Viscosity and Surface Tension of Fluorene and Perhydrofluorene Close to 0.1 MPa up to 573 K

In the present study, the liquid viscosity and surface tension of fluorene (H0-F) and its fully hydrogenated counterpart perhydrofluorene (H12-F), representing process-relevant byproducts of the liquid organic hydrogen carrier (LOHC) system based on diphenylmethane and dicyclohexylmethane and potentially interesting LOHC compounds by themselves, were determined close to 0.1 MPa using different experimental methods. Besides surface light scattering (SLS) allowing for a simultaneous access to both properties up to 573 K, conventional methods in the form of capillary viscometry and pendant-drop tensiometry were applied up to (473 and 523) K, respectively. Furthermore, the liquid density of H12-F was measured by vibrating-tube densimetry from (283 to 473) K. While agreement of the viscosity and surface tension results obtained by SLS and the conventional methods is found in the case of H12-F, this is not given for H0-F, especially with respect to its surface tension, which seems to be caused by SLS-specific effects. H0-F exhibiting a melting point of about 384 K shows larger values for density, surface tension, and viscosity compared to H12-F being liquid at 283 K. The latter compound features stereoisomerism which appears to have a pronounced effect on the thermophysical properties. This could be deduced by comparison with the very limited amount of experimental data for the fluorene-based substances that are available in the literature so far. The extension of the thermophysical property database for H0-F and H12-F under process-relevant conditions can be useful for future studies, particularly in the field of chemical hydrogen storage.