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THERMAL LIQUEFACTION OF LIGNIN TO AROMATICS: EFFICIENCY, SELECTIVITY AND PRODUCT ANALYSIS Evguenii I. Kozliak, Alena Kubatova, Anastasia A Artemyeva, Eric Nagel, Cheng Zhang, Rudresh B. Rajappagowda, and Alevtina L. Smirnova ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01046 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016
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THERMAL LIQUEFACTION OF LIGNIN SELECTIVITY AND PRODUCT ANALYSIS
TO
AROMATICS:
EFFICIENCY,
Evguenii I. Kozliak,a* Alena Kubátová,a Anastasia A. Artemyeva,a Eric Nagel,b Cheng Zhang,b Rudresh B. Rajappagowda,c Alevtina L. Smirnovac a
University of North Dakota, Department of Chemistry, 151 Cornell St. Stop 9024, Grand Forks, ND 58202, U.S.A. b
South Dakota State University, Department of Chemistry and Biochemistry, Avera Health Science Center 131, Brookings, SD 57007, U.S.A. c
South Dakota School of Mines & Technology, Department of Chemistry and Applied Biological Sciences, 501 E. Saint Joseph St., Rapid City, SD 57701, U.S.A. * Corresponding Author: voice: 701-777-2145, fax: 701-777-2331, Email:
[email protected] KEYWORDS Lignin liquefaction; protic solvents; characterization of lignin degradation products; repolymerization; sub- and supercritical fluids. ABBREVIATIONS GC – gas chromatography FID – flame ionization detections NMR – nuclear magnetic resonance FTIR – Fourier transform infrared spectroscopy TGA – thermogravimetric SEC – size exclusion chromatography OLP – organic liquid product FTICR MS – Fourier transform ion cyclotron resonance mass spectrometry MS – mass spectrometric ESI – electrospray ionization MALDI – matrix-assisted laser desorption ionization MW – molecular weight LC – liquid chromatography Mn – number-average molecular weight GPC – gel permeation chromatography MALS – multi-angle laser light scattering Pyr-GC/MS – Pyrolysis-gas chromatography-mass spectrometry TD – thermal desorption HHV – higher heating value
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ABSTRACT This perspective addresses efficiency and selectivity of high-temperature lignin liquefaction processes conducted in various reaction media as sub- and supercritical fluids. The challenges in efficient and selective production of high-value organic monomers from lignin are reviewed critically, along with analytical protocols essential for their accurate recovery after lignin degradation. The current approaches targeting the formation of phenolic monomers from lignin are discussed in terms of their re-polymerization, a process that decreases the reaction selectivity and yield of the dominant phenolic monomers. The potential to solve this grand challenge is analyzed in terms of acid and/or protic co-solvent application, reduction of the reaction temperature, “quenching” of the reactive lignin depolymerization intermediates and presence of heterogeneous catalysts, such as zeolites, metals, and metal oxides, sulfides and phosphides. SYNOPSIS This perspective emphasizes the main challenges in chemical analysis and production of aromatics from lignin and the main approaches to address them. Contents INTRODUCTION LIGNIN COMPOSITION AND CONVERSION PRODUCTS QUANTITATIVE AND QUALITATIVE ANALYSIS OF LIGNIN LIQUEFACTION PRODUCTS THE MAIN CHALLENGES IN LIGNIN LIQUEFACTION Breaking β-O-4 ether bonds Re-polymerization APPROACHES TO REDUCE RE-POLYMERIZATION Acid-base catalysis, use of protic solvents and ionic liquids Protic solvents Acid-base catalysis “Capping” agents Solubilizing solvents Redox approaches Lignin oxidation Lignin reduction Short residence time Noble vs. non-noble metal catalysts Metal oxides, sulfides and phosphides Zeolites Two step processes CONCLUSIONS ACKNOWLEDGEMENT REFERENCES
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INTRODUCTION Conversion of bio-renewable resources to high value products is receiving increased attention by the chemical industry. Among the potential sources of bio-renewable materials (e.g., the carbohydrate fraction of lignocellulosic biomass or triglyceride oil), lignin is known as the third most abundant biomass component (after cellulose and hemicellulose) accounting for up to 30% of biomass feedstocks,1-4 with its global production reaching 1.1 million metric tons per year.5 Being the most resilient of the three main feedstocks, lignin conversion into high-value chemicals presents a significant challenge in terms of yield, efficiency and selectivity. A few recent reviews emphasized thermal and hydrothermal conversion of lignin,1 oxidative treatments,6 gasification of biomass,2 biomass hydrothermal liquefaction aimed to improve the bio-oil yield,3 and processes targeting production of fuels, chemicals and polymers.7, 8 Bypassing the approaches that use lignin in its polymeric form, we will follow the current trend targeting its breakdown to monomer size units, which can be readily converted into usable chemicals. This perspective is specifically focused on lignin liquefaction processes, i.e., thermal decomposition conducted in aqueous or protic solvents and resulting in aromatic, primarily phenolic-based products. Thermal decomposition processes conducted in polar, particularly protic, solvents are expected to be more selective than solvent-free thermal treatments, for which more random free radical reactions are predominant.1, 9-11 To maintain the system homogeneity at temperatures high enough to break covalent C-O and C-C bonds (250–500 °C, depending on the presence and efficiency of catalysts), the solvent is typically either sub- or supercritical thereby significantly narrowing the suitable temperature and pressure ranges.12-14 Water is the default solvent regarding industrial applications, although other solvents and co-solvents are also considered as they may lower the subcritical system’s temperature and pressure. This perspective analyzes the published material on efficiency and selectivity of lignin conversion to aromatics and mechanisms of lignin liquefaction in the presence of homogeneous and heterogeneous acidic and basic catalysts compared to non-catalyzed systems (see Table 1 for representative processes). As shown, the current literature reveals two ‘bottlenecks’ hindering further progress, i.e., insufficient chemical analysis of products and lignin fragment repolymerization, which are further considered in detail. LIGNIN COMPOSITION AND CONVERSION PRODUCTS Lignin is an irregular aromatic biopolymer comprised of three main phenyl-propanoid monomers, i.e., p-coumaryl, sinapyl and coniferyl alcohols, which are linked by C-O and C-C bonds (Fig. 1) as a result of coupling reactions occurring during cell wall biosynthesis.15-18 The predominant linkage in lignin, comprising over 50% of all linkages, is the aryl-ether β-O-4 linkage16 (Fig. 1). Depending on whether or not the p-hydroxyl terminal group is connected to additional polymeric units, lignin is classified into either a phenolic or non-phenolic biopolymer. In addition to the β-O-4 bonds found in both softwood (45–50%) and hardwood (60–62%), other less frequent linkages (1–27%) such as 5-5, β-5, β-1, 4-O-5, β-β, dibenzodioxocin, and spirodienone were identified (Fig. 1).17, 18 Multiple covalent links per phenolic unit are responsible for a highly branched and complex biopolymer structure. Various lignin types are distinguished based on their natural source, as well as their isolation and purification technique and chemical modification of lignin after isolation (organosolv, kraft, lignosulfonate, pyrolysis, steam explosion, enzymatic soda, alkali, dilute acid lignin, etc.).19 Most of these treatments designed to separate lignin from the rest of the biomass 3 ACS Paragon Plus Environment
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are rather harsh and induce side reactions, resulting in products significantly different from both the original biopolymer and each other. Lignin “history” embedded in its chemical structure may thus influence the types of valuable products obtained as a result of its decomposition. This diversity in lignin isolation also leads to the lack of any ‘standard lignin’ for calibration and validation of analytical protocols. Lignin thermal conversion produces guaiacol and syringol as well as their alkylated derivatives, phenol, mono-, di- and trimethylated phenols, alkylphenols, alkoxyphenols, cycloalkanes. aromatics, catechols and furans.20-40 The reported lists of such monomeric and occasionally dimeric41 products may depend on the biomass source, processing conditions and be affected by the analytical method employed, as described in the next section. QUANTITATIVE AND QUALITATIVE ANALYSIS OF LIGNIN LIQUEFACTION PRODUCTS Assessment of process efficiency and selectivity requires the application of suitable and accurate analytical protocols that aim at 1) detection and quantification of specific structural moieties altered by decomposition reactions, 2) identification of individual compounds formed, and/or 3) attempts to close mass balance. The typical analysis protocols applied have been rather elaborate and diverse, using a suite of methods listed in Fig. 2.21-33, 42-50 The first essential step assessing mass balance closure was gravimetric analysis providing wt% of the solid and liquid fractions.23-33, 42-44, 51 This step was often supplemented by a gas phase product characterization employing gas chromatography (GC) with thermal conductivity22, 25-31, 42, 49-53 and flame ionization detections (FID)22, 29-31, 50, 53 typically conducted after a sample collection to Tedlar bags. The solid fraction or chars was analyzed employing structural characterization methods such as scanning electron microscopy, X-ray diffraction,54 nuclear magnetic resonance (NMR)27, 49, 51, 53, 55-58 and Fourier transform infrared spectroscopy (FTIR) spectroscopies28, 35, 43, 49, 53, 59-61 as well as thermogravimetric (TGA) and elemental analyses.23, 28, 33, 47-49, 53, 58, 62 Occasionally, further fractionation of the solid residue was performed by solvent extraction.43 For example, sequential extraction by dichloromethane and dimethyl sulfoxide was applied, given that the second solvent dissolves the remaining unconverted lignin.28, 62 Some studies also determined the molecular size of residual lignin using size exclusion chromatography (SEC).33, 49, 53, 62 The primary focus of most studies has been on the generation of renewable chemicals requiring a comprehensive analysis of the liquid phase, i.e., organic liquid product (OLP). The OLP characterization then consisted of several steps targeting the determination of “bio-oil” or “bio-crude-oil” content.22, 26, 33, 38, 62, 63 Bio-oil is typically defined as a fraction of OLP that is separated from the aqueous phase by extraction using an organic solvent. Bio-oil was usually separated by decantation and then often evaporated to be quantified using gravimetry.22, 25-27, 33, 38, 62 In addition, some studies determined the water content in bio-oil with Karl Fischer titration.25, 27, 32 Nevertheless, there is a concern with the definition of bio-oil due to the application of various reaction media (e.g., methanol,28, 39, 42, 63 ethanol,23, 24, 26, 32, 35, 38, 44, 51, 63 tert-butanol22) and extraction/washing solvents of varied polarity and miscibility with water (such as acetone,23, 25, 27 ethyl acetate33, 62 or dichloromethane).24, 28 As a result, it is difficult to compare quantitatively the yields and composition of the bio-oil fractions obtained across various studies. Given that the lignin chemical structure significantly depends on its source and can be modified by chemical processing, the possibility of artifacts due to the use of different solvents further increases given the differences in solubility of native lignin.40, 64, 65 4 ACS Paragon Plus Environment
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Gravimetric OLP measurements were often supplemented by comprehensive characterization methods, e.g., TGA25-27 and elemental analysis.22, 23, 25-27, 31, 33, 39, 47, 50, 62 TGA offers only a general idea on the changes that occur in lignin composition, as there is no ‘standard and certified’ lignin. The carbon, hydrogen, nitrogen and occasionally sulfur elemental analysis was used to determine the O/H and O/C ratios, sometimes using van Krevelen plots evaluating the extent of deoxygenation.23, 27, 39, 47 In most of these studies, though, the oxygen content was estimated by difference unless otherwise specified.26 A greater accuracy in determining the elemental composition can be achieved by using an informative yet high cost Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). This method features very high mass accuracy, providing a list of molecular formulas of individual compounds and thus allowing for an accurate determination of the O/C and O/H ratios.39 An opinion can be found in current literature that this approach may provide compound identification, even without the prior analyte chromatographic separation.39, 66 However, molecular ion fragmentation often becomes significant, even while using soft ionization, for both oligomeric standards67 and, particularly, high-MW species.68, 69 While mass spectrometric (MS) methods allow one to determine the oxygen content directly, one has to consider that different ionization techniques such as electrospray ionization (ESI) or matrix-assisted laser desorption ionization (MALDI) used within MS instruments may provide a varying response depending on the class of compounds and their functionalities.39 1 H or 13C NMR21, 25, 27-30, 32, 47, 48, 51, 55, 56, 58, 63, 70, 71 and FTIR20, 21, 29-32, 49, 50, 63, 71, 72 spectroscopies have commonly been used to evaluate structural changes occurring in the liquid fraction. These methods offer the benefit of a relatively fast data acquisition and, as such, are suitable for general material characterization. However, they do not allow for identification of individual compounds and their molecular weight (MW). Challenging is also the characterization of certain specific functional groups, such as hydroxyls, which may be attributed both to different structural units present in the sample and the aqueous/alcoholic solvent residue. This issue was resolved with a specialized 31P NMR application where the hydroxyls were phosphitylated, thus allowing one to distinguish not only the primary, secondary, tertiary, carboxylic, phenolic and benzylic OH groups45, 55, 73 but also those with different nearby structural features, e.g., guaiacyl, alkyl guaiacyl and syringyl moieties.21, 49, 54, 71, 73, 74 Besides the “bulk” methods involving NMR, FTIR and elemental analysis, various chromatographic approaches have been used for a more detailed OLP analysis. The separation methods employed included SEC, liquid chromatography (LC), as well as GC with various types of detectors. SEC21-23, 25-30, 32, 33, 38, 44, 51, 53, 62, 73, 75, 76 roughly estimates the polymer MW dispersion index by measuring the number-average molecular weight (Mn).21, 75 Typically, non-aqueous solvent based gel permeation chromatography (GPC) was used.21-23, 25-28, 32, 33, 49-51, 53, 56, 62, 73, 76, 77 However, SEC methods have severe limitations. The main issue is the reliance on the separation principle based on the molecular sieve, i.e., size exclusion effect to heteropolymeric lignin (or its decomposition products), thus not accounting for other separation factors resulting from the presence of various functional groups and potential steric effects specific for relatively rigid phenolic polymers.69, 75, 78 This issue may be magnified when polystyrenes are used as surrogate standards, which cannot emulate these interactions.75 Light scattering with a multi-angle laser light scattering (MALS) detector has frequently been used in combination with SEC to verify MW,79-84 however the resulting MW values might be skewed by polymer association.85 Another approach to MW determination was the application 5 ACS Paragon Plus Environment
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of MS methods, typically with MALDI24, 48, 58, 62, 69, 77 or ESI. However as mentioned above, the utility of these methods suffers from selective ionization and fragmentation. Also, as these methods are based on soft ionization, they often yield varying results on different instruments and thus are not standardized as with GPC. The detailed identification of individual compounds has typically been accomplished with GC-MS,22, 24, 26, 32, 33, 35, 38, 39, 42-52, 54, 56-58, 60-62, 77, 86 GC-FID,22, 24, 45, 48, 50, 60, 61, 87, 88 GC×GC-FID or MS,47, 49 GC-FID/MS,25, 27, 39, 44 LC-MS31, 36, 40, 89, 90 and, occasionally, pyrolysis-based PyrGC/MS.21, 35, 53, 54, 60, 61, 77 For identification of monomeric (single aromatic ring) compounds, chromatographic methods with MS detection were used most often.22, 25, 27, 28, 32, 33, 42, 62 In contrast to reliable identification afforded by this method, the quantitative characterization is limited due to the complexity of both the original lignin and products formed. The quantification of monomeric species by these methods is based on a set of available standards, which is often restricted to only a few compounds. However, the MS signal is species-specific, possibly hindering accurate quantification. The ionization efficiency in the MS source may vary, even for isomeric compounds having the same functional groups. To address the broad diversity of the compounds observed and the lack of a complete set of standards, advanced semi-quantitative methods were developed using response factors of several standards of varied size and functionalities.28 This approach is an excellent tool as long as it employs GC or GC×GC analysis with the FID detection, whose response is roughly proportional to the number of carbon atoms in the given molecule.25, 27, 28, 39, 44 For combined GC-FID/MS analysis, we showed that a correct set of standards must be used for biofuel FID quantification to address the variability in response due to differing functional groups and volatility;91 the simultaneous MS detection provides the added benefit of the confirmation of identification. The sum of lignin degradation products quantified by GC analysis often ends up being much smaller than the total mass of bio-oil or OLP determined by gravimetric analysis.27, 28 This discrepancy may be explained by typical GPC results revealing that even though the reaction products are of a lower MW than the native lignin, their large fraction is of a relatively high MW, >500 Da.27, 28, 38, 71 The point is that this fraction is not elutable by GC due to its high boiling point exceeding the limit of typical GC columns (about 300 °C). Consequently, the quantification based on the normalized total of all GC peaks across the chromatogram is problematic, as it misses a large fraction of higher-MW products in the bio-oil.32 LC-MS was one of the newest methods applied to the analysis of lignin decomposition products.31, 36, 40, 89 Unlike GC, it may be applicable to both the bio-oil and aqueous fractions (i.e., the entire OLP) and is expected to be an effective tool for simultaneous separation and identification of low and high MW species. LC with ESI MS detection have been shown to effectively separate monomeric and dimeric standard compounds.31, 36, 89, 92 However, caution must be used when standards are not available, as the identification and quantification become questionable due to the applied soft ionization, which is more prone to be affected by the matrix interactions and suppression. Furthermore, the obtained chromatograms often feature only a few identifiable peaks with a large and broad unresolved “hump.” Selection of suitable electrolytes, a thorough calibration and confirmation of the tentative MS identification with individual standards matching the peaks’ retention times are the essential tools for an LC-MS protocol implementation.89 This problem is also characteristic for other MS methods used without prior separation, such as MALDI and ESI, which have previously been used in combination with high resolution MS, e.g., time of flight or ICRMS.33, 36, 39, 48, 62, 69 While these methods provide a complementary 6 ACS Paragon Plus Environment
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sample characterization of both high- and low-MW fractions, the resulting mass spectra (including all compounds in a single spectrum) are extremely complex and challenging to interpret.69 This problem can be addressed (but only to some extent) by the application of MS/MS instruments, which allow for a sequential fragmentation of oligomer ions.69 Nevertheless, without prior separation these methods cannot distinguish whether the detected smaller ions represent the molecular ions of low-MW species or are actually the fragments of higher-MW oligomeric compounds. The data compiled in Table 1 show that the non-catalytic thermal lignin liquefaction usually yielded no more than 5–10 wt% of GC-elutable products, most of them being phenolic monomers. Dimers and trimers were also present, particularly when the process was conducted near the lower end of the standard 240–450 °C range.56 Matching this percentage with the OLP yield would provide the relative amount of oligomers, i.e., partially depolymerized or otherwise modified lignin. For example, after an alkaline treatment of lignin at 290–315 °C yielding 19% of monomers, oligomers and modified lignin accounted for 45–70% of the mass.45 This fraction remained significant when efficient catalysts were applied (Table 1). As a representative example, a treatment with CrCl3 on Pd/C at 280 °C yielded up to 78% OLP with 47% GCelutable products.30 The difference, i.e., 31% in this case, were phenolic oligomers, a potential source of valuable chemicals. Yet, the current methods of chemical analysis afford only a cursory characterization of this sizable oligomeric fraction. Pyr-GC-MS could provide valuable information on oligomers and so it was employed for characterization of native lignin93 and its degradation products.21, 35, 50, 54 The typical applications of Pyr-GC/MS using a single step pyrolysis were the determination of the syringol/guaiacol ratio in lignin to classify the hardness of wood and identification of lignin degradation compounds in hydrotreated samples.21, 35 However, besides being merely qualitative, this approach does not allow for a separation of monomeric and polymeric species, which will evolve and pyrolyze, respectively, during the same step yielding undistinguishable compounds. To address this problem, fractional thermal desorption (TD)/Pyr has recently been employed for characterization of lignin and its degradation products, where lower temperatures, ~200 °C, were employed for TD whereas higher temperatures (
