(583bp) Steam Catalytic Cracking of Naphtha Over Ce/SBA-15 and Ce/HZSM-5 for Production of Light Olefins

A series of Ce/SBA-15 catalysts with Ce content ranging from 0 to 10% were prepared and compared with Ce/HZSM-5 as conventional cracking catalysts. The structures of the catalysts were characterized using BET, SEM, TEM, XRD, TPR, NH₃-TPD, and pyridine-FTIR. SBA- 15 is by far the largest mesoporous material with highly ordered hexagonally arranged mesopores, thick wall and thus with better thermal and hydrothermal stability. However, pure siliceous SBA-15 does possess low acidity and therefore its application as a cracking catalyst is limited. The main advantages of adding cerium into the amorphous silica network are physicochemical properties such as Lewis and Bronsted acidity. The characterization results suggest that the active components were introduced without changing the mesostructure of SBA-15. By adding 2% cerium the total acidic sites on SBA-15 were nearly doubled (2.28 vs. 1.2 mmol NH₃/g_cat). The ethylene/propylene ratio is greater than one on SBA-15 compare to HZSM-5. The reason is that formation of ethylene by cracking of higher olefins via β-scission, oligomerization and isomerization involves favorable carbenium ion transition state. Although the total acidic sites on Ce/SBA-15 were lower than Ce/HZSM-5, the total yield of olefin was comparable. Higher surface area and larger pore volume of SBA-15 resulted in better dispersion of acidic sites, which consequently yielded to better accessibility of reactants for cracking reactions. In addition, the mass transfer imitation on SBA-15 is minimal.