Membrane Inherently Catalytic

As indicated in Chapter 8, the most compact configuration of a membrane reactor is where the membrane (selective layer) is also catalytic for the reaction involved. 

Many of the metal oxide materials used for making ceramic membranes, particularly the porous type, have also been used or studied as catalysts or catalyst supports. Thus, they are naturally suitable to be the membrane as well as the catalyst. For example, alumina surface is known to contain acidic sites which can catalyze some reactions. Alumina is inherently catalytic to the Claus reaction and the dehydration reaction for amine production. Silica is used for nitration of benzene and production of carbon bisulfide from methanol and sulfur. These and other examples are highlighted in Table 9.6. 

On the metallic membrane side, a well known type of material with this characteristic is Pd and certain Pd alloys. Palladium is known to be catalytic to many reactions including oxidation, hydrogenation and hydrocracking. It has been found that the catalytic activity of selected binary Pd alloys is higher than that of pure Pd. Silver catalyzes a number of oxidation reactions such as oxidation of ethylene and methanol. In addition, nickel is catalytic to many industrially important reactions. 

Examples of common inorganic membrane materials which are inherently catalytic to selected reactions 

Alumina Al203-Si02 Catalytic cracking for gasoline production 

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When the reaction between two reactants supplied from opposite membrane sides is very fast then a reaction interface is formed within the inherently catalytic membrane at a position which satisfies the mass balances. Any change in the reactant fluxes will cause a change in the location of the reaction interface which will be re-established at a new position within the... 

Figure 5. Different membrane-catalyst combinations (a) fixed-bed catalyst plus membrane (b) inherently catalytic membrane (c) membrane acting as support of a catalytically active phase.

There are a class of materials, although made by various methods, that deserve a separate discussion here due to their great promises for making very fine-pore and, in some cases, inherently catalytic membranes. They are molecular sieves which separate molecules based on whether the molecules can penetrate the regularly spaced channels of molecular dimensions (pore diameter in the 0.2 to 1 nm range) in the molecular sieve crystals. This "sieving" mechanism on the molecular level is the most distinct characteristics of these materials. 

Either inherently catalytic membranes or membranes tightly bound with catalysts are conveniently referred to as catalytic membranes or catalytically active membranes. If desired or necessary, the catalytic function of a catalytic membrane can be strengthened with a second catalyst in the form of pellets. 

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 nature of many high-temperature hydrocarbon reactions which potentially can benefit from inorganic membrane reactors (particularly catalytic membrane reactors) is such that the catalysts or the catalytic membranes are subject to poisoning over time. Deactivation and regeneration of many catalysts in the form of pellets are well known, but the same issues related to either catalyst-impregnated membranes or inherently catalytic membranes are new to industrial practitioners. They are addressed in this section. 

FIGURE 25.13 The main membrane/catalyst combinations (a) bed of catalyst on an inert membrane (b) catalyst dispersed in an inert membrane and (c) inherently catalytic membrane. (From Julbe, A., Farmsseng, D., and Guizard, C., J. Membr. ScL, 181, 3, 2001.)... 

Membrane reactors are defined here based on their membrane function and catalytic activity in a structured way, predominantly following Sanchez and Tsotsis [2]. The acronym used to define the type of membrane reactor applied at the reactor level can be set up as shown in Figure 10.4. The membrane reactor is abbreviated as MR and is placed at the end of the acronym. Because the word membrane suggests that it is permselective, an N is included in the acronym in case it is nonpermselective. When the membrane is inherently catalytically active, or a thin catalytic film is deposited on top of the membrane, a C (catalytic) is included. When catalytic activity is present besides the membrane, additional letters can be included to indicate the appearance of the catalyst, for example, packed bed (PB) or fluidized bed (FB). In the case of an inert and nonpermselective... 

The membrane is inherently catalytic (Figure 7.2b) or modified with catalytically active species distributed in or at the entrance of the membrane pores as individual particles or as a layer (Figure 7.2c). The catalytic activity is adjusted to the membrane (catalytically active membrane). In this way the strongest interaction between membrane transport properties and catalytic activity can be achieved. 

Figure 7X. Different membrane/catalyst combinations (a) the catalyst is packed next to the membrane, (b) the membrane is inherently catalytic and (c) the membrane is modified with catalytically active components.

It has been mentioned earlier that using porous membranes for product separation during the course of an equilibrium reaction, maximum attainable conversions are limited because of reactant permeation. This is the case where the membrane forms the wall of the reactor in which a catalyst is packed. It has also been mentioned that in this mode equilibrium conversions for some slow reactions could be increased by factors ranging between 1.3 and 2.3. Another important operation mode arises when the membrane is inherently catalytic or when the catalytically active species are placed within the membrane pores (catalytically active membrane as shown in Figure 7.2b and 7.2c). In this case, reaction and separation take place simultaneously and are combined in parallel rather than in series as was the case in the previous mode. 

As briefly mentioned earlier in this chapter, the porous matrix of an inorganic membrane can be applied as a well-engineered catalytic reaction zone where the catalyst is immobilize on the pore surface or the membrane is inherently catalytic to the reaction involved. 

In the second mode of incorporating a catalyst into a membrane reactor, the catalyst is attached to the membrane surface on the feed or permeate side or to the surfaces of the membrane pores. This case and the third mode where the membrane is inherently catalytic are often called catalytic membrane reactor (CMR). 

Catalytic membrane reactors represent the most compact and yet challenging membrane reactor design. The membrane material may be inherently catalytic or rendered catalytic by impregnating a catalyst on the surface of the membrane itself or the pores inside the membrane/support matrix. When the inner tube of a shell-and-tul reactor is a permselective and also catalytic membrane, the reactor is called catalytic membrane tubular reactor. Under this special circumstance, ibj = 0 = kf for Equations (10-36) to (10-37) and (10-44) to (10-45), assuming plug flows on both the tube and shell sides. The transport equations for the membrane zone. Equations (10-5) to (10-6), hold. 

In addition to space time which is indicative of the residence time of the feed (reactant) stream through the reactor length, the conuct time of the reactant(s) through the membrane pores can be very important. This is particularly true with inherently catalytic or catalyst-impregnated membranes. Here the residence time of the reactant(s) inside the membrane zone is expected to exert influence on the reaction involved. One obvious way of controlling the residence time, for a given pore size and tortuosity of the... 

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... 

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