Preparation and catalytic properties of Pd-, Rh- and Ru-salen complexes in faujasite-type zeolites

H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.)

Catalysis by Microporous Materials Studies in Surface Science and Catalysis, Vol. 94 9 1995 Elsevier Science B.V. All rights reserved.

479

P R E P A R A T I O N AND C A T A L Y T I C P R O P E R T I E S OF Pd-, Rh- AND R u - S A L E N C O M P L E X E S IN F A U J A S I T E - T Y P E Z E O L I T E S S. Ernst and O. Batr6au

Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany

ABSTRACT Palladium, rhodium and ruthenium complexes of the Schiff base salen are synthesized in the supercages of zeolite Y. The existence of intracrystalline transition metal-salen complexes is verified by a detailed physicochemical characterization. The catalytic properties of the prepared host/guest inclusion compounds are explored in the hydrogenation of hexene-(1) or an equimolar mixture of hexene-(1) and 2,4,4trimethylpentene-(1). 1. INTRODUCTION Zeolite-encapsulated transition metal complexes of the Schiff base salen (N,N'-bis(salicylidene)-ethylenediamine) are currently under intense study with respect to their properties as enzyme mimics [1-4], as heterogeneous catalysts [5-9] and in electrochemistry [10-12]. These host/guest inclusion compounds can be prepared via the so-called flexible-ligand method [2]. Its principle is that the free salen, i. e., the complete free ligand system, is flexible enough to enter the faujasite pore system through the 12-membered ring window giving access to the large supercages. The ligand reacts with transition metal ions already exchanged into the zeolite and forms a chelate with square planar configuration. Once formed, this complex is too bulky to escape from the intracrystalline pore system of the zeolite, hence it is entrapped in the supercage. The preparation of salen complexes of various transition metal cations in X- or Y-type zeolites [1-13] and of cobalt in zeolite EMT [3,4] has been reported in the literature. The resulting materials have been shown to act as oxygen carriers [1-4], as oxidation catalysts [8] or as catalysts for selective hydrogenation reactions [6,7,9]. Here we report on the preparation of palladium-, rhodium- and ruthenium-salen complexes in faujasite-type zeolites. The resulting host/guest-compounds are characterized by selected physicochemical methods. Their catalytic properties are explored in the hydrogenation of hexene-(1) or in the competitive hydrogenation of an equimolar mixture of hexene-(1) and 2,4,4-trimethylpentene-(1) as catalytic tests.

480 2. EXPERIMENTAL SECTION Palladium, rhodium and ruthenium were introduced into zeolite NaY (nsi/nA1 = 2.4) by ion exchange at 80 ~ with dilute aqueous solutions of [Pd(NH3)4]C12, RhC13 and RuC13, respectively, to achieve a final metal loading of two per unit cell (viz., one metal per 4 supercages). In addition, the metal loading was varied to one and four metals per unit cell for the case of ruthenium. The ion exchanged zeolites were first air dried at 100 ~ and then calcined in oxygen for 16 hours at 400 ~ The transition metal containing zeolites were then mixed in a glove box with the Schiff base salen (nsalen/ntransition metal = 2), transferred to a glass ampoule, evacuated, sealed and then heated for 20 hours at 140 ~ The samples prepared in this manner were soxhletextracted with acetone as solvent to remove uncomplexed salen or metal-salen complexes deposited at the external surface of the zeolite crystallites and afterwards subjected to an ion exchange with Na + cations to remove residual (i. e., uncomplexed) transition metal ions and acid sites formed during complexation. For comparative purposes, the free paUadium-salen complex was prepared as described previously [14]. Physicochemical characterization of the host/guest inclusion compounds was done by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), chemical analysis, and IR- and UV/VIS-spectroscopy. The thermal stability of the encapsulated complexes was studied using simultaneous thermogravimetric analysis and differential thermal analysis (TGA/DTA). For this purpose, the modified zeolites were heated in a purge of dry air ('(/air = 1.6 l/h) from room temperature to 900 ~ with a rate of 5K/min. The catalytic properties of the encaged complexes were explored in the gas phase hydrogenation of hexene-(1) or of an equimolar mixture of hexene-(1) and 2,4,4-trimethylpentene-(1), as described recently [15]. Typical reaction conditions were: atmospheric pressure; flow-type apparatus with fixed bed reactor from glass; reaction temperatures from 70 ~ to 140 ~ modified residence t i m e s (W/Falkene(s)) from 5 to 50 g.h/mol; mass of dry catalyst W = 0.2 g. Hydrogen was used as carrier gas and the partial pressure of the alkenes amounted to 7 kPa. Product analysis was achieved by on-line sampling and capillary gas chromatography.

3. RESULTS AND DISCUSSION Successful complexation of the exchanged transition metals by the tetradentate ligand salen was indicated by the typical yellow/brownish colour of the prepared inclusion compounds. X-ray powder diffraction and scanning electron microscopy reveal that the crystallinity of the zeolite host is virtually retained during the modification steps. Furthermore, no crystals of the salen ligand or of metal-salen complexes are visible on the external surface of the zeolite crystallites after careful purification via soxhlet extraction with acetone. Analysis of the used solvent by UV/VIS-spectroscopy revealed that only salen but no metal-salen complexes were removed from the zeolite during extraction. This indicates that the complexes are truly entrapped in the intracrystalline voids of the zeolite. It has been demonstrated [7] that the presence of salen and PdSalen in zeolite Y can be probed by m-spectroscopy in

481 the frequency range from 1400 cm-1 to 1600 cm-1. Pertinent results obtained with the PdSalenY samples prepared in the present study are compared in Table 1 with the IR data for the free salen and for PdSalen, respectively. It can be dearly seen that, after the complexation step, both salen and PdSalen are present in or on the zeolite. After soxhlet extraction with acetone, however, only those IR-bands which are typical for PdSalen remain. This indicates that the uncomplexed ligand can be practically completely removed by extraction with this solvent. As a whole, these results are in principal agreement with the earlier observations of De Vos and Jacobs [7] and they indicate that intrazeolitic PdSalen complexes were indeed formed. In priciple, similar results were also obtained for the materials containing encapsulated RhSalen and RuSalen complexes. Table 1 Positions of IR-bands of the free salen and PdSalen complexes and of PdSalenY before and after soxhlet extraction with acetone. compound salen PdSalen PdSalenY (before extraction) PdSalenY (after extraction)

frequency, cm-1 1578

-

1497

1455

-

1530

-

1450

1578

1532

1497

1452

-

1538

-

1450

Typical results of the characterization by UV/VIS-spectroscopy in the diffuse reflectance mode are depicted in Figure 1. Pronounced differences are observed between the ion exchanged and calcined samples (RhNaY and RuNaY, respectively) and the materials obtained after complexation and soxhlet extraction (RhSalenY and RuSalenY, respectively). From previous studies it is known that both PdSalen [7] and RhSalen [13 ] encapsulated in zeolite Y exhibit an absorption band around 400 nm. A similar band is observed in the present study for RhSalenY and RuSalenY. An additional band was reported by Balkus et al. [13] for some preparations of RhSalenY and attributed to Rh d-d transitions, i. e., the presence of uncomplexed rhodium. This band is absent in our sample of RhSalenY which suggests that rhodium was completely complexed under the experimental conditions applied in the present study. No unambiguous assignment of the other absorption bands appearing in the shown spectra can be presented at the present stage of this investigation.

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483 For catalytic applications it is important that the prepared materials possess a sufficiently high thermal stability, especially in an oxidizing environment, in order to avoid decomposition and/or combustion of the encapsulated complexes. Therefore the zeolite-encaged noble metal-salen complexes were characterized by thermogravimetric analysis combined with simultaneous differential thermal analysis. Pertinent results obtained for RhSalenY and RuSalenY, both containing approximately two metal-salen complexes per unit cell of the zeolite, are depicted in Figure 2. For RhSalenY, a considerable weight loss is observed upon heating from room temperature to ca. 200 ~ in air. Concomitantly, an endothermic effect can be detected in the same temperature range. This weight loss is therefore deliberately attributed to the desorption of water from the zeolite. With increasing temperature, an additional weight loss is observed with a strong exothermic peak around 380 ~ This is tentatively attributed to the combustion of the encapsulated RhSalen complexes. A third, very broad exothermic weight loss occurs in the temperature range from ca. 250 ~ to 350 ~ While at present, no unambiguous explanation can be given for this phenomenon, it could be due to the starting decomposition/combustion of rhodiumsalen complexes with a lower thermal stability, probably those with a bidentate coordination of salen. Depending on the preparation conditions, such a coordination could also occur in RhSalenY-zeolites [13]. Similar results are obtained with RuSalenY, however, this complex seems to be less stable and starts to decompose already at ca. 320 ~ 100

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Figure 3. Hydrogenation of hexene-(1) over RusalenY-zeolites with one, two or four RuSalen complexes per unit cell. Reaction conditions: T = 70 ~ W/Fhexene_(1)=7 g-h/mol, Phexene-(1)= 7 kPa.

484 It has been reported that PdSalen, either in homogeneous solution [14,16] or encapsulated in faujasite-type zeolites [6,7], is an active hydrogenation catalyst which may act as model for hydrogenase-type enzymes [14]. Therefore, hydrogenation of alkenes was selected as model reaction for a characterization of the catalytic properties of the zeolite-encapsulated noble metal-salen complexes prepared in this study. In Figure 3, the yield of the hydrogenation product from hexene-(1), viz. n-hexane, is plotted as a measure for the catalytic activity in dependence of time-on-stream. It can be seen that RuSalen complexes encapsulated in zeolite Y are active catalysts for alkene hydrogenation and that the activity of the catalyst increases with increasing amounts of encaged complex. However, the activity does not linearly increase with the concentration of RuSalen. This can be tentatively attributed to either a partial pore blockade which renders part of the active sites inaccessible for reactant molecules and/or to a rate limitation due to diffusional effects at the relatively low reaction temperatures. For all three cases, only a slowly deactivation with time-on-stream is observed. The reasons for this deactivation and possible methods for catalyst regeneration are currently under investigation in our laboratory. It has been claimed in the literature [6] that a size discriminating effect can be observed with PdSalen complexes encaged in faujasite-type zeolites: In the competitive hydrogenation of an equimolar mixture of hexene-(1) and cyclohexene in the liquid phase, hydrogenation of the slim 1-alkene is strongly prefered, whereas in the homogeneous phase both alkenes are hydrogenated with a comparable rate. In 100

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Figure 4. Competitive hydrogenation of an equimolar mixture of hexene-(1) and 2,4,4trimethylpentene-(1) over Pd-, Rh- and Ru-Salen complexes encaged in zeolite Y (nmetal salen/nunit cell = 2). Reaction conditions: T = 80 ~ W/Falkenes = 14 g-h/mol.

485 order to investigate the possibility of such a shape selectivity effect, an equimolar mixture of hexene-(1) and 2,4,4-tfimethylpentene-(1) was competitively converted over the faujasite-encapsulated metal-salen complexes prepared in the present study. Whereas hydrogenation is the sole reaction over the RhSalenY and RuSalenY catalysts, also double bond isomerization is observed over PdSalenY. This phenomenon was already reported in the literature and has been explained by an intrinsic isomerization activity of the PdSalen complex which is retained upon encapsulation [6]. However, residual Brtnsted-acid sites could also catalyze this type of reaction. For comparative purposes, only the yields of hydrogenated products are depicted in Figure 4. The following pertinent features can be observed: (i) Under comparable reaction conditions and for similar concentrations of the encapsulated complexes, RuSalenY is much more active than RhSalenY and PdSalenY, (ii) deactivation is much more severe with the more active catalysts and (iii) with all catalysts, hydrogenation of hexene-(1) is strongly preferred over that of 2,4,4trimethylpentene-(1). At first glance, one is tempted to attribute these differences in reactivity to shape selectivity effects, viz. a restricted access of the bulky tribranched alkene to the active sites in the zeolite cavities. However, if the same experiment is conducted over PdSalen deposited on alumina, the slim alkene is still preferentially hydrogenated. Hence, the observed selectivities are at least not exclusively due to shape selectivity effects but are most probably governed by an intrinsically preferred hydrogenation of the 1-alkene over the salen complexes.

CONCLUSIONS Palladium, rhodium and ruthenium complexes of the Schiff base salen can be synthesized and immobilized in the supercages of zeolite Y. Experimental evidences for true encapsulation are (i) the typical bands of the metal-salen complexes in the IRspectra, (ii) UV/VIS-spectra obtained in the diffuse reflectance mode suggesting the presence of intracrystaUine metal-salen complexes, (ii) decomposition/combustion of the entrapped complexes upon heating in air and (iv) catalytic activity for the gas phase hydrogenation of alkenes. The reactivity differences observed in the competitive hydrogenation of an equimolar hexene-(1)/2,4,4-trimethylpentene-(1) mixture cannot be attributed in a straightforward manner to shape selectivity effects. Further work in our laboratory is directed to elucidate into more detail the catalytic properties of these interesting host/guest compounds

ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie. O. Batrtau moreover is grateful to the Ecole Europeenne des Hautes Etudes des Industries Chimiques de Strasbourg, France, for financial support to cover his stay at Stuttgart. The authors thank Mrs. I. Eyb for valuable technical assistance.

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