Rhodium catalysts determination

Significant distinction in rate constants of MDASA and TPASA oxidation reactions by periodate ions at the presence of individual catalysts allow to use them for differential determination of platinum metals in complex mixtures. The range of concentration rations iridium (IV) rhodium (III) is determined where sinergetic effect of concentration of one catalyst on the rate of oxidation MDASA and TPASA by periodate ions at the presence of another is not observed. Optimal conditions of iridium (IV) and rhodium (III) determination are established at theirs simultaneous presence. Indicative oxidation reactions of MDASA and TPASA are applied to differential determination of iridium (IV) and rhodium (III) in artificial mixtures and a complex industrial sample by the method of the proportional equations. 

Such a complex, cw-Rh(CO)2I2, is the active species in the Monsanto process for low-pressure carbonylation of methanol to ethanoic acid. The reaction is first order in iodomethane and in the rhodium catalyst the rate-determining step is oxidative addition between these followed by... 

GP 8[ [R 7] Rhodium catalysts generally show no pronoimced activation phase as given for other catalysts in other reactions [3]. In the first 4 h of operation, methane conversion and hydrogen selectivity increases by only a few percent. After this short and non-pronounced formation phase, no significant changes in activity were determined in the experimental runs for more than 200 h. 

When a catalyst has sufficiently deactivated to justify taking some action is determined by economics. Both Gas and Liquid Recycle hydroformylation plants may be operated to give essentially constant production rates as the catalyst deactivates. Hydroformylation is approximately first order in both rhodium and alkene concentration. As the rhodium catalyst deactivates, the alkene concentration may be allowed to increase to compensate for the declining catalyst activity. Action is taken when the alkene efficiency declines to the point where it approximates or exceeds the cost of catalyst replacement or reactivation. 

Capture of Active Catalyst Using Solid Acidic Support with H2 Elution. The limit on the practical life of a catalyst solution may be determined by several factors including the presence of poisons or inhibitors, the buildup of less soluble materials such as the oxidation products of organophosphorus ligands, or an increase in the concentration of heavy aldehyde condensation products in the catalyst solution. In the latter case, there may be substantial amounts of active catalyst, but it is in a solvent that is unsuitable. Alternately, active rhodium catalyst may have been carried over with product. Technology has been disclosed [39] that permits the isolation of an active metal hydride catalyst. Steps include ... 

In contrast, the activity of supported rhodium catalysts is determined principally by the concentration of accessible surface Rh atoms, which catalyze methane decomposition, followed by CO2 reduction (186). As a result, the support plays a minimal role in the rhodium-containing catalysts. 

The argument of each sine contribution in (6-8) depends on k, which is known, r, which is to be determined, and the phase shift (f(k). The latter needs to be known before r can be determined. The phase shift is a characteristic property of the scattering atom in a certain environment, and is best derived from the EXAFS spectrum of a reference compound, for which all distances are known. For example, the phase shift for zero-valent rhodium atoms in the EXAFS spectrum of a supported rhodium catalyst is best determined from a spectrum of pure rhodium metal as in Fig. 6.13, while RI12O3 may provide a reference for the scattering contribution from oxygen neighbors in the metal support interface. 

The catalyst, the way it is operated, is about 25% faster than the Monsanto rhodium catalyst. In addition, it was assumed that the oxidative addition is no longer rate-determining and that now the migration of the methyl group to the co-ordinated carbon monoxide is rate-determining (Figure 6.3). 

To elucidate the use of TMS systems for the isomerizing hydroformylation, PC was chosen as the solvent for the rhodium catalyst, because the best selectivity to n-nonanal of 95% with a conversion on trans-4-octene of also 95% was achieved in this solvent under single-phase conditions. Dodecane was used as a non-polar solvent for the extraction of the product and p-xylene served as the mediator between the catalyst and the product phase [24]. Appropriate operation points for the reaction within this solvent system were determined by cloud titrations. 

The rhodium catalyst was recycled batch-wise four times. It was found that a short induction period occurred during the first reaction cycle. The following cycles showed a constant rate and no loss of activity was detected. A ligand-to-rhodium ratio of 5 1 led to a constant yield of 95% per cycle after 1 h. Within the four cycles a total turnover number of 1000 with a maximum turnover frequency of 234 h was achieved. The leaching of rhodium and phosphorus into the aqueous layer was determined by inductively coupled plasma atomic emission spectrometry. Rhodium leaching amounted to 14.2 ppm in the first run, then dropped to 3.6 ppm (second run) and reached values of 0.95 and 0.63 ppm in the third and fourth runs, respectively. 

The first example involving a rhodium catalyst in an ene reaction was reported by Schmitz in 1976. An intramolecular cyclization of a diene occurred to give a pyrrole when exposed to rhodium trichloride in isobutanol (Eq. 2) [15]. Subsequently to this work, Grigg utilized Wilkinson s catalyst to effect a similar cycloisomerization reaction (Eq. 3) [16]. Opplozer and Eurstner showed that a n -allyl-rhodium species could be formed from an allyl carbonate or acetate and intercepted intramolecularly by an alkene to afford 1,4-dienes (Eq. 4). Hydridotetrakis(triphenylphosphine)rhodium(l) proved to be the most efficient catalyst for this particular transformation. A direct comparison was made between this catalyst and palladium bis(dibenzylidene) acetone, in which it was determined that rhodium might offer an additional stereochemical perspective. In the latter case, this type of reaction is typically referred to as a metallo-ene reaction [17]. 

Disappointingly, the trimethylphosphite-modified Wilkinson catalyst, which had proven effective for the allylic substitution reaction [30], furnished only a trace amount of the PK product. By screening various rhodium catalysts for both reactions, it was determined that [RhCl(CO)dppp]2 was the optimum complex for the sequential pro-... 

Rate parameters of all unit reactions were determined by a differential reaction technique and are summarized in Table III for the Ni/A.C. catalyst. For methyl acetate formation, the reaction orders with respect to methyl iodide, methanol and carbon monoxide are 0.1, 0.6 and 0.7, respectively, which are remarkably different from those for the rhodium catalyst (1.0, 0 and 0, respectively)... 

Many of the reactions described above are seen to give less than quantitative recovery of the rhodium catalyst component. The amount of rhodium remaining in a catalyst solution was determined by atomic absorption spectroscopy, and is reported as the percent of the rhodium charged which remains soluble or suspended in the reaction mixture at the end of the reaction (95). After some experiments a wash procedure was employed to dissolve rhodium complexes possibly left in the reactor heating a charge of pure solvent in the reactor under H2/CO pressure sometimes dissolved substantial amounts of rhodium species (94-96, 104, 108, 109). High recoveries of rhodium are essential in a practical process because of the scarcity and high price of this metal (120, 121). 

The reaction is first order in rhodium catalyst concentration, first order in dihydrogen pressure and has an order of minus one in carbon monoxide pressure. In our Scheme 6.1 this would be in accord with a rate-determining step at the end of the reaction sequence, e.g. reaction 6. Since the reaction order in substrate is zero, the rhodium catalyst under the reaction conditions predominates as the alkyl or acyl species any appreciable amount of rhodium hydride occurring under fast pre-equilibria conditions would give rise to a positive depence of the rate of product formation on the alkene concentration. The minus one order in CO suggests that the acyl species rather than the alkyl species is dominant under the reaction conditions. The negative order in CO is explained [20] by equilibrium... 

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