James O. Titiloye

Atomistic simulation of the differences between calcite and dolomite surfaces

Atomistic simulation methods have been used to calculate and compare the surface structures and energies of the {l_brace}10{bar 1}4{r_brace}, {l_brace}0001{r_brace}, {l_brace}10{bar 1}0{r_brace}, {l_brace}11{bar 2}0{r_brace} and {l_brace}10{bar 1}1{r_brace} surfaces of calcite and dolomite and to evaluate their equilibrium morphologies. The calcite {l_brace}10{bar 1}4{r_brace} and the dolomite {l_brace}10{bar 1}0{r_brace} and {l_brace}11{bar 2}0{r_brace} surfaces are the most stable crystal planes. Investigation of the segregation of Mg and Ca ions in the dolomite crystal shows a clear preference for Ca{sup 2+} ions at the surface sites and for Mg{sup 2+} ions in the bulk sites and hence growth onto dolomite results in calcium carbonate or high magnesian calcite crystals which helps explain the difficulty in crystallizing dolomite vs. calcite under laboratory conditions.

Atomistic simulation of calcite surfaces and the influence of growth additives on their morphology

Abstract Atomistic simulation techniques based on the Born model of solids have been used to calculate the surface energies and to predict the equilibrium morphology of calcite crystals. The influence of Mg 2+ , Li + and HPO 2- 4 ion additives on the surface stability is reported. These results can be used to infer the effect on nucleation, and on the growth morphologies of calcite.

Computer simulation of the structure and dynamics of methane in hydrated Na-smectite clay

Abstract Monte Carlo and molecular dynamics computer simulation has been used to study the microscopic properties of aqueous fluids and methane in smectite clays, at the elevated temperatures and pressures that are encountered in sedimentary basins. Methane particles are solvated by approximately 12–13 water molecules, with six oxygen atoms from the clay surface completing the coordination shell. The tendency for hydrophobic methane–methane contacts reaches a maximum at 1 km (310 K and 150 bar), in close agreement with the behaviour observed in the bulk solutions. The self-diffusion coefficients are in excellent agreement with the experimental values for methane diffusion through near-surface shales.

Molecular dynamics simulation of methane in sodium montmorillonite clay hydrates at elevated pressures and temperatures

Computer simulation has been used to study the structure and dynamics of methane in hydrated sodium montmorillonite clays under conditions encountered in sedimentary basins. Systems containing approximately one, two, three and four molecuiar layers of water have followed gradients of 150 bar km−1 and 30Kkm−1, to a maximum burial depth of 6 km (900 bar and 460 K). Methane is coordinated to approximately 19 oxygen atoms, of which typically 6 are provided by the clay surface. Only in the three- and four-layer hydrates is methane able to leave the clay surface. Diffusion depends strongly on the porosity (water content) and burial deth self-diffusion coefficients are in the range 0.12 × 10−9m2S−1 < D < 12.65 × 10−9m2s−1 for water and 0.04 × 10−9m2s−1 < D < 8.64 × 10−9m2s−1 for methane. Bearing in mind that porosity decreases with burial depth, it is estimated that maximum diffusion occurs at around 3 km. This is in good agreement with the known location of methane reservoirs in sedimentary basins.

Atomistic simulation of zeolite surfaces

The aim of this paper is to describe the current state of atomistic simulation of zeolite surfaces by describing what has been achieved and to show how the surface structures are modelled. This is illustrated by using atomistic simulation techniques to model the {100} surface of zeolite LTA. The pure siliceous and aluminated CaNa-A and Na-A with Si/Al=1 structures were considered. The surface showed three stable terminations but the relative stability varied with composition. The resulting surface structures and geometries show extensive framework distortions, especially in the aluminated forms where the cations formed strong interaction with the zeolite framework thereby increasing their adsorption energies and stabilising their cation position.

Atomistic simulation of mineral surfaces: Studies of surface stability and growth

Abstract Atomistic simulation represents a valuable methodology for interpreting and predicting surface structures. The emphasis of our work is to develop and apply this approach to understanding the role of surface defects and additives in modifying the structure and stability of mineral surfaces. The basis of our approach is energy minimisation which allows us to evaluate the most stable surface configurations. The utility and limitations of this approach will be illustrated via a number of examples. These include describing the factors governing the stability of mineral surfaces and applying these considerations to understanding the surfaces of olivine and spinel. In addition, we are beginning to address the water-solid interface. We find a wide variation in the reactivity of the different surfaces of rock-salt oxides from (100) which show only physisorption, through stepped surfaces which show dissociative adsorption to (111) which forms the hydroxide. One way of determining the interaction between su...

Catalytic Combustion of Methane in a Monolith Reactor: Heat and Mass Transfer Under Laminar Flow and Pseudo-Steady-State Reaction Conditions

Abstract The catalytic combustion of methane was studied in monolith reactors, at atmospheric pressure and flow conditions corresponding to Reynolds numbers in the laminar flow region. The catalyst was palladium based and dispersed on a γ-alumina washcoal covering a 62 cell cm -2 ceramic support, with square-shaped cells. Because of the nature of the coating process the thickness of the washcoat varied from approximately 10 μn on the side to 150 μm in the corners. Chemical reaction kinetics were determined in a monolith of length 12.7 mm, while performance was measured for a number of lengths up to 151mm. The outside diameter of the monolith was 117 mm. Experimental conditions of temperature were between 607-845K with the fuel to air mass ratio fixed at 0.0053. It was shown that at temperatures > 770K, the reaction was limited by transport processes. For a 51mm long monolith, when 76% conversion was achieved, a one-dimensional flow model was used as a diagnostic tool to calculate Nu and Sh numbers from ex...

Modeling the similarities and differences between the sodalite cages (β-cages) in the generic materials

Initially in this review the sodalite framework [T12O24]6− (T=Al, Si] is modeled with regular tetrahedra and disordered T atoms. Equations are given for calculating atomic coordinates from the unit cell parameter a and the T—O distancet; the expansion or contraction of the sodaliteβ-cage is related quantitatively to changes ina through the cooperative twistsφ of TO4 tetrahedra about 4 axes and changes in < TOT bridge angles. The fully expanded cage hasφ=0° and the maximum value ofa. The equations are general for any framework formed by isomorphous substitution of T by atoms other than Al, Si and for any Si∶Al ratio. The model and equations are extended to the zeolite A framework, which can be built from fully expanded sodalite cages. With the cooperative tilt of the TO4 tetrahedra of zeolite A, described by Depmeier, the major variable features of the zeolite A framework are explained quantitatively. The faujasite framework has twistedβ-cages (φ>0), as in sodalite examples, and is quantitatively modeled most conveniently from sodalite examples with similarβ-cage contents. The review is extended to structures with T-ordering and distorted tetrahedra. Methods are given for estimating a for sodalites from a knowledge of the cavity contents, especially the sizes of cations and anions, and so on, present. Ways of predicting cavity sites in zeolite A as a function of cation size are presented, and the principal cavity sites in the faujasiteβ-cage region are discussed. Finally the review considers isomorphous replacement of T atoms (Si or Al) by B, Be, Fe, Ga, Ge, and P; many of these substituted frameworks are stabilized by templates, or guest molecules, which reside in the cavities. Templates also stabilize Si, Al frameworks with high Si∶Al ratios. The modeling approach reviewed here is tested on a range of isomorphously substituted frameworks isotypic with sodalite; observed and calculated values of twist and

The molecular structure of tetrakis(trimethylsilyl)methane studied by gas-phase electron diffraction and molecular mechanics

Abstract The molecular structure of the sterically overcrowded molecule tetrakis(trimethylsilyl)-methane has been studied using gas-phase electron diffraction data collected on the Balzers KDG2 instrument at UMIST. Molecular mechanics calculations were employed to assist in the structure determination and establish the minimum energy conformation, thus distinguishing between two alternative electron diffraction minima. The principal structural parameters of the minimum energy T d conformation were observed to be: C*Si = 1.931(3) A, SiMe = 1.896(2) A, CH = 1.102(3) A, ∠ C*SiMe = 113.1(2)°, ∠ SiCH = 112.3(4)° and torsion angle τ = 180—SiC*SiC = 18.4(2)°. The methyl group torsion angle C*SiCH determined by electron diffraction is 73.5(17)°, close to the molecular mechanics value of 59°.

Pyrolysis of Parinari polyandra Benth fruit shell for bio-oil production

Article history: Non-conventional agricultural residues such as Parinari polyandra Benth fruit shell (PPBFS) are potential sources of biomass feedstock that have not been investigated for bio oil production. In this study, PPBFS was pyrolyzed via an intermediate pyrolysis process for the production of bio oil. The bio oils were obtained using a fixed bed reactor within a temperature range of 375-550 o C and were characterized to determine their physicochemical properties. The most abundant organic compounds present were acetic acid, toluene, 2-cyclopenten-1-one, 2-furanmethanol, phenol, guaiacol and 2,6-dimethoxyphenol. The bio- oil produced at 550 o C possessed a higher quantity of desirable compounds than those produced at lower temperatures. The presence of acetic acids in the bio-oil suggested the need to upgrade the bio-oil before utilization as a fuel source.