Fernando Rey

A large-cavity zeolite with wide pore windows and potential as an oil refining catalyst

Crude oil is an important feedstock for the petrochemical industry and the dominant energy source driving the world economy, but known oil reserves will cover demand for no more than 50 years at the current rate of consumption. This situation calls for more efficient strategies for converting crude oil into fuel and petrochemical products. At present, more than 40% of oil conversion is achieved using catalysts based on faujasite; this zeolite requires extensive post-synthesis treatment to produce an ultrastable form, and has a large cavity accessible through four 0.74-nm-wide windows and thus limits the access of oil molecules to the catalytically active sites. The use of zeolites with better accessibility to their active sites should result in improved catalyst efficiency. To date, two zeolites with effective pore diameters exceeding that of faujasite have been reported, but their one-dimensional pore topology excludes use in oil refining. Similarly, zeolites with large pores and a three-dimensional pore topology have been reported, but in all these materials the pore openings are smaller than in faujasite. Here we report the synthesis of ITQ-21, a zeolite with a three-dimensional pore network containing 1.18-nm-wide cavities, each of which is accessible through six circular and 0.74-nm-wide windows. As expected for a zeolite with this structure, ITQ-21 exhibits high catalytic activity and selectivity for valuable products in preliminary oil refining tests.

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.

Determination of base properties of hydrotalcites: Condensation of benzaldehyde with ethyl acetoacetate

By carrying out the condensation of benzaldehyde with activated methylenic groups with different pK values, in the presence of a MgAl hydrotalcite catalyst, it has been found that this material shows basic sites with pK values up to 16.5. However, most of the basic sites show pK values in the range 10.7–13.3. In the use of zeolites as catalysts, the only reaction observed was Knoevenagel condensation, while on calcined hydrotalcite other reactions, the Michael-type addition and the Claisen condensation, also occur. These reactions require stronger basicities than the Knoevenagel condensation to occur. By carrying out the Knoevenagel condensation, using methylenic groups of different pK values, in the presence of increasing amounts of benzoic acid, it is possible to measure the total amount of basic sites and their strength distribution as determined under reaction conditions. By increasing the Mg/Al ratio in the hydrotalcite, the number of basic sites with 9.0 ≤ pK ≤ 13.3 increases, whereas the amount of basic sites within 13.3 ≤ pK ≤ 16.5 decreases.

A Miniaturized Linear pH Sensor Based on a Highly Photoluminescent Self-Assembled Europium(III) Metal–Organic Framework†

The field of lanthanide-based metal–organic frameworks (LnMOFs) with twoor three-dimensional structures is rapidly growing because of the discovery of new crystalline structures that exhibit interesting properties and have potential applications in catalysis, sensors, contrast agents, non-linear optics, 22] displays, and electroluminescent devices. For photoluminescence applications, it is necessary to prepare lanthanide-containing materials with high quantum efficiencies, in order to achieve the required miniaturization and reduce energy losses from undesirable quenching processes. Moreover, it is highly desirable to combine the properties of ligands and antennae in one organic moiety. A well-known powerful sensitizing ligand for Eu ions in solution is 1,10-phenanthroline-2,9-dicarboxylic acid (H2PhenDCA), in which both carboxylic and phenanthroline moieties may coordinate to the metal center. 27] The proximity between the coordinative parts means that this chelating agent has the tendency to form zero-dimensional (molecular) complexes that are useful in some solution-based analytical applications, but cannot be applied as solid sensors or light-emitting materials. Thus, it is of interest to obtain the twoor three-dimensional insoluble counterparts of these zero-dimensional water-soluble complexes. To achieve this goal, we have used hydrothermal synthesis, which is a powerful technique for the preparation of metastable compounds that may not be accessible by using conventional methods. 29] Hydrothermal synthesis also allows the use of chelating agents that are sparingly soluble in water at temperatures below 373 K, thus enhancing the lanthanidecoordinating ability of the ligand. Herein, we report the synthesis, structure, and sensing properties of a new Eu metal–organic framework ITQMOF-3-Eu (ITQMOF = Instituto de Tecnologia Quimica Metal Organic Framework) that contains the ligand 1,10-phenanthroline-2,9-dicarboxylic acid. The excellent balance between absorption, energy transfer, and emission rate of the Eu ITQMOF-3 (ITQMOF-3-Eu) allowed the fabrication of a miniaturized pH sensor prototype that functions in the biologically interesting range (5–7.5). By combining this material and an optical fiber, a linear photoluminescence response, which also allows the self-calibration of the emitting signal within this pH range, was achieved. The ITQMOF-3Eu material was obtained by reacting the H2PhenDCA ligand and the Eu salt or oxide under hydrothermal conditions (see the Supporting Information). The crystal has a strong red luminescence under ultraviolet light (see Figure 1 a). Chemical and elemental analyses showed that the formula of the material is [Eu3(C14H6N2O4)4(OH)(H2O)4]·2 H2O.

Thermal decomposition of hydrotalcites. An infrared and nuclear magnetic resonance spectroscopic study

Calcined hydrotalcites have been used extensively as catalysts for base-catalysed reactions. The calcination procedure in critical in determining the behaviour of the final material. The characteristics of the hydrotalcite, following the different stages of calcination, have been studied by means of thermogravimetric (TG), X-ray diffraction (XRD), IR, 1H and 27Al NMR techniques. We have shown that dehydroxylation begins within layers and then in a second stage occurs between adjacent layers, causing collapse of the structure. This process is accompanied by a change from octahedral to tetrahedral coordination of the aluminium. Decarbonation leads to the formation of micropores of radius <1.75 nm. Dehydroxylation and decarbonation are reversible processes, and their rates depend on the calcination temperature.

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.

ITQ-15: The first ultralarge pore zeolite with a bi-directional pore system formed by intersecting 14- and 12-ring channels, and its catalytic implications

The pore topology of ITQ-15 zeolite consists of an ultra-large 14-ring channel that is intersected perpendicularly by a 12-ring pore; acid sites have been introduced in its framework and this unique structure shows advantages over unidirectional ultralarge pore zeolites for diffusing and reacting large molecules.

Towards the rational design of efficient organic structure-directing agents for zeolite synthesis.

Zeolites are crystalline microporous materials with application in diverse fields, especially in catalysis. The ability to prepare zeolites with targeted physicochemical properties for a specific catalytic application is a matter of great interest, because it allows the efficiency of the entire chemical process to be increased (higher product yields, lower undesired by-products, less energy consumption, and cost savings, etc). Nevertheless, directing the zeolite crystallization towards the material with the desired framework topology, crystal size, or chemical composition is not an easy task, since several variables influence the nucleation and crystallization processes. The combination of accumulated knowledge, rationalization, and innovation has allowed the synthesis of unique zeolitic structures in the last few years. This is especially true in terms of the design of organic and inorganic structure-directing agents (SDAs). In this Minireview we will present the rationale we have followed in our studies to synthesize new zeolite structures, while putting this in perspective with the advances made by other researchers of the zeolite community.

Structure–functionality relationships of grafted Ti-MCM41 silicas. Spectroscopic and catalytic studies

Detailed spectroscopic measurements have been used to elucidate the nature of Ti(IV)-centred active sites that were anchored to a mesoporous silica surface (MCM41) by two distinct routes from titanocene precursors. The catalyst prepared in dry argon (Ti-MCM41 [Ar]) is more active than that prepared in air (in the presence of water vapour) in its activity in the epoxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The degree of loading of the titanium onto the silica support also influences the precise nature of the active sites. In Ti-MCM41 [Ar] samples containing less that 2 wt.% of Ti, the most abundant species are tetrahedrally bonded Ti(IV) active sites which absorb at 210–230 nm in diffuse reflectance (DR) UV–Vis spectra and exhibit an emission at 430 and 490 nm when excited with a 250 nm light. Samples of catalyst with greater than 4 wt.% loading are shown to contain TiO2-like microclusters. These species absorb at λ250 nm in the DR UV–Vis and emit very weakly in the 500–600 nm region. In the case of Ti-MCM41 [air] catalysts, which were prepared in the presence of atmospheric water at the MCM41 surface, even at the lowest Ti loading (e.g. ⩽0.5%) an incipient formation of oligomers occurs. It is proposed that dimers or very small oligomers which absorb at around 250 nm in the DR spectra are responsible for a very strong emission at around 500 nm in the photoluminescence spectra. The abundant presence of these species might well explain the lower catalytic performance of Ti-MCM41 [air] in comparison with that of Ti-MCM41 [Ar]. The anchoring of the Ti species at the surface silanol groups of MCM41 was in all cases followed by FTIR spectroscopy.

Structure and catalytic properties of the most complex intergrown zeolite ITQ-39 determined by electron crystallography

Porous materials such as zeolites contain well-defined pores in molecular dimensions and have important industrial applications in catalysis, sorption and separation. Aluminosilicates with intersecting 10- and 12-ring channels are particularly interesting as selective catalysts. Many porous materials, especially zeolites, form only nanosized powders and some are intergrowths of different structures, making structure determination very challenging. Here, we report the atomic structures of an aluminosilicate zeolite family, ITQ-39, solved from nanocrystals only a few unit cells in size by electron crystallography. ITQ-39 is an intergrowth of three different polymorphs, built from the same layer but with different stacking sequences. ITQ-39 contains stacking faults and twinning with nano-sized domains, being the most complex zeolite ever solved. The unique structure of ITQ-39, with a three-dimensional intersecting pairwise 12-ring and 10-ring pore system, makes it a promising catalyst for converting naphtha into diesel fuel, a process of emerging interest for the petrochemical industry.