Synthetic Control of the Defect Structure and Hierarchical Extra-Large-/Small-Pore Microporosity in Aluminosilicate Zeolite SWY.

The SWY-type aluminosilicate zeolite, STA-30, has been synthesized via different routes to understand its defect chemistry and solid acidity. The synthetic parameters varied were the gel aging, the Al source, and the organic structure directing agent. All syntheses give crystalline materials with similar Si/Al ratios (6-7) that are stable in the activated K,H-form and closely similar by powder X-ray diffraction. However, they exhibit major differences in the crystal morphology and in their intracrystalline porosity and silanol concentrations. The diDABCO-C82+ (1,1'-(octane-1,8-diyl)bis(1,4-diazabicyclo[2.2.2]octan)-1-ium)-templated STA-30 samples (but not those templated by bisquinuclidinium octane, diQuin-C82+) possess hierarchical microporosity, consisting of noncrystallographic extra-large micropores (13 Å) that connect with the characteristic swy and gme cages of the SWY structure. This results in pore volumes up to 30% greater than those measured in activated diQuin-C8_STA-30 as well as higher concentrations of silanols and fewer Brønsted acid sites (BASs). The hierarchical porosity is demonstrated by isopentane adsorption and the FTIR of adsorbed pyridine, which shows that up to 77% of the BASs are accessible (remarkable for a zeolite that has a small-pore crystal structure). A structural model of single can/d6r column vacancies is proposed for the extra-large micropores, which is revealed unambiguously by high-resolution scanning transmission electron microscopy. STA-30 can therefore be prepared as a hierarchically porous zeolite via direct synthesis. The additional noncrystallographic porosity and, subsequently, the amount of SiOHs in the zeolites can be enhanced or strongly reduced by the choice of crystallization conditions.

Machine learning accelerated high-throughput screening of zeolites for the selective adsorption of xylene isomers.

The production of widely used polymers such as polyester currently relies upon the chemical separation of and transformation of xylene isomers. The least valuable but most prevalent isomer is meta-xylene which can be selectively transformed into the more useful and expensive para-xylene isomer using a zeolite catalyst but at a high energy cost. In this work, high-throughput screening of existing and hypothetical zeolite databases containing more than two million structures was performed, using a combination of classical simulation and deep neural network methods to identify promising materials for selective adsorption of meta-xylene. Novel anomaly detection techniques were applied to the heavily biased classification task of identifying structures with a selectivity greater than that of the best performing existing zeolite, ZSM-5 (MFI topology). Eight hypothetical zeolite topologies are found to be several orders of magnitude more selective towards meta-xylene than ZSM-5 which may provide an impetus for synthetic efforts to realise these promising materials. Moreover, the leading hypothetical frameworks identified from the screening procedure require a markedly lower operating temperature to achieve the diffusion seen in existing materials, suggesting significant energetic savings if the frameworks can be realised.

Understanding the Anion-Templated, OSDA-Free, Interzeolite Conversion Synthesis of High Silica Zeolite ZK-5.

High silica zeolite ZK-5 (framework Si/Al=4.8) has been prepared by interzeolite conversion from ultrastable zeolite Y via a co-templating route using alkali metal cations and nitrate anions but without organic structure directing agents. The mechanism, which involves zeolite framework - alkali metal cation - nitrate anion ordering, has been established by a combination of chemical and thermal analyses, Raman spectroscopy, computational modelling, and X-ray powder diffraction. Ammonium exchange gives ZK-5 with occluded ammonium nitrate and subsequent heating gives microporous zeolite ZK-5.

Site-Specific Iron Substitution in STA-28, a Large Pore Aluminophosphate Zeotype Prepared by Using 1,10-Phenanthrolines as Framework-Bound Templates.

An AlPO4 zeotype has been prepared using the aromatic diamine 1,10-phenanthroline and some of its methylated analogues as templates. In each case the two template N atoms bind to a specific framework Al site to expand its coordination to the unusual octahedral AlO4 N2 environment. Furthermore, using this framework-bound template, Fe atoms can be included selectively at this site in the framework by direct synthesis, as confirmed by annular dark field scanning transmission electron microscopy and Rietveld refinement. Calcination removes the organic molecules to give large pore framework solids, with BET surface areas up to 540 m2 g-1 and two perpendicular sets of channels that intersect to give pore space connected by 12-ring openings along all crystallographic directions.

A family of molecular sieves containing framework-bound organic structure-directing agents.

Organic structure-directing agents (OSDAs), such as quaternary ammonium cations and amines, used in the synthesis of zeolites and related crystalline microporous oxides usually end up entrapped inside the void spaces of the crystallized inorganic host lattice. But none of them is known to form direct chemical bonds to the framework of these industrially important catalysts and adsorbents. We demonstrate that ECR-40, currently regarded as a typical silicoaluminophosphate molecular sieve, constitutes instead a new family of inorganic-organic hybrid networks in which the OSDAs are covalently bonded to the inorganic framework. ECR-40 crystallization begins with the formation of an Al-OSDA complex in the liquid phase in which the Al is octahedrally coordinated. This unit is incorporated in the crystallizing ECR-40. Subsequent removal of framework-bound OSDAs generates Al-O-Al linkages in a fully tetrahedrally coordinated framework.

An aluminophosphate molecular sieve with 36 crystallographically distinct tetrahedral sites.

The structure of the new medium-pore aluminophosphate molecular sieve PST-6 is determined by the combined use of rotation electron diffraction tomography, synchrotron X-ray powder diffraction, and computer modeling. PST-6 was prepared by calcination of another new aluminophosphate material with an unknown structure synthesized using diethylamine as a structure-directing agent, which is thought to contain bridging hydroxy groups. PST-6 has 36 crystallographically distinct tetrahedral sites in the asymmetric unit and is thus crystallographically the most complex zeolitic structure ever solved.