Spectroscopic tools for probing the isolated titanium centres
in MCM41 mesoporous catalysts (*)
L. MARCHESE(1), E. GIANOTTI(1), T. MASCHMEYER(2) G. MARTRA(1), S. COLUCCIA(1) and J. M. THOMAS(2)
(1) Dipartimento di Chimica IFM - via P. Giuria, 7, I-10125 Torino, Italy (2) Davy Faraday Research Laboratory, The Royal Institution of Great Britain
21 Albemarle Street, London W1X 4BS, UK
(ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)
Summary. — Catalytically active Ti(IV) centres anchored to the inner walls of MCM-41 mesoporous silicas have been investigated using a combination of FT-IR, diffuse reflectance UV-visible and luminescence spectroscopy supplemented by the use of H2O and NH3as molecular probes of the Ti coordination. The results re-affirm
and extend the conclusions reported earlier for the Ti-MCM41 novel epoxidation catalysts, see MASCHMEYERT., REYF., SANKARG. and THOMASJ. M., Nature, 378 (1995) 159, who, using in situ EXAFS studies, showed that the Ti(IV) centres exist largely as isolated active sites.
PACS 82.65.Jv – Heterogeneous catalysis at surfaces. PACS 81.05.Rm – Porous materials; granular materials. PACS 78.55 – Photoluminescence.
PACS 78.30 – Infrared and Raman spectra. PACS 01.30.Cc – Conference proceedings.
1. – Introduction
There is currently great interest in Ti(IV)-based siliceous catalysts for various kinds of selective oxidation. Starting with the seminal work of Taramasso et al. [1], who showed that the so-called titanosilicate TS-1 (a titanium silicalite) was capable of a wide range of low-temperature oxidations with H2O2, many others [2-5] have underlined the
importance and value of Ti-based silicate in catalytic contexts.
The synthesis of mesoporous siliceous materials with larger channel apertures, such as MCM-41 (pores in the 25–100 Å range) [6], has expanded considerably the scope for shape-selective Ti-based heterogeneous catalysts for bulky molecules [3, 5, 7-9]. For example the materials prepared by Maschmeyer et al. (anchoring organometallic complexes onto the inner walls of MCM-41), exhibit high catalytic performance in the
(*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.
molecular probes, which aid in the clarification of the coordination of titanium sites of Ti-MCM41 prepared by the grafting procedure of Maschmeyer et al. [7].
2. – Experimental
A Ti-MCM41, containing 2% wt Ti, was prepared with the strict exclusion of water under an argon atmosphere using conventional Schlenk line techniques and dehydrat-ing the MCM-41 at 250 7C under a dynamic vacuum prior to titanium loaddehydrat-ing.
FT-IR experiments on pelletized samples were recorded with a Bruker IFS88 spectrometer at a resolution of 4 cm21, and by means of specially designed cells which
were permanently connected to a vacuum line (ultimate pressure G1025torr ) to make
adsorption-desorption in situ experiments.
Diffuse reflectance UV-Vis spectra were recorded by means of a Perkin Elmer (Lambda 19) spectrometer equipped with an integrating sphere attachment and photoluminescence spectra by using SPEX FLUOROLOG-2 1680 spectrometer. The samples, in form of powder, were placed in quartz cells which, also in this case, were permanently connected to a vacuum line for in situ experiments.
All the samples were calcined in 100 Torr O2at 550 7C for 5 to 10 hours in order to
eliminate the organic fraction of the Ti-cyclopentadienyl complexes anchored onto the surface of MCM41; successively, the samples were also evacuated at 550 7C.
3. – Result and discussion
3.1. Diffuse reflectance UV data. – It is well established that the diffuse reflectance UV spectroscopy gives very effective information on the coordination state of Ti sites [13-17].
Figure 1 shows diffuse reflectance UV spectra of Ti-MCM41. The sample in vacuo (curve a) presents a band centred at around 230 nm which is gradually replaced, upon water adsorption, by a new broader band at ca. 240 nm (spectra b-d). The highest dose (curve d) has spectroscopic features which extend down to 320 nm and which cover also the region where the original tetrahedral Ti(IV) sites absorbed.
Similar results, obtained by studying Ti-silicalite, were explained by Boccuti et
al. [13] as due to tetrahedral Ti(IV) sites, in that case a band at 200–210 nm was found,
which undergoes a coordination change to octahedra by insertion of two molecules of water as extraligands (a band at 240 nm was observed). The bands were assigned to oxygen to tetrahedral titanium (IV) charge transfer (LMCT) according to the empirical
Fig. 1. – Diffuse reflectance UV spectra of Ti-MCM41 activated in vacuo at 550 7C (curve a) and upon adsorption of water (curves b-d); the spectrum of a pure MCM-41 is also reported.
optical electronegativity theory [14], which suggests that the (corrected) energy of the first Laporte-allowed CT (for TiX4 complexes it is related to pt1K de (p 1 s) t2K de
electron transfer) is given by equation
ncorr( cm21) 430 000[xopt( anion ) 2xopt( cation ) ] .
(1)
When reasonable values of optical electronegativities
(
OH24 3.45; tetr. Ti(IV)=1.85 and oct. Ti(IV)=2.05
)
are substituted in eq. (1), a fairly good agreement with the experimental data is obtained.However, an accurate inspection of the 200–210 nm CT band reveals that a strong shoulder at 230 nm is always present in dehydrated Ti-silicalites even in the case of samples in which, as suggested by accurate EXAFS, XANES [15, 16], XPS and1H and
29Si NMR [16] investigations, virtually all the titanium exists in tetrahedral
co-ordination. Le Noc et al. suggested that the two bands at 200-210 and 230 nm are due to two different framework sites
(
Ti(OH)(OSi)3and Ti(OSi)4, see structures 1 and 3in scheme 1
)
which have different TiOSi angles. The sites with the larger angles are responsible for the band at lower wavelengths [16].Such an assignment seems to contradict the results predicted by eq. (1) and raises a question whether the 230 nm band is due to tetrahedral or octahedral Ti(IV) sites. However, it should be considered that the relationship (1) allows to estimate the
energies for any transition metal ion complex, and it has also to be considered that various factors should be subtracted from the observed CT frequencies to get reliable (corrected ncorr) values [14].
In addition, it is reasonable to argue that complexes which have oxygen ligands with different electronegativity, as is the case of the structures depicted in scheme 1, will present more than one CT band at different energy. It is indeed well established that the oxygen of a siloxane bridge has a lower negative charge than that of a OH group [18], and, consequently, it will have an electron transfer at higher energy.
It is likely that tetrahedral Ti centres in Ti-anchored MCM-41 with both siloxy- and hydroxy-ligands (see structures 1 and 2 in scheme 1) are very abundant and this might well explain the presence of a very intense band centred at 230 nm rather than at 200–210 nm found on Ti-silicalites.
Therefore, it can be argued that the adsorption of two extra ligands (H2O), which
expands the Ti(IV) coordination sphere from tetrahedral to octahedral, occurs on Ti-MCM41 catalysts. The mechanism is represented in scheme 2.
Altogether these results confirm the XANES and EXAFS studies which suggested that a large fraction of titanium in Ti-anchored MCM-41 exists in tetrahedral co-ordination [7].
3.2. FT-IR data.
3.2.1. T h e n a t u r e o f s u r f a c e h y d r o x y l s . Figure 2 shows FTIR spectra of a typical pure-silica MCM-41 (a) and the Ti-MCM41 (b) calcined and outgassed at 550 7C. The MCM-41 has a narrow band at 3745 cm21 overlapped to a weak and very broad
absorption in the range 3700–3400 cm21; the assignment of such absorptions is
straightforward because bands in similar positions have been found on various amorphous silicas [18, 19 and references therein], as well as on silicalites [20] and can be attributed definitely the first to the O—H stretching of free silanols, and the second to silanols interacting by H-bonds (see scheme in the figure). TGA analysis revealed that, after heating at 600 7C, the MCM-41 has 2.0 OH/nm2, and it is indeed very likely that a fraction of hydroxyls are H-bonded.
In Ti-MCM41, besides free silanols (here found at 3743 cm21) and H-bonded
hydroxyls, a strong shoulder at ca. 3725 cm21 is also present and, even though a
clear-cut assignment of the precise nature of such band could not be done, it has to be noted that it is due to the presence of Ti sites onto the surface of the MCM-41. The Ti-MCM41 sample also has a high concentration of hydroxyls (1.8 OH/nm2) and it is
likely that some silanols are close enough to Ti Lewis acid centres, and/or to Ti-OH groups and this induces a downward shift of the stretching frequency with respect to that of the free silanols (see the structures depicted in scheme 3). Hydroxyls of this type should be more acidic than those on the pure MCM-41, and this is, in fact, what was found upon adsorption of ammonia (vide infra).
DFT calculations on H3Si-OH and (H3SiO)3-Ti-OH clusters suggest that the
stretching vibrations of silanols and Ti-O-H groups are, respectively, at 3722 and 3704 cm21[21]. It is remarkable that these values are downward shifted from the
experimental values of the hydroxyls found on the Ti-MCM41 by very similar amounts, 23 cm21for the free silanols and 21 cm21 for the band at 3725 cm21, so suggesting that
the band at 3725 cm21 should be assigned to Ti-OH species. However, we believe that
further experimental data and computational analysis is necessary to clarify the nature of the hydroxyls at 3725 cm21 more precisely. In particular, as DFT calculations are
normally associated with an error of about 6 5%, the good match between experiment and theory might be fortuitous.
3.2.2. T h e b a n d a t 9 3 5 c m21. A very broad band at 935 cm21 (see fig. 2) is also
observed in the IR spectrum of the Ti-MCM41 which is a clear evidence for the presence of Ti anchored onto the silica surface as similar absorptions have been found in TS-1 [13,22], Ti-MCM41 [4, 8, 9], and also in amorphous Ti-silicates [11]. In these materials, where Ti (IV) ions isomorphously substitute for Si within the siliceous
Fig. 2. – FTIR spectra of MCM-41 (curve a) and Ti-MCM41 (curve b) obtained after calcining and evacuating the samples at 550 7C.
matrix, a band centered mainly at 960 cm21 is constantly found, and, as proposed by
Boccuti et al. [13], related to vibrations in structures where TiO4 tetrahedra share
corners with SiO4tetrahedra (scheme 1).
Recent ab initio quantum chemical studies have confirmed the assignment of the absorption at 960 cm21 as being due to the antisymmetric Ti-O-Si stretching
vibration [23].
Interestingly, the band of the Ti-O-Si connectivity in the case of the Ti-anchored MCM-41 is found at lower frequency than that of the framework-substituted Ti-silicates and such an effect might be attributed to the presence of different ligands bound to the tetrahedral Ti sites. In the case of Ti-anchored MCM41, as suggested by UV-Vis reflectance data, it is more likely that one or two ligands are hydroxyls (structures 1 and 2 in the scheme 1) rather than SiO4tetrahedra: structures 3 in the scheme 1 should
be more abundant in framework-substituted Ti-silicates.
3.2.3. T h e a d s o r p t i o n o f N H3. The acidity of hydroxyl groups, both in MCM-41
and Ti-MCM41, as well as the coordination of titanium centres in Ti-MCM41, were probed by using NH3. On pure MCM-41, NH3 adsorbs on silanol groups by means
of H-bonds forming very weak complexes similar to the one depicted in the scheme of fig. 3 [18]. The spectroscopic features of these complexes are well known: i) a broad band centred at 3030 cm21 due to the stretching of the H-bonded silanols, ii) bands
at 3405 and 3320 cm21 respectively due to the NH
3 asymmetric and symmetric
stretching vibrations and iii) a band at 1635 cm21due to the NH
3 asymmetric bending
Fig. 3. – FTIR spectra of pure MCM-41 and Ti-MCM41 in vacuo at 550 7C (dotted lines) and upon adsorption of NH3 (curves 1, 30 Torr). Curves 1 to 16 for Ti-MCM41 and 1 to 13 for MCM-41
correspond to decreasing doses of NH3; the last spectra were obtained by evacuating the samples
at room temperature for 30 minutes.
fact that all the features of the complex completely disappear, and the narrow band at 3745 cm21 is restored, upon a simple desorption at room temperature.
However, bands at ca. 3500 (broad) and 1550 (very weak) cm21are still present once
the ammonia is desorbed from the surface of the MCM-41 and this clearly suggests that new surface species are formed. It is known that distorted surface siloxane bridges can react with ammonia giving Si-OH and Si-NH2[24, 25], and this would explain the
appearance of the bands at 3500 and 1550 cm21 which are associated with the newly
formed OH (stretching) and the NH2(bending) groups, respectively.
All the species formed upon adsorption of ammonia on MCM-41 are also formed on Ti-MCM41. However, in this latter case, the ammonia is not completely desorbed at room temperature after a prolonged evacuation, there still being present bands at 3391,
d NH1
4 — 1460
(a) This band cannot be measured with precision because it is ovelapped to the stronger absorption at 3391 cm21.
3292, 1605 and ca. 1460 cm21. The first three bands can be assigned to NH
3adsorbed on
Ti Lewis acid sites as similar complexes have been found also by UV-Vis spectroscopy: the LMCT absorption band at 230 nm of the tetrahedral Ti sites, similarly to the water adsorption experiment shown in fig. 1, is modified upon NH3adsorption and this effect
is irreversible at RT. Significantly, the stretching and bending modes of NH3adsorbed
on Ti Lewis sites are shifted to lower frequencies than the bands of NH3 adsorbed on
silanols: table I summarizes these results.
The band at 1460 cm21, not observed on pure MCM-41, can be assigned to the NH 4 1
species; the presence of these species suggests that on the surface of Ti-MCM41 there is a fraction of hydroxyls which are sufficiently acidic to protonate some ammonia molecules. Similar results have been also found for vanadia-based catalysts [26]. The nature of the hydroxyls responsible of such a reaction is still under investigation.
Finally, the silicon-oxygen-titanium vibration at 935 cm21(not shown for the sake of
brevity) is also deeply modified upon NH3 adsorption in that a blue shift up to ca.
990 cm21 occurs. The phenomenon has been attributed to the fact that when ammonia
molecules are bound to Ti the polarity of the Tid 1-Od 2-Si bonds is increased and,
consequently, more negative charge is accumulated on the [O3Si-O]d 2 so that the
absorption moves towards the values typical of the stretching of “isolated” Si-O groups [15, 19, 20, 22]. An example of isolated Si-O vibration is that of Si-OH groups which in pure MCM-41 absorb at 980 cm21(see fig. 2).
3.3. Photoluminescence data. – The photoluminescence technique has been applied to the study of anchored titanium oxide catalysts [27-29] and TS-2 [30], particularly by Anpo and co-workers. Photoemission spectra containing features in the range of 400 to 600 nm are reported and interpreted as deriving from isolated tetrahedral titanium centres. However, only in one paper [30] the corresponding reflectance spectra were reported and there they contain broad bands at l F300 nm which suggest that a significant fraction of Ti sites are present either with coordination ranging from 4 to 6 [13, 15, 17] or as TiO2-like phase [16, 31].
In order to get unambiguous data for the emitting sites in Ti-containing materials, it is vital to study samples, as in the present case, which have been characterized by in
situ X-ray absorption spectroscopy [7], complemented by reflectance UV-Vis and
FTIR spectroscopy (vide supra), and have been shown to contain a high degree of tetrahedral titanium sites.
The photoluminescence spectroscopy allows us to monitor both the ground and the excited states by collecting the absorption and the emission of light associated with processes such as those described in eqs. (2) and (3):
Ti4 1 OO2 21 hn K Ti3 1OO2, (2) Ti3 1 OO2K Ti4 1OO2 21 hn , (3)
with this technique we can probe the presence, if any, of different tetrahedral Ti sites (see scheme 1) in Ti-MCM41.
By exciting the Ti-MCM41 at 298 K with lex4 250 nm , a complex emission band
with two main maxima at 434 and 480 nm and two shoulders at ca. 400 and 525–550 nm is obtained (fig. 4A, curve a) and this reveals the presence of at least four different emitting sites. The overall emission band is greatly quenched upon exposure to O2 at
298 K (ca. 80% of the original intensity is lost, cf. curves d-f), meaning that a very large fraction of these sites are located on the MCM-41 silica surface; such an effect is nearly completely reversible when the oxygen is desorbed at 298 K (curve g). It might be stressed that no such emission bands are observed in the case of pure siliceous MCM-41 confirming that the emitting sites in Ti-MCM41 are the Ti(IV) centres.
The sites emitting at 434 and 480 nm have very similar excitation spectra (curves a and b) and both present a maxima at 248 nm and this strongly suggests that they stem from similar environments. The shift of the maximum in the excitation spectra (ca. 250 nm) as compared with the reflectance band (ca. 230 nm) is not surprising. In fact, it has to be considered that the absorption of tetrahedral Ti(IV) ions is complex [14] and that the maximum efficiency in exciting the emission does not necessarily coincide with the maximum absorption due to possible energy transfer processes among levels of the same sites.
The photoemission of light by Ti sites is a temperature-dependent phenomenon as can be clearly seen from an inspection of the section B of fig. 4, in which the spectrum recorded at 77 K (lex4 250 nm , curves a8) is 20-30 times more intense than that
recorded at 298 K (curve a). The relative intensities of the various components of the two spectra is also different being present, in that at low temperature, a very strong band at ca. 500 nm which arises probably from the great enhancement of the shoulder at 525–550 nm. The exposure to O2 at 77 K, 0.15 Torr in this case, nearly completely
quences the emission band (curve d8) confirming that within experimental error all the emitting sites are accessible to the O2and are, hence, on the surface.
The presence of the larger contribution of sites emitting at 525–550 nm in the low-temperature spectrum is also confirmed by the appearance in the excitation spectrum (lem4 520 nm , curve c8) of a broad shoulder at ca. 270 nm which is not present neither
Fig. 4. – Emission and excitation spectra of Ti-MCM41 at 298 (section A) and at 77 K (section B) under high vacuum (ultimate pressure G1025Torr) unless stated otherwise; section A: (a)
emission spectrum for lex4 250 nm, (b) lem4 434 nm vs. excitation from 200–350 nm, (c) lem4
480 nm vs. excitation from 200–350 nm, (d), (e) and (f) show the effect of O2 adsorption
corresponding to 1.5, 20 and 80 Torr O2 respectively on the emission spectra at lex4 250 nm,
(g) shows near reversibility of O2 adsorption when desorbing O2 for 1 h at 298 K (pressure
G1025Torr); section B: (a8) emission spectrum for lex4 250 nm (spectrum (a) at 298 K
shown, multiplied by 10, for comparison), (b8) lem4 420 nm vs. excitation from 200–350 nm,
(c8) lem4 520 nm vs. excitation from 200–350 nm, (d8) emission spectrum for
lex4 250 nm, corresponding to the adsorption of 0.15 Torr O2.
It should be stressed that even though a precise assignment of the various components cannot be made at this stage, the difference in the relative intensity of the bands at 77 and 298 K confirms that more than one tetrahedral titanium site is present in Ti-MCM41. In addition, similarly to other high surface area solids [32], energy transfers occur between the sites and such effects are particularly relevant at 298 K where the energy relaxation via non-radiative processes is also very important.
However, owing to the fact that the electron density of the oxygens in TiOH and on TiOSi is certainly different and, additionally, should change slightly when they are located in Ti(OH)(OSi)3 or Ti(OH)2(OSi)2surface sites (scheme 1), such groups should
have different spectroscopic features. We, therefore, propose that the various bands in the emission spectra are due to the relaxation of the different oxygen to tetrahedral Ti (IV) charge transfer transitions, requiring a minimum of two different tetrahedral titanium sites.
Finally, it is remarkable that the Ti-MCM41 catalysts have UV-Vis luminescence whose intensity is strictly related to the dispersion of the Ti(IV) ions onto the inner surface of MCM-41 [33] and such an effect has prompted us to correlate their catalytic activity/selectivity to their luminescence yield.
* * *
Italian ASP and CNR and British EPSRC are gratefully acknowledged for financial support. The authors thank Dr. R. OLDROYD for the help in preparing the Ti-MCM41 and Prof. A. ZECCHINAfor fruitful discussion.
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