MoVNb oxide

Ethylene conversion to acetic acid can be increased in at least two ways. One is making the catalyst surface more acidic, which will strengthen ethylene adsorption on the surface to give more time for its oxidation to acetic acid. A more acidic surface will also facilitate desorption of acetic acid thus reducing its surface over-oxidation to carbon oxides. This approach has been realized by adding small amounts of P, B and Te to the MoVNb oxide catalyst. Table 11.2 presents the results of adding phosphorus. 

Increasing the amount of phosphorus introduced into the MoVNh oxide catalyst decreased ethane conversion because of lower specific surface areas of P-promoted catalysts. Ethane conversion normalized to the catalyst surface area, as is seen from Table 11.2, did not change much. Selectivity to acetic acid (AA) increased and selectivity to ethylene decreased significantly with the amount of phosphorus in the catalyst, while their combined selectivity stayed roughly unchanged. These results imply that the phosphorus added to the MoVNb oxide catalyst indeed increased conversion of the intermediate ethylene to acetic acid. 

The comparison of P- and Pd-promoted catalysts shows that Pd-promoted catalysts are more selective towards the formation of acetic acid. Other advantages are that they do not produce CO as a reaction product and display better redox behavior. A study of catalyst redox property revealed that Pd facilitated both the reduction of the MoVNb oxide catalyst with ethane and its re-oxidation with oxygen. This is very important because the reaction occurs via the redox mechanism of a Mars-van Krevelen type. Such a conclusion comes from the data presented in Fig. 11.1. With increasing the amount of oxygen removed from the surface of the MoVNbPd... 

For the reasons listed above, a Pd-doped MoVNb oxide system has been picked as the basis for the development of industrial catalysts. This catalyst composition has been successfully scaled up in collaboration with a catalyst manufacturer and was commerciahzed in 2005 at the Ibn Rushd complex in Yanbu, Saudi Arabia. A block flow diagram for the ethane oxidation process is shown in Fig. 11.3. 

According to DD method, more than 85% wt. of the dried precursor prepared at pH = 3 dissolves in water (DW), the composition being MoiVo28Teoi7Nboo40n- With regard to XRD and FTIR data, we suppose HPA to be the main building block of DW. At pH = 4, along with DW (70%) there are also MoiTeog and a MoVNb oxide compound eiuiched with niobium. At pH below 3, the amount of DW decreases to 50% and formation of the MoVNb compound eiuiched with molybdenum is observed. Heat treatment at 320°C results in destruction of heteropolyanion and formation of nanosize particles with the stracture similar to that of M1 and M2 phases (Fig. 3). 

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. 

Symyx entered this competition in 1997 in collaboration with Hoechst with the goal of creating and validating primary and secondary synthesis and screening technologies and the use of this workflow to broadly explore mixed metal oxide compositions so as to discover and optimize new hits . The initial goal was a 10-fold increase in the space-time yield relative to the state-of-the-art MoVNb system for the ethane oxidative dehydrogenation reaction to ethylene. 

Fig. 1.2 Lineage of discoveries based on the MoVNb partial oxidation catalyst.

Fig. 3.17 Screening protocol showing library designs (top) and post reaction images of TLC detection wafers (bottom). Note that the white TLC plates appear black and the red spots appear white in the photo, (a) Binaries of redox active metal oxides (b) extension of binaries into ternaries by adding dopants (c) focus ternaries of best hits (d) noble metal doping of MoVNb ternary. Compositional details are given in the text. Reaction temperatures for (a-c) =375 °C, (d) =325 °C.

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