Physical separation, catalyst

Catalysts from Physical Mixtures. Two separate catalysts with different functions may be pulverized to fine powders and mixed to form a catalyst system that accomplishes a reaction sequence that neither of the two iadividual catalysts alone can achieve. For such catalyst systems, the reaction products of catalyst A become the feedstocks for catalyst B and vice versa. An example is the three-step isomerization of alkanes by a mixture of... 

A variety of mixed metal catalysts, either as fused oxides (42 7 8) or coprecipitated on supports (25 0) or as physical mixtures of separate catalysts (5P), have been tested in aniline reductions. In the hydrogenation of ethyl p-aminobenzoate, a coprccipitated 3% Pd, 2% Rh-on-C proved superior to 5% Rh-on-C, inasmuch as hydrogenolysis to ethyl cyclohexanecarboxylate was less (61) (Table 1). 

The continuous reaction system could be combined with solid acid-catalyzed in situ racemization of the slow-reacting alcohol enantiomer [149]. The racemiza-tion catalyst and the lipase (Novozym 435) were coated with ionic liquid and kept physically separate in the reaction vessel. Another variation on this theme, which has yet to be used in combination with biocatalysis, involves the use of scC02 as an anti-solvent in a pressure-dependent miscibility switch [150]. 

There was yet another possibility that the enhancement could be due to the fact that some stable products formed on the catalyst wafer, upon desorption into the void volume, underwent further sequential reaction with propane. If so, the enhancement would not require the immediate adjacency of the catalyst wafer and the void volume and should be observable when the catalyst and the void volume were physically separated. Such separation, however, would quench any desorbed reactive intermediates. This was tested. The wafer was separated from the void volume, and the two were separately heated to the appropriate temperatures. The result was that only a small enhancement was observed in the separated mode. This confirmed that the enhancement was due not to sequential reaction of stable products but to desorption of reactive intermediates from the catalyst surface. The small enhancement could be attributed to the higher temperature throughout the separately heated void volume in the separated mode than in the other mode. 

Physical separation of the catalyst from the bottoms product. Specific gravity, particle size, and magnetic susceptibility of the catalyst could possibly provide the means for catalyst separation. 

Sintering is an important mode of deactivation in supported metals. The high surface area support (carrier or substrate) in these catalysts serves several functions (l) to increase the dispersion and utilization of the catalytic metal phase, (2) to physically separate metal crystallites and to bind them to its surface, thereby enhancing their thermal stability towards agglomeration, and (3) in some cases to modify the catalytic properties of the metal and/or provide separate catalytic functions. The second function is key to the prevention or inhibition of thermal degradation of the catalytically active metal phase. 

Characterization thus involves analytical electron microscopy, ordinary microprobe analysis or other techniques for localizing elements or chemical compounds (Scanning Auger Spectroscopy, Raman Microprobe, Laser Microprobe Mass Spectrometry). It also requires, in most cases, some physical separation of the catalyst for separate analysis (e.g., near surface parts and center of pellets, by peeling or progressive abrasion pellets present at various heights in the catalyst bed, etc.). 

More complex reactors, like packed-bed reactors or catalytic monoliths, consist of many physically separated scales, with complex nonlinear interactions between the processes occurring at these scales. Figure 3 illustrates scale separation in a packed-bed reactor. The four length (and time) scales present in the system are the reactor, catalyst particle, pore scale, and molecular scale. The typical orders of magnitude of these four length scales are as follows reactor, lm catalyst particle, 10 2m (1cm) macropore scale, 1pm (10 6m) micro-pore/molecular scale, 10 A (10 9m). The corresponding time scales also vary widely. While the residence time in the reactor varies between 1 and 1000 s, the intraparticle diffusion time is of the order of 0.1s and is 10 5s inside the pores. The time scale associated with molecular phenomenon like adsorption is typically less than a microsecond and could be as small as a nanosecond. 

Where and how the catalyst is placed in the membrane reactor can have significant impact not only on the reaction conversion but also in some cases, the yield or selectivity. There are three primary modes of placing the catalyst (1) A bed of catalyst particles or pellets in a packed or fluidized state is physically separated but confined by the membrane as part of the reactor wall (2) The catalyst in e form of particles or monolithic layers is attached to the membrane surface or inside the membrane pores and (3) The membrane is inherently catalytic. Membranes operated in the first mode are sometimes referred to as the (catalytically) passive membranes. The other two modes of operation are associated with the so called (catalytically) active membranes. In most of the inorganic membrane reactor studies, it is assumed that the catalyst is distributed uniformly inside the catalyst pellets or membrane pores. As will be pointed out later, this assumption may lead to erroneous results. 

The different types of membrane reactor configurations can also be classified according to the relative placement of the two most important elements of this technology the membrane and the catalyst. Three main configurations can be considered (Figure 25.13) the catalyst is physically separated from the membrane the catalyst is dispersed in the membrane or the membrane is inherently catalytic. The first configuration is often called the inert membrane reactor (IMR), in contrast to the two other ones, which are catalytic membrane reactors (CMRs).5o... 

Multifimctionality is helpful to minimize process steps by combining several chemical transformations over the same catalyst bed. Further improvements can be expected with catalyst membranes that have the ability to combine chemical processing and physical separation steps. This special field of catalysis has great promise for the future. 

In the author s laboratory, extensive studies were undertaken not only to examine the reaction schemes proposed by Mills et al., but specifically to test the feasibility of catalytic cooperation by chemically unconnected, i.e., physically separate, catalytic components, wherein the intermediates are true gas phase species coupling the catalyst components through mass transport following the classical laws of gaseous diffusion, in hne with the principles and characteristics discussed in the preceding sections. Experimental work that makes use of physically distinct catalytic materials or components constitutes the most direct route to the testing and study of true polystep reaction mechanisms. 

As noted above, a stereoselective synthesis of the enamide is important. The azlac-tone method (Fig. 3) results in the preferential formation of the Z-enamide when an aromatic aldehyde is employed. In addition, this isomer usually precipitates from the reaction mixture and this simplifies purification. When an alkyl aldehyde is used, the ratio of enamide isomers is often 1 1 or close to this. In addition, many of these alkyl examples are not crystalline and physical separations such as chromatography have to be employed. This is obviously a limitation of the methodology when compared with catalysts that employ the DuPHOS ligands, and related ligand families where both isomers can be reduced down to the same enantiomer of the desired amino acid [12]. 

Resolution of compounds made as diastereoisomeric mixtures The synthesis of Jacobsen s Mn(III) epoxidation catalyst by resolution Resolution with half an equivalent of resolving agent Physical Separation of Enantiomers Chromatography on chiral columns Resolution of triazole fungicides by HPLC A commercial drug separation by chiral HPLC Differential Crystallisation or Entrainment of Racemates Conglomerates and racemic compounds Typical procedure for differential crystallisation (entrainment) Conventional resolution ofL-methyl DOPA Resolution ofL-methyl DOPA by differential crystallisation Finding a differential crystallisation approach to fenfluramine Resolution with Racemisation... 

Many catalysts are homogeneous, i.e. they react in the same phase as the substrates. One strategy to aid the separation is to immobilise the homogeneous catalyst onto a support that is heterogeneous to the reaction phase. The catalyst can then be physically separated easily. The important factors for an immobilised catalyst are ... 

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