UC Berkeley

Carbon dioxide separations are likely to play an important role in mitigating greenhouse gas emissions and preventing further increases in global temperature. To perform these separations efficiently at scale, new materials are needed with greater efficiencies in the capture and release of CO2 from the emissions of fossil fuel-fired power plants and industrial process streams. In recent years, metal–organic frameworks, constructed from inorganic ions or clusters connected by organic bridging units, have shown particular promise in improving the efficiency of CO2 separations. Specifically, a new class of metal–organic frameworks of the form M2(dobpdc)(diamine)2 (M = Mg, Mn, Fe, Co, Zn; dobpdc4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) have been found to capture CO2 through a cooperative mechanism involving the switch-like, reversible polymerization of CO2 in ammonium carbamate chains along the pore axis.

Chapter 1 introduces the concept of carbon capture and sequestration, beginning with an overview of common adsorbent classes employed for CO2 capture applications. Opportunities and challenges are subsequently discussed for CO2 removal from individual potential target streams, including the flue emissions of power plants, industrial process streams, and air. Specific reports are selected to highlight recent advances in overcoming stream-specific challenges, such as stability to common adsorbent poisons. The chapter concludes with a discussion of key needs from the materials research community to accelerate greater adoption of carbon capture technologies.

Chapter 2 describes the development of alkylethylenediamine-appended variants of Mg2(dobpdc) for carbon capture applications. Small modifications to the diamine structure are shown to shift the threshold pressure for cooperative CO2 adsorption by over 4 orders of magnitude at a given temperature. The observed trends are rationalized on the basis of crystal structures of the isostructural zinc frameworks obtained by in situ single-crystal X-ray diffraction experiments. The structure–activity relationships derived from these results are subsequently shown to enable the optimization of adsorbent design to match the conditions of a given CO2 separation process, thereby minimizing the energetic cost of CO2 capture.

Chapter 3 leverages the results of Chapter 2 in the design of a diamine-appended framework for cooperative CO2 capture from the flue emissions of natural gas combined cycle power plants. Due to growing adoption of natural gas as a fuel source, the emissions of gas-fired plants are contributing a growing portion of global CO2 emissions, but CO2 capture from these power plants is hindered by the low CO2 concentration and high oxygen and water content of the flue stream. In this chapter, functionalization of Mg2(dobpdc) with the cyclic diamine 2-(aminomethyl)piperidine (2-ampd) is shown to produce an adsorbent that is capable of >90% CO2 capture from a humid natural gas flue emission stream, as confirmed by breakthrough measurements. Multicomponent adsorption experiments, infrared spectroscopy, magic angle spinning solid-state NMR spectroscopy, and van der Waals-corrected density functional theory studies suggest that water enhances CO2 capture in 2-ampd–Mg2(dobpdc) through hydrogen-bonding interactions with the carbamate groups of the ammonium carbamate chains formed upon CO2 adsorption, thereby increasing the thermodynamic driving force for CO2 binding. The exceptional thermal, oxidative, and cycling stability of this material are subsequently demonstrated, indicating that 2-ampd–Mg2(dobpdc) is a promising adsorbent for this important separation.

Chapter 4 describes the development of a diamine-appended framework for CO2 removal from crude natural gas. Due to its low CO2 emission intensity compared to coal, natural gas is favored as a cleaner-burning fuel. However, for many natural gas reserves, CO2 contamination must be removed at the wellhead to meet pipeline specifications. In this chapter, the framework ee-2–Mg2(dobpdc) (ee-2 = N,N-diethylethylenediamine) is demonstrated as a next-generation CO2 capture material for high-pressure natural gas purification. This material can be readily regenerated with a minimal change in temperature or pressure and maintains its CO2 capacity in the presence of water. Moreover, consistent with the results in Chapter 3, breakthrough experiments reveal that water enhances the CO2 capture performance of ee-2–Mg2(dobpdc) by reducing or eliminating “slip” of CO2 prior to full breakthrough. As in Chapter 3, spectroscopic characterization and multicomponent isobars suggest that the enhanced performance under humid conditions arises from preferential stabilization of the CO2-inserted phase in the presence of water. The favorable performance of ee-2–Mg2(dobpdc) is further demonstrated through comparison with a benchmark material for this separation, zeolite 13X, as well as through extended pressure cycling experiments.

Finally, Chapter 5 builds upon the previous chapters in this work to advance a diamine-appended framework toward commercialization in upgrading crude biogas to biomethane, a renewable natural gas equivalent. Using the principles outlined in previous chapters, the material dmpn–Mg2(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane) is identified as a promising candidate for this separation, and its performance in capturing CO2 from CO2/CH4 mixtures is first demonstrated at the laboratory scale. Through a collaboration with Mosaic Materials, a start-up company working to commercialize cooperative adsorbents for CO2 separations, the performance of dmpn–Mg2(dobpdc) is then demonstrated in breakthrough experiments with composite pellets at the 30–50 g scale. Importantly, these experiments enable simultaneous monitoring of heat and mass transfer in the adsorbent bed, resulting in data suitable to inform the development of a process model. Finally, in partnership with the Davis Wastewater Treatment Plant, slipstream tests are conducted with a crude biogas stream containing water, oxygen, H2S, and siloxanes. These early results suggest that dmpn–Mg2(dobpdc) is relatively robust to H2S and can withstand short-term exposure to crude biogas feeds, representative of a process failure in upstream pretreatment units. These results represent a promising step toward the commercialization of cooperative adsorbents for CO2 separations.