Enhanced direct deoxygenation of anisole to benzene on SiO2-supported Ni-Ga alloy and intermetallic compound

Abstract

Herein, Ni/SiO2 and bimetallic NixGa/SiO2 (Ni/Ga atomic ratio x = 6 and 3) catalysts were prepared by the impregnation method followed by reduction at 550 °C and tested in the vapor hydrodeoxygenation of anisole at 0.1 MPa and 300 °C. Ni-Ga alloy and Ni3Ga intermetallic compound (IMC) formed in Ni6Ga/SiO2 and Ni3Ga/SiO2, respectively, where the Ga atoms break contiguous Ni ones reducing the ensembles of Ni atoms and the H2 uptakes. Also, a charge transfer from Ga to Ni increased the electron density of Ni, and hydrogen spill-over occurred on NixGa/SiO2. In contrast to Ni/SiO2, NixGa/SiO2 improved not only the hydrodeoxygenation activity but also the selectivity to benzene. At the similar anisole conversion (˜31%), the selectivity to benzene was 75.2%, 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 CArOCH3 bond cleavage (CAr represents the carbon in benzene ring) to benzene was more preferential on NixGa/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 CArO bond cleavage. Moreover, NixGa/SiO2 were less active for benzene hydrogenation and CC bond hydrogenolysis than Ni/SiO2, contributing to higher selectivity to benzene. Significantly, methanol, derived from the direct the CArOCH3 bond cleavage, dominatingly decomposed to CO and H2 and methanation scarcely occurred on NixGa/SiO2, however, it was mainly converted to methane on Ni/SiO2. Low activities for benzene hydrogenation, CC bond hydrogenolysis and methanation on NixGa/SiO2 (especially Ni3Ga/SiO2) are attributed to the geometric and electronic effects of Ga in alloy and IMC. The finding is significant in rationally designing the catalyst with high benzene yield and low H2 consumption.