Heterogeneous immobilized catalysts, flow reaction

Advantageously, SAPC as a technique with immobilized catalysts does not need devices for catalyst separation and recycling, since the reactions can in principle be carried out using standard flow reactors commonly used in heterogeneous catalysis. On the other hand, the presumed processes for the work-up of the constituents of the catalyst, the ligand (and - may be - the support) will be demanding and expensive, too. 

SCCO2 has also been used as a solvent with a silica-immobilized catalyst in metathesis reactions [25]. A heterogeneous catalytic process is developed, in which catalyst leaching is avoided but the reactivity is lower than when using a homogeneous catalyst This application has also been extended to continuous-flow processes for hydrogenation [26], Friedel-Crafts alkylations [27], etherification [28], and hydroformylation [29] reactions. 

A large number of heterogeneous catalysts have been tested under screening conditions (reaction parameters 60 °C, linoleic acid ethyl ester at an LHSV of 30 L/h, and a fixed carbon dioxide and hydrogen flow) to identify a suitable fixed-bed catalyst. We investigated a number of catalyst parameters such as palladium and platinum as precious metal (both in the form of supported metal and as immobilized metal complex catalysts), precious-metal content, precious-metal distribution (egg shell vs. uniform distribution), catalyst particle size, and different supports (activated carbon, alumina, Deloxan , silica, and titania). We found that Deloxan-supported precious-metal catalysts are at least two times more active than traditional supported precious-metal fixed-bed catalysts at a comparable particle size and precious-metal content. Experimental results are shown in Table 14.1 for supported palladium catalysts. The Deloxan-supported catalysts also led to superior linoleate selectivity and a lower cis/trans isomerization rate was found. The explanation for the superior behavior of Deloxan-supported precious-metal catalysts can be found in their unique chemical and physical properties—for example, high pore volume and specific surface area in combination with a meso- and macro-pore-size distribution, which is especially attractive for catalytic reactions (Wieland and Panster, 1995). The majority of our work has therefore focused on Deloxan-supported precious-metal catalysts. 

Chiral oxazoborolidine systems have been regularly used as catalysts for the enantioselective Diels-Alder reaction [173]. Such a catalyst has been immobilized by copolymerization of a sulfonamide-modified styrene monomer to produce polystyrene beads and subsequently reacted with borane to furnish catalyst 49 (Scheme 4.77). A column of 49 was cooled to —30 °C and a 1 1.5 methylacrolein cyclopentadiene solution was flowed in. Following aqueous workup and column chromatography, the desired product 50 was isolated to yield 138 mmol of product in 95% yield and 71% ee by using only 5.7 mmol of catalyst. This result was found to be comparable with the heterogeneous batch reactions that were also attempted [174]. 

Solid-catalyzed reactions are generally examined in continuous-flow reactors, while homogeneously catalyzed reactions (including reactions catalyzed by non-immobilized biocatalysts) are usually studied in batch reactors. An important difference between chemical catalysis and biocatalysis is that it is relatively easy to study the biocatalyst either as a homogeneous or a heterogeneous catalyst, as desired. 

The catalytic behavior of enzymes in immobilized form may dramatically differ from that of soluble homogeneous enzymes. In particular, mass transport effects (the transport of a substrate to the catalyst and diffusion of reaction products away from the catalyst matrix) may result in the reduction of the overall activity. Mass transport effects are usually divided into two categories - external and internal. External effects stem from the fact that substrates must be transported from the bulk solution to the surface of an immobilized enzyme. Internal diffusional limitations occur when a substrate penetrates inside the immobilized enzyme particle, such as porous carriers, polymeric microspheres, membranes, etc. The classical treatment of mass transfer in heterogeneous catalysis has been successfully applied to immobilized enzymes I27l There are several simple experimental criteria or tests that allow one to determine whether a reaction is limited by external diffusion. For example, if a reaction is completely limited by external diffusion, the rate of the process should not depend on pH or enzyme concentration. At the same time the rate of reaction will depend on the stirring in the batch reactor or on the flow rate of a substrate in the column reactor. 

Microreactors can be used for either gas-phase or liquid-phase reactions, whether catalyzed or uncatalyzed. Heterogeneous catalysts (or immobilized enzymes) can be coated onto the channel wall, although on occasion the metal wall itself can act as the catalyst. Gas-liquid contacting can be effected in the microchannels by either bubbly or slug flow of gas, an annular flow of liquid, or falling liquid films along the vertical channel walls. Contact between two immiscible liquids is also possible. The use of microreactor systems in the area of biotechnology shows much promise, not only for analytical purposes but also for small-scale production systems. 

Liquid-liquid-solid reactors are commonly used for biphasic reactions catalyzed by immobilized phase-transfer catalysts (which form the third, solid phase). Certain basic aspects of such reactors were considered in Chapter 19. Three-phase reactions of this type are also encountered in biological reactions, for example, the enzymatic synthesis of amino acid esters in polyphasic media (Vidaluc et al., 1983), and the production of L-phenylalanine utilizing enzymatic resolution in the presence of an organic solvent (Dahod and Empie, 1986). Predictably, the performance of these reactors is influenced by the usual kinetic and mass transfer aspects of heterogeneous systems (see Lilly, 1982 Chen et al., 1982 Woodley et al., 1991a,b). Additionally, performance is also influenced by the complex hydrodynamics associated with the flow of two liquids past a bed of solids (Mitarai and Kawakami, 1994 Huneke and Flaschel, 1998). It is noteworthy, for instance, that phase distribution within the reactor is different from that in the feed and is also a function of position within the reactor and within the voids of each pellet in the bed. More intensive research is needed before these reactors can be rationally designed. 

Joni et al. presented acidic SIL catalyst materials for slurry-phase alkylation reactions, in which the IL film remained free flowing on the support surface while being flxed to the support by capillary force and physical adsorption [23]. They pretreated the support material prior to impregnation with the acidic IL in order to obtain materials with strictly reproducible catalytic performance. Chemical pretreatment was achieved by contacting a solution of [EMIMlCl/AlClj = 1/2 in dichloromethane with the calcined support, followed by a washing procedure that removed all excess acid from the support. The pretreated supports themselves had no catalytic activity in the isomerization of diisopropylbenzenes (DIPBs), but provided a suitable support material for the immobilization of acidic chloroaluminate IL. The SIL materials obtained by this method proved to be excellent heterogeneous... 

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