Direct oxidation of ethane to acetic acid

The synthesis of intermediates and monomers from alkanes by means of oxidative processes, in part replacing alkenes and aromatics as the traditional building blocks for the chemical industry [2]. Besides the well-known oxidation of n-butane to maleic anhydride, examples of processes implemented at the industrial level are (i) the direct oxidation of ethane to acetic acid, developed by Sabic (ii) the ammoxidation of propane to acrylonitrile, developed by INEOS (former BP) and by Mitsubishi, and recently announced by Asahi to soon become commercial (iii) the partial oxidation of methane to syngas (a demonstration unit is being built by ENI). Many other reactions are currently being investigated, for example, (i) the... 

Although the direct oxidation of ethane to acetic acid is of increasing interest as an alternative route to acetic acid synthesis because of low-cost feedstock, this process has not been commercialized because state-of-the-art catalyst systems do not have sufficient activity and/or selectivity to acetic acid. A two-week high-throughput scoping effort (primary screening only) was run on this chemistry. The workflow for this effort consisted of a wafer-based automated evaporative synthesis station and parallel microfluidic reactor primary screen. If this were to be continued further, secondary scale hardware, an evaporative synthesis workflow as described above and a 48-channel fixed-bed reactor for screening, would be used. 

There seems to be no literature about the direct oxidation of ethane to acetic acid over heteropolycompounds catalysts. Nevertheless, there is a limited amount of literature[10,26-28] about direct oxidation of ethane to acetic acid over oxide catalysts at low temperature (200-350 C). It seems that vanadium and molybdenum are necessary to those catalysts, and the addition of water is useful to increase the production of acetic acid. Roy et al. [10] has proved that vanadium and molybdenum phosphates supported on Ti02-anatase were effective in the direct oxidation of ethane to acetic acid. Considering previous research results, it is suggested that other promoters, such as trcmsition-metal oxides, are necessary to enhance the catalytic activity of the activated H3PMol2O40(Py) in the direct oxidation of ethane to acetic acid. 

Direct oxidation of ethane to acetic acid is an attractive alternative to conventional processes for obtaining acetic acid. In the 1980s, Union Carbide researchers developed a process for the production of ethylene via the oxidative dehydrogenation of ethane with the co-production of acetic acid. Using MoVNbO mixed oxides as catalyst systems, different amounts of ethylene and acetic acid were obtained from ethane oxidation depending on the reaction conditions. In this way, selectivity to acetic acid of 26%, at ethane conversion of 5% was reported. After this, several patents were reported by Union Carbide (now known as Dow Chemical). 

One of the most important challenges in the modern chemical industry is represented by the development of new processes aimed at the exploitation of alternative raw materials, in replacement of technologies that make use of building blocks derived from oil (olefins and aromatics). This has led to a scientific activity devoted to the valorization of natural gas components, through catalytic, environmentally benign processes of transformation (1). Examples include the direct exoenthalpic transformation of methane to methanol, DME or formaldehyde, the oxidation of ethane to acetic acid or its oxychlorination to vinyl chloride, the oxidation of propane to acrylic acid or its ammoxidation to acrylonitrile, the oxidation of isobutane to... 

The direct synthesis of chemicals from alkanes is an attractive alternative to that via olefins. Alkanes are abundant and cheaper, while the elimination of the dehydrogenation unit allows simplified process designs and energy savings. Oxidations of several alkanes are of industrial interest and are being investigated (Table 3) at the most advanced stage are those of ethane to acetic acid and of propane to acrylonitrile. 

The Wacker process, the oxidation of ethylene to acetaldehyde, lost its original importance over the past 30 years. While at the beginning more than 40 factories with a total capacity of more than 2 million tons of acetaldehyde per year were installed, acetaldehyde as an industrial intermediate was replaced successively by other processes. For example, compounds such as butyraldehyde/butanol are produced by the oxo process from propylene, and acetic acid by the Monsanto process from methanol and CO or by direct oxidation of ethane. The way via acetaldehyde to these products is dependent on the price of ethylene. Petrochemical ethylene from cracking processes became considerably more expensive during these years. Thus, only few factories would be necessary to meet the demand for other derivatives of acetaldehyde such as alkyl amines, pyridines, glyoxal, and pentaerythritol. 

Recently, Sen has reported two catalytic systems, one heterogeneous and the other homogeneous, which simultaneously activate dioxygen and alkane C-H bonds, resulting in direct oxidations of alkanes. In the first system, metallic palladium was found to catalyze the oxidation of methane and ethane by dioxygen in aqueous medium at 70-110 °C in the presence of carbon monoxide [40]. In aqueous medium, formic acid was the observed oxidation product from methane while acetic acid, together with some formic acid, was formed from ethane [40 a]. No alkane oxidation was observed in the absence of added carbon monoxide. The essential role of carbon monoxide in achieving difficult alkane oxidation was shown by a competition experiment between ethane and ethanol, both in the presence and absence of carbon monoxide. In the absence of added carbon monoxide, only ethanol was oxidized. When carbon monoxide was added, almost half of the products were derived from ethane. Thus, the more inert ethane was oxidized only in the presence of added carbon monoxide. 

The paper overviews research carried out at SABIC Company in the last 15-20 years in the field of selective oxidation. Using different approaches, a number of effective catalysts were developed by proposing new or improving existing catalytic systems. On some of them reaction network and kinetics were studied that in combination with reaction engineering allowed elaborate process technology. The most advanced development is ethane direct oxidation to acetic acid which was commercialized at one of the SABIC plants. 

Figure 11.3. Block flow diagram for the SABIC process of direct ethane oxidation to acetic acid.

Ozone also reacts with ethane in the gas phase at room temperature. Rather than a direct molecular reaction, however, evidence points to the initiation of radical-chain reactions by the very small O-atom concentrations present in ozone at room temperature. Added oxygen scavenges the radicals and slows the build-up, leading to induction periods which may be in excess of 3 h. Recent advances in mechanistic investigations of gas-phase ozonolysis of alkanes have been reviewed. Oligomeric peroxides dominate the products of oxidation of nitrotoluenes with ozone in acetic acid. °... 

An impartial decision has not yet been given as to whether the ethane production depends upon a direct union of the anions or upon the oxidation of an intermediate product, like acetic acid, acetic anhydride, or acetyl superoxide. 

It is possible that some acetate radicals are formed by the direct discharge of the ions as, it will be seen shortly, is the case in non-aqueous solutions but an additional mechanism must be introduced, such as the one proposed above, to account for the influence of electrode material, catalysts for hydrogen peroxide decomposition, etc. It is significant that the anodes at which there is no Kolbe reaction consist of substances that are either themselves catalysts, or which become oxidized to compounds that are catalysts, for hydrogen peroxide decomposition. By diverting the hydroxyl radicals or the peroxide into an alternative path, viz., oxygen evolution, the efficiency of ethane formation is diminished. Under these conditions, as well as when access of acetate ions to the anode is prevented by the presence of foreign anions, the reactions mentioned above presumably do not occur, but instead peracetic acid is probably formed, thus,... 

The next higher hydroxy acid is hydroxy acetic acid CH2(OH)— COOH, known also as glycolic acid. It may be prepared (a) from chlor acetic acid, (b) from the cyan-hydrine obtained from formic aldehyde, or (c) hy the oxidation of ethylene glycol, by reactions which have been already discussed. Its relation to ethylene glycol gives it the name of glycolic acid. It may be considered as a direct oxidation product of ethane. 

Analogously, in the presence of silica-supported palladium catalysts, benzene is oxidized under ambient conditions to give phenol, benzoquinone, hydroquinone and catechol [37b]. Palladium chloride, used for the catalyst preparation, is believed to be converted into metallic palladium. The synthesis of phenol from benzene and molecular oxygen via direct activation of a C-H bond by the catalytic system Pd(OAc)2-phenanthroline in the presence of carbon monoxide has been described [38]. The proposed mechanism includes the electrophilic attack of benzene by an active palladium-containing species to to produce a a-phenyl complex of palladium(ll). Subsequent activation of dioxygen by the Pd-phen-CO complex to form a Pd-OPh complex and its reaction with acetic acid yields phenol. The oxidation of propenoidic phenols by molecular oxygen is catalyzed by [A,A"-bis(salicylidene)ethane-l,2-diaminato]cobalt(ll)[Co(salen)] [39]. 

Acetobacter are also capable of oxidizing acetic acid, but this reaction is inhibited by ethanol. It therefore does not exist in enological conditions. Acetic acid slows the second step, when it accumulates in the medium, in which case the ethanal concentration of the wine may increase. According to Asai (1968), this second step is a dis-mutation of ethanal into ethanol and acetic acid. In aerobiosis, up to 75% of the ethanal leads to the formation of acetic acid. In intense aeration conditions, the oxidation and the dismntation convert all of the ethanol into acetic acid. When the medium grows poorer in oxygen, ethanal accumulates in the medium. Furthermore, a pH-dependent metabolic regulation preferentially directs the pathway towards oxidation rather than towards dismu-tation in an acidic environment. 

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