Probe molecules Subject

As Fig. 7.58 indicates, our task was to explain the ordering of the eight probe molecules in the human in vivo target, but subjecting the eight probe molecules to each of the 50 PAMPA lipid models. For each PAMPA model, the regression correlation coefficient, r2, was used to assess the appropriateness of the model. 

Ammonia TPD is very simple and versatile. The use of propylamine as a probe molecule is starting to gain some popularity since it decomposes at the acid site to form ammonia and propene directly. This eliminates issues with surface adsorption observed with ammonia. The conversion of the TPD data into acid strength distribution can be influenced by the heating rate and can be subjective based on the selection of desorption temperatures for categorizing acid strength. Since basic molecules can adsorb on both Bronsted and Lewis acid sites, the TPD data may not necessarily be relevant for the specific catalytic reaction of interest because of the inability to distinguish between Bronsted and Lewis acid sites. 

Finally, dynamic structure-based pharmacophore models can be derived through a method first described by Carlson et al that uses multiple conformations of the target protein, which are obtained either by molecular dynamics simulation or by the use of multiple experimentally determined conformations. The binding sites of the respective snapshots are flooded with small molecular probes (e.g., methanol for hydrogen-bond interactions and benzene for aromatic hydrophobic interactions) and while the protein structure is held rigid the probe molecules are subjected to a low-temperature Monte Carlo minimization where they undergo multiple, simultaneous gas-phase... 

In adsorption microcalorimetry, surface equilibration depends not only on the chosen probe molecule but also on the adsorption temperature. It is worth mentioning that the literature contains some controversial articles on this subject [29]. 

Br0nsted acidity of zeolite protons is essential for catalytic reactions such as isomerization and cracking and has been studied extensively 15,264). Several characterization methods for acid sites in zeolites have been developed this subject has been covered in recent reviews (265,266). Pyridine and other basic molecules are often used in IR work as probe molecules for Brpnsted and Lewis acid sites (267). Trimethylphosphine has also been used as a probe for the determination of zeolite acidity by IR or NMR (96,268). 

Though, at first sight, lower accessibility of water or protic solvents to the ejected proton may imply a slower deprotonation rate, the actual situation depends on the probe used, The effect of different solvent mixtures on the ESPT process of various probe molecules has been the subject of several studies [35-... 

The process of adsorption and interaction of probe molecules such as ammonia, carbon monoxide as well as the whole spectrum of organic reactant molecules with zeolite catalysts has been the subject of numerous experimental and computational studies. These interaction processes are studied using several computational methods involving force fields (Monte Carlo, molecular dynamics emd energy minimization) or quantum chemical methods. Another paper [1] discusses the application of force field methods for studying several problems in zeolite chemistry. Theoretical quantum chemical studies on cluster models of zeolites help us to understand the electronic and catalytic properties of zeolite catalysts. Here we present a brief summary of the application of quantum chemical methods to understand the structure and reactivity of zeolites. 

In the determination of acidity by microcalorimetry, several factors play an important role, such as the adsorption temperature, the pretreatment temperature and the choice of the probe molecule. O er factors, more specific to zeolites, are the topology, the Si/Al ratio, the chemical composition, and the modifications to which the samples have been subjected. The shape of the differential heat curves versus coverage demonstrates quite explicitly that the number, the reactivity and the distribution of surface sites are significantly modified when the composition, or the pretreatment, of the samples are changed. However, the two main factors are the zeolite structure and the fi amework aluminum content. Most of the studies described herein are summarized as reference tables in a review by Cardona-Martinez et al. [6] or fiilly detailed in [4]. 

These methods suffer from the lack of complementarity, and thus the significance of results provided by any of them is limited. A standard practice to detect the Bronsted or Lewis character of surface sites is pyridine adsorption combined with FTIR measurements the number of Lewis or Bronsted sites is more difficult to count, however. Other titration methods use either color indicators and acid or base titrants in nonpolar solvents or the adsorption of gaseous acidic or basic probes. They do not, in general, give consistent quantitative information about the number of acid or base sites even when applied to the same sample. There are several reasons the applicability of titration methods is limited Either the state of the surface is different for different methods or adsorption equilibrium is not always achieved. Another more serious source of discrepancies between titration methods is that probe molecules of different basicities "see" different surface sites. The lack of a uniquely defined thermodynamic scale of acid strength of surface sites makes difficult any correlation between results obtained with different probe molecules. The use of standard catalytic tests for probing the so-called catalytic acidity is not always a better approach, because the mechanistic assumptions involved are neither straightforward nor subject to experimental proof. 

The methods discussed so far are particularly qualified for active site studies by offering the potential for in situ work—even in electron microscopy, an environmental version in which the sample is kept under some millibar of reactant pressure is available nowadays. Valuable insight into the structure of surfaces can be, however, also obtained by methods targeting adsorptive interactions of reactants or other probe molecules with the surface although these are usually performed in separate experiments, e.g., on catalysts previously subjected to reaction conditions. 

The adsorption properties of the Pt/Vulcan XC72 catalyst have been evaluated by means of different procedures using H2 and CO as probe molecules. The influence of the support surface groups on the adsorption processes has been analyzed on the catalyst when subjected to different thermal treatments. To enlighten the participation of the surface groups of the support in the adsorptive properties of the catalyst, exchange experiments with isotopically labeled molecules have been performed. 

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