Article pubs.acs.org/IECR
Kinetic Insights into the Hydrothermal Decomposition of Dihydroxyacetone: A Combined Experimental and Modeling Study Xiao Liang, Asanka Rahubadda, Brian S. Haynes, and Alejandro Montoya* School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, NSW 2006, Australia S Supporting Information *
ABSTRACT: The hydrothermal decomposition of dihydroxyacetone (DHA) is investigated under subcritical water conditions between 493 and 553 K at 200 bar using microtubular and pilot-scale flow reactors. The product samples contained a mixture of low molecular weight acids and aldehydes, such as glyceraldehyde, glycolic acid, lactic acid, acetic acid, and methylglyoxal. Quantification of products accounted for all the input carbon, within 95 ± 5% between 493 and 533 K. A thermodynamically consistent kinetic model using thermodynamics of species derived from static electronic structure computational methods and rate parameters obtained from experimental data at initial stages of DHA decomposition is developed. The reaction pathway for DHA decomposition involves isomerization producing glyceraldehyde; dehydration to methylglyoxal; fragmentation to glycolic acid; and further decomposition of methylglyoxal to lactic acid, which becomes relevant at 553 K. The DHA product speciation is kinetically, rather than thermodynamically, controlled.
1. INTRODUCTION Carbohydrate decomposition under hydrothermal conditions produces a complex mixture of oxygenates, among which trioses are formed through retro-aldol condensation of monomeric sugars.1−5 Dihydroxyacetone (DHA) is a triose sugar currently produced from glycerol either through a microbial fermentation6 or by an electrocatalytic oxidation in the presence of TEMPO.7 DHA is used as an active ingredient in sunless tanning lotions and as a precursor of lactic acid and its esters.2,8 Monomeric DHA establishes equilibrium with a hemiacetal dimer, which forms the basic building block of new biomaterials such as polycarbonate-acetals.9 Hydrothermal decomposition in batch reactors10 and under near-critical4,11 and supercritical water11 conditions in microflow reactors have shown that DHA stablishes an isomeric equilibrium with glyceraldehyde (GCA) and dehydrates to methylglyoxal (MGX). Available kinetic information for DHA decomposition under hydrothermal conditions suggests that the dehydration of DHA to MGX is faster than the isomerization of DHA to GCA.8,9 It is proposed that DHA dehydrates to MGX through α-elimination12,13 while GCA dehydrates to MGX via β-dehydration.14,15 MGX may further rehydrate to produce lactic acid (LA),4 possibly following a benzylic acid rearrangement.16 Acid-catalyzed degradation of DHA shows similar reaction pathways compared with the DHA degradation under subcritical and supercritical water conditions, but with differences in the relative rates of DHA isomerization and dehydration compared with those reported from hydrothermal studies using water alone.17,18 A general feature of experimental studies of carbohydrate reactions under hydrothermal conditions is that the reported carbon balance is poor, with only a relatively small fraction of the components in the complex product mixture of oxygenates being quantified or even identified. DHA has been used as a model compound to study the chemistry of biomass-derived products, and reasonable carbon balance during its decom© 2015 American Chemical Society
position is obtained at near-critical conditions below 1.5 s reaction time or between 453−513 K below 5 min reaction time,10,11 with the result that kinetic models are limited to primary decomposition products. A gap exists in DHA decomposition studies at intermediate reaction times (3−170 s) and intermediate reaction temperature (513−573 K), leaving a deficit in the understanding of primary and secondary reaction paths relevant to conditions of the actual hydrothermal process. Initial molecular modeling studies of trioses have concentrated on understanding the effect of electron correlation on energy differences and hydrogen bonding of carbohydrates.19−23 Recent attention is shifting to obtaining thermodynamics for reaction modeling. The free energy of DHA isomerization to GCA in gas phase was reported to be endoergic, in the range of 2.9−5.2 kcal mol−1 using B3LYP, G4, CCSD(T), and MP2 levels of theory,24 consistent with an equilibrium reaction that favors the presence of DHA, as observed in hydrothermal studies. Vasiliu et al.25 evaluated the thermodynamic parameters of a large number of oxygenates relevant to the hydrothermal decomposition of carbohydrates using B3LYP and G3MP2 approaches, among which the free energy of reaction of the DHA dehydration to MGX in gas phase was reported to be 15.2 kJ mol−1 exoergic. The mechanism of GCA dehydration and retro-aldol condensation in the absence and presence of one water molecule was studied using density functional theory (DFT) methods, showing that water enhanced both reactions by stabilizing the transition states with water-bridged ringlike structures.26 No information exists on the effect of water as a solvent on the thermodynamics of DHA decomposition to other oxygenates. Received: Revised: Accepted: Published: 8437
June 24, 2015 August 10, 2015 August 13, 2015 August 13, 2015 DOI: 10.1021/acs.iecr.5b02311 Ind. Eng. Chem. Res. 2015, 54, 8437−8447
Article
Industrial & Engineering Chemistry Research
Figure 1. Schematic representation of the continuous flow microchannel reactor system, containing two feedstock tanks, two HPLC pumps, one preheater, one reactor, one temperature-controlled oven, one tube heat exchanger, one backpressure regulator, and three pressure relief valves.
the reactor. The minimum flow was chosen to avoid laminar flow to better approximate plug flow conditions. All water used in the experiments was purified (Millipore Elix 5) and vacuum-filtered using 0.2 μm filter paper before each run. DHA (Sigma-Aldrich >97% purity) was dissolved in water to create the reagent feed which was diluted 1:5 (w/w) with preheated water in the mixer. The concentration of DHA in the mixed stream going to the reactor was approximately 0.018 M in all cases, unless otherwise stated. DHA decomposition was also studied in a continuous biomass slurry reactor system described elsewhere in its application to the hydrothermal processing of microalgae.27 In this pilot-scale system, the total feed (0.11 M DHA solution; 0.6−1.2 kg min−1) was preheated to ∼393 K before being introduced into the reactor which consists of 64 m × 3/8 in. tubing (internal diameter 6.2 mm, residence time 80−170 s) immersed in a heated fluidized sand bath (513 K). Final heating of the reaction stream to the bath temperature occurred over a significant length of the reactor, and the reaction conditions were therefore not truly isothermal. The temperature profile of the fluid inside the reactor coils (Tfluid) immersed in a fluidized sand bath at Tsand as a function of reactor length (L) was obtained from a heat balance expression (eq 1):
The focus of the current study is to gain further insights into the kinetics of DHA decomposition under hydrothermal conditions. Attention is placed on the identification of carbon products sufficient to account for nearly all the product formation and on the thermodynamic properties of identified species to develop a thermodynamically consistent kinetic model to determine the influence of reaction parameters on concentrations of species.
2. METHODS AND TECHNIQUES 2.1. Experimental Methods. The detailed kinetics of the hydrothermal decomposition of DHA were studied in a tubular flow reactor, as shown schematically in Figure 1. The essential feature of the setup was that, to achieve effectively instantaneous heating of the mixture, the reactant stream was created by mixing of a heated water stream (from the preheater in Figure 1) with a cold aqueous stream containing DHA; these streams were controlled precisely through the use of highperformance liquid chromatography (HPLC) pumps (Varian ProStar models 210 and 218 for DHA and water, respectively). Rapid mixing and heating (
