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Liquid-phase hydrocarbon oxidation using supported transition metal catalysts: Influence of the solid support
Chemical Engineering Science 54 (1999) 3563}3568
Liquid-phase hydrocarbon oxidation using supported transition metal catalysts: In#uence of the solid support G. Langhendries , G.V. Baron *, P.E. Neys, P.A. Jacobs Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium Center for Surface Chemistry and Catalysis, K.U. Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium
Abstract The cyclohexane oxidation into cyclohexanol and cyclohexanone using several zeolite-encapsulated and activated carbon supported iron-phthalocyanine catalysts was investigated in both batch and continuous reaction conditions. The catalytic activity and e$ciency were shown to be strongly dependent on the polarity of the solid support material. A substantial increase in the Si/Al ratio of the NaY zeolite resulted in a increase in hydrocarbon oxidation reaction rate. Adsorption equilibrium of the di!erent solid support materials was determined with an HPLC method. Initial reaction rate of the cyclohexane oxidation, obtained in a continuous reactor con"guration, correlated well with these adsorption data. A rapid catalyst deactivation was observed under continuous reaction conditions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Supported transition metal complex; Phthalocyanine; Selective oxidation; Adsorption
1. Introduction The immobilisation of catalytically active soluble transition metal complexes has become a timely and relevant research topic. Such novel catalysts combine the high selectivity of a homogeneous catalyst and the ease of separation of a heterogeneous one. However, the immobilisation of a transition metal complex often results in a decrease in catalyst activity due to additional mass transfer resistances (sorption and di!usion). Metallo-phthalocyanines and porphyrins are promising oxidation catalysts, but undergo rapid deactivation in homogeneous reaction conditions as a result of oxidative destruction. Consequently, several attempts were made to immobilise these complexes on a microporous solid support (silica, NaY zeolite, activated carbon, alumina, montmorillonite clay, poly(siloxane) polymer, etc.). In this presentation, the in#uence of the adsorption
* Corresponding author. Tel.:#32 2 629 32 50; fax:#32 2 629 32 48; e-mail: [email protected].
properties of the solid support on the catalyst activity and e$ciency will be demonstrated. Herron (1988) demonstrated the in-situ synthesis of these iron-phthalocyanine complexes into the supercage of a faujasite NaY and NaX zeolite (ship-in-a-bottle complex). As a result of the e!ective site isolation of the transition metal complexes into the micropore structure of the FePc}Y catalyst, any bimolecular reaction, leading to a oxidative destruction, is prevented. Although several interesting properties have been reported for these catalysts, a rapid catalyst deactivation was observed as a result of iodoxybenzene pore blocking (iodosobenzene oxidant is disproportioned into iodoxybenzene in the pore structure of the microporous zeolite). Parton et al. (1991) developed an improved synthesis procedure, and studied the room temperature oxidation of cyclohexane into cyclohexanol and cyclohexanone using an aqueous phase t-butylhydroperoxide (t-BOOH) oxidant and an acetone solvent. A conversion of respectively 15% has been reported for the improved zeoliteencaged FePc catalyst, whereas the homogeneous FePc complexes were rapidly oxidatively destroyed as shown
0009-2509/99/$ } see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 8 ) 0 0 3 9 6 - 0
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G. Langhendries et al. /Chemical Engineering Science 54 (1999) 3563}3568
by the colour change of the reaction solution. Parton et al. (1996b) also studied the in#uence of the reaction mode on the catalyst activity and selectivity. Using a fed batch reactor con"guration with continuous addition of aqueous t-BOOH, a conversion of 16% together with an organic peroxide e$ciency up to 61% could be achieved. Several other promising metallo-phthalocyanine and porphyrine supported catalysts have been reported during the last decade (Thibault-Starzyk et al., 1996; PeH rez-Bernal et al., 1991; Barloy et al., 1992; Hilal et al., 1996; Battioni et al., 1996). In this paper, the in#uence of the solid support on the catalyst activity and e$ciency will be discussed. The cyclohexane oxidation using several supported ironphthalocyanine catalysts has been studied in both batch and continuous reactor con"gurations. As a conclusion, a comparison between the support partition coe$cients and reaction rates was made.
temperature and atmospheric pressure. Batch experiments using 100 mg iron-phthalocyanine supported catalyst, 10 ml acetone solvent, 1 ml cyclohexane substrate, 1 ml t-butylhydroperoxide (70 wt% aqueous solution) and 0.1 ml chlorobenzene internal standard were performed in a magnetically stirred glass vial. Small samples are taken at regular time intervals and analysed using a Hewlett Packard 6890 gas chromatograph (HP-1 column and FID detector). Initial reaction rates were determined using a continuous laboratory scale reactor. A reaction mixture consisting of 100 ml acetone, 10 ml cyclohexane, 10 ml t-butylhydroperoxide (70 wt%) and 1 ml internal standard was pumped over a closely packed bed of ironphthalocyanine catalyst crystals (0.15 g) at a #ow rate of 0.03 ml/min (HP 1100 HPLC system).
4. HPLC method 2. Materials Cyclohexane (99.9#%) and acetone (99.9#%) were purchased from Sigma-Aldrich. Chlorobenzene (99%), cyclohexanone (p.a.) and t-butanol (p.a.) were acquired from Merck. Cyclohexanol (98%) was obtained from Janssen Chimica, n-hexane (99#%), 1,2-dicyanobenzene (98#%), dimethylformamide (99%) and ferrocene (98%) from Aldrich. t-butylhydroperoxide (70 wt% in water) was obtained from Sigma. Iron-phthalocyanine (98#%) was purchased from Strem Chemicals. Zeolite NaY (Si/Al ratio 2.47) was obtained from Zeocat, activated carbon type BL was kindly provided by Chemviron Carbon. Zeolite-encapsulated ironphthalocyanine catalyst (+1 lm particle size) is synthesised by the solid state adsorption of 0.575 g ferrocene on 5 g NaY (Parton et al., 1991). The ferrocene-loaded zeolite is then mixed with 3.15 g 1,2-dicyanobenzene and placed in an autoclave. The Te#on-lined autoclave is heated at 1803C for 17 h. Subsequently, the resulting blue-green solid is Soxhlet extracted with acetone, dimethylformamide and again acetone to remove unreacted reactants and intermediates from the microporous zeolite. The "nal catalyst is air dried at 703C. Iron-phthalocyanine complexes are deposited on the activated carbon (BL, Chemviron Carbon) by impregnation. 52 mg FePc was dissolved into 30 ml chloromethane solvent and 1 g of activated carbon was added. Subsequently, the solvent is evaporated at room temperature.
The HPLC method is a liquid chromatographic technique in which a carrier #ows through a column, packed with adsorbent or catalyst. A small pulse of a tracer component is then injected at the inlet of the column, while the concentration at the column outlet is monitored continuously using a suitable detector. The partition coe$cients can then be derived from the experimental response curve using the following equation (KaK rger and Ruthven, 1992): ¸ k" (e #(1!e ) K) (1) v D where k is the retention time, ¸ the column length, e the external porosity of the packed bed and v the super"cial D velocity. The partition coe$cient K is de"ned as the ratio of internal to external tracer component concentration. The measurements were performed at room temperature using a Hewlett Packard 1100 HPLC and a Hewlett Packard 1037A refractometer. The measured retention times have to be corrected for the contribution of the dead volume in the system. Typically a 1/4 stainless steel column (cross section S"16.6;10\ m) with a length of 50 mm was used. The zeolite NaY and catalyst crystals used in this study have an average diameter of 1 lm and were regenerated over night in a Shel Lab 1410 vacuum oven (1503C and 20 in Hg vacuum). The regeneration temperature is limited here due to the limited thermal stability of iron-phthalocyanine complexes.
5. Results and discussion 3. Reaction experiments The oxidation of cyclohexane to cyclohexanol and cyclohexanone is carried out in the liquid phase at room
The cyclohexane oxidation into cyclohexanol and cyclohexanone using an aqueous t-butylhydroperoxide oxidant was used as a model reaction system to study the
G. Langhendries et al. /Chemical Engineering Science 54 (1999) 3563}3568
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Table 1 Batch reactor experiments of several supported iron-phthalocyanine catalysts
FePc-Y (Si/Al"2.47) FePc-USY (Si/Al"28) FePc-USY (Si/Al"130) FePc-AC
lmol FePc
r
E$ciency
ol/on
9.20 9.20 9.20 9.15
7 10 25 35
11 14 18 23
1.4 1.1 0.9 0.7
lmol iron-phthalocyanine complexes in 0.1 g of catalyst. r, reaction rate: mol product/mol FePc.h (integrated reaction rate after 4 h reaction). Organic peroxide e$ciency, de"ned as the amount of t-BOOH used for the oxidation of cyclohexane and cyclohexanol to the total amount of organic peroxide converted (expressed in percentage). ol/on ratio: mol cyclohexanol/mol cyclohexanone.
Table 2 Continuous reactor experiments of the zeolite-encapsulated FePc catalysts
FePc-Y (Si/Al"2.47) FePc-USY (Si/Al"28) FePc-USY (Si/Al"130)
r
E$ciency
ol/one
38 190 463
6 11 16
1.4 1.1 0.9
r, initial reaction rate: mol product/mol FePc.h. Organic peroxide e$ciency, de"ned as the amount of t-BOOH used for the oxidation of cyclohexane and cyclohexanol to the total amount of organic peroxide converted (expressed in percentage). ol/on ratio: mol cyclohexanol/mol cyclohexanone.
Fig. 1. Oxidation product and oxidant concentration as a function of time.
catalytic activity and e$ciency of zeolite NaY and activated carbon supported iron-phthalocyanine catalysts. Fig. 1 shows the product (cyclohexanol and cyclohexanone) and t-BOOH concentration in a batch reactor experiment as a function of time. A catalyst activity of 7 (mol product. mol FePc\ h\) was observed for the zeolite-encapsulated iron-phthalocyanine catalyst (Si/Al"2.47). A drastic dealumination of the zeolite framework (Si/Al"130) resulted in an increase in reaction rate by a factor of three (Table 1). The immobilisation of iron-phthalocyanine on the activated carbon support (FePc-AC) resulted in an additional increase in catalyst activity. A similar trend was observed for the organic peroxide e$ciency (Table 1). An organic peroxide e$ciency of 11% indicates that for each mol of t-BOOH used for the oxidation of cyclohexane and cyclohexanol, there are approximately 8 mol of organic peroxide decomposed into molecular oxygen and t-BOH.
A similar reactivity trend was observed in the continuous reactor experiments (Table 2). A rapid deactivation was observed for the activated carbon supported ironphthalocyanine catalyst (Fig. 3), presumably a result of the release of transition metal complexes from the carbon support surface. No leaching occurred with the zeoliteencapsulated catalysts, since a simple regeneration, by washing with acetone and drying, restores the original activity of the zeolite-encapsulated iron-phthalocyanine catalyst (Parton et al., 1996b), which indicates that the phthalocyanine complexes are entrapped into the supercages of the faujasite zeolite (ship-in-a-bottle complexes). The initial reaction rates obtained in the continuous reactor con"guration (short contact time) are substantially higher than the integrated reaction rates in a batch reactor con"guration. The previous observation cannot only be explained by the depletion of cyclohexane and t-BOOH reactants in the batch reactor, since even after the extra addition of 1 ml t-BOOH to the reaction mixture, low cyclohexane oxidation and organic peroxide decomposition reaction rates are obtained. Consequently, it can be concluded that a catalyst deactivation
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occurs rapidly under such batch reactor conditions, presumably a result of oxidation product pore blocking. In order to explain the in#uence of the solid support on the catalyst activity and e$ciency of supported phthalocyanine complexes, the multicomponent adsorption in NaY and activated carbon was studied using a liquid chromatographic method. Table 3 shows the partition coe$cient of several tracer components using cyclohexane as a carrier liquid. In both ultrastable Y zeolite (Si/Al"130) and activated carbon type BL, polar components (acetone solvent and t-BOOH oxidant) are preferentially adsorbed as compared to the nonpolar cyclohexane substrate. Subsequently, the adsorption properties of the studied support materials were determined in acetone solvent (Table 4). As a result of the hydrophilic nature, acetone is strongly adsorbed into the micropore system of the catalyst, resulting in low tracer component partition coe$cients. Consequently as a result of the competitive adsorption of the polar components (acetone, t-BOOH, oxidation products, etc.) in the reaction mixture, the non-polar cyclohexane substrate is almost completely excluded from the active sites of the catalyst (FePc complexes), resulting in low reaction rates and catalyst e$ciency. Acetone dominates over the other polar components as could be expected from the results in Table 3. Only the organic peroxide and water are somewhat more strongly adsorbed in USY.
Table 3 Adsorption in USY zeolite and activated carbon type BL (cyclohexane carrier) Tracer component
K (USY)
K (AC)
n-Hexane Acetone Cyclohexanol Cyclohexanone t-Butanol t-BOOH
1.3 14 3.6 2.2 * 7.5
1.7 9.4 8.8 * 9.4 17
Table 4 Adsorption in USY zeolite and activated carbon type BL (acetone carrier) Tracer component
K (USY)
K (AC)
n-Hexane Cyclohexane Cyclohexanol Cyclohexanone t-Butanol t-BOOH Water
1.1 0.7 0.8 0.8 * 1.2 1.7
1.0 0.9 1.2 1.0 0.7 0.9 1.2
Additionally, the adsorption of acetone in NaY zeolite with a Si/Al ratio ranging from 2.47 to 130 was investigated (Table 5). An acetone partition coe$cient as high as 90 was observed for the microporous NaY (Si/Al"2.47) zeolite, and the partition coe$cient in aluminium rich zeolite was found to be approximately a factor of six higher as compared to the dealuminated zeolite, explaining the marked enhancement in reaction rate for the ultrastable FePc-USY catalyst (Si/Al"130). The acetone solvent is more strongly adsorbed in the FePc-Y (Si/Al"2.47) catalyst, excluding the hydrocarbon substrate from the active site of the catalyst. The higher acetone adsorption in ultrastable Y (Si/Al"130) as compared to the carbon support (Table 3) also explains the higher reaction rate obtained for the activated carbon supported iron-phthalocyanine catalyst in acetone solvent. The high organic peroxide e$ciency observed by Parton et al. (1996a, b) in the fed batch reactor con"guration can be explained from the adsorption experiments. The low t-butylhydroperoxide concentration in the external liquid phase results in a low adsorbed t-BOOH concentration and consequently a more balanced cyclohexane and t-BOOH concentration near the active sites of the catalyst, resulting in higher cyclohexane oxidation and lower organic peroxide decomposition reaction rates. To demonstrate the strong in#uence of adsorption on the catalyst activity, the acetone partition coe$cient of the di!erent zeolite-encapsulated FePc catalysts was correlated with the initial reaction rates, obtained in the laboratory scale packed bed reactor con"guration (Fig. 2). When the acetone solvent is strongly adsorbed into the (low Si/Al ratio) zeolite, excluding the cyclohexane substrate from the active sites of the catalyst, low hydrocarbon oxidation reaction rates are observed. On the other hand, higher reaction rates were observed when the acetone solvent is less strongly adsorbed into the high silica zeolite catalyst. Finally, the catalytic stability of the zeolite-encapsulated iron-phthalocyanine catalyst was investigated using a continuous reactor (Fig. 3). Although no leaching of FePc complexes was observed for the FePc-USY catalyst, a substantial decrease in cyclohexane conversion occurred. Almost no catalytic activity remained after
Table 5 Acetone partition coe$cient as a function of the Si/Al ratio of microporous zeolite NaY Si/Al ratio
K
2.47 13 41 130
91 78 48 14
G. Langhendries et al. /Chemical Engineering Science 54 (1999) 3563}3568
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immobilisation of the transition metal complex at the external surface of the solid support can substantially increase the catalyst stability, since oxidation product pore blocking is no longer a major problem in such catalytic materials.
6. Conclusion
Fig. 2. Adsorption-reaction correlation for the di!erent zeolite-encapsulated FePc catalysts.
Conclusively, it can be stated that a clear relationship between the polarity of the solid support material and the reaction rate of the iron-phthalocyanine supported catalyst was established. The reactant concentrations near the active sites of the catalyst, and consequently the reaction rates, are essentially controlled by the adsorption equilibrium of the solid support material. The rapid deactivation of the zeolite-encapsulated iron-phthalocyanine catalyst in continuous operation, presumably a result of oxidation product pore blocking, was demonstrated to be in good agreement with the adsorption data. Adsorption experiments were shown to be an interesting tool in the design of novel supported transition metal complex catalysts.
Acknowledgements
Fig. 3. Alcohol and ketone production rate as a function of time in a continuous reactor. (0.15 g catalyst, 0.05 ml/min, feed mixture consists of 100 ml acetone, 20 ml cyclohexane, 20 ml t-BOOH and 1 ml internal standard).
The authors acknowledge sponsoring from the Belgian Federal Government in the frame of a IUAP-PAI project on Supramolecular Chemistry and Catalysis and the Flemish Fund for Scienti"c Research (FWO KV.E2031996). G.L. and P.E.N. acknowledge &het Vlaams Instituut voor de bevordering van het wetenschappelijktechnologisch onderzoek in de industrie, IWT' for a grant as doctoral research fellow.
References
150 h on stream, due to the accumulation of polar oxidation products in the pore structure of the hydrophilic zeolite. The rapid catalyst deactivation, as a result of the oxidation product pore blocking or the leaching of ironphthalocyanine complexes from the solid support is the major drawback of these supported transition metal complex catalysts. Parton et al. (1994) immobilised nitro-substituted iron-phthalocyanine complexes at the external surface of zeolite NaY (FeTNPc-Y Si/Al"2.47), and this FeTNPc-Y catalyst material turned out to be superior as compared to the classic FePc-Y. In a batch reactor con"guration, the nitro-substituted catalyst is approximately ten times more active and no deactivation was observed even after 30 h of reaction. Therefore, the
Barloy, L., Lallier, J.P., Battioni, P., Mansuy, D., Pi!ard, Y., Tournoux, M., Valim, J.B., & Jones, W. (1992). Manganese porphyrins adsorbed or intercalated in di!erent mineral matrices: Preparation and compared properties as catalysts for alkene and alkane oxidation. New J. Chem., 16, 71}80. Battioni, P., Iwanejko, R., Mansuy, D., Mlodnicka, T., Poltowicz, J., & Sanchez, F. (1996). Reactivity of polyhalogenated and zeoliteencapsulated metalloporphyrins in oxidation with dioxygen. J. Mol. Catal. A: Chem., 109, 91}98. Herron, N. (1988). The selective partial oxidation of alkanes using zeolite based catalysts. Phthalocyanine (Pc) &&ship-in-bottle'' species. J. Coord. Chem., 19, 25}38. Hilal, H.S., Jondi, W., Khalaf, S., Keilani, A., Suleiman, M., & Schreiner, A.F. (1996). The catalytic activity of poly(siloxane)-supported metalloporphyrins in ole"n oxidation reactions: The e!ect of the support on the catalytic activity and selectivity. J. Mol. Catal. A: Chem., 113, 35}44. KaK rger, J., & Ruthven, D.M. (1992). Di+usion in zeolites and other microporous solids. New York: Wiley.
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Parton, R.F., Uytterhoeven, L., & Jacobs, P.A. (1991). Ironphthalocyanines encaged in zeolite Y and VPI-5 molecular sieve as catalysts for the oxyfunctionalization of n-alkanes. Stud. Surf. Sci. Catal., 59, 395}403. Parton, R.F., Bezoukhanova, C.P., Grobet, J., Grobet, P.J., & Jacobs, P.A. (1994). Synthesis, characterization and catalytic performance of nitro-substituted Fe-phthalocyanines on zeolite Y. Stud. Surf. Sci. Catal., 83, 371}378. Parton, R.F., Neys, P.E., Jacobs, P.A., Sosa, R.C., & Rouxhet, P.G. (1996a). Iron-phthalocyanine immobilized on activated carbon black: A selective catalyst for alkane oxidation. J. Catal., 164, 341}346.
Parton, R.F., Peere, G.J., Neys, P.E., Jacobs, P.A., Claessens, R., & Baron, G.V. (1996b). Cyclohexane oxidation with tertiary-butylhydroperoxide catalyzed by iron-phthalocyanines homogeneously and occluded in Y zeolite. J. Mol. Catal. A: Chem., 113, 445}454. PeH rez-Bernal, M.E., Ruano-Casero, R., & Pinnavaia, T.J. (1991). Catalytic autoxidation of 1-decanethiol by cobalt(II)phthalocyaninetetrasulfonate intercalated in a layered double hydroxide. Catal. ¸ett., 11, 55}62. Thibault-Starzyk, F., Van Puymbroeck, M., Parton, R.F., & Jacobs, P.A. (1996). Thiol-oxidation with zeolite encapsulated phthalocyanines: In#uence of the solid support. J. Mol. Catal. A: Chem. 109, 75}79.
Liquid-phase hydrocarbon oxidation using supported transition metal catalysts: In#uence of the solid support G. Langhendries , G.V. Baron *, P.E. Neys, P.A. Jacobs Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium Center for Surface Chemistry and Catalysis, K.U. Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium
Abstract The cyclohexane oxidation into cyclohexanol and cyclohexanone using several zeolite-encapsulated and activated carbon supported iron-phthalocyanine catalysts was investigated in both batch and continuous reaction conditions. The catalytic activity and e$ciency were shown to be strongly dependent on the polarity of the solid support material. A substantial increase in the Si/Al ratio of the NaY zeolite resulted in a increase in hydrocarbon oxidation reaction rate. Adsorption equilibrium of the di!erent solid support materials was determined with an HPLC method. Initial reaction rate of the cyclohexane oxidation, obtained in a continuous reactor con"guration, correlated well with these adsorption data. A rapid catalyst deactivation was observed under continuous reaction conditions. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Supported transition metal complex; Phthalocyanine; Selective oxidation; Adsorption
1. Introduction The immobilisation of catalytically active soluble transition metal complexes has become a timely and relevant research topic. Such novel catalysts combine the high selectivity of a homogeneous catalyst and the ease of separation of a heterogeneous one. However, the immobilisation of a transition metal complex often results in a decrease in catalyst activity due to additional mass transfer resistances (sorption and di!usion). Metallo-phthalocyanines and porphyrins are promising oxidation catalysts, but undergo rapid deactivation in homogeneous reaction conditions as a result of oxidative destruction. Consequently, several attempts were made to immobilise these complexes on a microporous solid support (silica, NaY zeolite, activated carbon, alumina, montmorillonite clay, poly(siloxane) polymer, etc.). In this presentation, the in#uence of the adsorption
* Corresponding author. Tel.:#32 2 629 32 50; fax:#32 2 629 32 48; e-mail: [email protected].
properties of the solid support on the catalyst activity and e$ciency will be demonstrated. Herron (1988) demonstrated the in-situ synthesis of these iron-phthalocyanine complexes into the supercage of a faujasite NaY and NaX zeolite (ship-in-a-bottle complex). As a result of the e!ective site isolation of the transition metal complexes into the micropore structure of the FePc}Y catalyst, any bimolecular reaction, leading to a oxidative destruction, is prevented. Although several interesting properties have been reported for these catalysts, a rapid catalyst deactivation was observed as a result of iodoxybenzene pore blocking (iodosobenzene oxidant is disproportioned into iodoxybenzene in the pore structure of the microporous zeolite). Parton et al. (1991) developed an improved synthesis procedure, and studied the room temperature oxidation of cyclohexane into cyclohexanol and cyclohexanone using an aqueous phase t-butylhydroperoxide (t-BOOH) oxidant and an acetone solvent. A conversion of respectively 15% has been reported for the improved zeoliteencaged FePc catalyst, whereas the homogeneous FePc complexes were rapidly oxidatively destroyed as shown
0009-2509/99/$ } see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 9 - 2 5 0 9 ( 9 8 ) 0 0 3 9 6 - 0
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G. Langhendries et al. /Chemical Engineering Science 54 (1999) 3563}3568
by the colour change of the reaction solution. Parton et al. (1996b) also studied the in#uence of the reaction mode on the catalyst activity and selectivity. Using a fed batch reactor con"guration with continuous addition of aqueous t-BOOH, a conversion of 16% together with an organic peroxide e$ciency up to 61% could be achieved. Several other promising metallo-phthalocyanine and porphyrine supported catalysts have been reported during the last decade (Thibault-Starzyk et al., 1996; PeH rez-Bernal et al., 1991; Barloy et al., 1992; Hilal et al., 1996; Battioni et al., 1996). In this paper, the in#uence of the solid support on the catalyst activity and e$ciency will be discussed. The cyclohexane oxidation using several supported ironphthalocyanine catalysts has been studied in both batch and continuous reactor con"gurations. As a conclusion, a comparison between the support partition coe$cients and reaction rates was made.
temperature and atmospheric pressure. Batch experiments using 100 mg iron-phthalocyanine supported catalyst, 10 ml acetone solvent, 1 ml cyclohexane substrate, 1 ml t-butylhydroperoxide (70 wt% aqueous solution) and 0.1 ml chlorobenzene internal standard were performed in a magnetically stirred glass vial. Small samples are taken at regular time intervals and analysed using a Hewlett Packard 6890 gas chromatograph (HP-1 column and FID detector). Initial reaction rates were determined using a continuous laboratory scale reactor. A reaction mixture consisting of 100 ml acetone, 10 ml cyclohexane, 10 ml t-butylhydroperoxide (70 wt%) and 1 ml internal standard was pumped over a closely packed bed of ironphthalocyanine catalyst crystals (0.15 g) at a #ow rate of 0.03 ml/min (HP 1100 HPLC system).
4. HPLC method 2. Materials Cyclohexane (99.9#%) and acetone (99.9#%) were purchased from Sigma-Aldrich. Chlorobenzene (99%), cyclohexanone (p.a.) and t-butanol (p.a.) were acquired from Merck. Cyclohexanol (98%) was obtained from Janssen Chimica, n-hexane (99#%), 1,2-dicyanobenzene (98#%), dimethylformamide (99%) and ferrocene (98%) from Aldrich. t-butylhydroperoxide (70 wt% in water) was obtained from Sigma. Iron-phthalocyanine (98#%) was purchased from Strem Chemicals. Zeolite NaY (Si/Al ratio 2.47) was obtained from Zeocat, activated carbon type BL was kindly provided by Chemviron Carbon. Zeolite-encapsulated ironphthalocyanine catalyst (+1 lm particle size) is synthesised by the solid state adsorption of 0.575 g ferrocene on 5 g NaY (Parton et al., 1991). The ferrocene-loaded zeolite is then mixed with 3.15 g 1,2-dicyanobenzene and placed in an autoclave. The Te#on-lined autoclave is heated at 1803C for 17 h. Subsequently, the resulting blue-green solid is Soxhlet extracted with acetone, dimethylformamide and again acetone to remove unreacted reactants and intermediates from the microporous zeolite. The "nal catalyst is air dried at 703C. Iron-phthalocyanine complexes are deposited on the activated carbon (BL, Chemviron Carbon) by impregnation. 52 mg FePc was dissolved into 30 ml chloromethane solvent and 1 g of activated carbon was added. Subsequently, the solvent is evaporated at room temperature.
The HPLC method is a liquid chromatographic technique in which a carrier #ows through a column, packed with adsorbent or catalyst. A small pulse of a tracer component is then injected at the inlet of the column, while the concentration at the column outlet is monitored continuously using a suitable detector. The partition coe$cients can then be derived from the experimental response curve using the following equation (KaK rger and Ruthven, 1992): ¸ k" (e #(1!e ) K) (1) v D where k is the retention time, ¸ the column length, e the external porosity of the packed bed and v the super"cial D velocity. The partition coe$cient K is de"ned as the ratio of internal to external tracer component concentration. The measurements were performed at room temperature using a Hewlett Packard 1100 HPLC and a Hewlett Packard 1037A refractometer. The measured retention times have to be corrected for the contribution of the dead volume in the system. Typically a 1/4 stainless steel column (cross section S"16.6;10\ m) with a length of 50 mm was used. The zeolite NaY and catalyst crystals used in this study have an average diameter of 1 lm and were regenerated over night in a Shel Lab 1410 vacuum oven (1503C and 20 in Hg vacuum). The regeneration temperature is limited here due to the limited thermal stability of iron-phthalocyanine complexes.
5. Results and discussion 3. Reaction experiments The oxidation of cyclohexane to cyclohexanol and cyclohexanone is carried out in the liquid phase at room
The cyclohexane oxidation into cyclohexanol and cyclohexanone using an aqueous t-butylhydroperoxide oxidant was used as a model reaction system to study the
G. Langhendries et al. /Chemical Engineering Science 54 (1999) 3563}3568
3565
Table 1 Batch reactor experiments of several supported iron-phthalocyanine catalysts
FePc-Y (Si/Al"2.47) FePc-USY (Si/Al"28) FePc-USY (Si/Al"130) FePc-AC
lmol FePc
r
E$ciency
ol/on
9.20 9.20 9.20 9.15
7 10 25 35
11 14 18 23
1.4 1.1 0.9 0.7
lmol iron-phthalocyanine complexes in 0.1 g of catalyst. r, reaction rate: mol product/mol FePc.h (integrated reaction rate after 4 h reaction). Organic peroxide e$ciency, de"ned as the amount of t-BOOH used for the oxidation of cyclohexane and cyclohexanol to the total amount of organic peroxide converted (expressed in percentage). ol/on ratio: mol cyclohexanol/mol cyclohexanone.
Table 2 Continuous reactor experiments of the zeolite-encapsulated FePc catalysts
FePc-Y (Si/Al"2.47) FePc-USY (Si/Al"28) FePc-USY (Si/Al"130)
r
E$ciency
ol/one
38 190 463
6 11 16
1.4 1.1 0.9
r, initial reaction rate: mol product/mol FePc.h. Organic peroxide e$ciency, de"ned as the amount of t-BOOH used for the oxidation of cyclohexane and cyclohexanol to the total amount of organic peroxide converted (expressed in percentage). ol/on ratio: mol cyclohexanol/mol cyclohexanone.
Fig. 1. Oxidation product and oxidant concentration as a function of time.
catalytic activity and e$ciency of zeolite NaY and activated carbon supported iron-phthalocyanine catalysts. Fig. 1 shows the product (cyclohexanol and cyclohexanone) and t-BOOH concentration in a batch reactor experiment as a function of time. A catalyst activity of 7 (mol product. mol FePc\ h\) was observed for the zeolite-encapsulated iron-phthalocyanine catalyst (Si/Al"2.47). A drastic dealumination of the zeolite framework (Si/Al"130) resulted in an increase in reaction rate by a factor of three (Table 1). The immobilisation of iron-phthalocyanine on the activated carbon support (FePc-AC) resulted in an additional increase in catalyst activity. A similar trend was observed for the organic peroxide e$ciency (Table 1). An organic peroxide e$ciency of 11% indicates that for each mol of t-BOOH used for the oxidation of cyclohexane and cyclohexanol, there are approximately 8 mol of organic peroxide decomposed into molecular oxygen and t-BOH.
A similar reactivity trend was observed in the continuous reactor experiments (Table 2). A rapid deactivation was observed for the activated carbon supported ironphthalocyanine catalyst (Fig. 3), presumably a result of the release of transition metal complexes from the carbon support surface. No leaching occurred with the zeoliteencapsulated catalysts, since a simple regeneration, by washing with acetone and drying, restores the original activity of the zeolite-encapsulated iron-phthalocyanine catalyst (Parton et al., 1996b), which indicates that the phthalocyanine complexes are entrapped into the supercages of the faujasite zeolite (ship-in-a-bottle complexes). The initial reaction rates obtained in the continuous reactor con"guration (short contact time) are substantially higher than the integrated reaction rates in a batch reactor con"guration. The previous observation cannot only be explained by the depletion of cyclohexane and t-BOOH reactants in the batch reactor, since even after the extra addition of 1 ml t-BOOH to the reaction mixture, low cyclohexane oxidation and organic peroxide decomposition reaction rates are obtained. Consequently, it can be concluded that a catalyst deactivation
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occurs rapidly under such batch reactor conditions, presumably a result of oxidation product pore blocking. In order to explain the in#uence of the solid support on the catalyst activity and e$ciency of supported phthalocyanine complexes, the multicomponent adsorption in NaY and activated carbon was studied using a liquid chromatographic method. Table 3 shows the partition coe$cient of several tracer components using cyclohexane as a carrier liquid. In both ultrastable Y zeolite (Si/Al"130) and activated carbon type BL, polar components (acetone solvent and t-BOOH oxidant) are preferentially adsorbed as compared to the nonpolar cyclohexane substrate. Subsequently, the adsorption properties of the studied support materials were determined in acetone solvent (Table 4). As a result of the hydrophilic nature, acetone is strongly adsorbed into the micropore system of the catalyst, resulting in low tracer component partition coe$cients. Consequently as a result of the competitive adsorption of the polar components (acetone, t-BOOH, oxidation products, etc.) in the reaction mixture, the non-polar cyclohexane substrate is almost completely excluded from the active sites of the catalyst (FePc complexes), resulting in low reaction rates and catalyst e$ciency. Acetone dominates over the other polar components as could be expected from the results in Table 3. Only the organic peroxide and water are somewhat more strongly adsorbed in USY.
Table 3 Adsorption in USY zeolite and activated carbon type BL (cyclohexane carrier) Tracer component
K (USY)
K (AC)
n-Hexane Acetone Cyclohexanol Cyclohexanone t-Butanol t-BOOH
1.3 14 3.6 2.2 * 7.5
1.7 9.4 8.8 * 9.4 17
Table 4 Adsorption in USY zeolite and activated carbon type BL (acetone carrier) Tracer component
K (USY)
K (AC)
n-Hexane Cyclohexane Cyclohexanol Cyclohexanone t-Butanol t-BOOH Water
1.1 0.7 0.8 0.8 * 1.2 1.7
1.0 0.9 1.2 1.0 0.7 0.9 1.2
Additionally, the adsorption of acetone in NaY zeolite with a Si/Al ratio ranging from 2.47 to 130 was investigated (Table 5). An acetone partition coe$cient as high as 90 was observed for the microporous NaY (Si/Al"2.47) zeolite, and the partition coe$cient in aluminium rich zeolite was found to be approximately a factor of six higher as compared to the dealuminated zeolite, explaining the marked enhancement in reaction rate for the ultrastable FePc-USY catalyst (Si/Al"130). The acetone solvent is more strongly adsorbed in the FePc-Y (Si/Al"2.47) catalyst, excluding the hydrocarbon substrate from the active site of the catalyst. The higher acetone adsorption in ultrastable Y (Si/Al"130) as compared to the carbon support (Table 3) also explains the higher reaction rate obtained for the activated carbon supported iron-phthalocyanine catalyst in acetone solvent. The high organic peroxide e$ciency observed by Parton et al. (1996a, b) in the fed batch reactor con"guration can be explained from the adsorption experiments. The low t-butylhydroperoxide concentration in the external liquid phase results in a low adsorbed t-BOOH concentration and consequently a more balanced cyclohexane and t-BOOH concentration near the active sites of the catalyst, resulting in higher cyclohexane oxidation and lower organic peroxide decomposition reaction rates. To demonstrate the strong in#uence of adsorption on the catalyst activity, the acetone partition coe$cient of the di!erent zeolite-encapsulated FePc catalysts was correlated with the initial reaction rates, obtained in the laboratory scale packed bed reactor con"guration (Fig. 2). When the acetone solvent is strongly adsorbed into the (low Si/Al ratio) zeolite, excluding the cyclohexane substrate from the active sites of the catalyst, low hydrocarbon oxidation reaction rates are observed. On the other hand, higher reaction rates were observed when the acetone solvent is less strongly adsorbed into the high silica zeolite catalyst. Finally, the catalytic stability of the zeolite-encapsulated iron-phthalocyanine catalyst was investigated using a continuous reactor (Fig. 3). Although no leaching of FePc complexes was observed for the FePc-USY catalyst, a substantial decrease in cyclohexane conversion occurred. Almost no catalytic activity remained after
Table 5 Acetone partition coe$cient as a function of the Si/Al ratio of microporous zeolite NaY Si/Al ratio
K
2.47 13 41 130
91 78 48 14
G. Langhendries et al. /Chemical Engineering Science 54 (1999) 3563}3568
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immobilisation of the transition metal complex at the external surface of the solid support can substantially increase the catalyst stability, since oxidation product pore blocking is no longer a major problem in such catalytic materials.
6. Conclusion
Fig. 2. Adsorption-reaction correlation for the di!erent zeolite-encapsulated FePc catalysts.
Conclusively, it can be stated that a clear relationship between the polarity of the solid support material and the reaction rate of the iron-phthalocyanine supported catalyst was established. The reactant concentrations near the active sites of the catalyst, and consequently the reaction rates, are essentially controlled by the adsorption equilibrium of the solid support material. The rapid deactivation of the zeolite-encapsulated iron-phthalocyanine catalyst in continuous operation, presumably a result of oxidation product pore blocking, was demonstrated to be in good agreement with the adsorption data. Adsorption experiments were shown to be an interesting tool in the design of novel supported transition metal complex catalysts.
Acknowledgements
Fig. 3. Alcohol and ketone production rate as a function of time in a continuous reactor. (0.15 g catalyst, 0.05 ml/min, feed mixture consists of 100 ml acetone, 20 ml cyclohexane, 20 ml t-BOOH and 1 ml internal standard).
The authors acknowledge sponsoring from the Belgian Federal Government in the frame of a IUAP-PAI project on Supramolecular Chemistry and Catalysis and the Flemish Fund for Scienti"c Research (FWO KV.E2031996). G.L. and P.E.N. acknowledge &het Vlaams Instituut voor de bevordering van het wetenschappelijktechnologisch onderzoek in de industrie, IWT' for a grant as doctoral research fellow.
References
150 h on stream, due to the accumulation of polar oxidation products in the pore structure of the hydrophilic zeolite. The rapid catalyst deactivation, as a result of the oxidation product pore blocking or the leaching of ironphthalocyanine complexes from the solid support is the major drawback of these supported transition metal complex catalysts. Parton et al. (1994) immobilised nitro-substituted iron-phthalocyanine complexes at the external surface of zeolite NaY (FeTNPc-Y Si/Al"2.47), and this FeTNPc-Y catalyst material turned out to be superior as compared to the classic FePc-Y. In a batch reactor con"guration, the nitro-substituted catalyst is approximately ten times more active and no deactivation was observed even after 30 h of reaction. Therefore, the
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