Tuning metal encapsulation in zeolites: Impact of crystal downsizing on liquid-phase hydrogenation reactions

Abstract

Pt- and Pd-encapsulated pure-silica MFI zeolites with tunable crystal sizes (80–350 nm) were synthesized by controlling the hydrothermal process temperature (80–175 °C). This approach enabled the systematic modulation of the textural and diffusional properties of the resulting materials. Comprehensive characterization confirmed the encapsulation of metals within the zeolite frameworks of varying crystal sizes. The impact of crystal size on catalytic behavior was evaluated through the selective liquid-phase hydrogenation of phenylacetylene, using it as a model reaction to assess diffusion constraints. Pd-MFI with smaller crystals exhibited markedly higher activity and selectivity toward styrene compared with its larger-crystal counterparts, underscoring the critical role of tailored textural and diffusional properties in catalyst design. However, nanocrystalline materials with larger external surfaces underwent partial metal sintering under liquid-phase conditions. Strategies to mitigate sintering, such as controlling structural defects or applying protective surface coatings, are essential for further improvements.