Studies on modification and structural ultra-stabilization of natural STI zeolite

Abstract

A natural STI zeolite, discovered in China, was treated in hot HCl solution with various concentrations for inspecting its acid-resistant property. The framework stability and the adsorption of four types of H-STI zeolite, which were prepared by modification on the natural zeolite with different routes of ion-exchange and calcination, were investigated with X-ray diffraction (XRD), temperature-programmed heating (TPH), 29Si and 27Al magic-angle spinning (MAS) nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FT-IR), in situ FT-IR, N 2 adsorption, X-ray fluorescence scattering spectroscopy (XRF), and energy dispersion X-ray analysis (EDX). The natural zeolite remains stable in hot HCl solution with the concentration ⩽2.0 mol dm −3. The framework compositions of Si and Al were determined and compared with both 29Si MAS NMR and EDX (or XRF) calibrated with 27Al MAS NMR. The modification with different routes leads to the variation in framework de-alumination and the distribution of the framework Si and Al atoms in the cell of the modified zeolites, and changes the contents of extra-framework aluminum (EFAL) and Si–OH defects, resulting in the modified zeolites with different thermal stabilities and adsorption property. A linear plot of Al atomic fraction in sum T (tetrahedral coordinative Si and Al) atoms vs the frequency of the internal asymmetric stretching vibration presents an IR spectra of the de-aluminated H-STI zeolite samples. High silica H-STI zeolites with the framework Si/Al ratios higher than 5.6, prepared with NH 4 + exchange and followed by calcination in fluid air, exhibit thermal stability up to 1273 K with open and perfect channel systems. Series H/M-STI (M = Li, Na, K, Ag, Ca) zeolite samples were prepared by ion-exchange from the high silica H-STI zeolites. The degree of ion-exchange and thermal stability were compared. The series zeolite samples calcined at 1173 K generally remain stable, among which H/Na and H/K form possess the highest thermal stability and H/Ag form shows the lowest thermal stability.