ISSN 09655441, Petroleum Chemistry, 2011, Vol. 51, No. 5, pp. 336–341. © Pleiades Publishing, Ltd., 2011. Original Russian Text © A.S. Berenblyum, T.A. Podoplelova, R.S. Shamsiev, E.A. Katsman, V.Ya. Danyushevsky, 2011, published in Neftekhimiya, 2011, Vol. 51, No. 5, pp. 342–347.
On the Mechanism of Catalytic Conversion of Fatty Acids | into Hydrocarbons in the Presence of Palladium Catalysts on Alumina A. S. Berenblyum*, T. A. Podoplelova, R. S. Shamsiev, E. A. Katsman, and V. Ya. Danyushevsky Lomonosov Moscow State Academy of Fine Chemical Technology, Moscow, Russia *email:
[email protected] Received February 28, 2011
Abstract—A conversion of stearic acid into hydrocarbons in the presence of palladium on alumina has been studied. It has been shown that heptadecane and carbon monoxide are formed as the main products, dihep tadecylketone is formed as a byproduct, and the contribution of the decarbonylation reaction increases as compared to decarboxylation in the presence of hydrogen with an increase in its pressure. The formation of heptadecene and formic acid as intermediate products has allowed the conclusion that the cleavage of the car bon–carbon bond in the stearic acid molecule R–COOH takes place in the Pd coordination sphere, resulting in the formation of formic acid (or its fragment associated with palladium) and the corresponding olefinic product. Depending on the reaction conditions, formic acid and/or its fragment decompose, yielding CO and H2O or CO2 and H2. The main routes of the reaction have been simulated using quantumchemical methods, and it has been shown that the reaction ratelimiting stage is the cleavage of C–C bond in the acid molecule. DOI: 10.1134/S0965544111050069
The reaction of conversion of fatty acids into the corresponding paraffins currently attracts heightened interest due to the fact that it can serve as a basis for industrial production of biodiesel (secondgeneration biodiesel or socalled “green biodiesel”) from renew able sources of nonfood origin, including algae [1–4]. A number of researchers have found that reactions of decarboxylation and decarbonylation of acids yield ing hydrocarbons take place in the presence of various catalysts, particularly palladiumbased, at a tempera ture of 300–350°С [1, 5, 6]. In recent years, sugges tions on the mechanism of these reactions have been put forward [7, 8]. According to Immer [7], a key stage of the process is the cleavage of the RCOO–H bond yielding RCOO–Pdn bond. Thus, the formation of palladium carboxylates on the catalyst surface has indeed been suggested, but then it is difficult to imag ine how the activation and the cleavage of the C–C bond in R–COO proceeds, which is required for the formation of CO and CO2. Unfortunately, this is not discussed in [7]. Boda et al. [8] believe that hydro genolysis of RСH2–COOH bond takes place sponta neously in the presence of hydrogen. The binding of the acid molecule via the interac tion of this saturated C–Cbond with two Pd–Pd atoms, as suggested by these authors, seems to be unlikely. The aim of this work is to study the mechanism of decarbonylation and decarboxylation reactions exem plified by the transformations of stearic acid (SA) in
the presence of aluminasupported palladium cata lysts. EXPERIMENTAL Reagents and catalyst. The following chemicals and solvents were used: dodecane (reagent grade), tridecane (reagent grade), hydrochloric acid (titration standard), stearic acid (99%, analytical grade, manu factured by VAG Chemie, Germany). To prepare 0.5% and 5.0% Pd/Al2O3 catalysts, palladium dichloride (reagent grade, metal content 59.5%) without addi tional purification, and γalumina (shaped, according to GOST 813685) were used. The palladium catalyst was prepared by impregnation of a preliminarily ground support (0.09–0.2 mm fraction) with an aque ous hydrochloric acid solution of palladium dichloride [9] of the appropriate concentration. Thus, catalysts with a Pd content of 0.5% and 5.0% were obtained. Methods of analysis. To determine the amount of unreacted SA, the reaction products were titrated with an alcoholic alkali solution in accordance with GOST [State Standard] 547680. The amount of saturated and unsaturated hydrocarbons in the product after preliminary methylation was determined by GLC on a Kristall 2000M gas chromatograph with FID and an HPultra2 fusedsilica capillary column. The amount of formed hydrocarbons was determined using an internal standard (tridecane).
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The gaseous products of deoxygenation were deter mined on an LHM80 chromatograph with a thermal conductivity detector and zeolite 5A (analysis for Н2, СН4, О2, N2, CO) and Porapak Q (СО2 analysis) packed columns. 1 H NMR studies were performed on Bruker DPX 300 and Bruker AM 400 spectrometers; the internal standard was CDCl3 (δ = 7.25 ppm). UV spectra were taken on a Beckman DU88 spectrophotometer. Catalytic experiments. Experiments on the decar boxylation/decarbonylation of SA were conducted in a 50ml Autoclave Engineers autoclave capacity equipped with a stirrer. The reactor was charged with 1 g of catalyst, and the catalyst was reduced in a hydro gen flow (4–6 l/h) under a pressure of 10 atm for 3 h at 200°С with stirring. Dodecane in an amount of 12 g was introduced through a special container into the autoclave at room temperature to cover the catalyst and thus protecting it from oxidation by air. Then, the autoclave was opened, 4 g of SA were added, and the reaction was run in a nitrogen or hydrogen atmosphere at a given pressure for 3 h at 350°С with vigorous stir ring (900–1100 rpm/min). Upon completion of the experiment, the cooled reactor was sampled for analy sis of the gas and liquid phases. Quantumchemical modeling of the reaction mech anism of decarbonylation and decarboxylation of fatty acids over palladium centers was performed using the scalarrelativistic approach of the densityfunctional theory under the program Priroda [10]. The exchangecorrelation functional PBE [11] and the Λ1 basis set [12] were used. Thermodynamic parameters were calculated for 298K, and the free energy values of noninteracting products were determined by the cou pledcluster method (CCSD) [13]. RESULTS AND DISCUSSION The main product of the reaction in a nitrogen atmosphere for 3 h at 350°С is heptadecane (HDCA), and the intermediate products are heptadecenes (HDCE), as shown in [5] in the presence of Pd on car bon. In our experiments, the SA conversion is 84.4% and the selectivity for C17 hydrocarbons is as low as 14.7%. According to the 1H NMR data, the main undesired product of the reaction is ketone, as evi denced by the signals of СН3 and СН2 groups in long alkyl chains (0.9 and 1.2 ppm, respectively), two mul tiplets near 1.6 ppm (СН2 in the βposition to the car bonyl group), and two triplets at 2.4 ppm (СН2 adja cent to carbonyl group). The assignment of the bands was made in accordance with the published data [14]. The ketone formation was also observed by other researchers [15, 16]. The ketone concentration calcu lated from the NMR data (naphthalene as internal standard) in this study was 65.3%, which is close to the value of 67.8% expected according to the GLC analy sis data within the experimental error.
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2RCOOH
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R–CO–R + H2O + CO2.
Thus, it may be assumed that ketone is the main undesired reaction product; therefore, to determine the amount of CO2 formed in the SA decarboxylation reaction, the amount of CO2 produced in the SA ketonization reaction was subtracted from the total amount of CO2 determined. In our experiments, the formation of hydrocarbons in a hydrogen atmosphere (350°С, 3 h) predominantly occurs via the decarbonylation reaction. Note that when hydrogen is not intentionally introduced to take part in the reaction, it is formed via dehydrogenation of the solvent. The characteristic bands for terminal olefins (180–190 nm) are not observed in the UV spectra of the products of a blank catalytic experiment (dodecane, without the addition of SA, 350°С, 3 h, 16 atm of nitrogen), but there are absorption bands at 260–270 nm that are commonly attributed to substi tuted alkylbenzenes [17]. It can be assumed that the solvent undergoes the dehydrocyclization reaction in our experiments. According to the GLC data, molec ular hydrogen is also produced (partial pressure of Н2 reaches 1.3 atm by the end of the run). According to Immer et al. [6], dodecene is formed from dodecane on a carbonsupported palladium catalyst. The following features are observed when the pro cess is conducted in an atmosphere of hydrogen intro duced intentionally. The main product of the catalytic reaction of SA conversion in the liquid phase is HDCA. The SA conversion achieves a value of 88– 100% at a hydrogen pressure of 6–14 atm, and the selectivity of its transformation into HDCA signifi cantly increases from 82.4 to 90.3% (table). Hepta decene is observed among the reaction products at a relatively low Н2 pressure (10 atm or less), whereas it is fully hydrogenated to the corresponding paraffin at relatively high pressures (more than 10 atm). Stearic acid is partially reduced to octadecane (ODC) at high hydrogen pressures (table). To confirm that the resulting heptadecene is hydro genated to the paraffin, we have carried out an experi ment over a copper catalyst (Cu/γAl2O3), which is sig nificantly less active in this reaction than the palla dium catalyst. It turned out that HDCE was observed among the reaction products in an amount of 65% of the total concentration of hydrocarbons even at a high Н2 pressure (14 atm, 350°С, 6 h). It is noteworthy that the reduction of SA is not observed in the presence of this catalyst and, thus, the formation of olefins cannot be explained by dehydration of the alcohols generated during the reduction of acids [18]. With an increase in hydrogen pressure, the contri bution of the decarbonylation reaction also increases, and at a hydrogen pressure of 10 atm and above, it achieves nearly 100% (table). A small amount of methane is produced via the CO methanation reaction [19]. Therefore, the total amount of CO and the meth ane produced from carbon monoxide was taken into
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Transformations of stearic acid over palladium catalysts (stearic acid, 4 g; dodecane, 12 g; catalyst, 1 g; 350°C, 3 h, H2) Conver S for paraf S for hep S for octa S for hep X(CO + CH4) X(CO2) sion, % fins tadecane decane tadecenes
No.
Catalyst
p, atm
1
0.5% Pd/Al2O3
6
87.6
83.9
83.3
0.6
2.1
91.1
8.9
2
0.5% Pd/Al2O3
8
97.0
88.6
87.4
1.2
1.3
97.2
2.8
3
0.5% Pd/Al2O3
10
97.5
84.5
82.4
2.1
0.5
~100
traces
4
0.5% Pd/Al2O3
14
~100
95.1
90.3
4.8
–
~100
traces
5
5% Pd/Al2O3
14
~100
91.0
9.0
–
~100
79.0
21.0
Note: S is selectivity, %; X is the volume fraction of carbon oxide and methane or carbon dioxide in the gas after experiment, %.
account when calculating the contribution of decar bonylation. It is important that the amount of the gas produced agrees well with its calculated value accord ing to the GLC data on the liquid reaction products. Thus, the selectivity on a heptadecane and hepta decene basis was 83%, and the selectivity on a CO basis was 82% for the experiment carried out at a hydrogen pressure of 10 atm in the presence of 0.5% Pd/Al2O3 at a conversion of 97.5%. The reaction of CO steam conversion is not observed under our experi mental conditions (СО2 is absent from the products). The reverse reaction cannot proceed for thermody namic reasons [20]. It was found that not only the pressure of hydrogen, but also the concentration of palladium in the catalyst affects the ratio of contributions of the decarboxyla tion and decarbonylation reactions. Thus, the increase in the palladium loading of the catalyst to 5% leads to an increase in the CO2 concentration to 21% of the total gas concentration even at a hydrogen pressure of 14 atm. All these data, especially the presence of the methyl ester of formic acid among the reaction products after their methylation, as was found recently in our 1H NMR study [1], and the known fact of intermediate olefin formation [1, 5, 19] allowed us to suggest that the cleavage of the C—C bond between carbon of the carboxyl group and the neighboring C atom in the SA molecule occurs in the coordination sphere of palla
dium, resulting in the formation of formic acid and/or its fragment associated with palladium, and the corre sponding olefin. Depending on the reaction condi tions, formic acid and/or its fragment decompose fol lowing one of the two characteristic pathways [21]: yielding CO and Н2О or СО2 and Н2. The olefin is reduced to the paraffin by hydrogen, which is formed via either acid decarboxylation or solvent dehydroge nation or when the reaction is carried out in a hydro gen atmosphere. It is noteworthy that the reaction reverse to decarboxylation, i.e., the reaction of an ole fin with formic acid yielding fatty acids, was observed in the presence of metal complex catalysts [22]. Possible routes of the decarboxylation and decar bonylation reactions were simulated using quantum chemical methods. A system containing a molecule of propionic acid and active centers in the form of clus ters Pd4 was selected as a model system. Our calcula tions have shown that the most favorable structure of such a cluster is a tetrahedron. The adsorption of the reacting acid molecule on the catalyst active site may lead to its coordination to the Pd atom via the interaction of one of the two oxy gen atoms and hydrogen atom at the βcarbon (scheme A1, A2). PETROLEUM CHEMISTRY
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CH3 CH3
Isolated products CO2 + C2H6 + Pd4 –26.9
O
HH H
H O
H
OH
H H 13.4
Pd Pd
H Pd
Pd Pd
A2
H HH H H
OH O
Pd
H H
11.1
Pd Pd
H Pd
H H H H
Pd
Pd
A1
Pd
Pd
B1
Pd
O C OH H
Pd
Pd
Pd
3 7.
Pd
C Pd O Pd Pd O 17.7 OH HO Pd P2 C O OH 7.4 21.2 O Pd Δ G 298 = –19.0 Pd H Pd Pd Pd Pd Pd Pd O H Pd O Pd Pd C B2 C2 D2 Isolated products C Pd OH14.2 CO + C2H4 + Pd H H HH H Pd Pd + H2O + Pd4 Pd O Pd H OH OH H –4.3 Pd HO H Pd H 4.4 C O O O 17.7 P3 ΔG298 = –20.4 D3 Pd H Pd H Pd
HH
C1
D1
Pd Pd
Pd Pd P1
Isolated products HCOOH + C2H4 + Pd4 +11.0
ΔG298 = +1.0
Scheme of transformation reactions of propionic acid over a palladium catalyst. * The calculated values of free activation energies of the reactions are given above the arrows, the calculated
values of free energies of the proposed intermediates are given under the formulas. In both cases, these values are calculated relative to the sum of free energies of noninteracting Pd4 and C2H5COOH.
The ability of Pd(0) to be inserted into a C–C bond, generating hydride complexes, is known for organometallic compounds [23], which gives grounds to suppose the formation of intermediates B1 and B2 (scheme). One can easily imagine the future transfor mation of complex B1 into C1, and complex B2 into C2 (the relative free energy of activation of these tran ⫽
sitions ΔG 298 does not exceed 4 kcal/mol). The published data on the reactions of Pd(0) com pounds [23] suggest that the next step will be the cleav age of the C–Cbond in acid molecule, resulting in the formation of the –COOH group and ethylene coordinated to the palladium center. The results of calculations indicate that the formed complexes D1, D2, and D3 are slightly more stable than their precur sors C1 and C2. The values of ΔG298 for the corre sponding complexes are as follows: –2.9 (D1), –4.0 (D2) –6.2 (D3), –0.7 (C1), and 1.3 (C2) kcal/mol. Further transformations are probably due to differ ent positions of the H atom on the face of the Pd4 tet rahedron and to the related pathways of the involved reactions. PETROLEUM CHEMISTRY
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Thus, the coordinated hydrogen atom’s attack on the –COOHgroup in D1 leads to the formation of formic acid and η2ethylenepalladium complex (P1). The formation of HCOOH as the final product is unlikely, because ΔG298 value for this route is rather large (11 kcal/mol). However, the thermodynamics of the decomposition and the known ability of formic acid for catalytic dehydration to carbon monoxide at temperatures of 300–350°С with high selectivity [21] show that such a route can be also realized in general. The result of the reaction of the coordinated hydro gen atom with ethylene in the D2 complex is ethyl, which is reduced to ethane by hydride transfer from the –COOHgroup. This group, in turn, yields CO2 after the detachment of the hydrogen atom (P2). The ΔG298 value for this route is –26.9 kcal/mol. Therefore, decarboxylation with the formation of CO2 is quite favorable from the thermodynamic point of view. In addition, there is another possibility of of the hydrogen atom attacking the hydroxyl group of the – COOH moiety in D3. The result is the formation of coordinated CO (the Pd–CO distance is 1.9 Å) and H2O molecules (P3). Calculation shows that the P3
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complex is quite stable at room temperature (ΔG298 is – 20.4 kcal/mol). However, it is obvious that the com plex can quite easily eliminate CO, making acid decarbonylation possible at a reaction temperature of 350°С. The scheme shows that the maximum values of the free activation energy are consistent with the steps of the formation of complexes D1 and D2; thus, the rate limiting step of the reaction is the C–C bond cleavage in the coordinated moiety of carboxylic acid. Since the complexes D1–D3 differ only in the position of the hydrogen atom, coordinated on the face of the tetra hedron, the transitions between them are accom plished without a significant consumption of energy. It is impossible to explain unambiguously the influ ence of hydrogen on the ratio of contributions of the decarboxylation and decarbonylation reactions at this point. However, it is obvious that the presence of Н2 should not affect the decarboxylation reaction, affect ing only the decarbonylation: R–COOH + H2
RH + CO + H2O.
The free energy of this reaction (ΔG 298 = –18.5 kcal/mol) is significantly lower than that for the decar bonylation in the absence of hydrogen (ΔG298= –4.3 kcal/mol): R–COOH
of CO to methane are observed. The formation of hep tadecene as an intermediate product, the presence of small amounts of methyl formate among the reaction products after their methylation, and quantumchem ical modeling of the main reaction routes make it pos sible to suppose how the acid catalytic transformations proceed in the presence of palladium catalysts. Appar ently, the SA molecule is adsorbed on the palladium active site due to the coordination between the oxygen atom of the carboxyl group and a hydrogen atom of the nearest methylene fragment. Then the palladium atom inserts into the corresponding C–Hbond followed by the insertion (during the ratelimiting step) into the C–Cbond of R–COOH. The formation of the main reaction products occurs in the coordination sphere of Pd as a result of the attack of hydrogen atom having different location on the face of the Pd4 tetrahedron on the fragments resulting from the C–Cbond cleavage in the acid molecule and coordinated to the active center. Although the decarboxylation reaction is the most favorable in terms of thermodynamics, it is likely that kinetic reasons make decarbonylation dominate during the conversion of acid molecules in the coordi nation sphere of palladium. Kinetic studies of this complex reaction can play an important role in under standing this phenomenon.
R'H + CO + H2O ,
where R’H is the corresponding olefin. This is associated with an energy gain due to the hydrogenation of olefin. Consequently, decarboxylation, rather than decar bonylation, seems to be preferable in thermodynamic terms. However, the decarbonylation reaction becomes preferable in the presence of a catalyst, apparently because of kinetic reasons. The results obtained in this study and the published experimental data [8] show that the reaction mainly proceeds towards decarbonylation in the catalytic experiments, especially, in a hydrogen atmosphere. As it has been mentioned above, formic acid, which we believe to be an intermediate product, also decom poses catalytically with high selectivity to CO at tem peratures of 300–350°С [21]. The results of this study using SA as an example and the analysis of the available literature data allow us to formulate the main features of the mechanism of cat alytic transformations of fatty acids in the presence of palladium catalysts. It has been found that depending on the conditions, the main reaction products are hep tadecane, carbon monoxide, and, partially, carbon dioxide, while the byproduct is diheptadecylketone. With the increasing hydrogen pressure, the contribu tion of the decarbonylation reaction increases and partial reduction of SA to octadecane and conversion
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