Zeolite characterization with spectroscopic methods

Studies in Surface Science and Catalysis 142 R. AieUo, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Zeolite characterization with spectroscopic m e t h o d s A. Zecchina 1'2, G. Spoto 1'2, G. Ricchiardi 1'2, S. Bordiga 1'2'3, F. Bonino 1, C. Prestipino 1'3 and C. Lamberti 1'2'3 1 Department of Inorganic, Physical and Material Chemistry, Via P. Giuria 7, 10125 Turin (I) 2 INSTM Unith di Torino 3 INFM Unit~ di Torino Universit~ (I)

Among the different spectroscopic techniques reported in the plenary lecture presented at the FEZA 2002 conference, in this work, we will focus on IR spectroscopy only, devoted to the characterization of the acid strength of the BrCnsted groups in zeolites In particular, in this brief review we will show how the systematic study of the spectroscopic manifestations observed by IR spectroscopy upon dosing to H-zeolites probe molecules with increasing proton affinity will allow to establish a spectroscopic acidity scale for the BrOnsted groups hosted in different zeolites. 1. INTRODUCTION It is universally recognized that the BrCnsted groups represent the most reactive species present in the channels and cavities and that most of the catalytic properties of the zeolites in the acidic form can be ascribed to these species [ 1-3]. The ability of the BrCnsted groups to interact with molecules entering the channels has been the subject of many investigations with physical techniques and, among them, vibrational spectroscopy has definitely played the major role in elucidating the interaction mechanisms and the structure of the formed species. In particular, one of the key questions associated with the activity of the BrCnsted groups present in the zeolite channels is related to the estimation of their acid strength and to the comparison with that of the most common mineral acids and superacids [4].To illustrate in a systematic way the results obtained by the application of vibrational spectroscopy, a useful approach is to describe the interaction of the BrCnsted site of zeolite 1~ (chosen as a prototype system) with molecules characterised by a proton affinity (PA) ranging in a very broad interval. To this end the following sequence of molecules (ordered following their PA) will be illustrated: N2 (PA = 118.2 kcal mol-1), CO (PA = 141.9 kcal mol-1), C2H4 (PA = 162.6 kcal mol-1), C3H6 (PA = 179.5 kcal mol-i), H20 (PA = 166.5 kcal mol-1), CH3CN (PA = 188.6 kcal mol-1), CH3OH (PA = 181.9 kcal mol-1), (CH3)20 (PA = 192.1 kcal mol-1), THF (PA = 196 kcal mol 1) and Py (PA = 204 kcal mo1-1) [5]. All these molecules have a basic character increasing along the sequence, and when appropriate temperature and pressure conditions are adopted, they can form hydrogen bonded adducts whit the BrCnsted groups, as shown in Scheme 1.

B!

i/i l

0,,.. /Ox, ..,,0

o':S

0 0

+B

0,,, /([Ix

o=S ,

,,0 Scheme 1

OO

The temperature conditions allowing the formation of the internal acid-base adducts change gradually in the above series from 77 K to 300 K. In fact the N2 and CO (which are very weak bases) adducts are stable only at very low T, while the adducts (or the salts) formed by interaction with the bases at the end of the series are stable at RT. The adducts formed by the acid-base reaction illustrated in scheme 1, have stretching and bending modes which differ from those of the original BrCnsted group because the hydrogen bonding perturbation is usually associated with profound modifications of the v(OH), 8(OH) and ),(OH) vibrational frequencies and minor changes of the internal modes of the bases B. In this brief review we want to show that the systematic study of these modifications form the basis of the so called spectroscopic method for the estimation of the acid strength of the BrCnsted groups in zeolites [6-15]. 2. D I S C U S S I O N

By starting from the molecules with lower PA, Figure la and lc report the modifications induced by the hydrogen bonding perturbation on the v(OH) stretching mode of the BrCnsted groups of 13 zeolite as function of the pressure of N2 and CO respectively. From the spectra it is clearly emerging that upon dosage of the base B the v(OH) mode of unperturbed groups (band at 3614 cm -1) is gradually consumed while that of the v(OH.-.B) vibration (shifted to lower frequency) simultaneously shows up. The clear isosbestic points observed in both spectra ensure that the 1:1 process illustrated in Scheme 1 is really occurring in a stoichiometric way. Other important observations are: (i) the negative shift A V(OH) increases on passing from N2 (AV = -126 c m -1) to C O (A~- = -319 cm-1), i.e. with the proton affinity of the base; (ii) the full width at half maximum (FWHM) of the v(OH) mode increases on passing from the unperturbed BrCnsted groups (FWHM -- 20 cm 1) to the N2 (FWHM -- 85 cm -1) and CO (FWHM -- 220 cm -1) adducts. It can be easily verified that the FWHM is roughly 90 of the shift AV. The resuks illustrated in a) and b) are the typical ones expected for the presence of linear hydrogen bonds [16,17] and represent further and clear demonstration of the formation of 1:1 adducts. Following the immense literature on the IR spectroscopy of the hydrogen-bonded systems [ 16-18], the shift to lower frequency and the increase of the bandwidth are due to the decrease of the force constant induced by the polarisation of the O-H bond and by coupling of the v(OH) with the v(O...B) modes of the adducts, which consequently can be better expressed as v(OH.--B) + v (O.-.B). As briefly mentioned before, the formation of the hydrogen bonded adducts can be accompanied also by a perturbation of the internal modes of the base (in the present case the N-N or the C-O stretching modes).

(a)

(b) 0.02

a.u.

!

3750 3 5 0 0 3250 W a v e n u m b e r (cm 4)

I

, (c)

2360

23'40'23'20

Wavenumber (cm-1)

(d)

i Io. a.u. i 1

I'll

i

3750 3 5 0 0 3250 Wavenumber (crn4)

' 22'oo ' 2~'so ' 21'oo Wavenumber (crn4)

Fig.1. IR spectra of increasing equilibrium pressures of N2 and CO adsorbed at liquid nitrogen temperature on activated H-I3 zeolite, parts (a,b) and (c,d) respectively. (a), (c): O-H stretching region. (b): N-N stretching region. (d) C-O stretching region. In each part the dotted line spectrum is that recorded before gas dosage.

As far as carbon monoxide is concerned, the perturbation of the v(CO) mode upon formation of the OH...CO adduct is shown in Figure ld. It is clear that the stretching frequency undergoes a consistent blue shift (AV (CO) = +34 cm 1) with respect to the gas phase value. This is the result of the CO bond polarisation subsequent to the hydrogen bond formation. An analogous result has been obtained for the adducts of CO with Na +, K +, Cs +, Ag + cations in zeolites, a subject which will be extensively discussed in refs. [ 19-26]. At highest filling conditions, also the silanols located on the external surfaces of the microcrystals or at internal defects form hydrogen bonded adducts with CO. The shift induced on the v(OH) stretching frequency of the silanols is definitely smaller (AV = -90 cm -1) than that observed for the BrCnsted sites: this is simply the consequence of the fact the shift of the v(OH) mode of the acid centres caused by the interaction with a given base is related to the acid strength of the group itself. In other words this different response simply reflect the fact that the OH groups of the structural BrCnsted sites are much stronger acid than the OH groups of the silanols. We will demonstrate in the following that this observation can be supported by a large amount of experimental observations obtained with different bases and different zeolites, so proving its general validity. We anticipate that this general correlation, which is the extension to heterogeneous systems of the well known Bellamy-Hallam-Williams (BHW) relation extensively documented in homogeneous phase [27-28], will form the basis of the spectroscopic method for the acid strength evaluation of the BrCnsted groups of zeolitic systems. Finally, notice also the band at 2138 cm -1 in Figure ld, favored at the highest dosages, which is due to liquid-like CO physically adsorbed in the channels and only interacting with the hydroxyl free, homopolar part of the internal surface [19-21]. Although very weak, mention must be made also of the peaks at 2230 cm -1 of adsorbed CO (Figure ld), because it indicates that in the 13-zeolite treated under vacuum at 673 K a small fraction of sites with very large polarising character are present which can represent potential sites for acid catalysed reactions. These sites, whose concentration is strongly influenced by the thermal treatments and can vary from one sample to the other, are A13+ ions in trigonal coordination deriving from the thermally induced migration of framework Al atoms into partial framework position [19-21]. Coming back to adsorbed nitrogen (Figure lab), it is worth noticing that although the interaction of nitrogen with the structural BrCnsted groups is very weak, the induced polarisation of the N-N bond (only RAMAN active in the gas phase) is sufficient to make the v(NN) mode of the OH.-.NN adduct slightly IR active and hence to originate an appreciable absorption in the 2400-2300 cm -1 range. The v(NN) stretching frequencies of the adducts with structural BrCnsted groups (2330 cm 1) and with silanols (2325 cm -1) are upward shifted as expected [29]. On the basis of the literature concerning homogeneous systems the formation of hydrogen bonded adducts should be accompanied by an upward of the ~5 and ~, modes. Unfortunately, due the overshadowing effect of the skeletal vibrations, it was not possible to measure the effect of the formation of the hydrogen bonded species on the ~5and T modes. In this review article it not possible to continue in the same detailed way the description of the spectra obtained with molecules like ethene, propene, acethylene, etc. which come immediately after N2 and CO in the PA scale. We consequently move to acetonitrile (PA = 188.6 kcal mol-1). The reasons of this choice are twice: i) the acetonitrile probe (CH3CN and CD3CN) has been studied extensively over a great variety of zeolites [4,30-36]; ii) the acetonitrile-zeolite complex is characterised by a complex spectroscopy generated by Fermi resonance effects. As these effects are dominant in the spectra of the adducts of structural BrCnsted sites with bases of medium-strong PA, their detailed illustration for the acetonitrile

complexes can be useful for the comprehension of a great variety of experiments involving different and stronger bases. The spectra of increasing doses of deuterated acetonotrile adsorbed on [~-zeolite [36] are illustrated in Figure 2.

l'+

0.1 a.u.

II I 41

++

db +t, I, +.

,l

41.

9

.+

i

ii

'%

.I ++41+

41111

i. 9

,i,. ,i+ ,i. l,

l,

,i.

+

::

3500

A ":. ..B " i

3000

2500

C

2000

1500

Wavenumber (crn Fig.2. IR spectra of increasing equilibrium pressures of CD3CN adsorbed on H-I~ zeolite. Solid line spectra 1-9 refer to CD3CN equilibrium pressures in the 0-10-1 Torr interval, while the dotted line one refers to a much higher pressure (30 Torr). Labels A, B and C denote the three components due to Fermi resonance effects (see text). As found before for N2 and CO we observe the progressive erosion of the structural BrCnsted groups because of the formation of hydrogen bonded adducts (full line spectra in Figure 2); at the highest filling conditions also the band due the silanol groups is eroded (dotted line spectrum). While upon interaction with the nitrile molecule the silanol band originates a broad peak shifted at lower frequency ( AV = -345 cm-1; FWHM = 260 cm-1), two absorptions with apparent maxima at 2856 and at 2452 cm -1 (hereafter named A and B respectively) originate from the structural BrCnsted peak (instead of the single one expected on the basis of the previous results). Other relevant features of the spectra illustrated in Figure 2 are: i) the v(CN) modes of the structural BrCnsted groups and of the weaker silanols are found at 2297 and 2275 cm 1 respectively (i.e. at frequencies higher than those of the free molecule); ii) a novel band at 1325 cm 1 shows up with coverage which can be ascribed to the 8 mode of the BrCnsted-acetonitrile group. The last result demonstrates that the interaction has become sufficiently strong to shift the 8 mode in a frequency range not dominated by the framework vibrations (a fact which makes it observable). The observation of the precise position of the 8 mode gives us the key for the explanation of the presence of A-B doublet. In fact as the minimum separating the A and B partners is observed at a frequency corresponding

to the twice of the 8 mode, it can be readily inferred that it corresponds to the Evans window generated by Fermi resonance effect between the v(OH...B) _+v(O-.-B) mode centred at 2680 cm "l (FWHM = 750 em "1) and the 28 overtone. This explanation finds justification in the abundant spectroscopic literature of hydrogen bonded systems [5,17,18,30] and on the observations concerning adduets with stronger bases (vide infra). In Figure 2 also a band at 1680 cm "1 (labeled with the symbol C) is clearly evident. A similar band is observed for aeetonitrile on H-ZSM-5 and H-MOR [30]. The assignment of this peak will be given in the following after a general introduction to the Fermi resonance effects in hydrogen bonded systems.

Fig.3. Qualitative representation of the IR spectra of weak (a-c), medium (d-f) and strong (g-h) A-H...B or A'---H-B§ H-bonded complexes. The grey areas correspond to regions obscured by the skeletal modes of the zeolite frameworks. For each spectrum the evolution of the proton potential as a function of the A-H distance is also schematically illustrated (fight).

To guide the understanding of the resuks obtained with other bases of larger proton affinity, we think that it is useful to represent schematically the dependence of the v, 8, )', 28, 2)' frequencies upon the O...B distance (Figure 3) used as measure of the strength of the acid base interaction. This dependence has been somewhat freely deduced from the literature data concerning homogeneous compounds and does not have fully quantitative meaning. Notwithstanding this fact, it can be successfully utilized to illustrate the IR spectroscopy of the hydrogen bonding interactions occurring in the zeolites. From this Figure 3 the following seven important points can be underlined: (i) The frequency of the v(OH.-.B) + v(O...B) mode decreases gradually following the well known curve established for homogenous compounds [15]; the shift AV is accompanied by a progressive broadening of the band (FWFM ___-90 AV); (ii) The frequencies of the ~5(OH-.B) and ),(OH...B) modes behaves in a opposite way and the same do the 28 and 2), overtones (the upward shifts are however definitely smaller); (iii) When the frequency of the 28 overtone falls within the stretching band a Fermi resonance occurs with formation of a Evans window and doubling of the peak (A and B bands) [37,38]; (iv) When both the frequencies of the 28 and 2), overtones fall within the v(OH...B) band, the broad v(OH-.-B) band is partitioned by the Fermi resonance into three peaks (named A, B and C) [38]. The relative intensity of the C band with respect to the A and B doublet increases as the strength of the hydrogen bond increases and becomes gradually dominant; (v) For the strongest hydrogen bonding interactions (i.e. for negative shifts of the stretching mode of the order of 2000 cm -1) the v, ~5 and y curves directly intersect. Under these circumstances, corresponding to a fiat potential well characterized by a single minimum [39], direct mixing is occurring and distinction between v, 8 and ), modes becomes impossible. Following the homogeneous literature [17,18] this condition corresponds to that of an hesitating proton; (vi) For strongest hydrogen bonds the downwards shifted v(OH) fall in the range typical of the internal modes of the base B: this fact can further complicate the assignment of the IR spectra; (vii) When the base B approaches proton affinity values near to 200 kcal mo1-1, proton tranfer occurs with formation of hydrogen bonded BH § This is for instance the case for NH3 [40] and Py [5,41] which lead to the formation of Z-.-.H-NH3 § and Z-...HPy+ adducts. As Z- is a weak base (the conjugated acid is strong) and NH4§ and PyH § are also weak acids, the acid-base hydrogen bonding interaction is weak and the shift A V of the v(NH) mode is consequently that typical of a weak interaction. As the PA of Z- is lower than that of NH3 and Py, it is quite conceivable that at the highest filling conditions also (H3N-H.-.NH3) § and (Py-H...Py) § dimers can be present in the zeolite channels. On the basis of these considerations we can now understand the essential features of the sequence of spectra reported in Figure 4, where the IR spectra of the interaction products of series of bases (ranging from N2 to Py) of increasing PA with zeolite H-13 are illustrated [5]. The gradual shift to lower frequency of the broad absorption associated with the perturbed OH group is well documented. The formation and the evolution of the A, B and C peaks and of their baricenter upon the change of the adsorbate and of the proton affinity is also clearly emerging. Notice how the greatest shift is occurring for THF (PA = 196 kcal mo1-1) and how the Py shifts is definitely smaller (because of protonation). Very similar resuks have been obtained on H-MOR, on HZSM-5 and on H-Y [30,41,42] so showing that the considerations illustrated before have general character. As for every adsorbate with proton affinity ranging from 118 to 196 kcal/mole we can determine the shift A V of the BrCnsted sites and of the silanols, we have the possibility to

10 plot them in a XY diagram (Figure 5) and to verify if the BHW relation, whose validity has been well established in solution [27,28], is also holding for the hydrogen bonding interactions occurring in the zeolite channels. 9

i

eta +

...

N2

CO H4

t J (D O t,-

[

CH,,CN _ /~

t~ L_

O r

OH

,<

HF Py

3500

3000

2500

2000

1500

Wavenumber (cm -1) Fig.4. Comparison of the background subtracted IR spectra of H-Beta/B adducts (B CO, C2H4, etc.). All the spectra were recorded at a H§ ratio equal to 1.

= N2,

11

15 16

2000 H-MOR H-ZSM-5 1600

H-Y 1200 10 1

17

800

H-F 89

J

SiOH

5 A

400

~ b . -tBBF" '

0

I

100

'

I

200

'

I

"

300

I

'

400

I

500

'

I

'

600

s (cm Fig.5. Plot of the shift (AV) of the v(OH)Br,~t~ frequencies in 1:10H...B complexes formed on H-Beta (v and v), H-ZSM-5 (O), H-Mor ( ) and H-Y (A) by interaction with different basis (B) v s the shift (AV) of SiOH groups in 1:1 complexes with the same basis. The data corresponding to FH.--B 1:1 adducts are also reported for comparison. Broken line correspond to the AVsioH v s AVsion plot. B is as follows: (1) 02; (2) N2; (3) N20; (4) CO2; (5) CO; (6) C4H4S, C2H2; (7) C2H4, C6H6, C4H6; (8) C4I-I40, C3H6; (9) HC2CH3; (10) H20; (11) CH3CN, CH3CO; (12) CH3OH; (13) CH3CH2OH; (14) (CH3)20; (16) THF; (17) NH3. The huge amount of data summarized in Figure 5 demonstrates that: i) the relation is linear for A V (BrCnsted sites) in the 0 - 1000 c m -1 interval; ii) the data obtained on H-13, H-ZSM-5, H-MOR are located on the same line, so indicating that the acid strength of the BrCnsted sites of these zeolites is identical or very similar; iii) the data obtained on H-Y are located on a line characterised by a smaller slope: this clearly shows that the acid strength of H-Y is smaller than that of the previous materials. It must be underlined that from the comparison of the slopes a quantification of the relative strength of the BrCnsted sites present in H-13, H-ZSM-5 and H-MOR on one side and H-Y on the other side can be estimated on the basis of the empirical relation first established by Paukshtis and Yurchenko [9] for the base CO. This relation is: PA (kJ mol 1) = 1390 - 442.5 log[ A V (OH)/A V (SiOH)]

(1)

12 where the ratio of the A V values is deduced from the slope of the straight lines of Figure 5 and is characteristic of each zeolite; iv) for shifts higher than 1000 cm 1 the data deviate from the straight line: this is not unexpected since the linear BHW plot is verified only for hydrogen bonds of small-medium strength. In turn deviation from the straight line can be considered as indication of presence of strong hydrogen bonds characterized by fiat potential walls where the proton is in so called "hesitating state". To qualitatively illustrate how the strength of the BrCnsted sites of zeolites are definitely higher than that of a common acid like HF, the HBW plot of the HF data obtained in Argon matrices [43-48] are also reported in Figure 5 A quantitative comparison of the slopes cannot be made in this case since the "solvents", i.e. the argon matrix on one side and the zeolitic framework on the other side, are too different. As a final comment of this brief review we shall dedicate a small space to the discussion of the IR spectroscopy of H20 adsorbed on BrCnsted sites and to the related question of whether and when proton transfer occurs. It is now well ascertained that the interaction of a Br0nsted site with a single molecule gives a hydrogen bonded species on H-ZSM-5, H-I3 and H-Mor, while the interaction with two or more molecules gives proton transfer with formation of solvated H30 § or H502§ [5,30]. This problem has been recalled because it shows clearly how co-operative effects between adsorbed molecules are able to promote reactions which are otherwise not possible with single molecules. This observation is related to the more general one concerning the cautions which must be always be considered when result obtained at low filling conditions are extrapolated to situations where the channels are filled with several species. 3. CONCLUSIONS In this brief review we have shown how, the acid strength of the strong Br0nsted sites of H-13,as probed by measuring the shift Av induced by the interaction with bases of proton affinity comprised in a wide interval, is found to be nearly identical to that of H-ZSM-5 and H-MORD, but higher than that of H-Y. Bases with PA < 200 kcal mo1-1 form hydrogenbonded 1:1 adducts, characterized by uncompleted proton transfer. Only for bases with PA > 200 kcal mo1-1 the true proton transfer is really observed with formation of ionic pairs. The basic IR spectroscopy of all of these complexes is discussed and compared with that of the analogous complexes in solution. The interaction of N2 and CO with the external OH of H20 adsorbed on strong Br0nsted sites indicates a substantial decrement of acid strength with respect to that of the original strong BrCnsted site of the zeolite. REFERENCES 1. W. H01derlich, M. Hesse and F. Niiumann, Angew. Chem., Int. Ed. Engl., 27 (1988) 226. 2. A. Corma, Chem. Rev., 95 (1995) 559. 3. A. Corma and A. Martinez, Adv. Mater., 7 (1995) 137. 4. R. Buzzoni, S. Bordiga, G. Ricchiardi, G. Spoto and A. Zecchina, J. Phys. Chem., 99 (1995) 11937. 5. C. Paz~, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina and G. Bellussi, J. Phys. Chem. B, 101 (1997)4740. 6. L. Kubelkovd, S. Beran and J. Lercher, Zeolites, 9 (1989) 539. 7. M.A. Makarova, A.F. Ojo, K. Karim, M. Hunger and J. Dwyer, J. Phys. Chem., 98 (1994) 3619.

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