Polybenzoxazine Aerogels. 2. Interpenetrating Networks with Iron Oxide and the Carbothermal Synthesis of Highly Porous Monolithic Pure Iron(0) Aerogels as Energetic Materials

There is a specific need for nanoporous monolithic pyrophoric metals as energetic materials and catalysts. Adapting modern-day blast furnace methodology, namely, direct reduction of highly porous iron oxide aerogels with H2 or CO, yielded coarse powders. Turning to smelting reduction, we used the acid environment of gelling [Fe(H2O)6]3+ sols to catalyze co-gelation of a second, extremely sturdy, carbonizable in high yield polybenzoxazine (PBO) network that plays the dual role of a reactive template. Formation of two independent gel networks was confirmed with rheology/dynamic mechanical analysis performed in tandem with the same sol and its gel, and results were correlated with data from microscopy (SEM, STEM) and small-angle X-ray scattering (SAXS) for the elucidation of the exact topological association of the two components. By probing the chemical interaction of the two networks with infrared, Mössbauer, XRD, and CHN analysis, we found out that iron(III) oxide undergoes pre-reduction to Fe3O4 and participates in the oxidation of PBO, which is a prerequisite for robust carbons suitable as structure-directing templates. Subsequently, interconnected submicrometer-size Fe3O4 nanoparticles undergo annealing at more than 800 °C below the melting point of the bulk oxide and are reduced to iron(0) at 800 °C, presumably via a solid (C)/liquid (Fe3O4) process. Carbothermal reduction, oxidative removal of residual carbon (air), and re-reduction (H2) of α-Fe2O3 formed in the previous step were all carried out as a single process by switching gases. The resulting pure iron(0) monoliths had a density of 0.54 ± 0.07 g cm–3 and were 93% porous. Infiltration with LiClO4 and ignition led to a new type of explosive behavior due to rapid heating and expansion of gases filling nanoporous space; annealing at 1200 °C reduced porosity to 66%, and those materials behaved as thermites. Ignition in a bomb calorimeter released 59 ± 9 kcal mol–1 of iron(0) reacted and is associated with oxidation to FeO (theoretical, 66.64 kcal mol–1).