A thermally stable 3 x 3 octahedral molecular sieve corresponding to natural todorokite (OMS-1) has been synthesized by autoclaving layer-structure manganese oxides, which are prepared by reactions of MnO(4)(-) and Mn(2+) under markedly alkaline conditions. The nature and thermal stability of products depend strongly on preparation parameters, such as the MnO(4)(-)/Mn(2+) ratio, pH, aging, and autoclave conditions. The purest and the most thermally stable todorokite is obtained at a ratio of 0.30 to 0.40. Autoclave treatments at about 150 degrees to 180 degrees C for more than 2 days yield OMS-1, which is as thermally stable (500 degrees C) as natural todorokite minerals. Adsorption data give a tunnel size of 6.9 angstroms and an increase of cyclohexane or carbon tetrachloride uptake with dehydration temperature up to 500 degrees C. At 600 degrees C, the tunnel structure collapses. Both Lewis and Brönsted acid sites have been observed in OMS-1. Particular applications of these materials include adsorption, electrochemical sensors, and oxidation catalysis.

Porous solids, such as zeolites and other molecular sieves, contain intracrystallite/framework cavities and channels that produce microporous (pore diameter, D < 2 nm), mesoporous (2 nm < D < 50 nm), and macroporous (D > 50 nm) structures, and have demonstrated excellent potential as materials for use in many separation and catalytic processes.[1] The pore sizes of these porous materials (especially the micropore sizes, which are close to molecular dimensions) may play a critical role in controlling separation and catalytic selectivity due to their shape and size selectivity. The production of materials with different microporosities has always been challenging. Natural and synthetic tunnel-structured manganese oxide octahedral molecular sieves (OMSs) make up a promising group of functional porous materials. They can exhibit various nanometer-scale tunnel sizes from 2.3 2.3 to 4.6 11.5 , which correspond to different micropore openings. As such, they constitute excellent model systems for studying the synthesis of materials with controlled microporosities. Moreover, the potential applications for synthetic manganese oxide OMS materials can be expanded by molecular modification or decoration of the tunnel structures.[2] There have been several attempts to synthesize manganese oxide OMSs with the same tunnel structures as those found in natural manganese oxides, or to create materials with new tunnel structures.[3±7]However, there has been very little work reported on the direct control of tunnel sizes. In this communication, we report the successful synthesis of manganese oxide OMS materials with controlled nanometer-scale tunnel sizes by controlling the pH of the hydrothermal transformation of layered manganese oxide precursors. Hydrothermal transformation of layered manganese oxide materials is one of the most effective methods of obtaining tunnel-structured manganese oxides. Due to the mixed-valent manganese framework, usually (+2, +3, and +4) or (+3 and +4), a small number of guest cations are usually required for charge balance in most layer-and tunnel-structured manganese oxides. These guest cations usually reside between the layers or inside the tunnels.[3±7] When layered manganese oxides are transformed into tunnel structures, the interlayer cations remain inside the tunnels. Therefore, the types and sizes of the guest cations in these layered manganese oxides might play critical roles as structure directors in templating different tunnel sizes during synthesis of tunnel structures. Since many cations are in hydrated form under aqueous/hydrothermal conditions, the template effect may actually result from the size of the hydrated cations rather than from that of the isolated cations. This is particularly intriguing since many cations can adopt different hydration states, and thus the sizes of the hydrated cations can be different. The corollary of this is that, if the sizes of hydrated cations can be controlled by varying the extent of hydration, it may be possible to synthesize materials with controlled tu...

Synthesis and characterization of a thermally stable analogue of synthetic todorokite, octahedral molecular sieve (OMS-I), is reported.

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