Preliminary resultsFigure 2 shows a photograph of a membrane. It is 25 mm in diameter and 3 mm thick. Only the alumina substrate is seen in the picture, the thin zeolite film can not be seen by the naked eye.

SEM images of a silicalite-1 membrane is shown in Figure 3. The images show that the film is polycrystalline and that there is no visible defects (e.g. pinholes or cracks). The top layer of the support has 100 nm pores and is visible in the lower part of the image. On top of the support there is a 500 nm thick zeolite film. The zeolite is intergrown with the support, but no zeolite depositions inside the support pores are visible in the image.

In Figure 4, typical porosimetry data of a silicalite-1 membrane are shown. It can be seen that the permeance drops drastically even at a very low partial pressure of n-hexane. At P/P0=0.01, only the zeolite pores are blocked by the adsorbing n-hexane, and all larger defects are still open. A large drop in permeance is thus indicative of a large fraction of the original permeance occurring through zeolite pores. In this case, the fraction was 99.6%. This in turn means that the amount of defects is small.

Methanol separation from synthesis gas is illustrated in Figure 5, where the synthesis gas is represented

Zeolite membranes for separation of carbon dioxide, methanol or ethanol for efficient production of renewable fuels from synthesis gas.

Linda StenbergSupervisor: Professor Jonas Hedlund

Div. of Chemical Technology, Luleå University of Technology, SE-97187 Luleå, Sweden

AcknowledgementsThe Swedish Research Counsil (VR) is acknowledged for financial support.

References[1] S.M. Auerbach, et al, Handbook of zeolite science and technology, 2003, New York, M. Dekker[2] J. Caro et al, Microporous and Mesoporous Mater. 38 (1) (2000) 3-24[3] C.N. Satterfield, Heterogeneous catalysis in industrial practice, 2 ed. 1996, Malabar, Krieger Pub. [4] J. Hedlund et al, Microporous and Mesoporous Mater. 52 (3) (2002) 179-189[5] S.A.S. Rezai et al, Microporous and Mesoporous Mater. (2007) In press

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AbstractThe project aims at designing zeolite membranes for gas phase separation as well as understanding the parameters controlling the separations. The membranes used consist of thin (500 nm) zeolite films on top of porous alumina substrates. Different zeolite structures (e.g. MFI and DDR) and different Si/Al ratios in the zeolite will be used to tailor the separation properties for each application. The membranes are characterized by SEM, XRD and single gas permeation measurements. A tool denoted porosimetry is used for defect characterization. Preliminary separation data for methanol/hydrogen show promising results.

IntroductionThe aim of the project is to design selective zeolite membranes for efficient gas phase separation of carbon dioxide and the renewable fuels methanol and ethanol. The aim is also to obtain fundamental understanding of the parameters controlling permeance and selectivity in these membranes, such as adsorption, diffusion and defects.

Zeolites are micro-porous minerals, consisting of silica and alumina tetrahedra [1]. These tetrahedraform a 2- or 3-dimensional network of pores. Zeolites have well defined pore sizes and high thermal and chemical stability [2]. They are used in industry e.g. for selective adsorption, ion-exchange and shape selective catalysis.

In order to obtain both high flux and mechanical stability, a zeolite membrane usually consists of a thin zeolite film on top of a porous support. Alumina is a common support material, but stainless steel is also used [2]. The selectivity of a zeolite is controlled by molecular sieving, diffusivity and/or adsorption [2]. Molecular sieving and diffusivity of a species in the membrane can be controlled by choosing a zeolite structure with the desired pore width. For example, the MFI structure has pores of about 0.5 nm while DDR has 0.4 nm pores [1]. The adsorption properties are tailored by controlling the aluminium content in the zeolite, and thereby the polarity. Both MFI and DDR have all-silica (silicalite-1 and DDR) as well as aluminium containing (ZSM-5 and sigma-1) analogues.

Separation of CO2 is necessary in many applications, but current techniques, such as absorption in amines, are expensive and zeolite membranes may cut the costs significantly. The zeolite structure DDR has small pores and could be suitable for CO2 separation e.g. from CH4 or synthesis gas.

Methanol synthesis from synthesis gas is carried out at temperatures around 250°C and is limited by equilibrium. Methanol is therefore separated from the reactor effluent by condensation, after which unreacted synthesis gas is reheated and recirculated to the reactor inlet [3]. The process could be improved significantly by separation of methanol directly after synthesis using a zeolite membrane. Separation at the process temperature reduces costs by excluding the cooling-reheating step. Higher alcohols, such as ethanol, can also be produced from synthesis gas, and a zeolite membrane could improve also this process.

ExperimentalMFI (silicalite-1 and ZSM-5) membranes are prepared as described below. The synthesis procedure for silicalite-1 membranes is described in detail in reference [4]. Synthesis of ZSM-5 membranes is described in [5]. In a later state of the project, membranes with other structures will be prepared.

Graded α-alumina discs (Inocermic GmbH, Germany) are used as supports. The discs are 25 mm in diameter, and 3 mm thick. The supports, apart from the top surface, are masked with wax in order to minimize support invasion (zeolite growth into the pores). The membranes are prepared using a seeding method, in which 60 nm silicalite-1 crystals (seeds) are first adsorbed on the support surface. The seeds are then grown into a dense film by hydrothermal treatment in a synthesis solution at 100°C. Different synthesis solutions and times were used for silicalite-1 and ZSM-5. The solutions contain a silicon source, an aluminium source (for ZSM-5) and an organic templating agent (TPA+). After synthesis, the membranes are calcined at 500°C to decompose the templating agent and the wax that are blocking the pores of the membrane.

The membranes are characterized with scanning electron microscopy, x-ray diffraction, single gas permeation measurements and porosimetry. In porosimetry, the permeance of a non-adsorbing gas is measured as a function of the partial pressure of an adsorbing gas. Pores and defects of different sizes are blocked by the adsorbing gas at different partial pressures, and porosimetry hence gives an estimate of the sizes and amount of defects [4].

The separation measurements are performed using a Wicke-Kallenbach apparatus. The feed gas is swept over the membrane surface, while a flow of pure helium on the permeate side maintains the pressure of the permeating species low. The driving force through the membrane is thus a partial pressure gradient. The stainless steel cell used for separation measurements and porosimetry is shown in figure 3. It is placed in an oven and equipped with a thermocouple to allow temperature control.

The separation factor, α, is defined as

Figure 2. Zeolite membrane, 25 mm diameter. Figure 3. SEM images of a) the surface morphology and b) the side view of a silicalite-1 membrane.

Figure 1. Stainless steel membrane cell forpermeation measurements.

Figure 5. Separation factor for methanol/hydrogen as a function of temperature for a silicalite-1 membrane.

Figure 4. Typical porosimetry data. Gases used are helium and n-hexane.

by hydrogen. This silicalite-1 membrane has a separation factor of 12 at room temperature. At low temperatures, methanol is adsorbing on the zeolite surface, and the hydrogen permeance through the pores is then blocked. The membrane is methanol selective up to about 50°C. Methanol synthesis is performed at 250°C and 5-10 MPa. Since adsorption is more pronounced at higher pressures, theseresults show that a silicalite-1 membrane probably is methanol selective at the synthesis conditions.

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