Selectivity of Rhodium-catalyzed Hydroformylation of 1-Octene in a Thermomorphic Solvent System
Maizatul S. Shaharun,
Binay K. Dutta
The use of temperature dependent multicomponent solvent (TMS) process, in which the catalyst remains as a residue in one of the liquid phases and the product goes preferably to the other liquid phase, can be an enabling strategy of commercial application of hydroformylation processes with high selectivity, efficiency and ease of product separation and catalyst recovery. This paper describes the isomer distribution of the formation of n-nonanal and 2-methyloctanal for the rhodium-catalyzed homogeneous hydroformylation of 1-octene using HRh(CO)[P(OPh)3]3 catalyst in TMS-system consisting of propylene carbonate (PC), dodecane and 1,4-dioxane. Effects of the concentration of 1-octene and catalyst, temperature, composition of syngas (H2/CO), total pressure of syngas and ligand to catalyst [P(OPh)3/HRh(CO)(PPh3)3] ratio on the selectivity of the nonylaldehyde products (n/iso ratio) have been measured. Highest selectivity for n-nonanal with average catalyst activity is observed at temperature of 363 K, equimolar of CO/H2, total syngas pressure of 1.5 MPa and ligand/catalyst ratio of 12 in TMS composition of 0.1/0.3/0.6. This study is practically useful for the optimization of n-nonanal production by hydroformylation of 1-octene.
October 18, 2010; Accepted: December 09, 2010;
Published: March 09, 2011
One of the widely studied homogeneous catalyzed reactions, the hydroformylation
reaction, provides a versatile route for the synthesis of a vast array of bulk
and specialty chemicals, such as industrial solvents, biodegradable detergents,
surfactants, lubricants and plasticizers (Bohnen and Cornils,
2003). The overall reaction can be represented by equation given below.
A cobalt- or rhodium-based catalyst is often used. The advantages of the rhodium
catalyst system are mild reaction conditions, higher n:i (normal to iso) ratio
of the products and higher activity. In general, a linear aldehyde is the desired
product. It is known that hydroformylation of 1-octene, in the presence of rhodium-based
catalyst, gives a mixture of two isomeric aldehydes namely, nonanal and 2-methyloctanal,
although, under certain conditions, small amounts of side products are formed.
The problem of directing this reaction to give preferential formation of n-nonanal,
owing to its greater practical importance, has mainly been mentioned in the
open and patent literature. Little has been reported on how to influence the
product distribution of isomeric aldehydes (Cornils, 1980;
Gholap et al., 1992; Piacenti
et al., 1991; Van Rooy et al., 1995;
Van Rooy et al., 1996; Tijani
and Ali, 2006). From the literature data, variations in the ratio of n-nonanal
to 2-methyloctanal from 1.6 to 10.1 were observed under widely different conditions
of temperature, catalyst concentration, ligand to catalyst mole ratio, solvent
and partial pressures of carbon monoxide and hydrogen. The most influential
parameter was found to be the partial pressure of carbon monoxide. The partial
pressure of hydrogen has a small but reproducible effect on the product distribution.
Temperature has a strong influence on the n/iso ratio, and conflicting results
have been reported for the effect of catalyst concentration. Despite these investigations
of the product distribution in the hydroformylation of 1-octene, the factors
influencing the n-nonanal/2-methyloctanal ratio are still rather obscure (Van
Rooy et al., 1995). Also, detailed kinetics of these individual
reactions is lacking. Therefore, as a first step toward understanding the more
complex problem of isomer distribution, the overall kinetics of the HRh(CO)[P(OPh)3]3
catalyzed hydroformylation of 1-octene in TMS-system consisting of propylene
carbonate (PC), dodecane and 1,4-dioxane was investigated and modeled (Shaharun
et al., 2010). The TMS-system is solvent mixture which changes thermally
from biphasic to monophasic with distribution of the products and of the catalyst
in the non-polar (dodecane) and polar phases (PC) thus simplifying the process
of separation and recycling of the catalyst (Behr et
al., 2005; Behr and Roll, 2005; Behr
and Fangewisch, 2003). We have also selected the optimum solvent composition
as a practical one for further study based on the selectivity to nonanal, TOF
value, percentage of Rh loss to nonpolar phase and yield of aldehyde (Shaharun
et al., 2010). In the present work, the objectives were to investigate
the effect of process variables on the isomer distribution during the hydroformylation
of 1-octene using HRh(CO)[P(OPh)3]3 as a catalyst precursor.
For this purpose, experiments were carried out under different operating variables
with HRh(CO)[P(OPh)3]3 catalyst in a TMS system consisting
of three components PC, n-dodecane and 1,4-dioxane. The variables included
temperature, 1-octene and catalyst concentrations, composition of syngas (H2/CO),
total pressure of syngas and ligand to catalyst [P (OPh) 3/HRh (CO)(PPh3)
MATERIALS AND METHODS
Materials: 1-Octene, triphenylphosphite, propylene carbonate, dodecane and 1,4-dioxane, purchased from Merck, were of purity 98-99%. The chemicals were used without further purification. GC analysis did not show any peaks of impurities. Nitrogen and syngas (1:1 CO/H2) were supplied by MOX, with a purity of 99.99%.
Experimental setup and procedure: The catalyst was prepared in situ
by mixing a rhodium catalyst precursor, Hrh(CO)(PPh3)3
(ABCR, Germany) and triphenylphosphite P(OPh)3. Experimental runs
were conducted at four different temperatures (353, 363, 373 and 383 K), three
total gas pressures (1.5, 2.0, 2.5 MPa), four CO/H2 pressure ratio
(0.3, 1, 2 and 3), four molar ratio of HRhCO(PPh3)3 catalyst
to P(OPh)3 ligand (10, 12, 14 and 16), catalyst concentrations varying
from 8.67x10-5 to 6.8x10-4 kmol m-3 (0.08 to
0.63 g L-1) and 1-octene concentration varying from 0.2 to 3.9 M.
Hydroformylation of 1-octene was carried out in a stirred high pressure reactor
(model: Parr 4843).
||Schematic of the experimental setup for kinetic studies: 1:
Nitrogen, 2: Syngas (CO/H2), 3: Cooling water in, 4: High pressure
reactor, 5: Stirrer, 6: Sampling valve, T: Thermocouple, P: Pressure gauge
A schematic of experimental set up is shown in Fig. 1.
The reactor was equipped with an automatic temperature control system, which consisted of an external electric heating jacket and an internal cooling loop. A pressure transducer-monitor system with high precision was also connected to the reactor for on-line measurement of reactor pressure in the course of semi-batch Hydroformylation reaction. The feed mixture (solvents, 1-octene and catalyst; volume = 220 mL) was taken in the preheated reactor. The reactor was purged with nitrogen and syngas successively, and then adjusted to the desired temperature. It was pressurized with an equimolar mixture of CO and H2 to a desired total pressure which was maintained constant during the whole run. The stirrer was set at 450 rpm while the reaction continued. Liquid samples were withdrawn and analyzed at regular time intervals to follow the progress of the reaction. The analysis of reactants and products was carried out on GC-2010 chromatograph equipped with flame ionization detector and a 5% phenylmethyl siloxane capillary column (30 m x 0.32 mm ID, 3.0 μm). Initial experiments were repeated three times to check for reproducibility. Measurements are, in general, reproducible within a maximum of 10% but mostly within a few percent.
RESULTS AND DISCUSSION
Effect of catalyst concentration: The regioselectivity for linear aldehyde
increased with the increase of catalyst loading. The n-product was favoured
under all conditions studied even though the regioselectivity to n-nonanal decreased
with time. Selectivity increased from 3.2 to 5.8 to 7.1 to 9 as HRh(CO)[P(OPh)3]3
increased from (0.0866 to 0.18 to 0.35 to 0.68)x10-3 kmol
m-3. This could be attributed to the ratio of phosphite ligand (L)
to rhodium metal affecting the coordination equilibrium of catalytic active
species. According to the catalytic cycle of hydroformylation, with the increase
of the ratio of phosphite ligand to rhodium metal, the rhodium metal complex
converted from HM(CO)2L to HM(CO)L2 and then to HM(CO)L3,
among which HM(CO)L2 was the main active species exhibiting higher
catalytic activity and selectivity. Similar observation was reported before
in the case of hydroformylation of 1-octene at 10 bar and 353 K, where the addition
of excess ligand (PPh3:Rh = 5:1 molar ratio) systematically led to
high rates of hydroformylation and high selectivity with reduced rates of isomerization
and hydrogenation of the olefin (Huang et al., 2004).
With 0.68 mM HRh(CO)(PPh3)3, the yield of total aldehyde
were 95% with n/iso ratio of 9.3 and 9.0 in dodecane and propylene carbonate
phases, respectively as shown in Fig. 2. In addition only
1.6% of 2-octene and 0.5 % of 1-octane were observed after 2.0 h. The percentage
yield of aldehyde distributed into the nonpolar phase was found to be in the
range of 89.5 to 93.3%.
Effect of temperature: Increases in temperature have a detrimental effect
on the n:iso ratio. The reaction temperature can significantly influence the
distribution of products due to enhanced isomerization of the double bond of
the substrate olefin and the dissociation of ligand from the catalytic complexes
at higher temperatures (Van Leeuwen, 2004). As presented
in Fig. 3, the selectivity of the reaction decreased from
11.5 to 10 to 8.5 to 8 (in dodecane phase) as temperature increased from 353
to 363 to 373 to 383 K. The percentage yield of aldehyde dropped significantly
from 90 to 72% with respect to an increase of temperature from 363 K to 373
K. In addition at the higher temperature (373 K) an orange colour develops in
the reaction mixture due to the decomposition of the triphenylphosphite ligand
which simultaneously affects the activity and stability of catalyst (Van
Leeuwen and Claver, 2000). A higher temperature decreases the n/iso ratio
as the isomerization of 1-octene into other internal octenes are favorable at
||Effect of HRh(CO)(PPh3)3 concentration
on the yield of total aldehyde and selectivity. Reaction conditions: P =
1.5 MPa, yH2/yCO = 1/1, T = 363 K, 1-octene = 1.9
kmol m-3, P(OPh)3/HRh(CO)(PPh3)3
= 12, PC/dodecane/1,4-dioxane= 0.30/0.10/0.60
||Effect of temperature on the yield of total aldehyde and selectivity.
Reaction conditions: P = 1.5 MPa, yH2/yCO = 1/1, HRh(CO)(PPh3)3
= 6.8x10-4 kmol.m-3, 1-octene = kmol m-3,
P (OPh)3/HRh(CO)(PPh3)3 = 12, PC/dodecane/1,4-0dioxane=0.30/0.10/0.60
||Effect of CO/H2 ratio on the yield of total aldehyde
and selectivity. Reaction conditions: p = 1.5 MPa, HRh(CO)(PPh3)3
= 2.1x10-4 kmol m-3, 1-octene = 1.9 kmol m-3,
P(OPh)3/HRh(CO)(PPh3)3 = 12, PC/deodecane/1,4-dioxane=0.30/0.10/0.6017,
temperature = 363 K
||Effect of 1-octene concentration on the yield of total aldehyde
and selectivity. Reaction conditions: p = 1.5 MPa, yH2/yCO
= 1/1, HRh(CO)(PPh3)3 = 1.73x10-4 kmol·m-3,
P(OPh)3/ Hrh(CO)(PPh3)3 = 12, PC/dodecane/1,4-dioxane=0.30/0.10/0.60,
temperature = 363 K
The resulting higher concentrations of the branched octenes adversely affect
the n/iso ratio.
Effect of CO/H2 ratio: The effect of composition of syngas
on the total yield of aldehyde and the selectivity are presented in Fig.
4. A decrease in the partial pressure of CO increased the selectivity of
the linear aldehyde but a decrease in the total yield of aldehydes was observed.
The selectivity of the reaction decreased from 10.8 to 9.45 to 9.0 to 8.9 (in
dodecane phase) as the molar ratio of CO/H2 changed from 0.3 to 1
to 2 to 3. A sudden increase of the total yield of aldehydes was observed when
equimolar ratio of CO/H2 was used. The negative-order dependence
with CO in a certain range of partial pressures appears to be a common feature
in hydroformylation reaction, using HRh(CO)(PPh3) as a catalyst.
As proposed by Evans et al. (1968) the inhibition
in the hydroformylation rates at higher CO pressures is mainly due to the formation
of (RCO)Rh(CO)2(PPh3)2 and/or (RCO)Rh(CO),(PPh,)
species which are unreactive toward hydrogen. These species are likely to be
formed by equilibrium reactions, thus reducing the effective concentration of
the active catalytic species. The equilibrium leading to the formation of unreactive
species will be more pronounced at higher pressures of CO causing a sharp decrease
in the total yield of aldehydes and rate of reaction as observed in this work.
At lower partial pressures of CO, the formation of these species is expected
to be negligible, and therefore a positive-order dependence of the rate on PCO
is observed. Regeneration of rhodium hydrides from dormant rhodium species formed
by impurities is another cause for the positive rate response to raising the
Effect of 1-Octene concentration: As shown in Fig. 5, a higher concentration of 1-octene increases the total yield of aldehydes but the n/iso ratio in the product or non-polar phase was in the range 6.0-6.5 and did not differ significantly. An increase in the 1-octene concentration resulted in a higher reaction rate and conversion of 1-octene, while the linear aldehyde selectivity remains unaffected. The increase in olefin concentration will predictably cause an increase of the addition of olefin to the active catalyst species, to form the alkyl complex and hence will cause enhancement in the rates of reaction as observed.
Effect of the total pressure of syngas: The effect of total pressure
(yH2/yCO = 1/1) on the total yield of aldehyde and the
selectivity is presented in Fig. 6. In general, an increase
in the total pressure decreased the selectivity of the linear aldehyde. Analysis
of the non-polar product phase revealed that the n/i ratio decreased from 7.7
at total pressure of 1.5 MPa to 4.23 at 2 MPa and 3.8 at 2.5 MPa.
||Effect of total pressure on the yield of total aldehyde and
selectivity. Reaction conditions: yH2/yCO = 1/1, HRh(CO)(PPh3)3
= 6.8x10-4 kmol m-3, 1-octene = 1.9 kmol m-3,
P(OPh)3/HRh(CO)(PPh3)3 = 12, PC/dodecane/1,4-dioxane=0.30/0.10/0.60,
temperature = 363 K
||Effect of P(OPh)3/HRh(CO)(PPh3)3
ratio on the yield of total aldehyde and selectivity. Reaction conditions:
p = 1.5 MPa, HRh(CO)(PPh3)3 = 2.1x10-4
kmol m-3, 1-octene = 1.9 kmol m-3, P(OPh)3/HRh(CO)(PPh3)3
= 12, PC/deodecane/1,4-dioxane=0.30/0.10/0.60, temperature = 363 K
According to Wilkinson, the increase in total pressure led to an increase of
catalytic activity (Evans et al., 1968). However,
side-hydrogenation and isomerization of isomerization of 1-octene also increased
at a pressure above 1.5 MPa, which made the selectivity for nonanal drop correspondingly.
Effect of ligand to catalyst [P(OPh)3/HRh(CO)(PPh3)3]
ratio: On the basis of the above catalytic results in 1-octene hydroformylation,
an excess of P(OPh)3 was added to HRh(CO)(PPh3)3,
and the effects of P(OPh)3 at 15 bar (H2/CO = 1) and 363
K was investigated. It is clearly seen from Fig. 7 that addition
of excess P(OPh)3 systematically leads to great increases in the
ratio of n/iso aldehydes when the molar ratio of P(OPh)3:Rh is in
the range of 10-12 and eventually dropped thereafter. The need for excess ligand
(L) arises from the facile Rh-L dissociation equilibrium shown in Fig.
8 (Van Leeuwen and Claver, 2000). As the concentration
of P(OPh)3 increases, the formation of inactive catalyst HRhL3
lowers the total yield. In addition, at high P(OPh)3 concentrations,
where the catalyst resting state is (P(OPh3)3)Rh(CO)H,
phosphite dissociation must occur to form the coordinatively unsaturated intermediates.
This dissociation is suppressed by increased P(OPh3)3
concentration, which serves to reduce the concentration of active Rh species
in the catalytic cycle.
Alkene complexation may also be accelerated or become more favored thermodynamically.
Migratory insertions are not particularly sensitive to electronic properties
of the ligand, but it is important to note that oxidative addition will slow
down when electron-withdrawing ligands are used (Broussard
et al., 1993). HRh(CO)(P(OPh)3)2 presents a
greater steric hindrance than Pph3 for the coordination of 1-octene
to the rhodium center and thus can lead to lower rates of the 1-octene reactions.
The greater steric hindrance relatively favours anti-Markownikov addition (Van
Leeuwen and Claver, 2000), which can produce more nonanal than 2-methyloctanal
and 2-octene. The addition of excess PPh3 makes a significant impact
on the enhancement of catalytic activity and selectivity of rhodium complexes
for 1-octene hydroformylation (Deshpande et al.,
1993). Consistent with the results reported with other rhodium complex systems
for 1-octene hydroformylation, our catalyst systems in the presence of excess
P(OPh)3 (P(OPh)3:Rh=12:1 molar ratio) show a marked increase
in the n/iso aldehyde ratio and great decreases in the activities for the formation
of 2-octene and n-octane.
|| HRh(CO)(PPh3)3 dissociation equilibrium
The increase in the n/iso aldehyde ratio and the suppression of the side reactions
of 1-octene are mainly due to strong steric effect of P(OPh)3. However,
the increase in the activity for the formation of aldehydes by addition of excess
PPh3 or P(OPh)3 remains unclear.
The selectivity behavior for the formation of isomeric nonyladehydes have been studied in the hydroformylation of 1-octene using the homogeneous HRh(CO)[P(OPh)3]3 catalyst precursor. The effects of various operating parameters on the n/iso ratio have been investigated. Experimental results show that the shift of equilibria between various active species with varied reaction conditions seems to be a determining factor in the hydroformylation reactions. Low ligand/metal ratios and high syngas pressure lead to the formation of unmodified rhodium carbon species with the characteristic of high reaction rates and low selectivities. Isomerization and hydroformylation were in competition in the reaction of 1-octene. Hydroformylation of isomerization products (2-octene, 3-octene) disturbs the initial linear aldehyde selectivity. This disturbance should be taken into consideration for process improvement. Moreover, the n/iso ratio was found to increase with increases in catalyst concentration and partial pressure of H2 but decreased with increase in temperature, total pressure and partial pressure of CO. No significant change was observed in the selectivity by varying the concentration of the substrate, 1-octene. The maximum n/iso ratio measured over the range of experimental conditions used was 11.5. In the hydroformylation of 1-octene using TMS-system of PC/dodecane/1,4-dioxane, highest selectivity for n-nonanal with average activity is observed at temperature of 363 K, equimolar of CO/H2, total syngas pressure of 1.5 MPa, ligand/catalyst ratio of 12 and TMS composition of 0.1/0.3/0.6. Such information is useful for the preferential production of n-nonanal.
Financial assistance from Ministry of Science, Technology and Innovation (MOSTI),Government of Malaysia, under the Research Project No.03-02-02-SF0019:Development of a green process for the production of higher aldehydes from olefins by hydroformylation is gratefully acknowledged.
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