MOLECULAR OR SUPRAMOLECULAR TEMPLATING - DEFINING THE ROLE OF SURFACTANT CHEMISTRY IN THE FORMATION OF MICROPOROUS AND MESOPOROUS MOLECULAR-SIEVES

We have explored the ability of alkyltrimethylammonium surfactants of the type CnH2n+1(CH3)(3)NBr to serve as structure directing agents, or templates, for the formation of microporous or mesoporous molecular s ieve frameworks. At equivalent gel compositions and reaction condition s, it was observed that the alkyl chain length of the surfactant molec ule dictated the nature of the silicate product obtained. Over the ent ire range of synthesis temperatures examined (100-200 degrees C), the shortest alkyl chain length surfactant (n = 6), produced amorphous or microporous zeolitic materials, such as ZSM-5. The zeolite contained t he intact surfactant cation consistent with a commonly observed molecu lar templating effect. At 100 degrees C, as the surfactant chain lengt h was increased (n = 8, 10, 12, 14, and 16), the formation of mesoporo us molecular sieves (MCM-41) was observed. In these cases, a combinati on of surfactant chain length and reaction conditions favor surfactant aggregation (micelles), and hence, the formation and utilization of s upramolecular templates. At synthesis temperatures of 200 degrees C, z eolitic and dense-phase products were obtained for even the higher alk yl chain lengths, suggesting that these supramolecular aggregates were disrupted and molecular structural direction dominated. C-13 CP/MAS d ata of MCM-41 and zeolitic materials prepared with identical surfactan ts 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 z eolite materials exhibit spectra that are indicative of a more rigid, isolated environment for the surfactant. The data are consistent with a hypothesis that single surfactant molecules serve to direct the form ation of microporous materials whereas mesoporous molecular sieves, su ch as MCM-41, are formed by surfactant aggregates. These results reinf orce the LCT (liquid-crystal templating) mechanism proposed for the fo rmation of the mesoporous MCM-41 materials and further add to our unde rstanding of the formation of inorganic porous materials.