16 Hydroformylation with Rhodium-Amine
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Complexes A N T H O N Y T. JUREWICZ, LOUIS D . R O L L M A N N , and D. D U A Y N E W H I T E H U R S T Mobil Research and Development Corp., Central Research Division, Princeton, N . J. 08540
Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium.
Hydroformvlation of olefins to aldehydes over cobalt carbonyl catalysts - * is the first step i n the industrial synthesis of oxo alcohols (1, 2). Reaction conditions require temperatures above 150 °C and pressures up to 3000 psig. Subsequent aldehyde hydrogénation occurs over supported cobalt or molybdenum disulfide catalysts. Addition of modifying ligands such as tributyl phosphine affords a one-step, cobalt-catalyzed synthesis of alcohols (at lower pressure), but accompanying olefin hydrogénation reduces yields (3). W i t h amine ligands, the effects are varied. Accelerated hydroformylation rates are possible with weak bases such as pyridine, but stronger bases ( piperidine or triethylamine, for example) retard or completely inhibit the reaction (4,5). In comparison with their cobalt analogs, rhodium carbonyls are significantly more active catalysts (6). As with cobalt, aldehyde is the A
1
240 Forster and Roth; Homogeneous Catalysis—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
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16.
jUREwicz E T A L .
Hydroformyhtion
241
principal product although some hydrogénation to alcohol has been reported at high temperature and pressure (7). In contrast to cobalt, rhodium permits a one-step oxo alcohol syn thesis i n the presence of certain monomeric and polymeric amines (8, 9, 10). Included in this group are triethylamine, N-alkylpiperidines, Nmethylpyrrolidine, and Ν,Ν-dimethylbenzylamine ( D M B A ) . Initial kinetic data on this amine-promoted alcohol synthesis (under severe re action conditions) have been reported by B. F e l l and coworkers (11), but no attempt has been made to characterize the catalytic species i n the reaction cycle. Data are presented to identify some of the important factors i n alde hyde hydrogénation and to characterize rhodium carbonyl chemistry under hydroformylation conditions. Comparison is made of the effects of monomeric and of polymeric amines, and a possible reaction mecha nism is examined in the light of the data. Experimental Materials. Rhodium carbonyl chloride was prepared by heating R h C l · 3 H 0 (40.0% R h , Matthey Bishop) i n a C O stream (12). The cluster carbonyls, R h ( C O ) i and R h ( C O ) i , were prepared according to C h i n i and Martinengo (13). Olefins (Humphrey, Aldrich) were perco lated over alumina prior to use. Amines (Aldrich) were, on occasion, fractionally distilled under nitrogen but were typically used without further purification. Other materials were reagent grade. The resins studied (Rohm and Haas and Ionac) were all functionalized, cross-linked, styrene polymers with the exception of poly(4-vinylpyridine). Porous, macroreticular resins included the polymeric analogs of Ν,Ν-dimethylbenzylamine (A21 p o l y D M B A , Rohm and Haas), N , N dimethylaniline, and l-phenyl-2- ( Ν,Ν-dimethylamino ) ethane ( polyA l i p h a m ) . The last two materials were prepared in this laboratory. Nitrogen content of the porous resins was 4.1, 2.5, and 2.6 mequiv/gram, respectively. Poly(4-vinylpyridine) (6.9 mequiv/gram) had a gel-type structure. Procedures. Batch oxo experiments were performed in a 300cc auto clave. The autoclave and contents were flushed with C O and brought to reaction temperature at the beginning of a run. The vessel was then pres surized with the desired mixture of C O and hydrogen and the run com menced. Reaction was followed by monitoring hydrogen and C O consumption and by periodic sampling for G L C analysis on tricresylphosphate and Carbowax 1000 columns. Rhodium carbonyl chemistry was monitored during the course of a run by attaching a high-pressure, heated I R cell to the autoclave. Cell design was similar to that reported by Noack (14). Sodium chloride plates were used. 3
2
4
2
6
6
Results Catalysis. W h e n 1-hexene reacts with C O and H over rhodium car bonyls, the two competing reactions are the formation of aldehyde and 2
Forster and Roth; Homogeneous Catalysis—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
242
HOMOGENEOUS
CATALYSIS
II
the olefin isomerization. The system is described schematically in Reac tion 1 (9). Under mild conditions, the aldehyde formation dominates with no further hydrogénation to alcohol. ^^heptanal
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1-hexene
(
)(
2- and 3-hexenes
(1)
^ b r a n c h e d C7-afdehyde The plot of data for the carbonyl, R h ( C O ) i , (Figure 1) shows that the rate of 1-hexene isomerization exceeds that of hydroformylation. A t olefin conversion in excess of 50%, little 1 isomer remains. A n increase in branched aldehyde relative to linear aldehyde accompanies the change in isomer distribution. The absence of aldehyde hydrogénation is com plete even at very high conversion levels using the conditions cited. The effect of D M B A and p o l y D M B A on the hydroformylation of 1-hexene is shown in Table I. Product distributions are reported at 80% total olefin conversion for various rhodium sources and olefin concentra tions. As shown, olefin isomerization is complete in all cases. In the presence of 2 M D M B A , significant aldehyde hydrogénation occurred producing nearly 30% alcohol. W i t h the exception of R h C l · 3 H 0 which is inactive, no differences were found between various rhodium sources. The aldehyde hydrogénation reaction is strongly dependent on amine basicity. The data in Table I obtained with D M B A demonstrate 6
6
3
Figure 1.
2
Hydroformylation of 1 -hexene
lOOcc 0.8Ml-hexene in hexane, 60 mg Rh (CO) , 100°C, 1000 psig 1:1 CO:H 6
16
Forster and Roth; Homogeneous Catalysis—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.
2
16.
juREWicz E T A L . Table I.
Hydroformylation
243
Effect of Amine on the Hydroformylation of 1-Hexene
a
Initial 1-Hexene Product Distribution Concentration, 2- and Cmolar 3-Hexenes Aldehyde C7-Alcohol b
7
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Catalyst
0.8 0.8 8" 8 8
Rh (CO) R h ( C O ) , + 2M D M B A RhCl -3H 0 5 % R h on Charcoal R h C l - 3 H 0 on p o l y D M B A a 6 c d e
6
16
6
6
3
2
3
2
20 20 NR« 20 20
c
80 51 NR 80 52
e
