Recycling of catalysts

In addition to the advantage of high heat transfer rates, fluidized beds are also useful in situations where catalyst particles need frequent regeneration. Under these circumstances, particles can be removed continuously from the bed, regenerated, and recycled back to the bed. In exothermic reactions, the recycling of catalyst can be... 

Alkyl chloride Olefins are chlorinated to alkyl chlorides in a single fluidized bed. In this process, HCl reacts with 02 over a copper chloride catalyst to form chlorine. The chlorine reacts with the olefin to form alkyl chloride. The process was developed by Shell Development Co. and uses a recycle of catalyst fines in aqueous HCl to control the temperature [Chem. Proc. 16 42 (1953)]. 

For the recyclability of catalyst 1, after completion of the oxidation of 4-methoxybenzyl alcohol, the reaction mixmre was treated with water (3 mL), and the organic layer, after drying (Na2S04) and GC analysis, was passed through a short pad of silica gel using ethyl acetate and hexane as eluent to afford analytically pure 4-methoxybenzaldehyde in quantitative yield. Evaporation of the aqueous layer afforded the copper complex 1 that was subsequently reused for the oxidation of 4-methoxybenzyl alcohol up to three times using fresh TEMPO whereupon no loss of activity was observed. 

Scheme 8.11 Recycling of catalyst and SPS following the copolymerization of C02 and CHO. Colors indicate species in low-polarity SPS (green), high-polarity SPS (yellow), and without solvent (colorless).

Systems have been developed that allow the recycling of catalysts. The first case study involved simple adsorption of proline onto silica gel [6], but the system suffered from a loss in enantioselectivity. More recently, promising results have been obtained with fluorous proline derivatives [64] used for aldol reactions the recycling of fluorous catalysts has been demonstrated using fluorous solid-liquid extraction. Solid phase-supported catalysts through covalent bonds [65] and through noncovalent interactions [66] were also used for aldol reactions. Proline and other catalysts can be recycled when ionic liquids or polyethylene glycol (PEG) were used as reaction solvents [67]. 

Disadvantages - Cumbersome separation recycling of catalyst - Not readily adapted to a continuous process - Heat transfer problems - Low activity and/or selectivity... 

In the process of catalytic cracking, characteristic reactions such as chain scission, hydrogen transfer and condensation take place under certain temperature and pressure conditions and when an appropriate catalyst is utilized, products with certain range of molecular weights and structures are obtained. Catalysts with surface acid sites and with the ability of hydrogen ion donation such as silica-alumina and molecular sieve catalyst have been already widely utilized. These catalysts can also enhance the isomerization of products and increase the yield of isomeric hydrocarbons. However, large amounts of coke will deposit on the surface of catalysts and consequently lead to their deactivation. Therefore, the recycling of catalysts is difficult to achieve. 

The asymmetric hydrogenation to A-acetyl-L-phenylalaninc (S/C = 10,000) is catalyzed by [Rh(COD)(9b)] BF4 at 40-kg scale in 99.5% ee and 100% conversion. The reduction is completed in 3-4 hours at 50°C under 150-225 psig hydrogen pressure. The TON can be further increased by the recycle of catalyst and ligand or by the attachment of the ligand to a solid support (9g) [72-74]. The asymmetric hydrogenation catalyst can be fine-tuned by the varia-... 

Choice of catalyst (efficiency separation and recycling of catalyst). 

Results of recycling of catalyst A were listed in Table 17.5. For the fresh catalyst, the molar ratio of EAHQ to the catalyst was 200 1, the conversion of... 

TABLE 17.6 Effect of the recycle of catalyst D on propylene epoxidation"... 

TABLE 17.9 Effect of Recycle of Catalyst D on Allyl Chloride Epoxidation°... 

The TEMPO-catalyzed oxidation of alcohols to carbonyl compounds with buffered aqueous NaOCl has found broad apphcation even in large-scale operations. Indeed, this selective methodology involves the use of safe and inexpensive inorganic reagents under mild reachon condihons. A supported TEMPO 7, which is soluble in CH2CI2 and acetic acid but insoluble in ethers and hexane, was prepared and proved to be an effective catalyst for the selective oxidahon of 1-octanol with various stoichiometric oxidants. When 7 was employed at 1 mol% as a catalyst with a stoichiometric amount of NaOCl, the aldehyde was obtained in 95% yield after only 30 min of reaction. The recycling of catalyst 7 was shown to be possible for seven reaction cycles in the oxidahon of 1-octanol, that occurred in undiminished conversion and selectivity under similar reachon conditions. 

Separation, recycling Separation via distillation, filtration or extraction. Catalyst recycling difficult (sometimes insufficient activity and/or productivity would require recycling to achieve acceptable costs). Relatively easy separation via filtration. Recovery and recycling of catalyst possible (BUT metal leaching can still be problematic). 

The products could be separated simply from the catalyst-water system, aud the catalysts could be reused at least six times without noticeably reducing catalytic activity. The methodology has the advantages of short reaction times, lack of organic solvent, recyclability of catalysts, and easy work-up for isolation of the products in good yields with high purity. 

Song CE, Shim WH, Roh El, Lee S, Choi JH (2001) Ionic liquids as powerful media in scandium triflate catalyzed Diels-Alder reactions signiflcant rate acceleration, selectivity improvement and easy recycling of catalyst. Chem Commun 1122-1123... 

The manufacture of most products from natural gas feed ultimately relies on a series of catalytically enhanced chemical reactions. For example, a typical 1000—metric ton/day ammonia plant has at least eight unit operations that make use of fixed-bed catalysts, with an overall catalyst volume of approxi-mately 310 m. The catalyst operations vary in useful economic life from 2 to over 10 years, depending on service. Historically, all of the catalysts were disposed of in sanitary landfills, since they are basically inert and pose no environmental or health problems. Today, with stricter regulation of the nation s landfills, under the Resource Conservation and Recovery Act (RCRA), greater attention is being given to recycling of catalyst for recovery of the main metal components. Today, numerous, cost-effective processes exist to reclaim valuable metal components from spent catalyst. Complete separation of the spent catalyst into its component parts, and subsequent reuse in the industry, leaves no environmental liability. 

Fig.2 Influence on the selectivity with the recycling of catalysts Pd/C and Pd / C+Bi (conditions described in Table 4)...

Two examples are highlighted below where precious metal catalysts are used to produce fine chemicals on an industrial scale via carbon-carbon bond forming reactions. The first (a) is rhodium-catalysed hydroformylation in the oxo-process , which is a well established industrial process. The second (b) highlights a new process developed by Lucite involving a palladium-catalysed methoxy-carbonyla-tion. Many of the points mentioned above in this article are illustrated in the examples, with efficient recycle of catalyst (precious metal) and the extra cost of ligands being justified by the costs savings of the novel chemistry. 

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