Porous membranes catalyst deposition

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

Another way of immobilizing catalyst complexes might be to trap them in the pores of solid particles, for instance by synthesizing the complex inside the pores of a zeolite ( ship in a bottle ). Another method could be to trap catalyst complexes in porous materials and deposit a membrane at the outer. surface. These methods of immobilizing a homogeneous catalyst do not involve chemical linkage between the catalyst and the carrier. The fixation is the result of steric hindrance. 

Modeling studies have also considered other aspects of CMRs. Sun and Khang [46] compared two types of CMRs, one with an inert (only separative) porous membrane associated with a fixed-bed catalyst, the other with the catalyst deposited within the porous framework of the membrane (thus leading to a catalytic membrane). For long contact times, the performance of the catalytic membrane is higher, due to the simultaneity of reaction and separation. 

Catalysts such as the platinum group metals can be used in dispersed or monolithic solid form. The catalyst can be deposited on the surface of a membrane (dense or porous) or, in the cases of catalyst particles, dispersed in the sub>surface layer or throughout the matrix of a porous membrane. 

Catalysts can be contained in the membrane pores or on the membrane surface by several impregnation and adsorption techniques commonly used for conventional catalyst preparation. To introduce catalyst inside the pores, for example. Sun aixl Khang [1988] use chloroplatinic acid solution to impregnate Ft catalyst in porous glass membrane pores. Ni, Fe Ag, and Pd catalysts of varying particle sizes can be deposited in porous alumina membranes by immersing the membrane in an electrolysis bath of a... 

Catalyst attached to membrane surface. When depositing catalyst particles on the surface of a catalytically inert, dense membrane, the membrane surface layer should be porous in nature to provide a high surface area catalyst support for strong adhesion of the catalyst particles. A layer or multi-layers of catalyst particles can be coated on inorganic membrane surfaces by several methods Pd by vapor deposition [Gryaznov et al, 1979], Pd and Pt by solution deposition [Gryaznov et al., 1983 Guther and Vielstich, 1982]. 

Based on matenal considerations, membrane reactors can be classified into (1) organic-membrane reactors, and (2) inorgamc-membrane reactors, with the latter class subdivided into dense (metals) membrane reactors and porous-membrane reactors Based on membrane type and mode of operation, Tsotsis et al. [15] classified membrane reactors as shown in Table 3. A CMR is a reactor whose permselective membrane is the catalytic type or has a catalyst deposited in or on it. A CNMR contains a catalytic membrane that reactants penetrate from both sides. PBMR and FBMR contain a permselective membrane that is not catalytic the catalyst is present in the form of a packed or a fluidized bed PBCMR and FBCMR differ from the foregoing reactors in that membranes are catalytic. 

Hazbum [2.129] in a U.S. patent, reported ethane and propane partial oxidation to ethylene and propylene oxides using a TiYSZ mixed O Velectron conducting membrane with silver deposited on the hydrocarbon side of the membrane as a catalyst. For the ethane partial oxidation reaction catalyst at 250-400 C, the ethylene oxide selectivity was (>75 %) but the ethane conversion was low (<10 %), limited by the low membrane oxygen flux at these temperatures. For the partial oxidation reaction to propylene oxide yields close to 5 % were reported. Using porous membranes Santamaria and coworkers (Mallada et al. [2.252, 2.253, 2.254, 2.255]), Mota et al. [2.256], and Xue and Ross [2.257] recently studied the oxidation of butane into maleic anhydride. 

Control of Contact between Catalyst and Reactants - In this use of catalytic membranes, the membrane is porous and in the CMR configuration, and is either intrinsically active or has had a catalyst deposited within the pores. The membrane geometry allows for a degree of control of the contact time. It is operated in the cross-flow mode, in which all of the reactant is forced to flow through the membrane by feeding it to one side with a closed exit. This is illustrated schematically in Figure 19. 

Porous metal membranes are commercially available in stainless steel and some other alloys (e.g.. Inconel, Hastelloy) and they are characterized by a macroporous structure. On the other hand, porous ceramic membranes can be found commercially in various oxides and combination of oxides (e.g., Al203,li02,Zr02, Si02) and pore size families in the mesopore and macropore ranges (e.g., from 1 nm to several microns). Most of the literature studies on three-phase catalytic membrane reactors have been carried out by developing catalytic ceramic membranes. The deposition techniques for the preparation of catalytic ceramic membranes involve methods widely used for the preparation of traditional supported catalysts (Pinna, 1998), and methods specifically developed for the preparation of structured catalysts (Cybulski and Moulijn, 2006). Other methods to introduce a catalytic species on a porous support include the chemical vapour deposition and physical vapour deposition (Daub et al, 2001). The catalyst deposition method has a strong influence on the catalytic membrane reactor performance. 

One way of forcing the reaction toward high conversion is to combine the catalyst and membrane technologies. The reaction is carried out in a porous membrane tube covered with the catalyst material. The principal concept is shown in Figure 9.18. The reaction proceeds on metal spots deposited on the membrane material. Simultaneously, the smallest product molecules (in this case H2) diffuse out of the system through the membrane. In this way, the equilibrium limitation is removed, and the process proceeds almost as an irreversible reaction in the ideal case. The additional benefit is that the components of the product gas are directly separated, and the construction of a specific separation unit is avoided. For the reactor construction, see Figure 9.19. From the reaction engineering... 

Catalytic membranes can be prepared by deposition of a catalytically active phase (e.g., a metal) in the inert porous membranes through various techniques commonly used for conventional catalyst preparation - such as wet impregnation, monolayer metal complexation, and ion exchange -followed by heat treatment (activation) [17], The conventional activation steps can also be avoided by direct deposition of solvated metal microclusters [21], The structural characteristics of the porous substrate, and in particular the relative laminar and Knudsen contributions to permeation, have a strong influence on the membrane performance [22], They make use of the membrane structure to optimize the access of disfavored reactants, or to control and rule the residence time and contact of species in the active zone. Furthermore, the porous membrane is required to present a rather homogeneous structure to avoid heterogeneities in the reactant-to-catalyst contact, and also to facilitate CMR operation control. 

In plate-type MMRs, the microchannels for catalyst deposition are fabricated on a porous silicon or stainless steel (i.e., SS 316L) plate, while the membrane layer is deposited on the back of the plate to perform separation (membrane/catalyst plate) as shown in Figure 8.1(a). In some cases the membrane layer, which also shows catalytic activity, is deposited on the wall of the microchannels for simultaneous reaction and separation... 

Membrane/catalyst combination modes (a) catalyst separated by the membrane (b) catalyst coated on the membrane and (c) catalyst deposited in porous layer. 

Heck C-C coupling reactions were also facilitated by the presence of a palladium catalyst when Pd was deposited on a tubular membrane of porous glass. Thus, the coupling of iodobenzene with allyl alcohol affording 3-phenylpropionaldehyde in the presence of this Pd catalyst had several advantages - the ease of catalyst manufacture, mechanical strength, thermal stability, and resistance to organic solvents [46],... 

The sheet of porous stainless steel with Re-carbon deposited film divided membrane reactor onto two equal parts. Cyclohexane vapors were fed to the surface of membrane with Re-carbon film (reaction part of membrane reactor) in argon flow from the thermostated bubler. The second part of reactor was flowed by argon and used for the removal of hydrogen, diffused through a membrane catalyst from the reaction zone. The products of reaction were benzene and hydrogen. 

Palladium and some of its alloys are catalytically active to many dehydrogenation reactions. In other cases where other catalysts arc required, they arc either impregnated in the porous support (for those composite membranes), deposited on the membrane surfaces as a coating or packed as pellets inside the membrane element. 

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