Synthesis and Characterization of Ammonium Decavanadate (V) M.R. Todorović 1,a, U.B. Mioč 2,b, I. Holclajtner-Antunović 2,c and D.

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1 Materials Science Forum Online: ISSN: , Vol. 494, pp doi:1.428/ 25 Trans Tech Publications, Switzerland Synthesis and Characterization of Ammonium Decavanadate (V) M.R. Todorović 1,a, U.B. Mioč 2,b, I. Holclajtner-Antunović 2,c and D. Šegan 1,d 1 Faculty of Chemistry, P.O.Box 158, 111 Belgrade, Serbia and Montenegro 2 Faculty of Physical Chemistry, P.O.Box 137, 111 Belgrade, Serbia and Montenegro a marijat@chem.bg.ac.yu, b ubavka@ffh.bg.ac.yu, c ivanka@ffh.bg.ac.yu, d segand@chem.bg.ac.yu Keywords: Ammonium Decavanadate, Conductivity, IR and Raman Spectra, Polyoxometalate (POM), Thermal Analysis, XRD. Abstract. It is known that various polyoxovanadates interact specifically with enzymes, which is the main way of their biochemical activity. Therefore we have synthesized ammonium decavanadate, (NH 4 ) 6 V 1 O 28 6H 2 O. The novel compound was characterized by elemental and thermal analysis, X-ray powder and single crystal diffraction and IR and Raman spectroscopy. Its conductive properties have been studied, too. The spectroscopic analysis has shown the presence of hydrogen bonds of different strengths. In order to improve the biochemical activity of this compound and having in mind the presence of strong hydrogen bonds, we essayed the synthesis of complex of polyoxovanadate with alanine. The obtained product was characterized by the mentioned methods. Introduction Polyoxometalates are a class of inorganic metal-oxygen cluster compounds. Their chemistry, dictated by their remarkable structural and electronic properties, is the subject of intense interdisciplinary research activities in some major scientific areas: catalysis, material science and medicine. Different polyoxometalates interact specifically with enzymes, which is the main way of their biochemical activity [1]. It is well known that there are difficulties to define the reactions of vanadate in aqueous solutions. Namely, oxovanadates in aqueous solutions exist in various forms and states, from simple protonated oxovanadate to decavanadate depending on solution ph. Therefore, the synthesis of the specific form of decavanadate should be performed under strictly determined conditions [2]. This paper deals with synthesis and characterization of ammonium decavanadate, (NH 4 ) 6 V 1 O 28 6H 2 O, (ADV-6). In order to improve the biochemical activity of this compound we synthesized a complex of oxovanadate with alanine. The characterization of the obtained product was performed, too. Besides biochemical activity, these compounds exhibit other useful properties important in catalysis and also in proton conductors. Sometimes they are incorporated in different substrates in order to improve their catalytic and conductive properties [3, 4]. Experimental Materials, methods and instrumentation ADV-6 was synthesized from NH 4 VO 3 (Merck), pa. Other reagents were Carlo Erba, pa. Characterization of the obtained compound was done by elemental analysis, X-ray powder and single crystal diffraction, IR and Raman spectroscopy, and thermal analysis. Its conductive properties were determined, too. IR spectra were recorded on a PerkinElmer 1725 spectrophotometer and Raman spectra on a Joban-Yvon-Horiba spectrometer. X-ray powder (XRP) diffraction patterns were obtained by using an automated Philips PW 171 diffractometer, with All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-1/5/16,19:33:32)

2 352 Current Research in Advanced Materials and Processes copper CuK α radiation and graphite monochromator. For single crystal, X-ray data were collected at 1 K on a Bruker-SMART three-circle diffractometer equipped with a CCD area detector using MoK α radiation. The permittivities in X-band were determined with a HP841C network analyzer. From obtained data proton conductivity was calculated in a function of frequency. Preparation of (NH 4 ) 6 V 1 O 26 6H 2 O ADV-6 was prepared by adding 6M HCl to a rapidly stirred solution of NH 4 VO 3 (5.g, 4.27mmol) in water (3ml) to a final ph=5.8. Solution was left to stir for a day and ph was adjusted with solid NH 4 Cl. Resulting orange solution was stirred for another day at room temperature and then placed in a refrigerator. Orange crystals suitable for X-ray diffraction were obtained after a week. Drying in air crystals maintained orange color; yield 3.5g (7%). Preparation of an alanine - oxovanadate complex As the attempt of synthesis of the complex of alanine with decavanadate was not successful, we performed synthesis by adding alanine into the starting solution of ammonim vanadate applying previously described procedure of ADV-6 synthesis. Results and Discussion The ADV-6 is formed readily in acidic solution but there are many oxovanadate species in equilibrium depending on medium conditions. The crystallization of ADV-6 is difficult, especially in concentrated solution and with temperature increase. Typical side product is (NH 4 ) 2 V 6 O 16, yellow powder which evolves if the temperature exceeds 3 C. Therefore, the crystallization was performed below room temperature. X-ray powder and single crystal diffraction analysis confirmed that the structure of the obtained compound corresponds to ADV-6 and has triclinic space group [2, 5]. The structure and single crystal view are presented in Fig. 1 a) and b). Parameters of elementary cell are given in Table 1. Fig. 1 View and structure of ADV-6 cage + IR spectra of ADV-6 were analyzed taking into account characteristic groups: NH 4, OH - and + V-O. The characteristic bands for NH 4 are at 147cm -1, bending and 2918/2851cm -1 stretching, and for OH - at about 1645 cm -1 (Fig. 2). The position and shape of bands characteristic of OH - and NH + 4, above 3cm -1 suggest the presence of hydrogen bonds of different strengths. Two

3 Materials Science Forum Vol characteristic sharp bands at 954cm -1 and 825cm -1 are assigned to the V-O terminal stretching and bands below 8cm -1 to the asymmetric and symmetric V-O bridge vibrations, respectively. Table 1 Characteristics of single crystal ADV-6 Chemical formula (NH 4 ) 2 V 6 O 28 6H 2 O Formula weight, g mol Temperature, K 1 Wavelength, Å.7173 Crystal system triclinic Space group P1 a(å) (2) b(å) (2) c(å) (3) α(º) (1) β(º) (2) γ(º) 71.47(1) Z 2 Volume, Å Crystal size, mm 3.7x.7x.6 Color orange The results of thermal analysis (Fig. 3) show that the crystal is thermo-stabile up to 19 C when the first dehydration process occurs. At this temperature, lower hydrate ADV-1 was formed. This crystal form is stable up to 175 C. Above this temperature, the crystal almost continually loses mass which can be attributed to the process of further dehydration and deamination. At about 4 C, the thermo-stable form, V 2 O 5, was obtained Residue 77.91% I Percentage III II IV 6 7 Microvolts Temperature( o C) Fig. 2 IR spectrum of ADV-6 Fig. 3 DTA and TGA curves of ADV-6 The conductivity is an important parameter for all polyoxometalates. We recorded the complex impedance in a wide frequency range from 1 to Hz at 3 C. On the basis of these results the conductivity was calculated (Fig. 4).

4 354 Current Research in Advanced Materials and Processes 1x1-4 σ ac (S/cm) T=33 K 1x f (Hz) Fig. 4 Conductivity of ADV-6 as a function of frequency a) b) c) It follows that the conductivity is slightly frequency dependent and ranges from to S/cm. It is evident that in the case of ADV-6, protons are not as mobile as in the case of other POMs [6]. The other part of the experiment was the characterization of the compound obtained in the reaction of ammonium vanadate and alanine. According to the elemental analysis of the carbon content it can be concluded that the obtained product is a coprecipitate of alanine and oxocavanadate. XRPD patterns of the starting components and coprecipitate are presented in Fig.5. The change in relative intensity of diffraction lines, shift of lines characteristic of alanine and ammonium vanadate and appearance of some new lines indicate the formation of the new product. IR and Raman spectra of ammonium vanadate, alanine and obtained coprecipitate are presented in Figs. 6 and 7, respectively. Fig. 5 XRPD patterns of the starting components, a) ammonium vanadate and b) alanine and c) coprecipitate

5 Materials Science Forum Vol I(a.u.) I(a.u.) b) c) a) a) ν(cm -1 ) b) c) Fig. 6 IR spectra of: a) ammonium vanadate, Fig. 7 Raman spectra of: a) ammonium b) alanine and c) coprecipitate vanadate, b) alanine and c) coprecipitate The changes in position and intensity of bands characteristic of the NH 4 + group (about 34, 167 and 14 cm -1 ) and V-O (below 1 cm -1 ) confirms the formation of coprecipitate of oxovanadate and alanine. Considerably stronger bands characteristic of hydrogen bonds, in the range from 34 to 23 cm -1 in the spectrum of the formed product, show that co-precipitate is formed by hydrogen bonding. The formation of coprecipitate could be followed more obviously from Raman spectra.

6 356 Current Research in Advanced Materials and Processes Conclusion On the basis of the presented results it can be concluded that the conditions for the synthesis of ammonium decavanadate hexahydrate are defined. The characterization of the obtained product was performed. It is shown that ammonium vanadate reacts with alanine forming oxovanadate coprecipitate by hydrogen bonds. The investigations of biochemical activity of the obtained products are in progress. By incorporation of ADV-6 in various substrates their conductivity is improved [7]. Acknowledgement This work was supported by the Ministry of Science and Environmental Protection of the Republic of Serbia under Project 173. We would also like to acknowledge Dr Anne Spasojević de-burre for help in controlling decavanadate synthesis. References [1] D. C. Crans, Comments Inorg. Chem. Vol. 16 (1994), p. 35. [2] C. Eglmeier, K. J. Range, A. Kleynhans, A. Heyns, S. Afr. J. Chem. Vol. 46 (1993), p. 7. [3] S. Sredić, T. Čajkovski, M. Davidović, V. Likar-Smiljanić, M. Marinović-Cincović, U. B. Mioč, Z. Nedić and R. Biljić, Material Science Forum Vol (24), p [4] U. B. Mioč, S. Milonjić, D. Malović, V. Stamenković, Ph. Colomban, M. Mitrović and R. Dimitrijević, Solid State Ionics Vol. 97 (1997), p [5] G. Bogdanović, N. Bošnjaković-Pavlović, A. Spasojević-Bire, N.E. Ghermani, to be published. [6] N. Tjapkin, Ph. Colomban and U. Mioč, Solid State Ionics Vol. 61 (1993), p [7] S. Sredić, M. Davidović, A. Spasojević-de-Bure, U. B. Mioč, M. Todorović, D. Šegan, D. Jovanović and P. Pissis, Solid State Proton Conductors-12, Upsala, Sweden, VIII 24., Proceedings, p. 17.

7 Current Research in Advanced Materials and Processes 1.428/ Synthesis and Characterization of Ammonium Decavanadate (V) 1.428/ DOI References [1] D. C. Crans, Comments Inorg. Chem. Vol. 16 (1994), p / [6] N. Tjapkin, Ph. Colomban and U. Mioč, Solid State Ionics Vol. 61 (1993), p / (93)9352-4