Quantitative Analysis of Aluminium in Zeolites by 27A1-NMR: Determination of Extra-Framework and Framework Species

P.J. Grobet et al. (Editors) IInnovation in Zeolite Materials Science © Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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QUANTITATIVE ANALYSIS OF ALUMINIUM IN ZEOLITES BY 27AI-NMR DETERMINATION OF EXTRA-FRAMEWORK AND FRAMEWORK SPECIES

C.FERNANDEZ, F.LEFEBVREl,J.B.NAGY and E.G.DEROUANE Laboratoire de Catalyse, Facultes Universitaires N.-D. de la Paix, 61 Rue de Bruxelles, 5000 NAMUR, Belgium. IPermanent address: Institut de Recherche sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69621 VIILEURBANNE, France.

ABSTRACT The feasibility of the quantitative determination of the AI content in various zeolites is critically examined. The optimized conditions for the NMR-parameters (adequate delay times and short pulse lengths, and powder spectra quantitative data) are derived for faujasite, mordenite and a-alumina. Tetrahedral framework aluminium in the presence of octahedral-extraframework aluminium can be determined from the initial slopes of the intensity signal vs. flip angle curves. A method is proposed to determine the relative amount of zeolite in a mechanical mixture of zeolite and a-alumina. It is based on the observation that for short delay times (0.1 s ), the 27AI-NMR intensity for a-alumina is drastically reduced with respect to that obtained with long delay times (lOs ). INTRODUCTION High resolution solid state 27AI-NMR spectroscopy applied to aluminosilicates, such as zeolites, provides useful information about the location and distribution of AI atoms in their framework and for the identification of extra-framework species. Our particular interest is in the ability to quantitate the various types of AI atoms in a given zeolite. The basic requirement for the quantitation of quadrupolar nuclei, such as aluminium in zeolites, to be achieved with a very high accuracy (less than 5% of error) includes the comparison of absolute intensities with those of Al in solutions using very low flip angles [ref. 1,2]. In the present work, the method is applied essentially to two kinds of problems which have practical interest in the catalytic applications of zeolites. First, the method is used to investigate dealuminated zeolites. Indeed, in acid catalysis by Bronsted acid sites, the catalytic activity is directly related to the framework aluminium atoms concentration [ref.2]. Dealumination appears as one of the most interesting ways to control the AI site density. However, after dealumination, aluminium can be located either in framework andlor in extra-framework positions [ref.3]. 27AI NMR allows a rapid quantitative analysis of each species. Secondly, the case of zeolites compounded with aluminium-containing binder is investigated. Indeed, in general, commercial zeolitic catalysts are obtained by combining the zeolite with an alumina or clay binder in order to increase their mechanical properties and hence their catalytic lifetime. As catalytic activity is related in most of the cases to the presence of zeolitic lattice aluminium, the distinction between support and lattice aluminium is of primary interest. The approach

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described below allows a discrimination of the two contributions using variable flip angles and scanning conditions. EXPERIMENTAL Materials The materials studied in this work are listed in Table 1. The chemical analysis of the zeolites (SiJAI ratio) was performed by PIGE (Proton Induced Gamma Ray Emission) and their water or ammonia content was determined by thermogravimetry. Techniques All 27 AI-NMR spectra were recorded at 20°C on a pulsed Fourier transform BRUKER CXP-200 spectrometer. The sample holder was a Delrin conical rotor containing about 250 mg of zeolite. For Al nuclei, the Larmor frequency is 52.1 MHz at 4.7 T magnetic field. The number of free induction decays (FIDs) accumulated was 100 for solutions and 1000-5000 for solids. The sweep width was 10 kHz for solutions and 125 kHz for solids. Quantitative analyses are achieved on non-rotating samples (powder or broad band condition) in order to avoid problems of intensity rejection in the spinning sidebands. RESULTS AND DISCUSSIONS Determination of the bulk Al content by 27 Al NMR The basic requirement for the quantitative analysis of Al nuclei by NMR is described in more details in ref. 1. Special care is devoted to limit possible losses in signal intensity. Typically, the delay time between pulses must be larger than the longitudinal relaxation time T 1, and corrections have to be made when the apparent relaxation time T 2' related to the width of the signal, is short compared to the instrumental dead-time. Indeed, the influence of the T 1 relaxation time becomes critical for solutions and even more in the case of alumina. On the other hand, the influence of T2 cannot be neglected when the spectra of TABLE 1. Al content ofthe samples

sample

SiJAI

Wt loss

2.4

37 17.7

9

%

Al content atglgf glgf

0.98 10- 3 3.50 1.42 1.96 1.15

10-3 10-3 10-2 10-2

26.46

9.45 3.83 5.29 3.10

10-2 10-2 10-1 10-1

aconcentration measured by titration bNH4 form prepared by exchange with a buffered NH 4Cl solution (pH=9) at room temperature CDealuminatedmordenite from NORTON Co. dcalcined a-alumina. ~echanical mixture of 50 wt% a-Al20 3 and 50 wt% NH 4-Y zeolite f AI amount in atg or g per g of sample

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Fig.I . Normalized 27Al NMR signal intensity vs.flip angle for various Al containing materials. (dead-time: 4 J1S, pulse length ofthe 90 o-pulsefor the aqueous solution: 9 J1S) solids are compared with those of solutions, because the widths of the lines vary over several orders of magnitude and lead to different intensity losses over the instrumental dead time. To resolve this problem, the intensity should always be extrapolated to the half pulse length [ref A]. Quantitative analyses are performed on non-spinning samples in order to avoid any intensity rejection into sidebands, which occurs in magic-angle-spinning conditions. Note also in agreement with Schmidt's calculation [ref.5] that the Al-intensity obtained in solution must be multiplied by a factor of 9/35 for a quantitative comparison between solid and liquid samples. Indeed, for a solid only the central transition is generally observed while for solutions all the transitions contribute to the total intensity. The 9/35 factor is the theoretical ratio of the central transition to the total intensity. The quantitation of Al consists in the comparison of the intensities obtained at short pulse length ( small flip angle) with those obtained with the same pulse length for a standard sample e.g. a solution of aluminium nitrate. Indeed, it was already demonstrated that, for quadrupolar nuclei, the intensities are directly proportional to the amount of nuclei only for small flip angles[ref.6,7] Experimental investigations of the variation of the integrated line intensity (normalized to the number of nuclei determined by chemical analysis) as a function of the flip angle for the different materials indicate clearly that the intensities are proportional to the number of Al only for small flip angles (see Fig. 1). It is worth noting that, in this case, all the Al atoms are thus detected in the sample, independent on their quadrupolar frequency or chemical states. Hence, using this approach, quantitative analysis is possible with a great accuracy (typically about 5%). Ouantitation of the framework and extraframework aluminium in dealuminated mordenite. The spectra of dealuminated mordenite recorded without MAS show that this sample contains two Al species characterized by two distinct NMR lines at @55 and @Oppm from AI(H20)6)3+ taken as reference (see Fig.2). These lines are respectively attributed to tetrahedrally-coordinated (framework AI) and octahedrally-coordinated Al species (extraframework Al ) [ref. 5,6].

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Fig.2. Dealuminated H-Mordenite powder spectra recorded at two different flip angles.

Figures 2. and 3. show the intensities of the total NMR line ( sum of the two Al species) and the respective intensities of the two contributions as a function of the flip angle. It must be noted, that for the two separate components, intensities are normalized with respect to the total amount of Al atoms in the sample. Fig.3 demonstrates clearly that the correct concentration of each species cannot be extracted from their respective NMR intensities at arbitrary flip angles. Indeed all tentatives to determine these concentrations at a given flip angle lead to irrelevant results because the ratio of the two line intensities is dependent on the pulse length. The respective quadrupolar frequencies are probably different as it can be seen from the different positions of the curves maxima [ref 6]. The proposed method for quantitative analysis is the direct comparison of the

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Fig.3. Normalized 27AI NMR signal intensity vs. flip angle for a dealuminated H-Mordenite compared with AI(N03)3 solution. Intensities corresponding to each species ( tetrahedral and octahedral Al atoms) are normalized to the total amount ofAl in the zeolite.

227

initial slopes. From the initial slopes, the ratio of the octahedral to the tetrahedral AI species, in the case under study, can be computed as: Slope (Octahedral species)

-------------------------------- = 27 % Slope (fetrahedral species) If this computation is made from intensities taken at other and different flip angles, this ratio is

39% at a 30°, or 75% at a 60° flip angle. Discrimination between the AI-content of a zeolite and that of a-alumina in a mechanically mixed ~

The line intensity for

~-alumina

is very dependent on the delay time between pulses as it is

shown on FigA, because its relaxation time T 1 is quite long. For zeolites a delay time of 100 ms leads to quantitative results. The initial slopes of the curves are all similar and proportional to the number of aluminium in the sample. However, for larger flip angles the use of shorter delay times leads to more asymetrical intensity vs. pulse angle curves. It can be thus concluded that this deviation from the sinusoidal shape of the curve for large flip angles is an indication of a too low value for the delay time. Indeed, for the optimum delay time, the curve must be close to a sinus wave. It is important to note that the intensity for a zeolite NH4Y is not affected by the delay time when it is greater than O.ls. This observation can be employed to discriminate between the contribution of the zeolitic (framework) material and that of an a-alumina binder. For example, a mechanical mixture (50-50 wt%) of a-alumina and NH 4Y was prepared. Figure 5 shows the shape of the spectra recorded at 30° with several delay times. If the delay time is short, the contribution of the a-alumina is drastically reduced but that of the zeolite is not affected. From the initial slope of the intensity vs. flip angle curve, at 10 s delay time (Fig.6), it is clear that the Al content of the alumina-zeolite mixture is slightly underestimated if the intensity are normalized for 1.15 atg/g of Al (see Table 1) calculated from the 50-50wt% ratio of the two materials in the mixture. It must be therefore corrected so that the inital slope for the mixture is equal to that of solutions. This correction gives a value equal to 1.28 10-2 atg/g of sample, which is higher than the


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expected 1.15 atg/g value. As the Al content in pure a-alumina and in NH 4Y zeolite are 1.96 10-2 atglg and 3.5 10-3 atg/g of sample, the relative concentration can be calculated from the formula: [AI20 3]x1.96 1O-2+[NH4Y]x3.5 10-3=1.28 10-2 [AI20 3]+[NH4Y]= 1

Hence, the actual concentrations are 58 wt% alumina and 42 wt% zeolite. These relative concentrations may also be extracted from the measurements performed at several delay times. For example, total intensities measured at 300 for the mixture are equal to 5.0 and 12.8 per g of sample for delay times of 0.1 and lOs respectivelly. The corresponding intensities for the pure alumina for these conditions are equal to 5.6 and 19.3 per g of sample respectivelly. The intensity for the NH 4Y zeolite is 4.0 per g of sample for both delay times.

Hence, the exact concentrations of a-alumina and NH 4Y in the mixture must fit the following

equations:

[AI20 3]xI9.3+[NH4Y]x4.0= 12.8 [AI20 3]x5.6+[NH4Y]x4.0=5.0 The fitted solutions are:

[AI20 3]+[NH4Y]= 1 [AI20 3]=58 wt%

[NH 4Y]=42 wt%

and show that the proportions of alumina and zeolite in the mixture calculated by this method are similar to those determined from the initial slope. This possibility directly impacts on the characterization of such complex systems for which it is known that Al migration can occur in certain condition from the alumina ( binder) phase to the zeolite [ref. 7].

229

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CONCLUSION It is shown that 27 AI-NMR is a very good tool to quantify the nature and concentration of different Al species in zeolites. However, this method of analysis should be used with great care in order to achieve high accuracy. The following recommendations emerge: 1) Avoid using MAS

for quantitative measurements because the appearance of sidebands

(typically when the lines are wide) can lead to apparent intensity losses. If MAS is used, the computation of adequate contribution of sidebands is required [ref. 8-10]. 2) Optimize the delay time between pulses in order to reach the maximum of intensity. For

example, the optimum delay time was found to be 100 times larger for a-alumina than for zeolites. 3) Obtain quantitative measurements from intensities at very small flip angle or from the initial

slope of the curves displaying the variation of intensity as a function of the flip angle (pulse length). This method is able to give the correct value of AI content within less than 5% accuracy. 4) Extract the relative concentration of framework and extraframework aluminium in a zeolite

from the ratio of the initial slopes ( short pulse length). 5) Determine the relative concentration of zeolite and alumina in a mechanical mixture or bound

(extruded) samples using different delay times, because, within a certain limit, this parameter influences only the intensity of the alumina contribution.

ACKNOWLEDGMENTS One of us (CF) acknowledges support from the EEC and Haldor Topsee AiS under Brite contract RI-lB-0066 DK(B). REFERENCES C.FERNANDEZ,F.LEFEBVRE,J.B.NAGY,E.G.DEROUANE, in preparation.

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2. E.G.DEROUANE, L.BALTUSIS,R.M.DESSAU, AND K.D.SCHMITT, Stud.Surf.Sci.CataI.,20,( 1985), 135-146 3. lSCHERZER, in T.E.WHYTE and coli. (Editor), Catalytic Materials,Relationship Between Structure and Reactivity, ACS Symposium Series No 248,1984, pp. 157-200 4. E.FUKUSHIMA,S.B.W.ROEDER, Experimental Pulse NMR: A Nuts and Bolts Approach, Addison-Wesley Publishing Company ,Massachusetts, 1981. 5. V.H.SCHMIDT, Proc.Ampere Int. Summer school II, Basko polje, 1972,pp. 75-83 6. D.FENZKE,D.FREUDE,T.FROHLICH,J.HAASE, Chem.Phys.Lett., 111, (1984),171-175 7. D.SHIHABI, W.E. GARWOOD, P.CHU,J.N.MIALE,R.M.LAGO,C.T.W.CHU, C.D.CHANG,J.CataI.,93,(1985),471-474 8. A.SAMOSON, E.LIPPMAA, Phys.Rev.,B28, (1983), 6567 9. E.LIPPMAA, A.SAMOSON, M.MAGI, J.Am.Chem.Soc., 108, (1986),1730-1735 10. A.SAMOSON, E.LIPPMAA, G.ENGELHARDT, U.LOHSE, H.-G.JERSCHKEWITZ, Chern. Phys. Lett. 134, (1987),589-592