Preparation And Application Of Novel Ni Based Catalysts In Hydrogenation Of Phenol

Bio-oil, obtained by fast pyrolysis of biomass, is more and more widely concerned nowadays as a promising alternative to liquid fossil fuels. However bio-oil has some disadvantages, such as high oxygen and water content, and poor stability, so it is necessary to upgrade bio-oil by hydrogenation. In order to prevent the loss and sintering of active component of supported non-noble metal catalysts for bio-oil upgrading, we prepared mesoporous Ni/SiO2(denoted as Ni/SBA-15-CO) composite catalysts with Ni incorporated into mesoporous silica walls, and investigated its stability in hydrogenation of phenol. In addition, the crystalline nickel catalysts are suffering from low catalytic activity under low temperature in the oil/water biphasic system of bio-oil. Here we prepared a supported amorphous NiB/SiO2-Al2O3 catalyst and studied its hydrogenation performance for phenol in oil/water biphasic system. The results were concluded as follows:(1) Ni/SBA-15-CO was prepared by Evaporation-Induced Self Assembly(EISA) method, using nonionic surfactant P123 as structure directing agent, tetraethyl orthosilicate and nickel nitrate as silica and nickel precursors. Ni/SBA-15-CO shows an ordered mesostructure and Ni nanoparticles distribute uniformly in the skeleton of silica supporter. The nanoparticle size and content of Ni are 30.8 nm and 2.2%. The specific surface area, pore volume and average pore diameter of Ni/SBA-15-CO are 286 m2/g, 0.49 cm3/g and 5.6 nm. As a contrast, Ni/SBA-15 was prepared by post-impregnated method. It also shows an ordered mesostructure. The active component Ni is dispersed in the pores of the carrier uniformly. The nanoparticle size and content of Ni are 15.4 nm and 3.1%. The specific surface area, pore volume and average pore diameter of Ni/SBA-15 are 339 m2/g, 0.60 cm3/g and 6.6 nm.(2) The stability of Ni/SBA-15-CO and Ni/SBA-15 catalysts was investigated using 40 mL decalin as solvent. Both catalysts exhibit high activity in hydrogenation of phenol with 96.6% and 95.5% of the initial phenol conversion. After four cycled experiments, the conversion of phenol decreases for both catalysts, and it is 73.1% for Ni/SBA-15-CO catalyst with slight decrease, while it is only 40.3% for Ni/SBA-15 catalyst with obvious decrease. After 4-cycles experiments, the size of Ni nanoparticle in Ni/SBA-15-CO is almost unchanged, while it increases 3.1 nm for Ni/SBA-15. Moreover, for the Ni/SBA-15 catalyst, the loss of Ni is 16.1%, but that is only 9.1% for Ni/SBA-15-CO. Compared with the fresh catalysts, the specific surface area and pore volume of Ni/SBA-15-CO-4 are reduced by 27.6% and 36.7%, but those of Ni/SBA-15-4 are reduced by 59.0% and 65.0%. The above results indicate that since the confined space of Ni for the Ni/SBA-15-CO catalyst, the aggregation and loss of Ni are inhibited effectively which is important to keep high stability of the catalyst. Moreover, since the acidity of the Ni/SBA-15-CO catalyst by EISA is stronger than the Ni/SBA-15 catalyst, cyclohexane is the primary hydrogenation product for Ni/SBA-15-CO.(3) The supported amorphous NiB/SiO2-Al2O3 catalyst was prepared by impregnation, following the chemical reduction with NaBH4 as the reducing agent, and it was used as the catalyst for hydrogenation of phenol in bio-oil water/oil biphasic system. Results show that the conversion of phenol is up to 84.5% under 200℃ for the amorphous NiB/SiO2-Al2O3 catalyst, while it is only 18.7% for the crystalline Ni/SiO2-Al2O3 catalyst, which indicates the higher activity of the amorphous catalyst. The conversion of phenol catalyzed by NiB/SiO2-Al2O3 first rises and then drops with the increasing temperature and it reaches the maximum(95.7%) at 250℃. As the temperature increases from 150℃ to 310℃, the selectivity of cyclohexanol reduces from 91.2% to 3.7%, while it increases from 0.2% to 58.4% for cyclohexene, so the selectivities of cyclohexanol and cyclohexene are controllable by adjusting reaction temperature. To prolong time from 1 h to 7 h, the selectivity of cyclohexanol decreases 20.1%, while it increases 21.4% for cyclohexene. It shows that the extension of the reaction time is in favor of the transformation of cyclohexanol to cyclohexene. Unlike the primary product is cyclohexanol or cyclohexene in oil/water biphasic system, cyclohexane is the major product in oil system, which means the selectivities of hydrogenation products can be controlled by regulating reaction solvent.