Rh oxidation

The well-known Adams platinum oxide can be prepared conveniently by the procedure of Adams et al. (2). Platinum oxides prepared in this way usually contain some traces of sodium, which in certain reactions can have an adverse effect. The sodium can be removed by washing with dilute acid (53). The Nishimuri catalyst (30% Pt, 70% Rh oxides) can be prepared by the same procedure as for platinum oxide or with variations from platinum and rhodium salts (64,65,66). This catalyst has much merit. It is usually most useful when hydrogenolysis is to be avoided (67,85,86). 

Figure 8.4 shows the steady-state effect of po2 and imposed catalyst potential Uwr on the rate of C2H4 oxidation and compares the results with the open-circuit kinetics. The sharp rate decline for high po2 values is due to the formation of surface Rh oxide.13 Increasing UWr causes a significant increase in the oxygen partial pressure, po2, where oxide forms and thus causes a dramatic increase in r for intermediate (1 to 2.5 kPa) Po2 values. For low P02 values (reduced surface) the effect of Uwr is moderate with p values up to 2. For highp02 values (po2>Po2 > oxidized surface) Uwr has practically no effect on the rate. 

The effect of Pc2H4 and Uwr on the rate of C2H4 oxidation is shown in Fig. 8.5. Increasing UWr causes a pronounced decrease in the ethylene partial pressure, pc2H4> necessary to reduce the surface Rh oxide and thus a dramatic, up to 100-fold, increase in reaction rate for intermediate PC2H4 values (p=100). 

It is worth noting the change in the reaction order with respect to ethylene, from positive to negative, upon positive current application. This shows the pronounced increase in C2H4 coverage induced by the positive potential and concomitant destabilization of surface Rh oxide.13... 

Conversely, since increasing Uwr and coverage of O8 stabilizes the Rh-C2H4 bond via enhanced 7t-electron donation to the metal, it follows that smaller pc2H4 values (Pc2h4) are required to reduce the surface Rh oxide as experimentally observed. 

This destabilization of surface Rh oxide formation with increasing catalyst potential or work function has been shown to be due to strong lateral repulsive interactions of the backpspillover O2 species and normally chemisorbed oxygen33 which causes a pronounced, up to leV, decrease in the chemisorptive bond strength of normally chemisorbed o.35,36... 

Suhonen, S., Polvinen, R., Valden, M. et al. (2002) Surface oxides on supported Rh particles thermal decomposition of Rh oxide under high vacuum conditions, Appl. Surf. Sci., 200, 48. 

Synthesis of ROOH by RH Oxidation in the Liquid Phase (p02 = 10s Pa)—continued... 

The kinetic scheme of a hydrocarbon RH oxidation catalyzed by PINO in the presence of initiator I includes the following elementary steps [90] ... 

Salts of transition metals are widely used in technological processes for the preparation of various oxygen-containing compounds from hydrocarbon raw materials. The principal mechanism of acceleration of RH oxidation by dioxygen in the presence of salts of heavy metals was discovered by Bawn [46 19] for benzaldehyde oxidation (see Chapter 1). Benzaldehyde was oxidized with dioxygen in a solution of acetic acid, with cobalt acetate as the catalyst. The oxidation rate was found to be [50] ... 

This region is determined by proportions between the rates of elementary reactions. For instance, when RH oxidation is chain-like and chains are terminated by the reaction R02 of with InH (mechanism III), the following six inequalities must be satisfied. 

Thus, the introduction of S into RH oxidized in the presence of InH leads to the following events. The concentration of hydroperoxide and the rate of autoinitiation decrease, whereas the duration of the InH-induced inhibitory period increases. When added at a sufficiently high concentration, S leads to a quasistationary regime of oxidation. If an inhibitory mechanism implies the occurrence of critical phenomena, the addition of S decreases the critical concentration [InH]cr (see Chapter 14). For mechanism III,... 

Essentially the C1=C2 bond is inserted into the C5-H bond. This suggests that the Rh oxidatively adds across the C5-H bond. Rh can do this with aldehydes. After oxidative addition to the C5-H bond to give a Rh(III) complex, insertion and reductive elimination give the product and regenerate Rh(I). Solvent molecules may be associating or dissociating at any point in the sequence. 

A proposed mechanism for the SiCaT reaction using the 1,6,11-triyne system as an example is illustrated in Scheme 7.21. The reaction proceeds through insertion of one of the terminal alkynes into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, generating an ethenyl-[Rh] intermediate, which undergoes addition to the second and third alkyne moieties to form intermediate III.2a. Subsequent carbocyclization followed by /9-hydride elimination gives the tricyclic silylbenzene derivative 70. Alternatively, ethenyl-[Rh] intermediate can be isomerized to the thermodynamically more... 

It is common practice to utilize forced degradation studies to accelerate degradation of the drug substance or drug product to get an understanding of its degradation profile. Potential environmental conditions that can be used include 40°C and 75% relative humidity (RH), 50°C and 75% RH, 70°C and 75% RH, or 80°C and 75% RH. Oxidation, reduction, and pH-related degradations are... 

With regard to other catalysts, little is known about the activity of palladium oxide catalysts for oxygen gas evolution. As discussed earlier in this section, it seems to be a poorer catalyst than Ru, Ir, or Rh oxides. A report by Razina and Gur eva [258] suggests that PdO has relatively good... 

Ethylbenzene (RH) oxidation was studied at 120°C in glass bubbling-type reactor in the presence of Ni(II)(acac)2 and additives of (L = N-methylpyrrolidone-2 (MP), HMPA MSt (M=Na, Li)) and PhOH. The selectivity Speh and RH conversion degree C of ethylbenzene into PEH oxidation were determined using the formulas Speh = [PEH] / A[RH]-100% and... 

The main difference between the Monsanto acetic acid process and Tennessee Eastman acetic anhydride process is the presence of water in the acetic acid process, which produces HI and acetic acid. In both reactions, a small amount of H2 is added to the CO stream to act as a reducing agent to keep the catalyst in the more active Rh oxidation state. An engineering problem with both processes is the highly corrosive nature of the Hl/iodide mixture, requiring the use of special chemically resistant alloys, pumps, and seals. 

In view of the relative thermal stabilities of the oxides (Rh203 > PdO Pt02), it is not surprising that under oxidizing conditions and at temperatures higher than 870 K, metallic Pt particles exist in combination with Rh oxide 132,133) and that PdO crystallites separate from Pt crystallites in Pd-Pt alloys in the temperature range 670-870 K 134). A more complicated behavior was found for Pd-Rh 135). 

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