Synthesis of mesoporous TiO2-Al2O3 composites supported NiW hydrotreating catalysts and their superior catalytic performance for heavy oil hydrodenitrogenation

Abstract

Mesoporous TiO2-Al2O3 composites with different Ti/Al molar ratios were successfully synthesized via pre-hydrolysis co-precipitation method and the corresponding NiW supported hydrotreating (HDT) catalysts were prepared via incipient wetness co-impregnation method. The effects of Ti modification on the physicochemical properties of the prepared supports and catalysts were determined by the characterization techniques such as XRD, FTIR, N2 physical adsorption–desorption, SEM-EDS, ICP-OES, NH3-TPD, H2-TPR, HRTEM and XPS. Finally, quinoline, indole and coker gas oil (CGO) were used respectively as probes to investigate the Ti modification on the hydrodenitrogenation (HDN) performances of the corresponding catalysts. The results show that the TiO2-Al2O3 samples synthesized by the pre-hydrolysis co-precipitation method achieved the level of molecular composition, and the crystalline structures, pore properties and acidity properties of the TiO2-Al2O3 supports varied with the Ti/Al molar ratios. Ti modification can modulate the interaction between the active metals and the supports (MSI), and promote the formation of more so-called “type II” NiWS active phases. Furthermore, the existence of Ti3+ species in catalysts NiW/TA1 and NiW/TA2 enhanced the sulfidation degrees of active metals and weakened the bond energy between the active metals and sulfur atoms and further promoted the formation of more coordinatively unsaturated sites (CUS). The catalytic assessment results show that the HDN activities of the catalysts were effectively improved after Ti modification and the improvement was largely due to the promotion of the cleavage of the C(sp3)-N bond of basic nitrides and the C(sp2)-N bond of non-basic nitrides. The NiW/TA1 catalyst exhibited the highest performance in the HDT process of heavy oil, with a 15.8% and 12.4% improvement in HDN rate and hydrodesulfurization (HDS) rate compared with the unmodified catalyst, respectively, which was mainly attributed to its suitable pore properties, moderate MSI, large number of CUS, moderate stacking and highest active metal sulfidation degree of the active phase.