Influence of cations on color and structure of ultramarine prepared from zeolite A

Studies in Surface Science and Catalysis, volume 158 J. (~ejka,N. Zilkovfiand P. Nachtigall (Editors) 9 2005 Elsevier B.V. All rights reserved.

215

Influence of cations on color and structure of ultramarine prepared from zeolite A S. Kowalak, A. Jankowska, S. L~czkowska A. Mickiewicz University, Faculty of Chemistry, Poznafi, Poland The ultramarine analogs with different coloration and different structure can be obtained by means of thermal treatment of synthetic zeolite A modified with various cations mixed with elemental sulfur and alkalis. The post-synthesis ion-exchange treatment of sodium forms of ultramarine analogs can also modified to some extent their properties, although the results are less spectacular than that attained upon the direct synthesis with various cations. The ultramarine analogs with SOD structure are less susceptible to cation modification than the colored products with retained LTA structure. 1. INTRODUCTION Ultramarine is a sodium aluminosilicate sodalite that contains sodium oligosulfides (mostly $3-) encapsulated inside the 13 cages [1]. The sulfur anion-radicals $3-play a role of chromophore responsible for the intense blue color of ultramarine. Another sulfur radicals (e.g. $2- and $4"-) can also contribute to the ultramarine color. The conventional production of ultramarine comprises an extended calcinations of the mixture of kaolin, sulfur, sodium carbonate and the reductive agents. The analogs of ultramarine can be also obtained from zeolites by means of thermal treatment with sulfur radical precursors [2,3]. Some attempts were undertaken to modify the color of the synthetic ultramarine by means of post-synthesis ion-exchange with various cations [4,5]. The reported results were not always consistent and they have not received any noticeable industrial meaning. Since the commercial pigments based on heavy metal compounds are recently being withdrawn from the market, the nontoxic ultramarine derivatives of various coloration could be considered as their potential substitutes. We have found that the coloration of the products obtained from zeolite A and sulfur radical precursors such as elemental sulfur and sodium carbonate or sodium oligosulfides can be modified from yellow to blue via green by tuning the alkalinity of the initial mixture (Na2/S ratio) [3]. The changes in alkalinity affected not only the color of the products, but also their structure. Generally, the low alkalinity mixtures led to the products with unchanged LTA structure, whereas the highly alkaline mixtures formed the SOD structure. The following study comprises syntheses of ultramarine analogs from zeolite A with contribution of cations other than sodium. The presence of various cations of different size and different electronegativity in a close vicinity to the sulfur radical can shift the light absorption frequency of oligosulfide chromophore. The nature of employed cation can affect a course of parent zeolite structure transformation. The chosen cations were introduced either to the zeolite NaA by means of extended ion-exchange procedure or they were admitted to the mixture with elemental sulfur as an alkalinity sources. Both options were combined in some experiments. Another aim of the study was to study a post-synthesis cation modification of

216 the ultramarine analogs prepared from zeolite exclusively with sodium cations. The ultramarine analogs with retained original LTA structure as well as the samples showing SOD structure or that of nepheline hydrate II were treated with aqueous solutions of various cations or were treated with respective salts at elevated temperature (solid state ion-exchange). The commercial ultramarine was modified with the above methods for comparison. 2. EXPERIMENTAL Zeolite NaA (donated by Atofina, Poland) was used as starting material for syntheses of ultramarine analogs. The elemental sulfur (P.O.Ch. Poland) was employed as radical precursor in all experiments and its content in the initial mixture always made 40 wt.% of zeolite. The parent sodium zeolite A was modified with cations such as K +, Li +, NH4+, Ca 2+ by means of conventional procedure with aqueous solutions of respective salts (KC1, LiC1, NH4C1, CaCI2). The 0.1M solutions were used for the cation modifications. The procedure was repeated three times in order to attain a high exchange degree. NazCO3 was the principal alkali source. Another alkalis applied were K2CO3, Li2CO3, CaO. The preparation procedure comprised a mixing and grinding of zeolite with sulfur and alkalis. The alkalis content (expressed as Na2/S or Me2/S if other cations applied) varied in the range of 0 - 1. The initial mixtures were maintained in covered ceramic crucibles and heated for 2 hours in the furnace in the range 5 0 0 - 800~ The samples were inserted into hot furnace. The products were cooled down in desiccator and then washed with water and dried. Another series of experiments comprised a post-synthesis modification of the ultramarine analogs with various cations. Usually 3 g of the sample was treated at room temperature with 30 ml of 0.1 M solutions of adequate salt. The procedure was repeated three times with fresh solutions after each 2 hour period. The same experiment with commercial ultramarine (provided by Prayon-Rupel) was conducted for comparison. The solid state ionexchange was carried out at elevated temperature with the salts of chosen cations. The modified samples were washes with water and dried. The characterization comprised the following methods: XRD (TUR-62M), UV-vis (Varian-Cary 100), FTIR (Bruker-Vector 22), ESR (Radiopan), SEM (Philips-SEM 515) 3. RESULTS AND DISCUSSION Properties of the samples prepared from cation modified zeolites A and sodium carbonate (as alkali source) are given in Table 2. The samples prepared from the mixtures of zeolites and sulfur without any additional alkalis differ very much from the samples obtained from unmodified NaA zeolites. Contrary to the latter, the products containing other alkali or alkali earth cations are colorless. The sample based on NH4A zeolite undergoes total amorphization. The amorphization is also noticed in the case of the products obtained from the Cu and Zn modified zeolites. The brown color of the sample prepared from CuA results from generation of copper sulfide. The admittance of some alkali (e.g. Na2/S = 0.2) results in coloration of the samples, but the colors are different than that of the sample obtained from unmodified zeolite. The structure of the products depends markedly on the cation introduced. For instance, LiA leads to forming of SOD already under low alkalinity, whereas the presence of other cations under study requires higher alkalinity to attain this structure. The differences in the properties of the products obtained from modified and unmodified zeolites become less distinct for the samples obtained from the more alkaline mixtures (e.g. Na2/S = 1.0). All the samples indicate the SOD structure (although in some cases the impurities of unknown structures are

217

noticeable). The intensity of the color is rather low. The shades of blue and green are prevailing. Only the sample modified with Cu remains brown. The declining influence of the cations introduced into zeolites on the properties of the products with growing alkalinity results from raising contribution of sodium (from Na2CO3) in the mixture. The samples prepared with zeolite NaA contain substantial content of sodium even if any NazCO3 is not added. The sample denoted as NaJS=0 contains as a matter of facts the amount of Na that corresponds to real Na2/S=0.23. The ion-exchange degree in modified samples was always -80%. For instance, the KA sample mixed with sulfur (Na2/S=0) contains some remnant of Na and the N a / K ratio is 0.3. In the samples containing Na2CO3 the contribution of sodium becomes prevailing (Table 1). Table 1 Molar proportions of the mixtures based on zeolite KA. Na2/S 0 0.2 0.4 Real Na2/S 0.05 0.25 0.5 Na/K 0.3 1.5 3

0.6 0.7 4

0.8 0.9 5

1 1.1 6.5

The differences in the products coloration are reflected in the UV-vis spectra (Fig. 1). In the case of samples based on alkaline cation modified zeolites the presence of sulfur radical is evident in ESR spectra (Fig. 2). The spectrum of the samples containing introduced Cu (or Zn) show only the signal resulting from metal cation, whereas the signals typical for the sulfur radicals are not seen.

1 36o

! NaA Na2/S=I ..__..._--------

~

LiA Naz/S=I

~

g=2,029 ~

I

I

f

I

! CuA ~ Na2/S=I

I

I

I

200 300 400 500 600 700 800 900 Wavelength (nm)

300 305 310 315 320 325 330 Field [mT]

Fig. 1. UV-vis spectra of selected samples obtained from zeolites A modified with indicated cations (Na2/S= 1).

Fig. 2. EPR spectra of the samples obtained from zeolites A modified with indicated cations (Na2/S= 1).

218

The products of thermal syntheses at 500~ (Table 3) also depend on nature of cations introduced into zeolites, although the thermal structure transformations are less pronounced than those at 800~ (Fig. 3,4). For example, the NH4A mixed with sulfur (Na2/S=0) retains the LTA structure after heating, while it undergoes an amorphization at 800~ Zeolite

Fig. 3. Structure transformations at ~ ~ j ~ :'~, ~~,'. . . . . ~,,;~I~:4~.~;~ 800~

Structure

NaA

l',

soo

LiA

.

:. :

SOD+U

'SOD

KA

SOD+C

C

'.. [ SOD;C' Iiiiis.;.:ci!i~liiiii!~ilili: ~4~/~i~!ii I:::::" :!:-:::} I:::::::-:::::: d;~:~N?))

NH 4A CaA

soo+,

CuA

SOD+U

ZnA Na2/S

,.o ....i .......

Nllll

I~i!~ii~::~i:!7/~':::i:i!71:!ii" ~

I o.; ........... 'o.4'1"'o12"

Zeolite

Structure

NaA !. . .SOD . . . . . .[.i.: i : i : i : i : i ~ : : : : : :9: :. :.:.:.:.~.i.~. . . LiA KA

. . . . . . . . . . . . . . . . .

~ ~

:'~~~J'~'~ ~ ! ~::::~ ~ , : ~ ~ i ~7)~~i : {

Fig. 4. Structure transformations at 500~

sod

CaA Na2/S

o 1

SOD+C 1.0

]

0.8

10.6

N~:<~IN ]0.4

[

0.2

0

The significant influence of heating temperature is conspicuous for the LiA based samples. It has not attained the SOD structure even at high alkalinity (Na2/S-1), while at 800~ the transformation to SOD was noticeable already after adding small amount (Na2/S=0.2) of sodium carbonate. Table 2 Color and structure of the products obtained at 800~ from the zeolites A, elemental sulfur and NazCO3. Na2/S ' 1.0 0.8 ........ 0.6 0.4 NaA Light blue Blue Turquoise Green LiA Light green Green, turq. Turquoise Blue KA Palett~.-gray L. turq.-gray Blue Blue NH4A Light blue Blue Blue L. turquoise CaA L. turquoise L. turquoise L. turquoise Light green CuA Brown Brown Brown Brown ZnA Blue Blue Blue White ,,

mixtures of cation modified 0.2 " 0 Green Yellow-green Light blue White Light yellow White Yellow-green White Light green White Brown Brown White White

The influence of the cations on properties of the products is much more pronounced in the syntheses involving the cation modified zeolites A and the alkalis containing the same cations (Tables 4-6). The potassium containing mixtures lead to the products of rather low color intensity. The yellow coloration prevails in most of samples. It is interesting to underline that none of the product attained the structure of SOD after the thermal procedure.

219 Table 3 Color and structure of the products obtained at 500~ from the mixtures of cation modified zeolites A, elemental sulfur and NazCO3. Na2/S ratio 1.0 0.8 0.6 0.4 0.2 0 NaA LiA KA NH4A CaA

Light green Green Green Dark green Light green Light green Turq.-green Green L. turq.- gray L. turq.-gray L. turq.-gray L. turq. Light blue Blue Turquoise Green L. turq. L. turq. Blue Blue

Blue Green Blue L. turq. Blue

White White White White White

The products obtained from the mixtures of high alkalinity (i.e. high potassium content) show a considerable contribution of kaliophilite in the products. The nepheline hydrate II is another structure detected. The high contribution of yellow chromophore (i.e. $2-, g-2.002) in the potassium modified samples can result from larger size of K + cations than Na + ions (Fig. 5,6). The bigger cations occupy considerable part of the B-cage, thus the smaller $2- anion-radicals can be favored better than $3 in accommodation inside the sodalite cages.

[

/

NaA + Na2CO3

600 ~

An

\

NaA+

J

r

KA+ K2CO3 LiA + Li2C..____ O3

g I

400

600

~

LiA +

~ ~

g=2,002 g=2,029

I

I

I

I

I

I

200 300 400 500 600 700 800 900 Wavelength (nm)

Fig. 5. UV-vis spectra of selected samples obtained at 800~ with Me~/S=0,4.

300

305

310

I

I

I

315

320

325

330

Field [mT]

Fig. 6. EPR spectra of selected samples obtained at 800~ with MeJS=0,4.

Table 4 Properties of the products obtained from KA, sulfur and K2CO3. K2/S ratio 1.0 0.8 0.6 0.4 500~ Pale yellow Pale yellow Pale yellow Pale yellow Structure N+K N+K N+K N 800~ Pale yellow L. yellow L. yellow Yellow Structure N+K N+K N LTA + N N-nepheline hydrate II, K- kaliophilite

0.2 Pale green LTA Yellow LTA

0 White LTA Pale yellow LTA

220

The samples prepared from LiA with lithium carbonate as alkali source show a green coloration at 500~ and blue at 800~ The blue coloration (800~ reflect the predominant contribution of $3- (g=2.029), which can result from small diameter of Li + cations and subsequently larger room for bulkier sulfur anion radicals. The green coloration of the samples prepared at 500~ can result from considerable content of elemental sulfur in the products, that acts as yellow chromophore (Fig. 5,6). The presence of lithium affects very much the structure transformation of zeolite. Contrary to the mixtures containing only sodium cations the structure of SOD is not formed. The LTA structure is retained (with admixture of nepheline hydrate II) upon heating at 500~ The nepheline hydrate is prevailing structure of the products obtained at 800~ Only in the samples of the lowest and the highest alkalinity show structure of carnegiete or unknown structure, respectively. No sodalite is formed at 800~ either. Table 5 properties of the samples obtained from LiA and Li2CO3. Li2/S ratio 1.0 0.8 0.6 0.4 500~ L. green,-gray L. green- gray L. green- gray L. green Structure LTA + N LTA + N LTA + N LTA + N 800~ White Light blue Blue Blue Structure U N+U N N N- nepheline hydrate II, C-carnegieite, U-unknown

0.2 L. green LTA Blue N

0 White LTA White C

The calcium modified zeolites mixed with sulfur and alkalized by CaO do not lead to any colored products. The structure of zeolites does not undergo any transformations at 500~ (only some crystalline impurities are noticeable in the sample of highest CaO content). The LTA structure is also retained at 800~ Only the sample without any CaO undergoes amorphization and some impurities are seen in the samples of high alkalinity (Tab. 6). Table 6 Prope.rties of the products obtained from CaA, sulfur and CaO. Ca/S ratio 1.0 0.8 0.6 0.4 500~ White White White White Structure LTA + U LTA LTA LTA 800~ White White White White Structure LTA + U LTA + U LTA + N LTA

0.2 White LTA White LTA

0 White LTA White Amorph.

It is likely that divalent cations prevent a forming of sulfur anion-radicals. One could consider that negative charge of the anion radical could be compensate by one positive charge of Ca +2 cation, whereas the other one could be bond to the framework anion (Fig. 7A). It seems, however, that probably both positive charges are compensated by sulfur anion-radicals. The close vicinity of two radicals combined with calcium results in their recombination (Fig. 7B). The above results indicate a significant influence of the cations employed for thermal syntheses from zeolite A. Both the coloration and the structure of resulting products depend markedly on nature and concentration of the cations.

221

S~

A

\ /

S--S

/ Ca \

S

S

S"

o

O\ca/O\

Q

/

si/

/\ QQ

/\ 0

....S

-S"

o

2+

\s, / /\

,,,S~

Q

S--S~s

c. Q

/s S

I

,S"

Fig. 7. Model of potential interaction of Ca +2 cations and sulfur anion-radicals. The color of ultramarine can be modified by means of post-synthesis treatment and the industrial production comprises such a modification in order to prepare the pink and violet ultramarine. The SOD structure of ultramarine is not suitable for cation-modification. The colored products prepared from zeolites can attain the SOD structure, nevertheless, they can also show the more opened structures such as nepheline hydrate II or of original zeolite (LTA). The latter structure are much more susceptible to a cation modification and the influence of the introduced cations on the product properties can be more conspicuous. The ion-exchange modification of the commercial ultramarine with most of applied cations causes only very minor changes in coloration of the pigment. Only the treatment with Fe(NO3)3 brings about some gray shade of the products (Table 7). Table 7 Results of post synthesis ion-exchange process with aqueous solutions, of respective salts. Cation Parent Li K Zn Co Cu Ca Fe NH4 Na

Color

Blue Comm. SOD

Blue

Blue

Blue

Blue

Blue

Blue

SOD

SOD

SOD

SOD

SOD

Structure

Olive green LTA

Olive green LTA

Olive green LTA

Olive green LTA

Olive gray LTA

SOD + U Olive gray LTA

Color

Turq.

Turq.

Turq.

Turq.

N

N

N

N

Turq. gray N

Green- L. turq. Turq. gray N N Amorph. N

Structure Color

Structure

Olive green LTA

Blue, gray Poor SOD Brown Poor LTA Brown

Blue Poor SOD Olive green LTA

The contact with acidic solution of the ferrous salt (pH-3) results in a noticeable evolution of HzS, which evidently suggests some deterioration of ultramarine, although the XRD still exhibits the SOD structure. The brown color of the suspension above the layer of modified ultramarine indicates that iron sulfide is formed. The results of the modification of the SOD ultramarine obtained from zeolite A (not indicated in the Table) were very much alike those of commercial ultramarine. Neither the coloration nor the structure of the colored material with LTA structure were considerable affected by the contact with alkaline cations. The transition cations caused more pronounced color changes, but the LTA structure was still retained. The modification with iron resulted in brown coloration and retained LTA structure. The structure of nepheline hydrate was not affected markedly by the cation treatment. Only the iron cation caused an amorphization. The color of the sample was drastically changed

222

after treatment with iron and somewhat with copper. The products modified with the other cations did not differ noticeably from the parent sample. The solid state modification affects the properties of ultramarine noticeably when transition metal cations were used. The SOD structure was considerably affected by treatment with Co, Cu, Fe. The crystallinity of the latter samples declined and some unknown phase was formed. The modification of ultramarine analogs of LTA or nepheline hydrate structures with alkaline cations did not cause any distinct color changes, neither the structure transformations. The transition metal cations resulted in more substantial color changes and they caused a noticeable amorphization of the samples. 4. CONCLUSIONS The presented results show that the color and structure of the ultramarine analogs prepared from zeolites A can be changed substantially by means of modification of the initial mixture composition with various cations. The cation modification of the parent zeolite affects the product properties very considerably in the case of the lowest alkalinity of the initial mixture (i.e. containing only zeolite and sulfur). Such low alkaline mixtures form the colored ultramarine analogs only in the case of the unmodified zeolite (i.e. the sodium form). The coloration of Cu containing mixture resulted rather from generation of CuS. The samples prepared from alkaline cation modified zeolite show the retained LTA structure, while those containing Zn, Cu and NH4+ lose their crystallinity. An increase in alkalinity of the starting mixtures by means of admitted NazCO3 diminishes the influence of cations introduced into zeolites, because the contribution of Na cations become predominant. The presence of Cu cations allows to form the SOD structure, but the sulfur radicals are not generated. The mixtures containing cation modified zeolites, sulfur and alkalis other than sodium carbonate (i.e. Li2CO3, KzCO3, CaO) resulted in products much different than the mixtures containing only sodium cations. The size of alkali cations influence the color of the products. The bigger potassium cations seem to favors forming smaller $ 2 (yellow) radicals, whereas smaller lithium cations facilitates generation of the bigger $ 3 radicals. The divalent calcium cations probably prevent generation of sulfur radicals and they probably facilitate their recombination by forming the ordinary sulfides. It is interesting to emphasize that the SOD structure was attained only when the thermally treated mixture contained sodium cations. The role of sodium seems crucial in the thermal recrystallization towards sodalite. The post-synthesis cation modification of ultramarine and its analogs obtained from zeolites is usually less efficient then the direct synthesis with contribution of chosen cations. The cation exchange with alkali metal salts does not affect markedly neither the commercial ultramarine nor the colored sodalite obtained from zeolite A. The transition metal salts affect the coloration and to some extent also the structure of the products. It results from partial deterioration of the original structure and forming the respective sulfides. The solid state modification seems to be more efficient procedure than the solution treatment. REFERENCES [1] F. Seel, Inorg. Chem., 5 (1984) 67 [2] S. Kowalak, S. Str6~3,k,M. Pawtowska, M. Milu~ka, J. Kania, Stud. Surf. Sci. Catal., 105 (1997) 237 [3] S. Kowalak, A. Jankowska, S. Lagzkowska, 14th International Zeolite Conference, (E. van Steen, L.H. Callanan, M. Claeys, Editors) Cape Town 2004, p. 608 [4] D.W. Breck, in Zeolite Molecular Sieves. Structure, Chemistry and use, J. Wiley & Sons (1974) [5] Y. Matsunaga, Can. J. Chem. 37 (1959) 994