- Email: [email protected]
Hydrothermal synthesis of microporous titanosilicates
Microporous and Mesoporous Materials 105 (2007) 232–238 www.elsevier.com/locate/micromeso
Hydrothermal synthesis of microporous titanosilicates V. Kostov-Kytin a
a,*
, S. Ferdov a, Yu. Kalvachev a, B. Mihailova b, O. Petrov
a
Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad G Bonchev Street 107, 1113 Sofia, Bulgaria b Mineralogisch-Petrographisches Institut, Grindelallee 48, Universita¨t Hamburg, D-20146 Hamburg, Germany Received 27 July 2006; received in revised form 29 January 2007; accepted 26 March 2007 Available online 2 April 2007
Abstract Titanosilicates of various framework topology are synthesized in the template-free Na2O–(K2O)–TiO2–SiO2–H2O system. The fields of crystallization of the as-synthesized phases are outlined and the role of certain physicochemical parameters on the framework type, crystal size and morphology as well as crystal aggregate texture of the run-products is studied. Kinetic investigations carried out in the potassium-free system give insights on the atomic arrangements in the amorphous sodium titanosilicate precursor powders, their evolution upon hydrothermal treatment and the related phase formation sequence. The synthesis results reveal possible mechanisms of increasing the structural diversity of synthetic heterosilicates with desired pore system and functionality by varying the composition of framework and extraframework cations and appropriate adjusting of the pH and temperature of reaction medium. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Hydrothermal synthesis; Titanosilicates; Crystallization fields; Kinetic investigations
1. Introduction Natural titanosilicates are mostly localized to the postmagmatic derivatives of peralkaline rocks [1] and more than 100 of these mineral species are already reported [2]. Among them the microporous titanosilicates are closely related to the natural and synthetic zeolites and lately draw interest for various applications. Their frameworks are built of tetrahedral (Si) fragments and transition elements (mainly Ti, Nb, Zr but also Ta, Sn, W, Fe, Mn, Zn, etc.) having six-fold, rarely five-fold coordination. Microporous titanosilicates often occur together with a large family of heterophyllosilicate minerals that contain the same set of major cations [3]. Materials of these types are promising ion exchangers, sorbents, catalysts or catalyst supports [4–9]. In recent years, suchlike phases with or without mineral analogues have been prepared by using various synthesis techniques [4], however, their number is still smaller than that one provided by Nature. *
Corresponding author. Tel.: +359 29797055; fax: +359 29797056. E-mail address: [email protected] (V. Kostov-Kytin).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.03.036
Aiming at further optimization of the synthesis conditions for preparation of titanosilicates with desired pore systems and functionality we have explored the system Na2O–(K2O)–TiO2–SiO2–H2O. The goal of our research was to synthesize as many as possible porous and layered phases of technological importance under similar conditions and without using organics by only varying the Na2O(K2O)/TiO2-ratio in the initial gel and thus to outline the fields of crystallization and the tendencies in the preparation of titanosilicate materials. In this paper, we present results on the effect of certain physicochemical parameters on the framework type, crystal size and morphology as well as crystal aggregate texture (the spatial arrangement and size of the microcrystals building the aggregates) of the run-products. Additionally, amorphous titanosilicate powders prepared from initial gels in the potassium-free system were investigated in order to study the interplay between the composition of the synthesis mixture and the predominant type of structural species in the amorphous precursors as well as the trends in the crystalline phase formation upon hydrothermal treatment of the corresponding gels.
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233
nous pressure. No organics were used as reactants or templates. The initial gels were obtained by mixing NaOH, KOH, SiO2, TiCl4 (Merck) and distilled water in appropriate ratios. In a typical procedure, SiO2 of 200 lm particle size was added to the alkaline aqueous solution and then mixed with the hydrolysed TiCl4 brought to the boiling point. The mixture was homogenized using a mechanical stirrer (200 rpm) for 60 min at room temperature. The gels
2. Experimental methods 2.1. Sample preparation Detailed synthesis studies were held in the system: aNa2O–bK2O–cTiO2–10SiO2–675H2O, where 0 6 a 6 9, 0 6 b 6 9, a + b = 9 or a + b = 5, 0.3 6 c 6 3.3, at temperature of 200 °C, crystallization time of 24 h and autoge-
Fig. 1. Powder XRD patterns of the as-synthesized titanosilicate phases with microporous, layered and dense structures.
Table 1 Synthesis conditions and chemical formulae for the as-prepared titanosilicate phases Phase name, structural type, mineral counterpart (if any)
Chemical composition of the initial gel, moles Na2O
K2O
TiO2
ETS-4, microporous, zorite GTS-1, microporous, isostructural with pharmacosiderite Microporous, sitinakite ETS-10, microporous, no mineral analogue STS, microporous, umbite AM-1, layered, no mineral analogue AM-4, layered, no mineral analogue Dense, paranatisite Dense, natisite
9 6
– 3
25–30 2.5 1.5 5–6 18 25 30
– 2.5 7.5 – – – –
a
See also Fig. 1.
Synthesis duration, h
Chemical formula
Ti coordination
Reference
1.2–2.8 1
24 24
6, 5 6
[5] [6,7]
3–3.5 0.6 0.3–1.8a 1–1.3 2.5 3.0 3.5
24 24 24 24 >96 24–48 >48
H2Ti4Si12O38(TiO)Na8 Æ 8.5H2O HM3Ti4O4(SiO4)3 Æ 4H2O (M = Na,K) Na2Ti2O3SiO4 Æ 2H2O (Na,K)2Si5TiO13 K2TiSi3O9 Æ H2O Na4Ti2Si8O22 Æ 4H2O Na3(Na,H)Ti2O2[Si2O6]2 Æ 2H2O Na8Ti3.5O2(OH)2(SiO4)4 Na2(TiO)(SiO4)
6 6 6 5 6 5 5
[8] [15] [13,14] [4,9] [4] [10,12] [11,12]
96 h
Natisite Natisite Natisite Natisite Sitinakite + paranatisite + natisite Sitinakite + natisite 2.55 4.26 0.28 0.21 8.5 9 S7 S8
0.30 0.35
3.8 4.5 5.4 7 7.4 S2 S3 S4 S5 S6
0.17 0.19 0.23 0.25 0.27
0.33 0.35 0.33 0.25 0.26
0.54 0.64 1.14 1.60 1.87
Amorphous ETS-4 + GTS-1 Amorphous + GTS-1 GTS-1 + amorphous GTS-1 + amorphous
Amorphous + AM-1 ETS-4 ETS-4 + GTS-1 GTS-1 + ETS-4 AM4 + paranatisite + natisite Natisite + paranatisite Natisite
AM-1 + quartz + ETS4 ETS-4 + AM-1 ETS-4 ETS-4 AM-4 + GTS-1 Natisite + AM-4
72 h 48 h
AM-1 + quartz AM-1
24 h TiO2/SiO2
0.21
Na2O/TiO2 TiO2/SiO2 Na2O/TiO2
Phase formation sequence Precursor powder Initial gel Sample no.
Table 2 Compositional ratios of the initial gels and precursor powders, and corresponding phase formation sequence
Fig. 2. Fields of crystallization of the as-synthesized titanosilicate phases. Empty circles designated from S1 to S8 compositionally correspond to the samples subjected to spectroscopic investigations (see Table 2). Filled circles a–f correspond to the compositions presented in Fig. 3; STS* – the ordinate is presented by (Na2O + K2O)/TiO2. Q – quartz; A – anatase; am – amorphous.
0.51
Initial characterization of the hydrothermally treated samples and phase identification were performed by powder X-ray diffraction analysis using a PW3710 diffractometer with CuKa radiation in the range 2h 5–60° or a DRON 3M diffractometer with Fe-filtered CoKa radiation in a step-scan regime (step 0.02° and time 3 s).
0.15
2.2. Sample characterization
3.7
were subsequently transferred into 10 ml Teflon-lined autoclaves. The hydrothermal treatment was terminated by quenching of the autoclaves in cold water. The run products were washed with distilled water and dried at 100 °C. Kinetic investigations were carried out in the system: aNa2O–cTiO2–10SiO2–675H2O, where 3 6 a 6 40, 0.5 6 c 6 5 and synthesis duration varied from 16 to 240 h. In addition, amorphous powder samples, which were further subjected to spectroscopic studies, were obtained by filtering the homogenized initial gels in the potassium-free system and subsequent drying at ambient temperature.
ETS-4 + AM1 + quartz ETS-4 + AM-1 ETS-4 + AM-4 AM-4 + ETS-4 AM-4 Natisite + AM-4
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The grain shape, morphology and size of the run-products were studied by scanning electron microscopy (SEM) using a Philips 515 scanning electron microscope. The chemical composition of tablets prepared from the precursor amorphous powders was determined by electron probe microanalysis using a Philips SEM 515 equipped with EDAX 9100/70 analytical system. A tablet of Na2O–SiO2–TiO2 of molecular ratio 1:1:1 was used as standard.
235
inakite and GTS-1 crystallized as nanosized products. Nanocrystalline Na–GTS-1 was hydrothermally synthesized at even 90 °C for 18 h [11]. The synthesis conditions for preparation of best quality samples and crystal chemical characterization of the as-synthesized phases are given in Table 1. Fig. 2 presents the fields of crystallization of the as-synthesized for 24 h titanosilicates. 3.2. Factors influencing the structural type and morphology of the run products
3. Results and discussion 3.1. Fields of crystallization Seven titanosilicate phases crystallized within the system aNa2O–cTiO2–10SiO2–675H2O (3 6 a 6 40, 0.5 6 c 6 5) under the described conditions: – the microporous materials ETS-4 [10], Na–GTS-1 [5,11] and the synthetic analogue of the mineral sitinakite [12], – the layered materials AM-1, also known as JDF-L1 [4,13] and AM-4 [4], and – two dense materials synthetic structural analogous of the minerals paranatisite [14] and natisite [15,16]. The microporous STS [17,18] (known also as AM-2 [4]), GTS-1 and ETS-4 were prepared in the system: aNa2O– bK2O–cTiO2–10SiO2–675H2O where a + b = 9 and the microporous ETS-10 [19] was obtained in the same system, however, at lower alkalinity, i.e., a + b = 5. With the exception of paranatisite all other titanosilicates were obtained as pure crystalline phases (Fig. 1). Sit-
In the course of the synthesis investigations it appeared that the ratio Na2O(K2O)/TiO2 in the initial gel dependably controls the pH of the reaction medium and together with the ratio TiO2/SiO2 gel plays a key role in the synthesis pathways. Complementary application of infrared, Raman and nuclear magnetic resonance spectroscopy revealed that these ratios influence the preferential atomic clustering in the initial mixtures prior to the hydrothermal treatment and, subsequently, on the type of the framework topology, size and morphology, and yield of the run-products [20]. Fig. 2 clearly outlines the tendency for formation of microporous phases with a low Ti content and large ˚ ) and layered hydrous titanosilipore size (ETS-10, 7.6 A cates (AM-1) for short synthesis duration (24 h) at low Na2O(K2O)/TiO2 and TiO2/SiO2 ratios. Microporous phases of higher Ti content (STS, ETS-4, GTS-1, sitinakite) ˚ and layered titanosilicates of and pore size of about 4 A comparatively lower water sorption capacity (AM-4) preferentially crystallize at intermediate ratios and generally need longer synthesis duration for complete crystallization (Tables 1 and 2). The high alkalinity and elevated Ti
Fig. 3. Micrographs of STS crystals and aggregates synthesized hydrothermally for 24 h in the system 2Na2O–7K2O–cTiO2–10SiO2–675H2O, where: (a) c = 0.3; (b) c = 0.6; (c) c = 0.9; (d) c = 1.2; (e) c = 1.5; (f) c = 1.8.
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content of the reaction medium facilitate the formation of titanosilicates with dense structures (paranatisite, natisite). It is interesting that within the investigated range of the Na2O/TiO2 ratio of the synthesis mixture, frameworks with five-coordinated titanium are obtained at the lowest (below 4) and highest (above 8.5) ratio values, whereas six-coordinated Ti4+ is predominant in frameworks prepared at intermediate ratio values. The role of the exchangeable cations as structure directing agents is evidenced by the formation of ETS-10 instead of AM-1 and STS instead of ETS-4 when part of Na is replaced by K (Fig. 2, Table 1). A noteworthy tendency related to the increase of TiO2/ SiO2 within the fields of crystallization of STS and ETS-4 synthesized for 24 h in the system aNa2O–bK2O–cTiO2– 10SiO2–675H2O where 0 6 a 6 9, 0 6 b 6 9, a + b = 9, 0.3 6 c 6 3.3 should be outlined. The increase only in the TiO2 content of the initial gel affects the morphology and crystal size of the run products as illustrated in Figs. 3 and 4. Transition from formation of large single crystals through intergrowths with decreasing degree of perfection to micro-crystals of random growth arrangement forming spherical aggregates is clearly observed. Apparently, the increase in titania enhances the formation of nuclei that further become centers of crystallization. This, in general, is accompanied by an increase in the run product yield. 3.3. Kinetic investigations and phase formation sequence Table 2 shows the composition of the initial synthesis mixture and the precursor powders as well as the phase formation sequence occurring upon hydrothermal treatment in the system aNa2O–cTiO2–10SiO2–675H2O, where 3 6 a 6 40, 0.5 6 c 6 5 and synthesis duration up to 96 h. The presented compositional ratios indicate the higher affinity of Ti to reside in the solid-gel phase as compared to Si and Na. This effect is significantly enhanced upon increasing the Na2O/TiO2 in the initial gel. Infrared, Raman and 29Si nuclear magnetic resonance investigations elucidated the atomic arrangements in the amorphous sodium titanosilicate precursor powders [20]. The spectroscopic data reveal that for gels with a lower Na2O/TiO2 ratio the precursor amorphous powders possess a lower degree of polymerisation of the SiO4 species as compared to the first crystallizing products. This means that the hydrothermal treatment induces the homocondensation of SiO4 groups. Contrary, for gels with a higher Na2O/ TiO2 ratio the hydrothermal treatment enhances the fragmentation of the Si–O system in the final run product as compared to the amorphous precursors. The kinetic investigations well confirm the validity of Ostwald’s rule of successive phase transformations leading to the appearance of more thermodynamically stable products in the following sequence: AM-1–ETS-4–GTS-1–AM4–sitinacite–paranatisite–natisite, i.e., from microporous and layered titanosilicates with a low Ti content up to dense, Ti-rich phases. This tendency is somehow contradic-
Fig. 4. Micrographs of ETS-4 synthesized hydrothermally for 24 h in the system aNa2O–bK2O–cTiO2–10SiO2–675H2O, where: (a) a = 4.5; b = 4.5; c = 1.65; (b) a = 6; b = 3; c = 2.15; (c) a = 7.5; b = 1.5; c = 2.65.
tory to that observed by Pekov and Chukanov [1]. These authors have reported that zeolite-like titanosilicates, e.g., members of the labuntsovite and lintisite groups and vinogradovite, crystallize later than Ti-containing minerals with higher framework density such as lorenzenite and titanite. This occurs if the mineral deposition with temperature lowering proceeds under high-alkaline conditions and, if alkalinity decreases the evolution proceeds in the direction of a condensation of structures. To interpret this mismatch one should take into consideration that the phase formation sequence reported here takes place in a thermodynamically closed system at constant temperature and alkalinity. In all syntheses the pH values are close to or exceed 13. This makes the reaction medium quite aggressive towards the configuration of the primary and secondary building units (PBU and SBU) in the precursor gel. With time, the
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Fig. 5. Micrographs and powder XRD patterns presenting the successive replacement of AM-4 by natisite: (a) AM-4 (96 h), (b) relics of AM-4 (122 h) with natisite and (c) pure natisite (240 h).
polymerized SiO2 and Ti(O,OH)4 PBU experience fragmentation that leads to increased TiO2/SiO2 values in the subsequently formed SBU and the resulting titanosilicate framework constructions (Tables 1 and 2). If the initially formed phase is not in equilibrium with the reaction medium, it may dissolve supplying PBU and SBU for crystallization of thermodynamically more stable phases (Fig. 5). 4. Conclusions Within the investigated range of Na2O/TiO2 ratios of the synthesis mixture, frameworks with five-coordinated titanium are obtained at the lowest (below 4) and highest (above 8.5) ratio values, while six-coordinated Ti4+ is predominant in structures prepared at intermediate ratio values. Layered and microporous titanosilicates crystallize as pure phases at lower Na2O(K2O)/TiO2 and TiO2/SiO2 ratios, while dense titanosilicates are preferentially formed at higher alkalinity. The sensitivity of the framework topology and phase morphology to tiny variations in the reaction medium allows precise tuning in the preparation of titanosilicates with tailored pore system, morphology and functionality.
To increase the structural diversity of synthetic heterosilicates with useful properties it is necessary to vary the precursor chemistry i.e., a choice of appropriate sources for framework and extraframework cations, mineralizers and co-solvents as well as to work in somewhat milder conditions concerning pH and temperature of the reaction medium. Acknowledgements Financial support by the National Science Fund – Bulgarian Ministry of Education and Science under contract No NT 1-02 is gratefully acknowledged. V.K. and Yu.K. are indebted to Joint Research Project with Academy of Sciences of the Czech Republic. B.M. and Yu.K. are indebted to the Alexender von Humboldt Fondation for equipment donation. References [1] [2] [3] [4]
I.V. Pekov, N.V. Chukanov, Rev. Mineral. Geochem. 57 (2005) 145. N.V. Chukanov, I.V. Pekov, Rev. Mineral. Geochem. 57 (2005) 105. G. Ferraris, A. Gula, Rev. Mineral. Geochem. 57 (2005) 69. J. Rocha, M. Anderson, Eur. J. Inorg. Chem. (2000) 801.
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[5] E.A. Behrens, A. Clearfield, Micropor. Mater. 11 (1997) 65. [6] L. Al-Attar, A. Dyer, A. Paajanen, R. Harjula, J. Mater. Chem. 13 (12) (2003) 2969. [7] C.M. Braunbarth, L.C. Boudreau, M. Tapatsis, J. Membr. Sci. 174 (2000) 31. [8] S.M. Kuznicki, V.A. Bell, S. Nair, H.W. Hillhouse, R.M. Jacubinas, C.M. Braunbarth, B.H. Toby, M. Tsapatsis, Nature 412 (2001) 720. [9] B. Shen, H. Li, W. Zhang, Ye Zhao, Z. Zhang, X. Wang, Sh. Shen, Catal. Today 106 (2005) 206. [10] S. Nair, H. Jeong, A. Chandrasekaran, C.M. Braunbarth, M. Tsapatsis, S.M. Kuznicki, Chem. Mater. 13 (2001) 4247. [11] S. Ferdov, C. Lengauer, V. Kostov-Kytin, O. Petrov, J. Mater. Sci. Lett. 39 (2004) 4343. [12] D.M. Poojary, R.A. Cahill, A. Clearfield, Chem. Mater. 6 (1994) 2364.
[13] S. Ferdov, V. Kostov-Kytin, O. Petrov, Chem. Commun. (2002) 1786. [14] E.V. Sokolova, N.A. Yamnova, Yu.K. Egorov-Tismenko, A.P. Khomyakov, Dokl. Akad. Nauk SSSR 284 (1985) 1136. [15] H. Nyman, M. O’Keffee, J.O. Bovin, Acta Cryst. B34 (1978) 905. [16] S. Ferdov, V. Kostov-Kytin, O. Petrov, Powder Diffr. 17 (2002) 234. [17] M.S. Dadachov, A. Le Bail, Eur. J. Solid State Chem. 34 (1997) 381. [18] J.L. Paillaud, V. Valchev, S. Mintova, H. Kessler, 12th International Zeolite Conference (1999) Materials Research Society 2525. [19] M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S.P. McKay, A. Ferreira, J. Rocha, S. Lidin, Nature (London) 367 (1994) 347. [20] V. Kostov-Kytin, B. Mihailova, Yu. Kalvachev, M. Tarassov, Micropor. Mesopor. Mater. 86 (2005) 223.
Hydrothermal synthesis of microporous titanosilicates V. Kostov-Kytin a
a,*
, S. Ferdov a, Yu. Kalvachev a, B. Mihailova b, O. Petrov
a
Central Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad G Bonchev Street 107, 1113 Sofia, Bulgaria b Mineralogisch-Petrographisches Institut, Grindelallee 48, Universita¨t Hamburg, D-20146 Hamburg, Germany Received 27 July 2006; received in revised form 29 January 2007; accepted 26 March 2007 Available online 2 April 2007
Abstract Titanosilicates of various framework topology are synthesized in the template-free Na2O–(K2O)–TiO2–SiO2–H2O system. The fields of crystallization of the as-synthesized phases are outlined and the role of certain physicochemical parameters on the framework type, crystal size and morphology as well as crystal aggregate texture of the run-products is studied. Kinetic investigations carried out in the potassium-free system give insights on the atomic arrangements in the amorphous sodium titanosilicate precursor powders, their evolution upon hydrothermal treatment and the related phase formation sequence. The synthesis results reveal possible mechanisms of increasing the structural diversity of synthetic heterosilicates with desired pore system and functionality by varying the composition of framework and extraframework cations and appropriate adjusting of the pH and temperature of reaction medium. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Hydrothermal synthesis; Titanosilicates; Crystallization fields; Kinetic investigations
1. Introduction Natural titanosilicates are mostly localized to the postmagmatic derivatives of peralkaline rocks [1] and more than 100 of these mineral species are already reported [2]. Among them the microporous titanosilicates are closely related to the natural and synthetic zeolites and lately draw interest for various applications. Their frameworks are built of tetrahedral (Si) fragments and transition elements (mainly Ti, Nb, Zr but also Ta, Sn, W, Fe, Mn, Zn, etc.) having six-fold, rarely five-fold coordination. Microporous titanosilicates often occur together with a large family of heterophyllosilicate minerals that contain the same set of major cations [3]. Materials of these types are promising ion exchangers, sorbents, catalysts or catalyst supports [4–9]. In recent years, suchlike phases with or without mineral analogues have been prepared by using various synthesis techniques [4], however, their number is still smaller than that one provided by Nature. *
Corresponding author. Tel.: +359 29797055; fax: +359 29797056. E-mail address: [email protected] (V. Kostov-Kytin).
1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.03.036
Aiming at further optimization of the synthesis conditions for preparation of titanosilicates with desired pore systems and functionality we have explored the system Na2O–(K2O)–TiO2–SiO2–H2O. The goal of our research was to synthesize as many as possible porous and layered phases of technological importance under similar conditions and without using organics by only varying the Na2O(K2O)/TiO2-ratio in the initial gel and thus to outline the fields of crystallization and the tendencies in the preparation of titanosilicate materials. In this paper, we present results on the effect of certain physicochemical parameters on the framework type, crystal size and morphology as well as crystal aggregate texture (the spatial arrangement and size of the microcrystals building the aggregates) of the run-products. Additionally, amorphous titanosilicate powders prepared from initial gels in the potassium-free system were investigated in order to study the interplay between the composition of the synthesis mixture and the predominant type of structural species in the amorphous precursors as well as the trends in the crystalline phase formation upon hydrothermal treatment of the corresponding gels.
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nous pressure. No organics were used as reactants or templates. The initial gels were obtained by mixing NaOH, KOH, SiO2, TiCl4 (Merck) and distilled water in appropriate ratios. In a typical procedure, SiO2 of 200 lm particle size was added to the alkaline aqueous solution and then mixed with the hydrolysed TiCl4 brought to the boiling point. The mixture was homogenized using a mechanical stirrer (200 rpm) for 60 min at room temperature. The gels
2. Experimental methods 2.1. Sample preparation Detailed synthesis studies were held in the system: aNa2O–bK2O–cTiO2–10SiO2–675H2O, where 0 6 a 6 9, 0 6 b 6 9, a + b = 9 or a + b = 5, 0.3 6 c 6 3.3, at temperature of 200 °C, crystallization time of 24 h and autoge-
Fig. 1. Powder XRD patterns of the as-synthesized titanosilicate phases with microporous, layered and dense structures.
Table 1 Synthesis conditions and chemical formulae for the as-prepared titanosilicate phases Phase name, structural type, mineral counterpart (if any)
Chemical composition of the initial gel, moles Na2O
K2O
TiO2
ETS-4, microporous, zorite GTS-1, microporous, isostructural with pharmacosiderite Microporous, sitinakite ETS-10, microporous, no mineral analogue STS, microporous, umbite AM-1, layered, no mineral analogue AM-4, layered, no mineral analogue Dense, paranatisite Dense, natisite
9 6
– 3
25–30 2.5 1.5 5–6 18 25 30
– 2.5 7.5 – – – –
a
See also Fig. 1.
Synthesis duration, h
Chemical formula
Ti coordination
Reference
1.2–2.8 1
24 24
6, 5 6
[5] [6,7]
3–3.5 0.6 0.3–1.8a 1–1.3 2.5 3.0 3.5
24 24 24 24 >96 24–48 >48
H2Ti4Si12O38(TiO)Na8 Æ 8.5H2O HM3Ti4O4(SiO4)3 Æ 4H2O (M = Na,K) Na2Ti2O3SiO4 Æ 2H2O (Na,K)2Si5TiO13 K2TiSi3O9 Æ H2O Na4Ti2Si8O22 Æ 4H2O Na3(Na,H)Ti2O2[Si2O6]2 Æ 2H2O Na8Ti3.5O2(OH)2(SiO4)4 Na2(TiO)(SiO4)
6 6 6 5 6 5 5
[8] [15] [13,14] [4,9] [4] [10,12] [11,12]
96 h
Natisite Natisite Natisite Natisite Sitinakite + paranatisite + natisite Sitinakite + natisite 2.55 4.26 0.28 0.21 8.5 9 S7 S8
0.30 0.35
3.8 4.5 5.4 7 7.4 S2 S3 S4 S5 S6
0.17 0.19 0.23 0.25 0.27
0.33 0.35 0.33 0.25 0.26
0.54 0.64 1.14 1.60 1.87
Amorphous ETS-4 + GTS-1 Amorphous + GTS-1 GTS-1 + amorphous GTS-1 + amorphous
Amorphous + AM-1 ETS-4 ETS-4 + GTS-1 GTS-1 + ETS-4 AM4 + paranatisite + natisite Natisite + paranatisite Natisite
AM-1 + quartz + ETS4 ETS-4 + AM-1 ETS-4 ETS-4 AM-4 + GTS-1 Natisite + AM-4
72 h 48 h
AM-1 + quartz AM-1
24 h TiO2/SiO2
0.21
Na2O/TiO2 TiO2/SiO2 Na2O/TiO2
Phase formation sequence Precursor powder Initial gel Sample no.
Table 2 Compositional ratios of the initial gels and precursor powders, and corresponding phase formation sequence
Fig. 2. Fields of crystallization of the as-synthesized titanosilicate phases. Empty circles designated from S1 to S8 compositionally correspond to the samples subjected to spectroscopic investigations (see Table 2). Filled circles a–f correspond to the compositions presented in Fig. 3; STS* – the ordinate is presented by (Na2O + K2O)/TiO2. Q – quartz; A – anatase; am – amorphous.
0.51
Initial characterization of the hydrothermally treated samples and phase identification were performed by powder X-ray diffraction analysis using a PW3710 diffractometer with CuKa radiation in the range 2h 5–60° or a DRON 3M diffractometer with Fe-filtered CoKa radiation in a step-scan regime (step 0.02° and time 3 s).
0.15
2.2. Sample characterization
3.7
were subsequently transferred into 10 ml Teflon-lined autoclaves. The hydrothermal treatment was terminated by quenching of the autoclaves in cold water. The run products were washed with distilled water and dried at 100 °C. Kinetic investigations were carried out in the system: aNa2O–cTiO2–10SiO2–675H2O, where 3 6 a 6 40, 0.5 6 c 6 5 and synthesis duration varied from 16 to 240 h. In addition, amorphous powder samples, which were further subjected to spectroscopic studies, were obtained by filtering the homogenized initial gels in the potassium-free system and subsequent drying at ambient temperature.
ETS-4 + AM1 + quartz ETS-4 + AM-1 ETS-4 + AM-4 AM-4 + ETS-4 AM-4 Natisite + AM-4
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The grain shape, morphology and size of the run-products were studied by scanning electron microscopy (SEM) using a Philips 515 scanning electron microscope. The chemical composition of tablets prepared from the precursor amorphous powders was determined by electron probe microanalysis using a Philips SEM 515 equipped with EDAX 9100/70 analytical system. A tablet of Na2O–SiO2–TiO2 of molecular ratio 1:1:1 was used as standard.
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inakite and GTS-1 crystallized as nanosized products. Nanocrystalline Na–GTS-1 was hydrothermally synthesized at even 90 °C for 18 h [11]. The synthesis conditions for preparation of best quality samples and crystal chemical characterization of the as-synthesized phases are given in Table 1. Fig. 2 presents the fields of crystallization of the as-synthesized for 24 h titanosilicates. 3.2. Factors influencing the structural type and morphology of the run products
3. Results and discussion 3.1. Fields of crystallization Seven titanosilicate phases crystallized within the system aNa2O–cTiO2–10SiO2–675H2O (3 6 a 6 40, 0.5 6 c 6 5) under the described conditions: – the microporous materials ETS-4 [10], Na–GTS-1 [5,11] and the synthetic analogue of the mineral sitinakite [12], – the layered materials AM-1, also known as JDF-L1 [4,13] and AM-4 [4], and – two dense materials synthetic structural analogous of the minerals paranatisite [14] and natisite [15,16]. The microporous STS [17,18] (known also as AM-2 [4]), GTS-1 and ETS-4 were prepared in the system: aNa2O– bK2O–cTiO2–10SiO2–675H2O where a + b = 9 and the microporous ETS-10 [19] was obtained in the same system, however, at lower alkalinity, i.e., a + b = 5. With the exception of paranatisite all other titanosilicates were obtained as pure crystalline phases (Fig. 1). Sit-
In the course of the synthesis investigations it appeared that the ratio Na2O(K2O)/TiO2 in the initial gel dependably controls the pH of the reaction medium and together with the ratio TiO2/SiO2 gel plays a key role in the synthesis pathways. Complementary application of infrared, Raman and nuclear magnetic resonance spectroscopy revealed that these ratios influence the preferential atomic clustering in the initial mixtures prior to the hydrothermal treatment and, subsequently, on the type of the framework topology, size and morphology, and yield of the run-products [20]. Fig. 2 clearly outlines the tendency for formation of microporous phases with a low Ti content and large ˚ ) and layered hydrous titanosilipore size (ETS-10, 7.6 A cates (AM-1) for short synthesis duration (24 h) at low Na2O(K2O)/TiO2 and TiO2/SiO2 ratios. Microporous phases of higher Ti content (STS, ETS-4, GTS-1, sitinakite) ˚ and layered titanosilicates of and pore size of about 4 A comparatively lower water sorption capacity (AM-4) preferentially crystallize at intermediate ratios and generally need longer synthesis duration for complete crystallization (Tables 1 and 2). The high alkalinity and elevated Ti
Fig. 3. Micrographs of STS crystals and aggregates synthesized hydrothermally for 24 h in the system 2Na2O–7K2O–cTiO2–10SiO2–675H2O, where: (a) c = 0.3; (b) c = 0.6; (c) c = 0.9; (d) c = 1.2; (e) c = 1.5; (f) c = 1.8.
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content of the reaction medium facilitate the formation of titanosilicates with dense structures (paranatisite, natisite). It is interesting that within the investigated range of the Na2O/TiO2 ratio of the synthesis mixture, frameworks with five-coordinated titanium are obtained at the lowest (below 4) and highest (above 8.5) ratio values, whereas six-coordinated Ti4+ is predominant in frameworks prepared at intermediate ratio values. The role of the exchangeable cations as structure directing agents is evidenced by the formation of ETS-10 instead of AM-1 and STS instead of ETS-4 when part of Na is replaced by K (Fig. 2, Table 1). A noteworthy tendency related to the increase of TiO2/ SiO2 within the fields of crystallization of STS and ETS-4 synthesized for 24 h in the system aNa2O–bK2O–cTiO2– 10SiO2–675H2O where 0 6 a 6 9, 0 6 b 6 9, a + b = 9, 0.3 6 c 6 3.3 should be outlined. The increase only in the TiO2 content of the initial gel affects the morphology and crystal size of the run products as illustrated in Figs. 3 and 4. Transition from formation of large single crystals through intergrowths with decreasing degree of perfection to micro-crystals of random growth arrangement forming spherical aggregates is clearly observed. Apparently, the increase in titania enhances the formation of nuclei that further become centers of crystallization. This, in general, is accompanied by an increase in the run product yield. 3.3. Kinetic investigations and phase formation sequence Table 2 shows the composition of the initial synthesis mixture and the precursor powders as well as the phase formation sequence occurring upon hydrothermal treatment in the system aNa2O–cTiO2–10SiO2–675H2O, where 3 6 a 6 40, 0.5 6 c 6 5 and synthesis duration up to 96 h. The presented compositional ratios indicate the higher affinity of Ti to reside in the solid-gel phase as compared to Si and Na. This effect is significantly enhanced upon increasing the Na2O/TiO2 in the initial gel. Infrared, Raman and 29Si nuclear magnetic resonance investigations elucidated the atomic arrangements in the amorphous sodium titanosilicate precursor powders [20]. The spectroscopic data reveal that for gels with a lower Na2O/TiO2 ratio the precursor amorphous powders possess a lower degree of polymerisation of the SiO4 species as compared to the first crystallizing products. This means that the hydrothermal treatment induces the homocondensation of SiO4 groups. Contrary, for gels with a higher Na2O/ TiO2 ratio the hydrothermal treatment enhances the fragmentation of the Si–O system in the final run product as compared to the amorphous precursors. The kinetic investigations well confirm the validity of Ostwald’s rule of successive phase transformations leading to the appearance of more thermodynamically stable products in the following sequence: AM-1–ETS-4–GTS-1–AM4–sitinacite–paranatisite–natisite, i.e., from microporous and layered titanosilicates with a low Ti content up to dense, Ti-rich phases. This tendency is somehow contradic-
Fig. 4. Micrographs of ETS-4 synthesized hydrothermally for 24 h in the system aNa2O–bK2O–cTiO2–10SiO2–675H2O, where: (a) a = 4.5; b = 4.5; c = 1.65; (b) a = 6; b = 3; c = 2.15; (c) a = 7.5; b = 1.5; c = 2.65.
tory to that observed by Pekov and Chukanov [1]. These authors have reported that zeolite-like titanosilicates, e.g., members of the labuntsovite and lintisite groups and vinogradovite, crystallize later than Ti-containing minerals with higher framework density such as lorenzenite and titanite. This occurs if the mineral deposition with temperature lowering proceeds under high-alkaline conditions and, if alkalinity decreases the evolution proceeds in the direction of a condensation of structures. To interpret this mismatch one should take into consideration that the phase formation sequence reported here takes place in a thermodynamically closed system at constant temperature and alkalinity. In all syntheses the pH values are close to or exceed 13. This makes the reaction medium quite aggressive towards the configuration of the primary and secondary building units (PBU and SBU) in the precursor gel. With time, the
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Fig. 5. Micrographs and powder XRD patterns presenting the successive replacement of AM-4 by natisite: (a) AM-4 (96 h), (b) relics of AM-4 (122 h) with natisite and (c) pure natisite (240 h).
polymerized SiO2 and Ti(O,OH)4 PBU experience fragmentation that leads to increased TiO2/SiO2 values in the subsequently formed SBU and the resulting titanosilicate framework constructions (Tables 1 and 2). If the initially formed phase is not in equilibrium with the reaction medium, it may dissolve supplying PBU and SBU for crystallization of thermodynamically more stable phases (Fig. 5). 4. Conclusions Within the investigated range of Na2O/TiO2 ratios of the synthesis mixture, frameworks with five-coordinated titanium are obtained at the lowest (below 4) and highest (above 8.5) ratio values, while six-coordinated Ti4+ is predominant in structures prepared at intermediate ratio values. Layered and microporous titanosilicates crystallize as pure phases at lower Na2O(K2O)/TiO2 and TiO2/SiO2 ratios, while dense titanosilicates are preferentially formed at higher alkalinity. The sensitivity of the framework topology and phase morphology to tiny variations in the reaction medium allows precise tuning in the preparation of titanosilicates with tailored pore system, morphology and functionality.
To increase the structural diversity of synthetic heterosilicates with useful properties it is necessary to vary the precursor chemistry i.e., a choice of appropriate sources for framework and extraframework cations, mineralizers and co-solvents as well as to work in somewhat milder conditions concerning pH and temperature of the reaction medium. Acknowledgements Financial support by the National Science Fund – Bulgarian Ministry of Education and Science under contract No NT 1-02 is gratefully acknowledged. V.K. and Yu.K. are indebted to Joint Research Project with Academy of Sciences of the Czech Republic. B.M. and Yu.K. are indebted to the Alexender von Humboldt Fondation for equipment donation. References [1] [2] [3] [4]
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