Catalytic Deoxygenation of C18 Fatty Acids Over ... - Springer Link

Top Catal (2011) 54:460–466 DOI 10.1007/s11244-011-9608-y

ORIGINAL PAPER

Catalytic Deoxygenation of C18 Fatty Acids Over Mesoporous Pd/C Catalyst for Synthesis of Biofuels Irina Simakova • Bartosz Rozmysłowicz Olga Simakova • Pa¨ivi Ma¨ki-Arvela • Andrey Simakov • Dmitry Yu. Murzin

•

Published online: 21 January 2011 Ó Springer Science+Business Media, LLC 2011

Abstract Deoxygenation was systematically investigated using either stearic, oleic or linoleic acids as a feedstock at 300 °C under 1 vol% hydrogen in argon over a mesoporous Pd/C (Sibunit) catalyst producing one less carbon containing, diesel-like hydrocarbons. The results revealed that catalyst activity and selectivity increased with less unsaturated feedstock. The main products in the case of stearic acid were desired C17 hydrocarbons, whereas the amounts of C17 aromatic compounds increased in case of oleic and linoleic acids. Catalyst deactivation was relatively prominent in linoleic acid deoxygenation giving only 3% conversion of fatty acids in 330 min. The deactivation originated from the formation of C17 aromatic compounds and fatty acid dimers, which was confirmed by size exclusion chromatographic analysis. The latter compounds were formed via Diels–Alder reaction. Keywords Catalytic deoxygenation Mesoporous carbon Pd/C Fatty acids

I. Simakova (&) O. Simakova Boreskov Institute of Catalysis, Novosibirsk, Russia e-mail: [email protected] B. Rozmysłowicz O. Simakova P. Ma¨ki-Arvela D. Yu. Murzin ˚ bo Akademi University, Process Chemistry Centre, A 20500 Turku, Finland B. Rozmysłowicz Faculty of Chemical Technology, Poznan University of Technology, Poznan, Poland A. Simakov Centro de Nanociencias y Nanotecnologı´a, UNAM, Ensenada, BC, Me´xico

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1 Introduction Development of new technologies to produce biofuel is of interest due to increasing energy demand in the future. Alternative feedstocks compared to fossil fuels should be found, especially in the transportation sector. Biomass gasification and further production of hydrocarbons and oxygenated products via Fischer–Tropsch method is one technology. Another technology for production of so-called biodiesel is transesterification of fatty acids applied especially for synthesis of fatty acid methyl esters. The diesel fuel is composed of long-chained hydrocarbons, and thus it would be beneficial to have biofuels also consisting of only hydrogen and carbon exhibiting thereby similar fuel properties as the conventional diesel. One already commercially available technology is NeXBTL, which utilizes hydrotreatment, the products being hydrocarbons containing the same carbon number as the feedstock. An alternative method, in which hydrogen is not a necessity is catalytic deoxygenation of fatty acids and their derivatives [1–10]. In this method the feedstock is deoxygenated and the main products are hydrocarbons having one carbon less than the original feed. Therefore, in addition to liquid phase products also CO and CO2 were formed as gas phase products. Catalytic deoxygenation for synthesis of biodiesel has been intensively investigated during the recent years [1–14]. Several feedstocks, such as triglycerides, tall oil fatty acids, and several saturated and unsaturated fatty acids and their esters were used as feedstocks. Stearic acid was used as a model compound in the catalytic deoxygenation of fatty acids and their derivatives and an intensive catalyst screening was performed at 300 °C under 6 bar helium over different catalysts prereduced with hydrogen [1]. The best catalysts were Pd and Pt supported on microporous active carbon. The catalytic deoxygenation

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of oleic and linoleic acids was also investigated over microporous Pd/C catalyst at 300 °C in a semibatch reactor under 5 vol% hydrogen in argon using the initial concentration of acid 0.83 mol/l in mesitylene as a solvent [6]. The results revealed that stearic acid was the most prominent product at 35% conversion of unsaturated acids. The selectivities to the desired C17 linear hydrocarbons after 6 h reaction time for oleic and for linoleic acids were 22 and 3%, respectively. One challenge in the catalytic deoxygenation of fatty acids over metal supported on microporous active carbon was the catalyst deactivation [8]. The reasons for catalyst deactivation are poisoning and coking. Poisoning of the active sites was originated from the strong adsorption of formed aromatic compounds on the metal surface. Recenlty, it was demonstrated, that palladium supported on mesoporous synthetic carbon (Sibunit) exhibited high activity and selectivity towards formation of linear hydrocarbons in the catalytic deoxygenation of different saturated fatty acids [9]. Furthermore, this catalyst was used in a continuous deoxygenation of neat stearic acid in a trickle bed reactor giving stable performance with time-on-stream [10]. The aim in this work was to investigate the effect of saturated stearic acid and unsaturated oleic and linoleic acids containing one and two double bonds, respectively, as a feedstock in catalytic deoxygenation over Pd supported on mesoporous carbon.

2 Experimental 2.1 Catalyst Preparation The 1 wt% Pd/C (A) catalyst was prepared by hydrolysis of H2PdCl4 (aqueous) with Na2CO3 (aqueous) at 1:21 molar ratio to form so-called polynuclear hydroxocomplexes of palladium (II) and thereafter depositing palladium(II) onto the carbon surface [11, 12]. The filtered catalyst was washed with distilled deionized water after 6 h of adsorption at room temperature. The catalyst washing was continued until no chloride-ions were present in the washing water. The catalyst was dried at 70 °C overnight and reduced in the hydrogen flow at 150 °C. The second 1 wt% Pd/C (B) (Sibunit) catalyst used only for deoxygenation of technical grade oleic acid was reduced using the following temperature programme: 10 °C/min–60 °C (60 min)– 10 °C/min–300 °C at 4.8 bar of hydrogen. 2.2 Catalyst Characterization The metal dispersion was measured by CO pulse chemisorption using Autochem 2900 (Micromeritics) apparatus.

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The catalyst was reduced prior to the measurement using the following temperature programme: 25 °C–5 °C/min– 300 °C (5 min) and thereafter flushed with helium at 300 °C for 60 min in order to remove the adsorbed hydrogen from the surface. The pulse chemisorption was performed at room temperature with the gas containing 10 vol% CO in helium. The stoichiometry for Pd:CO was assumed to be unity [13]. The metal particle size distribution of the 1 wt% Pd/C catalyst (B) used for only oleic acid deoxygenation was also investigated by TEM (not shown). The Pd content in catalyst samples was determined by Inductively Coupled Plasma Mass Spectrometry (ICP) using Optical Atomic-Emission Spectrometer Optima 4300 DV. For analysis of Pd content a catalyst sample was placed into nitric acid and Pd was dissolved from the catalyst sample by boiling in a sealed vessel during 3 h. Pd loading was practically the same (0.98 and 0.99 wt% for samples A and B, respectively) within the method accuracy. The specific surface area was measured by nitrogen adsorption with Carlo Erba 1900 apparatus. The samples were evacuated for 180 min at 150 °C prior to the nitrogen adsorption. The BET equation and Dollimore-Hill-method were used for determination of the specific surface area and the pore size distribution, respectively. 2.3 Reactor Set-up and Liquid Phase Analysis Different feedstocks (linoleic acid (Fluka), oleic acid, (p.a., Fluka), oleic acid (Fluka, 75096 containing 90 mol% oleic and 10 mol% palmitic acid) and stearic acid (Merck, 97%) were deoxygenated at 300 °C under 1 vol% H2 in argon over 1 wt% Pd/C (Sibunit) in a semibatch reactor using the initial concentration of the reactant 0.15 mol/l and dodecane as a solvent, if not otherwise stated. Technical grade oleic acid was deoxygenated using 5 vol% H2 in argon. In order to perform the experiments under kinetic regime the reaction was carried out under high stirring speed (1100 rpm) as well as using small catalyst particles (below 50 lm). The inert gas was purged through the reactor in order to flush the gaseous products out from the reactor. The liquid phase samples were silylated [8] and analyzed with a gas chromatograph equipped with a HP-5 column (length 30 m, internal diameter 0.32 mm, film thickness 0.25 lm). The following temperature programme was used for analysis: 70 °C (2 min)–05 °C/min–205 °C (40 min)–5 °C/min 285 °C (1 min). The products were confirmed by GC–MS. Size-exclusion chromatographic analysis were performed using three different columns: Jordi Protection column, Earth Igel DVB500A (7.8 mm 9 300 mm) and

123

462

(a)

100

80

mol%

TSK G3000HHR (7.8 mm 9 300 mm), respectively. The samples were diluted with tetrahydrofuran and filtered with 0.2 lm syringe filter (membrane material PTFE, Teflon). The molecules were detected using a LT-ELS detector (Sedex 85, U. S. Low Temperature Evaporative Light Scattering Detector). The quantification was performed using soybean oil as a standard. The data were recorded and analyzed with software (Shimadzu Class-VP (v. 6.12 SP5)).

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Linoleic acid Oleic acid Stearic acid Heptadecane Heptadecene Isomers Aromatics

60

40

20

3 Results and Discussion 0

3.1 Catalyst Characterization Results

0

60

120

180

240

300

360

Time (min)

(b)


100

80

mol%

Two different catalysts were used in the experiments. The pure linoleic, oleic and stearic acids were deoxygenated over catalyst A with the Pd dispersion of 37% and the BET specific surface area of 236 m2/g. The mean pore size of this catalyst was 2 nm. Catalyst B, used in the deoxygenation of technical grade oleic acid had the following properties: the BET specific surface area 379 m2/g, the metal dispersion 38% and the average metal particle sizes for the fresh catalyst was 2.5–2.7 nm according to TEM measurements (not shown). The volume of the pores ranging from 1.5 to 100 nm was 0.839 cm3/g corresponding to 85% of the total pore volume.

60

40

20

3.2 Qualitative Kinetics

0 0

123

120

180

240

300

360

Time(min)

(c)

100

80

mol%

For the catalytic deoxygenation of stearic, oleic and linoleic acids the overall kinetic plots are depicted in Fig. 1. Both oleic and linoleic acids were converted very fast to the primary products, which were C18 fatty acid isomers when using 1 vol% hydrogen in argon as a purge gas. Furthermore, hydrogenation and cyclisation of unsaturated components occurred. The main liquid phase by-products were C17 aromatic compounds, such as undecylbenzene. Traces of fatty acid dimers were also present in the liquid products for oleic or linoleic acids as feedstocks (see below). The most dominant primary products in case of stearic acid deoxygenation were C17 aliphatic hydrocarbons (straight chain alkanes and olefins, i.e. heptadecane and heptedecene), whereas only traces of C18 fatty acid isomers were formed from stearic acid. This result indicated that the dehydrogenation of stearic acid and heptadecene is negligible under hydrogen scarce conditions at 300 °C over Pd supported catalyst. Analogous results have been earlier obtained over microporous Pd/C catalysts [1].

60


60

40

20

0 0

60

120

180

240

300

360

Time (min) Fig. 1 Kinetic dependences of the deoxygenation of a linoleic, b oleic and c stearic acid over 1 wt% Pd/C (Sibunit) at 300 °C under 1 vol% hydrogen in argon. The initial concentration of the reactant was 0.15 mol/l in dodecane and the catalyst loading was 0.5 g

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Table 1 Area percentage of monomers, dimers and trimers presented in the reaction mixture in the catalytic deoxygenation of linoleic and oleic acid over 1 wt% Pd/C (Sibunit) using 1 vol% H2 in argon at 300 °C Species

Linoleic acid as a feed (area %)

Oleic acid as a feed (area %)

Monomers

95.6

98.3

Dimers

1.9

1.1

Trimers

2.4

0.6

(a)

100

Σ Fatty acids (mol%)

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95

90

85 Linoleic acid Oleic acid Stearic acid

3.3 Initial Reaction Rates and TOFs

3.4 Conversions After Prolonged Reaction Times The conversions of fatty acids after 330 min decreased in the following order: stearic acid [ oleic acid [ linoleic acid (Tables 1, 2). Furthermore, the transformation of oleic

Fig. 2 a Conversion of fatty acids, b formation of n-heptadecane and c n-heptadecene as a function of time and c selectivity to C17 aliphatic hydrocarbons as a function of fatty acid conversion in the catalytic deoxygenation of different acids over 1 wt% Pd/C (Sibunit) at 300 °C under 1 vol% hydrogen in argon. The initial concentration of the reactant was 0.15 mol/l in dodecane and the catalyst loading was 0.5 g

80 0

60

120

180

240

300

360

240

300

360

Time (min) Σ C17 aliphatic hydrocarbons (mol%)

(b)

20

Linoleic acid Oleic acid Stearic acid

15

10

5

0 0

60

120

180

Time (min)

(c) Selectivity to C17 aliphatic hydrocarbons (%)

Although the initial reaction rates (during 5 min) for oleic and linoleic acids were 150 fold that of stearic acid these unsaturated fatty acids reacted, however, primarily to either C18 fatty acid isomers or to stearic acid as the main products with deoxygenation being a subsequent step. When comparing the transformation rates of fatty acids (Fig. 2a) it can be stated that the rates for oleic and stearic aicds were about the same, whereas a very fast catalyst deactivation occurred in case of linoleic acid deoxygenation. It can be attributed to the presence of large amounts of dimers and trimers in the latter case (Table 1). The mechanism of the formation of dimers and trimers is discussed in Sect. 3.6. The TOF of the catalytic deoxygenation of stearic acid over the microporous and the mesoporous Pd supported carbon catalysts were compared. When stearic acid was deoxygenated over a microporous 5 wt% Pd/C (Aldrich) catalyst with a metal dispersion 18%, a conversion of stearic acid after 375 min using 0.2 g of catalyst in dodecane at 300 °C was 22% giving the TOF of 0.01 s-1 [4]. In the current case the conversion of stearic acid was 23% in 330 min using 0.5 g of the catalyst with the metal dispersion of 37% giving the same TOF as that of the microporous catalyst. This result indicates that the catalytic activities of 5 wt% Pd/C (Aldrich) and 1 wt% Pd/C (A) were thus about the same.

Linoleic acid Oleic acid Stearic acid

100

90

80

70

60 0

5

10

15

20

Conversion (mol%)

acid continued up to 137 min with the same reaction rate as for stearic acid, whereas linoleic acid transformation rate decreased substantially after 7 min (Fig. 2a).

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Table 2 Kinetic data for the deoxygenation of linoleic, oleic and stearic acids over 1 wt% Pd/C (Sibunit) at 300 °C under 1 vol% hydrogen in argon Selectivity to heptadecane and heptadecene (%)a

Ratio between n-heptadecane to n-heptadecenea

Ratio between n-heptadecane and n-heptadecene to C18 isomersa

Selectivity to aromatic compounds (%)a

3

58

0.25b

0.03

42

Oleic acid

12

78

1.0

0.29

22

Stearic acid

18

97

7.9

Large

3

Fatty acid

Linoleic acid

Conversion of fatty acids after 330 min (%)

The initial concentration of the reactant was 0.15 mol/l in dodecane and the catalyst loading was 0.5 g a

10% conversion of fatty acids

b

3% conversion of fatty acids

3.5 Product Selectivities The formation of linear C17 hydrocarbons as a function of reaction time correlates with the fatty acids conversion, i.e. the higher the conversion the more linear C17 hydrocarbons are formed (Fig. 2b). Furthermore, the selectivities as a function of fatty acid conversion increased with increasing conversion. In case of stearic acid high selectivities to the desired products, linear C17 hydrcarbons, were achieved due to the absence of unsaturated compounds after prolonged reaction times, since only traces of aromatics were initially formed, whereas the opposite situation was found for linoleic acid (Fig. 3a). In the transformation of fatty acids the main products were linear C17 hydrocarbons, but also aromatic C17 hydrocarbons were formed very fast after the deoxygenation. When plotting their formation versus each other, it can be stated that the initial slope for formation of C17 aromatic versus C17 linear hydrocarbons was the same for oleic and linoleic acids, whereas after a longer reaction time more aromatic compounds were formed compared to the case of oleic acid (Fig. 3b). The initial ratio for formation of C17 aromatics to C17 aliphatic hydrocarbons was calculated for each acid and this ratio was relatively high for linoleic and oleic acid, whereas for stearic acid it was only about half of those calculated for oleic and linoleic acids (Table 3). After seven minutes of reaction time, however, the ratio of the formation rate of aromatics to aliphatic C17 compounds was the highest for oleic acid (Table 3). These results originated from the very prominent catalyst deactivation after 5 min reaction time in case of linoleic acid. The formation of linear C17 hydrocarbons declined substantially after 5 min for linoleic acid, whereas the analogous trend was visible for oleic acid after 63 min. The formation of the desired product was relatively constant in the case of stearic acid (except below 5% conversion). The product selectivity was plotted as a function of fatty acid conversion, the trends for stearic and oleic acids were similar (Fig. 2c), whereas a declining selectivity as a

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function of conversion towards the desired products was achieved with linoleic acid. This is due to extensive catalyst deactivation under hydrogen scarce conditions, and high formation of aromatics. The aromatic formation as a function of time is depicted in Fig. 3a showing that the largest amount of aromatics was formed with oleic acid. If the aromatics concentration is plotted against fatty acid conversion, it can be seen that initially the slopes for aromatic compounds formation were the same for linoleic and oleic acids, whereas for stearic acid slightly less amount of aromatic compounds was formed at the same conversion (Fig. 3b). After 7 min of reaction time, however, the formation of aromatic compounds was 1.7 times higher for linoleic acid compared to oleic acid and no more aromatic compounds were formed with stearic acid as a feedstock after 192 min of reaction time (Fig. 3a). These results indicated very clearly that catalyst deactivation and formation of aromatic compounds were closely related to the number of double bonds in the fatty acid feedstock. Analogous trends were observed when plotting the formation of C17 aromatic compounds as a function of C17 linear hydrocarbons (Fig. 3c). 3.6 Reaction Mechanism A schematic reaction network for fatty acid deoxygenation is presented in Fig. 4. From the mechanistic point of view it can be stated that unsaturated fatty acids can be either isomerized or hydrogenated during the initial stages of the reaction. In the second step the primary products formed from the fatty acids were decarboxylated to the linear C17 hydrocarbons, which were both saturated and unsaturated. In addition to these decarboxylation products, also aromatic C17 hydrocarbons were formed from unsaturated linear C17 hydrocarbons via cyclisation and further dehydrogenation. Trace amounts of dimers and trimers were present in the liquid phase using either oleic or linoleic acids as feeds. It is known from litterature that two conjugated linoleic acid molecules can give a cyclohexene

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Aromatic compounds (mol%)

(a)

465

3.0

Table 3 The ratio between the formation rate of aromatic C17 compounds and the formation rate of linear C17 hydrocarbons obtained for different fatty acids

linoleic acid oleic acid stearic acid

2.5

Fatty acid

r0, arom./r0, aliph. (mmol/min)a

r1, arom./r1, aliph. (mmol/min)a

Linoleic acid

45

2.8

Oleic acid

38

8.9

Stearic acid

24

2.2

2.0 1.5 1.0

a

r0 initial rate, r1 after 7 min

0.5

C 17 aromatics

0.0 0

60

120

180

240

300

Time (min)


(b)

3

linoleic acid


-CO 2

oleic acid

+H2

stearic acid

-CO 2

n-heptadecane

Fig. 4 Schematic reaction route for catalytic deoxygenation of C18 fatty acids

2

1

0

5

10

15

20

Conversion of fatty acids (mol%)


+H2

dimers, trimers

0

(c)

n-heptadecene

360

3.0


2.5

based dicarboxylic acid containing 36 carbon atoms in a dimer via Diels–Alder reaction [15]. In addition to the formation of monocyclic compounds, also bicyclic linoleic acid molecules containing bicyclononene derivatives and spiroundecane structures have been reported to be formed from isomers of two linoleic acid molecules [16]. Furthermore, aliphatic dicarboxylic acids could be formed from two oleic acid molecules [17]. The amounts of dimers and trimers were quantified in the present work by size exclusion chromatography, but their structures were, however, not investigated in the current work. 3.7 Catalytic Deoxygenation of Technical Grade Oleic Acid

2.0 1.5 1.0 0.5 0.0 0

5

10

15

20

Σ C17 aliphatic hydrocarbons (mol%) Fig. 3 a Formation of aromatic C17 compounds as a function of time b as a function of fatty acid conversion, c the formation of aromatic C17 hydrocarbons as a function of the formation of aliphatic C17 hydrocarbons in the catalytic deoxygenation of different acids over 1 wt% Pd/C (Sibunit) at 300 °C under 1 vol% hydrogen in argon. The initial concentration of the reactant was 0.15 mol/l in dodecane and the catalyst loading was 0.5 g

Oleic acid containing also palmitic acid as the main impurity was deoxygenated over catalyst B at 300 °C under 5 vol% hydrogen in argon atmosphere (Fig. 5). Close to 87% of oleic acid converted to stearic acid within the first 5 min and only 3 mol% of n-heptadecane was formed during this time. After 5 min of reaction time, when about 83 mol% of stearic acid was present in the liquid phase, the formation of n-heptadecane became more prominent. Furthermore, palmitic acid (C16) being present in the feed, was deoxygenated to n-pentadecane. The total conversion of fatty acids to one carbon less hydrocarbons was 98% within 570 min. When plotting the formation of n-heptadecane versus the formation of n-pentadecane (the graph is not presented here) a linear relationship was acheived, as expected, since an earlier comparison of the deoxygenation rates of stearic and palmitic acids [9] showed that the deoxygenation rate

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Top Catal (2011) 54:460–466 1.0

Molar fraction

0.8 pentadecane heptadecane palmitic acid stearic acid oleic acid

0.6

0.4

0.2

occurred as a consecutive reaction and presence of hydrogen plays the important role for the selective deoxygenation of unsaturated fatty acids. As a comparison, deoxygenation of technical grade oleic acid was investigated at 300 °C in the presence of hydrogen rich gas atmosphere. This feedstock contained besides oleic acid also 10 mol% of palmitic acid as the main impurity. The main products were thus n-heptadecane and n-pentadecane originated from the two fatty acids presented in the initial feedstock. No C18 fatty acid isomers were obtained in the product mixture, since 5 vol% hydrogen was enough to hydrogenate oleic acid to stearic acid followed by its selective decarboxylation to n-heptadecane.

0.0 0

100

200

300

600

Time (min) Fig. 5 Deoxygenation of oleic acid, which contained palmitic acid as an impurity. Conditions: the inital concentration of acids 0.1 M, dodecane as a solvent, 1 wt% Pd/C(Sibunit), catalyst loading 1.4 g, liquid phase volume 70 ml, 300 °C, 12.5 bar of 5 vol% H2 in argon

of fatty acids was not dependent on the hydrocarbon chain length. The main difference in the oleic acid deoxygenation presented in Fig. 1b and in Fig. 5 is the catalyst activity, since in the former case the main product was stearic acid, whereas in the latter case it was n-heptadecane. This result besides the difference on intrinsic activity and lower concentration of oleic acid can be explained by the fact that 1 vol% hydrogen in argon was not enough to hydrogenate oleic acid in the former case. In addition, no isomers of C18 fatty acids were observed with 5 vol% hydrogen, since the hydrogenation reaction proceeded fast.

4 Conclusions Catalytic deoxygenation of C18 fatty acids containing either one or two double bonds or saturated ones, was investigated over 1 wt% Pd supported on mesoporous carbon at 300 °C under 1 vol% hydrogen in argon in dodecane as a solvent. The results revealed that the deoxygenation rates for oleic and stearic acids were the same, whereas in case of linoleic acid as a feedstock the catalyst deactivation occurred already after 5 min reaction time. For oleic acid the transformation rate declined, however, after 140 min due to more extensive catalyst deactivation compared to stearic acid. The reason for deactivation was the formation of C17 aromatic hydrocarbons, as well as in case of oleic and linoleic acids also oligomerisation via Diels–Alder reaction. The main product in case of stearic acid was n-heptadecane, whereas C18 fatty acid isomers and stearic acid were as main products for linoleic and oleic acid, respectively. The formation of the desired product, n-heptadecane

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˚ bo Akademi Acknowledgments This work is part of activities at the A Process Chemistry Centre of Excellence Programmes (2000–2011) financed by the Academy of Finland. Authors are grateful to Mr. Jarl Hemming performing the size exclusion analysis in Process Chemistry ˚ bo Akademi University. The research work was supported in Centre, A part by RFBR Grants No. 08-03-91758-AF, 10-03-01005-a, 11-0394001_INIS_a. The work was also partly supported by DGAPAPAPIIT (UNAM, Mexico) through grant N 224510.

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