Advantages of the solventless hydroformylation of olefins

Article abstract of DOI:10.1016/j.molcata.2015.07.021

Abstract The hydroformylation of olefins using Rh(acac)(CO)₂ as a catalyst with the excess of PPh₃ was investigated at the temperature of 80°C within the pressure range from 4 to 12 bar in a neat substrate, without a solvent. The conversion of 1-hexene was complete, with a linear-to-branched aldehyde ratio of ca. 10. Very good results were also obtained for 1-pentene and 1-octene. The catalytic performance of the Rh(acac)(CO)₂/PPh₃ catalytic system in hydroformylation under solventless conditions was better than that in toluene, owing to the high concentration of the reactants. Recycling experiments confirmed the good stability of the catalyst and its constant activity.

Full text of DOI:10.1016/j.molcata.2015.07.021

Journal of Molecular Catalysis A: Chemical 408 (2015) 147–151
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Advantages of the solventless hydroformylation of olefins
∗
W. Alsalahi, A.M. Trzeciak
Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The hydroformylation of olefins using Rh(acac)(CO) as a catalyst with the excess of PPh was investigated
at the temperature of 80 C within the pressure range from 4 to 12 bar in a neat substrate, without
a solvent. The conversion of 1-hexene was complete, with a linear-to-branched aldehyde ratio of ca.
2
3
Received 14 March 2015
Received in revised form 9 July 2015
Accepted 18 July 2015
◦
1
0. Very good results were also obtained for 1-pentene and 1-octene. The catalytic performance of the
Available online 27 July 2015
Rh(acac)(CO)2/PPh3 catalytic system in hydroformylation under solventless conditions was better than
that in toluene, owing to the high concentration of the reactants. Recycling experiments confirmed the
good stability of the catalyst and its constant activity.
Keywords:
Rhodium
Hydroformylation
Solventless
Olefin
©
2015 Elsevier B.V. All rights reserved.
1
. Introduction
Monnereau et al. reported the application of the hemispherical
Rh(acac)(1,3-calix-diphosphite) complex for this reaction with a
very good activity. For example, 33% of 1-nonanol was obtained
at 80 C and 20 bar with TOF of 17,290 h [6]. Colbey, Klosin, and
Whiteker used a rhodium catalyst with (R,S)-BINAPHOS and (S,S)-
kelliphos for the asymmetric hydroformylation of allyl cyanide
under solvent-free conditions and found a high reaction rate [12].
Olefin hydroformylation, the addition of synthesis gas (CO/H2)
◦
−1
to a carbon–carbon double bond in the presence of a transition
metal catalyst, is one of the most important homogeneously cat-
alyzed industrial processes. The hydroformylation of 1-olefin yields
a mixture of products, a linear n-aldehyde and a branched iso-
aldehyde. These aldehydes can be easily converted into many
industrially important secondary products, such as solvents,
biodegradable detergents, soaps, fragrance, flavors, perfumes,
pharmaceuticals, surfactants, lubricants, and plasticizers [1–7]. The
hydroformylation reaction has been carried out in different reac-
tion media such as organic solvents [8], aqueous-biphasic [9],
supercritical CO2 [10], or ionic liquids [5,11].
Clarke and Roff used Rh(acac)(CO) with 1,3,5,7-tetramethyl-2,4,8-
2
trioxa-6-phosphaadamantane for the hydroformylation of methyl
acrylate under solvent-free conditions receiving high selectivity to
the branched product [13].
In this paper, we report on the solventless hydroformylation of
olefins using Rh(acac)(CO) as a catalyst with an excess of PPh . This
2
3
catalytic system is well known for the hydroformylation of olefins.
However, all reported studies have been performed in organic sol-
vents. It was interesting for us to check whether any improvement
of the catalytic activity can be achieved by the elimination of the
solvent from the catalytic system.
Solvent-free synthesis is one of the most important methods of
green chemistry, which has recently become increasingly attrac-
tive. Among the expected benefits of solvent-free processes are
cost savings, a high reaction rate, decreased energy consumption,
the easy separation and purification of products, and a significant
reduction in reactor size and capital investment. The solvent-free
method has also been applied to hydroformylation [6,12]. In 2010,
2. Experimental
2.1. Materials
The rhodium complex Rh(acac)(CO) was prepared according to
2
the literature [14].
∗ Corresponding author.
E-mail address: anna.trzeciak@chem.uni.wroc.pl (A.M. Trzeciak).
Triphenylphosphine (PPh ) was purchased from Avocado; 1-
hexene and 1-pentene were purchased from Merck; 1-octene was
3
http://dx.doi.org/10.1016/j.molcata.2015.07.021
1
381-1169/© 2015 Elsevier B.V. All rights reserved.

1
48
W. Alsalahi, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 147–151
Table 1
Hydroformylation of 1-hexene using Rh(acac)(CO)2/PPh3.
TOF, (h 1
−
)
Entry
Conversion (%)
Hexane (g)
2-Hexene (%)
l
b
l/b
Acid (%)
(
%)
(%)
1a
99.2
99.5
100
4
2
1.5
6.4
10
4.5
69.7
77
81
18.7
8.5
11
3.7
9.1
7.4
0.4
2
2
438
970
1330
b
2
c
3
Reaction conditions: 1-hexene (0.012 mol, 1.5 mL), [Rh] (2.9 × 10 mol), (PPh3 = 1.6 × 10−4 mol) [1-hexene]/[Rh] = 420, [L]/[Rh] = 6.8,T = 80 C, (H2:CO)(1:1) = 10 bar, t = 40 min.
−
5
◦
a
Toluene as a solvent, t = 90 min.
b
Solventless.
c
Recycled catalyst from entry 2.
purchased from Sigma–Aldrich; Styrene was purchased from Alfa
3. Results and discussion
Aesar; hydrogen (H , 99.999%) and carbon monoxide (CO, 99.97%)
2
were procured from Air Products. All chemicals were used without
any additional purification.
3.1. Hydroformylation of 1-hexene
The hydroformylation of 1-hexene was investigated under sol-
2.2. Hydroformylation reaction
ventless conditions using Rh(acac)(CO) as a catalyst with a 6.8-fold
2
excess of PPh as a ligand. The reaction was very fast, and the com-
3
Hydroformylation experiments were carried out in 50 mL and
plete conversion of the olefin was already achieved after 40 min.
The selectivity to aldehyde was ca. 86%, the l/b ratio was 9.1, and
TOF was 970 (Table 1, entry 2). The recycled catalyst in the presence
of an excess of PPh3 afforded the complete conversion of 1-hexene
with a high yield of the aldehyde, 92%, (l/b ratio = 7.4), and a high
in 100 mL stainless steel autoclaves provided with a manome-
ter, a thermostat, and a magnetic stirrer. The 100 mL autoclave
was equipped with a Teflon vessel. The catalyst Rh(acac)(CO)2,
PPh , and the substrate were introduced to the autoclave under
3
−
1
a nitrogen atmosphere. The autoclave was closed, flushed with
TOF, 1330 h (Table 1, entry 3). Under the same conditions, the
reaction was performed using toluene as a solvent in order to com-
pare the reaction rate and the catalytic outcome. The conversion of
1-hexene and the selectivity to aldehydes were high, ca. 99% and
88%, however, the l/b ratio was only 3.7 (Table 1, entry1). The reac-
hydrogen (5 bar) three times, and then pressurized with synthe-
◦
sis gas (H :CO = 1:1) to 10 bar and heated to 80 C for a specified
2
time. After the reaction was finished, the autoclave was cooled to
room temperature and depressurized. The products were analyzed
by means GC and GC–MS.
−
1
tion time was 90 min and the turnover frequency was 438 h . The
catalytic activity in the absence of a solvent was higher than in the
experiment carried out in toluene, as was noted by other authors
2
.3. Recycling of the catalyst
[
6,15].
Fig. 1 shows the reaction rate of 1-hexene hydroformylation,
When the hydroformylation reaction was completed, the
organic reaction products were separated from the catalyst by “vac-
uum transfer”. New portion of olefin was added to the residue and
the resulting mixture was introduced to the autoclave together
estimated by the pressure drop.
The effect of synthesis gas pressure on the hydroformylation of
1-hexene under solventless conditions was studied at 4, 6, and 8 bar
◦
with PPh . The autoclave was closed, flushed with hydrogen (5 bar)
at a temperature of 80 C. The results, summarized in Table 2, show
3
three times, and then pressurized with synthesis gas (H :CO = 1:1)
to 10 bar and heated to 80 C for a specified time.
that the conversion of 1-hexene decreased from 87% to 44% when
the CO/H2 pressure changed from 8 to 2 bar. Simultaneously, the
yield of aldehydes decreased from 66.4% to 22% with an increase
in isomerization reaction yield due to the not sufficient amount of
synthesis gas. However, the l/b ratio increased from 10.2 to 22 when
2
◦
2.4. Turnover frequency (TOF)
The turnover number (TON) is defined as the number of moles of
the CO/H pressure was reduced from 8 to 2 bar. Such an effect was
2
substrate that a mole of catalyst can convert before becoming inac-
tivated. Turnover frequency (TOF) is defined as molecules reacting
per active site in unit time.
The TOF values were calculated as moles of the aldehyde/([mol
of catalyst] × time); time (h) was estimated from the linear part of
the graph presenting pressure drop vs. time. For example the TOF
value in Table 1 entry 2 was calculated as following:
expected, because linear aldehyde formation is favored at a lower
pressure [16].
In order to study the effect of the [1-hexene]/[Rh] ratio on the
reaction course, the amount of the olefin changed to achieve a
1
2
amount of substrate (moles)
amount of catalyst (moles)
0.012
.86 × 10−
10
8
TON =
=
= 420
5
2
y = -0.5807x + 12.989
The method of estimation time from the linear part of the graph
presenting pressure drop vs. time is illustrated in Fig. 1.
6
Toluene
Solventless
4
y = 0.5807x + 12.989
x = 22.4 min = 0.37 h
The esꢀmated ꢀme
2
9
0, 1.8
4
0, 1.5
0
0
10 20 30 40 50 60 70 80 90 100
actualTON
time(h)
359.1
0.37
= 970 h−
1
TOF =
=
time, min
The actual TON at the 85.5% of aldehyde yield is:
Fig. 1. Pressure drop during hydroformylation of 1-hexene in a solventless sys-
−
5
tem and in toluene at [1-hexene]/[Rh] = 420, [Rh] = (2.9 × 10 mol), [L]/[Rh] = 6.8,
◦
actualTON = 420 × 0.855 = 359.1
T = 80 C, (H2:CO)(1:1) = 10 bar. The estimated time means the time used for calcu-
lations of TOF value.

W. Alsalahi, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 147–151
149
Table 2
Effect of the pressure on the hydroformylation of 1-hexene under solventless conditions.
−
Entry
P(bar)
Conversion (%)
2-Hexene (%)
l
(
b
(%)
l/b
Acid(%)
0.6
TOF, (h 1
)
%)
1
2
3
8
6
4
86.5
69
44
13
22
21
66.4
44
22
6.5
3
1
10.2
14.7
22
757
322
163
Reaction conditions: 1-hexene (0.012 mol, 1.5 mL), [Rh] (2.9 × 10−5 mol), PPh3 = (1.6 × 10−4 mol) [1-hexene]/[Rh] = 420, [PPh3]/[Rh] = 6.8,T = 80 C, (H2:CO)(1:1), t = 40 min.
◦
Table 3
Effect of the [1-hexene]/[Rh] ratio on the hydroformylation of 1-hexene under solventless conditions.
TOF, (h 1
−
)
Entry
[1-Hexene]/[Rh]
Conversion (%)
Hexane (%)
2-Hexene (%)
l
b
l/b
Acid(%)
(
%)
(%)
a
1
2
3
4
5
6
7
1000
1500
2000
3000
3000
4000
5000
98
99
97
100
100
99
11
12
8
22
3
2
74
68
71
56
72
72
71
10
17
15
16
23
23
23
7.4
4
3
2
3
2
1
1
1
1945
2327
4062
3299
4353
2403
3056
a
a
4.7
3.5
3.1
3.1
3.1
a
4
1
1
2
b
b
b
99
2
◦
Reaction conditions: [PPh3]/[Rh] = 13, T = 80 C, (H2:CO)(1:1) = 10 bar.
a
t = 60 min, autoclave 50 mL.
t = 80 min, autoclave 100 mL.
b
1
000–5000-fold excess over rhodium. The obtained results listed in
14
Table 3 indicate that the conversion of 1-hexene was very high and
practically not influenced by the [1-hexene]/[Rh] ratio. Selectivity
to the aldehyde increased with the increasing amount of 1-hexene
mainly because the formation of 2-hexene, the isomerization prod-
uct diminished. The l/b ratio decreased from 7.4 to 3.1 when the
1
1
2
0
8
6
4
2
0
1
000
1500
[
1-hexene]/[Rh] ratio increased. It is worth to note here that results
2
000
000
obtained in two autoclaves (50 mL and 100 mL) are slightly differ-
3
ent as a result of different volume of CO/H and different rate of heat
2
3000
4000
transfer. It is illustrated by the curves of pressure changes in time
for two experiments performed at S/C = 3000 (Table 3, Fig. 2). The
products composition was also different and, for example, more of
5000
2
-hexene was formed in smaller autoclave (Table 3).
Fig. 2 shows the pressure drop vs. time at different [1-
0
20
40
60
80
100
hexene]/[Rh] ratios. As expected, the pressure drop was faster at
a lower [1-hexene]/[Rh] ratio, 1000–3000. Next, due to the low
concentration of the catalyst, the reaction rate decreased, and the
prolongation of the reaction time to ca. 80 min. was necessary to
achieve a high conversion Fig. 2.
time, min
Fig. 2. Graph depicting pressure drops with time for the hydroformylation of 1-
hexene catalyzed by Rh(acac)(CO)2/PPh3 under solventless conditions at different
[1-hexene]/[Rh] ratios. [PPh3]/[Rh] = 13, T = 80 C, (H2:CO)(1:1) = 10 bar. Reactions
were performed in 50 mL and 100 mL autoclaves according to Table 3.
◦
3.2. Hydroformylation of 1-octene
octene, which indicate the good stability of the catalyst with no
significant loss in activity and selectivity during five sequential
runs.
The solventless hydroformylation of 1-octene was carried out
using the same catalytic system, Rh(acac)(CO) /PPh . When the
2
3
first run was finished (entry 1), the products were removed by “vac-
uum transfer”, and new portions of 1-octene and PPh3 were added
to the remaining catalyst to start the second run (entry 2). The pro-
cedure was repeated and next reaction was performed with the
same catalyst (entry 3). Table 4 summarizes the obtained results
for recycling the catalyst in the solventless hydroformylation of 1-
The catalyst recovered was investigated for the hydroformy-
lation of 1-octene under solventless conditions five times, the
conversion of 1-octene slightly decreased from 97% (Table 4, entry
1) to 81% (Table 4, entry 5). Selectivity to a linear aldehyde was
almost the same, changing from about 72% to 66%. The yield of the
branched aldehyde was 17% (Table 4, entry 1) and then decreased to
Table 4
Hydroformylation of 1-octene using Rh(acac)(CO)2/PPh3 without a solvent.
Entry
Conversion (%)
Octane (%)
2-Octene (%)
l
(
b
(%)
l/b
Acid
(%)
TOF
%)
(h−1)
1
2
3
4
5
96
94
92
90
81
2
5
7
4
5
5
5
71
72
73
71
66
72
17
12
12
10
9
4
6
6.1
7.1
7.3
3.6
1
3
3
4
1
703
606
421
356
330
545
a
6
100
3
20
Reaction conditions: 1-octene (0.01 mol, 1.5 mL), [Rh] (2.3 × 10 mol), (PPh3 = 1.6 × 10−4 mol), [1-octene]/[Rh] = 440, [PPh3]/[Rh] = 6.8, T = 80 C, (H2:CO)(1:1) = 10 bar, t = 1 h.
−
5
◦
a
Toluene as solvent.

1
50
W. Alsalahi, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 147–151
00
1
90
80
70
60
50
40
30
20
10
0
conversion (%)
linear (%)
branched (%)
l/b raꢀo
0
1
2
3
4
5
cycles
Fig. 3. Plot of the conversion of 1-octene, yield of aldehydes (linear and branched), and the l/b ratio vs. the number of cycles, for the hydroformylation of 1-octene under
−
5
−4
◦
solventless conditions. 1-octene (0.01 mol, 1.5 mL), [Rh] (2.3 × 10 mol), (PPh3 = 1.6 × 10 mol), [1-octene]/[Rh] = 440, [PPh3]/[Rh] = 6.8, T = 80 C, (H2:CO) (1:1) = 10 bar,
t = 1 h.
Table 5
Hydroformylation of 1-pentene using Rh(acac)(CO)2 without a solvent.
Entry
Conversion
%)
Time (h)
Pentane
(%)
2-Pentene
(%)
b
(%)
l
l/b
Acid
(%)
TOF
(
(%)
(h−1)
1
2
3
4
95.4
99.5
96
97.8
96
1
0.5
0.1
0.4
0.1
2
5.4
1.8
3.7
1.5
5
11.5
12.7
11.7
11.1
19
76.4
80.6
77.9
83.7
70
6.6
6.4
6.7
7.6
3.7
1.6
4.3
2.3
1.4
1512
634
526
379
1237
1.5
1.5
2
a
5
1
Reaction condition: 1-pentene (0.0137 mol, 1.5 mL), [Rh] (1.7 × 10 mol), [PPh3] (1.03 × 10−4 mol), [1-pentene]/[Rh] = 800, [PPh3]/[Rh] = 6, T = 80 C, (H2:CO)(1:1) = 10 bar.
−
5
◦
a
Toluene as solvent.
Table 6
.
Solventless hydroformylation of styrene using Rh(acac)(CO)2/PPh3
TOF (h 1
−
)
Entry
Conversion (%)
P (bar)
Ethylbenzene (%)
l (%)
b (%)
l/b
1
2
3
68
97
96
10
12
12
7
2
2
22
35
33
39
60
61
0.6
0.6
0.5
250
524
415
a
−
4
◦
Reaction condition: styrene (0.013 mol, 1.5 mL), [Rh] (2.9 × 10−5 mol), [PPh3] (2 × 10 mol) [styrene]/[Rh] = 440, [PPh3]/[Rh] = 6.8, T = 80 C, (H2:CO)(1:1), t = 2 h
a
Toluene as solvent.
9
–10% (Table 4, entries 4 and 5). As a result, the linear-to-branched
aldehyde yield in the first run and 95% in the second one performed
with the same catalyst. The l/b ratio was 0.6, whereas it was 0.5 in
reaction in toluene Table 6
ratio increased from 4 to 7.3, probably due to the increasing excess
of the ligand in the system (Fig. 3).
3.3. Hydroformylation of 1-pentene
3.5. Rhodium species formed during hydroformylation
The hydroformylation of 1-pentene was performed under
solventless conditions using the same catalytic system,
Rh(acac)(CO) /PPh . Similarly as in the case of other olefins,
In order to identify rhodium species present in the reac-
31
2
3
tion mixture, NMR measurements were performed. The P NMR
(300 MHz) spectrum measured after hydroformylation reaction
showed the presence of two doublets at 40 ppm (J(Rh-P) = 156 Hz)
and at 32.2 ppm (J(Rh-P) = 135 Hz), assigned to HRh(CO)(PPh3)3
and to HRh(CO) (PPh ) respectively. The intensity of the doublet
the catalyst was reused four times with the addition of PPh3 in
each run. The results presented in Table 5 show that the conversion
of 1-pentene and selectivity to aldehydes remained approximately
constant in this series. It is worth noting that good conversion was
also obtained in toluene; however, the l/b value was remarkably
lower, amounting to 3.7. Under solventless conditions, l/b varied
from 6.4 to 7.6.
2
3 2
originated from HRh(CO)(PPh3)3 increased during storage of the
solution for a few hours under nitrogen atmosphere. Conversely,
signals of HRh(CO) (PPh ) were observed in a fresh sample.
2
3 2
3.4. Hydroformylation of styrene
As expected, the hydroformylation of styrene using Rh-
phosphine as a catalyst led principally to the branched aldehyde
(
2-phenylpropanal) [6,17]. The solventless hydroformylation of
◦
styrene at 80 C and 10 or 12 bar of CO/H resulted in 61%
2

W. Alsalahi, A.M. Trzeciak / Journal of Molecular Catalysis A: Chemical 408 (2015) 147–151
151
4
. Conclusions
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◦
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