Determination of the mass loss through evaporation of supported ionic liquids by a contactless microwave-based method

Titelangaben

Anke, Marie-Luise ; Moos, Ralf ; Jess, Andreas:
Determination of the mass loss through evaporation of supported ionic liquids by a contactless microwave-based method.
2016
Veranstaltung: 49. Jahrestreffen Deutscher Katalytiker , 16. - 18. März 2016 , Weimar.
(Veranstaltungsbeitrag: Kongress/Konferenz/Symposium/Tagung , Poster )

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Abstract

In the last years, the interest in ionic liquids (ILs) caused by their unique and tunable physical and chemical properties, has grown rapidly. For example ILs have a low vapor pressure and a high thermal stability. To utilize efficiently the expensive ILs, they are immobilized on solid catalysts. In this case, the ionic liquids modify both the chemical and physical properties of the catalyst and its selectivity, which is important for consecutive reactions where the intermediate is desired. For some applications, the detection of the pore filling degree or of the mass loss through evaporation and/or thermal decomposition is important. Typically, the stability of ILs is analyzed ex situ by thermogravimetric analysis [1]. In this work, an alternative method is presented where the IL loading is measured in situ with a contactless microwave-based method. This approach was already successfully investigated and applied to detect the coking of catalysts.

The measurement setup, consists of an aluminum resonator with an inner radius of 92 mm and a height of 80 mm. Through two inductive couplers, an electromagnetic field is excited in the cylindrical cavity resonator. In this work a specific resonance mode in analyzed. The electric component of this mode is homogeneous and has a maximum along the central axis of the resonator. For maximal sensitivity, a quartz reactor with an inner diameter of 10 mm is placed there. Owing to its low electrical conductivity, the quartz tube affects the electromagnetic field only negligibly. The material under study is located in the heatable quartz reactor. Shifts of the resonance behavior affected by the electrical properties, such as the effective conductivity of the sample, could be analyzed by detecting the transmission parameters with a network analyzer (Anritsu VNA Master MS2028B).

Here the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethyl¬sulfonyl)¬imide, in short form [EMIM] [NTf2], is used as a well-studied model substance. Its vapor pressure is 0.365 Pa (250°C) and the maximum operation temperature is 260 °C. This IL is immobilized in variable amounts (and hence filling degrees) on Silica 150. For an initial evaporation measurement, the effective conductivity of the supported model IL is analyzed during an evaporation period of 10 hours at a temperature of 250 °C. The pore filling degree at the beginning of the test is 50%. During the whole measurement, 185 l/h (NTP) nitrogen flow over the sample and the transmission parameters are recorded. As a result, the calculated effective conductivity of the supported ionic liquid decreases continuously with time. Because the electrical conductivity of pure [EMIM] [NTf2] is much higher than the conductivity of the porous Silica, a decrease in the signal indicates an IL mass loss, in this case through evaporation. To verify the change in pore filling degree, a thermogravimetric analysis is used. The TGA data correlate well with the microwave-derived data. These initial experiments show that it is possible to detect the IL mass loss through evaporation in situ by using a contactless microwave-based method.