Preparation and Potential Catalytic Applications of Hierarchically Structured Zeolites with Macropores

Abstract

Hierarchically structured zeolites can mitigate problems arising from the slow transport in the micropores, such as diffusion limitations. Hence, a manifold of different preparation methods for hierarchical zeolites have been developed over the years and the corresponding zeolites showed superior performance due to enhanced mass transport caused by higher diffusivities in the additional pore system and reduced diffusion path length in the micropores. The dimensions of the additional secondary “transport” pores in these materials is a major factor affecting their diffusion properties and consequently their catalytic performance. Interestingly almost all reports focus on the generation of additional mesopores and study of catalytic performance of mesoporous zeolites. Larger macropores with pore diameters of more than 50 nm, however, have some intrinsic advantages over mesopores that make them very attractive for catalytic applications. In the last years, methods for the preparation of zeolites and zeolite composites with additional macropores have been developed. These developments have brought about a novel and promising class of hierarchically structured materials; zeolites with macropores. This review gives an overview about different preparation routes for zeolites with inter or intra crystalline macropores as well as zeolite composites with macropores and summarizes the first studies regarding catalytic their performance.

Regioselective Surface Assembly of Mesoporous Carbon on Zeolites Creating Anisotropic Wettability for Biphasic Interface Catalysis

“…It is worth noting that the S-1@Pt/mC catalyst shows a higher reaction rate than the Pt/S-1@mC catalyst, because of the faster diffusion of the reactants to the active sites in the mesopores than to those in micropores. 36 In stark contrast, the Pt/mC, Pt/S-1, and commercial Pt/C catalysts show very low conversion efficiency (<10%) in the same period (Figure S31), because the low oil/water interfacial area hinders the reaction kinetics. By comparison, the nitrobenzene conversion is less than 2% after 180 min when the NaBH 4 solution is replaced by the atmospheric H 2 , indicating that the in situ generated H 2 from NaBH 4 hydrolysis is much more efficient than the conventional gaseous H 2 for the reduction of the nitroarenes.…”

“…In the presence of either Pt/S-1@mC or S-1@Pt/mC catalysts, the nitrobenzene conversion could reach 100% in the initial 2 h due to the formation of a stable emulsion (Figure b). It is worth noting that the S-1@Pt/mC catalyst shows a higher reaction rate than the Pt/S-1@mC catalyst, because of the faster diffusion of the reactants to the active sites in the mesopores than to those in micropores . In stark contrast, the Pt/mC, Pt/S-1, and commercial Pt/C catalysts show very low conversion efficiency (<10%) in the same period (Figure S31), because the low oil/water interfacial area hinders the reaction kinetics.…”

Application of Mesoporous/Hierarchical Zeolites as Catalysts for the Conversion of Nitrogen Pollutants: A Review

“…In this case, the overall reaction rate is limited by the diffusion rate, and the reaction may proceed mainly on the outer surface of the catalyst grains. Another problem related to diffusion limitations is the blocking of pores due to the formation of carbon deposits, resulting in catalyst deactivation [1][2][3].…”

Design of Bifunctional Nanocatalysts Based on Zeolites for Biomass Processing

“…It is worth noting that besides the major types of zeolites discussed above, new modifications were introduced, including layer-like hierarchical [ 93 , 94 , 95 , 96 ] and mesoporous zeolites [ 97 , 98 , 99 , 100 ]. A recent example shows a remarkably different outcome of the catalytic reaction, depending on the zeolite type, including porosity.…”