Abstract
A hierarchical zeolite CaA with microporous, mesoporous and macroporous structure was hydrothermally synthesized by a ”Bond-Blocking” method using organo-functionalized mesoporous silica (MS) as a silica source. The characterization by XRD, SEM/TEM and N2 adsorption/desorption techniques showed that the prepared material had well-crystalline zeolite Linde Type A (LTA) topological structure, microspherical particle morphologies, and hierarchically intracrystalline micro-meso-macropores structure. With the Bond-Blocking principle, the external surface area and macro-mesoporosity of the hierarchical zeolite CaA can be adjusted by varying the organo-functionalized degree of the mesoporous silica surface. Similarly, the distribution of the micro-meso-macroporous structure in the zeolite CaA can be controlled purposely. Compared with the conventional microporous zeolite CaA, the hierarchical zeolite CaA as a catalyst in the conversion of methanol to dimethyl ether (DME), exhibited complete DME selectivity and stable catalytic activity with high methanol conversion. The catalytic performances of the hierarchical zeolite CaA results clearly from the micro-meso-macroporous structure, improving diffusion properties, favoring the access to the active surface and avoiding secondary reactions (no hydrocarbon products were detected after 3 h of reaction).
Highlights
Zeolites are crystalline, microporous aluminosilicates from interlinked tetrahedra of alumina (AlO4 ) and silica (SiO4 ) with a relatively open, three-dimensional crystal structure, cavities and channels of molecular dimensions in the different directions
We report the preparation of hierarchical zeolite CaA single crystals with micro, meso- and macroporous structure by using organo-functionalized mesoporous silica (MS)
21.6 synthesized samples characteristic diffraction located at 2θ value
Summary
Microporous aluminosilicates from interlinked tetrahedra of alumina (AlO4 ) and silica (SiO4 ) with a relatively open, three-dimensional crystal structure, cavities and channels of molecular dimensions in the different directions. The presence of active sites in the zeolite micropores gives rise to shape-selective catalysis, but imposes steric limitations to the diffusion of the reactant molecular to the interior of the pores and reduces the diffusion rate of the reaction products outside the inner cavities, rendering a large portion of the actual zeolite crystals ineffective and the occurrence of undesirable second reaction. It is well-known that the incorporation of additional meso(macro)pores in zeolite can enhance the accessibility to the active sites, overcoming the steric limitations and shortening diffusion pathways.
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