Separation of complex mixtures of potential LOHC’s
Rabya Aslam, Karsten Müller, Wolfgang Arlt
Institute of Separation Science & Technology, Friedrich-Alexander-Universität Erlangen-Nuremberg
Liquid organic hydrogen carriers (LOHC) are potential compounds that can facilitate chemical energy storage of hydrogen, a pre-requisite for the storage of renewable energy, through reversible hydrogenation in a reasonable quantity. Among others, dibenzyltoulene (Marlotherm SH®, H0-MSH) is a prominent LOHC which can store up to 6.2 mass % of hydrogen in the form of perhydro-dibenzyltoluene (H18-MSH). High thermal stability even at harsh conditions (> 300°C) and reasonable hydrogen storage capacity also make it a favorable choice as a possible LOHC. For the application of MSH as LOHC, it is necessary to gather its thermo-physical data both in its lean (fully dehydrogenated) and rich forms (fully hydrogenated). However, MSH is not a pure compound but a mixture of various isomers of dibenzyltoluene. In the reversible cycle of hydrogenation and dehydrogenation reactions, it furthermore forms various stable intermediates with a different degree of hydrogenation. More than 24 stable isomers and intermediate compounds have been observed in partially hydrogenated mixtures formed in the reactions. These intermediates need to be separated into pure compounds and fractions (H0-MSH, H6-MSH, H12-MSH, and H18-MSH) not only for the measurement of thermo-physical data and toxicological analysis but also to study the detailed kinetics of hydrogenation and dehydrogenation reactions.
In this contribution, a process comprising distillation and chromatography, for the separation of a partially hydrogenated reaction mixture into fractions of isomers with the same degree of hydrogenation with more than 98% purity is presented. A pre-separation is done by a high vacuum distillation column with Sulzer packing. The developed HPLC method is successfully scaled up to a semi-preparative HPLC column (250 mm x 50 mm I.D.). The optimal method was developed on an analytical scale HPLC using phenylhexyl Silica as stationary phase and acetone/water mixture as eluent. Further purification of the separated fractions need different schemes. Fractions obtained from the semi preparative HPLC experiments have been used to measure various thermo-physical properties of the isomers like viscosity, density, refractive index, vapor pressure, heat capacity, and heat of vaporization.