Comparison of the OKα x-ray emission bands in micro- and mesoporous silica materials and in α-quartz

Abstract

X-ray emission spectroscopy (XES) at the O Kalpha threshold has been used to investigate the electronic structure of a microporous pure calcined zeolite with the crystal structure of the MFI-type framework (silicalite), a deboronated MFI zeolite (DB-MFI), a pure mesoporous cubic MCM-48 material, a MCM-48 loaded with copper and zinc oxide nanoparticles (CuZnO-MCM-48), and a crystalline layered silicic acid H-RUB-18. For comparison, the XES O Kalpha spectrum of pure alpha-quartz has also been recorded. In the nonresonant energy regime the XES O Kalpha spectra for all these compounds look very similar indicating that the electronic structure of the micro- and mesoporous silica materials is very similar to that of quartz. In the resonant regime, however, the spectra exhibit significant differences. In all the materials under study, the resonant XES O Kalpha spectra recorded at photon energies close to the positions of the O K edges show Raman-type inelastic peaks with an energy loss of 11 eV, originating from electronic excitations within these insulating materials. The prominent features in the XES O Kalpha spectra of alpha-quartz and H-RUB-18 are analyzed by means of quantum chemical ab initio cluster calculations.