Diffusion of n -alkanes in mesoporous 5A zeolites by ZLC method

  • PDF / 643,597 Bytes
  • 8 Pages / 595.276 x 790.866 pts Page_size
  • 10 Downloads / 168 Views

DOWNLOAD

REPORT


Diffusion of n-alkanes in mesoporous 5A zeolites by ZLC method Zhiping Liu · Weiming Fan · Zhaoteng Xue · Jinghong Ma · Ruifeng Li

Received: 23 May 2012 / Accepted: 26 October 2012 / Published online: 8 November 2012 © Springer Science+Business Media New York 2012

Abstract Diffusion properties of mesostructured zeolite 5A were investigated by employing n-alkanes as probe molecules using the zero length column (ZLC) method. The mesopores were found to enhance molecule diffusion. Moreover, the effective diffusion time constant (Deff /R 2 ) increased with mesoporosity in the zeolites between 308 K and 393 K, whereas the activation energy decreased with increasing mesopore volume. The effective diffusivity values of n-alkanes in mesoporous zeolite 5A were generally higher than that the microporous zeolite 5A sample. This clearly implied the important role of the mesopore in zeolites crystals in facilitating the transport of reaction molecules due to shorter average diffusion path length and less steric hindrance. Keywords Mesoporous zeolite · 5A zeolite · ZLC method · Diffusion · Activation energy

Electronic supplementary material The online version of this article (doi:10.1007/s10450-012-9441-z) contains supplementary material, which is available to authorized users. Z. Liu · R. Li () College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, China e-mail: [email protected] W. Fan · Z. Xue · J. Ma · R. Li Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, China J. Ma e-mail: [email protected]

1 Introduction Zeolite is a microporous crystalline material with ordered pore structures which are composed of channels or cages with diameters ranging from 0.3 to 1.2 nm. Many remarkable features of zeolites such as narrow pore size distribution (or shape selectivity), high surface area, as well as high thermal and hydrothermal stability, render its wide applications in heterogeneous catalysis and in the separation and purification of gases. In most applications, molecular diffusion plays an important role. The intracrystalline diffusivity of molecular species involved in these processes is often the rate limiting factor in the overall kinetics of the processes (Gunadi and Brandani 2006). In many cases, the adsorption or reaction rate is limited by the low effective diffusivity in zeolite crystals, which is as a result of the size similarity of the target molecules and the micropore diameter of zeolites (Huang et al. 2006; Zheng et al. 2010). Diffusion limitation can be minimized by reducing intracrystalline diffusion path length or creating mesopores in zeolite crystals. Diffusion capability of large molecules in zeolite channels has thereby been improved (Groen et al. 2006; Zhao et al. 2008). Mesostructured zeolite materials have recently attracted much attention because they have the advantages of both mesoporous and microporous zeolites. However, there are only a few articles discussing the kinetics and the diffusion of hydrocarbons in mesopor