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Novel clay intercalated metal catalysts: a study of the hydrogenation of styrene and 1-octene on clay intercalated Pd catalysts
Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All rights reserved.
477
Novel clay intercalated metal catalysts: a study of the hydrogenation of styrene and 1-octene on clay intercalated Pd catalysts A. Mastalir ^ F. Notheisz ^ Z. Kiraly \ M. Bartok ^ and L Dekany ^ ''Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, JozsefAttila University, Dom ter 8, H-6720 Szeged, Hungary ^Department of Colloid Chemistry, JozsefAttila University, Aradi vertanuk tere 1, H-6720 Szeged, Hungary
Pd-hexadecylammonium montmorillonite (Pd-HDAM) catalysts have been prepared by a novel synthetic route. Sample characterization including XRD and TEM measurements confirmed the existence of interlayer Pd nanocrystallites which occupy clay particle defect sites. The catalytic activities of Pd-HDAM samples were tested by hydrogenation of 1-octene and styrene in the liquid phase. The reaction of styrene was found to be less dependent on the dispersion of Pd than that of 1-octene. The highest activities were observed for samples of low and medium Pd content. The application of various solvents made it possible to estabhsh a correlation between the activities and the basal spacings dL of Pd-HDAM samples. When the value of dL exceeded 3 nm, interlamellar active sites became more accessible for reactants.
1. INTRODUCTION The research on the catalytic applications of clay minerals has been the subject of considerable interest for some time [1-2]. On account of their lamellar structure, clays can be excellent host materials for various metals, and thus the precursors of clay intercalated metal catalysts [3]. This paper reports on the catalytic behaviour of novel Pd intercalated montmorillonites. Montmorillonite, a 2:1 type layered silicate belongs to the group of smectite clays. Each layer of montmorillonite is composed of an octahedral A106 layer sandwiched between two tetrahedral Si04 layers. The basal spacing dL can be obtained from XRD measurements and may vary between 1-3 nm, depending on the amount of water molecules in the interlamellar space [1]. Replacement of Al^^ by Mg^^ and Si"^^ by Al^^ resuks in a net negative charge smeared over the layers and balanced by interlayer hydrated cations like Na^, K^, Ca^^ and Mg^^. Such charge balancing cations may be forther substituted by others, making montmorillonite a cation exchanger [4]. When organic cations are employed for replacement (e. g. cationic tensides), clay organo-complexes are formed which, unlike hydropMlic clays, may readily be dispersed in organic media applied in liquid phase hydrogenation reactions.
478
The catalytic application of montmorillonites may either be related to their acidity [5-7], or their swelling properties [8-9]. Swelling, induced by careful selection of both the reaction medium and interlayer cations, enables the reactant molecules to enter the interlamellar region and undergo catalytic reaction on interstitial active sites. Moreover, the layered structure of montmorillonite may induce a substrate selectivity depending on the reactant size. Therefore, clay intercalated metal catalysts offer potential applications in the preparation of fine chemicals.
2. EXPERIMENTAL The synthesis of Pd-hexadecylammonium montmorillonite (HDAM) samples was performed by the in situ reduction of Pd(0Ac)2 in the interlamellar space, with a bulk composition of ethanol: toluene = 5 : 95. The method of preparation was designed according to preliminary liquid sorption and XRD measurements. At small ethanol concentrations in toluene, the selective interlamellar sorption of ethanol was established, which ensured the diffiision of Pd^^ ions from the bulk phase to the interlamellar space, where reduction to Pd^ took place. Samples of 2.5, 4.2, 6.5 and 10.2% Pd loadings were synthesized and characterized by N2 adsorption, TEM, XRD and SAXS measurements. A detailed description of the preparation method and instrumental analysis has been published recently [10]. The catalytic hydrogenations of 1-octene and styrene were effected in a conventional hydrogenation apparatus with a molar ratio of Pd : substrate = 1 : 500 at 298 K and atmospheric pressure. In each case, 5 x lO'^ g sample was pretreated in H2 at 298 K for 1 h and subsequently in 1 cm^ of solvent, under stirring, for another 45 min. After introduction of the reactant, a vigorous stirring was applied (1240 rpm), and the reaction was conducted until the theoretical volume of H2 has been consumed. After removing the catalyst by filtration, the products were identified and analyzed by GC.
3. RESULTS AND DISCUSSION The structure investigation of Pd-HDAM samples revealed that the orientation of the clay lamellae was substantially modified via formation of internal Pd particles. However, the occurrence of regular intercalation was found to be unlikely [10]. Therefore, we propose that the particles occupy defect sites of montmorillonite, similarly to a model introduced by Gianneliset al [11]. The hydrogenation of 1-octene was studied in toluene. According to XRD measurements, toluene provides a considerable expansion of the clay lamellae (dL = 4.07 nm), which opens access for reactant molecules to interlayer active sites. As a resuh, the reaction of 1-octene may take place in the interlamellar space of Pd-HDAM samples. The decrease of the dispersion values indicated by Table 1 is attributed to the effect of Pd aggregation. However, the amount of exposed metal atoms displayed no particular change. In contrast to supported Pd catalysts, usually considered as structure insensitive in alkene hydrogenations [13], the initial rates and the turnover fi-equencies obtained for Pd-HDAM samples considerably vary with the dispersion values. This suggests that in the hydrogenation of 1-octene, Pd-HDAM catalysts act in a different way. The difference observed may be
479 related to the structure of Pd-organoclay samples. For the samples of higher Pd loadings, the particle size increases considerably. Table 1 Characteristic data of Pd-HDAM samples of different metal content and initial rates and turnover frequencies obtained for the hydrogenation of 1-octene on Pd-HDAM catalysts sample [%Pd]
d"" [nm]
Pd^/Pd ^
Pds' [%]
2.5 4.2 6.5
3.5 6.3
0.25 0.14 0.085 0.069
0.62 0.59 0.55
10.2
10.4 12.8
0.7
initial rates [cm'xmin'^xg'^]
560 160 100 60
TOP [s'^] 6.41 1.97 1.31 0.61
a: mean particle diameters of Pd crystallites b: dispersion values of Pd-HDAM catalysts, calculated from TEM data (P4/Pd = 0.885/d [12]) c: exposed metal contents of Pd-HDAMs.
Large interlayer particles are likely to block the interlamellar region, which results in a limited access of reactant molecules to internal active sites. Such a hindering effect may be accounted for the lower activities obtained for the samples of 6.5 and 10.2% Pd content. As hydrogenation progresses in time, decreasing reaction rates can be observed for all samples (Figure 1). The most pronounced effect is obtained for the sample of 2.5 % Pd content. In order to interpret such behaviour, a comparative experiment was carried out on 3% Pd/Cab-0-Sil, a conventional supported catalyst. Under the same experimental conditions, the reaction rate did not diminish with reaction time, and the values of the initial rate and the turnover frequency were 40 cm^ x min"^ x g"^ and 0.509, respectively. Considering the above resuhs, the specific structural properties of HDAM are accounted for the loss of reaction rates experienced for Pd-montmorillonites. In this respect, diffusion control may have an influence on the progress of hydrogenation. The interlamellar transformation of 1-octene involves the chemisorption and hydrogenation of reactant molecules on interlayer Pd atoms as active sites. Reactants can be supplied via diffiision from the bulk phase if the product molecules leave the interlamellar space. However, octane molecules formed tend to interact with the hydrophobic alkyl chains of HDAM, which results in the displacement of the solvent and prevents the entrance of further reactant molecules. If so, transport phenomena become predominant and the reaction slows down. Meanwhile, the accumulation of octane in the interlamellar space decreases the basal spacing (dL = 1.78 nm in pure octane where no swelling occurs at all), and thereby makes the internal Pd sites less accessible. In the reaction of styrene hydrogenation in toluene, interlayer solvent displacement and clay shrinkage do not occur, as the product ethylbenzene is very similar in character to the solvent molecules. Data collected in Table 2 indicate that all Pd-montmorillonite samples were active catalysts in styrene hydrogenation. This is not surprising, since both the surface and the internal alkyl
480
Figure 1. Conversion of 1-octene as a function of reaction time, on Pd-HDAM samples of different Pd content.
100
Figure 2. Conversion of styrene as a function of reaction time, on Pd-HDAM samples of different metal content.
481 chains of HDAM are suitably solvated by the liquid components of the reactant mixture. Of all samples, the highest activities were measured for those of medium Pd content (4.2 and 6.5%). The activities of Pd-HDAM catalysts seem to be less dependent on the metal dispersion than for the reaction of 1-octene. Table 2 Initial rates and turnover frequencies obtained for the hydrogenation of styrene on Pd-HDAM catalysts of different metal content TOF initial rates sample [cm^ X min"^ x g"^] [%Pd] [s-^] 100 1.16 2.5 240 2.96 4.2 200 6.5 2.63 100 10.2 1.03 Figure 2 demonstrates that the reaction rates did not decrease with time to any appreciable extent. For the samples of low Pd content, hydrogenation was completed in a remarkably short time. The Pd content of the samples of higher loadings formed aggregates which, as in the case of 1-octene, tend to block the interlamellar space and restrict conversion to the surface active sites. This particularly holds for 10.2% Pd-HDAM, which was the least effective sample in the reaction. Similarly to conventional Pd supported catalysts, styrene hydrogenation conducted on Pd-HDAM samples was found to be a moderately structure sensitive reaction. The selection of solvent is an important factor for most liquid phase reactions, those including swelling type catalysts in particular. For Pd-HDAM samples, solvent variation directly affects catalytic activity through swelling of the clay host. In order to establish a correlation between the catalytic activities and the basal spacings, we performed the hydrogenation of styrene in various solvents. The basal spacings of the solvent-HDAM systems were determined from XRD patterns (Figure 3). Table 3 Data obtained for styrene hydrogenation on 4.2 % PdHDAM, in different solvents, basal spacings determined from XRD measurements TOF solvent time"" dL [nm] [min] [s-^] 10.5 toluene 4.05 2.96 9 tetrahydrofurane 3.72 1.97 3.12 2.96 16 ethanol 38 1.76 1.23 cyclohexane a: reaction time at 50% conversion For its high activity, the sample selected for comparative investigations was 4.2% PdHDAM. Experimental data are listed in Table 3. Although the values of the basal spacings dL vary to some extent, it is noticeable that the application of toluene, ethanol or
482
1200
1200
toluene, 4.05 nm
tetrahydrofurane, 3.72 nm
800
cyclohexane, 1.76 nm ethanol, 3.12 nm
C 9i
400
0
3
6
9
0
3
6
9
Figure 3. X-ray diffractograms of HDAM organocomplexes in suspensions of organic solvents.
/^xl"" poor solvent
good solvent
\
V \
nt
Id
Figure 4. Schematic illustration of the aggregation and swelling of HDAM in poor and in good solvents.
483 tetrahydrofurane had a similar effect on the catalytic activity and the reaction progress, as the TOF values obtained were relatively high and hydrogenation was completed in a fairly short time. On the other hand, when the reaction was conducted in cyclohexane, the catalytic activity decreased and the reaction time increased considerably. The experimental evidence can be interpreted on the basis of Figure 4. In a poor solvent (cyclohexane), the clay lamella packages form aggregates due to adhesion of the secondary particles (a card house structure is formed). Furthermore, the value of di is very close to the thickness of the silicate layer plus the cross sections of two alkyl chains. It follows that in a poor solvent, the alkyl chains are laying flat on the surface and no swelling takes place. In that case, the interstitial sites are not available for reactants and reaction proceeds on the external surface sites. In a good solvent (toluene, ethanol, or tetrahydrofurane), the clay particles undergo desaggregation and swelling. The distance between the clay lamellae increases and the alkyl chains are lifted away from the basal planes. As a result, the reactants can penetrate the interlamellar space and interlayer reaction may occur [14]. Despite aggregation and compactness of clay lamellae in cyclohexane, the interlamellar space is not completely inaccessible, as styrene, even at small concentrations, may induce swelling. However, the hydrogenation rate may be limited by transport phenomena and diffision control may develop. It is established that dL > 3 nm represents a sufficiently large distance between the clay lamellae for the reactants to enter the interlayer space and undergo hydrogenation there. Nevertheless, the reaction rate may also be influenced by solvation effects, i. e. the adsorption equilibrium of the liquid mixture (reactant, product and reaction medium) at the solid-liquid interface, which determines the concentration of each component in both the adsorption layer and the bulk phase.
4. CONCLUSIONS Novel type Pd-montmorillonite catalysts were prepared by controlled colloid synthesis. The samples exhibited a remarkable catalytic activity in the hydrogenations of 1-octene and styrene. The catalytic performance was found to be closely related to the particle size of Pd and the swelling properties of the clay host. In the solvents toluene, ethanol or tetrahydrofurane, active sites composed of interlamellar Pd particles were involved in hydrogenation. Since a considerable shape selectivity may also be achieved through swelling, Pd-HDAM samples may be regarded as promising consumer designed catalysts.
ACKNOWLEDGEMENT Financial support through Grant OTKA T016109 and T007530 is gratefully acknowledged.
484 REFERENCES 1. 2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
P. Laszlo, Science, 235 (1987) 1473. M. Balogh and P. Laszlo, "Organic Reactions using Clays", K. Hafner, C. W. Rees, B. M.Trost, J-M. Lehn, P. vonRague Schleyer, R. Zahradnik(Eds), Springer, Berlin, 1993. T. J. Pinnavaia, Science, 220 (1983) 365. P. Ravindranathan, P. B. Mall, S. Komameni and R. Roy, Catal. Lett., 6 (1990) 401. D. Fishman, J. T. Klug and A. Shani, Synthesis, 1981 (1981) 137. P. G. Gassman and D. A. Singleton, J. Am. Chem. Soc, 106 (1984) 7993. P. Laszlo and L. Lucchetti, Tetrahedron Lett., 25 (1984) 4387. R. Raythatha and T. J. Pinnavaia, J. Organomet. Chem., 218 (1981) 115. M. Choudary and P. Bharathi, J. Chem. Soc. Chem. Comm., (1987) 1505. Z. Kiraly, L Dekany, A. Mastalir and M. Bartok, J. Catal., 161 (1996) 401. E. P. Giannelis, E. G. Rightor and T. J. Pinnavaia, J. Am. Chem. Soc, 110 (1988) 3880. P. C. Aben, J. Catal., 10 (1968) 224. R. Burch, "Catalysis", G. C. Bond, G Webb (Eds), The Royal Society of Chemistry, London, 1985. I. Dekany, Pure and Appl. Chem., 64 (1992) 1499.
477
Novel clay intercalated metal catalysts: a study of the hydrogenation of styrene and 1-octene on clay intercalated Pd catalysts A. Mastalir ^ F. Notheisz ^ Z. Kiraly \ M. Bartok ^ and L Dekany ^ ''Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, JozsefAttila University, Dom ter 8, H-6720 Szeged, Hungary ^Department of Colloid Chemistry, JozsefAttila University, Aradi vertanuk tere 1, H-6720 Szeged, Hungary
Pd-hexadecylammonium montmorillonite (Pd-HDAM) catalysts have been prepared by a novel synthetic route. Sample characterization including XRD and TEM measurements confirmed the existence of interlayer Pd nanocrystallites which occupy clay particle defect sites. The catalytic activities of Pd-HDAM samples were tested by hydrogenation of 1-octene and styrene in the liquid phase. The reaction of styrene was found to be less dependent on the dispersion of Pd than that of 1-octene. The highest activities were observed for samples of low and medium Pd content. The application of various solvents made it possible to estabhsh a correlation between the activities and the basal spacings dL of Pd-HDAM samples. When the value of dL exceeded 3 nm, interlamellar active sites became more accessible for reactants.
1. INTRODUCTION The research on the catalytic applications of clay minerals has been the subject of considerable interest for some time [1-2]. On account of their lamellar structure, clays can be excellent host materials for various metals, and thus the precursors of clay intercalated metal catalysts [3]. This paper reports on the catalytic behaviour of novel Pd intercalated montmorillonites. Montmorillonite, a 2:1 type layered silicate belongs to the group of smectite clays. Each layer of montmorillonite is composed of an octahedral A106 layer sandwiched between two tetrahedral Si04 layers. The basal spacing dL can be obtained from XRD measurements and may vary between 1-3 nm, depending on the amount of water molecules in the interlamellar space [1]. Replacement of Al^^ by Mg^^ and Si"^^ by Al^^ resuks in a net negative charge smeared over the layers and balanced by interlayer hydrated cations like Na^, K^, Ca^^ and Mg^^. Such charge balancing cations may be forther substituted by others, making montmorillonite a cation exchanger [4]. When organic cations are employed for replacement (e. g. cationic tensides), clay organo-complexes are formed which, unlike hydropMlic clays, may readily be dispersed in organic media applied in liquid phase hydrogenation reactions.
478
The catalytic application of montmorillonites may either be related to their acidity [5-7], or their swelling properties [8-9]. Swelling, induced by careful selection of both the reaction medium and interlayer cations, enables the reactant molecules to enter the interlamellar region and undergo catalytic reaction on interstitial active sites. Moreover, the layered structure of montmorillonite may induce a substrate selectivity depending on the reactant size. Therefore, clay intercalated metal catalysts offer potential applications in the preparation of fine chemicals.
2. EXPERIMENTAL The synthesis of Pd-hexadecylammonium montmorillonite (HDAM) samples was performed by the in situ reduction of Pd(0Ac)2 in the interlamellar space, with a bulk composition of ethanol: toluene = 5 : 95. The method of preparation was designed according to preliminary liquid sorption and XRD measurements. At small ethanol concentrations in toluene, the selective interlamellar sorption of ethanol was established, which ensured the diffiision of Pd^^ ions from the bulk phase to the interlamellar space, where reduction to Pd^ took place. Samples of 2.5, 4.2, 6.5 and 10.2% Pd loadings were synthesized and characterized by N2 adsorption, TEM, XRD and SAXS measurements. A detailed description of the preparation method and instrumental analysis has been published recently [10]. The catalytic hydrogenations of 1-octene and styrene were effected in a conventional hydrogenation apparatus with a molar ratio of Pd : substrate = 1 : 500 at 298 K and atmospheric pressure. In each case, 5 x lO'^ g sample was pretreated in H2 at 298 K for 1 h and subsequently in 1 cm^ of solvent, under stirring, for another 45 min. After introduction of the reactant, a vigorous stirring was applied (1240 rpm), and the reaction was conducted until the theoretical volume of H2 has been consumed. After removing the catalyst by filtration, the products were identified and analyzed by GC.
3. RESULTS AND DISCUSSION The structure investigation of Pd-HDAM samples revealed that the orientation of the clay lamellae was substantially modified via formation of internal Pd particles. However, the occurrence of regular intercalation was found to be unlikely [10]. Therefore, we propose that the particles occupy defect sites of montmorillonite, similarly to a model introduced by Gianneliset al [11]. The hydrogenation of 1-octene was studied in toluene. According to XRD measurements, toluene provides a considerable expansion of the clay lamellae (dL = 4.07 nm), which opens access for reactant molecules to interlayer active sites. As a resuh, the reaction of 1-octene may take place in the interlamellar space of Pd-HDAM samples. The decrease of the dispersion values indicated by Table 1 is attributed to the effect of Pd aggregation. However, the amount of exposed metal atoms displayed no particular change. In contrast to supported Pd catalysts, usually considered as structure insensitive in alkene hydrogenations [13], the initial rates and the turnover fi-equencies obtained for Pd-HDAM samples considerably vary with the dispersion values. This suggests that in the hydrogenation of 1-octene, Pd-HDAM catalysts act in a different way. The difference observed may be
479 related to the structure of Pd-organoclay samples. For the samples of higher Pd loadings, the particle size increases considerably. Table 1 Characteristic data of Pd-HDAM samples of different metal content and initial rates and turnover frequencies obtained for the hydrogenation of 1-octene on Pd-HDAM catalysts sample [%Pd]
d"" [nm]
Pd^/Pd ^
Pds' [%]
2.5 4.2 6.5
3.5 6.3
0.25 0.14 0.085 0.069
0.62 0.59 0.55
10.2
10.4 12.8
0.7
initial rates [cm'xmin'^xg'^]
560 160 100 60
TOP [s'^] 6.41 1.97 1.31 0.61
a: mean particle diameters of Pd crystallites b: dispersion values of Pd-HDAM catalysts, calculated from TEM data (P4/Pd = 0.885/d [12]) c: exposed metal contents of Pd-HDAMs.
Large interlayer particles are likely to block the interlamellar region, which results in a limited access of reactant molecules to internal active sites. Such a hindering effect may be accounted for the lower activities obtained for the samples of 6.5 and 10.2% Pd content. As hydrogenation progresses in time, decreasing reaction rates can be observed for all samples (Figure 1). The most pronounced effect is obtained for the sample of 2.5 % Pd content. In order to interpret such behaviour, a comparative experiment was carried out on 3% Pd/Cab-0-Sil, a conventional supported catalyst. Under the same experimental conditions, the reaction rate did not diminish with reaction time, and the values of the initial rate and the turnover frequency were 40 cm^ x min"^ x g"^ and 0.509, respectively. Considering the above resuhs, the specific structural properties of HDAM are accounted for the loss of reaction rates experienced for Pd-montmorillonites. In this respect, diffusion control may have an influence on the progress of hydrogenation. The interlamellar transformation of 1-octene involves the chemisorption and hydrogenation of reactant molecules on interlayer Pd atoms as active sites. Reactants can be supplied via diffiision from the bulk phase if the product molecules leave the interlamellar space. However, octane molecules formed tend to interact with the hydrophobic alkyl chains of HDAM, which results in the displacement of the solvent and prevents the entrance of further reactant molecules. If so, transport phenomena become predominant and the reaction slows down. Meanwhile, the accumulation of octane in the interlamellar space decreases the basal spacing (dL = 1.78 nm in pure octane where no swelling occurs at all), and thereby makes the internal Pd sites less accessible. In the reaction of styrene hydrogenation in toluene, interlayer solvent displacement and clay shrinkage do not occur, as the product ethylbenzene is very similar in character to the solvent molecules. Data collected in Table 2 indicate that all Pd-montmorillonite samples were active catalysts in styrene hydrogenation. This is not surprising, since both the surface and the internal alkyl
480
Figure 1. Conversion of 1-octene as a function of reaction time, on Pd-HDAM samples of different Pd content.
100
Figure 2. Conversion of styrene as a function of reaction time, on Pd-HDAM samples of different metal content.
481 chains of HDAM are suitably solvated by the liquid components of the reactant mixture. Of all samples, the highest activities were measured for those of medium Pd content (4.2 and 6.5%). The activities of Pd-HDAM catalysts seem to be less dependent on the metal dispersion than for the reaction of 1-octene. Table 2 Initial rates and turnover frequencies obtained for the hydrogenation of styrene on Pd-HDAM catalysts of different metal content TOF initial rates sample [cm^ X min"^ x g"^] [%Pd] [s-^] 100 1.16 2.5 240 2.96 4.2 200 6.5 2.63 100 10.2 1.03 Figure 2 demonstrates that the reaction rates did not decrease with time to any appreciable extent. For the samples of low Pd content, hydrogenation was completed in a remarkably short time. The Pd content of the samples of higher loadings formed aggregates which, as in the case of 1-octene, tend to block the interlamellar space and restrict conversion to the surface active sites. This particularly holds for 10.2% Pd-HDAM, which was the least effective sample in the reaction. Similarly to conventional Pd supported catalysts, styrene hydrogenation conducted on Pd-HDAM samples was found to be a moderately structure sensitive reaction. The selection of solvent is an important factor for most liquid phase reactions, those including swelling type catalysts in particular. For Pd-HDAM samples, solvent variation directly affects catalytic activity through swelling of the clay host. In order to establish a correlation between the catalytic activities and the basal spacings, we performed the hydrogenation of styrene in various solvents. The basal spacings of the solvent-HDAM systems were determined from XRD patterns (Figure 3). Table 3 Data obtained for styrene hydrogenation on 4.2 % PdHDAM, in different solvents, basal spacings determined from XRD measurements TOF solvent time"" dL [nm] [min] [s-^] 10.5 toluene 4.05 2.96 9 tetrahydrofurane 3.72 1.97 3.12 2.96 16 ethanol 38 1.76 1.23 cyclohexane a: reaction time at 50% conversion For its high activity, the sample selected for comparative investigations was 4.2% PdHDAM. Experimental data are listed in Table 3. Although the values of the basal spacings dL vary to some extent, it is noticeable that the application of toluene, ethanol or
482
1200
1200
toluene, 4.05 nm
tetrahydrofurane, 3.72 nm
800
cyclohexane, 1.76 nm ethanol, 3.12 nm
C 9i
400
0
3
6
9
0
3
6
9
Figure 3. X-ray diffractograms of HDAM organocomplexes in suspensions of organic solvents.
/^xl"" poor solvent
good solvent
\
V \
nt
Id
Figure 4. Schematic illustration of the aggregation and swelling of HDAM in poor and in good solvents.
483 tetrahydrofurane had a similar effect on the catalytic activity and the reaction progress, as the TOF values obtained were relatively high and hydrogenation was completed in a fairly short time. On the other hand, when the reaction was conducted in cyclohexane, the catalytic activity decreased and the reaction time increased considerably. The experimental evidence can be interpreted on the basis of Figure 4. In a poor solvent (cyclohexane), the clay lamella packages form aggregates due to adhesion of the secondary particles (a card house structure is formed). Furthermore, the value of di is very close to the thickness of the silicate layer plus the cross sections of two alkyl chains. It follows that in a poor solvent, the alkyl chains are laying flat on the surface and no swelling takes place. In that case, the interstitial sites are not available for reactants and reaction proceeds on the external surface sites. In a good solvent (toluene, ethanol, or tetrahydrofurane), the clay particles undergo desaggregation and swelling. The distance between the clay lamellae increases and the alkyl chains are lifted away from the basal planes. As a result, the reactants can penetrate the interlamellar space and interlayer reaction may occur [14]. Despite aggregation and compactness of clay lamellae in cyclohexane, the interlamellar space is not completely inaccessible, as styrene, even at small concentrations, may induce swelling. However, the hydrogenation rate may be limited by transport phenomena and diffision control may develop. It is established that dL > 3 nm represents a sufficiently large distance between the clay lamellae for the reactants to enter the interlayer space and undergo hydrogenation there. Nevertheless, the reaction rate may also be influenced by solvation effects, i. e. the adsorption equilibrium of the liquid mixture (reactant, product and reaction medium) at the solid-liquid interface, which determines the concentration of each component in both the adsorption layer and the bulk phase.
4. CONCLUSIONS Novel type Pd-montmorillonite catalysts were prepared by controlled colloid synthesis. The samples exhibited a remarkable catalytic activity in the hydrogenations of 1-octene and styrene. The catalytic performance was found to be closely related to the particle size of Pd and the swelling properties of the clay host. In the solvents toluene, ethanol or tetrahydrofurane, active sites composed of interlamellar Pd particles were involved in hydrogenation. Since a considerable shape selectivity may also be achieved through swelling, Pd-HDAM samples may be regarded as promising consumer designed catalysts.
ACKNOWLEDGEMENT Financial support through Grant OTKA T016109 and T007530 is gratefully acknowledged.
484 REFERENCES 1. 2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
P. Laszlo, Science, 235 (1987) 1473. M. Balogh and P. Laszlo, "Organic Reactions using Clays", K. Hafner, C. W. Rees, B. M.Trost, J-M. Lehn, P. vonRague Schleyer, R. Zahradnik(Eds), Springer, Berlin, 1993. T. J. Pinnavaia, Science, 220 (1983) 365. P. Ravindranathan, P. B. Mall, S. Komameni and R. Roy, Catal. Lett., 6 (1990) 401. D. Fishman, J. T. Klug and A. Shani, Synthesis, 1981 (1981) 137. P. G. Gassman and D. A. Singleton, J. Am. Chem. Soc, 106 (1984) 7993. P. Laszlo and L. Lucchetti, Tetrahedron Lett., 25 (1984) 4387. R. Raythatha and T. J. Pinnavaia, J. Organomet. Chem., 218 (1981) 115. M. Choudary and P. Bharathi, J. Chem. Soc. Chem. Comm., (1987) 1505. Z. Kiraly, L Dekany, A. Mastalir and M. Bartok, J. Catal., 161 (1996) 401. E. P. Giannelis, E. G. Rightor and T. J. Pinnavaia, J. Am. Chem. Soc, 110 (1988) 3880. P. C. Aben, J. Catal., 10 (1968) 224. R. Burch, "Catalysis", G. C. Bond, G Webb (Eds), The Royal Society of Chemistry, London, 1985. I. Dekany, Pure and Appl. Chem., 64 (1992) 1499.