Hydrothermal Carbonization of Glucose, Fructose, and Xylose

Research Article pubs.acs.org/journal/ascecg

Hydrothermal Carbonization of Glucose, Fructose, and XyloseIdentification of Organic Products with Medium Molecular Masses Juergen Poerschmann, Barbara Weiner,* Robert Koehler, and Frank-Dieter Kopinke Department of Environmental Engineering, Helmholtz Centre for Environmental Research - UFZ, Permoserstr. 15, D-04318 Leipzig, Germany S Supporting Information *

ABSTRACT: The monosaccharides glucose, fructose, and xylose were subjected to hydrothermal carbonization in aqueous solution at temperatures of 180, 220, and 250 °C for different operating times (30 min to 16 h). Here, 68% to 78% of the organic carbon was converted into hydrochar at 220 °C with glucose and fructose as feedstock, whereas hydrothermal treatment of xylose did not result in significant hydrochar formation under these conditions. The main topic of this contribution was the identification of stable organic products in the process water in the molecular mass range between 120 and 300 Da by means of GC-MS analysis using several derivatization agents. Special attention was paid to polar OH- and COOH-functionalized compounds. The overwhelming majority of identified organic compounds had cyclic structures of which a prominent group included hydroxylated benzofurans. However, the combined yield of products, which might be potential substrates for liquid biofuels, turned out very low. KEYWORDS: Hydrothermal carbonization, Monosaccharides, GC-MS analysis, Cyclic intermediates, Benzofurans

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INTRODUCTION Hydrothermal carbonization (HTC) using water as an environmentally friendly reaction medium has developed into a promising thermochemical approach to produce hydrochars as well as to upgrade waste lignocellulosic biomass into valuable chemicals and fuels.1,2 As a result of HTC, a solid hydrochar containing commonly around 60−75% of the organic carbon (OC) of the biomass feedstock, process water with 20−25% of the biomass OC, and a minor gas fraction mainly consisting of carbon dioxide are formed. Carbohydrates are the most significant source of hydrochars.3 The formation of char is associated with reactions including hydrolysis, deoxygenation (dehydration, decarboxylation), and aldol condensation.4 Subsequent to degradation into smaller molecules and defunctionalization reactions, the formed intermediates (re)polymerize to form spherical nuclei. Hydrochars formed from glucose and xylose as carbon feedstocks were previously characterized by particle morphology, scanning electron microscopy particle morphology, and 13C solid state NMR spectroscopy.5 Structure and morphology proved very similar irrespective of the nature of the hexose, whereas differences between pentoses and hexoses were observed. The formation of water-soluble monomers and oligomers proceeds concomitantly to that of hydrochar. Thus, low molecular mass breakdown products of carbohydrates and analytes having molecular masses between 90 and 290 Da have been considered main precursors of the solid hydrochar. At the beginning of the HTC activities relaunched by Antonietti et al.,6 emphasis was put on the solid hydrochar, whereas the © 2017 American Chemical Society

process water was considered an undesirable product stream. At present, the dissolved organic matter (DOM) fraction is considered a potential alternative source of platform chemicals and biofuels in the future.7 To accomplish this goal, a detailed, molecular level knowledge of the organic products in the process water is necessary. Structural assignment of organic products in process water is also required from the perspective of hydrochar formation. Consequently, DOM studies should be embedded into the structural analysis of hydrochars. In order to investigate its potential, the elucidation of process water components is significant for a further reason: the functionalized conversion products constitute potentially hazardous chemicals, which necessitates appropriate treatment in case of process water discharge. Further products unknown so far may also be potentially toxic. To date, the knowledge of DOM on the molecular level has been scarce. Previous studies provided evidence that simple organic molecules such as levulinic acid, furfural, 5-hydroxymethyl furfural (5-HMF), and phenolic products (determined commonly by gas chromatography), as well as lactic, formic, and acetic acid (typically determined by ion chromatography) arise from hydrothermal treatment of biomasses rich in carbohydrates.8 The content of carbohydrates in biomasses scatters widely. Typical lignocellulosic biomasses have cellulose contents of about 30% and hemicellulose contents of about Received: January 25, 2017 Revised: June 12, 2017 Published: June 14, 2017 6420

DOI: 10.1021/acssuschemeng.7b00276 ACS Sustainable Chem. Eng. 2017, 5, 6420−6428

Research Article

ACS Sustainable Chemistry & Engineering

an HTC process temperature of 220 °C was applied under autogenous pressure. For methodological reasons, operating temperatures of 180 and 250 °C were also applied. After the indicated reaction time, the autoclaves were cooled to room temperature and opened, and the glass tubes were removed. Solid and liquid products were separated by filtration with a folded cellulose paper filter (Whatman, grade 1). The solid hydrochar was removed from the filter and then dried at 105 °C overnight. Elemental analysis (C, H, and N) was carried out by an elemental analyzer (Perkin-Elmer Corp., Norwalk, CT). Process Water Pretreatment. Aqueous process waters were diluted with water (1:1), spiked with internal standards (500 μg of phenanthrene-d10 g−1 referred to as carbohydrate mass input, 2000 μg g−1 of acetic acid-d4, 2000 μg g−1 of sodium lactate-d3), then extracted twice with chloroform (2 mL × 1 mL) using a vertical shaker (HS 250 basic) at 250 rpm for 10 min.13 Although the phenanthrene internal standard concentration is higher than the water solubility, the partition of phenanthrene onto the macromolecular DOM in the process water enhances to an apparent water solubility far beyond the theoretical water solubility devoid of macromolecular DOM. The combined extracts were dried over sodium sulfate and then rotary evaporated to give a final volume of approximately 200 μL. The purification of solvent extracts was performed by solid phase extraction using Supelclean LC-Si cartridges (3 mL volume; 500 mg sorbent; Supelco Visiprep for manipulations). The cartridges were initially conditioned with 5 mL toluene/acetone (75:25, v/v). They were then loaded with the chloroform extract. The analytes were eluted by gravity with 3 mL chloroform/acetone (75:25, v/v), then subjected to volume reduction. Analysis of Bulk Parameters. The pH was measured with a pH meter (MP 225, Mettler Toledo, Gieβen, Germany). DOC was analyzed using a total organic carbon analyzer TOC 600 (Shimadzu, Germany). GC-MS Analysis. Extracts of process waters were subjected to a variety of derivatization reactions including silylation to form trimethylsilyl ethers/esters (TMS) and mild methylation with trimethyl chlorosilane/methanol to form methyl esters, as well as acylation with acetic acid anhydride to form acetates. Silylation was performed by adding ∼100 μL BSTFA (BSTFA-d9) to the volume reduced solvent extract, which was then allowed to stand for 2 h at 80 °C. Methylation using trimethyl chlorosilane/methanol (1:10, v/v) was carried out according to Poerschmann et al.14 Acylation was performed with acetic acid anhydride/pyridine (5:1, v/v) at 80 °C for 90 min. Both the derivatized extracts and the native solvent extract were injected into the GC-MS without any further purification. Solvent extracts were injected using a pulsed splitless mode at 280 °C into a GC-MS (HP 5973B) system equipped with a DB-5MS fused silica capillary column (5% phenyl-, 95% methylpolysiloxane, 30 m × 0.25 mm, film thickness 0.50 μm). Helium served as the carrier gas at a constant flow rate of 1.0 mL min−1. The temperature program started at 40 °C for 2 min, then the temperature was raised to 295 °C at 6 °C/ min and held for 5 min. Ion source and transfer line temperatures were 200 and 270 °C, respectively. Quantification of analytes (m/z 130 to 350 Da) was done by external calibration using 5-HMF, which was calibrated against the deuterated standard phenanthrene-d10. Both analytes in water (six 5-HMF concentrations covering a range of 10− 500 μg mL−1, with a concentration of 10 μg mL−1 of phenanthrene-d10 in each sample) were extracted with chloroform, then dried over sodium sulfate, rotary evaporated, and subjected to derivatization procedures. Calibration was based on six points, with three replicates at each level. The calibration factor obtained by dividing the intensity of the most abundant ion m/z = 183 amu for the TMS derivative of 5HMF against the ion m/z = 188 amu for phenanthrene-d10 was used for all analytes except low molecular mass acids, where acetic acid-d4 and sodium lactate-d3 were used. Quantitation following this protocol can be conceived semiquantitative only by two reasons: variable chloroform extraction yields, in particular, for highly polar compounds, and variable response factors in the GC-MS detection. 5-HMF was selected as a reference compound because it is a main HTC product representing the fraction of polar solutes. The NBS 98 mass spectral library was used for identification.

19%.9 The formation of those products under hydrothermal conditions was studied on a time-resolved basis for fructose10 and xylose11 as substrates. The main focus of the study was furfural, as it was the most abundant product. In addition, polar, low molecular mass analytes such as acetol, glyceraldehyde, and glycolaldehyde could be identified by GC. However, detailed information about organic products with molecular masses beyond 5-HMF (126 Da) is still lacking. It should be pointed out that functionalized analytes containing hydroxyl and carboxyl groups are difficult to analyze by GC due to their low volatility and high polarity. A previous contribution addressed volatile HTC products by means of headspace GC.12 Highly polar analytes with low volatility and low Henry’s law coefficients cannot be subjected to headspace GC analysis in their native form. Another shortcoming inherent to previous research is that there are very few papers which are devoted to quantification, e.g., solid data on concentrations of known breakdown products referred to (dry) mass of the feedstock are lacking. Given the above-mentioned reasons, comprehensive studies devoted to the structure elucidation of dissolved products on the molecular level are necessary. In a previous contribution, lignin-derived feedstocks such as organosolv wastewater were studied to reveal structures of organic HTC products in the process water fraction.13 Herein, three carbohydrates including glucose (building block of cellulose; synonymous with aldohexoses), fructose (ketohexose, product of glucose isomerization), and xylose (aldopentose, prominent building block of hemicellulose), which represent the carbohydrate fraction in biomasses, were subjected to HTC. The impact of reaction time and temperature on the yield of hydrochar and the DOM fraction was studied. The main focus of this contribution was the structural assignment of stable organic products formed under hydrothermal conditions. Identification of stable products was primarily performed by GC-MS analysis in combination with common derivatization reactions of solvent extracts.

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EXPERIMENTAL SECTION

Chemicals. Glucose, fructose, and xylose (purity beyond 99% each) were purchased from Sigma-Aldrich (Munich, Germany). All other chemicals and solvents (analytical grade), as well as derivatization agents nonlabeled BSTFA (>99.5% purity) and acetic anhydride (>99.5% purity), were purchased from Sigma-Aldrich. The isotopically labeled derivatization agent BSTFA-d9, as well as internal standards including phenanthrene-d10 (to quantify the identified compounds listed in Table 1 and Table S1), acetic acid-d4 (to quantify C2−C6 carboxylic acids), and sodium lactate-d3 (to quantify shortchain hydroxyl carboxylic acids), were purchased from CDN-Isotopes (Pointe-Claire, Canada). Isotopic purity was beyond 99 atom % D. Authentic standards included (i) organic acids such as 4-OH benzoic acid and short-chain acids (lactic acid, glycolic acid, oxalic acid, C2nC6 acid mix, crotonic acid, and 2-pentenoic acid), (ii) phenols including phenol, all cresol isomers, catechol, and hydroquinone, (iii) “furanics” such as 2-methyl furan (2-M-furan), furfural, furfuryl alcohol, 5-HMF, furan-3(2H)-one, furan-2(5H)-one, furancarboxylic acid, (5-M-2-furyl)methanol, 1-(2-furyl)ethanol, 2,2′-furoin, and maltol (3-OH-2-methyl-pyranone), as well as (iv) cyclopenten-1-one and 2M- and 3M-cyclopenten-1-one, hydroxyacetone, 1,3-dihydroxyacetone, and γ-valerolactone. Hydrothermal Carbonization. An aliquot of a given carbohydrate (300 mg) in a glass tube was mixed with deionized water (3 mL). The glass tubes were placed inside an in-house designed stainless steel autoclave (inner diameter, 8 mm; outer diameter, 12 mm; length, 22 cm; volume, 25 mL), sealed and placed into a GC-oven. In most cases, 6421


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Table 1. Organic Compounds Extracted from HTC Process Waters of Glucose, Fructose, and Xylose along with Their Yieldsa Yields [μg g−1 input]

Formic acidb Acetic acidb 2-Butanone Glycolic acidb Furan-3(2H)-one Butyrolactone Lactic acidb 1,3-Dihydroxyacetone Phenol Furfural 3-M-2-cyclopenten-1-one 5M-2-Furanone 2-Furylmethanol γ-Valerolactone 4-Pentenoic acid 2-Pentenoic acid 2,4-Dihydroxybut-2-enal 2-Acetylfuran 5M-2-Furfural ∑ (1,2- and 1,4-Benzenediol) 5-M-2-Furanmethanol 2,5-Hexanedione 2,5-Dihydroxypenta-2,4-dienal Levulinic acid Diacetone alcohol 2-Furanacrolein 4-OH benzaldehyde 2,5-Furandicarboxaldehyde 5-M-2-Acetylfuran Isomaltol Furyl hydroxymethyl ketone 5-(Hydroxymethyl)-2-furfural 1,2,4-Benzenetriol 3,6-Heptanedione Bis(hydroxymethyl)furfural ∑ (OH-acetophenones) OH-isomaltol 2,5,6-Trihydroxyhexa-2,4-dienal ∑ (Dihydroxy benzofurans) 2-(Hydroxymethyl)benzofuranol Sorbitan 1-(2-Furyl)-1,4-pentanedione 4-(Furan-2-yl)buta-1,3-diene-1,2,4-triol Benzofuran-2,3-diyldimethanol 4-(Furan-2-yl)-2,3,4-trihydroxybutanal Acetyl-hydroxybenzofuran-3-one 1-(5-Dihydroxymethyl)furan−2-yl)-2-ethoxyethanone (5-OH-hydroxyphenyl) methyl)furan-2-ol (Dihydroxy-2,3-dihydro-benzofuran-yl)propan-2-one Bis(hydroxymethyl)naphtha-lene-1,4-dione 1-(3-OH-2-(hydroxymethyl)benzofuran-4-yl)propan-2-one 1-(Trihydroxy-2,3-dihydro-benzofuran-2-yl)propan-2-one 1-(2,3-Bis(hydroxymethyl) benzofuran-4-yl)propan-2-one 4-(5-Hydroxymethylfuran-2-yloxy)catechol 2-OH-1,2-Bis(5-hydroxy-methyl)furan-2-yl)ethanone 1,1′-(2,3-Bis(hydroxyl-methyl)benzofurandiyl)- dipropan-2-one

Molecular mass [Da]

Molecular formula

46 60 72 76 84 86 90 90 94 96 96 98 98 100 100 100 102 110 110 110 112 114 114 116 116 122 122 124 124 126 126 126 126 128 128 136 142 144 150 164 164 166 168 178 186 192 200 206 208 218 220 224 234 234 252 290

C1H2O2 C2H4O2 C4H8O C2H4O3 C4H4O2 C4H6O2 C3H6O3 C3H6O3 C6H6O C5H4O2 C6H8O C5H6O2 C5H6O2 C5H8O2 C5H8O2 C5H8O2 C4H6O3 C6H6O2 C6H6O2 C6H6O2 C6H8O2 C6H10O2 C5H6O3 C5H8O3 C6H12O2 C6H4O3 C7H6O2 C6H4O3 C7H8O2 C6H6O3 C6H6O3 C6H6O3 C6H6O3 C7H12O2 C5H4O4 C8H8O2 C6H6O4 C6H8O4 C8H6O3 C9H8O3 C6H12O5 C9H10O3 C8H8O4 C10H10O3 C8H10O5 C11H12O3 C9H12O5 C11H10O4 C11H12O4 C12H10O4 C12H12O4 C11H12O5 C13H14O4 C12H10O5 C12H12O6 C16H18O5

∑(OHCOOH) (TMS)

OH-groups (HAC)

2

2 2 1

n.d.c 1

1

n.d.c

1 1 2

n.d.c

2 1

2 1

2 1 1

2

1

1

1 1 1 3

n.d. 1 1 3

2 1 2 3 2 2 4

2 1 2

3 2 3 1 2 3 2 2 2 3 2 3 3 2

n.d.c 2 n.d.c 1 2 3 2 2 2 3 2 n.d.c 3 2

1

2 2 n.d.c

Glucose

Fructose

Xylose

27650 4560 1440 14880 630 510 16210 870 270 5700 120 710 n.d. 760 910 630 n.d. 1160 1950 570 1860 630 n.d. 29600 n.d. n.d. 140 680 410 520 385 9830 720 310 430 275 430 650 240 910 240 310 n.d. 420 250 240 215 225 590 550 3850 2650 3530 540 450 1770

30480 3920 1780 16040 910 3580 17650 750 385 6600 190 650 n.d. 620 580 325 n.d. 1570 1820 490 1520 1110 n.d. 28450 320 n.d. 190 555 700 370 475 12420 950 480 570 425 485 905 350 800 210 450 n.d. 925 380 385 270 370 735 770 5750 3100 5150 785 310 2230

10560 6630 120 2200 n.d. n.d. 30500 80 125 285000 n.d. n.d. 650 n.d. 155 90 770 195 215 n.d. 210 2340 6100 750 1170 1750 n.d. 75 105 n.d. 730 535 n.d. n.d. n.d. 2370 n.d. n.d. n.d. n.d. n.d. n.d. 870 n.d. n.d. n.d. n.d. 80 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

a

HTC at 220°C for 2 h. bAcids analyzed by ion chromatography (see text). cAcylation of aliphatic di- and tri-alcohols results in acetates which do not give diagnostic mass spectra (molecular ions of acetates are mostly lacking or have low abundance). 6422


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ACS Sustainable Chemistry & Engineering Table 2. Hydrochar Mass Yields and Organic Carbon Yields from Glucose and Fructose as HTC Substratesa Glucose

a

Temp. [°C]

Reaction time [h]

Mass yields [%]

180 180 220 220 220 220 220 220 220 220 250

2 4 0.5 1 1.5 2 3 4 6 16 2

no char no char 41 43 44 44 44 43 43 40 35

Carbon yields and content in hydrochar [%]

Fructose Oxygen yields and content in hydrochar [%]

Mass yields [%]

Carbon yields and content in hydrochar [%]

23/30 23/29 22/27 21/26 20/24 19/24 19/24 18/24 15/22

18 40 38 41 42 43 42 42 42 39 36

30/66 66/66 63/66 69/68 72/69 75/70 75/71 74/71 76/72 69/71 66/73

68/66 73/67 74/68 77/70 78/72 77/72 77/72 72/72 65/74

Relative standard deviations of single values ranging from 3% to 7%.

for reaction times between 1 and 6 h at 220 °C (Table 1) indicating both efficient and fast defunctionalization (see also below: oxygen yields). More severe carbonization conditions (e.g., 250 °C, 2 h) resulted in lower organic carbon yields, mainly due to gas formation. The organic carbon contents in the hydrochars of both hexoses were enhanced at higher process temperatures due to a higher degree of defunctionalization (Table 1). Clearly, the reaction temperature has a stronger impact on the organic carbon content in the hydrochars as compared to operation time. The differences in carbon balances between hexoses on one side and xylose on the other side will be addressed in a forthcoming contribution. Defunctionalization can also be traced by oxygen yields. Oxygen content was calculated on the basis of carbon and hydrogen yields obtained by elemental analysis. As expected, oxygen yields decreased at longer process times at a given temperature (e.g., 220 °C in Table 1) and on switching to higher temperatures. The oxygen balance confirmed very rapid defunctionalization as previously deduced from the carbon balance. Both glucose and fructose produced hydrochar at 220 °C after an operating time of as low as 30 min. In the case of glucose, only 30% of the original biomass oxygen was recovered in the hydrochar (fructose data, not reported here, were almost identical). The oxygen content in the hydrochars dropped only slightly from about 30% at 30 min to 24% after 16 h reaction time, pointing to the defunctionalization process leveling off after 30 min. Even at the very low HTC temperature of 180 °C, hydrochar formation took place with fructose. Literature data provide evidence that for many biomass feedstocks a lower fraction of organic carbon is converted into hydrochar compared to glucose and fructose.5 Clearly, hydrochar yields from proteins and lignin are lower compared to those for carbohydrates,16 whereas lipids do not significantly contribute to hydrochar formation.17 Our preliminary results, the subject of a forthcoming publication, provided evidence that hydrochar organic carbon recoveries from glucose and cellulose were similar for operating temperatures of 220 and 250 °C, whereas at 180 °C hydrothermal treatment of cellulose resulted in lower yields. These data confirm previous findings based on NMR spectroscopy and scanning electron microscopy, according to which there are no substantial differences between monosaccharide- and polysaccharide-derived carbon speciation in hydrochars.5

Ion Chromatography. To quantify polar low molecular mass organic acids, the HTC solution was diluted 1:100 and subjected to anion chromatography using an ion chromatograph (DX600, Thermo) equipped with an anion suppressor (ASRS 300), conductivity detector (CD20), and an IonPac AS18 analytical column (4 mm × 250 mm) connected to an AG18 precolumn. Gradient elution using the eluent generator EG40 was used with the following program: 2 mM KOH from 0 to 11 min, ramp to 18 mM KOH over 6 min, hold at 18 mM KOH for 1 min, followed by an increase to 35 mM KOH over 12 min and held for 10 min.

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RESULTS AND DISCUSSION Hydrochar Yields and Distribution of Organic Carbon between Hydrochar and Process Water. Table 2 summarizes data based on gravimetric hydrochar determination. Data provide evidence that hydrochar formation proceeds rapidly with fructose and glucose: a clear phase separation between hydrochar and process water was observed for fructose after a reaction time as short as 30 min at 220 °C. In the case of glucose, hydrochar was also formed (again after 30 min at 220 °C), but phase separation was difficult to manage. Clear-cut phase boundaries for glucose were observed for longer operation times at 220 °C. Hydrochar mass yields at 220 °C proved high and independent of the operation time between 1 and 6 h for both substrates (43% to 44% for glucose, 41 to 43% for fructose). Data showed good reproducibilities (3% to 6% relative standard deviations, n = 3). The similarity in hydrochar yields over a wide range of process times was most likely related to the offsetting effects of counteracting processes such as defunctionalization (which should give rise to higher hydrochar yields) versus mass loss attributed to splitting off of fragment (gas formation). Hydrochar yields declined at higher temperatures such as 250 °C due to the increased significance of gas formation which were not further quantified.15 The high hydrochar mass yields can be explained considering a theoretical stoichiometry of hexose dehydration according to the reaction C6H12O6 = C6H2O + 5 H2O. This reaction would give rise to a char mass yield of 50%. However, such a simplified reaction scheme does not represent the reality, as it does not account for dissolved organic matter and gas formation.5 As shown below, a significant amount of the organic carbon of the hexoses (∼20%) was in fact converted into DOM. In the face of these assumptions, the difference to the yields just given (41% to 44%, Table 1) is small. The hydrochar organic carbon recoveries turned out very high (73% to 78% in case of glucose, 69% to 76% for fructose) 6423


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ACS Sustainable Chemistry & Engineering Data for xylose, a basic constituent of hemicellulose, are mostly lacking in Table 2 because hydrochar formation proceeded only under severe reaction conditions (e.g., 220 °C/16 h or 250 °C/2 h). Thus, it can be concluded that dissolved organic matter in HTC process water under commonly used HTC conditions originates mostly from hemicellulose rather than cellulose. Hemicellulose, being a less stable polymer compared to cellulose, can be hydrothermally converted at lower temperatures. The tendency of cellulose and hemicellulose to form solid hydrochars is likely to be tied to their main intermediates (see below). The most abundant xylose-based intermediate furfural does not produce hydrochar as readily as the more reactive 5-HMF, the most abundant intermediate of glucose and fructose. Solid state 13C NMR spectroscopy confirmed that polyfuranic chains dominate the hydrochar structure at low HTC temperatures such as 180 °C.18 These oxygen-rich chains are subjected to further dehydration and defunctionalization reactions to enhance aromatic moieties (see the Identified Intermediates and End Products section). Figure 1 details the dissolved organic carbon (DOC) yields for both hexoses. As a rule of thumb, 17−23% of the organic

(i) Methylation of carboxylic groups followed by silylation of alcohols allowed distinguishing between carboxylic acids and phenols when these chromatograms were compared with chromatograms obtained by silylation alone (both carboxylic and phenolic groups were converted in the latter case). (ii) Silylation with both nonlabeled BSTFA and deuterated BSTFA-d9 allowed drawing conclusions about the number of OH groups in a molecule. Acylation with acetic acid anhydride served to confirm the molecular mass of the parent molecule and the number of OH groups. In most cases, the molecular formula of the parent molecule could be derived using these combined findings. The structural identification was further refined by tracing diagnostic fragment ions to recognize distinctive structural units. As an example, a diagnostic fragment ion at m/z = 147 amu ((CH3)3−Si−O−Si(CH3)2)+, m/z = 162 amu in case of BSTFA-d9) is characteristic for derivatives with more than one TMS group. Another diagnostic fragment ion at m/z = 103 amu originates from the α-cleavage of C−C bonds, its typical high abundance being due to a stabilization of the positive charge by the unshared electron pairs on the oxygen atoms (see the scheme below):

The strategy to identify structures is exemplified with two analytes: the linear intermediate 2,5,6-trihydroxyhexa-2,4-dienal (Table 1: parental molecular mass, 144 Da; C6H8O4; TMS, 360 Da) and the hydroxylated benzofuran, benzofuran-2,3-diylmethanol (parental molecular mass, 178 Da; C10H10O3; TMS, 322 Da). For the first compound, silylation with nonlabeled BSTFA vs BSTFA-d9 pointed to three donor hydrogen atoms, which was confirmed by acylation. Thus, a parental molecular mass of 144 Da could be concluded. Second, the proton abstraction (ion at m/z = 359 amu) pointed to an aldehyde. Consequently, the parental molecular formula turned out to be C6H8O4 rather than C7H12O3, which was also suggested by the retention behavior (molecules with three TMS groups and a C7-structure are expected to elute later). Third, mass spectral fragmentation (Figure 2) pointed to the structure listed in Table 1 and depicted in Figure S2. For the second compound, silylation and acylation indicated that the parental analyte had two donor hydrogen atoms; thus, the parental molecular mass turned out to be 178 Da (Table 1; Table S1). Mass spectral fragmentation pointed to an aromatic

Figure 1. Recovery of dissolved organic carbon from glucose and fructose as substrates. HTC temperature 220 °C if not otherwise stated.

carbon of a wide array of biomasses were found to be converted to DOC.19 As observed with hydrochar yields, the DOC yields did not significantly depend on the kind of hexose (Table 1). The DOC recovery was reduced with longer residence times and higher operating temperatures. The combined organic carbon yields of hydrochars and DOC in process waters did not result in a closed balance. Clearly, the formation of gaseous HTC products, the extent of which increases with longer operating times, as well as other potential inaccuracies accounted for this gap to 100%. Methodological Considerations on Structural Assignment of Organic Products in Process Water. Structural assignments of organic compounds were primarily performed by combining information based on derivatizations and on interpretation of mass spectra emphasizing the role of diagnostic fragments. The well-established derivatization reactions proved very useful for structural assignments:

Figure 2. Tentative identification of 2,5,6-trihydroxyhexa-2,4-dienal by characteristic mass fragments. 6424


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ACS Sustainable Chemistry & Engineering system with a benzofuran scaffold, as well as to at least one R− CH2−OH moiety (abundant fragment ion at m/z = 103 amu). The radical cation at m/z = 234 amu (Table S1) points to an ortho-effect, as the Retro-Diels−Alder reactions, McLafferty rearrangements, and double charged cations, all of which may also give even-numbered radical cations, are very unlikely to occur. Thus, a benzofuran-2,3-diyl structure was assumed, although structures such as (hydroxymethyl)-methyl-benzofuranols or benzofuran-diylmethanols (2,4 or 2,5 or 2,6 or 2,7diylmethanol) might also be possible. This strategy was applied to further structural assignments. It is outside the scope of this contribution to detail all structural assignments. For further information, Figure S2 exemplifies the fragment at m/z = 95 amu, which is indicative for a furfuraloriginating structure. In addition to the identification route shown above, mass spectral fragmentation patterns of TMS derivatives of monosaccharides, sugar alcohols, and anhydrosugars, along with their Kovats retention indices20 and mass spectra of TMS derivatives of “furanics”21 are available, facilitating structural assignments without using authentic standards. Identified Intermediates and End Products. Dissolved organic matter in HTC process water is formed in parallel to the (re)polymerization of the formed intermediates to produce spherical hydrochar particles. This formation is controlled by degradation kinetics and (re)polymerization kinetics of carbohydrates (cellulose and hemicellulose, as well as monosaccharides such as glucose, fructose, xylose, and intermediates thereof). Depending on the process parameters and the feedstock composition, hydrochar formation is highly variable.22 The variability of hydrochar characteristics (elemental composition, particle size, char texture, aggregation, etc.) is expected to be associated with the pattern of dissolved conversion products, which can serve as hydrochar precursors. Table 1 summarizes a multitude of conversion products identified in the framework of this contribution along with their concentrations in HTC process waters. Herein, quantification of analytes was restricted to process waters from HTC of the three monosaccharides under study at 220 °C for 2 h. The distribution of the identified analytes across different process times and operating temperatures to reveal degradation kinetics should be dealt with in a forthcoming contribution. Structures of identified analytes are depicted in Figure S1 (Supporting Information). Figure 3 exemplifies a segment of the total ion chromatogram (TIC) of the BSTFA-derivatized solvent extract from HTC of fructose at 220 °C. Structural assignment was primarily based on data given in Table S1 (Supporting Information). A large body of references targeted simple organic products such as furfural, 5-HMF, levulinic acid, 1,3-dihydroxyacetone, and 2-butanone, as well as low molecular mass organic acids such as lactic acid, formic acid, and acetic acid, which have been previously recognized as intermediates and end products of hydrothermal treatments of biomasses and model substrates (Introduction). Their formation pathways have been studied extensively. As an example, a pathway based on glucose to form the key intermediate 5-HMF and further conversion products is given in Figure S3. Following the reaction cascade shown in this Figure, levulinic acid formed from 5-HMF decomposes under hydrothermal conditions to α-angelica lactone,23 which in turn decomposes to dihydroxyacetone.24 As expected, levulinic acid, 5-HMF, angelica lactone, and dihydroxyacetone could be identified as abundant analytes

Figure 3. TIC of a BSTFA-derivatized solvent extract. Sample: Fructose subjected to HTC at 220 °C for 1 h. Peak labels: Molecular mass of the parental compound followed by molecular mass of the corresponding TMS-derivative (Table 1; Table S1).

(Table 1). The 5-HMF intermediate proved very abundant for hydrothermal degradation of hexoses, whereas furfural was mainly formed from pentoses. It should be pointed out here that the concentration of 5-HMF was more than 1 order of magnitude higher as compared to data in Table 1 belonging to 220 °C/2 h when turning to shorter reaction times such as 30 min at 220 °C. Thus, the formation of 5-HMF, synonymous with the removal of three water molecules from the parent sugar molecule, proceeds rapidly. Further dehydration of 5HMF is also possible by polycondensation to form “polyfuranics” (see below). Levulinic acid represented an abundant intermediate across all samples: more than 3 % of the organic carbon of either hexose was converted into that acid (Table 1). Levulinic acid as a diagnostic intermediate of hexose dehydration was found alongside 5-HMF as the second most abundant compound from hexoses. The product pattern from xylose is relatively simple because it contains basically only a single compound, furfural. The very high concentration of furfural in the HTC process water (29% of the total DOC) is associated with the finding that no hydrochar was formed. The low tendency of furfural to be converted into hydrochar is based on its low reactivity with respect to polycondensation, which is due to the stabilization of the aldehyde moiety and the furan ring.25 Furfural may also be formed by conversion of 5-HMF. However, while this reaction was significant in supercritical water,26 only low yields were obtained in subcritical water.27 Regarding low molecular mass organic acids, ion chromatographic analysis revealed that formic acid, lactic acid, glycolic acid, and acetic acid (in this abundance order) were the most abundant surrogates for all monosaccharides under study (Table 1). In HTC of glucose (220 °C, 2 h), the total low molecular mass acid concentration was 64 mg g−1 glucose, which translates into 51 mg OCTotal Acid/g OCGlucose (assuming an average organic carbon content of 32% across the four acid molecules). Total acid concentrations were found to increase with increasing HTC severity, which should be detailed in a forthcoming contribution. These acids exert an impact on hydrothermal processes, for example, by stimulating hydrolysis due to the increased hydronium ion concentration. Most recent findings indicate that formic acid promotes conversion of fructose into 5-HMF.28 Residual glucose, fructose, and xylose could not be identified. The same holds true for well-known hydrothermal decay 6425


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ACS Sustainable Chemistry & Engineering products of glucose identified by Knezevic et al. including levoglucosan, erythrose (as tris-TMS derivative of erythrofuranose), and glycolaldehyde (commonly analyzed in the dimer form).15 Clearly, they were converted into more stable intermediates such as 5-HMF, levulinic acid, furfural, and short-chain carboxylic acids, as well as into gas. The aldehyde 2,4-dihydroxybut-2-enal (parent molecule: 102 Da, Table 1) is likely to originate from erythrose, which in turn may be formed from glycolaldehyde. Likewise, polycyclic aromatic hydrocarbons (PAH) could not be detected in process water. They are known to be formed during dry thermal treatment of biomasses at 400−500 °C.29 Due to their high hydrophobicity, PAH would mostly sorb onto hydrochar. However, solvent extraction of hydrochars did not reveal any PAH (LOD = 100 ng per g of hydrochar for phenanthrene and anthracene). As revealed in Table 1, the concentrations of phenol, cresols, catechol, etc., known for inducing environmental hazards and inhibiting crop germination when hydrochar is used as soil amendment, were very low.30 Phenols/benzenediols in the process water of biomasses were shown to be mainly associated with lignin-based feedstocks.13 Beyond the array of simple organic molecules which were identified as hydrothermal conversion products of carbohydrates in the framework of previous papers (Introduction), a multitude of analytes within a molecular mass range between 120 and 300 Da could be detected herein. Figure 3 details a subsection of the total ion chromatogram of a silylated solvent extract of fructose conversion (HTC at 220 °C, 1 h). Table 1 along with structures depicted in Figure S1 illustrate that the overwhelming majority of analytes with parental molecular masses above 150 Da are cyclic in nature. Among the cyclic compounds, the aromatic benzofurans accounted for a significant group of intermediates. The identification of benzofuran-2,3-diyldimethanol is detailed above. A multitude of other benzofuran-based compounds are listed in Table 1. As an example, the compound 1-(3-hydroxy-2-(hydroxymethyl) benzofuran-4-yl)propan-2-one (MMParent = 220 Da, MMTMS = 364) is based on benzofuran-2,3-diyldimethanol (Table 1; Table S1, Figure S4). Structural similarities of 1-(3-hydroxy-2(hydroxymethyl) benzofuran-4-yl)propan-2-one with the compound having MMParent = 234 Da and MMTMS = 378 Da (Table 1; Table S1 and Figure S1) are striking. On the basis of the hypothesis that the quality of hydrochar depends on the dissolved organic matter in the process water and vice versa, the identification of hydroxylated benzofurans confirms former findings, according to which these structures were found in the carbonaceous scaffold of hydrochar produced by HTC of glucose31 and cellulose32 as substrates. Some other identified analytes beyond benzofurans along with their presumed formation pathways should be mentioned in addition to benzofurans: (i) Furyl hydroxymethyl ketone (2-(2-hydroxyacetyl)furan, parental molecular mass 126 Da) is a precursor of furfural.33 (ii) Isomaltol (126 Da), which was produced by isomerization of fructose to the 2,3-enediol,23, can be converted into acetic acid and furan-3(2H)-one (both compounds listed in Table 1). (iii) The formation of 1,2,4-benzenetriol (126 Da) is expected to result also from the conversion of 5-HMF as demonstrated previously by the hydrothermal treatment of fructose at 290 °C.26

(iv) Low abundance sorbitan (MMParent = 164 Da, C6H12O5, four OH groups) could tentatively be detected. This compound originates from the glucose intermediate sorbitol (6 OH groups), which was subjected to dehydration.34 Further dehydration products such as isosorbide (C6H10O4, MMParent = 146 Da) could not be detected. (v) The triol with a molecular mass of 168 Da, tentatively identified as 4-(furan-2-yl)buta-1,3-diene-1,2,4-triol, is expected to be derived from a tetrol (presumably 4furan-2-yl)but-1-ene-1,2,3,4-tetrol) by dehydration, which in turn originates from Aldol condensation of two C4-units. (vi) The formation of the linear intermediate 2,5,6-trihydroxyhexa-2,4-dienal (C6H8O4, see the Methodological Considerations on Structural Assignment of Organic Products in Process Water section) is due to the removal of two water molecules from hexoses (C6H12O6). Hence, it can be conceived as a 5-HMF precursor. On the other hand, it can also be formed by water addition to 5-HMF followed by ring opening and isomerization. Similar to the C6-dienal, the intermediate 2,5-dihydroxypenta-2,4dienal (Table 1: MMParent = 114, C5H6O3) is associated with the dehydration of xylose, obviously serving as a precursor of furfural. (vii) The formation of bis(5-hydroxymethyl)-2,2′-furoin (2OH-1,2-bis(5-hydroxymethyl) furan-2-yl)ethanone, MMParent = 252 Da, MMTMS = 468 Da, Table 1; Table S1 and Figure S1) originates from a self-benzoin condensation of 5-HMF. Intriguingly, this reaction has been previously known to proceed only catalytically in organic solvents as well as in aqueous solutions under mild conditions on immobilized catalysts35 rather than on hydrothermal treatment. Furoin (available as authentic standard), the self-condensation product of furfural, could not be detected in any sample. The same holds true for the cross-condensation product of furfural and 5-HMF. Quantification results revealed that the total yield of carbohydrate conversion products beyond the molecular mass of 5-HMF (126 Da) amounted to 2% for glucose and 3% for fructose. Under the assumption that the averaged organic carbon content of these compounds is about 60% higher than that of glucose/fructose, even when considering methodological inaccuracies in the quantification work, it remains clear that no more than 5% of the organic carbon of the carbohydrate can be converted into dissolved organic matter of this type, which may serve as substrate to produce biofuels. It fits into this consideration that the concentration of γvalerolactone (conversion product of levulinic acid), which can be converted by solid phase catalysis into biofuel components furans and hydrocarbons,36 proved very low. Conversion products listed in Table 1 arise from the common cascade of reaction pathways consisting of the following steps: (i) hydrolysis to monosaccharides, (ii) formation of “furanics”, and (iii) further defunctionalization into oxygen-depleted (aromatic) moieties. These compounds are considered the precursors which fine-tune the chemical structure of the solid hydrochar. The studies presented herein to address dissolved organic matter should be embedded into the structural analysis of hydrochars. The suitability of benzofuran derivatives as feedstocks of platform chemicals or 6426


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(3) Sevilla, M.; Fuertes, A. B. Chemical and structural properties of carbonaceous products obtained by hydrothermal carbonization of saccharides. Chem. - Eur. J. 2009, 15, 4195−4203. (4) Sevilla, M.; Fuertes, A. B. The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 2009, 47, 2281−2289. (5) Titirici, M.-M.; Antonietti, M.; Baccile, N. Hydrothermal carbon from biomass: a comparison of the local structure from poly- to monosaccharides and pentose/hexoses. Green Chem. 2008, 10, 1204− 1212. (6) Titirici, M.-M.; Thomas, A.; Antonietti, M. Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem. New J. Chem. 2007, 31, 787−789. (7) Luterbacher, J. S.; Alonso, D. M.; Dumesic, J. A. Targeted chemical upgrading of lignocellulosic biomass to platform molecules. Green Chem. 2014, 16, 4816−4838. (8) Cao, X.; Peng, X.; Sun, S.; Zhong, L.; Sun, R. Hydrothermal conversion of bamboo: identification and distribution of the components in solid residue, water-soluble and acetone-soluble fractions. J. Agric. Food Chem. 2014, 62, 12360−12365. (9) Godin, B.; Agneessens, R.; Gerin, P. A.; Delcarte, J. Composition of structural carbohydrates in biomass: precision of a liquid chromatography method using a neutral detergent extraction and a charged aerosol detector. Talanta 2011, 85, 2014−2026. (10) Antal, J. M.; Mok, W. S. L.; Richards, G. N. Mechanism of formation of 5-(hydroxymethyl)-2-furaldehyde from image-fructose and sucrose. Carbohydr. Res. 1990, 199, 91−109. (11) Antal, J. M.; Leesomboon, T.; Mok, W. S.; Richards, G. N. Mechanism of formation of 2-furaldehyde from D-xylose. Carbohydr. Res. 1991, 217, 71−85. (12) Becker, R.; Dorgerloh, U.; Helmis, M.; Mumme, J.; Diakité, M.; Nehls, I. Hydrothermally carbonized plant materials: Patterns of volatile organic compounds detected by gas chromatography. Bioresour. Technol. 2013, 130, 621−628. (13) Poerschmann, J.; Gorecki, T. Molecular-level based analysis of Organosolv wastewater. Current Chromatography 2017, in press. (14) Poerschmann, J.; Spijkerman, E.; Langer, U. Fatty acid patterns in Chlamydomonas sp. as a marker for nutritional regimes and temperature under extremely acidic conditions. Microb. Ecol. 2004, 48, 78−89. (15) Knezevic, D.; van Swaaij, W.; Kersten, S. Hydrothermal conversion of biomass. II. Conversion of wood, pyrolysis oil, and glucose in hot compressed water. Ind. Eng. Chem. Res. 2010, 49, 104− 112. (16) Hoekman, S. K.; Broch, A.; Robbins, C.; Zielinska, B.; Felix, L. Hydrothermal carbonization (HTC) of selected woody and herbaceous biomass feedstocks. Biomass Convers. Biorefin. 2013, 3, 113−126. (17) Broch, A.; Jena, U.; Hoekman, S. K.; Langford, J. Analysis of solid and aqueous phase products from hydrothermal carbonization of whole and lipid-extracted algae. Energies 2014, 7, 62−79. (18) Falco, C.; Perez Caballero, F.; Babonneau, F.; Gervais, C.; Laurent, G.; Titirici, M.-M.; Baccile, N. Hydrothermal carbon from biomass: structural differences between hydrothermal and pyrolyzed carbons via 13C solid state NMR. Langmuir 2011, 27, 14460−14471. (19) Oliveira, I.; Bloehse, D.; Ramke, H. G. Hydrothermal carbonization of agricultural residues. Bioresour. Technol. 2013, 142, 138−146. (20) Medeiros, P. M.; Simoneit, B. R. T. Analysis of sugars in environmental samples by gas chromatography−mass spectrometry. J. Chromatogr. A 2007, 1141, 271−278. (21) Fabbri, D.; Chiavari, G.; Prati, S.; Vassura, I.; Vangelista, M. Gas chromatography/mass spectrometric characterisation of pyrolysis/ silylation products of glucose and cellulose. Rapid Commun. Mass Spectrom. 2002, 16, 2349−2355. (22) Wiedner, K.; Naisse, C.; Rumpel, C.; Pozzi, A.; Wieczorek, P.; Glaser, B. Chemical modification of biomass residues during hydrothermal carbonization − What makes the difference, temperature or feedstock? Org. Geochem. 2013, 54, 91−100.

liquid biofuels needs to be studied. The yield of those compounds under HTC conditions could most likely be increased in the presence of efficient hydrogen donors (e.g., methanol, formic acid). This contribution demonstrates that a multitude of organic HTC conversion products in the molecular mass range between 120 and 300 Da, which have not yet been characterized, was produced in addition to the limited array of well-known low molecular mass compounds. It should be acknowledged that the structural assignment (Table 1) is tentative in some cases as outlined above, as the sole application of mass spectra does not commonly allow for a final structural assignment of complex organic molecules. Further activities should be directed to refine/confirm the structural considerations gathered herein.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00276. GC-MS data, structures, and identification of compounds(PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 3412351573. ORCID

Barbara Weiner: 0000-0003-2747-8648 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work (B.W.) was supported by funds of the Federal Ministry of Food and Agriculture (BMEL) based on a decision of the Parliament of the Federal Republic of Germany via the Federal Office for Agriculture and Food (BLE) under the innovation support program. We thank M. Wunderlich (UFZ, Analytical Department) for IC and K. Lehmann (UFZ, Department of Environmental Engineering) for elemental analyses.

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ABBREVIATIONS DOC, dissolved organic carbon; DOM, dissolved organic matter; HMF, hydroxymethylfurfural; HTC, hydrothermal carbonization; OC, organic carbon; TMS, trimethylsilyl ether/ester

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REFERENCES

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Hydrothermal Carbonization of Glucose, Fructose, and Xylose

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