Kinetics of hydrodeoxygenation of octanol over supported nickel catalysts: A mechanistic study

Article abstract of DOI:10.1039/c4ra06826b

The hydrodeoxygenation (HDO) of 1-octanol as a model aliphatic alcohol of bio-oil was investigated in a continuous down-flow fixed-bed reactor over γ-Al₂O₃, SiO₂, and HZSM-5 supported nickel catalysts in the temperature range of 488-533 K. The supported nickel catalysts were prepared by incipient wetness impregnation method and characterized by BET, XRD, TPR, TPD, H₂pulse chemisorption, and UV-vis spectroscopy. Characterization of supported nickel (or nickel oxide) catalysts revealed existence of dispersed as well as bulk nickel (or nickel oxide) depending on the extent of nickel loading and the nature of the support. The acidity of γ-Al₂O₃supported nickel catalysts decreased with increasing the nickel loading on γ-Al₂O₃. n-Heptane, n-octane, di-n-octyl ether, 1-octanal, isomers of heptene and octene, tetradecane, and hexadecane were identified as products of HDO of 1-octanol. The C₇hydrocarbons were observed as primary products for catalysts with active metal sites such as γ-Al₂O₃and SiO₂supported nickel catalysts. However, C₈hydrocarbons were primarily formed over acidic catalysts such as pure HZSM-5 and HZSM-5 supported nickel catalyst. The 1-octanol conversion increased with increasing nickel loading on γ-Al₂O₃, and temperature and decreasing pressure and WHSV. The selectivity to products was strongly influenced by temperature, nickel loading on γ-Al₂O₃, pressure, and types of carrier gases (nitrogen and hydrogen). The selectivity to C₇hydrocarbons was favoured over catalysts with increased nickel loading on γ-Al₂O₃at elevated temperature and lower pressure. A comprehensive reaction mechanism of HDO of 1-octanol was delineated based on product distribution under various process conditions over different catalysts. This journal is

Full text of DOI:10.1039/c4ra06826b

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Kinetics of hydrodeoxygenation of octanol over
supported nickel catalysts: a mechanistic study†
Cite this: RSC Adv., 2014, 4, 41612
*
Venkata Chandra Sekhar Palla, Debaprasad Shee and Sunil K. Maity
The hydrodeoxygenation (HDO) of 1-octanol as a model aliphatic alcohol of bio-oil was investigated in a
continuous down-flow fixed-bed reactor over g-Al2O3, SiO2, and HZSM-5 supported nickel catalysts in
the temperature range of 488–533 K. The supported nickel catalysts were prepared by incipient wetness
impregnation method and characterized by BET, XRD, TPR, TPD, H2 pulse chemisorption, and UV-vis
spectroscopy. Characterization of supported nickel (or nickel oxide) catalysts revealed existence of
dispersed as well as bulk nickel (or nickel oxide) depending on the extent of nickel loading and the
nature of the support. The acidity of g-Al2O3 supported nickel catalysts decreased with increasing the
nickel loading on g-Al2O3. n-Heptane, n-octane, di-n-octyl ether, 1-octanal, isomers of heptene and
octene, tetradecane, and hexadecane were identified as products of HDO of 1-octanol. The C7
hydrocarbons were observed as primary products for catalysts with active metal sites such as g-Al2O3
and SiO2 supported nickel catalysts. However, C8 hydrocarbons were primarily formed over acidic
catalysts such as pure HZSM-5 and HZSM-5 supported nickel catalyst. The 1-octanol conversion
increased with increasing nickel loading on g-Al2O3, and temperature and decreasing pressure and
WHSV. The selectivity to products was strongly influenced by temperature, nickel loading on g-Al2O3,
pressure, and types of carrier gases (nitrogen and hydrogen). The selectivity to C7 hydrocarbons was
favoured over catalysts with increased nickel loading on g-Al2O3 at elevated temperature and lower
pressure. A comprehensive reaction mechanism of HDO of 1-octanol was delineated based on product
distribution under various process conditions over different catalysts.
Received 8th July 2014
Accepted 21st August 2014
DOI: 10.1039/c4ra06826b
www.rsc.org/advances
fossil fuels, has enormous potential to deliver all forms of
energy (heat, electricity, and transportation fuels) as well as
1 Introduction
basic organic chemicals in an integrated biorenery.
At present our society is heavily dependent on nite fossil fuels
to meet the growing demands of energy and organic chemicals.
Currently, more than 80% of energy and about 90% of organic
chemicals of the world are derived from fossil fuels alone.1 The
continuous decline of fossil fuels reserves, increase of fuel pri-
ces, and deterioration of environmental cleanliness due to large
scale usage of fossil fuels have stimulated intense research
worldwide to pursue carbon neutral renewable sources of
energy and organic chemicals to reduce reliance on limited
fossil fuels. Several renewable energies including hydro, solar,
wind, geothermal, and biomass were thus developed in the past
to alleviate the fossil fuel crisis. However, with the exception of
biomass, all these renewable energies can only full the global
energy demands to some extent in the form of heat and elec-
tricity only. On the other hand, biomass, being the origin of
Several biomass conversion technologies including gasi-
cation, fast pyrolysis, transesterication, and fermentation have
thus been progressed considerably to full goals of integrated
biorenery. The fast pyrolysis is one such promising technology
for thermochemical conversion of world's most abundant
lignocellulosic biomass directly into liquid products commonly
known as bio-oil.2,3 Low capital investments and economic
viability at small scale make fast pyrolysis an attractive choice
for decentralized biorenery. The bio-oil, being a mixture of
more than 300 identied oxygenated hydrocarbons, could be
considered as a potential feedstock for basic organic chemicals
as well as transportation fuels.4 However, separation of the
compounds of very low concentration from mixtures of large
number of chemical compounds of many classes is practically
impossible and hence bio-oil is inappropriate as feedstock for
organic chemicals. The bio-oil is also unsuitable for direct usage
as transportation fuels in internal combustion engines because
of its high water and oxygen contents, immiscibility with
petroleum fuels, low heating value (ꢀ40–45% of hydrocarbon
fuels), poor storage stability due to unsaturated compounds,
and high corrosiveness due to organic acids mainly acetic and
Department of Chemical Engineering, Indian Institute of Technology Hyderabad,
Ordnance Factory Estate, Yeddumailaram 502205, Medak, Telengana, India. E-mail:
dshee@iith.ac.in; Fax: +91 40 2301 6003; Tel: +91 40 2301 6109
† Electronic supplementary information (ESI) available: HDO of 1-octanal,
1-octanol, 1-heptanol and 1-hexanol; Effects of H2 ow rate on HDO of
1-octanol; vapour pressure of pure components; GCMS chromatogram of liquid
sample HDO of 1-octanol, 1-hexanol and 1-heptanol. See DOI: 10.1039/c4ra06826b
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formic acid.5–7 The removal of oxygen from bio-oils is thus hexahydrate (Ni(NO3)2$6H2O, Merck, >97% purity) as precursor.
indispensable for production of liquid transportation fuels. The nickel precursor solution was rst prepared by dissolving
The hydrodeoxygenation (HDO) is a propitious technology measured amount of precursor corresponding to desired nickel
for removal of oxygen from bio-oils thereby increasing volatility loading in distilled water of volume equal to or slight excess of
and thermal stability and reducing viscosity.8 Knowledge of total pore volume of the support. The precursor solution was
reaction mechanism plays crucial role for design of suitable then added to the support and resultant mixture was agitated
catalysts for cost-effective HDO of bio-oils. Understanding continuously for 2 h for uniform distribution of metal precursor
mechanism of HDO of whole bio-oils is however highly chal- solution into the support. The wet material was then dried
lenging because of its diverse functionalities (phenolics, alde- overnight at 353 K for 12 h. The dried material was nally
hydes, ketones, alcohols, organic acids, and others). The calcined at 823 K for 6 h. The supported nickel oxide catalysts
present work was therefore commenced to explore fundamental obtained aer calcination was referred as oxidized catalyst
understanding of HDO of 1-octanol as a model aliphatic alco- throughout the article. The g-Al2O3 supported nickel catalysts
holic functionality of bio-oil.
were designated as xNiAl (x ¼ 5–20 wt% nickel). The HZSM-5
Enormous research efforts have been made in the past on (Zeolyst) with Si/Al ratio of 23 and 55 were designated as
mechanistic understanding of HDO of whole bio-oil2,3 and ZSM23 and ZSM55 respectively. The 15 wt% nickel supported
various oxygenated compounds of bio-oil such as phenol, ani- on SiO2, ZSM23, and ZSM55 were represented as 15NiSi,
sole, guaiacol,9–13 aliphatic aldehydes like heptanal,14–16 15NiZSM23, and 15NiZSM55 respectively.
aromatic aldehydes like furfural,17,18 benzaldehyde, 2-phenyl-
propionaldehyde, cinnamaldehyde, 3-phenylpropionaldehyde,
and 4-isopropylbenzaldehyde,16 and various ketones such as 6-
2.2 Catalyst characterization
undecanone, 2-dodecanone, 2,4-dimethyl-3-petnanone, 2,2,4,4-
2.2.1 Brunauer–Emmett–Teller
(BET)
surface
area
tetramethyl-3-pentanone,14 cyclopentanone,19 and methyl iso- measurement. The nitrogen adsorption–desorption studies
butyl ketone.20 However, limited information is available in were performed in Micromeritics ASAP 2020 physisorption
open literatures on HDO of aliphatic alcohols.14,21–24 The HDO of analyzer at liquid nitrogen temperature, 77 K. The sample was
1-heptanol was rst reported over commercial sulphided NiMo/ rst degassed at 473 K for 5 h under vacuum (ꢀ5 ꢁ 10ꢂ5 mmHg)
g-Al2O3 and CoMo/g-Al2O3 catalysts in both ow (at 523 K and to remove adsorbed moisture and other impurities present, if
15 bars of H2) and batch (at 523 K and 75 bars of H2 or N2) any. The specic surface area was determined using adsorption
reactor using m-xylene or n-dodecane as solvent.21,22 The work branch of isotherm data in the relative pressure (P/P0) range of
was further extended in a ow reactor using two different H2S 0.05 to 0.30 using multi-point BET equation. The pore volume
concentration in the feed.23 Heptane and heptenes together was considered as the volume of liquid nitrogen adsorbed at
with small quantity of diheptyl ether were observed as prod- P/P0 ¼ ca. 1.
ucts.21–23 Donnis et al. reported HDO of 3-heptanol in a micro-
2.2.2 Powder X-ray diffraction (XRD). The XRD patterns of
scale reactor over commercial sulded NiMo catalyst at 453 and the supported nickel catalysts and pure supports were obtained
483 K and 50 bars of H2 using n-octane as solvent.13 Very in PANalytical X'Pert pro instrument in the 2q range of 10 to 90ꢃ
˚
recently, Peng et al. reported HDO of aqueous 1- and 2-propanol using Cu-Ka radiation (l ¼ 1.541 A, 45 kV, and 30 mA) and
in a batch reactor in presence of 3 wt% Pt/Al2O3 at 473 K and 40 scanning speed of 2ꢃ minꢂ1
.
bars of H2.23 Propane, ethane, CO2, and acetone (in case of 2-
2.2.3 UV-vis spectroscopy studies. The UV-vis spectra of the
propanol only) were observed as products. The HDO of 2- oxidized catalysts were acquired in PerkinElmer UV-vis NIR
propanol was also carried out in a xed-bed reactor under vapor spectrophotometer (Model no.: Lamda 1050) equipped with
phase at different temperatures (393–523 K) using hydrogen as Harrick scientic diffuse reectance accessory (Praying Mantis,
carrier gas.
It is quite evident from above discussion that detailed study nm using BaSO4 as reference under ambient condition.
on HDO of neat aliphatic alcohols was not attempted in the past
2.2.4 Temperature programmed reduction using hydrogen
Model no.: DRK-4-P72) in the wavelength range of 200 to 800
over inexpensive nickel based catalysts to establish a compre- (H2-TPR). H2-TPR studies were carried out in Micromeritics
hensive reaction mechanism. A systematic investigation was AutoChem II 2920 chemisorption analyzer to understand
therefore commenced in the present work on HDO of 1-octanol reducibility of the oxidized catalysts. The sample was rst
in a xed-bed reactor under various process conditions over degassed at 473 K for 1 h under ow of He (20 mL minꢂ1) and
several supported (g-Al2O3, SiO2, and HZSM-5) nickel catalysts then cooled down to room temperature. Subsequently gas ow
with different degree of acidity to delineate a holistic reaction was switched over to 10 vol% H2–Ar (50 mL minꢂ1). The sample
mechanism.
was then heated up to 1073 K with a heating rate of 10 K minꢂ1
.
The rate of hydrogen consumption was monitored using
thermal conductivity detector (TCD). Pulse calibration studies
were performed for quantication of hydrogen consumption.
2.2.5 H2 pulse chemisorption. H2 Pulse chemisorption
2 Experimental
2.1 Catalyst preparation
g-Al2O3 (Alfa Aesar, 99.9% purity), SiO2 (Aerosil-200), and studies were performed in a Micromeritics AutoChem II 2920
HZSM-5 (Zeolyst) supported nickel catalysts were prepared by chemisorption analyzer. The oxidized catalyst was rst reduced
incipient wetness impregnation method using nickel nitrate by 10 vol% H2–Ar (20 mL minꢂ1) at 923 K for 3 h. The gas ow
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was then switched over to pure helium (50 mL minꢂ1) and consisting of two heating zones (preheating and reaction). The
sample was allowed to cool down to 323 K. The helium ow was temperature of the reaction zone was controlled by a PID
continued for another 1 h to ensure complete removal of controller within ꢄ1 K. The ow rate of all gases was controlled
gaseous hydrogen from entire chemisorption setup and phys- by individual mass ow controllers (Bronkhorst, EL-Flow). The
isorbed hydrogen from catalysts surface, if any. Several pulses of oxidized catalysts were rst reduced by hydrogen (25 mL minꢂ1
)
measured volume of 10 vol% H2–Ar gas mixture were then at 923 K for 3 h. The supported nickel catalysts obtained aer
introduced into the reactor till three consecutive H2 peaks were reduction were referred as reduced catalyst throughout the
similar. The hydrogen chemisorption was monitored quantita- article. The reactor was then cooled down to desired reaction
tively using a TCD detector. Metal dispersion (MD) and active temperature and 1-octanol was pumped into the reactor at
metal surface area (SM) were determined based on amount of specied ow rate using a HPLC pump (Lab alliance, Series-I).
hydrogen chemisorbed.
The product gases leaving the reactor were then passed
2.2.6 Temperature programmed desorption studies using through a vertical condenser. The non-condensable gases
NH3 (NH3-TPD). NH3-TPD studies were performed using separated in a gas-liquid separator were subsequently analyzed
Micromeritics AutoChem II 2920 chemisorption analyzer. The online by gas chromatograph (GC) equipped with TCD detector
sample was rst degassed at 473 K for 1 h under the ow of pure and Carbosieve-S-II packed column (Chromatopak, 3 m ꢁ 1/800)
helium (20 mL minꢂ1) to remove adsorbed moisture and other using argon as carrier gas. Liquid samples were collected at
impurities present, if any. The sample was then cooled down to regular interval of time aer reaching steady state condition.
373 K. Subsequently, 10 vol% NH3–He gas mixture (20 mL The products were identied using GC attached with mass
minꢂ1) was introduced into the reactor and continued the ow spectroscopy (Shimadzu, GCMS-QP2010 Ultra). Occurrence of
for 1 h to ensure complete chemisorption of ammonia. The ow the products was conrmed by comparing standard mass
of gas mixture was then switched over to pure He (50 mL minꢂ1
)
fragment data available with data acquisition soware (Wiley
and continued the ow for 1 h to remove physically adsorbed and NIST standard libraries). The products were quantied by
ammonia from the sample. The sample was consequently GC (Shimadzu, GC-2014) using capillary column (ZB-5HT
heated up to 1073 K with a heating rate of 10 K minꢂ1 and Inferno, Phenomenex, 30 m ꢁ 0.32 mm ꢁ 0.10 mm) connected
amount of NH3 desorbed was monitored using a TCD detector. with ame ionization detector (FID). The following GC oven
The quantity of acid sites was calculated based on amount of temperature programming was used to obtain well resolved
NH3 desorbed and pulse calibration data collected separately chromatogram: Initially oven was kept at 313 K for 3 minutes
using same gas mixture.
and then temperature was increased to 493 K with a ramping
rate of 15 K minꢂ1, and held at this temperature for 3 min.
Injector and detector were maintained at 493 K and 523 K
respectively. The material balance was checked for all experi-
mental runs and errors were within ꢄ3%.
2.3 Experimental setup and procedure
The HDO of 1-octanol was performed in an isothermally oper-
ated continuous down-ow xed-bed reactor using either
nitrogen or hydrogen as carrier gas. The known amount of
oxidized catalyst diluted with appropriate quantity of quartz
beads was placed inside a stainless steel tubular reactor (1/200
3 Results and discussion
3.1 Catalyst characterization
O.D. and 40 cm length) reinforced by quartz wool from either The BET surface area and pore volume of the oxidized catalysts
side.25 The reactor was then kept inside a tubular furnace and pure supports are shown in Table 1. The surface area of the
Table 1 Physico-chemical properties of supported nickel catalysts and pure supportsa
Particle sizec, nm
Sa
PVa
MDb
SMb
MH2
MNH3
Oxidized
Reduced
a
b
Catalyst
Al2O3
5NiAl
104
88
78
68
69
209
164
318
370
232
310
8.1
8.3
8.0
7.9
7.7
—
—
—
—
—
—
—
0.35
0.23
0.21
0.17
0.17
—
—
—
—
—
0.21
0.36
0.15
0.16
—
0.29
—
—
1.42
2.43
0.98
1.09
—
1.96
—
—
0.54
2.06
2.91
3.83
—
1.93
—
—
10NiAl
15NiAl
20NiAl
SiO2
15NiSi
ZSM23
ZSM55
15NiZSM23
15NiZSM55
9.0
22.0
31.8
—
19.1
—
—
23.9
20.5
9.0
29.0
36.0
—
26.2
—
—
28.8
26.2
—
1.09
0.64
0.90
0.46
0.18
0.37
1.21
2.48
1.83
1.91
—
a
S ¼ surface area, m2 gꢂ1; PV ¼ pore volume, mL gꢂ1; MD ¼ metal dispersion, %; SM ¼ metallic surface area, m2 gꢂ1; MH2 ¼ total amount of H2
consumed mmol gꢂ1 of cat.; MNH3 ¼ total amount of NH3 desorbed, mmol gꢂ1
.
a Oxidized catalysts. b Reduced catalyst. c Calculated using Debye–
Scherer equation.
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oxidized catalysts was somewhat lesser than pure supports. The
decrease of surface area might be due to blocking of pores and
difference in atomic weight of nickel and supports. Further-
more, surface area of oxidized catalysts was observed to
decrease with increasing nickel loading up to 15 wt% on g-Al2O3
and levelled off thereaer. The pore volume was also decreased
slightly with increasing nickel loading. MD and SM determined
from H2 chemisorption results was increased with increasing
nickel loading till 10 wt% and levelled off thereaer.
The XRD patterns of both oxidized and reduced catalysts and
pure supports are shown in Fig. 1A and B respectively. Analysis
of XRD patterns of oxidized and reduced xNiAl catalysts
conrmed presence of bulk nickel oxide (PDF#780643) and
nickel (PDF #882326) respectively with nickel loading of 10 wt%
or more on g-Al2O3.26 It was further observed that intensity of
bulk nickel oxide (or nickel) peaks was increased with
increasing nickel loading on g-Al2O3. However, only dispersed
nickel oxide (or nickel) was present in xNiAl with nickel loading
below 10 wt%. XRD patterns of 15NiSi, 15NiZSM23, and
15NiZSM55 also revealed presence of bulk nickel oxide and
Fig. 2 UV-vis spectra of oxidized xNiAl, 15NiSi, 15NiZSM23, and
15NiZSM55 catalysts.
nickel in oxidized and reduced catalysts respectively. From
these results it may be concluded that characteristic bulk nickel
peak appeared in XRD pattern of reduced catalysts were most
likely originated by reduction of bulk nickel oxide present in the
oxidized catalysts. Particle size of the reduced and oxidized
catalysts was determined using Debye–Scherer equation. The
size of nickel oxide or nickel depends on the nickel loading and
nature of support. In case of xNiAl catalysts, the particle size was
observed to increase with increasing nickel loading.
The UV-vis DRS spectra of the oxidized catalysts under
ambient condition are shown in Fig. 2. The spectra at lower
wavelength region are however not shown due to presence of
strong charge transfer band that gives very high value of
Kubelka Munk function. The band appeared at 280–320 nm (not
shown in Fig. 2) for all catalysts is attributed to O2ꢂ / Ni+2
charge transfer.27 For xNiAl catalysts, position of this band
shied to higher wavelength with increasing nickel loading. It
may be due to increased concentration of bulk nickel oxide.
Additional band appeared for xNiAl at 380 and 646 nm are
characteristics of nickel aluminate (NiAlO4) which is a solid
solution of nickel oxide and g-Al2O3.27 Evolution of DRS bands
at 380, 415 and 715–720 nm was due to bulk nickel oxide with
octahedral geometry.27 For xNiAl, these bands were observed for
10 wt% and more nickel loading on g-Al2O3.
TPR results of the oxidized catalysts are shown in Fig. 3. The
peak appeared at 699 K for xNiAl was due to reduction of
dispersed nickel oxide possessing strong interaction with the
support.26 The peak observed at 627–650 K for 15NiAl and
20NiAl was due to reduction of bulk nickel oxide possessing no
or little interaction with support.26 TPR prole of bulk nickel
oxide further conrms above argument as shown in the same
gure. Moreover, intensity of the reduction peaks correspond-
ing to both dispersed and bulk nickel oxide was increased with
increasing nickel loading on g-Al2O3. The XRD and UV-vis
spectroscopy results also conrmed presence of bulk as well
as dispersed nickel oxide for 15NiAl and 20NiAl. However, for
10NiAl, the reduction peak corresponding to bulk nickel oxide
Fig. 1 XRD patterns of (A) oxidized and (B) reduced g-Al2O3, SiO2,
ZSM23, and ZSM55 supported nickel catalysts.
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Fig. 4 NH3-TPD profiles of xNiAl, 15NiSi, 15NiZSM23, 15NiZSM55
Fig.
3 TPR profiles of oxidized xNiAl, 15NiSi, 15NiZSM23, and
catalysts, and pure supports.
15NiZSM55 catalysts.
decreasing Si/Al ratio. Upon deposition of nickel on HZSM-5,
intensity of the peaks was decreased signicantly together
with shiing of peaks to lower temperature (570–578 K). These
results indicate that strength and number of acid sites decrease
on deposition of nickel on zeolites.
was appeared as weak shoulder in TPR proles though it was
very prominent in XRD pattern and UV-vis spectra. This might
be due to either presence of insignicant amount of bulk nickel
oxide in 10NiAl or TPR studies are less sensitive compared to
XRD and UV-vis spectroscopy for the detection of specically
small amount of bulk NiO species. The oxidized 15ZSM23 and
15ZSM55 also showed characteristic reduction peaks of both
bulk and dispersed nickel oxide. In contrast, 15NiSi showed
only a broad reduction peak centered at 605 K. The XRD pattern
and UV-vis spectroscopy conrmed presence of bulk nickel
oxide in 15NiSi. Thus it can be concluded that reduction of
different surface nickel oxide species in 15NiSi might happen
within a small range of temperature giving rise to a broad
reduction peak centred at 605 K. The H2 consumption during
TPR studies was calculated (Table 1) and values were matched
closely with theoretical amounts of H2 required to reduce nickel
oxide for xNiAl catalysts. Thus, these results indicate that only
nickel oxides (dispersed and bulk) are present in the supported
nickel catalysts. Contrary to xNiAl, H2 consumption values for
15NiSi, 15NiZSM23, and 15NiZSM55 were lower than theoreti-
cally required amounts thereby suggesting the presence of
differently coordinated nickel species or a fraction of the nickel
may not be accessible to hydrogen during reduction.
NH3-TPD results of reduced catalysts and pure supports are
shown in Fig. 4 and Table 1. Pure g-Al2O3 showed a distinct
ammonia desorption peak at 445 K corresponding to weak/or
moderate acid sites.26 The intensity of this peak was sup-
pressed gradually with increasing nickel loading indicating
reduction of acidity with increasing nickel loading on g-Al2O3.26
The ammonia desorption peak appeared at 913 K and above
may be due to dehydroxylation of hydroxyl group present in g-
Al2O3. For zeolites, strength and number of acid sites were
signicantly more compared to pure g-Al2O3 with characteristic
ammonia desorption peaks at 445–461 K and 645–657 K. The
intensity of these peaks was enhanced with decreasing Si/Al
ratio suggesting increase of number of acid sites with
3.2 Reaction mechanism
The n-heptane, n-octane, isomers of heptene (heptenes) and
octene (octenes), 1-octanal, di-n-octyl ether (DOE), tetradecane
(TETD), and hexadecane (HEXD) were identied by GC-MS in
the liquid samples for HDO of 1-octanol (Fig. S1†). Small
quantities of octanoic acid (OA) and octyl caprylate (OC, octyl
octanoate, C7H15COOC8H17) were also identied in few cases.
Furthermore, liquid samples were found to undergo phase
separation (aqueous and organic) especially at high conversion
of 1-octanol (or freezing the samples below 273 K) demon-
strating formation of water as one of the product. A compre-
hensive reaction mechanism of HDO of 1-octanol was
delineated based on products distribution as shown in
Scheme 1. 1-Octanol undergoes HDO following two different
routes on two different sites of the catalyst: acid and metal.
In the rst route, 1-octanol undergoes dehydration to
octenes either directly or through intermediate etherication
reaction via DOE with elimination of one mole of water in each
step. The octenes subsequently undergo either hydrogenation
to n-octane or oligomerization followed by hydrogenation to
HEXD. The acidic sites of catalysts might be responsible for
etherication, dehydration, and oligomerization reactions.
DOE, n-octane, and octenes were major products for highly
acidic pure HZSM-5 and HZSM-5 supported nickel catalysts and
their selectivity were also increased with increasing acidity of
the catalysts thereby justifying above statements. The dehy-
dration of alcohols, for example ethanol and butanol, was
reported previously over acidic catalysts in the temperature
range of 498–748 K.28 The oligomerization of olens, for
instance 1-hexene and 1-octene, was also described over various
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Scheme 1 Reaction mechanism for HDO of 1-octanol.
solid acid catalysts such as solid phosphoric acid, ZSM-5, 1-octanal was performed over 15NiAl catalyst using hydrogen
zeolite-Y, and cation exchange resins.29,30 For HDO of 1-hepta- as carrier gas (Table S1†). In this particular experiment, n-
nol using sulphided NiMo/g-Al2O3 and CoMo/g-Al2O3 catalysts, heptane, 1-octanol, and heptenes were identied as major
dehydration of alcohol to alkene followed by its hydrogenation products together with small amounts of DOE, n-octane,
to alkane were reported too.21–23
octenes, HEXD, TETD, and OA. The formation of 1-octanol
Following second route, 1-octanol undergoes dehydrogena- suggests that dehydrogenation of 1-octanol to 1-octanal is a
tion reaction over active metal sites of the catalysts leading to reversible reaction. Small amount of CO2 and CH4 were also
formation of 1-octanal. The 1-octanal is consequently converted detected for HDO of 1-octanol under hydrogen as carrier gas.
to either n-heptane by decarbonylation reaction or heptenes by Formation of CO2 and CH4 may be due to water gas shi
dehydroformylation reaction. The heptenes are then either reaction and methanation of CO/CO2 respectively as shown in
hydrogenated to n-heptane or oligomerized followed by hydro- Scheme 1. Formation of OA was however not well understood
genated to TETD. The GC analysis of gas sample under nitrogen and might be formed by disproportionation of 1-octanal. The
as carrier gas additionally conrmed formation of both OA formed further undergoes esterication reaction with 1-
hydrogen and carbon monoxide thereby justifying above claim. octanol producing OC. The reaction mechanism is generally
Moreover, 1-octanal, n-heptane, and heptenes were primary conrmed directly through in situ or operando spectroscopic
products for g-Al2O3 supported nickel catalysts as discussed in studies. However, it is beyond the scope of present study and
subsequent section. Furthermore, their combined selectivity le to the researchers as an open challenge problem for future
was observed to increase with increasing nickel loading on g- study.
Al2O3 demonstrating role of metal sites for dehydrogenation,
In order to demonstrate the proposed mechanism as
decarbonylation, and dehydroformylation reactions. For cata- generalized one, study was further extended to HDO of 1-hep-
lytic HDO of C3 alcohols, Peng et al. also suggested that alcohol tanol and 1-hexanol over 15NiAl at 518 K. The products distri-
was rst dehydrogenated to aldehyde over metal sites.24 The CO butions for both alcohols were fully in line with proposed
underwent water gas shi reaction to give CO2 and H2.
mechanism thereby validating the proposed mechanism (Table
To conrm proposed pathways of conversion of 1-octanal S1, Fig S2 and S3†). All subsequent studies were performed with
to various products, an independent experiment of HDO of 1-octanol only.
0
@
1
A
Rate of moles of n-heptane; heptenes;
n-octane; octenes; 1-octanal;
or octanoic acid formed
0
@
1
A
Selectivity to n-heptane
heptenes; n-octane; octenes ; % ¼ 100 ꢁ
(1)
Rate of moles of 1-octanol reacted
octanal; or octanoic acid
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0
@
1
A
Rate of moles of di-n-octyl ether;
tetradecane; hexadecane;
or octyl caprylate formed
0
1
Selectivity to
di-n-octyl ether;
tetradecane; hexadecane;
or octyl caprylate
B
B
@
C
C
A
; % ¼ 2 ꢁ 100 ꢁ
(2)
ðRate of mole of 1-octanol reactedÞ
hydrogen ow rates (20 and 40 mL minꢂ1) maintaining 1-
octanol ow rates and WHSV constant. However, 1-octanol
conversion and selectivity to products remained practically
unaffected with increasing hydrogen ow rate (Table S3†).
Therefore, all subsequent experiments were carried out with 20
mL minꢂ1 of hydrogen ow rate.
3.3.2 Catalyst stability study. The stability of supported
nickel catalysts was tested for HDO of 1-octanol over 15NiAl
catalyst. The conversion of 1-octanol was calculated based on
collected reaction data at different time-on-stream (TOS) up to
600 min to know the variation of catalytic activity with TOS as
shown in Fig. 5. As observed from gure, the catalyst was highly
active initially and catalytic activity was decreased gradually
with increasing TOS. Aer about 240 min of TOS, variation of
conversion of 1-octanol was however found to be insignicant
(ꢀ0.7% variation) with TOS up to 600 min. Moreover, the vari-
ation of selectivity of major products was also insignicant aer
240 min of TOS. From this result it can be safely concluded that
the catalyst is quite stable aer about 240 min of TOS. Hence,
conversion and selectivity calculations for all experimental runs
were performed aer reaching steady state conditions.
3.3 Reaction studies
The HDO of 1-octanol was investigated in a down-ow xed-bed
reactor over g-Al2O3, SiO2, ZSM23, and ZSM55 supported nickel
catalysts using either nitrogen or hydrogen as carrier gas under
wide range of pressures (1–10 bars), nickel loadings on g-Al2O3
(0–20 wt%), weight hourly space velocities (WHSV) (2.5–9.99
hꢂ1), and temperatures (488–530 K). n-Heptane, heptenes, 1-
octanal, DOE, n-octane, and octenes were identied as primary
products of HDO of 1-octanol in presence of both nitrogen and
hydrogen as carrier gas. The TETD, HEXD, OA, and OC were
obtained as minor products of the reaction. The selectivity to n-
heptane, heptenes, n-octane, octenes, 1-octanal, and OA used
throughout the article is dened by eqn (1). The selectivity to
DOE, TETD, HEXD, and OC is dened by eqn (2). It may be
noted that all isomers of heptene (1-heptene, 2-heptene, and 3-
heptene) and octene (1-octene, 2-octene, and 3-octene) are
included within the selectivity to heptenes and octenes
respectively.
3.3.1 Effects of carrier gases. In order to capture effects of
carrier gas on conversion of 1-octanol and selectivity to various
products, two independent experiments were conducted using
nitrogen and hydrogen as carrier gas over 15NiAl catalyst. The
conversion of 1-octanol was observed to be considerably higher
for hydrogen as carrier gas compared to that of nitrogen (Table
S2†). It may be due to the fact that hydrogenation of interme-
diate alkenes drives the reactions in forward direction leading
to higher conversion of 1-octanol under hydrogen as carrier gas.
The selectivity to alkanes (n-heptane and n-octane) was higher
for hydrogen as carrier gas. The selectivity to alkenes (heptenes
and octenes), oligomer products (TETD and HEXD), and 1-
octanal were however higher for nitrogen as carrier gas. For the
reaction with hydrogen as carrier gas, the intermediate alkenes
undergo hydrogenation preferentially to alkanes under the
reaction conditions resulting in higher selectivity to alkanes.
For reaction with nitrogen as carrier gas, the alkenes however
can only undertake oligomerization route leading to higher
selectivity to TETD and HEXD. The reversible dehydrogenation
of 1-octanol to 1-octanal is also favourable under nitrogen as
carrier gas leading to higher selectivity of 1-octanal. Good
amount of alkanes were also formed under nitrogen as carrier
gas. The dehydrogenation of 1-octanol to 1-octanal is the source
of hydrogen for hydrogenation of intermediate alkenes. Since
higher conversion of 1-octanol was observed for hydrogen as a
carrier gas; further studies were carried out using hydrogen as
carrier gas.
3.3.3 Effects of pressure. The investigation of effects of
pressure is very much important to understand performance of
HDO of 1-octanol under two different phases (vapor and liquid).
The vapor pressure of 1-octanol and products of HDO of 1-
octanol were calculated at several operating temperatures using
Fig. 5 Variation of activity and selectivity of major products for HDO
In order to determine critical ow rate of hydrogen, two
independent experiments were conducted for two different
of 1-octanol with TOS. Reaction conditions: catalyst
¼
15NiAl,
temperature ¼ 488 K, pressure ¼ 1 bar, WHSV ¼ 3.99 hꢂ1
.
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Table 2 Effects of pressure on HDO of 1-octanola
Selectivity, %
Conversion of
Pressure, bar
1-octanol, %
n-Heptane
Heptenes
n-Octane
Octenes
1-Octanal
DOE
HEXD
TETD
OC
OA
1
5
10
30.4
17.5
7.5
43.4
24.8
11.2
22.9
1.7
0.4
0.5
1.2
2.6
0.4
0.3
0.6
15.4
6.7
5.4
13.9
61.1
65.6
0.4
0.2
2.6
0.7
0.7
5.1
1.7
2.4
4.2
0.7
0.9
2.3
a
Reaction conditions: temperature ¼ 503 K, catalyst ¼ 15NiAl, WHSV ¼ 3.99 hꢂ1, TOS ¼ 240 min.
Table 3 Effects of temperature on HDO of 1-octanola
Selectivity, %
Conversion of
Temp., K
1-octanol, %
n-Heptane
Heptenes
n-Octane
Octenes
1-Octanal
DOE
HEXD
TETD
OC
OA
488
503
518
533
13.8
30.4
45.1
53.2
40.2
43.4
50.9
57.0
6.0
22.9
24.9
23.1
0.4
0.5
0.7
1.0
0.3
0.4
0.4
0.4
26.6
15.4
10.7
9.1
22.1
13.9
10.1
7.4
0.3
0.4
0.4
0.4
1.1
0.7
0.3
0.3
2.0
1.7
1.2
1.0
1.0
0.7
0.4
0.3
a
Reaction conditions: pressure ¼ 1 bar, catalyst ¼ 15NiAl, WHSV ¼ 3.99 hꢂ1, TOS ¼ 240 min.
Table 4 Effects of nickel loading on Al2O3 on HDO of 1-octanola
Selectivity, %
Ni loading,
%
Conversion of
1-octanol, %
n-Heptane
Heptenes
n-Octane
Octenes
1-Octanal
DOE
HEXD
TETD
OC
OA
0
5
10
15
20
9.8
10.4
16.7
30.4
20.7
0
0
0
17.5
2.4
0.9
0.4
0.3
0.8
28.1
28.4
15.4
18.2
81.7
32.4
25.5
13.9
15.5
0
0
0
0
12.1
18.6
43.4
47.7
15.5
20.3
22.9
14.4
1.7
1.1
0.5
1.3
0.5
0.4
0.4
0.3
1.5
1.2
0.7
0.7
4.3
2.4
1.7
0.9
1.5
1.2
0.7
0.7
a
Reaction conditions: temperature ¼ 503 K, pressure ¼ 1 bar, WHSV ¼ 3.99 hꢂ1, TOS ¼ 240 min.
Table 5 Effects of supports on HDO of 1-octanola
Selectivity, %
Conversion of
Catalysts
1-octanol, %
n-Heptane
Heptenes
n-Octane
Octenes
1-Octanal
DOE
HEXD
TETD
OC
OA
SiO2
ZSM23
ZSM55
15NiAl
15NiSi
15NiZSM23
15NiZSM55
1.7
9.3
4.0
2.3
50.9
52.7
8.2
0
0
0
24.9
32.8
0
12.4
16.4
9.8
0.7
0.3
54.9
77.8
85.5
0.4
0.2
73.2
69.5
9.7
1.0
1.0
10.7
10.8
6.2
2.9
0.8
1.5
10.1
2.6
6.5
0
0
0
0.4
0
0
0
0
0.3
0
0.2
0
0
1.2
0
0
10.6
0
0
0.4
0.6
0
55.6
87.4
45.1
43
40.2
61.5
5.9
4.0
0
2.6
0
0
10.6
0
6.2
6.9
0.1
0.1
a
Reaction conditions: temperature ¼ 518 K, pressure ¼ 1 bar, WHSV ¼ 3.99 hꢂ1, TOS ¼ 240 min.
Aspen Plus®, Aspen Tech™ soware as presented in Table S4.† however remain in liquid phase even at atmospheric pressure
1-Octanol remains in vapor phase at atmospheric pressure for entire range of working temperatures.
under the range of operating temperatures as observed from the
The HDO of 1-octanol was accordingly designed at three
table. At 3.30 bars and above, 1-octanol transformed to liquid different pressures, 1, 5, and 10 bars as presented in Table 2.
phase at the maximum operating temperature of 518 K. On the The conversion of 1-octanol was decreased with increasing
other hand, C7 and C8 hydrocarbon products require high pressure indicating that reaction was unfavourable at higher
pressure to persist in liquid phase. DOE, TETD, and HEXD pressure. With increasing pressure, selectivity to 1-octanal, n-
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Table 6 Effects of WHSV on HDO of 1-octanola
Selectivity, %
Conversion of
WHSV, hꢂ1
1-octanol, %
n-Heptane
Heptenes
n-Octane
Octenes
1-Octanal
DOE
HEXD
TETD
OC
OA
10.0
5.0
3.3
2.5
15.2
25.0
31.7
37.2
49.4
45.0
45.2
41.9
21.4
22.3
23.4
21.8
0.7
0.6
0.6
0.6
0.4
0.4
0.6
0.4
14.8
16.5
15.1
12.5
9.5
11.7
12.2
18.5
0.4
0.5
0.4
0.6
1.1
0.6
0.6
0.6
1.1
1.8
1.3
2.6
1.2
0.6
0.6
0.5
a
Reaction conditions: temperature ¼ 503 K, pressure ¼ 1 bar, catalyst ¼ 15NiAl, TOS ¼ 240 min.
heptane, and heptenes were decreased; while selectivity to DOE, (responsible for hydrogenation/dehydrogenation and decar-
n-octane, octenes, TETD, and HEXD were increased. According bonylation) in the catalyst.
to Le Chatelier's principle, vapor phase reactions with positive
In order to delineate role of supports on the reaction, HDO of
change in number of moles such as dehydrogenation, dehy- 1-octanol was conducted over pure supports (g-Al2O3, SiO2,
droformylation, decarbonylation are unfavourable at high ZSM23 and ZSM55) as well (Tables 4 and 5). 1-Octanol mainly
pressures and vice versa. This leads to lower selectivity to 1- undergoes dehydration and etherication reaction in presence
octanal, n-heptane, and heptenes at higher pressure. The of pure supports resulting in formation of n-octane, octenes,
etherication and oligomerization reactions are however and DOE as foremost products together with only small quan-
favourable at high pressures leading to increasing trends of tity of 1-octanal. The relative proportion of dehydration and
selectivity to n-octane, octenes, DOE, TETD, and HEXD with etherication products strongly depends on strength and
pressures. All subsequent experiments were carried out under nature of acid sites (Lewis and Bronstead). Strongly acidic
vapor phase at atmospheric pressure.
supports promote dehydration of alcohols resulting in the
3.3.4 Effects of temperature. HDO of 1-octanol was studied formation of corresponding olens as major products. In
at four different temperatures, 488, 503, 518, and 533 K as general, Lewis acid sites of g-Al2O3 are responsible for ether-
shown in Table 3. As observed from the table, 1-octanol ication reaction of 1-octanol yielding DOE as major product
conversion was increased with increasing temperature as together with lesser amount of octenes. The octenes thus
expected. The combined selectivity to n-heptane, heptenes, and formed however cannot easily undergo hydrogenation reaction
1-octanal were increased with increasing temperatures. From even in presence of hydrogen due to non-availability of active
these results it may be concluded that higher temperature metal sites in the catalyst. Hence, poor selectivity to alkanes was
favors dehydrogenation of 1-octanol to 1-octanal more than observed for pure supports.
dehydration of 1-octanol to DOE and octenes. However, selec-
The 15 wt% nickel loaded on g-Al2O3, SiO2, ZSM23, and
tivity to 1-octanal was decreased with increasing temperature. ZSM55 catalysts were also tested for HDO of 1-octanol (Table 5).
These results clearly show that increase of rate of dehy- As observed from the table, C8 hydrocarbons and DOE were
droformylation and decarbonylation of 1-octanal with temper- primary products over highly acidic catalysts (15ZSM23 and
ature is much more than dehydrogenation of 1-octanol to 1- 15ZSM55).31 On the other hand, C7 hydrocarbons and DOE were
octanal. The variation of selectivity to oligomerized products, main products for 15NiAl and 15NiSi. From these results it may
HEXD and TETD was remained insignicant with increasing be concluded that HDO of 1-octanol mainly proceeds through
reaction temperature.
dehydration pathway in presence of acidic catalysts and dehy-
3.3.5 Effects of nickel loading and supports. The nickel drogenation pathway for metallic catalysts.
loading on g-Al2O3 were varied from 0 to 20 wt%. The results of
3.3.6 Effects of WHSV. The effects of WHSV on conversion
nickel loading on g-Al2O3 on conversion of 1-octanol and of 1-octanol and selectivity to products are shown in Table 6. As
selectivity to products are shown in Table 4. As observed from observed from table, 1-octanol conversion was decreasing with
table, 1-octanol conversion were increased with increasing increasing WHSV. This is due to decrease of space time of
nickel loading on g-Al2O3 up to 15 wt% and levelled off there- reactants in the reactor. However, selectivity to products was
aer. Therefore, 15 wt% nickel loading on g-Al2O3 was delib- persisted practically with increasing WHSV. Therefore, it may
erated as critical nickel loading and henceforth all subsequent be concluded from these results that WHSV affects only
experiments were conducted using 15 wt% nickel loading on g- conversion of 1-octanol without disturbing selectivity to prod-
Al2O3. The increase of nickel loadings on g-Al2O3 has signicant ucts much.
impacts on selectivity to products. The selectivity to n-octane,
octenes, and DOE were decreased with increasing nickel
loading on g-Al2O3. This may be due to decrease of acidity of the
4 Conclusions
catalysts with increasing nickel loading on g-Al2O3 as observed
from ammonia TPD (Table 1). With increasing nickel loading
on g-Al2O3, the combined selectivity to 1-octanal, n-heptane,
and heptenes was increased due to increase of nickel contents
The HDO of 1-octanol was investigated in a down-ow xed-bed
reactor using g-Al2O3, SiO2, and HZSM-5 supported nickel
catalysts prepared by incipient wetness impregnation method.
The prepared catalysts together with pure supports were
41620 | RSC Adv., 2014, 4, 41612–41621
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characterized by BET, XRD, UV-vis, TPR, TPD, and H2 chemi-
sorption. The supported nickel (or nickel oxide) catalysts were
observed to be associated with bulk nickel (or nickel oxide) as
well beyond 10 wt% nickel loading on the supports. The
strength and number of acid sites of the supported nickel
catalysts were somewhat lesser compared to their respective
pure supports. HDO of 1-octanol follows two different pathways
depending on nature of the catalysts. HDO reaction proceeds
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formation of DOE, isomers of octene, and n-octane as primary
products. The dehydrogenation route becomes dominant over
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conversion of 1-octanol was increased with increasing nickel
Saidi,
F.
Samimi,
D.
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T. Nimmanwudipong, B. C. Gates and M. R. Rahimpour,
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Catal., 2009, 52, 229–240.
ˇ
´
´
´
´
ˇ
combined selectivity to n-octane, octenes, and DOE was however
J. Cejka, Appl. Catal. A: Gen., 2007, 332, 56–64.
16 M. R. D. Brimont, C. Dupont, A. Daudin, C. Geantet and
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observed to follow reverse trend.
17 S. Sitthisa, T. Sooknoi, Y. Ma, P. B. Balbuena and
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Nomenclatures
18 S. Sitthisa and D. E. Resasco, Catal. Lett., 2011, 141, 784–791.
19 J. Huang, W. Long, P. K. Agrawal and C. W. Jones, J. Phys.
Chem. C, 2009, 113, 16702–16710.
20 M. A. Alotaibi, E. F. Kozhevnikova and I. V. Kozhevnikov,
Chem. Commun., 2012, 48, 7194–7196.
21 E.-M. Ryymin, M. L. Honkela, T.-R. Viljava and
A. O. I. Krause, Appl. Catal., A, 2009, 358, 42–48.
22 O. ¸Senol, E.-M. Ryymin, T.-R. Viljava and A. O. I. Krause, J.
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23 O. ¸Senol, T.-R. Viljava and A. O. I. Krause, Appl. Catal., A,
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HDO
WHSV
xNiAl
15NiSi
ZSMy
15NiZSMy
TETD
HEXD
DOE
Hydrodeoxygenation
Weight hourly space velocity
x wt% nickel supported on g-Al2O3
15 wt% nickel supported on SiO2
HZSM-5 with Si/Al ¼ y, where y ¼ 23 and 55
15 wt% nickel supported on HZSM-5
Tetradecane
Hexadecane
Di-n-octyl ether
Octyl caprylate
Octanoic acid
OC
OA
TOS
´
24 B. Peng, C. Zhao, I. Mejıa-Centeno, G. A. Fuentes, A. Jentys
Time-on-stream
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Acknowledgements
The authors gratefully acknowledge the nancial support from
the Department of Science and Technology (DST), India.
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