High field 27Al ENDOR reveals the coordination mode of Cu2+ in low Si/Al zeolites.

The siting of Cu2+ in zeolites with low exchange levels has been a subject for debate due to the lack of experimental evidence that provide directly the interaction between the Cu2+ ion and the zeolite framework. High field 27Al ENDOR provided highly resolved orientation selective ENDOR spectra from which both the 27Al hyperfine and quadrupole principal components and orientations relative to the g tensor principal axis system were determined for a dehydrated Cu2+ exchanged zeolite X with Si/Al = 1. The results show that all three Cu-Al distances in the six-member ring are equivalent, in contrast to DFT predictions using cluster models.

Methylcyclohexane ring-contraction: A sensitive solid acidity and shape selectivity probe reaction.

In this paper we describe the utility of an acid-catalyzed isomerization reaction, specifically, ring-contraction of methylcyclohexane to an isomeric mixture of alkylcyclopentanes as a tool for characterizing the acidic properties of a wide range of platinum-loaded solid acids. Methylcyclohexane isomerization is particularly useful as a solid acidity probe reaction since it is a simple molecule containing one six-membered ring and a single methyl group substituent. As a solid acidity probe molecule methylcyclohexane has a number of advantages over cyclohexane. Ring-contraction of cyclohexane produces a single product, methlycyclopentane. Methylcyclohexane ring-contraction, in contrast, yields a richer and thus more informative product mixture including ethylcyclopentane, and five isomeric dimethylcyclopentanes. For the first time it will be shown that variations in the three primary descriptors of solid acids, acid site density, acid site strength, and shape selectivity, within a wide range of amorphous and crystalline solid acids can be simultaneously ranked using a single component probe reaction, namely, methylcyclohexane ring-contraction.

Fine structures of zeolite-Linde-L (LTL): surface structures, growth unit and defects.

High-resolution electron microscopy (HREM) has been used to image the surface structure of nano- and micrometer-sized synthetic crystals of zeolite-Linde-L (LTL). Columnar holes and rotational, nano-sized, wheel-like defects were observed within the crystals, where the hole has a minimum size equal to that of the rotational defect. Predictions of surface structure from atomistic computer simulation concur with the observations from HREM and provide insight into the crystal growth mechanism of perfect and defective LTL. Analysis of the energetics of the formation of rotational defect structures reveals that the driving force for defect creation is thermodynamic and furthermore, the rotational defects could be created in high concentrations. Formation of a columnar hole is found to be slightly energetically unfavourable and therefore we speculate that the incidence of both rotational and nano-sized vacancy defects is strongly dependent on kinetic factors and reaction conditions. The morphology of nano- and microcrystalline LTL is contradistinct and we use insights from simulation to propose an explanation of the disparity in crystal shape.

Structure of the first silicate molecular sieve with 18-ring pore openings, ECR-34.

The three-dimensional microporosity of zeolite frameworks have allowed their widespread use in industry as heterogeneous catalysts, absorbents, and ion-exchangers. While the phosphate analogues of zeolites having up to 24 tetrahedral atoms in the pore openings are known, silicate-based zeolites have, until now, been limited to 14-membered ring pore openings. We now disclose the structure and characterization of the synthetic zeolite ECR-34, which can be prepared from a mixed alkali metal reaction gel containing tetraethylammonium (TEA) cations. Its structure has been determined from powder diffraction data and shows ECR-34 to be hexagonal with the dimensions a, b = 21.030(1) A, c = 8.530(1) A, containing one-dimensional, 18-ring pores with 10 A diameter free openings. ECR-34 is stable to 800 degrees C and is able to absorb and ion-exchange large organic molecules. The existence of ECR-34 suggests the potential of preparing other thermally stable silicate molecular sieves with extra-large pores.

Constraining asymmetric organometallic catalysts within mesoporous supports boosts their enantioselectivity.

By constraining tethered asymmetric organometallic catalysts within the nanopores of silica supports so as to increase the interaction between the pore wall and the active center (and hence to restrict access of the reactant to the catalyst), a significant improvement in enantioselectivity is achieved. A schematic illustration of a cationic chiral, organometallic catalyst, [Rh(I)(COD)PMP] {(S)-(+)-1-(2-pyrrolidinylmethyl)-pyrrolidine and cyclooctadiene}, which is noncovalently anchored via a N-H...F hydrogen bond with the triflate ion, CF3SO3-, to the curved inner surface of a 38 A diameter pore of a silica support for the asymmetric hydrogenation of the C=O bond in methyl benzoylformate to the corresponding methyl mandelate is shown. Rh (purple); N (blue); H (white); F (green); C (gray); S (yellow); O (red).