecane (HEPD) and n-pentadecane (PEND)
were reported as products [29]. Lestari et al. latter extended the
work using palladium nanoparticles anchored in SBA-15 as cata-
lyst in dodecane solvent at 573 K and 17 bars of 5 vol.% H2 in argon
pressure in a semi-batch reactor [19]. HEPD and trace amounts of
isomers of n-heptadecenes were reported as products. Simakova
et al. studied deoxygenation of mixture of palmitic and SA (59 mol%
palmitic and 40 mol% SA) in presence of palladium supported on
synthetic carbon (Sibunit) in the temperature range of 533–573 K
at 17.5 bars of 5 vol.% hydrogen in helium pressure in a semi-batch
reactor using n-dodecane as solvent and HEPD and PEND were
reported as products [30]. Ping et al. reported catalytic decarboxyl-
ation of SA over palladium nanoparticles supported on ultra-porous
silica mesocellular foam at 573 K under nitrogen atmosphere in
batch reactor [31]. Immer et al. reported liquid phase deoxygen-
ation of SA using 5 wt.% Pd/C catalysts using on-line quadrupole
mass spectrometry under He or 10% H2 [16]. Berenblyum et al.
reported catalytic conversion of SA over palladium supported on
alumina at 623 K and 6–14 bars of hydrogen pressure in a stirred
autoclave [32]. HEPD was reported as main product with dihep-
tadecylketone as by-product. It was reported that conversion of SA
remained almost unaffected beyond 8 bars of hydrogen pressure.
It is evident from above discussion that HDO of fatty acids and
vegetables oils have been carried out over expensive and less abun-
dance novel metal catalysts. Extremely high cost and insufficient
abundance of noble metals catalysts are one of the serious con-
straints for probable large scale applications. Thus development
of relatively active and inexpensive metal catalysts such as nickel
0NiZSM 10 wt.% Ni/HZSM-5
synthesis gas with enhanced hydrogen content; economics how-
ever favors use of hydrocarbons and inexpensive coal. The fast
pyrolysis is another propitious technology for thermal disintegra-
tion of biomass in absence of oxygen or in presence of significantly
less oxygen required for complete combustion in the temperature
range of 623–773 K to produce liquid products normally known
as bio-oil. The bio-oils is however incompatible for direct use
as fuel because of its high oxygen content (low calorific value)
and viscosity, immiscibility with petroleum derived fuels, partial
phase separation, and long term storage instability. The bio-ethanol
[
5,6] and bio-butanol [7,8] produced by microbial fermentation of
sugars derived from sugarcane, sugar beet, and corn or lignocellu-
losic biomass and biodiesel [9] manufactured by transesterification
of triglycerides with methanol using NaOH as catalyst are other
promising bio-fuels. However, properties of these oxygenated bio-
fuels especially bio-ethanol and bio-diesel allows for blending with
petroleum derived fuels to limited extent only. Therefore, there is a
strong need to pursue alternative hydrocarbon analogous bio-fuels
fully compatible with present internal combustion engines.
The triglycerides (vegetable oils, animal fats, waste cooking
oils, and microalgal oils) are promising renewable feedstock for
production of hydrocarbon fuels because of its low functionality
and oxygen content compared to cellulosic biomass. Moreover,
the triglycerides are composed of linear C –C24 fatty acids with
8
majority of C16 and C18 fatty acids [10]. Therefore, removal of oxy-
gen heteroatoms from triglycerides provides hydrocarbon fuels
resembling petroleum derived fuels in the range of gasoline, diesel,
and jet fuels generally known as green-gasoline, green-diesel, and
green-jet fuels respectively. Significant research efforts have thus
been devoted in the past on thermal and catalytic cracking of
triglycerides to produce hydrocarbon fuels [11]. However, ther-
mal and catalytic cracking of triglycerides suffers from drawback
of low yield of liquid fuels and huge loss of carbon in the form of
gaseous products. The hydrodeoxygenation (HDO), similar to exist-
ing hydrotreatment technology of petroleum refinery, is another
promising technology for elimination of oxygen heteroatoms from
triglycerides and fatty acids in the form of water to produce liquid
hydrocarbon fuels with high yield. The possibility of co-processing
and use of existing petroleum refinery infrastructures are addi-
tional advantages of this technology. Recognizing importance, the
present work was commenced on HDO of stearic acid (SA) as model
compound. The triglycerides are commercially hydrolyzed to corre-
sponding fatty acids and glycerol either in absence of any catalysts
is desirable for HDO applications. The performance of Ni/␥-Al O3
2
catalysts of varying nickel loading was therefore investigated in the
present work for HDO of SA. In order to understand the effects of
support and acidity of the catalysts, nickel catalysts supported on
silica (SiO ) and zeolite (ZSM-5) were also prepared and tested for
2
HDO of SA. Moreover, detailed reaction mechanism of HDO of SA
was proposed based on products distribution observed for differ-
ent catalysts. An empirical kinetic model was developed based on
proposed mechanism to correlate experimental data.
2. Experimental
(
at 483 K and high pressure) or in presence of small amounts of
2.1. Chemicals
sulfuric acid or, more usually, zinc oxide (423 K) [12]. Following
hydrolysis, the fatty acids mixture separated by distillation could
be used as feedstock for production of green-diesel [13].
Nickel (II) nitrate hexahydrate [Ni(NO ) ·6H O, extra pure,
3
2
2
≥97%], SA (SG, ≥97%), and carbon tetrachloride (purity 99.8%) were