Molecular Dynamics Simulation Of Benzene Adsorption In Palygorskite And Sepiolite

Volatile organic compounds(VOC)are the most common air pollutants in chemical,pharmaceutical,printing and other industries.Because of their toxicity,odor,mutagenicity and carcinogenicity,their emissions have become one of the most serious environmental problems.Among all kinds of volatile organic compounds,Benzene series is considered as the most important treatment object because of its high toxicity and universality.With the development of technology,the demand for waste purification and precious chemical recovery is also growing.Therefore,the demand for porous adsorbents with different application characteristics is also increasing.Sepiolite and palygorskite are fibrous clay minerals.Because of their structure and physicochemical properties,they become useful adsorbents.In this paper,the structural models of palygorskite,sepiolite and adsorbate benzene were constructed and optimized to make the model closer to the existing state in nature.Then,the slit model was constructed and the adsorption of benzene was simulated at 268 k,278k,288 k,298K and 308 k,respectively.The adsorption capacity and adsorption heat under different conditions were compared and analyzed.After that,palygorskite and sepiolite were modified by the way of de zeolite water and acid treatment respectively,and the modified model was constructed.Under the same conditions,benzene molecules were adsorbed again,and the corresponding adsorption data were obtained.On this basis,the above adsorption results were simulated by molecular dynamics,and analyzed from the aspects of concentration distribution function,radial distribution function and self diffusion coefficient.The conclusions are as follows:(1)The average adsorption capacity of palygorskite and Sepiolite for benzene reached the maximum at 278 k with the increase of temperature.The variation of zeolite water content in the structural channel leads to the fluctuation of adsorption capacity.At different temperatures,the average adsorption capacity of palygorskite for benzene is higher than that of sepiolite,and the adsorption capacity of palygorskite for benzene is better than that of sepiolite.The isosteric adsorption calorific value of palygorskite and Sepiolite for benzene is lower than 42 k J·mol-1,which is physical adsorption,and palygorskite has higher adsorption potential and more stable adsorption products.With the increase of temperature,the adsorption potential of palygorskite and sepiolite decreased,and the adsorption performance also decreased.(2)After the removal of zeolite water,the average benzene adsorption capacity of palygorskite and sepiolite decreased with the increase of temperature,and reached the lowest value at 308 K.Compared with the non zeolite water adsorption system,the adsorption capacity does not fluctuate,and the zeolite water content in the structural channel does not change.The benzene adsorption capacity of palygorskite and sepiolite increased after zeolite removal.Because the pore area of sepiolite is larger than that of palygorskite,the adsorption performance of sepiolite is improved greatly after the removal of zeolite water.The average isosteric adsorption heat of benzene on the two minerals decreased with the increase of temperature,the adsorption potential decreased,and the adsorption performance decreased gradually.The average adsorption heat of palygorskite was lower than that of sepiolite at different temperatures,and the stability of sepiolite adsorption system was better than that of palygorskite.(3)After acidification,the average adsorption capacity of palygorskite for benzene increased first and then decreased with the increase of temperature,and reached the maximum at 288 k.The average adsorption capacity of acid treated sepiolite decreased with the increase of temperature.Acidification results in the dissolution of part of the octahedron,the change of surface functional groups,the enlargement of the original pore diameter and the appearance of new nano pore structure.The adsorption capacity of palygorskite and Sepiolite for benzene was improved.After acid treatment,palygorskite was more stable than sepiolite.The improvement of adsorption performance of palygorskite by acid treatment is more obvious than that of sepiolite.(4)Compared with de zeolite water,the improvement of adsorption performance of palygorskite by acid treatment is more obvious,while the improvement of adsorption performance of Sepiolite by acid treatment is close to that of de zeolite water.Considering that the self diffusion coefficient of benzene in acid treated sepiolite is much higher than that in de zeolite water treated sepiolite,while the self diffusion coefficient of benzene in acid treated palygorskite is generally lower than that in de zeolite water treated sepiolite,it is considered that de zeolite water can improve the performance of sepiolite more obviously.(5)Palygorskite and sepiolite have a certain adsorption effect on benzene.When they are used for benzene adsorption,palygorskite is more suitable to improve the adsorption capacity by acidification treatment,while sepiolite is more suitable to improve the adsorption capacity by de zeolite water treatment.