Influence of calcination temperature on Brønsted acidity and activity of amorphous silica-alumina for n-hexadecane hydroisomerization

A series of amorphous silica-aluminas (ASAs) were prepared by altering the initial and final calcination temperatures. Multi-characterization techniques were utilized to investigate the influence of these temperatures on the textural characteristics, coordination state, and acidity properties. The BET external surface area and pore volume showed a slight initial increase with increasing final calcination temperatures, but decreased significantly beyond 700 °C due to partial collapse and blockage of mesopores. Appropriately high calcination temperatures favored the production of metastable Al species, while ultra-high temperatures could disrupt the ASAs network, causing Al atoms to detach from their original structure. The resulting “isolated” Al IV and Al V species were inefficient in forming Brønsted acid sites (BAS), and the Al V species in them are relatively unstable, tending to convert into Al VI -enriched alumina. In contrast, the network-bound Al V species were more responsible for BAS generation, providing deeper insights into the role of Al V in BAS formation. Among all cases, SA-IC400 calcined at 700 °C with an initial calcination temperature of 400 °C, exhibited optimal physicochemical properties. The corresponding Pt/SA-IC400 catalyst demonstrated superior activity in n -hexadecane hydroisomerization, outperforming that of HBeta-based catalyst and achieving a maximum multi-branched isomer yield of 49.4%. These results are attributed to the amorphous topology and enhanced acidic properties, which play a significant role in mitigating spatial hindrance around acid sites and lowering reaction temperature. This controlled calcined treatment provides a facile method to synthesize ASAs with enhanced acidity and textural properties, providing a valuable reference for tailoring efficient hydroisomerization catalysts.