Oxidation of Ethane to Acetic Acid

The major conventional processes for the production of acetic acid include the carbonylation of methanol (originally developed by Monsanto, and now carried out by several companies, such as Celanese-ACID OPTIMIZATION, BP-CATIVA, etc.), the liquid-phase oxidation of acetaldehyde, still carried out by a few companies, and the liquid-phase oxidation of n-butane and naphtha. More recent developments include the gas-phase oxidation of ethylene, developed by Showa Denko K.K., and the liquid-phase oxidation of butenes, developed by Wacker [2a], 

The synthesis of acetic add by direct gas-phase oxidation of ethane  

Catalysts similar to those claimed by Union Carbide were later studied by Bordes and coworkers [4], and by Burch and coworkers [5]. Merzouki et al. [4a, b] proposed that the Mo/V/Nb/O catalyst is made up of (VNbMo)5014-type microdomains in a M0O3 matrix. At 200 °C, a selectivity of 45% to acetic acid and 45% to ethylene was obtained at 25% ethane conversion an increase of temperature caused a loss in selectivity to acetic acid in favor of that to ethylene. Burch and Swarnakar [5a] compared the reactivity of Mo/V/O and Mo/V/Nb/O systems. The former contained M0O3, Mo6V9O40 and Mo4V6025 crystalline compounds, while the latter also contained Mo3Nb2On, the most intense diffraction line of which occurred at 4.01 A The addition of Nb increased both activity and selectivity, and the formation of Mo3Nb201 i was proposed to account for the increase in performance. The product distribution was independent of the conversion, indicating the absence of consecutive reactions. 

Researchers from Union Carbide proposed a mechanism in which ethane is first adsorbed on Mo6 + or Vs + [5c] sites to form an ethoxide species the latter transforms into ethylene via [3-elimination. However, the surface ethoxide can also be oxidized further and by a-elimination form acetaldehyde and then a surface acetate, which 

The catalyst claimed by Saudi Basic is in large part amorphous, but shows a few diffraction lines relative to a crystalline compound characterized by d values at 4.03 (100% I), 3.57, 2.01 and 1.86 A. Doping the compound with P improves the yield to acetic acid this was attributed to an enhancement of surface acidity, which facilitates the ethylene adsorption and the acetic acid desorption. A detailed investigation of catalyst composition confirmed that the former model originally proposed by Merzouki et al. [4a, b] was the most likely [4c]. Therefore, the excellent properties 

Different classes of catalysts have been claimed for the oxidation of ethane to acetic acid [3], but the catalyst that gives the best performance is made of a mixed oxide of Mo/V/Nb (plus other components in minor amounts). This compound was first described in a paper by Thorsteinson et cd. 3a] - a paper that is considered nowadays a milestone in the field of the selective oxidation of alkanes, in view of the number of active phases that have been developed starting from catalysts described therein. Several patents were also issued by Union Carbide [3a-f], now Dow Chemical, regarding this system and the ETHOXENE process. The activity in ethane oxidation was attributed to the development of a crystalline phase characterized by a broad X-ray diffraction reflection at d = 4.0 A. The best composition was claimed to be Moo.73Vo.i8Nbo.o90 c, which reached 10% conversion of ethane at 286 °C with almost total selectivity to ethylene the selectivity decreased with increasing temperature, due to the formation of carbon oxides. The main peculiarity of this catalyst is its capability to activate the paraffin at low temperatures ( 250 °C). 

The process and the catalyst claimed were developed for ethane oxidative dehydrogenation, and acetic acid was only a minor by-product of the reaction however, the use of pressures above atmospheric enhanced the selectivity of acetic acid. 

9 Recent Achievements and Challenges for a Greener Chemical Industry 

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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 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... 

Figure 9.1 Mechanism proposed in the literature for the oxidation of ethane to acetic acid [2b],...

Table 9.1 summarizes catalyst compositions and corresponding performances. The oxidation of ethane to acetic acid is now commercial an industrial plant is installed, with the technology developed by Saudi Basic. Elements that have contributed to the successful development of the process are (1) the discovery of a catalytically active compound, the multifunctional properties of which can be modified and tuned to be adapted to reaction conditions through incorporation of various elements (2) the stability of the main products, ethylene and acetic acid, which do not undergo extensive consecutive degradation reactions (3) the possibility of recycling the unconverted reactant and the major by-product, ethylene (4) the use of reaction conditions that minimize the formation of CO and (5) an acceptable overall process yield. 

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. 

Micro structured wells (2 mm x 2 mm x 0.2 mm) on the catalyst quartz wafer were manufactured by sandblasting with alumina powder through steel masks [7]. Each well was filled with mg catalyst. This 16 x 16 array of micro reactors was supplied with reagents by a micro fabricated gas distribution wafer, which also acted as a pressure restriction. The products were trapped on an absorbent plate by chemical reaction, condensation or absorption. The absorbent array was removed from the reactor and sprayed with dye solution to obtain a color reaction, which was then used for the detection of active catalysts by a CCD camera. Alternatively, the analysis was also carried out with a scanning mass spectrometer. The above-described reactor configuration was used for the primary screening of the oxidative dehydrogenation of ethane to ethylene, the selective oxidation of ethane to acetic acid, and the selective ammonoxidation of propane to acrylonitrile. 

Volpe, A. F., Weinberg, W. H., Woo, L., Zysk, J., Combinatorial heterogeneous catalysis oxidative dehydrogenation of ethane to ethylene, selective oxidation of ethane to acetic acid, and selective ammonoxidation of propane to acrylonitrile, Top. Catal. 2003, 23, 65-79. 

The oxidation of ethane to acetic acid is believed to proceed via the intermediate formation of ethylene (Equation Al). The catalysts are multicomponent mixed oxides, having optimized compositions for ethane oxydehydro-genation (Mo-V oxides) and ethylene oxidation (Pd, Nb oxide). Reported... 

Vanadyl pyrophosphate is widely considered to play an important catalytic role in the oxidation of -butane to MA, specifically the (100) face (Figure 18b), which is retained from the topotactic transformation (6,43,84—86) of the catalyst precursor phase (Figure 18a). Furthermore, this active phase has been reported to be an efficient catalyst for the oxyfimctionalization of light paraffins (a) for the oxidation of ethane to acetic acid (3,87), (b) for the oxidation and ammoxidation of propane to acrylic acid (88) and acrylonitrile (89,90), respectively, and (c) for the oxidation of n-pentane to maleic and phthalic anhydrides (90-102). 

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. 

Catalytic aerobic oxidation of ethane to acetic acid was successfully performed through a catalytic radical process using NHPl derivatives combined with a Co(II) salt in acetonitrile or propanoic acid. Among the catalysts examined, N,N-dihydroxypyr-omellitimide (NDHPI) was found to be the best, for instance, when a mixture of ethane (20 atm) and air (20 atm) in acetonitrile was allowed to react in the presence of NDHPI (100 xmol) and Co(OAc)2 (30 gmol) at 150 °C for 15 h, 830 gmol of acetic acid was obtained, and the turnover number (TON) of NDHPI reached 8.3 (Eq. (6.2)). In this reaction, other products such as ethanol or acetaldehyde were not detected at all. 

Karim, K., Al-Hazmi, M. and Mamedov, E. (2000). Catalysts for the Oxidation of Ethane to Acetic Acid, Processes of Making Same and Processes of Using Same, US Patent 6,013,597. Karim, K., Mamedov, E., Al-Hazmi, M., et al. (2000). Catalysts for Producing Acetic Acid from Ethane Oxidation, Processes of Making Same and Methods of Using Same, US Patent 6,030,920. Roussel, M., Barama, S., Karim, K., et al. (2009). MoV-based Catalysts in Ethane Oxidation to Acetic Acid Influence of Additives on Redox Chemistry, Catal. Today, 141, pp. 288-293. Fierro, J., Karim, K. and Mamedov, E. (1997). Unpubhshed data. 

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). 

Li, X. and Iglesia, E. (2008). Support and Promoter Effects in the Selective Oxidation of Ethane to Acetic Acid Catalyzed by Mo-V-Nb Oxides, Appl. Catal. A Gen., 334, pp. 339-347. 

Karim, K., Al-Hazmi, M. and Khan, A. (2000). US Patent 6060421, Catalysts for the oxidation of ethane to acetic acid, methods of making and using the same (Saudi Basic Industries Corporation, Saudi Arabia.). 

Linke, D., Wolf, D., Baems, M., et al. (2002). Catalytic Partial Oxidation of Ethane to Acetic Acid over MojVo 25Nl>o.i2Pdo.00050x- Catalyst Performance and Reaction Mechanism, J. Catal, 205, pp. 16-31. 

Merzouki, M., Bordes, E., Taouk, B., Monceaux, L., and Courtine, P. Correlation between catalytic and structural properties of modified molybdenum and vanadium oxides in the oxidation of ethane to acetic acid or ethylene. In Proceedings of the 10th International Congress on Catalysis. Budapest (Hungary), July 19-24, 1992 New Frontiers in Catalysis, Guczi, L., Solymosi, F. and Tetenyi, P. Eds. 1993, pp. 753-764. 

Roussel, M., Bouchard, M., Bordes-Richard, E., and Karim, K. Oxidation of ethane to acetic acid and ethylene by MoVNbO catalysts. Catal. Today 2005, 99,11-%1. 

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