Construction Of Highly Stable Metal-organic Frameworks And Application Study In Catalysis And Sensing

Metal-organic frameworks?MOFs?are a class of newly developed porous solid materials,constructed from organic linkers and metal-containing nodes through coordination bonds.These materials have attracted intense attention over last two decades due to their structural variety and promising applications in luminescence,sensing,separation,catalysis,gas storage,etc.However,most of reported MOFs are not stable in aqueous media,which restricting their applications in the presence of water.Thus,it is an important subject to construct MOFs with highly hydrothermal stability.In the chapter 2 of this thesis,a pyrazolate-based porohyrinic MOF with extraordinary base-resistance is constructed by a top-down topological analysis.Moreover,under the guidance of the reticular synthesis strategy,a base-resistant porphyrin MOF with large pores is constructed and applied in C-H bond halogenation under Lewis base condition as a catalyst,which is introduced in the chapter 3.In the chapter 4,two structurally flexible and chemically robust MOFs are prepared and used for luminescent pH sensing and molecule recognition.Finally in the chapter 5,we explored a linker rigidization strategy for enhancing the stability of MOFs.The corresponding results are briefly described as follows:?1?Construction of a pyrazolate-based porphyrinic MOF with extraordinary base-resistance.Guided by a top-down topological analysis,a MOF constructed by pyrazolate-based porphyrinic ligand,namely PCN-601,has been rationally designed and synthesized,which exhibits excellent stability in alkali solutions.It is,to the best of our knowledge,the first identified MOF that can retain its crystallinity and porosity in saturated sodium hydroxide solution at 100oC.This almost pushes base-resistance of porphyrinic MOFs to the limit in aqueous media and greatly extends the range of their potential applications.In this part,we also try to interpret the stability of PCN-601from both thermodynamic and kinetic perspectives.?2?Construction of a base-resistant metalloporphyrin MOF for catalysis in C-H bond halogenation.A base-resistant porphyrin MOF,namely PCN-602 has been constructed with 12-connected[Ni₈?OH?₄?H₂O?₂Pz₁₂]?Pz=pyrazolate?cluster and a newly designed pyrazolate-based porphyrin ligand,5,10,15,20-tetrakis?4-?pyrazolate-4-yl?phenyl?porphyrin under the guidance of the reticular synthesis strategy.Besides its robustness in hydroxide solution,PCN-602 also shows excellent stability in aqueous solutions of F^–,CO₃^2–,and PO₄^3–ions.Interestingly,the Mn³⁺-porphyrinic PCN-602,as a recyclable MOF catalyst,presents high catalytic activity for the C–H bond halogenation reaction in a basic system,significantly outperforming its homogeneous counterpart.To the best of our knowledge,for the first time,a porphyrinic MOF is used as an efficient catalyst in a basic solution.?3?Structurally flexible and chemically robust MOFs for aqueous solution pH sensing and molecule recognition.Stimuli-responsive MOFs are promising advanced materials in chemical sensing,but advancing their applications in aqueous media is challenging.Two isoreticular fluorescent MOFs,Zr₆O₄?OH?₈?H₂O?₄?CTTB?₂?BUT-62?and Zr₆O₄?OH?₈?H₂O?₄?CTNA?₂?BUT-63?were synthesized with newly-designed carbazole core-based ligands,4,4',4'',4'''-?9H-carbazole-1,3,6,8-tetrayl?tetrabenzoate(CTTB^4–)and6,6',6'',6'''-?9H-carbazole-1,3,6,8-tetrayl?tetrakis?2-naphthoate?(CTNA^4–).BUT-62 and-63 are highly porous,while showing excellent stability in aqueous solutions with a wide range of pH values.Interestingly,both MOFs exhibit structural breathing behavior upon the variation of guest molecules inside their frameworks due to the topologically allowed flexibility.As configurational change of the ligands during breathing of frameworks,the two MOFs represent sensitive fluorescence change mainly on shifting emission peaks rather than only intensity change,as response.Based on the structural flexibility and chemical robustness of their frameworks,fluorescent BUT-62 and-63 have been demonstrated to be functional for pH sensing in aqueous media.Particularly,the wavelengths of maximum emission(?_max)of BUT-63 are well dependent on pH values of aqueous solutions ranging from 1 to 8in a linear relationship,qualifying this MOF for quantitatively sensing pH value of an aqueous solution by the?_max of its fluorescence.The sensitive fluorescent response of the MOFs can also be triggered by the adsorption of most common solvent molecules in term of different emission shifts.They are thus potentially useful for recognizing solvent molecules by the?_max of fluorescent emission.?4?Linker rigidization strategy for enhancing stability of MOFs.Three tetranaphtholic acid ligands with the same four-fold connectivity:6,6',6'',6'''-?[1,1'-biphenyl]-3,3',5,5'-tetrayl?tetrakis?2-naphthoicacid??H₄BTNA?,6,6',6'',6'''-?2,2',4,4',6,6'-hexamethyl-[1,1'-biphenyl]-3,3',5,5'-tetrayl?tetrakis?2-naphthoicacid??H₄MTNA?and6,6',6'',6'''-?pyrene-1,3,6,8-tetrayl?tetrakis?2-naphthoicacid??H₄PTNA?,have been prepared.These ligands have quite different flexibility,which is due to the distinct nature of their cores.It has been found that four different MOFs?BUT-90,-91,-92,-93?,constructed with the flexible ligand H₄BTNA,are unstable in aqueous solution.In contrast,the MOFs?BUT-94 and-95?constructed from the rigid ligands H₄PTNA and H₄MTNA,show excellent hydrothermal stability.Moreover,correspondingflexibleligand5',5''-bis?4-carboxyphenyl?-[1,1':3',1'':3'',1'''-quaterphenyl]-4,4'''-dicarboxylic acid?H₄BCQD?,and rigid ligands 5',5''-bis?4-carboxyphenyl?-2',2'',4',4'',6',6''-hexamethyl-[1,1':3',1'':3'',1'''-quaterphenyl]-4,4'''-dicarboxylic acid?H₄MTTB?and 4,4',4'',4'''-?pyrene-1,3,6,8-tetrayl?tetrabenzoic acid?H₄PTTB?have been prepared through the replacement of naphthyl groups of these ligands by phenyl groups.It has been also found that the hydrothermal stability of the MOFs?BUT-97 and NU-1000?constructed from the rigid ligands,is superior to that of the MOF?BUT-96?which is constructed with the flexible ligands.Especially,the strategy can be also applied in some other unstable systems,BUT-98 was synthesized by a rigid ligand pyrene-2,7-dicarboxylic acid?H₂PDBC?.It is as expected that the hydrothermal stability of BUT-98 has been obviously enhanced relative to that of UiO-67.These systematic results have demonstrated that ligand rigidization is an efficient and general method for enhancing the stability of MOFs.