Iridium-alumina catalysts

The dehydrocyclization activity of rhodium-alumina is lower than that of platinum-alumina. Hydrogenolysis predominates over all the other reactions with this catalyst (57). The effect of temperature on the anthracene/phenanthrene ratio in the product from 2- -butylnaphthalene is the same over iridium-alumina catalyst as that observed over platinum-alumina more phenanthrene and less anthracene are formed at high temperatures (58). 

The studies of n-heptane and methylcyclopentane conversion provide insight into the advantages of platinum-iridium and platinum-rhenium catalysts over catalysts containing only one of the transition metal components, that is, platinum, iridium, or rhenium. If, for example, we consider an iridium-alumina catalyst for the reforming of a petroleum naphtha fraction, we find that it produces a substantially higher octane number reformate than a platinum on alumina catalyst under normal reforming conditions. The iridium-alumina catalyst will also exhibit a lower rate of formation of carbonaceous residues on the surface, with the result that the maintenance of activity with time will be much superior to that of a platinum-alumina catalyst. 

On the basis of activity and activity maintenance, the use of an iridium-alumina catalyst in reforming appears very reasonable (35,36). However, in our experience, the yields of low molecular weight alkanes (methane and ethane) are higher with an iridium-alumina catalyst than with a platinum-alumina catalyst, resulting in lower yields of C5+ reformate. Because of the higher value of C5+ reformate relative to products such as methane and ethane, the iridium-alumina catalyst is not used, despite its higher activity and better activity maintenance. 

Erivanskaya and co-workers also studied the dehydrocyclization of 2-n-butylnaphthalene over supported palladium, rhodium, and iridium catalysts (56-55). Palladium-alumina showed the lowest C6-dehydrocyclization activity, but was the most active for the C5-dehydrocyclization of 2-n-butyl-naphthalene. A later study showed, however, that this enhanced activity was due to the high chlorine content of the palladium-alumina catalyst and not to some mysterious inherent catalytic activity of palladium (56). 

Similar results have been obtained in the hydrogation of the n-pentenes in the liquid phase using iridium-alumina as catalyst (58),... 

Figure 4.19 Hydrogen chemisorption, at room temperature and 10 cm Hg pressure, as a function of the total content of platinum and iridium in catalysts comprising equal weights of platinum and iridium dispersed on alumina or silica (4). (Reprinted with permission from Academic Press, Inc.)...

The use of the ethane hydrogenolysis reaction as a chemical probe in the characterization of platinum-iridium on alumina catalysts is illustrated by the data in Figure 4.21. Rates of hydrogenolysis per iridium atom are shown over a range of temperatures for a series of catalysts containing 0.3 wt% iridium... 

Data on rates of dehydrocyclization rD and cracking rc of n-heptane at 495°C and 14.6 atm are given in Table 5.2 for platinum-iridium on alumina and platinum-rhenium on alumina catalysts, and also for catalysts containing platinum or iridium alone on alumina (33). The rate rD refers to the rate of production of toluene and C7 cycloalkanes, the latter consisting primarily of methylcyclohexane and dimethylcyclopentanes. The rate of cracking is the rate of conversion of n-heptane to C6 and lower carbon number alkanes. 

Results of some extended naphtha reforming runs illustrate further the differences between platinum-iridium on alumina and platinum-rhenium on alumina catalysts observed in the reforming of pure hydrocarbons (33). The results considered here should be regarded as illustrative only, since they are limited to the reforming of a series of Persian Gulf naphtha fractions in a particular range of operating conditions of commercial interest. The catalysts used in these runs contained 0.3 wt% platinum and 0.3 wt% of either iridium or rhenium. 

To understand the role of the y-alumina support, two experiments were carried out (i) variation ofthe molar ratio ofIr/Re and (ii) the direct addition ofy-alumina to the system. Increasing the molar amount of the Re20y catalyst (molar ratio of 1 1.8) led to a better activity. An additional amount of y-alumina provided better productivity, but parallel studies on either Ir-2(H2) or supported Ir-17(C2H4) with the olefin metathesis catalyst Mo-1 in the presence of y-alumina were found to be detrimental to the reaction selectivity [143]. All of these results strongly support that the adsorption of iridium pincer catalysts on y-alumina could prevent the... 

McVicker and al, 1980, Chemisorption properties of iridium on alumina catalysts , J.Catal., 65,207. 

Figure 2. Photo of a channel of a 400 cpsi cordierite monolith (a) coated by y-alumina and (b) impregnated by iridium (first catalyst set). EDS spectrum of the circle inside.

Conditions cited for Rh on alumina hydrogenation of MDA are much less severe, 117 °C and 760 kPA (110 psi) (26). With 550 kPa (80 psi) ammonia partial pressure present ia the hydrogenation of twice-distilled MDA employing 2-propanol solvent at 121°C and 1.3 MPa (190 psi) total pressure, the supported Rh catalyst could be extensively reused (27). Medium pressure (3.9 MPa = 566 psi) and temperature (80°C) hydrogenation usiag iridium yields low trans trans isomer MDCHA (28). Improved selectivity to aUcychc diamine from MDA has been claimed (29) for alumina-supported iridium and rhodium by iatroduciag the tertiary amines l,4-diazabicyclo[2.2.2]octane [280-57-9] and quiaucHdine [100-76-5]. 

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

---