- Email: [email protected]
Synthesis of sapo-34 from the lamellar alpo-kanemite
966
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.
SYNTHESIS OF SAPO-34 FROM THE LAMELLAR ALPO-KANEMITE Albuquerque, A . \ Coluccia, S.^, Marchese, L.^'^ and Pastore, H.O.^ ^Institute de Quimica, Universidade Estadual de Campinas, C. P. 6154, CEP 13084-971, Campinas-SP, Brazil. E-mail:gpmmm(g)iqm.unicamp.br ^Dipartimento di Chimica IFM, Universita di Torino, v. P. Giuria 7, 10125, Torino, Italy. E-mail:salvatore.coluccia(a)unito.it ^Dipartimento di Scienze e Tecnologie Avanzate, Universita del Piemonte Orientale "Amedeo Avogadro", C.so Borsalino 54,1-15100, Alessandria, Italy. E-mail: [email protected] ABSTRACT This work describes the preparation of SAPO-34 from a aluminophosphate lamelar analogue of kanemite structure, herein named ALPO-kan. The transformation occurs through a novel zeolitic silicoaluminophosphato named CAL-1 and was promoted by the addition of silica and hexamethyleneimine (HMI) as structure directing agent into a suspension of ALPO-kan. Calcination of CAL-1 affords H-SAPO34-kan. The material was characterized by X-rays diffraction, scanning electronic microscopy, thermogravimetry, infrared spectroscopy in the hydroxyls region and Raman spectroscopy. The particles morphology is rather different from that of SAPO-34 prepared from a conventional gel synthesis and is composed of thin plates and pseudo-rombohedral particles that show remainings of a lamelar structure. This indicates that the transformation has occurred through a solid state reaction. INTRODUCTION Since the discovery of the microporous ALPO [1] and SAPO [2] molecular sieves, it has been known that they are synthesized hydrothermally at temperatures in the range of 100 to 200°C from reactive mixtures containing an organic amine or a quaternary ammonium compound as structure directing agent (SDA). The SDA species are retained in the pores after reaction. Reactive species are used as sources of aluminun, phosphorous and silicon in these synthesis. The presence of the organic species has always be paramount to the process of the synthesis of theses porous materials since in their absence or, in some cases, upon its calcination, dense ALP04-phases appear. Among the small-pore silicoaluminophosphates SAPO-17 (ERI), SAPO-34 (CHA), SAPO-35 (LEV), SAPO-42 (LTA) and SAPO-44 (CHA), the most studied one is the chabazite-like SAPO-34, already described by Flanigen in 1984[2] in consideration of the commercial application as acid catalyst in the methanol-to-oleflns (MTO) process [3-5]. In fact, the synthesis of this structure is directed by n-propylamine, i-propylamine, tetraethylammonium hydroxide[6], morpholine[7], piperidine [7], l,3,3-trimethyl-6azabicyclo[3.2.1]octane and 4-piperidinopiperidine [8], diethylamine [9], and triethylamine [6]. The crystallinity of the material was found to increase when morpholine was used in association with HF and in the triethylamine/HF system [6]. In this last case, the amount of amine, and hence pH, is capable of directing the synthesis to produce SAPO-11, a medium pore mulecular sieve obtained in the lower concentration range, SAPO-5, a large pore material in the range from 0.65 < Et3N/(Si+Al+P) < 0.75, and finally SAPO-34 when the concentration of EtsN becomes large. Some structure directing agents were reported as responsible for the formation of silica patches or aluminosilicate domains in the SAPO-34 structure [7,10,11], however it seems reasonably accepted that the nature of the SDA governs the distribution of the silicon atom in the framework [12-14]. For a given structure topology, which gives the maximun number of silicon incorporated as isolated species, the template molecules or ions determine the maximum charge possible. This controls the way in which silicon is distributed, as isolated or non-isolated Si(4Al) species or as silica islands. In addition, it was shown that isolated silicon, that is without another silicon atom until the second coordination sphere, cannot exist in SAPO-34 above an atomic fraction of around 0.11 [12]. When the template is enough for the uptake of all the silicon atoms in the structure, the mechanism by which the silicon substitutes phosphorus is the only one operating up to a silicon molar fraction of 0.18.
967 Layered precursors were suspected in the synthesis of SAPO and ALPO structures such as, CoSAPO-44 and CoAPO-44 [15,16], and were proven to be valid sources of T atoms for the synthesis of ALPO-5, ALPO-22, ALPO-16 and SAPO-35 at varying concentrations of hexamethyleneimine, in aqueous media and in ethyleneglycol [17]. Although in the initial reseach on the CHA aluminophosphates templated by morpholine lamellar phases or prephases were also invoked[7], it was not until very recently that the preparation of SAPO-34 from a layered prephase was reported. Indeed, Vistad et al. [18] isolated a layered precursor of SAPO-34 from a mixture of pseudobohemite, phosphorous acid, coloidal silca, and morpholine in the presence of HF, after Ih at 170°C. From the layered prephase, pure triclinic SAPO-34 was isolated after 4h. This very particular prephase contained a large amount of fluorine when as synthesized (5h) and silicon below the detection limit. Longer synthesis times favoured the incorporation of silicon in the layered precursor. If the prephase was the starting material, the addition of water and treatment at 180°C for 24h yielded pure ALPO-34. In an attempt to describe the transformation from the prephase to the final SAPO-34 structure, the dissolution of the prephase followed by nucleation and growth of another intermediate phase was proposed while the solid-solid transition was considered less probable because of the large differences between the proposed structures of the prephase itself and the chabasite topology for the triclinic SAPO-34. Among the layered aluminophosphates, ALPO-kanemite had its synthesis published very recently [19] and is one of the few aluminophosphates analogues of naturally occurring lamellar aluminosilicates. In this work, we show for the first time how a layered aluminophosphate with kanemite-type structure, ALPO-kan, may be transformed via a solid-state reaction to a silicoaluminophosphate that we named CAL-1 (CAmpinas, ALessandria), by the addition of a silicon source and HMI as structure directing agent. CAL-1 is a new zeolitic phase which does not find correspondence with any of the IZA collected structures and transforms to acid SAPO-34 catalyst by calcination. It is important to remember that SAPO-34 is not obtained from a gel mixture formed by HMI, phosphoric acid, aluminum hydroxide and silica. EXPERIMENTAL PART ALPO-kanemite, [AlP03(OH)2(C4H9NH2)]: The synthesis of the ALPO-kanemite was performed according to the procedure already in the literature [19] with slight modifications. In a polypropylene beaker, a suspension of 23.0 mL of water and 15.7 g of the pseudo-bohemite (Catapal-B, Vista Chemicals, 72% AI2O3) was mechanically stirred until homogeneous. To this suspension 15.0 mL of phosphoric acid (Merck, 85%) were added dropwise, followed by 30.0 mL of water. After 2 hours of stirring, 22.4 mL of nbutylamine (NBA, Riedel, 99%) were added dropwise. The molar composition of the gel was: 1.0 AI2O3: 1.0 P2O5: 2 NBA: 30 H2O. The mixture was stirred for 2 more hours before it was loaded into Teflon-lined stainless steel autoclaves and heated at 473 K for 48 hours. The resultant solid was washed with distilled water thoroughly and dried at ambient temperature. CAL-1: In a one-neck round-bottomed flask containing 11.4 mL of water, 5.0 g of the as-synthesized ALPO-kanemite [A1P03(0H)2(C4H9NH2)] were slowly added. After 3 h stirring, 0.3 Ig of Si02 (Aerosil 200, Degussa) was slowly added. The mixture was stirred for another 30 min after which 2.2 mL of hexamethyleneimine (HMI, Aldrich, 99%) were added dropwise. The molar composition of the gel was 2 ALPO-kan: 0.4 Si02: 1.5 HMI: 50 H2O. The final mixture was aged at room temperature for 48 h. Crystallization was performed at 473 K for various amounts of time. After crystallization, the product was filtered, washed thoroughly and dried at ambient temperature. H-SAPO-34: was prepared by heating the as-synthesized CAL-1 slowly to 873 K under argon and maintaining this temperature for six hours under O2 for removal of the organic template. X-ray powder diffractograms were recorded at room temperature from hand-pressed wafers in a Shimadzu XRD 6000 diffractometer CuKa radiation, 40 kV, 30 mA, at a rate of 2"" 26min"^ using 0.5mm, 0.5mm and 0.3° slits for scattering, divergence and reception. The thermogravimetry was performed under argon (100 mL min'^) on a Du Pont 2000 Thermal Analyser, from 303 to 1273 K, at a heating rate of 10 K min"^ Scanning electron microscopy images were obtained in a JEOL microscope, model JVA 840A. Samples were covered with a carbon conductive film by a Balzer metalizer, model MED 020. Nuclear magnetic resonance spectra of nucleae ^^Si, ^^Al and '^^P, were obtained in a Bruker AC300P from samples packed in zirconia rotors at 59.63 MHz for ^^Si, 121.50 MHz for ^^P and 78.21 MHz for ^^Al.
968 RESULTS AND DISCUSSION X-rays diffraction, Figure 1, showed that after 12 h crystallization, along with the ALPO-kan, a zeolite phase begins to appear with main peaks at 13.2, 15.0, 21.3, 23.6, 25.5, 29.5, 30.5, 32.0° 20 which are related to a chabasite-type structure. This zeolitic phase progressively develops until 48h crystallization time while ALPO-kan simultaneously disappears. At this point, new XRD reflections are also found (main peaks at 11.8, 16.0, 17.7, 20.7, 24.8, 30.7° 26), which disappear upon subsequent calcination, leading to the typical XRD pattern of SAPO-34 (20).
2000 cps calcined JUAJLJL.JL.
.^1
I
10
II
20 29/degrees
ALPO-kan
30
40
Figure 1. X-ray diffraction patterns of solids obtained at different reaction times, of ALPO-kan and of the calcined material. The scanning electron micrographs of CAL-1, both before and after calcination (Figure 2 reports the calcined one), show the presence of lamellae whose morphology is very much like the one of the ALPO-kan however more extended and regular. Some lamellae are packed in particles with a pseudo-rhombohedral morphology (see the one in the dotted square), typical of the chabasite structure. The lamellar morphology is very unusual for SAPO-34 crystals [21] and is a prove of the fact that CAL-1, the SAPO-34 precursor, is formed via a true solid-state reaction from ALPO-kan in the presence of silica and HMI.
Figure 2. Scanning electron micrograph of the calcined solid which has the SAPO-34 structure. Silicon atoms are included into the structure during the reaction, as confirmed by infrared spectroscopy in the hydroxy 1 region of the calcined product. The bridging hydroxy 1 groups of SAPO-34 produced from
969
ALPO-kan appear at 3626 and 3600 cm ^ in the same region where these signals are found for SAPO-34 prepared from a gel mixture of reactants [7,8,21]. Nuclear magneti resonance confirmed the presence of silicon into the structure of SAPO-34 kan; a peak at -88.68 ppm from TMS (Figure 3c) indicates thata silicon is surrounded by four aluminum atoms. ^''AI and ^'P -MAS-NMR of SAPO-34 prepared from ALPO-kan, Figures 3a and 3b, respectively, agree very well with the profiles already reported for SAPO-34 prepared by the conventional method, from the gel [22]. The presence and state of the structure directing agent in the as-synthesized material was revealed by thermogravimetry in combination with Raman and infrared spectroscopies. The derivative of mass loss (DTG) curves (Figure 4) show that only after 12h reaction, the HMI is seen at around 723 K and the nbutilammine, the structure directing agent of kanemite, is still present (523 K) in the same position as in the pure ALPO-kanemite. This confirms the XRD conclusions that a zeolite-like structure begins to appear after 12h. CAL-1, formed after 48h reaction, displays DTG signals of HMI essentially at the same position as they are found in MCM-22 (in Figure 4, for comparison). The progressive exchange of n-butylamine with HMI as the reaction proceeds was monitored by Raman spectra which showed the HMI ring breathing mode at 736 cm'\ progressively increasing in intensity. The parallel increase of NH2 vibrational mode at 1600 cm'^ in the FTIR spectra suggested that the HMI is in the protonated form within the zeolitic cages. These spectroscopic results, along with the DTG, indicate that the mass loss at 723K is due to the decomposition of protonated HMI. XRD had pointed out that the zeolite-like structure appearance starts after 12h reaction, leading to the conclusion that it is the protonated amine the responsible for the formation of this structure and its development to CAL-1 after 48h crystallization.
;l
zm
r—^
r—1
too
0 (a)
1
r™"
4m ppm
m
2«
fi
..4i -m
»2# -m -m -m -m^
(b)
-m} -um -no ~i4
Figure 3. Nuclear magnetic resonance spectra of (a) ^^Al, (b) ^^P and (c) ^^Si.
|0.1%K'
ALPO-kan 3h 12h 48h MCM-22 400
600
800
1000
Temperature/K
Figure 4. DTG profiles of the solids obtained at increasing reaction times, along with ALPO-kanemite and MCM-22 for comparison. It is important to underline that when a gel mixture of phosphoric acid, aluminum hydroxide, silica and both structure directing agents (n-butylamine and HMI) was submitted to the same crystallization conditions.
970 neither ALPO-kan nor CAL-1 were formed; SAPO-35 and SAPO-34 were obtained instead. This result indicates that these two products were formed independently directed by HMI and n-butilamine respectively. In summary, the addition of silica and HMI into an aqueous suspension of ALPO-kanemite affords CAL1 a new silicoaluminophosphate precursor of SAPO-34. X-rays diffraction showed that the transformation of ALPO-kanemite into CAL-1 does not occur through an amorphous phase, but is a solid-state transformation. This conclusion is supported by the close observation of the pseudo-rhombohedral particles surface, which shows the memory of the kanemite-type layers. These features would not be found if a complete dissolution of kanemite had occurred before the crystallization of CAL-1. Despite the apparent complexity of this new synthesis procedure, it affords a SAPO-34 material with an unusual morphology resembling that of a layered material, not previously observed when using other synthesis procedures. The SAPO-34 material thus prepared might allow a more effective gas diffusion through the zeolitic structure during a catalytic reaction; this would be of interest in the use of this molecular sieve as catalyst for the production of light olefins. The relevance of this work stands also in the field of mechanistic studies where the extent of silicon incorportion and its siting in the framework might shed light on all the potential silicoaluminophosphate structures possibly prepared by a solid-state reaction from a lamellar precursor of CAL-1 type or other. ACKNOWLEDGEMENTS This work was funded by the Brazilian FAPESP ("Funda9ao e Amparo a Pesquisa no Estado de Sao Paulo") and the Italian MURST ("PRIN, Cofm2001").
REFERENCES 1. Wilson, S.T., Lok, B. M., Messina, C. A., Cannan, T. R., Flanigen, E. M., J. Am. Chem. Soc. 104 (1982) 1146-1147. 2. Lok, B. M., Messina, C. A., Patton, R. L., Gajek, R. T., Cannan, T. R., Flanigen, E. M., J. Am. Chem. Soc. 106(1984)6092-6093. 3. Stocker, M., Micropor. Mesopor. Mater. 29 (1999) 3-48. 4. Frache, A., Gianotti, E., Marchese, L., Catal. Today 77 (2003) 371-384. 5. Smith, L., Marchese, L., Cheetham, A.K., Thomas, J. M., Wright, P. A., Chen, J., Morris, R E., Science 271(1996)799-802. 6. Xu, Y., Maddox, P. J., Couves, J. W., J. Chem. Soc, Faraday Trans. 86 (1990) 425-429. 7. Girnus, I., Loffler, E., Lohse, U., Neissendorfer, F., Collect. Czech. Chem. Commun. 57 (1992) 946-958. 8. Wyles, J. K., Sankar, G., Lewis, D. W., Catlow, C. R. A., Thomas, J. M., Proceedings of the 12"^ International Zeolite Conference, vol III, Baltimore, 1998; J. M. M. Treacy, B. K. Marcus, M. E. Bisher, J. B. Higgins, Eds.; Materials Research Society: Warrendale, Pensylvannia, 1998, p. 1723-1730. 9. Chen, J. S., Thomas, J. M., Catal. Lett. 11 (1991) 199-207. 10. Prakash, A. M., Unnikrishnan, S., J. Chem. Soc, Faraday Trans. 90 (1994) 2291-2296. 11. Ashtekar, S., Chilukuri, S. V. V., Chakrabarty, D. K., J. Phys. Chem. 98 (1994) 4878-4883. 12. Barthomeuf, D., J. Phys. Chem. 97 (1993) 10092-10096. 13. Vomscheid, R., Briend, M., Peltre, M. J., Man, P. P., Barthomeuf, D., J. Phys. Chem. 98 (1994) 96149618. 14. Briend, M., Vomscheid, R., Peltre, M. J., Man, P. P., Barthomeuf, D., J. Phys. Chem. 99 (1995) 82708276. 15. Batista, J., Kaucic, V., Rajic, N., Stojakovic, D., Zeolites 12 (1992) 925-928. 16. Lee, Y.-J., Chon, H., Microporous Mater. 11 (1997) 253-260. 17. Venkatathri, N., Hegde, S. G., Ramaswamy, V., Sivasanker, S., Micropor. Mesopor. Mater. 23 (1998) 277-285. 18. Vistad, 0 . B., Akporiaye, D. E., Lillerud, K. P., J. Phys. Chem. B 105 (2001) 12437-12447. 19. Cheng, S. F., Tzeng, J.-N., Hsu, B.-Y., Chem. Mater. 9 (1997) 1788-1796. 20. M. M. J. Treacy, J. B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, Elsevier, Fourth Revised Edition, 2001, 100-101. 21. L. Marchese, A. Frache, E. Gianotti, G. Martra, M. Causa, S. Coluccia, Micropor. Mesopor. Mater. 30 (1999) 145-153.
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.
SYNTHESIS OF SAPO-34 FROM THE LAMELLAR ALPO-KANEMITE Albuquerque, A . \ Coluccia, S.^, Marchese, L.^'^ and Pastore, H.O.^ ^Institute de Quimica, Universidade Estadual de Campinas, C. P. 6154, CEP 13084-971, Campinas-SP, Brazil. E-mail:gpmmm(g)iqm.unicamp.br ^Dipartimento di Chimica IFM, Universita di Torino, v. P. Giuria 7, 10125, Torino, Italy. E-mail:salvatore.coluccia(a)unito.it ^Dipartimento di Scienze e Tecnologie Avanzate, Universita del Piemonte Orientale "Amedeo Avogadro", C.so Borsalino 54,1-15100, Alessandria, Italy. E-mail: [email protected] ABSTRACT This work describes the preparation of SAPO-34 from a aluminophosphate lamelar analogue of kanemite structure, herein named ALPO-kan. The transformation occurs through a novel zeolitic silicoaluminophosphato named CAL-1 and was promoted by the addition of silica and hexamethyleneimine (HMI) as structure directing agent into a suspension of ALPO-kan. Calcination of CAL-1 affords H-SAPO34-kan. The material was characterized by X-rays diffraction, scanning electronic microscopy, thermogravimetry, infrared spectroscopy in the hydroxyls region and Raman spectroscopy. The particles morphology is rather different from that of SAPO-34 prepared from a conventional gel synthesis and is composed of thin plates and pseudo-rombohedral particles that show remainings of a lamelar structure. This indicates that the transformation has occurred through a solid state reaction. INTRODUCTION Since the discovery of the microporous ALPO [1] and SAPO [2] molecular sieves, it has been known that they are synthesized hydrothermally at temperatures in the range of 100 to 200°C from reactive mixtures containing an organic amine or a quaternary ammonium compound as structure directing agent (SDA). The SDA species are retained in the pores after reaction. Reactive species are used as sources of aluminun, phosphorous and silicon in these synthesis. The presence of the organic species has always be paramount to the process of the synthesis of theses porous materials since in their absence or, in some cases, upon its calcination, dense ALP04-phases appear. Among the small-pore silicoaluminophosphates SAPO-17 (ERI), SAPO-34 (CHA), SAPO-35 (LEV), SAPO-42 (LTA) and SAPO-44 (CHA), the most studied one is the chabazite-like SAPO-34, already described by Flanigen in 1984[2] in consideration of the commercial application as acid catalyst in the methanol-to-oleflns (MTO) process [3-5]. In fact, the synthesis of this structure is directed by n-propylamine, i-propylamine, tetraethylammonium hydroxide[6], morpholine[7], piperidine [7], l,3,3-trimethyl-6azabicyclo[3.2.1]octane and 4-piperidinopiperidine [8], diethylamine [9], and triethylamine [6]. The crystallinity of the material was found to increase when morpholine was used in association with HF and in the triethylamine/HF system [6]. In this last case, the amount of amine, and hence pH, is capable of directing the synthesis to produce SAPO-11, a medium pore mulecular sieve obtained in the lower concentration range, SAPO-5, a large pore material in the range from 0.65 < Et3N/(Si+Al+P) < 0.75, and finally SAPO-34 when the concentration of EtsN becomes large. Some structure directing agents were reported as responsible for the formation of silica patches or aluminosilicate domains in the SAPO-34 structure [7,10,11], however it seems reasonably accepted that the nature of the SDA governs the distribution of the silicon atom in the framework [12-14]. For a given structure topology, which gives the maximun number of silicon incorporated as isolated species, the template molecules or ions determine the maximum charge possible. This controls the way in which silicon is distributed, as isolated or non-isolated Si(4Al) species or as silica islands. In addition, it was shown that isolated silicon, that is without another silicon atom until the second coordination sphere, cannot exist in SAPO-34 above an atomic fraction of around 0.11 [12]. When the template is enough for the uptake of all the silicon atoms in the structure, the mechanism by which the silicon substitutes phosphorus is the only one operating up to a silicon molar fraction of 0.18.
967 Layered precursors were suspected in the synthesis of SAPO and ALPO structures such as, CoSAPO-44 and CoAPO-44 [15,16], and were proven to be valid sources of T atoms for the synthesis of ALPO-5, ALPO-22, ALPO-16 and SAPO-35 at varying concentrations of hexamethyleneimine, in aqueous media and in ethyleneglycol [17]. Although in the initial reseach on the CHA aluminophosphates templated by morpholine lamellar phases or prephases were also invoked[7], it was not until very recently that the preparation of SAPO-34 from a layered prephase was reported. Indeed, Vistad et al. [18] isolated a layered precursor of SAPO-34 from a mixture of pseudobohemite, phosphorous acid, coloidal silca, and morpholine in the presence of HF, after Ih at 170°C. From the layered prephase, pure triclinic SAPO-34 was isolated after 4h. This very particular prephase contained a large amount of fluorine when as synthesized (5h) and silicon below the detection limit. Longer synthesis times favoured the incorporation of silicon in the layered precursor. If the prephase was the starting material, the addition of water and treatment at 180°C for 24h yielded pure ALPO-34. In an attempt to describe the transformation from the prephase to the final SAPO-34 structure, the dissolution of the prephase followed by nucleation and growth of another intermediate phase was proposed while the solid-solid transition was considered less probable because of the large differences between the proposed structures of the prephase itself and the chabasite topology for the triclinic SAPO-34. Among the layered aluminophosphates, ALPO-kanemite had its synthesis published very recently [19] and is one of the few aluminophosphates analogues of naturally occurring lamellar aluminosilicates. In this work, we show for the first time how a layered aluminophosphate with kanemite-type structure, ALPO-kan, may be transformed via a solid-state reaction to a silicoaluminophosphate that we named CAL-1 (CAmpinas, ALessandria), by the addition of a silicon source and HMI as structure directing agent. CAL-1 is a new zeolitic phase which does not find correspondence with any of the IZA collected structures and transforms to acid SAPO-34 catalyst by calcination. It is important to remember that SAPO-34 is not obtained from a gel mixture formed by HMI, phosphoric acid, aluminum hydroxide and silica. EXPERIMENTAL PART ALPO-kanemite, [AlP03(OH)2(C4H9NH2)]: The synthesis of the ALPO-kanemite was performed according to the procedure already in the literature [19] with slight modifications. In a polypropylene beaker, a suspension of 23.0 mL of water and 15.7 g of the pseudo-bohemite (Catapal-B, Vista Chemicals, 72% AI2O3) was mechanically stirred until homogeneous. To this suspension 15.0 mL of phosphoric acid (Merck, 85%) were added dropwise, followed by 30.0 mL of water. After 2 hours of stirring, 22.4 mL of nbutylamine (NBA, Riedel, 99%) were added dropwise. The molar composition of the gel was: 1.0 AI2O3: 1.0 P2O5: 2 NBA: 30 H2O. The mixture was stirred for 2 more hours before it was loaded into Teflon-lined stainless steel autoclaves and heated at 473 K for 48 hours. The resultant solid was washed with distilled water thoroughly and dried at ambient temperature. CAL-1: In a one-neck round-bottomed flask containing 11.4 mL of water, 5.0 g of the as-synthesized ALPO-kanemite [A1P03(0H)2(C4H9NH2)] were slowly added. After 3 h stirring, 0.3 Ig of Si02 (Aerosil 200, Degussa) was slowly added. The mixture was stirred for another 30 min after which 2.2 mL of hexamethyleneimine (HMI, Aldrich, 99%) were added dropwise. The molar composition of the gel was 2 ALPO-kan: 0.4 Si02: 1.5 HMI: 50 H2O. The final mixture was aged at room temperature for 48 h. Crystallization was performed at 473 K for various amounts of time. After crystallization, the product was filtered, washed thoroughly and dried at ambient temperature. H-SAPO-34: was prepared by heating the as-synthesized CAL-1 slowly to 873 K under argon and maintaining this temperature for six hours under O2 for removal of the organic template. X-ray powder diffractograms were recorded at room temperature from hand-pressed wafers in a Shimadzu XRD 6000 diffractometer CuKa radiation, 40 kV, 30 mA, at a rate of 2"" 26min"^ using 0.5mm, 0.5mm and 0.3° slits for scattering, divergence and reception. The thermogravimetry was performed under argon (100 mL min'^) on a Du Pont 2000 Thermal Analyser, from 303 to 1273 K, at a heating rate of 10 K min"^ Scanning electron microscopy images were obtained in a JEOL microscope, model JVA 840A. Samples were covered with a carbon conductive film by a Balzer metalizer, model MED 020. Nuclear magnetic resonance spectra of nucleae ^^Si, ^^Al and '^^P, were obtained in a Bruker AC300P from samples packed in zirconia rotors at 59.63 MHz for ^^Si, 121.50 MHz for ^^P and 78.21 MHz for ^^Al.
968 RESULTS AND DISCUSSION X-rays diffraction, Figure 1, showed that after 12 h crystallization, along with the ALPO-kan, a zeolite phase begins to appear with main peaks at 13.2, 15.0, 21.3, 23.6, 25.5, 29.5, 30.5, 32.0° 20 which are related to a chabasite-type structure. This zeolitic phase progressively develops until 48h crystallization time while ALPO-kan simultaneously disappears. At this point, new XRD reflections are also found (main peaks at 11.8, 16.0, 17.7, 20.7, 24.8, 30.7° 26), which disappear upon subsequent calcination, leading to the typical XRD pattern of SAPO-34 (20).
2000 cps calcined JUAJLJL.JL.
.^1
I
10
II
20 29/degrees
ALPO-kan
30
40
Figure 1. X-ray diffraction patterns of solids obtained at different reaction times, of ALPO-kan and of the calcined material. The scanning electron micrographs of CAL-1, both before and after calcination (Figure 2 reports the calcined one), show the presence of lamellae whose morphology is very much like the one of the ALPO-kan however more extended and regular. Some lamellae are packed in particles with a pseudo-rhombohedral morphology (see the one in the dotted square), typical of the chabasite structure. The lamellar morphology is very unusual for SAPO-34 crystals [21] and is a prove of the fact that CAL-1, the SAPO-34 precursor, is formed via a true solid-state reaction from ALPO-kan in the presence of silica and HMI.
Figure 2. Scanning electron micrograph of the calcined solid which has the SAPO-34 structure. Silicon atoms are included into the structure during the reaction, as confirmed by infrared spectroscopy in the hydroxy 1 region of the calcined product. The bridging hydroxy 1 groups of SAPO-34 produced from
969
ALPO-kan appear at 3626 and 3600 cm ^ in the same region where these signals are found for SAPO-34 prepared from a gel mixture of reactants [7,8,21]. Nuclear magneti resonance confirmed the presence of silicon into the structure of SAPO-34 kan; a peak at -88.68 ppm from TMS (Figure 3c) indicates thata silicon is surrounded by four aluminum atoms. ^''AI and ^'P -MAS-NMR of SAPO-34 prepared from ALPO-kan, Figures 3a and 3b, respectively, agree very well with the profiles already reported for SAPO-34 prepared by the conventional method, from the gel [22]. The presence and state of the structure directing agent in the as-synthesized material was revealed by thermogravimetry in combination with Raman and infrared spectroscopies. The derivative of mass loss (DTG) curves (Figure 4) show that only after 12h reaction, the HMI is seen at around 723 K and the nbutilammine, the structure directing agent of kanemite, is still present (523 K) in the same position as in the pure ALPO-kanemite. This confirms the XRD conclusions that a zeolite-like structure begins to appear after 12h. CAL-1, formed after 48h reaction, displays DTG signals of HMI essentially at the same position as they are found in MCM-22 (in Figure 4, for comparison). The progressive exchange of n-butylamine with HMI as the reaction proceeds was monitored by Raman spectra which showed the HMI ring breathing mode at 736 cm'\ progressively increasing in intensity. The parallel increase of NH2 vibrational mode at 1600 cm'^ in the FTIR spectra suggested that the HMI is in the protonated form within the zeolitic cages. These spectroscopic results, along with the DTG, indicate that the mass loss at 723K is due to the decomposition of protonated HMI. XRD had pointed out that the zeolite-like structure appearance starts after 12h reaction, leading to the conclusion that it is the protonated amine the responsible for the formation of this structure and its development to CAL-1 after 48h crystallization.
;l
zm
r—^
r—1
too
0 (a)
1
r™"
4m ppm
m
2«
fi
..4i -m
»2# -m -m -m -m^
(b)
-m} -um -no ~i4
Figure 3. Nuclear magnetic resonance spectra of (a) ^^Al, (b) ^^P and (c) ^^Si.
|0.1%K'
ALPO-kan 3h 12h 48h MCM-22 400
600
800
1000
Temperature/K
Figure 4. DTG profiles of the solids obtained at increasing reaction times, along with ALPO-kanemite and MCM-22 for comparison. It is important to underline that when a gel mixture of phosphoric acid, aluminum hydroxide, silica and both structure directing agents (n-butylamine and HMI) was submitted to the same crystallization conditions.
970 neither ALPO-kan nor CAL-1 were formed; SAPO-35 and SAPO-34 were obtained instead. This result indicates that these two products were formed independently directed by HMI and n-butilamine respectively. In summary, the addition of silica and HMI into an aqueous suspension of ALPO-kanemite affords CAL1 a new silicoaluminophosphate precursor of SAPO-34. X-rays diffraction showed that the transformation of ALPO-kanemite into CAL-1 does not occur through an amorphous phase, but is a solid-state transformation. This conclusion is supported by the close observation of the pseudo-rhombohedral particles surface, which shows the memory of the kanemite-type layers. These features would not be found if a complete dissolution of kanemite had occurred before the crystallization of CAL-1. Despite the apparent complexity of this new synthesis procedure, it affords a SAPO-34 material with an unusual morphology resembling that of a layered material, not previously observed when using other synthesis procedures. The SAPO-34 material thus prepared might allow a more effective gas diffusion through the zeolitic structure during a catalytic reaction; this would be of interest in the use of this molecular sieve as catalyst for the production of light olefins. The relevance of this work stands also in the field of mechanistic studies where the extent of silicon incorportion and its siting in the framework might shed light on all the potential silicoaluminophosphate structures possibly prepared by a solid-state reaction from a lamellar precursor of CAL-1 type or other. ACKNOWLEDGEMENTS This work was funded by the Brazilian FAPESP ("Funda9ao e Amparo a Pesquisa no Estado de Sao Paulo") and the Italian MURST ("PRIN, Cofm2001").
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