Reaction Pathway Analysis of Ethyl Levulinate and 5

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Reaction Pathway Analysis of Ethyl levulinate and 5Ethoxymethylfurfural from D-Fructose Acid Hydrolysis in Ethanol Thomas Flannelly, Stephen Dooley, and James J. Leahy Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 22 Sep 2015 Downloaded from http://pubs.acs.org on October 9, 2015

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Reaction Pathway Analysis of Ethyl levulinate and 5-Ethoxymethylfurfural from DFructose Acid Hydrolysis in Ethanol

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Thomas Flannelly, Stephen Dooley, J.J Leahy

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Department of Chemical and Environmental Sciences, University of Limerick, Ireland.

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Abstract

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This study utilises numerical modelling to provide a mechanistic discussion of the synthesis

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of the advanced biofuel candidates, ethyl levulinate and 5-ethoxymethylfurfural, from α/β-D-

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fructopyranose (D-fructose) in a condensed phase homogeneous ethanol system at 351 K

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catalysed by hydrogen cations. A mechanistic comprehension is pursued by detailed

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measurements of reactant, intermediate and product species temporal evolutions, as a

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function of H2SO4 (0.09, 0.22, 0.32 mol/L) and

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concentration, also considering the addition of water to the ethanol media (0, 12, 24 mass %

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water in ethanol).

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levulinate, and several other intermediate species are quantified as major species fractions at

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45-85 % of the initial D-fructose mass. To inform the mechanistic discussion mass-conserved

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chemically authentic kinetic models and empirical rate constants are derived each assuming a

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first order relationship to the hydrogen cation concentration. The optimal synthesised

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fractions of ethyl levulinate and 5-ethoxymethylfurfural considered as fuel components,

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achieve a mass yield of 63 % with respect to the fructose mass and a volumetric energy

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valorisation (∆HCombustion, kcal/mL) of 215 % with respect to the ethanol consumed, indicating

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the viability of the synthesis.

D-fructose,

D-fructose

(0.14, 0.29, 0.43 mol/L)

5-hydroxymethylfurfural, 5-ethoxymethylfurfural, ethyl

25 26

Keywords. D-fructose, 5-ethoxymethylfurfural, ethyl levulinate, reaction mechanism, kinetic

27

model.

1 ACS Paragon Plus Environment

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1. Introduction

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Presently there is a growing effort to find renewable and sustainable alternatives to petroleum

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derived bulk chemicals and fuels. The catalytic conversion of biomass derived cellulose and

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hemicellulose to platform chemicals has been widely recognised as an opportunity to develop

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a carbohydrate based chemical industry.1-2 Lignocellulosic derived hexose and pentose sugars

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have potential as a sustainable alternative to the carbohydrates derived from edible crop

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matter. The United States Department of Energy have identified promising renewable

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chemical building blocks that may be produced from such biomass derived sugars.3 Ethyl

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levulinate and 5-ethoxymethylfurfural, Figure 1, are two such promising furan derived

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chemicals for potential use as transportation fuels.

38 39

Ethyl levulinate has received a significant amount of attention purporting its potential use as a

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fuel.4-5 Chemical systems reporting its synthesis include; ethylation of levulinic acid, furfuryl

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alcohol and 5-chloromethylfurfural.6–12 Ethyl levulinate production from biomasses have

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been reported13–15 however; yields have been modest at 40-50 %. There have also been

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considerable difficulties in its synthesis from glucose and cellulose with 44.8 mol % the

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highest yield reported to date using glucose as a starting material in an ethanol/H2SO4

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system.16

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5-ethoxymethylfurfural has received more limited suggestions as a fuel component17 despite

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its high volumetric density of 1.099 g/mL (298 K). Gruter and Dautzenberg18 suggest an

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enthalpy of formation of 120.1 kcal/mol, corresponding to an enthalpy of combustion of 7.87

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kcal/g (see Table 1). These terms correspond to a volumetric energy density of 8.66 kcal/mL

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(36.24 MJ/L at 298 K), thus being advantageous over those of other oxygenated fuel

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components, such as ethanol (7.11 kcal/mL), and ethyl levulinate (7.53 kcal/mL). Indeed the


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value is comparable to petroleum derived diesel and gasoline (8.53 kcal/mL for the gasoline

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primary reference fuel, iso-octane). Mascal and Nikitin19 use 5-chloromethylfurfural to

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convert 5-hydroxymethylfurfural to 5-ethoxymethylfurfural. Other reports use aluminium

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chloride as a catalyst to transform glucose to 5-ethoxymethylfurfural in an ethanol/water

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medium20 and also from using a mixture of Sn-Beta and amberlyst catalysts.21

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Like ethyl levulinate, the production of 5-ethoxymethylfurfural from glucose or other

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cellulosic sugars presents challenges. The desired etherification of glucose is suppressed by

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side reactions of various polymerisations and acetalizations producing the recalcitrant humic

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substances.22,23 40-50 % of lignocellulosic biomass is made-up of cellulosic glucose

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polymers, accounting for the largest proportion of hexoses that may be obtained from

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biomass.24 As such glucose-like hexose sugars are cheaper and more readily available than

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pentose sugars.

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It is well understood that the steric and electronic configurations of the hydroxyl groups of

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sugars, significantly affect yields of esterification products from monosaccharides.25 As a

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consequence, fructose is much easier to convert into furan related products than glucose. The

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desired isomerisation from glucose is known to be facilitated by an aqueous/organic media

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where the equilibrium population have high proportions of labile α and β‐fructofuranose

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structures.26 There is a general consensus that in order for glucose to be efficiently converted

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into furanic derivatives, it must initially tautomerize into fructose species.24,27 There are

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several recent reports describing reaction conditions that promote glucose isomerisation to

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fructose in chemical media.11,28–30 For example, Despax et al.30 reported on the use of

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heterogeneous catalysts in organic solvent mixtures showing ~ 68 % conversion of glucose to

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fructose. The successful isomerisation of glucose to fructose in an alcohol medium by the


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deliberate formation of methyl fructoside species as intermediates between glucose

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conversions to fructose is of particular potential significance for the production of ethyl

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levulinate and 5-ethoxymethylfurfural. Saravanamurugan et al.30,32

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conversion from glucose to fructose by a one hour reaction in methanol at 120 C using an H-

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USY zeolite. This suggests that an alcohol may be employed as both a solvent and alkylating

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agent simultaneously, whilst also facilitating the required isomerization of glucose to

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fructose. In this way, ethyl levulinate and 5-ethoxymethylfurfural may be synthesised

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directly, rather than relying on the intermediary ethylation of the levulinic acid produced in

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an aqueous system.

achieved 55 %

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In addition to this sugar inter-conversion, the subsequent mechanism of fructose consumption

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is the obvious further limiting step in achieving viable yields of furanic derivatives.33 Given

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this prevailing position in the literature an improved mechanistic understanding of ethyl

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levulinate and 5-ethoxymethylfurfural synthesis from fructose, as well as from glucose is

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sought. There are recent reports of kinetic studies conducted on the purported H+ (hydrogen

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cation) homogeneously catalysed dehydration of D-fructose to 5-hydroxymethylfurfural in

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water.34–36 However, little is known of the analogous D-fructose dehydration in the presence

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of ethanol and H+. Plausible reaction pathways have been suggested,27,37 but no quantitative

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kinetic data for 5-ethoxymethylfurfural and ethyl levulinate synthesis have been reported.

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We pursue an improved mechanistic comprehension employing a hierarchical modelling

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approach that studies one sugar sub-mechanism at a time. In this context, we study the bottle-

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neck α/β-D-fructopyranose (D-fructose) sub-mechanism initially, which once understood

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would allow the more complex D-glucose and cellulose sub models to be developed in a

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hierarchical manner. In order to develop realistic reaction kinetics, it is necessarily to limit


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the modelling complexity. To do so, we choose a homogeneous catalytic system of α/β-D-

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fructose/H2SO4/ethanol, thereby minimising the mass transfer complexities of multiphase

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heterogeneous reactions. The aim is to establish the main mechanistic relationships between

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reactant, intermediate and intended product species such as to inform mechanistic discussion

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and to test the validity of a viable reaction mechanism by the derivation of empirical rate

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constants. By so doing the viability of preferentially producing one proposed fuel component

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over the other, or to what extent this is achievable may be determined in a rigorous scientific

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manner.

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2. Experimental Materials

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Ethanol, normal-octanol, acetone, (99 % purity), α/β-D-fructopyranose (CAS 57-48-7, 99%

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purity), α/β-D-glucopyranose (CAS 50-99-7, 99 % purity) α/β-D-mannopyranose, (3458-28-4,

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99 % purity), hence forth “D-fructose” “D-glucose” and “D-mannose” respectively, sulphuric

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acid (H2SO4, 95-97% purity), 5-hydroxymethylfurfural (CAS 67-47-0, 99 % purity) furfural

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(CAS 98-08-1, 98 % purity),and ethyl levulinate (CAS 539-88-8, 97 % purity) are each

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obtained from Sigma Aldrich Ireland. Ethyl-α-D-glucopyranoside (CAS 34625-23-5, 98 %

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purity) is obtained from Carbosyth Ltd. UK, and 5-ethoxymethylfurfural (CAS 1917-65-3,

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96-97 % purity) is purchased from Akos Organics Gmbh, Germany.

122 123

Experimental Design and Procedure

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Reactions are performed in a 20 cm3 spherical reactor at isothermal conditions of 351 ±1 K at

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atmospheric pressure. The reactor is heated by an external oil bath. The reaction temperature

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is independently controlled and monitored by a thermocouple array (Stuart™ SCT1

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temperature controller) and an in-situ magnetic propeller ensures that the reaction mixture (D-


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fructose/H2SO4/ethanol) is well mixed and homogeneous. Atmospheric pressure is regulated

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by fitting the main reactor exit with an open-ended condensing unit (~ 277 K), thus allowing

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the reaction to be at reflux. For the test conditions reported here, Table 2, a heating time of 16

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minutes is required for the reacting mixture to be heated from ambient to the prescribed

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reaction temperature of 351 ±1 K. Reaction conditions are selected (Table 2) to parameterise

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the influence of [H2SO4] and [D-fructose] on the reaction mechanism, whilst also considering

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three scenarios of ethanol/water as reaction media. Reaction progress is monitored by

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removing and analysing a 50 mg sample of the bulk reaction (0.104 g of D-fructose in 15.78 g

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of ethanol) every hour for 480 minutes, resulting in a small cumulative perturbation to the

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overall system mass. Control tests at the most severe conditions of Table 2, replacing D-

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fructose with ethyl levulinate, show ethyl levulinate degradation to be within the estimated

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experimental uncertainty, indicating it as a stable end-product. Control reactions are also

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performed substituting 5-hydroxymethylfurfural and 5-ethoxymethylfurfural as starting

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materials for the purposes of identifying the origins of various intermediate species, as

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elaborated later.

143 144

Analytical Methods

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The concentrations of ethyl levulinate and 5-ethoxymethylfurfural, are analysed by gas

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chromatography (GC, Agilient Technologies 7820 A GC system) fitted with a Restek

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Stabilwax capillary column (30 m, 0.25 mm ID, 0.25 µm), employing hydrogen carrier gas

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and a flame ionisation detector. Species are identified by matching retention-times to known

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standards, and quantified by calibration of detector response to known concentrations (using

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n-octanol as internal standard). The injection port is maintained at 523 K, a temperature

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sufficiently high to ensure the full vaporisation of the expected reaction components. A

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temperature program of 40 K increasing to 493 K at a rate of 20 K per minute, remaining


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isothermal at 493 K for 5 minutes is found to achieve adequate separation of these species

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from the ethanol/water media. GC-MS analysis is also employed for the identification of

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sample species using an Agilient 5975C MSD, which uses a HP-5MS column (30 m, 0.25

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mm ID, 0.25 µm) otherwise employing the same variables as for GC–FID analysis. For GC

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analysis, a known mass (50 ± 5 mg ) of analyte is extracted from the reaction media into 0.4

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g of room temperature acetone and 0.8 g of 0.16 mg/g n-octanol in acetone, this is followed

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by the neutralisation of any remaining acid by the addition of 50 mg of NaHCO3. This

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dilution and cooling procedure ensures that the chemical reaction is effectively quenched.

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This sample is then filtered through 13 mm thick, 2 µm pore size syringe filters (Acrodisc) to

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remove any insoluble humic substances that may have been formed, and 1µl of the resulting

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solution is injected into the sample inlet port of the GC.

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Identification and quantification of D-fructose, D-glucose, 5-hydroxymethylfurfural and the

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various sugar-type derivatives is performed on an ion exchange liquid chromatography

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system (IC) system (Dionex Corp., Sunnydale, CA) equipped with a pulsed amperometric

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detector (AS, 10 µL sample loop, Dionex Corp., Sunnydale, CA). Analysis is performed at

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291 K by isocratic elution with deionised water (18.2 MΩ.cm at a flow rate of 1.1 mL/min)

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using a Dionex CarboPac PA1 carbohydrate column. The column is reconditioned using a

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mixture of 0.4 mol/L sodium hydroxide and 0.24 mol/L sodium acetate after each analysis. A

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25 mg portion of the sampled reaction media is diluted with 1.0 g of deionised water. As

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before, 50 mg of NaHCO3 is added to neutralise any acid present. This sample is filtered as

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described above before being analysed. D-fructose, D-glucose, and 5-hydroxymethylfurfural

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concentrations are determined by detector calibration to mass prepared standard solutions.

176


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In all experiments the quantity of “humins” formed is very small, and so are only determined

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at the completion of each reaction, when the reaction mixture is filtered through glass fibre

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paper (Whatman, grade GF/B 2.7 µm). The filter paper is subsequently washed with ethanol

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and placed in an oven for 24 hours at 378 K. The mass of the material remaining on the paper

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is determined by difference and referred to as “humins”.

182 183

The pH of the reaction samples is also determined in order to measure hydrogen cation

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concentrations [H+]. An Orion pH Ag/AgCl glass electrode fitted to a VWR Symphony

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SB70P pH meter is employed. For pH measurements, 0.4 g samples taken from the reaction

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media are diluted with 10 g of deionised water. The pH meter is calibrated against buffer

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solutions (VWR 32032.291) of known [H+]. The pH of samples is variable with reaction time

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and condition but is always in the range of 1.7-2.4.

189 190

Measurement Uncertainties

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A reproducibility and repeatability study of Test #1 shows the overall experiment-to-

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experiment variability to be ± 12 %, which is comparable to the majority of the uncertainty

193

estimates below. For GC analysis, experimental measurement uncertainties are; ethyl

194

levulinate (± 9.6 %), 5-ethoxymethylfurfural (± 8.2 %),

195

hydroxymethylfurfural (± 10.1 %). Uncertainties in reported [H+] are generally ± 4.5 %. In

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addition to the several identified chemical species discussed in Section 3.1, five discrete

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components separated and detected by IC analysis cannot be identified by retention time

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matching to expected sugar derivatives for which analytical standards are available. Figure

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SI1 marks these detections at 1.66, 1.81, 2.11, and 2.41 and 15.27 minutes for a

200

representative chromatogram. These species are termed “unknown # 1-5” for the purposes of

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discussion. By testing and elimination, it is determined that these detections are not due to the

D-fructose

(± 5.6 %) and 5-


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following compounds; ethyl levulinate, 5-ethoxymethylfurfural, levulinic acid, formic acid,

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dihydroxyacetone, ethyl formate, ethyl α/β-D-glucopyranoside, H2SO4, furfural, or any

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species that result from a series of dummy reactions comprising; H2SO4/ethanol, and each of

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5-hydroxymethylfurfural, 5-ethoxymethylfurfural, and ethyl levulinate at 351 K for 480

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minutes at the most extreme reaction conditions listed in Table 2. In this way, it is determined

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that the unknown species originate from the reaction of D-fructose. As they account for a

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considerable amount of the total ion chromatograph signal (see Table 3), their identity is

209

worthy of some speculation.

210 211

To separate carbohydrates, the PA1 Carbopac column exploits their weakly acidic nature. At

212

high pH values (supplied by the sodium hydroxide mobile phase) the carbohydrates are

213

partially ionised and can be separated by the anion exchange mechanisms embedded on the

214

column. More acidic carbohydrates bind more strongly to the column and are retained for

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longer time. Table SI1 in the Supporting Information demonstrates the correlation of sugar

216

pKa to retention time for a series of standard carbohydrates tested.

217 218

Of the five unknowns marked in Figure SI1 (1.61, 1.81, 2.11, 2.41 and 15.27 mins), it may

219

thus be concluded that #1-4 are of higher pKa than D-fructose or D-glucose. By analogy to the

220

glucose/methanol/acid studies of Saravanamurugan et al.38, who provide evidence of the

221

formation of various methylated pyranosides and furanosides as intermediate species; it is

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speculated that the species that are eluted before 2 minutes are ethyl fructopyranoside or

223

fructofuranoside species. It is clear from the temporal evolution of these identities that they

224

are intermediates in the formation of the desired fuel components. Only Ethyl α-D-

225

glucopyranoside (CAS 34625-23-5) and ethyl β-D-glucopyranoside analytical standards are

226

presently available (Carbosynth Ltd.) and show very similar retention times of 1.68 min and


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1.72 minutes respectively under the conditions of separation. This behaviour is consistent

228

with the elutions at 1.61 and 1.81 minutes being similar such C8H16O6 ethyl pyranoside or

229

analogous ethyl furanoside isomers (for which standards are not available). As only very

230

small quantities of D-glucose are detected in any of the experimental tests, it suggests that

231

these entities are more likely fructose derived ethyl fructofuranoside and/or fructopyranoside,

232

C8H16O6 isomers. Henceforth these substances will be termed “ethyl fructosides”. As ethyl

233

fructoside analytical standards are unavailable, these moieties are quantified by standard

234

preparations of ethyl α-D-glucopyranoside and ethyl-β-D-glucopyranoside, which show

235

exactly equivalent response factors on the pulsed amperometry detector (see SI).

236 237

Correlation of pKa to retention time infers that unknown #5 (15.27 mins) is more acidic in

238

nature than

239

speculate that unknown #5 is a C6 sugar species intermediate between

240

hydroxymethylfurfural. A detailed discussion supporting this speculation is provided as SI.

D-fructose

(12.5 mins) and is thus likely be a stable sugar intermediate. We D-fructose

and 5-

241 242

The corresponding quantitative uncertainties are estimated as; unknowns #1 & #2 (ethyl

243

fructosides, ±5 %), unknowns #3 & #4 (±24 %) and unknown #5, ±12 %.

244 245 246

3. Results and Discussion Experimental Observations and Reaction Mechanism

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Figure 2 shows a general reaction mechanism for the formation of ethyl levulinate and 5-

248

ethoxymethylfurfural from

249

measurements reported in this study and those conducted by others. Figures 3-5 show the

250

temporal evolution of the major species for representative ethanol and ethanol/water

D-fructose

that is derived from both the experimental


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conditions. The data for the other tests closely follow this detail and are available as

252

Supporting Information (SI3-SI6).

253 254

Trace Species, Not Considered in Mechanistic Analysis

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Trace amounts of D-mannose, D-glucose, dihydroxyacetone, and 5.5’(oxybis(methylene)bis-

256

2-furfural are observed in each test condition #1-7. Levulinic acid and furfural are also

257

present at

Reaction Pathway Analysis of Ethyl Levulinate and 5

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