Preparation Of Novel Ag/ZSM-5 Adsorbents And The Investigation Of The Adsorption Performance For Trace Ethylene

As a gaseous hormone,ethylene is able to promote the growth and maturity of plants.However,during the storage and transportation of postharvest fruits and vegetables,the existence of trace ethylene could shorten the shelf life significantly,resulting in food waste.Adsorbents is often used to remove the ethylene that exists in the storage and transportation environment of fruits and vegetables,but the materials,such as activated carbon and zeolite,generally face the problems of poor selectivity,small capacity,and poor reusability.Therefore,by adjusting several properties（metal valence,surface acidity,pore structure and defects degree）of HZSM-5 zeolite,a supported chemical adsorbent that capable of removing trace ethylene has good reusability was prepared.The"structure-activity"relationship between the structure of the novel Ag/ZSM-5 adsorbents and their adsorption performance was explored.The main research contents are as follows:A series of Ag/ZSM-5 adsorbents with different Si/Al ratios（38,85,130,200 and 300）were prepared via a vacuum assisted incipient impregnation method.It was found that the adsorbents with the Si/Al ratio of 130 possessed the best trace ethylene adsorption performance,which can remove 100 ppm ethylene completely at 25℃,showing the adsorption capacity of 4.89 m L·g_ads^-1,and still retained more than 80%of the adsorption capacity after six cycles.The effects of silver loading contents and calcination temperature on the adsorption ability were investigated,and the optimum preparation conditions were as follows:1 wt.%of silver loading contents,550℃of calcination temperature.A postharvest banana storage experiment was conducted,confirming the ability of prolonging the preservation period of banana.The characterization results indicated that the Si/Al ratio of zeolite does not affect the pore structure of the adsorbents.However,it changes the surface metal valence and surface acidity of the adsorbents.The adsorbents that have more Ag⁺content and higher proportion of weak Lewis acid sites possess stronger adsorption ability for ethylene.In Ag/ZSM-5（130）,the proportion of Ag⁺was 63.3%,and the number of weak Lewis acid sites was 3.83 times higher than that of Br（?）nsted acid sites.The synergistic effect between the metal valence and surface acidity makes the adsorbents have the strongest electron-acceptability and show the best adsorption performance.Different amount of ammonia（1.5,2.0,2.5,3.0 and 3.5 m L）was added for the modification of Ag/ZSM-5（130）.The adsorption capacity of the adsorbents that were modified by 2.5 m L of ammonia increased to 5.62 m L·g_ads^-1,confirming that its ethylene adsorption ability increased by 15%after modification.The evaluation results showed that the modified adsorbents possess a better adsorption affinity for ethylene,which could effectively remove trace ethylene,and the adsorption of ethylene is dominated by chemisorption.According to the characterization results,the modification of ammonia introduced mesoporous into the adsorbents,forming a hierarchical porous structure.Therefore,the diffusion resistance is decreased,while the accessibility of adsorption sites is increased.The IHF of Ag/NZSM-5（2.5）was calculated to be 0.65.In addition,the desilication of ammonia increases the defect degree of the adsorbents,resulting in stronger metal-support interaction,which enhances the dispersion of silver.The metal dispersion rate of Ag/NZSM-5（2.5）was 20.77%.The modification of ammonia introduces hierarchical porous structure and defect sites into the adsorbents,and the synergistic effects of these two factors strengthen the ethylene adsorption ability.