Catalyst in membrane reactors

5 CATALYSIS IN POROUS MEMBRANE REACTORS 2.5.1 Catalyst in Membrane Reactors 

For most chemical reactions, a catalyst is required to obtain the desired product. The catalyst performance in MRs can be influenced positively or negatively by the use of a membrane. It is well known that the withdrawal of hydrogen during dehydrogenation in an MR will likely favor coking processes, promoting deactivation of the catalyst. This implies that MRs require catalysts with improved stability. However, in the Fischer-Tropsch synthesis, the use of membranes to extract water from the reaction zone may protect the catalyst from 

When the catalyst is coated on the membrane surface or dispersed inside the membrane pores, the membrane body will exhibit catalytic activity and participate directly in chemical reactions. To obtain high performance, the porous membrane should provide high surface area and strong adhesion of the catalyst. The reactants permeate from one side or opposite sides of the membrane into the catalyst layer where reactions take place. The catalyst in porous membranes may benefit from the better transfer and membrane active role in promoting the contact of reactants. An appropriate thickness of the catalytic layer is necessary to enhance the reaction selectivity [20]. 

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Terminal palladium-complexed, phosphane-functionalised carbosilane dendrimers have been used as potential catalysts in membrane reactors [87]. 

The Van Koten group has developed an interesting approach to the assessment of the permeability of nanofiltration membranes for the application of metallodendrimer catalysts in membrane reactors. They have selectively grafted dendrons to organometallic pincers with sensory properties and have used these as dyes in a colorimetric monitoring procedure. 

Further work with supported catalysts in membrane reactors is needed to raise the enantioselectivity to commercially useful levels. Care should be used to pick supports, polymers or otherwise, that are very stable under the conditions of the reaction so that they will be longlived. 

Figure 6.14.8 Example of a molecularly enlarged triphenylphosphine-type ligand obtained by core functionalization of a carbosilane dendrimer with triphenylphosphine ligands of this type have been used for the immobilization of a Rh-hydroformylation catalyst in membrane reactors. Adapted from Oosterom et al. (2002).

Deepen our theoretical and experimental investigation into the design and control of membrane permeability to match the catalytic activity of catalysts in membrane reactors. This is critical for the design of industrial membrane reactors. Experimental procedures in combination with mathematical modeling will provide a better understanding. 

Deepening theoretical and experimental investigation into the design and control of membrane permeability to match the catalytic activity of catalysts in membrane reactors. 

Morbidelli M, Gavriilidis A and Varma A (2001) Catalyst Design Optimal Distribution of Catalyst in Pellets, Reactors, and Membranes, Cambridge University Press. 

Homogeneous Catalysts Applied in Membrane Reactors N.J. RONDE AND D. VOGT... 

Membranes can also be used as a reactor where catalysts are used frequently. The membrane may physically segregate the catalyst in the reactor, or have the catalyst immobilized in the porous/microporous structure or on the membrane surface. The membrane having the catalyst immobilized in/on it acts almost in the same way as a catalyst particle in a reactor does, except that separation of the product(s) takes place, in addition, through the membrane to the permeate side. All such configurations involve the bulk flow of the reaction mixture along the reactor length while diffusion of the reactants/products takes place generally in a perpendicular direction to/from the porous/microporous catalyst. 

Nonporous silver membrane tube (99.99 wt.% Ag), (in double pipe configurationX thickness 100/im. Feed enters the reactor at shell side, oxygen at tube side. Oxidation of ammonia. Silver catalyst in membrane form (see previous column). Oxidation of ethanol to acetaldehyde. Silver catalyst in membrane form (see previous column). r- 250-380°C. The yield of nitrogen was 40%, the yield of nitrogen monoxide was 25%. r- 250-380°C. The yield of acetaldehyde was 83%. The yield with bulk powdered silver catalyst was 56%. Gryaznov, Vedernikov and Guryanova (1986)... 

Reaction engineering helps in characterization and application of chemical and biological catalysts. Both types of catalyst can be retained in membrane reactors, resulting in a significant reduction of the product-specific catalyst consumption. The application of membrane reactors allows the use of non-immobilized biocatalysts with high volumetric productivities. Biocatalysts can also be immobilized in the aqueous phase of an aqueous-organic two-phase system. Here the choice of the enzyme-solvent combination and the process parameters are crucial for a successful application. 

For example, POPAM dendrimers of 1,3-diaminopropane type have been used in membrane reactors as supports for palladium-phosphine complexes serving as catalysts for allylic substitution in a continuously operated chemical membrane reactor. Good recovery of the dendritic catalyst support is of advantage in the case of expensive catalyst components [9]. It is accomplished here by ultra-or nanofiltration (Fig. 8.2). 

All peptide-catalyzed enone epoxidations described so far were performed using insoluble, statistically polymerized materials (neat or on solid supports). One can, on the other hand, envisage (i) generation of solubilized poly-amino acids by attachment to polyethylene glycols (PEG) and (ii) selective construction of amino acid oligomers by standard peptide synthesis-linked to a solid support, to a soluble PEG, or neat as a well-defined oligopeptide. Both approaches have been used. The former affords synthetically useful and soluble catalysts with the interesting feature that the materials can be kept in membrane reactors for continuously oper-... 

Among the different heterogenization strategies, the entrapping of catalysts in membranes or, in general, the use of a catalyst confined by a membrane in the reactor, offers new possibility for the design of new catalytic processes. 

Fig. 7 Schematic representation of compartmentalization of nanosized catalysts in membrane-covered vial reactors...

Whilst the enhancement of unwanted side reactions through excessive distortion of the concentration profiles is an effect that has been reported elsewhere (e.g., in reactive distillation [40] or the formation of acetylenes in membrane reactors for the dehydrogenation of alkanes to olefins [41]), the possible negative feedback of adsorption on catalytic activity through the reaction medium composition has attracted less attention. As with the chromatographic distortions introduced by the Claus catalyst, the underlying problem arises because the catalyst is being operated under unsteady-state conditions. One could modify the catalyst to compensate for this, but the optimal activity over the course of the whole cycle would be comprised as a consequence. 

Internal diffusion limitations can generally be easily avoided in laboratory slurry reactors as the minimum of the pellet diameter is determined by the maximum of the pore size of the membrane filter used to maintain the catalyst in the reactor. Membrane filters allowing the use of pellets with a diameter of 1 pm are commercially available. 

By "inert it means that the membrane is a separator but not a catalyst. Many membrane reactor modeling studies consider only those cases where the membrane is catalydcally inert and the catalyst is packed most often in the tubular (feed) region but sometimes in the annular (permeate) region. When it is assumed that no reaction takes place in the membrane or membrane/support matrix, the governing equations for the membrane/support matrix are usually eliminated. The overall eff ect of membrane permeation can be accounted for by a permeation term which appears in the macroscopic balance equations for both the feed and permeate sides. Thus, the diffusional gradient term... 

The polymeric oxazaborolidine prepared from the linear copolymer of 29 and styrene was used in membrane reactor and resulted in high total turnover number with high enantioselectivity [44]. Another polystyrene-based soluble polymeric oxazaborolidine 38 was used in the same system. Polysiloxanes are also useful polymeric supports of catalyst 39 for the same purpose [45]. 

Kikuchi, E., Menoto, Y., Kajiwara, M., Uemiya, S., Kojima, T. (2000). Steam reforming of methane in membrane reactors comparison of electroless-plating and CVD membranes and catalyst packing methods. Catalysis Today 56, 75-81. 

Finally, some authors [31,130,131] employed 7-AI2O3 thin supported layers (pore size 4 nm) for ethylene partial oxidation in membrane reactors with separate feed of reactants. In such cases the membrane material had a specific surface area high enough to guarantee a direct catalyst support. 

The heterogenization of catalysts in membrane is particularly suitable for catalyst design at the atomic and molecular level. One of the main advantages of the membrane reactors, compared to traditional reactors, is the possibility to recycle easily the catalyst. Moreover, the selective transport properties of the membranes can be used to shift the equilibrium conversion (e.g., esterihcation reaction), to remove selectively products and by-products from the reaction mixture, to supply selectively the reagents (e.g., oxygen for partial oxidation reactions). 

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