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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.45 n.3 Concepción set. 2000

http://dx.doi.org/10.4067/S0366-16442000000300003 

Kinetics of the Water Gas Shift Reaction Catalyzed by
[Rh(COD)(4-Picoline)2]PF6 Immobilized On
Poly(4-vinylpyridine)

A. J. Pardey1*, M. Fernández1, J. Alvarez1, M. C. Ortega1, M.
Canestrari1, C. Longo2, P. Aguirre3, S. A. Moya3*, E. Lujano4, P. J.
Baricelli4

1Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela,
Caracas, Venezuela
2Centro de Investigación y Desarrollo de Radiofármacos, Facultad de Farmacia,
Universidad Central de Venezuela, Caracas, Venezuela
3Departamento de Química Aplicada, Facultad de Química y Biología,
Universidad de Santiago de Chile, Casilla 40, Santiago 33, Chile
4Centro de Investigaciones Químicas, Facultad de Ingeniería,
Universidad de Carabobo, Valencia, Venezuela

(Received:March 22, 2000 - Accepted: May 11, 2000)

ABSTRACT

Kinetic studies of the water gas shift reaction have been carried Æt in the presence of [Rh(COD)(4-picoline)2]PF6, (COD = 1,5-cyclooctadiene) as catalyst, immobilized on poly(4-vinylpyridine) 2% cross linked with divinylbenzene in contact with a mixture of 80% aqueous 2-ethoxyethanol. The rhodium complex was anchored to the pyridine groups of the organic polymer. Analyses of kinetic results show a first order dependence on carbon monoxide pressure, a nonlinear dependence on rhodium total concentration and a segmented Arrhenius plot. These data are discussed in terms of a possible mechanism.

KEYWORDS: Water gas shift reaction, rhodium, poly(4-vinylpyridine).

RESUMEN

Estudios cinéticos de la reacción de desplazamiento del gas de agua se llevaron a cabo en presencia del complejo [Rh(COD)(4-picolina)2]PF6, (COD = 1,5-ciclooctadieno) como catalizador, inmovilizado en poli(4-vinilpiridina) entrecruzado con 2% de divinilbenceno en contacto con una mezcla de 80% de 2-etoxietanol acuoso. El complejo de rodio fue anclado a los grupos piridínicos del polímero orgánico. Los análisis de los resultados de los estudios cinéticos muestran que la reacción en el sistema heterogenizado sigue una dependencia de primer orden con respecto a la presión de monóxido de carbono, una dependencia no lineal con respecto a la concentración de rodio total y muestra una curva de Arrhenius segmentada. Estos resultados son discutidos en términos de un posible mecanismo.

PALABRAS CLAVES: Reacción de Desplazamiento del Gas de Agua, rodio, poli(4-vinilpiridina).

INTRODUCTION

Organometallic polymer anchored catalysts for the water gas shift reaction (WGSR, Eq. [1]) have been reported1-9).

 

CO + H2O ®
¬
CO2 + H2
Eq. [1]

Research in our group10) has shown that rhodium(I) complexes, [Rh(COD)(amine)2]PF6 (COD = 1,5-cyclooctadiene and amine = 4-picoline, 3-picoline, 2-picoline, pyridine, 3,5-lutidine or 2,6-lutidine) immobilized on poly(4-vinylpyridine) in contact with a mixture of aqueous 2-ethoxyethanol, 2/8, v/v, generates a pseudo-homogeneous WGSR catalytic system at [Rh] = 1.9 wt. %, P(CO) = 0.9 atm at 100 ºC. The catalytic activity defined as hydrogen turnover frequency (TF(H2) = moles of H2(mole of Rh**day)-1) depends on the nature of amine ligand and follows the order: 4-picoline(11.9) > 3-picoline(9.9) > 2-picoline(5.7) ³ pyridine(5.4) ³ 3,5-lutidine(5.2) > 2,6-lutidine(3.3), being the 4-picoline complex the most active.

This report presents the results of the quantitative studies of P(CO), [Rh] and T variation on the WGSR catalyzed by [Rh(COD)(4-pic)2]PF6 anchored to the pyridine groups of the P(4-VP) in contact with aqueous 2-ethoxyethanol.

Experimental

Materials:

4-Picoline (4-pic) was obtained from Aldrich and distilled over KOH. Rhodium trichloride was obtained from Aldrich. All gases and gas mixtures CO, N2, He/H2 (91.4/8.6%), CO/CH4 (95.8/4.2%), CO/CH4/CO2/H2 (84.8/5.1/5.3/4.8%) were purchased from BOC Gases and were used as received. 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 and [Rh(COD)(4-pic)2]PF6 was prepared by the method reported by Denise and Pannetier11).

Instrumentation:

Analyses of gas samples from catalysis runs were performed as described previously12, 13) on a Hewlett-Packard 5890 Series II programmable (ChemStation) gas chromatograph fitted with a thermal conductivity detector. The column used was Carbosieve-B (80-100) mesh obtained from Hewlett-Packard. The column temperature was programmed from 60-175 ºC at a flow rate of 50 ml.min-1 using a He/H2 mixture as the carrier gas. The gas mixture CO/CH4/CO2/H2 was used to generate calibration curves for H2, CO, CO2, and CH4. Analyses of Rh concentrations in catalysis solutions were performed on a Perkin-Elmer Lambda 10 UV-Visible spectrophotometer in a 1.0 cm quartz cell.

Catalyst preparation:

A 0.5 g sample of poly(4-vinylpyridine) and a known amount (typically 1x10-4 mol) of the rhodium complex [Rh(COD)(4-pic)2]PF6 were stirred for 120 h in 10 mL of 2-ethoxyethanol/water (8/2, v/v) 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 a mixture of 2-ethoxyethanol/water (5 mL) to remove the unabsorbed rhodium which concentration was determined by UV-Visible spectrophotometry, dried at room temperature for about two hours after which it was ready to use in the kinetic runs.

Kinetics measurements:

The procedure employed in the kinetics measurements was the same as previously detailed in Ref. 10. Kinetic runs were conducted in all-glass reactor vessels consisting of a 100 mL round bottom flask connected to an "O" ring sealed joint to a two-way Rotoflow Teflon stopcock attached to the vacuum line12). In a typical kinetic experiment, 0.5 g of the loaded polymer beads (typically 1.9 wt. % of Rh) and 10 mL of 80% aqueous 2-ethoxyethanol were added to the glass reactor vessel, then the solution was degassed by three freeze-pump-cycles. The reaction vessel was charged with CO/CH4 mixture at the desired pressure (typically 0.9 atm), then suspended in circulating glycerol oil bath fitted with an analog temperature controller (Cole-Palmer, Model 71). The specified temperature (typically 100 ºC) was maintained at ± 0.5 °C by continuously and vigorously stirring the oil bath as well as the reaction mixture which was provided with a Teflon-coated magnetic stirring bar.

Gas samples of 1.0 mL were removed at the end of heating time from the reaction vessel at bath temperature with Pressure Lok Series A-2 gas syringes (Dynatech Precision Sampling Corporation), analyzed by gas chromatography and corrected for small background signals. The CH4 was used as internal standard to allow calculation of absolute quantities of CO consumed and CO2 produced, during a time interval. In addition, calibration curves were prepared periodically for CO, CH4, H2, and CO2. The validities of the calibration were repeatedly checked by analyzing known gas mixtures and individual analyses were found to be accurate and reproducible to better than 10%. The reaction vessel was flushed out and then recharged with CO/CH4 in a similar manner to that used for the initiation of the run.

Results and Discussion

We recently reported10) that the rhodium(I) complex, [Rh(COD)(4-pic)2]PF6 immobilized on P(4-VP) in contact with aqueous 2-ethoxyethanol forms a system which shows the highest catalytic activity in the WGSR than those found for the other ligands (4-picoline, 3-picoline, 2-picoline, pyridine, 3,5-lutidine or 2,6-lutidine) under the same reaction conditions. Furthermore, H2 and CO2 formation equaled stoichiometrically (within experimental uncertainties) as required by Eq. [1]. In addition, FT-IR characterization studies of an active Rh(4-pic)/P(4-VP) catalyst, after being exposed to CO, displays two bands in the nCO region at 1994 (carbonyl-linear Rh species) and 1832 cm-1 (carbonyl-bridged Rh species) suggesting the presence of rhodium carbonyl compounds with different nuclearities anchored to the nitrogen-functionalized polymer through the pyridine groups of the P(4-VP) as reaction intermediates10).

This immobilized catalyst was also characterized by scanning electron microscope (SEM), UV/Visible reflectance, electron paramagnetic spectroscopy (EPR) and X-ray photoelectron spectroscopy (XPS)14). The FT-IR, UV/Visible reflectance, EPR and SEM results indicate that the rhodium ions are covalently anchored to the nitrogen-functionalized polymer. Furthermore, XPS studies suggest that the immobilization of the rhodium(I) [Rh(COD)(4-pic)2]PF6 on the aminated polymer yields non-uniform catalyst systems, in which various rhodium oxidation states coexist (e. g., Rh(I), Rh(II) and Rh(-I)). Finally, based on the characterization studies, we are able to infer that the observed WGSR catalysis is the result of the presence of both cationic mononuclear and anionic polynuclear anchored amino-carbonyl rhodium complexes that act as catalytic species under reaction conditions.

For this highly active Rh(4-pic)/P(4-VP) system the effects of varying the carbon monoxide pressure (P(CO)), the rhodium concentration ([Rh]), and the temperature (T) on the catalysis of the WGSR were explored and these results are reported in Table 1-3 and they represent the average value of duplicate runs deriving for the same experimental conditions. The calculated kinetics activity defined as TF(H2) was reproducible to within less than 10% for a series of experimental runs. In addition, the TF(H2) for the kinetics runs were determined for short periods (5 h or less) where [H2O] and P(CO) were essentially constant, diminishing by less than 15% overall. Hence, possible shifts in [H2O] and P(CO) dependent equilibria among the catalyst component during a run were minimized owing to the near constancy of [H2O] and P(CO).

Effect of carbon monoxide pressure:

The effect of varying the CO pressure is summarized in Table I. The plots of TF(H2) values vs. P(CO) for [Rh] = 1.9 wt. % at 100 and 120 ºC shown in Fig. 1, are linear. In addition the plot of log TF(H2) values vs. log P(CO) are also linear with slopes of ca. 1.0. This behavior was confirmed by plotting the log TF(aniline) vs. log P(CO) in which a slope with values of 1.0 ± 0.1 were observed at 100 and 120 ºC respectively, indicating that the reaction is first order in [CO] at these temperatures in the 0 - 1.9 atm range. Pardey and Ford12) and Eisenberg et al.15) reported similar behavior in the homogeneous catalysis of the WGSR by rhodium complexes and they also concluded that reaction rate is first order in P(CO).

Table I. Carbon monoxide pressure, rhodium concentration and temperature effects on WGSR catalysis by [Rh(COD)(4-pic)2]PF6 immobilized on P(4-VP) in contact with 2-ethoxyethanol/watera.

FIG. 1 Plot of TF(H2) vs. P(CO) for [Rh] 1.9 wt. %, 0.5 g P(4-VP) at, 100 and 120 ºC, in contact with 2-ethoxyethanol/water, 8,2, v/v. (Lines drawn for ilustrative purposes only).

This behavior suggests the formation of polymer anchored carbonyl-rhodium species followed by slower step to give H2 and CO2 (Eq. [2]).

 
k1
 
k2, H2O
   
P-[Rh]+ + CO
®
P-[Rh-CO]+
®
P-[Rh]+ + H2 + CO2
Eq. [2]

P = P(4-VP)

The WGSR rate law for such behavior would be:

WGSR rate = k1k2P(CO)[Rh]tot
Eq. [3]

Where [Rh]tot = P-[Rh]+ + P-[Rh-CO]+ and k1 includes the solubility of CO in the medium and k2 the [H2O]. The above expression, Eq. [3] can be reduced to:

TF(H2) = k1k2P(CO)
Eq. [4]

For this kinetics model, plots of TF(H2) vs. P(CO) should be linear with slopes of k1k2 and zero intercept. Indeed these plots are linear at 100 and 120 °C with nearly zero intercept value as predicted by Eq. [4].

Effect of rhodium concentration:

Catalytic runs were carried out for a series of different rhodium concentrations (wt. %) over the range 0.9 to 9.7 wt. % (Table II). A typical run involved determining TF(H2) as a function of [Rh] under P(CO) = 0.9 atm at 100 ºC. Figure 2 shows the TF(H2) values vs. [Rh] plot. An increase in [Rh] from 0.9 to 4.0 wt. % resulted in a decrease in TF(H2), followed by nearly constant values at higher [Rh]. The results indicate that catalyst activity is not first order in the [Rh] range of 5.9 to 9.7 wt. % and suggest an equilibrium of active species having different nuclearities (mononuclear and polynuclear).

Table II. Rhodium concentration effects on WGSR catalysis by [Rh(COD)(4-pic)2]PF6 immobilized on P(4-VP) in contact with 2-ethoxyethanol/watera.

FIG 2. Plot of TF (H2) vs. Rh (wt. %) for P(CO) = 0.9 atm at 100 ºC, 1.5 g of P(4-VP), in contact with 2-ethoxyethanol/water, 8/2 v/v. (Lines drawn for ilustrative purposes only).

Also, our conclusion is strongly supported by the in situ FT-IR, EPS and XPS spectral analyses of the Rh(4-pic)/P(4-VP)/CO system described above, which shows the presence of rhodium species with different nuclearities (mononuclear and polynuclear) and oxidation states ((I),(0) or (–I)) and by the results of an earlier full characterization study by Fachinetti et al.16) on the homogeneous WGSR catalysis by cis-[Rh(CO)2(py)2]PF6 dissolved in aqueous pyridine (py), which showed that the active species were the mononuclear cationic, cis-[Rh(CO)2(py)2]+ and [Rh(CO)(py)3]+ and the polynuclear anionic [Rh5(CO)13(py)2]- complexes.

Effect of temperature:

To determine the activation parameters, TF(H2) values for this Rh/P(4-VP) system were measured at various temperatures (Table III). Figure 3 displays the TF(H2) values against T plot for [Rh] = 1.9 wt. % and P(CO) = 0.9 atm. Arrhenius plot of TF(H2) values were nonlinear in the 70 to 120 ºC range, giving convex curves. The apparent activation energies obtained from the slopes of the respective segments are 216.3 kJ/mol·K at temperatures < 90 ºC and 88.1 kJ/mol·K at temperatures >> 90 ºC.

Table III. Temperature effects on WGSR catalysis by [Rh(COD)(4-pic)2]PF6 immobilized on P(4-VP) in contact with 2-ethoxyethanol/watera.

FIG. 3. An arrthenius plot from the date obtained for P(CO) = 0.9 atm, [Rh] = 1.9 wt. % 0.5 g P(4-VP) in contact with 2-ethoxyyethanol/water, 8/2, v/v.

Non-linear Arrhenius plot similar to the one reported here has previously been observed for the hydrogenation, hydroformylation and isomerization of 1-hexene, catalyzed by polymer-anchored rhodium trichloride under WGSR conditions18). The temperature dependence did not follow simple Arrhenius behavior, but appeared segmented. Also, Eisenberg et al. reported in their mechanistic studies of the homogeneous catalysis of the WGSR by rhodium carbonyl iodide complex that the logarithm of the WGSR rate was not linear with (1/T) over the 55 - 100 °C temperature range. A concave downward plot was observed indicating a change in the rate-determining step with changes in temperature15).

Other factors, such as change with temperature of an equilibrium constant, can be responsible for curvature in an Arrhenius plot19). In the present situation the observed segmentation is the result of the change of the equilibrium constant of the mononuclear and polynuclear rhodium complexes, previously suggested based on the concentration study, both acting as WGSR catalysts, with different energy pathways, i.e., the path related to polynuclear Rh complexes has the higher Ea and it is rate limiting at lower temperature, while the path related to mononuclear Rh complexes with the lower Ea is rate limiting at higher temperature.

Mechanistic consideration:

Scheme illustrates the proposed mechanism for the more active WGSR mononuclear Rh(I) species. As mentioned before, earlier characterization studies10, 14) of Rh(4-pic)/P(4-VP) catalyzed WGSR had shown the anchoring of the rhodium(I) complex [Rh(COD)(4-pic)2]+ through the pyridine groups of the P(4-VP) by displacing the coordinated COD (1a) and the air oxidation of the Rh(I) to Rh(II) species. The latter is reduced to Rh(I) and Rh(-I) by the CO/H2O couple at temperatures over 70 ºC (1b).

The dependence of TF(H2) on P(CO) was attributed to addition of CO to the anchored P(4-VP)-[Rh(4-pic)2]+ complex to give the electrophilic P(4-VP)-[Rh(4-pic)2CO]+ specie (1c). Nucleophilic attack by H2O on the coordinated CO, perhaps assisted by free P(4-VP) through its general base character, yields the hydroxycarbonyl P(4-VP)-[Rh(4-pic)2(CO2H)] and the protonated polymer [P(4-VP)]H+ (1d). Extrusion of CO2 from the former complex gives the hydride P(4-VP)-[HRh(4-pic)2] complex (1e), which upon protonation by the pyridinium moiety [P(4-VP)]H+ through its general acid character, eliminates H2 and regenerates P(4-VP)-[Rh(4-pic)2]+ complex (1f). So, the aminated polymer acts as insoluble ligand and plays the double functions as a general acid and a general base when these two characters are separately required7, 18). A similar mechanism can be envisioned for the polynuclear species.

Acknowledgments

This work was supported by CDCH-UCV (03.12.3424.98 and AI-03.12.4232.98) and CONICIT-Venezuela (S1-95001662). SAM thanks to DICYT-USACH and FONDECYT (Chile). PB thanks to CODECIHT-UC for support. Poly(4-vinylpyridine) Cross-Linked lot Nº. 70515AA was donated by Reilly Industries INC.

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