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Transformation of light alkenes over templated and non-templated ZSM-5 zeolites
Applied Catalysis A: General 177 (1999) 245±255
Transformation of light alkenes over templated and non-templated ZSM-5 zeolites I.P. Dzikha,b, J.M. Lopesa, F. Lemosa, F. RamoÃa Ribeiroa,* a
b
Centro de Engenharia BioloÂgica e QuõÂmica, Instituto Superior TeÂcnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal Petroleum Re®nery Engineering Department, State University ``Lvivska Polytechnica'', Bandera St., 12, 290646 Lviv-13, Ukraine Received 27 March 1998; received in revised form 3 August 1998; accepted 5 August 1998
Abstract The transformations of ethene, propene and i-butene over templated (CVN) and non-templated (CVM) ZSM-5 zeolites at 3508C were examined. The initial activity values obtained for the conversion of C2H4 were signi®cantly higher with H-CVM than with H-CVN zeolite, in accordance with the stronger acidity of its sites. In contrast, for the transformation of C3H6 and iC4H8 the opposite order was observed: the more acidic catalyst presents a lower activity. It was also observed that, in the case of CVM, the sodium-exchanged CVM samples have higher activities than the H form. These observations can be related to the extremely high acidity of the CVM samples. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ethene; Propene; i-Butene; Templated and non-templated ZSM-5; Acidity; Catalytic activity
1. Introduction MFI type zeolite (ZSM-5) is conventionally prepared by the hydrothermal crystallisation of aluminosilicate gel using various kinds of organic bases as templating agents (for example, tetrapropyl ammonium bromide, 1,6-diaminohexane, 1,3-diaminopropane, etc.) [1]. However, the template is corrosive, expensive, usually unrecoverable, and requires high temperature treatments for its decomposition and removal from the zeolite. ZSM-5 zeolite has also been synthesised in the absence of an organic template, that is, by employing ammonia as the templating agent [2± 4]. In this case, the production of the protonic form of the zeolite involves only a simple calcination. *Corresponding author. Fax: +351-1841-7246.
Acidity is one of the most important characteristics of zeolites, and the one that makes them extremely important materials in catalytic applications. The acidity of zeolites is known to depend on several factors: structure, preparation method, chemical composition, impurities, Si/Al ratio, additives and poisons. A lot of attention has been given to the relationship between the acidity of zeolites and their catalytic activity and review papers by RamoÃa Ribeiro et al. [5] and Auroux [6] have been recently published on this subject. ZSM-5 samples, synthesised with and without template, present very different acidity and consequently different catalytic properties [7,8]. However, for nontemplated ZSM-5 zeolite this relationship has been insuf®ciently examined [9,10]. The transformation of light alkenes over ZSM-5 catalysts is important in the various petrochemical
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00272-5
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processes such as MOGD, MTG, Cyclar, etc., and it is also a good test reaction to probe the acidity of these materials. Thus, it is of considerable interest to compare the catalytic behaviours of templated and nontemplated ZSM-5 zeolites for the transformation of light alkenes. Light alkene transformation within the temperature range from 2008C to 5008C, over ZSM-5 zeolites, has been the subject of numerous studies [11±25], especially in connection with the conversion of methanol to gasoline and, recently, with the production of aromatics by light alkane aromatisation. However, in these works authors used mainly templated ZSM5 catalysts. The aim of the present work is to study the in¯uence of zeolite preparation method on the total acidity of samples, on the activity and on the product distribution of the transformation of ethene, propene and i-butene over templated and non-templated ZSM-5 zeolites. 2. Experimental 2.1. Catalyst preparation The starting forms of the templated (CVN) and nontemplated (CVM) ZSM-5 zeolites, in ammonium form, were obtained from Angarsk Oil±Chemical Company (Russia). Details of the preparation of NH4±ZSM-5 zeolites have been previously described [26,27]. The acidic forms of ZSM-5 were prepared by the procedures described below. CVN sample. The template cations were decomposed at 5508C under a ¯ow of dry nitrogen during 3 h, after which a calcination under a ¯ow of dry air for 8 h was performed. CVM sample. The samples were calcined at 5008C for 8 h under a ¯ow of dry air. Departing from these acidic forms of CVM and CVN, several samples with different sodium contents were prepared by successive ion exchanges with 1 M sodium nitrate solutions. Two exchanges were performed at room temperature for 4 h. The corresponding acidic forms, HNa-ZSM-5, were obtained by calcination at 5008C for 8 h under a ¯ow of dry air.
2.2. Catalyst characterisation Characterisation studies were performed using elemental chemical analysis, X-ray diffraction (XRD) in a Rigaku diffractometer with Cu K radiation, temperature programmed desorption (TPD) of NH3, scanning electron microscopy (SEM) with a JEOL JSM840 instrument. Solid state MAS NMR measurements (for 29 Si and 27 Al) were also carried out, using a Bruker MSL 300 spectrometer. The H-ZSM-5 (CVM and CVN) zeolites were analysed for phase purity by using an X-ray powder diffractometer. The investigation of the acidity of the zeolite catalysts was performed by TPD of ammonia. These experiments were carried out after activation of a 0.2 g sample in a Pyrex reactor for 7 h at 5008C in dry nitrogen ¯ow. The sample was then cooled to 908C and ammonia adsorption was performed by feeding pulses of reactant grade NH3 to the reactor. The sample was then ¯ushed with helium at 908C for 2 h to remove excess ammonia and decrease the amount of ammonia that was physically adsorbed on the samples. The ammonia was then thermally desorbed with a heating rate of 108C/min in a helium ¯ow (60 ml/min) from 908C to 6008C. The amount of NH3 desorbed was measured by a TCD detector. The electrical signals from the detector and the thermocouple, which measures the temperature inside the cell containing the catalyst, were digitised by a CR3A integrator and sent to a computer. The morphology of the zeolite catalysts was investigated by means of SEM. A silicon to aluminium ratio was determined by 27 Al and 29 Si MAS NMR. 2.3. Catalyst testing The transformation of pure ethene, propene and ibutene over the catalysts of the two series HNa-ZSM-5 (CVM and CVN) was carried out in a continuous ¯ow ®xed-bed reactor at 3508C, at total pressure of 1 bar with a nitrogen-to-hydrocarbon molar ratio equal to 9. The reactant ¯ow rate was kept at 500 ml (STP)/h and the weight of catalyst that was used was 150 mg for CVM and 20 mg for CVN. The reactor ef¯uent products were analysed by on-line gas chromatography using a PLOT Al2O3±KCl capillary column with a FID detector.
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Table 1 Chemical composition of HNa±ZSM-5 samples (for CVM±Si/ Al26 and for CVN±Si/Al42) Sample
Na level (% of total cationic positions)
H-CVM HNa-CVM (62) HNa-CVM (89) H-CVN HNa-CVN (60) HNa-CVN (72)
<5 62 89 <6 60 72
Prior to the catalytic transformation of the reactants, the catalysts were pre-treated in situ at 4508C for 12 h under a ¯ow of dry nitrogen (60 ml/min). 3. Results and discussion 3.1. Physico-chemical characteristics of ZSM-5 catalysts The chemical composition of the two H-ZSM-5 and their sodium-exchanged ZSM-5 samples are listed in Table 1. Both series of samples show good crystallinity and the XRD patterns agree very well with those reported in the literature for ZSM-5. The Si/Al ratio, as estimated from MAS NMR was found to be around 26 for CVM and 42 for CVN zeolites (see caption in Table 1), i.e., Al concentration in two samples differs by approximately 1.6 times. The analysis of 27 Al MAS NMR spectra shows a singlet around 111 ppm for H-CVM sample and 112 ppm for H-CVN sample, indicating that the aluminium atoms are in tetrahedral coordination. These spectra have also shown that there is no signi®cant non-framework aluminium in any of these zeolites. The TPD of ammonia thermograms, as recorded for the two series of catalysts, the templated and the nontemplated ones, are shown in Fig. 1. The TPD curves were very similar to those usually found for ZSM-5 zeolite [28]. The peak maxima in the ammonia evolution pro®les were observed at 1658C and 3008C for HCVN sample and at 2008C and 3508C for H-CVM sample. So, on the basis of the recorded TPD spectra
Fig. 1. TPD spectra of NH3 for CVM and CVN samples.
of ammonia, it can be seen that H-CVM has a higher acid strength, as well as a larger amount of acid sites, than H-CVN zeolite. It is generally accepted that acid site concentration is proportional to the number of Al atoms in the elementary lattice [6] and that, for zeolites with a high silica content, site strength does not depend signi®cantly on the Si/Al ratio. This is consistent with the difference in the number of acid sites between H-CVM and H-CVN but the difference in acid site strength is certainly related to other aspects. The comparison of the TPD curves of all samples indicates that the high temperature peak may correspond to the desorption of ammonia from structural OH groups of the ZSM-5. It was also observed that the sodium-exchanged CVM and CVN catalysts exhibited a lower acidity than their parent zeolite samples, as it would be expected from the replacement of some of the protons in the zeolite by sodium cations [29]. The SEM micrographs of these zeolite catalysts show the typical crystal size and form of the ZSM-5 zeolite. The CVM sample, prepared without template, appears to have smaller particles (0.5±1 mm) than the CVN sample, which was prepared with template, and that consists of larger and elongated particles (3±4 mm). This difference in crystallite size can account for some of the differences that were observed in the TPD thermogram, since it is generally expected that small crystallites may show a higher apparent acidity (mainly due to larger accessibility of the acid sites) than large crystallites, for similar Si/Al ratios [30].
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3.2. Catalytic transformation of ethene, propene and i-butene The activity and selectivity of all catalysts were studied in the conversion of ethene, propene and i-butene at 3508C. From these alkenes the main reaction products that are observed are alkenes in the range of C3±C5. The distribution of the products remained practically unchanged with time-on-stream for all the samples that were inspected. For the duration of the runs, approximately 60 min, no signi®cant deactivation was observed for any of the samples. The spent samples were only light grey, not black, indicating only a moderate deposition of coke, in accordance with the absence of deactivation during the run. 3.2.1. Activity Fig. 2 shows the catalytic activities versus time-onstream for the C2H4, C3H6 and i-C4H8 conversion over CVM, CVN and their sodium-exchanged samples. It can be seen that the initial activity values obtained for the ethene transformation were higher with H-CVM than with H-CVN zeolite, in accordance with the higher acidity of its sites, as measured by thermal desorption of ammonia. In contrast, for the C3H6 and i-C4H8 transformation the opposite order was observed: the more acidic catalyst presents a lower activity. The results obtained for the transformation of ethene are consistent with the higher acidity that was observed for the H-CVM zeolite. In fact, the total number of acid sites, as well as the strength of these sites, as measured by the temperature programmed desorption of ammonia, is far larger in H-CVM samples than on H-CVN, and the rate for the transformation of ethene is around 6 times larger in H-CVM than in H-CVN. Also, the site density seems to favour the higher activity of the CVM sample. In fact, in a ®rst approximation we could expect a higher site density in case of CVM zeolite (also due to its lower Si/Al ratio). Observations corroborating the role of acid site density on the activity have been noted earlier by Giannetto, who has shown that it in¯uenced the catalytic activity of H-[Al]±ZSM-5 catalyst for propene oligomerisation [31] and also on the mode of alkane cracking [32].
For the transformation of ethene, as expected, when protons are removed and exchanged with sodium cations, the activity decreases, due to the removal of a certain number of acid sites, usually the most acidic ones. Similarly, Ono and coworkers [33] found that a small amount of alkali cations in H-ZSM-5 greatly reduced the catalytic activity for the acidcatalysed reactions of cyclopropane isomerisation and hexane cracking. The behaviour however, is quite different when we look at the conversion of propene. In this case, not only is the activity order reversed, H-CVN is about 6 times more active than H-CVM, but also, when sodium is removed from these catalysts, the activity increases for the CVM samples. It must be noted that propene, being larger than ethene, produces carbocations which are much more stable than the latter, namely because it can easily produce a secondary carbocation. A positive contribution of Lewis acid±base pairs [34] (sodium-cation and the neighbouring basic oxygen) in the conversion of propene and isobutene over these catalysts can also be considered. Recently it was found that the propene aromatisation over alkaliexchanged ZSM-5 zeolites occur on the strong Lewis acid sites and weak Lewis basic sites [23]. Similarly, these have been regarded by Huang and Kaliaguine [23] as acid±base pairs of alkali-cation and neighbouring basic oxygen. This could help to explain the increase in activity when sodium is introduced, but would not explain why H-CVM shows a lower activity than H-CVN and, in any case, should represent a minor contribution. It is generally accepted that active sites which possess an overly strong interaction with the reactants and/or products, generates very little catalytic activity and that optimum catalytic sites should have a ``moderate'' interaction with both reactants and products. We have observed that H-CVM samples possess acid sites with very strong acidity, as con®rmed both by the temperature programmed desorption of ammonia and by the extremely higher rates of transformation of ethene, which is a molecule which is very dif®cult to protonate due to its need to produce primary carbocations. Sabatier's rule would not only explain the relative activity order between H-CVM and H-CVN samples, but also why the activity for the CVM series increases when the proton content is
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Fig. 2. Catalytic activities versus time-on-stream for the transformation of C2H4 (a), C3H6 (b) and i-C4H8 (c) over CVM and CVN catalysts.
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decreased. The partial titration of acid sites in CVM with sodium cations would possibly remove the stronger acid sites, the ones that would irreversibly adsorb propene, leaving sites with moderate activity and that would, thus, be more effective for the transformation. From Fig. 2 we can also see that the behaviour in relation to isobutene transformation follows similar lines to the one of propene. Isobutene is much easier to protonate than propene, because it readily produces a tertiary carbocation. This usually implies that its rate of transformation is higher than that of propene. In fact, for CVN samples the activity for the i-C4H8 conversion is higher than for C3H6, which is in accordance with the expectations. It should be noted, that the observed increase in reactivity could even be under-rated in relation to what would be expected, due to stereochemical aspects. In the case of i-C4H8 the shape selective effect of the ZSM-5 [35] will make it dif®cult to form dibranched and other rami®ed isomers. ZSM-5 zeolite has a bi-directional system of intersecting 10 MR channels between 5.35.6 and Ê diameter and formation of branched 5.15.5 A C10±C12 oligomers inside its channels also should be strongly hindered. However, for the isobutene transformation over HCVM zeolite the observations are similar to the ones made for propene and, in fact, the rate of conversion of isobutene is lower than that of propene, whilst one would expect that the small crystal size of CVM zeolite would reduce the shape selective effects and should, consequently, increase of the activity for i-C4H8 conversion. Sabatier's rule, however, is in accordance with all these observations. CVN, possessing moderate acid sites shows an increase in activity when going from propene to isobutene, whilst CVM samples, possessing overly strong acid sites, produces the reverse effect, all due to the higher stability of the tertiary carbocation. As for propene, the replacement of acid sites by sodium cations induces an increase in activity for CVM samples and a decrease for CVN samples. 3.2.2. Selectivity From C3H6 (Fig. 3) the main reaction products are butenes and pentenes. Ethene, C3±C6 alkanes, hexenes and aromatics (BTX) are observed in lower amounts. From ethene and isobutene (Figs. 4 and 5) the main
products are propene, butenes and pentenes; C3±C7 alkanes, hexenes, heptenes, aliphatic hydrocarbons C8 and aromatics are also observed. For all catalysts the trans-2-butene and cis-2-butene are formed in approximately equimolar proportions. Methane, ethane and aromatics are obtained in minor quantities in all cases. The concentration of C8 products was relatively low. No aliphatic hydrocarbons higher than C8 are observed in the reaction products. The observed values for the transformation of C2H4 over CVN catalysts did not differ from the values obtained for its thermal conversion in blank experiments using either an empty reactor or a reactor with an inactive material (like silica). It must be emphasised that for isobutene transformation the propene/pentenes molar ratio is smaller than 1. In contrast, for ethene transformation this ratio is greater than 1. This could be related to the fact that some of the pentenes and propene are also formed by cracking reactions of intermediate species other than C8 carbenium ions. The formation of products with 6 and 7 carbon atoms may be explained if we consider the recombination of C3 and C4 fragments. Similar observations and conclusions have been reported by Xu et al. [36] in order to explain the formation of hexenes and heptenes during the isomerisation of n-butenes over ZSM-23 zeolite. The reactions of C4 and C5 intermediate species with propene will lead to the formation of C7 and C8 hydrocarbons that will ®nally produce toluene and xylenes by dehydrocyclisation and dehydrogenation reactions. The absence of aliphatic hydrocarbons with more than nine atoms of carbon and the relatively low concentration of octenes in the reaction products can be related to the high cracking activity of the strong acid sites of ZSM-5 towards the transformation of these species and also to the strong limitations that they encounter when diffusing through the 10-membered ring windows. The formation of aromatics probably proceeds by several successive steps: dehydrogenation of C6±C8 oligomers to dienes, diene cyclisation with formation of cyclic dialkenes and aromatics via hydrogen transfer on the acid zeolite sites, according with a kinetic model for C2H4 and C3H6 aromatisation, recently reported by Lukyanov et al. [25]. It must be underlined that for propene and ethene transformation over CVM catalysts the selectivity
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Fig. 3. Selectivity plots for C3H6 transformation over CVM and CVN catalysts for different time-on-stream: (a), (b) 6 min, (c), (d) 24 min and (e), (f) 60 min. C2ethylene, C3 propane, C4 paraffins, C4olefins, C5 paraffins, C5olefins, C6 paraffin, C6olefins, and C7 aliphatics and aromatics.
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Fig. 4. Selectivity plots for i-C4H8 transformation over CVM and CVN catalysts for different time-on-stream: (a), (b) 6 min, (c), (d) 24 min and (e), (f) 60 min. C3 propane, C3propylene, C4 paraffins, C4olefins, C5 paraffins, C5olefins, C6 paraffin, C6olefins, C7 paraffins, C7olefins, and C8 aliphatics and aromatics.
to butenes and pentenes are lower than those over CVN. The selectivity towards C3±C6 alkanes and aromatics were signi®cantly higher with CVM than with CVN samples. In addition, the alkene/alkane ratio is
slightly greater with CVN than with CVM catalysts. These facts indicated that CVM zeolite, which presents the highest acidity and site density, favours hydride transfer reactions between carbenium ions and alkene molecules. On the other hand, for the
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Fig. 5. Selectivity plots for C2H4 transformation over CVM catalysts for different time-on-stream: (a), (b) 6 min, (c), (d) 24 min and (e), (f) 60 min. C1 methane, C2 ethane, C3 propane, C3propylene, C4 paraffins, C4olefins, C5 paraffins, C5olefins, C6 paraffin, C6olefins, and C6(C6C7 aliphatics) and aromatics.
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two series of catalysts, HNa-CVM and HNa-CVN the increase of sodium level leads to the relative increase of selectivity towards alkenes and a corresponding decrease of selectivity to alkanes, as it can be seen in Figs. 3±5. This reinforces the data previously discussed which indicates that H-CVM is, indeed, much more acidic and possesses a much higher site density than H-CVN and, thus, favours hydride transfer. Nayak and Moffat [37] cracked C6±C8 alkenes over ZSM-5 zeolites prepared with tetrapropyl ammonium bromide and ammonia as template, and found that the activity and selectivity are similar for the two templating agents employed in the preparation of catalyst. It must be emphasised that the authors used the samples with little difference of total acidity and other properties. Obviously, it is the reason of absence of signi®cant difference between two zeolites concerning their activity and selectivity. 3.3. Reaction scheme considerations From the experimental results, and also based on the data published in the literature, it is possible to make some considerations regarding the reaction scheme for the transformation of C2±C4 light alkenes. The ®rst step involved in all these transformations is the protonation of the ole®ns. In the initial moments this will be done by direct interaction between an ole®n molecule and an acid site, but for longer times on stream it is likely that there can be direct interactions between a gas-phase ole®n molecule and an adsorbed carbocation. Unlike the case of alkane transformation, the production of adsorbed carbocations from ole®ns is relatively easy.The main steps that can be considered afterwards are [25,38±40]: oligomerisation, or alkylation of one gas-phase alkene molecule by an adsorbed carbocation; isomerisation of the oligomers; cracking of the intermediate carbenium ions. From the relatively high selectivity to propene, butenes and pentenes we can see that the oligomerisation is only likely to proceed up to a certain extent. For instances, if we consider the transformation of ethene, we will have a stepwise production of butene, then hexene, then octene, and so forth. However, as the number of carbon atoms in the oligomer increases, the ease of isomerisation and of cracking by -scission
increases, partially because the ease of cracking increases with the increase of the degree of branching [41±43]. For oligomers with higher number of carbon atoms, the cracking steps are faster than the oligomerisation ones, so that very little of heavier products are usually observed. Several other authors have corroborated this; for example, a similar behaviour has been observed recently by Guisnet et al. [39] during skeletal isomerisation of n-butenes over a H-FER zeolite. It must be emphasised that most likely the C8 and C9 carbenium ions are the real intermediates in the formation of ®nal reaction products. It has also been shown that the rate of isomerisation of C5±C8 alkenes was faster than the rate of their cracking over H-ZSM-zeolite [43]. Indeed, the formation of isomers in relative abundance prior to appreciable cracking taking place when the feed is an alkene indicates that an alkene typically undergoes numerous chemisorption and desorption steps before cracking [43]. It is also clear that the main products (propene, butenes and pentenes) of light alkene transformation can be formed by various pathways. Propene and pentenes can result, for instances, from scission of trimethylpentane, from dimethylpentane and from methylhexane, the same being true for butene. The most-favourable reactions are the ones that involve the conversion of a secondary carbocation into a tertiary and from a tertiary to a secondary one [43]; scission reactions involving other carbocations tend to be much slower [39]. The formation of tribranched isomers can occur at speci®c sites but its net production will be hindered by transport limitations of these species in ZSM-5 channels [43]. 4. Conclusions From experimental results obtained over templated (CVN) and non-templated (CVM) ZSM-5 zeolites the following conclusions can be drawn: 1. The initial activity values obtained for the C2H4 transformation were signi®cantly higher with HCVM than with H-CVN zeolite, in accordance with the higher acidity of its sites as observed by TPD of ammonia. In contrast, for the C3H6 and i-C4H8 transformation the opposite order was observed: the more acidic catalyst (H-CVM) presents a lower activity.
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2. The formation of the main reaction products (C3±C5 alkenes) can be rationalised by the accepted mechanism involving oligomerisation steps followed by isomerisation of these oligomers and cracking. The oligomerisation steps are slower than the isomerisation and cracking steps for species with more than eight carbon atoms. 3. For the C3H6 and i-C4H8 transformation the selectivity to C4±C6 alkanes and aromatics were significantly higher with H-CVM than with H-CVN zeolite; hydrogen transfer processes are more effective in H-CVM sample which presents the highest number and strength of the acid sites. 4. The shape selectivity of ZSM-5 is probably responsible for the inhibition of the diffusion of multibranched oligomers in the narrow pore of zeolite and limits the formation of propene and pentenes from isobutene. Acknowledgements Financial support by the Junta National de Investigac,aÄo Cientõ®ca e TecnoloÂgica (JNICT) of Portugal (Project PBIC/C/QUI/2378/95) is gratefully acknowledged. IPD thanks NATO Scienti®c Affairs Division and JNICT for a postdoctoral fellowship and the State University ``Lvivska Polytechnica'' for a leave of absence during this period. The authors also thank Prof. JoaÄo Rocha from Universidade de Aveiro (Portugal) for collecting MAS NMR spectra. References [1] P.A. Jacobs, J.A. Martens, Stud. Surf. Sci. Catal. 33 (1987) 113. [2] V.P. Shiralkar, A. Clearfield, Zeolites 9 (1989) 363. [3] G. Bellussi, G. Perego, A. Carati, U. Cornaro, V. Fattore, Stud. Surf. Sci. Catal. 37 (1988) 37. [4] E. Narita, K. Sato, T. Okabe, Chem. Lett. (1984) 1055. [5] F.R. Ribeiro, F. Alvarez, C. Henriques, F. Lemos, J.M. Lopes, M.F. Ribeiro, J. Mol. Catal. A 96 (1995) 245. [6] A. Auroux, Topics Catal. 4 (1997) 71. [7] S. Narayanan, A. Sultana, P. MeÂriaudeau, C. Naccache, Catal. Lett. 34 (1995) 129. [8] S. Narayanan, A. Sultana, P. MeÂriaudeau, C. Naccache, A. Auroux, C. Viornery, Appl. Catal. A 143 (1996) 337. [9] A. Tissler, P. Polanek, V. Girrbach, V. MuÈller, K.K. Unger, Stud. Surf. Sci. Catal. 46 (1989) 399.
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[10] P.A. Jacobs, J.A. Martens, Stud. Surf. Sci. Catal. 33 (1987) 134. [11] P. Dejaifve, J.C. VeÂdrine, V. Bolis, E.G. Derouane, J. Catal. 63 (1980) 331. [12] J.C. VeÂdrine, P. Dejaifve, E.D. Garbowski, E.G. Derouane, Stud. Surf. Sci. Catal. 5 (1980) 29. [13] R.M. Dessau, R.B. LaPierre, J. Catal. 78 (1982) 136. [14] V.S. Nayak, V.R. Choudary, Appl. Catal. 9 (1984) 251. [15] R.J. Quann, L.A. Green, S.A. Tabak, F.J. Krambeck, Ind. Eng. Chem. Res. 27 (1988) 565. [16] W.O. Haag, R.M. Lago, P.B. Weisz, J. Chem. Soc., Faraday Discuss 72 (1982) 317. [17] W.O. Haag, R.M. Lago, P.G. Rodewald, J. Mol. Catal. 17 (1982) 161. [18] K.G. Wilshier, P. Smart, R. Western, T. Mole, T. Behrsing, Appl. Catal. 31 (1987) 339. [19] S.J. Miller, Stud. Surf. Sci. Catal. 38 (1987) 187. [20] S. Bessell, D. Seddon, J. Catal. 105 (1987) 270. [21] J.M. Baker, S. Bessell, D. Seddon, Appl. Catal. 45 (1989) L1. [22] M.D. Ralph, N.J. Edison, US Patent 5 026 919 (1991), to Mobil Oil Corporation. [23] M. Huang, S. Kaliaguine, J. Mol. Catal. 81 (1993) 37. [24] M. Guisnet, N.S. Gnep, D. Aittaleb, Y.J. Doyemet, Appl. Catal. A 87 (1992) 255. [25] D.B. Lukyanov, N.S. Gnep, M.R. Guisnet, Ind. Eng. Chem. Res. 33 (1994) 223. [26] E.D. Radchenko, Khimia tverdogo topliva, (1982) 51 (in Russian). [27] N.E. Gorbatkina, L.D. Konovalchikov, B.K. Nefedov, B.L. Khusid, Khimia i technologia topliv i masel, (1989) 5 (in Russian). [28] N.-J. Topsùe, K. Pedersen, E.G. Derouane, J. Catal. 70 (1981) 41. [29] G. Lietz, K.H. Schnabel, Ch. Peuker, Th. Gross, W. Storek, J. VoÈlter, J. Catal. 148 (1994) 562. [30] V.P. Shiralkar, P.N. Joshi, M.J. Eapen, B.S. Rao, Zeolites 11 (1991) 511. [31] F. Yanez, J. Papa, G. Giannetto, M. Lopez, B. Mendez, F. Galarraga, in: Actas de XV Simp. Iberoamer. de Catal., CoÂrdoba (Argentina), 1996, p. 989. [32] G. Giannetto, S. Sansare, M. Guisnet, J. Chem. Soc., Chem. Commun. (1986) 1302. [33] T. Baba, Y. Inoue, Y. Ono, J. Catal. 159 (1996) 230. [34] D. Barthomeuf, Catal. Rev.-Sci. Eng. 38(4) (1996) 521. [35] M.A. Asensi, A. Corma, A. MartõÂnez, J. Catal. 158 (1996) 561. [36] W.-Q. Xu, S.L. Suib, Ch.L. O'Young, J. Catal. 150 (1994) 34. [37] V.S. Nayak, J.B. Moffat, Appl. Catal. 60 (1990) 87. [38] M. Guisnet, N.S. Gnep, Appl. Catal. A 146 (1996) 33. [39] M. Guisnet, P. Andy, N.S. Gnep, E. Benazzi, C. Travers, J. Catal. 158 (1996) 551. [40] G. Giannetto, R. Monque, R. Galiasso, Catal. Rev.-Sci. Eng. 36(2) (1994) 271. [41] J. Weitkamp, P.A. Jacobs, J.A. Martens, Appl. Catal. 8 (1983) 123. [42] J. Abbot, B.W. Wojciechowski, Can. J. Chem. Eng. 63 (1985) 462. [43] J.S. Buchanan, J.G. Santiesteban, W.O. Haag, J. Catal. 158 (1996) 279.
Transformation of light alkenes over templated and non-templated ZSM-5 zeolites I.P. Dzikha,b, J.M. Lopesa, F. Lemosa, F. RamoÃa Ribeiroa,* a
b
Centro de Engenharia BioloÂgica e QuõÂmica, Instituto Superior TeÂcnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal Petroleum Re®nery Engineering Department, State University ``Lvivska Polytechnica'', Bandera St., 12, 290646 Lviv-13, Ukraine Received 27 March 1998; received in revised form 3 August 1998; accepted 5 August 1998
Abstract The transformations of ethene, propene and i-butene over templated (CVN) and non-templated (CVM) ZSM-5 zeolites at 3508C were examined. The initial activity values obtained for the conversion of C2H4 were signi®cantly higher with H-CVM than with H-CVN zeolite, in accordance with the stronger acidity of its sites. In contrast, for the transformation of C3H6 and iC4H8 the opposite order was observed: the more acidic catalyst presents a lower activity. It was also observed that, in the case of CVM, the sodium-exchanged CVM samples have higher activities than the H form. These observations can be related to the extremely high acidity of the CVM samples. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Ethene; Propene; i-Butene; Templated and non-templated ZSM-5; Acidity; Catalytic activity
1. Introduction MFI type zeolite (ZSM-5) is conventionally prepared by the hydrothermal crystallisation of aluminosilicate gel using various kinds of organic bases as templating agents (for example, tetrapropyl ammonium bromide, 1,6-diaminohexane, 1,3-diaminopropane, etc.) [1]. However, the template is corrosive, expensive, usually unrecoverable, and requires high temperature treatments for its decomposition and removal from the zeolite. ZSM-5 zeolite has also been synthesised in the absence of an organic template, that is, by employing ammonia as the templating agent [2± 4]. In this case, the production of the protonic form of the zeolite involves only a simple calcination. *Corresponding author. Fax: +351-1841-7246.
Acidity is one of the most important characteristics of zeolites, and the one that makes them extremely important materials in catalytic applications. The acidity of zeolites is known to depend on several factors: structure, preparation method, chemical composition, impurities, Si/Al ratio, additives and poisons. A lot of attention has been given to the relationship between the acidity of zeolites and their catalytic activity and review papers by RamoÃa Ribeiro et al. [5] and Auroux [6] have been recently published on this subject. ZSM-5 samples, synthesised with and without template, present very different acidity and consequently different catalytic properties [7,8]. However, for nontemplated ZSM-5 zeolite this relationship has been insuf®ciently examined [9,10]. The transformation of light alkenes over ZSM-5 catalysts is important in the various petrochemical
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00272-5
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processes such as MOGD, MTG, Cyclar, etc., and it is also a good test reaction to probe the acidity of these materials. Thus, it is of considerable interest to compare the catalytic behaviours of templated and nontemplated ZSM-5 zeolites for the transformation of light alkenes. Light alkene transformation within the temperature range from 2008C to 5008C, over ZSM-5 zeolites, has been the subject of numerous studies [11±25], especially in connection with the conversion of methanol to gasoline and, recently, with the production of aromatics by light alkane aromatisation. However, in these works authors used mainly templated ZSM5 catalysts. The aim of the present work is to study the in¯uence of zeolite preparation method on the total acidity of samples, on the activity and on the product distribution of the transformation of ethene, propene and i-butene over templated and non-templated ZSM-5 zeolites. 2. Experimental 2.1. Catalyst preparation The starting forms of the templated (CVN) and nontemplated (CVM) ZSM-5 zeolites, in ammonium form, were obtained from Angarsk Oil±Chemical Company (Russia). Details of the preparation of NH4±ZSM-5 zeolites have been previously described [26,27]. The acidic forms of ZSM-5 were prepared by the procedures described below. CVN sample. The template cations were decomposed at 5508C under a ¯ow of dry nitrogen during 3 h, after which a calcination under a ¯ow of dry air for 8 h was performed. CVM sample. The samples were calcined at 5008C for 8 h under a ¯ow of dry air. Departing from these acidic forms of CVM and CVN, several samples with different sodium contents were prepared by successive ion exchanges with 1 M sodium nitrate solutions. Two exchanges were performed at room temperature for 4 h. The corresponding acidic forms, HNa-ZSM-5, were obtained by calcination at 5008C for 8 h under a ¯ow of dry air.
2.2. Catalyst characterisation Characterisation studies were performed using elemental chemical analysis, X-ray diffraction (XRD) in a Rigaku diffractometer with Cu K radiation, temperature programmed desorption (TPD) of NH3, scanning electron microscopy (SEM) with a JEOL JSM840 instrument. Solid state MAS NMR measurements (for 29 Si and 27 Al) were also carried out, using a Bruker MSL 300 spectrometer. The H-ZSM-5 (CVM and CVN) zeolites were analysed for phase purity by using an X-ray powder diffractometer. The investigation of the acidity of the zeolite catalysts was performed by TPD of ammonia. These experiments were carried out after activation of a 0.2 g sample in a Pyrex reactor for 7 h at 5008C in dry nitrogen ¯ow. The sample was then cooled to 908C and ammonia adsorption was performed by feeding pulses of reactant grade NH3 to the reactor. The sample was then ¯ushed with helium at 908C for 2 h to remove excess ammonia and decrease the amount of ammonia that was physically adsorbed on the samples. The ammonia was then thermally desorbed with a heating rate of 108C/min in a helium ¯ow (60 ml/min) from 908C to 6008C. The amount of NH3 desorbed was measured by a TCD detector. The electrical signals from the detector and the thermocouple, which measures the temperature inside the cell containing the catalyst, were digitised by a CR3A integrator and sent to a computer. The morphology of the zeolite catalysts was investigated by means of SEM. A silicon to aluminium ratio was determined by 27 Al and 29 Si MAS NMR. 2.3. Catalyst testing The transformation of pure ethene, propene and ibutene over the catalysts of the two series HNa-ZSM-5 (CVM and CVN) was carried out in a continuous ¯ow ®xed-bed reactor at 3508C, at total pressure of 1 bar with a nitrogen-to-hydrocarbon molar ratio equal to 9. The reactant ¯ow rate was kept at 500 ml (STP)/h and the weight of catalyst that was used was 150 mg for CVM and 20 mg for CVN. The reactor ef¯uent products were analysed by on-line gas chromatography using a PLOT Al2O3±KCl capillary column with a FID detector.
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Table 1 Chemical composition of HNa±ZSM-5 samples (for CVM±Si/ Al26 and for CVN±Si/Al42) Sample
Na level (% of total cationic positions)
H-CVM HNa-CVM (62) HNa-CVM (89) H-CVN HNa-CVN (60) HNa-CVN (72)
<5 62 89 <6 60 72
Prior to the catalytic transformation of the reactants, the catalysts were pre-treated in situ at 4508C for 12 h under a ¯ow of dry nitrogen (60 ml/min). 3. Results and discussion 3.1. Physico-chemical characteristics of ZSM-5 catalysts The chemical composition of the two H-ZSM-5 and their sodium-exchanged ZSM-5 samples are listed in Table 1. Both series of samples show good crystallinity and the XRD patterns agree very well with those reported in the literature for ZSM-5. The Si/Al ratio, as estimated from MAS NMR was found to be around 26 for CVM and 42 for CVN zeolites (see caption in Table 1), i.e., Al concentration in two samples differs by approximately 1.6 times. The analysis of 27 Al MAS NMR spectra shows a singlet around 111 ppm for H-CVM sample and 112 ppm for H-CVN sample, indicating that the aluminium atoms are in tetrahedral coordination. These spectra have also shown that there is no signi®cant non-framework aluminium in any of these zeolites. The TPD of ammonia thermograms, as recorded for the two series of catalysts, the templated and the nontemplated ones, are shown in Fig. 1. The TPD curves were very similar to those usually found for ZSM-5 zeolite [28]. The peak maxima in the ammonia evolution pro®les were observed at 1658C and 3008C for HCVN sample and at 2008C and 3508C for H-CVM sample. So, on the basis of the recorded TPD spectra
Fig. 1. TPD spectra of NH3 for CVM and CVN samples.
of ammonia, it can be seen that H-CVM has a higher acid strength, as well as a larger amount of acid sites, than H-CVN zeolite. It is generally accepted that acid site concentration is proportional to the number of Al atoms in the elementary lattice [6] and that, for zeolites with a high silica content, site strength does not depend signi®cantly on the Si/Al ratio. This is consistent with the difference in the number of acid sites between H-CVM and H-CVN but the difference in acid site strength is certainly related to other aspects. The comparison of the TPD curves of all samples indicates that the high temperature peak may correspond to the desorption of ammonia from structural OH groups of the ZSM-5. It was also observed that the sodium-exchanged CVM and CVN catalysts exhibited a lower acidity than their parent zeolite samples, as it would be expected from the replacement of some of the protons in the zeolite by sodium cations [29]. The SEM micrographs of these zeolite catalysts show the typical crystal size and form of the ZSM-5 zeolite. The CVM sample, prepared without template, appears to have smaller particles (0.5±1 mm) than the CVN sample, which was prepared with template, and that consists of larger and elongated particles (3±4 mm). This difference in crystallite size can account for some of the differences that were observed in the TPD thermogram, since it is generally expected that small crystallites may show a higher apparent acidity (mainly due to larger accessibility of the acid sites) than large crystallites, for similar Si/Al ratios [30].
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3.2. Catalytic transformation of ethene, propene and i-butene The activity and selectivity of all catalysts were studied in the conversion of ethene, propene and i-butene at 3508C. From these alkenes the main reaction products that are observed are alkenes in the range of C3±C5. The distribution of the products remained practically unchanged with time-on-stream for all the samples that were inspected. For the duration of the runs, approximately 60 min, no signi®cant deactivation was observed for any of the samples. The spent samples were only light grey, not black, indicating only a moderate deposition of coke, in accordance with the absence of deactivation during the run. 3.2.1. Activity Fig. 2 shows the catalytic activities versus time-onstream for the C2H4, C3H6 and i-C4H8 conversion over CVM, CVN and their sodium-exchanged samples. It can be seen that the initial activity values obtained for the ethene transformation were higher with H-CVM than with H-CVN zeolite, in accordance with the higher acidity of its sites, as measured by thermal desorption of ammonia. In contrast, for the C3H6 and i-C4H8 transformation the opposite order was observed: the more acidic catalyst presents a lower activity. The results obtained for the transformation of ethene are consistent with the higher acidity that was observed for the H-CVM zeolite. In fact, the total number of acid sites, as well as the strength of these sites, as measured by the temperature programmed desorption of ammonia, is far larger in H-CVM samples than on H-CVN, and the rate for the transformation of ethene is around 6 times larger in H-CVM than in H-CVN. Also, the site density seems to favour the higher activity of the CVM sample. In fact, in a ®rst approximation we could expect a higher site density in case of CVM zeolite (also due to its lower Si/Al ratio). Observations corroborating the role of acid site density on the activity have been noted earlier by Giannetto, who has shown that it in¯uenced the catalytic activity of H-[Al]±ZSM-5 catalyst for propene oligomerisation [31] and also on the mode of alkane cracking [32].
For the transformation of ethene, as expected, when protons are removed and exchanged with sodium cations, the activity decreases, due to the removal of a certain number of acid sites, usually the most acidic ones. Similarly, Ono and coworkers [33] found that a small amount of alkali cations in H-ZSM-5 greatly reduced the catalytic activity for the acidcatalysed reactions of cyclopropane isomerisation and hexane cracking. The behaviour however, is quite different when we look at the conversion of propene. In this case, not only is the activity order reversed, H-CVN is about 6 times more active than H-CVM, but also, when sodium is removed from these catalysts, the activity increases for the CVM samples. It must be noted that propene, being larger than ethene, produces carbocations which are much more stable than the latter, namely because it can easily produce a secondary carbocation. A positive contribution of Lewis acid±base pairs [34] (sodium-cation and the neighbouring basic oxygen) in the conversion of propene and isobutene over these catalysts can also be considered. Recently it was found that the propene aromatisation over alkaliexchanged ZSM-5 zeolites occur on the strong Lewis acid sites and weak Lewis basic sites [23]. Similarly, these have been regarded by Huang and Kaliaguine [23] as acid±base pairs of alkali-cation and neighbouring basic oxygen. This could help to explain the increase in activity when sodium is introduced, but would not explain why H-CVM shows a lower activity than H-CVN and, in any case, should represent a minor contribution. It is generally accepted that active sites which possess an overly strong interaction with the reactants and/or products, generates very little catalytic activity and that optimum catalytic sites should have a ``moderate'' interaction with both reactants and products. We have observed that H-CVM samples possess acid sites with very strong acidity, as con®rmed both by the temperature programmed desorption of ammonia and by the extremely higher rates of transformation of ethene, which is a molecule which is very dif®cult to protonate due to its need to produce primary carbocations. Sabatier's rule would not only explain the relative activity order between H-CVM and H-CVN samples, but also why the activity for the CVM series increases when the proton content is
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Fig. 2. Catalytic activities versus time-on-stream for the transformation of C2H4 (a), C3H6 (b) and i-C4H8 (c) over CVM and CVN catalysts.
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decreased. The partial titration of acid sites in CVM with sodium cations would possibly remove the stronger acid sites, the ones that would irreversibly adsorb propene, leaving sites with moderate activity and that would, thus, be more effective for the transformation. From Fig. 2 we can also see that the behaviour in relation to isobutene transformation follows similar lines to the one of propene. Isobutene is much easier to protonate than propene, because it readily produces a tertiary carbocation. This usually implies that its rate of transformation is higher than that of propene. In fact, for CVN samples the activity for the i-C4H8 conversion is higher than for C3H6, which is in accordance with the expectations. It should be noted, that the observed increase in reactivity could even be under-rated in relation to what would be expected, due to stereochemical aspects. In the case of i-C4H8 the shape selective effect of the ZSM-5 [35] will make it dif®cult to form dibranched and other rami®ed isomers. ZSM-5 zeolite has a bi-directional system of intersecting 10 MR channels between 5.35.6 and Ê diameter and formation of branched 5.15.5 A C10±C12 oligomers inside its channels also should be strongly hindered. However, for the isobutene transformation over HCVM zeolite the observations are similar to the ones made for propene and, in fact, the rate of conversion of isobutene is lower than that of propene, whilst one would expect that the small crystal size of CVM zeolite would reduce the shape selective effects and should, consequently, increase of the activity for i-C4H8 conversion. Sabatier's rule, however, is in accordance with all these observations. CVN, possessing moderate acid sites shows an increase in activity when going from propene to isobutene, whilst CVM samples, possessing overly strong acid sites, produces the reverse effect, all due to the higher stability of the tertiary carbocation. As for propene, the replacement of acid sites by sodium cations induces an increase in activity for CVM samples and a decrease for CVN samples. 3.2.2. Selectivity From C3H6 (Fig. 3) the main reaction products are butenes and pentenes. Ethene, C3±C6 alkanes, hexenes and aromatics (BTX) are observed in lower amounts. From ethene and isobutene (Figs. 4 and 5) the main
products are propene, butenes and pentenes; C3±C7 alkanes, hexenes, heptenes, aliphatic hydrocarbons C8 and aromatics are also observed. For all catalysts the trans-2-butene and cis-2-butene are formed in approximately equimolar proportions. Methane, ethane and aromatics are obtained in minor quantities in all cases. The concentration of C8 products was relatively low. No aliphatic hydrocarbons higher than C8 are observed in the reaction products. The observed values for the transformation of C2H4 over CVN catalysts did not differ from the values obtained for its thermal conversion in blank experiments using either an empty reactor or a reactor with an inactive material (like silica). It must be emphasised that for isobutene transformation the propene/pentenes molar ratio is smaller than 1. In contrast, for ethene transformation this ratio is greater than 1. This could be related to the fact that some of the pentenes and propene are also formed by cracking reactions of intermediate species other than C8 carbenium ions. The formation of products with 6 and 7 carbon atoms may be explained if we consider the recombination of C3 and C4 fragments. Similar observations and conclusions have been reported by Xu et al. [36] in order to explain the formation of hexenes and heptenes during the isomerisation of n-butenes over ZSM-23 zeolite. The reactions of C4 and C5 intermediate species with propene will lead to the formation of C7 and C8 hydrocarbons that will ®nally produce toluene and xylenes by dehydrocyclisation and dehydrogenation reactions. The absence of aliphatic hydrocarbons with more than nine atoms of carbon and the relatively low concentration of octenes in the reaction products can be related to the high cracking activity of the strong acid sites of ZSM-5 towards the transformation of these species and also to the strong limitations that they encounter when diffusing through the 10-membered ring windows. The formation of aromatics probably proceeds by several successive steps: dehydrogenation of C6±C8 oligomers to dienes, diene cyclisation with formation of cyclic dialkenes and aromatics via hydrogen transfer on the acid zeolite sites, according with a kinetic model for C2H4 and C3H6 aromatisation, recently reported by Lukyanov et al. [25]. It must be underlined that for propene and ethene transformation over CVM catalysts the selectivity
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Fig. 3. Selectivity plots for C3H6 transformation over CVM and CVN catalysts for different time-on-stream: (a), (b) 6 min, (c), (d) 24 min and (e), (f) 60 min. C2ethylene, C3 propane, C4 paraffins, C4olefins, C5 paraffins, C5olefins, C6 paraffin, C6olefins, and C7 aliphatics and aromatics.
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Fig. 4. Selectivity plots for i-C4H8 transformation over CVM and CVN catalysts for different time-on-stream: (a), (b) 6 min, (c), (d) 24 min and (e), (f) 60 min. C3 propane, C3propylene, C4 paraffins, C4olefins, C5 paraffins, C5olefins, C6 paraffin, C6olefins, C7 paraffins, C7olefins, and C8 aliphatics and aromatics.
to butenes and pentenes are lower than those over CVN. The selectivity towards C3±C6 alkanes and aromatics were signi®cantly higher with CVM than with CVN samples. In addition, the alkene/alkane ratio is
slightly greater with CVN than with CVM catalysts. These facts indicated that CVM zeolite, which presents the highest acidity and site density, favours hydride transfer reactions between carbenium ions and alkene molecules. On the other hand, for the
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Fig. 5. Selectivity plots for C2H4 transformation over CVM catalysts for different time-on-stream: (a), (b) 6 min, (c), (d) 24 min and (e), (f) 60 min. C1 methane, C2 ethane, C3 propane, C3propylene, C4 paraffins, C4olefins, C5 paraffins, C5olefins, C6 paraffin, C6olefins, and C6(C6C7 aliphatics) and aromatics.
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two series of catalysts, HNa-CVM and HNa-CVN the increase of sodium level leads to the relative increase of selectivity towards alkenes and a corresponding decrease of selectivity to alkanes, as it can be seen in Figs. 3±5. This reinforces the data previously discussed which indicates that H-CVM is, indeed, much more acidic and possesses a much higher site density than H-CVN and, thus, favours hydride transfer. Nayak and Moffat [37] cracked C6±C8 alkenes over ZSM-5 zeolites prepared with tetrapropyl ammonium bromide and ammonia as template, and found that the activity and selectivity are similar for the two templating agents employed in the preparation of catalyst. It must be emphasised that the authors used the samples with little difference of total acidity and other properties. Obviously, it is the reason of absence of signi®cant difference between two zeolites concerning their activity and selectivity. 3.3. Reaction scheme considerations From the experimental results, and also based on the data published in the literature, it is possible to make some considerations regarding the reaction scheme for the transformation of C2±C4 light alkenes. The ®rst step involved in all these transformations is the protonation of the ole®ns. In the initial moments this will be done by direct interaction between an ole®n molecule and an acid site, but for longer times on stream it is likely that there can be direct interactions between a gas-phase ole®n molecule and an adsorbed carbocation. Unlike the case of alkane transformation, the production of adsorbed carbocations from ole®ns is relatively easy.The main steps that can be considered afterwards are [25,38±40]: oligomerisation, or alkylation of one gas-phase alkene molecule by an adsorbed carbocation; isomerisation of the oligomers; cracking of the intermediate carbenium ions. From the relatively high selectivity to propene, butenes and pentenes we can see that the oligomerisation is only likely to proceed up to a certain extent. For instances, if we consider the transformation of ethene, we will have a stepwise production of butene, then hexene, then octene, and so forth. However, as the number of carbon atoms in the oligomer increases, the ease of isomerisation and of cracking by -scission
increases, partially because the ease of cracking increases with the increase of the degree of branching [41±43]. For oligomers with higher number of carbon atoms, the cracking steps are faster than the oligomerisation ones, so that very little of heavier products are usually observed. Several other authors have corroborated this; for example, a similar behaviour has been observed recently by Guisnet et al. [39] during skeletal isomerisation of n-butenes over a H-FER zeolite. It must be emphasised that most likely the C8 and C9 carbenium ions are the real intermediates in the formation of ®nal reaction products. It has also been shown that the rate of isomerisation of C5±C8 alkenes was faster than the rate of their cracking over H-ZSM-zeolite [43]. Indeed, the formation of isomers in relative abundance prior to appreciable cracking taking place when the feed is an alkene indicates that an alkene typically undergoes numerous chemisorption and desorption steps before cracking [43]. It is also clear that the main products (propene, butenes and pentenes) of light alkene transformation can be formed by various pathways. Propene and pentenes can result, for instances, from scission of trimethylpentane, from dimethylpentane and from methylhexane, the same being true for butene. The most-favourable reactions are the ones that involve the conversion of a secondary carbocation into a tertiary and from a tertiary to a secondary one [43]; scission reactions involving other carbocations tend to be much slower [39]. The formation of tribranched isomers can occur at speci®c sites but its net production will be hindered by transport limitations of these species in ZSM-5 channels [43]. 4. Conclusions From experimental results obtained over templated (CVN) and non-templated (CVM) ZSM-5 zeolites the following conclusions can be drawn: 1. The initial activity values obtained for the C2H4 transformation were signi®cantly higher with HCVM than with H-CVN zeolite, in accordance with the higher acidity of its sites as observed by TPD of ammonia. In contrast, for the C3H6 and i-C4H8 transformation the opposite order was observed: the more acidic catalyst (H-CVM) presents a lower activity.
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2. The formation of the main reaction products (C3±C5 alkenes) can be rationalised by the accepted mechanism involving oligomerisation steps followed by isomerisation of these oligomers and cracking. The oligomerisation steps are slower than the isomerisation and cracking steps for species with more than eight carbon atoms. 3. For the C3H6 and i-C4H8 transformation the selectivity to C4±C6 alkanes and aromatics were significantly higher with H-CVM than with H-CVN zeolite; hydrogen transfer processes are more effective in H-CVM sample which presents the highest number and strength of the acid sites. 4. The shape selectivity of ZSM-5 is probably responsible for the inhibition of the diffusion of multibranched oligomers in the narrow pore of zeolite and limits the formation of propene and pentenes from isobutene. Acknowledgements Financial support by the Junta National de Investigac,aÄo Cientõ®ca e TecnoloÂgica (JNICT) of Portugal (Project PBIC/C/QUI/2378/95) is gratefully acknowledged. IPD thanks NATO Scienti®c Affairs Division and JNICT for a postdoctoral fellowship and the State University ``Lvivska Polytechnica'' for a leave of absence during this period. The authors also thank Prof. JoaÄo Rocha from Universidade de Aveiro (Portugal) for collecting MAS NMR spectra. References [1] P.A. Jacobs, J.A. Martens, Stud. Surf. Sci. Catal. 33 (1987) 113. [2] V.P. Shiralkar, A. Clearfield, Zeolites 9 (1989) 363. [3] G. Bellussi, G. Perego, A. Carati, U. Cornaro, V. Fattore, Stud. Surf. Sci. Catal. 37 (1988) 37. [4] E. Narita, K. Sato, T. Okabe, Chem. Lett. (1984) 1055. [5] F.R. Ribeiro, F. Alvarez, C. Henriques, F. Lemos, J.M. Lopes, M.F. Ribeiro, J. Mol. Catal. A 96 (1995) 245. [6] A. Auroux, Topics Catal. 4 (1997) 71. [7] S. Narayanan, A. Sultana, P. MeÂriaudeau, C. Naccache, Catal. Lett. 34 (1995) 129. [8] S. Narayanan, A. Sultana, P. MeÂriaudeau, C. Naccache, A. Auroux, C. Viornery, Appl. Catal. A 143 (1996) 337. [9] A. Tissler, P. Polanek, V. Girrbach, V. MuÈller, K.K. Unger, Stud. Surf. Sci. Catal. 46 (1989) 399.
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