Characterization of framework and extra-framework aluminum species in non-hydrated zeolites Y by 27Al spin-echo, high-speed MAS, and MQMAS NMRspectroscopy at B0 = 9.4 to 17.6 T

Abstract

²⁷Al spin-echo, high-speed MAS (ν_rot = 30 kHz), and MQMAS NMR spectroscopy in magnetic fields of B₀ = 9.4, 14.1, and 17.6 T were applied for the study of aluminum species at framework and extra-framework positions in non-hydrated zeolites Y. Non-hydrated γ-Al₂O₃ and non-hydrated aluminum-exchanged zeolite Y (Al,Na-Y) and zeolite H,Na-Y were utilized as reference materials. The solid-state ²⁷Al NMR spectra of steamed zeolite deH,Na-Y/81.5 were found to consist of four signals. The broad low-field signal is caused by a superposition of the signals of framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminum cations (δ_iso = 70 ± 10 ppm, C_QCC = 15.0 ± 1.0 MHz). The second signal is due to a superposition of the signals of framework aluminum atoms compensated by sodium cations and tetrahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (δ_iso = 65 ± 5 ppm, C_QCC = 8.0 ± 0.5 MHz). The residual two signals were attributed to aluminum cations (δ_iso = 35 ± 5 ppm, C_QCC = 7.5 ± 0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (δ_iso = 10 ± 5 ppm, C_QCC = 5.0 ± 0.5 MHz). By chemical analysis and evaluating the relative solid-state ²⁷Al NMR intensities of the different signals of aluminum species occurring in zeolite deH,Na-Y/81.5 in the non-hydrated state, the aluminum distribution in this material was determined.