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Dry synthesis of B-MFI, MTN- and MTW-type materials
MICROPOROUS MATERIALS Microporous
Materials
9 ( 1997) 287-290
Short Communication
Dry synthesis of B-MFI, MTN- and MTW-type materials U. Deforth a,*, K.K. Unger a, F. Schiith b a Institut fiir Anorganische b Institut jiir Anorganische
Chemie und Analytische Chemie. Johannes Gutenberg- Oilicersitiit Main=, Becherweg 24, 55099 Main:. Germany Chemie, Johann Wolfgang Goethe-Unicersitat. Marie-Curie-Str. II, 60439 Frankfurt/Main, Germanl
Received 26 September
1996; revision 28 October 1996: accepted 5 November
1996
Abstract B-MFI and MTN-type materials were synthesized via the fluoride route from dry powders in the complete absence of a solution phase. Amorphous precursors obtained by drying SiO,’ A&O, gels at 700°C were transformed into MTN-type material in the presence of dried NH,F and TMACl. Amorphous precursors obtained by drying SiO, gels at 700’C were transformed into B-MFI in the presence of dried NH,F, B203 and TPABr. Water is formed as a reaction product. However, the water vapor pressure is appreciably below the water vapor saturation pressure under the given reaction conditions. In addition. syntheses using seed crystals were carried out successfully. 0 1997 Elsevier Science B.V. Keywords:
B-MFI;
Dry
synthesis;
MTN;
Seed crystals
1. Introduction
The synthesis of zeolites is traditionally performed under hydrothermal (solvothermal ) conditions using water or organic alcohols as a solvent. Based on the fluoride route [ 11, we developed previously a procedure which did not contain any solvent as a reactant [2]. So far. only pure silica zeolites such as Silicalite I [2], AST [ 31 and NON [3] clathrasils were synthesized by means of the dry synthesis. Here we report the synthesis of MTN-type material, another pure silica material, and the first incorporation of boron into an MFItype zeolite using the dry process. * Corresponding +61 31 392710;
author. Tel.: +61 31 395745/392107: e-mail: uweaak-unger.chemie.uni-mainz.de
0927-6513:97/$17.00 G 1997 Elsevier PII SO927-6513(96)00118-6
Fax:
Science B.V. All rights reserved
The essential feature of the dry method is that the water content in the reaction mixture under synthesis conditions is always kept lower than the water vapor saturation pressure to prevent the formation of a liquid phase. The reaction is assumed to take place via a gas phase transport process. The mobile species are probably formed by transformation of the amorphous SiOZ precursor according to: SiO, + 4NH,F-+SiF,
+ 2H,O +4NH,.
Therefore, so far only those zeolitic structures whose T-atoms can form volatile fluorides can be formed. Thus, in the presence of aluminum, the synthesis of MFI-type zeolites always leads to the pure silica MFI structure, Silicalite I [2]. In order to generalize the dry synthesis of zeo-
288
c’. Drforrh
rt al. : Microporous
lites we extended our studies by considering the following aspects: (1) synthesis of other pure silica zeolites using the dry fluoride process; zeolites using other (2) synthesis of MFI-type T-atoms, which form volatile fluorides, e.g., boron; (3) synthesis of MFI-type zeolites or other structures using seed crystals.
2. Experimental
MTN and B-MFI crystals were obtained according to the dry synthesis procedure described elsewhere [2]. In all systems a precursor was needed. For the preparation of the precursor for the MTN synthesis, aluminum nitrate (Aldrich) and fumed silica (Aerosil 200; Degussa) were mixed and stirred in 200 ml of water for 1 h. The obtained slurry was placed on a porcelain dish and dehydrated in a preheated oven at 700’C for 24 h. The precursor for the B-MFI synthesis was formed by the same procedure using only fumed silica (Aerosil 200; Degussa). Seed crystals were synthesized following the hydrothermal fluoride method [ 11. Before the addition to the reaction mixture the resulting crystals were ground in a mortar. The seed crystals, the prepared precursor, NH,F (Aldrich) and the ternplating agent, TMACl (Aldrich) or TPABr (Aldrich), were mixed and ground in a mortar for 1 min. The composition of the reaction mixture for the MTN synthesis was: 1 SiO, : 0.04 Al,O, 1 NH,F
: 0.5 TMACl
:
: O-O.9 wt.% seed crystals.
The reaction mixture was placed in a 50-ml teflonlined stainless-steel autoclave and sealed before heating it to the reaction temperature of 210°C for 144 h. After quench cooling, the products were washed, dried at 12O’C and kept for characterization. The water vapor partial pressures were calculated to be between 10 and 18 bar under reaction conditions, assuming ideal behavior which overestimates the pressure [4]. The water vapor
Materids
9 ( 1997) 287-290
saturation pressure was 19 bar at the reaction temperature of 210°C. The composition of the reaction mixture for the B-MFI synthesis was: 1 SiOZ : 0.186 B,O, (Aldrich) 1.59 NH,F
: O-3.6
: 0.048 TPABr :
wt.% seed crystals.
The reaction mixture was placed in a 50-ml teflonlined stainless-steel autoclave and sealed before heating it to the reaction temperature of 200°C for 72 h. After quench cooling, the products were washed, dried at 120°C and kept for characterization. The water vapor partial pressures were calculated to be between 8 and 14 bar under reaction conditions. The water vapor saturation pressure was 15 bar at the reaction temperature of 200°C. The products were characterized by X-ray powder diffraction (XRD), X-ray fluorescence analysis (RFA). thermogravimetry (TG) and differential thermal analysis (DTA) and scanning electron microscopy (SEM ). The incorporation of boron into MFI-type zeolite was proven by induced coupled plasma (ICP) measurements, XRD measurements (refinement of lattice constants) and “B-NMR.
3. Results and discussions
It was possible to obtain pure MTN of high crystallinity using the dry reaction process. The X-ray diffraction pattern shows no traces of byproducts or indications for amorphous material. RFA measurements showed an Si/Al ratio of 225. Aluminum is not incorporated into the framework. However, aluminum-containing precursors proved to give better crystalline samples and made the syntheses more reproducible than pure silica precursors. The crystals had a nearly perfect octahedral shape with an average size of 40 pm (Fig. 1). When the water content of the synthesis was reduced while all other parameters were kept constant, the particle size decreased from about 150 pm for a conventional hydrothermal synthesis to about 80 pm for a synthesis from dry precursors in which enough water could be released to form a liquid phase to the above-mentioned value for
U Deforth
et ul. / Microporous
Materials
9 ( 1997) 287-290
289
Fig. 1. SEM picture of MTN synthesized using the dry method without seed crystals.
Fig. 3. SEM picture of B-MFI synthesized using the dry method without seed crystals.
the solvent-free synthesis. The addition of seed crystals improved both the yield and the reproducibility of the reaction. However, the seeding induced a strong intergrowth of the crystals (Fig. 2). The attempt to synthesize isomorphously substituted ZSM-5 (MFI) was also successful. B-MFI was obtained without any phase impurities, if B203 was used as the boron source. The crystals had the typical MFI-twinning and a length of 30 urn (Fig. 3). Using H,BO,, only poorly crystalline samples were obtained in two of about 15 experiments. B20, resulted in higher quality crystals and a reproducible synthesis. As described above in case of MTN, the synthesis with addition
of seeds produced intergrown crystals of about 6 urn length (Fig. 4). ICP-measurements of the bulk material showed an average concentration of 0.88 wt.% boron in the samples. The zeolites contained an average amount of 8.8 wt.% of organic template as measured by means of TG and DTA. Evaluation of XRD data proved the incorporation of boron into the MFI framework. The volume of the unit cell decreases linearly from 5335 A” with increasing boron concentration down to a plateau level of 5220 A” at a boron concentration of 5 B/tic. [5]. From this relationship one
Fig. 2. SEM picture of MTN synthesized using the dry method with seed crystals.
Fig. 4. SEM picture of B-MFI synthesized using the dry method with seed crystals.
290
U. Deforth
et al. / Microporous
can calculate boron contents of the framework to be around 4 B/u.c., which corresponds well with the chemical analysis. NMR analysis also proved that more than 90% of the boron is incorporated into the framework (line at -3.8 ppm) [6]. An attempt was also made to synthesize B-MTW. However, this synthesis system proved to be poorly reproducible. Only in some of the experiments using an optimized reaction composition: 1 SiO, : 0.0075 Al,Oj
Materials
9 (1997)
287-290
focus on the incorporation of germanium and vanadium, which should form volatile fluorides under the applied conditions. Using this synthesis route, we plan to grow crystals directly from the gas phase in a specially designed gas reactor. The reaction will be realized by a continuous introduction of the volatile reaction components into a reaction cell. This could open new and fast synthesis pathways for zeolitic materials.
: 0.116 B,O, :
0.985 NH4F : 0.875 Dabco@ and B-MTW seed crystals (3.33 wt.%) obtained from a hydrothermal synthesis, was B-MTW formed together with amorphous material. Crystals were needle shaped and between 10 and 50 pm long. Synthesis of B-MTW is more difficult than that of the other systems because even the hydrothermal route is not straightforward and to our knowledge no synthesis had been reported before.
Acknowledgment
We gratefully acknowledge ‘Stiftung RheinlandPfalz fiir Innovation’ for financial support. We also would like to thank J. Patarin for NMRmeasurements. Thanks for ICP-measurements to ALSI Penta Zeolith GmbH, Schwandorf. Especially we would like to thank U. Ciesla and U. Junges and C. Borgmann for the inspiring discussions.
References 4. Conclusionsand perspectives
It was shown that B-MFI and MTN can be synthesized using the dry process. The addition of seed crystals improved both the yield and the reproducibility of the reaction. The powder method can now be considered a common method to synthesize zeosiles, e.g., Silicalite I [2], AST [3], NON [ 31, MTN and also an isomorphously substituted zeolite, B-MFI. Further investigations will
[I] J.L. Guth, H. Kessler, J.M. Higel, J.M. Lamblin and J. Patarin, ACS Symp. Ser., 398 ( 1989) 176. [2] R. Althoff, K. Unger and F. Schiith, Microporous Mater., 2 (1994) 557. [3] G. van de Goor, B. Lindlar, J. Felsche and P. Behrens, J. Chem. Sot. Chem. Commun.. (1995) 2559. [4] R. Althoff, Thesis, University of Mainz, 1995. [5] J.C. Jansen, E. Biron and H. van Bekkum, Stud. Surf. Sci. Catal., 37 (1988) 133. [6] K.F.M.G.J. Scholle and W.S. Veemann, Zeolites, 5 (1985) 118.
Materials
9 ( 1997) 287-290
Short Communication
Dry synthesis of B-MFI, MTN- and MTW-type materials U. Deforth a,*, K.K. Unger a, F. Schiith b a Institut fiir Anorganische b Institut jiir Anorganische
Chemie und Analytische Chemie. Johannes Gutenberg- Oilicersitiit Main=, Becherweg 24, 55099 Main:. Germany Chemie, Johann Wolfgang Goethe-Unicersitat. Marie-Curie-Str. II, 60439 Frankfurt/Main, Germanl
Received 26 September
1996; revision 28 October 1996: accepted 5 November
1996
Abstract B-MFI and MTN-type materials were synthesized via the fluoride route from dry powders in the complete absence of a solution phase. Amorphous precursors obtained by drying SiO,’ A&O, gels at 700°C were transformed into MTN-type material in the presence of dried NH,F and TMACl. Amorphous precursors obtained by drying SiO, gels at 700’C were transformed into B-MFI in the presence of dried NH,F, B203 and TPABr. Water is formed as a reaction product. However, the water vapor pressure is appreciably below the water vapor saturation pressure under the given reaction conditions. In addition. syntheses using seed crystals were carried out successfully. 0 1997 Elsevier Science B.V. Keywords:
B-MFI;
Dry
synthesis;
MTN;
Seed crystals
1. Introduction
The synthesis of zeolites is traditionally performed under hydrothermal (solvothermal ) conditions using water or organic alcohols as a solvent. Based on the fluoride route [ 11, we developed previously a procedure which did not contain any solvent as a reactant [2]. So far. only pure silica zeolites such as Silicalite I [2], AST [ 31 and NON [3] clathrasils were synthesized by means of the dry synthesis. Here we report the synthesis of MTN-type material, another pure silica material, and the first incorporation of boron into an MFItype zeolite using the dry process. * Corresponding +61 31 392710;
author. Tel.: +61 31 395745/392107: e-mail: uweaak-unger.chemie.uni-mainz.de
0927-6513:97/$17.00 G 1997 Elsevier PII SO927-6513(96)00118-6
Fax:
Science B.V. All rights reserved
The essential feature of the dry method is that the water content in the reaction mixture under synthesis conditions is always kept lower than the water vapor saturation pressure to prevent the formation of a liquid phase. The reaction is assumed to take place via a gas phase transport process. The mobile species are probably formed by transformation of the amorphous SiOZ precursor according to: SiO, + 4NH,F-+SiF,
+ 2H,O +4NH,.
Therefore, so far only those zeolitic structures whose T-atoms can form volatile fluorides can be formed. Thus, in the presence of aluminum, the synthesis of MFI-type zeolites always leads to the pure silica MFI structure, Silicalite I [2]. In order to generalize the dry synthesis of zeo-
288
c’. Drforrh
rt al. : Microporous
lites we extended our studies by considering the following aspects: (1) synthesis of other pure silica zeolites using the dry fluoride process; zeolites using other (2) synthesis of MFI-type T-atoms, which form volatile fluorides, e.g., boron; (3) synthesis of MFI-type zeolites or other structures using seed crystals.
2. Experimental
MTN and B-MFI crystals were obtained according to the dry synthesis procedure described elsewhere [2]. In all systems a precursor was needed. For the preparation of the precursor for the MTN synthesis, aluminum nitrate (Aldrich) and fumed silica (Aerosil 200; Degussa) were mixed and stirred in 200 ml of water for 1 h. The obtained slurry was placed on a porcelain dish and dehydrated in a preheated oven at 700’C for 24 h. The precursor for the B-MFI synthesis was formed by the same procedure using only fumed silica (Aerosil 200; Degussa). Seed crystals were synthesized following the hydrothermal fluoride method [ 11. Before the addition to the reaction mixture the resulting crystals were ground in a mortar. The seed crystals, the prepared precursor, NH,F (Aldrich) and the ternplating agent, TMACl (Aldrich) or TPABr (Aldrich), were mixed and ground in a mortar for 1 min. The composition of the reaction mixture for the MTN synthesis was: 1 SiO, : 0.04 Al,O, 1 NH,F
: 0.5 TMACl
:
: O-O.9 wt.% seed crystals.
The reaction mixture was placed in a 50-ml teflonlined stainless-steel autoclave and sealed before heating it to the reaction temperature of 210°C for 144 h. After quench cooling, the products were washed, dried at 12O’C and kept for characterization. The water vapor partial pressures were calculated to be between 10 and 18 bar under reaction conditions, assuming ideal behavior which overestimates the pressure [4]. The water vapor
Materids
9 ( 1997) 287-290
saturation pressure was 19 bar at the reaction temperature of 210°C. The composition of the reaction mixture for the B-MFI synthesis was: 1 SiOZ : 0.186 B,O, (Aldrich) 1.59 NH,F
: O-3.6
: 0.048 TPABr :
wt.% seed crystals.
The reaction mixture was placed in a 50-ml teflonlined stainless-steel autoclave and sealed before heating it to the reaction temperature of 200°C for 72 h. After quench cooling, the products were washed, dried at 120°C and kept for characterization. The water vapor partial pressures were calculated to be between 8 and 14 bar under reaction conditions. The water vapor saturation pressure was 15 bar at the reaction temperature of 200°C. The products were characterized by X-ray powder diffraction (XRD), X-ray fluorescence analysis (RFA). thermogravimetry (TG) and differential thermal analysis (DTA) and scanning electron microscopy (SEM ). The incorporation of boron into MFI-type zeolite was proven by induced coupled plasma (ICP) measurements, XRD measurements (refinement of lattice constants) and “B-NMR.
3. Results and discussions
It was possible to obtain pure MTN of high crystallinity using the dry reaction process. The X-ray diffraction pattern shows no traces of byproducts or indications for amorphous material. RFA measurements showed an Si/Al ratio of 225. Aluminum is not incorporated into the framework. However, aluminum-containing precursors proved to give better crystalline samples and made the syntheses more reproducible than pure silica precursors. The crystals had a nearly perfect octahedral shape with an average size of 40 pm (Fig. 1). When the water content of the synthesis was reduced while all other parameters were kept constant, the particle size decreased from about 150 pm for a conventional hydrothermal synthesis to about 80 pm for a synthesis from dry precursors in which enough water could be released to form a liquid phase to the above-mentioned value for
U Deforth
et ul. / Microporous
Materials
9 ( 1997) 287-290
289
Fig. 1. SEM picture of MTN synthesized using the dry method without seed crystals.
Fig. 3. SEM picture of B-MFI synthesized using the dry method without seed crystals.
the solvent-free synthesis. The addition of seed crystals improved both the yield and the reproducibility of the reaction. However, the seeding induced a strong intergrowth of the crystals (Fig. 2). The attempt to synthesize isomorphously substituted ZSM-5 (MFI) was also successful. B-MFI was obtained without any phase impurities, if B203 was used as the boron source. The crystals had the typical MFI-twinning and a length of 30 urn (Fig. 3). Using H,BO,, only poorly crystalline samples were obtained in two of about 15 experiments. B20, resulted in higher quality crystals and a reproducible synthesis. As described above in case of MTN, the synthesis with addition
of seeds produced intergrown crystals of about 6 urn length (Fig. 4). ICP-measurements of the bulk material showed an average concentration of 0.88 wt.% boron in the samples. The zeolites contained an average amount of 8.8 wt.% of organic template as measured by means of TG and DTA. Evaluation of XRD data proved the incorporation of boron into the MFI framework. The volume of the unit cell decreases linearly from 5335 A” with increasing boron concentration down to a plateau level of 5220 A” at a boron concentration of 5 B/tic. [5]. From this relationship one
Fig. 2. SEM picture of MTN synthesized using the dry method with seed crystals.
Fig. 4. SEM picture of B-MFI synthesized using the dry method with seed crystals.
290
U. Deforth
et al. / Microporous
can calculate boron contents of the framework to be around 4 B/u.c., which corresponds well with the chemical analysis. NMR analysis also proved that more than 90% of the boron is incorporated into the framework (line at -3.8 ppm) [6]. An attempt was also made to synthesize B-MTW. However, this synthesis system proved to be poorly reproducible. Only in some of the experiments using an optimized reaction composition: 1 SiO, : 0.0075 Al,Oj
Materials
9 (1997)
287-290
focus on the incorporation of germanium and vanadium, which should form volatile fluorides under the applied conditions. Using this synthesis route, we plan to grow crystals directly from the gas phase in a specially designed gas reactor. The reaction will be realized by a continuous introduction of the volatile reaction components into a reaction cell. This could open new and fast synthesis pathways for zeolitic materials.
: 0.116 B,O, :
0.985 NH4F : 0.875 Dabco@ and B-MTW seed crystals (3.33 wt.%) obtained from a hydrothermal synthesis, was B-MTW formed together with amorphous material. Crystals were needle shaped and between 10 and 50 pm long. Synthesis of B-MTW is more difficult than that of the other systems because even the hydrothermal route is not straightforward and to our knowledge no synthesis had been reported before.
Acknowledgment
We gratefully acknowledge ‘Stiftung RheinlandPfalz fiir Innovation’ for financial support. We also would like to thank J. Patarin for NMRmeasurements. Thanks for ICP-measurements to ALSI Penta Zeolith GmbH, Schwandorf. Especially we would like to thank U. Ciesla and U. Junges and C. Borgmann for the inspiring discussions.
References 4. Conclusionsand perspectives
It was shown that B-MFI and MTN can be synthesized using the dry process. The addition of seed crystals improved both the yield and the reproducibility of the reaction. The powder method can now be considered a common method to synthesize zeosiles, e.g., Silicalite I [2], AST [3], NON [ 31, MTN and also an isomorphously substituted zeolite, B-MFI. Further investigations will
[I] J.L. Guth, H. Kessler, J.M. Higel, J.M. Lamblin and J. Patarin, ACS Symp. Ser., 398 ( 1989) 176. [2] R. Althoff, K. Unger and F. Schiith, Microporous Mater., 2 (1994) 557. [3] G. van de Goor, B. Lindlar, J. Felsche and P. Behrens, J. Chem. Sot. Chem. Commun.. (1995) 2559. [4] R. Althoff, Thesis, University of Mainz, 1995. [5] J.C. Jansen, E. Biron and H. van Bekkum, Stud. Surf. Sci. Catal., 37 (1988) 133. [6] K.F.M.G.J. Scholle and W.S. Veemann, Zeolites, 5 (1985) 118.