Direct synthesis of well-ordered and unusually reactive MnSBA-15 mesoporous molecular sieves with high manganese content

Abstract

The mesoporous MnSBA-15 materials with different nsi/n Mn ratios of 4, 8, 20, and SO have been synthesized, for the first time, using manganese nitrate tetrahydrate and Pluronic 123 triblock polymer [(EO)20(PO)70-(EO)20] by simply adjusting the molar ratio of water to hydrochloric acid (nH2o/nHCl) under direct hydrothermal conditions. For the effect of structural and textural properties with incorporation of manganese, the MnSBA-15 has been synthesized with different synthesis temperatures at the fixed molar ratios of n Si/nMn = 4 and nH2o/nHCl = 295 in the synthesis gel. The hydrothermal and thermal stabilities of MnSBA-15 have also been investigated. The calcined MnSBA-15 materials prepared have been characterized by ICP-AES, XRD, N2 adsorption, ESR, FE-SEM, and TEM. The ICP-AES studies show a higher amount of manganese incorporation on the silica pore walls, as MnSBA-15 with a nSi/nMn ratio up to 2.2 can be successfully prepared at a fixed nH2o/nHCl molar ratio of 295 by adjusting the ratios of nSi/nMn in the synthesis gel. The structural and textural properties of calcined MnSBA-15 prepared can be found by the results of XRD and N2 adsorption. The investigation of ESR results clearly describe the effect of structure and Mn species coordination on the SB A-15 silica pore walls while the uniform pore diameter and rope-like hexagonal mesoporous structure of MnSBA-15 can be identified by TEM and FE-SEM images. With increasing synthesis temperature, an increase the unit cell parameter, pore size, and pore volume and a decrease the specific surface area and pore wall thickness of MnSBA-15 can be obviously noted by the results of XRD and N2 adsorption. The hexagonal MnSBA-15 materials prepared could be tested as catalysts in epoxidation of trans-stilbene to produce trans-stilbene oxide under various optimal conditions while their catalytic properties could also be compared to the results of MnMCM-41 and ZrMnMCM-41. © 2006 American Chemical Society.