Ultrasound catalyst preparation

Use of ultrasounds in catalyst preparation leads to higher penetration of the active metal inside the pores of the support and greatly increases the metal dispersion on the support [185]. Major advances in ultrasonic technology have increased the acoustic power and sensitivity of transducers. 

Use of electromagnetic and acoustic irradiation to enhance heterogeneous catalytic reactions has been recently reviewed [83]. Alternative use of ultrasound and microwave irradiation for hydrocarbon oxidation and catalyst preparation, to improve selectivity, was also recently reported [84]. 

Table 5.4), prepared from reduction of Pd(II) salts with potassium graphite. The results suggested that this catalyst was not very active. However, some years later Jikei and Kakimoto [73] prepared a more active Pd/CGr based on a smaller crystallite size. In 2002, Kohler et al. [74] studied a variety of Pd/C catalysts with different properhes (Pd dispersion, oxidation state, water content, conditions of catalysts preparation etc.) in the Heck reaction of aryl bromides with olefins (entry 4, Table 5.4). The authors pointed out the hypothesis that the leached Pd from the support is the active species and the solid Pd/C catalyst acts as a reservoir that delivers catalytically active Pd species into solution. All catalysts were obtained by wet impregnation (5% Pd loading). The Heck reaction can also be conducted in ionic liquids through promotion by microwave irradiation. Moreover the reaction of iodobenzene with methylacrylate in NMP was reported to be accelerated by ultrasound [75]. The ionic liquid containing the catalyst system was used five consecutive times with only a slight loss of activity (entry 5, Table 5.4) [76]. Perosa [77] reported the addition of a phase transfer catalyst to an ionic liquid as a method to accelerate the C-C coupling reaction. As far as we know, only by using ionic liquids has Pd on carbon been recovered and reused with success.

Ultrasound irradiation has been used in the process of catalyst preparation. Acoustic irradiation increases the dispersion of the active metal on the support,depassivates the metal, and reduces the particle size to nanometer scale.In the case of palladium supported on active carbon prepared under ultrasound with extremely high surface area, not only was a greater metal dispersion achieved, but a larger penetration of metal inside the pores of the support and an easier elimination of chloride ion were observed as well. ... 

Ultrasound-assisted catalyst preparation has been reviewed elsewhere some recent developments in the production of catalysts that were not covered in these review are discussed here. 

Figure 1. Installation for catalyst preparation and polymerization reactions under ultrasonic irradiation (50-100 kHz) (1-Reaction vessel 2-Stirrer 3-Ultrasound bath 4-Transducer 5-Optional heating.

Table 1 - Dispersion data of Pd catalysts prepared with and without ultrasound...

Moreover, the unsonicated samples prepared from PdCk show a Dm% value much lower than the catalysts prepared in the absence of ultrasoimd with the same metal contents from Pd(N03)2 2H2O. The chlorine atoms probably bind to the metallic sites and prevent the hydrogen chemisorption on the surface, affecting the metal dispersion measurements. On the contrary, the sonicated samples show similar Dm% values as if chlorine had no effect on the H2 uptake ultrasound probably cleans the sample surface by removing chlorine. This hypothesis was confirmed by ion-chromatographic measurements (Table 2) performed on a series of Pd catalysts. 

Figure 5. Catalytic activities of precipitated manganese dioxide with and without ultrasound of high intensity. The graph shows the time to develop 10 ml of oxygen and to dissolute the catalyst. Under ultrasound influence prepared catalysts develop the oxygen generally in shorter time. They are more active.

The control of surface functionality by proper selection of the composition of the LB films and/or the self-assembling (amphiphatic) molecular systems can mimic many functions of a biologically active membrane. An informative comparison is that between inverted erythrocyte ghosts (Dinno et al., 1991 Matthews et al., 1993) and their synthetic mimics when environmental stresses are imposed on both systems. These model systems can assist in mechanistic studies to understand the functional alterations that result from ultrasound, EM fields, and UV radiation. The behavior of carrier molecules and receptor site functionality must be mimicked properly along with simulating disturbances in the proton motive force (PMF) of viable cells. Use of ion/electron transport ionomers in membrane-catalyst preparations is beneficial for programs such as electro-enzymatic synthesis and metabolic pathway emulation (Fisher et al., 2000 Chen et al., 2004). Development of new membranes used in artificial organs and advances in micelle reaction systems have resulted from these efforts. 

Unsymmetrical as well as symmetrical anhydrides are often prepared by the treatment of an acyl halide with a carboxylic acid salt. The compound C0CI2 has been used as a catalyst. If a metallic salt is used, Na , K , or Ag are the most common cations, but more often pyridine or another tertiary amine is added to the free acid and the salt thus formed is treated with the acyl halide. Mixed formic anhydrides are prepared from sodium formate and an aryl halide, by use of a solid-phase copolymer of pyridine-l-oxide. Symmetrical anhydrides can be prepared by reaction of the acyl halide with aqueous NaOH or NaHCOa under phase-transfer conditions, or with sodium bicarbonate with ultrasound. 

Figure 2 schematically presents a synthetic strategy for the preparation of the structured catalyst with ME-derived palladium nanoparticles. After the particles formation in a reverse ME [23], the hydrocarbon is evaporated and methanol is added to dissolve a surfactant and flocculate nanoparticles, which are subsequently isolated by centrifugation. Flocculated nanoparticles are redispersed in water by ultrasound giving macroscopically homogeneous solution. This can be used for the incipient wetness impregnation of the support. By varying a water-to-surfactant ratio in the initial ME, catalysts with size-controlled monodispersed nanoparticles may be obtained. 

The original patent uses platinum as the catalyst and calls for temperatures of 100-300° and pressures of 45-115 psi(47). We found that such rigorous conditions are not required for the hydrosilation reaction with most commercial sources of platinum on carbon. Usually vigorous stirring at slightly elevated temperatures, 40-80°, at 15 psi will give moderate yields of the product. The rates and yields are usually highly dependent on the method of preparation of the catalyst. However, ultrasound permits the reaction to occur at a useful rate at 30° at atmospheric pressure(48) ... 

In a more recent study, Wang and coworkers have discussed microwave-assisted Suzuki couplings employing a reusable polymer-supported palladium complex [141]. The supported catalyst was prepared from commercial Merrifield polystyrene resin under ultrasound Bonification. In a typical procedure for biaryl synthesis, 1 mmol of the requisite aryl bromide together with 1.1 equivalents of the phenyl-boronic acid, 2.5 equivalents of potassium carbonate, and 10 mg of the polystyrene-... 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

The ease of oxidahon of the surface iron carbonyl species has been shown in the preparation of Fe/MCM-41 catalysts. A method of preparation with ultrasound that led to subcarbonyl confined species in the case of Cr, Mn or Co catalysts rendered Fe203 when Fe2(CO)9 was used as precursor [23]. 

Acyl cyanides1708 can be prepared by treatment of acyl halides with copper cyanide. The mechanism is not known and might be free-radical or nucleophilic substitution. The reaction has also been accomplished with thallium(I) cyanide,1709 with MejSiCN and an SnCl4 catalyst,1710 and with Bu3SnCN,1711 but these reagents are successful only when R = aryl or tertiary alkyl. KCN has also been used, along with ultrasound,1712 as has NaCN with phase transfer catalysts.1713 OS III, 119. 

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