Membrane technology catalysts

Membrane Reactor. Another area of current activity uses membranes in ethane dehydrogenation to shift the ethane to ethylene equiUbrium. The use of membranes is not new, and has been used in many separation processes. However, these membranes, which are mostly biomembranes, are not suitable for dehydrogenation reactions that require high temperatures. Technology has improved to produce ceramic and other inorganic (90) membranes that can be used at high temperatures (600°C and above). In addition, the suitable catalysts can be coated without blocking the pores of the membrane. Therefore, catalyst-coated membranes can be used for reaction and separation. 

The previous discussion asserts that design, fabrication, and implementation of stable and inexpensive materials for membranes and catalyst layers are the most important technological challenges for PEFC developers. A profound insight based on theory and modeling of the pertinent materials will advise us how fuel cell components with optimal specifications can be made and how they can be integrated into operating cells. 

Membrane Systems for Improved Chemical Synthesis 529 Tab. 13.2 Summary of homogeneous catalyst recycling using membrane technology [3]. 

The properties associated with the amphiphilic monomer units are strongly exemplified in thermosensitive water-soluble polymers, typical examples of which are shown in Scheme 5. Thermosensitive polymers possess a lower critical solution temperature (LCST) in water solutions. Due to their sharp response to temperature variation, they are widely used in various scientific and technological applications. Drug and gene delivery [1-3], chromatographic [9,10], membrane technology [11,12], and catalyst immobiliza-... 

Membrane technology could offer interesting possibilities in order to overcome these limitations and to improve the advantages of catalysis mediated by the decatungstate by the multiturnover recycling associated to heterogeneous supports, the selectivity tuning as a function of the substrate affinity towards the membrane, the effect of the polymeric microenvironment on catalyst stability and activity. 

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... 

An innovative potential application of membrane technology in catalysis and in CMRs might be the possibility to produce catalytic crystals with a well-dehned size, size distribution, and shape by membrane crystallization [19,20] (Figure 43.5). Membrane crystallization is particularly attractive for the preparation of heat-sensitive catalysts such as enzymes. 

Membrane technology is a recent development to separate (or concentrate) water-soluble catalysts (mainly hydroformylation catalysts) [147, 149], although a prior art is known [194, 195]. There are proposals for the use of immobilized or re-immobilized aqueous phases for large-scale processes (cf. Ref. [222] and Section 3.1.1.6). Carbon dioxide as a solvent for biphasic hydroformylations has been described by Rathke and Klinger [184], although the use of CO2 for hydroformylation purposes was described earlier [185]. For the use of supercritical CO2 cf. Section 3.1.13 with non-aqueous ionic liquids cf. Section 3.1.1.2.2. Investigations with supercritical water are in an early state (e. g., Ref. [223]). 

In the application of membrane technology (cf. Section 3.2.3) for the separation of the Rh complexes and the re-immobilized ligands after the reaction, a further remarkable enlargement of the ligands was desirable. Unfortunately, the combination of diamines with TPPTS yields highly crosslinked polymeric materials, which cannot be handled. A reduction of the degree of crosslinking is possible by use of the disulfonated TPPDS (cf. Section 3.1.1.1). So, in a combination with TCD-diamine (tricyclodecane diamine) a salt was formed that was partly soluble in toluene and soluble in THF. The same was the case with the use of MA -dimethyl-TCD-diamine. These salts may be useful in water-free two-phase catalyst systems. 

Despite the attractive perspectives of membrane technology, many basic problems have still to be solved. Beyond the optimization of reaction conditions and catalysts, the chief obstacles to the scaling-up are membrane fragility, deterioration, high cost and manufacture complexity, which restrict, for the moment, this technique to an experimental level. 

The use of a sohd or hquid membrane to separate products and reactants is most attractive as it lends itself to continuous operations. The main problems are the efficiency of the separation, the speed of the separation, and the dma-bility of the membrane. Membrane separation was also one of the first ways linear soluble polymer-bound catalysts were separated from products. However, separation efficiencies in those first examples were not as high as present ones, as membrane technology has substantially improved over the past 35 years [135]. Thus, it is not surprising that many recent examples using membranes to recover polymer-bound catalysts and to separate them from products have been reported. This technique seems particularly apt for dendrimers because of their overall globular structure [136]. However, improved membranes can also be useful with hnear soluble polymer-bound catalysts. A recent review summarizes much of this work [137]. 

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