Dispersion of Inorganic Salts into Zeolites and Their Pore Modification

Abstract

A series of samples, prepared from mechanical mixtures of inorganic salts (MoO3, CuCl2, NiCl26H2O, and Ni(NO3)26H2O) with zeolites (NaZSM-5, NaY, NaY, and MCM-41), have been investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), the adsorption of probing molecules, and the determination of pore size distribution. After the heating of MoO3with zeolites at 723 K for 24 h, the samples (MoO3/NaZSM-5/NaY/MCM-41=0–0.088/0–0.24/0–0.45 g/g) show only those X-ray peaks assigned to zeolites, the characteristic peaks of MoO3having disappeared completely, which suggests that MoO3is highly dispersed in the pores of zeolites. Similar phenomena are observed for the samples of CuCl2/NaZSM-5, CuCl2/NaY, NiCl2/NaZSM-5, NiCl26H2O/NaY, and Ni(NO3)26H2O/NaY. In contrast, the inorganic salts of NiCl26H2O and Ni(NO3)26H2O cannot be dispersed into the channels of NaZSM-5 zeolite due to the limitation of the channel size. These results suggest that the dispersion only occurs under the condition that the diameter of the inorganic salts is less or similar to the pore size of the zeolites. Also, we observed that the dispersion capacity of the inorganic salts is strongly related to the pore size of the zeolites. Furthermore, the isotherms for probing molecules show that the dispersion of inorganic salts into zeolites leads to significant change in the pore size of zeolites. The MoO3/NaY samples exhibit that the pore size was selectively formed at 6.8–8.0, 6.0–6.8, 4.3–6.0, and 3.0–4.3 Å with MoO3loading at 0.08, 0.16, 0.21, and 0.24 g/g, respectively. Therefore, it is proposed that we could modify the pore sizes of zeolites to various degrees, which may be very useful to design suitable pores of zeolites for the catalysts in catalytic reactions. Catalytic data in selective reduction of NO by propylene at the temperature of 300°C show that CuCl2/HZSM-5 catalyst prepared from the dispersion method exhibits much higher catalytic conversion (39%: N2yield) than that (21%: N2yield) of CuZSM-5 catalyst prepared from the ion-exchange method.