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

X-ray emission spectroscopy (XES) at the $OKα$ 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 (⁠$Cu∕ZnO$-MCM-48), and a crystalline layered silicic acid H-RUB-18. For comparison, the XES $OKα$ spectrum of pure $α$-quartz has also been recorded. In the nonresonant energy regime the XES $OKα$ 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 $OKα$ spectra recorded at photon energies close to the positions of the $OK$ edges show Raman-type inelastic peaks with an energy loss of $11eV$⁠, originating from electronic excitations within these insulating materials. The prominent features in the XES $OKα$ spectra of $α$-quartz and H-RUB-18 are analyzed by means of quantum chemical ab initio cluster calculations.