Quantum Chemistry Analysis of Reaction Thermodynamics for

Oct 9, 2017 - The complexity and variability of lignin polymers has stimulated the use of simple lignin model compounds to study reactions of lignin.(...
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Quantum Chemistry Analysis of Reaction Thermodynamics for Hydrogenation and Hydrogenolysis of Aromatic Biomass Model Compounds Laurene Petitjean, Raphael Gagne, Evan S. Beach, Jason An, Paul T. Anastas, and Dequan Xiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02384 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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Quantum Chemistry Analysis of Reaction Thermodynamics for Hydrogenation and Hydrogenolysis of Aromatic Biomass Model Compounds Laurene Petitjeanb, Raphael Gagnea, Evan S. Beachb, Jason Ana, Paul T. Anastasb*, and Dequan Xiaoa* a

Laboratory for Integrative Materials Discovery, Department of Chemistry and Chemical Engineering, University of New Haven, 300 Boston Post Road, West Haven, CT 06516 b Center for Green Chemistry and Green Engineering, Yale University, 370 Prospect Street, New Haven, CT 06520 *Corresponding authors: [email protected], [email protected] KEYWORDS: Gibbs Free Energy, Lignin, Hydrogenolysis, Hydrogenation, Biomass ABSTRACT: Designing effective and selective reactions at sustainable or mild conditions is key for the valorization or refinery of lignin biomass using H2 reduction methods. However, it remains unclear what are the feasible mildest conditions for the reductive valorization of lignin, at which transformations can be designed. Here, we aim to exploit this critically important question using quantum chemistry calculations to systematically analyze the thermodynamics of hydrogenation and hydrogenolysis of typical functional groups found in lignin based on a set of aromatic model compounds. Our results show that it is thermodynamically feasible to break ether linkages and remove oxygen content in the model compounds even at room temperature, room pressure, and in aqueous solvent (i.e., the global mildest conditions). Interestingly, the potential influence on the thermodynamics by reaction variables is ranked in the order of temperature > H2 pressure > solvent dielectric constant; a strategically chosen solvent may enable increased selectivity for hydrogenolysis over hydrogenation. Our predicted reaction thermodynamics is consistent with our experimental findings of probed reaction pathways. This work may inspire researchers to pursue the design of ‘ultimate’ green biomass conversion processes closer to the global mildest conditions.

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SYNOPSIS Predicting the mildest thermodynamics conditions for the transformation of biomass into valueadded chemicals will empower the development of more sustainable chemical processes targeting renewable resources.

ARTICLE TEXT INTRODUCTION As a renewable carbon source, biomass (consisting of mainly lignin, cellulose, and hemicellose) has the potential to provide energy and platform chemicals,1 therefore mitigating society’s unsustainable consumption of finite resources, and decreasing the environmental impacts of the latter’s extraction, distribution and use.2-3 Lignin comprises up to 25-35 % of plant biomass and consists of methoxylated and hydroxylated 4-propylphenol units coupled as ethers and occasionally cross-linked with carbon–carbon bonds.4 Whereas cellulose and hemicellulose have been extensively studied for the generation of bioethanol and other platform chemicals, lignin remains a challenge to valorize and is currently still discarded or burnt after separation from the glycopolymers. The valorization of lignin, however, could provide a renewable source of aromatic monomers, which could serve as drop-in replacements to those that are petroleum derived. Designing processes that work under sustainable conditions is the key for the valorization or refinery of lignin using H2 reduction methods.5-7 Reductive approaches to depolymerize lignin are advantageous for their ability to quench char-forming radical species, limit unwanted repolymerization, and thus potentially increase the yield and selectivity of targeted aromatic products. Efficient and selective approaches reduce economic and environmental costs associated with the production of waste, and purification or separation

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processes. The splitting of water, furthermore, can allow hydrogen to be renewably sourced. However, it remains unclear what are the feasible mild conditions for the reductive valorization of lignin for which methodologies can be designed. Here, we aim to exploit this critically important question using quantum chemistry calculations to systematically analyze the reaction thermodynamics of hydrogenation and hydrogenolysis based on a set of aromatic model compounds representing lignin. Our goal is to evidence the mildest thermodynamic conditions (lowest energy input, most benign solvent, least reagent inputs) at which these transformations can be effected for lignin valorization. Reaction of lignin with hydrogen can produce small organic molecules through the breakage of aromatic ether bonds (dominant) or C-C bonds, lower the oxygen content, and increase the hydrogen/carbon ratio, a valuable route for the production of biofuels. Three key reaction variables: temperature (T), H2 pressure (PH2) and solvent dielectric constants (εs) play a significant role on reaction efficiency and selectivity, and are commonly and easily altered to optimize reactions. However, there remains a lack of knowledge of the thermodynamics of hydrogenation or hydrogenolysis of lignin with systematic variation of T, PH2, and εs. The early literature has some interesting findings for the thermodynamics of hydrogenation of benzene8 that is relevant to lignin model compounds. For example, the formation of cyclohexa-1,3-diene by adding one H2 molecule to benzene has a positive Gibbs free energy in the temperature range of 298.15 – 1003 K, while the formation of cyclohexane by adding three H2 molecules has a negative Gibbs free energy.8 In this work, we used quantum chemistry calculations at the density functional theory (DFT) level9 to study reaction thermodynamics for hydrogenation or hydrogenolysis of six typical functional groups in lignin: benzylic aldehyde (F1), aliphatic double-bond (F2), aromatic

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double-bond (F3), benzylic ketone (F4), aromatic ether (F5), and benzylic hydroxyl (F6) (Scheme 1a), with the systematic variation of T, PH2, and εs. These functional groups were represented in four biomass model compounds for the calculations (Scheme 1b). The complexity and variability of lignin polymers has stimulated the use of simple lignin model compounds to study reactions of lignin.10 These compounds are also found as reactive lignin depolymerization intermediates, and are thus important to study. We present the two-dimensional (2D) contours plots of Gibbs free energy (ΔrG) of reaction as a function of varying PH2 and T, εs and T, and εs and PH2, respectively. We show these functional groups have different thermodynamic properties in terms of reactions with hydrogen, and our calculated thermodynamics results are consistent with the experimental studies of the model compounds’ H2-reductive reactions with or without a Cu-doped porous metal oxide (Cu-PMO) catalyst. Cu-PMO has the advantage of being a heterogeneous catalyst composed of earth abundant elements. It is thus easily recyclable, easier to implement on industrial scale, and relatively cheap to synthesize. This work provides the theoretical foundation for the feasibility of designing more sustainable processes for lignin valorization, potentially utilizing green catalysts that function nearer the global mildest conditions. We assume that introducing catalysts to the reactions will not change reaction thermodynamics, but affect the reaction kinetics, leading to potential expedition of reactions. In addition, we will identify possible reaction pathways using known catalysts (Cu-PMO) in experiments, serving as evidence of the thermodynamic feasibility of specific reactions. This study does not focus on exploring the detailed kinetics of catalysis and the design of actual catalysts, which will be the subject of future work.

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Hydrogenation

Hydrogenolysis

O

O R

R F1

R

F2

F3

H2

H2

H2

H

H

H

O R

R

H

R'

R

R

R'

+ H 2O

OH F6

H2

H H

H

O F5

2H2

R

H

F4

R

R'

2H2

OH

R

H

+ H 2O

+ R'H

(a) F5

F1 O

O HO

OH

HO

F3 Compound 1

Compound 2

O

O

F2 HO Compound 3

F6

O

(b)

F4 O

HO Compound 4

Scheme 1. (a) Six functional groups for the first-principle thermodynamics study of hydrogenation (F1-F3) and hydrogenolysis (F4-F6), where R or R’ denotes aliphatic or aromatic functional group in lignin polymers or model compounds. (b) Four lignin model compounds used to study the six functional groups. In (b), the waved line denotes the position of bond cleavage. EXPERIMENTAL SECTION The high-performance computational chemistry software NWChem11 was used to determine the energy of different reaction pathways of the hydrogenation or hydrogenolysis of four biomass model compounds. The structures were built in .xyz format using the model building program Avogadro.12 The .xyz geometries were then used in NWChem to obtain their respective optimized geometries, as seen in Section 4 of the Electronic Supporting Information (i.e., ESI). This ensured the inter-molecular energy minimization of each of the structures. The optimized structures were then used to perform vibrational frequency analysis and solvation energy calculations. All DFT calculations were performed at the B3LYP/6-31G* level. We performed additional calculations with different basis sets (6-311g* and cc-pvdz) for the aldehyde moiety of the vanillin model compound (F1), at 1 bar and 571.15 K (see ESI, Table S2) to confirm our

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method. Gibbs energy calculations in continuum solvation model showed an error of 5 kcal/mol for neutral organic compounds.13 This general error is smaller than all results discussed in the present paper, unless otherwise specified. The output files of the frequency calculations provided the following information: Zero-point correction to energy (E1), thermal correction to enthalpy (H) and total entropy (S). The solvation calculation provided the total DFT energy (E0) and the electrostatic solvation energy (Es). Equation 1 was used to determine the change in Gibbs free energy (G). 𝑑𝐺 = 𝐻 − 𝑇𝑆

(1)

This information was then used to find Gibbs free energy (G) of each structure determined by equation 2. 𝐺 = 𝑑𝐺 + 𝐸* + 𝐸+ + 𝐸,

(2)

The Gibbs free energy of each structure was then used to determine the total change in Gibbs free energy for each reaction pathway. For example, the hydrogenation reaction can be generalized by the following chemical equation in solution 𝑅(𝑠𝑜𝑙𝑢) + 𝐻4 (𝑠𝑜𝑙𝑢) ↔ 𝑅𝐻4 (𝑠𝑜𝑙𝑢)

(3)

The Gibbs free energy change of this reaction in gas phase 𝑅(𝑔) + 𝐻4 (𝑔) ↔ 𝑅𝐻4 (𝑔) is ∆𝐺8 = 𝐺8,:;< − 𝐺8,;< − 𝐺8,:

(4)

After that, reaction ΔrG in solution is computed using the following Born-Haber cycle (5): ΔGg R(g) + H 2 (g) → RH 2 (g) ΔGs,R

ΔGs,H 2

(5)

ΔGs,RH 2

R(solu) + H 2 (solu) → RH 2 (solu)

ΔGsolu

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The solvation energy ∆𝐺,,= for each species is computed using the COSMO solvation model.14 The reaction free energy in solution ∆𝐺,>?@ is computed by ∆𝐺,>?@ = ∆𝐺8 + ∆𝐺,,:;< − ∆𝐺,,: − ∆𝐺,,;
95%

F6 - benzylic hydroxyl

Predicted: ΔrG < 0 Yield A : 15% Yield B: > 95%

HO F2 - aliphatic double bond

O O HO Compound 4 Acetovanillone

453.15 K, 18 hours MeOH (0.21 M) H 2 (40 bars)

O HO F4 - benzylic ketone

Predicted: ΔrG < 0 Yield A : 0% Yield B: > 95%

Scheme 2. Reaction yields for the reductive treatment of six lignin functional groups, with and without Cu-PMO catalyst. 10

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O O HO

O H

O

H2 ΔG > 0

Vanillin

HO

H H

O

H2 - H 2O

HO

Vanillyl Alcohol

Creosol

H+

O O HO

H H

A

H2 -MeOH

Cu-PMO H2

MeOH No Cu-PMO

O

OMe

HO

H 2 (high P) high T

B

O HO

OMe C

H 2 (low P) lower T OMe O HO

OMe D

Scheme 3: Alternative pathway for the the formation of vanillyl from alcohol vanillin Scheme 3. Alternative pathway for formation of alcohol vanillyl from vanillin

As one of the applications, our predicted thermodynamics of hydrogenation or hydrogenolysis can provide new insights for the design of processes for lignin degradation. For example, Figure 4 illustrates the experimental working conditions of PH2 and T for lignin depolymerization for a set of known metallic catalysts, including heterogeneous noble metals, heterogeneous transition metals and homogeneous metallic catalysts. Near the global mildest condition (i.e., room temperature and room pressure), we define an “ideal” region where PH2 = 110 bar and T = 298.15 – 393.15 K. So far, no process utilizing heterogeneous catalysts fall in the “ideal” region. Hence, it is a future challenge to design heterogeneous transition-metal catalysts that enable new lignin valorization processes to fall into this region.

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Figure 4. Working conditions of a number of known catalysts in the literature: Cu/Cr,21 Ni/Mo/S,22 Cu/Fe/Sn/S,23 Cu-PMO 1,24 Pd/C,25 PdC+ZnCl2,26 and Cu-PMO 2.27 In summary, we studied reaction Gibbs energy for the hydrogenation/hydrogenolysis reactions of six typical functional groups in lignin model compounds, with systematic changes of T, PH2, and εs, based on quantum chemistry calculations. Our predicted thermodynamic results are consistent with the probed reaction pathways in experiments, apart from the hydrogenation of vanillin (F1), which was evidenced to proceed through a different pathway than the one-step hydrogenation with H2. We find that the influence to ΔrG is ranked in the order of T > PH2 > εs. Even though εs could be important in influencing reaction kinetics and selectivity by biasing the solvation of chemical species, it plays only a subtle role in changing ΔrG for the reactions here when εs > 5. This is a helpful insight for selection of green solvents for biomass conversion. In the ranges of T (273.15 – 573.15 K), PH2 (1-60 bar), and εs (0-80), the thermodynamically favored reactions include hydrogenation of aliphatic double bonds and hydrogenolysis of carbonyl, aromatic ether, and hydroxyl groups in the set of studied aromatic model compounds. However, hydrogenation of the first aromatic double-bond in a phenyl ring is thermodynamically disfavored in the studied model compounds. Based on the feasibility of hydrogenolysis of aromatic ether in vanillin, we anticipate that it may be feasible to perform

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lignin depolymerization and upgrading even at the global mildest condition through the breaking of aromatic ether linkages and the removal of oxygen contents via hydrogenolysis of carbonyl and hydroxyl groups. Currently known methods for lignin depolymerization do not yet reach the predicted mildest conditions. Our study provides the theoretical foundation for the feasibility of designing novel reactions, for which new green catalysts (e.g., transition metal catalysts) will be crucial, for lignin conversion near the global mildest conditions, i.e., room temperature, room pressure, and aqueous solvent. Our work will hopefully inspire and enable the design and development of new-generation processes for biomass valorization.

ASSOCIATED CONTENT Supporting Information. General experimental, synthesis of Cu-PMO catalyst, synthesis of 2methoxy-4-(methoxymethyl)phenol, general procedure for reduction of lignin model compounds, model compound reduction reactions, 1H NMR spectra of crude reduction reaction mixtures, optimized geometries for model compounds, characterization of isolated reaction products, temperature dependence of the dielectric constant for methanol, reaction Gibbs Free Energy for the hydrogenation of vanillin with varying basis sets, solvation free energies for reactions F1-F6 in methanol at 303 K (PDF).

AUTHOR INFORMATION Corresponding Authors *Dequan Xiao. Laboratory for Integrative Materials Discovery, Department of Chemistry and Chemical Engineering, University of New Haven, West Haven, CT 06516; [email protected] *Paul T. Anastas. Center for Green Chemistry and Green Engineering, Yale University, New Haven, CT 06520

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[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The summer research grant and research fellowship (2014 & 2015) of the University of New Haven, the faculty start-up fund of the University of New Haven, and QAFCO, helped fund this work.

ACKNOWLEDGMENTS D.X. thanks the faculty start-up fund and summer research grant and research fellowship (in 2014 and 2015) of the University of New Haven. P.A. would like to thank QAFCO for continued funding support. The Center for Green Chemistry and Green Engineering is thankful to the School of Forestry and Environmental Studies for its support.

ABBREVIATIONS density functional theory (DFT)

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TOC/GRAPHICAL ABSTRACT

Hydrogenation O R

R F1

F2

F3

Hydrogenolysis O R

R'

R

F4

O F5

R'

R

OH F6

Synopsis: The predicted thermodynamics here for transforming biomass model compounds will inspire the design of more sustainable catalytic processes targeting renewable resources.

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