Production of liquefied fuel from depolymerization of kraft lignin over a novel modified nickel/H-beta catalyst
Graphical abstract
Introduction
As environmental pollution from fossil energy intensifies, the conversion of biomass to renewable energy has attracted extensive attention in recent years (Kristianto et al., 2017, Liu et al., 2018, Luo et al., 2018). Lignocellulosic biomass is composed of three kinds of components (cellulose, hemicelluloses and lignin) (Shao et al., 2018). So far, cellulose and hemicelluloses have been utilized efficiently in many industrial processes while lignin is not widely used; only 2% of isolated lignins are used in commercial products (Das et al., 2017, Linger et al., 2014). However, lignin is abundant in nature. The pulp and paper industry generates 40–50 million tons per years, most of which is kraft lignin (Hu et al., 2018, Shao et al., 2017). Most of the kraft lignin is burnt as a low-value fuel in the chemical recovery process. While the chemical recovery is vital to the kraft process, about 10–15% of the kraft lignin can be isolated for higher value uses without adversely affect the chemical recovery process in a kraft pulp mill. Lignin is a rich source of aromatic compounds and can be converted into liquefied fuel (phenolic compounds) by depolymerization (Kusumoto and Nozaki, 2015, Long et al., 2014, Lu et al., 2016, Ma et al., 2018, Opris et al., 2016). Depolymerization is an important starting point for the sustainable utilization of kraft lignin (Chen et al., 2017).
In recent years, various approaches of lignin depolymerization have been reported, including mainly hydrolysis, oxidation, and reduction. Aqueous solutions of alkali metal hydroxides or carbonates have been used to hydrolyze ether bonds in lignin, which lead to phenol derivatives. However, lignin degradation using homogeneous catalyst suffers from tedious separation procedures (Long et al., 2015, Toledano et al., 2014). The oxidation reaction proceeds mainly by oxidative cleavage of C–H bonds and/or C–C bonds adjacent to C–O–C linkages, producing aromatic acids, aldehydes, phenols, etc. (Lancefield et al., 2015, Rahimi et al., 2014, Wang et al., 2016, Zhang et al., 2017a). These oxidation reactions usually cause oxidative damage to the aromatic structure of lignin, resulting in the formation of wasteful final oxidation products COX and H2O (Gaspar et al., 2007). Besides, the products obtained from oxidation reactions have high oxygen content and are not suitable for use as fuel. Reductive depolymerization has been considered as a promising method for the conversion of lignin to liquefied fuel. The ether bonds are selectively cleaved into low molecular weight phenolic compounds by hydrogenolysis methods (Laskar et al., 2014, Shu et al., 2016, Xiao et al., 2017). The reduction method can partially avoid condensation of intermediates into repolymerized products, a key issue of lignin depolymerization (Prasomsri et al., 2013, Saidi et al., 2014). However, the reduction method usually needs a high reaction temperature. Thus, the study of novel catalytic technologies of lignin depolymerization is very important.
In recent years, catalytic depolymerization of kraft lignin has been widely investigated. It is found that the conversion of lignin into liquefied fuel is mainly hampered by two challenges. The first challenge is the complex lignin structure that makes lignin difficult to depolymerize. The second challenge is the prevention of the condensation of lignin degradation intermediates, which are prone to produce stable repolymerized products. Overcoming these challenges is necessary to obtain high grade liquefied fuel from lignin. Hence, a variety of catalysts have been developed in the past years with noble metal catalysts being particularly noteworthy. Wang et al. (2017) simultaneously employed a dual acid-base catalyst S2O82−-KNO3/TiO2 and Ru/C as a mixture to investigate lignin depolymerization. In his work, the S2O82−-KNO3/TiO2 catalyst could efficiently degrade lignin and Ru/C could prevent the depolymerized products from re-polymerization. The combination of the two catalysts achieved high conversion of lignin to monomeric and dimeric products. Klein et al. (2016) used ZnCl2 and Pd/C as a synergic catalyst, taking advantage of the synergy of acid catalyzed depolymerization and hydrogenolysis to degrade lignin model compounds to monomers. These results indicate that the bond breakage and the stability of intermediate products play an important role in the process of lignin depolymerization. Nevertheless, it is hypothesized that instead of physical mixing of two different functional catalysts, better results may be obtained if the two active centers are properly spaced and close enough on the same support material, resulting in a potential synergistic effect between acidic sites and hydrogen binding sites. Inspired by the regularity and porosity of the zeolite skeleton and the ease of aluminum removal by acid, a part of the aluminum from the zeolite skeleton was replaced by adding hydrogenation metals into the vacancy, so that good synergy can be formed between the hydrogenation metal and the remaining aluminum (Song et al., 2017).
In addition to preparation method optimization, screening of active metals is also a key step in the preparation of catalysts that can efficiently depolymerize lignin. Compared with the aforementioned noble metal catalysts, non-noble metal catalysts are gaining more and more attention. Since the production of liquefied fuel is expected to be a large scale process, the use of noble metals-based catalysts will greatly increase the production costs (Zhang et al., 2017b). Thus, it is more reasonable to use non-noble metal-based catalyst for lignin to liquefied fuel conversion due to its lower cost and high efficiency. It is well known that nickel-based catalysts have shown excellent chemoselectivity for aromatic products (De et al., 2016, Gao et al., 2016, Wang et al., 2018, Xin et al., 2014, Zuliani et al., 2017). Based on multiple considerations, a novel bifunctional Ni/DeAl-beta catalyst was prepared for lignin depolymerization.
The novel catalyst was characterized by various instruments and the upgraded liquid fuel from lignin was analyzed by GC/MS and GC. The catalytic performance of Ni/DeAl-beta catalyst was mainly assessed by the yield of liquefied fuel, the yield of petroleum ether soluble product, and the higher heating values (HHV). The effects of nickel content, reaction temperature, and reaction time of the catalyst on the yield of products were investigated.
Section snippets
Chemicals
The lignin used in this experiment was Indulin AT™ that was purchased from Meadwestvaco (Shanghai, China). Nickel (II) acetate tetrahydrate and oxalic acid dihydrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Commercial H-beta (ID: NKF-6-25YY) with an nSiO2/nAl2O3 ratio of 25 was purchased from Nankai University Catalyst Co., Ltd (Tianjin, China). All organic solvents used in this experiment were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai,
Catalyst characterization
The results of XRD showed that both the dealumination-desilicication and the nickel incorporation processes did not appreciably affect beta zeolite topology. It has been reported that dealumination and metal incorporation processes are accompanied by lattice contraction/expansion of the zeolite framework, as indicated by a change of the position of (3 0 2) the narrow main diffraction peak around 2θ of 22.50° (Śrębowata et al., 2014). A significant XRD peak movement from 22.42° to 22.54° was
Conclusions
Ni/DeAl-beta is a powerful catalyst for lignin to liquefied fuel conversion. The synergistic effect of Ni and Al significantly improved the catalytic activity of Ni/DeAl-beta. A maximum yield of 64.5% of petroleum ether soluble product and the HHV increase from 24.9 MJ/kg to 32.0 MJ/kg were achieved after reaction for 36 h at 300 °C. The potential of widespread application of Ni/DeAl-beta catalyst in lignin depolymerization is strengthened by its high activity and low cost. More importantly, no
Acknowledgments
This work was supported by the Program of National Natural Science Foundation of China (51676178), the National Key Technology R&D Program of China (NO. 2015BAD15B06), Transformational Technologies for Clean Energy and Demonstration, Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDA 21060101. The authors appreciate the beamline BL07W in the National Synchrotron Radiation Laboratory (NSRL) for help in X-ray CT characterizations.
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