Lignin-enriched waste hydrothermal liquefaction with ZVMs and metal-supported Al2O3 catalyst
Graphical abstract
Introduction
Lignin is one of the most important components of lignocellulosic biomass and it is also the only renewable resource of aromatics. An amount of 50–70 million tons of technical lignin wastes such as kraft, lignosulfonate, soda and organosolv lignin was produced annually and most of them was simply disposed or directly burnt to produce energy [1,2]. To valorize this waste producing bio-fuels and chemicals several techniques including pyrolysis [3], solvolysis [4], catalytic oxidative depolymerization [5] and catalytic reductive depolymerization [6] were widely employed. Among them, catalytic reductive depolymerization was regarded as one of the most promising technique, generally conducted under hydrogen atmosphere with the presence of suitable solvent and redox catalysts [7]. During this process, the lignin reactive intermediates were reductively stabilized preventing their repolymerization into unreactive condensed derivatives [8,9]. The presence of hydrogen could significantly reduce the tar/coke yield, consequently increasing the bio-crude yield [7]. Non-noble metal catalysts especially Ni could promote the hydrogenation reaction [6]. Wang et al. obtained 54% bio-crude yield from pine wood lignin liquefaction under 4 MPa H2 and water solvent with ZnCl2 and Ru/C catalyst at 200 °C for 6 h. The main compounds of bio-crude were alkylphenols in the absence of Ru/C catalyst, while after its addition, phenolic compounds were further converted into cyclic hydrocarbons via hydrodeoxygenation [10]. Zeng et al. conducted hydrogenolysis of sugarcane bagasse lignin over Fe–Pd/HZSM-5 catalyst using ethanol-water co-solvents under 1 MPa H2 atmosphere at 320 °C for 120 min and the lignin conversion and aromatic monomers yield reached 98.2% and 27.9%, respectively [11].
Compared to gaseous hydrogen which usage is associated with several security and availability issues, hydrogen donors such as methanol, isopropanol and formic acid are considered promising for its substitution [12]. Lower strength of C–H bonds compared to H–H bonds makes hydrogen donor more efficient than the gaseous hydrogen [13]. Some researchers investigated the reductive depolymerization of lignin with only the hydrogen donors rather than gaseous hydrogen. Catalytic hydroprocessing of stubborn lignin with Cu/CuMgAlOx catalyst was conducted with supercritical methanol acting as solvent and hydrogen donor. After reaction at 300 °C for 6 h, the yield of monomers reached 37.8% with cyclohexanols and 4-alkyl phenolics as main products [14]. Kong et al. studied the alcoholysis of kraft lignin over Ni–Cu supported zeolites (HZSM-5, MCM-41, H-Beta, SAPO-11 and MAS-7) catalysts in isopropanol solvent. The maximum bio-crude yield of 98.8% and monomer yield of 50.8% were obtained over Ni–Cu/H-Beta catalyst with reaction at 330 °C for 3 h [15].
Zero valent metals (ZVMs) could also be a promising green hydrogen source, in fact, under hydrothermal condition, they react with water and release hydrogen in situ. The use of ZVMs as hydrogen producer limits the use of organic solvent being only water needed and reducing, in this way, the operation cost and environmental impact of the process. However, the reductive depolymerization of lignin using ZVMs-water redox system as hydrogen source has rarely been studied before. In author's previous work, Fe and Zn acting as hydrogen producer inhibited the condensation of oak wood HTL intermediates, Ni and Co were added as hydrogenation catalysts and used successfully in combination with Fe and Zn to exploit their hydrogenation ability [16]. The function of ZVMs on cellulose HTL was also investigated, showing a promotion on the aromatic compounds and 2-cyclopenten-1-ones in the bio-crudes [17]. Objectives of the work are to better understand the complex lignocellulosic biomass liquefaction mechanism studying the decomposition of pure lignin and to find a viable and efficient process for the valorization of lignin. In this work hydrothermal liquefaction in presence of ZVMs and hydrogenation catalysts is proposed as lignin depolymerization process to produce high quality bio-crude. First, the optimization of the reaction condition of lignin waste HTL was conducted. Then the effect of ZVMs (Zn, Fe, Ni and Co) on the lignin waste HTL was studied. Finally, different transition metal supported Al2O3 catalysts (Ni/Al2O3, Fe/Al2O3, Co/Al2O3 and Cu/Al2O3) were employed to further exploit the hydrogenation potential of Zn and Fe. A test with gaseous hydrogen and hydrogenation catalyst was also done for comparison. In our previous study Al2O3 supported nickel catalyst present high catalytic efficiency on the guaiacol hydrodeoxygenation, therefore Al2O3 was selected as the catalyst support for lignin HTL tests [18]. In fact, the peculiar textural properties of Al2O3 such as high surface area, moderate pore-size distribution, suitable acid/base characteristics and high hydrothermal stability make it an excellent catalyst support [19].
Section snippets
Materials
The lignin enriched solid waste was kindly supplied by a plant for second generation bio-ethanol production, the moisture content is 8.2 wt%. The elemental analysis result of this feedstock is shown in Table S1. Ni (purity >99.9%; particle size <7 μm), Co (purity >99.8%; particle size <2 μm), Fe (purity >99.5%; particle size <10 μm), and Zn (purity >99.9%; particle size <10 μm) were purchased from Sigma-Aldrich and used as received. γ-Al2O3 was purchased from Merck. Nickel (II) nitrate
XRD analysis
XRD spectra of metal supported Al2O3 catalysts are displayed in Fig. 1. Diffraction peaks corresponding to the Al2O3 support were seen at 37.44°, 39.67°, 42.82°, 45.79° and 67.30° in the spectra of supported catalysts with JCPDS No. 00-004-0880. This means that the structure of alumina support was maintained after metal loading. The diffraction peaks at 2θ = 43.3°, 50.44°, 74.12 are assigned to the cubic phase of copper (JCPDS No. 01-085-1326) in the Cu/Al2O3 sample. The peaks at 2θ values of
Conclusions
Hydrothermal liquefaction of lignin-enriched waste was conducted in the presence of ZVMs (Zn, Fe, Ni and Co) and metal-supported Al2O3 catalysts (Ni/Al2O3, Fe/Al2O3, Co/Al2O3 and Cu/Al2O3). First, blank tests were performed to optimize operative conditions and at 330 °C and 10 min the highest bio-crude yield was obtained. Prolonging the reaction time reduced the bio-crude yield due to the repolymerization of lignin degraded intermediates, but improved the oil quality. ZVMs enhanced the yield
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