Teresa Blasco

Selective and Shape-Selective Baeyer–Villiger Oxidations of Aromatic Aldehydes and Cyclic Ketones with Sn-Beta Zeolites and H2O2

Sn-Beta is used as a heterogeneous catalyst for the Baeyer-Villiger reaction with hydrogen peroxide. Cyclic ketones are transformed into the corresponding lactones, while unsaturated ketones are oxidized to the corresponding unsaturated lactones with very high chemoselectivity. The catalyst is also selective for the oxidation of aromatic aldehydes with H2O2, producing the formate ester or the corresponding hydrolyzed product, that is the alcohol. Shape-selective oxidations are observed for isomeric reactants with different molecular shapes. The catalytic Sn sites have been characterized by 119Sn MAS-NMR spectroscopy, and tetrahedral incorporation into the zeolite framework has been demonstrated. In situ IR spectroscopy and 18O labeling experiments have shown that the oxidation mechanism involves an intermediate of the Criegee type.

Unseeded synthesis of Al-free Ti-β zeolite in fluoride medium: a hydrophobic selective oxidation catalyst

Al-free Ti-β zeolite spontaneously nucleates and grows in a reaction mixture containing tetraethylammonium and fluoride ions at near neutral pH, and shows an enhanced crystallinity and thermal and hydrothermal stability as well as a significant hydrophobic character.

Ammonia-Containing Species Formed in Cu-Chabazite As Per In Situ EPR, Solid-State NMR, and DFT Calculations.

Nowadays, the most attractive technology for the elimination of nitric oxides from the exhaust gas of diesel vehicles is the selective catalytic reduction with ammonia (NH3-SCR-NOx) using Cu zeolite with the chabazite structure as the catalyst. Isolated copper species are the active sites, but the reaction intermediates and the overall reaction mechanism are still under debate. Here, we study the interaction of ammonia with zeolite Cu-SSZ-13 (CHA topology) with a uniform distribution of Cu(2+) sites prepared in one pot and a conventional Cu-ZSM-5 (MFI topology) for comparison. In situ EPR and solid-state NMR spectroscopies combined with DFT calculations have allowed the identification of NH4(+), [Cu(NH3)5](2+), [Cu(Of)2(NH3)2](2+), [Cu(Of)3NH3](2+), [Cu(NH3)2](+), and [CuOf(NH3)](+) (Of being framework oxygen) under different conditions. The results demonstrate that ammonia is able to reduce Cu(2+) to Cu(+) and provide new information on the species formed in Cu-SSZ-13, which have important implications for the elucidation of the SCR reaction mechanism.

X-Ray photoelectron spectroscopy of Ti-Beta zeolite

Abstract A series of Ti-Beta zeolites with different titanium contents were investigated by X-ray photoelectron spectroscopy (XPS). Analysis of the Ti(2p) peaks from the Ti-Beta samples and comparison with the spectra obtained for two naturally occurring titano-silicates gave information on the chemical environment of the titanium atoms in the samples.

Magnetic resonance studies on V-containing, and V,Mg-containing AFI aluminophosphates

Abstract As-synthesised V-containing and V,Mg-containing aluminophosphates of the AlPO 4 -5 type (AFI structure) with different compositions were characterised with the use of NMR and ESR spectroscopic techniques, with the aim of determining if V and/or Mg have been successfully incorporated into the framework sites. The identification of different structural P ( n Al, 4− n Mg) units by 31 P-MAS NMR is a clear evidence of the substitution of framework Al 3+ by Mg 2+ in V,Mg-containing samples. The relative site population shows that Mg 2+ is randomly distributed and that above ca. 2 wt.% a part is outside the framework. The ESR spectra of V- and V,Mg-containing samples indicate the presence of isolated (VO) 2+ ions in square pyramidal or distorted octahedral environment, and the spin density accounts for 25–35% of the total. The contribution of a broad resonance of associated V 4+ species in the spectra of the samples with Mg is attributed to the presence of an extra-framework oxidic phase. The location of the well dispersed vanadyl species has been deduced from the 31 P T 1 longitudinal relaxation times measured by NMR. The shorter 31 P T 1 in V-containing aluminophosphates is consistent with the presence of vanadyl cations at framework sites, whereas in V,Mg-containing aluminophosphates, V 4+ occurs preferentially outside the framework as aggregated and dispersed species. The 51 V-NMR spectra indicate the presence of both tetrahedral and octahedral V 5+ in V-containing and octahedral V 5+ in V,Mg-containing aluminophosphates, although nothing can be concluded about its incorporation into the framework sites.

Chapter Four - Structural Characterization of Zeolites by Advanced Solid State NMR Spectroscopic Methods

This review overviews the application of solid state NMR spectroscopy to the structural characterization of silicate, and acidic aluminosilicate and borosilicate zeolites focusing on the utilization of new developed methods. We summarize recent results on the investigations of the distribution of silicon atoms into the zeolite framework, as well as the location of F− anions and formation of pentacoordinated silicon species in zeolites synthesized in fluoride medium. Special attention is paid here to the characterization of framework and extraframework boron and most specially aluminium species and the acid sites in zeolites. In Section 6 of this review, we present the basis and practical aspects of advanced NMR experiments used in the characterization of microporous solids paying special attention to quadrupolar nuclei and other methods based on dipolar interactions.

Characterization of Ga-substituted zeolite Beta by X-ray absorption spectroscopy

The incorporation of Ga into the framework of zeolite Beta has been evidenced by X-ray absorption spectroscopy. Analysis of extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data reveals the atomic environment of Ga in zeolite Beta. Both techniques provide evidence that calcination and dehydration of the synthesized zeolites provokes changes in the Ga atomic environment, which can be understood as due to the migration of Ga from framework sites to extra-framework positions. The XANES results allow calculation of the atomic fraction at both positions.

An NMR study on the adsorption and reactivity of chloroform over alkali exchanged zeolites X and Y

Multinuclear solid state NMR has been applied to investigate the use of chloroform as a probe molecule to characterise the basic properties of zeolites. The characteristics of the 1H signals of adsorbed chloroform are consistent with highly mobile molecules leading to an average chemical shift that reflects the interaction of the molecule with the host zeolite through hydrogen bonding with basic framework oxygen atoms. The 1H chemical shift of CHCl3 adsorbed on alkali exchanged zeolites X and Y, as well as on other less basic zeolites with different topology and zeotypes, correlates with the mean negative charge over the framework oxygen as calculated by the method of Sanderson, and agrees with the results derived from the use of chloroform as an infrared probe molecule. The use of 13C enriched 13CHCl3 has allowed us to establish that the 13C chemical shift position is also a measure of framework basicity for zeolites X and Y. The changes observed on the 23Na magic angle spinning (MAS) NMR spectra with the amount of adsorbed chloroform are consistent with the decrease in the quadrupolar coupling constants of sodium at sites SII and SI′. 13 C MAS NMR of 13CHCl3 shows that this molecule decomposes at room temperature over alkali exchanged zeolite X to give small amounts of dichloromethoxy species bound to the framework. Further reaction at 423 K gives rise to the formation of 13CO accompanied by framework destruction. These processes also occur over alkali exchanged zeolite Y but much more slowly, and the reactivity does not only depend on the zeolite basicity.

Synthesis of SiVPI-5 with enhanced activity in acid catalysed reactions

A novel method for the synthesis of silicon-substituted aluminophosphate VPI-5 with enhanced acid catalytic activity is described.

Modelling active sites for the Beckmann rearrangement reaction in boron-containing zeolites and their interaction with probe molecules.

Theoretical calculations and in situ solid state NMR spectroscopy have been combined to get insight on the nature of the active sites for the Beckmann rearrangement reaction in borosilicate zeolites. The interaction of a B site in zeolite Beta with a series of probe molecules (ammonia, pyridine, acetone and water) has been modelled and the (15)N and (11)B NMR isotropic chemical shift of the resulting complexes calculated and compared with experimental in situ NMR results. This approach has allowed validation of the methodology to model the adsorption on a zeolite boron site of molecules of varying basicity which are either protonated or non-protonated. The limitation is that theoretical calculations overestimate the effect of molecular adsorption through hydrogen bonds on the calculated isotropic (11)B NMR chemical shift.Theoretical and experimental results on the adsorption of acetophenone and cyclohexanone oximes on zeolite B-Beta indicate that Brønsted acid sites protonate the oximes, changing the boron coordination from trigonal to tetrahedral. Comparison of theoretical and experimental (15)N NMR chemical shifts of the adsorbed amides (acetanilide and epsilon-caprolactam) indicates that they are non-protonated, and the (11)B NMR spectra show that, as expected, boron remains in trigonal coordination with an isotropic delta(11)B(exp) which differs from the calculated value delta(11)B(calc).