Activity of solids

Catalytic activity of solid acids in hydrocarbon conversions is often correlated with their acidity. Problems arise from the difficulty to bridge the gap between the equilibrium thermodynamic concept of acidity and the composite kinetic concept of catalytic activity [1], The correlation is meaningful if connected parameters are related to each other, namely, intrinsic activities are correlated with intrinsic acidities or relationship is established between corresponding apparent parameters. 

Microwaves are frequently used in the laboratory by synthetic organic chemists for regeneration and activation of solids such as molecular sieves, silica gel or alumina when fast and complete drying is required. 

As compared to conventional petrochemicals, the significant hindrance of carbohydrates induces many diffusional limitations and activity of solid catalysts is obviously strictly governed by the accessibility of the catalytic sites. In this context, the porosity of commonly used siliceous-based catalysts or metal oxides is not crucial since, because of the steric hindrance of carbohydrates, the catalytic reaction mainly takes place on the catalyst surface. In the case of organic polymers, utilization of flexible polymeric chains considerably improves the accessibility of the catalytic sites. 

One of the aims of research in catalysis is to identify the factors that contribute to the catalytic activity of solids. Among the various factors, a distinction is usually made between the geometric factor and the electronic factor. According to Balandin (1969), who emphasized the role of the geometric factor, activity depends on the presence of a... 

Despite various attempts, no single universal correlation between bulk properties and catalytic activity of solids has been found. It is now recognized that the geometric factor and the electronic factor cannot be separated from one another and that catalytic activity should be considered along with catalyst selectivity to arrive at an understanding of heterogeneous catalysis (Sachtler, 1981). 

Ramadan EM, Abd Ed-Gawad AH, Nouh AT. Bioavailaibility and erosive activity of solid dispersions of some non-steroidal anti-inflammatory drugs. Pharm Ind 1987 49 508-513. 

Local chemical composition from areas less than 1 nm in diameter can be measured by energy dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS). Such spectroscopic information may be presented in 2D maps showing the spatial element distribution in the specimen (13). Furthermore, information about the local density of unoccupied electron states of a specific element can be extracted from EELS data and used to estimate the oxidation state and the local coordination geometry of the excited atoms (14). In some favorable cases, electronic structure information with a resolution of about 1 eV from individual atomic columns has been attained (15,16). Recent developments of monochromators and spectrometers have brought the resolution down to 0.1 eV (17,18), and this capability may offer new opportunities to determine relationships between electronic structure information, the atomic arrangements and the catalytic activities of solids. 

Recently Schwab (70) also emphasized the important role of electron defects for catalytic activities of solid oxides. Following this concept, he tried to develop a mechanism for the decomposition of H2O2 and for the oxidation of CO using inverse spinels as catalysts. Since the mechanism of the electron disorder and conductivity in spinels is much more complicated than in simple cubic oxides, it is not surprising that a satisfactory interpretation of these catalytic effects is still lacking,... 

If we neglect the (unit) activities of solid metals and approximate the ion activities by molarities [cf. (8.22a)],... 

Fig. 45. Catalytic activities of solid acids for reaction of liquids Alkylation of 1,3,5-tri-methylbenzene with cyclohexene (373 K) alkylation of phenol with 1-dodecene rearrangement of benzopinacol. (From Ref. 249.)...

Underwood PA, Steele JG, Dalton BA (1993) Effects of polystyrene surface-chemistry on the biological-activity of solid-phase fibronectin and vitronectin, analyzed with monoclonal-antibodies. J Cell Sci 104 793-803... 

Activation of solid catalysts under well-specified conditions is a key step for obtaining the desired catalytic performance. It is particularly the case with zeolites, which are hygroscopic solids and for which the efficiency can be significantly reduced by the presence of water (e.g. change in the characteristics of the protonic acid sites, loss of reactant by hydrolysis). Polar organic molecules (even present in low amounts in the atmosphere of the chemical laboratories) can also be rapidly and strongly adsorbed over zeolites causing a decrease of their catalytic efficiency. Pretreatment of the zeolite in the reactor is preferable. This in situ pretreatment is easier to carry out in fixed bed than in batch reactors. 

The term mechanical activation should only be used for processes where a solid is pretreated prior to a subsequent reaction. The mechanical activation of solids is often used on an industrial scale to effect the calcination times of ceramics, or the solubility of ores. For a review of hydrometallurgical applications, see Ref. [38],... 

The activity of solid potassium carbonate can be enhanced in presence of quaternary ammonium PTCs627. An additional advantage is the fact that solid carbonates do not hydrolyze esters and hence can be used with CC14 to chlorinate malonates and similar compounds. 

The activity of solid catalysts is usually proportional to the active surface area per unit volume of catalyst A high activity per unit volume consequently calls for (extremely) small particles Since most active species... 

Table 4. Activities of solid acid catalysts for acylation.

Subtract the blank absorbance from the sample absorbance (the difference should be between 0.100 and 1.000). Determine the acid phosphatase activity (HFU/mL) from the standard curve, and multiply by the dilution factor. For the activity of solid samples, use the following equation ... 

Enhancing the Activity of Solid Acid Catalysts with Supercritical Reaction Media Experiments and Theory ... 

Shortcomings of the method are the formation of large amounts of organic wastes and high temperatures of synthesis. The application of mechanical activation of solids allows to avoid liquid wastes during the synthesis and to decrease the temperature. 

Table 11 Catalytic activities of solid and liquid acids for hydrolysis of 2-methylpbenyl acetate...

Reactions at gas/solid interfaces may lead to sorption of radionuelides from the gas phase or they may eause loss of activity of solid samples by interaction with the gas phase. For example, Ba " C03 exhibits loss of " C02 to moist air due to the isotope exchange C03"(s)/ C02(g) in the presence of water vapour. The specific activity of solid samples may be reduced appreciably by such reactions. 

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