Thirsty MOF sucks more water from the air

Researchers have reported a record-setting performance for a water-trapping metal-organic framework (MOF) that could one day be used for water harvesting in arid environments or for more efficient air-conditioning (ACS Cent. Sci. 2017, DOI: 10.1021/acscentsci.7b00186). Porous materials can spontaneously pull water out of the air even at low humidity if their pores are the right size for the reversible adsorption of a large amount of water and their interior surfaces are hydrophilic. Building on their previous work, researchers led by Mircea Dincă at Massachusetts Institute of Technology tested a group of MOFs with pores approximately 2 nm in diameter made with manganese, cobalt, or nickel ions bound to triazolate linkers. The researchers found that water spontaneously enters the pores at as low as 28% relative humidity. At 30% relative humidity, consistent with nighttime conditions in arid climates, the cobalt MOF adsorbs almost 90% of its own weight in water, approximately double that of the next-best known material, one of a group of zirconium-based MOFs. The researchers calculated that if the cobalt-containing MOF were used in a hypothetical adsorption heat pump, the trapped water could be stripped from the material at just 55 °C. This means the device could be powered by waste heat from a car engine, for example. The MIT team also predicts that if the cobalt MOF were used in a recently reported device that uses solar energy to capture water from the air (C&EN, April 17, page 8), the device would be able to deliver almost three times as much water per cycle.