Abstract
The hydrodeoxygenation of lignin-derived bio-oil is the key to its upgrading into biofuels. Currently, the catalytic transfer hydrodeoxygenation (CTHDO) has been widely studied due to the avoidance of the use of hydrogen gas. In this work, cobalt-glycerolates nanospheres derived Co@C-T catalysts were synthesized via the solvothermal method, followed by the carbothermic method at different calcination temperatures in a nitrogen atmosphere. The obtained Co@C-450 catalyst exhibited the best vanillin CTHDO performance and afforded nearly 100 % conversion of vanillin and 87 % yield of 2-methoxy-4-methylphenol (MMP) under the conditions of 140 °C, 1 MPa N2 and 3 h using isopropanol as H-donor. Based on the various characterizations, the Co@C-450 catalyst possessed the appropriate metallic Co0 sites, acid sites (Co(II)), and carbon defects, and the high catalytic activity was attributed to the synergistic effects of them. During the CTHDO process, the Co0 sites were responsible for carbonyl hydrogenation of vanillin to generate 4-(hydroxymethyl)-2-methoxyphenol (HMP) and the generation of active hydrogen species from isopropanol, while the acid sites (Co(II)) were responsible for the cleavage of the CO bond in HMP to form MMP. In addition, the Co@C-450 catalyst had good applicability to other lignin-derived monomers, and its performance did not decrease after 11 runs. This work could provide some insights for upgrading bio-oil.
Graphical abstract
Introduction
Currently, global chemicals and transport fuels are mainly produced from the non-renewable fossil resources. The overexploitation and utilization of fossil resources have resulted in significant environmental pollution and substantial CO2 emissions [1], [2], [3]. Lignin is the abundant renewable organic carbon resource and can serve as a substitute for fossil resources to produce high-valued chemicals or fuels, which can help alleviate the energy crisis and environmental pollution [4]. Through fast pyrolysis, it can be decomposed into phenolic compounds with a high oxygen content (named bio-oil), which possesses the properties of low heating value, high viscosity, unstable combustion, and inability to direct use [5], [6]. Hydrodeoxygenation (HDO) has been widely applied to upgrade the lignin-derived oxygen-containing compounds to form cycloalkane or aromatics due to its advantage of mild reaction conditions [7], [8].
Vanillin (VAN) is an important platform chemical from lignin depolymerization, which can undergo the hydrogenation of aldehyde group to 4-(hydroxymethyl)-2-methoxyphenol (HMP) and hydrodeoxygenation to 2-methoxy-4-methylphenol (MMP) [9], [10]. Among them, MMP is a valuable chemical, which is commonly used to produce spices, pharmaceuticals, and other organic synthesis intermediates [11]. Most hydrodeoxygenation of VAN to MMP are conducted over various heterogeneous catalysts. The noble metal-based catalysts have already been studied due to its excellent catalytic performance. For example, Yue Xiaokang et al. prepared the porous organic polymer supported bimetallic PdAg nanoparticles catalysts, and the Pd9Ag1/BA-M catalyst exhibited excellent catalytic activity for HDO of vanillin under the conditions of 140 °C and 0.5 MPa H2 [12]. Zeng Yongjian and co-workers constructed Ru nanoparticles anchored on NiAl layered double oxides catalyst for selective hydrodeoxygenation of vanillin, affording 94.2 % yield of MMP at 130 °C in methanol solvent [13]. Although the noble metal-based catalysts displayed good performance, they usually suffer from high cost and coke formation [14], [15]. Thus, in recent years, researchers have made tremendous efforts in developing low-cost and high-efficiency catalysts, such as Ni, Co and Cu catalysts, which also demonstrate catalytic performance comparable to precious metal catalysts. Li Hao et al. conducted the HDO of vanillin over Ni/T-Nb2O5 catalyst with 79.2 % selectivity of MMP at 180 °C and 1 MPa H2 and the high performance was attributed to the presence of abundant Ov of Ni/T-Nb2O5 [16]. Gao Zhi et al. constructed Cu/Zn-Al-Sn layered double hydroxide for catalytic transfer hydrodeoxygenation of vanillin to MMP, affording nearly 100 % yield MMP under the conditions of 180 °C and 4 h [17]. Wang Dongdong and coworkers prepared NiCo alloy nanoparticles encapsulated in carbon nanotubes as HDO catalysts, which possessed outstanding performance for selective conversion of vanillin to MMP under milder conditions (120 °C, 2 MPa H2 and 6 h). It was reported that the introduction of cobalt could create a synergistic effect to improve the selective adsorption and activation of CO, and desorption of the activated hydrogen species [18]. Jaehoon Kim developed ZnO/Co@CNTs catalysts and vanillin could be completely transformed into the promising MMP under mild reaction conditions 150 °C, 0.7 MPa H2 and 2 h [19]. Compared to Ni-based and Cu-based catalysts, it seems that cobalt is more conducive to the hydrodeoxygenation of vanillin under milder conditions. Therefore, it is worth exploring the construction of stable and highly active cobalt-based catalysts for the transformation of vanillin to high-valued MMP.
Catalytic transfer hydrodeoxygenation (CTHDO) has been widely used in the upgrading of biomass derived oxygen-containing compounds because it avoids the use of dangerous hydrogen gas [20], [21]. Yang Huanghuang et al. conducted the CTHDO of vanillin over an N-doped Co@C catalyst with formic acid as the hydrogen donor (H-donor), affording > 95 % vanillin conversion with MMP as the main product [19]. However, the formic acid could corrode equipment and make the cost increase [22]. Non corrosive low-carbon alcohols, such as methanol, ethanol, and isopropanol, are the best alternative because they can be produced from renewable biomass, and the ketones and aldehydes produced can be easily separated from the products [23]. Li Hao et al. designed a yolk-shell structure Ni2P@YSS catalyst, which exhibited 76.2 % selectivity of MMP in an ethanol environment at 180 °C for 5 h [24]. Wu Xue and co-workers reported CTHDO of vanillin in the isopropanol solvent over N-doped lignin-MOFs derived Co-based catalysts, which showed good performance with 100 % conversion of vanillin and 92 % yield of MMP at 180 °C and 2 h [25]. When isopropanol is used as the hydrogen donor, it can generate the active hydrogen species through cleavage the OH and the αCH bonds to form acetone. Therefore, isopropanol, as a green reaction solvent, has great industrial application potential in the upgrading of lignin-derived oxygen-containing chemicals in the future.
Glycerol, as a byproduct of the biodiesel industry, is cost-effective and abundant in reserves, and it can exhibit suitable coordination properties through deprotonation reactions, and the generated glycerolates are better ligands that can coordinate with metal ions to produce structural nanomaterials. Metal-glycerolates (M-glycerolates) derived materials, such as metal oxides, metal sulfides, and metal phosphides, have already been applied in supercapacitors and batteries [26], [27]. However, their application in the HDO of lignin and its derivatives is rarely reported. In this work, we used solvothermal method to synthesize the Co-glycerolates nanospheres, which subsequently underwent a self-carbothermal reduction process at different calcination temperatures in a N2 atmosphere. The catalytic activity of the obtained Co@C-T catalysts for the CTHDO of vanillin was evaluated in the isopropanol H-donor solvent. It was confirmed that the Co@C-450 catalyst showed the best vanillin conversion and MMP yield at a lower reaction temperature of 140 °C. Combined with characterizations, the synergistic effects of the appropriate metallic Co0 sites, acid sites (Co(II)), and carbon defects were the key to the high catalytic performance. In addition, we studied the CTHDO pathways of vanillin and the stability and substrate applicability of the catalysts and finally investigated the reaction mechanism during the vanillin CTHDO process.
Section snippets
Material
All chemicals used in this study were of analytical grade. Glycerol (AR, ≥99 %), isopropanol (AR, ≥99.7 %), Co(NO3)2·6H2O (AR, 99 %), methanol (AR, ≥99.9 %), ethanol (AR, ≥99.9 %), vanillin (AR, 99 %) were purchased from Titan Science Co., Ltd. Shanghai, China. Deionized (DI) water used in the experiments was obtained from the ultrapure water polishing system. N2 gas (99.999 %) was provided by Nanjing Lianyang Gas Co., Ltd. All the reagents and chemicals were used directly without any further
Catalysts characterizations
The schematic illustration for obtaining the spherical Co@C-T catalysts was shown in Scheme 1. Firstly, the Co-glycerolates microspheres were prepared by a solvothermal method, where the Co2+ ions could self-assemble with glycerolate ions produced by deprotonation of glycerol to form precursors with a spherical structure [26]. Then, the obtained Co-glycerolates were transformed into spherical Co@C-T catalysts with the ultrasmall metal nanoparticles dispersed on the carbon matrix after the
Conclusion
In this work, a series of Co-glycerolates nanospheres derived Co@C-T catalysts were synthesized via a simple solvothermal method, followed by a self-carbothermal reduction process at N2 atmosphere. Among these catalysts, the obtained Co@C-450 catalyst exhibited the best catalytic transfer hydrodeoxygenation (CTHDO) performance of vanillin and afforded nearly 100 % conversion rate and 87 % yield of 2-methoxy-4-methylphenol (MMP) under milder reaction conditions of 140 °C, 1 MPa N2 and 3 h using
CRediT authorship contribution statement
Fei Ge: Writing – original draft. Peng Liu: Software, Resources. Wenlin Xu: Validation, Methodology. Jianchun Jiang: Methodology, Investigation. Minghao Zhou: Writing – review & editing, Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors are grateful for the financial support from National Natural Science Foundation of China (32301548) and the Young Talent Promotion Project of Jiangsu Association for Science and Technology (JSTJ-2023-XH019).
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