Search

The roles of organic additives in the assembly and crystallisation of zeolites are still not fully understood. This is important when attempting to prepare novel frameworks to produce new zeolites. We consider 18-crown-6 ether (18C6) as an additive, which has previously been shown to differentiate between the zeolite EMC-2 (EMT) and faujasite (FAU) frameworks. However, it is unclear whether this distinction is dictated by influences on the metastable free-energy landscape or geometric templating. Using high-pressure synchrotron X-ray diffraction, we have observed that the presence of 18C6 does not impact the EMT framework flexibility-agreeing with our previous geometric simulations and suggesting that 18C6 does not behave as a geometric template. This was further studied by computational modelling using solid-state density-functional theory and lattice dynamics calculations. It is shown that the lattice energy of FAU is lower than EMT, but is strongly impacted by the presence of solvent/guest molecules in the framework. Furthermore, the EMT topology possesses a greater vibrational entropy and is stabilised by free energy at a finite temperature. Overall, these findings demonstrate that the role of the 18C6 additive is to influence the free energy of crystallisation to assemble the EMT framework as opposed to FAU.

This work demonstrates the potential of zeolite Y supported nickel phosphide materials as highly active catalysts for the upgrading of bio-oil as an improved alternative to noble metal and transition metal sulphide systems. Our systematic work studied the effect of using different counterions (NH 4 + , H + , K + ) and Si/Al ratios (2.56 and 15) of the zeolite Y. It demonstrates that whilst the zeolite counterion itself has little impact on the catalytic activity of the bare Y-zeolite, it has a strong influence on the activity of the resulting nickel phosphide catalysts. This effect is related to the nature of the nickel phases formed during the synthesis process Zeolites containing K + ) and Si/Al ratios (2.56 and 15) of the zeolite Y. It demonstrates that whilst the zeolite counterion itself has little impact on the catalytic activity of the bare Y-zeolite, it has a strong influence on the activity of the resulting nickel phosphide catalysts. This effect is related to the nature of the nickel phases formed during the synthesis process Zeolites containing K + and Na + favour the formation of a mixed Ni 12 P 5 /Ni 2 P phase, H + Y produces both Ni 2 P and metallic Ni, whereas NH 4 + Y produces pure Ni 2 P, which can be attributed to the strength of the phosphorus-aluminium interaction and the metal reduction temperature. Using quinoline as a model for the nitrogen-containing compounds in bio-oils, it is shown that the hydrodenitrogenation activity increases in the order Ni 2 P > Ni 0 > Ni 12 P 5 . While significant research has been dedicated to the development of bio-oils produced by thermal liquefaction of biomass, surprisingly little work has been conducted on the subsequent catalytic upgrading of these oils to reduce their heteroatom content and enable processing in conventional petrochemical refineries. This work provides important insights for the design and deployment of novel active transition metal catalysts to enable the incorporation of bio-oils into refineries.

Zeolites with the LTL framework topology are attractive materials for use in optoelectronics, gas adsorption and as chemical reactors. This is due to their unique, one-dimensional (1D) channel systems which are large enough to act as hosts for organic dye molecules and other guest materials. Here, we use high-resolution X-ray diffraction to show the effect of cation exchange on the pore geometry of LTL-type zeolites. The nature of the exchanging cation is shown to influence the free access diameter, volume and water content of the 12-ring (12R) channel systems. As such, cation exchange can be used to tune the molecular sieving and adsorption properties of LTL-type zeolites. This offers new possibilities for these materials in technologically relevant applications. [ABSTRACT FROM AUTHOR]

The use of organic additives presents the greatest versatility and control of zeolite synthesis in order to prepare novel architectures for desired applications. Despite this prospect, there is little clarity of how organic additives are involved in framework assembly and the range of behaviours that are available. To address this issue, we have considered zeolites RHO and ZK-5 which can both be prepared using 18-crown-6 ether as an additive. Previously, this additive has shown to employ different structure directing behaviours to assemble a variety of zeolites. We have used high resolution powder X-ray diffraction and Rietveld refinement to determine structural models for zeolites RHO and ZK-5 with 18-crown-6 ether occluded in the framework. In doing so, we can observe the identity, location and orientation of the occluded additive and reason the structure directing behaviour in synthesis. We report that the isolated 18-crown-6 ether molecule is involved in the assembly of zeolite RHO, and for zeolite ZK-5 it is the K+ coordinated macrocation. In both cases the relevant additive is disordered in the framework, suggesting that they behave as space-filling species that stabilise the formation of the α-cage. [ABSTRACT FROM AUTHOR]

Chabazite (CHA), one of the most common zeolite framework types, has a remarkable capacity to accommodate a wide range of different cations within the unique CHA framework. This has led to CHA being applied extensively in ion exchange, and studied for highly selective gas sorption, most notably through a trapdoor mechanism. Here, we report the systematic study of a series of six chabazite zeolites (i.e. K-CHA, Cs-CHA, Ca-CHA, Ba-CHA, Sr-CHA and Zn-CHA) obtained by subjecting the parent chabazite (KNa-CHA) to exchange operations with cations of different valences and atomic radii. These samples were examined using numerous techniques and it was found that the differences in valence and size between extra-framework cations exert a significant effect on the abundance of these cations positioned in the framework, resulting in differing nitrogen sorption ability measured in the synthesised chabazite zeolites. These findings will help to understand how the zeolite counter-cation affects the ability of the CHA material to selectively sequester and separate gases through the use of the trapdoor mechanism. [ABSTRACT FROM AUTHOR]

Framework materials have structures containing strongly bonded polyhedral groups of atoms connected through their vertices. Typically the energy cost for variations of the inter-polyhedral geometry is much less than the cost of distortions of the polyhedra themselves - as in the case of silicates, where the geometry of the SiO4 tetrahedral group is much more strongly constrained than the Si-O-Si bridging angle. As a result, framework materials frequently display intrinsic flexibility, and their dynamic and static properties are strongly influenced by low-energy collective motions of the polyhedra. Insight into these motions can be obtained in reciprocal space through the `rigid unit mode' (RUM) model, and in real-space through template-based geometric simulations. We briefly review the framework flexibility phenomena in energy-relevant materials, including ionic conductors, perovskites and zeolites. In particular we examine the `flexibility window' phenomenon in zeolites and present novel results on the flexibility window of the EMT framework, which shed light on the role of structure-directing agents. Our key finding is that the crown ether, despite its steric bulk, does not limit the geometric flexibility of the framework. [ABSTRACT FROM AUTHOR]

Template-based geometric simulation is a specialised method for modelling flexible framework structures made up of rigid units using a simplified, localised physical model. The strengths of the method are its ability to handle large all-atom structural models rapidly and at minimal computational expense, and to provide insights into the links between local bonding and steric geometry and global flexibility. We review the implementation of geometric simulation in the ‘GASP’ software, and its application to the study of materials including zeolites, perovskites and metal–organic frameworks. The latest version (5) of GASP has significant improvements and extensions, in particular an improved algorithm for relaxation of atomic positions, and the capacity to handle both polyhedral and molecular structural units. GASP is freely available to researchers. [ABSTRACT FROM AUTHOR]

We present geometric simulations on a zeolite framework (faujasite) with extra-framework methanol and water contents explicitly present. We distinguish the intrinsic flexibility window of the framework from the newly defined extrinsic window limited by host–guest steric interactions. The extrinsic flexibility window can be limited not only in compression, but also in expansion, as the beta-cages in a maximally expanded framework lack the flexibility to adapt bulky contents such as a combination of methanol and water molecules. Our simulations suggest a reinterpretation of extra-framework content nominally refined as water sites in compression experiments. [ABSTRACT FROM AUTHOR]

Alkali metal cations play an important role in directing formation of zeolite frameworks in the absence of organic structure directing agents. The interplay between Na and Cs cations in directing formation of zeolite RHO is the main focus of this study. Depending on the ratio of Na and Cs in the precursor gel, with no organic structure directing agents, several different zeolites form during synthesis. This indicates that pure zeolite RHO can only be obtained at a very specific Cs to Na ratio. More generally, this may shed light on the bigger question of how zeolite pores form in the absence of organic structure directing agents. [ABSTRACT FROM AUTHOR]

Silicalite. a high-silica zeolite with MFI topology, undergoes pressure-induced amorph i sal i on (PIA) when compressed with nonpenetrating pressure media. PIA is prevented by penetrating pressure media. In several zeolites, links have recently been found between pressure-induced phase transitions and ihe flexibility window, a geometric property of the framework identified by geometric simulation. We have analyzed structural dala from compression experiments on silicalite, and we find that PIA occurs while the structure lies within its flexibility window. Penetrating media, which prevent PIA. push the structure outside the flexibility window. Thus framework flexibility is required for PIA to occur in silicalite. This link between amorphisation and flexibility is further evidence of the deep connections between framework geometry, flexibility, and the physical properties of zeolites. [ABSTRACT FROM AUTHOR]

Today synthetic zeolites are the most important catalysts in petrochemical refineries because of their high internal surface areas and molecular-sieving properties. There have been considerable efforts to synthesize new zeolites with specific pore geometries, to add to the 167 available at present. Millions of hypothetical structures have been generated on the basis of energy minimization, and there is an ongoing search for criteria capable of predicting new zeolite structures. Here we show, by geometric simulation, that all realizable zeolite framework structures show a flexibility window over a range of densities. We conjecture that this flexibility window is a necessary structural feature that enables zeolite synthesis, and therefore provides a valuable selection criterion when evaluating hypothetical zeolite framework structures as potential synthetic targets. We show that it is a general feature that experimental densities of silica zeolites lie at the low-density edge of this window—as the pores are driven to their maximum volume by Coulomb inflation. This is in contrast to most solids, which have the highest density consistent with the local chemical and geometrical constraints. [ABSTRACT FROM AUTHOR]

Every year millions of tons of zeolites are produced, being used as molecular sieves, hydrocracking catalysts, gas-capture materials and for emerging novel applications. There is a demand to synthesise new zeolites with bespoke frameworks, which are tailor-made for a chosen application. To achieve these 'designer zeolites' it is crucial to fully understand the host-guest interactions between organic additives and zeolitic frameworks. Here we have studied four different zeolites, synthesised with the same organic additive, 18-crown-6 ether, which show observable differences in the host-guest interactions. We demonstrate that the framework geometry dominates the decomposition temperature, enthalpy and mechanism. The zeolites show unique decomposition features, emphasising experimental differences in how the organic additive and framework interact. Image 1 • Thermogravimetric and differential thermal analysis methods were used to study the crown ether decomposition in zeolites. • There is a clear distinction in the decomposition of the 18C6 molecule in the different frameworks. • Measurable differences are seen in the host-guest interactions between organic additives and zeolite hosts. • Stronger host-guest confinement interactions are seen in zeolites with smaller pores. • Our methodology can be applied to other zeolites to study the host-guest interactions. [ABSTRACT FROM AUTHOR]