Fejes Pál

Kiricsi Imre

Tasi Gyula

Varga K.

English

University of Szeged

ISOMERIZATION OF CYCLOPROPANE OVER Co(II)-EXCHANGED A-TYPE ZEOLITES P. FEJES, I. Kiricsi, Gy. Tasi, K. Varga Department of Applied Chemistry, Jozsef Attila University, Szeged, Hungary ABSTRACT Study of the skeletal isomerization of cyclopropane over Co(Na)A zeolites exchanged to different levels reveals that the reaction takes place irreversibly and follows first-order kinetics. The rate is pro-2+ portional to the number of accessible Co ions in the large cavities. This value is the same as the number of adsorbed propylene molecules at full coverage. In contrast to the situation with Bronsted acids as catalysts, the reaction intermediate is a it-adsorbed complex produced 2+ by the interaction of cyclopropane adsorbed "face-on" over Co ions. 2+ As Co ions are inactive in side-reactions such as oligomerization and cracking, the kinetics of isomerization can be studied under "clean" conditionso INTRODUCTION It is widely accepted that the skeletal isomerization of cyclo-propane on solid acids is one of the most simple catalytic transforma-tions. The reaction takes place irreversibly, following apparent first-order kinetics, and edge-protonated cyclopropane is regarded as the reaction intermediate [1-5]. The product propylene is not stable in contact with Bronsted acids, as it undergoes further reactions leading to different oligomeric and cracking products [6-8]. These un-desired side-reactions can be avoided and the kinetic picture is accord-ingly simplified if alkaline earth or transition metal ion-exchanged zeolites, as Lewis acids, are used as catalysts [9]. With Co(Na)A zeolites, for example, it was possible to follow the kinetics by i.r. spectroscopy, measurements in a static reactor permitting estimation of the relevant rate coefficients of the surface reaction [10]. The 2+ i.r. spectroscopic studies revealed that the active centres are Co ions interacting with cyclopropane adsorbed in a "face-on" mode [11]. The reactor experiments resulted in a strict correlation between the 2+ true kinetic parameters and the number of accessible Co ions in the 405 zeolitic framework. This paper is intended to shed light on some details of the ki-netics of cyclopropane isomerization over Co(Na)A zeolites exchanged to various extents. EXPERIMENTAL The Co(Na)A specimens were obtained from binderless Linde 4A zeolite by exchange with Co(NO^^ solution. Their compositions are given in Table 1. Table 1 Unit cell compositions of catalysts used îîo Composition 1 Na12 - A (A = Al12Si12°48) 2 C°0.24Na11.52 - A 3 C°0.85Na10.2 - A 4 Co1.38Na9.24 - A 5 Co3.6 Na4.8 - A 6 ' Co3.7 Na4.6 - A 7 Co4.02Na3„96 - A 8 Co4.55Na1.9 - A The zeolite samples were characterized by XRD, IR spectroscopy and TG. For adsorption and kinetic studies, the catalyst samples were heat-treated at 673 K, in vacuo, for 4 h. (The final pressure at-_2 tained was better than 10 Pa.) A description of the kinetic equip-ment used and the product GC analysis is to be found in [12]. RESULTS Adsorption experiments with cyclopropane run into difficulties, because the kinetic diameter of the cyclopropane molecule (around 0.52 nm) is larger than the pore openings in NaA. On the other hand, cyclopropane undergoes a quite fast transformation over Co(Na)A, even at ambient temperature. These obstacles are not encountered with pro-pylene as adsorptive. Figure 1 shows the adsorption isotherms of propylene on NaA zeolite. The isotherms cannot be described correctly by either the Langmuir or the Freundlich isotherm equation: at low temperatures they are rather of the Langmuir type, while at higher temperatures 406 the Freundlich isotherm equation seems to be valid, with a continuous transition between the two types at intermediate temperatures. Similar 2+ results have been obtained with samples with low Co contents. Fig. 1. Adsorption isotherms of propylene on NaA zeolite. (Temperatures: a 332 K; b 377 K; c 398 K; d 438 K; e 453 K; f 473 K.) Figure 2 depicts the isosteric heats of adsorption at different coverages, as obtained from the isotherms for NaA and a low-level ex-changed specimen (No. 2). It is clearly seen that, after some de-crease, near a =0, the curves level off at the same value (Q , = ' - I s st = 47 kJ mol ) for the two adsorbents. The quite high value of Q implies weak chemisorption, as found by Schirmer in the case of Ca(Na)A [13]. With ethylene as adsorptive the character of the curves for a 2+ specimen containing 1 Co ion/u.c. is similar; nevertheless, the levelling-off occurs at a lower isosteric heat of adsorption (Qst = = 42 kJ mol"1 [14]). 2+ At high Co ion-exchange levels (see specimens No. 6 and No. 7), the isosteric heat curves are similar to each other (see Fig. 3), but differ markedly from those seen in Fig. 2. At higher coverages a constant value of Q = 75 kJ mol-1 is reached here too. The adsorption heat curves in Fig. 2 and 3 clearly demonstrate that no simple adsorption model can be valid for the adsorption of 2 + propylene on Co -exchanged NaA zeolites. The kinetic curves of cyclopropane isomerization, as measured VI e 701 50 *»5Bot > H > ' 10 as10/moMcg <3 70 50 10 dslO/moMcg Fig. 2. Isosteric heat of ad-sorption at different cov-erages for NaA (o) and sample No. 2 (•). Fig. 3. Isosteric heat of ad-sorption at different coverages for samples No. 6 (o) and No. 7 in a recirculatory flow reactor using catalyst samples with varying 2+ Co contents, are shown in Fig. 4. NaA turns out to be a practically 10 t/ks Fig. 4. Kinetic curves of cyclopropane isomerization. (Reaction -4 temperature 473 K; mass of catalyst 2'10 kg; reactor volume 0.14 dm3.) inactive catalyst, mainly due to sieving effects. The curves reflect 2+ a similar change between specific activity and Co content. The ac-tivation energies computed from the kinetic curves are listed in 408 Table 2 Activation energies for cyclopropane isomerization Zeolite Nq> 4 N o > 5 N o > samples Activation energies 47.2 64.3 65.7 kJ mol-1 DISCUSSION The adsorption properties of NaA for different hydrocarbons have been investigated by several authors [15-17]. From the results it could be concluded that through exchange of the bulky Na ions for 2+ 2+ 2+ ions of increasingly smaller size (e.g. Ca , Mg , Co , etc.) the pore openings could be widened (the eight-ring windows are blocked by ions of smaller diameter), permitting the penetration of bulkier hydro-carbon molecules. Though the amount of propylene adsorbed increases with the de-2+ gree of Co ion-exchange, no direct proportionality can be observed (see Fig. 5). Breck came to a similar conclusion concerning ethylene adsorption on KNaA zeolites [16]. The first-order rate constants for the isomerization reaction can be seen in Fig. 5 too. The similarity of these two sets of data proves that in the chemisorption and in the catalytic reaction the same centres are involved, their number and distribution (accessi-bility) being identical. The kinetic curves in Fig. 4 for both cyclopropane consumption and propylene production are apparently first order in the case of high-level exchange. No induction period whatsoever is discernible in the curves, in contrast to NaY and NaCaY zeolites [12], and no oligo-meric species or cracking products could be identified by GC. The ac-tivation energy for these samples (E+ = 65 kj niol agrees well with that found previously [18]. At low exchange levels the character of the kinetic curves dif-fers markedly from that found before. The activation energy (for sam-ple No. 4) drops from 65 kJ mol 1 to E+ = 47 kJ mol this value be-3L ing equal to the isosteric heat of adsorption for propylene at high coverages on the same samples. Propylene preadsorbed on the activated catalyst does not influ-ence the rate of cyclopropane skeletal isomerization. Allene, admit-A.AQ ted to the catalyst before commencement of the experiment, causes re-tardation. This is probable due strong chemisorption, or else is caused by its isomerization product, methylacetylene, rendering the active sites inaccessible for cyclopropane and/or propylene. These findings are opposite to those found earlier on NaCaY •zeolites exhibiting weak BrcJnsted acidity, where small amounts of pre-sorbed propylene or allene reduced the length of the induction period by creating C^H^ carbocations as ionic chain carriers. This behaviour is characteristic for promoters. Indirectly, this result suggests that the Co(Na)A samples are free of any Bronsted acidity. Forster and co-workers came to a similar conclusion from their investigation of the i.r. spectra of CoNaA zeolites in the OH region [19]. Fig. 5. Comparison of the adsorption and kinetic data On the basis of what has been stated about the correlation found between specific activity (expressed as the first-order rate constant 2+ of the surface reaction) and Co content (see Fig. 5), it seems very probable that, starting from NaA, with adsorption practically nil for cyclopropane, the sieving effect diminishes progressively with in-creasing levels of exchange, permitting access to active sites in the zeolite cavities. This point is stressed, for there are a few cases in the literature where a similar trend was observed with Bronsted acid sites as active centres, which necessitates another explanation [20]. 410 REFERENCES 1. Habgood, H.W., George, Z.M., Molecular Sieves, Soc. Chem. Ind. London, 1968. p. 130. 2. Bassett, D.W., Habgood, H.W., J. Phys. Chem. 64, 769 (1960). 3. George, Z.M., Habgood, H.W., J. Phys. Chem. J2_, 1689 (1968), 74, 1502 (1970). 4. Hightower, J.W., Hall, W.K., J. Am. Chem. Soc. 90, 851 (1968), J. Phys. 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Breck, D.W., Eversole, W.G., Milton, R.M., Reed, T.B., Thomas, T.L., J. Am. Chem. Soc. 78, 5963 (1956) 17. Schöllner, R., Ulber, G., Platzdasch, K., Chem. Techn. _34, 583 648 (1982) 18. Fejes, P., Förster, H., Kiricsi, I., Seebode J., Zeolites 4_, 259 (1984 ) 19. Förster, H., Seelemann, R., J. Chem. Soc. Faraday I., 71_, 1359 (1981) 20. Jacobs, P.A., "Carboniogenic Activity of Zeolites". Elsevier, Amsterdam, 1977, p. 128. 411 

Isomerization of cyclopropane over Ci(II)-exchanged a-type zeolites

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Isomerization of cyclopropane over Ci(II)-exchanged a-type zeolites

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