The coated cylinders could be installed below the photocatalytically-active insert. Thus, for a single pass of the reaction solution the photocatalytic step would occur before the acid-catalyzed step. An overview of the amount of solid acid coated onto the cylinders for different procedures is given in Table . A simple weighting of the cylinders before and after coating did not yield reasonable results due to the weight loss by dissolving PP in the boiling xylene. Thus, to obtain correct solid acid loading, the total mass of the utilized solid acid was compared before and after the experiment, revealing a total mass loss of 139.4 mg. This mass was then divided by the total number of cylinders that were coated, which was 26. By assuming a homogeneous coating of all cylinders, a total amount of 5.4 mg solid acid per cylinder was calculated. The nitrobenzene reduction was conducted as feasibility study with differently coated cylinders. The results are summarized in Figure and Table . For measurements without adding any acid, with trifluoromethanesulfonate (triflate) as homogenous acid catalyst and a suspension of the sulfonated mesoporous SiO2, initial reaction rates of the nitrobenzene reduction of 1.39 nmol s -1 , 2.42 nmol s -1 and 1.25 nmol s -1 were registered. For all experiments with immobilized solid acid, a reaction rate larger than for the reference measurement with a suspension of the solid acids was found. Conversions after 3 h of operation were similar for all experiments except for the use of triflate as catalyst, for which an about 20 % larger conversion and also a significant faster initial reaction rate was observed. Taking the experiment with triflate and without any acid as upper and lower reference and considering an experimental error of 0.2 nmol s -1 for the initial reaction rates, the activity of the solid acid coated cylinders could be clearly demonstrated. The results also give some evidence on the effect of the applied solid acid amount, even though it is not statistically significant. A more frequent coating of the acid catalyst or the use of 8 instead of 6 cylinders, resulting in larger amounts of coated solid acid on the cylinders, increases the initial reaction rate.