J. Markert et al. / Applied Catalysis A: General 462–463 (2013) 287–295
293
Fig. 9. Response to composition and pressure perturbations (a) introduced after 15 min from p0 = 15 bar = const., p = p = 12.5 bar to p = 25 bar with H2 and (b) starting
0
0
tot
CO
H
2
◦
with 15 bar H2. Two perturbations were introduced after 15 min to p = 25 (H2) bar and after 45 min to p = 35 bar (CO/H2) (T = 110 C, Cat/Ole = 1/10,000, Cat/Lig = 1/3.3, ligand
biphephos).
3
.3.2. Response to perturbations of hydrogen
Further perturbations were performed to identify the effect
hydrogenation of isomers corresponding to the formation of “iso-
aldehyde”. Thus, a direct formation of “iso-aldehyde” from the
isomers could be occurring. The reaction rate with respect to back-
isomerization and subsequent hydroformylation, which was also
observed in [28], seems to be very low under this conditions inves-
tigated. Furthermore, very small amounts of the terminal aldehyde
were formed without any remaining 1-dodecene in the reaction
mixture. This observation confirms the fact that the isomerization
is an equilibrium limited reaction. However, the formation of “iso-
aldehyde” is more favored under the conditions considered.
In summary on the basis of all dynamic perturbation exper-
iments carried out six reactions could be identified to be most
relevant for the reaction system investigated. They were consid-
ered in the formation of a reduced reaction network, which will
be described in the following section. Furthermore, an inhibiting
effect of carbon monoxide leading to inactive catalyst species (Rh-
dimers and Rh-dicarbonyls), which were even identified in [12],
was clearly recognized. It will be included in a hypothesis regarding
the catalytic cycle for the hydroformylation of 1-dodecene in the
thermomorphic solvent system using a Rh-biphephos-catalyst.
of hydrogen on the courses of the isomerization and hydrogena-
tion reactions of terminal and/or branched aldehydes. Therefore,
a perturbation experiment was performed with hydrogen after
feeding initially synthesis gas (Fig. 9a). The increase of the par-
tial pressure of hydrogen had under this reaction conditions a
slightly positive effect on the formation of the favored product
tridecanal. When 1-dodecene was totally consumed, only a small
further enhancement of the tridecanal formation is observed. This
indicates that tridecanal is formed primarily from 1-dodecene.
When 1-dodecene was consumed also the pseudo-component “iso-
aldehyde” could be detected in a lesser amount. In general, the
perturbation with hydrogen intensified the isomerization reac-
tion. The observed intermediate behavior of the “iso-dodecene”
indicates clearly that the hydrogenations of 1-dodecene and of
“
iso-dodecene” proceed. The latter could also be a combination of
back-isomerization and secondary hydrogenation. An experimen-
tal clarification of this aspect is not possible with the equipment
and techniques used due to instant conversion of the 1-dodecene
after back-isomerization.
4. Discussion
3.3.3. Combined perturbations of hydrogen and synthesis gas
Finally a more sophisticated perturbation experiment was
Based on all experimental observations made during the batch
designed for further clarification of the reaction network. The reac-
tor was initially pressurized with hydrogen up to H = 15 bar for
and semi-batch experiments, and accepting the limited analytical
information regarding the composition of the isomers, the set of 22
reactions given in Fig. 1 was reduced to six essential parallel-series
reactions. This reduced network is illustrated in Fig. 10.
2
1
5 min (batch conditions). Subsequently the reactor was pertur-
bated increasing the hydrogen amount up to 25 bar for 30 min.
Finally, the gas phase composition and the total pressure were
changed to synthesis gas (1:1) and 35 bar. Among all perturbation
experiments carried out, the enhancement of the hydrogen pres-
sure combined with a subsequent feeding of synthesis gas provided
the most sensitive effect on the reaction course (Fig. 9b). With this
experiment in particular the consecutive reaction of the dodecene
isomers could be evaluated. The increase of hydrogen in the feed
led to a higher reaction rate of the isomerization. Consequently, less
dodecane was formed during the first 15 min. Increasing the pres-
sure of hydrogen reduced the amount of “iso-dodecene” formed,
hydrogenating these isomers directly to dodecane. This demon-
strates that the catalyst was able to hydrogenate the branched
isomers as well as the linear dodecene. A coupled mechanism of
back-isomerization and additional hydrogenation of the terminal
olefin is also possible as mentioned in Section 3.3.2.
The main and desired reaction is for the considered catalytic
and solvent systems and the reaction conditions investigated the
hydroformylation of 1-dodecene (1) leading to tridecanal. As a
second main and important side reaction the reversible isomeriza-
tion to the pseudo-component “iso-dodecene” (2) was identified.
Without more knowledge about the individual isomers the ques-
tion, which isomer reacts to the corresponding or next internal
isomer as shown in Fig. 1, cannot be answered. Thus, a precise
description of motion of internal bonds is not reasonable for 1-
dodecene. The rates of the parallel reactions hydroformylation (1)
and isomerization (2) are approximately of the same magnitude
◦
at the temperature considered in this paper (T = 110 C). The yield
with respect to the desired product tridecanal can be particularly
enhanced by reducing the isomerization using an optimal pro-
cess trajectory manipulating the process variables (T, pCO) during
the reaction. The formed “iso-dodecene” can be converted in a
reversible reaction back to 1-dodecene (2) as reported by [28,34].
The second perturbation performed during this experiment, i.e.
the introduction of synthesis gas, caused a reduced rate of the