Ammonium Metatungstate, (NH4)6[H2W12O40]: Crystallization and Thermal Behavior of Various Hydrous Species

Felix Eder,Matthias Weil,Wolf-Dieter Schubert,Christian Gierl-Mayer

doi:10.1021/acs.cgd.1c00639

Abstract

Various hydrous ammonium metatungstate phases (NH4)6[H2W12O40]·XH2O (AMT-X) have been obtained in the form of single crystals, using supersaturated solutions, antisolvent crystallization, and partial dehydration as strategies for crystal growth. On the basis of laboratory X-ray diffraction data, the crystal structures of the hydrous phases with X = 22, 12.5, 9.5, 8.5, 6, 4, and 2 as well of the cocrystals with X = 12-ethanol and X = 4-2acetone have been determined for the first time. AMT-22 forms the solid solution series (NH4)6–xKx[H2W12O40]·22H2O with the potassium salt (PMT-22) without a miscibility gap. Its tetragonal crystal structure shows a distorted cubic close-packed arrangement of the spherical α-Keggin-type [H2W12O40]6– anions with disordered ammonium N and water O atoms in the voids. The crystal structures of the other, less-hydrated AMT phases can be derived from distorted hexagonal rod packings of the α-Keggin anions, likewise with the cations and crystal water molecules in the voids. The thermal behavior of AMT-22 crystals reveals a quick dehydration, with AMT-9.5 as an intermediate and AMT-4 as a stable hydrate phase under ambient conditions. Upon further heating, the material decomposes with stepwise formation of AMT-2, AMT-1, AMT-0, and an amorphous phase, before orthorhombic WO3 forms above 400 °C. One of the commercially available AMT phases, “(NH4)6[H2W12O40]·XH2O” (with X = unspecified) has a water content of X = 4 but crystallizes in a structure other than AMT-4. Consequently, the 4-hydrate shows polymorphic behavior.

Highlights

In aqueous solution and with a dependence on the pH value, different species with tungsten(VI) in an anionic moiety can exist
CONCLUSIONS ammonium metatungstate (AMT) and its hydrated phases have been known for nearly 200 years, details of their crystal structures have remained unknown so far
With a quality sufficient for standard X-ray structure analysis were grown and their crystal structures determined for the first time

Summary

Introduction

In aqueous solution and with a dependence on the pH value, different species with tungsten(VI) in an anionic moiety can exist. In addition to the orthotungstate anion WO42− there are more complex isopolyoxotungstate(VI) anions known, the most and tihmepomrteatnattuonngesstabteein[gHt2hWe 1p2Oara40t]u6n−gstaantieon[Hs.12WT12hOe 42p]1H0−-. Dependent formation and decomposition of the paratungstate anion from and into orthotungstate can be summarized by reaction eqs 1 and 2, respectively. F [H2W12O42]10−(aq) [1] 14OH−(aq) 2− (aq) [2]

Objectives

Results

Conclusion