Mechanistic Insights into Formaldehyde-Blocked Lignin Condensation

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Mechanistic Insight into FormaldehydeBlocked Lignin Condensation: a DFT Study Xueli Mu, Zhe Han, Chengbu Liu, and Dongju Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00247 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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The Journal of Physical Chemistry

Mechanistic Insight into Formaldehyde-Blocked Lignin Condensation: a DFT Study

Xueli Mu,a, b Hanzhe,c Chengbu Liu,a and Dongju Zhang*a

aKey

Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of

Theoretical Chemistry, Shandong University, Jinan 250100, P. R. China bDepartment

of Chemistry and Chemical Engineering, Jining University, Qufu, 273155, Shandong, China

cEnvironmental

Engineering Materials, Advanced Materials Institute, Shandong Academy of Sciences, Jinan 250014, China

Corresponding author: E-mail: [email protected]

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTEACT: The formaldehyde (FA)-prevented lignin condensation reported by Luterbacher et al. [Science 2015, 354 (6310), 329–333] provides an efficient strategy for obtaining lignin monomer with near theoretical production. Our work presents a DFT calculation study that reveals the intrinsic mechanism why FA can efficiently hinder lignin condensation. Using veratrylglycerol-β-guaiacyl ether (VG) as a lignin model compound, we found that the formation of C–C linkage between two VG units is highly exothermic (–43.7 kcal/mol) and involves a medium-heighted barrier (20.1 kcal/mol), explaining the extensive and irreversible lignin condensation observed during lignin depolymerization. In the situation with FA, VG prefers to react with FA with much lower barrier (13.1~20.7 kcal/mol) to form 1,3-dioxane structures, which protect the reactive sites of VG (1,3-diols groups and the ortho/para positions to methoxyl groups on lignin aromatic rings) and thereby hinder lignin condensation. The theoretical results provide a clear picture of how FA hinders lignin condensation.

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1. INTRODUCTION Lignin, as an important component of lignocellulosic biomass (15~30% by weight, 40% by energy), is an abundant nature renewable carbon-containing polymer.1-3 It is a three-dimensional irregular phenolic (aromatic) biopolymer consisting of diversely bonded “hydroxyl-” and “methoxy-” substituted phenylpropane units.4,5 Lignin-derived phenolic monomers have significant potential as useful renewable aromatic building blocks on which the petrochemical industry so heavily depends.6-9 In recent years, lignin depolymerization has received great attention to obtain various value-added lignin-derived phenolic chemicals.10-17 Utile now, there are mainly two tactics for lignin depolymerization. One is the direct hydrogenolysis of native lignin, obtaining 45~55 mol% of lignin-derived phenolic monomers via cleaving ether bonds with near theoretical yields. But this tactic suffers from catalyst recovery and mass transfer problems, which limits the large-scale application of lignin.18 In contract, another tactic, hydrogenolysis of extracted lignin, can avoid these problems. Unfortunately, this tactic usually requires either harsh reaction conditions, such as high temperature and pressure (>200°C and >5 MPa), strong acidic environments, transition metal catalysts, or special starting materials, such as cellulolytic enzyme lignin.19 In particular, under high temperature and/or strong acidic conditions, lignin suffers from extensive and irreversible re-polymerization, reducing the monomer yields dramatically. The condensation of lignin involves the formation of a benzylic carbocation intermediate which subsequently electrophilically attacks the carbon atom of an electron-rich aromatic ring to form a strong and recalcitrant interunit carbon-carbon (C–C) linkage.11,12 Therefore, hindering the formation of interunit C–C linkages during lignin extraction process is crucial to depolymerize lignin efficiently.

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Recently, Luterbacher and co-authors20 developed an efficient formaldehyde-stabilized lignin condensation method. It was found that under acidic conditions, by adding formaldehyde (FA) into the reactor during biomass pretreatment, lignin can be converted to guaiacyl and syringyl monomers at near theoretical yields (three to seven times higher than those obtained using analogous method but without FA) (Scheme 1). The proposed mechanism by Luterbacher et al.20 involves the reactions of FA with both 1,3-diols groups on lignin side-chains and the electron-rich sites at the ortho- and para-positions to methoxyl groups on lignin aromatic rings. The former results in a six-membered 1,3-dioxane structure (Scheme 1, A), blocking the formation of benzylic carbocation intermediate. The latter forms a hydroxymethyl-functionalized 1,3-dioxane structure (Scheme 1, B), blocking the reactive sites on aromatic ring (Scheme 1, in blue color). These two intermediates have been confirmed through two-dimensional heteronuclear single-quantum coherence nuclear magnetic resonance (2D HSQC NMR) spectrum analysis during depolymerization of the lignin model compound, veratrylglycerol-β-guaiacyl ether (Scheme 1, VG). Moreover, it was found that A was formed quickly at the initial stage, and then B was produced slowly (Figure 1).

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Scheme 1 Formation of lignin monolignols from lignin with and without presence of formaldehyde (FA), re-produced from Luterbacher et al .20

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Figure

1.

Reaction

kinetics

of

the

depolymerization

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of

lignin

model

compound

(veratrylglycerol-β-guaiacyl ether, VG), re-produced from Luterbacher et al.20 Here, we present a systematical density functional theory (DFT) study to obtain insight into the molecular mechanism of the FA-blocked lignin condensation and to rationalize the intriguing kinetic observations. Based on the calculated results, we expected to answer: (i) how does the lignin condensation reaction happen under acidic conditions without presence of FA; (ii) what is the formation mechanism of intermediates A and B in the presence of FA; (iii) why FA can efficiently hinder the formation of interunit C–C linkages during lignin extraction process. These mechanistic insights might be of potential importance for understanding the nature of FA-blocked lignin condensation and also for designing biomass depolymerization strategies. 2. COMPUTATIONAL DETAILS VG in Figure 1, the model lignin dimer used experimentally, was employed as the model compound 6


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of lignin in the current work. A hydronium ion (H3O+) is added to the reaction system to mimic the lignin model compound depolymerization under acidic conditions, as done in previous work.21-23 All calculations were carried out using the Gaussian 09 package24 with Truhlar’s M06-2X meta-GGA-hybrid functional, which has been confirmed to be accurate enough for describing π-π stacking and hydrogen bond interactions, as involved in lignin. This functional has been applied for studies of lignin model compounds, including VG.25-27 The 6-31g(d) basis set was used for all the atoms except for reactive atoms/molecules (hydrogens on the aromatic rings, α-H, α-OH, β-H and γ-OH of VG, H2O, H3O+ and HCHO), for which the 6-311g(d,p) basis set was applied. Each stationary point was classified as a minimum (no imaginary frequencies) or as a transition state (only one imaginary frequency) by vibrational frequency analysis. Intrinsic reaction coordinate (IRC) calculations at the same level were also carried out to verify the correct connections between a transition state and its forward and reverse minima.28,29 To take the solvent effects into account, solvation model density (SMD) solvent model was applied for the single-point calculations by employing 1,4-dioxane (ε=2.2099), the solvent used experimentally, as implicit solvent. All SCRF calculations were conducted at the M06-2X/6-311+G(d,p) level. Noncovalent interactions in real space for some key structures were conducted based on the analysis of the electron densities and their reduced density gradient (RDG) isosurfaces, as implemented by Multiwfn30 and VMD31 packages. 3. RESULTS AND DISCUSSIONS 3.1. Geometries of VG. As shown in Scheme 1, there are two chair carbon atom centers (Cα and Cβ) in VG, resulting in four kinds of stereochemically distinct configurations. To confirm the most preferred configuration of 7



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VG, we first performed scan calculations for each configuration in the space of dihedral angle, Cary–Cα–Cβ–O (θ), which determines the relative orientations of phenyl and phenoxyl groups on VG. The results are presented in Figure 2. Low-energy conformers identified from the scan calculations were subsequently fully optimized to obtain the corresponding minima. Figure 3 shows optimized geometries based on the four lowest-energy conformers, denoted as VGRR, VGRS, VGSS, and VGSR respectively, where the two subscript letters present the chirality of Cα and Cβ respectively. It is found that VGRR is the most stable diastereoisomer of VG, which is energetically more favorable by 1.5~6.6 kcal/mol in comparison with other minima. The high stabilization of VGRR may be ascribed to the effective H−bond interaction between the α–OH and γ–OH groups, as indicated by a distance of 1.982 Å. In the following discussion, VGRR is chosen as the initial structure of lignin model compound, and for easy description, VGRR is abbreviated as VG.

Figure 2. Relaxed scan (step size: 10º) of the dihedral angle (Cary–Cα–Cβ–O, θ) of different veratrylglycerol-β-guaiacyl ether (VG) configurations: a) VGRR, b) VGRS, c) VGSR, and d) VGSS, where two subscript letters present the chiralities of Cα and Cβ in VG. 8


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Figure 3. Optimized geometries based on the lowest-energy conformers of VG shown in Figure 2. Dihedral angles (in pink) are in degrees, and relative energies (in parentheses) are in kcal/mol. 3.2. Mechanism of VG condensation. We began our investigation by addressing the question why the interunit C−C linkages easily form during lignin extraction under acidic conditions. Figure 4 shows the calculated Gibbs free energy profile for the condensation of VG under acidic conditions. The reaction starts with coordination of a hydronium ion (H3O+) to the α-hydroxyl group of VG via a strong H−bond interaction with a distance of 1.032 Å, forming the initial complex Int1. Then benzylic carbocation intermediate, Int2, is afforded via a protonation-dehydration transition state TS1-2 with a barrier of 20.0 kcal/mol. Subsequently, the carbocation intermediate links to the electron-rich carbon atom at the para-position of methoxyl group on the aromatic ring of the second VG via electrostatic interaction. This process results in Int3, which then deprotonates by a water molecule via TS3-4 with a barrier of 20.1 kcal/mol, resulting in Int4, a trimolecular complex among the condensed VG (CVG), H2O, 9



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and H3O+. The transformation from Int3 to Int4 can be described as an electrophilic aromatic substitution process. As shown in Figure 4, the condensation of VG under acidic conditions is estimated to involve an overall barrier of 20.1 kcal/mol with an energy release of 43.7 kcal/mol. These results indicate that the formation of interunit C–C linkages of VG easily occur under acidic conditions, explaining the observed lignin condensation during biomass pretreatment.20 Our calculations show that the condensation of VG occurs via a carbocation intermediate, here Int 2. However, it is noted that in a very recent report of the Ni-Al2O3-catalyzed lignin-first

fractionation, Sels et al.32 observed the formation of a double bond intermediate after dehydration of the alpha hydroxyl group. For the present acid catalyzed system, the double bond intermediate is expected to be formed via deprotonation of the carbocation intermediate (Int 2 in Figure 4). However, our calculations show that under acidic conditions such a double bond intermediate always returns to Int2 upon structural optimization calculations, implying that the reaction could occur via the carbocation intermediate rather than the double bond intermediate.

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Figure 4. Calculated energy profiles with schematic structures for the consideration mechanism of veratrylglycerol-β-guaiacyl ether (VG) under acidic conditions. Bond distances are in Å and key geometrical parameters in transition states are highlighted by red color for visual clarity. It should be noticed that the protonation-dehydration process discussed above occurs at the α–OH of VG. Whether the γ–OH of VG may compete with the α–OH for the protonation-dehydration process? To answer such a question, we compared the basicity of α–OH and γ–OH by calculating their electrostatic potential ( V ) and average local ionization energy ( I ). As shown in Figure 5, both the V and I values of γ–OH are calculated to be larger than those of α−OH of VG (–32.34 vs. –42.14 kcal/mol, and 13.02 vs. 12.97 eV), indicating its weaker basicity and hence lower activity

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toward the protonation-dehydration process. The relatively higher reactivity of α–OH can be mainly attributed to its smaller steric hindrance and higher electrophilic inductive effect from the aromatic ring of VG. Thereafter, we exclude the pathway that forms CVG from the γ–OH of VG.

Figure 5. Calculated molecular surface properties at the 0.001 au contour of the electron density for veratrylglycerol-β-guaiacyl ether (VG) with the corresponding values of oxygen atoms on α–OH and γ–OH groups (denoted by black arrows): a) electrostatic potential, and b) average local ionization energy. 3.3. Reaction of VG with FA. Experimentally, Luterbacher et al.20 observed the generations of two 1,3-dioxane structures, A and B (Figure 1), via 2D HSQC NMR spectrum analysis when FA was introduced into the reactor. Moreover, it is noted that A is first formed rapidly at the initial stage and then evolves into B partially, instead of completely. To rationalize these observations, we calculated potential pathways for formations of A and B. Formation of structure A. The present work studied three potential pathways for the formation of A, denoted as pathways I-IV, in Figures 6–8. Pathway I (Figure 6) describes the formation of A via the intermolecular cyclization between Int2 and FA. The reaction occurs via TS5-6 with a barrier of 27.6 kcal/mol, and is calculated to be exothermic by 40.6 kcal/mol. Compared with the 12


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transformation of Int2 to CVG with a barrier of 20.1 kcal/mol and an exothermicity of 43.7 kcal/mol (Figure 4), this process is energetically less favorable both kinetically and thermodynamically. This is not consistent with the experiment where A rather than CVG was formed fast at the initial stage of the reaction. Therefore, we exclude the pathway forming A via the reaction of Int2 with FA.

Figure 6. Calculated energy profile with schematic structures for the formation of the 1,3-dioxane structure A (Scheme 1) along pathway I, the reaction of Int2 with FA in an acidic environment. Bond distances are in Å. Pathways II and III (Figure 7) involve the reaction of VG and FA with and without assistance of water molecules. The reaction consists of two elementary steps: nucleophilic addition and cyclization-dehydration. Figure 7 shows the calculated results for the nucleophilic addition process. It is found that without assistance of water molecules (pathway II), the barrier of the nucleophilic addition is calculated to be as high as 45.4 kcal/mol. The high energy barrier may originate from the 13



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large ring strain in TS7-8. In contrast, in presence of two water molecules, the reaction proceeds along pathway III, and the barrier is reduced to 28.3 kcal/mol. In the H2O-assisted nucleophilic addition process, the water molecules play the role of proton shuttles.17,33-37 However, the barrier of 28.3 kcal/mol is still 8.2 kcal/mol higher in free energy as compared to the overall barrier (20.1 kcal/mol) involved in the VG condensation pathway (Figure 4). This result can also not explain the fast formation of A. Thus, both pathways II and III are not viable pathways for the formation of A. In this sense, the further calculations along these two pathways were not carried out.

Figure 7. Calculated energy profiles with schematic structures for the nucleophilic addition process along pathways II and III. Bond distances are in Å. Figure 8 shows the calculated results along pathway IV, which mimics the formation of structure

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A via the reaction of VG and FA in an acidic environment. Initially, a tetramolecular complex, Int7, among VG, FA, H2O and H3O+ is first formed. Subsequently, we located a less stable intermediate Int8, which is a formal addition product of the carbonyl group of FA at the α–OH of VG. However, we failed to locate a transition structure connecting Int7 and Int8, and the hypothetical transition state structure always collapses into Int8 upon optimization. This indicates that the α–OH of VG is very reactive towards the carbonyl group of FA.

Figure 8. Calculated energy profiles with schematic structures for the formation of 1,3-dioxane structure (A) via the reaction of the α–OH of veratrylglycerol-β-guaiacyl ether (VG) with formaldehyde (FA) in an acidic environment. Bond distances are in Å.

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Int8 then undergoes a cyclization-dehydration process through a concerted transition state TS8-9 to form the 1,3-dioxane structure (A), as indicated in Int9, a tetramolecular complex of structure A, H3O+ and two water molecules. In TS8-9, the C1–O2 bond is forming and the C1–O3 bond is broking with the assistance of H2O and H3O+, which act as Brönsted base and acid, respectively. As shown in Figure 8, the overall barrier is calculated to be 13.1 kcal/mol as compared to Int7. It should be noted that the formation of A is calculated to be exergonic by 52.0 kcal/mol, which is thermodynamically favorable for lignin depolymerization. This pathway is identified as the energetically viable pathway for the generation of A from VG with presence of FA. The reaction can also occur at the γ–OH of VG. The calculated results are shown in Figure S1. The reaction is calculated to be exothermic by 52.0 kcal/mol with an overall barrier of 20.5 kcal/mol. In comparison with the results shown in Figure 8, the reaction at the γ–OH of VG is energetically less favorable, indicating that the γ–OH of VG is less reactive than its α–OH towards FA. This can be attributed to the smaller steric hindrance at the α–OH position and also to the stronger electrophilic inductive effect of the aromatic ring of VG at the α–OH position, which facilities the nucleophilic addition of FA. Formation of structure B. Structure A undergoes the second FA insertion affording the hydroxylmethyl functionalized-1,3-dioxane, structure B in Figure 1. The calculated results are shown in Figure 9. Starting from Int9, the coupling of A with the second FA generates complex Int10, where the second FA electrophilically attacks the electron-rich carbon atom (C2) at the para position to methyl group on the aromatic ring via TS10-11 with a barrier of 19.1 kcal/mol. The IRC calculations from TS10-11 show the forward minimum Int11. Finally, the water molecule as a Brönsted base abstracts the H-proton on C2 atom through TS11-12, resulting in Int12, which denotes 16


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a bimolecular complex of structure B and H3O+. The generation of B from A occurs with a barrier of 20.7 kcal/mol and is calculated to be slightly exothermic by 4.4 kcal/mol, indicating the transformation from A to B is almost a thermoneutral process.

Figure 9. Calculated energy profile with schematic structures for the formation of hydroxylmethyl functionalized-1,3-dioxane structure (B) from the reaction of 1,3-dioxane structure (A) with formaldehyde (FA) in an acidic environment. Bond distances are in Å. On the basis of calculations, we summary a simplified illustration for the FA-prevented lignin condensation mechanism in Scheme 2, where the reaction of VG with FA forms A and B, which protect the reactive sites of VG (1,3-diols groups and the ortho/para positions to methoxyl groups on lignin aromatic rings).

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Scheme 2 Sketch of the formaldehyde (FA)-prevented lignin condensation mechanism proposed from the present calculations.

Figure 10. Simplified energy profile for the formation of CVG (red color), A and B (blue color). 3.4. Explanation for the FA-blocked lignin condensation. Figure 10 shows the simplified energy profiles for the formations of CVG, A and B, and Table 1 collects the corresponding Gibbs free energy barriers and reaction entropies. Clearly, TS8-9, the transition state involved in the generation of A is 7.0 kcal/mol energetically more favorable than

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TS1-2, the transition state involved in the generation of CVG. This accounts for the FA-blocked lignin condensation observed by Luterbacher et al.20 Table 1. Calculated Gibbs free energy barriers (∆Gsol) and reaction entropies (∆Hsol) for the formations of the condensed veratrylglycerol--guaiacyl (CVG) and 1,3-dioxane structures (A and B). Species ∆Gsol (kcal/mol) ∆Hsol (kcal/mol) CVG

20.1

–43.7

A

13.1

–52.0

B

20.7

–56.4

On the other hand, for the formation of A, the cyclization-dehydration step (TS8-9) with a barrier of 13.1 kcal/mol is identified as the rate-determining step. In contrast, the H-migration step is crucial for the formation of B, which occurs via TS11-12 with an overall barrier of 20.7 kcal/mol. These results suggest that the generation of A is more favorable than that of B, which explains the experimental observation (Figure 1) that A is produced rapidly at the initial stage of the reaction, and then is transformed to B slowly and partly. In addition, from a thermodynamic point of view, the formations of A and B are also more favorable than that of CVG, as indicated by calculated reaction heats, 52.0~56.4 kcal/mol vs. 43.7 kcal/mol. To understand the higher thermal stability of A and B in comparison with CVG, Figure 11 compared calculated isodensity surfaces of HOMOs and weak interaction for Int4, Int9, and Int12. Several important results are observed. Firstly, there are strong π-π interactions in intermediates Int9 and Int12, while there are only weak C-H…π interactions in Int4. Secondly, both Int9 and Int12 have a cloud between the π-system of the aromatic ring and the p-orbitals of O4 19



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atom, but these features are not observed in Int4. Thirdly, both Int9 and Int12 feature a smaller steric hindrance effect than that in Int4. Thereafter, in the presence of FA, VG is more likely to react with FA to generate A and B rather than the self-condensation to form CVG.

Figure 11. The isodensity surfaces of HOMOs for Int4, Int9 and Int12 with an isodensity value of 0.02, together with the corresponding noncovalent interaction analysis (blue, strong interaction; green, weak interaction; red, strong repulsion). To obtain a general conclusion for aldehyde-based protection strategy in lignin depolymerization, our calculations extend to acetaldehyde-, propionaldehyde-, and benzaldehyde-containing systems. For the formation of the 1,3-dioxane structure (A), the calculated barrier is 14.2 kcal/mol for the acetaldehyde-containing system, and 15.9 kcal/mol for the propionaldehyde-containing system. These two barriers are smaller than the barrier involved in the condensation of VG, 20.1 kcal/mol, and also comparative with the barrier involved in the formaldehyde-containing system, 13.1 kcal/mol. In contrast, for the benzaldehyde-containing system, the barrier is calculated to be 22.4

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kcal/mol, which is clearly higher than the barrier involved in the condensation of VG, 20.1 kcal/mol. From these results, it seems that aliphatic aldehydes can effectively prevent lignin condensation but aromatic aldehyde cannot. The theoretical results are in good agreement with the experimental observation by Luterbacher et al.38 that acetaldehyde and propionaldehyde are equally effective like formaldehyde for preventing lignin condensation but benzaldehyde is less effective. The theoretical results above on the model lignin dimer provide a preliminary picture of how FA hinders lignin condensation and also rationalized the observed kinetics of lignin depolymerization well.20 We expect that the conclusion is still true when a real lignin substrate or a lignin oligomer is considered because the dimer substrate represents the predominant phenol-etherified -O-4 units of lignin with their characteristic and available α- and γ-hydroxyl groups.20 4. CONCLUSIONS In summary, we have presented a systematic theoretical study of how and why FA can efficiently prevent the condensation of lignin model compound, VG. Theoretical calculations first clarified the detailed mechanism of lignin condensation, finding that CVG is produced by forming interunit C–C linkages with a Gibbs free energy barrier of 20.1 kcal/mol. Then we explored formation mechanism of the six-membered (hydroxymethyl-functionalized) 1,3-dioxane structures (A and B), rationalizing the puzzling phenomena that VG can covert to A and B with presence of FA. In our proposed mechanism, FA as a protecting agent blocks the reactive sites of VG, forming A and B with high thermodynamic and kinetic stability, which may be attributed to the small steric hindrance and strong π-π and d-π conjugate interactions. The net energy change forming A and B is the mainly overall thermodynamic driving force for lignin depolymerization. The theoretical results provide a clear picture of the FA-blocked lignin condensation at molecular level, and rationalize the 21



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good performances of FA for preventing lignin condensation during biomass depolymerization. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: _________________________. Calculated results and Cartesian coordinates of optimized intermediates and transition states (PDF). AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] ORCID Xueli Mu: 0000-0003-3286-3786 Notes There are no conflicts to declare. ACKNOWLEDGEMENTS The authors acknowledge the financial support from National Natural Science Foundation of China (21773139, 21833004, 21433006, and 21703123). REFERENCES (1) Zakzeski, J.; Bruijnincx, P. C.; Jongerius; A. L.; Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110, 3552–3599. (2) Tadesse, H.; Luque, R. Advances on Biomass Pretreatment Using Ionic Liquids: an Overview. 22


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TOC

HO H3CO HO

O

OCH3 OCH3

Why ?

CVG

OCH3 OCH3 VG

H3CO O

HO H3CO HO

HO

O

without HCHO

OCH3 OCH3

O H3CO O HCHO

O

O H CO 3 O HCHO

O

slow

fast OCH3 OCH3 A

28


CH2OH OCH3 OCH3

B

Mechanistic Insights into Formaldehyde-Blocked Lignin Condensation

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