Structural analysis of catalysts

The future trends in XAFS spectroscopy relevant to characterization of catalysts in reactive atmospheres will thus be a combination of gm-ns time-resolved XAFS spectroscopy, time-resolved and spatially resolved XAFS spectroscopy, and state-resolved XAFS observations of the local structures of working catalysts. These more precise and definitive measurements, when coupled with advances in theory, will lead to more reliable structural analysis of catalysts and the ability to definitively resolve the structures in mixed-phase catalysts. It is indeed an exciting and continuously evolving field. 

FIGURE 32.9. X-ray structural analysis of catalyst-bound imine structure (a phenyl substituent on silicon has been removed for clarity). (Reprinted with permission from Reference 7. Copyright 2007, American Chemical Society.)... 

Due to these virtues, solid state NMR is finding increasing use in the structural analysis of polymers, ceramics and glasses, composites, catalysts, and surfaces. 

Corey et al. reported that the catalyst 19, prepared from trimethylaluminum and the bis-trifluorosulfonamide of stilbenediamine (stien), with generation of methane, is a suitable catalyst for the Diels-Alder reaction of 3-acryloyl, and 3-crotonoyl-l,3-oxazo-lidin-2-ones, giving the cycloadducts in high optical purity [28] (Scheme 1.35, Table 1.14). X-ray structure analysis of the catalyst and and NMR studies revealed that... 

Li, S., O Brien, R.J., Meitzner, G.D., Hamdeh, H., Davis, B.H., and Iglesia, E. 2001. Structural analysis of unpromoted Fe-based Fischer-Tropsch catalysts using x-ray absorption spectroscopy. Appl. Catal. A Gen. 219 215-22. 

In spite of the large success of XRD in routine structural analysis of solids, this technique does present some limitations when applied to catalysis [1,9]. First, it can only detect crystalline phases, and fails to provide useful information on the amorphous or highly dispersed solid phases so common in catalysts [22], Second, due to its low sensitivity, the concentration of the crystalline phase in the sample needs to be reasonably high in order to be detected. Third, XRD probes bulk phases,... 

Tsogoeva and co-workers explored the catalytic potential of pyridyl- and imida-zoyl-containing thiourea derivatives (e.g., thiourea 92 and 93) in the asymmetric model Strecker reactions [157] of N-benzyl- and benzhydryl-protected benzaldi-mine with HCN [258], The observed enantioselectivities were consistently very low (4—14% ee) for all catalyst candidates and were far below synthetically useful levels, while imidazoyl-thiourea 93 was reported to be highly active and displayed 100% conversion (at 7% ee) of the N-benzhydryl-protected benzaldimine (Scheme 6.99). X-ray structure analysis of a pyridyl-thiourea revealed an intramolecular hydrogen-bond between the basic ring nitrogen and one amide proton. This could make this... 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

In the foregoing discussion, the emphasis has been mainly on the properties of the catalyst, but it is evident that these must be regarded in close connection with the nature of the adsorbed hydrocarbons. Important information about this interaction can be gained from structure analysis of adsorption and reaction complexes, as well as adsorption measurements. 

The use of physical constants is, however, limited in the case of more complicated chemical processes the more complex the chemical change, the larger the number of physical properties necessary to investigate completely the chemical transformations. This especially holds when catalysts are involved in the reactions. When studying catalytic reactions we are dealing with catalysts as mixtures of a far more complicated nature than is the case, for example, in the structural analysis of hydrocarbon mixtures. The latter can be described, as was shown in the preceding sections, by means of a limited number of physical constants, from which either the chemical composition of the mixture or a series of other physical constants can easily be derived. For the characterization of catalysts completely different principles have to be applied even in simple cases, because in the case of a catalyst it is not chiefly its chemical composition that is important, but its chemical activity, which determines the result obtained by its chemical action. 

Cundari, T.R., Deng, J., Pop, H.F. and Sarbu, C. (2000) Structural analysis of transition metal beta-X substituent interactions. Toward the use of soft computing methods for catalyst modelling. J. Chem. Inf. Comp. Sci., 40, 1052. 

Here, I focus on application of ruthenium complexes as catalysts for the cyclopropanation of olefins with diazoesters to describe their catalytic activity, stereoselectivity, and enantioselectivity together with structural analysis of intermediary carbene complexes, especially with nitrogen-based ligands including porphyrin derivatives [4,5]. 

Such defect-driven structural transformations are effectively investigated by powder diffraction analysis of samples kept in reactive atmospheres. As solid catalysts are dynamic systems, the phase inventory and the defect ordering (real structure) may well change as a result of changes of chemical potential of a constituent in a reactive environment. Some of the changes are irreversible and can be detected by pre- and postoperation analysis of catalysts, but many are reversible and will not be evident in such experiments. 

This example brings up an interesting point regarding XAFS analysis of catalysts in the working state. Rarely, if ever, is there an example in the literature in which reproducibility of the catalytic performance is given, never mind the reproducibility of the structural analysis. Unfortunately, presumably because of the manner in which these data are collected (beam time assigned for a few days at a time every few months), time is of the essence and, given the complexity of the experiments, it is unlikely that many researchers prioritize time for repeat measurements. 

The first chiral aluminum catalyst for effecting asymmetric Michael addition reactions was reported by Shibasaki and coworkers in 1986 [82], The catalyst was prepared by addition of two equivalents of (i )-BINOL to lithium aluminum hydride which gave the heterobimetallic complex 394. The structure of 394 was supported by X-ray structure analysis of its complex with cyclohexenone in which it was found that the carbonyl oxygen of the enone is coordinated to the lithium. This catalyst was found to result in excellent induction in the Michael addition of malonic esters to cyclic enones, as indicated in Sch. 51. It had previously been reported that a heterobimetallic catalyst prepared from (i )-BINOL and sodium and lanthanum was also effective in similar Michael additions [83-85]. Although the LaNaBINOL catalyst was faster, the LiAlBINOL catalyst 394 (ALB) led to higher asymmetric induction. 

The above analyses of platinum in its reduced state show the advantages of EXAFS in providing in situ information under high temperature conditions and under reactive gases. This type of analysis enables a more realistic approach to the characterisation of this type of solid. It is also possible to envisage an analysis of catalysts under conditions similar to their use, in contact with hydrocarbon molecules. On contact of these catalysts with a hydrocarbon charge, it is possible to obser c the reversible formation of Pt -C bonds (Fig. 11.13), The number of carbon atoms in contact with a platinum atom (in this. study 1.4 on average) and their distance (1.96 A) can then be determined. In the case studied here, the structure of the metallic particle is not altered. 

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