Alkanes, diffusivity

Asfour-Dullien developed a relation for predicting alkane diffusivities at moderate concentrations that employs ... 

Leung, K.M., and R. P. Linstedt. 1995. Detailed kinetic modeling of C1-C3 alkane diffusion flames. Combustion Flame 102 129-60. 

It has been suggested that linear alkanes diffuse through the holes of membranes by alignment with the segments of the organic polymer chains. Such alignments are more difficult for branched alkanes so that they diffuse more slowly. 

K.M. Leung and R.P. Lindstedt. Detailed Kinetic Modeling of Ci to C3 Alkane Diffusion Flames. Combust. Flame, 102 129-160,1995. 

The observations of the n-alkane diffusion triggered extensive theoretical investigations, mostly employing Monte Carlo (MC) and Molecular Dynamics (MD) simulations [63-69]. From systematic simulations of a series of n-alkanes (n-CsHs, n-CeHi4, n-CioH22 and n-C2oH42) on W(001) the trends observed experimentally on Ru(001) were confirmed in the 300-1000 K... 

The corresponding diffusion coefficient in the liquid state (L), DLderived from Ds.ji and Dc. Starting from the reference class of n-alkanes, diffusion coefficients can be estimated for any solute and matrix with corresponding specific values for the critical temperature and critical pressure of the matrix and the critical molar volume of the solute. 

When one plots the D values of various different compounds versus their relative molecular mass, M, for a given polymer, then the values for the n-alkanes lie on the upper limit of diffusion (Fig. 9-8). This result means in practice that one has a maximum value for a variety of compounds with similar molecular weights when the n-alkane diffusion coefficients are known. In a first approximation one can assume the ratio of the D values between a gas, for example carbon dioxide, and a n-alkane with the molecular weight Mr to be constant and independent of the polymer. Given the existence of values for simple gases in a large number of polymers (Table 9-2) the... 

A number of methods are used for studying the sorption of basic probe molecules on zeolites to learn more about zeolite acidity. A common disadvantage of all the examinations is that adsorbed basic probe increases the electron density on the solid and, thereby, change the acidic properties of the sites examined. From this aspect it seems advantageous to probe the acid sites with a weak base, e. g., with a hydrocarbon. It was shown that adsorption of alkanes is localized to the strong Brdnsted acid sites of H-zeolites [1, 2]. However, recent results suggest that usually the diffusion in the micropores controls the rate of hydrocarbon transport [3-5]. Obviously, the probe suitable for the batch FR examination of the sites has to be non-reactive and the sorption dynamics must control the rate of mass transport. The present work shows that alkanes can not be used because, due to their weak interaction with the H-zeolites, the diffusion is the slowest step of their transport. In contrast, acetylene was found suitable to probe the zeolitic acid sites. The results are discussed in comparison with those obtained using ammonia as probe. Moreover, it is demonstrated that fundamental information can be obtained about the alkane diffusivity in H-zeolites... 

Al isopropoxide, MCM-41 secondary treatment Alkane diffusion, silicalite-l 19-P-lO... 

Figure 8 displays some typical FR data of Ci - Ce n-alkanes diffusing in coffin shaped crystals of silicalite-1 (40 x 40 x 260 p,m ). All the spectra in Fig. 8a-f,l can be fitted by the theoretical in-phase and out-of-phase characteristic function curves of the single diffusion model described by Eqs. 3-6, implying that only a simple, single diffusion process is involved in these systems. The diffusivities calculated from the best fit are presented in Fig. 9 and Tables 1 and 2. Equations 5 and 6 were applied since the channel framework structure of sihcahte-1 is comprised of near circular (0.54 x 0.56 nm)... 

In this work, the RHS theory was firstly applied to -alkane diffusion data. In Figure 3b, the linear relation between D/T and molar volume of the solvent for diffusion in -C64 suggests that the functional form of eq. (3) is confirmed by our results. The same linear relationship was also observed for all the solute - -alkane systems. In Figure 4 a eomparison between the simulated D and the predieted D from the RHS theory for the -alkanes is shown. For all the studied -alkanes, the % average absolute deviation (% AAD) is 4.7 for H2, 6.8 for CO and 5.9 % for H2O, thus eonfirming the very good agreement between the two sets of calculations. 

This sieving mechanism is rare but may be highly selective. One case where it is definitely involved is in transport of linear and branched alkanes into zeolites. The linear alkanes diffuse into the zeolites about fifteen times faster than the branched ones do. The reason is that the linear alkanes have a smaller cross-section which can fit into the pores in the zeolite crystal. This difference in diffusion is exploited in some forms of pressure swing adsorption. If thin zeolite layers can be produced commercially, they will have considerable value. 

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