Preparation of C9-aldehyde via aldol condensation reactions in ionic liquid media

Article abstract of DOI:10.1039/b203068c

C₉-aldehyde has been prepared via aldol condensation reactions in ionic liquid media; catalyst investigation showed enhanced product selectivity for the desired aldehyde in ionic liquid media than in conventional solvent systems.

Full text of DOI:10.1039/b203068c

Preparation of C9-aldehyde via aldol condensation reactions in ionic
liquid media
Christian P. Mehnert,*a Nicholas C. Dispenziereb and Raymond A. Cooka
a ExxonMobil Research and Engineering Company, Corporate Strategic Research, 1545 Route 22 East,
Annandale, NJ 08801, USA. E-mail: christian.p.mehnert@exxonmobil.com
b ExxonMobil Chemical Company, Basic Chemicals & Intermediates Technology, 1545 Route 22 East,
Annandale, NJ 08801, USA
Received (in Corvallis, OR, USA) 26th March 2002, Accepted 7th May 2002
First published as an Advance Article on the web 26th June 2002
C9-aldehyde has been prepared via aldol condensation
reactions in ionic liquid media; catalyst investigation showed
enhanced product selectivity for the desired aldehyde in
ionic liquid media than in conventional solvent systems.
temperatures and can be utilized for a wide variety of
applications. The liquid phases exhibit very low vapor pressure,
tunable polarity, and high thermal stability. Depending on the
application the ionic fragments can be designed to accom-
modate catalysis or separation in the most efficient way. Fig. 2
shows imidazolium based ionic liquids containing tetra-
fluoroborate and hexafluorophosphate anions, which have been
investigated in our study.
For the catalyst preparation† the ionic liquid phases were
treated with a concentrated solution of sodium hydroxide. The
resulting mixtures were investigated in parallel with the
aqueous solution‡ of the base. The hydroxyl concentration of
the aqueous solution and the ionic liquid phases were adjusted
to 1 molar, which corresponds to a conventional concentration
for an aldol catalyst system (4 wt% NaOH).
Our first investigation (Table 1) focused on the evaluation of
the self-aldol condensation reaction of propanal to form
2-methylpent-2-enal (Aldol I). The reaction progresses through
an aldol intermediate and produces the unsaturated aldehyde
under the applied reaction conditions. We investigated two
types of ionic liquid phases consisting of [bmim] and [bdmim]
cations with tetrafluoroborate and hexafluorophosphate anions.
Most of the reactions reached quantitative conversion of the
propanal after a reaction time of 3 h. The highest product
selectivity (82%) was found for the ionic liquid [bdmim][PF6],
which is identical to the selectivity obtained for our aqueous
base case study. Although the conversion values of the other
ionic liquid phases were comparable to water, their product
selectivities were reduced. In most of these reactions a large
amount of higher boiling aldehydes were produced. Due to the
increased solubility of the formed C6-aldehydes in the ionic
liquids higher boiling side-products are formed via cross-aldol
condensation reactions. In contrast, if the reaction is carried out
in an aqueous solution the resulting C6-aldehydes have only a
limited solubility and therefore side-reactions are limited.
Similar to our results for the Aldol I investigation the catalyst
evaluation of Aldol II reactions showed comparable conversion
values for the basic ionic liquids and the aqueous sodium
hydroxide system. However, the selectivity towards the con-
densation products were significantly increased for the sodium
hydroxide containing ionic liquid phases. In our investigation of
basic [bmim][BF4] (entry 7) the selectivity towards the desired
2,4-dimethylhept-2-enal was 20% higher than in the sodium
hydroxide–water system. This increased selectivity is attributed
to the higher solubility of the starting material 2-methylpentanal
in the ionic liquid phase.
The aldolization1 of ‘oxo’ aldehydes is a large volume process
and important for the production of plasticizers. This reaction2
utilizes n-butyraldehyde as a carbon building block and is
carried out with a world production capacity of more than 2.3 3
106 t a21. After the self-aldol condensation reaction of n-
butyraldehyde, followed by hydrogenation, the resulting prod-
uct 2-ethylhexanol is used for the production of dioctyl
phthalate plasticizer. Alternatively, propanal can be used as a
carbon building block and be converted via two consecutive
aldol condensation reactions (Fig. 1) to generate 2,4-dime-
thylhept-2-enal. The unsaturated aldehyde can be hydrogenated
and further used as a precursor for the preparation of phthalate
ester.
The first step involves the self-aldol condensation of propanal
to form 2-methylpent-2-enal (Aldol I). Followed by the
selective hydrogenation of the condensation product the
resulting complex 2-methylpentanal is further reacted with
propanal in a cross-aldol condensation reaction (Aldol II). The
hydrogenation of the resulting aldehyde gives the alcohol
2,4-dimethylheptanol, which is a potential feed for the produc-
tion of phthalate ester.
Next to solventless reaction systems,3 many aldol condensa-
tion reactions are carried out in the liquid-phase using aqueous
base-catalysis (e.g. NaOH or basic ion exchanged resins).
Although water is clearly preferred as a solvent, other criteria
like recyclability, waste water contamination, and selectivity of
the aldol reaction have to be considered when choosing the most
appropriate solvent. Therefore we investigated ionic liquid
phases as a potential solvent medium for aldol condensation
reactions.
Ionic liquids4 are currently investigated for a variety of
different applications, e.g. solvent media for homogeneous
catalysis,5 extraction processes,6 membrane technology,7 oligo-
merization and polymerization catalysis.8 These ionic systems
are salts that have melting points at or below ambient
In the cross-aldol condensation reaction (Aldol II) it is very
important that both substrates have high enough solubility in the
Fig. 1 Self-aldol condensation reaction of propanal to form 2-methylpent-
2-enal (Aldol I) and cross-aldol condensation reaction of propanal and
2-methylpentanal to form 2,4-dimethylhept-2-enal (Aldol II).
Fig. 2 Imidazolium based ionic liquid phases.
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Table 1 Aldol condensation reaction carried out in sodium hydroxide containing ionic liquid media
Reaction
Typec
Temperature/ Conversiond/ Selectivitye
Selectivityf
C6/wt%
Selectivityg
C9= /wt%
Selectivityh
> C9/wt%
Entrya
Reaction Mediab
°C
wt%
C6= /wt%
Comments
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
[bmim][BF4]
[bmim][PF6]
[bdmim][BF4]
[bdmim][PF6]
H2O
[emim][BF4]
[bmin][BF4]
[bmim][BF4]
[bmim][BF4]
[bmim][BF4]
[hmim][BF4]
[omim][BF4]
[bmim][PF6]
[bdmim][BF4]
[bdmim][PF6]
H2O
Aldol I
Aldol I
Aldol I
Aldol I
Aldol I
Aldol II
Aldol II
Aldol II
Aldol II
Aldol II
Aldol II
Aldol II
Aldol II
Aldol II
Aldol II
Aldol II
80
80
80
80
80
90
95
96
99
97
90
90
96
95
95
90
99
99
99
100
100
100
100
99
100
99
100
100
99
64
74
68
82
82
4
3
10
8
2
1
1
1
0
6
3
9
12
7
15
19
13
9
20
0
—
—
—
—
—
69
80
62
48
26
57
60
59
54
45
59
33
25
31
17
18
21
14
19
32
61
16
21
15
21
24
5
Base case
1st recycle
2nd recycle
3rd recycle
6
12
10
13
15
11
36
96
99
100
Base case
a Reaction time of 3 h. b Ionic liquid media: [emim] 1-ethyl-3-methylimidazolium, [bmim] 1-butyl-3-methylimidazolium, [hmim] 1-hexyl-3-methylimidazo-
lium, [omim] 1-methyl-3-octylimidazolium, [bdmim] 1-butyl-2,3-dimethylimidazolium. c Aldol I - self-aldol condensation reaction of propanal, Aldol II -
=
cross-aldol condensation reaction of propanal and 2-methylpentanal. d Conversions of propanal. e C6 — 2-methylpent-2-enal. f C6 — 3-hydroxy-
2-methylpentanal and other C6-derivatives. g C9 — 2,4-dimethylhept-2-enal. h > C9 — higher boiling aldehydes and oligomers.
=
reaction phase. In the aqueous sodium hydroxide system the
substrate propanal is fully water-soluble while the other
Notes and references
†
Aldol I catalyst evaluation: (a) the ionic liquid (40.0 mmol) was treated
substrate 2-methylpentanal exhibits only limited solubility.
Although the reaction is already carried out with a four-fold
excess of 2-methylpentanal to compensate for the low sol-
ubility, the reaction only reaches a product selectivity of 60%.
The major side reaction occurs through the self-condensation of
propanal to form 2-methylpent-2-enal with a selectivity of 36%.
By carrying out the same reaction under identical conditions in
the basic ionic liquid phase [bmim][BF4] the side reaction was
reduced to only 4% and the product selectivity for the desired
2,4-dimethylhept-2-enal increased to 80%. This improvement
of the selectivity is attributed to the higher solubility of the
substrate 2-methylpentanal in the ionic liquid reaction phase.
Further studies on varying the polarity of the ionic liquid phase
[bmim][BF4] revealed that the butyl substituent produced the
highest selectivity while hexyl-, octyl-, and ethyl-groups
showed slightly decreased values. The polarities of these ionic
liquid phases can be further adjusted through structural
variations and further improvements of the selectivity could be
expected through fine-tuning of the system.
After the conclusion of the reaction the products are
conveniently isolated by distillation. The resulting ionic liquid
phases were freed from organic sodium salts and reused as
solvent media for additional aldol reactions. The formation of
organic sodium salts (mostly carboxylic sodium derivatives) is
a side-reaction in the aldol condensation process and is apparent
in both the ionic liquid and aqueous sodium hydroxide
system.
with an aqueous solution (1 mL) of sodium hydroxide (8.8 mmol, 0.35 g)
and decane (internal standard) (5.6 mmol, 0.7 g). After the resulting mixture
was heated to 80 °C, propanal (232.4 mmol, 13.5 g) was added. The reaction
mixture was kept under reflux conditions for 3 h; (b) an aqueous solution of
sodium hydroxide (1 M) (8.8 mmol, 0.35 g) and decane (internal
standard)(5.6 mmol, 0.8 g) was heated to 80 °C. After the addition of
propanal (232 mmol, 13.5 g) the reaction mixture was refluxed for an
additional 3 h.
Aldol II catalyst evaluation: (a) the ionic liquid (40.0 mmol) was treated
with an aqueous solution (1 mL) of sodium hydroxide (8.8 mmol, 0.35 g),
2-methylpentanal (180 mmol, 18.0 g), and nonane (internal standard)(7.8
mmol, 1.0 g). After the resulting mixture was heated to 90 °C, propanal
(44.8 mmol, 2.6 g) was added. The reaction mixture was kept under reflux
conditions for 3 h; (b) the aldehyde 2-methylpentanal (175 mmol, 17.5 g)
and nonane (internal standard)(8.7 mmol, 1.1 g) were added to an aqueous
solution of sodium hydroxide (1 M) (8.8 mmol, 0.35 g). The resulting
mixture was heated to 90 °C and treated with propanal (44.8 mmol, 2.6 g).
The reaction mixture was refluxed for an additional 3 h. Product isolation
was carried out via distillation under reduced pressure. Conversion and
selectivity were determined by GC and GC/MS techniques.
‡ Aldol condensation reactions in aqueous sodium hydroxide solutions can
be influenced by phase transfer catalysis (e.g. the usage of surfactants like
methyltrioctylammonium chloride can accelerate the reaction and change
the selectivity).
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A recycle study of the ionic liquid medium [bmim][BF4] was
carried out for the cross-aldol condensation reaction (entries
7–10). Each of the four runs showed greater than 99%
conversion under the reaction conditions applied. Due to the
fact that the heavier reaction products were accumulated in the
ionic liquid phase the relative product selectivity decreased
from 80% to 26%. Therefore, these relative product selectivities
need to be evaluated accordingly.
The investigated ionic liquid media offer an interesting
alternative to carry out aldol condensation reactions. In light of
their enhanced product selectivity and the ease of recycle, these
ionic phases could compliment traditional aldol catalyst
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