Catalytic cracking of coal-tar model compounds over ZrO_2/Al_2O_3 and Ni-Ce/ Al_2O_3 catalysts under steam atmosphere

摘要

A series of mesoporous ZrO2/Al2O3 and Ni-Ce/Al2O3 catalysts were prepared using mechanochemical method and characterized by XRD, N-2 adsorption, NH3-TPD, O-2-TPD and XPS. Catalytic cracking of toluene/pyrene, a model compound of coal tar, was investigated under steam atmosphere in a fixed-bed reactor where ZrO2/Al2O3 and Ni-Ce/Al2O3 catalysts were sequentially layered. The results show that ZrAlO solid solution with a new pore framework is formed in prepared ZrO2/Al2O3 catalysts, resulting in higher BET surface area, larger pore volume and more adsorption sites than pure ZrO2 and Al2O3. Meanwhile, the formation of ZrAlO inhibits the transformation of tetragonal ZrO2 to monoclinic ZrO2 and obviously generates strong acid sites. The use of ZrO2/ Al2O3 catalyst increases cracking efficiency of pyrene due to promoting active hydrogen formation from H2O and converting pyrene to monocyclic, dicyclic and tricyclic aromatics, which can be further decomposed by Ni-Ce/Al2O3 catalysts to form naphthalene. ZrO2/Al2O3 with a certain amount of Zr favors the cracking of pyrene and inhibits total yield of carbon deposition. When Zr content exceeds 50 wt%, a deep cracking of pyrene occurs, resulting in serious carbon deposition and decrease in liquid and naphthalene yields. The cracking of toluene and pyrene contribute to the formation of naphthalene, whereof, the latter is the primary way. Different distribution of liquid products for cracking of either toluene or toluene/pyrene was observed over ZrO2/Al2O3 and Ni-Ce/Al2O3, respectively. It was also demonstrates that the carbon formed on the spent ZrO2/Al2O3 catalysts is amorphous, whereas that deposited on the spent Ni-Ce/Al2O3 is carbon filaments.