(704b) Effect of PdCu Alloy Composition on Reactivity and Selectivity for Ethylene Acetoxidation to Vinyl Acetate | AIChE

Other Sites & Tools

Technical Groups

(704b) Effect of PdCu Alloy Composition on Reactivity and Selectivity for Ethylene Acetoxidation to Vinyl Acetate

Authors 

Deshlahra, P. - Presenter, Tufts University
Leelavathi, A., Tufts University
Zha, Z.
Effect of PdCu Alloy Composition on Reactivity and Selectivity for Ethylene Acetoxidation to Vinyl Acetate

Zhaoru Zha, Leelavathi Annamalai, Prashant Deshlahra, Tufts University

Vinyl Acetate (VA), a valuable bulk chemical, is typically produced from acetic acid (AA), ethylene (EY) and oxygen (O2) using Pd and PdAu based catalysts.1 Here, kinetic and mechanistic details of VA formation on monometallic Pd and bimetallic PdCu catalysts of different atomic ratios (Pd2Cu, Pd0.2Cu, Pd0.036Cu, Pd0.0036Cu) are compared. The metals are dispersed on SiO2 using incipient wetness impregnation and characterized via transmission electron microscopy and chemisorption probes.

The Pd and PdCu samples exhibit similar effects of EY and AA on VA formation rates and selectivity. Rates and selectivity increase with increasing EY and AA pressures at low pressures, due to increased opportunities of the two reactants to couple with each other instead of undergoing combustion reaction separately, and saturate at high pressures.2 In contrast, the O2 pressure effects change significantly with the catalyst composition. These changes are highlighted in Figures 1 and 2 by ratios of VA formation rate on bimetallic to that on monometallic catalysts (rPdCu/rPd) and VA selectivity, respectively.

The VA selectivity can be low at high O2 pressures that favor EY combustion, while the reaction rates are low at low O2 pressures because the absence of surface O-atoms inhibits H-abstraction steps required for the reaction;3 thus intermediate O2 pressures can be optimal. The rPdCu/rPd values increase with decreasing O2 pressures, suggesting that relative reactivity of PdCu samples to Pd samples is increasingly higher at lower O2 pressures. These trends are consistent with more negative density functional theory (DFT) derived adsorption energy of O-atoms on Cu than on Pd, which leads to greater O-atom coverage at Pd-Cu interfaces than monometallic Pd. The VA selectivity on monometallic samples increases with O2 pressures and then levels-off and exhibits a slight decrease. PdCu samples exhibit much higher selectivity than Pd at low O2 pressure, but the selectivity decreases more strongly with increasing O2 pressure. The low VA selectivity in monometallic Pd at low O2 pressure is a consequence of acetic acid decomposition indicated by detection of CO in the reactor effluent. Such decompositions are likely to require Pd-atom pairs that are absent in PdCu samples with low Pd content. The isolated nature of Pd in such samples was confirmed by exclusive detection of linear CO-adsorption features by infrared. The PdCu samples with low Pd loading exhibited increase in rates with time on stream at low O2 pressure, suggesting an activation of samples possibly due to migration of Pd atoms to the surface of PdCu particles or formation of acetate phases in such particles. These mechanistic details apparent from the measurements will be probed further using DFT calculations.

The results described here suggest that PdCu samples with low Pd content can exhibit high VA selectivity at low O2 pressures, which may lead to efficient catalysts with lower costs than PdAu catalysts used currently in industrial processes.

Â

Figure 1. Ratio of VA formation rates on PdCu to Pd catalysts (rPdCu/rPd) as a function of O2 pressure (40 kPa EY, 10 kPa AA, 433 K, EY conversion < 2%).

Â

Figure 2. VA selectivity on PdCu and Pd catalysts as a function of O2 pressure (40 kPa EY, 10 kPa AA, 433 K, EY conversion < 2%).

Â

References

  1. Stacchiola, D.; Calaza, F.; Burkholder, L.; Schwabacher, A.W.; Neurock, M.; Tysoe, W.T. Elucidation of the Reaction Mechanism for the Palladiumâ€Catalyzed Synthesis of Vinyl Acetate. Angew. Chem. Int. Ed., 2005, 44, 4572-4574.
  2. Augustine, S.M.; Blitz, J.P. The Use of DRIFTS-MS and Kinetic Studies to Determine the Role of Acetic Acid in the Palladium-Catalyzed Vapor-Phase Synthesis of Vinyl Acetate. J. Catal., 1993, 142, 312-324.
  3. Han, Y. F.; Wang, J. H.; Kumar, D.; Yan, Z.; Goodman, D. W. A Kinetic Study of Vinyl Acetate Synthesis Over Pd-based catalysts: Kinetics of Vinyl Acetate Synthesis over Pd–Au/SiO2 and Pd/SiO2 catalysts. J. Catal. 2005, 232, 467-475.
Effect
of PdCu Alloy Composition on Reactivity and
Selectivity for Ethylene Acetoxidation to Vinyl
Acetate

Zhaoru Zha, Leelavathi Annamalai, Prashant Deshlahra,
Tufts University

Vinyl
Acetate (VA), a valuable bulk chemical, is typically produced from acetic acid
(AA), ethylene (EY) and oxygen (O2) using Pd
and PdAu based catalysts.1 Here, kinetic
and mechanistic details of VA formation on monometallic Pd
and bimetallic PdCu catalysts of different atomic ratios (Pd2Cu, Pd0.2Cu,
Pd0.036Cu, Pd0.0036Cu) are compared. The metals are dispersed
on SiO2 using incipient wetness impregnation and characterized via
transmission electron microscopy and chemisorption probes.

The Pd and PdCu samples exhibit similar effects of EY and AA on VA
formation rates and selectivity. Rates and selectivity increase with increasing
EY and AA pressures at low pressures, due to increased opportunities of the two
reactants to couple with each other instead of undergoing combustion reaction
separately, and saturate at high pressures.2 In contrast, the O2
pressure effects change significantly with the catalyst composition. These
changes are highlighted in Figures 1 and 2 by ratios of VA formation rate on
bimetallic to that on monometallic catalysts (rPdCu/rPd) and VA selectivity, respectively.

The VA selectivity can be low at high O2
pressures that favor EY combustion, while the reaction rates are low at low O2
pressures because the absence of surface O-atoms inhibits H-abstraction steps required
for the reaction;3 thus intermediate O2 pressures can be
optimal. The rPdCu/rPd
values increase with decreasing O2 pressures, suggesting that
relative reactivity of PdCu samples to Pd samples is increasingly higher at lower O2
pressures. These trends are consistent with more negative density functional
theory (DFT) derived adsorption energy of O-atoms on Cu than on Pd, which leads to greater O-atom coverage at Pd-Cu interfaces than monometallic Pd. The VA selectivity
on monometallic samples increases with O2 pressures and then
levels-off and exhibits a slight decrease. PdCu
samples exhibit much higher selectivity than Pd at
low O2 pressure, but the selectivity decreases more strongly with
increasing O2 pressure. The low VA selectivity in monometallic Pd at low O2 pressure is a consequence of acetic
acid decomposition indicated by detection of CO in the reactor effluent. Such
decompositions are likely to require Pd-atom pairs that
are absent in PdCu samples with low Pd content. The isolated nature of Pd
in such samples was confirmed by exclusive detection of linear CO-adsorption
features by infrared. The PdCu samples with low Pd loading exhibited increase in rates with time on stream
at low O2 pressure, suggesting an activation of samples possibly due
to migration of Pd atoms to the surface of PdCu particles or formation of acetate phases in such
particles. These mechanistic details apparent from the measurements will be
probed further using DFT calculations.

       The results described here suggest that PdCu samples with low Pd content
can exhibit high VA selectivity at low O2 pressures, which may lead
to efficient catalysts with lower costs than PdAu
catalysts used currently in industrial processes.

Figure 1. Ratio of VA formation rates on PdCu
to Pd catalysts (rPdCu/rPd) as a function of O2 pressure (40
kPa EY, 10 kPa AA, 433 K, EY conversion < 2%).

Figure
2.
VA selectivity on PdCu and Pd
catalysts as a function of O2 pressure (40 kPa EY, 10 kPa AA, 433 K,
EY conversion < 2%).

References

1.
Stacchiola, D.; Calaza, F.;
Burkholder, L.; Schwabacher, A.W.; Neurock, M.; Tysoe, W.T.
Elucidation of the Reaction Mechanism for the Palladium‐Catalyzed
Synthesis of Vinyl Acetate. Angew.
Chem. Int. Ed., 2005, 44,
4572-4574.

2.
Augustine, S.M.; Blitz, J.P. The Use of DRIFTS-MS and Kinetic Studies to
Determine the Role of Acetic Acid in the Palladium-Catalyzed Vapor-Phase
Synthesis of Vinyl Acetate. J. Catal., 1993, 142, 312-324.

3.
Han, Y. F.; Wang, J. H.; Kumar, D.; Yan, Z.; Goodman, D. W. A Kinetic Study of
Vinyl Acetate Synthesis Over Pd-based catalysts: Kinetics
of Vinyl Acetate Synthesis over Pd–Au/SiO2
and Pd/SiO2 catalysts. J. Catal. 2005, 232,
467-475.

Topics 

Catalysis

More Conference Links