The effect of accessibility to acid sites in Y zeolites on ring opening reaction in light cycle oil hydrocracking

The light cycle oil (LCO) hydrocracking process converts polycyclic aromatics into highly valuable light aromatics such as benzene, toluene, xylene (BTX), in accordance with the requirements of low-carbon development and high-quality transformation from oil refining to chemical industry. The accessibility of acid sites is a critical factor that impacts LCO conversion and BTX yield. Initially, the fine structure and molecular size of the typical polycyclic aromatics in LCO and their hydrogenation reaction intermediates were investigated through gas chromatography-mass spectrometry (GC-MS) analysis and density functional theory (DFT) calculations. Three porous Y zeolites with comparable Si/Al molar atomic ratios and pyridine (Py)-measured Brønsted acid amounts were chosen as an acidic component to prepare NiMo/(Al2O3 + HY) catalysts. The acid accessibility of HY zeolite was characterized via dual-beam infrared spectroscopy using 2,4,6-tri-tert-butylpyridine (2,4,6-TTBPy) and trihexylamine (THA) as probe molecules, and the LCO hydrocracking performance was evaluated on a fixed-bed reactor. The results revealed that bicyclic aromatic hydrocarbons featuring multiple, short side chains such as dimethylnaphthalene and trimethylnaphthalene are the main components of LCO, with sizes larger than those of HY zeolite micropores. There is a strong positive correlation between LCO conversion and ring-opening rate of polycyclic aromatics and cycloalkanes in LCO. Among these three HY zeolite catalysts, the ring-opening rates of polycyclic aromatics and cycloalkanes, and the yields of C6-C12 aromatics and BTX increased in the order of HY1 < HY2 < HY3, which is consistent with external surface acidity measured by THA as the probe molecule.