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Broensted acidity in zeolites (*)

J. DWYER, A. KHODAKOV, S. BATES, M. MAKAROVAand V. ZHOLOBENKO

Centre for Microporous Materials, Chemistry Department, UMIST - Manchester M60 1QD, UK (ricevuto il 28 Febbraio 1997; approvato l’8 Maggio 1997)

Summary. — Theoretical and experimental investigations of intrinsic and enhanced

Broensted acidity i) show no evidence for a significant effect on intrinsic acidity arising from neutralisation of some of the acid sites, ii) suggest a suitable scale for protic acidity using CO as a probe, iii) provide a fundamental functional linearity for the correlation of shift in hydroxyl frequency and changes in band intensity, iv) allow deconstruction of complex infrared spectra of zeolite OH region, and v) demonstrate the basis for generating Broensted sites of enhanced acidity by interaction of Lewis sites with proximate Broensted sites in zeolites.

PACS 82.30 – Specific chemical reactions; reaction mechanisms. PACS 01.30.Cc – Conference proceedings.

1. – Introduction

For some considerable time there has been discussion concerning the strength of Broensted acid sites in zeolites and methods for measurement [1, 2]. Additionally, there have been differences in viewpoint regarding the nature of long-range vs. short-range influences on Broensted acidity in zeolites. For example, the notion of an intrinsic factor in zeolite acidity, which can be modified by neutralisation of a few framework sites represents an extreme view of co-operative effects whereas other reports emphasise the role of local composition and structural features [2, 3]. In this paper we present the results of recent studies on zeolites carried out in the Centre for Microporous Materials at UMIST.

2. – Experimental

Zeolites ZSM-5 (Si/Al416, BP Chemicals Ltd), US-Y (Si/Al framework 45.4, Crosfield Chemicals) and Mordenite (Si/Al47.7, Laporte Chemicals) were used in this (*) Paper presented at the “First International Workshop on Reactivity of Oxide Materials. Theory and Experiment”, Como, 8, 9 November 1996.

J.DWYER,A.KHODAKOV,S.BATES,M.MAKAROVAandV.ZHOLOBENKO 1674

work. Details of FT-IR and catalytic experiments are described in refs. [4-6]. Calculations were made using the Gaussian-94 suite of programs.

3. – Results and discussion

3.1. Intrinsic acidity in zeolites. – The interaction of base (B) with a zeolitic Broensted site can be envisaged as follows:

i) ZOH(s) KH1_{(g) 1OZ}2_(s), ii) H1_{(g) 1BKHB}1_(g), iii) HB1_{(g) 1OZ}2_{(s) K[HB}+ Q Q Q OZ2_](s). Overall, ZOH(s) 1B(g) K [ HB1_{Q Q Q OZ}2_](s).

Steps i) and iii) involve relaxation of structure and step iii) involves interaction of the protonated base with surface atoms of the zeolite. Consequently, if we want to establish experimentally an “intrinsic” scale for zeolite acidity we need to maximise interaction with the protic hydrogen and minimise both extended interaction with the surface and relaxation energies.

Previous theoretical work [7] established the preferred single-point adsorption of carbon monoxide, on Broensted sites, via the carbon lone pair and also demonstrated that perturbation of the site geometry was minimal. Moreover, good correlation between shifts in the hydroxyl stretching frequency, on adsorption of CO, with strength of the Broensted sites in acid zeolites has been reported [7]. As a result CO provides a suitable probe to assess intrinsic acidity.

The IR spectra of carbon monoxide sorbed on NH4OH-ZSM-5 containing different

amounts of pre-adsorbed ammonia demonstrate a linear correlation between the shifts in hydroxyl stretch (DnOH) and hydroxyl coverage, which is independent of the extent

of neutralisation of bridging OH-groups by ammonia. This implies that the intrinsic acidity of the remaining Broensted acid sites is not significantly affected by the amount of preadsorbed ammonia [8].

Kinetic results for n-hexane cracking also demonstrate no profound effect on the activity of acid sites at low surface coverage of ammonia [8]. The absence of significant change in intrinsic acidity on exchange of protic hydrogens for cations in H-ZSM-5 is also supported by previous work on Cs/H-ZSM-5 [9]. Consequently, the significant poisoning effect of traces of ammonia is better explained by the neutralisation of a few strong acid sites which show enhanced activity, an alternative explanation also considered by Hall et al. [3].

3.2. Hydrogen bonding in solid acids. – Although perturbation of Broensted acid sites in solid acids by bases such as CO provides a good measure of acidity, there remains a problem concerning the quantification of spectral intensities. Well-established correlations for changes in frequency of hydroxyls perturbed by hydrogen bonding and corresponding changes in intensities are widely available and understood for acids in solution, but not for solid acids. We used a series of sorbates to establish a fundamental correlation between frequency shifts and enhancement of intensities resulting from hydrogen bonding of sorbates to OH-groups in a solid acid where acid sites are accepted to be homogeneous (H-ZSM-5, Si/Al 416). The results presented in

Fig. 1. – Weak perturbation of the OH band.

fig. 1 provide a linear relation between the change in integral intensity, DA, and the frequency shift, Dn , for OH bands in the IR spectra: DA/A04 0 .018 Q Dn.

This correlation establishes the fundamental point that hydrogen bonding involving solid acid-base interactions follows the linear pattern observed previously for acids in solution. Consequently, it can provide a basis for estimating relative extinction coefficients, for example for hydroxyls located in different channels of zeolites.

The experimentally observed linear relation between DA/A0and Dn was supported

by molecular orbital calculations using small cluster models of zeolites and a series of sorbates [8].

3.3. Sites of enhanced Broensted acidity in zeolites. – The hydroxyl region of IR spectra of zeolites HY and HEMT consists of two distinct bands associated with the OH stretch of Broensted acid sites vibrating in the large cages (A3640 cm21_{) and in the}

small cages (A3550 cm21_{). The lower frequency vibration is attributed to hydrogen}

bonding between the hydrogen of the OH group and proximate oxygens in the small cage. As the framework becomes more siliceous, the frequency of the hydroxyl stretch in the large cages decreases towards a value around 3620 cm21_.

By comparison with HY zeolite, the IR spectrum of the hydroxyl region of US-Y zeolite, obtained by hydrothermal treatment, is very complex. Some attempts have been made to deconvolute this region of the IR spectrum using numerical methods, but this approach is unsatisfactory. Recently [6], we utilised stepwise thermodesorption of ammonia to deconstruct the complex hydroxyl region of US-Y zeolite. The results (fig. 2) reveal two hydroxyl bands in the large cage (at 3627 cm21_{and 3599 cm}21_{) and}

two bands in the small cages (at 3554 cm21_{and 3524 cm}21_{). These bands are assigned}

to hydroxyls in both types of cages, which are either isolated ( 3627 cm21 _and

3554 cm21_{) or perturbed by extraframework aluminium species ( 3599 cm}21 _and

J.DWYER,A.KHODAKOV,S.BATES,M.MAKAROVAandV.ZHOLOBENKO 1676

Fig. 2. – Decomposition of the hydroxyl band in US-Y zeolite, (Si/Al )f4 5.4.

Location of the hydroxyls of different types was confirmed using low-temperature CO adsorption. The bands at 3554 cm21 _{and 3524 cm}21 _{were not perturbed, since}

carbon monoxide does not enter the small cages at the temperatures and pressures used. The hydroxyls associated with bands at 3627 cm21_{and 3599 cm}21_{interacted with}

CO, confirming their presence in the large cages, with shifts of DnOH4 327 and

382 cm21_{, respectively. The shift value of 327 cm}21 _{is typical for framework acid sites}

in siliceous Y zeolites and is consistent with the assignment of the band at 3599 cm21_to

the sites of enhanced acidity generated by interaction of dislodged Al with framework acid sites.

Clearly, it is possible to quantify the relative amounts of both types of non-perturbed hydroxyls vibrating at 3627 cm21 _{and 3554 cm}21 _{using the linear}

correlation described above. However, there remains a question concerning hydroxyls vibrating at 3599 cm21 _{and 3524 cm}21 _{which result from perturbation of the oxygen}

atom of bridging hydroxyls by dislodged aluminium. It is difficult, experimentally, to determine the effect of perturbation of the oxygen atom of Broensted OH-groups by extra-framework Al, but theoretical MO calculations using small clusters [8] suggest that changes in the infrared band intensity due to this interaction are much smaller than changes produced by hydrogen bonding. As a first approximation we can, therefore, ignore changes in hydroxyl intensities arising from the perturbation of the oxygen atom by Lewis sites.

3.4. Modelling Broensted sites having enhanced acidity. – Since enhanced Broensted acidity is observed in hydrothermally treated zeolites which contain extraframework aluminium and strong Lewis acid sites, it is widely assumed that acidity enhancement arises from interaction between Broensted and Lewis acid sites. However, until recently this has never been demonstrated. In the previous work on the BF3-HEMT zeolite system [4] two complexes were observed upon BF3 adsorption on

the Broensted acid sites of the zeolite. The first complex giving rise to a hydroxyl band at 3472 cm21 _{is stable up to 240 7C, whereas the second (at 3267 cm}21_{) can be}

Fig. 3. – Complexes of BF3with cluster models.

removed only by heating to room temperature. The latter complex was assigned to interaction between the zeolite framework and the Lewis acid, BF3. A series of MO

calculations, using clusters of four and more tetrahedra, indicated that, where possible, BF3prefers to form a six-membered ring rather than a larger ring as suggested earlier

for a unit containing a terminal silanol. An example of complexes with BF3is shown in

fig. 3 and results of calculations are given in table I.

The results show that complexation with a Lewis acid can lead to enhancement of acidity for neighbouring hydroxyls. The effect is localised and is largest for TABLEI. – Results of ab initio MO calculations on models in fig. 3 (3-21G Basis).

Model cluster a) b)

Charge on acid hydrogen

Harmonic frequencies (uncorrected) of OH stretch, nOHOcm21

Shift in nOHon complexing with CO, DnOHOcm21 2

C-O stretch, nCOOcm21

DnCOOcm21(*) 1 0.4895 3876 216 2 2392 77 1 0.4771 3918 177 2386 71 (*)(DnCO2 nCO( gas )), nCO( gas ) 42315 cm21(3-21G Basis).

J.DWYER,A.KHODAKOV,S.BATES,M.MAKAROVAandV.ZHOLOBENKO 1678

perturbation of the oxygen of the Broensted site. These results, confirmed, in the case of the six-ring complex, by calculations using more extensive basis sets, are in agreement with experimental data for zeolite EMT which showed [4] that sorption of BF3 into the zeolite resulted in enhancement in acidity of the remaining hydroxyls

uncomplexed with BF3. The maximum experimental frequency shift of the hydroxyl

stretch due to interaction with CO in the BF3-HEMT system was identical with the

value obtained for the sites of enhanced acidity in US-Y zeolite.

Models shown in fig. 3 are not to be taken as good representations of zeolite structures but rather as a means to illustrate the influence of a Lewis site on a proximate Broensted acid site.

Both experimental results and molecular modelling indicated that formation of the appropriate complexes are heavily restricted in HZSM-5, so that acidity enhancement should not be expected.

* * *

We are grateful to the EEC (BRITE EURAM grant 4633) for financial support and to our industrial partners for providing the materials. We also thank Dr. N. A. BURTON,

University of Manchester Chemistry Department, for helpful discussion.

R E F E R E N C E S

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[6] MAKAROVAM. A., GARFORTHA., ZHOLOBENKOV. L., DWYERJ., EARLG. J. and RAWLENCED. J., Stud. Surf. Sci. Catal., 84 (1994) 365.

[7] BATESS. and DWYERJ., J. Phys. Chem., 97 (1993) 5897.

[8] DWYER J., ZHOLOBENKOV., KHODAKOV A., BATES S. and MAKAROVA M. A., Stud. Surf. Sci. Catal., 105 (1997) 2307.

[9] LAGOR. M., HAAGW. O., MIKOVSKYR. J., OLSOND. H., HELLRINGS. D., SCHMITTK. D. and KERR G. T., Proceedings of the Conference “New Developments in Zeolite Science and Technology” (Kodansha Ltd., Tokyo) 1986, p. 677.