Directional and integrated conversion of whole components in biomass for levulinates and phenolics with biphasic system
Graphical abstract
Introduction
Biomass is the largest sustainable carbon resources in nature that can be converted into bio-fuels and fine chemicals (Ennaert et al., 2016, Zhang et al., 2017). Waste biomass resources such as crop straws, forestry logging residues and furniture manufacturing waste are enriched and underutilized. As the growing concerns on protect environment and the increasing requirement for renewable chemicals, conversion of waste biomass to bio-chemicals has attracted multiple focus (Li et al., 2016, Gaudino et al., 2019, Cai et al., 2020). Efficient transforming the biomass to liquid products could obviously increase the density of energy and partially replace fossil fuels. The liquid products was considered as the sustainable energy carrier and renewable bio-fuel (Pang, 2019). However, as the intricacy of cell wall structure, the intrinsic heterogeneity and highly complex molecular structure of three components in biomass cannot be overcome (Zhao et al., 2019), the liquid products have complex components and unstable physicochemical properties. Therefore, it is still a challenge to obtain high-quality and low-cost chemicals from the simultaneous conversion of whole components in biomass.
In order to achieve efficient and simultaneous conversion of the three components in biomass, the connection bonds between the components in the cell wall and the crystal structure of cellulose must be destroyed. Thermochemical conversion such as hydrolysis or alcoholysis of waste biomass for fine chemicals with organic solvents or water under moderate condition has become a research hotspot for recent biomass utilization (Zhu et al., 2017, Gollakota et al., 2018). These conversion process can synthesize high yields of products (75 ~ 85%) with some improved levels and grades as low viscosity and acidity. Process development research on transformation of biomass to biofuels and chemicals with co-solvent system has been widely investigated (Mellmer et al., 2018). These systems take advantage of the differences in solubility of materials and products, typically contribute to higher product yields than with pure solvent system (Tyagi et al., 2019, Zang et al., 2020). Usually, two or several kinds of solvents such as water, alcohols, γ-valerolactone (GVL), and dioxane are used to form a composite solvent with specific functions to degrade the cellulose, hemicellulose, and lignin of biomass into levulinic acid (ester), furfurals, phenolics, etc. chemicals.
Catalytic deep eutectic solvents or ionic liquids can efficiently convert cellulose, hemicellulose and lignin into high added-value chemicals with the advantages of low cost, toxicity and biodegradability (Cheng et al., 2019, Kohli et al., 2020, Liu et al., 2020a, Liu et al., 2020b). However, there are problems such as unstable product properties and complex separation processes. Relative to the yield of bio-crude product, the efficiency of biomass liquefaction in the three media systems abided by the following sequence: water/ethanol co-solvent » pure water > pure ethanol regardless of the feedstocks. The products transformed from cellulose mainly included esters, furfurals and furfurals derivatives, and the products derived from lignin mainly involved aromatic compounds (Feng et al., 2018). The lignocellulosic residues were converted into levulinic acid more than 60 mol % in the biochemicals under polar aprotic GVL/H2O media systems (Liu et al., 2020a, Liu et al., 2020b). Cellulose in biomass was directly dehydrated by glucose and L-glucan (LGA) to form 5-hydroxymethylfurfural (5-HMF) when water/tetrahydrofuran (THF) was used as the reaction medium, while in pure THF, cellulose is mainly converted to L-glucosone by LGA dehydration. It was also found that the hydrolysis conversion process of the compound in H2O/GVL, H2O/cyclopentyl methyl ether, H2O/DMSO and other composite solvents showed that the yield of 5-HMF in water/THF was relatively high, and the yield of target products increases as the acidity of the catalyst increases. Xu et al have studied the preparation of furfural compounds from wheat straw using CrPO4 catalyst in water/THF co-solvent system, 32% 5-hydroxy-methylfurfural and 67% furfural were simultaneously achieved from cellulose and hemicellulose in wheat straw (Xu et al., 2018). Chen et al studied the synchronous lignocellulose conversion and fractionation with effective biphasic solvent system consisted of an aqueous methyl isobutyl ketone/choline chloride. Under optimal conditions (0.6 wt% H2SO4, 170 °C, 60 min), the yields of xylan and furfural were 96% and 84.04% from switchgrass, alternative extract lignin with a 93.1% high purity (Chen et al., 2019). These results indicated that different co-solvents and acidic catalysts could affect the yield of the target products. And the solvent effect produced by a suitable composite solvent, may affect the reaction rate of process, the composition and properties of liquid product (Walker et al., 2018). Therefore, in the process of comprehensive biomass degradation and transformation, solvents with specific chemical properties can be selected to form co-solvent system. And its regulation effect on the conversion of three components, the reactivity of reactants and the solubility of products should be investigated in detail.
Directional liquefied conversion of waste biomass is one of the important ways to achieve effective utilization and produce structurally determined chemicals (Xu et al., 2016), such as levulinic ester and small molecule phenolics, which were widely used in medicine, food, cosmetics, liquid fuel and fuel additives. The active carboxyl, carbonyl and hydroxyl groups in the structure of levulinic ester and phenolics endow them with good reactivity and functional application prospects (Mika et al., 2018). Levulinates as high value-added chemicals were applied as fuel additives with non-corrosive nature and good stability, and can also be used as a platform compound to prepare GVL, 1,4-pentanediol and other fine products (Zhang et al., 2019). Many researches have been made to replace the petroleum-based phenolic compounds in phenolic resin foams of cheap phenolic compounds derived from renewable biomass (Banu et al., 2019). Meanwhile, phenolic substances were applied to antibacterial agents and antioxidants, and can be converted into fuels such as liquid alkanes withe hydrodeoxygenation, or various functionalized phenolic resin (Ji et al., 2018). Therefore, levulinates and phenolic compounds can be used as high value-added chemicals in liquid fuels with upgrading process. Efficient use of these chemicals from biomass can alleviate problems such as the greenhouse effect and environmental pollution.
Taking into account all the above, an integrated and simple process that incorporates directional conversion of whole components in waste biomass for producing two renewable platform compounds with biphasic system has been layout. The influences of co-solvent systems coupling different solvent were assessed and investigated for optimizing the conversion of biomass and maximizing the yield of target levulinates and phenolics. The METH/DMM biphasic cosolvent has an initiative synergic effect for the directional and synchronous conversion of three components in biomass. This method means that target phenolics and levulinates produced from lignin and holocellulose in biomass can be gradually separation, and realizes the comprehensive utilization of liquefied products in different synthetic directions according to their analogous molecular structure and chemical solubility, respectively.
Section snippets
Materials
In this study, waste biomass rice straw was collected from Sichuan (China), mechanically crushed and passed via a 100 mesh (150 μm) sieve. Then the feedstocks were dried at 105 °C overnight for an absolute dried weight and kept in a sealed bag until used. Chemical reagents such as alcohols, catalysts, etc. were analytical pure grade and used without further modification in the experiment.
Analysis ways
The chemical structures of liquefied liquid products were determined by GC–MS (Agilent 5975C VL MSD). The
Results and discussion
The influences of different reaction parameters including different types of catalyst, composition and ratio of co-solvent systems, catalyst amounts, holding time and temperature were studied to evaluate the effect of process variables on the yields of levulinates and phenolics, optimize the conversion of straw at suitable condition. As a reaction solvent, low alcohols can dissolve the products of biomass degradation, thereby inhibiting the polymerization of the products. The –OH as the
Conclusion
Directional and integrated conversion with biphasic system were studied for producing levulinates and DL from straw with suitable conditions (straw 4 g, METH/DMM 25/15 g, A15 0.002 mol, 120 min, 200 °C). Levulinates were transformed from simultaneous conversion of hemicellulose and cellulose in biphasic co-solvent, phenolics was produced from efficient depolymerization of lignin. Absolute-content of extracted levulinates was 87.5 wt% and properties meet the requirements of fuel additives. DL
CRediT authorship contribution statement
Junfeng Feng: Data curation, Conceptualization, Writing - original draft. Le Tong: Investigation. Changyue Ma: Validation. Yangyang Xu: Methodology. Jianchun Jiang: Visualization, Supervision. Zhongzhi Yang: Software. Hui Pan: Writing - review & editing.
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.
Acknowledgments
The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (31530010) and Research Grant of Jiangsu Province Biomass Energy and Materials Laboratory (JSBEM201903) for this investigation. The authors also thank the Nanjing Forestry University scientific research start-up funds (GXL2018037) for this investigation.
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