(594f) Designing Ti-Zeolites with Gradients in Heteroatom Composition for Improved Olefins Epoxidation
AIChE Annual Meeting
2023
2023 AIChE Annual Meeting
Catalysis and Reaction Engineering Division
Catalyst Design, Synthesis, and Characterization II: Control of Catalytic Site Speciation and Distribution
Wednesday, November 8, 2023 - 9:30am to 9:48am
There is growing interest in the design of zeolite catalysts with tunable composition using a
combination of framework or extra-framework Lewis acids as sole active sites or in tandem with
traditional Al Brønsted acid sites. Replacing framework Si and Al by other elements can lead to
improvements in zeolite acidity or adsorption capacity, while potentially influencing properties
such as crystal size and shape. Several elements like B, Ge, V, Sn, Ti, and Ga, among others,
have been introduced into different zeolite frameworks as heteroatoms. Notably, Ti-based
zeolites have garnered much interest in recent years due to their acidic properties that are ideal
for oxidation reactions, such as olefin epoxidation. One of the most frequently studied zeolitic
materials is TS-1 (titanium silicalite-1) where Ti sites are distributed homogeneously throughout
the zeolite crystal. TS-1 is an excellent catalyst for hydrogen peroxide olefins epoxidation
reactions; however, overcoming the low epoxide selectivity on traditional bulk TS-1 is a major
challenge. Here, we discuss alternative design schemes of TS-1 catalysts wherein the distribution
of Ti sites is altered to increase access to active sites while minimizing mass transport limitations
within zeolite pores. We designed TS-1 crystals with both core-shell (silicalite-1@TS-1) and
egg-shell (TS-1@silicalite-1) configurations using secondary growth protocols that were
previously established with ZSM-5. Tests of egg-shell samples reveal an improved selectivity of
olefins epoxidation compared to bulk TS-1. We also produced a series of finned
zeolites where nanosized protrusions of TS-1 (30 - 50 nm in average dimension) were epitaxially
grown on the surface of catalytically inactive silicalite-1 seeds. Our findings reveal interesting
trends in structure-performance relationships, which we compare against previous work with
ZSM-5 showing that fins enhance the internal diffusion of molecules, thereby reducing external
coking and extending the lifetime of the catalyst. The egg-shell architecture of TS-1 functions as
a pseudo nanosheet with enhanced mass transfer, whereas the reverse core-shell configuration
presents a passivated exterior rim that eliminates non-selective reactions at external surface sites.
combination of framework or extra-framework Lewis acids as sole active sites or in tandem with
traditional Al Brønsted acid sites. Replacing framework Si and Al by other elements can lead to
improvements in zeolite acidity or adsorption capacity, while potentially influencing properties
such as crystal size and shape. Several elements like B, Ge, V, Sn, Ti, and Ga, among others,
have been introduced into different zeolite frameworks as heteroatoms. Notably, Ti-based
zeolites have garnered much interest in recent years due to their acidic properties that are ideal
for oxidation reactions, such as olefin epoxidation. One of the most frequently studied zeolitic
materials is TS-1 (titanium silicalite-1) where Ti sites are distributed homogeneously throughout
the zeolite crystal. TS-1 is an excellent catalyst for hydrogen peroxide olefins epoxidation
reactions; however, overcoming the low epoxide selectivity on traditional bulk TS-1 is a major
challenge. Here, we discuss alternative design schemes of TS-1 catalysts wherein the distribution
of Ti sites is altered to increase access to active sites while minimizing mass transport limitations
within zeolite pores. We designed TS-1 crystals with both core-shell (silicalite-1@TS-1) and
egg-shell (TS-1@silicalite-1) configurations using secondary growth protocols that were
previously established with ZSM-5. Tests of egg-shell samples reveal an improved selectivity of
olefins epoxidation compared to bulk TS-1. We also produced a series of finned
zeolites where nanosized protrusions of TS-1 (30 - 50 nm in average dimension) were epitaxially
grown on the surface of catalytically inactive silicalite-1 seeds. Our findings reveal interesting
trends in structure-performance relationships, which we compare against previous work with
ZSM-5 showing that fins enhance the internal diffusion of molecules, thereby reducing external
coking and extending the lifetime of the catalyst. The egg-shell architecture of TS-1 functions as
a pseudo nanosheet with enhanced mass transfer, whereas the reverse core-shell configuration
presents a passivated exterior rim that eliminates non-selective reactions at external surface sites.