关键词: 分子筛, 晶粒尺寸, 催化剂, 加氢, 中油选择性

Abstract:

Y molecular sieves of high n(SiO₂)/n(Al₂O₃) ratio were prepared by the seeding method, and then different crystal size(515mn、317nm、220nm) Y molecular sieves were synthesized by changing the aging temperature of the seeding gel. The prepared Y molecular sieves with different crystal sizes and amorphous silica-alumina were mixed as supports, and the loaded NiW/(Y+ASA) hydrocracking catalysts were prepared by the incipient impregnation method. The physicochemical properties of the Y molecular sieves with different crystal sizes and corresponding catalysts were analyzed by scanning electron morphology(SEM), X-ray diffraction(XRD), N₂ sorption-desorption, and NH₃ temperature-programmed desorption(NH₃-TPD). The active metals morphology, dispersion, and sulfation of the catalysts were characterized by H₂ temperature-programmed reduction (H₂-TPR), transmission electron microscopy(TEM), X-ray photoelectron spectroscopy(XPS), and other characterization methods. The results showed that the external specific surface area and pore size increased with the decrease of Y molecular sieves grain size. Meanwhile, as the n(SiO₂)/n(Al₂O₃) ratio increases, the weak and medium acid acidity decreases, and the number of acid decreases. The results of hydrocracking of n-hexadecane showed that the reduction of Y molecular sieve crystal size was beneficial to improve the yield and selectivity of the middle distillate products (C₈—C₁₂). Since the small crystal size of Y molecular sieve can reduce the residence time of reactant molecules in the pore channel and improve the accessibility to the active phase on the catalyst surface, over cracking can be avoided and higher yields of middle distillates can be obtained. Therefore, the highest yield of C₈—C₁₂ products was obtained for the Y zeolite catalyst NiW/(Y₃+ASA) with a crystal size of 220nm.

Key words: molecular sieves, crystal size, catalyst, hydrogenation, middle distillate selectivity