RECENT DEVELOPMENT IN THE ZEOLITE MEMBRANE SYNTHESIS

RECENT DEVELOPMENT IN THE ZEOLITE MEMBRANE SYNTHESIS L. GÓRA, J.C. JANSEN, A.W.C. van den BERG and Th. MASCHMEYER Laboratory for Applied Organic Chemistry and Catalysis, DelftChemTech, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands e-mail: L.Gora@stm.tudelft.nl INTRODUCTION Zeolites offer the advantage of uniforme pore diameters in the size range of many interesting gas and liquid species, high stability in many liquid environments, and high thermal stability. Therefore, zeolite membranes are of great interest for separation processes. Catalytic zeolite membranes offer an extension of possible applications. They could act as simultaneous (catalytic) reactor/separator, and increase yields or conversions in some otherwise systems. A conceptual process design of such an assembly of zeolite types and functions in one reactor for the hydroisomerization of alkanes has been made recently and indicates feasibility [1]. In all cases the quality of the zeolite phase in a membrane configuration must be excellent to obtain high selectivity as well as high yield. The main criterion for high-flux zeolitic membranes is a continuous thin layer of zeolite on a macroporous support. In such a configuration the zeolite channels are the only diffusion paths for molecules. The separation occurs on the basis of adsorption and configurational diffusion. However, obtaining a perfect membrane is a difficult task. The separation performance of zeolite membranes significantly depends on the membrane quality because any defects between the crystals reduce selectivity [2,3]. Therefore, mono-crystal-layered membranes composed of small well-intergrown crystals are preferred as they allow for a decrease of the membrane thickness as well as an increase in the membrane integrity, and therefore an increase of fluxes. Syntheses of zeolite monolayered as well as bi-layered membranes composed of different types of zeolites, (assembling a membrane and catalyst phase) are important for process intensification. For the formation of bi-layered systems two synthesis strategies are used i) the composition of the synthesis mixture slightly changes upon the formation of the first layer, thus inducing the crystallisation of the second layer and ii) just sequential synthesis with two different recipes. The bi-layer systems can be build up by, both, a rough permselective layer and a fine selective layer, or by a catalytic layer (rough

permselective or not selective at all) and a membrane layer. To realise these systems, refined synthesis procedures were studied as a first approach. In this study, we report the successful and reproducible preparation, using various techniques, of silicalite-1, zeolite A, Y, beta and of a composite two layers membrane, actually zeolite A with silicalite-1 on top. For these syntheses the use of seeds seems a key factor. The performance of these membranes will be shortly reported for molecular separations of industrial importance. EXPERIMENTAL SECTION The synthesis mixtures with molar composition: for silicalite-1 membranes-100.7 SiO 2 :32.68 TPA :32.68 OH :12000 H 2 O (reaction mixture I) or 100 SiO 2 :123 TPA :63.7 OH :14200 H 2 O (reaction mixture II), zeolite A membranes-1 SiO 2 :0.225 Al 2 O 3 :16.5 OH:180 H 2 O, zeolite Y membranes- 10 SiO 2 :1 Al 2 O 3 :4 Na 2 O:180 H 2 O and zeolite beta membranes-100 SiO 2 :1.97 Al 2 O 3 :49 TEA :1.64 OH :5.9 Cl :1500 H 2 O were prepared as was described elsewhere [2, 4-9]. Zeolite membranes were grown on the titania side of a TRUMEM TM porous support, which consists of a porous sintered stainless steel layer coated by a 15 µm top TiO 2 layer. Two synthesis approaches were used: i) single synthesis (zeolite A and silicalite-1), ii) seeded reaction mixture or seeded surface syntheses in varied conditions (zeolite Y, beta, silicalite-1, single and on top of zeolite A membrane or beta membranes). Silicalite-1 seeds having a mean size 700 nm and 220 nm were prepared based on the recipe of Jansen et al. [5] and Person et al. [6]. The silicalite-1 suspension was applied on the support, dried at 373 K and used in the syntheses. Zeolite beta seeds were synthesised from the same reaction mixture as later zeolite beta membranes were grown. The autoclaves were placed in the reactors at the synthesis temperature (353-483 K) for the needed reaction time (3-142 h). After heating, the autoclave was removed from the oven, and cooled with water. The samples were washed with distilled water and dried. Membranes containing silicalite-1 were calcined for 16 h in air at 673 K with a heating and cooling rate of 1 O C/min. An optical microscope was used for the estimation of macroscopic integrity, and the presence of cracks in the layer before and after calcination (silicalite-1 and beta). The membranes were examined by scanning electron microscopy (SEM) using a Philips XL20 microscope. SEM was also used to estimate the thickness of the zeolite membrane (cross sections). X-ray diffraction analysis was carried out on the zeolite membranes using a Philips PW 1830 X-ray diffractometer. Permeation measurements using single components of n-butane, i-butane, CO 2, N 2 and binary mixtures of n/i-butane as well as CO 2 /N 2 were performed by the Wicke-Kallenbach method at 303 K. The pervaporation tests for EtOH/water mixture were carried out at 45 O C. Feed liquid at composition 95% EtOH and 5% of water, was supplied to the zeolite side of the

membrane at constant flow. The other side of the membrane was evacuated through a vacuum line. The permeate vapour was collected by a cold trap cooled by nitrogen. Zeolite membranes were glued with epoxy resin (Varian- Torr Seal) onto a flange, which had a 2.54 cm 2 window, hardening at 303 K for 120 minutes. Directly thereafter the permeation measurements were carried out. The feed and permeate sides were kept at atmospheric pressure. Helium was used as sweep gas with a flow rate of 100 ml min -1. Feed, retentate and permeate streams were analysed with a mass spectrometer of Multi Gas-Spectra. RESULTS AND DISCUSSION Four different types of single-phase membranes, zeolite A (figure 1.a), Y (figure 1.b) beta (figure 1.c) and silicalite-1 (figures 1.d) were prepared in the course of the study. The zeolite A membrane could be prepared without seeding. Its thickness was a function of the crystallization temperature and the synthesis time. The thickness of the zeolite A membrane could be increased by the same thickness with each next synthesis (for example 2.5 µm at crystallisation temperature equal to 80 O C). Each next layer was composed of the new population of crystals, which were growing on top of the existing layer. a b c d Fig. 1. Zeolite membranes. a. Zeolite A; b. Zeolite Y; c. Zeolite beta; d. Silicalite-1

Increasing the crystallization temperature from 80 O C to 115 O C resulted in a double membrane thickness from 2.5 µm to 5 µm. In the first three hours of crystallization at 80 O C the zeolite A membrane increased in thickness linearly. Longer crystallization than 3 hours or crystallization at a temperature higher than 140 O C resulted in the formation of hydroxysodalite type zeolite. The zeolite A membrane synthesized at 80 O C was used for EtOH/H 2 O pervaporation. The flux was calculated by weighing the condensed permeate. The separation factor was determined as α A/B=(Y A /Y B )/(X A /X B ), where X A,X B, Y A and Y B denote the mass fractions of components A and B in the feed and permeate sides. It was possible to separate H 2 O from 95%/5% EtOH/H 2 O mixture at 45 O C with selectivity of between 15000-1900 and flux 0.19-1.02 kg/m 2 h (see table1). We have shown recently that a coating of zeolite Y on a cleaned common stainless steel could be achieved with a new synthesis procedure, using a seeded synthesis mixture [7]. We employed this technique to make a zeolite Y membrane [8]. A high quality zeolite Y membrane can be synthesized by combining two synthesis steps. With this method firstly a seeded gel layer is deposited on the support upon dipping it in the Tab. 1. Pervaporation with supported zeolite A membranes Sample number 12.7 16.2 18.1 19.1 19.2 16.4 15.1 Flux [kg/m 2 h] 0.13 0.49 0.67 0.83 0.59 0.60 1.02 EtOH/H20 (5% H2O) Separation factor 14896.0 7438.5 1905.0 1969.7 2466.8 6609.9 2460.0 synthesis mixture. The seeds initially present in the synthesis mixture are also part of the supported gel layer. Therefore their growth occurs directly on the support and this gives rise to a dense layer when exposing the system to a second synthesis step. The use of a seeded mixture was found to be mandatory for a dense coating. The behaviour of the zeolite Y membrane was tested in the separation of the light molecules N 2 and CO 2, since membranes able to separate these gases are not commonly or readily available. XRD evidenced that the zeolite Y free of impurities was present on the surface of the Trumem support. SEM images showed the layer was well intergrown (figure 1 b). This layer was c.a. 0.7 µm thick and permeation measurements confirmed that a closed layer had been synthesized. The zeolite Y membrane could separate CO 2 from N 2 with a higher selectivity (see table 2) than that which can be derived from the Knudsen value.

Tab. 2. Separation with supported zeolite Y membranes Composition CO 2 /N 2 single 1:1 CO 2 /N 2 CO 2 /N 2 Knudsen diff. Temperature and Pressure [K], [kpa] 303, 101 303, 101 --------- Flux of first component [mmol/m 2 s] 95 33.1 -------- Selectivity 2.1 4.0 1.2 Seeded and unseeded syntheses of silicalite-1 membranes were performed. In the case of seeded syntheses porous TiO 2 /stainless steel supports were coated with a layer of i) 700 nm (see figure 1.d) and ii) 220 nm silicalite particles. Despite of the used synthesis techniques the crystallization temperature was the major variable affecting the zeolite membrane formation. In all cases, the supports were fully covered by silicalite-1 crystals. Silicalite-1 layers were firmly bound to the surface of titania covering the supports: the membrane cross-section preparation did not destroy the top zeolite layer. The thickness of the synthesised silicalite-1 membranes varies from 1 to 35 µm depending on the membrane synthesis conditions. It was possible to decrease the membrane thickness by using smaller seeds (220 nm instead 700 nm). Such membranes were about 1-2 µm. In order to evaluate the performance of the membranes synthesized at different temperatures, the separation of a binary mixture of n-butane/ibutane was studied. Table 3 summarizes the binary permeation data of butane isomers at 303 K for several membranes that were synthesized at two temperatures 393 and 453 K with (700 nm seeds) and without using seeds. The membranes synthesized at 453 K give better performance (highest flux/selectivity). The selectivity for an equimolar n-butane/i-butane mixture is as high as 55. Such a high selectivity of n-butane/i-butane at 303 K indicates that the transport is governed by the molecular size and shape. It is interesting to note that whereas the selectivity is alike at 453 K or higher at 393 K, when seeded membranes or unseeded ones are compared, the fluxes are higher on seeded membranes due to their lower thickness. Indeed, there is a difference in the thickness of zeolite membranes synthesized in the same conditions with and without seeds. A zeolite layer grown on the seeded support at 453 K was about 10 µm thick compared to 32-40 µm for the membranes synthesized on non-seeded support. Mathematical modelling of permeation through silicalite-1 membranes indicates that the selectivity does not change vs. thickness, only the fluxes become lower (but this effect levels [3]. This prediction is in good agreement with our experimental observations.

Tab. 3. Flux of n-butane and n/i-butane selectivity for various silicalite-1 membranes. 1:1 mixture of n-butane and i-butane at 303 K Synthesis conditions Temp. [ O C], time [h] 120, 114, seeds 1 120, 115 no seeds 180, 17, seeds 1 180, 17.25, no seeds Thickness [µm] n-butane flux [mmol/m 2 s] n-butane/i-butane selectivity 12 10 10 32 5.68 5.23 3.40 2.60 16.2 6.0 47.2 55.0 1 Seeds 700 nm in size. The selectivity of n-butane/i-butane improved with reducing pinholes in the intercrystalline boundaries between the crystals forming the membrane, by increasing the synthesis temperature. Furthermore, there is not a big influence of the thickness of the membranes on the fluxes of n-butane. The n-butane flux slightly decreases with increasing the membrane thickness and reaches values equal to 2.6 mmol m- 2 s -1 for samples synthesized at 453 K on unseeded support compared to 3.4 mmol m -2 s -1 for samples prepared on seeded support from a synthesis with the same temperature. Improvement in the selectivity with increase of the synthesis temperature indicates that defects/imperfections are minimized. Silicalite-1 membranes were also used to separate n-hexane from 2-methyl-pentane and selectivity was as high as 86. Base on the refined synthesis recipes for single phases a combination of possible bi-layer syntheses involving silicalite-1, zeolite A and Y were performed. However, only combinations zeolite A/silicalite-1 and zeolite beta/silicalite-1 resulted in the formation of the second layer. For all other syntheses the first layer dissolved. This may be caused by the rates of dissolution of the first layer and the formation of the second layer. Thus the combination of syntheses mixtures of certain zeolite types as well the order of application to form the layer seem relevant. As an example, a combination of silicalite-1 with a zeolite A membrane is shown on figure 2. Zeolite A and silicalite-1 -layers in the sandwich mode were 2.5-5 µm and 3-15 µm thick, respectively. With this successful synthesis of silicalite-1 on top of a zeolite A membrane possibilities are created for process densification and operation. The possibility of applications of a bi-layer in separation with a rough preselective layer and a fine selective layer and in catalytic membrane application with a separation layer (A or silicalite-1) and a catalytic layer or coating (beta or Y) becomes apparent.

Silicalite- Zeolite Fig. 2. Zeolite membranes combined zeolite A (5 µm layer) with silicalite-1 on top. Silicalite-1 grown using 700 nm seeds, 180 O C, 17 h, reaction mixture II. CONCLUSIONS Single layer silicalite-1, zeolite A, and Y membranes were grown on porous Trumem TM supports. The prepared membranes are of a good structural quality. The zeolite films were firmly bound to the TiO 2 top-layer of the support. The various zeolites required specific treatments to achieve a dense membrane. Zeolite A was relatively easy to synthesise and the thickness of the membrane could be modified by simply increasing the number of successive syntheses. Zeolite Y could be obtained only after the use of a seeded synthesis mixture. It is possible to achieve a membrane using silicalite-1 without any seeding, but the use of seeds was efficient to reduce the final thickness of the membrane without any loss in the selectivity and somewhat higher fluxes. In the silicalite-1 syntheses the increase in the growth rate (achieved by increasing the crystallization temperature) also influences the quality of the interfaces between the crystals that form the membrane. Closing pinholes in the intercrystalline boundaries improves the membrane performance. The highest selectivity/flux results are given by membranes synthesized in the temperature range of between 453-463 K. However, in the dual layer synthesis cases when it is difficult to obtain complete coverage of the support, a seeding technique can be chosen. These membranes were prepared in a highly reproducible manner. Hence although seeding is often unnecessary to obtain an efficient membrane, its use can be advisable in many cases, notably because it assists the presence of crystals directly on the support in the first moments of the synthesis and thus assists the formation of a dense system.

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