Theoretical study on 31P NMR chemical shifts of phosphorus-modified CHA zeolites

Abstract

Abstract The 31P MAS NMR spectra of phosphorus-modified chabazite (P-CHA) zeolites have been observed during the hydrothermal treatment to probe the structural changes of phosphorus species in zeolites. Characteristic changes of the spectra were observed in the range of −27 ~ −42 ppm, which correlates to the hydrothermal structure changes in P-CHA zeolites. Theoretical calculations on the 31P and 27Al NMR chemical shifts have been systematically performed to disclose the possible phosphorus species of intra- and extra-framework and the structural changes during the hydrothermal treatment. The shift of the 31P resonances toward higher field were identified for condensed phosphates and aluminophosphates. Based on the calculated 31P NMR chemical shifts, the peak with increased intensity at −42 ppm in the initial stage of the hydrothermal treatment is mainly assigned to the formation of siliconoaluminophosphate (SAPO) species in the 6-membered ring of the zeolite framework, while the peak with increased intensity at −29 ppm in the later stage of treatment is ascribed to the accumulation of the extra-framework condensed phosphate or aluminophosphate species after the partial framework decomposition. The dominant peak at −33 ppm in all 31P NMR spectra is assigned to the phosphates in the framework.