Studies on crystallography, stability, acidity and skeletal isomerization of C 5 olefins of THF–FER zeolite

Abstract

The crystallography characterization such as the morphology, the crystal size and the orientation, the composition and the cell parameters of high silica FER-type zeolite, which was hydrothermally synthesized in the reactant system of tetrahydrofuran (THF)–Na 2O–SiO 2–Al 2O 3–H 2O, were investigated with powder X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM) and selected-area electron diffraction (SAED), and compared with that of the siliceous FER zeolite synthesized with pyridine as the template under organothermal conditions. H-form FER zeolite samples, H-FER-I and H-FER-II were prepared with the procedure of ion exchange with H + for getting rid of alkaline cations followed with calcination for removing template, and with the procedure of removing template by calcination and then getting rid of alkaline cations with ion exchange, respectively, due to the positions of the Na + cations and THF molecules trapped in the zeolite channels and cages, respectively. 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR), in situ Fourier transform infra-red spectroscopy (FT-IR) and nitrogen adsorption were used to investigate the stability of the framework structure and the framework composition, the solid acidity, and the microporous properties on both samples of H-FER-I and H-FER-II treated in saturated steam at 1073 K for 10 h. The investigations indicate that THF-FER zeolite crystals with 3 × 2 μm in size possess the morphology and the preferential orientation very close to those of siliceous FER zeolite single crystals. H-FER-I exhibits better stability, perfect framework structure with less defects, more open channel systems, and more Brönsted acid sites than those of H-FER-II, resulting in excellent catalysis performance on skeletal isomerization of n-C 5 olefins to iso-C 5 olefins for the feed of C 5 raw mixture gas.