Preparation Of Metal-organic Frameworks And Their Composites For Liquid Phase Adsorption

In chemical and pharmaceutical industries,separation is the key process to produce pure compounds,and a large part cost of products comes from this operation unit.Distillation is frequently used in chemical separation process;however,the effectiveness of distillation is limited when separating compounds that possess similar chemical properties,particularly,the excessive consumption of energy may be very large.As an environmental-friendly and energy-saving separation method,adsorption separation is increasingly used in industrial production.The efficiency of adsorption separation largely depends on the properties of adsorbents.Metal-organic frameworks(MOFs),as new type of porous functional materials,show excellent adsorption performance due to their unique porosity and structure tenability.However,the poor stability and high cost of MOFs limit their practical application.On the other hand,MOFs based composites can relieve the contradiction to a certain extent.In this work,MOFs and their composites were utilized for removal of different contamination pollutants in liquid phase.The adsorption behaviors of MOFs for various systems were investigated systematically,so that new MOF composites can be synthesized for removal of target pollutants according to adsorption mechanisms.The main results are outlined as follows:1.The adsorption process is always accompanied by van der Waals force.It is important to investigate the influence of adsorbent pore size on van der Waals force during the absor:ption processes.UiO-66,which possesses well moisture susceptibility,was chosen as model MOF material to explore the effect of pore size on adsorption velocity and adsorption capacity of big organic molecules in liquid phase.Microporous UiO-66(0.5-0.7 nm)and three kinds of mesoporous UiO-66(pore size:4.0 nm,9.6 nm,12.3nm)were synthesized according to the methods in references.The synthesized UiO-66s were used for investigating the adsorption capacity of two dye molecules(methyle orange(MO)and direct blue 86(DB86)),rifampicin,bovine serum albumin and spherical MOP molecule with different contact time.The results demonstrate that the closer of pore size of adsorbent to the molecular dimension of adsorbate,the faster the adsorption rate,as long as the pore is accessible for the adsorbate.The adsorption capacity achieves the highest when the pore size of MOFs is 2 to 3 times larger than the dimension of adsorbate molecules.2.The complete removal of highly concentrated dye molecules from water remains a great challenge for porous materials.Concerning this issue,NH2-MIL-101(A1)was selected to adsorb DB86 in aqueous solution based on its mesoporous structure and high BET specific surface area.Removal rate reaches 100%even in the concentration of 1200 mg·L-1.The maximum adsorption capacity of DB86 on NH2-MIL-101(Al)is 1531 mg g-l,surpassing other reported materials in the literature.The excellent adsorption performance is attributed to the high specific surface area and the strong electrostatic interaction between anionic DB86 molecule and the protonated-NH2 in NH2-MIL-101(Al).This illustrates NH2-MIL-101(Al)is a good candidate for anionic dye removal.3.Considering high cost and poor stability of MOFs,low-cost carbon nanotubes(CNTs)was chosen as addictive materials,and a series of CNT@MIL-68(Al)composites were prepared.Benefited from the incorporation of CNTs,the particle size of MIL-68(Al)in composites is small and uniformlly dispersed compared with pure MIL-68(Al).Then 0.75 wt%CNT@MIL-68(Al)was utilized for phenol removal.In the industrial concentration range of phenol,the adsorption capacity of CNT@MIL-68(Al)was improved up to 188.7%,higher than the pristine MIL-68(Al),which may be ascribed to strong adsorptive force through the increase of small micropores and large exposed surface of small particles after the incorporation of CNTs.In addition,the composite shows good regenerability,which can used for separation and purification in the future.4.The incorporation of long line shaped CNTs may limit the dispersibility of CNT@MOF composites,which will further weaken the adsorption performance.With this consideration,nano silicon dioxide(SiO2)particles were further used as addictive material to prepare SiO2@MIL-68 to increase the dispersibility of MIL-68 composites.The composites also possess small well-distributed particles and small micropores(11-12 A).Besides,the adsorption of aniline on SiO2?MIL-68(Al)composites with 7 wt%of SiO2 were investigated,demonstrating a relatively high adsorption capacity(531.9 mg g"')and an ultrafast adsorption dynamics(reaching equilibrium within 40 s).Besides the strong adsorptive force caused by small micropores,the fast adsorption dynamics may be that MOF composites prepared from nanoscale SiO2 spheres are consist of single independent particles,which can provide much surface to contact aniline molecules,thus promoting the mass transfer in the adsorption process.