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Selective formation of SAPO-5 and SAPO-34 molecular sieves with microwave irradiation and hydrothermal heating
Microporous and Mesoporous Materials 64 (2003) 33–39 www.elsevier.com/locate/micromeso
Selective formation of SAPO-5 and SAPO-34 molecular sieves with microwave irradiation and hydrothermal heating Sung Hwa Jhung a, Jong-San Chang a
a,*
, Jin Soo Hwang a, Sang-Eon Park
a,b,*
Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology, P.O. Box, 107, Yusung, Taejon 305-600, South Korea b Department of Chemistry, Inha University, Nam-Gu, Incheon, South Korea Received 27 February 2003; received in revised form 10 July 2003; accepted 25 July 2003
Abstract SAPO-5 and SAPO-34 molecular sieves can be selectively formed with microwave irradiation and hydrothermal heating, respectively, of the same gel irrespective of the acidity or the type of the templates such as triethylamine and N,N,N0 ,N0 -tetraethylethylenediamine. The SAPO-5 structure may transform into the SAPO-34 structure with increase of crystallization time probably due to the relative stability of the two phases at the reaction conditions. Crystallization with microwave irradiation can be used as a phase selective synthesis method for unstable material because of fast crystallization. 2003 Elsevier Inc. All rights reserved. Keywords: SAPO-5 molecular sieve; SAPO-34 molecular sieve; Phase selectivity; Microwave irradiation; Hydrothermal heating
1. Introduction Microporous materials with pore sizes near molecular dimensions, such as zeolites and aluminophosphate molecular sieves (AlPO), are widely used in catalysis and separation, and are being developed for new applications in membranes, sensors, optics etc. [1]. The facile and effective synthesis of AlPOtype microporous materials is therefore very important. Since the first report [2,3] on the synthesis of aluminophosphate molecular sieves by Flanigen
*
Corresponding authors. Fax: +82-42-860-7676. E-mail addresses: [email protected] (J.-S. [email protected] (S.-E. Park).
Chang),
et al., numerous studies including synthesis [4], catalysis, application and modification etc. on the aluminophosphate type molecular sieves, such as AlPO, silicoaluminophosphate (SAPO) and metal aluminophosphate (MeAPO), have been undertaken. AFI-type molecular sieves (AFI) [5] such as AlPO-5 and SAPO-5 with one-dimensional channels of 0.73 nm have attracted much interest because of catalysis, separation and possible new applications such as nonlinear optics [6] and use as containers for the smallest single-walled carbon nanotubes [7]. CHA-type molecular sieves (CHA) [8] such as SAPO-34 with small channel of 0.38 nm have the potential to be used as a catalyst in the petrochemical industry e.g. in the Ômethanol to olefinÕ (MTO) process [9,10]. The AFI and CHA
1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1387-1811(03)00501-8
34
S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
molecular sieves have hexagonal and trigonal structures, respectively [5,8]. Up to now microporous materials have been synthesized mainly with conventional hydrothermal heating. Since the early 1990s, however, numerous studies have been undertaken to synthesize and modify the properties of microporous materials by the groups of van Bekkum [11], Cundy [12] and Komarneni [13] etc. Synthesis of zeolites and AlPOs with microwave has many advantages such as fast crystallization [11–13], increased phase purity [14] and narrow particle size distribution [15]. We have reported on the morphology control of porous materials such as TS-1 [16], MCM-41 [17], SBA-16 [18] and AFI [19] under microwave irradiation and have shown that microwave is very efficient to control the morphology of microporous materials. To our knowledge, phase selectivity obtained with microwave irradiation is not well known. Preferential formation of zeolite NaY at high temperature has been reported by Zhao et al. [14]. The preferential formation of NaY over zeolite P has been explained with the relative stabilities (kinetic effect) of zeolite P and zeolite NaY. NaY can be transformed into zeolite P if the crystallization time is long. NaY can be selectively formed at short crystallization times by utilizing microwave irradiation, even though the stability of zeolite NaY is lower than that of zeolite P. It is known that the CHA and AFI structures compete [20–24] and the content of CHA increases with an increase of pH or concentration of the template and heteroatoms [20–24]. The synthesis of phase-pure molecular sieves is very important and interesting for characterization and application. Herein, we report on the preferential formation of SAPO-5 and SAPO-34 from the same gel with microwave irradiation and hydrothermal heating,
respectively. The SAPO-5 may transform into SAPO-34 with prolonged crystallization, but SAPO-5 can be selectively formed by fast crystallization of alkaline or acidic reactant gels.
2. Experimental SAPO molecular sieves were synthesized using pseudoboehmite (Catapal A, Vista), phosphoric acid (85 wt.%, Aldrich), silica sol (Ludox HS-40, 40 wt.%, Aldrich) and deionized water. Triethylamine (99.5%, Aldrich) or N,N,N0 ,N0 -tetraethylethylene diamine (98 %, Aldrich) was used as a structure-directing agent. Pseudoboehmite was added to the diluted phosphoric acid solution, and stirred until a white uniform gel was obtained. Silica sol and the template, triethylamine (TEA) or N,N,N0 ,N0 -tetraethylethylene diamine (TEEDA), were added successively to the gel, which was stirred to a uniform reaction mixture. The reaction conditions are summarized in Table 1. Thirty to sixty g of the gel was loaded in a 100 ml Teflon autoclave, which was sealed and placed in a microwave oven (Mars-5, CEM, maximum power of 1200 W). The reaction mixture was heated to the reaction temperature of 180–200 C and kept for a predetermined time. For hydrothermal crystallization, the gel was loaded in a Teflon lined autoclave and put in a preheated electric oven for a fixed time without agitation. After the reaction, the autoclave was cooled to room temperature, and the solid product was recovered with centrifugation and dried overnight at 105 C. The molecular sieves were calcined at 550 C in an electric furnace. There was no extra separation to remove impurities or amorphous gel. The phase and crystallinity of the samples was determined with an X-ray diffractometer (Rigaku,
Table 1 Reaction conditions for synthesis of SAPO-5 and SAPO-34 molecular sieves Sample number
Composition
pH of reactant
Heating method
Temperature (C)
Time (h)
A B C D
Al2 O3 :0.8P2 O5 :SiO2 :3.5TEA:50H2 O Al2 O3 :0.8P2 O5 :SiO2 :3.5TEA:50H2 O Al2 O3 :1.0P2 O5 :1/3SiO2 :1.0TEEDA:100H2 O Al2 O3 :1.0P2 O5 :1/3SiO2 :1.0TEEDA:100H2 O
9.7 9.7 5.8 5.8
Microwave Hydrothermal Microwave Hydrothermal
180 180 190 190
2 24 1 48
S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
(d) Intensity (arb. unit)
D/MAX IIIB, CuKa radiation). The morphology and composition (Al, P, Si) were analyzed with a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (Philips, XL30S FEG). The composition was cross-checked with an inductively coupled plasma spectrometer (Jovin Yvon Ultima-C). The level of silicon incorporation in the solid product is the ratio of silicon concentrations in the solid product to those in reactant gel, and calculated from the obtained chemical composition. The yield of the solid product was calculated by comparing the amount of recovered solid with the total weight of TO4 (in the gel) such as Al2 O3 , P2 O5 and SiO2 . The contribution of the template and water was corrected with a weight loss at 700 C, determined with a TGA system (Du Pont 9900).
35
(c)
(b)
(a) 5
10
15
20
25
30
35
40
Two theta (degree) Fig. 1. XRD patterns of SAPO-5 and SAPO-34 with templates and heating methods: (a) TEA/microwave, sample no. A; (b) TEA/hydrothermal, sample no. B; (c) TEEDA/microwave, sample no. C; (d) TEEDA/hydrothermal, sample no. D.
3. Results and discussion 3.1. Selective formation of SAPO-5 and SAPO-34 molecular sieves Fig. 1 and Table 2 show that SAPO-5 and SAPO-34 can be selectively formed with microwave irradiation and hydrothermal heating, respectively. The XRD patterns correspond well with those of SAPO-5 and SAPO-34 structures reported in the literature [25,26]. The obtained SAPO-5 molecular sieves show the hexagonal habits (Fig. 2a, c) in harmony with the hexagonal crystal structure [27–29]. The CHA molecular sieves have the cubic or orthorhombic morphology (Fig. 2b, d) irrespective of the template.
It is known that the CHA and AFI structures compete and that the content of CHA increases with an increase of the pH or the concentration of template TEA [20–24]. Moreover, the CHA content increases with the concentration of heteroatoms (such as Mg, Co, Si etc.) [20–24]. In this study, it has been confirmed that SAPO-5 can be synthesized even from an alkaline gel by utilizing microwave irradiation. Contrary to the microwave irradiation, the SAPO-34 structure can be obtained from alkaline gel by hydrothermal heating as reported previously [21,22,26]. This phase selectivity with microwave irradiation and hydrothermal heating is also found in acidic conditions using TEEDA as
Table 2 Properties of the synthesized SAPO molecular sieves Sample number
Phase
Crystallization yield (%)
Composition (Al:P:Si, mol%)
Si incorporation (%)a
A B C D
SAPO-5 SAPO-34 SAPO-5 SAPO-34
18 31 81 43
48.6:49.0:2.5 44.6:45.6:9.8 48.7:48.2:3.1 47.7:41.8:10.5
11.5 45.1 40.3 136.5
a Si incorporation is the ratio of Si concentration (molar ratio Si/(Si + Al + P) in solid product to the Si concentration in reactant gel).
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S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
Fig. 2. SEM images of SAPO-5 and SAPO-34 with templates and heating methods: (a) TEA/microwave, sample no. A; (b) TEA/ hydrothermal, sample no. B; (c) TEEDA/microwave, sample no. C; (d) TEEDA/hydrothermal, sample no. D. The magnifications are 5000· for (a) and 1000· for (b), (c) and (d).
the template as shown in Fig. 1 and Table 2. SAPO-5 and SAPO-34 can be formed with microwave irradiation and hydrothermal heating, respectively, even though it has been reported that TEEDA can yield AFI [3] or CHA [30] with hydrothermal heating. This phase selectivity is also observed in the crystallization of alkaline gels with increased concentration of TEEDA (TEEDA/ Al2 O3 ¼ 2.0 instead of 1.0; data are not shown). Table 2 shows that the silicon incorporation level is very high for the obtained SAPO-34 compared with that for the obtained SAPO-5 (Sample B vs. A, Sample D vs. C). However, it is not clear whether the transformation of SAPO-5 to SAPO34 is related to the increased silicon incorporation or not even though it is known that the CHA structure is prevailing with the increase of the concentration of heteroatoms [20–24]. The silicon incorporation level is higher for the crystals synthesized from gel in acidic condition (Samples C, D) than for those crystallized in alkaline condition (Samples A, B).
3.2. Phase transformation Nieto et al. [20] have reported that AFI (MgAPO-5) can be transformed into CHA (MgAPO-34) with long crystallization due to the high solubility of AFI structure. Inoue et al. [31] also observed the phase transformation of SAPO-5 to SAPO-34 with the progress of reaction or increase of reaction temperature. Upon increasing the reaction time in microwave irradiation, SAPO-34 forms in small amount in the later part of the crystallization (Figs. 3a and 4a). The concentration of SAPO-34 increases also with the crystallization time of hydrothermal heating. However, the SAPO-5 forms only in the initial part of crystallization by hydrothermal heating, and disappears as the time increases (Figs. 3b and 4b). This phase transformation may indicate that SAPO-34 is more stable than SAPO-5 at the reaction condition as reported by Nieto et al. [20] and Inoue et al. [30] though AFI has a more dense 3 ) than CHA (14.6 T/1000 structure (17.5 T/1000 A
100
(a) 80 60 40 20
37
100
Crystallization yield (%)
Crystallization yield (%)
S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
(b) 80 60 40 20
0
0 0
1
2
3
4
5
6
7
0
30
Crystallization time (h) SAPO-34
60
90
120
150
Crystallization time (h)
SAPO-5
SAPO-34
SAPO-5
100
Crystallization yield (%)
Crystallization yield (%)
Fig. 3. Competitive formation of SAPO-5 and SAPO-34 with (a) microwave irradiation and (b) hydrothermal heating. TEA was used as a template and the reaction temperature was 200 C.
(a)
80 60 40 20
100
(b)
80 60 40 20 0
0 0
0.5
1
Crystallization time (h) SAPO-34
SAPO-5
0
30
60
90
120
150
Crystallization time (h) SAPO-34
SAPO-5
Fig. 4. Competitive formation of SAPO-5 and SAPO-34 with (a) microwave irradiation and (b) hydrothermal heating. TEEDA was used as a template and the reaction temperature was 200 C.
3 ) [4,7]. This study shows that SAPO-5 and A SAPO-34 may be obtained selectively by short (microwave) and long (hydrothermal) crystallization time, respectively. The SAPO-5 may transform into SAPO-34 with the progress of reaction time irrespective of crystallization method such as microwave irradiation and electrical hydrothermal crystallization. The SAPO-5 and SAPO-34 in this study may correspond to the NaY and P, respectively, of the study [14] by Cundy et al. Lillerud et al. [32] have reported that the heating rate is important for the selective formation of SAPO-34 and SAPO-5 from
gel containing morpholine as a structure-directing agent. In this study, the heating rate of microwave irradiation was also changed from 90 C/min (normal condition) to 1.6 C/min to check the effect of heating rate. However, the selective formation of SAPO-5 with microwave irradiation does not change (Fig. 5) upon decreasing the heating rate. Moreover, the pure SAPO-34 cannot be obtained with microwave irradiation in short time by adding SAPO-34 seeds (SAPO-34 0.3 g + gel 29.7 g) before crystallization. The product contained not only cubic-like SAPO-34 but also hexagonal SAPO-5 even with the seeding (Fig. 6).
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S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
phase selective method for preparation of unstable materials because of fast crystallization. Acknowledgements
Fig. 5. SEM image of SAPO-5 obtained with microwave irradiation. The reaction conditions were similar to that of sample A except the heating rate which was decreased from 90 to 1.6 C/min.
This work was supported by the Korea Ministry of Science and Technology through Research Center for Nanocatalysis (KN-0292), one of the National Science Programs for Key Nanotechnology and Institutional Research Program (KK0302-G0). The authors thank Dr. Young Kyu Hwang and Dr. K. Patil for helpful discussions and Mr. Ji Woong Yoon and Mr. Jin Ho Lee for much assistance. References
Fig. 6. SEM image of SAPO-34/SAPO-5 obtained with microwave irradiation. The reaction conditions were similar to that of sample A except 1% of SAPO-34 crystals that were added to the gel before crystallization and the crystallization time was 4 h instead of 2 h.
4. Conclusion The microwave irradiation technique can be used to obtain unstable microporous materials utilizing the fast crystallization of the process. SAPO-5 and SAPO-34 molecular sieves can be selectively formed with microwave irradiation and hydrothermal heating, respectively, of the same gel irrespective of the acidity or the type of the templates such as TEA and TEEDA. The SAPO-5 structure may transform into the SAPO-34 structure with the increase of crystallization time probably due to the relative stability of the two phases at the reaction condition. Crystallization with microwave irradiation can be suggested as a
[1] M.E. Davis, Nature 417 (2002) 813. [2] S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, J. Am. Chem. Soc. 104 (1982) 1146. [3] J.M. Bennett, J.P. Cohen, E.M. Flanigen, J.J. Pluth, J.V. Smith, in: G.D. Stucky, F.G. Dwyer (Eds.), Intrazeolite Chemistry, ACS Symp. Series, vol. 218, 1983, p. 109. [4] S.T. Wilson, Stud. Surf. Sci. Catal. 137 (2001) 229. [5] W.M. Meier, D.H. Olson, Atlas of Zeolite Structure Types, Butterworths, London, 1987, p. 18. [6] J. Caro, F. Marlow, K. Hoffmann, C. Striebel, J. Kornatowski, I. Girnus, M. Noack, P. Kolsch, Stud. Surf. Sci. Catal. 105 (1997) 2171. [7] D. Wang, Z.K. Tang, G.D. Li, J.S. Chen, Nature 408 (2000) 50. [8] W.M. Meier, D.H. Olson, Atlas of Zeolite Structure Types, Butterworths, London, 1987, p. 42. [9] T. Inui, Stud. Surf. Sci. Catal. 105 (1997) 1441. [10] B.V. Bora, T.L. Marker, P.T. Barger, H.R. Nilsen, S. Kvisle, T. Fuglerud, Stud. Surf. Sci. Catal. 107 (1997) 87. [11] A. Arafat, J.C. Jansen, A.R. Ebaid, H. van Bekkum, Zeolites 13 (1990) 162. [12] C.S. Cundy, Collect. Czech. Chem. Commum. 63 (1998) 1699. [13] M. Park, S. Komarneni, Micropor. Mesopor. Mater. 20 (1998) 39. [14] J.P. Zhao, C. Cundy, J. Dwyer, Stud. Surf. Sci. Catal. 105 (1997) 181. [15] X. Xu, W. Yang, J. Liu, L. Lin, Adv. Mater. 12 (2000) 195. [16] D.S. Kim, J.-S. Chang, J.S. Hwang, Y.K. Hwang, J.M. Kim, S.-E. Park, submitted for publication. [17] S.-E. Park, D.S. Kim, J.-S. Chang, W.Y. Kim, Catal. Today 44 (1998) 301. [18] Y.K. Hwang, J.-S. Chang, Y.-U. Kwon, S.-E. Park, Stud. Surf. Sci. Catal. 146 (2003) 101. [19] S.H. Jhung, J.-S. Chang, Y.K. Hwang, S.-E. Park, submitted for publication.
S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39 [20] P. Concepci on, J.M.L. Nieto, A. Mifsud, J. Perez-Pariente, Zeolites 16 (1996) 54. [21] C. He, Z. Liu, L. Yang, G. Cai, Tianranqi Huagong 18 (1993) 14 (in Chinese). [22] Y. Xu, J. Maddox, J.W. Couves, J. Chem. Soc. Faraday Trans. 86 (1990) 425. [23] U. Lohse, R. Bertram, K. Jancke, I. Kurzawaski, B. Parlitz, E. L€ offler, E. Schreier, J. Chem. Soc. Faraday Trans. 91 (1995) 1163. [24] M.G. Uytterhoeven, R.A. Schoonheydt, Micropor. Mater. 3 (1994) 265. [25] M.M.J. Treacy, J.B. Higgins, R. von Ballmoos, Collection of Simulated XRD Powder Patterns for Zeolites, Elsevier, New York, 1996, pp. 352, 410.
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[26] J. Tan, Z. Liu, X. Bao, X. Liu, X. Han, C. He, R. Zhai, Micropor. Mesopor. Mater. 53 (2002) 97. [27] I. Girnus, K. Jancke, R. Vetter, J. Richter-Mendau, J. Caro, Zeolites 15 (1995) 33. [28] D. Demuth, G.D. Stucky, K.K. Unger, F. Sch€ uth, Micropor. Mater. 3 (1995) 473. [29] G. Finger, J. Richter-Mendau, M. B€ ulow, J. Kornatowski, Zeolites 11 (1991) 443. [30] S.H. Jhung, Ph.D. thesis, Korea Adv. Inst. Sci. Tech., Seoul, South Korea, 1990. [31] M. Inoue, P. Dhupatemiya, S. Phatanasri, T. Inui, Micropor. Mesopor. Mater. 28 (1999) 19. [32] B. Vistad, D.E. Akporiaye, K.P. Lillerud, J. Phys. Chem. B 105 (2001) 12437.
Selective formation of SAPO-5 and SAPO-34 molecular sieves with microwave irradiation and hydrothermal heating Sung Hwa Jhung a, Jong-San Chang a
a,*
, Jin Soo Hwang a, Sang-Eon Park
a,b,*
Catalysis Center for Molecular Engineering, Korea Research Institute of Chemical Technology, P.O. Box, 107, Yusung, Taejon 305-600, South Korea b Department of Chemistry, Inha University, Nam-Gu, Incheon, South Korea Received 27 February 2003; received in revised form 10 July 2003; accepted 25 July 2003
Abstract SAPO-5 and SAPO-34 molecular sieves can be selectively formed with microwave irradiation and hydrothermal heating, respectively, of the same gel irrespective of the acidity or the type of the templates such as triethylamine and N,N,N0 ,N0 -tetraethylethylenediamine. The SAPO-5 structure may transform into the SAPO-34 structure with increase of crystallization time probably due to the relative stability of the two phases at the reaction conditions. Crystallization with microwave irradiation can be used as a phase selective synthesis method for unstable material because of fast crystallization. 2003 Elsevier Inc. All rights reserved. Keywords: SAPO-5 molecular sieve; SAPO-34 molecular sieve; Phase selectivity; Microwave irradiation; Hydrothermal heating
1. Introduction Microporous materials with pore sizes near molecular dimensions, such as zeolites and aluminophosphate molecular sieves (AlPO), are widely used in catalysis and separation, and are being developed for new applications in membranes, sensors, optics etc. [1]. The facile and effective synthesis of AlPOtype microporous materials is therefore very important. Since the first report [2,3] on the synthesis of aluminophosphate molecular sieves by Flanigen
*
Corresponding authors. Fax: +82-42-860-7676. E-mail addresses: [email protected] (J.-S. [email protected] (S.-E. Park).
Chang),
et al., numerous studies including synthesis [4], catalysis, application and modification etc. on the aluminophosphate type molecular sieves, such as AlPO, silicoaluminophosphate (SAPO) and metal aluminophosphate (MeAPO), have been undertaken. AFI-type molecular sieves (AFI) [5] such as AlPO-5 and SAPO-5 with one-dimensional channels of 0.73 nm have attracted much interest because of catalysis, separation and possible new applications such as nonlinear optics [6] and use as containers for the smallest single-walled carbon nanotubes [7]. CHA-type molecular sieves (CHA) [8] such as SAPO-34 with small channel of 0.38 nm have the potential to be used as a catalyst in the petrochemical industry e.g. in the Ômethanol to olefinÕ (MTO) process [9,10]. The AFI and CHA
1387-1811/$ - see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1387-1811(03)00501-8
34
S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
molecular sieves have hexagonal and trigonal structures, respectively [5,8]. Up to now microporous materials have been synthesized mainly with conventional hydrothermal heating. Since the early 1990s, however, numerous studies have been undertaken to synthesize and modify the properties of microporous materials by the groups of van Bekkum [11], Cundy [12] and Komarneni [13] etc. Synthesis of zeolites and AlPOs with microwave has many advantages such as fast crystallization [11–13], increased phase purity [14] and narrow particle size distribution [15]. We have reported on the morphology control of porous materials such as TS-1 [16], MCM-41 [17], SBA-16 [18] and AFI [19] under microwave irradiation and have shown that microwave is very efficient to control the morphology of microporous materials. To our knowledge, phase selectivity obtained with microwave irradiation is not well known. Preferential formation of zeolite NaY at high temperature has been reported by Zhao et al. [14]. The preferential formation of NaY over zeolite P has been explained with the relative stabilities (kinetic effect) of zeolite P and zeolite NaY. NaY can be transformed into zeolite P if the crystallization time is long. NaY can be selectively formed at short crystallization times by utilizing microwave irradiation, even though the stability of zeolite NaY is lower than that of zeolite P. It is known that the CHA and AFI structures compete [20–24] and the content of CHA increases with an increase of pH or concentration of the template and heteroatoms [20–24]. The synthesis of phase-pure molecular sieves is very important and interesting for characterization and application. Herein, we report on the preferential formation of SAPO-5 and SAPO-34 from the same gel with microwave irradiation and hydrothermal heating,
respectively. The SAPO-5 may transform into SAPO-34 with prolonged crystallization, but SAPO-5 can be selectively formed by fast crystallization of alkaline or acidic reactant gels.
2. Experimental SAPO molecular sieves were synthesized using pseudoboehmite (Catapal A, Vista), phosphoric acid (85 wt.%, Aldrich), silica sol (Ludox HS-40, 40 wt.%, Aldrich) and deionized water. Triethylamine (99.5%, Aldrich) or N,N,N0 ,N0 -tetraethylethylene diamine (98 %, Aldrich) was used as a structure-directing agent. Pseudoboehmite was added to the diluted phosphoric acid solution, and stirred until a white uniform gel was obtained. Silica sol and the template, triethylamine (TEA) or N,N,N0 ,N0 -tetraethylethylene diamine (TEEDA), were added successively to the gel, which was stirred to a uniform reaction mixture. The reaction conditions are summarized in Table 1. Thirty to sixty g of the gel was loaded in a 100 ml Teflon autoclave, which was sealed and placed in a microwave oven (Mars-5, CEM, maximum power of 1200 W). The reaction mixture was heated to the reaction temperature of 180–200 C and kept for a predetermined time. For hydrothermal crystallization, the gel was loaded in a Teflon lined autoclave and put in a preheated electric oven for a fixed time without agitation. After the reaction, the autoclave was cooled to room temperature, and the solid product was recovered with centrifugation and dried overnight at 105 C. The molecular sieves were calcined at 550 C in an electric furnace. There was no extra separation to remove impurities or amorphous gel. The phase and crystallinity of the samples was determined with an X-ray diffractometer (Rigaku,
Table 1 Reaction conditions for synthesis of SAPO-5 and SAPO-34 molecular sieves Sample number
Composition
pH of reactant
Heating method
Temperature (C)
Time (h)
A B C D
Al2 O3 :0.8P2 O5 :SiO2 :3.5TEA:50H2 O Al2 O3 :0.8P2 O5 :SiO2 :3.5TEA:50H2 O Al2 O3 :1.0P2 O5 :1/3SiO2 :1.0TEEDA:100H2 O Al2 O3 :1.0P2 O5 :1/3SiO2 :1.0TEEDA:100H2 O
9.7 9.7 5.8 5.8
Microwave Hydrothermal Microwave Hydrothermal
180 180 190 190
2 24 1 48
S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
(d) Intensity (arb. unit)
D/MAX IIIB, CuKa radiation). The morphology and composition (Al, P, Si) were analyzed with a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (Philips, XL30S FEG). The composition was cross-checked with an inductively coupled plasma spectrometer (Jovin Yvon Ultima-C). The level of silicon incorporation in the solid product is the ratio of silicon concentrations in the solid product to those in reactant gel, and calculated from the obtained chemical composition. The yield of the solid product was calculated by comparing the amount of recovered solid with the total weight of TO4 (in the gel) such as Al2 O3 , P2 O5 and SiO2 . The contribution of the template and water was corrected with a weight loss at 700 C, determined with a TGA system (Du Pont 9900).
35
(c)
(b)
(a) 5
10
15
20
25
30
35
40
Two theta (degree) Fig. 1. XRD patterns of SAPO-5 and SAPO-34 with templates and heating methods: (a) TEA/microwave, sample no. A; (b) TEA/hydrothermal, sample no. B; (c) TEEDA/microwave, sample no. C; (d) TEEDA/hydrothermal, sample no. D.
3. Results and discussion 3.1. Selective formation of SAPO-5 and SAPO-34 molecular sieves Fig. 1 and Table 2 show that SAPO-5 and SAPO-34 can be selectively formed with microwave irradiation and hydrothermal heating, respectively. The XRD patterns correspond well with those of SAPO-5 and SAPO-34 structures reported in the literature [25,26]. The obtained SAPO-5 molecular sieves show the hexagonal habits (Fig. 2a, c) in harmony with the hexagonal crystal structure [27–29]. The CHA molecular sieves have the cubic or orthorhombic morphology (Fig. 2b, d) irrespective of the template.
It is known that the CHA and AFI structures compete and that the content of CHA increases with an increase of the pH or the concentration of template TEA [20–24]. Moreover, the CHA content increases with the concentration of heteroatoms (such as Mg, Co, Si etc.) [20–24]. In this study, it has been confirmed that SAPO-5 can be synthesized even from an alkaline gel by utilizing microwave irradiation. Contrary to the microwave irradiation, the SAPO-34 structure can be obtained from alkaline gel by hydrothermal heating as reported previously [21,22,26]. This phase selectivity with microwave irradiation and hydrothermal heating is also found in acidic conditions using TEEDA as
Table 2 Properties of the synthesized SAPO molecular sieves Sample number
Phase
Crystallization yield (%)
Composition (Al:P:Si, mol%)
Si incorporation (%)a
A B C D
SAPO-5 SAPO-34 SAPO-5 SAPO-34
18 31 81 43
48.6:49.0:2.5 44.6:45.6:9.8 48.7:48.2:3.1 47.7:41.8:10.5
11.5 45.1 40.3 136.5
a Si incorporation is the ratio of Si concentration (molar ratio Si/(Si + Al + P) in solid product to the Si concentration in reactant gel).
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S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
Fig. 2. SEM images of SAPO-5 and SAPO-34 with templates and heating methods: (a) TEA/microwave, sample no. A; (b) TEA/ hydrothermal, sample no. B; (c) TEEDA/microwave, sample no. C; (d) TEEDA/hydrothermal, sample no. D. The magnifications are 5000· for (a) and 1000· for (b), (c) and (d).
the template as shown in Fig. 1 and Table 2. SAPO-5 and SAPO-34 can be formed with microwave irradiation and hydrothermal heating, respectively, even though it has been reported that TEEDA can yield AFI [3] or CHA [30] with hydrothermal heating. This phase selectivity is also observed in the crystallization of alkaline gels with increased concentration of TEEDA (TEEDA/ Al2 O3 ¼ 2.0 instead of 1.0; data are not shown). Table 2 shows that the silicon incorporation level is very high for the obtained SAPO-34 compared with that for the obtained SAPO-5 (Sample B vs. A, Sample D vs. C). However, it is not clear whether the transformation of SAPO-5 to SAPO34 is related to the increased silicon incorporation or not even though it is known that the CHA structure is prevailing with the increase of the concentration of heteroatoms [20–24]. The silicon incorporation level is higher for the crystals synthesized from gel in acidic condition (Samples C, D) than for those crystallized in alkaline condition (Samples A, B).
3.2. Phase transformation Nieto et al. [20] have reported that AFI (MgAPO-5) can be transformed into CHA (MgAPO-34) with long crystallization due to the high solubility of AFI structure. Inoue et al. [31] also observed the phase transformation of SAPO-5 to SAPO-34 with the progress of reaction or increase of reaction temperature. Upon increasing the reaction time in microwave irradiation, SAPO-34 forms in small amount in the later part of the crystallization (Figs. 3a and 4a). The concentration of SAPO-34 increases also with the crystallization time of hydrothermal heating. However, the SAPO-5 forms only in the initial part of crystallization by hydrothermal heating, and disappears as the time increases (Figs. 3b and 4b). This phase transformation may indicate that SAPO-34 is more stable than SAPO-5 at the reaction condition as reported by Nieto et al. [20] and Inoue et al. [30] though AFI has a more dense 3 ) than CHA (14.6 T/1000 structure (17.5 T/1000 A
100
(a) 80 60 40 20
37
100
Crystallization yield (%)
Crystallization yield (%)
S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
(b) 80 60 40 20
0
0 0
1
2
3
4
5
6
7
0
30
Crystallization time (h) SAPO-34
60
90
120
150
Crystallization time (h)
SAPO-5
SAPO-34
SAPO-5
100
Crystallization yield (%)
Crystallization yield (%)
Fig. 3. Competitive formation of SAPO-5 and SAPO-34 with (a) microwave irradiation and (b) hydrothermal heating. TEA was used as a template and the reaction temperature was 200 C.
(a)
80 60 40 20
100
(b)
80 60 40 20 0
0 0
0.5
1
Crystallization time (h) SAPO-34
SAPO-5
0
30
60
90
120
150
Crystallization time (h) SAPO-34
SAPO-5
Fig. 4. Competitive formation of SAPO-5 and SAPO-34 with (a) microwave irradiation and (b) hydrothermal heating. TEEDA was used as a template and the reaction temperature was 200 C.
3 ) [4,7]. This study shows that SAPO-5 and A SAPO-34 may be obtained selectively by short (microwave) and long (hydrothermal) crystallization time, respectively. The SAPO-5 may transform into SAPO-34 with the progress of reaction time irrespective of crystallization method such as microwave irradiation and electrical hydrothermal crystallization. The SAPO-5 and SAPO-34 in this study may correspond to the NaY and P, respectively, of the study [14] by Cundy et al. Lillerud et al. [32] have reported that the heating rate is important for the selective formation of SAPO-34 and SAPO-5 from
gel containing morpholine as a structure-directing agent. In this study, the heating rate of microwave irradiation was also changed from 90 C/min (normal condition) to 1.6 C/min to check the effect of heating rate. However, the selective formation of SAPO-5 with microwave irradiation does not change (Fig. 5) upon decreasing the heating rate. Moreover, the pure SAPO-34 cannot be obtained with microwave irradiation in short time by adding SAPO-34 seeds (SAPO-34 0.3 g + gel 29.7 g) before crystallization. The product contained not only cubic-like SAPO-34 but also hexagonal SAPO-5 even with the seeding (Fig. 6).
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S.H. Jhung et al. / Microporous and Mesoporous Materials 64 (2003) 33–39
phase selective method for preparation of unstable materials because of fast crystallization. Acknowledgements
Fig. 5. SEM image of SAPO-5 obtained with microwave irradiation. The reaction conditions were similar to that of sample A except the heating rate which was decreased from 90 to 1.6 C/min.
This work was supported by the Korea Ministry of Science and Technology through Research Center for Nanocatalysis (KN-0292), one of the National Science Programs for Key Nanotechnology and Institutional Research Program (KK0302-G0). The authors thank Dr. Young Kyu Hwang and Dr. K. Patil for helpful discussions and Mr. Ji Woong Yoon and Mr. Jin Ho Lee for much assistance. References
Fig. 6. SEM image of SAPO-34/SAPO-5 obtained with microwave irradiation. The reaction conditions were similar to that of sample A except 1% of SAPO-34 crystals that were added to the gel before crystallization and the crystallization time was 4 h instead of 2 h.
4. Conclusion The microwave irradiation technique can be used to obtain unstable microporous materials utilizing the fast crystallization of the process. SAPO-5 and SAPO-34 molecular sieves can be selectively formed with microwave irradiation and hydrothermal heating, respectively, of the same gel irrespective of the acidity or the type of the templates such as TEA and TEEDA. The SAPO-5 structure may transform into the SAPO-34 structure with the increase of crystallization time probably due to the relative stability of the two phases at the reaction condition. Crystallization with microwave irradiation can be suggested as a
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