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Cracking of 1,3,5-triisopropylbenzene over deeply dealuminated Y zeolite
Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.
417
Cracking of 1 , 3 , 5 - triisopropylbenzene over deeply dealuminated Y zeolite E.Falabella S-Aguiar 1'2, M.L.Murta-Valle 1, E.V. Sobrinho 3, D. Cardoso 3. Petrobr/ls / CENPES - Ilha do Fund~o, Q7, 21949-900, Rio de Janeiro, Brazil. Fax: 55-21-5986626 2Escola de Quimica/UFRJ, Rio de Janeiro, Brazil, Tel: 55-21-5903192. 3DEQ / UFSCar, S~o Carlos, Brazil, Fax: 55-162-748266.
ABSTRACT Cracking of a rather volmninous molecule, 1,3,5-triisopropylbenzene, was carried out over several zeolites with different degrees of dealumination. Very crystalline zeolites were prepared via combined steam/acid leaching treatments, being afterwards characterized by various techniques. IR-OH region showed that highly condensed EFAL, rather than non-condensed EFAL, was removed during the acid leaching. Nevertheless, MAS/27A1 NMR clearly demonstrated that EFAL-free zeolites are never obtained, regardless of the pH of the acid leaching step. Mesopores surface areas, determined by t-plot increased with increasing number of treatments, as well as the strength of acid sites. Initial rate of cracking of 1,3,5-triisopropylbenzene was plotted against the number of A1 atoms per unit cell, a maximum being obtained for 11 A1/u.c.. Since this molecule has a kinetic diameter larger than 8.0 A, it will not penetrate the zeolitic micropores. After dealumination, mesopores are generated and the reactant is allowed to diffuse. Therefore, both accessibility and acidity seem to control the rate of reaction.
1. INTRODUCTION During the last three decades, extensive work has been carried out aiming at the correlation between the acidity of several dealuminated zeolites and their catalytic activity in the cracking of hydrocarbons. Various molecules such as 2,3dimethylbutane [1], n-pentane [2], n-hexane [3], n-heptane [4], isooctane [5], decalin [6] and cumene [7] have been used in model reactions. Nevertheless, the use of heavier feedstocks in FCC processes has greatly increased the interest for the cracking chemistry of more voluminous molecules. However, not much has been
418 published regarding model reactions with bulky molecules. Previous studies developed in our group [8] have emphasized the importance of external surface area of small crystallite zeolites in the cracking of molecules with kinetic diameter larger than 8.0A such as 1,3,5-triisopropylbenzene (TIPB), showing that acid strength and site density are not the only parameters controlling activity and selectivity during catalytic cracking. The aim of the present work is to demonstrate that the presence of mesopores generated by acid leaching may also play an important role in the cracking of the same molecule (TIPB).
2. EXPERIMENTAL Parent NaY (Si/AI=2.8) zeolite was ion-exchanged in a 11 wt% NH4C1 solution, at 70~ After the ion-exchange, steaming treatments took place in a cylindrical reactor containing 60 g of the zeolite, with 20 g/h flow rate of 100% saturated steam (200~ at 650~ for 90 minutes. Acid leachings were carried out at 70~ for 30 minutes, with pH between 1.0 and 2.5, generating DAZ samples. Treatments were repeated three times and the sodium values were reduced to values below 0.1% Na20. X-ray diffraction (XRD) took place in a Phillips PW1729 diffractometer with CuKo~ radiation and the relative crystallinity of the samples was estimated from the integrated areas of the peaks with Miller indexes 220, 311, 33 l, 333, 440, 533, 642, 660, 751 and 664. Chemical composition was determined by X-ray fluorescence (XRF) in a Phillips PW1407 spectrometer with CrKct radiation. X-ray photoelectron spectroscopy (XPS) was carried out in a VG-Scientific ESCALAB MK II spectrometer with MgKtx radiation, using bands of Al(2p), Si(2p) and Na(ls) to determine surface chemical composition. MAS/NMR spectra of 29Si and 27A1 were obtained on a Varian VXR 300 spectrometer working under a 7.05T magnetic field, being 27A1 spectra collected after impregnation with acetylacetone. Infrared measurements were done on a FTIR Nicolet 60 SXR spectrometer and the OH region (3400-3800 cm -1) was observed. Nitrogen adsorption isotherms were determined using a Micromeritics ASAP2400. From the isotherms, BET surface area, micropores volume (t-plot) and volume of mesopores (BJH) were calculated. Cracking of 1,3,5-triisopropylbenzene (FLUKA 92075) was carried out in a differential fixed-bed gas phase plug flow reactor, with a 15 mm bed containing 5 wt% zeolite diluted in kaolin. Reaction products were analyzed by means of GC-MS (HP5890 and HP5970-B), column HP-PONA (50 m, 0.2 mm, 0.5 pro) for liquid phase and A1203/KC1 column (50 m, 0.3 mm, 0.5 ~rn) for the gas phase.
3. RESULTS AND DISCUSSION Table 1 presents the main characteristics of the parent NaY zeolite and the dealuminated samples prepared thereof. It is clear that dealumination generated very crystalline samples, as a good indication that the process took place without extensive
419 destruction of the framework. It is also evident that progressive dealumination was performed along the cycles, since A o values decreased considerably whereas Si/A1 ratio in the framework (measured by MAS/29Si NMR) increased accordingly. The comparison between Si/A1 ratios obtained by XRF and XPS indicates that aluminum surface contents (XPS) in the samples obtained by acid leaching are lower than the values obtained by XRF and similar to those obtained by 29Si NMR (framework aluminum contents). This reveals a preferential outer surface leaching of the extra framework aluminum by acid treatment [9-11].
Table 1 Main characteristics of the NaY zeolite and dealuminated samples Sample XRD XRF XPS
29Si NMR
Cryst.(%)
Ao(A )
Si/Alg
Si/Als
Si/Alf
NaY
100
24.63
2.8
3.0
2.7
DAZ 1
110
24.50
3.8
4.0
5.6
DAZ 2
119
24.34
11.2
15.0
16.2
DAZ 3
126
24.29
27.8
35.0
33.5
*g=global, s=superficial, f=framework
Results of textural properties of the samples are depicted in table 2, as well as 27A1 NMR relative intensities of aluminum peaks after acetylacetone impregnation. Besides the increasing values of mesopore volume with the increasing dealumination conditions, one can also verify that micropore volume is growing, being larger than the value encountered for the NaY zeolite. This fact indicates that in addition to the characteristic micropores of the parent zeolite, dealuminated samples present supermicropores with diameter between 10 and 20A [9]. The 27A1 NMR spectra display two distinct peaks for tetrahedral (framework) and octahedral (non-framework) aluminum [9-12]. As showed in figure 1, all samples contain extraframework aluminum species (EFAL), regardless of the acid leaching step. Although acid leaching reduces EFAL contents, results of the table 2 indicate that dealuminated samples contain around 30% of EFAL, as an evidence that diffusional limitations in the removal of aluminum species are taking place. In fact, infrared spectra in the OH region after acid leaching revealed the disappearance of the band at 3690 cm l , which is ascribed to highly condensed EFAL located in the larger cavities [ 13]. However, the band at 3600 cm 1, related to low
420
Table 2 Textural properties and 27A1 NMR relative intensifies of the NaY zeolite and dealuminated samples Sample 27A1NMR MiPV a
MePV b
% tetrahedral
%octahedral
NaY
0.326
0.031
100
DAZ 1
0.326
0.092
68.5
31.5
DAZ 2
0.358
0.191
70.7
29.3
DAZ 3
0.360
0.188
64.8
35.2
a MiPV = volume of micropores (cm3/g) b MePV = volume ofmesopores (cm3/g)
DAZ 1
DAZ 2
DAZ 3
O
100
0
100
100
0
-100
100
0
-100
--9 p.p.m. Figure 1. MAS/27A1 NMR spectra for DAZ samples. (T) Tetrahedral, (O) Octahedral
condensation non-framework ahanina located at smaller cavities, remained almost intact. This confmn our assumption that non-accessible EFAL is hardly removed by acid treatment. Rates of 1,3,5-triisopropylbenzene cracking were calculated assuming differential behavior. Plots of rate of TIPB disappearance against time-on-stream were fit to the classical Voohries equation (r = r o t -n) [ 14], allowing one to estimate initial rates of reaction. Initial rates of TIPB disappearance were then plotted against the number of aluminum atoms per unit cell (A1/u.c.), according to NMR results. Such graph is
421 presented in figure 2, for three different temperatures (400, 425 e 450~ For all temperatures, a maximum is achieved for about 11 A1/u.c.. These results contradict previous published data which found a maximum for 30 A1/u.c. in the cracking of, nhexane [3],for instance. Although extensive published literature has often encountered a maximum of activity between 30 - 40 A1/u.c., it must be borne in mind that all model reactions employed in previous studies used small molecules such as linear paraffins (hexane, heptane) or light aromatics (cumene). Explanations for maximum occurrence were based upon changes in the strength, type and concentration of acid sites [3,9,15,16]. However, when voluminous molecules are used, the effect of the formation of mesopores (and thus changes in the external surface area of the zeolites) must not be disregarded. The increase in surface area would certainly be important for diffusion limited reactions, which is apparently the case of the cracking of TIPB. Thus, DAZ 2 and DAZ 3 zeolites possess similar values of micro and mesopore volumes, which, in turn, are higher than the values obtained for DAZ 1 (table 2). Nevertheless, DAZ 2 is more active than DAZ 3 because of the higher concentration of acid sites thereof. This fact confLrms that in the cracking of bulky molecules, both acidity and diffusion are playing an important role in the rate determining step.
3.60
3.20
2.80
~
2.40 -
2.00
1.60
~ 6
l 10
,
f ~ l 18 20 AI/u.c. (NMR)
~
1
l
28
30
Figure 2. Initial cracking rates of 1,3,5-TIPB as a function of A1/u.c. (NMR). (a) 400~ (o) 425~ and ( . ) 450~
422 4. CONCLUSIONS Cracking of TIPB was studied over several acid leached steam treated zeolites and initial rates of disappearance were plotted against the number of A1/u.c.. For three different temperatures, a maximum of activity was obtained for 11 A1/u.c., contradicting previous results. Apparently, as not yet observed in the cracking of smaller molecules, the accessibility of the larger molecule to the internal zeolitic pores is controlling the rate of reaction. Therefore, maxima are encountered for zeolites which display both high mesoporosity and considerable acid site concentration. In the case of acid leaching of ultra-stable samples this is a compromise between acidity and accessibility, since the combined treatments generate mesoporosity but also reduce the acid site concentration.
REFERENCES
1 2 3 4 5 6
G.R. Bamwenda, Y.X. Zhao, B.W. Wojciechowski. J. Catal. 150(1994)243. P.V. Shertukde, W.K. Hall, J.M. Dereppe, G. Marcelin. J. Catal. 139(1993)468. J.R. Sohn, S.J. DeCanio, P.O. Fritz, J.H. Lunsford. J. Phys. Chem. 90(1986)4847 A. Corma, J.Planelles, J.Sanches-Marin, F.Tomfis. J. Catal. 93(1985)30. R. Beamnont, D. Barthomeuf. J. Catal. 30(1973)288. E. Falabella S-Aguiar, M.P. Silva, M.L.Murta-Valle, D.F. Silva. ACS Division of Petroleum Chemistry, Inc. Preprints, 39-3(1994)356. 7 S.J. DeCanio, J.R. Sohn, P.O. Fritz, J.H. Lunsford. J. Catal. 101(1986)132. 8 E. Falabella S-Aguiar, M.L.Murta-Valle, M.P. Silva, D.F. Silva. Proc. of the 1st EUROPACAT, Montpellier, 1(1994) 104. 9 J. Scherzer in T.E. Whyte Jr.(Editor). The Preparation and Characterization of Aluminum Deficient Zeolites (ACS Symp.Series 248), New York, 1984, p.157. 10 G. Fleisch, B.L. Meyers, G.J. Ray, C.L. Marshall. J. Catal. 99(1986)117. 11 E.V. Sobrinho, D. Cardoso, E. Falabella S-Aguiar, J.G. Silva. XIV Simp. Iberoamericano Catal, Concepci6n, 1994, p.433. 12 G. Engelhardt in H.V. Bekkun (Editor). Solid State NMR Spectroscopy Applied to Zeolites, Elsevier, 1991, p.285. 13 R.D. Shannon, K.H. Gardner, R.H. Staley, G. Bergeret, P. Gallezot, A.Auroux. J. Phys. Chem. 89(1985)4778. 14 A. Voorhies. Ind. Eng. Chem. 37(1945)318. 15 D. Barthomeuf. Materials Chem. Phys. 17(1978)49. 16 H. Stach. Catal. Letters 13(1992)339.
417
Cracking of 1 , 3 , 5 - triisopropylbenzene over deeply dealuminated Y zeolite E.Falabella S-Aguiar 1'2, M.L.Murta-Valle 1, E.V. Sobrinho 3, D. Cardoso 3. Petrobr/ls / CENPES - Ilha do Fund~o, Q7, 21949-900, Rio de Janeiro, Brazil. Fax: 55-21-5986626 2Escola de Quimica/UFRJ, Rio de Janeiro, Brazil, Tel: 55-21-5903192. 3DEQ / UFSCar, S~o Carlos, Brazil, Fax: 55-162-748266.
ABSTRACT Cracking of a rather volmninous molecule, 1,3,5-triisopropylbenzene, was carried out over several zeolites with different degrees of dealumination. Very crystalline zeolites were prepared via combined steam/acid leaching treatments, being afterwards characterized by various techniques. IR-OH region showed that highly condensed EFAL, rather than non-condensed EFAL, was removed during the acid leaching. Nevertheless, MAS/27A1 NMR clearly demonstrated that EFAL-free zeolites are never obtained, regardless of the pH of the acid leaching step. Mesopores surface areas, determined by t-plot increased with increasing number of treatments, as well as the strength of acid sites. Initial rate of cracking of 1,3,5-triisopropylbenzene was plotted against the number of A1 atoms per unit cell, a maximum being obtained for 11 A1/u.c.. Since this molecule has a kinetic diameter larger than 8.0 A, it will not penetrate the zeolitic micropores. After dealumination, mesopores are generated and the reactant is allowed to diffuse. Therefore, both accessibility and acidity seem to control the rate of reaction.
1. INTRODUCTION During the last three decades, extensive work has been carried out aiming at the correlation between the acidity of several dealuminated zeolites and their catalytic activity in the cracking of hydrocarbons. Various molecules such as 2,3dimethylbutane [1], n-pentane [2], n-hexane [3], n-heptane [4], isooctane [5], decalin [6] and cumene [7] have been used in model reactions. Nevertheless, the use of heavier feedstocks in FCC processes has greatly increased the interest for the cracking chemistry of more voluminous molecules. However, not much has been
418 published regarding model reactions with bulky molecules. Previous studies developed in our group [8] have emphasized the importance of external surface area of small crystallite zeolites in the cracking of molecules with kinetic diameter larger than 8.0A such as 1,3,5-triisopropylbenzene (TIPB), showing that acid strength and site density are not the only parameters controlling activity and selectivity during catalytic cracking. The aim of the present work is to demonstrate that the presence of mesopores generated by acid leaching may also play an important role in the cracking of the same molecule (TIPB).
2. EXPERIMENTAL Parent NaY (Si/AI=2.8) zeolite was ion-exchanged in a 11 wt% NH4C1 solution, at 70~ After the ion-exchange, steaming treatments took place in a cylindrical reactor containing 60 g of the zeolite, with 20 g/h flow rate of 100% saturated steam (200~ at 650~ for 90 minutes. Acid leachings were carried out at 70~ for 30 minutes, with pH between 1.0 and 2.5, generating DAZ samples. Treatments were repeated three times and the sodium values were reduced to values below 0.1% Na20. X-ray diffraction (XRD) took place in a Phillips PW1729 diffractometer with CuKo~ radiation and the relative crystallinity of the samples was estimated from the integrated areas of the peaks with Miller indexes 220, 311, 33 l, 333, 440, 533, 642, 660, 751 and 664. Chemical composition was determined by X-ray fluorescence (XRF) in a Phillips PW1407 spectrometer with CrKct radiation. X-ray photoelectron spectroscopy (XPS) was carried out in a VG-Scientific ESCALAB MK II spectrometer with MgKtx radiation, using bands of Al(2p), Si(2p) and Na(ls) to determine surface chemical composition. MAS/NMR spectra of 29Si and 27A1 were obtained on a Varian VXR 300 spectrometer working under a 7.05T magnetic field, being 27A1 spectra collected after impregnation with acetylacetone. Infrared measurements were done on a FTIR Nicolet 60 SXR spectrometer and the OH region (3400-3800 cm -1) was observed. Nitrogen adsorption isotherms were determined using a Micromeritics ASAP2400. From the isotherms, BET surface area, micropores volume (t-plot) and volume of mesopores (BJH) were calculated. Cracking of 1,3,5-triisopropylbenzene (FLUKA 92075) was carried out in a differential fixed-bed gas phase plug flow reactor, with a 15 mm bed containing 5 wt% zeolite diluted in kaolin. Reaction products were analyzed by means of GC-MS (HP5890 and HP5970-B), column HP-PONA (50 m, 0.2 mm, 0.5 pro) for liquid phase and A1203/KC1 column (50 m, 0.3 mm, 0.5 ~rn) for the gas phase.
3. RESULTS AND DISCUSSION Table 1 presents the main characteristics of the parent NaY zeolite and the dealuminated samples prepared thereof. It is clear that dealumination generated very crystalline samples, as a good indication that the process took place without extensive
419 destruction of the framework. It is also evident that progressive dealumination was performed along the cycles, since A o values decreased considerably whereas Si/A1 ratio in the framework (measured by MAS/29Si NMR) increased accordingly. The comparison between Si/A1 ratios obtained by XRF and XPS indicates that aluminum surface contents (XPS) in the samples obtained by acid leaching are lower than the values obtained by XRF and similar to those obtained by 29Si NMR (framework aluminum contents). This reveals a preferential outer surface leaching of the extra framework aluminum by acid treatment [9-11].
Table 1 Main characteristics of the NaY zeolite and dealuminated samples Sample XRD XRF XPS
29Si NMR
Cryst.(%)
Ao(A )
Si/Alg
Si/Als
Si/Alf
NaY
100
24.63
2.8
3.0
2.7
DAZ 1
110
24.50
3.8
4.0
5.6
DAZ 2
119
24.34
11.2
15.0
16.2
DAZ 3
126
24.29
27.8
35.0
33.5
*g=global, s=superficial, f=framework
Results of textural properties of the samples are depicted in table 2, as well as 27A1 NMR relative intensities of aluminum peaks after acetylacetone impregnation. Besides the increasing values of mesopore volume with the increasing dealumination conditions, one can also verify that micropore volume is growing, being larger than the value encountered for the NaY zeolite. This fact indicates that in addition to the characteristic micropores of the parent zeolite, dealuminated samples present supermicropores with diameter between 10 and 20A [9]. The 27A1 NMR spectra display two distinct peaks for tetrahedral (framework) and octahedral (non-framework) aluminum [9-12]. As showed in figure 1, all samples contain extraframework aluminum species (EFAL), regardless of the acid leaching step. Although acid leaching reduces EFAL contents, results of the table 2 indicate that dealuminated samples contain around 30% of EFAL, as an evidence that diffusional limitations in the removal of aluminum species are taking place. In fact, infrared spectra in the OH region after acid leaching revealed the disappearance of the band at 3690 cm l , which is ascribed to highly condensed EFAL located in the larger cavities [ 13]. However, the band at 3600 cm 1, related to low
420
Table 2 Textural properties and 27A1 NMR relative intensifies of the NaY zeolite and dealuminated samples Sample 27A1NMR MiPV a
MePV b
% tetrahedral
%octahedral
NaY
0.326
0.031
100
DAZ 1
0.326
0.092
68.5
31.5
DAZ 2
0.358
0.191
70.7
29.3
DAZ 3
0.360
0.188
64.8
35.2
a MiPV = volume of micropores (cm3/g) b MePV = volume ofmesopores (cm3/g)
DAZ 1
DAZ 2
DAZ 3
O
100
0
100
100
0
-100
100
0
-100
--9 p.p.m. Figure 1. MAS/27A1 NMR spectra for DAZ samples. (T) Tetrahedral, (O) Octahedral
condensation non-framework ahanina located at smaller cavities, remained almost intact. This confmn our assumption that non-accessible EFAL is hardly removed by acid treatment. Rates of 1,3,5-triisopropylbenzene cracking were calculated assuming differential behavior. Plots of rate of TIPB disappearance against time-on-stream were fit to the classical Voohries equation (r = r o t -n) [ 14], allowing one to estimate initial rates of reaction. Initial rates of TIPB disappearance were then plotted against the number of aluminum atoms per unit cell (A1/u.c.), according to NMR results. Such graph is
421 presented in figure 2, for three different temperatures (400, 425 e 450~ For all temperatures, a maximum is achieved for about 11 A1/u.c.. These results contradict previous published data which found a maximum for 30 A1/u.c. in the cracking of, nhexane [3],for instance. Although extensive published literature has often encountered a maximum of activity between 30 - 40 A1/u.c., it must be borne in mind that all model reactions employed in previous studies used small molecules such as linear paraffins (hexane, heptane) or light aromatics (cumene). Explanations for maximum occurrence were based upon changes in the strength, type and concentration of acid sites [3,9,15,16]. However, when voluminous molecules are used, the effect of the formation of mesopores (and thus changes in the external surface area of the zeolites) must not be disregarded. The increase in surface area would certainly be important for diffusion limited reactions, which is apparently the case of the cracking of TIPB. Thus, DAZ 2 and DAZ 3 zeolites possess similar values of micro and mesopore volumes, which, in turn, are higher than the values obtained for DAZ 1 (table 2). Nevertheless, DAZ 2 is more active than DAZ 3 because of the higher concentration of acid sites thereof. This fact confLrms that in the cracking of bulky molecules, both acidity and diffusion are playing an important role in the rate determining step.
3.60
3.20
2.80
~
2.40 -
2.00
1.60
~ 6
l 10
,
f ~ l 18 20 AI/u.c. (NMR)
~
1
l
28
30
Figure 2. Initial cracking rates of 1,3,5-TIPB as a function of A1/u.c. (NMR). (a) 400~ (o) 425~ and ( . ) 450~
422 4. CONCLUSIONS Cracking of TIPB was studied over several acid leached steam treated zeolites and initial rates of disappearance were plotted against the number of A1/u.c.. For three different temperatures, a maximum of activity was obtained for 11 A1/u.c., contradicting previous results. Apparently, as not yet observed in the cracking of smaller molecules, the accessibility of the larger molecule to the internal zeolitic pores is controlling the rate of reaction. Therefore, maxima are encountered for zeolites which display both high mesoporosity and considerable acid site concentration. In the case of acid leaching of ultra-stable samples this is a compromise between acidity and accessibility, since the combined treatments generate mesoporosity but also reduce the acid site concentration.
REFERENCES
1 2 3 4 5 6
G.R. Bamwenda, Y.X. Zhao, B.W. Wojciechowski. J. Catal. 150(1994)243. P.V. Shertukde, W.K. Hall, J.M. Dereppe, G. Marcelin. J. Catal. 139(1993)468. J.R. Sohn, S.J. DeCanio, P.O. Fritz, J.H. Lunsford. J. Phys. Chem. 90(1986)4847 A. Corma, J.Planelles, J.Sanches-Marin, F.Tomfis. J. Catal. 93(1985)30. R. Beamnont, D. Barthomeuf. J. Catal. 30(1973)288. E. Falabella S-Aguiar, M.P. Silva, M.L.Murta-Valle, D.F. Silva. ACS Division of Petroleum Chemistry, Inc. Preprints, 39-3(1994)356. 7 S.J. DeCanio, J.R. Sohn, P.O. Fritz, J.H. Lunsford. J. Catal. 101(1986)132. 8 E. Falabella S-Aguiar, M.L.Murta-Valle, M.P. Silva, D.F. Silva. Proc. of the 1st EUROPACAT, Montpellier, 1(1994) 104. 9 J. Scherzer in T.E. Whyte Jr.(Editor). The Preparation and Characterization of Aluminum Deficient Zeolites (ACS Symp.Series 248), New York, 1984, p.157. 10 G. Fleisch, B.L. Meyers, G.J. Ray, C.L. Marshall. J. Catal. 99(1986)117. 11 E.V. Sobrinho, D. Cardoso, E. Falabella S-Aguiar, J.G. Silva. XIV Simp. Iberoamericano Catal, Concepci6n, 1994, p.433. 12 G. Engelhardt in H.V. Bekkun (Editor). Solid State NMR Spectroscopy Applied to Zeolites, Elsevier, 1991, p.285. 13 R.D. Shannon, K.H. Gardner, R.H. Staley, G. Bergeret, P. Gallezot, A.Auroux. J. Phys. Chem. 89(1985)4778. 14 A. Voorhies. Ind. Eng. Chem. 37(1945)318. 15 D. Barthomeuf. Materials Chem. Phys. 17(1978)49. 16 H. Stach. Catal. Letters 13(1992)339.