The effect of acid treatment on the structure of clinoptilolite

The influence of nitric acid treatment on the composition and structure of clinoptilolite (HEU) was analyzed by various characterization techniques, such as element analysis (ICP-AES), N₂ physical adsorption, X-ray powder diffraction (XRD), ²⁷Al/²⁹Si MAS NMR and DFT calculation.The Si/Al ratio and BET surface area of Na-K-HEU and NH₄-HEU were significantly increased after nitric acid treatment, and meanwhile, their relative crystallinity decreased to below 50% when the concentration of nitric acid reached 4 mol/L, illustrating the severe distortion of zeolite framework.However, the structure of H-HEU was more stable and it could maintain a high relative crystallinity of 94.8% even the concentration of nitric acid reached 6 mol/L.The results of ²⁷Al/²⁹Si MAS NMR indicated that some extra-framework aluminum species appeared in H-HEU and they could protect the framework aluminum from further distortion, whereas this protective effect did not exist on Na-K-HEU and NH₄-HEU, as they have little extra-framework aluminum species.It can be further proved that through back-exchanging the H-HEU to Na-HEU and K-HEU, their resistance to nitric acid treatment could be markedly improved.