On the performance of c-oriented MFI zeolite Membranes treated by rapid thermal processing

Journal of Membrane Science 436 (2013) 79–89

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On the performance of c-oriented MFI zeolite Membranes treated by rapid thermal processing Taehee Lee a, Jungkyu Choi a,n, Michael Tsapatsis b,nn a b

Department of Chemical & Biological Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713 Republic of Korea Department of Chemical Engineering and Materials Science, University of Minnesota-Twin Cities, 421 Washington Avenue SE, Minneapolis, MN 55455 USA

a r t i c l e i n f o

abstract

Article history: Received 21 November 2012 Received in revised form 29 January 2013 Accepted 2 February 2013 Available online 20 February 2013

Rapid thermal processing (RTP) was shown to be effective in reducing defects, especially grain boundary defects in c-oriented MFI membranes (Choi et al., Science 325 (2009) 590-593). In this study, the xylene and butane separation performances of RTP-treated c-oriented MFI membranes, which were now synthesized with a shorter secondary growth time (from 2 d to 1 d), were investigated. These membranes achieved a good performance for separating both p-/o-xylene (  88 separation factor (SF)) and n-/i-butane (  30 SF) isomers. In addition, fluorescence confocal optical microscopy was conducted to explore the defect structure by varying the contact time with the dye solution. This characterization revealed that the accessibility of dye molecules to grain boundary defects at the molecular level can be correlated with separation performance. Finally, the fluxes of xylene and butane isomers across RTPtreated c-oriented MFI membranes were deciphered by the Maxwell-Stefan formulation to obtain their diffusion coefficients. The diffusivities of the slow permeating component (i.e., o-xylene) were estimated to be lower than the literature values obtained by a zero length column method; this accounted for the high p-/o-xylene SF. We considered the combination of adsorbate-adsorbate and adsorbate-zeolite interactions to calculate the diffusivities of n-/i-butane isomers. & 2013 Elsevier B.V. All rights reserved.

Keywords: MFI zeolite membranes Rapid thermal processing Xylene Butane Maxwell-Stefan diffusivities

1. Introduction In the era of high-cost petroleum, zeolite membrane technology, developed in an attempt to replace or at least supplement thermally driven energy-intensive separation units (distillation, crystallization, etc.), has become increasingly important. In fact, zeolite membranes have been demonstrated to be effective in separating industrially important mixtures such as H2/CO2 [1,2], CO2/CH4 [3,4], and p-/o-xylene [5,6]. Among them, MFI type zeolites are known for shape-selectivity, for example, favoring the production of p-xylene over the other isomers [7]. Mirth et al. reported that the ratio of the diffusion coefficients of p-, o-, and m-xylene isomers inside MFI zeolite was 1000:10:1 [8]. The intrinsic molecular sieving behavior has led researchers to adopt the MFI zeolite as a membrane material, especially for the separation of commercially demanding p-xylene from the other isomers as an alternative to cost-intensive separation processes. The xylene separation performance of MFI membranes has been reported by many researchers [5,9–13]. Among the available microstructures, 15–30 mm thick columnar c-oriented MFI membranes n

Corresponding author. Tel.: þ82 2 3290 4854; fax: þ82 2 926 6102. Corresponding author. Tel.: þ 1 612 626 0920; fax: þ 1 612 626 7246. E-mail addresses: [email protected] (J. Choi), [email protected] (M. Tsapatsis). nn

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.02.028

are attractive for large scale industrial use, due to their simple and highly reproducible synthesis. However, although permselectivity (or ideal selectivity) of p-xylene over o-xylene through these c-oriented MFI membranes is high (up to 100), the corresponding separation factor (SF) for the binary mixture is lowered significantly to  2-4 [14]. Poor performance has been attributed to the presence of extrazeolitic paths (i.e., defects) in the membranes [5,14] and/or the defect opening possibly due to the local distortion of the MFI microstructure upon the adsorption of p-xylene [15,16]. These defects are very often non-selective to all permeating species in mixtures, thereby deteriorating the separation performance. The defect structures in c-oriented MFI zeolite membranes were visually characterized via fluorescence confocal optical microscopy (FCOM) [17,18], which was based on the use of an appropriate dye molecule that is accessible to extra-zeolitic parts, but not to zeolitic parts [19]. Defects that seemingly penetrated along the film thickness of the c-oriented MFI membrane and reached the interface with the a-Al2O3 disc were associated with the poor performance for separating p-xylene from o- xylene [17,18]. A quantitative information of defect structures in c-oriented MFI membranes with the FCOM methodology, as recently demonstrated for NaX membranes [20], will provide a more fundamental correlation between separation performance and defect characteristics. In general, the mismatch of thermal expansion behaviors between MFI membranes and porous supports during calcination is believed to result in the formation of defects [21,22]. Instead of

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conventional calcinations, alternative methods to remove organic structure directing agents (SDAs) have been proposed [23,24]. However, zeolite membranes’ attractive feature of high thermal resistance makes the alternative approaches less desirable. In addition, post-treatment methods have been developed by selectively blocking extra-zeolitic parts through silica deposition [14], coke treatment [25], and so on. Additional treatments are undesirable as well because of the added post-treatment cost and flux reduction. It was demonstrated that the role of grain boundary defects in c-oriented MFI membranes could be reduced by rapid thermal processing (RTP) [26]. Accordingly, RTP-treated c-oriented MFI membranes showed marked separation performance for p-/oxylene separations. The secondary growth of a randomly oriented seed layer, comprised of 100 nm globular MFI seeds, was conducted at 175 1C for 2 d, resulting in highly c-oriented MFI membranes. One time hydrothermal growth of 2 d instead of the two consecutive growths of 1 d as reported before [15,27] was chosen to avoid the complicated processes involved in the additional hydrothermal growth. Provided that only one time RTP treatment is sufficient to fully activate as-synthesized zeolite membranes, the adoption of the RTP technique in place of conventional slow calcination will be beneficial in providing the faster processing (more than  100 fold) and saving the corresponding demanding energy (more than  10 fold) for large scale production. In this study, we further characterized the xylene and butane separation performance of RTP-treated c-oriented MFI membranes, which were also fabricated with a shorter secondary growth time (from 2 d to 1 d) than that in the previous study [26]. The shorter synthesis time was adopted in an attempt to see an effect, if any, of the film thickness of c-oriented MFI membranes on overall separation performance. RTP-treated c-oriented MFI membranes exhibited an improved performance for separating both p-/o-xylene and n-/i-butane isomers, which is in good agreement with the previous report [26]. In addition, FCOM was employed to characterize defects in RTP-treated c-oriented MFI membranes after gradual exposure to the dye solution for up to 15 d. Under an assumption of negligible transport through nonzeolitic parts, the fluxes of xylene and butane isomers were analyzed by the Maxwell-Stefan (M-S) formulation. This analysis provided diffusion information of permeating p-/o-xylene and n-/i-butane binary components.

2. Experimental 2.1. c-Oriented MFI film fabrication Home-made a-Al2O3 discs (diameter: 22 mm; pore size: 170 nm; and membrane area: 3.8  10  4 m2) were used as supports in this study. Prior to seed deposition, one side of each disc was polished and was subsequently dip-coated with silica. The silica-coated a-Al2O3 discs were calcined at 480 1C for 4 h, with a heating ramp rate of 1 1C/min ramp under 150 mL/min air flow. They were then dried overnight in an oven preheated at 120 1C before seed deposition. Globular MFI crystals (  100 nm in diameter) were synthesized as per the method in the literature [28]. They were calcined in the presence of a polymer that played a diffusion barrier role to prevent crystals from being aggregated, based on a method reported by the Yan group [29]. The calcined globular MFI crystals were chemically attached to the silicacoated surface using a sonication-assisted deposition method [30]. This was followed by the hydrothermal growth of seeded substrates with a molar composition of 40 SiO2: 9 TPAOH: 9500 H2O (C4 composition) at 175 1C for 1 or 2 d to close the

gaps between deposited globular MFI seeds, thus forming a columnar, continuous MFI film. Although a seed layer was randomly oriented due to the globular shape of MFI seeds, preferentially c-out-of-plane oriented MFI films were achieved after secondary growth. The detailed procedures for c-oriented MFI film fabrication including a-Al2O3 disc preparation, silica synthesis and coating, MFI particle deposition, and secondary growth are described elsewhere [31]. 2.2. Rapid thermal processing (RTP) A rapid thermal processing (RTP) system, purchased from Research Inc., consisted of a temperature controller, an infrared heating chamber, and a water circulator (IR chamber: model E410 and Controller: model 915). RTP was performed by controlling ramp rates and soaking times at a certain target temperature similar to normal temperature programming, with the only difference being the fast heating rate up to several hundred degrees Centigrade within several minutes or seconds. For RTP, a MFI zeolite membrane was calcined by heating to nominally 700 1C within 1 min, holding that temperature for 30 s, and cooling by water circulation. This was followed by conventional slow calcination (480 1C for 10 h with a heating ramp rate of 0.5 1C/min under 150 mL/min air flow) to ensure the complete removal of SDAs. For convenience, this membrane is named RSx where R represents RTP, S stands for slow calcination (0.5 1C/min), and x denotes the synthesis time in days at 175 1C with C4 composition. Though two consecutive RTPs on  15-20 mm thick as-synthesized c-oriented MFI membranes were shown to remove all SDAs in the previous study [26], one time RTP was not sufficient for their full pore activation, and thus required additional slow calcinations. Additionally, as-synthesized MFI films were also calcined with different heating ramp rates in a box furnace (Thermolyne 48000): Sx and Fx represent slow and fast calcinations at 480 1C with heating ramp rates of 0.5 1C/min and 30 1C/min, respectively. c-Oriented MFI membranes, calcined in the above-mentioned three different ways, were already tested for xylene and butane separations [26] and are accordingly equivalent to membranes RS2, F2, and S2 in this study. For convenience, these membranes in the literature [26] are referred to as membranes RS2, F2, and S2. 2.3. Microstructural characterization Scanning electron microscopy (SEM) was performed using a JEOL 6500 microscope to visualize MFI zeolite powder and films. X-ray diffraction (XRD) patterns were obtained with a Bruker-AXS (Siemens) D5005, and pole figure analysis was conducted in a Bruker-AXS Microdiffractometer. In addition, fluorescence confocal optical microscopy (FCOM) images were obtained to envisage the microstructure of MFI films using a laser scanning confocal microscope (Olympus, FluoView FV1000 IX2 inverted confocal). The membrane side of each sample was contacted with a fluorescent molecular probe dye solution (1 mM fluorescein-Na in DI water solution: fluorescein-Na salt (F6377) was purchased from Sigma-Aldrich and its 1 mM solution was made in the lab) by using an ‘‘osmosis-type’’ home-made glass cell. The other side of the a-Al2O3 disc was contacted with DI water. Viton O-rings above and below an a-Al2O3 supported MFI membrane were used for sealing. During the contact time (up to 15 d), dye molecules were allowed to pass through the MFI membrane at room temperature in the quiescent condition. A schematic of the glass cell is shown in Scheme. S1 (supporting information). In FCOM images, bright spots represent the presence of dye molecules and thus designate extra-zeolitic areas (i.e., defects), while dark spots represent the absence of dye molecules and indicate

T. Lee et al. / Journal of Membrane Science 436 (2013) 79–89

81

well-intergrown zeolitic parts. A resolution limit in the FCOM characterization was estimated to be as small as  1-10 nm [20]. Xylene and butane isomer binary mixtures were used to examine the separation performance of MFI films calcined with different heat processes. For permeation measurements, the Wicke-Kallenbach mode was adopted with the both feed and permeate sides maintained at 1 atm total pressure. The permeation data were recorded at each temperature after waiting at least 12 h for xylene and 3 h for butane isomer feeds. The feed pressures of binary mixtures were 0.5 kPa/0.5 kPa for p-/o-xylene, and 50 kPa/50 kPa for n-/i-butane. To meet the desired partial pressures of the binary mixture,  38 and 51 mL  min-1 helium flow rates were fed to two gas bubblers that contained p- and oxylene, respectively, and  50 mL  min-1 flow rates were used for both n- and i-butanes. For sweeping, 100 mL  min-1 helium flow rates were used in both cases. The detailed permeation set-up is described elsewhere [31].

3. Theory

Scheme 1. Schematic of a c-oriented MFI membrane fabricated on an a-Al2O3 disc.

The molar flux of binary mixtures that pass through zeolite membranes at steady states can be expressed by [32,33] 3 " # " sat #" #2 y1 ŒŒ122 1 þ y1 ŒŒ212 q1 0 N1 Œ1 0 4 5  ¼r 0 qsat N2 0 Œ2 y2 ŒŒ211 1þ y2 ŒŒ121 2

and for the strong confinement scenarios, they were decreased linearly with the fractional coverage of the binary mixture:

1 1 þ y1 ŒŒ212

2y

1

4 P1 þ y2 ŒŒ121 0

0 y2

P2

32 54

dP 1 dz dP 2 dz

3 5

ð1Þ

where Ni is the molar flux of component i, r is the density of zeolite (1.79 kg  m-3 for silicalite-1), qsat is the saturated capacity i of component i, Œi is the Maxwell-Stefan (M-S) diffusivity of component i, Œij ¼ Œji is the M-S i-j pair diffusivity, yi is the fractional coverage of component i, and P i is the partial pressure of component i. In Eq. (1), partial pressure is assumed to be equal to fugacity, which is satisfied at moderate pressure. The detailed information of Maxwell-Stefan analysis to elucidate the interaction between guest molecules that move along micro-pores inside zeolites and between guest molecules and zeolite frameworks is well described elsewhere [34,35]. The boundary conditions and related information are shown in Scheme 1. Given the fractional coverage of the binary mixture, three unknowns remain in Eq. (1): Œ1 , Œ2 , and Œ12 ð ¼ Œ21 Þ. The M-S i-j pair diffusivities are symmetric, and for the binary mixture case, it can be approximated by q1

q2

Œ12 ¼ Œ11 q1 þ q2 :Œ22 q1 þ q2

ð2Þ

where qi is the adsorbed loading or amount of component i [34]. Accordingly, this relation allows for the reduction of unknowns to two M-S main diffusivities (Œi ). The two unknowns can be acquired by equating the two solutions of Eq. (1) to the experimentally obtained flux. When the interaction between two adsorbed components is weak, the M-S i-j pair diffusivity becomes Œ12 ð ¼ Œ21 Þ-1:

ð3Þ

This physically represents the facile interexchange of the two components. The interaction between an adsorbate and a zeolite framework has been rigorously considered to determine the exact loading dependence of Œi [36,37]. In this study, two limiting cases for the loading dependence of Œi were used for facile calculation. For the weak confinement scenario, the M-S diffusivities of a binary mixture were independent of loading: Œi ¼ Œi ð0Þ

ð4Þ

Œi ¼ Œi ðyÞ ¼ Œi ð1y1 y2 Þ:

ð5Þ

For the weak confinement scenario, Eq. (1) remains same, and for the strong confinement scenario, the insertion of Eq. (5) into Eq. (1) results in the following formulation for molar fluxes: " # " sat #" # q1 0 N1 0 Œ10 ð1y1 y2 Þ  ¼r 0 qsat 0 Œ20 ð1y1 y2 Þ N2 2 2 3 Œ20 ð1y1 y2 Þ Œ20 ð1y1 y2 Þ y1 1þ y1 Œ21 Œ12 4 5 Œ10 ð1y1 y2 Þ y1 y2 Þ y2 1 þ y2 Œ10 ð1 Œ21 Œ12 2y

1

1

4 P1 y1 y2 Þ Œ10 ð1y1 y2 Þ 0 þ y 1 þ y1 Œ20 ð1 2 Œ Œ 21

12

0 y2

P2

32 54

dP 1 dz dP 2 dz

3 5:

ð6Þ

The molecular dynamic results of n- and i-butane isomers in MFI zeolites [36] showed that the main M-S diffusivities of both n-butane and i-butane in MFI were strongly dependent upon their fractional coverage. Although the M-S diffusivities of n-butane and i-butane could be more accurately described based on the Reed and Ehrlich model [36–38], the strong confinement scenario appeared to adequately account for the dependency of the fractional coverage on M-S main diffusivities for n-butane and i-butane in the temperature range of 373 to 463 K [36]. Specifically, the strong confinement scenario, as reflected by the linear dependence of the M-S main diffusivities on the fractional vacancy, held well for both n-butane and i-butane up to the feed pressure (50 kPa). For butane isomers, both strong and weak confinement scenarios were considered for comparison purpose. On the contrary, since the dependence of M-S diffusivities of xylene isomers on the fractional coverage has not been known, the simplest case, i.e., the weak confinement scenario was adopted in an attempt to approximate the order of magnitude in the M-S diffusivities of p- and o-xylene as previously reported by Daramola et al. [39]. The partial pressures of the binary mixture at the end of the aAl2O3 disc (i.e., P1,iii and P2,iii in Scheme 1) were estimated by considering the molar flow rates of the permeating components and the sweep gas (here, helium). In addition, for the boundary conditions at the interface of the zeolite membrane and a-Al2O3 disc (i.e., P1,ii and P2,ii in Scheme 1), the Knudsen diffusion

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mechanism was employed to describe the flux through the a-Al2O3 disc: 2e Ni ¼  r pore 3t

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 8 1 dP i pRT Mi dz

yi ¼ ð7Þ

where e is the porosity, t is the tortuosity, r pore is the pore radius, R is the gas constant (8.314 J  K  1 mol  1), T is the absolute temperature in Kelvin, and M i is the molecular weight of component i. In this study, r pore was 85 nm as estimated by mercury porosimetry. The ratio of the porosity to the tortuosity was chosen to be  0.2, which was obtained by equating the flux of permanent gases (He, H2, CO2, and N2) with Eq. (7). The porosity was estimated from mercury porosimetry and found to be 0.42. Therefore, the tortuosity in the a-Al2O3 disc was  2.1. The thickness of the a-Al2O3 disc was 2.0 mm, while the film thicknesses of membranes RS1 and RS2 were 13–15 and 18 mm, respectively. The boundary conditions at the feed side (i.e., P 1,i and P 2,i ) were 0.5 kPa/0.5 kPa for p-/o-xylene isomers, and 50 kPa/ 50 kPa for n-/i-butane isomers. To extract diffusion information of permeating components, the thermodynamic contribution, represented in Eq. (1) in terms of fractional coverage, should be independently obtained. It has been recognized that xylene adsorption isotherms in MFI zeolites at low temperatures are too complicated to be expressed analytically. It prevents the effective use of the M-S formulation especially when the ideal adsorbed solution theory (IAST) [40] is considered. In this study, we calculated M-S diffusion coefficients at 100 and 150 1C, while using the adsorption isotherms of p-xylene and o-xylene single components found in the literature [41–44]. For butane isomers, the single component adsorption isotherms reported by Zhu and co-workers [45] were adopted for predicting the adsorption isotherms of their binary mixture above 100 1C. All single component adsorptions isotherms were assumed to obey the single-site Langmuir adsorption isotherms. We employed both the extended Langmuir adsorption isotherm and IAST to predict the adsorption isotherm of the p-/o-xylene mixture, while the IAST was only used to estimate the adsorption isotherm of the n-/i-butane mixture. Once the adsorption isotherms of the binary mixture were known, their fractional coverage could be easily calculated at given partial pressures. The IAST [40] was useful to estimate the fractional coverage of the binary components based on their single component adsorption isotherms. In the IAST, the following four conditions should be satisfied: Z tð ¼ P1 =x1 Þ 0

q1 ðt Þdln t ¼

y1 ¼

x1 qt , qsat 1

y2 ¼

ð1x1 Þqt , and qsat 2

Z tð ¼ P2 =ð1x1 ÞÞ 0

respectively. The extended Langmuir isotherms (EL) were described by

q2 ðt Þdln t,

1 1 1 : þ  ¼ qt q1 ðP1 =x1 Þ q2 P 2 =ð1x2 Þ For the single-site Langmuir adsorption bi P i qi ¼ qsat i 1 þ bi P, Eqs. (8) and (11) can be expressed as     b1 P 1 b2 P2 and ¼ qsat qsat 1 ln 1 þ 2 ln 1 þ x1 ð1x1 Þ 1 x1 ðx1 þ b1 PÞ ð1x1 Þðð1x1 Þ þ b2 PÞ þ ¼ , qt qsat qsat 1 b1 P 2 b2 P 2

ð8Þ

ð9Þ

ð10Þ

ð11Þ isotherms,

ð12Þ

ð13Þ

qi bi Pi ¼ : 1þ b P1 þ b2 P 2 qsat 1 i

ð14Þ

Given M-S main diffusivities, the numerical solution of Eq. (1) with the thermodynamical contribution described by Eqs. (9), (10), (12), and (13) for the IAST or by Eq. (14) for the EL could provide the partial pressure (i.e., fugacity) profile and accordingly, fractional coverage profile of the p-/o-xylene or n-/i-butane binary mixture along the membrane thickness. In this study, the criterion of less than 1% difference between the experimental partial pressure at the interface (i.e., P1,ii and P 2,ii at zii in Scheme 1) and the numerically calculated partial pressure was adopted to determine M-S main diffusivities of the abovementioned binary mixture.

4. Results and discussion Fig. 1(a) and (b) show top and cross-sectional view SEM images of a RTP-treated and further slowly calcined MFI film (membrane RS1). The columnar morphology of membrane RS1 at the surface, typical in c-out-of-plane oriented MFI films [27,46,47], suggested the achievement of the c-out-of-plane orientation. The cross-sectional view SEM image revealed that the film thickness of membrane RS1 was  13 mm, which was comparable to previously reported values [14]. The c-out-of-plane orientation was further verified by the XRD pattern in Fig. 1(c) and by pole figure analysis in Fig. 1(d). In our previous study, the electron probe microanalysis (EPMA) showed that the chemical distribution of Si and Al in membrane RS1 was similar to that in a slowly calcined c-oriented MFI film (membrane S1) [26]. In summary, SEM, XRD, and pole figure analysis characterizations along with the EPMA result confirmed that the microstructure of membrane RS was virtually identical to that of slowly calcined counterparts. Membranes RS1 and S1 shared common microstructural characteristics: grain shape and size, out-of-plane orientation, film thickness, Al and Si chemical distribution, etc. Further, we investigated vapor and gas permeation behaviors of membranes RS and S. We also tested the separation performance of c-oriented MFI membranes calcined by fast ramp rates (membrane F). First, the permeation rates of p-xylene and o-xylene single components were measured for membranes S, F, and RS, as shown in Fig. S1 (supporting information). Despite distinct permeation behaviors of p-xylene and o-xylene, the resulting p-/o-xylene ideal selectivities were comparable among the three different membranes with the maximum separation factor (SF) of 30-50. Fig. 2 shows the permeation rates of the p-/o-xylene mixture passing through c-oriented MFI membranes. Membranes S1 and F1 showed a maximum p-/o-xylene SF of 3-4, which is close to the reported SF value [14]. Membrane RS1, however, provided a maximum of 88 p-/o-xylene SF at 125 1C. At this point, it is worth mentioning that despite the similar SF trends of membranes S1 and F1, the permeation behaviors were considerably different from each other. On the other hand, membrane RS1 showed the monotonic increase and decrease of p-xylene and o-xylene permeances up to 125 1C, respectively. This resulted in the corresponding SF of up to  88 at 125 1C. Furthermore, the p-/o-xylene SF for membrane RS1 even at 195 1C was as high as 20, while p-/o-xylene SFs of membranes S1 and F1 decreased to 1. It was noted that the p-/o-xylene SFs of membrane RS1 were almost comparable to or, at some temperatures, superior to that of b-oriented MFI membranes [5]. In general, membranes S1 and RS1 showed permeation behavior comparable to those of membranes S2 and RS2, respectively [26].

T. Lee et al. / Journal of Membrane Science 436 (2013) 79–89

83

5 μm

5 μm

MFI zeolite -Alumina

1.0

(002)

0.8 0.6 0.4 0.2

(102) (103)

(101)

*

0.0 5

10

15

1000

Membrane RS1

Intensity (a.u.)

Normalized Intensity

1.2

20

25

800

(002) Membrane RS1

600 400 200 0

30

2 θ (°)

0 10 20 30 40 50 60 70 80 90

Tilt Angle (°)

Fig. 1. Scanning electron microscopy (SEM) (a) top view and (b) cross-sectional view of membrane RS1, and (c) X-ray diffraction (XRD) pattern of membrane RS1 along with (d) its (002) pole figure line plot. The asterisk (*) in the XRD pattern in (c) marks the peak from an a-Al2O3 disc.

However, membrane F2 provided an improved performance with a p-/o-xylene SF as high as  20 [26], while membrane F1 showed a maximum SF of  3. Membranes RS1 and RS2 showed marked improvement in xylene separation performance (88 and  130 [26] p-/o-xylene maximum SF, respectively). As an intermediate condition between membranes RS and S, an as-synthesized c-oriented MFI membrane was heated by RTP to 500 1C within 1 min, held at that temperature for 30 s and 120 s, and then further slowly calcined. This soaking temperature was chosen to match conventional calcination temperatures (480-500 1C). A membrane, soaked for 120 s, showed a modestly increased p-/o-xylene SF ( 28) as shown in Fig. S2, which was still much higher than the maximum p-/o-xylene SF ( 3-4) of membrane S1. In addition, butane permeation rates of n-/i-butane isomers through membranes RS and F were measured along with membrane S as a reference for comparison. Fig. 2 also shows the butane separation performance of membranes S1, F1, and RS1. While membrane S1 exhibited a monotonic SF decrease (from 45 to 6) with temperature as in the previous study [47], membrane RS1 showed an almost constant SF (  24-30) at all temperatures up to 190 1C. On the other hand, membrane F1 had a SF as high as about 24 at 200 1C, but at temperatures higher than 200 1C, a lower SF could be expected from the monotonically decreasing trend. For all membranes, the permeance trend of the slower permeating component, i.e., i-butane, determined the overall SF. In particular, the unique permeation behavior of i-butane through membrane RS1 at high temperatures suggested a difference in access to the extra-zeolitic parts (i.e., defects) compared to membrane S1. In general, the permeation behaviors of membranes S1 and RS1 were similar to those of membranes S2 and RS2, respectively reported in the previous study [26]. A minor difference was that membrane F2 had a lower SF of  10 at the high temperature (200 1C), compared to  24 obtained through membrane F1. In Fig. 3, the xylene and butane separation performances of various MFI zeolite membranes [9–11,27,47–49] are summarized

along with the performance of membranes S, F, and RS. Only MFI type membranes that were tested for the separation of both p-/oxylene and n-/i-butane isomers were considered for Fig. 3. Compared to other MFI membranes, membrane RS showed good performances for separating both p-/o-xylene ( 88 and  130 [26] SFs for membrane RS1 and RS2, respectively) and n-/i-butane (  25 and 30 [26] SFs at 190 1C for membranes RS1 and RS2, respectively) isomers. It is worth mentioning that a c-oriented MFI membrane can serve as a molecular sieve with SF as high as  130, if defect formation is appropriately suppressed. The permeances of p-xylene in membrane RS (  15 mm thick) were lower than those in b-oriented MFI membranes (  1 mm thick), largely because of the higher film thickness. In particular, recent work [50,51] demonstrated that sub-micrometer thick, randomly oriented MFI membranes can separate p-/o-xylene isomers as comparably as b-oriented MFI membranes can. This suggested that given a defect-free thin film, the tortuous MFI channels in the random direction (e.g., a tortuosity of  3 with respect to the straight channel along the b-axis [52]) could serve as an effective molecular sieve. Although it was not pronounced, membranes F1 and F2 showed a modest trade-off between xylene (3 vs. 20 SF at  100 1C) and butane separations (24 vs. 10 SF at  200 1C). Given the subtle difference in the film thicknesses of membranes F1 and F2, this trade-off possibly reflects on the non-negligible discrepancy in their as-synthesized structures and even the same calcination process could not result in the identical final structure. The detailed information of MFI membranes used in Fig. 3 is summarized in Table.S1. In addition, fluorescence confocal optical microscopy (FCOM) was employed to elucidate the permeation results via improved understanding of the microstructure of membrane RS. In the previous study [26], FCOM results of membranes S2 and RS2 were reported after contacting with the dye solution for  2 d. In membrane S2, grain boundary defects, beyond the resolution of the SEM technique, were widespread throughout the surface and were propagated along the film thickness toward the a-Al2O3

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Fig. 2. (Top) the permeances of p-xylene (filled) and o-xylene (open), and the p-/o-xylene separation factor (semi-filled) vs. temperature for membranes S1 (left), F1 (middle), and RS1 (right). (Bottom) the permeances of n-butane (filled) and i-butane (open), and n-/i-butane separation factor (semi-filled) vs. temperature for membranes S1 (left), F1 (middle), and RS1 (right).

Fig. 3. (a) p-/o-Xylene separation factors vs. p-xylene permeance, and (b) n-/i-butane separation factors vs. n-butane permeance for membranes S, F, and RS. Previously reported data for permeances and the corresponding separation factors through MFI membranes are also included: b and c represent b-oriented and c-oriented MFI membranes, respectively while t stands for a thin MFI membrane.

disc. In contrast, membrane RS2 had no clear grain boundary defects, but cracks were still visible throughout the surface. In this study, the accessibility of dye molecules to defects was further investigated by increasing the contact time with the dye solution from 2 d to 7 or 15 d. Fig. 4 shows cross-sections and slices taken at the lines (horizontal white lines) of membranes RS2 after contact times of 7 d and 15 d. Grain boundary defects near the membrane surface, though hardly observed on membrane RS2 after 2 d contact time, became pronounced as the contact time was increased from 2 d to 7 d (Fig. 4(a) and (b)). Dye molecules were still not detected all the way down to the bottom of membrane RS2, as shown in Fig. 4(a). However, a longer exposure time (15 d) resulted in clearly visible grain boundary defects, indicating their presence along the film thickness

(Fig. 4(f)). The time study suggested no access or penetration of dye molecules to grain boundaries in membrane RS for up to 2 d exposure, but their full penetration to grain boundaries in membrane S after 2 d contact time. If time was extended, grain boundaries were reached by the dye eventually (after 15 d) in membrane RS, indicating that grain boundary transport pathways were still present (not eliminated completely). However, the accessibility of dye molecules (  1 nm vs.  0.6 nm of MFI zeolite pores) to grain boundaries was considerably reduced at least by 7.5 fold (15 d/2 d). The observation that membrane RS showed high performance for separating both xylene (maximum SF of  88-130) and butane (maximum SF of 25 at 190 1C) isomers, along with the FCOM observations supports the existence of a structural difference at the molecular level in the grain boundary

T. Lee et al. / Journal of Membrane Science 436 (2013) 79–89

20 μm (b) (c) (d) (e) Interface

85

20 μm (g) (h) (i) (j) Interface

Fig. 4. FCOM cross-sections of membranes RS2. The membrane sides of membranes RS2 were contacted with the dye solution for (a) 7 d and (f) 15 d. The corresponding slices were taken at approximately 2 mm (b, g), 6 mm (c, h), 10 mm (d, i), and 18 mm (e, j) below the surface, as indicated by horizontal lines in (a) and (f).

structure in membranes RS and S and underscores the importance of engineering defects as desired [32,34]. Finally, we extracted diffusion information of permeating species to understand their mass transport across MFI zeolite membranes, under the assumption of impermeable pathway through defects. For this purpose, we adopted the MaxwellStefan (M-S) formulation [34]. The calculated M-S diffusion coefficients of p-xylene and o-xylene are summarized in Table 1. They are plotted in Fig. 5(a) along with xylene diffusion coefficients reproduced from Ref. [52] and references therein, for comparison. Although the single component adsorption isotherms used in this study were extracted from different references [41–44], the calculated p-xylene diffusion coefficients for membranes RS and F were in good agreement with the values reported in the literature as shown in Fig. 5(a). In contrast,

o-xylene diffusion coefficients in membranes RS1 and RS2 were considerably lower than the reported data by more than one order of magnitude. This slow diffusion of o-xylene is responsible for the marked increase in the xylene SF of membranes RS (up to  88–130 SF) and F (  20 SF). Despite the possible structural change of MFI structure upon the adsorption of p- and o-xylene, the difference between the corresponding M-S diffusivities was more than one order of magnitude, indicating the realization of the intrinsic molecular sieving in favor of the p-xylene diffusion. In addition, the difference between the M-S diffusivities of p- and o-xylene was in a good agreement with the previous result [8]. Although models accounting for permeation through defects exist [53,54], their implementation requires the introduction of additional parameters, which are not justified by the current membrane microstructure. Therefore, the current assumption of the

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negligible permeation through defects can be regarded as reasonable. The partial pressure (i.e., fugacity) and fractional coverage of p- and o-xylene mixture, passing across membrane RS1 at 100 1C, are shown in Fig. S3. It appeared that the fractional coverage of o-xylene toward the interface between membrane RS1 and the a-Al2O3 disc decreased faster than that of p-xylene as expected from a favored molecular transport of p-xylene through MFI zeolite pore channels over o-xylene [8]. In addition to xylene diffusivities, the M-S diffusivities of the n- and i-butane mixture were calculated. Fig. 5(b) shows the calculated M-S diffusion coefficients of the n- and i-butane mixture as a function of inverse temperature. Calculated M-S diffusion coefficients are summarized in Table 2. Specifically, three combinations of adsorbate-adsorbate and adsorbatezeolite framework interactions are considered: (1) weak confinement scenario for main M-S diffusion coefficients with the facile exchange, (2) weak confinement scenario for main M-S diffusion coefficients with the finite exchange, and (3) strong confinement scenario for main M-S diffusion coefficients with the finite exchange. Physically, (i) case (1) could be interpreted as an absence of interactions among adsorbates and zeolites, (ii) case (2) corresponded to a scenario where there was no interaction between adsorbates and zeolites, but interactions existed between adsorbates, and (iii) case (3) corresponded to the presence of interactions among adsorbates and zeolites [32,36,55]. In the cases of (1) and (2), n-butane diffusion across membrane RS appeared to follow an activated process, and its activation energies were estimated to be  16–23 and  23–32 kJ/mol, respectively (Table 3). As shown in Fig. S4, the other membrane RS1 showed a similar result. Fig. S5(a) shows the n-butane Table 1 Maxwell-Stefan diffusion coefficients of p-xylene (p-X) and o-xylene (o-X) permeating through membranes RS1, RS2, and F2 shown in Fig. 5(a). Sample 100 1C

150 1C 13

RS1-1 RS1-2 RS2 F2 a b c

15

c

p-X  10 (m2  s  1)

o-X  10 (m2  s  1)

p-X  1013 (m2  s  1)

o-X  1015 (m2  s  1)

ELa

IASTb

EL

IAST

EL

IAST

EL

IAST

2.2 2.4 3.3 2.5

2.1 2.2 3.1 2.4

3.2 4.4 3.1 9.3

3.3 4.7 3.2 9.7

17 15 47 21

14 13 39 17

5.1 6.3 7.8 46

5.3 6.5 8.1 48

EL: Extended Langmuir Isotherm. IAST: Ideal Adsorbed Solution Theory. An adsorption isotherm of o-xylene at 160 1C is adopted from Ref. [41].

diffusivities reproduced from Ref. [56] and references therein along with the n-butane diffusivities obtained in this study. The reported activation energies for n-butane diffusion spanned a wide range of 4–45 kJ/mol; this indicates the significant discrepancies in the reported n-butane diffusivities. In contrast, the M-S diffusivities of i-butane were not activated and showed minimum values around 463 K (Fig. 5(b)). A similar result was observed in the other membrane RS1 (Fig. S4). Compared to cases (1) and (2), case (3) was more realistic to reflect the adsorbateadsorbate and adsorbate-zeolite framework interactions. In case (3), the estimated diffusivities of both n-butane and i-butane did not appear to obey an activated process. In particular, the activation energy for n-butane diffusion, if fitted, was estimated to be 5–8 kJ/mol, but was not creditable due to the much larger confidence intervals (Table 3). We noted that the M-S diffusivities of i-butane did not follow the activated process in all three cases. In a previous study, Paschek and Krishna estimated the M-S diffusivities of i-butane [57] by analyzing its molar flux through a MFI membrane that Millot and co-workers reported [58]. The dependence of i-butane diffusivities on temperature was similar to the results in all three cases, as shown in Fig. S5(b), with a pronounced deviation in case (3). Paschek and Krishna showed that the nearest neighbor repulsion for the molecular transport of i-butane accounted for the observed diffusivity data in general [57]. The partial pressure (i.e., fugacity), fractional coverage, and M-S main diffusivities of n-/i-butane mixture at 373 K are shown in Fig. S6. Obviously, the higher fractional coverage and larger diffusivity of n-butane favor the permeation of n-butane over that of i-butane.

5. Conclusions RTP-treated c-oriented MFI membranes, which were fabricated with a shorter secondary growth time (from 2 d to 1 d), showed a good performance for separating both xylene and butane isomers. Specifically, RTP-treated c-oriented MFI membranes (membrane RS1) showed a good performance for separating both p-/o-xylene and n-/i-butane isomers (  88 p-/o-xylene separation factor (SF) at 125 1C and 25 n-/i-butane SF at 190 1C). However, c-oriented MFI membranes calcined at a 30 1C/min ramp rate (membrane F1) did not show an improved SF (  3 p-/o-xylene SF). In addition, fluorescence confocal optical microscopy (FCOM) was performed to investigate defect structures in membrane RS2 by varying the contact time with the dye solution (from 2 d to 15 d). After the longer exposure time (15 d), the grain boundaries in membrane RS2 were eventually reached by dye molecules, revealing grain

Fig. 5. (a) Maxwell-Stefan (M-S) diffusion coefficients of p-xylene (p-X, filled) and o-xylene (o-X, semi-filled) in membranes F2, RS1, and RS2; (b) Maxwell-Stefan diffusion coefficients of n-butane (n-C4, filled) and i-butane (i-C4, open) in membrane RS1. Reported diffusion coefficients of p- and o-xylene isomers reproduced from Ref. [52]are included in (a) for comparison: ZLC, TZLC, Grav., and membrane represent the measurement methods of zero length column, tracer zero length column, gravimetric uptake, and membrane permeation, respectively. In (b), the fitted lines, whose slopes are proportional to the activation energies, are included for M-S diffusivities of n-butane in the cases of (1) and (2), while the other M-S diffusivities are connected to guide the eye.

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87

Table 2 Maxwell-Stefan diffusion coefficients of n-butane and i-butane permeating through membranes RS1 and RS2 shown in Fig. 5(b). Sample

Temp. (1C)

n-butane  1010 (m2  s  1)

i-butane  1010 (m2  s-1)

WC w/Œ12 -1a

WC w/Œ12 ¼ Œ12 ðyÞb

F w/Œ12 ¼ Œ12 ðyÞc

WC w/Œ12 -1

WC w/Œ12 ¼ Œ12 ðyÞ

F w/Œ12 ¼ Œ12 ðyÞ

RS1-1

100 130 160 190

0.65 1.2 1.9 2.7

0.67 1.5 3.2 4.7

5.5 5.5 7.2 7.9

0.18 0.11 0.10 0.12

0.16 0.09 0.08 0.10

1.3 0.33 0.18 0.16

RS1-2

100 130 160 190

0.86 1.6 2.6 3.6

0.90 2.0 4.2 6.3

8.0 7.7 9.8 11

0.22 0.14 0.12 0.14

0.20 0.11 0.10 0.11

1.7 0.44 0.23 0.19

RS2

100 125 175 200

1.1 2.0 2.6 3.7

1.2 2.5 4.5 6.4

10 9.4 10 11

0.25 0.15 0.11 0.14

0.22 0.13 0.09 0.11

1.9 0.47 0.20 0.18

a b c

WC w/ Œ12 -1: weak confinement scenario with the facile interexchange (i.e., Œi ¼ Œi ð0Þ and Œ12 ð ¼ Œ21 Þ-1). q1 q2 WC w/ Œ12 ¼ Œ12 ðyÞ: weak confinement scenario with the finite interexchange diffusion coefficients (i.e., Œi ¼ Œi ð0Þ and Œ12 ð ¼ Œ21 Þ ¼ Œ11 q1 þ q2 :Œ22q q1 þ q2 ). q 1 2 F w/ Œ12 ¼ Œ12 ðyÞ: strong confinement scenario with the finite interexchange diffusion coefficients (i.e., Œi ¼ Œi ð1y1 y2 Þ and Œ12 ð ¼ Œ21 Þ ¼ Œ11 q1 þ q2 :Œ22 q1 þ q2 ).

Table 3 Estimated activation energies for M-S diffusivities of n-butane in the cases of three adsorbate-adsorbate and adsorbate-zeolite interactions. Sample

RS1-1 RS1-2 RS2

defects should be made along with a steady endeavor for developing a simple, reliable synthesis protocol of MFI zeolite membranes with desired microstructures.

Activation energy for n-butane diffusion (kJ/mol)a WC w/Œ12 -1b

WC w/Œ12 ¼ Œ12 ðyÞc

F w/Œ12 ¼ Œ12 ðyÞd

237 4 237 4 167 11

32 79 32 78 23 711

67 8 57 8 57 52

a 95% confidence intervals are included next to the estimated activation energies. b WC w/ Œ12 -1: weak confinement scenario with the facile interexchange (i.e., Œi ¼ Œi ð0Þ and Œ12 ð ¼ Œ21 Þ-1). c WC w/ Œ12 ¼ Œ12 ðyÞ: weak confinement scenario withq the finite interexchange q2 1 diffusion coefficients (i.e., Œi ¼ Œi ð0Þ and Œ12 ð ¼ Œ21 Þ ¼ Œ11 q1 þ q2 :Œ22 q1 þ q2 ). d F w/ Œ12 ¼ Œ12 ðyÞ: strong confinement scenario with the finite interexchange q1 q2 diffusion coefficients (i.e., Œi ¼ Œi ð1y1 y2 Þ and Œ12 ð ¼ Œ21 Þ ¼ Œ11 q1 þ q2 :Œ22 q1 þ q2 ).

boundary accessibility with reduced (at least 7.5 fold) dye diffusivity. The improvement in the separation performance of membrane RS2 could be thus associated with the significantly reduced accessibility of dye molecules to grain boundaries as compared to the accessibility in the case of membrane S2. FCOM and permeation characterizations indicated the subtle but manifest difference in defect structures between membranes RS2 and S2 at the molecular level. Finally, the fluxes of xylene and butane isomers across membrane RS were interpreted by the MaxwellStefan formulation in order to acquire their diffusion coefficients. For xylene isomers, the diffusion coefficient of the larger o-xylene in membrane RS was estimated to be much lower than values reported in the literature [52]; this indicates its critical role in the resulting high p-/o-xylene separation performance. For butane isomers, three combinations of adsorbate-adsorbate and adsorbate-zeolite interactions were considered, and their corresponding Maxwell-Stefan diffusivities were calculated. The current findings point to the sensitivity of the microstructures of MFI membranes to overall separation performance. In particular, grain boundary defects and cracks at the molecular level along with other major microstructural characteristics (e.g., out-of-plane orientation, film thickness, grain shape and size, and Si/Al ratio) played critical roles in determining the overall separation performance. Therefore, an effort to avoid and remove

Acknowledgements This work was supported in part by the Initiative for Renewable Energy and Environment (IREE) and National Science Foundation (NSF, Grant NSF-NIRT CMMI 0707610), and also by the National Research Foundation (NRF) funded by the Korea government (MEST) (2012R1A1A1042450). Portions of this work were conducted at the University of Minnesota Characterization Facility, which receives partial support from the NSF through the NNIN program.

Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2013. 02.028.

Nomenclature Symbols bi fi Œi Œij Lzeolite LaAl2 O3 Mi Ni Pi r pore qi qsat i qt R

Single-site Langmuir adsorption constant (Pa  1) Fugacity of component i (Pa) Maxwell-Stefan (M-S) diffusivity of component i (m2  s  1) Maxwell-Stefan (M-S) i-j pair diffusivity (m2  s  1) Thickness of a zeolite membrane (m) Thickness of an a-Al2O3 disc (m) Molecular weight of component i (kg  mol  1) Molar flux of component i (mol  m  2  s  1) Partial pressure of component i (Pa) Pore radius (m) Loading of component i (mol  kg  1) Saturated capacity of component i (mol  kg  1) Total adsorbed amount of the binary mixture in the IAST (mol  kg  1) Gas constant (J  K  1  mol  1)

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Greek letters

e t r yi yV

Porosity Tortuosity Density (kg  m-3) Fractional coverage of component i Fractional vacancy

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List of Acronyms (1) Membrane RSx: a c-oriented MFI membrane was synthesized at 175 1C for x day with C4 composition. The as-synthesized membrane was heat-treated at

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700 1C with the nominal ramp rate of 700 1C/min (rapid thermal processing), and was further calcined at 480 1C with a ramp rate of 0.5 1C/min (slow calcination).; (2) Membrane Fx: a c-oriented MFI membrane was synthesized at 175 1C for x day with C4 composition, and the membrane was calcined at 480 1C with a ramp rate of 30 1C/min (fast calcination).; (3) Membrane Sx: a c-oriented MFI membrane was synthesized at 175 1C for x day with C4 composition, and the membrane was calcined at 480 1C with a ramp rate of 0.5 1C/min (slow calcination).; (4) C4: the molar composition of 40 SiO2: 9 Tetrapropylammonium hydroxide: 9500 H2O: 240 Ethanol.