High-Loading Nickel Phosphide Catalysts Supported on SiO2–TiO2

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High-Loading Nickel Phosphide Catalysts Supported on SiO2−TiO2 for Hydrodeoxygenation of Guaiacol Peng Zhang,†,∥ Yu Sun,†,∥ Mohong Lu,*,† Jie Zhu,† Mingshi Li,*,† Yuhua Shan,† Jianyi Shen,‡ and Chunshan Song§

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†

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, and School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China ‡ Laboratory of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China § Clean Fuels and Catalysis Program, EMS Energy Institute, Departments of Energy & Mineral Engineering and of Chemical Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, State College, Pennsylvania 16802, United States ABSTRACT: High-loading NixPy catalysts supported on SiO2−TiO2 binary oxides with different Si/Ti atomic ratios were prepared by liquid-phase phosphidation using triphenylphosphine (PPh3) and valued for the hydrodeoxygenation (HDO) of guaiacol. The samples synthesized were characterized by N2 adsorption, X-ray diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), and ammonia temperature-programmed desorption. The XRD and XPS results showed that the Ni2P/Ni12P5 mixed phase was formed when SiO2−TiO2 binary oxides were employed as supports, whereas only Ni2P and Ni12P5 phases were produced on SiO2 and TiO2 single oxides, respectively. XPS analysis showed that TiO2−x species were generated on the surface of the catalyst during the reduction of the precursor, and the electron transfer occurred from TiO2−x species to the NixPy surface. TiO2−x species on the NixPy surface would contribute to the high HDO activity of guaiacol on NixPy catalysts because the presence of TiO2−x species promoted the adsorption of guaiacol and activation of the C−O bond in the guaiacol molecule. HDO reaction of guaiacol on NixPy catalysts dramatically followed the hydrogenation path owing to the high hydrogenation activity of NixPy catalysts.

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INTRODUCTION The effort on seeking alternative energy sources has been becoming the focus in the past decades owing to the excessive consumption of limited petroleum resources and the increase of environmental pollution.1 Biofuel appears to be a potential resource alternative to fossil fuel since it is a renewable clean energy that can help alleviate global environmental problems.2,3 However, the high oxygen content in the bio-oil brings many disadvantage.4 Hydrodeoxygenation (HDO) is a potential technology for removing oxygen from bio-oils and transforming them into commercial biofuels.3,5 In recent years, transition-metal phosphide catalysts have received much attention due to their excellent HDO catalytic performance.6−9 Oyama et al. evaluated different phosphides supported on silica for HDO of guaiacol and drew a conclusion that the Ni2P catalyst exhibits the highest HDO activity. The main reaction products are high-valued chemicals (benzene, phenol, and anisole). Thus, the Ni2P catalyst is regarded as a very promising alternative for upgrading bio-oil.10 The temperature-programmed reduction (TPR) method was widely used to prepare nickel phosphide catalysts. However, because the P−O bond in PO43− is too strong to be reduced by H2, a high reduction temperature (>600 °C) is required during the TPR process and results in the formation of Ni2P particles with a large crystal size, which may suffer from the fading in catalytic activity of Ni2P catalysts.11−13 Therefore, the invention of a novel method for preparing Ni2P catalysts is necessary. In our previous work, high-loading Ni2P/ SiO2, Ni2P/Al2O3, and Ni2P/ZrO2−Al2O3 catalysts were © XXXX American Chemical Society

synthesized by using 4 wt % PPh3 solution as the P source when Ni catalyst precursors were prereduced under a H2 atmosphere at a relatively low temperature. As-synthesized Ni2P particles by the solution-phase method are highly dispersed on the support and exhibit higher activity for the hydrotreating reactions than phosphidation with highly toxic PH3.11 It is also noteworthy that the coordination agents can be avoided in this method.14−16 It is well known that the specific surface area of bulk Ni2P is very small ( Ni x P y /SiO 2 −TiO 2 (1:1) > Ni x P y /SiO 2 − TiO2(1:2) > Ni12P5/TiO2 ≈ Ni2P/SiO2, indicating that the HDO activity of nickel phosphide catalysts supported on SiO2−TiO2 is higher than that on Ni12P5/TiO2 and Ni2P/ SiO2. There are some factors responsible for the activity sequence. First of all, TiO2 may play a pivotal role in enhanced HDO activity for NixPy/SiO2−TiO2 catalysts. A wealth of literature reported that the incorporation of TiO2 can promote the HDO, HDS, and HDN performance of nickel phosphide catalysts.17,22,46,50 The partially reduced TiO2−x produced in the high-temperature reduction process can migrate to the surface of the catalysts, and the excess electrons of Ti3+ can be transferred directly into nickel phosphide particles through the conduction band. The increase of electron density on the surface of nickel phosphide will facilitate the dissociation of active hydrogen molecules on nickel phosphide nanoparticles, and the highly active discrete H donor will enhance the hydrogenation reaction pathway.17 The electron in Ti3+ can transfer into the anti-π orbit of Ni species to weaken the Ni−P bonds and even to break it. Then, the P− anions produced will attack the C−O bonds of the guaiacol molecule, and the dangling bonds of Ni and P species on the surface of the catalysts can also enhance the adsorption of the guaiacol

Table 3. Quantity of NH3 Released (μmol/g) from NH3TPD Experiments μmol(NH3) (g) catalysts

below 400 °C

above 400 °C

total

Ni12P5/TiO2 NixPy/SiO2−TiO2(1:2) NixPy/SiO2−TiO2(1:1) NixPy/SiO2−TiO2(2:1) Ni2P/SiO2

164 228 290 305 213

372 715 774 871 803

536 943 1064 1176 1016

chemisorbed NH3 is compiled in Table 3. The desorbed peak at relatively low temperature (160−230 °C) can be assigned to the weak Brønsted acid sites arising from P−OH groups.3,4 The high-temperature NH3 desorbed peak centered at 558−610 °C suggests the existence of strong acid sites, which can be attributed to the Lewis acidity caused by unreduced Ni2+ species and Niδ+ sites.2,5,47 The total amount of acid sites of all of the catalysts followed the order NixPy/ SiO2−TiO2(2:1) > NixPy/SiO2−TiO2(1:1) > Ni2P/SiO2 > Ni1×Py/SiO2−TiO2(1:2)> Ni12P5/TiO2, suggesting that the E

DOI: 10.1021/acs.energyfuels.9b01538 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

can reduce the charge transfer resistance of Ni2P/Ni12P5 and promote the electron transfer rate between the BH4− donor and the 4-nitrophenol acceptor, which is responsible for the enhancement in the reduction activity of catalysts.37 The results obtained from XPS show the occurrence of charge transfer from TiO2−x species to NixPy. The electron-enriched surface would facilitate the cleavage of the C−O bond in the guaiacol molecule by a nucleophilic substitution reaction. Therefore, it is reasonable that the Ni2P/Ni12P5 mixed phase in NixPy/SiO2−TiO2 catalysts can act as an ideal media for electron transfer between TiO2−x species and the NixPy surface, resulting in high conversion of guaiacol on the NixPy/SiO2− TiO2 catalysts. Then, the acidity of catalysts may have contributed to the HDO activity to a certain extent. Wu et al. considered that the aromatic ring of guaiacol, which contains a Lewis base, can be chemisorbed on Lewis acid sites of the catalysts and then HDO reaction was carried out.47 Ayako et al. deduced that Si− OH and P−OH groups on the surface of the catalysts, which provide Brønsted acid sites, can adsorb 2-methyltetrahydrofuran by a hydrogen bond or physisorption. The adsorbed reactants will deoxygenate to form n-pentane.52 Zhang et al. also reported that the acid sites can catalyze the rupture of the C−O bond.53−55 The acid amount of Ni12P5/TiO2 is the lowest as we can see from the NH3-TPD (Table 3), which might result in relatively lower TOF of cyclohexane compared with that of the NixPy/SiO2−TiO2 catalysts. However, the acid amount of Ni2P/SiO2 is more than that of NixPy/SiO2− TiO2(1:2) and Ni12P5/TiO2 catalysts, whereas the TOF of cyclohexane on the Ni2P/SiO2 catalyst is the lowest (Figure 6), suggesting that the acidity of the catalyst is not the only factor determining the deoxygenation activity of guaiacol in this case. The selectivity of the HDO products is shown in Table 4. Cyclohexane, cyclohexanol, and 2-methoxycyclohexanol are the main products over all of the catalysts in the temperature range from 200 to 260 °C. The higher selectivity of cyclohexane on NixPy/SiO2−TiO2 catalysts than Ni2P/SiO2

molecule. This synergistic effect can promote the C−O bond rupture.17,24 In our previous work, the strong interaction between nickel particles and TiO2−x can produce Niδ−−OV− Ti3+ interface sites, and the guaiacol molecule can be absorbed and activated through the interaction between the oxophilic sites (OV−Ti3+ defect sites) and oxygen in guaiacol, which favors the hydrogenolysis of the C−O bond.27,51 Herein, we infer that the OV−Ti3+ defect sites produced by the strong interaction of nickel phosphide and TiO2−x may interact with the C−O bond of the guaiacol molecule and weaken the C−O bond, which would result in the promotional hydrogenolysis of the C−O bond by reacting with dissociative hydrogen supplied from the nickel phosphide surface. The TOF of cyclohexane (Figure 6) followed the order NixPy/SiO2−TiO2(1:2) > NixPy/

Figure 6. TOF of cyclohexane at different temperatures on nickel phosphide catalysts.

SiO2−TiO2(1:1) > NixPy/SiO2−TiO2(2:1) ≈ Ni12P5/TiO2 > Ni2P/SiO2. It demonstrates that TiO2−x species are a key factor affecting the HDO reaction activity of nickel phosphide catalysts. Moreover, it should be noted that the NixPy/SiO2−TiO2 catalysts possess a Ni2P/Ni12P5 biphase heterostructure. Tian et al. considered that an n−n semiconductor heterojunction can be formed between Ni2P and Ni12P5 phases. This structure

Table 4. Selectivity of Products at Reaction Temperature over Different Catalysts selectivity (%) catalysts Ni12P5/TiO2

NixPy/SiO2−TiO2(1:2)

NixPy/SiO2−TiO2(1:1)

NixPy/SiO2−TiO2(2:1)

Ni2P/SiO2

T (°C)

cyclohexane

benzene

cyclohexanol

anisole

phenol

2-methoxycyclohexanol

200 220 240 260 200 220 240 260 200 220 240 260 200 220 240 260 200 220 240 260

34.2 47.0 58.8 89.8 45.9 72.1 92.1 98.6 30.3 61.7 81.4 92.3 22.3 36.0 77.2 93.3 17.8 29.3 44.2 75.0

0.1 0.4 0.56 0.4 0 0.6 0.8 1.4 0.3 0.2 0.2 0.2 0.1 0 0.1 0 0 0.8 1.0 1.6

24.4 21.1 19.4 5.9 22.6 11.7 3.2 0 25.8 16.0 8.8 4.3 33.1 25.4 10.6 3.8 27.0 21.3 18.0 10.3

2.3 2.1 1.5 0 5.0 3.7 1.2 0 1.4 1.2 1.1 0.7 0.3 0.3 0.2 0.1 4.0 4.5 4.9 2.8

1.1 0.6 0.2 0 4.9 3.2 1.1 0 2.3 1.7 1.0 0.4 0 0 0 0 6.8 5.9 3.8 0

37.7 28.5 19.4 3.8 21.4 8.5 1.5 0 39.5 19.0 7.4 1.8 44.1 38.2 11.7 2.7 44.2 38.1 28.0 10.1

F

DOI: 10.1021/acs.energyfuels.9b01538 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels and Ni12P5/TiO2 would be attributed to the strong interaction between TiO2−x and nickel phosphide particles, which can enhance adsorption and activation of the C−O bond in the guaiacol molecule. SiO2 in SiO2−TiO2 can promote the dispersion of TiO2, resulting in the enhancement of the interaction between nickel phosphide and TiO2−x as we discussed in the previous work.27 Small amounts of benzene, phenol, and anisole were detected for all of the catalysts (no phenol was found on the NixPy/SiO2−TiO2(2:1) catalyst). The HDO reaction network of guaiacol on Ni−P catalysts is proposed in Scheme 1 on the basis of the product distribution.

Jianyi Shen: 0000-0002-5146-6316 Chunshan Song: 0000-0003-2344-9911 Author Contributions ∥

P.Z. and Y.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Natural Science Foundation of China (21761132006 and 21676029), the Natural Science Foundation of Jiangsu Higher Education Institutions (grant no. 12KJB530001), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for the financial support.

Scheme 1. Hydrodeoxygenation Reaction Network of Guaiacol Proposed on Nickel Phosphide Catalysts

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(1) Li, Y.; Fu, J.; Chen, B. Highly selective hydrodeoxygenation of anisole, phenol and guaiacol to benzene over nickel phosphide. RSC Adv. 2017, 7, 15272−15277. (2) Chen, J.; Shi, H.; Li, L.; Li, K. Deoxygenation of methyl laurate as a model compound to hydrocarbons on transition metal phosphide catalysts. Appl. Catal., B 2014, 144, 870−884. (3) Infantes-Molina, A.; Gralberg, E.; Cecilia, J. A.; Finocchio, E.; Rodríguez-Castellón, E. Nickel and cobalt phosphides as effective catalysts for oxygen removal of dibenzofuran: role of contact time, hydrogen pressure and hydrogen/feed molar ratio. Catal. Sci. Technol. 2015, 5, 3403−3415. (4) Zhang, Z.; Tang, M.; Chen, J. Effects of P/Ni ratio and Ni content on performance of γ-Al 2 O 3 -supported nickel phosphides for deoxygenation of methyl laurate to hydrocarbons. Appl. Surf. Sci. 2016, 360, 353−364. (5) Wu, S.-K.; Lai, P.-C.; Lin, Y.-C.; Wan, H.-P.; Lee, H.-T.; Chang, Y.-H. Atmospheric Hydrodeoxygenation of Guaiacol over Alumina-, Zirconia-, and Silica-Supported Nickel Phosphide Catalysts. ACS Sustainable Chem. Eng. 2013, 1, 349−358. (6) Lan, X.; Hensen, E. J. M.; Weber, T. Hydrodeoxygenation of guaiacol over Ni 2P/SiO2-reaction mechanism and catalyst deactivation. Appl. Catal., A 2018, 550, 57−66. (7) Cecilia, J. A.; Infantes-Molina, A.; Rodríguez-Castellón, E.; Jiménez-López, A.; Oyama, S. T. Oxygen-removal of dibenzofuran as a model compound in biomass derived bio-oil on nickel phosphide catalysts: Role of phosphorus. Appl. Catal., B 2013, 136−137, 140− 149. (8) Feitosa, L. F.; Berhault, G.; Laurenti, D.; Davies, T. E.; Teixeira da Silva, V. Synthesis and hydrodeoxygenation activity of Ni 2 P/CEffect of the palladium salt on lowering the nickel phosphide synthesis temperature. J. Catal. 2016, 340, 154−165. (9) Gonçalves, V. O. O.; de Souza, P. M.; da Silva, V. T.; Noronha, F. B.; Richard, F. Kinetics of the hydrodeoxygenation of cresol isomers over Ni 2 P/SiO 2: Proposals of nature of deoxygenation active sites based on an experimental study. Appl. Catal., B 2017, 205, 357−367. (10) Zhao, H. Y.; Li, D.; Bui, P.; Oyama, S. T. Hydrodeoxygenation of guaiacol as model compound for pyrolysis oil on transition metal phosphide hydroprocessing catalysts. Appl. Catal., A 2011, 391, 305− 310. (11) Zhao, Y.; Xue, M.; Cao, M.; Shen, J. A highly loaded and dispersed Ni2P/SiO2 catalyst for the hydrotreating reactions. Appl. Catal., B 2011, 104, 229−233. (12) Song, H.; Dai, M.; Song, H.-L.; Wan, X.; Xu, X.-W.; Jin, Z.-S. A solution-phase synthesis of supported Ni2P catalysts with high activity for hydrodesulfurization of dibenzothiophene. J. Mol. Catal. A: Chem. 2014, 385, 149−159. (13) Bussell, M. E. New methods for the preparation of nanoscale nickel phosphide catalysts for heteroatom removal reactions. React. Chem. Eng. 2017, 2, 628−635.

The large amount of 2-methoxycyclohexanol formed at the initial temperature indicates that guaiacol hydrogenation took place to 2-methoxycyclohexanol followed by demethoxylation to cyclohexanol, and further to cyclohexane through the cleavage of the C−O bond. A trace amount of phenol was formed via the extraction of the methoxy group from guaiacol or the cleavage of the C−O bond from anisole.56,57 Benzene can be formed through the Caromatic−O bond breakage of guaiacol, phenol, and anisole, and it can be eventually converted to cyclohexane by a hydrogenation process.1,5

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CONCLUSIONS In the present study, a series of highly loading NixPy/SiO2− TiO2 catalysts were prepared by liquid-phase phosphidation by PPh3. The Ni2P/Ni12P5 mixed phase was formed on SiO2− TiO2 composite oxide supports, whereas pure Ni2P and Ni12P5 phases were formed on SiO2 and TiO2 single oxides, respectively. The surface of NixPy was covered partially by TiO2−x species formed during the reduction of the precursor. The existence of a strong interaction between TiO2−x species and NixPy particles results in electron transfer from TiO2−x species to the NixPy surface, and TiO2−x species on the NixPy surface could enhance the adsorption and activation of the C− O bond in the guaiacol molecule, which would be responsible for the promotional HDO activity of NixPy/SiO2−TiO2 catalysts. HDO reaction of guaiacol on NixPy catalysts dramatically follows the hydrogenation path as a result of the high hydrogenation activity of NixPy catalysts.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.L.). *E-mail: [email protected] (M.L.). ORCID

Mohong Lu: 0000-0002-3641-2619 Mingshi Li: 0000-0001-9404-7894 G

DOI: 10.1021/acs.energyfuels.9b01538 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.9b01538 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.9b01538 Energy Fuels XXXX, XXX, XXX−XXX