Active site counting method

Landis and coworkers [140] have developed an active-site counting method based on H-labelling, for the metallocene-catalyzed alkene polymerization. After quenching the reaction by addition of methanol, the polymer is analyzed by NMR, which allows the quantification of Zr-alkyl sites. A typical NMR of quenched polymer is shown in Scheme 1.7 (label is found at terminal positions only). This technique has been applied to the polymerization of 1-hexene catalyzed by [Zr(rac-C2H4(l-indenyl)2)Me][MeB(QF5)3], 91. As shown in Scheme 1.7, there are two possible approaches ... 

Scheme 1.7 Active-site counting method based on H-labelling, in the zirconocene-catalyzed polymerization of 1-hexene. Lower left typical NMR of the quenched polymer according to method A. The integrals allow the quantification of the Zr-alkyl active sites. All labels are found in the terminal position. Lower right comparison of fractional active-site counts using Method A (open circles, o) or Method B (diamonds ).

The plot shown in Scheme 1.7 shows that both labeling methods yield similar active-site counts. From this observation it can be concluded that the catalyst 91 does not deactivate during the time scale of the experiments. 

The value of the turnover frequency can be reproduced in different laboratories, if the method of measurement of the rate and the counting of sites are kept the same. Moreover, the use of turnover frequency allows the comparison between two catalysts that differ in metal or size for a specific reaction. The great advantage of such a comparison is that the activity of different catalysts is compared at active site level without the considerations of catalyst arrangement. To be more specific, using turnover frequency, we can compare the activity of the pure active site, ignoring the specific area of the catalyst. 

It is emphasized that each method of measuring active-site density has limitations, and some common techniques are actually invalid. For example, counting the number of chains formed provides no useful information, because Cr/silica is not a living system under industrial conditions and therefore each site produces many chains each second. Nor can kinetics be used in a stand-alone way, because the measured activity is always the product of the number of sites times the rate per site, both of which are unknown. Poisoning experiments give an upper bound. Below is a discussion in more detail of those techniques that the author believes provide most insight into the commercial catalyst. 

How can a theoretical method decide between proposed mechanisms, and how can the origin of the enzymatic power be identified This review will try to answer these questions for one particular theoretical approach, the one where an active site model is treated by accurate quantum mechanical (QM) methods. The main idea in the QM active site approach is to make sure that the computational results have the required accuracy. During the last decade the accuracy of density functional methods (DFT) has been dramatically improved, and in particular the hybrid B3LYP functional has achieved a remarkable accuracy [8, 9]. The use of DFT has also made it possible to treat dramatically larger molecular systems than can be done with conventional wave-function methods of similar accuracy. In spite of this important development, DFT models have usually been limited to 50-60 atoms, but more recently systems with more than 100 atoms have been treated efficiently. Still, even 100 atoms is a very small part of the total number of 8,300 atoms in yeast ODCase, not counting hydrogens or surrounding water molecules. Thus a very severe selection has to be made when the enzyme model is set up, and an important task is to select the residues required to solve the mechanism and to analyze all important contributions. 

In the case of irreversible inhibition, if the reaction is catalytic, its rate will reach zero value when all the sites responsible for catalytic action have been destroyed by the inhibitor or poison. The addition of known successive doses of poison then provides a method of counting the number of active sites on the enzyme or solid catalyst, provided that each molecule of poison destroys a constant number of sites. But in the case of a chain reaction, if the irreversible inhibitor is present in the reacting mixture from the very start, the reaction will be severely retarded until the inhibitor has all reacted away with the active centers produced in the initiation step. Thus during an induction period, the inhibitor disappears at a rate which provides a very useful measure of the rate of initiation. 

Deutscher, R.L. and Fletcher, S. (1988) Nucleation on active sites part IV. Invention of an electronic method of counting the number of crystals as a function of time and the discovery of nucleation rate dispersion. Journal of Eiectroanaiyticai Chemistry, 239, 17—54. 

In summary, there are readily available and easy to use XRD and chemisorption techniques that are applicable to metal catalysts, in particular, and to other catalysts, in general. If available, TEM can add additional information. Chemisorption is the most sensitive method, and all kinetic studies of metal catalysts should be accompanied by a measurement of the metal surface area and dispersion via a standard adsorption procedure. For non-metallic catalysts, adsorption sites can still be counted in many situations by finding the appropriate region of temperature and pressure to measure adsorption of one of the reactants, and some (or all) of these sites would be expected to be active sites under reaction conditions. Examples of such efforts have been reported for N2O, NO and O2 adsorption on Mn203 and... 

In principle, this chemisorption method should enable the investigator to count surface sites that are catalytically active. In practice, this does not appear to be the case for most amines. Even in the case of a highly... 

Several techniques to measure air concentrations are outlined by Breslin (1980). Most of the techniques for measuring radon use the fact that both radon-222 and the short-lived daughters are alpha- emitting nuclides. The sample is collected and taken back to the laboratory for "alpha-counting" or an alpha-detector is taken to the field for on-site measurement. There are several ways to measure alpha decay. A scintillation flask is one of the oldest and most commonly used methods. The flask is equipped with valves which are lined with a phosphor (silver-activated zinc sulfide) and emit light flashes when bombarded with alpha particles. Other methods draw the air through a filter (or filters) for a variety of time intervals and then count the number of alpha-decays occurring on the filter. EPA (1986) and NCRP (1988) reports provide more in-depth discussions of these methods. 

There have been attempts to count active centers ever since their existence was postulated in 1925. Normally site densities, where the site density is the number of active centers per unit rea< are thought to be near the maximum value, 10 cm , but in some cases values which are several orders of magnitude smaller have been suggested. A direct method of determining site density is one which depends on results of kinetic studies. Several direct methods, including one using transition state theory, are described results are presented. Many indirect methods, along with results, are also discussed. 

An indirect method is one in which it is assumed that the group of active centers is that group of surface sites which has a certain characteristic, say, the ability to adsorb a certain kind of molecule. Thus, counting those sites is the same as determining the site density. But as long ago as 1929 Taylor ( ) stated that sites on the same surface could qualitatively or quantitatively differ with respect to catalytic activity. Sometimes one is justified in assuming that a certain set of sites, such as all metal atoms or all sites which adsorb a certain poison molecule, is the same as the set of sites which catalyzes a reaction. But unfortunately this assumption is not always justified. 

Other indirect methods, those not involving poisoning or reactant chemisorption, have been used to determine site densities. Perhaps the most common of these methods is to count the surface metal atoms, with either pure metal crystals or metal supported on oxides, with the counting carried out by determination of the amount of O2 or CO2 which chemisorbs ( ) the assumption has then sometimes been made that all such atoms are catalytically active. 

The method indicated in this section works equally well for ions and radicals, ionic or not. In the case of negative ions, the excess electrons must be specified as well, since electrons are distinct from all elements similarly, in the case of positive ions, the deficit in electrons must be indicated. This will ensure that charge is always properly preserved. When different isotopes of an element occur, they must be counted separately along with the different components they occur in. Sometimes unconventional elements such as active catalyst sites are used. Sometimes subcomponents can be used as pseudoelements (water in hydrates, building blocks of polymers, amino acids). 

Background reduction by removal of the sources of radionuclides within the materials of the detector and surroundings and siting the detector in a location of low background are referred to as passive methods. Methods that deduce which detector counts can be identified as originating from background and prevent them being recorded by the MCA system are termed active methods. These will be discussed in Section 13.5. 

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