POLYMER SUPPORTED HOMOGENEOUS CATALYSTS FOR REACTIONS IN SUPERCRITICAL CARBON DIOXIDE

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1 POLYMER SUPPORTED HOMOGENEOUS TLYSTS FOR RETIONS IN SUPERRITIL RON DIOXIDE. german *, R. Flores, Z.. Lopez-astillo, I. ani, and J. P. Facler, Jr. hemical Engineering Dept., Department of hemistry Texas &M University, ollege Station, TX Fax: (979) Introduction Organometallic alysts in homogeneous media offer high reactivity, selectivity, and tunability. Supercritical carbon dioxide, near the critical region, is easily tunable by small variation in pressure and/or temperature allowing the possibility of engineering the reaction environment to improve the reactivity and selectivity [-3]. Limitations of sco as solvent are associated to its wea solvent strength. Efforts have been directed to the design of alyst ligands that are soluble in sco under mild conditions [4-6]. Particularly, fluoroacrylate polymers have been moderately soluble in sco. Therefore, a new alyst was synthesized by attaching a homogeneous rhodium alyst to a fluoroacrylate polymer bacbone maing it soluble in sco. The polymer was synthesized from polymerization of H,H,H,Hheptadecafluoradecyl acrylate monomer (zonyl TN) and N-acryloxysuccinimide (NSI), the former increasing the solubility in sco and the latter providing attachment sites for the alyst. Diphenylphosphino-propylamine, NH (H ) 3 PPh, (DPP) was used to exchange the N-acryloxysuccinimide groups in the polymer, which was then reacted with [Rhl(OD)] to obtain the alyst. The activity of the alyst was evaluated in hydrogenation and hydroformylation of different olefins and a inetic model was proposed for each reaction. In special cases, statistical methods were applied to discriminate among rival models. I Materials and Methods Organic substances were purchased from Sigma-ldrich and used as received. arbon dioxide and hydrogen were bought from razos Valley Welding Supply and used as received. The preparation and characterization of the alyst has been explained in full detail elsewhere [7]. number of alysts with various copolymer composition and rhodium to copolymer molar ratio were prepared. The experimental equipments and procedure used for the experimentation have been described elsewhere [8]. small visual cell reactor with sapphire windows, 8 ml, was used to visually confirm the solubility of the alyst and a single-phase of the reaction mixture at experimental conditions. ml batch reactor with sampling online was set up for taing samples at different times. Finally, it we also used a 7 ml visual cell reactor with sampling on-line for both taing samples at different times and to confirm one phase of reaction mixture. fter each experiment the reactors were carefully washed and blan reaction tests were made to verify total removal of the alyst. If any conversion was observed during the blan test, the cleaning procedure was repeated until there was no conversion. The samples taen from the reactors were analyzed by gas chromatography (HP-

2 FID 89) using a % phenyl polyxilosane capillary column. The chromatograph was calibrated using pure samples of the products and reactants. The hydrogenation of -octene was studied in the ml reactor at 7 and 9º and pressures from 7 to 4 bar. The alyst employed for this study had a copolymer with a zonyl TN to NSI molar ratio of, Rhl(TN DPP) 3, with % of oxidized phosphines. The initial mole fractions of -octene and hydrogen were ept constant in all experiments at. and.33, respectively. The substrate to alyst molar ratio employed was 8. Figure shows the experimental results obtained at different conditions. 8 7º 8 9º Figure. Hydrogenation of -octene at 7 and 9º using the alyst Rhl(TN DPP) 3 with % of oxidized phosphines. Pressure: (?) 7 bar, ( ) 7 bar, (? ) 4 bar, ( ) predicted. The main product was n-octane, and the side products were (E)-octene and (Z)-octene in all the experimental conditions. Deactivation of the alyst was also noticed. Increasing the pressure enhanced the activity of the reaction for both hydrogenation and isomerization. However, increasing the pressure also faster deactivated the alyst. reaction mechanism was proposed, which involved the formation of several alytic complexes [9]. It was assumed that the alyst first reacts with hydrogen to form mono- and dihydride alytic species that further promote the isomerization and hydrogenation of the olefin, respectively. lso, it was considered that alyst was deactivated due to the formation of an unsaturated alytic olefin complex. The cycle for the generation of isomer must be repeated twice with different rate constants, one for the production of (E)-octene, and the other for the production of (Z)-octene. Many inetic models were proposed by assuming different rate-controlling steps in the hydrogenation and isomerization cycles. Some models were immediately discarded because of the lac of fitting with the experimental data. The remaining models were discriminated by applying the Method of Lielihood Ratio as a Discrimination riterion and the Method of Non-Intrinsic Parameters []. Finally, the optimal inetic model considered the insertion of hydrogen into the olefin as rate-determining step for hydrogenation reaction and the coordination of the olefin to the monohydride alytic species (steps 6 and ) for the isomerization reactions. The following inetic equations are obtained:

3 d P d D ( + ) + ( + ) + d d ( + ) + ( + ) 4 + where, : -octene, : hydrogen, P: n-octane, : (E)-octene, D: (Z)-octene The value of the parameters at and 7º and all operating pressures are presented in Table and. Figure also presents the comparison of predicted and experimental values for the hydrogenation of -octene at all the studied experimental conditions. It can be concluded the inetic model is consistent with the total conversion of -octene. Table. Parameter values of the inetic model for the hydrogenation of -octene at 7º Parameter 7 bar 7 bar 4 bar 3 [] h -.9E4 ±.6E3.47E4 ±.86E3.83E ±.9E4 6 [] L mole - h - 44 ± ± E4 ±.33E3 [] L mole - h - 4 ± ± E4 ±.86E3 4 [] L mole - h - ± 9 ±.9 89 ± 7. 4 [] h - [] L mole ±.3 [] L mole -.3 ±.9 [] L / mole -/ 7.96 ±.79 Table. Parameter values of the inetic model for the hydrogenation of -octene at 9º Parameter 7 bar 7 bar 4 bar 3 [] h -.7E ±.6E4.E ±.49E4 4.9E±.83E4 6 [] L mole - h - 4.3E4 ±.3E3.84E4 ±.E3.7E ±.3E4 [] L mole - h -.84E4±.E3 3.8E4 ±.7E3 7.4E4 ±.7E3 4 [] L mole - h -.E3 ±.89E3.E3 ±.6E3 4.E3 ±.4E3 4 [] h - [] L mole ±.89 [] L mole -.44 ±.4 [] L / mole -/ 8.38 ±.8 4' d The same inetic mechanism was also employed to determine the inetic parameters for the hydrogenation of cyclohexene using the alyst Rhl(TN DPP) 3 with 3% of oxidized phosphines. The temperature range studied was 7-º at 7 bar. The substrate to alyst molar ratio was 4 at 7º, and 7 at 9 and º. The initial mole fraction of reactants was ept constant in all the experiments at different temperatures. The only product of the reaction was cyclohexane but the formation of monohydride alytic species is taen into consideration for modeling the reaction. This was done in order to eep the same mechanism. The double bond in cyclohexene may be shifting around, but it did not produce a distinct new isomer. The following inetic equations are obtained: d P + 3 ( + ) + d + ( + ) 4 + 4' d 3

4 where is cyclohexene and is hydrogen. Table 3 presents the values of the parameters for the hydrogenation of cyclohexene at 7 bar and different temperatures and Figure shows the comparison of predicted and experiments results for these experiments. Once more, the calculated values fit very well the experimental data. Table 3. Parameter values of the inetic model for the hydrogenation of cyclohexene at 7 bar Parameter 7º 9º º 3 [] h - 39 ± E4 ±.E3 9.6E4 ±.36E4 4 [] L mole - h ±.4 49 ± ± [] h - [] L mole -.6 ±.7.9 ±..7 ±.6 [] L mole -.94 ±.7. ± ±. [] L / mole -/ 7.96 ± ± ±.8 The hydroformylation of -octene was studied at -7º and 7-4 bar in the ml batch reactor. The alyst employed in this study was Rhl(TN DPP) 3, with % of oxidized phosphines. The experimental results are presented in Figure. Isomerization and hydroformylation of the substrate, and hydroformylation of the isomers were obtained at all conditions studied. The main product was -methyloctanal, and the side products were n- nonanal, isomers of -octene, and their respective hydroformylation products. The effect of pressure at º was more evident than at 7º. The reaction was modeled with a pseudo-first order expression. Table 4 presents the value of the rate constant at different experimental conditions and Figure 3 also shows the fitting of the inetic model with the experimental data. Table 4. Parameter values of the inetic model for the hydroformylation of -octene Parameter 7 bar 4 bar º 7º º 7º '. ±.9.7 ±.4.7 ±.7. ±.8 The hydroformylation of styrene was studied at and 7º, and between 7 and 4 bar in the 7 ml visual cell reactor. The alyst employed in this study was Rhl(TN DPP) 3, with % of oxidized phosphines. The initial mole fractions of styrene, carbon monoxide and hydrogen were.4,.636 and.636, respectively. The substrate to alyst mole ratio was in all the experiments. The main product was - phenylpropionaldehyde with small amounts of 3-phenylpropionaldehyde and traces of ethylbenzene. The experimental results are presented in Figure 4. This reaction was modeled using the dissociative mechanism for hydroformylation of olefins presented by (ornils and Herrmann, 996). efore the alytic cycle, several reactions have to proceed to finally generate the ey intermediate HRh(O) L. ecause the great excess of O and H in the reaction system, the concentration of both compounds are considered constant during the reaction. Therefore, the final inetic equation was as given below where S is styrene. d S 8 S + 9 S 4

5 6 4 3 Figure. Hydrogenation of cyclohexene at 4 bar using the alyst Rhl(TN DPP) 3 with 3% of oxidized phosphines. Temperature: (?) 7º, (? ) 9º, ( ) º, ( ) predicted º 3 º 3 Figure 3. Hydroformylation of -octene at and 7º using the alyst Rhl(TN DPP) 3 with % of oxidized phosphines. Pressure: (?) 7 bar, ( ) 4 bar, ( ) predicted. The values of the parameters for inetic model are reported in Table and 6. Figure 6 also shows the fitting of the inetic model with the experimental data. It can be concluded the inetic model is consistent with the general profile of all the products. Table. Parameter values of the inetic model for the hydroformylation of styrene at º Parameter 7 bar 7 bar 4 bar [] L mole - h ± 7 78 ± ± [] 3.7 ±.8 9 [] L mole -.98 ±.37 [].9 ±.9

6 º 3 7º 3 Figure 4. Hydroformylation of styrene at and 7º using the alyst Rhl(TN DPP) 3 with % of oxidized phosphines. Pressure: (?) 7 bar, ( ) 4 bar, ( ) predicted. Table 6. Parameter values of the inetic model for the hydroformylation of styrene at 7º Parameter 7 bar 7 bar 4 bar [] L mole - h - 39 ± ± ± 34 8 [].7 ±.9 9 [] L mole ±.6 [] 3.73 ±.7 References [] Jessop, P. G., T. Iariya, and R. Noyori, hem. Rev., Vol. 99, 999. p. 47 [] Leitner, W., Top. urr. hem., Vol. 6, 999, p. 7 [3] Oaes, R. S.,.. lifford, and. M. Rayner, J. hem. Soc. Perin Trans.,, p.97 [4] hen, W., L. Xu, and J. Xiao, Org. Lett., Vol.,, p. 67 [] ainz, S., D. och, W. aumann, W. Leitner, ngew. hem. Int. Ed. Eng., Vol. 36, 997, p. 68 [6] Sarbu, T., T. J. Styranec, and E. J. ecman, Vol. 4,, p. 6 [7] ani, I.; M..Omary, M..Rawashdeh-Omary, Z..Lopez-astillo, R. Flores,.german, J. P.Facler, Jr., Tetrahedron, Vol. 8,, p. 393 [8] Lopez-astillo, Z..; R. Flores, I. ani,, J. P. Facler, Jr., and. german, Ind. Eng. hem. Res., Vol. 4,, p. 37 [9] Flores, R. Fluoroacrylate opolymer Grafted Rhodium atalysts for Hydrogenation Reactions In Supercritical Fluids. Ph. D. Dissertation, Texas &M University (3). [] Froment, G. M., and L. Hosten. In TLYSIS - Science and Technology; nderson, J. R., and M oudart, Eds.; Springler-Verlag erlin Heidelberg, Germany, Vol., p. 97 (98). [] ornils,., and W.. Herrmann, Eds., pplied Homogeneous atalysis with Organometallic ompounds; VH Publishers: Weinheim (996). 6