KARGEN Technology Blog | MOFs for Dehumidification in Air Water Harvesting

Classification Topics: MOFs, Metal-Organic Frameworks, Dehumidification, Desiccants, Water Vapor Adsorption, Air Water Harvesting, Water Treatment, Freshwater Production

01. The Necessity of Air Water Harvesting

1.1 Overview of Water Scarcity

According to the UN's 2023 World Water Development Report, global water usage has been increasing by about 1% annually over the past 40 years, with most of this growth occurring in developing countries. Climate change exacerbates the issue by causing seasonal water shortages in previously water-rich areas and worsening the situation in already arid regions. By 2050, it is projected that about 10% of the global population will live in areas experiencing severe water scarcity. Water-scarce regions are categorized into three types based on geographic, climatic, and economic differences: resource-based scarcity, quality-based scarcity, and infrastructure-based scarcity.

Resource-based scarcity occurs when the availability of water resources is less than the demand due to regional distribution differences. In China, such areas are mainly in the northwest, north, and Liaodong Peninsula; globally, these regions include North Africa, the Middle East, the northwest USA, and the Sahel. To alleviate this, measures like improving water use efficiency and inter-basin water transfer projects (e.g., the South-to-North Water Diversion Project) are necessary.

Quality-based scarcity often affects water-rich regions like Jiangsu-Zhejiang and the Pearl River Delta in China, and parts of Mexico and India. Pollution from urban waste and industrial wastewater exceeds the natural self-purification capacity of these waters, leading to contamination. Addressing this requires pollution control measures such as wastewater treatment and desalination.

Infrastructure-based scarcity arises in areas with inadequate water management infrastructure. Regions like the Yangtze and Pearl River basins in China, and parts of South Asia and Africa, face this issue. Effective solutions include building reservoirs, improving water management facilities, and enhancing urban water supply networks.

1.2 Advantages and Potential of Atmospheric Water Harvesting

The atmosphere holds about 10% of the Earth's freshwater, equivalent to 13 trillion liters, representing a significant water resource. Historically, atmospheric water harvesting has been limited to collecting dew and fog, which only works in high relative humidity environments and is not ideal for large-scale applications. With the advent of MOFs (Metal-Organic Frameworks), researchers have discovered that MOFs exhibit excellent water vapor adsorption even in low humidity conditions, and can achieve desorption at low temperatures (~65°C). MOF-based small-scale atmospheric water harvesting devices can be powered directly by solar energy, enabling scalable deployment at the household level.

Compared to traditional water treatment methods like seawater desalination and wastewater sedimentation, atmospheric water harvesting, though producing less water per unit time, has a simpler process, lower energy consumption, and higher-quality water that can be used directly for household and industrial purposes. Importantly, while traditional methods are suited for water quality issues in regions with natural water sources or wastewater, MOF-based systems can operate in arid regions with high temperatures and low relative humidity, and their low-temperature desorption capabilities reduce or eliminate the need for additional energy. This makes them suitable for addressing water shortages in regions with energy deficits or inadequate infrastructure.

02. What is Atmospheric Water Harvesting?

2.1 Principles and Classification

As illustrated in Figure 1, atmospheric water harvesting technologies can be classified based on their principles and materials used into four types:

1.Desiccant Passive Collection Method

2.Thermo-responsive Hydrophilic Switching Method

3.Hygroscopic Salt Osmosis Method

4.Active Air Cooling Method

Among these, the thermo-responsive hydrophilic switching method and the hygroscopic salt osmosis method are relatively new concepts that are still in the laboratory testing phase, so they will be briefly introduced. In contrast, the desiccant passive collection method and the active air cooling method are more developed and are gradually moving towards practical application. A detailed comparison of these methods will be provided in the following sections.

Thermo-responsive Hydrophilic Switching Method: This method employs polymer materials, such as polyisopropylacrylamide gels, whose hydrophilicity decreases with increasing temperature. At night, the polymer materials are hydrophilic and capture water vapor, which is then gradually released as vapor when the temperature rises during the day. The advantage of this method is that the captured water vapor can be recovered directly in liquid form rather than as vapor, making the thermoresponsive hydrophilic materials also known as "molecular reservoirs." A key challenge in this technology is that most materials are only effective in capturing and releasing water vapor in medium to high humidity environments. In low humidity conditions, higher temperatures are required for vapor release, necessitating significant additional energy input [5].

Hygroscopic Salt Reverse Osmosis Method: This method utilizes hygroscopic salts (e.g., LiCl, CaCl₂) that have a strong affinity for water vapor, combined with reverse osmosis principles. After the hygroscopic salts absorb moisture and form concentrated salt solutions, freshwater is recovered through pressure-driven reverse osmosis. The advantage of this method is its relative energy efficiency; however, it heavily relies on the development of high-pressure reverse osmosis technology and the performance of permeable membranes. This technology also struggles in low humidity environments and only achieves high water yields in high relative humidity conditions.

2.1.1 Active Air Cooling Method

The active air cooling method is similar to the principle of air conditioning compressors. This is introduced to readers through a simplified humidity diagram (Figure 2).

Simplified Humidity Diagram

● Absolute Humidity: Also known as moisture content, it refers to the weight of water molecules per unit volume of air at a given pressure and temperature, representing the actual water content in the air.

● Relative Humidity: Refers to the ratio of the actual vapor pressure of water in the air to the saturation vapor pressure at a given temperature.

● Dew Point Temperature: At a constant moisture content, it is the temperature to which the air must be cooled for its relative humidity to reach 100%, causing water vapor to undergo a phase change and begin to condense into liquid droplets.

In the active air cooling method, humid air is blown over an evaporator where it exchanges thermal energy with a refrigerant. The temperature is lowered to the dew point or slightly below it, causing some of the water vapor in the air to condense on the walls of the evaporator and subsequently be collected in a reservoir. As shown in the process from a→b in Figure 2, at 30°C and 20% RH, the air temperature needs to be reduced to 4°C to extract fresh water from the air. Comparing processes a→b and c→d, it can be observed that, at the same temperature, air with higher RH has a higher dew point temperature and is easier to extract water from. The advantages of the active air cooling method include a large water output, potentially reaching 1000 ^m³/day[7]; in regions with abundant green energy sources such as wind or solar power, renewable energy can be used to power the compressor for green production; and the technology is relatively mature, with large-scale equipment already in use. However, the drawbacks are also significant. As shown in Figure 2, air humidity greatly affects the energy consumption of the equipment, and in arid regions, a large amount of fossil energy is needed to extract water from the air. Additionally, apart from cooling, fans are required to drive air circulation and provide sufficient air to the cooling surfaces. Therefore, such equipment tends to be large and incurs high operating costs.

2.1.2 Drying Agent Passive Collection Method

The drying agent passive collection method has a long history. Initially, drying agents were used to capture moisture from the air in shaded areas. The moisture was then desorbed and collected by exposing the drying agents to sunlight. The regenerated drying agents were subsequently cooled in the shade and used for the next water capture cycle. Today, a simplified system has been proposed: drying agent materials absorb moisture from humid air at night and regenerate using solar heat during the day. In this scenario, the drying agents can be cooled by the air itself at night. Compared to active air cooling systems, drying agent-based air water collection devices are simpler to operate, achieving a cycle efficiency of >90% with higher thermal efficiency. New desiccant materials, such as MOFs, can capture moisture even at low RH and achieve desorption under low energy input conditions. For practical applications, air water collection technologies should be efficient, economical, scalable, and stable. Among the four air water collection methods mentioned, drying agent-based systems are the best choice for desert and arid regions because they maintain moisture absorption performance regardless of environmental conditions and have lower energy requirements.

2.2 Drying Agent Material Selection

In air water collection systems, the choice of drying agent is crucial for achieving high energy efficiency and low cost. Besides the adsorption capacity, the adsorption speed and operational cycle of the drying agent need to be considered. The choice of drying agent should meet conditions such as hydrophilicity, chemical stability, and adjustable pore size for fine-tuning adsorption curves and kinetics.

2.2.1 Liquid Drying Agents

Common industrial liquid drying agents include glycol-based solutions, halide solutions, and salt solutions, such as CaCl₂ and LiBr solutions. However, in air water collection systems, glycol-based substances are excluded due to their strong volatility. LiCl solution, although stable and exhibiting medium to low humidity adsorption (30-40%), is relatively expensive. Halide salts (such as LiBr) can dry air to RH 6%, showing outstanding performance, but their corrosiveness can cause irreversible damage to the equipment. Weak organic acid salts, such as sodium formate, potassium salts, or acetate salts, have lower volatility, corrosiveness, and viscosity, making them potential alternatives to the aforementioned liquid drying agents in many applications. However, these materials only reduce air RH to around 30%. Therefore, common liquid drying agents are not suitable for air water collection. Despite their lower regeneration temperature (about 65-80°C), their adsorption performance under low humidity conditions is limited, and their water production capability is restricted. Additionally, devices based on liquid drying agents often use combinations of liquid drying agents with materials like sand or activated carbon, which have not yet been developed into highly integrated, scalable devices and still require improvement.

2.2.2 Solid Drying Agents

Traditional drying agents (such as silica gel and zeolites) were used in air water collection technology due to their good hydrophilicity. However, their water adsorption capacity is relatively low (about 0.2 g/g), and their high hydrophilicity means that high energy consumption is required to release the collected water. Additionally, these materials have slow adsorption and desorption kinetics, leading to longer water collection cycles, making them unsuitable for air water collection technology.

MOFs (Metal-Organic Frameworks) have become promising drying agents for air water collection due to their highly tunable and permanently porous structure. MOFs outperform traditional drying agents in performance. When selecting MOFs for air water collection devices, materials with adsorption isotherm inflection points in the RH 10-30% range should be preferred because they provide both high hydrophilicity and low-temperature regeneration properties.

Guangdong Advanced Carbon Materials Co., Ltd.is the first domestic company to achieve ton-level production capacity of MOFs and offers several MOF products with mature preparation processes and excellent performance. Figure 3 shows the adsorption kinetics test results of Carbon Language MOF products at RH 20% and adsorption curves at 25°C, indicating that the material achieves an adsorption amount of 0.25 g/g within 10 minutes, with better adsorption performance and speed than traditional adsorbents under low humidity conditions.

Adsorption Isotherms and Kinetics Curves of Carbon Language MOF Products

2.3 Air Water Collection Equipment

Professor Omar M. Yaghi's team, known as the "fathers of MOFs," has conducted extensive research on the application of MOFs for air water collection. Their goal was to design materials and build equipment to collect fresh water from desert air, successfully demonstrating that MOFs can capture and produce collectable liquid water in hot, low RH outdoor environments. Figure 4 illustrates the development process of air water collection devices from concept validation to commercialization, showing the evolution of MOF-based systems from simple prototypes to standardized devices, with production rates increasing from 0.001 to 100 L/kg over five years. Figure 5 outlines the basic design concept of the device: it consists of a water adsorption unit containing MOF materials and a water collector with an outer shell. At night, the box cover is opened to allow the MOFs to absorb moisture from the air. During the day, the cover is closed, sealing the system. The humid hot air carrying water vapor disperses from the MOFs to the surrounding environment, cooling to the dew point temperature, leading to condensation of the water vapor, which then collects at the bottom of the outer shell.

Development Process of MOF-Based Air Water Collection Devices

MOF-Based Desert Air Water Collection Device

In testing, the MOF material used was MOF-801. At RH 10%, MOF-801 can be regenerated at 65°C, but the released water vapor only condenses when the temperature is below 20°C. To create a significant temperature gradient, the water adsorption unit was designed as an insulated body, and all surfaces except the MOF surface were coated with an infrared-reflective coating. This design ensures that only the MOF surface is exposed to solar radiation, which helps heat the MOF while keeping the condenser temperature low, facilitating efficient collection of condensed water.

The performance of MOF materials is crucial in establishing and using air water collection devices. Professor Yaghi's team has recently developed various MOFs for water vapor capture in low humidity environments. MOF-303, synthesized from aluminum salts and pyrazine-2,3-dicarboxylate, features hydrophilic one-dimensional pores with a diameter of 6 Å and has a maximum adsorption capacity of about 0.48 g/g. By mixing MOF-303 with 33 wt% of non-porous graphite, its thermodynamic performance was enhanced, and experiments showed that each kilogram of MOF-303 can collect 175 g of pure water from desert air. The team also developed MOF-LA2-1 from MOF-303 using a long-arm (LA) linker extension strategy with aluminum salts and furan-2,5-dicarboxylate. At RH 20%, MOF-LA2-1 demonstrated an adsorption capacity of 0.45 g/g and excellent cyclic performance, indicating its potential application in air water collection in the future.

03. Relevant Companies

3.1  Drupps

Drupps was established in 2019 as a green technology startup based in Sweden, primarily focusing on water recovery from industrial air. Drupps has developed a bromide-lithium magnesium salt composite liquid adsorption material called Absium, which effectively recovers and purifies water from industrial air streams.

Figure 6 illustrates the Drupps Atmo system based on Absium: During the production process, waste steam is condensed, and the released heat is recovered. Meanwhile, water vapor in the air above the factory is absorbed by Absium and transferred to an evaporator. The recovered heat provides the energy for desorption, resulting in purified water. Testing has shown that the operational cost of this system is 3 €/m³.

Schematic of Drupps Atmo System

3.2  Watergen

Watergen, established in Israel in 2009, is a company specializing in air-to-water devices. Its core technology, the "GENius" water extraction system, is the first air-to-water heat exchanger made from food-grade polymers. Watergen’s air-to-water devices can quickly process air through the GENius system and produce water via active cooling (see Figure 7). The equipment requires only power to operate, with the cost of producing one liter of drinking water being about one yuan. The largest model, GEN-M PRO, can produce up to 900 liters of water per day. These devices are best suited for environments with RH > 40%, making them more suitable for humid and hot regions such as Southeast Asia and Latin America, rather than arid desert regions.

Operating Flow Diagram of Watergen Water Extraction System

3.3  Atoco

To further advance the commercial application of MOFs, Omar Yaghi, the founder of MOFs, established Atoco in 2023. Atoco is dedicated to developing sustainable atmospheric water collection and carbon dioxide capture solutions through the design and development of MOFs, aiming to address issues of water scarcity and global warming. Atoco's achievements are based on the research conducted by Yaghi's team, which has developed materials capable of efficiently capturing and generating pure water from the atmosphere even under dry conditions with RH ≤ 20%. Additionally, the water collection devices developed by Atoco can operate in a passive mode without using electricity, achieving zero carbon footprint and off-grid operation.

3.4 Hurrain

Hurrain is a domestic innovation-driven company based on the Advanced Manufacturing and Research Center of Tsinghua University. The core technology, developed by Tsinghua University's team, is an air water capture technology that significantly reduces the heat loss of water vapor at low humidity through micro-topology control and functional composite moisture-absorbing materials. Combined with advanced heat pump technology, the system adjusts based on the enthalpy and humidity changes of water vapor to produce clean water rapidly under low humidity conditions. The company's Yunhai series products are expected to reach a production capacity of up to ten thousand tons, with a single container or shell-type device capable of continuously capturing over 10,000 tons of pure water annually in low-temperature, low-humidity environments, with water production energy consumption ranging from 80 to 180 kWh/t. Additionally, Hurrain has also developed a range of household water production machines with capacities from 10 to 100 liters per day, achieving water production energy consumption as low as 0.3 kWh/L.

04. Conclusion

Air-to-water technology is an emerging method for water collection that, with the development of MOFs as dehumidification materials, is gaining increasing attention. Compared to other water collection technologies, MOFs require less energy input for water adsorption-desorption cycles and are effective across a wider range of temperatures and humidity levels, offering new possibilities for addressing water supply challenges in hot and arid regions. However, current air-to-water devices on the market have yet to use MOFs, possibly due to the extensive research on water-adsorptive MOFs and a lack of exploration into integrating these materials with devices. Nonetheless, we believe MOFs hold significant potential for the air-to-water field in the future.

Guangdong Advanced Carbon Materials Co., Ltd. is the first domestic company to achieve ton-scale production of MOFs, with hundreds of functional MOFs developed. Our team has substantial technical expertise in MOF synthesis and applications. For more information on MOF applications and customization, please contact the KARGEN team. Follow us to learn more about MOF applications and cutting-edge technology developments.

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