Ethylene epoxidation catalyst selectivity

Linic S, Jankowiak J, Barteau MA (2004) Selectivity driven design of bimetallic ethylene epoxidation catalysts from first-principles. J Catal 226 245... 

Table 1 Intrinsic Rate of Ethylene Epoxidation and Selectivity Toward Ethylene Oxide as a Eunction of Mean Silver Particle Size for ETnpromoted Ag/Al203 Supported Catalysts... 

To test the prediction that Cu would improve the selectivity of Ag as an ethylene epoxidation catalyst, Linic, Jankowiak, and Barteau tested several Cu/ Ag catalysts on porous a-Al203 monoliths.56 The Cu concentrations in the catalysts varied from 0 to 0.8 at.%, so only a single bimetallic phase was expected. The catalysts were tested under conditions where the temperature, the feed composition of ethylene and oxygen, and the conversion of ethylene were held constant. Under these conditions, the selectivity of the catalyst increased significantly as the Cu content was increased from 0 to 0.2 at.%. This is a dramatic success the DFT calculations of transition states for competing reactions on bimetallic catalysts lead directly to the experimental identification of an improved catalyst. 

The commonly held view of the uniqueness of Ag for ethylene epoxidation may soon change in view both of the propene epoxidation work of Haruta and coworkers on Au/Ti02 catalysts upon cofeeding H2 123 and also in view of the recent demonstration by Lambert and coworkers124 126 that Cu(lll) and Cu(110) surfaces are both extremely efficient in the epoxidation of styrene and butadiene to the corresponding epoxides. In fact Cu was found to be more selective than Ag under UHV conditions with selectivities approaching 100%.124-126 The epoxidation mechanism appears to be rather similar with that on Ag as both systems involve O-assisted alkene adsorption and it remains to be seen if appropriately promoted Cu124 126 can maintain its spectacular selectivity under process conditions. 

Figure 4.42. Ethylene epoxidation on Ag/p"-Al203.101 Steady-state effect of catalyst potential on the selectivity to ethylene oxide at various levels of gas-phase dichloroethane (a) and 3-dimensional representation of the effect of dichloroethane concentration, catalyst potential and corresponding Na coverage on the selectivity to ethylene oxide (b).101 Reprinted with permission from Academic Press.

Figure 9,11. Ethylene epoxidation on Ag/fT-AhC Transient effect of a negative applied current (Na supply to the catalyst) on catalyst potential, Na coverage and selectivity to ethylene oxide22 Conditions as in Fig. 9.10. Reprinted with permission from Academic Press.

Electrochemical promotion, or non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA) came as a rather unexpected discovery in 1980 when with my student Mike Stoukides at MIT we were trying to influence in situ the rate and selectivity of ethylene epoxidation by fixing the oxygen activity on a Ag catalyst film deposited on a ceramic O2 conductor via electrical potential application between the catalyst and a counter electrode. 

Other TUD-l catalysts proven for selective oxidation (16) include Au/Ti-Si-TUD-1 for converting propylene to propylene oxide (96% selectivity at 3.5% conversion see also (17), Ag/Ti-Si-TUD-1 for oxidizing ethylene to ethylene oxide (29% selectivity at 19.8% conversion), and Cr-Si-TUD-1 for cyclohexene to cyclohexene epoxide (94% selectivity at 46% conversion). 

Modifying the selectivity for a particular product is a more challenging task. To understand why Ag is the most selective catalyst for ethylene epoxidation, an highly important reaction practiced industrially for decades, Linic et al. performed detailed spectroscopic and kinetic isotope experiments and DFT calculations, and they concluded that the selectivity between the partial and total oxidation of ethylene on Ag(l 11) is controlled by the relative stability of two different transition states (TS s) that are both accessible to a common oxametallacycle intermediate One results in the closure of the epoxide ring and ethylene oxide (EO), while the other leads to acetaldehyde (AC) via intra-molecular H shift and eventually combustion. The authors... 

Linic and co-workers provided two additional examples of modifying selectivity for the ethylene epoxidation reaction. They demonstrated the promotional effect of Cs on EO formation using DFT calculations Cs atoms increase AEa by up to 0.2 eV vs. Ag only, via an induced electric field that interacts with the different dipoles of the two TS s." More recently Christopher et al. synthesized and tested (100) facet-dominated Ag nanowire catalysts, based on the DFT results that AEa is ca. 0.1 eV larger on Ag(lOO) than on Ag(lll), because of the extra elongation of the Ag-adsorbate bonds required to form the TS to AC on Ag(lOO). The Ag nanowire catalysts were indeed more selective than conventional Ag catalysts, in which Ag particles mainly exposes the (111) facet." Incidentally,... 

The optimal distribution of silver catalyst in a-Al203 pellets is investigated experimentally for the ethylene epoxidation reaction network, using a novel single-pellet reactor. Previous theoretical work suggests that a Dirac-delta type distribution of the catalyst is optimal. This distribution is approximated in practice by a step-distribution of narrow width. The effect of the location and width of the active layer on the conversion of ethylene and the selectivity to ethylene oxide, for various ethylene feed concentrations and reaction temperatures, is discussed. The results clearly demonstrate that for optimum selectivity, the silver catalyst should be placed in a thin layer at the external surface of the pellet. 

The ethylene epoxidation reaction network, occurring in a Dirac-type catalyst, has previously been studied theoretically (7-8). Both studies showed that the selectivity to eAylene oxide is maximized when the catalyst is located at the external surface of the pellet, i.e. for an egg-shell type catalyst. A systematic experimental investigation of the performance of such catalysts for this industrially important reaction network has recently been reported (9). A summary of this work, as well as some new results, are presented in this paper. 

Silver is an important metallic catalyst for the selective oxidation of ethylene. The silver catalyst is used to selectively convert ethylene to ethylene epoxide, an important intermediate for antifreeze. Whereas the epoxidation of ethylene proceeds with high selectivity on oxidic silver phases, metallic silver surfaces give only total oxidation of ethylene. Electron-deficient O is created on oxidized silver surfaces and this readily inserts into the electron-rich ethylene bond. 

This section describes in detail three topics in heterogeneous catalysis to which DFT calculations have recently been applied with great effect, the prediction of CO oxidation rates over RuO2(110), the prediction of ammonia synthesis rates by supported nanoparticles of Ru, and the DFT-based design of new selective catalysts for ethylene epoxidation. All three examples involve the careful application of DFT calculations and other appropriate theoretical methods to make quantitative predictions about the performance of heterogeneous catalysts under realistic operating conditions. 

In addition, silver is used on a very large scale in industry as an catalyst for the selective oxidation of ethylene to ethylene oxide ( an epoxide ) . The selectivity of silver catalysts can be enhanced by addition of trace amounts of alkali... 

J. T. Jankowiak, M. A. Barteau, Ethylene epoxidation over silver and copper-silver bimetallic catalysts 1. Kinetics and selectivity, /. Catal. 236 (2005) 366. 

Investigation of the Origins of Selectivity in Ethylene Epoxidation on Promoted and Unpromoted Ag/a-Al2O3 Catalysts A Detailed Kinetic, Mechanistic and Adsorptive Study... 

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

Origins of Selectivity in Ethylene Epoxidation on Ag/a-Al2O3 Catalysts... 

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