Catalyst encapsulation

Figure 5.1 The use of multiple catalysts encapsulated in star polymers to enable a one-pot cascade reaction containing interfering catalysts. Catalyst 11 is formed via the protonation of imidazolidinone (9) by immobilized PSTA (10), and is responsible for the nucleophilicaddition ofN-methylindole (14)...

Having already examined the use of the LbL method to make various nanocapsules, including polymer nanocapsules, and having already encountered the use of star polymers for catalyst encapsulation, we turn our attention to other methods for the formation of polymeric nanocapsules. Useful reviews of the formation of these capsules using various methods are available [78-84]. 

Figure 3.11. Schematic representation of MCM materials functionalised at the outside with diphenylsilane and with the rhodium catalyst encapsulated at the inside of the pores...

We chose to organize the discussion of catalyst encapsulation by separating examples into catalyst type as opposed to encapsulation method. As previously, we do not intend to present a comprehensive treatment of the literature and discuss only examples we consider particularly illustrative. 

New Supramolecular Approaches in Transition Metal Catalysis Template-Ligand Assisted Catalyst Encapsulation, Self-Assembled Ligands and Supramolecular Catalyst Immobilization... 

We started this templated approach to catalyst encapsulation using tris(mefa-pyridyl)phosphine and zinc(ii)TPP (TPP = tetraphenylporphyrin) [7]. From NMR and UV/Vis titration experiments we found a selective assembly process via coordination of the nitrogen to the zinc(ii)TPP, rendering the phosphine donor atom completely encapsulated by the three porphyrin components (Figure 8.4). The phosphine center is still available for coordination to transition metals, providing... 

Controlled release. For practical use the active ingredient must be released. A drug must be delivered with well defined kinetics. Sometimes it is not the encapsulated ingredient that is released but a by-product. This is the case when the encapsulated product is an enzyme or a catalyst. Encapsulation may have the objective to limit release, but in some cases to make it more rapidly available. A typical example is an instant powder consisting of aggregates made of fine particles that are insoluble, in a very soluble matrix. 

Since several supported catalytic systems have sufficient activity for the POM reaction, the main topic of research is the stability of the catalysts. There are two main processes for the deactivation of the catalyst carbon deposition and sintering of the metal. Carbon deposition is due to the process of decomposition of CH4 and CO (reactions (3) and (4)). Two different kinds of carbon can be formed on the surface of the catalyst encapsulated carbon, which covers the metal particle and is the reason for physical-chemical deactivation and whiskers of carbon, which do not deactivate the particle directly but may produce mechanical plugging of the catalytic bed. 

Transition metal catalysts encapsulated within the ligand-template nanoreactor G, P(Py)s [ZnJs, have been applied to catalyze industrially relevant processes such as hydroformylation and Heck reaction. Nanoreactor [G 3 Rh(CO)(acac)] encapsulates a Rh-species that contains only one tris(meta-pyridyl)phosphine ligand, P(m-Py)3, surrounded by three Zn-porphyrins or Zn-salphens. Under syngas pressure (H2/CO), rhodium species like Rh(CO)(acac)P(Py)3 transform into a complex of type HRh(CO)3P(Py)3, which is the active species for the hydroformylation reaction, hi this reaction terminal alkenes are converted into linear and/or branched aldehydes, and the ratio of these products strongly depends on the specific catalyst applied. Hydroformylation of 1-octene by encapsulated rhodium, [G 3 HRh(CO)3], resulted in a 10-fold rate enhancanent compared... 

Polystyrene supported versions of catalyst Id have been reported/ In addition, Frechet and coworkers demonstrated that catalyst Id can be encapsulated in the core of a soluble star polymer, which enables reagents to diffuse, allowing catalysis to take place/ Interestingly, the concurrent mixing of different chiral and achiral catalysts encapsulated in a similar manner onto different star polymers enabled one-pot multicomponent asymmetric cascade reactions of otherwise incompatible catalysts. 

Ruthenium [RuCp(PMe3)(MeCN)2][PFg] catalyst encapsulated into a water-soluble supramolecular assembly (274) promoted isomerization of allyl alcohol r1CH=CHCH(OH)R2 to R CH2CH2C0R2 with a turnover number The... 

Several groups have described the fabrication of microcapsules loaded with a catalyst. Catalysts encapsulated include palladium(II) acetate (86) and osmium tetraoxide (87). Microencapsulated catalysts are described as effective, easily isolated from a reaction system by filtration, and reusable. Shchukin and co-workers (88) recognized that chemical processes can be performed within microcapsules to produce imique products that are retained within the microcapsules. They produced crystalline WO3 nanoparticles inside microcapsules with a polyelectrolyte shell. 

Silicone 3112 RTV Low viscosity two catalyst Encapsulation/ potting Dow Coming... 

Park JN, Forman JA, Tang W, Cheng J, Hu YS, Lin H, McFarland EW (2008) Highly active and sinter-resistant Pd-nanoparticle catalysts encapsulated in silica. Small 10 1694-1697... 

Styrene oxide to 2-phenyl ethanol Pd-Cu catalyst encapsulated with poly urea 100% conversion, 100% selectivity 51... 

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