ZnO Clusters Encapsulated inside Micropores of Zeolites Studied by UV Raman and Laser-Induced Luminescence Spectroscopies

Abstract

Using microporous zeolites as host, sub-nanometric ZnO clusters were prepared in the micropores of the host by the incipient wetness impregnation method. A small amount of sub-nanometric ZnO clusters were introduced into the channels of HZSM-5 zeolite, whereas a large quantity of sub-nanometric ZnO clusters can be accommodated in the supercages of HY zeolite and no macrocrystalline ZnO exists on the extra surface of the HY material. The vibrations of the zeolite framework and ZnO were characterized by UV Raman spectroscopy. The optical properties of these ZnO clusters were studied by UV-visible absorption spectroscopy and laser-induced luminescence spectroscopy. It is found that there are strong host-guest interactions between the framework oxygen atoms of zeolite and ZnO clusters influencing the motions of the framework oxygen atoms. The interaction may be the reason why ZnO clusters are stabilized in the pores of zeolites. Different from bulk ZnO materials, these sub-nanometric ZnO clusters exhibit their absorption onset below 265 nm and show a purple luminescence band (centered at 410-445 nm) that possesses high quantum efficiency and quantum size effect. This purple luminescence band most likely originates from the coordinatively unsaturated Zn sites in sub-nanometric ZnO clusters. On the other hand, the differences in the pore structure between HZSM-5 and HY zeolites cause the absorption edge and the purple luminescence band of ZnO clusters in ZnO/HZSM-5 show a red shift in comparison with those of ZnO clusters in ZnO/HY.

Significant Influence of Zn on Activation of the C‐H Bonds of Small Alkanes by Brønsted Acid Sites of Zeolite

“…UV/Vis diffuse reflectance spectra of Zn/H-BEA and Zn/H-ZSM-5 have shown the presence of sub-nanometric ZnO clusters and macrocrystalline particles of ZnO on the extra surface of the zeolite, exhibiting bands at 265 and 370 nm, respectively. [52] Simple estimations derived from XRD data and FTIR data suggest that most of the Zn species in both samples is present in the form of sub-nanomeric ZnO clusters inside the zeolitic pores. The estimates of the acidic OH group quantity were made from the analysis of the intensities of corresponding signals of SiOHAl groups in 1 H MAS NMR spectra of zeolites with adsorbed methane as internal standard.…”

Transformation of Isobutyl Alcohol to Aromatics over Zeolite-Based Catalysts

“…16,23 As shown in UV−vis spectra ( Figure 5), a weak band appears at ∼370 nm on 0.8% Zn/ZSM-5, corresponding to the ZnO crystal on the external surface of ZSM-5. 24,26 This band obviously develops with the increase in the Zn loading. The intensive band on 5.1% Zn/ZSM-5 and 9.0% Zn/ZSM-5 is indicative of the formation of large ZnO particles, which are also clearly observed in the TEM images (Supporting Information Figure S7).…”

On the deactivation of Mo/HZSM-5 in the methane dehydroaromatization reaction

“…The Raman signal upon 244 nm excitation mostly derives from the species present at the external surface of the zeolite crystals [12]. The Raman spectrum of HZSM-5 shows bands that are typical for MFI zeolite [18]. The most prominent band at 380 cm −1 can be assigned to the double-five-ring vibration of the MFI framework.…”