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Microwave assisted depolymerization of alkaline lignin over hydrotalcite-based CuNiAl mixed oxides Minghao Zhou, Brajendra K. Sharma, Peng Liu, Haihong Xia, Junming Xu, and Jian-Chun Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01697 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018
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ACS Sustainable Chemistry & Engineering
Microwave assisted depolymerization of alkaline lignin over hydrotalcite-based CuNiAl mixed oxides Minghao Zhouab, Brajendra K Sharmab, Peng Liua, Haihong Xiaa, Junming Xua*, Jian-chun Jianga a. Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry (CAF), No.16, Suojin Five Village, Nanjing 210042, China b. Illinois Sustainable Technology Center, Prairie Research Institute, one Hazelwood Dr. Champaign, University of Illinois at Urbana-Champaign, IL 61820, USA * Corresponding Author: E-mail:
[email protected] Abstract In this study, microwave assisted depolymerization of alkaline lignin into bio-oil was investigated in presence of CuNiAl hydrotalcite based solid base catalysts in methanol. The effect of catalysts, reaction temperature, time, and heating methods were all studied in detail to improve the bio-oil yield. The promotion effect of microwave heating and Cu amount in the CuNiAl based catalysts was obviously observed during degradation of lignin, leading to improved bio-oil yield and lower molecular weight. The highest yield of bio-oil was up to 60.1%, when the depolymerization was conducted at 160 ℃ for 80 min over CuNiAl based catalyst (metal ratio of 1.5:4.5:2). The monomers and oligomers were identified by GC-MS and MALDI-TOF MS, finding p-hydroxyacetophenone (H2), guaiacol (G1), p-hydroxyacetovanillon (G5) and syringaldehyde (S3) as main monophenols in bio-oil. The possible chemical
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structures for oligomers with molecular weight of 288 m/z, 306 m/z, 316 m/z, 330 m/z, 412 m/z, 426 m/z, 456 m/z were obtained, and plausible depolymerization pathways were proposed in order to facilitate the understanding of possible relationship for the formation of prominent oligomers and monomers. This study confirmed that heterogeneous base catalysts coupled with microwave heating could provide a promising technique to convert lignin to liquid fuels. Keywords: lignin; microwave assisted depolymerization; bio-oil; MALDI-TOF MS
Introduction Lignin is the third major constituent of lignocellulosic biomass, which is an abundant aromatic biopolymer in nature, and regarded as a promising and sustainable source of aromatic chemicals
1-3
. The main phenylpropane units presented in lignin,
including hydroxyl cinnamyl, guaiacyl and syringyl units, make it a valuable resource for the production of either macromolecules (e.g. phenol-formaldehyde resins) or other high-value added chemicals (e.g. vanillin, guaiacol, etc)
4,5
. However, lignin is
usually left as residual waste and burnt to generate heat after the utilization of cellulose and hemicelluloses of biomass. Due to the large amount of lignin produced through the kraft pulping process 6, lignin valorization, aiming to develop practical technologies capable of depolymerization of lignin to high-value added chemicals has recently attracted increasing attention from the scientific community 6, 7. A wide range of chemical transformation pathways have been proposed for the conversion of lignin to useful chemicals, which can be generally based on three main approaches: oxidation 8, pyrolysis
9-11
and hydrotreating
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12-15
. However, those
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depolymerization processes are always achieved under relatively harsh reaction conditions (high temperature and/or high reaction pressure)
16-18
. Additionally, both
oxidation and hydrotreating processes mainly depend on homogeneous acidic or basic catalysts (e.g. H2SO4, NaOH), although some studies associated with heterogeneous catalysts have also been reported
15, 22, 23
. Hydrotreating process is always in need of
external hydrogen and recycling and/or purification of unreacted hydrogen gas, which would not only add to the cost for the conversion, but also lead to those low-valued chemicals 15, 24. In comparison with those conventional methods, microwave assisted depolymerization of lignin exhibits to be a promising and alternative method for 25-27
lignin utilization
. Microwave provides heat at molecular level to the reaction
system, which could prevent the formation of other undesired products
28
. The
interaction between microwave and the reaction media is usually based on the mechanism of dipole rotation and ionic conduction. Microwave absorption ability is an important factor, influencing the microwave effectiveness. Therefore, certain microwave receptors should be added to the lignin depolymerization system due to the low microwave absorption ability of lignin
29
. Polar solvent coupled with
microwave heating, had been successfully introduced to the lignin depolymerization in literatures
30, 31
, which was confirmed to be effective for the partial deconstruction
of lignin under mild reaction conditions using polar hydrogen-donor solvents over suitable catalysts, with the assistance of microwave heating. Microwave assisted solvolysis of lignin and other biomass have been widely
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investigated over suitable catalysts under relatively mild reaction conditions (
