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Indian Journal of Chemical Technology Vol. 5, May 1998, pp. 109-123 Acidity in zeolites and their characterization by different spectroscopic methods Ramesh Ch. Deka Catalysis Division, National Chemical Laboratory, Pune 411 008, India Received 19 September 1997; accepted 7 January 1998 An overview of the different acidic sites in zeolites is presented here. It is followed by a discussion of the methods for identification of the various acidic sites and their distribution in some widely used zeolites such as faujasite, ZSM-5 and mordenite. Emphasis is laid on the discussion of various techniques, their relative advantages and disadvantages and several example studies, where these techniques are applied. The techniques discussed include titration in aprotic solvents, temperature-programmed desorption, microcalorimetric measurement of the heats of adsorption of probe molecules, IR spectroscopy with and without probe molecules, MAS NMR, photoelectron spectroscopy and positioQ annihilation spectroscopy. From this study it appears that a single technique is not sufficient for the characterization and interpretation of zeolite acid sites and for the prediction of catalytic activity. Combination of different techniques such as NMR with IR can provide more information about the structure of acid sites of zeolites. Zeolites are three-dimensional crystalline inorganic polymers whose structures are formed by comer sharing of Si044- and AIOt tetrahedra. Owing to the difference in valencies between Si and AI, each framework Al creates a negative charge in the lattice that requires the presence of a charge balancing cation to ensure the electroneutrality of the solid. The ionic nature of the bond between these counter cations and the zeolite framework allows the exchange of a cation by other cations without altering the crystalline structure ofthe solidl-3. In zeolites, the tetrahedral primary building blocks are linked through oxygens producing a three-dimensional network containing channels and cavities of molecular dimensions. The catalytic properties of zeolites depend on a variety of factors, including (i)/ the regular crystalline structure and uniform pore size which allows only molecules below a certain size to diffuse to the active sites and (ii) the presence of strongly acidic hydroxyl groups, which can initiate carbenium ion reactions. The acidity and acid strength of a zeolite can be modified by changing the sample pretreatment or preparation method, exchanging the cations, modifying the Si/Al ratio or by isomorphous substitution of Al and Si4,5. Both theory and experiments have contributed significantly to the understanding of the zeolitic OH bond, the different species (framework & non framework ) present and the interaction of zeolites with reactant molecules6.7. Quantum chemical ca1culationss.13 indicate that the acid strength of bridging hydroxyls depends on the geometry of the bridge (bond lengths and angles) and on the number of neighbouring Al atoms. In zeolites where all hydroxyls have the same bridge geometry such as zeolite A and where there is the same number of nearby Al atoms, the bridging hydroxyls are "homogeneous", i.e., they have the same acid strength. On the other hand, in zeolites containing hydroxyls with different bridge geometries, such as ZSM-5 and in materials with different numbers of Al in the vicinity of the bridge, such as zeolite Y, OH groups with different acid strengths are simultaneously presene4. A relationship between the acid strengths of the different Bronsted acid sites and their chemical shift has been derived from quantum chemical calculations in the case of acidic 1516 Th .. f h zeo Ites '. e spectroscopIC propertIes 0 te zeoli tic protons attached to the lattice oxygen atoms that bridge tetrahedrally coordinated trivalent (AI, Ga, B etc.) and tetravalent (Si, Ge) ions have been studied mainli by infrared and NMR spectroscopiesl7• I. NMR spectroscopy has provided

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  • Indian Journal of Chemical Technology

    Vol. 5, May 1998, pp. 109-123

    Acidity in zeolites and their characterization by differentspectroscopic methods

    Ramesh Ch. Deka

    Catalysis Division, National Chemical Laboratory, Pune 411 008, India

    Received 19 September 1997; accepted 7 January 1998

    An overview of the differentacidic sites in zeolites is presented here. It is followed by a discussionof the methods for identification of the various acidic sites and their distribution in some widely usedzeolites such as faujasite, ZSM-5 and mordenite. Emphasis is laid on the discussion of various techniques,their relative advantages and disadvantages and several example studies, where these techniques areapplied. The techniques discussed include titration in aprotic solvents, temperature-programmeddesorption, microcalorimetricmeasurement of the heats of adsorption of probe molecules, IR spectroscopywith and without probe molecules, MAS NMR, photoelectron spectroscopy and positioQ annihilationspectroscopy. From this study it appears that a single technique is not sufficient for the characterizationand interpretation of zeolite acid sites and for the prediction of catalytic activity. Combination of differenttechniques such as NMR with IR can provide more informationabout the structureof acid sites of zeolites.

    Zeolites are three-dimensional crystalline inorganicpolymers whose structures are formed by comersharing of Si044- and AIOt tetrahedra. Owing tothe difference in valencies between Si and AI, eachframework Al creates a negative charge in the latticethat requires the presence of a charge balancingcation to ensure the electroneutrality of the solid.The ionic nature of the bond between these countercations and the zeolite framework allows the

    exchange of a cation by other cations withoutaltering the crystalline structure ofthe solidl-3.

    In zeolites, the tetrahedral primary buildingblocks are linked through oxygens producing athree-dimensional network containing channels andcavities of molecular dimensions. The catalytic

    properties of zeolites depend on a variety of factors,including (i)/ the regular crystalline structure anduniform pore size which allows only moleculesbelow a certain size to diffuse to the active sites and

    (ii) the presence of strongly acidic hydroxyl groups,which can initiate carbenium ion reactions. The

    acidity and acid strength of a zeolite can bemodified by changing the sample pretreatment orpreparation method, exchanging the cations,modifying the Si/Al ratio or by isomorphoussubstitution of Al and Si4,5.

    Both theory and experiments have contributed

    significantly to the understanding of the zeolitic OHbond, the different species (framework & nonframework ) present and the interaction of zeoliteswith reactant molecules6.7. Quantum chemicalca1culationss.13 indicate that the acid strength ofbridging hydroxyls depends on the geometry of thebridge (bond lengths and angles) and on the numberof neighbouring Al atoms. In zeolites where allhydroxyls have the same bridge geometry such aszeolite A and where there is the same number of

    nearby Al atoms, the bridging hydroxyls are"homogeneous", i.e., they have the same acidstrength. On the other hand, in zeolites containinghydroxyls with different bridge geometries, such asZSM-5 and in materials with different numbers of

    Al in the vicinity of the bridge, such as zeolite Y,OH groups with different acid strengths aresimultaneously presene4. A relationship between theacid strengths of the different Bronsted acid sitesand their chemical shift has been derived fromquantum chemical calculations in the case of acidic

    1 1516 Th .. f hzeo Ites '. e spectroscopIC propertIes 0 t ezeoli tic protons attached to the lattice oxygen atomsthat bridge tetrahedrally coordinated trivalent (AI,Ga, B etc.) and tetravalent (Si, Ge) ions have been

    studied mainli by infrared and NMRspectroscopiesl7 I. NMR spectroscopy has provided

  • ~ ,_,>" ., ._,,~ . ,... ;,.~ ... , I

    removed from the zeolite framework, according tothe scheme shown in Fig. 2a. Later this mechanismwas found to be incorrect, since according to thenumerous solid state high resolution NMR data, thedehydroxylation of hydrogen forms of faujasitesresults in dealumination instead of the formation of

    latticeLewis sites (Fig. 2b).

    Importance of acidity in zeolites and its characterization

    Both the Bronsted (OH) and Lewis (= AI) acidsites are often present simultaneously in thestructure at high temperatures. There are a largenumber of reactions which are catalyzed by the acidsites of zeolites. However, in order to design andoptimize a zeolite catalyst for a specific reaction, theaspects to be considered are-the type of acid sitesneeded, the acid strength required to activate thereactant molecule and the way in which the numberof the required acid sites in a given zeolite catalystbe maximized.

    Different techniques have been developed todetermine the nature and strength of acid sites ofzeolites and are being described briefly. The conceptof acid strength distribution to characterize aparticular sample is still valid, even though recentmodels of acidity involve the added concept ofcation and proton mobility27. Such migration wouldaffect the strength of individual sites but would notalter the overall energy distribution of the system.Modifications to the zeolite by processes such ascation exchange, thermal treatment, aluminiumremoval and isomorphous substitution do affect thisdistribution markedly.

    The ideal method for the measurement of surface

    acidity should cover all the aspects discussed aboveand should also study the surface as nearly aspossible under reaction conditions. No singlemethod is available that provides all the informationand so several approaches must be combined. Manytechniques have been described in recentpublications28.33. These include titration methods,adsorption and temperature-programmed desorption

    of base molecules, microcalorimetry, vibrational

    spectroscopy, nuclear magnetic resonance,photoelectron spectroscopy, positron annihilationmethods, catalytic reactions, etc.

    Techniques for characterization of acidity in zeolitesTitration methods-Titration in aqueous medium

    d' hid' 3435 b h' .were use 10 t e ear y stu les' ut t IS IS not auseful approach because of the modification of

    acidity in the presence of water. However, titration

    in a non-aqueous medium does provide ven' usefulinformation on the acidity of the surface36,3 !Whenthe indicator (B) reacts with a Bronsted acid (All) toform the corresponding conjugate acid (BEt) andbase (A"), the acid strength is expressed by theHammett acidity function as,Ho=pKa-log[BH 1/[B] ... (1)It can be seen from Eq. (l) that Ho is equivalent tothe well known acidity function pH when diluteaqueous solutions are involved. Ho may be extendedto include the Lewis concept of acidity given inEq. (2) where [B] is the concentration of the neutralbase reacting with a Lewis acid or an electron pairacceptor, A.Ho=pKa-log[AB]/[B] ... (2)

    The problem with the Hammett indicators is thatboth Bronsted and Lewis acidity are measured

    without any distinction. Since the catalytic activityof silica-aluminas and zeolites for reactions such asisomerization and cracking of hydrocarbons hasbeen strongly linked to the presence of Bronstedacid sites38-4o an indicator that is 'proton specific'

    , 4\would be more useful. Hirschler suggestedsubstances that form stable, coloured carbonium

    ions by protonation and established the use of aseries of arylmethanols as indicators as given inEq. (3). This equilibrium is defined by another acidityfunction, I-k, given by Deno et al.42-44 as shown in Eq.(4) and hence the two surface acidity functions arerelated by Eq. (5).ROH+W +H20 ... (3)

    HR = -logaH+ + logaH2o + log(f; / fRoH)... (4)

    where aH+,a H20 are the respective activity and

    f ~,fROH are respective activity coefficients.HR = Ho + 10gaHzo -log(fRoHf ;HI fBf;)

    ... (5)

    Even when all experimental precautions aretaken the titration method suffers from a largenumber of limitations, one of which is that the

    equilibrium is rarely achieved45.46 and thereforeacidity data from indicator methods should not beused directly to predict catalytic behaviour. Anotherproblem arises from the inaccuracies associated withvisual determination of the colour changes. In orderto alleviate this problem, spectrophotometric

  • 112 INDIAN 1. CHEM. TECHNOL., MAY 1998

    h d h b 2130 .met 0 save een used' However, It has beenfound47'49 that the traditional Hammettmeasurements based on the determination of the

    position of the protonation equilibrium of anindicator by UV-visible spectroscopy are notapplicable to real~life catalysts. When indicatormethods are used, there is the possibility that thecolour or the band arising from the acid form of theindicator may also be produced by surface siteswhich are not catalytically active. It appears thenthat if a zeolite has 'a high acid strength, as measuredwith indicators, this does not necessarily imply that

    it is going to be catalytically very active. Indeed, theuse of the Ho function is adequate for homogeneousaqueous solutions of acids, but in the case ofheterogeneous acids, neither aH + nor the surfaceacidity function Ho have an explicit physicalmeaning. Therefore, the application of Hammettindicators to characterize the catalytic properties ofsolid acids can be misleading. Thus several othernew techniques were developed.

    Temperature programmed desorption (TPD) ofbase molecules-Adsorption of volatile amines suchas NH3, pyridine, n-butylamine, quinoline etc., isfrequently used to measure the number and strengthof acid site~ on solid catalysts40 The strength andamount of acid sites are reflected in the desorptiontemperature and the peak area, respectively, in aTPD plot. However, it is difficult to express thestrength in a definite scale and the count as thenumber of sites quantitatively. Relative strengthsand relative numbers of acid sites on the different

    catalysts can be estimated by carrying out the TPDexperiments under the same conditions.

    Temperature programmed desorption of ammo-nia typically involves saturation of the surface withammonia under some set of adsorption conditions,followed by linear ramping of the temperature of thesample in a flowing inert gas stream. Ammoniaconcentration in the effluent gas may be followed byabsorption, titration, mass spectrometry using acatherometer or a simple gas chromatographic set-up. Alternatively, the experiment may be carried outin a microbalance and changes in sample mass maybe followed continuously. It is worth noting that therate of desorption of the probe molecules might beaffected by diffusion restrictions. Furthermore, themeasurements may depend on experimentalconditions such as the geometry of the arrangement,heating rate, reliable temperature control of thesample versus oven etc. The factors indicated above

    must be carefully taken into account. However, aranking of the acid strength of members of a seriesof acidic zeolites under identical conditions is

    usually feasible.Often, during the characterization of acidic sites

    in zeolites by TPD, the amount of ammonia (basemolecule) desorbing above a certain temperature istaken as the acid-site concentration and the peakdesorption temperatures of the TPD spectra areassumed to be measures of the strengths (relative) ofthe sites. However, the spectra are often relatively

    broad and moreover, TPD by itself does not provide

    any information about the nature of the sites fromwhich the probe molecule desorbs. Therefore,Yushchenko et al.50 Dima and Rees51, Hashimoto

    152 d 'd 53a het a. an Karge and Don ur ave evaluated theTPD spectra in combination with other methods,such as IR spectroscopy, to identify the types ofsites releasing the probe molecules. Due to thesimplicity involved in the TPD set-up there arenumerous studies applying this technique53b.e,

    Microcalorimetry-Calorimetry provides a directmethod for measuring the enthalpies of the acid-base interactions in zeolitei4 The usual methodinvolves dosing of aliquots of the reference baseonto the solid held at a given temperature. Theresulting heat pulse is recorded and integrated.Dosing is continued until saturation coverages arereached. The calorimetric data are usually displayedas a plot of enthalpy of adsorption versus coverage.The differential heats of adsorption as a function ofcoverage for ammonia, pyridine and isopropylaminein H-Yare shown55 in Fig. 3. The heats of adsorp-tion are almost constant (-150-190 kJ/mol),which corresponds to one molecule per Bronsted

    !I!I0 D0 00 0

    ~ .~~ 0

    000 0 0 00 00 0 ] 00...I I


    ....... /.

    Fig. 3-Variation of heats of adsorption of ammonia (0),pyridine (0) and isopropylamine ~ on H-Y with respect tocoverage. ( Reprinted from ref 55. Copyright 1993 AmericanChemical Society).


    I II I 'II 1 1 1'1 'I 1'1 "II!!'llJ ~.~ J~!:.,"-",. "~, ".,_,.J....l!l"JlU~;.!;." "".iW1;JU"LJJlIJ.i._, .JI "..,li ..,1 II




    1630 1489

    II~ I 14541 _ /


    IIIoz~ Iba:oIII1IICl

    .'\-' . -- MY/0.17--(N.1HY/~2 '.

    ,. -NeI4Y/O.07 f \

    Fi.g. 5-IR spectra of (A) quinoline and (B) pyridine adsorbedon NH4Y: (a) degassed at 523 K and 102 Pa; (b) degassed at623 K and 10-2 Pa; (c) degassed at 673 K and 10-2 Pa (for Ih).

    (Reprinted from ref. 40. Copyright 1995 American ChemicalSociety).

    Fig. 4-Infrared absorption spectra of NaRY with differentHlNa ratios and HY. (Reprinted from ref 59. Copyright 1992Elsevier.)

    3!lOO 3600 3700Way. numb.r, canl



    A 1700 1500em'


    IIIoz~ Iba:oIIIGICl

    concentration of Bronsted and Lewis acid sites

    simultaneously.When there are different types of hydroxyl

    groups in a zeolite, it is possible by looking at thestretching bands of the hydroxyl groups before andafter the pyridine adsorption, as well as afterdesorption at different temperatures, to determinewhich hydroxyls are acidic and to characterize theirrelative acid strengths64.67.Quinoline is a more basicand larger molecule than pyridine and can alsosimultaneously distinguish between Bronsted and

    ;Lewis acids68. The spectra given40 in Fig. 5 showsthe characteristic bands associated with the presence

    acid site, and then fall sharply at higher coverages.This method is very useful in order to understandthe effect of structure on the strength of an acidsite. Correlation between heats of adsorption andgas-phase proton affinities, provides a useful start-ing point for a more complete description of thethermochemistry of proton transfer reactions inzeolites. There are, however, several inherentlimitations to the technique which require that it beused in combination with other characterizationmethods. A combined study of calorimetric and IRmeasurement to characterize the acid sites of zeolite

    H- Y has been carried out by Mitani et a[ 56.Infrared (IR) spectroscopic Methods-Infrared

    spectroscopy (IR) is the starting point for acomprehensive examination of zeolite acid sites. IRspectroscopy is a very powerful technique since itallows one to measure directly the OH bond strengthand hence it has traditionally been used as a meansof characterizing the intrinsic acid strengths ofzeolites. A band at 3745 cm'l is characteristic of

    terminal Si-OH groups. For the bridging OHgroups, usually two bands are detected for faujasitetype zeolites: a sharp high frequency (HF) band at3660 cm'l and a broad low frequency (LF) bandaround 3556 cml. There is some variation in these

    frequencies, depending on the composition and the[5758 . d hstructure type. Jacobs et a .. have asslgne t e

    HF band to OH groups freely vibrating inside thelarge cavities.

    In Fig. 4, the infrared spectra of NaRY withdifferent H/Na ratios is compared59. Withdecreasing proton concentrations a small shift tohigher frequencies is found. In principle, theconcentration of hydroxyl groups, and therefore theconcentration of potential Bronsted acid sites, couldbe obtained from the intensity of the correspondingIR band. However, for quantitative estimation, theextinction coefficients of the different types ofhydfoxyls contributing to the IR bands are required,something which is seldom possible.

    The measurement of the concentrations of

    different hydroxyl groups on the basis of theassignment of O-H stretching frequencies tospecific types ofhydroxyls59.63, has met with limitedsuccess so far. It is therefore not surprising that mostof the information on the acidity of zeolite catalystsobtained from IR methods comes from studies of

    adsorbed molecules. IR spectra of sorbed moleculeslike pyridine, substituted pyridine, quinoline,diazines etc. ,are very useful for determining the

  • 114 INDIAN J. CHEM. TECHNOL., MAY 1998

    Table I-IR bands observed forN2 adsorption on H-Mordenite at 135 K (cm,l) (Reprinted from ref73. Copyright 1993Elsevier.)

    ve4N2) V(15N2) ratio site

    Adsorbed species 23522335




    Fig. 6-Spectral change of the v(OH) bands of H-mordenitewith the increase of N2 pressure at 135 K. (Reprinted from ref73. Copyright 1993 Elsevier).

    of Bronsted and Lewis acid sites. Because of its

    larger size, quinoline can be used in some casesonly, and under controlled conditions, to measurethe amount of external acid sites in microporousacid materials, with pores smaller than - 6A.

    NH3 can also be used to distinguish betweenBronsted and Lewis acid sites, since it gives

    characteristic IR bands associated with NH; and

    NH3 coordinated to Lewis acid sites69. Moreover,the adsorbed species are quite stable and IRspectroscopy can differentiate and quantify theamounts adsorbed on Br6nsted and Lewis sites.

    However, as the above probe molecules are strongbases when compared to the reactants whichparticipate in typical catalytic reactions, theyinteract even with the weak acid sites which may notbe responsible for the reactions 70. Thus, weakerbases have been used as probe molecules for zeolitecatalysts, and quantitative IR data for systemsinvolving CO, SH2, acetone, benzene, olefins,

    . '1 H h b d7! 72acetomtn e, 2, etc. ave een reporte '.N2 molecule has also been found to give

    characteristic IR bands when it specifically interacts



    with Bronsted or Lewis acid sites of zeolites at low

    temperatures regardless of the zeolite structure73. N2has a lower proton affinity (494.5 kJ/mol) comparedto pyridine (924 kJ/moI) and ammonia (853.5kJ/mol) and is regarded as a very weak base74.Further, it is smaller than pyridine in moleculardiameter to make it an excellent candidate as a

    probe of acid sites in zeolites cages where the sterichindrance is effective. Nitrogen adsorbed on H-mordenite at 135 K gave two bands at 2352 and2335 cm-I in the v(N=N) region. These bandsshowed73 exact isotope shift when ]5N2 was used asadsorbate (Table I). Therefore, these were assignedto the v (N=N) bands of N2 species adsorbed on H-mordenite. They exhibited distinct dependence onthe N2 pressure and the substrate temperature;suggesting that the 2352 em'] species is more stablethan the 2335 em'] species and that they originatedfrom N2 species on different sites. From the effect ofD20 pretreatment on the y (N=N) bands at differentevacuation temperature it was attributed that the y(N=N) band at 2352 cm,l to the N2 species adsorbedon Lewis acid sites while the 2335 em'] band wasattributed N2 species adsorbed on Bronsted acidsites through hydrogen bonding.

    The y(OH) bands of H-mordenite exhibitedsignificant change with the increase of N2 pressureas shown73 in Fig. 6. On increasing N2 pressure, theintensity of the y(OH) band at 3616 em'! whichoriginates from the bridging OH groups decreasedand a new band, which is characteristic of thehydrogen bonded OH groups75, grew at 3510 em'].This fact suggests that hydrogen bonding betweenthe bridging OH groups and the adsorbed N2 speciesoccurred. However, the terminal silanol groups,virtually remained undisturbed on N2 adsorption,consistent with the low acidity of the silanol groups.

    The shifts of hydroxyl bands upon adsorption ofcarbon monoxide, benzene and ethylene on zeolitesindicate the strength of the Bronsted acid sites76,80.Hydrogen sulfide has also proved to be an excellentprobe molecule to characterize strong acid sites inzeolites80. Finally, acetonitrile which is a small







    molecule with low basicity, pKt, = 24 and deuteratedacetonitrile which gives a less complex spectrumhave been found to be of great use in monitoringBronsted and Lewis acid strengths81-84.

    Nuclear Magnetic Resonance Methods-NuclearMagnetic Resonance (NMR) is a powerfultechnique to study the catalytic activity of zeolitesl4.Advanced NMR methods such as cross-polarization(CP), magic angle spinning (MAS) of zeolites, highresolution and pulse field gradient 'magneticresonance have increased the capability of thistechnique to study acid sites in zeolite catalysts.Both the total number of acid sites and their relative

    strength can be measured by proton MAS NMRtechniques8589 High-resolution 1H MAS NMR is apowerful tool for the measurement of zeoliticacidity since the intensity (area) of a NMR line isdirectly proportional to the concentration of thecorresponding nuclei.

    Apart from the analysis of chemical shift of theacidic protons, analysis of structure of the bridginghydroxyl groups and their chemical environments

    fiovide additional in~ormation on, zeolite acidity.Si MAS NMR studies have proVided many new

    insights into the structure and chemistry of zeolites.Signals originating from crystallographic allyinequivalent silicon atoms can be resolved andrelated to structural parameters. Pioneering studiesin high-resolution solid state 29 Si NMR spectroscopyhave been performed by Lippmaa and co-

    k 90-92 h ' d h fiwor ers , w 0 came out t e lrstcomprehensive investigation of a variety of silicatesand aluminosilicates.

    In principle27, Al is a very favourable nucleus forNMR because of its 100% natural abundance and a

    chemical shift range of about 450 ppm. However, itis a quadrupolar nucleus (spin 5/2) and thequadrupole interaction is usually large, whichbroadens and shifts the resonance lines. Hence, the

    detection and quantification of aluminium in solidsby NMR is often difficult. This has seriouslyhindered NMR studies of the AIP04 molecularsieves, in which very large quadrupolar effects aswell as strong interactions with water and otheradsorbates are observed. The development of doublerotation is a major advance in the study ofquadrupolar nuclei as it removes not only first-orderbroadening effects such as chemical shiftanisotropy, but also second-order quadrupolar, ,9394 Th d bl ' hn' hmteractlons '. e ou e rotatlon tec lque asbeen applied to study the large pore

    aluminophosphates, VPI-5, AlP04-11 and AIP04-21/2s'(Ref. 95).

    There is no doubt that with the help of NMRmany additional problems in zeolite chemistryrelating to their catalytic properties can be solved.Some of these are: (i) Si, Al ordering in the zeoliticframework, (ii) Crystallographically inequivalenttetrahedral environments of the various Si and Al

    sites, (iii) Factors governing the acidity of zeolites,(iv) The mechanism ofdealumination of the zeoliticframework and the nature of the extra-framework

    AI, (v) The location and mobility of exchangeablecations and (vi) The mobility, diffusivity andconfiguration of the adsorbed molecules.

    Composition of aluminosi/icate and nature ofextraframework aluminium-29Si and 27Al NMR areof considerable assistance in determining theordering of Si and Al atoms and can also be used todetermine theSiiAI ratio in the zeolitic framework.The amount of extraframework aluminium whichbehaves as Lewis acid. sites in zeolites can be

    estimated by comparing (SilAI~MR values with theresults of chemical analysis. Framework Si inzeolites is tetrahedrally coordinated and there arefive different possible environments for frameworksilicon atoms. These are denoted as Si (nAl) where n(~ 4) signifies the number of aluminium atomsconnected via oxygens to a silicon. Each type of Si

    (nA~ building block corresponds to a definite rangeof 2 Si chemical shift. When a 29 Si MAS NMR

    spectrum of a zeolite (a) contains more than onesignal and (b) is correctly assigned in terms of Si(nAl) units, the SilAl ratio in the zeolitic frameworkmay be calculated from the spectrum alone. Thismethod is valid because in the absence of

    AI-O-Al linkages the environment of every Alatom is AI(4Si). Therefore, each Si-O-Allinkagein an Si (nAI) unit therefore incorporates 0.25 Alatoms and the whole unit 0.25n Al atoms. Thus, theSilAl ratio in the alumino silicate framework may becalculated directly from the 29 Si MAS NMR

    . 'h ~ I 9697spectrum usmg t e lormu a ' ,. 14+13+12+11+10

    (SI/ AI)NMR= --------14 +0.7513 +0.5/2 +0.2511

    " .(6)

    where In denotes the intensity (peak area) of theNMR signal attributable to Si(nAl) units. Bycomparing (SilAl~MR values with the results ofchemical analysis, which gives bulk composition,the amount of extraframework aluminium can be

  • _.. "". ," "". ",."",,""""!'Il"'n"-' 'I.II'II.'I.lrl~Ir.,t'Il'II"" II-,-:lj:li"t 50)ratio the sample shows a sharp Si (OAI) peak at-106.9 ppm (compared with -107.4 ppm in quartz)and very small broad signal at ca. - 101.3 ppm,attributable to the residual Si (lAI) units (Fig 8d).27AI MAS NMR shows directly how the occludedsix-coordinated Al builds up at the expense of thefour-coordinated Al in the framework. The spectrumof the sample with Si/Al > 50 contains a broadresidual tetrahedral signal and an extremely sharpoctahedral signal due to motionally free AI(RzO)/+,not removed by leaching with acid. Other zeolitesalso undergo ultrastabilization during which Al isisomorphously replaced by Se2.

    Three questions concerning ultrastabilizationneed to be answered. They are regarding the precisemechanism of Al removal, the nature of theintermediate defect structure and the origin of thesilicon needed for framework reconstruction. Gas

    sorption studieslO2 indicate that ultrastable zeolite Ycontains a secondary mesopore system with poreradii in the range 15-19A, suggesting thattetrahedral sites are reconstituted with silicon which,contrary to earlier speculations, does not come onlyfrom .the surface or from amorphous parts ofsample, but also from its bulk, which may involvethe elimination of the entire sodalite cages.

    1 '" 'i !!' I 11"I'!""I"1 1'1' I'


    _,~c.~ ._''''".'""~'" --.


    Line eat 2.6 - 3.6 ppm, is due to AI-OH groupsattached to nonframework AI.

    '" k 85-89Followmg the work of Freude and cowor ers ,the ratio between the number of protons involved ina particular process (n) and the mean residence timeof the proton at a lattice oxygen ion ('t), can be usedas a relative measure of the acidity of OH groups.Moreover, lH MAS NMR is a convenient tool forthe determination of the AI-H distances, becausethe second moment of the broad-line proton spectra,which is inversely proportional to the sixth power ofthe distance between the dipole-coupled nuclei, isdominated by the 1HYAI interaction. An argumentbased on van Vleck's fonnulalO4 can be usedlO5to

    calculate the aluminium-proton distance, rAJ-H,fromthe second moment, M2

    rAI_H = V126.09 / M2 (7)

    where rAI-H is in A and M2 in units of 10-8 T.Stevensonl06 took advantage of this in the study ofthe location of protons in dehydrated H-Y zeolite.He measured the average M2 = (0.71 0.04 ) x 10-8T and found that M2 is virtually independent of thesodium content which indicates that the distancebetween IH and the residual 23Na in the zeolite islarge. The only interaction which may beresponsible for dipolar broadening are thus thehomonuclear H-H and the heteronuclear H-Al

    interactions. In order to estimate the magnitude ofthe former, deuterium, which has a much smallermagnetic moment, was successively substituted forthe proton. It was found that the homonuclear

    interaction was very small and estimated the rH-Has > 4.5 A, indicating that the protons are located inthe faujasitic supercages. From Eq. 7 rH-Al=238

    0.03 A was estimated. Taking the X-ray valuesrH_o=0.93-1.03 A and rAI-O=1.72 A, the AI-O-Hangle of 1160 was observed, close to the 1200required for the complete si hybridization of theoxygen atom. It was suggestedlO6 that the acidity ofzeolites may be due to the decreased p character ofthe bridging oxygen in the bond.

    In H-ZSM-5, the calculated second moment M2 is

    (0.54 0.05) 10-8 T, which corresponds to rAI-H =2.48 0.04 A for the bridging hydroxyl groups. Thestructure of the active site in H-ZSM-5 is shown22 in

    Fig. 9. The AI-H distance is larger than in zeoliteH-Y because of the smaller T-0-T angles in aframework q~mposed mostly of 5-member rings.

    I 107 108 d I MA dScho Ie et al. ' use H S NMR to stu ythe acidity of the hydroxyl groups in H-ZSM-5 andits borosilicate "equivalent", known as H-boralite, atvarious water contents. They were able todistinguish terminal and water hydroxyls fromacidic hydroxyl groups in the framework. H-ZSM-5was found to be more acidic than H-boralite.

    Recently, two NMR techniques, 2H MAS NMR andecho Fourier 27Al NMR have been applied to theinvestigation of Bronsted sites in dehydratedhydrogen forms of zeolites I 09. For a dehydratedsample of the H-ZSM-5 zeolite, the 2H MAS NMRshowed that in addition to the values of the chemical

    shifts, the quadrupole coupling constants (Cqcc) canbe studied in order to characterize the hydroxylgroups. The values of Cqcc of the bridging hydroxylgroups increase along with an increase in theframework Al content of faujasite owing to thedecreased acid strength of the bridging OH groups.

    Interaction of base molecules with acid sites-The NMR technique can also be used to characterizethe interaction of amines with acid sites, providedthat the spectra of the protonated and unprotonatedforms are significantly different. In the case of the


    probe molecules, C MAS NMR allows accuratecalculation of the sum of Lewis and Br6nsted acidsites. However, the shift measurements are notprecise enough to separate the two types of acidsites. Better results for the discrimination betweenBr6nsted and Lewis acid sites are obtained from 15Nand 31P NMR of Nand P surface-bound

    I 110-117Th '" '" I Icomp exes . e mtrogen atom m mo ecu essuch as ammonia and pyridine has a lone pair ofelectrons and binds directly to the surface site.Therefore, one observes large effects on a nucleuswith a wide (ca. 900 ppm) range of chemical shifts,rather than an indirect influence as in the case of

    13C.In ultrastabilized samples, strong association ofammonia molecules occurs even at low coverages

    leading to a constant chemical shift. At lowcoverages, the resonance shift of ]5NH3 on zeoliteH-Y remains constant and is close to that for

    NH; solutions, which shows that all ammoniamolecules are converted into ammonium cations as

    a consequence of interaction with structuralhydroxyl groups. Studies on the formation ofpyridinium ions in ultrastable zeolites has lead to thedirect determination of the number of interacting

    hydroxyl groupSI!l, Acetonitrile can be convenientlyused for the characterization of interactions with the

  • 118INDIAN 1. CHEM. TECHNOL., MAY 1998




    3650 3550v. cm-l





    Fig, 9-"Thc geometry of the Bmnsted acid site in II,ZSM-S(Reprinted from ref 22. Copyright 1988 Royal Society ofChemistry),

    h bl . d I . 'd' '16 J 17 Thexc angea e catIons an _eWIsaCl Sttes' .. eelectron acceptor strength of ultrastable zeolitesincreases with increased temperature of activation,the rise being particularly drastic in the region 300-400C.

    Trimethylphosphine (TMP) has also beendl1811~ 1 'h f h 'd" fuse . to pro Je t e nature 0 t e aCI te SItes 0

    zeolite H-Y by 3Ip MAS NTvlR studIes. When asample was activated at 400"C, the spectrum wasdominated by the resonance at approximately (3 = -2ppm due to (CH,)3PH+ complexes fom1ed by thechemisorption of TMP on the Bronsted acid sites inzeolite H- Y. At least two types of such complexeswere detected, an immobilized complex coordinatedto hydroxyl protons and a highly mobile one whichis desorbed at 300C. The resonances observed at

    approximately -32 to 58 ppm were ascribed to ThfPmolecules bound to the Lewis acid sites created byh I 120t e ca cmatton process .

    Combined NMR-IR methodsA combined study of the IR spectra ofOH groups

    in zeolites with 29Si MAS NMR spectra providesinformation on the geometry of the bridginghydroxyls and the number of nearby Al atoms. InMAS NMR spectra of zeolites A and X (Si/Al = 1),only the Si (4AI) signal is present90.10), so onlyAI3Si-OR-AISi3 hydroxy Is may exit. By contrast,zeolite Y (Si/ Al ~ 2.5) gives separate 29 Si signalsfrom Si{3AI), Si(2Al), Si(1AI) and Si(OAl) units, thealuminium content being too low for the number ofSi(4AI) units to be significant. As Si(OAl) unitscannot create bridging OH groups, there are threepossible kinds of hydroxy Is. Three kinds of bridginghydroxyls of different acid strengths are indeed

    Fig. ](}----IR spectra of hydroxyl groups in zeolite X (A) andzeolite Y (8) with different Si/A I ratios, (Reprinted from ref122. Copyright \ 994 J\ meriean Chemical Society).

    detected by IR spectroscopy in NaH- Y (Si/ Al ;:::;2.56)121. The intensity ratio of Si(nAI) si!:,'Tlalsin29Si MAS NMR depends on the Si/AI ratio.The question arises whether the relative popula-tions of AI3Si-()H--AISi3, AI2SiSi--OH--AlSi,.AISi2Si-OH--AISi, and Si3Si-OH-A1Si3hydroxyls depend on the Si/Al ratio in a similarfashion. The IR spectra of OH groups in zeolites Xand zeolites Y with different Si/Al ratios are shown

    in Fig. 10 (A) and (B)122. Only one Si-01H-AIband (at 3652 - 3660 em-I) is present in the spectraof zeolites X (Si/ Al

  • --~.-".~~--------------





    ~3350. 3250

    S'/A"~ /(1~33:lo. 3250


    I.'f~~'0.4 AI

    N> 0_0",'" -0..5


    -10.3450. 3350

    'I, em-I













    N> 1.-/~ -o.J'"


    Fig. 12--(J\.) IR spectra af OH graups in zeolite Y with differentSiiAl ratias interacting with benzene and chlorobenzene(narmalized to. the same band area); (BHE) secand derivativesaf the spectra. (Reprinted fram ref 122. Capyright 1994American Chemical Saciety).


    N O.3L 51/Ar'I.06>1:1,

    N 03F\'I.''n3~ N>~ ~

    ~ -03L . c {-0.8 ~3500 34:la 340.0. 3350

    04[=~"'~ $'1."167 ~ 0.,.2'"ao[ '"-0.2'" N"-04 ~. ~ 0. "3:lOC3450. 3400 3350.--~----

    '"c'y.> . > Co." ", ,r1- -aJ '" -0.4'0 E ~-0.3 -0..835(:0 3450. 3400 33:lo.

    v. crn-l Y, em-I

    Fig. II--(A) IR spectra afOH graups in zealite X with differentSil Al ratias interacting with benzene and chlaroben~ne(narmalized to. the same band area); (B)-(E) secand derivatives

    af the spectra .. (Reprinted fram ref 122. Capyright 1994American Chemical Saciety).

    bands of hydrogen bonded hydroxyl groups arecomposed of several submaxima. The frequencies ofthese were estimated from the positions of minimain the second derivative plots: 3440, 3350, 3380,

    3330, 3270 cm-l ~enzene sorption) and 3470, 3420,3460, 3290 em' (chlorobenzene sorption). Fourkinds of Si--OJ H-AI groups of different acidstrengths are predicted by comparing these resultswith 29Si MAS NMR spectra122 These OH groupscan be referred to as OH(t), OH(2), OH(3) andOR( 4). In zeolite X only 2 or 3 groups with low acidstrengths can be seen. Three submaximacorresponding to high acid strengths, OH(2), OH(3)and OR(4) are detected in zeolite Y. It is reasonableto assume that the most acidic hydroxyl groups, i.e.,OH(4) groups correspond to the Si3Si-OH-AISi3qluster in which aluminium is surrounded by thelargest number of Si atoms and thus to the SiClAl)NMR signal. Similarly, the less acidic OR(l) groups

    correspond to the AI3Si-OH-AlSi3 cluster andthus to the SiC4AI) NMR signal.

    The population of OH(1)-OH(4) groupsdepends on the Sil Al ratio in the same way as thepopulation of Si(nAI) units. In zeolite X with SilAl =1.03 there is only one Si(4AI) signal and only onekind of bridging hydroxyls: AI3Si-OH-AlSi3 i.e.

    OR(l). As the Si/AI ratio increases, the NMRsignals from Si(3AI), Si(2Al) and Si(1Al) as well asthe IR bands from OR(2), OH(3) and OH(4) grow inintensity. In highly dealuminated samples, theSi(OAl) and Si(lAI) signals and IR bands of OH(4)groups dominate.

    The strength of acidity of bndging OH groups

    depends on the Sanderson intermediateelectronegativity which may be changed through asubstitution of aluminium by phosphorus orboron'23 and by the silicon-to-aluminium ratio. In

  • HI' .11~ljl,i'I.:ldll~' 11~!11 lit, .,.JI II" -'11111 \\,." ., .

    ]20 INDIAN J. CHEM. TECHNOL., MAY 1998

    Fig. 13, a plot of VOH as a function of the SilAl ratio

    is given 103. In the same figure the chemical shift

    8H of line (b) (from bridging OR groups) in theH MAS NMR spectra of hydrogen zeolites

    (faujasites, mordenite and ZSM-5) are presented. Asone can see, after a sharp increase, the chemical

    shift 4I remains constant within the limits ofaccuracy ( 0.1 ppm) for silicon-to-aluminiumratios greater than :::;10.



    ~1-~ 4 ~----l -



    3640 i~o620_ 600

    : I 10" 20~" 30 '"$,/"'1

    Fig. 13--1R high-frequency OH stretching frequency (rightaxis) and chemical shift (left axis) bH line (b) in the lH MASNMR spectra of various zeolites as a function of the Si/AIratio. (Reprinted from ref 103. Copyright 1986 Elsevier).

    Fig. 14--Plot of the line-shape (S) parameter obtained inpositron annihilation spectroscopy versus relative specificacidity (H) of the NH/-lSM-5 sample. (Reprinted from ref125. Copyright 1994 Baltzer Science Publishers BV).


    In conclusion, it appears that the indicators can beused only to get an approximate idea on the acidstrength distribution of a given zeolite. Whenindicators are used together with titration of organicbases the number of acid sites with different

    strengths could be determined. However, if thoseresults are compared with catalytic activity, good

    measurement of the positron or positronium life-time specttum can provide information aboutthe chemical and physical properties of themedium 126,127. Positron lifetime measurements ofNaY and HY zeolites show that the lifetime of o-Ps

    in HY is much shoiter than those in NaY, indicatingthat the annihilation of o-Ps is accelerated by itsoxidation reaction with the strong Bronsted acidityin the zeolite as shown by

    W+o-Ps~H+e+ ... (8)Fig. 14 presents125 a plot of line-shape parameter Sversus the relative specific acidity (relative surface

    proton concentration denoted by H) of NH: ~ZSM-

    5 zeolite. It clearly demonstrates a linear correlationbetween the Hand S values.


    R.latlv, SpecifiC aClivjlylHl


    III.:I023iZ. 0.22"&... 0.21..J

    Photoelectron spectroscopy methodsSince XPS measures the kinetic energy of

    photoelectrons emitted from the core levels ofsurface atoms upon X-ray irradiation of theuppermost atomic layers, it can be used tocharacterize surface acid sites, in combination with


    probe molecules .

    When pyridine is sorbed on acid forms of

    zeolites, the N(ls) peak is substantially broader than

    the corresponding Si(2p), Al(2p) and O(1s) peaksl24

    suggesting a composite peak for nitrogen. The N(1s)for pyridine on H-ZSM-5 can be deconvoluted intothree components having different binding energiesbut similar peak widths. The peaks at higher bindingenergy are assigned to two Br6nsted sites and thethird peak to a Lewis site. However, it can berealized that this technique cannot be as powerful a$IR and NMR spectroscopies. Indeed, owing to thecharging and contamination problems, it is verydifficult to obtain accurate measurements.

    Moreover, it cannot be used to determine the totalacidity of microporous solid acids. On the otherhand, it can be used to monitor acidity at externalsurfaces and on solids which are opaque to IRradiation.

    Positron annihilation spectroscopyPositron annihilation spectroscopy (PAS) has

    been scarcely applied to the study of catalyticmaterials 125. Positrons are antiparticles of electronSthat can be formed from the decay of neutron-deficient radioisotopes such as 22Na. They combine

    with electrons of the medium to form positroniumatoms (Ps). Ps exists in two ground states, thesinglet Ps (p- Ps) and the triplet Ps (0- Ps). Theintrinsic lifetime of p-Ps is so short that once it isformed it annihilates nearly independently of themedium. The lifetime of o-Ps is long enough forpositronium-medium interactions. The interactionsshorten the intrinsic lifetime of o-Ps and an accurate

    I III'/"1 I'" I I " I' I"


    correlations are not necessarily obtained.Furthermore, only a small fraction of the total acidsites measured are usually active for a givenreaction, and the low temperature at which acidity ismeasured favours indiscriminate adsorption of thebasic molecules on all acidic sites. This can be

    partially overcome by using adsorption-desorptionmethods. IR methods are particularly useful todetermine simultaneously both Bnzmsted and Lewisacid sites. NMR, and XPS are useful techniques toknow the structure, concentration, strength andaccessibility of the Bf0nsted and Lewis acid sitesand details of their interaction with adsorbed organicmolecules. In general, it appears that a singletechnique is not sufficient for the characterizationand interpretation of zeolite acid sites and for theprediction of catalytic activity and selectivity. Moreinformation about the structure of acidic sites ofzeolites can be obtained when NMR is combinedwith other methods such as IR. IR and NMR,including the use of 15N MAS NMR are particularlyuseful to determine simultaneously both Bronstedand Lewis acid sites. Positron annihilation

    spectroscopy can also be a useful tool for the studyof zeolite acidity. From the lifetimes and intensitiesmeasurement, it is possible to obtain interestinginformation about some important properties of thezeolites, such as the Bronsted acidity and theelectrostatic field gradient. Since positron lifetimemeasurements can be carried out at elevated

    temperatures, this kind of spectroscopy may providean approximate in situ estimation of the electric-field gradients in zeolites at catalytic conditions.

    All the experimental techniques that have beenused to probe acid sites in zeolites have importantcontributions to make toward the elaboration of a

    clear picture of the active sites in zeolites. No doubt,at present, it is not easy to design new catalysts forspecific reactions based mainly on experimentalsurface science investigations, or on purelytheoretical considerations. At best, a combination oftheoretical methods such as molecular mechanics,

    molecular dynamics, ab initio quantum mechanicsand different experimental techniques can shortenthe time required for the design and development ofa new catalyst. These methods have already provedto be useful for the optimization of existing

    catalysts. Hence, the combination of the theoreticalmethods with experimental techniques such as IR,NMR, XPS and PAS will lead to a powerfultechnique for providing new insights into the driving

    forces for intrazeolite reactions as well as the

    catalytic activity of zeolites.

    AcknowledgementThanks are due to Dr R Vetrivel and Dr S

    Sivasanker for fruitful discussion and encourage-

    ment. The financial support received from theCouncil of Scientific and Industrial Research

    (CSIR), New Delhi in the form of a researchfellowship is also acknowledged.

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