Zeolites are crystalline materials with a regular and microporous structure. Their pore size and the chemical composition in their framework, according to Al/Si ratio, give them distinctive properties and selective interaction with adsorbed molecules [1, 2]. These properties are important to obtain zeolite membranes with good features for gas separation [3, 4].
Zeolite membranes are conventionally prepared by in situ hydrothermal synthesis on porous supports, which are autoclaved together with a zeolite precursor gel at temperatures ranging from 80 to 200°C and lengths of time from a few hours to several days . This method has two main disadvantages: firstly, it cannot avoid the influence of the homogeneous synthesis of the resulting membrane and secondly, the synthesis employs an excess of reagents such as silicon and aluminum sources and organic compounds .
Since past decade, many efforts have been devoted to the development of methods for the reproducible preparation of high-quality zeolite membranes. A variety of synthesis methods for silicalite membranes has been reported . These methods not only differ in the gel composition and the supports employed, but also with respect to the procedure used for bringing into contact the support and the solution . The Dry Gel Converted (DGC) method is a good alternative to synthesize zeolites and zeolite membranes due to reduction of the quantities of reagents used [6, 7, 9].
Zeolite membranes are mainly characterized by pure inert gas or by small hydrocarbon permeation such as n-butane or isobutane . The permeance ratio of n-butane to isobutane has been widely used as an indication of the compactness of zeolite membranes.
The recovery of hydrocarbons from natural gas is desirable for a number of reasons. For instance, there is a clear economic motivation, since the price of hydrocarbons is considerably higher than that of methane. More compelling still is the necessity to remove hydrocarbons due to practical problems: condensates from higher molecular weight hydrocarbons produce liquid slugs and give rise to partial dissolution/softening of plastic pipes and meters .
In this work, silicalite membranes were prepared on stainless steel tubes by the dry gel method and tested in gas separation.
Silicalite membranes were synthesized by the DGC method on porous tubes (7 mm i.d. and 10 mm o.d.) stainless steel (Mott, Co.) with pore size of 500 nm. The permeation zone was approximately 5 cm long. A precursor gel was prepared with the following composition : 0.22Na2O: 10SiO2: 280H2O: 0.5TPABr, where TPABr stands for tetrapropylammonium bromide, and was used as the structure-directing agent (SDA). First, the NaOH was dissolved in distilled water and then colloidal silica (Ludox AS-40) was slowly added under stirring. After stirring for one hour the solution was kept to rest for 24h giving rise to a transparent silicate gel. To prepare a membrane, the tubular supports were dipped into the clear gel for 1h, and then dried at 100ºC approximately for one hour. This procedure was repeated 4 or 5 times, then, the dry gel on the support was crystallized in a teflon-lined autoclave in the presence of steam at 170°C for 5 days. The crystallization step was repeated 3 or 4 times and each time the gas permeation was measured. The template (SDA) in the zeolite pores was removed by calcination at 440°C for 6h. After calcination, the membranes were characterized by XRD, SEM and gas permeation experiments at room temperature. The relative separation factor was determined using a mixture of N2/SF6 and natural gas.
Results and Discussion
Figure 1 shows the XRD patterns of the stainless steel and the zeolite on the tube. In both cases the XRD patterns were measured directly over the tube. As can be clearly seen in Figure 1a, peaks appearing at 43.8° and 50.8° were attributed to stainless steel. In Figure 1b the main peaks corresponding to silicalite phase at 7.9°, 8.8°, 14.8°, 23.2°, 23.9° and 30º (JCPDS=48-0136) are shown.
These results were also confirmed by SEM (Figures 2a and 2b). In Figure 2a, two phases can be observed. One clear zone that corresponds to stainless steel and other dark zone belonging to dry gel phase. In Figure 2b, zeolite crystals both on the stainless steel grains and between the pores of the support were identified.
The gas permeation results are shown in Figures 3 and 4. In the first case (Figure 3), the zeolite membranes were selective to small molecules of N2 (diameter=3.64 Å) compared with SF6 (diameter=5.4 Å)  and the relative separation factor for this system was 11 to 15 times. This result is clear evidence that the diffusion of N2 was faster than that of the SF6. In the case of the permanent gases the diffusion is governed by Knudsen type, which is controlled by the size and the molecular weight. The calculated theoretical separation factor of the mixture N2/SF6 = 2.28, but our results showed a separation factor which is 5 times higher. This behavior could be explained by the compactness of the membrane indicating that the gas separation was carried out mainly by the zeolite cavities rather than through the intercrystalline grains .
The separation of hydrocarbons from a natural gas sample is illustrated in Figure 4. It was observed that the amount of each hydrocarbon permeated from the mixture of hydrocarbons was as follows: n-C4H10>i-C4H10> C3H8>C2H6>CH4. Note that the n-butane showed the highest concentration compared to the others compounds, in spite of the fact that it is bigger and heavier than the other hydrocarbons. Probably the diffusion as well as the permeation is strongly controlled by the surface adsorption and slightly by the size and molecular weight . Further, the adsorption force could increase with the interaction of compounds such as n-butane and it could block partially the pores, decreasing the permeation of the light hydrocarbons . In summary our results may be explained in two separate ways. The first one concerning the permanent gases on the surface of the zeolite, in which the most important parameters are the molecular size, molecular weight and the pore size of the membrane. The second one is regarding condensable gases on the surface, and the controlling step is the adsorption properties of the zeolite.
Zeolite membranes can be synthesized using the dry gel method with a lower amount of chemical reagents compared with the conventional hydrothermal method. The dry gel method led to good compactness, homogeneity and adherence on the stainless steel tube.
The separation factor measured for the N2/SF6 ratio was 5 times higher than the theoretical one. This improved selectivity to N2 was explained in terms of the properties such as the pore size of the membrane, controlling a Knudsen-type diffusion. In hydrocarbon separation of natural gas, n-butane showed the higher concentration in the permeated side of the membrane. In this case a remarkable separation effect assigned to the adsorption properties of the zeolite was proposed.
CGPI project 2005-0370, COFAA-IPN
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