SELECTIVE DIMERIZATION OF ETHYLENE ON Pt ORGANOMETALLIC COMPLEX ENCAPSULATED IN Y ZEOLITE

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1 SELECTIVE DIMERIZATIO OF ETHYLEE O Pt ORGAOMETALLIC COMPLEX ECAPSULATED I Y ZEOLITE R. Zavoianu, E. Angelescu*, C. enu 1,. astase 2 *University of Bucharest, Dept. of Chemical Technology and Catalysis, bd. Carol I no. 13, Bucharest, Romania, fax , e-angelescu@upb.chim.ro 1 ZECASI S.A., Spl. Independentei, o 202A, Sect. 6, Bucharest, Romania 2 Romanian Academy, Institute of Physical Chemistry I.G. Murgulescu, Spl. Independentei, o 202, Sect. 6, Bucharest, Romania ITRODUCTIO owadays, zeolite encapsulated transition metal complexes are studied as functional mimics of enzymes, heterogeneous catalysts and as electrocatalysts [1, 2]. The encapsulation technique allows to the complex compound to have a molecular dispersion, preserving its structure and composition, but also enhancing its catalytic performances even at mild temperature and pressure [3, 4]. This paper continues our previous researches concerning encapsulated 2,2 -bipyridine complexes of transitional metals [5], and presents the results on ship in the bottle synthesis of Pt(2,2 -bipyridine) 2 (K) encapsulated in Y zeolite and its activity in C 2 H 4 selective dimerization to linear butenes. EXPERIMETAL The catalysts were prepared using a parent zeolite a(h)y (SiO 2 /Al 2 O 3 = 6 and 2 wt% a), that was used to obtain two series of encapsulated complex catalysts by ship in the bottle technique. First, we submitted a(h)y to ionic exchange with an aqueous solution of Pt 2 (0,01M) to obtain two catalysts: Pt 1.8 a(h)y and Pt 0.7 a(h)y (indexes 1,8 and 0.7 respectively, represent the concentration of Pt in wt% as determined by AAS). Afterwards the former two solids were stirred with an excess of melted 2,2 bipyridine during 2 h at 348 K and then submitted to Soxhlet extraction with benzene in inert atmosphere to remove the excess ligand and the complex adsorbed on the zeolite surface. In order to remove the uncomplexed Pt species, the solids were dried and treated under stirring at 353 K during 4 h with a 10wt% aqueous solution. The slurries were then filtered, washed with distilled water and dried at 393 K during 12 h. In this manner we obtained two encapsulated complex catalysts designated as K Pt 1.8 a(h)y and K Pt 0.7a(H)Y, (indexes represent the starting material for preparation of the encapsulated complex). The catalysts were characterised by diffuse-reflectance UV-VIS spectroscopy, FTIR spectroscopy and DTA/TG analysis. The UV-VIS diffuse reflectance spectra were obtained with a Carl Zeiss-Jena SPECORD M40, spectrophotometer, equipped with a diffuse reflection device 45/0, using as standard MgO. Infrared spectra were recorded on a Perkin Elmer 1600 FTIR spectrometer. Samples were prepared using the KBr discs technique. A resolution of 4 cm -1 and 64 scans were used. Thermal analysis was performed on a Setaram LabSys apparatus. Samples 447

2 (50 mg) were heated, under flowing air (45 ml min -1 ), from room temperature up to 1073 K and then cooled to room temperature. The heating and cooling rates were 10 and 25 K min -1, respectively. α-al 2 O 3 was used as reference material. Ethylene dimerization was performed in a continuos flow microreactor, in the range of temperature K at WHSV = 2 h -1. RESULTS Figure 1 presents the UV-VIS spectra of the free complex K comparatively to that of the encapsulated complex K Pt1.8 a(h)y having the higher concentration of Pt.. Fig.1. UV-VIS diffuse reflectance spectra of K and K Pt1.8 a(h)y In a previous work [5], we showed that the bipyridine ligand presents absorption maximums at 44000; 33000; and cm -1. From the above presented spectra we noticed that the free complex presents absorption maximums at 47250; 37000; 32000; 25200; and respectively 18770cm -1. Thus, as a consequence of the coordination of the atoms from bipyridine on Pt 2+ cations, the maxima at cm -1 disappears because the electronic transition n-π* is shifted to wavenumbers situated outside the range investigated. The other four bands of the ligand are shifted towards higher energies as a consequence of Pt coordination. Due to the electronic structure of Pt 2+ cations, the maximum at cm -1 will be splitted in two bands due to the strong coupling spin-orbital. Another band atributed to charge transitions at high energy is noticed in the spectra of the free complex. After the encapsulation, the spectrum of the complex is not significantly altered with respect to the position of the absorption maxima. The absorption bands for the encapsulated complex appear at 47280; 39730; 34470; 26960; and respectively cm -1. However, in visible domain, we notice an increase for the 448

3 intensities of the bands corresponding to d-d transitions of the cations. This increase may be correlated to the distorsion of the complex under the influence of the solid field. The FTIR spectrum of the free complex (fig. 2- a) presents the characteristic maxims at: cm -1 corresponding to ring stretching vibrations, 1415 cm -1 corresponding to A 1 symmetry, 1245 and 1320cm -1 corresponding to ring stretching mode B 1, cm -1 corresponding to ring stretching bending motion, cm -1 and cm -1 characteristic to aromatic out of plane substitution dependent hydrogen modes, cm -1 corresponding to ring bending motions, 462 cm -1 corresponding to Pt- stretching, 411 cm -1 corresponding to torsional ring motion. Other bands corresponding to Pt- + Pt- + bipyridine ring stretching, Pt- stretching and bending are localised in the far infrared region of the spectra between cm -1 and are not presented in fig. 2 [6]. Comparing the FTIR spectrum of the sample modified by ionic exchange with Pt (II) (Pt1.8a(H)Y), (fig. 2-c) with the spectrum of the parent zeolite (fig. 2b) we notice that Pt has a strong interaction with the zeolite. Thus, the bands characteristic for Si-O vibrations are shifted from 1000 and respectively 1100cm -1 to 1100 and 1200cm -1. The intensity of the band corresponding to D6R, D4R vibration from 570 cm -1 decreases dramatically while the two bands corresponding to Al-O vibrations at 700 and 780 cm -1 are much enlarged and shifted to 800 and 950cm -1. Also, the bands at 440 and 460cm -1 are not demarked anymore and are shifted to 478 cm -1 value corresponding to Pt-O bonds vibrations [7]. These effects may be attributed to the bonding of Pt cations to the O atoms of the zeolitic framework [8] established as in the scheme 1. a + a + O - O O O - O O O - O O - O a + Si Al Si Si Al Si Si Al Si Al O OO O O O O O O O O O O O O O O O O O H a(h)y Si O 2a Pt 2 Pt +2 [Pt] + O - O O O - O O O - O O - O Al Si Si Al Si Si Al Si Al OO O O O O O O O O O O O O O O O O O H Pt-a(H)Y SCHEME 1. Ionic exchange between a(h)y zeolite and Pt 2 Pt-a(H)Y 2,2'-bipy Si O Pt-2,2 bipyy Pt 2+ Pt 2+ O - O O O - O O O - O O - O Al Si Si Al Si Si Al Si Al OO O O O O O O O O O O O O O O O O O H + a Si O Pt 2+ Pt 2+ K-a(H)Y a + a + O - O O O - O O O - O O - Al Si Si Al Si Si Al Si Al O a + OO O O O O O O O O O O O O O O O O O H SCHEME 2. Encapsulation of Pt(2,2 -bipyridine) 2, (K), in a(h)y zeolite 449

4 We supposed that after the synthesis of the encapsulated complex, the bond Pt-O is split, and the coordinative bonds Pt-ligand (respectively the two atoms in 2,2 -bipyridine molecule - scheme 2) replace it. Most of the characteristic bands of the free complex (K) are ecranated by the spectrum of the zeolite. The bond Pt- is not visible in the spectrum because the absorption maxima appears outer the domain registered (331 and 310 cm -1 ). However, in the spectra of the encapsulated complex (fig. 2-d) we notice three maxims characteristic to the complex at 600, 650 and 1460 cm -1. On the other hand, we notice an increase of the intensity of the band at 570 cm -1 characteristic to the parent zeolite that was very attenuated after the ionic exchange. These charactersitics of the spectrum, sustain our supposition that after the last a treatment, the bonds Pt-O disappear and the complex remains encapsulated in the zeolite matrix. a b 2,0 1,5 40 Transmittance (a.u.) Wavenumber (cm -1 ) c d TG; DTG [µg ; (dµg/dt)] 1,0 0,5 0,0-0,5-1,0-1,5 TG DTG -2, HF DHF HF; DHF [J ;dj/dt)] Fig. 2. FTIR spectra of K (a), a(h)y (b), Pt 1.8 a(h)y (c) and K Pt1.8 a(h)y (d) Fig. 3. Thermal analysis of K Pt1.8 a(h)y Results of DTA/TG analysis presented in fig. 3. revealed that the encapsulated complex is stable up to 573 K that made it suitable to be used in reactions taking place under this temperature. The variation of the catalytic performances for ethylene dimerization as a function of the reaction temperature is presented in Fig. 4 (a-d). From the below presented results we notice that the catalytic activity of the parent zeolite is increased as a consequence of its modification by ionic exchange with Pt 2+. Thus, on Pt 0.7 a(h)y and Pt1.8a(H)Y the ethylene conversion increase up to 25-30% at 473 K comparatively to ca. 8% on a(h)y. The selectivity to n-butenes decreases as the temperature increases due to the increased frequency of disproportionation reactions. On the ionic exchanged 450

5 Conversion (%) a(h)y Pt 0.7 a(h)y Pt 1.8 a(h)y K Pt0.7 a(h)y K Pt1.8 a(h)y (a) 0 Selectivity to n-c4h8 (%) (b) Temperature ( K) 2-C4H8/1-C4H (c) 2-trans-C4H8/2-cis-C4H (d) Fig. 4. The variation of the catalytic performances as a function of the reaction temperature in the case of ethylene conversion, WHSV = 2 h -1 (a) - conversion; (b) - selectivity to n-c 4 H 8 ; (c) 2-C 4 H 8 /1-C 4 H 8 ; (d) 2-trans-C 4 H 8 /2-cis-C 4 H 8 catalysts, the best results for ethylene dimerization are obtained at 423 K when the conversion is ca. 20% and the selectivity to n-butenes reaches 75%. The increase of the reaction temperature favorises the formation of 2-C 4 H 8 without influencing significantly the value of the ratio between the trans and the cis isomers. An interesting aspect is that the zeolite with the lower concentration of Pt has a better activity than the one with higher concentration of Pt. This behaviour may be a consequence of the fact that in a first step, the reaction mechanism implies the formation of carbocations on the acid sites of the zeolite and part of these sites may be blocked when higher concentrations of Pt 2+ are used for the ionic exchange according to the reaction presented in Scheme 1. The encapsulated complex catalysts are more active than the ionic exchanged zeolites, the ethylene conversion being higher at all reaction temperatures. For these catalysts, the selectivity to n-butenes is even 100% at temperatures lower than 423 K. Also, the production of 1-butene is favourised comparatively to 2-C 4 H 8 and in the fraction of 2-C 4 H 8, the isomer cis is prevailing comparing to the isomer trans. We may associate these performances with the diffusion constraints of the molecular traffic, that allow to the encapsulated complex catalysts to have a better selectivity than the ionic 451

6 exchanged ones. This effect is more pregnant with the increase of the Pt concentration. However, as in the case of the catalysts prepared by ionuc exchange, the global catalytic activity of the catalyst with higher concentration of Pt is lower than that for that with the lower concentration of transitional metal. We may suggest that the encapsulated complex catalyst with the higher concentration of Pt has a lower activity than that with the lower concentration of Pt probably due to the lower dispersion of Pt species as a result of the production of dimeric complex species: Pt Pt COCLUSIOS Taking into account the above presented results, we may conclude that Pt(2,2 - bipyridine) 2 encapsulated in Y zeolite is a selective catalyst for ethylene dimerization to linear butenes at low temperatures up to 423 K. In the reaction product obtained with the encapsulated complex catalyst, the concentration of 1-C 4 H 8 is greater than that obtained for the same reaction conditions when using ionic exchanged zeolite. Also, in the composition of 2-C 4 H 8 fraction, cis isomer is prevailing over trans when using encapsulated complex catalysts. This allows us to conclude that due to the diffusion constrains induced by the encapsulated complex, the distribution of the reaction products is modified comparatively to the one obtained when using conventional catalysts. The steric effect of the complex is evident also in the UV-VIS diffuse reflectance and in the FTIR spectra. REFERECES 1. S.P. Varkey, C. Ratnasamy, P. Ratnasamy,. J. Mol. Catal., 135 (1998), K. J. Balkus Jr., C. D. Hargis, S. Kowalak, ACS Symp. Ser. 499, (1992), D. Chatterjee, H. C. Bajaj, A. Das, K. Bhatt, J. Mol. Catal., 92 (1994), L Herron, J. Coord. Chem., 19, (1986), R. Zavoianu, E. Angelescu, T. Mocanu, I. Voin, C. enu, Pd(bipyridine) 2 on H(a)Y zeolite, catalyst for ethylene dimerization - Proceedings of EUROPACAT III, Krakow, Poland, vol. I (1997), p J. S. Strukl, J. L. Walter, Spectrochimica Acta, (1971), 27A, ; K. akamoto, «Infrared and Raman Spectra of Inorganic and Coordination Compounds» - V-th Edition, John Wiley & Sons Inc. Eds., ew York-London, (1997), part B, E. M. Flanigen, H. A. Khatami, H. A. Symanski, Molecular Sieve Zeolites Adv. Chem. Series, (1971), 101, Am.. Chem. Soc., Washington D.C.,