(416a) Use of Supported Group IB-Pd Bimetallic Catalysts Prepared By Electroless Deposition for the Selective Hydrogenation of Acetylene and 1,3-Butadiene

Use of Supported Group IB-Pd Bimetallic Catalysts
Prepared by Electroless Deposition for the Selective Hydrogenation of Acetylene
and 1,3-Butadiene

Yunya Zhang, Christopher T.
Williams and John R. Monnier

Department of Chemical
Engineering, University of South Carolina, Columbia SC 29208 USA

Ethylene is used as a starting
point for many chemical intermediates in the petrochemical industry. The yearly
demand for ethylene is over 150 million tons with a growth rate of 3.5% [1].
Ethylene obtained from steam cracking of higher hydrocarbons contains up to 2%
acetylene, which acts as a poison for ethylene polymerization catalysts at even
ppm concentrations (> 5 ppm) [2]. Thus, selective hydrogenation of acetylene
to ethylene over low weight loading Pd catalysts is industrially used to purify
ethylene streams. Due to inferior selectivity at high acetylene conversion and
the formation of green oil during this reaction, various additives, such as Ag,
Ni, Cu, Au, Pb, Tl, Cr and K have been added to Pd to improve the performance
of current generation catalysts [3-6]. However, the bimetallic effects of the
above additives are not yet well understood, possibly because the conventional
methods of catalyst preparation (i.e., co-impregnation, successive
impregnation) result in both monometallic and bimetallic particles with varying
compositions. This in turn makes it difficult to determine the position of the
two metallic components, since the bimetallic interaction occurs only when the
two metallic components form bimetallic surface compositions instead of
separate particles.

We use electroless deposition
(ED) to prepare Ag- and Au-Pd/SiO₂ bimetallic catalysts with controlled
and proximal contact between the two metals. ED uses a controlled redox
chemical reaction to deposit Ag or Au onto Pd surface from a solution
containing a reducible Ag or Au salt and a reducing agent. Because the reducing
agent is catalytically activated by Pd surface atoms to produce an active
hydrogen species, Ag or Au is only deposited on Pd surface atoms. Since ED is
kinetically controlled, the final composition of a particular bimetallic
catalyst can be controlled to give rather precise combinations of the two
metallic components. Our goals are not only to discuss the improvements in
catalysts selectivity of acetylene to ethylene, but also to illustrate the
roles of Ag and Au additives during acetylene hydrogenation. If the promoting
effect of Ag or Au is to modify the electronic structure of the catalytically-active
Pd component, we should expect different behavior for the Ag-Pd and Au-Pd
bimetallic catalysts. On the other hand, if Ag or Au only dilutes the Pd
surface into different ensemble sizes that change the adsorption modes of
acetylene on Pd to allow the hydrogenation of acetylene to ethylene and not
ethylene to ethane, Ag-Pd and Au-Pd bimetallic catalysts should behave
similarly.

Figure 1A shows that the
conversion of acetylene for Ag-Pd/SiO₂ bimetallic catalysts
decreased sharply at ¦È_Ag  >
0.7 while the selectivity for acetylene conversion to ethylene increased to 90%
between ¦È_Ag = 0.7 ¨C 0.95. It is proposed that at low coverages of Ag
on Pd, there is an abundance of contiguous Pd surface sites where the strongly
adsorbed acetylene and ethylene are adsorbed as ethylidyne. This surface
species has been observed in the direct formation of ethane in the following
sequential hydrogenation process: ethylidyne °ú ethylidene °ú ethyl °ú ethane [7, 8].
As a result, this type of adsorption is not selective for hydrogenation to
ethylene. At high coverages where Ag atoms cover most of the Pd surface, the
remaining ensembles of Pd sites are too small to allow the formation of
ethylidyne. In this case, acetylene is more weakly adsorbed as a ¦Ð-bonded
species requiring only a single Pd surface site, which favors formation of
ethylene.The turnover frequencies (TOF) results shown in Figure 1B are
consistent with the above assumptions. At low Ag coverages, since ethylidyne
requires more Pd surface sites than adsorbed ¦Ð-bonded species at high coverage,
there are less C₂H₂ molecules reacted per Pd surface
site.  Thus, TOF of C₂H₂
conversion was lower at low Ag coverage. On the other hand, hydrogenation of
these strongly bound ethylidyne species leads to the preferable formation of C₂H₆
while hydrogenation of ¦Ð-complex species prefers the formation of ethane,
so TOF of C₂H₆ formation was higher at low Ag coverage.The
kinetics was then investigated to verify the above hypotheses. In Figure 2, analyses
for un-promoted Pd and ¦È_{Ag on Pd}= 0.92 gave C₂H₂
pressure dependences of ¨C0.67 and ¨C0.20, respectively, consistent with much
more strongly adsorbed C₂H₂ for large Pd ensembles. Families
of Au-Pd/SiO₂ bimetallic catalysts have similar performance trend as
Ag-Pd/SiO₂ catalysts, indicating the bimetallic effect for these
catalysts is not electronic but geometric in nature.

We
are now using the same Ag- and Au-Pd/SiO₂ bimetallic catalysts to
investigate the reaction mechanism of selective hydrogenation of 1,3-butadiene.
Similar to acetylene, 1,3-butadiene, along with other diolefins also
contaminates the ethylene streams and poisons the catalyst used for further
polymerization of ethylene. In addition to evaluation and kinetics study, the
mechanism of this reaction will be investigated by studying the adsorption of
reactants and products on the surface of catalysts using Fourier transform infrared
(FTIR) spectroscopy. The elucidation of reaction mechanism will advance our
understanding of the catalytic phenomena and allow us to relate the variations
in catalytic behavior to the properties of metals. In the future research, it
will be interesting to study hydrogenation of 2-methyl-1,3-butadiene
(isoprene), 1,3-pentadiene (isomer of isoprene) and 2,3-dimethyl-1,3-butadiene,
which are electronically similar to 1,3-butadiene but have more steric
hindrance. Comparison between the results of acetylene and diolefin
hydrogenation will give us a better idea to understand how Ag- or Au-Pd/SiO₂
bimetallic catalysts affect the reaction pathways to control the product
distribution during hydrogenation of unsaturated hydrocarbons.

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