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Multiple field solid state MAS NMR structural investigations of ultrastable Zeolite-Y and Aluminophosphate materials Bretherton, Jeremy L.

Abstract

Solid state MAS NMR investigations of ultrastable zeolite-Y (USY) materials were performed at magnetic fields of 18.8 T, 17.6 T, 14.4 T and 9.4 T. ²⁷Al 3QMAS and quantitative high field ²⁷Al MAS NMR spectra were acquired and can be interpreted in terms of four distinct aluminium environments. Simulation of the ²⁷Al MAS NMR spectra at multiple fields enabled the accurate parameterisation of the aluminium environments, as demonstrated by the good agreement between calculated and experimental isotropic ²⁷Al 3QMAS shifts at both high and low field. Quantitative ²⁷Al MAS NMR spectroscopic data is supported by ²⁹Si MAS NMR spectroscopy, powder X-ray diffraction patterns and nitrogen physisorption isotherms in the study of several series of USY materials. The effects of acid washing and steam calcination on the structure of USY and the proportions and nature of the aluminium present were investigated. The effects of repeated ammonium exchange and calcination on the structure of USY are shown to be consistent with a new model describing the progressive structural changes taking place during hydrothermal treatments. This information is used to provide a preliminary interpretation of high temperature (540°C) n-hexane catalytic cracking data acquired for the USY materials. These multiple field ²⁷Al MAS NMR techniques were applied to other mixed crystallineamorphous systems. In the study of a series of complex aluminophosphate ceramic materials, the quantitative reliability of these techniques was further demonstrated by the agreement of the results of multiple field ²⁷Al MAS NMR and ³¹P MAS NMR experiments. ²⁷Al MAS NMR spectroscopy has enabled the quantification of all crystalline and amorphous phases present in these mixtures, which is not possible using any other techniques. Multiple field ²⁷Al and ³¹P MAS NMR spectra and powder X-ray diffraction data were obtained for calcined, hydrated AlPO₄-18 molecular sieve. Al-P bonding connectivities were derived from ²⁷Al -³¹P refocused INEPT MAS NMR spectra. These data have facilitated the assignment of the crystallographic space group and the unit cell parameters and the number of possible assignments of the NMR resonances has been reduced from 5.2 x 10⁴ to eight.

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