German Sastre

Multifunctional Hybrid Organic−Inorganic Catalytic Materials with a Hierarchical System of Well-Defined Micro- and Mesopores

Novel layered zeolitic organic-inorganic materials (MWW-BTEB) have been synthesized by intercalation and stabilization of arylic silsesquioxane molecules between inorganic zeolitic MWW layers. The organic linkers are conformed by two condensed silyl-arylic groups from disilane molecules, such as 1,4-bis(triethoxysilyl)benzene (BTEB), which react with the external silanol groups of the zeolitic layers. The hybrids contain micropores within the inorganic layers and a well-defined mesoporous system in between the organic linkers. An amination post-treatment introduces basic groups in the organic linkers close to the acid sites present in the structural inorganic counterpart. Through this methodology it has been possible to prepare bifunctional acid-base catalysts where the acid sites are of zeolitic nature located in the inorganic building blocks and the basic sites are part of the organic structure. The resultant materials can act as bifunctional catalysts for performing a two-step cascade reaction that involves the catalytic conversion of benzaldehyde dimethylacetal into benzylidene malononitrile.

A zeolitic structure (ITQ-34) with connected 9- and 10-ring channels obtained with phosphonium cations as structure directing agents.

Zeolites are materials with a large applied interest. Here we present ITQ-34 a new zeolitic material, obtained combining the use of the highly stable tetraalkylphosphonium cations as structure directing agents with the incorporation of Ge atoms in the structure.

Modular Organic Structure-Directing Agents for the Synthesis of Zeolites

Routes to Rare Zeolites Zeolites are microporous crystalline solids with well-defined structures. Although many naturally occurring ones have been obtained in laboratory synthesis, some have remained elusive. One of these, boggsite, is of interest for catalytic reactions because it has large channels defined by rings of 10 or 12 atoms that intersect within its crystalline lattice. Simancas et al. (p. 1219) report the synthesis of boggsite by using phosphazenes as the organic groups that directed the formation of rings during synthesis. These reagents can be readily modified—a feature that should allow greater flexibility in synthesis routes. Phosphazene molecules enable the synthesis of a rare naturally occurring zeolite. Organic structure-directing agents (OSDAs) are used to guide the formation of particular types of pores and channels during the synthesis of zeolites. We report that the use of highly versatile OSDAs based on phosphazenes has been successfully introduced for the synthesis of zeolites. This approach has made possible the synthesis of the elusive boggsite zeolite, which is formed by 10- and 12-ring intersecting channels. This topology and these pore dimensions present interesting opportunities for catalysis in reactions of industrial relevance.

Modelling of Brønsted acidity in AFI and CHA zeotypes

The Bronsted acidity of AFI and CHA zeotypes in their aluminosilicate (zeolite) and silico-aluminophosphate (SAPO) forms has been modelled using potential-based computational methods. We find that the gradient of the electrostatic potential at the proton correlates with the OH stretching frequency, providing a method for characterising the acidity of these materials. Structural effects are shown to be one of the factors influencing acidity through the study of acid centres on the different oxygen atoms attached to the same T site. The influence of chemical composition on acidity is also analysed through the simulation of Bronsted acid pairs, silicon islands and aluminosilicate regions in SAPOs. We present a general discussion on the relative acidity of these aggregates, demonstrating how strong acid centres in SAPOs can appear in SiAl regions, although the highest acidity in SAPOs is found at the borders of the silicon islands.

On the shape selective acylation of 2-methoxynaphthalene over polymorph C of Beta (ITQ-17)

Abstract 2-Methoxynaphthalene (2-MN) has been acylated with acetic anhydride on polymorph C of Beta (ITQ-17), and 2-acetylmethoxynaphthalene (2-AMN) and 1-acetylmethoxynaphthalene (1-AMN) have been obtained. The diffusivity of the two isomer products, as determined by molecular dynamics, shows that the ratio of the diffusion coefficients for 2-AMN/1-AMN is one order of magnitude larger for polymorph C than for polymorphs A and B (Beta zeolite). This corresponds very well with experimental acylation results that show a higher selectivity for 2-AMN on polymorph C when reaction is carried out in batch as well as in a fixed-bed continuous reactor.

Mechanisms of silicon incorporation in aluminophosphate molecular sieves

The mechanisms of Si incorporation into aluminophosphate molecular sieves has been investigated using lattice simulation techniques. We consider the relative stability of dispersed Si and Si islands, along with changes in the substitution mechanisms in the different structure types. Two mechanisms are considered: P substitution by Si (SM2) and the replacement of adjacent Al and P with 2Si (SM3). We demonstrate that aggregation of silicon will only occur above certain thresholds determined by the structure type. Our previous work has demonstrated that silicon island formation by means of SM3 is energetically unfavourable, and thus 2Si and 4Si islands are not expected to form in SAPO structures. In contrast, the formation of silicon islands by means of a combination of SM2 and SM3 has proved to be energetically favourable, leading to 5Si and 8Si islands being stable in the SAPO structure. The instability of [Si-O-P] linkages is therefore demonstrated. We also investigate the stability of Si islands in a number of SAPO structures. We find differences in island stability in these different structures, which can be explained in terms of the connectivity of the framework structure.

RELATION BETWEEN STRUCTURE AND LEWIS ACIDITY OF TI-BETA AND TS-1 ZEOLITES.A QUANTUM-CHEMICAL STUDY

Abstract Ab-initio Hartree–Fock and density-functional-theory quantum-chemical calculations have been carried out to characterise the geometry and LUMO energies of two zeolites, Ti-Beta and TS-1. The cluster model has been used and results from classical force-field calculations have also been included for comparison and to guide the definition of the cluster boundaries. The LUMO energy, which is related to the Lewis acidity, is shown to be different for the different T-sites, and, in general, Ti-Beta shows higher Lewis acidity than TS-1. This can explain their behaviour for epoxidation of olefins as well as their catalytic properties for Lewis acid-catalysed reactions such as Meerwein–Ponndorf–Verley reactions.

ZeoTsites: a code for topological and crystallographic tetrahedral sites analysis in zeolites and zeotypes

Abstract The program zeoTsites has been developed as a complementary tool to some solid state computer codes with the aim of obtaining detailed analyses of TO and TT distances, and TOT angles in zeotypes (T ≡ four connected atom), as well as T atom connectivity and vertex symbols. From the crystallographic data the program finds the zeotype geometry for individual and average TOT units. Two kinds of averages are considered by taking into account the topologic and the crystallographic nature of the T sites. The framework distortions upon isomorphic substitution can be analyzed from the results of computational calculations or experimental data. The coordination sequences of the T atoms and the vertex symbols are also calculated by the program. Connected atoms in the first and second coordination shells are also calculated for assistance in the interpretation of NMR spectra of pure AlPO 4 and SiO 2 structures as well as chemically substituted forms.

Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites

Brønsted acidity of H-chabazite (CHA) zeolites (Si : Al(2) = 4.2) was investigated by means of ammonia infrared-mass spectrometry/temperature-programmed desorption (IRMS-TPD) methods and density functional calculations. Four IR bands were observed at 3644, 3616, 3575 and 3538 cm(-1), and they were ascribable to the acidic OH groups on four nonequivalent oxygen sites in the CHA structure. The absorption band at 3538 cm(-1) was attributed to the O(4)H in the 6-membered ring (MR), and ammonia adsorption energy (DeltaU) of this OH group was the lowest among the 4 kinds of OH groups. The other 3 bands were assigned to the acidic OH groups in 8MR. It was observed that the DeltaU in 8 and 6MR were 131 (+/-3) and 101 kJ mol(-1), respectively. On the other hand, the density functional theory (DFT) calculations within periodic boundary conditions yielded the adsorption energies on these OH groups in 8 and 6MR to be ca. 130 and 110 kJ mol(-1), respectively, in good agreement with the experimentally-observed values.

Confinement effects in the hydrogen adsorption on paddle wheel containing metal–organic frameworks

The confinement effects upon hydrogen adsorption in Cu(II)-paddle wheel containing metal-organic frameworks (MOFs) were evaluated and rationalized in terms of the structural properties (cavity types and pore diameters) of PCN-12, HKUST-1, MOF-505, NOTT-103 and NOTT-112. First-principles calculations were employed to identify the strongest adsorption positions at the paddle wheel inorganic building unit (IBU). The adsorption centres due to confinement were located through analysis of 3D occupancy maps obtained from the hydrogen trajectories computed via molecular dynamics simulations. It was found that the confinement enhances the adsorption on the weakest adsorption centres around the IBU in regions close to the narrowest windows and promotes the formation of new adsorption regions into the small cavities. Our results indicate that at low pressure, the high H(2) uptake in these materials is partly due to the presence of small cavities (5.3-8.5 Å) or narrow windows where the long-range contribution to the adsorption becomes important. Conversely, confinement effects in cavities with diameters >12 Å were not observed.