Deoxygenation of palm oil to bio-hydrogenated diesel over metal and metal sulfide catalysts in a trickle-bed reactor

Abstract

The deoxygenation of palm oil to bio-hydrogenated diesel (BHD) or so-called green diesel over the γ-Al2O3-supported metal and metal sulfide catalysts prepared by impregnation method was conducted in a trickle-bed reactor. The studies were divided into 4 parts. Firstly, the effect of reaction parameters (temperature, pressure, LHSV, and H2/oil ratio) on the deoxygenation of palm oil over NiMoS2 was investigated to optimize the operating conditions. The results demonstrated that hydrodeoxygenation (HDO), decarbonylation (DCO), and decarboxylation (DCO2) reactions actively and competitively occurred at each condition, and had different optimal and limiting conditions. Secondly, the roles of monometallic catalysts were studied. Metallic sites of the catalysts were found to be formed after pre-reduction in H2 with differences in metal particle size and metal dispersion. These properties played important roles in the palm oil deoxygenation, resulting in the activity turnover frequency (TOF) with the order of Co > Pd > Pt > Ni. Oleic acid was used as a model compound to get the basic information on the reaction pathway. Consequently, a reaction network for the deoxygenation of palm oil was developed and discussed. Thirdly, the comparisons of the metal (Ni, Co, Mo, NiMo, and CoMo) and metal sulfide (NiSx, CoSx, MoS2, NiMoS2, and CoMoS2) catalysts on activity, selectivity, and stability were studied. The DCO reaction was dominant over metallic Ni catalyst, whereas, the HDO was dominant when the reaction was catalyzed by NiMoS2 and CoMoS2 catalysts. Interestingly, the contribution of DCO was nearly comparable to that of the HDO over metallic Co catalyst. The catalytic stability of the metal sulfides was superior to that of the metal catalysts with the order of NiMoS2 > CoMoS2 > Ni > Co catalysts. Finally, the deactivation and regeneration behaviors of the metallic Ni and Co catalysts were examined. The catalysts exhibited the stable performance for 100 h on-stream. Nevertheless, the product yield over metallic Ni catalyst gradually decreased, whereas, the dramatic decline in product yield could be noticed over metallic Co catalyst after 150 h on-stream. The carbon deposition was found to be the main cause of the catalyst deactivation, while the metal sintering was a minor reason. The catalyst regeneration by thermal treatment in air, followed by H2 reduction could completely restore the catalyst activity.