Catalyst attached to membrane

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). 

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. 

There is evidence that, in some cases of gas-phase or multiphase (e.g., gas-liquid) catalytic reactions, attachment of catalyst to the membrane in some form is more desirable than having the catalyst bed separated from the membrane in their catalytic performance [Sun and Khang, 1988 Harold et al., 1989 Zaspalis et al., 1991a]. Zaspalis et al. [199la] estimated that, when impregnated in membrane pores, a catalyst could be ten times more active than in the form of pellets. As long as the pore size is sufficiently large, the reactant(s) and produces) can be transported to and from the catalytic sites by convection which is more efficient than diffusion. 

Figufe 9.8 Comparison of IMRCF, CMR and FBR with uniform and Dirac delta catalyst activity distributions for (a) product purity on the feed side, (b) product purity on the permeate side, (c) product molar flow rate on the feed side, and (d) product molar flow rate on the permeate side as a function of the dimensionless residence time H eung et al., 1994] 

Beside the benefit of having less mass transfer resistance from the catalyst particles to the membrane, attachment of catalyst to the membrane is also preferred over a bed of catalyst in certain reactions where the contact between the reactant(s) and the catalyst is controlled. An example is the partial oxidation or oxidative coupling of methane [Eng and Stoukides, 1991]. In this case, the reaction conversion and selectivity have been observed to depend on the chemical form of the reactant oxygen and its rate of delivery to the reaction zone through the membrane. This approach has also been demonstrated through an analysis for utilizing a catalytically impregnated porous membrane tube as a catalyst for gas-liquid reactions [Harold et al. 1989]. 

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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]. 

Catalyst attached to membrane pore surface. The final distribution of the catalyst in the membrane pores can significantly impact the reactor performance. The optimal form of the catalyst distribution for maximizing conversion was studied mathematically by Keller et al. [1984]. They determined that the optimal distribution of the catalyst concentration is of the Dirac delta function. 

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