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In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/advances Page 1 of 27 RSC Advances Hydrodeoxygenation of phenol as a bio-oil model compound on intimate contact noble metal-Ni 2P/SiO 2 catalyst Yunhua Li *, Xin Yang, Linhui Zhu, Hua Zhang, Binghui Chen Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, Xiamen University, Xiamen 361005, PR China. Abstract: This study investigates phenol hydrodeoxygenation on supported Ni 2P from the sol-gel and TPR method and noble metal (Pd, Pt and Ru)-Ni 2P catalysts from Ni 2P partially in-situ reducing noble metal precursor. Manuscript 10%Ni 2P/SiO 2 had the relatively uniform distribution of Ni 2P nanoparticle and high active activity for phenol hydrodeoxygenation. Phenol conversion increased with the increasing in reaction temperature and the main products on noble metal-Ni 2P/SiO 2 also changed from cyclohexanol at 453K to cyclohexane at 493K. In comparison with supported Ni 2P or noble metal catalysts and their physical mixture, Pd-Ni 2P/SiO 2 presented the Accepted highest conversion activity and cyclohexane selectivity. Physiochemical characterizations showed that the active sites over catalysts increased and electron transferring occurred from Ni 2P to noble metal due to intimate contact between Ni 2P and noble metal. Synergistic effect of deoxygenation and carbonyl hydrogenation from Ni 2P and hydrodeoxygenation from Pd resulted in improvement of catalytic activity and differences of selectivity on catalysts. Advances Keywords: Hydrodeoxygenation, Phenol, Bio-oil, Nickel phosphide, Noble metal RSC 1. Introduction Most effort has focused on production of renewable and clear biofuels and chemicals due to energy consumption * Corresponding author. E-mail: [email protected] . 1 RSC Advances Page 2 of 27 and environmental deterioration. Lignin, typically composed of methoxy-substituted phenyl propanoid units, is the second most abundant component in biomass and can be regarded as a potential resource for the production of biofuels and special chemicals 1. Bio-oil from lignin, however, cannot be used without prior refining due to its high viscosity, low heating value and corrosiveness. Thus, hydrodeoxygenation (HDO) has been developed to convert this biofuel with these undesirable properties into feasible vehicle fuel. Up to now, hydrodeoxygenation catalysts mainly include the traditional MoS 2, noble metal and transition metal phosphide based catalysts 2-4. Among them, nickel phosphide has been prepared from the traditional temperature programmed reduction (TPR) method. However, there are still problems such as high preparation temperature and the lager and uncontrollable particle size of nickel phosphide in this method. For this reason, Manuscript several other processes have been developed, such as the solution-phase method 5, the decomposition of metal hypophosphites 6 and converting metal nanoparticles into metal phosphides 7. At the same time, a series of composite catalysts for hydrodeoxygenation of guaiacol or phenol, the common Accepted model chemicals in biofuel, have also been studied 8, 9. For example, Yan et al. 10 catalyzed hydrodeoxygenation of phenols using Ru nanoparticle catalysts combined with brønsted acidic ionic liquids. Hong et al. 11 prepared Pd/WOx/γ-Al 2O3 for guaiacol hydrodeoxygenation. With respect to bimetallic catalysts containing transition metal phosphide, Pd-Ni 2P/C has presented 1.5 times higher power density than commercial Pd for direct formic acid fuel 12 13 Advances cells . Ni 2P could also enhance the activity and durability of the Pt anode catalyst in direct methanol fuel cells . In addition, some researchers also noticed that programmed reduction temperature for phosphate decreased and the 14 RSC hydrodesulfurization activity increased with the addition of Pd or Pt into Ni 2P supported catalyst . However, these catalysts have usually been obtained by impregnating passivated phosphide or phosphate with noble metal precursor solution. It is difficult to ensure the directional loading of noble metal and the formation of intimate contact between phosphide and noble metal due to nonselective deposition of noble metal on support or phosphate. 2 Page 3 of 27 RSC Advances It is noteworthy that transition metal phosphide requires further passivation to avoid the drastic oxidation of nickel phosphide when it is exposure to air. This means that phosphide has certain reducibility. Herein, supported nickel phosphide was firstly prepared from the sol-gel and the traditional TPR method. Thus, Ni 2P was uniformly dispersed over SiO 2. The as-prepared supported nickel phosphide was then impregnated by noble metal precursor solution to obtain noble metal-Ni 2P catalyst. This preparation process not only favors the distribution of noble metal and minimizes its usage due to controllable distribution of reductant Ni 2P, but also facilitates the formation of intimate contact structure and intensifies the interaction between noble metal and Ni 2P. The catalytic HDO evaluation and catalyst characterizations were conducted on supported Ni 2P, noble metal and Manuscript noble metal-Ni 2P catalysts. Results showed that noble metal-Ni 2P/SiO 2 has the higher catalytic performances for phenol hydrodeoxygenation due to the more active site and electron transfer from Ni 2P to Pd than supported Ni 2P or noble metal catalysts. Based on these outcomes, relations between structure and catalytic performance were also discussed. The result may provide valuable information for developing Ni 2P or noble metal based catalyst to Accepted upgrade bio-crude-oil to feasible fuel. 2. Experimental 2.1. General Chemical reagents (GC grade) used for analysis, such as benzene, phenol, cyclohexanol, cyclohexane, Advances cyclohexene, cyclohexanone, isooctane, docecane and noble metal precursors were from Aladdin Chemistry Co., Ltd. All other chemicals were of analytical grade from sinopharm chemical reagent RSC Co.,Ltd and were used as received without any further purification. 2.2. Preparation of Ni 2P/SiO 2 catalysts and noble metal-Ni 2P/SiO 2 7 Ni 2P/SiO 2 was prepared via a sol-gel process according to the reference . In the typical preparation, a 3 RSC Advances Page 4 of 27 homogeneous solution of 2.62 g Ni(NO 3)2·6H 2O, 4.2 g urea and 225 ml H 2O was kept in a conical flask at 303 K, and its pH value was adjusted at 2.5 using diluted HNO 3 solution. A solution of 22.4 ml tetraethyl orthosilicate (TEOS) and 20 ml ethanol was dropwise added to the above solution under the stirring. The as-prepared sample was then heated to 363 K, filtered at pH 8.0, dried at 393K for 12h and calcined in U-tube quartz reactor at 823K for 5h to obtain Ni/SiO 2. Ni 2P/SiO 2 was further prepared from the traditional TPR method using NH 4H2PO 4 solution impregnating the as-prepared Ni/SiO 2 sample. The nominal Ni/P ratio of the catalyst was 1.0. After impregnation for 12h, the sample was calcined at 823K in air atmosphere for 4h. After cooled to room temperature, the sample was reduced at 923K in a H 2 flow for 3h and passivated under 1.5% O 2/N 2 for 6h at room temperature. Manuscript The noble metal-Ni 2P/SiO 2 was prepared by in-situ reduction of noble metal precursors. The detailed procedure is as follows. After the preparation of fresh Ni 2P/SiO 2 in U-tube quartz reactor was completed, a H 2 flow was switched into a N 2 atmosphere. This reactor filled with N 2 was placed in the ultrasonic cleaner (KQ-250DE). A certain concentration of noble metal precursor, such as PdCl 2, H 2PtCl 6, RuCl 3, solution was injected with a Accepted syringe into the freshly prepared Ni 2P/SiO 2 in U-tube quartz reactor under the ultrasound condition. The concentration of noble metal precursor was 0.01g/mL and corresponding liquid volume used is based on the nominal loading of noble metal, 1%. After 40min, the sample was washed, dried under H 2 at 393K for 2h and passivated under 1.5% O 2/N 2 for 3h. Advances For comparison, supported pure noble metal catalysts were prepared from impregnation method. The preparation of support SiO 2 was similar to that of Ni/SiO 2 except on addition of nickel precursor. After RSC impregnating noble metal precursor solution for 12h, the samples were calcined under H 2 at 723K for 3h. 2.3. Physiochemical Characterization The X-ray powder diffraction (XRD) measurements were carried out on a PAN-alytical X-ray diffractometer 4 Page 5 of 27 RSC Advances with Cu Ka (λ = 0.15406 nm) radiation. A continuous scan mode was used to collect 2θ data from 10° to 90° with a step width of 0.0167°. Transmission electron microscopy (TEM) images were collected using a JEOL JEM-1400 instrument. The samples were prepared by dispersing the powders in ethanol and then dropping a small volume onto a carbon-coated copper grid.