Selective hydrogenation of acetylene on zeolite-supported bimetallic catalysts

Abstract

Novel catalysts have been synthesized and evaluated by supporting Pd-based bimetallic nano catalysts on zeolites to achieve higher selectivity for the selective hydrogenation of acetylene in a stream containing excess ethylene at relatively low temperatures (300-339K). Low-temperature hydrogenation offers the opportunity of using competitive adsorption to achieve preferential hydrogenation of acetylene. Previous work from Chen group has found that bimetallic catalysts favor low temperature hydrogenation. Results from many other groups have also shown that Pd is a good catalyst for the selective hydrogenation of alkynes in excess ethylene. Therefore the strategy of the present work was to modify Pd catalysts and to embed bimetallic nanoparticles in an environment that is highly selective for acetylene hydrogenation.

Cation-π interaction offers the potential for selective adsorption of acetylene on the zeolite supports. In the current work we used ion-exchanged zeolite as the support of the bimetallic catalysts. The zeolite structure should have multiple dimensions and contain large pores, in order to house the bimetallic nanoparticles inside the pores. In this study, we began with zeolite β, and compared with zeolite Y and zeolite L.

Supported catalysts were prepared by the incipient impregnation method. Flow reactor studies using Gas Chromatography (GC), batch reactor studies using Fourier transform infrared (FTIR) have been performed. Transmission Electron Microscope (TEM) and Extended X-ray Absorption Fine Structure (EXAFS) measurements confirmed the formation of Pd-Ag bimetallic particles. CO/H ₂-Chemisorption has been used to determine the catalysts' dispersion.

Batch reactor studies carried out using infrared spectroscopy (IR) show that Pd-Ag bimetallic catalysts have higher selectivity for acetylene hydrogenation in the presence of ethylene than either Pd or Ag monometallic catalysts, while the Pd-Ni bimetallic catalyst behaved similarly to the Pd catalyst. Kinetic modeling of the reaction data reveals significant differences in the hydrogenation rate constants and adsorption equilibrium constants. The influence of the Na⁺/K⁺-β-zeolite support is compared with traditional γ-Al₂O₃ supported catalysts. The Na ⁺/K⁺-β-zeolites supported catalysts exhibit higher selectivity than their γ-Al₂O₃ counterparts. Overall, K⁺-β-zeolite supported Pd-Ag bimetallic catalyst is found to have the highest selectivity among the catalysts studied in the current work.

Flow reactor studies of the selective hydrogenation of acetylene in the presence of ethylene have been performed on Na⁺/K⁺ exchanged β-zeolite supported Pd, Ag and PdAg catalysts, as an extension of the batch reactor studies. Results from flow reactor studies show that the PdAg/Na⁺(K⁺)-β-zeolite bimetallic catalyst has lower activity than Pd/Na⁺(K⁺)-β-zeolite monometallic catalyst, while Ag/Na⁺(K⁺)-β-zeolite does not show any activity for acetylene hydrogenation. However, the selectivity for the PdAg bimetallic catalyst is much higher than that for either the Pd catalyst or Ag catalyst. The selectivity to byproduct (ethane) is greatly inhibited on PdAg bimetallic catalyst as well.

The supported PdAg bimetallic catalysts were also optimized in this study. Cation effects, zeolite structure effects and promoter effects for the selective hydrogenation of acetylene in the presence of ethylene were studied and optimized. Results from flow reactor studies show that K⁺-β-zeolite supported PdAg catalyst has the lowest activity but highest selectivity comparing to the γ-Al₂O₃ support and other alkaline metal exchanged β-zeolite supports. Zeolite structure does not affect acetylene conversion, selectivity to ethylene and selectivity to ethane at steady state.

The K promoter effect on γ-Al₂O₃ was also tested and it is found that adding K⁺ to γ-Al₂O ₃ increases activity and selectivity comparing to γ-Al₂O ₃ supported PdAg catalysts. The reason that K promotes the selectivity and activity of acetylene is probably that adding K to the catalysts favors the ethylene desorption from the catalysts surface. Therefore, the activity of acetylene and selectivity to ethylene are both increased.