Designed Development of Ultra-microporous, Amine-rich and Hydrophobic Metal Organic Frameworks for Humid CO2 Capture and Natural Gas Purification

Abstract

Carbon dioxide (CO2) is a prominent greenhouse gas responsible for global warming, primarily driven by human activities such as deforestation and the combustion of fossil fuels. Natural processes, including respiration and volcanic eruptions, also contribute to CO2 emissions. Recent data from NASA (2017) indicate that atmospheric CO2 levels have reached alarming concentrations, particularly in the Mid Troposphere, where CO2 concentrations have risen by 110 ppm over the past decade and a half, from 365 ppm to 475 ppm. In response to this pressing issue, there is an urgent need to develop innovative technologies to reduce atmospheric CO2 levels. Such efforts have environmental benefits and hold economic potential, especially in coal-based power production. Carbon Capture and Storage (CCS) is a viable approach among the promising solutions. While current industrial CO2 capture methods rely on liquid amine-based sorbents, energy-efficient and cost-effective adsorptive separation techniques employing solid sorbents have emerged as the preferred choice. Large-scale adsorptive separation of CO2 from industrial gas mixtures effectively utilizes materials such as zeolites, porous carbons, and silica gels as sorbents. Developing next-generation sorbents with improved efficiency is crucial to meet the ever-growing demand for more energy-efficient, cost-effective, and environmental friendly gas separation processes.

Furthermore, transitioning from coal to natural gas as an energy source holds significant potential for drastic reductions in CO2 emissions. However, the cost of natural gas production is exacerbated by the challenge posed by the separation of methane (CH4) from nitrogen (N2), the two major components of crude natural gas. Notably, both have comparable physical properties, making their separation hard. Addressing this challenge is critical in unearthing natural gas's full environmental and economic advantages as a cleaner energy alternative.

Metal Organic Frameworks (MOFs), made up of metal ions/metal-oxo clusters connected via organic linkers, have been recognized as well-suited materials for the abovementioned application. Several reports on gas separation using both Micro/Ultra-micro porous MOFs have appeared in the last decade. In our research endeavors, we have crafted a series of ultramicroporous Metal-Organic Frameworks (MOFs) exhibiting compelling CO2 capture characteristics. The idea is to link simple metal nodes with short, readily available organic linkers to form 3D frameworks. Notably, Fe(pyc)2(OH), known as IISERP-MOF22d, represents a flexible and ultramicroporous MOF demonstrating impressive CO2 capture capabilities, evidenced by a distinct stepped isotherm behavior exclusively with CO2. This unique feature sets it apart as other investigated gases fail to access its pores. Also, coordination flexibility assisted gate-opening solely by CO2 is quite unexpected from a rigid ultramicroporous framework.

Furthermore, we prepared another Cu-based MOF referred to as IISERP-MOF22. This MOF was synthesized using readily available and cost-effective starting materials and showcased commendable CO2 uptake, along with a moderate heat of adsorption and notable recyclability. Importantly, despite having multiple CO2 binding sites, the MOF is hydrophobic, making it suitable for humid CO2 capture. Learning from this Cu-MOF, we built another amine-functionalized Zinc-MOF designated as IISERP-MOF33, wherein we employ zinc to ensure superior oxidative stability. It has high CO2 capacity, and due to its lowered interaction with water, it exhibits selective uptake of CO2 even under high relative humidities. IISERP-MOF33's exceptional CO2 capture attributes makes it a promising candidate for addressing CO2 capture from humid flue gas.

Additionally, we developed an amine- & carboxylate-functionalized microporous Fe-based MOF that can upgrade natural gas to meet pipeline specifications, particularly from the complex CO2/N2/CH4 ternary mixture. These material advancements can significantly impact sorption based CO2-separation processes offering promising solutions for addressing CO2 management in various industrial sectors.