Catalysts reduction step

Entrained catalyst is removed from the product off-gas by means of cyclones. The catalyst circulates continuously from the reactor to the regenerator and vice versa by means of transfer lines. Coke deposited on the catalyst is burnt off in the regenerator however, because the amount of coke is relatively small, additional fuel must be burnt in the regenerator to satisfy the thermal requirements of the endothermic dehydrogenation reaction. However, while this approach is similar to that in the Houdry process, FED does not have a catalyst reduction step with hydrogen before proceeding to the dehydrogenation cycle lack of this step is believed to be somewhat detrimental to the overall performance of the process. 

The catalyst reduction steps are steps (10.20)-(10.22), already written for the NH3-NO reacting system, were still assumed for the Fast SCR reaction, while for the reoxidation steps, a new step (10.47) was added to step (10.23), which considered the catalyst reoxidation by oxygen, to include the possibility for nitrates to reoxidize the catalytic sites ... 

The catalyst is previously prepared in an apparatus for catalytic hydrogenation, in which are placed 0.5 g. of palladous chloride, 3.0 g. of Norite, and 20 ml. of distilled water. The bottle is swept out with hydrogen and then shaken with hydrogen for 2-3 hours at 2-3 atmospheres (40 lb.) pressure. The palladium on carbon is collected on a Biichner funnel, washed with five 50-ml. portions of distilled water, then with five 50-ml. portions of 95% ethanol, and finally twice with ether. Upon drying, about 3 g. of the catalyst is obtained. It is stored in a vacuum desiccator over solid sodium hydroxide. If the reduction of the chloro-lepidine does not proceed normally, the used catalyst should be removed by suction filtration and a fresh 3-g. portion of catalyst added. Failure of the reduction step is usually due to an inactive catalyst or to impurities in the acetic acid or chlorolepidine. The palladium catalysts, prepared as described elsewhere in this volume, are presumably also satisfactory for the reduction of 2-chlorolepidine (p. 77). 

When dicobalt octacarbonyl, [Co(CO)4]2, is the catalyst, the species that actually adds to the double bond is tricarbonylhydrocobalt, HCo(CO)3. Carbonylation, RCo(CO)3- -CO—>RCo(CO)4, takes place, followed by a rearrangement and a reduction of the C—Co bond, similar to steps 4 and 5 of the nickel carbonyl mechanism shown in 15-30. The reducing agent in the reduction step is tetra-carbonylhydrocobalt HCo(CO)4, ° or, under some conditions, H2. When HCo(CO)4 was the agent used to hydroformylate styrene, the observation of CIDNP indicated that the mechanism is different, and involves free radicals. Alcohols can be obtained by allowing the reduction to continue after all the carbon monoxide is... 

Figures 3 and 5 give examples of hydrogen TPD profiles observed on both catalysts reduced at 473K. The more intense patterns concern the desorption of the hydrogen fixed on the solids during the reduction step (curve a) while the weaker peaks (curve b) are related to the hydrogen retained after readsorption at room temperature. Hydrogen adsorption is thus an activated process.

When a calcined Cr(VI)/Si02 catalyst is fed with ethylene at 373-423 K, an induction time is observed prior to the onset of the polymerization. This is attributed to a reduction phase, during which chromium is reduced and ethylene is oxidized [4]. Baker and Garrick obtained a conversion of 85-96% to Cr(II) for a catalyst exposed to ethylene at 400 K formaldehyde was the main by-product [44]. Water and other oxidation products have been also observed in the gas phase. These reduction products are very reactive and consequently can partially cover the surface. The same can occur for reduced chromium sites. Consequently, the state of sihca surface and of chromium after this reduction step is not well known. Besides the reduction with ethylene of Cr(Vl) precursors (adopted in the industrial process), four alternative approaches have been used to produce supported chromium in a reduced state ... 

Figure 5.2 Environmental scores of reduction steps in routes B and C (Scheme 5.1) according to the Eco-lndicator95 evaluation method (Figure 2 in reference [11 ]). Ml = catalyst, M2 = reduction, M3 = catalyst removal, M4 = extraction, M5 = solvent drain off, M5 = rectification, M7 = enantiomeric purification, M8 = solvent recycling. Reproduced from Jodicke [11 ], Copyright 1 999, with permission from Elsevier.

As always in chemisorption measurements, pretreatment of the samples should be done with care. For metal catalysts prepared from oxides in particular this is experimentally troublesome because a reduction step is always needed in the preparation of the metal catalyst. Hydrogen or hydrogen diluted with an inert gas is usually used for the reduction but it is difficult to remove adsorbed H2 from the surface completely. So, after reduction the metal surfaces contains (unknown) amounts of H atoms, which are strongly retained by the surface and, as a consequence, it is not easy to find reliable values for the dispersion from H2 chemisorption data. 

While it was felt that some of the individual issues above could be addressed using the same synthetic sequence (e.g., alternate catalysts for the reduction step) it seemed unlikely that all the above would be solvable, especially as efforts to replace sodium azide with other nucleophiles had failed. Based on this assessment the team felt it would be necessary to evaluate a fundamentally new approach to taranabant and, in particular, to look for a method for installation of the chiral centers without the intermediacy of an alcohol. 

The effect of different operating conditions (e.g. temperature, presence of C02 and water) on the reduction process by H2 was studied in a large temperature range (200—400"C) over the ternary Pt—Ba/y-Al203 catalyst. In all cases, the catalyst was previously saturated with N0/02 mixtures at the same temperature used for the reduction step (Sections 1 and 6 in Chapter 3). 

When the Ba loading increased up to 10% (w/w) (Pt—Ba/ y-AljC 1/11.5/100 w/w sample, Figure 6.18c), the reduction step was longer than in the previous case, in line with the higher amounts of NOx species that the catalyst was able to store during the previous adsorption phase (see Figure 6.6). Also in this case, low amounts of NO were found at the reactor outlet so that the process presented a selectivity rate close to 100% (Figure 6.19). 

Equivalent amounts of aldehydes and alkoxytrimethylsilanes react to form unsymmetrical ethers in near quantitative yields in the presence of either trimethylsilane or triethylsilane and catalytic amounts (ca. 10 mol%) of TMSI in dichloromethane.329,333,334,341 The procedure is particularly convenient experimentally when trimethylsilane is used with TMSI because the catalyst provides its own color indicator for the reduction step (color change from deep violet to vivid red-gold) and the only silicon-containing product following aqueous workup is the volatile hexamethyldisiloxane (bp 99-100°). It is possible to introduce trimethylsilane (bp 7°) either as a previously prepared solution in dichloromethane or by bubbling it directly into the reaction mixture. Cyclohexyloxytrimethylsilane and n-butanal react by this method to give a 93% isolated yield of n-butyl cyclohexyl ether (Eq. 183).334... 

Hydride reductions of C = N groups are well known in organic chemistry. It was therefore obvious to try to use chiral auxiliaries in order to render the reducing agent enantioselective [88]. The chiral catalyst is prepared by addition of a chiral diol or amino alcohol, and the active species is formed by reaction of OH or NH groups of the chiral auxiliary with the metal hydride. A major drawback of most hydride reduction methods is the fact that stoichiometric or higher amounts of chiral material are needed and that the hydrolyzed borates and aluminates must be disposed of, which leads to increased costs for the reduction step. 

The first step in this reaction mechanism is the catalyst reduction. Cat-0 represents an oxidized catalyst, which is attacked by a reductant (Red). The catalyst itself undergoes reduction, while the... 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

In summary, the examples given above demonstrate that immobilization of metal salts in a block copolymer micellar system followed by a reduction step is a suitable method to synthesize stable colloids with small particle sizes and narrow size distributions. Moreover, such systems are very interesting for catalytic applications because they offer the possibility of designing tailored catalysts for special demands and can be easily tuned by the choice and combination of different polymer block types and lengths, different types of the metal precursor and of the reduction method used. Additional introduction of further functionalities such as charges or chiral groups could make these catalyst systems even more versatile and effective. 

Recently another family of dendrimers has become commercially available. These polyamines were developed by Meijer and de Brabander-van den Berg of DSM Research and are based on Vogtle s initial synthesis [7]. In this case the troublesome reduction step was performed using a Raney cobalt hydrogenation catalyst and other process improvements have permitted this synthesis to be continued up to the fifth generation with multikilogram quantities available. 

Ir-catalyzed alkylation with a nitro compound was applied in a synthesis of flS,2R)-tra s-2-phenylcyclopentanamine, a compound with antidepressant activity (Scheme 9.41) [45]. The reaction of cinnamyl methyl carbonate with 4-nitro-l-butene gave the substitution product with 93% ee in 82% yield. A Grubbs I catalyst sufficed for the subsequent RCM. Further epimerization with NEts yielded a trans-cyclopentene in 83% yield via the two steps, while additional reduction steps proceeded in 90% yield. 

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