Catalysis monolithic

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

Control of emissions of CO, VOC, and NOj, is high on the agenda. Heterogeneous catalysis plays a key role and in most cases structured reactors, in particular monoliths, outperform packed beds because of (i) low pressure drop, (ii) flexibility in design for fast reactions, that is, thin catalytic layers with large geometric surface area are optimal, and (iii) attrition resistance [17]. For power plants the large flow... 

The next level is that of shaped catalysts, in the form of extrudates, spheres, or monoliths on length scales varying from millimeters to centimeters, and occasionally even larger. Such matters are to a large extent the province of materials science. Typical issues of interest are porosity, strength, and attrition resistance such that catalysts are able to survive the conditions inside industrial reactors. This area of catalysis is mainly (though not exclusively) dealt with by industry, in particular by catalyst manufacturers. Consequently, much of the knowledge is covered by patents. 

Structured catalysts, including monoliths, are very promising as far as pressure drop and high performance for selective reactions are concerned. The perspectives for the use of monolithic catalysts in heterogeneous catalysis have been analysed by Cybulski and Moulijn (1994) and are further discus.sed in Section 5.4.7.7. 

Concerning the hydrodynamics and the dimensioning of the test reactor, some rules of thumb are a valuable aid for the experimentalist. It is important that the reactor is operated under plug-flow conditions in order to avoid axial dispersion and diffusion limitation phenomena. Again, it has to be made clear that in many cases testing of monolithic bodies such as metal gauzes, foam ceramics, or monoliths used for environmental catalysis, often needs to be performed in the laminar flow regime. 

Although the history of rigid monolithic polymers is relatively short, a number of applications have already been explored. These applications cover a rather broad range of fields from heterogeneous catalysis and solid-phase extraction, to polymer-supported chemistry and a variety of separation processes. 

Because the monoliths allow total convection of the mobile phase through their pores, the overall mass transfer is dramatically accelerated compared to conventional porous structures. Based on the morphology and porous properties of the molded monoliths, which allow fast flow of substrate solutions, it can be safely anticipated that they would also provide outstanding supports for immobilization of biocatalysts, thus extending the original concept of monolithic materials to the area of catalysis. 

Fig. 8.2 Representative microstructures of monoliths used in heterogeneous catalysis.

Applications of Functionalized Metathesis-based Monoliths in Catalysis... 

These highly porous glasses retain a rigid and exposed interfacial surface area (typically 300-1000 m g ), whereas conventional organic polymer beads swell and shrink in different solvents, often with unpredictable effects on catalysis Functionalization of a monolithic (largest dimension 1 mm) gel affords a bulk catalyst sample. This obviates the need for filtration to recover the catalyst tweezers can be used instead ... 

Deleuze et al. used the same approach for the synthesis and functionalization of emulsion-derived polymeric foams [119]. Alternatively, a post-synthesis grafting method recently developed in our group offers access to high-capacity functionalized monolithic systems. Such high capacity monoliths are vital to various applications such as catalysis, extraction of environmental contaminants, extraction of biochemicals for either pharmaceutical or clinical purposes or, more generally, separation techniques [100]. With these systems, amounts of grafted monomers can exceed 1 mmol/g [94]. 

These reactions are responsive to both acid and base catalysis, and can be manipulated to give a variety of silica products, e.g., discrete particles, monolithic gels, films, and fibers. This technique of materials synthesis via alkoxide hydrolysis has become known as sol-gel processing (17). It should be noted, however, that under certain conditions, gelation may be confined only to the interior of discrete particles (base-catalyzed systems), while the sol may consist of polymeric networks rather than individual particles (acid-catalyzed systems). 

L.L. Raja, R J. Kee, O. Deutschmann, J. Wamatz, and LD. Schmidt. A Critical Evaluation of Navier-Stokes, Boundary-Layer, and Plug-Flow Models of the Flow and Chemistry in a Catalytic-Combustion Monolith. Catalysis Today, 59 47-60,2000. 

Buchmeiser, M.R., Lubbad, S., Mayr, M. and Wurst, K., Access to silica- and monolithic polymer supported C-C-coupling catalysts via ROMP applications in high-throughput screening, reactor technology and biphasic catalysis, Inorg. Chim. Acta, 2003, 345, 145. 

M. Marek, Evolution of spatio-temporal temperature patterns in monolithic catalytic reactor. Catalysis Today, 70 (2001) 393-409. 

Converters for cars are usually ceramic monoliths and occasionally metal based. Without much exaggeration, they can be claimed to be one of the major successes of recent decades in the area of chemical engineering and catalysis. In the beginning, the catalytic converter was placed underbody, where sufficient space was available and where the temperature was expected to be mild. There was no need... 

In addition to dense monolithic ceramics, porous silicon nitrides are gaining more importance in technological applications [24], Some porous silicon nitrides with high specific surface area have already been applied as catalysis supports, hot gas filters and biomaterials [25], There is an emerging tendency to facilitate silicon nitride as biomaterial, because of specific mechanical properties that are important for medical applications [25], Moreover, in a recent study it was shown that silicon nitride is a non-toxic, biocompatible ceramic which has the ability to propagate human bone cells in vitro [25], Bioglass and silicon nitride composites have already been realized to combine... 

Then, a survey of micro reactors for heterogeneous catalyst screening introduces the technological methods used for screening. The description of microstructured reactors will be supplemented by other, conventional small-scale equipment such as mini-batch and fixed-bed reactors and small monoliths. For each of these reactors, exemplary applications will be given in order to demonstrate the properties of small-scale operation. Among a number of examples, methane oxidation as a sample reaction will be considered in detail. In a detailed case study, some intrinsic theoretical aspects of micro devices are discussed with respect to reactor design and experimental evaluation under the transient mode of reactor operation. It will be shown that, as soon as fluid dynamic information is added to the pure experimental data, more complex aspects of catalysis are derivable from overall conversion data, such as the intrinsic reaction kinetics. 

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