Formation of Hierarchically Organized Zeolites by Sequential Intergrowth

Hierarchically organized porous materials can provide multidimensional spatial networks on different length scales with improved characteristics relevant to molecular diffusion. Zeolites that are microporous crystalline materials having pores and channels at molecular dimensions are of great importance for industrial applications. However, the presence of only micropores in zeolite frameworks often limits the molecular diffusion and therefore, restricts the transport of bulky molecules. This problem can be resolved by shortening the effective diffusion path lengths, which has been achieved by miniaturizing zeolite crystals, delaminating or exfoliating layered zeolites, synthesizing zeolite nanosheets, and introducing mesopores into zeolite particles. Among these solutions, the fabrication of hierarchical zeolites with microand mesoporosity is of interest because it combines intrinsic micropores with bypass-interconnected mesopores, and therefore, enhances both the micropore accessibility and molecular traffic within zeolite particles. Hierarchical zeolites have been produced using several techniques, including top-down desilication by alkali postsynthetic treatment and bottom-up directed assembly by hard or soft templates. The hard-template method requires multistep procedures and is therefore unfavorable for large-scale production. Alternatively, the direct introduction of organic mesopore-generating agents (mesoporogens) during zeolite synthesis can create uniform mesopores. The use of such mesoporogens is currently one of the most promising methods for the single-step construction of hierarchical zeolites. Progress has been made using well-designed mesoporogens composed of long hydrophobic chains and hydrophilic zeolitic structure-directing groups to generate zeolites with tunable mesoporosity and to direct the hierarchical assembly of zeolite nanosheets, yielding mesoporous zeolites with house-of-cards-like structures. These hierarchical nanosheets showed excellent catalytic performance in several important reactions because the presence of thin layers with specific crystalline faces facilitates catalysis at the exteriors or pore mouths. Such mesoporogens are probably necessary for the direct, singlestep synthesis of hierarchical zeolites. Herein we report an alternative, mesoporogen-free approach for the construction of hierarchically organized MFI zeolites by sequential intergrowth using a simple organic structure-directing agent (OSDA). The selection of an appropriate OSDA and optimized synthesis conditions that can form plate-like zeolites with enhanced 908 rotational intergrowths seems to be a key to achieving a hierarchical structure with three classes of porosity in one structure: the intrinsic microporosity of the zeolite framework together with mesoporosity existing within the zeolite plates and macroporosity stemming from the complex intergrown structure. Epitaxial and rotational intergrowths are commonly observed in many zeolites. We have hypothesized that by engineering the zeolite intergrowths, hierarchically organized zeolites can be constructed without the need for mesoporogens. In particular, we have focused on the MFI zeolite because it is an excellent catalyst in many industrial processes and a promising material for membrane separation. MFI zeolite that contains sinusoidal channels along the a axis interconnected with straight channels along the b axis is often formed with 908 rotational intergrowths, in which substantial (h00) faces are epitaxially overgrown on the (0k0)