M. Arend et al. / Applied Catalysis A: General 399 (2011) 198–204
201
1
00
100
90
Product mass balance
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
80
Converison
C17 Selectivity
Selectivity Heptadecane
Selectivity Heptadecenes
7
6
0
0
50
4
3
2
1
0
0
0
0
0
20
40
60
80
100
120
140
160
180
60
120
180
240
Time on stream [min]
Time on stream [min]
Fig. 4. Mass balance of all collected liquid products over time on stream; reaction
conditions: T = 380 C, V = 50 ml/ min, m˙ OA = 6.2 ml/h, mCatalyst = 5 g.
◦
˙
Fig. 3. Conversion and C17 selectivity of a reference reaction plotted over time on
stream; C17 selectivity is the combination of the selectivities to heptadecane and all
H2
◦
˙
heptadecene isomers (grey lines); reaction conditions: T = 380 C, VH = 50 ml/ min,
2
m˙ OA = 6.2 ml/h, mCatalyst = 3 g.
combination with the smoothed particle shape, resulting in lower
activity.
The time dependant mass balance was comparable in all exper-
iments. A single experiment using 5 g of the above mentioned
catalyst was conducted to determine the loss of mass in detail. Fig. 4
shows the overall mass balance of this experiment.
tion could be e.g. thermal cracking or hydrogenation of the reactant
oleic acid. A marginal conversion of less than 10 mol% was observed
in the blank test. However, no desired C17 products (heptadecane
and heptadecenes) were formed. Stearic acid was the only product
found which was formed by hydrogenation of oleic acid. Conse-
quently, a low hydrogenation performance of the stainless steel
tube was concluded if pure hydrogen was used as reactant and
carrier gas, respectively.
After 20 min TOS, a low mass balance of about 20 wt% was
observed which was probably due to adsorption processes on the
charcoal surface and the reactor. Additionally, we assume that the
catalyst showed an incubation period in which the catalytically
most active sites were most probably deactivated by the deposi-
tion of high molecular weight compounds and by coke formation.
After 60 min the mass balance increased to about 70–80 mol% and
finally reached a steady state at about 90% after 2–3 h. A complete
decarboxylation of oleic acid would result in a mass loss of 14.8 wt%,
caused by the elimination of CO2. As the highest obtained decar-
boxylation rate was about 37 mol%, the mass loss by the formation
of CO2 was at most 5.5 wt%, which, partially, explains the loss of
mass of the reaction. The mass balance was calculated by means of
all collected liquid products (21.42 g) after 180 min. The initial mass
of the charcoal catalyst (Pd/C), 5 g, gained 0.39 g of weight during
the reaction due to the adsorption processes. Additionally, 1.10 g of
3.4. Reference reaction and mass balance of the reaction
Fig. 3 shows the conversion and selectivity of a reference reac-
◦
tion over time on stream (T: 380 C, H -flow: 50 ml/min, Catalyst:
2
3
g Pd/C). In the beginning of the reaction, oleic acid was converted
completely. After 180 min time on stream (TOS), the conversion
rate dropped to 80 mol% and decreased further with longer TOS.
We assume that the decrease in conversion was caused by
coking and accumulation of different compounds on the cata-
lyst surface, e.g. oleic acid or heptadecane, leading to catalyst
deactivation by product inhibition as shown by TGA measure-
ment. The main product, found to be stearic acid, was formed by
hydrogenation of oleic acid. Stearic acid and the C17 products (hep-
tadecane and heptadecenes) were the only products found within
the reaction mixture. Typically, higher amounts of heptadecane
were found in the beginning of the reaction, whereas the selectivity
to all heptdadecene isomers increased with longer time on stream.
This is probably due to decreasing hydrogenation performance by
the abovementioned deactivation mechanisms. Gas phase analysis
showed the existence of CO2 and very low amounts of CO. Exact
gas phase composition could not be determined due to technical
limitations.
The selectivity to C17 products increased slightly between 60
and 120 min TOS. The selectivity reached a maximum of 37 mol%
after about 120 min and dropped afterwards. After 180 min, the
selectivity to the desired C17 products was about 26 mol%. We infer
that the decreasing selectivity after 120 min TOS was caused by cok-
ing and the accumulation of different compounds on the catalyst
surface, covering the surface and thus suppressing the reaction at
the catalytic centres of the catalyst. This assumption was supported
by the fact that the initial BET surface area dropped by more than
◦
gaseous product was liquefied at -196 C, using liquid nitrogen.
All products were considered for the calculation of the over-
all mass balance. Hence, the difference between feed (24.88 g) and
recovered reaction products (22.90 g) was due to liquid residues in
the reactor or in the cooling trap, coke formation, polymerization
or the formation of gaseous products which were not liquefied in
liquid nitrogen. The overall loss of mass was 1.98 g, corresponding
to 7.9 wt%. However, it cannot be assured that all gaseous prod-
ucts were liquefied. In particular, the boiling point of CO is close
to the temperature of liquid nitrogen. This loss of mass is also a
reason for the highly problematic quantification of the gas phase
composition, as the calculation of the individual component flow
rates would not be accurate. Therefore, qualitative analysis was
the only possibility to determine the composition of the gas phase.
Additionally, water–gas shift reaction (WGSR) or reverse WGSR can
occur in hydrogen atmosphere at elevated temperatures and would
therefore affect the determined ratio of CO2/CO. Consequently, the
CO2/CO ratio was considered to be rather uncertain and not used for
a clarification of the reaction mechanism and catalyst deactivation.
3.5. Reaction optimization
9
9%. Consequently, less surface area resulted in less available cat-
alytic centres. As the Pd crystals were initially irregularly shaped
with sharp edges, which became all round and smoothed after the
reaction, we conclude that rounded Pd particles provide less sur-
face area, compared to the surface area before the reaction, thus, in
The influence of temperature, catalyst amount and hydrogen
flow on conversion rate and selectivity were investigated in an
experimental design which was generated by the statistical soft-
ware Design-Expert, Version 5.0.8, Stat-Ease Inc. The Influence