Alon V. McCormick

Hierarchical nanofabrication of microporous crystals with ordered mesoporosity

Shaped zeolite nanocrystals and larger zeolite particles with three-dimensionally ordered mesoporous (3DOm) features hold exciting technological implications for manufacturing thin, oriented molecular sieve films and realizing new selective, molecularly accessible and robust catalysts. A recognized means for controlled synthesis of such nanoparticulate and imprinted materials revolves around templating approaches, yet identification of an appropriately versatile template has remained elusive. Because of their highly interconnected pore space, ordered mesoporous carbon replicas serve as conceptually attractive materials for carrying out confined synthesis of zeolite crystals. Here, we demonstrate how a wide range of crystal morphologies can be realized through such confined growth within 3DOm carbon, synthesized by replication of colloidal crystals composed of size-tunable (about 10-40 nm) silica nanoparticles. Confined crystal growth within these templates leads to size-tunable, uniformly shaped silicalite-1 nanocrystals as well as 3DOm-imprinted single-crystal zeolite particles. In addition, novel crystal morphologies, consisting of faceted crystal outgrowths from primary crystalline particles have been discovered, providing new insight into constricted crystal growth mechanisms underlying confined synthesis.

Chemistry of zirconia and its use in chromatography.

The purpose of this review is to shed some light on the complex properties of zirconias surface chemistry in order to better understand its behaviour under chromatographic conditions. We emphasize the great differences between the much better known chemistry of a silica surface and the chemistry of zirconias surface. The review describes both the physical and chemical properties of zirconium dioxide from a chromatographic point of view. The chemistry of monoclinic zirconia surface is developed from its underlying crystalline structure. The paper describes the dependence of the specific surface area, pore volume, porosity and mechanical strength on thermal treatment. Methods of synthesis of chromatographically useful zirconia are outlined. The review also covers the adsorption properties of zirconia at both gas-solid and liquid-solid interfaces. Adsorption of water, carbon dioxide, carbon monoxide and ammonia are described and the controversies concerning the surface concentration of adsorption sites are presented. The complex chemistry of a zirconia surface is pointed out and the importance of ligand exchange reactions is emphasized. In contrast to a silica surface, ligand exchange plays an important role in liquid chromatographic applications of zirconia. Strong, hard Lewis acid sites, present on a zirconia surface, can interact with hard Lewis bases and these interactions, sometimes troublesome, can be successfully exploited even for protein separations. Zirconias surface can be modified in many ways: dynamically, by addition of competing Lewis bases to the mobile phase, or permanently, by covering its surface with polymers or by depositing carbon. The review also shows that the main difficulty in achieving a wider variety of applications is probably our lack of knowledge and poor understanding of zirconias surface chemistry.

Part I. Chromatography using ultra-stable metal oxide-based stationary phases for HPLC.

The first part of the review contrasts the main drawbacks of silica-based packings such as their relative thermal and chemical instability with excellent stability of metal oxides. The paper concerns mainly ZrO2, TiO2 and Al2O3. Methods of preparation of spherical particles for HPLC are described. Surface chemistry of the oxides is, however, very different from that of silica. Ability of the oxides to ion- and ligand exchange is discussed from a chromatographic point of view.

Dispersible exfoliated zeolite nanosheets and their application as a selective membrane

Thin zeolite films prepared through a polymer exfoliation method were used as selective membranes. Thin zeolite films are attractive for a wide range of applications, including molecular sieve membranes, catalytic membrane reactors, permeation barriers, and low-dielectric-constant materials. Synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets is desirable because of the packing and processing advantages of the nanosheets over isotropic zeolite nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation of their lamellar precursors have been hampered because of their structure deterioration and morphological damage (fragmentation, curling, and aggregation). We demonstrated the synthesis and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI. The purity and morphological integrity of these nanosheets allow them to pack well on porous supports, facilitating the fabrication of molecular sieve membranes.

EVIDENCE FOR SINGLE FILE DIFFUSION OF ETHANE IN THE MOLECULAR SIEVE ALPO4-5

Abstract We measure the diffusion of ethane in very large crystals of the molecular sieve AlPO 4 -5 (provided by M. Davis) using pulsed field gradient (PFG) NMR. Thanks to the large crystal size and the long longitudinal relaxation time, the mean square displacement could be monitored over times that arranged over an order of magnitude. The mean square displacement was proportional to the square root of time, providing strong evidence that the ethane molecules are moving in a single file, i.e. they are unable to pass each other. This is the first direct experimental evidence for single-file diffusive motion — a phenomenon that is probably widely prevalent in both industrial catalytic and biological systems.

A Closer Look at an Aggregation Model of the Stober Process

Abstract Using experimentally measured nucleation and ionic strength transients, we confirm that an aggregation model can predict the particle growth profiles of Stober systems for a range of conditions. In addition, we examine the characteristics of the evolving size distribution. Systems giving the largest particles are characterized, after a short transient, by a size distribution wherein the dominant aggregation event is between the freshly generated nucleus and a large aggregate.

Flash nanoprecipitation: particle structure and stability.

Flash nanoprecipitation (FNP) is a process that, through rapid mixing, stabilizes an insoluble low molecular weight compound in a nanosized, polymer-stabilized delivery vehicle. The polymeric components are typically amphiphilic diblock copolymers (BCPs). In order to fully exploit the potential of FNP, factors affecting particle structure, size, and stability must be understood. Here we show that polymer type, hydrophobicity and crystallinity of the small molecule, and small molecule loading levels all affect particle size and stability. Of the four block copolymers (BCP) that we have studied here, poly(ethylene glycol)-b-poly(lactic-co-glycolic acid) (PEG-b-PLGA) was most suitable for potential drug delivery applications due to its ability to give rise to stable nanoparticles, its biocompatibility, and its degradability. We found little difference in particle size when using PLGA block sizes over the range of 5 to 15 kDa. The choice of hydrophobic small molecule was important, as molecules with a calculated water-octanol partition coefficient (clogP) below 6 gave rise to particles that were unstable and underwent rapid Ostwald ripening. Studies probing the internal structure of nanoparticles were also performed. Analysis of differential scanning calorimetry (DSC), cryogenic transmission electron microscopy (cryo-TEM), and (1)H NMR experiments support a three-layer core-shell-corona nanoparticle structure.

Unidirectional and single-file diffusion in AlPO4-5: molecular dynamics investigations

Canonical ensemble molecular dynamics simulations of Lennard-Jones methane and ethane are conducted in an atomistic model of AlPO4-5, a molecular sieve with approximately cylindrical channels of diameter 7·3 A. Methane molecules are able to pass each other in the nanopore and exhibit unidirectional but otherwise ordinary diffusion along the channel axis, with the mean-square displacement directly proportional to time, and a diffusion coefficient calculated at a loading of 0·7 molecules per unit cell at 295 K of 4·70 × 10-4 cm2 s-1. Ethane molecules cannot pass each other easily in the nanopore and for short times exhibit single-file diffusion, i.e., the mean-square displacement is proportional to the square root of the time. After longer times, contributions of ordinary unidirectional diffusion are observed due to the nonzero probability of passing. A slightly larger molecule exhibits pure single-file diffusion. The single-file mobility for the large molecule at 0·7 molecule per unit cell and 295 K is 1·5...

Superselectivity and solvation forces of a two component fluid adsorbed in slit micropores

We use the grand canonical Monte Carlo simulation technique to calculate adsorption of mixtures in molecularly narrow slit pores immersed in a two‐component bath of spherical molecules that are different in size. The composition of the pore fluid oscillates strongly with the pore width. The oscillations reflect the differing ability of each molecule to pack as layers in the pore. Even in pores wide enough to admit both components, this difference in packing ability leads to a shut‐out of the smaller component. Trends in the calculated solvation force agree with both experiment and theory.

The Effect of Nanopore Shape on the Structure and Isotherms of Adsorbed Fluids

A Grand Canonical Monte Carlo simulation method is used to determine the adsorption isotherms, interaction energies, entropies, and density distribution of a Lennard-Jones fluid adsorbed in smooth-walled nanopores of varying size and shape. We specifically include very crowded pores, where packing effects are important. Differences in the isotherms of slit, cylindrical, and spherical nanopores of varying sizes can be explained in terms of the adsorbate-adsorbate interaction energy, the adsorbate-pore interaction energy, and the density profiles, which influence the balance between the former and the latter energy contributions. The expectation from low loading studies that the most energetically favorable adsorbate-pore interactions maximize adsorption is not borne out at intermediate and higher loadings. Instead, the relationships between adsorbed amounts and pore size and shape are found to be strong functions of the depth and steepness of the external potential, the extent to which adsorbate-adsorbate repulsion establishes short range fluid order, and the accessible pore volume. This study has implications for high pore density processes in nanoporous materials, such as zeolite catalysis, separations, and templating in zeolite synthesis.