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Ind. Eng. Chem. Res. 2008, 47, 7184–7189
Syngas Production from Catalytic Partial Oxidation of n-Butane: Comparison between Incipient Wetness and Sol-gel Prepared Pt/Al2O3 Rainer J. Bass, Timothy M. Dunn, Yu-Chuan Lin, and Keith L. Hohn* Department of Chemical Engineering, Kansas State UniVersity 1005 Durland Hall, Manhattan, Kansas 66506-5102
Catalytic partial oxidation (CPO) of n-butane under autothermal condition was studied over both incipient wetness (IW) and sol-gel (SG) prepared Pt/Al2O3 catalysts. These catalysts gave different results, with SG Pt/Al2O3 producing more H2 and CO than IW Pt/Al2O3 under the same reaction conditions. The difference between these two may be attributed to the particle size of platinum: the observed Pt particle sizes on both freshly prepared and used SG Pt/Al2O3 was 50% smaller than those on IW Pt/Al2O3. 1. Introduction Syngas, a mixture of CO and H2, is highly valued as a building block for organic chemicals like paraffinic liquid fuels through Fischer-Tropsch chemistry and ethylene glycol through glycol synthesis.1 It is also an intermediate in the production of high-purity hydrogen for use in fuel cells.2 Numerous methods have been studied to convert hydrocarbons to syngas. These include catalytic partial oxidation (CPO), steam reforming, autothermal reforming, and CO2 reforming.3,4 CPO is attractive because it is exothermic and can achieve a high production rate of syngas with a compact design.5-8 Because of the interest in on-board generation of hydrogen to power portable fuel cell power systems, liquid fuels are of interest because of their high energy density and established distribution infrastructure.9 Butane is attractive for portable power generation since it can be easily liquefied. There have been only a few studies on CPO of n-butane to syngas: C4H10 + 2O2 f 4CO + 5H2
(1)
2,10-12
studied this reaction at Schmidt and his collaborators short contact times with noble metal coated monoliths. They found that rhodium gave higher yield of syngas, whereas platinum produced mostly cracking products such as alkanes and olefins. Aksoylu’s group13,14 used a bimetallic catalyst comprised of platinum and nickel for CPO of liquefied petroleum gas (LPG). They found that dual-bed and single-bed catalyst configurations made up of Pt and Ni were inferior to a single bed of bimetallic platinum and nickel catalysts. Although nickel-based catalysts were widely adopted in CPO of hydrocarbons under autothermal conditions, their catalytic deactivation remains a critical issue.15-17 Rhodium has been found to give a higher yield of syngas than platinum;18,19 however, the soaring price decreases its attractiveness.20-22 For this reason, platinum may be one of the promising candidates in autothermal CPO if its selectivity to syngas can be improved. One way to do so may be through tailoring the structure of catalyst via its preparation method.23 Several papers have indicated that Pt dispersion may play a role in CPO.24,25 Veser and his co-workers26 speculated that smaller particles may lead to higher selectivity by controlling the surface concentration of oxygen. They suggested that oxygen coverage is important in determining whether complete or partial oxidation occurs. The same research group found that platinum nanoparticles were * Corresponding author. Tel.: +1-785-532-4315. Fax: +1-785-5327372. E-mail:
[email protected].
more selective in methane partial oxidation.25 It is highly plausible that butane CPO will likewise be improved by the use of platinum nanoparticles. In this work, the effect of preparation method on CPO of butane under autothermal conditions is studied. Al2O3 was synthesized via a sol-gel method, and platinum was loaded either by incorporating a platinum salt into the gel or by using an incipient wetness technique. The resulting catalysts were characterized to obtain the Pt particle size and dispersion and then tested in CPO of butane for different butane-to-oxygen ratios. 2. Experimental Section 2.1. Catalyst Preparation. Both incipient wetness (IW) and sol-gel (SG) prepared Pt/Al2O3 were used in this study. The difference between these two is how the Pt was incorporated into the alumina support. Details on preparing amorphous alumina can be found elsewhere.27 Briefly, 25 mL of aluminum trisec-butoxide (ATB, Fluka, >95%) was dissolved in 10 mL of deionized water and ∼100 mL of ethanol. This mixture was then stirred and aged in a sealed, water bath-heated flask at 40 °C until it turned into a transparent solution (Solution A). By dissolving a designated amount of dihydrogen hexachloroplatinate hexahydrate (H2PtCl6 · 6H2O, Alfa Aesar, 99.9%) into 4 mL of ethanol, the aqueous platinum precursor could be made (Solution B). 2.1.1. Incipient Wetness Prepared Pt/Al2O3 (IW Pt/ Al2O3). Solution A was dried in a rotavapor (Brinkmann Instruments, R110) at 60 °C. The remaining paste was calcined at 500 °C overnight, yielding alumina powder. Solution B was then dripped on the powder at room temperature. After air-drying, the sample was dried at 100 °C overnight, followed by calcination at 500 °C in a 10% O2/Ar stream for 8 h. 2.1.2. Sol-gel Prepared Pt/Al2O3 (SG Pt/Al2O3). By slowly dripping Solution B into Solution A, a clear, yellowish mixture was obtained. This mixture was aged at 60 °C for a day with continuous rotation in the rotavapor. Subsequently, the solvent was removed under vacuum at 80 °C for 1 h and finally at 100 °C for another 1 h. The resulting paste was then dried and calcined in the same way as IW Pt/Al2O3. These steps are similar to the technique reported by Balakrishna and Gonzalez.28 2.2. Catalyst Characterization. The dispersions of platinum on both freshly prepared and used catalysts were measured on an AMI-200 (Altamira Instruments) by hydrogen chemisorption. About 0.1 g of sample was used for each trial. The sample was first heated in a furnace with a 10 °C/min heating rate from
10.1021/ie8007506 CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 7185
Figure 1. TEM images of 2 wt % freshly prepared (a) IW and (b) SG Pt/Al2O3; 2 wt % used (c) IW and (d) SG Pt/Al2O3. Note that the arrows indicate Pt particles.
ambient temperature to 500 °C in a 10% O2/Ar stream. That temperature was maintained for 1 h. Next, the sample was purged with argon and reduced in 5% H2/Ar for 1 h. The chemisorption of H2 was carried out by dosing pulses (519 µL each) of 5% H2/Ar through the sample at room temperature until saturation, allowing the dispersion of Pt to be estimated.29 Transmission electron microscopy (TEM) images were collected by Dan Boyle at the Microscopy and Image Processing Facility of Kansas State University using a Philips CM 100 TEM. Ground Pt/Al2O3 powder was loaded onto a carbon-coated copper grid for imaging. 2.3. Catalytic Reactivity Testing. The reactor setup and online gas chromatography system were essentially identical to those described previously.30,31 To start the reaction, a Bunsen burner was used to heat the catalyst bed to its ignition temperature. After attaining light-off, no external heat was required and the reaction zone was insulated with packed alumina mat (Zircar). Approximately 0.19 g of sample with particle size of 40-45 mesh was used for each test. Prior to all runs, the samples were pretreated at 500 °C in a flow of 5% H2/Ar for 1 h. The temperature of the catalyst bed was measured with a type-K thermocouple placed against the back edge of the catalyst bed. All gases were acquired from Linweld. The inlet gases were composed of ∼60 mol % N2 (99.9%) with O2 (99.0%) and n-C4H10 (99.9%) in appropriate amounts to vary the O/C ratio
from 0.5 to 1.5. All trials were kept at gas hourly space velocity (GHSV) equal to ∼10 800 h-1 (total volumetric flow rate of 0.255 standard liters per minute (SLPM)). N2 was used as an internal standard. CO, CO2, H2, and H2O were the main products, with lesser amounts of light hydrocarbons. For the data reported, the carbon and hydrogen mass balances closed within 10%. 3. Results and Discussion 3.1. Catalyst Morphology. Figure 1 shows the TEM micrographs of freshly prepared and used IW and SG Pt/Al2O3 catalysts with 2 wt % loading; the dark spots are the Pt particles. For the IW sample, the platinum particles are clearly seen on the surface of alumina. The observed particle size for the freshly prepared IW sample (Figure 1a) was estimated by measuring the diameter of 25 particles in two TEM micrographs looking at two different locations and was found to be 6.6 ( 3.7 nm. The micrograph for the freshly prepared SG sample is much more complicated. The dark spots for the Pt particles (indicated with arrows in Figure 1b) are less clearly defined, probably because the particles are partly or completely encapsulated by the alumina support. The observed particle size was estimated by measuring the diameters of 20 particles in micrographs of two different locations on the catalysts and was found to be 4.5 ( 1.4 nm. The difficulty in distinguishing the Pt particles
7186 Ind. Eng. Chem. Res., Vol. 47, No. 19, 2008 Table 1. Surface Areas and Dispersions of Freshly Prepared and Used Pt/Al2O3 freshly prepared
used
1 wt % Pt 2
IW SG
2 wt % Pt 2
1 wt % Pt 2
2 wt % Pt 2
S.A. (m /g)
disper. (%)
S.A. (m /g)
disper. (%)
S.A. (m /g)
disper. (%)
S.A. (m /g)
disper. (%)
118.8 281.2
26.8 50.0
137.2 180.1
32.3 57.0
110.4 201.3
3.2 3.3
100.4 187.9
7.8 4.7
from the support makes the exact particle size distribution difficult to measure; however, it can be concluded based on the large changes in the TEM micrographs that the particle size of the fresh SG sample is both smaller and more monodisperse than that of the fresh IW sample. Table 1 lists the surface areas and dispersions for both freshly prepared and used (reacted at O/C ) 1 for 16 h) catalysts. The dispersions for fresh SG Pt/Al2O3 were about twice that of IW Pt/Al2O3, even though some of the Pt is expected to be encapsulated. It is, therefore, highly likely that the rest of the nonencapsulated Pt particles on SG Pt/Al2O3 were very small and finely dispersed, in agreement with the observation from TEM. The decreased dispersions for samples after reaction were probably due to changes in the structure of the catalyst32 or coke deposition33 under autothermal conditions, as discussed later. 3.2. Catalytic Results. Figures 2, 3, and 4 plot the selectivities of CO and H2 as functions of time-on-stream at O/C of 0.5, 1.0, and 1.5, respectively. As seen in these figures, some of the samples show a slight descending trend in selectivity over time, particularly the 1 wt % SG sample. Selectivities are generally higher for 2 wt % samples than 1 wt % samples. CO and CO2 make up 95% or more of the carbon-containing products. Generally, methane was the most prevalent minor product. For hydrogen selectivity, hydrogen and water were the predominant products, while hydrocarbon products contribute only a few percent to the hydrogen selectivity. Selectivity of hydrogen was generally low (under 50%), especially for O/C ) 0.5. This is not surprising for platinum: Huff and Schmidt11 reported hydrogen selectivities of
