4.0: Introduction

Water from Air: A Chemistry Breakthrough for Clean Drinking Water

                                    

Figure \(\PageIndex{1}\): A graphic with basic facts about Omar M. Yaghi, inventor of metal-organic frameworks (MOFs). MOFs have since been used in the applications of greenhouse gas capture, drug delivery, catalysis, and water harvesting. Image from Wikimedia Commons, with permission under Creative Commons license CC-BY.

In this chapter, we’ll explore the properties of gases, including how their volume, pressure, temperature, and composition influence their behaviour. One of the most remarkable recent advances in chemistry uses that knowledge to tackle a global challenge: water scarcity.

Chemist Omar M. Yaghi grew up in Jordan, where water was delivered to his neighbourhood only once a week, sometimes for just a few hours. That experience shaped his vision as a scientist: to use chemistry to create new materials that meet basic human needs. In 1995, Yaghi and his lab invented an entirely new class of materials called metal-organic frameworks, or MOFs.

These extended structures (polymers) are made from metal ions connected by organic linkers. MOFs contain vast networks of nanoscopic pores, giving them incredibly high surface areas: a single gram of MOF can have enough surface area to cover a football field. Their size, shape, and chemical composition can be tuned to capture and store specific gas molecules through intermolecular interactions. These gas molecules can later be released by increasing temperature or pressure.

 

Figure \(\PageIndex{2}\): Illustration of a MOF framework that demonstrates how water molecules can be entrapped in the structure’s nanopores. Adapted from Xu and Yaghi, ACS Central Science 2020, 6(8) with permission under Creative Commons license CC-BY; copyright 2020 American Chemical Society.

MOFs were originally designed to store gases like hydrogen and capture carbon dioxide from industrial emissions, but in 2014 Yaghi’s team discovered that certain MOFs also absorbed water vapour from the air, even in very dry conditions. This inspired Yaghi to develop MOFs specifically for harvesting water from air. Yaghi’s team built a simple prototype: an inner box containing a MOF called MOF‑801, nested inside a lidded outer box. At night, the lid opened to expose the MOF to the air for water capture; during the day, it closed to trap heat from the sun. The rise in temperature caused the MOF to release the captured water vapour, which condensed inside the box and was collected as liquid water. In the Mojave Desert, this device harvested up to 1 L of drinkable water per kilogram of MOF per day, using only ambient temperature and pressure, no electricity required.

Figure \(\PageIndex{3}\): An illustration of the simple device used by Yaghi and his team for water harvesting from desert air under ambient temperature and pressure. The nighttime configuration for water vapour collection by the MOF exposed to open air is shown on the left. On the right, the daytime configuration is shown. The lid of the outer box is on so that heat from the sunlight causes vapour release, condensation, and liquid water collection. Adapted from Dao, Chien, and Strek VNUHCM Journal of Science and Technology Development 2020, 23(2) with permission under Creative Commons license CC-BY.

Yaghi’s team has since developed improved MOFs like MOF‑303 and partnered with companies to scale up production. With further optimization, these materials could produce tens of litres per kilogram of MOF per day, offering a low-cost, low-energy source of renewable freshwater, even in arid regions. Thirty years after his first MOF paper, Yaghi’s vision is helping meet one of the most fundamental needs of life through his water harvesting technology.

While the development of MOFs is rooted in materials chemistry, their ability to trap and release gas molecules depends on temperature, pressure, and other gas properties. In this chapter, we explore the principles that help us describe and predict gas behaviour.

 

Sources:

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5. University of California - Berkeley. In desert trials, next-generation water harvester delivers fresh water from air. Phys.Org. https://phys.org/news/2018-06-trials-next-generation-harvester-fresh-air.html.

6. University of California - Berkeley. Omar M. Yaghi. UC Berkeley Research. https://vcresearch.berkeley.edu/faculty/omar-yaghi

7. Xu, W.; Yaghi, O. M. Metal–Organic Frameworks for Water Harvesting from Air, Anywhere, Anytime. ACS Cent. Sci. 2020, 6 (8), 1348–1354. https://doi.org/10.1021/acscentsci.0c00678.