30
I. Simakova et al. / Catalysis Today 150 (2010) 28–31
Table 3
corresponding to 85% of the total pore volume. Furthermore, the
relative volumes for pores in different ranges are given in Table 1.
The specific surface area and the metal dispersion of the fresh
catalyst were 379 m2/g and 38%, respectively. The average metal
particle sizes for the fresh and spent catalysts (see below) were
2.5–2.7 nm according to TEM measurements indicating that the
metal dispersion remained constant during the catalytic experi-
ments (Fig. 1).
Decarboxylation of fatty acids was performed over 1 wt.% Pd/C
(Sibunit) catalyst using five different fatty acids, C17, C18, C19, C20
and C22 as feedstocks at 300 8C in dodecane (Fluka) under argon
atmosphere at 17 bar total pressure. Catalyst activity and stability
during the deoxygenation of fatty acids is very much dependent on
the acid-to-catalyst amount ratio [3]. In the current work catalytic
deoxygenation of C18, C19, C20 and C22 (Entries 2, 3, 5 and 7 in
Table 2) was performed using the same ratio of the initial acid
concentration to catalyst mass and the results are presented in
Fig. 2. Intuitively the initial rates for decarboxylation of fatty acids
should be about the same over the same catalyst, since the
hydrocarbon chain in fatty acid is inert towards catalyst surface,
whereas the carboxylic group is adsorbed on the catalyst surface
resulting in the formation of a hydrocarbon exhibiting one carbon
atom shorter chain compared to the original carboxylic acid
according to the following scheme:
Characterization of the spent 1 wt.% Pd/C (Sibunit) catalysts after using in
decarboxylation of different fatty acids.
Spent catalyst (1 wt.% Pd/C)
used in the decarboxylation
of fatty acid
Metal
d Pd
(nm)
BET specific
surface area,
(m2/g)
dispersiona
(%)
Heptadecanoic acid
Stearic acid
8
4
2
8
3
14
27
52
13
40
330
331
222
305
242
Nonadecanoic acid
Arachidic acid
Behenic acid
a
Measured by CO-chemisorption.
reason for the lower decarboxylation rates for C19 and C22 can be
explained by the presence of impurities.
The amounts of Ca, Mg, Fe and P were investigated in
nonadecanoic and behenic acids by ICP-techniques. Especially,
in behenic acid the phosphorus content was relatively high being
266 mg/kg nonadecanoic acid. The ratio between the molar
amount of phosphorus to molar amount of surface Pd was
calculated by taking into account Pd dispersion and assuming that
all phosphorus would adsorb on Pd. The P/Pd ratio was 0.42
indicating that about 40% of surface active Pd can be covered by
phosphorus. This high phosphorus level is thus explaining the low
decarboxylation activity achieved for behenic acid. Furthermore,
the spent 1 wt.% Pd/C catalysts used in the decarboxylaiton of
different fatty acids were characterized by determining the
apparent Pd dispersion and the specific BET surface areas
(Table 3). Very low values of apparent metal dispersion (note
that HRTEM did not show any significant variations in the cluster
size) and BET specific surface area were achieved for the catalysts
used in behenic acid decarboxylation indicating both poisoning of
Pd as well as coking. The other impurities in the behenic acid were
according to GC–MS arachidic and palmitic acids, since the purity
of behenic acid was 80%. These compounds should not, however,
decrease the decarboxylation activity. On the other hand, their
decarboxylation products were also obtained as products. In the
case of nonadecanoic acid, which had the purity of 90%, the main
impurities were behenic acid and palmitic acid. The amount of
phosphorus in nonadecanoinc acid was only about 56% of the level
found in behenic acid. Thus the low decarboxylation rate of
nonadecanoic acid cannot be explained in exactly the same way as
in case of behenic acid. One possible reason for low decarboxyla-
tion rate could be the presence of unsaturated fatty acids, which
could undergo dimerization and Diels-Alder type of cyclisation
[12]. Under lack of hydrogen Pd is also known to act as a
dehydrogenation catalyst [13] and thus aromatic compounds
could be formed. No unsaturated fatty acids were, however, found
in GC–MS measurements in nonadecanoic acid. Coking was,
however, indirectly confirmed by recording low BET specific
surface area in the spent catalyst used for nonadecanoic acid
decarboxylation (Table 3). Furthermore, the apparent metal
dispersion measured by CO-chemisorption was very low for the
catalysts used in nonadecanoic acid decarboxylation, which most
probably can be explained by limited access to metal clusters due
to pore blocking by coke rather than sintering.
RCOOH ! RH þ CO2
Such experimental results confirming a possibility of neglecting
a potential geometric effect were reported recently in case of
palmitic and stearic acids decarboxylation [11].
The decarboxylation rates are not, however, the same for all
acids in this study. The same initial decarboxylation rates for C18
and C20 were achieved (Fig. 2), whereas for nonadecanoic and
behenic acids slow decarboxylation rates were obtained. The
Table 2
Kinetic data from catalytic deoxygenation of fatty acids at 300 8C under 17 bar
argon in dodecane.
Entry
Fatty acid
cFA (mol/l)/mcat. (g)
Conversion
after 150 min (%)
1
2
3
4
5
6
7
C17
C18
C19
C19
C20
C20
C22
0.05
0.1
91
93
19
90
83
98
23
0.1
0.02
0.1
0.07
0.1
The effect of the ratio between the initial concentration of fatty
acid to mass of catalyst was also compared. With a lower ratio
typically higher decarboxylation rates for fatty acids were
achieved. Similar results were also achieved for nonadecanoic
acid (Fig. 3) and for arachidic acid (Table 2). Decarboxylation of
heptadecanoic acid was performed using the initial concentration
to catalyst amount ratio of 0.05 (Table 2, Entry 1). For proper
comparison the results obtained with different ratios substrate/
catalyst should be plotted using a normalized abscissa, time
multiplied by catalyst mass divided by the initial concentration of
Fig. 2. Decarboxylation of stearic (&), nonadecanoic (^), arachidic (~) and behenic
(*) acid at 300 8C in dodecane using 1.0 g 1 wt.% Pd/C (Sibunit) catalyst. The initial
concentration of acid is 0.1 mol/l.
Simakova, Irina
