Förster, Horst

Seebode, J.

English

Förster Horst

Seebode J.

University of Szeged

SPECTROSCOPIC AND KINETIC STUDIES ON THE CYCLOPROPANE ISOMERIZATI ON OVER MORDENI TES OF DIFFERENT ACIDITY H.FÖRSTER, J . SEEBODE Institute of Physical Chemistry, University of Hamburg, Laufgraben 24, D-2000 Hamburg 13, Federal Republic of Germany ABSTRACT Isomerization of cyclopropane over mordenites of different acidity has been studied by UV-VIS-IR spectroscopy and reactor kinetic measurements. Apart from the beginning and at high degrees of conversion the kinetics follows the law of a first order irreversible reaction. With increasing proton content of the zeolite the rate of isomeriza-tion is enhanced and the activation energy is decreased. The forma-tion of allylic and polyenylic carbocations in Bronsted acidic mordenites responsible for the production of oligomers can be proved by UV-VIS as well as by IR spectroscopy. By-products of this kind are missing on the Na-form, although there is evidence for a low residual surface acidity, the confirmation of which seems to be more likely by kinetic experiments or conversion of selected probe mole-cules than by vibrational spectroscopy. INTRODUCTION In spite of the spectacular transformation of coal or organic waste to hydrocarbons over silica-rich zeolites, there is still much industrial interest and effort in isomerization, which runs parallel to cat-cracking and upgrades the benzine f r a c t i o n . As this type of reaction is known to be catalyzed by strong acids in homogeneous media, the intermediacy of carbonium ions also with zeolites should be established and their role in parallel or consecutive reaction steps should be e x p l o r e d . Due to its higher m o d u l e , mordenite is of greater stability compared to faujasites and seems to be well-suited in the H-exchanged form for fundamental investigations. As the test reaction isomerization of cyclopropane was chosen, details of which are well-established in the literature /1/, stating the rate determining step 413 being the formation of the non-classical protonated cyclopropane intermediate. The objective of this paper was to examine the depen-dence of the kinetic parameters for the isomerization of cyclo-propane upon the proton content of the zeolite and the influence of other compounds enhancing the allylic ion g e n e r a t i o n . EXPERIMENTAL Materia 1s. The base material was synthetic mordenite Zeolon 100-Na (Norton) designated as Zeolon. A part of this sample was stirred for 16 hours at 350 K in an excess of 1 molar NaCl solution in order to achieve the homo-ionoc Na-form, named NaM. Starting from Zeolon, mordenites of different ammonium content were prepared by stirring with stoichiometric amounts of 0.1 molar solutions of ammonium chloride at 350 K for 16 hours. The compositions were determined by atomic absorption spectrophotometry (AAS) and neutron activation analysis. Samples will be designated as NaxHyM, where M denotes A l g S i ^ O g g . Cyclopropane (Merck, stated purity 9 9 . 9 9 8 % ) , allene (Matheson, 97%), ammonia (Matheson, 99.998%) and propene (Messer-Griesheim, 99.98%) were used without further purification. Apparatus. A stainless steel recircu1atory flow reactor was used for the kinetic experiments, a more detailed description of which will be given elsewhere /2/. For product analysis the system was connected via a motor-driven sampling valve to a gas chromato-graph (VARIAN AEROGRAPH 1520). The FID (flame ionization d e t e c t o r ) signal is controlled by an 8-bit microcomputer (COMMODORE CBM 8032) and was stored for further processing on a d i s k . The FID response was calibrated by known amounts of cyclopropane and propene, respec-tively. Baseline correction, determination of the rate constants by linear regression and calculation of the apparent activation ener-gies were performed by the aid of computer p r o g r a m s . The .spectra were recorded on a CARY 17, DIGILAB FTS 14 or DIGILAB FTS 20 spectrometer, respectively. Details of the instru-ments and procedures employed have been described previously / 3 , 4 / . Pretreatment. For reactor measurements 0.5 g of the 0.3-0.5 mm sieve fraction of pelletized zeolites were used and activated in a helium stream of 0.58 ccm/s at 750 K for 13 h o u r s . For the spectroscopic studies, thin self-supporting wafers were applied and outgassed at 750 K under high vacuum conditions (see also /3,4/). RESULTS AND DISCUSSION UV-VIS spectroscopic studies. Isomerization of cyclopropane over Y-type zeolites has been studied by Fejes et al. by means of reactor kinetic measurements /5/. Their kinetic curves showed a pronounced induction period ' indicating the formation of a reactive intermediate on the catalyst prior to isomerization. Upon pre-adsorption of traces of propene or allene this induction period decreased or d i s a p p e a r e d , respectively. Allylic cations were assumed to be these intermediates. By c o - o p e r a t i o n , we have studied the formation of carbocations in mordenites of different Bronsted acidity using UV-VIS-NIR spec-troscopy. The results are summarized as follows /4/:.Upon adsorption of allene, propene, cyclopropane, isopropanol, and acetone, respec-tively, similar electronic bands in the region 200-600 nm were observed. Their development with time or upon heating up to 370 K decreased in the sequence given above (allene being exceptionally reactive) as well as with decreasing proton content of the zeolites. The bands were divided into four groups and assigned to the IT-IT* transitions of m o n o - , di-, tri-, and tetraenylic carbenium ions. Mechanistically, their formation can be explained by the following steps (see /4/): (i) protonation (and dehydration) of the adsorbates leads to the propyl or allyl ion, respectively; (ii) hydride ion abstraction (by a propyl ion, e.g.) from propene produces the allyl ion; (iii) dienylic carbenium ions are formed by reaction of the allyl ion with allene directly, whereas the reaction of the allyl ion with a mono- or diene to a di- or trienylic carbocation requires consecutive deprotonation and hydride ion abstraction. In adddition to the published results, we recently investi-gated the adsorption of allene and cyclopropane on the base material, Zeolon, expecting no formation of carbenium ions, due to the lack of Bronsted acidity. In case of cyclopropane this expecta-tion was proved; no bands were detected even after heating up to 500 K. S u r p r i s i n g l y , upon admission of allene at room temperature very weak absorptions arose immediately, increasing in intensity upon short heating to 400 K. Analysis of this sample by AAS indicated a slight deficiency of Na. Therefore, we assume that this gap is balanced by OH g r o u p s , even though they were not detectable by IR spectroscopy. This conclusion agrees with the results and the pro-posed mechanism of carbocation formation and is supported by further IR spectroscopic and kinetic studies discussed later. In c o n c l u s i o n , the formation of allylic carbenium ions - even upon adsorption of cyclopropane - in Bronsted acidic mordenites was proved by UV-VIS spectroscopy. But n e v e r t h e l e s s , an important question is still open: Is the allylic ion involved in the cyclopro-pane isomerization or does it produce oligomers via the observed polyenylic carbocations only ? IR spectroscopic studies. Unfortunately, using the transmis-sion technique, strong lattice vibrational modes of the zeolites obscure bands of adsorbed species in the range below 1300 c m " 1 . Furthermore, the sharp increase of light scattering at higher wave-numbers leads to spectra of poorer quality in the region above 3000 c n f K Therefore, and for shortening the d i s c u s s i o n , the spectra presented here will be restricted to the region 2300-1300 c m - 1 . Nevertheless, our conclusions are confirmed by the bands in the range up to 4000 c m - 1 as well ! The positions and intensities of the bands of propene adsorbed in NaM at room temperature are very similar to those obser-ved on A-type zeolites /3/. The absorptions at 1455 (CH 3 asym. def.) and 1635 cm"1 (C=C stretch) are the most intense (very strong) followed by those at 1435, 1382 (strong) and 1417 cm" 1 (medium) (CH d e f . m o d e s ) . As the C=C stretching mode shows a cation-sensitive downscale shift on A-type zeolites /3/ and its position on NaM is identical to that on NaA, sorption of the propene molecules in front of sodium ions is c o n c l u d e d . The spec-trum does neither change with time nor does it indicate the formation of any other spe-cies. The spectra of cyclopropane adsorbed in Zeolon at room temperature (Figure 1) prove the slow isomerization on this zeo-lite, confirmed by the increase of the bands of propene with time and upon short heating of the loaded s a m p l e . In the spectral region shown, cyclopropane exhibits only two b a n d s , one of them accidental-ly at the same position as propene (1435 c m " 1 ) and the other one ( 1462 cm" 1 ), being a doubly degenerate and only raman active mode of the free m o l e c u l e , becomes IR active due to symmetry lowering upon Fig. 1. IR spectra of cyclopropane ad-sorbed in Z e o l o n . (r.t. = room temp.) adsorption. As this mode does not split into two bands (parallel to the corresponding absorptions in the CH stretch region) a "face on" interaction of the cyclopropane molecules with sodium ions has to be assumed (compare /3/). Fig. 2 . Dynamic IR spectra of allene adsorbed in Zeolon at room temperature. Sequence from bottom to top (0-10 h after admission). As allene turned out to be the most reactive substrate concerning the formation of carbenium ions, its adsorption in Zeolon at room temperature was investigated by IR spectroscopy as well (Figure 2). Most of the bands, observed in the region 1300-4000 c m " 1 are unambigously assigned to allene and its isomer, propine. Concerning Figure 2 this is mainly confirmed by the decreasing intensities of the absorptions at .1935, 1750, and 1400 cm" 1 (C=C) stretch, combination of bending m o d e s , and CH d e f . of allene) and by the increase of the bands at 2102, 1442, and 1388 c m " 1 (C=C stretch and CH d e f . of propine). But in addition a broad absorption at 1510 c m " 1 arises with t i m e . In agreement with our UV-VIS results and published IR data we assign this band to the CCC stretch of allyl type cations. Deno /6/ and Evans /7/ reported this absorption of monoenylic ions to appear around 1530 c m " 1 , shifting downscale upon further alkylation and / or conjugation. Furthermore it is generally observed that most of the CH and especially C=C bands of hydrocar-bons are shifted to lower wavenumbers upon adsorption in z e o l i t e s . So, we conclude that allene isomerizes to propine via the allylic cation, even though Bronsted acidic hydroxyl groups are not detec-table on this sample by IR spectroscopy. But kinetic studies (see below) and recent IR studies on the acidity of our zeolites by adsorption of ammonia /8/ revealed that even the Zeolon exhibits a low residual Bronsted acidity. The spectra of propene adsorbed in HM (Ha, , 2 H 7 . 8 M ) indicate fast oligomerization. ^ Only during the first 10 minutes after ad-mission propene bands at 1455, 1435 (shoul-ders, CH d e f . ) , and 1613 cm - 1 (C = C stretch of propene interacting with OH groups) are visible. The assignment of the latter is confirmed by the large shift of the OH stretching band, from 3610 to 3480 c m " 1 . The most intense bands are those of oligomers at 1465, 1385, and 1370 c m " 1 , which do not vanish upon heat treatment under vacuum. Immediately after adsorption of propene, the CCC stretching mode of allylic cations at 1535 c m " 1 is present and that of dienylic carbenium ions arises at 1505 c m " 1 with t i m e . So,.the spectra prove the fast oligo-merization of propene in HM via allylic and dienylic carbenium ions in agreement with the UV-VIS results. Upon adsorption of cyclopropane in HM at room temperature (Figure 3) bands of propene at 1635, 1613, 1455, and 1435 cm" 1 are detectable only during the first few minutes after admission. In course of time new absorptions at 1535, 1505 crtf* (CCC stretches of mono- and dienylic carbenium ion$), and of oligomers in the region below 1450 cm~^ arise. After heating the loaded sample to 370 K for one hour even the intense C=C stretching bands of propene totally disappeared and those of the carbocations and the oligomers became the most intense o n e s . Thus, a fast isomerization of cyclo-propane to propene in HM and consecutive (or parallel) oligomeriza-tion of propene via allylic and dienylic carbocations is c o n c l u d e d . Fig. 3 . IR spectra of cyclopropane ad-sorbed in HM. i.1« Concerning the adsorption of cyclopropane or propene in morde-nites of lower proton content, the generation of the bands discussed above is c o m p a r a b l e , but the rate of formation of oligomers dimini-shes with decreasing Bronsted acidity. On samples of small proton content (e.g Na^H^M) carbocations are not detectable by IR spectro-scopy. Here the bands of the adsorbed species are more clearly assigned to cyclopropane or propene as they are very similar to those observed on N a M . Time (ks) Fig. 4 . Kinetic curves (partial pressure vs. time) of the cyclopro-pane isomerization over Zeolon. (Triangles / squares = experimental cyclopropane / propene data; mass of catalyst 0.5 g) Kinetic Studies. In Figure 4 a typical set of kinetic curves of the cyclopropane isomerization over Zeolon (reaction temperature: 473 K) is p r e s e n t e d . (The cyclopropane curve is calculated by con-verting In p(cp) vs. time, obtained by linear regression, to the exponential f u n c t i o n , whereas the propene data were fitted by a polynom). In contrast to studies of Fejes et al. on Y-type zeolites /5/ our curves do not indicate any induction period. Deviations from the law of a first order irreversible reaction are detectable at high degrees of conversion only (compare Figure 4: 9 ks = 70% con-version), due to blocking of the active sites by adsorbed propene. At lower reaction temperatures, further small deviations (experimen-tal cyclopropane data higher than the calculated ones) at the very beginning of the reaction (up to 15 minutes) are due to the process of approaching the sorption equilibrium. These features were ob-tained on A-type zeolites as well /9/. 419 For convenience all samples were activated overnight (13 hours, exactly)., at 750 K. But as the UV-VIS spectra proved the formation of carbenium ions upon adsorption of allene on the starting m a t e r i a l , Zeolon, which can be interpreted by Bronsted acidic groups o n l y , the influence of different activation times was investigated. The rate constants of the isomerization measured at 473 K reaction temperature decreased linearly with increasing the activation time from 3 to 18 hours, and a further small decrease was observed upon extending this time to 40 hours. Thus we c o n c l u d e , that the low residual Bronsted acidity of this sample was reduced by dehydroxylation, even though acidic OH groups were not detectable by IR spectroscopy. Some kinetic data of the cyclopropane isomerization over mordenites of different Bronsted acidity are summarized in table 1. Table 1 Rate constants (k) and apparent activation energies (Ea) k (443 K) Ea_ . t e m p , range number Sample (s-1) (kJ/mol) (K) of data N a 4 H 4 M 1.04 10 - 0 _ A 72 + - 4 383 • - 443 4 N a g H 2 M 5.00 10 c 74 + - 1Q 286 • - 443 3 Zeolon 2.28 10 - D 102 + - 2 443 • - 473 4 NaM 1.90 10 -6 111 + - 3 443 • • 523 5 k values are given for using 0.5 g of z e o l i t e . Temp, range and number of data refer to calculation of Ea Table 1 shows that the rate constants are enhanced with in-creasing Broensted acidity of the samples whereas the apparent activation energies d e c r e a s e . Whether the rate constants are propor-tional to the proton content cannot-be decided c l e a r l y , due to the limitations of the analysis of the samples exhibiting very low acidity, i.e. Zeolon and N a M . Nevertheless, we conclude that the rate determining step of the cyclopropane isomerization over acidic mordenites is the protonation of the adsórbate, leading to a non-classical carbonium ion intermediate. 420 As UV-VIS and IR spectroscopy prove the formation of oligomers inside the framework of the zeolites, their formation was studied by gas chromatography as w e l l . At a reaction temperature of 403 K in case of NaM there are no by-products detectable even after complete conversion of cyclopropane to propene, whereas in case of Na^H^M weak peaks of by-products were detected in the gas phase even at the very beginning of the isomerization , i.e. about 20% c o n v e r s i o n . Nevertheless they amount only to less than 0.1% of propene and cyclopropane, respectively. Only after full conversion their amount is remarkably enhanced. So, they do not affect the accuracy of the kinetic m e a s u r e m e n t s . SUMMARY AND CONCLUSIONS Isomerization of cyclopropane to propene over mordenites is proved by IR spectroscopy and reactor kinetic m e a s u r e m e n t s . On samples of negligible Bronsted acidity no by-products are detectable by IR spectroscopic and gas chromatographic m e a s u r e m e n t s . The kinetics follows the law of a first order irreversible reaction without any indication of an induction period. On acidic mordenites the rate constants are enhanced and apparent activation energies decrease with increasing proton content of the samples, indicating the rate determining step to be the protonation of cyclopropane to a non-classical carbonium ion. The formation of oligomeric by-products via allylic and polyenylic carbenium ions on acidic mordenites is proved by UV-VIS and IR spectroscopy. Both IR spectroscopy as well as gas chromatographic studies show their generation during isomeri-zation, the rate of their formation being very small compared to the rate of isomerization. Thus, the formation of the by-products is concluded to be a negligible side reaction. The question, whether the proven allylic cation is involved in cyclopropane isomerization is still open, as its formation can be rationalized via small amounts of propene, formed at the very beginning of the isomeriza-tion, as w e l 1 . ACKNOWLEDGEMENT The financial support of this work by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Hungarian Academy of Sciences is gratefully acknowledged. 421 REFERENCES 1. Jacobs, P.A.: Carboniogenic Activity of Z e o l i t e s . Elsevier S e i . P u b l . C o . , Amsterdam, 1977, p. 97 2. Förster, H., Seebode, J., to be published 3 . Förster, H., Seebode, J., Zeolites 3, 1983, 63 4 . Fejes, P., Förster, H., Kiricsi, I., S e e b o d e , J . in: Structure and Reactivity of Modified Zeolites, J a c o b s , P . A . et al. (Eds.), Elsevier Sei. Publ. B.V., Amsterdam, 1984, p. 91 5. Fejes, P., Hannus, I., Kiricsi, I., V a r g a , K., Acta P h y s . C h e m . Szeged 24, 1970, 1 19 6. Deno, N.C. in: Carbonium Ions; Olah, G . A . , S c h l e y e r , R.P. (Eds.) W i l e y - I n t e r s c i e n c e , New York, 1970, V o l . II, p. 783 7. Evans, J.C. in: Carbonium Ions; Olah, G . A . , S c h l e y è r , R . P . , (Eds.), W i l e y - I n t e r s c i e n c e , New York, 1968, V o l . I, p . 223 8 . Förster, H., Jacubasch, H., to be published 9. Fejes, P., Kiricsi, I., Förster, H., S e e b o d e , J . , Zeolites 4 , 1984, 259 422 

Spectroscopic and kinetic studies on the cyclopropane isomerization over modernites of differnet acidity

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Spectroscopic and kinetic studies on the cyclopropane isomerization over modernites of differnet acidity

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