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IR Spectra of CO Adsorbed at Low Temperature (77 K) On Titaniumsilicalite, H-ZSM5 and Silicalite
G. Ohlmann et al. (Editors), Catalysis and Adsorption by Zeolites 0 1991 Elsevier Science Publishers B.V., Amsterdam
671
IR SPECTRA OF CO ADSORBED AT LOW TEMPERATURE (77 K) ON TITANIUMSILICALITE, €I-ZBMS AND SILICALITE
A. ZECCHINA~,G. SPOTO~,s. BORDIGA~,M. PA DO VAN^, G. LEO FAN TI^ and G. PETRINI~ 'Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Via P. Giuria 7, 10125 Torino (Italy)
'Montedipe, Unit& di Ricerca, Via San Pietro (MI) (Italy)
50,
28100 Bollate
ABSTRACT
The IR band at 960 cm'l observed on TS is due to a local stretching mode of a [Ti04] unit in the silicalite framework. Framework Ti does not show Lewis acidity as probed by CO adsorption at 77 K. TiOH and SiOH have undistinguishable IR properties. At 77 K they form with CO very weak 1:l OH...CO adducts. Small amount of extralattice Ti in form of Ti02 can be probed by CO adsorption at 77 K. INTRODUCTION
The substitution of Ti and A1 for Si in zeolites of the pentasil family (Silicalite) leads to Ti-Silicalite and ZSMS which are two important catalysts for oxidation with HZOZ (refs. 1-3) and acid catalyzed reactions (ref. 4 ) . The characterization of Titanium-Silicalite (TS) by means of physical methods has been recently published (ref. 5). However, a few problems are still open concerning: i) the presence of extralattice Ti (as Ti02 microparticles); ii) the interpretation of the vibrational spectrum of the solid; iii) the crystallinity of TS with respect to pure S and ZSMS; iv) the presence of titanols in the channel8 and cavities. The CO molecule is an efficient probe of Lewis and Broensted acidity and can be used to explore the acidity of OH groups in the channels (TiOH and SiOH in TS, (Si,Al)OH, SiOH and AlOH in H-ZSM5). On dehydroxylated samples coordinatively unsaturated Ti4+ and A13+ ions in lattice and extralattice positions can be revealed as well, because they can form Lewis adducts with CO. In this contribution we report on the IR spectra of CO adsorbed at 77 K on TS, s and ZSMS. The vibrational spectra of s,
TS and Na-ZSM5 are also compared and discussed. EXPERIMENTAL
Silicalite and Titanium-Silicalite have been synthesized in Montedipe laboratories following the method described in ref. 2; Na-ZSM5 and H-ZSM5 (external surface area 60 m’9-l) have been provided by the same laboratory. The ER spectra have been obtained on a Bruker IFS 113V FTIR spectrometer using a specially designed silica cell permanently attached to a vacuum manifold and allowing in situ outgassing procedures at temperatures in the 273-1073 K interval and gas dosing at 7 7 K. The samples were either in form of thin pellets or of films deposited on KBr or Si plates. RESULTS AND DISCUSSION IR modes associated with framework Ti
The spectra recorded at 7 7 K of S, TS and ZSM5 (sodium form; Si/A1=35) films outgassed at 573 K are compared in Fig.1. The major difference between S and ZSM5 on one side and TS on the other side is represented by the presence in the spectrum of TS of a (finger print) peak at 960 cm-l. In order to give a correct assignment of this band the following considerations have to be made: a) it cannot be assigned to an optical mode of TiOZ microparticles entrapped into the channels or at their intersection because neither rutile nor anatase show strong IR bands at this frequency; b) despite the similarity with the IR manifestations of homogeneous titanyl containing analogs , the 960 cm-’ peak cannot be attributed to the stretching mode of internal (Ti=O)’+ because: i) it is not perturbed by filling the pores and channels with CO (results not reported), which, being a weak Lewis base, is expected to interact with the positive centres and to perturb them; ii) it does not have the expected counterpart in the UV-Vis diffuse reflectance spectrum (absorption in the 25000-35000 cm’l range) (ref. 6); iii) it cannot be removed by reductive treatments in H2 and CO even at very high temperature or under plasma discharge conditions (Ha). On the basis of the previous **negativet* evidences, the hypothesis has been made that the 960 cm-l band is due to a local mode of the Titanium-doped pentasilic structure (essentially a stretching mode of a [Si04] unit perturbed by an adjacent Ti) (ref. 5 ) .
673
0.5
A
F'LI / I
1
I0
YRVENUHBER C H - I
Fig.1. IR absorbance spectra (recorded at 77 K) of: a) Silicalite, b) Na-ZSM5 (Si/A1=35) and c) Ti-Silicalite. In the inset: W-Vis reflectance spectra of a) Silicalite and c) Ti-Silicalite. A full assignment of the IR spectra of s, TS and ZSM5 requires a detailed unit cell vibrational analysis and is outside the scope of this contribution. However, an explanation of the presence of the extra-absorption in the TS spectrum can be equally given on the basis of the following qualitative considerations. The vibrational representation of the stretching modes of an llisolatedll tetrahedral [ S i O , ] unit is:
rstret.= T2 *I (TZ: IR active; A1: IR inactive). Packing of the [Sio,] (by sharing the corners) to form the zeolitic structure has two consequences: i) the local *'site1@symmetry of each [SiO,] primary +
674
building unit is lowered to C2 (ref. 7) so that the triply degenerated T2 mode is splitted into three, one weak (A) and two strong (B), components and ii) due to the high number of [SiO,] units forming the unit cell (where 9 6 tetrahedral units are present) broad bands are expected because of the further splitting of each component into a maximum of 96 sub-components. On these basis, the spectrum of S in the stretching region can be interpreted as follows: i) the two clear absorption at 1235 (mw) and 1200-1050 (vs) cm-l, are essentially associated with the A and B modes of the primary unit broadened by unit cell splitting effects; ii) the complex (vw) band centered at =770 cm-l derives from the A1 (IR inactive) mode (the fine structure observed at 77 K being associated with the unit cell sub-components). The bands at lower frequency belong to skeletal modes having bending character and will not be considered here for sake of brevity. We only mention that the peak at ~ 5 5 0cm-l is absent on amorphous silica and is considered as a gfcrystallinityfg (structure sensitive) band (ref. 5). As far as TS is concerned, some 1-2% of the [SiO,] building blocks of pure silicalite framework are substituted for Igheavierff [TiO,] units. The substitution does not dramatically change the overall IR spectrum other than for the appearence of localized fgimpuritygg modes associated with the [TiO,] dopant units. In the stretching region four additional local modes are, in principle, expected, three (A+2B) deriving from the T2 degenerate vibrations of the free [TiO,] unit and one (A) from the A1. These modes should be shifted to lower frequencies with respect to those of Silicalite, because of the mass effect. Actually only the band deriving from the strongest absorption at 1120 cm-l (presumably of B simmetry) has enough intensity and is enough shifted to show up clearly in the gap between the 1250-1050 and 820-750 cm-l absorptions. The previous analysis predicts that, due to the small difference in the mass of A1 and Sir the impurity modes associated with [AlO,] should be not osservable at all. This explains the great similarity between the Silicalite and Na-ZSM5 spectra. The assignment of the 9 6 0 cm" peak to local modes of [TiO,] units is in agreement with: i) the characteristic reflectance spectrum of TS (inset of Fig.1) where an high intensity band at 48000 cm-', with charge transfer character, is indicative of the presence of [TiO,] units (ref. 5 and references therein) ; ii)
675
the IR spectrum of Si02/Ti02 glasses where a similar, although broader, band is observed (refs. 8,9). Of course these considerations, based only on the mass effect, are an heavy approximation. In fact, due to the higher ionicity of the Ti-0 bond, the Ti-0 and the Si-0 stretching constants are not the same. This assignment is slightly different with respect to that given in ref. 5, where a larger ionicity of the Ti-0 bond was assumed. It can be easily demonstrated that they transform the one into the other when the ionicity of the Ti-0 bond is gradually changed from pure covalent to ionic. Ir spectra of CO adsorbed on Silicalite, Ti-Silicalite and HZSM5: assianment and comarison The spectra of CO adsorbed at 77 K on S and TS samples outgassed at 573 K under vacuo are reported in Fig.2a (3800-3000 cm-l region) , and Fig.2b (2250-2050 cm-l) and Fig.3a and 3b respectively. Similar spectra have been obtained for samples outgassed at lower and higher temperature. They are not described in detail for sake of brevity. The following can be commented. Silicalite The IR spectrum in the OH stretching region (Fig.2a) is characterized by two main absorptions at 3750-3700 cm-' (composit with narrow components at 3750 and 3730 cm-l and a broader one at These two bands 3710 cm-l) and at 3460 c m ' l (0))112=130 cm") essentially correspond to free (isolated and terminal, external and internal) and hydrogen bonded silanols respectively. Upon CO dosage the bands due to free silanols (both isolated and terminal) are eroded, while two new peaks are formed at 3640 and 3585 .''nrc The presence of a clear isosbestic point indicates that the free OH (isolated and terminal) are transformed into hydrogen bonded species (because of formation of 1:l OH...CO adducts) as illustrated in the following scheme:
-
OH.. .CO
I
Si
(isolated)
Upon CO adsorption the peak at ic changes, in agreement with small but clear shift at lower reinforcement of the hydrogen
.
OH...OH...OH...CO
I
si
l
Si
l
si
(terminal)
3460 cm" does not undergo dramatthe given assignment. However, a frequency is indicative of a small bond between adjacent OH groups.
676
t
a
a 2
Fig.2. IR spectra of CO adsorbed at 77 K (increasing doses) on Silicalite outgassed at 573 K. a) OH stretching region: the evolution of'the bands is indicated by the arrows; Aindicate the isosbestic points. b) CO region. The spectrum of CO adsorbed on H-ZSM5 (at 77 K and maximum coverage) outgassed at the same temperature is reported for comparison.
This can be easily explained on the basis of the previous scheme: in fact the formation of the 1:l adducts involving terminal OH groups and CO induces the polarization in the terminal 0-H bond and this in turn reinforces the hydrogen bonding between the vicinal groups of the chain. Similar effects have been observed also in homogeneous conditions (ref. 10). The previous assignment is confirmed by the following other experiments: i) by outgassing Silicalite at 973 K the peaks at 3460 and 3710 cm-' simultaneously disappear; ii) the adsorption of CO on samples containing only isolated OH both on silica and silicalite gives the 3750 cm-' peak only. In the part b of the figure the parallel spectra in the CO stretching region is illustrated. Two main absorptions are clearly observed at 2159 and 2137 cm-'. The first peak, with frequency slightly higher than that of the CO gas (a7=16cm-l) , is due to
677
the CO stretch of the OH...CO (1:l) adducts (both isolated and terminal). The second peak (with frequency nearly coincident with that of (CO)liq) is due to CO molecules physically adsorbed (or in a liquid-like state) in the channels and pores. In the same figure (2b) the spectrum of CO on H-ZSM5 (Si02/A1203=28) outgassed at the same temperature is reported for sake of comparison. We can immediately notice that the two type of spectra are very similar but for the peak at 2170 cm-' present only on H-ZSM5: this peak, characterized by a blue shift of 27 cm" with respect to CO gas, is associated with Broensted acid-CO complexes. This comparison shows that CO is a good probe of the strong Broensted acidity of H-ZSM5 and that SiOH groups are much less acidic in agreement with ref. 11. Titanium-Silicalite An identical experiment carried out on TS outgassed at 573 K gives the spectra reported in Fig.3a and 3b (OH and CO stretching region respectively). The great similarity with the spectra of Fig.2 is immediately evident. This observation has two main consequences: i) titanols (either on framework or extra-framework Ti) cannot be easily distinguished from silanols by IR spectroscopy (especially when the Titanium content is small) and ii) titanols (either framework or extraframework) have Broensted acidity not substantially different from silanols. In conclusion, the presence of Ti does not substantially modify the OH distribution and their acidity. In these conditions we can ask wether TiOH groups are really present (specially if we consider that if Ti is totally framework and if the Ti-0-Si bridges are not at least partially hydrolyzed TiOH should be absent). For the time being we cannot answer this question. However, we shall come back again on this problem later on, on the basis of further data. As known from indipendent experiments on the Ti02/C0 system (ref. 12) , the adsorption of CO on microparticles of Ti02 outgassed at temperature t 873 K, gives surface Ti4+cus-C0 (cus: coordinatively unsaturated) species characterized by a well defined high intensity peak at 2179 cm-'. The presence, or the absence, of this peak on properly outgassed TS samples can be so used as a text of the presence or the absence of extraframework Ti in form of Ti02 microparticles. The results of this experiments on a TS sample outgassed in vacuo at 873 K are reported in Fig.4a and 4b. In the same figure the spectrum of CO (CO-stretch-
678
1
t TO'
a
Ii1
n
b
0.5
-z
r 0 3
Y
u
z U
m w
0 VI
U m
3888
3688
3400
YRVCNUHBtR CM-I
3288
380022
YRVt"ljHBtR
CH-I
8
Fig. 3. IR spectra of CO adsorbed at 77 K (increasing doses) on Ti-Sil icalite outgassed at 573 K. a) OH stretching region. The arrows and A have the same meaning as in Fig.2. b) CO region.
ing region) adsorbed at 77 K on TiOZ (anatase) outgassed at the same temperature is reported for sake of comparison. From these figures we can conclude that: i) the bands at 3450 and 3700 cm" disappear simultaneously upon outgassing, in agreement with the given assignment ; ii) isolated silanols (either external or internal) are the main species present on the sample and give the expected OH...CO 1:l complexes characterized by peaks at 3650 cm" (OH) and 2155 cm'l (CO); iii) very low intensity band at ~ 2 1 8 0cm-l reveals that Ti4+cus-C0 adducts are indeed present on the sample outgassed at high temperature. However, comparison with the spectrum of CO adsorbed on Ti02 clearly indicates that the amount of extralattice Ti under form of Ti02 microparticles is negligible. The last observation confirm that the major part of Ti is in framework position and does not have relevant Lewis acidity. We can now go back to the question concerning the presence or
679
a
I!
h '
I
3600
3400 YRVENUHBER
3200
CM-I
31
12250
2200 2150 2100 YRVENUMBER C H - I
2
0
Fig.4. IR spectra of CO adsorbed at 77 K (increasing doses) on Ti-Silicalite outgassed at 873 K. a) OH stretching region. The arrows and A h a v e the same meaning as in the previous figures. b) CO region. The spectrum of CO adsorbed at 77 K (maximum coverage) on pure Ti02 outgassed at the same temperature is reported for comparison.
absence of TiOH groups. First of all we can now say with confidence that TioH or Ti02 microparticles do not play any relevant role in the spectrum of TS (OH stretching region), and secondly that TioH (if present) must comes only from partial hydrolysis of polar Si-0-Ti bridges, as already hypothesized in ref. 5. In order to prove or reject the hypothesis that Si-0-Ti bridges exposed in the channels and cavities are preferential sites for H20 adsorption, the following comparative experiment has been designed. Silicalite and Ti-Silicalite samples were outgassed at 973 K in high vacuum for 4 hrs to eliminate all the hydrogen bonded hydroxyl groups. After this treatment the IR spectra in the OH stretching region were substantially undistinguishable (with only one peak left at 3720 cm-' similar to that shown in Fig.4a). H20 vapuor was then dosed (10 torr) on the sample and
680
left to stand for 15 minuts. After this dehydration step the unreacted H20 was pumped off at 573 K and the hydration state of the surface monitored by recording the IR spectrum in the OH stretching region. The result was as follows: after the thermal treatment Silicalite shows hydrofobic character, as H20 dosage does not restore the original hydroxyl concentration; on the contrary, Ti-Silicalite is slightly more hydrophylic and can be partially rehydrated. We think that this behaviour could be a consequence of the presence of Si-0-Ti bridges which, being more polar, can easily undergo H20 attack giving silanols and titanols in vicinal position. CONCLUSIONS
The IR band observed in the spectrum of TS at 960 cm-', associated with framework Ti, is due to a local impurity stretching mode of the [Ti04] unit in the Silicalite lattice. The small fraction of extralattice Ti in form of Tio2 microparticles does not contribute to the IR spectrum of TS. TiOH and SiOH cannot be distinguished by IR spectroscopy and have very similar acidity (as tested by CO adsorption at 77 K). Moreover, comparison with the spectra of OH...CO adducts on H-ZSM5 shows that their acidity is much lower. Lewis acidity associated with Ti4+ ions is also pratically absent even on samples outgassed at 973 K. REFERENCE8
W. Holderich, M. Messe and F. Naumann, Angew. Chem. Int. Ed. Engl., 27 (1988) 26. C. Neri, A. Esposito, B. Anfossi and F. Buonomo, Eur. Pat. 100 119.
C. Neri, M. Taramasso and F. Buonomo, U.K. Pat. 102 665. J.A. Rabo, Catal. Rev. Sci. Technol., 24 (1982) 202. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrinil Spectroscopic characterization of Silicalite and Titanium-Silicalite, in: C. Morterra, A. Zecchina and G. Costa (Eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, pp. 133-144. P. Comba and A . Merbach, Inorg. Chem., 26 (1987) 1325. A. Miecznikowski and J. Hamuza, Zeolites, 7 (1987) 249. M.F. Best and R.A. Condrate, J. Mat. Sci. Letters, 4 (1985) 994. B.G. Varshal, V.N. Denisov, B.N. Maurin, G . A . Paulova, V.B. Podobedov and K.E. Stebin, Opt. Spectrosc. (USSR), 47 (1979) 344. 10 G.C. Pimentel and A.L. McClellan, The hydrogen bond, W.H. Freeman and Co., San Francisco and London, 1960. 11 V.B. Kazanskii, Kinet. Catal., 28 (1987) 482. 12 G. Spoto, C. Morterra, L. Marchese, L. Orio and A. Zecchina, Vacuum, in press.
671
IR SPECTRA OF CO ADSORBED AT LOW TEMPERATURE (77 K) ON TITANIUMSILICALITE, €I-ZBMS AND SILICALITE
A. ZECCHINA~,G. SPOTO~,s. BORDIGA~,M. PA DO VAN^, G. LEO FAN TI^ and G. PETRINI~ 'Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Via P. Giuria 7, 10125 Torino (Italy)
'Montedipe, Unit& di Ricerca, Via San Pietro (MI) (Italy)
50,
28100 Bollate
ABSTRACT
The IR band at 960 cm'l observed on TS is due to a local stretching mode of a [Ti04] unit in the silicalite framework. Framework Ti does not show Lewis acidity as probed by CO adsorption at 77 K. TiOH and SiOH have undistinguishable IR properties. At 77 K they form with CO very weak 1:l OH...CO adducts. Small amount of extralattice Ti in form of Ti02 can be probed by CO adsorption at 77 K. INTRODUCTION
The substitution of Ti and A1 for Si in zeolites of the pentasil family (Silicalite) leads to Ti-Silicalite and ZSMS which are two important catalysts for oxidation with HZOZ (refs. 1-3) and acid catalyzed reactions (ref. 4 ) . The characterization of Titanium-Silicalite (TS) by means of physical methods has been recently published (ref. 5). However, a few problems are still open concerning: i) the presence of extralattice Ti (as Ti02 microparticles); ii) the interpretation of the vibrational spectrum of the solid; iii) the crystallinity of TS with respect to pure S and ZSMS; iv) the presence of titanols in the channel8 and cavities. The CO molecule is an efficient probe of Lewis and Broensted acidity and can be used to explore the acidity of OH groups in the channels (TiOH and SiOH in TS, (Si,Al)OH, SiOH and AlOH in H-ZSM5). On dehydroxylated samples coordinatively unsaturated Ti4+ and A13+ ions in lattice and extralattice positions can be revealed as well, because they can form Lewis adducts with CO. In this contribution we report on the IR spectra of CO adsorbed at 77 K on TS, s and ZSMS. The vibrational spectra of s,
TS and Na-ZSM5 are also compared and discussed. EXPERIMENTAL
Silicalite and Titanium-Silicalite have been synthesized in Montedipe laboratories following the method described in ref. 2; Na-ZSM5 and H-ZSM5 (external surface area 60 m’9-l) have been provided by the same laboratory. The ER spectra have been obtained on a Bruker IFS 113V FTIR spectrometer using a specially designed silica cell permanently attached to a vacuum manifold and allowing in situ outgassing procedures at temperatures in the 273-1073 K interval and gas dosing at 7 7 K. The samples were either in form of thin pellets or of films deposited on KBr or Si plates. RESULTS AND DISCUSSION IR modes associated with framework Ti
The spectra recorded at 7 7 K of S, TS and ZSM5 (sodium form; Si/A1=35) films outgassed at 573 K are compared in Fig.1. The major difference between S and ZSM5 on one side and TS on the other side is represented by the presence in the spectrum of TS of a (finger print) peak at 960 cm-l. In order to give a correct assignment of this band the following considerations have to be made: a) it cannot be assigned to an optical mode of TiOZ microparticles entrapped into the channels or at their intersection because neither rutile nor anatase show strong IR bands at this frequency; b) despite the similarity with the IR manifestations of homogeneous titanyl containing analogs , the 960 cm-’ peak cannot be attributed to the stretching mode of internal (Ti=O)’+ because: i) it is not perturbed by filling the pores and channels with CO (results not reported), which, being a weak Lewis base, is expected to interact with the positive centres and to perturb them; ii) it does not have the expected counterpart in the UV-Vis diffuse reflectance spectrum (absorption in the 25000-35000 cm’l range) (ref. 6); iii) it cannot be removed by reductive treatments in H2 and CO even at very high temperature or under plasma discharge conditions (Ha). On the basis of the previous **negativet* evidences, the hypothesis has been made that the 960 cm-l band is due to a local mode of the Titanium-doped pentasilic structure (essentially a stretching mode of a [Si04] unit perturbed by an adjacent Ti) (ref. 5 ) .
673
0.5
A
F'LI / I
1
I0
YRVENUHBER C H - I
Fig.1. IR absorbance spectra (recorded at 77 K) of: a) Silicalite, b) Na-ZSM5 (Si/A1=35) and c) Ti-Silicalite. In the inset: W-Vis reflectance spectra of a) Silicalite and c) Ti-Silicalite. A full assignment of the IR spectra of s, TS and ZSM5 requires a detailed unit cell vibrational analysis and is outside the scope of this contribution. However, an explanation of the presence of the extra-absorption in the TS spectrum can be equally given on the basis of the following qualitative considerations. The vibrational representation of the stretching modes of an llisolatedll tetrahedral [ S i O , ] unit is:
rstret.= T2 *I (TZ: IR active; A1: IR inactive). Packing of the [Sio,] (by sharing the corners) to form the zeolitic structure has two consequences: i) the local *'site1@symmetry of each [SiO,] primary +
674
building unit is lowered to C2 (ref. 7) so that the triply degenerated T2 mode is splitted into three, one weak (A) and two strong (B), components and ii) due to the high number of [SiO,] units forming the unit cell (where 9 6 tetrahedral units are present) broad bands are expected because of the further splitting of each component into a maximum of 96 sub-components. On these basis, the spectrum of S in the stretching region can be interpreted as follows: i) the two clear absorption at 1235 (mw) and 1200-1050 (vs) cm-l, are essentially associated with the A and B modes of the primary unit broadened by unit cell splitting effects; ii) the complex (vw) band centered at =770 cm-l derives from the A1 (IR inactive) mode (the fine structure observed at 77 K being associated with the unit cell sub-components). The bands at lower frequency belong to skeletal modes having bending character and will not be considered here for sake of brevity. We only mention that the peak at ~ 5 5 0cm-l is absent on amorphous silica and is considered as a gfcrystallinityfg (structure sensitive) band (ref. 5). As far as TS is concerned, some 1-2% of the [SiO,] building blocks of pure silicalite framework are substituted for Igheavierff [TiO,] units. The substitution does not dramatically change the overall IR spectrum other than for the appearence of localized fgimpuritygg modes associated with the [TiO,] dopant units. In the stretching region four additional local modes are, in principle, expected, three (A+2B) deriving from the T2 degenerate vibrations of the free [TiO,] unit and one (A) from the A1. These modes should be shifted to lower frequencies with respect to those of Silicalite, because of the mass effect. Actually only the band deriving from the strongest absorption at 1120 cm-l (presumably of B simmetry) has enough intensity and is enough shifted to show up clearly in the gap between the 1250-1050 and 820-750 cm-l absorptions. The previous analysis predicts that, due to the small difference in the mass of A1 and Sir the impurity modes associated with [AlO,] should be not osservable at all. This explains the great similarity between the Silicalite and Na-ZSM5 spectra. The assignment of the 9 6 0 cm" peak to local modes of [TiO,] units is in agreement with: i) the characteristic reflectance spectrum of TS (inset of Fig.1) where an high intensity band at 48000 cm-', with charge transfer character, is indicative of the presence of [TiO,] units (ref. 5 and references therein) ; ii)
675
the IR spectrum of Si02/Ti02 glasses where a similar, although broader, band is observed (refs. 8,9). Of course these considerations, based only on the mass effect, are an heavy approximation. In fact, due to the higher ionicity of the Ti-0 bond, the Ti-0 and the Si-0 stretching constants are not the same. This assignment is slightly different with respect to that given in ref. 5, where a larger ionicity of the Ti-0 bond was assumed. It can be easily demonstrated that they transform the one into the other when the ionicity of the Ti-0 bond is gradually changed from pure covalent to ionic. Ir spectra of CO adsorbed on Silicalite, Ti-Silicalite and HZSM5: assianment and comarison The spectra of CO adsorbed at 77 K on S and TS samples outgassed at 573 K under vacuo are reported in Fig.2a (3800-3000 cm-l region) , and Fig.2b (2250-2050 cm-l) and Fig.3a and 3b respectively. Similar spectra have been obtained for samples outgassed at lower and higher temperature. They are not described in detail for sake of brevity. The following can be commented. Silicalite The IR spectrum in the OH stretching region (Fig.2a) is characterized by two main absorptions at 3750-3700 cm-' (composit with narrow components at 3750 and 3730 cm-l and a broader one at These two bands 3710 cm-l) and at 3460 c m ' l (0))112=130 cm") essentially correspond to free (isolated and terminal, external and internal) and hydrogen bonded silanols respectively. Upon CO dosage the bands due to free silanols (both isolated and terminal) are eroded, while two new peaks are formed at 3640 and 3585 .''nrc The presence of a clear isosbestic point indicates that the free OH (isolated and terminal) are transformed into hydrogen bonded species (because of formation of 1:l OH...CO adducts) as illustrated in the following scheme:
-
OH.. .CO
I
Si
(isolated)
Upon CO adsorption the peak at ic changes, in agreement with small but clear shift at lower reinforcement of the hydrogen
.
OH...OH...OH...CO
I
si
l
Si
l
si
(terminal)
3460 cm" does not undergo dramatthe given assignment. However, a frequency is indicative of a small bond between adjacent OH groups.
676
t
a
a 2
Fig.2. IR spectra of CO adsorbed at 77 K (increasing doses) on Silicalite outgassed at 573 K. a) OH stretching region: the evolution of'the bands is indicated by the arrows; Aindicate the isosbestic points. b) CO region. The spectrum of CO adsorbed on H-ZSM5 (at 77 K and maximum coverage) outgassed at the same temperature is reported for comparison.
This can be easily explained on the basis of the previous scheme: in fact the formation of the 1:l adducts involving terminal OH groups and CO induces the polarization in the terminal 0-H bond and this in turn reinforces the hydrogen bonding between the vicinal groups of the chain. Similar effects have been observed also in homogeneous conditions (ref. 10). The previous assignment is confirmed by the following other experiments: i) by outgassing Silicalite at 973 K the peaks at 3460 and 3710 cm-' simultaneously disappear; ii) the adsorption of CO on samples containing only isolated OH both on silica and silicalite gives the 3750 cm-' peak only. In the part b of the figure the parallel spectra in the CO stretching region is illustrated. Two main absorptions are clearly observed at 2159 and 2137 cm-'. The first peak, with frequency slightly higher than that of the CO gas (a7=16cm-l) , is due to
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the CO stretch of the OH...CO (1:l) adducts (both isolated and terminal). The second peak (with frequency nearly coincident with that of (CO)liq) is due to CO molecules physically adsorbed (or in a liquid-like state) in the channels and pores. In the same figure (2b) the spectrum of CO on H-ZSM5 (Si02/A1203=28) outgassed at the same temperature is reported for sake of comparison. We can immediately notice that the two type of spectra are very similar but for the peak at 2170 cm-' present only on H-ZSM5: this peak, characterized by a blue shift of 27 cm" with respect to CO gas, is associated with Broensted acid-CO complexes. This comparison shows that CO is a good probe of the strong Broensted acidity of H-ZSM5 and that SiOH groups are much less acidic in agreement with ref. 11. Titanium-Silicalite An identical experiment carried out on TS outgassed at 573 K gives the spectra reported in Fig.3a and 3b (OH and CO stretching region respectively). The great similarity with the spectra of Fig.2 is immediately evident. This observation has two main consequences: i) titanols (either on framework or extra-framework Ti) cannot be easily distinguished from silanols by IR spectroscopy (especially when the Titanium content is small) and ii) titanols (either framework or extraframework) have Broensted acidity not substantially different from silanols. In conclusion, the presence of Ti does not substantially modify the OH distribution and their acidity. In these conditions we can ask wether TiOH groups are really present (specially if we consider that if Ti is totally framework and if the Ti-0-Si bridges are not at least partially hydrolyzed TiOH should be absent). For the time being we cannot answer this question. However, we shall come back again on this problem later on, on the basis of further data. As known from indipendent experiments on the Ti02/C0 system (ref. 12) , the adsorption of CO on microparticles of Ti02 outgassed at temperature t 873 K, gives surface Ti4+cus-C0 (cus: coordinatively unsaturated) species characterized by a well defined high intensity peak at 2179 cm-'. The presence, or the absence, of this peak on properly outgassed TS samples can be so used as a text of the presence or the absence of extraframework Ti in form of Ti02 microparticles. The results of this experiments on a TS sample outgassed in vacuo at 873 K are reported in Fig.4a and 4b. In the same figure the spectrum of CO (CO-stretch-
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1
t TO'
a
Ii1
n
b
0.5
-z
r 0 3
Y
u
z U
m w
0 VI
U m
3888
3688
3400
YRVCNUHBtR CM-I
3288
380022
YRVt"ljHBtR
CH-I
8
Fig. 3. IR spectra of CO adsorbed at 77 K (increasing doses) on Ti-Sil icalite outgassed at 573 K. a) OH stretching region. The arrows and A have the same meaning as in Fig.2. b) CO region.
ing region) adsorbed at 77 K on TiOZ (anatase) outgassed at the same temperature is reported for sake of comparison. From these figures we can conclude that: i) the bands at 3450 and 3700 cm" disappear simultaneously upon outgassing, in agreement with the given assignment ; ii) isolated silanols (either external or internal) are the main species present on the sample and give the expected OH...CO 1:l complexes characterized by peaks at 3650 cm" (OH) and 2155 cm'l (CO); iii) very low intensity band at ~ 2 1 8 0cm-l reveals that Ti4+cus-C0 adducts are indeed present on the sample outgassed at high temperature. However, comparison with the spectrum of CO adsorbed on Ti02 clearly indicates that the amount of extralattice Ti under form of Ti02 microparticles is negligible. The last observation confirm that the major part of Ti is in framework position and does not have relevant Lewis acidity. We can now go back to the question concerning the presence or
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a
I!
h '
I
3600
3400 YRVENUHBER
3200
CM-I
31
12250
2200 2150 2100 YRVENUMBER C H - I
2
0
Fig.4. IR spectra of CO adsorbed at 77 K (increasing doses) on Ti-Silicalite outgassed at 873 K. a) OH stretching region. The arrows and A h a v e the same meaning as in the previous figures. b) CO region. The spectrum of CO adsorbed at 77 K (maximum coverage) on pure Ti02 outgassed at the same temperature is reported for comparison.
absence of TiOH groups. First of all we can now say with confidence that TioH or Ti02 microparticles do not play any relevant role in the spectrum of TS (OH stretching region), and secondly that TioH (if present) must comes only from partial hydrolysis of polar Si-0-Ti bridges, as already hypothesized in ref. 5. In order to prove or reject the hypothesis that Si-0-Ti bridges exposed in the channels and cavities are preferential sites for H20 adsorption, the following comparative experiment has been designed. Silicalite and Ti-Silicalite samples were outgassed at 973 K in high vacuum for 4 hrs to eliminate all the hydrogen bonded hydroxyl groups. After this treatment the IR spectra in the OH stretching region were substantially undistinguishable (with only one peak left at 3720 cm-' similar to that shown in Fig.4a). H20 vapuor was then dosed (10 torr) on the sample and
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left to stand for 15 minuts. After this dehydration step the unreacted H20 was pumped off at 573 K and the hydration state of the surface monitored by recording the IR spectrum in the OH stretching region. The result was as follows: after the thermal treatment Silicalite shows hydrofobic character, as H20 dosage does not restore the original hydroxyl concentration; on the contrary, Ti-Silicalite is slightly more hydrophylic and can be partially rehydrated. We think that this behaviour could be a consequence of the presence of Si-0-Ti bridges which, being more polar, can easily undergo H20 attack giving silanols and titanols in vicinal position. CONCLUSIONS
The IR band observed in the spectrum of TS at 960 cm-', associated with framework Ti, is due to a local impurity stretching mode of the [Ti04] unit in the Silicalite lattice. The small fraction of extralattice Ti in form of Tio2 microparticles does not contribute to the IR spectrum of TS. TiOH and SiOH cannot be distinguished by IR spectroscopy and have very similar acidity (as tested by CO adsorption at 77 K). Moreover, comparison with the spectra of OH...CO adducts on H-ZSM5 shows that their acidity is much lower. Lewis acidity associated with Ti4+ ions is also pratically absent even on samples outgassed at 973 K. REFERENCE8
W. Holderich, M. Messe and F. Naumann, Angew. Chem. Int. Ed. Engl., 27 (1988) 26. C. Neri, A. Esposito, B. Anfossi and F. Buonomo, Eur. Pat. 100 119.
C. Neri, M. Taramasso and F. Buonomo, U.K. Pat. 102 665. J.A. Rabo, Catal. Rev. Sci. Technol., 24 (1982) 202. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrinil Spectroscopic characterization of Silicalite and Titanium-Silicalite, in: C. Morterra, A. Zecchina and G. Costa (Eds.), Structure and Reactivity of Surfaces, Elsevier, Amsterdam, 1989, pp. 133-144. P. Comba and A . Merbach, Inorg. Chem., 26 (1987) 1325. A. Miecznikowski and J. Hamuza, Zeolites, 7 (1987) 249. M.F. Best and R.A. Condrate, J. Mat. Sci. Letters, 4 (1985) 994. B.G. Varshal, V.N. Denisov, B.N. Maurin, G . A . Paulova, V.B. Podobedov and K.E. Stebin, Opt. Spectrosc. (USSR), 47 (1979) 344. 10 G.C. Pimentel and A.L. McClellan, The hydrogen bond, W.H. Freeman and Co., San Francisco and London, 1960. 11 V.B. Kazanskii, Kinet. Catal., 28 (1987) 482. 12 G. Spoto, C. Morterra, L. Marchese, L. Orio and A. Zecchina, Vacuum, in press.