A Multitechnique Characterization of Lignin Softening and Pyrolysis

Research Article pubs.acs.org/journal/ascecg

A Multitechnique Characterization of Lignin Softening and Pyrolysis Binod Shrestha,§ Yann le Brech,§ Thierry Ghislain,§ Sébastien Leclerc,† Vincent Carré,‡ Frédéric Aubriet,‡ Sandrine Hoppe,§ Philippe Marchal,§ Steve Pontvianne,§ Nicolas Brosse,∥ and Anthony Dufour*,§ §

LRGP, CNRS, Université de Lorraine, ENSIC, 1 rue Grandville 54000 Nancy, France LEMTA, CNRS, Université de Lorraine, 2 avenue de la Forêt de Haye, 54518 Vandœuvre-lès Nancy, France ‡ LCPA2MC, ICPM, Université de Lorraine, 1 Boulevard Dominique François Arago, 57000 Metz, France ∥ Faculté des sciences et technologies, LERMAB, Université de Lorraine, Boulevard des Aiguillettes, 54500 Vandœuvre-lès Nancy, France †

S Supporting Information *

ABSTRACT: The understanding of lignin softening and pyrolysis is important for developing lignocellulosic biorefinery in order to produce carbon fibers, polymers additives, green aromatics, or biofuels. Protobind lignin (produced by soda pulping of a wheat straw) was characterized by thermogravimetry, calorimetry (for glass transition temperature and heat of pyrolysis reactions), in situ 1H NMR (for the analysis of the mobility of protons upon lignin thermal conversion), and solution-state 13C and 31P NMR (determination of functional groups in lignin). In situ rheology reveals the real-time viscoelastic behavior of lignin as a function of temperature. Upon heating, lignin undergoes softening, through glass transition overlapped with depolymerization, and is followed by the solidification of the softened material by cross-linking reactions. The lignin residues were quenched within the rheometer at the midpoint temperatures of softening and solidification regions and were further analyzed by elemental analysis, GPC-UV of acetylated THF soluble fractions, FTIR, solid 13C NMR, and laser desorption ionization (LDI) combined with very high-resolution mass spectrometry (HRMS). We present the first report on lignin biochars analysis by LDI-HRMS. NMR and FTIR analyses provide the evolution of functional moieties in lignin residues. 13C NMR, GPC-UV, and LDI FTICRMS analyses depict the depolymerization mechanism combined with cross-linking and demethoxylation reactions. An overall physical and chemical mechanism for the thermal conversion of alkali lignin is proposed based on these complementary analyses. KEYWORDS: lignin, thermochemical, processing, softening, char liquefaction process.2−4 Analytical methods based on Fourier transform infrared (FTIR), gel permeation chromatography (GPC), 13C and 31P nuclear magnetic resonance (NMR) have been developed to characterize various lignins and lignin derived products.18−24 Two dimensional 13C−1H NMR methods employed at solution state (after lignin swelling by a solvent)25,26 or at solid state27 (on cross-linked solids after lignin pyrolysis) have shown interesting insights into the mechanisms of lignin thermo-chemical conversion. An advanced methodology combining FTIR, pyrolysis-gas chromatography/mass spectrometry (py-GC/MS), GPC, NMR, and laser desorption ionization (LDI)-MS (among other methods) has been developed to fully characterize the structure of pyrolytic lignin (the water insoluble fraction of pyrolysis biooil) and to elucidate its chemical structure.28,29 LDI-MS has

1. INTRODUCTION Lignin is nature’s dominant aromatic polymer and is found in lignocellulosic biomass in the range of 15−40% dry weight. It is a renewable resource produced by the pulp and paper industry and by lignocellulosic biofuels’ production plants. The valorization of lignin yields promising opportunities to produce carbon fibers, polymers, commodity chemicals (such as benzene or phenols), and fuels.1−3 It has been the subject of widespread interest as a consequence of its availability and potential. Lignin can be valorized by pyrolysis4−7 or by depolymerization in liquid media,8−12 followed by further up-grading of the vapors for pyrolysis13,14 or of the condensed bio-oils.15−17 The first important task prior to the development of any lignin conversion process is the analysis of lignin structure by appropriate analytical methods. Indeed, lignin composition greatly depends on biomass types and on the fractionation process (e.g., soda pulping, Organosolv, Kraft, etc.), and this will notably impact the selectivity of the pyrolysis or © 2017 American Chemical Society

Received: April 13, 2017 Revised: June 14, 2017 Published: June 24, 2017 6940

DOI: 10.1021/acssuschemeng.7b01130 ACS Sustainable Chem. Eng. 2017, 5, 6940−6949

Research Article

ACS Sustainable Chemistry & Engineering

is pivotal to study the evolution of the lignin-based liquid intermediate to the solid char. The softening properties of lignin have been extensively studied in the scope of biomass pulping,55 lignin-based carbon fibers,56−58 or composite materials.59 The glass transition temperature has been analyzed by calorimetry (which is an in situ non-intrusive analysis) and the viscoelastic properties of softened lignin by in situ rheology.56,59,60 Furthermore, the rheological properties of lignin provide important data to optimize processing conditions for the production of lignin fibers or composites. But these comprehensive studies56,59,60 are usually limited to temperatures lower than 200 °C in order to reduce lignin degradation during melt spinning. Despite all these studies, the mechanisms of lignin softening followed by char solidification are still poorly understood. In situ rheology was first applied on biomass, cellulose, and lignin (organosolv) by our group to investigate the formation of the softened matrix and its conversion to the solid char between 200 and 400 °C.31 Here, we complete this approach on a different lignin (soda pulping) with the analysis by various methods (NMR, FTIR, GPC-UV, LDI-MS) of the pristine lignin and its residues quenched at key temperatures, in order to unravel the physical-chemical mechanisms of lignin softening and pyrolysis. To the best of our knowledge, all these techniques are combined for the first time, and, furthermore, LDI-MS (very high resolution MS) is applied for the first time to analyze the chemical structure of lignin chars. The methodology presented here could be applied to all other lignins or softening polymers.

been shown to be an interesting method to analyze ligninderived products,29,30 but to the best of our knowledge, it has not yet been applied to lignin solid residues obtained after different pyrolysis temperatures. The most important challenge to tackle for the development of lignin pyrolysis technologies is the formation of agglomerates in reactors (such as fluidized bed reactors) due to lignin softening and the subsequent formation of a sticky and fluffy char.4,31 Different strategies have been developed to process lignin in fluidized bed reactors5,32 such as the mechanically fluidized reactor (MFR). In the MFR, the mechanical stirrer provides a good mixing between lignin and the bed material, avoids the use of a carrier fluidization gas, and notably breaks any possible agglomerate.32 However, it is essential to better understand the fundamentals of lignin pyrolysis to optimize the reactors. The studies on lignin pyrolysis by using thermogravimetric analysis (TGA), 33−39 differential scanning calorimetry (DSC),36,37,40,41 or Py-GC/MS42−49 have dealt with the global mass loss, heats of reactions, or the composition of volatiles, respectively. Nevertheless, there are few studies dealing with the fundamental mechanisms of lignin char formation by structural analysis. The evolution of the chemical moieties in chars from lignin pyrolysis has been elucidated by 13C NMR and FTIR analysis.19,32,33 Interesting NMR data have also been obtained for the carbonization of lignin fibers at temperatures higher than 400 °C,50 i.e., after the softening and the subsequent solidification of char.31 The analysis of lignin or biomass chars by scanning electron microscopy (SEM) has shown the formation of a softened surface from an “intermediate liquid” material.19,48,51−53 SEM analysis of lignin char produced at 250 °C showed that “the lignin particles soften and fuse into a mass of matrix and vesicles”. The vesicles are the result of volatile gases released within the softened lignin matrix, and they maintain the expanded shape after cooling.19 Figure 1 exemplifies this structure of a lignin char. The scope of this present study is to better understand how this char structure is formed upon softening and pyrolysis.

2. MATERIALS AND METHODS 2.1. Global Methodology Developed in This Work. The lignin used for this study was produced by soda pulping of a wheat straw and was supplied by Green Value (Switzerland). This lignin was then characterized by the various complementary methods presented in Table 1. Each method is presented further in the following sections. 2.2. TGA and DSC Analysis. Thermogravimetric experiments were performed in a TGA1 from Mettler Toledo with analytical nitrogen as a carrier gas and a flow rate of 50 mL min−1. Lignin (6 mg) was analyzed using a heating rate of 5 K min−1 from 25 to 900 °C. The maximum rate of decomposition temperature was analyzed by the dTG curve. Differential scanning calorimetry (DSC) analysis of the lignin (10 mg) was analyzed using Mettler Toledo DSC. The DSC analysis was carried out after a first heating step (“annealing”, heating the lignin sample within the aluminum pan from 25 to 100 °C with the heating rate of 5 K min−1, then cooling down to 25 °C with the heating rate of −25 K min−1). The lignin was further heated from 25 to 500 °C at a heating rate of 5 K min−1. The glass transition temperature and the heat of pyrolysis were studied by DSC. 2.3. In Situ 1H NMR Analysis. An in situ 1H NMR analysis was carried out in a Bruker 300 MHz NMR instrument in conjunction with a high-temperature probe as previously described.61 Briefly, 31.6 mg of lignin was placed in the NMR quartz tube equipped with a capillary tube that flushes the sample with analytical argon (10 mL min−1). Pyrolysis was carried out with a heating rate of the NMR probe of 5 K min−1. The 1H NMR spectra were deconvoluted using a Matlab program into Gaussian (solid-like) and Lorentzian (liquid-like) distribution functions. The fraction of mobile hydrogen or fluid phase (%HL) is calculated from the following relation:

Figure 1. Structure of a lignin char (Protobind lignin, 500 °C, 5 K/ min) by SEM54 showing vesicles formed by volatiles pushing a viscoelastic (softened) matrix, like a “balloon”. The expanded shape is maintained after cooling.

The aforementioned methods (FTIR, NMR, and SEM) were conducted ex situ after the cooling of char and obviously, the physical-chemical properties of the “intermediate liquid” at real pyrolysis temperature differs from the one obtained after cooling. For this reason, the development of in situ analysis (non-intrusive analysis at real temperature during the pyrolysis)

%HL = AL /(AL + A G)

(1)

where AL and AG are the areas of Lorentzian and Gaussian functions, respectively. More details about the fundamentals of this technique can be found in our previous work.31 6941


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ACS Sustainable Chemistry & Engineering Table 1. Analytical Methods Used To Study the Softening and Pyrolysis Behavior of Lignin analytical method TGA DSC in situ 1H NMR solution-state (13C NMR, 31P NMR)

in situ rheology

elemental analysis LDI FTICR-MS GPC-UV FTIR, solid-state 13C NMR

description mass loss as a function of temperature glass transition and heat of pyrolysis reaction evolution of the mobility of protons (or “fluidity development”) (in real-time and -temperature). Mobile protons are formed during lignin thermal conversion. characterization of chemical structure of lignin by solution-state NMR. evolution of elastic and viscous moduli of lignin softening and solidification during pyrolysis (real-time and high temperature). swelling and shrinking behaviors of the lignin pellet during softening and solidification obtained by the displacement of the parallel plates of the rheometer. samples heated in the rheometer were quenched at various key temperatures (after softening and solidification) and are so-called “lignin residues” in the following text. analysis of C, H, O content of lignin and lignin residues elemental formulas of 100−600 Da molecules in lignin residues desorbed and ionized by a laser and analyzed by a high-resolution mass spectrometer molecular weight distribution of THF soluble acetylated lignin and lignin residues characterization of chemical structure of lignin and lignin residues. evolution of chemical moieties in lignin residues by solid-state NMR and FTIR.

response on the top plate of the rheometer31 and combined to define tan(δ) = G″/G′, where δ is the phase angle between stress and strain (δ = 0 for an elastic solid, δ = π/2 for a purely viscous fluid and 0 < δ < π/2 for a viscoelastic material). The rheological data were analyzed by using Trios software. 2.6. LDI FTICR-MS Analysis. The mass spectrometric analysis of lignin and lignin residues quenched at key temperature in the rheometer was carried out in a 9.4 T Fourier transform ion cyclotron resonance mass spectrometer (FTICRMS, IonSpec, Lake Forest, CA, USA) in a laser desorption ionization mode. A minilite air-cooled Nd:YAG Laser (wavelength 266 nm, forth harmonic, pulse duration 5 ns, maximum output energy 4 mJ-Continuum, San Jose, CA, USA) was used to produce ions by laser matter interaction. The ions generated in an external laser/desorption ion source were transferred into the FTICR ion cell. A very high mass resolution (higher than 400 000 at m/z 200) was achieved with an average mass measurement accuracy close to 0.5 ppm. The elemental compositions (O number and C number) are derived from the mass spectrometric data. 2.7. GPC-UV Analysis. The molecular weight distribution of lignin and lignin residues was determined by GPC after acetylation. The acetylation of lignin and lignin residues was carried out as in ref 20. Briefly, the lignin and lignin residue samples (20 mg) were dissolved in a 1:1 acetic anhydride/pyridine mixture (1 mL) and stirred for 24 h at room temperature. Ethanol (25 mL) was added to the reaction mixture, left for 30 min, and then removed with a rotary evaporator. The addition and removal of ethanol were repeated several times to ensure complete removal of acetic acid and pyridine from the sample. Afterward, the acetylated lignin and residues were dissolved in chloroform (2 mL) and added dropwise to diethyl ether (100 mL), followed by centrifugation. The precipitate was washed three times with diethyl ether and dried under vacuum at 40 °C for 24 h. The acetylated sample was dissolved in THF (1 mg·mL−1), the solution was filtered (pore size = 0.45 μm), and 20 μL of the solution was then injected in the GPC/UV. GPC analysis was performed using a Schimadzu HPLC system equipped with a variable-wavelength detector (UV, λ = 254 nm) on a column sequence consisting of a PLgel 5 μm guard column, Agilent PLgel 5 μm mixed-C, and Agilent PLgel 5 μm 100 Å. These columns were operated at 30 °C and eluted with tetrahydrofuran (THF) at a flow rate of 1 mL min−1. The calibration was made with the polystyrene standards ranging from 162 to 19540 g·mol−1. We are aware that the hydrodynamic volumes of the lignin and the polystyrene differ,21 therefore the GPC profiles are only discussed based on a qualitative comparison data collected and analyzed with Lab solution software. 2.8. FTIR Spectroscopy. The methods and results for FTIR analysis of the lignin and lignin residues are presented in the Supporting Information.

2.4. Solution-State 13C and 31P NMR Analysis. The methods and results for lignin characterization by 13C and 31P solution-state NMR are presented in the Supporting Information. 2.5. In Situ Rheology Analysis. The basics of in situ rheology analysis have been previously presented.31 The rheological analysis of the lignin has been performed with ARES G2 high-torque controlled strain rheometer. Lignin pellets (250 mg, 2.5 mm thickness, and 10 mm diameter) were prepared applying 2 ton force using a mechanical pelletizer. The lignin pellet was placed between two serrated plates (in stainless steel, built at CNRS Nancy) to reduce slippage (Figure 2).

Figure 2. Scheme of the rheometer with lignin pellet positioned between two parallel plates. Lignin and plates are located in a furnace swept by N2. Rheology analysis is conducted in situ during the pyrolysis of lignin. The dimensions of the pellet have been optimized in order to obtain a good signal on the rheometer and to avoid overflow during softening from the parallel plates’ configuration. The lignin pellet, the plates, and rheometer’s axes were positioned inside a furnace flushed by N2 (analytical purity). Plates were heated through the convection of N2, and the temperature is monitored by thermocouples placed inside the bottom plate and in the furnace. The lignin residues were quenched at key temperatures within the parallel plates by N2 flow. For instance, the quenching of the lignin residues at 400 °C was reached below 100 °C within 3 min. Tests were performed with 1% strain amplitude (γ, amplitude of oscillation divided by disk thickness) and a frequency (ω) of 1 Hz (6.28 rad·s−1) with the following heating conditions: initial temperature (35 °C), final temperature (400 °C), and heating rate (5 K· min−1). A constant normal force of 200 g was applied throughout the test to the sample from the top plate to reduce slippage. Therefore, the thickness of the sample was allowed to change during the analysis. The elastic (G′) and viscous (G″) moduli are obtained from the stress 6942


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ACS Sustainable Chemistry & Engineering 2.9. Solid-State CP/MAS 13C NMR Spectroscopy. Solid-state CP/MAS 13C NMR spectra of the lignin and lignin residues were recorded at 25 °C using a Bruker Avance III 600 spectrometer. The NMR probe used was a 4 mm DVT magic angle spinning probe. The acquisition time was 0.02 s with number of scans of about 10,000. All the spectra were run with a relaxation delay of 4 s, CP ramp of 50− 100% and contact time of 2 ms, spectral width of 60 kHz, and spinning rates of 10 kHz. No corrections for differences in the rates of crosspolarization have been made. The rotational side bands have not been taken into account. The spectra were deconvoluted using DMFIT (Gaus/Lor model) from CEMHTI, CNRS (Orléans, France).62 As curve fitting does not give a unique possibility because of the multiple overlapping signals, the spectra has been divided into broad classes. The NMR spectrum of Protobind lignin was first fitted with Gaussian curves. The NMR-signal parameters (line width, chemical shift) were kept to decompose spectra of pyrolyzed samples. Additional peaks were added in order to consider the formation of new chemicals structures. The yield in each deconvoluted moieties was calculated based on char yield (from TGA) and elemental analysis of char (as in ref 63). We are aware that common cross-polarization methods are not quantitative.63 For this reason the relative evolutions of the different moieties (e.g., mol of Caro in char/mol of Caro in lignin) are presented.

Figure 4. Fluidity development from in situ 1H NMR analysis (in dotted black line) and DSC curve showing glass transition temperature and heat of pyrolysis, as a function of temperature (in solid red line).

the glass transition temperature defined as the midpoint temperature of the step change in the heat capacity is found to be 135 °C. An endothermic signal around 100 °C is observed for all DSC analyses carried out without annealing (not shown). The shift in the heat capacity is not easily detectable with this large endothermic peak. Annealing of lignin makes the determination of Tg easier because no endothermic peak from drying is observed for the annealed lignin (Figure 4). The glass transition temperature of the same lignin has been determined by conventional DSC with annealing, modulated temperature DSC, TMA, and rheology to be 162, 163, 164, and 179 °C, respectively.65 This difference may be explained by a different DSC procedure.65 The heat of lignin pyrolysis is exothermic from 150 to 500 °C (Figure 4). The exothermic peak maximum is at 420 °C. Upon pyrolysis, bonds are broken, but the endothermicity of bonds breakage is overlapped by the important exothermicity of cross-linking reactions forming char (cross-linked aromatic residues). This feature is in agreement with previous studies.36,41 The exothermic DSC signal highlights the progressive cross-linking reactions starting from temperature as low as 150 °C. 3.1.2. In Situ 1H NMR. The in situ 1H NMR shows the molecular mobility of lignin during pyrolysis. Figure 4 presents the fluidity development (fraction of mobile protons, see eq 1) of lignin as a function of temperature. The fluidity starts from around 140 °C and reaches 50% at 175 °C and further increases to 100% at 200 °C. The fluidity remains 100% until 325 °C. This means that 100% of the protons are fluid within the lignin/char matrix on this range of temperatures (200−325 °C). Then the fluidity drops down to 50% at 350 °C. This lignin (soda pulping from straw) exhibits a similar pattern as an organosolv lignin extracted from miscanthus.31 3.1.4. In Situ High-Temperature Rheology. The rheological signature of lignin pyrolysis is presented in Figure 3. It shows the evolution of elastic and viscous moduli as a function of temperature. At around 130 °C, the elastic and viscous moduli both decrease with the increase in temperature and reach a minimum at around 225 °C. Then, they increase with the increase in temperature further. At 370 °C, the viscous modulus decreases significantly, and the elastic modulus stops increasing. The physical modifications undergone by lignin during pyrolysis are defined by the onsets of softening and solidification (Figure 3a). These are the points at which the elastic modulus equals the viscous modulus. Two distinct temperature ranges from 140 to 250 °C and from 250 to 370

3. RESULTS AND DISCUSSION 3.1. Characterization of Lignin. 3.1.1. TGA and DSC Analysis. The rate of mass loss during lignin pyrolysis as a function of temperature (5 K min−1), as obtained by TGA, is presented in the Figure 3a. After drying, the mass loss starts at

Figure 3. Evolution of (a) elastic and viscous moduli (from high temperature in situ rheology) and mass loss (obtained by TGA). (b) Displacement between plates of the rheometer (ΔL) and tan(δ) as a function of temperature (5K·min−1).

about 130 °C. The significant mass loss of lignin occurs between 250 and 400 °C. The temperature of maximum weight loss is 345 °C. The char yield at 900 °C is 32% (Supporting Information). For the same lignin, the temperature of maximum decomposition rate (341 °C) and a final char yield at 800 °C of 37.7% have been determined for a heating rate of 20 K min−1.64 The glass transition temperature and the heat of lignin pyrolysis reaction were studied by DSC. As shown in Figure 4, 6943


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ACS Sustainable Chemistry & Engineering °C are defined as the softening and resolidification regions, respectively. The temperatures 195 and 308 °C represent the midpoint temperatures of the softening and solidification regions, respectively. Two other experiments were conducted to quench the lignin residues at these targeted temperatures of 195 and 308 °C. These residues along with the initial lignin and final lignin residues (400 °C) were further characterized by various ex situ analysis techniques (Table 1) in order to figure out the chemical changes undergone by lignin during pyrolysis. The vertical displacement of the parallel plates normalized by the initial thickness of the lignin pellet depicts the real time swelling and shrinking behavior of the lignin upon thermal conversion as presented in Figure 3b. A linear evolution of ΔL as a function of temperature before 105 °C and after 250 °C is essentially related to the thermal dilatation/contraction of the sample when it is in a solid state (lignin before softening and char after solidification) and, to a lower extent, to the dilatation of the parallel plates of the rheometer (about 1 μm K−1). The swelling of the pellets occurred in the temperature range from 110 to 125 °C. A significant decrease in gap is observed at a temperature range of 125−225 °C due to the glass transition and softening under the normal force imposed by the upper plate (200 g). The mechanism of lignin “softening” will be explained further in the following sections. Concerning the evolution of tan(δ) (Figure 3b, tan(δ) = viscous modulus/elastic modulus), it is higher than 1 from 140 to 250 °C, which represents the soft and viscous nature of the softening lignin undergoing thermal conversion. From 250 °C, tan(δ) becomes lower than 1 which indicates the dominant elastic behavior of the resolidified lignin residue. 3.2. Characterization of Lignin Residues Quenched at Key Temperatures within the Rheometer. 3.2.1. Elemental Analysis of Lignin Residues. The results of elemental analysis are presented in Supporting Information, and they are briefly discussed hereafter. A decrease in the oxygen and hydrogen contents and a gradual enrichment of carbon in the lignin residues are observed with the increasing temperature. Both the H/C and O/C ratios decrease by about 40% to the final temperature. A significant decrease of H/C ratio occurs after 308 °C, while the decrease of O/C ratio occurs after 195 °C. This is in agreement with previous work.19 A reduction of the H/C and O/C ratios have been accounted for the pyrolysis lignin chars obtained at 250 and 550 °C.19 3.2.2. LDI-FTICRMS Analysis of Lignin and Lignin Residues. To the best of our knowledge, we present here the first LDIFTICRMS analysis of lignin and lignin residues quenched at key temperatures. Our group has previously shown the interest of LDI-FTICRMS to analyze lignin oligomers in bio-oils30 but not yet for chars. The mass spectra obtained for lignin and for lignin residues produced at 195, 308, and 400 °C are presented in the Supporting Information. All attributed species are presented in the Supporting Information for the four samples. About 600 species were attributed for lignin or residues produced (at 195 and 308 °C). A systematic study has been conducted in order to optimize the conditions of the LDI (laser wavelength and power).66 Under these tailored LDI conditions, the mobile species quenched in the matrix of the material are desorbed and ionized with reduced thermal effects from the laser because soft and selective LDI conditions have been used for this study. Figure 5 presents the distribution of the species as analyzed by LDI-MS as a function of their oxygen number and carbon number.

Figure 5. LDI-FTICRMS analysis of lignin and lignin residues showing the number of O and C atoms present in the species desorbed and ionized by the laser from the aromatic matrix.

The surface area of the circles is function of the relative abundance of the species. The main species analyzed by this method are between C10 to C40 and O1 to O8. These compounds may be mobile oligomers (as revealed by in situ 1H NMR and GPC analysis) formed from lignin pyrolysis, quenched and entrapped in the matrix. Figure 6 presents a simplified scheme to depict the complementarity between LDI-MS, 1H in situ NMR, and GPC analyses. 1H NMR analyzes in situ all the mobile protons. LDI-MS analyzes ex situ the molecular species that can be desorbed by the laser from the solid (quenched) matrix. These species contribute in part to the mobility of protons during pyrolysis. GPC analysis gives the molecular weight distribution of species soluble in THF after acetylation. Therefore, these three methods present an interesting complementarity in order to analyze the “liquid-like” species entrapped in the “char” upon lignin conversion. With the increase in temperature, the LDI-MS analysis shows that the oxygen number of the abundant compounds decreases and the carbon number increases (Figure 5), demonstrating a progressive deoxygenation and cross-linking of these species. The range of oxygen numbers also decreases with temperature. The signal detected at 400 °C is not presented because of the 6944


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Figure 6. Simplified scheme illustrating the complementarity between in situ 1H NMR, LDI-MS, and THF extraction followed by GPC analysis for the analysis of the intermediate “liquid-like” structure formed during lignin pyrolysis. Circles represent aromatic subunits which are covalently linked together or not (in the 3D network of lignin and char). The matrix is composed of cross-linked (“solid-like”) structures forming “solid char” and mobile species (“liquid-like” at temperature of reactions) which can be analyzed by these three methods. 1H NMR quantifies in situ all the mobile protons at the temperature of reactions. LDI-FTICRMS gives (after quenching) the elemental composition of “oligomers” desorbed by the laser. GPC gives the molecular weight distribution of acetylated species which are soluble in THF and therefore extracted by THF from the matrix.

highly cross-linked nature of the solid residue which leads to a very weak signal, in agreement with in situ 1H NMR and GPC analysis. 3.2.3. GPC-UV Analysis. The GPC-UV analysis of the acetylated lignin and residues gives a bimodal molecular weight distribution as presented in Figure 7. The lignin and lignin

Figure 8. CP-MAS 13C NMR spectra of lignin and lignin residues quenched at key temperatures in the rheometer.

Figure 7. GPC-UV of the THF-soluble fraction of acetylated lignin and lignin residues quenched at key temperatures in the rheometer.

residues quenched at 195 °C are completely soluble in THF (after acetylation), while the lignin residue at 308 °C is partly soluble (after acetylation), and the lignin residue at 400 °C is sparingly soluble. As shown in the Figure 7, a higher molecular weight fraction is present in the lignin residue at 195 °C compared with the initial lignin. This finding demonstrates that cross-linking reactions have already occurred at 195 °C in agreement with the DSC signal (exothermic behavior from 150 °C). At 308 °C, the soluble fractions exhibit a similar pattern as the extracted fraction at 195 °C but at a lower yield due to the more crosslinked nature of the material. 3.2.4. Solid 13C CP-MAS NMR. The 13C carbon NMR spectra of lignin and lignin residues quenched at key temperatures in the rheometer are presented in Figure 8. The lignin substructures remain almost intact until 300 °C. Concerning the residue at higher temperature (400 °C), the spectrum shows an important depletion of lignin substructures. The fitting profiles and deconvolution data (chemical shift, line width) are presented in Supporting Information. The evolution of the deconvoluted moieties as a function of temperature is displayed in Figure 9. The changes in the aromaticity during pyrolysis are accounted by monitoring the changes in protonated (124− 102 ppm), condensed (142−124 ppm) and oxygenated aromatic carbons (160−142 ppm) as a function of temperature.

Figure 9. Evolution of structural moieties (from CP-MAS 13C NMR) as a function of temperature (a) aromatic carbons and (b) aliphatic carbons.

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in situ rheology are complemented by the other analyses to give an overall outlook of the mechanism of lignin pyrolysis. First, the swelling of the lignin pellet in the rheometer is observed from 110 to 125 °C. The onset of softening (where elastic modulus becomes equal to viscous modulus) is determined at 140 °C which nearly coincides with the glass transition temperature (135 °C), as depicted by DSC analysis. Along with the softening, the 1H fluidity starts (at around 140 °C), as revealed by in situ 1H NMR analysis with 50% fluidity reached at 175 °C. Lignin pellet shrinks between 140 and 250 °C due to the softening, as analyzed by the displacement between the parallel plates of the rheometer. This softening region is marked by the decrease in the elastic modulus and viscous modulus. Both moduli reach a minimum value at 225 °C. Lignin residue quenched at 195 °C clearly represents the softened lignin. The analysis by CP MAS 13C NMR spectra of lignin residue at 195 °C does not show a significant cleavage of the side chain of α, β, and γ origin (Cα, Cβ, Cγ). This cleavage has not been distinguished by FTIR. However, the GPC-UV analysis of the same residue shows the increased intensity of the higher molecular weight fraction indicating that some crosslinking reactions have occurred. The immediate growth of a broad exothermic peak from the DSC curve also supports this cross-linking behavior. The significant mass loss occurs from 250 to 400 °C with the maximum rate of decomposition at 345 °C (30% mass loss). The 1H fluidity increases to 100% at around 200 °C, which remains constant until 325 °C. This is consistent with the GPC and LDI-FTICR-MS analyses which show that there are still mobile species at 308 °C (desorbed by LDI or extracted by THF). The elastic modulus and the viscous modulus increase and cross at 250 °C, which is marked as the onset of solidification. After the onset of solidification, the elastic modulus and the viscous modulus increase until 370 °C. Even though the solidification starts at 250 °C, the 1H fluidity remains 100% until 325 °C. Mobile species (analyzed by 1H NMR, GPC and LDI-FTICRMS) are still present in “cavities” formed by the solid matrix of char (mainly elastic, as analyzed by rheology), in agreement with our previous work.31 An interesting finding based on 13C NMR is that the side chain carbons are mainly converted between 195 and 308 °C, within the matrix rich in mobile protons, whereas there are still few modifications of the aromatic structures until 308 °C. The aromatic structures are rather converted after the solidification of the matrix, within a matrix with less mobile protons. Mobile protons stabilize the radical intermediates formed during side chain cleavage, and the aromatic structure is not really modified within the softened matrix (rich in H-donor species). When the 1 H mobility decreases, the matrix becomes more “rigid”, and the condensation of aromatic structures is favored. This mechanism is in agreement with previous work on lignin67,68 or coal69 pyrolysis, but this present work is the first one to clearly evidence this mechanism based on in situ analysis and ex situ 13C NMR. Then the structure of the residue at 400 °C is mainly composed of aromatic clusters. The condensation of aromatics as analyzed by 13C NMR coincides with the heat of charring represented by the exothermic peak analyzed by DSC with a maximum at 420 °C. To summarize, upon heating lignin produces a soft material which is depolymerized within a mobile (liquid-like) matrix. This soft matrix is progressively enriched in carbon, becomes

The evolution of the total aromatic content (160−102 ppm) remains stable for the residues up to 308 °C and slightly decreases for the residue produced at 400 °C (Figure 9a). This observation is in agreement with previous work.19 The protonated aromatic carbon signals decrease slightly for the lignin residue at 308 °C and then more significantly at 400 °C. It is interesting to notice that the composition of the aromatic structure is not significantly modified until 308 °C, which is in agreement with LDI-MS analysis. The main deoxygenation occurs after 308 °C, thus within the stiffened matrix. Indeed, the oxygenated aromatic carbons decrease significantly at 400 °C. On the other hand, the condensed aromatic carbon signals increase at 400 °C. These observations highlight the aromatization of the lignin structure. This aromatization process of char is counterbalanced by the devolatilization of aromatic compounds. The residue at 400 °C shows the appearance of a broad aromatic signal centered at 126 ppm, as already observed in lignin chars.19 The decrease of aliphatic carbons (Figure 9b) is more marked at the lower temperatures (195 and 308 °C residues) than the one of aromatic carbons. At 195 °C, a slight decrease of carbonyl moieties and increase of alkyl and alkenyl carbons can be noticed. The cleavage of the side chain carbons of α, β, and γ origins (Cα, Cβ, Cγ, detected at 95−70 ppm) is clearly evidenced from 308 °C and further decreases at 400 °C. This finding highlights that the depolymerization of lignin mainly occurs within the softened matrix with few modifications of the aromatic structures, and then the aromatic structures are rather converted after the solidification of the matrix. The methoxyl carbon signal (56 ppm) decreases more progressively than side chain carbons. Further studies are needed to explain this decrease of methoxyl carbons which could be attributed to demethoxylation reactions and to the devolatilization of methoxyl aromatic species. The alkyl and alkenyl aliphatic carbons decrease from 308 to 400 °C, which is in agreement with the previous investigation of Sharma et al.19 3.3. Discussion on the Physical-Chemical Mechanism of Lignin Pyrolysis. The scheme (Figure 10) of the physicalchemical mechanism of lignin pyrolysis is revealed by means of our complementary techniques. High-temperature in situ rheology is a pivotal method for studying lignin pyrolysis. Therefore, our findings are illustrated along with the rheological signature of the lignin pyrolysis in Figure 10. The results from

Figure 10. Scheme of physical-chemical mechanism of lignin thermal conversion based on the rheological signature of lignin (curves). 6946


ACS Sustainable Chemistry & Engineering more rigid, and produces a condensed aromatic structure. Finally it gives rise to the fused and solid char as illustrated in Figure 1.

ACKNOWLEDGMENTS

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REFERENCES

(1) Holladay, J. E.; White, J. F.; Bozell, J. J.; Johnson, D. Top ValueAdded Chemicals from Biomass - Vol. II - Results of Screening for Potential Candidates from Biorefinery Lignin; PNNL-16983; Pacific Northwest National Laboratory: Richland, WA, 2007. (2) Zakzeski, J.; Bruijnincx, P. C. A.; Jongerius, A. L.; Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 2010, 110 (6), 3552−3599. (3) Ragauskas, A. J.; Beckham, G. T.; Biddy, M. J.; Chandra, R.; Chen, F.; Davis, M. F.; Davison, B. H.; Dixon, R. A.; Gilna, P.; Keller, M.; Langan, P.; Naskar, A. K.; Saddler, J. N.; Tschaplinski, T. J.; Tuskan, G. A.; Wyman, C. E. Lignin Valorization: Improving Lignin Processing in the Biorefinery. Science 2014, 344 (6185), 1246843. (4) Nowakowski, D. J.; Bridgwater, A. V.; Elliott, D. C.; Meier, D.; de Wild, P. Lignin Fast Pyrolysis: Results from an International Collaboration. J. Anal. Appl. Pyrolysis 2010, 88 (1), 53−72. (5) de Wild, P. J.; Huijgen, W. J. J.; Heeres, H. J. Pyrolysis of Wheat Straw-Derived Organosolv Lignin. J. Anal. Appl. Pyrolysis 2012, 93, 95−103. (6) Ma, Z.; Troussard, E.; van Bokhoven, J. A. Controlling the Selectivity to Chemicals from Lignin via Catalytic Fast Pyrolysis. Appl. Catal., A 2012, 423−424, 130−136. (7) Li, D.; Briens, C.; Berruti, F. Improved Lignin Pyrolysis for Phenolics Production in a Bubbling Bed Reactor − Effect of Bed Materials. Bioresour. Technol. 2015, 189, 7−14. (8) Goheen, D. W.. Hydrogenation of Lignin by the Noguchi Process. In Lignin Structure and Reactions; Advances in Chemistry; American Chemical Society: Washington, DC, 1966; Vol. 59, pp 205− 225. (9) Vigneault, A.; Johnson, D. K.; Chornet, E. Base-Catalyzed Depolymerization of Lignin: Separation of Monomers. Can. J. Chem. Eng. 2007, 85 (6), 906−916. (10) Jongerius, A. L.; Jastrzebski, R.; Bruijnincx, P. C. A.; Weckhuysen, B. M. CoMo Sulfide-Catalyzed Hydrodeoxygenation of Lignin Model Compounds: An Extended Reaction Network for the Conversion of Monomeric and Dimeric Substrates. J. Catal. 2012, 285 (1), 315−323. (11) Song, Q.; Wang, F.; Cai, J.; Wang, Y.; Zhang, J.; Yu, W.; Xu, J. Lignin Depolymerization (LDP) in Alcohol over Nickel-Based Catalysts via a Fragmentation−hydrogenolysis Process. Energy Environ. Sci. 2013, 6 (3), 994−1007. (12) Oregui Bengoechea, M.; Hertzberg, A.; Miletić, N.; Arias, P. L.; Barth, T. Simultaneous Catalytic De-Polymerization and Hydrodeoxygenation of Lignin in Water/Formic Acid Media with Rh/ Al2O3, Ru/Al2O3 and Pd/Al2O3 as Bifunctional Catalysts. J. Anal. Appl. Pyrolysis 2015, 113, 713−722. (13) Olcese, R. N.; Francois, J.; Bettahar, M. M.; Petitjean, D.; Dufour, A. Hydrodeoxygenation of Guaiacol, A Surrogate of Lignin Pyrolysis Vapors, Over Iron Based Catalysts: Kinetics and Modeling of the Lignin to Aromatics Integrated Process. Energy Fuels 2013, 27 (2), 975−984. (14) Olcese, R. N.; Lardier, G.; Bettahar, M.; Ghanbaja, J.; Fontana, S.; Carré, V.; Aubriet, F.; Petitjean, D.; Dufour, A. Aromatic Chemicals by Iron-Catalyzed Hydrotreatment of Lignin Pyrolysis Vapor. ChemSusChem 2013, 6 (8), 1490−1499. (15) de Wild, P.; Van der Laan, R.; Kloekhorst, A.; Heeres, E. Lignin Valorization for Chemicals and (Transportation) Fuels via (Catalytic) Pyrolysis and Hydrodeoxygenation. Environ. Prog. Sustainable Energy 2009, 28 (3), 461−469.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01130. Thermogravimetric analysis, methods and results for solution-state 13C and 31P NMR characterization of the lignin; elemental analysis (C, H, O, and ashes) and Van Krevelen diagram of lignin and residues, deconvolution of solid-state 13C NMR spectra, FTIR analysis, LDI FTICRMS analysis of the lignin and lignin residues (PDF)

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The authors gratefully acknowledge the financial support of the French research national agency thorough the project “PYRAIM” (ANR-11BS09-003), the CNRS interdisciplinary program and INSIS institute of CNRS thorough the “FORêVER” project, the CPER Lorraine “Valorisation énergétique de la biomasse”, and the TGE-TGIR CNRS “FT-ICR”.

4. CONCLUSION To the best of our knowledge, such a multitechnique complementary development is presented for the first time in order to understand the physical-chemical mechanism of lignin pyrolysis. The viscoelastic behavior (swelling, softening, shrinking and solidification) is depicted by in situ hightemperature rheology. The DSC analyzes the glass transition temperature (at 135 °C) along with the heat of pyrolysis showing the exothermic peak maxima at 420 °C due to crosslinking reactions. The fluidity measured by in situ 1H NMR shows that the fluidity starts at 150 °C and reaches 100% at around 200 °C until 325 °C, which then decreases. Elemental analysis and the LDI-FTICRMS analysis shows a decline of the O and H numbers along with the gradual increase in the C numbers as a function of temperature. LDI-FTICRMS figures out an average composition of the complex intermediate mobile structures entrapped in the quenched solid matrix. FTIR accounts insignificant changes in the structural moieties for the lignin residue at 195 °C. However, it accounts intense loss of the structural moieties for the lignin residue at 308 and 400 °C. 13 C CP MAS NMR depicts the increase of condensed aromatic carbons and the decrease of protonated and oxygenated aromatic carbons as a function of temperature. It also highlights the gradual decline of side chain carbons and methoxyl carbon. This multitechnique analysis will contribute to the development of a detailed mechanism of lignin pyrolysis. This study is also useful to design feeders and reactors for the thermo-chemical conversion of lignin into materials or chemicals. On practical aspects, this study provides information to set the most appropriate thermal conditions for the production of carbon fibers by melt spinning of lignin and also to design pyrolysis reactors by managing agglomeration or plugging issues from lignin softening and swelled char formation. The present methodology could be applied to faster heating conditions and to other biomasses, lignins, or polymers.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Binod Shrestha: 0000-0002-5828-2501 Notes

The authors declare no competing financial interest. 6947


Research Article

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A Multitechnique Characterization of Lignin Softening and Pyrolysis

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