Subcritical Water Reactions of a Hardwood ... - ACS Publications

Jun 4, 2012 - Meredith A. Hill Bembenic*. ,† and Caroline E. Burgess Clifford. ‡. †. John and Willie Leone Family Department of Energy and Miner...
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Subcritical Water Reactions of a Hardwood Derived Organosolv Lignin 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, The Pennsylvania State University, 225 Academic Projects Building, University Park, Pennsylvania 16802, United States ‡ The EMS Energy Institute and John and Willie Leone Family Department of Energy and Mineral Engineering, The Pennsylvania State University, C205 Coal Utilization Laboratory, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: Subcritical H2O at 365 °C is considered for lignin conversion, because H2O exhibits unusual properties at higher temperatures (i.e., decreased ion product and static dielectric constant), such that there is a high solubility for organic compounds. This high solubility for organic compounds is expected to apply to lignin for its conversion into high value transportation fuels, which may prove the effectiveness of integrated biorefineries. Experiments were conducted with hardwood derived Organosolv lignin, subcritical H2O (defined here as H2O at 365 °C and autogenous pressure), and various industrial gases (N2, H2, CO, and CO2 at a cold pressure of 500 psi) for 30 min to determine both lignin’s potential to generate valueadded products (e.g., monomer compounds and methanol) without the need for a catalyst and the roles (if any) of the H2O and the gases in the reactions. The behavior of H2O at temperature (365 °C) and pressure within this research is expected to be similar to the behavior of supercritical H2O (374 °C and 3205 psi), without the need to maintain supercritical conditions. Different characterization techniques were used for the products collected including primarily gas chromatography with flame ionization detection and thermal conductivity detection (GC/FID-TCD) of the evolved gases, GC/MS analysis of the organic liquids, solid phase microextraction analysis of the recovered H2O, and solid state 13C NMR analysis of the solid residues. The reactor pressure at temperature was shown to influence the outcome of products, and the highest conversions (≈54−62%) were obtained when adding gas. The collected solids from the N2, H2, and CO reactions appeared to be the most reacted (i.e., the most changed from the unreacted lignin) according to solid state 13C NMR analysis, and the widest variety of products (methoxy-substituted phenolic compounds) were also obtained when using CO, according to GC/MS analysis.

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reaction with organics.4−6 There has been much research concerning experiments between lignin and supercritical H2O (374 °C and 3205 psi) that have focused on further decomposition to gases (e.g., H2, CH4, and CO2) where nearly no char formation is expected in the presence of a catalyst. However, the conditions required for supercritical H2O are difficult to maintain, catalysts can be expensive, and gases are not favorable to the current liquid fuel infrastructure. Lignin is a complex macromolecular biopolymer that must be broken down into smaller monomeric units in order to produce value-added liquids and fuels from it; to do so efficiently using the best reaction chemistry, it is important to know how the monomeric units are connected and what linkages must be broken. The absence and/or presence of the aforementioned branched, cross-linked monomer alcohols within the lignin structure indicate the origin and possible structure of lignin: hardwood lignin generally consists of both sinapyl alcohol and coniferyl alcohol, softwood lignin generally consists of coniferyl alcohol, and grass lignin consists of all three monomers.1−3 The dominant intermolecular linkages of lignin comprised of these

INTRODUCTION Biofuels, such as cellulosic ethanol, may only be cost-effective if the lignin byproduct is upgraded to value-added products. However, effective conversion is hindered by the inherent aromatic structure of lignin, which consists of highly branched, intermolecular cross-linkages of phenylpropane units (i.e., monomer alcohols) including p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol (shown in Figure I).1−3 Water is considered for lignin conversion, because the changes in the physical properties of H2O (e.g., decreased ion product and static dielectric constant) as temperature is increased suggest that H2O at the elevated temperatures is more compatible to

Received: March 31, 2012 Revised: May 30, 2012 Published: June 4, 2012

Figure I. Primary monomers of lignin: (a) p-coumaryl alcohol, (b) coniferyl alcohol, and (c) sinapyl alcohol.1−3 © 2012 American Chemical Society

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however, diphenyl ether and dibenzofuran were both unreactive under aqueous and thermal conditions. Siskin and Katritzky also describe how condensation reactions can occur through formation of C−C bonds at the ring carbons by the hydroxyl methyl substituents.4,5,11,12 For example, the reaction of benzyl phenyl ether in H2O at 250 °C forms phenol and benzyl alcohol, followed by condensation to form di- and tribenzylated phenolic compounds. In particular, the ring carbon atom of phenol behaves like a nucleophile, which is being attacked by a C-electrophile (i.e., hydroxyl methyl).4,5,11,12 Wahyudiono et al. (2009) studied the conversion of catechol (a lignin monomer model compound) in high temperature H2O (370−420 °C) for 5−240 min. (autogenous pressure), and they found that both conversion and liquids yields were greater for the reactions at the higher temperatures. However, phenol was detected in the liquid products of each reaction studied proving that decomposition of catechols is possible in high temperature H2O.13 Early results from Bobleter and Concin (1979) concerning the degradation of lignin in high temperature H2O (270−372 °C) revealed two types of reactions occurring consecutively: (1) a fast degradation phase that forms soluble lignin fragments and (2) a slower reaction phase where the fragments recondense creating more insoluble products.14 Secondary cross-linking was also observed to occur between the lignin degradation monomers. Lignin solubilization in H2O at lower temperatures (occurring as temperature increased from ≈25 °C to ≈225 °C over 90 min.) has been studied with results concluding that lignin depolymerization was occurring primarily via β−O−4 ether bond breakage to generate low molecular weight compounds such as guaiacol (≈22% yield by mass) and syringol (≈23% yield by mass).15 Results from Wahyudiono et al. (2008), who treated Organosolv lignin with high temperature H2O (350 °C) for 5−60 min without catalysts, showed small amounts of small molecular weight products (e.g., catechol, phenol, cresols) present in the methanol soluble products; the group observed a slight increase in these low molecular weight compounds from 5 min reaction time (≈20%) to 60 min (≈40%), as well as an increase in methanol insoluble products.16 Cleavage of β−O−4 lignin ether bonds under acidic conditions was shown to generate monomeric compounds such as vanillin and methoxyphenol, as well as formaldehyde.17 The formaldehyde was theorized to form from the terminal hydroxymethyl groups in the lignin phenylpropane side chains. Recent work theorizes that intermediate compounds forming from the reaction of lignin with high temperature and supercritical H2O, particularly formaldehyde,18−21 may aid in char formation and recondensation (i.e., cross-linking) without a catalyst present.22 However, Siskin and Katritzky (2001) describe autocatalysis as a pathway for depolymerization of resource materials (such as kerogen) where ether cleavage releases water-soluble products (e.g., formaldehyde and formic acid) that can act as transfer agents for hydride ions.12 Kratzl (1948) further showed that cleavage of the α−β linkage of spruce lignin occurred under alkaline conditions to form vanillin and acetaldehyde (after the β−O−4 ether cleavage under acidic conditions).23 Kleinert and Barth (2008) used formic acid/alcohol mixtures to depolymerize lignin into liquid oils (e.g., monomeric alkylated phenols and hydrocarbons) via deoxygenation in a one-step process.24,25 They determined that better conversion occurs above 350 °C for reaction times over 4 h.

primary phenylpropane lignin monomers are shown in Figure II.3,7 The arylglycerol-β-aryl ether linkage, or β−O−4 linkage, is

Figure II. The dominant intermolecular linkages of lignin (comprised from the primary phenylpropane lignin monomers) include (a) ethers (≈80% intermolecular linkages), (b) direct coupling (≈10% intermolecular linkages), and (c) direct aryl−alkyl (≈8% intermolecular linkages). Adapted from refs 3, 8, and 7.

the most common monomeric linkage found in lignin,2 representing over 50% of the interunit linkages.1,8 The ether bonds constitute approximately 80% of the intermolecular linkages present in lignin, followed by direct coupling (e.g., 5− 5) to form biphenyl bonds at approximately 10% and direct aryl-alkyl intermolecular linkages (e.g., β−1) at approximately 8%.8 Additional sources that propose and review structural models of lignin include Nimz (1974),9 Adler (1977),3 and Fengel and Wegener (1984).1 The aromatic structure and random intermolecular linkages of lignin make it chemically and physically difficult to predict its reaction behavior after separation from a bulk wood sample. Complete degradation of lignin can only occur after cleavage of the aromatic rings.8 Further complexity arises when the lignin has been generated as a byproduct from an industrial application. For example, different chemical pulping processes exist (i.e., various sulfite pulping, kraft pulping, solvent-based pulping) yielding different types of lignin wastes. Similarly, there are diverse preparation methods (e.g., solvent extraction, oxidation, ball milling) required for analysis of the lignin, which can lead to varying differences in its chemical and physical structure. The isolated lignin is suggested to be more condensed and cross-linked due to its highly reactive nature compared to the original lignin.10 This is due to secondary condensation, or secondary polymerization, and is similar to the initial formation of lignin from its monomers. Therefore, it is necessary to characterize the ligninbased material that will be fed into the reactor after separation from the cellulosic biomass. Siskin and Katritzky collaborated to study the conversion of various organic compounds (similar to lignin monomers) including aromatic ethers in high temperature H2O, and they published several review papers regarding these types of reactions.4,5,11,12 Cleavage of 4-phenoxyphenol (a diaryl ether), in particular, at 250 °C in H2O formed phenol; 4541

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In this paper, experiments using Organosolv lignin, subcritical H2O at 365 °C (abbreviated hereafter as sub-H2O) and various industrial gases (N2, H2, CO, and CO2 at cold pressure of 500 psi) for 30 min were examined to determine both lignin’s potential to generate value-added products (e.g., monomer compounds and methanol) and the role (if any) of the sub-H2O and the gases during the reactions. The behavior of sub-H2O at reaction temperature and pressure is expected to be similar to the behavior of supercritical H2O without the need to maintain supercritical conditions. Different characterization techniques were used for the products collected including primarily gas chromatography with flame ionization detection and thermal conductivity detection (GC/FID-TCD) of the evolved gases, GC/MS analysis of the organic liquids, solid phase microextraction analysis of the water, and solid state 13C NMR analysis of the residues.

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Figure III. Product separation and analyses after experiments were quenched. subsequently evaporated using rotary evaporation so that the organic liquid fraction mass could be determined. For subsequent organic liquids analysis via GC/MS, the evaporated filtrate was dissolved in methanol (20 mg of sample in 1 mL of solvent). Average conversion percentage is defined as

EXPERIMENTAL SECTION

The Organosolv lignin (purchased from Sigma Aldrich, CAS no. 806803-9) was used as received, and initial analyses of it using py-GC/MS and solid state 13C NMR were performed to determine its woody biomass origin. Pyrolysis-GC/MS using tetramethylammonium hydroxide (TMAH) thermochemolysis (10% aqueous TMAH, pyrolysis temperature of 310 °C) of the unreacted Organosolv lignin based on Clifford et al. (1995)26 showed presence of principally syringyl and guaiacyl derivatives along with a p-hydroxyphenyl derivative (Figure S-I of Supporting Information). Solid state 13C NMR analysis of the unreacted Organosolv lignin based on Orem and Hatcher (1987)27 showed the presence of particular syringyl derivative units indicative of hardwood sources.28−31 The results from both pyGC/MS and solid state 13C NMR analyses of the unreacted Organosolv lignin suggested that this lignin is derived from a hardwood source. Each baseline gas experiment requires Organosolv lignin (1 g), H2O (4 g), and industrial gases (500 psi of either N2, H2, CO, or CO2). The lignin and H2O were loaded into the tubing reactors (316 stainless steel, ≈25 mL). The mass loading ratio for lignin-to-water was 1:4, which balances the maximum allowable water loading for these reactors with desired water density (0.16 g/cm3).32,33 The reactor was purged with N2 gas (1000 psi) three times for all experiments performed, and then, the selected gas (either N2, H2, CO, or CO2) was added to the reactor (500 psi). A baseline tubing reaction experiment (BTR) between lignin and H2O without additional gas pressure was also conducted for comparison to these baseline gas experiments. The reactors were 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 the experiment in a cold water bath. The reactor mass was recorded before and after each experiment for mass balance purposes. Figure III outlines the subsequent product separation and associated analyses of the solid, liquid, and gas products. Evolved gases were collected in gas bags for later qualitative analysis by gas chromatography (GC). The reactor mass was recorded and then opened to remove the products, which were a mixture of water, organic liquids, and solids. The products were removed from the reactor barrel using dichloromethane (DCM) and a metal spatula. The use of a modified Dean−Stark apparatus (35 mL capacity) with DCM during the subsequent product workup eliminated the need for several processing steps and allowed for better separation of the organic liquid product (DCM-solubles) from the water fraction.28,29,34,35 The water fraction is then removed from the apparatus, weighed, and subsequently stored in a refrigerator until ready for analysis by solid phase microextraction (SPME). The remaining products, a mixture of solids and DCM-soluble liquids, were filtered using Whatman filter papers (Grades 54 and 50 with particle retentions of 20−25 and 2.7 μm, respectively). DCM within the organic liquid filtrate was

initial mass lignin (g) − final mass solids (g) × 100 initial mass lignin (g) and the average DCM-soluble liquid yields is defined as

DCM‐solubles (g) × 100 initial mass lignin (g) A baseline Dean−Stark run (BDS run) was also conducted to mimic the product workup methods to determine any influence of the Dean− Stark method on products collected. This BDS run involved only the unreacted lignin, water, and DCM (i.e., there was no tubing reactor, heating in a sandbath, or incorporation of gas). Evolved gases were characterized using headspace GC (Shimadzu GC17A) with FID and TCD. The concentration of compounds within the recovered gases was estimated using standards for each type of detector: a paraffin standard (C1−C6 compounds) was used for the FID and a permanent gas standard (CO, CO2, H2, CH4, O2, and N2) was used for the TCD. The GC program parameters are described elsewhere.29 These analyses are qualitative, not quantitative, since it was not possible with the current experimental setup to both sample the gas and determine its volume before and after reaction. Solid state 13C NMR spectra of selected recovered solids were obtained with a Bruker AV-300 (7T) spectrometer (parameters are given in the Supporting Information) using cross-polarization magicangle spinning (CPMAS)27 and total suppression of sidebands (TOSS). All obtained spectral information was processed using the SpinWorks software.36 Deconvolution of the spectra was performed using Thermo Scientific’s GRAMS/AI spectroscopy software (version 7.02), and the type of peak fitting used was a mix of Gaussian and Lorentzian.37 All recovered organic liquids were characterized using GC/MS (Shimadzu GC17A with QP5000 MS). The GC program settings are described elsewhere.29 The National Institute of Standards and Technology (NIST) database was used to help identify the generated peaks based on spectral information. Other liquids analyses (e.g., liquid chromatography with mass spectrometry, time-of-flight mass spectrometry) were performed for selected experiments, as detailed elsewhere.29 SPME was used for analysis of the collected water fractions. A manual headspace sampling (10 min) and analysis method was used with a Shimadzu 2010 GC/MS instrument.38,39 A carboxen/ polydimethylsiloxane (PDMS) fiber with 85 μm thickness and StableFlex/SS (Supelco cat no. 57334-U) was used for targeted adsorption and desorption of low molecular weight analytes.39,40 All samples were diluted with distilled H2O, but the extent of dilution varied depending on the sample. The sampling and subsequent analysis of each particular sample of interest was conducted in duplicate or triplicate. A polar column was used to detect any polar 4542

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analytes, specifically methanol, within the water fractions. An external calibration method was used to determine the concentration of methanol in the water fraction samples. Methanol yields (%) are defined as

mass methanol (g) × 100 initial lignin mass (g) and methanol in total liquids (%) is defined as mass methanol (g) × 100 mass methanol (g) + DCM‐solubles mass (g) Further specifics of the sampling procedure and the GC program settings are reported elsewhere.29

Figure IV. Average conversion (wt %) and average highest evolved pressure (psi) for baseline gas reactions with and without added pressure.

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RESULTS AND DISCUSSION The reactions with N2, H2, CO, and CO2 initially seem to show similar reactivity, as seen in the conversion and liquids yields shown in Table I. For all gas experiments, the standard

decreased ion product and static dielectric constant),4−6 combined with the higher, evolved pressure from the added gas, may be allowing the H2O to decompose the lignin more effectively. Conversely, the lower, evolved pressure from the BTR experiments may not be decomposing the lignin as effectively. Additionally, Yu and Eser (1997) studied the thermal decomposition kinetics of C10−C14 n-alkanes at three different temperatures (400, 425, and 450 °C) and different times at near and supercritical conditions. They observed that pressure affected conversion (first positively and then negatively), which they attributed to the fluid exhibiting high compressibility at the higher temperature and pressure (i.e., the rate constant may be influenced by the pressure).41 The consequent pressure from the addition of gas to the sub-H2O experiments with lignin is affecting the products generated. However, the reactivity of the gases at the conditions studied is not clearly defined. The Supporting Information provides more detail about the composition of the collected gases obtained with GC-FID/ TCD. The majority of permanent gases detected are the initial gases themselves for their respective reactions (i.e., N2 was the primary gas generated from the N2 gas runs, H2 was the primary gas generated from the H2 runs, etc.). The majority of paraffin gases detected include CH4 and C2H6. Since neither the volume nor the concentration of gas was measured before or after each experiment, it was not possible to determine how the concentration of initial gas changed. Further research on the reactivity of the gases under the conditions used in the present research is recommended. However, experiments with sub-H2O and each of the different gases (without lignin) did not show the permanent formation of H2 or CO2 in the gaseous products.29 There were also few compounds detected within the DCMsoluble liquids, as shown with the GC/MS data in Figure V and Table II. For all baseline gas runs, GC/MS identified compounds within the DCM-soluble liquid products that were generally either phenolic and/or methoxy-substituted benzenes. The products from GC/MS also had a noticeable absence of ethers and propyl groups, both of which are inherently characteristic for lignin and lignin monomers. The cleavage of the ethers (via hydrolysis) to generate phenolic products (as seen in the GC/MS results) may be contributing to autocatalysis from water-soluble intermediate reaction products such as formaldehyde and formic acid.12 However, confirmation of the intermediate products was not possible during this research. The greatest variety of products was seen with CO (GC/MS showed there are 10 compounds observed

Table I. Conversion and Liquids Yields of BTR Runs (No Added Gas) and Baseline Gas Runs with Subcritical H2Oa expt name (n= no. of runs) CO2 (n = 4) CO (n = 6) H2 (n = 6) N2 (n = 6) baseline tubing reaction (BTR) (n = 2)

lignin conversion (avg wt %)

standard deviation (s)

DCM-soluble yields (avg wt %)

standard deviation (s)

53.9 58.2 59.4 62.4 39.6

8.8 7.3 4.5 2.0 1.3b

48.2 46.9 52.4 60.8 23.4

13.8 18.9 26.8 15.4 2.4b

a 365 °C, 500 psi (cold) of selected gas, 30 min. bRange (high value − low value) is reported instead of standard deviation here, since there were only two runs studied for this particular experiment.

deviation calculated for lignin conversion was lower than the standard deviation calculated for the DCM-soluble liquids yields, which is a reflection of the calculations used to determine the conversion and liquids yields themselves (i.e., the calculation for lignin conversion relies upon the initial and final mass of solids collected, while the DCM-soluble liquids yield relies upon the initial mass of solids and the mass of DCM-solubles where the DCM-solubles are determined after DCM evaporation). A range is calculated for the BTR run because there were not enough data points to justify determining standard deviation. The BTR run had both the lowest conversion and liquids yields compared to the results obtained for the gas runs. The gas runs had similar conversion (≈54−62%) and liquids yields (≈47−61%) after accounting for their respective standard deviations. The average evolved pressure at reaction temperature shown in Figure IV indicates that added pressure influenced the reactivity. The baseline gas experiments with N2, H2, CO, and CO2 had average evolved reaction pressures of at least 1325 psi, and they also had similar lignin conversion (≈54−62%). The BTR experiment without added pressure had the lowest evolved reaction pressure of up to 500 psi and the lowest lignin conversion (≈40%). The high conversion when using N2 was not expected, since N2 should be inert at these reaction conditions. The increased evolved pressure (from the gas and the H2O in the reactor) is likely the reason for the higher lignin conversions and liquid yields observed. The changes in the physical properties of H2O as temperature is increased (i.e., 4543

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compounds. The value of generating a single compound (such as 2,6-dimethoxyphenol) would need to be evaluated economically, but economics have not been completed on this reaction at this point. 2,6-Dimethoxyphenol was the primary compound detected in all runs, followed by 1,2,3-trimethoxybenzene. Using alkali metals and low polarity solvents, selective reduction of 1,2,3trimethoxybenzene at position 2 with a hydrogen atom (reductive demethoxylation) via indirect electrophilic substitution has been shown to occur along with competing cleavage of the alkyloxygen bond (reductive demethylation).45,46 However, an equimolecular mixture of 2,3- and 2,6-dimethoxyphenol (60% yield) is obtained after ionic reaction of 1,2,3-trimethoxybenzene with sodium and hexamethylphosphoramide (HMPA).47 Both 1,2,3-trimethoxybenzene and 2,6-dimethoxyphenol were determined to be stable under supercritical methanol conditions (239 °C, 1173.35 psi, 1−20 min reaction time), since more than 97% and 91%, respectively, of these products resulted after 10 min.48 The yields only decreased slightly at higher temperature (350 °C) and pressure (6236.63 psi) in methanol for 1,2,3-trimethoxybenzene (82%) and 2,6-dimethoxyphenol (71%), which indicates that these compounds are still considered stable in supercritical methanol.49 Thus, the 1,2,3-trimethoxybenzene and 2,6-dimethoxyphenol are not affected by any generated methanol. The BTR run (no gas added) does show a variety of compounds detected from GC/MS analysis, but there are low lignin conversions and low liquids yields. These products could be present due to a low level of lignin degradation (i.e., dealkylation) or, more likely, from hydrolysis of ether bonds.15,16 Zakzeski and Weckhuysen (2011) also showed that a representative model compound with the 5−5′ carbon bond was converted into 4-methyl-2-methoxyphenol (7%), which is then converted into 3-methoxyphenol (13%) in H2O at 225 °C (using a Pt/Al2O3 catalyst in presence of H2SO4 and 420 psi He for 90 min). The increase in these compounds, combined with the increase in the insoluble products, is suggested to be from the formation of high molecular weight compounds from the simultaneous liberation of low molecular weight compounds.16 The low conversion and low liquids yields for the BTR run suggest that the products are likely generated from a small amount of lignin degradation.

Figure V. GC/MS spectra of DCM-soluble liquids from baseline gas runs with subcritical H2O, compared to the BTR experiment (i.e., no added gas). Data corresponds to relative area percentages seen in Table II.

for the CO run, while the other gas runs generated from 1 to 6 compounds) suggesting that CO provides the best environment for these conditions in generating lower molecular weight compounds from lignin. The least reactive gas seems to be CO2, since only syringol (2,6-dimethoxyphenol) was observed in the DCM-solubles by GC/MS. Acid-catalyzed hydrolysis was expected to occur with the addition of CO2 as H2O and CO2 react to form carbonic acid (H2CO3). The physical nature of CO2 at temperature may explain why there were few compounds detected with GC/MS, yet there was still over 50% conversion and nearly 50% liquids yields. Studies with pure H2O and CO2 show that CO2 solubility generally decreases as temperature is increased, while its solubility generally increases as pressure is increased.42,43 The behavior of CO2 at reaction temperature combined with the inherent stability of CO2 may lead to less opportunity for the CO2 to react with any liberated lignin monomers. It may also be possible that a higher rate of acid-catalyzed reactions is occurring44 that could be affecting condensation. However, there could be advantages to generating a single compound since there may not be the need to separate a suite of

Table II. Relative Area Percentage of Compounds Detected in DCM-Soluble Liquids (From Baseline Gas Runs with Subcritical H2O) Using GC/MSa relative area percentage (wt %) retention time (min)

compd

no.

14.99 17.58 18.74 21.08 21.73 24.15 27.18 29.58 32.02 36.37

4-methoxyphenol (mequinol) 1,2,3,4-tetrahydronaphthalene (tetralin) 3-methoxy-2-methylphenol 2-methoxy-1,3-benzenediol 4-ethyl-2-methoxyphenol 2,6-dimethoxyphenol (syringol) 1,2,3-trimethoxybenzene 1,2,3-trimethoxy-5-methylbenzene 2,3-hexadienedoic acid, 3-methyl-4-propyl-,dimethyl ester (E,E) 2,5-dimethoxy benzenemethanol acetate

1 2 3 4 5 6 7 8 9 10

a

CO2 CO H2 N2 expt no. 26 expt no. 21 expt no. 20 expt no. 18 0.0 0.0 0.0 0.0 0.0 100.0 0.0 0.0 0.0 0.0

6.0 2.0 3.8 6.2 2.6 49.7 18.8 5.6 3.4 2.0

0.0 0.0 0.0 0.0 0.0 64.8 23.7 7.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 63.7 28.2 8.1 0.0 0.0

BTR expt no. 28 0.0 0.0 0.0 4.2 1.4 45.8 34.4 9.3 4.9 0.0

Data corresponds to spectra seen in Figure V. 4544

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Figure VI. Chromatograms of results obtained using LC/MS with assistance of Shimadzu Corporation for (a) positive mode scans and (b) negative mode scans.

the DCM-soluble liquids from the BDS (black line), N2 (pink line), and CO (blue line) runs.28 There are some high molecular weight compounds present in the N2 and CO runs at reaction temperature, but there are fewer of these compounds compared to the number in the BDS run. These high molecular weight compounds could be a result of incomplete lignin conversion associated with the particular gas used and/or the recondensation of any liberated lower molecular weight compounds. The LC/MS spectrum for N2 does visually show the presence of more compounds compared to the spectrum for CO, which may indicate that lignin conversion is better with CO as the reaction gas. The spectrum for BDS visually shows an even greater presence of high molecular weight compounds compared to the N2 and CO products, which may indicate that the lignin is solubilized in the DCM. For example, previous research showed agricultural residues were processed by DCM (205 °C for rice husks and 202 °C for corncobs in stainless steel batch reactors) into lignin-derived compounds.50 However, the absence of any low molecular weight compounds in the GC/MS spectrum of the DCM-solubles from the BDS runs indicates that decomposition did not occur at these studied conditions; that is, the DCM did not affect the product work-up, particularly when using the Dean−Stark method.

Even though higher yields and higher evolved pressures are seen for the N2 and H2 gas runs, high pressures at temperature do not necessarily indicate that lignin is decomposing into low molecular weight compounds, as shown with the GC/MS data. The reaction temperature and consequent high pressure is suggested to cause the higher conversion (and liquids yields), as shown, but the products could be high molecular weight compounds caused from the solubilization of lignin and/or recondensation of reactive low molecular weight compounds.16,19,21 The BDS run to determine the possible influence of the Dean−Stark method on products collected showed no compounds detected with GC/MS analysis.29 However, the solvation and liquids yields were high (≈74% and ≈86%, respectively), so the lack of compounds detected with GC/MS indicates the absence of GC amenable compounds; that is, there must be higher molecular weight compounds present even though no tubing reaction took place. Similarly, the fewer number of low molecular weight compounds observed within the DCM-soluble liquids from the BTR, N2, H2, and CO2 runs compared to the CO run likely indicates the presence of higher molecular weight material that is not amenable to GC/MS analysis. The liquid chromatography (LC)/MS results in Figure VI show the presence of high molecular weight compounds in 4545

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The highest methanol yields, shown in Table III, were observed from the baseline runs with N2, H2, and CO (≈0.35

Time-of-flight mass spectroscopy (TOFMS) has also been performed on the DCM-soluble liquids from the BDS and N2 runs (Figure VII). A range of masses (from 1400 m/

Table III. Methanol Yields and Methanol in Total Liquids from Collected Water Fractions for Baseline Gas Runs of Organosolv Lignin Reacted with Subcritical H2O Using SPME Analysis expt name (n = no. of runs)

methanol yields (wt %)

range

methanol in total liquids (wt %)

range

CO2 (n = 2) CO (n = 2) H2 (n = 2) N2 (n = 2) BTR (n = 2)

0.13 0.35 0.34 0.35 0.22

0.1 0.0 0.4 0.2 0.1

0.34 0.80 0.79 0.83 0.91

0.4 0.1 0.2 0.4 0.2

wt %), while the lowest methanol yields were observed with CO2 (0.13 wt %) and when no pressure was added (BTR runs) (0.22 wt %). The methanol in total liquids, which is based on the mass of methanol and DCM-solubles collected, provides some insight about the distribution of liquid products within the total liquids. Similar percentages of methanol in total liquids was generated for the baseline runs with N2, H2, and CO (≈0.80 wt %), while the runs with CO2 generated the least methanol in total liquids (0.34 wt %). The BTR runs had the greatest percentage of methanol in total liquids (0.91 wt %). The methanol observed is attributed to the depolymerization/ degradation of lignin in H2O and also to the demethoxylation of the ethers within lignin itself.15,51 It was originally thought that more methanol could be generated from the addition of CO or CO2 due to the water−gas shift reaction followed by CO2 hydrogenation, but these results did not show enhanced methanol formation. Separate reactions with only sub-H2O and the separate gases (365 °C, 500 psi (cold) selected gas, 30 min) showed that no methanol was formed when lignin was not present.29 Methanol was also not observed in the BDS extraction,29 which further indicates that lignin solubilization, and not reaction chemistry, is likely responsible for the high lignin conversion and liquids yields observed in the products collected from the BDS runs. Analysis of the solids proved to be helpful in understanding how the reactions progressed by comparing the collected residues with the unreacted lignin. There are definite changes in the spectra of the solid products from all of the baseline runs with Organosolv lignin and sub-H2O (with and without cold gas pressure) compared to the unreacted lignin as shown in the solid state 13C NMR analyses in Figure VIII (the Supporting Information contains peak deconvolutions and statistics generated by GRAMS/AI37 spectroscopy software). The primary changes for all reacted solids include loss of lignin side chains (74 ppm), representative syringyl unit components (104 ppm, 153 ppm), aromatic olefins (117 ppm), and different carbonyl groups (174 ppm, 182 ppm, 201 ppm). The primary functionality remaining for the reacted solids, though less defined than the unreacted lignin, include saturated alkanes (15 ppm), methoxy groups (56 ppm), and representative components of both syringyl (134 ppm) and guaiacyl units (147 ppm).10 The reactions involving CO, H2, and N2 show the fewest number of peaks, an indication that the reactions with these gases could have the greatest reactivity toward liquid formation. The CO2 and BTR runs also have an additional peak at ≈45 ppm, which is indicative of saturated alkanes and/or aryl carbons. Table IV shows how the percentages of aliphatic and

Figure VII. Mass spectrometry data from TOFMS with electrospray ionization (ESI) for liquid products from a (a) BDS run and (b) N2 run.

z) was observed for both products with little difference shown between the two products. These high molecular masses (compared to those masses observed with GC/MS) likely represent a combination of both oligomers of lignin monomers and those compounds detected with GC/MS with typical lignin substituents (e.g., methoxyl-, hydroxyl-, methyl-, and carboxylgroups). For this research, the effects of lignin solubilization on products generated, and subsequently detected with GC/MS, is noted and interesting, but further investigation on lignin solubilization in solvents is suggested. Furthermore, LC/MS analysis of all the DCM-soluble liquids, along with identification of the compounds, would provide more complete information about the extent of solubilization and degradation and/or recondensation. The combination of TOFMS and LC/ MS, though limited for this research, with GC/MS helped provide a broader understanding of the extent of lignin reaction. The TOFMS and LC/MS results showed presence of high molecular weight compounds that are not GC amenable. However, these methods did not provide useful compositional information, which is attributed to the inherently complex composition of lignin that is making separation of products difficult. 4546

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Figure VIII. Solid state 13C NMR analyses of the collected solids from Organosolv lignin reactions with subcritical H2O with and without added gas pressure compared to unreacted Organosolv lignin (the Supporting Information shows the parameters).

(if any) of the H2O and the gases throughout the reactions. The recovered solids, liquids, and gaseous products from the reaction of lignin with sub-H2O (with and without added pressure) were successfully characterized using different analytical methods. In particular, this research addresses analysis of the solids (with solid state 13C NMR), the organic liquids (with GC/MS, LC/MS, and TOFMS) and the water fractions (with SPME), which 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. The findings in this research indicate that sub-H2O (365 °C) can be used to depolymerize lignin into GC amenable compounds with or without added cold gas pressures. However, adding gas (and therefore increasing the pressure during the experiment) will further modify the types of products generated. The roles of sub-H2O and the added gases are not confirmed yet based on these results, but the reactions with gases showed the most change based on the conversion and the solid state 13C NMR results. Depolymerization occurred in the presence of sub-H2O based on the variety of phenolic compounds detected in the DCM-soluble liquids (with GC/MS), the significant change in reacted solids from the unreacted lignin (with solid state 13C NMR), and the low amount of high molecular weight compounds present in the DCM-soluble liquids products compared to the liquids from the BDS runs (based on LC/MS and TOFMS). SPME confirms the suspected presence of methanol in the water fractions, which corresponds to the loss of methoxy groups, but its formation may be slightly negatively affected by the added pressure. SPME is an emerging technique for the purposes of solvent-free sampling of gases and/or liquids, and this research proves its usefulness for analysis of H2O that contains miscible organic compounds. There has been limited work reported on the reaction chemistry of lignin and sub-H2O (particularly at 365 °C), and this research defines the chemistry of lignin conversion at such conditions.

Table IV. Percentages of Aliphatic and Aromatic Compoundsa of the Collected Solids from Organosolv Lignin Reaction with Subcritical H2O with and without Added Gas Pressure Compared to Unreacted Organosolv Lignin Using Solid State 13C NMR expt name

% aromatics

% aliphatics

Organosolv lignin (unreacted) CO2 run no. 26 CO run no. 21 H2 run no. 20 N2 run no. 18 BTR run no. 28

64.2 74.3 84.2 82.1 81.3 75.9

35.8 25.7 15.8 17.9 18.7 24.1

a

Determined from deconvolution of peaks of solid state 13C NMR, as shown in the Supporting Information.

aromatic compounds changed from the unreacted lignin (determined from deconvolution of the peaks). All gas runs show a decrease in aliphatic compounds and an increase in aromaticity compared to the unreacted lignin. The N2, H2, and CO runs show the most change from the unreacted lignin, and they have similar amounts of aliphatic (≈16−19%) and aromatic (≈81−84%) components. The decrease in aliphatic compounds compared to the unreacted lignin is likely due to the loss of saturated alkanes and methoxy groups, while the increase in aromaticity corresponds to char formation.

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CONCLUSIONS Sub-H2O was considered as a means for lignin conversion into value-added products. Water is environmentally benign, and it exhibits unusual properties at higher temperatures (particularly at and near its supercritical point of 374 °C and 3205 psi), such that there is a high solubility for organic compounds, such as lignin. Experiments with Organosolv lignin, sub-H2O (365 °C), and various industrial gases (N2, H2, CO, and CO2 at a cold pressure of 500 psi) for 30 min were examined to determine both lignin’s potential to generate value-added products (e.g., monomer compounds and methanol) and to establish the role 4547

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Based on results from these baseline gas experiments, further tubing reactor experiments at 365 °C were explored and will be reported in forthcoming publications: (1) The effect of CO pressure (300 psi, 500 psi, and 800 psi) and time (15 min, 30 min, and 60 min.) using Organosolv lignin; and (2) The role of H2O and the gases (N2, H2, CO, and CO2 at a cold pressure of 500 psi) using lignin model compounds (i.e., aromatic aldehydes, aromatic ketones, and aromatic ethers).

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ASSOCIATED CONTENT

S Supporting Information *

Analysis of Organosolv lignin using py-GC/MS with TMAH thermochemolysis; characterization of evolved gases by GCFID/TCD; solid state 13C NMR parameters; peak deconvolutions of the solid state 13C NMR spectra, as shown in Figure VIII. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (814) 863-3571. Fax: (814) 863-7432. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported through several sources including the Agriculture and Food Research Initiative Competitive Grant No. 2009-10002-05127 from the United States Department of Agriculture National Institute of Food and Agriculture, Ford Foundation Predoctoral Diversity Fellowship, National Science Foundation Transforming Earth System Science Education Fellowship, Alfred P. Sloan Foundation Scholarship, International Association for the Exchange of Students for Technical Experience through Association for International Practical Training, and Pennsylvania State Center for Environmental Chemistry and Geochemistry Summer Fellowship. The authors also thank R. Wasco at Penn State’s EMS Energy Institute for his ongoing support in the laboratories, Dr. D. A. AlvarezFonseca at Penn State’s EMS Energy Institute for her ongoing analytical assistance and useful discussions, Dr. W. Luo and Dr. A. J. Benesi at Penn State’s NMR Facility for their help in acquiring NMR results, G. Oishi at Supelco for his useful discussions about SPME, Dr. F. Hays and E. Manning at Shimadzu for ongoing analytical assistance, and various undergraduate students, namely, R. Gilligan, K. Greene, and D. Reynolds.

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