| Metal-Organic Frameworks(MOFs),as a kind of crystalline porous material of which the inner structures are arranged orderly,have attracted great attentions from chemists worldwidely,due to their fascinating topologies and potential applications,particularly in gas storage and separation,drug delivery,fluorescence and catalysis.With the continuous development of the study of MOFs,now the vision and study emphasis of the related scientists have gradually turned from the simple and mechanical synthesis of the MOFs to rational design and assembly of MOFs to achieve MOFs with excellent properties and expected structures.For the rational design and assembly of MOFs,in the early 1990s,Robson initially introduced the"node and spacer" approach.Later on,another breakthrough was made by O'Keeffe and Yaghi by using inorganic clusters as the augments of vertices in a given net to construct MOFs,and reticular chemistry was formed.Zaworotko and Yaghi further developed the idea of metal-organic polyhedra(MOPs),as supermolecular building blocks(SBBs)to extend MOFs.By these strategies,numerous MOFs have been synthesized and reported.However,chemists are still always trying to break the traditions and open up the novel territory of MOFs study.In the process,the study of liquid adsorption of MOFs appeared as a new study area.The adsorbents and environment of the liquid adsorption are different with the traditional gas adsorption.Specifically speaking,how to encapsulate the functional particles(such as drug molecules,dye molecules,luminescence molecules)into the pores of the MOFs based on the specific host-guest interaction is a promising study point,and the focus on the enrichment and inspection of trace elements such as iodine is still rare.The original intention of this paper is to from the square pyrazole-bridged cyclic dinuclear metal complex based on the pyrazole-3,5-dicarboxylate units and metal ions,combining the acylamide group which is of good flexibility and affinity to CO2,and the MOP-1 SBB(supermolecular building block)constructed from isophthalic acid units to build a new(4,24)-c cage-like structure which shall be of good CO2 adsorption.We designed the 1,5-pyrazole acylamide tetracid and tried to get the expected structure by the solvothermal reaction of this ligand and various metals.But accidentally,the structures we got are isoreticular and not(4,24)-c cage-like structures.In the structure,the two pyrazole-3,5-diacylamide units with the metal ions don't form the square pyrazole-bridged cyclic dinuclear metal complex,and only half of the oxygen atoms of the acylamide groups coordinate to the metal ions which lead to a dinuclear metal complexwith the dihedral angle being 113.25°.As a result of the disturbance of this factor and maybe some other factors,the isophthalic acid units in the ligands don't synergistically form the 24-c cage-like SBUs(Secondary Building Units)but to form the simple paddle-wheel SBUs or coordinate to the former dinuclear metal clusters.Surprisingly,while the spatial arrangements of the SBUs seemed disordered,in the whole structure there exist parallel square nano-tubular channels(6.7×6.7A)along the b axis where nearly all the ligands are paved levelly in the surface and many metal clusters with coordinated water molecules exist.The size of the channel is bigger than the kinetics diameters of any common molecules and the porosity of all the complexis about 64.7%,the calculated BET surfaces of these complexare about 5140 m2/g.Accordingly,we deduced that there will be unsaturated metal sites and CO2 binding sites in the channel of these complex after activation which thus will be of good gas adsorption capacity.But unfortunately,the complex are of no meaningful N2 adsorption in low pressure range as we tested.We deduced that the structures of these complex might have collapsed after activation which resulted in the sharp decrease of the surfaces of the skeletons.Later,we tested the CO2 adsorption of these complex and the adsorption amounts(the highest is 24.6 cm3 g-1 at 273 K,15.7 cm3 g-1 at 298 K for complex 1)were times lower than the expected ones Afterwards,we tested the Powder X-Ray Diffraction of these complex after activation and found that the structures were probably collapsed upon activation.According to the data and some literatures,we deduced that the relatively instable pyrazole-bridged cyclic binuclear avulsed by surface tension of solvent molecules upon evacuation,which leaded to the blockage of the channels but in the collapsed structures existed many scattered acylamide groups and UMCs(unsatured metal sites)so these complex could still adsorb considerable amount of CO2.In the case of failure of gas adsorption characterization of these complex,we were planned to take complex 1 for example and test its liquid adsorption capacity.As we know,different from the criteria of traditional gas adsorption,the frameworks of the related MOFs in the liquid adsorption shall be stable in different solutions rather than under evacuation.In the gas adsorption,all the solvent molecules and some coordinated water molecules in the frameworks shall be removed,while in the liquid adsorption,these molecules shall be replaced.Above all,in the literatures we found that many MOFs of finely porous structures on the whole are unstable under evacuation and of no good gas adsorption capacities which may be attributed to some "weak points" in the structures,but these MOFs are of excellent performance in liquid adsorption.At the same time,the adsorbents and mechanism of adsorption of both kinds of adsorption are different,which prompt us to adopt new strategies and criteria when we design and evaluate MOFs for liquid adsorption.Considering the size of the micropores in the complex 1 is 6.7 × 6.7 A,and among many adsorbents in liquid adsorption,the van der waals diameter of I2 molecule is about 3.96 A,combing the advantages that many secondary bonds of ?…I and N-H…I will form between the aromatic rings(pyrazole and benzene rings)of the ligands paved along the channels and I2 molecules which will lead to strong host-guest interaction between the framework and I2 molecules.So we deduced that complex 1 is of fine adsorption capacity of iodine.After a series of characterization of iodine adsorption in complex 1,we found that complex 1 is of certain iodine adsorption and desorption speed and excellent performance in several cycles of reversible adsorption of I2,and the adsorption amount is as high as 0.598g/g,at the same time,we speculated the adsorption state and mechanism of complex 1 reasonably.As a result,the experiments verified our deduction on the structure-property of complex 1.We were intended to combine several new SBUs of MOFs to construct a complex of novel topology with a designed ligand(acylamide tetracid of pyrazole)in this paper.But after solvothermal reaction of the ligands with many different metals,the complex we got are of isoreticular and microporous structure with another two kinds of metal clusters inside.Though the complex are of no excellent gas adsorption capacity due to their inferior framework stability,we deduced that the complex shall be of good performance in liquid adsorption.Subsequently,we found complex 1 is of good capacity in several cycles of reversible adsorption of I2.The series of investigation process made us to comprehend and experience the relationships between design and synthesize of MOFs and gas and liquid adsorption,especially broaden our design strategies and criteria of evaluation of MOFs and blaze a trail for more comprehensive and Innovative study and applications of MOFs. |
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