Production of Sustainable Diesel via Decarboxylation of PalmStearin Basic SoapsMeiti Pratiwi,*,†,‡ Oki Muraza,*,§ Godlief F. Neonufa,∥ Ronny Purwadi,† Tirto Prakoso,†,‡

and Tatang H. Soerawidjaja*,†

†Department of Chemical Engineering, Institut Teknologi Bandung, Jalan Ganesha 10, Bandung 40132, Indonesia‡Department of Bioenergy and Chemurgy Engineering, Institut Teknologi Bandung, Jalan Let. Jen. Purn. Dr. (HC) Mashudi No.1/Jalan Raya Jatinangor km 20.75, Sumedang 45363, Indonesia§Chemical Engineering Department and Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum andMinerals, Dhahran 30261, Saudi Arabia∥Department of Agriculture Product Technology, Universitas Kristen Artha Wacana, Jalan Adisucipto 147, Oesapa, Kupang 85000,Indonesia

ABSTRACT: Production of sustainable diesel was conducted via hydrogen-free decarboxylation of palm stearin basic soaps.Metal soaps are alkaline earth and transition-metal salts combined with carboxylic acids with 7−22 carbon atoms. Stearin basicsoaps were prepared by direct reaction of palm stearin and mixed metal (Ca, Mg, and Zn) hydroxides. The stearin basic soapswere decarboxylated at 370 °C for 5 h to produce liquid crude bio-hydrocarbon, also known as sustainable diesel. The stearinbasic soaps were characterized by Fourier transform infrared (FTIR) and thermogravimetric analysis, and the resulted liquidbio-hydrocarbons were analyzed by GC equipped with a flame ion detector. The hydroxyl band at 3678 cm−1 observed fromFTIR spectroscopy indicated that the Ca/Mg/Zn ions were associated with the −OH ions in the compounds. This proved thatthe soaps produced from this work were basic metal soaps. The thermal stability of the soaps was examined up to 1000 °C, andthe decomposition of stearin basic soaps was observed in the range of 300−500 °C. The metal contained in the basic soapsaffected their thermal characteristics. Liquid crude bio-hydrocarbon with carbon chain length between 8 and 20 has beenobtained from decarboxylation of stearin basic soaps. In this study, the decarboxylation of stearin basic soaps resulted insustainable diesel as the main product. This promising process is expected to open a plethora of opportunities in the productionof sustainable diesel.

1. INTRODUCTION

Nowadays, the development of sustainable biofuel productionto mitigate greenhouse gas emissions and global warmingissues received significant attention.1 In the last decade, theworld biofuel production has increased exponentially.2

Production of first-generation biofuels is mainly based onagriculture raw material such as edible3 and nonedible plants.4

The first generation of biodiesel which consists of fatty acidmethyl esters (FAME) has limitation in the blending volumewith the diesel oil. These issues have attracted researchers tosynthesize alternative liquid fuel that has similar structure tothe petroleum-derived diesel, which is the bio-hydrocarbon,also known as green diesel.The most common bio-hydrocarbon production process is

hydrodeoxygenation (HDO). The HDO process has beenstudied intensively and commercially applied by UOP/Eni andNeste oil.5,6 However, this process is expensive and onlyapplicable for large scale because this method requireshydrogen gas, uses noble catalysts, for example, Pd7,8 orPt,9,10 and needs high-pressure operation. Besides, bio-hydrocarbon produced through decarboxylation of fatty acidsis also studied widely.11−14 The principal reaction ofdecarboxylation of fatty acid is the removal of carboxyl groupsof fatty acid chain to obtain hydrocarbons. For example,

palmitic acid (C16:0) decarboxylation generates pentadecane(C15), diesel fuel constituent (see reaction 1).

− − → − − +CH (CH ) COOH CH (CH ) CH CO3 2 14 3 2 13 3 2(1)

The decarboxylation of fatty acids takes place using the samecatalyst as the HDO catalyst, Pd,11−13 and Pt.15 Besides thesetwo noble metals, Ni can be employed as a deoxygenationcatalyst for vegetable oil/fats and fatty acids.16 However, toobtain high conversion and selectivity of bio-hydrocarbon, thisprocess still requires hydrogen to establish the reaction. Thehydrogen-free decarboxylation of fatty acids potentiallygenerates products that are still contaminated with thereactant.14

Because of those drawbacks, several researchers proposed analternative pathway to produce bio-hydrocarbon by decarbox-ylation of basic metal soap [formula: M(RCOO)(OH); M =divalent metal; R = alkyl] which was carried out at milderreaction condition compared with HDO of triglycerides andcatalytic decarboxylation of fatty acids. The decarboxylation ofmetal basic soap reaction is written in reaction 2 and the

Received: July 23, 2019Revised: October 13, 2019Published: October 14, 2019

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reactions involved in this process is shown in Figure 1 usingtripalmitin as saponification feed.

→ +M(RCOO)(OH) RH CO2 (2)

Liquid bio-hydrocarbon was obtained via decarboxylation ofcalcium soap from rapeseed, peanut, and tung oil.17 Thedecarboxylation of basic metal soaps has been observed byNeonufa et al.18 at atmospheric pressure, with the combinationof the Mg−Fe and Mg−Zn metal has shown that Zn gave apositive effect to the decarboxylation process. The basic metalsoaps used in their experiment were basic soaps from themetathesis process. Metathesis reaction or double decom-position reactions is the common method for the metal soapsaponification process. This reaction produces metal soaps byreacting an aqueous solution of water-soluble metal salt withsodium or potassium salt of oil/fat or fatty acids. Thedevelopment and commercialization of the metathesis processis difficult because this process generated a large amount ofwaste water in the purification step19 and it could create waterpollution problem. Therefore, in this research, saponificationwas performed by direct reaction of palm stearin and mixedmetal hydroxides, as described elsewhere.20

The methodology of this study is shown in Figure 2. Thefirst stage is the saponification process of palm stearin and Ca/Mg/Zn hydroxides. Then, proceeded with the purificationprocess of basic metal soaps to remove glycerol, fatty acids, andwater. We studied the combination of Ca/Mg/Zn basic metalsoaps. The metals used in this experiment have their respectiveroles. Magnesium (Mg) as the decarboxylation catalyst21 andzinc (Zn) as the isomerization catalyst.22 Calcium soap has ahigh decomposition temperature of 315−490 °C for Ca-(CH3COO)2,

23 thus having a positive effect on thedecarboxylation process, which is expected to withstand thebasicity of metal soaps at decarboxylation temperature (300−400 °C).The aims of this research were to obtain basic metal soaps,

via direct reaction, to investigate the characteristics of basicmetal soaps produced, and to determine the effect of differentmetals to the type of bio-hydrocarbon resulted. Results fromFourier transform infrared (FTIR) and thermogravimetric

analysis (TGA) of basic metal soaps, and also compoundidentification of the liquid bio-hydrocarbon product werediscussed.

2. EXPERIMENTAL SECTION2.1. Materials. Palm stearin was obtained from local oil palm

plantation in Indonesia. Acid value (AV) and saponification value(SV) of palm stearin were determined using the AOCS method (Cd3d-63 and Cd 3c-91). The fatty acid composition of palm stearin wasdetermined with Shimadzu GC-2010 gas chromatography with flameion detector (FID) and capillary column FAMEWAX with length of30 m and inner diameter of 0.25 mm and using FAME mixSUPELCO (C8:0−C22:0) as standard. The fatty acids in this palmstearin are myristic acid (1.45% mol), palmitic acid (68.24% mol),stearic acid (5% mol), oleic acid (20.63% mol), and linoleic acid(4.68% mol) with AV and SV analysis results of 0.62 and 206 mgKOH/g, respectively.

The Ca/Mg/Zn, or mixed metals, hydroxides (Mmix(OH)2) wereprepared by co-precipitation of Ca(NO3)2·4H2O, Mg(NO3)2·6H2O,and Zn(NO3)2·6H2O with NaOH (99%, Merck), modified fromprevious research.20 Ca/Mg/Zn−stearin basic soaps (Mmix−stearin)were synthesized by direct reaction of palm stearin with Mmix(OH)2.The metal compositions of Mmix−stearin were made based on % mol,with fixed mole of Zn as 50% mol, which are shown in Table 1.

2.2. Saponification Process of Palm Stearin Basic Soaps.The saponification reaction was performed in 100 mL SS autoclaveand modified from the Blachford method.24 A typical 20 g of palmstearin, an excess of Ca/Mg/Zn metal hydroxides, and 10 mL of waterwere fed to the saponification reactor. After closing the reactor,nitrogen was flowed into the reactor to remove the remaining airinside the reactor. The saponification reaction was carried out at 185°C for 3 h under stirring. The saponification reaction of stearin basicsoaps by direct reaction is also shown in Figure 1.

Figure 1. Schematic sustainable diesel production from metal basic soaps, illustrated using tripalmitin as saponification reaction feed.

Figure 2. Research methodology.

Table 1. Composition of Ca/Mg/Zn in % mol

sample Ca Mg Zn

A 7.5 42.5 50B 17.5 32.5 50C 25 25 50D 32.5 17.5 50E 42.5 7.5 50

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Because glycerol is formed as a byproduct and is soluble in waterand free fatty acids were soluble in methanol, the saponificationproduct was filtered using a Buchner funnel. To remove most of theglycerol and free fatty acids, wet basic soaps were washed with hotwater and then with methanol. The wet basic soaps were dried in theoven at 60 °C, overnight. The dried Mmix−stearin basic soaps wereground using a mortar.2.3. Characterization of Palm Stearin Basic Soaps. The FTIR

instrument, Bruker Alpha-platinum FTIR spectrometer with platinumdiamond sampling, was used to identify the functional groupsconsisted in Mmix−stearin basic soaps. This analysis used the Opusprogram to process the infrared spectra at wavelength 4000−500cm−1. The thermal characteristic of basic soaps was determined byTGA (Linseie Simultaneous Thermal Analysis, platinum series), inwhich basic soap samples (8−10 mg) were loaded into crucible andheated from room temperature to 900 °C at 10 K/min under 40 mL/min nitrogen flow.2.4. Decarboxylation of Palm Stearin Basic Soaps. The

decarboxylation reaction of Mmix−stearin basic soaps was performedin a 100 mL glass batch reactor (destructive distillation apparatus) atatmospheric pressure. The decarboxylation process was carried outfrom room temperature to 370 °C and it was maintained for 5 h. Theheat was supplied and maintained in the reactor by using an automaticheater system with thermocouples and controller. To remove theremaining air in the decarboxylation reactor, nitrogen was flushed intothe reactor. When the decarboxylation temperature was reached, theproducts evaporated into the vapor and gaseous phase. Thoseproducts were cooled by a condenser tube to get the liquid products.These products were stored in a beaker glass.Palmitic acid (C16:0), which is the major fatty acid in palm stearin,

approximately 68.24% mol. Hypothetically, the dominant product ofMmix−palmitic basic soap decarboxylation is normal pentadecane (n-C15); n-pentadecane is one of the main hydrocarbons in diesel-fuel.2.5. Analysis of Decarboxylation Liquid Product. The liquid

crude bio-hydrocarbon was analyzed using the Ferrox paper test inorder to provide qualitative information on oxygen functional groupsor or oxygenates, such as alcohol, aldehyde, ketone, and so forth. Thefilter paper was immersed into ferric thiocyanate solution in methanolto obtain dark green paper, and then it was dried at roomtemperature. The paper was cut into a small square (5 × 5 mm).In a test tube, a piece of Ferrox paper was stirred with a few drops ofthe liquid. The appearance of liquid became thick red wine (orreddish purple), which indicated that the components in the liquidcontained oxygen. The hydrocarbon compositions of liquid productswere determined by gas chromatography Shimadzu 2010 equippedwith a FID and a Restex-1 capillary column with dimensions of 30 ×0.25 mm × 0.25 μm, using He as a carrier gas. Injector and detectorport temperature was 340 °C. The column temperature wasprogrammed from 40 to 300 °C at a rate of 5 °C/min andsubsequently was raised to 315 °C at a rate 1 °C/min and kept at thattemperature for 10 min. Standard alkane (C8−C20) was used for

identification of retention time and peak area calibration. Thedetermination of 1-alkene based on Maher et al.25 and determinationof i-paraffin (isomer) based on Zeng et al.26

3. RESULTS AND DISCUSSION

3.1. Palm Stearin Basic Soaps. The stearin basic soapsobtained from direct reaction had hard solid form at roomtemperature and exhibited yellowish-white color. After washingand drying processes, it was easier to ground the saponificationproducts. The saponification reaction was carried out at thetemperature of 185 °C because this reaction could only becarried out at the temperature above the melting point of themetal soaps.14 The AV of saponification products (beforewashing and drying) was slightly higher than AV of palmstearin. This confirmed that the saponification reaction occursin the two stages of reactions: hydrolysis reaction andsaponification reaction, whereas the triglycerides are hydro-lyzed into free fatty acids and the reaction between fatty acidsand metal hydroxides to form metal basic soaps. The overallreaction from these reactions is summarized in Figure 1 (seeSection 2.2).Figure 3 shows the infrared spectra of five (5) Mmix−stearin

basic soaps. The spectra peaks of Mmix−stearin basic soaps atabout 1568 and 1453 cm−1 were observed for all samples.These bands were assigned to asymmetric and symmetricvibration stretching of the carboxylic group coordinated to themetal ions (νaCOO

− and νsCOO−), respectively. Antisym-

metric and symmetric methylene stretching bands (νaCH2 andνsCH2) were observed at about 2915 and 2849 cm−1.27 Thecarboxylic group and methylene as alkyl chain are thefunctional groups consisted in the Mmix−stearin basic soapsstructure.The broad band between 3000 and 4000 cm−1 appears as

the hydroxyl group stretching (νOH) of zinc hydroxides andphysically adsorbed water.28 The broad band observedbetween 3000 and 3600 cm−1, centered at about 3391 cm−1

is attributed to the hydroxyl (−OH) bond of absorbed water.The hydroxyl group stretching vibration in the crystal structureof magnesium hydroxide and calcium hydroxide is at 3668 and3642 cm−1.29 The small peak observed at around 3678 cm−1

belongs to the hydroxyl bond stretching vibration in thesamples structure. The band at about 717 cm−1 can be relatedto the hydroxyl bending (δOH).

28 The hydroxyl groupstretching and bending detected in samples structure showsthat the metal soaps produced are basic metal soaps.

Figure 3. IR spectra of Mmix basic soaps.

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The change in mass applied as a function of temperature isindicated, highlighting the difference between the behaviors ofthe samples when subjected to a constant heating rate (10 K/min). Figure 4 shows the thermal decomposition of the Mmix−stearin basic soaps. The small mass loss around 4−5% massfrom 100 to 115 °C is attributed to moisture loss associatedwith the soaps. The main mass loss, or the first decompositionprocess, of these basic soaps occurred in the temperature rangeof 300−500 °C, except for sample E, whereas the main weightloss started occurring at the higher temperature of 400 °C. Thelatter sample has higher decomposition temperature than othersamples because sample E has the highest Ca content in themetal soaps. The first decomposition process of basic metalsoaps involved the formation of hydrocarbons and metalcarbonates. The decomposition temperature of magnesiumcarbonate and zinc carbonate into magnesium oxide and zincoxide are above 35030 and 250 °C.31 Metal soaps with excessof Mmix(OH)2 was employed in this experiment, thereby in thistemperature range also occurred decomposition of Zn(OH)2at 260−350 °C, Mg(OH)2 at 285−380 °C, dan Ca(OH)2 at325−415 °C.23 This result shows that there were threedecomposition reactions occurring in the first decompositionprocess, the decomposition reaction of Mmix−stearin basicsoaps, decomposition reaction of magnesium carbonate andzinc carbonate, and Ca/Mg/Zn-hydroxides.The second decomposition process occurred in the

temperature range of 600−700 °C, whereas the decompositionreaction of calcium carbonate into calcium oxide and carbondioxide occurred, it was noted that CaCO3 decomposed intoCaO at 605−725 °C.23 The mass of the sample was stablewhen the temperature was above 700 °C. The highest residualmass of the Mmix−stearin basic soaps, which consist ofrespective metals oxides, was sample E. This residual masshas a high content of metal oxides because the saponificationreaction was held with excess metal hydroxides.

3.2. Decarboxylation Products. The products of Mmix−stearin basic soap decarboxylation are liquid bio-hydrocarbons,water, solid residues, and others, including gas. Table 2presents the composition of Mmix−stearin basic soapdecarboxylation products. The similar products were alsoobserved decarboxylation products show that Ca/Mg/Znmetal combination has a good catalytic activity to decarbox-ylation reaction of Mmix−stearin basic soaps into bio-hydrocarbons. This research proves that decarboxylation ofbasic metal soaps could produce bio-hydrocarbon and thisprocess does not depend on the noble catalyst (Pt and Pd) andthe reaction was carried out at atmospheric pressure.This process also produced liquid bio-hydrocarbons (52−

58% by weight) as the main product. The yield of liquid bio-hydrocarbons was slightly lower than liquid bio-hydrocarbonsgenerated from decarboxylation of basic Mg−Zn stearin32

because of the decarboxylation temperature difference. Thisstudy was carried out at temperature above 20 °C fromdecarboxylation reaction temperature performed by Neonufaet al.,32 so it might result in higher gas formation which led to ahigh amount of gas and loss products in this study (11−23%by weight).The Ferrox paper test of these crude liquid bio-hydro-

carbons gave a positive result. This result indicates that theliquid products contain oxygenated compounds. The oxy-genated compounds that were predicted to be present in theliquid product was traces of ketone. The trace of ketone mightcome from decarboxylation side reaction and presumed to beformed from the free fatty acids contained in the basic metalsoaps and at a temperature above about 340 °C, MgO wascapable to catalyze the decarboxylation of free fatty acids intoketones.33 Mmix−stearin basic soaps contained an amount offree fatty acids (AV > 5 mg KOH/g sample), thus allowing theoccurrence of side reaction as described by Tani et al.34 asfollows

+ ′ → ′ + +RCOOH R COOH RCOOR H O CO2 2 (3)

Figure 4. TGA curve of Mmix basic soaps.

Table 2. Mass Balance for the Decarboxylation Products Formed after a 5 h Reaction at 370 °C

sample liquid biohydrocarbon (% wt) water (% wt) solid residues (% wt) balancea (% wt) biohydrocarbon yield (% wt)

A 31.1 ± 2.8 1.6 ± 0.4 44.4 ± 5.3 22.8 ± 2.1 57.7 ± 5.1B 29.9 ± 3.8 0.6 ± 0.1 53.3 ± 2.3 16.2 ± 1.4 55.8 ± 7.1C 28.1 ± 4.5 0.3 ± 0.1 51.7 ± 6.4 19.9 ± 1.8 52.8 ± 8.4D 30.5 ± 0.4 2.8 ± 0.2 45.0 ± 2.7 23.0 ± 3.8 57.5 ± 0.7E 27.1 ± 0.9 0.4 ± 0.1 57.5 ± 1.7 11.6 ± 2.8 51.5 ± 1.7

aBalance = gas products and product loss. The values presented: average ± standard error, duplicate runs.

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Liquid crude bio-hydrocarbon obtained from decarboxyla-tion of Mmix−stearin basic soaps at 370 °C for 5 h was analyzedusing GC-FID. Figure 5 shows the typical bio-hydrocarbonproducts, decarboxylation of Mmix−stearin basic soapsproduced liquid bio-hydrocarbon with carbon chain lengthfrom C8 to C19. The liquid bio-hydrocarbon productsobtained from decarboxylation of basic soap are divided intotwo types of hydrocarbon, paraffinic (n-alkane and i-alkane),and olefins (1-alkenes). The distribution of liquid bio-hydrocarbon based on the type of hydrocarbons is shown inFigure 6.

The i-paraffin group is the most desirable hydrocarbongroup in the diesel fuel because though the cetane number of i-paraffin is lower than its n-paraffin parent its freezing point ismuch lower. ZnO was the hydrocarbon isomerizationcatalyst35 and relatively the constant level of i-paraffin, whichwas unaffected by Ca and Mg content, confirmed that amongthe three metals present in the hydroxide mixture, Zn activelycatalyzed the isomerization of paraffin.Considering that the main fatty acids of palm stearin are

palmitic and stearic acids, if only the decarboxylation reactionoccurs, the amount of n-paraffin in bio-hydrocarbons should be

significantly greater than the amount of the olefin. Thus, theincrease of olefin amount close to the n-paraffin amount mustbe due to reaction other than decarboxylation. Thedehydration of magnesium basic soaps generated partiallydehydrated magnesium basic soaps (and water), and thedecarboxylation of these basic soaps will produce a mixture ofalkanes and alkenes, depending on its basic soap dehydrationdegree.36

The quantity of olefin produced in this study was greaterthan previous experiment,18 which could be due to differentdecarboxylation operation conditions. Nevertheless, olefinscontained in bio-hydrocarbon liquid products were single-bond alkene with a lower freezing point. Single-bond olefinsfrom C12 to C20 have a slightly lower cetane number ascompared with the n-paraffin parent.37,38 Liquid bio-hydro-carbons with single-bond alkene about 35% are still accept-able.39

Figure 7 shows that the liquid bio-hydrocarbons product wasdistributed from C8 to C19, with the main compound in theliquid bio-hydrocarbon products was n-C15, n-pentadecane(11−13% mol) for all samples. It has been predicted to be themain product of palm stearin-based decarboxylation, as themajor fatty acids of palm stearin were palmitic acids (seeFigure 1). However, the amount of n-pentadecane obtainedwas considerably below the palmitic acid content in the palmstearin (63% mol). This result indicated that decarboxylationreaction of Mmix−stearin basic soaps occurred, and it wasaccompanied with other reactions.The formation of small alkanes and alkenes (<C15) was

probably because of the further decomposition of the long-chain bio-hydrocarbons that were generated previously. Whenan organic compound (e.g., hydrocarbon) decomposes, it wasbroken up into two or more radicals depending on the numberof bonds in the molecule then it may either decompose into anolefin and a smaller free radical or it may react with thesurrounding molecules to form hydrocarbons and generate anew free radical.40 Palm stearin composed of even chain fattyacids, but the decarboxylation products were composed of odd

Figure 5. GC-FID chromatogram showing the typical bio-hydrocarbon products, n-C15 as the main product of Mmix−stearin basic soapsdecarboxylation. The reaction was conducted in atmospheric pressure and at 370 °C for 5 h.

Figure 6. Types of liquid bio-hydrocarbon from Mmix−stearin basicsoap decarboxylation. Bars are the averages; error bars represent themaximum and minimum of duplicate runs.

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and even hydrocarbons. Apparently, there has been arearrangement or disproportionation reaction. The lower bio-hydrocarbon formed by primary decomposition (decarbox-ylation reaction of metal basic soaps) followed by secondarydecomposition, such as cracking of bio-hydrocarbon, and thehigher bio-hydrocarbon formed immediately after secondarydecomposition occurred, followed by tertiary reactions,polymerization reactions, of short-formed carbon molecules.

4. CONCLUSIONSBased on the infra-red analysis, the direct reaction of palmstearin and mixed Ca/Mg/Zn hydroxides has been successfullysynthesized by the metal basic soaps. These basic soaps havedecomposition temperature at 300−500 °C, except for sampleE, which has a higher initial decomposition temperature at 400°C. The increase of calcium content in basic soaps increasedthe initial decomposition temperature. Decarboxylation ofMmix−stearin basic soaps produced the liquid crude bio-hydrocarbons and the obtained products contained C15 (greendiesel) as the main product. The decarboxylation wasaccompanied by other related reactions such as dehydrogen-ation. The work will open further research to optimize thecontent of mixed metals in basic soap decarboxylationphenomenon and/or the decarboxylation condition.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: mei@che.itb.ac.id.*E-mail: tatanghs@che.itb.ac.id.*E-mail: omuraza@kfupm.edu.sa.

ORCIDOki Muraza: 0000-0002-8348-8085NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This publication was supported by Institut Teknologi Bandungresearch grant awarded in the scheme of Research, CommunityService and Innovation Program 2018.

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Figure 7. Distribution of liquid bio-hydrocarbon from Mmix−stearin basic soaps decarboxylation. Bars are the averages; error bars represent themaximum and minimum of duplicate runs.

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Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.9b02427Energy Fuels 2019, 33, 11648−11654

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