Lignin conversion into C4 dicarboxylic acids by catalytic wet peroxide oxidation using titanium silicalite-1
Introduction
Lignin is obtained as a by-product in pulp mills, where it is burned in the recovery boiler to produce energy and heat. However, this step is usually considered a bottleneck in pulp production (Ahmad et al., 2020; Mathias, 1993). Consequently, using part of this residual lignin as a raw material for conversion to added-value products would improve the sustainability of the pulping industry without affecting pulp production and the energy balance of the mill (Junghans et al., 2020; Rodrigues et al., 2018; Silva et al., 2009).
The physical and chemical properties of technical lignins change depending on the feedstock source, the pulping process, and the isolation method (Kienberger et al., 2021). Lignin structure is based on three monomers: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), which are present in different ratios and linked differently depending on the biomass origin (Kamm et al., 2008; Upton and Kasko, 2016). Softwood lignin usually has a high G:H ratio and no S units. Units are linked by β-O-4 (43–50 %), 5−5’ (10–25 %), β-5 + α-O-4 (9–12 %) linkages, among others. Hardwood is an S:G:H lignin with a higher S content. Units are linked by β-O-4 (50–65 %), 5−5’ (4–10 %), α-O-5 (4–8 %), and α-O-5′ (6–8 %) linkages, among others (Rodrigues Pinto et al., 2011). These structural differences introduce challenges for the depolymerisation and upgrading of the recalcitrant lignin matrix. Cooking methods focused on high-quality cellulose, such as kraft, sulfite, and alkaline processes, use harsh conditions to achieve their objective, causing strong modifications in the lignin structure (Sun et al., 2018). The alkali process cleaves many of the ester and ether bonds between lignin and carbohydrates, and also some of the lignin C-C bonds (Kim et al., 2016). Kraft process produces a highly modified and partially fragmented lignin, with α-aryl ether and β-aryl ether linkages cleavage, recondensation reactions, and sulphur incorporation (Li et al., 2015; Sun et al., 2018). Other milder extraction processes, such as ionic liquid extraction, organosolv, and milled-wood lignin, cause fewer modifications to the lignin structure. Organosolv uses organic solvents, such as primary alcohols, alone or mixed with water at high temperatures and pressures, and produces lignins with structures closer to the native one, with slight modifications like cleavage of β-O-4 linkages, repolymerizations, and formation of new C-C bonds (Sun et al., 2018).
Depolymerisation is an important route to achieve new products from the lignin. Oxidative lignin depolymerisation has several advantages, leading to the incorporation of new functional groups (e.g., alcohol, aldehydes, and carboxylic groups) and production of low-molecular-weight fractions, giving rise to added-value products such as phenolics, dicarboxylic acids, and others. However, it remains challenging to overcome the low selectivity towards target products and circumvent the over-oxidation to gaseous compounds (Ahmad et al., 2020; Liu et al., 2019). C4 dicarboxylic acids, currently derived from petrochemical sources and carbohydrates fermentation routes, are used in several fields as final products (e.g. food additives) and as raw materials for pharmaceuticals and polymer industries (Zhang et al., 2020). They are also pointed out as the biomass-derived building blocks for the future development of a greener chemistry (Jong et al., 2011; Werpy and Petersen, 2004). Therefore, achieving C4 dicarboxylic acids from lignin would allow the attainment of added-value products without competing with food and fossil resources.
Non-catalytic lignin oxidative depolymerisation has been previously reported with succinic and maleic acids yields lower than 3% (Abdelaziz et al., 2019; Demesa et al., 2015; Figueirêdo et al., 2019; Hasegawa et al., 2011; Vega-Aguilar et al., 2021). Better results were obtained with catalytic oxidation, especially in the presence of heterogeneous catalysts, such as perovskite-type oxides, chalcopyrite, and heteropoly acids (i.e., phosphotungstic and phosphomolybdic acids) (Ansaloni et al., 2017; Bi et al., 2018; Cronin et al., 2017; Demesa et al., 2017; Ma et al., 2014). The use of Fenton reagent (Fe2+/H2O2) gave rise up to 8% maleic acid with phenol (Faisal, 2009), although with other lignin model compounds, the yield was lower than 2% (Kang et al., 2019). When lignin was used, no C4-DCA were obtained (Zeng et al., 2015), suggesting that this oxidation methodology is not an efficient strategy for lignin depolymerisation towards C4-DCA. Perovskite-type oxides have also been tested due to their oxidative activity and the presence in their structure of transition metals with two different oxidation states. However, catalysts like LaFeO3, LaMnO3, and CeFeO3 showed no improvement for C4-DCA production comparatively with non-catalysed experiments (Ansaloni et al., 2017). Chalcopyrite (CuFeS2) has been used with lignin and lignin model compounds with good results. Namely, catechol oxidation yielded 6% malic acid and 8% succinic acid, and diluted-acid corn stove lignin oxidation yielded 7% succinic acid and 1% malic acid (Ma et al., 2014). Chalcopyrite nanoparticles in acidic pH enhanced succinic acid yield up to 12 % when an industrial lignin was oxidised, accompanied by low fumaric and maleic acids yields (Bi et al., 2018), confirming that acid yields are dependent on the used oxidation reaction conditions. Heteropoly acids (i.e., phosphotungstic acid (H3PW12O40) and phosphomolybdic acid (H3PMo12O40)) showed low improvement for succinic acid yield, with disadvantages such as higher costs and recovering difficulties (Demesa et al., 2017). Other catalysts based on V and Mo oxides or pyrophosphates (deposited in materials like Al2O3, TiO2, and HZSM-5), used in lignin gas-phase oxidation gave rise to no more than 2% of succinic acid (Lotfi et al., 2016, 2015). In most of these studies, significant amounts of oxalic, formic, and acetic acids were obtained (as degradation products of lignin over-oxidation). Still, the final yield and C4-DCA type varied according to the used catalyst and applied reaction conditions. In most of these works, succinic acid yield varied between trace levels up to 2−3 wt% (lignin-basis), while maleic and fumaric acids were reported to be obtained at contents less than 1%.
Different oxidants, such as O2, H2O2, O3, and peroxy acids, have enough oxidative power to break down most ether linkages and some C-C linkages (Ma et al., 2018a). However, a strong oxidant is needed to cause ring-opening reactions to achieve dicarboxylic acids (Sun and Argyropoulos, 1996). Molecular oxygen oxidation occurs only in alkaline conditions and is usually intended for mild oxidation to achieve aldehydes. Its oxidative power is relatively low to origin ring-opening reactions at high yields (Cabral Almada et al., 2020; Li et al., 2015). Peroxide oxidation is more active than O2 oxidation by eliminating liquid/gas mass transfer barriers and releasing free radicals, allowing faster oxidations in milder and cleaner conditions (Bhargava et al., 2006; Cheng et al., 2017). Hydrogen peroxide oxidation causes the cleavage of β-O-4 linkages, forming non-condensed structures, especially in softwood lignins. In hardwood lignins, demethoxylation of S units to G units happens, which could be repolymerised, increasing the condensed OH structures (Ahmad et al., 2020). These small fragments can further have the aromatic ring cleaved to dicarboxylic acids. Ozone can also be used for lignin oxidation, leading to ring-opening reactions and generating C4-DCA (Figueirêdo et al., 2019). However, O3 handling can be complex since it has to be generated in situ (Ahmad et al., 2020). Recently, electrochemical depolymerisation was used for lignin oxidation, releasing O2 and H2O2 in situ; however, this technique leads to small amounts of C4-DCA (Di Marino et al., 2019).
Among the catalysts enhancing H2O2 oxidative power, titanium silicalite-1 catalyst (TS-1) has been used for lignin model compounds peroxide oxidation to C4-DCA with good results. Namely, it was reported that guaiacol is oxidised mainly to maleic acid, while vanillic acid is primarily converted to succinic acid, an important feedstock for renewable polymer production (Su et al., 2014; Vega-Aguilar et al., 2020). This catalyst has an MFI zeolite structure, with less than 3% TiO2 and hydrophobic properties, allowing the oxidation of non-polar compounds in aqueous medium, such as phenolic compounds (Clerici, 2015; Gamba et al., 2009). The active species in the TS-1 catalysed peroxide oxidation is the Ti−OOH group, which shows better reactivity than H2O2 (Clerici, 2015).
Under the current state of the art, the catalytic peroxide oxidation of lignin to C4-DCA using TS-1 catalyst is still lacking. Given the reported advantages of this catalyst towards the production of C4-DCA, namely the previous studies conducted with model compounds, its use to improve the production yield of target acids (e.g., succinic acid) from lignin is a promising strategy towards a circular economy. Therefore, in this work, lignins from different origins (Indulin AT, Lignol, alkali, and E. globulus kraft lignins) were oxidised using H2O2 and TS-1 catalyst focusing C4-DCA production, especially succinic acid. Moreover, the stability of the TS-1 catalyst to the applied reaction conditions, taking as reference five consecutive utilizations, was also be studied.
Section snippets
Materials
All chemical reagents were purchased from commercial sources and used without further purification: formic acid (Chem-labs, >99 %), sulfuric acid (Chem-labs, 95–97 % p.a.), sodium hydroxide (Merck, p.a.), hydrogen peroxide solution (Fluka, >30 % p.a.), N,N-dimethylformamide (VWR, ≥99.9 %) and lithium chloride (VWR, AnalaR NORMAPUR). Catalyst TS-1 (ref. #: MSTS1001, lot number: 130,117; H+ cation) was acquired from ACS Materials, LLC.
Four different lignins were studied: alkali lignin (ALK),
Lignin characterisation
The studied lignins correspond to products available from commercial processes. Alkali (ALK), Indulin AT (IAT), and ethanol organosolv lignin (EOL) were received in powder form, and no further purification or treatments were done. Regarding E. globulus kraft lignin (EKL), received as black liquor, it was isolated by the acid precipitation method (Costa et al., 2018).
The selected lignins are products of different pulping processes and proceed from distinct biomass feedstocks. Lignins IAT and EKL
Conclusions
Lignin catalytic peroxide oxidation using TS-1 catalyst proved that C4-DCA could be obtained at attractive yields. Succinic and malic acids were the main products, while maleic, fumaric, and tartaric acids were present in small amounts. Compared to the alkali lignin, kraft and organosolv lignins lead to higher conversion towards C4-DCA. Succinic acid, a valuable product for bio-based polymers, was obtained at higher yields for the catalysed reaction than the non-catalysed one, being Indulin AT
Funding sources
This work was financially supported by: Base Funding - UIDB/50,020/2020 of the Associate Laboratory LSRE-LCM - funded by national funds through FCT/MCTES (PIDDAC); Base Funding - UIDB/00,690/2020 of CIMO - Centro de Investigação de Montanha—funded by national funds through FCT/MCTES (PIDDAC). COST Action LignoCOST (CA17128). Costa Rica Science, Technology and Telecommunications Ministry for the PhD. Scholarship MICITT-PINN-CON-2-1-4-17-1-002.
CRediT authorship contribution statement
Carlos A. Vega-Aguilar: Conceptualization, Methodology, Investigation, Formal analysis, Data curation, Writing - original draft. M. Filomena Barreiro: Conceptualization, Resources, Writing - review & editing, Supervision. Alírio E. Rodrigues: Conceptualization, Resources, Writing - review & editing, Supervision.
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
This work was financially supported by: Base Funding - UIDB/50020/2020 of the Associate Laboratory LSRE-LCM - funded by national funds through FCT/MCTES (PIDDAC); Base Funding - UIDB/00690/2020 of CIMO - Centro de Investigação de Montanha—funded by national funds through FCT/MCTES (PIDDAC). COST Action LignoCOST (CA17128). Carlos Vega-Aguilar thanks the Costa Rica Science, Technology and Telecommunications Ministry for the PhD. Scholarship MICITT-PINN-CON-2-1-4-17-1-002. The authors thank Dr.
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