EMM-17, a New Three-Dimensional Zeolite with Unique 11-Ring Channels and Superior Catalytic Isomerization Performance.

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.

Spectral editing in 13C solid-state NMR at high magnetic field using fast MAS and spin-echo dephasing.

A simple method is proposed for separating NMR resonances from protonated and non-protonated aromatic carbons in solids under fast magic angle spinning (MAS). The approach uses a MAS-synchronized spin-echo to exploit the differences in rotational recoupling of the dipolar interactions while fully refocusing the isotropic chemical shifts. This strategy extends the relevant time scale of spin evolution to milliseconds and circumvents the limitation of the traditional dipolar dephasing method, which in fast rotating solids is disrupted by rotational refocusing. The proposed approach can be used for quantitative measurement of carbon aromaticities in complex solids with poorly resolved spectra, as demonstrated for model compounds.

Determination of 27Al-29Si connectivities in zeolites with 2D 27Al-->29Si RAPT-CPMG-HETCOR NMR.

The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr-Purcell-Meiboom-Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of approximately 4, which enabled acquisition of a high quality two-dimensional 27Al-29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.

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.

P-derived organic cations as structure-directing agents: synthesis of a high-silica zeolite (ITQ-27) with a two-dimensional 12-ring channel system.

Recently, efforts have been made to synthesize large-pore, multidimensional zeolite frameworks as a basis for new catalysts to improve various hydrocarbon conversions. A new aluminosilicate zeolite, ITQ-27, has been prepared using the phosphorus-containing structure-directing agent, dimethyldiphenylphosphonium. Its crystal structure was determined in its calcined form by direct methods (FOCUS) on synchrotron powder diffraction data (lambda = 0.8702 A) after the unit cell and space group were determined from tilt electron diffraction experiments on individual microcrystals. The material crystallizes in space group Fmmm, where a = 27.7508(5) A, b = 25.2969(7) A, and c = 13.7923(4) A. The final model, refined by Rietveld methods, comprises seven unique T-sites forming a framework with straight 12-MR channels that are connected by 14-MR openings between them. (Corresponding 12-ring pore dimension is 6.94 A x 6.20 A.) Since access from one 14-MR opening to the next is through the 12-MR channel, the structure is best described as a two-dimensional, 12-MR framework.

Crystal structure of MCM-70: A microporous material with high framework density.

The crystal structure of the borosilicate MCM-70 (siliceous framework formula Si12O24) was determined from synchrotron powder diffraction data with the program FOCUS. The framework crystallizes in space group Pmn2(1), where a = 13.663, b = 4.779, c = 8.723 A, and forms 1D ellipsoidal 10-ring channels with the following dimensions: 5.0 x 3.1 A. Rietveld refinement of the model against synchrotron powder data from solvated material gives Rwp = 0.15, R(F2) = 0.11. In addition to the four tetrahedral sites and seven framework oxygens, one potassium position is found during this refinement. Because of the unreasonable geometry of five putative extraframework oxygen sites, another synchrotron pattern was obtained from a dehydrated specimen. A refinement in space group P1n1 (removing the mirror operation of Pmn2(1)), where a = 13.670, b = 4.781, c = 8.687 A, and beta = 90.24 degrees , verified that the previous framework geometry is preserved as well as the potassium position. One extraframework oxygen was located that would yield a reasonable K-O distance. The existence of potassium is verified by electron energy dispersive spectroscopic measurements as well as quantitative elemental analysis. (There are approximately 2.35 K sites per 12 Si in the unit cell.) It is likely that the constricted channels occlude KOH. 11B and 29Si MAS NMR measurements indicate a framework SiO2/B2O3 of approximately 40:1, which is consistent with a wavelength dispersive spectroscopic measurement. The silicate framework density is 2.10 gm/cm3. The resulting framework density for T sites, 21.1, is unusually high for a zeolite, just below the value for paracelsian (21.4) or scapolite (21.8), each of which also has a smallest four-ring loop. The 1H --> 29Si CP MAS measurements suggest sample heterogeneity, that is, a portion of the sample that is strongly coupled to hydrogen and efficiently cross polarizes and another portion that does not.

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.