Acrylic acid, mixed oxide catalysts

Figure 1.18 Survey and expanded V 2p and Mo 3d XPS spectra form a Mo-V-Sb-Nb mixed oxide catalyst after calcination in nitrogen (a) and air (b) atmospheres [145]. The data indicate a lesser degree of oxidation in nitrogen, a result that was correlated with the promotion of reactions leading to the production of propene and acrylic acid rather than acetic acid, the main product obtained with the fully oxidized sample. (Reproduced with permission from Elsevier.)...

It is doubtful whether a single-step process is at present competitive with the two-step process currently used in industry. In the latter, the oxidation of acrolein to acrylic acid is carried out with high selectivity over mixed-oxide catalysts based, for example, on M0O3—V2Os or Mo03— Te02 [160],... 

Tu X, Furuta N, Sumida Y, Takahashi M, Niiduma H. A new approach to the preparation of MoVNbTe mixed oxide catalysts for the oxidation of propane to acrylic acid. Catalysis Today. 2006 117(l-3) 259-264. 

Table 5 Mixed oxide catalysts for propane oxidation to acrylic acid...

Among various mixed oxide catalysts supposedly active for the oxidation of propane to acrylic acid (equation 7) in patents and literature (Table Mo-V-Te-Nb oxides... 

Giebeler, L., Kampe, R, Wirth, A., et al. (2006). Structural Changes of Vanadium-Molybdenum-Tungsten Mixed Oxide Catalysts during the Selective Oxidation of Acrolein to Acrylic Acid, J. Mol. Catal. A Chem., 259, pp. 309-318. 

The second most active and selective mixed oxide catalyst for the propane oxidation to acrylic acid is based again on Mo-V-Te-Nb-Ox, in which Te is substituted by Sb [61]. Test results on this catalyst suggested that Sb-based catalyst is less active and selective than its Te analog, but the overall performance was still quite good (16% acrylic acid yield). Takahashi et al. [63] proposed further improvements to this catalyst system by introducing potassium into the catalyst, resulting in raise of the yield to 25%. 

Catalytic experiments were performed in a continuous flow fixed bed microreactor using SiC diluted catalyst to obtain predetermined total volume of catalyst bed (i.e., GHSV, h ). The feed composition comprises propane, oxygen, and, in some cases, steam or ammonia. In the case of mixed oxide catalyst and Ga-ZSM-5, the condensable products (acrylic and acetic acids, acrolein, acetone, acrylonitrile,... 

Therefore, the combined presence of (i) correct crystalline structure, (ii) partial reduction of the transition metal oxides, and (hi) low Lewis acidity, are crucial elements for successful propane oxidation to acrylic acid over Mo-V-Sb-Nb mixed oxide catalyst. 

For propane oxidation to acrylic acid, mixed metal oxides appear to be the most promising catalysts in presence of steam. The presence of two phases such as Ml and M2 turn out to be determining. 

Botella, P, Concepcion, R, Lopez-Nieto, J.M., and Solsona, B. Effect of potassium doping on the catalytic behavior of Mo-V-Sb mixed oxide catalysts in the oxidation of propane to acrylic acid. Catal Lett. 2003, 89, 249-253. 

Novakova E.K. Ph.D. thesis. Catalytic Studies of Propane Oxidation to Acrylic Acid on MchA CSlrdifb Mixed Oxides Catalysts. University of Liverpool, 2002. 

Attenlion should be drawn to ihe use of tin oxide systems as heterogeneous catalysts. The oldest and mosi extensively patented systems are the mixed lin-vanadium oxide catalysis for the oxidation of aromatic compounds such as benzene, toluene, xylenes and naphthalene in the. synthesis of organic acids and acid anhydride.s. More recenily mixed lin-aniimony oxides have been applied lo the selective oxidaiion and ammoxidaiion of propylene to acrolein, acrylic acid and acrylonilrile. 

Partial oxidation of propylene results in acrolein, H2C=CHCHO, an important intermediate for acrylic acid, H2C=CHCOOH, or in the presence of NH3, in acrylonitrile, H2C=CHCN, the monomer for acrylic fibers. Mixed metal oxides are used as the catalysts [B.C. Gates, Catalytic Chemistry (1992), Wiley, New York]. 

Acrolein, in turn, can be oxidized further to acrylic acid. The catalyst for this step is a mixed vanadium-molybdenum oxide. 

More than a decade after the publication of the MoVNb catalyst system, scientists at Mitsubishi Chemical reported that modifying this family of mixed metal oxides with Te produced a catalyst for the amoxidation of propane to acrylonitrile [4] and the oxidation of propane to acrylic acid [5], Modification of the Union Carbide catalyst system with Te was probably not a random choice as it is a known propylene activator [5 b] and the molybdate phase TeMoO oxidizes propylene into acrolein and ammoxidizes propylene to acrylonitrile [6], a key intermediate in the commercial production of acrylic acid using Mo-based oxides. Significant efforts to optimize this and related mixed metal oxides continues for the production of both acrylic acid and acrylonitrile, with the main participants being Asahi, Rohm Hass, BASF, and BP. 

The direct oxidation of propane has fewer restrictions on plant location since the alkane is easier to ship over long distances as the compressed liquid. Its oxidation to acrolein, acrylic acid and acrylonitrile is the subject of numerous studies. The synthesis of acrylonitrile has already been developed to the stage of a demonstration plant. Catalysts are based on V-Sb mixed oxides, with additional metal promoters. Propylene is generally recognized as the intermediate through which acrylonitrile is obtained. Selectivities are close to 50-60% at ca. 20% propane conversion. 

One-step partial oxidation of propane to acrylic acid (an essential chemical widely used for the production of esters, polyesters, amides, anilides, etc.) has been investigated so far on three types of catalysts, namely, vanadium phosphorus oxides, heteropolycompounds and, more successfully, on mixed metal oxides. The active catalysts generally consist of Mo and V elements, which are also found in catalysts used for the oxidation of propene to acrolein and that of acrolein to acrylic acid. 

The aim of this review is to describe the reactivity of three catalytic systems whieh have been widely studied in recent years for the oxidative tran.sformation of light paraffins i) vanadyl pyrophosphate, which is the industrial catalyst for the oxidation of /i-butane, but has also been elaimed to be selective in the oxidation of n-pentane to maleie and phthalic anhydrides (18-22), ii) heteropolyeompounds, whieh are currently being studied for the oxidation of isobutane and propane to the corresponding unsaturated acids (methacrylic acid and acrylic acid) (5,23-29), and whose composition can be tuned to change the acidic and oxidizing properties and iii) rutile-based mixed oxides, which can act as the matrix to host various metal components, and whieh have been claimed as optimal eatalysts for the ammoxidation of propane to acrylonitrile (15,30-33). 

Partial oxidation of propane was investigated in the presence of molybdenum oxide based catalysts. We have shown the existence of a synergetic effect between the two phases aNiMo04 and aMoOs. Indeed activity and selectivity towards acetic acid and acrylic acid were maximal with a ratio aMo03 / (aNiMo04 + aMoOj) close to 0.25. These results could be explained by an interaction and a mutual covering of the two phases. The addition of bismuth to these mixed systems led to a total or a partial inhibition in the production of acetic acid and an increase in the formation of acrolein and acrylic acid. 

The present study showed that the major reaction pathway on Cs2.5Hi.5PViMoii.xWx04o heteropoly compounds is the formation of acetic acid via acetone, while the formation of acrylic acid through acrolein is a minor reaction pathway. In both cases, the major primary product is propene. The formation of acetic acid could be assigned to the strong Bronsted acidity of the catalysts compared to that of MoNb(Sb)TeV mixed oxides known to be the best catalysts for propane to acrylic acid in the presence of water vapour. 

Acrylic acid catalyst, based on Mo/V mixed oxide copper lowers reaction temperature... 

The principal monomer used in the manufacture of superabsorbent polymers is acrylic acid. Acrylic acid is made by the oxidation of propene in two steps (5). First, propene is oxidized to acrolein, and then the acrolein is further oxidized to aciylic acid. Different mixed metal oxide catalysts are used for each step to optimize the yield and selectivity of the oxidation reactions. Technical-grade acrylic acid is isolated from the steam-quenched reaction gas by means of solvent extraction and distillation, and is used principally in the fiirther preparation of acrylate esters. The technical-grade acrylic acid is further purified by distillation or by crystallization from the melt to afford the polymerization-grade monomer. 

The carbonylative oxidation of alkenes catalyzed by palladium catalysts has been extensively studied owing to its industrial importance. The conversion of ethylene to acrylic acid has been developed into a commercial process by Union Oil (Scheme 10). The reaction is performed in a mixed solvent of acetic acid and acetic anhydride in the presence of a Wacker catalyst under high pressure of ethylene. 

In general, mixed-metal oxides are at present the most important catalytic system for the partial oxidation of C2-C5 alkanes into 0-containing partial oxidation products. However, different characteristics should be considered among the several catalysts within this group. For instance, VPO- and MoVO-based catalysts are the most effective systems for -butane (or -butene) oxidation to MA and propane (or propene) oxidation to acrylic acid, respectively. It must be indicated that, although some similarities are observed for the ammoxidation " over MoVO- or SbVO-based catalyts, the latter show very low selectivity during the partial oxidation of propane and no results with other alkanes have been reported. " Therefore, SbVO-based catalysts will not be explicitly included in the following discussion. 

At this point it is interesting to compare the evolution of propylene adsorption over catalysts with different surface acid characteristics, i.e. a MoVTeNbO catalyst (active and selective in the partial oxidation of propane to acrylic acid), an alumina-supported vanadium oxide (active in the ODH of propane to propylene), or a MoVNbO mixed oxide (active in the oxidative transformation of propane to propylene and acetic acid). The final products observed in each case were related to the characteristics of the adsorbed intermediates (Fig. 24.7) (i) a ir-allylic compound, interacting with a redox site intermediate in the selective oxidation of... 

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. ... 

Acrylic acid has traditionally been used as the raw material for acrylic estos, polyacrylates, cross-linked polyacrylates, and copolymers. The global acrylic acid capacity was ca. 4.7 million tons in 2006, with an estimated average growth of 4%. Nowadays, acrylic acid is industrially obtained from a physically separated two-step process with propylene as the starting raw material. Firstly, propylene is selectively oxidized to acrolein at 300-350 C employing multicomponent catalysts based on metallic mixed oxides, i.e. MoBiO, FeSbO, or SnSbO. Then, the acrylic acid is obtained in a second step from acrolein oxidation at 200-260°C using multicomponent catalysts based on Mo-V-W mixed oxides. Thus, an overall acrylic acid yield of 85-90% is reached. 

It must also be indicated that the presence of water in the reaction gas mixture is essential to obtain high acrylic acid yields over these mixed-metal oxide catalysts. An IR spectra of propylene on these catalysts suggest that jt-allyl intermediate (key for the acrylic acid formation) is not formed in the absence of water, while it is clearly observed when both propylene and water are present ... 

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