EXAFS and XANES investigation of the ETS-10 microporous titanosilicate

In this work, we report state-of-the-art analysis of both Ti K-edge high-resolution XANES and EXAFS data collected on the ETS-10 molecular sieve at the GILDA BM8 beamline of the ESRF facility. The interatomic distances and the angles obtained in our EXAFS study are in fair agreement with the single-crystal XRD data of Wang and Jacobson (Chem. Commun. 1999, 973) and with the recent ab initio periodic study of Damin et al. (J. Phys. Chem. B 2004, 108, 1328) Differently from previous EXAFS work (J. Phys. Chem. 1996, 100, 449), our study supports a model of ETS-10 where the Ti atoms are bonded with two equivalent axial oxygen atoms. This model is also able to reproduce the edge and the post-edge region of the XANES spectrum. Conversely, the weak but well-defined pre-edge peak at 4971.3 eV can be explained only by assuming that a fraction of Ti atoms are in a local geometry similar to that of the pentacoordinated Ti sites in the ETS-4 structure. These Ti atoms in ETS-10 should be the terminal of the -Ti-O-Ti-O-Ti- chains, of which the actual number is strongly increased by the high crystal defectivity (Ti vacancies).     

Paramagnetic nitrosyliron adducts in pentasilic zeolites: an EPR study

The interaction of nitrogen monoxide with various types of iron-containing pentasilic zeolites has been investigated by EPR spectroscopy. The systems investigated were iron-silicalite, ion-exchanged ZSM-5 and a bare silicalite impregnated with iron ions at the external surface only. NO forms paramagnetic adducts on all these systems essentially reacting with Fe(II) species. Three distinct types of nytrosyl adducts have been identified all lacking the hyperfine structure. Two of them are in doublet (S=1/2) state while the third one, observed in ZSM-5 samples only and already reported in the literature, is a quadruplet (S=3/2). While all activated samples exhibit EPR spectra (due to Fe(III) ions) very similar one to each other, their reactivity towards NO is different in each case. This allows some advance in understanding the state of the activated samples which, due to the high importance of iron-containing pentasilic zeolites in heterogeneous catalysis, is the object of a active debate in the literature.     

A Binaphthyl‐Bridged Salen Zirconium Catalyst Affording Atactic Poly(propylene) and Isotactic Poly(α‐olefins)

Summary: A binaphthyl‐bridged salen dichlorozirconium (IV ) complex that displays an octahedral structure with a trans‐O, cis‐N, and cis‐Cl arrangement was synthesized and tested as a precatalyst for ethylene and α‐olefin polymerization. While use of methylaluminoxane (MAO) cocatalyst afforded poor catalytic activity, activation by mixtures of aluminium alkyls such as AlⁱBu₃ and either MAO or [CPh₃][B(C₆F₅)₄] resulted in reasonable polymerization activities for ethylene, propene, and higher α‐olefins. Quite unexpectedly, while the polymerization of propene results in the production of a high‐molecular‐weight stereoirregular polymer, highly isotactic polymers are obtained under similar conditions from polymerization of 1‐butene, 1‐pentene, and 1‐hexene.

Polymerization employing the binaphthyl‐bridged salen dichlorozirconium (IV ) complex gave unexpected different stereospecificities for the polymerization of propene and higher α‐olefins, to yield ultrahigh‐molecular‐weight atactic poly(propylene) and highly isotactic polymers, respectively.     

Spectroscopic and adsorptive studies of a thermally robust pyrazolato-based PCP

The pyrazolato-based PCP [Ni(8)(μ(4)-OH)(4)(μ(4)-OH(2))(2)(μ(4)-PBP)(6)] (NiPBP, H(2)PBP = 4,4'-bis(1H-pyrazol-4-yl)biphenyl), whose 3-D architecture is built upon octametallic hydroxo clusters reciprocally connected by organic spacers, is a very promising candidate for gas adsorption applications, owing to its remarkable thermal stability (up to 400 °C in air) and its high void volume (70%). As such, NiPBP was selected as a proof-of-concept material to demonstrate how an optimized set of solid state techniques can concur to create a comprehensive and coherent picture, relating (average and local) structural features to adsorptive properties. To this aim, the response of NiPBP toward different gases, retrieved by gas adsorption measurements (N(2) at 77 K, in the low pressure region; H(2) at 77 K, in the high pressure region), was explained in terms of local-level details, as emerged by coupling electronic, X-ray (absorption and emission), and variable temperature IR spectroscopy.     

Coordination chemistry and reactivity of zinc complexes supported by a phosphido pincer ligand

The preparation and characterization of new Zn(II) complexes of the type [(PPP)ZnR] in which R = Et (1) or N(SiMe(3))(2) (2) and PPP is a tridentate monoanionic phosphido ligand (PPP-H = bis(2-diphenylphosphinophenyl)phosphine) are reported. Reaction of ZnEt(2) and Zn[N(SiMe(3))(2)](2) with one equivalent of proligand PPP-H produced the corresponding tetrahedral zinc ethyl (1) and zinc amido (2) complexes in high yield. Homoleptic (PPP)(2) Zn complex 3 was obtained by reaction of the precursors with two equivalents of the proligand. Structural characterization of 1-3 was achieved by multinuclear NMR spectroscopy ((1)H, (13)C, and (31)P) and X-ray crystallography (3). Variable-temperature (1)H and (31)P?NMR studies highlighted marked flexibility of the phosphido pincer ligand in coordination at the metal center. A DFT calculation on the compounds provided theoretical support for this behavior. The activities of 1 and 2 toward the ring-opening polymerization of ε-caprolactone and of L- and rac-lactide were investigated, also in combination with an alcohol as external chain-transfer agent. Polyesters with controlled molecular parameters (M(n), end groups) and low polydispersities were obtained. A DFT study on ring-opening polymerization promoted by these complexes highlighted the importance of the coordinative flexibility of the ancillary ligand to promote monomer coordination at the reactive zinc center. Preliminary investigations showed the ability of these complexes to promote copolymerization of L-lactide and ε-caprolactone to achieve random copolymers whose microstructure reproduces the composition of the monomer feed.     

Representation of the eigenvalues of the p ‐Laplacian by means of a surface integral

We prove a natural generalization to the p ‐Laplacian of the celebrated Rellich identity. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A simple model for a two-stage chemical reaction with diffusion

We analyse the evolution of a two-stage chemical reaction betweentwo neighbouring plumes of reactants. Under the assumption thatthe plumes are approximately Gaussian we derive a system ofordinary differential equations for the total amount, the centroidand the variance of each reactant. We compare the solution ofthese equations with full numerical simulation of the reaction.Excellent agreement is obtained, with solution of the near-Gaussianmodel requiring considerably less computational effort thanthe full simulations. Of key importance is the yield of thereaction, and we discuss this feature in particular.