Reduction/reoxidation activation

Experiments whereby reduced enzyme was reoxidized by O2 enriched in (which has a magnetic nucleus) showed that an oxygen species ended up close to the Ni-based unpaired spin in the ready as well as in the unready state (Van der Zwaan et al. 1990). It could only be removed by full reduction and activation of the enzyme. The crystal structure (see Chapter 6) shows an oxygen atom close to nickel and it is... 

By this time it was demonstrated that the [3Fe-4S]W+ form of aconitase is inactive, while the [4Fe-4S]2+ form is active. How is the activity of the enzyme affected by the oxidation state of the [4Fe-4S] cluster Because the active enzyme contains a [4Fe-4S]2+ cluster, either the 3+ or 1+ oxidation states may also be stable. The 3+ state is unstable since oxidation of the [4Fe-4S]2+ resulted in the immediate loss of a ferrous ion and conversion to a [3Fe-4S]i+ cluster (46,47). However, reduction of active aconitase by sodium dithionite or photoreduction in the presence of deazaflavin produced in high yields an EPR signal characteristic for [4Fe-4S]l+ clusters (47). When active enzyme within an anaerobic assay cuvette was photoreduced, the activity of the enzyme dropped to 1/3 of its initial value. Further photoreduction resulted in cluster destruction. Then, if the enzyme is reoxidized with air, the activity returned to its original value. This demonstrated that the redox state of the cluster can modulate the enzyme activity. A scheme summarizing the cluster interconversions and various redox states of the Fe-S cluster of aconitase is shown below. 

The most effective molybdenum-based oxide catalyst for propane ammoxidation is the Mo-V-Nb-Te-0 catalyst system discovered and patented by Mitsubishi Chemical Corp., Japan, U.S.A. (140). Under single-pass process conditions, acrylonitrile yields of up to 59% are reported, whereas under recycle process feed conditions, the acrylonitrile selectivity is 62% at 25% propane conversion (141). Although the latter results show that the catalyst operates effectively under recycle feed conditions, the catalyst system was originally disclosed for propane ammoxidation under single-pass process conditions. The catalyst was derived from the Mo-V-Nb-0 catalyst developed by Union Carbide Corp. for the selective oxidation of ethane to ethylene and acetic acid (142). The early work by Mitsubishi Chemical Corp. used tellurium as an additive to the Union Carbide catalyst. The yields of acrylonitrile from propane using this catalyst were around 25% with a selectivity to acrylonitrile of 44% (143). The catalyst was also tested for use in a regenerative process mode much like that developed earlier by Monsanto (144) (see above and Fig. 8). Operation under cyclic reduction/reoxidation conditions revealed that the performance of the catalyst improved when it was partially reduced in the reduction cycle of the process. Selectivity to acrylonitrile reached 67%, albeit with propane conversions of less than 10%, since activity in... 

The modification of palladium-based catalysts by addition of various promoters and additives, usually metals or metal oxides, has been investigated. Studies have shown improved catalytic performance for the total oxidation of light alkanes, usually leading to higher conversions and lower deactivation. The reason for this promotion is still under discussion, since the metal oxide additives alone usually show relatively low activity for alkane oxidation over the range of reaction temperatures. Alloying phenomena, modification of the properties of the support, modification of the PdO particle size, variations in the Pd oxidation states or an enhanced reduction-reoxidation cycle are considered as the most likely factors for the enhancement of activity. For example, if Pd/Al203 catalysts are modified with titania, a... 

In agreement with the TPR results, the hydrogen chemisorption/pulse reoxidation data provided in Table 8.3 indicate that, indeed, the extents of reduction for the air calcined samples are -20% higher upon standard reduction at 350°C (compare 02 uptake values). Yet in spite of the higher extent of reduction, the H2 desorption amounts, which probe the active site densities (assume H Co = 1 1), indicate that the activated nitric oxide calcined samples have higher site densities on a per gram of catalyst basis. This is due to the much smaller crystallite that is formed. The estimated diameters of the activated air calcined samples are between 27 and 40 nm, while the H2-reduced nitric oxide calcined catalysts result in clusters between 10 and 20 nm, as measured by chemisorption/pulse reoxidation. 

The results confirm that the novel metal nitrate conversion method using nitric oxide in place of air advocated by Sietsma et al. in patent applications WO 2008029177 and WO 2007071899 leads to, after activation in H2, catalysts with smaller cobalt crystallites, as measured by EXAFS and hydrogen chemisorption/ pulse reoxidation. In spite of the lower extent of cobalt reduction for H2-activated nitric oxide calcined catalysts, which was recorded by TPR, XANES, EXAFS,... 

Reports by Li and Zuberbuhler were in support of the formation of Cu(I) as an intermediate (16). It was confirmed that Cu(I) and Cu(II) show the same catalytic activity and the reaction is first-order in [Cu(I) or (II)] and [02] in the presence of 0.6-1.5M acetonitrile and above pH 2.2. The oxygen consumption deviated from the strictly first-order pattern at lower pH and the corresponding kinetic traces were excluded from the evaluation of the data. The rate law was found to be identical with the one obtained for the autoxidation of Cu(I) in the absence of Cu(II) under similar conditions (17). Thus, the proposed kinetic model is centered around the reduction of Cu(II) by ascorbic acid and reoxidation of Cu(I) to Cu(II) by dioxygen ... 

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