Hierarchical Mesoporous Zeolites: Direct Self‐Assembly Synthesis in a Conventional Surfactant Solution by Kinetic Control over the Zeolite Seed Formation

Abstract

By kinetic control over the zeolite seed formation, we report the direct fabrication of hierarchical mesoporous zeolites using hexadecyl trimethyl ammonium bromide (CTAB) as the soft template in a conventional solution route. Nanometer-sized, subnanocrystal-type zeolite seeds with a high degree of polymerization are essential to prevent the formation of a separate amorphous mesoporous phase and the phase separation between the mesophase and zeolite crystals in the presence of CTAB and a certain amount of ethanol. The mechanisms for the formation of hierarchically porous zeolites in the solution process, including the effect of mother liquid aging, formation of subnanocrystal zeolite seeds and their self-assembly effect with CTAB, and the role of ethanol are proposed and discussed in detail. The prepared mesoporous ZSM-5 zeolite showed much higher catalytic activity than conventional counterparts for aldol condensations involving large molecules, especially in the synthesis of vesidryl.

Rapid room-temperature synthesis of hierarchical porous zeolitic imidazolate frameworks with high space-time yield

“…3d, f), confirming that these materials have high crystallinity [30]. These results indicate that the entire particle is crystalline with well-ordered mesopores penetrating through the microporous framework [31]. Thus, the introduced organic amines (surfactant) act as templates-termed structuredirecting agents (SDAs) that direct the formation of mesopores.…”

Hierarchy concepts: classification and preparation strategies for zeolite containing materials with hierarchical porosity

“…78,79 Alternatively, secondary methods have been developed, which in contrast to the primary methods utilise conventional surfactants like CTAB to assist the assembly of subnano pre-crystalline zeolite species induced by surfactant arrays into zeolitic assemblies exhibiting intercrystalline or intracrystalline mesoporosity. 64 This has been realised via a two-step synthesis procedure. In the first step, zeolite synthesis gel was allowed to age in the absence of the surfactant at 100 1C for different periods of time to form seeds (subnanocrystals) with different degree of polymerisation.…”

“…10, depending on the aging time (t age ) of the initial gel, either a mixture of zeolite and mesoporous silica (when the aging time was too short) or cubic shaped crystals of MFI-type zeolites perforated with mesopores (after 2-3 days of aging time) or just aggregates of nanocrystals with intercrystalline mesopores (when the aging time was too long) were obtained. 64 In addition to the soft templating role, surfactants have also been used to introduce additional porosity in zeolites via the generation of a biphasic emulsion. The main principle behind the emulsion approaches is a phase-separation process during the zeolite formation: A mixture consisting of an organic and an aqueous phase undergoes a phase-separation under the applied synthesis conditions in such a way that the zeolite mixture (aqueous phase) and the organic fraction (future pore/ void) form a stable, biphasic (and potentially bicontinuous) emulsion.…”

Direct Synthesis of Functional Zeolitic Materials

“…This hierarchical material showed enhanced catalytic activity in cracking and esterification reactions. Zhu et al have also described the synthesis of hierarchical mesoporous zeolites by kinetic control on the zeolite seed formation [113]. They first aged the gel containing the zeolitic precursors at low temperature, favoring the formation of zeolite subnanocrystals, and later, soft template CTMA was introduced in the medium (see Figure 7), and the resulting gel heated at 150…”