Supported Poly(ethyleneimine) Adsorbents for CO2 Removal from Air
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The simultaneous desire for global economic growth and the threat of climate change poses a unique challenge to mankind. Despite warnings from the scientific community, the decarbonization of the energy and transportation sectors continues to progress at too slow a pace for complete mitigation of the risk of climate change. As such, strategies to geoengineer the climate to mitigate such risks have been proposed. One of these is direct CO2 capture from air using engineering chemicals, or ‘air capture’. Air capture is a particularly challenging technical problem due to the ultra-dilute nature of CO2 in air, currently at a concentration of ~400 parts per million (ppm). Any technology intended to separate CO2 from air at a scale relevant to global climate change must perform the separation with with near infinite selectivity for CO2, and have the capacity to process enormous volumes of air rapidly and at low cost. Amine based solid adsorbents are excellent potential candidates for this application, most notably due to their advantageous interaction thermodynamics with CO2. These ‘supported amines adsorbents’ are able to selectively concentrate CO2 from air, and rapidly liberate it upon a temperature increase to ~100 oC. Our research group has worked in the development of these materials for air capture and other applications, partnering with Global Thermostat, LLC and Corning, Inc to that end. Global Thermostat has proposed a temperature swing adsorption process for CO2 extraction from air that utilizes supported amine adsorbents dispersed in a honeycomb monolith structure, and steam to drive desorption. This particular technology is the subject of this dissertation, where the goal was to progress the materials development. The specific areas addressed were i) sorbent stability, ii) tuning of oxide properties, iii) tuning of organic properties, and iii) amine incorporation into honeycomb monoliths. The operating lifetime of any technology is essential to its economic viability, meaning that the performance of adsorbents should last for many thousands of cycles. An understanding of the mechanisms of degradation of target adsorbents under relevant operating conditions is thus very important. In chapter 2, the hydrothermal stability of alumina/poly(ethyleneimine) (PEI) adsorbents are studied in detail. It is shown that under relevant timescales of steam exposure, the adsorbents partially hydrate from γ-alumina to boehmite. It is further shown that his hydration does not affect the porosity or efficiency of impregnated PEI to adsorb CO2, the most important properties of the adsorbent. This firm understanding of degradation and its relation to performance provides confidence in the alumina/PEI composition as a practical choice for commercial deployment. Supported amines have a high degree of tunability in their materials design. These include the properties of the host oxide, as well as those of the active organic amine. In chapter 3, the relative importance of oxide surface properties in improving the amine efficiency of PEI are studied in relation to altered textural properties. It is shown that changes to the pore size, particle size and pore surface texture have a more substantial effect on the amine efficiency of PEI than the acid/base properties of the surface. In chapter 4, the effect of various additive/PEI mixtures are assessed in the context of their improvement in the amine efficiency of air capture sorbents. Low molecular weight poly(ethyleneglycol) (PEG) is shown to be particularly effective, and target PEG/PEI contents that have improved kinetic and thermodynamic performance are identified. These studies both provide insight into the efficacy of particular material design levers in improving adsorption performance. Finally, in order to utilize performance levers such as those studied in chapters 3 and 4 in honeycomb monoliths, a firm understanding of how to prepare such monoliths and how their performance compares with analogous powders is necessary. In chapter 5, PEI functionalized alumina powders, small monolith pieces, and a large monolith are prepared, characterized and assessed relative to one another in CO2 adsorption experiments. Despite differences in the deposition of PEI in particular pore size regimes observed in monoliths compared to powders, their adsorption performance at each scale was similar. A lone exception was the equilibration rate of the large monolith sample, suggesting that a study of transport phenomena in such a structure will be an important research topic in the near future.