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

The P-31 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 similar to -42 ppm, which correlates to the hydrothermal structure changes in P-CHA zeolites. Theoretical calculations on the P-31 and Al-27 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 P-31 resonances toward higher field were identified for condensed phosphates and aluminophosphates. Based on the calculated P-31 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 P-31 NMR spectra is assigned to the phosphates in the framework.