(349ah) Structure-Property Relationship of Metal Encapsulated in MFI Zeolites for CO Hydrogenation

CO hydrogenation over metal-supported zeolites via the Fischer-Tropsch process is an alternative route to petroleum-based process for producing value-added chemicals such as light olefins (ethylene and propylene).[1] However, fixing metal nanoparticles within zeolites remains a challenge because metal nanoparticles randomly deposit within the pores and on the external zeolite surfaces, where they can be subject to sintering. The use of a core-shell catalyst with a metal-supported zeolite core and zeolite shell is one way of ensuring that metal nanoparticles are fixed within zeolites.[2] Another advantage of the core-shell structure is the secondary reactions that occur within the shell zeolites (cracking, isomerization, and aromatization) during CO hydrogenation, thus improving the catalyst selectivity to the desired olefins, aromatics, and higher hydrocarbons.[1-2]

Here, we investigate the catalytic effect of Fe encapsulated within MFI zeolites (ZSM-5 and S-1) in a core-shell structure (Fe@MFI). During the synthesis of the core-shell catalyst, Fe impregnated ZSM-5 is used as nucleating seeds for the shell zeolite. The catalysts are characterized by various analytical techniques including Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), CO pulse chemisorption, and X-ray powder diffraction (XRD) to correlate the structure of the catalysts with the performance. XRD and TEM results show the maintenance of the catalyst crystallinity after encapsulation. When compared to Fe impregnated ZSM-5 catalysts, Fe@MFI shows improved selectivity to light olefins and higher hydrocarbon during CO hydrogenation, with the acidic catalyst Fe@ZSM-5, showing higher selectivity to light olefins than Fe@S-1. However, the catalysts suffer from deactivation, possibly due to slow transportation of the products and intermediates through the microporous zeolite double layers. The introduction of mesopores within the shell zeolite may minimize deactivation and enhance the catalyst activity.

1. Wang, Chengtao, et al., Journal of the American Chemical Society 141.21 (2019): 8482-8488.
2. Zhang, Jian, et al., Nature Catalysis 1.7 (2018): 540-546