Susana Valencia

Sn-zeolite beta as a heterogeneous chemoselective catalyst for Baeyer-Villiger oxidations.

The Baeyer–Villiger oxidation, first reported more than 100 years ago, has evolved into a versatile reaction widely used to convert ketones—readily available building blocks in organic chemistry—into more complex and valuable esters and lactones. Catalytic versions of the Baeyer–Villiger oxidation are particularly attractive for practical applications, because catalytic transformations simplify processing conditions while minimizing reactant use as well as waste production. Further benefits are expected from replacing peracids, the traditionally used oxidant, by cheaper and less polluting hydrogen peroxide. Dissolved platinum complexes and solid acids, such as zeolites or sulphonated resins, efficiently activate ketone oxidation by hydrogen peroxide. But these catalysts lack sufficient selectivity for the desired product if the starting material contains functional groups other than the ketone group; they perform especially poorly in the presence of carbon–carbon double bonds. Here we show that upon incorporation of 1.6 weight per cent tin into its framework, zeolite beta acts as an efficient and stable heterogeneous catalyst for the Baeyer–Villiger oxidation of saturated as well as unsaturated ketones by hydrogen peroxide, with the desired lactones forming more than 98% of the reaction products. We ascribe this high selectivity to direct activation of the ketone group, whereas other catalysts first activate hydrogen peroxide, which can then interact with the ketone group as well as other functional groups.

Supramolecular self-assembled molecules as organic directing agent for synthesis of zeolites.

Solid materials with uniform micropores, such as zeolites, can act as selective catalysts and adsorbents for molecular mixtures by separating those molecules small enough to enter their pores while leaving the larger molecules behind. Zeolite A is a microporous material with a high void volume. Despite its widespread industrial use in, for example, molecular separations and in detergency, its capability as a petroleum-refining material is limited owing to its poor acid-catalytic activity and hydrothermal stability, and its low hydrophobicity. These characteristics are ultimately a consequence of the low framework Si/Al ratio (normally around one) and the resulting high cationic fraction within the pores and cavities. Researchers have modified the properties of type-A zeolites by increasing the Si/Al compositions up to a ratio of three. Here we describe the synthesis of zeolite A structures exhibiting high Si/Al ratios up to infinity (pure silica). We synthesize these materials, named ITQ-29, using a supramolecular organic structure-directing agent obtained by the self-assembly, through π–π type interactions, of two identical organic cationic moieties. The highly hydrophobic pure-silica zeolite A can be used for hydrocarbon separations that avoid oligomerization reactions, whereas materials with high Si/Al ratios give excellent shape-selective cracking additives for increasing propylene yield in fluid catalytic cracking operations. We have also extended the use of our supramolecular structure-directing agents to the synthesis of a range of other zeolites.

New Insights on CO2−Methane Separation Using LTA Zeolites with Different Si/Al Ratios and a First Comparison with MOFs

LTA zeolites can be synthesized with tailored adsorption properties by controlling the Al content in the framework. In this work, we have demonstrated that it is possible to adjust the polarity of the zeolitic adsorbent to optimize its thermodynamic adsorption properties for the energetically relevant CO(2)/CH(4) separation process. The thermodynamic study has been made from the corresponding adsorption isotherms of the pure gases carried out at different pressures and temperatures, as well as breakthrough separation experiments of CO(2)/CH(4) mixtures and the results were compared to those reported on MOFs. The separation values obtained allow us to conclude that LTA zeolites offer unique possibilities for CH(4) upgrading from natural gas.

Synthesis in fluoride media and characterisation of aluminosilicate zeolite beta

Zeolite beta has been synthesised for the first time in a wide range of initial Al/(Al+Si) ratios (0–0.14) in the presence of F– and TEA+ at near neutral pH. The samples have been characterised by a number of techniques and compared to those synthesised in basic medium. An increase in crystal size and crystallinity as the Si/Al ratio increases together with an enhanced resolution of the XRD pattern and of the IR and 29Si MAS NMR spectra are apparent. This is attributed to an enhanced local order as Si substitutes for Al in the zeolite framework. IR and 27Al MAS NMR demonstrate dealumination is more severe the lower the Si/Al ratio. The presence of F– in the as-made samples induces significant changes in the IR spectrum in the region of characteristic framework vibrations (500–650 cm–1) probably due to a distortion of the framework. The determination of the micropore volume has been carried out using different methods, the t-plot method giving the more reliable results. Apparently, thermal analysis allows one to distinguish between TEA+ cations compensating for F– anions from those compensating for framework Al, as the latter appear to be oxidised at a higher temperature.

Synthesis of nanocrystalline zeolite beta in the absence of alkali metal cations

Contrarily to previous reports, pure aluminosilicate zeolite Beta can be synthesized in the complete absence of alkali cations without the use of seeds. Thus, the presence of alkali cations is not necessary for the nucleation and crystal growth to occur, although they greatly affect the crystallization kinetics. By this synthesis method stable suspensions of colloidal zeolite Beta particles in the nanoscale size (100 to 10 nm) were prepared. It is also possible to synthesize 100% crystalline zeolite Beta with up to 7.6 framework Al atoms per unit cell (Si/Al=7.4).

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.

Synthesis and catalytic activity of aluminium-free zeolite Ti-β oxidation catalysts

Al-free Ti-β zeolite is synthesized and it shows an enhanced activity and selectivity to the epoxide in the oxidation of alkenes with H2O2 in protic solvents with respect to the previously reported (Al)-Ti-β samples.

Spontaneous nucleation and growth of pure silica zeolite-β free of connectivity defects

A microporous SiO2 material isomorphous to zeolite-β showing an enhanced crystallinity and thermal stability due to the absence of connectivity defects, spontaneously nucleates and grows in a hydrothermal system containing tetraethylammonium and fluoride ions at near neutral pH.

On the researching of a new zeolite structure for the selective catalytic reduction of NO: The possibilities of Cu-exchanged IM5

A new zeolite (IM5) whose structure has not been resolved yet is tested for the selective catalytic reduction (SCR) of NO with C3H8 and characterized by in situ X-ray photoelectron spectroscopy. The catalytic behavior of copper exchanged IM5 zeolite is compared with that of copper exchanged ZSM5 zeolites. The influence of the partial pressure of oxygen, reaction temperature and exchange level of the metal in the catalyst is similar to that observed with copper exchanged ZSM5 zeolite, but Cu–IM5 is a more active and hydrothermically more stable catalyst than Cu–ZSM5.

A new highly efficient method for the synthesis of Ti-Beta zeolite oxidation catalyst

Abstract A new method for the efficient synthesis of zeolite Ti-Beta oxidation catalysts is reported. High yields of zeolite are obtained by carrying out the synthesis in a dense system, by crystallizing a mixture obtained by wetness impregnation of TiO2 SiO2 cogels. This method requires lower amounts of organic structure directing agent, and materials with low aluminium content can be synthesized. The crystallization mechanism involves transport in the liquid phase, indicating that a direct transformation of the cogel does not occur. Spectroscopic characterization techniques and catalytic tests demonstrate that Ti is incorporated into framework sites.