Early methods of surface passivation commercialized by DuPont and Mobil used chemical vapor or liquid deposition of silanes. development of bulky aromatics and graphitic hydrocarbons) owing to the passivation of external acid sites.Īn analogous approach to passivate the external surfaces of zeolite catalysts involves post-synthesis processes wherein a silicon-rich layer of either amorphous silicate or siliceous zeolite is grown on the surface of a parent zeolite crystal. Among reported cases of zoning, the majority involve Al-zoned MFI-type zeolites Weckhuysen and coworkers reported the direct synthesis of Si-zoned MFI zeolite and demonstrated by operando MTH testing of single crystals that the siliceous exterior suppresses the formation of external coke (i.e. Prior studies have reported the synthesis of naturally zoned zeolites where the exterior rim is enriched in either aluminum or silicon. Si/Al ratio) and assessing their effects on catalyst performance using the methanol-to-hydrocarbons (MTH) reaction. In this manuscript, we combine these approaches in catalyst design by focusing on the preparation of zeolites with tailored mesoscopic gradients in aluminum composition (i.e. There has also been growing interest in the development of methods to control aluminum distribution in zeolite frameworks on both a microscopic and mesoscopic level. Many synthetic approaches to optimize the physicochemical properties of zeolite catalysts focus on the design of hierarchical materials with nano-sized microporous domains that reduce diffusion limitations, thereby enhancing overall catalytic performance. Collectively, this study demonstrated that mesoscopic gradients in acid concentration via the design of core–shell and egg-shell zeolites significantly improve catalyst performance over conventional analogues for hydrocarbon upgrading. This inverse design of the egg-shell created pseudo nanosheets with total turnovers that were markedly higher than their homogeneous counterparts. Moreover, we prepared egg-shell configurations of each zeolite, and, comprised of an inert core and catalytically active shell. In comparison, prepared MFI core-shells ( ) showed similar enhancement in catalyst performance. Time-resolved acid titration of core and core–shell catalysts confirmed that the siliceous shell introduces a hydrophobic exterior that impacts molecular diffusion. Catalytic testing using the methanol-to-hydrocarbon (MTH) reaction showed that core–shell zeolites exhibit longer lifetimes, higher total turnovers, and an unexpected promotion of the aromatic cycle in the hydrocarbon pool mechanism. Our findings revealed that particles with ultrathin shells (<10 nm) have enhanced mass transport, characteristic of relatively smaller particles, compared to the corresponding ZSM-11 core.
CORE SHELL SERIES
A series of core–shell MEL-type zeolites were synthesized with catalytically active ZSM-11 cores and passivated silicalite-2 shells of varying thickness.
Here, we systematically assessed the impact of mesoscopic gradients in acid site concentration, which has generally received little attention in the design of zeolite catalysts for hydrocarbon upgrading. Developing structure-performance relationships with the underlying goal of optimizing known zeolite catalysts involves the manipulation of their physicochemical properties.
0 kommentar(er)
