Jeffrey Scott Beck

Recent advances in the synthesis, characterization and applications of mesoporous molecular sieves

Abstract The past year has seen an explosive growth in research on mesoporous molecular sieves. Major advances in understanding and exploiting the synthesis protocols and mechanism of formation of these materials have allowed designed tailoring of their composition, pore size, structure and texture. The concept of ‘supramolecular templating’ with molecular aggregates of surfactants, which has been proposed as a mechanistic step in the formation of molecular sieves, has expanded our idea of the classical single molecular interaction. Well documented, simplified synthesis preparation of these materials have allowed study of their application in fields ranging from catalytic conversion of large molecules to their functioning as nanoscopic homes where polymers, atomic arrays of metal atoms, and electronic materials can reside.

Development of a formation mechanism for M41S materials

1. Summary The influence of surfactant/silica molar ratio(Sur/Si) in M41S syntheses was studied in the simple synthesis system consisting of tetraethylorthosilicate(TEOS), water, and the cetyltrimethylammonium(CTMA) cation at 100°C. As the Sur/Si increased from 0.5 to 2, the siliceous products obtained were identified and could be classified into four separate groups: MCM-4 1(hexagonal), MCM-48(cubic), thermally unstable M4 1 S, and a molecular species, the organic octamer [(CTMA)SiO2, 5]8. One of the thermally unstable structures has been identified as a lamellar phase. These results are consistent with micellar phase transformations that occur at various surfactant concentrations and reinforce the concept that micelle structures serve as templating agents for the formation of M41S type materials.

Designed Synthesis of Mesoporous Molecular Sieve Systems Using Surfactant-Directing Agents

The use of cationic surfactants as structure directing agents has resulted in the discovery of M41S, the first, ordered mesoporous molecular sieves. Herein we describe the ability of the surfactant molecules to interact with silicate counterions resulting in the formation of organosilicate-surfactant composite arrays which exhibit hexagonal, cubic or lamellar structure.

Molecular Or Supramolecular Templating: Defining The Role of Surfactant Chemistry In the Formation of M41S and Zeolitic Molecular Sieves

Publisher Summary The chapter explores the ability of alkyltrimethylammonium surfactants of the type C n H 2n+I (CH 3 ) 3 NBr to serve as structure directing agents, or templates, for the formation of microporous or mesoporous molecular sieves frameworks. At equivalent gel compositions and reaction conditions, it was observed that the alkyl chain length of the surfactant molecule dictated the nature of the silicate product obtained as indicated by the X-ray diffraction patterns. Over the entire range of synthesis temperatures examined (100–200°C) the shortest alkyl chain length surfactant (n=6), produced amorphous or microporous zeolitic materials, such as ZSM-5. The zeolite contained the intact surfactant cation consistent with a commonly observed molecular templating effect. At 100°C as the surfactant chain length was increased (n=8, 10, 12, 14, and 16), the formation of mesoporous molecular sieves (MCM-41) was observed. In these cases, a combination of surfactant chain length and reaction conditions favor surfactant aggregation (micelles), and hence, the formation and utilization of supramolecular templates.