Shape-selective sieving layers on an oxide catalyst surface

Abstract

New porous materials such as zeolites, metal-organic frameworks and mesostructured oxides are of immense practical utility for gas storage, separations and heterogeneous catalysis. Their extended pore structures enable selective uptake of molecules or can modify the product selectivity (regioselectivity or enantioselectivity) of catalyst sites contained within. However, diffusion within pores can be problematic for biomass and fine chemicals, and not all catalyst classes can be readily synthesized with pores of the correct dimensions. Here, we present a novel approach that adds reactant selectivity to existing, non-porous oxide catalysts by first grafting the catalyst particles with single-molecule sacrificial templates, then partially overcoating the catalyst with a second oxide through atomic layer deposition. This technique is used to create sieving layers of Al(2)O(3) (thickness, 0.4-0.7 nm) with 'nanocavities' (<2 nm in diameter) on a TiO(2) photocatalyst. The additional layers result in selectivity (up to 9:1) towards less hindered reactants in otherwise unselective, competitive photocatalytic oxidations and transfer hydrogenations.

Improving Heterogeneous Catalyst Stability for Liquid-phase Biomass Conversion and Reforming

“…Furthermore, controlling pore size openings within these overcoats could introduce shape selectivity and be used to sieve out undesired reactants as was recently demonstrated for an alumina overcoat on a titania catalyst used for photocatalysis. [84] Finally, it will become increasingly important to use real biomass-derived streams for testing catalytic stability. The negative effects of using liquid or aqueous conditions can be compounded by the presence of numerous, perhaps yet unidentified, inorganic or biogenic impurities.…”

Scalable Production of Nanostructured Particles using Atomic Layer Deposition

“…The most commonly used passivation materials are oxides such as Al 2 O 3 and SiO 2 , with the required thickness ranging from values as low as 0.5 nm up to 50-nm films, depending on the application. Special treatment can tailor the nature of the protective layer to induce a certain accessibility (Canlas et al, 2012;Lu et al, 2012). Moreover, supported active multi-component materials (e.g.…”

“…In recent years, researchers have proposed a wide range of novel nanostructured materials for applications such as catalysis (Canlas et al, 2012;Li and Somorjai, 2010) and photovoltaic devices (Jancar et al, 2010;Kamat, 2008). Often, such materials use nanostructured particles as building blocks.…”

Review Article: Catalysts design and synthesis via selective atomic layer deposition

“…89 For example, protecting the active sites by bulky organic moiety can prevent the ALD process. Then, ALD was performed to deposit a film, but the resulting layer grew only on the support surface and not on the blocking group.…”

“…Notestein and coworkers demonstrated tuning of pore size and depth of ALD Al 2 O 3 films on TiO 2 substrate by choosing the steric diameter of the blocking SAMs molecules. 89 In this case, calixarene was used to block parts of the TiO 2 surface to achieve the selective deposition of Al 2 O 3 . The formed Al 2 O 3 nanobowl structure (<2 nm in diameter) resulted in the enhanced selectivity (up to 9:1) toward less hindered reactants in competitive photocatalytic oxidations and transfer hydrogenations.…”