Mechanism of NaZSM-5 zeolite in enhancing the yield of humic acid from straws via hydrothermal process
The hydrothermal preparation of humic acid from straws is a significant method for utilizing straw re-sources,but it is often hindered by low humic acid yields.This study explored the impact of incorporating NaZSM-5 zeolite into the hydrothermal treatment of straws on humic acid yield.The experimental results indicated that adding NaZSM-5 zeolite significantly enhanced the yield of humic acid.Specifically,as the Si/Al molar ratio of NaZSM-5 in-creased,the yield of humic acid increased first and then decreased,with a peak yield of 16.04%observed at a Si/Al molar ratio of 50.To elucidate the mechanism of NaZSM-5 zeolite to improve humic acid yield,we employed several characterization techniques,including X-ray photoelectron spectroscopy,nuclear magnetic resonance spectroscopy,NH3 adsorption-desorption,quaternary ammonium salt adsorption-desorption,and infrared spectroscopy.The findings revealed that NaZSM-5 zeolite with high Si/Al molar ratios(200 and 85)contained low to medium strength Lewis acid sites.As the Si/Al molar ratio decreased(from 200 and 85 to 38 and 25),medium-strength Lewis acid sites gradually converted to high-strength Lewis acid sites.The NaZSM-5 zeolite with a Si/Al molar ratio of 50 exhibited the highest content of medium-strength Lewis acid sites.These medium-strength sites effectively stabilized the quaternary ammonium nitrogen structure(R4 N)formed during the hydrothermal process,thus facilitating humic acid formation.Conversely,NaZSM-5 with high Si/Al molar ratios showed weaker adsorption strength for the quaternary ammonium nitrogen structure,which hindered its con-version to humic acid.On the other hand,NaZSM-5 with lower Si/Al molar ratios exhibited stronger adsorption,which could lead to excessive coupling of the quaternary ammonium nitrogen structure,thus reducing the yield of humic acid.This study presents an effective method for improving the yield of humic acid through straw hydrothermal treatment,thereby ad-vancing the resource utilization of straws.
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