Jul 17, 2023
Simulações moleculares multiescala para solvatação de lignina em líquidos iônicos
Scientific Reports volume 13, Artigo número: 271 (2023) Citar este artigo 2202 Acessos 5 Citações 3 Detalhes da Altmetric Metrics A lignina, o segundo biopolímero mais abundante encontrado na natureza, tem
Scientific Reports volume 13, Artigo número: 271 (2023) Citar este artigo
2202 Acessos
5 citações
3 Altmétrico
Detalhes das métricas
A lignina, o segundo biopolímero mais abundante encontrado na natureza, emergiu como uma fonte potencial de combustíveis, produtos químicos e materiais sustentáveis. Encontrar solventes adequados, bem como tecnologias para dissolução e despolimerização eficientes e acessíveis da lignina, são grandes obstáculos na conversão da lignina em produtos de valor agregado. Certos líquidos iônicos (ILs) são capazes de dissolver e despolimerizar a lignina, mas projetar e desenvolver um IL eficaz para a dissolução da lignina permanece bastante desafiador. Para resolver esse problema, o modelo COnductor-like Screening MOdel for Real Solvents (COSMO-RS) foi usado para rastrear 5670 ILs calculando coeficientes de atividade logarítmica (ln (γ)) e entalpias de excesso (HE) de lignina, respectivamente. Com base nas propriedades termodinâmicas computadas do COSMO-RS (ln(γ) e HE) da lignina, prevê-se que ânions como acetato, carbonato de metila, octanoato, glicinato, alaninato e lisinato em combinação com cátions como tetraalquilamônio, tetraalquilfosfônio e piridínio ser solventes adequados para dissolução da lignina. As propriedades de dissolução, como energia de interação entre ânion e cátion, viscosidade, parâmetros de solubilidade de Hansen, constantes de dissociação e parâmetros Kamlet-Taft de ILs selecionados foram avaliadas para avaliar sua propensão à dissolução da lignina. Além disso, simulações de dinâmica molecular (MD) foram realizadas para compreender as propriedades estruturais e dinâmicas de ILs e misturas de lignina à base de tetrabutilamônio [TBA]+ e para esclarecer os mecanismos envolvidos na dissolução da lignina. Os resultados da simulação MD sugeriram que os ILs baseados em [TBA]+ têm o potencial de dissolver a lignina devido à sua maior probabilidade de contato e energias de interação com a lignina quando comparados ao lisinato de colínio.
Na corrida pela energia sustentável, estima-se que a biomassa lignocelulósica será capaz de fornecer 20% da procura mundial de energia até 20501. Um total de 170 mil milhões de toneladas métricas de biomassa lignocelulósica são produzidos todos os anos em todo o mundo2. A fim de concretizar todo o potencial da biomassa lignocelulósica, a lignina (cerca de 20-30% em peso da composição inicial da biomassa) ainda precisa ser utilizada de forma eficiente em escala industrial . A dissolução e despolimerização da lignina durante a desconstrução da biomassa lignocelulósica é uma etapa crítica na produção de biocombustíveis e bioprodutos à base de lignina6,7,8. A lignina não apenas previne a degradação enzimática da biomassa9,10, mas para tornar as biorrefinarias economicamente viáveis, ela deve ser usada para produzir bioprodutos com valor agregado11. Após a remoção da lignina, a celulose e a hemicelulose podem ser posteriormente transformadas em precursores de biocombustíveis12,13,14, enquanto a lignina dissolvida pode servir como um valioso precursor para fibras de carbono15, colóides16, termoplásticos17,18, produtos químicos aromáticos19, nanomateriais à base de lignina4,20, adesivos comerciais21 e mais22.
A espinha dorsal aromática da lignina é composta por três unidades fenilpropanóides principais, p-hidroxifenol (H), guaiacil (G) e siringil (S)7,23. Esses monômeros fenilpropanóides são ligados entre si por éter (C – O – C: β‒O‒4, α‒O‒4 e 4‒O‒5) e C – C (β‒β, β‒5, β‒1 e 5‒5) ligações formadas durante a biossíntese23,24,25. A lignina é altamente resistente à degradação enzimática devido à sua heterogeneidade, hidrofobicidade e estrutura reticulada26. Além disso, a lignina contém um número abundante de ligações de hidrogênio intramoleculares e interações π-π entre os anéis aromáticos rígidos, resultando em uma rede tridimensional compacta de ligações H, aumentando sua recalcitrância à despolimerização enzimática. A lignina também é muito insolúvel em água e nos solventes orgânicos mais comuns, limitando assim seu uso potencial em aplicações químicas de alto valor. Como resultado, as biorrefinarias sustentáveis requerem um solvente que seja capaz de remover a lignina e tornar efetivamente a biomassa passível de desconstrução em fragmentos atualizáveis.
0) are situated at the top portion and right side of Figs. 2 and 3. The anions such as acetate, methyl carbonate, octanoate, glycinate, alaninate, and lysinate are predicted to dissolve lignin more efficiently when in combination with cations like tetraalkylammonium, tetraalkylphosphonium, and pyridinium. This is due to the fact that effective ILs form strong H-bonds, C‒H…π, and cation‒π interactions with lignin. However, the anions such as triflate, gentisate, histidinate, and bis(trifluoromethylsulfonyl)imide have high positive values of ln(γ). The ln(γ) and HE values of lignin in 5670 ILs are provided in Tables S3 and S4. As the alkyl chain length of anion or cation increases, the ln(γ) and HE of lignin were seen to be decreasing. For example, in a comparison between the imidazolium-based cations ([12Dmim]+, [Amim]+, [Emim]+, [Bmim]+, [Hmim]+, and [Omim]+) with all 90 investigated anions, the ln(γ) of lignin decreased (i.e., more negative) as alkyl chain length of the cation increased from [Emim]+ to [Omim]+ (Fig. S3a). A similar observation was made in the case of anions (Fig. S3b). A contrary observation was reported in the literature, where Wang et al.38 reported that the solubility of lignin decreases with increase in the alkyl chain length of cations. This discrepancy may be due to the viscosity of ionic liquids. As the alkyl chain length of cation increases, the viscosity of IL also increased68. This higher viscosity of IL restricts the mass transfer rate of liquids which results in the lowering of lignin solubility. A similar observation was also noticed in our previous studies63, where spermidine and spermine showed stronger interaction with lignin, but results in lower biomass delignification due to their higher viscosity. Therefore, the combination of longer alkyl chain length of both the cation and anion resulted in weaker solvent for lignin solvation as compared to that of the combination of a highly polar and less polar ions. Based on these observations, the ions of IL should obey the following successive criteria: (1) either of the ions should be a good hydrogen bond acceptor or donor, and (b) another ion to be slightly polar (to weakly coordinate with counter ion thereby reduces the cross interactions between anion and cation). According to this thumb rule, the cations such as tetraalkylammonium, tetraalkylphosphonium, and alkylpyridinium are less polar, and the anions such as acetate, methyl carbonate, glycinate, alaninate, and lysinate are highly polar in nature. The interaction between the polar anion and the lignin is energetically much stronger than the interaction between anion and cation, resulting in a high lignin solvation capability./p> + 0.01 e/Å2), H-bond acceptor (red: σ < − 0.01 e/Å2), and H-bond donor (blue: σ > + 0.01 e/Å2) regions (see Fig. 4). The sigma potential, μ(σ), of lignin is negative in both the negative and positive charge density regions (σ < − 0.01 e/Å2 and σ > + 0.01 e/Å2), indicating that lignin has a tendency to interact with both negative and positive polar surfaces of molecule (i.e., H-bond donors and acceptors in the solvent) (Fig. 4a). On the negative screening charge densities side (σ > − 0.01 e/Å2), the σ-potential value of anions is negative (− 1.5 to − 2.4 kcal/mol Å2), implying that anions have more affinity to interact with the positive surface charge density of lignin or cation (i.e., H-bond donors) (Fig. 4a). In contrast, the σ-potential value of anions is positive (0.5–1.0 kcal/mol Å2) in the region of positive screening charge densities (σ > + 0.01 e/Å2), which reflects that the anions lack electron donor surfaces. For cations such as [Ch]+ and [Emim]+, the μ(σ) value is negative in the positive screening charge density side (σ > + 0.01 e/Å2), indicating that [Ch]+ and [Emim]+ cations have higher affinity toward negatively charged surfaces of lignin and anions (Fig. 4). For system composed of lignin and [Ch]+ or [Emim]+-based ILs, the anions are more likely to interact with the cations than lignin since [Ch]+ and [Emim]+ are polar cations with higher tendency to interact with negatively charged surfaces thereby forming stronger electrostatic and hydrogen bondings. Therefore, the logarithmic activity coefficient and excess enthalpy of lignin in such ILs are weaker. However, in the case of [TBA]+, [TBP]+, and [DPrPyrr]+, the cations lack both positive and negative surfaces (positive μ(σ) value), implying these cations have less tendency to interact with anions i.e., the interaction between anion and cation is weaker. Thus, both the anions and cations have higher odds of interacting with lignin, thereby having a stronger interaction with lignin. Anions interacting through electrostatic and H-bonding interactions with lignin and the [TBA]+, [TBP]+, and [DPrPyrr]+ cations interact with lignin through vdW and cation-π interactions./p> 1, the affinity between the IL and lignin is weaker and leads to lower biomass delignification. From the RED point of view, [TBA]+, [TBP]+, and [DprPyrr]+-based ionic liquids had lower RED values of lignin than [Ch]+ and [Emim]+-based ILs. The ILs based on [Ch]+ and [Emim]+ had higher polar and hydrogen-bonded contributions, which is due to the stronger polarity and hydrogen-bonding capability of [Ch]+ and [Emim]+-based ionic liquids. The larger RED values of [Ch]+ and [Emim]+-based ILs are due to the higher polarity and hydrogen bonding contributions, which further leads to stronger interactions between anion and cation of the IL. The stronger anion and cation interactions (Fig. S5) result in higher viscosity (Fig. 5) and weaker solvents for lignin dissolution. The RED ranking of anions with [TBA]+, [TBP]+, and [DprPyrr]+-based cations for lignin dissolution was as follows: [Ala]− > [Ace]− > [Gly]− > [Lys]− > [Val]−./p> [TBA][Val] > [TBP][Ace] > [TBP][Lys] > [Ch][Lys]./p> 14 Å), lignin adopted an extended polymer-like structure, while if the Rg value of lignin is lower than 13 Å, lignin forms collapsed compact conformation which was generally obtained in the water environment86. Representative snapshots of lignin (i.e., coil-like structure) from the simulations in ILs were shown in Fig. S8./p> 0.334, reflective a good solvent in polymer theory88. On the other hand, SASA for a given lignin polymer is largely determined by its molecular weight and variation within SASA values within a trajectory is comparatively smaller than it was for Rg. For a sphere of uniform density, the SASA should increase with the M2/3sphere. The scaling parameter ν > 0.67 (i.e., 0.94), indicating that there is an excess surface area for real lignin polymers relative to an ideal spherical globule87,88. Mass-fractal dimensions α, closer to 1 (i.e., below 2) are “good” solvents while fractal dimensions near 3 or above are “bad/poor” solvents. From the literature, in water, the α value of lignin is greater than 3 and thus water is a poor solvent for lignin dissolution87,89./p>
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