Microporous and Mesoporous Materials 105 (2007) 140–148 www.elsevier.com/locate/micromeso
A comparative study on permeation and mechanical properties of random and oriented MFI-type zeolite membranes Jessica O’Brien-Abraham, Masakoto Kanezashi, Y.S. Lin
*
Department of Chemical Engineering, Arizona State University, Tempe, AZ 85287, USA Received 17 January 2007; received in revised form 19 May 2007; accepted 23 May 2007 Available online 12 June 2007
Abstract Changes to microstructure of a zeolite membrane can affect defect formation, degree of intergrowth, stability, and performance of the membrane. In this work, random, c- and h,0,h-oriented membranes are synthesized under similar conditions (i.e. same structure directing agent, seed layer solution, calcination procedure) and characterized with performance based methods. Moderate ideal selectivities for H2/SF6 around 20 were observed for those membranes most effected by defects induced during the template removal step including both the random and h,0,h-oriented membranes. In contrast, the c-oriented membrane was less susceptible to crack formation and showed H2/SF6 selectivity around 46. Single component pervaporation studies yielded no ideal selectivity for p-xylene over o-xylene for random membranes while a selectivity of 2–3 was observed for the oriented membranes. Furthermore, membrane degradation was observed in all microstructures of MFI membranes that were subjected to pervaporation experiments and subsequent heat treatment to remove the adsorbed xylenes. It is believed that this effect is due structural deformations induced by high loadings of adsorbed xylene which upon burn out cause permanent damage to the membrane. 2007 Elsevier Inc. All rights reserved. Keywords: MFI-type zeolite membranes; Microstructure; Gas permeation; Pervaporation
1. Introduction In recent years there has been a strong focus on developing and optimizing zeolite membranes for operations that require resilience in harsh environments because of their attractive chemical and thermal stability. MFI-type (ZSM-5 and aluminum-free silicalite) zeolites have been shown to be stable up to temperatures of 1300 C and under highly acidic conditions [1]. MFI-type zeolites are crystalline aluminosilicates with a pore structure consisting of two channels: a straight channel along the b-axis with circular openings of 0.54 · 0.56 nm and a sinusoidal channel along the a-axis with elliptical openings of 0.51 · 0.55 nm. Diffusion along the channel intersections in the c-direction is also possible [1]. Because the pore sizes are on the order of many molecules, the material expresses *
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[email protected] (Y.S. Lin).
1387-1811/$ - see front matter 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.05.045
a molecular sieving capability that is based on size exclusion and shape selectivity. Two methods are commonly used to synthesize MFI zeolite membranes. The first is in situ crystallization where the substrate is placed in contact with a precursor solution and zeolite film formation occurs during hydrothermal treatment at a specified temperature and duration [2]. The second is a secondary growth technique developed by Lovallo and Tsapatsis [3]. In this method the nucleation and film growth step are decoupled by first depositing a thin seed layer onto the support and then subjecting it to hydrothermal growth. The advantages of this method are increased flexibility and greater control of final film microstructure [4]. As testament to the flexibility of the secondary growth method, a variety of synthesis conditions have been employed to synthesize MFI membranes. Hedlund et al. [5] grew thin (0.5 lm) silicalite membranes on seeded masked a-Al2O3 supports after hydrothermal growth of
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the silicalite for 30 h at 100 C (atmospheric pressure). Lovallo and Tsapatsis [3] synthesized silicalite membranes on a-Al2O3 supports and glass slides after 8 hours hydrothermal growth at 130 C in an autoclave. Lai et al. [6] synthesized ZSM-5 membranes at temperatures between 110 and 150 C and 4–16 h. Tsapatsis and co-workers [4] identified conditions where the crystal growth proceeded in a preferentially oriented out-of-plane direction despite the fact that the original seed layer was randomly oriented. This growth mechanism is explained by the evolutionary Van-der-Drifts columnar growth where the crystal grains with the highest vertical velocity are most likely to dominate the structure [7]. Essentially these grains that grow fastest perpendicular to the surface will bury the slower growing grains and over time the membrane will adopt a preferred orientation [3]. Lovallo et al. [4] first reported the synthesis of c-oriented MFI membranes after 24 h synthesis at 175 C. In these membranes the c-axis is aligned normal to the substrate surface and the b- and a-axes are parallel to the surface which results in a highly columnar microstructure. Gouzinis and Tsapatsis [8] reported the synthesis of both c-oriented and h,0,h-oriented MFI membranes. The h,0,h-oriented membranes have the c-axis at a 34 angle normal to the surface and can be synthesized at temperatures below 140 C for extended growth times [8]. More recently, Lai et al. [9] synthesized b-axis out-of-plane oriented MFI membranes by changing the structure directing agent (SDA) in the hydrothermal growth solution from tetraproplyammonium (TPA) to a trimer-TPA. The a-axis out-of-plane can also be synthesized by using the trimer-TPA to synthesize the seeds used for deposition and conducting hydrothermal growth with TPA as the SDA, as published by Choi et al. [10]. It is well known that varying synthesis conditions yield variations in membrane properties. Aside from microstructure there can be changes to defect formation, degree of intergrowth, and stability. Each of these will change the observed transport properties of the membrane which determines the overall membrane selectivity. For both random and preferentially oriented membranes significant characterization studies have been performed to evaluate performance of each membrane type. Table 1 lists the results of a number of studies that have been conducted on MFI membranes of varying microstructure. Much
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attention has been focused on xylene separation due to its use as a characterization tool and industrial significance [17]. Typically characterization of membrane quality is carried out using performance based methods. Popular molecular probing techniques include permeation and pervaporation [14]. Differences in the experimental conditions of such methods make it difficult to compare the results of different studies directly and as a result the role of microstructure on membrane transport and quality is not well understood. In fact, the direct comparison of microstructures has been the subject of only a few works. Xomeritakis et al. [11] evaluated the permeation differences for vapor permeation of xylene isomers through h,0,h- and c-oriented membranes. Wong et al. [15] studied h,0,h-oriented membranes and discussed relationship between grain size/shape and permeability the more oriented the membrane became. However, these studies do not focus on the role that the microstructural variations play in membrane performance. If this fundamental relationship can be understood it will be possible to present a more detailed look into the transport occurring through MFI-type membranes. It will also provide the information needed to determine the optimal microstructure for various applications such as hydrocarbon or gas separation. The objective of this work is to show the differences in mass transport through random and oriented MFI membranes as well as to demonstrate the affect of microstructure on selectivity, permeance and quality. Random, c- and h,0,h-oriented membranes are synthesized under similar conditions (i.e. same SDA, seed layer solution, calcination procedure) and characterized with performance based methods. This study addresses the affect of microstructure on the sensitivity to defect formation during calcination, gas separation performance, and xylene pervaporation performance. 2. Experimental 2.1. Preparation and characterization of random and oriented MFI membranes MFI membranes were synthesized on homemade a-Al2O3 disks 20 mm in diameter and 2 mm thick (average pore diameter: 0.2 lm, porosity 45%) [17] made with
Table 1 Past work conducted on random and oriented MFI membranes Orientation
Growth method
Separation (method)
Selectivity
Reference
c-Oriented c-Oriented c-Oriented h,0,h-Oriented h,0,h-oriented Random Random Random
Secondary Secondary Secondary Secondary Secondary Secondary In situ In situ
H2/CO2 (gas permeation) H2/N2 (gas permeation) p-Xylene/o-xylene (vapor permeation) p-Xylene/o-xylene (vapor permeation) p-Xylene/o-xylene (vapor permeation) p-Xylene/o-xylene (pervaporation) p-Xylene/o-xylene (vapor permeation) p-Xylene/o-xylene (pervaporation)
2.5–4.5 1–34 2–40 10 35–278 7–40 18–73 1–1.2
[4] [4] [11] [11] [29] [12] [30] [12]
growth growth growth growth growth (defect sealing) growth (template free)
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Table 2 Secondary growth conditions for various MFI membrane microstructures Orientation
c-Oriented h,0,hOriented Random
Secondary growth conditions
Average thickness (lm)
Temperature (C)
Duration (h)
Number of growths
175 130
24 24
2 2
30 11
175
4
1
15
calcined alumina powder (Alcoa, A-16). Supports were prepared in the same manner as reported in a previous publication [13]. The supports were dip-coated in 1– 2 wt% silicalite suspensions synthesized from a solution of molar composition of 10 SiO2:2.4 TPAOH:1 NaOH:110 H2O that had been treated hydrothermally at 125 C for 8 h. Following dip-coating, the supports were dried for two days at 40 C. Procedures were repeated three times each to ensure adequate coverage with seed layer [17]. Preparation of the secondary growth solution followed the procedures of Xomeritakis et al. [18] and consisted of dissolving potassium hydroxide (KOH, 85% Sigma– Aldrich) into solution of de-ionized water and ethanol. The mixture was heated to 80 C and tetraethylorthosilicate (TEOS, 98% Sigma–Aldrich) was added. The solution was stirred until clear; the final molar composition was 1 KOH:1 TPABr:4.5 SiO2:16 C2H5O:1000 H2O. The clear solution and seeded supports were added to Teflon-lined stainless steel autoclaves for hydrothermal syntheses. Membranes were held vertically with a Teflon holder. The autoclaves were placed in an oven at the specified temperatures for the duration of the synthesis to obtain the desired microstructure. Table 2 outlines the secondary growth conditions for each membrane orientation type. Upon completion of the synthesis, the autoclave was cooled to room temperature by quenching; the membranes were removed from the autoclave and washed with distilled water. After drying for two days the membranes were calcined at 525 C for 5 h at a heating/cooling rate of 0.3 C min 1. Characterization of membrane microstructure was evaluated by X-ray diffraction (XRD) (Bruker AXS-D8, Cu Ka radiation). XRD measurements were performed by step-wise scanning (2h step-size: 0.015, 5 < 2h < 45). Thickness and morphology were examined by scanning electron microscopy (SEM) (Philips, XL 30). 2.2. Permeation and pervaporation experiments Single gas permeation measurements were taken using a transient gas permeation apparatus as described elsewhere [17]. The membrane was sealed in a stainless steel cell with the top layer facing upstream and placed within an oven to vary the temperature. The upstream pressure was controlled through use of a needle valve while the downstream side was maintained at atmospheric pressure. The gas feed
flow rate was controlled with mass flow rate controllers and the permeate flow rate was measured with a bubble flow meter. Heating/cooling rates were maintained at 1 C min 1 due to the relatively low temperature range (25–150 C). Pervaporation experiments were conducted with pure component p-xylene (PX, 99% Aldrich) and o-xylene (OX, 99% Aldrich) at temperatures from 25 C to 100 C. The membrane was sealed in the vertical stainless steel cell (top layer upwards) wrapped in heating tape. The liquid feed was maintained at atmospheric pressure and contained in the steel reservoir above while vacuum was applied to the downstream side. Permeate vapors were caught in a liquid nitrogen cold trap and measurements were taken by weighing the trap before and after each run. Ideal selectivity was determined by taking the ratio of the pure component fluxes. 3. Results and discussion 3.1. Membrane microstructure and single gas permeation For the synthesis of the each different microstructure of the MFI membrane it was desired to maintain as similar growth conditions as possible. Therefore, the seed layer preparation/deposition, secondary growth solutions, and membrane placement were kept the same in all syntheses. Different microstructures were obtained by varying only the synthesis duration and temperature as shown in Table 2. It is known that the development of preferential orientation occurs due to the competitive growth during long durations of growth [3]. The random oriented membrane was synthesized by reducing the synthesis time from two 24 h growths to one 4 h growth at 175 C. In this manner the grains did not have time to orient themselves [6] but similar conditions of initial seed layer, growth solution composition, and synthesis temperature (for random and c-oriented membranes) could be maintained. To synthesize the h,0,h-oriented membranes the synthesis temperature was reduced to 130 C while maintaining the same seed layer conditions as well as secondary growth duration and solution. Each membrane orientation was confirmed by XRD analysis. Fig. 1 shows the XRD spectra obtained. The typical dominant peaks are present in the XRD spectra for the oriented membranes. For the c-orientation the characteristic (0 0 2) peak is observed and the characteristic (1 0 1) peak for the h,0,h-oriented membrane can be seen. As has been reported previously the various orientation yield differences in surface characteristics [11]. SEM images of the surfaces of each type of membrane are also shown in Fig. 2. The random membrane appears to exhibit an entirely different morphology yielding the roughest of the three surfaces. The c-oriented membrane shows a higher degree of intergrowth than the random but the h,0, h-oriented membrane is the most smooth. These results
J. O’Brien-Abraham et al. / Microporous and Mesoporous Materials 105 (2007) 140–148 (a)
(002)
(101)
(200)
*
(501)
*
Relative Intensity (-)
143
*
*
(002)
(b)
(101)
(c)
5
10
15
20
25
30
35
40
45
2-theta (º)
Fig. 1. XRD spectra of (a) random MFI membranes, (b) c-oriented MFI membranes and (c) h,0,h-oriented membranes (* indicates a-Al2O3 peaks).
Fig. 2. SEM images of the surface (a) and cross-section (b) of c-oriented, surface (c) and cross-section (d) of h,0,h-oriented, and surface (e) and crosssection (f) random MFI membranes.
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-9
Permeance (10 mol.m-2.s-1.Pa-1)
1000
H2 He CO N2 CO 2 SF 6
100
10 2
2.5
3
3.5
1000/T (K-1)
Fig. 3. Single component gas permeation of H2, He, CO, N2, CO2, and SF6 results for random MFI membrane for temperatures ranging from 25 C to 150 C.
H2 He
100
CO N2 CO 2
-9
Permeance (10 mol.m-2.s-1.Pa-1)
1000
SF 6
10
1 2
2.5
3
3.5
1000/T (K -1)
Fig. 4. Single component gas permeation of H2, He, CO, N2, CO2, and SF6 results for c-oriented MFI membrane for temperatures ranging from 25 C to 150 C.
1000 H2 He CO N2 CO 2
100
-9
are in agreement with past work conducted by Lovallo and co-workers on membranes with these two preferred orientations [11]. Mass transport of single component gas molecules through zeolite membranes are controlled mainly by the diffusivity of the gas molecule through the pore and the adsorption affinity between the gas and the zeolite framework [19]. For high quality MFI-zeolite membranes either Knudsen or configurational diffusion is representative of the flow. According to Xiao and Wei [20], the type of diffusion exhibited by a particular molecule in within the MFI-type zeolite framework is determined by the kinetic diameter of the molecule (dm) as well as the diameter of the pore channel (dc). They define a parameter k as the ratio of dm to dc. The transition from Knudsen to configurational diffusion occurs in MFI-type zeolite at 0.6 < k < 0.8 at 300 K; at higher temperatures (500– 700 K) the transition occurs at k P 0.8. Small molecules such as H2 (dm = 0.289 nm) and He (dm = 0.260 nm) can be expected to diffuse in a Knudsen-like manner whereas larger molecules will exhibit configurational behavior in the MFI framework [20]. Single component gas (H2, He, CO, N2, CO2 and SF6) measurements were taken on c-oriented, h,0,h-oriented, and random membranes. Due to the fact that these measurements are conducted at relatively low temperatures, it is expected that the larger but more strongly adsorbing components (i.e. CO2, SF6) will exhibit higher permeances and that Knudsen diffusion for each of the molecules should be observed [18]. Figs. 3–5 show the gas permeation results for the various membranes as a function of temperature. For the random membrane (Fig. 3) each of the gases exhibits Knudsen-like temperature dependency (T 0.5), with the exception of SF6 which shows activated permeation. Similar behavior is exhibited by the h,0,h-oriented membrane but with lower permeances overall. Bonilla et al. [21] reported the presence of straight and open grain boundaries that run parallel to the c-axis in
Permeance (10 mol.m-2.s-1.Pa-1)
144
SF 6
10
1 2
2.5
3
3.5
1000/T (K -1)
Fig. 5. Single component gas permeation of H2, He, CO, N2, CO2, and SF6 results for h,0,h-oriented MFI membrane for temperatures ranging from 25 C to 150 C.
the c-oriented membranes identified by fluorescent confocal optical microscopy. The fastest diffusion pathways through the MFI framework exist through the a- and b-axis, but because (in the case of c-oriented membranes) the c-axis is aligned perpendicular to the trans-membrane direction molecules are forced to take the more tortuous route [22]. In a study on the modeling of permeation through c-oriented membranes Nelson et al. [22] discusses the fact that the presence of these intrinsic intercrystalline pores along the grain boundaries presents a ‘‘short cut’’ for molecular diffusion. They also attribute the relatively high fluxes in these membranes to an effective thickness that is much smaller than the actual thickness of the membrane. In a pervaporation study performed by our group [16] it was found that despite a significant thickness difference between the template-free random membrane (3 lm) and the c-oriented membrane (20 lm), the two membranes exhibited comparable fluxes of xylenes. However, the observed ideal selectivity for the random membrane
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in terms of PX over OX was an order of magnitude higher than the c-oriented membrane, indicating that while permeances are comparable the gaps do not aid in selectivity. In this work we see the effect that these grain boundaries also have on the permeability of light gases. For both the c-oriented (Fig. 4) and h,0,h-oriented (Fig. 5) membranes at room temperature it can be observed that He, CO, N2, and CO2 permeate at the same rate while H2 permeates faster. It appears that the diffusion pathways present through the oriented membranes tend to enhance the permeation of the relatively larger molecules. However there is a departure in the behavior of the molecules through the c-oriented membranes as the temperature increases. We begin to see the start of activated permeation of He and H2 at 150 C as well as a sharper decline in the permeance of CO2. 3.2. Microstructure and template removal effects Transport through MFI zeolite membranes occurs through three pathways: zeolitic micropores (0.55 nm), intercrystalline microporous defect (<2 nm), and larger meso- and macroporous defects (>2 nm). The molecular sieving capability of the MFI membrane is dependent on the relative flux through the zeolitic pores and non-selective microporous and larger defects [19]. As such, the H2/SF6 ideal selectivity is a qualitative indication of the ratio flow through zeolitic to non-zeolitic pores. Being that H2 is such a small molecule it can be expected to travel through all openings with little difficulty. However, the kinetic diameter of SF6 (0.6 nm) is approximately the same size as the pore opening which reduces its mobility within the MFI zeolite framework. While not excluded entirely from zeolite pores [23], the majority of its passage is expected to go through intercrystalline gaps, grain boundaries, and other small defects. For the random and h,0,h-oriented membranes the H2/SF6 selectivity is in the range of 20–24 and the c-oriented membrane 46. All three microstructures exhibit H2/SF6 ratios much greater than Knudsen (8.54) which indicates a dominant role of molecular sieving effect despite the presence of defects. The differences in selectivity can be explained in terms of the combined effect of the individual microstructures and the template removal step. Each membrane type was synthesized via seeded growth method involving a templated secondary growth solution. As a consequence, it is necessary to calcine the membranes (500 C) in order to remove the TPA cation from the occluded zeolite pores. Prior to the template removal step all three membrane types had room temperature He permeance on the order of 10 10–5 · 10 9 mol m 2 s 1 Pa 1 indicating that the membranes were free of intercrystalline gaps at this stage. Dong et al. [24] found that for random oriented MFI zeolite membranes synthesized in situ and supported on a-Al2O3 the removal of the template caused formation and enlargement of the intercrystalline gaps due to the combined effect of shrinkage of the crystallites and thermal
Fig. 6. SEM image of the type of crack formation observed in h,0, h-oriented membranes as a result of calcination.
expansion of the support. This is in agreement with the results found in this work for the random membranes synthesized with seeded growth method as well. Gues and van Bekkum [25] conducted a study on the thermal behavior of silicalite crystals upon calcination and found that while there was an overall contraction of the framework by 0.5–0.9%, the a- and c-axes experienced contraction but the b-axis experienced expansion. In comparing the microstructures in this work we find that the c-oriented membrane is the least affected by the calcination procedure as indicated by the higher H2/SF6 ideal selectivity. Within this particular orientation the a- and b-axes are aligned parallel to the support surface. It is speculated that the expansion and compression forces are held nearly in balance in the transmembrane direction limiting the formation/expansion of the intercrystalline gaps. This was not the case for the h,0,h-orientation whose microstructure made it very susceptible to crack formation as shown in the example in Fig. 6. These results are in agreement with those of Tsapatsis and co-workers [18]. They explained that the different distribution of stresses within the h,0,h-oriented membranes affect the crack formation exhibited. The membrane imaged in this figure had no further tests performed on it due to the extensive crack formation observed. It is suspected that while these defects do occur in all h,0,h-oriented membranes, not all of them had the same large number of such defects. 3.3. Single component xylene pervaporation results Separation of xylene isomers by MFI zeolite membranes occurs via molecular sieving and shape selectivity. PX (dm = 0.585 nm) is the only isomer small enough to pass through the zeolitic pores. The bulkier OX and MX (dm = 0.68 nm) are limited to transport through microporous defect and intercrystalline gaps. As such, the ability
J. O’Brien-Abraham et al. / Microporous and Mesoporous Materials 105 (2007) 140–148 1.4
random
Flux (kg.m-2.hr-1)
1.2 1 0.8 0.6
c-oriented
0.4 h,0,h-oriented
0.2 0 0
20
40
60
80
100
120
Temperature (ºC)
Fig. 7. Pervaporation flux of PX as a function of temperature for MFI membranes of varying microstructure.
in random and c-oriented membranes occur at 75 C. Studies on the adsorption/diffusion behavior of PX through silicalite crystals show that at temperatures above 50 C the adsorption equilibrium loading and the surface coverage decreases [26]. However, even at moderate loadings PX (i.e. 4 molecules per unit cell) diffusivity can be greatly reduced due to tight fitting of the sorbate and strong sorbate–sorbate interactions [27]. The increase in flux from 25 C to 75 C is likely due to an increase in the equilibrium loading in both the zeolitic pores and microporous defects. At 100 C, however, the contribution to the flux from adsorption is reduced and due to high loadings the diffusivity through the MFI pores is hindered. 3.4. Membrane degradation Upon exposure to xylenes, discoloration was observed on the surface of the MFI membranes. After heat treatment at 350 C for 10 h [13], at a heating/cooling rate of 0.3 C min 1, the oriented membranes were no longer selective for xylene isomers. It is well known that the loading of PX into MFI type zeolite induces structural changes 4.5 4 3.5 3 2.5
-7
of a membrane to separate PX from OX or MX is an indication of the relative amount of non-zeolitic porosity and membrane quality. Table 3 shows the single component pervaporation results for PX and OX at 25 C for each membrane type. Multiple runs of each type are presented to show reproducibility of results. For the random membranes we see that there is only minimal selectivity for PX over OX and fluxes that are double that of the oriented membranes. Such high fluxes can only be explained in terms of flow through defects which correlates well with the lack of selectivity. The results are in agreement with those of Wegner et al. [13] who conducted pervaporation of xylene isomers through random oriented MFI zeolite membranes synthesized via in situ hydrothermal synthesis. They found the membranes to be non-selective for PX over OX in both binary and single component studies. This was attributed to the defects formed during the calcination process. We present a similar argument here. For the c-oriented membranes the selectivity obtained was around 3.5; h,0,h-oriented membranes showed selectivity around 2.5. The fluxes through these membranes are approximately the same as one another and about half the flux through the random membranes. Again we see that despite undergoing the same calcination procedure, the oriented membranes sustain less damage in terms of intercrystalline gap formation as evidenced by the observed selectivity. No other group has reported on xylene pervaporation results through oriented membranes. However, significant study of vapor permeation through oriented membranes has been conducted. C-oriented membranes were found by Xomeritakis et al. [11] to exhibit PX/OX selectivity of 9–18 at partial pressures of PX less than 0.06 kPa; above this pressure there was a significant reduction in performance. Membranes with the h,0,h – orientation were found to have PX/OX selectivity around 12 but when ‘‘repaired’’ with a surfactant-templated silica sol, which filled cracks and defects, separation factors of around 300 were reported with a binary feed (0.45 kPa P, 0.35 kPa O) at 175 C [11]. Fig. 7 shows the affect of temperature on the flux of PX for temperatures ranging from 25 C to 100 C. Random and c-oriented membranes show the strongest temperature dependency however, similar behavior is observed in all three membrane types. The highest flux observed for PX
Permeance (10 mol.m-2.s-1.Pa-1)
146
Table 3 Pervaporation results of single component PX and OX performed at 25 C Orientation Random Random c-Oriented c-Oriented h,0,h-Oriented h,0,h-Oriented
PX flux (10 2 kg m 88.0 71.0 22.9 35.2 24.0 32.6
2
h 1)
OX flux (10 2 kg m 57.0 39.0 6.3 9.9 9.9 11.5
2
h 1)
Ideal PX/OX selectivity (–) 1.5 1.8 3.6 3.6 2.4 2.8
2 1.5 1 0.5 0
N2
H2
SF 6
Fig. 8. Single gas permeation results of respective gas for fresh membranes (line-shaded) and heat-treated (line-unshaded) membranes after xylene exposure for c-oriented MFI membranes.
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147
Post-heat treatment *
Relative Intensity (-)
*
*
Pre-heat treatment *
*
Fresh c-oriented membrane *
* 10
15
*
20
25
30
* 35
40
2-theta (º)
Fig. 9. XRD spectra for various stages of experimentation with c-oriented membrane. Analysis conducted on fresh (bottom) membrane, after exposure to xylene through pervaporation experimentation but before heat treatment (middle) and after heat treatment to remove adsorbed xylenes (* indicates a-Al2O3 peaks).
that deform the pore openings of the framework in order to incorporate an even higher loading [28]. The saturation capacity of PX in the MFI framework is reported to be approximately 7.8 molecules per unit cell. Upon loading of silicalite the first transition occurs at approximately 4 molecules per unit cell and consists of a monoclinic M to orthrombic O1 transition. In this phase the PX molecules are situated in the zig–zag and straight channel intersections. As the loading increases to saturation, a second phase transition occurs to orthrombic O2 which is accompanied by the packing of PX within the sinusoidal channels as well as the channel intersections [28,31]. This ‘‘high loading’’ condition causes displacement of framework atoms which increase the maximal sinusoidal pore dimension from 0.55 and 0.56 nm to 0.59 and 0.58 nm, while leaving the straight channel dimensions relatively unchanged [31,32]. When compared to permeation, pervaporation feed-side coverages can be much higher which may correlate to higher loadings of the membrane. As a result, during heat treatment it is possible that the adsorbed xylenes distorting the framework actually cause permanent damage upon burn-out during heat treatment. Fig. 8 demonstrates this effect with single component permeances through a c-oriented fresh membrane and permeances through the membrane after it has been subjected to xylene pervaporation and heat treatment. Similar observations were seen in membranes of all three microstructures (not shown). From these results we can see there is significant increase in the H2 and N2 permeance and only a slight increase in the SF6 permeance indicating that the damage was more significant within the zeolitic pores rather than within the defects. Additionally, Fig. 9 shows the XRD spectra for the progression of a c-oriented membrane during experimentation. Analysis was conducted on the fresh membrane, on the membrane after exposure to xylenes but
before heat treatment, and on the membrane after heat treatment. From these results we can see evidence of the peak shift and broadening of the (0 0 2) peak which indicates structural damage has occurred. 4. Conclusions Differences in transport properties through random and oriented (c- and h,0,h-) MFI membranes were demonstrated by single component gas permeation and single component xylene separation. Synthesis and experimental conditions were designed to be as similar as possible in order to determine the role of microstructure on membrane performance. It was found that moderate ideal selectivities for H2/SF6 around 20 were observed for those membranes most affected by defects induced during the template removal step including both the random and h,0,h-oriented membranes. In contrast, the c-oriented membrane was less susceptible to crack formation and showed H2/SF6 selectivity around 46. Single component pervaporation experiments yielded no selectivity for PX/OX for random membranes while a selectivity of 2–3 was observed for the oriented membranes. This work shows that microstructure affects not only membrane quality but subsequent performance. Acknowledgment The authors would like to thank the Department of Energy for their support (DE-PS36-03GO93007). References [1] S.M. Auderbach, K.A. Carredo, P.K. Dutta (Eds.), Handbook of Zeolite Science and Technology, Marcel Dekker, New York, 2003. [2] M. Pan, Y.S. Lin, Micropor. Mesopor. Mater. 43 (2001) 319.
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