Article pubs.acs.org/IECR
Manufacture of Levulinic Acid from Pine Wood Hemicelluloses: A Kinetic Assessment Sandra Rivas,†,‡ María Jesús González-Muñoz,†,‡ Carlos Vila,†,‡ Valentín Santos,*,†,‡ and Juan Carlos Parajó†,‡ †
Department of Chemical Engineering, Faculty of Science, University of Vigo (Campus Ourense), As Lagoas, 32004 Ourense, Spain CITI (Centro de Investigación, Transferencia e Innovación), University of Vigo, Tecnopole, San Cibrao das Viñas, 32900 Ourense, Spain
‡
S Supporting Information *
ABSTRACT: Pinus pinaster wood samples were subjected to two consecutive treatments with hot, compressed water, in order to remove water-solubles in the first step and to cause hemicellulose solubilization in the second. The liquid phase from the second stage, containing hemicellulose-derived saccharides (mainly of oligomeric nature), was mixed with sulfuric acid and heated to convert the saccharides into levulinic acid. Experiments were carried out at different acid concentrations, temperatures, and reaction times. The concentration profiles were interpreted using a model involving the following major steps: conversion of oligomers into monosaccharides, conversion of hexoses into hydroxymethyl furfural, decomposition of this latter into levulinic and formic acids, dehydration of pentoses into furfural, and conversion of this latter into formic acid. Parasitic reactions limited the theoretical yields in the target products. The kinetic coefficients governing the various reactions were determined by analysis of data. Under the best conditions assayed, the yield in levulinic acid accounted for 66% of the stoichiometric value.
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INTRODUCTION A wide range of technologies have been proposed in the last years for producing chemicals, materials, or goods from renewable sources following the biorefinery concept. Biorefineries follow a principle analogous to oil refineries: the raw material is fractionated into a number of components that can be used for obtaining a variety of fuels, chemicals, and/or biobased-materials,1−3 including levulinic acid. Levulinic acid (4-oxopentanoic acid, denoted LevA) is a lowmolecular weight carboxylic acid considered as a new platform chemical, owing to its possible conversion into a number of commercial compounds, including solvents, food flavoring agents, monomers for synthesis of resins or polymers, plasticizers, herbicides, and antifreeze and pharmaceutical agents.4 As it is well-known, LevA can be produced from sugars in acidic media. Softwoods are the dominant source of lignocellulosic materials in the Northern hemisphere,5 and present hemicelluloses containing hexoses and pentoses. Schemes for softwood fractionation may include the separation of hemicelluloses as a first processing stage.6,7 Pinus pinaster wood hemicelluloses are mainly made up of glucomannans substituted with acetyl and galactosyl moieties.8 The backbone of pine mannans [which include glucomannans (GM) or galactoglucomannans (GGM), depending on the abundance of galactosyl substituents] is made up of randomly distributed D-glucose and D-mannose structural units linked by β (1→4) glycosidic bonds, which can be substituted with irregularly distributed acetyl groups.9,10 Soluble saccharides of polymeric or oligomeric nature can be obtained from pine wood hemicelluloses by treatment with compressed water under selected conditions, leading to reaction media that also contain monosacharides, acetic acid © 2013 American Chemical Society
(coming from acetyl groups linked to hemicelluloses), and small quantities of furfural and hydroxymethylfurfural (5hydroxymethyl-2-furaldehyde, HMF).11 Hexose decomposition in acidic media leads to the formation of formic acid (FA) and LevA, through a mechanism in which HMF participates as an intermediate, which is formed by hexose dehydration. In the case of fructose, a direct route was proposed,4 explaining the superior HMF yields obtained when using this sugar.12 Oppositely, glucose and other hexoses are converted into HMF by formation of the enediol, which is further transformed to HMF via additional intermediates.13 The LevA yield is decreased by parasitic reactions leading to the formation of dark, colored solids usually referred to as humins.14,15 Patil and Lund16 studied the formation of humins by acid-catalyzed conversion of HMF, and proposed that humin polymers are derived from HMF but not from LevA or FA. Girisuta et al.14 evaluated the effects reached by different acids (H3PO4, oxalic acid, HCl, H2SO4, and HI) on the conversion of HMF into LevA, finding that H3PO4 and oxalic acid provided limited conversions, HI enabled high HMF conversion (but not into LevA), and that HCl and H2SO4 provided the best HMF conversions and LevA yields. LevA has also been produced by acidic processing of biomass containing cellulose, starch or sucrose.4,17−20 Following a related mechanism, dehydration of pentoses in acid-catalyzed media leads to the formation of furfural, which in turn can be converted into FA,4 again with the participation of side reactions. Similarly, the processing of substrates containing Received: Revised: Accepted: Published: 3951
July 14, 2012 February 7, 2013 February 24, 2013 February 24, 2013 dx.doi.org/10.1021/ie3018725 | Ind. Eng. Chem. Res. 2013, 52, 3951−3957
Industrial & Engineering Chemistry Research
Article
Analytical Methods. The compositions of both autohydrolysis liquors and solutions obtained in acid hydrolysis treatments were determined by HPLC using an Agilent 1100 instrument fitted with a refractive index detector. Samples were filtered through 0.45 μm cellulose acetate membranes before analysis. Monosaccharides (glucose, xylose, galactose, arabinose, and mannose) were analyzed using a CARBOsep CHO 682 column with a guard column (Transgenomic) kept at 80 °C, employing distilled water as a mobile phase (flow rate, 0.4 mL/min). Before analysis in the CARBOsep CHO 682 column, posthydrolysis samples and liquors from acid hydrolysis were diluted, neutralized with BaCO3, centrifuged, and filtered. Furfural, HMF, FA, acetic acid, and LevA were determined using an Aminex HPX-87H column (BioRad, Life Science Group Hercules, CA), using the following conditions: mobile phase, 0.006 N H2SO4; flow rate, 0.6 mL/min; and temperature, 60 °C. Concentrations of oligosaccharides (OS) and bound acetyl groups were determined on the basis of the increases in the concentrations of monosaccharides and acetic acid after a quantitative posthydrolysis (performed with 4% H2SO4 for 20 min). Concerning the experimental error, linear relationships between detector responses and concentrations were found at concentrations up to 1 g/L for all determined compounds. In some experiments, the concentrations of one or several target compounds were below 1 mmol/L. To ensure the reliability of the analytical method under these conditions, the reproducibility of the analytical methods under these conditions was confirmed in additional experiments. Calibration curves were determined using five different concentrations evenly distributed along the considered concentration range, and the fitting calculations led to regression coefficients ≥0.995 for all the considered compounds. Replicates of samples with typical concentrations of glucose, xylose, arabinose, levulinic acid, and furfural (0.8 mmol/L) presented standard deviations
