Synthesis and characterization of chromium silicalite

Abstract

Infrared spectroscopy (mid-IR), 29Si and 27Al 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 27Al MAS-NMR. The mid-IR spectra are very similar to the zeolite ZSM-5 with the addition of a weak band at ~6 8 0 cm-l, which we tentatively assign to symmetric stretching of (Cr-0-Si), 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(V1) 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.

The effect of active sites’ nature on the photo-catalytic performance of Cr-TUD-1 in the oxidation of C1–C3 hydrocarbons

“…The prepared samples were characterized by means instrumental neutron activation analysis (INAA), nitrogen phsyisorption, powder X-ray diffraction (XRD), UV-vis spectroscopy, Raman spectroscopy (under ambient and under in-situ heating), 29 Si nuclear magnetic resonance (NMR), high-resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Detailed characterization procedures can be found in the Supporting information section.…”

“…All the experiments were carried out at least for three times, the results presented here is the average of the three with a standard deviation does not exceed than 3%. 29 Si MAS NMR was performed to study the interaction between Si and Cr in the Cr-TUD-1 sample. The 29 Si MAS NMR spectrum of the calcined siliceous TUD-1 sample is compared with the Cr-5 spectrum in Fig.…”

“…29 Si MAS NMR was performed to study the interaction between Si and Cr in the Cr-TUD-1 sample. The 29 Si MAS NMR spectrum of the calcined siliceous TUD-1 sample is compared with the Cr-5 spectrum in Fig. 1.…”

“…The spectra are dominated by: two intense bands around 270 and 350 nm, a shoulder around 440 nm and a weak band around 600 nm are observed only in high Cr-loading samples. The adsorption bands centred at 270 and 350 nm are usually assigned to the charge transfer from O 2− to Cr 6+ of the tetrahedrally coordinated Cr 6+ and indicate the presence of highly isolated Cr 6+ atoms inside the silica matrix[28,29]. The shoulder around 440 nm is characteristic of Cr 6+ polychromate ( Cr O Cr )n in the framework.…”

Synthesis of Imprinted Polysiloxanes for Immobilization of Metal ions

“…It was observed that prior to washing there was a peak near 698 cm -1 which suggests that Cr 3+ is coordinated by oxygen moieties of polysiloxanes network [13,14] and thus indicated successful imprinting. After through washing of polymers, the metal-oxygen peak was missing in spectrum indicating the removal of template ions.…”