Monolithic catalysts precipitation

The LD configuration typically is applied in the plants where the residual dust content and the resistivity of the dust are compatible with the use of hot electrostatic precipitators. One of the advantages of this system is the reduced deterioration of the monolithic catalyst. 

An alternative method to deposit the oxidic layer or precursors of the active phase is precipitation or coprecipitation. This is widely used in conventional catalyst manufacture. An advantage is that a high loading of the active phase can be reached. As in monolithic reactions, catalyst loading is a point of concern. It is not surprising that precipitation methods are often applied in monolithic catalyst synthesis. 

Sometimes, due to a low melting point of a salt, e.g., when a nitrate is chosen as the precursor of the active phase, homogeneous distribution is difficult to obtain. Then, the deposition precipitation method might be appreciated [91-93]. This method is illustrated for the synthesis of a nickel monolithic catalyst. 

From model calculations it appeared that the pore structure is of great importance for the activity. To improve the activity of the monolith catalyst, silica was used as base material for the monolith. The pore structure was managed by the technology used. Silica was coated with titania upon which vanadia was precipitated. The activity of this new catalyst type improved according to the calculated 50%. 

Effective preparation methods of hexaaluminates for catalytic applications, such as the hydrolysis of alkoxides and the co-precipitation in aqueous medium, ensure high interspersion of the constituents in the precursor. This allows the formation of single phase materials with layered-alumina structure at reasonably low temperature (1100-1200 °C) and with high surface area. The hydrolysis of alkoxides was extensively studied and used for the industrial scale-up in the production of catalysts in the monolith shape. However, the co-precipitation in aqueous medium has much potential in view of the possible commercialization of these materials due to its simplicity and low cost. 

A detailed discussion of the deposition of metals on monolithic supports was presented by Vergimst et al. (75). The most popular methods are the same as those applied in typical catalyst synthesis, namely, impregnation, ion exchange, and deposition precipitation. 

As the deposited oxide layer is well mixed, strong interaction between the oxides is expected, leading often to mechanically strong materials, but pretreatment procedures can be hindered. For instance, in the preparation of a metal-based catalyst, a reduced reducibility of the precursor is often encountered, and, as a result, a reduced availability of the catalytically active phase is encountered. Moreover, under strongly acidic or basic conditions, some support materials, e.g., alumina, may be dissolved, as mentioned before. Furthermore, the adhesion of the precipitated layer with the monolith substrate is often a point of concern, especially during drying and heat treatment. 

The precipitate was separated from the excess liquid by centrifugation and washed once with acetone. The precipitate was dried at 120 °C for 10 h. The dried powder was consecutively calcined at 1000 °C and then at 1200 °C for 4 h each. Samples were taken out after each calcination. Finally a small fraction of the powder was aged at 1400 °C in 15% steam for 10 h. The main fraction of the powder was ball-milled and washcoated onto cordierite monoliths (Coming 400 cpsi) by dip-coating technique. The washcoated catalysts were then calcined at 1000 °C for 4 h. 

Mayoral [90] also demonstrated that the PyBox-Ru supported complexes on macroporous monolithic polymers were efficient catalysts for the cyclopropanation reaction. PyBox monolithic minireactors were prepared in a stainless column by radical copolymerization of 4-vinyl or 4-styryl PyBox in presence of styrene and DVB using 10% toluene/50% dodecanol as the precipitating porogeneous mixture (polymers 94 and 95 Scheme 44). To form the Ru complex, a solution of [RuCl2(p-cymene)]2 was passed through the column at low flow then washed with dichloromethane to remove the non complexed Ru. 

Then, for a catalyst carrier that has been coated onto the surface, and not for a ready-made catalyst, the active species of the catalyst has to be impregnated onto the carrier. For porous monoliths and other high surface area materials such as glass fibres [122], a catalyst carrier is not required and thus the active species may be impregnated immediately onto the uncoated substrate. Other methods of incorporating active species are crystallisation and precipitation, to name but a few. 

Unfortunately, the widely used flow reactors (for example, microreactors, multi-cell flow reactors, and disk reactors) do not tolerate solid particulates and precipitates, which would clog the miniaturized flow devices. Therefore, immobilization of the reagents/catalysts by a solid support is of significant importance in this field. Hence, a broad range of solid supports have been employed to incorporate the reagent/ catalyst into the reactors, including packed-beds, monoliths, and other systems that exploit the high surface-to-volume areas obtained in microchannel devices. 

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