Energy Conversion and Management 47 (2006) 3344–3350

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Decomposition of a long chain saturated fatty acidwith some additives in hot compressed water

Masaru Watanabe, Toru Iida, Hiroshi Inomata *

Research Center of Supercritical Fluid Technology, Tohoku University, 6-6-11-404 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan

Received 19 July 2005; accepted 30 January 2006Available online 15 March 2006

Abstract

Stearic acid (C17H35COOH: C17-acid) was treated using a batch reactor with supercritical water (SCW) of 673 K and0.17 g/cm3 for 30 min. In SCW, C17-acid was stable (2% conversion) and the main products were CO2 and C16 alkene. Anaddition of alkali hydroxide (NaOH and KOH) in the SCW reaction enhanced the decarboxylation of C17-acid with themain products being CO2 and C17 alkane. Metal oxides (CeO2, Y2O3 and ZrO2) enhanced the decarboxylation of C17-acidand the main products were CO2 and C16 alkene. Based on the results, the reaction mechanism was proposed.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Stearic acid; Decarboxylation; SCW; Metal oxide catalyst; ZrO2; CeO2; Y2O3

1. Introduction

Vegetable and animal fats/oils, which mainly consist of triglyceride of straight chain fatty acid, are organicchemicals made from solar energy, CO2 and water. Biodiesel made from the fats/oils has been noted as anecological fuel because the fats/oils are a sustainable energy resource. One of methods of biodiesel productionfrom the fats/oils is trans-esterification of triglyceride by methanol (methanolysis) to make methyl esters of thestraight chain fatty acid. Recently, methanolysis was found to proceed without catalyst in supercriticalmethanol [1–6].

Further utilization of the fats/oils may be for fuels and chemical resources as petroleum alternatives. Thefats/oils can be converted to petroleum compounds by effectively separating the carboxylic group withoutbreaking the hydrocarbon chain. Hot compressed water is one of the candidates for treating the fats/oils toproduce petroleum (long chain hydrocarbon) because of its capability to hydrolyze triglyceride into free fattyacid and glycerol without catalyst [7,8]. However, a free fatty acid is stable in subcritical water [7]. Thus, todevelop the petroleum recovery process from fats/oils, decarboxylation of a free fatty acid is a key reaction.

0196-8904/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2006.01.009

* Corresponding author. Tel.: +81 22 795 7283; fax: +81 22 795 7282.E-mail address: inomata@scf.che.tohoku.ac.jp (H. Inomata).

M. Watanabe et al. / Energy Conversion and Management 47 (2006) 3344–3350 3345

Decarboxylation of organic acids in sub- and supercritical water has been studied with and without catalyst[9–11]. The reaction rates of some carboxylic acids in subcritical water were enhanced by alkali hydroxideaddition for the case that the stability of the anionic form of carboxylic acid is lower than that of the non-anionic form [9]. In the previous study, we confirmed that CH3COOH conversion into CH4 and CO2 (Eq.(1)) through monomolecular decarboxylation was enhanced by KOH addition in supercritical water (SCW)[10]. The enhancement was attributed to the promotion of dissociation of CH3COOH into CH3COO� andH+ by adding an amount of alkali hydroxide [12], and decarboxylation of the anionic form is faster than thatof the molecular form.

CH3COOH!CH4 + CO2 ð1Þ

To develop an environmentally benign process, a solid base catalyst that can enhance the decarboxylation ofCH3COOH was favored. We employed ZrO2 as a solid base catalyst because it is stable in SCW [13] and hasbase sites on the surface from CO2-TPD (temperature programmed desorption) measurement [14]. As a result,by adding ZrO2, bimolecular decarboxylation (Eq. (2)) was found to occur with high selectivity in SCW [10].The selective bimolecular decarboxylation was supposed that ZrO2 worked not only as base catalyst but alsoas acid catalyst because of its acidity and basicity on the surface [14].

2CH3COOH!CH3C(O) CH3 + CO2 + H2O ð2Þ

This catalytic reaction can be applied to decarboxylation of longer fatty acids to produce petroleum-like com-pounds (namely hydrocarbon that has a lower oxygen in the structure). If a free fatty acid would be stabilized insupercritical water, even at fairly high temperature, and also activate the selective decarboxylation in the pres-ence of catalyst without decomposing the main structure (straight chain hydrocarbon group), then vegetableand animal fats/oils might be used in the probable production of the diesel fraction (long chain hydrocarbon).

In this study, we conducted catalytic decarboxylation of stearic acid (C17-acid) with alkali hydroxides andsome metal oxides in SCW. C17-acid has a saturated chain in the structure. Normally, the main structure oftriglyceride in a vegetable oil is an unsaturated chain free fatty acid such as oleic, linoleic, linoleinic and so on,however, it is difficult to get a high purity unsaturated free fatty acid as a commercial reagent. Therefore, toknow the decarboxylation mechanism of the long chain free fatty acid, we employed C17-acid in this study. Inaddition to ZrO2, ceria (CeO2) and yttria (Y2O3) were also employed because CeO2 and Y2O3 also have acidand base sites on the surface [15,16]. For understanding the effect of SCW, we also conducted C17-acid pyro-lysis experiments (without water and catalyst). We focused on the conversion of C17-acid and the formation ofCO and CO2 to know the decarboxylation efficiency, which means how much of the amount of oxygen atomsin the C17-acid is released as gaseous compounds. The effects of alkali hydroxide and the metal oxides on thedecarboxylation of stearic acid in SCW were suggested.

2. Experimental

Ceria (CeO2) was obtained from Merck. Yttria (Y2O3) was purchased from Wako Pure Chemicals Co.Zirconia catalyst (ZrO2) was prepared by calcination of zirconium hydroxide at 673 K for 3 h. Zirconiumhydroxide (ZrO2 Æ xH2O) was purchased from Nakarai Tesque Inc. Potassium hydroxide and sodiumhydroxide (both 1 M aqueous solutions) and stearic acid (C17-acid: +95% purity) were purchased from WakoPure Chemicals Co. and used without further purification. Tetrahydrofuran (THF) with a stabilizer was alsoobtained from Wako Pure Chemicals Co. Pure water that was distilled after deionization was obtained by awater distillation apparatus (Yamato Co., model WG-220).

The reaction was performed by use of a SS 316 tube bomb reactor with an inner volume of 6 cm3 as describedin the previous publication [17]. The loaded amount of C17-acid was 0.3 g, and the loaded amount of water was1.0 g (0.167 g/cm3: water pressure at 673 K is about 25 MPa). Metal oxide catalyst (CeO2, Y2O3 and ZrO2) wasintroduced with 0.3 g. In the case of addition of KOH or NaOH, the 1 M aqueous solution was introduced tothe reactor instead of pure water. After loading C17-acid, water and the catalyst, argon gas (Ar) was introducedand pressurized at 1 MPa after purging the air in the reactor. Some experiments without water and catalyst werealso conducted for comparison. The reactor was submerged in a fluidized sand bath (Takabayashi Riko Co.,model TK-3) whose temperature was controlled to keep at the reaction temperature (673 K). The reaction times

3346 M. Watanabe et al. / Energy Conversion and Management 47 (2006) 3344–3350

ranged from 15 min to 120 min. The reactor was taken out of the bath and rapidly quenched in a water bathafter a given reaction time. After the reactor reached room temperature, the stop valve connected to the reactorwas joined to a syringe that was equipped with gas samplers to collect the produced gas and measure its volume.The reactor was opened after the sampling or displacement of the gaseous products and washed with THF torecover the liquid products and solid, including the catalyst. The recovered solution that contained the catalystwas filtered with a membrane to separate the liquid products and the catalyst.

The identification and quantification of the gaseous products were accomplished by GC–TCD (thermalconductivity detector) (Shimadzu, model GC-7A, and Hitachi, model GC163). An external standard wasadded to each water solution for analytical purposes. Identification and quantification of the liquid productsin the THF solution were conducted by GC–MS (mass spectrometry) (JEOL, model JMS-700) and GC–FID(flame ionization detector) (Hewlett Packard, model 6890). The capillary column for GC–MS and FID is HP-5 (30 m · 0.32 mm · 0.25 lm). Conversion of the reactant was evaluated from the amount of reactant recov-ered and the amount loaded. In order to know the amount of oxygen atom in C17-acid released to gaseouscompounds, the yields of CO and CO2 were evaluated as oxygen atom yield, mol%, based on the amountof oxygen atom in CO/CO2 and that in C17-acid. The metal oxide catalysts before and after the reaction wereanalyzed by an X-ray diffractometer (XRD) (Rigaku, model MiniFlex) with Cu Ka radiation.

3. Results and discussion

3.1. Effect of supercritical water on stearic acid decomposition

In order to elucidate the effect of SCW on the C17-acid reaction, we conducted experiments with and with-out SCW. Fig. 1 shows the FID chromatograms of the liquid products of the C17-acid reaction in the absence(Fig. 1a) and the presence of SCW (Fig. 1b). The number displayed next to the peak of C17-acid is the con-version of C17-acid. As shown in Fig. 1, the conversion of C17-acid decomposition in the Ar atmosphere (with-out SCW) was 50%, while that in SCW was 2%. All products were THF soluble and gaseous. This resultsuggests that C17-acid is energetically stabilized by SCW, the same as seen in acetic acid decomposition [10].

The gaseous products were mainly CO and CO2 and the liquid products were large amounts of hydrocar-bons (alkane and alkene) and some amounts of carbonyl compounds (aldehydes, ketone and carboxylic acid)for all the cases. The largest peak on the chromatogram in the case of the Ar atmosphere (Fig. 1a) was C17

alkane, while that in the presence of SCW (Fig. 1b) was C16 alkene. There were two peaks (only alkaneand alkene) at each peak group on the chromatogram in SCW. Several peaks (sometimes four or more peaks)at a peak group, by contrast, were found on the chromatogram in the Ar atmosphere. Fig. 2 shows the effect ofSCW on the CO and CO2 yields. The amounts of CO and CO2 in the Ar atmosphere were almost the same,and the total amount of those was about 8% of the oxygen base of C17-acid. In SCW, the yield of CO plus CO2

was about 0.4%, and the CO2 yield was higher than that of CO.

3.2. Effect of alkali hydroxides on stearic acid decomposition in supercritical water

Experiments in the presence of a KOH or NaOH were conducted to know the effect of the alkali hydroxideon the C17-acid decomposition in SCW. The conversion of C17-acid was increased by adding alkali hydroxide(13% for NaOH and 32% for KOH, see Fig. 1c and d). Precipitation of NaOH or KOH in the recovered THFsolution was found, however, all the organic compounds were possibly dissolved in the THF solution.

Figs. 1 and 2 also show the FID chromatogram and COx (namely CO and CO2) yield, respectively. Themain product in the recovered THF solution was C17 alkane in the case of the KOH and NaOH additions(Fig. 1c and d). Furthermore, the small peaks of carbonyl compounds were found on the chromatogram inthe case of NaOH. The total yield of CO and CO2 in the presence of NaOH (about 0.6%) was as low as thatin the absence of NaOH (0.4%) as shown in Fig. 2, although the conversion of C17-acid in NaOH solution(13%) was six times higher than that without NaOH (2%). In the case of the addition of KOH, the total yieldof CO and CO2 (1.9%) was much higher than that without KOH (0.4%), however, the CO and CO2 yields werestill low, taking into account the difference of conversion between the presence (32%) and absence of KOH(2%).

b) SCW

a) Ar atmosphere

d) KOH

c) NaOH

e) CeO2

16

175~15

f) Y2O3

g) ZrO2

Stearic acid

Retention time

Inte

nsity

2ND

1617

1617

1617

16

17

17

17

16

16

50 %

2 %

13 %

32 %

30 %

62 %

68 %

Fig. 1. FID chromatogram of THF solubles of stearic acid conversion (673 K, 30 min).

0 2 4 6 8 10 12

ZrO2

Y2O3

CeO2

KOH

NaOH

SCW

Ar atmosphere

Oxygen atom yield [%]

COCO2

COCO2

Fig. 2. CO and CO2 yields of stearic acid conversion (673 K, 30 min).

M. Watanabe et al. / Energy Conversion and Management 47 (2006) 3344–3350 3347

3.3. Effect of metal oxides on stearic acid decomposition in supercritical water

A metal oxide such as ZrO2 affects the conversion of acetic acid in SCW [10]. In this study, CeO2, Y2O3 andZrO2 were employed on C17-acid conversion because these metal oxides have acid and base sites on the surface[15,16]. The conversions of C17-acid in the presence of CeO2, Y2O3 and ZrO2 were 30%, 62% and 68%, respec-tively (see Fig. 1e–g, respectively). In the cases of Y2O3 and ZrO2, 2-nonadecanone (2ND: C17H35COCH3)formed in addition to CO, CO2, hydrocarbons and some small carbonyl compounds.

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The FID chromatogram and COx (CO and CO2) yields are shown in Figs. 1 and 2, respectively. The largestproducts peak at the FID chromatogram was C16 alkene for all the cases of addition of the metal oxides(Fig. 1e–g). Formation of CO was less than 2% by adding the metal oxides. The CO2 yields were 1%(CeO2), 8% (Y2O3) and 9% (ZrO2) as indicated in Fig. 2. The order of the CO2 formation was the same asthat of the conversion. This implies that the C17-acid converted with formation of CO2 with the metal oxidesin SCW.

After the reaction, the structure of Y2O3 was found to change into YOOH from XRD (X-ray diffraction)analysis, while the structure of ZrO2 (mainly monoclinic) and CeO2 (cubic) were not significantly changed.The stable form of Y2O3 in high pressure water at 673 K was YOOH [13], and thus, the structure change couldoccur during the reaction. At this moment, it cannot be found whether the promotion of C17-acid decarbox-ylation was by Y2O3 or YOOH. The effect of YOOH on the reaction and the stability of YOOH will bestudied.

3.4. Discussion about mechanism of stearic acid conversion in supercritical water with and without additives

Without SCW, the formation of CO and a lot of carbonyl compounds were found. This shows that the C17-acid mainly decomposed with the dissociating carboxylic group as shown in Fig. 3, not decarboxylation. Theformed long chain carbonyl radical further decomposed into CO or a shorter chain carbonyl compound(Fig. 3). In contrast, by adding SCW, the carboxyl group was stabilized, and the C17-acid decomposed intoCH3COOH and C16 alkene at a slow reaction rate. Since decarboxylation of CH3COOH is very slow inSCW at 673 K [10], the CO2 yield was very low (0.4%) compared with the conversion (2%). In this study,CH3COOH was not observed because the peak of CH3COOH in the FID chromatogram cannot be separatedfrom the large THF peak. Further study will be conducted by focusing on CH3COOH formation in the nearfuture.

In the presence of NaOH and KOH aqueous solutions, C17 alkane was the main product as well as the casewithout SCW. CO formation was accompanied with the C17-acid conversion by adding NaOH. This probablyindicates phase separation between NaOH rich water (liquid like SCW) and C17-acid (bp 560 K; gas). Thus, inthe gas phase, pyrolysis of C17-acid probably proceeded to form CO and a little amount of carbonyl com-pound in the case of NaOH addition. In contrast, C17-acid with KOH-SCW converted into CO2 and C17

alkane mainly. Fig. 4 shows the possible mechanism of C17-acid conversion with and without additives inSCW at 673 K. As shown in Fig. 4, monomolecular decarboxylation of C17-acid proceeded by addingKOH in SCW, as the same as the case of CH3COOH [10].

Fig. 4 also shows the effect of ZrO2 on C17-acid conversion. Since CO2 and C16 alkene formation wasmainly observed in the case of ZrO2, the main part of the C17-acid reaction with ZrO2 was probably decom-position into CH3COOH and C16 alkene. In addition, 2-ND was produced by adding ZrO2. In the case ofCH3COOH decomposition with ZrO2 [10], acetone and CO2 selectively formed via bimolecular decarboxyl-ation between two molecules of CH3COOH. Thus, 2-ND was assumed to be produced by bimolecular decar-boxylation between CH3COOH and C17-acid and by that between two molecules of C17-acid (C35 ketoneformed via the decarboxylation between two molecules of C17-acid further decomposed into C16 alkene and2-ND, possibly).

Now, we are studying the character of hydrocarbons recovered from the decarboxylation as diesel fuel.Also, we are trying to get fuels from the waste vegetable and animal fats/oils by use of the SCW decarboxyl-ation technique.

COOH

CO

CO+

RCHORCOR

Fig. 3. Stearic acid conversion without supercritical water (Ar atmosphere).

2-Nonadecanone

C CH3

O

Low molecularhydrocarbon

CH3CO2H

Acetic acid

CO2 Monomoleculardecarboxylation

Low molecularhydrocarbon

C17 Alkane

C16 Alkene

CO2 + H2O

C

OC35 Ketone

CO2H

Stearic acid

Bimoleculardecarboxylation

Fig. 4. Stearic acid conversion in supercritical water with and without additives.

M. Watanabe et al. / Energy Conversion and Management 47 (2006) 3344–3350 3349

4. Conclusion

Supercritical water stabilized C17-acid as well as CH3COOH. Thus, catalysts were required to promote thedecarboxylation of C17-acid. Alkali hydroxides (NaOH and KOH) were attempted to promote the decarbox-ylation. As the result, KOH was effective for monomolecular decarboxylation into C17 alkane and CO2. Todevelop a green process for decarboxylation of the fatty acid, we also attempted to use metal oxides. As a con-sequence, ZrO2 promoted the decarboxylation, which was probably bimolecular decarboxylation because longchain ketone was obtained.

Acknowledgement

We gratefully acknowledge the NEDO project (ID: 02A44001d) for financial support for this work.

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