"On water" hydroformylation of 1-hexene using Rh/PAA (PAA = polyacrylic acid) as catalyst

Article abstract of DOI:10.1039/c4ra03568b

A new rhodium catalyst, Rh/PAA, obtained by the immobilization of Rh(acac)(CO)₂ on polyacrylic acid (PAA), was successfully applied for the hydroformylation of 1-hexene in a water medium. Spectroscopic analysis evidenced that rhodium in Rh/PAA was chemically bonded to polyacrylic acid and formed a hydrido-carbonyl rhodium compound in reaction with H₂/CO. Excellent results (98% conversion, TOF 1000) were obtained in the "on water" hydroformylation of 1-hexene when Rh/PAA was used together with a hydrophobic phosphine (triphenylphosphine, tri-p-tolylphosphine, or diphenyl(2-methoxyphenyl) phosphine). A similar efficiency was also obtained for a system composed of Rh(acac)(CO)₂ and PPh₃, tested in the same conditions in water. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra03568b

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“On water” hydroformylation of 1-hexene using
Rh/PAA (PAA ¼ polyacrylic acid) as catalyst
Cite this: RSC Adv., 2014, 4, 30384
*
W. Alsalahi and A. M. Trzeciak
A new rhodium catalyst, Rh/PAA, obtained by the immobilization of Rh(acac)(CO)2 on polyacrylic acid (PAA),
was successfully applied for the hydroformylation of 1-hexene in a water medium. Spectroscopic analysis
evidenced that rhodium in Rh/PAA was chemically bonded to polyacrylic acid and formed a hydrido-
carbonyl rhodium compound in reaction with H2/CO. Excellent results (98% conversion, TOF 1000) were
obtained in the “on water” hydroformylation of 1-hexene when Rh/PAA was used together with a
hydrophobic phosphine (triphenylphosphine, tri-p-tolylphosphine, or diphenyl(2-methoxyphenyl)
phosphine). A similar efficiency was also obtained for a system composed of Rh(acac)(CO)2 and PPh3,
tested in the same conditions in water.
Received 19th April 2014
Accepted 30th June 2014
DOI: 10.1039/c4ra03568b
www.rsc.org/advances
hydrophobic interface has a free dangling hydroxyl group that
protrudes into the organic phase, plays a key role in catalyzing
1. Introduction
reactions, and demonstrates an extraordinary reaction rate
acceleration.9,12 Such interactions were also studied in catalytic
systems for hydroformylation.13–15
The hydroformylation reaction of olens is a very important
industrial process used for the production of aldehydes.1 This
reaction was discovered in 1938 by Otto Roelen for a cobalt
complex at Ruhrchemie in Germany.1 The aqueous–organic
biphasic process for propene and butene hydroformylation was
commercialized at Ruhrchemie-Rhone Poulenc in 1984. The
main advantage offered by this process is the efficient separa-
tion of the catalyst from the organic products. This is possible
because the catalyst bearing a water-soluble phosphine is kept
in the aqueous phase, which is not miscible with the organic
phase containing aldehydes.1
In this paper, we report a new catalytic system for an on-
water hydroformylation process by applying of a water-soluble
immobilized catalyst, Rh/PAA, and a hydrophobic phosphine.
The second system studied in the same conditions contained a
water-insoluble complex, Rh(acac)(CO)2, as the rhodium source.
These systems, applied for the rst time in the hydro-
formylation of 1-hexene in water, gave very promising results.
Until now there has been no detailed reports on the hydro-
formylation of higher olens using the on-water methodology.
Water is the most plentiful, non-environmentally harmful,
nontoxic, and nonammable solvent used by nature for bio-
logical transformations, very attractive from an economical
point of view. However, water also possesses the extraordinary
ability to catalyze chemical transformations between some
insoluble organic reactants.2–6 Moreover, unique reactivity and
selectivity is observed in water when compared with conven-
tional organic solvents.7–10 In 1980, Rideout and Breslow
discovered rate acceleration of Diels–Alder reactions between
nonpolar compounds in homogeneous aqueous solutions
compared with the same reactions in organic solvents.11 In
2005, Sharpless and his co-workers reported examples of
organic syntheses performed in a water medium with reactants
insoluble in water.9 Those reactions, termed “on water”, gave
higher yields and faster kinetics than in any organic solvent. An
on-water reaction is the chemical process that takes place at the
organic/water phase boundary (emulsion). The formation of
hydrogen bonds in interfacial water molecules at the
2. Experimental
2.1. Materials
The rhodium complex Rh(acac)(CO)2 was prepared according to
the literature.16
Polyacrylic acids (Mw ꢀ 1800 and 450 000) were purchased
from Sigma-Aldrich; triphenylphosphine (PPh3) was purchased
from Avocado; 1-hexene was purchased from Merck; 1-octene
was purchased from Sigma-Aldrich; hydrogen (H2, 99.999%)
and carbon monoxide (CO, 99.97%) were procured from Air
Products. All chemicals were used without any additional
purication. Distilled water was used as the reaction medium.
2.2. Synthesis of Rh/PAA (Mw ꢀ 1800)17
A solution of PAA (Mw ꢀ 1800, 1.5 g) dissolved in water (30 ml)
was added to a suspension of Rh(acac)(CO)2 (1 g) in water
(20 ml). Aer stirring for 24 h at room temperature, a second
portion of PAA (1.5 g) in water (20 ml) was added and stirring
continued for the next 24 h. During that time, the color of the
Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław,
Poland. E-mail: anna.trzeciak@chem.uni.wroc.pl
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mixture changed from green to black. Next, the mixture was temperature and depressurized. The organic products were
ltered off, and the resulting ltrate was evaporated using a separated by a vacuum transfer procedure and analyzed by
rotary evaporator yielding a black lm of Rh/PAA containing means GC and GC-MS.
5.4% of Rh. A second synthesis, performed according to the
same procedure but at the temperature of 35 ꢁC, gave a product
2.8. Turnover frequency (TOF)
containing 10.2% of Rh.
The turnover number (TON) is dened as the number of moles
of substrate that a mole of the catalyst can convert before
becoming inactivated. The turnover frequency (TOF) is dened
2.3. Synthesis of Rh/PAA (Mw ꢀ 450 000)
A solution of PAA (Mw ꢀ 450 000) (1.5 g) in water (30 ml) was
as the number of molecules reacting per active site in unit time.
added to a suspension of Rh(acac)(CO)2 (1 g) in water (20 ml),
and the mixture was stirred for 24 h at room temperature. The
color changed from green to black. The mixture was ltered off,
and the resulting ltrate was evaporated using a rotary evapo-
rator yielding a black lm of Rh/PAA containing 0.1% of Rh.
The TOF values were calculated as moles of the aldehyde [mol of
catalyst]ꢂ1
the graph presenting pressure vs. time.
t
ꢂ1; t ¼ time (h) was estimated from the linear part of
3. Results and discussion
3.1. Characterization of Rh/PAA by IR
2.4. Synthesis of HRh(CO)(PPh3)3
The IR spectrum of Rh(acac)(CO)2 presented two peaks of
carbonyl stretching frequencies (n(CO)) at 2006 and 2064 cmꢂ1
and two peaks originating from a coordinated acac ligand at
1253 and 1559 cmꢂ1. The IR spectrum of Rh/PAA showed only
one n(CO) peak at 2064 cmꢂ1 and no peaks for acac, indicating
its removal from the coordination sphere of rhodium during
reaction with PAA. In order to recognize reactivity towards
substrates of hydroformylation, a solid sample of Rh/PAA was
treated with H2 (5 bar), CO (5 bar), and H2 + CO (10 bar). The
reaction with H2 was also performed in benzene. The IR spectra
of Rh/PAA, measured aer treatment with H2 or CO, showed
unchanged bands in the region of ca. 2000 cmꢂ1. In contrast,
the IR spectrum of Rh/PAA aer reaction with syngas (H2–CO,
1 : 1) showed two new bands at 2100 and 2000 cmꢂ1, attributed
to rhodium-hydride and carbonyl vibrations (Table 1). More-
over, the shiing of n(CO) from 2064 to 2073 cmꢂ1 was observed
(Table 1).
The TEM analysis of Rh/PAA did not show any Rh(0) nano-
particles; however, nanoparticles appeared aer reaction of
Rh/PAA with H2/CO. Small, ca. 4 nm, as well as aggregated Rh(0)
nanoparticles were found in different places of the TEM grids
(Fig. 1). A quite different result was obtained when Rh/PAA
reacted with H2/CO in water. The size of Rh(0) nanoparticles
was smaller, about 2 nm.
A mixture of Rh(acac)(CO)2 (0.00762 g) and PPh3 (0.05136 g) in
water (2 ml) was stirred under hydrogen gas (1 bar) at 38 ꢁC for
ca. 24 h. The yellow precipitate was ltered off and dried.
31P NMR (C6D6): d 40.60 ppm (d, JRh–P ¼ 151.75 Hz). IR: nRh–H
¼ 2036 cmꢂ1, nCO ¼ 1921 cmꢂ1
.
2.5. Preparation of samples for ICP measurements
A 0.03 g sample of Rh/PAA was placed in a 25 ml volumetric
ask, and 0.5 ml of concentrated nitric acid and 1.5 of
concentrated hydrochloric acid were added. The ask was lled
to 25 ml with distilled water, and the contents were mixed until
complete dissolution.
2.6. Characterization techniques
The IR spectra of the samples were recorded in the range from
400 to 4000 cmꢂ1 with a Bruker Vector 22 IR spectrometer. NMR
spectra were recorded at room temperature on a Bruker Avance
500 MHz spectrometer. Transition electron microscopy (TEM)
was performed at 200 kV using a FEI Tecnai 20 X-TWIN electron
microscope. The Rh/PAA samples were dispersed in water and
then suspended on a carbon grid. The Rh content in the Rh/PAA
catalyst was determined using inductively coupled plasma
atomic emission spectroscopy (ICP-AES) analysis (ARL Model
3410, Fisons Instruments). The reaction products were identi-
ed and analyzed with a Hewlett-Packard 5890 II gas chro-
matograph equipped with an HP-5 column (25 m ꢃ 0.2 mm)
using helium as the carrier gas. Mass spectra were obtained
using an HP 5971A mass selective detector.
3.2. Hydroformylation of 1-hexene
The hydroformylation of 1-hexene was investigated in different
solvents with Rh/PAA as the catalyst and a 13-fold excess of a
water-insoluble phosphine (PPh3). Aldehydes were formed as
the main reaction products, namely a linear aldehyde (1-hep-
tanal) and a branched aldehyde (2-methyl-hexanal) (Scheme 1).
2.7. Hydroformylation of 1-hexene
Hydroformylation experiments were carried out with 100, 50, Small amounts of the isomerization product, 2-hexene (1–4%),
and 35 ml stainless steel autoclaves provided with manometers, were also found. In some experiments, performed in methanol,
thermostats, and magnetic stirrers. Weighted samples of Rh/ traces of an acetalization product, 1,1-dimethoxy-heptane, were
PAA and PPh3 were placed in an autoclave under a nitrogen identied.
atmosphere, and the solvent (toluene, methanol, or water) and
In reactions performed without a solvent, in neat 1-hexene,
1-hexene were introduced. The autoclave was closed, lled with only traces of aldehydes were formed. Similarly, a very low
hydrogen (5 bar) 3 times, and then pressurized with synthesis conversion of 1-hexene was noted in toluene, although PPh3
gas (H2–CO ¼ 1 : 1) to 10 bar and heated to 80 ꢁC. Aer the and 1-hexene are soluble in that solvent. Much better results
reaction was nished, the autoclave was cooled to room were obtained in methanol and in a methanol–water mixture. In
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Table 1 IR data (in cmꢂ1) of the Rh/PAA after reactions with H2, CO and H2 + CO
Sample
nCO
nCO or nRh-H
nCO(acac), nCO(PAA), nCH(PAA)
Rh(acac)(CO)2
2006vs, 2064vs
1526vs, 1559s
PAA
Rh/PAA
Rh/PAA + H2
Rh/PAA + CO
Rh/PAA + H2 + CO (4h)
Rh/PAA + H2 + CO (6h)
Rh/PAA + H2 + CO + water
Rh/PAA + H2 + benzene
1714vs
1714vs
1714vs
1718s
1718s
1711s
1713s
1711vs
1454w
1455w
1455w
1457w
1453w
1453w
1454m
1457w
1415w
1412w
1415w
1405w
1413w
1413w
1416w
1415w
1269m, 1165m
1271m, 1175m
1272m, 1177m
1263m, 1172m
1262m, 1176m
1269m, 1176m
1262m, 1171m
1262m, 1172m
2064w
2064w
2065w
2069w
2073w
2080m
2070w
2000vw, 2100vw
2006vw, 2104vw
2023vw, 1983vw
Fig. 1 Morphology and rhodium nanoparticle size after reaction of Rh/PAA with: (a) syngas (H2–CO), 10 bar, 80 ꢁC and (b) syngas (H2–CO), 10
bar, 80 ꢁC in water.
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l/b ratio from 5.3 to 30. This is in agreement with earlier
observations that a good linearity of aldehydes is favored at low
pressure.1 Lower pressure favors the formation of a linear alkyl
rhodium species which is next transformed to a linear acyl
intermediate and nally to a normal aldehyde.
Scheme 1
As can be deduced from the data presented on Fig. 2, the
effect of temperature on hydroformylation selectivity is rather
small. For instance, the l/b value changed from 4.3 to 6.9 when
the temperature rose from 50 to 90 ꢁC. In contrast, the pressure
has a decisive effect on the yield of aldehydes as well as on
hydroformylation selectivity. At 4 bar, the l/b ratio was 22.2, and
at 2 bar, it reached 30.
The effect of the [PPh3]/[Rh] ratio on the reaction course was
studied in the range from 0 to 13 at 80 ꢁC at 10 bar. The results
presented in Table 4 show that in the absence of phosphine Rh/
PAA catalyzes only the isomerization of 1-hexene to 2-hexene.
However, already a 5-fold excess of PPh3 made it possible to get
57% of aldehydes. With an increase in the [PPh3]/[Rh] ratio, the
conversion of 1-hexene increased to 92% together with an
increase in the l/b ratio to 5.7. Interestingly, high catalytic
activity and good selectivity towards aldehydes were also
obtained with the application of other water-insoluble phos-
phines, namely tri-p-tolylphosphine and diphenyl(2-methoxy-
phenyl) phosphine with Rh/PAA (Table 4). However, it should be
noted that the kind of phosphine has a remarkable inuence on
the TOF values, and the highest TOF was obtained for PPh3.
The hydroformylation of 1-hexene was also performed with
particular, the l/b ratio increased to a good value, 7.3. The
addition of methanol to toluene resulted in a signicant
increase in 1-hexene conversion to aldehydes. Interestingly,
when a toluene–water mixture was used, both conversion and
selectivity were even better than for a toluene–methanol
mixture. Such an effect was surprising because 1-hexene, Rh/
PAA, and PPh3 are soluble in methanol and only Rh/PAA is
soluble in water. As a rule, a faster reaction is expected in a
homogeneous system than in a multiphase one. However, the
most spectacular results, in respect to conversion, selectivity,
and the reaction rate, were obtained in water only (Table 2). It is
worth noting that the volume of water inuences the reaction
course only very slightly, and almost the same results were
obtained using 0.5 or 1 ml of water.
Consequently, further experiments were performed using
water as the reaction medium. Such a procedure can be named
“on water” because 1-hexene and PPh3 are insoluble in water.
The effect of temperature on the hydroformylation of 1-hexene
using the catalytic system Rh/PAA + PPh3 was studied at 50–
90 ꢁC and a constant pressure of 10 bar (Table 3). An increase in
1-hexene conversion was observed with an increase in temper-
ature. Thus, when temperature increased from 50 to 80 ꢁC, the
yield of aldehydes increased from 21 to 94% with a decrease
in the reaction time from 3 h to 1 h and a slight increase in the
l/b ratio from 4.3 to 6.9. At 90 ꢁC, the yield of aldehydes
decreases to 87%, and 6% of 2-hexene, an isomerization
product, was formed. Consequently, aldehyde formation was
another Rh/PAA catalyst containing polyacrylic acid, Mw
¼
450 000. The rhodium content in this catalyst was signicantly
lower than in Rh/PAA immobilized on a polymer of Mw ¼ 1800.
Nevertheless, this catalysts also gave quite good results, 97% of
aldehydes with l/b ¼ 4.6 aer 150 min.
In order to recognize the possibility of the in situ formation
of an Rh/PAA catalyst, Rh(acac)(CO)2 and PPh3 were used in the
next experiment together with PAA added to the reaction
mixture. The reaction was even faster than with Rh/PAA;
however, a similar yield of the products was obtained. Next,
Rh(acac)(CO)2 was used with PPh3 only, without PAA, and,
surprisingly, the catalytic result was very promising (Table 5).
The nal composition of the products was similar to that
obtained with Rh/PAA, but the TOF values were higher. At a
ꢁ
preferred at 80 C.
The effect of pressure on the catalytic activity of a Rh/PAA +
PPh3 system was studied at 2–10 bar and at 80 ꢁC, and the
achieved results are shown in Table 3. When the pressure of the
synthesis gas decreased from 10 to 2 bar, the conversion of 1-
hexene to the products decreased from 97 to 37% with a
decrease in aldehyde yield from 94 to 31%. However, lowering
the synthesis gas pressure caused a remarkable increase in the
Table 2 Hydroformylation of 1-hexene catalyzed by Rh/PAA + PPh3 in organic solvents and in watera
Aldehydes
Solvent
Conversion (%)
2-Hexene (%)
l (%)
b (%)
l/b
TOF, hꢂ1
—
0.6
3
0.2
0.6
2
3
4
3
3
4
0.3
1.7
59
76
72
80
79
73
0.1
0.6
15
16
11
12
14
10
Toluene (1.5 ml)
Toluene–MeOH (1 : 0.5 ml)b
Toluene–H2O (1 : 0.5 ml)
MeOH (1.5 ml)b
H2O (0.5 ml)
2.8
3.9
4.8
6.5
6.7
5.6
7.3
77
96
92
96
97
96
296
940
530
877
849
827
H2O (1 ml)
MeOH–H2O (0.75 : 0.75 ml)
a
Reaction conditions: 1-hexene 1.5 ml (0.012 mol), [1-hexene]/[Rh] ¼ 800, [PPh3]/[Rh] ¼ 13, T ¼ 80 ꢁC, H2–CO (1 : 1) ¼ 10 bar, time ¼ 1 h, autoclave
b
50 ml. Time ¼ 2 h.
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Table 3 Effect of temperature and pressure on hydroformylation of 1-hexene catalyzed by Rh/PAA + PPh3 in watera
Aldehydes
T
ꢁC
P, bar
t, min
Conversion (%)
2-Hexene (%)
l (%)
b(%)
l/b
4.3
4.7
5.1
5.3
6.9
9.7
TOF, hꢂ1
50
60
70
80
90
80
80
80
80b
10
10
10
10
10
8
6
4
2
180
180
60
60
60
60
60
62
240
24
65
70
97
98
83
62
48
37
1
2
3
3
6
7
6
8
6
17
52
56
79
76
68
53
38
30
4
11
11
15
11
7
3
1.7
1
56
168
536
803
1003
677
436
287
63
17.7
22.2
30
a
Reaction conditions: 1-hexene 1.5 ml (0.012 mol), water 1.5 ml, [Rh] (1.5 ꢃ 10ꢂ5 mol), [1-hexene]/[Rh] ¼ 800, [PPh3]/[Rh] ¼ 13, autoclave 50 ml.
b
Autoclave 100 ml, 1-hexene 3 ml, water 3 ml.
With these improved reaction conditions in hand, we decided
to investigate the on-water hydroformylation of other alkenes.
1-Octene showed good conversion, 98% (Table 6, entry 1). The
on-water hydroformylation reaction using 2-hexene and styrene
as the substrates gave 76% and 67.0% conversion and 76% and
64% aldehyde selectivity. However, the l/b ratio was expectedly
poor, 0.07 and 0.7, respectively (Table 6).
3.3. Recycling of the catalyst
The reusability of Rh/PAA in the on-water hydroformylation of
1-hexene and 1-octene was studied according to two different
protocols. When the reaction with 1-hexene was completed, the
organic phase was separated from the aqueous phase by
decantation, and the aqueous phase was placed again in an
autoclave. All liquid components were removed from the
organic phase by “vacuum transfer”, a new portion of olen and
PPh3 were added to the residue, and the resulting mixture was
introduced to the autoclave. Thus, in such recycling, practically
the entire amount of rhodium was used in the next catalytic
cycle. The results presented in Table 7 indicate that recycling
was efficient, although the third reaction was slower than the
second one. An increase in the l/b ratio from 6.5 to 8.1 was
noted.
In reaction with 1-octene as the substrate, only the water
Fig. 2 Graphs depicting the influence of 1 : 1 CO–H2 pressure and
temperature on the selectivity of 1-hexene hydroformylation catalyzed
by Rh/PAA + PPh3; [PPh3]/[Rh] ¼ 13, [1-hexene]/[Rh] ¼ 800.
phase was used for the rst recycling producing 70.9% of
aldehydes. For the next run, all rhodium was used again,
according to the procedure described for 1-hexene.
lower pressure, 4 or 6 bar, l/b increased to attractive values,
16 and 21. Interestingly, the hydrophobic complex
3.4. The nature of the catalyst
HRh(CO)(PPh3)3 can also be successfully applied for the The results of recycling experiments indicated the leaching of
hydroformylation of 1-hexene in water giving the highest TOF, rhodium from the water to the organic phase. To verify that
1903 hꢂ1, noted in these studies.
conclusion, the 31P NMR spectrum of the organic phase sepa-
Fig. 3 shows the H2/CO pressure drop during the hydro- rated aer hydroformylation with Rh/PAA + PPh3 was measured.
formylation of 1-hexene catalyzed by Rh/PAA, Rh(acac)(CO)2, It showed a broad doublet at 32.2 ppm with JRh–P 133.8 Hz
and HRh(CO)(PPh3)3. As the last two systems are totally conrming the presence of a rhodium species. In addition,
hydrophobic, the name “on water” is suitable for their according to the ICP analysis, ca. 50% of rhodium was trans-
description.
ferred to the organic phase during the catalytic process.
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Table 4 Effect of [L]/[Rh] ratio (L ¼ phosphine) on hydroformylation of 1-hexene with Rh/PAA in waterd
Aldehydes
Entry
[L]/[Rh]
Time (min)
Conversion (%)
2-Hexene (%)
l (%)
b (%)
l/b
TOF, hꢂ1
1a
2a
3a
4a
5b
6c
0
5
10
13
13
13
240
240
174
120
120
120
8
62
90
92
90
85
8
5
4
4
4
2
44
70
88
66
62
13
16
15
20
20
3.4
4.4
5.7
3.2
3.1
86
382
718
538
344
a
L ¼ PPh3. b L ¼ tri-p-tolylphosphine. c L ¼ diphenyl(2-methoxyphenyl)phosphine. d Reaction conditions: 1-hexene 1 ml (0.008 mol), water 1 ml,
[Rh] 1 ꢃ 10ꢂ5 mol, [1-hexene]/[Rh] ¼ 800, 80 ꢁC, H2–CO(1 : 1) ¼ 10 bar, autoclave 35 ml.
Table 5 Hydroformylation of 1-hexene catalyzed by different rhodium complexes in watera
Aldehydes
Catalyst precursor
P bar
t min
Conversion (%)
2-Hexene (%)
l (%)
b (%)
l/b
4.6
15.4
25.2
37
4.9
16
21
TOF, hꢂ1
Rh/PAA (Mw ꢀ 450 000)a
10
10
6
4
10
6
150
40
40
40
65
70
75
60
97
97
76
62
5
11
18
20
4
13
16.1
8
74
77
51
37
16
5
2
495
1658
967
574
1153
967
b
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
Rh(acac)(CO)2
b
b
1
d
99
78
16
3.5
1.8
6
d
73.2
55.4
98
56.7
37.5
82
d
4
10
374
1903
c
HRh(CO)(PPh3)3
13.7
a
a,d
b,c
Reaction conditions:
1-hexene 1 ml (0.008 mol), water 1 ml, autoclave 35 ml,
1-hexene 1.5 ml (0.012 mol), water 1.5 ml, autoclave
a,b,c
d
50 ml,
[1-hexene]/[Rh] ¼ 800, [PPh3]/[Rh] ¼ 13, [1-hexene]/[Rh] ¼ 420, [PPh3]/[Rh] ¼ 6.8. T ¼ 80 ꢁC.
isopropanol. Unexpectedly, when
a suspension of Rh-
(acac)(CO)2 and PPh3 in water was treated with H2 (1 bar), the
complex HRh(CO)(PPh3)3 was formed in a stoichiometric
amount (Scheme 2).
Such a synthetic procedure differs remarkably from those
reported in the literature.18–20 The most efficient syntheses of
HRh(CO)(PPh3)3 apply KOH, NaBH4, or hydrazine in an ethanol
solution.18
4. Conclusions
Fig. 3 Changes of H2/CO pressure vs. time during hydroformylation
of 1-hexene catalyzed by Rh/PAA, Rh(acac)(CO)2 and HRh(CO)(PPh3)3
in the presence of PPh3 ([PPh3]/[Rh] ¼ 13) in water. [1-Hexene]/[Rh] ¼
800, T ¼ 80 ꢁC, H2–CO (1 : 1) ¼ 10 bar. Linear approximation used for
determination of TOF was shown for reaction with HRh(CO)(PPh3)3.
It was demonstrated that the hydroformylation of 1-hexene
can be very efficiently performed in water using water-soluble
Table 6 On water hydroformylation of different alkenes catalyzed by
b
Rh/PAA + PPh3
For a better understanding of rhodium transformations in
the studied system, the reaction of Rh(acac)(CO)2 with PPh3 and
H2 (1 bar) was performed in water. It should be mentioned that
PPh3 reacts with Rh(acac)(CO)2 in any organic solvent
producing Rh(acac)(CO)(PPh3), which in the presence of a PPh3
excess and H2/CO forms a catalytically active hydrido-carbonyl
species. Reaction with H2/CO takes place only under elevated
pressure, and no reaction was observed at atmospheric
pressure in benzene or toluene. In contrast, formation of
HRh(CO)(PPh3)3 was observed in the presence of methanol or
Aldehydes
Substrate
t, min
Conv.a (%)
l (%)
b (%)
l/b
TOF, hꢂ1
1-Octene
2-Hexene
Styrene
120
180
90
98
76
67
77
5
26
13
71
38
5.9
0.07
0.7
760
216
389
a
Rh(acac)(CO)2 as catalyst, [1-hexene]/[Rh] ¼ 420, [PPh3]/[Rh] ¼ 6.5, p ¼
1 bar. b Reaction conditions: Substrate 1.5 ml, water 1.5 ml, [sub]/[Rh] ¼
800, L ¼ PPh3 [PPh3]/[Rh] ¼ 13, T ¼ 80 ꢁC, P(H2–CO) ¼ 1 : 1, p ¼ 10 bar.
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Table 7 Recycling Rh/PAA catalyst in the hydroformylation of 1-hexene and 1-octene in watera
Substrate
1-Hexene
Time min
Conversion (%)
2-Hexene (%)
Aldehydes %
l/b
TOF, hꢂ1
70
120
180
120
120
120
95.7
95.6
96.3
97.7
77
4.8
4.1
4.5
3.7
2.4
2.2
90.9
90.1
91.1
92.4
70.9
91.4
6.5
6.7
8.1
3.8
4.1
3.9
752
751
306
1-Octene
95.2
a
Reaction conditions: [sub]/[Rh] ¼ 800, L is PPh3 [PPh3]/[Rh] ¼ 13, T ¼ 80 ꢁC, p(H2–CO) ¼ 1 : 1, p ¼ 10 bar.
Acknowledgements
Authors thank Prof. Yuri Varshavsky for important contribution
in the synthesis of Rh/PAA and prof. David Cole–Hamilton for
very inspiring discussions.
Scheme 2
Rh/PAA or water insoluble Rh(acac)(CO)2 complexes with an
excess of hydrophobic PPh3. Such “on water” methodology is
very simple and, in particular, the application of an expensive
water soluble phosphine is not needed to perform hydro-
formylation. Also, the organic solvent is eliminated from the
system. Moreover, aldehydes were formed as the main
products under relatively mild conditions, 80 C and 10 bar.
At a lower pressure, a linear aldehyde can be obtained with
excellent selectivity.
The presented systems can be used for any higher olen, as
was demonstrated for 1-hexene, 2-hexene, and 1-octene.
Recycling experiments showed that a rhodium catalyst can
be recovered without a loss of its catalytic activity. However,
during hydroformylation rhodium is leached to the organic
phase, which complicates the efficient separation of alde-
hydes from the catalyst. We are now working on the
improvement of this step and preliminary results show that
aer the catalytic cycle rhodium can be transferred back to
the water phase.
Explanation of the high catalytic activity of the presented
“on water” system can only in part be based on specic
interactions between the reactants, similarly as it was
described in the literature. In our opinion, reactions of the
rhodium precursor with H2/CO are remarkably inuenced by
the presence of water, which is not only a solvent but also a
reactant. As was shown, in a water medium, a catalytically
active HRh(CO)(PPh3)3 complex was formed in high yield
under very mild conditions as a result of efficient H2 activa-
tion. Such activation was realized without any additives, in
particular in the absence of a base that could eventually
facilitate heterolytic splitting of H2.
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RSC Adv., 2014, 4, 30384–30391 | 30391