LIQUIDꢀPHASE HYDROGENATION OF ACETYLENE ON THE Pd/SIBUNIT CATALYST
255
In the liquidꢀphase reaction, the concentration of (31.0% altogether). The mechanism of the formation
acetylene, which is involved in an adsorption equilibꢀ of these compounds is beyond the scope of this study.
rium with palladium, is increased owing to the high In addition,
solubility of acetylene in ꢀmethylpyrrolidone and is products were detected, namely,
about 7 cm /cm under the reaction conditions. This (16.5%) and ꢀmethylpropanamide (7.5%).
N
ꢀmethylpyrrolidone hydrogenation
N
Nꢀmethylacetamide
3
3
N
can slow down the reaction. Along with acetylene,
carbon monoxide also competes with hydrogen for
adsorption sites on the palladium surface, and,
accordingly, the reaction rate in the presence of CO is
always lower. It can be assumed that the strongly
bound CO adsorption species are not displaced by
acetylene owing to their heat of adsorption far exceedꢀ
When the reaction is conducted in the presence of
CO, the resulting liquid phase again contains aromatic
compounds, namely, benzene, ethylbenzene, and styꢀ
rene (~27% altogether). Linear and cyclic unsaturated
C hydrocarbons are also detected, whereas hydrooliꢀ
8
gomerization in the absence of CO involves at most
three acetylene molecules. Chain propagation in the
presence of CO is possible owing to the decrease in the
hydrogenation rate because of the decrease in the
amount of surface hydrogen, which is replaced by carꢀ
ing the Нads of C H (see above). As a consequence,
Δ
2 2
there are fewer active sites available for acetylene and
the reaction slows down.
The selectivity of the reaction depends considerꢀ bon monoxide. Note that no solvent conversion prodꢀ
ably on the acetylene concentration (Fig. 2b). As the ucts were detected in the presence of CO.
acetylene concentration is raised, SC2H4 decreases and
SC2H6 increases. According to Borodzinski and Bond
The effect of the hydrogen concentration on the
characteristics of the reactions in the system is illusꢀ
trated by Fig. 3. The ethane selectivity remains almost
[
4], there are two types of active sites on the palladium
surface: soꢀcalled Aꢀsites, on which only acetylene can
invariable over the CH2 = 30–90% range and is about
be hydrogenated, and Eꢀsites, on which the hydrogeꢀ 1%. In the same CH2 range, SC2H4 increases by a factor
nation of both acetylene and ethylene can take place. of about 2 and the proportion of acetylene involved in
Carbon monoxide primarily blocks Eꢀsites, thus
increasing the ethylene selectivity. Acetylene displaces
hydrooligomerization (SCnHm ) decreases by a factor of
2
.4. At low hydrogen concentrations, the dominant
CO from adsorption sites on the palladium surface,
thereby shifting the Pd–CO + C H2 d–C H +
route is acetylene hydrooligomerization, which
accounts for 60% of the reacted C H .
P
2
2
2
2
2
CO equilibrium to the right. The displacement of CO
from Eꢀsites raises the ethane selectivity because the
ethylene resulting from acetylene hydrogenation on the catalytic activity increases in proportion to
In the hydrogen concentration range of 30–90%,
C
H2
these sites does not desorb and undergoes further (Fig. 3a). Acetylene hydrogenation to ethylene is
hydrogenation to ethane, while the ethylene forming known to be firstꢀorder with respect to hydrogen [8].
on Aꢀsites desorbs because of their small size. This is Two reactions—hydrogenation and hydrooligomerꢀ
how the increase in the ethane selectivity is explained. ization—take place in the system. The overall order of
This effect is more pronounced at the lower CO conꢀ the reaction with respect to hydrogen, calculated from
centration (2%) because the equilibrium in the comꢀ the observed acetylene conversion rate, is also unity. It
petition between acetylene and carbon monoxide for is, therefore, likely that the hydrooligomerization
active sites of the catalysts is more strongly shifted to reaction is zeroꢀorder with respect to hydrogen. It is
the right.
possible that its rateꢀlimiting step is one of the chain
propagation steps, e.g., the insertion of an acetylene
molecule into the Pd–C bond of the intermediate
compound.
Raising the acetylene concentration increases the
contribution from acetylene hydrooligomerization.
The proportion of acetylene converted into the
hydrooligomers can be as large as 40%.
The identification of reaction products by the GCꢀ
MS method demonstrated that the composition of the
acetylene hydrogenation products depends on
whether CO is present in the feed. When the reaction
is conducted in the absence of CO, the gaseous prodꢀ
ucts consist largely of С2 hydrocarbons (87.5%) and
The temperature of the reaction medium exerts a
significant effect both on the catalytic activity and on
the selectivities of the reactions in the C H –H –CO
2
2
2
system (Fig. 4).
The Arrhenius apparent activation energy in the
0–80 range is 22 kJ/mol. This value is close to the
Еа of the diffusion of molecules in the liquid phase,
5
°С
contain a considerable amount of butenes (6.3%). In which is 19–46 kJ/mol [19]. This fact suggests that the
the presence of CO, the total amount of С2 hydrocarꢀ rateꢀlimiting process under our experimental condiꢀ
bons is again fairly large (69.1%), acetylene hydrooliꢀ tions is the diffusion of substrate and product compoꢀ
gomerization is more pronounced, and the proportion nents through the solvent layer adjoining the catalyst
of 2ꢀbutene is 3 times higher.
According to the GCꢀMS analyses of the liquid
phase, in the absence of CO the liquid product consists
particle. Lowering the reaction temperature decreases
the ethylene selectivity by a factor of ~2, from 86.1 to
41.8%, in spite of the presence of CO in the gas mixꢀ
largely of ethylcyclohexadiene (39.2%) and contains ture. Simultaneously,
S
2
C H increases by a factor of ~3
6
considerable amounts of benzene and its derivatives and the proportion of acetylene converted into
KINETICS AND CATALYSIS Vol. 52
No. 2
2011