Phenolic aerogels containing oxygen and other polymeric aerogels containing both oxygen and nitrogen (polybenzoxazine and a polyamide‐polyimide‐polyurea co‐polymer) are converted to carbon aerogels (800 °C/Ar), and are etched with CO2 (1000 °C). Etching opens closed pores and increases micropore volumes and size. Heteroatoms are retained in the etched samples. All carbon aerogels are evaluated as CO2 absorbers in terms of their capacity and selectivity toward CH4, H2, and N2. CO2 adsorption capacity is linked to microporosity. In most cases, monolayer coverage of micropore walls is enough to explain CO2 uptake quantitatively. The interaction of CO2 with micropore walls is evaluated via isosteric heats of adsorption, and is stronger with carbons containing only oxygen heteroatoms. The adsorption capacity of those carbons (5–6 mmol g−1) is on par with the best carbon and polymeric CO2 adsorbers known in the literature, with one exception however: etched carbon aerogels from low‐density resorcinol‐formaldehyde aerogels show a very high CO2 uptake (14.8 ± 3.9 mmol g−1 at 273 K, 1 bar) attributed to a pore‐filling process that proceeds beyond monolayer coverage, whereas surface phenoxides engage in a thermally neutral carbonate forming reaction (surface‐O– + CO2 → surface‐O–(CO)–O–) that continues until micropores are filled.

Polymeric aerogels (PA-xx) were synthesized via room-temperature reaction of an aromatic triisocyanate (tris(4-isocyanatophenyl) methane) with pyromellitic acid. Using solid-state CPMAS C andN NMR, it was found that the skeletal framework of PA-xx was a statistical copolymer of polyamide, polyurea, polyimide, and of the primary condensation product of the two reactants, a carbamic-anhydride adduct. Stepwise pyrolytic decomposition of those components yielded carbon aerogels with both open and closed microporosity. The open micropore surface area increased from <15 m g in PA-xx to 340 m g in the carbons. Next, reactive etching at 1,000 °C with CO opened access to the closed pores and the micropore area increased by almost 4× to 1150 m g (out of 1750 m g of a total BET surface area). At 0 °C, etched carbon aerogels demonstrated a good balance of adsorption capacity for CO (up to 4.9 mmol g), and selectivity toward other gases (via Henry's law). The selectivity for CO versus H (up to 928:1) is suitable for precombustion fuel purification. Relevant to postcombustion CO capture and sequestration (CCS), the selectivity for CO versus N was in the 17:1 to 31:1 range. In addition to typical factors involved in gas sorption (kinetic diameters, quadrupole moments and polarizabilities of the adsorbates), it is also suggested that CO is preferentially engaged by surface pyridinic and pyridonic N on carbon (identified with XPS) in an energy-neutral surface reaction. Relatively high uptake of CH (2.16 mmol g at 0 °C/1 bar) was attributed to its low polarizability, and that finding paves the way for further studies on adsorption of higher (i.e., more polarizable) hydrocarbons. Overall, high CO selectivities, in combination with attractive CO adsorption capacities, low monomer cost, and the innate physicochemical stability of carbon render the materials of this study reasonable candidates for further practical consideration.

Isocyanates react with carboxylic acids and yield amides. As reported herewith, however, transferring that reaction to a range of mineral acids (anhydrous H3BO3, H3PO4, H3PO3, H2SeO3, H6TeO6, H5IO6, and H3AuO3) yields urea. The model system for this study was a triisocyanate, tris­(4-isocyanatophenyl)­methane (TIPM), reacting with boric acid in DMF at room temperature, yielding nanoporous polyurea networks that were dried with supercritical fluid CO2 to robust aerogels (referred to as BPUA-xx). BPUA-xx were structurally (CHN, solid-state 13C NMR) and nanoscopically (SEM, SAXS, N2-sorption) identical to the reaction product of the same triisocyanate (TIPM) and water (referred to as PUA-yy). Minute differences were detected in the primary particle radius (6.2–7.5 nm for BPUA-xx versus 7.0–9.0 nm for PUA-yy), the micropore size within primary particles (6.0–8.5 Å for BPUA-xx versus 8.0–10 Å for PUA-yy), and the solid-state 15N NMR whereas PUA-yy showed some dangling −NH2. All data together were consistent with exhaustive reaction in the BPUA-xx case, bringing polymeric strands closer together. Residual boron in BPUA-xx aerogels was quantified with prompt gamma neutron activation analysis (PGNAA). It was found very low (≤0.05% w/w) and was shown to be primarily from B2O3 (by 11B NMR). Thus, any mechanism for systematic incorporation of boric acid in the polymeric chain, by analogy to carboxylic acids, was ruled out. (In fact, it is shown mathematically that boron-terminated star polyurea from TIPM should contain ≥3.3% w/w B, irrespective of size.) Retrospectively, it was fortuitous that this work was conducted with aerogels, and the model system used H3BO3, whereas the byproduct, B2O3, could be removed easily from the porous network, leaving behind pure polyurea. With other mineral acids, results could have been misleading, because the corresponding oxides are insoluble and remain within the polymer (via skeletal density determinations and EDS). On the positive side, the latter is a convenient method for in situ doping robust porous polymeric networks with oxide or pure metal nanoparticles (Au in the case of H3AuO3) for possible applications in catalysis.

Morphology is a qualitative property of nanostructured matter and is articulated by visual inspection of micrographs. For deterministic procedures that relate nanomorphology to synthetic conditions, it is necessary to express nano- and microstructures numerically. Selecting polyurea aerogels as a model system with demonstrated potential for rich nanomorphology and guided by a statistical design-of-experiments model, we prepared a large array of materials (208) with identical chemical composition but quite different nanostructures. By reflecting on SEM imaging, it was realized that our first preverbal impression about a nanostructure is related to its openness and texture; the former is quantified by porosity (Π), and the latter is oftentimes related to hydrophobicity, which, in turn, is quantified by the contact angle (θ) of water droplets resting on the material. Herewith, the θ-to-Π ratio is referred to as the K-index, and it was noticed that all polyurea samples of this study could be put in eight K-index groups with separate nanomorphologies ranging from caterpillar-like assemblies of nanoparticles, to thin nanofibers, to cocoon-like structures, to large bald microspheres. A first validation of the K-index as a morphology descriptor was based on compressing samples to different strains: it was observed that as the porosity decreases, the water-contact angle decreases proportionally, and thereby the K-index remains constant. The predictive power of the K-index was demonstrated with 20 polyurea aerogels prepared in 8 binary solvent systems. Subsequently, several material properties were correlated to nanomorphology through the K-index and that, in turn, provided insight about the root cause of the diversity of the nanostructure in polyurea aerogels. Finally, using response surface methodology, K-indexes and other material properties of practical interest were correlated to the monomer, water, and catalyst concentrations as well as the three Hansen solubility parameters of the sol. That enabled the synthesis of materials with up to six prescribed properties at a time, including nanomorphology, bulk density, BET surface area, elastic modulus, ultimate compressive strength, and thermal conductivity.

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