INCORPORATION OF OXYGEN IN CRYSTALLINE ZEOLITIC CHROMOSILICATES - OPTICAL-IDENTIFICATION OF CHROMIUM(VI) BY PHOTOACOUSTIC-SPECTROSCOPY

Incorporation of oxygen to crystalline zeolitic chromosilicates, with oxidation of anchored Cr(III) to non-interacting Cr(VI) species, has been confirmed by photoacoustic spectroscopy; the dichromate anion being extracted from the chromosilicate with water and identified by precipitation of AgCrO4 and oxidation to CrO5.1492292

SYNTHESIS AND CHARACTERIZATION OF CHROMIUM SILICALITE

Infrared spectroscopy (mid-IR), Si-29 and Al-27 magic angle spinning nuclear magnetic resonance (MAS-NMR), powder X-ray diffraction (PXRD), electron paramagnetic resonance (EPR), and photoacoustic spectroscopy (PAS), in combination, have produced useful information about a series of crystalline MFI-type chromium silicalite, synthesized in a fluoride medium. Aluminum is an impurity and occupies framework sites, as evaluated by the Al-27 MAS-NMR. The mid-IR spectra are very similar to the zeolite ZSM-5 with the addition of a weak band at congruent-to 680 cm-1, which we tentatively assign to symmetric stretching of (Cr-O-Si)n groups. The samples are crystalline materials and have expanded unit cells, as PXRD measurements pointed out. The as-synthesized sample shows a distribution of Cr sites, detected by EPR, comprising structure and channel occlusion sites; the nonstructural sites being extensively oxidized to Cr(VI) species after calcination in the presence of dioxygen. The changes in the photoacoustically measured physical properties, such as nonradiative relaxation time, the thermal diffusivity and optical absorption coefficient of both calcined and noncalcined chromium silicalite samples are also reported.5216617

ELECTRON-PARAMAGNETIC RESONANCE STUDY OF CHROMOSILICALITE

An electron paramagnetic resonance (EPR) study of crystalline chromosilicalite has been presented with the purpose of characterization of the distribution of Cr3+ sites in the solid. In addition, a deconvolution process is described whereby the EPR parameters can be analysed.88142071207

Metal Complexes as Structure-Directing Agents for Zeolites and Related Microporous Materials

Metal complexes can act as structure-directing agents (SDAs) for zeolites and zeotypes, either alone or together with additional SDAs in dual-templating approaches. Such complexes include organometallic cobaltocenium ions, alkali metal crown ether complexes, first-row transition-metal (Fe, Co, Ni, Cu) poly-amines and thiol-complexed second-and third-row transition metals (Pd, Pt). Their inclusion has been demonstrated in some cases by crystallographic methods but more commonly by spectroscopy (UV-visible, X-ray absorption, M€ossbauer). The unique feature of this class of template is that they can not only direct crystallisation but also give solids with homogeneously distributed metal cations or metal oxide species upon calcination, precluding the need for an additional post-synthesis modification step. Materials prepared via this ‘one-pot’ synthetic route have been shown to give shape-selective catalysts for reactions such as the selective catalytic reduction of NOx with ammonia and the hydrogenation, dehydration and oxidative dehydrogenation of small hydrocarbons and oxygenates.</p