Molecular Sieves

Announcing the 2025 Nobel Prize in Chemistry last week, won by Susumu Kitagawa, Richard Robson and Omar M. Yaghi for work on metal-organic frameworks, one of the judges described MOFs as ‘kind of like Hermione’s charmed handbag in the Harry Potter series’: an object that appears to be ‘small on the outside’ but ‘very, very large on the inside’. Elsewhere, MOFs have been described as ‘magic mops’ and ‘sponge crystals’. They are materials that link organic molecules and metals in a repeating pattern to form an orderly molecular lattice – their use of space is so efficient that a couple of grams can have as much surface area as a football pitch. Neat trick.

The obvious utility – always a concern for the Nobel committee, especially in chemistry, especially in recent years – is to use that vast surface to capture and store other molecules: water or carbon in the air; pollutants in the environment; drugs or toxins in the body. There are startups and clinical trials exploring these uses, but nothing yet at scale.

That said, they are unquestionably fascinating materials. During the 1980s, Robson was exploring how various substances can form crystals – as in the densely ordered network of carbon atoms that makes up a diamond. Diamonds are hard and impermeable, but not all crystals need to be so. Robson found that with the right laboratory coaxing, he could form crystals of a molecule made up of a metal (copper) and an organic compound (tetracyanotetraphenylmethane) with evenly ordered porous spaces in its structure.

The paper he published on it in 1990 is bracing in its clear view of the new terrain ahead, announcing ‘the deliberate design and construction of a new and potentially extensive class of solid materials with infinite framework structures resembling scaffolding’. It notes that these materials could work as a molecular sieve (or mop), selectively admitting or expelling various chemicals. But most important, it anticipates MOFs modular (or tunable, as most chemistry literature puts it) nature. By using a longer, shorter or differently shaped organic compound in the synthesis (or a different metal), the structure and selectivity of the pores in the network could be customised.

Yaghi devised new methods and components to make Robson’s original, somewhat fragile, crystals more stable and useful – and coined the name ‘metal-organic framework’. Kitagawa also made technical advances, especially with MOFs that could trap and diffuse gases. The promise of endless customisability appealed to chemists – who tend to be inveterate and enthusiastic tinkerers – and the field has exploded over the past two decades. Some ten thousand papers on MOFs were published last year, and there are now more than 100,000 different structures listed as MOFs in the Cambridge Structural Database. The vast majority are laboratory noodling (the Financial Times last week called MOFs ‘molecular lego’), but a few have shown real promise.

One has been used to bind sulphur dioxide, separating it from a mix of chemicals found in a factory flue stack. Once saturated, the MOF can be taken elsewhere, the sulphur dioxide can be removed, and the MOF returned to start again. There are trials using relatively large amounts of MOF product (several kilograms) to trap potable water from the air – even in extremely arid conditions. There is work on the loading and release of drugs in the body, and in creating vast microscopic reaction chambers within MOFs to make electrochemical processes – such as in a battery – more efficient.

The caveat, of course, is that none of the trials have progressed to a scale that could be called useful – whether in a commercial or planetary sense. MOFs suffer from the usual problems of scientific innovations trying to achieve escape velocity from the laboratory: they are difficult or expensive to make in large quantities, can be more fragile than expected, or may not work as advertised in the harsh and uncontrolled conditions of the real world.

As with any subfield whose hype cycle has extended beyond a decade, it isn’t hard to find chemists who are pessimistic, even snooty, about MOFs, which have pulled in huge numbers of researchers, millions in funding and now the Nobel Prize. In general, the detractors say that MOFs are interesting but largely unrealised. It is probably true that too much of the supposed importance of the laureates’ discoveries is based on a promise yet to be fulfilled. But it is fundamentally interesting chemistry, and in a field that is thriving and active now – not a keystone advance of the past. Given that last year’s Chemistry Nobel went to researchers at Google for an AI tool, really straining the bounds of the category, this one feels about right.