Abstract: Currently, the development of technologies for ethylene production is an important task, since ethylene acts as a raw material for petrochemistry and is used for obtaining important industrial products (acetaldehyde, ethylene oxide, acetic acid, etc.). One of the promising ways to obtain ethylene from natural and associated petroleum gas is the GTE technology ("gas-to-ethylene"). Natural gas is subjected to oxidative pyrolysis to produce C2H2, which is absorbed by a selective solvent and converted to C2H4 by hydrogenation on palladium catalysts [1]. In order to increase the selectivity of the process and the resistance of the samples to deactivation, Pd is modified with a second metal (IB, IIB or IIIA elements) [2, 3]. However, the issue of the relationship between the composition, the structure of active component for such systems and their properties in studied process is not sufficiently covered in the literature. Therefore, the purpose of this work was to study the effect of the silver amount on the active component state and catalytic properties of 0.5%Pd-Ag/Sibunit catalysts in the liquid-phase hydrogenation of acetylene to ethylene. By using instrumental methods of analysis, the phase composition (XRD), local structure (EXAFS) and electronic state of Pd and Ag (XPS) for studied samples were determined. The morphology of the catalysts was studied by transmission electron microscopy (TEM). Carbon composite Sibunit (Sib) (SBET = 325 m2/g) was used as a support. Samples of the catalysts were prepared by an incipient wetness impregnation with joint aqueous solution of palladium and silver nitrates with following stages of drying and reduction in a hydrogen stream (5 h, 500°C). The modifier content was varied from Pd:Ag = 1:0.1 to 1:4. Hydrogenation of the model gas mixture (4 % vol. C2H2 and 96 % vol. H2) was carried out in N-methylpyrrolidone medium in the shaker-type flow reactor in the temperature range from 25 to 75°C. The introduction of silver into Pd/Sib has a significant effect on its catalytic properties. As the silver content increases, the acetylene conversion (X(C2H2) = 50%) shifts to a higher temperature range (from 27 to 77°C), which indicates a depression in catalytic activity. At the same time, the increase in Pd:Ag molar ratio to 1:1 is accompanied with the growth in the ethylene selectivity (up to 81%) compared with a Pd/Sib (68%), due to a decrease in the selectivity of ethane and C4+ oligomers. XRD, EXAFS and TEM data showed that the addition of Ag to Pd/Sib leads to the interaction of the supported components with the formation of dispersed bimetallic nanoalloys (dav = 5.6 nm [4]) with different degrees of enrichment with the silver component Pd(1-x)Agx. The calculation of its composition in accordance with the Vegard’s law showed that the Pd:Ag ratio in the nanoalloys is determined by the initial Pd:Ag molar ratio, and the optimum on the ethylene selectivity is achieved on the catalysts containing bimetal particles similar in composition to the equimolar one. In the case of a smaller silver amount, a significant part of the active component is presented as a monometallic Pd particles and in the case of a higher Pd:Ag molar ratio, Ag exist in the sample in a noticeable amount. The process selectivity on Pd-Ag/Sib is raised due to increasing the distance between neighboring palladium atoms from 2.72 Å (Pd/Sib) to 2.80 Å (Pd-Ag(1:1)/Sib). It leads to a change in the C2H2 adsorption geometry [2] and, accordingly, to an increase in the yield of the target product. Using XPS, it has been shown that an increase in the amount of Ag in Pd-Ag/Sib is accompanied by an amplification in the degree of electron density donation from Ag to Pd. This suppresses the affinity of Pd to the sorption of unsaturated compounds [2], reduces the activity, but it greatly facilitates the C2H4 desorption, preventing its full hydrogenation to C2H6. Based on the TEM results, it can be concluded that a decrease in activity for the Pd:Ag ≥ 1:1 samples can additionally be caused by local blocking of the active centers by large Ag particles (up to 9 nm).

Cite: Glyzdova D.V. , Afonasenko T.N. , Leont’eva N.N. , Khramov E.V. , Shlyapin D.A.
Study of the silver addition influence on the Pd/Sibunit catalyst of selective acetylene hydrogenation
In compilation Catalysis without Borders: book of Abstracts (short lectures) of 14th EuropaCat – European Congress on Catalysis, Aachen, Germany, 18-23 august 2019. 2018.

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