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.