Reticular Materials

Reticular chemistry is a branch of chemistry that focuses on the design and synthesis of crystalline, highly ordered structures by connecting molecular building blocks through strong bonds, such as covalent or coordination bonds, to make open frameworks.  This field was pioneered by Omar M. Yaghi, who has been recognized by the community for his groundbreaking contributions. Reticular chemistry is at the intersection of inorganic chemistry, organic chemistry, and materials science, revolutionizing how functional materials are developed.

Key features of reticular materials

The most notable examples of reticular chemistry are metal–organic frameworks (MOFs) which consist of metal ions or clusters connected by anionic organic linkers and covalent organic frameworks (COFs) that consist of organic molecules linked via covalent bonds. Another example includes zeolitic imidazolate frameworks (ZIFs). Overarching key features of reticular materials are the following:

Order and porosity

Reticular materials are highly ordered and often porous, making them suitable for applications like gas storage, catalysis, and drug delivery.

Customizable frameworks

The design of reticular materials can be tailored to specific needs through the choice of nodes (metal ions, organic molecules) and linkers (organic ligands, covalent bonds). This tunability allows precise control over physical, chemical, and mechanical properties.

Exceptional surface area

Many reticular materials feature extremely high internal surface areas. One gram of a reticular material can have the internal surface area equivalent to several football fields. These properties enhance their effectiveness in adsorption, separation, and energy storage applications.

Stability and robustness

Depending on the choice of materials and synthesis methods, reticular materials, because of the strong bonds, can be designed to withstand extreme temperatures and harsh chemical environments over extended periods of time, making them suitable for demanding industrial processes.

Commercialization and applications

Reticular materials are being developed and commercialized for a wide range of applications, from environmental solutions to advanced applications in medicine and electronics.

Carbon Capture

Reticular materials are increasingly being used in carbon capture technologies, where their robustness and their ability to adsorb large volumes of gases make them suitable for trapping carbon dioxide (CO₂). Companies like Atoco, Nuada and BASF are pioneering the development of technologies based on reticular materials for CO₂ capture, leveraging their ability to selectively adsorb and capture carbon dioxide molecules from the atmosphere (Direct Air Capture, DAC) or industrial exhaust gases (Post Combustion Capture, PCC). These advancements in reticular materials are expected to significantly improve the cost-efficiency of carbon capture solutions.

Atmospheric Water Generation

The ability of reticular materials to adsorb and desorb water molecules from air makes them suitable for the development of technologies for atmospheric water generation (AWG), a process that extracts moisture from the air to provide fresh water, even in arid regions. This technology holds promise for addressing water scarcity in areas where traditional water resources are limited, offering an environmentally friendly solution for water harvesting and storage. Companies like Air Joule and Atoco are advancing the use of reticular materials in the fight against water scarcity.

Gas Separation And Storage

Reticular materials are particularly well-suited for gas separation and storage, owing to their highly porous structures and the ability to selectively adsorb specific gases. This application is critical in industries such as energy, where efficient hydrogen storage is essential for clean fuel technologies. H2MOF, for instance, uses reticular materials to store hydrogen gas in solid state at high densities, making them viable for use in fuel cells and other hydrogen-based energy systems. Reticular materials are also used for the separation of gases like methane, nitrogen, and carbon dioxide, helping improve efficiency in natural gas processing, air separation, and other industrial processes. Companies like ExxonMobil, UniSieve, and Porous Liquid Technologies are advancing the use of reticular materials in the context of gas separation and storage.

Chemical Protection

Reticular materials are applied in chemical protection, especially in protective equipment such as gas masks. Companies like Numat and Tetramer are utilizing MOFs and other reticular materials in the development of advanced filtration systems. These materials can adsorb hazardous gases and chemicals, offering enhanced protection for individuals in toxic or hazardous environments. Their high surface area and tunable pore sizes make them highly effective at capturing a wide range of harmful substances, including chemical warfare agents, industrial chemicals, and other toxic compounds, making them a valuable component in personal protective equipment (PPE).

Electronics

Reticular materials are also making an impact in the electronics industry, particularly in the development of advanced electronic devices. The unique properties of reticular materials enable the development of flexible and high-performance components such as capacitors, transistors, and photodetectors. The reticular materials’ ability to store and release charge, coupled with their tunable electronic properties, positions them as promising candidates for next-generation electronic devices and sensors technologies.

Biomedical

In the medical field, reticular materials, particularly MOFs, are being assessed and tested for a variety of use-cases, from drug delivery systems to medical imaging. Their high surface area and biocompatibility allow them to be used as carriers for controlled release of therapeutic agents, offering targeted treatments with reduced side effects. Additionally, reticular materials are being explored for applications in diagnostics, such as imaging agents for magnetic resonance imaging (MRI) or as biosensors for detecting disease markers. Their versatility and ability to be tailored to specific medical applications make them a key focus of ongoing research in the biomedical field. Commercial players working on the integration of reticular materials include Vector Bioscience Cambridge, Gilead Sciences and Medtronic.

Sensors

The use of reticular materials in sensor technologies is rapidly expanding due to their ability to selectively adsorb and interact with various gases, liquids, and ions. These properties make them highly effective in detecting and measuring specific substances in the environment. For example, MOFs and COFs are being developed for use in chemical sensors, gas detectors, and humidity sensors. These sensors can be employed in a variety of applications, from environmental monitoring to industrial safety, providing real-time data for detecting pollutants, toxic gases, or changes in environmental conditions. The adaptability and sensitivity of reticular materials make them crucial for advancing sensor technologies in both commercial and industrial settings. Companies working on the implementation of reticular materials in the context of sensors include AstraZeneca, AMGEN, and CSL Behring.

A Selection of Notable Reticular Materials

MOF-5

MOF-5 is one of the earliest metal-organic frameworks (MOFs) discovered, composed of zinc oxide (Zn₄O) clusters and terephthalic acid (BDC) ligands. It is known for its large surface area and high porosity, making it a promising material for gas storage applications, particularly hydrogen storage and carbon dioxide capture. As a foundational MOF, MOF-5 has played a significant role in the development of porous materials for energy and environmental applications.

MOF-74

MOF-74 is a metal-organic framework (MOF) characterized by its one-dimensional (1D) channel structure and honeycomb-like network with high porosity. It is composed of metal ions, such as magnesium (Mg), cobalt (Co), or nickel (Ni), connected by 2,5-dihydroxyterephthalic acid (DHTA) linkers. MOF-74 is particularly notable for its exceptional gas adsorption properties, especially for hydrogen and carbon dioxide. Its high surface area and open metal sites enable efficient gas storage, making it a promising material for energy storage and environmental applications.

MIL-101

MIL-101 is a metal-organic framework (MOF) composed of chromium (Cr) nodes and terephthalic acid (BDC) linkers. It is distinguished by its exceptionally large pore size and high surface area, making it one of the most porous MOFs synthesized. Due to these properties, MIL-101 is used in gas storage, separation, and catalysis, offering potential applications in energy storage and environmental remediation.

MOF-177

MOF-177 is a metal-organic framework (MOF) composed of zinc (Zn) nodes and terephthalate linkers. It is distinguished by its exceptionally large pore volume and high surface area, making it one of the most porous MOFs synthesized. These properties make MOF-177 highly effective for gas storage and separation, particularly for applications involving hydrogen and carbon dioxide capture. Its extensive porosity has contributed to its recognition as a benchmark material in the field of porous frameworks.

UiO-66 (University of Oslo-66)

UiO-66 is a metal-organic framework (MOF) composed of zirconium (Zr) nodes and terephthalic acid (BDC) ligands. It is known for its exceptional chemical and thermal stability, making it a highly versatile material for various applications. UiO-66 is widely studied for gas storage, separation, and catalysis, with its robust structure enabling efficient performance in harsh conditions. Its stability and tunability have made it one of the most extensively researched MOFs in the field of porous materials.

CALF-20

CALF-20 is a metal-organic framework (MOF) composed of zinc ions coordinated with triazolate and oxalate ligands, forming a three-dimensional porous structure. This unique architecture allows CALF-20 to selectively adsorb carbon dioxide (CO₂) over water, making it highly effective for CO₂ capture applications. Notably, CALF-20 exhibits exceptional stability under harsh conditions, including exposure to steam, wet acid gases, and prolonged contact with direct flue gas from natural gas combustion. Its robustness and scalability position CALF-20 as a promising material for industrial CO₂ capture and storage, with potential applications in sectors such as cement production.

CD-MOF

Most MOFs are synthesized through solvothermal methods, but alternative approaches like microwave-assisted, electrochemical, mechanochemical, and sonochemical synthesis also exist. However, traditional MOFs are often unsuitable for in-vivo applications due to potential toxicity from their metal ions and organic linkers. To address this, green synthesis methods using biocompatible metals (e.g., Ca, K, Ti) and safe organic linkers (e.g., peptides, carbohydrates, amino acids, cyclodextrin derivatives) have been developed to reduce health risks and enable a broader range of applications.

Cyclodextrin-based metal-organic frameworks (CD-MOFs), composed of γ-cyclodextrin (γ-CD) and alkali metal cations, are edible MOFs that can be efficiently synthesized on a large scale from natural carbohydrates. Due to their biocompatibility and scalability, CD-MOFs have been explored for applications such as drug delivery, CO2 capture, separation/purification, adsorption, sensors, food packaging, electrical conductors, memristors, photocatalysis, and polymerization.

COF-1

COF-1 (Covalent Organic Framework-1) is the first covalent organic framework. It was synthesized using tetrahedral boronate ester (B–O–B) linkages and biphenyl-based linkers. It is distinguished by its highly crystalline structure and large surface area, making it an excellent material for gas storage and catalysis. As the first COF, COF-1 has played a crucial role in the development of porous organic materials for advanced applications.

COF-108

COF-108 is the first three-dimensional (3D) covalent organic framework (COF), featuring a highly ordered and crystalline structure. It is constructed from tetrahedral boron nodes and pyridine-based linkers, forming an extensive 3D porous network. This architecture provides COF-108 with a significant surface area and open channels, making it a promising material for applications such as hydrogen storage.

COF-505

COF-505 is a notable covalent organic framework (COF) recognized as the first example of molecular weaving. This innovative design involves a 3D interpenetrated framework, where molecular strands are woven together in a manner similar to fabric, creating a highly ordered and stable structure. The molecular weaving approach enhances the mechanical strength and flexibility of COF-505, opening new possibilities for designing durable and functional porous materials for advanced applications.

COF-999

COF-999 is a covalent organic framework (COF) specifically engineered for efficient carbon dioxide (CO₂) capture from ambient air. Discovered by Omar Yaghi and his team at Berkeley, its structure features olefin linkages and is post-synthetically modified with covalently attached amine initiators, leading to the formation of polyamines within its pores. This design enables COF-999 to achieve a CO₂ adsorption capacity of 0.96 mmol g⁻¹ under dry conditions and 2.05 mmol g⁻¹ at 50% relative humidity, both at a CO₂ concentration of 400 ppm. Notably, the material maintains its performance over more than 100 adsorption–desorption cycles in open air, demonstrating exceptional stability. Additionally, COF-999 allows for CO₂ desorption at a relatively low regeneration temperature of 60 °C, which is advantageous for energy efficiency. These properties position COF-999 as a promising material for direct air capture applications, offering a combination of high capacity, rapid kinetics, and durability.

ZIF-8

ZIF-8 (Zeolitic Imidazolate Framework-8) is composed of zinc (Zn) nodes and imidazolate linkers. It is known for its exceptional thermal and chemical stability, as well as structural flexibility. Due to its porous nature, ZIF-8 is widely studied for applications in gas storage and separation, particularly for CO₂ capture. Its high selectivity for CO₂ over other gases makes it a promising material for carbon capture and environmental remediation.

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