Adsorbate-Induced Expansion of Silicalite-1 Crystals

Abstract

Optical microscopy and XRD measurements show that silicalite-1 crystals expand when C 4 -C 8 alkanes and i-butane adsorb in the MFI structure at room temperature. The c-direction linear expansion was 0.20-0.45%, and was measured with XRD and optical microscopy for 200 µm crystals. n-Pentane, n-hexane, and n-heptane had the maximum linear expansion of approximately 0.54% in the b-direction, and n-hexane and n-heptane had the maximum volume expansion of 1.2%. In contrast, benzene did not cause a significant change in the unit cell volume. The a-direction expanded least for the molecules studied except i-butane. n-Hexane expanded the silicalite-1 crystal more at 180 K than at room temperature. Crystal expansion due to n-alkane adsorption decreased the size of defects in a polycrystalline MFI membrane, and changes in the fluxes of i-octane through defects were measured by transient permeation. Loadings as low as 0.5 molecule/unit cell decreased the flux through defects at 358-473 K, but benzene had no effect. Higher loadings increased crystal expansion and decreased the defect size more. Crystal expansion was reversible.

Adsorption-induced deformation of nanoporous materials—A review

“…21 None of these studies, however, presented strain isotherms, but rather gave a comparison of XRD patterns before loading and after loading of the adsorbate. Later, Noble and Falconer investigated the change of the unit cell as a function of adsorption of various species [31][32][33][34] (these works are further discussed in Section IV C). They presented a strain isotherm for NaA zeolite membrane for a i-butane/methanol mixture (as a function of methanol activity) in Ref.…”

Inhibiting crystal swelling in MFI zeolite membranes

“…The XRD measurements of changes in unit cell parameters correlated well with the changes in flux through defects due to changes in defect size [11,15,16]. Permporometry, vapor permeation, pervaporation, and temperature-programmed desorption measurements all confirmed that the flux through defects decreased orders of magnitude when n-alkanes and other molecules adsorbed in MFI pores and swelled the crystals [10,[12][13][14].…”

“…This type of behavior has been reported previously for MFI membranes [12,14,26] and arises because n-hexane swells MFI crystals and almost closes off defects. XRD measurements showed that MFI crystals expand about 1.15 vol% at saturation n-hexane loading, whereas benzene does not expand MFI crystals significantly [11,15,16]. Although CD decreased the helium flux for membrane A, the fraction of helium flow through defects was still 65%.…”

“…The smaller percentage increase in single-gas SF 6 flux compared to the percentage increase in He flux during permporometry with n-hexane may result because SF 6 is much larger than He, and because SF 6 swells MFI crystals less than n-hexane. Sulfur hexafluoride decreased the helium flux less than n-hexane during permporometry on other MFI membranes [12], and XRD measurements showed that MFI crystals expanded by 0.75 vol% for SF 6 at saturation and 1.15 vol% for n-hexane at saturation [11,16]. Additionally, n-hexane swelled MFI crystals at lower percentages of saturation loading than SF 6 [11].…”

“…Our recent studies [11][12][13][14][15][16] showed that n-alkanes, SF 6 , ibutane, ethanol, p-xylene and 2-propanol swell MFI crystals by 0.5-1.5 vol%, whereas benzene does not cause significant swelling.…”