Catalytic Deoxygenation of Fatty Acids and Their ... - ACS Publications

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Energy & Fuels 2007, 21, 30-41

Catalytic Deoxygenation of Fatty Acids and Their Derivatives Pa¨ivi Ma¨ki-Arvela, Iva Kubickova, Mathias Snåre, Kari Era¨nen, and Dmitry Yu. Murzin* Process Chemistry Centre, A° bo Akademi UniVersity, Biskopsgatan 8, FIN-20500 Turku, Finland ReceiVed September 7, 2006. ReVised Manuscript ReceiVed October 9, 2006

Catalytic deoxygenation including decarboxylation/decarbonylation of fatty acids and their esters has been investigated over Pd supported on active carbons in a semibatch reactor. The main studied parameters were catalyst acidity, type of feed, effect of solvent, and gas atmosphere. High yields of the desired product, n-heptadecane, were achieved in the decarboxylation of stearic acid at 300 °C under helium. The results demonstrate that the catalytic transformation of fatty acids proceeded mainly via decarboxylation, whereas decarbonylation was the main route for esters, according to gas-phase analysis. The gas atmosphere and the acidity of the catalyst were important factors for determining product selectivity. Over alkaline catalysts as well as under a hydrogen atmosphere, higher yields of n-heptadecane were obtained. The decarboxylation of unsaturated fatty acids leads to hydrogenated products, which reacted further to hydrocarbons.

1. Introduction The utilization of renewable raw materials for the production of diesel components is an important research area, because the global energy consumption is expected to increase from 350 EJ in 1995 to 900 EJ in 2040 (E stands for exa, equal to 1018).1 According to a sustainable growth scenario, part of the fossil fuel should be replaced by renewable sources, such as wood, grain, and oilseeds.2 The share of alternative fuel should be 5.75% in 2010.3 Potential feedstocks for the synthesis of biodiesel from saturated fatty acids are, e.g., palm oil and triglycerides. Saturated fatty acids are more attractive constituents in vegetable oils than naturally occurring unsaturated ones, because the latter ones are less prone to autoxidation.4 Under the term biodiesel, different types of products, such as fatty esters5 and hydrocarbons, are considered.6 Fatty esters can be produced via several methods; e.g., transesterification of oils with alcohols can be performed over homogeneous acid and alkaline catalysts,7 heterogeneous catalysts,8 or with enzymes.7 Hydrocarbons have been previously synthesized from carboxylic acids by using homogeneous transition complexes as catalysts9 or recently over heterogenized homogeneous metal complexes.10 Heterogeneous catalysts have been applied in pyrolysis11 or * To whom correspondence should be addressed. E-mail: [email protected]. (1) Okkerse, C.; van Bekkum, H. From fossil to green. Green Chem. 1999, 107-114. (2) Eggersdorfer, M.; Meijer, J.; Eckers, P. FEMS Microbiol. ReV. 1992, 103, 355-364. (3) Scho¨pe, M. Macroeconomic EValuation of Rape CultiVation for Biodiesel Production in Germany; Scho¨pe, M., Britschkat, G., Eds.; Ifo Schnelldients, 27.03.2002. (4) Falk, O.; Meyer-Pittroff, R. The effect of fatty acid composition on biodiesel oxidative stability. Eur. J. Lipid Technol. 2004, 106, 837-843. (5) Ramadhas, A. S.; Jayaraj, S.; Muraleedharan, C. Biodiesel production from high FFA rubber seed oil. Fuel 2005, 84, 335-340. (6) Kubickova, I.; Snåre, M.; Era¨nen, K.; Ma¨ki-Arvela, P.; Murzin, D. Yu. Hydrocarbons for diesel fuel via decarboxylation of vegetable oils. Catal. Today 2005, 106, 197-200. (7) Fangrui, M.; Milford, A. H. Biodiesel production: A review. Bioresour. Technol. 1999, 70, 1-15. (8) Xie, W.; Oeng, H.; Chen, L. Transesterification of soybean oil catalyzed by potassium loaded on alumina as a solid-base catalyst. Appl. Catal., A 2006, 300, 67-74. (9) Foglia, T. A.; Barr, P. A. Decarbonylation of fatty acids to alkenes in the presence of transition metal complexes. J. Am. Oil Chem. Soc. 1976, 53, 737-741.

catalytic decarboxylation of fatty acids or esters for the production of hydrocarbons.6,12 Fatty acid methyl esters have been pyrolyzed at 400 °C to linear hydrocarbons over activated alumina,11 whereas catalytic decarboxylation of fatty acids has been demonstrated in ref 6 to occur at around 300-360 °C over Pd/C catalysts. Furthermore, it can be mentioned that thermodynamically stable microemulsion of water, vegetable oil, methanol, and surfactant has been applied as diesel fuel.13 The quality of biodiesel is defined by the cetane number, which gives a dimensionless measure of the ignition delay time of a diesel fuel from injection into the combustion chamber.14 A high cetane number (100) exhibiting the high combustion quality of a fuel is given to n-hexadecane. When the biodiesel quality of fatty esters is compared to hydrocarbons, it can be observed that branched esters of vegetable oils, such as isopropyl soyate, exhibit relatively low cetane numbers, i.e., 52.6,15 although higher cetane numbers have been reported for stearic acid methyl and 2-ethylhexenal esters, 101 and 115.5, respectively.14 Hydrocarbons have high cetane numbers, and they are decreasing linearly with the carbon number.16 Catalytic deoxygenation of vegetable oils is an important reaction for the production of biodiesel. This reaction has been, however, scarcely investigated.6,12,17 Decarboxylation of hep(10) Tanaka, S.; Shimizu, K.; Yamamoto, I. Synthesis of 1-nonene from decanoic acid by polymer-bound palladium complexes. Chem. Lett. 1997, 12, 1277-1278. (11) Boocock, D. G. B.; Konar, S. K.; Glaser, G. The formation of petrodiesel by the pyrolysis of fatty acid methyl esters over activated alumina. Prog. Thermochem. Biomass ConVers., 5th 2001, 2, 1517-1524. (12) Snåre, M.; Kubickova, I.; Ma¨ki-Arvela, P.; Era¨nen, K.; Murzin, D. Yu. Heterogeneous catalytic deoxygenation of stearic acid for production of biodiesel. Ind. Eng. Chem. Res. 2006, 45, 5708-5715. (13) Knothe, G.; Dunn, R. O.; Bagby, M. O. Biodiesel: The Use of Vegetable Oils and Their DeriVatiVes as AlternatiVe Fuels, Fuels and Chemicals from Biomass; Washington, D.C., 1997. (14) Knothe, G.; Matheaus, A. C.; Ryan, T. W., III Cetane numbers of branched and straight-chain fatty esters determined in an ignition quality tester. Fuel 2003, 82, 971-975. (15) Zhang, Y.; van Gerpen, J. H. Combustion analysis of esters of soybean oil in a diesel engine. Performance of alternative fuels for SI and CI engines, SAE technical paper series 96075, SP-1160; Society of Automotive Engineers, 1996; (special publication) SP, pp 1-15. (16) Heck, S. M.; Pritchard, H. O.; Griffiths, J. F. Cetane number vs. structure in paraffin hydrocarbons. J. Chem. Soc., Faraday Trans. 1998, 94, 1725-1727.

10.1021/ef060455v CCC: $37.00 © 2007 American Chemical Society Published on Web 12/15/2006

Catalytic Deoxygenation of Fatty Acids

Energy & Fuels, Vol. 21, No. 1, 2007 31

Table 1. Catalyst Characterization Results

d

code

catalyst

BET-specific surface area of the catalyst (m2/gcat)

I II III IV

Pd/C (Aldrich) Pd/C (Degussa) Pd/C (BAC) Pd/C-JM

1126 715 826 1007d

pH of the catalyst slurry

D (%) (by CO chemisorption)

10.2 7.2 5.9 6.4a

18a, 9b 18a, 29c 11a, 18c 24b, 12e

a Reduced at 200 °C. b Reduced at 350 °C. c Reduced at 100 °C. Support. e Reduced at 300 °C.

tanoic acid to octane was demonstrated to occur in the gas phase over Pd/SiO2 and Ni/Al2O3 catalysts.17 In the former case, the yield of octane from the decarboxylation of heptanoic acid was 98% at 330 °C, whereas over the latter catalyst, the yield of octane was 64%, when the reaction temperature was 180 °C.17 In our recent work, decarboxylation of stearic acid and ethyl stearate in liquid phase over the Pd/C catalyst within a temperature range of 300-360 °C in dodecane was demonstrated.6 The highest yields of n-heptadecane in the decarboxylation of stearic acid were close to 60% in a hydrogen/argon atmosphere at 300 °C.6 The reaction atmosphere had an influence on the catalyst activity, because in the presence of hydrogen, higher conversions of ethyl stearate were obtained compared to under an inert atmosphere. The selection of an active metal was studied in ref 12. Supported Pd and Pt catalysts were superior metals compared to other tested metals. The aim in this work is to investigate several types of palladium catalysts supported on active carbons. Other parameters studied in this work are solvent selection, type of feed, and reaction atmosphere. In particular, to elucidate further the reaction mechanism, several feeds were tested. Both saturated (behenic, stearic, and nonanoic acid) and unsaturated acids (oleic acid) and esters (ethyl stearate) were studied. Additionally, analysis of gaseous components as a function of conversion was performed, providing information on the reaction mechanism. 2. Experimental Section 2.1. Catalyst Preparation. Carbon (Johnson Matthey-peat based) was crushed and sieved to obtain a particle size below 90 µm and used as supplied. For the synthesis of H2PdCl4 (0.375 M), palladium chloride (PdCl2, Aldrich, 59.5 wt % Pd) was dissolved in a stoichiometric amount of hydrochloric acid (10 M). Carbon support (4.75 g) was immersed in 100 mL of demineralized water, in which an aqueous solution of NaOH (0.375 M) was added in double-molar excess. This slurry was stirred overnight to achieve the complete wetting of the carbon pores. When NaOH was added into the carbon solution, the pH of the solution increased from initial 1.6 to 10. The amount corresponding to the nominal loading of 5 wt % Pd on the catalyst was then added dropwise under the flow of nitrogen. The stirring of the slurry continued for an additional 24 h. Then, the catalyst was filtrated, washed several times by demineralized water, and dried for 24 h (75 °C, in nitrogen). Three other 5 wt % Pd/C catalysts were used in this work, namely, Pd/C (Aldrich), Pd/C (Degussa), and Pd/C (BAC, birch activated carbon), denoted as I, II, and III (Table 1). These catalysts were reduced in situ prior to the experiments with flowing hydrogen at 300 °C. 2.2. Catalyst Characterization. Before impregnation, surface areas were determined by the isothermal adsorption of nitrogen (77 K) using Carlo Erba Sorptomatic 1900. The catalysts were outgassed (4 h, 200 °C) before the measurement. The metal dispersion was measured by the CO pulse chemisorption method using Autochem 2910, Micromeritics. The catalysts (17) Maier, W.; Roth, W.; Thies, I.; van Rague´ Schleyer, P. Gas phase decarboxylation of carboxylic acids. Chem. Ber. 1982, 115, 808-812.

were reduced in situ with flowing hydrogen at various temperatures between 100 and 350 °C; thereafter, they were flushed with helium. The specific reduction temperatures are given in the Results and Discussion. The CO pulse chemisorption measurements were performed at room temperature by introducing 10% CO in helium. The dispersion was calculated by using a stoichiometry of CO/Pd equal to unity.18 The pH of the catalyst/water slurry was measured by using a pH electrode (Metrohm). A total of 50 mg of catalyst dispersed in deionized water was used in the measurement, and the measurement continued until a constant pH level was achieved. 2.3. Reactor Setup. Decarboxylation experiments were carried out in a 300 mL semibatch reactor with a liquid-phase volume of 100 mL (Figure 1). The pressure and mass flow controllers in the inlet and outlet of the reactor maintained the reaction pressure and gas flow during the reaction. The stirring speed was typically 1100 rpm. Several feeds were investigated: stearic acid (Fluka, 97%), ethyl stearate (Aldrich, >97%), behenic acid (Aldrich, 89%), nonanoic acid (Acros, 90%), and oleic acid (Riedel-de-HaNn, technical grade). Either dodecane (Aldrich, 99%) or mesitylene (Fluka, >98%) were used as solvents. The temperature and pressure ranges were 300-320 °C and 6-17.5 bar (depending essentially on the solvent vapor pressure). The reaction atmosphere was either helium [American Gas Association (AGA), 99.996%], nitrogen (AGA, 99.999%), argon (AGA, 99.999%), hydrogen (AGA, 99.999%), or a mixture thereof. In a typical kinetic experiment, 45 g of stearic acid, 55 mL of dodecane, and 1 g of catalyst were used, although the initial concentrations were varied in a range of 0.1-1.6 mol/L. 2.4. Analysis. The liquid products were analyzed by a gas chromatograph (HP 6890) equipped with a FI detector and a capillary column DB-5 (length, 60 m; internal diameter, 0.32 mm; film thickness, 0.5 um). The samples were dissolved in pyridine or 2-propanol (in the case of ethyl stearate) and silylated by N,Obis(trimethylsilyl)-trifluoroacetamide (BSTFA) in an oven at 60 °C for 30 min. The following temperature program was used for the analysis of stearic acid: 110 °C at 1 °C/min, 126 °C at 30 °C/min, 185 °C at 1 °C/min, 200 °C at 5 °C/min, and 300 °C. The products were confirmed by gas chromatography-mass spectrometry (GCMS). The gaseous products were identified by a gas chromatograph (HP 5890) equipped with a thermal conductivity (TC) detector, flame ionization (FI) detector, and a capillary column (Porapack Q, 30 m, 0.534 mm) coupled with molecular sieves (5 A, 30 m). The gaseous compounds have been identified and quantified by using the following calibration gases: 1 vol % CO2, 1 vol % C2H6, 0.1 vol % C2H4, 1 vol % CH4 in helium and 203 ppm of CO2, and 1 mol % CO in helium supplied by AGA. It should be pointed out here that the kinetic results are based on the liquid-phase concentrations of the products. In some cases, gas-phase analyses have been taken. The light liquid-phase compounds, which have been present in the gas phase because of their high vapor pressures have been removed from the gas phase with a liquid trap prior to the gasphase analysis with a gas chromatograph. Thus, the presence of, e.g., light aldehydes and alcohols could not be confirmed.

3. Results 3.1. Catalyst Characterization Results. The four different Pd/C catalysts were characterized by nitrogen adsorption, by measuring the pH of the catalyst slurry and by CO chemisorption (Table 1). The Brunauer-Emmett-Teller (BET)-specific surface areas and the metal dispersions varied in the range of 7151126 m2/gcat and 18-29%, respectively. For all of the Pdsupported carbon catalysts as expected, the metal dispersions decreased with an increasing reduction temperature. It can be seen from Table 1 that only catalyst III exhibited a lower metal (18) Gurrath, M.; Kuretzky, T.; Boehm, H. P.; Okhlopkova, L. B.; Lisitsyn, V. A.; Likholobov, V. A. Palladium catalysts on activated carbon supports: Influence of reduction temperature, origin of the support and pretreatments of the carbon surface. Carbon 2000, 38, 1241-1255.

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Ma¨ki-ArVela et al.

Figure 1. Reactor setup.

acid concentration was 0.16 mol/L. The initial decarboxylation rate was very low, 0.015 mmol/min, and the conversion within 360 min was 5%. The main products were unsaturated C17 isomers, with a selectivity of 51%, whereas the selectivity toward n-heptadecane was ca. 7%. The formation of large amounts of unsaturated C17 isomers is caused by the hydrophobic nature of the solvent, dodecane, as well as the presence of an inert atmosphere, helium. Typically, 1-heptadecene, 3-heptadecene, and 8-heptadecene, as well as undecyl benzene and 1-methyldecyl benzene, were observed among C17 products. 3.2.2. Preliminary Catalytic Experiments with Stearic Acid. 3.2.2.1. Effect of Catalyst Mass in the Decarboxylation of Stearic Acid. The effect of catalyst mass on initial reaction rates for stearic acid decarboxylation was studied at 300 °C under 6 bar of helium in dodecane by using three different amounts of catalyst I. The initial reaction rates with 0.2, 0.5, and 1 g of catalyst were 7.0, 7.9, and 5.9 mmol min-1 gcat-1, respectively, indicating that the initial rates increased linearly with the mass of the catalyst. When the kinetics for stearic acid decarboxylation

Figure 2. (a) Decarboxylation kinetics for stearic acid and (b) selectivity to n-heptadecane in the decarboxylation of stearic acid at 300 °C under helium with a total pressure of 6 bar over Pd/C catalyst in dodecane. (b) 0.2 g, ([) 0.5 g, and (9) 1 g of catalyst. The initial stearic acid concentration was 0.16 mol/L.

dispersion compared to the others. The pH of the catalyst/water slurries increased in the following order: III < II < I < IV. 3.2. Decarboxylation of Carboxylic Acids. 3.2.1. Thermal Decarboxylation of Stearic Acid. Thermal decarboxylation of stearic acid was investigated at 300 °C in dodecane under an inert atmosphere at a total pressure of 6 bar. The initial stearic

Figure 3. Kinetics for decarboxylation of stearic acid over catalyst I at 300 °C under 6 bar of helium in dodecane. Conditions: catalyst, 1 g; initial stearic acid concentration, 0.16 mol/L. ([) Stearic acid, (9) n-heptadecane, (2) C17 saturated hydrocarbons, and (b) unsaturated C17 hydrocarbons.



Table 2. Kinetic Data for the Decarboxylation of Stearic Acid over Catalyst I with Two Different Concentrations of Stearic Acid in Dodecane at 300 °C in He under a Total Pressure of 17 bar concentration of stearic acid (mol/L)

initial rate (mmol min-1 gcat-1)

conversion after 300 min (%)

S (n-heptadecane)a (%)b

1.59 0.79

6.3 2.4

40 100

80 82

a

Selectivity. b At a conversion of 30%.

Figure 4. (a) Decarboxylation of stearic acid and (b) selectivity to n-heptadecane over catalyst I at 300 °C and 17 bar of helium in dodecane. Conditions: catalyst, 1 g. The initial stearic acid concentration is (9) 1.6 mol/L and ([) 0.8 mol/L.

was plotted as a function of normalized abscissa mcatreaction time, the kinetic curves should coincide when working in the absence of external mass-transfer limitations. This was not, however, the case in the decarboxylation of stearic acid (Figure 2a). The catalyst deactivated more rapidly with a smaller amount of catalyst, when the initial concentration of stearic acid was kept constant. A typical kinetic plot for the decarboxylation of stearic acid over a Pd/C catalyst at 300 °C is depicted in Figure 3. The main product was n-heptadecane, with a molar fraction of 95%. Other products formed were unsaturated isomers containing 17 carbon atoms (3%). When the decarboxylation experiments are carried out in a semibatch reactor, it is important to know how close to 100% the experimental mass balance is when comparing it to the theoretical one, according to the reaction mechanism proposed in eq 1 (see section 4). The mass balance with a 0.16 mol/L initial concentration of stearic acid gave the value of 97%, indicating that only a minor amount of light liquidphase products, such as ethanol, can be formed (see the Experimental Section).

The selectivity to n-heptadecane was dependent upon the catalyst amount: slightly lower selectivities to n-heptadecane were achieved with 0.5 g of catalyst compared to 1 g of catalyst (Figure 2b). When only 0.2 g of catalyst was used, the selectivity to n-heptadecane decreased with an increasing conversion and aromatic C17 compounds were formed. The retardation of the catalyst activity can thus be explained by the formation of aromatic compounds, which in turn enhance catalyst deactivation, because they are strongly adsorbed on the metal surface. 3.2.2.2. Effect of the Stirring Speed on the Decarboxylation Kinetics. The kinetic curves for decarboxylation of stearic acid in dodecane at 300 °C under 7 bar of helium coincided with two stirring speeds, 1100 and 1800 rpm, indicating that the reaction was performed under the absence of the external masstransfer limitations. 3.2.3. Effect of the Initial Concentration of Stearic Acid on Its Decarboxylation. The initial reaction rate in the decarboxylation of stearic acid over catalyst I at 300 °C under helium with a total pressure of 17 bar in dodecane was increased with an increasing initial concentration of stearic acid; i.e., with an initial concentration of 0.8 mol/L, the reaction rate was 2.6fold lower than achieved with c0,A ) 1.6 mol/L (Table 2). A complete conversion of stearic acid was achieved after 180 min in the former case, whereas it was only 40% after 300 min in the latter case. Selectivities to n-heptadecane were only slightly higher with the lower initial stearic acid concentration at a conversion level of 30% (Table 2 and Figure 4), corresponding to the conditions when the catalyst kept its activity for a longer time. Unsaturated C17 compounds were the second largest product group, pointing out that, when the reaction is carried out under an inert atmosphere, in this case helium, and the solvent, dodecane, is not an effective hydrogen donor, unsaturated decarboxylated products are formed in large amounts. 3.3. Decarboxylation of Fatty Acid Esters. 3.3.1. Hydrothermal Decarbonylation/Decarboxylation of Ethyl Stearate. Thermal decarbonylation/decarboxylation of ethyl stearate was investigated at 320 °C under a hydrogen atmosphere with a total pressure of 7.5 bar in dodecane (Figure 5). The initial reaction rate was 0.1 mmol/min, corresponding to the conversion level of 17% after a 360 min reaction time. The main product was stearic acid, with a selectivity of 88% after 360 min, and other products observed were stearone (C17H34)2CO, n-heptadecane, and aromatic C17 compounds. The following consecutive reaction schemes could be suggested:



Figure 5. Thermal transformation of ethyl stearate at 320 °C under hydrogen with a total pressure of 7.5 bar. The solvent was dodecane. ([) Ethyl strearate, (9) stearic acid, (2) stearone, and (×) n-heptadecane. The initial ethyl stearate concentration was 0.1 mol/L. Table 3. Kinetic Data in the Decarboxylation of Ethyl Stearate over Three Different Pd/C Catalysts under 17 bar of Nitrogen at 300 °Ca catalyst I II III a



0.411 0.115 0.0948

30 20 13

Conditions: catalyst, 1 g; initial ethyl stearate concentration, 1.6 mol/L.

When, however, the formed amounts of stearic acid were plotted versus n-heptadecane, a linear correlation could be observed and the ratio between the initial formation rates of stearic acid and n-heptadecane was 28. This result suggested that the thermal reaction of ethyl stearate proceeds not only consecutively to n-heptadecane from stearic acid but also directly from ethyl stearate. Reaction 1 was observed to occur already previously19 at 315 °C, where ethyl stearate was pyrolyzed to stearic acid and ethylene. Analogously, in ref 20, the pyrolysis of ethyl stearate produced stearic acid (22%) and ethylene in the gas phase. Moreover, large amounts of low boiling point liquids were obtained at a much higher temperature (600 °C).20 3.3.2. Testing Different Pd/C Catalysts in the Decarbonylation/Decarboxylation of Ethyl Stearate. Three different Pd/C catalyst were tested in the decarboxylation of ethyl stearate under a nitrogen atmosphere. The initial reaction rates of ethyl stearate increased with an increasing pH of the catalyst slurry (see Tables 1 and 3); i.e., alkalinity in the catalyst enhanced the initial reaction rate. This result can be correlated well with the gasphase decarbonylation results of methyl formate over alkali metal catalysts supported on active carbon.21 Not only the initial activity but also the conversion of ethyl stearate after 300 min increased also with an increased pH of the catalyst slurry (Table 3 and Figure 6a). The selectivities to n-heptadecane followed the analogous trends as initial rate and conversion (Table 3 and Figure 6b). The reason for higher selectivities to n-heptadecane over alkaline catalysts was the capability of stearic acid to react further over such catalysts (19) Colson, A. Formation of ethylene hydrocarbon from esters. C. R. Acad. Sci., Ser. IIc: Chim. 1999, 147, 1054-1057. (20) Bailey, W. J.; Turek, W. N. Synthesis and purification of fatty acids by the pyrolysis of esters. J. Am. Oil Chem. Soc. 1956, 3, 317-319. (21) Bowker, M.; Morgan, C. Couves, Acetic acid adsorption and decomposition on Pd(110). J. Surf. Sci. 2004, 555, 145-156.

(Figure 6c), whereas over a more acidic catalyst, stearic acid remained unreacted. The product distribution in the decarboxylation of ethyl stearate obtained with three different Pd/C catalysts was compared at the conversion level of 13% (Table 4). Over the most acidic catalyst, III, the main product was stearic acid and only trace amounts of aromatic side products were formed (Figure 6d). The initial rate for the formation of stearic acid over catalyst III was 3.8 times higher than the rate for the formation of n-heptadecane (Table 5). This value was also 10 times higher than the corresponding ratio for catalyst I of 0.38, indicating that the least acidic catalyst was initially favoring decarbonylation over the formation of stearic acid under an inert atmosphere. The third catalyst, II, with the pH of the catalyst slurry being 7.3, gave the corresponding ratio for the initial formation of stearic acid to n-heptadecane of about 12.2, indicating that there is no direct correlation with the pH of the catalyst slurry and the selection of either stearic acid or n-heptadecane formation. The initial formation rate for both unsaturated C17 as well as aromatic C17 compounds was enhanced by the alkalinity of the catalyst (Tables 4 and 5). It should be pointed out here that in the current work no analysis of the light liquid-phase products was available; thus, we cannot exclude the formation of small amounts of ethanol or acetaldehyde. Their amounts could not, however, be very large based on the liquid phase trapped before GC analysis of gaseous compounds (see the Experimental Section). 3.3.3. Effect of the Reaction Atmosphere in the Decarbonylation/Decarboxylation of Ethyl Stearate. The effect of the reaction atmosphere was systematically studied in the current work, because during decarboxylation under helium, the catalytic activity decreased not only in the decarboxylation of stearic acid but also in ethyl stearate decarboxylation in mesitylene. Furthermore, in our previous publication, higher rates and conversion levels were achieved in the case of a hydrogen compared to a He atmosphere in the decarboxylation of ethyl stearate at 300 °C over the Pd/C catalyst.6 Because the amount of unsaturated C17 compounds could be decreased with an increasing amount of hydrogen in the gas feed,6 it was decided to investigate systematically the effect of hydrogen in the gas feed. Decarboxylation of ethyl stearate was investigated over catalyst IV at 320 °C and 17 bar of total pressure in dodecane



Figure 6. (a) Decarboxylation of ethyl stearate, (b) molar fraction of stearic acid versus n-heptadecane, and (c) selectivity to n-heptadecane and molar fraction of C17 aromatic compounds over three different Pd/C catalysts [(*) I, (b) II, and (2) III] at 300 °C and under 17 bar of nitrogen. Conditions: initial ethyl stearate concentration, 1.6 mol/L; solvent, dodecane; catalyst, 1 g. Table 4. Initial Formation Rates of Different Products in the Decarboxylation of Ethyl Stearate over Three Different Pd/C Catalysts under 17 bar of Nitrogen at 300 °Ca

a

catalyst

initial rate for stearic acid (mmol min-1 gcat-1)

initial rate for n-heptadecane (mmol min-1 gcat-1)

initial rate for n-heptadecene (mmol min-1 gcat-1)

initial rate for aromatic C17 compounds (mmol min-1 gcat-1)

I II III

0.06 0.08 0.06

0.16 0.007 0.005

0.02 0.0002 0.02

0.18 0.004 0.003

Conditions: catalyst, 1 g; initial ethyl stearate concentration, 1.6 mol/L.

Table 5. Product Selectivities Achieved in the Decarbonylation of Ethyl Stearate over Three Different Pd/C Catalysts at 300 °C and under Nitrogen with a Total Pressure of 17 bara

catalyst

stearic acid (mol %)

n-heptadecane (mol %)

aromatics C17 (mol %)

unsaturated C17 (mol %)

I II III

2 15 61

46 38 26

36 31 0

6 5 1

a The conversion level is 13%. Conditions: catalyst, 1 g; initial ethyl stearate concentration, 1.6 mol/L.

as the solvent in hydrogen and hydrogen/helium mixtures; the following ratios of H2/He were used: 3:1, 1:1, and 0.5:1. (Table 6 and Figure 7a). The initial decarboxylation rate was 2.7 times higher in pure hydrogen compared to H2/He with the ratio of 3:1, whereas the initial rates with the two lower H2/He ratios were about the same, being about 80% of the rates obtained for the H2/He ratio of 3:1 (Table 7). An analogous effect of the reaction atmosphere was observed for the conversion levels after 360 min. Hydrogen had a beneficial effect on the conversion after prolonged reaction times (Table 6 and Figure 7a). This result is analogous to the one achieved in the gas-phase decarboxylation of fatty acids over the Pd/SiO2 catalyst,17 where it was stated that hydrogen

is important in the activation of Pd. Furthermore, hydrogen can help the organic species desorb from the metal surface. The ratio between initial formation rates for n-heptadecane and stearic acid is compared as a function of the H2/He ratio in Table 8. Under hydrogen, this ratio was the highest, 1.6, while it decreased with a decreasing amount of hydrogen in the reaction atmosphere. This result indicated that decarbonylation of ethyl stearate, confirmed by gas-phase analysis (Figure 9), is enhanced in the presence of hydrogen, whereas under hydrogen-poor conditions, it forms stearic acid. The formation of stearic acid (Figure 7b) and n-heptadecane (Figure 7c) was enhanced with an increasing amount of hydrogen in the reaction atmosphere. The parallel formation of stearic acid and n-heptadecane is depicted in Figure 8a, showing that the highest molar fractions of stearic acid, close to 20 mol %, were obtained under a H2/He ratio of 0.5. Stearic acid decarboxylated further to n-heptadecane, because a typical consecutive pattern for the molar fractions of stearic acid and n-heptadecane can be seen in Figure 8a. The selectivities to n-heptadecane declined with an increasing the amount of helium in the reaction atmosphere, but the opposite effect was observed in the selectivites of stearic acid (Figure 8b and Table 6). Very small amounts of aromatic and unsaturated C17 compounds (below 1 mol %) were formed



Table 6. Initial Formation Rates of Different Products in Ethyl Stearate Decarboxylation over Catalyst I in Two Different Solvents, Mesitylene and Dodecane, at 300 °C in He under a Total Pressure of 17.5 bara

solvent

initial rate for the formation of stearic acid (mmol min-1 gcat-1)

initial rate for the formation of n-heptadecane (mmol min-1 gcat-1)

initial rate for the formation of n-heptadecene (mmol min-1 gcat-1)

initial rate for the formation of C17 aromatics (mmol min-1 gcat-1)

mesitylene dodecane

0.007 0.018

0.002 0.002

low low

low low

a

Conditions: catalyst, 0.5 g; initial ethyl stearate concentration, 0.1 mol/L.

Figure 7. (a) Kinetics in decarboxylation of ethyl stearate, (b) formation of stearic acid, and (c) n-heptadecane on catalyst IV in dodecane at 320 °C and 17 bar. (b) Hydrogen, ratio between hydrogen and helium (*) 3.0, ([) 1.0, and (9) 0.5; initial ethyl stearate concentration, 0.1 mol/L; catalyst, 0.5 g.

during these experiments, and there was a trend for the formation of smaller amounts of these compounds with an increasing amount of hydrogen. This result supported the fact that hydrogen in the reaction atmosphere retarded the catalyst deactivation by coke formation (see below).

The decarboxylation of ethyl stearate in helium was studied over catalyst I at 300 °C with two different initial concentrations of ethyl stearate, 0.1 and 1.6 mol/L (Table 6). In the former case, the conversion was only 18% after a 360 min reaction time. When this result was compared with the result obtained with the H2/He ratio of 0.5:1 over catalyst IV at the same conversion level (18%), it can, however, be observed that the stearic acid selectivity increased and the n-heptadecane selectivity decreased with an increasing amount of inert gas. In our previous publication, ethyl stearate transformation was studied over catalyst I with a 16 times higher initial ethyl stearate concentration (Table 6).6 In that case, the main product in ethyl stearate decarboxylation was n-heptadecane (18 mol %) at a conversion of 34%. Additionally, a substantial amount of unsaturated C17 olefins were formed under an inert atmosphere (about 10 mol %). Moreover, about 3 mol % of C17 aromatic compounds were formed after 360 min, but only traces of stearic acid were formed. The main difference in the product distribution with a higher initial ethyl stearate concentration under an inert atmosphere compared to the results with different gas mixtures of H2/He under more diluted initial reactant concentrations was the large formation of unsaturated C17 olefins. n-Heptadecane and C17 olefins were initially formed parallel with the same initial rates (0.32 mmol min-1 gcat-1), but after a 90 min reaction time, the formation of n-heptadecane continued with the rate of 0.07 mmol min-1 gcat-1, whereas the corresponding formation rate for unsaturated C17 aromatics was only 0.02 mmol min-1 gcat-1, indicating that the selectivity of n-heptadecane increased with an increasing reaction time.6 The parallel formation of nheptadecane and unsaturated C17 olefins over catalyst I was observed under high initial ethyl stearate concentrations, and under diluted reactant concentrations, the amount of formed unsaturated C17 olefins under a helium atmosphere was very minor. It can be concluded that stearic acid formation was enhanced with an increasing amount of inert gas in the gas mixture, whereas the opposite effect is valid for the formation of the desired product under diluted reactant concentrations. When the initial ethyl stearate concentration was high, the amount of unsaturated C17 olefins was enhanced under inert conditions. It should, however, be pointed out that, over a more acidic catalyst, such as III, no aromatic C17 compounds were formed under nitrogen with high initial concentrations of ethyl stearate (Table 5), indicating that the acidity (corresponding to the pH of the catalyst slurry) is a very important factor determining the product distribution in ethyl stearate transformations. Gas-phase analyses were taken during the decarboxylation of ethyl stearate from two different experiments corresponding to the hydrogen/helium ratios of 3:1 and 0.5:1, respectively (parts a and b of Figure 9). The two major gaseous products from the decarbonylation/decarboxylation of ethyl stearate were CO and ethane. In the beginning of the decarbonylation, C2H4 was also observed. Under hydrogen-rich conditions (H2/He ) 3), its relative amount declined with a rate of 8.1 µmol/min, whereas under the ratio of H2/He ) 0.5, the corresponding



Table 7. Kinetics in the Decarboxylation of Ethyl Stearate over Catalyst IV at 320 °C under 17 bar of Total Pressure in Dodecanea atmosphere



S (stearic acid)b (%)c

S (n-heptadecane)b (%)c

H2 3:1 H2/He 1:1 H2/He 0.5:1 H2/He He He N2

0.12 0.044 0.035 0.036 0.027e 0.32f 0.41h

78 65 58 56 18 33f 13i

17 27 27 42 (56d) 60d

Catalytic Deoxygenation of Fatty Acids and Their ... - ACS Publications

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