Rhodium occurrence

Rhenium(VII) complexes, 196-202 amides, 196 deuterides, 201 halides, 201 hydrides, 201 imides, 196 nitrides, 196 thiols, 201 Rhodium occurrence, 902... 

The occurrence of multinuclear catalysts in hydrogenations catalyzed by rhodium-DIOP systems seems unlikely, although the trans-RhCl(CO)(DIOP) complex 43 is dimeric (276), and in basic methanolic solution the 1 1 diphos complex exists as Rh3(diphos)3(OMe)2 + (138a, Section II, B, 1). 

Structural, thermodynamic, and kinetic studies have shown that hydroxo-bridged polynuclear complexes of (diromium(III), cobalt(III), rhodium(III), and iridium(III) have many general features in common. Structurally, the four metal ions exhibit an almost identical pattern, and in particular the occurrence of many well-characterized oligomers... 

There has been great interest in recent years in methods for the generation of azomethine ylides and in exploitation of these reactive species in tandem/cascade processes for the rapid assembly of polyaza, polycyclic, multifunctional systems. a-Diazo ketones have featured greatly in such studies, treatment with a catalytic amount of rhodium(II) acetate generating transient rhodium carbenoids. A very common feature of many investigations of this type is the occurrence of quite unexpected reactions. For example, treatment of the diazo ketone 1 with a catalytic amount of... 

In general, carboxylic acids are hydrogenated with difficulty under mild conditions over usual metallic catalysts. However, it has been recognized that acetic acid may be reduced over platinum oxide under very mild conditions in the presence of perchloric acid.1 Kaplan observed that acetic acid was reduced rapidly over rhodium catalysts at room temperature and pressure, but the reduction stopped abruptly after a very small conversion.2 The occurrence of these reductions may lead to appreciable errors in the amounts of absorbed hydrogen when hydrogenations are carried out in acetic acid as solvent, although the reductions usually proceed only to limited extents, probably due to formation of some poisonous products. These undesired reductions of acetic acid solvent may be depressed in the presence of a substrate that may be adsorbed more strongly than acetic acid. 

Occurrence and History.—Rhodium occurs as an alloy in platinum ore and osmiridium. An alloy with gold, known as rhodite or rhodium gold, contains from 30 to 43 per cent, of rhodium, and has a density of 15-5 to 16-8.1 The metal was discovered by Wollaston,2 and so named from the Greek p6Sov, a rose, in recognition of the colour of aqueous solutions of its salts. 

Occurrence and History of Rhodium—Preparation—Proportion- Colloidal Rhodium—Rhodium Black—Uses—Atoraio Weight —Alloys. 

The relatively low partial pressure of carbon monoxide in the flash-tank has implications for catalyst stability. Since the rhodium catalyst exists principally as iodocarbonyl complexes (e.g., [Rh(CO)2l2] and [RtKCO LJ-), loss of CO ligands and precipitation of insoluble species (e.g., Rhl3) can be problematic. The conventional Monsanto process operates with a relatively high water concentration (10-15%, w/w) that helps to maintain catalyst stability and solubility (as discussed later). However, this operation results in a costly separation process to dry the product, typically requiring three distillation columns. The presence of water also results in the occurrence of the WGS reaction (Equation (2)), in competition with the desired carbonylation process, resulting in a lower utilization of CO. 

The first infrared evidence of the possible occurrence of formate ions as reaction intermediates on the surface of metals resulted from the research of Hirota and his colleagues (46, 56), who showed formate ions to be present on powders of silver, copper, nickel, palladium, rhodium, platinum, and zinc, after adsorption of formic acid at room temperature. Moreover, in the far-infrared region they observed bands at 410 and 130 cm-1, which is an indication of bonding between metal atoms and formate ions via oxygen atoms [Hirota and Nakai (57)]. 

The dynamic phenomena associated with the rhodium-catalyzed oxidation of carbon monoxide, methane and propane have been studied by in-situ infrared thermography. High-resolution temperature maps of the reacting catalyst revealed the mobility of the reaction front during ignition and extinction of the CO oxidation, and the development of thermokinetic oscillations. The catalytic oxidation of methane and propane produced weaker dynamics. Chemisorption and kinetic experiments suggest that the competitive adsorption of the reactants and the occurrence of self-inhibition, represent key factors in the development of the observed transient effects. 

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