EPR STUDIES OF THE FORMATION MECHANISM OF THE MESOPOROUS MATERIALS MCM-41 AND MCM-50

The formation mechanism of the hexagonal, MCM-41, and the lamellar, MC M-50, mesoporous materials, prepared at room temperature with the surf actant cetyltrimethylammonium chloride (CTAC) and tetraethylorthosilic on (TEOS), was studied by in situ EPR spectroscopy using the spin prob e xadecyl)ammonium-2,2,6,6-tetramethylpiperidinyloxy iodide(CAT16). Th is probe has a structure similar to that of the surfactant molecules w ith the nitroxyl radical situated at the head group. Accordingly, it p robes the interface between the organic and inorganic phases during th e formation of M41S materials. The EPR spectrum of CAT16 in the reacti on gel, prior to the addition of TEOS, consists of a superposition of two subspectra due to spin probe: molecules in micelles and in the aqu eous phase, respectively. For a gel composition which forms MCM-41, th e addition of TEOS leads to a gradual transformation of the micellar s ubspectrum into a characteristic rigid limit spectrum. This observatio n provides direct evidence that micellar structures present in the ini tial reaction mixture serve as precursors for the final mesoporous pro duct. The temporal evolution of the spectrum is characteristic of an i sotropic system undergoing a gradual increase in the microviscosity. T he isotropic nature of the spectrum is a consequence of the specific g eometry of the CAT16 head group and its motion in the interface region . Comparison of the temporal evolution of the EPR spectrum with that o f the X-ray diffraction pattern indicates that the hexagonal long-rang e order is formed already 5-8 min after mixing the reagents, whereas t he formation of the inorganic phase, which is apparently responsible f or the slowdown of the spin probe motion, is considerably slower (>1.5 h). The latter process begins only after a critical amount of TEOS is added to the mixture. These results are consistent with a mechanism w hereby the addition of TEOS initially forms clusters of rodlike micell es coated with silicate anions, followed by the condensation of the si licate anions at the interface to yield the final product. By monitori ng the peak height of the central EPR line, phenomenological kinetic p rofiles of the reaction were obtained. These curves were quite differe nt for MCM-41;md MCM-50 and they provide qualitative information regar ding the sequence of transformations which occur during the reaction. Specifically, these curves show that while no intermediate phases occu r during the formation of MCM-41, several phase transformations take p lace when MCM-50 is formed and the reaction is significantly slower.