Browsing by Author "Jiao, Jian"

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    Quantitative characterization of aluminum in non-hydrated zeolite catalysts by multi-nuclear solid state NMR spectroscopy
    (2006) Jiao, Jian; Hunger, Michael (Prof. Dr.)
    To study aluminum species in dealuminated zeolites, a series of dealuminated zeolites Y with framework nSi/nAl ratios of 2.8 to 6.0 was prepared by steaming. The steaming of zeolite H,Na-Y was performed under water vapor pressures of 3.4 to 81.5 kPa and at a temperature of 748 K. As determined by X-ray diffraction (XRD), the crystallinity of zeolites was well preserved after the above-mentioned treatments. The hydrated materials were investigated by 29Si MAS NMR, 27Al MQMAS NMR, high-field 27Al MAS NMR, and 1H MAS NMR spectroscopy. Upon adsorption of ammonia on the steamed zeolites H,Na-Y, a reversible change of octahedrally coordinated to tetrahedrally coordinated aluminum atoms was found. Quantitative 29Si, 27Al, and 1H MAS NMR measurements indicated that this coordination change is accompanied by the formation of bridging OH groups (SiOHAl) in the dehydrated materials, while only a weak decrease in the amount of silanol (SiOH) groups and no systematic change of AlOH groups occurred. Based on these results, a model for the reversible coordination change of aluminum atoms in the framework of hydrothermally treated zeolites H-Y is proposed assuming local structures consisting of threefold-coordinated framework aluminum atoms with SiO- defect sites in their vicinity, which are coordinated to extra-framework aluminum species. After adsorption of ammonia at the threefold-coordinated framework aluminum atoms, the SiO- defect sites are healed to Si-O-Al- bridges leading to a transformation of the threefold-coordinated aluminum atoms to tetrahedrally coordinated atoms. Upon thermal decomposition of the ammonium ions formed at these Si-O-Al- bridges, SiOHAl groups occur. It is also found that framework and extra-framework aluminum species in zeolite Y were strongly influenced upon rehydration. Therefore, the investigations of these materials in non-hydrated state, i.e., without hydration after the dealumination, are required. To obtain dealuminated zeolites in non-hydrated state, these dealuminated materials were immediately filled into glass containers under dry nitrogen in an air-lock after steaming. By 29Si MAS NMR spectroscopy, a strong high-field shift of the signals of Si(3Al) and Si(2Al) sites in the spectra of non-hydrated zeolites Y in comparison with those of the hydrated samples was observed. These high-field shifts of the Si(nAl) signals of 2 to 5 ppm occurring in the 29Si MAS NMR spectra of non-hydrated zeolites Y were discussed to be caused by i) a variation of the local structure of neighboring AlO4 tetrahedra or ii) the presence of multivalent extra-framework aluminum cations. To clarify the reasons for these resonance shifts, zeolites Y with different H- and Al-exchange degrees were investigated by solid-state NMR spectroscopy. The experimental results indicate that the primary reason for the high-field shift of 29Si MAS NMR signals of silicon atoms in non-hydrated state is the change of O-Al-O bond angles and Al-O bond lengths during the dehydration of AlO4 tetrahedra in the framework. In contrast, the presence of extra-framework aluminum cations leads only to a strong broadening of the Si(nAl) signals, probably due to 29Si-27Al couplings, and a weak high-field shift of ca. 1 ppm. With increasing water vapor pressure during the steaming of zeolite Y, a systematic decrease of the total amounts of framework aluminum atoms in the non-hydrated materials was found by 29Si MAS NMR spectroscopy. The amounts of threefold coordinated framework aluminum atoms in non-hydrated zeolites Y were determined by the increase of the concentrations of bridging OH groups after an ammonia adsorption/desorption treatment and by application of 1H MAS NMR spectroscopy. By a quantitative comparison of the amounts of tetrahedrally coordinated framework aluminum atoms, responsible for the occurrence of negative framework charges, and the amounts of charge-compensating residual sodium cations and bridging hydroxyl protons, the mean cationic charge of extra-framework aluminum atoms was calculated. This means that the cationic charge per extra-framework aluminum atom was found to vary from ca. +2 to ca. +0.5 for weakly and strongly dealuminated zeolites Y samples, respectively. Solid-state NMR characterization of zeolite catalysts in the hydrated state is often accompanied by an uncontrolled hydrolysis of the framework and a variation in the coordination of aluminum species. It is demonstrated that the limitations occurring for 29Si and 27Al MAS NMR spectroscopy of non-hydrated zeolites Y, such as strong decrease of resolution and significant line broadening, can be overcome by loading these materials with ammonia. In the 29Si MAS NMR spectra of non-hydrated and ammonia-loaded zeolites Y, no dehydration-induced high-field shift of Si(nAl) signals (n = 3, 2, 1) occurs, which is generally responsible for the loss of resolution in the spectra of non-hydrated materials. The 27Al MAS NMR spectra of the non-hydrated and ammonia-loaded zeolites Y consist exclusively of signals of the tetrahedrally coordinated framework aluminum atoms with spectroscopic parameters similar to those of framework aluminum atoms in hydrated samples. The framework nSi/nAl ratios obtained by quantitative evaluation of both 29Si and 27Al MAS NMR spectra of the non-hydrated and ammonia-loaded zeolites Y agree well with each other. Beside adsorption of ammonia, the adsorption of other probe molecules, such as pyridine, acetone, and acetonitrile, was utilized to conquer the above-mentioned limitations occurring for 29Si and 27Al MAS NMR spectroscopy of non-hydrated zeolites Y. It is interestingly to note that the base strength of probe molecules is reflected by the variation of the second-order quadrupolar effect (SOQE) value of the framework aluminum atoms in the non-hydrated zeolites. The occurrence of a proton transfer from the catalyst to the probe molecules is necessary to detect the framework aluminum by 27Al MAS NMR spectroscopy in moderate magnetic fields (ca. 9.4 T). It is demonstrated that a proton affinity (PA) of the adsorbate molecules of ca. 850 kJ/mol is required to induce a proton transfer from the zeolite framework to the adsorbate compounds. In addition, other effects could influence the observation of framework aluminum by 27Al MAS NMR spectroscopy, such as, the size of probe molecules. The quantitative evaluation shows that the adsorption capacity of pyridine is hindering the detection of all framework aluminum atoms by both 29Si and 27Al MAS NMR spectroscopy in moderate magnetic fields. 27Al spin-echo, high-speed MAS (nrot = 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 extra-framework positions in non-hydrated zeolites Y. Non-hydrated g-Al2O3, aluminum-exchanged zeolite Y (Al,Na-Y) and parent 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 tetrahedrally coordinated framework aluminum atoms in the vicinity of bridging hydroxyl protons and framework aluminum atoms compensated in their negative charge by aluminium cations (diso = 70±10 ppm, CQCC = 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 (diso = 65±5 ppm, CQCC = 8.0±0.5 MHz). The residual two signals were attributed to aluminum cations (diso = 35±5 ppm, CQCC = 7.5±0.5 MHz) and octahedrally coordinated aluminum atoms in neutral extra-framework aluminum oxide clusters (diso = 10±5 ppm, CQCC = 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 non-hydrated zeolite deH,Na-Y/81.5, the aluminum distribution in this material could be determined.