Catalysts solid-phase

Transformations of heterocycles on solid-phase catalysts 97T5643. 

In 1998, Kurt and Halm reported the preparation of resin-based bis(sulfo-namides) ligands in order to extend the precedent methodology to the solid phase. Therefore, the solid-phase catalyst depicted in Scheme 6.21 was found to be able to mediate the Simmons-Smith cyclopropanation of cinnamyl alcohol with an enantioselectivity of 65% ee. 

So far, no systematic work has been done on the use of recyclable, solid-phase catalysts in cross-coupling reactions. Most of the examples have been obtained for cross-couplings with either arylboronic acids or terminal acetylenes. It should be noted, however, that due care should be exercised when interpreting results on the cross-coupling of arylboronic acids with aryl iodides, as this extremely facile reaction can be catalyzed by practically any palladium-containing material, including trivial Pd black,481 e.g., as a sediment on the reaction vessel. Therefore, this reaction cannot serve as a reliable test for comparison between different catalytic systems. 

Three-phase reactor systems are ideally suited for methanol production because of the ability to provide intimate contact between the gaseous phase reactants and the solid phase catalysts and to remove the large amounts of heat created by the high heats of reaction. In the three-phase system, an inert liquid phase circulates between the reactor and an external... 

There are at least three reasons for attempting to prepare solid-phase catalysts that resemble enzymes. Synthetic procedures would generally be simplified. Catalytic groups are fixed on the support so that they cannot interact with one another, for example, thiols cannot deactivate by forming disulfides and metal ions cannot deactivate by forming binuclear structures. Finally, if the successful catalyst is eventually made, it will almost certainly be used in heterogeneous systems. 

Problems such as diffusional limitations and the analysis of catalyst composition occur with solid-phase catalysts. Much work has been done on diffusion in bound enzymes (for reviews, see 24 and 88). In our work we used ninhydrin, which is a reagent ideal for surface analysis amino acid analysis is used wherever possible. Amine depletion as followed by ninhydrin is not exact, but some quantitative guides are obtained. Certainly synthetic catalysts must be made with bonds other than amide bonds and components other than those compounds that are detectable on the amino acid analyzer. 

High-Loading Dendritic Solid Phase Catalysts ... 

Scheme 9. HKR of 4-hydroxy-1-butene oxide with solid phase catalyst.

A number of modified reaction conditions have been developed. One involves addition of silver salts, which activate the halide toward displacement.94 Use of sodium bicarbonate or sodium carbonate in the presence of a phase-transfer catalyst permits especially mild conditions to be used for many systems.95 Tetraalkylammonium salts often accelerate reaction.96 Solid-phase catalysts in which the palladium is complexed by polymer-bound phosphine groups have also been developed.97 Aryl chlorides are not very reactive under normal Heck reaction conditions, but reaction can be achieved by inclusion of triphenylphosphonium salts with Pd(OAc)2 or PdCl2 as the catalyst.98... 

Some interesting solid-phase catalysts were developed last year by the introduction of Co(III) or Mn(ni) ions into the precursor gels of molecular sieves before calcining. These novel complexes... 

The polymer complex, Ru(PSt-bpy) (bpy)2+, is stable under irradiation, and shows almost the same sensitizing ability in the photoreduction of MV2+ in solution 30,31). The solubility of the polymer complex, quite different from the monomeric Ru(bpy)2+ allows its use as solid phase catalyst in water as described in the next section 31). 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

The first array-based technique was designed specifically to study reactions on solid phase catalysts as IR thermography.9,19 This approach utilizes IR sensitive FPA detectors to measure the temperature of catalysts under reaction conditions. This approach has the advantages of a theoretical high thermal sensitivity, typically several tens of millikelvin, and the ability to study both endothermic and exothermic reactions. The main disadvantage of this approach, however, is the lack of chemical information. It must be assumed that the temperature change is associated entirely with the desired reaction pathway. The presence of unexpected side reactions will not be detected in this approach, as long as they have similar thermal behavior as the reaction under study. 

In the above mechanism, (g) shows that the respective species is in the gas phase and (s) in the solid phase (catalyst). We symbolise with C the concentrations in the gas phase and 6 the concentration in the solid phase, usually in units of mol g l. 

C jimlfPFe] Pd(OAc)2 Pd(PPh3)4 Et3N k2co3 100-150 °C. Supported ionic liquid phase catalysis (SILP) with silica as solid phase catalyst relatively stable for at least 5 runs product decanted and solid phase washed with hexane less than 0.24% catalyst loss. [87]... 

Monoliths are mainly produced by extrusion, although other methods are applied, in particular for the production of metal monoliths from thin corrugated sheets. The size of the channels and the wall thickness can be varied independently, and the optimal values depend on the particular application. Therefore, an optimum can be established between the amount of the solid phase (catalyst loading), the void space in the monolith, and the wall thickness. As a consequence of the extrusion process and the use of plasticizers, the channel walls are not completely dense but possess a macroscopic porosity, t)q)ically 30-40%. Thus, the thermal expansion properties can also be adjusted. 

Polymeric catalysts are also developed. For example, phospholene oxide modified divinylbenzene/styrene copolymers, as well as a polystyrene anchored triph-enylarsine oxide catalyst were prepared. The solid phase catalysts can be removed by filtration after partial conversion of an isocyanate to the carbodiimide. Such a catalyst is useful for the preparation of carbodiimide modified liquid MDl (4,4 -diisocyanatodiphenylmethane) products, which are of considerable commercial interest. 

Although infrared absorption spectrometry can be used to observe rotational spectra or even electronic spectra, we will limit ourselves here to examining molecular vibrational spectra, since these are the most frequently encountered in studying solid phase catalysts. 

Effective catalysts for preparing the polyformals were p-toluenesulfonic acid, camphorsulfonic acid, methanedisulfonic acid, and perchloric acid. Various other acidic compounds were evaluated as catalysts with tetramethylcyclobutanediol. In these experiments, 0.5 to 1.0 gram of acidic compound per mole of tetramethylcyclobutanediol was normally added. If insufficient water was obtained, more catalyst was added. If the prepolymer was obtained but an appreciable amount of brown color was present, less catalyst was then used. Compounds which did not catalyze the reaction (no water obtained) were phosphoric acid, zinc chloride, trifluoroacetic acid, and heptafluorobutyric acid. Incomplete reactions (insufficient water) took place with concentrated hydrochloric acid, concentrated nitric acid, zinc fluoroborate, or Amberlite IRC-50 ion exchange resin as catalyst. A prepolymer was obtained when boron trifluoride etherate was used, but buildup did not take place in the solid phase (catalyst probably too volatile). Brown or speckled-brown polymers (after solid-phase buildup) were obtained with catalysts containing sulfonic acid groups (benzenesulfonic, dodecylbenzenesulfonic, sulfo-acetic, methanetrisulfonic, sulfuric, p-toluenesulfonic, camphorsulfonic, and methanedisulfonic acids). To obtain white polymers from tetramethylcyclobutanediol it was necessary to treat the solvent and prepolymer reaction mixture as previously described. (White polyformals were obtained from the other diols without this treatment.)... 

Those inunobilization procedures generally yielding supported solid-phase catalysts (SSPCs) have already been described in the preceding sections this section deals with catalytically active species (e. g., Wilkinson s or Vaska s complex) that are dissolved in liquids (therefore the catalyst is exactly the same as in homogeneous catalysis) or are even liquids themselves supported on porous solids. The principal structure of such an SLPC and the principal difference from an SAPC (Supported Aqueous-Phase Catalyst) is shown in Figure 2. 

Comely, A. C., Gibson, S. E., Hales, N. J. Polymer-supported cobalt carbonyl complexes as novel solid-phase catalysts of the Pauson-Khand reaction. Chem. Common. 2000, 305-306. 

Heterogeneous reduction processes still involve the reaction of gases, but in these cases the reaction occurs in the presence of a suitable solid phase catalyst. Sulfur dioxide may be reduced to sulfur with hydrogen sulfide, if this is available, and the sulfur vapor condensed out of the gas stream by cooling, as in the second half of the Claus process (Eq. 3.17). 

In some enzymatic reactions and, particnlarly, in those involving lipases, solvent is not necessary (i.e., the snbstrates and prodncts perform as a solvent). However, in cases where glycerides or fatty acyl snbstrates are present at significant concentration, the temperatnre mnst be elevated (approximately 50 to 80°C) in order to rednce the medinm viscosity to allow effective dispersion of a solid-phase catalyst. 

The synthesis of ammonia, an important industrial product, is facilitated by a solid phase catalyst (the Haber process). H2 and Nj bind to the surface, their bonds are weakened, dissociation and reformation as ammonia occur, and the newly formed ammonia molecules leave the surface. This process is repeated over and over, with no change in the catalyst. 

A catalyst has no effect on the equilibrium composition. A catalyst increases the rates of both forward and reverse reactions to the same extent. The equilibrium composition and equilibrium concentration do not change when a catalyst is used, but the equilibrium composition is achieved in a shorter time. The role of a solid-phase catalyst in the synthesis of ammonia is shown in Figure 8.13. 

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