The temperature gradient through the reactor wall is evaluated by recording the outer wall temperature and the temperature in the inner tube center at the height of the catalyst bed. The thermocouple at the reactor wall has the same diameter as the kerf it is placed in, therefore it is in ideal contact with the outer reactor wall and the aluminum jacket. Visual confirmation of the correct positioning is assured due to the transparency of the reactor wall, where the catalyst bed is visible, and by measuring the insertion length of the thermocouple in the inner capillary. Since the inner tube is made of stainless steel with a thin wall, heat transfer limitations are negligible and the temperature measured in the inner tube is assumed identical to the mean temperature of the catalyst bed.