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- Lignin valorization for added-value chemicals: kraft lignin versus lignin fractionsPublication . Costa, Carina A.E.; Casimiro, Filipa M.; Rodrigues, Alírio; Vega-Aguilar, CarlosLignin is a raw material that can potentially be converted into valuable compounds through depolymerization reactions in addition to its use as a polymer or material. However, the chemical recalcitrance and the heterogeneous composition and structure of lignin make it challenging to establish processes that add value to this complex aromatic biopolymer. In this work, solvent fractionation was applied to obtain lignin fractions with a narrowed molecular weight and specific structural characteristics, improving its homogeneity and purity. A kraft lignin was submitted to fractionation using different ratios of acetone, ranging from 60 to 15% v/v, in aqueous mixtures. The composition, structure, and molecular weight of each fraction were studied and their potential applications were evaluated. The most water-soluble fraction has more phenolic OH, less aliphatic OH groups, and shows the lowest content of aryl-ether linkages, which is in accordance with its highest degree of condensation. On the other hand, the insoluble fraction from the mixture with 60% of acetone has the lowest molecular weight and the highest content of inorganic material. Radar plots were applied for lignin fractions evaluation and the fraction with the highest potential (IF 30:70) was submitted to alkaline oxidation with O-2. The results were compared with the products yielded from kraft lignin. An increase of about 13 and 19% was found for vanillin and syringaldehyde, respectively, when the fraction IF 30:70 was submitted to oxidation. In conclusion, the proposed fractionation process showed to be an effective method to obtain lignin fractions with specific composition and structural characteristics that could improve its potential as a source of high added-value monomeric phenolic compounds.
- Microwave-assisted lignin wet peroxide oxidation to C-4 dicarboxylic acidsPublication . Vega-Aguilar, Carlos; Costa, Carina A.E.; Barreiro, M.F.; Rodrigues, AlírioInnovative methodologies, such as microwaveassisted reaction, can help to valorize lignin with higher productivity and better energy efficiency. In this work, microwave heating was tested in the wet peroxide oxidation of three lignins (Indulin AT, Lignol, and Eucalyptus globulus lignins) as a novel methodology to obtain C4 dicarboxylic acids. The effect of temperature, time, and catalyst type (TS-1 or Fe-TS1) was evaluated in the production of these acids. The TS-1 catalyst improved succinic acid yield, achieving up to 9.4 wt % for Lignol lignin. Moreover, the microwave heating specifically enhanced Lignol conversion to malic acid (34 wt %), even without catalyst, showing to be an attractive path for the future valorization of organosolv lignins. Overall, compared to conventional heating, microwave heating originated a rapid lignin conversion. Nevertheless, for prolonged times, conventional heating led to better results for some target products, e.g., malic and succinic acids.
- Hardwood and softwood lignins from sulfite liquors: structural characterization and valorization through depolymerizationPublication . Casimiro, Filipa M.; Costa, Carina A.E.; Vega-Aguilar, Carlos; Rodrigues, AlírioThis work aims to evaluate the structural characteristics and study the oxidative depolymerization of lignins obtained from hardwood and softwood sulfite liquors. Lignins were obtained after ultrafiltration and freeze- drying of the sulfite liquors and characterized based on inorganic content, nitrobenzene oxidation, 13C NMR, and molecular weight determination. The structural characteristics achieved allow evaluating the potential of each lignin through oxidative depolymerization to produce added-value phenolic monomers. Hardwood and softwood lignins were submitted to alkaline oxidation with oxygen and the reaction conditions optimized to obtain a final oxidation mixture with the maximum yield of phenolic monomers. Through oxidation with O2, hardwood lignin generates mostly syringaldehyde while lignin from softwood biomass mainly produces vanillin; moreover, a lower reaction time and the interruption of O2 admission avoid the degradation of the oxidation products in the final mixture for both lignins, more evidenced to hardwood lignin due to its higher reactivity. From the results, it is possible to conclude that a phenolic aldehyde-rich oxidation mixture could be obtained, confirming the viability of lignin as raw material to produce added-value products as vanillin and syringaldehyde.
- Effect of methoxy substituents on wet peroxide oxidation of lignin and lignin model compounds: understanding the pathway to c4 dicarboxylic acidsPublication . Rodrigues, Alírio; Vega-Aguilar, Carlos; Barreiro, M.F.Lignin depolymerization through peroxide oxidation produces dicarboxylic acids (DCA), especially C4-DCA, like succinic acid. In this work, the effect of methoxy substituents on C4-DCA production using peroxide oxidation of lignin model compounds (p-hydroxybenzoic acid, vanillic acid, and syringic acid) and hardwood and softwood lignin samples was studied. It was concluded that methoxy substituents increased the reactivity toward peroxide oxidation. The succinic acid yield was higher for the model compounds with fewer methoxy groups, achieving 5.8 wt % of succinic acid for p-hydroxybenzoic acid. For Eucalyptus globulus kraft lignin (hardwood lignin with guaiacyl and syringyl units), an increased reactivity was verified, and more succinic acid (3.5 wt %) was produced in a shorter time, comparatively with Indulin AT lignin (softwood lignin, with only guaiacyl units), which produced 2.7 wt %. This evidence suggests that E. globulus kraft lignin might be a better raw material than Indulin AT for succinic acid production by peroxide oxidation.
- Lignin conversion into C4 dicarboxylic acids by catalytic wet peroxide oxidation using titanium silicalite-1Publication . Vega-Aguilar, Carlos Alberto; Barreiro, M.F.; Rodrigues, AlírioLignin valorisation towards added-value products has become a relevant topic to consolidate a future circular bioeconomy. In this context lignin oxidation to C4 dicarboxylic acids (C4-DCA) by catalytic wet peroxide oxidation is emerging as a value-added strategy, supported by the extensive use of these building blocks in several industrial fields. In this work, lignins from different sources and processes (Indulin AT, Lignol, alkali and E. globulus kraft lignins) were oxidised using H2O2 and titanium silicalite-1 catalyst (TS-1) under different operating conditions (temperature, pH, time, H2O2, and TS-1 load). Indulin AT was the lignin leading to the highest succinic acid yield (11.3 wt%), and TS-1 catalyst enhanced its production four times over the noncatalysed reaction. Malic acid was also produced at high yields, especially for Lignol lignin. The other lignins (E. globulus kraft, and alkali lignins) also produced these C4 acids but at lower yields. The catalyst remained stable at the used experimental conditions, and showed potential to be reused for several cycles without being deactivated. Overall, the catalytic conversion of lignin to C4-DCA can help to guide the pathway to renewable chemicals production.
- Catalytic wet peroxide oxidation of vanillic acid as a lignin model compound towards the renewable production of dicarboxylic acidsPublication . Vega-Aguilar, Carlos; Barreiro, M.F.; Rodrigues, AlíriotLignin can be depolymerised and used as a feedstock to obtain renewable raw-materials,providing a green alternative to fossil counterparts. Among others, C4dicarboxylic acids(DCA), like succinic, malic, maleic and fumaric acids, which can find applications in phar-maceuticals, food industry, and act as solvents, can be obtained from lignin oxidation. Toinvestigate their formation, the oxidation of vanillic acid (VA), a lignin model compound, wasstudied under catalytic wet peroxide oxidation (CWPO) conditions, using titanium silicalite-1 (TS-1) as the catalyst. The effect of temperature, pH, and reaction time were studied. Ina second phase, catalyst modification with transition metal oxides (Fe, Co, Cu) was tested.Results showed that oxidation under pH = 10.5 gives rise to complete VA conversion withhydroxylated DCA, namely malic (15 mol%) and tartaric (5 mol%) acids, as the main products.At pH = 4.0, the production of succinic acid was improved (7.4 mol%), with VA conversionachieving 78% after 2.0 h of reaction. At alkaline pH, H2O2reactivity is higher, leading to C4-DCA degradation to low-molecular weight compounds. Catalyst desilication was observed,pointed out for the convenience of using neutral and acidic pH. In acidic pH, Fe and Cu cat-alysts enhanced VA conversion, and Fe catalyst was more selective towards succinic acidproduction.
- Added-value chemicals from lignin oxidationPublication . Costa, Carina A.E.; Vega-Aguilar, Carlos; Rodrigues, AlírioLignin is the second most abundant component, next to cellulose, in lignocellulosic biomass. Large amounts of this polymer are produced annually in the pulp and paper industries as a coproduct from the cooking process-most of it burned as fuel for energy. Strategies regarding lignin valorization have attracted significant attention over the recent decades due to lignin’s aromatic structure. Oxidative depolymerization allows converting lignin into added-value compounds, as phenolic monomers and/or dicarboxylic acids, which could be an excellent alternative to aromatic petrochemicals. However, the major challenge is to enhance the reactivity and selectivity of the lignin structure towards depolymerization and prevent condensation reactions. This review includes a comprehensive overview of the main contributions of lignin valorization through oxidative depolymerization to produce added-value compounds (vanillin and syringaldehyde) that have been developed over the recent decades in the LSRE group. An evaluation of the valuable products obtained from oxidation in an alkaline medium with oxygen of lignins and liquors from different sources and delignification processes is also provided. A review of C4 dicarboxylic acids obtained from lignin oxidation is also included, emphasizing catalytic conversion by O2 or H2O2 oxidation.