Spectroscopic monitoring for the formation of mesoporous MCM-41 materials upon microwave irradiation

The mesoporous MCM-41 materials were prepared in very short crystallization time (∼40 min) upon microwave irradiation in comparison with conventional hydrothermal heating method. With both microwave irradiation and hydrothermal heating, the MCM-41 formation via supramolecular templating method has been monitored by fluorescence and electron spin resonance (ESR) spectroscopy. Pyrene as a fluorescence probe and 4-(N,N-dimethyl-N-hexadecyl)ammonium-2,2,6,6-tetramethyl piperidinyloxy iodide (CAT16) as a spin probe were respectively dissolved into the micelle solutions to form the MCM-41 precursor gels. These probes allow the monitoring of the supramolecular interaction between the anionic silicate species and the cationic surfactant molecules during the MCM-41 formation. Analyses of fluorescence and ESR spectra indicate that the fast increase of hydrophobicity and microviscosity at the solubilzation sites of the probes results from the accelerated condensation of silicates onto the micelle surface upon microwave irradiation. The fluorescence change from the silicate L-center in the MCM-41 precursor gel also probes the fast silicate condensation upon microwave irradiation. It seems that the fast formation of MCM-41 upon microwave irradiation is ascribed to the microwave-susceptible head groups of surfactant molecules in addition to fast dissolution of the precursor gel.