Study On Properties And Heavy Oil Catalytic Cracking Activity Of Ti Modified USY Zeolite

As an efficient catalytic conversion technology of heavy oil and the only way toproduce the clean oil, which can meet the Euro IV standards, the hydrocracking ofheavy oil receives increasingly attentions. Ti modified USY zeolite exhibits higheryield of middle oil and it is favorable to enhance catalyst service life in the heavy oilcracking process. The reaserch on the influence of modification on acidity of carrieras well as the catalytic activity of zeolite in cracking of heavy oil could provide animportant theoretical support for the deisgn and development of new, efficient andlow-cost catalyst. In this paper, molecular simulation combined with experimentalmethods are employed to study the structural characteristics, acidity and the catalyticcracking activity of Ti modified USY zeolite.The structural characteristics and electronic properties of Ti modified USY zeoliteare investigated by molecular simulation method. The calculation results on Ti atomsubstituted in framework atom of USY zeolite show that the substitution energy of Tiatom to replace Al atom is13.9kcal/mol, whereas the minimum substitution energy ofTi atom to replace Si atom is41.3kcal/mol, which indicates that Ti atom is easier toreplace the Al atom in framework of USY zeolite. The results of TiO2extra-framework modified USY zeolite show that there exist two kinds of interactionways between TiO2and zeolite, which named as ring-form configuration (L) andbridge-form configuration (J) respectively. After the extra-modification of TiO2, the Hproton provided the activity of cracking is transferred from the pore surface of USYzeolite to TiO2cluster.The DFT method is employed to analyze the influence of different modificationmethods on acidity of USY zeolite. The calculations on the charges of H proton, thedeprotonation energy of USY zeolite and the bond order of H-O bond show that thedeprotonation energy is changed little after the substitution of Al and Si atoms by Tiatoms in framework of USY zeolite, which suggests that there is little influence on theacidity of USY zeolite after the substitution of the framework atoms. The Ti-OHacidic site is formed after the extra-framework modification of TiO2in the USYzeoilte, thus the acidity distribution is changed. By comparing with the Al-OHstructure in unmodified USY zeolite, the changes of H proton decrease, and the deprotonation energy of USY zeolite and the bond order of H-O bond in Ti-OHincrease, which indicates a weaker B acidic strength than Al-OH. Theextra-framework modification of TiO2significantly alters the intensity distribution ofthe USY zeolite acid centers, which might be more suitable for catalytic cracking ofheavy oil.The performance of TiO2modified USY zeolite in cracking of heavy oil isinvestigated with the DFT method. The results show that the modification of Tireduces the HOMO-LUMO gap value, which indicates a reduction of the difficultiesfor electronic transfer between zeolite and heavy oil model compound. By thecalculation of adsorption geometry and adsorption energies, it is found that Timodification enhances the adsorption of reactant and reduces the bond order of C-Cbond in naphthenic ring, which makes it easier to break up. The activation energy ofinitial cracking reaction of heavy oil model compound is decreased from74.8kcal/molbefore the modification to49.3kcal/mol after the modification.The Ti modified USY zeolite is prepared by impregnation methods.Temperature-programmed desorption, infrared spectrometric and transmissionelectron microscopy methods are adopted to investigate the structure and acidity of Timodified USY zeolite. The characteristic results of infrared spectroscopy andtransmission electron microscopy show that there exist TiO2nano-clusters in theextra-framwork of USY zeolite, and there is an interaction between Ti atom and Oatom in the framework of zeolite. The results of Pyridine-IR and Pyridine-TPD showthat the distribution of acidic strength is changed because of the TiO2modification.The catalytic cracking activity of Ti modified USY zeolite in the model compound ofheavy oil is also discussed in a fixed bed reactor. The results show that the conversionof tetralin is increased from42.3%on USY zeolite to71.1%on the Ti modified USYzeolite after one hour reactions, i.e. the modification of Ti improves the catalyticcracking activity of USY zeolite.