Sulfated Zr-pillared saponite: preparation, properties and thermal stability

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

903

Sulfated Zr-pillared saponite : preparation, properties and thermal stability L. Bergaoui a, A. Ghorbel a and J.-F. Lambert b aLaboratoire de Chimie des MatEriaux et Catalyse, Facult6 des Sciences de Tunis, Universit6 E1 Manar, Campus Universitaire, 1060 Le B61ved~re, Tunis, Tunisie bLaboratoire de R6activit6 de Surface (URA 1106), Tour 54-55, 2~me 6tage Universit6 Pierre et Marie Curie, 4, Place Jussieu, 75252 - Paris cedex 05, France The intercalation of a saponite with zirconium oligomers containing variable amounts of sulfate has been studied (SO4:Zr molar ratio between 0 and 0.3). Well-ordered intercalated clays with basal spacing between 18 and 20 A were obtained. For the higher SO4:Zr ratio, a highly polymerized species is intercalated, giving a nanocomposite material with a low surface area (50 m2/g). For the lower ratios, higher surface areas are obtained (160 m2/g). After calcination of those clays, the presence of sulfate induces a loss of cristallinity but surface areas remain important. Thermal evolution of the intercalated compound was followed by mass spectroscopy showing that sulfur is not eliminated from the solids before 450~ (under helium). Above this temperature, two distinct thermal events were observed, suggesting two different modes of linking of sulfates with the polycation, in agreement with Raman data on the SO4-Zr intercalating solution. 1. INTRODUCTION The modification of swelling clays by "pillaring" is a simple way to prepare materials with new physical and chemical properties. The intercalation of large size inorganic cations (pillars precursors) between the clay layers, followed by calcination, allows the preparation of thermally stable microporous solids [1-4]. The chemical properties of the pillared clays obtained in this way depend on the constitution of the clay layers [5] and the nature of the intercalating agent [6]. The polycations intercalated during the first step of the synthesis are obtained by partial hydrolysis of aqueous solution of multivalent cations. In particular, to prepare zirconium-pillared clays, zirconium oxychloride solutions are commonly used as the pillaring precursors. It has been shown that the predominent polycations in these solutions have a structure built up from [Zr4(OH)8(H20)16] 8+ tetramers [7]. Since the promotion of zirconium oxide by sulfates is widely used to prepare strong acidic solids [8], it was tempting to try similar modifications on zirconium-pillared clays. Indeed, the introduction of sulfate ions into a zirconium pillared clay has been shown to enhance the acidity of this solid but it also decreases its thermal stability and its porosity [9].

904 In the present work, we have tested a new preparation method to optimise the structural properties of sulfate-promoted zirconium-pillared clays. In particular, the influence of the amount of sulfate on the structure and the thermal stability of Zr-pillared clays has been studied. 2. E X P E R I M E N T A L SECTION

2.1. Materials A sample of saponite from Ballarat (USA) was obtained from the Source Clays Minerals Repository and its < 2 [tm fraction was separated by gravity sedimentation. It was then exchanged three times with a 1 mol/L NaC1 solution and washed thoroughly. Chemical analysis yielded the following formula for the Na-exchanged saponite : Nao.600Cao.028K0.006(Si7.262A10.738) (Mg5.920Fe2+0.104Tio.006Mn0.002)O20(OH)4 There are only minor differences with the formula proposed by Prost [ 10] for the Ballarat saponite. 2.2. Intercalation and pillaring procedures Ammonium sulfate was added to freshly prepared 0.05 mol/L ZrOC12 solutions, with SO4/Zr molar ratios varying from 0 to 0.3. Intercalated saponites were then prepared by adding the SO4/Zr solution dropwise to 10 g/L clay suspension until an Zr/Clay ratio of 5 mmol/g was reached. The slurry was stirred overnight at 50~ washed by successive dialyses, and dried in air at room temperature, giving SO4/Zr-intercalated clays. The samples were then calcined in flowing oxygen. The temperature was raised at 60~ to 400~ and the final temperature was maintained for 4 h. The term of " pillared " clays will here be reserved to samples stabilised by calcination. The intercalated (uncalcined) saponites will be called ZrI-r, and the corresponding pillared saponites will be called ZrP-r (where r is the SO4/Zr molar ratio in the intercalation solution). 2.3. Samples characterisation A SIEMENS X-Ray D 500 diffractometer using Cu Kc~ radiation was used to record powder diffractograms. Oriented ZrI-r samples were prepared by air drying of a clay slurry on a glass plate. The ZrP-r powder samples were pressed onto a glass holder. Surface area measurements were performed by nitrogen physisorption at 77 K using a static volumetric apparatus (Micromeritic ASAP 2000 adsorption analyzer) ; the BET equation was applied to the adsorption isotherm. All samples pretreated in vacuum at 110~ prior to nitrogen physisorption. Elemental analyses for zirconium and sulfur were performed at the Centre d'Analyse du CNRS (Vernaison, France). Thermal treatments of the intercalated solids from room temperature to 1000 ~ were carried out in quartz reactors under helium flow, with a flow rate of 3 cc/mn. A Hiden Analytical HPR 20 Mass spectrometer (MS) was used to conduct evolved gas analysis upon thermal treatment, in the mass range between 1 and 200 a.m.u. A capillary leak maintained at 170 ~ was used to divert a fraction of the gas flow to the analysis chamber. 2.4. Raman Raman measurements were performed using the 514.5 nm line of a coherent argon ion laser. The laser beam power was 80-100 mW and the resolution was set at 1 cm -1. The spectra were recorded using a Jobin-Yvon U1000 spectrometer over two frequency ranges 850-1200 cm-1 for sulfate vibrations and 300-600 cm-1 for Zr-O vibrations.

905

3. R E S U L T S AND D I S C U S S I O N

3.1. Solution Raman study Figure 1 shows the Raman spectrum of the intercalation solution with a SO4:Zr ratio = 0.2 in the 300-600 cm -1 region. The low zirconium concentration results in a poor resolution because of interference with the bands of water [ 11 ]. Nevertheless, the band at 430 cm -1 can be attributed to the presence of the tetrameric form [Zr4(OH)8(H20)16] 8+ and/or of a more hydrolyzed species in solution [ 11 ]. In the higher wavelength region a sharp band is apparent at 983 cm-1 for the 10-2 mol/L (NH4)2SO4 solution (figure 2-a). The 5.10 -2 mol/L ZrOC12 solution with a SO4:Zr ratio = 0.2 shows a non symmetric band at 1012 cm -1 (figure 2-b). In spite of the intensity of the noise, we can see two shoulders at 986 cm -1 and at 999 cm -1. The band at 983-986 cm -1 for both solutions can be assigned to a free sulfate in solution. It seems reasonable to attribute the bands at 1012 cm -1 and the shoulder at 999 cm -1 for the SO4-Zr solution to two types of interaction of (SO4) 2- with the zirconium polycations. 3.2. Amount of fixed zirconium and sulfate As seen in table 1, the amount of zirconium fixed by ZrI-0 is 20.7 wt % (or 28 wt % of ZrO2). This amount corresponds to one pillar per 2.3 unit cells. If we suppose that all the sodium ions have been exchanged with hydroxy-zirconium cations, the positive charge per intercalated Zr4-polycation should be close to 1.4 to compensate the negative layer charge. Former studies of Zr-pillared clays have reported very different amounts of intercalated Zr, depending on the host clay and the pillaring method used. In the classical work of Yamanaka and Brindly, the amount of zirconium fixed was about 13.9 wt % of ZrO2 [12]. The intercalation reaction was carried out at room temperature in this case. The higher amount of fixed zirconium in our work is probably do to the temperature of intercalation ; in fact, an amount of fixed Zr closer to our value has been reported by Farfan-Torres et al., where the suspension was stirred at 40 ~ during the intercalation [ 13]. When the SO4:Zr ratio increases, the amount of fixed zirconium increases (table 1). This trend is not surprising if sulfate anions are cointercalated, since the Zr-containing polycations would have to compensate for their negative charge as well. Table 1 also shows the S:Zr molar ratios in the intercalated clays as a function of the SO4:Zr ratio in solution. For ZrI-0.1 and ZrI-0.15 we find the same ratios in the intercalated solids as in the intercalating solutions : apparently, most of the sulfate is bonded to the Zr polycations in the solution. On the other hand, for the higher SO4:Zr ratio, part of the sulfate seems to remain free in solution, in agreement with the Raman data. Table 1. Quantities of zirconium and sulfur fixed on intercalation of saponite for different SO4:Zr ratio. SO4:Zr molar ratio in solution wc % Zr wt. % S S:Zr molar ratio in solid 0

20.7

< 0.1

0

0.1

21.2

0.65

0.09

0.15

22.3

1.15

0.15

0.2

22.5

1.25

0.16

0.3

26.8

2.27

0.24

906

1012 0 9 9 9 ~d

430

I

987'

. ,...q r~

~ _ _ . . . . . ~ ' "

300

'I " '

'I "

"l

"

"

I"

"

I' " '

I' "

400 500 600 Wavelenght, crn1

Figure 1. Raman spectrum of ZrOC12 solution with SO4:Zr ratio = 0.2.

'''

850

'1''''1

l

b

~_.

''''l''

.....

''l''"'

.

._

.a

I''''1'"''

950 1050 1150 Wavelength, cni s

Figure 2. Raman spectra of (NH4)2SO4 solution (a) and ZrOC12 solution with SO4:Zr ratio = 0.2 (b).

3.3. X-Ray diffraction and N2 physisorption The XRD pattems of the intercalated samples are shown in figure 3. In the case of ZrI-0, two main peaks are observed at 20.2 A and 11.4 A. Those peaks could correspond to the first and second order, respectively, of the (001) family of planes ; the difference between the apparent d001 and 2d002 could be explained by interstratification of intercalated and unintercalated interlayers. Similar results are reported by Yamanaka and Brindly [12] who suggest that the intercalated species is [Zr4(OH)16_n(H20)8+n] n+. When (SO4)2" is added to the solution of intercalation, the d001 and d002 peaks shift to higher theta values, their intensity decreases and their width increases. Thus, the higher the amount of fixed sulfate, the lower the cristallinity. After calcination at 400~ the ZrP-0 shows a d001 basal spacing at 19.4 A (figure 4-a), slightly lower than after intercalation. This is probably due to the departure of the hydration water of polycations. This could be sufficient to induce some loss of crystallinity, but it is likely that the pillars mostly retain their structure upon moderate heating, as happen for aluminum intercalated clays [14]. For those Al-clays, [All3] polycations release proton from some of terminal (H20) ligands but the rest of the polycation structure is not much affected [15]. Indeed, Mieh6-Brendl6 et al. [ 16] have shown by EXAFS that the nearly square frame of Zr4 zirconyl units is preserved after calcination at moderate temperatures. For samples prepared with sulfates (figure 4-b to 4-e) the dool peaks are broad after calcination, indicative of a loss of structural organization.

907

20.2/k

11.4.&

.9]k .;.%:..----;

a :

--.,

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---

.

L _ / ~ .gA f4i" 10.7A L % ~ " : - : -

I'"l'"l'"t"'l" 2

6

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18

2O Figure 3. XRD patterns of ZrI-r: (a) r = 0, (b) r = 0.1, (c) r = 0.15, (d) r = 0.2 and (e) r = 0.3.

2

6

10

14

18

20 Figure 4. XRD patterns of ZrP-r: (a) r = 0, (b) r = 0.1, (c) r = 0.15, (d) r = 0.2 and (e) r = 0.3.

Table 2. BET surface areas of pillared saponite for different SO4:Zr ratio. SO4:Zr molar ratio in solutioz Surface area (m2/g) Before calcination After calcination 0 178 191 0.1 160 154 0.15 155 157 0.2 139 153 0.3 59 70

Table 2 shows the BET surface areas of intercalated and pillared saponites. When the quantity of sulfate increases, the surface area decreases but remains rather high except for the sample prepared with the highest SO4:Zr ratio where the decrease is more drastic. The surface area does not change much upon calcination. All of these data indicate successful intercalation of Zr polycations, at least for SO4:Zr ratios lower than 0.3 which have both high interlayer distances (9 to 10 ]k) and high surface areas (between 160 and 139 m2/g). The state of the intercalated sulfate cannot be ascertained from textural data alone, but a preliminary comment can be made. No significant difference in d001 is apparent as a function of the SO4:Zr ratio in the solution, suggesting that the intercalated species all have approximately the same gyration radius. This is not unlikely since the solid contain less than one sulfate ion per Zr4 polycation on average 9 therefore, the most likely intercalated species should consist in sulfate-free polycations, and polycations with at least one sulfate attached. The calcination affect differently those species that induces

908 a deterioration of the XRD pattern but surface areas remain high: the layers do not collapse at 400 ~ For a SO4:Zr ratio = 0.3, on the other hand, the low surface area together with a high interlayer distance (about 9 A before calcination) could be explained by extensive intercalation of bulkier species, clogging up the interlayer space. 3.4. Thermal stability of sulfate Thermal treatment of the ZrI-0.1 sample under an inert gas (helium) between 50 and 1000 ~ resulted in MS peaks corresponding to the masses : 16 (NH2 +, O+), 17 (OH+), 18 (H2 O+ and NH4+), 32 (O2+), 48 (SO +) and 64 (SO2+). Figure 5 shows the evolution of the peaks at 16, 17 and 18 a.m.u., which exhibit similar patterns for all pillared clays samples. The departure of a large quantity of physisorbed water is observed at 100 ~ as expected. These peaks show a secondary maximum at 800~ corresponding to the condensation of hydroxy groups of the clay octahedral sheets. Water evolution only stops at very high temperatures (> 900~ Featureless H20 loss between 300 and 700~ can arise from dehydration and dehydroxylation processes of the zirconiumcontaining pillars. The evolution with temperature of MS peaks at 48 a.m.u. (SO +) and 64 a.m.u. (SO:z+) is shown in figure 6 for the ZrI-0.1 sample. The elimination of sulfur-containing compounds only occurs above 450~ proving the high thermal stability of sulfate species. Two separate thermal events can be observed : the first one has a maximum at 550~ and the second one at 750 ~ This profile could be explained by the existence of two interaction modes between (SO4) 2- ions and the zirconium tetramers. Figure 7 compares the profiles of the signal at 64 a.m.u. (SO2 +) for the samples prepared with solutions with different SO4:Zr ratios (0.1, 0.15, 0.2 and 0.3). As the amount of sulfate increases, the relative intensity of the second peak (at 750 ~ with respect to the first one also increases. Sample ZrI-0.3 shows a more complex phenomenon (figure 7-d) where the two peaks are larger and hardly distinguishable.

_t

~'''l'''l'''l"'l' 100 300 500 700 900 Temperature (~ Figure 5. Profile of the MS signals at 18 a.m.u. (a), 17 a.m.u (b) and 16 a.m.u. (c) upon heating the ZrI-0.1 sample.

200

400 600 800 Temperature (~

1000

Figure 6. Profile of the MS signals at 64 a.m.u. (a) and 48 a.m.u (b) upon heating the ZrI-0.1 sample.

909

400

500

600 700 Temperature (~

800

Figure 7 : Profile of the MS signals at 64 a.m.u, for ZrI-r: (a) r = 0.1, (b) r = 0.15, (c) r = 0.2, and (d) r = 0.3. At this point of the discussion, we may try to put forward some hypotheses on the nature of the intercalated species, although they will remain temptative due to the lack of quantitative data and the complexity of these systems [17]. In solution, the zirconium tetrameric polycation (figure 8, species a) seems to exist in equilibrium with a sulfated tetramer. Raman spectroscopy and mass spectroscopy reveal the presence of two modes of interaction between sulfate ions and zirconium tetramers. It is possible to present some simple ideas based on our observations. These two modes of interaction may correspond to (SO4) 2- linked to one tetramer (figure 8, species b) and (SO4) 2- bridging two tetramers (figure 8, species c), respectively. Zr~

Zr~ j Zr ~ , . ,

j Zr

,.,/Zr~

j Zr

/ / Zr-v / / ~ ZJ~ . .Zr Z. .r.-[- 0\0/S~ 0~ r/Z ~~ / / ~Zr\ 0 U / S \ / t o O ~ ! ~ r ~ / / ~ ~ Zr

~~.~ Zr Zr4 tetramer

zU

Zr.

0 /~Z~~

Oz Z .zr

j

z

o,,

Zr

o

II

Z

r- O - S - 0G Z

r-O--

O--

r

0 Z (a)

(b)

(c)

Figure 8. Schematic view of different species existing in a zirconyl solution with a low SO4:Zr ratio.

910 4. CONCLUSION The intercalation of saponite clays with sulfated zirconium tetramer has been clearly evidenced, even though the chemical nature of the intercalating species remains somewhat hypothetical. Adding mixed SO4:Zr solutions to a saponite suspension gives an intercalated clay with a surface area of about 150 m2/g and a d001 peak around 18 A. At this point, the interlayer contains heterogeneous species : non-sulfated Zr tetramers, and sulfated polymers with two different modes of sulfate/polycation binding. After calcination at 400~ the surface area is still important but a loss of cristallinity is observed, probably because the sulfated and the non-sulfated zirconium species react differently up calcination. The sulfated species in these solids are not eliminated before at least 450~ ; the two modes of sulfate/polycation binding have significantly different stabilities. High SO4:Zr ratios should be avoided, because they result in the intercalation of bulky species, giving solids with a low surface area. In this paper, we have established a correlation between the properties of the intercalant solution and the textural properties of the resultant solids. The description of the acid sites needs more techniques (FTIR, model catalytic reaction). After calcination of the solid, the interaction between sulfate ions and zirconium polycations changes and the correlation with study of the solution will not be perhaps evident. References

1. D.E.W. Vaughan, Catal.Today, 2 (1988) 187. 2. F. Figueras, Catal. Rev. Sci. Eng., 30 (1988) 457. 3. M. L. Occelli, Physicochemical proporties of pillared clay catalysts. In: Keynotes in Energy-Related Catalysis. S. Kaliaguine (eds.), Elsevier, Amesterdam. Stud. Surf. Sci. Catal., 35 (1988) 101. 4. I. V. Michell, Pillared Layered Structure: Current Trends and Applications. Elsevier, London, (1990). 5. L. Bergaoui, I. Mrad, J.-F. Lambert and A. Ghorbel, J. phys. Chem. B, 103 (1999) 2897. 6. J. -F. Lambert and G. Poncelet, Top. Catal. 4 (1997) 43. 7. A. Clearfield and P. Vaughan, Acta Crystallogr. 9 (1956) 555. 8. G. D. Yadave and J. J. Nair, Microporous Mesoporous Materials, 33 (1999) 1. 9. E. M. Farfan-Torres, E. Sham and P. Grange, Catal.Today, 15 (1992) 515. 10. J. L. Post, Clays Clay Miner., 32 (1984) 147. 11. S. Hannane, F. Bertin and J. Bouix, Bull. Soc. Chim. Fr., 127 (1990) 43. 12. S. Yamanaka and G. W. Brindly, Clays Clay Miner, 27 (1979) 119. 13. E. M. Farfan-Torres, O. Dedeykcker and P. Grange, Preparation Of Catalysts V, G.Poncelet, P. A. Jacobs, P. Grange and B. Delmon, (eds.), Elsevier, Amsterdam, (1990) 337. 14. J.-F. Lambert, S Chevalier, R. Franck, H. Suquet and D. Barthomeuf, J. Chem. Soc., Faraday Trans. 90 (1994) 675. 15. L. Bergaoui, J.-F. Lambert, R. Franck and H. Suquet, J. Chem. Soc., Faraday Trans. 91 (1995) 229. 16. J. Mieh6-Brendl6, L. Khouchaf, J. Baron, R. Le Dred and M. -H. Tuilier, Microporous Materials, 11 (1997) 171. 17. A. Clearfield, Rev. Pur. App. Chem, 14 (1964) 91.