Directing Zeolite Structures into Hierarchically Nanoporous Architectures

Abstract

Crystalline mesoporous molecular sieves have long been sought as solid acid catalysts for organic reactions involving large molecules. We synthesized a series of mesoporous molecular sieves that possess crystalline microporous walls with zeolitelike frameworks, extending the application of zeolites to the mesoporous range of 2 to 50 nanometers. Hexagonally ordered or disordered mesopores are generated by surfactant aggregates, whereas multiple cationic moieties in the surfactant head groups direct the crystallization of microporous aluminosilicate frameworks. The wall thicknesses, framework topologies, and mesopore sizes can be controlled with different surfactants. The molecular sieves are highly active as catalysts for various acid-catalyzed reactions of bulky molecular substrates, compared with conventional zeolites and ordered mesoporous amorphous materials.

Multimodal Zr-Silicalite-1 zeolite nanocrystal aggregates with interconnected hierarchically micro-meso-macroporous architecture and enhanced mass transport property

“…There have been many successful approaches to hierarchical MFI-type zeolites [16]. The synthetic techniques involve (i) chemical and/or physical posttreatment, such as the steaming or acid leaching methods [17][18][19]; (ii) hard templating synthesis, such as carbon and colloid templating [20][21][22]; (iii) soft templating synthesis, such as the use of amphiphilic organosilanes [23,24]. However, for increased efficiency within industrial processes, hierarchically porous zeolitic catalysts displaying both micro-meso-macroporosity and strongly active sites are more desirable.…”

Size-Controlled Synthesis of Nano-Zeolites and Their Application to Light Olefin Synthesis

“…Because the molecular size of polyolefins is larger than the pore size, cracking of polyolefins occurs mainly over the acid sites on the external surface, followed by the -20-formation of lighter olefins on the internal pore surface. Recently, hierarchical-structured zeolites have attracted attention because nano-pores (called as nanomorphic zeolite) [44,45] and meso-pores [46][47][48][49][50][51][52] are formed among nano-zeolite crystals. The hierarchical materials composed of nano-sized zeolites possess two main advantages: a large external surface area ascribable to nano-zeolite, and larger pores than the zeolitic pore allowing large molecules to reach active sites.…”

π–π interaction of aromatic groups in amphiphilic molecules directing for single-crystalline mesostructured zeolite nanosheets

“…Long-chain alkyl-quaternary ammonium molecules are universally used to synthesize mesoporous silica materials through the self-assembled liquid crystal templating route [20][21][22] . Recently, Ryoo and coworkers 8 succeeded in synthesizing polycrystalline mesostructured MFI (zeolite framework code by International Zeolite Association) zeolite using designed bifunctional surfactants 7,8,23 , in which the connection of the head groups of the multiple quaternary ammoniums to hydrophobic alkyl chains is the indispensable factor for the effective structure-directing agent to simultaneously form MFI zeolite and a mesoscale micellar structure. However, directing the structure of zeolite using longer alkyl chain lengths and simple one-head quaternary molecules is generally an energetically unfavourable event compared with the use of multiple quaternary ammonium head groups 8,20,24 .…”

“…There are several methods to prepare hierarchical zeolites such as top-down desilication by alkali post-synthetic modification 5 , bottom-up directed assembly by self-assembly of various zeolite seeds 6 and by templates. Much attention has been focused on the synthesis of zeolites with desired pore structure using soft and hard templates, such as surfactants [7][8][9] , cationic polymer 10 , polystyrene beads 11 , mesoporous silica 12 and carbon materials [13][14][15] . Although much considerable progress has been made in developing strategies to direct the formation of porous zeolites, designing a templating strategy for obtaining ordered hierarchical pore structures with crystallographically ordered zeolite remains an unsolved challenge.…”