J. CHEM. SOC. FARADAY TRANS., 1993, 89(11), 1843-1855 1843

Acetylene, Methylacetylene and Ethylacetylene Polymerization on H-ZSM5 : A Spectroscopic Study

Silvia Bordiga, Gabriele Ricchiardi, Giuseppe Spoto, Domenica Scarano, Luca Carnelli and Adriano Zecchina" Dipartimento di Chimica lnorganica Chimica Fisica e dei Materiali, Via Pietro Giuria 7,l-10125 Torino, Italy Carlos Otero Arean Departamento de Quimica, Universidad de las lslas Baleares, 07071 Palma de Majorca, Spain

Acetylene, methylacetylene and ethylacetylene interact with t h e Brernsted acid sites of H-ZSMS with t h e forma- tion of hydrogen-bonded (precursor) species characterized by well defined IR properties. These precursors are then protonated to give intensely coloured carbocationic species. The speed of protonation is in t h e order: C,H5-C'CH > CH3-C=CH > HCECH. Insertion of the monomer into t h e first protonation product, leads to an oligomeric species with carbocationic character. The IR and UV-VIS spectra of the carbocationic species are discussed in detail and found to correspond to those of analogous species generated in the homogeneous phase. The n-delocalization of the positive charge on t h e backbone is responsible for t h e peculiar and intense spectroscopic manifestations both in the IR and UV-VIS. The positive charge in the carbocationic species can be captured by bases (NH, , pyridine) and subtracted to n-delocalization. This is accompanied by the disap- pearance of the spectroscopic manifestations associated with t h e positive charge and by the appearance of new absorptions associated with neutral oligomeric chains. The process can be fully reversed.

The dimensions and shape of t h e oligomers are discussed in t e rms of the steric constraints imposed by the zeolitic framework, by means of computer graphics and molecular mechanics modelling.

Polyacetylene, an insulator in its pure state, can be con- sidered a prototype of a number of conjugated polymers which develop high electrical conductivity ( 102-103 ' cm- ') when doped with oxidizing or reducing agents.'P2 Other relevant properties of organic polymers with conju- gated n-electron backbones include low-energy optical tran- sitions, low ionisation potential and high electron affmities. This results in a class of materials with potential applications in a number of technological fields, such as electrical switches and b a t t e r i e ~ , ~ . ~ semiconducting and electronic device^,^-^ non-linear optical and chemical sensor^.^*'^ For many of these applications controlled polymerization of the monomeric unit is needed. This should aim at obtaining highly oriented polymer chains and controlling secondary processes which could lead to loss of conjugation, branching and cross-linking. A possible strategy is to effect poly- merization inside the pores of a zeolite. In principle, isolated linear polyenes could thus be obtained, and the resulting host-guest composite material can be conveniently oriented by alignment in an electrical field.' Moreover, encapsulation of the polymer would increase the stability.

H-ZSM5 is a highly acidic zeolite which can protonate acetylene and initiate a carbocationic polymer chain. The zeolite pore structure consists of two intersecting sets of tubular channels ca. 0.55 nm in diameter which allow internal space for the polymer to grow. Pereira et ~ 1 . ' ~ have recently reported polymerization of acetylene in an H-ZSM5 sample with Si : A1 = 35 : 1. They found a negligible reaction rate at temperatures lower than 425 K. Polymerization of methyl- acetylene in several H-zeolites (including H-ZSM5) was reported by Cox and Stucky.I3 COY and NiY zeolites were also used for acetylene polymerization, l4 but the presence of metal ions in the resulting composite material can adversely affect optical and electronic properties. We present here a comparative study on the polymerization of acetylene, methylacetylene and ethylacetylene in an H-ZSM5 sample with Si : A1 = 14 : 1 and a crystallite size in the 40-100 nm range.

Experiment a1 Details on the synthesis and characterization of this zeolite have been given elsewhere." We mention only that the Si: A1 ratio in our samples was 14: 1. which corresponds roughly to a proton for each channel crossing.

Polymerization was monitored by IR and UV-VIS spec- troscopy. For IR studies, a Brucker 48 FTIR spectrometer was used, operated in the transmission mode at 2 cm-' resolution. UV-VIS spectra were obtained with a Cary 5 spectrophotometer. Before gas dosage, the zeolite sample (in the form of a self-supporting wafer for IR experiments, or as a powder in UV-VIS experiments) was fired in vacuum at 623 K for 30 min. Suitable cells were used, which allowed both high-temperature activation and room-temperature spectroscopic measurements to be performed in situ. Com- puter graphics and molecular mechanics/dynamics were obtained with the software programs Insight11 and Discover, respectively. Both programs are distributed by Biosym Tec- nology Inc. and were run on a SGI 4D/35 workstation. Mechanics and dynamics studies were performed using the CVFF force field.I6

Results and Discussion Accessible Space in H-ZSM5

The geometric structure of the pentasilic skeleton is reported in Fig. l(a). This representation is very useful for illustrating the crystallographic features of the lattice and the presence of straight channels running along the [OlO] direction (the zig-zag channels, on the other hand, cannot be shown in the same representation). However, the purely stick representa- tion is not very informative about the internal space and the internal surfaces really accessible to molecules moving (or growing) inside the channels.

In order to represent the accessible (internal) surfaces (and hence the accessible space), the Connolly algorithm' orig- inally developed for biological molecules, has been used here

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1844 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

(001 1

Fig. 1 (a) Stick representation of the ZMSS structure seen along the [OlO] direction. (b) The solvent-accessible surface of a ZSMS slab, calculated by the Connolly algorithm. View along the main crystallo- graphic directions.

for zeolitic systems. The Connolly surface is the boundary of the volume from which a probe molecule is excluded if it is not to experience van der Waals overlap with the atoms of the zeolite framework. Fig. l(b) shows the accessible surface of a zeolite slab, seen along the main crystallographic direc- tions. The radius of the probe sphere used was 1.4 A (conventional radius for water) while the van der Waals radii of Si and 0 were 1.7 and 1.35 A, respectively. The shape of the surface is quite insensitive to the radius value adopted for the silicon atom. The effective dimensions of the pores (and hence of the available internal space) are thus represented in a realistic way. A further advantage of the Connolly method (applied to zeolites) is its capacity to show the simultaneous presence in the structure of intersecting straight and sinus- oidal channels.

Acetylene Polymerization I R Spectra of the Oligomers; The Reversible Interaction with ND, Fig. 2 shows the IR spectrum of adsorbed acetylene imme- diately after dosage at room temperature (contact time 7 s). The sharp band at 1950 cm-' is assigned to the C S C stretching of hydrogen-bonded C2H2 molecules. Hydrogen bonding renders the Raman mode of free acetylene at 1974 Cm-l 18 IR-active, and causes a bathochromic shift of 24 cm-' (because electronic charge is subtracted from the 7~ orbitals). The corresponding C- H stretching expected at 3287 cm-' l 8 appears (as a narrow component) at 3250 cm-', superimposed on a broader composite spectrum due to hydroxy groups perturbed by the hydrogen bonding. The

10.1

I L

wavenumber/cm - I

3 500 3000 2500 2000 1500

Fig. 2 C2H2 contact: (---) background, (-) 7 s after C2H2 dosage

IR spectra of H-ZSMS outgassed at 673 K before and after

hydroxy groups involved are absorbing at 3670 cm- ' (Al-OH extraframework species) and at 3610 cm-' (bridged framework hydroxy groups with Brernsted character).' 5 b 9 1 9 , 2 0

Silanols (peaks at 3750-3740 cm- ') are unaffected by C2H2 adsorption at room temperature. The shifts caused by hydro- gen bonding are A7 = 290 cm-' and 350 cm-' for bridges and extralattice OH groups, respectively.

From these shifts it is inferred that the hydroxy group absorbing at 3610 cm- are slightly more acidic than those in the extralattice position as already discussed in ref. 15(b). In conclusion, we can state that the most abundant species formed during the initial stages of C2H2 adsorption is the 1 : 1 complex formed following Scheme 1.

3610 cm-I ( H

Scheme 1

This complex is located mainly at the channel intersections where the accessible space is slightly larger than inside the channels.

Upon standing at the IR-beam equilibrium temperature [ca. 320 K ; spectra reported after background subtraction in Fig. 3(a)], the band at 1950 cm- ' gradually decreases to 60% of the initial intensity, while a complex group of bands develops in the 1700-1500 cm-' range, i.e. the range typical of the C=C stretching modes of neutral and charged poly- enes of variable length (spectra 1-4).2'-25 At the same time a deep blue-violet colour develops gradually.

The most significant absorptions are observed at 1702 (very weak, vw), 1640 (vw), 1580-1565 (doublet of medium intensity) and ca. 1500 cm-' (intense and broad, probably composite). It is most noticeable that the 1702 cm-' band decreases slightly upon standing at room temperature. By increasing the temperature to ca. 220 K (spectra 5 and 6), the broad adsorption centred at ca. 1500 cm-' becomes the dominating feature of the spectrum, while the very weak peaks at 1702 and 1640 cm-' and the 1580-1565 cm-' doublet disappear. At the end of this stage the intensity of the peak at 1950 cm- ', attributed to hydrogen-bonded species, is only 30% of the initial value. The colour of the sample is now

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 1845

n

2000 I900 1800 1700 1600 I500 1400 1

wavenum ber/cm - ' Fig. 3 (a) IR spectra of H-ZSMS in contact with C2H2 (2000-1300 cm-' range): evolution with time at room temperature (1-4), and at 420 K (5, 6). (b) Effect of ND, on preadsorbed C2H2 (1700-1300 cm-' range). 1, H-ZSM5-C,H2 system heated for 30 min at 420 K and pumped at room temperature for 5 min; 2, effect of 5 Torr [l Torr = (101 325/760) Pa] of ND, dosage; 3, after ND, outgassing for 5 min at room temperature; 4, after outgassing for 1 h at 420 K. All spectra are reported after background subtraction.

deep blue. This colour does not disappear upon exposure to the atmosphere.

In Fig. 4 the evolution of the spectrum in the 3800-2800 cm-' range [v(OH) and v(CH) stretching region] is also shown. (Only the most significant spectra are reported.)

It is worth mentioning that, particularly for samples heated at ca. 420 K, bands associated with aliphatic groups at 2950- 2850 cm-' are observed. These bands are attributed to methyl and methylenic groups. The intensity of these bands remains quite low throughout the experiment.

Gas removal at this stage of the experiment causes only the immediate disappearance of the bands of the hydrogen-

3

10.1

10 3600 3400 3200 3000 2800 wavenumber/cm -'

Fig. 4 Evolution with time of the IR spectra of the H-ZSMS- acetylene system in the 3800-2800 cm-' range: (a) background; (b) after 7 s of contact; ( c ) after heating for 1 h at 420 K ; (d) outgassed for 1 min at room temperature

bonded species and the partial restoration of the peaks of acidic OH groups, while the other parts of the spectrum are substantially unaffected. In general terms, these data indicate that the 1 : 1 hydrogen-bonded complexes with Bronsted sites are slowly consumed, with the formation of oligomeric chains trapped in the channels, following Scheme 2.

HCZCH

hydrogen-bonded protonation step formation of

carbocat ions precursor conjugated polyenic

Scheme 2

The gradual filling of the pores with polymeric structures is accompanied (as is usually observed when the zeolitic chan- nels are filled with adsorbates) by a small but definite fre- quency shift of the overtone of the skeletal band at 1900 cm-'. This explains the appearance in the difference spectra of the sinusoidal features indicated with a bold arrow in Fig. 3(a). These features can be considered as pore-filling indica- tors: their presence is a direct proof of oligomer formation in the pores. It is most remarkable that substantial pore filling has also been measured by Pereira et al.I2 under similar polymerization conditions. We can hypothesize that the C2nH:,,+l (n = 1, 2, ...) carbocationic species are mainly linear and so can be represented as shown in Scheme 3.

In this scheme, only the transoid form is considered for the sake of simplicity. These species do not carry unpaired elec- trons in agreement with the absence of EPR signals after C2H2 dosages. In all these structures, the positive charge is expected to be essentially localized on the terminal part of the chain because of the attractive effect of the negative counterpart of the zeolite skeleton. When monomers C 2 H l are considered, the positive charge is obviously confined to the single n-bond present in the moiety. For longer oligo- mers, the extent of the delocalization will be larger. In doped p o l y a c e t y l e n e ~ ~ ~ . ~ ~ ~ ~ ~ with n = cc the positive charge associ- ated with the doping agent is assumed to be delocalized over

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1846

monomer

C2H3*

dimer

C4H5+

trimer

C6H7+

polymer

(C2"H2"+ I)'

H

H.c"C--H +

+ H -

v @ k C ) = 1702 cm-' z+* = 3900038000 Cm-'

Scheme 3

v(C=C) = ca.1500 cm-' z+* = 18000-8000 cm-'

a maximum of eight carbon atoms (four n-bonds). The same can be expected for the present case. This means that in poly- meric chains characterized by n > 4, two regions are present: in one region (not extending for more than four C=C distances), the positive charge is effectively delocalized and confers the backbone carbocationic nature; in the second region, the polymer is neutral and similar to undoped polya- cetylene.

The formation of CH, can be accounted for by further protonation as shown in Scheme 4:

+ I I I I I ' ' - H H H H H ' H I

Scheme 4

J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

Since the intensity of the aliphatic stretching bands is small, doubly protonated species must be considered as minor products of the polymerization at least at room tem- perature. The absence of bands associated with Brsnsted sites involved in hydrogen bonding with alkene double bonds, suggests that the protonation of terminal double bonds of growing chains is fast. In fact, these bands, expected in the 3400-3200 cm-' interval, are not visible at all in the spec- trum shown as Fig. 4(d). This is in agreement with a similar observation, recently made in our laboratory, concerning ethene polymerization in H-ZSM5.28 Of course, a second protonation interrupting the conjugation acts as a powerful limitation of the effective length of the chain where the posi- tive charge is delocalized.

The carbocationic character of the polymeric species formed inside the channels implies a strict similarity with polyacetylene doped with electron acceptors, because in both cases electron-deficient sites are present in the backbone (in physics terminology : charged solitons). It has been demonstrated' 3-2 that electron-deficien t sites (charged solitons) can be described in terms of charged conjugational defects and are associated with strong IR modes in the C=C stretching region (1400-1500 cm- '). These bands are totally absent or very weak in neutral (undoped) polyacetylene.21.22

These IR-active modes essentially correspond to the asym- metric stretching of the two carbon-carbon bonds of the elec tr on-deficien t site (Scheme 5).

H H H H H

Scheme 5

The frequency of this strong peak depends upon the length of the conjugated chain. In infinite chains the main absorp- tion is observed at ca. 1380 cm-', while in shorter chains (containing up to 12 double bonds) it moves gradually towards 1500 cm-1.23 When very short chains are con- sidered, the frequency is expected to move further on to higher wavenumbers, the limiting value being represented by the stretching of the CH=CH; carbocationic species (expected at ca. 1700 cm-'). It is most interesting that com- puter experiments concerning a chain of 22 carbon atoms in the C22Hz3 entity show a strong IR peak at ca. 1500 cm-' 2 5 and that this absorption is totally absent in the C22H24 neutral molecule. This definitely suggests that the broad band with a maximum at 1500 cm-' is associated with a family of medium-sized carbocationic species. The broad- ness of the band = 200 cm-') is due to the size dis- tribution of the carbocations. Those having high polymerization number are responsible for the tail at lower frequency (which extends at frequencies even lower than 1400 cm- '). Those having a polymerization number approaching unity are responsible for the higher frequency tail and for the discrete components observed at 1702, 1640, 1580 and 1565 cm-'. The transient nature of the components suggests that the 1702, 1640, 1580 and 1565 cm-' bands are associated with monomeric (C2H,f), dimeric (C4H,f), trimeric (C,Hq) and tetrameric (C,H;) species, respectively, while the bands of the charged oligomers with n > 4 (where n is the number of monomeric units) are all collected in a small frequency interval to form the broad band centred at 1500 cm-'. For the reasons described above, when the polymerization number increases, also the (weaker) IR absorptions character- istic of the neutral part of the chain should become observ-

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 1847

able. In this respect we remember that infinite chains of trans and cis polyacetylene show a strong Raman absorption at ca. 1474 cm-' and 1550 cm-l, respectively,20.21 which is absent in the IR spectra. A strong Raman absorption centred at 1500 cm-' due to polyacetylene has been observed on TiO, and on A120,.29,30 Owing to the presence of the zeolitic framework, this transition is expected to gain intensity in the IR. As a matter of fact, the major intensity increase is observed at ca. 1450 cm- ' when the temperature is increased to 420 K: we thus infer that this could indeed be the v(C=C) contribution of the double bonds located in the central portion of long chains. For shorter chains, a contribution of terminal -CH=CH, groups at ca. 1600-1700 cm-' should also become progressively more important. On the basis of the results illustrated in Fig. 3(a), we cannot conclude about this last point, because the IR manifestations of the carbo- cationic species are overwhelming. In the following, we shall more deeply discuss this point on the basis of the data derived from interaction with bases.

As already mentioned in ref. 13 for charged methyl- acetylene oligomers formed in acidic zeolites, dosage of NH, (or ND,) leads to a deep modification of the IR spectrum. This important observation led us to repeat the same experi- ment for acetylene oligomers. In Fig. 3(b) the effect of dosing ND, on the C=C stretching bands of the charged oligomers is illustrated (spectrum 2). ND, has been used instead of NH, because it does not have IR bands in the 1800-1700 cm-' region and consequently does not interfere spectroscopically to any great extent. Note that the overall (C=C) spectrum greatly decreases in intensity, as expected if the positive charge present on the carbocationic chain has been captured or neutralized by electron-donor ND, molecules. The process can be described as shown in Scheme 6, leading to quatern- ary ammonium cation formation. In these cations, the positive charge, being totally localized on the ammonium moiety, is no longer delocalized on the carbocationic chains : conse- quently, the IR manifestations of the carbocationic part of the chain disappear almost completely, leaving only the much weaker manifestations of the neutral chains (bands at 1650- 1570 and 1510-1430 cm-'). These results allow us to con- clude that the absorption at 1510-1430 cm-' does indeed correspond to the stretching of C-C bonds located in central positions of the neutral oligomeric chains, while the complex envelope in the 1650-1570 cm-' range is associated with the terminal part of the chains and/or with the shortest oligomers.

Outgassing at temperatures in the 300-420 K interval leads to the progressive restoration of the original spectrum (curves 3 and 4). The experiment can be repeated several times, so demonstrating the reversible nature of the process and its basic simplicity.

U V-VIS-NIR Spectra of the Oligomers before and after Interactions with NH , The evolution with time and the effect of temperature on the UV-VIS-NIR spectra of adsorbed acetylene are shown in Fig. 5(a)-(d).

0 ' 40000 35000 30000 25000 20000 16000 10000 6000

wavenumber/cm -' Fig. 5 UV-VIS-NIR spectra of acetylene polymerized on H-ZSM5 with increasing times of contact: (a) 5 min, (b) 15 min, (c) 3 h, and after heating for 1 h at 420 K (6)

The spectra at room temperature [Fig. 5(a)-(c)] are very complex and structured with distinct components at 39 000- 38 OOO cm- ' (doublet), 33 000-29 000 cm- (doublet), 25 OOO- 23 500 cm- ' (doublet), 20 500-19 500 cm- ' (doublet), 18 500 cm-' and a tail extending down to 8000 cm-'. In this tail, several unresolved components (shoulders) are observable. The time of contact at constant temperature affects the differ- ent components in a different way. In particular, long expo- sure times favour the species absorbing at low frequencies : this clearly indicates that oligomeric C2,Hln+ species of increasing n absorb at progressively lower frequencies. On this basis and following ref. 31 and 32, we can tentatively associate the absorptions at 39 000-38 0o0, 33 000-29 000, 25 000-23 500, 20 500-19 500, 18 500 and 18 000-8000 cm- ' (5 < n < GO), respectively. Heating at 420 K [Fig. 5(d)], besides causing the broadening of the bands, increases prefer- entially the absorption in the 18000-12000 cm-' range, so indicating that the polymerization degree is increasing. This experiment fully confirms the previous assignment.

From the envelopes of Fig. 6A(c) and B(d) we can conclude that the most intense absorption in the samples treated at 420 K corresponds to optical transitions in carbocationic species where the positive charge is delocalized over three to six conjugated double bonds, while in samples treated at room temperature it corresponds to three to five double bonds. From the purely spectroscopic point of view, these coloured species could be either short carbocationic chains containing no more than six double bonds or longer chains where the positive charge is effectively delocalized only on the 6-terminal double bonds of the chain. However, the corre- spondence between the spectra of charged and uncharged oli- gomers shown in Fig. 6A (vide infra) indicates that the first possibility predominates over the second. It must be kept in mind that as the molar absorption coefficient of the carbo- cationic species is not necessarily constant, the intensity profile in Fig. 6A and B does not give the real distribution of the charged oligomeric species.

to C*H;, C,H,f, C,Hq, CgH;, CIoHTI and C,nH:n+1

H H H H

Scheme 6

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1848 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

n = 1 n = 2

main absorption of

n = 3 n = 4

n = 5

main absorption of

(A/); n = 1

n = 2 n = 3

n = 4 n = - n = 5 n = - -

C .- 4-

C

Y +

5 1 5 r Q) 0 3 Y

0 1 I I 1 I 4 40;1000 35b00 30000 25000 20000 15000 10000 5000

wavenumber/cm-‘

n=oo - - n = 7

- n = 6 n = 5

n = 4 ___ n = 3 main absorption of n = 2

(-1 ”

wavenumber/cm -’

- n=7 - n = 6 n = 5

n = 4 n = 3 main absorption of

n = 2 (W) n

10

B, at Fig. 6 UV-VIS-NIR spectra of acetylene polymerized on H-ZSMS: A, at room temperature ( c ) before and (c’) after contact with NH,; 420 K before (61 and after contact with NH, (a). At the toD and bottom of parts A and B schematic representations of the (n-n*) transitions in

\ I J \ I

and in neutral3 1 * 3 3 species, respectively, are illustrated.

The effect of NH3 dosage on C,H, polymerized at room temperature and at 420 K is illustrated in Fig. 6A(c’) and 6B(d’), respectively. In the same figure, schematic illustrations of the (n-n*) transitions occurring in ~ h a r g e d ~ ’ . ~ ~ and n e ~ t r a l ~ ’ . ~ ~ reference species are also given. We can see that: (a) the main bands of the spectrum associated with carbo- cationic properties disappear almost completely; (b) a weaker spectrum remains, which can be attributed to the ‘neutralized’ oligomers.

This representation allows correlation of the spectra of charged and uncharged polymers containing the same number of conjugated double bonds.

In spectra (c’) and (d’) we can clearly distinguish: (a) a band at 39 000-37 000 cm- ’ associated with three conjugated double bonds; (b) a broad absorption centred at ca. 32500 corresponding to a distribution of polyenes peaking at four to five double bonds; (c) a tail extending from 27500 to 17500 cm-’ for (c’), and to loo00 cm-’ for (d’), correspond- ing to a distribution of polyenes up to ca. 14 double bonds.

The remarkable correspondence between the distributions emerging from the two types of spectra represents an impor- tant proof of the consistency of the whole picture emerging from this investigation.

Besides these general observations, the following more spe- cific conclusions can be derived: (a) the polymerization degree is larger on the sample treated at 420 K in C,H, : in fact, the edge at 12000 cm-’ characteristic of well poly- merized neutral polyacetylene3 is clearly seen after contact with NH, (the same edge is not present on the sample treated at room temperature where the lowest frequency absorption corresponds to chains containing 14 or 15 double bonds); (b)

on samples polymerized at 420 K the polymerization number, as deduced from spectroscopic measurements, clearly corresponds to that reported in ref. 12. It is worth mentioning that exposure to the atmosphere does not alter the spectrum substantially.

A representation of polymeric chains growing in linear and sinusoidal channels is given in Fig. 7. Note that the ZSM5 pore dimension is well suited to stabilize a single growing chain and there is no space for a second chain. Moreover, it can be seen that there is a limitation for the growth of chains. In sinusoidal channels the limiting number of conjugated double bonds suggested by computer graphics is ca. 10. If straight channels are considered, there is no such apparent limitation. However, the chain growth will be stopped at

Fig. 7 (left) and sinusoidal (right) channels

Polyacetylene oligomer with 12 carbon atoms inside straight

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channel intersections by other growing chains or further protonation.

Finally, in Fig. SA(c’”(c’’’) and B(d)-(d“‘) the effect of out- gassing NH, is reported. We can immediately see that for the sample treated at 420 K in C2H2 (Fig. 8B), the recovery of the original spectrum is nearly complete (so, once again, demonstrating the reversibility of the phenomenon and the validity of the whole picture). For the sample treated at room temperature in C2H2, the situation is more complex. In fact, while a recovery of the original intensity in the 30000-8000 cm- range is observed, no identification is possible from the frequencies of the maxima (which are more similar to those of the sample treated at 420 K). This is due to the thermal treat- ment which not only favours NH, removal but also induces a further increase of the chain length of some oligomers at the expense of the shorter ones.

Note that parallel experiments (not reported for sake of brevity), utilizing pyridine instead of NH, , gave very similar results. In conclusion, the results illustrated above are consis- tent with the following picture: (a) the Brernsted acid sites are the catalytic centres for C,H, polymerization; (b) the length of the charged oligomers formed at room temperature does not exceed 14 or 15 double bonds, with a maximum probabil- ity for oligomers containing 3-5 double bonds; (c) at 420 K, long chains with the spectroscopic properties of polyacety- lene are formed, probably in the straight channels and at the channel mouths; (d) the carbocationic species interact with strong bases (NH, , pyridine) and are insensitive towards H 2 0 vapour and oxygen.

The assignment of the IR and UV-VIS bands before and after interaction with NH, is summarized in Table 1.

I A

40000 35000 30000 E6OOO 20000 15000 10000 5(

wavenumber/cm -’ 0

B I

3 2

f m r o al 3 Y

f O 0 0 0 36600 30000 25600 ZOO00 15000 10000 5000

wavenumber/crn -’ Fig. 8 Recovery of the UV-VIS-NIR spectra after NH, dosage: A, (c) and (c’) as in Fig. 6A; (c”) and (c”‘) after NH, outgassing in the 300-420 K temperature range. B, (6) and (d’) as in Fig. 6B; (8) and (d”‘) after NH, outgassing in the 300-420 K temperature range.

Methylacetylene Polymerization

IR Spectra of the Oligomers; The Reversible Interaction with

IR experiments have been conducted following a sequence similar to that adopted for acetylene: consequently, we shall not describe the experimental details. The only difference deserving a comment is associated with the much higher speed of the reaction between methylacetylene and acidic hydroxy groups (Brmsted sites).

In Fig. 9, the IR spectrum in the 3800-2800 cm- range of the H-ZSM5-methyacetylene system 0.5 s after the opening of the communication between the gas reservoir and the sample [Fig. 9(b)] and after 30 s [Fig. 9(c)] are compared. Note that Brnrnsted sites are immediately consumed with the formation of hydrogen-bonded species (I) whose spectro- scopic characteristics are given in Scheme 7(a).

ND3

I

H I - 3090 cm-l n

Scheme 7(a)

At this stage of the experiment the system has not yet reached the equilibrium pore filling situation because of diffu- sion limitations.

As deduced from Fig 9(c), the amount of species I present quickly decreases upon standing because of the protonation reaction, while other species simultaneously appear which are hydrogen bonded to silanols (11) and/or are simply physically adsorbed. After 30 s the sample is orange-brown in colour.

10.1

1 I 1

3800 3600 3400 3200 3000 2800 wavenumber/cm -’

Fig. 9 Evolution with time of the IR spectra of the H-ZSM5- methylacetylene system in the 3800-2800 cm-’ range: (a) H-ZSM5 outgassed at 673 K; (b) after 0.5 s of contact; (c) after 30 s; (d ) out- gassed for 1 min at room temperature

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1850 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

Table 1 Summary of IR and UV-VIS bands

carbocationic neutralized species v(C=C)/m- (n-n*)/cm- species v(C=C)/cm- (n-n*)/cm-

acetylene C2H:

ethylacetylene C2H5C2H:

(C2H 5)2C~H:

(C2H5)3C6Hz

1702 1640 1580

1565

ca. 1550

ca. 1500

1680 1626 1515

1480-1440

1480-1440

1675 1620- 16 10 1520-1 5 10

1520-1 5 10

39 000-38 000 33 000-29 000 25 000-23 500

20 500-19 500

18 500

8000

38 500-25 000 30 000-27 500 24 700-21 300

19 300

16 500

37 000-33 000 28 000-25 000

23 000

19 000

1645 1650-1 570 1650-1 570 1510-1430 1650-1570 15 10-1430 1650-1570 1510-1430 1510-1430 1650-1 570

1655 1660-1590 1660-1590 1510-1450 1660-1 590 15 10-1450 1660-1 590 15 10-1450

1636 1650-1 590 1650-1590

1650-1 590

(60 000-50 000) 45 500-42 000 39 OOO-36 000

36 OOO-32 000

33 000-29 000

(50 000-40 OOO) 42 000-38 000 35 000-33 000

(sh) 28 000

(sh) 22 000

(50 000-40 OOO) 43 000-38 000 36 000-35 000

(sh) 27 500

The structure and spectroscopic characteristics of the hydrogen-bonded species (11) are given in Scheme 7(b).

In Fig. 9(d), the spectrum obtained after removing the gas phase is shown; besides the small residual peak due to unre-

\ Scheme 7(b)

~ ~ ~~~~

acted methylacetylene, the bands in the 3150-2800 interval are indicative of the formation of polymeric species. Unlike the C,H,-ZSMS system, outgassing does not lead to partial restoration of the original Brnnsted sites, which are fully con- sumed in ca. 30 s.

In Fig. lqa), the evolution with time of the spectrum in the 2200-1300 cm- ' range is reported (spectra after background subtraction).

The main results deriving from the experiment illustrated in Fig. lqa) are: (a) The v(CEC) mode of the hydrogen- bonded precursor species is at 1950 cm-' (i.e. ca. 100 cm-' lower than in the gas phase.18 Its intensity is low even in the very initial stages and tends rapidly to zero. This enhanced reactivity is associated with the inductive effect of the methyl group, which, by causing an electron density increment at the

2200 2100 2000 1900 1800 1700 1600 1500 1400 wavenumber/cm -' u 1700 1600 1500 1400

Fig. 10 (a) IR spectra of H-ZSM5 in contact with CH,-CECH (2000-1300 cm-' range): evolution with time at room temperature (1-4). (b) Effect of ND, on preadsorbed methylacetylene (1700-1300 cm-' range). 1, H-ZSM5-CH3-C=CH system after 30 min of contact and pumping at room temperature for 1 min. 2, Effect of 5 Torr of ND3 dosage; 3, after ND3 outgassing for 5 min at room temperature; and 4, for 1 h at 420 K. All spectra are reported after background subtraction.

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 1851

C=C triple bond, increases the strength of the hydrogen bond (reflected in an increased downward shift of both the OH and C Z C stretching modes) and ultimately favours the evolution of adduct I towards the fully protonated species. (b) The v(C=C) stretching mode of CH,-C=CH hydrogen bonded to silanols fSi-OH groups is observed at 2115 cm- ' (structure 11). (c) The v(CGC) stretching mode of physi- cally adsorbed CH,-CeCH is at 2126 cm-'. (d) Intense and narrow peaks are observed at 1680, 1626 and 1515 cm-'. Their intensity is time dependent, the growth speed being in the order 1515 > 1626 > 1680 cm-'. They are readily as- signed to carbocationic species, which are responsible for the orange-brown colour of the sample. (e) A very broad absorp- tion centred at 1460 cm-' also gradually grows with time (underneath the narrower absorptions at 1515, 1439 and 1380 cm-'). (f) The modes 6,(CH,) and 6,(CH,) are observed at 1439 and 1380 cm-'. (9) The background perturbation indi- cated by the bold arrow (as for the acetylene-H-ZSM5 system) is indicative of the progressive filling of the channels.

All the observations so far reported are consistent with the formation inside the channels of a distribution of carbo- cationic species of the type shown in Scheme 8, where the maximum degree of oligomerization does not exceed 5.13

These carbocationic species are not destroyed by exposure to the atmosphere. Possible assignments of the v(CEC) modes are given in Table 1.

dimer

- IC-d = 30000-27500 cm-' >Si'O'A<

I I

Scheme 8

The interaction with ND, is very similar to that observed for C2H2 oligomers. In fact [Fig. lqb) 21, the IR manifesta- tion of carbocationic species is readily destroyed, leaving only the bands centred at ca. 1630 and CQ. 1465 cm-', due to neutral species. Also, in this case, vibrations associated with long chains (1510-1450 cm-') and with terminal groups (and/or short chains, 1660-1590 cm- ') are clearly observable after ND, adsorption.

In comparison with acetylene, the bands at 1510-1450 cm- ' are less intense, so once more indicating a lower degree of oligomerization. As found for C2H2 oligomers, the process can be fully reversed by outgassing ND, at 420 K [Fig. w ) 41.

UV-VZS-NIR Spectra of the Oligomers before and after Interaction with NH, The evolution with time of the UV-VIS-NIR spectra upon methylacetylene dosage is shown in Fig. 1 l(u)-(c). Several

3 A

0 1 40000 35000 30000 26000 20000 15000 10000 6000

wavenumber/cm -' UV-VIS-NIR spectra of methylacetylene polymerized on Fig. 11

H-ZSM5 for increasing contact times: (a) 1, (b) 5, (c) 10 min

bands are observed at 38 500-35 OOO, 30 000-27 500 (complex and asymmetric), 24 700-21 300 (complex), ca. 19 300 (shoulder) and 16 500 cm- ' (shoulder).

All bands grow with time and the orange-brown colour of the sample also increases in intensity. The speed of formation is 36000 > 29000 > 23000 cm-': this indicates that the bands belong to different species with increasing degrees of polymerization. Unlike acetylene, further temperature increments do not appreciably modify the situation.

Following the path outlined in the previous paragraph, we can advance the following interpretation : on going from 36000 to 16500 cm-' the observed bands are assigned to monomeric (38 500-35 000), dimeric (30 000-27 500), trimeric (24 700-21 300) and polymeric carbocationic species (19 500- 16 000), respectively. This interpretation finds support in the results of the interaction with NH, shown in Fig. 12(c'): in fact, upon NH, dosage, all the bands associated with carbo- cationic species disappear. As already documented for C2H2, the spectrum after NH, dosage gives information about the neutralized polymers which so result, absorbing at fre- quencies in the 50000-20000 cm-' interval (with a maximum intensity above 40 OOO cm- '). It is evident that with respect to the neutral polyacetylenic species, the neutral methylacetylenic polymers absorb at distinctly higher fre- quency. This can be readily interpreted in terms of the lower degree of polymerization. However, it is known34 that poly- methylacetylene (n > 100) is an orange compound absorbing at ca. 33000 cm-' and that the above phenomenon has been interpreted in terms of steric effects of methyl groups. This suggests that the presence of bands at higher v may be due more to steric effects than to a lower polymerization degree.

" 1 .- Y E l

A

0 40000 36000 30000 26000 20000 16000 10000 51 00

wavenumber/cm -' Fig. 12 H-ZSM5 (c) before and (c') after contact with NH,

UV-VIS-NIR spectra of methylacetylene polymerized on

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1852 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

However, this conclusion cannot be considered as completely convincing, because the effect of branching has not been con- sidered in ref. 34.

In this respect, it has been firmly established that the series of neutral dimethylpolyenes converge towards a limiting 7t--n* value of 22000 cm-’ 35*26 (ie. despite the presence of two methyl groups, this limiting value is lower than the ca. 33000 cm-’ edge of methylacetylene deduced from ref. 34 which is not possible). In conclusion, the higher UV-VIS bands of methylacetylene are definitely indicative of a lower polymerization degree.

Finally, the NH, desorption experiment is reported in Fig. 13(c)-(c”’). This experiment shows that the carbocationic species can be fully restored by NH, elimination. NH, dosage and desorption experiments can be repeated several times, so demonstrating that this type of interaction is fully reversible. Similar experiments (not reported for the sake of brevity) using pyridine as base, gave nearly coincident results.

In conclusion, methylacetylene readily oligomerizes in the H-ZSM5 channels giving a distribution of carbocationic species which are insensitive towards exposure to the atmo- sphere. From the intensity and positions of the IR and UV-VIS maxima it is inferred that the predominant species contains two or three double bonds. From this it is deduced that they are prevalently formed at channel intersections, as is shown in Fig. 14. This picture shows a neutral methyl- acetylene trimer, placed in a low-energy configuration obtained by molecular mechanics (with a fixed zeolite framework). The molecule is accommodated in the channel intersection, but seems not to be able to diffuse (as calculated by molecular dynamics at various temperature^.,^ We can see here the utility of computer graphics and molecular mechanics to explain and support the experimental data. Longer chains containing four to five double bonds are also present in very minor amounts. If due consideration is made of the fact that all protons are consumed upon methyl- acetylene contact, we can also conclude that every intersec- tion carries one n-chromophoric species containing two to three n bonds. A simplified illustration of the energy- minimized situation is shown graphically in Fig. l q b ) . These results are in full agreement with the quantitative data reported in ref. 13. The whole set of assignments are sum- marized in Table 1.

Ethylacetylene Polymerization

IR Spectra of Oligomers before and after interaction with ND, The time-resolved spectra (in the 3800-2800 cm- range), are essentially similar to those reported for methylacetylene and

I0

wavenumber/cm - ’ Fig. 13 Recovery of the original UV-VIS-NIR spectra after NH, dosage: (c) and (c’) as in Fig. 12; (c”) and (c”’) after NH, outgassing in the 300-420 K temperature range

Fig. 14 sinusoidal channels : (a) individual molecule ; (b) periodic model

Methylacetylene trimer at the intersection of straight and

are shown in Fig. 15. The following observations can be made: (a) 0.5 s after the opening of the valve connecting the sample with the gas reservoir, the Brernsted sites are destroyed. No sign of hydrogen-bonded species is found: this means that the protonation is so fast that no hydrogen- bonded intermediate can be detected at room temperature. (6) After 30 s, the gas has filled the pores and formed the 1 : 1 adducts with silanols which are readily consumed. At this stage the sample has an intense orange colour. The process is fully reversible and the isolated ZSi-OH groups can be re- stored by outgassing at room temperature. The structure of the adduct and the proposed IR assignment are shown in Scheme 9. All the frequencies and assignments are sum- marized in Table 1. (c) The stretching modes of ethyl groups are observed at 2982,2943 and 2884 cm-

\ Scheme 9

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 1853

n ln

C 1

c .-

4 n v

al C

e 2 n

I 1 I I I

I

3800 3600 3400 3200 3000 2E wavenumber/cm -'

3

Fig. 15 IR spectra of the H-ZSMSkthylacetylene system in the 3800-2800 cm-' range: (a) H-ZSMS outgassed at 673 K; (b) after 0.5 s of contact with methylacetylene; (c) after 30 s; (d) outgassing for 1 min at room temperature

In the 2200-1300 cm-' region [Fig. 16(a)] the situation is as follows: (i) The bending modes of the ethyl groups in bound ethylacetylene are centred at 1462 and 1385 cm-'. (ii) The orange carbocationic species absorb at 1675, 1620-1610 (composite) and 1511 cm-'. The 1675 cm-' band is not appreciably time dependent, while those at lower v grow with speeds in the order: 1615 > 1511 cm-'. (iii) There is no trace of bands due to v(C=C) perturbed by hydrogen bonding to Br~rnsted sites, even in the initial stages of polymerization. This means that the protonation is very fast as already con- cluded from Fig. 15. (iv) The v(C=C) stretching of hydrogen-

h

c ln

C 3

.-

ui n v

al

C m -s 2 n

2

bonded species involving silanols is at 2110 ern-', while the physisorbed species absorbs at 2118 cm-'. (v) The pore- filling indicator (bold arrow) is particularly strong and grows with time. Similar to the other systems investigated, exposure to the atmosphere does not have any effect on the coloured species.

With respect to methylacetylene, the most relevant differ- ence is represented by the intensity of the carbocationic species at ca. 1511 cm-', which is distinctly lower in the present case. This suggests a lower degree of oligomerization, associated with steric hindrance.

Interaction with ND, [Fig. 16(b) 21 leads to the elimi- nation of all the carbocationic bands, leaving (besides the unaltered ethyl modes) a group of v(C-C) bands due to neutral species of very low polymerization degree at 1650- 1590 cm-'. This result is similar to that found for acetylene and methylacetylene. In those cases, a broad band centred at ca. 1450 cm- ' was observed. This band, assigned to v(C=C) of groups in non-terminal positions, is indicative of a high degree of oligomerization (in agreement with the UV-VIS spectra). In the present case, the band at 1450 cm- ' is practi- cally absent: this definitely indicates that the oligomers are shorter than those observed for methylacetylene.

By outgassing ND, [Fig. 1qb) 3, 41 the original spectrum is gradually recovered, with restoration of carbocationic bands. Similar results were obtained by using pyridine instead of ammonia.

In conclusion, upon ethylacetylene dosage, carbocationic species containing one to three double bonds are readily formed. These species give strong IR bands in the C=C stretching region. The positive charge of the carbocationic species is immediately captured by dosing with ND, (or pyridine). The process is reversible. The positive species are resistant to oxygen and H,O vapour.

UV-VIS-NIR Spectra of Oligomers before and after Interaction with NH, Upon ethylacetylene dosage a complex UV-VIS-NIR spec- trum is immediately observed which does not change very much with time. In Fig. 17, only the spectrum (a) obtained after 30 min of contact is illustrated for the sake of simplicity. Bands are observed at 37 000-33 OOO, 28 000-25 OOO, 23 000

10 2100 2000 1900 1800 1700 I600 1500 1400 17 1

O 1600 1500 1400

wavenumber/cm -' Fig. 16 (a) IR spectra of H-ZSMS in contact with C2H5-CECH in the 2200-1300 cm-' range: evolution with time at room temperature (1-4). (b) Effect of ND, dosages on preadsorbed ethylacetylene in the 1700-1300 cm-' range. 1, H-ZSMS-C,H5-C=CH system after 30 min of contact and pumping at room temperature for 5 min; 2, effect of 5 Torr of ND, dosage; 3, after ND, outgassing for 1 rnin at room temperature; and 4, for 1 h at 420 K. All spectra are reported after background subtraction.

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1854 J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89

wavenurnber/crn -’ Fig. 17 UV-VIS-NIR spectra of ethylacetylene polymerized on H-ZSM5 (a) before and (af) after contact with NH,

and 19 000 cm-’. On the basis of the previous considerations they can be attributed to monomeric, dimeric, trimeric and tetrameric carbocationic species, respectively, the dimeric species being the most abundant. As already documented in the discussion of the IR spectra, some limiting factor must exist which prevents the formation and stabilization of long oligomers. This factor is essentially steric: in fact, suffcient space for dimer formation can be found only at channel inter- sections. Further evidence of the space restriction in the case of the ethylacetylene-ZSM5 system is obtained by consider- ing the effect of dosing with NH,. Spectrum (af ) after NH, dosage is reported in the same figure. Note that: (i) the inter- action of NH, with the carbocationic species is not complete (unlike acetylene and methylacetylene) : in fact, the absorp- tions due to trimeric and tetrameric carbocations (23 000 and

19000 cm-’) are not completely destroyed (as expected if spatial constraints are present); (ii) the growth of a strong absorption in the 40 000-25 000 cm- range indicates the for- mation of neutral ethylacetylene oligomers mainly containing two double bonds. More precisely, the absorption centred at 40000 cm-’ can be associated with the dimer, while the shoulders at 36 000-35 000 and 27 500 cm- correspond to minor fractions of the trimer and tetramer, respectively.

The effect of pumping is illustrated in Fig. 18(u)-(a”’). We observe, as a major effect, the nearly complete restoration of the original carbocations. A partial persistence of the absorp- tion at highest frequencies indicates, however, a residual pres- ence of ‘neutralized’ species. These observations are easily explained by considering the steric reasons limiting the NH, removal.

I

wavenurnber/crn-’

Fig. 18 300-420 K temperature range

Recovery of the original UV-VIS-NIR spectra after NH, dosage: (a) and (a’) as in Fig. 17; (a“) and (a”’) after NH, outgassing in the

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J. CHEM. SOC. FARADAY TRANS., 1993, VOL. 89 1855

Conclusions The first species formed by interaction of R-C=CH (R-H, CH, , C2H5) with H-ZSM5, is the hydrogen-bonded complex with Brsnsted acid sites. At room temperature, the acetylene complex is stable and evolves only slowly towards the proto- nated species. The methylacetylene protonation, in contrast, is faster and the hydrogen-bonded precursor can be seen only as a transient intermediate. For ethylacetylene, the proto- nation is so rapid that IR spectroscopy of transient hydrogen-bonded precursors is precluded.

Formation of the hydrogen-bonded complex is revealed by: (a) a shift of v(0H) (CH=CH: A7 = -350 cm-'; CH,CECH : A7 = -520 cm-'); (b) a shift of v(C=C) (CHECH : A7 = -24 cm-'; CH3C"-CH : A7 = -100 cm-'; ( c ) a shift of v(C-H) (CH=CH : A7 = -37 cm-'; CH,C'-CH : Aij = - 139 cm-I).

At room temperature, the formation of Si-OH.-.(R-CECH) 1 : 1 complexes (R = CH,; C,H,) with silanols is revealed by IR spectroscopy.

The protonated species insert new monomeric molecules to form carbocationic oligomers. These species, which are stable towards exposure to oxygen and H,O vapour, are responsible for the appearance of strong IR and UV-VIS absorptions and for the strong colours observed. These spec- tral characteristics compare very well with those of singular positive species prepared in cryogenic matrices.

Upon dosage of bases (NH, and pyridine) the strong IR and UV-VIS manifestations disappear, leaving only less intense bands associated with neutral oligomers. This is due to the capture of the positive charge by the base and its sub- sequent subtraction from the conjugated double-bond sequence.

The distribution of acetylene oligomers is bimodal and shows two maxima: one for n = 3-6 (which is the most important) and one for 14 < n < co (especially for poly- merization at 420 K). Note that the number of carbon atoms contained in the first distribution is 6-12.

The methylacetylene and ethylacetylene distributions peak at three to four and two to three double bonds, respectively (which correspond to carbocations containing 9-12 and 8-12 carbon atoms). Comparison of the whole set of results leads to the remarkable observation that the maximum number of carbon atoms (n,) contained in the carbocationic species more frequently observed in H-ZSM5, is quite invariant (n, = 12). In other words, the dimensions of the charged species are constant. This indicates that steric effects associ- ated with the microporous nature of the adsorbent essentially determine the size of the trapped species.

The observed distributions are interpreted and modelled by means of computer graphics and molecular mechanics in terms of oligomer formation at channel intersections (distributions corresponding to two to six double bonds are well justified on this basis) and formation of linear chains in straight channels (only in the case of acetylene).

2 3

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This investigation has been supported by CNR (Progetto Finalizzato Chimica Fine 11) and MURST.

Paper 2/06820F; Received 23rd December, 1992 References 1 C. K. Chiang, C. R. Fisher, Y. W. Park, A. J. Heeger, H. Shira-

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