Small-pore framework zirconium and hafnium silicates with the structure of mineral tumchaite

Microporous and Mesoporous Materials 76 (2004) 99–104 www.elsevier.com/locate/micromeso

Small-pore framework zirconium and hafnium silicates with the structure of mineral tumchaite Zhi Lin *, Joa˜o Rocha Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Received 20 May 2004; received in revised form 29 July 2004; accepted 31 July 2004 Available online 7 October 2004

Abstract Small-pore framework sodium silicates (AV-14) Na2MSi4O11 Æ 2H2O (M = Zr and Hf) possessing the structure of the rare mineral tumchaite have been prepared via hydrothermal synthesis. The AV-14 framework consists of corner-sharing MO6 (M = Zr, Hf) octahedra and SiO4 tetrahedra. The [Si4O11] silicate sheets may be described as a combination of the spiral chains formed from the corner-sharing SiO4 tetrahedra. These sheets are connected by MO6 octahedra, resulting in a three dimensional framework. AV-14 materials are the first examples of synthetic microporous zirconium and hafnium silicates whose structures are built up from silicate sheets. Hf- and Zr-AV-14 materials and the parakeldyshite-type phase, which forms at 1100 °C from the decomposition of Zr-AV-14, have been studied by bulk chemical analysis, powder X-ray diffraction, scanning electron microscopy, 23Na and 29Si magic-angle spinning NMR spectroscopy and thermogravimetry. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Microporous; Silicates; Zirconium silicates; Hafnium silicates; Synthesis

1. Introduction Microporous materials have many potential applications, for example, in catalysis, ion exchange and gas adsorption and separation. The synthesis of new microporous silicates with mixed tetrahedral–pentahedral– octahedral frameworks has broadened the scope of applications of zeolite-type materials [1], the most prominent example being titanosilicate ETS-10 [2]. Zirconium silicates occur widely in nature and their synthesis has been accomplished via hydrothermal and high temperature solid-state methods. More than 20 natural occurring or synthetic such materials are known [3]. The hydrothermal syntheses of synthetic analogues of zirconosilicate minerals such as umbite (AM-2) [4–6], kostylevite (AV-8) [7], gaidonnayite (AV-4) [4,5], petar*

Corresponding author. E-mail addresses: [email protected] (Z. Lin), [email protected] (J. Rocha). 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.07.037

asite (AV-3) [4] and elpidite [5] have been reported recently. All these solids possess structures with silicate rings or chains interconnected by ZrO6 octahedra. Penkvilksite is a microporous sodium titanosilicate mineral with the ideal formula Na4Ti2Si8O22 Æ 4H2O [8], occurring in nature in two polytypic modifications, orthorhombic (penkvilksite-2O) and monoclinic (penkvilksite-1M). Polytypes 2O and 1M contain the same building blocks and have the same atoms, labeled in the same way, in the asymmetric unit. They differ only in the way the building blocks are stacked. The synthetic analogues of both penkvilksite polytypes have been reported and are known as AM-3 materials [9,10]. Interestingly, among zirconium silicates it seems that only the 1M polytype occurs in nature, in the Vuoriyarvi alkali-ultrabasic massif, Murmansk region, (Russia), known as tumchaite (ideal formula Na2ZrSi4O11 Æ 2H2O) [11]. Here, we wish to report the hydrothermal synthesis and structural characterisation of Zr and Hf synthetic analogues of mineral tumchaite, which we

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have named Zr(Hf)-AV-14 (Aveiro microporous material no. 14). Despite our best efforts, so far we were unable to prepare AV-14 in the form of polytype 2O, even if the titanium analogue, AM-3(2O) is easily prepared in the laboratory. However, it is likely that AV-14 samples (polytype 1M) reported here are contaminated with small amounts (<10%) of polytype 2O.

a recycle delay of 2 s. Chemical shifts are quoted in ppm from 1 M aqueous NaCl. Thermogravimetry curves (TGA) were measured with a TG-50 Shimadzu analyser. The samples were heated in air with a rate of 5 °C/min.

3. Results and discussion 2. Experimental 2.1. Typical AV-14 synthesis An alkaline solution was made by dissolving 20.25 g of sodium silicate solution (27% m/m SiO2, 8% m/m Na2O, Merck), 3.11 g NaOH (Merck), 3.61 g NaCl (Panreac) and 1.77 g KCl (Merck) into 23.69 g H2O. 4.21 g ZrCl4 (Riedel-de Ra¨en) was added to the alkaline solution with stirring thoroughly. 0.20 g AM-3(2O) seeds were added to this mixture. Without seeding the gel with this material or with AV-14 crystals (once the solid is available from a previous synthesis) we were unable to obtain AV-14 materials. This gel, with a molar composition 5.4 Na2O : 0.7 K2O : 5.2 SiO2 : 1.0 ZrO2 : 116 H2O, was transferred to a Teflon-lined autoclave and treated at 230 °C for 4–31 days under autogenous pressure without agitation. Hf-AV-14 analogues were obtained by using 5.78 g HfCl4, in place of the Zr source, and allowing 21 days at 230 °C. 0.45 g Zr-AV-14 seeds were used in the synthesis of Hf-AV-14. The product was filtered off, washed at room temperature with distilled water, and dried at 70 °C overnight, the final product being an off-white microcrystalline powder. The as-synthesized Zr-AV-14 materials were calcined under air with a heating rate of 10 °C/min from room temperature to up to 1200 °C and held for 5 h at the final temperature. 2.2. Materials characterisation Powder X-ray diffraction (XRD) was performed between 5° and 50° 2h on a Philips Xpert MPD diffractometer using Cu Ka radiation. The unit cell parameters were refined with the programs PowderX [12] and Powder Cell (PCW) [13]. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectrometry (EDS) were performed on a Hitachi S-4100 microscope. 23 Na and 29Si magic-angle spinning (MAS) NMR spectra were recorded at 105.85 and 79.49 MHz, respectively, on a Bruker Avance 400 (9.4 T, wide-bore) spectrometer. Before measurements, the samples were fully hydrated. 29Si MAS NMR spectra were recorded with 40° pulses, a spinning rate of 5.0 kHz and 60 s recycle delays. Chemical shifts are quoted in ppm from TMS. 23 Na MAS NMR spectra were measured using short and powerful radiofrequency pulses (0.6 ls, equivalent to a 15° pulse angle), a spinning rate of 32 kHz and

Fig. 1 displays the SEM images of an Hf-AV-14 sample and Zr-AV-14 samples obtained after different synthesis time. AV-14 crystallizes as plates, a habit similar to that of penkvilksite-1M crystals [9]. Zr-AV-14 samples crystallized for 14 or more days exhibit similar habits. Powder XRD reveals that after four days of synthesis, samples are amorphous with small amounts of AM-3 seeds, while samples reacted for seven days contain a poorly crystalline AV-14 phase with a significant amount of amorphous material. The powder XRD patterns of Zr-AV-14 reacted for 21 and 31 days are identical. These patterns and the calculated tumchaite pattern are very similar. The ideal formula of tumchaite is Na2ZrSi4O11 Æ 2H2O. In fair agreement, EDS analysis yielded the following molar ratios: Si/Zr, Na/Zr 3.8 and 1.7 for Zr-AV-14, and Si/Hf, Na/Hf 4.3 and 2.1 for Hf-AV-14, respectively. The unit cell parameters of Zr-AV-14 have been calculated with PowderX using 18 well-resolved lines from powder XRD pattern and the ˚, cell dimensions are: a = 9.152, b = 8.831, c = 7.555 A b = 113.29°, which are very similar to those reported ˚, b= for tumchaite a = 9.144, b = 8.818, c = 7.537 A 113.22° [11]. Using these unit cell parameters and the atomic coordinates of tumchaite, and assuming a monoclinic unit cell, space group P21/c, the powder XRD pattern was simulated by PCW. The experimental and simulated powder XRD patterns of Zr-AV-14 (Fig. 2a and b) are identical, except for the faint peaks at 15.42°, 18.72°, 26.32° and 34.37° 2h, which are absent from the pattern of the sample synthesized from AV14 seeds. These extra reflections may be ascribed to polytype 2O. To test this possibility, the powder XRD pattern of the hypothetical Zr-AV-14 polytype 2O was simulated with program PCW. Assuming lattice para˚ , most polytypes meters a = 16.80, b = 8.85, c = 7.50 A 2O and 1M peaks overlap, except for the four peaks observed at 15.47°, 18.75°, 26.30° and 34.34° 2h (tick marks in Fig. 2b). In principle, the four extra reflections could also be due to AM-3(2O) seeds. However, this is unlikely because: (i) only a minor amount of seeds (2.3 wt%) were added to the synthesis gel; (ii) the AM-3(2O) lattice parameters are different (a = 16.372, ˚ [10]). b = 8.749, c = 7.402 A The unit cell parameters of Hf-AV-14 (space group P21/c) were refined yielding: a = 9.135, b = 8.813, ˚ , b = 113.27°. The slight shrinkage in cell c = 7.536 A dimensions relatively to Zr-AV-14 has also been

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Fig. 1. SEM images of Zr-AV-14 samples obtained after (a) 7, (b) 14, (c) 31 days of synthesis and (d) Hf-AV-14 sample.

d

c

b

a

9

19

29

39

2 θ/ ° Fig. 2. Simulated (a,c) and experimental (b,d) powder XRD patterns of Zr-AV-14 (a,b) and Hf-AV-14 (c,d). The tick marks indicate the extra peaks calculated for polytype 2O.

observed previously, for example for AV-13 [Na2+xZrSi3O9Clx Æ 2.5H2O (M = Zr, Hf, Sn)] and umbite materials [14,15]. The experimental and simulated powder XRD patterns of Hf-AV-14 (Fig. 2c and d) are identical except for three small peaks at approximately 12.33°,

17.88° and 19.28° 2h, which may result from a poorly crystalline impurity phase with petarasite structure [4]. In the structure of AV-14 materials and tumchaite (Fig. 3), the wave-like silicate sheets [Si4O11], which are connected by MO6 (M = Zr, Hf) octahedra, are parallel to (1 0 0). The silicate sheets form when adjacent spiral chains fuse by sharing two SiO4 tetrahedra. Adjacent spiral chains are orientated in alternate clockwise and counter-clockwise fashion. The spiral chains of corner-sharing tetrahedra develop along [0 1 0] with a periodicity of six tetrahedral units, only two of which are crystallographically non-equivalent. The narrow, sixmember ring channels in AV-14 run along the [0 1 0] and [0 0 1] directions and house sodium atoms and the water species. 29 Si and 23Na solid-state NMR data support the structure proposed for AV-14 materials. The Zr-AV-14 29 Si MAS NMR spectrum (Fig. 4a) displays two main resonances at 94.9 and 101.8 ppm with equal intensity (the small peak at approximately 100.7 ppm is given by an unknown impurity). The crystal structure of tumchaite calls for the presence of two unique Si sites with equal populations. Si1 connects to two Zr and two Si via oxygens [Si(2Si, 2Zr)], while Si2 links to one Zr and three Si via oxygens [Si(3Si, 1Zr)]. In general, Si(2Si, 2Zr) 29Si MAS NMR resonances are shifted to high frequency relatively to the Si(3Si, 1Zr) peaks. Previously reported zirconium silicates with Si(2Si, 2Zr) chemical environments resonate in the range 86 to 95 ppm [4]. Therefore, we assign the Zr-AV-14 peaks at 94.9 and 101.8 ppm to the Si(2Si, 2Zr) and

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material of any seeds added to the synthesis gel. Moreover, AM-3(1M) has been reported to give two resonances at 95.7 and 101.1 ppm [9]. We conclude that it may not be possible to distinguish one polytype from the other on the basis of 29Si MAS NMR spectroscopy. The Hf-AV-14 29Si MAS NMR spectrum (Fig. 4b) displays two resonances at 91.6 and 100.4 ppm with equal intensity, which are assigned to Si(2Si, 2Hf) and Si(3Si, 1Hf) chemical environments. The two weak resonances at approximately 94.8 and 101.8 ppm are due to Zr-AV-14 seeds. The faint peak at approximately 91.2 ppm is given by a small amount of unknown impurity. Few NMR studies are available on microporous hafnium silicates. For example, the Si(2Si, 2Hf) 29Si MAS NMR peaks of Hf-AV-12 [15] and Hf-AV-13 [14] shift 2–3 ppm to higher frequency relatively to the Si(2Si, 2Zr) sites of the zirconium analogues. The 23Na MAS NMR spectra of Zr- and Hf-AV-14 materials (Fig. 5b and d) display characteristic secondorder quadrupole powder patterns given by the single non-framework cation site. Simulation of these patterns [16] (Fig. 5a and c) yields the following parameters for Zr-AV-14 and Hf-AV-14, respectively: quadrupole coupling constant, CQ, 2.74 and 2.76 MHz, asymmetry parameter, g, 0.31 and isotropic chemical shift, diso, 3.2 and 3.5 ppm. For comparison, the following parameters have been reported for AM-3(2O): CQ = 3.35, g = 0.51, diso = 4.8 ppm. Thus, although of the same order, the quadrupole parameters and isotropic chemi-

b a a c

Fig. 3. Projections of the structure of AV-14 along the [0 0 1] and [0 1 0] directions.

d b

c 17

0

-17 δ (ppm)

-34

-51

a

-86

Fig. 4.

-90

-94

-98

δ (ppm)

-102

-106

-110

29

Si MAS NMR spectra of Zr-AV-14 (a) and Hf-AV-14 (b).

b a 17

Si(3Si, 1Zr) chemical environments, respectively. AM3(2O) resonates at 95.8 and 100.1 ppm [10] and, thus, NMR does not detect the presence in the final Zr-AV-14

0

-17

-34

-51

δ (ppm) Fig. 5. Simulated (a,c) and experimental (b,d) spectra of Zr-AV-14 (a,b) and Hf-AV-14 (c,d).

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Na MAS NMR

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cal shifts of AM-3(2O) and Zr-, Hf-AV-14(1M) materials are significantly different. It is not clear whether this is due to the presence of a different metal in the framework or to the fact that two different polytypes are being studied. If the latter hypothesis holds, then the quadrupole interaction provides an interesting tool for studying closely related polytype structures. TGA (Fig. 6) provides further evidence that the structures of AV-14 materials and tumchaite are very similar. The total Zr-AV-14 mass loss (ascertained by thermogravimetry) between 30 and 800 °C (Fig. 6a) is about 8.0 wt% and is due to the release of molecular water. Ideally, the water loss of tumchaite is 7.8 wt%. The TGA curve of Fig. 6b and the powder XRD pattern of ZrAV-14 rehydrated for 24 h after dehydration at 550 °C show an almost reversible water loss (7.5 wt%) for the material. Like AM-3(1M) materials, most water is lost below 200 °C, [9]. The small differences observed between TGA curves a and b may result from adsorption of extra water or silanol groups on defect sites and at the external surface of the crystallites. The total Hf-AV-14 mass loss between 30 and 800 °C (Fig. 6c) is about 7.5 wt%. A first stage of dehydration is observed between 25 and 450 °C, with a mass loss of 7.1 wt%, due to the release of molecular water, structural or adsorbed. This corresponds to 2.2 water molecules and is in excess of the two water molecules revealed by the crystal structure analysis. Thus, the solid contains adsorbed water. Between 450 and 800 °C a second dehydration step is seen (mass loss of 0.4 wt%), probably due

100 Hf

Mass loss (wt %)

c

d

103

to the loss of silanol groups on defect sites. The TGA curve of Fig. 6d and powder XRD pattern of Hf-AV14 rehydrated for 24 h after dehydration at 550 °C show that the water loss (6.6 wt%) is almost reversible. However, the second weight loss step is not observed. AM-3(2O) is stable up to 600 °C, being completely amorphous at 700 °C and re-crystallising at 800 °C as synthetic narsarsukite. AM-3(1M) is also stable only up to 600 °C and converts to a dense phase at 700 °C [9]. In order to study its thermal stability, Zr-AV-14 was treated at temperatures up to 1200 °C. Powder XRD (Fig. 7) reveals that the framework of Zr-AV-14 is stable up to approximately 800 °C and converts to an amorphous phase at 900 °C. A sample calcined at 1000 °C for 5 h contains mainly amorphous material and some new reflections from an unknown phase. Heating the sample at 1100–1200 °C for 5 h reveals a poorly crystalline phase and some amorphous material. Although the chemical formulas of Zr-AV-14 and mineral vlasovite are related, this is not the phase which re-crystallizes from amorphous material. The tick marks in Fig. 7 indicate the reflections from sodium zirconium silicate mineral parakeldyshite, Na2ZrSi2O7, (PDF card: 39–209). This seems to be the new phase formed. However, the 29Si MAS NMR spectrum of the sample heat treated at 1100 °C for 5 h displays only a very broad signal centred at 96 ppm, a typical spectrum of amorphous siliceous material (not shown). The ideal formula of parakeldyshite is Na2ZrSi2O7. Since ZrO2 already start to crystallize at this temperature, and assuming the remaining zirconium atoms are in the parakeldyshite crystallites, more than 50% of silicon is in amorphous or secondary siliceous phase. Therefore, parakeldyshite crystallization is accompanied by the presence of a significant amount of siliceous material. Because this synthetic parakeldyshite is still poorly crystallized it is not easy to be detected by NMR. In conclusion, small-pore zirconium and hafnium sodium silicate AV-14 materials, possessing the structure

92 100 Zr 1200 ºC 1000 ºC 900 ºC

a

b 800 ºC

As-prepared

90

5 25

175

325 475 Temperature (ºC)

625

775

Fig. 6. TGA of as-synthesized AV-14 (a,c); rehydrated sample after dehydration at 550 °C for 4 h (b,d).

15

25

35

2θ / º Fig. 7. Powder XRD patterns of Zr-AV-14 calcined at the temperatures indicated. The tick marks indicate the reflections from parakeldyshite (PDF card: 39–209).

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of rare mineral tumchaite, were successfully synthesized under hydrothermal conditions. This is the first report of synthetic microporous zirconium and hafnium silicates whose structures are built up from silicate sheets. Zr-AV-14 structure is stable up to 800 °C and a parakeldyshite-type phase begins to crystallize at 1100 °C. Acknowledgments This work was supported by FCT, POCTI and FEDER. References [1] J. Rocha, M.W. Anderson, Eur. J. Inorg. Chem. (2000) 801. [2] M.W. Anderson, O. Terasaki, T. Ohsuma, A. Philippou, S.P. MacKay, A. Ferreira, J. Rocha, S. Lidin, Nature 367 (1994) 347. [3] A.I. Nortun, L.N. Bortun, A. Clearfield, Chem. Mater. 9 (1997) 1854. [4] Z. Lin, J. Rocha, P. Ferreira, A. Thursfield, J.R. Agger, M.W. Anderson, J. Phys. Chem. B 103 (1999) 957. [5] S.R. Jale, A. Ojo, F.R. Fitch, Chem. Commun. (1999) 411.

[6] D.M. Poojary, A.I. Bartun, L.N. Bartun, A. Clearfield, Inorg. Chem. 36 (1997) 3072. [7] P. Ferreira, A. Ferreira, J. Rocha, M.R. Rosario, Chem. Mater. 13 (2000) 355. [8] S. Merlino, M. Pasero, G. Artioli, A.P. Khomyakov, Am. Miner. 79 (1994) 1185. [9] Y. Liu, H. Du, Y. Xu, H. Ding, W. Pang, Y. Yue, Micropor. Mesopor. Mater. 28 (1999) 511. [10] Z. Lin, J. Rocha, P. Branda˜o, A. Ferreira, A.P. Esculcas, J.D. Pedrosa de Jesus, A. Philippou, M.W. Anderson, J. Phys. Chem. B 101 (1997) 7114. [11] V.V. Subbotin, S. Merlino, D.Yu. Pushcharovsky, Y.A. Pakhomovsky, O. Ferro, A.N. Bogdanova, A.V. Voloshin, N.V. Sorokhtina, N.V. Zubkova, Am. Miner. 85 (2000) 1516. [12] C. Dong, J. Appl. Cryst. 32 (1999) 838. [13] W. Kraus, G. Nolze, Federal Institute for Materials Research and Testing, Rudower Chaussee 5, 12489, Berlin, Germany. [14] A. Ferreira, Z. Lin, M.R. Soares, J. Rocha, Inorg. Chim. Acta 356 (2003) 19. [15] Z. Lin, J. Rocha, Impact of zeolites and other porous materials on the new technologies at the beginning of the new millennium, in: R. Aiello, G. Giordano, F. Testa (Eds.), Studies in Surface Science and Catalysis, vol. 142, Elsevier, Amsterdam, 2002, p. 319. [16] D. Massiot, F. Fayon, M. Capron, I. King, S. Le Calve´, B. Alonso, J.-O. Durand, B. Bujoli, Z. Gan, G. Hoatson, Magn. Reson. Chem. 40 (2002) 70.