Abstract
Ni-based catalysts as replacement for noble metal catalysts are of particular interest in the catalytic conversion of biomass due to their cheap and satisfactory catalytic activity. The Ni/SiO2 catalyst has been studied for the hydrogenolysis of glycerol, and doping with phosphorus (P) found to improve the catalytic performance significantly because of the formation of Ni2P alloys. However, in the present work we disclose a different catalytic phenomenon for the P-doped Ni/Al2O3 catalyst. We found that doping with P has a significant effect on the state of the active Ni species, and thus improves the selectivity to 1,2-propanediol (1,2-PDO) significantly in the hydrogenolysis of glycerol, although Ni-P alloys were not observed in our catalytic system. The structure and selectivity correlations were determined from the experimental data, combining the results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR) and ammonia temperature-programmed desorption (NH3-TPD). The presence of NiO species, formed from P-doped Ni/Al2O3 catalyst, was shown to benefit the formation of 1,2-PDO. This was supported by the results of the Ni/Al2O3 catalyst containing NiO species with incomplete reduction. Furthermore, the role the NiO species played in the reaction and the potential reaction mechanism over the P-doped Ni/Al2O3 catalyst is discussed. The new findings in the present work open a new vision for Ni catalysis and will benefit researchers in designing Ni-based catalysts.
Highlights
Over the last decade, biodiesel, a potential fuel, has attracted much attention
1,2-PDO and ethylene glycol (EG) were produced as the main products over the Ni-based catalysts accompanied by minor amounts of byproducts of 1-propanol, ethanol, methanol, and acetol in the hydrogenolysis of glycerol; we mainly discuss the factors which affected the selectivity to 1,2-PDO and EG
Based on the above results, we propose that both the acidity and NiO species have an influence on the production of 1,2-PDO; the amount of surface acid sites is not the decisive factor, but the NiO species is the main factor for the enhancement of selectivity to 1,2-PDO and the decrease in the reaction conversion in the hydrogenolysis of glycerol
Summary
Biodiesel, a potential fuel, has attracted much attention. high cost in production makes it difficult for plants to take a step forward. The noble metal catalysts are effective for the hydrogenolysis of glycerol at a relatively low temperature, they do not fit the requirements of industrial application due to their lower selectivity and high cost. Huang et al, reported that the Ni/Al2O3 showed high selectivity to CH4 and CO for vapor phase glycerol hydrogenolysis in a fixed-bed reactor [13]. The effects of promoters like Al, Ba and Zn on the Cu-Cr catalysts in the hydrogenolysis of glycerol was investigated in 2-propanol at 220 °C, in the presence of Ba, the highest selectivity to 1,2-PDO reached 85% at 34% conversion, it was suggested that the increased acid sites and the BaCrO4 phase might be responsible for the improved activity and selectivity [17]. It was confirmed that the doping of phosphorus could induce the formation of NiO species in Ni-P/Al2O3 catalysts, which was found to be responsible for the improved selectivity to 1,2-PDO (91%)
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