Ethylene selective oxidation catalyzed

R.B. Grant, and R.M. Lambert, A single crystal study of the silver-catalyzed selective oxidation and total oxidation of ethylene, 7. Catal. 92, 364-375 (1985). 

More recently, direct catalytic oxidative condensation of methane to ethane (with metal oxides),57,82-84 as well as to ethylene and acetylene (via high-temperature chlorinative conversion) was explored.76 In all these processes, however, a significant portion of methane is lost by further oxidation and soot formation. The selectivity in obtaining ethane and ethylene (or acetylene), respectively, the first C2 products, is low. There has, however, been much progress in metal-oxide-catalyzed oxidative condensation to ethane. 

The Au-catalyzed glycerol oxidation was influenced by the kind of support, the size of Au particles and the reaction conditions such as concentration of glycerol, p02 and molar ratio of NaOH to glycerol. As metal oxide supports showed inferior selectivity to glyceric acid compared to carbons, due to successive oxidation and C—C bond cleavage to form di-adds such as tartronic acid and glycolic acid, research has focused on Au NPs supported on carbon, as in the case of ethylene glycol oxidation [182]. Indeed, the catalytic activity was influenced by the kind of carbon support in terms of porous texture [183]. 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

Among the various reactions catalyzed by bases, we have selected (i) the polymerization of ethylene oxide catalyzed by MgO (260), (ii) the closely related reaction of ethylene oxide with alcohols on basic hydrotalcites to give polyoxyethylene polymer (the Henkel process), (iii) the phenol alkylation with methanol on MgO (General Electric and BASF processes) (261), and (iv) phenol animation to give aniline on MgO (the USS process) (262). 

In industry many selective oxidations are carried out in a homogeneously catalyzed process. Heterogeneous catalysts are also applied in a number of processes, e.g. total combustion for emission control, oxidative coupling of methane, the synthesis of maleic acid from butanes, the epoxidation of ethylene. Here we focus upon heterogeneous catalysis and of the many examples we have selected one. We will illustrate the characteristics of catalytic oxidation on the basis of the epoxidation of ethylene. It has been chosen because it illustrates well the underlying chemistry in many selective oxidation processes. 

Among the several types of homogeneously catalyzed reactions, oxidation is perhaps the most relevant and applicable to chemical industry. The well-known Wacker oxidation of ethylene to ethylene oxide is the classic example, although this is not a true catalytic process since the palladium (II) ion becomes reduced to metallic palladium unless an oxygen carrier is present. Related to this is the commercial reaction of ethylene and acetic acid to form vinyl acetate, although the mechanism of this reaction does not seem to have yet been discussed publicly. Attempts to achieve selective oxidation of olefins or hydrocarbons heterogeneously do not seem very successful. 

Platinum (metal)- and acid (oxide)-catalyzed processes were developed to convert petroleum to high-octane fuels. Hydrodesulfurization catalysis removed sulfur from the crude to prevent catalyst deactivation. The discovery of microporous crystalline alumina silicates (zeolites) provided more selective and active catalysts for many reactions, including cracking, hydrocracking, alkylation, isomerization, and oligomerization. Catalysts that polymerize ethylene, propylene, and other molecules were discovered. A new generation of bimetallic catalysts that were dispersed on high-surface-area (100-400 m /g) oxides was synthesized. 

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

Zeolite MCM-22 in its Br0nsted-acid form has been described in the hterature as a useful catalyst for a variety of acid-catalyzed reactions, such as iso-alkane/olefin alkylation [e.g.40,41],skeletal and double-bond isomerization of olefins [42] and ethylbenzene synthesis via alkylation of benzene with ethylene [43], to name merely a few. Moreover, due to its very large intracrystalline cavities, zeolite MCM-22 has also been demonstrated to be a suitable host material for a variety of catalytically active guests, e.g. transition metal complexes which are useful in selective oxidation [44] or hydrogenation [45] reactions. Due to these interesting properties it seems worthwhile to focus on the synthesis features of MCM-22 (see below). 

In the literature numerous studies exist on the kinetics and reaction network of the direct ethylene oxidation to ethylene oxide catalyzed by silver (see Example 6.12.1). The surface reaction is considered to proceed via the Langmuir-Hinshelwood mechanism (adsorption of ethylene and oxygen —> surface reaction desorption of ethylene oxide, see Section 4.5.2). The rate expression for the selective oxidation can be expressed in the following equation ... 

Ml phase " represents the clearest example of a multifunctional catalyst in which each element, in close geometrical and electronic synergy with the surrounding elements, plays a specific role in turn, as an isolated active site, in every reaction step for the alkane transformation into the partial oxidation product desired. The flexibility of the structure allows modification of the catalyst composition and hence its catalytic behavior. Moreover, this type of mixed-metal oxide catalyst has the ability to catalyze other different oxidation reactions starting from alkanes, such as propane oxidation to acrylic acid, " oxidative dehydrogenation of ethane to ethylene, and n-butane selective oxidation. ... 

Silver oxide catalyzes the hydration of ethylene with steam in a vapor phase.Over a temperature range of 370 — 430 K, silver oxide on an alumina carrier gave conversions to glycol ranging from 20% —30%, with selectivity of 80% —90%. This yield is affected by catalyst age, increasing to an approximately constant value of 80% after 5 h of operation. 

The difference between a catalytic and a stoichiometric reaction is illustrated by the selective oxidation of ethylene to ethylene epoxide, where we compare the silver-catalyzed ethylene epoxidation with the stoichiometric epichlorohydrine process. Ethylene epoxide (oxirane) has industrial importance as a starter material for the production of ethylene glycol (antifreeze) and many other products (poly ethers, polyurethanes). 

Figure 9 shows a selection of (111) diffraction profiles from Pd-Rh alloy films (deposited and annealed at 400°C) which were used to catalyze ethylene oxidation (60) at 150°-200°C. The profile for the film with 24.6% Rh is symmetrical, and inspection of the (222) profile (not illustrated) after resolution of the ai-a doublet showed no evidence of phase... 

Propylene Oxide. Unlike ethylene, propylene cannot be selectively transformed to propylene oxide by silver-catalyzed oxidation. Instead, indirect oxidations (the peracid and the hydroperoxide routes) are employed.912-915... 

A soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

Two selective processes are important in the oxidation of ethylene the production of ethylene oxide and acetaldehyde. The first process is specifically catalyzed by silver, the second one by palladium-based catalysts. Silver catalysts are unique and selective for the oxidation of ethylene. No similar situation exists for higher olefins. The effect of palladium catalysts shows a resemblance to the liquid phase oxidation of ethylene in the Wacker process, in which Pd—C2H4 coordination complexes are involved. The high selectivity of the liquid phase process (95%), however, is not matched by the gas phase route at present. 

Excluding the effects of recombination of oxygen atoms and combustion of ethylene oxide, the maximum selectivity in such a coupled system is 85.7%. Such a high value is never reached, which is understandable because ethylene oxide can oxidize further, a process which is also catalyzed by silver. 

The epoxidation of propene is analogous to that of ethylene catalyzed by silver. However, the selectivity is much lower. Due to the pronounced oxidation sensitivity of the allyl CH3-group, excessive combustion occurs as a side reaction. The heterogeneous process has no practical significance, therefore, as it has to compete with a highly selective liquid phase epoxidation process. 

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