Influence of Hydrogen in Catalytic Deoxygenation of Fatty Acids and

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Influence of Hydrogen in Catalytic Deoxygenation of Fatty Acids and Their Derivatives over Pd/C Bartosz Rozmyszowicz,† P€aivi M€aki-Arvela,† Anton Tokarev,† Anne-Riikka Leino,‡ Kari Er€anen,† and Dmitry Yu. Murzin*,† †

Laboratory of Industrial Chemistry and Reaction Engineering, Process Chemistry Centre, Åbo Akademi University, FI-20500 Turku/Abo, Finland ‡ Laboratory of Microelectronics and Materials Physics, University of Oulu, PL 4500, 90014 Oulu, Finland ABSTRACT: The influence of hydrogen on the catalytic deoxygenation of fatty acids was investigated in the semibatch reactor over a mesoporous 5 wt % Pd/C (Sibunit) catalyst. Transformations of a model fatty acid (lauric acid) and reaction intermediates (lauric aldehyde and lauryl alcohol) were performed under inert and hydrogen rich atmosphere. Analysis of the liquid and gas phases of the reaction mixture revealed that different reactions occurred depending on hydrogen content in the reaction atmosphere. A higher yield of hydrocarbons (59 mol %) was obtained in the fatty acid deoxygenation under hydrogen pressure compared to an experiment under argon where the yield of hydrocarbons reached 39 mol % after 300 min of the reaction. Different product distributions, intermediates and reaction pathways depending on hydrogen content in the reaction atmosphere, are discussed.

1. INTRODUCTION The energy consumption in the world is expected to be increased simultaneously with the depletion of fossil fuel reservoirs. There is especially a need to develop technologies to produce liquid transportation fuels. In addition to biodiesel, the most common biofuel after ethanol, which is produced from triglycerides and is not fully compatible with current motor technology due to its high oxygen content, there are also other processes developed for production of diesel-type liquid fuels. These processes are hydrodeoxygenation reaction over conventional hydrodesulfurization catalysts,1 such as Ni
Mo and Co
Mo and catalytic deoxygenation over noble metal supported catalysts,2 especially Pd and Pt supported on active and mesoporous carbon. Long-chain paraffins derived from triglycerides and fatty acids are suitable biofuels to be used in the current motors. In catalytic deoxygenation over noble metals, the main liquid phase product is one less carbon containing hydrocarbon compared to the original feedstock, for example n-heptadecane from stearic acid. This method is relatively carbon efficient, since only CO2 and CO are formed as gaseous products starting from stearic acid. During recent years there has been intensive research in catalytic deoxygenation of triglycerides and fatty acids.3
8 The reaction network is not, however, fully understood until today, since the main emphasis has been in investigation of metal dispersion, optimization of reaction conditions, and selection of the reactor type. From the mechanistic point of view, it has been proposed that the reaction occurs either via decarboxylation or decarbonylation. However, the effect of the reaction atmosphere, namely presence of hydrogen, has been only briefly mentioned and was not studied from the mechanistic viewpoint. A previous work9 shows higher activity of the palladium catalyst under hydrogen rich conditions. Therefore, experiments with a model fatty acid (lauric acid) and its derivatives (lauric aldehyde and lauryl alcohol) were performed in different reaction r 2011 American Chemical Society

atmospheres. Kinetic results were applied to elucidate the reaction network, as demonstrated below.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Deposition-precipitation method was used for preparation of 5 wt % Pd catalyst on a mesoporous carbon Sibunit (400 m2/g
calculated with Dubinin
Radushkevich equation) obtained from Boreskov Institute of Catalysis (Novosibirsk), Russia. Sibunit carbon was crushed and sieved to obtain particles below 70 μm to prevent internal mass transfer limitations. Prior to Pd deposition, the support was treated with 5 wt % HNO3 overnight and thereafter dried at 80 °C for 8 h. The Pd precursor, H2PdCl4, was hydrolyzed in aqueous solution by adding Na2CO3. The formed polynuclear hydroxycomplexes of Pd(II) were deposited onto carbon surface.10 After complete Pd adsorption on a carbon surface, the catalyst was filtered and washed with deionized water until no Cl
ions could be detected in the washing water. Thereafter the catalyst was calcined in air at 200 °C for 2 h. 2.2. Deoxygenation Reaction. Experiments were performed in the 300 mL semibatch reactor coupled to a heating jacket and a condenser. Temperature in all of the experiments was kept at 300 °C. The reaction was performed at the total pressure of 2 MPa (of hydrogen or argon) and at the flow of 30 mL/min controlled by Brooks 58505S pressure controller and Brooks 5866 flow controller, respectively. The reaction mixture in the experiments contained a 100 mL solution of either lauric acid Special Issue: CAMURE 8 and ISMR 7 Received: October 21, 2011 Accepted: December 14, 2011 Revised: December 12, 2011 Published: December 14, 2011 8922

dx.doi.org/10.1021/ie202421x | Ind. Eng. Chem. Res. 2012, 51, 8922–8927

Industrial & Engineering Chemistry Research (Sigma, 99%), lauric aldehyde (SAFC, g95) or lauric alcohol (Fluka, g98.5) diluted in hexadecane (0.05 M). Prior to the experiment, 100 mg of 5 wt % Pd/C was reduced in situ with pure hydrogen using temperature program as follows: 10 °C/min to 200 °C, 1 h at 200 °C and ramp 5 °C/min to 300 °C. In the experiments with inert atmosphere, catalyst was flushed after reduction with argon for 0.5 h, at the reaction temperature, to remove all remaining hydrogen from the reactor system. The stirring speed during reaction was kept at 1200 rpm to avoid external mass transfer limitations. The reaction time was 5 h, and the samples were periodically withdrawn from the reactor, during this time. 2.3. Liquid Phase Analysis. Analysis of the liquid phase was performed with gas chromatograph (HP 5890) containing HP5 column (60 m, 0.32 mm, 0.25 μm). Sample was prepared by mixing 0.1 mL of reaction mixture with 0.1 mL of silylation agent (BSTFA + TMCS 99:1, Supelco), 0.1 mL of eicosane (Acros Organics, 99%) dissolved in hexadecane (2 mg/mL), as an internal standard, in 1 mL of pyridine (Sigma-Aldrich, g99.9%). Thereafter, the samples were left in the oven at 70 °C for 1 h to ensure proper silylation, before injection to the GC. The same sample preparation procedure was also performed before qualitative analysis in GC-MS (GC-6890N, MS-5973) with the chromatographic column DB-PETRO (100 m, 0.25 mm, 0.5 μm). 2.4. Gas Phase Analysis. Gas phase analysis was performed with a mass spectrometer (Omnistar GSD 300, Balzers Instruments) coupled to the outlet of the reactor as well as with a Micro GC (Agilent 3000) instrument. The mass spectrometer was calibrated prior to each experiment with a calibration mixture of 5 mol % hydrogen, 0.5 mol % CO, and 1 mol % CO2 in argon (AGA). For the Micro GC system each gas sample was analyzed 10 times to ensure correct peak integration. The instrument was calibrated for numerous different gases, including hydrogen, CO, CO2, and light hydrocarbons. The samples for Micro GC were collected in 50 mL bottles from the reactor outlet immediately after taking the liquid sample. 2.5. Catalyst Characterization. The specific surface area of the fresh catalyst was measured with nitrogen physisorption by Sorptometer 1900 (Carlo Erba instruments) apparatus. Prior analysis sample was degassed at 200 °C for 3 h. The specific surface area was calculated by Dubinin
Radushkevich equation, and pore distribution was obtained with Dollimore-Heal correlation. Palladium dispersion was measured by pulse CO chemisorption with the Autochem 2910 apparatus (Micromeridics). Prior analysis the catalyst was dried overnight in the oven at 100 °C. Thereafter, it was reduced in hydrogen flow at the same temperature as in the reaction, and after cooling it to the room temperature pulses of 10% CO were injected until no more adsorption occurred. Palladium particle size distribution was measured by high resolution transmission electron microscopy (LEO 912 Omega, voltage 120 kV). Histograms of the particle size distribution were obtained by counting at least 100 particles on the micrographs for the sample.

3. RESULTS 3.1. Results of Catalyst Characterization. The specific surface area of 5 wt % Pd/C catalyst was 357 m2/g calculated with Dubinin
Radushkevich equation, while the pores diameter was between 1
5 nm, obtained with Dollimore-Heal correlation.

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Figure 1. a) TEM picture and b) palladium particle distribution of fresh 5 wt % Pd/C (Sibunit) catalyst.

Figure 2. Transformation of 0.05 M lauric acid in hexadecane under a) hydrogen and b) argon over 0.1 g of 5 wt % Pd/C catalyst, at 300 °C and 2 MPa of total pressure.

The TEM results (Figure 1) show that palladium particles had average size of 2.4 ( 1.4 nm, which was confirmed with CO chemisorption measurements. Sintering and leaching of palladium on Sibunit at the same reaction conditions was shown previously to be minor.11 Therefore, influences of these phenomena can be neglected and will be not discussed in this work. 3.2. Deoxygenation of Lauric Acid. Deoxygenation of lauric acid was performed in inert and hydrogen rich atmosphere (Figure 2). The main product was undecane, which was obtained by decarboxylation/decarbonylation of lauric acid over Pd/C catalyst. A higher yield of hydrocarbons at the end of the reaction was obtained in hydrogen rich atmosphere (TOF = 0.07 1/s 8923

dx.doi.org/10.1021/ie202421x |Ind. Eng. Chem. Res. 2012, 51, 8922–8927

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Figure 3. a) Yield of hydrocarbons (undecane, undecenes, and dodecane) and b) selectivity vs conversion in the reaction of 0.05 M lauric acid in hexadecane over 0.1 g of 5 wt % Pd/C catalyst, at 300 °C and 2 MPa of hydrogen or argon total pressure.

[molsubstrate/molPd‑surface/s]), compared to the experiment in inert atmosphere where the initial deoxygenation rate was higher during the first 50 min (TOF = 0.12 1/s, calculated for 0
50 min), but its activity decreased with time keeping stable rate after 100 min (TOF = 0.045 1/s, calculated for 100
300 min) (Figure 3a). Selectivity toward desired hydrocarbons in the inert atmosphere increased gradually with the conversion of lauric acid from around 82 mol % to 91 mol % at the end of the reaction. Similar behavior was observed in the experiments with hydrogen where selectivity rises from 36 mol % at the beginning of the reaction to eventually 90 mol % at its end (Figure 3b). For reaction in hydrogen atmosphere, low selectivity at the low conversion level and increase of it, with increasing conversion, can be explained by initial high formation of intermediates which are gradually converted into products. In the reaction under argon atmosphere traces of aromatic compounds were found in the reaction mixture which is in line with the results reported in the literature previously.2,9 The gas phase analysis during lauric acid transformation under different gas atmospheres revealed different pathways of the reaction, depending on the hydrogen content in the reaction media. Previous studies demonstrated that without hydrogen lauric acid deoxygenates predominantly via decarboxylation generating CO2.2,11 Transformation in the presence of hydrogen proceeds through decarbonylation reaction which generates CO (Figure 4a), which is clearly visible from the results obtained with decreasing hydrogen content in the reaction atmosphere. In the beginning of the reaction hydrogen level was at 4% of gas

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Figure 4. Gas products distribution in the reaction of 0.05 M lauric acid in hexadecane under a) 100% hydrogen and b) initial 4% of hydrogen in argon over 0.1 g of 5 wt % Pd/C catalyst, at 300 °C and 2 MPa of total pressure.

atmosphere decreasing with time when applying 30 mL/min argon flow. Correspondingly, decarbonylation was the main reaction at the beginning of the experiment, but its rate decreases with time. Simultaneously, decarboxylation rate increases with time becoming the main reaction (Figure 4b). The amount of hydrocarbons varies depending on hydrogen content (Table 1). When hydrogen is present, the main products of lauric acid transformation are saturated hydrocarbons, predominantly undecane, a product of lauric acid decarbonylation, and dodecane which is a product of carboxylic group reduction. The yield of dodecane reached 2.5 mol % at the lauric acid conversion level of 65 mol %, which indicates that decarboxylation/decarbonylation reaction is predominant to carboxylic group reduction at 300 °C over Pd/C catalyst. In experiments without hydrogen in the reaction system, as expected, no dodecane was noticed. Moreover, contrary to the experiments in hydrogen rich atmosphere, significant yields of unsaturated hydrocarbons were observed (Table 1). Lauryl alcohol was observed in the reaction mixture under hydrogen rich atmosphere. Its presence was expected as it is an intermediate of carboxylic group hydrogenation reaction. However, only traces of lauric aldehyde were observed in the samples during the reaction, indicating high reactivity of aldehyde hydrogenation over Pd/C catalyst. 3.3. Deoxygenation of Lauric Aldehyde. Similar to lauric acid deoxygenation, experiments with lauric aldehyde were performed in inert and hydrogen rich atmospheres. These experiments revealed that the rate of aldehyde deoxygenation over Pd/C catalyst was extremely fast, showing the total conversion of lauric aldehyde to hydrocarbons during the first 5 min, 8924

dx.doi.org/10.1021/ie202421x |Ind. Eng. Chem. Res. 2012, 51, 8922–8927

Industrial & Engineering Chemistry Research

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Table 1. Deoxygenation of 0.05 M Lauric Acid, Lauric Aldehyde, and Lauryl Alcohol, over 0.1 g 5 wt % Pd/C, at 300 °C and 2 MPa of Total Pressure yield of hydrocarbons % selectivity to hydrocarbons %

undecane (C11)

undecene (C11)

dodecane (C12)

conversion % after 300 min at 40% conversion after 300 min at 40% conversion after 300 min at 40% conversion after 300 min at 40% conversion Lauric Acid Ar

43

91

88

30

27

8

8