View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Kyoto University Research Information Repository Retro-aldol-type fragmentation of reducing sugars Title preferentially occurring in polyether at high temperature: Role of the ether oxygen as a base catalyst Author(s) Matsuoka, Seiji; Kawamoto, Haruo; Saka, Shiro Citation Journal of Analytical and Applied Pyrolysis (2012), 93: 24-32 Issue Date 2012-01 URL http://hdl.handle.net/2433/152376 Right © 2011 Elsevier B.V. Type Journal Article Textversion author Kyoto University Retro-aldol-type Fragmentation of Reducing Sugars Preferentially Occurring in Polyether at High Temperature: Role of the Ether Oxygen as a Base Catalyst Seiji Matsuoka, Haruo Kawamoto*, Shiro Saka * Corresponding author: Haruo Kawamoto Graduate School of Energy Science, Kyoto University Yoshida-honmachi, Sakyo-ku, Kyoto 606-8501, Japan Tel/Fax: +81-75-753-4737 E-mail address: [email protected] 1 Abstract: 1 The pyrolysis behavior of reducing monosaccharides was compared in the 2 presence and absence of tetraethyleneglycol dimethylether (TEGDE), a polyether 3 (N2/150-250 °C). The pyrolytic pathways changed drastically in TEGDE. Glucose 4 started to decompose at >160 °C under the neat conditions, and polysaccharides, 5 anhydrosugars (levoglucosan and 1,6-anhydroglucofuranose), a colored substance and 6 char were the major products. However, glucose was completely stabilized against 7 decomposition in TEGDE and instead converted into fragmentation products including 8 formaldehyde, glycolaldehyde, glyceraldehyde, 1,3-dihydroxyacetone, erythrose and 9 erythrulose at higher temperatures. The total yield of the fragmentation products 10 reached a 74.9 wt % at 250 °C. An aldose-ketose isomerization and retro-aldol 11 fragmentation including a six-membered cyclic transition state were suggested as the 12 principle mechanisms. Several other polyethers gave similar results. This unique 13 property of polyether can be explained by the basicity of the ether oxygen which acts as 14 a proton acceptor for the hydroxyl groups in the sugar. This H-bonding between the 15 polyether and glucose may prevent inter- and intramolecular H-bonding (H-donation to 16 the oxygen atoms) of glucose, which results in stabilization against transglycosylation 17 and dehydration reactions. Such inter- and intramolecular H-bonding (H-donation) may 18 also be involved in the thermal decomposition of the melt sugar as an activation (acid 19 catalysis) mechanism. 20 21 Keywords: 22 Reducing sugar; Controlled pyrolysis; Polyether; Retro-aldol fragmentation; Hydrogen 23 bond. 24 2 25 1. Introduction 26 Pyrolysis is an effective method for obtaining fuels and chemicals from organic 27 resources. Methanol used to be produced from wood pyrolyzates [1]. Catalytic cracking 28 is currently available for production of gaseous and liquid fuels and commodity 29 chemicals from petroleum [2,3]. Biomass has great potential as a future renewable 30 resource for fuels and chemicals production [1,4]. Thermochemical methods such as 31 fast pyrolysis [5,6] and gasification [5,7] are promising ways for converting biomass 32 into fuels and chemicals. However, low product selectivity arising from the complex 33 reactions of biomass pyrolysis makes the application of these pyrolysis-based processes 34 difficult. Understanding the molecular mechanisms of biomass pyrolysis and their 35 control would be useful for improving the product selectivity. 36 Carbohydrates are a major component of biomass resources, and hence, 37 pyrolysis of reducing [8,9] and non-reducing sugars [10-12] and polysaccharides 38 [13-15] has been studied extensively. Reducing monosaccharides are known to be 39 degraded at a much lower temperature range (> 160 °C) than the non-reducing sugars, 40 and their thermal decomposition has been discussed in connection with a caramelization 41 process [8]. Thermal glycosylation into polysaccharides and formation of furanic 42 compounds, char and colored substances through dehydration are reported to be 43 pyrolytic reactions occurring during caramelization [8]. At temperatures higher than > 44 250 °C, formation of low molecular weight (MW) products including anhydrosugars 45 (levoglucosan (1,6-anhydro--D-glucopyranose), 1,6-anhydro--D-glucofuranose), 46 furanic compounds (furfural, 5-hydroxymethylfurfural), fragmentation products 47 (hydroxyacetone, glycolaldehyde) and organic acids (acetic acid, formic acid, etc.) 48 becomes more important [8,16,17]. 49 Recently, Hosoya et al. [18] have reported that levoglucosan, an important 50 intermediate in cellulose pyrolysis, was selectively converted into gaseous products (CO 51 and CO2) in the gas phase at 400 °C, while it was converted into polysaccharides, char 3 52 and low MW products as described above in the liquid phase. Thus, the pyrolytic 53 pathway varies depending on the phase of pyrolysis, that is, gas or liquid phase. As a 54 related phenomenon, Hosoya et al. [19] also reported that levoglucosan was stabilized 55 up to 350°C in aromatic substances, and they related this unexpected feature to the 56 dispersion of levoglucosan molecules in the aromatic substance through C-H/ 57 interactions. 58 Because of the drastic differences in the reactivity reported for some pyrolysis 59 conditions, we hypothesize that inter- and intramolecular hydrogen bonding initiates the 60 pyrolysis reactions such as glycosylation and dehydration of sugars. Hydrogen-donation 61 to the oxygen atoms of the sugar may act as an acid catalysis at high temperature. To 62 confirm this hypothesis, polyether, which is expected to act only as a hydrogen acceptor, 63 was tested. Most of the experiments were conducted with tetraethyleneglycol 64 dimethylether (TEGDE) as the model polyether because this has a high boiling point 65 (280 °C) and can solubilize glucose up to 2 wt % (150 °C). Pyrolysis of aldo (glucose 66 and glyceraldehyde) and keto (fructose and 1,3-dihydroxyacetone) sugars was 67 conducted in TEGDE. The roles of intra and intermolecular hydrogen bonding in sugar 68 pyrolysis are discussed. 69 70 71 2. Experimental 72 2.1. Materials 73 Glucose, fructose, and glyceraldehyde were from Nacalai Tesque Inc., Japan. 74 Glycolaldehyde, 1,3-dihydroxyacetone, erythrose, and erythrulose were from 75 Sigma-Aldrich Co., USA. Tetraethyleneglycol dimethylether, 18-crown-6, isosorbide 76 dimethylether, diethyleneglycol dibutylether, and levoglucosan were from Tokyo 77 Chemical Industry Co. Ltd., Japan. Cellotriose and cellohexaose were purchased from 78 Seikagaku Biobusiness Corporation, Japan. 1,6-Anhydro--D-glucofuranose was 4 79 prepared by heat treatment of glucose in sulfolane (tetramethylene sulfone) at 250 ˚C. 80 81 2.2. Method of sugar pyrolysis 82 Sugar (20 mg), polyether (2 g) including 3,5-dibuthyl hydroxyl toluene (BHT, 83 0.5 wt % as a stabilizer), and a glass-coated stir bar were placed at the bottom of a 30 ml 84 flask. A condenser, a three-way tap, and a nitrogen balloon were connected to the flask. 85 After the air inside the reactor was replaced with N2 by using an aspirator connected 86 through the three-way tap, the flask was heated in an oil bath which was preheated at 87 150-250 °C, and the mixture was stirred with a magnetic stirrer for 10-60 min. After 88 heating, the flask was removed from the oil bath and cooled with flowing air (30 s) and 89 then in cold water (30 s). BHT was used as a stabilizer for polyether. However, 90 influences of the addition of BHT on the products composition were only small under 91 the present pyrolysis conditions. Neat sugar pyrolysis was also conducted without 92 addition of polyether, BHT, and the stir bar. 93 94 2.3. Recovery of the products and unreacted sugar 95 To remove polyether and BHT, n-hexane (20 mL) was added to the reaction 96 mixture. The mixture was left for 1 h at room temperature to give a colorless crystalline 97 substance or a syrup as the precipitate, which was recovered by centrifugation at 8000 98 rpm for 10 min. After washing with another 20 mL of n-hexane and subsequent drying 99 in air, the precipitate was dissolved in 2 mL of water, and 0.1 mL of the resulting 100 solution was dried in a vacuum desiccator and subsequently trimethylsilylated with a 101 0.1 mL of silylation reagent (BSTFA: TMCS: Pyridine = 2:1:7) at 60 °C for 10 min. 102 Recovery of the unreacted sugar was measured by GC-FID analysis. The GC analysis 103 was performed on a Shimadzu GC-14B with the following chromatographic conditions, 104 column: CBP5-M25-O25 (25 m, 0.22 mm in diameter), injector temperature: 250 °C, 105 detector temperature: 250 °C, column temperature: 160→250 °C (0→9 min), 250 °C 5 106 (9→15 min), carrier gas: He, flow rate: 1.5 mL/min, detector: FID. 107 108 2.4. Isolation and identification of the products 109 The mixtures obtained as the precipitates were purified further using silica gel 110 column chromatography; eluent: chloroform and then 20% MeOH/chloroform. 111 Formaldehyde was recovered according to the method as described below. 112 Glycolaldehyde, glyceraldehyde, 1,3-dihydroxyacetone and erythrose were identified by 113 comparing the 1H-NMR spectra of these oxime derivatives (with hydroxylamine) with 114 those of the authentic compounds. Chemical shift and coupling constant were shown as 1 115 and Hz, respectively. The H-NMR spectra were measured in D2O on a Varian 116 AVANCE 400 spectrometer (400MHz): Glycolaldehyde oxime (Z, E-isomer): 7.49 (t, 117 J = 4.8, 1 H, H-C=N, E), 6.90 (t, J = 4.0, 1 H, H-C=N, Z), 4.35 (t, J = 4.0, 2 H, 118 -CH2-OD, Z), 4.13 (d, J = 4.8, 2 H, -CH2-OD, E); 1,3-dihydroxyacetone oxime: 4,44 119 (s, 2 H), 4.25 (s, 2 H); glyceraldehyde oxime (E-isomer): 7.42 (d, J = 6.0, 1 H, C1-H), 120 4.27 (ddd, J = 4.8, 6.0, 6.0, 1 H, C2-H), 3.65 (dd, J = 4.8, 12.0, 1 H, C3-H), 3.61 (dd, J = 121 6.0, 11.6, 1 H, C3-H); erythrose oxime (E-isomer): 7.47 (d, J = 6.4, 1 H, C1-H), 4.18 (t, 122 J = 6.4, 1 H, C2-H), 3.75-3.79 (m, 1 H, C3-H), 3.67 (dd, J = 4.0, 12.0, 1 H, C4-H), 3.55 1 123 (dd, J = 6.4, 12.0, 1 H, C4-H).