Scorching heat turns MOF into discerning hydrogenation catalyst

A catalyst that helps convert alkynes into alkenes could offer a more efficient and greener way to achieve the tricky industrial transformation (ACS Catal. 2024, DOI: 10.1021/acscatal.4c00310).

The alkenes used to manufacture polymers often contain troublesome traces of alkynes. To purify these feedstocks, chemists use semihydrogenation to turn carbon-carbon triple bonds into double bonds. This process requires a catalyst that preferentially binds and hydrogenates alkynes but shuns alkenes, sparing double-bonded carbons from further hydrogenation into unwanted alkanes.

One option is a palladium-based Lindlar catalyst, which contains additives such as lead that dial down palladium’s activity. But this type of catalyst sometimes suffers from poor stability and can cause overhydrogenation. And using toxic lead on an industrial scale is far from ideal.

A team led by Pascual Oña Burgos at the Institute of Chemical Technology (ITQ) in Valencia, Spain, has now created a semihydrogenation catalyst based on a metal-organic framework (MOF), a type of porous material containing a scaffold of metal-based nodes holding together organic molecular struts.

Researchers had already found that when MOFs are pyrolyzed at several hundred degrees Celsius, they can form metal nanoparticles embedded in porous carbon, potentially making stable and active catalysts. Oña Burgos’s team has now shown that chemical pretreatment of a MOF before pyrolysis can produce an even more effective catalyst.

The researchers reacted a palladium-indium MOF with aniline and then pyrolyzed the material at 800 °C. That caused the MOF’s metals to form palladium-indium nanoparticles and its organic molecules to degrade into porous carbon peppered with nitrogen atoms. Compared with pyrolyzed MOFs that didn’t get the pretreatment, the strategy produced smaller nanoparticles—and hence a higher density of catalytic sites. Indium improves palladium’s preference for binding alkynes, and nitrogen helps to activate incoming hydrogen molecules.

The catalyst semihydrogenates phenylacetylene to styrene with 96% selectivity, and with 96% conversion at ambient temperature and pressure. In contrast, a commercial palladium-on-carbon catalyst offers only 50% selectivity, generating loads of the unwanted alkane product.

Having proved that their catalyst works well with liquid alkynes, “the next step is to move to the gas phase for industrial applications like the semihydrogenation of acetylene to ethylene,” Oña Burgos says.