Surface Properties of Fluorine-Functionalized Metal–Organic Frameworks Based on Inverse Gas Chromatography

Abstract

The introduction of the concept of surface properties can help us to better analyze the basic physicochemical property changes of metal−organic framework (MOF) materials before and after fluorine functional group treatment. In this study, several polar and nonpolar probes were selected to determine the surface properties, including surface-dispersive free energy, Lewis acid−base constants of Ni-MOF-74, and perfluoro carboxylic acid-modified Ni-MOF-74-Fn (n = 3, 5, and 7) in the range of 343.15−383.15 K by inverse gas chromatography (IGC). It was observed that the surface energy of the treated Ni-MOF-74-Fn showed a substantial decrease with the growth of the perfluorocarbon alkyl chains and the increase in surface roughness. In addition, Lewis acidic sites exposed by the Ni-MOF-74 material after adopting modification with fluorine functional groups increased with the increase of perfluorinated carboxylic acid chains, and their surface properties changed from amphiphilic acidic to strongly acidic. These results not only enrich the basic physical property data of Ni-MOF-74 but also provide more theoretical basis for the fluorinated functionalized custom-designed MOFs and enrich their applications in the fields of multiphase catalysis, gas adsorption, and chromatographic separation.

Some Irregularities in the Evaluation of Surface Parameters of Solid Materials by Inverse Gas Chromatography

“…The results obtained by these authors − ,,,,− cannot be considered as confident results, because of the following reasons:…”

“…Many authors − ,,,,− recently studied the London dispersive energy and the Lewis’s acid–base properties of some solid surfaces by using IGC technique and the classical methods of Dorris–Gray and Schultz et al that were proved to give wrong results. Furthermore, in their study they used constant values of the surface areas of organic solvents and also supposed as constant the dispersive component of the surface tension of the various organic molecules. − ,− …”

“…Table gives the different equations γ l d ( T ) of the different nonpolar molecules . It was observed that many authors − considered γ l d ( T ) as constant for the various n -alkanes.…”

“…(2) Many scientists − ,− ,− supposed a –CH2– = 0.06 nm 2 . One proved in a previous study that a –CH2– ( T ) is function of the temperature.…”

“…(3) The surface areas of n -alkanes used in literature − ,− to determine the dispersive component of the surface energy of solid surfaces were that of Kiselev (Table ). However, in previous papers, − several ways were proposed to calculate the surface area of organic molecules by using different methods and models.…”