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The oxidative degradation rates of a CO 2 sorbent composed of a mesoporous alumina impregnated with poly-(ethylenimine) (PEI) are measured under systematically varied conditions and a reaction rate law is created. Good agreement is shown between the rate of oxidation obtained via in situ calorimetric heat measurement during oxidative degradation reactions and the loss of CO 2 capture performance presented as amine efficiency (mol CO 2 /mol amine). PEI mass loss and elemental composition are tracked over the course of the reaction and used in conjunction with the oxidation rate measurements to shed insight into the oxidation reaction(s). These data, in combination with measurements of the heat of reaction, suggest a common reaction set across the range of temperatures, oxygen concentrations, and sorbent compositions tested. The data are consistent with the basic autoxidation scheme (BAS), the accepted mechanism of autoxidation of aliphatic polymers. We propose a lumped kinetic model to describe the oxidation reaction set and estimate an activation energy of 105 kJ/mol and an oxygen reaction order of 0.5−0.7 from the data accordingly. These parameters can be incorporated into process cycle models to estimate the material lifetime, a critical uncertainty in the deployment of DAC technologies.

Zeolitic nanotubes Nanotubes generally have solid walls, but a low-dimensional version of zeolites now introduces porosity into such structures. Korde et al . used a structure-directing agent with a hydrophobic biphenyl group center connecting two long alkyl chains bearing hydrophilic bulky quaternary ammonium head groups to direct hydrothermal synthesis with silicon-rich precursors (see the Perspective by Fan and Dong). The nanotubes have a mesoporous central channel of approximately 3 nanometers and zeolitic walls with micropores less than 0.6 nanometers. Electron microscopy and modeling showed that the outer surface is a projection of a large-pore zeolite and the inner surface is a projection of a medium-pore zeolite. —PDS

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The catalytic condensation of ethanol to n-butanol and higher alcohols, known collectively as Guerbet reactions, has attracted attention in recent years as ethanol becomes increasingly available as a biorenewable feedstock. Results are presented here for the continuous, condensed-phase conversion of ethanol to higher alcohols using Ni/La2O3/γ-Al2O3 catalysts and for catalysts containing a second metal (Cu, Co, Pd, Pt, Fe, Mo) in addition to nickel. Detailed characterization of the catalyst surface and bulk properties has been carried out and is correlated to catalyst activity and selectivity. The best results obtained for nickel catalysts are a selectivity to higher alcohols of 75–80% and a turnover frequency of 200 mol ethanol/mol Ni site/h at 230 °C. The addition of cobalt nearly doubles the ethanol conversion rate relative to Ni alone, with only a slight reduction in higher alcohol selectivity. Results of catalyst characterization, a simple kinetic model, and experiments with reaction intermediates support the initial dehydrogenation of ethanol as the rate-limiting step of the condensed-phase reaction.