Thanks to their sponge-like nanostructure, zeolites—crystals comprising aluminum, silicon and oxygen—have found uses in many things from washing powder to cat litter. In addition to acting as a molecular sieve, zeolites can also be used to stabilize metal nanocatalysts such as those used in the ‘cracking’ of crude oil into useful products like gasoline.
As useful as they are, the effectiveness of metal-zeolite catalysts is largely confined to their surface as reactants diffuse very slowly into the center of the zeolite particle. To increase the efficiency of metal-zeolite catalysts, a team of researchers at A*STAR’s Institute of Chemical and Engineering Sciences (ICES) and the National University of Singapore (NUS) encapsulated iron nanoparticles inside a hollow, single-crystal zeolite shell to produce a nanocatalyst called Fe@h-ZSM5. Chen’s colleague Kelvin Kwok at NUS played a key role in synthesizing Fe@h-ZSM5.
“By precisely embedding tiny iron nanoparticles within hollow hierarchical zeolite crystals, we are able to utilize the ZSM-5 zeolite shell for sieving and cracking activities. At the same time, this catalyst can reduce undesirable effects such as sintering and carbon deposition, processes that often affect the performance of traditional catalysts,” explained study author Luwei Chen. This effect is beneficial given that sintering, which is the loss of surface area of the catalyst, causes a significant drop in catalyst activity.
To demonstrate the functionality of their catalysts, Chen’s team tested the performance of Fe@h-ZSM5 against two non-hollow iron catalysts in the Fischer−Trøpsch process, whereby a mixture of hydrogen and carbon monoxide gas is converted into products like kerosene or gasoline. “We showed that our Fe@h-ZSM5 catalyst had a higher selectivity and higher long-term stability compared to catalysts made from the same materials but without hollow structure,” Chen said.
Hua Chun Zeng of NUS, a collaborator on the project, explained that the hollow zeolite system is very flexible and can be adapted to other chemical processes by using other metals. “For example, by incorporating nickel, the catalyst can be used for methane production, while the copper-zinc catalyst can be used for methanol or dimethylether production,” he said.
Going forward, the team plans to design different hollow zeolite structures and test them in other reactions such as carbon dioxide hydrogenation.
The A*STAR-affiliated researchers contributing to this research are from the Institute of Chemical and Engineering Sciences (ICES).