Cellular Ceramics / 5

464 Part 5 Applications

ium–sulfur compounds in the presence of sulfur oxides. Such compounds are very stable up to high temperatures and reduce the NOx storage capacity of the catalyst. Therefore, reduction of the sulfur content in the fuel is essential for the function of the NOx storage system.

5.4.3.2

Diesel Engine Catalysts

With progress in the emission control of gasoline-fueled vehicles, the comparatively small pollutant emissions of diesel vehicles moved in the last few years into the center of attention. In addition, the number of diesel-engined cars increased due to their lower fuel consumption. Diesel engines operate with excess oxygen under lean conditions. Solutions for this working area of the catalysts were already discussed in the preceding section. Additionally, diesel engines emit soot, soluble organic fractions (SOF), and SO2. The cleaning of the exhaust gases of diesel engines can be divided into three tasks: 1) total oxidation of HCs, SOF, CO, aldehydes, and SO2; 2) trapping of particles and removal of NOx, and 3) reduction of NOx to nitrogen (DENOXing). The removal of sulfur trioxide is necessary due to its capability to form sulphate, which deactivates the precious metals or attacks the support material [55]. Due to the lower temperature of diesel exhaust gas, the oxidation catalyst must be highly active at temperatures below 250 C [56]. Common catalysts are composed of Pt/CeO2/Al2O3 with a tailored porous system that allows adsorption at lower temperatures for removal of SOF below the light-off temperature [55]. At present great efforts are dedicated to the development of catalytically active diesel soot traps based on honeycomb or foam monoliths to remove soot and NOx simultaneously [57–69]. A more detailed description of particulate diesel traps, materials used for their preparation, and working principle can be found in Chapter 5.2 of this book.

After conversion of CO, HC, SOF, and soot, removal of nitrous oxide under oxygen excess, especially for systems with continuous regeneration trap (CRT), is necessary. Thus, typical lean-burn diesel engines require procedures for reduction of nitrogen oxides in the presence of oxygen. Technically important DENOX processes are selective catalytic reduction (SCR) with ammonia (NH3-SCR) [70, 71] or hydrocarbons (HC-SCR) [72] and the NOx storage–reduction catalysts (NSR) [53] (see Section 5.4.3.1).

NH3-SCR is driven by the reaction of NO and NH3 to form nitrogen and water when NO is the major NOx component (Eq. (1)):

In the case of equimolar amounts of NO and NO2 in the exhaust gas, the reaction can be described by Eq. (2) [73, 74]:

At temperatures below 200 C the formation of ammonium nitrate is a serious problem for NH3-SCR. During its decomposition at higher temperatures ammonium nitrate

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 465

can form the stable N2O, or on initial ignition explosive decomposition can occur [70]. Generally, honeycomb monoliths covered with TiO2/WO3/V2O5 wash coat have been used for SCR of NOx with ammonia. For safety reasons ammonia is produced onboard diesel trucks by hydrolysis of urea in a preheated catalyst zone (Fig. 8).

Urea solution

Fig. 8 Scheme of NH3-SCR [51] with urea injection. (Reprinted from Catalysis Today, 77, J. Kaspar, P. Fornasiero and N. Hickey, Automotive catalytic converters: current status and some perspectives, 419–449, Copyright 2002, with permission from Elsevier.)

The HC-SCR process is similar to NH3-SCR, except that hydrocarbons are employed as reducing agents, commonly in combination with copper-loaded zeolite catalysts (Cu-ZSM-5) [72, 75].

5.4.3.3

Catalytic Combustion for Gas Turbines

The catalytic reduction of NOx to nitrogen in power plant exhaust gas based on the SCR method is described in Ref. [76]. A relatively new application for monolith reactors is catalytic combustion in gas turbines. In conventional gas turbines fuel is burnt by flame combustion at around 1800 C. Because of materials limitations it is necessary to cool the hot gases with bypassed compressed air having temperatures of 1100–1500 C. Due to the high flame temperature the combustion leads to large amount of NOx in the exhaust gas. For this reason catalytic combustion at lower temperature is desirable [77]. The catalyst should withstand continuously temperatures up to 1500 C and thermal shocks of 1000 K s–1. The existing solutions therefore consist of hybrid combustion systems [78] in which the heterogeneously catalyzed conversion has the function of preheating the fuel and air for the downstream homogeneous combustion.

5.4.3.4

Applications of Honeycomb Catalysts for Other Gas Phase Reactions

Apart from the large and well-established area of exhaust gas purification, only a few other examples for uses of monolithic catalysts in gas-phase reactions have become established. Corning developed a novel extrusion method to make bulk tran-

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 467

The strongly exothermic nature of Fischer–Tropsch synthesis requires effective heat transfer for successful reactor operation. The authors also compared different materials of monolithic structures (cordierite, c-alumina, and steel) in Fischer–Tropsch synthesis [81]. Each was compared to corresponding powder catalysts. Cordierite monoliths were as active and selective to C5+ (pentane and higher hydrocarbons) as comparable powder catalysts, steel monoliths were found to have lower activity and C5+ selectivity than comparable powder catalysts and cordierite monoliths, and alumina monoliths gave comparable selectivities to powder catalysts but lower activity.

Nicolau et al. (Celanese International Corporation) synthesized vinyl acetate by vapor-phase reaction of ethylene, oxygen, and acetic acid using a ceramic honeycomb as reactor. The catalysts consist of Pd and Au deposited on silica-coated cordierite or mullite monoliths. Catalyst space–time yields for vinyl acetate production of 434 g L–1 h–1 at 192 C have been reported [82, 83].

For the numerical simulation of monolithic catalysts, several proposals have been made in recent years. A newly developed computational tool is DETCHEMMONOLITH

for the transient twoand three-dimensional simulation of catalytic combustion monoliths, which is especially useful for spatially structured monolithic catalysts in which the timescales of variation of the gas phase are much smaller than those of the thermal changes in the monolithic structure, for example, for catalytic combustion, conversion of natural gas, and automotive catalytic converters. The developed computer code for the first time offers the possibility of performing transient 2D and 3D monolith calculations with such detailed models for transport and chemistry in the individual channels. As an example, the application of the tool to the hydro- gen-assisted catalytic combustion of methane in a platinum-coated honeycomb monolith is demonstrated in Ref. [84].

5.4.3.5

Honeycomb Catalysts for Gas/Liquid-Phase Reactions

Since the introduction of the monolith multiphase reactor several investigations have been carried out to increase the utilization of monolithic structures as catalyst support in multiphase reactions. Current research is mainly focused on hydrogenation reactions. These multiphase applications are mostly operated in cocurrent mode [85]. In monoliths, different flow regimes can occur. Most interesting are Taylor (bubble or slug flow) and film flow. Figure 10 demonstrates bubble or slug flow in a capillary. In this type of flow gas bubbles and liquid slugs move with constant velocity through the monolith channels, and the gas is separated from the catalyst only by a thin liquid film. For this flow regime a higher mass transfer of gas to the catalyst is expected compared to that of a trickle-bed reactor. The better plug-flow behavior (gas and liquid have sharp residence-time distribution) is expected to result in higher selectivities towards the desired product in conversions with unwanted consecutive reactions [86]. A more detailed description of fluid flow through cellular ceramics can be found in Chapter 4.2 of this book.

468 Part 5 Applications

Fig. 10 Taylor (bubble or slug) flow in a capillary. The CFD picture (left) shows the liquid circulation patterns [86]. (Reprinted from Chemical Engineering Science, 56, T.A. Nijhuis, M.T. Kreutzer, A.C.J. Romijn, F. Kapteijn and J.A. Moulijn, Monolithic catalysts as efficient three-phase reactors, 823–829, Copyright 2001, with permission from Elsevier.)

Regarding multiphase reactions advantages of monolithic reactors are:

Despite these advantages, only a few structured reactors are used in chemical industry for two reasons. The structured reactors contain less catalyst than a fixed bed and hence require a more stable catalyst than fixed bed or batch process. In three-phase processes extremely good mixing is required to bring together the reactants in the gas phase and those in the liquid phase at the active site of the catalyst to produce good productivity and selectivity. This is the justification for all the work devoted to understanding the hydrodynamics of structured reactors.

Presently, there is only one large-scale industrial application of monolithic catalysts in a multiphase process [38]. Akzo developed a monolith-based process for H2O2 production in which the hydrogenation step of the anthraquinone (AQ) autoxidation process is performed under relatively mild conditions (a few bar, 40–70 C) [87, 88]. Instead of conventional monolith coated with a porous wash coat, reinforced amorphous silica was used throughout the wall [89]. The Pd active phase was deposited by electroless deposition, which involves chemical reduction of a Pd-containing solution [87]. The main reason for choosing a monolithic catalyst was to avoid the transport of fine catalyst particles with the liquid to the oxidation reactor, which results in the decomposition of hydrogen peroxide [7]. The concept of using a monolithic catalyst on a large scale was successful. Anthraquinones dissolved in an ordinary organic solvent could be hydrogenated with extremely high selectivity and high productivity. The idea of using a monolith for this reaction was put forward by researchers at the Chalmers University of Technology, G.teborg and was picked up by Eka Chemicals. The first large-scale reactor was brought on stream in the early 1990s.

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 469

Today the new type of reactor is used in plants with a total annual capacity of 200 000 t [87].

Kapteijn, Moulijn et al. built a pilot-scale setup and investigated two model reactions [86]: the hydrogenation of a-methylstyrene as a (hydrogen) mass-transfer-lim- ited reaction and the hydrogenation of benzaldehyde to benzyl alcohol as a conversion with an unwanted consecutive reaction. In comparison with a trickle-bed reactor higher productivities for the mass-transfer-limited reaction and higher selectivities for the hydrogenation of benzaldehyde were found. For the former reaction the overall mass transfer of 0.5–1.5 s–1 indicates that excellent mass transfer can be achieved for gas–liquid–solid reactions in monolithic reactors [90].

One of the first examples of research on monolithic catalysts was a monolithic reactor investigated by the Moulijn group in cooperation with DSM Research, Netherlands, in the selective hydrogenation of unsaturated hydrocarbons. A mixture of styrene and 1-octene in toluene, representative for hydrocarbon mixtures subjected to hydrotreating, was hydrogenated in a cordierite monolith with alumina wash coat impregnated with Pd [91]. Operating the monolithic reactor in the Taylor flow regime gave considerably higher reaction rates than those reported in the literature so far. Nevertheless, external mass-transfer limitation was present under the conditions investigated. The fact that reaction rates could be strongly enhanced by operating at higher liquid loadings indicated that nonuniform or incomplete catalyst wetting may partially control conversion. Proper design of the gas–liquid distributor was found to be the critical factor.

In comparisons of monolith and slurry reactors, the former has the added advantage of allowing easy catalyst separation. Comparative analysis of monolithic reactors and slurry reactors were done by Moulijn et al. [92], Winterbottom et al. [93], and Hatziantoniou et al. [94]. The Moulijn group investigated the liquid-phase hydrogenation of 3-hydroxypropanal to 1,3-propanediol. Mathematical modeling of both reactors over a broad range of operating conditions showed that the monolithic reactor performed better in terms of both productivity and selectivity [92]. The hydrogenation of 2-butyne-1,4-diol to cis-2-butene-1,4-diol was additionally compared with a stirred tank reactor (STR) [93]. The selectivity to cis-2-butene-1,4-diol, an important intermediate in the production of Endosulfan (insecticide) and vitamins A and B6, was significantly higher for the monolith and the slurry reactor (98.0–99.3 %) than for the STR (90–95 %) at conversions of 2-butyne-1,4-diol approaching 100 %. The hydrogenation of mixtures of nitrobenzene and m-nitrotoluene was investigated by Hatziantoniou et al. [94]. The activity of the catalyst was so high that the mass-trans- fer steps were rate-determining, and mass transfer of hydrogen directly from the gas plugs to the channel wall was found to be the dominant transport step. The decreased selectivity of aniline formation found in the monolithic Pd catalyst was explained by the influence of film-transport resistance near the channel wall. The catalytic hydrogenation of nitrobenzoic acid to aminobenzoic acid was used also for a quantitative study of influences of operating conditions on the observed reaction rate in a single-channel monolith reactor operated in Taylor flow regime [95]. An improvement in a process for hydrogenating nitroaromatics by employing a monolithic catalyst system was claimed by Machado et al. of Air Products and Chemicals

470Part 5 Applications

[96].The same authors also claimed a gas–liquid reaction process including a liquidmotive ejector as a gas–liquid distributor, and monolith catalyst. The invention relates to process for carrying out gas–liquid reactions such as those employed in the hydrogenation or oxidation of organic compounds [97, 98].

Other examples for hydrogenation reactions are the hydrogenation of dimethyl succinate over cordierite-based monolithic copper and nickel catalysts prepared by the wash coat technique, studied by Cybulski et al. [99], and the Friedel–Crafts acylation of aromatics using zeolite-coated (BEA and FAU) monoliths [100]. The viability of using a monolithic catalyst support system was also demonstrated for a condensation/polymerization reaction. For the production of siloxane fluids tripotassium phosphate was used as catalyst, and the experiments were performed in a singlechannel flow reactor with 15 mm i.d. and 500 mm catalyst-coated length [101].

A multifunctional reactor integrating reaction and a separation process based on a monolithic system was developed by Moulijn et al. Internally finned monolith coated with solid acids (zeolite BEA or the ion-exchange resin Nafion, both prepared by dip coating with 5 wt % with gas and liquid flowing countercurrently) were used in the esterification of 1-octanol with hexanoic acid, with removal of the side product water from the liquid reaction mixture by means of reactive stripping [102, 103]. This special type of monolith, which is optimally suited for stripping operations, is shown in Fig. 11. The fins stabilize the flow and provide additional surface area for the catalyst.

The reactive-stripping operation in a monolithic reactor results in a significantly better performance of the catalyst, since not only is the inhibiting effect of water reduced, but also conversions beyond equilibrium conversion are possible. This type of monolithic reactor was originally developed by the same authors for deep-HDS (hydrodesulfurization), in which the H2S produced has a strong inhibiting effect on the catalyst activity, and countercurrent operation is therefore highly attractive.

A rotating monolith reactor was used for catalytic propane dehydrogenation by Stitt et al. [104]. The reaction is equilibrium-limited, strongly endothermic, and normally carried out at high temperatures. Catalyst deactivation due to the deposition of carbonaceous species on the surface (catalyst time on line for a given cycle is on the order of 10–10 000 min) is conventionally countered by subjecting the catalyst to periodic regeneration. The authors developed a rotating monolith reactor, in which a cylindrical block of honeycomb monolith rotates past various feed zones subjecting the catalyst successively to propane and regenerating gas. The exothermic nature of the regeneration reactions is used at least in part to provide heat to the endothermic dehydrogenation reaction via regenerative heat transfer facilitated by the movement of the solid monolith. The catalyst exhibits very high activity and selectivity in the period shortly after regeneration. Process modeling shows the design to be feasible in terms of matching the heats of reactions and achieving high conversions, but questions were raised over its practicability from the viewpoints of mechanical design and process stability. A rotating monolith was also used by Moulijn et al. to achieve alternate contact of the monolith with gas and liquid phase in the bottom of the reactor and gas phase in the top of the reactor [105]. Another alternative to mixing of the gas and liquid phase inside the reactor is saturating the liquid with gas,

472 Part 5 Applications

liths in multiphase reactions were investigated: oxidation of cyclohexanol to adipic acid [104], hydrogenation of cinnamaldehyde to cinnamyl alcohol [110, 111], and selective oxidation of cyclohexanone [112, 113]. More details of preparation and characterization can be found in Refs. [114, 115]. Another application could be found in patents and patent applications published by Nordquist et al. of Air products and Chemicals concerning carbon coated monolith catalysts for hydrogenation in a monolithic reactor under conditions of immiscible liquid phases [116–118].

The following conclusions could be drawn. Regarding multiphase reactions, advantages of monolithic reactors could be proved, but they are still seldom used because of lack of experience, especially in larger scale processes. Compared to conventional agitated slurry reactors, the productivity of a monolithic reactor is usually much higher, while the energy costs for mixing/circulation are much lower than for a mechanically agitated slurry reactor. Heat removal from the monolithic reactor is simpler since no operation with a slurry is needed. Operation of a monolithic reactor is safer because 1) a smaller amount of hazardous materials is handled in the reaction zone, 2) the reactants can be separated immediately from the catalyst in the case of a process interrupt, and 3) filtration of a catalyst is avoided. Slurry reactors are superior to monolithic reactors when a catalyst is rapidly deactivated: replacement of the used catalyst with a fresh one is simpler and faster in this reactor [92].

5.4.3.6

Other Research Applications of Honeycomb Catalysts

Ceramic monoliths, as special type of micro/minireactors, can be applied for highthroughput screening of heterogeneous catalysts [119–120]. If a monolith material is chosen that is impermeable for gases and fluids (e.g., Cordierite 410, Inocermic GmbH, Hermsdorf, Germany), each channel of the monolith represents a single fixed-bed reactor and can contain a different catalyst material, so that up to 200 catalyst compositions can be tested in parallel (channel diameter 2.6 mm (72 cpsi), channel length 75 mm) [120]. The monolith was coated uniformly with the catalyst support material (Al2O3, SiO2, TiO2, etc.) by a wash coat procedure. It could be shown that it was possible to prepare in a reproducible manner catalytically active coatings on the wall of single channels of the monoliths by a channel-by-channel procedure. Thus, these multichannel reactors, coupled with mass spectrometry or gas chromatography, could be used for fast screening of heterogeneous catalysts. Because temporally and spatially resolved sampling is needed and complex gaseous mixtures must be analyzed, a special 3D positioning system, which allows measurements at high temperatures, is needed as the central element of the equipment. The high efficiency and reliability of the channel-by-channel preparation and the developed screening method was demonstrated for the total oxidation of hydrocarbons and carbon monoxide in the presence of further components such as O2, H2O, CO2, NO, SO2, and inert gases over precious metal catalysts.

5.4 Heterogeneously Catalyzed Processes with Porous Cellular Ceramic Monoliths 473

5.4.4

Catalytic Processes with Ceramic Foam Catalysts

While ceramic honeycomb structures with their unidirectional channel systems are well established in mobile and industrial catalytic processes (see Section 5.4.3), ceramic foam supports have been the subject of an increasing number of investigations in the last ten years. A reason for the “late discovery” of ceramic foams for catalytic applications might be seen in the unsatisfactory mechanical properties at the time when the first honeycomb structures where used in heterogeneous catalysis.

From the viewpoint of catalytic performance ceramic foams can be superior in some of the applications using honeycomb monoliths or packed-bed catalysts. While in some special cases in honeycomb structures plug flow can cause a temperature profile within a single channel, and in packed catalyst beds hot spots with an offset of up to 300 K can occur, in rigid, open-cell foam structures excellent mixing in the radial direction occurs as a consequence of the flow tortuosity, and mass and heat transfer in radial directions is excellent. Compared to catalyst pellets with an equivalent surface area, heat transfer in ceramic foams can be as much as five times higher [121]. The use of ceramic foam structures in heterogeneous catalysis was discussed with respect to advantages for the following categories of chemical processes in Ref. [122]:

. Heat-transfer-limited processes: many processes with heat-transfer limitation are carried out in long, small-diameter reactor tubes in order to achieve the desired temperature by heat exchange through the reactor walls. A significant pressure drop is the consequence. Larger voids between the pellets, realized by the use of ceramic foams, can reduce pressure drop, enhance radial heat transfer, and increase the effectiveness factor, if other parameters such as the catalytically active surface and the residence times are comparable.

. Pore-diffusion-limited processes: processes with pore-diffusion limitation are mostly carried out with small-diameter catalyst pellets which result in a high pressure drop. By using ceramic foams with a coated surface the reactants have direct access to the catalyst without or with significantly decreased inner diffusion. The consequence is a reduced pressure drop and an improved effectiveness factor.

. Processes with selectivity-control problems: when an intermediate product is desired, short residence times realized by high flow rates are necessary. This operational mode always causes a high pressure drop, and ceramic foams are the ideal candidates to overcome such problems.

The pressure drop can be reduced significantly by the use of ceramic foams rather than catalyst pellets. In fact, catalyst pellets can provide a significantly higher surface area. By modification of the monoliths by several coating procedures the surface area of foams and honeycombs can be increased drastically without changing the cellular performance of a monolith. Coating procedures, however, lead to an increase in surface roughness, which affects the pressure drop. The pressure drop increased with increasing surface area [8]. Thin homogeneous layers of catalytically