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Cesium Bearing Ultramarine Prepared From Zeolites
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
193
Cesium Bearing Ultramarine Prepared From Zeolites Aldona Jankowska, Stanislaw Kowalak Faculty of Chemistry, A. Mickiewicz University, Pozna, Grunwaldzka 6, 60780 Pozna
Abstract Pigments analogous to ultramarine with various colors including pink have been prepared mainly from zeolites A and also from X by a thermal treatment with sulfur and various amounts of Cs2CO3. The bulky Cs+ cations introduced into zeolites reduce markedly a size of their pore openings and that of inner voids and subsequently affect the type and distribution of generated sulfur chromophores. It is likely that the chromophores in cesium-rich samples occupy rather the shrunken voids of -cavities but not the sodalite units. Keywords: zeolites, pigments, cesium, ultramarine
1. Introduction The ultramarine analogs can be obtained by a thermal treatment of zeolites (particularly A type) with sulfur radical precursors such as alkali oligosulfudes or elemental sulfur and alkalis [1-3]. We found that the nature of alkali cation affects the coloration and structure of resulted products. The size of cations is important in formation of given sulfur anion-radicals (S3.- or S2.-) inside the sodalite cages. We noticed a predominance of yellow chromophore (S2.-) in the pigments obtained from mixtures of zeolite A, sulfur and K2CO3, whereas the mixtures with Li2CO3 led to blue (S3.-) products [3]. Transformation of zeolite structure during the thermal treatment with sulfur radical precursors depends very much on the nature of contributing alkali cations. The formation of the SOD structure takes place only in the presence of sodium. The cesium cations (with diameter 0.34 nm) are almost twice as big as sodium cations (0.19 nm) [4]. Their introduction into zeolite A by means of ion-exchange replacement of Na+ cations causes a contraction of pore mouth diameter below 0.3 nm [5], which means to the level comparable to six-member ring in a standard sodalite cages. Moreover, the presence of Cs+ cations inside the large cavities of zeolite A reduces their volume markedly [5,6]. Thus, the shrunken cavities of the Cs-modified zeolite A could attain the volume comparable to sodalite cages. It is matter of discussion, whether Cs+ cations are enable of reaching the sodalite cages [6]. The diminished zeolite pore opening could protect potential sulfur radicals or other unstable sulfur species generated inside the cavities against a contact with reactive compounds (e.g. water, oxygen).
2. Experimental Zeolites NaA (Arkema) and zeolites NaX (Aldrich) were used as principal reagents. Elemental sulfur (POCh) was used as sulfur source and Cs2CO3 (Aldrich) as alkali source. Usually zeolites (1 g) were mixed with sulfur (0.4g) and various amounts of Cs2CO3. The mixture was ground and then heated in covered ceramic crucibles at 800, 500 or 400oC (selected mixtures) for 2 hours. Some samples were obtained by initial thermal pretreatment of zeolites with sulfur and then by mixing the sulfur saturated zeolites with Cs2CO3 and heating. The resulting samples were washed with water and
194
A. Jankowska and S. Kowalak
characterized by means of XRD (D8 Advance, Bruker), FTIR (Vector 22, Bruker), UVvis (Cary 100, Varian), EPR (SE/X 2547, Radiopan) at room and liquid nitrogen temperatures.
3. Results and discussion The products obtained from zeolite A with cesium carbonate (Table 1) show rather low intensity of color compared to series obtained with sodium carbonate. The samples prepared at 800oC are light green, light blue or colorless. The samples originated from low alkaline mixtures maintain their LTA structure, although some impurities are noticeable. The mixtures of high alkalinity form the structure of non porous cesium aluminosilicate. The samples heated at 500oC generally preserves the LTA structure but their crystallinity is decreased. The samples show various shades of green coloration except for the sample generated from high alkaline mixture (Cs2/S = 1) which is pink. The further increase in Cs loading in the starting mixture (up to Cs2/S = 1.5) did not result in any increase in pink color intensity after heating at 500oC. Instead, the sample turned orange and they became beige after washing. The samples heated at 400oC show well preserved LTA crystallinity and turquoise (in case of intermediate alkalinity) or beige (for high alkaline mixture) coloration. Table 1: Properties of typical products Cs2/S 0 0.2 800oC-2h
l. green very pale green #
0.4 l. blue #
Structure
LTA
LTA + imp
o
white
l. blue with turq. shade green
500 C-2h
#
LTA + imp
#
0.6
0.8
1
pale green
white
white
CsAlSiO4
CsAlSiO4
CsAlSiO4
green
pale green
l. pink
#
#
Structure
LTA
LTA
LTA
LTA
LTA
LTA#
400oC
-
l. turquoise
l. turquoise
-
-
pale beige
Structure
-
LTA
LTA
-
-
LTA
#- poor crystallinity,
The electronic spectra of the products (Fig. 1) show very weak absorption bands at ~600 nm for the samples prepared at 800oC (Fig. 1A). Only the spectrum of the sample with Cs2/S = 0.4 exhibits a distinct peak reflecting the blue chromophore (S3-.). The bands from yellow chromophores (mostly S2-.) appear at different positions (350 – 420 nm) and show various intensities. The spectra of the samples prepared at 500oC show more distinct bands at ~600 nm, particularly for the samples with low alkalinity (Fig. 1B). The most intriguing is an appearance of band at 480 nm for the pink sample (Cs2/S=1), which reflects the presence of new chromophore (e.g. S4-.). The samples prepared at 500oC of some lower alkalinity show also the peaks in the range 430 – 480 nm but less intense. The samples obtained at 400oC indicated distinct bands at 350-420 and at 600 nm (low and medium alkalinity) and intense peak at 455 nm for Cs-rich sample. It is likely that the latter band reflects the presence of the same or similar sulfur species as that in the Cs-rich sample obtained at 500oC. The influence of Cs+ cations on properties of the products obtained from zeolite NaX is much less conspicuous. The products obtained at 500oC preserve the original FAU structure but they show very poor coloration or remain colorless. The syntheses at
195
Cesium bearing ultramarine prepared from zeolites
800oC do not improve the intensity of color, whereas the structure of zeolite is transformed to non porous aluminosilicates. A
B
365
360
Cs2/S=0
C
Cs2/S=0
350
420 600
F(R)
F(R)
Cs2/S=0.2
F(R)
420
Cs2/S=0.2 600
Cs2/S=0.4
600
420
Cs2/S=0.4
Cs2/S=0.6 Cs2/S=0.8
400
500
600
700
800
455
Cs2/S=0.5
Cs2/S=0.8
480
Cs2/S=1 300
Cs2/S=0.2
365
Cs2/S=0.6
Cs2/S=1
Cs2/S=1 300
900
400
500
600
700
800
300
900
400
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
500
Figure 1. UV-vis spectra of the samples prepared from mixtures of zeolite A, sulfur and Cs2CO3 at 800oC (A), 500oC, 400oC (C).
T=293K
T=293K
A
T=77K
B
C Cs2/S=0
Cs2/S=0
Cs2/S=0.2
Cs2/S=0.2
Cs2/S=0.2
Cs2/S=0.6
Intensity
Cs2/S=0.5
Intensity
Intensity
Cs2/S=0.4 Cs2/S=0.4
Cs2/S=0.6
Cs2/S=0.8 Cs2/S=0.8 Cs2/S=1
Cs2/S=1 Cs2/S=1
300
310
320
Field [mT]
330
300
310
320
Field [mT]
330
300
310
320
330
Field [mT]
Figure 2. EPR spectra of the samples prepared at 400oC recorded at R.T (A), spectra of the samples obtained at 500oC and recorded at R.T (B) or in liquid nitrogen (C).
The EPR spectra recorded at room temperature of the pigments obtained at 400oC show isotropic signal typical of S3-. (g = 2.03) for the sample with low alkalinity, while the Cs-rich samples indicate some anisotropy (Fig. 2A). The anisotropy is also noticeable for the Cs-richest sample prepared at 500oC. The maximum of the latter signal is markedly shifted compared to the others of this series (Fig. 2B). The low temperature measurement for the series prepared at 500oC indicated the anisotropic signals. The pink sample (Cs2/S=1) shows two distinct signals. The g tensor (g=2.006) of the main signal can reflect the presence of S4-. radicals, responsible for the red coloration. The signals at the similar field value, and perhaps of the same origin are seen for the samples with lower alkalinity. The colorless Cs-free sample does not indicate any EPR signal.
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A. Jankowska and S. Kowalak
It is likely that the chromophores NaS2, and NaS3 in the samples with low alkalinity are formed with contribution of remaining sodium cations and they probably occupy the cages. In the case of cesium rich samples a penetration of Cs+ cations into sodalite units seems less possible than for the low alkaline samples. Therefore, it is conceivable that the pink chromophore S4-. is located inside the -cage and is combined with Cs+. Despite of the contraction caused by Cs+ cations its size is still larger than the standard sodalite cage and it can accommodate the bulkier sulfur moieties (S4-.). The introduction of Cs+ cations into zeolite upon the thermal treatment (solid state ion-exchange) can hinder a diffusion of sulfur inwards the cavities of zeolites, and could cause a relatively low color intensity of most of samples. Considering the above another series of experiments involved first a saturation of zeolites with elemental sulfur (500oC, 2h) and then mixing and grinding with respective content of Cs2CO3 and finally heating of the mixture. The resulted products differed markedly from the earlier series [7], particularly these prepared at lowest temperatures. The Cs-rich sample obtained at 500oC showed beige (not pink) color, although the UV-vis bands at 470 nm and the ESR signal were similar as those for conventionally prepared samples. It is likely that this time a diffusion of Cs cations inwards the voids is hindered by sulfur introduced into zeolites.
4. Conclusion The use of Cs2CO3 as alkaline source affects substantially the structure and coloration of the products obtained by thermal treatment of mixtures with zeolite A and sulfur. The most spectacular effects are noticed for the samples obtained at low temperature (especially at 500oC). All the samples maintain the LTA structure. These of low and medium alkalinity exhibit bluish or greenish coloration, while the sample with high Cs loading indicate a pink color. The Cs-rich samples obtained at 400oC are beige. It is likely that the red chromophores are accommodated inside the -cavity of zeolite, since a penetration of Cs+ cations as well as sulfur species into the sodalite cages is considerably hindered. The ESR spectrum of the red sample is much different that the regular spectra of ultramarine or its analogs obtained from zeolites with a contribution of sodium cations. It is possible that S4-.radicals are generated and they are responsible for red coloration. The red coloration does not appear after heating at 800oC, where the Cs-rich mixtures undergo the zeolite structure transformation into condensed aluminosilicate. The saturation of zeolite A with sulfur before mixing with Cs2CO3 lead to formation of the products much different than those prepared by means of conventional method. Zeolite X applied for mixtures with sulfur and Cs2CO3 result in formation of very pale or colorless products. The influence of cesium was much less conspicuous than in syntheses involving zeolites A.
References [1 ]S.Kowalak, A.Jankowska, Microp. Mesop. Mat., 61( 2003) 213. [2] S.Kowalak, A.Jankowska, S.Zeidler, Microp. Mesop. Mat., 93 (2006) 111. [31] S.Kowalak, A.Jankowska, Europ. Journal of Mineralogy, 17 (2005) 861. [4] E.R. Nightingale, J. Phys. Chem., 63 (1959) 1381. [5] E. M. Flanigen, Pure & Appl.Chem., 52 (1980) 2l9l. [6] K.S. Ryu, M.N. Bae, Y. Kim, K. Seff, Microp. Mesop. Mat., 71( 2004) 65. [7] S. Kowalak, A. Jankowska, to be published.
193
Cesium Bearing Ultramarine Prepared From Zeolites Aldona Jankowska, Stanislaw Kowalak Faculty of Chemistry, A. Mickiewicz University, Pozna, Grunwaldzka 6, 60780 Pozna
Abstract Pigments analogous to ultramarine with various colors including pink have been prepared mainly from zeolites A and also from X by a thermal treatment with sulfur and various amounts of Cs2CO3. The bulky Cs+ cations introduced into zeolites reduce markedly a size of their pore openings and that of inner voids and subsequently affect the type and distribution of generated sulfur chromophores. It is likely that the chromophores in cesium-rich samples occupy rather the shrunken voids of -cavities but not the sodalite units. Keywords: zeolites, pigments, cesium, ultramarine
1. Introduction The ultramarine analogs can be obtained by a thermal treatment of zeolites (particularly A type) with sulfur radical precursors such as alkali oligosulfudes or elemental sulfur and alkalis [1-3]. We found that the nature of alkali cation affects the coloration and structure of resulted products. The size of cations is important in formation of given sulfur anion-radicals (S3.- or S2.-) inside the sodalite cages. We noticed a predominance of yellow chromophore (S2.-) in the pigments obtained from mixtures of zeolite A, sulfur and K2CO3, whereas the mixtures with Li2CO3 led to blue (S3.-) products [3]. Transformation of zeolite structure during the thermal treatment with sulfur radical precursors depends very much on the nature of contributing alkali cations. The formation of the SOD structure takes place only in the presence of sodium. The cesium cations (with diameter 0.34 nm) are almost twice as big as sodium cations (0.19 nm) [4]. Their introduction into zeolite A by means of ion-exchange replacement of Na+ cations causes a contraction of pore mouth diameter below 0.3 nm [5], which means to the level comparable to six-member ring in a standard sodalite cages. Moreover, the presence of Cs+ cations inside the large cavities of zeolite A reduces their volume markedly [5,6]. Thus, the shrunken cavities of the Cs-modified zeolite A could attain the volume comparable to sodalite cages. It is matter of discussion, whether Cs+ cations are enable of reaching the sodalite cages [6]. The diminished zeolite pore opening could protect potential sulfur radicals or other unstable sulfur species generated inside the cavities against a contact with reactive compounds (e.g. water, oxygen).
2. Experimental Zeolites NaA (Arkema) and zeolites NaX (Aldrich) were used as principal reagents. Elemental sulfur (POCh) was used as sulfur source and Cs2CO3 (Aldrich) as alkali source. Usually zeolites (1 g) were mixed with sulfur (0.4g) and various amounts of Cs2CO3. The mixture was ground and then heated in covered ceramic crucibles at 800, 500 or 400oC (selected mixtures) for 2 hours. Some samples were obtained by initial thermal pretreatment of zeolites with sulfur and then by mixing the sulfur saturated zeolites with Cs2CO3 and heating. The resulting samples were washed with water and
194
A. Jankowska and S. Kowalak
characterized by means of XRD (D8 Advance, Bruker), FTIR (Vector 22, Bruker), UVvis (Cary 100, Varian), EPR (SE/X 2547, Radiopan) at room and liquid nitrogen temperatures.
3. Results and discussion The products obtained from zeolite A with cesium carbonate (Table 1) show rather low intensity of color compared to series obtained with sodium carbonate. The samples prepared at 800oC are light green, light blue or colorless. The samples originated from low alkaline mixtures maintain their LTA structure, although some impurities are noticeable. The mixtures of high alkalinity form the structure of non porous cesium aluminosilicate. The samples heated at 500oC generally preserves the LTA structure but their crystallinity is decreased. The samples show various shades of green coloration except for the sample generated from high alkaline mixture (Cs2/S = 1) which is pink. The further increase in Cs loading in the starting mixture (up to Cs2/S = 1.5) did not result in any increase in pink color intensity after heating at 500oC. Instead, the sample turned orange and they became beige after washing. The samples heated at 400oC show well preserved LTA crystallinity and turquoise (in case of intermediate alkalinity) or beige (for high alkaline mixture) coloration. Table 1: Properties of typical products Cs2/S 0 0.2 800oC-2h
l. green very pale green #
0.4 l. blue #
Structure
LTA
LTA + imp
o
white
l. blue with turq. shade green
500 C-2h
#
LTA + imp
#
0.6
0.8
1
pale green
white
white
CsAlSiO4
CsAlSiO4
CsAlSiO4
green
pale green
l. pink
#
#
Structure
LTA
LTA
LTA
LTA
LTA
LTA#
400oC
-
l. turquoise
l. turquoise
-
-
pale beige
Structure
-
LTA
LTA
-
-
LTA
#- poor crystallinity,
The electronic spectra of the products (Fig. 1) show very weak absorption bands at ~600 nm for the samples prepared at 800oC (Fig. 1A). Only the spectrum of the sample with Cs2/S = 0.4 exhibits a distinct peak reflecting the blue chromophore (S3-.). The bands from yellow chromophores (mostly S2-.) appear at different positions (350 – 420 nm) and show various intensities. The spectra of the samples prepared at 500oC show more distinct bands at ~600 nm, particularly for the samples with low alkalinity (Fig. 1B). The most intriguing is an appearance of band at 480 nm for the pink sample (Cs2/S=1), which reflects the presence of new chromophore (e.g. S4-.). The samples prepared at 500oC of some lower alkalinity show also the peaks in the range 430 – 480 nm but less intense. The samples obtained at 400oC indicated distinct bands at 350-420 and at 600 nm (low and medium alkalinity) and intense peak at 455 nm for Cs-rich sample. It is likely that the latter band reflects the presence of the same or similar sulfur species as that in the Cs-rich sample obtained at 500oC. The influence of Cs+ cations on properties of the products obtained from zeolite NaX is much less conspicuous. The products obtained at 500oC preserve the original FAU structure but they show very poor coloration or remain colorless. The syntheses at
195
Cesium bearing ultramarine prepared from zeolites
800oC do not improve the intensity of color, whereas the structure of zeolite is transformed to non porous aluminosilicates. A
B
365
360
Cs2/S=0
C
Cs2/S=0
350
420 600
F(R)
F(R)
Cs2/S=0.2
F(R)
420
Cs2/S=0.2 600
Cs2/S=0.4
600
420
Cs2/S=0.4
Cs2/S=0.6 Cs2/S=0.8
400
500
600
700
800
455
Cs2/S=0.5
Cs2/S=0.8
480
Cs2/S=1 300
Cs2/S=0.2
365
Cs2/S=0.6
Cs2/S=1
Cs2/S=1 300
900
400
500
600
700
800
300
900
400
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
500
Figure 1. UV-vis spectra of the samples prepared from mixtures of zeolite A, sulfur and Cs2CO3 at 800oC (A), 500oC, 400oC (C).
T=293K
T=293K
A
T=77K
B
C Cs2/S=0
Cs2/S=0
Cs2/S=0.2
Cs2/S=0.2
Cs2/S=0.2
Cs2/S=0.6
Intensity
Cs2/S=0.5
Intensity
Intensity
Cs2/S=0.4 Cs2/S=0.4
Cs2/S=0.6
Cs2/S=0.8 Cs2/S=0.8 Cs2/S=1
Cs2/S=1 Cs2/S=1
300
310
320
Field [mT]
330
300
310
320
Field [mT]
330
300
310
320
330
Field [mT]
Figure 2. EPR spectra of the samples prepared at 400oC recorded at R.T (A), spectra of the samples obtained at 500oC and recorded at R.T (B) or in liquid nitrogen (C).
The EPR spectra recorded at room temperature of the pigments obtained at 400oC show isotropic signal typical of S3-. (g = 2.03) for the sample with low alkalinity, while the Cs-rich samples indicate some anisotropy (Fig. 2A). The anisotropy is also noticeable for the Cs-richest sample prepared at 500oC. The maximum of the latter signal is markedly shifted compared to the others of this series (Fig. 2B). The low temperature measurement for the series prepared at 500oC indicated the anisotropic signals. The pink sample (Cs2/S=1) shows two distinct signals. The g tensor (g=2.006) of the main signal can reflect the presence of S4-. radicals, responsible for the red coloration. The signals at the similar field value, and perhaps of the same origin are seen for the samples with lower alkalinity. The colorless Cs-free sample does not indicate any EPR signal.
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A. Jankowska and S. Kowalak
It is likely that the chromophores NaS2, and NaS3 in the samples with low alkalinity are formed with contribution of remaining sodium cations and they probably occupy the cages. In the case of cesium rich samples a penetration of Cs+ cations into sodalite units seems less possible than for the low alkaline samples. Therefore, it is conceivable that the pink chromophore S4-. is located inside the -cage and is combined with Cs+. Despite of the contraction caused by Cs+ cations its size is still larger than the standard sodalite cage and it can accommodate the bulkier sulfur moieties (S4-.). The introduction of Cs+ cations into zeolite upon the thermal treatment (solid state ion-exchange) can hinder a diffusion of sulfur inwards the cavities of zeolites, and could cause a relatively low color intensity of most of samples. Considering the above another series of experiments involved first a saturation of zeolites with elemental sulfur (500oC, 2h) and then mixing and grinding with respective content of Cs2CO3 and finally heating of the mixture. The resulted products differed markedly from the earlier series [7], particularly these prepared at lowest temperatures. The Cs-rich sample obtained at 500oC showed beige (not pink) color, although the UV-vis bands at 470 nm and the ESR signal were similar as those for conventionally prepared samples. It is likely that this time a diffusion of Cs cations inwards the voids is hindered by sulfur introduced into zeolites.
4. Conclusion The use of Cs2CO3 as alkaline source affects substantially the structure and coloration of the products obtained by thermal treatment of mixtures with zeolite A and sulfur. The most spectacular effects are noticed for the samples obtained at low temperature (especially at 500oC). All the samples maintain the LTA structure. These of low and medium alkalinity exhibit bluish or greenish coloration, while the sample with high Cs loading indicate a pink color. The Cs-rich samples obtained at 400oC are beige. It is likely that the red chromophores are accommodated inside the -cavity of zeolite, since a penetration of Cs+ cations as well as sulfur species into the sodalite cages is considerably hindered. The ESR spectrum of the red sample is much different that the regular spectra of ultramarine or its analogs obtained from zeolites with a contribution of sodium cations. It is possible that S4-.radicals are generated and they are responsible for red coloration. The red coloration does not appear after heating at 800oC, where the Cs-rich mixtures undergo the zeolite structure transformation into condensed aluminosilicate. The saturation of zeolite A with sulfur before mixing with Cs2CO3 lead to formation of the products much different than those prepared by means of conventional method. Zeolite X applied for mixtures with sulfur and Cs2CO3 result in formation of very pale or colorless products. The influence of cesium was much less conspicuous than in syntheses involving zeolites A.
References [1 ]S.Kowalak, A.Jankowska, Microp. Mesop. Mat., 61( 2003) 213. [2] S.Kowalak, A.Jankowska, S.Zeidler, Microp. Mesop. Mat., 93 (2006) 111. [31] S.Kowalak, A.Jankowska, Europ. Journal of Mineralogy, 17 (2005) 861. [4] E.R. Nightingale, J. Phys. Chem., 63 (1959) 1381. [5] E. M. Flanigen, Pure & Appl.Chem., 52 (1980) 2l9l. [6] K.S. Ryu, M.N. Bae, Y. Kim, K. Seff, Microp. Mesop. Mat., 71( 2004) 65. [7] S. Kowalak, A. Jankowska, to be published.