Furio Corà

Modeling the framework stability and catalytic activity of pure and transition metal-doped zeotypes

We present a thorough computational study of transition metal-doped zeolite and aluminophosphate (AIPO) frameworks. The structural and electronic chemistry of the dopants is examined with ab initio quantum mechanical calculations, and the results correlated with the Bronsted and Lewis acid strength, and with the redox potential of the dopant ions in the framework. The energetics of doping is provided, and is employed to analyze the mode of dopant incorporation, and its site ordering in the microporous framework. In total, 23 dopant ions are examined in the isostructural framework of chabasite and AlPO-34. These cover most of the isomorphous framework replacements known to occur experimentally, but also framework replacements that have not yet been achieved. In this case, ab initio modeling techniques are employed in a predictive way. Finally, we present a computational study of the alkene epoxidation on titanosilicates, that covers the whole catalytic cycle

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.

On the nature of iron species in iron substituted aluminophosphates

The local structure of the iron centres in the as-synthesised and calcined FAPO-5 and FAPO-18 catalysts has been investigated using a combination of techniques, including X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Mossbauer spectroscopy and computational modelling. The calcination of the as-synthesised materials has been followed by combined in situ XRD/QuEXAFS measurements, which reveal the different nature of the iron species in the two catalysts. In the as-synthesised FAPO-5 catalyst all the iron centres are present as octahedrally coordinated Fe(III) ions; upon calcination, two water molecules are desorbed when the temperature is raised above 120u2006°C to yield tetrahedrally coordinated Fe(III) ions in the active catalyst. In contrast, as-synthesised FAPO-18 contains a mixture of tetrahedrally coordinated Fe(II) ions and octahedral Fe(III) centres. Two independent processes occur upon calcination of FAPO-18: the desorption of water from the octahedral Fe(III) ions at ca. 120u2006°C, and the oxidation of the tetrahedral Fe(II) ions at temperatures above 300u2006°C; both materials possess tetrahedrally coordinated Fe(III) ions in the calcined state. The three dimensional structure and electronic state of the tetrahedral Fe(III) ions in the activated catalysts has been determined viaab initio quantum mechanical calculations and X-ray absorption spectroscopy.

Structural and magnetic phase transitions in simple oxides using hybrid functionals

We present the structural as well as elastic properties of the alkaline earth oxides and FeO, calculated using hybrid exchange functionals within DFT. We show that by empirically fitting the amount of Fock-exchange in the hybrid functionals, we can accurately reproduce the pressure-induced phase transitions for MgO, CaO, SrO and BaO. For FeO the hybrid functionals predict an insulator↔metal transition at ca. 150u2009GPa, associated with an i-B8↔B8 structural phase transition. The structural phase transition is accompanied by a spin transition from a high- to low-spin electron configuration on the Fe2+ ions. Hence, FeO undergoes a magnetic phase transition from an anti-ferromagnetic to non-magnetic structure. We also find that as the ionicity of the polymorphs increases a higher fraction of Fock-exchange is required to reproduce the structural volumes reported from experiments.

A computational modelling study of oxygen vacancies at LaCoO3 perovskite surfaces

Atomistic computational modelling of the surface structure of the catalytically-active perovskite LaCoO(3) has been undertaken in order to develop better models of the processes involved during catalytic oxidation processes. In particular, the energetics of creating oxygen ion vacancies at the surface have been investigated for the three low index faces (100), (110) and (111). Two mechanisms for vacancy creation have been considered involving dopant Sr(2+) cations at the La(3+) site and reduction of Co(3+) to Co(2+). For both mechanisms, there is a general tendency that the smaller the cation defect separation, the lower the energy of the cluster, as would be expected from simple electrostatic considerations. In addition, there are clear indications that oxygen vacancies are more easily created at the surface than in the bulk. The results also confirm that the presence of defects strongly influences crystal morphology and surface chemistry. The importance of individual crystal surfaces in catalysis is discussed in terms of the energetics for the creation of oxygen vacancies.

Acid and redox properties of Co-substituted aluminium phosphates

Abstract Ab initio quantum chemical techniques are applied to the investigation of structural and bonding properties of microporous aluminophosphates and gallophosphates. The calculations find a close measure of agreement with experimental structural data. The bonding in the materials is shown to be of ‘molecular-ionic’ character, i.e. comprising Al 3+ (Ga 3+ ) and PO 4 3− ions. Calculated redox energies are reported for Co-substituted materials.

Design of microporous transition metal oxide catalysts and investigation of their synthesis conditions

Microporous materials based on cations of medium acidic strength. such as, the alumino-silicate zeolites, represent one of the most widely used heterogeneous catalysts. Synthesising novel materials that could combine the molecular-sieve characteristics of Si-zeolites. with the much higher acidic strength and redox properties of early transition metal cations, would be a major breakthrough in catalysis. In this paper we examine this topic employing computer modelling in advance of experiment: we design suitable microporous structures of MoO3 and WO3, compare their relative energy with those of the known dense polymorphs of the materials. and finally explore the possibility of their synthesis via a template/host approach previously developed for zeolitic materials

Site ordering of dopant ions in microporous aluminophosphates: size effects

We apply computational techniques based on interatomic potentials, to investigate the site ordering of dopant ions in microporous aluminophosphate (AlPO) catalysts. We show that the larger the difference in ionic radius between host ion and dopant, the more site-ordered phases become energetically stable. We relate this result to the topological features of the host AlPO framework.

Electronic state and three-dimensional structure of Mn(III) active sites in manganese-containing aluminophosphate molecular sieve catalysts for the oxyfunctionalisation of alkanes

Combined in situ X-ray absorption spectroscopy measurements and quantum mechanical calculations yield a quantitative three-dimensional structure of the tetrahedrally coordinated Mn(III) active sites in MnAlPO catalysts.

Quantum Mechanical Investigations on the Insertion Compounds of Early Transition Metal Oxides

We present the results of electronic structure ab initio Hartree-Fock calculations performed on the perovskite, hexagonal, pyrochlore and layered polymorphic structures of MoO3 and WO3. We also examine their sodium and potassium insertion compounds, i.e. the molybdenum and tungsten bronzes; the materials examined are at the basis of applications in electrochromic devices and rechargeable batteries. Results of the calculations allow to gain insight into the processes occurring at the atomic level during the insertion process: we examine the relative stability of the host MoO3 and WO3 frameworks under addition of electrons to the conduction band, and extraframework ions in the empty interstices of the host lattice, which occur upon electrochemical insertion. The extraframework ions show a templating behaviour for the structure of the host transition metal oxide framework, reminiscent of zeolite synthesis. We exploit the knowledge gained in the latter field of research to examine the feasibility of the synthesis of novel microporous transition metal oxide lattices with composition MoO3 and WO3.