On determining the nuclearity of iron sites in Fe-ZSM-5--a critical evaluation

In order to interpret catalytic data on iron zeolites in terms of structure-activity relationships, reliable characterisation methods are needed. In particular, the nuclearity of the iron species is an important issue, since it is often invoked to explain catalytic activity. In the present contribution, we address the problem of the nuclearity of the iron species in Fe-ZSM-5 by a combination of techniques, that is, UV-Vis, EXAFS, Magnetic Circular Dichroism (MCD) and magnetisation measurements. Based on an in-depth analysis of these data, we show that some of the current interpretations of UV-Vis and EXAFS spectra need to be revised.     

Analysis of in situ EXAFS data of supported metal catalysts using the third and fourth cumulant

X-Ray absorption spectra of supported Pt catalysts with various Pt cluster sizes were collected between 77 and 673 K, in inert and hydrogen atmospheres. When analyzing these spectra with the standard EXAFS equation, a Pt-Pt bond contraction and a large increase in the inner potential correction were observed with increasing temperature. These errors are up to 0.08 A and 10 eV for clusters of 1 nm diameter. They were corrected by including the third and fourth cumulants as fit parameters. Fit guidelines were developed to analyze EXAFS data of supported metal catalysts collected at elevated temperatures, allowing for asymmetry and broadening or sharpening of the pair distribution function. These comprise fixing fit parameters, using different k-weightings and identifying trends in a series of experiments. By fitting the EXAFS spectra using these guidelines, it was determined that in small Pt clusters the Pt-Pt bond is 0.10 A shorter than in bulk Pt. These contracted bonds relax to distances near that of bulk Pt upon hydrogen chemisorption.     

Photoregulation of polymer conformation by photochromic moieties. I. Anionic ligand to an anionic polymer

Abstract— Photoregulated biological processes appear to make use of membrane-bound photochromic macromolecules. In order to elucidate various physicochemical pathways by which these processes can be triggered, model studies have been undertaken employing photochromic moieties bound to synthetic macromolecules with a labile fold structure. Inspired by Lovrien's 1967 work, attention was first focused on the anionic dis-azo stilbene dye chrysophenine (CHP) and poly(methacrylic acid) (PMA) in aqueous solution. This ligand is found to bind to and unfold PMA only if the degree of ionization of the polymer is below 0·75. Viscosity as well as equilibrium dialysis data indicate that maximally one CHP per ten monomer units PMA is bound. The apparent degree of ionization conferred to PMA by the ionic CHP ligand leading to polymer unfolding is the same as the known real degree of ionization leading to polymer unfolding in the absence of CHP. Upon trans→cis photoisomerization, the ligand either desorbs or creates a higher local dielectric constant because of the large cis-azo dipole moments. As a result some refolding to a more compact hydrodynamic volume occurs, as deduced from the viscosity measurements at a low degree of ionization. Simultaneously a lowering of the pK_app by 0·1 pK unit is observed at degrees of ionization below 0·075. Photoregulation of conformation as well as of pK_app indicates two possible pathways for the regulation of ionic fluxes such as have been postulated for photobiological transducers.     

Hydrogenolysis of benzylic alcohols on rhodium catalysts

Alumina-supported catalysts from various sources and with different rhodium dispersions predominantly yield the hydrogenation product perhydro-1-indanol in the liquid-phase hydrogenation of 1-indanol, while carbonsupported catalysts mainly give the C-O bond scission - hydrogenation product perhydroindane. Addition of organic or inorganic bases to the reaction mixture suppresses C-O bond scission. To distinguish between the two possible pathways for C-O bond scission of direct hydrogenolysis or dehydration followed by hydrogenation, deuteration studies have been carried out with carbon-supported catalysts. Not only 1-indanol but also indane and indene (the two possible mechanistic intermediates in the C-O bond scission routes) were deuterated. Information about the actual pathway has been obtained by determining the degree of deuteration and the positions at which deuterium is incorporated in the resulting perhydroindane product by means of mass spectrometry and 13C NMR spectroscopy. The results prove that C-O bond scission takes place primarily through the direct hydrogenolysis pathway on the carbon-supported catalysts. Direct hydrogenolysis occurs on the carbon support because of the formation of a better leaving group (OH2+) from the benzylic hydroxy group and its subsequent substitution by spillover hydrogen.