Synthesis and Characterization of Octahedral Molecular Sieves (OMS-2) Having the Hollandite Structure

Cr-hollandite: Breaking tradition with todorokite-type manganese oxides

“…The resulting black slurry was allowed to age for at least one week. The aged sample of Na-birn(Cr) was then filtered, washed 3 times with water, immediately slurried in 50 mL of 0.25 M Cr(NO 3 ) 3 , and stirred overnight. The resulting Cr 3+ -exchanged Cr-buse(Cr) was filtered and washed 3 times with water.…”

“…Na-birn(0) was prepared, aged, and isolated similarly to Na-birn(Cr) above but without addition of CrCl 3 ·6H 2 O. Ion exchange was carried out in 20 mL of 1.0 M Cr(NO 3 ) 3 with stirring for 24 h. The resulting Cr-buse(0) was filtered without washing and treated hydrothermally at 160 °C for 1 week. Grey-black Cr-holl(0) was isolated by filtration, washed 3 times with water, and dried at 110 °C.…”

“…Given the prevalence of cryptomelane (K-holl) in hollandite chemistry, K-holl(Cr) was targeted to ascertain if Cr 3+ as a framework dopant would have a disruptive effect on tunnel formation. Calcination of K-birnessite is a known synthetic route to cryptomelane [3,22] so similar treatment was applied to K-birn(Cr) after preparation by ion exchange with Na-birn(Cr). Heating at 500 °C for 2 h produced crystalline K-holl(Cr), demonstrating that framework Cr 3+ does not affect the formation of 2x2 hollandite with interstitial K + .…”

“…K-holl(Cr) can also be obtained by hydrothermal treatment of a K-birn(Cr) slurry in 0.1 M HCl/1 M KCl for one week at 160 °C. Acidic conditions are conducive to hollandite formation [3] and excess potassium ion minimizes proton exchange during the reaction. The morphology is similar to that of K-holl(Cr) from calcination.…”

“…It can be prepared by a variety of methods, including reflux heating, hydrothermal treatment, and sol-gel processing. [3][4][5] It is also generated in syntheses of hierarchical urchin-like nanostructured manganese oxides. [6,7] The 3x3 todorokite structure is typically synthesized from layered buserite precursors under hydrothermal conditions.…”