EXAFS study of Pd/Sibunit and Pd–Ga/Sibunit catalysts for liquid-phase hydrogenation of acetylene to ethylene
Introduction
Conversion of natural and associated petroleum gas into more valuable hydrocarbons (particularly ethylene) is an important scientific and technological challenge. For large-scale production of inexpensive ethylene, an experimental technology from Synfuels Int. Inc. may be of promise. The technology consists in the production of acetylene by oxidative pyrolysis of natural gas, the absorption of acetylene in a solvent and its subsequent selective hydrogenation to ethylene in the liquid phase [1], [2]. The most important step in this process is hydrogenation of acetylene. This reaction is carried out in a solvent to avoid overheating of the reaction mixture and to enhance the process selectivity. The general composition of hydrogenation catalysts includes supported palladium modified with Ga, In, Ag, Mn, or Zn [2], [3]. The most effective modifiers are the elements of group IIIA (Ga, In) [3], but their modifying effect has been little studied.
In recent years, intermetallic compounds of palladium and gallium have become of considerable interest as catalysts for the selective gas-phase hydrogenation of acetylene to ethylene, for the steam reforming of methanol, and for the synthesis of methanol from CO2 and H2 [4], [5], [6], [7], [8], [9], [10], [11], [12]. It has been shown that the Pd–Ga intermetallic compounds are more selective and resistant to deactivation in the gas-phase hydrogenation of acetylene than conventional catalysts Pd/Al2O3 [4], [6]. Of practical interest are Pd–Ga systems deposited on different supports (activated carbon, Al2O3, SiO2, etc.) [13], [14]. Supported bimetallic Pd–Ga particles can be obtained by coprecipitation [15], coreduction of precursors of palladium and gallium in an organic medium [16], or by impregnation of the support with water [17] or alcohol [10] solutions of precursors. The last two methods are the simplest to implement.
Previously, we have studied the catalytic and structural properties of Pd/Ga2O3 [18]. On the basis of EXAFS data, it was shown that the bimetallic Pd–Ga phase is present in these catalysts in the subsurface layers of palladium particles. The presence of such an inhomogeneous alloy increases the activity of catalysts in the liquid-phase hydrogenation of acetylene. We have also attempted to obtain bimetallic Pd–Ga compounds on Sibunit via its impregnation with joint aqueous solutions of palladium nitrate and gallium nitrate (followed by drying and reduction [19]). However, despite the presence of contacts between palladium and gallium compounds on a support surface, a large part of the components located separately [19], which made it impossible to fully reveal the effect of modification of supported palladium with gallium. In the present paper, to enhance the interaction between palladium and gallium, we used a method, according to which supported bimetallic catalysts were prepared by impregnation with alcoholic solutions [10].
The aim of this work was to study the structure and catalytic properties of such catalysts in dependence on ratio Pd:Ga. Carbon composite Sibunit was selected as a support because of almost complete absence (in comparison with alumina) on its surface of strong Lewis acid sites, which promote the formation of oligomers [20].
Section snippets
Preparation of catalysts
Catalysts Pd/Sibunit and Pd–Ga/Sibunit were prepared by impregnating the support with a joint alcoholic solution of palladium and gallium [10]. An aqueous solution of palladium nitrate was dried to form brown crystals. After that, exact amount of Ga(NO3)3·8H2O (reagent grade) was added, and 80 mL of ethanol was poured under stirring to obtain full dissolution of salts. After a minute of stirring, a specific amount of Sibunit (SBET = 200 m2/g, fraction 0.09–0.07 mm) was added into the resulting
Catalytic properties of Pd/Sibunit and Pd–Ga/Sibunit
A series of catalysts with a molar ratio Pd:Ga of 1:0.1, 1:0.25, 1:0.5, 1:1, 1:2, and 1:4 (No. 1–7, Table 1) was tested in the liquid-phase hydrogenation of acetylene. The main reaction products under these conditions were ethylene and C4+ oligomers. The selectivity toward ethane was zero. The catalyst without gallium (Pd/Sibunit) was used as a reference sample. A sample that contained only gallium (without palladium) was inactive in the hydrogenation of acetylene.
As seen from Table 1, the
Conclusions
During this work, we studied the state of the active component and catalytic properties of the samples Pd/Sibunit and Pd–Ga/Sibunit prepared by support impregnation with joint alcohol solutions of precursors. In this work, it is shown that the transition from gallium oxide as carrier to a carbon composite Sibunit which does not have the Lewis acid sites, does not change the nature of the active component.
According to EXAFS data, in the catalysts modified with gallium, the active component
Acknowledgements
Authors thank E.A. Anoshkina for conducting catalytic tests.
Physicochemical studies were partially performed in the Omsk Regional Center for collective use of SB RAS (Omsk). EXAFS spectra were recorded using the equipment of the Siberian Synchrotron and Terahertz Radiation Center (Novosibirsk) under partial support by the Ministry of Education of Russia and by the Russian Science Foundation. Authors gratefully acknowledge the financial support from the RFBR, grant No. 14-03-90032 Bel_a.
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