Study On Enhanced Adsorption Rubidium Ion Onto Typical MOFs Materials By Lewis-acid-base Function

The separation and purification of rare metal rubidium with high added value from the salt lake is one of the hotspots of current research.In this paper,the properties of the adsorption of rubidium ions onto MOFs after the surface chemical modification of three typical MOFs materials,MIL-101?Cr?,Cu-BTC and MIL-53?A1?,were studied by means of the Lewis acid-base function.It mainly includes:1.Using phenol?PH?as the modifier,the surface chemical properties of MIL-101?Cr?were modified by impregnation method,and the PH@MIL-101?Cr?material was prepared by modifying the pore channel.MIL-101?Cr?and PH@MIL-101?Cr?were characterized by N2 adsorption-desorption,XRD,SEM,FT-IR and TGA.The results showed that the modification of phenol did not change the basic structure of MIL-101?Cr?,and the specific surface areas decreased from 3451m2/g to 2205m2/g,which has decreased by 36%.PH@MIL-101?Cr?is still a microporous positive regular octahedron crystal structure.Because of the modification of phenol,the phenol hydroxyl expansion vibration peak of PH@MIL-101?Cr?is obviously enhanced at the wavelength of 748cm-1,and the thermal stability of the material reaches 400?.The acid-base function of phenol hydroxyl Lewis base and Rb⁺ Lewis acid makes the adsorption capacity of rubidium ions onto PH@MIL-101?Cr?reaches to 93mg/g,which is 28%higher than that of basal MIL-101?Cr?.The batch adsorption process conforms to the Freundlich adsorption equation and the pseudo-second-order kinetic model.All of Na⁺,K⁺ and Cs⁺ can produce competitive adsorption of Rb⁺,and the interference degree of K⁺ is the strongest.2.Using phosphomolybdic acid?PMA?as the modifier,PMA was encapsulated into the Cu-BTC pore structure by impregnation method,and PMA@Cu-BTC was successfully prepared.The N2 adsorption-desorption,XRD,SEM,FT-IR and TGA were used to characterize the Cu-BTC before and after loading with PMA.The results showed that the doping of PMA did not destroy the basic morphology characteristics of Cu-BTC;the specific surface areas of Cu-BTC and PMA@Cu-BTC were 1030 and 244m2/g,respectively,and the ratio of micropores was 61%.Similar to basal Cu-BTC,PMA@Cu-BTC shows regular octahedron crystal structure,and its thermal stability reaches 350?.The wavelength positions of 965 and 791cm-1 in FT-IR spectrum graph corresponds to the characteristic peaks of P-O and Mo-O-Mo.The acid-base function of PMA Lewis base and Rb⁺ Lewis acid makes PMA@Cu-BTC adsorption capacity of rubidium ion reach 100mg/g,which is 30%higher than that of basal Cu-BTC.The batch adsorption process is in accordance with the Freundlich isothermal model and the pseudo-second-order kinetic model.The adsorption of Rb⁺was disturbed by K⁺,Cs⁺ and Na⁺,and the order of the interference intensity of those ions ranks as K⁺>Cs⁺>Na⁺.3.Using mercaptoacetic acid and iron oxide?HS-Fe₃O₄?as modifiers,MIL-53?Al?was modified by impregnation method,and HS-Fe₃O₄@MIL-53?Al?was subsequently prepared.N2 adsorption-desorption,XRD,SEM,and FT-IR were used to characterize the MIL-53?Al?before and after the modification.The results showed that the structure and morphology of MIL-53?Al?modified by mercaptoacetic acid and Fe₃O₄ remained almost unchanged.MIL-53?Al?and HS-Fe₃O₄@MIL-53?Al?were irregular bulky crystal structures.The specific surface areas were 824 and 537m2/g,correspondingly.The acid-base function of sulfhydryl Lewis base and Rb⁺Lewis acid can make the uptake capacity of rubidium ion onto HS-Fe₃O₄@MIL-53?Al?reach to 85mg/g which increases by 46%.The presence of magnetic Fe₃O₄ is conducive to the solid-liquid separation operation.The results of batch adsorption experiments show that the adsorption process of Rb⁺ is in accordance with the Freundlich isothermal model and the pseudo-second-order kinetic model.Compared with Na⁺ and Cs⁺,K⁺ still shows strong competitive adsorption with Rb⁺.