Ethene, oxidation

Cobalt carbonyls are the oldest catalysts for hydroformylation and they have been used in industry for many years. They are used either as unmodified carbonyls, or modified with alkylphosphines (Shell process). For propene hydroformylation, they have been replaced by rhodium (Union Carbide, Mitsubishi, Ruhrchemie-Rhone Poulenc). For higher alkenes, cobalt is still the catalyst of choice. Internal alkenes can be used as the substrate as cobalt has a propensity for causing isomerization under a pressure of CO and high preference for the formation of linear aldehydes. Recently a new process was introduced for the hydroformylation of ethene oxide using a cobalt catalyst modified with a diphosphine. In the following we will focus on relevant complexes that have been identified and recently reported reactions of interest. 

To probe the origin of acetaldehyde in ethene oxidation, ethene oxide was admitted to the (TS-I/H2O2) system containing TiOOH groups. The formation of acetaldehyde was negligible even under the influence of UV-visible irradiation. Hence, the significant amount (10%) of acetaldehyde formed in the reaction of ethene with TS-1/H202 could not have been the product of the further reaction of ethene oxide. It is rather a primary product of oxidation at the vinylic carbon atom. 

Zador, J., Pilling, M. J., Wagner, V., and Wirtz, K. Quantitative assessment of uncertainties for a model of tropospheric ethene oxidation using the European Photochemical Reactor, Atmos. Environ., submitted, 2004. 

If the initial intermediate or the original fuel is a large monoolefin, the radicals will abstract H from those carbon atoms that are singly bonded because the CH bond strengths of doubly bonded carbons are large (see Appendix D). Thus, the evidence [12, 32] is building that, during oxidation, all nonaromatic hydrocarbons primarily form ethene and propene (and some butene and isobutene) and that the oxidative attack that eventually leads to CO is almost solely from these small intermediates. Thus the study of ethene oxidation is crucially important for all alkyl hydrocarbons. 

Catalytic Oxidation of Ethene to Acetaldehyde and Acetic Acid. -Evnin et al120 studied Pd-doped V2 Os catalysts for the vapor-phase oxidation of ethene to acetaldehyde in a heterogeneous type of Wacker process. From a mechanistic study they establish a redox mechanism with Pd both as the site of the ethene oxidation and of the reoxidation of the catalyst. On the basis of the role of the V4+ ions proposed by these authors, Forni and Gilardi121 substantiated this mechanism by adding tetra- and hexa-valent dopants to the V2 05 and studying the effects on the catalytic reaction. 

Figure 1. Schematic catalytic cycle of the Wacker-Hoechst ethene oxidation.

K. Svensson, and C.J. Calleman. Evaluation of genetic risks of alkylating agents. III. Alkylation of haemoglobin after metabolic conversion of ethene to ethene oxide in vivo. Mutat. Res. 45 175-184,... 

The nature of the working silver catalyst was different during methanol oxidation and ethene oxidation reactions as a result of variations in the reaction conditions (Wachs, 2002). In methanol oxidation (at 600 °C), H 2 was a major by-product, and Raman spectroscopy showed that the silver catalyst was essentially reduced and contained only trace amounts of atomic oxygen in the subsurface. During ethene oxidation (at ca. 230 °C), H2 was not formed as a byproduct. The absence of H2 and the lower reaction temperature during ethene oxidation result in a silver surface with atomic oxygen species. 

The state of silver supported on CaCOs was monitored by Lu et al. (2005) during exposure to a feed for ethene or propene oxidation at 473 K. Reduction of an initially present Ag+ component was observed in C3H6/ 02/He, whereas in C2H4/02/He some oxygen appeared to be retained by the silver. The authors considered a change of the silver particle shape under ethene oxidation conditions because of a shift of the surface plas-mon resonance. 

The decrease of the rate of ethene oxide formation with respect to time observed at temperatures >470 K can be explained by the accumulation of oxygen species embedded in the silver surface that decrease the surface area available for the formation of the reactive species. Nucleophilic and electrophilic oxygen, which are the major surface species at 420 K, are still present on the silver surface at 470 K however, they are rapidly removed in the absence of oxygen in the gas phase (see the difference spectrum in Figure 18). 

The presence of nucleophilic and electrophilic oxygen on the active silver surface suggests that they participate in the ethene oxidation reaction. This observation is in agreement with the mechanisms of ethene epoxidation proposed by the authors previously on the basis of the experiments with bulk silver (Bukhtiyarov et al., 1994,1999) ... 

FIGURE 19 Variation of the PTRMS signals of ethene oxide with temperature, measured in reaction mixtures of (A) PC2h = 0.0065 mbar and P0i — 0.065 mbar and (B) PCiH = 0.1 mbar and P0 = 0.95 mbar in the presence of a polycrystalline silver foil. The last part of the high-pressure curve was measured after stopping the oxygen flow. 

Moreover, analysis of the XPS and PTRMS data gave a linear correlation between the yield of ethene oxide and the abundance of electrophilic oxygen (Figure 21). 

FIGURE 21 Yield of ethene oxide as a function of the abundance of the electrophilic oxygen measured at various temperatures of a silver foil 425 K (circles) and 475 K (squares). 

The main use of ethene oxide is as ethylene glycol, half of which is used in as anti-freeze and the other half is used in the production of polyesters. Other products are glycol ethers, polyurethanes and polyethylene glycols). We find these in many consumer products such as fibres, foils, bottles, solvents, plasticizers, adhesives, detergents, brake fluids, etc. 

Nine million tons of ethene oxide are produced annually. 

Another process, the chlorohydrin process, is now obsolete. On a weight basis this process produced more calcium chloride than ethene oxide ... 

Functionalization of hydrocarbons from petroleum sources is mainly concerned with the introduction of oxygen into the hydrocarbon molecule. In general, two ways are open to achieve oxygen functionalization oxidation and carbonylation. Oxidation is commonly encountered in the synthesis of aromatic acids, acrolein, maleic anhydride, ethene oxide, propene oxide, and acetaldehyde. Hydroformylation (CO/H2) (older literature and the technical literature refer to the oxo reaction) is employed for the large-scale preparation of butanol, 2-ethylhexanol, and detergent alcohols. The main use of 2-ethylhexanol is in phthalate esters which are softeners in PVC. The catalysts applied are based on cobalt and rhodium. (For a general review see ref. 3.)... 

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