The post-reaction material was analyzed by optical methods. For the PFA reactor, some changes were already visible from the outside due to the excellent transparency of this material. The opened reactors showed some obvious changes at the inner tube wall being a dark powder and some embossments with the size of the catalyst particles (see Figure ). These observations were only made for the zone in which the catalyst and the inert glass beads were mounted. The microscopic images (Figure ) were recorded in different light modes to highlight different details.