Dry-Gel Synthesis of Mesoporous MCM-41 Materials with Modified Pore Structure

Abstract

Abstract For the first time, the dry-gel conversion method was applied for the preparation of MCM-41 materials with partially crystalline framework. The X-ray diffraction pattern of the material obtained after a dry-gel conversion time of 36 h consists of reflections of both long-range ordered MCM-41 material and zeolite ZSM-5. The nitrogen adsorption/desorption isotherm confirms the presence of micropores and uniform mesopores with a narrow pore size distribution. By 29Si MAS NMR spectroscopy it was evidenced that a significant portion of the amorphous MCM-41 framework was transformed into a partially crystalline framework. TEM and SEM images indicate the occurrence of mesopores with disturbed arrangements, but no formation of separate zeolite ZSM-5 crystals occurred. By 1H MAS NMR spectroscopy of the above-mentioned material, the formation of hydroxyl groups with a chemical shift of ca. 4 ppm and a concentration of ca. 0.1 mmol/g was observed. The acid strength of these hydroxyl groups, as studied by adsorption of the 13C MAS NMR probe molecule 13C-2-acetone, was found to agree with that of bridging OH groups in zeolite H-ZSM-5. After conversion times of 48 h and 72 h, an extensive to total transformation of the dry gel into zeolite ZSM-5 occurred.