Oxygen ethylene epoxide

Wagner was first to propose the use of solid electrolytes to measure in situ the thermodynamic activity of oxygen on metal catalysts.17 This led to the technique of solid electrolyte potentiometry.18 Huggins, Mason and Giir were the first to use solid electrolyte cells to carry out electrocatalytic reactions such as NO decomposition.19,20 The use of solid electrolyte cells for chemical cogeneration , that is, for the simultaneous production of electrical power and industrial chemicals, was first demonstrated in 1980.21 The first non-Faradaic enhancement in heterogeneous catalysis was reported in 1981 for the case of ethylene epoxidation on Ag electrodes,2 3 but it was only... 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

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... 

One of the most striking results is that of C2H4 oxidation on Pt5 where (xads,o ctact = -1, i.e. the decreases in reaction activation energy and in the chemisorptive bond strength of oxygen induced by increasing work function ethylene epoxidation and deep oxidation on Ag.5... 

Figure 8.38. Steady state effect of current on the increase in the rates of ethylene epoxidation (rj) and deep oxidation to CO2 (r2) of C2H4 on Ag and comparison with the rate Go2=I/4F of electrochemical oxygen supply42 pC2H4=l-6 kPa, pO2=10 kPa, T=400°C intrinsic (1=0) selectivity 0.5, 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. 

For the epoxidation of propylene with HO—ONO, both the QCISD and CISD calculations result in a Markovnikov-type transition structure, where the electrophile is slightly skewed toward the least substituted carbon, with a small difference in the bond lengths between the spiro-oxygen and the double-bond carbons (0.106 and 0.043 A, respectively Figure 8). The B3LYP calculations also lead to an unsymmetrical Markovnikov-type transition structure (b). However, the MP2/6-31G geometry optimization results in an anti-Markovnikov-type structure (c) (Figure 8). The CCSD(T)/6-31G and BD(T)/6-31G barriers for ethylene epoxidation with HO—ONO calculated with the QCISD/6-31G ... 

Like CO oxidation on Ru, the understanding for ethylene epoxidation on Ag has continued to evolve. Many questions remain open, including the reaction mechanism on the Ag structures, and the role of intercalated oxygen atoms. Another dimension that is little explored so far is the surface states in a combined oxygen-ethylene atmosphere. Greeley et al. have reported recently that an ethylenedioxy intermediate may be present at appreciable coverage under industrial reaction conditions, the effect of which on the structure of the surface is unknown. More importantly, the implication of a dynamic co-existence of various surface oxides under reaction conditions for the reaction mechanism needs to be explored and understood at greater depth. 

As alluded to before, the adsorption of atoms and molecules may also induce segregation in alloys. Upon revisiting the thermodynamic behavior of the improved Cu-Ag alloy catalysts for ethylene epoxidation synthesized by Linic et al, (section 2.1) Piccinin et al. calculated that, while in the absence of oxygen Cu prefers to stay in the subsurface layers, oxygen adsorption causes it to segregate to the surface which then phase-separates into clean Ag(lll) and various Cu surface oxides under typical industrial conditions (Fig. 7). This casts doubt on the active state of the previous Cu-Ag catalysts being a well-mixed surface Ag-Cu alloy. 

Transient response techniques are used to investigate the activation of silver powder for ethylene epoxidation at vacuum and atmospheric pressures. Results indicate that the activation process is qualitatively the same in both pressure regimes. Numerical simulation of the process indicates that activation involves the concurrent incorporation of oxygen into surface and subsurface sites. The reaction selectivity parallels the incorporation of oxygen into the subsurface. 

Ethylene epoxide (EO) is an important intermediate in the chemical industry and the mechanism of its formation has been studied in detail [94-98]. For the industrial aspects see Chapter 2. EO is produced by the selective oxidation of ethylene with oxygen ... 

Tn previous work it has been shown that a competition exists during - ozonation of olefins between ozonolysis and epoxide formation (I). As steric hindrance increases around the double bond, the yield of epoxide or subsequent rearrangement products increases. This is illustrated with both old (1) and new examples in Table I for purely aliphatic olefins and in Table II for aryl substituted ethylenes. It was suggested that the initial attack of ozone on an olefinic double bond involves w (pi) complex formation for which there were two fates (a) entrance into 1,3-dipolar cycloaddition (to a 1,2,3-trioxolane adduct), resulting in ozonolysis products (b) conversion to a o- (sigma) complex followed by loss of molecular oxygen and epoxide formation (Scheme 1). As the bulk... 

The goal of using solid-state electrolytic reactors is not only to generate electrical power, but also to combine this with an industrially important catalytic reaction, such as dissociation of oxygen-containing compounds like NO [40,41], quantitative oxidation of NH3 to NO [42-44], oxidation of SO2 [45], and methanol [46], ethylene epoxidation [46], or Fischer-Tropsch synthesis [47]. The cross-flow reactor used in this type of study (Fig. 10) [48,49] has a solid electrolyte consisting of yttria-doped zirconia. The plates are electrically connected in series, with a varying number of plates in parallel. The oxidant flow channels... 

---