Title

Synthesis and characterization of nanocrystalline mordenite, high silica zeolite RHO, and copper faujasite

Date of Completion

January 2005

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Mordenite is a zeolite that has been used as a selective adsorbent and as a catalyst. In reactions where the diffusion of reagents into the pore system is the rate-determining step, nanoparticles of the catalyst improve the reaction rate. Mordenite with a crystal diameter smaller than 100 nm has been prepared by the modification of different synthetic parameters such as the source of aluminum, the presence of seeds, the use of low temperatures (150°C vs. 170°C), longer crystallization times (24 h vs. 96 h), and different silica to alumina ratios (10–30). The decrease in the crystal diameter of the prepared mordenite was monitored by the application of the Scherrer equation that relates the broadness of the X-ray diffraction peaks to crystal sizes. ^ Zeolite RHO with an initial silica to alumina ratio (SAR) higher than 20 has been prepared. EDTA, citric acid, and tartaric acid have been used as complexing agents in the synthesis of zeolite RHO. Crystallization time increases (from 48 h to 900 h) with increasing the silica to alumina ratios (SAR) of the initial gel (SAR: 10.8 to 30) and by adding complexing agents. Complexing agents favor the formation of small crystals (0.8 μm) with increased silica to alumina ratio (final SAR: 4.5 vs. 4.0 without complexing agents). The products were characterized by XRD, FESEM, EDX, FTIR, and in-situ XRD. ^ Copper containing faujasite has been successfully prepared for the first time using a direct synthesis method. Ammonium hydroxide was used to form a copper complex that was later mixed with the reacting gel. Crystallization took place at 85°C for 11 days. The copper containing faujasite obtained was characterized by XRD, FESEM, EDX, EPR, FTIR, TPR, and BET. According to the XRD pattern only FAU type zeolite was obtained. According to TPR experiments, the reduction temperature for Cu2+ ions present in Cu-FAU prepared by direct synthesis was 70 K higher than for Cu-FAU prepared by ion-exchange. This difference can be due to the different location of the copper ions in the supercages or in the sodalite cages of the faujasite. ^