Foam monoliths

New developments in the field of ceramic foam monoliths could also potentially provide new catalytic process technology for the conversion of methane into synthesis gas. For example, workers at Minnesota University [9] have achieved high synthesis gas yields at both high temperatures and space velocities using rhodium supported on a ceramic foam. 

GP 8[ [R 7[ Syngas generation with commercial Pt-Rh gauzes, metal-coated foam monoliths and extruded monoliths has been reported. For similar process pressure, process temperature, and reaction mixture composition, methane conversions are considerably lower in the conventional reactors (CH4/O2 2.0 22 vol.-% methane, 11 vol.-% oxygen, 66 vol.-% inert species 0.14—0.155 MPa 1100 °C) [3]. They amount to about 60%, whereas 90% was reached with the rhodium micro reactor. A much higher H2 selectivity is reached in the micro reactor the CO selectivity was comparable. The micro channels outlet temperatures dropped on increasing the amount of inert gas. 

Three basic types of catalysts were studied in these experiments Pt-10% Rh gauzes, foam monoliths, and extruded monoliths. The gauze catalysts were 40 mesh (40 wires per inch) or 80 mesh R-10% Rh woven wire samples which were cut into 18 mm diameter circles and stacked together to form a single gauze pack 1 to 10 layers thick. Gauze catalysts are used industrially in the oxidation of hftls to NO for HNO3 production and in the synthesis of HCN from NH3, CH4, and air. 

The foam monoliths were a-Al203 samples with an open cellular, spongelike structure. We used samples with nominally 30 to 80 pores per inch (ppi) which were cut into 17 mm diameter cylinders 2 to 20 mm long. A 12 to 20 wt.% coating of Pt or Rh was then applied directly to the alumina by an organometallic deposition. 

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. 

Figure 1. Selectivities (defined in equations 6 and 7), fractional conversion of CH4, and product gas temperatures for CH4 oxidation over a 7 mm long, 12 wt.% Pt, 50 ppi foam monolith (circles) and a 10 mm long, 9.83 wt.% Rh, 80 ppi foam monolith (squares) with 4 slpm of feed gases and 2 different feed gas temperatures (open symbols = 25°C and filled symbols = 460°C).

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. 

Tonkovich et al. [81] compared the performance ofa commercial ruthenium/zirconia powder catalyst from Degussa with a laboratory-made ruthenium/zirconia catalyst prepared on a nickel foam monolith for the water-gas shift reaction. Methane formation occurred for the powder catalyst, which was much less pronounced for the monolith. The selectivity towards methane could be reduced at shorter residence times. However, the activity of the laboratory-made catalyst was lower, which was partially attributed to the lower catalyst mass (modified residence time). 

A bench-scale evaporator was built first, consisting of the nickel foam monolith and heat exchanger plates 5.7 cm wide and 7 cm long. The stainless-steel channels fabricated by EDM were 254 pm deep and the vapor channel depth was varied from aspect ratios of 4 to 18, the latter being the optimum value determined by experiments. 

Mayer et al. [43] compared their results generated at a micro structured monolith (see Section 2.4.3) with literature data [137]. The degree of conversion and the hydrogen selectivity of the rhodium monolith outperformed both metal-coated foam monoliths, Pt-Rh gauzes and extruded monoliths. This was partially attributed to the higher activity of the rhodium monolith, but also to the lower cross-sectional channel area of the metallic monolith, which reduced mass transfer limitations, and to the improved heat conductivity. 

A representative comparison of the effect of the catalyst bed geometry on methane conversion and product selectivity over a range of methane/air ratios is shown in Fig 4 Unlike typical supported catalysts, where the catalyst is well-dispersed and submicrometer-sized, the noble-metal catalysts in these methane oxidation reactions were basically films with micrometer-sized surface features (Other tests on both extruded cordiente and alumina foam monoliths with lower catalyst loading resulted in similar carbon monoxide production but lower hydrogen yields than those illustrated in the figure, which provided evidence that the reaction is catalyst-dependent and not initiated by the monoliths or gas... 

Figure 4 Product selectivities and CH4 conversion for the following samples five layers of 40-mesh Pt-10% Rh gauze a 7-mm-long, 14-wt%-Pt extruded monolith with 400 cells/in. a 7-mm-long, 12-wt%-Pt, 50-pores-per-inch (ppi) foam monolith and a 7-mm-long, 9.9-wt%-Pt and 9.9-wt%-Rh, 50-ppi foam monolith. All experiments were performed at 4-5 slpm total flow. (From Ref. 7.)...

Table 8 Optimal Selectivity of HCN Synthesis for Gauze, Extruded Monolith, and Foam Monolith Catalysts at 1400 K...

Bitsch-Larsen, A., Degenstein, N.L, and Schmidt, L.D. Effect of sulfur in catalytic partial oxidation of methane over Rh-Ce coated foam monoliths. Applied Catalysis. B, Environmental, 2008, 78 (3—4), 364. 

Ethene ceramic foam monoliths at short contact time, 5 olefins) ... 

Figure 3. Conversion, selectivity, and yield of ethylene formation by ethane oxidation on Pt coated alumina ceramic foam monoliths. Up to 70% C2H4 selectivity is obtained at -70% C2H6 conversion for a single pass yield of -55%. Addition of Sn to Pt increases the selectivity and alkane conversion significantly.

We have also examined olefin formation from higher alkanes. Propane and butane also produce up to 70% selectivity to olefins on Pt monolith ceramic foam monoliths at nearly 100% O2 conversion with alkane conversions of typically 80% at comparable flow rates and catalyst temperatures to those used for C2H6. However, olefins from these higher alkanes exhibit considerable cracking, with C2H4 the dominant product except at low temperatures and excess fuel. However, isobutane produces primarily isobutylene and C3H6 with litde C2H4. 

Chromium oxide-containing catalysts are promising for partial oxidation of methane to synthesis gas (POM). TTie known preparation procedure of the foam monoliths from chromium oxide includes the use of gels prepared from alkoxides of metals forming matrix for the monolith catalysts [1]. In the present work, the preparation procedure of ceramometal monoliths from the metallic chromium and aluminum alloy blend has been described. The main stages of the preparation procedure and properties of the porous monolith have been studied. The catalysts performmce in the POM has been tested. 

Seven samples were prepared for evaluation. Six of the samples under consideration (No. 1 to No. 6) were small cordierite (Coming EX-80 type) monoliths (diameter 24.8 mm, length 52 mm, cell density 200 cpi) and one sample was an alumina foam monolith (diameter 25.2 mm, length 53 mm). Table 3 shows the catalyst applied to each sample tested. The catalyst formulations were similar to those used in 2.1 above in terms of the active components and their relative loading. 

The permeability of the foam monolith was calculated using the Darcy-Forchheimer law and the permeability and Forchheimer coefficient of the filter are determined by regression using equation (2). Table 4 summarizes the results for each filter sample tested. 

Poroplastic and Sustrelle ultramicroporous cellulose triacetate Porous polymeric substrates and foams Monolithic systems... 

Foam monoliths can be produced from a broad variety of materials. Commercially available are ceramic foams made of alumina, mullite, cordierite, silicon carbide, or zirconia, but numerous other oxides, carbides, nitrides, and... 

The alternative to foam replication is manufacturing by foaming of slurries or gels. The foaming agent may be either gas supplied from external sources or gas produced in situ through reaction. In catalytic applications, foamed monoliths are less useful than replication products because, at least in the present state of the art, they have lower porosities and contain considerably more closed cells. Therefore, these techniques are not treated here, but reference is made to a review by Binner [38]. 

Kraushaar-Czarnetzki, B. (2003) Ceramic foam monoliths as catalyst carriers. 1. Adjustment and description of the morphology. Ind. Eng. Chem. Res., 42 (9), 1863-1869. 

The CNFs can form layers on stmctured materials such as foams, monoliths, or felts this helps to keep diffusion distances short. The structured materials of choice obviously will also determine the hydrodynamic behavior of the reactor [8]. 

Adachi et al. [168] developed a model for a natural gas fuel processor composed of an ATR designed as metallic foam monolith coated with catalyst and two-stage WGS reactors also designed as foam monoliths followed by two-stage ceramic monoliths for the preferential oxidation of carbon monoxide as shown in Figure 14.27. Figure 14.28 shows the course of temperature and gas composition of feed and reformate as calculated for... 

Hickman and Schmidt were amongst the first to propose partial oxidation of methane over noble metal catalysts with short contact times of few milliseconds [225]. They used platinum and rhodium coated onto porous alumina foam monoliths. Rhodium showed superior performance, namely higher activity and hydrogen... 

In Fig. 2.16, a close-up picture of the reactor with the capillary for temperature profiles is displayed. The FHS, the capillary inserted into one channel and the ceramic fiber paper covering FHS, catalyst, and BHS are visible. The setup can be used for in situ measurements, where axial temperature and concentration profiles can be detected from one channel of the monohthic catalyst. Figure 2.17 shows the micro volume tee mounted onto the motorized linear stage used for moving the capillary. The setup is similar to the one introduced for foam monoliths by Horn et al. (2006a,b) and then adapted to honeycomb monoliths by Donazzi et al. (2011a,b). A novel feature of our... 

Metal gauzes, foam monoliths, sponges and traditional honeycomb monoliths have been successfully applied as catalysts or catalyst supports. The main advantages offered by the structured catalysts with high void fractions are represented by the reduced pressure drop and, in the case of foams, by the even distribution of the reactant flow across the fixed bed. 

Very high olefin yields were reported by Schmidt and co-workers,who first proposed the oxidative dehydrogenation of hght alkanes over insulated noble metal coated monoliths at contact times of a few milhseconds. This new concept of catalytic reactor had been previously applied by the same group to methane partial oxidation and was extended to test the reactivity of C2-C6 alkane/air fuel-rich feeds. Ceramic foam monoliths (with 45 and 80 ppi) were mostly studied as supports of noble metals and bimetallic catalysts. 

Foam monoliths behaved better than extruded or metal monoliths, which was explained on the basis of the higher mass transfer for the foam compared to straight channelled monoliths. The effect of mass transfer was confirmed by observing that HCN selectivity improved over foam monoliths with smaller pore size and extruded monoliths with higher cell density. 

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