Effects of niobium addition on ZSM-5 studied by thermal and spectroscopy methods

Incorporation of niobium into microporous molecular sieves brings up new properties for these materials. ZSM-5 supported catalysts containing 2, 5, 13 and 19 wt.% of niobium pentoxide were prepared by aqueous impregnation and characterized by the combination of thermal (TG/DTA) and spectroscopic methods such as FTIR, FT-Raman, DRIFTS, and 29Si and 27Al MAS and 29Si CP-MAS-NMR. In addition, X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and measurements of specific surface area by nitrogen adsorption (BET method) were carried out in order to obtain structural, textural and stability information about the solids. The samples were studied at different temperatures and heating time to optimize these parameters and to avoid a possible reduction of acidic properties by dehydroxylation. MAS-NMR and XRD results showed that the zeolite structure containing Nb did not undergo dealumination after thermal treatment, according to the presence of SiONb units on the zeolites structure in FTIR and Raman studies. Thermal analysis confirmed that Nb2O5 impregnation reduced zeolite ZSM-5 dehydroxylation and thus promoted thermal stabilization. Nitrogen adsorption isotherms by the BET method indicated that surface area and pore volume decreased with the increase of Nb loading on the calcined samples at 450 °C. In addition, the nature of the zeolite active sites was investigated by spectroscopic and thermal analysis after pyridine adsorption. It was proposed that there are two different acidic sites (Brønsted and hydrogen bond site types) on the studied systems.