The first example of a small-pore framework hafnium silicate

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

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The First Example of a Small-pore F r a m e w o r k Hafnium Silicate Zhi Lin and Jo~o Rocha* Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal

The first example of a mieroporous framework potassium hafnium silicate (AV-12, Aveiro microporous solid no.12) with a known structure is reported. AV-12 and the rare potassium zirconium silicate mineral umbite possess similar structures, composed of comer sharing MO6 octahedra and SiO4 tetrahedra forming a three-dimensional framework with eightmembered ring channels. The material has been characterized by powder X-ray diffraction, scanning electron microscopy, 29Si magic-angle spinning NMR spectroscopy, FT Raman and infrared spectroscopies and thermogravimetry.

1. I N T R O D U C T I O N In recent years, there has been an upsurge of interest in the synthesis and characterization of novel titanosilicates and their zirconium and tin analogues. TM These materials possess novel structures and display important physical and chemical properties. The isomorphous substitution of various metal ions into crystalline inorganic frameworks has been a very active area of research in the last two decades, allowing the fine tuning of properties of a given material. Although hafnium and zirconium ions have similar radii, it is of interest to prepare microporous hafnium silicates because the pore sizes and ion-exchange and adsorption properties (among others) of the materials may be slightly different. However, no research is presently available on the synthesis of such materials. In the course of a systematic study, we have obtained (to the best of our knowledge) the first example of a microporous hafnium silicate. AV-12 possesses the structure of the rare zirconium silicate mineral umbite. Umbite occurs in the Khibiny alkaline massif (Russia) and has ideal formula K2ZrSi309.H20. 5 Titanium and tin analogues of umbite have been prepared in the laboratory. 3'6'7 Here, we wish to report preliminary results of the synthesis and characterization of AV-12 by powder XRD, SEM, 29Si MAS NMR, TGA, FTIR and Raman. The natural occurrence of a hafnium silicate analogue of umbite is unknown. * Corresponding author. E-mail: [email protected]; Fax: 351 34 370084

320 2. E X P E R I M E N T A L

2.1. Synthesis Typical A V-12 synthesis. An alkaline solution was made by dissolving 1.50 g of precipitated silica (93 m/m%, Riedel-deHa6n) and 5.71 g KOH (85 m/m%, Aldrich) into 16.04 g n20. 2.55 g nf~14 (98 m/m%, Aldrich) was added to the alkaline solution while stirring thoroughly. This gel, with a molar composition 5.5 K20 : 3.0 SiO2 : 1.0 Hff)2 : 120 1-120, was transferred to a Teflon-lined autoclave and treated at 230 ~ for 1-4 days under autogenous pressure without agitation. The sample synthesized at 230~ for 1 day contains significant amount of amorphous materials. The crystalline product was filtered off, washed at room temperature with distilled water, and dried at 70 ~ overnight, the final product being an offwhite microcrystaUine powder.

2.2. Techniques Powder X-ray diffraction (XRD) was performed between 5-50~ in 20 on a Philips X'pert MPD diffractometer using CuKt~ radiation. The unit cell parameters were refined with programs PowderX s and Powder Cell. 9 Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analysis were carried out on a Hitachi S-4100 microscope. Thermogravimetry curves (TGA) were measured with a TG-50 Shimadzu analyser. The samples were heated in air with a rate of 5 ~ min1. 29Si MAS NMR spectra were recorded on a Bruker Avance 400 spectrometer. Samples were spun at the magic-angle in double-bearing zirconia rotors. Spectra were recorded at 79.49 MHz using 40 ~ radiofrequency pulses, interpulse delays of 60 s and spinning rates of 5 kHz. Before measurement, the samples were fully hydrated. Chemical shifts are quoted in ppm from tetramethylsilane (TMS). Fourier transformed infrared (FTIR) transmission spectra were measured on a Mattson 7000 FTIR spectrometer in the range of 400-4000 cm1 using a KBr wafer, resolution 1 cm1 and 32 scans. Fourier transform Raman spectra were measured on a Bruker RFS 100/S spectrometer in the range of 50-3600 cm"1 using a Nd : YAG laser (1064 nm), resolution 4 cm 1 and 200 scans.

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

The SEM micrograph in Figure 1 shows that AV-12 consists of plate crystals ca. 5x5xl.5 l.tm. The powder XRD patterns of this material (Figure 2) and the previously reported potassium zirconium silicate umbite are very similar.2 Within experimental error, chemical analysis by EDS indicates a K/Hf ratio of 2. Because of Hf and Si peak overlap in

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Figure 1. SEM image of AV-12.

EDS spectra, the Si/Hf ratio is difficult to estimate. On the other hand, the yield calculated according to the formula K2HfSi309 is nearly 100% based on the Hf source. The following AV-12 unit cell parameters have been calculated using the first 20 wellresolved powder X R reflections and program PowderX: a = 13.2665, b = 10.2333, c = 7.1726 A, with reasonable figures of merit (M20 = 14, F20 = 22 and M30 = 10, F30 = 19). Using these unit cell parameters, the atomic coordinates and space group (P212121) of mineral umbite, 5 the powder XRD pattern of AV-12 material has been simulated with program Powder

Experimental

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Figure 2. Experimental and simulated powder XRD patterns of AV-12.

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322 Cell. The axes a and b were exchanged in agreement with the unit cell ofumbite. The unit cell parameters have been refined between 10 and 50~ 20: a = 10.2768, b = 13.3084, c = 7.1999 A, similar to the cell parameters of mineral umbite 5 (a = 10.207, b = 13.241, c = 7.174 A). The experimental and simulated powder XRD patterns of AV-12, shown in Figure 2, are in good accord. Figure 3 shows polyhedral representations of the umbite 5 and, hence, AV-12 structure viewed along [001] and [100]. In this structure the metal M octahedra, MO6, and the T tetrahedra, SiO4, form a three-dimensional MT-condensed framework. The M oetahedron is coordinated by six T. In addition to the M-O-T bonds, these tetrahedra form also T-O-T links with each other. The resulting T radical has an identity period of three T tetrahedra and forms infinite chains, along the c axis, which are connected by M06 octahedra. Three infinite chains connect one, two and three Si04 tetrahedra, respectively, to a M06 octahedron. Channels form along the c axis with eight-membered tings comaining -O-Si-O-M-O- linkages. Potassium ions and water molecules (omitted in Figure 3 for clarity) reside in these channels.

Figure 3. Polyhedral representations of the umbite and AV-12 structure viewed along [001] and [ 100]. For clarity, the water molecules have been omitted.

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-84 ~i (ppm)

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Figure 4. Experimental and simulated 29Si MAS NMR spectra of AV-12.

The 29Si MAS NMR spectrum of AV-12 material, shown in Figure 4, contains two overlapping peaks, which can be deconvoluted in two resonances at ca. 8 -83.8 and -84.4 in a 2 : 1 intensity ratio. Now, the crystal structure of umbite calls for the presence of three unique Si sites with on a 1 : 1 : 1 intensity ratio and, therefore, two peaks probably overlap in the resonance at ca. ~ -83.8. Ti-, Zr- and Sn-umbite materials give 29Si MAS NMR resonances between-84.6 and -87.3. 2-3 Peak overlapping was also observed for Zr and Ti-umbite: the former gives only one broad peak while the latter exhibits two resolved peaks. 2 The FTIR spectra of zirconium umbite and AV-12, shown in Figure 5, are very similar. Only small shitts and band intensity changes are observed. Above 900 cm 1, the spectrum of zirconium umbite contains bands at 1097, 1035, 964, 948, 938 and 905 cm 1. These bands are most likely attributed to the asymmetric and symmetric vibrations of the SiO4 framework polyhedm, v The specmun of AV-12 displays all these bands but they are slightly (ca. 5 cm1) shifted to high frequency. Bands under 900 cm1 are essentially unchanged. FTIR and Raman (not shown) spectra exlfibit similar band shiits. The total AV-12 mass loss, ascertained by TGA analysis, between 30 and 700 ~ is ca. 4.9 wt%. The TGA curves in Figure 6 and powder XRD patterns of AV-12 rehydrated for 24 hours after dehydration at 500 ~ show an almost reversible water loss. The small differences observed in the TGA curves may result from slow rehydmtion or silanol group on defect sites.

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AV-12

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Figure 5. FTIR spectra of zirconium umbite and AV-12.

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Temperature (~ Figure 6. TGA of (a) as-synthesized AV-12; (b) rehydrated sample after dehydration at 500 ~ for 2 hours.

325 Dehydration begins immediately on heating and is completed by c a . 500 ~ between 250 and 400 ~ as in synthetic zirconium umbite.

Most water is lost

4. C O N C L U S I O N S

The synthesis and characterization of the first example of a microporous potassium hafnium silicate, possessing the structure of the rare zirconium silicate mineral umbite, has been reported. The synthesis conditions are very similar to those reported for zirconium and titanium silicate umbite. 2"6 This suggests the possibility of preparing solid solutions among Zr, Ti and Hf, perhaps allowing the fine tuning of ion-exchange and adsorption properties. This work and the Rietveld refinement of the AM-12 structure are in progress in our laboratory.

5. A C K N O W L E D G M E N T S This work was supported by FCT, PRAXIS XXI, SAPIENS and FEDER.

6. REFERENCES 1 J. Rocha and M. W. Anderson, Eur. J. Inorg. Chem., (2000) 801. 2 Z. Lin, J. Rocha, P. Ferreira, A. Thursfield, J. R. Agger and M. W. Anderson, J. Phys. Chem., B 103 (1999) 957. 3 Z. Lin, J. Rocha and A. Valente, Chem. Commun., (1999) 2489. 4 Z. Lin, J. Rocha, Jfilio D. Pedrosa de Jesus and A. Ferreira, J. Mater. Chem., 10 (2000) 1353. 5 G. D. Ilyushin, Inorg. Mater., 29 (1993) 853. 6 Z. Lin, J. Rocha, P. Brand,o, A. Ferreira, Ana. P. Esculcas, Jfilio D. Pedrosa de Jesus, A. Philippou, M. W. Anderson, J. Phys. Chem. B 101 (1997) 7114. 7 A. I. Bortun, L. N. Bortun, D. M. Poojary, O. Xiang and A. Clearfield, Chem. Mater., 12 (2000) 294. 8 C. Dong, J. Appl. Cryst., 32 (1999) 838. 9 W. Kraus and G. Nolze, Federal Institute for Materials Research and Testing, Rudower Chaussee 5, 12489, Berlin, Germany. 10 S. R. Jale, A. Ojo and F. R. Fitch, Chem. Commun., (1999) 411.