Large-Pore Mesoporous Materials with Semi-Crystalline Zeolitic Frameworks

Abstract

Mesoporous molecular sieves with well-defined pore diameters of 20 ± 500 [1±3] overcome the pore diameter constraint of zeolites (`10 ) [4] and allow the diffusion of larger molecules. Furthermore, high surface area and tunable pore size are among the many desirable properties that have made such materials the focus of great interest. However, these materials do not show short-range order and, from this point of view, are closer to amorphous than zeolitic materials. As a consequence, the mesoporous silica ± alumina have sites that are less acidic than those of zeolites, and do not exhibit the spectacular catalytic properties of acidic zeolites. Moreover, their hydrothermal stability is low and their industrial use as catalysts is rather limited to date. [1±3, 5, 6] Therefore, to upgrade the performances of mesoporous molecular sieves and zeolites, attempts have been made to synthesize a new type of material, which would combine the advantages of these two kinds of molecular sieves. To date, only a few articles concerning this study have been published. [7±12] For example, Kloetstra et al. [7] prepared MCM-41/FAU composites which gave a clear improvement in catalytic activity in the cracking of vacuum gasoil. Huang et al. [9] reported the synthesis of an MCM-41/ZSM-5 composite containing an interconnected mesopore and micropore structure. Attempts to crystallize the mesopore walls of mesostructured materials resulted in the formation of materials with increased thermal stability and catalytic activity. [10±14] A synthesis of a partially crystalline bimodal pore system with combined micro-and mesopores, for example, ITQ, has been achieved by delaminating the layered zeolite MCM-22 and ferrierite (FER). [10] Entrapped unit cells of ZSM5 contained in Al-MCM-41 and Al-HMS were prepared by ion exchange of the mesoporous aluminosilicates with tetrapropylammonium cation (TPA ) as an MFI-structure directing agent and then digestion of the mesoporous materials in glycerol at 120 8C. [11] Pinnavaia and co-workers [12,17] and Zhang et al. [18] recently reported steamstable aluminosilicate mesostructures assembled from zeolitetype seeds such as zeolite Y, ZSM-5, and zeolite beta. In the latter cases, the X-ray diffraction (XRD) patterns did not show crystalline features. Evidence for the presence of zeolite units cells may however be provided by the acid catalytic activity of such materials toward cumene cracking. [11,12] Herein, we describe a new general method for the production of a new type of material with semi-crystalline zeolitic mesopore walls. This procedure involves a templated solid-state secondary crystallization of zeolites starting from the amorphous mesoporous materials of corresponding elemental composition. It is important that the walls of the amorphous mesoporous precursor material are as thick as possible. References [2, 3] provide methods for the preparation of mesoporous precursors that are useful in this context including the synthesis and characterization of the mesoporous materials with semi-cryst...

Multimodal Zeolite‐Beta‐Based Catalysts with a Hierarchical, Three‐level Pore Structure

“…The desilication process occurs in the quasi‐solid state system in the presence of a strong alkaline because of the existence of the structure‐directing agent tetraethylammonium hydroxide (TEAOH). However, the desilication rate is very slow in the glycerin medium, which is different from the conventional hydrothermal condition,17 resulting in the generation of a few intracrystalline mesopores with a small pore size of approximately 2.5 nm in each Beta nanocrystal through a long‐time reaction process. Hence, there is a dual‐mode mesoporous system in the final MMM‐Beta, including the larger interconnected mesopores, formed by the aggregation of zeolite Beta nanocrystals, and the smaller ones, distributed in each zeolite Beta nanocrystal, which was further confirmed by N 2 adsorption–desorption investigations.…”

Overview and Industrial Assessment of Synthesis Strategies towards Zeolites with Mesopores

“…Thus, as soon as crystallization of the mesopore walls occurs, the constraints often involve distortion and even collapse of the mesostructure. To avoid this phenomenon, several measures can be taken including using more stable mesoporous materials, that is, with thicker walls, such as SBA‐15,102, 115 using milder conditions like lower temperatures or other solvents, such as glycerol, to limit silica dissolution and moderate zeolite growth 103. 107, 116 An alternative is to fill the mesopores with an organic compound which, upon pyrolysis under nitrogen, will give a CMK‐type carbon material that maintains the mesostructure during the crystallization 117.…”

Synthesis, Characterization and Catalytic Performance of Well-Ordered Crystalline Heteroatom Mesoporous MCM-41

“…However, the resulting materials are composites of zeolite crystallites embedded in a disordered mesoporous matrix. Kaliaguine et al described a general method for the production of a new type of material with semi-crystalline zeolite mesopore walls [13]. Other attempts to crystallize the mesopore walls of mesostructure materials have resulted in the formation of materials with increased thermal stability and catalytic activity.…”