Solid-state catalysts

The final step of the whole reaction process is the desorption of the products. This step is essential not only for the practical purpose of collecting and storing the desired output, but also for the regeneration of the catalytic active sites of the surface. Most reactions have at least one rate-hmiting step, which frequently makes the reaction prohibitively slow for practical purposes when, e.g., it is intended for homogeneous (gas or fluid) media. The role of a good solid-state catalyst is to obtain an acceptable... 

Figure 1.6. Common features of Heterogeneous Catalysis, Fuel Cell operation, Electrolysis and Electrochemical Promotion 1. Solid state catalyst, 2. Adsorption, 3. AG < 0, 4. Yield control via DC current or voltage application (Adapted from N. A. Anastasijevic).

With the use of a solid-state catalyst, hydrogen can be added to carbon-carbon double bonds in a hydrogenation reaction ... 

Senkan, S.M. (1998) High-throughput screening of solid-state catalyst libraries. Nature, 394, 350. 

Keywords porous materials, silicoaluminophosphates, solid-state catalysts, structuredirecting agents... 

K. Soai, M. Watanabe, Chiral Quaternary Ammonium Salts as Solid State Catalysts for the Enantioselective Addition of Diethylzinc to Aldehydes , J. Chem Soc, Chern. Commun 1990, 43-44. 

Especially for inorganic functional materials, it is evident that for the majority of cases the situation is completely different. Even though today a full range of analytical methods is at hand, it is impossible to fully characterize a solid-state material. This does not even take into account that catalysis (with the help of solid-state catalysts) is an interface phenomenon and traces of impurities very... 

Third, many solid state catalysts offered several active polymerization sites due to differences in the precise structure at and about the active sites. This resulted in an average stereoregular product being formed. 

The new soluble catalysts offer a solution to these three problems. First, the smaller size of the active site, and associated molecules, allows the growing chains to take advantage of a natural tendency for the growing polymer chain to form a regular helical structure (in comparison to polymers formed from solid state catalysts). 

Second, the solution catalysts allow the synthesis of polymers that contain little or no catalytic agents, allowing the elimination of the typical additional clean-up steps necessary for polymers produced from solid state catalysts. 

The summation of these affects is the production of polymers with increased strength and tensile properties. For PE the use of these soluble catalysts allows the synthesis of PE chains with less branching compared to those produced using solid state catalysts such as the ZNCs. PE produced employing soluble catalysts also shows increased properties in comparison with PE produced by solid catalysts. Table 5.2 gives some comparisons of the PEs produced using the ZNCs with those produced with soluble catalysts. 

What are the difficulties associated with the use of solid state catalysts in the production of polymers ... 

Sulfonated EPDMs are formulated to form a number of rubbery products including adhesives for footwear, garden hoses, and in the formation of calendered sheets. Perfluori-nated ionomers marketed as Nation (DuPont) are used for membrane applications including chemical-processing separations, spent-acid regeneration, electrochemical fuel cells, ion-selective separations, electrodialysis, and in the production of chlorine. It is also employed as a solid -state catalyst in chemical synthesis and processing. lonomers are also used in blends with other polymers. 

During the last two decades it has been found that there is a special group of chemical reactions, essentially redox reactions, for which the catalytic influence of solids can be interpreted in terms of the catalyst s electronic structure and the controlled variations of that structure. The study of single-phase catalysts and the relationship between function and electronic structure of solid state catalysts show that redox reactions may be divided into two classes. Donor reactions are reactions in which the rate-determining step involves an electron transition from the reactant molecule to the catalyst acceptor reactions are those where the reactant must accept electrons from the catalyst in order to form the activated state. Broadly speaking, donor reactions mobilize reducing agents like... 

The activity of the particles in unsaturated positions have also a purely physical and crystallographic effect. At places with high activity the recrystallization that usually is inadvantageous for catalytic processes occurs at lower temperatures or at constant temperatures with higher velocity than at less active places on the surface of the solid-state catalyst. 

Recently, a profound interest in studies of properties of granulated metals, structures constituted by metallic nanoparticles, has been aroused. Problems associated with the application of these structures in the development of new nanoelectronic devices [1], devices for ultrahigh-density magnetic recording [2], new functional coatings [3], and high-efficiency solid-state catalysts [4] are widely discussed in the literature. This chapter is concerned with catalytic properties of metallic nanostructures. 

Further projects dealt with the fabrication of heterogeneous, basic or acidic solid-state catalysts or adsorbents carrying, for instance, amino or sulfonic acid groups. Amino-functionalized silicas were prepared and analyzed for the catalytic activity in Knoevenagel condensation reactions of aldehydes or ketones with ethyl cyanoace-tate ions by Macquarrie et al.154 155 Recently, Zhang et al.156,157 reported on the successful preparation of amino-functionalized silica thin films by means of the EISA approach. 

Geoffrey Coates, Cornell University Using a solid-state catalyst to incorporate carbon dioxide into polycarbonates. 

Titanium-based solid-state catalysts for the industrial production of polyolefin materials were discovered in the early 1950 s and have been continually improved since then (see Section 7.3). Due to the high degree to which they have been perfected for the production of large-volume polyolefin commodities, they continue to dominate the processes presently used for polyolefin production. Despite (or because of) this product-oriented perfection, only limited degrees of variability with regard to some relevant polymer properties appear to be inherent in these solid-state catalysts. 

Due to the use of advanced, highly active and selective solid-state catalysts (sections 7.3), the processes described above produce polymers from which neither stereoirregular polymer components nor catalyst residues need be removed. This has resulted in substantial reductions in the costs of investments, energy and maintenance, compared to slurry processes with first-generation catalysts. Ongoing developments are aimed at increased process flexibility and at process adaptation to the use of supported metallocene catalysts (Section 7.4). 

Polymerization catalysis with soluble complexes of group IV transition metals, in particular with hydrocarbon-soluble titanocene complexes, was discovered in the 1950 s, shortly after the appearance of Ziegler s and Natta s reports on solid-state catalysts, and rather thoroughly studied from then on. Alkylalu-minium compounds, such as AlEt2Cl, are required to activate also these soluble catalysts. In distinction to their solid-state counterparts, however, early soluble catalysts were able to polymerize only ethylene, and not any of its higher homologues. After their activation by methylalumoxanes had been discovered (Section 7.4.1), soluble catalysts became as efficient as solid-state catalysts - in... 

Due to this chain-migration process ethylene is polymerized to macromolecules containing multiple branches - rather than to the linearly enchained polymer obtained with classical solid-state catalysts. In propylene polymerization with these catalysts 1,2-insertions give the normal methyl-substituted polymer chains, but after each 2,1-insertion the metal centre is blocked by the bulky secondary alkyl unit and can apparently not insert a further propylene. Instead the metal must then first migrate to the terminal, primary C atom before chain growth can continue by further propylene insertions. By this process, also called 1,CO-enchainment or polymer straightening, some of the methyl or (in the case of higher olefins) alkyl substituents are incorporated into the chain. 

Fig. 26. Design of the MAS rotor (a) and the turbine (b) modified for in-situ MAS NMR investigations of solid-state catalysts (Reproduced by permission of The Royal Society of Chemistry, Cambridge).

The special properties of surfaces also account for the action of solid-state catalysts. The action of a catalyst often depends on its ability to hold molecules in a particular orientation that will facilitate the reaction. In the gas or liquid phase, a collision at just the right orientation is a function of chance. If the target molecule is held in a favorable orientation on a surface, the chances of a productive encounter are improved. 

Enzymes are capable of the kind of selectivity and rate enhancements discussed above because their active sites exhibit a number of distinctive features compared to the active sites employed by soluble transition metal complexes and solid state catalysts multi-point contact with the substrate, which is very hard to engineer in a synthetic catalyst the structural flexibility to undergo collective and rapid changes in structure to facilitate catalysis of a reaction and a unique ability to combine apparently incompatible features in catalysis, such as simultaneous acid and base catalysis and hydrophobic/hydrophilic interactions [62,63]. These points are discussed in more detail in the following sections. 

To minimize the indane, 99, formation, dimerization was conducted in two-phase systems containing toluenesulfonic acid,354 sulfuric acid,355 356 electrophilic transition-metal complexes,357 the polymeric solid-state acid Nafion,358 359 metal oxide solid-state catalysts such as tungstophosphoric acid,360 various zeolites,361 362 mixed oxides,363 and montmorillonite clay in the presence of organic solvents.364 365 The major limitation of the cationic approach, however, is the unavoidable formation of internal isomer 100. Since isomer 100 is inert in radical polymerization, the lower the content of isomer 100, the higher activity of the 98 mixture. Even in the very best cases, its presence is never less than 5—15%. 

A 10-mL aliquot of the catalyst solution was diluted with 100 ml of freshly distilled toluene in a beaker. The solution was stirred for five minutes, and then the membrane sample was placed in the solution. The catalyst solution immediately penetrated the microporous structure. The sample was allowed to soak for about five minutes and then was allowed to dry in the box under blowing nitrogen for 10-15 minutes, giving a membrane with the solid state catalyst components deposited within the pore structure of the polypropylene. Weight of the sample plus the deposited catalyst was 0.8872 grams, or a wt % gain of 4.7%. The sample was placed in an empty Schlenk tube and removed from the dry box. 

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