J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2827-2835 IR Study of Ethene and Propene Oligomerization on H-ZSM-5: Hydrogen-bonded Precursor Formation, Initiation and Propagation Mechanisms and Structure of the Entrapped Oligomers Giuseppe Spoto, Silvia Bordiga, Gabriele Ricchiardi, Domenica Scarano, Adriano Zecchina" and Enzo Borello Dipartimento di Chimica lnorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, Via Pietro Giuria 7,l-10125 Torino, Italy The oligomerization reaction of ethene and propene on H-ZSM-5 has been studied by fast FTIR spectroscopy. Oligomerization proceeds through : (i)formation of short-lived hydrogen-bonded precursors by interaction of the alkene with the internal acidic Brsnsted sites, (ii) a protonation step and (iii) a chain-growth step.The relative strength of the hydrogen bonds in the ethenMH and propenMH n-complexes (precursors) is estimated on the basis of the downward shift of both the v(0H) and v(C=C) frequencies (-389 and -11 cm-' for ethene and -539 and -19 cm-' for propene). For both molecules, the protonation of the precursors is the rate-determining step of the oligomerization process. The chain-growth mechanism and the structure of the entrapped oligomers are discussed on the basis of computer graphic and molecular dynamics simulations. Mainly linear or low branched products are formed whose length and structure is essentially determined by the steric hindrance imposed by the zeolitic framework. In a recent paper devoted to a IR and UV-VIS investigation of the proton-catalysed oligomerization of acetylene, methyl- acetylene and ethylacetylene in H-ZSM-5 channels,' we have shown that the acetylenic hydrocarbons form hydrogen-bonded precursors by interaction with the Brsnsted-acid sites of the zeolite, which are then slowly protonated and oligo- merized to give short conjugated carbocationic chains.The lifetime of the hydrogen-bonded precursors was sufficiently long to allow the detection of their vibrational spectrum. On the basis of the shift and half-width of the perturbed OH stretching bands it was possible to calculate the strength of the hydrogen bonds between the triple bond and the Brsnsted sites, and to conclude that there is a clear relation- ship between the strength and the rates of protonation and oligomerization.Methyl and ethyl group substitution also favours nucleophilic attack. The effect of the steric constraints, constituted by the three- dimensional array of intersecting channels typical of the MFI structure of ZSM-5, was also studied in detail, leading to the conclusion that the positively charged chains are character- ized by an average oligomerization number that is essentially dictated by the distance between the channel crossings. This leads to conjugated products whose main TC -,n* electronic transitions are located in the 30 000-25 OOO cm-range. On the basis of these results, it is conceivable that ethene and propene oligomerization (which are known to occur in H-ZSM-5 channel^^.^) could be preceded by the formation of hydrogen-bonded complexes and that the strength of the hydrogen bonds could be evaluated by measuring the pertur- bation of the OH and C=C stretching bands.Similarly, it is also expected that the three-dimensional array of the inter- secting channels should have a distinct effect on the length and on the branching of the positively charged oligomers. This effect could be studied in detail by means of IR spectros- copy, taking advantage of the prolific literature concerning the IR spectra of saturated hydrocarbons4 and by compari- son with the 13C NMR results.' In this paper we report the IR spectra, recorded under reaction conditions by in situ FTIR spectroscopy, of the hydrogen-bonded precursors and growing oligomers formed by interaction of ethene and propene with H-ZSM-5 zeolite.For propene, the hydrogen-bonded species are found to have a very short life (of the order of few seconds); the vibrational spectrum is hence investigated by using fast scanning condi- tions. Experimental The H-ZSM-5 samples used in these experiments were syn- thesized in the ENICHEM, Centro di Ricerche di Bollate, laboratories. Most of the experimental data reported here were obtained on samples with high external surface areas (crystallites with dimensions in the 20-50 nm range) and low (ca. 20) Si :A1 ratios. Full characterization of this material is given in ref. 6. Before they were dosed with ethene or propene, H-ZSM-5 samples (in the form of self-supporting wafers suitable for IR transmission measurements) were outgassed under high vacuum (< 10-Torr) at 673 K for 3 h to remove water and other adsorbed impurities. The thermal treatment was carried out in the same cell as was used for the transmission IR measurements.Ethene and propene (Matheson, high-purity grade), pre- viously purified by repeated freeze-pumpthaw cycles, were dosed from a vacuum line permanently attached to the IR cell. The IR spectra were recorded (at 4 cm-' resolution) on Bruker IFS48 or IFS88 FT instruments. For the fast acquisi- tion of the interferograms the Bruker LC/CG IR software package was used, running on an Aspect 1000 computer and allowing the recording of ca. 7 interferograms s-(at 4cm-' resolution).Computer graphics and modelling of the structure of the zeolite and of the entrapped oligomers, and molecular dynamic calculations were performed with software programs from Biosym Technology Inc. running on a SGI4D35 work-station. Results and Discussion H-ZSM-5-Ethene System As the IR spectrum of the H-ZSM-5-C2H4 system changes with time because of the occurrence of oligomerization, it is useful to divide the whole set of experimental data into three 2828 0.5 h UJ +d.-C 3 r; V." v _. 1600 1400al tm e ns 0.0 3800 3600 3400 3200 3000 2800 wavenumber/cm -' Fig. 1 IR absorbance spectra showing the formation of hydrogen- bonded precursors by interaction of ethene with H-ZSM-5.(-) Pure H-ZSM-5 outgassed at 673 K; (----) after dosage of C2H4 (5 Torr); (-.-.) effect of increasing the C2H4 pressure to 10 Torr. The spectra in presence of ethene were obtained for contact times 6 10 s. At the bottom the spectrum of C2H4 in the gas phase is shown for comparison. parts (A-C). In part A the spectra obtained immediately after dosage of two different amounts of ethene (and for a total contact time not exceeding 10 s) are considered, with the aim of elucidating the structure of the primary interaction pro- ducts (precursors). In part B the sequence of spectra recorded at 6.8 s intervals for a total contact time of ca. 1.5 min is reported and discussed. This sequence is expected to give information on the first stages of oligomerization and on the structure of the shortest oligomers.Finally, part C is devoted to the discussion of the sequence of spectra obtained for contact times in the 1.5 min to several hours interval, where chain propagation is supposed to be the predominant phenomenon. This sequence is expected to give information about the mechanism of chain propagation under the space restrictions imposed by the zeolitic framework and about the structure of the oligomeric chains under conditions approaching pore filling. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Hydrogen-bonded Precursors (Contact Timed 10 s) The IR spectrum of pure H-ZSM-5 (previously outgassed at 673 K for 3 h under high vacuum) is shown in Fig. 1, together with the spectra obtained at room temperature in the pres- ence of two doses (5 and 10 Torr) of ethene (and for a total contact time of ca.10 s). In the same figure the spectrum of pure ethene in the gas phase is also shown for comparison. The spectrum of the pure zeolite is characterized by a triplet in the OH stretching region at 3746, 3660 and 3609 cm-' which is assigned as follows:6 (i) 3746 cm-: v(0H) of Si-OH groups (silanols) mainly located on the external surface of the zeolite particles. This peak is particularly intense because of the small dimensions (20-50 nm) of the microcrystals constituting the zeolite sample used in this experiment; (ii) 3660 cm-': v(0H) of A1-OH groups in defective (partially extra lattice) positions; (iii) 3609 cm-': v(0H) of structural -Si-(OH)-Al- groups (structural Brrnsted-acid sites).Dosage of ethene results in the gradual disappearance of the 3660 and 3609 cm-' absorptions with simultaneous for- mation of two much broader bands (FWHM = 200 cm-') at 3369 and 3220 cm-'. Note that the process is accompanied by the appearance of a clear isosbestic point at 3560 cm-' (Fig. 1). Analogous experiments (results not reported in detail for the sake of brevity) performed on zeolite samples charac- terized by different Si:Al ratios, where the intensity ratio of the bands at 3660 and 3609 cm-' (outgassed samples) and consequently of the bands at 3369 and 3220 cm-' (samples contacted with ethene) is different, imply that the band at 3369 cm-' originates from perturbation of that at 3660 cm-' (with a downward shift of 291 cm-') and the band at 3220 cm-' from perturbation of that at 3609 cm-' (downward shift of 389 cm-').On all samples the 3746 cm-' absorption is hardly affected. As far as the spectroscopic features of adsorbed ethene (which are compared in Table 1 with those of the ethene mol- ecule in the gas phase') are concerned, the following com- ments can be made: (i) the vasym(CH2) (B2J and vsy,(CH2) (B1J modes (which are observed, respectively, at 3106 and 2990 cm-' in the gas phase) are shifted to lower frequency after interaction, giving rise to new bands at 3095 and 2974 cm-;a small downward shift from 1444 (gas phase) to 1440 cm-' (adsorbed state) is also found for the 6(CH,) (B1J mode which also gains intensity; (ii) the v(CC) (Alg) and Table 1 Most relevant spectroscopic features of H-ZSM-5 [v(OHj region], C2H4, C,H,, C2HJH-ZSM-5 and C,H,/H-ZSM-5 (in cm-'); owing to the uncertainty in measuring the positions of the bands, only a few frequencies are reported for the n-complex of C,H, C2H4-OH n-complex C,H,-OH n-complex H-ZSM-5 C2H4 (gas) C,H, (gas) V Ai V Ai assignment' 3476 3476 0 3660 3369 29 1 2609 3220 389 3106 308 1 3095 11 - 3012 - - 2990 2979 2974 16 - 2960 - - - 2916 - - -_ 2852 - - 1623 1647 1612 11 - 1472 - - - 1448 - - 1444 1416 1440 4 - 1399 - - 1342 1340 2 Ref. 6 and 7.J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 6(CH,) (Alg) modes, which are IR-inactive in the free mol- ecule and are responsible for two Raman lines at 1623 and 1342 cm-', become IR-active in the adsorbed state, giving two weak bands at 1612 and 1340 cm-l.Both the shift to low frequency of the OH stretching bands and the perturbation of the vibrational modes of adsorbed ethene can easily be explained on the basis of the formation of the 1 : 1 n complexes HZCTCH2 a In particular, owing to the rehdced symmetry of adsorued C2H, and to the reduced density of charge of the carbon- carbon double bond, structures I and I1 account for the IR activation and for the small downward shift (Av = -11 cm-') of the v(C=C) mode. Formation of hydrogen-bonded n-complexes was pre-viously evidenced by IR spectroscopy for the interaction of ethene with hydrogen halides*-" and with the acidic Brsnsted sites of Y zeolite^.^'-'^ In the latter case the shifts of v(0H) were in the range 370-350 cm-l.The higher value observed on H-ZSM-5 for complex I(389 cm-') is caused by the higher acidity of the Brsnsted sites in the MFI structure. The different acidity of the two zeolites is also demonstrated by the fact that, unlike H-ZSM-5, ethene is not oligomerized by H-Y zeoliteg (only weakly adsorbed species are formed). Note that under the temperature and pressure conditions 2829 adopted here, no formation of hydrogen-bonded complexes with the silanols (responsible for the band at 3746 cm-') is observed. This type of species is found only by lowering the temperature or by increasing the gas pressure (spectra not illustrated for brevity). This behaviour is fully consistent with the low acidity of silanols.Based on the extent of the v(0H) shifts upon ethene adsorption, the Brsnsted acidity is in the order : -Si(OH)Al-> -Al(OH)( b -Si-OH) in agreement with what was found by adsorption of CO at 77 K. It has been reported that the hydrogen-bonded species I are associated with a formation enthalpy of CQ. 38 kJ mol-'. Of course not all this enthalpy is related to hydrogen-bond formation. In fact, AH can be roughly split into two terms, AHa and AHb: AHa is associated with the pure hydrogen-bonding interaction (involving the Bransted sites and the C=C bond), while AHb is due to the van der Waals interaction mainly associated with the CH, groups.AHb can be estimated from the adsorption enthalpy of n-hexane5 and is ca. 11 kJ mol-'. We infer that the enthalpy associated with the pure hydrogen-bonding interaction is of the order of 16 kJ mol-', in reasonable agreement with the literature data concerning quantum-chemical calculations of the protonation of alkenes by acidic OH groups of iso- morphously substituted zeolite^.'^" Protonation Mechanism and Structure of the Shortest Oligo-mers (Contact Time 0-130 s) The IR spectra (recorded at the temperature of the beam and at intervals of 6.8 s) of the C,H4-H-ZSM-5 system for contact times between 0 and ca. 2 min are shown in Fig. 2. k h T0.03 ?its) 1 L I I I II r I I I1 I r I 14 3000 2900 2800 1600 1400 wavenumber/cm-' wavenumber cm-' Fig.2 IR absorbance spectra showing the initial stages of C,H, oligomerization on H-ZSM-5. The spectral sequence (1-19), obtained by recording one spectrum every 6.8 s, covers the contact time interval 0-130 s: (a) v(CH,) and v(CH,) region; (b) v(C-C), 6(CH,) and 6(CH,) regon. The spectrum of gaseous ethene is shown at the bottom for comparison. After ca. 7 s of contact (curve 1 in Fig. 2) the spectrum is characterized by the peaks, already discussed, due to the hydrogen-bonded precursors (bands at 2974, 1612, 1440 and 1340 cm-'). In the successive spectra new weak features, assigned to saturated CH, and CH, groups, develop at 2960 and 2876 cm-l [v,,,,(CH,) and vsym(CH3)], 2940 and 2866 cm-' [vasym(CH2) and vsy,(CH,)], 1460 and 1382 cm-' [G,,,,(CH,) and G,,,(CH,)] and 1469 and 1442 cm-' [dasymm(CH2)and 6,,,(CH2)].The appearance of the charac- teristic modes of saturated (CH,) and CH,) groups is a clear indication that a protonation and polymerization process occurrs following the Scheme 1, where (because of the rela- H2C =;'CH2 I Y Y Scheme 1 tively small amount of defective Al-OH groups) only chains growing on Si-(OH)-A1 sites are shown. C,H, stands here for the possible isomers illustrated in Scheme 2, which are in chemical equilibrium. Similarly, C,H ,represents different isomers in chemical equilibrium, and so on. In agreement with ref. 14-18 structures (a), (b), (c) and (d)have deliberately \si.o.T' 0 been written in covalent form, even if a certain degree of ion- icity cannot be excluded (which could be enhanced by the 'solvating' effect of the zeolite framework). As already pointed out by Van den Berg et al.,' because of the con- straints imposed by the zeolite superstructure, the distribu- tion (in chemical equilibrium conditions) of the possible isomers in the channels is not necessarily identical to that observed in the gas phase or in solution (where the more branched and energetically more stable isomers are preferred). Detailed information concerning the reaction mechanism and the nature of the products can be gained by closer inspection of the spectral sequence of Fig. 2. From this, we can first infer that the number of oligomeric chains formed in the time interval considered here must be very small, as demonstrated by the negligible decrease of the bands of the hydrogen-bonded precursors (see in particular the peaks at 1612, 1440 and at 1340 cm-l).Furthermore, if the very early stages are considered (spectra 1-4 in Fig. 2 corresponding to contact times between ca. 7 and ca. 30 s) it can be concluded that as far as the first-formed products are concerned, there is no sign of -CH(CH,), and -C(CH,), chain branching (which should contribute an extra, characteristic peak at ca. 1368 cm-').19 It is also most noticeable that in the sequence J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 of Fig. 2, the intensity ratio I(CH,)/I(CH,) of the (CH,) and (CH,) stretching modes is constantly greater than 1.As the specific intensity of the (CH,) stretching modes is known to be always larger than that of the (CH,) groups,20-22 this observation [together with the absence of branched products containing -CH(CH,), and/or -C(CH,), groups] indicates that the only species formed in the 7-30 s time interval are small, linear -0-C,H,,+ oligomers. Moreover, as I(CH,)/I(CH,) is between 1 and 2, it is also inferred that structures of type (c) are less important. Whether spectrum 2 of Fig. 2 corresponds to a -0-CH,CH, species only or, more probably, to a mixture of -0-CH,CH, plus linear -0-(CH,),CH, species cannot be stated with confidence because a true reference compound is not available and because the considerations based on the intensity ratios must always be used with caution.In any case, due to the predominace of short, linear structures the 7-30 s interval can be considered as essentially characterized by the protonation reaction (initiation step). In principle the -O-c& species could also be under the branched forms (b) and (4,formed through isomerization of linear species. The experimentally ascertained absence of branched forms of the -O7c4H9 dimer in the very early stages of oligomerization implies that the isomerization rate is lower than the protonation (initiation) rate (as we shall see, the propagation rate is, however, larger). This observation is intriguing: at least for the smallest oligomers formed, for instance, at the channel intersections, no space restrictions limiting the isomerization rate to the (b) and (d) type oligo- mers are expected to exist.A plausible explanation for the experimentally observed low propensity to form branched oligomers can be given, however, if partial covalent character of the oligomers is considered. While it is well known that fully ionic C4H9+ species should readily give the (CH,),C+ carbocationic species, the same does not hold for more cova- lent structures, even in absence of steric constraints. In con- clusion, the experimental results essentially confirm those obtained from quantum-chemical ~alculations,'~-' suggest- ing a high degree of covalent character of the C-0 bond. In the section devoted to propene polymerization we shall add further arguments in favour of the hypothesis that the rate of chain propagation is larger than the rate of protonation (initiation) and isomerization.Based on these spectroscopic results, the evolution of the C,H,-H-ZSM-5 system in the 0-30 s interval is graphically represented in Plate 1 (where the zeolite framework is oriented along the [OlO] crystallographic direction). Con- cerning the zeolitic structure, in Plate 1 we have chosen a representation which is informative about the internal space accessible to molecules moving inside the channels and where the crystallographic features of the zeolitic lattice are omitted. This goal was achieved using the Connolly algorithm,, which allows the 'Connolly surface' to be constructed.This surface represents the boundary of the volume from which a probe molecule (H,O in this case) is excluded if it experiences van der Waals overlap with the atoms of the zeolite frame- work. In more detail, Plate l(a) shows the situation before admission of ethene (pure zeolite): the accessibility in the intersecting straight and sinusoidal channels (running along the [OlO] and [lo01 directions, respectively) of the Brnmsted- acid sites (represented as red balls for which the Li+ van der Waals radius was adopted) is evidenced. Plate l(b) shows the complete transformation of the Brmsted sites into the hydrogen-bonded 1 : 1 complexes through interaction with ethene. Finally, in Plate l(c) the situation after ca.30 s of contact is shown. In this last picture randomly distributed, short, linear -C,H, and -C,H, oligomers growing in the straight and sinusoidal channels are seen, together with the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 1 Computer graphic models showing: (a)the H-ZSM-5 channel array (Connolly representation) and distribution of Brmsted-acid sites (red balls); (b) the formation of 1 : 1 n-complexes upon interaction with ethene; (c) the formation of entrapped short and linear protonated species G. Spoto et al. (Facing p. 2830) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Plate 2 Computer graphic model illustrating the location (as obtained by molecular dynamic calculations) of: linear (A), containing terminal isopropyl groups (B), and (CH,),CH oligomers (C) inside the silicalite framework Plate 3 Computer graphic simulations showing the distribution inside the H-ZSM-5 framework of unreacted precursors and oligomerization products after 130 s of contact with ethene.(a)View along the [OlO] crystallographic direction; (b) view along the [loo] direction J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2831 vast majority of unreacted hydrogen-bonded precursors (in agreement with the experimental observations). The implicit assumption used to construct the previous representations is that the hydrogen-bonded precursors are all equivalent, because they are formed through interaction with Br~rnsted sites of very similar acidity. Following this simplified hypoth- esis, the fact that a very small fraction of these complexes has evolved into protonated species in this time interval is simply due to the kinetic effects associated with the high activation barrier for the protonation process and not to the existence of a small fraction of more acid sites.When the contact time increases beyond 30 s (Fig. 2, curves 5-19), the intensity of the v(CH,) and v(CH,) bands grows proportionally. Moreover, a weak peak at 1366 cm-' is observed to emerge from the background: this is a clear indi~ation'~that, as well as the formation of trimeric and longer oligomers of the linear type, branching occurs with possible formation of species like (e) and (f).Based on simple (n= 0,l .......) (n = 0,l .......) (8 1 (f 1 geometrical considerations, one suspects that the growth in the H-ZSM-5 channels of oligomers containing terminal iso- propyl (e) and tert-butyl (f) groups can suffer in different way from the space restrictions imposed by the framework (because of their different dimensions compared to the channel diameter). To clarify this point we decided to invest- igate the mobility (at different simulated temperatures) of en- trapped hydrocarbons of the type CH,(CH,CH,), CH(CH,), and (CH,),CH (as models of those formed inside the H-ZSM-5 channels) by means of molecular dynamics calcu- lation~.,~For the sake of simplicity, silicalite (i.e.a pure sili- ceous material having the same structure and channel diameter as ZSM-5) was chosen as the model for the zeolite structure.The result was that, while there are no constraints limiting the diffusion of isomers containing the isopropyl group (or less branched species), the (CH,),CH species cannot enter the channels and can only be located at the channel intersections, where it almost completely fills the available space. A graphical representation of the results obtained by molecular mechanics (by considering a fixed zeolite framework and by allowing the different neutral isomers to reach the lowest-energy configuration) is shown in Plate 2. It follows that the -0-C(CH,), species derived from isomerization of the -0-C4H, moieties, once formed cannot add further ethene molecules. On one hand they com- pletely fill the space at the channel intersections (preventing further C2H4 molecules from reaching the catalytic site) and, on the other hand, they are too large so they cannot grow and penetrate the channels.In other words, the only possible value of n for the hypothetical -0-(CH,CH,),C(CH,) species (f)is zero and the entrapped 0-C(CH,), species behave as dead oligomers. By comparison of the intensities of the v(CH,) and v(CH,) bands in the stretching and bending regions (Fig. 2) with those of model hydrocarbons,20-22 we infer that the oligo- mers formed for contact times in the 20-100 s interval are characterized by a CH,: CH, ratio in the (2-4) : 1 range. Although this stoichiometry is very approximate, some general conclusions can be derived.For instance, if we con- sider that the CH, :CH, ratio in isobutyl and isopentyl struc- tures is, respectively, 1 : 2 and 2 : 3 (in comparison with the value 2-4 : 1 observed experimentally), it is concluded that not only are the (CH,),C-groups, possibly formed at the channel intersections (and on the external surface as well), small or negligble in number, but also the oligomers (e) con-taining the isopropyl group must represent a minor fraction. After ca. 130 s of contact (considered somewhat arbitrarily as the time interval where initiation is more important than propagation) CH,:CH, finally reaches 4-5 : 1 [Fig. 2(a), curve 191. Note that the number of chains (equivalent to the number of hydrogen-bonded precursors consumed) formed at this time must still be very small, as demonstrated by the small (ca.10%) decrease in the 1612 cm-' band (characteristic of the ethene-OH n-complex). When the overall integrated intensity of the bands due to the stretching vibrations of the CH, and CH, groups (3050-2800 cm-') is plotted as a function of time, Fig. 3 is obtained. The integrated intensity in the 1395-1355 cm-' range is also shown in Fig. 3. Note that in the latter region only the CH, bending modes contribute; hence from the examination of this spectral interval we can obtain an insight as to the time evolution of the CH, groups. As these groups give a qualit- ative indication of chain termination, and hence of the approximate number of chains formed, from Fig.3 it can be inferred that in the 0-130 s time interval not only do we have growth of the oligomeric chains but also the parallel and con- tinuous formation of new species through the protonation of the unreacted hydrogen-bonded precursors. Based on the previous discussion, the situation after 130 s is represented graphically in Plate 3 (as obtained using the correct van der Waals surfaces of the polymeric chains). The following comments can be made: (a) notwithstanding the small dimensions of zeolitic pores and the sinusoidal behav- iour of some of them, linear polymeric chains can grow freely in the straight and the sinusoidal channels without any hin- drance. Polyethylene is flexible enough to follow any channel shape. (b) As noted previously, branched oligomers contain- ing the -CH(CH,), group fit exactly the space available in the channels, while the bulkier tert-butyl residues can be accommodated only at the channel intersections.Owing to the experimentally ascertained low branching rate, the con- centration of these species is, however, very low (and corres- pondingly only one of these groups is reported in the simulations for the sake of illustration). (c) The small channel dimensions prevent the presence of more than one chain inside the same channel portion. This means that when a living polymeric chain reaches a channel intersection where another polymerization centre is functional, it must stop its growth. This observation is in agreement with the spectro- scopic results which indicate the presence of chains with a limited number of carbon atoms.(d)When the growing oligo- meric chains reach portions of the framework where unre- acted precursors are present, owing to the limited space available they can interact strongly with the n-bonded ethene molecules. As will be shown in the following, this results in ethene being displaced from the complex. Structure of the Oligomers after Prolonged Contact Time The evolution of the spectra for prolonged contact time is illustrated in Fig. 4. The most relevant results are: (i) the bands associated with the (CH,) groups gradually take over and become the predominant features [in both the C-H stretching, Fig. *a), and bending, Fig. 4(b), regions]. At the end of the time interval shown in Fig.4, the intensity ratio between the CH, and CH, bands points towards an average CH, :CH, z 7-8 : 1 stoichiometry. This implies that if only linear polymers are considered, the average chain is made of ca. eight carbon atoms. It is interesting to note that a chain containing this number of carbon atoms almost exactly fills the space between two channel crossings (Plate 3). The pre- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2.0 1.5 r I 5 m m m v mV m 1.0 2 ms? 0 -0 +-h 13, +-. .-0 3.5 3I I 0 0.10 0.20 0.30 0.401 42 43 4 Fig. 3 Integrated intensity of the IR bands in the 3000-2800 and 1395-1355 m-’ regions as a function of the ethene-H-ZSM-5 contact time I 1I.. i . I I 3600 2900 2ioo 1600 1400 wavenum ber/cm -’ wavenumber/cm-’ Fig. 4 IR (absorbance) spectra of the ethene oligomerization products formed over a prolonged contact time. The spectral sequence covers 1:he 40(spectrum 1)-1040 (spectrum 26) s interval. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 vious hypotheses on the length of the chains have been made without considering the possible presence of isopropyl and tert-butyl terminations [the latter, containing three CH, groups, could account for a considerable fraction of the I(CH,) intensity even when present in very small amounts]. Consequently, the figures previously given for the chain lengths must be considered as underestimated. (ii) The rate of growth of the bands in the 3050-2800 cm-range (mainly due to CH, formation) decreases gradually (Fig.3 and 4); this is a clear indication of the progressive filling of the empty space where the oligomeric chains are forced to grow and is in agreement with the previously advanced hypothesis that the average number of carbon atoms in the chains is ca. eight. The growth of the chains is necessarily stopped at the channels intersections because they encounter either other growing oligomers or dead species that fill the voids at the intersections (as will be shown in the following, the number of sites at which protonation and oli-gomerization occur is ca. 20% of the total). (iii) The IR bands of the hydrogen-bonded precursors diminish progressively and at the end of the experiment they are reduced to ca.40% of their original value [see the intens-ity of the bands at 1612, 1440 and 1340 cm-' in Fig. 4(b)]. The decrease of concentration of the hydrogen-bonded pre-cursors is also demonstrated by the changes occurring in the OH stretching region illustrated in Fig. 5. As the reaction continues the absorption centred at 3220 cm-' gradually declines in favour of a new broad and complex band with a maximum at ca. 3470 cm-'. As will be demonstrated later, this new absorption is due to OH groups interacting with the polymeric chains. The residual presence of a considerable quantity of unre-acted hydrogen-bonded precursors even after prolonged contact is demonstrated by the results shown in Fig.6, where the effect of removing the gas phase is illustrated. When the sample is outgassed at room temperature, the bands charac-teristic of the n-complex at 1612, 1440 and 1340 cm-' and the broad band at 3220 cm-' disappear rapidly while the peak at 3609 cm-', characteristic of the unperturbed Si(0H)Al groups, simultaneously shows up. In addition, the spectrum in the OH stretching region shows a novel feature at ca. 3470 cm-' (i.e. at the same frequency as the band appearing after prolonged oligomerization time). The origin of this band can be readily understood if consideration is made of the results of the adsorption experiment of a saturat-I 10.0 , I ,J 3800 3600 3400 3200 wavenumber/crn -' Fig. 5 IR (absorbance) spectra illustrating the changes occurring in the v(0H) region during ethene oligomerization (full lines).Broken line: spectrum of pure H-ZSM-5. 2833 0.71 II 1 1I 1I V." 3800 3600 3400 3iOO wavenumber/cm -' Fig. 6 Effect of outgassing the ethene-H-ZSM-5 system at room temperature. Curve 1: after 1040 s of contact. Curves 2-4: after out-gassing for 1 (2), 5 (3) and 30 (4) s. Broken line: spectrum of pure H-ZSM-5. ed hydrocarbon (n-heptane) on H-ZSM-5 illustrated in Fig. 7. We note that upon heptane adsorption: (i) the band at 3609 cm-' (bridged OH groups with Brsnsted-acid character) is shifted to CQ. 3474 cm-(Av = -135 cm-') with broadening (FWHM x 160 cm-'); (ii) in the presence of an excess of n-heptane (spectrum not shown), the band at 3746 cm-' (silanols) is shifted to 3700 cm-' (Av = -46 cm-') and is broadened (FWHM z 100 cm-').This experiment undoubtedly shows that saturated hydrocarbons perturb the Si(0H)Al and -SOH oscillators and that the effect in term of induced shift and band broadening (change of half-width) is larger for the more acidic group (as expected). As the fre-quency of the OH groups perturbed by n-heptane is identical to that observed after polymerization, this experiment also demonstrates that the saturated hydrocarbon chains growing inside the zeolite channels interact with the framework acidic OH groups, displacing the weakly adsorbed C,H, molecules. The process can be represented as follows: H2CICH2 I From this observation two further important conclusions can be drawn.First, the observed decrement of the band of the hydrogen-bonded precursors (1612 cm-I) observed when the ethene is left to stand in contact with H-ZSM-5 for a long time is not totally due to the transformation of the n-complexes into the protonated and oligomerized form. Sec-ondly, by comparison of the intensity of the residual peak at 3474 cm-' with that obtained in the blank experiment with n-heptane (where all the Brnrnsted sites are perturbed) we can infer that, contrary to the initial expectations, a minority (only ca. 20%) of the Brnrnsted-acid sites have undergone protonation, so acting as a true catalytic centre where chain initiation and propagation are occurring. This also demon-strates that the oligomeric chains formed on one-fifth of the Brnrnsted sites are sufficiently long to perturb nearly all the other (unreacted) Brnrnsted sites, displacing adsorbed C2H4 and decreasing the concentration of precursors. The decrease 1.! hIn c..-C 4 v a, C -e2 a 0.0 3800 3600 3400 3200 3000 2800 wavenumber/cm-’ Fig.7 IR (absorbance) spectra showing the perturbation of the v(0H) bands of H-ZSM-5 upon dosage of n-heptane.Broken line: pure H-ZSM-5. Full line: after dosage with n-heptane. in concentration of the precursors induced by pore filling also justifies the decrement of the oligomerization rate. Finally, this confirms the view that protonation is the slowest process and that a high activation barrier is present.Propene-H-ZSM-5 System When the sample is contacted with propene (4 Torr) at room temperature and the IR spectrum is recorded following the T A J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 procedure already described for the ethene-H-ZSM-5 system (i.e. with acquisition times of the order of 7 s), strong bands in the 3050-2800 cm-’ range due to CH, and CH, groups of saturated oligomeric species are the only features observed. This indicates that protonation and oligomerization are so fast that the hydrogen-bonded precursors and protonated (initiation) intermediates escape observation. In order to detect these short-lived intermediates, we decided to record the spectra following the Bruker GC/LC (gas/liquid chromatography) fast acquisition program which allows the collection of 7 interferograms per second at ‘4 cm-’ resolution.The results are shown in Fig. 8 (where each spec- trum is the average of 10 interferograms). Only the difference spectra (obtained by using the spectrum of the pure zeolite as background) are reported : in this representation the bands appearing as negative peaks belong to species which are con- sumed, while those appearing as positive peaks belong to species which are formed during the adsorption and oligo- merization. The main results emerging from this experiment are: (i) the hydrogen-bonded precursor ti& ICHCH3 I is characterized by v(0H) = 3070 cm-’ (with a downward shift of 539 em-’ with respect to the unperturbed species) 17.5x 10” (arb.units) I n 1 L I \I I I 1 I 1t . 3600 3200 2800 1600 1400 wavenumber/cm-’ Fig.8 IR (absorbance, background-subtracted) spectra illustrating propene (initial pressure 4 Torr) oligomerization on H-ZSM-5. The spectral sequence covers the 0-90 s time interval. The time interval between two successive spectra is 1.5 s. Each spectrum corresponds to the average of 10 interferograms. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 strate that teristic of the band at 3470 cm-', the CH, -CH(CH,), meric chains of the type siderations are based hydrogen formed similar acidity). All the HM of 420 cm-'. These dat 2835 and a FW a undoubtedly demon- grams to investigate transient species residing on the surface the hydrogen bond formed by interaction of for times not longer than tenths of a second.This timescale with the Brsnsted-acid sites is stronger th an that indicates that the observed time evolution is associated with y interaction with ethene (as etive effect of the CH, group) xpected on the basis of kinetic effects only and that this spectroscopy cannot be con- the inducpropene formed b . This in turn explains sidered as time-resolved in the usual sense (i.e. on the time- this experiment. A strong the higher (with respect to ethene) pro perturbation tonation rate sition of the b is also charac- scale dictated by the molecular and electronic motions). inferred fto the v(wards w rom the examination of the po and due Conclusions C=C) mode of the precursors, ith respect to the free molecule (Av = -19 cm-l) di which is shifted down- The three steps in the oligomerization of ethene and propene on H-ZSM-5 (formation of hydrogen-bonded precursors, ).(ii) The hydrogen-bonded w seconds (Fig. 8). This is d precursors sappear ue both to their con- protonation and chain propagation) can be studied individ- after a fe(Table 1displacembefore). (the avera protonation-oligomerization) and to ually by IR spectroscopy provided that a sufficiently fast sumption (caused by ated by the gr by the chains growing owth of acquisition rate is used (of the order of 7 s for ethene and 1.5 s for propene). characteristic annels (as is clearly demonstrd by the saturated hydrocarboent of the propene molecules in the chperturbe of the Brsnst ed sites n chains, as discussed The relative strength of the hydrogen bonding in the C,H,-OH and C,H,-OH n-complexes (precursors) as deter- iii) At the end of the experim ent (i.e.after c a. 90 s), mined by the shift of \(OH) and v(C=C) is much higher for ge CH, :CH, ratio (as estima ted by the inte nsity of propene (because of the inductive effect of the methyl groups). and CH, bands) is ca. 1 : 1 and no signs of This also accounts for the much higher oligomerization rate. and -C(CH,), groups aby the absence of the characteristic ban re observed (as argued d at ca. 1366 c m-'). A For both molecules, the slowest step of the reaction is found to be the protonation of the precursors. o is almost exactly the same 1 : 1 rati as expected f or poly- The steric constraint imposed by the three-dimensional array of channels of the zeolite framework strongly influences the nature (degree of branching, distribution and length) of (CH2CHCH3),CH2CHI 2CH, the products.References be used 1 S. Bordiga, G. Ricchiardi, G. Spoto, D. Scarano, L. Carnelli, A. Zecchina and C. Otero Arean, J. Chem. SOC., Faraday Trans., the ethene case, the methyl g of the modes of the terminal groups. groups are effroups cannot . This means tthe intensity of the band at ca. 3470 cm-' due to the stretching modes of the unreacted groups perturbed by saturated chains can be inferred that also in this case only (ca. 25%) of Brsnsted-acid Unlike here as indicators of chain terminationnot possible to estimate the length of by means From sites) it fraction the oligomeric chains (which is initiated at different ectively hat it is Brsnsted-acid a small 2 3 4 5 6 1993,89, 1843.J. R. Anderson, T. Mole and V. Christov, J. Catal., 1980,61,477. V. Bolis, J. C. Vedrine, J. P. Van den Berg, J. P. Wolthuizen and E. G. Derouane, J.Chem. Soc., Faraday Trans. I, 1980,76,1606. L. J. Bellamy, The Infrared Spectra of Complex Molecules, Chapman and Hall, London, 2nd edn., 1980, vol. 11. J. P. Van den Berg, J. P. Wolthuizen, A. D. H. Clague, G. R. Hays, R. Huis and J. H. C. Van Hooff, J. Catal., 1983,80, 130. A. Zecchina, S. Bordiga, G. Spoto, D. Scarano, G. Petrini, G. Leofanti, M. Padovan and C. Otero Arean, J. Chem. SOC., in protonation.In other wordte is increased greatly with rethe internal available space of the protonic sites have ef spect to ethene s, although the proto- is already filled when fectively been involved , it is so 7 Faraday Trans., 1992,88,2959, and references therin. K. Nakamoto, IR and Raman Spectra of Inorganic and Coordi- nation Compounds, John Wiley, New York, 4th edn., 1986; G. Herzberg, Infrared and Raman Spectra of Polyatomic Molecules, D. Van Nostrand, New York, 1947. in protonengaged nation rahigh that only 25% case the previous con- 8 A. J. Barnes, J. B. Davies, H. E. Hollam and J. D. R. Howells, J. on the implicit precursors are all equivalation. Of course, also in this assumption te hat the nt (because they are 9 Chem. SOC.,Faraday Trans. I, 1973,69,246.L. Andrews, G. L. Johnson and B. J. Kelsell, J. Chem. Phys., through interaction with B rsnsted sites of very 1982,76,5767. 10 L. Andrews, G. L. Johnson and B. J. Kelsall, J. Chem. Phys., observations made are in mechanism shown in Scheme 3. The exFig. 8 also demonstrates the utility o agreement with periment illustrated the f fast acquisition pro- in 11 12 1982,104,6180. V. B. Liengine and K. Hall, Trans. Faraday SOC., 1966,62,3229. L. Kubelkova, J. Novarova, Z. DolejSek and P. Jiri, Collect. Czech. Chem. Commun., 1980,45,3101. 13 14 15 16 N. W. Cant and K. Hall, J. Catal., 1972,25, 161. P. Viruela-Martin, C. M. Zicovich-Wilson and A. Coma, J. Phys. Chem., 1993,97,13713. V. B. Kasansky, Acc. Chem. Res., 1991,24, 379. I. N. Senchenya, N. D. Chuvylkin and V. B. Kazansky, Kinet. Catal., 1985,26,926. CH2CHCH2CH2CH3 17 18 19 1. N. Senchenya and V. B. Kazansky, Kinet. Catal., 1987,28,490. V. B. Kazansky and I. N. Senchenya, J. Catal., 1989,119,108. N. B. Colthup, L. H. Daley and S. E. Wiberly, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, II H&=CHCHs 1975. CH3 CH3 I I CH2CHCH2CHCH2CH2CH3I \,SiCo'Al< I I H&=CHCH, - ............ 20 21 22 23 24 S. A. Francis, J. Chem. Phys., 1950,18,861. S. H. Hastings, A. T. Watson, R. B. Williams and J. A. Anderson Jr., Anal. Chem., 1952,24,612. H. Luther and G. Czerwony, Z. Phys. Chem., NF, 1956,6,286. M. L. Connolly, Science, 1983,221, 709. Insight 11 and Discover User Guides, Biosym Technology, San Diego, 1992; P. Dauber-Osgurthorpe, V. A. Roberts, D. J. Osgurthorpe, J. Wolff, M. Genest and A. T. Hagler, Proteins: Struct., Funct. Genet., 1988, 4, 31. Scheme 3 Paper 4/01492H; Received 14th March, 1994