Gordon J. Kennedy

Multinuclear MAS NMR studies of zeolites MCM-22 and MCM-49

Abstract MCM-22 and MCM-49 comprise a new class of molecular sieves that crystallizes as thin sheets or platelets and has many unusual structural features. MCM-22 is first synthesized hydrothermally as a precursor, MCM-22(P), that condenses upon calcination to a three-dimensional framework (MCM-22). Its framework topology is comprised of two independent pore systems, both accessible through 10-membered rings. One of these pore systems is defined by two-dimensional sinusoidal channels, which maintain an effective 10-ring diameter throughout the structure. The other consists of large supercages whose inner free diameter, 7.1xa0A, is defined by 12 rings with inner height of 18.2xa0A. MCM-49 has the same framework topology as MCM-22, but is synthesized directly in the reaction gel and therefore still contains the organic template. Multinuclear MAS NMR studies of MCM-22 (P), MCM-22, and MCM-49 are presented and discussed in light of the proposed structure and provide new insights into this novel class of materials. The structural information obtained from this NMR investigation is complementary to and consistent with the structure proposed from X-ray diffraction measurements. 13 C NMR data support the existence of different dual pore systems within both MCM-22(P) and MCM-49. 27 Al MAS NMR spectra exhibit three distinct T d resonances that can be interpreted in terms of the proposed framework topology. 29 Si MAS studies of a highly siliceous MCM-22 prepared by hydrothermal dealumination confirm the presence of at least one buried T-site in its framework structure that is not accessible to a channel wall, favor the orthorhombic form of the proposed structure, and support the presence of the modified dodecasil-1H cage.

Toward the rational design of zeolite synthesis: The synthesis of zeolite ZSM-18☆

Abstract ZSM-18 has, until now, only been made using one trisquaternary ammonium ion, 2,3,4,5,6,7,8,9-octahydro-2,2,5,5,8,8-hexamethyl-1 H -benzo[1,2-c:3,4-c′:5,6-c″]tripyrrolium, 1 . This trisquaternary is difficult to make and destabilizes the zeolite during calcination. We used modeling to aid the design and selection of new templates for ZSM-18. The geometry and charge distribution of these templates were chosen to best fit our estimate of “templating.” Template 2 , tris -(2-trimethylamonioethyl)amine, is easy to synthesize in high yield and can be routinely calcined to produce highly sorptive ZSM-18. Template 3 , tris -(2-trimethylamonioethyl)methane, produces ZSM-18 only when seeded. Si n.m.r. and Al n.m.r, indicate that aluminum is not located exclusively in the three rings as has been predicted theoretically. [H]-ZSM-18 can be steam-dealuminated. The Si n.m.r, of the dealuminated material agrees with the number of sites and relative occupancies expected.

Sorbate-induced changes in the 29Si MAS NMR spectrum of highly siliceous MCM-22

Abstract Sorption of organics, such as 2,4-pentanedione and toluene, cause distinct and reversible changes in the 29 Si MAS NMR spectrum of highly siliceous MCM-22 that are characteristic of perturbations in local geometry. These subtle spectral features further substantiate the presence of the modified dodecasil-1H cage in the MCM-22 framework.

Crystal structure of MCM-71 : a new zeolite in the mordenite group

Abstract The crystal structure of the aluminosilicate, MCM-71, was determined by a match of the powder X-ray diffraction pattern to one calculated from a theoretically predicted framework model. The unit cell contains 48 T-sites and 96 framework oxygens, where T = 41.1 Si, 6.9 Al. It crystallizes in space group Cmca, a = 7.4422(2), b = 18.5324(5), c = 19.1877(5) Å. The framework contains an elliptical 10-membered ring channel (4.3 × 6.5 Å) and an orthogonal undulating 8-membered ring channel (3.6 × 4.7 Å) to constitute a two-dimensional network of channels. Considering the aluminosilicate framework as a silicate, the crystal density is 1.77 g/cm3 and the T-site framework density is 17.8 T/1000 A3. The structure was refined against Debye-Scherrer and Bragg-Brentano powder synchrotron data by the Rietveld procedure. While non-framework oxygen atoms, consistent with water content, could be justified by TGA measurements, it was not possible to derive an accurate extra-framework model using synchrotron data from hydrated specimens. Refinement against powder data from an anhydrous specimen subsequently detected the presence of extra-framework aluminum, identified also by solid state NMR measurements.

Multifield magic-angle spinning and double-rotation nuclear magnetic resonance studies of a hydrated aluminophosphate molecular sieve: AlPO4-H2

AlPO4-H2 is a microporous hydrated aluminophosphate, structurally related to VPI-5, whose framework has highly elliptical 10-ring channels (2.9 x 7.6 A) parallel to the c crystallographic axis. To resolve a previously reported discrepancy between nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) data a highly crystalline AlPO4-H2 has been further characterized with 27Al and 31P magic-angle spinning (MAS) NMR at 11.7 T and 27Al double-rotation (DOR) NMR at 4.7 T. These present NMR data definitively show that the true space group symmetry of the AlPO4-H2 framework structure is triclinic rather than the higher orthorhombic symmetry proposed earlier from XRD studies.

Examination of the growth dynamics of zeolites ZSM-5 and mordenite from inorganic reactant compositions

Abstract Zeolite frameworks formed from the reactant composition ranges of (15–40)SiO 2 · Al 2 O 3 · (1.5–12.0)Na 2 O · 1500H 2 O heated to 190°C for time periods varying from 20 to 1000 h were examined. Mordenite, ZSM-5, ferrierite and quartz dominated the crystallization fields. Pure ZSM-5 existed only over a narrow range of compositions. Raman and magic-angle spinning (MAS) nuclear magnetic resonnnce (NMR) studies of precursors to ZSM-5 and mordenite showed that these crystals evolved from gels with similar structures, but with the mordenite gel being more depolymerized (Si-O − ) and aluminous. A particular composition was discovered which showed interesting crystallization dynamics: ZSM-5 → amorphous → mordenite + ZSM-5 → amorphous → mordenite as a function of heating time. It was determined that the insoluble nature of the aluminum source (pseudoboehmite) and the particular choice of reactant composition at the boundary between the crystallization of ZSM-5 and mordenite led to this dynamic behavior. Spectroscopic studies also showed that the crystal → amorphous change was brought about by disruption of the five-membered rings.