A. Behr et al. / Applied Catalysis A: General 505 (2015) 243–248
247
In order to draw further conclusions regarding the effects of
using inorganic salts and additives in iridium-catalyzed hydro-
into consideration (Table 4).
1-dodecene provided similar results with aldehyde selectivities
higher than 90% (Table 4, entries 1, 3 and 5). Experiments with
internal olefins such as 4-octene and 2-pentene and cyclohexene
afforded chemoselectivities up to 93% as well; however, the alkene
conversion was low (Table 4, entries 2, 4 and 6). Iridium-catalyzed
hydroformylation using internal olefins is thus very difficult to
achieve. Experiments with 2-pentene and 4-octene produced an
l:b ratio of 1:99 and did not result in isomerized hydroformylation.
stimulating isomerized hydroformylation. With the substrate 3,3-
dimethyl-1-butene, which is unable to isomerize, a l:b-ratio of 95:5
with a high aldehyde selectivity of 97% and a conversion of 59% was
reached (Table 4, entry 7).
Fig. 4. Results of varying the LiCl concentration.
8.91 mmol 1-octene, 4 g NMP, 0.0713 mmol Ir(cod)(acac), 0.1579 mmol PPh3, LiCl,
30 bar (CO/H2 = 2/1), 100 ◦C, 16 h, 700 min−1
.
4. Conclusion
The use of NaCl, with its low price, could pose a particularly sig-
nificant economic opportunity for potential industrial processes.
Consequently, NaCl was chosen as an additive for further inves-
tigations besides LiCl. Furthermore, no significant dependence on
different anions was observed, too. However, the strong effect and
improvement on the chemoselectivity by addition of each salt can
be validated very well.
In summary, the addition of inorganic salts, e.g., LiCl, LiBr or
NaCl, had strong effects on the suppression of hydrogenation prod-
ucts in iridium-catalyzed hydroformylation. Even small amounts
of the additives were sufficient to achieve high chemoselectivities
towards the desired aldehydes of up to 94%, which represents an
improvement of more than 20%. Our investigations with different
additives led to the assumption of a preforming effect caused by
both a cationic and an anionic influence. First kinetic investigations
resulted in high reaction rates and a conversion of 90% after only
7 h. In addition to using the precursor Ir(cod)(acac), also other Ir(I)
complexes can be applied. The optimized catalyst concentration
was in the range of 0.8 mol%. With optimized reaction parameters,
converting other substrates such as 1-pentene, 1-dodecene and 3,3-
dimethyl-1-butene to high yields of the desired aldehydes was also
possible. Due to the high suppression of undesired alkanes, the
described selective catalyst system for iridium-catalyzed hydro-
formylation was investigated on a continuously operated miniplant
scale, too. The results of these experiments will be published in a
separate paper soon.
Therefore, the presence of anions e.g., Cl− in the solution is
very important to achieve high hydroformylation performances.
Based on the precursor Ir(cod)(acac), the presence of anions might
have a strong preforming effect to the formation of the appro-
priate hydroformylation complex containing the desired ligands
PPh3, CO and even Cl− or hydride. Especially, the chloride anion
is able to coordinate to the iridium and prevents the formation
of undesired hydrogenation species. The presence of chloride in
an active iridium precursor can be seen with the Vaska com-
plex (IrCl(CO)(PPh3)2) (chapter 3.2, Table 2, entry 5). In this case,
good hydroformylation performances could be observed already
without the addition of LiCl. This effect was also observed with
IrH(CO)(PPh3)3, although this precursor contains no chloride. This
could indicate, that the chloride only causes the preforming and
coordination of the desired ligands PPh3, CO and hydride to an
appropriate intermediate as described above. Another possibility is
ers may interact with Ir(cod)(acac) and effect the formation of an
active hydroformylation complex as well.
Acknowledgements
The described studies were realized within the scope of the
“PROFORMING” project (no. 03X3559) financed by German Fed-
eral Ministry of Education and Research (BMBF). The authors would
like to thank Evonik Industries and Leibniz Institute of Catalysis
(LIKAT) for their cooperation and helpful discussions. The authors
would also like to thank Umicore AG & Co. KG for donation of metal
precursors.
Therefore, chlorine-based additives with different cations were
investigated (Table 3, entries 8–13). The results showed high
chemoselectivities 2 + 3 at about 90% with a maximum conversion
of 89%. However, in some cases, e.g., with Al- and Fe-salts, the con-
version was much lower than using alkali or alkaline earth metals.
Thus, the best combination seems to use alkali salts with halogen
ions such as Cl− or Br−.
References
Although, no strict dependency on different anions or cations
could be figured out with our results, the crucial effect of additives
on the hydroformylation performance was well validated. The pos-
itive influence on the chemoselectivity may be the result of adding
both anions and cations to the reaction as already assumed by
Haukka et al. [22]. In our further studies, we preferred the use of LiCl
as an additive in the iridium-catalyzed hydroformylation due to the
high selectivities of 94% and conversion of 86%. However, from an
economical point of view the application of NaCl as an alternative
additive should remain in mind.