Two years ago, I reported on a really cool device developed by Dr. Omar Yaghi.  One that is able to produce pure water right from the air.  (OK. It does require that there be humidity- water vapor- in the air.  We are not talking about producing something from nothing.)

To be honest, I was disappointed I hadn’t heard more about the invention.  Because the design was elegant (albeit pricey) and could solve a major issue around the world- a plethora of water shortages.  You do recall that by the year 2025, there will be 1.8 billion folks without sufficient water- a 50% increase in water shortages since 2014.

The device relied upon a class of organic compounds developed by Dr. Richard Robson (University of Melbourne) called ‘coordination polymers’.  (I won’t get very technical, but these materials use metals linked together with ligand [long chains of organic molecules].  Normally, a polymer would be built around a central carbon atom- the metal in this case is what is different- and the number of ligands can be altered by choosing a different metal.  The big issue- these polymers were highly reactive [susceptible to combinations with other materials] and, therefore, not very stable.)

That’s where Dr. Yaghi made his improvements.  Employing negatively charged ligands joined by clusters of metal atoms (which he called metal organic frameworks [MOF]), the bonds were much stronger and the compounds far more stable.  His MOF-5, based upon zinc, was stable to 300C.  But that wasn’t the coolest thing about this MOF- it’s that it had an internal surface area approaching 2900 square meters per gram.  (That’s bigger than ½ a football field!  Which means- I kid you not- that if you had a flask with MOF-5, it could hold more gas than the flask could contain without any MOF-5 present- at the same pressure.  Yes, this MOF is a sort of ‘super-sponge’.  And, it turns out these MOF are pretty selective- the pore size determines which gases can be contained and which will be barred.)

Dr. Yaghi also produced a zirconium based MOF (which he named MOF-801) that had the perfect size to allow water vapor in (or out).  Teaming with Dr. Evelyn Wang (MIT), they produced a palm-sized device capable of harvesting water from the air.  (The MOF-801 crystals were pressed into a sheet of copper, with the entire mess encased in a plastic box.)

As I reported in 2017, the concept has box  left open overnight, when the MOF sucks water molecules from the air.  Replacing the top during the day when the sun beats down upon it means the temperature is raised and the device ‘vomits’ the water out of the pores.  This prototype worked fine with only 20% relative humidity (which is pretty close to what obtains in deserts).

The next step was to scale it up.  Because the prototype only produced 140 ml (about 5 ounces) of water per kg of MOF per day.

But, there was yet another development issue- his device used zirconium!  Sure, that metal resists corrosion and can survive high temperatures, but it’s pretty darned expensive- running about $ 150 a kilogram.

So, now, he is basing his work on MOF-303, which is an aluminum polymer.  (He patented this compound, which employs aluminum cations connected to 3,5, parazoledicarboxylic acid ligands.) The MOF is produced in water without any catalyst- and the process can be easily scaled up for major production.

Thankfully, this design seems capable of producing 230 ml of water per kilogram.  (Although this is twice as much as his prototype, he is pretty sure he can up the production to 2 liters, if he can connect solar panels and use that energy to heat and cool the box multiple times during the day.  (Yes, this means the device is no longer going to rely on the 24 hour passive cycle of day and night.)

Yaghi also instituted a company (Water Harvesting, Inc.), so we can finally expect the product to start popping up around the world.

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