Study On Performance Of SiO₂-supported Ni-Ga Bimetallic Catalysts For Hydrodeoxygenation Of Anisole And Methanol Steam Reforming

It is a sustainable approach to yielding fuels and chemicals from bio-oils.Therein,benzene series compounds can be produced from lignin-based bio-oil via hydrodeoxygenation.Low-cost metallic Ni catalysts are very active for the hydrodeoxygenation,however,they also have high activities for benzene ring hydrogenation,C-C bond hydrogenolysis and methanation,which increase the hydrogen consumption and reduce the yield of benzene series compounds.To overcome these shortcomings,here,geometric and electronic properties of Ni were modified by Ga to improve the catalyst reactivity.Also,the performance of Ni₃Ga intermetallic compound for methanol steam reforming to hydrogen was investigated.Ni/SiO₂ and Ni_xGa/SiO₂（Ni/Ga atomic ratio x=6 and 3）were prepared by the incipient impregnation method,and Ni-SiO₂and Ni₃Ga-SiO₂ were prepared by the sol-gel method.Catalysts structure and HDO mechanism were studied by H₂-TPR,XRD,H₂chemisorption,H₂-TPD,NH₃-TPD,XPS,N₂ physical adsorption-desorption,D₂ isotope tracing,anisole-TPD and anisole-TPSR.Their performances for the anisole HDO and methanol steam reforming were evaluated on a quartz fixed-bed reactor.The relationship between catalyst structure and performance is discussed.It was found that Ni-Ga alloy and Ni₃Ga intermetallic compound（IMC）formed in Ni₆Ga/SiO₂ and Ni₃Ga/SiO₂ after reduction at 550 ℃,respectively,where the Ga atoms break contiguous Ni ones reducing the ensembles of Ni atoms and the H₂uptakes.Also,a charge transfer from Ga to Ni increased the electron density of Ni,and hydrogen spill-over occurred on Ni_xGa/SiO₂.In contrast to Ni/SiO₂,Ni_xGa/SiO₂improved not only the hydrodeoxygenation activity but also the selectivity to benzene.At the similar anisole conversion（～31%）,the selectivity to benzene was75.2%,83.0% and 92.6% on Ni/SiO₂,Ni₆Ga/SiO₂ and Ni₃Ga/SiO₂,respectively.Reactivity evaluation,anisole-TPD and TPSR results show that the direct C_Ar-OCH₃bond cleavage(C_Ar represents the carbon in benzene ring)to benzene was more preferential on Ni_xGa/SiO₂ than on Ni/SiO₂.Isotope tracing experiment indicates that the spilt-over hydrogen at the interface between the Ni₃Ga particles and support participated in the reaction.We suggest that the synergetic effect between Ni and Ga facilitated the direct C_Ar-O bond cleavage.Moreover,Ni_xGa/SiO₂were less active for benzene hydrogenation and C-C bond hydrogenolysis than Ni/SiO₂,contributing to higher selectivity to benzene.Significantly,methanol,derived from the direct C_Ar-OCH₃ bond cleavage,dominatingly decomposed to CO and H₂ and methanation scarcely occurred on Ni_xGa/SiO₂,however,it was mainly converted to methane on Ni/SiO₂.Low activities for benzene hydrogenation,C-C bond hydrogenolysis and methanation on Ni_xGa/SiO₂（especially Ni₃Ga/SiO₂）are attributed to the geometric and electronic effects of Ga in alloy and IMC.Giving that Ni₃Ga intermetallic compound has extremely low methanation activity,it was tested for the methanol steam reforming.The Ni₃Ga intermetallic catalyst showed lower activity than the metal Ni one,while its lower methanation activity increased the H₂ selectivity.Compared with Ni₃Ga/SiO₂,Ni₃Ga-SiO₂ had higher activity,which is related to its higher Ni₃Ga dispersion.However,the Ni₃ Ga-based catalysts mainly catalyzed the methanol decomposition to CO and H₂ and were very low active for water gas shift reaction.Although the H₂ selectivity can be increased by increasing the water-to-methanol molar ratio and reducing the methanol space velocity,the CO selectivity was still higher than 80%.Thus,the water gas shift activity of the Ni₃ Ga-based catalysts is required to further improve.