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Decomposition of cellulose in hot-compressed water: detailed analysis of the products and effect of operating conditions Felipe Buendia-Kandia, Guillain Mauviel, Emmanuel Guedon, Emmanuel Rondags, Dominique Petitjean, and Anthony Dufour Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02994 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017
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Decomposition of cellulose in hotcompressed water: detailed analysis of the products and effect of operating conditions Felipe BUENDIA-KANDIA, Guillain MAUVIEL, Emmanuel GUEDON, Emmanuel RONDAGS,
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Dominique PETITJEAN, Anthony DUFOUR* *
[email protected] 8
LRGP, CNRS, Université de Lorraine, 54000 Nancy, France
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ABSTRACT 12
Understanding the reaction pathways of cellulose hydrolysis in hot-compressed water (HCW) is crucial for the optimization of fermentable sugars and chemicals production. Advanced analytical
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strategies are required in order to better assess the wide range of products from cellulose conversion in HCW. In this work, cellulose conversion in HCW was conducted in an autoclave with sampling upon
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the reaction time under isothermal conditions (180°C, 220°C, 260°C, 0 to 120 min). Total watersoluble carbohydrates were quantified (phenol/sulfuric acid method). These products were first
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characterized by size exclusion chromatography coupled to mass spectrometry and evaporative light scattering detector (SEC-ELSD-MS). SEC is useful for screening the molecular weight distribution of
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soluble products. Then the chemical structure of water solubles has been attributed by hydrophilic interaction liquid chromatography coupled to a linear trap quadrupole Orbitrap mass spectrometer
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(HILIC-LTQ-Orbitrap-MS). This method notably evidences the formation of a cellobiose conformer under some HCW conditions. A specific high performance anion exchange chromatography with
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pulsed amperometric detection (HPAEC-PAD) method has been developed. This method allows for a selective separation of 5-hydroxy-methyl-furgural (5-HMF), glucose, fructose, mannose and
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oligomers up to cellopentaose. Carboxylic acids were quantified by high performance liquid chromatography with ultraviolet detection (HPLC-UV). Solid residues obtained after HCW conversion
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were characterized by X-ray diffraction (XRD) and permanent gas by micro-gas chromatography. The global reaction mechanism of cellulose liquefaction in HCW is rationalized based on these
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complementary methods. Cellulose conversion first proceeds with heterogeneous hydrolysis (fiber surface) to produce soluble oligomers in competition with pyrolysis (inner fibers with limited mass
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transfer of water) producing levoglucosan (promoted at higher temperature). Soluble oligomers produce glucose and isomers by homogeneous hydrolysis (liquid phase). C6 sugars can then undergo
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further conversion to produce notably 5HMF and erythrose.
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1 Introduction The depletion of fossil fuels represents one of the major concerns of our society. Moreover, their use
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produces high quantities of carbon dioxide emissions that are the main factor in the global warming. As a consequence, bio-fuels are developed in order to replace fossil fuels. Bioethanol is mainly
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produced from carbohydrates rich materials (sugarcane, sugar beet, maize) that are normally used for food production. This represents a controversy problem to society.1 Lignocellulosic biomass
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(wood, straw…) is a promising source of renewable carbon that does not compete with food production. It could be processed into second generation biofuels.2,3 This resource is composed
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mainly of cellulose (40-60 wt. %), hemicelluloses (20-40 wt. %) and lignin (15-25 wt. %) tightly bonded in a complex matrix.4–6 Cellulose is the most abundant polymer on earth and its valorization
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to produce biofuels or chemical is widely studied. Cellulose can be depolymerized into glucose and soluble oligosaccharides by a wide variety of
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technologies. Acid hydrolysis was developed to obtain glucose from cellulose. Several methods and technologies have been studied, reporting that high sugar yields and low reaction times can be
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achieved with strong acid media and high temperatures.7–10 Cellulose can also be depolymerized into levoglucosan and other anhydro-oligosaccharides with high yields by fast pyrolysis.11–15 The main
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drawback is that these anhydro-saccharides need to be hydrolyzed into saccharides in order to be suitable for fermentation, that allows to transform them into biofuels and other chemical products of
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interest.16 Different hydrothermal treatment technologies for cellulose/biomass have been developed. The operating conditions have a strong effect on water properties.17 The ionization of
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water (H2O) into OH- and H3O+ increases with temperature allowing the water to become the acid catalyst for cellulose hydrolysis. That is why the solvation power is different, and water at high
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temperatures near the critical point can dissolve organic compounds, unlike the ambient temperature water that will only dissolve the polar compounds.18,19 Supercritical water treatment
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ensures a fast solubilisation of cellulose and a subsequent hydrolysis of the primary products into
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simple molecules.20–22 Nevertheless, working with supercritical water will increase considerably both 62
the operational and reactor cost.20,23,24 Supercritical water is highly corrosive and expensive materials will be needed for the reaction system. This is because of the high concentration of acidic
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hydroniums H3O+ generate by the autoionization of water at this temperature.18 Another alternative technology is liquefaction in hot-compressed water (subcritical conditions), that normally operates at
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conditions between 150-300ᵒC at around 10 MPa. Even if water at these conditions has a reactivity that is considerably lower than at supercritical conditions, this technology presents an interesting
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alternative to perform cellulose depolymerization into profitable carbohydrates for subsequent fermentation into biofuels and/or chemicals.25,26 Significant progress has been made in
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cellulose/lignocellulose pretreatment by hot-compressed water (HCW). Several reaction systems in continuous,20,21,27 semi-continuous28–30 and batch19,27 mode have been developed.
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Cellulose hydrolysis products are often characterized by HPLC methods using conventional ionexchange or aqueous size exclusion columns that allow the separation of monomers but not
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oligomers.20,31,21 High-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) have been studied and used as a powerful tool for the analysis of
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carbohydrates.32–35 Several methods have been developed according to the targeted compounds and now is currently the most common and efficient analytical method for the quantification of cellulose
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derived compounds.28,23,36–39 Yu et al.40 and Tolonen et. al22 have developed interesting methods coupling HPAEC-PAD with mass spectrometry (MS), allowing the identification of isomers and
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cellulose derived compounds with a high degree of polymerization, for which there is no standard available in the current market. MALDI-TOF is commonly used for the analysis of molecules with a
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high molecular weight.41,42 An accurate and comprehensive analysis of liquid products is fundamental for the understanding of the reaction mechanisms of cellulose decomposition in HCW. In the same
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way, the latter is fundamental for improving efficiency of this technology. Sasaki et al.20,21,29,43,44 have intensively studied the hydrolysis of cellulose in HCW. These studies concluded that cellulose
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depolymerization follows two major reaction pathways: 1) dehydration of reducing-end glucose via
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pyrolytic cleavage of the glycosidic bond in cellulose (or partly retro-aldol reaction as described later) 88
and 2) hydrolysis of the glycosidic bond via swelling and dissolution of cellulose. Several other studies on cellulose hydrolysis in HCW confirm that the main products of cellulose depolymerization in HCW
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are: glucose, fructose, 5-hydroxymethyfurfural (5-HMF), glycolaldehyde, dihydroxyacetone, erythrose, levoglucosan, cello-oligosaccharides (cellobiose, cellotriose, cellotetraose, cellopentaose,
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etc.).18,22–27,38,44–51 The main reaction pathways have been summarized by Matsumura et al.25. This work highlights that a heterogeneous hydrolysis occurred at the surface of the cellulose fibers while
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pyrolysis takes place inside the fibers. Then the soluble products undergo secondary reactions designated as “homogeneous hydrolysis” (Figure 1). Despite all the extensive studies on cellulose
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conversion in HCW, there is still an important lack of a comprehensive characterization of the products using various analytical techniques on the same liquid sample in order to account for the
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global mechanism of cellulose conversion. The last two decades have witness important advances in analytical chemistry that can be used as a
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powerful tool in the characterization of cellulose hydrolysis products. In this work, an advanced methodology is proposed in order to characterize the products from cellulose conversion in HCW.
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Solid, liquid (water-soluble compounds) and gaseous products were recovered to determine the mass balance. Liquids were characterized for the first time by a combination of:
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1) Size exclusion chromatography with evaporative light scattering detection and mass spectrometry (SEC-ELSD-MS) for assessing the distribution of molecular weight of species
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soluble in water; 2) Hydrophilic interaction liquid chromatography with linear trap quadrupole Orbitrap mass
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spectrometry (HILIC-LTQ-Orbitrap MS) for identifying the chemical structure of some important products (a cellobiose isomer has been identified);
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3) A specific high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) method. This HAPEC-PAD method allows the characterization of both
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monomeric and oligomeric saccharides and is able to discriminate glucose isomers (mannose and fructose) and 5-(hydroxy)methyfurfural that often escape detection or are poorly
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quantified with regular chromatographic methods. The predominant products and reactions as evidenced by all these complementary methods are
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discussed. This work provides a better understanding of the evolution of the products of cellulose liquefaction in HCW as a function of temperature and reaction time.
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2 Materials and Methods 2.1 Reactants
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Microcrystalline cellulose Avicel PH-101 (particle size < 5µm, Sigma Aldrich, San Luis, USA) was used for all the experiments. Glucose (99.5 %), 5-hydroxymethyl furfural (5-HMF, 99 %), glycolaldehyde
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(99 %), fructose (99 %), mannose (99 %), erythrose (75 %) standards were purchased from SigmaAldrich. Levoglucosan (98 %), cellobiose (98 %), cellobiosan (95 %), cellotriose (95 %), cellotetraose
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(95 %) and cellopentaose (95 %) standards were purchased from Carbosynth (Compton, United Kingdom).
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2.2 Liquefaction in hot-compressed water The overall experimental and analysis procedure is illustrated in Figure 2. A batch reactor system was
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used to study the cellulose depolymerization. A stirred reactor (stainless steel 316, Parr Instrument Company) of 300 mL was filled up with 200 mL of de-ionized water and 10 g of cellulose. Reactor was
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purged 2 times with nitrogen. Then 10 bar of nitrogen atmosphere was used to maintain the water in liquid phase and as a tracer for the quantification of permanent gases. Reactor was heated to the
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target temperature (180°C, 220°C or 260°C) at 5 K/min, with a final pressure of 14 bar, 24 bar and 50 bar respectively. When the target temperature was reached, reaction time started and 7 samples
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were taken every 20 min for the liquid phase analysis (from initial time to 120min). The sample volume was 3 mL. 2 purges were first sampled followed by the sample for further analysis (leading to
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9 mL of total sampled volume per sampling). The sampling of this small volume does not impact the pressure and temperature inside the autoclave (pressure and temperature constant within 1bar and
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3ᵒC respectively thorough the test). At the end of each experiment (2h), the reactor was rapidly quenched (with cold water) to ambient temperature (3-4min.). Specific experiments were conducted
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without sampling in order to quantify mass balances. The solid/liquid mixture was filtered using a glass microfiber filter (Whatman®, pore size: 0.7 µm)
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with a Büchner under vacuum suction. The solid residue was washed with de-ionized water and subsequently dried at 105ᵒC for 24h. Then it was cool down in a desiccator to be weighted and it was
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finally stored for further XRD analysis. The recovered liquid was immediately analyzed by liquid chromatography and total carbohydrates determination. Then it was stored in a freezer at -80ᵒC.
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Once the liquid is completely frozen, the water was removed by freeze drying at -40ᵒC,
