Mesoporous materials with zeolitic crystal

Zhang and co-workers reported partial conversion of a mesoporous starting material (SBA-15) into a mesoporous aluminosilicate with zeolitic characteristics in a so-called vapour phase transport method.[82] In this process, Al is firstly introduced onto the mesoporous surface, followed by a filling of the mesopores with a carbonaceous species, and finally a partial recrystallization of aluminosilicate in the vapour of the SDA is conducted. The advantage of this method, compared with the hydrothermal recrystallization method of Kloetstra et al., lies in the fact that the mesopore structure collapses to a lesser extent as the crystallization is limited to the surface of the mesoporous precursor. 

A nanomaterial can be loosely defined to be any material containing heterogeneity at the nanoscale in one or more dimensions. In the broadest sense, then, the following are nanomaterials phase-separated glasses or crystals with domains in the nanoregime, zeolites and mesoporous materials with pores of nanometer dimensions, clays with nanometer sized alternations of aluminosilicate layers and interlayer hydrated cations, and nanoscale leach layers at the mineral-water interface. 

It is well known that the elements in framework of zeolite molecular sieves greatly influence the properties and behaviors of these materials [1-3], The introduction of heteroatoms into the framework has become one of most active fields in study of zeolites. The investigations were mostly focused on the methods to introduce heteroatoms into the framework (for examples, hydrothermal synthesis and post-synthesis), the mechanisms for incorporations, the effect of heteroatoms on the acid-base properties and the catalytic features of modified samples [1-10]. Relatively less attention was paid to the effect of treatment process on the porous properties of samples although the incorporation of heteroatoms, especially by the so-called post-synthesis, frequently changes the distribution of pore size. Recently, we incorporated Al, Ga and B atoms into zeolites (3 by the post-synthesis in an alkaline medium named alumination, galliation and boronation, respectively. It was found that different trivalent elements inserted into the [3 framework at quite different level. The heteroatoms with unsuitable atom size and poor stability in framework were less introduced, leading to that a considerable amount of framework silicon were dissolved under the action of base and the mesopores in zeolite crystal were developed. As a typical case, the boronation of zeolites (3 and the accompanied formation of mesopores are reported in the present paper. 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

The mesoporous materials discussed here comprise silicates and aluminosilicates that are formed from synthesis conditions comparable to zeolites. The main difference with the latter is the use of supra-molecular assemblies of surfactant molecules, e.g. alkyltrimethylammonium ions, as the structure directing agents. Due to the large dimensions of these spherical or cylindrical micelles, the framework of the silicates is not so well crystallized as in the case of zeolites. This causes lower framework stability as well as weaker Bronsted acidity. Upon calcination, large pores of uniform diameter (say 4-10 nm, depending on the surfactants used) become accessible. 

The main difference between the synthesis of MCM-41 mesoporous material and traditional synthesis of zeolite or silica molecular sieve is the use of different templates. An individual organic molecule or metal cation is used for the traditional synthesis of silica microporous molecular sieve. For example, the typical template for ZSM-5 synthesis is tetrapropylammonium ion the crystal is formed through the condensation of silicate species around the template molecule, while for the formation of MCM-41, the typical template is the assembly of large molecules containing one hydrophobic chain with more than 10 carbons. 

Zeolites are currently manu ctured as micron size crystals and compacted into millimeter size pellets for applications as packed beds. In many catalytic and adsorptive applications, mass and heat transfer properties could potentially be improved by structuring the zeolite in a different way. Research in this area led already to significant achievements. Alternatively structured zeolite matter are e.g. delaminated zeolites [1], supported zeolite films and membranes [2], hybrid structures with microporosity in walls of ordered mesoporous materials [3-6] and nanosized zeolites such as those synthesized in confined space [7]. The common property of these alternative zeolites is that at least in one direction, the zeolite framework has a dimension of around a nanometer. 

The blossoming of new materials has been so rapid that the nomenclature of ordered mesoporous materials is in the wild state in which zeolite nomenclature was thirty years ago. A set of rules for writing a standardized crystal chemical formula for both microporous and mesoporous materials has been established by the Physical Chemistry Division of the International Union for Pure and Applied Chemistry, through its Commission on Colloid and Surface Chemistry including Catalysis [65]. The impact of this nomenclature on the activity of the scientists dealing with mesoporous materials has still to be verified. 

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