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Catalytic Deoxygenation of Stearic Acid in a Continuous Reactor over a Mesoporous Carbon-Supported Pd Catalyst Siswati Lestari,†,‡ Pa¨ivi Ma¨ki-Arvela,† Heidi Bernas,† Olga Simakova,†,§ Rainer Sjo¨holm,† Jorge Beltramini,‡ G. Q. Max Lu,‡ Jukka Myllyoja,| Irina Simakova,§ and Dmitry Yu. Murzin*,† Process Chemistry Centre, Åbo Akademi UniVersity, Turku FI-20500, Finland, ARC Centre of Excellence for Functional Nanomaterials, UniVersity of Queensland, Brisbane, Queensland 4072, Australia, BoreskoV Institute of Catalysis, NoVosibirsk 630090, Russia, and Neste Oil Oy, PorVoo 06101, Finland ReceiVed February 10, 2009. ReVised Manuscript ReceiVed May 25, 2009
Catalytic deoxygenation of neat stearic acid was studied at 360 °C under 10 bar argon or 5 vol % hydrogen in argon in a fixed-bed reactor (down flow) using mesoporous-supported Pd/C (Sibunit) beads as a catalyst. The results showed stable catalyst performance, giving about 15% conversion level of stearic acid. The main liquid-phase product was heptadecane, while the main gaseous products were CO and CO2.
1. Introduction It is necessary to use renewable and carbon-neutral biofuels to ensure the access of energy at an affordable price and to prevent environmental damage. Examples on biofuels produced industrially are biodiesel (fatty acid methyl esters), bioethanol, and NexBTL (Neste Oil). However, to improve the production technology used today and to find new types of fuels, extensive research still has to be made. We have focused our research on NExBTL, which is a fuel consisting of n-paraffinic compounds obtained through hydrotreating of vegetable oils and animal fats.1 NExBTL has a very high cetane number and is free from aromatics and sulfur. It has been commercially available since 2007, produced by Neste Oil in Porvoo, Finland. The current production technology, where alkanes are produced through hydrotreating, requires large volumes of hydrogen. To avoid the use of hydrogen and, consequently, ensure a higher safety and reduce the production costs, the feedstock can be deoxygenated under an inert atmosphere (alternatively inert gas with 1-5% hydrogen). Deoxygenation experiments in laboratory scale under an inert atmosphere or with small amounts of hydrogen present have been performed in previous studies.2-7 * To whom correspondence should be addressed. E-mail: dmurzin@ abo.fi. † Åbo Akademi University. ‡ University of Queensland. § Boreskov Institute of Catalysis. | Neste Oil Oy. (1) Neste Oil. NExBTL diesel, http://www.nesteoil.com/default.asp? path)1,41,11991,12243,12335, 14.4.2009. (2) Snåre, M.; Kubickova´, I.; Ma¨ki-Arvela, P.; Era¨nen, K.; Wa¨rnå, J.; Murzin, D. Yu. Production of diesel fuel from renewable feeds: Kinetics of ethyl stearate decarboxylation. Chem. Eng. J. 2007, 134, 29–34. (3) Ma¨ki-Arvela, P.; Kubickova, I.; Era¨nen, K.; Snåre, M.; Murzin, D. Yu. Catalytic deoxygenation of fatty acids and their derivatives. Energy Fuels 2007, 21, 30–41. (4) 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. (5) 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.
The catalytic deoxygenation of fatty acids and their derivatives has been intensively investigated in the liquid phase by screening different heterogeneous-supported metal catalysts.4 The most promising catalysts were commercial Pt and Pd on an active microporous carbon support. Several kinds of raw materials, including fatty acids from C12 to C22, both saturated and unsaturated, as well as their esters and triglycerides, were applied as feedstock.6 Recently, mesoporous synthetic carbon-supported palladium catalysts were applied for fatty acid deoxygenation in a semibatch reactor.7 Four different 1 wt % Pd on synthetic mesoporous carbon, (Sibunit) catalysts with varying Pd dispersion in a range of 19-72%, were investigated using a mixture of stearic acid and palmitic acid as reactants diluted with dodecane at 260-300 °C under 17.5 bar H2/Ar. The optimum metal dispersion was found to be 47%. The synthetic carbon support has high mechanical strength, large mesoporous volume, tunable pore sizes, and narrow pore size distribution. It is commercially available for both batch and fixed-bed application.8 To study catalyst deactivation, it is necessary to perform deoxygenation in continuous mode, because the investigation of the catalyst deactivation in the batch mode is very difficult. When a batch reactor is used, the separation of the kinetics and catalyst deactivation is challenging. The first attempts in continuous deoxygenation of fatty acids were performed using diluted lauric acid (C12) as a feedstock over a microporous commercial Pd/C catalyst in a up-flow fixed-bed reactor (no gas flow).9 The results showed, however, a severe catalyst deactivation within the first 20-30 min time-on-stream. (6) Murzin, D. Yu.; Kubickova, I.; Snåre, M.; Ma¨ki-Arvela, P.; Myllyoja, J. Method for the manufacture of hydrocarbons. WO 2006075057, 2006; p 21. (7) Simakova, I.; Simakova, O.; Ma¨ki-Arvela, P.; Simakov, A.; Estrada, M.; Murzin, D. Yu. Deoxygenation of stearic acid over supported Pd catalysts: Effect of metal dispersion. Appl. Catal., A 2009, 355, 100–108. (8) Simakova, I. L.; Simakova, O.; Romanenko, A. V.; Murzin, D. Yu. Hydrogenation of vegetable oils over Pd on nanocomposite carbon catalysts. Ind. Eng. Chem. Res. 2008, 47, 7219–7225. (9) Ma¨ki-Arvela, P.; Snåre, M.; Era¨nen, K.; Myllyoja, J.; Murzin, D. Yu. Continuous decarboxylation of lauric acid over Pd/C catalyst. Fuel 2008, 87, 3543–3549.
10.1021/ef900110t CCC: $40.75 2009 American Chemical Society Published on Web 07/07/2009
Catalytic Deoxygenation of Stearic Acid
Figure 1. Schematic picture of the reactor system.
The aim of the present work was to develop a method for the catalytic liquid-phase deoxygenation of stearic acid in a laboratory-scale fixed-bed reactor (down flow) and to determine the activity and the selectivity, as well as the catalyst stability and possible deactivation. In this study, pure stearic acid is used as a model component; in future research using industrial feedstock, stearic acid can be present in the feed or formed during the reaction as an intermediate product. A 5 wt % Pd/C (Sibunit) catalyst was used, and argon or a mixture of 5 vol % hydrogen in argon was used as the reaction atmosphere. Moreover, a commercial Pd/C catalyst (for fixed-bed application) was tested. 2. Experimental Section 2.1. Materials. Stearic acid (Merck, 97%) was used as a feedstock; dodecane (Fluka) was used as a solvent (for wetting the catalyst, dilution of the samples, and washing of the reactor system); and N,O-bis(trimethyl)-trifluoroacetamide (BSTFA) was used as a silylation agent. 2.2. Catalyst Preparation Method. The Pd/C Sibunit catalyst with 5 wt % metal (Pd) loading was prepared by hydrolysis of H2PdCl4 at pH 5-6 to yield so-called polynuclear hydroxocomplexes of palladium followed by their adsorption on carbon beads and increasing of the pH of the slurry up to a Na/Pd ratio of 1:2.10 The carbon Sibunit support was preoxidized by treatment with 5 wt % HNO3 for 17 h, then washed by distilled water, and dried. The Pd deposition was performed via sequential adsorption and hydrolysis of H2PdCl4 and Na2CO3. Thereafter, the catalysts were washed with water and dried at 70 °C. 2.3. Experimental Procedure. The catalytic deoxygenation experiments were carried out in a fixed-bed reactor (down flow). The reactor length was 150 mm, and the diameter 15.9 mm. Except for the fixed bed, the reactant feed vessel, pump, lines, sampling valve, and collector of the residue were also heated (schematic picture of the reactor system in Figure 1). In a typical experiment, 10 g of the catalyst was placed between layers of quartz sand and quartz wool. A high-performance liquid chromatography pump (Eldex Pump 1 SMP) was used for pumping of the reactant (a heater (10) Simonov, P.; Troitskii, S.; Likholobov, V. Preparation of the Pd/C catalysts: A molecular-level study of active site formation. Kinet. Catal. 2000, 41, 255–269.
Energy & Fuels, Vol. 23, 2009 3843 block was put around the pump head, which was heated using a heating cartridge). Mass flow and pressure controllers were manufactured by Brooks Instrument. At the beginning of the experiment, dodecane was pumped into the reactor at a volumetric flow of 0.1 mL/min. The catalyst reduction was carried out under H2 and dodecane flow at the following conditions: 100 °C for 1 h under 50 mL/min hydrogen flow. Thereafter, the reactor was flushed with Ar and pressurized to the reaction pressure, and the temperature was increased by 10 °C/min to the desired reaction temperature. The reaction was started by pumping 0.075 mL/min of neat stearic acid (Merck, 97%). Several liquid-phase samples were withdrawn from the reactor via a sampling valve during experiments. The amount of CO and CO2 was continuously measured using IR-active component Siemens Ultramat 6 continuous process gas analytics. 2.4. Product Analysis. Typically, 100-200 mg of liquid-phase samples was dissolved in 4 mL of dodecane, and then 100 µL of this mixture was silylated with 100 wt % excess of N,Obis(trimethylsilyl)-trifluoroacetamide (BSTFA) for GC analyses. The internal standard eicosane (C20H42) was also added for quantitative calculations. After the addition of the silylation agent, the samples were kept in an oven at 60 °C for 1 h. The samples were analyzed with a gas chromatograph (HP 5890) equipped with a nonpolar column (HP-1, with the dimensions of 60 m × 0.32 mm and film thickness of 0.5 µm) and a flame ionization detector. A 1 µL sample was injected into the GC with a split ratio of 50:1 and He as a carrier gas. The injector and detector temperatures were 280 and 290 °C, respectively. The following temperature program was used for analysis: 130-169 °C (1 °C/min) hold for 15 min, 246 °C (5 °C/min) hold for 3 min, and 300 °C (10 °C/min). The product identification was validated with a gas chromatograph-mass spectrometer (GC-MS). Some liquid-phase samples were analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy (Bruker AV-600) to detect the presence of aromatic compounds. The presence of phosphorus, calcium, and sulfur in fatty acids was investigated by ICP-MS. The gas phase was analyzed off-line using a GC (FP 6890) with the following temperature program: 40 °C (7.5 min), 25 °C/min, 80 °C (7 min), 25 °C/min, and 140 °C (5.5 min) using a Porapack Q column (length, 71 m; diameter, 500 µm; film thickness, 50 µm). The gaseous compounds were quantified using the following calibration gases: propene (393 ppm), propane (406 ppm), carbon monoxide (362 ppm), nitrogen (531 ppm), carbon dioxide (12%) in He (AGA) and CO at 499 ppm, ethane at 0.098%, and methane at 0.2126% in He (AGA). 2.5. Materials Characterization. The specific surface area measurements were conducted with a physisorption/chemisorption instrument sorptometer 1900 (Carlo Erba instruments). The specific surface area was calculated from the nitrogen adsorption-desorption isotherms using the Brunauer-Emmett-Teller (BET) equation. The pore size distribution was obtained from the Dollimore-Heal correlation. The possible leaching of palladium was determined by an inductively couple plasma-optical emission spectrometer (ICP-OES) Perkin-Elmer Optima 5300 DV optical emission spectrometer. To approximately 0.2 g of a liquid-phase sample, 5 mL of HNO3 (65 wt % purity) and 1 mL of H2O2 (30 wt % purity) were added and heated using a microwave oven. The samples were diluted to 100 mL and analyzed by ICP-OES. The distribution of Pd in the catalyst bead was investigated by a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) using the laser ablation system New Wave UP213 and the ICP-MS Perkin-Elmer Sciex Elan 6100 DRC Plus. The catalyst bead was cut and put into epoxy glue on the glass plate.
3. Results and Discussion 3.1. Decarboxylation of Stearic Acid. The effect of the reaction atmosphere on the catalytic deoxygenation of concentrated stearic acid was studied under argon atmosphere and in
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Lestari et al. Table 1. Specific Surface Area and Pore Volume of Fresh and Spent Catalysta surface areab pore volumec total adsorption (m2/g) (cm3/g) volume (cm3/g)
sample C Sibunit support 5 wt % Pd/C Sibunit fresh 5 wt % Pd/C Sibunit spent
504 358 122
1.23 0.75 0.24
907 545 319
a Spent catalyst from the deoxygenation of neat stearic acid. b Surface area calculated by the BET method. c Pore volume measured at P/P0 ) 0.9999.
Table 2. Pore Size Distribution of Fresh and Spent Catalyst (%) macropores and mesopores
Figure 2. Molar fraction versus time-on-stream for the deoxygenation of neat stearic acid (liquid sum is the sum of heptadecane and heptadecene). Conditions: 5 wt % Pd/Sibunit; reaction temperature, 360 °C; reaction pressure, 10 bar (first argon and then 5% hydrogen in argon); volumetric flow rate of stearic acid, 0.075 mL/min.
Figure 3. Molar flow of liquid-phase heptadecane and heptadecene and the gas-phase CO and CO2 versus time-on-stream for the deoxygenation of neat stearic acid (liquid sum is the sum of heptadecane and heptadecene). Conditions: 5 wt % Pd/Sibunit; reaction temperature, 360 °C; reaction pressure, 10 bar (first argon and then 5% hydrogen in argon); volumetric flow rate of stearic acid, 0.075 mL/min.
a gas mixture of 5 vol % hydrogen in argon. In the first run under argon atmosphere, the reactor was initially filled with dodecane and stearic acid was pumped with 0.075 mL/min of volumetric flow. Because of the reactor dynamics, the concentration of stearic acid was very low in the beginning and then it gradually increased by increasing time-on-stream (Figure 2). Therefore, the concentration of the main product, n-heptadecane, was initially high, and it decreased with increasing time-onstream until 20.5 h (1230 min) without reaching the steadystate conditions giving 15% conversion. The main gaseous products were CO and CO2, and their levels were constant after 20.5 h (1230 min) time-on-stream, which could indicate that the steady-state operation would have been reached also in the liquid phase (Figure 3). The consecutive experiment was started by releasing the argon atmosphere while keeping the reaction temperature constant. After the reactor was pressurized with 5 vol % hydrogen in argon, the pumping of stearic acid was started. The same conversion level was achieved compared to the first experiment, indicating a stable catalyst performance (Figure 2). The catalytic activity remained stable from 25 h (1500 min) to 45 h (2700 min) time-on-stream under these conditions. The off-line gas-phase analysis showed, similar to the online analysis, more CO than CO2 in the gas. Moreover, methanation
micropores
catalyst
100-10 nm
10-5 nm
5-2 nm
