Research Article pubs.acs.org/journal/ascecg
Decomposition Reactions and Kinetics of Alkaline-Earth Metal Levulinate and Formate Salts Scott J. Eaton† and M. Clayton Wheeler*,‡,§ †
Marine Engine Testing and Emissions Laboratory, Maine Maritime Academy, 1 Pleasant Street, Castine, Maine 04420 United States Department of Chemical and Biological Engineering and §Forest Bioproducts Research Institute, University of Maine, 5737 Jenness Hall, Orono, Maine 04469, United States
‡
ABSTRACT: Decomposition of biomass-derived alkalineearth metal levulinate and formate salts above 700 K, commonly called thermal deoxygenation (TDO), has previously been demonstrated to produce deoxygenated crude oil at 80% of theoretical yield suitable for refining to gasoline, jet, and diesel fuel. Decomposition in thermogravimetric experiments involves low-temperature (423−623 K) and hightemperature reaction (623−723 K) regimes. The current work presents results of isothermal decomposition and infrared techniques which provide insights into the low-temperature, or Stage I, reactions. Stage I reactions are shown to be primarily addition/condensation reactions between levulinate ketone groups evolving water which equates to 8−15 wt % of the sample. These Stage I reactions account for one-third of theoretical oxygen removal of the levulinate and formate salts while also increasing the molecular weight of the organic fraction. Therefore, Stage I is an important reaction step in the TDO process. KEYWORDS: Levulinic acid, Formic acid, Thermal deoxygenation, Biofuels, Hydrolysis, Condensation
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INTRODUCTION Thermal deoxygenation (TDO) of levulinate and formate salts of Group II metals is a promising way to produce biorenewable liquid hydrocarbons. Levulinic acid (LA) is a target biomolecule for the production of biorenewable goods, chemicals, and fuels1−6 which can be obtained at high yields at 1:1 molar proportion with formic acid (FA) by acid hydrolysis of cellulose.7−10 The TDO process utilizes slow pyrolysis of calcium-neutralized hydrolyzates at 723 K to yield a mixture of highly deoxygenated organics consisting substantially of aromatics.11 The oil-phase product separates from water and contains less than 1 wt % oxygen with higher heating values greater than 38 MJ/kg. The resulting organics can be obtained at 80% theoretical carbon yield and are suitable as refinery feedstock for liquid transportation fuels as demonstrated by hydrotreating and fractionation experiments.12 The TDO reactions are not well understood. Simple organic salts, such as calcium propionate or calcium acetate, are known to undergo decarboxylation around 723 K forming CaCO3 and the respective ketones, 3-pentanone, or 2-propanone.13−17 In contrast, thermal decomposition of calcium levulinate (CaLA) does not form CaCO3 and 2,5,8-nonanetrione but instead forms a water-soluble mixture of aromatics and cycloketones along with CaCO3 and char.18 Schwartz et al. speculated that calcium levulinate underwent ketonic decarboxylation, but then radical chemistry at elevated temperatures led to increased aromaticity and molecular weight. This hypothesis, however, did not incorporate two reaction regimes which were observed © XXXX American Chemical Society
in thermogravimetric experiments showing approximately 10% of the mass was lost below 623 K, and approximately 27% of the mass was lost between 623 and 823 K. Thermal decomposition of calcium formate (CaFA) occurs at approximately 700 K and produces CO and H2 product gases.19−23 Multiple studies have suggested formate-derived hydrogen can participate in hydrogenation and hydrodeoxygenation reactions in complex systems. Case et al. demonstrated that CaFA can improve the quality of pyrolytic bio-oils by impregnating pine sawdust at a loading of 1.4:1 (wtCaFA/wt-pine) prior to pyrolysis at 773 K.24 The resulting biooil’s hydrogen content increased from 6.5 to 7.3 wt %, and oxygen content decreased from 25.7 to 16.3 wt % compared to untreated pine sawdust. Since there were no transition metals capable of dissociating hydrogen to catalyze the hydrogenation or hydrodeoxygenation in these experiments, transfer hydrogenation involving CaFA was speculated. Transfer hydrogenation was also speculated by Ramsurn and Gupta during switchgrass deoxy-liquifaction in supercritical water with CaFA.25 A comparison of hydrothermal liquefaction products resulting from 1 g dry switch grass in 20 mL deionized water alone, and with 2−5 g CaFA, in an 80 mL reactor between 350 and 450 °C at 160−270 bar for 30 min showed evidence of hydrogenation and hydrodeoxygenation. Careful examination Received: November 8, 2016 Revised: February 24, 2017 Published: March 3, 2017 A
DOI: 10.1021/acssuschemeng.6b02704 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Table I. Elemental Composition of Levulinate and Levulinate/Formate Salt Mixtures Comprised of Calcium or Magnesium sample cation Mg
2+
Ca2+
elemental composition, wt % result/expected organic
Mg
Ca
C
H
O
(C5H7O3)2 (C5H7O3), (COOH) (C5H7O3)2 (C5H7O3), (COOH)
8.6/9.6 13.5/13.2 0.1/− 0.1/−
0.1/− 0.2/− 14.5/14.8 19.3/20.0
45.7/47.2 38.4/39.1 43.0/44.5 35.6/36.0
6.3/5.5 4.7/4.4 5.4/5.2 4.1/3.1
39.8/37.8 43.5/43.4 37.0/35.5 40.9/40.0
Table II. Infrared Absorbance Bands of Primary Carbonyl Functional Groups Identified for Calcium and Magnesium Salts frequency cm−1 magnesium functional group
band description
carboxylate
OCO (SSt) OCO (ASt) OCO (AD) C−H (St) C−H (B) CO (St)
aldehyde aliphatic a
a
calcium
formate
levulinate
mixed
formate
levulinate
mixed
1344 1606 1402 2893 793
1370 1587 1440
1362 1592 1442/1411 2908 787 1714
1358 1616 1384 2875 791
1363 1568 1421
1362 1564 1420 2875 789 1712
1712
S = symmetric, A = asymmetric, St = stretch, B = bend, D = deformation.
of product hydrogen revealed that 52% of the CaFA hydrogen was incorporated into the biomass. The resulting bio-oils obtained higher heating values in excess of 31 MJ/kg and improved yields by approximately 10 wt % when CaFA was incorporated. Co-pyrolysis of CaFA and CaLA salts by Case et al. in semibatch reactions also exhibited hydrogenation and hydrodeoxygenation activity when compared to results of CaLA alone by Schwartz et al.26 Copyrolysis of the salts increased product carbon yields (g-Cproducts/g-Cfeed) with an optimal yield at a formate:levulinate molar ratio of 1:1. Case et al. also reported a distinct oil-phase product containing primarily aromatics, but with increased hydrogen-to-carbon ratio and heating values above 40 MJ/kg. Thermogravimetric experiments of the salts indicated the existence of two reaction regimes: 423−623 K (Stage I) and 623−723 K (Stage II).18,26 The current work improves the understanding of LA and FA salt decomposition pathways by providing reaction data for Stage I reactions using in situ infrared (IR) and isothermal kinetic experiments for calcium and magnesium. Volatile and residual products are identified over the temperature range of 373−623 K using GC-MS techniques. The salts undergo a solid-to-plastic transition at temperatures between 373 and 473 K. Temperatures above 623 K induce LA addition/ condensation reactions producing water accompanied by swelling and resolidification of the sample resulting in a highly porous, brittle, material (Stage I). This reaction is responsible for as much as one-third theoretical oxygen removal from the biomass-derived organics and is the first report of a new poly salt material important for biofuel synthesis. Temperatures above 723 K induce ketonic decarboxylation reactions (poly salt decomposition) producing hydrocarbon vapors and char/ metal oxide/metal carbonate solids (Stage II). Detailed hydrocarbon analyses of Stage II reaction products for semibatch experiments have been reported by Eaton et al.11,12 and Schwartz et al.18 and are not repeated here for brevity. Understanding of the Stage I reaction pathways will enable process optimization for improved yields and potentially new products.
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EXPERIMENTAL SECTION
Salt Preparation. Calcium and magnesium salts of levulinic acid and a 1:1 molar mixture levulinate/formate (LA/FA) were prepared in an aqueous solution at the grams scale. Levulinic acid (>96% purity) and formic acid (>95% purity) and the hydroxides of magnesium (>95% purity), calcium (>96% purity) were obtained from SigmaAldrich and used without further purification. A 20 g portion of the requisite hydroxide were placed in 200 mL of distilled water. The acids were dropwise added over 30 min in a cold water bath until stoichiometric proportion. The neutralized solutions were transferred to aluminum drying pans and dried in air at 393 K for 24 h. The resulting salts, having the appearance of glass, were crushed and ground to a fine powder and stored with desiccant prior to experiments. Elemental Analysis. Metal concentrations were determined using a model 750 Thermo Jarrell Ash Corp. inductively coupled plasma optical emission spectrometer (ICP-OES). Samples weighing approximately 0.4 g were microwave digested in 8 mL of 1:3 aqua regia (1 part HNO3-to-3 parts HCl) and brought up to 100 mL final volume with deionized water. CHNS/O concentrations were measured using a Thermo Scientific Flash 2000 elemental analyzer conforming to ASTM D5291. The instrument was calibrated using 2,5-bis(5-tertbutyl-benzoxazol-2-yl)thiophene (BBOT) for both CHNS and direct oxygen measurements. All samples were analyzed in duplicate. Elemental compositions of prepared salts are presented in Table I with good agreement to expected values. Diffuse Reflectance Infrared Fourier Transform Analysis. Infrared spectra of LA, FA and LA/FA salts of magnesium and calcium were obtained using a Nicolet FTIR equipped with a Harrick DRIFTS chamber over the range 4000−500 cm−1 with resolution of 1 cm−1 under vacuum over 373−673 K, in 50 K increments. Sample powder was loaded onto a KBr bed and dried at 393 K for 2 h prior to spectrum collection. At each temperature increment, the sample was soaked for 20 min and cooled to 298 K for IR spectra collection. A total of 128 spectral scans were collected and averaged. Background reference spectra and single-beam data were collected at 298 K after each temperature treatment for postprocessing and analysis. Table II is a summary of IR absorption bands for FA, LA, and LA/FA of Ca and Mg. Formates have five primary bands, three bands corresponding to carboxylate functionality and two associated with the C−H aldehyde vibrations and in good agreement with the literature.21,27 The IR spectra of levulinates have strong carbonyl absorbance at 1712 cm−1 consistent with aliphatic ketones. Spectra obtained for the mixed salts exhibited a near superposition of the single species spectra. B
DOI: 10.1021/acssuschemeng.6b02704 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Schematic of test apparatus used in isothermal salt reaction experiments. Isothermal Kinetic Experiments. A Mellen horizontal tube furnace (model TT12-2x24) was used for isothermal decomposition experiments at temperatures from 423 to 723 K in 50 K increments. The furnace was equipped with an 80 cm long, 2.8 cm OD quartz tube, sealed on either end and purged with N2 at 0.5 L/min. Nominally, 0.5 g of sample powder was loaded into a ceramic crucible and placed in a conditioning chamber extending off the quartz tube, purged with N2, and maintained at room temperature. The sample was pushed into the preheated furnace and heated for either 2, 5, 10, 20, 30, 45, or 60 min, and the salt conversion was determined. LA and FA fractional conversion was determined by acidifying nominally 0.2 g of residual material in 5 mL of 0.5 M solution of sulfuric acid at room temperature. Organics were extracted into 3 mL of 1:1 molar solution 2-ethyl hexanol and trihexylamine for GC-MS speciation and quantification. This solvent system has been previously demonstrated to extract formic acid from aqueous formate salt solutions and confirmed here to obtain a distribution ratio of 24.1:1 and 53.4:1 for formic acid and levulinic acid, respectively, with starting salt concentrations of 0.93 for each sample. Magnitude of the kinetic parameters suggests transport limitations are important in calcium salts achieving similar rate constants to diffusioncontrolled processes encountered in epoxy curing, polymer cross-linking, and single crystal diffusion which have preexponential factors on the order of 10−1−104 s−1.30,31 Magnesium salt pre-exponential factors, however, are consistent with unimolecular reactions of 107−1010 s−1. To elucidate the relationship between LA conversion and water evolution, water yield data from Figure 3 was normalized to LA conversion and is shown in Figure 4. Water yields at temperatures between 423 and 623 K support the hypothesis of LA addition/condensation reactions involving the methylketone group. At LA conversion below 50%, a 1:2 molar ratio (water/LA) was observed, consistent with proposed reactions R.1 and R.2. The condensation products, a chelated α,βunsaturated ketone and water, could result from either intramolecular reactions R.1 or intermolecular reactions R.2 resulting in a cross-linked poly salt. Cross-linking may be an important parameter affecting product organic molecular weight distribution during decarboxylation. At increasing LA conversion, water stoichiometry approaches 1:1. This signifies possible secondary condensation reactions between LA methylketone and the product α,β-unsaturated ketone. Such a reaction, if driven to completion, would result in
(1)
where [L] is the normalized abundance of residual LA (mol), k is the Arrhenius rate constant (mol/s−1), A is the preexponential factor (s−1), Ea is the activation energy (kJ/mol), T D
DOI: 10.1021/acssuschemeng.6b02704 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 4. Normalized water yields versus levulinate (LA) conversion for the data points in Figure 3 showing the reaction stoichiometry of LA addition/condensation reactions.
Figure 5. Mass spectrum of a compound which was isolated from acidified MgLA reaction products is consistent with the product of R.4, suggesting the presence of the salt shown in R.3.
with a LA addition/condensation monomer. This monomer undergoes cyclization and lactone formation during acidification/extraction33 by intramolecular addition at the βcarbon (R.3).34 The resulting compound would be susceptible a final ratio of 3:4. The complete loss of noncarboxylate levulinate oxygen indicates possible tertiary condensations. Such a pathway would increase the degree of salt polymerization significantly. Theoretical volatile yield of 13 wt % for CaLA is in agreement with observed volatile yields at 623 K. Triple ketone condensation reactions have been explored by Li et al.32 who showed such reactions yield aromatic hydrocarbons and complete dehydration of ketonic oxygen. Such a mechanism is consistent with aromatic species observed in TDO reactions.11,12,26 Although such a mechanism cannot be confirmed in this work, successive condensation of methylketones within the solid-phase is a significant observation elucidating the deoxygenation pathway present during Stage I volatilizations and determining that evolved water is responsible for as much as one-third theoretical oxygen removal from the biomass-derived organics. Addition/condensation products were identified by GC-MS in residual solids. Six observable peaks are present in the chromatograms at retention times between 75 and 85 min. The predominant peak, 75.5 min RT, had a normalized peak area inversely correlated with LA concentration. The compound was isolated from MgLA salts treated at 523 K, which had a LA conversion of approximately 75%, by column chromatography using a 60 cm long, 2 cm ID buret packed with 29 g silica gel in 60/40 vol/vol ethyl acetate/methylene chloride with 2 g sand beds on each end. The column was loaded with 3 mL of acidified residual product in ethyl acetate solution and fractionated every 10 mL (26 total) and analyzed by GC-MS. The fractions containing the isolated compound were combined and concentrated at 323 K. The mass spectrum of the target compound is presented in Figure 5 and is consistent
to intramolecular condensation under acidic conditions (R.4) which is consistent with the compound identified in the mass spectrum presented in Figure 5. Solid-Phase Reactions in the Infrared. To explore the hypothesis of LA addition/condensation reaction, DRIFTS spectra were obtained under vacuum conditions between 373 and 773 K. The resulting spectra support the hypothesis. During thermal treatment, carbonyl reactivity is observed. As an example, Figure 6 is an overlay of IR spectra obtained from CaLA/FA salts thermally treated between 373 and 673 K. The 373 K spectrum shows a strong absorbance of the LA methylketone carbonyl at 1712 cm−1 (and its overtone at 3417 cm−1) as well as peaks associated with carboxylate and aldehyde carbonyls. At 473 K, absorbance of the methylketone carbonyl, as well as the associated carbon chain skeletal vibrations at 1164 cm−1, are diminished, and new carbonyl E
DOI: 10.1021/acssuschemeng.6b02704 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
ACS Sustainable Chemistry & Engineering
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CONCLUSION Alkaline−earth metal salts comprised of either levulinate or levulinate/formate mixtures are shown to undergo a series of morphological changes and chemical reactions during TDO processing. Slow pyrolysis of the salts produces two stages of reaction. Stage 1 (
