UC Berkeley

Delayed mitigation of anthropogenic climate change has necessitated the deployment of large-scale carbon dioxide removal (CDR) strategies to achieve stringent global warming targets. Within the past few years, direct air capture (DAC) with sequestration has garnered significant attention due to its modularity and potential for minimal strain on arable land and freshwater resources. Although a large number of DAC companies have already formed with the intent of megatonne- or even gigatonne-scale CO2 removal, the separation itself remains costly and energy intensive. This is fundamentally a consequence of the low (~420 ppm) atmospheric concentration of CO2. Yet despite this ultradilute concentration, DAC is technologically possible and could become cost-competitive with other climate change mitigation strategies in the near-term. Toward reducing the cost and energy expense of DAC, significant attention has been paid toward the development and testing of new materials and process designs. This dissertation seeks to provide a multifaceted view on the technoeconomics of DAC, contextualizing its unique capabilities with respect to other CDR strategies. Significant attention will be paid to the synthesis and characterization of the metal–organic framework Mg2(dobpdc) (dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), which when appended with amines is a strong candidate for commercial DAC. In motivating large-scale DAC, an understanding of the current greenhouse gas emission portfolio must first be obtained. Chapter 1 focuses on CO2 emissions from the power generation sector and explores the application of point-source carbon capture and storage (CCS) to mitigate these emissions. In spite of its potential for decarbonization of the power generation and industrial sectors, CCS designs commonly seek to capture only 90% of the CO2 produced from these sources, as this capture fraction was previously deemed the optimal condition for aqueous amine scrubbers. Recognition of the shrinking global carbon budget, along with advances in carbon capture material and process designs, have since rendered the 90% capture fraction an artificial limit. Rather than mitigate the remaining ~10% of point-source CO2 emissions with CDR, it is often technoeconomically more desirable to capture these residual emissions with CCS. We estimate that targeting capture fractions significantly above 90%, which we term “deep CCS,” could avoid ~1 Gt/yr of CDR. Chapter 2 explores the technoeconomics of DAC in particular, focusing on the concurrent capture of CO2 and water from air. Although DAC processes are primarily concerned with the capture of atmospheric CO2, it is essential to recognize that water is typically present at much higher concentration than CO2 in air. Water can both enhance and hinder the thermodynamics and kinetics of DAC in candidate materials, and these materials can often capture large amounts of atmospheric water in addition to CO2. As climate change intensifies, regional water scarcity is projected to affect an increasing portion of the biosphere, and as such it is intriguing to consider the prospect of DAC processes that intentionally harvest large amounts of atmospheric water in addition to CO2. Indeed, the recovery of atmospheric water could offset part of the intrinsic costs of performing DAC and warrants further exploration. A wide variety of materials capable of performing DAC already exist, but there remains ample potential for improvements in the capacities, regeneration energies, and stabilities of these materials. Metal–organic frameworks are among the most attractive candidates for DAC because they can offer extremely high internal surface areas, which can facilitate the capture and recovery of large amounts of CO2 from air in each adsorption-desorption cycle. The attainment of high DAC working capacities is especially attractive with frameworks that feature cooperative adsorption of CO2, such as amine-appended Mg2(dobpdc). Although diamine-appended variants of Mg2(dobpdc) can capture large quantities of CO2 from air, these materials often degrade at high temperatures and high humidity due to the volatilization of appended diamines. Tetraamine-appended variants of Mg2(dobpdc) offer enhanced resistance to volatilization due to the higher molecular weight and multiple points of attachment of the appended tetraamines. In Chapter 3, the potential for DAC with tetraamine-appended Mg2(dobpdc) is explored via a combination of experimental and computational techniques. These findings collectively demonstrate that the material Mg2(dobpdc)(3-4-3) (3-4-3 = N,N′-bis(3-aminopropyl)-1,4-diaminobutane) is capable of DAC over consecutive adsorption-desorption cycles when exposed to simulated air conditions. Additionally, this material can capture significant quantities of atmospheric water alongside CO2 at minimal regeneration expense.