Monolithic catalyst support,

Some catalyst supports rely on a relatively low surface area stmctural member coated with a layer of a higher surface area support material. The automotive catalytic converter monolith support is an example of this technology. In this appHcation, a central core of multichanneled, low surface area, extmded ceramic about 10 cm in diameter is coated with high surface area partially hydrated alumina onto which are deposited small amounts of precious metals as the active catalytic species. 

J. Howitt, Thin Wall Ceramics as Monolithic Catalyst Supports, SAE 910611, Society of Automotive Engineers, Warrendale, Pa., 1991. 

A ceramic monolith catalyst support, cordierite, consisting of silica, alumina and magnesium oxide. The purpose of this is to provide support, strength and stability over a wide temperature range. 

The conventional selective reduction of NOx for car passengers still competes but the efficient SCR with ammonia on V205/Ti02 for stationary sources is not available for mobile sources due to the toxicity of vanadium and its lower intrinsic activity than that of noble metals, which may imply higher amount of active phase for compensation. As illustrated in Figure 10.9, such a solution does not seem relevant because a subsequent increase in vanadium enhances the formation of undesirable nitrous oxide at low temperature. Presently, various attempts for the replacement of vanadium did not succeed regarding the complete conversion of NO into N2 at low temperature. Suarez et al. [87] who investigated the reduction of NO with NH3 on CuO-supported monolithic catalysts... 

The transformation of straw and agrofood residues with high sulfur and ash content requires the development of materials for sulfur abatement at high temperature, tar cracking and as monolith for syngas production by exothermic or autothermal processes thanks to catalysts supported on materials with a high thermal conductivity. 

A structured ruthenium catalyst (metal monolith supported) was investigated by Rabe et al. [70] in the ATR of methane using pure oxygen as oxidant. The catalytic activity tests were carried out at low temperature (<800 ° C) and high steam-to-carbon ratios (between 1.3 and 4). It was found that the lower operating temperature reduced the overall methane conversion and thus the reforming efficiency. However, the catalyst was stable during time on-stream tests without apparent carbon formation. 

The cordierite extruded monoliths, having 400 square cellsAn, were similar to those used in automobile catalytic converters. However, instead of using an alumina washcoat as in the catalytic converter, these catalyst supports were loaded directly with 12 to 14 wt.% Pt in the same manner as the foam monoliths. Because these extruded monoliths consist of several straight, parallel channels, the flow in these monoliths is laminar (with entrance effects) at the flow rates studied. 

Similarly, a Rh foam monolith with 0.56 wt.% Rh gave a lower optimal H2 selectivity than a Rh foam monolith with 9.83 wt.% Rh (75% vs. 87%). In both the Pt and the Rh experiments, the samples with the lower metal loa gs had significantly higher adiabatic reaction temperatures because of the heat generated by the formation of H2O. As demonstrated by these experiments, the formation of H2 occurs on the noble metal surface, not in the gas phase or on the catalyst support. 

Ceramic honeycomb monoliths are porous macro-structured supports consisting of parallel channels. On the walls a thin layer of active material can be applied (Figure 1). Honeycomb catalyst supports were originally developed for use in automotive... 

Hoveyda and co-workers immobilized an olefin metathesis catalyst on monolithic sol-gel and claimed that the catalytic material is easily recyclable. Barrett and co-workersprepared a recyclable boomerang polymer supported catalyst for olefin methathesis by grafting the preformed catalyst to a polystyrene... 

A variety of photocatalyst supports has been examined experimentally. Dip-coated glass slides or plates have been used in many experimental systems as a simple lab-scale supported photocatalyst system. Coated glass offers many of tlte important features of a supported photocatalyst while still offering relatively simple preparation. Honeycomb monoliths, widely used as commercial catalyst supports for a variety of gas-phase applications, have also been examined as photocatalyst supports (Fig. 3). Although these monoliths offer good stability and excellent throughput, providing illumination for the photocatalyst inside the monolith channels can be problematic [41,42]. Randomly structured support materials, like fiber-based filters, reticulated foams, and similar materials, have been used... 

N. Reinecke, D. Mewes, Oscillatory transient two-phase flows in single channels with reference to monolithic catalyst supports, Int. J. Multiphase Flow 25 (6-7) (1999) 1373-1393. 

Groppi, G., Tronconi, E., Design of novel monolith catalyst supports for gas/solid reactions with heat exchange, Chem. Eng. Sci. 2000, 55, 2161-2171. 

In principle, any catalyst bed used for reactive distillation or trickle bed operation can also be applied in reactive stripping. The performance will depend mainly on the optimal ratio between catalyst hold-up, the gas-liquid and the liquid-solid interface. However, recycling of the strip gas flow makes a low pressure drop (and therefore a high voidage) especially beneficial. In countercurrent operation, flooding - a well-known problem - must be avoided. The present studies have focused on structured catalyst supports, developed for either reactive distillation or reactive stripping, with a particular emphasis being placed on the use of so-called film-flow monoliths. 

Although common catalyst support materials such as alumina, silica, and carbon are also used for the production of monoliths, the most common material for commercial monolith production is cordierite, because of its high thermal stability and low coefficient of expansion. 

Coating of a Monolith with Catalyst Support Material... 

A bare monolithic structure can be coated with a catalyst support layer in several ways. Figure 21 shows a SEM image of a typical commercial cordierite monolith structure. Washcoating can be done by (partly) filling the pores of the macroporous walls with the washcoat material or by depositing a washcoat as a layer on top of the walls. These methods are shown schematically in Figure 22. 

Besides ceramic monoliths, metallic monoliths are available (64). In comparison with ceramic monoliths, metallic monoliths can be produced in more advanced structures, for example, to create turbulence in the flow in the channels (65). Several structured catalyst supports, such as solid foams or Sulzer packings, are usually made from metal. The surface area of the metal itself will be usually too low for practical applications. 

Monoliths that were anodized extensively (72) had an anodization thickness of up to 25 pm with a BET surface area of 40 m /g, which is sufficient for many applications. However, because this layer contained only mesopores (pore diameters up to 20 nm) and no macropores, internal diffusion limitations can easily be a problem. An extensive report on the anodization of aluminum monoliths, with the aim of using the anodization layer as catalyst support, was provided by Burgos et al. (73). 

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. 

FIGURE 32 Initial rates of reaction in catalysis by free lipase and immobilized lipase (Novozyme, and catalyst supported on 200 cpsi carbon monolith) in the acylation of butanol with vinyl acetate in an organic medium at 300 K. 

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