Continuous processing catalyst immobilization

Harmer and co-workers996,997 tested various Nafion-based samples in the dimerization of a-methylstyrene to form isomeric pentenes and a cyclic dimer, which are of industrial interest 13% and 40% Nafion-silica nanocomposites exhibited near-complete conversion and gave the cyclic dimer 300 with high selectivity [Eq. (5.360)], whereas compound 299a was the main product over Nafion NR50. In sharp contrast, isomeric pentenes 299 could be obtained in a continuous process (86% selectivity at conversion >95%). Similar findings were reported with Nafion immobilized in MCM-41 mesoporous silica. In kinetic studies, 13% Nafion-silica and the catalyst with... 

Recently, Feng [274] reported a continuous process for the transesterification of EC with methanol in a flow reactor over a dibutyl amine catalyst immobilized on a MCM-41 molecular sieve (n-Bu2N-MCM-41). The catalyst performed well and afforded 25.5% and 41.7% EC conversions at 283 and 323 K, respectively, and also exhibited a good stability. Arai and coworkers [275, 276] subsequently performed the preparation of DMC in two steps (Scheme 7.18). 

For preparative applications, especially in the case of continuous processes, means for the recovery of the valuable enzymes, the cofactors, and the redox catalysts in the reactor must be developed. One attractive possibility is the immobilization of the enzyme and sometimes also the cofactor and the redox catalyst at the electrode surface. However, the formation of enzyme-modified electrodes has also some practical drawbacks ... 

In the present study, we describe the methods of preparing the silica hybrids of linear and branched fiinctional polysiloxanes which could be used as a support for metal complex catalysts. The way in which the catalyst operates when it is attached to the polysiloxane moiety of the hybrid suspended in a polysiloxane solvent should be similar to the way it operates when in solution. Thus, its high catalytic activity is expected. On the other hand, it is easily separated from the reaction products and may be recycled or used in the continuous process. A high catalytic activity and specificity may be achieved if a polymer with a highly branched structure is used for the immobilization of catalysts [1-3]. Considerable amounts of catalytic groups may be placed in the external part of the branched macromolecule. 

Finally it must be pointed out that the aldol condensation reaction is reversible thus, in a one-pot reaction the yield for this reaction step is increased by permanently removing the condensation product. Since the immobilized catalyst system permits a continuous process, all advantages of a heterogenized multifunctional catalyst system can be utilized with the catalyst described. 

In a first approximation, the new methods correspond to the conventional solvent techniques of supported catalysts (cf Section 3.1.1.3), liquid biphasic catalysis (cf Section 3.1.1.1), and thermomorphic ( smart ) catalysts. One major difference relates to the number of reaction phases and the mass transfer between them. Owing to their miscibility with reaction gases, the use of an SCF will reduce the number of phases and potential mass transfer barriers in processes such as hydrogenation, carbonylations, oxidation, etc. For example, hydroformylation in a conventional liquid biphasic system is in fact a three-phase reaction (g/1/1), whereas it is a two-phase process (sc/1) if an SCF is used. The resulting elimination of mass transfer limitations can lead to increased reaction rates and selectiv-ities and can also facilitate continuous flow processes. Most importantly, however, the techniques summarized in Table 2 can provide entirely new solutions to catalyst immobilization which are not available with the established set of liquid solvents. 

Resin-bound amino alcohols also served to load diethyl zinc which then was employed in continuous flow additions to aromatic aldehydes [33]. Both diethyl zinc and p-chlorobenzaldehyde were added simultaneously at a slow rate under nitrogen into a cooled column loaded with functionalized polymer 6a, which afforded l-(p-chlorophenyl)-propanol with good enantiopurity (94% ee). The authors note that 58 mmol of the optically active alcohol were prepared in a continuous process by only employing 0.7 mmol of the immobilized catalyst. Similar results were reported for immobilized ephedrine, so it was concluded that continuous flow processes are often superior in efficiency and practicability compared to batch processes. In some cases, it was foimd that enantioselectivities were higher for convective flow processes than for the corresponding batch systems [34]. 

As has been shown already on an Industrial scale, fermentation can be substituted by heterogeneous catalysts with resting microbial cells Immobilized In polymeric carriers. Repeated use of the once formed biomass, continuous process operation, and elimination of costly separation steps of product solution from biomass are obvious advantages of this new technology. Some principal aspects of a) Immobilization methodology, b) catalyst effectiveness, and c) operational stability shall be outlined In this contribution. 

The environmentally acceptable nature of supercritical carbon dioxide has already been discussed in this chapter, but another type of solvent which is generating great interest as a possible replacement for volatile organic compoimds is ionic hquids. A detailed account of ionic liquids and their application for catalyst immobilization is the subject of Chapter 5, edited by H. Olivier, in this handbook. In essence, ionic Hquids are involatile, of low toxicity, and very stable, and are therefore seen as having a low environmental impact. The very different properties of supercritical carbon dioxide and ionic liquids makes them ideally suited for use in combination to provide an environmentally acceptable form of two-phase catalysis, which might be carried out as a continuous-flow process. 

What could be expected in catalysis of the reaction by fixing the metal complexes with polymers Firstly, an increase of their activity. This is due to the formation of a higher proportion of monoperoxide complexes and their greater stability. Catalysts obtained by immobilization of molybdenyl groups on modified polyvinylalcohol or furflirolidine resin, are active and stable over 500 h of a continuous process [139]. In this way effective catalysts have been made for the epoxidation of cyclopentane, cyclohexene, st Tene, etc. by tert-butyl hydroperoxide... 

Enzymes are exploited as catalysts in many industrial, biomedical, and analytical processes. There has been considerable interest in the development of carrier systems for enzyme immobilization because immobilized enzymes have enhanced stability compared to soluble enzymes, and can easily be separated from the reaction. This leads to significant savings in terms of reduced enzyme consumption, and the ability to use such enzymes in continuous processes. The activity and stability of enzymes depends largely on the particular operating and storage conditions, and is strongly influenced by factors such as the chemical environment, temperature, pH, and solvent properties. Most enzymes are water soluble and a certain amount of water is always required for their solubilization. Additionally, many enzymes require the presence of a cofactor, which may be attached firmly to the enzyme or may need to be added separately as a coenzyme. When the immobilization of an enzyme... 

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