1
40
X. Besse et al. / Applied Catalysis A: General 524 (2016) 139–148
acid conversion. In a comparative study of the conversion of stearic
acid (C17H35-COOH) and oleic acid (C17H33-COOH, with one C
bond) under similar conditions (5% Pd/C catalyst, in dodecane, no
H2 added), the reaction of oleic acid was found very slow relative
to stearic acid and decarbonylation was the main reaction pathway
studies. A kinetic model of the reaction is proposed, that provides
relevant insight in the reaction pathway leading to the observed
reaction products.
C
[
12]. The solvent (dodecane) was also shown to undergo partial
dehydrogenation and to transfer hydrogen to the reactants. Under
H2 pressure, oleic acid and linoleic acid (C17H31-COOH, two C
2
. Experimental
C
2.1. Catalyst preparation
bonds) were first hydrogenated into stearic acid before decarboxy-
lation could proceed at a significant rate [12].
Pt/C and Ru/C catalysts were prepared by wet impregnation of
The catalytic hydrothermal conversion of fatty acids has also
been studied in supercritical water in the absence of reductant.
Watanabe et al. reported that zirconia promoted the decarboxyla-
tion of stearic acid in supercritical water; the main products formed
were CO2 and C16-alkenes, and presumably acetic acid [13]. Oleic
acid has also been converted in sub- and supercritical water in the
presence of Ni-based catalysts [14]. The conversion of oleic acid
Pt(NH ) (OH) and Ru(III) nitrosylnitrate precursors (Alfa Aesar).
3
4
2
−1
The carbon support (Engelhard, surface area 1000 m g ) was pre-
◦
viously calcined at 500 C under N2 for 5 h. The metal precursors
dissolved in water were deposited on the supports by wet impreg-
nation in order to obtain a similar molar loading, close to 0.01 mol.%
for both metals (1 wt.% of Ru and 2 wt.% of Pt on carbon). Water was
evaporated in a rotatory evaporator and the catalyst was calcined
◦
◦
was 13–15% at 350 C (sub-critical conditions) and 30–33% at 400 C
supercritical conditions) with heptadecenes as the main products;
◦
at 500 C in air to decompose the metal salts. The metals were then
(
◦
reduced under hydrogen at 300 C for 2 h.
decarboxylation was proposed as the main reaction pathway. The
addition of glycerol to provide in-situ H2 improved the conver-
sion of oleic acid and the selectivity for heptadecane [14]. Similarly,
the hydrothermal conversion of triglycerides into C17 hydrocarbons
over a 5 wt.%Pd/C catalyst was improved when glycerol or methanol
were added as reagents providing in-situ hydrogen through aque-
ous phase reforming [15]. Savage and co-workers obtained high
yields (>80%) in Cn-1 hydrocarbons from different saturated fatty
2
.2. Catalyst characterization
The Ru and Pt loadings on the carbon support were analysed by
ICP-OES. The 2 wt.% Pt/C catalyst used in kinetic studies had a spe-
2
−1
cific surface area of 970 m g , with a mean particle size of 16 m
measured by Laser diffraction particle size analysis). The catalyst
(
acids, stearic (C17H35-COOH), palmitic (C15H31-COOH) and lauric
porosity was characterized by nitrogen physisorption using a 3Flex
Micromeritics analyzer. The catalyst was essentially microporous
◦
(
C11H23-COOH), in water at 330 C using a 5%Pt/C commercial cata-
lyst [16,17]. However, only low yields of hydrocarbons (<20%) were
formed from unsaturated C18 fatty acids (oleic and linoleic), while
heptadecane was the main product. The authors showed that a frac-
(
≈80% of BET surface area, pore size around 0.6 nm) with a small
amount of mesopores with a pore size around 2.3 nm, actually very
close to micropores. Pt leaching was assessed by hydrothermal age-
tion of oleic acid decomposed into H , CO , CH and C H , and
2
2
4
2
6
◦
ing of Pt/C catalyst in water at 320 C for 3 h. The Pt content in water
the hydrogen formed in-situ allowed the reduction of oleic acid
into stearic acid, which subsequently underwent decarboxylation
to yield heptadecane. PtSnx/C catalysts were also developed for the
hydrothermal conversion of stearic, oleic and linoleic acids in the
absence of reductant [18]. Nearly quantitative yields of heptade-
cane were obtained from stearic acid, but oleic and linoleic acids
were found more difficult to convert into hydrocarbons than stearic
acid, i.e. the yield in hydrocarbons did not exceed 15% with linoleic
acid.
after ageing analysed by ICP-OES was 0.1 ppm, which represented
less than 0.006% of the total Pt amount. HAADF-STEM micrographs
were obtained with a FEI Titan ETEM microscope.
2.3. Reactor and kinetic studies
Linoleic acid (>99%, Sigma-Aldrich) and ethanol (99.8%, Sigma-
Aldrich) were used as received. The reactions were performed in
a 250 mL Hastelloy autoclave (Parr) equipped with high-pressure
valves for liquid or gas introduction and sampling, under autoge-
nous pressure. In a typical experiment, the reactor was loaded with
water and the desired amount of catalyst, then purged by bubbling
nitrogen in the water for 10 min in order to remove gaseous air and
dissolved oxygen. The heating system was then started and sta-
bilized at the desired temperature in about 30 min. The mixture of
linoleic acid in ethanol was injected in the autoclave using a prepar-
ative HPLC pump. Injection time was set to 2 min. The total liquid
volume loaded in the autoclave was always 150 mL. The stirring
rate was set at 300 rpm. In tests performed at 600 rpm the linoleic
acid disappearance rate was not modified, showing therefore that
the reaction was not limited by external mass transfer at 300 rpm.
Cyclohexene, tetralin, formic acid and organic alcohols have
been known for a long time as hydrogen-donor compounds, that
can be used as solvents to perform liquid phase catalytic transfer
hydrogenation reactions such as the reduction of organic multi-
ple bonds (alkenes, alkynes, carbonyls, nitriles) and hydrogenolysis
[
19]. More recently, catalytic transfer hydrogenation has been
shown to convert efficiently biomass-derived compounds, such
as levulinic acid, 5-hydroxymethylfurfural (HMF), glycerol, to
partially deoxygenated compounds [20]. Carbonyl groups are selec-
tively hydrogenated into alcohols, but the hydrogenolysis of C
O
bonds can also lead to partial deoxygenation through elimination of
CO2 or CO. Using H-donor solvents for hydrogenation is an attrac-
tive alternative to using high-pressure hydrogen, which presents
important safety and handling issues. In addition, alcohols, and
particularly ethanol, can be produced from renewable sources.
In the present work, the deoxygenation of linoleic acid into
hydrocarbons has been investigated. The reactions were run in
water/ethanol mixtures, ethanol being used as hydrogen-donor
solvent, in the presence of Pt/C or Ru/C catalysts, which were pre-
viously found the best catalysts under similar reaction conditions
◦
The Weisz-Prater criterion, calculated at 300 C using a diffu-
−
10
2
sion coefficient of 3.87 10
m /s for linoleic acid in water [22],
was found <0.05 indicating that internal diffusion limitations were
negligible.
◦
Kinetic studies were carried out between 250 and 350 C, with
a linoleic acid concentration between 1 and 2 g/L and using a cata-
lyst/linoleic acid ratio between 0.0625 and 0.125. The initial volume
of solution was always 150 mL. Liquid samples (1 mL) were period-
ically collected through a liquid sampling valve and were analyzed
by GC–MS.
[
21]. The hydrogenation process using ethanol was found highly
selective towards the formation of hydrocarbons. The reaction time
was accurately controlled by injecting the reactants under pres-
sure in a pre-heated autoclave, which allowed carrying out kinetic