Base catalysis probe molecules

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Complementing this contribution, Haw and Xu present a detailed assessment of the nature of acidic surface sites (most in zeolites) and their interactions with probe molecules, as assessed in NMR experiments. Their comprehensive approach sheds light on a number of timely issues in acid-base catalysis and demonstrates how successfully NMR spectroscopy has been used recently to understand surface and catalytic phenomena. 

The use of CeOs-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO. O2, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of the role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium catalysts/additives in diesel soot abatement technology. 

P-13 - Heterogeneous base catalysis characterization of zeolites and mixed oxides using nitromethane as a NMR probe molecule and activity in the Michael condensation of nitromethane and cyclohex-2-en-l-one... 

Fig. 11. Study of acidity and basicity with adsorbed probe molecules. Left column N Is lines of pyridine chemisorbed on Al-ZSM-22 (A),Fe-ZSM-22 (B),Al-ZSM-5 (C),Fe-ZSM-5 (D),B-ZSM-5 ( ) right column N Is lines of pyrrole chemisorbed on alkali-exchanged X. Taken from Borade RB, Huang M, Adnot A, Sayari A, Kaliaguine S (1993) Acid-base properties of zeolites an XPS approach using pyridine and pyrrole probe molecule. In New frontiers in catalysis, Proc 10th Int Congr Catal, Budapest, Hungary 1992, p 1625, with kind permission from Elsevier Science NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netiierlands...

Characterization of Acidity/Basicity. The acid-base properties of a metal oxide play a crucial part in catalysis and are generally a function of pretreat-ment/preparation variables and catalyst morphology. It is important to be able to fully characterize the surface in terms of the strength and number of acid and base sites. Frequently, TPD methods involving probe molecules have been applied. However, caution must be exercised to simple TDP measurements, as it is often not clear whether the probe molecule is behaving in the acid-base manner assumed. An illustrative example of this is provided in a study of desorption of NH3 from CaO, in which the high temperature of desorption may lead one to believe that CaO is a strong acid (280). The explanation for this... 

The presence of acidic sites on the POM pillars enables the LDH-POM to act as acid/base bifimctional catalysts or even as strongly acidic catalysts (251-253,468). As mentioned previously, isopropanol is a probe molecule for distinguishing between acidic and basic catalysis. For example, ZnAl-SiWiiM04o-LDH (M = Mn, Fe, Co, Ni, Cu, and Zn divalent ions) has been used as the catalyst for the decomposition of isopropanol to give propene almost exclusively (98%, reaction (B)) at 90% conversion (251). 

Nevertheless, solid-state NMR can be usefiil for active site studies, which is nowadays predominantly employed for acid catalysis, e.g., with zeolites [32]. Chemical shifts obtained are related to the electronic state of the atom H-NMR data have been, for instance, used to determine acidity of Brpnsted sites in zeolites, NMR can discriminate in various environments in supported catalysts [33]. As long as the sites are sufficiently dilute, NMR intensity is directly related to concentration. Structural information is accessible by echo techniques which allow determining distances between spin sites. By analogy with IR methodology, adsorption sites have been investigated with probe molecules also in NMR studies. While work with N bases may require isotopic enrichment of the probe, there are opportunities to detect subtle structural features by cross-polarization and doubleresonance (e.g., " N/ A1) techniques [34]. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

In non-electrochemical heterogeneous catalysis, the interface between the catalyst and the gas phase can often be characterized using a wide variety of spectroscopic probes. Differences between reaction conditions and the UHV conditions used in many studies have been probed extensively 8 as have differences between polycrystalline and single-crystalline materials. Nevertheless, the adsorbate-substrate interactions can often be characterized in the absence of pressure effects. Therefore, UHY based surface science techniques are able to elucidate the surface structures and energetics of the heterogeneous catalysis of gas phase molecules. 

In the field of heterogeneous catalysis, pyridine is frequently employed as a molecule probe of the surface acid sites of zeolites and other oxide catalysts, with which the amount and the strength of the acid sites are determined, for instance by infrared measurements of the intensity and the peah position of pyridine adsorbed on zeolite surfaces [9]. Despite its importance in this respect, details of the structure and the molecule orientation within the adsorbed pyridine layer on these nonconductive substrates are not known, due to the lack of techniques to yield such data before the advent of AFM, The series of our present work constitutes the first report on the determination, by means of. A.FM. of the array structure of the pyridine base adlayers on zeolite surfaces, and the estimation of the molecular orientation within the adsorbed layer. 

---