Mobae Afeworki

EMM-23 : A Stable High-Silica Multidimensional Zeolite with Extra-Large Trilobe-Shaped Channels.

Stable, multidimensional, and extra-large pore zeolites are desirable by industry for catalysis and separation of bulky molecules. Here we report EMM-23, the first stable, three-dimensional extra-large pore aluminosilicate zeolite. The structure of EMM-23 was determined from submicron-sized crystals by combining electron crystallography, solid-state nuclear magnetic resonance (NMR), and powder X-ray diffraction. The framework contains highly unusual trilobe-shaped pores that are bound by 21-24 tetrahedral atoms. These extra-large pores are intersected perpendicularly by a two-dimensional 10-ring channel system. Unlike most ideal zeolite frameworks that have tetrahedral sites with four next-nearest tetrahedral neighbors (Q(4) species), this unusual zeolite possesses a high density of Q(2) and Q(3) silicon species. It is the first zeolite prepared directly with Q(2) species that are intrinsic to the framework. EMM-23 is stable after calcination at 540 °C. The formation of this highly interrupted structure is facilitated by the high density of extra framework positive charge introduced by the dicationic structure directing agent.

1H MAS NMR (Magic-Angle Spinning Nuclear Magnetic Resonance) Techniques for the Quantitative Determination of Hydrogen Types in Solid Catalysts and Supports

Distinct hydrogen species are present in important inorganic solids such as zeolites, silicoaluminophosphates (SAPOs), mesoporous materials, amorphous silicas, and aluminas. These H species include hydrogens associated with acidic sites such as Al(OH)Si, non-framework aluminum sites, silanols, and surface functionalities. Direct and quantitative methodology to identify, measure, and monitor these hydrogen species are key to monitoring catalyst activity, optimizing synthesis conditions, tracking post-synthesis structural modifications, and in the preparation of novel catalytic materials. Many workers have developed several techniques to address these issues, including 1H MAS NMR (magic-angle spinning nuclear magnetic resonance). 1H MAS NMR offers many potential advantages over other techniques, but care is needed in recognizing experimental limitations and developing sample handling and NMR methodology to obtain quantitatively reliable data. A simplified approach is described that permits vacuum dehydration of multiple samples simultaneously and directly in the MAS rotor without the need for epoxy, flame sealing, or extensive glovebox use. We have found that careful optimization of important NMR conditions, such as magnetic field homogeneity and magic angle setting are necessary to acquire quantitative, high-resolution spectra that accurately measure the concentrations of the different hydrogen species present. Details of this 1H MAS NMR methodology with representative applications to zeolites, SAPOs, M41S, and silicas as a function of synthesis conditions and post-synthesis treatments (i.e., steaming, thermal dehydroxylation, and functionalization) are presented.

Structure of an Aluminophosphate EMM-8: a Multi-Technique Approach

The crystal structure of an aluminophosphate, EMM-8 (ExxonMobil Material #8), was determined in its calcined, anhydrous form from synchrotron powder diffraction data using the computer program FOCUS. A linkage of double four-ring (D4R) building units forms a two-dimensional framework with 12-MR and 8-MR channels, and differs from a similar SAPO-40 (AFR) framework only by the relationship between paired D4R units. Rietveld refinement reveals a fit of the model to the observed synchrotron data by Rwp=0.1118, R(F2)=0.1769. Local environments of the tetrahedral phosphorus and aluminium sites were established by solid-state NMR, which detects distinct differences between as-synthesized and calcined materials. Distinct, reversible changes in the local symmetry of the P and Al atoms were observed by NMR upon calcination and subsequent hydration. These NMR data provided important constraints on the number of tetrahedral (T) atoms per unit cell and the connectivities of the T atoms. Detailed local structural information obtained by solid-state NMR thereby guided the ultimate determination of the structure of AlPO EMM-8 from the powder data. Comparisons are made to the recently published crystal structure of the fluoride-containing, as-synthesized SSZ-51, indicating that the unit-cell symmetry, axial dimensions and framework structure are preserved after calcination.

Synthesis and structure of ECR-40: An ordered sapo having the MEI framework

Abstract The structure of the silicoaluminophosphate ECR-40 has been determined and refined from synchrotron powder X-ray diffraction data. Its framework is the same as the aluminosilicate ZSM-18, which has the MEI structure. ECR-40 is the first example of a MEI structure with a non-aluminosilicate composition. Due to unique ordering of the silicon, phosphorous, and aluminum atoms into specific T-atom positions, ECR-40 contains odd-numbered rings, which are not present in other tetrahedral SAPO frameworks. 1H, 27Al, 31P, and 29Si MAS NMR data are consistent with the MEI structure. The structure also contains an Al-O-Al connection which imparts very strong acidity to the material.

Probing the non-random aluminum distribution in zeolite merlinoite with ultra-high-field (18.8 T) 27Al and 29Si MAS NMR

Abstract Two crystallographically distinct tetrahedral Al sites in zeolite merlinoite are directly identified by ultra-high-field (18.8 T) 27 Al MAS NMR spectroscopy. The experimental chemical shifts and peak areas indicate a preferential siting of Al in site TII. This non-random aluminum distribution is independently confirmed by ultra-high-field (18.8 T) 29 Si MAS NMR spectroscopy.

The crystal structures of polymorphic SUZ-4

Abstract SUZ-4 is an aluminosilicate zeolite originally synthesized at British Petroleum and known to be related somehow to ferrierite. Although a model had been suggested for the framework structure no quantitative proof of its existence has been given in previous work. It is now found to be polymorphic. The originally identified ambient, hydrated form crystallizes in space group Imma where a = 18.882(1), b = 14.872(1), c = 14.211(1) Å. A model constructed by trial and error refines by Rietveld methods to give Rwp = 0.1643, R(F2) = 0.1665. Powder data collected at 100 °C, on the other hand, correspond to the Cmmm model proposed earlier by Lawton et al., where a = 18.790(1), b = 14.2305(8), c = 7.4511(4) Å. The Rietveld match gives Rwp = 0.1473, R(F2) = 0.1508. Although the two forms crystallize in different space groups, the framework topology remains the same, as revealed by vertex symbols and coordination sequences. Evidence is seen for potassium ions that cannot be completely removed by ammonium exchange. Later, the origin of the polymorphic change was discovered to be hydration of microporous channels rather than a thermal effect. Solid-state NMR measurements of the hydrated and anhydrous forms show that a more symmetric environment around the Al species is observed without affecting the coordination of the Al sites. Synchrotron powder data collected at room temperature from anhydrous material conformed to the Cmmm structural model, where a = 18.8064(4), b = 14.2298(3), c = 7.4548(2) Å. Rietveld refinement, including three potassium sites gives Rwp = 0.1127, R(F2) = 0.1296. (Elemental analysis indicates a stoichiometric assembly: K4T36O72; the Rietveld refinement finds 3.86 of the 4.00 potassium atoms.) The material represents a one-dimensional 10-membered ring framework.

Accurate structure determination of a borosilicate zeolite EMM-26 with two-dimensional 10 × 10 ring channels using rotation electron diffraction

A new borosilicate zeolite |N2H36C16|[Si22B2O48]·H2O, denoted as EMM-26, has been synthesized by employing a linear dicationic organic structure directing agent 1,6-bis(N-methylpyrrolidinium)hexane (OSDA). EMM-26 has a novel zeolite framework and contains two-dimensional (2D) intersecting 10 × 10-ring channels. Its structure was solved from sub-micrometer sized crystals using rotation electron diffraction (RED) and refined against both the RED and synchrotron powder diffraction data. We have shown for the first time that RED data alone can be used to accurately determine zeolite structures. The OSDAs can be removed from the framework generating permanent pores. EMM-26 shows good CO2 uptake and CO2/CH4 selectivity.

Supported metal catalysts : Some interesting new leads in an old field

1. Abstract We report on a new approach for preparing Ru metal supported on silica. We show that partial decomposition of Ru impregnates on silica, formed using bifunctional organic additives, creates strongly interacting complexes that bond to and spread on silica. Reducing these complexes produces 12-15A Ru crystallites homogeneously distributed on the support. This homogeneous distribution leads to enhanced resistance to reductive sintering.

High-Throughput Synthesis and Structure of Zeolite ZSM-43 with Two-Directional 8-Ring Channels

The aluminosilicate zeolite ZSM-43 (where ZSM = Zeolite Socony Mobil) was first synthesized more than 3 decades ago, but its chemical structure remained unsolved because of its poor crystallinity and small crystal size. Here we present optimization of the ZSM-43 synthesis using a high-throughput approach and subsequent structure determination by the combination of electron crystallographic methods and powder X-ray diffraction. The synthesis required the use of a combination of both inorganic (Cs+ and K+) and organic (choline) structure-directing agents. High-throughput synthesis enabled a screening of the synthesis conditions, which made it possible to optimize the synthesis, despite its complexity, in order to obtain a material with significantly improved crystallinity. When both rotation electron diffraction and high-resolution transmission electron microscopy imaging techniques are applied, the structure of ZSM-43 could be determined. The structure of ZSM-43 is a new zeolite framework type and possesses a unique two-dimensional channel system limited by 8-ring channels. ZSM-43 is stable upon calcination, and sorption measurements show that the material is suitable for adsorption of carbon dioxide as well as methane.

Synthesis and Structure of a Layered Fluoroaluminophosphate and Its Transformation to a Three-Dimensional Zeotype Framework

Two-dimensional zeolitic materials have drawn increasing attention because of their structural diversity, high accessible surface areas, and potential as precursors to form novel three-dimensional (3D) structures. Here we report a new layered fluoroaluminophosphate, denoted as EMM-9 (ExxonMobil Material #9), synthesized in the same synthesis system as that for a previously reported 3D framework structure EMM-8 (framework-type code: SFO) using an F- medium and 4-(dimethylamino)pyridine (DMAP) as the organic structure-directing agent. The structure of EMM-9 was solved from rotation electron diffraction data and refined against synchrotron powder X-ray diffraction data. The fluoroaluminophosphate layer of EMM-9 is composed of sti composite building units. The DMAP cations are located between the layers. π-π interactions between the DMAP cations and hydrogen bonding between the DMAP cations and layers were identified. The layered EMM-9 structure is closely related to the 3D framework structure of EMM-8 and can be transformed to EMM-8 by calcination.