Solid acids such as sulfonated mesoporous SiO2 can be used for the quinaldine synthesis and can be used either as suspension or immobilized. Due to the laborious synthesis of these materials and the thus accessible small amounts of the solid acid, the immobilization approach used for TiO2, i.e. the fabrication of filaments containing the solid acid, is unsuited. Consequently, an alternative immobilization strategy was developed. The 3D-printed cylinders made of PP were immersed in a boiling suspension of the solid acid in xylene (referred to as "dip coating" method). Microscopy images of the cylinders obtained by this procedure are shown in Figure . The color of the solid changed from white/yellowish to brown, indicating changes to the material. Furthermore, it is obvious that only low amounts of the solid acid were deposited on the columns. The dip coating procedure was optimized by varying the number of coating steps and the time the cylinders were solvated in boiling xylene.