Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy

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Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy Study of Carbohydrate Decomposition by Hydrothermal Liquefaction Treatment: A Modeling Approach on Bio-oil Production from Organic Wastes Annamaria Croce,† Ezio Battistel,‡ Stefano Chiaberge,§ Silvia Spera,§ Samantha Reale,*,† and Francesco De Angelis† †

Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi di L’Aquila, Via Vetoio, 67100 L’Aquila, Italy Centro di Ricerca per la Chimica Verde, eni Versalis, via Fauser 4, 28100 Novara, Italy § Renewable Energy and Environmental R&D, Istituto eni Donegani, via Fauser 4, 28100 Novara, Italy ‡

S Supporting Information *

ABSTRACT: Glucose and cellulose as model compounds were treated under hydrothermal liquefaction (HTL) conditions to describe the main reaction pathways that are involved in the process. The HTL-derived phases (gas phase, bio-oil, aqueous phase, and solid residue) were fully characterized by a combination of analytical techniques [i.e., elemental analysis (EA), gas chromatography−mass spectrometry (GC−MS), electrospray ionization/atmospheric pressure photoionization Fourier transform ion cyclotron resonance−mass spectrometry (ESI/APPI FTICR−MS), and 13C cross-polarization−magic angle spinning nuclear magnetic resonance (CP−MAS NMR)], and a comprehensive HTL degradation mechanism was proposed. A wide range of different reactions (dehydration, decarboxylation, retro-aldol, aromatization, condensation, oxidation, and reduction) were found to be involved in the formation of the different compounds detected in the four phases. The main identified products in both glucose and cellulose HTL bio-oils were furfural derivatives, which further react leading to several phenolic and aliphatic compounds. Oligomers arising from the condensation of furfural derivatives were also found, and their polymerization finally results in a solid residue whose characterization confirmed the presence of polyfuranic networks together with graphite-like domains. Finally, glucose and cellulose showed a similar behavior considering the product yields and phase composition, suggesting that the polymerization degree does not significantly affect the HTL process.

1. INTRODUCTION In recent years, biomass transformation has gained increasing interest as a promising alternative to biofuel production for covering the impelling need to readily available, renewable, sustainable, and environmental friendly energy sources. For this purpose, biomass is first transformed into a liquid bio-oil, a water-insoluble oil-like material with a lower heteroatom content (especially oxygen and nitrogen) and a higher carbon content than the starting biomass. Accordingly, the obtained bio-oil can then be upgraded to a biofuel, which is a viable alternative to normal petroleum-based fossil fuels. In this context, the hydrothermal liquefaction (HTL) treatment, which exploits subcritical liquid water as the reaction medium,1 has been applied to wet biomass transformation, showing it to be a highly promising, efficient process for bio-oil production. Together with bio-oil, such treatment also produces a gas phase, an aqueous phase, and a solid residue.2,3 Typically, during the HTL process, water is subjected to high temperature (250−350 °C) and high pressure (5−15 MPa) conditions, creating an efficient reaction environment that is able to break down the biomass components and convert them into value-added products,4 lowering environmental pollution at the same time.1,5 Quite recently, we became interested in this field by studying the composition of a waste biomass-derived bio-oil6,7 as well as © XXXX American Chemical Society

the chemical transformation undergone by some primary components of the biomass (amino acids and fatty acids).8 Such kinds of studies are motivated by the necessity of obtaining information on the types of heteroatom-containing organic molecules in the bio-oil, because a high nitrogen and oxygen presence (especially related to aromatic structures) and a high organic acid content are critical issues for the next upgrading steps. Accordingly, the complete characterization of the bio-oil composition and the knowledge of the biomass degradation mechanisms involved during the HTL treatment are fundamental for selecting the most proper operating conditions and for planning the following steps necessary to improve the technological properties of the final product. HTL-derived bio-oil is characterized by an extremely complex composition, strictly dependent upon the quality and nature of the biomass used in the process. For this reason, a model approach for studying in detail the mechanisms involved in the HTL treatment of the biomass appears to be a very promising tool. Indeed, several reaction Received: May 29, 2015 Revised: July 31, 2015

A

DOI: 10.1021/acs.energyfuels.5b01204 Energy Fuels XXXX, XXX, XXX−XXX

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Information). At 300 °C, the internal pressure is of about 90 bar (which is the endogenous pressure mainly as a result of the vapor pressure of water). It is likely that, even during the heating time, several reactions are already taking place, especially when the temperature is near the HTL temperature range. We have investigated different heating rates, and actually, the composition of the HTL products does not change. Furthermore, although we have not analyzed the carbohydrate HTL products during that heating time, this does not represent a limitation in our study. In fact, because the heating time was the same for all of the experiments, we can assume a similar effect on carbohydrate degradation. After 2, 5, 10, or 20 min of reaction time (calculated once the temperature of 300 °C is reached), the reaction was stopped by inserting the autoclave in a water bath. After quenching, the value of the final pressure was registered and a sample of the gas phase was taken (using a gas sample bag) for the analysis. The autoclave was then disconnected and opened. The reaction mixture was moved into a pot, and the autoclave was rinsed with 10 mL of ethyl acetate 3 times. The solid residue (SR) was recovered by filtration (on 0.45 μm pore size hydrophobic polymer filters), washed with 40 mL of ethyl acetate, then dried overnight at 105 °C, and finally weighted. The filtrate was composed by two different phases: an aqueous phase (arising from the water loaded at the beginning of the reaction and the one eventually formed during the process) and a solvent phase (as a result of ethyl acetate added to wash the reactor and the solid residue). A centrifuge at 7500 rpm for 10 min optimized the separation between these two phases. On the basis of their own properties, all of the components formed during the HTL process were distributed between the aqueous and ethyl acetate phases. In this work, every compound soluble in ethyl acetate was considered as bio-oil, whereas all of the components that remained in the aqueous phase (after solvent addition) are hereafter called water-soluble organic (WSO) compounds. Bio-oil was obtained by recovering the ethyl acetate phase (by syringe aspiration) and further solvent evaporation (by rotavapor). The final solvent-free bio-oil amount was determined by weighing. Unfortunately, it was not possible to quantify the amount of the WSO compounds because of their high volatility and the strong interference of ethyl acetate, which is partially miscible with water. 2.2. Product Mass Yields. The weight percentage (wt %) yield of each phase obtained by the HTL process of glucose and cellulose was calculated according to eq 1

pathways of biomass degradation were studied and identified in this way.1,8−10 With regard to carbohydrates, they are one of the most abundant biomass components and can be appropriate simple molecules for model studies. Accordingly, their behavior under thermal treatments was extensively studied in the past. The reaction path scheme for glucose and other monosaccharides (such as D-mannose, D-fructose, and D-galactose) to their primary decay products is reported by several researchers.11−15 With regard to polysaccharides, they are first broken up into fragments by hydrolysis and then degraded into smaller compounds by various reaction mechanisms, including dehydration, dehydrogenation, deoxygenation, and decarboxylation, depending upon the catalytic conditions employed.14 In their turn, such small and highly reactive fragments can recombine and condense into novel molecules, eventually building up oligomeric and composite compounds with higher molecular weights.16 Although progressively growing, the exploration of the chemical complexity of the HTL-derived products still requires a more refined set of data, especially on the composition of the other phases (aqueous, gaseous, and solid).10 For instance, the characterization of the aqueous phase has been rarely addressed in the literature using model compounds, with the exception of cellulose.3 Furthermore, another important analytical challenge is related to the detection of non-eluting and less volatile compounds, which cannot be determined by conventional gas chromatography−mass spectrometry (GC−MS) analysis.10 A rather challenging and powerful analytical approach is thus required. Fourier transform ion cyclotron resonance mass spectrometry coupled with an atmospheric pressure photoionization ion source (APPI FTICR−MS) can be used in the characterization of this fraction, because it has been recently applied to characterize bio-oil6,17 but not yet used to identify HTL products of model compounds. In this work, glucose and cellulose were chosen as model compounds and treated under HTL conditions. To the best of our knowledge, for the first time, all four of the HTL-derived phases (gas phase, bio-oil, aqueous phase, and solid residue) were fully characterized and the main degradation pathways were proposed accordingly. By integration of conventional techniques [elemental analysis (EA), GC−MS, and 13C crosspolarization−magic angle spinning nuclear magnetic resonance (CP-MAS NMR)] with high-resolution APPI FTICR−MS, it was possible to characterize the structure of a large number of glucose HTL-derived compounds and, most importantly, to figure out the formation mechanism of higher molecular weight non-volatile compounds, with particular focus on the aromatic intermediates and their progressive composite polycondensates.

Y (wt%) = (m gas phase/bio‐oil/SR) × 100/(m starting compound)

(1)

where m is the mass (in grams) of the gas phase, the bio-oil, or the solid residue and the starting compound (glucose or cellulose). Gravimetric data were used for all compounds and phases (glucose, cellulose, bio-oil, and SR), except for the gas phase, whose mass was calculated according to the ideal gas law (eq 2)

m gas phase = PV MWCO2/RT

(2)

where P is the pressure, V is the volume, T is the temperature of the gas, R is the ideal gas constant, and MWCO2 is the molecular weight of carbon dioxide. For this calculation, the gas phase was considered composed by CO2 only (see gas phase composition in Table S1). The amount of water-soluble organics was calculated by the mass difference. 2.3. EA. The C, H, and O contents of bio-oil and SR were determined by the elemental analyzer ThermoQuest Instrument EA1100. A calibration procedure was performed before each measurement. Each sample was analyzed in duplicate, and the final value was given as an average value. The resulting data were expressed as weight percent. The O content was determined by mass difference. 2.4. Gas-Phase Analysis. The gas sample was analyzed using GC (Varian Micro GC CP-4900) equipped with two analytical columns:

2. EXPERIMENTAL SECTION 2.1. Product Recovery and Workup. The HTL experimental scheme is shown in Figure S1 of the Supporting Information. Different reactor volumes and shapes were tested to optimize the heating rate and the reaction product recovery. Finally, a 45 mL internal volume, stainless-steel autoclave was chosen to perform all of the experiments reported here. The reactor was loaded with 1 g of the model compound and 9 g of pure water (no catalysts were used). Once closed, the autoclave was flushed with nitrogen (to remove the air present in the reactor) and then heated (by immersion in a fluidized sand bed heated by an electrical oven and pre- heated air). The final reaction temperature was 300 °C, reached in about 8 min with an average heating rate of 35 °C/min (see Figure S2 of the Supporting B


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Table 1. Mass Yield Distributions (in Terms of wt %) of Products Obtained after HTL Treatment of Glucose and Cellulose at 300 °C and Different Reaction Timesa glucose

a

cellulose

reaction time (min)

gas (wt %)

oil (wt %)

SR (wt %)

WSO (wt %)

gas (wt %)

oil (wt %)

SR (wt %)

WSO (wt %)

2 5 10 20

7.3 7.9 8.9 9.6

12.0 11.5 8.6 6.0

26.9 28.5 30.2 31.6

53.8 52.1 52.2 52.8

8.1 9.6 11.6 13.0

12.0 9.3 8.4 5.5

31.0 33.9 36.5 37.7

49.0 47.2 43.8 43.7

Gas, gas phase; oil, bio-oil; SR, solid residue; and WSO, water-soluble organics. and a final isotherm at 320 °C for 20 min. Conditions: injector temperature, 250 °C; FID temperature, 350 °C; carrier gas, He at 1.92 mL/min; and split ratio, 25. The sample preparation was the same as for bio-oil GC−MS analysis. 2.7. ESI/APPI FTICR−MS Analysis. The aqueous-phase SPE extracts in methanol were analyzed by electrospray ionization in negative-ion mode, ESI(−), by flow injection with a rate of 5 μL/min. Conditions: source voltage, 3.3 kV; capillary voltage, 27 V; tube lens voltage, 55 V; capillary temperature, 270 °C; sheath gas, 60 μL/min; and auxiliary gas, 10 μL/min. Bio-oil samples were analyzed by atmospheric pressure photoionization in positive-ion mode, APPI(+). Bio-oil samples (0.02 mg/ mL) in 50:50 (v/v) toluene/methanol (where toluene acts as both a solvent and dopant in the APPI process) was directly injected with a flow rate of 50 μL/min in the mass spectrometer. The ion source was equipped with a vacuum ultraviolet gas (Kripton) discharge lamp. Nitrogen was used as nebulizing gas. The mass spectrometer analyzer was a 7 T FTICR (LTQ-FT Ultra Thermo Scientific). ESI(−) and APPI(+) mass spectra were recorded at a mass range of m/z 100−1000 and with an average resolving power of 400 000 at m/z 400 in profile mode. Conditions: microscan, 1; maximum injection time, 1000; and automatic gain control on the ICR cell, 106. A minimum of 100 scans were collected and averaged for each analysis to improve the signal-to-noise ratio. The molecular formulas were assigned to those peaks presenting relative intensities higher than 0.2%. Data were then processed by the software Xcalibur (Thermo Scientific), after setting the following restrictions to the element ranges: 1−60 12C, 0−2 13C, 10−100 1H, and 0−30 16O, while the error range was set at ±2.5 ppm. The list of masses and the corresponding molecular formulas were then grouped with custom-designed software (ISOMASS),18 and the mass peaks as a result of isotopic distributions were deleted. The signals were categorized according to the number of heteroatoms and the aromatic degree, expressed as double bond equivalent (DBE) values.19 The DBE value was calculated according the following equation for CcHhNnOoSs:

Molsieve 5A (10 m) and PPQ (10 m), using He as a carrier gas. A total of 10 different gases were quantified: H2, N2, O2, CO2, CO, CH4, C2H4, C2H6, C3H6, and C3H8. A calibration procedure was performed before each measurement. Each sample was analyzed 3 times, and the final value was given as an average value. The resulting data were expressed as percentage of the total gas composition. CO2 and CO were the main gases identified in all glucose- and cellulose-derived gasphase samples. Cellulose produced also a little quantity of H2. Gas analysis data for glucose- and cellulose-derived gas phases are reported in Table S1 of the Supporting Information. 2.5. Solid-Phase Extraction of WSO. The WSO components were recovered and concentrated from the aqueous-phase samples (after ethyl acetate extraction) by solid-phase extraction (SPE), using SPE Bond Elut Plexa cartridges (Agilent). Every SPE device was conditioning with 4 mL of methanol (or acetonitrile) and then with 4 mL of deionized water. Then, 1 mL of the HTL aqueous phase was loaded, washed with 4 mL of deionized water (to remove salts), and finally eluted with 4 mL of methanol (or acetonitrile). The SPE extract in methanol was analyzed by Fourier transform ion cyclotron resonance mass spectrometry coupled with an electrospray ionization ion source (ESI FTICR−MS), whereas the SPE extract in acetonitrile was silylated and analyzed by GC−MS analysis (see below). 2.6. GC−MS/GC−FID Analysis. WSO and bio-oil GC−MS analysis was performed on a Finnigan Trace DSQ (Thermo) interfaced to a Finnigan Trace GC Ultra equipped with a capillary column SUPELCO MDN-5S (length, 30 m; inner diameter, 0.32 μm; and film thickness, 0.25 μm). The temperature program was 40 °C for 4 min, then to 320 °C with a 10 °C/min rate, and then a final isotherm of 20 min. Conditions: injector temperature, 280 °C; transfer line temperature, 300 °C; ion source temperature, 250 °C; carrier gas, He at 1.0 mL/min; delay time, 5 min; and splitless mode. The mass spectra were obtained by electron ionization (EI) at 70 eV. Data were acquired and processed using Xcalibur software. Peak identification was based on EI mass spectra interpretation and by comparison to National Institute of Standards and Technology (NIST) libraries. Sample preparation was performed by dissolving bio-oil in ethyl acetate to a final concentration of 8 mg/mL; subsequently, an internal standard (3-nitrobenzyl alcohol) was added to each sample at the final concentration of 0.03 wt %. Conversely, for GC−MS analysis of WSO, a silylation procedure was necessary: a small amount of SPE acetonitrile-extracted aqueous-phase sample (100 μL) was added to 100 μL of N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) and 50 μL of pyridine and kept at 60 °C for 1 h. The final volume was adjusted to 1 mL, and the samples were injected. No internal standard was added to the samples. The main detected compounds in the bio-oils (2-furaldehyde, pyruvaldehyde, 2,5-hexanedione, phenol, levulinic acid, 1,2-dihydroxybenzene, 5-hydroxymethylfurfural, 1,4-dihydroxybenzene, and 1,2,4benzenetriol) were also confirmed by authentic commercial compounds (Sigma-Aldrich). A quantitative analysis of the bio-oils was performed by gas chromatography coupled with a flame ionization detector (GC−FID). GC8000 TOP (EL980) GC−FID (Carlo Erba Instruments) was used. A capillary column SUPELCO PTE-5 (length, 30 m; inner diameter, 0.32 μm; and film thickness, 1.00 μm) was used. The temperature program was 40 °C for 4 min, then to 320 °C with a 10 °C/min rate,

DBE = c − h/2 + n/2 + 1 2.8. NMR Spectroscopy Analysis. 13C CP−MAS NMR spectra for the solid residues were acquired with a Bruker Avance 400 spectrometer for solid state (wide bore) using 4 mm zirconia rotors, spinning at a MAS frequency νMAS of 10 kHz. Cross-polarization spectra were acquired with a cross-polarization contact time of 5 ms, a recycle delay of 5 s, and a number of scans of 20 000.

3. RESULTS AND DISCUSSION 3.1. Product Mass Yields. The mass yields of the various phases (see Table 1) are similar for glucose and cellulose treatments. The bio-oil yield is higher at shorter reaction times, whereas the amount of SR increases with the reaction time. In the case of cellulose, the quantity of SR is higher than glucose. This result may be explained by considering the hypothesis made by Baccile et al.20,21 According to their studies, the mechanism for SR formation is different for cellulose than for glucose. In fact, two competing degradation pathways occur in cellulose by HTL treatment: (i) a direct transformation into C


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and dihydroxyacetone, precursors of pyruvaldehyde and lowmolecular-weight acids, such as lactic acid.1,13,25,26 3.3. Bio-oil Characterization. Bio-oils were analyzed with two different and complementary approaches. GC−MS was used for determination of the most volatile compounds, with GC−FID being used for their quantification. APPI FTICR−MS was employed, conversely, for the less volatile and aromatic components by means of a petroleomic approach.17,19 APPI was used as an ion source because it was found to be the best ionization technique in bio-oil characterization, for both the high number of identified species and also because the data are in good agreement with bulk elemental analysis.7 3.3.1. Bio-oil GC−MS Analysis. Both glucose and cellulose produce in the bio-oils the same compounds at equal reaction times, although with different concentrations. The list of the identified products after 2 min at 300 °C is showed in Table 3. Most of the detected products were confirmed by comparison of authentic commercial compounds (as indicated in Table 3). The remaining compounds were identified by comparison of the experimental EI mass spectra to those of the NIST MS library. Compounds with higher molecular weight were also present in the bio-oil samples; however, they presented very low-intensity peaks with uninterpretable mass spectra. Three main categories of compounds were identified: furfural-based compounds, phenolic compounds, and aliphatic oxygenated compounds. These compounds are likely originated by a series of competitive/consecutive reactions. Dehydration, decarboxylation, and aromatization are the most important reactions, whereas condensation, oxidation, and reduction can be considered side reactions. It is remarkable the presence of species with seven carbon atoms that are presumably originated by alkylation reactions involving a methyl (one C unit) transfer. The most abundant compound was 5-hydroxymethylfurfural (5-HMF),2,12−14,27,28 followed by levulinic acid,26,29,30 1,2,4benzenetriol,11,26,29,30 and furfural.14,22,31 Glucose under hydrothermal conditions undergoes rapidly the Lobry de Bruyn−Alberda van Ekenstein isomerization, which finally leads to the formation of fructose.1,15,24 Then, fructose forms 5-HMF via three consecutive dehydration reactions.22,31 In its turn, 5-HMF forms levulinc acid and 1,2,4-benzenetriol but also other phenolic compounds.11 Further experiments showed that the same compounds were also found in bio-oil from 5-HMF, suggesting that depolymerization (for cellulose) and dehydration (for both glucose and cellulose) are very fast reactions, leading to 5-HMF, which further reacts to form both the bio-oil and the SR components. The proposed reaction mechanisms based on HTL product formation from 5-HMF are shown in Figure 2. In agreement with the quantitative results obtained by GC− FID, the total amount of detected compounds in the bio-oils is about 40 and 30% for glucose and cellulose, respectively (see Table S6 of the Supporting Information). This result confirms that GC−MS analysis is limited to the most volatile compounds, which actually represent only a fraction of the total bio-oil mixture. In fact, even if a silylation procedure was used in bio-oil GC−MS analysis, any improvement was obtained for the quality/quantity of the detected compounds. 3.3.2. Bio-oil APPI FTICR−MS Analysis. APPI FTICR−MS investigation was performed on the 2 min of reaction of glucose and cellulose HTL bio-oils, which gave rise to the highest yields in bio-oil. The mass spectra were elaborated, giving more than 400 different molecular formulas (data not shown) in the m/z

an aromatic carbon network and (ii) a depolymerization/ dehydration step, leading to furfural derivatives that, eventually, in turn, condense to form the same aromatic network. In the case of glucose, only the latter pathway occurs. According the product yields (see Table 1), the different polymerization degree has a negligible effect, because glucoseand cellulose-derived product streams are very similar. This peculiar behavior can be explained if we consider that, despite a further depolymerization step required, cellulose degradation is more activated than glucose degradation, especially at a higher temperature region.1 EA of glucose and cellulose bio-oils and solid residues (see Tables S2−S5 of the Supporting Information) reveals an increased carbon content which unfortunately leads to a small energy recovery (especially for bio-oils). Furthermore, in comparison of O/C and H/C ratios for glucose and cellulose and their HTL-derived bio-oils and solid residues, a strong decrease of both the ratios can be detected, and this suggests that dehydration is the main reaction involved in oxygen removal. 3.2. WSO Characterization. The nature of WSO components in the aqueous phases (after ethyl acetate extraction) obtained after 2 min of reaction of glucose and cellulose (corresponding to the highest amount of soluble organic compounds) was determined by GC−MS and ESI FTICR−MS. Ethylene glycol, glycolic acid, lactic acid, and 3-methyl-2pentenoic acid were determined as major components by GC− MS analysis (data not shown). Noticeably, a larger list of compounds was identified by ESI FTICR−MS (see Table 2), Table 2. Compounds Identified through ESI FTICR−MS Analysis of WSO Obtained after HTL Treatment of Glucose and Cellulose at 300 °C for 2 min compound name 1,6-anhydro-β-D-glucose (levoglucosan) ribofuranuronic acid glucuronic acid 2,5-hexandione 2-pentenedioic acid (glutaconic acid) quinic acid reduced glutaconic acid 4-oxopentanoic acid (levulinic acid) 1,2,4-benzenetriol

molecular weight (g/mol)

DBE

molecular formula

162

2

C6H10O5

164 194 114 130

2 2 2 3

C5H8O6 C6H10O7 C6H10O2 C5H6O4

192 114 116

2 3 2

C7H12O6 C5H6O3 C5H8O3

126

4

C6H6O3

mainly related to oxygenated organic compounds, most of them having an acidic nature. Levoglucosan was found to be one of the most abundant species (considering the relative abundance of the peaks in the ESI FTICR mass spectra) in both the glucose and cellulose WSO. The main compounds are formed according to the reaction mechanism proposed in Figure 1.2,12,13,22−24 A number of different reactions are involved in product formation. We can consider that dehydration, retro-aldol reactions, and oxidation/reduction reactions are the most important reactions involved in the formation of WSO components (on the basis of the relative abundance of the peaks in the ESI FTICR mass spectra). Retro-aldol reactions are especially responsible for the formation of glyceraldehyde D


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Figure 1. Proposed degradation mechanisms involved in the formation of WSO compounds obtained after HTL treatment of glucose and cellulose at 300 °C for different reaction times. OX, oxidation; RA, retro-aldol reaction.

Table 3. Compounds Identified by GC−MS Analysis of Bio-oils Obtained after HTL Treatment of Glucose and Cellulose for 2 min at 300 °Ca compound name

molecular weight (g/mol)

DBE

molecular formula

glucose oil (wt %)

cellulose oil (wt %)

2-furaldehyde (furfural)b pyruvaldehydeb 5-methyl-2(3H)-furanone 2,5-hexanedioneb 5-methyl-2-furaldehyde phenolb 2,5-heptanedione 3-methyl-1,2-cyclopentenolone 4-oxopentanoic acid (levulinic acid)b 2,5-furandicarboxaldehyde 2-acetyl-5-methylfuran 1,2-dihydroxybenzene (pyrocatechol)b 5-hydroxymethylfurfuralb 1,4-dihydroxybenzene (hydroquinone)b 1,2,4-benzenetriol (hydrohydroxyquinone)b

96 72 98 114 110 94 128 112 116 124 124 110 126 110 126

4 2 3 2 4 4 2 3 2 5 4 4 4 4 4

C5H4O2 C3H4O2 C5H6O2 C6H10O2 C6H6O2 C6H6O C7H12O2 C6H8O2 C5H8O3 C6H4O3 C7H8O2 C6H6O2 C6H6O3 C6H6O2 C6H6O3

1.97 1.00 0.21 0.20 0.50 0.09 0.17 0.30 3.80 0.18 0.27 0.10 28.49 0.22 0.00

3.34 0.63 0.06 0.17 0.63 0.07 0.09 0.41 3.38 0.14 0.21 0.07 15.14 0.30 1.84

a

The weight percent of each compound was determined by GC−FID analysis. bThe compounds were confirmed by the GC−MS analysis of the corresponding authentic compounds.

the structure (for instance, the CH class includes only hydrocarbons, whereas On classes consist of molecules that have carbon, hydrogen, and a different number of oxygen atoms in their structure). Figure 3 shows the distribution and abundance of the main identified classes for carbohydratederived bio-oils. Although the class distribution is similar for glucose and cellulose, cellulose bio-oil contains a higher amount of hydrocarbons (CH class) and a lower amount of compounds belonging to the most oxygenated classes (O4, O5, and O6) with respect to the glucose bio-oil. These data are consistent with the elemental analysis data (see Tables S2 and S4 of the Supporting Information), where the oxygen content was found to be higher for the glucose bio-oil than for the cellulose bio-oil. The most representative classes (CH, O3, O4, and O5) were then analyzed in detail for the determination of the main

100−500 range for both samples. This is an outstanding result by considering that the starting material is a relatively simple molecule. To validate this result, a 2 min reaction HTL experiment was performed for an aqueous solution of 13C6-D-glucose (SigmaAldrich, 99 atom % 13C). The analysis by GC−MS and APPI FTICR−MS of the thus obtained bio-oil showed the same composition of the natural glucose HTL bio-oil, and all compounds in the mass spectrum showed the corresponding mass shifts. Because of the huge number of peaks contained in a FTICR mass spectrum (each of them corresponds to a specific molecular formula, determined by its accurate mass), we approached this study categorizing all of the identified compounds in different classes, according to their heteroatom content, specifying kind and number of heteroatoms present in E


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Figure 2. Proposed degradation mechanisms involved in the formation of bio-oil compounds obtained after HTL treatment of glucose and cellulose at 300 °C for different reaction times. 5-HMF is the starting molecule whose decomposition lead to the formation of almost all bio-oil compounds. 5-HMF, 5-hydroxymethylfurfural; OX, oxidation; and RID, reduction.

(estimator of the molecular weight), where the relative abundance of each mass peak is related to the size of the spot in the diagram (see Figures 4 and 5). The CH class distribution is similar for both glucose and cellulose bio-oils (see Figure 4). The main compounds show a DBE value higher than 6. These compounds are likely related to alkyl-aromatic compounds, in particular to alkyl-anthracenes (DBE of 10), alkyl-fluorenes (DBE of 9), alkyl-dihydronaphtalenes (DBE of 8), and alkyl-naphtalenes (DBE of 7). Considering the most abundant compounds, a very narrow homologues series (compounds belonging to the same class with the same DBE value and different carbon atom number) can be found. These homologues are related to short alkyl substituents (2−5 CH2 units) linked to aromatic structures. This result suggests a low content of alkylating agents in the reaction mixture. Concerning the oxygen-containing compounds, as shown in Figure 5, the product distributions are similar between glucose and cellulose bio-oils. The main compounds are included in the 4−12 DBE range. The average carbon atom number is in the

Figure 3. Relative abundances (%) of the main classes identified through APPI FTICR−MS analysis of bio-oils obtained after HTL treatment of glucose and cellulose at 300 °C for 2 min.

molecular structures. Each class was plotted as DBE (estimator of the number of insaturations) versus carbon atom number

Figure 4. APPI FTICR−MS data elaboration: DBE versus carbon atom number distributions for compounds belonging to the CH class in bio-oils obtained after HTL treatment of glucose and cellulose at 300 °C for 2 min. F


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Figure 5. APPI FTICR−MS data elaboration: DBE versus carbon atom number distributions for compounds belonging to O3, O4, and O5 classes in bio-oils obtained after HTL treatment of glucose and cellulose at 300 °C for 2 min.

Figure 6. Correlation between DBE and average carbon atom number for compounds belonging to O3, O4, and O5 classes in the bio-oil obtained after HTL treatment of glucose at 300 °C for 2 min.

main identified compounds (see Figure S3 of the Supporting Information). On the basis of DBE versus carbon atom number distributions for all of the oxygenated classes, higher molecular weight compounds are likely formed by condensation of two or three furfural derivative molecules. Furthermore, in comparison

range of 8−25, except for the O3 class, where the peak related to 5-HMF at carbon atom number 6 is very abundant. According to the GC−MS and APPI FTICR−MS data (which give the molecular formulas and the relevant DBE values), a series of molecular structures were proposed for the G


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Energy & Fuels of the distribution of class difference of one oxygen atom (O3 versus O4, O4 versus O5, and O5 versus O6), it is possible to infer that the extra oxygen atom is due to a hydroxyl or a carbonyl group based on the same polyfuranic structure. These results suggest that oxygen might be contained partially in furanic structures and is partially present as hydroxyl and carbonyl groups. Considering the more oxygenated classes O4 and O5, a DBE limit of 12 is found in all of the plots, suggesting that the theoretical limit of furan-linked structures is 4 (because each furanic cycle has a DBE of 3) for both glucose and cellulose bio-oils. Higher molecular weight compounds with more than 4 furan-linked cycles might also be present in the bio-oil (class O6) but cannot be detected efficiently with this analytical approach. To find further proof for polymerization of furfural derivatives, APPI FTICR−MS data for glucose and cellulose bio-oils were elaborated as follows: the average number of carbon atoms was calculated for all compounds with the same DBE value and belonging to the O3, O4, and O5 classes. The obtained data were then plotted in a DBE versus average carbon atom number diagram (see Figure 6). Figure 6 clearly shows that compounds in the On class with DBE value = x and compounds in the O(n + 1) class with DBE value = x + 3 differ, in all cases, by 4 or 5 carbon atoms, corresponding to the units A and B, respectively, reported in Figure 6. These findings are in agreement with gel permeation chromatography−refractive index detector (GPC−RID) analysis data for glucose bio-oil (see Figure S4 of the Supporting Information): GPC−RID main components showed a difference in their molecular weights of about 80 g/mol, which corresponds to a furfural derivative unit, such as structure B in Figure 6. Figure 7 displays the H/C versus O/C van Krevelen diagram for all assigned molecular formulas obtained by GC−MS and

Noticeably, in our case, the about 400 molecular formulas assigned by APPI FTICR−MS for glucose and cellulose bio-oils are reduced to a total number of about 60. This is due to the same H/C and O/C ratios shown by several compounds. Considering the van Krevelen plot, it can be pointed out that the starting material (glucose) is located in the region of high H/C and O/C ratios, while the HTL bio-oil compounds are shifted to lower H/C and O/C ratios. In particular, the GC− MS compounds are mainly located in the intermediate zone of the diagram, whereas the APPI FTICR−MS-assigned formulas are mainly located at lower O/C ratios. Therefore, these results confirmed that, by the HTL process, the oxygen content of the bio-oil is drastically reduced, with its caloric content thus being increased (compare data in Tables S2 and S4 of the Supporting Information). 3.4. SR Characterization. HTL treatment of glucose and cellulose produces a large amount of SR, a hygroscopic carbonaceous black material. To obtain more information about its composition, 13C CP−MAS NMR was performed for both glucose and cellulose solid residues at different reaction times. The SRs obtained after HTL of glucose and cellulose at equal reaction times show similar 13C CP−MAS NMR spectra (see data elaboration shown in Table 4). Each 13C CP−MAS NMR spectrum shows six different broad signals in three specific chemical shift regions (see Figure 8). In the region related to carbonyl groups (170−220 ppm), two main signals can be found: the first signal is due to aldehydic or ketonic carbons (signal 190−220 ppm), and the second signal is related to carboxylic carbons (signal at about 180 ppm). In the aromatic region (110−160 ppm), the main signals are due to oxygen-bonded aromatic carbons (signals at 150 and 110 ppm) and to aromatic carbons (signal at 130 ppm). In more detail, the components at about 150 and 110 ppm are assigned to α and β carbons of the furan derivative unit, respectively (in agreement with the high-molecular-weight compounds found in the bio-oil samples by APPI FTICR− MS). The aromatic carbon signals in the range of 130−120 ppm are due to graphite-like domains. Finally, in the aliphatic region (below 70 ppm), the main signals are related to aliphatic carbons (signal at about 30 ppm) and to terminal methyl groups (signal at about 14 ppm). Cross-polarization spectra are not quantitative. However, a comparison among the integral values of the 13C CP−MAS NMR signals can be made because the spectra were acquired with the same experimental parameters and quite long contact times, and considering that all of the samples arise from the same experimental procedure (differing only by the reaction time), we can expect that they have a similar mobility behavior.20 By the evaluation of the areas of the 13C CP− MAS NMR signals, the significant aromatic nature of the SRs was confirmed. In particular, the percentage of aromatic carbons over the total carbons (Fα) ranges between 57 and 59% in all of the SR samples (see Table 4), with slightly higher values for longer reaction times. Although the composition of the SRs at the different reaction times is very similar, by increasing the reaction times, the signals related to the polyfuranic components decrease, whereas the signals related to aromatic carbons increase (see Table 4). According to these results, the intermolecular condensation of polyfuranic chains followed by aromatization (which leads to a graphite-like material) is likely to be the main reaction involved in the SR evolution. These results are consistent with

Figure 7. H/C versus O/C van Krevelen diagram for bio-oil components obtained after HTL treatment of glucose at 300 °C for 2 min: (▲) compounds identified by GC−MS, (•) compounds identified by APPI FTICR−MS, and (●) glucose.

APPI FTICR−MS. This is a diagram commonly used to elucidate structural information from the very high number of molecular formulas present in a sample. The correlation between this kind of plot and the higher heating value (HHV) of a fuel or a biomass has been demonstrated.32 Additionally, the location of a molecular formula on the van Krevelen diagram can also be correlated to the biochemical classes of a biomass.33 H


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Table 4. Percent Peak Areas of the Different Signals Observed in the 13C CP−MAS NMR Spectra of the Solid Residues of Cellulose and Glucose for Different Reaction Times at 300 °Ca aromatic carbons reaction time (min)

carbonyl carbons (%)

carboxyl carbons (%)

furan-like (%)

graphite-like (%)

aliphatic carbons (%)

Fα (%)

28.8 28.1 30.8 32.6

32.0 32.6 31.6 32.1

56.9 57.8 58.5 59.3

24.1 28.7 31.3 37.3

34.9 34.4 33.7 34.4

57.5 58.2 58.1 58.7

Glucose SRs

a

2 5 10 20

8.4 7.6 7.4 6.7

2.7 2.0 2.5 1.9

2 5 10 20

6.4 6.3 6.7 6.0

1.2 1.1 1.5 0.9

28.1 29.7 27.7 26.7 Cellulose SRs 33.4 29.5 26.8 21.4

The data are reported as a percentage with respect to the total area.

Figure 8. 13C CP−MAS NMR spectrum of the solid residue obtained after HTL treatment of glucose at 300 °C for 2 min.

Furthermore, the APPI FTICR−MS analysis allowed us to describe for the first time the less volatile component of bio-oils derived by biomass model compounds, showing the connection between the more volatile compounds identified through GC− MS of bio-oils and WSO compounds and the higher molecular weight components identified through 13C CP−MAS NMR of SR samples. Although this model approach based on a single model compound cannot mimic the behavior of a complex biomass, it allowed for the identification of the degradation products arising from HTL treatment of carbohydrates. Furthermore, the identification of the thus obtained products becomes crucial because they are involved in further complex reactions, involving other biomass components and, in particular, the nitrogen-containing compounds arising by the degradation of other biological material.

the mechanism proposed by studying SR formation from glucose and cellulose as a function of the temperature.20,21,34

4. CONCLUSION A detailed study was performed on glucose and cellulose to figure out the behavior of the glucosidic part of the real biomass under HTL conditions. A comprehensive analytical approach allowed for a detailed characterization of all of the HTL phases, revealing that the different polymerization degrees barely affect the kinetics of the main reactions, without changing the general degradation mechanism. Dehydration and decarboxylation were both important reactions. Nevertheless, other important reactions were found to be involved in the degradative processes: retro-aldol, aromatization, condensation, oxidation, and reduction products were also found. Both glucose and cellulose showed a marked tendency to form aromatic compounds. Indeed, depolymerization (for cellulose) and dehydration (for both glucose and cellulose) are very fast reactions that lead to 5-HMF. According to the results shown in this paper, 5-HMF favors polymerization, leading to the formation of the higher molecular weight molecules identified through APPI FTICR−MS and, finally, to the insoluble polyfuranic domains detected in the SR.

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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/acs.energyfuels.5b01204. Scheme of the experimental setup used in the HTL treatment (Figure S1), heating curves relevant to HTL treatment of glucose and cellulose (Figure S2), APPI I


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FTICR−MS data elaboration for bio-oils from glucose and cellulose: several hypothesized structures for the main higher molecular weight compounds (Figure S3), molecular weight distributions through GPC−RID analysis of bio-oils and solid residues obtained after HTL treatment of glucose (Figure S4), composition of the gas phases obtained after HTL treatment of glucose and cellulose (Table S1), elemental composition and properties of bio-oils and solid residues obtained after HTL treatment of glucose and cellulose (Tables S2−S5), and quantitative data for the compounds identified through GC−FID analysis of bio-oils obtained after HTL treatment of glucose and cellulose (Table S6) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOMENCLATURE 5-HMF = 5-hydroxymethylfurfural APPI = atmospheric pressure photoionization CP−MAS NMR = cross-polarization−magic angle spinning nuclear magnetic resonance DBE = double bond equivalent ESI = electrospray ionization FTICR−MS = Fourier transform ion cyclotron resonance− mass spectrometry GC−FID = gas chromatography−flame ionization detector GC−MS = gas chromatography−mass spectrometry GPC−RID = gel permeation chromatography−refractive index detector HHV = higher heating value HTL = hydrothermal liquefaction SPE = solid-phase extraction

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Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy

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