L. Völkl et al. / Journal of Catalysis 329 (2015) 547–559
551
In contrast, the Pd-IMes system yielded full conversion after
.5 h for the pure 1,3-butadiene feed. Compared to the explanation
applied, which both should be chemically inert in the telomeriza-
tion of butadiene with methanol. Iso-butene, one of the main com-
4
for the TPP ligand, this could stem from the fact that the palladium
is not able to coordinate more than one bulky IMes ligand and
therefore the equilibrium between species A and B is in favor of
the active species B. Even more surprisingly, the conversion of
the IMes-modified palladium catalyst increased when diluted
4
ponents of the sCC mixture, was tested as well. The results for
both modified catalysts are shown in Fig. 3. Additionally, n-butane
and 1-butene were used as inert compounds for the IMes-modified
palladium catalyst showing similar results as those for iso-butene
(see ESI, Fig. S2).
sCC
4
feed was used and full conversion was obtained within 4 h.
For the Pd-TPP system, the highest activity was obtained for
pure 1,3-butadiene as substrate, and all diluted substrates showed
reduced activity similar to the one for sCC . No difference between
4
This behavior was reproduced several times and the trend has also
been seen using a reactor with a fivefold larger reaction volume.
By plotting the amount of converted 1,3-butadiene versus reac-
tion time, the resulting rate for the Pd-IMes catalyst is independent
of the applied feed in the first hours. With the diluted feed, the rate
levels off after around 3.5 h. The amount of converted
an inert solvent or iso-butene could be observed. The values for the
selectivity to the main product, the chemoselectivity as well as the
n:iso ratios were nearly the same for hexane, toluene and iso-bu-
tene as additional solvent. The diluted systems showed a minor
incubation period of approx. 1 h, after which the reaction pro-
ceeded with higher rate. With progressing reaction, the even lower
substrate concentration in the batch reactor caused the activity of
all diluted systems to level off around 80% conversion after 6 h.
For the IMes-modified catalyst, the use of pure 1,3-butadiene
feed resulted in the lowest reaction rates again, while all other
1
,3-butadiene is lower due to the lower amount of 1,3-butadiene
present in the diluted feed. For the Pd-TPP catalyst the reaction
rate with pure butadiene is higher than with sCC
assumed dilution effect.
4
confirming the
As summarized in Table 1, the selectivity to the desired product
was significantly higher for the IMes-modified catalyst system
1
and seemed to be independent of the applied feed. For TPP, the
selectivity to 1 differed slightly for the two feeds and was higher
for pure butadiene.
4
diluting solvents and the sCC gave higher reaction rates. Within
the error margin the effect of all diluting solvents on the reaction
rate was identical and the rates were the same. The inerts n-butane
and 1-butene also behaved similar to iso-butene (see ESI, Fig. S2)
and the inert solvents toluene and hexane. This behavior indicates
that the Pd-IMes system is either deactivated by too high concen-
trations of 1,3-butadiene or follows a zero-order reaction kinetics
with respect to butadiene.
Compared to the TPP ligand, the IMes-modified systems
showed a distinctively lower selectivity to the byproducts 2, 3
and vinylcyclohexene 4, resulting in a higher n:iso ratio. The forma-
tion of byproducts with Pd-TPP was probably due to the higher
sensitivity of the TPP system to an excess of ligand due to its lower
steric demand [9]. For the TPP system, the coordination of a second
ligand is facilitated leading to lower 1-Mode selectivity as reported
by Vollmüller et al. [10].
Interestingly, all other studied Pd-NHC catalysts also showed
higher activity for the diluted sCC
4
compared to pure
1,3-butadiene (see ESI, Figs. S3 and S4). This indicates that the
observed effect is not limited to IMes-modified palladium com-
plexes, but represents a rather general effect for NHC ligands.
The influence of 1,3-butadiene concentration was further stud-
ied by varying the molar ratio of methanol to 1,3-butadiene. As no
In order to investigate the origin of the higher activity of the
diluted feed, pure 1,3-butadiene was combined with (a) an inert
solvent and (b) a component of the sCC
4
to mimic the dilution of
butadiene in sCC
4
. For these experiments hexane and toluene were
Table 1
Telomerization results obtained for two feeds and two different palladium catalysts after 6 h reaction time.
Feed
Ligand
X (%)
Y
1
(%)
Y
2
(%)
Y
3
(%)
S
1
(%)
S
1+2 (%)
n:iso
TON
1
sCC
,3-Butadiene
TPP
TPP
IMes
IMes
95.7
76.2
99.1
99.9
83.2
64.4
96.7
97.3
8.2
6.5
1.7
1.6
3.9
4.9
0.4
0.4
87
95.5
93.1
99.4
99.1
10.2
9.9
55.5
59.7
38311
30562
39676
39925
4
84.5
97.6
97.5
1
,3-Butadiene
sCC
4
Definition of abbreviations: X = conversion, Y = yield, S = selectivity, TON = turnover number.
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.0
0.8
0.6
0.4
0.2
0.0
0
2
4
6
0
2
4
6
time / h
time / h
Fig. 3. Conversion in Pd-TPP (left) and Pd-IMes (right) catalyzed telomerization of pure 1,3-butadiene (&), sCC
4
(h) and 1,3-butadiene diluted with hexane (M), toluene and
1
ꢁ
iso-butene (X). Reaction conditions: 70 °C, 15 bar, Vreaction = 140 ml, nbutadiene:nMeOH = 0.5, nbutadiene:nPd = 40,000, nLig:nPd = 4, nbutadiene:nbase = 400, cPd,0 = 0.15 mmol l and
ꢁ
1
ꢁ1
ꢁ1
c
4
butadiene,0 = 6.1 mol l for experiment with pure butadiene, cPd,0 = 0.086 mmol l and cbutadiene,0 = 3.4 mol l for experiments with sCC /diluted feeds.