Improved selectivity of carbon-supported palladium catalysts for the hydrogenation of acetylene in excess ethylene
Graphical abstract
Introduction
Ethylene is a high volume commodity that is important for the polymer industry. It is used to produce chemical compounds such as ethylene oxide, polyethylene, and ethylene dichloride, which are precursors for many consumer products including surfactants, detergents, plastic bags, films and piping. Ethylene is produced by the steam cracking of hydrocarbons and acetylene is a byproduct of this process. The 0.5–2% of acetylene in the ethylene is enough to poison the catalysts used to polymerize ethylene into polyethylene [1]. Palladium (Pd) catalysts have been shown to be effective to selectively hydrogenate acetylene to ethylene while preventing over hydrogenation to ethane [2]. Catalysts used for this reaction must retain high selectivity to ethylene at high acetylene conversions since it is essential to reduce to amount of acetylene to 5–10 ppm in order to protect the polymerization catalysts [3].
In current industrial practice, Pd catalysts are modified with an additive such as silver, silicon, germanium or lead [4], [5], [6] and α-alumina is often used as a support. The additives help to improve the selectivity, but there still many challenges in the industrial application of these selective hydrogenation catalysts. The catalyst experiences deactivation over time and undergoes restructuring during regeneration which affects its performance. There is often carbon monoxide (CO) in the feed stream which improves selectivity [7] but the CO leads to the undesirable formation of oligomers, often referred to as green oil. These green oils build up on the catalyst surface and cause deactivation [8], [9], [10].
Studt et al. [11] performed density functional calculation studies that suggest that ethylene selectivity is related to the binding energies of the adsorbates onto the metal surface. This study showed that the activation barrier energy for ethylene hydrogenation for Pd alone is about the same as the ethylene desorption energy. Their calculations show the addition of silver (Ag) increases the activation barrier for the hydrogenation of ethylene to an energy that allows for desorption of ethylene before it can be further hydrogenated. This work suggests that selectivity in the acetylene hydrogenation reaction is strictly a result of binding of the adsorbates to the metal surface and is unaffected by the catalyst support. However, previous studies have shown that the support may play a role in selectivity [12]. Bauer et al. [13] studied selective hydrogenation of propyne and found that Pd supported on alumina was not selective to propene when conversion of propyne approached 100%. In contrast, the Pd supported on carbon nanotubes was more selective. The samples they studied had a broad particle size distribution, but they also found that particle size influenced selectivity. Therefore, to study the role of the support, we feel it is important to keep other parameters such as catalyst precursor and metal particle size constant.
It is difficult to synthesize catalysts having the same particle size (or dispersion) on different supports, using aqueous precursors. This is because supports such as SiO2 or Al2O3 have different points of zero charge (PZC) which influence the adsorption of metal salts [14]. The PZC is 1.1 and 7.9 for SiO2 and Al2O3, respectively, influencing the resulting particle size for catalysts prepared using incipient wetness impregnation [15]. For another commonly used support zinc oxide (ZnO), the strongly acidic solutions of the precursor make it difficult to prepare catalysts without modifying the support morphology [16]. Hence, there is a need to use a synthesis method that has the ability to deposit similar sized particles on different supports starting from the same precursor. There are colloidal methods that use ligands and capping agents to generate uniformly sized particles but these ligands must be removed otherwise they will interfere with the catalyst activity [17], [18]. The removal of the ligands often involves high temperatures and harsh conditions that can cause particle sintering and also add an extra costly and time consuming step during synthesis, and could influence the support and/or metal support interface. The ligand removal step could also influence the selectivity and activity of the metal phase [19], [20].
In this work we used a method recently developed by Burton et al. [21] where methanol is used as the reducing agent for a Pd acetate precursor at room temperature. Using this method, Pd nanoparticles can be deposited on both oxide and carbon supports while maintaining similar particle sizes. This synthesis method allowed us to generate highly dispersed Pd particles (∼1 nm in diameter) so as to maximize the metal-support interaction. We prepared Pd particles with an average diameter less than 1 nm on carbon, Al2O3 and magnesium oxide (MgO). The Pd catalysts were tested for the hydrogenation of acetylene in excess ethylene, and the results show that the catalytic performance varies markedly with the support, with the carbon support providing the highest selectivity under conditions where the conversion of acetylene is near 100%.
Section snippets
Preparation of supported catalyst
Supported 1 wt% Pd nanoparticle catalysts were synthesized via a method developed by Burton et al. [21]. The alcohol reduction method is carried out by dissolving anhydrous Pd(OAc)2 (20 mg) in anhydrous methanol (30 mL) in a glass vial to get 3 mM Pd acetate in methanol. This was done under ambient pressure and temperature. The vial was capped and placed in a ultrasonic bath for ∼10 min, until the Pd(OAc)2 was completely dissolved. The solution was then added to 1 g of the support in a round bottom
Transmission electron microscopy
Aberration corrected electron microscopy (ACEM) was used to determine the particle size and morphology of as-prepared samples. A representative image of each sample, Pd/C, Pd/MgO, and Pd/Al2O3, is shown in Fig. 1. The image contrast in this high angle annular dark field (HAADF) mode is atomic number dependent (∼Z1.7), hence the bright regions represent Pd particles while the less bright regions represent the support. All of the images are shown at the same magnification. The Pd particles range
Discussion
The goal of this study was to prepare catalysts with similar particles sizes on a variety of supports. The electron microscopy data confirms that the majority of the particles are between 0.5 and 1 nm in diameter. EXAFS was used to corroborate the information obtained via STEM. We studied only the as-prepared samples to capture the characteristics that represent the sample as it was loaded into the reactor, representing the precursor to the working catalyst. Samples after reaction were not
Conclusions
We have deposited a narrow size distribution of Pd nanoparticles on 3 different supports, one carbon and two oxides (MgO and Al2O3), using identical synthesis methods and Pd precursor. This allows us to eliminate the effects that particle size, ligands, or capping agents may have on the reaction and allow us to isolate the role of the support. We have shown carbon-supported Pd is more selective toward the formation of ethylene during the hydrogenation of acetylene (in excess ethylene) than
Acknowledgments
The electron microscopy was performed at the Environmental Molecular Sciences Laboratory (EMSL), a user facility operated by the DOE at Pacific Northwest National Laboratory. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. The research was supported NSF grants OISE 0730277 and DOE grant
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Present address: Sandia National Laboratories, Chemical and Biological Systems Department, PO Box 5800, Albuquerque, NM 87185-0734, USA.