Thermal Decomposition Kinetics And Mechanism Analysis Of Several Metal-organic Frameworks

Metal organic frameworks(MOFs)have become a promising candidate in the energy field because of its unique properties such as periodic arrangement of metal ions and organic ligands,high porosity,large specific surface areas and adjustable pore size structure.As an ideal precursor,the pyrolized MOFs-derived materials on the certain condition can well inherit the original structural advantages of MOFs and provide highly active and ultrafine nanoparticles.Therefore,the pyrolized MOFsderived materials have been extensively explored in the fields of catalysis,energy storage,separation,etc.The study of the thermal stability and decomposition kinetics of MOFs is of great significance to provide theoretical and experimental basis for the thermochemical application of MOFs at high temperatures.Herein,this study focused on the thermal decomposition mechanism of MOF s unveiled by a combination technology of Thermogravimetric analysis-Fourier transform infrared spectroscopy-Gas chromatographyMass spectroscopy(TG-FTIR-GC-MS).The Flynn-Wall-Ozawa method and Starink method were employed to calculate the activation energy(Ea),and the most probable mechanism function of the thermal decomposition process was determined by the general integration method.The kinetic compensation equation and the pre-exponential factor(A)were obtained from the Coats-Redfern equation according to the kinetic compensation effect.1.The pyrolysis of Zn-p-phthalic acid(Zn-BDC)MOFs exhibits a two-step decomposition feature.The TG-FTIR-GC-MS analysis indicated the emerging of DMF,CO₂ during the first decomposition process,and the CO,CO₂,DMF,C₆H₆ and C₁₂H₁₀ were subsequently produced at the second decomposition process.The remained solid products after decomposition were ZnO and carbon materials.The activation energy Ea of the first step was calculated to be 78.33 kJ·mol^-1,and pre-exponential factor A is 9.07×105 s^-1.The thermal decomposition process follows the phase interfacial reaction R3 reaction mechanism and most probable mechanism function is G(?)=1-(1-?)^1/3,The kinetic equation for the decomposion process is (d?)/(dt)=2.72 × 10⁶ × exp[(-7.83×10⁴)/(RT)](1-?)^2/3.Ea of the second process was determined as 213.94 kJ·mol^-1,A is about 2.96×10¹¹ s^-1.The thermal decomposition process follows the random nucleation and subsequent growth Johnson-Mehl-Avrami reaction mechanism and most probable mechanism function is G(a)=[-ln(1-?)]ⁿ,n=0.57.The kinetic equation for the thermal decomposition process is (d?)/(dt)?5.19×10¹¹×exp[(-2,14×10⁵)/(RT)(1-?)[-In(1-?)]^0.43.2.The thermal decomposition of Cu-trimellitic acid(Cu-BTC)MOFs was studied by TG-FTIR-GC-MS,which can be divided into three steps.The first two step corresponded to the expulsion of H₂O,C₂H₅OH and DMF,where H₂O,DMF,CO,CO₂,C₆H₆ and C₁₂H₁₀ were detected during the third decomposition process.The solid products after decomposition were Cu,Cu₂O and carbon materials.It is worth noting that the third decomposition process contains one simple reaction,with a determined Ea is 167.51 kJ·mol^-1,and the values of A is 6.76×10¹¹ s^-1.The thermal decomposition process follows the random nucleation and subsequent growth A2 reaction mechanism and most probable mechanism function is G(a)=[-ln(1-?)]^1/2,The kinetic equation for the thermal decompotion process is (d?)/(dt)=1.35×10¹²×exp[(-1.68×10⁵)/(RT)](1-?)[-ln(1-?)]^1/2.However,when changing the metal from Cu to Ni,the pyrolysis of Ni-trimellitic acid(Ni-BTC)MOFs migrated to a two-step mechanism.The TG-FTIR-GC-MS analysis revealed the production of H₂O during the first decomposition process,and the generation of H₂,H₂O,CO,CO₂,C₆H₆ and C₁₂H₁₀ during the second decomposition process.The solid products after decomposition were Ni and carbon materials.At first step elimination of coordinated water,the activation energy Ea is 82.78 kJ·mol^-1,and pre-exponential factor A is 4.82×107s^-1.The thermal decomposition process follows the random nucleation and subsequent growth A1 reaction mechanism and most probable mechanism function is G(a)=-ln(1-?),The kinetic equation for the thermal descomposition process is (d?)/(dt)=4.82 × 10⁷ × exp[(-8.28×10⁴)/(RT)](1-?).At second step the activation energy Ea is 224.70 kJ·mol^-1,pre-exponential factor A is 7.75 × 10¹³ s^-1.The thermal decomposition process follows the random nucleation and subsequent growth reaction mechanism and most probable mechanism function is G(a)=[-ln(1-?)]^2/5,The kinetic equation for the decomposition process is d?/dt=1.94 × 10¹⁴ ×(1?)× exp[(-2.25×10⁵)/(RT)][-ln(1-?)]^3/5.