A microporous silicoaluminophosphate with a novel topology type, STA-20, has been prepared via a dual templating method using hexamethylene bisdiazabicyclooctane (diDABCO-C6) and trimethylamine as ...cotemplates. Its structure has been solved and confirmed using a multitechnique approach that included the use of a hypothetical zeolite database to obtain a candidate starting structure, followed by scanning transmission electron microscopy with annular dark field imaging and Rietveld refinement. STA-20 is a member of the ABC-6 family of zeotype structures. The structure has trigonal symmetry, P-31c, with a = 13.15497(18) Å and c = 30.5833(4) Å in the calcined form. It has a 12-layer stacking sequence of 6-rings (6Rs), AABAABAACAAC(A), which contains single and double 6R units. In addition to d6r, can, and gme cages, STA-20 possesses the longest cage observed in an ordered ABC-6 material, giving a 3D-connected pore system limited by 8R windows. Models for the location of the templates within cages of the framework were obtained by combining elemental analysis, 13C MAS NMR, computer modeling, and Rietveld refinement.
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 AlO4N2 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.
The aromatic diamine 1,10‐phenanthroline acts as a framework‐bound template for the large‐pore aluminophosphate zeotype STA‐28 and can be used to introduce Fe for Al at a single crystallographic site, as verified by Rietveld refinement and annular dark field scanning transmission electron microscopy. Removal of the template leaves a crystalline microporous solid with accessible iron cations.
A new catalytically active zeolite, designated EMM-17 (ExxonMobil Material-17), with a three-dimensional (3D) 11 × 10 × 10-ring topology has been discovered from high throughput experiments while ...evaluating a family of new organic structure directing agents (OSDAs), 1-alkyl-4-(pyrrolidin-1-yl)pyridin-1-ium hydroxide. The framework structure was determined by model building techniques and confirmed by diffraction calculations. The EMM-17 structure is a random intergrowth of two polymorphs which have a 3D arrangement of intersecting 11 × 10 × 10-ring pores. EMM-17 is stable to calcination to remove the OSDA and can be reproducibly synthesized in the presence of fluoride using common, inexpensive reagents over a wide Si/Al range from 15 to infinity, enabling the catalyst acidity to be tailored to almost any petrochemical application. Unlike OSDAs for many new zeolite structures, the OSDAs for EMM-17 are prepared in one simple alkylation step, making EMM-17 an easy to prepare, highly accessible, catalytically active zeolite. Zeolites containing odd numbered channel sizes are rare, and this is the first confirmed example of a 3D 11-ring aluminosilicate zeolite with a pore size in between those of the commercially important 10- and 12-ring zeolites such as ZSM-5 and Zeolite-Y, respectively. Catalysts prepared from EMM-17 exhibit significantly higher activity for catalytic isomerization with no loss in selectivity than current state of the art catalysts. Catalytic isomerization of linear to branched alkanes is a critical component of commercial dewaxing, allowing for the improvement of cold flow properties of hydrocarbon fuels and lubricants through selective hydroisomerization of normal paraffins.
Liquid-phase exfoliation of layered materials offers a large-scale approach toward the synthesis of 2D nanostructures. Structural properties of materials can however change during transition from ...bulk to the 2D state. Any such changes must be examined and understood for successful implementation of 2D nanostructures. In this work, we demonstrate nonbulk stacking sequences in the few-layer MoS2 and WS2 nanoflakes produced by liquid-phase exfoliation. Our analysis shows that nonbulk stacking sequences can be derived from its bulk counterparts by translational shifts of the layers. No structural changes within the layers were observed. Twenty-seven MoS2 and five WS2 nanoflakes were imaged and analyzed. Nine MoS2 and four WS2 nanoflakes displayed nonbulk stacking. Such dominance of the nonbulk stacking suggests high possibility of unusual stacking sequences in other 2D nanostructures. Notably, the electronic structure of some non bulk stacked bilayers presents characteristics which are uncommon to either the bulk phase or the single monolayer, for instance, a spin-split conduction band bottom. Our main characterization technique was annular dark-field scanning transmission electron microscopy, which offers direct and reliable imaging of atomic columns. The stacking characterization approach employed here can be readily applied toward other few-layer transition metal chalcogenides and oxides.
We have studied the exfoliation and dispersion of hexabenzocoronene (HBC) in 28 different solvents. We see a wide range of dispersed concentrations and aggregation states, all of which can be related ...to the solvent properties. To a first approximation, the dispersed concentration is maximized for solvents with Hildebrand solubility parameter close to 21 MPa1/2, similar to graphitic materials such as nanotubes and graphene. We have also studied the concentration dependence of the absorbance and photoluminescence of HBC for both a good solvent, cyclohexyl pyrrolidone (CHP), and a poor solvent, tetrahydrofuran (THF). In both cases, we observe features that can be associated with either individual molecules or aggregates, allowing us to establish metrics both for aggregate and individual molecule content. While the aggregate content always increases with concentration, good solvents disperse individual molecules at relatively high concentrations while poor solvents display aggregation even at low concentrations. Using these metrics, we determine that large populations of individual molecules are present at low concentrations in certain solvents with Hildebrand solubility parameters close to 21 MPa1/2. However, the aggregation state of HBC is considerably more sensitive to solvent Hildebrand parameter for halogenated solvents than for amide solvents. We find a combination of high overall concentrations and large populations of individual molecules in four solvents: cyclohexyl pyrrolidone, 1-chloronaphthalene, 1-bromonaphthalene, and 1,2,4-trichlorobenzene. Scanning tunnelling microscopy (STM) measurements show the formation of self-assembled monolayers at the interface between a HBC–solvent dispersion and a highly oriented pyrolytic graphite (HOPG) substrate. Similar structures were observed on ultrathin supports by aberration-corrected transmission electron microscopy (TEM). Also observed were graphitic objects of size ∼1 nm consistent with monomers or aggregated stacks of very few monomers. We believe this is strong evidence of the presence of individual molecules in dispersions prepared with appropriate solvents.
The high selectivity of iron molybdate catalysts for the production of formaldehyde from methanol is somewhat surprising in view of the very different behaviour of the individual oxides of Fe and Mo ...for this reaction. The former, on its own, is a complete combustor of methanol, whereas the latter is highly selective. Here we use STEM (scanning transmission electron microscopy) at high resolution to image the surface of small particles of the catalyst and to show that this high selectivity is due to the dominance of the surface region by molybdenum oxide.
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 AlO4N2 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.
The aromatic diamine 1,10‐phenanthroline acts as a framework‐bound template for the large‐pore aluminophosphate zeotype STA‐28 and can be used to introduce Fe for Al at a single crystallographic site, as verified by Rietveld refinement and annular dark field scanning transmission electron microscopy. Removal of the template leaves a crystalline microporous solid with accessible iron cations.
The location and distribution of the promoters in reduced and passivated cobalt catalysts were investigated using the “SuperSTEM” aberration-corrected scanning transmission electron microscope, which ...passes the 1-Å resolution threshold. Atomic number contrast imaging was achieved using a high-angle annular dark-field detector and elemental spectroscopy, with close to 0.1 nm spatial resolution, using electron energy-loss spectroscopy. The catalyst investigated was the newly developed 20% Co-on-alumina Fischer–Tropsch catalyst prepared via the cobalt ammine carbonate deposition-precipitation route. The catalyst was postimpregnated, after drying, with Pt, Ir, Re, or Ru promoter precursors. Whereas only Ru, Pt, and Ir were effective in lowering the temperature for the conversion of Co
3O
4 to CoO, all four elements promoted the further reduction to metallic cobalt. All of the promoters applied increased the Co surface area, the greatest improvement being with Ir. The microscopy results show the Re, Pt, and Ir promoters mainly as isolated atoms, whereas Ru was present in regions of high concentration within Co particles. Because even promoter-free cobalt particles (within the promoted samples) seem to undergo reduction promotion, the results are consistent with hydrogen spillover as a mechanism for reduction promotion.
Fischer–Tropsch catalysts, based on cobalt, are complex materials containing the catalyst support and reduction and structural promoters in addition to the cobalt metal itself. While this basic ...description covers both modern FT catalyst and those first commercialised, in Germany in 1930s, modern FT catalysts have vastly improved performance. While such improvements owe much to modern materials and manufacturing methods, the role of advanced characterisation techniques cannot be underestimated. Modern techniques discern details of catalyst structure that were not available during the early pioneering work, they guide current developments and offer the prospect of designing tailored catalysts.
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