Effects of Types of Metal Oxides on Hydrothermal Gasification of Phenol over Novel Metal Oxide–carbon Composite Supported Ni Catalysts Prepared by Sol-gel Method

Abstract

Novel carbon and oxide-supported Ni (Ni/C/TiO₂, SiO₂ or ZrO₂) catalysts were prepared by the sol-gel method and investigated for hydrothermal gasification of aqueous solution of phenol (phenol water) as a model of wastewater, and compared with carbon and Al₂O₃ supported Ni catalysts. Catalysts were prepared by the combination of titanium tetraisopropoxide (TIP), tetraethylorthosilicate (TEOS) or zirconium tetrabutoxide (ZB) and polyethylene glycol (PEG) as an organic template, with the addition of active metal, nickel nitrate hexahydrate, at the preparation of catalysts. After calcination under a nitrogen atmosphere, larger amounts of active metal species were deposited on the carbon skeleton in the catalysts. The introduction of PEG dispersed metal Ni with high loading on carbon derived from PEG. Hydrothermal gasification was performed under the following conditions: 350 °C, pressure 20 MPa, phenol water 2-20 g/L, catalyst 5 mL, LHSV 48 h⁻¹. 16N11C73Z catalyst, which represents 16 wt% Ni, 11 wt% carbon in PEG, and 73 wt% ZrO₂ at the preparation of the catalyst, showed the highest activity among the 16N11C73Oxide catalysts (Oxide: TiO₂, SiO₂, ZrO₂ or Al₂O₃). The conversion of phenol decreased in the order 16N11C73Z>16N11C73A>16N11C73T>16N11C73S (16N: 16 wt% Ni; 11C: 11 wt% carbon in PEG; 73Z, A, T or S: 73 wt% ZrO₂, Al₂O₃, TiO₂ or SiO₂). As the amount of PEG added was increased, TiO₂, ZrO₂ and Al₂O₃ containing catalysts showed 100 % of conversion whereas the yields of carbon and methane decreased in the order 16N63C21A>16N53C31Z>16N53C31T (16N: 16 wt% Ni; 53 or 63C: 53 or 63 wt% carbon in PEG; 21 or 31A, Z or T: 21 or 31 wt% Al₂O₃, ZrO₂ or TiO₂). 16N53C31Z and 16N53C31T did not show change in pore structure after the reaction, although the sizes of Ni species increased.