Hydrothermal and Solvothermal Synthesis of Cerium-Zirconium Oxides for Catalyst Applications

Cerium oxide (CeO₂) is a rare earth metal oxide that has high oxygen storage capacity at low temperature. In order to enhance this capacity, as well as its thermal stability, it is necessary to combine CeO₂ with zirconium oxide (ZrO₂). This work focuses on the synthesis of cerium-zirconium oxides by hydrothermal and solvothermal treatment at low temperature to obtain ones suitable for catalyst applications. The possibility of the application of ceria-zirconia oxide to the delignification reaction was investigated. The experiments were conducted at a constant pressure of 5 MPa, constant temperature of 150^oC, and constant synthesis time of 2 h, in an autoclave reactor made of SUS 316 with an internal volume of 100 mL. Precursor was prepared from Ce(NO₃)₃ and ZrO(NO₃)₂ at 0.06 M concentration, dissolved in various solvents. The solvents used were water, water/ethanol (70:30 vol/vol), and water/ethylene glycol (70:30 vol/vol). After hydrothermal and solvothermal synthesis, the colloid products were dried at 60^oC for 6 h and then calcined at 500^oC for 6 h. The characterizations of the particle products were analyzed using SEM and XRD. Furthermore, these products were used for the hydrothermal delignification process of wood biomass. The addition of ceria-zirconia particles dramatically increased the percentage of lignin removal from rapeseed wood up to 97.58%. Based on the results, ceria-zirconia oxide particles are effective for the pre-treatment of wood biomass in bio-refinery applications. Moreover, ceria-zirconia oxides may reduce the use of chemical compounds in the delignification process.

Cerium oxide; Delignification; Hydrothermal; Solvothermal; Zirconium oxide

Cerium oxide has a beneficial redox property, and therefore it is applied in reactions in many industries and at the lab scale as an eco-friendly redox catalyst; for example, in hydrogen production, water gas shift reactions, and automotive exhaust gas conversion. Recently, investigations have been conducted into the application of cerium oxide as an adsorbent for CO₂ capture (Kusrini et al., 2018). Many catalytic reactions have employed cerium oxide at lower operating temperatures compared to other oxides, especially in reactions with oxygen as a reactant, due to its redox nature (Kwon et al., 2011). It is established that cerium has the capacity to change between two oxidation states, +3 and +4 that characterizes its redox property. Cerium keeps the oxygen in  reserve in  aerobic conditions and discharges it in anaerobic conditions in order to gratify its stoichiometry. This characteristic is responsible for the oxygen storage capacity (OSC) property of cerium. However, pure cerium oxide has limitations in OSC and thermal stability due to the effect on its surface area at higher temperatures of the growth of nucleation and crystallites within the cerium pores (Lundberg et al., 2002).

Several attempts have been made to counter the restrictions of pure cerium oxide. Some rare earth metal oxides, such as lanthanum, yttrium, zirconia and silica, have been added to improve the OSC and thermal stability of cerium oxide by raising its pore parameters (Suda et al., 2001). It is well-known that the ionic conductivity of cerium oxide can be increased by the ion doping of the rare earth metal; moreover, cerium oxide catalytic activity can also be elevated by supplying lattice oxygen from the bulk to the surface (Zhou & Gorte, 2008). Generally, cerium contains both Ce⁴⁺ and Ce³⁺ ions, which coexist in its lattice. The tetravalent cerium cation has a lower ionic radius of 0.97 A° than the trivalent, which has a 1.14 A° ionic radius. As a result, the close fluorite framework of the cerium is deformed. The tetravalent cations consolidate into the cerium lattice, the diffusivity capacity of oxygen can be changed, and the ion mobility inside the lattice will also be altered, with the establishment of subsidiary deformations. These deformations can be reduced by the generation of damage solid oxides structurally with smaller size tetravalent cations, such as Zr⁴⁺ (Reddy et al., 2007). Further, the combination of cerium with zirconium was formulated and the composite of CeO₂-ZrO₂ solid oxides exhibited appropriate thermal stability and redox capacity, together with certain characteristic features. The composite of CeO₂-ZrO₂ solid oxides with various compositions could improve the OSC (Machmudah et al., 2018).

In the last few years, cerium-zirconium oxides have become a key factor in many heterogeneous catalysis applications; moreover, they meet the zero-emission target of automotive exhaust gas. Cerium-zirconium oxides have been investigated in many areas of chemistry, such as the synthesis of butyl acetate from acetic acid and butanol (Yucai, 2006); the oxidation of landfill leachate (Aneggi et al., 2012); its application as a catalyst for the steam reforming of raw bio-ethanol (Palma et al., 2016); and as a catalyst for hydrogen production from methane reformation (Gil-Calvo et al., 2017). Delignification is the removal of lignin from woody tissue by natural enzymatic or chemical processes, such as oxidation reaction (Park et al., 2015). The delignification of biomass is lucrative and important in ethanol production from lignocellulosic biomass. In order to improve the removal of lignin from the biomass, it is necessary to find a simple process with a catalyst to enhance the oxidation reaction. From the literature review, it appears that the application of cerium-zirconium oxides in the delignification process has yet to be reported.

Cerium-zirconium oxides have been synthesized by the sol-gel process at ambient temperature (Rumruangwong & Wongkasemjit, 2006). In this process, various chemicals need to be used for the preparation of the precursor, and a long synthesis time is needed. Numerous methods for cerium oxide synthesis have been reported, such as solution precipitation, thermal decomposition, ball milling, hydrothermal synthesis, solvothermal synthesis, spray pyrolysis, and thermal hydrolysis (Dhall & Self, 2018). As with the sol-gel process, solution precipitation requires various chemicals and needs a long synthesis time. In ball milling, it is difficult to control particle size, while the other methods need high temperatures to form the particles.    

In this work, the application of cerium-zirconium oxide particles in the delignification process is investigated. The cerium-zirconium oxides were synthesized by hydrothermal and solvothermal treatment, which are modest techniques for the synthesis of cerium-zirconium oxides particles. These methods allow the formation of particles in water or an organic solvent at temperatures above the boiling point of the solvent, as well as at pressures higher than the solvent vapor pressure. Synthesis was performed with water, ethanol/water (70:30 vol/vol), and water/ethylene glycol (70:30 vol/vol) as the solvent, at a temperature of 150^oC. The delignification process was conducted by hydrothermal treatment at 150^oC, with the addition of cerium-zirconium oxide particles synthesized by hydrothermal and solvothermal treatment. The effect of the addition of the cerium-zirconium particles on the lignin removal yield was also determined.

Cerium-zirconium oxide particles were successfully synthesized with hydrothermal and solvothermal treatment at 150^oC. Solvent significantly influenced the morphology and crystallinity of the particles. The synthesized particles have an XRD pattern similar to the composite cerium-zirconium oxides of Ce_0.18Zr_0.19O₂. As a catalyst, cerium-zirconium oxides were applied in the delignification process. The addition of cerium-zirconium oxide particles in the hydrothermal delignification process resulted in an increase in lignin removal from the rapeseed wood, from 5.11% without catalyst to 97.58% using the cerium-zirconium oxide catalyst. This indicates that ceria-zirconium oxides are effective in the hydrothermal delignification process and may reduce the use of chemical compounds for the pre-treatment of wood biomass in bio-refinery applications.

The authors acknowledge funding from the Directorate General of Research and Development Strengthening, Ministry of Research, Technology, and Higher Education of the Republic of Indonesia through a PDUPT research grant (contract nos. 616/PKS/ITS/2017 and 925/PKS/ITS/2018).

Aneggi, E., Cabbai, V., Trovarelli, A., Goi, D., 2012. Potential of Ceria-based Catalysts for the Oxidation of Landfill Leachate by Heterogeneous Fenton Process. International Journal of Photoenergy, Volume 2012, Article ID 694721, p. 8

Chelliah, M., Rayappan, J.B.B., Krishnan, U.M., 2012. Synthesis and Characterization of Cerium Oxide Nanoparticles by Hydroxide Mediated Approach. Journal of Applied Sciences, Volume 12(16), pp. 1734–1737

Dhall, A., Self, W., 2018. Cerium Oxide Nanoparticles: A Brief Review of their Synthesis Methods and Biomedical Applications. Antioxidants, Volume 7(8), pp. 1–13

Gil-Calvo, M., Jimenez-Gonzalez, C., De Rivas, B., Gutierrez-Ortiz, J., Lopez-Fonseca, R., 2017. Hydrogen Production by Reforming of Methane over NiAl₂O₄/Ce_xZr_1-xO₂ Catalysts. Chemical Engineering Transactions, Volume 57, pp. 901–906

Kanie, K., Seino, Y., Matsubara, M., Nakaya, M., Muramatsu, A., 2014. Hydrothermal Synthesis of BaZrO₃ Fine Particles Controlled in Size and Shape and Fluorescence Behavior by Europium Doping. New Journal of Chemistry, Volume 38(8), pp. 3548–3555

Kusrini, E., Saleh, M.I., Lecomte, C., 2009. Coordination of Ce(III) and Nd(III) with Pentaethylene Glycol in The Presence of Picrate Anion: Spectroscopic and X-ray Structural Studies. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, Volume 74(1), pp. 120–126

Kusrini, E., Saleh, M.I., Adnan, R., Fun, H-K., Yamin, B.M., 2010. Triclinic Structural Isomer of Cerium(III)–picrate Complexes with Triethylene Glycol. Journal of Coordination Chemistry, Volume 63(3), pp. 484–497

Kusrini, E., Utami, C.S., Usman, A., Nasruddin., Tito, K.A., 2018. CO₂ Capture using Graphite Waste Composites and Ceria. International Journal of Technology, Volume 9(2), pp. 287–296

Kwon, K., Lee, K.H., Jin, S., You, D.J., Pak, C., 2011. Ceria-promoted Oxygen Reduction Reaction in Pd-based Electrocatalysts. Electrochemistry Communications, Volume 13(10), pp. 1067–1069

Lundberg, M., Skarman, B., Cesar, F., Wallenberg, L.R., 2002. Mesoporous Thin Films of High-surface-area Crystalline Cerium Dioxide. Microporous Mesoporous Materials, Volume 54(1-2), pp. 97–103

Machmudah, S., Prastuti, O.P., Widiyastuti, Winardi, S., Wahyudiono, Kanda, H., Goto, M., 2016. Macroporous Zirconia Particles Prepared by Subcritical Water in Batch and Flow Processes. Research on Chemical Intermediates, Volume 42, pp. 5367–5385

Machmudah, S., Widiyastuti, Winardi, S., Wahyudiono, Kanda, H., Goto, M., 2018. Synthesis of Ceria Zirconia Oxides using Solvothermal Treatment. MATEC Web of Conferences, Volume 156, 05014

Palma, V., Ruocco, C., Ricca, A., 2016. Low-Temperature Steam Reforming of Raw Bio-Ethanol Over Ceria-Zirconia Supported Catalysts. Chemical Engineering Transactions, Volume 52, pp. 193–198

Park, J., Shin, H., Yoo, S., Zoppe, J.O., Park, S., 2015. Delignification of Lignocellulosic Biomass and Its Effect on Subsequent Enzymatic Hydrolysis. Bioresources, Volume 10(2), pp. 2732–2743

Reddy, B.M., Bharali, P., Saikia, P., Khan, A., Loridan, S., Muhler, M., Grünert, W., 2007. Hafnium Doped Ceria Nanocomposite Oxide as a Novel Redox Additive for Three-way Catalysts. The Journal of Physical Chemistry C, Volume 111(5), pp. 1878–1881

Rumruangwong M., Wongkasemjit S., 2006. Synthesis of Ceria–zirconia Mixed Oxide from Cerium and Zirconium Glycolates via Sol–gel Process and Its Reduction Property. Applied Organometallic Chemistry, Volume 20(10), pp. 615–625

Santolalla, C., Chavez-Esquivel, G., de los Reyes-Heredia, J.A., Alvarez-Ramirez, J., 2013. Fractal Correlation Analysis of X-rayDiffraction Patterns with Broad Background. Industrial & Engineering Chemistry Research, Volume 52(24), pp. 8346–8353

Suda, A., Sobukawa, H., Suzuki, T., Kandori, T., Ukyo, Y., Sugiura, M., 2001. Effect of Ordered Arrangement of Ce and Zr Ions on Oxygen Storage Capacity of Ceria-Zirconia Solid Solution. Journal of the Ceramic Society of Japan, Volume 112(1311), pp. 586–589

Yucai, H., 2006. Hydrothermal Synthesis of Nano Ce–Zr–Y Oxide Solid Solution for Automotive Three-way Catalyst. Journal of the American Ceramic Society, Volume 89, pp. 2949–2951

Zhou, G., Gorte, R.J., 2008. Thermodynamic Investigation of the Redox Properties for Ceria?Hafnia, Ceria?Terbia, and Ceria?Praseodymia Solid Solutions. The Journal of Physical Chemistry B, Volume 112, pp. 9869–9875