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Catalysts – Examples of services

Initial growth of phthalocyanines on oxide substrates

Zeolites have a micro-porous structure. When the micro-pores are of a precise and uniform size, these materials areknown as “molecular sieves”. Their very
regular structure, with pores of molecular dimensions running in specific directions throughout the material, give them the ability to selectively sort and retain
molecules based primarily on their size. Zeolites find extensive use in industry and at home as the materials for purifying water, separation of gases, acceleration of chemical reactions (catalysis) and a constituent of detergents. Synthetic zeolites are widely used as calatysts in petrochemical industry for breaking down complex organic molecules into simpler molecules.

The zeolite known as ZSM-5 (from Zeolite Socony Mobile Five, also MFI) has a pore size of the order of the dimensions of organic molecules. This property makes ZSM-5 very interesting for industrial use. One application among many is the separation of para-xylene from C8 aromatic fractions from oil refineries. Para-xylene is used in the preparation of polyethylene terephthalate, PET, the material from which recyclable plastic bottles and artificial fibers are made. ZSM-5 prepared from silica-alumina gel in the presence of alkali (here ammonia) and organic templates (the inclusion of the template in the synthesis mixture determines which zeolite is formed) forms prisms, which in view of their clarity, fine morphology and straight edges look like single crystals (i. e., a solid made up of regularly ordered subunits).
However, closer inspection of the prisms in an optical microscope under crossed polarizers reveal typical hourglass-like features, indicating that the samples are not single crystals but rather composed of conjoined differently orientated single crystals (Fig. 1), in which the pores run in different directions. By treating prisms of both tetra-n propylammonium- templated (TPA) ZSM-5 (Si:Al=126) and TPA-Silicalite-1 with H2O2 /NaOH at 120 °C in a microwave oven, and successive ultrasound treatment in de-ionized water, it is possible to obtain pyramidal segments which appear to be single crystals under the polarizing microscope. Single crystals are ideal for study because their structures are well defined. The three-dimensional atomic compositions of these small pyramidal segments were determined using synchrotron radiation at ANKA in order to gain a better understanding of the working of the catalysts. In each case, the pyramidal segments were found to be single crystals with the straight pores running in one direction.

Figure 1

 

 

 

 

 

 

 

Figure 1: Two orthogonal views of MFI-Prisms (left and right)

 

 

 

 

Chemical treatment and optical investigations indicate that the type I and type III segments have the same orientation, and that the straight pores of the type II segments are rotated by 90° to these about the unique c axis. A consequence of this is that the straight pores are only exposed at the gable ends of the pyramidal segments of the original ZSM-5 prisms. Due to the accuracy of the diffraction data obtained at ANKA, it was possible to locate the hydroxyl ions necessary to balance the positive charges of the tetrapropylammonium cations, not previously found in the crystal structures of TPA-templated Silicalite and ZSM-5 prisms. Both the hydroxyl anions and the tetra-propylammonium cations are absent from the crystal structure of the calcined TPA-templated ZSM-5 pyramidal segments. Work is currently in progress to isolate the type II segments and shed further light on how the prisms are formed and function as catalysts.

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Mechanistic Insights into a Homogeneous Catalyzed Reaction

Introducing oxygen into hydrocarbons is one of the main goals of organic synthesis, making hydrocarbons accessible as starting compounds of countless chemical reactions. Epoxides are one of the most important classes of functionalised compounds, as many other chemicals, such as alcohols, diols and ketones can be prepared from this educt. All these compounds are indispensable in chemistry, e.g. to prepare pharma­ceutical products.

Figure 2

 

 

 

Figure 2: XANES spectra of the solid Mn(III)-salen complex  1 (X = bromide, black line) and a Mn(V) reference (red line) in comparison to the solution of 1 in acetonitrile, oxidized by mCPBA (blue line).

 

 

 

 

Specific enantiomeric epoxides, i.e. compounds with a certain geometric arrangement of the different functional groups, are needed especially for the latter purpose. The manganese-catalyzed enantioselective oxidation of olefins according to Jacobsen and Katsuki has become a famous tool synthesizing such particular epoxides.
The most commonly used catalyst is a so-called salen complex of  Mn(III) (1, X = Cl or Br), which has been proposed to be oxi­dized by meta-chloroperoxybenzoic acid  (mCPBA) to a Mn(V) intermediate (2)  with a Mn=O double bond according to the scheme below. This complex is usually considered the catalytically active species. The pro­posed reaction mecha­nism, and especially the existence of the Mn=O dou­ble bond as well as the oxidation state of the Mn atom, are matters of debate. Designing highly efficient catalysts requires detailed knowledge of these points, however, many methods have been employed to identify the oxidation states and structures of the intermediates of the reaction, e.g. EPR, IR/Raman, UV-Vis spectroscopy and mass spectrometry.3 Yet, none of these methods provided direct information about the local structure around the manganese center and its oxidation state. We therefore studied this system by X-ray absorption spectroscopy (XAS) and  analyzed the XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure). In addition, time-dependent  measurements, which allow the catalyst to be studied under working conditions,  were performed in the quick-EXAFS mode.  In the first step of our studies, the catalytically active species was investigated with static in-situ measurements of the solid Mn(III)-salen catalyst 1 in a solution of acetonitrile. The product 2, proposed in the literature, has a square-pyramidal penta- coordination of Mn(V), which would cause a pronounced prepeak in the XANES region of the manganese absorption edge. Figure 1 shows the XANES spectra of the solid Mn(III)-salen bromide complex 1, and a reference with a Mn-O bond instead of a Mn=O bond, but of the same geometry. They are compared with a solution of 1, to which mCPBA was added as an oxidant.
Two results are obvious from this comparison. While the edge position of the oxidized solution corresponds with an oxidation state of +V, the very weak prepeak excludes a square-pyramidal penta-coordination with a Mn=O group. This result is confirmed by EXAFS analysis of the oxidized solution.
In the picture on the top left, the bromide complex 1 is shown, where the signal of the Br atom is marked by an arrow, while the spectrum of the Mn-O reference is shown on the top right. The bond distance of the Mn-O bond is only slightly shorter than that of a Mn=O bond and its signal is clearly visible as a shoulder, marked by an arrow. In the spectrum of the oxidized solution, shown as a large picture, both the shoulder at short distance and the bromine signal are absent. This indicates that the halide atom is abstracted in solution, and also that no Mn=O bond formation occurs. Instead, a longer oxygen bond is found which is characteristic of a Mn-O single bond.
Moreover, a solvent molecule is coordinated to the Mn(V) center. For in-operando studies, a special instrumental setup was designed in our group which allows studies to be performed under real reaction conditions, i.e., reactants can be added in the course of measurements. This, plus the unique technical characteristics of the ANKA XAS beamline provided new insights into the catalytic cycle by quick-EXAFS measurements. This experiment allowed the state of oxidation to be monitored over the whole reaction time after the oxidizing agent mCPBA had been added to a solution of complex 1 (X = Cl) and cyclohexene as an olefin in acetonitrile.
Surprisingly, the time evolution of the oxidation state shows a very uncommon behaviour. A stepwise reduction of the initially formed Mn(V) species to the final state +III via the intermediate oxidation state +IV is found. The synthesis of the epoxide in the Jacobsen-Katsuki reaction starts with the formation of a Mn(V) species that does not contain a Mn=O group. After a reaction time of about 2000 seconds the oxidation state of the manganese atom changes from Mn(V) to the relatively stable Mn(IV) compound. Such a +IV complex was also proposed in earlier studies as intermediate, but could not be structurally characterized. From the time evolution of the oxidation state of the manganese it can be deduced that it should be possible to determine the local structure of this intermediate by conventional EXAFS measurements.

 

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