Dry-gel synthesis of mesoporous MFI zeolite nanosponges using a structure-directing surfactant

Abstract

The synthesis of mesoporous MFI zeolite nanosponges was investigated in a dry-gel conversion process using C18H37N+(CH3)2C6H12N+(CH3)2C4H9 as a micropore-mesopore hierarchical structure-directing surfactant. The initial synthesis mixture was converted at 333 K to a mesoporous material exhibiting a structure similar to that of MCM-41. The mesoporous material was filtered, dried at 373 K, and subsequently heated at 423 K in an autoclave with controlled humidity. In this process, precise control of the relative chamber humidity and the pH of the precursor gel was necessary to achieve full crystallization of the zeolite. The final product was composed of three-dimensional networks of 2.5-nm thick MFI nanolayers and exhibited a high surface area and narrow distribution of mesopore diameters centered at 4 nm. The zeolite crystallization behavior at 423 K was investigated after various heating times with X-ray diffraction, gas adsorption, and electron microscopy. The results indicated that under controlled humidity, crystallization took place through a pseudomorphic transformation, where the initial gel morphology was maintained throughout. The overall quality of the dry-gel product was similar to that of a conventional hydrothermal product. The Si/Al ratios of the zeolite could be controlled over the range of 50 ∼ ∞. Furthermore, in a 100-g synthesis batch, the dry-gel method had the advantage of compact autoclave size for large-scale synthesis.