Selectivity to Ethylene

Oxalic acid produced from syngas can be esteiified (eq. 20) and reduced with hydrogen to form ethylene glycol with recovery of the esterification alcohol (eq. 21). Hydrogenation requires a copper catalyst giving 100% conversion with selectivities to ethylene glycol of 95% (15). 

An early source of glycols was from hydrogenation of sugars obtained from formaldehyde condensation (18,19). Selectivities to ethylene glycol were low with a number of other glycols and polyols produced. Biomass continues to be evaluated as a feedstock for glycol production (20). 

Catalyst Selectivity. Selectivity is the property of a catalyst that determines what fraction of a reactant will be converted to a particular product under specified conditions. A catalyst designer must find ways to obtain optimum selectivity from any particular catalyst. For example, in the oxidation of ethylene to ethylene oxide over metallic silver supported on alumina, ethylene is converted both to ethylene oxide and to carbon dioxide and water. In addition, some of the ethylene oxide formed is lost to complete oxidation to carbon dioxide and water. The selectivity to ethylene oxide in this example is defined as the molar fraction of the ethylene converted to ethylene oxide as opposed to carbon dioxide. 

Methanol to Ethylene. Methanol to ethylene economics track the economics of methane to ethylene. Methanol to gasoline has been flilly developed and, during this development, specific catalysts to produce ethylene were discovered. The economics of this process have been discussed, and a catalyst (Ni/SAPO 34) with almost 95% selectivity to ethylene has been claimed (99). Methanol is converted to dimethyl ether, which decomposes to ethylene and water the method of preparation of the catalyst rather than the active ingredient of the catalyst has made the significant improvement in yield (100). By optimizing the catalyst and process conditions, it is claimed that yields of ethylene, propylene, or both are maximized. This is still in the bench-scale stage. 

Unsteady-State Direct Oxidation Process. Periodic iatermption of the feeds can be used to reduce the sharp temperature gradients associated with the conventional oxidation of ethylene over a silver catalyst (209). Steady and periodic operation of a packed-bed reactor has been iavestigated for the production of ethylene oxide (210). By periodically varyiag the inlet feed concentration of ethylene or oxygen, or both, considerable improvements ia the selectivity to ethylene oxide were claimed. 

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

Ethylene is currently converted to ethylene oxide with a selectivity of more than 80% under commercial conditions. Typical operating conditions are temperatures in the range 470 to 600 K with total pressures of 1 to 3 Mpa. In order to attain high selectivity to ethylene oxide (>80%), alkali promoters (e.g Rb or Cs) are added to the silver catalyst and ppm levels of chlorinated hydrocarbons (moderators) are added to the gas phase. Recently the addition of Re to the metal and of ppm levels of NOx to the gas phase has been found to further enhance the selectivity to ethylene oxide. 

Figure 4.41. Effect of Ag/YSZ catalyst potential, work function and feed partial pressure of dichloroethane on the selectivity to ethylene oxide (a) and to acetaldehyde (b). T=270°C, P=500 kPa, 8.5% 02,7.8% C2H4.77 Reprinted with permission from Academic Press.

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.

At lower temperatures (260°C) higher operating pressures (5 bar) and high C2H4 to 02 ratios (Fig. 8.42) ethylene oxide formation and C02 formation both exhibit electrophobic behaviour over the entire Uwr range 47 Both rates vary by a factor of 200 as UWr is varied by 0.6 V (p varies between 3 and 0.015). The selectivity to ethylene oxide exhibits two local maxima 47 More interestingly, acetaldehyde appears as a new product47... 

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.

In Figure 4.42 we have seen already the effect of catalyst potential UWr, corresponding sodium coverage 0n3 and C2H4CI2 partial pressure on the selectivity to ethylene oxide. For UWr = -0.25 V and Pc2H4Ci2=l-0 ppm the selectivity to ethylene oxide is 88%, which is one of the highest values reported for this important reaction.22... 

Despite the existing uncertainty about the exact nature of the oxygen species present during reaction several different approaches have been reported which lead to an increase in the selectivity to ethylene oxide. These include alloying silver with other metals (14) and using different catalyst supports and various promoters (6,15). It is also known and industrially proven that the addition of few PPM of chlorinated hydrocarbon "moderators" to the gas feed improves the selectivity to ethylene oxide but decreases the catalyst activity (15). It has also been found recently by Carberry et al that selectivity increases with y-ir-radiation of the catalyst (16). 

While chlorine is a poison for the ammonia synthesis over iron, it serves as a promoter in the epoxidation of ethylene over silver catalysts, where it increases the selectivity to ethylene oxide at the cost of the undesired total combustion to C02. In this case an interesting correlation was observed between the AgCl27Cl ratio from SIMS, which reflects the extent to which silver is chlorinated, and the selectivity towards ethylene oxide [16]. In both examples, the molecular clusters reveal which elements are in contact in the surface region of the catalyst. 

Figure 9. Selectivity to ethylene-d4 oxide and oxygen uptake as a function of pulse number.

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. 

As noted earlier, previous theoretical studies (7-8) have shown that the selectivity to ethylene oxide is maximized, when the active material is located at the external surface of the pellet. This behavior results primarily from the fact that the main undesired reaction 2 has a higher activation energy than the desired one. Therefore, intraphase temperature gradients are detrimental to the selectivity. Indeed, in Figure 2, where results for Type 1 pellets are presented, it is shown that for all the temperatures studied, selectivity decreases when the active layer is located deeper inside the pellet. This behavior was observed for all the inlet ethylene concentrations investigated. 

Addition of various promoters to ruthenium-containing solutions can increase the overall rate of CO reduction, but the most remarkable effect is the change in selectivity to ethylene glycol. The effects of several potassium salts are illustrated in Table XVII. The acetate, phosphate, and fluoride... 

Several processes based on air or oxygen have been developed.890-895 Oxidation with air (260-280°C) or oxygen (230°C) is carried out at about 15-25 atm at a limited conversion (about 10-15%) to achieve the highest selectivity.896-898 High-purity, sulfur-free ethylene is required to avoid poisoning of the catalyst. Ethylene concentration is about 20-30 vol% or 5 vol% when oxygen or air, respectively, is used as oxidants. The main byproducts are C02 and H20, and a very small amount of acetaldehyde is formed via isomerization of ethylene oxide. Selectivity to ethylene oxide is 65-75% (air process) or 70-80% (02 process).867... 

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