A study on structural features of bimetallic Pd-M/C (M: Zn, Ga, Ag) catalysts for liquid-phase selective hydrogenation of acetylene
Graphical abstract
Introduction
Nowadays, palladium supported on different carriers is one of the most intensively studied catalytic systems. Pd-based catalysts exhibit excellent activity in such important from practical point of view processes as hydrogenation of organic acids and oils [1,2] and halogen-substituted organic compounds [3,4], selective hydrogenation of polyunsaturated compounds [5] and acetylene [[6], [7], [8], [9], [10], [11], [12]], reduction of aromatic nitrocompounds [13], oxidation of methane [[14], [15], [16]] and carbon monoxide [[17], [18], [19], [20]], etc. At the same time, a tendency towards using the bimetallic [[21], [22], [23], [24], [25], [26], [27], [28], [29]] or even trimetallic [30] systems instead of Pd-only catalysts has to be stated. Doping the Pd-based catalysts with second metal creates a new type of active sites consisting of palladium and modifying metal atoms. As a rule, such sites facilitate an improvement of the process selectivity thus increasing the yield of the target product. In the case of bimetallic catalysts for the selective hydrogenation of acetylene to ethylene, a key role is considered to be played by a dilution of Pd atoms with atoms of modifying metal within the structure of the nanophases being formed. This leads to an increase of the distance between neighboring atoms of palladium from 2.75 Å for metallic Pd to approximately 2.82–2.99 Å depending on the applied modifier [27,[31], [32], [33]]. Such structure of the active sites practically rules out the possibility for strong multisite adsorption of acetylene and, thereby, prevents the realization of secondary reactions such as the hydrogenation of ethylene to ethane and formation of C4+ oligomers [21,32]. At the same time, a contribution of electron interaction of Pd with modifier is of significant importance. Atoms of second metal act as donors of electron density for vacant d-orbitals of palladium that suppresses an affinity of Pd towards adsorption of unsaturated compounds, facilitates desorption of ethylene and impedes its complete hydrogenation to ethane [21,34].
Among the promising catalysts for the selective hydrogenation of acetylene, Pd-containing catalytic systems doped with silver, gold, zinc, gallium, indium, copper and manganese should be mentioned [21,22,34]. These catalysts were reported to be efficient as for industrial gas-phase purification of ethylene from acetylene traces, as for hydrogenation of relatively large amounts of C2H2 (above 4 vol.%). For example, the latter process is used in an experimental technology patented by Synfuels International Inc., where acetylene obtained via pyrolysis of natural and associated oil gases is subjected to hydrogenation procedure in order to produce marketable ethylene and/or high-octane components of engine fuels [35,36]. Because the process is strongly exothermic (ΔH = 174.5 kJ/mol), this requires to be performed in a solvent medium for the safety reasons. This approach allows one not only removing the reaction heat but facilitates an improvement in selectivity towards ethylene. On the other hand, a description in the literature of the bimetallic catalysts applied for the considered process is limited by the mentioned patents [35,36]. Thereby, a detailed research in this field attracts both the fundamental and practical interests.
Recently we have already reported about carbon-supported Pd-Ga catalysts for the liquid-phase hydrogenation of acetylene [6,[37], [38], [39]]. Porous carbonaceous material Sibunit was used as a support, which choice was stipulated, first of all, by absence of strong surface acid sites responsible for oligomerization of acetylene [40,41]. Moreover, carbon materials are more preferable than oxide supports (for instance, Al2O3) because in the case of the latter a part of modifier’s atoms can be lost due to their incorporation into the support bulk with formation of solid solutions [42]. Carbon materials, in their turn, were reported to show a weak or even negligible metal-support interaction [43]. It is obvious that elimination of the negative metal-support interaction should intensify the efficiency of Pd-M interaction (where M is second metal) resulting in the formation of more active and selective bimetallic phases [38]. As it was shown for Pd-Ga/C catalysts, an increase of selectivity (on about 20 rel.%) in relation to the monometallic Pd/C catalyst is connected with the presence of alloyed Pd-Ga particles being formed during the catalyst preparation at the stage of its reductive treatment in a hydrogen flow [6,44]. Possibility in principle for the formation of Pd-Zn and Pd-Ag bimetallic systems characterized with enhanced selectivity in the hydrogenation of acetylene to ethylene was described elsewhere [44]. In the present research, the structural features of carbon-supported bimetallic Pd-Ag, Pd-Zn and Pd-Ga catalysts prepared by an incipient wetness impregnation of Sibunit with solutions of corresponding nitrates are studied in detail. A special attention was paid to intercommunication between the data of transmission electron microscopy (TEM), and X-ray absorption spectroscopy and the catalytic performance of the samples in the liquid-phase hydrogenation of acetylene to ethylene.
Section snippets
Preparation of the catalysts
In the present study, porous carbonaceous material Sibunit (fraction of 0.07–0.09 mm) with a specific surface area of 335 m2/g was used as a support [45]. It should be noted that Sibunit is able to reduce palladium in the solution of its salts with formation of a particulate palladium black. In order to prevent this undesired process, the support was preliminary treated with a 5% solution of nitric acid with subsequent evaporation of the latter on a water bath. Then, the support was dried on
Catalytic performance of the carbon-supported palladium catalysts
The results of catalytic tests of the bimetallic Pd-Zn/C, Pd-Ga/C and Pd-Ag/C catalysts as well as the reference monometallic Pd/C sample are presented in Table 1. It is obvious that the modifying effect is significantly determined by the nature of the modifier used. All the doped catalysts are more active than the reference Pd/C catalyst. Thus, an introduction of silver increases the acetylene conversion in 1.4 times, while doping with gallium or zinc has led to more significant growth of the
Conclusions
As it was shown in the present research, carbon-supported palladium catalysts modified with zinc, gallium and silver and prepared by an incipient wetness impregnation exhibit high efficiency in the liquid-phase hydrogenation of acetylene to ethylene. The yield of the target product obtained at 55 °C increases in the following order: Pd/C (19%) < Pd-Ag/C (34%) < Pd-Ga/C (42%) < Pd-Zn/C (53%). According to the TEM and XAS data, a rise of hydrogenation selectivity is facilitated by formation of
Acknowledgements
The present research was performed using the equipment of SRC “Kurchatov Institute” (Moscow) and Omsk regional center of collective use SB RAS (Omsk). Financial support from the Russian Foundation for Basic Research (grant number 17-33-50019) is acknowledged with attitude. Authors are grateful to Y.V. Zubavichus, A.A. Saraev and E.V. Khramov for their support in the XAS studies. The experimental calculations were partly carried out at Tomsk Polytechnic University within the framework of Tomsk
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