Nevertheless, the mesoporous structure of Siralox 30 with its larger pores is advantageous for the condensation as it is suitable for larger products without deactivation. For the production of the ASA of the Siralox family, silica is added to high-purity aluminas during synthesis. Depending on the amount of silica, the silica-to-alumina ratio (SAR) is varied and thus material properties such as acidity (strength and density) and thermostability can be tuned. To assess the influence of the SAR on the self-condensation of acetone, Siralox materials from 1-70 % SiO2 were studied under the optimum reaction conditions of 260 °C and 12.5 gcat h mol -1 and presented based on the steady-state conversion over 10 hours-on-stream in Figure . With silicaloading, the mesitylene yield (from 2.8 to 10.5 %) and acetone conversion (from 19 to 42 %) increase to a maximum at 30 wt% SiO2. For a further increase in silica-loadings, the conversion and yields decrease and are virtually zero for pure silica. Especially for the initial doping with 1 wt% silica, a strong increase in activity is observed for both mesitylene yield (7.5 %) and acetone conversion (28 %). The isophorone yield decreases with silica-content. Since bases promote the condensation to isophorone, the amphoteric nature of alumina exhibiting basic sites is relevant at low silica loadings. The yields of mesityl oxide and oligomers increase up to 10 wt% silica, showing a maximum of 4.8 and 3 %, respectively. With higher silica content, both yields decrease but remain constant up to 70 wt% SiO2. Overall, an increase in mesitylene yield is accompanied by an increase in acetone conversion. This confirms that the mesitylene increase mainly stems from the consecutive reaction of mesityl oxide with acetone and subsequent aromatization. Based on the findings, a silica-content of 20-40 wt% gives the highest mesitylene yields with an optimum at 30 wt%. The trends observed in the flow reactor are supported by batch experiments at 230 °C for 3 h (see experimental details in SI and Figure ). At these longer catalyst contact times, the maximum mesitylene yield at 30 wt% is more pronounced. The low activities of pure γ-Al2O3 and pure SiO2 found under flow and batch conditions highlight the effect of the silica-doping on the catalytic activity as additional active sites are created. Indeed, very large silica-loadings of 40 wt% and higher lead to a primarily silica surface with generally fewer surface acid sites. 31 Under the optimized reaction conditions, all tested Siralox materials were stable at steady-state conditions for more than 10 hours-on-stream (Figure ). The silica-content therefore does not play a significant role for the stability of the catalysts under reaction conditions. The TGA measurements of the spent ASA from the flow reactor show that the mass loss is about 7 wt% for silica-loadings of 10-40 wt% (Figure ) after reaction at 260 °C. For very high and silicaloadings, the mass loss is lower than 7 wt% due to decreased catalyst activity. For flow conditions, ~0.5 wt% less carbonaceous deposits are found compared to batch reactions which shows the efficiency of the continuous process with constant product removal from the catalyst bed. The structural stability of the Siralox materials was assessed by XRD of the spent catalysts. The diffractograms (Figure ) show that for the majority of materials, no significant change is visible. For Siralox 20 and 40, a small reflex at 49.2° can be observed, hinting to minor formation of the γ-AlO(OH) boehmite likely due to water formed in the aldol condensation. Surprisingly, Siralox 30 does not show the formation of boehmite which supports its suitability for scale-up in a continuous flow process.