Atmospheric Hydrodeoxygenation of Guaiacol over Nickel Phosphide Catalysts: Effect of Phosphorus Composition
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The phosphorous composition of nickel phosphide catalysts in atmospheric guaiacol hydrodeoxygenation (HDO) was investigated. Various initial Ni/P molar ratios, ranging from 0 to ∞, were used to prepare the catalysts. The physicochemical properties of the catalysts were characterized. The catalytic performances were evaluated in a continuous fixed-bed system at 300 °C with different contact times under atmospheric pressure. The phosphorous contents affected both the active phase compositions and catalytic performances of nickel phosphide catalysts. Ni2P, Ni12P5, and Ni3P were identified as the major active phases on the Ni/P = 1, 2, and 3 samples, respectively. Mostly, the P-rich catalyst exhibited lower guaiacol conversions but higher amounts of deoxygenated products (e.g. benzene) and less coke than the P-lean sample did. A guaiacol HDO network containing demethylation (DME), demethoxylation (DMO), direct deoxygenation (DDO), hydrogenation (HYD), and transalkylation (TRA) was proposed. The intrinsic activity decreased following the order as Ni/P = 3 > 1 > 2 at short contact times, and Ni/P = 1 and 2 samples are likely to enhance the rate of hydrogen transfer at long contact times. The pseudo-first-order kinetics analysis also indicated that phosphorous content has a negative effect on guaiacol conversion. The product selectivity at zero guaiacol conversion revealed that the Ni/P = 1 sample promoted guaiacol DMO and phenol DDO. The samples with Ni/P = 2 and 3 enhanced guaiacol DME and TRA. The bifunctional property (HYD on Ni and protonation on PO–H) of Ni2P/SiO2 and the Brønsted acidity of Ni12P5/SiO2 and Ni3P/SiO2 affected their catalytic behaviors. Lifetime testings showed that all catalysts deactivated in long-term operations but with different extents. The Ni/P = 1 sample displayed the highest deactivation rate of guaiacol conversion (~78 %) while the Ni/P = 3 sample had the lowest (~46 %). Coking and phosphide leaching are attributed to cause the deactivation of nickel phosphide catalysts in guaiacol HDO.