Selection of Probe Molecules

Essentially, any molecule could potentially be used as a probe molecule. However, practically, a useful probe molecule should meet certain minimum criteria. Such criteria for the selection of an ideal probe molecule were first developed by Paukshtis and Yurchenko [104]. These have been refined by several research groups and summarized by Knozinger and Huber [86]. The criteria are summarized in Table 4.3. Based on these criteria, small, weakly interacting probe molecules are recommended for zeolites particularly when you want to probe sites within the pore structure. 

As mentioned above, many different probe molecules have been used to measure the acidity of zeolites. These molecules vary over a wide range of proton affinities, size (kinetic diameter) and shape. Table 4.4 lists these properties for several of the more commonly used probes. 

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Table 4.1. Conceptional criteria for the selection of probe molecule to characterize solid acids (from ref. [31])...

In view of catalytic potential applications, there is a need for a convenient means of characterization of the porosity of new catalyst materials in order to quickly target the potential industrial catalytic applications of the studied catalysts. The use of model test reactions is a characterization tool of first choice, since this method has been very successful with zeolites where it precisely reflects shape-selectivity effects imposed by the porous structure of tested materials. Adsorption of probe molecules is another attractive approach. Both types of approaches will be presented in this work. The methodology developed in this work on zeolites Beta, USY and silica-alumina may be appropriate for determination of accessible mesoporosity in other types of dealuminated zeolites as well as in hierarchical materials presenting combinations of various types of pores. 

The determination of acidity in FCCs from adsorption microcalorimetry of probe molecules was the object of a review article by Shen and Auroux [105], Adsorption microcalorimetry results obtained using anunonia as a probe molecule revealed that, as long as Lewis acid sites with strength greater than 100 kJ/mol are present and as long as these sites are available to gas oil, FCCs can retain their useful cracking activity and selectivity properties [221],... 

These examples and many others have provided evidence of significant changes in catalyst structures resulting from changes in operating conditions. Techniques are thus necessary that can be applied to catalysts in the presence of probe molecules, in reactive environments (e.g., when catalysts undergo reduction, oxidation, etc.), and under catalytic reaction conditions. Moreover, the simultaneous determination of catalyst structure and activity or selectivity is needed to establish structure-activity or structure-selectivity relationships, which provide a basis for improvement and development of catalysts (Banares, 2005 Thomas, 1980 Thomas, 1999 Topsoe, 2000 Wachs, 2005). The need for characterization of catalysts during... 

Raman spectroscopy has been used frequently to investigate the chemisorption of probe molecules (Cooney et al., 1975 Weber, 2000). Several groups reported variable Raman cells in which the temperature of the sample and the environment can be controlled so that catalytic reaction conditions can be simulated (Abdelouahab et al., 1992 Brown et al., 1977 Chan and Bell, 1984 Cheng et al., 1980 Lunsford et al., 1993 Mestl et al., 1997a Vedrine and Derouane, 2000). In these investigations, conversion and selectivity values were not measured simultaneously with the spectra. The developments of these Raman experiments have been reviewed elsewhere (Banares, 2004 Knozinger and Mestl, 1999 Vedrine and Derouane, 2000). 

Examination of the reaction pathways of probe molecules has also been applied to the characterization of basicity. In this method, a suitable probe molecule is selected with the intention that base sites will convert it uniquely to a given product. The conversion of isopropanol has been widely applied in this context. 

Proton Affinities, Vertical Ionization Potentials, and Acidic Dissociation Constants of Selected Basic Probe Molecules"... 

A problem, different in nature, that needs additional attention refers to the characterization of the active centers involved in adsorption and catalytic processes and particularly to the estimation of the number of metallic centers and the exposed surface in supported and unsupported perovskites. A number of chemical and physical methods have been used for metals and oxides, and those based on selective chemisorption of probe molecules seem to be the most promising for this purpose (307). However, while considerable progress has been made for supported metals, no method has been accepted for oxides. This has been caused by the comparatively complex nature of these latter compounds where oxide ions and metal ions of different oxidation states may be present. As probe molecules, O2, CO, and NO were the most frequently used (307) the 02 chemisorption presents the problems inherent to any method based on gas adsorption at low temperatures (a large fraction of physisorbed gas accompanying the chemisorption). On the other hand, its symmetric character renders this molecule unamenable to study by IR spectroscopy. Nonetheless, this method has been used with some success by Weller et al. (308-310) on simple oxides, and its possible application to perovskites and other mixed oxides should be explored. Previous chemisorption work... 

Dynamic vapour phase techniques are interesting tools for the determination of these properties. When compared to standard wettability experiments, they provide two main benefits. They can easily and reproducibly be applied to powders and a wide variety of probe molecules can be selected. In the current study dynamic gravimetric vapour sorption (DVS) and inverse gas chromatography (IGC) have been used to characterise the energetic and acid-base properties of a calcined ruthenium oxide / MCM41 catalyst as well as the corresponding MCM41 support. 

Most of the computational studies of Type II CSPs do not consider the CSP directly. Instead, regression models are constructed to explain how a set of probe molecules interact with the CSP. We now present selected examples from the literature illustrating the diversity of such computational methodology used by chemists to address how these CSPs work. 

In this study, the product selectivity and product distribution following photooxidation was found to depend on the zeolite used. All of the zeolites that exhibited a loss of product selectivity for toluene photooxidation are susceptible to acid site formation during cation exchange and subsequent pretreatment or calcination. The colorimetric test described by Ramamurthy and coworkers was utilized to detect Bransted acid sites in the zeolites used in this study (10, 11). The colorimetric test is based on differences in the electronic absorption properties of protonated and unprotonated forms of probe molecules, such as retinol and retinol acetate. Both retinol and retinol acetate form a blue retinyl cation in an acid solution. The zeolites were activated by heating to 300°C under vacuum for approximately 12 hrs. Samples of BaZSM-5 (12) and CaY both turned dark blue when a dilute solution of retinol was added to the activated zeolite indicating the presence of Bronsted acid sites. Activated NaZSM-5 turned blue and activated BaY zeolite turned light blue... 

The main fields of application for spin probes are complex materials that consist of different components. Often, suitably selected spin probes can be used as tracers for one or even several of these components. Examples of such components are the counterions of polyelectrolytes " " or ionomers " " and surfactants. " This approach may also involve synthetic effort, as the spin probe should mimic the component of interest as closely as possible otherwise it might distribute in the material in a different way or it might even perturb the structure. To avoid the latter complication, one should use the smallest fraction of probe molecules that provides a sufficiently large signal in the intended experiments. It is good practice to consider, estimate, and discuss possible perturbations introduced by probing or labeling. 

All the work described above has established XPS firmly as a flexible method for the evaluation of the acid-base properties of homopolymers, but the technologically more important advantage of XPS is its ability to analyze thin modified or segregated layers not amenable to traditional forms of analysis [129]. The molecular probe technique has been used to good effect by Shahidzadeh et al. in the study of the plasma treatment of poly(propylene) film [130-132]. Their work identified the need to select basic probe molecules for the assessment of acidic surfaces, and dimethyl sulfoxide was shown to be a good choice, although the photoelectron cross section for sulfur is low, as it is for the Cl 2p core level used for trichloromethane this puts an effective limit on the detectability of the molecular probe, i.e., the number (but not the strength) of the acid-base pairs detectable. 

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