For technical applications of supported ionic liquids (ILs), the stability of the IL layers both with regard to thermal decomposition and to losses by evaporation is of great importance. An innovative radio frequency-based method is presented to determine the pore filling degree of supported ILs in situ and in a contactless way. As an example, the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, which was supported on the internal surface of porous silica, was selected. The complex permittivity of the porous solid coated with the IL increases linearly with the IL pore filling degree. Therefore, the evaporation rate of the IL in a fixed bed could be measured in situ in the reactor.
The
analysis of the thermal stability of supported ionic liquids (ILs)
is of great interest for their application in catalysis. However,
thermogravimetric (TG) measurements are very time-consuming, destructive,
and cannot be conducted operando. Therefore, a new radio-frequency
(RF)-based method is presented that analyzes the electrical properties
of supported ILs in the microwave range and can detect a possible
IL mass loss caused by evaporation or decomposition. In this study,
the decomposition of supported 1-butyl-3-methylimidazolium dimethylphosphate
([BMIM][DMP]) with and without palladium (as an active metal) is investigated
operando during the selective hydrogenation of 1,3-butadiene. In addition
to volatile decomposition products, solid products are formed, which
remain on the carrier. These solid products impair the activity of
the Pd catalyst. Using the RF-based method, a distinction can be made
between “intact” IL and the solid decomposition products
because the electrical properties of both substances differ substantially.
In contrast, the destructive TG analysis only measures the mass loss
by the formation of gaseous decomposition products and thus cannot
distinguish between “intact” IL and the solid decomposition
products of [BMIM][DMP]. In addition, a model of the thermal decomposition
which depicts the measured mass losses well is presented.
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