Stability Of Inclusion Compounds Research Articles

Some basic correlations in the thermal (kinetic) stability of inclusion compounds on the basis of microporous metal–organic frameworks

Metal–organic frameworks (MOFs) have promising practical applications in gas storage, separation and purification, and catalysis. The standard process for MOF production begins with the synthesis of the inclusion compound. The molecules of the organic solvent used are caught in the channels and caves of the MOF structure. These primary inclusion guest molecules are excluded further by the evacuation or by the heating. We investigate a series of inclusion compounds and study the correlation between their thermal (kinetic) stability and the framework and guest molecule properties. Thermogravimetric curves are used for the kinetic studies. Kinetic parameters of decomposition are estimated within the approaches of non-isothermal kinetics (“model-free” kinetics and nonlinear regression methods). We discuss guest molecular kinetic diameters, guest molecule sizes, shape and polarity, the guest phase state (fluid or solid) within the framework pores, the flexibility of the framework structures, the enthalpy and entropy contributions to the kinetic stability and the inclusion compound properties with very similar frameworks (channel walls with similar chemical constitutions), but with different channels length.

Thermal (kinetic) stability of inclusion compounds on the basis of porous metal–organic frameworks

Metal–organic frameworks (MOFs) have promising practical applications in gas storage, in separation and purification of substances, and in catalysis. The standard process of the MOF production begins from the synthesis of the inclusion compound; the molecules of the used organic solvent are caught in the channels and caves of the MOF structure. These primary included guest molecules are excluded further by the evacuation or by the heating. We studied the correlation between the thermal (kinetic) stability of the inclusion compounds and the framework and guest molecules properties. The thermogravimetric curves were used for the kinetic studies; kinetic parameters of decomposition were estimated within the approaches of the non-isothermal kinetics (“model-free” kinetics and nonlinear regression methods). Studied compounds series: [Zn2(bdc)2(dabco)]·4DMF and [Zn4(DMF)(ur)2(ndc)4]·5DMF, [Zn4(DMF)(ur)2(ndc)4]·6benzene and [Zn4(DMF)(ur)2(ndc)4]·5toluene (bdc = bdc2− = terephthalate, dabco = 1,4-diazabicyclo[2.2.2]octane, DMF = dimethylformamide; ur = hexamethylenetetramine, ndc2− = 2,6-naphthalenedicarboxylate); [Mn(HCOO)2]·0.33dioxane and [Li2(H2btc)]·dioxane (H4btc = 1,2,4,5-benzenetetracarboxylic acid). The values of commonly used molecular kinetic diameters do not take into consideration the real molecular form and the size (e.g. benzene and toluene molecules have the same kinetic diameters 5.85 A). Therefore, the ease of removal of guest molecules does not correlate with them directly.

Thermal (kinetic) stability of the inclusion compound on the base of Li-contain MOF [Li2(H2btc)]·dioxane

The inclusion compounds, based on the metal-organic frameworks (MOFs), have promising practical application in gas storage, separation and fine purification of substances, and also in catalysis. These MOFs are crystalline compounds consisting of metal ions coordinated by bridging organic ligands with the formation of porous structures. We study the kinetic stability of the inclusion compound: [Li2(H2btc)]·dioxane (H4btc = 1,2,4,5-benzenetetracarboxylic acid, 1,4–dioxane = C4H8O2). The connection between the kinetic stability of inclusion compounds and the properties of the host matrix and of the guest molecules is considered. So as the centrosymmetric dioxane molecule can easily transform the chair conformation to the bath conformation, it can have the influence on the steric hindrance (as well as on the activation barrier) for the guest molecules removal. Therefore, the entropy contribution is as favorable factor, as the energetic one in the kinetic stability of the supramolecular compounds.

The stability of inclusion compounds under heating

The thermal decomposition of three inclusion compounds: [Zn 2 (camph) 2 dabco]·DMF·H 2 O, [Zn 2 (camph) 2 bipy]·3DMF·H 2 O and [Zn 2 (camph) 2 bpe]·5DMF·H 2 O was studied in the inert atmosphere. TG and DTG curves confirm multi-step decomposition process, the dehydration being the first step. Thermogravimetric data (obtained at different rates of linear heating) were processed with computer program (with 'Model-free' approach). Kinetic parameters of decomposition were calculated for the DMF multi-step removal, the processes are described by Avrami-Erofeev equations. The connection between the kinetic parameters and structural features of the host frameworks (ligand linker lengths and porous-free volumes) are discussed.

Vapour pressure of guest and thermodynamic stability of inclusion compounds [Ni(DBM)2Py2]*2G (DBM = dibenzoylmethanate anion, G = pyridine, tetrahydrofurane and chloroform)

Temperature dependences of the equilibrium vapour pressure of the guest molecules (G) over inclusion compounds [Ni(DBM)2Py2]*2G (DBM = C6H5COCHCOC6H5 −—dibenzoylmethanate anion; Py = pyridine; G = pyridine, tetrahydrofurane and chloroform) were investigated by means of the static membrane procedure. It was established using TGA and DTA methods that both of two guest molecules get detached at the first step of decomposition 1/2[Ni(DBM)2Py2]*2Gs = 1/2[Ni(DBM)2Py2]s + Ggas. The changes of thermodynamic parameters \( (\Updelta {\text{H}}^{0}_{\text{av}} , \Updelta {\text{G}}^{0}_{ 2 9 8} \;{\text{and}}\;\Updelta {\text{S}}^{0}_{\text{av}} ) \) in this process were determined on the basis of the experimental data.

Thermodynamic and kinetic stability of inclusion compounds under heating

Thermodynamic and kinetic stability of inclusion compounds (so called supermolecular compounds) is discussed. Compounds under study and discussion are clathrates (with coordination compounds matrices) and intercalates (with fluorinated graphite matrices).

The stability of inclusion compounds under heating

The thermal decomposition of two inclusion compounds Mn(HCOO)2·1/3C4H8O2 and Mn(HCOO)2·1/3C4H8O was studied in the inert atmosphere. The process of both dioxane, and tetrahydrofurane removal has two steps; the intermediate phase is unstable and kinetically hindered. Manganese formate is stable up to 330°C. Thermogravimetric data (obtained at different rates of linear heating) were processed with computer program (with ‘Model free’ approach). Kinetic parameters were calculated for the first decomposition step, and the process is described by equation of n-order reaction with autocatalysis.

In vitro nitrosation of ephedrine in the presence of cyclodextrins

β-, and γ-cyclodextrin and heptakis-2,6-di-O-methyl-β-cyclodextrin enhance the nitrosation rate of l-ephedrine if the nitrosation assay procedure (NAP test) is applied. During this reaction with β-cyclodextrin a solid inclusion compound of β-cyclodextrin andN-nitrosoephedrine precipitates. Solubilities and stabilities of inclusion compounds of the cyclodextrins with ephedrine and nitrosoephedrine, respectively, explain especially the catalytic effects of some cyclodextrins on ephedrine.

A Unifying Approach to the Thermal Behavior of Inclusion Compounds

A rational approach to the understanding of the thermal stability of inclusion compounds requires an examination of all the involved phases, in particular the solid and the liquid phase. Equations ...