Christopher P. Nicholas

Formation Pathway for LTA Zeolite Crystals Synthesized via a Charge Density Mismatch Approach

A solid understanding of the molecular-level mechanisms responsible for zeolite crystallization remains one of the most challenging issues in modern zeolite science. Here we investigated the formation pathway for high-silica LTA zeolite crystals in the simultaneous presence of tetraethylammonium (TEA(+)), tetramethylammonium (TMA(+)), and Na(+) ions as structure-directing agents (SDAs) with the goal of better understanding the charge density mismatch synthesis approach, which was designed to foster cooperation between two or more different SDAs. Nucleation was found to begin with the formation of lta-cages rather than the notably smaller sod and d4r-cages, with concomitant incorporation of TMA(+) and Na(+) into a very small amount of the solid phase with a low Si/Al ratio (ca. 2.5). The overall characterization results of our work demonstrate that sod-cages are first built around the preorganized lta-cages and that d4r-cages are in turn constructed by the progressive addition of low-molecular-weight (alumino)silicate species, which promotes the formation and growth of embryonic LTA zeolite crystals. We also show that the crystal growth may take place by a similar process in which TEA(+) is also incorporated, forming a single LTA zeolite phase with a higher Si/Al ratio (ca. 3.3).

On the track to silica-supported tungsten oxo metathesis catalysts: input from 17O solid-state NMR.

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

Single-Face/All-cis Arene Hydrogenation by a Supported Single-Site d0 Organozirconium Catalyst

The single-site supported organozirconium catalyst Cp*ZrBz2 /ZrS (Cp*=Me5 C5 , Bz=benzyl, ZrS=sulfated zirconia) catalyzes the single-face/all-cis hydrogenation of a large series of alkylated and fused arene derivatives to the corresponding all-cis-cyclohexanes. Kinetic/mechanistic and DFT analysis argue that stereoselection involves rapid, sequential H2 delivery to a single catalyst-bound arene face, versus any competing intramolecular arene π-face interchange.

A Complex Zeolite Containing Multiple Ring Sizes in a Single Channel: One-Dimensional Zeolite UZM-55

Zeolites are porous aluminosilicate materials utilized in a variety of sorption, separation, and catalytic applications. The oil refining industry in particular has seen a number of significant advances due to the introduction of new technologies enabled by new zeolites. Of particular importance are zeolites with 10- or 12-membered ring pores, resulting in pore shapes and sizes appropriate for the interaction with small hydrocarbon molecules. Here, the synthesis of a new zeolite UZM-55 is reported and the idealized structure thereof is presented. The most complex structure solved to date, UZM-55 possesses a large triclinic unit cell containing 52 T-sites. The material uniquely contains both 10- and 12-membered ring pores in a single, undulating one-dimensional channel, the first example in a zeolitic material of multiple delimiting rings in a single channel. This discovery opens new opportunities in shape-selective adsorption and catalysis. Demonstrated here is the unique adsorption behavior of UZM-55, shown both experimentally and computationally to adsorb one nonane molecule per unit cell in a linear conformation.