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Al2 ratio as measured by temperature programmed desorption of ammonia
Microporous and Mesoporous Materials 40 (2000) 271±281
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Acidity of b zeolite with dierent Si/Al2 ratio as measured by temperature programmed desorption of ammonia Yasunobu Miyamoto, Naonobu Katada, Miki Niwa * Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680-8552, Japan Received 2 February 2000; accepted 23 June 2000
Abstract High quality b zeolite (BEA) with a Si/Al2 ratio of 30:70 was readily prepared by a dry gel conversion method. Acidity of the thus prepared b zeolite was measured by an improved technique of temperature programmed desorption of ammonia. Concentration of acid site, measured from the desorbed ammonia, was nearly equal to that of aluminum in the zeolite, and the enthalpy change of ammonia desorption, i.e., the strength of acidity, was 124±127 kJ molÿ1 , and independent of the concentration of acid site. The long tailing desorption of ammonia was distinct at higher temperature, and this was characteristic of BEA. The tail-like desorption spectrum may be correlated with the presence of strong acid site due to the defect or the tetrahedral site with dierent structural environments; the conclusion was supported by the characterization data using NMR, IR, and test reaction. Thus found solid acidity was compared with that of the commercially available b zeolite; the observed small dierence was explained due to the presence of extraframework Al. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: b Zeolite; TPD of ammonia; Acidity; Dry gel conversion
1. Introduction Recently, much attention has been paid to b zeolite, because it has a unique structure, acidity, and activity as a solid acid catalyst [1±4]. Because of dierent stacks of layers, b zeolite is not a single pure crystal, and termed as *BEA, having an asterisk, to show that pure end members have not been obtained. Recent studies using MAS NMR [5±8] have indicated its uniqueness of aluminum
* Corresponding author. Tel.: +81-857-315256; fax: +81-857315256. E-mail address: [email protected] (M. Niwa).
con®guration due to the unavoidable defect, and even the Al(VI) cation in the zeolite framework is hypothesized. Because the solid acidity is directly related with the aluminum cation in the zeolite, the unique con®guration of b zeolite brings about the unique acidity distribution which is not encountered in other kinds of zeolites. Acidity of b zeolite, therefore, is a new target of investigation on the zeolite acidity. Signi®cance of extra-framework aluminum to the generation of strong acid sites has been reported in the study of alkylation of hydrocarbons on b zeolite [9]. Strong Lewis acid site in b zeolite has been shown in the study of the Meerwein±Ponndorf±Verley reduction [2]. On the other hand, b zeolite is potentially active for acylation [10] using organic acid, which may be a
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reaction catalyzed by strong Brùnsted acid. Recently, there are many investigations reported on the catalytic reaction using b zeolite [11±17]. Hydrothermal synthesis of b zeolite, however, does not seem to be easy in comparison to that of MFI, and some kinds of b zeolite with broad range of Si/Al2 ratio are not always available. On the other hand, Rao and Matsukata [18] invented a new method of dry gel conversion (DGC); they reported that, using this method, b zeolite was readily synthesized up to a high SiO2 /Al2 O3 ratio. They proposed this method in order to prepare thin ®lm of zeolite, however, the DGC method could be used for the production of b zeolite powder. In the present investigation, the DGC method will be applied to produce b zeolite powder with various SiO2 /Al2 O3 ratios. These b zeolites will be used for the measurements of solid acidity using various techniques, particularly temperature programmed desorption (TPD) of ammonia. We are recently studying the solid acidity of zeolites using TPD of ammonia in order to ®nd out the principle of the generation of zeolite acidity [19,20]. The number and strength of the acid site in zeolites was measured using this method. We concluded the following fundamental principles of acidity generation [21±23]: (1) number of acid sites is equal to the aluminum tetrahedrally coordinated in the framework, unless the concentration exceeds the limit; (2) strength of acidity is independent of the number of acid sites, and determined by the structure. Zeolite crystals are ordered as per the strength, MOR > MFI > BEA > FAU. However, the study on b zeolite [22] was insucient, because dierent kinds of species of b zeolite were not available so far, and our study was limited to commercially available ones. In this investigation, our proposed novel techniques of TPD of ammonia, i.e., water vapor treatment to remove the low temperature peak [23] and curve ®tting method to determine the strength of acidity [21], will be applied to b zeolite prepared by the DGC method. The purpose of the present investigation is therefore not only to determine the number and strength of acid sites of b zeolite, but also to reveal its uniqueness, based on the measurements of TPD of ammonia.
2. Experimental methods 2.1. Dry gel conversion Tetraethylammonium hydroxide solution (10 wt.% in water), NaOH (4 mol dmÿ3 ), and colloidal silica (Aldrich, Ludox HS-40; SiO2 , 40 wt.%) were mixed at room temperature, to which Al2 (SO4 )3 solution was added to obtain the mixture with composition, SiO2 :0.37TEAOH:0.05±0.01Al2 O3 : 0.22NaOH. This mixture was vigorously stirred for 2 h at room temperature. Subsequently, the mixture was thoroughly mixed with a Te¯onsealed stirrer at 353 K in a hot bath to ®nally obtain the white dry gel. Next, the dry gel was set in a glass tube, and it was made to stand against the wall of the autoclave with 10±40 cm3 water, as shown in Fig. 1. The capacity of autoclave was 0.1 dm3 , and the size of the glass tube was 30 mm in diameter and 50 mm in length. The autoclave was kept for 18±108 h at 433 K in the temperaturecontrolled oven. After the DGC procedure, the sample in the autoclave was cooled to room temperature, and washed with water three times. After it was dried in the oven at 373 K for 12±24 h, the sample was calcined; in this step, the temperature was increased slowly (1 K minÿ1 ) in a ¯ow of N2 up to 773 K, and the ¯owing gas was switched to oxygen for calcination. Thus obtained Na-type zeolite was ion exchanged with NH4 NO3 at 343 K for 24 h, washed, dried and ®nally calcined at 773 K in an oxygen ¯ow.
Fig. 1. Experimental apparatus for the DGC.
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2.2. Characterization Powder X-ray diraction (XRD) was recorded by a Rigaku Mini¯ex Plus diractometer with 0.45 kW CuKa X-ray source (30 kV, 15 mA). Composition of the zeolite was measured by inductively coupled plasma photo-emission spectroscopy after digestion in HF solution. Adsorption isotherm of nitrogen was measured at 77 K under 0.1±90 kPa of the nitrogen pressure p=p0 10ÿ6 ±0.9). MAS NMR of 29 Si was recorded at 79.45 MHz on a Varian INOVA-400. The spinning frequency was 5000 Hz, and the pulse width was 5.7 ls. Measurements were repeated 4000 times with a time interval of 10 s. Infrared spectrum was collected on the self-supporting disk with 10 mm of the diameter molded from 10 to 20 mg of the zeolite powder in an in situ cell by a JASCO FT/IR-5300 spectrometer. The sample was evacuated at 773 K in vacuo, and pyridine was adsorbed at room temperature. Infrared spectrum was taken after the evacuation at 373±773 K. Cracking of octane isomers was performed using the usual pulse technique. Octane or 2,2,4-trimethylpentane liquid (1 mm3 ) was injected into the zeolite (5 mg) at 573 K under the helium carrier gas (30 cm3 minÿ1 ), and the products were analyzed by gas chromatography with silicone SE-30 column and FID detector. 2.3. Temperature programmed desorption of ammonia The equipment and the procedure were described in detail previously [20]. A sample of 0.1 g was set in a quartz cell and evacuated at 773 K for 1 h to reach less than 0.4 Pa. Ammonia (13.3 kPa) was introduced into the cell, and the pressure was kept at 373 K for 30 min. After the evacuation, water vapor (2±4 kPa) was admitted into the cell, and exposed to the sample for 30 min at 373 K. Evacuation and addition of water was repeated twice as usual; this step is important to remove unnecessary low temperature peak (l-peak), since the l-peak was ascribable to ammonia desorbed from non-acid sites [20,23]. The adsorbed ammonia was desorbed in helium ¯ow (0.044 mmol sÿ1 ) under 13.3 kPa of the total pressure with 10
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K minÿ1 of the heating rate from 373 K. The desorbed ammonia was detected by a mass spectrometer (ULVAC, UPM-ST-200P) at m=e 16. The theory and procedure for the curve-®tting method has been described elsewhere [21].
3. Results 3.1. Synthesis of b zeolite In the range of Si/Al2 ratio of 20±100, b zeolite was synthesized by the DGC method. Fig. 2 shows the XRD of the prepared zeolite after it was ion exchanged. Based on the diraction pattern, b zeolite was identi®ed for the samples, BEA 20, 30, 50, and 70 (number refers to the Si/Al2 molar ratio of the synthesis gel). However, the diraction pattern of the sample with Si/Al2 ratio of 100 was dierent from others. The obtained zeolite seemed to contain not only BEA but also MTW (i.e., ZSM-12) zeolites, and this is shown as BEA MTW100 in Fig. 2. Transformation of BEA zeolite into MTW, OU-1 or ZSM-5 during the DGC has been reported previously [24,25]. These zeolites were prepared for 108 h of the synthesis time with 40 ml of water sealed in the autoclave. When the synthesis was performed under conditions of a shorter time of 18 h and a smaller amount of water, 10 ml, the XRD obtained showed the
Fig. 2. XRD of the prepared zeolites.
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3.2. Nitrogen adsorption
Table 1 Chemical composition of prepared zeolites Sample
SiO2 / Al2 O3
[Al] (mol kgÿ1 )
[Na] (mol kgÿ1 )
BEA20 BEA30 BEA50 BEA70 BEA100 BEA MTW100
22 37 56 68 92 91
1.43 0.86 0.58 0.49 0.30 0.37
0.01 0.01 0.01 0.01 0.09 0.01
structure of b zeolite, as shown by BEA100 in Fig. 2. However, the diractions with small intensity ascribable to MTW were also observed on the BEA100. Therefore, the BEA100 contained a small amount of MTW zeolite as an impurity phase. Table 1 shows chemical composition of the ionexchanged zeolites. The SiO2 /Al2 O3 ratio ®nally obtained was nearly the same as in the synthesis gel. The concentration of Na cation was negligibly small. The dierence in concentrations (Al ÿ Na) will be compared with number of acid sites measured by TPD of NH3 , as shown below. d-Spacing size which was calculated from the diraction of (3 0 2) plane was plotted against the concentration of aluminum, as shown in Fig. 3. The linear increase of d-spacing with Al concentration shows the incorporation of Al cation in the framework.
Fig. 3. Plot of d-spacing against the concentration of aluminum.
Fig. 4 shows the adsorption isotherms of nitrogen at 77 K on the zeolite samples. These showed a typical pro®le of type I adsorption thus proving the microporosity of the material. However, the adsorbed amount of nitrogen on BEA20 was exceptionally small, and it was only about half of the amount on other samples. The BEA20 thereby contained the zeolitic solid by 50% only. Pore volume of b zeolite was 0.26 (BEA30) to 0.22 (BEA50) cm3 gÿ1 , and close to or 20% smaller than the reported value (0.28 cm3 gÿ1 )2 . High crystallinity of b zeolite, prepared by the DGC, was therefore estimated. Adsorption experiment therefore showed that b zeolite prepared in the present study was identi®ed as a highly crystallized material. Deviation from type I isotherm became clear at the high partial pressure; based on the pro®le, the formation of mesopore or macropore was estimated. SEM of the samples showed the very small particle (not shown), and the size of crystal seemed to be 0.2±0.4 lm. 3.3. Temperature programmed desorption of NH3 Fig. 5 shows the TPD of NH3 on the prepared H-type BEA zeolites. Usually, a main desorption peak was observed at 600 K except for BEA100 (E). However, the desorption of ammonia con-
Fig. 4. Adsorption isotherm of nitrogen on BEA 20 (d), 30 (s), 50 (m), 70 (n), 100 ( ), and BEA MTW100 ( ).
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tinued up to higher temperature, thus, forming an additional tail-like TPD spectrum. The tailing desorption was characteristic of BEA zeolite, and not encountered in the measurements of ZSM-5 zeolite. The curve ®tting method based on the proposed equation [19] was applied to the main desorption
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in order to determine the parameter of acidity DH on BEA 30, 50, and 70, as shown in Fig. 5B±D, respectively. On the other hand, the amount of acid sites A0 was determined from the total amount of desorbed ammonia, since the long tailing desorption appeared. Thus determined parameters for acidity are summarized in Table 2.
Fig. 5. TPD of NH3 (shown by the solid line) on BEA (A) 20, (B) 30, (C) 50, (D) 70, (E) 100 and (F) BEA MTW100. Dotted line in (A) shows the TPD on BEA (PQ) and those on B±D and F are simulated spectra.
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Table 2 Solid acidity of BEA zeolites measured by TPD of NH3 a Sample
SiO2 /Al2 O3
A0 (mol kgÿ1 )
DH (kJ molÿ1 )
r (kJ molÿ1 )
BEA20 BEA30 BEA50 BEA70 BEA100 BEA MTW100 BEA (PQ)
22 37 56 68 92 91 25
0.80 0.81 0.56 0.42 0.20 0.30 0.61
± 124 127 127 ± 133 124
± 8 8 9 ± 9 12
a
Parameters of acidity: A0 , concentration; DH, enthalpy change of desorption; r, its distribution.
Enthalpy changes of desorption DH obtained on BEA 30, 50, and 70 were 124±127 kJ molÿ1 , and almost independent of the aluminum concentration. From the observation, it was concluded also for the BEA zeolite that the strength of acidity was independent of the concentration of Al, and determined by the structure of zeolite. On the other hand, the DH of BEA MTW100 was calculated to be 133 kJ molÿ1 , and it was larger than on BEA, 30±70. The number of acid sites was plotted against the amount of contained (Al ÿ Na) in Fig. 6. A linear 1:1 relationship was observed between them on the BEA 30±100. The principle of acidity generation due to the aluminum cation was, thereby, con-
Fig. 6. Correlation between the concentration of (Al ÿ Na) and the total number of acid sites; number shows Si/Al2 ratio of the BEA, and BEA MTW100 is MTW containing BEA with a Si/ Al2 ratio of 100. PQ shows the commercially available BEA.
®rmed on these samples. However, the number of acid sites on BEA20 was smaller than that expected from the Al concentration. The smaller concentration of acid sites on the BEA20 could be understood from the nitrogen isotherm shown above as being due to the adsorbed amount of nitrogen which was about half of the expected value. Therefore, it was considered that both silicon and aluminum atoms were dislodged from the zeolite framework to a similar extent, and these oxides formed the non-zeolitic and non-acidic solid. TPD on the commercially available BEA zeolite with a Si/Al2 ratio of 25, prepared by PQ Corporation, was measured for comparison. As shown by the dotted line in Fig. 5A, the pro®le of TPD spectrum on the BEA (PQ) was fundamentally similar to those on the present BEA zeolite. However, an additional desorption was outstanding at 780 K as a shoulder. Such a shoulder spectrum has been observed for the HZSM-5 containing extra-framework Al, and termed h peak [26]. The strength of acid site, measured from the main desorption at 600 K, was 124 kJ molÿ1 , and nearly equal to the value obtained in the prepared BEA. On the other hand, the number of acid sites was smaller than the value expected from the concentration of aluminum, as shown in Fig. 6. The BEA100 showed an unusual pro®le of ammonia desorption, as shown in Fig. 5E. The temperature range of desorption peak was not only very broad but also higher than on other BEA species. From this point of view, the BEA100 has an unusual character of the solid acidity, unlike on other samples of BEA.
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3.4.
29
Si MAS NMR measurements
29
Si NMR was measured on BEA 30 and 70, as shown in Fig. 7. The NMR spectra were analyzed based on the usual Gaussian distribution, and the resultant peak positions and areas are shown in Table 3. Four dierent components centered at ca. ÿ115, ÿ111, ÿ105 and ÿ102 ppm could be distinguished. Obviously, two components of Si (4Si) were identi®ed at ca. ÿ115 and ÿ111 ppm on both samples. The dierent types of Si sites surrounded by 4 Si±O are characteristic of BEA zeolite, as has already been reported by Corma et al. [27]. The presence of two kinds of 29 Si can be directly correlated with the Si element with the dierent tetrahedral environments, because the chemical shift was related with the angle of Si±O±Si in the zeolite framework [28]. Furthermore, the presence of ÿ101 ppm line of BEA70 can be explained by the species Si(3Si, 1OH), and then the ÿ105 ppm line can be identi®ed as the species of Si(3Si, 1Al). The same analysis of spectrum resolution was applied also to BEA30, although the dierence between Si(3Si, 1OH) and Si(3Si, 1Al) species was not dis-
Fig. 7. Table 3 Characterization by Peak
29
29
tinct. The Si/Al2 ratio obtained from the NMR measurements, 27 and 42 on BEA 30 and 70, respectively, was smaller than the value of chemical analysis in Table 1. This could be explained by the over-estimation of intensity of Si(3Si, 1Al), because of the disturbance of the presence of Si(3Si, 1OH). The disagreement of NMR derived Si/Al2 ratio with the chemical analyzed one was reported also by Corma et al. [27]. When comparison was made between two spectra, one can identify that the larger the Si/Al2 ratio, the higher the fraction of Si(3Si, 1OH). The higher fraction of the species of Si(3Si, 1OH) could be related to the larger concentration of defects in the framework of BEA70 of the more siliceous BEA zeolite. 3.5. IR measurements of BEA and adsorbed pyridine Infrared spectrum of BEA30 revealed hydroxide bands at 3745 and 3610 cmÿ1 , as shown in Fig. 8a. One more absorption with the weak intensity was observed at 3785 cmÿ1 , as has been indicated by the previous study on BEA zeolite [3]. Pyridine
Si MAS NMR of BEA 30 and 70.
Si MAS NMR
BEA30, Si/Al2 ratio from NMR 27.4
Peak Peak Peak Peak
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1 2 3 4
41.9
Position (ppm)
Area (%)
Species
Position (ppm)
Area (%)
Species
ÿ116.1 ÿ112.4 ÿ106.4 ÿ103.3
9.90 47.8 29.5 12.9
Si(4Si) Si(4Si) Si(3Si, 1Al) Si(3Si, 1OH)
ÿ114.0 ÿ110.6 ÿ104.7 ÿ101.4
17.2 40.1 19.1 23.7
Si(4Si) Si(4Si) Si(3Si, 1Al) Si(3Si, 1OH)
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Fig. 8. (a) IR spectra of BEA30 evacuated at 773 K (A) and those of adsorbed pyridine after evacuation at 373 (B), 473 (C), 573 (D), 673 (E), 773 (F) and 873 (G) K. (b) IR spectra of BEA100 evacuated at 773 K (A) and those of adsorbed pyridine after evacuation at 373 (B), 473 (C), 573 (D), 673 (E), 773 (F) and 873 (G) K.
adsorption brought about the decrease of the hydroxide bands, and adsorbed pyridine band appeared. The absorption at 1547 cmÿ1 was ascribable to Brùnsted acid sites, while the doublet at 1450 cmÿ1 to molecular pyridine coordinated or hydrogen bonded. On evacuation by increasing the temperature, these absorptions were removed gradually. Two absorption bands at 1598 and 1446 cmÿ1 disappeared to a similar degree upon evacuation at 373±573 K thus indicating two absorptions caused by an adsorbed species. On evacuation at 673 K (Fig. 8a, curve E), another small absorption appeared at 1463 cmÿ1 .
In Fig. 8b, the infrared spectrum for BEA100 that showed unusual TPD is shown. Compared with BEA30, only the OH band at 3745 cmÿ1 was observed and the band at 3600 cmÿ1 of Brùnsted acid site was not seen. On adsorption of pyridine, absorptions at 1598 and 1446 cmÿ1 were distinct, and disappeared gradually during the evacuation. As on BEA30, three kinds of absorptions were identi®ed at 1440±1470 cmÿ1 (Fig. 8b, curve E). The bands at 1444 and 1455 cmÿ1 were identi®ed as hydrogen-bonded pyridine and pyridine adsorbed on Lewis acid site, respectively. On the other hand, the band at 1463 cmÿ1 could be as-
Y. Miyamoto et al. / Microporous and Mesoporous Materials 40 (2000) 271±281
cribed to pyridine strongly bonded to Lewis-acid site. Thus, the BEA100 was characterized to have, mainly, Lewis-type acidity. 3.6. Catalytic cracking Cracking of octane isomers was performed using a pulse technique to test the catalytic activity. Fig. 9 shows the relation between the conversion of the isomers and the concentration of acid site measured by TPD of ammonia. BEA100 showed a small degree of conversion, while it increased with the concentration of acid sites, as observed on BEA 70±20. The BEA MTW100 showed a higher activity than BEA zeolites; this could be understood from the higher value of DH of BEA MTW than of BEA. On all tested zeolites, octane reacted with a higher degree of conversion than 2,2,4-trimethylpentane. The preferential conversion of octane can be related to the relatively large pore size of BEA, and agrees with the expected value from the constraint index [29] of BEA (0.6±2) in comparison with ZSM-5 (6±8.3).
Fig. 9. Dependence of conversion of cracking of octane (d) and 2,2,4-trimethylpentane (s) on concentration of acid site. The notation in the ®gure is referred to in Fig. 6.
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4. Discussion The present investigation showed that the DGC was a potential method to produce b zeolite with a small size of crystal. By this method, b zeolite was prepared readily in the range of 30±70 of Si/Al2 ratio. Nitrogen adsorption showed the typical type-I isotherm, and the pore volume was nearly the same as that of usual b zeolite. It was clear from the XRD that the aluminum cation was located inside the zeolite framework. TPD of ammonia showed a simpler behavior than that of commercially available one, and superior solid acidity was indicated. Thus, the high quality of the present b zeolite could be indicated, when compared with that prepared under hydrothermal conditions. Note that the present study was carried out on high quality BEA zeolite and the ®ndings are more precise than those obtained on the commercially available b zeolite. TPD spectra showed the unique acid property of b zeolite which was not observed on MOR and MFI zeolites, because the long tailing desorption at higher temperatures appeared distinctively. A discussion will be given, at ®rst, on the main desorption peak and total amount of desorbed ammonia, and then extended to the identi®cation of the high temperature tail-like desorption. The strength of acid site, i.e., the DH of ammonia desorption, was calculated from the principal peak, and the value, 124±127 kJ molÿ1 , was obtained on BEA 30±70. The acid strength depends on the structure, but not on the concentration of aluminum, as has already been observed on MOR and MFI [19,20]. The sequence of zeolite species from the value of DH is FAU < BEA < MFI < MOR. Relatively weak property of b zeolite acid site will be an important character, when it is used in the catalytic reaction. On the other hand, the number of acid sites, measured from the total desorbed amount, was nearly equal to the amount of (Al ÿ Na). It was thereby concluded that all the aluminum cations inside b zeolite showed strong acidity. In this study, it was identi®ed that b zeolite had the distribution of strength of acidity, which was not observed on the MOR and MFI species. The distribution of acid strength, therefore, is the most
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important character of b zeolite acidity. In our previous study on the ZSM-5 zeolite, however, a small peak of desorption (h -peak) appeared at the temperature higher than on usual ZSM-5 species [26], and it was identi®ed as very strong acid sites due to the extra-framework Al. The impregnation of aluminum cation onto the ZSM-5 formed this site, and by the modi®cation, Lewis type property was enhanced. Structure of the acid site was estimated from the observation; i.e., a combination of extra-framework aluminum with the usual Brùnsted acid site creates the strong acid site, as has been previously indicated [30±32]. Two aluminum cations are therefore required for the strong acid site. On the other hand, the coincidence between concentrations of Al and acid site was clearly observed in the present b zeolite, as shown in Fig. 5. Change of d-spacing with Al concentration suggested the incorporation of Al in the framework. The presence of very strong acid due to the extra framework is, therefore, not a plausible explanation for b zeolite. Furthermore, on the MFI zeolite, the very strong acid site (h -peak) appears at a high concentration of metal cations such as Al and Ga [33], because the dislodging of framework cation became distinct at higher concentrations. On the other hand, the tailing desorption of BEA, contrarily, became distinct at the small concentration of Al. MAS NMR of 29 Si also showed that the extent of defect site was high on the siliceous b zeolite, as shown in Fig. 7. Thus, the tailing desorption of BEA must be explained based on the generation mechanism dierent from the h -peak of HZSM-5. In the previous papers, the presence of aluminum with a unique con®guration was reported by the MAS NMR study on b zeolite [3±7]. These studies showed that the aluminum had the unique con®guration in the zeolite framework due to the defect structure of b zeolite. The NMR spectra in the present study showed the ®ndings, which fundamentally agreed with the previous one. An interesting observation is the presence of Si(3Si, 1OH) and two kinds of Si(4Si). The formation of Si(4Si) doublet shows the existence of, at least, two kinds of Si sites with dierent con®gurations, whereas the Si(3Si, 1OH) shows the defect site in the framework of BEA zeolite. The comment on
the latter structure is supported by the IR measurements, because the isolated SiOH was outstanding on the BEA. These characteristics of BEA zeolite structure may be correlated with the acid site distribution. We could indicate, thereby, two possible explanations for the species to show the higher temperature tailing desorption. First, the aluminum attached to the defect site may show strong Lewis acidity, when it loses the hydroxide. Alternatively, when the Al occupies the T(Si) site at ÿ115 ppm in Si NMR, it could show a stronger acidity than the usual Al in the Si site of ÿ110 ppm. Because the strong acid site showed Lewis acidity, as shown by the IR measurement, the former explanation seems to be more plausible. The commercially available b zeolite showed a somewhat dierent TPD spectrum. The h -peak appeared, and the concentration of acid site was smaller than that expected from the Al concentration. We could explain this behavior on the basis of the presence of extra-framework Al interacting with the usual acid site. The very strong acid site to show the h -peak was created from a combination of extra-framework Al and usual acid site, and thereby the concentration of acid site decreased by the formation of extra-framework Al cation. In other words, both ®ndings, i.e., h -peak and small concentration, commonly indicated the dislocation of framework Al in the commercially available BEA (PQ). On the other hand, b zeolite prepared in the present DGC method may not contain the extra-framework Al, as shown above. However, we do not simply arrive at the conclusion that the DGC method is better than the hydrothermal synthesis in preparing the high quality BEA. In our recent study, the appearance of h peak in the HZSM-5 depends on the whole procedure of synthesis. For example, the step of template removal from as-synthesized zeolite and the gel condition of prepared zeolite strongly aect the formation of h -peak [34]. Therefore, the present b zeolite seems to be prepared under relatively appropriate conditions, and the DGC method itself is one of them. The BEA100 species only showed an extremely unusual TPD pro®le, although the XRD pattern was identi®ed as the structure of b. When the gel
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was heated for a longer time such as 108 h, the zeolite partially transformed into the MTW. Therefore, the BEA100 could be regarded as a transitionally formed species, and the distribution of aluminum seemed to be dierent from that on other prepared species. The BEA100 showed the strength of acidity which was greater than others, but the Lewis type acidity was outstanding. Maybe, this is the reason for the poor cracking activity of octane isomers, and the Brùnsted acid site seems to play the role.
5. Conclusion b zeolite was prepared by the method of DGC, and the acidity parameters of thus prepared zeolite, concentration (A0 ) and strength (DH), were determined by TPD of ammonia. A0 was equal to the aluminum concentration in the zeolite. DH was independent of the concentration of acid sites, and determined by the structure of zeolite. The generation mechanism of the zeolite acidity was con®rmed also for b zeolite. However, the stronger acid site was outstanding, and the site could be ascribed to the framework Al with the defect structure or in dierent tetrahedral environments. This observation is characteristic of b zeolite, and became distinct on the more siliceous composition.
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[8] F. Deng, Y. Yue, C. Ye, J. Phys. Chem. 102 (1998) 5252. [9] A. Corma, A. Martinez, P.A. Arroyo, J.L.F. Monteiro, E.F. Sousa-Aguiar, Appl. Catal. A: General 142 (1996) 139. [10] K. Smith, Z. Zhenhua, P.K.G. Hodgson, J. Mol. Catal. 134 (1998) 121. [11] M.A. Camblor, A. Corma, S. Valencia, Micropor. Mesopor. Mater. 25 (1998) 59. [12 R. Loenders, P.A. Jacobs, J.A. Martens, J. Catal. 176 (1998) 545. [13] S. Kotrel, M.P. Rosynek, J.H. Lunsford, J. Phys. Chem. 103 (1999) 818. [14] S. Kotrel, M.P. Rosynek, J.H. Lunsford, J. Catal. 182 (1999) 278. [15] W. Zhang, P.G. Smirniotis, J. Catal. 182 (1999) 400. [16] G.P. Heitmann, G. Dahlho, W.F. Hoelderich, Appl. Catal. A: General 185 (1999) 99. [17] F. Collingnon, R. Loenders, J.A. Martens, P.A. Jacobs, G. Poncelet, J. Catal. 182 (1999) 302. [18] P.R.H.P. Rao, M. Matsukata, Chem. Commun. (1996) 1441. [19] M. Niwa, N. Katada, M. Sawa, Y. Murakami, J. Phys. Chem. 99 (1995) 8812. [20] M. Niwa, N. Katada, Catal. Surv. Jpn 1 (1997) 215. [21] N. Katada, H. Igi, J.-H. Kim, M. Niwa, J. Phys. Chem. B 101 (1997) 5969. [22] N. Katada, S. Iijima, H. Igi, M. Niwa, Stud. Surf. Sci. Catal. 105 (1996) 1227. [23] H. Igi, N. Katada, M. Niwa, in: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.), Proceedings of the 12th International Zeolite Conference, MRS, Warrendale, 1999, p. IV-2643. [24] H.P. Rao, K. Ueyama, E. Kikuchi, M. Matsukata, Chem. Lett. (1998) 311. [25] R. Bandyopadhyay, Y. Kubota, Y. Sugi, Chem. Lett. (1998) 813. [26] T. Kunieda, N. Katada, M. Niwa, in: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins (Eds.), Proceedings of the 12th International Zeolite Conference, MRS, Warrendale, 1999, p. IV-2549. [27] J. Perez-Pariente, J. Sanz, V. Fornes, A. Corma, J. Catal. 124 (1990) 217. [28] S. Ramdas, J. Klinowski, Nature 308 (1984) 521. [29] N.Y. Chen, W.E. Garwood, F.G. Dwyer, Shape Selective Catalysis in Industrial Applications, Marcel Dekker, New York, 1996, p. 23. [30] C. Mirodatos, D. Barthomeuf, J. Chem. Soc. Chem. Commun. (1981) 39. [31] F. Lonyi, J.H. Lunsford, J. Catal. 136 (1992) 566. [32] A. Corma, A. Martinez, C. Martinez, Appl. Catal. A: General 134 (1996) 169. [33] T. Miyamoto, N. Katada, J.-H. Kim, M. Niwa, J. Phys. Chem. B 102 (1998) 6738. [34] A.Kohara, N. Katada, M. Niwa, unpublished data.
www.elsevier.nl/locate/micromeso
Acidity of b zeolite with dierent Si/Al2 ratio as measured by temperature programmed desorption of ammonia Yasunobu Miyamoto, Naonobu Katada, Miki Niwa * Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680-8552, Japan Received 2 February 2000; accepted 23 June 2000
Abstract High quality b zeolite (BEA) with a Si/Al2 ratio of 30:70 was readily prepared by a dry gel conversion method. Acidity of the thus prepared b zeolite was measured by an improved technique of temperature programmed desorption of ammonia. Concentration of acid site, measured from the desorbed ammonia, was nearly equal to that of aluminum in the zeolite, and the enthalpy change of ammonia desorption, i.e., the strength of acidity, was 124±127 kJ molÿ1 , and independent of the concentration of acid site. The long tailing desorption of ammonia was distinct at higher temperature, and this was characteristic of BEA. The tail-like desorption spectrum may be correlated with the presence of strong acid site due to the defect or the tetrahedral site with dierent structural environments; the conclusion was supported by the characterization data using NMR, IR, and test reaction. Thus found solid acidity was compared with that of the commercially available b zeolite; the observed small dierence was explained due to the presence of extraframework Al. Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: b Zeolite; TPD of ammonia; Acidity; Dry gel conversion
1. Introduction Recently, much attention has been paid to b zeolite, because it has a unique structure, acidity, and activity as a solid acid catalyst [1±4]. Because of dierent stacks of layers, b zeolite is not a single pure crystal, and termed as *BEA, having an asterisk, to show that pure end members have not been obtained. Recent studies using MAS NMR [5±8] have indicated its uniqueness of aluminum
* Corresponding author. Tel.: +81-857-315256; fax: +81-857315256. E-mail address: [email protected] (M. Niwa).
con®guration due to the unavoidable defect, and even the Al(VI) cation in the zeolite framework is hypothesized. Because the solid acidity is directly related with the aluminum cation in the zeolite, the unique con®guration of b zeolite brings about the unique acidity distribution which is not encountered in other kinds of zeolites. Acidity of b zeolite, therefore, is a new target of investigation on the zeolite acidity. Signi®cance of extra-framework aluminum to the generation of strong acid sites has been reported in the study of alkylation of hydrocarbons on b zeolite [9]. Strong Lewis acid site in b zeolite has been shown in the study of the Meerwein±Ponndorf±Verley reduction [2]. On the other hand, b zeolite is potentially active for acylation [10] using organic acid, which may be a
1387-1811/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 0 ) 0 0 2 6 4 - X
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reaction catalyzed by strong Brùnsted acid. Recently, there are many investigations reported on the catalytic reaction using b zeolite [11±17]. Hydrothermal synthesis of b zeolite, however, does not seem to be easy in comparison to that of MFI, and some kinds of b zeolite with broad range of Si/Al2 ratio are not always available. On the other hand, Rao and Matsukata [18] invented a new method of dry gel conversion (DGC); they reported that, using this method, b zeolite was readily synthesized up to a high SiO2 /Al2 O3 ratio. They proposed this method in order to prepare thin ®lm of zeolite, however, the DGC method could be used for the production of b zeolite powder. In the present investigation, the DGC method will be applied to produce b zeolite powder with various SiO2 /Al2 O3 ratios. These b zeolites will be used for the measurements of solid acidity using various techniques, particularly temperature programmed desorption (TPD) of ammonia. We are recently studying the solid acidity of zeolites using TPD of ammonia in order to ®nd out the principle of the generation of zeolite acidity [19,20]. The number and strength of the acid site in zeolites was measured using this method. We concluded the following fundamental principles of acidity generation [21±23]: (1) number of acid sites is equal to the aluminum tetrahedrally coordinated in the framework, unless the concentration exceeds the limit; (2) strength of acidity is independent of the number of acid sites, and determined by the structure. Zeolite crystals are ordered as per the strength, MOR > MFI > BEA > FAU. However, the study on b zeolite [22] was insucient, because dierent kinds of species of b zeolite were not available so far, and our study was limited to commercially available ones. In this investigation, our proposed novel techniques of TPD of ammonia, i.e., water vapor treatment to remove the low temperature peak [23] and curve ®tting method to determine the strength of acidity [21], will be applied to b zeolite prepared by the DGC method. The purpose of the present investigation is therefore not only to determine the number and strength of acid sites of b zeolite, but also to reveal its uniqueness, based on the measurements of TPD of ammonia.
2. Experimental methods 2.1. Dry gel conversion Tetraethylammonium hydroxide solution (10 wt.% in water), NaOH (4 mol dmÿ3 ), and colloidal silica (Aldrich, Ludox HS-40; SiO2 , 40 wt.%) were mixed at room temperature, to which Al2 (SO4 )3 solution was added to obtain the mixture with composition, SiO2 :0.37TEAOH:0.05±0.01Al2 O3 : 0.22NaOH. This mixture was vigorously stirred for 2 h at room temperature. Subsequently, the mixture was thoroughly mixed with a Te¯onsealed stirrer at 353 K in a hot bath to ®nally obtain the white dry gel. Next, the dry gel was set in a glass tube, and it was made to stand against the wall of the autoclave with 10±40 cm3 water, as shown in Fig. 1. The capacity of autoclave was 0.1 dm3 , and the size of the glass tube was 30 mm in diameter and 50 mm in length. The autoclave was kept for 18±108 h at 433 K in the temperaturecontrolled oven. After the DGC procedure, the sample in the autoclave was cooled to room temperature, and washed with water three times. After it was dried in the oven at 373 K for 12±24 h, the sample was calcined; in this step, the temperature was increased slowly (1 K minÿ1 ) in a ¯ow of N2 up to 773 K, and the ¯owing gas was switched to oxygen for calcination. Thus obtained Na-type zeolite was ion exchanged with NH4 NO3 at 343 K for 24 h, washed, dried and ®nally calcined at 773 K in an oxygen ¯ow.
Fig. 1. Experimental apparatus for the DGC.
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2.2. Characterization Powder X-ray diraction (XRD) was recorded by a Rigaku Mini¯ex Plus diractometer with 0.45 kW CuKa X-ray source (30 kV, 15 mA). Composition of the zeolite was measured by inductively coupled plasma photo-emission spectroscopy after digestion in HF solution. Adsorption isotherm of nitrogen was measured at 77 K under 0.1±90 kPa of the nitrogen pressure p=p0 10ÿ6 ±0.9). MAS NMR of 29 Si was recorded at 79.45 MHz on a Varian INOVA-400. The spinning frequency was 5000 Hz, and the pulse width was 5.7 ls. Measurements were repeated 4000 times with a time interval of 10 s. Infrared spectrum was collected on the self-supporting disk with 10 mm of the diameter molded from 10 to 20 mg of the zeolite powder in an in situ cell by a JASCO FT/IR-5300 spectrometer. The sample was evacuated at 773 K in vacuo, and pyridine was adsorbed at room temperature. Infrared spectrum was taken after the evacuation at 373±773 K. Cracking of octane isomers was performed using the usual pulse technique. Octane or 2,2,4-trimethylpentane liquid (1 mm3 ) was injected into the zeolite (5 mg) at 573 K under the helium carrier gas (30 cm3 minÿ1 ), and the products were analyzed by gas chromatography with silicone SE-30 column and FID detector. 2.3. Temperature programmed desorption of ammonia The equipment and the procedure were described in detail previously [20]. A sample of 0.1 g was set in a quartz cell and evacuated at 773 K for 1 h to reach less than 0.4 Pa. Ammonia (13.3 kPa) was introduced into the cell, and the pressure was kept at 373 K for 30 min. After the evacuation, water vapor (2±4 kPa) was admitted into the cell, and exposed to the sample for 30 min at 373 K. Evacuation and addition of water was repeated twice as usual; this step is important to remove unnecessary low temperature peak (l-peak), since the l-peak was ascribable to ammonia desorbed from non-acid sites [20,23]. The adsorbed ammonia was desorbed in helium ¯ow (0.044 mmol sÿ1 ) under 13.3 kPa of the total pressure with 10
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K minÿ1 of the heating rate from 373 K. The desorbed ammonia was detected by a mass spectrometer (ULVAC, UPM-ST-200P) at m=e 16. The theory and procedure for the curve-®tting method has been described elsewhere [21].
3. Results 3.1. Synthesis of b zeolite In the range of Si/Al2 ratio of 20±100, b zeolite was synthesized by the DGC method. Fig. 2 shows the XRD of the prepared zeolite after it was ion exchanged. Based on the diraction pattern, b zeolite was identi®ed for the samples, BEA 20, 30, 50, and 70 (number refers to the Si/Al2 molar ratio of the synthesis gel). However, the diraction pattern of the sample with Si/Al2 ratio of 100 was dierent from others. The obtained zeolite seemed to contain not only BEA but also MTW (i.e., ZSM-12) zeolites, and this is shown as BEA MTW100 in Fig. 2. Transformation of BEA zeolite into MTW, OU-1 or ZSM-5 during the DGC has been reported previously [24,25]. These zeolites were prepared for 108 h of the synthesis time with 40 ml of water sealed in the autoclave. When the synthesis was performed under conditions of a shorter time of 18 h and a smaller amount of water, 10 ml, the XRD obtained showed the
Fig. 2. XRD of the prepared zeolites.
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3.2. Nitrogen adsorption
Table 1 Chemical composition of prepared zeolites Sample
SiO2 / Al2 O3
[Al] (mol kgÿ1 )
[Na] (mol kgÿ1 )
BEA20 BEA30 BEA50 BEA70 BEA100 BEA MTW100
22 37 56 68 92 91
1.43 0.86 0.58 0.49 0.30 0.37
0.01 0.01 0.01 0.01 0.09 0.01
structure of b zeolite, as shown by BEA100 in Fig. 2. However, the diractions with small intensity ascribable to MTW were also observed on the BEA100. Therefore, the BEA100 contained a small amount of MTW zeolite as an impurity phase. Table 1 shows chemical composition of the ionexchanged zeolites. The SiO2 /Al2 O3 ratio ®nally obtained was nearly the same as in the synthesis gel. The concentration of Na cation was negligibly small. The dierence in concentrations (Al ÿ Na) will be compared with number of acid sites measured by TPD of NH3 , as shown below. d-Spacing size which was calculated from the diraction of (3 0 2) plane was plotted against the concentration of aluminum, as shown in Fig. 3. The linear increase of d-spacing with Al concentration shows the incorporation of Al cation in the framework.
Fig. 3. Plot of d-spacing against the concentration of aluminum.
Fig. 4 shows the adsorption isotherms of nitrogen at 77 K on the zeolite samples. These showed a typical pro®le of type I adsorption thus proving the microporosity of the material. However, the adsorbed amount of nitrogen on BEA20 was exceptionally small, and it was only about half of the amount on other samples. The BEA20 thereby contained the zeolitic solid by 50% only. Pore volume of b zeolite was 0.26 (BEA30) to 0.22 (BEA50) cm3 gÿ1 , and close to or 20% smaller than the reported value (0.28 cm3 gÿ1 )2 . High crystallinity of b zeolite, prepared by the DGC, was therefore estimated. Adsorption experiment therefore showed that b zeolite prepared in the present study was identi®ed as a highly crystallized material. Deviation from type I isotherm became clear at the high partial pressure; based on the pro®le, the formation of mesopore or macropore was estimated. SEM of the samples showed the very small particle (not shown), and the size of crystal seemed to be 0.2±0.4 lm. 3.3. Temperature programmed desorption of NH3 Fig. 5 shows the TPD of NH3 on the prepared H-type BEA zeolites. Usually, a main desorption peak was observed at 600 K except for BEA100 (E). However, the desorption of ammonia con-
Fig. 4. Adsorption isotherm of nitrogen on BEA 20 (d), 30 (s), 50 (m), 70 (n), 100 ( ), and BEA MTW100 ( ).
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tinued up to higher temperature, thus, forming an additional tail-like TPD spectrum. The tailing desorption was characteristic of BEA zeolite, and not encountered in the measurements of ZSM-5 zeolite. The curve ®tting method based on the proposed equation [19] was applied to the main desorption
275
in order to determine the parameter of acidity DH on BEA 30, 50, and 70, as shown in Fig. 5B±D, respectively. On the other hand, the amount of acid sites A0 was determined from the total amount of desorbed ammonia, since the long tailing desorption appeared. Thus determined parameters for acidity are summarized in Table 2.
Fig. 5. TPD of NH3 (shown by the solid line) on BEA (A) 20, (B) 30, (C) 50, (D) 70, (E) 100 and (F) BEA MTW100. Dotted line in (A) shows the TPD on BEA (PQ) and those on B±D and F are simulated spectra.
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Table 2 Solid acidity of BEA zeolites measured by TPD of NH3 a Sample
SiO2 /Al2 O3
A0 (mol kgÿ1 )
DH (kJ molÿ1 )
r (kJ molÿ1 )
BEA20 BEA30 BEA50 BEA70 BEA100 BEA MTW100 BEA (PQ)
22 37 56 68 92 91 25
0.80 0.81 0.56 0.42 0.20 0.30 0.61
± 124 127 127 ± 133 124
± 8 8 9 ± 9 12
a
Parameters of acidity: A0 , concentration; DH, enthalpy change of desorption; r, its distribution.
Enthalpy changes of desorption DH obtained on BEA 30, 50, and 70 were 124±127 kJ molÿ1 , and almost independent of the aluminum concentration. From the observation, it was concluded also for the BEA zeolite that the strength of acidity was independent of the concentration of Al, and determined by the structure of zeolite. On the other hand, the DH of BEA MTW100 was calculated to be 133 kJ molÿ1 , and it was larger than on BEA, 30±70. The number of acid sites was plotted against the amount of contained (Al ÿ Na) in Fig. 6. A linear 1:1 relationship was observed between them on the BEA 30±100. The principle of acidity generation due to the aluminum cation was, thereby, con-
Fig. 6. Correlation between the concentration of (Al ÿ Na) and the total number of acid sites; number shows Si/Al2 ratio of the BEA, and BEA MTW100 is MTW containing BEA with a Si/ Al2 ratio of 100. PQ shows the commercially available BEA.
®rmed on these samples. However, the number of acid sites on BEA20 was smaller than that expected from the Al concentration. The smaller concentration of acid sites on the BEA20 could be understood from the nitrogen isotherm shown above as being due to the adsorbed amount of nitrogen which was about half of the expected value. Therefore, it was considered that both silicon and aluminum atoms were dislodged from the zeolite framework to a similar extent, and these oxides formed the non-zeolitic and non-acidic solid. TPD on the commercially available BEA zeolite with a Si/Al2 ratio of 25, prepared by PQ Corporation, was measured for comparison. As shown by the dotted line in Fig. 5A, the pro®le of TPD spectrum on the BEA (PQ) was fundamentally similar to those on the present BEA zeolite. However, an additional desorption was outstanding at 780 K as a shoulder. Such a shoulder spectrum has been observed for the HZSM-5 containing extra-framework Al, and termed h peak [26]. The strength of acid site, measured from the main desorption at 600 K, was 124 kJ molÿ1 , and nearly equal to the value obtained in the prepared BEA. On the other hand, the number of acid sites was smaller than the value expected from the concentration of aluminum, as shown in Fig. 6. The BEA100 showed an unusual pro®le of ammonia desorption, as shown in Fig. 5E. The temperature range of desorption peak was not only very broad but also higher than on other BEA species. From this point of view, the BEA100 has an unusual character of the solid acidity, unlike on other samples of BEA.
Y. Miyamoto et al. / Microporous and Mesoporous Materials 40 (2000) 271±281
3.4.
29
Si MAS NMR measurements
29
Si NMR was measured on BEA 30 and 70, as shown in Fig. 7. The NMR spectra were analyzed based on the usual Gaussian distribution, and the resultant peak positions and areas are shown in Table 3. Four dierent components centered at ca. ÿ115, ÿ111, ÿ105 and ÿ102 ppm could be distinguished. Obviously, two components of Si (4Si) were identi®ed at ca. ÿ115 and ÿ111 ppm on both samples. The dierent types of Si sites surrounded by 4 Si±O are characteristic of BEA zeolite, as has already been reported by Corma et al. [27]. The presence of two kinds of 29 Si can be directly correlated with the Si element with the dierent tetrahedral environments, because the chemical shift was related with the angle of Si±O±Si in the zeolite framework [28]. Furthermore, the presence of ÿ101 ppm line of BEA70 can be explained by the species Si(3Si, 1OH), and then the ÿ105 ppm line can be identi®ed as the species of Si(3Si, 1Al). The same analysis of spectrum resolution was applied also to BEA30, although the dierence between Si(3Si, 1OH) and Si(3Si, 1Al) species was not dis-
Fig. 7. Table 3 Characterization by Peak
29
29
tinct. The Si/Al2 ratio obtained from the NMR measurements, 27 and 42 on BEA 30 and 70, respectively, was smaller than the value of chemical analysis in Table 1. This could be explained by the over-estimation of intensity of Si(3Si, 1Al), because of the disturbance of the presence of Si(3Si, 1OH). The disagreement of NMR derived Si/Al2 ratio with the chemical analyzed one was reported also by Corma et al. [27]. When comparison was made between two spectra, one can identify that the larger the Si/Al2 ratio, the higher the fraction of Si(3Si, 1OH). The higher fraction of the species of Si(3Si, 1OH) could be related to the larger concentration of defects in the framework of BEA70 of the more siliceous BEA zeolite. 3.5. IR measurements of BEA and adsorbed pyridine Infrared spectrum of BEA30 revealed hydroxide bands at 3745 and 3610 cmÿ1 , as shown in Fig. 8a. One more absorption with the weak intensity was observed at 3785 cmÿ1 , as has been indicated by the previous study on BEA zeolite [3]. Pyridine
Si MAS NMR of BEA 30 and 70.
Si MAS NMR
BEA30, Si/Al2 ratio from NMR 27.4
Peak Peak Peak Peak
277
1 2 3 4
41.9
Position (ppm)
Area (%)
Species
Position (ppm)
Area (%)
Species
ÿ116.1 ÿ112.4 ÿ106.4 ÿ103.3
9.90 47.8 29.5 12.9
Si(4Si) Si(4Si) Si(3Si, 1Al) Si(3Si, 1OH)
ÿ114.0 ÿ110.6 ÿ104.7 ÿ101.4
17.2 40.1 19.1 23.7
Si(4Si) Si(4Si) Si(3Si, 1Al) Si(3Si, 1OH)
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Fig. 8. (a) IR spectra of BEA30 evacuated at 773 K (A) and those of adsorbed pyridine after evacuation at 373 (B), 473 (C), 573 (D), 673 (E), 773 (F) and 873 (G) K. (b) IR spectra of BEA100 evacuated at 773 K (A) and those of adsorbed pyridine after evacuation at 373 (B), 473 (C), 573 (D), 673 (E), 773 (F) and 873 (G) K.
adsorption brought about the decrease of the hydroxide bands, and adsorbed pyridine band appeared. The absorption at 1547 cmÿ1 was ascribable to Brùnsted acid sites, while the doublet at 1450 cmÿ1 to molecular pyridine coordinated or hydrogen bonded. On evacuation by increasing the temperature, these absorptions were removed gradually. Two absorption bands at 1598 and 1446 cmÿ1 disappeared to a similar degree upon evacuation at 373±573 K thus indicating two absorptions caused by an adsorbed species. On evacuation at 673 K (Fig. 8a, curve E), another small absorption appeared at 1463 cmÿ1 .
In Fig. 8b, the infrared spectrum for BEA100 that showed unusual TPD is shown. Compared with BEA30, only the OH band at 3745 cmÿ1 was observed and the band at 3600 cmÿ1 of Brùnsted acid site was not seen. On adsorption of pyridine, absorptions at 1598 and 1446 cmÿ1 were distinct, and disappeared gradually during the evacuation. As on BEA30, three kinds of absorptions were identi®ed at 1440±1470 cmÿ1 (Fig. 8b, curve E). The bands at 1444 and 1455 cmÿ1 were identi®ed as hydrogen-bonded pyridine and pyridine adsorbed on Lewis acid site, respectively. On the other hand, the band at 1463 cmÿ1 could be as-
Y. Miyamoto et al. / Microporous and Mesoporous Materials 40 (2000) 271±281
cribed to pyridine strongly bonded to Lewis-acid site. Thus, the BEA100 was characterized to have, mainly, Lewis-type acidity. 3.6. Catalytic cracking Cracking of octane isomers was performed using a pulse technique to test the catalytic activity. Fig. 9 shows the relation between the conversion of the isomers and the concentration of acid site measured by TPD of ammonia. BEA100 showed a small degree of conversion, while it increased with the concentration of acid sites, as observed on BEA 70±20. The BEA MTW100 showed a higher activity than BEA zeolites; this could be understood from the higher value of DH of BEA MTW than of BEA. On all tested zeolites, octane reacted with a higher degree of conversion than 2,2,4-trimethylpentane. The preferential conversion of octane can be related to the relatively large pore size of BEA, and agrees with the expected value from the constraint index [29] of BEA (0.6±2) in comparison with ZSM-5 (6±8.3).
Fig. 9. Dependence of conversion of cracking of octane (d) and 2,2,4-trimethylpentane (s) on concentration of acid site. The notation in the ®gure is referred to in Fig. 6.
279
4. Discussion The present investigation showed that the DGC was a potential method to produce b zeolite with a small size of crystal. By this method, b zeolite was prepared readily in the range of 30±70 of Si/Al2 ratio. Nitrogen adsorption showed the typical type-I isotherm, and the pore volume was nearly the same as that of usual b zeolite. It was clear from the XRD that the aluminum cation was located inside the zeolite framework. TPD of ammonia showed a simpler behavior than that of commercially available one, and superior solid acidity was indicated. Thus, the high quality of the present b zeolite could be indicated, when compared with that prepared under hydrothermal conditions. Note that the present study was carried out on high quality BEA zeolite and the ®ndings are more precise than those obtained on the commercially available b zeolite. TPD spectra showed the unique acid property of b zeolite which was not observed on MOR and MFI zeolites, because the long tailing desorption at higher temperatures appeared distinctively. A discussion will be given, at ®rst, on the main desorption peak and total amount of desorbed ammonia, and then extended to the identi®cation of the high temperature tail-like desorption. The strength of acid site, i.e., the DH of ammonia desorption, was calculated from the principal peak, and the value, 124±127 kJ molÿ1 , was obtained on BEA 30±70. The acid strength depends on the structure, but not on the concentration of aluminum, as has already been observed on MOR and MFI [19,20]. The sequence of zeolite species from the value of DH is FAU < BEA < MFI < MOR. Relatively weak property of b zeolite acid site will be an important character, when it is used in the catalytic reaction. On the other hand, the number of acid sites, measured from the total desorbed amount, was nearly equal to the amount of (Al ÿ Na). It was thereby concluded that all the aluminum cations inside b zeolite showed strong acidity. In this study, it was identi®ed that b zeolite had the distribution of strength of acidity, which was not observed on the MOR and MFI species. The distribution of acid strength, therefore, is the most
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important character of b zeolite acidity. In our previous study on the ZSM-5 zeolite, however, a small peak of desorption (h -peak) appeared at the temperature higher than on usual ZSM-5 species [26], and it was identi®ed as very strong acid sites due to the extra-framework Al. The impregnation of aluminum cation onto the ZSM-5 formed this site, and by the modi®cation, Lewis type property was enhanced. Structure of the acid site was estimated from the observation; i.e., a combination of extra-framework aluminum with the usual Brùnsted acid site creates the strong acid site, as has been previously indicated [30±32]. Two aluminum cations are therefore required for the strong acid site. On the other hand, the coincidence between concentrations of Al and acid site was clearly observed in the present b zeolite, as shown in Fig. 5. Change of d-spacing with Al concentration suggested the incorporation of Al in the framework. The presence of very strong acid due to the extra framework is, therefore, not a plausible explanation for b zeolite. Furthermore, on the MFI zeolite, the very strong acid site (h -peak) appears at a high concentration of metal cations such as Al and Ga [33], because the dislodging of framework cation became distinct at higher concentrations. On the other hand, the tailing desorption of BEA, contrarily, became distinct at the small concentration of Al. MAS NMR of 29 Si also showed that the extent of defect site was high on the siliceous b zeolite, as shown in Fig. 7. Thus, the tailing desorption of BEA must be explained based on the generation mechanism dierent from the h -peak of HZSM-5. In the previous papers, the presence of aluminum with a unique con®guration was reported by the MAS NMR study on b zeolite [3±7]. These studies showed that the aluminum had the unique con®guration in the zeolite framework due to the defect structure of b zeolite. The NMR spectra in the present study showed the ®ndings, which fundamentally agreed with the previous one. An interesting observation is the presence of Si(3Si, 1OH) and two kinds of Si(4Si). The formation of Si(4Si) doublet shows the existence of, at least, two kinds of Si sites with dierent con®gurations, whereas the Si(3Si, 1OH) shows the defect site in the framework of BEA zeolite. The comment on
the latter structure is supported by the IR measurements, because the isolated SiOH was outstanding on the BEA. These characteristics of BEA zeolite structure may be correlated with the acid site distribution. We could indicate, thereby, two possible explanations for the species to show the higher temperature tailing desorption. First, the aluminum attached to the defect site may show strong Lewis acidity, when it loses the hydroxide. Alternatively, when the Al occupies the T(Si) site at ÿ115 ppm in Si NMR, it could show a stronger acidity than the usual Al in the Si site of ÿ110 ppm. Because the strong acid site showed Lewis acidity, as shown by the IR measurement, the former explanation seems to be more plausible. The commercially available b zeolite showed a somewhat dierent TPD spectrum. The h -peak appeared, and the concentration of acid site was smaller than that expected from the Al concentration. We could explain this behavior on the basis of the presence of extra-framework Al interacting with the usual acid site. The very strong acid site to show the h -peak was created from a combination of extra-framework Al and usual acid site, and thereby the concentration of acid site decreased by the formation of extra-framework Al cation. In other words, both ®ndings, i.e., h -peak and small concentration, commonly indicated the dislocation of framework Al in the commercially available BEA (PQ). On the other hand, b zeolite prepared in the present DGC method may not contain the extra-framework Al, as shown above. However, we do not simply arrive at the conclusion that the DGC method is better than the hydrothermal synthesis in preparing the high quality BEA. In our recent study, the appearance of h peak in the HZSM-5 depends on the whole procedure of synthesis. For example, the step of template removal from as-synthesized zeolite and the gel condition of prepared zeolite strongly aect the formation of h -peak [34]. Therefore, the present b zeolite seems to be prepared under relatively appropriate conditions, and the DGC method itself is one of them. The BEA100 species only showed an extremely unusual TPD pro®le, although the XRD pattern was identi®ed as the structure of b. When the gel
Y. Miyamoto et al. / Microporous and Mesoporous Materials 40 (2000) 271±281
was heated for a longer time such as 108 h, the zeolite partially transformed into the MTW. Therefore, the BEA100 could be regarded as a transitionally formed species, and the distribution of aluminum seemed to be dierent from that on other prepared species. The BEA100 showed the strength of acidity which was greater than others, but the Lewis type acidity was outstanding. Maybe, this is the reason for the poor cracking activity of octane isomers, and the Brùnsted acid site seems to play the role.
5. Conclusion b zeolite was prepared by the method of DGC, and the acidity parameters of thus prepared zeolite, concentration (A0 ) and strength (DH), were determined by TPD of ammonia. A0 was equal to the aluminum concentration in the zeolite. DH was independent of the concentration of acid sites, and determined by the structure of zeolite. The generation mechanism of the zeolite acidity was con®rmed also for b zeolite. However, the stronger acid site was outstanding, and the site could be ascribed to the framework Al with the defect structure or in dierent tetrahedral environments. This observation is characteristic of b zeolite, and became distinct on the more siliceous composition.
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