Add heat to MOFs for highly selective catalysts

A catalyst that converts carbon dioxide into carbon monoxide gas with exceptional selectivity could reduce the cost of producing CO as a chemical feedstock.

This is the claim of researchers in Saudi Arabia and Spain whose novel approach to making catalysts involves heating bimetallic metal organic frameworks (MOFs) until their porous structure collapses into nanoparticles.

KAUST catalyst promises sustainable CO2 conversion

The benefit of this method pioneered at KAUST (King Abdullah University of Science and Technology, Saudi Arabia) is that it can generate mixed metal catalytic nanoparticles that have proven challenging or impossible to make by conventional means.

Capturing CO2 emissions and catalytically converting the greenhouse gas into CO, a valuable chemical feedstock, is one option for reducing greenhouse gases associated with climate change. Precious metals can catalyse this reaction, but they are costly and supplies are limited, said Samy Ould-Chikh, a research engineer in KAUST.

“Iron oxide catalysts are an inexpensive alternative,” Ould-Chikh said in a statement. “However, in the presence of CO, the iron is carburised forming iron carbide, which leads to by-product formation and catalyst deactivation.”

According to KAUST, adding titanium to the catalyst particles could stabilise iron oxide against carburisation. Chemical incompatibilities between iron and titanium precursors, however, had made it impossible to synthesise nanoparticles incorporating a homogenous mixture of the two metals in the necessary ratio. To overcome this limitation, the team turned to MOFs, porous materials made from metal ions connected together by carbon-based linkers.

“The use of MOFs allows us to perfectly control the iron-titanium ratio on the parent MOF,” said research engineer Adrian Ramirez Galilea. Heating decomposes the organic part of the MOF, leaving the two metals behind, homogenously mixed in the desired ratio and in neat octahedral nanoparticles that mirror the structure of the parent MOF.

The team said that nanoparticles converted CO2  to CO with 100 per cent selectivity, with no sign of deactivation after several days of use. “Our initial calculations suggested that nanoparticles with such atomic ratios should be able to do the job; however, the results far exceeded our original expectations,” said Gascon.

As well as continuing to explore the properties and reactivity of the iron-titanium nanocatalyst, the team is examining other metal catalyst systems made from MOFs in the same way. “The use of MOFs opens the way to synthesise new catalysts that were not possible to make using conventional approaches,” said Ramirez Galilea.

“We are looking at different metal combinations for applications ranging from traditional thermal catalysis to photo and photothermal catalysis,” said Jorge Gascon, who led the research. “This paper is just the tip of the iceberg.”

The team, including researchers from the Universitat de València, Spain and Université Grenoble Alpes, France, have had their findings detailed in Chem Catalysis.