Selective catalytic membrane

Selective Catalytic Membrane Process Linked with a Separation... 

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

Ozone decomposition in airplanes Selective catalytic reduction of NOx Arrays of corrugated plates Arrays of fibers Gauzes Ag Methanol -> formaldehyde Pt/Rh NO production from ammonia HCN production from methane Foams Catalytic membranes reactors... 

The membrane has the premier function in the process of biogenesis. It allows for individual ownership and retention of biocatalysts, and thereby for up to a million fold increases in catalytic activity. Substrate/enzyme ratios in cells may approach unity and thus enzymes can actually change the equilibrium of some reactions. Clearly, membranes are essential and the hurdle for nascent life is the need for a selectively permeable membrane... that means a membrane that contains, suspended in its lipid layers, the first communication proteins.13,14 The cell must breathe at once if there is to be any future and that again equalizes units from different clones. Is it surprising then that all life forms have membranes Shapeless wafting life is a thing of poor science fiction. Membrane formation is the moment when life became competitive, it... 

These Hyflon-based catalytic membranes have been used to catalyze the photooxidation of ethylbenzene (neat), showing superior catalytic performance with higher turnover number and better selectivity when compared to homogeneous catalysts [47]. When dispersed in the Hyflon matrix, the efficacy of fluoro-containing decatungstate depends on the specific electrochemical environment of the catalytic sites, the high solubility of 02 in the Hyflon matrix, and the selectivity of the perfluorinated polymeric material towards the ethylbenzene reagent and the products 2-phenylethanol and acetophenone. 

The possibility of having membrane systems also as tools for a better design of chemical transformation is today becoming attractive and realistic. Catalytic membranes and membrane reactors are the subject of significant research efforts at both academic and industrial levels. For biological applications, synthetic membranes provide an ideal support to catalyst immobilization due to their biomimic capacity enzymes are retained in the reaction side, do not pollute the products and can be continuously reused. The catalytic action of enzymes is extremely efficient, selective and highly stereospecific if compared with chemical catalysts moreover, immobilization procedures have been proven to enhance the enzyme stability. In addition, membrane bioreactors are particularly attractive in terms of eco-compatibility, because they do not require additives, are able to operate at moderate temperature and pressure, and reduce the formation of by-products. 

The conditions are substantially more favorable for the microporous catalytic membrane reactor concept. In this case the membrane wall consists of catalyti-cally active, microporous material. If a simple reaction A -> B takes place and no permeate is withdrawn, the concentration profiles are identical to those in a catalyst slab (Fig. 29a). By purging the permeate side with an inert gas or by applying a small total pressure difference, a permeate with a composition similar to that in the center of the catalyst pellet can be obtained (Fig. 29b). In this case almost 100% conversion over a reaction length of only a few millimeters is possible. The advantages are even more pronounced, if a selectivity-limited reaction is considered. This is shown with the simple consecutive reaction A- B- C where B is the desired product. Pore diffusion reduces the yield of B since in a catalyst slab B has to diffuse backwards from the place where it was formed, thereby being partly converted to C (Fig. 29c). This is the reason why in practice rapid consecutive reactions like partial oxidations are often run in pellets composed of a thin shell of active catalyst on an inert support [30],... 

In a catalytic membrane reactor the pressure difference between feed and permeate could be adjusted such that high selectivity and high conversion in a once through process is obtained (Fig. 29d). The amount of catalyst necessary and the required residence time of the gas would be less than in conventional fixed bed reactors since the diffusional resistance is overcome by the external pressure gradient. The above advantages are already partly exploited by the use of macro-porous catalyst pellets, mentioned in Section 10.1.2.3 [19, 20]. 

Note C=conversion, selectivity, K=yield, subscript m denotes the maximum value, subscript e indicates the equilibrium value without membrane, CMR=catalytic membrane reactor, /MRCF=catalytic ly inert membrane with catalyst pellets on the feed side... 

There has been a large volume of data showing the benefit of having thin dense membranes (mostly Pd-based) on the hydrogen permeation rate and therefore the reaction conversion. An example is catalytic dehydrogenation of propane using a ZSM-5 based zeolite as the catalyst and a Pd-based membrane. Clayson et al. [1987] selected a membrane thickness of 100 m and achieved a yield of aromatics of 38% compared to approximately 80% when a 8.6 pm thick membrane is used [Uemiya et al., 1990]. 

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. 

There are, however, some studies demonstrating another concept of using porous membranes. In this concept, the permselective property of a membrane is immaterial and not utilized. Instead, the well sU uctured porous maU ix of the membrane serves to provide a well controlled reaction zone. Specifically, two reactants are fed separately to the opposing sides of a catalytic membrane. For those reactions the rate of which is faster than the diffusion rate of the reactants in the membrane, the reaction can take place inside the caialytically active membrane. This type of membrane reactor, where the membranes are catalytic but not selective, are called catalytic non-pcrmselective membrane reactors (CNMR). 

When the catalyst is immobilized within the pores of an inert membrane (Figure 25.13b), the catalytic and separation functions are engineered in a very compact fashion. In classical reactors, the reaction conversion is often limited by the diffusion of reactants into the pores of the catalyst or catalyst carrier pellets. If the catalyst is inside the pores of the membrane, the combination of the open pore path and transmembrane pressure provides easier access for the reactants to the catalyst. Two contactor configurations—forced-flow mode or opposing reactant mode—can be used with these catalytic membranes, which do not necessarily need to be permselective. It is estimated that a membrane catalyst could be 10 times more active than in the form of pellets, provided that the membrane thickness and porous texture, as well as the quantity and location of the catalyst in the membrane, are adapted to the kinetics of the reaction. For biphasic applications (gas/catalyst), the porous texture of the membrane must favor gas-wall (catalyst) interactions to ensure a maximum contact of the reactant with the catalyst surface. In the case of catalytic consecutive-parallel reaction systems, such as the selective oxidation of hydrocarbons, the gas-gas molecular interactions must be limited because they are nonselective and lead to a total oxidation of reactants and products. For these reasons, small-pore mesoporous or microporous... 

The insertion of catalytically active guests, such as transition metal ions, is an example of the potentialities of zeolite membranes for applications in catalytic membrane reactors. The well-known catalytic properties of supported vanadium oxides for selective oxidations have recently prompted a number of studies on the possibility of inserting vanadium in the framework of crystalline microporous silica and aluminosilicate powders. " ... 

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