Preparation And Adsorption Of Metal-organic Framework Composite Adsorbent On N-hexane Vapor

Oil vapor is a component of classic Volatile organic compounds（VOCs）,which is one kind of the main sources of atmospheric pollution.The major components of oil vapor include alkanes,aromatic hydrocarbons and alcoholic organic compounds.Normally,n-hexane is a typical representative of alkanes,which plays an important role in daily production and life.Therefore,the control of n-hexane pollution is imminent.The current optimal choice for the treatment of n-hexane is adsorption,and the performance of the adsorbent is the dominant factor determining the efficiency of the adsorption.Metal-organic frameworks（MOFs）are considered as promising adsorbent materials,due to their large specific surface area and pore volume,adjustable pore size,and good thermal stability.However,despite a series of advantages,MOFs still have some shortcomings that limit their practical application in the industrial field.Except for a few MOFs such as MIL-101（Cr）,most MOFs have the disadvantage of poor water stability.In addition,MOFs are mostly crystalline and exist in the form of powder.In practical application,especially in the process of cyclic adsorption,it is inevitable that the powder blockage the adsorption bed,even causing accidents.In view of the above problems,the Cu-BTC was selected as a target,which modified with activated carbon（AC）,forming composite AC/Cu-BTC.Furthermore,a series of M-BTC materials were compounded with alginate hydrogel to obtain granular products.The main researches are as follows:（1）The Cu-BTC was synthesized by hydrothermal method.A hand of tests such as N₂ adsorption-desorption isotherm,fourier infrared diffraction（FT-IR）,X-Ray diffraction（XRD）,and scanning electron microscopy（SEM）were performed on the Cu-BTC.373 K,10 h is a suitable condition for the growth of the Cu-BTC.The Cu-BTC synthesized under this condition has a type I isotherm,is a microporous material。The BET specific surface area of the Cu-BTC is 1663 m²·g^-1.The XRD results and FT-IR of the Cu-BTC show that it has the characteristic peaks and functional groups.The thermal stability of the Cu-BTC is great,which can maintain its framework at600 K.It suggests that the impurities remain in the Cu-BTC,such as H₂O and solvent molecule,can be removed by heating,therefore,400 K is optimal heat temperature before using the Cu-BTC.（2）A stable activated carbon was incorporated into the Cu-BTC by the in-situ method to yield a composite material AC/Cu-BTC with high water stability.A certain amount of the AC powder is 5%,7%,10%mass ratio of the Cu-BTC.The BET specific surface area of the AC/Cu-BTC was obtained by N₂ adsorption-desorption isotherms.XRD images shows that the addition of the AC did not hinder the formation of the Cu-BTC crystal.The FT-IR images exhibits that no new functional groups were formed between the Cu-BTC and AC.The n-hexane vapor adsorption experiments were performed on the AC/Cu-BTC.Then,the n-hexane vapor adsorption experiments were carried out before adsorbing water vapor.XRD and SEM tests were performed on the AC/Cu-BTC after water adsorption.The Cu-BTC loses its crystal characteristic peaks,while the AC_7%/Cu-BTC still intact retains its crystal characteristic peaks.Moreover,the SEM image of the Cu-BTC after water adsorption indicates that the Cu-BTC crystal appears cracks,meaning the crystal have collapsed.At the same time,the AC_7%/Cu-BTC crystal image displays that only some water erosion marks on the surface,no obvious cracks.The adsorption capacity of the fresh Cu-BTC on n-hexane vapor is greater than that of the AC/Cu-BTC.However,the adsorption capacity of the AC_7%/Cu-BTC on n-hexane vapor after having adsorbed water is three times that of the Cu-BTC.The AC/Cu-BTC has better water stability than the Cu-BTC.（3）Based on the principle of ionic cross-linking,several MOFs materials,for example,Cu-BTC,Zn-BTC,Cr-BTC and Fe-BTC with different concentrations(10,15,20 mg·mL^-1)of alginate were compound to form the M-BTC/alginate（M stands for metal center ion）composite materials.The M-BTC/alginates were dried at 353 K.The specific surface area and pore size parameters of M-BTC/alginate was obtained by N₂ adsorption-desorption isotherm analysis.Alginate concentration has an effect on the specific surface parameters of the M-BTC/alginates.FT-IR results show that the characteristic peaks of the M-BTC/alginate shift to higher wavelengths,from 1602 to 1636,1644,1628,and 1628 cm^-1,the possible reason is that the metal center ion crosslink with the network of alginate.The M-BTC/alginates were used to adsorb n-hexane vapor.The Fe-BTC/alginate has the best adsorption performance,while the Cr-BTC/alginate has the worst adsorption amount.After 5 cycles of adsorption-desorption,the Fe-BTC/alginate still retains 70%of the initial adsorption capacity,while the other three M-BTC/alginates are unable to keep such a high repeatability.The adsorption capacity of M-BTC/alginate is determined by specific surface area,affinity with n-hexane molecules,and material stability.（4）A kind of graphene oxide（GO）was compounded with the Cu-BTC.GO was added at 0.5%,2%,and 5%of the mass ratio of the Cu-BTC to obtain the GO/Cu-BTC composite material.The specific surface area parameters of the GO/Cu-BTC were obtained from the N₂adsorption-desorption isotherms at 77 K.The presence of GO increases the BET specific surface area of the Cu-BTC,thanks to that GO is a kind of two-dimensional sheet material,which can provide a substrate for the growth of the Cu-BTC crystals,leading to the specific surface area of the Cu-BTC is increased.XRD images give evidence that the addition of GO does not affect the formation of the Cu-BTC crystals.The GO/Cu-BTC was used to adsorb methanol vapor,ethanol vapor,and CO₂.The adsorption capacity of the GO/Cu-BTC for methanol vapor was 9.22%higher than that of the Cu-BTC.The adsorption capacity for ethanol vapor was 116.45%of that of the Cu-BTC.However,the adsorption capacity of the GO/Cu-BTC for CO₂ has not been significantly improved,due to the selected GO is polar.The Cu-BTC connect with GO via functional group.Therefore,the polarity of the Cu-BTC can be strengthened.Thanks to that both methanol and ethanol are polar organics,the adsorption capacity of the GO/Cu-BTC on methanol vapor and ethanol vapor can be improved.CO₂ is a non-polar molecule,the polarity of the GO/Cu-BTC has been improved,however,the adsorption capacity for non-polar molecules may has not been enhanced.As a result,the adsorption capacity of the GO/Cu-BTC on CO₂ performs poorly.