Nickel-based Catalysts Derived From Layered Double Hydroxide And Their Hydrodeoxygenation Performance

Facing the depletion of fossil energy resources associated with increasing energy demand and ubiquitous environmental concerns from fossil fuels, it is imperative to develop sustainable energy from alternative sources. Biomass, a kind of extensive, regenerable and economical resources from agricultural and forest, has garnered much attention. Nonetheless, the high oxygen content(10-50 wt. %) of crude bio-oils derived from lignocellulose biomass via fast pyrolysis, gasification and liquefaction afford poor engine performance, due to their chemical and thermal instability, low heating value, high viscosity and acidity. Fortunately, the high quality of liquid fuel can be obtained by catalytic hydrodeoxygenation, which is considered to be an exceedingly effective route for bio-oils upgrading. Nickel-based catalysts seem to be a promising non-noble metal catalytic system for anisole because of the high HDO degree, sulfur-free and low cost, which have been discussed in depth and further modified from the bimetal and the support. In this study, we launched an original strategy to obtain a series of highly active catalysts, in which the catalysts were prepared by calcination and reduction of layered double hydroxide(LDH) precursors. Compared with the traditional methods for the synthesis of supported metal catalysts, there are three advantages of LDH precursors:(i) uniform distribution of active particles—upon calcination and subsequent reduction could promote the formation of highly dispersed metallic species at the atomic level;(ii) strong metal–support interactions between the oxide and metal particles could inhibit the aggregation of the metal particles during the reaction; and(iii) controlled particle size and specific morphology allows the modulation by the in-situ transformation into mixed metal oxides of the topotactic of LDHs. The process is uncomplicated, inexpensive and possible to scale up for industrial production.The catalytic performance of Ni-based catalysts derived from layered double hydroxide(LDH) precursors with nickel ions incorporated into the brucite-like layers was investigated for the hydrodeoxygenation(HDO) of anisole as a model compound of the lignin. The synthesized LDH precursors and nickel supported catalysts were characterized. The samples derived from the LDH precursors indicated a strong interaction between the metal nanoparticles and the support, which also illustrated that the filling sequence of Ni2+ cations into the octahedral and tetrahedral coordination at high loading under the current preparation condition. Upon calcination and subsequent reduction, the well-crystallized phase and highly dispersed Ni nanoparticles(3~10 nm) on the support were obtained. The catalysts exhibited high activity toward the hydrogenolysis of the C-O bonds and the hydrogenation saturation of aromatic ring at the low temperature(200-280 °C) and suitable hydrogen pressure(2 MPa). And the HDO performance of the high loading of Ni-based catalysts derived from LDH precursors with different Ni/Al molar ratios was investigated. In addition, the catalysts are highly resistant to coking and easy to be reactivated by the utilization of the layered double hydroxide precursors.In order to investigate the effect of the molybdenum content on the HDO performance, NiMoAl catalysts were prepared by the reduction of mixed-oxides obtained by the calcination of NiAl-heptamolybdate LDHs. The latter was obtained by ion-exchange with ammonium heptamolybdate of NiAl-terephthalate LDHs. In combination with the quantitatively analysis of molybdenum content, it was revealed that an enhanced molybdenum content in the catalysts contributes to the HDO conversion of anisole, which may be due to the synergistic effects between MoO3 and Ni controlled by the Mars-van Krevelen molybdenum oxide redox cycle mechanism, or the well dispersed active Ni nanoparticles with small diameter based on “pinning effect” of Mo around the Ni NPs. And the catalysts also exhibit superior stability. The conversion intensively increased with the temperature in a certain region, implying that the major reaction pathway may be changed. Meanwhile, we conducted the pseudofirst-order reaction kinetic analysis to prove our assumption─the molybdenum oxide can decrease the HDO activation energies on the catalysts. And when the temperature is over 280 ℃, the hydrogenation reaction direction of benzen to cyclohexane goes into the reverse.