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Ethylene polymerisation catalyzed by caesium in zeolite NaY
571
Surface Science 203 (1988) 571-586 North-Holland, Amsterdam
ETHYLENE POLY~~SA~ON IN ZEOLITE NaY Fritz BLATTER
CATALYZED
BY CAIESIUM
and Ernst SCHUMACHER
Institute for Inorganic, Analytical and Physical Chemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Received 7 January 1988; accepted for publication 18 May 1988
Alkali metals adsorbed on dehydrated zeolite NaY or a-Al,O, produce well known chromophores which are parama~etic and similar to F-centres. Caesium and ~bidium sorbates in zeolite NaY are good catalysts for ethylene pol~e~sation at 50 bar. High molecular straight chain paraffins result which completely wrap the microcrystals. Polyethylene is extracted from the “composite” by acid digestion of the aluminosilicate and characterized by FTIR spectroscopy, X-ray diffraction, and stating electron microscopy. A negative polyethylene replica of the zeolite crystals is observed in the SEM. The chemical properties of dispersed caesium are compared to those of the bulk which produces only monoolefinic oligomers. Na, K, and Rb are compared to Cs, propene and butadiene to ethene, and zeolites to OL-A~~O~.Plausible mechanisms and applications are discussed.
In recent years the chemical and catalytical reactivities of bare metal clusters have attracted much scientific [l-3] and applied interest [4-81. The paramount question is whether (and how) metal particles as a function of diminishing size begin to exhibit reactive properties distinctly different from bulk behaviour. Earlier studies with supported Pt-particles on non-metallic supports (aluminosilicates) showed a l/r (r = particle radius) dependence of specific catalytic activity which leveled off when “‘atomic” dispersion was reached 191. This is to be expected if no size dependence exists and solely the availability of the catalyst atoms for the reaction is responsible for the size effect. However, more recent experiments with much smaller clusters in the gas phase show dramatically selective effects: One atom more or less e.g. with Fe,/Fe,, or Fe&Fe,, causes a change in the rate of hydrogen sorption by more than an order of magnitude [lo-121. These rate changes are pertinent to heterogeneous catalytic activity since adsorption is a prerequisite for it. Similar observations with supported catalysts have not yet appeared. Metal clusters on supports usually have broad, ill defined size distributions which might mask specific activities of certain size ranges 1131. Moreover, the cluster support 0039-6028/88/$03.~0 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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/ Cs catalyzed ethylene polymerisation
interaction may cause insensitivity to the very question asked. Synthesis or isolation of nearly monodisperse metal clusters of various sizes and their study on specific surfaces will be essential to the clarification of the issue. Meanwhile, metal clusters in zeolite channels offer a well reproducible reactive system [14]. The structured and confined space for the deposition or in situ generation of metal clusters produces a much narrower size dist~bution than usually encountered on more open supports like alumina, silica or carbon molecular sieves [15]. It is common to add precursors for metal particle formation by salt impregnation or by ion exchange [16]. Reduction or pyrolysis in situ to form the metallic state is usually performed at temperatures ranging from 250 to 45O’C. This may create migration and sintering which can lead to the formation of larger clusters with 100 to 1000 atoms at the surface. Therefore, quantitative information at small cluster sizes is still sparse or lacking. The present paper shows size and reactivity dependence and perhaps size-support interaction effects during the intracrystal polyme~sation of ethylene initiated by the heavy alkali metals. It also offers a methodology for protecting chemically highly reactive sorbed species in zeolites from atmospheric corrosion. Finally, the produced completely intertwined inorganic porous system with organic polymer chains is an interesting model for studying composites with its own set of applications.
2. Alkali metal dusters
in zeolites
We use the synthetic zeolites NaY and NaX [17-201. The commercial sources (Strem Chemicals, Union Carbide) and the samples synthesized from analytically pure chemicals in our laboratory [21] give identical results. Differences are only essential for ESR spectroscopy which is performed with our chemically pure and crystallographically uniform zeolites. Alkali metal clusters are prepared by adsorption of metal vapour on dehydrated zeolite powder under high vacuum and thermal equilibrium. Coioured samples result as soon as the metal is admitted. These chromophores were first observed by Rabo et al. [22]. They attributed the red colour of Na in NaY to Na:+ , and the blue colour of the same metal in zeolite NaX to Nazt particles. This assignment was derived from ESR measurements and later verified by other authors [23-281. In the case of zeolite NaY at least four different experiments seem to lead to the same 13-line ESR spectrum attributed to Nay : - adsorption of very small quantities of metal vapour; - irradiation of the pure zeolite with y- or X-rays; - inducing a tesla coil gas discharge near the pure zeolite in the presence of small quantities of hydrogen (- 0.1 Torr);
F. Blatter, E. Schumacher
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/ Cs catalyzed ethylene polymerisation
- thermal decomposition of included NaN, [27]. ESR-measurements of zeolite NaX as a function of Na content, producing colours between blue and black, exhibit first the Na:+ spectra which then become more complex and finally show conduction electron signals [25,28]. These results from ESR and ENDOR investigations will be published elsewhere [28]. It is, of course, highly probable that the 19-line feature belongs to lower charged odd electron species like Nai+ and even Nai rather than to Nai+ [22] in the heavily metal doped zeolites. Similarly Na: is much more likely than Nay in NaY with a high content of sorbed sodium. In addition the state of the sorbed metal may include neutral clusters which do not announce themselves with ESR signals, however. These chromophores are extremely sensitive to air and lose their colour in seconds if they are exposed to an atmosphere containing only traces of oxygen in argon forming alkali metal superoxides [22]. The optical reflection spectrum (fig. 1) and the ESR signals are very similar to F-centres [29]. The ESR spectrum of a trapped electron in an H--vacancy of crystalline sodium hydride shows 19 hyperfine lines (6 nearest neighbours with I = 3/2) and almost the same coupling constant (26 gauss) as observed in the zeolites NaY and NaX with sorbed Na [30]. The colours for the different alkali metals at comparable molar concentration in zeolites are characteristic for the metal, and the zeolite, and the exchangeable cations (if they are from another chemical element than the sorbed metal). Quantum chemical calculations with the extended Hiickel model have led to detailed information about the interaction of Ag, and Cu atoms as well as Ag+ and Cu+ ion clusters with the zeolite cavities of the 4/4, 6/6 subunits,
500
1000 WAVELENGH
Fig. 1. UV-VIS
spectrum caesium
1500
2000
[nm]
of CsIO [Nay]. The chemical formula means that atoms per elementary cell of the zeolite are adsorbed.
an average
of 10
574
F. Blatter, E. Schumacher
/ Cs catalyzed ethylene polymerisation
and of the P-cage [31]. The same methodology (full charge iteration) applied to alkali atoms in 4/4 and 6/6 subunits indicate that - 0.9 elementary charges are transferred from the metal atom to the zeolite lattice [32]. This transfer will be even more complete with clusters because their ionization potential decreases strongly with size [2]. The calculations do, at present, not include the interaction with the exchangeable alkali ions of the zeolite lattice but they lend support to the view that the chromophores mentioned above can indeed be interpreted as F-centres.
3. Experimental 3.1. The preparation
of the metal clusters
Zeolite samples are dried for 48 h at 450 o C in a Pyrex glass high vacuum system under thermogravimetric control. The alkali metals are prepared by decomposition of their azides and purified by vacuum distillation [33] in the same sealed glass system. The quantity of alkali metal admitted to the zeolite corresponds stoichiometrically to the weight of the azide. Zeolite and metal are finally united in the same sealed-off glass tube. This is placed in a thermostat at temperatures between ambient and 150° C. Red, blue or green colours develop immediately depending on zeolite, cation and metal content. Homogeneous colouration announces sorption equilibrium in the different parts of the zeolite powder. The sorbates are chemically stable at room temperature in vacua. Adsorption of alkali metals (except Li) on completely dehydrated zeolites is reversible. If dehydration is not complete, some of the remaining lattice hydroxyls are reduced by the sorbed metals producing a small hydrogen pressure and fading of the colour. This can be avoided by proper drying and is only observed at very low metal loads. The formula M, [Nay] means that x metal atoms per unit cell are adsorbed. The analytical error of x is less than kO.2 atoms. x can be understood as the average metal content of each a-cage which is the most likely locus of the sorbate particles. The standard error of the size distribution for x 2 8 is probably quite small (+ 1) if thermodynamical equilibrium is attained. For small metal content Poisson distribution has to be applied. The following observations are relevant to an understanding of the results of this paper: Caesium is very rapidly sorbed already at room temperature whereas rubidium and the lower alkalis need 50-60 ’ C, and between 100 and 150 o C respectively and longer equilibration times. With caesium 10% metal by weight are adsorbed in less than one minute by zeolite NaY or NaX if the microcrystals touch the liquid metal phase. Very dramatic phenomena are observable at the interface. The zeolite crystals in contact with the metal become immediately black and the metal is rapidly soaked up. Further
F. Blatter, E. Schumacher
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/ Cs catalyzed ethylenepolymerisation
distribution of the metal is obtained by shaking of the black coloured with the unloaded white crystals. Cs is transferred over the crystal planes touching each other probably assisted by vapour phase diffusion. In a couple of minutes a homogeneous blue, green or red colour is achieved depending on zeolite and exchangeable lattice cation. The colouration extends evenly through the crystal interior as well. The spontaneous atomization of macroscopic Cs metal into the pore system of zeolite crystals is, of course, driven by the free energy changes of the metal support interaction and cluster formation within the cages. The totally nonmetallic aspect of the pigment can be reversed by heating whereby a metal mirror condenses on the surface of the glass tube and a white zeolite powder is obtained. 3.2. Preparation
of polyethylene
4.0 g zeolite Cs,, [Nay] sealed in a small glass bulb under vacuum is transferred into a stainless steel reactor. This is a cylinder with 35 mm bore and 180 mm length; it is closed by a Balzers CF35 HV-flange, into which two Nupro valves are welded, leading respectively to a manometer (Wika, O-60 bar range) and a glass-to-metal joint to connect the reaction tube with the vacuum line. A stainless steel cone is brazed into the bottom of the tube for breaking the glass bulb. The reactor is connected to the vacuum system and ethylene (99.95% purity CARBA AG) is condensed into it after having passed through a bed of dehydrated molecular sieve grains (Linde 4A). The quantity transferred is calculated from the ideal gas law to reach a terminal pressure of - 50 bars at room temperature.
CM-1
Fig. 2. IR-spectra of polyethylene extracted from zeolite, with small contaminations solved decomposition products from the zeolite (1000-1300 cm-‘). Arrows indicate the infrared absorptions of standard polyethylene, e.g. Merck p.a.
of undispositions of
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/ Cs catalyzed ethylene polymerisation
The reaction is started by tilting the detached reactor so that the glass bulb crashes on the stainless steel cone. After 24 h the reactor is emptied of its ethylene content and opened in an argon dry-box. The solid is transferred into a weighing shell; the weight gain of the zeolite by the reaction products is 20.5 & 0.5 mg/g zeolite or 18.7 f 0.5 CH, per unit cell. The powder is analysed with a BOMEM DA3.02 FTIR spectrometer (3% zeolite powder in KBr). Polyethylene Merck p.A., spectroscopic quality, 0.5% in KBr, is used as reference, fig. 2. Using a 1 : 1 mixture of 40% hydrofluoricand 37% hydrochloric acid the zeolite is dissolved in a teflon vessel during six days. The isolated product is polyethylene, which is again analysed by IRspectroscopy, electron microscopy and X-ray diffraction.
4. Results 4.1. The polymerisation
of ethene in zeolite NaY
initiated by Cs
Upon breaking the Cs filled glass bulb, the ethene pressure immediately drops by 2% because of the added dead volume from the zeolite pores and glass bulb. No further reduction of the pressure/ temperature ratio is observed even at 60 o C reaction temperature. The zeolite cavities are filled at the given pressure of ethylene and the polymerization through the zeolite channels and on the surface begins. No evolution of hydrogen could be detected nor are other residual gases present if the reactor is cooled by liquid N,. Therefore, dehydrogenation reactions of the olefin or reduction to methane are excluded. The infrared spectrum shows the six main bands of polyethylene slightly shifted (less than 1 wavenumber) in comparison to the reference substance (fig. 2). The same spectrum is observed with the intact zeolite before and with the almost completely destroyed aluminosilicate after the treatment by the acid mixture. Since the vibrational spectra of polyethylene and n-paraffins are well understood, the identification of polyethylene is sufficiently unique using only IR-spectroscopy and the determination of the melting point [34]. The chain length can be estimated in the usual way be comparing the end-group (-CH,) to the methylene-group (-CH,-) intensities. A further comparison with the spectra of C,,, C,,, and C,, n-alkanes allows the conclusion that a lower limit for the sequence of unbranched CH,-chain links is about 50. Chain branches with that order of length cannot be totally excluded. In addition, large aliphatic macrocycles C,H,, n > 50 may be formed inside of the zeolite channel structure. Both varieties of polymer are probably the cause for the fact that the zeolite cannot be completely digested in a strong HF/HCl acid mixture and that the dissolution kinetics is retarded by more than three orders of magnitude compared to an untreated zeolite. The polyethylene thus released
F. Bfatter, E. Sehu~c~er
/ Cs catalyzed ethylene ~olyme~~satia~
577
Fig. 3. Guinier-de Wolff powder X-ray diffraction pattern, Cu K, line, 3 h. Spectra from top to bottom: zeotite NaY (hydrated), polyethylene powder, Merck p.a. for spectroscopic uses, polyethylene powder from synthesis in {Cs} ,,, [Nay], extracted by HF/HCl acid mixture.
remains in flakes with a melting point of 135-140 o C. It still contains traces of undissolved aluminosilicate (as shown by the IR-spectrum) after several days. They are well shielded from acid attack by an unbroken polyethylene rnicrocover. The X-ray diffraction pattern, fig. 3, obtained for the polyethylene crystals shows the first two reflexes slightly broadened in comparison to the reference substance but with all other lines identical. Small differences in the degree of polyme~sation and of c~st~li~ty are responsible for this. Exposure of the ethylene treated zeolite to air leads to slow bleaching of the metal chromophores with a half life of approximately one hour, in contrast to the untreated zeolite which reacts in less than a second. A network of polyethylene chains through the outer layers of zeolite cavities and a film on the surface protect the remaining metal (- 70-80%) from oxidation. 4.2. The reaction of ethylene with pure metallic caesium For the purpose of comparing bulk behaviour with the preceding results the reaction of 1.0 g caesium metal ~99.99~~ purity 1331) with ethene was done in the same apparatus and under the same conditions. In contrast to the experiment with zeolite, the pressure diminishes from 50 bar at the begirming to 5 bar after 48 h. A faint yellowish liquid containing a mixture of monoolefinic hydrocarbons is produced. This is distilled in vacua at 60 o C from the reaction tube into a liquid nitrogen cold trap where a colourless liquid results. The products are identified by gas chromatographic and mass spectrometric analysis on a Ribermag R-10 GC-MS system. The distribution of ethylene oligomers found is shown on table 1 The mixture consists of several straight chain and also branched isomers for each of the given formulas. A similar experiment was carried out many years ago by Clusius and Mollet [35], who found iso-octenes as the main products with more than 70% yield after about 24 h. We find a similar compound distribution with isomeric octenes the most abundant after 24 h. However, the distribution is not stable in time. Evidently the oligomerisation reaction can continue to form higher molecular units as long as the surface of caesium metal remains uncon-
578
Table 1 Distribution caesium
F. Hatter,
E. Schumacher / 6.3 catalyzed eth.~levlenepolymerisation
of ethylene-oligomers
as formed by ethylene polymerisation
with metallic bulk
Substances
After 24 h
After 48 h
Remaining pressure of unused ethylene Hexenes C,H,z Octenes Cs% Decenes C,oHzo Dodecenes C,, H 24 Tetradecenes t&H,, Hexadecenes C,,H,2
25% 7% 32% 27% 6% 1% -
10% 3% 22% 29% 27% 7% 1%
Sum detected
98%
99%
taminated by products of side reactions. It takes about one day more to shift the maximum to C,, at 20 o C. Clusius and Mollet used fractional distillation, molecular weight determination, and refraction to identify the products. In view of the complex mixtures produced, table 1, a detailed analysis was impossible 35 years ago. Even earlier studies report that caesium metal is a catalyst for olefin hydrogenation 1364. Hydrogen transfer reactions do indeed take place in the bulk caesium catalyzed oligomerisation while they are only possible in
zeolite/Cs polymerisations if branched polymers are formed (no direct evidence). Alkali metals dispersed on aluminium oxide catalyse the dehydrogenation or cracking of saturated hydrocarbons readily in the range of C, n-alkanes even at room temperature [16,28]:
GH&‘) -,WIZ(~) + H,(g), and similarly 5% Cs on {Al,O, C&I&‘)
--+C,H&)
}
+ CH,(g).
As soon as the catalyst is dropped into the n-hexane (water-fry) evolution takes place. GC/MS reveals H, and CH,.
rapid gas
5. Discussion 5.1. Mechanism of the polymerisation: bulk-cluster Traditionally [37] (a-)olefins are considered to be well activated for polymerisation either by a free radical initiator or by an organometallic coordination system allowing insertion of the unsaturated C, unit or moiety into a growing d-metal atom terminated C-chain (Ziegler-Natta type). For the
F. Blatter, E. ~c~u~c~~r
/ Cs catalyzed ethylene po~~erisati~n
579
higher cY-alkenes the second initiator becomes the preferred one. Alkali metal initiated ethylene polymerisation is usually discussed parallel to that of alkali-alkyl(ide) or sodium naphthalide catalysis. Radical intermediates are excludes because they are considered to be energetically too unfavourable [38].
Model 1 Only the first step needs the formation of an anion radical through electron transfer from an alkali atom to the ethene molecule (slow process). Next, fast dimerisation of two anion radicals leads to a closed shell 1,2- (with hydrogen transfer) or 1,4-butanedianion which is the smallest chain propagating unit. From here on the m-acidity of ethene drives the chain propagation by addition to the polymeric carbanion. This is directed along the channels of the zeoiite and stops as soon as the polymer chain comes to an outer crystal surface because of lack of charge compensating cations. These ~guments follow closely the description of anionic polymerisation in polar solvents like THF: Cs + CH,=CH, CS@~CH,-CH,.
--, CS@~CH,-CH,. + . CH,-CH,
=Cs
,
0) + Cs=%H,-CH,-CH,-CH,=‘Cs, (2)
Cs@@CH2CH,CH,C
1eCH,=CH,
-+ Cs=‘*CH,CH,CH,CH,CH,CHze*Cs.
A second hypothesis follows from less traditional thinking and modifies step 3 of the schematic above: Since the growing polymer has to propagate its anionic end together with a cation along its path electrostatic constraints exist: the olefin does not offer a dielectric medium in which cations could be dislodged from their negatively charged lattice site and move in the zeolite. Chain growth according to model 1 can, therefore, only take place by electron movement through the zeolite in the opposite direction. This may easily be a~omp~shed at every cluster location which represents a source of electrons to the environment as assumed by the identification as F-centre. We have macroscopic proof that Cs moves rapidly within the zeolite already at 20 o C. Hence it is likely that there are always a few Cs atoms around where a new ethene is attached. One of them (or the ensemble in the sense of delocalized cluster states) jumps into the role of the compensating cation while the Cs,f cluster at the second to last growth location (about 3 A down-chain) gains an
580
F. Bfatter, E. Schumacher / Cs catalyzed ethylenepolymerisation
electron. This mechanism could easily move the chain within and over the whole outer surface of the zeolite microcrystal and let it dive again into any pore of suitable size and Cs content. This would form the observed total coverage of the crystals. Model 3 A third hypothesis breaks with the taboo of the high energy free radicals and modifies steps (2) and (3) of the schematic. In zeolites with alkali clusters ESR spectra prove the existence of a constant concentration of free electron spins. The hyperfine splittings indicate that those are continuously probing between 4 and 6 alkali nuclei with nuclear spin 3/Z. No larger odd electron clusters have been identified to date. The reason for this is probably, that the alkali atoms condense preferentially around the exchangeable lattice cations because these are the deepest traps in the sense that they offer the highest gain in energy of polarization. Depending on the type of zeolite four (in Nay) or six (in NaX) atom cluster can be accommodated within the geometrical constraints of the cage and its cation positions (and content). The first step of the interaction of the clusters with ethene is again a charge transfer and formation of a radical anion. This does not dimerize since it can be stabilized through participation in bond~g of the cluster which is a radical itself. Chain propagation follows now along a pure radical mechanism. There is no charge movement involved, only on anchor site for each chain necessary, and chain termination by radical recombination (radical disproportionation is not possible because no polymers with double bonds have been detected). These chains could grow into the gas phase as long as monomer feed is available thereby bridging two microcrystals. They are able to form reentrant loops on crystal surfaces and bizarre meshes of intertwined polymer threads. How can one identify the most plausible hypothesis on the basis of existing information or additional experiments ? Model 1 will not support surface coverage by polyethylene chains, model 2 allows surface coverage but cannot explain intercrystal polymer chains, whereas both are enabled in model 3. From the scanning-electron micrographs, fig. 4, we observe that the micromorphology of a polyethylene flake extracted from the “composite” by dissolving the inorganic matrix resembles strongly that of the host zeolite crystals, especially their surfaces. Furthermore, the mimicked (negative) crystal skeleton of the zeolite formed by the polyethylene mesh is totally connected over many crystalreplicas. Both observations favour model 3. From the photographs of fig. 4 it becomes clear that both the commercial and the synthesised polyethylenes are transformed to the same habitus after melting and resolidification. A definitive proof for a sustained existence of hydrocarbon radicals could be obtained by repeating the polymerisation experiment in a high pressure cell within the cavity of an ESR-spectrometer [28].
F. Blatter, E. Schumacher
/ Cs catalyzed ethylene polymerisation
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F. Blatter, E. Schumacher
/ Cs catalyzed ethylene polymerisation
5.2. Comparison of different a~~a~~metals Our data show that both bulk metallic caesium and caesium clusters in zeolites catalyse the polymerisation of ethylene. Whereas with bulk caesium a mixture of many unsaturated oligomeric compounds is formed the reaction of ethylene in caesium loaded zeolite leads to polyethylene of high molecular weight alone. With other alkalis differences are expected resulting from the changes in the ionisation potentials and polar&abilities from Li to Cs. In experiments with zeolites containing 10 atoms of metallic sodium or potassium per unit cell only traces of high molecular paraffins resulted. The reaction of Rb,,[NaY] zeolite under the same conditions as in 3.1 leads to high molecular paraffins with a yield of 10% polymers by weight, i.e. 5 times larger than with Cs. However, the resulting paraffins showed a lower average degree of polymerisation as indicated by a melting range from 105 to 110 o C. The reason for the higher yield is probably the lower space filling by the same number of Rb atoms in the zeolite channels and hence more free volume for adsorption of ethylene rather than enhanced reactivity of the rubidium clusters.
5.3. Metal support interactions and use of Al,O, Adsorption of alkali metals on dehydrated alumina produces blue or green-blue chromophores. Because of the broader pore size distribution of the a-alumina used the colouration has probably a component of Mie scattering at small metallic particles in the range of 50 A diameter. ESR-measurements show typical conduction electron signals similar to those of the highly metal loaded zeolites [24,28]. The same polyme~sation experiments with caesium and rubidium dispersed on porous cr-alumina lead to a different product spectrum: From unsaturated compounds of molecular weight around C i0 up to long paraffins with more than 100 carbon atoms all sizes are obtained.
5.4. Chemical reactivity of metal clusters in zeolites Additional informations on the chemical reactivities of alkali metal clusters in zeolites are pertinent to this discussion: Stoichiometric intrazeolite reactions like Wurtz-Fittig coupling and related reactions with chlorinated hydrocarbons give evidence for the different chemical behaviour of clusters and bulk [32]. Geismar and Westphal [25] claim that alkali metals dispersed in zeolites are less reactive than in bulk. As a consequence fewer radicals would be initiated and thus longer polymer chains formed by dispersed caesium, in agreement with the present work. Experiments with propylene at 70 o C and 40 bar did not give polymer with
F. Blatter, E. Schumacher / Cs catalyzed ethylene po!vmerisation
583
zeolite/Cs.
a-alum.ina/Cs yielded very small amounts of oligomeric products. Butadiene, which is even more reactive than ethene, did not show a reaction with zeolite/Cs at normal pressure but at 10 bar a broad distribution of ~saturated oligomers was produced. In both cases a sorption barrier may exist towards the partially Cs filled zeolite so that in a short reaction period only Cs clusters near the crystal surface are accessible,
6. Conclusions
We are aware of two other recent papers on olefin reactions in zeolites: O~gome~sation was observed in zeolite H-ZSM-5, and dimerisation of ethylene with propylene in Cr3+ exchanged zeolite NaY [39]. In both cases only low molecular products resulted and the mechanism was considered as a cationic induction. To date the heavy alkali-olefin reactions in NaY are rather unique. It is interesting to point this out in perspective and conclude with possible applications. We shall concentrate on Cs alone. The Cs on NaY sorbates with an average content of 10 Cs per unit cell contain a very dilute but clustered form of Cs. ESR spectra reveal an association of the clusters with lattice cations to form delocalized electronic states probing between 4 and 6 nuclear spins. This arrangement is probably quite fluid and does promote ovation of uncharged Cs within the channel system. In contact with a Cs cluster ethene obtains an electron thus becoming an anion radical. Within the spatial constraints of the zeolite this reactive intermediate can grow an aliphatic chain by end-on reaction with the olefin propagating its f localized odd electron together with its front end. Chain termination occurs, when two growing chains meet within or outside of the zeolite crystal by radical recombination. This scheme works well if the olefin in the pore system is dense enough to form queues which support a rapid radical propagation. Therefore, several dozen bars of ethene pressure are necessary to obtain long chain paraffins. The property of the Cs sorbate to initiate this sequence of reactions is pecuIiar to the cluster and the supporting zeolite. Neither of them alone nor the bulk phase produces it. Reaction stops several seconds after admitting ethene to the NaY/Cs because the polymer chains seal the crystal completely from the outside environment. The plugged average volume of between 10 to 30% of each cubic crystal (determined from the rem~~ng active Cs> fills a surface shell of depth between 2 and 6% of the cube’s side. Thermal movements of the polymer chains always allow small molecules to migrate along them. Therefore, Cs in the inner part of the zeolite is bleached by oxygen after several hours exposure to air. Mechanically the polyethylene mesh within and over the surface of each zeolite crystal has some remarkable properties akin to bone tissue. A large part
584
F. Hatter,
E. Schumacher
/ Cs catalyzed ethylene polymerisation
of the polyethylene is not removable from the crystal without destroying the latter. This makes a perfect hydrophobic nonbleeding phase for chromatography. - By means of their securely anchored polyethylene surfaces, zeolite crystals can be molded into thermoplastics, glued onto thin plastic webs and, if desired, mech~cally opened up at the outer side to their unchang~ 70% irmer volume which remains an active adsorbent. - The zeolite obtains almost complete acid resistivity by the polyethylene wrap. This allows chemical surface treatment and derivatization. - Since the polymerisation of ethene is promoted by a small amount of sorbed Cs in the surface layer of the zeolite, the inner part of the crystals may be preloaded by a different sorbate, possibly an air sensitive material. This can be protected by creating a tight polymer wrap initiated by Cs. - The list of possible applications of this and similar systems is very large, indeed. Acknowledgements
We thank the Swiss National Foundation, grant No.2.431.84/2.273.86, and the “Kommission zur Forderung der Wissenschaftlichen Forschung”, grant No. 1467/1211, for financial support; Professor R. Giovanoli, M. Faller, and B. Frey (University of Bern) for X-ray diffraction and electron microscopy; Professor P. Wachter, and PD Dr. E. Kaldis (ETH, Ztirich) for UV/VIS diffuse reflectance spectroscopy; Professor U.P. Schlunegger, and Hans Gfeller (diversity of Bern) for help in GC-MS problems; Professor M. Neuenschwander (University of Bern) for discussions; Professor A. von Zelevsky, and Eva Moser (University of Fribourg), PD Dr. A. Schweiger, and Susanne Pfenninger (ETH Ztirich) for ESR measurements. References [l] M.M. Kappes and E. Schumacher, Surface Sci. 156 (1985) 1. [2] M.M. Kappes, M. S&k, P. Radi and E. Schumacher, 3. Chem. Phys. 84 (1986) 1863; M.M. Kappes, M. Schk, U. R~t~isberger, Ch. Yeretzian and E. Schumacher, Chem. Phys. Letters 143 (1988) 251. [3] A.W. Castleman and R.G. Keesee, Ann. Rev. Phys. Chem. 37 (1986) 525; G.C. Bond, Surface Sci. 156 (1985) 966. [4] M. Boudart, A.W. Aldag, L.D. Ptak and J.E. Benson, J. Catalysis 11 (1968) 35. [5] K. Foger and J.R. Anderson, J. Catalysis 54 (1978) 318. [6] H.A. Benesi, R.M. Curtis and HP. Studer, J. Catalysis 10 (1968) 328. [7) R.A. Dalla Betta and M. Boudart, in: Proc. 5th Intern. Congr. of Catalysis (1972) p. 1329. [8] P. Gallezot, Surface Sci. 106 (1981) 459. [9] M. Boudart, J. Mol. Catalysis 30 (1985) 27. [lo] R. Whetten, M. Z&in, D. Cox and A. Kaldor, J. Chem. Phys. 85 (1986) 1697. [ll] M. Geusic, M. Morse and R. Smalley, J. Chem. Phys. 82 (1985) 590, 2292.
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/ Cs catalyzed ethyienep~~merisat~on
585
[I21 S. Richtsmeier, E. Parks, K. Liu, K. Pobo and S. Riley, J. Chem. Phys. 82 (1985) 3659. [13] E. Schumacher, F. Blatter and T. Irlet, 191st ACS Meeting New York, April 1986. [14] J.A. Rabo, Zeolite Chemistry and Catalysis, Vol. 171 of ACS Series (Am. Chem. Sot., Washington, 1976). [15] P. Fotis Jr, US Patent No. 2887472 (1959); G.A. Ozin, L.F. Nazar and F. Hugues, European Patent No. 0115 188 (1983); C. Tiby, German Patent DE 3341566 Al (1985). [16] B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud, Eds., Catalysis by Zeolites, Vol. 5 of Studies in Surface Science and Catalysis (Elsevier Science Publishers, Amsterd~, 1980); B. Dehnon, P. Grange, P.A. Jacobs and G. Poncelet, Eds., Preparation of Catalysts VI, Vol. 31 of Studies in Surface Science and Catalysis (Elsevier Science Publishers, Amsterdam, 1987). [17] R.M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves (Academic Press, New York, 1978). [18] W.M. Meier and J.B. Uytterhoeven, Molecular Sieves, Vol. 121 of ACS Series (Am. Chem. Sot., Washington, 1973); W.M. Meier, Atlas of Zeolite Structure Types (Structure Commission of the International Zeolite Association, 1978). [19] G.D. Stucky and F.G. Dwyer, Zeolite Chemistry, Vol. 218 of ACS Monograph Series (Am. Chem. Sot., Washington, 1983). [20] S. Ramdas, J.M. Thomas, P.W. Betteridge, A.C. Cheetham and E.K. Davies, Angew. Chem. 96 (1984) 629. [21] F. Blatter, Laboratory Synthesis of Pure NaY, in preparation. [22] J.A. Rabo, C.L. Angell, P.H. Kasai and V. Schomaker, Disc. Faraday Sot. 41 (1966) 328. [23] P.H. Kasai and R.J. Bishop, Jr., 3. Phys. Chem. 77 (1973) 2309; Y. Ben Taarit, C. Naccache, M. Che and A.J. Tenth, Chem. Phys. Letters 24 (1973) 41. [24] M.R. Harrison, P.P. Edwards, J. Klinowski, D.C. Johnson, C.J. Page and J.M. Thomas, J. Solid State Chem. 54 (1984) 330. [25] G. Geismar and U. Westphal, Z. Allg. Anorg. Chem. 504 (1984) 165; Chem. Z. 111 (1987) 217. [26] A. Herrmann, Dissertation, Bern, 1978; F. Blatter, Diploma Thesis, Bern, 1984. [27] P. Fejes, I. Kirisci, I. Harmus, T. Tibanyi and A. Kiss, in: Catalysis by Zeolites, Eds. B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud, Vol. 5 of Studies in Surface Science and Catalysis (Elsevier Science Publishers, Amsterdam, 1980) p. 135; L.R.M. Martens, W.J.M. Vermeiren, P.J. Grobet and P.A. Jacobs, in: Preparation of Catalysts VI, Eds. B. Delmon, P. Grange, P.A. Jacobs and G. Poncelet, Vol. 31 of Studies in Surface Science and Catalysis (Elsevier Science Publishers, Amsterdam, 1987) p. 531; L.R.M. Martens, P.J. Grobet, W.J.M. Vermeiren and P.A. Jacobs, in: New Developments in Zeolite Science and Technology, Eds. Y. Murakami, A. Iijima and J. W. Ward, Vol. 28 of Studies in Surface Science and Catalysis (Elsevier Science Publishers, Amsterdam, 1986) p. 935. (281 F. Blatter, E. Schumacher, S. Pfenninger and A. Schweiger, to be published; K.W. Blazey, K.A. Mtiller, F. Blatter and E. Schumacher, Europhys. Letters 4 (1987) 8.57. [29] C. Kittel, Introduction to Solid State Physics (Wiley, New York, 1976). [30] W.T. Doyle and W.R. Williams, Phys. Rev. Letters 6 (1961) 537. [31] G. Calzaferri and L. Forss, Helv. Chim. Acta 69 (1986) 873. (321 F. Blatter, Diploma Thesis, Bern, 1984, to be published; R. Kunz, Postdoctoral work, University of Bern, 1980.
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[33] F. Blatter and E. Schumacher, J. Less-Common Metals 115 (1986) 307; R. Suhrnxum and K. Clusius, Z. Allgem. Anorg. Chem. 151 (1926) 52. [34] S. Krimm, Forts&r. Hochpolymere Forsch. 2 (1960) 51; P.J. Hendra, H.P. Jobic, E.P. Marsden and D. Bloor, Spectrochim. Acta 33A (1970) 445. [35] K. Clusius and H. Mollet, Helv. Chim. Acta 34 (1956) 363. [36] D.G. Hill and G.B. Kistiakowsky, J. Am. Chem. Sot. 52 (1930) 892. [37] J. Boor, Ziegler-Natta Catalysts and Polymerisations (Academic Press, New York, 1979); B. Vollmert, Polymer Chemistry (Springer, New York, 1973). [38] G. Henrici-Olive and S. Olive, Polymerisation (Verlag Chemie, Weinheim, 1969); H. Pines and W.M. Stalick, Base Catalyzed Reactions of Hydrocarbons and Related Compounds Academic Press, New York, 1977). [39] J.P. van den Bergh, J.P. Wolthuizen, A.P.H. Clague, G.R. Hays, R. Huis and J.C. van Hooff, J. Catalysis (1983) 130; N. Taskahashi, Y. Fujiwara and A. Mijin, Zeolites 5 (1986) 363.
Surface Science 203 (1988) 571-586 North-Holland, Amsterdam
ETHYLENE POLY~~SA~ON IN ZEOLITE NaY Fritz BLATTER
CATALYZED
BY CAIESIUM
and Ernst SCHUMACHER
Institute for Inorganic, Analytical and Physical Chemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland Received 7 January 1988; accepted for publication 18 May 1988
Alkali metals adsorbed on dehydrated zeolite NaY or a-Al,O, produce well known chromophores which are parama~etic and similar to F-centres. Caesium and ~bidium sorbates in zeolite NaY are good catalysts for ethylene pol~e~sation at 50 bar. High molecular straight chain paraffins result which completely wrap the microcrystals. Polyethylene is extracted from the “composite” by acid digestion of the aluminosilicate and characterized by FTIR spectroscopy, X-ray diffraction, and stating electron microscopy. A negative polyethylene replica of the zeolite crystals is observed in the SEM. The chemical properties of dispersed caesium are compared to those of the bulk which produces only monoolefinic oligomers. Na, K, and Rb are compared to Cs, propene and butadiene to ethene, and zeolites to OL-A~~O~.Plausible mechanisms and applications are discussed.
In recent years the chemical and catalytical reactivities of bare metal clusters have attracted much scientific [l-3] and applied interest [4-81. The paramount question is whether (and how) metal particles as a function of diminishing size begin to exhibit reactive properties distinctly different from bulk behaviour. Earlier studies with supported Pt-particles on non-metallic supports (aluminosilicates) showed a l/r (r = particle radius) dependence of specific catalytic activity which leveled off when “‘atomic” dispersion was reached 191. This is to be expected if no size dependence exists and solely the availability of the catalyst atoms for the reaction is responsible for the size effect. However, more recent experiments with much smaller clusters in the gas phase show dramatically selective effects: One atom more or less e.g. with Fe,/Fe,, or Fe&Fe,, causes a change in the rate of hydrogen sorption by more than an order of magnitude [lo-121. These rate changes are pertinent to heterogeneous catalytic activity since adsorption is a prerequisite for it. Similar observations with supported catalysts have not yet appeared. Metal clusters on supports usually have broad, ill defined size distributions which might mask specific activities of certain size ranges 1131. Moreover, the cluster support 0039-6028/88/$03.~0 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
512
F. Hatter,
E. Schumacher
/ Cs catalyzed ethylene polymerisation
interaction may cause insensitivity to the very question asked. Synthesis or isolation of nearly monodisperse metal clusters of various sizes and their study on specific surfaces will be essential to the clarification of the issue. Meanwhile, metal clusters in zeolite channels offer a well reproducible reactive system [14]. The structured and confined space for the deposition or in situ generation of metal clusters produces a much narrower size dist~bution than usually encountered on more open supports like alumina, silica or carbon molecular sieves [15]. It is common to add precursors for metal particle formation by salt impregnation or by ion exchange [16]. Reduction or pyrolysis in situ to form the metallic state is usually performed at temperatures ranging from 250 to 45O’C. This may create migration and sintering which can lead to the formation of larger clusters with 100 to 1000 atoms at the surface. Therefore, quantitative information at small cluster sizes is still sparse or lacking. The present paper shows size and reactivity dependence and perhaps size-support interaction effects during the intracrystal polyme~sation of ethylene initiated by the heavy alkali metals. It also offers a methodology for protecting chemically highly reactive sorbed species in zeolites from atmospheric corrosion. Finally, the produced completely intertwined inorganic porous system with organic polymer chains is an interesting model for studying composites with its own set of applications.
2. Alkali metal dusters
in zeolites
We use the synthetic zeolites NaY and NaX [17-201. The commercial sources (Strem Chemicals, Union Carbide) and the samples synthesized from analytically pure chemicals in our laboratory [21] give identical results. Differences are only essential for ESR spectroscopy which is performed with our chemically pure and crystallographically uniform zeolites. Alkali metal clusters are prepared by adsorption of metal vapour on dehydrated zeolite powder under high vacuum and thermal equilibrium. Coioured samples result as soon as the metal is admitted. These chromophores were first observed by Rabo et al. [22]. They attributed the red colour of Na in NaY to Na:+ , and the blue colour of the same metal in zeolite NaX to Nazt particles. This assignment was derived from ESR measurements and later verified by other authors [23-281. In the case of zeolite NaY at least four different experiments seem to lead to the same 13-line ESR spectrum attributed to Nay : - adsorption of very small quantities of metal vapour; - irradiation of the pure zeolite with y- or X-rays; - inducing a tesla coil gas discharge near the pure zeolite in the presence of small quantities of hydrogen (- 0.1 Torr);
F. Blatter, E. Schumacher
573
/ Cs catalyzed ethylene polymerisation
- thermal decomposition of included NaN, [27]. ESR-measurements of zeolite NaX as a function of Na content, producing colours between blue and black, exhibit first the Na:+ spectra which then become more complex and finally show conduction electron signals [25,28]. These results from ESR and ENDOR investigations will be published elsewhere [28]. It is, of course, highly probable that the 19-line feature belongs to lower charged odd electron species like Nai+ and even Nai rather than to Nai+ [22] in the heavily metal doped zeolites. Similarly Na: is much more likely than Nay in NaY with a high content of sorbed sodium. In addition the state of the sorbed metal may include neutral clusters which do not announce themselves with ESR signals, however. These chromophores are extremely sensitive to air and lose their colour in seconds if they are exposed to an atmosphere containing only traces of oxygen in argon forming alkali metal superoxides [22]. The optical reflection spectrum (fig. 1) and the ESR signals are very similar to F-centres [29]. The ESR spectrum of a trapped electron in an H--vacancy of crystalline sodium hydride shows 19 hyperfine lines (6 nearest neighbours with I = 3/2) and almost the same coupling constant (26 gauss) as observed in the zeolites NaY and NaX with sorbed Na [30]. The colours for the different alkali metals at comparable molar concentration in zeolites are characteristic for the metal, and the zeolite, and the exchangeable cations (if they are from another chemical element than the sorbed metal). Quantum chemical calculations with the extended Hiickel model have led to detailed information about the interaction of Ag, and Cu atoms as well as Ag+ and Cu+ ion clusters with the zeolite cavities of the 4/4, 6/6 subunits,
500
1000 WAVELENGH
Fig. 1. UV-VIS
spectrum caesium
1500
2000
[nm]
of CsIO [Nay]. The chemical formula means that atoms per elementary cell of the zeolite are adsorbed.
an average
of 10
574
F. Blatter, E. Schumacher
/ Cs catalyzed ethylene polymerisation
and of the P-cage [31]. The same methodology (full charge iteration) applied to alkali atoms in 4/4 and 6/6 subunits indicate that - 0.9 elementary charges are transferred from the metal atom to the zeolite lattice [32]. This transfer will be even more complete with clusters because their ionization potential decreases strongly with size [2]. The calculations do, at present, not include the interaction with the exchangeable alkali ions of the zeolite lattice but they lend support to the view that the chromophores mentioned above can indeed be interpreted as F-centres.
3. Experimental 3.1. The preparation
of the metal clusters
Zeolite samples are dried for 48 h at 450 o C in a Pyrex glass high vacuum system under thermogravimetric control. The alkali metals are prepared by decomposition of their azides and purified by vacuum distillation [33] in the same sealed glass system. The quantity of alkali metal admitted to the zeolite corresponds stoichiometrically to the weight of the azide. Zeolite and metal are finally united in the same sealed-off glass tube. This is placed in a thermostat at temperatures between ambient and 150° C. Red, blue or green colours develop immediately depending on zeolite, cation and metal content. Homogeneous colouration announces sorption equilibrium in the different parts of the zeolite powder. The sorbates are chemically stable at room temperature in vacua. Adsorption of alkali metals (except Li) on completely dehydrated zeolites is reversible. If dehydration is not complete, some of the remaining lattice hydroxyls are reduced by the sorbed metals producing a small hydrogen pressure and fading of the colour. This can be avoided by proper drying and is only observed at very low metal loads. The formula M, [Nay] means that x metal atoms per unit cell are adsorbed. The analytical error of x is less than kO.2 atoms. x can be understood as the average metal content of each a-cage which is the most likely locus of the sorbate particles. The standard error of the size distribution for x 2 8 is probably quite small (+ 1) if thermodynamical equilibrium is attained. For small metal content Poisson distribution has to be applied. The following observations are relevant to an understanding of the results of this paper: Caesium is very rapidly sorbed already at room temperature whereas rubidium and the lower alkalis need 50-60 ’ C, and between 100 and 150 o C respectively and longer equilibration times. With caesium 10% metal by weight are adsorbed in less than one minute by zeolite NaY or NaX if the microcrystals touch the liquid metal phase. Very dramatic phenomena are observable at the interface. The zeolite crystals in contact with the metal become immediately black and the metal is rapidly soaked up. Further
F. Blatter, E. Schumacher
515
/ Cs catalyzed ethylenepolymerisation
distribution of the metal is obtained by shaking of the black coloured with the unloaded white crystals. Cs is transferred over the crystal planes touching each other probably assisted by vapour phase diffusion. In a couple of minutes a homogeneous blue, green or red colour is achieved depending on zeolite and exchangeable lattice cation. The colouration extends evenly through the crystal interior as well. The spontaneous atomization of macroscopic Cs metal into the pore system of zeolite crystals is, of course, driven by the free energy changes of the metal support interaction and cluster formation within the cages. The totally nonmetallic aspect of the pigment can be reversed by heating whereby a metal mirror condenses on the surface of the glass tube and a white zeolite powder is obtained. 3.2. Preparation
of polyethylene
4.0 g zeolite Cs,, [Nay] sealed in a small glass bulb under vacuum is transferred into a stainless steel reactor. This is a cylinder with 35 mm bore and 180 mm length; it is closed by a Balzers CF35 HV-flange, into which two Nupro valves are welded, leading respectively to a manometer (Wika, O-60 bar range) and a glass-to-metal joint to connect the reaction tube with the vacuum line. A stainless steel cone is brazed into the bottom of the tube for breaking the glass bulb. The reactor is connected to the vacuum system and ethylene (99.95% purity CARBA AG) is condensed into it after having passed through a bed of dehydrated molecular sieve grains (Linde 4A). The quantity transferred is calculated from the ideal gas law to reach a terminal pressure of - 50 bars at room temperature.
CM-1
Fig. 2. IR-spectra of polyethylene extracted from zeolite, with small contaminations solved decomposition products from the zeolite (1000-1300 cm-‘). Arrows indicate the infrared absorptions of standard polyethylene, e.g. Merck p.a.
of undispositions of
516
F. Blaiter, E. Schumacher
/ Cs catalyzed ethylene polymerisation
The reaction is started by tilting the detached reactor so that the glass bulb crashes on the stainless steel cone. After 24 h the reactor is emptied of its ethylene content and opened in an argon dry-box. The solid is transferred into a weighing shell; the weight gain of the zeolite by the reaction products is 20.5 & 0.5 mg/g zeolite or 18.7 f 0.5 CH, per unit cell. The powder is analysed with a BOMEM DA3.02 FTIR spectrometer (3% zeolite powder in KBr). Polyethylene Merck p.A., spectroscopic quality, 0.5% in KBr, is used as reference, fig. 2. Using a 1 : 1 mixture of 40% hydrofluoricand 37% hydrochloric acid the zeolite is dissolved in a teflon vessel during six days. The isolated product is polyethylene, which is again analysed by IRspectroscopy, electron microscopy and X-ray diffraction.
4. Results 4.1. The polymerisation
of ethene in zeolite NaY
initiated by Cs
Upon breaking the Cs filled glass bulb, the ethene pressure immediately drops by 2% because of the added dead volume from the zeolite pores and glass bulb. No further reduction of the pressure/ temperature ratio is observed even at 60 o C reaction temperature. The zeolite cavities are filled at the given pressure of ethylene and the polymerization through the zeolite channels and on the surface begins. No evolution of hydrogen could be detected nor are other residual gases present if the reactor is cooled by liquid N,. Therefore, dehydrogenation reactions of the olefin or reduction to methane are excluded. The infrared spectrum shows the six main bands of polyethylene slightly shifted (less than 1 wavenumber) in comparison to the reference substance (fig. 2). The same spectrum is observed with the intact zeolite before and with the almost completely destroyed aluminosilicate after the treatment by the acid mixture. Since the vibrational spectra of polyethylene and n-paraffins are well understood, the identification of polyethylene is sufficiently unique using only IR-spectroscopy and the determination of the melting point [34]. The chain length can be estimated in the usual way be comparing the end-group (-CH,) to the methylene-group (-CH,-) intensities. A further comparison with the spectra of C,,, C,,, and C,, n-alkanes allows the conclusion that a lower limit for the sequence of unbranched CH,-chain links is about 50. Chain branches with that order of length cannot be totally excluded. In addition, large aliphatic macrocycles C,H,, n > 50 may be formed inside of the zeolite channel structure. Both varieties of polymer are probably the cause for the fact that the zeolite cannot be completely digested in a strong HF/HCl acid mixture and that the dissolution kinetics is retarded by more than three orders of magnitude compared to an untreated zeolite. The polyethylene thus released
F. Bfatter, E. Sehu~c~er
/ Cs catalyzed ethylene ~olyme~~satia~
577
Fig. 3. Guinier-de Wolff powder X-ray diffraction pattern, Cu K, line, 3 h. Spectra from top to bottom: zeotite NaY (hydrated), polyethylene powder, Merck p.a. for spectroscopic uses, polyethylene powder from synthesis in {Cs} ,,, [Nay], extracted by HF/HCl acid mixture.
remains in flakes with a melting point of 135-140 o C. It still contains traces of undissolved aluminosilicate (as shown by the IR-spectrum) after several days. They are well shielded from acid attack by an unbroken polyethylene rnicrocover. The X-ray diffraction pattern, fig. 3, obtained for the polyethylene crystals shows the first two reflexes slightly broadened in comparison to the reference substance but with all other lines identical. Small differences in the degree of polyme~sation and of c~st~li~ty are responsible for this. Exposure of the ethylene treated zeolite to air leads to slow bleaching of the metal chromophores with a half life of approximately one hour, in contrast to the untreated zeolite which reacts in less than a second. A network of polyethylene chains through the outer layers of zeolite cavities and a film on the surface protect the remaining metal (- 70-80%) from oxidation. 4.2. The reaction of ethylene with pure metallic caesium For the purpose of comparing bulk behaviour with the preceding results the reaction of 1.0 g caesium metal ~99.99~~ purity 1331) with ethene was done in the same apparatus and under the same conditions. In contrast to the experiment with zeolite, the pressure diminishes from 50 bar at the begirming to 5 bar after 48 h. A faint yellowish liquid containing a mixture of monoolefinic hydrocarbons is produced. This is distilled in vacua at 60 o C from the reaction tube into a liquid nitrogen cold trap where a colourless liquid results. The products are identified by gas chromatographic and mass spectrometric analysis on a Ribermag R-10 GC-MS system. The distribution of ethylene oligomers found is shown on table 1 The mixture consists of several straight chain and also branched isomers for each of the given formulas. A similar experiment was carried out many years ago by Clusius and Mollet [35], who found iso-octenes as the main products with more than 70% yield after about 24 h. We find a similar compound distribution with isomeric octenes the most abundant after 24 h. However, the distribution is not stable in time. Evidently the oligomerisation reaction can continue to form higher molecular units as long as the surface of caesium metal remains uncon-
578
Table 1 Distribution caesium
F. Hatter,
E. Schumacher / 6.3 catalyzed eth.~levlenepolymerisation
of ethylene-oligomers
as formed by ethylene polymerisation
with metallic bulk
Substances
After 24 h
After 48 h
Remaining pressure of unused ethylene Hexenes C,H,z Octenes Cs% Decenes C,oHzo Dodecenes C,, H 24 Tetradecenes t&H,, Hexadecenes C,,H,2
25% 7% 32% 27% 6% 1% -
10% 3% 22% 29% 27% 7% 1%
Sum detected
98%
99%
taminated by products of side reactions. It takes about one day more to shift the maximum to C,, at 20 o C. Clusius and Mollet used fractional distillation, molecular weight determination, and refraction to identify the products. In view of the complex mixtures produced, table 1, a detailed analysis was impossible 35 years ago. Even earlier studies report that caesium metal is a catalyst for olefin hydrogenation 1364. Hydrogen transfer reactions do indeed take place in the bulk caesium catalyzed oligomerisation while they are only possible in
zeolite/Cs polymerisations if branched polymers are formed (no direct evidence). Alkali metals dispersed on aluminium oxide catalyse the dehydrogenation or cracking of saturated hydrocarbons readily in the range of C, n-alkanes even at room temperature [16,28]:
GH&‘) -,WIZ(~) + H,(g), and similarly 5% Cs on {Al,O, C&I&‘)
--+C,H&)
}
+ CH,(g).
As soon as the catalyst is dropped into the n-hexane (water-fry) evolution takes place. GC/MS reveals H, and CH,.
rapid gas
5. Discussion 5.1. Mechanism of the polymerisation: bulk-cluster Traditionally [37] (a-)olefins are considered to be well activated for polymerisation either by a free radical initiator or by an organometallic coordination system allowing insertion of the unsaturated C, unit or moiety into a growing d-metal atom terminated C-chain (Ziegler-Natta type). For the
F. Blatter, E. ~c~u~c~~r
/ Cs catalyzed ethylene po~~erisati~n
579
higher cY-alkenes the second initiator becomes the preferred one. Alkali metal initiated ethylene polymerisation is usually discussed parallel to that of alkali-alkyl(ide) or sodium naphthalide catalysis. Radical intermediates are excludes because they are considered to be energetically too unfavourable [38].
Model 1 Only the first step needs the formation of an anion radical through electron transfer from an alkali atom to the ethene molecule (slow process). Next, fast dimerisation of two anion radicals leads to a closed shell 1,2- (with hydrogen transfer) or 1,4-butanedianion which is the smallest chain propagating unit. From here on the m-acidity of ethene drives the chain propagation by addition to the polymeric carbanion. This is directed along the channels of the zeoiite and stops as soon as the polymer chain comes to an outer crystal surface because of lack of charge compensating cations. These ~guments follow closely the description of anionic polymerisation in polar solvents like THF: Cs + CH,=CH, CS@~CH,-CH,.
--, CS@~CH,-CH,. + . CH,-CH,
=Cs
,
0) + Cs=%H,-CH,-CH,-CH,=‘Cs, (2)
Cs@@CH2CH,CH,C
1eCH,=CH,
-+ Cs=‘*CH,CH,CH,CH,CH,CHze*Cs.
A second hypothesis follows from less traditional thinking and modifies step 3 of the schematic above: Since the growing polymer has to propagate its anionic end together with a cation along its path electrostatic constraints exist: the olefin does not offer a dielectric medium in which cations could be dislodged from their negatively charged lattice site and move in the zeolite. Chain growth according to model 1 can, therefore, only take place by electron movement through the zeolite in the opposite direction. This may easily be a~omp~shed at every cluster location which represents a source of electrons to the environment as assumed by the identification as F-centre. We have macroscopic proof that Cs moves rapidly within the zeolite already at 20 o C. Hence it is likely that there are always a few Cs atoms around where a new ethene is attached. One of them (or the ensemble in the sense of delocalized cluster states) jumps into the role of the compensating cation while the Cs,f cluster at the second to last growth location (about 3 A down-chain) gains an
580
F. Bfatter, E. Schumacher / Cs catalyzed ethylenepolymerisation
electron. This mechanism could easily move the chain within and over the whole outer surface of the zeolite microcrystal and let it dive again into any pore of suitable size and Cs content. This would form the observed total coverage of the crystals. Model 3 A third hypothesis breaks with the taboo of the high energy free radicals and modifies steps (2) and (3) of the schematic. In zeolites with alkali clusters ESR spectra prove the existence of a constant concentration of free electron spins. The hyperfine splittings indicate that those are continuously probing between 4 and 6 alkali nuclei with nuclear spin 3/Z. No larger odd electron clusters have been identified to date. The reason for this is probably, that the alkali atoms condense preferentially around the exchangeable lattice cations because these are the deepest traps in the sense that they offer the highest gain in energy of polarization. Depending on the type of zeolite four (in Nay) or six (in NaX) atom cluster can be accommodated within the geometrical constraints of the cage and its cation positions (and content). The first step of the interaction of the clusters with ethene is again a charge transfer and formation of a radical anion. This does not dimerize since it can be stabilized through participation in bond~g of the cluster which is a radical itself. Chain propagation follows now along a pure radical mechanism. There is no charge movement involved, only on anchor site for each chain necessary, and chain termination by radical recombination (radical disproportionation is not possible because no polymers with double bonds have been detected). These chains could grow into the gas phase as long as monomer feed is available thereby bridging two microcrystals. They are able to form reentrant loops on crystal surfaces and bizarre meshes of intertwined polymer threads. How can one identify the most plausible hypothesis on the basis of existing information or additional experiments ? Model 1 will not support surface coverage by polyethylene chains, model 2 allows surface coverage but cannot explain intercrystal polymer chains, whereas both are enabled in model 3. From the scanning-electron micrographs, fig. 4, we observe that the micromorphology of a polyethylene flake extracted from the “composite” by dissolving the inorganic matrix resembles strongly that of the host zeolite crystals, especially their surfaces. Furthermore, the mimicked (negative) crystal skeleton of the zeolite formed by the polyethylene mesh is totally connected over many crystalreplicas. Both observations favour model 3. From the photographs of fig. 4 it becomes clear that both the commercial and the synthesised polyethylenes are transformed to the same habitus after melting and resolidification. A definitive proof for a sustained existence of hydrocarbon radicals could be obtained by repeating the polymerisation experiment in a high pressure cell within the cavity of an ESR-spectrometer [28].
F. Blatter, E. Schumacher
/ Cs catalyzed ethylene polymerisation
581
582
F. Blatter, E. Schumacher
/ Cs catalyzed ethylene polymerisation
5.2. Comparison of different a~~a~~metals Our data show that both bulk metallic caesium and caesium clusters in zeolites catalyse the polymerisation of ethylene. Whereas with bulk caesium a mixture of many unsaturated oligomeric compounds is formed the reaction of ethylene in caesium loaded zeolite leads to polyethylene of high molecular weight alone. With other alkalis differences are expected resulting from the changes in the ionisation potentials and polar&abilities from Li to Cs. In experiments with zeolites containing 10 atoms of metallic sodium or potassium per unit cell only traces of high molecular paraffins resulted. The reaction of Rb,,[NaY] zeolite under the same conditions as in 3.1 leads to high molecular paraffins with a yield of 10% polymers by weight, i.e. 5 times larger than with Cs. However, the resulting paraffins showed a lower average degree of polymerisation as indicated by a melting range from 105 to 110 o C. The reason for the higher yield is probably the lower space filling by the same number of Rb atoms in the zeolite channels and hence more free volume for adsorption of ethylene rather than enhanced reactivity of the rubidium clusters.
5.3. Metal support interactions and use of Al,O, Adsorption of alkali metals on dehydrated alumina produces blue or green-blue chromophores. Because of the broader pore size distribution of the a-alumina used the colouration has probably a component of Mie scattering at small metallic particles in the range of 50 A diameter. ESR-measurements show typical conduction electron signals similar to those of the highly metal loaded zeolites [24,28]. The same polyme~sation experiments with caesium and rubidium dispersed on porous cr-alumina lead to a different product spectrum: From unsaturated compounds of molecular weight around C i0 up to long paraffins with more than 100 carbon atoms all sizes are obtained.
5.4. Chemical reactivity of metal clusters in zeolites Additional informations on the chemical reactivities of alkali metal clusters in zeolites are pertinent to this discussion: Stoichiometric intrazeolite reactions like Wurtz-Fittig coupling and related reactions with chlorinated hydrocarbons give evidence for the different chemical behaviour of clusters and bulk [32]. Geismar and Westphal [25] claim that alkali metals dispersed in zeolites are less reactive than in bulk. As a consequence fewer radicals would be initiated and thus longer polymer chains formed by dispersed caesium, in agreement with the present work. Experiments with propylene at 70 o C and 40 bar did not give polymer with
F. Blatter, E. Schumacher / Cs catalyzed ethylene po!vmerisation
583
zeolite/Cs.
a-alum.ina/Cs yielded very small amounts of oligomeric products. Butadiene, which is even more reactive than ethene, did not show a reaction with zeolite/Cs at normal pressure but at 10 bar a broad distribution of ~saturated oligomers was produced. In both cases a sorption barrier may exist towards the partially Cs filled zeolite so that in a short reaction period only Cs clusters near the crystal surface are accessible,
6. Conclusions
We are aware of two other recent papers on olefin reactions in zeolites: O~gome~sation was observed in zeolite H-ZSM-5, and dimerisation of ethylene with propylene in Cr3+ exchanged zeolite NaY [39]. In both cases only low molecular products resulted and the mechanism was considered as a cationic induction. To date the heavy alkali-olefin reactions in NaY are rather unique. It is interesting to point this out in perspective and conclude with possible applications. We shall concentrate on Cs alone. The Cs on NaY sorbates with an average content of 10 Cs per unit cell contain a very dilute but clustered form of Cs. ESR spectra reveal an association of the clusters with lattice cations to form delocalized electronic states probing between 4 and 6 nuclear spins. This arrangement is probably quite fluid and does promote ovation of uncharged Cs within the channel system. In contact with a Cs cluster ethene obtains an electron thus becoming an anion radical. Within the spatial constraints of the zeolite this reactive intermediate can grow an aliphatic chain by end-on reaction with the olefin propagating its f localized odd electron together with its front end. Chain termination occurs, when two growing chains meet within or outside of the zeolite crystal by radical recombination. This scheme works well if the olefin in the pore system is dense enough to form queues which support a rapid radical propagation. Therefore, several dozen bars of ethene pressure are necessary to obtain long chain paraffins. The property of the Cs sorbate to initiate this sequence of reactions is pecuIiar to the cluster and the supporting zeolite. Neither of them alone nor the bulk phase produces it. Reaction stops several seconds after admitting ethene to the NaY/Cs because the polymer chains seal the crystal completely from the outside environment. The plugged average volume of between 10 to 30% of each cubic crystal (determined from the rem~~ng active Cs> fills a surface shell of depth between 2 and 6% of the cube’s side. Thermal movements of the polymer chains always allow small molecules to migrate along them. Therefore, Cs in the inner part of the zeolite is bleached by oxygen after several hours exposure to air. Mechanically the polyethylene mesh within and over the surface of each zeolite crystal has some remarkable properties akin to bone tissue. A large part
584
F. Hatter,
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/ Cs catalyzed ethylene polymerisation
of the polyethylene is not removable from the crystal without destroying the latter. This makes a perfect hydrophobic nonbleeding phase for chromatography. - By means of their securely anchored polyethylene surfaces, zeolite crystals can be molded into thermoplastics, glued onto thin plastic webs and, if desired, mech~cally opened up at the outer side to their unchang~ 70% irmer volume which remains an active adsorbent. - The zeolite obtains almost complete acid resistivity by the polyethylene wrap. This allows chemical surface treatment and derivatization. - Since the polymerisation of ethene is promoted by a small amount of sorbed Cs in the surface layer of the zeolite, the inner part of the crystals may be preloaded by a different sorbate, possibly an air sensitive material. This can be protected by creating a tight polymer wrap initiated by Cs. - The list of possible applications of this and similar systems is very large, indeed. Acknowledgements
We thank the Swiss National Foundation, grant No.2.431.84/2.273.86, and the “Kommission zur Forderung der Wissenschaftlichen Forschung”, grant No. 1467/1211, for financial support; Professor R. Giovanoli, M. Faller, and B. Frey (University of Bern) for X-ray diffraction and electron microscopy; Professor P. Wachter, and PD Dr. E. Kaldis (ETH, Ztirich) for UV/VIS diffuse reflectance spectroscopy; Professor U.P. Schlunegger, and Hans Gfeller (diversity of Bern) for help in GC-MS problems; Professor M. Neuenschwander (University of Bern) for discussions; Professor A. von Zelevsky, and Eva Moser (University of Fribourg), PD Dr. A. Schweiger, and Susanne Pfenninger (ETH Ztirich) for ESR measurements. References [l] M.M. Kappes and E. Schumacher, Surface Sci. 156 (1985) 1. [2] M.M. Kappes, M. S&k, P. Radi and E. Schumacher, 3. Chem. Phys. 84 (1986) 1863; M.M. Kappes, M. Schk, U. R~t~isberger, Ch. Yeretzian and E. Schumacher, Chem. Phys. Letters 143 (1988) 251. [3] A.W. Castleman and R.G. Keesee, Ann. Rev. Phys. Chem. 37 (1986) 525; G.C. Bond, Surface Sci. 156 (1985) 966. [4] M. Boudart, A.W. Aldag, L.D. Ptak and J.E. Benson, J. Catalysis 11 (1968) 35. [5] K. Foger and J.R. Anderson, J. Catalysis 54 (1978) 318. [6] H.A. Benesi, R.M. Curtis and HP. Studer, J. Catalysis 10 (1968) 328. [7) R.A. Dalla Betta and M. Boudart, in: Proc. 5th Intern. Congr. of Catalysis (1972) p. 1329. [8] P. Gallezot, Surface Sci. 106 (1981) 459. [9] M. Boudart, J. Mol. Catalysis 30 (1985) 27. [lo] R. Whetten, M. Z&in, D. Cox and A. Kaldor, J. Chem. Phys. 85 (1986) 1697. [ll] M. Geusic, M. Morse and R. Smalley, J. Chem. Phys. 82 (1985) 590, 2292.
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