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Synthesis of boron-beta zeolite from near-neutral fluoride-containing gels
MICROPOROUS MATERIALS ELSEVIER
Microporous Materials 10 (1997) 181 188
Synthesis of boron-beta zeolite from near-neutral fluoride-containing gels S. Kallus, J. Patarin, P. Caullet *, A.C. Faust 1,ahoratoire de MatOriau v MinOraux U R A - C N R S 428, Ecole Nationale Sup~rieure de Chimie de Mulhouse-UHA, 3, rue AlJred Werner, F-68093 Mulhouse, Cedex, France
Received 18 November 1996: accepted 7 January 1997
Abstract
A borosilicate beta zeolite was prepared from almost neutral fluoride aqueous gels in the presence of 1,4-diazabicyclo[2.2.2]octane and methylamine as the templates. The influence of several physical and chemical parameters was studied. In comparison with the (Si,AI/ system, this (Si,B) system appears less 'reactive" with longer crystallization times. Moreover~ the products obtained display a narrower range of compositions. Only boron-rich (Si/B-21 and fluoride-poor media (F/Si=2) led to well-crystallized samples of beta zeolite characterized by a Si/B molar ratio close to 14. The differences observed between the two systems might be related to a stronger complexation of boron by the fluoride anions. The crystallization of the boron zeolite beta occurs probably through condensation reactions after hydrolysis of fluoroborate (BF,~), hydroxyfluoroborate (BF3OH and BF2(OH )2) and fluorosilicate species. Zeolite beta was characterized by XRD, SEM, chemical analysis, thermal analysis and I~B, 19F and t3C MAS NMR spectroscopy. The latter technique shows that the as-made beta samples contain, beside the dabconium cation, a polymeric compound identified as polyethylene piperazine. ~ 1997 Elsevier Science B.V. I~2,vwor&v Beta zeolite: Borosilicate; DABCO: d.t.a., t.g.: Fluoride media; Solid state NMR spectroscopy; Synthesis
1. Introduction
Zeolite beta was first synthesized in 1967 from basic aqueous aluminosdicate gels with sodium and tetraethylammonium as the templates [1]. Afterwards the use of other templating agents such as dibenzyl 1,4-diazabicyclo[2.2.2]octane [2] or dibenzyldimethylammonium cation [3] was also claimed. This high-silica zeolite (Si/A1 molar ratio from ca. 10 to several hundreds) displays a threedimensional network of channels delimited by
* Corresponding author. Tel.,'tax: + 33 399428730. 0927-0513,, 97 $17 00 ,c, 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 6 5 1 3 1 9 7 ) 0 0 0 1 0 - 2
twelve-membered rings. Due to its great complexity, the structure remained unresolved for over 21) years, zeolite beta being actually an intergrown hybrid of two distinct but closely related polymorphs [4, 5]. Partial or complete substitution of aluminum by iron [6], gallium [7,8] or boron [9-14] could be achieved in the framework of zeolite beta by direct synthesis from basic aqueous gels. The borosilicate beta zeolite is mostly prepared in the presence of the tetraethylammonium cation as the templating agent, except in Refs. [13,14] where the template is the 4,4'-trimethylene dipiperidine [13] or the dibenzyldimethylammonium cation [14]. Depend-
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ing on the nature of the template, samples of boron-beta were obtained with a Si/B molar ratio ranging from about 10 [12] to several hundreds [14]. We reported recently on the synthesis of (Si,A1) beta zeolite from near neutral fluoride-containing media in the presence of 1,4-diazabicyclo[2.2.2]octane and methylamine as the templates [15]. In such non-alkaline reaction mixtures, the hydroxide ion is replaced by the fluoride ion as the mineralizing agent [16]. In the present work, these experimental synthesis conditions were used to prepare a borosilicate beta zeolite. The samples were characterized by XRD, SEM, chemical analysis, thermal analysis and 13C, t~B and 19F MAS N M R spectroscopy.
2. Experimental 2.1. Reactants The reactants were 1,4-diaza-bicyclo[2.2.2]octane (DABCO, Aldrich, 98%), methylamine (Fluka, 40wt.% in water), hydrofluoric acid (Prolabo, normapur, 40 wt.% in water) and ammonium fluoride (Prolabo, rectapur). The silicon and the boron sources were a precipitated silica (Merck, 94% SiO2) and boric acid (Merck, 99%).
2.2. Synthesis procedure The samples were obtained by hydrothermal synthesis at 170r'C. The molar composition of the starting mixtures was: 1SiO2:xB203:1DABCO: 1CH3NH2:2HF:yNH4F: 10H20 with x = 0.1 or 0.25 and y = 0, 4 or 8. The synthesis procedure was similar to that used for synthesizing the alumino-silicate beta samples [15], seed crystals (crushed crystals of (Si,AI) zeolite beta) being generally added to the reaction mixtures. As an example, for sample D (Table 1 ), l l . 2 g of DABCO and 7.8 g of methylamine were first dissolved in 6.9 g of distilled water. Then, 10 g of
Table 1 Description of the most representative syntheses Sample
Aa B C D E F G
Molar ratios of the gels Si/B
F /Si
2 2 2 2 2 5 2
6 6 2 2 2 2 10
Crystallization time (days)
X R D results (estimated crystallinity %)
21 21 14 21 31 31 21
Amb+MTN Beta (70%) Amb +((Beta)y Beta (100%) Beta (100%) A m + B e t a (50%) MF1 + ( ( C d ) ) + A m
aWithout seeds.hAm = amorphous material.°(()) = traces.aC = cristoba[ite.
hydrofluoric acid and 3.1 g of boric acid (x =0.25 ) were added to the resulting solution under stirring. Finally, the silica source (6.42g) was slowly poured in the thoroughly stirred solution. The resulting gel was then stirred for about 1 h. The pH of the starting mixture was close to 9. Seeds of zeolite beta prepared using the classical alkaline route [1] were introduced (3 wt.% with respect to the silica content)just before the gel was transferred into a PTFE-lined stainless-steel autoclave and heated at 170~'C for 21 days. After reaction (pH ~ 9 ) , the solid obtained was filtered, washed with distilled boiling water until the pH of the filtrate was neutral and then dried at 60"C overnight. The calcination of the samples was performed under air at 550°C for 6 h, the heating rate being 2 C r' min-1 and an additional holding step being programmed at 200~'C for 2 h.
2.3. Chemical analysis The analysis for Si and B was performed by atomic absorption spectroscopy. F - was determined by using a fluoride ion-selective electrode after mineralization. The amount of water and organic species was obtained by thermogravimetry. The analysis of C and N was also performed by coulometric and catharometric determinations, respectively, after calcination of the samples.
S, Kallus et aL / Microporous Materials 10 (1997) 181 188
2.4. Powder X-ray diffraction The powder patterns were obtained with Cu KT~ radiation on a STOE STADI-P diffractometer equipped with a curved germanium (111 ) primary monochromator and a linear positionsensitive detector. The estimated crystallinity of the BEA-type samples was determined from the height of the peak at 20 close to 22.5 °. Sample D in Table 1 was taken as the 100% reference. 2.5. 13C and I1B solid-state M A S N M R
spectroscopy The spectra were recorded on a Bruker MSL 300 spectrometer under the following conditions: • for 1H-13C CP MAS NMR: chemical shift s t a n d a r d = T M S ; frequency=75.5 MHz; pulse length = 4.6 tas; pulse interval = 5 s; contact time = 2 ms; spinning rate = 4000 Hz; number of scans = 750; • for I~B MAS NMR: chemical shift standard= (C2Hs)2OBF3; frequency=96.3 MHz; pulse length=0.7 ~ts; pulse interval=3 s; spinning rate = 8000 Hz; number of scans = 110.
2.6. Thermal analysis Prior to analysis, the solids were kept in a wet atmosphere over a saturated aqueous solution of NH4C1 (p/po=0.85) during 24h in order to set the hydration state. T.g. was performed on a Mettler 1 thermoanalyser by heating in air at. 4"C m i n - l D.t.a, was carried out in air on a BDLSetaram M2 apparatus and was programmed to scan from 20 to 700~C at a heating rate of 10 C ~' m i n - t
3. Results and discussion
3.1. Influence of some co,stallization parameters The most representative syntheses are described in Table 1. Systematically one observes a strong increase of the viscosity of the reaction mixture
183
during the synthesis, which was already noticed for (Si,A1) beta zeolite [15]. This phenomenon was attributed to the polymerization process involving the DABCO and methylamine molecules (see Section 3.4). The presence of seeds is critical. Indeed, without seeds, no crystallization of zeolite beta was observed after 21 days of reaction from a gel characterized by Si/B and F - / S i molar ratios, respectively, equal to 2 and 6 (sample A), the product recovered consisting of MTN-type zeolite and residual gel. A similar experiment (sample B) performed in the presence of seeds led to a mixture of beta zeolite and gel, the estimated crystallinity (It,l) being close to 70%. Taking this into account, all other experiments were made in the presence of seeds. Two other chemical parameters were mainly studied, i.e., the Si/B and F - / S i molar ratios of the gels. For instance when Si/B=2 and F - / S i = 2 , a well-crystallized sample of beta (Ire~=100%) was obtained after 21 days of reaction (sample D), no further change occurring after 31 days (sample E). When the crystallization time is shorter, i.e., 14 days (sample C), only traces of beta zeolite are detected. This is indicative of a slow crystallization rate. When the Si/B is increased (sample F ) or the F - / S i is increased (samples B and G) the formation of beta zeolite is unfavored (samples B and F ) or even completely hindered (sample G). Indeed, in the latter case the final product consists of MTNand MFI-type materials (with unreacted gel). It is thus clear that the formation of borosilicate zeolite beta is strongly dependent on the boron content of the gel. Actually this (Si,B) system behaves similarly as the (Si,A1) system [15] under otherwise comparable experimental conditions. However, it appears that the (Si,B) system is less 'reactive', with slower crystallization rates. In fact, a synthesis experiment analogous to experiment F, but with boron replaced by aluminum (Si/AI molar ratio=5), led to a completely crystallized sample of beta within 10-15 days. Moreover the range of Si/T m molar ratio ( T = B or A1) suitable to the formation of zeolite beta shifts towards lower values in the case of boron. Thus, with Si/B = 2, a well-crystallized zeolite beta is obtained (Sample D), whereas no
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S. Kallus et al. / Microporous Materials 10 (1997) 181 188
crystallization occurs in the corresponding (Si,A1) experiment. The differences observed between the two systems might be related to a stronger complexation of boron by the fluoride ions. Such a hypothesis is strengthened by the observations made during the synthesis of MFI-type zeolites from fluoride media, where boron is less incorporated than aluminum [16]. Indeed, MFI-type zeolite samples with a Si/T m molar ratio (TIn=A1 or B) close to 30 are obtained from an aluminum-poor (A1/Si=0.14) and fluoride-rich (F/Si = 0.5) aluminosilicate gel or from a boron-rich ( B / S i = I ) and fluoride-poor ( F/Si = 0.125 ) borosilicate gel (the other experimental conditions being the same). Concerning the nature of the soluble boron species, some data were reported in the literature [17,18]. In particular a 19F N M R study [18] showed the presence of fluoroborate (BF 4) and hydroxyfluoroborate (BF3OH and BF2(OH)2) species in the liquid phase of boron MFI-type synthesis mixture (the only silicate species found is the SiFZ6- ion). In our case, the same species are probably present too. We can imagine that the crystallization of the zeolite beta occurs through condensation reactions after hydrolysis of these species. Micrographs of the beta crystals (sample D) are shown in Fig. 1. The crystals display a truncated square bipyramidal morphology, with irregularities on the tops. The average size of the crystals is close to 1 rtm.
3.2. X-ray diffraction results Fig. 1. Micrographs of boron zeolite beta (sample D). Typical XRD patterns of the as-synthesized and calcined zeolite beta (sample D) are shown in Figs. 2(a) and 2(b), respectively. The X-ray diffraction data (d values and relative intensities) are listed in Table 2. Only few small differences are visible. In the XRD pattern of the as-made material the first broad peak is less intense and a few minor peaks (d= 7.53, 6.53 and 6.05 A) are missing. It is worthy to note that these three peaks are present in the corresponding pattern of as-synthesized zeolite beta prepared from basic fluoride-free gels containing sodium and tetraethylammonium cations [1]. Compared to the XRD pattern of an assynthesized aluminosilicate beta zeolite (with the same Si/T In molar ratio) [15], all the peaks are
shifted towards higher 20 values for the as-made boron sample. This means that, in the latter case, the unit cell volume is lower than that of the corresponding aluminum sample. Finally, this observation is consistent with the incorporation of boron in the framework.
3.3. Chemical analysis The elemental analysis of the D is the following (in wt.%): Si, 0.085: C, 12.3; N, 4.8: F, 1.0. The Si/B molar ratio close to than the corresponding ratio (i.e.,
as-made sample 35.2; B. 1.0: A1, 14 is far higher 2) of the starting
185
S. Kallus et al. ,; Microporous Materials 10 (1997) 181-188
Table 2 Powder X-ray diffraction data of boron-beta zeolite (sample D) prepared from fluoride media
x 103 2.0
I(cps)
As-synthesized form d (A) LO 11.82
2.0
b
LO
t\
4.92 4.14 3.944 3.490 3.244 3.093 2.991 2.927 2.668 2.552 2.480 2.397 2.070 2.040
Calcined form Relative intensity (1/I o × 100) 16 (b)
5 (vb) 22 100 9 10 6 10 5 8 4 4 3 (b) 9 3 (b)
d (,~)
Relative Intensity (L/lo x 100)
11.64 7.53 6.53 6.05 4.80 4.13 3.937 3.492 3.267 3.085 2.996 2.920 2.666 2.557 2.471 2.397 2.065 2.040
63 (b) 7 (b) 14 7 (b) 5 (vb) 17 100 16 13 12 12 7 8 4 5 4 10 2 (b)
b, broad: vb, very broad. tO.O
20.0
30.O
40.0
20 (degrees)
Fig. 2. X-ray powder diffraction patterns (Cu K¢q) of boron zeolite beta (sample D): as-synthesized form (a), calcined form (b).
the dabconium cation, a polymeric compound resulting from the transformation of DABCO. Nevertheless, the amount of occluded organics is expressed in the following molar unit-cell formula (based on 64 T atoms) assuming DABCO as the occluded organic: 8.1 DABCO 8i59.45 B4. 4 Alo.15 O128 2.6F-; xH20
mixture, which confirms the strong complexation of boron by fluoride anions. According to the XRD results (discussed above) and to the ~B MAS NMR spectroscopy (see below) all boron is incorporated in the framework. The detected amount of AI is very low and probably arises from the seeds of (Si,A1) beta. The corresponding bulk Si/A1 molar ratio is close to 400. The content in organics is close to 20 wt.%, which fits well the usual values found for beta samples. The C/N molar ratio equal to 3 is characteristic of the DABCO molecule. However, according to the lsC MAS NMR spectroscopy (see below), the main incorporated organic species is, beside
Finally, to get a satisfactory charge balance, about half of the amine functions should be protonated. In spite of the presence of 2.6 F - ions per unitcell, no characteristic 19F NMR signals corresponding to F - in ion-pairs (e.g., in the - 6 0 to -80ppm/CFC13 range) [19] are visible, the F - ion being distributed over a large variety of species. 3.4. lsC CP M A S N M R spectroscopy
Fig. 3 displays the 13C CP MAS NMR spectrum of the as-made sample D. Two broad peaks occur at about 46 and 55 ppm.
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S. Kallus et al. /Microporous Materials 10 (1997) 181 188
R may be a proton or a methyl cation arising from the breaking of the methylamine. 3.5.
I
NMR
spectroscopy
The spectra of sample D in its as-made and calcined-rehydrated forms are given in Fig. 4. Both spectra display only one peak at - 4 . 1 (as-made sample) and - 3 . 7 p p m (calcined sample) from BF30(C2Hs) 2, which is characteristic of BO4 tetrahedra in a crystalline borosilicate structure [21,22]. There is no evidence in Fig. 4(a) (as-made sample) for another resonance line at c a = - 0 . 3 p p m as observed by Gabelica [21] and attributed to B O 4 units present in an amorphous phase. Moreover, for the calcined sample the line at ca. - 4 p p m is narrower. The reason for this reduced line may be
j 80
11B M A S
I
I
60
40
20
,~ppm/TMS Fig. 3. ~3CCP MAS NMR spectrum of an as-made boron zeolite beta (sample D).
The signal at ca. 46 p p m can be assigned to the carbon atoms of the D A B C O molecule. According to the p K a values of the D A B C O molecule (about 8.6 and 2.5) [20] and the p H value of the reaction mixture, which is close to 9, D A B C O is probably occluded in its m o n o p r o t o n a t e d form. The main signal at about 55 p p m corresponds to a transformation product of the organic species originally present. This signal was previously assigned, in the case of the synthesis of (Si,AI) beta zeolite [15], to the polyethylenepiperazine compound arising from the D A B C O (and the methylamine) molecules via the following schematic way: I
T
20
I
L
0
I
I
-20
8ppm/(C2H5)2OBF 3 etc
.,.
Fig. 4. 11B MAS NMR spectra of boron zeolite beta (sample D): as-synthesized form (a), calcined form (b).
s. Kallus et al. /Microporous Materials 10 (1997) 181-188
due to the suppression of dipolar interactions with other nuclei such as N and F which are eliminated by calcination. The change in chemical environment is also responsible for the small shift to lower field. Concerning the 298i MAS N M R spectroscopy, the spectrum (not reported) of the as-synthesized sample D displays only one signal at about - 110 ppm (reference TMS). As already observed for borosilicate samples prepared from alkaline media [12], no additional component corresponding to Si(OSi)3(OB) is visible. 3.6.
T h e r m a l analysis
The d.t.a, and t.g. curves of the as-synthesized sample D performed under air are given in Fig. 5. On the t.g. curve, three weight-losses are observed at about 80, 350 and 500°C. The first one (2.6 wt.%) corresponds to the loss of the adsorbed water, which is observed as a weak endothermic broad peak on the d.t.a, curve. The two other weight losses (ca. 20 wt.%) correspond to the thermal decomposition of the organic species occluded in the structure.
T G wt Ic6s l°/ol
lendo
U
10
187
On the d.t.a, curve this decomposition is characterized by three exothermic peaks with maxima occurring at 340, 420 and 580~C. The residues are completely white, no loss of crystallinity being observed after these thermal treatments.
4. Conclusion
Boron zeolite beta was synthesized using the 'fluoride route' from gels containing DABCO and methylamine as the templates. As a general rule the crystallization rates are very low. For instance a fully crystallized material with a Si/B molar ratio close to 14 could only be obtained after 20 days of reaction at 170°C from a boron-rich starting mixture ( S i / B = 2 and F / S i = 2 ) . For boronpoorer and/or fluoride-richer gels, the formation of zeolite beta is unfavored or even prevented. Compared to the (Si,A1) system, the (Si,B) system appears less "reactive' with larger crystallization times. Moreover, gels with high Si/AI molar ratios can be used. This might be related to a stronger complexation of boron by F - ions, which is less available for the building of the framework. It is worthy to note that similar observations were made for syntheses performed from alkaline fluoride-free gels. The crystallization of the boron-beta zeolite in fuoride media occurs probably through condensation reactions after hydrolysis of fluoroborate (BF4), hydroxyfluoroborate ( B F 3 O H and BFz(OH)2) and fluorosilicate species. According to the HB MAS N M R spectroscopy and to the X R D results, all the boron seems to be incorporated in the framework. According to the ~3C CP MAS N M R spectroscopy, in the (Si,B) beta samples, the occluded organic species is mainly a polymeric compound resulting from DABCO and methylamine, in addition to the dabconium cation.
20
Acknowledgment I
I
200
I
i
400
I
i
600
Fig. 5. Thermal analysis of the as-synthesizedboron zeolitebeta (sample D): t.g. (a) and d.t.a. (bl curves recorded under air.
The authors would like to thank Dr. H. Kessler for fruitful discussions, Dr. L. Delmotte for running the N M R spectra and S. Einhorn for taking the micrographs. S.K. is grateful for the financial
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S. Kallus et al. / Microporous Materials 10 (1997) 181-188
support provided by the European Union (Human Capital
and
CHRX-CT
Mobility
Program,
Contract
No.
93-0289 ).
References [1] R.L. Wadlinger, G.T. Kerr and E.J. Rosinski. US Pat. 3308069 (1967). [2] M.K. Rubin, Eur. Pat. Appl. 159847 (1985). [3] R.B. Calvert, C.D. Chang, M.K. Rubin and E.W. Valyocsik, US Pat. 4642226 (1986). [4] J.M. Newsam, M.M.J. Treacy, W.T. Koetsier, C.B. de Gruyter, Proc. R. Soc. London A420 (1988) 375. [5] J.B. Higgins, R.B. La Pierre, J.L. Schlenker, A.C. Rohrmann, J.D. Wood, G.T. Kerr, W.J. Rohrbaugh, Zeolites 8 (6) (1988) 446. [6] R. Kumar, A. Thangaraj, R.N. Bhat, P. Ratnasamy, Zeolites 10 (2) (1990) 85. [7] M.A. Camblor, J. Perez-Pariente, V. Fornes, Zeolites 12 (31 (1992) 280. [8] F. Schueth, R. Spichtinger, Prep.-Am. Chem. Soc., Div. Pet. Chim. 36 (4) ( 1991 ) 677. [9] M. Taramasso, G. Perego and B. Notari, in L.V.C. Rees (Ed.), Proceedings of the fifth International Conference on Zeolites, Heyden, London, 1980, p. 40. [10] T.F. Degman, J.D. Luther and N.Y. Chen, US Pat, 4788169 (1988).
[11 ] S.I. Zones, D.L. Holtermann, L.W. Jossens, D.S. Santilli, A. Rainis and J.N. Ziemer, PCT WO 9100777 (1991). [12] R. de Ruiter, K. Pamin, A.P.M. Kentgens, J.C. Jansen, H. van Bekkum, Zeolites 13 (8) (1993) 611. [13] B. Marler, R. BOhme and H. Gies, in R. yon Ballmoos. J.B. Higgins and M.J. Treacy (Eds.), Proceedings of the Ninth International Conference on Zeolites. ButterworthHeinemann, Boston, 1993, p. 425. [14] J.C. van der Waal, M.S. Rigotto, H. van Bekkum. J. Chem. Soc., Chem. Commun. ?? (1994) 1241. [15] P. Caullet, J. Hazm, J i . Guth, J.F. Joly, J. Lynch, F. Raatz, Zeolites 12 (3) (1992) 240. [16] J.L. Guth, H. Kessler and R. Wey, in Y. Murakami, A. lijima and J.W. Ward (Eds.), New Developments in Zeolite Science and Technology, Elsevier, Amsterdam, 1986, p. 121. [17] R.E. Mesmer, K.M. Palen, C.F. Baes, Inorg. Chem. 12 (1973) 89. [18] F. Marcuccilli-Hoffner, PhD thesis, Mulhouse, France, (1992). [19] E. Klock, L+ Delmotte, M. Soulard and J.k. Guth. in R. yon Ballmoos, J.B. Higgins and M.M.J. Treacy (Eds+), Proceedings of the Ninth International Zeolite Conference, Butterworth-Heinemann, Boston, 1993, p. 611. [20] D.J. Cralk, G.C. Levy, A. Lombardo, J. Phys. Chem. 86 (1982 J 3893. [2l] Z. Gabelica, G. Debras, J.B. Nagy, J. Stud. Surf. Sci. Catal. 19 (1984) 113. [22] E.G. Derouane, L. Baltusis, R.M. Dessau, K.D. Schmitt, Stud. Surf. Sci. Catal. 20 (1985) 135.
Microporous Materials 10 (1997) 181 188
Synthesis of boron-beta zeolite from near-neutral fluoride-containing gels S. Kallus, J. Patarin, P. Caullet *, A.C. Faust 1,ahoratoire de MatOriau v MinOraux U R A - C N R S 428, Ecole Nationale Sup~rieure de Chimie de Mulhouse-UHA, 3, rue AlJred Werner, F-68093 Mulhouse, Cedex, France
Received 18 November 1996: accepted 7 January 1997
Abstract
A borosilicate beta zeolite was prepared from almost neutral fluoride aqueous gels in the presence of 1,4-diazabicyclo[2.2.2]octane and methylamine as the templates. The influence of several physical and chemical parameters was studied. In comparison with the (Si,AI/ system, this (Si,B) system appears less 'reactive" with longer crystallization times. Moreover~ the products obtained display a narrower range of compositions. Only boron-rich (Si/B-21 and fluoride-poor media (F/Si=2) led to well-crystallized samples of beta zeolite characterized by a Si/B molar ratio close to 14. The differences observed between the two systems might be related to a stronger complexation of boron by the fluoride anions. The crystallization of the boron zeolite beta occurs probably through condensation reactions after hydrolysis of fluoroborate (BF,~), hydroxyfluoroborate (BF3OH and BF2(OH )2) and fluorosilicate species. Zeolite beta was characterized by XRD, SEM, chemical analysis, thermal analysis and I~B, 19F and t3C MAS NMR spectroscopy. The latter technique shows that the as-made beta samples contain, beside the dabconium cation, a polymeric compound identified as polyethylene piperazine. ~ 1997 Elsevier Science B.V. I~2,vwor&v Beta zeolite: Borosilicate; DABCO: d.t.a., t.g.: Fluoride media; Solid state NMR spectroscopy; Synthesis
1. Introduction
Zeolite beta was first synthesized in 1967 from basic aqueous aluminosdicate gels with sodium and tetraethylammonium as the templates [1]. Afterwards the use of other templating agents such as dibenzyl 1,4-diazabicyclo[2.2.2]octane [2] or dibenzyldimethylammonium cation [3] was also claimed. This high-silica zeolite (Si/A1 molar ratio from ca. 10 to several hundreds) displays a threedimensional network of channels delimited by
* Corresponding author. Tel.,'tax: + 33 399428730. 0927-0513,, 97 $17 00 ,c, 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 6 5 1 3 1 9 7 ) 0 0 0 1 0 - 2
twelve-membered rings. Due to its great complexity, the structure remained unresolved for over 21) years, zeolite beta being actually an intergrown hybrid of two distinct but closely related polymorphs [4, 5]. Partial or complete substitution of aluminum by iron [6], gallium [7,8] or boron [9-14] could be achieved in the framework of zeolite beta by direct synthesis from basic aqueous gels. The borosilicate beta zeolite is mostly prepared in the presence of the tetraethylammonium cation as the templating agent, except in Refs. [13,14] where the template is the 4,4'-trimethylene dipiperidine [13] or the dibenzyldimethylammonium cation [14]. Depend-
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S. Kallus et al. / Microporous Materials 10 ( 1997i 181-188
ing on the nature of the template, samples of boron-beta were obtained with a Si/B molar ratio ranging from about 10 [12] to several hundreds [14]. We reported recently on the synthesis of (Si,A1) beta zeolite from near neutral fluoride-containing media in the presence of 1,4-diazabicyclo[2.2.2]octane and methylamine as the templates [15]. In such non-alkaline reaction mixtures, the hydroxide ion is replaced by the fluoride ion as the mineralizing agent [16]. In the present work, these experimental synthesis conditions were used to prepare a borosilicate beta zeolite. The samples were characterized by XRD, SEM, chemical analysis, thermal analysis and 13C, t~B and 19F MAS N M R spectroscopy.
2. Experimental 2.1. Reactants The reactants were 1,4-diaza-bicyclo[2.2.2]octane (DABCO, Aldrich, 98%), methylamine (Fluka, 40wt.% in water), hydrofluoric acid (Prolabo, normapur, 40 wt.% in water) and ammonium fluoride (Prolabo, rectapur). The silicon and the boron sources were a precipitated silica (Merck, 94% SiO2) and boric acid (Merck, 99%).
2.2. Synthesis procedure The samples were obtained by hydrothermal synthesis at 170r'C. The molar composition of the starting mixtures was: 1SiO2:xB203:1DABCO: 1CH3NH2:2HF:yNH4F: 10H20 with x = 0.1 or 0.25 and y = 0, 4 or 8. The synthesis procedure was similar to that used for synthesizing the alumino-silicate beta samples [15], seed crystals (crushed crystals of (Si,AI) zeolite beta) being generally added to the reaction mixtures. As an example, for sample D (Table 1 ), l l . 2 g of DABCO and 7.8 g of methylamine were first dissolved in 6.9 g of distilled water. Then, 10 g of
Table 1 Description of the most representative syntheses Sample
Aa B C D E F G
Molar ratios of the gels Si/B
F /Si
2 2 2 2 2 5 2
6 6 2 2 2 2 10
Crystallization time (days)
X R D results (estimated crystallinity %)
21 21 14 21 31 31 21
Amb+MTN Beta (70%) Amb +((Beta)y Beta (100%) Beta (100%) A m + B e t a (50%) MF1 + ( ( C d ) ) + A m
aWithout seeds.hAm = amorphous material.°(()) = traces.aC = cristoba[ite.
hydrofluoric acid and 3.1 g of boric acid (x =0.25 ) were added to the resulting solution under stirring. Finally, the silica source (6.42g) was slowly poured in the thoroughly stirred solution. The resulting gel was then stirred for about 1 h. The pH of the starting mixture was close to 9. Seeds of zeolite beta prepared using the classical alkaline route [1] were introduced (3 wt.% with respect to the silica content)just before the gel was transferred into a PTFE-lined stainless-steel autoclave and heated at 170~'C for 21 days. After reaction (pH ~ 9 ) , the solid obtained was filtered, washed with distilled boiling water until the pH of the filtrate was neutral and then dried at 60"C overnight. The calcination of the samples was performed under air at 550°C for 6 h, the heating rate being 2 C r' min-1 and an additional holding step being programmed at 200~'C for 2 h.
2.3. Chemical analysis The analysis for Si and B was performed by atomic absorption spectroscopy. F - was determined by using a fluoride ion-selective electrode after mineralization. The amount of water and organic species was obtained by thermogravimetry. The analysis of C and N was also performed by coulometric and catharometric determinations, respectively, after calcination of the samples.
S, Kallus et aL / Microporous Materials 10 (1997) 181 188
2.4. Powder X-ray diffraction The powder patterns were obtained with Cu KT~ radiation on a STOE STADI-P diffractometer equipped with a curved germanium (111 ) primary monochromator and a linear positionsensitive detector. The estimated crystallinity of the BEA-type samples was determined from the height of the peak at 20 close to 22.5 °. Sample D in Table 1 was taken as the 100% reference. 2.5. 13C and I1B solid-state M A S N M R
spectroscopy The spectra were recorded on a Bruker MSL 300 spectrometer under the following conditions: • for 1H-13C CP MAS NMR: chemical shift s t a n d a r d = T M S ; frequency=75.5 MHz; pulse length = 4.6 tas; pulse interval = 5 s; contact time = 2 ms; spinning rate = 4000 Hz; number of scans = 750; • for I~B MAS NMR: chemical shift standard= (C2Hs)2OBF3; frequency=96.3 MHz; pulse length=0.7 ~ts; pulse interval=3 s; spinning rate = 8000 Hz; number of scans = 110.
2.6. Thermal analysis Prior to analysis, the solids were kept in a wet atmosphere over a saturated aqueous solution of NH4C1 (p/po=0.85) during 24h in order to set the hydration state. T.g. was performed on a Mettler 1 thermoanalyser by heating in air at. 4"C m i n - l D.t.a, was carried out in air on a BDLSetaram M2 apparatus and was programmed to scan from 20 to 700~C at a heating rate of 10 C ~' m i n - t
3. Results and discussion
3.1. Influence of some co,stallization parameters The most representative syntheses are described in Table 1. Systematically one observes a strong increase of the viscosity of the reaction mixture
183
during the synthesis, which was already noticed for (Si,A1) beta zeolite [15]. This phenomenon was attributed to the polymerization process involving the DABCO and methylamine molecules (see Section 3.4). The presence of seeds is critical. Indeed, without seeds, no crystallization of zeolite beta was observed after 21 days of reaction from a gel characterized by Si/B and F - / S i molar ratios, respectively, equal to 2 and 6 (sample A), the product recovered consisting of MTN-type zeolite and residual gel. A similar experiment (sample B) performed in the presence of seeds led to a mixture of beta zeolite and gel, the estimated crystallinity (It,l) being close to 70%. Taking this into account, all other experiments were made in the presence of seeds. Two other chemical parameters were mainly studied, i.e., the Si/B and F - / S i molar ratios of the gels. For instance when Si/B=2 and F - / S i = 2 , a well-crystallized sample of beta (Ire~=100%) was obtained after 21 days of reaction (sample D), no further change occurring after 31 days (sample E). When the crystallization time is shorter, i.e., 14 days (sample C), only traces of beta zeolite are detected. This is indicative of a slow crystallization rate. When the Si/B is increased (sample F ) or the F - / S i is increased (samples B and G) the formation of beta zeolite is unfavored (samples B and F ) or even completely hindered (sample G). Indeed, in the latter case the final product consists of MTNand MFI-type materials (with unreacted gel). It is thus clear that the formation of borosilicate zeolite beta is strongly dependent on the boron content of the gel. Actually this (Si,B) system behaves similarly as the (Si,A1) system [15] under otherwise comparable experimental conditions. However, it appears that the (Si,B) system is less 'reactive', with slower crystallization rates. In fact, a synthesis experiment analogous to experiment F, but with boron replaced by aluminum (Si/AI molar ratio=5), led to a completely crystallized sample of beta within 10-15 days. Moreover the range of Si/T m molar ratio ( T = B or A1) suitable to the formation of zeolite beta shifts towards lower values in the case of boron. Thus, with Si/B = 2, a well-crystallized zeolite beta is obtained (Sample D), whereas no
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S. Kallus et al. / Microporous Materials 10 (1997) 181 188
crystallization occurs in the corresponding (Si,A1) experiment. The differences observed between the two systems might be related to a stronger complexation of boron by the fluoride ions. Such a hypothesis is strengthened by the observations made during the synthesis of MFI-type zeolites from fluoride media, where boron is less incorporated than aluminum [16]. Indeed, MFI-type zeolite samples with a Si/T m molar ratio (TIn=A1 or B) close to 30 are obtained from an aluminum-poor (A1/Si=0.14) and fluoride-rich (F/Si = 0.5) aluminosilicate gel or from a boron-rich ( B / S i = I ) and fluoride-poor ( F/Si = 0.125 ) borosilicate gel (the other experimental conditions being the same). Concerning the nature of the soluble boron species, some data were reported in the literature [17,18]. In particular a 19F N M R study [18] showed the presence of fluoroborate (BF 4) and hydroxyfluoroborate (BF3OH and BF2(OH)2) species in the liquid phase of boron MFI-type synthesis mixture (the only silicate species found is the SiFZ6- ion). In our case, the same species are probably present too. We can imagine that the crystallization of the zeolite beta occurs through condensation reactions after hydrolysis of these species. Micrographs of the beta crystals (sample D) are shown in Fig. 1. The crystals display a truncated square bipyramidal morphology, with irregularities on the tops. The average size of the crystals is close to 1 rtm.
3.2. X-ray diffraction results Fig. 1. Micrographs of boron zeolite beta (sample D). Typical XRD patterns of the as-synthesized and calcined zeolite beta (sample D) are shown in Figs. 2(a) and 2(b), respectively. The X-ray diffraction data (d values and relative intensities) are listed in Table 2. Only few small differences are visible. In the XRD pattern of the as-made material the first broad peak is less intense and a few minor peaks (d= 7.53, 6.53 and 6.05 A) are missing. It is worthy to note that these three peaks are present in the corresponding pattern of as-synthesized zeolite beta prepared from basic fluoride-free gels containing sodium and tetraethylammonium cations [1]. Compared to the XRD pattern of an assynthesized aluminosilicate beta zeolite (with the same Si/T In molar ratio) [15], all the peaks are
shifted towards higher 20 values for the as-made boron sample. This means that, in the latter case, the unit cell volume is lower than that of the corresponding aluminum sample. Finally, this observation is consistent with the incorporation of boron in the framework.
3.3. Chemical analysis The elemental analysis of the D is the following (in wt.%): Si, 0.085: C, 12.3; N, 4.8: F, 1.0. The Si/B molar ratio close to than the corresponding ratio (i.e.,
as-made sample 35.2; B. 1.0: A1, 14 is far higher 2) of the starting
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S. Kallus et al. ,; Microporous Materials 10 (1997) 181-188
Table 2 Powder X-ray diffraction data of boron-beta zeolite (sample D) prepared from fluoride media
x 103 2.0
I(cps)
As-synthesized form d (A) LO 11.82
2.0
b
LO
t\
4.92 4.14 3.944 3.490 3.244 3.093 2.991 2.927 2.668 2.552 2.480 2.397 2.070 2.040
Calcined form Relative intensity (1/I o × 100) 16 (b)
5 (vb) 22 100 9 10 6 10 5 8 4 4 3 (b) 9 3 (b)
d (,~)
Relative Intensity (L/lo x 100)
11.64 7.53 6.53 6.05 4.80 4.13 3.937 3.492 3.267 3.085 2.996 2.920 2.666 2.557 2.471 2.397 2.065 2.040
63 (b) 7 (b) 14 7 (b) 5 (vb) 17 100 16 13 12 12 7 8 4 5 4 10 2 (b)
b, broad: vb, very broad. tO.O
20.0
30.O
40.0
20 (degrees)
Fig. 2. X-ray powder diffraction patterns (Cu K¢q) of boron zeolite beta (sample D): as-synthesized form (a), calcined form (b).
the dabconium cation, a polymeric compound resulting from the transformation of DABCO. Nevertheless, the amount of occluded organics is expressed in the following molar unit-cell formula (based on 64 T atoms) assuming DABCO as the occluded organic: 8.1 DABCO 8i59.45 B4. 4 Alo.15 O128 2.6F-; xH20
mixture, which confirms the strong complexation of boron by fluoride anions. According to the XRD results (discussed above) and to the ~B MAS NMR spectroscopy (see below) all boron is incorporated in the framework. The detected amount of AI is very low and probably arises from the seeds of (Si,A1) beta. The corresponding bulk Si/A1 molar ratio is close to 400. The content in organics is close to 20 wt.%, which fits well the usual values found for beta samples. The C/N molar ratio equal to 3 is characteristic of the DABCO molecule. However, according to the lsC MAS NMR spectroscopy (see below), the main incorporated organic species is, beside
Finally, to get a satisfactory charge balance, about half of the amine functions should be protonated. In spite of the presence of 2.6 F - ions per unitcell, no characteristic 19F NMR signals corresponding to F - in ion-pairs (e.g., in the - 6 0 to -80ppm/CFC13 range) [19] are visible, the F - ion being distributed over a large variety of species. 3.4. lsC CP M A S N M R spectroscopy
Fig. 3 displays the 13C CP MAS NMR spectrum of the as-made sample D. Two broad peaks occur at about 46 and 55 ppm.
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S. Kallus et al. /Microporous Materials 10 (1997) 181 188
R may be a proton or a methyl cation arising from the breaking of the methylamine. 3.5.
I
NMR
spectroscopy
The spectra of sample D in its as-made and calcined-rehydrated forms are given in Fig. 4. Both spectra display only one peak at - 4 . 1 (as-made sample) and - 3 . 7 p p m (calcined sample) from BF30(C2Hs) 2, which is characteristic of BO4 tetrahedra in a crystalline borosilicate structure [21,22]. There is no evidence in Fig. 4(a) (as-made sample) for another resonance line at c a = - 0 . 3 p p m as observed by Gabelica [21] and attributed to B O 4 units present in an amorphous phase. Moreover, for the calcined sample the line at ca. - 4 p p m is narrower. The reason for this reduced line may be
j 80
11B M A S
I
I
60
40
20
,~ppm/TMS Fig. 3. ~3CCP MAS NMR spectrum of an as-made boron zeolite beta (sample D).
The signal at ca. 46 p p m can be assigned to the carbon atoms of the D A B C O molecule. According to the p K a values of the D A B C O molecule (about 8.6 and 2.5) [20] and the p H value of the reaction mixture, which is close to 9, D A B C O is probably occluded in its m o n o p r o t o n a t e d form. The main signal at about 55 p p m corresponds to a transformation product of the organic species originally present. This signal was previously assigned, in the case of the synthesis of (Si,AI) beta zeolite [15], to the polyethylenepiperazine compound arising from the D A B C O (and the methylamine) molecules via the following schematic way: I
T
20
I
L
0
I
I
-20
8ppm/(C2H5)2OBF 3 etc
.,.
Fig. 4. 11B MAS NMR spectra of boron zeolite beta (sample D): as-synthesized form (a), calcined form (b).
s. Kallus et al. /Microporous Materials 10 (1997) 181-188
due to the suppression of dipolar interactions with other nuclei such as N and F which are eliminated by calcination. The change in chemical environment is also responsible for the small shift to lower field. Concerning the 298i MAS N M R spectroscopy, the spectrum (not reported) of the as-synthesized sample D displays only one signal at about - 110 ppm (reference TMS). As already observed for borosilicate samples prepared from alkaline media [12], no additional component corresponding to Si(OSi)3(OB) is visible. 3.6.
T h e r m a l analysis
The d.t.a, and t.g. curves of the as-synthesized sample D performed under air are given in Fig. 5. On the t.g. curve, three weight-losses are observed at about 80, 350 and 500°C. The first one (2.6 wt.%) corresponds to the loss of the adsorbed water, which is observed as a weak endothermic broad peak on the d.t.a, curve. The two other weight losses (ca. 20 wt.%) correspond to the thermal decomposition of the organic species occluded in the structure.
T G wt Ic6s l°/ol
lendo
U
10
187
On the d.t.a, curve this decomposition is characterized by three exothermic peaks with maxima occurring at 340, 420 and 580~C. The residues are completely white, no loss of crystallinity being observed after these thermal treatments.
4. Conclusion
Boron zeolite beta was synthesized using the 'fluoride route' from gels containing DABCO and methylamine as the templates. As a general rule the crystallization rates are very low. For instance a fully crystallized material with a Si/B molar ratio close to 14 could only be obtained after 20 days of reaction at 170°C from a boron-rich starting mixture ( S i / B = 2 and F / S i = 2 ) . For boronpoorer and/or fluoride-richer gels, the formation of zeolite beta is unfavored or even prevented. Compared to the (Si,A1) system, the (Si,B) system appears less "reactive' with larger crystallization times. Moreover, gels with high Si/AI molar ratios can be used. This might be related to a stronger complexation of boron by F - ions, which is less available for the building of the framework. It is worthy to note that similar observations were made for syntheses performed from alkaline fluoride-free gels. The crystallization of the boron-beta zeolite in fuoride media occurs probably through condensation reactions after hydrolysis of fluoroborate (BF4), hydroxyfluoroborate ( B F 3 O H and BFz(OH)2) and fluorosilicate species. According to the HB MAS N M R spectroscopy and to the X R D results, all the boron seems to be incorporated in the framework. According to the ~3C CP MAS N M R spectroscopy, in the (Si,B) beta samples, the occluded organic species is mainly a polymeric compound resulting from DABCO and methylamine, in addition to the dabconium cation.
20
Acknowledgment I
I
200
I
i
400
I
i
600
Fig. 5. Thermal analysis of the as-synthesizedboron zeolitebeta (sample D): t.g. (a) and d.t.a. (bl curves recorded under air.
The authors would like to thank Dr. H. Kessler for fruitful discussions, Dr. L. Delmotte for running the N M R spectra and S. Einhorn for taking the micrographs. S.K. is grateful for the financial
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S. Kallus et al. / Microporous Materials 10 (1997) 181-188
support provided by the European Union (Human Capital
and
CHRX-CT
Mobility
Program,
Contract
No.
93-0289 ).
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[11 ] S.I. Zones, D.L. Holtermann, L.W. Jossens, D.S. Santilli, A. Rainis and J.N. Ziemer, PCT WO 9100777 (1991). [12] R. de Ruiter, K. Pamin, A.P.M. Kentgens, J.C. Jansen, H. van Bekkum, Zeolites 13 (8) (1993) 611. [13] B. Marler, R. BOhme and H. Gies, in R. yon Ballmoos. J.B. Higgins and M.J. Treacy (Eds.), Proceedings of the Ninth International Conference on Zeolites. ButterworthHeinemann, Boston, 1993, p. 425. [14] J.C. van der Waal, M.S. Rigotto, H. van Bekkum. J. Chem. Soc., Chem. Commun. ?? (1994) 1241. [15] P. Caullet, J. Hazm, J i . Guth, J.F. Joly, J. Lynch, F. Raatz, Zeolites 12 (3) (1992) 240. [16] J.L. Guth, H. Kessler and R. Wey, in Y. Murakami, A. lijima and J.W. Ward (Eds.), New Developments in Zeolite Science and Technology, Elsevier, Amsterdam, 1986, p. 121. [17] R.E. Mesmer, K.M. Palen, C.F. Baes, Inorg. Chem. 12 (1973) 89. [18] F. Marcuccilli-Hoffner, PhD thesis, Mulhouse, France, (1992). [19] E. Klock, L+ Delmotte, M. Soulard and J.k. Guth. in R. yon Ballmoos, J.B. Higgins and M.M.J. Treacy (Eds+), Proceedings of the Ninth International Zeolite Conference, Butterworth-Heinemann, Boston, 1993, p. 611. [20] D.J. Cralk, G.C. Levy, A. Lombardo, J. Phys. Chem. 86 (1982 J 3893. [2l] Z. Gabelica, G. Debras, J.B. Nagy, J. Stud. Surf. Sci. Catal. 19 (1984) 113. [22] E.G. Derouane, L. Baltusis, R.M. Dessau, K.D. Schmitt, Stud. Surf. Sci. Catal. 20 (1985) 135.