Article pubs.acs.org/EF
Subcritical Water Reactions of Lignin-Related Model Compounds with Nitrogen, Hydrogen, Carbon Monoxide, and Carbon Dioxide Gases Meredith A. Hill Bembenic*,† and Caroline E. Burgess Clifford‡ †
John and Willie Leone Family Department of Energy and Mineral Engineering and the EMS Energy Institute, Penn State University, 225 Academic Projects Building, University Park, Pennsylvania 16802, United States ‡ The EMS Energy Institute and the John and Willie Leone Family Department of Energy and Mineral Engineering, Penn State University, C205 Coal Utilization Laboratory, University Park, Pennsylvania 16802, United States S Supporting Information *
ABSTRACT: Experiments using subcritical H2O (365 °C) and various industrial gases (500 psi cold pressure of N2, H2, CO, or CO2) have been considered previously for the conversion of Organosolv lignin (representative of lignin-derived compounds generated from the production of bioethanol from woody biomass sources) into value-added products. The findings indicated that subcritical H2O can be used to depolymerize lignin into GC amenable compounds with or without added gas pressures, though adding gas further modified the types of products generated. However, the roles of the combined effect of subcritical H2O and the gases were not confirmed, as the reaction chemistry for lignin depolymerization at these conditions is not intuitive. Thus, experiments with subcritical H2O, the aforementioned industrial gases and lignin-related model compounds (i.e., aromatic aldehydes represented by vanillin and syringaldehyde, aromatic ketones represented by acetovanillone and acetosyringone, and aromatic ethers represented by dibenzyl ether and 2-phenethyl phenyl ether), were completed to study this. From these results, the suggested reaction pathway of Organosolv lignin reactions in subcritical H2O with and without added pressure appears to proceed in two stages. The first stage appears to be a hydrolysis reaction that breaks the ether linkages between lignin aromatic monomers with typical methoxy and hydroxyl functional groups and either an aldehyde or ketone (i.e., syringaldehyde, acetophenone). A second stage appears to remove the aldehyde/ketone so that the main products are methoxy phenols.
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INTRODUCTION Cellulosic bioethanol technologies may only be cost-effective if the lignin-based byproduct is upgraded to value-added products (e.g., gases, chemicals, materials, fuels). However, effective conversion of the lignin-based byproducts is hindered due to its inherent aromatic structure consisting of highly branched, intermolecular cross-linkages of phenylpropane units (i.e., pcoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol).1−3 In particular, the chemical and physical reactions are difficult to predict, and augmenting this is the suggestion that byproduct lignin is even more condensed and cross-linked than native lignin.4 Subcritical H2O (sub-H2O) at 365 °C has been considered previously for the conversion of Organosolv lignin into valueadded products using different industrial gases including N2, H2, CO, and CO2 (500 psi cold pressure).5−8 There was a focus on broad characterization of the products to include solids (with solid state 13C NMR), organic liquids (with GC/MS, LC/MS, and TOFMS), and the water fraction (with solid phase microextraction). These products are not often reported in the literature as extensively as the gaseous products for similar research concerning the reaction of lignin-derived sources with H2O. In our previous research, lignin was depolymerized into GC-amenable compounds using sub-H2O with or without the added cold gas pressures.5−8 The sub-H2O was expected to cleave lignin ether bonds, as this has been demonstrated in water at 250−350 °C for similar organic compounds.9−11 The © 2013 American Chemical Society
addition of the separate gases was expected to prevent retrogressive cross-linking reactions, which would suppress the formation of solids. 12 The addition of gas (and consequently the increase in evolved reaction pressure) positively affected conversion and modified the types of products generated.5−8 The products generated from a previous sub-H2O reaction included primarily 2,6-dimethoxyphenol and trimethoxybenzene.5−8 From these results and the literature, ethers can be cleaved by sub-H2O,9−11,13,14 but the role of the types of gases is not clear yet. Lignin-related aromatic monomer compounds are used in this research as a means to simplify the products generated: Aromatic ethers are represented by dibenzyl ether (DBE) and 2-phenethyl phenyl ether (PPE), aromatic aldehydes are represented by vanillin and syringaldehyde, and aromatic ketones are represented by acetovanillone and acetosyringone. The experiments use the same reaction conditions as the aforementioned experiments involving Organosolv lignin, subH2O, and different gases.5−8 This research is not intended to cover all types of lignin-related model compounds (e.g., C−C linkages), but compounds were chosen based on availability, use by previous researchers, knowledge of wood structure, and awareness about the types of reactions (and products) Received: June 13, 2013 Revised: October 14, 2013 Published: November 12, 2013 6681
dx.doi.org/10.1021/ef401113a | Energy Fuels 2013, 27, 6681−6694
Energy & Fuels
Article
Figure 1. Model compounds used in the subcritical water reactions (365 °C) with nitrogen, hydrogen, carbon monoxide, and carbon dioxide gases.
Figure 2. Product separation and analyses after experiments were quenched.
viously observed in lignin, and they can also be viewed as building blocks to larger, complex molecules (i.e., dimeric and tetrameric ones). The ease of obtaining monomeric compounds compared to the synthesis of the more complex ones is noted, and more representative results are expected from compounds that address one or more of the primary linkages observed in lignin. However, the selected lignin-related model compounds are expected to supplement the literature concerning reactions with lignin from different sources These lignin-related model compounds also address the aromaticity of lignin, which is perhaps the most important physical and chemical characteristic of lignin since it is required for complete degradation of lignin.20 It is thought that both ionic and free radical reactions may be occurring competitively at near supercritical conditions due to the low density of water used with the high temperature and high pressure conditions studied in this research.21−24 The results from subsequent experiments among sub-H2O, gases, and lignin-related model compounds are expected to clarify whether ionic reactions (i.e., hydrolysis) or free radical
generated from the reaction of lignin with sub-H2O. For example, Funazukuri et al. (1997) chose DBE as a model compound for “coal or related substances”, such as lignin (i.e., lignin is a precursor to coal).15 González and Montané (2005) also used DBE as a representative ether linkage of lignin.16 Britt et al. (1995) chose PPE as a representative lignin and low rank coal model compound for their work involving thermolysis at 330−425 °C. 17 Hawkes et al. (1993) used vanillin, acetovanillone, and syringaldehyde in their research that compared structures of lignin and lignin model compounds using solution and solid state 13C NMR.18 These compounds were chosen to represent the phenolic portions of lignin.18 Wong et al. (2010) have also shown that oxygen delignification of different woody biomass sources (like pine, eucalyptus pulp, and wheat straw pulp) generated different aromatic aldehydes (vanillin and syringaldehyde) and aromatic ketones (acetovanillone and acetosyringone).19 The structure of lignin cannot be represented solely using simple monomeric units. However, the selected monomers are representative of particular fundamental functionalities pre6682
dx.doi.org/10.1021/ef401113a | Energy Fuels 2013, 27, 6681−6694
Energy & Fuels
Article
Figure 3. Spectra from GC/MS for dibenzyl ether reacted at baseline conditions with and without gas (365 °C, 500 psi, 30 min) with corresponding information seen in Table 1. collected in gas bags for later analysis by GC. The reactor was then opened, and the products were removed from the reactor barrel using dicholoromethane (DCM). A modified Dean−Stark apparatus (35 mL capacity) was used with DCM during the subsequent product workup to separate the organic liquid product (DCM-solubles) from the water fraction.5−8,26 The water fraction is then removed from the apparatus, weighed, and stored in a refrigerator until ready for analysis by solid phase microextraction (SPME). The DCM within the organic liquid filtrate was subsequently evaporated using rotary evaporation so that the organic liquid fraction mass could be determined. The evaporated filtrate is then dissolved in methanol (20 mg of sample in 1 mL of solvent) for later analysis using GC/MS. Further filtration of the organic filtrate samples was required prior to GC/MS analysis using Iso-Disc syringe tip filters (purchased from Supelco, nylon membrane, pore size 0.2 μm). There were so little solids collected that further analysis of any residual solids was not performed. Evolved gases were characterized using headspace GC (Shimadzu GC17A) with FID and TCD as described elsewhere.5,7 Results from these analyses are presented in the Supporting Information. All recovered organic liquids were characterized using GC/MS (Shimadzu GC17A with QP5000 MS) and quantified as area percentage of peaks. Further information regarding the GC program settings and compound identification are described elsewhere.5,7 SPME was used for analysis of the collected water fractions. Methanol yields (%) are defined as
reactions (i.e., thermolysis) dominate. The roles of the combined effect of sub-H2O and the gases are also explored.
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EXPERIMENTAL SECTION
The lignin-related model compounds, shown in Figure 1, were used as received. Dibenzyl ether (purchased from Sigma Aldrich, CAS Number: 103-50-4) and 2-phenethyl ether (purchased from Frinton Laboratories, Inc., CAS Number: 40515-89-7) were chosen to represent aromatic ethers. Vanillin (purchased from Sigma Aldrich, CAS Number: 121-33-5) and syringaldehyde (purchased from Sigma Aldrich, CAS Number: 134-96-3) were chosen to represent aromatic aldehydes. Acetovanillone (purchased from Sigma Aldrich, CAS Number: 498-02-2) and acetosyringone (purchased from Sigma Aldrich, CAS Number: 2478-38-8) were chosen to represent aromatic ketones. Each model compound gas experiment requires a model compound (0.571 g), H2O (4 g), and industrial gases (500 psi of either N2, H2, CO, or CO2).5−8 The lignin-related model compound and H2O were loaded into the tubing reactors (stainless steel, ≈ 25 mL) such that the mass loading ratio for model compound-to-water was 1:7, which mimics the mass loading ratio of Siskin et al. (1990) work with model compounds and H2O.25 The reactor was purged with N2 gas (1000 psi) three times, and then the selected gas (either N2, H2, CO, or CO2) was added to the reactor (500 psi). Experiments between a model compound and H2O without additional gas pressure were also conducted for comparison to the model compound gas experiments. The fully loaded reactor was placed into a 365 °C preheated, fluidizing sand bath for 33 min (this time includes 3 min of initial heat-up time of the reactor). The actual experiment time is 30 min. The pressure of the tubing reactor was monitored over the duration of the experiment (at 5 min intervals) and also after quenching of the experiment in a cold water bath. For mass balance purposes, the tubing reactor mass was recorded at several instances before and after each experiment: 1) before adding any model compound, water, and/or gas, 2) after addition of model compound, water, and/or gas, 3) after reaction is complete, and 4) after gases are collected into gas bags. The determination of water mass collected is discussed in detail in the following paragraph. Figure 2 outlines the subsequent product separation and analyses that followed after quenching of the experiments. Evolved gases were
mass methanol (g) × 100 initial model compound mass (g) and methanol in total liquids (%) is defined as
mass methanol (g) × 100 mass methanol(g) + DCM‐solubles mass (g) Further specifics about the SPME sampling procedure and the GC program settings are reported elsewhere.5,7
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RESULTS AND DISCUSSION Lignin-related aromatic monomer compounds were used in this research based on results from previous experiments involving 6683
dx.doi.org/10.1021/ef401113a | Energy Fuels 2013, 27, 6681−6694
Energy & Fuels
Article
Organosolv lignin, sub-H2O, and different gases.5−8 The previous research showed that lignin depolymerization occurred in the presence of sub-H2O, specifically as follows: 1) a variety of phenolic compounds were detected in the DCMsoluble liquids using GC/MS (i.e., phenolic and/or methoxysubstituted benzenes and noticeable absence of ethers and propyl groups), 2) a significant change in reacted solids from the unreacted lignin was observed using solid state 13C NMR (i.e., decrease in aliphatic compounds compared to the unreacted lignin likely due to the loss of saturated alkanes and methoxy groups, and an increase in aromaticity likely corresponding to char formation), and 3) a low amount of high molecular weight compounds were present in the DCM-soluble liquids products based on LC/MS and TOFMS.5−8 The primary conversion of lignin into low molecular weight compounds is suggested to be from the hydrolysis of the βO-4 ether bond,5−8 which is similar to reactions between supercritical H2O and lignin. Adding an initial pressure improved conversion (54−62% conversion) compared to when there was no initial pressure (40% conversion). This added pressure also selectively modified the types of products generated. The combined roles of sub-H2O and the added gases were not confirmed yet based on these results, but the reactions with gases seemed to be showing the most reactivity based on the solid state 13C NMR results.5−8 The following results from reactions with sub-H2O, gases, and different lignin-related model compounds (i.e., aromatic ethers, aromatic aldehydes, and aromatic ketones) are expected to clarify whether the reactions with sub-H2O and/or these particular gases are primarily ionic, free radical, or both. Aromatic Ethers. Alcohols are primarily expected to be in the liquid products if the ether linkage were cleaved by H2O via ionic reactions (like hydrolysis).27−30 However, alcohols and large amounts of char and high molecular weight compounds are expected if the ether linkage were cleaved by pyrolysis (free radical reactions).31−34 Dibenzyl Ether (DBE). Figure 3 and Table 1 respectively show the GC/MS spectra and corresponding information about
previous experiment used with these same reactors) was looked into, but product carryover did not prove to be an obvious problem. When adding an initial gas, the primary compound detected becomes benzyl alcohol. For the runs with CO and CO2, benzyl alcohol (≈76−77% relative area) and DBE (≈22% relative area) were the primary compounds detected. The runs with N2 and H2 primarily had benzyl alcohol (≈61.5% and ≈80.1% relative area, respectively) and DBE (≈37% and ≈17% relative area, respectively). Townsend and Klein (1985) and Townsend et al. (1988) suggested that DBE cleavage in high temperature H2O proceeds two ways via parallel reaction pathways: 1) hydrolysis of DBE into two moles of benzyl alcohol and 2) neat thermolysis of DBE into toluene and benzaldehyde.13,14 The presence of primarily benzyl alcohol, compared to the low or no presence of benzaldehyde and toluene, for the reaction of DBE with sub-H2O and different gases suggests that hydrolysis is primarily occurring. Without a gas, the low presence of benzyl alcohol, benzaldehyde, and 2-phenethyl phenyl ether (≈ 10% relative area combined) suggests that the potential for pyrolysis exists. Tsao et al. (1992) reported low yields (0.06 mols product/mol reacted) of benzaldehyde in the products from the reaction of benzyl alcohol and supercritical H2O (400 °C, 3872.5 psi) without a catalyst.35 They also reported low yields of benzyl alcohol (
