J . Phys. Chem. 1991, 95, 283-288
283
Investigation of the Distribution of Acidity Strength in Zeolites by Temperature-Programmed Desorption of Probe Molecules. 2. Dealuminated Y-Type Zeolites Hellmut G. Karge,* Vera Dondur,? and J. Weitkamp* Fritz- Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 1000 Berlin 33, West, FRG (Received: April 9, 1990)
The acidity of dealuminated hydrogen forms of Y-type zeolites (Si/AI = 2.4-8.6) is determined by temperature-programmed desorption of ammonia or pyridine, which is monitored through a mass spectrometer. Four types of acidic sites are indicated by ammonia, viz., weak Brernsted and/or Lewis centers and medium and strong Brernsted and strong Lewis sites. In contrast, pyridine, after sample activation at 675 K, probed only two types of sites, Le., medium and strong Brernsted sites. This difference is ascribed to different accessibility of sites for the two probe molecules. From the desorption spectra (i) the fractional coverage of the various sites, (ii) the most frequent energies of activation, Ed,for desorption, and (iii) the probability functions of the activation energies are derived by using a previously described method of evaluation.
1. Introduction Over the past two decades, the acidity of Y-type zeolites has been extensively studied by using a variety of experimental techniques. Among them were IR spectroscopy, titration, microcalorimetry, and frequently, temperature-programmed desorption (TPD) of basic probe However, in nearly all of the pertinent studies the acidity strength was characterized simply by the temperature of the desorption peaks! Since in most cases zeolites exhibit a remarkable heterogeneity of acidity, a more detailed and quantitative determination of the acidity distribution is desirable. There were only a few attempts made to determine the distribution of strength of acidic sites (see, eg., refs 5 and 6). In a preceding study,' which was devoted to the investigation of the acidity of dealuminated mordenites, it was shown that an appropriate mathematical analysis of the TPD spectra is able to provide such information about the distribution of acidity strength. The pethod outlined in ref 7 renders possible a characterization of the acidity strength through the distribution of the respective desorption energies, the width of this distribution, the relative density, and the population of the various types of sites. This presentation of acidity strength distribution, viz., through the distribution of the activation energies of probe molecule desorption, is similar to that advanced by Hashimoto et ale6 The nature and absolute number of these sites, however, had to be identified by complementary measurements, for instance, by IR spectroscopic and gas chromatographic techniques, respectively; IR spectroscopy provides bands typical of Brernsted or Lewis sites, and gas chromatography measures the absolute amounts of desorbed probe molecules. Since the acidity of dealuminated Y-type zeolites is interesting, both under the aspects of fundamental research and in view of using these zeolites in catalysis, it seemed worthwhile to apply the methods developed in ref 7 to such materials as well. Therefore, in the present study a series of dealuminated NH4Y zeolites were investigated by TPD of ammonia and pyridine, and the results evaluated in terms of (i) desorption energies, (ii) their distribution with respect to the strength of the sites from which desorption occurred, and (iii) the relative population of the respective centers. 2. Experimental Section 1 . Materials. N a y , provided by Union Carbide Corp., Tarrytown, NY, was repeatedly ion-exchanged with NH4Cl solution to give NH,Y with a 90% degree of exchange. The NH4Y charge was dealuminated via the procedure described by Skeels and *To whom correspondence should be addressed. 'On leave from the University of Belgrade, Belgrade, Yugoslavia. 'University of Stuttgart, Stuttgart, FRG.
0022-3654/9 1/2095-0283$02.50/0
Brecks using (NH4)#F6 as the dealuminating agent. Details of the preparation and chemical and structure analyses are reported el~ewhere.~The chemical composition of the NH4Y and the dealuminated NH,Y-type zeolites is presented in Table I. X-ray diffraction patterns showed that samples 2-4 remained highly crystalline, whereas sample 5 exhibited a considerable loss (about 50%) of crystallinity. Pyridine and ammonia (99.8 vol %), which were used as adsorbates, were purchased from Merck, Darmstadt, FRG, and Messer Griesheim, Dusseldorf, FRG, respectively. Pyridine was purified by three freeze-pumpthaw cycles and subsequently dried over activated Linde 3A sieve under careful exclusion of the ambient atmosphere; ammonia was used without further purification. 2. Apparatus and Procedures. TPD of ammonia from dealuminated NH,Y samples and TPD of ammonia or pyridine from dealuminated HY samples, loaded with those bases prior to the desorption experiment, were carried out in a stainless steel ultrahigh-vacuum system equipped with a turbomolecular pump, gas-dosing system, device for volumetric adsorption measurements, and a mass spectrometer for monitoring the desorbing species. A detailed drawing and description of the experimental setup was presented in ref 7. Also, the procedures were illustrated in that publication. After adsorption, weakly held ammonia or pyridine was pumped off for 1 h at the temperature (usually 375 K) at which the temperature-programmed desorption was subsequently started. During temperature-programmed desorption, the ammonia or pyridine evolved was trapped at 77 K in a trap of known volume. (1) Ward, J. W. Infrared Studies of Zeolite Surfaces and Surface Reactions. In Zeolite Chemistry and Catalysis; ACS Monograph 171; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976; p 118. (2) Barthomeuf, D. In Proceedings of the 4th International Conference on Molecular Sieves 11, Chicago, Ill., April 18-22, 1977; Katzer, J., Ed.; American Chemical Society: Washington, DC, 1977; ACS Symp. Ser. No. 40, p 453. (3) Jacobs, P. A. Carboniogenic Activity of Zeolites; Elsevier Scientific: Amsterdam, 1977. (4) Ped, N.; Noller, H. In Proceedings of a Symposium on Zeolites, Sreged, Hungary, Sept 11-14, 1978; Fejes, P.,Ed.; Acta Physica et Chemica, Nova Series, Acta Universitatis Szegediensis: Szeged, 1978; p 267. ( 5 ) Dima, E.; Rees, L. V. C. Zeolites 1987, 7, 219. (6) Hashimoto, K.; Masuda, T.; Mori, T. In New Developments in Zeolite Science and Technology; Proceedings of the 7th International Zeolite Conference, Tokyo, Japan, Aug. 17-22,1986 Murakami, y.; Iijima, A,; Ward, J. W., Eds.; Kodansha: Tokyo, 1986; p 503. (7) Karge, H. G.; Dondur, V. J. Phys. Chem. 1990, 94, 765. (8) Skeels, G. W.; Breck, D. W. In Proceedings of the 6th International Zeolite Conference, Reno, NV, July 10-15, 1983; Olson, D. H., Bisio, A., Eds.; Buttersworths: London, 1984; p 87. (9) Neuber, M.; Dondur, V.; Karge, H.G.; Pacheco, L.; Ernst, S.; Weitkamp, J. In Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, Belgium, Sept. 13-1 7 , 1987. Grobet, P. J., Mortier, W. J., Vasant, E. F., Schulz-Ekloff, G., Eds. Stud. Surf. Sci. Catal. 1988, 37, 461.
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The Journal of Physical Chemistry, Vol. 95, No. 1 , 1991
TABLE I: Chemical Compositions and Degrees of Dealumination of the Starting Material, NH,Y, and Dealumi~tedN b Y Zeolites nb
samplc
zeolite
1
NH4Y NH4Y-D I NH4Y-D2 NH4Y-D3 N H,Y-D4
2 3 4 5
chem comp Na4(NH4)~o(A102)56(Si02) 136 Na3(NH4)36(A102)40(Si02)152
NH3 3.18 2.34 1.60 1.55 1.38
8,o %
Si/A1 2.4 3.8 5.6 Na~(NH4)26(A102)29(Si02)163 N ~ ~ ( N H , ) ~ , ( A ~ O * ) ~ B ( S ~ O ~5.8 )I~~ 8.6 Na2(NH4)24(A102)2o(Si02)172
29 48 50 64
PY 2.20 1.53 1.12 1.09 0.97
degree of dealumination, expressed as percentage of AI removed. Amounts of NH, or pyridine desorbed during temperature-programmed desorption, in mmol/g. After reevaporation into a known volume, the amounts of ammonia or pyridine were calculated from the pressure gain. Four series of experiments were conducted: (i) TPD-MS of NH3 from initial samples 1-5 of Table I (deammoniation). (ii) TPD-MS of NH3 from deammoniated samples 1-5 after reloading with large amounts of NH3 (i.e., after complete reammoniation). (iii) TPD-MS of NH, from deammoniated samples 1-5 after reloading with small amounts of NH, (Le,, after partial reammoniation). (iv) TPD-MS of pyridine from deammoniated samples 1-5 after loading with small amounts of pyridine (Le., after partial coverage of the acidic centers).
-
I
I
I
I
I
A
W
3. Theory and Evaluation Evaluation of the data obtained by TPD was based on the assumption that parallel and independent desorption processes occur with respect to n different types of sites populated by the species prior to desorption. A first-order relationship with respect to the coverage, 0 , was adopted for the rate of the individual desorption process; this was confirmed by isothermal desorption measurements. Finally, a Gaussian distribution of the desorption energies was introduced into the overall rate expression, even though a Weibull distribution proved to be equally suitable. Thus, the probability densities, P(E,), for the activation energies of desorption, E,, from sites of type n, were calculated according to eq 1 where E, stands for the most probable energy of desorption P(&) = [1/(2xan)'/*] exp[-(E, - En)2/2a,2]
(1)
and U, for the variance as a measure of the width of distribution. Again, details of the theoretical treatment are provided in ref 7. Computational evaluation of the data was carried out on a Model Cyber 180 computer (Control Data) with the help of the NAG-BASS library and fitting programs based on the analytical Jacobian. It was confirmed that the method used provided a unique deconvolution of the experimental TPD spectra. As in ref 7, a preexponential factor of A = 0.15 X IO8 min-' was used in all calculations.
4. Results and Discussion I . TPD-MS Spectra of Dealuminated NH4Y Zeolites. First, the as-prepared dealuminated NH4Y samples 2-4 were submitted to temperature-programmed heating. For the sake of comparison, the nondealuminated NH4Y starting material was investigated under the same conditions. The samples were maintained for 1 h at 325 K under a dynamic pressure of lo4 Pa and then heated at 20 K/min to about 1050 K. Signal intensities of m / e = 16 (NH2 and 0),17 (NH, and HO), 18 (HzO), 19 ( H 3 0 and F), and 20 (HF) were continuously monitored. The (corrected) TPD spectra for NH, (via the corrected signal intensity of m / e = 16), HzO and H F are shown in Figure 1. There is a considerable overlap of removal of physisorbed HzO ( m / e = 18, dehydration peak around 425 K, additional peaks between 500 and 600 K) and deammoniation ( m / e = 16, main peak 520-600 K). Around 925 K a prominent signal ( m / e = 18) appeared indicating dehydroxylation of the deammoniated NH4Y (Le., HY) samples. An increasing degree of dealumination or Si/AI ratio corresponds to (i) increasing hydrophobicity and (ii) decreasing density of AI04,,- tetrahedra in the framework (decreasing density of charge-compensating NH4+or, after deammoniation, hydroxyl groups). A consequence is that in the series from NH,Y through NH,Y-D4 (i) the intensity of the peak around 425 K (due to
VI
rr
w
+ w E
0
a c
U
w
n. VI
VI V I i
U
I
.-=!yo7 p i m i
,,-'
1
400
600
800
I
1000 1200
T E M P E R A T U R E [K]
Figure 1. TPD spectra of NH,, H20, F, and HF ( m / e = 16, 18, 19, and 20, respectively) evolved during deammoniation of NH4Y and (via (NH4)2SiF6 treatment) dealuminated NH4Y zeolites with increasing
Si/AI ratios. physisorbed HzO) decreases and the peak p i t i o n is slightly shifted to lower temperatures and (ii) the intensities of the deammoniation peak ( m / e = 16) at about 525 K and the dehydroxylation peak ( m / e = 18) at 925-1000 K significantly decrease. Moreover, the deammoniation peak is shifted to higher temperatures as the degree of dealumination is enhanced. This indicates increasing strength of the sites (from which NH3 desorbs) with increasing Si/AI ratio. Similarly, the dehydroxylation peak moves to higher temperatures as the Si/AI ratio of the zeolites increases. Sample 5, NH4Y-D4,exhibited a somewhat deviating behavior in that the dehydration peaks a t 510 and 575 K were very pronounced (nearly coinciding with two well-resolved NH, peaks), dehydroxylation proceeded over a wide temperature range, and finally relatively large amounts of H F were detected. However, as mentioned in the Experimental Section, the highly dealuminated NH4Y-D4 sample suffered from a considerable loss of crystallinity and significant portions of the dealuminating agent, (NH,)*SiF,, were incorporated. In effect this rendered the surface of NH4Y-D4 more heterogeneous and different from that of the other samples, Le., a fraction of the surface would have properties similar to those of amorphous silica, alumina, and/or silica/alumina. In Figure 2 the NH, TPD-MS spectrum is redrawn for sample 2 (NH,Y-DI), and the result of deconvolution presented. 2. TPD-MS Spectra of NH3 Desorbing from Dealuminated HY Zeolites. After deammoniation, the dealuminated NH4Y samples were contacted with NH, and, thus, reammoniated. Preliminary tests showed that contact with NH3 at 500 Pa and 375 K resulted in maximum loading, indicating that under these conditions probably all of the available acidic sites were covered.
The Journal of Physical Chemistry, Vol. 95, No. I, 1991 285
Acidity Strength in Zeolites
TABLE 11: IR Spectroscopic Data Obtained before and after Pyridine Adsorption on Dealuminated NH4Y (NH4Y-D1) as a Function of Activation Temperature"
K
A[OHIc (3640 cm-I)
A[OHId (3550 cm-I)
A[OHIc (3550 cm-I)
A[ PyH+]f ( 1 545 cm-')
A[PYLP (1455 cm-I)
1.84 1.61 I .68 0.68 0.03
1.48 1.35 1.38 0.49
1.07 0.85 0.92 0.36
0.99 0.90 0.9 1 0.52 0.07 0.03
0.16 0.19 0.32 0.94 1.66 1.79
675 725 775 825 875 925
"Self-supported NH4Y-D1 wafers (about 6 mg were activated in high vacuum Pa, 2 h); pyridine (0.6 kPa) was adsorbed at 475 K (2 h) followed by removal of weakly held pyridine by evacuation at the same temperature (IO4 Pa, 2 h). *Temperature of activation. CAbsorbanceof the high-frequency O H band around 3640 cm-' before pyridine adsorption (was in all experiments zero after pyridine adsorption). dAbsorbance of the low-frequency OH band around 3550 cm-' before pyridine adsorption. 'Absorbance of the low-frequency O H band around 3550 cm-' after pyridine adsorption. /Absorbance of the pyridinium ion band around 1545 cm-I due to Bransted sites. ZAbsorbance of the band around 1455 cm-' due to pyridine coordinatively bound to strong Lewis sites.
TABLE Ill: Parameters of Acidity Distribution in NH4Y and Dealuminated NH,Y after Deammoniation and Subsequent Reloading with NH, (Complete Coverage) Obtained by TPD-MS of Ammonia" weak sites, SI medium B sites, S2[B] strong B-sites, S3[B] zeolite N HPY N H4Y-D 1 NH,Y-D2 NH4Y-D3 NH4Y-D4
Si/AI 2.4 3.8 5.6 5.8 8.7
kJ/mol 79.6 79.0 82.0 82.0 83.5
Eld.
(I'
XI
3.5 4.0 5.5 5.0 7.5
0.448 0.369 0.370 0.397 0.410
E2d,
kJ/mol 87.5 91.0 95.8 95.9 98.0
kJ/mol 104.0 104.5 112.5 110.9 125.0
E3d.
(12
X2
3.5 4.0 4.4 4.4 6.5
0.467 0.552 0.536 0.530 0.470
(I3
4.0 5.0 5.5 5.5 8.5
x3 0.085 0.079 0.094 0.073 0.120
End= most frequent activation energy of desorption in kJ mol-'; x, = fractional population of site of type S,; B = Bransted acid; L = Lewis acid; = width of distribution of activation energies, data after deammoniation of initial N H 4 zeolite samples at 675 K.
a
(I,
Subsequently, weakly held ammonia was removed via evacuation at 375 K by pumping (lod Pa) for 1 h and the temperatureprogrammed heating (20 K/min) was initiated. Figure 3 shows, as an illustrative example, the experimental TPD-MS data obtained for sample 2, Le., deammoniated NH4Y-D1. The shape of the NH3 peak (experimental data, m / e = 16) looks very much like that obtained with the initial dealuminated NH4Y-D sample (compare Figures 2 and 3). Moreover, the temperatures of the main peak maxima found in the experiments of Figures 2 and 3 are close together, viz., 540 and 550 K, respectively. In line with earlier measurementsio of temperature-programmed pyridine desorption from a similar but not identical nondealuminated HY sample, the experimental data displayed in Figure 2 suggest that their overall contour would surround at least three peaks corresponding to three types of sites from which NH3 is released. This was confirmed by deconvolution of the NH3 TPD-MS spectra using the kinetic modeL7 When three independent types of parallel desorption processes were assumed, the theoretical desorption curves for the individual peak components summed to an overall contour that was well-correlated to the experimental data (compare Figures 2 and 3). The mean standard deviations amounted to less than 3%. Analogous spectra were obtained for samples 1 and 3-5. Even though the peak shapes and maximum temperatures, Tmax, depended to a certain degree on experimental conditions such as heating rate, initial coverage, etc., the NH3 TPD-MS experiments provided a reproducible and reliable comparative characterization of the samples under investigation when identical conditions were applied. The deconvoluted spectra demonstrated in all cases that three types of adsorption sites were operative. The low-temperature NH3 peak around 450 K is attributable to weak (Brransted and/or Lewis) acid sites7 Experiments with H Y samples partially dehydroxylated (at pretreatment temperatures of 675-1075 K) prior t o loading with NH3 revealed that the intensity of the second and third peak (around 580 and 650 K, respectively) decreased and a new peak appeared around 780 K (see Figure 4). Dehydroxylation of hydrogen forms of zeolites converts Bransted to Lewis acid sites,'-3 which was confirmed for the present case via separate and well-reproduced IR measurements using pyridine as a probe (see, e g , Table 11). Therefore, the above-mentioned (IO) Dondur, V.;Karge, H. G. Surf.Sci. 1987, 189/190, 873.
300
500 600 TEMPERATURE
400
700
800
[Kl
Figure 2. T P D spectrum of N H , evolved during deammoniation of NH4Y-DI; experimental data (0);calculated curves for desorption from different types of sites, see text (---); theoretical curve of overall desorption (-). 81
'
I
'
I
'
I
'
I
I
T E M P E R A T U R E [Kl
Figure 3. T P D spectrum of N H , desorbed from deammoniated NH4YD1 after subsequent reloading with NH, to complete coverage under 500 Pa; experimental data (e);calculated curves for desorption from different types of sites, see text (- - -); theoretical curve of overall desorption (-).
second and third peaks of the NH3 TPD-MS spectra were assigned to medium and strong Bransted acid centers, whereas the new signal encountered with dehydroxylated HY adsorbents was ascribed to Lewis acid sites.
286
The Journal of Physical Chemistry, Vol. 95, No. I, 1991
Karge et al.
TABLE IV: Parameters of Acidity Distribution in NH4Y and Dealuminated NH4Y after Deammoniation and Subsequent Reloading with NH3 (Low Coverage, 0.4 mmol/g) Obtained by TPD-MS of Ammoniaa weak sites, S , medium B sites, S2[B] strong B sites, SJB] strong L sites, S,[L]
El,+ kJ/mol
zeolite N H4Y N H4Y-D I N H,Y-D2 NH4Y-D3 N H,Y-D4 u,
79.0 78.7 82.5
xI
ul
6.5 6.5 6.2
&d,
kJ/mol 80.5 83.4 89.9 91.9 97.5
0.153 0.173 0.183
u2
x2
E,& kJ/mol
u,
x,
EM, kJ/mol
u4
x4
4.0 4.7 5.5 5.5 7.8
0.542 0.537 0.427 0.490 0.488
97.5 102.5 108.3 107.7 115.7
7.6 8.4 6.5 6.5 7.8
0.323 0.295 0.243 0.243 0.150
130.8 133.0 133.0 133.0 135.0
7.7 7.8 8.0 8.0 8.0
0.135 0.168 0.143 0.094 0.178
= most frequent activation energy of desorption in kJ mol-'; x, = fractional population of site of type S,; B = Bransted acid; L = Lewis acid; = width of distribution of activation energies, data after deammoniation of initial NH, zeolite samples at 675 K. I
I
I
1
I 1'
0
I
K W
c W
I 0 a
c V W
n v)
,,44?,
.-.-._
600 800 1000 T E M P E R A T U R E [K] Figure 4. TPD spectra of N H , ( m / e = 16) and H 2 0 ( m / e = 18) desorbing from deammoniated and progressively dehydroxylated NH4Y-D2 after reloading with NH, to complete coverage; experimental data ( 0 ) : calculated curves for N H , desorption from different types of sites, see text (---); evolution of H 2 0 (-*-): theoretical curve of overall NH,
400
desorption (-). The results of computational analysis of the NH3 TPD-MS spectra according to ref 7 were summarized in Table I11 for the whole set of (reammoniated) Y-type samples. The data listed for the (i) fractional population of the various sites, xi, prior to desorption, (ii) width of the distribution of the activation energies for desorption, u, and (iii) most frequent value of this energy, Ed, were very close to those evaluated from the spectra of the initial NH4Y and dealuminated NH4Y samples. Therefore, the data for the corresponding set of deammoniation experiments with initial NH4Y zeolites (see section 4.2) are not presented, In fact, the u values derived from the experiments of section 4.2 were larger by about 40% and the Edvalues lower by about I-2% compared to those obtained by the deammoniation experiments (section 4.1). This narrowing of the peaks and shift of Ed to higher values is most likely caused by dehydroxylation, which occurred to some extent during the first heating of the NH4Y samples. Dehydroxylation would remove part of the Bransted sites and strengthen the remaining ones. Lewis-type sites, which must have formed during such dehydroxylation, were not detected because of their low density and overlap of the corresponding TPD peak with that of the strong Bransted sites (compare Figure 4). They were observed, however, when the deammoniated (and slightly dehydroxylated) NH4Y were reammoniated under a low NH3 pressure yielding a small coverage (0.4 mmol/g, vide infra). The most relevant result of series 1 and 2 of the experiments is the ranking of the zeolite samples 1-5 with respect to the strength of the sites measured by the most frequent activation
v)
v)
a
I T E M P E R A T U R E [Kl Figure 5. TPD spectra of N H 3 desorbed from NH4Y and dealuminated NH,Y-D2 after deammoniation and subsequent reloading of H Y and HY-D2 with NH, to low coverage (0.4 mmol/g); experimental data (0); calculated curves for desorption from different types of sites, see text (- - -): theoretical curve of overall desorption (-). energy of ammonia desorption, Ed. In all cases, the strength of the sites increased with increasing degree of dealumination or Si/AI ratio. Simultaneously, the fractional population of the medium and strong Bransted sites, x2 and x3,increased at the expense of that of the weak sites, x l . The increase of strength of sites with increasing degree of dealumination observed in our studies seems to be at variance with a statement by Lohse et al." and results reported by Corma et al.I2 However, at least some of the calorimetric measurements by Lohse et al." show, in fact, also an increase of strength (increase of differential heat of ammonia adsorption) with increasing dealumination. Moreover, neither of the studies'IJ2 provides a distribution of acidity strength for particular types of sites of each sample investigated, and the authors used different techniques of dealumination, viz., deep bed calcination or SiCI, treatment. Series 1 and 2 of the TPD experiments started with complete coverage of the zeolite samples with ammonia. To improve the resolution of the TPD spectra, a further series of desorption experiments was undertaken with samples that were initially loaded (at 375 K) with only 0.4 mmol/g. Figure 5 presents for samples 1 and 3 the experimental data, the individual components as the result of deconvolution, and the theoretical spectrum obtained by summing the individual peaks. Superposition of the individual components leads to a good fit of the experimental data. Table ( 1 1) Lohse, U.; Parlitz, (12) Corma, A.; Melo,
B.; Patzelovi, V.J. Phys. Chem. 1989, 93, 3677. F. V.; Herrero, J. Zeolifes 1987, 7, 559.
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 287
Acidity Strength in Zeolites
TABLE V: Parameters of Acidity Distribution in NHIY and Dealuminated NH,Y after Deammoniation and Subsequent Loading with Pyridine (Low Coverage. 0.27 mmol/g at 475 K) Obtained by TPD-MS of Pyridine"
weak Bronsted sites, S2[B] zeolite N H4Y NHdY-Dl NH4Y-D2 NH4Y-D3 N H,Y-D4'
Si/AI
E,,, kJ/mol
01
XI
2.4 3.8 5.6 5.8 8.7
111.5 116.5 118.5 120.9 120.1
7.2 8.0 8.0 9.5 11.0
0.507 0.556 0.484 0.496 0.692
strong Bransted sites, SJB] kJ/mol a,
I?,,,
130.0 137.5 139.2 139.2 142.9
5.0 7.8 6.3 5.8 6.5
X,
0.493 0.444 0.516 0.504 0.308
= most frequent activation energy of desorption in kJ mol-l; x, = fractional population of site of type S,,; B = Bransted acid; a,, = width of distribution of activation energies, data after deammoniation of initial NH, zeolite samples at 675 K. b0.14 mmol pyridine/g zeolite.
IV summarizes the relevant data for the whole set of samples. For more detailed illustration, the distributions of the activation energies of desorption, Ed, which represents a measure of the acidity strength, were evaluated according to ref 7 and are displayed for samples 1-5 in Figure 6. From this figure one recognizes the change in strength distribution with increasing degree of dealumination, in particular the variation of the population of the various sites (appearance of the weak Lewis and/or Brsnsted sites of type SI in the case of HY-D2, HY-D3 and HY-D4), the shift of the position of the peaks and increase in the width of the distribution curves. From comparison of, for instance, Figures 1 and 5 it is evident that, indeed, lower initial coverage of the samples resulted in better resolution. Interestingly, the NH3 TPD-MS spectra of all samples exhibited a flat broad peak around 800 K ascribable to strong Lewis sites (vide supra). This peak was obviously obscured in the case of higher loadings (series 2). On the other hand, peaks indicating weakly acidic sites (SI,T,, around 480 K) are missing or exhibit only minor fractional population, x, due to low initial coverage (compare Tables 111 and IV). However, the maximum temperatures, T,,,,,, of those peaks, which were detected in series 3, are in general shifted by 10-20 K to lower temperatures. This holds for the peaks indicating stronger sites, S2and S3, as well. Also, the energy values, Ed, obtained upon lower initial coverage are lower by 2-5 kJ/mol than the energies obtained from series 1 or 2 (compare Tables 111 and IV). Both findings may indicate that, in the case of complete coverage (series 1 and 2), effects of diffusivity and readsorption of the released NH3 molecules are not negligible. Table I V also presents the energy values, E d , obtained from experiments of series 3. In fact, the Edvalues for the Lewis-type sites, S4. which were detected upon low initial coverage, are less accurate because of the small fractional population, x4, and disturbances caused by the effect of dehydroxylation commencing around 750 K. However, the Ed values in general increase with increasing %/AI ratio. This is particularly evident for the medium and strong Bransted sites, S2and S3. Thus, the results of series 1 and 2, Le., increasing strength of these sites with progressively higher dealumination, were confirmed. Li et aI.,I3 who carried out temperature-programmed desorption of ammonia and pyridine from H-Y samples, derived Edvalues that were suprisingly low (Ed[NH3] = 11.9 or 43.8 kJ mol-' and Ed[pyridine] = 13.5 or 25.9 kJ mol-') compared to those presented here or in pertinent references (see, e.g., refs 5 and 6). Furthermore, they observed exclusively lower Ed values for Lewis than for Bransted sites (compare, however, Table 111). It seems possible that these severe deviations originate from the oversimplified mathematical treatment by Li et aI.,l3 who for instance, did not consider a distribution of acidity strength. 3. TPD-IUS Spectra of Pyridine Desorbingfrom Dealuminated H Y Zeolites. The series 4 experiments were carried out on deammoniated samples 1-5 with a low initial coverage by pyridine (0.27 mmol/g) to minimize interference.of diffusion and dehydroxylation as mentioned in the last paragraph of section 4.2. Such effects were supposed to be even more serious in the case of the ( I 3) Li, Q.; Zhang, R.; Xue, Z. In New Developments in Zeolite Science and Technology; Proceedings of the 7th International Zeolite Conference, Tokyo, Japan, Aug 17-22, 1986 Murakami, Y., Iijima, A,, Ward, J. W., JUS.; Kodansha Tokyo, 1986; p 487.
z W
ez W
z W z
0.03 0.01
-
0 I-
o
-a
L
A
0.03
0.01 1
0
e a.
0.01
60
80
100
120
140
160
E [KJImol]
Figure 6. Calculated distributions of the activation energies of desorption of ammonia from various types of sites of deammoniated NH4Y (Le., HY) and NH4Y-D1 through NH4Y-D4 (Le., HY-DI through HYD4); compare text and Table IV).
bulkier and more strongly held pyridine probe molecule, which requires higher temperatures for complete desorption than ammonia. In Figure 7 the experimental data for temperature-programmed desorption of pyridine from deammoniated samples 1 (HY) and 2 (HY-D1) is depicted together with the result of the deconvolution and the fitting curve. The latter resulted as the sum of the calculated components of the deconvolution and provided a satisfactory fit of the experimental data. Again, the assignment of the TPD peaks was based on comparison with IR results, which showed that both peaks were due to pyridine molecules desorbing from Bransted sites.I0 According to ref IO these sites were labeled (Table V) as S2 (weak) and S3(strong). Sites SIand S4 (weak and strong Lewis centers) were not observed under the experimental conditions of series 4; SI sites are indicated only upon higher loadings with the probe (compare ref 10 and, particularly for samples 1 and 2, Table IV). The set of distribution curves for the activation energies, Ed, of pyridine desorption from deammoniated samples 1 and 2 is shown in Figure 8. In a previous study,9 pyridine adsorption on deammoniated samples 1-5 was investigated by IR. It was shown that the accessibility of those O H groups, which are located in the 8-cages and indicated by the low-frequency (LF) band at 3550 cm-I, is significantly improved by progressive dealumination. This, however, is not reflected in Figures 7 or 8. Two peaks occur even in the TPD spectrum of pyridine desorbing from HY (sample I), where only a few, if any, LF O H groups were involved in pyridine adsorption. Thus, we have to conclude that both high-frequency (3650 cm-I) and low-frequency (3550 cm-I) O H groups give rise
288 The Journal of Physical Chemistry, Vol. 95, No. 1, 1991
Karge et al. 1
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Figure 7. TPD spectra of pyridine desorbed from NH4Y and dealuminated NH4Y-DI after deammoniation and subsequent loading of HY and HY-DI with pyridine: experimental data ( 0 ) ;calculated curves for desorption from two different types of sites, see text (- - -); theoretical curve for overall desorption (-). to two TPD peaks upon pyridine desorption; in other words, w a g e as well as P-cage hydroxyls comprise weak Bransted sites ( S , ) and strong Bransted sites (S,). In contrast to this, some other authors (see, e.g., ref 14) tend to ascribe the high-frequency (HF) and the low-frequency (LF) IR bands to OH groups of higher and lower acidity strength, respectively. In excellent agreement with the results obtained via TPD-MS of ammonia (section 4.1 and 4.2), Figures 7 and 8 and the data of Table V for TPD-MS of pyridine also demonstrate a significant increase of the activation energies for desorption, E d , with increasing Si/AI ratio; in other words, an enhanced acidity strength of both types of Bransted sites (S2,S,) with progressive dealumination. 5. Conclusions Temperature-programmed desorption (TPD) of ammonia indicated four types of acidic sites on hydrogen forms of dealuminated Y-type zeolites that were previously activated at 675 K. Comparison with results on partially dehydroxylated samples (14) Macedo, A.; Raatz, F.; Boulet, R.; Janin, A.; Lavallcy, J. C. In Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, Belgium, Sept 13-17,1987; Grobet, P. J., Mortier, W. J., Vasant, E. F., Schulz-Ekloff, G., Eds.; Stud. Surf. Sci. Carol. 1988, 37, 375.
Figure 8. Calculated distributions of the activation energies of desorption of pyridine from two types of sites of deammoniated NH4Y (Le., HY) and NH4Y-D1 through NH4H-D4 ( k , HY-Dl through HY-D4); compare text and Table V. provided evidence that these sites are ascribable to weak Bransted and/or Lewis centers (S,), medium and strong Bransted sites S2[B] and S3[B], and, finally, to strong Lewis sites (S4[L]). The fact that, under equal conditions, pyridine as a probe indicated only two types of Bransted sites, viz. S,[B] and S,[B], is explained by the restricted accessibility of the other sites for the bulkier pyridine and is, with respect to the strong Lewis sites, S4[L], also due to the low concentration of these centers. As a most important result, the TPD spectra suggest that the strength of the medium and strong Bransted sites increases with increasing degree of dealumination, Le., from Si/AI = 2.4 through Si/A1 = 8.6, as indicated by the shift of the peaks toward higher temperatures. However, the activation energy for desorption of the probe molecules, Ed, evaluated from the spectra on the basis of a previously tested theoretical model, turned out to be an even more suitable measure for ranking the zeolite samples with respect to acidity, because Ed can be determined after reliable deconvolution of the rather complex TPD spectra. The distribution functions of the activation energies rendered possible a more detailed and quantitative determination of the acidity distribution in dealuminated and nondealuminated hydrogen forms of Y-type zeolites.
Acknowledgment. We greatly appreciate the excellent experimental assistance of Mrs. Erika Popovie and Mr. Walter Wachsmann. Registry No. NH,, 7664-41-7; pyridine, 110-86-1.
