- Email: [email protected]
Al ratio of zeolite omega by boron adulteration
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.
217
IMPROVING THE Si/Al RATIO OF ZEOLITE OMEGA BY BORON ADULTERATION Chen, J., Liu, E., Song, Y., Li, L., Zhang, S. and Guan, N. Cooperative Institute of Nankai and Tianjin University, Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, P.R.China. *E-mail: [email protected]
ABSTRACT Boron substituted omega (MAZ) was directly synthesized and the synthesis conditions were systemically investigated, focusing on reducing the template usage by employing ageing and seeding processes. Boron substituted zeolite omega was characterized by ^^B, ^^Al, ^^Si and 2D ^^Al 3Q MAS NMR to study the structure and dealuminization process. The result suggested that boron adulteration is a good way to improve the Si/Al ratio of omega. Keywords: Boron substituted; Zeolite; Omega; MAZ; Synthesis; INTRODUCTION Zeolite omega (MAZ) is a large-pore zeolite (7.5A) with hexagonal symmetry and intermediate range Si/Al ratio (2.5-5). Omega framework consists of columns of gmelinite cages bridged by oxygen atoms to give a 12-membered cylindrical channel system along the crystallographic c-axis. Its catalytic properties have been evaluated for gas-oil cracking ^^\ hydrocracking ^^\ aromatic alkylations ^^''^\ isomerizations ^^\ olefin hydration ^^\ and hydroconversion of alkanes ^^l A good technique to modify the acidity and improve the stability of zeolites, which are essential to their catalytic ability, is to increase the Si/Al ratio of zeolite. Traditionally, hydrothermal treatment, acid treatment, SiCU and NH4SiF4 dealuminization then reinsertion of Si were applied to modify zeolites. But all these methods have shortcoming of causing partly structure breakage. Boron adulteration and then deboronation, a method to improve the Si/Al ratio but contain the framework structure of zeolite in the maximum extent was applied to zeolite omega in present work. Due to the interesting catalytic activity of boron substituted zeolite, many boron substituted zeolites were studied ^^'^^\ Some boron substituted zeolites were hydrothermal synthesis directly ^^^'^^^ and some were obtained by modifying aluminosilicate with boron compounds ^^^^^^-^'^\ j h e indirect way, by substituting Al with B in a small amount, results in changes of the acidic property of zeolites. From the hydrothermal synthesis, zeolites with larger amounts of framework boron could be obtained until free of Al. Because boron atom is smaller than aluminium, it can more easily remove from the zeolite structure. When boron release from the structure, second-class mesoporous structures were created which are beneficial to some large molecular catalytic reactions. During the investigation of boron substituted zeolites, low content of template was tried to reduce the template consumption, and meanwhile, the employment of seeding method further reduce the amount of template. In deboronation process, water steam treatment was then applied to remove the boron and aluminium from the framework. Solid state MAS NMR was also performed to characterize the samples. EXPERIMENTAL Sodium metaaluminate. Boric acid, silica gel, tetra-methyl ammonium hydroxide and sodium hydroxide were mixed under stirring at room temperature for ten hours. The gel was aged at 50°C for 10 days. The crystallization was carried out at different conditions in autoclaves. The molar composition of the gel was (4.5-5)Na20: (x)Al203: (l-x)B203: 14Si02: 2.2TMAOH: 22OH2O (x=0.3—0.7). The aged gel can also be added as seeds to an as made gel with the same composition but without template TMAOH. Then the new gel was crystallization at the same condition. The product was washed and then calcined at 450°C for 4 to 5 hours to remove the organic compounds. The Na-omega was exchanged with 0.5M NH4CI at 90 °C for 4
218 times and then calcined at 450°C for 4 hours to get H-omega. Zeolites H-omega were then heated to 450°C, 500°C or 550°C in a quartz boat within a quartz cannulation under water steam for 3 hours. XRD determination was carried out on a Rigaku D/Max 2500 X-ray diffractometer at 40kV and 100mA with Cu Ka radiation.^^B, 27Al and 29Si MAS NMR spectra were obtained from a Varian UNITY plus-400M NMR. The nominal frequency of ^^B was 128.36 MHz, of ^^Al was 104.26 and of ^^Si was 79.47MHz. RESULT AND DISCUSSION Synthesis of zeolite omega The optimal condition to synthesize omega was selected by a series of experiments. That is to say, the composition aNa20: AI2O3: bSi02: cTMAOH: 22OH2O, the crystallization temperature and time were optimized (Table 1). Table 1. Crystallization temperature and time of synthesizing zeolite omega. Temperature Time
100°C 48h
110°C 34h
120°C 24h
130°C lOh
140°C 8h
145°C 8h
Zeolite omega with less than 30% impurity was synthesized when a=4—7.5, b=10—21, c=0.5—4.5, crystallization time 8h, crystallization temperature 135 °C. Pure zeolite omega was synthesized under conditions that a = 5 , b = 14, c=2.2, crystallization time 8h, crystallization temperature 135°C. Reducing the usage of template is important in synthesizing zeolite. Template was essential because omega could not be obtained without template till now. So, template is the main cost of zeolite synthesis on the one hand. And on the other hand, template is usually hard to remove from the framework of zeolite ^^^\ If it was calcined at inertia gas, carbonation will occur. While it was calcined in air, "hot site" arose and may cause losing of pore volume ^^^l A good way to synthesize zeolite is to use the seeding technique. The amount of omega seeds was studied below: At the proportion of 5Na20: AI2O3: 14Si02: 2.2TMAOH: 22OH2O, the gel was airproofed at 50°C for 10 days to produce seed crystals. And then at the same proportion but without template TMAOH, the mixture was stirred rapidly for 10 hours to make silicon-aluminum gel. And then different amounts of seed crystals were add to the gel to crystallization at 135''C for 8 hours. The result was showed below (Table 2): Table 2. Crystallization results of different content of seeds. Omega seed crystals used 2% 5% 10% 15%
Crystallization results No zeolite omega Zeolite omega with many other crystals and impurity Zeolite omega and about 30% other crystals Zeolite omega and less than 2% other crystals
The thermostability of zeolite omega synthesized by this method was also studied (showed in Table 3). Table 3. Calcination temperature and the relative crystallinity*. Calcination temperature 550°C 500^ 600 °C 88% 58% 95% Relative crystallinity *The XRD pattern of the as-synthesized sample dried at 120°C was used as standard (100%).
650°C No crystals
The process of synthesizing of boron-substituted zeolite is much similar as non-boron zeolite, using borate to substitute part of aluminum source in the precursor. The amount of boron used and the synthetic condition was studied (showed in Table 4) and it was relative harder to obtain zeolite omega out of these ranges.
219 Table 4. Composition of the starting mixture of boron-substituted zeolite omega aNa20: bAl203: CB2O3: dSi02: 2.2TMAOH: 22OH2O. a 4.9—4.6 4.8-4.5 4.7-4.5 4.7-4.5 4.6, 4.5
b 0.7 0.6 0.5
c 0.3 0.4 0.5
d 14 14 14
0.4
0.6
14
0.3
0.7
14
Characterization of zeolite omega The framework boron was investigated using chemical analysis, XRD pattern and ^^B MAS NMR. The chemical compositions of zeolite omega with 40% and 60% boron substituted (in synthetic precursor) were studied by chemical analysis with Si02/Al203 ratios 7.34 and 9.47, which shows that more boron atoms may substitute the aluminum atoms in the framework. In the XRD patterns, when zeolite was boron adulterated, the peaks move to low angle, which suggested the decrease of unit cell dimension after boron adulteration.
^^' 450 i asteam treatment
.-^A
Na-Al-B-omega
-200 PPm
Figure 1. '^B MAS NMR spectrum of B-Al- Q .
120
HO
40
0
mm)
Figure 2. ^^Al MAS NMR spectra of B-Al- Q with different boron content (in precursor).
Boron atoms in zeolite framework are mostly four-coordinated, which show sharp peaks at about ^ p p m in ^^B MAS NMR spectra (fig. 1 A). And the extra-framework three-coordinated boron is a broad peak (fig. IB). The spectra suggest that a large amount of boron atoms have removed from framework during detemplate process. And after water steam treatment at 450°C, almost all boron atoms removed from the omega framework. The ^^Al MAS NMR spectra of boron-adulterated omega (fig. 2) show that the behavior of aluminium atoms in zeolite framework is much the same as non-boron zeolite, which means, the aluminium atoms have the tendency to stay at the B site of zeolite omega ^^'^\ The ^^Si MAS NMR spectra of different zeolite omega were studied to show the distribution of Si, B and Al atoms in the framework of omega structure (fig. 3). In zeolite omega, Si(nAl)i(n=0—4, I = A or B) was used to indicate Si atoms with different circumstance. The chemical shifts were proved to have a linear relationship with the atoms distance and the Si—O—T (T=Si or Al) angles ^^^\ Ramdas and Klinowski ^^^^ gave the expression of: Si(ppm)=7.95n+143.O3-2O.34[3.37n+3.24(4-n)]sin(0i/2). The n in the expression denote the same as Si(nAl)i, and 0i denote the average angle of Si—O—T (T=^Si or Al). In the fig. 3, the peak at — 100.70ppm indicate the signal of Si(lAl)A, and the peak at -113.18 represent the signal of Si(OAl)B. Replace the Si(lAl)A=-100.70, Si(OAl)B=-l 13.18 into the expression show above, we can get that the angles of S I A " O — T and Sie—O—T were 141.91° and 152.79°. So the chemical shifts of other peaks were calculated with that expression (see table 5).
220
—I— -80
— I
-100
ppm
Figure 3. ^^Si MAS NMR spectra of A1-Q,B-A1-Q andB-Al-Q after 550°C steam treating for 3h. Table 5. The ascription of peaks in ^^Si MAS NMR spectra. Samples
Q
B-Q(50%) B-Q550°C steam 3h *M: Al or B or Al+B
Angles of bonds
T site
Si4(M*)
Si3(M)
Si2(M)
Sil(M)
SiO(M)
141.91
A(ppm)
-84.35
-89.8
-95.25
-100.70
-106.15
152.79
B(ppm)
-91.66
-97.04
-102.42
-107.8
-113.18
141.61
A(ppm)
-84.03
-89.48
-95.02
-100.47
-105.92
154.15
B(ppm)
-92.41
-97.78
-103.15
-108.53
-113.90
143.63 159.82
A(ppm) B(ppm)
-85.66 -95.11
-91.10 -100.46
-96.53 -105.81
-101.97 -111.15
-107.41 -116.50
Then, a self-written computer simulation program was used to get the strength of the peaks (table 6). Table 6. The result of the computer simulation of the ^^Si MAS NMR spectra. Samples
Si/M*
A/B(M)
Q
4.23
1.61
B-Q 4.91 (50%) B-Q550°C 7.92 steam treated 3h *M: Al or B or Al+B
2.01 /
T site A(%) B(%) A(%) B(%)
Si(4M) 0 0 0 0
Si(3M) 2.28 1.28 4.36 2.77
Si(2M) 14.83 7.47 9.08 1.99
Si(lM) 28.53 10.83 26.08 11.96
Si(OM) 22.20 12.58 27.09 16.65
A(%)
0
0
3.78
16.86
33.88
B(%)
0
0
5.69
14.66
25.11
The boron substituted zeolite omega had a more dramatic increase in Si/Al ratio when treated by high temperature steam then non-boron omega, which indicates that boron atoms remove from the structure easier than aluminium atoms. The boron atoms removed from the framework thoroughly when treated by steam, but the dealuminization also occurred. ^^Al MAS NMR spectra of the samples treated with steam were studied (fig. 4). The two peaks around 55ppm stand for the structure four-coordinated Al, and the peak relatively sharper at 5ppm represent extra-structure six-coordinated Al. While the broad peak at 40—20ppm, shows the
221 existence of five-coordinated or twisted four-coordinated Al, which was also observed in zeoUte Y, ZMS-5 and mordenite ^^^\ When treated with 450°C water steam, the number of aluminium atoms in site A was more intensively decreased than B site. While treated with 500 °C and 550 °C steam, there was no remarkable change, which suggested that at the beginning of dealuminization, the aluminium atoms in site A were easy to remove from the structure than in site B, which was in accordance with the result of ^^Si MAS NMR.
100
80
60
40
-2 0
20
-4 0
ppm
Figure 4. ^^Al MAS NMR spectra of B-Al-omega (50%) after steam treatment (a) HB-Al-omega (b)450°C 3h(c)500°C 3h(d)550°C 3h. ppl
a
I
ppl I
HBOmega
-60 -4a
HBOmega-450
-40
B \
-2(J 0"
'
-60
n(^C W^^^"^^--^^
^L.
A ^^\
20-
]>v-i\\.
\ V
60"
B
0"
\
A
r
20"
^'^^^yc^ ^''
4 0"
-20
y
'K
'^"Vlx^ I-
4 0~
), 0. \^ i
60"
\c
8 0"
JV"k^ ^^ —D
0
c
8 0" 100
80
60
40
20
0
-20
-40
ppm
100
80
60
40
20
0
-20
ppi I
-40
ppr
HBOmega-550
-60 -40^ -20
B^ 0"
\ K
A^
^^^^ D
20"
•
60"
^M^^
ii
4 0"
'
c
8 0" ^ — 1 — ^
100
80
60
40
20
0
-20
-40
ppm
^ E
100
—
1
—
80
'
•
— 1 — ^
60
40
20
0
-20
-40
ppm
Figure 5. 2D ^^Al 3Q MAS NMR spectra of boron-containing zeolite omega after steam treatment (a)HB-Al-Omega and(b)450°C (c)500°C (d)550°C steam treated.
222
The 2D ^^Al 3Q MAS NMR spectra were obtained to further study the dealuminization of boron substituted omega (fig. 5). Signal A and B denote the aluminium atoms in site A and B of zeolite omega. Signal D denotes the extra-structure six-coordinated aluminium atoms. Signal C can be associated to twist four-coordinated aluminum atom according to its chemical shift. The signal of twist four-coordinated aluminium comes from dealuminization appears after the process of detemplate, and which increase after 450 °C water steam treatment. Further dealuminization at 500°C steam treatment bring on a new signal E, which denotes extra-structure five-coordinated aluminium. The signal E increase when treated by 550°C steam. The broad peak of 20-40 ppm on the one-dimension MAS NMR spectra was the overlap-add of twist four-coordinated aluminum and extra-structure five-coordinated aluminium comparing to the 2D ^^Al 3Q MAS NMR spectra, which also shows that the extra-structure aluminium come from twist four-coordinated aluminium. CONCLUSION Boron adulteration as a good way to improve the Si/Al ratio of zeolite omega was systematically investigated, including the synthesis conditions in which aging and seeding process was applied in order to reduce the template consumption. The samples were dealuminated and deboronated under relatively milder steam treatment conditions and showed good MAZ structure as well as high Si/Al ratio. ACKNOWLEDGEMENT This work was a Joint Project between Nankai University and Tianjin University sponsored by the Ministry of Education, P. R. China and was financially supported by open project of China Petrochemical Corporation. It was alsofinanciallysupported by National Natural Science Fund of China: 20233030.
REFERENCES 1. Perrotta, A. J.; Kibby, C; Mitchell, B. R.; Tucci, E. R. J. Catal. 240(1978) :55. 2. Cole, J. F.; Kouwenhoven, H. W. AdV. Chem. Ser. (1973) :121. 3. Bowes, E.; Wise, J. J. U.S Pat 3,578,728, 1971. 4. Flanigen, E. M.; Kellberg, E. R. U.S. Patent 4,241,036, 1980. 5. Solinas, V.; Monaci, R.; Marongiu, B.; Fomi, L. Appl. Catal. 171(1983) :5. 6. Fajula, F.; Ibarra, R.; Figueras, F.; Gueguen, C. J. Catal. 89(1984) :64. 7. Coq, B.; Figueras, F.; Raja Fanova J. Catal. 114(1988) :321. 8. W.W. Kaeding, C. Chu, L.B. Young, S.A. Butter, J. Catal. 69 (1981) :392. 9. C.T.W. Chu, G.H. Kuehl, R.M. Lago, CD. Chang, J. Catal. 93 (1985) :451. 10. M.G. Howdeu, Zeolites 5 (1985) :334. 11. P. Ratnasamy, S.G. Hegde, A.J. Chandwadkar, J. Catal. 102 (1986) :467. 12. Shang Xiao-yu, Wang Yan-ji, Zhao Xin-qiang, etal.. Engineering Chemistry & Matallurgy(Chinese), 21(2000):98 13. M.A. Camblor, J. Perez-Pariente, V. Fomes, Zeolites 12 (1992) :280. 14. M. Taramasso, G. Perego, B. Notari, Proceedings of the Fifth IZC, 1980, p. 40. 15. G. Bellussi, R. Millini, A. Carati, G. Maddinelli, A. Gervasini, Zeolites 10 (1990) :642. 16. H. Kessler, J.M. Chezeau, J.L. Guth, H. Strub, G. Coudurier, Zeolites 7 (1987) :360. 17. J.C. Jansen, R. de Ruiter, E. Biron, H. van Bekkum, in: P.A. Jacobs, R.A. van Santen, Zeolites: Facts, Figures, Future, Elsevier, Amsterdam, 1989, p. 679. 18. CO. Arean, G.T. Palomino, F. Geobaldo, A. Zecchina, J. Phys. Chem. 100 (1996): 6678. 19. S.G. Hegde, R.A. Abdullah, R.N. Bhat, P. Ratnasamy, Zeolites 12 (1992) :951. 20. K.J. Chao, S.P. Sheu, L.-H. Lin, M.J. Genet, M.H. Feng, Zeolites 18 (1997) :18. 21. Z. Fu, D. Yin, D. Yin, Q. Li, L. Zhang, Y. Zhang, Microporous and Mesoporous Materials. 29 (1999): 351. 22. Rajib Bandyopadhyay, Yoshihiro Kubota, Noriaki Sugimoto,Yoshiaki Fukushima, Yoshihiro Sugi, Microporous and Mesoporous Materials. 32 (1999) :81. 23. X. Liu, J.M. Thomas, J. Chem. Soc, Chem. Commun. (1985) :1544. 24. B. Sulikowski, Klinowski, J. Zeolite Synthesis, ACS Symp. Ser. 398 (1989) :394. 25. K. Pat. Application GB 2175890A.
223
26. 27. 28. 29. 30.
J. F. Cole. Advan. Chem. Ser., 1973 :383. J. Klinowski, M. W. Anderson. J. Chem. Soc, Faraday Trans., 1986,1, 82, 569. G. Debraetal.. Zeolite, 6(1986), 161. S. Ramdas, J. Klinowski, Nature, (308)1984, 521. T.H. Chen, B.H. Wouters, P.J. Grobet, Euro. J. Tnorg. Chem., 2000, 281.
217
IMPROVING THE Si/Al RATIO OF ZEOLITE OMEGA BY BORON ADULTERATION Chen, J., Liu, E., Song, Y., Li, L., Zhang, S. and Guan, N. Cooperative Institute of Nankai and Tianjin University, Institute of New Catalytic Materials Science, College of Chemistry, Nankai University, Tianjin 300071, P.R.China. *E-mail: [email protected]
ABSTRACT Boron substituted omega (MAZ) was directly synthesized and the synthesis conditions were systemically investigated, focusing on reducing the template usage by employing ageing and seeding processes. Boron substituted zeolite omega was characterized by ^^B, ^^Al, ^^Si and 2D ^^Al 3Q MAS NMR to study the structure and dealuminization process. The result suggested that boron adulteration is a good way to improve the Si/Al ratio of omega. Keywords: Boron substituted; Zeolite; Omega; MAZ; Synthesis; INTRODUCTION Zeolite omega (MAZ) is a large-pore zeolite (7.5A) with hexagonal symmetry and intermediate range Si/Al ratio (2.5-5). Omega framework consists of columns of gmelinite cages bridged by oxygen atoms to give a 12-membered cylindrical channel system along the crystallographic c-axis. Its catalytic properties have been evaluated for gas-oil cracking ^^\ hydrocracking ^^\ aromatic alkylations ^^''^\ isomerizations ^^\ olefin hydration ^^\ and hydroconversion of alkanes ^^l A good technique to modify the acidity and improve the stability of zeolites, which are essential to their catalytic ability, is to increase the Si/Al ratio of zeolite. Traditionally, hydrothermal treatment, acid treatment, SiCU and NH4SiF4 dealuminization then reinsertion of Si were applied to modify zeolites. But all these methods have shortcoming of causing partly structure breakage. Boron adulteration and then deboronation, a method to improve the Si/Al ratio but contain the framework structure of zeolite in the maximum extent was applied to zeolite omega in present work. Due to the interesting catalytic activity of boron substituted zeolite, many boron substituted zeolites were studied ^^'^^\ Some boron substituted zeolites were hydrothermal synthesis directly ^^^'^^^ and some were obtained by modifying aluminosilicate with boron compounds ^^^^^^-^'^\ j h e indirect way, by substituting Al with B in a small amount, results in changes of the acidic property of zeolites. From the hydrothermal synthesis, zeolites with larger amounts of framework boron could be obtained until free of Al. Because boron atom is smaller than aluminium, it can more easily remove from the zeolite structure. When boron release from the structure, second-class mesoporous structures were created which are beneficial to some large molecular catalytic reactions. During the investigation of boron substituted zeolites, low content of template was tried to reduce the template consumption, and meanwhile, the employment of seeding method further reduce the amount of template. In deboronation process, water steam treatment was then applied to remove the boron and aluminium from the framework. Solid state MAS NMR was also performed to characterize the samples. EXPERIMENTAL Sodium metaaluminate. Boric acid, silica gel, tetra-methyl ammonium hydroxide and sodium hydroxide were mixed under stirring at room temperature for ten hours. The gel was aged at 50°C for 10 days. The crystallization was carried out at different conditions in autoclaves. The molar composition of the gel was (4.5-5)Na20: (x)Al203: (l-x)B203: 14Si02: 2.2TMAOH: 22OH2O (x=0.3—0.7). The aged gel can also be added as seeds to an as made gel with the same composition but without template TMAOH. Then the new gel was crystallization at the same condition. The product was washed and then calcined at 450°C for 4 to 5 hours to remove the organic compounds. The Na-omega was exchanged with 0.5M NH4CI at 90 °C for 4
218 times and then calcined at 450°C for 4 hours to get H-omega. Zeolites H-omega were then heated to 450°C, 500°C or 550°C in a quartz boat within a quartz cannulation under water steam for 3 hours. XRD determination was carried out on a Rigaku D/Max 2500 X-ray diffractometer at 40kV and 100mA with Cu Ka radiation.^^B, 27Al and 29Si MAS NMR spectra were obtained from a Varian UNITY plus-400M NMR. The nominal frequency of ^^B was 128.36 MHz, of ^^Al was 104.26 and of ^^Si was 79.47MHz. RESULT AND DISCUSSION Synthesis of zeolite omega The optimal condition to synthesize omega was selected by a series of experiments. That is to say, the composition aNa20: AI2O3: bSi02: cTMAOH: 22OH2O, the crystallization temperature and time were optimized (Table 1). Table 1. Crystallization temperature and time of synthesizing zeolite omega. Temperature Time
100°C 48h
110°C 34h
120°C 24h
130°C lOh
140°C 8h
145°C 8h
Zeolite omega with less than 30% impurity was synthesized when a=4—7.5, b=10—21, c=0.5—4.5, crystallization time 8h, crystallization temperature 135 °C. Pure zeolite omega was synthesized under conditions that a = 5 , b = 14, c=2.2, crystallization time 8h, crystallization temperature 135°C. Reducing the usage of template is important in synthesizing zeolite. Template was essential because omega could not be obtained without template till now. So, template is the main cost of zeolite synthesis on the one hand. And on the other hand, template is usually hard to remove from the framework of zeolite ^^^\ If it was calcined at inertia gas, carbonation will occur. While it was calcined in air, "hot site" arose and may cause losing of pore volume ^^^l A good way to synthesize zeolite is to use the seeding technique. The amount of omega seeds was studied below: At the proportion of 5Na20: AI2O3: 14Si02: 2.2TMAOH: 22OH2O, the gel was airproofed at 50°C for 10 days to produce seed crystals. And then at the same proportion but without template TMAOH, the mixture was stirred rapidly for 10 hours to make silicon-aluminum gel. And then different amounts of seed crystals were add to the gel to crystallization at 135''C for 8 hours. The result was showed below (Table 2): Table 2. Crystallization results of different content of seeds. Omega seed crystals used 2% 5% 10% 15%
Crystallization results No zeolite omega Zeolite omega with many other crystals and impurity Zeolite omega and about 30% other crystals Zeolite omega and less than 2% other crystals
The thermostability of zeolite omega synthesized by this method was also studied (showed in Table 3). Table 3. Calcination temperature and the relative crystallinity*. Calcination temperature 550°C 500^ 600 °C 88% 58% 95% Relative crystallinity *The XRD pattern of the as-synthesized sample dried at 120°C was used as standard (100%).
650°C No crystals
The process of synthesizing of boron-substituted zeolite is much similar as non-boron zeolite, using borate to substitute part of aluminum source in the precursor. The amount of boron used and the synthetic condition was studied (showed in Table 4) and it was relative harder to obtain zeolite omega out of these ranges.
219 Table 4. Composition of the starting mixture of boron-substituted zeolite omega aNa20: bAl203: CB2O3: dSi02: 2.2TMAOH: 22OH2O. a 4.9—4.6 4.8-4.5 4.7-4.5 4.7-4.5 4.6, 4.5
b 0.7 0.6 0.5
c 0.3 0.4 0.5
d 14 14 14
0.4
0.6
14
0.3
0.7
14
Characterization of zeolite omega The framework boron was investigated using chemical analysis, XRD pattern and ^^B MAS NMR. The chemical compositions of zeolite omega with 40% and 60% boron substituted (in synthetic precursor) were studied by chemical analysis with Si02/Al203 ratios 7.34 and 9.47, which shows that more boron atoms may substitute the aluminum atoms in the framework. In the XRD patterns, when zeolite was boron adulterated, the peaks move to low angle, which suggested the decrease of unit cell dimension after boron adulteration.
^^' 450 i asteam treatment
.-^A
Na-Al-B-omega
-200 PPm
Figure 1. '^B MAS NMR spectrum of B-Al- Q .
120
HO
40
0
mm)
Figure 2. ^^Al MAS NMR spectra of B-Al- Q with different boron content (in precursor).
Boron atoms in zeolite framework are mostly four-coordinated, which show sharp peaks at about ^ p p m in ^^B MAS NMR spectra (fig. 1 A). And the extra-framework three-coordinated boron is a broad peak (fig. IB). The spectra suggest that a large amount of boron atoms have removed from framework during detemplate process. And after water steam treatment at 450°C, almost all boron atoms removed from the omega framework. The ^^Al MAS NMR spectra of boron-adulterated omega (fig. 2) show that the behavior of aluminium atoms in zeolite framework is much the same as non-boron zeolite, which means, the aluminium atoms have the tendency to stay at the B site of zeolite omega ^^'^\ The ^^Si MAS NMR spectra of different zeolite omega were studied to show the distribution of Si, B and Al atoms in the framework of omega structure (fig. 3). In zeolite omega, Si(nAl)i(n=0—4, I = A or B) was used to indicate Si atoms with different circumstance. The chemical shifts were proved to have a linear relationship with the atoms distance and the Si—O—T (T=Si or Al) angles ^^^\ Ramdas and Klinowski ^^^^ gave the expression of: Si(ppm)=7.95n+143.O3-2O.34[3.37n+3.24(4-n)]sin(0i/2). The n in the expression denote the same as Si(nAl)i, and 0i denote the average angle of Si—O—T (T=^Si or Al). In the fig. 3, the peak at — 100.70ppm indicate the signal of Si(lAl)A, and the peak at -113.18 represent the signal of Si(OAl)B. Replace the Si(lAl)A=-100.70, Si(OAl)B=-l 13.18 into the expression show above, we can get that the angles of S I A " O — T and Sie—O—T were 141.91° and 152.79°. So the chemical shifts of other peaks were calculated with that expression (see table 5).
220
—I— -80
— I
-100
ppm
Figure 3. ^^Si MAS NMR spectra of A1-Q,B-A1-Q andB-Al-Q after 550°C steam treating for 3h. Table 5. The ascription of peaks in ^^Si MAS NMR spectra. Samples
Q
B-Q(50%) B-Q550°C steam 3h *M: Al or B or Al+B
Angles of bonds
T site
Si4(M*)
Si3(M)
Si2(M)
Sil(M)
SiO(M)
141.91
A(ppm)
-84.35
-89.8
-95.25
-100.70
-106.15
152.79
B(ppm)
-91.66
-97.04
-102.42
-107.8
-113.18
141.61
A(ppm)
-84.03
-89.48
-95.02
-100.47
-105.92
154.15
B(ppm)
-92.41
-97.78
-103.15
-108.53
-113.90
143.63 159.82
A(ppm) B(ppm)
-85.66 -95.11
-91.10 -100.46
-96.53 -105.81
-101.97 -111.15
-107.41 -116.50
Then, a self-written computer simulation program was used to get the strength of the peaks (table 6). Table 6. The result of the computer simulation of the ^^Si MAS NMR spectra. Samples
Si/M*
A/B(M)
Q
4.23
1.61
B-Q 4.91 (50%) B-Q550°C 7.92 steam treated 3h *M: Al or B or Al+B
2.01 /
T site A(%) B(%) A(%) B(%)
Si(4M) 0 0 0 0
Si(3M) 2.28 1.28 4.36 2.77
Si(2M) 14.83 7.47 9.08 1.99
Si(lM) 28.53 10.83 26.08 11.96
Si(OM) 22.20 12.58 27.09 16.65
A(%)
0
0
3.78
16.86
33.88
B(%)
0
0
5.69
14.66
25.11
The boron substituted zeolite omega had a more dramatic increase in Si/Al ratio when treated by high temperature steam then non-boron omega, which indicates that boron atoms remove from the structure easier than aluminium atoms. The boron atoms removed from the framework thoroughly when treated by steam, but the dealuminization also occurred. ^^Al MAS NMR spectra of the samples treated with steam were studied (fig. 4). The two peaks around 55ppm stand for the structure four-coordinated Al, and the peak relatively sharper at 5ppm represent extra-structure six-coordinated Al. While the broad peak at 40—20ppm, shows the
221 existence of five-coordinated or twisted four-coordinated Al, which was also observed in zeoUte Y, ZMS-5 and mordenite ^^^\ When treated with 450°C water steam, the number of aluminium atoms in site A was more intensively decreased than B site. While treated with 500 °C and 550 °C steam, there was no remarkable change, which suggested that at the beginning of dealuminization, the aluminium atoms in site A were easy to remove from the structure than in site B, which was in accordance with the result of ^^Si MAS NMR.
100
80
60
40
-2 0
20
-4 0
ppm
Figure 4. ^^Al MAS NMR spectra of B-Al-omega (50%) after steam treatment (a) HB-Al-omega (b)450°C 3h(c)500°C 3h(d)550°C 3h. ppl
a
I
ppl I
HBOmega
-60 -4a
HBOmega-450
-40
B \
-2(J 0"
'
-60
n(^C W^^^"^^--^^
^L.
A ^^\
20-
]>v-i\\.
\ V
60"
B
0"
\
A
r
20"
^'^^^yc^ ^''
4 0"
-20
y
'K
'^"Vlx^ I-
4 0~
), 0. \^ i
60"
\c
8 0"
JV"k^ ^^ —D
0
c
8 0" 100
80
60
40
20
0
-20
-40
ppm
100
80
60
40
20
0
-20
ppi I
-40
ppr
HBOmega-550
-60 -40^ -20
B^ 0"
\ K
A^
^^^^ D
20"
•
60"
^M^^
ii
4 0"
'
c
8 0" ^ — 1 — ^
100
80
60
40
20
0
-20
-40
ppm
^ E
100
—
1
—
80
'
•
— 1 — ^
60
40
20
0
-20
-40
ppm
Figure 5. 2D ^^Al 3Q MAS NMR spectra of boron-containing zeolite omega after steam treatment (a)HB-Al-Omega and(b)450°C (c)500°C (d)550°C steam treated.
222
The 2D ^^Al 3Q MAS NMR spectra were obtained to further study the dealuminization of boron substituted omega (fig. 5). Signal A and B denote the aluminium atoms in site A and B of zeolite omega. Signal D denotes the extra-structure six-coordinated aluminium atoms. Signal C can be associated to twist four-coordinated aluminum atom according to its chemical shift. The signal of twist four-coordinated aluminium comes from dealuminization appears after the process of detemplate, and which increase after 450 °C water steam treatment. Further dealuminization at 500°C steam treatment bring on a new signal E, which denotes extra-structure five-coordinated aluminium. The signal E increase when treated by 550°C steam. The broad peak of 20-40 ppm on the one-dimension MAS NMR spectra was the overlap-add of twist four-coordinated aluminum and extra-structure five-coordinated aluminium comparing to the 2D ^^Al 3Q MAS NMR spectra, which also shows that the extra-structure aluminium come from twist four-coordinated aluminium. CONCLUSION Boron adulteration as a good way to improve the Si/Al ratio of zeolite omega was systematically investigated, including the synthesis conditions in which aging and seeding process was applied in order to reduce the template consumption. The samples were dealuminated and deboronated under relatively milder steam treatment conditions and showed good MAZ structure as well as high Si/Al ratio. ACKNOWLEDGEMENT This work was a Joint Project between Nankai University and Tianjin University sponsored by the Ministry of Education, P. R. China and was financially supported by open project of China Petrochemical Corporation. It was alsofinanciallysupported by National Natural Science Fund of China: 20233030.
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