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Journal of Membrane Science
journal homepage: www.elsevier.com/locate/memsci
Preparation of graded silicalite-1 substrates for all-zeolite membranes ^CrossMark with excellent C02/H2 separation performance
Farid Akhtara'b'*, Erik Sjöbergc, Danil Korelskiyc, Mark Raysond, Jonas Hedlundc, Lennart Bergström a,n
a Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden b Division of Materials Science, Luleä University of Technology, SE-97187 Luleä, Sweden c Chemical Technology, Luleä University of Technology, SE-97187 Luleä, Sweden d Department of Chemistry, The University of Surrey, Guildford, Surrey GU2 7XH, UK
ARTICLE INFO
Article history: Received 18 March 2015 Received in revised form 3 June 2015 Accepted 6 June 2015 Available online 23 June 2015
Keywords:
All-zeolite membranes Membrane separation Gas separation Cracking Thermal cycling
ABSTRACT
Graded silicalite-1 substrates with a high gas permeability and low surface roughness have been produced by pulsed current processing of a thin coating of a submicron silicalite-1 powder onto a powder body of coarser silicalite-1 crystals. Thin zeolite films have been hydrothermally grown onto the graded silicalite-1 support and the all-zeolite membranes display an excellent CO2/H2 separation factor of 12 at 0 °C and a CO2 permeance of 21.3 x 10~7 mol m~2 s~1 Pa~1 for an equimolar CO2/H2 feed at 505 kPa and 101 kPa helium sweep gas. Thermal cracking estimates based on calculated surface energies and measured thermal expansion coefficients suggest that all-zeolite membranes with a minimal thermal expansion mismatch between the graded substrate and the zeolite film should remain crack-free during thermal cycling and the critical calcination step.
© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND
license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Inorganic zeolite membranes consisting of highly crystalline microporous aluminosilicate films supported onto a porous substrate [1-3] have shown promise for energy-efficient production and upgrading of biofuels [4,5], carbon dioxide separation from CO2/CH4, CO2/N2, and CO2/H2 gas mixtures [6-9], and pervapora-tion [4]. Zeolite membranes have been prepared of a limited number of framework types; FAU [8], DDR [10], LTA [11], MOR [12,13] and MFI [6], where MFI zeolite membranes have gained a large research interest due to a high thermal, chemical and mechanical stability [7,14,15].
The performance of zeolite membranes is primarily controlled by the properties of the zeolite films that should have a well-defined pore size and shape, be as thin as possible to maximize the flux, and be free from macroscopic defects and cracks. The presence of defects and microcracks in the zeolite film is problematic as they greatly reduce the separation performance of the membrane. The zeolite film is usually supported onto a substrate that provides sufficient mechanical strength for handling and
* Corresponding authors at: Department of Materials and Environmental Chemistry, Stockholm University, SE-10691 Stockholm, Sweden. Tel.: +46 81 63568; fax: + 46 81 52781.
E-mail addresses: farid.akhtar@mmk.su.se (F. Akhtar), lennart.bergstrom@mmk.su.se (L. Bergström).
thermal or pressure cycling. Commonly used membrane supports for MFI film are a and y alumina [6,7,16,17], stainless steel [18], clay [19], cordierite [20], glass [21] and carbon [22]. While substrate materials like alumina have a high strength and are relatively easy to manufacture, differences in coefficient of thermal expansion (CTE) between the support and the MFI film may build up thermal stresses during thermal cycling in zeolite film, which could lead to grain boundary openings and cracks. Many zeolites have in fact negative CTE, the CTE of MFI is e.g. - (1-3) x 10-6 °C-1 [23], while commonly used alumina substrate alumina have positive CTE; ranging between 8.0 and 8.8 x 10-6 °C-1 [24].
Recent advances in zeolite membrane research have shown significant improvements in membrane synthesis and separation performance [7,14]. Important advances have been made to minimize and treat defects and cracks including e.g. tailoring the microstructure during film crystallization [25,26], selective chemical vapour deposition of coke and silica at defects [27,28], and condensation of Si-OH groups by rapid thermal treatment [14]. Although these advances show promise, there is a need to engineer the membranes to minimize the thermally induced stresses.
Thermal cracking of laminated systems can be reduced by either reducing the film thickness or by minimizing or eliminating the difference in the CTE between the zeolite film and membrane support [29,30]. Reducing the zeolite film thickness to tens of nanometres compared to current state-of-the-art zeolite films of
http://dx.doi.org/10.1016/j.memsci.2015.06.020
0376-7388/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
500 nm [25] is difficult using synthesis routes based on seeding followed by hydrothermal growth [15]. In this study, we demonstrate how all-zeolite MFI membranes consisting of an MFI zeolite film grown on a graded MFI zeolite support with a similar CTE and high gas permeability can be produced. The preparation and characterization of the graded MFI support and the CO2/H2 separation performance of the all-zeolite membranes are reported.
2. Experimental
2.1. Materials
Silicalite-1 powders (Sud-Chemie AG, Bruckmuhl, Germany) with silica-to-alumina ratios of 1200 and 400 and a particle size of 5 mm were used. Commercially available graded a-alumina supports (Fraunhofer IKTS, Munich, Germany) with a diameter of 25 mm and a thickness of 3 mm thickness were used as reference.
2.2. Processing of graded silicalite-1 membrane supports
The silicalite-1 powder was gently ball milled in deionized water with zirconia balls of 6 mm as milling media in a polymer container to prepare a water-based suspension of fine silicalite-1 particles [31]. Polyethylene glycol (PEG) was added to the aqueous silicalite-1 dispersion to yield dispersions with a solid loading of 10-15 wt% and a PEG content of 3 dwb%. Thin silicalite-1 film with a thickness of 40-60 mm were drop casted from aqueous silicalite-1/PEG dispersions on graphite papers with a diameter of 25 mm. The silicalite-1 coated graphite paper was placed in a cylindrical graphite die and 1.8 g of dry silicalite-1 particles were added. The powder body was pulsed current processed (PCP) in a so called spark plasma sintering machine (Dr. Sinter 2050, Sumitomo Coal Mining Co., Ltd., Japan) The powder assemblies were first heated to a temperature of 600 °C at a heating rate of 200 °C min~1 and then heated to 1200 °C at a heating rate of 100 °Cmin"1. The powder assemblies were kept at 1200 °C for 3 min. The PCP was performed following a procedure described in our previous work on silicalite-1 [24].
2.3. Film growth
Supported silicalite-1 films with a thickness of ca. 0.5 mm were prepared mainly following a method described earlier [25] that is briefly summarized below. Prior to film synthesis, the graded supports were masked [32]. This process involves rinsing of the disks with acetone, followed by covering the wet top surface with PMMA CM205 (Polykemi). After drying, the pores in the graded supports were filled with Sasol wax C-105 (Carbona AB). In order to grow an ultra-thin uniform zeolite film, the masked supports were seeded with a monolayer of colloidal silicalite-1 crystals of ca. 50 nm in size. The film synthesis was carried out hydrother-mally at 88 °C for 96 h. The molar composition of the synthesis mixture was 3TPA0H:25Si02:1450H20:100C2H50H. After the synthesis, the membranes were rinsed in a 0.1 M solution of ammonia for 24 h and calcined at 500 °C for 6 h. The heating and cooling rates during calcination were 0.2 °C minand 0.3 °C min _ 1, respectively.
2.4. Characterization
The microstructure of the graded membrane supports were characterised on a field emission gun scanning electron microscope (FEG-SEM), JSM-7000F (JEOL, Tokyo, Japan). The biaxial flexural strength of the membrane supports was determined using a cylinder on 3 balls geometry on a Zwick Z050 machine (Zwick
GmBH Co & KG, Ulm, Germany). Mercury intrusion porosimetry (MIP) was performed within the pore diameter range of 125 ^m > 0 > 3 nm on an Auto Pore III 9410 (Micromeritics, Norcross GA, USA) to determine macropore volume and the pore size distribution. Brunauer, Emmett and Teller (BET) surface area was obtained from nitrogen adsorption and desorption data collected on an ASAP 2020 analyzer (Micromeritics, Norcross GA, USA) within the 0.05-0.15 p/po relative pressure region. A degassing of as-received powder or membrane supports was performed prior to nitrogen adsorption at 300 °C for 10 h
Single component permeation of H2, He and N2, through the graded zeolite supports and all-zeolite membranes, were measured on a volumetric flow measurement device with a digital output (ADM2000 Flowmeter, Agilent). Prior to measurements, the membrane supports were kept at 300 °C for 6 h for degassing. The pressure of 1 bar was maintained on the permeate side of membrane support and all-zeolite membranes and the applied pressure difference (AP) was varied from 0.1 to 1 bar at 25 °C. The CTE of graded silicalite-1 membrane support and alumina supports was measured using a DIL 402 C, horizontal push rod dilatometer (NETZSCH-Geratebau GmBH, Selb, Germany). The CTE measurements were performed on specimens with a length of 24 mm and a square cross-section 3 x 3 mm. The CTE was determined from 200 °C to 800 °C.
2.5. Gas separation measurements
The gas separation performance of the all-zeolite membranes was evaluated at 0 °C using an equimolar mixture of CO2 and H2. The total feed volumetric flow rate was 3 l min~1. The total feed pressure was varied stepwise from 1 to 5 bar whereas the total permeate pressure was atmospheric. All experiments were performed using a 101 kPa helium sweep gas at a volumetric flow rate of 0.75 l min_ 1. Prior to the separation experiments, the membrane was flushed with helium overnight using feed and permeate volumetric flow rates of 0.3lmin~1 at atmospheric pressure in order to desorb moisture. The permeate volumetric flow rate was measured with a drum-type gas meter (TG Series, Ritter Apparatebau GmbH) and the permeate composition was analysed by a mass spectrometer (GAM 400, InProcess Instruments) connected on-line. The permeance of component i (mol sm~2 Pa_ 1) was estimated from the measured molar flow of the corresponding component through the membrane Fi (mol s_ 1) as
n = Fi /(AAPi), (1)
where A is the membrane area (m2) and APi (Pa) is the partial pressure difference of component i across the membrane.
The separation factor acO2/H2 was estimated as
a yCO2 /yH2 2
Pco2/h2 = Xco2 /XH2 ' (2)
where x and y in above equation represent molar fractions in the feed and permeate, respectively.
2.6. Modeling
An orthorhombic lattice was used with lattice parameters fixed at the values experimentally determined [24] at a temperature of 200 °C [a=20.11 A, b=19.92A and c= 13.39 A]. The a-direction was taken to be normal to the zeolite membrane surface. Therefore, cleaved surface normals are perpendicular to the a-direction. Considering cleaved surfaces with b and c normals and choosing the planar cleaved surface to minimize the number of broken bonds, one can estimate the surface energy, using a single SiO2 bond energy to be 444/(6.20 x 1023) kJ bond"1 [33], to be 1.06J m"2 and 1.43 J m~2 for the b and c normal surfaces, respectively. We concentrated on the
b normal surface. The periodically repeated unit cell consisted of 288 atoms of silicalite-1. All calculations were performed using density functional theory (DFT) as implemented in the AIMPRO code [34,35] with the PBE exchange-correlation functional [36] and the Grimme van der Waals correction [37] (see SI, Section S2 for details).
3. Results and discussions
Graded silicalite-1 supports were prepared by co-sintering a thin layer of fine silicalite-1 particles (200 nm), that have been deposited by drop casting onto a graphite paper, onto a layer of coarse silicalite-1 particles (5 ^m) by rapidly heating and subjecting the powder body to a compressive stress by pulsed current processing (PCP). It was previously shown that by identifying the material specific temperature range in combination with a com-pressive stress it is possible to produce strong porous supports with minimal loss of the micro-porosity of zeolite crystals by PCP [24,38-40]. The scanning electron microscope (SEM) images in Fig. 1 show that the top layer has a thickness of about 60 mm with a smooth and sharp interface without any evidence of trapping of small (200 nm) particles in the interstitial spaces between the larger zeolite crystals. The microstructure of the graded silicalite-1 membrane support (Fig. 1) is similar to commercially available alumina membrane supports (SI, Fig. 1S). BET surface area (Table 1) of the graded MFI (302 m2 g~1) is lower compared to the unprocessed powders (390 m2 g_ 1). The reduction in BET surface area of zeolite crystals happens during PCP-treatment due to
localized sintering of zeolite crystals at the contact points [24,38-42].
Mercury intrusion porosimetry data shows that the pore size distribution of the PCP-consolidated graded silicalite-1 supports (Fig. 2, Table 1) is bimodal. The average pore size of 0.1 ^m in the thin top layer of silicalite-1 substrate is sufficiently small for zeolite film growth [32] and sufficiently large to not significantly reduce the mass transport of molecules [43]. Indeed, the silicalite-1 supports show a smoother surface finish compared to commercial alumina supports (SI, Figs. 3S and 4S), which is beneficial for the formation of smooth and defect-free silicalite-1 films. The graded MFI membrane supports show reasonably high single gas permeabilities (Table 2) and a mechanical strength of 15 MPa (Table 1). The mechanical strength of the graded substrates is lower than the alumina supports but sufficiently high to sustain large pressure gradients.
Thin zeolite films were grown onto the top surface of the graded silicalite-1 supports by a combined seeding and hydrothermal synthesis route [25]. Fig. 3 shows that the calcined MFI film grown onto the graded MFI porous support was uniform, crack free and with no evidence of pinhole defects. The cross-sectional SEM image in Fig. 3a of an all-zeolite membrane shows that the MFI film has a thickness of 500 nm. Invasion of the seeding particles and growth of the zeolite film into the support pores is small and the macroporosity of the top layer of the graded support is retained.
The high resolution SEM image of the cross section (Fig. 3a) indicates that the MFI seed crystals grow in favorable orientation all the way from the substrate to the top surface of the film. Seed
Fig. 1. SEM micrographs of PCP-consolidated graded silicalite-1 support, (a) cross-sectional view showing top and bottom layer; (b) top layer; (c) bottom layer; (d) interface between top and bottom layer.
Table 1
Properties of PCP-consolidated Silicalite-1 membrane supports.
Specimen BET surface areaa Total pore volumeb Macro-pore volumec Porosityc Biaxial Median pore Co-efficient of thermal
(m2g-1) (cm3g-1) (cm3g-1) (%) strength (MPa) diameterc (^m) expansiond (10-6/°C)
Silicalite-1 powders Graded silicalite-
1 support Alumina substrate
0.23 0.18
0.42 0.18
15 102
0.75, 0.1 2.15, 0.1
- 0.70 8.98
a BET surface area (m2 g-1) was calculated from nitrogen adsorption isotherms within the relative pressure range: 0.05-0.15 p/po. b Single point adsorption total pore volume determined from nitrogen adsorption data at a relative pressure of 0.98 p/po. c Porosity, macropore volume and median pore diameter were determined by mercury intrusion porosimetry. d Determined by dilatometry in the temperature range from 200 °C to 800 °C.
Fig. 2. Mercury intrusion porosimetry (Incremental pore volume vs mean diameter of pores) of a) PCP consolidated graded silicalite-1 membrane support.
Table 2
Single gas permeation properties of graded silicalite-1 support and all-zeolite membranes calcined at 500 °C for 6 h.
Support/ Gas Flow [ml min 1] at Permeance
membrane 0.8 bar [10-7 mol s-1 m-2 Pa-1]
Support He 184 48
Support H2 325 86
Support N2 135 36
Support CO2 130 34
Membrane He 177 47
Membrane H2 316 83
Membrane N2 133 35
Membrane CO2 130 34
crystals with no favorable orientation for growth will be encapsulated in the film. This competitive growth process corresponds well to previous observations of silicalite-1 films grown on other membrane supports [16-22]' [44]. Note that the support was masked with hydrocarbon wax during growth of the film, which prohibited growth of the zeolite crystals into the support. The single gas permeances measured for the all-zeolite membranes comprised of the thin zeolite film and the support are close to the gas permeances measured for the zeolite supports (Table 2), which shows that the mass transfer of these light molecules through the membrane is mainly limited by the support.
Fig. 4 shows the CO2 permeance at 1 bar feed pressure for the all-zeolite membrane was 10.3 x 10-7 mol s-1 m-2 Pa-1. The CO2 permeance is somewhat lower than what is obtained for a similar membrane prepared on a graded alumina support [6], which is probably a result of higher mass transfer resistance of the zeolite support. The CO2 permeance increases with increasing feed pressure up to 21.3 x 10-7 mol s-1 m-2 Pa-1 at 5 bar. The increase in
the CO2 permeance with increasing feed pressure is most likely caused by the reduced helium sweep counter-flux through the membrane at high pressure, as also reported by van de Graaf et al. [45]. In addition, the increase in the permeance may also be a result of some water being desorbed from the membrane as the feed pressure increases. In contrast, the H2 permeance decreases from 6.0 x 10"7 mols"1 m"2 Pa"1 to 1.8 x 10"7 mol s"1 m"2 Pa"1 when the feed pressure increases from 1 bar to 5 bar. The decrease in H2 permeance with increasing feed pressure can be related to an increased adsorption of CO2 in the MFI film that reduce or block the transport of H2 through the membrane [6]. Hence, we find that the CO2/H2 separation factor estimated by Eq. (2) increases from 2 to 12 with increasing feed pressure. The CO2/H2 separation factor for the all-zeolite membranes is comparable to the separation factors achieved for previously reported MFI membranes evaluated for gas mixture separation, see Table 3.
We have calculated the surface energy of the silicalite-1 film by density functional theory. We found the relaxed surface energy of the surface perpendicular to the a-direction of MFI crystals after charge balancing to be 1.14J m"2 (see SI, Section S2). We note that the calculated value is significantly higher than the commonly quoted value of 0.1 J m"2 for silica surfaces [51].
The CTE of the graded silicalite-1 support (aSiiiCame-1 = -0.7 x 10"6 °C"1) and the alumina support (aAl2O3 = +8.98 x 10"6 °C"1) was determined by dilatometer and is reported in Table 1. The CTE of a silicalite-1 film (af= -2.00 x 10 "6 °C"1) was taken from the literature [23]. We adopted the laminate model of stress relaxation from our previous work [24] to predict the critical film thickness of silicalite-1 to avoid cracking in a thermal cycling process. The model assumes that cracks appear in the thin film when the stored energy exceeds the surface energy of the film material. We used the DFT-based estimate of surface energy (1.14J m"2) of silicalite-1, a Poison's ratio of 0.175, and an elastic modulus of 40 x 109 Pa [52,53] together with CTE of silicalite-1 film (af= "2.00 x 10"6 T"1) [23] and measured CTE of the graded silicalite-1 substrate (aSiUcaUt:e_1 = "0.7 x 10"6 T"1) and alumina substrate (aaAl2O3 = + 8.98 x 10"6 °C"1) to estimate the critical fi m thickness.
Fig. 5 shows that all-zeolite membranes with zeolite films as thick as 5 ^m can sustain temperature differences as high as 800 °C without cracking, while the critical film thickness of a silicalite-1 film onto an alumina substrate is around 500 nm at a moderate AT of 400 degrees, which is lower temperature cycle limit for most zeolite membranes [54]. Hence, while our estimates suggest that typical calcination processes at 400-600 °C may be sufficient to produce defects such as crack and open grain boundaries in alumina-supported zeolite films, all-zeolite membranes may provide defect free membranes that not only can avoid defect formation during the critical calcination step at 400-600 °C but also sustain high temperature thermal cycling.
Fig. 3. SEM micrographs of all-zeolite membrane, (a) cross section and (b) top view (see SI, Fig. 5S for magnified images).
- 30 -'(A O
1 2 3 4 5
Feed Pressure (bar)
Fig. 4. Permeances of CO2 and H2 as a function of feed pressure at 0 °C. CO2 permeance, ▼—H2 permeance, and CO2/H2 separation factor as a function of feed pressure.
Table 3
Summary of studies on CO2 separation from H2 gas using MFI membranes prepared on non-zeolite supports.
Feed Feed Temp. CO2 permeance CO2/H2 Ref.
mixture pressure (°C) (107 mol m 2 s~1 Pa_1) separation
(bar) factor
CO2/H2 1 0 10.3 2 This
CO2/H2 5 0 21.3 12 This
CO2/H2 1 22 3.8 12 [46]
CO2/H2 1 20 7 17 [47]
CO2/H2 3.3 25 1.2 10 [48]
CO2/H2 1 25 12 3.4 [49]
CO2/H2 1 22 13 2.2 [49]
CO2/H2a 1 25 11.7 6.2 [50]
CO2/H2 10 23 93 15 [6]
a Saturated with water at 20 °C.
4. Conclusions
All-zeolite membranes were prepared to minimize the thermal
expansion mismatch between the graded support and the zeolite film to minimize crack formation during calcination and thermal
cycling. We prepared graded silicalite-1 supports with a tailored porous architecture with good permeability and low surface roughness suitable for silicalite-1 film growth by a modified
0 200 400 600 800 1000
AT (°C)
Fig. 5. A comparison of critical film thickness to avoid thermal cracking during thermal cycling of temperature difference (AT) between alumina-supported zeolite membrane and all-zeolite membrane.
pulsed current powder processing route. Thin (500 nm) silicalite-1 films were grown on the graded zeolite supports using a previously develop route based on masking and seeded hydrothermal growth. The resulting all-zeolite membranes showed a high permeance and CO2/H2 separation selectivity. Estimates of thermal cracking based on calculated surface energies and measured thermal expansion coefficients of the support show that all-zeolite membranes could remain crack-free during the critical calcination step and can undergo thermal cycling from temperatures exceeding 500 °C to room temperature without cracking. Tailoring the properties of all-zeolite membranes could extend the application of zeolite membranes in demanding separation processes involving with high temperature thermal cycling. The method of preparing zeolite membrane supports is generic and can be implemented to produce other framework types of all-zeolite and possibly also all-metal organic framework membranes.
Acknowledgements
The Swedish Foundation for Strategic Research is gratefully acknowledged for financially supporting this work. M. Rayson thanks the Swedish Research Council (VR) for financial support for the project 2012-3174. F. Akhtar acknowledges Christina Schütz for AFM measurements, Segey Istomin and Gunnar Svensson for thermal expansion measurements, Damian Hodel and Sajid Alvi for experimental assistance in developing silicalite-1 membrane supports.
Appendix A. Supplementary information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.06. 020.
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