Diffusivities of linear alkanes

Matthews-Akgerman The free-volume approach of Hildebrand was shown to be valid for binary, dilute liquid paraffin mixtures (as well as self-diffusion), consisting of solutes from Cg to Cig and solvents of Cg and C o- The term they referred to as the diffusion volume was simply correlated with the critical volume, as = 0.308 V. We can infer from Table 5-15 that this is approximately related to the volume at the melting point as = 0.945 V, . Their correlation was vahd for diffusion of linear alkanes at temperatures up to 300°C and pressures up to 3.45 MPa. Matthews et al. and Erkey and Akger-man completea similar studies of diffusion of alkanes, restricted to /1-hexadecane and /i-octane, respectively, as the solvents. 

Fig. 12 Diffusivities of linear alkanes in different zeolites obtained at 475 K as a function of the carbon number a self-diffusivities derived from PPG NMR in Na-X zeolite, b self-diffusivities measured by QENS in ZSM-5, c transport diffusivities obtained by NSE in 5A [48]...

Kortunov et al [55] have used the PFG NMR technique to measure the diffusion of linear alkanes within the crystals and within the macropores of HY and REY based cracking catalysts. At 600°C Dmacro/Dmicro 10 but, since the crystal size is about 1 xm while the particle size is about 100 pm the ratio of the diffusional... 

A good example for reactant shape selectivity includes the use of catalysts with ERI framework type for selective cracking of linear alkanes, while excluding branched alkanes with relatively large kinetic diameters from the active sites within the narrow 8-MR zeolite channels [61, 62]. Here molecular sieving occurs both because of the low Henry coefficient for branched alkanes and because of the intracrystalline diffusion limitations that develop from slow diffusivities for branched alkane feed molecules. 

Dumont and Bougeard (68, 69) reported MD calculations of the diffusion of n-alkanes up to propane as well as ethene and ethyne in silicalite. Thirteen independent sets of 4 molecules per unit cell were considered, to bolster the statistics of the results. The framework was held rigid, but the hydrocarbon molecules were flexible. The internal coordinates that were allowed to vary were as follows bond stretching, planar angular deformation, linear bending (ethyne), out-of-plane bending (ethene), and bond torsion. The potential parameters governing intermolecular interactions were optimized to reproduce infrared spectra (68). 

Adsorption and diffusion of linear and branched Ce alkanes in silicalite-1 were investigated by Zhu et al. (34,35). They also developed a mathematical model taking into account the thermodynamical factor for intercrystalline diffusivities, enabling the determination of intracrystalline diffusivity from the uptake curve operated outside the linear adsorption range, van Donk et al. (36) also made transient uptake measurements to investigate the diffusivity of -hexane in Pt/H-mordenite. 

Variation of Diffusivity with Chain Length of Linear Alkanes... 

Zhu, W. Kapteijn, F. Moulijn, J.A., Diffusion of linear and branched C6 alkanes in silicalite-1 studied by the tapered element oscillating microbalance. Micropor. Mesopor. Mater. 47 (2001) pp. 157-171. 

Jobic, H. 2000. Diffusion of linear and branched alkanes in ZSM-5. A quasi-elastic neutron scattering study J. Mol. Catal. Vol. 158 pp 135-142. 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

Figure 13.15 illustrates the diffusivity by ZLC for various branched and linear Cs alkanes and n-Cs, as compiled by Ruthven and Post [18]. The results indicate that diffusivities vary by four orders of magnitude between different isomers. Linear alkanes show lower activation energy than branched alkanes. The absence... 

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. 

Liu and Ruckenstein [17] presented a semiempirical equation to estimate diffusivities under supercritical conditions that is based on the Stokes-Einstein relation and the long-range correlation, respectively. The parameter 20 was estimated from the Peng-Robinson equation of state. In addition, f = 2.72 — 0.3445 TcB/TcA for most solutes, but for C5 through C14 linear alkanes,/ = 3.046 — 0.786 l cBl l cA. In both cases Ta is the species critical temperature. They compared their estimates to 33 pairs, with a total of598 data points, and achieved lower deviations (5.7 percent) than the Sun-Chen correlation (13.3 percent) and the Catchpole-King equation (11.0 percent). 

Among the chemical reactions of interest catalyzed by zeolites, those involving alkanes are specially important from the technological point of view. Thus, some alkane molecules were selected and a systematic study was conducted, on the various steps of the process (diffusion, adsorption and chemical reaction), in order to develop adequate methodologies to investigate such catalytic reactions. Linear alkanes, from methane to n-butane, as well as isobutane and neopentane, chosen as prototypes for branched alkanes, were considered in the diffusion and adsorption studies. Since the chemical step requires the use of the more time demanding quantum-mechanical techniques, only methane, ethane, propane and isobutane were considered. 

---