Various nano-size iron oxide polymorphs on mesoporous supports I -: New mesoporous catalyst supports synthesized in acidic and alkaline media

A fairly large number (altogether 12) of new mesoporous silica catalyst supports have been synthesized mainly in alkaline media using ionic (CTAB) and non-ionic Aldrich-type triblock poly(alkylene oxide) copolymers and polyglycol mixtures. Preference was given to water-glass as the silicon source. This study was aiming at setting up an own catalyst carrier repertory to be able to select supports with "good" characteristics for the investigation of deconvoluted Mossbauer and EPR spectra. This task could not be carried out on compact oxides. The physical characterization (N-2 adsorption, XRD, SEM, HRTEM, SAED, Mossbauer, EPR and Si-29-NMR spectroscopy) was supplemented by activity testing, as well, in the N2O-mediated selective oxidation of benzene and synthesis of (carbon) nanotubes at very high temperature. The first part of the planned two publications deals with the description of various synthesis recipes of silica carriers and a few novel observations obtained during their characterization. It seems to be important to know that the so called amorphous silicas are not always and never fully amorphous. Their structure consists of crystalline SiO2 polymorph grains (beta-tridymite or quartz) embedded and randomly oriented in the agglomeration of amorphous silica particles of similar (15-20 nm) size. This final state of genesis is preceded during the synthesis by the separation of viscous, tacky liquid droplets from the aqueous phase when the "surfactant + silicate" complex becomes insoluble in the slurry. Later on the droplets solidify as such or experience a slight deformation if the ordering continues even after their separation. The amorphous silica grains have no hexagonal pore structure, thus, pore walls either. Their extreme heat-and hydrothermal stability is due to this fact. They can be synthesized from water-glass in a wide range of specific surface areas and pore diameters, and in the possession of activity testing data and information obtained from the respective deconvoluted EPR and Mossbauer spectra, the selection of a "good" carrier from the literature or rather from an own carrier repertory seems to be a promising perspective. (C) 2007 Elsevier Inc. All rights reserved.