Potential applications of vanadosilicate AM-6 in optoelectronics and photocatalysis have motivated research on the synthesis of AM-6 crystals. Currently, seeds of isostructural titanosilicate ETS-10 are used to hydrothermally grow AM-6 [1]. This can be expected to result in a “core-shell” crystal morphology with titanium-rich core and vanadium-rich shell (if AM-6 grows epitaxially on ETS-10 seeds), or yield products containing a mixture of ETS-10 and AM-6 crystals. Vanadium-substituted ETS-10 crystals with V/Ti≤0.95 were hydrothermally synthesized using mixtures containing sources of both vanadium and titanium [2-4]. Synthesis of AM-6 from the pure vanadosilicate mixtures in the absence of ETS-10 seeds has not been reported. The availability of titanium-free AM-6 should facilitate characterization of the structure and properties of the “pure” vanadosilicate framework containing hexa-coordinated vanadium.

Unseeded hydrothermal syntheses in a vanadosilicate system were investigated at 503 K, and explored the effects of mixture Si/V ratio, initial pH, and addition of tetramethylammonium bromide (TMABr) as a “template”. In the absence of a “template”, neither the Si/V ratio (varied between 4.7-6.2), nor the initial pH, (varied between 9.5-12.3) produced AM-6. However, addition of TMABr to mixtures with the initial pH of approximately 11.0 and the Si/V ratios of 4.85, 5.5, and 6.2 produced AM-6 crystals. For each Si/V ratio investigated, the content of AM-6 in the final product increased with increasing TMABr content. For TMABr/V≥2.0, mixtures with Si/V ratios of 4.85 and 5.5 produced nearly pure (>95%) AM-6 products. AM-6 crystals in these products were accompanied by small amounts of the needle-like crystals of an unidentified phase. AM-6 crystals synthesized from mixtures with Si/V=6.2 and the TMABr/V ratios of 1.0 and 2.0 were accompanied by quartz and platelets of the unidentified impurity phase. Synthesized AM-6 crystals had square truncated bipyramidal morphology, various irregularities/“steps” on the trapezoidal faces, and multiple small, square shaped island-like structures and terraces on the square faces. These morphological and topographic features are similar to those of ETS-10 crystals, and suggest a layer-by-layer crystal growth mechanism for AM-6 akin to that deduced before for ETS-10 [5, 6]. The size and aspect ratio (i.e., combined height of two pyramids divided by the width of the shared base) of AM-6 crystals depended on the mixture Si/V ratio and TMABr content, but did not exceed 4 µm and 0.5, respectively. The initial pH of synthesis mixture appears to play an important role in the unseeded synthesis of AM-6. When the initial pH of mixtures with Si/V=4.85 and the TMABr/V ratios of 2 and 3, which at a pH of approximately 11.0 yielded nearly pure AM-6 products, was adjusted to approximately 10.5, the resulting products contained a mixture of AM-6 and unidentified phase(s) (for TMABr/V=2) or were devoid of AM-6 (for TMABr/V=3). The AM-6 products calcined in air at 623-673 K typically retained ~70-90% of the crystallinity as determined by X-ray diffraction. However, calcination at 698 K rendered as synthesized AM-6 crystals completely amorphous. The Si/V ratio in AM-6 crystals measured by EDX appeared to depend on the Si/V ratio of the synthesis mixture. AM-6 crystals obtained from mixtures with the Si/V ratios of 4.85, 5.5, and 6.2 at a constant TMABr/V=2 had the EDX-determined Si/V ratios of 6.9±0.7, 9.0±0.8, and 11±1, respectively. These values are higher than the theoretical value of 5.0 for the Si/Ti ratio in isostructural ETS-10 crystals. This suggests large number of vanadium vacancies in the AM-6 framework, consistent with the topographical features of AM-6 crystals prominently displaying large number of the stepped square shaped islands/terraces. Since defects form where the spreading islands meet during crystal growth, the large number of the islands on AM-6 suggests the high concentration of defects, which is consistent with high Si/V ratios measured in AM-6 crystals. UV-vis spectroscopy of the AM-6 product showed absorbance at ~420-440 nm associated with a charge transfer transition in octahedral vanadium. This suggests AM-6 may be photocatalytically active in the visible light.

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