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vol.48 número2CHEMICAL CHARACTERIZATION OF THE INHALABLE PARTICULATE MATTER IN CITY OF CHILLAN, CHILECATALYTIC ACTIVITY OF SULFUR RESISTANT CATALYSTS ON THE ISOMERIZATION OF A MIXTURE OF PINENES POISONED WITH THIOPHENE AS CRUDE SULFATE TURPENTINE MODEL índice de autoresíndice de materiabúsqueda de artículos
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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.2 Concepción jun. 2003

http://dx.doi.org/10.4067/S0717-97072003000200010 

J. Chil. Chem. Soc., 48, N 2 (2003)

Hydroformylation and Isomerization of Alkenes Catalyzed by [Rh(COD)(Amine)2]PF6 Complexes Immobilized on
Poly(4-vinylpyridine) Under CO/H2O Conditions

A.J. Pardey1*, J. Brito1, A.B. Rivas1, M.C. Ortega1, C. Longo2,
P.J. Baricelli3, E. Lujano3, M. Yañez 4 ,C. Zuñiga4 ,R. Lopez4, S.A. Moya4*

1Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela.
2Centro de Investigación y Desarrollo de Radiofármacos, Facultad de Farmacia, Universidad Central de Venezuela,
Caracas, Venezuela.
3Centro de Investigaciones Químicas, Facultad de Ingeniería, Universidad de Carabobo, Valencia, Venezuela.
4Departamento de Química Aplicada, Facultad de Química y Biología,
Universidad de Santiago de Chile, Santiago de Chile, Chile.

( Received : September 23, 2002 ­ Accepted : March 10, 2003 )

ABSTRACT

Reported here is the catalytic studies of the hydroformylation and the isomerization of 1-hexene to aldehydes (heptanal and 2-methyl-hexanal) and 2-hexene, respectively by [Rh(COD)(amine)2]PF6 complexes (COD = 1,5-cyclooctadiene, amine = 4-picoline, 2-picoline or 2,6-lutidine) immobilized on poly(4-vinylpyridine) in contact with 10 mL of 80% aqueous 2-ethoxyethanol, under CO atmosphere (0.9 atm at 100 C). The results of these studies show the following reactivity order defined as Turnover Frequencies for aldehydes production: 2-picoline {2.9} > 4-picoline {2.6} > 2,6-lutidine {2.4}. The electronics and steric effects induced by the amine ligands apparently influence on the observed catalytic activities. The [Rh(COD)(2-picoline)2]PF6/poly(4-vinylpyridine) system also catalyzed the hydroformylation and isomerization of 1-octene.

Key Words: Hydroformylation, isomerization, 1-hexene, rhodium catalyst.

INTRODUCTION

The hydroformylation reaction is the principal method of incorporation of oxygen-containing functional group into olefins and addition of formyl groups and hydrogen atoms to double bonds. The modern industrial hydroformylation is carried out in the presence of soluble rhodium complexes1). The main shortcoming of rhodium catalysts is their high cost, therefore the loss of rhodium catalyst should be minimized. This limitation stimulates the search for immobilized catalysts for the hydroformylation reaction. The main goal of this approach is the preparation of catalytic systems displaying the good activity, selectivity and reproducibility typical of homogeneous catalyst, combined with the easy separation and recovery characteristic of heterogeneous catalyst2-4).

Catalytic hydroformylation of olefins to aldehydes (Eq. [1]) under the water gas shift reaction conditions (WGSR, Eq. [2]) by transition metal complexes has been reported5,6). Water and carbon monoxide serve as a hydrogen source for the formation of a metal-alkyl intermediate, which later reacts, with CO to give aldehydes7-10).

R-CH=CH2 + 2CO + H2O ®R-CH2CH2(HCO) + CO2

Eq. [1]

H2O + CO Û H2 + CO2

Eq. [2]

Researchin our group has shown the catalytic activity toward both the WGSR11,12) and nitrobenzene reduction13,14) by rhodium(I) amino complexes of the type [Rh(COD)(amine)2](PF6) (2 wt %) immobilized on P(4-VP) (0.5 g) in contact with 10 mL of 80% aqueous 2-ethoxyethanol under 0.9 atm of CO at 100 C. For example, the WGSR catalytic activity defined as TF(H2) (mol gas (mol Rh x 24 h)-1) production followed the order: 4-picoline (11.9) > 3-picoline (9.9) > 2-picoline (5.7) > pyridine (5.4) > 3,5-lutidine (5.2) > 2,6-lutidine (3.3) under the above described reaction conditions.

On the other hand, the selective reduction of nitrobenzene to aniline production (milimole/3 h) depends on the nature of the coordinated pyridine or substituted pyridine and decrease in the following order: 2-picoline (0.65) > 4-picoline (0.59) > 3-picoline (0.56) > pyridine (0.49) > 3,5-lutidine (0.38) > 2,6-lutidine (0.34). The WGSR and nitrobenzene reduction catalytic activities observed in these systems were explained in terms of a fine balance between electronic and steric effects introduced by the methyl groups.

As a continuation of our systematic investigation of the catalytic properties of this type of complexes, we now report the hydroformylation and isomerization of 1-hexene in the presence of CO and H2O.

EXPERIMENTAL

Materials. 1-Hexene and 1-octene (Aldrich) were simple distilled. 2-Picoline (2-pic), 4-picoline (4-pic) and 2,6-lutidine (2,6-lut) were obtained from Aldrich and were distilled from KOH before use. 2-Ethoxyethanol (Aldrich) was distilled from anhydrous stannous chloride. Water was doubly distilled. Poly(4-vinylpyridine) (P(4-VP)) 2% cross linked was used as provided by Reilly Industries. All gas mixtures CO/CH4 (95.8%/4.2%, v/v) and CO/CH4/H2/CO2 (84.8%/5.1%/4.8%/5.3%) were purchased from BOC Gases and were used as received. The rhodium complex [Rh(COD)(Amine)2]PF6 were prepared as reported15). The characterization studies of the solids formed with the 2-picoline and 4-picoline ligands by UV-Vis/DR, FT-IR, EPR, SEM and XPS techniques suggest the presence of immobilized Rh(amine)22+ species14,16).

Catalyst preparation: A 0.50 g sample of P(4-VP) and a known amount of [Rh(COD)(Amine)2]PF6 (typically 1.0x10-4 mol) were stirred for 72 h in 10 mL of 2-ethoxyethanol until almost all the rhodium was extracted by the polymer from the solution as marked by the presence of a colorless clear solution above the polymer beads. The yellow polymer was filtered, washed with 2-ethoxyethanol (15 mL) to remove the unabsorbed rhodium complex which concentration was determined by UV/Vis spectrophotometry. The catalytic solid was dried at room temperature under vacuum for more than 4 h.

Catalyst testing: 1-Hexene and 1-octene hydroformylation was performed as follows: A given sample of the alkene substrate was added to a 150 mL magnetically stirred glass reaction vessel (equipped with devices for collecting gas and liquid samples during the reaction) after the catalytic mixture (0.5 g of P(4-VP)/2.0 wt. % of Rh in contact with 10 mL of 80% aqueous 2-ethoxyethanol) reached a constant WGSR activity. The detailed procedure of the WGSR experiment has been described earlier11). Subsequent to addition of the alkene and heptane (internal standard), the glass reactor vessel was charged with CO/CH4 (0.9 atm of CO at 100 C) and placed in an electrically heated (typically, at 100 ± 0.5 C) and mechanically stirred glycerol bath for 24 h. At the end of reaction time gas samples were removed from the reactor and analyzed by GC as previously described for the WGSR test7). CH4 was used as internal standard. This procedure allowed calculation of absolute quantities of CO consumed and H2 and CO2 produced in 24 hours. Quantities of CO2, H2, and CO in the sample were determined from GC calibrations made by using variously sized samples of the standardized gas mixture CO/CH4/H2/CO2. Also liquid samples were removed and analyzed by GC and GC-MS.

Instrumentation: Gas sample analyses from catalytic runs were performed on a Hewlett-Packard 5890 Series II programmable gas chromatograph fitted with Carbosieve-B (mesh 80-100) column and thermal conductivity detector. Liquid samples analyses were done on a Hewlett-Packard 5890 Series II programmable gas chromatograph fitted with a Porapak-p column and a flame ionization detector. The column temperature was programmed from 100 to 240 C (18 C/min) with helium carrier-gas flow rate of 25 mL/min. Calibration curves were generate for heptane (internal standard), 1-hexene, 2-hexene, 2-methyl-hexanal and heptanal. Also, calibration curves were generate for 1-octene, 2-octene, 2-methyl-octanal and nonenal. GC-MS data were recorded on a GC-MS Hewlett-Packard 5995 gas chromatograph-mass spectrometer equipped with a DB5 (30 m x 0.53 mm) column. Analyses of Rh concentration in the filtered solution were performed on a Perkin-Elmer Lambda 10 UV-Vis spectrophotometer. Analyses of Rh concentrations in catalysis solutions were performed on a GBC Avanta atomic absorption spectrometer operated in the flame mode.

RESULTS AND DISCUSSION

The catalytic activities for the hydroformylation/isomerization of 1-hexene by [Rh(COD)(amine)2]PF6/P(4-VP)/aqueous 2-ethoxiethanol system under P(CO) = 0.9 atm at 100C, [Rh] = 2.0 wt. % and 1-hexene/Rh = 105, were evaluated. Under these reaction conditions the GC-MS and GC results show that the only carbonylated organic products observed were heptanal and 2-methyl-hexanal. However, formation of 2-hexene (a cis- and trans-2-hexene mixture) and small amounts of molecular H2 are detected, the latter coming from the WGSR. Despite the formation of H2, the hydrogenation of 1-hexene and/or 2-hexene to hexanes and the heptanal and/or 2-methyl-hexanal to the corresponding alcohol was not observed.

In addition, the solution left after a catalytic run was analyzed by atomic absorption spectroscopy and less than 0.2% of rhodium free was detected in the solution, and it shows that not significant metal leaching occurs under catalytic conditions. Also this homogeneous solution exhibited no activity toward WGSR and 1-hexene hydroformylation and/or isomerization when tested in the absence of the loaded aminated P(4-VP). Control experiments show that the homogeneous system formed by [Rh(COD)(amine)2]PF6 (0.1 mmol) dissolved in 10 mL of 80% of aqueous 2-ethoxyethanol is not active for the hydroformylation of 1-hexene under similar reaction conditions. In addition, activity toward the WGSR was observed as reported before by Pardey et al.11) when the catalytic mixtures were tested in the absence of 1-hexene or 1-octene.

For this catalytic system the effects of varying the nature of amine ligand (2-picoline, 4-picoline or 2,6-lutidine) were explored. These results are summarized in Table I. Also, the atomic absorption measurement values reveal that the anchoring of the rhodium complexes on 0.5 g of P(4-VP) is greater than 99% ([Rh] = 2.0 wt. %). The calculated catalytic activity defined as turnover frequencies (TF(product) = mol product(mol Rh*24 h)-1) was reproducible within less than 10% for a series of experimental runs.

TABLE I. Hydroformylation and isomerization of 1­hexene in the presence of [Rh(COD)(amine)2]PF6 complexes immobilized on poly


Amine (pKa)b

TF(CO2)c

TF(H2)c

TF(Iso)d

TF(Ald)d (n/i)e


2-Picoline
(5,97)

20.1

1.2

16.1

2.88 (0.94)

4-Picoline
( 6.00)

18.1

1.1

14.6

2.57 (0.85)

2,6-Lutidine
(6.75)

17.4

0.9

13.9

2.40 (1.27)


a0.5 g of P(4-VP); P(CO) = 0.9 atm at 100 °C for 24 h; 10 mL of 80%
2­ethoxyethanol/H2O; [Rh] = 2,0 wt. % (1.0 x 10-4 mol); 1.3 mL of 1-hexene (1.05 x 10-2 mol); 1­hexene/Rh =105.
bK. Schofield. "Hetero-Aromatic Nitrogen Compounds", Plenum Press, New York, 1967, pp. 146-148.
cTF(gas) = mol of gas (mol Rh x 24 h)-1. Experimental uncertainty < 10%.
dTF(product) = mol of product (mol Rh x 24 h)-1. Experimental uncertainty < 10%.
Iso = isomerization. Ald = aldehydes.
en/i = normal to branched molar ratio.

The hydroformylation and isomerization reactions depends on the nature of the amine and decreases in the following order: 2-picoline > 4-picoline > 2,6-lutidine. The electronics and steric effects induced by the amine ligands apparently influence on the observed catalytic activities. Similar behavior was observed in the catalysis of the WGSR and nitrobenzene reduction to aniline by these immobilized complexes13,14) and the explanations there were similar. On the other hand, it cannot be ruled out that the small differences in the reactivity between the catalysts formed from different amines may be to subtle differences in catalyst preparation.

The 2-hexene formed under the catalytic conditions is hydroformylated to 2-methyl-hexanal. This result suggests that Rh(amine)/P(4-VP) system catalyzed the isomerization of 1-hexene faster than the hydroformylation reaction and this behavior is consistent with the reported by Kalck et al.7)

Catalytic hydroformylation, isomerization and hydrogenation of 1-hexene under the water gas shift reaction conditions by RhCl3 heterogenized on nitrogen-functionalized polymers has been reported by Ford et al.6). In the case of our Rh(amine)2/P(4-VP) system the hydrogenation reaction is inhibited due to the presence of the amine ligands.

Further, the studies related to the influence of the reaction conditions variation (1-hexene/rhodium content = 16 - 105, temperature = 70 ­ 110 C and carbon monoxide pressure = 0.6 ­ 1.8 atm) on the catalytic hydroformylation of 1-hexene to aldehydes (heptanal and 2-methyl-hexanal) by the rhodium(I) complex, [Rh(COD)(2-pic)2]PF6 immobilized on poly(4-vinylpyridine) in contact with 10 mL of 80% aqueous 2-ethoxyethanol, under water gas shift reaction conditions has been reported17). These studies show that the hydroformylation and isomerization rate of 1-hexene increase with P(CO) to ca. 1.2 atm. Above 1.2 atm of CO, the isomerization of 1-hexene is inhibited and the formation of aldehydes appears to be independent of CO pressure in the 1.2 to 1.8 atm range. In addition, the hydroformylation process shows a segmented concave upward Arrhenius plot. The turnover frequency of aldehydes formation increased from 1.1 (1-hexene/rhodium = 16) to 2.8 (24 h)-1 (1-hexene/rhodium = 105). Further studies of the Reppe hydroformylation of 1-octene by the [Rh(COD)(amine)2]PF6 (amine = 4-picoline, 3-picoline, pyridine or 2,6-lutidine) immobilized on P(4-VP) catalyst system are in progress and will they be published elsewhere.

We have also investigated the catalytic hydroformylation of 1-octene by the [Rh(COD)(2-pic)2]PF6/P(4-VP) system in contact with 10 mL of 80% aqueous 2-ethoxiethanol under P(CO) = 0.9 atm at 100C for 24 h, [Rh] = 2.0 wt. %, 1-octene (1.6 mL, 1.0 x 10-2 mol) and 1-octene/Rh = 100. Under these reaction conditions the GC-MS and GC results show formation of nonenal, 2-methyl-octanal and 2-octene. The TF values are given in Table II. It notable from Table II that hydroformylation and isomerization reaction rates are faster for 1-hexene than for 1-octene. However, the WGSR rate is higher for the Rh/1-octene than the Rh/1-hexene system. These results suggested the existence of an apparent competition between hydroformylation and the WGSR. Also, this is an indication that the concentration of the Rh species responsible for the catalysis of the WGSR decreased more in presence of 1-hexene than 1-octene due to the differences between the stability of the alkene-Rh complex (alkene = 1-hexene or 1-octene) formed under the catalytic conditions, which depends on the nature of the alkene18). The general trend is that any increment of the size of the alkene decreased the stability of the complex formed. In our case, this trend is confirmed based on the catalytic results. Finally, despite the formation of H2, the hydrogenation of 1-alkenes and/or 2-alkenes to alkanes and the reduction of aldehydes to the corresponding alcohols was not observed.

TABLE II. Comparison of hydroformylation and isomerization rates for 1-hexene and 1-octene catalyzed by [Rh(COD)(2-pic)2]PF6 complex immobilized on poly(4­vinylpyridine) as catalyst precursora


Substrate
(S/Rh)

TF(CO2)b

TF(H2)b

TF(Iso)c

TF(Ald)c
(n/i)d


1-hexene
(105)

20.1

1.2

16.1

2.88 (0.94)


1-octene
(100)

17.6

1.8

13.4

2.35 (1.29)


a0.5 g of P(4-VP); P(CO) = 0.9 atm at 100 °C for 24 h; 10 mL of 80%
2­ethoxyethanol/H2O, [Rh] = 2.0 wt. %.
bTF(gas) = mol of gas (mol Rh x 24 h)-1. Experimental uncertainty < 10%.
cTF(product) = mol of product (mol Rh x 24 h)-1. Experimental uncertainty < 10%.
Iso = isomerization. Ald = aldehydes.
dn/i = normal to branched molar ratio.

ACKNOWLEDGMENTS

The present work was carried out under the research programs CDCH-UCV-Venezuela (Proy. 06.10.4654.2000), CONICIT-Venezuela (S1-95001662), FONDECYT-Chile Proy. 1020076 (SAM), dicyt-usach Proy. 02141 LB (RL) and CODECIHT-UC, (PJB). We thank Reilly Industries INC by donating the poly(4-vinylpyridine) cross-linked polymer (lot N. 70515AA). Also, we thank Mr. Alberto Fuentes, IVIC-Venezuela, for the GC-MS measurements.

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