Luis Gómez-Hortigüela

Supramolecular chemistry in the structure direction of microporous materials from aromatic structure-directing agents.

A combination of fluorescence spectroscopy, thermogravimetric analysis, and molecular mechanics calculations has been used to study the structure-directing effect of the aromatic benzylpyrrolidine (BP) molecule (and its monofluorinated derivatives), and (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) in the synthesis of the microporous AFI structure. The results clearly show that, while all molecules form supramolecular aggregates in concentrated water solution, BPM molecules have a much more pronounced trend to aggregate as dimers within the AFI structure due to the development of interdimer H-bond interactions. Instead, BP (and its ortho- and meta-fluorinated derivatives) SDAs tend to incorporate in the AFI structure as monomers but with the simultaneous occlusion of water molecules, while para-fluorinated BP derivatives do not form compact dimers able to be accommodated in the AFI structure. We propose a crystallization mechanism where the presence of dimers is required for the nucleation step to occur, while crystal growth takes place through the simultaneous occlusion of SDA monomers and water (when the synthesis is performed with BP and derivatives) or through the occlusion of SDA dimers (in the synthesis with BPM).

Catalytic Reaction Mechanism of Mn‐Doped Nanoporous Aluminophosphates for the Aerobic Oxidation of Hydrocarbons

In this work we apply state-of-the-art electronic-structure-based computational methods based on hybrid-exchange density functional theory to study the mechanism of the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates (Mn-AlPOs). We compare our results with available experimental data. We show that the catalytic efficiency of Mn-AlPOs in oxidation reactions is intrinsically linked to 1) the Mn redox activity, in particular between 2+ and 3+ oxidation states, and 2) the coordinative insaturation of tetrahedral Mn embedded in AlPO frameworks, which facilitates the reaction by stabilising oxo-type radicals through the formation of Mn complexes. Our mechanism demonstrates the crucial role of both Mn(III) and Mn(II) in the reaction mechanism: Mn(III) sites undergo an initial reaction cycle that leads to the production of the alkyl hydroperoxide intermediate, which can only be transformed into the oxidative products (alcohol, aldehyde and acid) by Mn(II). A preactivation step is required to yield the reduced Mn(II) sites able to decompose the hydroperoxide intermediates; this step takes place through a transformation of the hydrocarbon into the corresponding peroxo-derivative, stabilised by forming a complex with Mn(III) and yielding at the same time reduced Mn(II) sites. Both species enter a subsequent propagation cycle in which Mn(II) catalyses the dissociation of the hydroperoxide that proceeds until the formation of the oxidative products by two parallel pathways, through alkoxy- or hydroxy-radical-like intermediates, whilst the Mn(III)-peroxo complex enables further production of the hydroperoxide intermediate.

Dopant-siting selectivity in nanoporous catalysts: control of proton accessibility in zeolite catalysts through the rational use of templates

A computational study is performed to rationalize the effect of the organic template molecules used in the synthesis of zeolites on their catalytic behaviour. Apart from being structure-directing, these molecules influence the location of heteroatoms. Molecules bearing acidic protons susceptible to forming H-bonds with framework oxygens show the strongest dopant-siting direction.

Computational study of a chiral supramolecular arrangement of organic structure directing molecules for the AFI structure

Molecular mechanics computational methods have been employed to study the structure directing effect of S-(-)-1-benzyl-2-pyrrolidiniummethanol molecules towards microporous aluminophosphate materials with the AFI structure. These chiral molecules form dimers inside the one-dimensional AFI channel, which are the active structure-directing agents in the synthesis. Four different conformers of the S-(-)-1-benzyl-2-pyrrolidiniummethanol molecule are in principle available; of these, the S,S-trans shows a marked stability in dimeric form. Self-assembly between adjacent dimers generates a helicoidal, and hence chiral arrangement of the organic molecules, which extends with the same direction of rotation through the whole solid, and may thus be employed to introduce chirality in the microporous material.

Molecular insights into the self-aggregation of aromatic molecules in the synthesis of nanoporous aluminophosphates: a multilevel approach.

Fluorescence spectroscopy and a range of computer simulation techniques are used to study the structure directing effect of benzylpyrrolidine (BP) and (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) in the synthesis of nanoporous aluminophosphate frameworks with AFI (one-dimensional channels) and SAO (three-dimensional interconnected channels) topologies. We study the supramolecular chemistry of BP and BPM molecules in aqueous solution and compare it with the aggregation state of the molecules found when they are inside the AlPO nanopores after crystallization. The aggregation of the molecules within the structures can be explained by a combination of thermodynamic and kinetic effects. The former are given by the stability of the molecular species interacting with the oxide networks relative to their stability in solution; the latter depend on the aggregation behavior of the molecules in the synthesis gels prior to crystallization. Whereas BPM only forms one type of aggregate in solution, which has the appropriate conformation to match the empty channels of the forming nanoporous frameworks, BP forms aggregates with different molecular orientations, of which only one matches the framework interstices. This different supramolecular chemistry, together with the higher interaction of BPM with the oxide networks, makes BPM a better structure directing agent (SDA); it is also responsible for the higher incorporation of BPM as dimers in the frameworks, especially in the AFI structure, observed experimentally. The concentration of the SDA molecules in the gels, and so the density per volume of the SDAs, determines the exclusion zone from which the pores and/or cavities of the framework will arise, and so the porous network of the formed material. A clear relationship between the SDA density in solution and in the framework is observed, thus enabling an eventual control of the material density by adjusting the SDA concentration in the gels. The topological instability intrinsic to these open framework structures is compensated by a high host-guest interaction energy; the SAO topology is further stabilized by doping with Zn. Our computational results account for and rationalize all the effects observed experimentally, providing a complete picture of the mode of structure direction of these aromatic molecules in the synthesis of nanoporous aluminophosphates.

Insights into Structure Direction of Microporous Aluminophosphates: Competition between Organic Molecules and Water

A combination of experimental characterisation techniques and computational modelling has allowed us to gain insight into the molecular features governing structure direction in the synthesis of microporous aluminophosphates. The occlusion of three different structure-directing agents (SDAs), triethylamine (TEA), benzylpyrrolidine (BP) and (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM), within the AFI structure during its crystallisation, together with the simultaneous incorporation of water, has been experimentally measured. We found a higher incorporation of organic molecules in the structure obtained with BPM, while a higher water (and lower organic) content is found for the ones obtained with TEA and BP as SDAs. The computational study provides a thermodynamic explanation for the observed behaviour in terms of the relative stabilisation energy of the SDAs and water molecules within the AFI framework compared with when they are in aqueous solution, and demonstrates that a competition for preferential occupation exists between water and organic SDAs, which is a function of the interaction with the inorganic framework. The lower interaction of TEA and BP molecules with the AFI structure promotes the simultaneous incorporation of water molecules in the 12-membered-ring (MR) channel, to increase the host-guest interaction energy and thus the thermodynamic stability. The presence of strongly interacting methanol groups in the BPM molecules leads to the incorporation of only organic molecules within the 12-MR channels. Our results demonstrate the essential role that water molecules play in the stabilisation of hydrophilic microporous aluminophosphates; a minimum amount of organic SDA is, however, essential for a templating role of the microporous architecture.

Aerobic oxidation of hydrocarbons in Mn-doped aluminophosphates: a computational perspective to understand mechanism and selectivity

We discuss the mechanism and energetics for the aerobic oxidation of hydrocarbons catalysed by Mn-doped nanoporous aluminophosphates with the AFI structure (Mn-APO-5), obtained computationally using electronic structure techniques. Calculations have been performed employing hybrid exchange density functional theory methods under periodic boundary conditions. The active sites of the catalyst are tetrahedral Mn ions isomorphously replacing Al in the microporous crystalline framework of the AlPO host. Since all Al sites in AFI are symmetry equivalent, all Mn dopants are in an identical chemical and structural environment, and hence satisfy the definition of a single-site heterogeneous catalyst. We focus in particular on the atomic-level origin of selectivity in this catalytic reaction.

Controlled growth of hydroxyapatite on the surface of natural stilbite from Ethiopia: application in mitigation of fluorosis

This contribution reports the preparation of a composite material based on a natural zeolite rich in calcium, stilbite, on which surface nanosized hydroxyapatite is grown under controlled conditions. Low crystallisation temperatures enable the crystallisation of nano-sized hydroxyapatite crystals, which display a very high intrinsic capacity (based on weight of hydroxyapatite) of fluoride removal. The low cost and easy accessibility of the materials employed for its preparation, as well as the simplicity of the synthesis procedure, make these materials suitable candidates for their use in fluoride removal from drinking water.

Formation of a Nonlinear Optical Host-Guest Hybrid Material by Tight Confinement of LDS 722 into Aluminophosphate 1D Nanochannels.

In this work, hemicyanine dye LDS 722 is encapsulated into the 1D elliptical nanochannels of MgAPO-11 aluminophosphate by a crystallization inclusion method. The synthesis of the hybrid material has been optimized through a systematic variation of the crystallization conditions in order to obtain pure and large crystals (around 20 μm×30 μm) suitable for optical applications. The tight fitting between the molecular size of the guest dye and the pore dimensions of the host has favored a rigid planar conformation of the dye, restricting its inherent flexibility, which is confirmed by molecular simulations. Consequently, the encapsulation of LDS 722 into MgAPO-11 has led to an astonishing enhancement of the fluorescence with respect to the dye into MgAPO-5, with slightly larger cylindrical channels, and with respect to the dye in solution. Moreover, the perfect alignment of LDS 722 (dye with intrinsic nonlinear-optical properties) along the channels of MgAPO-11 has revealed attractive second-order nonlinear properties, such as second harmonic generation, proven through microscopy measurements in single crystals.

Synthesis of Zn-containing microporous aluminophosphate with the STA-1 structure

Zn atoms have been incorporated into the STA-1 microporous framework (SAO framework type) for the first time. Pure Zn-containing STA-1 aluminophosphate has been obtained in the presence of either (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) or benzylpyrrolidine (BP) as structure-directing agent. Characterisation results have demonstrated that the organic molecules remain intact within the microporous framework after the hydrothermal treatment. The X-ray diffraction pattern has been indexed in the tetragonal space group I ̅4m2 (a = 13.7828(1) Å and c = 21.9444(4) Å). Chemical analysis and (31)P MAS NMR spectroscopy confirm the incorporation of ca. 4.5 Zn atoms per unit cell by isomorphous substitution of Al. This large-pore aluminophosphate has one of the lowest framework densities among the known zeolite structures, and is therefore of potential interest for applications in adsorption or catalytic processes involving bulky molecules.