Structured foam catalysts

Selective oxidations over structured foam catalysts... 

Twigg MV, Richardson JT. Fundamentals and applications of structured ceramic foam catalysts. Industrial Engineering Chemistry Research. 2007 46(12) 4166-4177. 

Foams were proved to be highly suitable as catalytic carrier when low pressure drop is mandatory. In comparison to monoliths, they allow radial mixing of the fluid combined with enhanced heat transfer properties because of the solid continuous phase of the foam structure. Catalytic foams are successfully used for partial oxidation of hydrocarbons, catalytic combustion, and removal of soot from diesel engines [14]. The integration of foam catalysts in combination with microstructured devices was reported by Yu et al. [15]. The authors used metal foams as catalyst support for a microstructured methanol reformer and studied the influence of the foam material on the catalytic selectivity and activity. Moritz et al. [16] constructed a ceramic MSR with an inserted SiC-foam. The electric conductive material can be used as internal heating elements and allows a very rapid heating up to temperatures of 800-1000°C. In addition, heat conductivity of metal or SiC foams avoids axial and radial temperature profiles facilitating isothermal reactor operation. 

A comparison between ceramic foams and metallic microstructured monoliths of Fecralloy was performed for the OSR and CPO of propane under similar conditions of total catalyst amount, catalyst composition and modified catalyst residence time [40], A major difference in this investigation was, however, that uncoated foam was located before and after the catalytically coated foam to prevent gas-phase ignition. Irrespective of this difference, two main conclusions can be drawn first, the catalyst supported on the microchaimel structure did not deactivate like the foam catalyst under OSR conditions. This could be due to the fact that peak temperature... 

In comparison to monoliths, applications of open-celled foam structures to the chemical process industry are still at an earlier developmental stage. We report fundamental and applied investigations demonstrating opportunities for the implementation of foam catalysts in the same two main areas of millisecond contact time processes, and of fixed-bed reactors with enhanced heat management. 

Indeed, a major obstacle to the development of foam catalysts is the lack of reliable engineering correlations for the relevant morphological and transport properties, which also prevents a conclusive appraisal of their potential in comparison to other more or less conventional structures. Four interlinked areas are of interest in this respect, associated with the description of the foam geometry, as well as of pressure drop, gas/solid mass and heat transfer, and overall (axial and radial) heat transfer in foam catalytic structures, respectively. An exhaustive review of the technical literature on foam catalysts is beyond the scope of the present chapter, so only the most significant published contributions to these topics are summarized in the next paragraphs. 

Alumina foams coated with Rh/MgAl204 spinel active phases have been produced to be used as catalysts in steam reforming processes with improved thermal transfers and limited pressure drops. Those foam-supported catalysts are here fully characterised before and after aging in water-rich atmosphere at elevated temperatures. It is shown that they are stable at any architectural scale macro- (foam), micro- (coating) and nano-(Rh active phase) structures. Such catalysts are then very promising catalytic loads to be further implemented in industrial units instead of standard loads. 

Foams may be produced from these resins by addition of 65 35 TDI, water, a catalyst, an emulsifier, a structure modifier and paraffin oil which helps to control pore size and prevents splitting of the foams. 

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

The laboratory prototype of the Dinex electrochemically promoted catalyst unit is shown in Figure 12.12 and the assembled unit schematically in Fig. 12.13. It consists (Fig. 12.14) of a tubular bundle porous (ceramic foam) structure made of CeOa-GcfeOj (CGO) which is an O2" conductor with ionic conductivity significantly higher than YSZ at temperatures below 500°C... 

Recently, many papers have been published on fiber catalysts and foam structures (Figure 9.2). Although, strictly speaking, fibers and foams might not be considered as structured systems, beds of such catalysts exhibit typical features of structured catalysts, namely, low pressure drop, uniform fiow, a good and uniform access to the catalytic surface, and they are definitely nonrandom. Therefore, we have included them in this chapter. 

Figure 9.2 Fiber and foam structures, (a) Knitted silica fibers catalyst. (Reprinted from [7].) (b) Woven active carbon fiber catalyst. (Reprinted from [8].) (c) Aluminum foam. (Reprinted from [9].)...

For the amounts of Fe below x=l, the sheath-like structures form mostly (Fig. 2d). This proceeds likely so when the Fe amount is low enough, the catalyst does not get to deeper layers of onions and pyrrole polymerises already in outer layers, which hinders the access of further monomer molecules to the onions inside. Use of still smaller amounts of the Fe catalyst results in formation of carbon (e.g., OCM-.NO.25) consisting of both foam- and sheath-like structures (Fig. 2c). The XPS analysis reveals that 0.43 wt.% Si and 0.5 wt.% Fe remain in the surface layer of OCM-.NO.25. This sample as well as CMK-3N1.25 and CMK-3N2.00 do not bum up totally (Table 1, Fig. 3). 

Cobalt-on-alumina catalysts with increased dispersion and catalytic activity are prepared by addition of mannitol to the cobalt nitrate solution prior to impregnation. Thermogravimetric analysis (TGA) and in situ visible microscopy of the impregnation solution show that the organic compound reacts with cobalt nitrate, forming a foam. The foam forms because significant amounts of gas are released through a viscous liquid. The structure of the foam is retained in the final calcined product. It is this effect that is responsible for the increased dispersion. 

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. 

Moreover, flexible foams are characterized by utilization of special emulsifiers in their synthesis yielding an open-cell architecture, whereas for rigid foams emulsifiers are chosen that create more closed-cell structures. As diisocyanate for both types, the commercially available mixture of 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate is especially suitable. If foam formation is to take place at room temperature, and especially when hydroxy compounds with secondary hydroxy groups are used [poly(propylene glycol)s], the presence of a catalyst is generally required (see Sect. 4.2.1). 

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