Sulfur Pigments Synthesized from Zeolite LTA under Vacuum and in

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Ind. Eng. Chem. Res. 2010, 49, 8192–8199

Sulfur Pigments Synthesized from Zeolite LTA under Vacuum and in Air. XRD and Spectroscopic (UV-vis, FTIR, Raman, ESR, ESE) Characterization Stanisław K. Hoffmann,*,† Janina Goslar,† Stefan Lijewski,† Iwona Olejniczak,† Aldona Jankowska,‡ Anna Werbin´ska,‡ and Stanisław Kowalak‡ Institute of Molecular Physics, Polish Academy of Sciences, 60-179 Poznan´, Poland, and Adam Mickiewicz UniVersity, Faculty of Chemistry, 60-780 Poznan´, Poland

X-ray, UV-vis, FT-infrared, Raman, continuous wave, and pulsed ESR spectroscopy were applied to characterize pigments prepared at 500 °C under vacuum or in air from mixtures of zeolite A (LTA) with sulfur (40 wt %) and anhydrous sodium sulfide (Na2/S ) 0.1-0.4). The samples maintained the LTA structure. Sample color results from an S3 absorption band and the shoulder of a UV band. The fingerprint and asymmetric stretch region of the IR spectra show that the pigments synthesized in air have more disordered structures. The S2 /S3 ratio was estimated from Raman spectra. The ESR spectra of S3 radicals show the existence of 3+ several phases visible as five different ESR components. Two components dominate: ideal [Na4Sclusters 3] and clusters existing in a disordered environment. The samples synthesized in air show a substantial distortion of local structure. Synthesis in vacuum leads to products containing about 20% of the ordered crystalline phase. 1. Introduction The color of ultramarine analogs can be obtained from various zeolites by thermal treatment with sulfur radical precursors such as alkaline oligosulfides or elemental sulfur and alkalis.1 The chemistry of sulfur radical formation still remains not fully explained. Various ring and chain sulfur anions S- to S9 have been identified by photoelectron spectroscopy.2 It is likely that in oxygen free conditions the paramagnetic S˙3 radicals can be formed by homolytic scission of S-S bond in oligosulfides: -

S-S-S-S-S-S- f S-S-S˙- + S-S-S˙-

or in the presence of oxygen by a mild oxidation of respective oligosulfides: 1 2˙2S23 + /2O2 f 2S3 + O

The aim of the present study is to control color and quality of pigments by comparing the spectroscopic properties of the pigments obtained from zeolite LTA and sodium oligosulfides having various Na2/S ratios either in evacuated sealed tubes or in covered crucibles with some access of air. We have noticed some differences in appearance and properties of zeolite pigments prepared from the same mixtures (with sulfur radical precursors) regarding the lack or presence of oxygen during the thermal treatment.3 The powder X-ray diffraction was used to check the parent zeolite structure maintained after thermal treatment, as well as UV-vis spectroscopy to control the relative amount of the various chromphores responsible for sample coloration. The FTIR and Raman spectra were recorded to follow the zeolite structure transformation and the presence of various sulfur species. Special attention is drawn to the analysis of the electron spin resonance (ESR) spectra of S3 radicals which are very sensitive to radical molecule reorientations and to local disorder. * To whom correspondence should be addressed. Tel.: +48 61 86 95 207. Fax: + 48 61 86 84 524. E-mail: [email protected]. † Institute of Molecular Physics. ‡ Adam Mickiewicz University.

The S3 anion-radical has the largest dimension of 0.34 nm. Thus, in principle, it can be located in many sites of zeolite structure. The preferable location is in the β cage, where the radical is encapsulated in the form of tetrahedral cluster 3+ [Na4S3 ] . The radical is well stabilized and rigid at low temperatures (usually below 70 K) giving a well shaped threepeak powder ESR spectrum described by a rhombic g-tensor. With increased temperature, thermally activated jumps of Na+ ions take place between possible orientations in the β cage. This results in a continuous ESR line shift and line broadening with final collapse into a single ESR line above room temperature.4 For this reason only the rigid lattice ESR parameters are adequate for analysis. We have analyzed the ESR spectra of S3 recorded at 10 K and below. The rigidly localized S3 with its well resolved ESR spectrum is observed in the nondistorted zeolite structure. We have also found this for samples with LTA structure calcinated at 800 °C for 2 h.4 In most cases, however, some degree of local disorder or amorphization in the zeolite structure can be expected. Such a disorder around the S3 is observed in the ESR spectra. We show in this paper that besides 3+ the ideal [Na4Sgeometry which has been calculated by DFT 3] 5 method, the weakly distorted geometries also exist and they lead to a distribution of measured parameters. Moreover, a deficiency of sodium atoms in a Na-tetrahedron or freely rotating radical molecules can exist even at low temperatures. Preparation of ultramarine analogs from zeolites and sodium oligosulfides usually involves a contribution of water, when Na2S · 9H2O is used as the starting compound treated with respective content of elemental sulfur to achieve desired oligosulfides. Therefore, we use the anhydrous alkali sulfides which can be easily obtained by reaction of sodium and sulfur in liquid ammonia. Although our samples were synthesized from anhydrous sodium sulfide and activated zeolite, some water can be adsorbed during postsynthesis washing procedure. A trace of water can be detected using electron spin echo (ESE) spectroscopy. The Fourier transform of the ESE decay gives FT-ESE spectrum which can display peaks from protons of water molecules at a distance shorter than 0.5 nm from the radical.

10.1021/ie100983m  2010 American Chemical Society Published on Web 07/27/2010

Ind. Eng. Chem. Res., Vol. 49, No. 17, 2010

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Table 1. Properties of the Studied Pigments sample

A(0.1)

A(0.14)

A(0.17)

A(0.2)

A(0.4)

synthesis Na2/S thermal treatment

vacuum 0.1 evacuated ampule, anhydrous olive

vacuum 0.14 evacuated ampule, anhydrous olive

vacuum 0.17 evacuated ampule, anhydrous green-olive

air 0.2 covered crucible, hydrated Na2S turquoise

air 0.4 covered crucible, hydrated Na2S turquoise

color

2. Experimental Section 2.1. Materials. Anhydrous sodium sulfide was prepared by reaction of Na and S in liquid ammonia followed by evaporation of the ammonia. The preactivated (calcinated at 350 °C for 5 h) zeolite LTA was mixed with anhydrous sodium sulfide and various amounts of elemental sulfur corresponding to Na2/S ratios of 0.1, 0.14, 0.17, 0.2, and 0.4. The samples are labeled as A(Na2/S) through the text. The total content of the sulfur in the mixture was always 40 wt % of zeolite. Some mixtures were evacuated in quartz ampules and sealed. Other samples were heated in lid-covered ceramic crucibles allowing some contact with oxygen of the ambient air. Then samples were heated at 500 °C for 5 h. The products were washed with water and dried afterward. 2.2. Characterization. 2.2.a. X-ray Diffraction. A Bruker D 8 Advance diffractometer was used with filtered KR radiation (λ ) 0.154056 nm, Ni filter) in angle range 2Θ of 4-50°. 2.2.b. Optical, IR, and Raman Spectroscopy. Diffusive reflectance UV-vis spectra were recorded using a Cary 100 Varian spectrometer. Infrared absorption measurements in the frequency range 400-12000 cm-1 were performed using a Bruker Equinox 55 FT-IR spectrometer equipped with a Hyperion 1000 infrared microscope; samples were prepared using the KBr pellet method. Raman spectra were measured with a Raman Labram HR800 spectrometer using a He-Ne laser (λ ) 632.8 nm); the power of laser beam was about 1 mW. The scattered light was collected in a backward scattering geometry; the spectral resolution was 4 cm-1. 2.2.c. ESR Spectroscopy. ESR spectra were recorded with an X-band Bruker spectrometer ESP 380E FT/CW with a dielectric resonator TE011, equipped with an Oxford CF935 cryostat. Low microwave power was used to avoid the saturation effect. The ESR spectra were simulated with the SimFonia Bruker routine. In pulsed ESR experiments were made for recording of the echo detected spectra (ED-ESR), and threepulse modulations (ESEEM spectra) of the electron spin echo decay were recorded, as described previously.4

structure of zeolite LTA. The structure is distorted toward lower symmetry and the XRD pattern resembles the patterns of the hexagonal nepheline hydrate II with two-dimensional pore system in the bc-plane as described by Barrer in an early work.8 The background baseline in the diffraction patterns of Figure 1 is not perfectly flat, especially for samples A(0.2) and A(0.4) synthesized in air. This indicates that a fraction of the samples is in a noncrystalline (amorphous) state.9 This is consistent with ESR results indicating local disorder as discussed in a next section. 3.2. UV-vis Spectroscopy. The samples show some differences in colors (see Table 1), which is reflected in the UV-vis spectra. The spectra of the samples examined do not differ considerably one from another (Figure 2). The spectra were deconvoluted using eight Gaussian absorption bands (showed as dashed lines in the A(0.17) spectrum) centered at 213, 257 (weak), 293 (strong), 350 (very weak), 362 (weak), 406, 410, and 600 nm. The band at 600 nm is due to the S3 anions giving a blue component to the sample color. Position of this band is practically the same in various matrices indicating that the electronic structure of the S3 radical is not influenced by its environment. Especially the energy of the 2B1 f 2A2 transition (responsible for the 600 nm band) between π-orbitals of the radical molecule is nonsensitive to the environment as we indicated previously.4 Thus, one cannot expect that a change in local structure around the radical molecule will influence a sample color except a change resulting from the radical concentration. The relative intensity (ratio of the shaded areas under the curves in Figure 2) of the S3 and S2 bands was kept constant and equal to 1/0.13 in all the studied samples as evaluated from Raman spectra discussed below. The S2 band is dominated by a closely positioned band at 406 nm of an unknown origin. For this reason UV-vis spectra should not be used as an evaluation of S2 concentration. Generally, a color of ultramarine pigments results from the 600 nm band of S3 and a shoulder of the UV-bands whereas the contribution from 9 the S2 band can be insignificant, as was already also suggested.

3. Results and Discussion 3.1. X-ray Diffraction (XRD). The colored products (Table 1) obtained after heating at 500 °C are crystalline, however the original structure of zeolite A (space group Fm3jc) is affected by the thermal treatment of the mixture containing sulfur compounds. The XRD spectra of the parent zeolite LTA and the resulted pigments, with peaks marked with appropriate (hkl) indices of the crystal planes,6 are compared in Figure 1. A characteristic feature of the zeolite LTA is the systematic absence of XRD peaks for the indices being the odd numbering indices. Similarly as in other pigments prepared from zeolite A, the recorded XRD spectra indicate the presence of reflections attributed to zeolite LTA structure.7 However the intensity of some low angle reflections is diminished. It should be noticed that the amplitude of the reflections from the (hk0) planes decreases, whereas amplitude of the (hkl) reflections remains unchanged. This suggests that after the sulfur and sodium atoms are introduced into the host there is no pure phase of the original

Figure 1. X-ray diffraction patterns of pure zeolite LTA and sulfur treated samples labeled with symbols used in Table 1. The reflections are marked by (hkl) indices according to ref 6.

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Figure 2. UV-vis spectra of samples prepared under vacuum (left) and in air (right). The dashed lines are the Gaussian components of the representative spectra. The absorption peaks of S2 (yellow chromophore) and S3 (blue chromophore) are indicated by arrows.

This UV-region of Figure 2 is dominated by the 293 nm band in all species. This band is not assigned but dominates also in pure blue ultramarine pigment.4 In Figure 2 (right) a spectrum of a sample A(0.2) prepared at 800 °C which we studied previously is displayed.4 This spectrum differs in shape from the other spectra of Figure 2 because of the different relative intensity of the absorption bands. The large relative intensity of the band at 410 nm leads to the light green color of the A(0.2) 800 °C sample. It should be mentioned that computer fitting routines do not give unique results in the case when the number of bands is not known, but is treated as a fitting parameter. The bands in the ultraviolet region in zeolite-type pigments are generally not well assigned. They can be attributed to the occluded elemental sulfur (350 nm),10 to the S4 molecule (530 nm-cis and 620 nm-trans)11 and the S4 anion (520 and 529 nm).12 It is well-known that at a relatively low synthesis temperature, the S8 crow like ring is stable and well stabilized at 300 °C. Thus, the S8 species can still remain when synthesis is performed at 500 °C, with the absorption bands theoretically calculated at 95, 137, and 247 nm.13 Theory gives also absorption band for S7 molecules at 103, 146, and 354 nm.14 The presence of anionic radicals requires additional charge compensation. By assuming that only the additional Na+ ions introduced in the form of sodium polysulfide should compensate the negative charge of anion-radical molecules, one can evaluate the number of sodium ions needed for this charge compensation. Radical anions are localized in the β cages of the zeolite 3+ structure in the form of tetrahedral clusters of [Na4Sor 3] 3+ [Na4S] (refs 4, 5). The positive charge of the clusters has to 2 be compensated by the negatively charged host framework. Neglecting a small amount of S2 , the Na2/S ratio needed for 3+ the [Na4Scluster formation is equal to 0.66. The pigments 3] examined have a lower Na2/S ratio, i.e., an excess of the sulfur exists (although some part of the sulfur can be oxidized to SO2). This appears as intensive absorption bands in the UV-region due to diamagnetic forms of sulfur. The UV-vis spectra of samples prepared under various conditions do not differ significantly, but they indicate that the samples are not single-phase systems since they contain various forms of sulfur. 3.3. Infrared and Raman Spectroscopy. The FT-mid-IR and Raman spectra of sample A(0.17) synthesized under vacuum are compared in Figure 3. Three bands νmax/cm-1 466, 552, and 993 dominate the IR spectrum and they are in the range typical for zeolite materials.15-17 The IR spectra recorded in the range 400-1300 cm-1, where vibrations characteristic for

Figure 3. Infrared and Raman spectra of sample A(0.17) synthesized under vacuum. The peaks ascribed to sulfur radicals are marked.

Figure 4. Experimental mid-infrared spectra of the studied samples A compared with pure LTA zeolite, LTA with Na2/S ) 1, ultramarine blue, and with theoretical spectrum of LTA.

zeolite framework appear, are shown in Figure 4. Besides the spectra of the samples studied, the spectrum of parent pure zeolite A (marked as LTA), zeolite A with Na2/S ) 1 (marked

Ind. Eng. Chem. Res., Vol. 49, No. 17, 2010 -1

Figure 5. Fingerprint region of the intermolecular vibrations for four samples. A(1) sample having an excess of sodium with Na2/S ) 1 is added.

as A(1)), blue ultramarine UM (having sodalite structure), and theoretical LTA spectrum calculated using Molecular Dynamics theory,18 are shown as a reference data. The IR spectra are not sensitive to the sodium and sulfur introduced into the zeolite structure for samples having a small amount of sulfur compared to Na (with Na2/S < 0.2). Only the 550 cm-1 band is split into a doublet. The lower energy band of the doublet at 552 cm-1 can be assigned to double ring vibrations15,19 and a higher energy band at 575 cm-1 can be assigned to the ν3 stretching 16,20 vibration of the S-S bond in the blue S3 chromophore. The peaks due to S3 both in IR and Raman spectra are marked in Figure 3. The lowest energy band at 466 cm-1 can be assigned to the T-O bending mode (T ) Si, Al) of TO4 tetrahedra and this band appears in all zeolite structures. There are two regions of vibrations sensitive to structural changes. In the “fingerprint” region 500-800 cm-1 weak bands appear due to external linkage between TO4. This region of vibrations allows identification of various phases in mixed and multiphase zeolite materials and is shown in Figure 5. Pure LTA structure does not have a very characteristic fingerprint region, only a single broad peak at 670 cm-1. In LTA-pigments with Na2/S < 0.2 this peak remains and an additional weak broad band at 755 cm-1 appears. For samples with Na2/S g 0.2 three bands exist at νmax/cm-1 650, 665, and 755 whereas for sample A(1) with strong deficiency of sulfur a well formed three-peak region exists with band at νmax/cm-1 665, 710, and 730. These three bands are characteristic of a sodalite structure and correspond to the Si-O-Al symmetric stretching mode.21 A comparison of the above fingerprint regions with the regions for pure phases of ultramarine blue and for sodalite, cancrinite, and nepheline in Table 2, shows that these simple structures do not appear in the studied pigments. The second characteristic region with strong bands exists around 1000 cm-1 where asymmetric stretch vibrations of Si-O-Al are expected.21 Data of Table 2 and Figure 4 show that the spectra of LTA-pigments with Na2/S g 0.2 contain more bands compared to pure LTA and the other samples. This region decomposes into Lorentzian bands for samples with Na2/S ) 0.14 and Na2/S ) 0.2 as shown in Figure 6. An increase in the number of bands for samples synthesized in air indicates that they contain more structural phases and/or a larger degree of a local disorder or residual sulfur derivatives. This conclusion will be confirmed by ESR data presented below. Raman spectra are typical for ultramarine-type pigments24,25 with the dominant peak at ν1 ) 550 cm-1 attributed to the symmetric stretching mode of S3 anion. This peak is ac-

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companied by a weak peak at ν1′ ) 580 cm which can be assigned as the S2 symmetric stretch (Figure 7). The symmetric -1 bending mode of S3 gives the peak at ν2 ) 262 cm . Moreover, the overtone 2ν1 and combination peaks at ν1 + ν2 and 2ν1 + ν2 are observed (Figure 3). From the relative intensity of the ν1 and ν1′ bands, the relative concentration ratio S2-/S3- can be determined. This ratio (S2 /S3 ) is practically the same for all studied samples and equals 0.13((0.01). We used this value in the interpretation of the UV-vis spectra. 3.4. ESR Spectroscopy. The S2 and S3 radicals are paramagnetic and in principle can be studied by electron spin resonance (ESR). However, only the ESR spectrum of S3 is observed. The S2 spectra are undetectably broadened by radical dynamics as already suggested,4,5 although their static g-factor values are theoretically evaluated.5 ESR spectra of the samples are shown in Figures 8 and 9. Low temperature spectra (below 80 K) display higher resolution compared to the spectra recorded at room temperature. They 4,5,26-30 are typical multicomponent spectra of SAn 3 radicals. increase in temperature gives rise to a line shift and line broadening due to radical reorientations which collapse into a single broad resonance line as we previously described.4,30 For this reason the low temperature (below 10 K) spectra have been analyzed. The collection of the spectra recorded at 6 K is shown in Figure 8. The spectra of samples prepared in vacuum, i.e., in sealed ampules (upper three spectra) are similar to one another and are composed of more lines than the spectra of the samples synthesized in air (in covered crucible) which are similar one to another also. Therefore, we analyze in details the spectra of A(0.14)-vacuum, and A(0.4)-air samples only. The spectra consist of two different types of components: anisotropic three-peak spectrum of rhombic symmetry (three g-factors) and an isotropic line (isotropic g-factor). The components can be distinguished by means of computer simulations of the continuous wave ESR (cw-ESR) spectra or in the spectra recorded using the electron spin echo (ESE) pulse techniques. The isotropic ESR lines do not appear in the echo detected spectra (ED-ESR) since these lines do not participate in the ESE formation because they are homogeneously broadened. Thus the first derivative of the ED-spectrum gives anisotropic components usually dominating in a spectrum at low temperatures. The first derivative is compared with the corresponding ESR spectra in Figure 9. It is visible that the central line existing in the cw-ESR spectrum vanishes in the ED-ESR spectrum. The isotropic components can be discriminated by their temperature dependence and the species responsible for its existence appear with different concentration in various ultramarine-type pigments we have studied so far. Thus a total ESR spectrum can be computer simulated with high uniqueness. The existence of different components of the S3 ESR spectra seems to be not expected since the radical is a rather rigid molecule, weakly influenced by its environment with electronic structure determined by intra molecular charge distribution (Figure 10). ESR parameters of the components are in the range expected for S3 -radical suggesting that there exist some modifications of the parameters by molecular dynamics or local disorder. The rigid radical localized in the undistorted tetrahedral 3+ environment [Na4Sin a β cage has an ideal ESR spectrum 3] which we call spectrum A. This spectrum can be considered as a model spectrum and it is shown in the bottom of Figure 8. It is a typical three-component ESR powder spectrum, described by the rhombic Zeeman spin-Hamiltonian H ) µB(gxSxBx + gySyBy + gzSzBz) with principal g-tensor components giving three peaks in the powder ESR spectrum. Typical experimental values

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Table 2. Assignment of Bands in the IR Spectra of LTA-Pigments, LTA, Ultramarine Blue (UM), and Pure Zeolite Structures: Sodalite (SOD), Cancrinite (CAN), and Nepheline Hydrate (NEPH) A-samples (LTA-pigments) vibration T-O bend double ring S3- stretch

vacuum

theory18,19

exper. 466 557

466 552 572

466 552 572

425-445 500

670 755

650 665 755

620 680 750w

fingerprint region 500-800 cm-1

asymmetric stretch of T-O and T-O-T

air

Zeolite LTA

943 1000 1054w 1119 1263

938 986 1064 1106w 1137w 1197 1303

of g-factors of spectrum A in various solids are4 gx ) 2.000-2.005 (2.002), gy ) 2.045-2.060 (2.060), and gz ) 2.02-2.04 (2.039)sthe values in parentheses are derived from DFT calculations.5 The average g-factor value is 2.029 as for ultramarine blue where the ESR spectrum is exchange averaged and consists of a narrow single Lorentzian line. Experimental values of g-factors in various materials are very similar confirming that the electronic structure of the S3 radical is very stable and practically not influenced by its local environment but determined by the charge distribution within the radical molecule. Thus, one cannot expect a manipulation of a sample color by a modification of the S3 radical environment. We have previously found the spectrum A as dominant in pigments having the LTA structure and prepared at 800 °C.4 Unpaired electron of S3 occupies antibonding ground state of the 2B1(π)-symmetry shown by lobes in Figure 10. The three lowest-lying excited electronic states are 2A1(σ), 2 B2(σ), and 2A2(π). The 2B1 f 2A2 transition (16500 cm-1 )

UM 451

SOD17,21,22

CAN17,22

NEPH23

460 632

455 565

449 510

665 711 737

565 629 691

575 625 715

980

960 1020

945

584vw

1055

670

665 698 750

974 1002 1047sh 1090sh

1013 1038 1134w

600 nm) gives a strong band in absorption optical spectrum and is responsible for the blue color of ultramarine pigments. The details of the electronic structure of S3 and the MO-theory of the g-factors we previously described in detail.4,30 The appearance of spectrum A indicates that S3 exist in well ordered (structurally uniform) phase in nondistorted environment (sodium tetrahedron) in β cages.

Figure 7. Raman spectra of the samples recorded in the range 400-650 cm-1 compared with ultramarine blue.

Figure 6. Region of asymmetric stretch of the T-O vibrations. The spectrum is deconvoluted into individual Lorentzian peaks (dashed lines).

Figure 8. Low-temperature ESR spectra of the studied samples. The lower spectrum is a pure spectrum we have recorded for the sample prepared at 800 °C.


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Table 3. g-Factors, Line Width ∆Bpp, and Relative Intensities of the Five ESR Spectra at 10 K (L and G Mark Lorentzian and Gaussian Line Shapes, Respectively) vacuum synthesized spectrum

g-factors

A

gx ) 2.0015(5) gy ) 2.0496 (5) gz ) 2.0346 (8) 〈g〉 ) 2.029 gx ) 2.0010(6) gy ) 2.0510 (5) gz ) 2.0360 (6) 〈g〉 ) 2.029 g ) 2.029(1) g ) 2.0348(5) g ) 2.0138(5)

B

C D E Figure 9. ESR spectra recorded at room temperature and at low temperature. Echo-detected (ED-ESR) spectra recorded in liquid helium, measurement temperatures are indicated.

Figure 10. Open (C2v-symmetry) form of the free S3 radical molecule with the 2B1-symmetry ground state molecular orbital lobes and x, y, z reference frame. The planar molecule lies in the yz-plane with the negative charge -0.7e on the terminal sulfur atoms. The S-S bond length is r ) 0.20 nm and the S-S-S bond angle is 116°.

air synthesized

line width relative line width relative ∆Bpp (mT) intensity ∆Bpp (mT) intensity 0.7 (G) 1.0 (G) 1.0 (G)

1

2.0 (G) 1.9 (G) 3.5 (G)

3.9

1.5 1.8 3.3

1

1.6 (L) 2.2 (L) 1.9 (L)

0.04 0.2 0.1

1.6 1.3

0.15 0.03

-

-

result of this strong broadening the line at gz apparently “disappears” in ESR spectrum. The effect of the line broadening along the z-direction on the spectrum A is shown in Figure 12, where the line at gz is successively broadened from 1mT (as in the spectrum A) to 4 mT (as in the spectrum B). In ultramarine pigment the spectrum A can be transformed into spectrum B, as it is in the samples prepared in air (Figure 11 left) or both spectra can coexist as in samples synthesized in vacuum (Figure 11 right). It is very visible in ED-ESR spectra (Figure 9) which do not contain isotropic lines superimposed on spectrum A or spectrum B. The broadening effect has been considered as a result of a distribution of the gz values due to a local disorder around the 4,30 SFrom orbital 3 disturbing the radical molecule geometry. energy dependence on the bent angle φ (Figure 10) we have evaluated that the distribution of (0.5 deg in the φ-values will produce Gaussian ESR line with line width ∆Bpp(z) ) 4 mT. Thus, even a small disturbance of the radical geometry by its environment can result in a significant effect in ESR spectrum. Thus, the spectrum B indicates an existence of a local disorder in the host lattice (disordered or amorphous phase) and this spectrum, without the spectrum A, dominates in air synthesized pigments. In the zeolite framework composed of alternating SiO4 and AlO4 corner-sharing tetrahedra a disorder can be due to the observed random distribution of Si4+ and Al3+ in pigments obtained at high temperatures.31,32 Spectra A and B arise from well localized radicals, whereas the other spectra (C, D, and E) are produced by rapidly reorienting S3 molecules. Spectrum C is a spatially averaged spectrum A or spectrum B as indicated by the average g-factor value (see Table 3). The

Figure 11. Comparison of the ESR spectra and their simulated components for sample prepared in air and for sample prepared under vacuum. The spectrum “sum” is the total simulated spectrum being sum of the component (dashed lines) added with relative intensity shown in Table 1.

Computer simulations of the experimental spectra allowed distinguishing five different spectral components (assigned as spectra A-E) as presented by dashed lines in Figure 11. The g-factors of different spectra and simulation parameters are collected in Table 3. Spectrum A appears in the vacuum synthesized samples only, i.e., in A(0.1), A(0.14), and A(0.17) as is visible in Figure 11 for A(0.14). Only about 20% of radicals exist in this perfect environment in these samples. Other spectra (B-E) we have identified previously in various zeolite A pigment samples.4,30 Spectrum B has practically the same g-factors as spectrum A, but the lines are broader (see Table 3). Especially the line at gz is several times broader than the lines at gx and gy. As the

Figure 12. Evolution of the spectrum A (with peak-to-peak line width along the z-axis) toward the spectrum B (with ∆Bpp ) 4 mT).

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number of the radicals producing spectrum C is small in samples produced under vacuum and such a spectrum does not exist in samples produced in air (see Figure 11). The g-factor averaging is a result of rapid reorientations or tunneling between possible 33,34 SSuch freely 3 molecule orientations in the β cage. reoriented radical molecules can be also localized in R cage or in voids of the zeolite lattice. Another quite probable possibility is an existence of small aggregates or clusters of the sulfur radicals.34 An interaction between clustered radicals gives an exchange averaged ESR spectrum consisting of a single Lorentzian line similarly to that in ultramarine blue. The isotropic line D is also a motionally averaged spectrum resulting from fast spatial reorientations of S3- radicals. The g-factor value is, however, different from the average g-factor of the spectrum A and B, i.e., it is the spectrum different from the isotropic spectrum C. The line D exists even at very low temperatures indicating fast jumps or tunneling of the radical. Such fast low-temperature dynamics of the radical molecule in β-cage can be expected only when S3 radicals have fewer than four Na+ anions in the nearest environment. The spectrum D always accompanies spectrum B, i.e., it appears in locally disordered samples and when Na2/S ratio is lower than 0.6. Spectrum E is due to another sulfur radical center. The spectrum is a single isotropic Lorentzian homogeneously broadened line with g ) 2.0138, which is close to the average theoretical g-factor value of the closed triangular form of S3 radicals with D3 h-symmetry.5 The samples synthesized in crucible show only the ESR spectra B, D, and E, whereas for samples synthesized at vacuum the experimental spectra are similar but consist of the A, B, C, D, and E components. Thus, in contrast to the UV-vis spectroscopy, ESR distinguishes between samples prepared under vacuum and in air. Although only three ESR spectra (three types of radical centers) exist in samples synthesized in a crucible, the radical centers are distorted in all sites, as it is indicated by the absence of the spectrum A. In the samples synthesized under vacuum (in sealed ampules), the number of various radical centers (and ESR spectra) is larger but the existence of spectrum A and its fast reorienting form indicates that the local structure is locally ordered (nondistorted) for about 20% of the S3 radicals in the samples. This indicates that the samples prepared at 500 °C under vacuum show some degree of crystallinity coexisting with amorphous or disordered phases. The host zeolite crystallites structure or grain surfaces can be contaminated by water molecules. To check the presence of water in our samples we have performed pulsed ESR measurements using electron spin echo (ESE) techniques. It was made for our A(0.1) sample and compared with LTA previously studied30 synthesized with K2CO3 and sulfur having K2/S ) 0.2 (sample marked as AK(0.2)). Three-pulse electron spin echo decay measured at low temperatures show strong echo modulation amplitude. The modulated echo decay and a subtracted modulation function for A(0.1) sample at 6 K are shown in the inset of Figure 13. The Fourier transform of modulation function gives ENDOR-type spectrum with peaks at Larmor frequency of modulating nuclei surrounding the radical center. The spectra of the samples A(0.1) and AK(0.2) are compared in Figure 13. Only a single peak at 3.8 MHz appears in the A(0.1) sample (and in other studied samples). The peak is due to the 27Al surrounding S3 . There are no peaks at about 15 MHz expected when a water impurity exists. Such a peak arising from protons of water molecules is clearly visible for AK(0.2) sample. Thus, all our samples do not contain water molecule, or more generally protons in the radical center vicinity (up to of about 0.5 nm).

Figure 13. Fourier transform (FT) spectra of ESE decay modulations of the A(0.21) sample. For a comparison the FT-spectrum of zeolite LTA sample (AK(0.2)) treated with K2CO3 (instead of Na2S) and elemental sulfur at 500 °C is added. Peaks at 3.8 MHz are due to the surrounding sodium and aluminum nuclei, whereas the peak at 15 MHz is due to water molecules. The three-pulse ESE amplitude decay and subtracted modulation function are shown in the inset.

4. Conclusions Ultramarine analogs synthesized from zeolite LTA and anhydrous sodium oligosulfides with various alkali content at 500 °C in a crucible or in a sealed ampule under vacuum do not differ significantly according to XRD or UV-vis spectral data. The differences appear in infrared and ESR spectra. Intermolecular vibrations observed in the fingerprint region and zeolite framework vibrations appearing around 1000 cm-1 show a multiphase structure of the studied samples. Apart from the parent zeolite LTA structure new phases appear. The new phases cannot be identified with simple zeolite structures but occur due to the doped sodium and sulfur atoms. Raman spectra show a small amount of S2- radicals compared to S3- radicals (13% only). At first glance, this is not consistent with UV-vis spectra where both radicals have bands with comparable intensity. However, detailed analysis of the UV-vis spectra shows that an additional band at 410 cm-1 is superimposed on the S2 band. This suggests that UV-vis spectra should rather not be used for a quantitative determination of the S2 /S3 concentration ratio. ESR spectroscopy gives information on local structure around the S3 radicals and on the dynamics of the radical molecules. The ESR results show that the samples consist of several phases (represented by different type of the ESR spectra) dispersed through the samples. The samples synthesized with excess air show a strong distortion of the local structure (spectrum B dominates) suggesting a strong local disorder in the material. Synthesis under vacuum also gives multiphase samples, but although the disordered phase (spectrum B) dominates, the significant (of about 20%) amounts of radicals exist in well ordered (crystalline) phase (spectrum A). Except for these two predominant types of radical existing in a distorted or in the ideal tetrahedral sodium environment, there also exist radicals combined with fewer than four Na+ anions in the nearest environment (spectrum D), another type of S3 radicals (spectrum E), and fast reorienting or aggregated radicals similarly as in ultramarine blue (spectrum C). The general conclusion is that the samples of zeolite LTA pigments are of better quality, i.e., they indicate a higher degree of crystalline when synthesized under vacuum, whereas samples synthesized in air are locally disordered although the parent zeolite framework is maintained. Literature Cited (1) Kowalak, S.; Jankowska, A. Inorganic Sulphur Pigments Based on Nanoporous Materials. In Ordered Porous Solids; Valtchev, V., Mintova, S., Tsapatsis, M., Eds.; Elsevier: New York, 2009; p 575.

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ReceiVed for reView April 29, 2010 ReVised manuscript receiVed June 24, 2010 Accepted July 5, 2010 IE100983M

Sulfur Pigments Synthesized from Zeolite LTA under Vacuum and in

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