J. Phys. Chem. 1990, 94, 165-112 high Si/AI ratio^.^^,^^ In our case, this observation could indicate that, at the first stages of P, zeolite formation, the Si and AI availabilities are similar, as a consequence of the dissolution of phases with Si/AI = 1 ratio, but later the increase of Si polymerized species in the solution favors the incorporation of a higher amount of silicon in this zeolite. However, during the synthesis of P, zeolite from 980 OC heated kaolinite, the fast increase of Si polymerized species in the liquid phase ensures that the P,zeolite formed is more siliceous and homogeneous than those obtained in the previous cases. The observed differences show the importance that the control of the degree of polymerization of silica has during the synthesis of a given zeolite.
Conclusions NMR study (27Aland %i signals) of the solid and liquid phases corresponding to the alkali treatment of kaolinites heated previously at different temperatures (400 < T < 1000 "C) has evidenced the strong influence that the thermal preactivation of kaolinite has on the nature of zeolite formed. (22) Weeks, T. J.; Passoja, E. D. Clays Clay Miner. 1977, 25, 211. (23) Von Ballmoos, R.; Meier, W. M. Nature 1981, 289, 782.
765
The NaOH leaching of kaolinite produces the slow dissolution of this silicate and the formation from dissolved species of nuclei of H S zeolite. On the other hand, the dehydroxylation of kaolinite (metakaolinite) favors the NaOH attack and increases the amount of AI(OH)4- and SO4"- monomers in the solution. The polymerization of these species allows the formation of an amorphous gel, where nuclei of Na-A zeolite are formed. Finally, the thermal treatment of kaolinite at 980 "C produces the separation of amorphous silica and an Al-rich cubic phase. The fast dissolution of the amorphous silica permits the accumulation and polymerization of Si species in the solution which directly gives P, zeolite nuclei (%/AI > 1). During prolonged alkali treatment of kaolinite and metakaolinite the HS and Na-A zeolites initially formed (Si/AI = 1) transform into the more stable and siliceous P, zeolite. In this process a polymerization of dissolved silicon species is always required to form the zeolite precursors. In order to assure the composition and homogeneity of this zeolite, the control of the Si polymerization in the liquid phase is extremely important. Registry No. Na6(A@i&)-nH20, 12251-30-8; AI(OH),, 1448539-3; 17181-37-2; kaolinite, 1318-74-7; metakaolinite, 1512381-6.
Investigation of the Distribution of Acidity in Zeolites by Temperature-Programmed Desorption of Probe Molecules. 1. Dealuminated Mordenites Hellmut C. Karge* and Vera Dondud Fritz-Haber-Institut der Max-Planck-Cesellschaft, Faradayweg 4-6, 1000 Berlin 33, West, FRG (Received: January 3, 1989; In Final Form: July 5, 1989)
The acidity of dealuminated hydrogen mordenites (Si/A1 = 12-39) is characterized by the temperature-programmed desorption of ammonia or pyridine which is monitored through a mass spectrometer. Four types of sites are indicated by ammonia desorption, viz., weak and strong sites of Br~nstedand Lewis type, whereas pyridine desorption reveals only the existence of three types of acidic sites which are tentatively assigned to one sort of Br~nstedsite and weak and strong Lewis centers. Evaluation of the desorption spectra provides the temperatures of maximum rate of desorption, the population of the respective sites, the desorption rates as a function of the activation energies of desorption,and, finally, probability functions of the activation energies. The results are discussed in terms of coverage, Si/Al ratio, and accessibility.
1. Introduction The acidity of zeolites is an interesting phenomenon both in view of fundamental research and in application of acidic zeolites as catalysts in industrialized processes because catalysis of numerous important reactions proceeds on acidic active centers. A considerable number of investigations have been, therefore, devoted to the problems of nature, number, and strength of acidic sites in zeolites (e.g., see the reviews1-j). Spectroscopic techniques such as IR and MAS-NMR have proved to be particularly powerful tools in characterization of the nature (e.g., Br~nsted and Lewis acid type) of sites4-" Similarly, spectroscopic procedures as well as the titration of acid sites in aprotic solvent^,'^-^^ neutralization of sites via gas-phase adsorption of b a ~ e s , ' ~and J~ rate measurements of acid-catalyzed test reaction^'^-'^ are well-established techniques to determine the density of the respective sites. However, there is considerable evidence of nonuniformity of the acidic sites. Therefore, several experimental methods have been proposed in order to characterize the strength of acid centers in zeolites. This has turned out to be a relatively difficult task mainly because of the need to establish a reliable scale of acidity strength. *To whom correspondence should be addressed. On leave from the University of Belgrade, Belgrade, Yugoslavia.
0022-3654/90/2094-0165$02.50/0
The use of Hammetzl or Hirschler indicatorsz2 assumes a number of fundamental premises. It is, for instance, questionable (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 rhe 4th International Conference on Molecular Sieues II, Chicago, IL,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 Acriuity of Zeolites; Elsevier Scientific: Amsterdam, 1977. (4) Uytterhoeven, J.; Christner, L. G.; Hall, W. K. J . Phys. Chem. 1965, 69, 21 17. ( 5 ) Hughes, Th. R.; White, H. M. J. Phys. Chem. 1967, 71, 2192. (6) Ward, J. W. J. Colloid Interface Sci. 1968, 28, 269. (7) Karge, H. 2.Phys. Chem. (Munich) 1971, 76, 133. (8) Ripmeester, J. A. J. Am. Chem. SOC.1983, 105, 2925. (9) Earl, W. L.; Fritz, P. 0.;Gibson, A. A.; Lunsford, J. H. J. Phys. Chem. 1987, 91, 2091. (10) Freude, D.; Hunger, M.; Pfeifer, H. Z . Phys. Chem. (Munich) 1987, 152. - - ,171. ~
(1 1) Ernst, H.; Freude, D.; Hunger, M.; Pfeifer, H.; Seiffert, B. Z . Phys. Chem. ( h i p z i g ) 1987, 268, 304. (12) Tanabe, K. Solid Acids and Bases; Academic Press: New York, 1970. (13) Fiedorow, R.; Dudzik, Z. W a d . Chem. 1978, 32, 309. (14) Kittelmann, K. U. Dissertation (Ph.D. Thesis), Technische Hochschule Darmstadt, 1979.
0 1 9 9 0 American Chemical Society
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TABLE I: Characterization of the Dealuminated Mordenites" sample Si/AI A(OH), au A(BPy), au A(LPy), au AI,,, AI, HMD-I 12 0.35 0.32 0.43 2.27 1.54 HMD-2 17 0.22 0.27 0.36 1.48 1.46 HMD-3 30 0.20 0.28 0.25 0.94 0.79 HMD-4 39 0.25 0.23 0.19 0.81 0.69 a A(0H) = absorbance of the OH band of acidic hydroxyl groups, around 3600 cm-I; A(BPy) = absorbance of the pyridinium ion band at 1540 cm-' due to Bronsted sites; A(LPy) = absorbance of the band at 1450 cm-' due to pyridine adsorbed on Lewis sites; AI,,,, = number of tetrahedrally coordinated aluminum atoms per unit cell; AI,, = number of octahedrally coordinated aluminum atoms per unit cell.
r---
whether the scale of the pK, values which was established for aqueous solutions can be adopted or titrations in aprotic solvents. The titration technique may work with the large-pore faujasite-type zeolite^,^^-^^ but even here some severe objections have been advanced in the literature.28 With nondealuminated mordenites only a small fraction of the total number of sites were indicated, and no reliable acidity spectrum could be ~ b t a i n e d . ' ~ - ~ ~ - ~ ~ In more recent MAS-NMR work a promising method has E-F appeared, viz., the measurement of the shift of the proton resonance of the (acidic) OH groups in zeolites.lo*llAnother method that was successfully applied in some cases is the determination of the acidity distribution by calorimetric measurements of the differential heat of adsorption when acidic zeolites are equilibrated with basic probe molecules such as ammonia and ~ y r i d i n e . ~ ] ? ~ ~ Such measurements are time-consuming, and particular care must be taken to render possible a sufficient mobility of the adsorbate. Figure 1. (A) Scheme of the apparatus for temperature-programmed Usually the calorimeter must be operated at higher temperatures. desorption: 1, roughing pump; 2, bellows; 3, turbo molecular pump; 4, swing gate valves; 5, ionization gauge and control unit; 6, sample holder Thus, a still widely used technique to characterize the acidity and heating device (pan, see (B)); 7, temperature programmer; 8, ion strength of zeolites is the temperature-programmed desorption source and mass filter; 9, secondary electron multiplier (SEM/MS); 10, of basic probe molecules. Thereby, the desorption of the adsorbate dosing valves; 1 1, Baratron gauge; 12, Baratron control unit; 13, pyridine may be monitored by b a l a n ~ e , pressure ~ ~ . ~ ~ gauge,34 gas chroreservoir; 14, cylinder with ammonia; 15, calibrated volume; 16, cooling matography (GC),35*36 or mass spectrometry (MS).37-39 finger. (B) Sample holder and heater (stainless steel): 1, feed through; setllon
2, heating wire; 3, heating block (copper); 4, thermocouple; 5, CF 35 flange. (15) Steinberg, K. H.; Bremer, H.; Falke, P. Z . Chem. 1974, 14, 110. (16) Thoang, H. Sh.; Topchieva, K. V.; Romanowskii, B. V. Kinet. Katal. 1974, 15, 1053. (17) Turkevich, J.; Nozaki, F.; Stamires, D. Proc. Int. Congr. Catal., 3rd 1965, I , 586. (1 8) Karge, H. G.; Ladebeck, J.; Sarbak, Z.; Hatada, K. Zeolites 1982, 2, 95. (19) Karge, H. G.; Kbsters, H. In Proceedings of the 6th International Zeolire Conference, Reno, NV, July 1&15, 1983; Olson, D., Bisio, A., Eds.; Butterworths: London, 1984, p 308. (20) Guisnet, M. In Catalysis by Acids and Solids; Proceedings of an International Symposium, France, Villeurbanne (Lyon), Sept 25-27, 1984; Imelik, B., et al., Eds.; Elsevier: Amsterdam; Stud. Surf. Sei. Catal. 1985, 20, 283. (21) Benesi, H. A. J . Phys. Chem. 1956, 61, 5490. (22) Hirschler, A. E. J. Catal. 1963, 2, 428. (23) Beaumont, R.; Barthomeuf, D. J. Catal. 1972, 26, 218. (24) Beaumont, R.; Barthomeuf, D. J. Catal. 1972, 27, 45. (25) Barthomeuf, D. J. Phys. Chem. 1979,83,766. (26) Karge, H. G.; Hatada, K.; Zhang, Y . ; Fiedorow, R. Zeolites 1983, 3, 13. (27) Gosh, A. K.; Curthoys, G. In Catalysis on the Energy Scene; Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, Canada,Sept 30-0ct 3,1984; Kaliaguine, S., Mahay, A., Eds.; Elsevier: Amsterdam, 1984; Stud. Surf. Sei. Catal. 1984, 19, 147. (28) Deeba, M.; Hall, W. K. J. Catal. 1979, 60, 417. (29) Kladnig, W. J . Phys. Chem. 1979,83, 765. (30) Schweckendiek, J. Dissertation (Ph.D. Thesis), Fritz-Haber-Institut der MPG and Universitit Bremen, 1982. (31) Klyachko, A. L.; Brueva, T. R.; Mishin, I. V.; Kapustin, G. I.; Rubinstein, A. M. In Proceedings of the Symposium on Zeolites, Hungary, Szeged, Sept 11-14, 1978; Fejes, P. et al., Eds.; Petbfi Nyomda: KecskemCt; Acta Phys. Chem., Nova Ser. 1978, 24, 183. (32) Auroux, A.; Bolis, V.; Wierzchowski, P.; Gravelle, P. C.; Vedrine, J. C. J . Chem. Soc., Faraday Trans. I 1979, 75, 2544. (33) Benesi, H. A. J . Caral. 1967, 8, 368. (34) Mirodatos, C.; Barthomeuf, D. J. Catal. 1979, 57, 136. (35) Cvetanovif, R. J.; Amenomiya, Y . Adu. Catal. 1967, 17, 103.
Frequently, the strength of the acidic sites from which the probe molecules desorb is simply characterized by the temperature of the peak maxima of the desorption spectra. More information is obtained by evaluating the energies of desorption and, if possible, their distribution function.Since the desorption spectra are often poorly resolved, it is necessary to employ curve deconvolution techniques. Even then it remains difficult to compare results that were obtained with different adsorbate-zeolite systems. In many cases it cannot be excluded that the desorption spectra are affected by diffusion limitations. Furthermore, the data derived from the desorption spectra might depend on the proper choice of the probe molecule and/or coverage of the zeolite surface. To minimize such complications, a series of zeolite adsorbents with systematically changed properties but closely related structures should be used. The effects of the nature of the probe molecule and the coverage should be evaluated by a systematic variation of both factors. (36) Steinberg, K.-H.; Bremer, H.; Falke, P. Z . Phys. Chem. ( k i p z i g ) 1977, 258, 305. (37) Karge, H. G.; Schweckendiek, J. In Proceedings of the 5th International Symposium on Heterogeneous Catalysis; Varna Bulgaria, Oct 3-6, 1983; Shopov, D., et al., Eds.; Publishing House of the Bulgarian Acad. Sci.: Sofia, 1983; p 429. (38) Lercher, J. A.; Rumplmayer, G. Z. Phys. Chem. (Munich) 1985,146, 113. (39) Parker, M. L.; Bibby, D. M.; Meinhold, R. H. Zeolites 1985,5, 384. (40) Richards, R. E.; Rees, L. V. C. Zeolites 1986, 6, 17. (41) Hunger, B.; Hoffman, J. Thermochim. Acta 1986, 106, 133. (42) Dima, E.; Rees, L. V. C. Zeolites 1987, 7, 219. (43) Hidalgo, C. V.; Itoh, H.; Hattori, T.; Niwa, M.; Murakami, Y . J . Catal. 1984, 85, 362. (44) Hashimoto, K.; Masuda, T.; Mori, T. In New Developments in Zeolites Science and Technology; Proceedings of the 7th International Zeolite Conference, Japan, Tokyo, Aug 17-22, 1986; Murakami, Y.,Iijima, A., Ward, J. W., Eds.; Kodansha: Tokyo, 1986; p 503.
Acidity of Dealuminated Mordenites
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 767
TABLE II: Parameters of Acidity Distribution in Dealuminated Hydrogen Mordenites Obtained bv T P D M S of Ammonia" ~
initial
coverage, zeolite HMD-I HMD-2 HMD-3 H M D-4 HMD-Ib HMD-4b
mmo1.g-I 2.49 2.05 1.65 1.39 C C
weak L sites, Sl[Ll 01 x1
El
81.6 82.6 82.6 82.6 84.6 84.6
6.2 7.0 8.0 8.0 8.0 8.8
0.09 0.09 0.11 0.10 0.28 0.16
weaker B sites, S2[B]
stronger B sites, S,[B]
strong L sites, S4[L]
E7
07
X,
E?
b?
X?
E A
bA
X.4
99.5 99.5 103.5 103.5 105.5
8.5 8.5 8.5 8.4 8.3 9.5
0.22 0.20 0.21 0.21 0.26 0.35
115.2 114.5 115.0 117.0
7.6 8.0 8.0 7.7
0.48 0.45 0.50
138.0 137.0 140.0 142.5 129.0 127.0
10.0 10.5 10.5 10.5 8.5 8.5
0.21 0.25 0.18 0.19 0.46 0.49
102.0
0.50
a E, = most frequent activation energy of desorption; x, = fractional population of site of type S,; B = Bronsted acid; L = Lewis acid; un = width of distribution of activation energies. bAfter activation at 1075 K (dehydroxylation). CNot determined.
TABLE 111: Parameters of Acidity Distribution in Dealuminated Hydrogen Mordenites Obtained by TPD-MS of Pyridine" initial coverage, zeolite HMD-1 HMD-1 HMD-1 HMD-I HMD-2 HMD-3 HMD-4
mmo1.g-I 0.15 0.30 0.60
1.01 0.86 0.57 0.38
weak L sites, S,[L]
E, 106.5 105.5 105.5 105.6 109.6
61
9.0 8.0 8.0 8.0 11.5
B sites, S2[BIb
strong L sites, S,[L]
XI
E*
02
x2
E4
04
x4
9.5 10.5 9.7 9.0 9.5 10.5 12.5
0.32 0.47 0.49 0.30 0.35 0.40 0.22
159.0 158.8 158.6 155.9 155.9 155.9 164.9
10.0
0.12 0.30 0.25 0.20 0.28
131.5 132.8 130.7 126.5 127.5 131.5 134.5
0.68 0.54 0.39 0.40 0.40 0.40 0.50
11.5 11.8 15.5 15.5 14.0 14.0
"E, = most frequent activation energy of desorption; x, = fractional population of site of type S,; B = Bronsted acid; L = Lewis acid; u, = width of distribution of activation energies. bAssignment of these sites is discussed in section 4.3.
2. Experimental Section 2.1. Materials. A series of dealuminated hydrogen mordenites were prepared via acid leaching of the starting material, Le., hydrogen mordenite (Zeolon H), which was purchased from the Norton Co., Worchester, MA. Progressively higher degrees of dealumination were obtained by repeated treatment with 6 N HCI at 370 K. The resulting samples were characterized by AAS, standard IR methods with and without probe molecules, and MAS-NMR. Details of the IR and N M R procedures were described e l s e ~ h e r e . ' * ~ The ~ , ~ ~results of IR and N M R characterization agreed in the general trends of the effects of dealumination (see Table I). An obvious exception was the increase in A ( 0 H ) from HMD-3 to HMD-4 while the tetrahedrally coordinated AI exhibited a continuous decrease throughout the whole series of samples. This point needs further clarification. Pyridine and ammonia (99.8 vol %), which were used as adsorbates in the TPD-MS study, were purchased from Merck, Darmstadt, FRG, and Messer Griesheim, Diisseldorf, FRG; respectively. Pyridine was purified by three freeze-pumpthaw cycles prior to use and dried over activated Linde 3A molecular sieve; ammonia was used without further purification. 2.2. Apparatus. The TPD measurements were carried out in an ultrahigh-vacuum apparatus which is schematically represented by Figure 1A. The main constituents of this all-metal device were (i) a turbo molecular pump (Balzers, Model TPU 330), (ii) a recipient equipped with pressure gauges (ionization gauges and gauges for a Baratron manometer (Model 220) BHS purchased from MKS Co., Munich, FRG), the sample holder/heater, and a gas dosing system, and (iii) a quadrupole mass spectrometer (Model QMG 311, Balzers, Nordenstadt, FRG), with a mass discriminator (Model PP 200, Leyboldt-Heraeus, Koln, FRG). The stainless steel sample holder/heater and feed throughs for
the heaters and thermocouple are schematically described by Figure 1B. It is worthwhile to mention that no soldering was used in order to avoid any contamination; all necessary joints were made by welding. 2.3. Procedure. The zeolite samples were used in the form of thin platelets with a thickness of about 0.1 mm or 10 mg.cm-z; a mass of 10 mg was employed. Prior to adsorption of the probe molecules the samples were usually pretreated of 2 h at 675 K under lod Pa; exceptions are indicated in the text. The mass spectra after this pretreatment indicated sufficient degassing of the samples; e.g., the partial pressure of HzO was below IO4 Pa. Ammonia or pyridine was then admitted from reservoirs of known volume into the calibrated chamber. The amounts of ammonia or pyridine adsorbed by the pretreated sample were calculated from the observed pressure drop. The pressures before and after admission from the calibrated volume to the chamber were monitored by a Baratron manometer. The coverage of the samples with ammonia or pyridine (after contact with ammonia at 375 K or pyridine at 475 K and prior to the start of the TPD) was determined as follows: the excess of ammonia and pyridine was removed at 375 and 475 K, respectively, trapped into the cooling finger (16), evaporated with the valves (10) closed, and determined from the pressure increase in the calibrated volume (1 5). Thus, the determined excess was subtracted from the initial coverage giving the loading of the sample prior to the onset of the TPD. The respective data (see Tables I1 and 111) were very reproducible. In another type of experiment the coverage was not really measured but characterized by the stationary pressure of the adsorbate established prior to the desorption experiment and at the starting temperature, by means of a leaking valve. In this way especially low loadings were obtained, e.g., under dynamic pressures of 1 X to 9 X Pa of NH3. The.temperature-programmed desorption was usually carried out with a linear heating rate of 20 Kmin-I. In preliminary experiments, this rate has been found optimum with respect t o resolution of the TPD spectra. The desorbing species were continuously monitored by the mass spectrometer, and the corresponding signal intensities were recorded by a multiple-channel recorder (Model 316, W + W Elektronik AG, Basel, Switzerland). In recording the TPD spectra, masses originating from the parent molecule, e.g., m / e = 79, pyridine, or from fragmentation, m/e = 52 (C4H4from CsHsN) and m/e = 16 (NH2 from NH,), were monitored.
(45) Karge, H. G. 2.Phys. Chem. (Munich) 1980, 122, 103. (46) Beyer, H. K.; BorKly, G.; Karge, H. G.Manuscript in preparation.
3. Theory and Evaluation Parallel and independent desorption processes are assumed4'
Therefore, in the present study a series of dealuminated mordenites prepared from the same starting material were used. Two different basic probe molecules, viz., ammonia and pyridine, were employed, either as the sole adsorbate or after successive adsorption using various coverages. It was the aim of the work to investigate whether the acidity of the zeolites under study could be sufficiently characterized and related to their catalytic behavior when suitable methods of evaluation of the TPD spectra were employed.
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The Journal of Physical Chemistry, Yol. 94, No. 2, I990
Karge and Dondur
where 8 is the coverage, t the time, n the number of the type of site (S,) from which the desorption occurs, and x, the fraction of the total population that is due to coverage of sites of type S,, with Ex, = 1 n
from a through e
curve f
(2)
A
For the rate of desorption a first-order relationship with respect to coverage was adopted; it was confirmed by isothermal measurements of the desorption
-($I
= k,6, n
(3)
The rate constant k , is related to the molar energy of desorption, E,, through the Arrhenius equation
k , = A,, exp(-E,/RT)
increasing initial coverages,
dehydroxylated hydrogen mordenite
(4)
When the distribution of the desorption energies, P ( E ) , is introduced, the rate of desorption from sites of type S, is finally described by (see ref 48) I I
In order to simplify the evaluation of the experimental desorption spectra, two assumptions were made: (i) the preexponential factor is constant, and (ii) the desorption from different kinds of sites, S,, occurs through a nonassociative mechanism. The following distribution functions for the probability density P(E,) were tried in eq 1 and 5: Dirac distribution function 6(E-E’) =
I
1 ifE=E’ 0 i f E f E’
where E‘ is the constant activation energy of desorption. Gauss distribution function I
P ( E , ) = -exp[-(E, - E,)*/2un2] (2aff,)1/2
(7)
where E , stands for the most probable energy of desorption and c,, for the variance. Weibull distribution function P(EA = 2a,(E, - b n ) exP[-an(En - bn)*I
(8)
with a, and b, being parameter^.^^ The experimentally obtained TPD spectra could be well fitted by all of the three distribution functions; the mean standard deviations were between 1% and 3%. However, application of the Dirac &function (eq 6) required an unreasonably low preexponential factor A, and hence, the activation energies of desorption obtained with the Dirac function did not agree with the energies evaluated with the other two functions. In the case of pyridine ~-~’ of the TPD spectra desorption from H-Y z e ~ l i t e , ~evaluation resulted in differential heats of desorption that were close to calorimetrically determined valuesSoonly when eq 7 or 8 was used. The Dirac function was therefore abandoned. The results presented in the next section were uniformly obtained after introduction of the Gauss distribution function, eq 7. The distribution P(E,) of the activation energies for the desorption from sites of type S, originates from a corresponding heterogeneity of sites S,. Preexponential factors ANH3= 0.15 X IO8 min-l (for N H 3 desorption) and Ab = 0.15 X IO8 min-’ (for pyridine desorption) (47) (48) (49) (50) (51)
Dondur, V.; RakiE, V.; Karge, H. G. Manuscript in preparation. Dondur, V.; Fidler, D. Surf. Sci. 1980, 150, 480. Weibull, W. J . Appl. Mech. 1951, 18, 293. Karge, H. G.; Dondur, V . Part 11, manuscript in preparation. Dondur, V.; Karge, H. G . Surf. Sci. 1987, 189/190, 873.
I
I
100
600
I
I
800
T E M P E R A T U R E [K] Figure 2. TPD spectra of NH3 desorption from dealuminated hydrogen mordenite, HMD-I, with increasing initial coverages (a-e) due to different exposure: (a) 1 X IO-’ Pa, 15 min; (b) 9 X Pa, 15 min; (e) 9 X lo-’ Pa, 30 min; (d) 9 X lo-’ Pa, 60 min; (e, f) static initial pressure 5 X I O 2 Pa; pretreatment at 675 K with the exception of (f), where T,, = 1025 K.
were derived from isothermal desorption experiments. All the computations necessary to evaluate the data with the help of eq I , 5, and 7 were carried out on a computer (Model Cyber 180, Control Data), using the NAG-BASS library and fitting programs based on the analytical Jacobian. The numerical method for deconvolution of the experimental TPD curves minimizes the nonlinear sum of the squares of deviations; Le., the function of standard deviations for a given set of parameters is analyzed and the numerical minimum of the residue obtained. This method is precise and reliably provides a unique deconvolution of the experimental curves. 4. Results and Discussion 4.1. TPD Spectra of Ammonia Desorption. In preliminary experiments, viz., without removal of excess adsorbate at 375 K,
a low-temperature peak appeared with a maximum temperature T, of about 385 K. This was ascribed to weakly physisorbed NH,. Due to the procedure employed in the subsequent experiments (see Experimental Section), this low-temperature peak was no longer recorded. Therefore, the following results and discussions refer to TPD peaks in the range from about 475 to 875 K, originating from chemisorbed probe molecules. Figure 2 presents a set of TPD spectra recorded during desorption of NH, from HMD- l . The spectra were better resolved and exhibited two separated peaks at about 620 and 8 15 K, when the samples had been contacted with N H 3 under low dynamic and 9 X pressures (1 X Pa) for only 15 min prior to the onset of desorption (spectra a and b in Figure 2). With increasing coverages (Figure 2, spectra c-e) the peaks showed considerable overlapping and only one main peak with T, = 675 K seemed to remain. However, this peak was not symmetric and possessed at least two shoulders, one around 520 K and the other around 825 K . Therefore, it appeared that ammonia was released from
Acidity of Dealuminated Mordenites
The Journal of Physical Chemistry, Vol. 94, NO. 2, 1990 769
computed curve
4-
- 3-
n HMD-1
-3a 2-. W
+ a
a
i-
00 F 52
o
:
b
computed curve
4-
400 500 T E M P E R A T U R E [K]
Figure 3. TPD spectra of NH, desorption from dealuminated hydrogen mordenites, HMD-I through HMD-4; initial exposure to 5 X IO2 Pa of ammonia.
at least three different types of sites with significantly different activation energies of desorption. When the adsorbent (HMD-1) was pretreated at 1075 K instead of 675 K with all the other steps of the procedure remaining unchanged, the resulting TPD spectrum was significantly altered (Figure 2, curve f). The peak at 675 K was markedly weakened, and instead, partially resolved peaks appeared at about 500 and 775 K. Pretreatment at 1075 K almost completely dehydroxylated the hydrogen form of the mordenite? Le., removed acid Bransted sites, and formed Lewis type sites instead. Therefore, the peak around 675 K was ascribed to one kind of Brmsted sites or even to two (compare Figure 2, spectrum d) whereas the peaks at 500 and 775 K were assigned to weak and strong Lewis acid sites, essentially in agreement with earlier results from combined IR/ MS-TPD measurements by Karge et The TPD spectra of the more dealuminated samples looked very similar to that observed with HMD-1. These are shown in Figure 3 where the spectrum of TPD of NH, from HMD-1 is repeated to facilitate the comparison. However, deconvolution of the spectra with use of the evaluation method outlined in section 3 provided further information and revealed significant changes in acidity, i.e., number, population, and strength of the various types of sites, as a function of dealumination. In Figure 4 two examples of spectrum analysis are shown, viz., for desorption of NH, from HMD-1 and HMD-4, the least and most dealuminated samples, respectively. In order to make the comparison easier, the areas under the experimental desorption spectra (curves with filled circles) were normalized. The experimental curves could be fitted only when independent N H 3 desorption from four different types of sites, SI through S4, was assumed. Under this assumption, however, a very good fit was obtained. On the basis of the above discussion, S I and S4 were ascribed to weak and strong Lewis sites, respectively, which must be associated with remarkably differing desorption energies. The peaks S2and S, were ascribed to two types of Bransted sites whose desorption energies for ammonia are more similar than is the case of the Lewis sites. These peaks were not resolved in our earlier Inspection of Figure 4 reveals that the maximum temperatures of the peaks assigned t o desorption from Bransted sites shift to higher values (580-620 and 675-690 K) when the Si/AI ratio
600
700
800
900
T E M P E R A T U R E [K]
Figure 4. Evaluation of the TPD spectra of N H , desorption from dealuminated hydrogen mordenites, HMD-1 and HMD-4, after initial exposure to 5 X lo2 Pa at 375 K: experimental data (e);calculated curves for desorption from different types of sites, see text (---); theoretical curve of overall desorption (-).
is increased from 12 (HMD-1) to 39 (HMD-4). More detailed information is obtained from Table I1 where the mean activation energies for desorption, the width of the energy distribution, and the fractional population of the various sites are listed. In this table the results obtained with samples HMD-2 and HMD-3 are included. From the evaluation of the TPD spectra, all of the four HMD samples are shown to possess the four types of sites present in HMD-1 and HMD-4. Moreover, the data for the samples pretreated at 1075 K show that one type of site, viz., S3, was completely removed and the fractional density of two others, viz., SI and S4, considerably enhanced (see fifth and sixth lines of Table 11). These findings confirm the former assignments of S3 to Bransted sites and S, and S4 to Lewis-type sites. The initial overall coverages (seeSection 2.3) are shown in Table I1 to decrease from HMD-1 through HMD-4. This behavior is expected for the Bransted sites in the case of decreasing aluminum content of the zeolite framework, since one tetrahedrally bonded AI corresponds to one acidic O H group. In fact, the fractional population of SI sites is essentially constant and that of the S4 sites first increases and then, i.e., from HMD-2 through HMD-4, decreases. Multiplication with the respective coverage, however, results for both cases in a decrease, Le., in a decrease of the absolute number of weak and strong L sites indicated by desorbed N H 3 molecules. Corresponding to the reverse behavior of the strong Lewis sites of type S4, the fractional occupancy of the Bransted centers of types S2and S3first decreases and then, from HMD-2 to HMD-4, increases. The widths of the acidity distributions, u1 through u4,do not significantly change with change of the Si/AI ratio. Some information about the strength of the various acidic sites as a function of dealumination is provided through the evaluation of the most frequent energies of desorption of ammonia, E, through E,. These energies were altered by dealumination, but in a different way for the various kinds of sites. From HMD-1 to HMD-2, the most frequent energy of desorption from weak Lewis sites, E,, slightly increases, similar to the corresponding parameter for the weaker Brernsted centers, E,. Both the energies of desorption from strong Bransted and Lewis sites, E , and E4, respectively, appear somewhat lower for HMD-2 than for HMD-I. In fact, the differences between sites of HMD-I and the corre-
770
Karge and Dondur
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 I ~
41
1
ru
2 500
700
900 TEMPERATURE
from a through d tncrearing initial coverages
1100
[Kl
Figure 5. TPD spectra of pyridine desorption from dealuminated hydrogen mordenite; HMD- 1, with increasing initial coverages, in mmol-g-I: (a) 0.15; (b) 0.30; (c) 0.60; (d) 1.01; (e) exposure to 5 X IO2 Pa; pretreatment at 675 K under high vacuum, with the exception of (e), where T,,, = 875 K.
HMD - 1 : SiiAl = 12
'\
,'
0
experimental data :
-j.*...*......
4t
2
computed curve /
\
sponding ones of HMD-2 might be within or close to the limit of experimental error. However, in the sequence of HMD-2, HMD-3, and HMD-4 3 a substantial increase in the most frequent energies of desorption is observed for strong Lewis centers, E4, and similarly for the stronger Bransted sites, E3. The relevant result is that the strength of the Bransted sites )I, 2 1 considerably increases a t higher degrees of dealumination and, , /' ', 0 'I ,x, '\ simultaneously, the fractional density of these centers remains high, viz., x2 = 0.21 and x3 = 0.50. Hence, it seems possible that 500 700 900 1100 the overall loss of acidic sites due to dealumination is compensated or even overcompensated as far as Bransted acidity is involved. T E M P E R A T U R E [K] This view is supported by more recent measurements of the activity Figure 6. Evaluation of the TPD spectra of pyridine desorption from of the same samples in Bransted acid catalyzed reactions such dealuminated hydrogen mordenite; HMD-1: experimental data ( 0 ) ; as ethylbenzene or ethylene conversion and coke f o r m a t i ~ n . ~ * , ~ ~calculated curves for desorption from different types of sites, see text (---); theoretical curve of overall desorption (-). Initial coverages in Here, the initial decrease in activity was followed by a reenmmo1.g': (a) 0.15, (b) 0.30, (c) 0.60, (d) 1.01. hancement of the reaction rate when the samples HMD-3 and HMD-4 were employed as catalysts. This seems to be at some In Figure 6 the set of TPD spectra a-d of Figure 5 are revariance with predictions derived when the Sanderson theory of produced after normalization to the same peak area of desorbed intermediate electronegativity was applied to acidic zeolites. pyridine; i.e., the peak area below the experimental TPD contours Accordingly, the acidic strength of hydrogen forms of zeolites is equalized for all of the four loadings. In addition, the single should not increase any longer when a ratio of Si/AI = 10 is peaks into which the experimental spectrum was deconvoluted reached.54 However, this presumes that no cationic aluminum (according to section 3) as well as their superposition on the species reside on extralattice sites of the hydrogen form. But the "theoretical curves" are incorporated in Figure 6. Even though IR and NMR results of Table I provide evidence that all of the the experimental curve of Figure 6b appears to be symmetrical, samples under study (HMD-I through HMD-4) contain signifit is in fact not, and the deconvolution results in two different single icant amounts of extralattice aluminum, viz., intrinsic true Lewis peaks. This set of spectra clearly shows that at lower coverages sites. only two types of sites were involved (see also Table 111, first and 4.2. TPD Spectra of Pyridine Desorption. Similar to the second line) whereas at higher coverages up to saturation defindings with ammonia, preliminary experiments with pyridine sorption from a third type of site (low-temperature peak) occurred without removal of excess adsorbate (at 475 K) showed that as well. Furthermore, it is evident from the deconvoluted spectra pyridine can be physisorbed at low temperatures. The physithat the relative occupancy of the two types of sites, indicated by sorption of pyridine gave rise to a sharp peak at 430-450 K. Due the peaks at higher temperatures, changed as a function of preto the generally employed procedure (see section 2.3), however, loading. The relevant data of this set of TPD spectra (Figure 6, this peak did not appear in the TPD spectra. Therefore, in what curves a-d) and the corresponding parameters of similar spectra follows the description of the results as well as the discussion is for samples HMD-2 to HMD-4, which were obtained after full restricted to the high-temperature pyridine peaks due to desorption coverage with chemisorbed pyridine, are listed in Table 111. of chemisorbed molecules. Pretreatment of HMD-I at 875 K, which resulted in partial TPD spectra of pyridine obtained during desorption from dehydroxylation, considerably weakened the center of the broad HMD- 1 samples with progressively higher preloading are plotted peak; instead, two resolved maxima at 670 and 875 K appeared in Figure 5 (spectra a-d). When lower coverages prior to de(see Figure 5, spectrum e). Consequently, peaks around these sorption were employed, two distinct peaks were observed. At temperatures obtained by temperature-programmed pyridine the highest preloading, Le., with full coverage, a very broad peak desorption were ascribed to Lewis-type sites whereas contributions resulted. However, the contour of this peak (one maximum at to the center of the peak at 775 K in spectrum a of Figure 5 or 670 K and two shoulders around 775 and 925 K) still suggested contributions to the broad peaks at higher coverages (spectra W) the presence of more than one type of site from which pyridine must be due to pyridine desorption from Bransted sites. desorption occurred. In Figure 7 the probability distributions of the activation energies for desorption of pyridine from the various sites are plotted as they were derived according to section 2.3 from the desorption ( 5 2 ) Karge, H. G.; Boldingh, E. P. Catal. Today 1988, 3, 379. spectra. From the figure it is evident that the most frequent (53) Karge, H. G.; Boldingh, E. P. Catal. Today 1988, 3, 5 3 . energies of desorption from Br~nstedcenters (middle peak) shift (54) Mortier, W. J . Catal. 1978, 55, 138.
.
The Journal of Physical Chemistry, Vol. 94, No. 2, I990 771
Acidity of Dealuminated Mordenites
three Werent desorption Utes
a Y
0.0 0.016 0.012
2
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F
0.004 0.0
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-
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t
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900
700
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three different
,'
0.0
60
100
'
.:\,
140
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TEMPERATURE
desorption sites
.)
180
220
260
E [KJIMOL] Figure 7. Calculated distributions of the activation energies of desorption of pyridine from various sites of dealuminated hydrogen mordenites, HMD-1 through HMD-4.
to higher values when the degree of dealumination increases, viz., from about 126 kJ.mo1-' for HMD-1 to about 135 kEmol-' for HMD-4. These data, along with those for the Lewis type sites, are also incorporated into Table 111. The data of Table 111 show for HMD-1 that, with the nonsignificant exception of the smallest coverage of sites S2,the most frequent energy of desorption decreases with increasing amounts of initially chemisorbed pyridine. The effect of dealumination is, therefore, evaluated with respect to those results obtained when the temperature-programmed desorption was started after full coverage of the various samples with chemisorbed pyridine (see Table I). Here, an interesting feature is the continuous increase of the most frequent energy of pyridine desorption from Bransted sites of type S2,whereas the desorption energies for Lewis sites, SIand S4, are constant in the case of HMD-1, HMD-2, and HMD-3 and are enhanced only for HMD-4. Also, there is a continuous increase of the width of the acidity distribution with Bransted but not with Lewis sites, except for the weak ones of HMD-4. Thus, it is only in the case of the Bransted sites that the acidity strength becomes more pronounced with progressive dealumination. This effect is certainly not dramatic, but it is significant and might overcompensate for the effect of decreasing number of acid sites. In this respect the results obtained with pyridine corroborate the findings with ammonia. There is, however, an interesting difference between the TPD results obtained with ammonia and with pyridine. In the latter case only one type of Bransted site is indicated; the second type, called S3(Bransted sites) in the TPD spectra of ammonia, seemed to be missing. The question arises whether this is due to a poorer resolution, since the width of distribution is broader with pyridine than with ammonia, or to a lower accessibility of S3sites for the more bulky pyridine molecule. Experiments with sequential pyridine and ammonia adsorption and subsequent TPD were carried out to elucidate this point. 4.3. TPD Spectra of Ammonia and Pyridine after Successive Adsorption of Both Probe Molecules. Both thermodesorption maxima of ammonia ( m / e = 16) and pyridine ( m / e = 79) appeared in the TPD spectra after successive adsorption of pyridine and NH3, as is demonstrated by Figure 8. Pyridine was
[K]
Figure 8. TPD spectra of desorption of ammonia ( m / e = 16) and pyridine ( m / e = 79) after successive adsorption of (i) pyridine and (ii) ammonia (for details see text). I
I
500
600
I
400
I
I
I
I
700
800
900
lo00
T E M P E R A T U R E IK]
Figure 9. Evaluation of TPD spectra obtained after successive adsorption of (i) pyridine and (ii) ammonia (for details see text): calculated curves for desorption of -) ammonia ( m / e = 16) and (- - -) pyridine ( m / e = 79); overall desorption curves (-). (e
-
preadsorbed at 475 K under 0.5 kPa for 1 h; the excess was pumped off. Subsequently, the sample was exposed to N H 3 (0.5 kPa) at 375 K followed by 1-h degassing at the same temperature prior to starting the temperature-programmed desorption. The total density of acidic sites, estimated via the sum of the ammonia and pyridine peak areas, obviously decreased from HMD-1 through HMD-4 as a result of dealumination. In the same sequence, the maximum of the overall TPD contour for NH3 desorption shifted from higher to lower temperatures (665,655, 645, and 635 K for HMD-1, HMD-2, HMD-3, and HMD-4, respectively), whereas the fraction of acidic sites occupied by N H 3 was continuously diminished from HMD- 1 through HMD-3 but then inclined again. If there are sites that are inaccessible for pyridine but accessible for ammonia, these findings show that their contribution to the acidity of the mordenite samples would change with the degree of dealumination. The complex TPD spectra observed after sequential adsorption and desorption of pyridine and ammonia could be deconvoluted in the same manner as for single adsorption (see sections 4.1 and 4.2). One example is provided by Figure 9. Here, a relatively low degree of preloading with pyridine, viz., 0.30 mmo1.g-' zeolite, was employed corresponding to about 45% and 75% coverage of
172
Karge and Dondur
The Journal of Physical Chemistry, Vol. 94, No. 2, 1990 1 .o
kn\ I
I
I
I
I
S,IB1
,S,“
I81
1
P R E ADSORBED P Y R I D I N E [ m m o l . g ’ l
Figure 10. Effect of pyridine preadsorption on NH, coverage of the various sites, S,through S4,by subsequent adsorption of ammonia: sites S I ,weak Lewis sites (0); sites S1,weaker Bronsted sites (m); sites S,, stronger Bronsted sites (0); sites S,, strong Lewis sites ( 0 ) .
S2[B] and S,[L], respectively. The evaluation of the TPD spectrum gave similar results, as in the case of simple ammonia or pyridine experiments, in that four types of sites were indicated by the former probe molecule (compare Figure 4) and only two types by the latter (compare Figure 6b). However, systematic variation of precoverage showed that the occupancy of sites of types SI through S4by subsequently adsorbed NH, was significantly affected as the amount of preadsorbed pyridine increased. This is demonstrated by Figure 10 for the case of HMD-1. Hence, strong Lewis centers of type S4 and Bransted sites of type S2 were probed by N H 3 to a decreasing extent when the amount of preadsorbed pyridine increased. On a surface of HMD-I fully covered with chemisorbed pyridine, adsorption of NH, on sites of type S2 or S, no longer occured. Similarly, adsorption of NH, on sites of type S, (weak Lewis centers) was considerably hindered by preadsorbed pyridine. The same observation was made when the NH, pressure and the contact time of NH, with the pyridine-covered sample were increased. Obviously, ammonia was not able to replace pyridine under the prevailing experimental conditions. Compared to the situation with sites of types SI, Sz, and S4, indication of sites of type S3 by TPD of NH, was much less affected by preadsorbed pyridine. If therefore appears that a larger fraction of sites of this type are located in such parts of the mordenite structure where they are easily approached by NH, molecules but difficult to reach by the bulky pyridine. Because of the different response of S2[B] and S3[B] sites to preadsorption of pyridine (Figure 10) the labeling of sites, viz., S,, S2,and S4, was chosen as in Table 111, where the results of TPD of pyridine were discussed. Despite this designation, however, it is not ruled out that a fraction of S,[B] type sites indicated by ammonia (Table 11) contributes to the number of Bransted sites indicated by pyridine (Table 111). Note that u2 in Table I11 is relatively large and increases considerably from HMD- 1 through HMD-4. Concomitantly, the measure of the site strength, E2, increases. On the basis of the present study it cannot be completely ruled out that diffusional effects influence the experimental results. However, the parameters of the TPD spectra of NH, remained essentially unchanged in the presence of the bulky pyridine molecules, which were in general more strongly bonded and desorbed only at higher temperatures than N H 3 (see Figure 8). If diffusion limitations were operative, the presence of the large pyridine molecules in the channels of the mordenite structure after partial pyridine coverage of S2[B] and S,[L] sites should affect much more strongly the rate constant of NH, desorption. But, the most frequent energies of desorption of NH, from S1,S2,S,, and S, were 80.0, 99.0, 116.8, and 137.0 kJ.mo1-l in the case of
a sample HMD-I preloaded with 0.30 mmo1.g-I pyridine, i.e., close to the values obtained with a pyridine-free sample (compare Table 11). Also, the preexponential factor A (see eq 4) remained unchanged. Furthermore, the results did not depend on the heating rate of the temperature program. Finally, the cr, values were only slightly affected. The same was true for the parameters of concomitant pyridine desorption where, for example, E2 = 132.8 and E4 = 159.0 kJ.mo1-I were found which compare with 132.8 and 158.8 kJ-mol-‘ in the case of sole pyridine desorption (compare Table 111). All these findings suggest that the results obtained in the present investigation was not seriously affected by diffusion phenomena. 5. Conclusions (i) Spectra of temperature-programmed desorption (TPD) of ammonia and pyridine, desorbing from dealuminated hydrogen mordenites, can be successfully deconvoluted and evaluated on the assumption of parallel and independent desorption processes of first order in coverage and by application of a Gaussian distribution of activation energies. (ii) TPD spectra of ammonia reveal the presence of four distinguishable acidic sites in hydrogen mordenite, viz., two Lewis-type (S,[L], S,[L]) and two Bransted-type sites (S2[B],S,[B]). (iii) Disregarding the sample with the lowest degree of dealumination, the most frequent activation energies for NH, desorption from the two Bransted sites and of the strong Lewis sites increase with increasing degree of dealumination; the most frequent activation energies for the weaker Lewis-type sites S,[L] do not depend on the degree of dealumination. (iv) Evaluation of the TPD spectra of pyridine leads to the conclusion that pyridine as a probe molecule is able to indicate only three distinct types of sites, viz., a weaker (S,[L]) and a stronger (S,[L]) Lewis-type site and only one Bransted-type (S2[B]). A second Bronsted-type site with higher energy of activation, Le., (S3[B]) in the case of ammonia spectra, is not discriminated by pyridine. (v) Analysis of TPD spectra of ammonia obtained from samples that were preloaded with pyridine suggests that at least a larger fraction of the sites of the second Bransted type (S,[B]) is difficult to reach by the more bulky pyridine molecules and is therefore not indicated by the latter probe. (vi) Similar to the results obtained with ammonia, the most frequent energy of activation for desorption of pyridine from Brmsted sites of type S2[B] (initially fully covered with chemisorbed pyridine) continuously increases with the degree of dealumination. The corresponding energies for the Lewis-type sites, S1[L] and S,[L], are constant with the exception of maximum dealumination. (vii) The results obtained through TPD both of ammonia and pyridine from dealuminated hydrogen mordenites with Si/A1 ratios from 12 to 39 suggest that the influence of the strength of Bransted acidity becomes more significant with progressive dealumination. This effect might even overcompensate for the decrease in the number of sites at higher Si/A1 ratios and would explain the catalytic behavior of the same hydrogen mordenites in some Brmsted acid catalyzed reactions.
Acknowledgment. The authors are indebted to Professor Fetting (Technische Hochschule Darmstadt) for providing the dealuminated mordenite samples and their chemical analysis. They thank Dr. Hermann K. Beyer (Central Research Institute for Chemistry of the Hungarian Academy of Science) for characterization of the samples via MAS-NMR spectroscopy. Thanks are due to Mr. Walter Wachsmann (FHI) and Mrs. Erika PopoviE (FHI) for excellent assistance throughout the experimental work. Financial support of the Bundesminister fur Forschung und Technologie (Project No. 03C2311) is gratefully acknowledged. Registry No. Py, 110-86-1; NH,, 7664-41-7.
