Lovat V. C. Rees

NMR Studies of Single-File Diffusion in Unidimensional Channel Zeolites

Single-file diffusion is the restricted propagation of particles that cannot pass each other. The occurrence of this phenomenon should be reflected by a change in the time dependence of the mean particle displacement in comparison with ordinary diffusion. Although this process is considered to be the rate-controlling mechanism in a large variety of processes, so far no direct evidence of this phenomenon has been provided. Diffusion measurements made with pulsed field gradient nuclear magnetic resonance (NMR) in unidimensional pore systems (zeolites AlPO4-5 and Theta-1) reflect the expected time dependence of single-file diffusion.

Diffusivities of benzene in HZSM-5, silicalite-I, and NaX determined by frequency-response techniques

The single-step frequency-response technique, a recent development from our laboratory/ and the full frequency-response method have been used to follow the diffusion of benzene in HZSM-5, silicalite-I, and NaX zeolites. Diffusivities of benzene in HZSM-5 was found to be a factor of 2–3 smaller than in silicalite-I. Both diffusivities at 400 K were in close agreement with some previously published diffusivities of ~ 10−13 m2/s. In the more open channel structure of NaX, the diffusivity of benzene at 450 K was ~ 10−10 m2/s i.e., three orders of magnitude larger than in the pentasils

Experimental and molecular simulation studies of adsorption and diffusion of cyclic hydrocarbons in silicalite-1

The adsorption and diffusion of benzene, p-xylene, cyclohexane, cis- and trans-1,4-dimethylcyclohexane (c- and t-DMCH) in silicalite-1 zeolite have been investigated using the frequency response (FR) experimental method and the simulation techniques such as the canonical ensemble Monte Carlo simulation, the force field minimisation and dynamics. It has been found by the FR measurements that two independent fluxes of p-xylene exist in the two channels of silicalite-1 respectively at low loadings and low temperatures, while at high temperatures, only one single, pure diffusion process is observed. The diffusivity of benzene is slower than that of p-xylene and the saturated hydrocarbons diffuse much more slowly than their aromatic equivalents. As found with benzene and p-xylene, t-DMCH has been found to diffuse much more rapidly than cyclohexane. The c-DMCH, on the other hand, diffuses extremely slowly. The theoretical calculations give rational interpretations to these interesting experimental results.

Model compounds for microbial iron-transport compounds. Part 1. Solution chemistry and Mössbauer study of iron(II) and iron(III) complexes from phenolic and catecholic systems

The iron complexes found in the systems FeCl3–phenol, FeCl3–catechol, FeCl3-2,3-dihydroxybenzaldehyde, FeCl3–2,3-dihydroxybenzoic acid, and FeCl3–salicylic acid have been investigated by pH and conductance titrations together with Mossbauer spectroscopy. On the basis of further studies using ascorbic acid and salicylaldehyde, a mechanism for the reduction of FeIII to FeII and the reverse oxidation via a phenolic or catecholic radical is discussed. The properties of enterobactin are explored in the light of the chemistry of the model systems. The Mossbauer spectra of frozen aqueous solutions of hydroxybenzene compounds containing iron cations show that those complexes that are green or blue at acid pH contain FeII, whereas those that are purple or red contain FeIII. It is proposed that these results make the colour of the complex a good indication of the oxidation state of the iron in phenolic and catecholic systems.

Adsorption and diffusion of n-butane and 2-butyne in silicalite-I

The sorption and diffusion of n -butane and 2-butyne in silicalite-I zeolite have been measured. Both n -butane and 2-butyne exhibited type I sorption isotherms, but due to its triple bond, the sorbed amount of 2-butyne exceeded that of n -butane by about 20% under the same conditions. The isosteric heats of sorption showed that at lower coverage ( n -butane Frequency-response measurements showed that for the “coffin-shaped” silicalite-I the phase lag and characteristic function vs. frequency curves could not be fitted by a single diffusion coefficient. However, when two diffusion coefficients were considered, good fits could be obtained. Thus, diffusion in the straight and sinusoidal channels could be followed for the first time. At 323 K and 1.5 Torr, the diffusion coefficients of 2-butyne in silicalite-I are 5.1 × 10 −10 and 5.9 × 10 −11 m 2 s −1 in the straight and sinusoidal channels, respectively, whereas those of n -butane are 1.2 × 10 −10 and 2.0 × 10 −11 m 2 s −1 . Thus, the diffusivities of 2-butyne are 2–5 times faster than those of n -butane. The frequency-response (FR) parameters indicated that diffusion in the sinusoidal channels contributed about 25% to the total diffusion flux

ADSORPTION AND TRANSPORT OF N-HEXANE IN SILICALITE-1 BY THE FREQUENCY RESPONSE TECHNIQUE

The adsorption and mass-transfer behaviour of n-hexane in silicalite have been investigated using a gravimetric balance and the frequency response (FR) method, respectively. Deviations of the isotherms of this system from the type-I (Langmuir model) at low temperatures have been observed and attributed to the immobilisation of n-hexane molecules in the sinusoidal channels of silicalite-1. It was found that the FR spectra of the system can be described by a model involving two independent diffusion processes or a diffusion–rearrangement model, rather than a non-isothermal diffusion model. The first two models may be two special cases of a model which assumes that three processes occur in the n-hexane–silicalite-1 system, two diffusion processes in the straight and sinusoidal channels, respectively, and a mass exchange process between the two channels. Surface barriers have been observed from the FR data of the system and the development of more complicated models describing better the overall processes taking place in the system has been suggested. Comparisons between the diffusivities and activation energies of n-hexane diffusion in silicalite-1 determined by both FR and pulsed field gradient (PFG) NMR methods are also discussed.

Formation of zeolite from the system Na20-Al2O3-SiO2-H2O in alkaline medium (pH > 10)

Abstract The hydrothermal reactions of three aluminosilicates, i.e., kaolinite, metakaolinite, and sodium aluminosilicate gel, with aqueous sodium hydroxide have been reexamined. The molar composition of the quaternary system Na 2 O-Al 2 O 3 -SiO 2 -H 2 O was maintained constant in the three systems. Kaolinite gave a partially rehydrated nonbasic hydroxysodalite, whereas zeolite 4A is the product from metakaolinite and gel. The kinetics of the reaction and the crystal habit of the zeolite formed in the metakaolinite-alkali system appear to be different from that of conventional gel route of synthesis. The metakaolinite slowly dissolves in alkali to form a gel that is the direct precursor of zeolite 4A. However, the crystallization seems to start even before the metakaolinite gel conversion is completed. The ion-exchange and water-adsorption capacity and the chemical assay of the precursors and products have been used to substantiate the XRD and spectroscopic (i.r. and MAS n.m.r.) data in establishing the mechanism of the reaction.

Characterization of microporous sorbents by frequency-response methods☆

The principal features of the frequency-response (FR) apparatus developed at Imperial College are described. The apparatus has been used in both its (a) full and (b) single-step frequency modes to determine the diffusivities of various hydrocarbons in zeolites. The diffusion coefficients and activation energies of benzene, ethylbenzene and p-xylene in silicalite-I are reported and compared. The very fast diffusion of benzene in NaX at 440 and 468 K has been measured by both FR methods. The simpler single-step method is shown to give accurate diffusion coefficients.

Sorption of N2, CH4 and CO2 in Silicalite-1

An isosteric sorption apparatus has been used to obtain data for the sorption of N2, CH4 and CO2 in Silicalite-1 over the temperature range 0–70°C. Isotherms have also been determined in a high pressure sorption balance at equilibrium pressures up to 20 atm. Isosteric heats of sorption have been calculated from these sorption data. Separation factors calculated from Henrys Law constants determined from the initial slopes of the single-component isotherms have been found to be in good agreement with experimental separation factors. The single-component sorption data have been fitted to various isotherm models and the Ideal Adsorbed Solution theory has been used to predict a binary sorption isotherm from the respective single-component data. The predicted isotherm is in good agreement with the corresponding experimental isotherm.

Adsorption of ethane and propane in silicalite-1 and zeolite NaY: determination of single components, mixture and partial adsorption data using an isosteric system

An isosteric method has been used to measure the adsorption of ethane and propane in silicalite-1 and zeolite NaY. A full description of their binary mixture behaviour is presented for three mixtures of different percentage mole fraction. Single-component, mixture and partial thermodynamic data have been calculated and compared.