Size control, metal substitution, and catalytic application of cryptomelane nanomaterials prepared using cross-linking reagents

A novel sol-gel-assisted solid-state synthetic method has been developed to prepare pure and transition-metal-substituted cryptomelane 1D nanomaterials with controlled particle sizes. Different cross-linking reagents (PVA, glycerol, or glucose) have been used to prepare precursor powder materials and to produce nanorods, nanoneedles, or nanowires. The phase transformation, crystal structures, and properties of these nanomaterials have been investigated using a variety of characterization techniques including XRD, TEM, SEM, FT-IR, TGA, and BET surface area measurement. Both the cross-linking reagents and nitrate were found to affect the crystalline phase transformation of the nanomaterials. The cryptomelane phase was formed around 500-600 degreesC in the nanomaterials depending on the type of cross-linking reagent used. The nanoscale materials exhibit long-range ordered structures along the b axis. No cross-linking reagent residues were found in the final products after the reactions were complete. Several types of transition metal cations (Fe3+, Co2+, Ni2+, and Cu2+) were used to substitute into cryptomelane nanomaterials. XRD data show that Fe(III) cations have been substituted into the materials without the formation of additional amorphous or crystalline phases, while the other three cations caused the formation of impure amorphous or crystalline phases. These nanomaterials showed thermal stability up to 700-800 degreesC. Catalytic application of these nanoscale manganese oxide materials has been explored in the green oxidation of toluene to produce benzyl alcohol, benzylaldehyde, and benzoic acid. The nanoscale catalysts showed unique catalytic activity for this oxidation compared to those catalysts conventionally used for this reaction.