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

摘要

We have explored the ability of alkyltrimethylammonium surfactants of the type CnH2n+l(CH3)3NBr 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 alky! chain length of the surfactant molecule dictated the nature of the silicate product obtained as indicated by the X-ray diffraction patterns shown in Figure 1. 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, At synthesis temperatures of 200°C, zeolitic and dense phase products were obtained for even the higher alkyl chain lengths, suggesting that these supramolecular aggregates were disrupted and molecular structural direction dominated. 13C CP/MAS data of MCM-41 and zeolitic materials prepared with identical surfactants indicates that the role of the organic directing agent is different in the formation of these two classes of materials. MCM-41 materials have NMR spectra that suggest a micellar array of surfactant and the zeolite materials exhibit spectra that are indicative of a more rigid, isolated environment. The data are consistent with a hypothesis that single surfactant molecules serve to direct the formation of microporous materials whereas mesoporous molecular sieves, such as MCM-41, are formed by surfactant aggregates. These results reinforce the LCT (Liquid Crystal Templating) mechanism proposed for the formation of the mesoporous MCM-41 materials and further add to our understanding of the formation of inorganic porous materials.