Spectroscopic study of phosphorus modified H-ZSM-5

A series of H-ZSM-5 samples (Si/Al 14) were modified by impregnation with phosphoric acid and consecutive calcination. The influence of this modification was investigated by Fr-IR and Al-27 and P-31 MAS NMR spectroscopy. temperature-programmed desorption of ammonia with coupled in situ FT-IR. Phosphoric acid treatment influences strongly the textural properties of H-ZSM-5. A decreasing optical density indicates that phosphoric acid treatment is accompanied by a reduction of the short distance order caused by the reaction between phosphoric acid and zeolite framework species during calcination. Al-27 MAS NMR exhibited a clear decrease of framework aluminum forming intermediated pentacoordinated aluminum and finally non-classical octahedral coordinated aluminum. Classical extra-framework aluminum disappears with increasing phosphorus content. A frequency shift for silanol groups was observed due to the modification of the environment of initially free silanol groups. Strong evidence for this assumption is the appearance of a vibration band at 1557 cm(-1) which is typical for Si-NH2 groups resulting from the reaction of NH3 with modified silanol groups. (C) 2010 Elsevier Inc. All rights reserved.