Luis J. Smith

On the Nature of Water Bound to a Solid Acid Catalyst

The nature of the species formed when water interacts with Brønsted acid sites in a microporous solid acid catalyst, HSAPO-34, was studied by powder neutron diffraction and infrared spectroscopy. Previous infrared and computational studies had failed to unambiguously establish whether water is protonated to form hydronium (H3O+) ions or is merely hydrogen-bonded to acid sites inside the zeolite. Our experiments clearly showed that both species are present: An H3O+ ion is found in the eight-ring channel of the zeolite and a second water molecule is hydrogen-bonded to an acid site on the six-ring.

A quantitative description of the active sites in the dehydrated acid catalyst HSAPO-34 for the conversion of methanol to olefins

A neutron diffraction study of the deuterated form of a microcrystalline specimen of the H+ SAPO-34 catalyst for converting methanol to light olefins reveals two distinct bridging hydroxyl Brønsted sites. These sites are also identified by IR spectroscopy which distinguishes their acid strength from the shift in O-H frequencies following adsorption of CO at 77 K. Protons attached to the O(4) bridging oxygen (OH distance 0.95 Å) exhibit greater acidity than those attached to O(2) (OH distance 0.91 Å).

Molecular dynamics of glucose in solution: A quasielastic neutron scattering study

The molecular dynamics of glucose dissolved in heavy water have been investigated at 280 K by the technique of quasielastic neutron scattering. The scattering was described by a dynamic structure factor that accounts for decoupled diffusive jumps and free rotational motions of the glucose molecules. With increasing glucose concentration, the diffusion constant decreases by a factor five and the time between jumps increases considerably. Our observations validate theoretical predictions concerning the impact of concentration on the environment of a glucose molecule and the formation of cages made by neighboring glucose molecules at higher concentrations.

Cross-relaxation dynamics between laser-polarized xenon and a surface species using a simple three-spin model

Abstract We present an analysis of the cross-relaxation dynamics between optically polarized 129 Xe and surface 13 C and 1 H spins in methanol adsorbed on a SiO 2 surface. Analytical expressions for the 13 C polarization are derived, which show that the 13 C polarization is determined by cross-relaxation with both 129 Xe and 1 H spins. The 1 H spins act as a polarization sink; however, saturation of the protons results in a further increase in signal intensity. Replacing the 1 H spin reservoir with 2 H increases the 13 C signal by a factor of 12 (25 as compared to room-temperature data). Further improvements are anticipated using higher-powered lasers.

Rapid Microwave-Assisted Grafting of Layered Perovskites with n-Alcohols

Dion-Jacobson layered niobates have been extensively researched in recent years because of a variety of useful properties such as dielectric behavior, proton conduction, and solid acid catalysis. The behavior of these materials is strongly dependent on the elemental composition and, more specifically, the interlayer surface environment. A novel method of partial grafting of n-alcohols into the interlayer of HSr2Nb3O10 with approximately 40% conversion has been developed using microwave irradiation to generate high temperatures. This method has reduced the grafting reaction time by more than 97% while maintaining conversion rates consistent with previous methods.

A unified approach to the dynamics of a polymer melt

Stretched exponentials are often used to describe quasi-elastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) relaxation data from polymer melts. In this paper, we attempt to derive a more physically meaningful model of the local (∼0.1 nm), short-time (∼10 ps) dynamics of linear polymers that takes into account (i) orientational diffusion along the polymer chain, (ii) local conformational transitions, and (iii) long-time, large-scale motions. The model takes into account the spatial component of the local dynamics, described in terms of the scattering vector Q. The model is applied to QENS results on highly entangled polyethylene oxide (PEO) melt at 373 K. We find the Q dependences of the three correlation times of the model to be consistent with Q°, Q -2 and Q -4 power laws, respectively. The high-Q limit of the model closely resembles the NMR-based DLM model (Dejean de la Batie et al 1988 Macromolecules 21 2045) but the physical interpretation is different. At 373 K, the polymer dynamics is described in terms of transverse motions of the chain segments over a distance of a few nm, with a local monomeric diffusion coefficient of 1.78 x 10 -9 m 2 s -1 . From this value, we derive a monomeric friction coefficient ξ 0 = 2.89 x 10 -12 N s m -1 that, used as numerical input to the Doi-Edwards theory, leads to a chain centre-of-mass diffusion coefficient D cm = 9.4 x 10 -15 m 2 s -1 . This value is in good agreement with pulsed field gradient NMR data (Appel and Fleischer 1993 Macromolecules 26 5520) and validates the proposed model.

Incomplete double frequency sweeps to select small quadrupolar coupling static powder patterns.

Difference spectra based on the magnitude of the quadrupolar coupling of a site has been observed under static conditions utilizing a double frequency sweep pulse sequence to enhance the central transition of a small electric field gradient (EFG) environment. Through the use of convergent sweeps that only cover the inner satellite transitions of the smaller EFG site, an echo spectrum results that favors the smaller site, which can then be used in conjunction with normal echo spectra to create a difference spectrum that consists primarily of the smaller site. The simplification of the static lineshape data permits simulation for the extraction of chemical shift anisotropy (CSA) information for the site. The method is demonstrated using (93)Nb NMR for samples with multiple niobium environments due to mixtures of compounds, MgNb(2)O(6)/LiNbO(3), or due to crystallographic structure, KCa(2)Nb(3)O(10).

Characterization of Polymer Clay Nanocomposite Electrolyte Motions via Combined NMR and Neutron Scattering Studies

The activation energies for poly(ethylene oxide) motion in a polymer clay composite are reported for the polymer intercalated and external to the clay. PEO intercalated into the clay is found to have a lower activation energy for motion but also a larger Arrhenius prefactor, by almost two orders of magnitude, than for PEO found external to the clay. Neutron scattering measurements confirm the presence of two environments and the effects of confinement on the mean square displacement of the PEO.

Triazole Functionalized Sol-Gel Membranes, Effect of Crosslink Density and Heterocycle Content on Water Free Proton Conduction and Membrane Mechanical Properties

Freestanding, ion conducting, membranes were synthesized by incorporating triazole-containing tetracyclosiloxanes into a polyethylene glycol-tetraethyl orthosilicate (PEG-TEOS) based sol-gel matrix. These membranes show comparable or higher proton conductivities than their linear, liquid, polysiloxane analogs and fall within an order of magnitude of the target ion mobilities for use in proton exchange membrane fuel cells (PEMFCs). The absence of any unbound PEG or cyclic siloxane was confirmed by proton nuclear magnetic resonance (1H-NMR), while the chemical structure and composition of the membranes was corroborated by Fourier transform infrared (FTIR) spectroscopy. Thermogravimetric analysis (TGA) indicated that the membranes are stable up to 180°C and differential scanning calorimetry (DSC) analysis showed complete suppression of PEG crystallization after incorporation of the triazole-functionalized cyclosiloxanes. An increase in the molecular weight of the PEG chains used to create the sol-gel matrix produced membranes with increased flexibility and higher proton conductivities at temperatures below 100 °C. Pulse field gradient echo (PFG) NMR studies showed an increase in the apparent diffusion coefficient of the sol-gel threaded cyclosiloxane motifs compared to the linear polysiloxanes, indicating a significant reduction on the coupling between mechanical strength and ion transport capability.

An Analytic Method for Estimating the Kinetic Parameters of Active Conductances from Current-Clamp Data

Detailed compartmental models of individual neurons require information on the kinetic parameters of voltage-gated conductances present on their membranes. Since voltage-clamp and whole cell patch-clamp methods do not easily permit a characterization of active conductances as well as other aspects of neuronal physiology on the same cell, a method of obtaining some information on the kinetics of voltage-gated conductances from current-clamp data is introducted.