Synthesis, characterization and thermogravimetric decomposition of 2-ethylpyridinium metatungstate monohydrate. Kinetic study of its thermal dehydration

Abstract

2-Ethylpyridinium dodecatungstate(Vl) monohydrate was synthetized and characterized as ~-(CsH4NHC2Hs)sH [HzW1204o]. H20 by chemical analysis and electronic, IR and ~H NMR spectral studies.The UV spectra showed the characteristic maximum absorption at 260 nm for the metatungstate anion. The two central protons in the anion were also characterized via I R and J H NMR spectroscopy.The thermal behaviour of this compound was investigated by thermogravimetric and derivative thermogravimetric methods. The TG curves revealed four main steps of decomposition, viz. dehydration, partial organic base decomposition and pyrolysis of the organic base, leading to the formation of WO 3 in the final stage.The intermediates obtained at the end of the various thermal decomposition steps were studied via the IR spectral data on the residues obtained by isothermal heating of the compound at different temperatures.Kinetic parameters for the dehydration reaction were obtained through two methods of analysis.At temperatures lower than 100 ~ the acidification of alkali metal tungstate solutions results in the formation of various polynuclear species. The results of a great number of studies to characterize the polytungstate species have been exhaustively reviewed by several authors [1][2][3][4].The pH inflection observed at H+/WO42-= 1.5 in potentiometric titrations of aqueous alkali metal tungstates corresponds to the formation of pseudometatungstate (0). The latter anion is formed rapidly, but is metastable in solution and is slowly transformed first into "tungstate-X" and finally into true metatungstate, e-[H2W1204o] 6-, the thermodynamically stable product. At 50 ~ and 5 mM tungstate concentration, some 15 days are required for complete conversion to meta-

Thermal analysis of solvatomorphic decakis (dimethylammonium) dihydrogendodecatungstate hydrates

“…Since in our compounds there are strongly reducing cations and oxidizing anions, one can expect a thermally induced redox reaction between them. The occurrence of such kind of redox reactions within one compound having oxidizing and reducing components is well documented in the literature [11][12][13][14][15][16][17]. Since in inert atmosphere the decomposition steps are endothermic, one can conclude that the energy required for elimination of water and dimethylamine as well as the disruption of the lattice exceeds the exothermicity of the redox interaction.…”

“…8), the intensities of dimethylamine peaks compared with those of water decrease with simultaneous appearance of oxidative condensation products of dimethylamine (m/z = 58 and 59). Since the water of crystallization is eliminated from polyoxometalates at temperatures below 250 °C, and the dehydroxylation temperatures [ [10], [14], [18] ] of the polytungstate cages are above 400 °C, the source of water at around 300 °C can only be the polytungstate cage via the expected redox reaction between the amine/ammonium and W=O functionalities. Further evidence of the redox reaction is that according to the XRD pattern, the residue formed by heating up to 700 °C in inert atmosphere is reduced tungsten oxides like WO~2 .93 .…”

Deuteration and Vibrational Spectra of Dimethylammonium Paratungstate‐B Hydrates

“…Curve fitting of the bands resulted in four components. The lowest-frequency band centered at 549 corresponds to the ρ(OH) int libration, [20] whereas the other three bands that we think belong to the mixed ν(W-O-W) modes are centered at 573, 600 and 644 cm -1 (Figure 2). The lattice modes are all located below 300 cm -1 as it can be seen in the far-IR spectra of compounds 1, 2, 2-D ( Figures S1-S3, respectively, Supporting Information) and also in the Raman spectrum of compound 1 (Figure S4, Supporting Information).…”

“…Curve fitting of the bands resulted in four components. The lowest‐frequency band centered at 549 corresponds to the ρ(OH) int libration, [ 20 ] whereas the other three bands that we think belong to the mixed ν(W–O–W) modes are centered at 573, 600 and 644 cm –1 (Figure 2).…”

“…The 955 cm -1 band probably contains the δ(OH) mode of the internal OH groups which appears exactly at the same frequency for both compounds 1 and 2. [20] The positions of these OH bands cannot be changed by deuteration, because, as explained above, the hydrogen atoms of the internal OH groups in polytungstates are not deuterium exchangeable, so further resolution of the peaks is not possible. [15][16][17] The ν[W(=O) 2 ] and the δ[W(=O) 2 ] modes can be assigned to be at 942 and 943/933 cm -1 as well as at 471 and 474/ 458 cm -1 in the IR spectra of the compounds 1 and 2, respectively.…”