Collaborative Research: Investigation of Mass and Energy Transfer Mechanisms in Stimuli-Responsive Smart Sorbents for Direct Air Capture

减轻和消除大气中的二氧化碳等温室气体排放是当今最紧迫的重大挑战之一。解决这一挑战的一种可能方法是通过直接空气捕获技术（DAC）。DAC技术可以直接从大气中提取二氧化碳并永久储存。分离气体混合物的传统方法包括将高压气体吸附到固体表面并在压力降低时释放（解吸）它们（称为变压吸附）或使用温度变化来实现分离（称为变温吸附）。然而，这些方法不适合DAC系统，因为驱动CO2传质的浓度梯度非常小。因此，这些方法在能源使用方面效率非常低。此外，目前最先进的基于胺或离子液体的吸附剂材料需要大量的能量来解吸二氧化碳并再生吸附剂。此外，由于大多数吸附剂材料具有低导热性，外部加热再生效率低下，并导致额外的热损失。开发新的材料和技术来解决这些缺点，并使大规模DAC系统的成功实施至关重要。该项目将研究一类二氧化碳吸附剂材料，该材料可以通过施加外部磁场来诱导释放吸附的二氧化碳。磁场在材料内部产生局部热量，因此不需要外部能量输入。该研究将对这些磁场响应吸附剂（MF-RSs）的基本能量和传质机制产生新的见解。该项目还将为密苏里科技大学和南加州大学的本科生提供研究经验、课程开发和K-12 STEM推广的机会。这项工作的目的是获得用于DAC系统的MF-RSs中的能量和传质机制的基本理解，即F3O4磁性纳米颗粒与微孔金属有机框架（F3O4/MOF-amine）或介孔氨基硅酸盐（Fe3O4/SiO2-amine）的复合材料。由于磁性纳米颗粒的静态磁滞和动态磁芯损耗，外加磁场产生局部热。吸附的CO2在没有外部加热的情况下解吸，克服了大多数吸附剂材料导热系数低的问题，避免了外部加热方法带来的热损失。将进行计算和实验研究，以了解影响MF-RSs中CO2释放和系统再生的因素。在暴露于外部磁场时，导致磁性吸附剂低能量释放CO2的分子间吸引将被表征。具体而言，研究将探讨磁场感应下电子转移扰动的程度。该研究还将阐明热容-磁化权衡对扩散热传递和分子传递的影响。最后，研究了吸附剂在氧气、氮气和水存在下再生过程中磁场触发的CO2输运机制。大量的实验和计算技术将被应用于揭示在外部磁场存在下，从MF-RSs吸附和解吸CO2的能量和质量传递机制。这些技术包括分子水平的原位光谱测量和瞬态解吸测试，如电子顺磁共振（EPR）光谱、频域热反射（FDTR）、零长度柱（ZLC）和磁感应摆动吸附（MISA），这些技术将与密度泛函数理论（DFT）和纳米级分子动力学模拟相结合。该研究将为开发低能耗吸附剂再生系统开辟新的途径。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Mitigating and removing greenhouse gas emissions such as carbon dioxide (CO2) from the atmosphere is one of today's most pressing grand challenges. One possible approach to address this challenge is through direct air capture technologies (DAC). DAC technologies can extract CO2 directly from the atmosphere to be stored permanently. Traditional methods for separating gaseous mixtures involve either adsorbing high-pressure gases onto a solid surface and releasing (desorbing) them when the pressure is reduced (known as pressure swing adsorption) or using temperature changes to achieve separation (known as temperature swing adsorption). However, these methods are unsuitable for DAC systems because the concentration gradient, which drives the mass transfer of CO2, is very small. As a result, these methods are highly inefficient in terms of energy usage. Additionally, the current state-of-the-art sorbent materials based on amines or ionic liquids require a lot of energy to desorb the CO2 and regenerate the sorbents. Furthermore, since most sorbent materials have low thermal conductivity, externally heating them for regeneration is inefficient and leads to additional heat losses. It is crucial to develop new materials and technologies that can address these drawbacks and enable the successful implementation of large-scale DAC systems. This project will investigate a class of CO2 sorbent materials that can be induced to release the adsorbed CO2 by applying an external magnetic field. The magnetic field generates local heat within the material, so external energy input is not required. The research will yield new insights into the fundamental energy and mass transfer mechanisms in these magnetic field-responsive sorbents (MF-RSs). The project will also provide opportunities for undergraduate student research experiences, curriculum development, and K-12 STEM outreach at the Missouri University of Science & Technology and the University of Southern California.The purpose of this work is to gain a fundamental understanding of energy and mass transfer mechanisms in MF-RSs for use in DAC systems, namely, composites of F3O4 magnetic nanoparticles and microporous metal-organic frameworks (F3O4/MOF-amine) or mesoporous aminosilicates (Fe3O4/SiO2-amine). The external magnetic field generates local heat due to the static hysteresis and dynamic core losses of the magnetic nanoparticles. The adsorbed CO2 is desorbed without external heating, overcoming the issue of low thermal conductivity of most sorbent materials and avoiding the heat losses accompanying externally heated methods. Computational and experimental investigations will be conducted to understand the factors affecting CO2 release and system regeneration in MF-RSs. The intermolecular attractions that result in the low-energy release of CO2 from magnetic sorbents upon exposure to an external magnetic field will be characterized. Specifically, the research will probe the extent of electron transfer perturbation upon magnetic field induction. The study will also elucidate the effects of heat capacity-magnetization tradeoffs on diffusive thermal and molecular transfers. Finally, the magnetic field-triggered CO2 transport mechanisms during sorbent regeneration in the presence of oxygen, nitrogen, and water will be investigated. A host of experimental and computational techniques will be applied to reveal the energy and mass transfer mechanisms of CO2 adsorption and desorption from MF-RSs in the presence of an external magnetic field. These techniques include molecular-level in-situ spectroscopic measurements and transient desorption tests such as electron paramagnetic resonance (EPR) spectroscopy, frequency-domain thermoreflectance (FDTR), zero-length column (ZLC), and magnetic induction swing adsorption (MISA), which will be combined with density-functional theory (DFT) and nanoscale molecular dynamics simulations. The investigation will open new avenues for developing low-energy sorbent regeneration systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.