Dewi W. Lewis

Zeolitic Polyoxometalate-Based Metal−Organic Frameworks (Z-POMOFs): Computational Evaluation of Hypothetical Polymorphs and the Successful Targeted Synthesis of the Redox-Active Z-POMOF1

The targeted design and simulation of a new family of zeolitic metal-organic frameworks (MOFs) based on benzenedicarboxylate (BDC) as the ligand and epsilon-type Keggin polyoxometalates (POMs) as building units, named here Z-POMOFs, have been performed. A key feature is the use of the analogy between the connectivity of silicon in dense minerals and zeolites with that of the epsilon-type Keggin POMs capped with Zn(II) ions. Handling the epsilon-Keggin as a building block, a selection of 21 zeotype structures, together with a series of dense minerals were constructed and their relative stabilities computed. Among these Z-POMOFs, the cristobalite-like structure was predicted to be the most stable structure. This prediction has been experimentally validated by the targeted synthesis of the first experimental Z-POMOF structure, which was strikingly found to possess the cristobalite topology, with three interpenetrated networks. Crystals of [NBu(4)](3)[PMo(V)(8)Mo(VI)(4)O(36)(OH)(4)Zn(4)(BDC)(2)].2H(2)O (Z-POMOF1) have been isolated under hydrothermal conditions from the reduction of ammonium heptamolybdate in the presence of phosphorous acid and Zn(II) ions. Tetrabutylammonium cations play the role of counterions and space-filling agents in this tridimensional interpenetrated framework. Moreover, the electrochemistry of the epsilon-Keggin POM is maintained and can be exploited in the insoluble Z-POMOF1 framework, as demonstrated by the electrocatalytic reduction of bromate.

Modelling of structure and reactivity in zeolites

Publisher Summary This chapter highlights recent applications using both forcefield and electronic structure techniques to the study of problems in structure, sorption, synthesis, and reactivity of both zeolites and microporous aluminophosphates. In particular, the chapter describes how simulation techniques may yield detailed and accurate information of the structure, and energies associated with framework modification by hetero-atoms; on the diffusion coefficients and migration mechanisms of sorbed molecules; on the nature of the interactions between host structures and templates used in zeolite synthesis; and on the use of electronic structure techniques in throwing light on the crucial questions of acid site–molecule interactions in solid acid catalysts. The chapter also summarizes the range and status of the current computational methodologies before describing the recent illustrative applications. Forcefield methods have been used extensively in modeling the structures of microporous solids, and in investigating docking and diffusion of sorbed molecules. The full range of simulation techniques—Energy Minimisation (EM), Monte Carlo (MC), and Molecular Dynamics (MD) have been employed, as have combinations of these methods, as in the widely used “docking” procedure for identifying low-energy sites for sorbed molecules. The chapter concludes with consideration of the likely future direction for the field.

Zeolitic imidazole frameworks: structural and energetics trends compared with their zeolite analogues

We use periodic DFT calculations to compute the total energy of known zeolitic imidazole frameworks (ZIFs) together with those of hypothetical porous ZIFs. We show that the total energy of ZIFs decreases with increasing density, in a similar fashion to the alumino-silicate zeolites, but with a more complex energy landscape. The computational evaluation of the stability of hypothetical ZIFs is useful in the search for viable synthesis targets. Our results suggest that a number of hitherto undiscovered nanoporous topologies should be amenable to synthesis (CAN, ATN) and that even the most open framework types might be obtained with appropriately substituted ligands.

Solvent-free routes to clean technology.

A major aim for the chemical technology of the future is the avoidance of noxious and environmentally unacceptable organic solvents. In this concept article we discuss more environmentally friendly and highly selective alternatives which we have evolved for carrying out a number of important chemical conversions. These entail the use of porous heterogeneous catalysts in which the active sites have been atomically engineered and fully characterized. Such solid catalysts operate under solvent-free conditions and usually entail one-step processes.

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.

Zeolite-Modified Discriminating Gas Sensors

The use of zeolites as transformation layers to enhance the response and discriminating power of solid-state metal-oxide-semiconductor gas sensors is demonstrated. Thick film sensors were prepared by screen printing layers of tungsten trioxide or chromium titanium oxide with various zeolites as overlayers. The sensors gas response was tested against carbon monoxide and ethanol in varying concentrations. Experimental result.,; show that it is possible to dramatically alter the response behavior of the devices: in the instance of ethanol gas with a zeolite Y-modified sensor. the response was increased by 2 orders of magnitude compared to the unmodified sensor. Computational modelling studies show that a combination of catalytic reaction and diffusion behavior are responsible for these changes. Such discriminatory behavior should prove useful in electronic noses and sensor arrays. (c) 2009 The Electrochemical Society. [DOI: 10.1149/1.3065436] All rights reserved.

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.

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 120 °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. 120 °C, and the oxidation of the tetrahedral Fe(II) ions at temperatures above 300 °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.

Application of computer modelling to the mechanisms of synthesis of microporous catalytic materials

We present results of a study of species present during the hydrothermal synthesis of zeotype materials. We have studied the interactions of typical silica fragments, which condense to form the framework structure, with both solvent (water) and the organic molecules used as structure-directing agents. The hydrophobic nature of the fragments results in the collapse of the fragment structure on solvation, resulting in geometries far removed from their configuration in the crystalline material. The presence of the organic template provides stabilisation for the hydrated fragments, effectively shielding them from the water and allowing them to maintain their open structure. The impact of these results on possible mechanisms of crystallisation is discussed.

Discrimination Effects in Zeolite Modified Metal Oxide Semiconductor Gas Sensors

The use of zeolites to enhance the response and discriminating power of solid state metal oxide semiconductor gas sensors is demonstrated. Thick film sensors were prepared by screen printing layers of chromium titanium oxide (CTO) or tungsten trioxide with various zeolites as over-layers. The sensors gas response was tested against two similar gases; ethanol and isopropyl alcohol. The modified sensors effectively discriminated between the two gases. An understanding of this discriminating behavior is elucidated through computer modeling of diffusion and reaction processes occurring in the zeolite transformation layer and sensor element. Our analysis suggests that the discrimination is the result of the size and shape selective behavior.