Noble metal catalyst, recycling

Supported noble metal catalysts (Pt, Pd, Ag, Rh, Ni, etc.) are an important class of catalysts. Depositing noble metals on high-area oxide supports (alumina, silica, zeolites) disperses the metal over the surface so that nearly every metal atom is on the surface. A critical property of supported catalysts is that they have high dispersion (fraction of atoms on the surface), and this is a strong function of support, method of preparation, and treatment conditions. Since noble metals are very expensive, this reduces the cost of catalyst. It is fairly common to have situations where the noble metals in a catalyst cost more than 100,000 in a typical reactor. Fortunately, these metals can usually be recovered and recycled when the catalyst has become deactivated and needs to be replaced. 

Regeneration -hollow fibers [HOLLOW-FIBERMEMBRANES] (Vol 13) -of catalysts [CATALYSIS] (Vol 5) -of noble metal catalysts [CATALYSTS - REGENERATION - NOBLE METAL AND BASE METAL CATALYSTS] (Vol 5) -of nonferrous metals [RECYCLING - METALS - NONFERROUS METALS] (Vol 20)... 

The crude MNB is washed to remove residual acid and the impurities formed during the nitration reaction. The product is then distilled and residual benzene is recovered and recycled. Purified MNB is fed, together with hydrogen, into a liquid phase plug-flow hydrogenation reactor that contains a DuPont proprietary catalyst. The supported noble metal catalyst has a high selectivity and the MNB conversion per pass is 100%. 

Description The fresh paraffin feedstock is combined with paraffin recycle and internally generated steam. After preheating, the feed is sent to the reaction section. This section consists of an externally fired tubular fixed-bed reactor (Uhde reformer) connected in series with an adiabatic fixed-bed oxyreactor (secondary reformer type). In the reformer, the endothermic dehydrogenation reaction takes place over a proprietary, noble metal catalyst. 

This conversion is conducted at moderate temperature and pressure (lOO C, IS. 10 Pa absolute), and possibly in the presence of a hydrocarbon diluent, for better control of the temperature rise in the catalyst beds, due to the high exothennicity of the reaction, which is itself related to the high diolefmic content of the initial Cj cut As a rule, the feed is introduced in a downflow stream into the reactor, which contains several beds of a noble metal catalyst on alumina. Quench by recycling and diluent injection is carried out between the beds. The diluent is recovered, by distillation in a depentanizer, after flash to eiimioate the inert compounds introduced with hydrogen gas at the same time as the feedstock. The leading licensors include FP and Shett, etc... 

Nevertheless, the use of a supported noble metal catalyst (usually Pt) for soot oxidation under loose contact conditions (proposed to be closer to the practical condition) results in a significant decrease in the soot oxidation temperature.76,94,97,98 Thus, incorporation of the soot in a Pt/SiC foam catalyst allows the soot oxidation rate to be doubled (and also to decrease the maximum rate temperature) with respect to a non-catalysed situation in which the soot is incorporated into the Pt-free SiC foam (with Pt/SiC foam located upstream to promote NO oxidation). In turn, a considerable decrease in the maximum rate temperature is observed when employing NO + O2 instead of O2 as oxidant in the Pt/SiC-soot configuration.98 On the basis mainly of these results, a catalytic role for NO is proposed in a recycle reaction as follows ... 

In some cases, temperature-induced phase separation does not result in the required quantitative separation of all components or damages temperature sensitive catalysts. This is especially true for catalytic reactions with expensive noble metal catalysts. Such reactions often require more than 99% recovery of the catalyst in order to avoid severe economic losses due to the extremely high costs of such catalysts. Here, ultrafiltration is a suitable tool for quantitative catalyst recovery under mild conditions. In general the same conditions for reaction and ultrafiltration should be chosen in order to recycle the catalyst in its active form. 

Especially for mass applications such as automotive, a full life cycle assessment (LCA) is absolutely critical. Since there are a lot of embedded resources such as noble metal catalysts and energy for manufacturing fuel cells, LCA very much depends on aspects such as recyclability and lifetime. For the operation period itself, cost, performance, and durability issues are important. But a complete picture must also consider aspects of specific applications including how they are connected in future energy scenarios. 

The unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

CAMET control catalyst was shown to obtain 80% NO reduction and 95% carbon monoxide reduction in this appHcation in the Santa Maria, California cogeneration project. The catalyst consists of a cormgated metal substrate onto which the active noble metal is evenly deposited with a washcoat. Unlike the typical 20 on titania turbine exhaust catalysts used eadier in these appHcations, the CAMET catalyst is recyclable (52). 

The widespread application of enantioselective catalysis, be it with chiral metal complexes or enzymes, raises another issue. These catalysts are often very expensive. Chiral metal complexes generally comprise expensive noble metals in combination with even more expensive chiral ligands. A key issue is, therefore, to minimise the cost contribution of the catalyst to the total cost price of the product a rule of thumb is that it should not be more than ca. 5%. This can be achieved either by developing an extremely productive catalyst, as in the metachlor example, or by efficient recovery and recycling of the catalyst. Hence, much attention has been devoted in recent years to the development of effective methods for the immobilisation of metal complexes [130, 131] and enzymes [132]. This is discussed in more detail in Chapter 9. 

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