Growth of a faujasite-type zeolite membrane and its application in the separation of saturated/unsaturated hydrocarbon mixtures

Abstract

Faujasite type zeolite membranes were synthesized on porous ceramic alumina supports by using direct (in situ) and secondary (seeded) growth methods. In the secondary growth method a seed layer of ZSM-2 nanocrystals (prepared according to a report by Schoeman et al. J. Colloid Interface Sci. 1995, 170, 449–456) was deposited on the surface of the support before the hydrothermal growth. For both in situ and secondary growth, the mixture composition was 4.17 Na₂O:1.0 Al₂O₃:10 TEA (triethanol ammonium):1.87 SiO₂:460 H₂O. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron microprobe analysis (EPMA), indicate well intergrown 5–30 μm thick FAU films with Si/Al ∼1–1.5. The separation of saturated/unsaturated hydrocarbon mixtures is demonstrated over a range of temperatures (40–160°C). The mixtures examined (and the corresponding equimolar mixture separation factors) are benzene/cyclohexane (160), benzene/n-hexane (144), toluene/n-heptane (45), propylene/propane (6.2), and ethylene/methane (8.4). In all cases, the membranes are unsaturated hydrocarbon permselective. With equimolar feed mixtures (5 kPa/5 kPa benzene/cyclohexane) and in the temperature range 65–160°C, the membranes exhibit separation factor of 20–160 with the benzene flux in the range 10⁻⁴–10⁻³ mol m⁻² s⁻¹. Decreasing the total feed partial pressure (0.31/0.31 kPa benzene/cyclohexane) reduces both separation factor (12) and benzene flux. Similar trend is observed when the benzene/cyclohexane ratio in the feed mixture (0.5/9.5 kPa benzene/cyclohexane) is reduced. A sorption diffusion model based on the Stefan–Maxwell formulation has also been employed to show that the benzene/cyclohexane separation can mainly be attributed to differences of their adsorption properties.

Introduction

Efforts towards zeolite membrane preparation are receiving an increasing amount of attention world-wide. Their well defined pore structures with sizes of molecular dimensions enable them to discriminate between sub-nanometer molecules. Thus, zeolite films synthesized on porous supports can find applications in separations of a variety of gas, vapor or liquid mixtures. Several examples of separations using different zeolite (MFI, LTA, FAU) films have been reported in the literature [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13].

Faujasite is a large pore zeolite (∼7.4 Å) [14], hence, it can be used in applications involving larger molecules compared to those that can be accommodated in other zeolites (e.g. the MFI pore diameter is 5.5 Å). Depending on their Si to Al ratio, faujasites are called X (Si/Al=1–1.5) and Y (Si/Al>2) even though they both have topologically the same framework. Furthermore, modification of the faujasite crystals, either by ion exchange or by de-alumination, can be used to control the adsorption or intracrystalline diffusion properties, providing a way of tailoring the membranes to specific applications. The synthesis of faujasite (Na–Y or Na–X) membranes on porous α-alumina supports has already been demonstrated by several research groups [7], [15], [16], [17], [18].

Kusakabe et al. [7], [17] synthesized Na–Y and Na–X zeolite membranes on porous α-alumina support tubes. The outer surface of the support tube was first rubbed with Na–X or Na–Y zeolite particles, and then treated hydrothermally at 90°C for 24 h. The initial molar composition of the reaction mixture was 1 Al₂O₃:xSiO₂:17 Na₂O:975 H₂O, with x varying between 1 and 25.6. The membrane quality was tested in separations of different binary gas mixtures, such as CO₂/N₂, CH₄/H₂, C₂H₆/H₂, C₃H₈/H₂, CO₂/H₂. The corresponding separation factors were 39.0, 1.7, 5.3, 9.7, and 27.8. The effect of permeation temperature and ion exchange on CO₂/N₂ separation factor was also studied. It was found that the Na–Y membranes are stable over a temperature range of 0–400°C, with the separation factor decreasing with temperature. It was also found that at 35°C, the K–Y membranes demonstrated the highest separation factor.

Kita et al. [18] have also synthesized a Na–Y zeolite membrane on the surface of porous alumina support tubes which were first coated with commercially available Na–Y crystals and then treated hydrothermally at 100°C for ∼5 h. The synthesis gel had molar composition 1 Al₂O₃:10 SiO₂:14 Na₂O:840 H₂O. Pervaporation experiments were carried out with the following feed mixtures: water/ethanol, methanol/benzene, methanol/methyl tert-butyl ether (MTBE), ethanol/benzene and ethanol/cyclohexane. The separation factors for all the mixtures were larger than 100, with the best results observed for the methanol/MTBE mixture (separation factor ∼7500).

Zeolite A or faujasite membranes were also synthesized on porous α-alumina disk supports under identical conditions, with the only difference being the seed crystals that have been used (Zeolite A or Y crystals) [15]. The membranes were synthesized at 85°C for 1–25 h from a clear mixture that had a molar composition of 1 Al₂O₃:9 SiO₂:80 Na₂O:5000 H₂O. The separation performance of the membranes was measured by pervaporation of 90/10 by volume water/ethanol mixtures at 30°C, reporting selectivities greater than 10,000 for zeolite A and ∼100 for faujasite.

Here, we present a method for synthesizing faujasite Na–X membranes on alumina supports by using in situ and secondary (seeded) growth methods. We further demonstrate the ability of the membranes to separate unsaturated from saturated hydrocarbons. Examples examined include a variety of gas or vapor hydrocarbon mixtures, such as benzene/cyclohexane (C₆H₆/C₆H₁₂), benzene/n-hexane (C₆H₆/n-C₆H₁₄), toluene/n-heptane (C₇H₈/n-C₇H₁₆), propylene/propane (C₃H₈/C₃H₆), and ethylene/methane (C₂H₄/CH₄).

The separation of cyclohexane and benzene mixtures is challenging [19] since these hydrocarbons form a close-boiling point system over all ranges of composition. They also form an azeotrope at 45% by volume cyclohexane. Processes such as azeotropic or extractive distillation are currently used in industry to separate these mixtures. All these processes are limited by the vapor–liquid equilibrium and are practical over a narrow range of feed compositions. A third component is also added and removed from the system, thus increasing the capital and operating costs. Benzene and cyclohexane also have similar kinetic diameters (0.6 nm for cyclohexane and 0.585 nm for benzene) making a molecular sieving separation difficult. The present work is mainly focused on this separation while limited permeation data are given for the other separations.