Monolithic supports, advantages

In principle, deposition of an active phase (metal and/or oxide) on a monolithic catalyst support can be carried out in a manner similar to that used to prepare a t) ical catalyst. However, the large dimension of a monolith can easily enhance problems of nonhomogeneous deposition. For example, if in the preparation of conventional catalyst particles the active phase would be deposited at the external surface of the support, the result would be an egg-shell-t) e catalyst, which for many processes can be advantageous. However, if this pattern of deposition were applied to a monolithic support, it could result in a monolith with only the outer charmels of the structure having a significant catalytic activity, resulting in a dramatically poor catalytic reactor. The critical steps in the s)mthesis process are the deposition and drying steps, which are discussed separately below. Calcination, reduction, etc. for monolith catalysts are not different from those used to manufacture t) ical catalysts, and these steps are therefore not discussed here. 

A relatively easy method of placing an active phase homogeneously on a monolithic support is by deposition precipitation (76). The advantage of this method is that an insoluble metal salt is deposited on the support, which usually cannot spread readily during the drying step. Satisfactory results are obtained when supersaturation of the liquid occurs everywhere in the monolith at the same rate. This goal can be achieved by a homogeneous deposition precipitation procedure in which both the... 

The monolithic supports typically have higher geometric surface areas than do packed beds this is specifically advantageous for the SCR process. Due to its high rates, the DeNO reaction suffers from strong intraporous diffusional limitations and is confined only to a thin outer layer of catalyst, so that the NOx reduction efficiency is controlled by the catalyst geometric area rather than by its volume. 

In an another effort to utilize the advantages of monoliths to control selectivity of product, a patent [24] was issued for monolithic supported catalysts for selectively converting... 

Monolithic supports were obtained om CTI Company (Salindres, France). The chosen material is mullite, 3Al203-2Si02, and the monolith displays 7x7 square channels of 1 mm internal side length. The overall length of the decomposition chamber is 15 mm. The advantages of mullite are good thermal stability and mechanical properties. The monoliths are put for 1 h in a solution of nitric acid 1 mol.L then heated at 300°C for 1 h. 

For example, we present here the monolithic support (Figure 13.8) which is an inert material (2MgO x 5Si02 x 2AI2O3). It is highly resistant and has been used in different processes. The main advantage is the low residence time or contact time of the order of milliseconds. It presents 400 cells in (D = 12 mm, L= 8 mm) as shown in Figure 13.8. 

The optimization of the catalyst formulation is relevant not only to the active species but also to the structure of the support. Indeed, structured catalysts in the form of monolith or foam offer great advantages over pellet catalysts, the most important one being the low pressure drop associated with the high flow rates that are common in environmental applications. 

The main advantage of the grid approach is that the catalyst position is well defined. This is easy to do with solid catalysts, but not with homogeneous ones in solution. One way of solving this problem is to attach the homogeneous catalysts to a solid support, such as polymer beads [45] peptide scaffolds [46], or inorganic monoliths [47]. 

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