Improved selectivity of carbon-supported palladium catalysts for the hydrogenation of acetylene in excess ethylene

doi:10.1016/j.apcata.2014.05.027

Applied Catalysis A: General

Volume 482, 22 July 2014, Pages 108-115

https://doi.org/10.1016/j.apcata.2014.05.027 Get rights and content

Highlights

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Novel method of preparation ensures similar particle size.
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EXAFS and AC-STEM confirm similar particle sizes.
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Highest selectivity for acetylene hydrogenation seen on Pd/C.
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Oxide supports such as MgO or Al₂O₃ lead to over hydrogenation.
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Electronic properties of Pd influenced by the carbon support.

Abstract

In this study we examine the role of the support for selective hydrogenation of acetylene. Palladium (Pd) nanoparticles with a narrow size distribution were deposited on three supports, carbon, alumina and magnesia. The Pd particles ranged from 0.5 to 1.0 nm in diameter. A novel synthesis based on room temperature alcohol reduction of the Pd acetate precursor allowed us to deposit similar sized Pd particles on all three supports. We used electron microscopy and X-ray absorption spectroscopy (EXAFS) to characterize these samples and to confirm the similarity of the distribution and the size of the nanoparticles on all three supports. The carbon-supported Pd yielded a higher selectivity to ethylene at 100% acetylene conversions (from acetylene/ethylene mixtures) when compared to the oxide-supported samples. This work provides clear evidence that the support can play an important role in the selective hydrogenation of acetylene. While alumina is extensively used in industry as the support for Pd and Pd alloys, considerable improvements in selectivity could be made by the use of carbon supports.

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 (∼Z^1.7), 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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¹: Present address: Sandia National Laboratories, Chemical and Biological Systems Department, PO Box 5800, Albuquerque, NM 87185-0734, USA.

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Improved selectivity of carbon-supported palladium catalysts for the hydrogenation of acetylene in excess ethylene

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of supported catalyst

Transmission electron microscopy

Discussion

Conclusions

Acknowledgments

Catal. Today

Appl. Catal., A: Gen.

J. Catal.

Appl. Catal., A: Gen.

Appl. Catal., A: Gen.

J. Catal.

Appl. Catal., A: Gen.

J. Catal.

Appl. Catal., A: Gen.

Catal. Today

J. Catal.

J. Catal.

J. Catal.

J. Catal.

J. Catal.

Nanostruct. Mater.

J. Chem. Soc., Faraday Trans.

Trans. Faraday Soc.