Carbon Nanotube Selective Membranes with Subnanometer

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Carbon Nanotube Selective Membranes with Sub-Nanometer, Vertically Aligned Pores, and Enhanced Gas Transport Properties Anastasios Labropoulos, Charitomeni Veziri, Maria Kapsi, George Pilatos, Vlassis Likodimos, Michael Tsapatsis, Nick K. Kanellopoulos, George E. Romanos, and Georgios N. Karanikolos Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01946 • Publication Date (Web): 17 Nov 2015 Downloaded from http://pubs.acs.org on November 24, 2015

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Chemistry of Materials

Carbon Nanotube Selective Membranes with Sub-Nanometer, Vertically Aligned Pores, and Enhanced Gas Transport Properties Anastasios Labropoulos1*, Charitomeni Veziri1, Maria Kapsi1, George Pilatos1, Vlassis Likodimos1,2, Michael Tsapatsis3, Nick K. Kanellopoulos1, George E. Romanos1, Georgios N. Karanikolos1,4,* 1

Institute of Nanoscience & Nanotechnology (INN), Demokritos National Research Center, Athens 153 10, Greece 2

Solid State Physics Department, Faculty of Physics, University of Athens, Panepistimioupoli, 157 84 Athens, Greece

3

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, USA

4

Department of Chemical Engineering, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates * [email protected], [email protected]

Abstract Membranes consisting of ultra-thin, oriented, single wall carbon nanotube (SWCNT) micropores with a diameter of ∼ 4 Å were developed. c-Oriented AFI-type aluminophosphate (AlPO) films (AlPO4-5 and CoAPO-5), consisting of parallel channels of 7.3 Å in diameter, were first fabricated by seeded growth on macroporous alumina supports, and used as templates for synthesis of CNTs inside the zeolitic channels by thermal treatment, utilizing the structure directing agent (amine) occluded in the channels as carbon source. Incorporation of CNTs inside the AFI channels altered the transport mechanism of all permeating gases tested, and imposed a substantial increase in their permeation rates in comparison to the 1 ACS Paragon Plus Environment


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AlPO4-5 membrane, despite the pore size reduction due to nanotube growth. The enhancement of the permeation rates is attributed to repulsive potentials between gas molecules and occluded nanotubes, which limit adsorption strength and enhance diffusivity, coupled to the smooth SWCNT surface that enables fast diffusion through the nanotube interior. Separation ability, evaluated with respect to H2 and CO2 gases, was enhanced by using polysterene as defect-blocking medium on both AlPO and CNT/AlPO membranes, and was preserved after CNT growth.

1. Introduction The importance to efficiently separate chemical substances from mixtures has been recognized long ago. For instance, the ability to efficiently and economically separate CO2 from power plant flue gases could revolutionize efforts to reduce greenhouse gas emissions. In almost every chemical and/or petrochemical process, the separation of the products accounts for a major part of the capital costs mainly in the form of energy consumption. It is generally accepted that the use of membranes can significantly reduce the energy requirement of the separation stages compared to traditional separation methods, such as distillation or absorption 1. High molecular flux combined with high chemical selectivity is the most critical performance element that a membrane should exhibit. Most membranes in use industrially today are polymeric, and their fabrication is well developed, yet polymeric membranes for gas separations show a near-universal tradeoff between flux and selectivity 2. Materials with high throughput also have low selectivity, and vice versa. Polymeric membranes are also typically unstable for high–temperature applications. New membrane materials that can overcome these fundamental hurdles could ultimately drastically reduce the energy required in future separation operations. The regularly structured pores and cages of zeolites and their thermal

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and chemical stability has enabled fabrication of thin films, coatings and membranes that can be potentially utilized for industrial separations 3-6. Recently, membranes consisting of carbon nanotubes (CNTs) attracted significant attention because of theoretical studies indicating very high fluxes. Molecular dynamics simulations

7-8

have shown that transport rates through CNTs are orders of magnitude higher

than in zeolites with comparable pore sizes or in any nanoporous material for which experimental data are available. These high transport rates are shown to be a result of the inherent smoothness of nanotubes. Experimentally, aligned carbon nanotubes have been embedded in a silicon nitride matrix 9 and in a polymer matrix 10-11, or forming closely-packed arrays

12

, and the resulting membranes exhibited rapid transport of single gases, confirming

theoretical predictions. Although

selective operation has not been demonstrated

experimentally up to date, selectivity can be achieved by preferential adsorption, as recently demonstrated by molecular simulations

13-14

, and/or by reducing the CNTs inner diameter

down to the molecular dimensions so that sieving performance is enabled. The aforementioned techniques are based on first synthesizing CNT arrays via chemical vapor deposition (CVD) and subsequently embed the nanotubes in dense matrix in order to fill the gaps between them, or form closely packed arrays with the nanotube interstitial spaces also available for transport. However, size control of CVD-grown CNTs is rather limited and formation of nanotube membranes with pore diameters below 1nm, which is required for sizeselective molecular separations, is still to be realized. Such size control is also desirable because it is expected that selectivity will vary with the curvature of the nanotubes

13

. In

addition, CNTs grown by CVD are typically non-uniform in thickness, a factor that can compromise the molecular sieving accuracy of the resulting membranes. In this work we developed single wall CNT (SWCNT) membranes by growing subnanometer diameter CNTs in the interior of one-dimensional, monodisperse pores of c3 ACS Paragon Plus Environment


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oriented aluminophoshate (AlPO) molecular sieve films (AlPO4-5 and CoAPO-5). Following the template approach is inspired by our previous work of developing aligned CNTs with larger diameters in the channels of anodized alumina membranes

15-16

. The use of AlPO as

CNT templates was motivated by the structure of the AFI-type framework, which consists of one-dimensional, uniformly sized channels of 7.3 Å in diameter extended parallel to the c-axis of the crystals. Theoretical studies have predicted that such an arrangement existing in the AFI-type zeolites can exhibit faster molecular transport than through similarly sized multidimensional zeolites 17. Additionally, an unusual size-selectivity pattern can take place, which is attributed to the transition to single file transport resulting in considerable changes in diffusivity. In contrast to ordinary diffusion, in “single-file” transport, the diffusivity of an isolated molecule can be up to two orders of magnitude larger than the largest intra-crystalline diffusivities in zeolites so far observed 18. The use of AlPO films as hosts of carbon nanotubes was motivated by previous studies that demonstrate that SWCNTs of ~4 Å in diameter can be formed inside the micropores of AlPO4-5 powder crystals by pyrolysis of triethylamine occluded inside the zeolitic channels 19 and builds upon our expertise to grow well-intergrown and appropriately oriented AlPO-based AFI films

20-22

. To this extent, c-oriented AlPO films

were fabricated on macroporous alumina by seeded growth, and the oriented channels were subsequently used as templates for growth of SWCNTs yielding membranes with a CNT pore diameter of ∼4 Å. Both AlPO/CNT and AlPO films were coated with polysterene (PS) after growth as to enable blocking of inter-crystal gaps. The effect of SWCNTs incorporation inside CoAPO-5 zeolitic channels on the transport and permeation rates of several probe gases was investigated with regard to the permeation properties of the AlPO4-5/PS membrane. The alteration of the transport mechanism of the selected gases as a result of the CNT growth and pore size reduction was elucidated and the separation efficiency of both membranes with respect to H2 and CO2 was evaluated.


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2. Experimental Section

2.1. AlPO4-5 membrane fabrication via seeding AlPO4-5 crystals to be used as seeds were prepared through hydrothermal growth using a precursor mixture with a composition of 1.0Al2O3:1.3P2O5:1.2TEA:100H2O. Aluminum isopropoxide (98%, Aldrich) was hydrolyzed in de-ionized (DI) water for 4.5 h under stirring followed by dropwise addition of phosphoric acid (85%, Aldrich). Following homogenization, triethylamine (TEA, Aldrich), as structure directing agent (SDA), was added dropwise and further allowed to homogenize for 12 h under stirring. The final gel was added to Teflon liners, sealed in stainless steel autoclaves, and heated up for 72 h at 150°C. After crystallization, the autoclaves were quenched in cold water followed by centrifugation and drying to recover the powder. In order to produce seeds, AlPO4-5 crystals were subsequently broken by sonication in dilute hydrochloric acid. Home-made alumina discs (diameter: 22 mm, surface area: 3.8 cm2) from high purity a-alumina powder (Baikowski, CR-6) were used as supports for membrane fabrication. The detailed preparation procedure can be found elsewhere 22-23. One side of the support was smoothed by polishing with 1000 grit sandpaper. The disc was cleaned with DI water and dried at 100oC overnight before use. Seeds were deposited onto the supports through an attachment-by-sonication procedure, according to which a-alumina supports were functionalized using a silane coupling agent, 3chloropropyltrimethoxysilane (3-CPTMS), and the functionalized supports were subjected to sonication between two glass slides in an AlPO4-5 seed suspension in toluene under dry conditions to facilitate formation of a seed monolayer

20, 22

. In order to accomplish film

formation, the seeded supports were vertically placed in Teflon liners and loaded in stainless steel autoclaves for hydrothermal growth at 150 °C for 10 h. The secondary growth precursor solution composition was 1.0Al2O3:1.3P2O5:1.2TEA:400H2O. Upon completion, the 5 ACS Paragon Plus Environment


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autoclave was quenched in water at room temperature and the support was removed and carefully washed with DI water. Calcination was conducted under flowing air in order to remove the SDA from the pores of the AlPO membranes.

2.2. SWCNT growth CoAPO-5 crystals of columnar morphology both as powder and as oriented film were used. Co was added in the AlPO framework as to catalytically enhance CNT growth. Growth of CoAPO seeds/crystals/films was based on the procedure described above, yet by adding cobalt acetate tetrahydrate (Aldrich) as cobalt source in the reaction mixture after the addition of TEA, at a molar ratio CoO:Al2O3 of 0.025

24

. For CNT growth in the AFI channels, the

samples were heated up to a maximum temperature of 750 °C under flow of helium and H2, and the temperature was maintained at this value for 60 min. The heating and cooling ramp rate was 0.5°C/min. In order to enhance CNT growth by offering a carbon source in addition to the channel-occluded triethylamine molecules, a stream of C2H2 was also used with a flow rate of 15 ml/min. It should be noted however, that growth experiments that took place in inert atmosphere or in vacuum without the use of external carbon precursor also yielded SWCNTs, indicating that the SDA occluded in the AFI channels constitutes a sufficient source for nanotube growth. After growth of both AlPO and AlPO/CNT membranes, spin coating with polystyrene (molecular weight 250,000 g/mol) took place as to fill the gaps between the crystals. A solution of 20 wt% polystyrene in toluene was added dropwise on the membrane surface until a uniform coverage was achieved, and the polymer was allowed to diffuse through. After removing the excess of polymer by toluene, the membranes were spin-coating for 1 minute at 3000 rpm.


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2.3. Characterization Film morphology was evaluated by scanning electron microscopy (SEM) using a JEOL JSM-7401F field-emission gun (FEG) SEM at typical conditions of 10 mA emission current and 5 kV operating voltage. X-ray diffraction (XRD) was performed to check film orientation using a Siemens D500 X-ray diffractometer in a θ/2θ geometry. SWCNT formation inside the AFI channels was confirmed by Raman spectroscopy using an inVia Reflex (Renishaw) micro-Raman spectrometer with a focal point of 1–2 µm2 and a crystal laser excitation of 785 nm, operating at 10 mW (~10% of full power). Liquid N2 adsorption isotherms at 77 K were obtained, using an Autosorb-1 MP (Quantachrome) porosimeter. Before each measurement, the samples were degassed, under high vacuum 10-5 mbar for 24 h at the outgassing station of the instrument.

2.4. Membrane performance evaluation The gas permeation properties of the fabricated membranes were evaluated against theoretical descriptions of the gas transport mechanisms in porous inorganic membranes, which are provided in the Supporting Information. Single-gas permeance measurements were conducted for both AlPO and AlPO/CNT membranes in the dead-end configuration using the following probe gases: He (99.999 %), H2 (99.999 %), CO2 (99.998 %), O2, (99.999 %), N2 (99.999 %) and CH4 (99.995 %). For each employed gas, the permeances were acquired after attainment of the steady state. Mixed-gas permeance measurements were performed in the Wicke – Kallenbach (WK) configuration, for binary mixtures of N2 / TIPB vapours and N2 / Heptane vapours, using a N2 flow rate of 20 cm3·min-1. The principles of the dead-end and WK techniques together with the equations used to determine single-gas or mixed-gas permeances are provided in the Supporting Information. A detailed description of the experiments for conducting the two types of permeance measurements, together with an 7 ACS Paragon Plus Environment


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illustration of the experimental set-up (Fig. S1) are also included in the Supporting Information.

3. Results and Discussion 3.1. Membrane morphology Fig. 1a shows the AlPO4-5 film morphology after the seeded growth processing. The film consists of hexagonally-shaped, columnar crystals oriented perpendicular to the support. The XRD pattern (Fig. 1b) confirms the maintenance of the AFI topology upon detemplation and indicates film orientation. The degree of orientation of the film can be estimated by the crystallographic preferential orientation (CPO) index, comparing the relative XRD intensities of the (002) (preferred orientation – channels perpendicular to the support) and (100) reflections. The following formula was used25, where (S) refers to the supported membrane (Fig. 1b) and (P) refers to the powder (Fig. 2a): CPO = ([I002/I100]S - [I002/I100]P)/[I002/I100]S). Given the above formula, for a perfectly oriented film a CPO index value of almost 1 would be expected. The CPO of our membrane is 0.9 indicating good orientation, in agreement to the SEM images. Upon PS coating, plugging of the film’s intercrystalline gaps is observed (Fig. 1c), yet the fact that the upper face of the crystals is evident reveals the efficiency of the spin coating procedure applied, causing PS to be mostly located in the gaps thus allowing the crystals to be directly accessible for gas molecules. SWCNT growth took place via pyrolysis of the organic SDA occluded inside the (b)

zeolitic channels. Decomposition of triethylamine or tripropylamine molecules generally occurs at 350-450 °C while transition to graphitic nanotubes takes place at 500-800oC

19, 26

,

CoAPO-5 instead of AlPO4-5 was employed for CNT growth as Co2+ generates Brønsted acid sites that are anticipated to favor catalytic decomposition and enhance CNT formation.


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Moreover, the quality of SWCNTs is sensitive to the adsorption potential between the channel walls and the organic agent. To this extent, replacement of Al3+ by a divalent cation such as Co2+, yields a negatively charged framework which enhances amine interaction with the zeolitic channels, thus increasing SWCNT density and quality. The morphology of the CoAPO-5 columnar membrane is maintained after SWCNTs growth (Fig. 1d). According to SEM observations (Fig. 1e), spin-coating of the CoAPO/SWCNTs with PS successfully plugged the gaps, while the upper surface of the crystals is evident demonstrating their availability towards gas flow. 3.2. XRD and unpolarized Raman spectrum of AFI/CNT composite crystals The XRD pattern of CoAPO-5 crystals in powder form after growing SWNTs in the channels (Fig. 2a) confirms the maintenance of AFI topology at processing temperatures as high as 750°C. Notably, based on our previous XRD results24, CoAPO-5 crystals upon calcination under oxidative conditions have shown a destabilization of the AFI microstructure that starts at 650oC and transforms the material into dense tridymite. Under inert atmosphere therefore, as used in the present work, the stability of the AFI framework is extended to higher temperatures, which is attributed to the CNTs formed inside the channels. The formation and structural characteristics of the synthesized SWCNTs was studied by unpolarized Raman scattering. Raman spectra were collected for the synthesized CoAPO/SWCNTs crystals at room temperature (Fig. 2b). The Raman spectrum extends in a frequency region from 100-1700 cm-1 exhibiting three main zones, at low (200-800 cm-1), intermediate (1000-1500 cm-1), and high (1500-1620 cm-1) frequencies. In the high frequency zone, a graphitic tangential Raman mode at 1598 cm-1 is evident. The Raman modes observed in the intermediate frequency region (1000-1500 cm-1) are associated with the presence of Dband or finite-size effects in sp2 carbons. This intermediate region can be divided into two regions. In the first region (1310-1480 cm-1), the peak at 1424 cm-1 is related to D band, while 9 ACS Paragon Plus Environment


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the fact that its relative intensity is low indicates high quality CNTs. The peak at 1377 cm-1, is related to G- band. In the second region (1200-1300 cm-1), a mode is evident at ~1278 cm-1 which is attributed to the D band 27-28. The low- frequency Raman mode centered at 554 cm-1 is the radial breathing mode (RBM). Since this frequency is in the silent region for graphite and other carbon fragments, it is a good trademark for specifying the carbon nanotube structure 26. The observed high frequency shift of the RBM band from that expected for a nanotube with a radius of about 3 Å is attributed to the interaction between the nanotube wall and the pore wall of the AFI framework 29. The three RBMs of the SWCNTs are located at 514, 542 and 598 cm1

which are attributed to the (4,2), (5,0) and (3,3) tubes respectively. RBM mode is sensitive to

nanotube radius, with a linear relationship to the reciprocal of carbon nanotube diameter according to the expression ω(dt) = (a/dt), where ω(dt) is the RBM frequency, dt is the tube diameter, and a is the proportional constant, which for (10,10) armchair nanotubes has been estimated to be 224 nm cm-1 30.Based on the RBM, the diameter of our SWCNTs is estimated to be ~0.4 nm. This value is in agreement to high resolution transmission electron microscope (HR-TEM) based measurements for CNTs after removing the AFI framework

31-33

, according

to which nanotube diameter was determined by measuring the separation between the paired dark fringes using the spacing between the neighboring parallel fringes of graphite (≈ 0.34 nm) as an internal reference. Furthermore, mechanics models predict an effective wall thickness of a continuous SWCNT smaller than the theoretical diameter of a carbon atom (1.42 Å) 34-35. By combining the above prediction, with the estimated SWNT diameter based on RBM values and reported HR-TEM observations we can conclude that SWCNTs with a 4 Å inner diameter and ~1.5 Å wall thickness are accommodated inside the 7.3 Å AFI channels of the CoAPO/SWCNT hybrid material. To investigate whether external carbon source is required for SWCNT growth, we conducted growth experiments by pyrolysis of CoAPO-5 crystals in vacuum conditions (10-2


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mbar) at 750 °C and 1h, as well as under inert atmosphere (He) at the same growth temperature and time. In both cases, Raman spectroscopy revealed analogous features (Fig S4 in the Supporting Information) as the ones reported in the previous paragraph, indicating that the TEA molecules occluded in the AFI channels constitute a sufficient source for SWCNT growth.

3.3 Permeance results – Elucidation of gas transport mechanism

3.3.1 AlPO4-5/PS composite membrane In the case of the AlPO4-5/PS composite membrane, gas transport could take place through both the zeolitic channels36 and the polymeric plugs. Gas transport through the latter should follow the typical solution – diffusion mechanism. Fig. 3 depicts the permeances of He, H2, CO2, O2, N2 and CH4 single gases through the AlPO4-5/PS membrane as a function of temperature. Physical properties of the aforementioned gases are listed in Table 1. At low temperatures, the determined permeances for O2 and N2 were 3 or 4 orders of magnitude higher than the respective ones determined for polymeric films consisted of polystyrene or polystyrene block copolymers 37-40, whereas comparable permeance values for both gases have been reported for AlPO4 zeolitic membranes

41

. Furthermore, the highest permeance values

were determined for H2 gas, despite the fact that it is one of the least soluble gases in the membrane’s polymeric component. The gas permeation rates through the composite membrane decrease in the order: H2 > He > CH4 > N2 > O2 > CO2. Taking into account the higher solubility and permeability of CO2 through thin PS films in comparison to O2 and N2 gases as reported in the literature, and considering the quite low CO2 and the much higher He and H2 permeation rates through the AlPO4-5/PS composite membrane (in opposition to the permeation behaviour expected for dense polymeric membranes), the total gas transport through the composite membrane can be deduced to be 11 ACS Paragon Plus Environment


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controlled by the zeolitic channels. The observed gas transport behaviour also implies that the surface of the zeolitic layer is not covered by a dense PS film and the zeolitic channels are directly accessible to the gas molecules, as also evidenced by the SEM analysis. Additional evidence for this claim can be provided by the correlation of the permeance of H2, CO2, O2 and N2 probe gases with their kinetic diameter, which presents identical trend with results reported for unmodified AlPO4 zeolitic membranes

41

. Furthermore, the inter-crystalline gaps seem to

have been completely occluded by PS dense plugs, since the permeance values of the selected probe gases would be expected to be much higher in the presence of open gaps or macroporous defects, which would result in non-selective Poiseuille flow. According to Weh et al. 42, separation of small gas molecules like H2, CO2, O2, N2 and CH4 can occur in the large pore system of faujasite (NaX) zeolites (0.74 nm pore opening diameter) only by interactions (adsorption and diffusion) but not by size exclusion. Since CO2 is a quadrupolar gas, it can interact electrostatically with the polar sites on the surface of NaX channels and as a result, CO2 transport is inhibited owing to the strong adsorption forces. Keizer et al.’s model

43

for separation through MFI membranes suggests that both molecular

size, relative to the zeolite pore, and the relative adsorption strengths determine the faster permeating species in a binary mixture. The low CO2 permeance values indicate stronger adsorption on the zeolitic pore surface in comparison to the other probe gases. The decreasing tendency of permeance with rising temperature, which was found for all gases over the range of 30-80oC, can be explained by considering the combined effect of surface diffusion and activated Knudsen diffusion components on the total gas transport. For a micropore diameter of 7.3 Å, the surface coverage of the pore walls by the adsorbed particles of a given gas at sufficiently low temperatures is high. For such high pore surface loadings, the surface flux increases with rising temperature owing to mobility enhancement of the physically adsorbed species, as indicated by Eq.(S4),


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even though the concentration of the adsorbed phase, Cs, decreases. Eventually when a specific temperature is reached, the impact of the decline in θ (and Csat) prevails despite the continuously increasing surface diffusivity, Ds 44-46.

3.3.2 Mixed-gas permeance results The results of N2 mixed-gas permeance measurements for the binary mixtures of N2 / TIPB vapours and N2 / n-Heptane vapours are presented in Fig. 4(a). The composite membrane AlPO4-5/PS was essentially impermeable to TIPB vapours since no traces of the organic solute could be detected in the permeate stream. TIPB molecules with a kinetic diameter of 8.5 Å, are size-excluded from the channels of AlPO4 crystals and thereby can only dissolve and diffuse into the PS plugs. The slow increase of N2 permeance (up to 46 %, as shown in Fig. 4(b)) after an initial interval of ∼ 850 min of constant permeation rate can be attributed to the swelling of the polymeric matrix owing to the dissolution of the quite soluble (in polystyrene) TIPB vapours. On the contrary, the supply of n-C7H16 vapours into the membrane module resulted in a substantial drop of N2 permeance down to 32.4 % of its initial value. Since the n-C7H16 molecules (dk = 4.3 Å) fit to pass through the channels of the AlPO4 crystals, this reduction can be ascribed to the pore blocking effect of the paraffin which reduces and eventually diminishes N2 permeation rate through the zeolitic pores. On the other hand, n-heptane is a poor solvent for PS

47

and thus the slightly soluble n-heptane vapours are almost non-

permeable through the polymeric plugs. Consequently, regarding the transport of n-C7H16, the zeolitic micropore diffusion is dominant (similar to the case of N2 gas) and the mutual hindering suppresses both n-C7H16 and N2 mixed-gas permeance. As shown in Fig. 5, the final n-C7H16 permeance is by 2.53 times lower than the respective one after 44 min of vapours supply in the membrane. Besides, considering the negligible permeability of TIPB vapours



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(not detected in the permeate stream), it is plausible to conclude that the n-heptane / TIPB ideal selectivity should be extremely high.

3.3.3 CoAPO-5/PS/CNTs composite membrane Regarding the CoAPO-5/PS/CNTs composite membrane, the permeation rates of He, H2, CO2, O2, N2 and CH4 probe gases were studied with respect to the temperature and transmembrane pressure, ∆P. The permeance results with respect to ∆P at the temperatures of 35, 50, 60 and 80oC are presented in Fig. 6. Single-gas permeances of He, H2, CO2 and N2 gases were also performed at the temperature of 80 oC, 12 months after the growth of the membranes with the purpose to examine the membrane’s long-term performance stability and the repeatability of the permeance results. Fig. 7 presents the results of the long-term repeatability investigation together with the average permeance values, which include the first results obtained for the freshly produced membrane. The permeance results were obtained after attainment of the steady state, as indicated in Fig. S2 in Supporting Information for CO2 at 80oC over the range of pressure gradients of 0.5 – 2 bar. The extent of formation of CNTs inside the zeolitic channels was investigated by performing N2 porosimetry at 77 K on a sample of CoAPO-5/CNTs powder calcined at 300oC under helium flow for 15 hours as well as a CoAPO-5 sample in powder form, heated in air at 550oC for 48 hours. By interpreting the results of N2 porosimetry we have unveiled the possible existence of a fraction of the zeolitic channels of the CoAPO-5/PS/CNTs composite membrane that was not filled with carbon nanotubes. In specific, the N2 adsorption isotherms for the two samples are presented in Fig. S3 in the Supporting Information. A non-trivial uptake of N2 vapours by the CoAPO-5/CNTs powder is observed in the range of very low N2 relative pressures, indicating that the fraction of open micropores (channels not containing CNTs), which are accessible to N2 molecules, is small.


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With the purpose to evidence the possibility of existing open channels in the zeolitic matrix, single gas permeance measurements for SF6 (99.99 %) through the CoAPO-5/PS/CNTs membrane were conducted with the dead-end technique, at the temperatures of 35, 50, 60 and 80oC, as in the case of the other studied gases. It is noted that the kinetic diameter of SF6 is 5.5 Å and therefore, no permeation is expected to occur through the channels of the CoAPO5/PS/CNTs membrane, since the CNTs diameter is smaller (∼ 4 Å). However, as shown in Table 2, SF6 permeance ranged between 1.7 and 2.1⋅10-8 mol⋅m-2⋅s-1⋅Pa-1, that is lower than the permeances of all probe gases studied yet not negligible. The observed SF6 permeances can be ascribed to the existence of open zeolitic channels, which are accessible to the SF6 molecules since their diameter is 7.3 Å. Therefore, incomplete conversion of the amine precursor to carbon nanotubes during the pyrolysis step, owing to partial volatilization, can be deduced. Moreover, the ideal selectivities for SF6 with respect to each employed gas are higher than the respective ones calculated considering Knudsen diffusion for all probe gases, as shown in Table S4 in the Supporting Information. This enhancement in the experimentally observed ideal selectivities can be ascribed to the size-exclusion of SF6 molecules from the CNTs-filled channels, thus resulting in substantially reduced SF6 gas flux. The good separation performance for SF6 is indicative for the small fraction of open zeolitic channels (exempted from intra-grown carbon nanotubes) and shows that the substitution of Al by Co in the zeolite framework is highly beneficial for the formation of CNTs inside the channels. Indeed in a recent study, ultra-small SWCNTs have been synthesized inside the channels of AlPO4-5 single crystals by pyrolyzing tripropylamine (Pr3N) molecules incorporated in the channels as organic precursor/templates during the synthesis of the AlPO4-5 crystal 26. When Al3+ is replaced by divalent cations such as Co2+, the framework is negatively charged and Brønsted acid sites are generated. Zhai et al.

48

performed

substitutions adding Co2+ atoms on the formation of 0.4 nm SWNTs in the channels of single



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CoAPO-5 crystals; the formation energies of SWCNTs were significantly lower than those of AlPO4-5 crystals, indicating that the growth of carbon nanotubes is more favourable in the CoAPO-5 crystals. The negatively-charged framework produced by the Co2+ substitution (of Al3+) enhanced the adsorption forces of the channel walls on the amine molecules. Owing to the Brønsted sites and negatively-charged framework, the conversion of the organic precursor Pr3N to carbon nanotubes during the pyrolysis process was more complete in CoAPO-5 crystals than that in the AlPO4-5 crystals. In an effort to further evaluate the relative contribution of the flux through the open zeolitic channels and the CNT-filled channels respectively on the overall flux through the membrane, we further examined our results for each gas in relation to the expected behaviour if the entire number of the AlPO4-5 channels were filled with SWCNTs having an open core diameter of 4.2 Å. In the later case, micropore diffusion in the CNTs should be considered as the predominant mechanism of gas transport. Gas-pore wall interactions determine diffusion mechanisms and permeation properties of microporous and especially molecular sieving membranes. Therefore, in order to elucidate transport of the employed probe gases in the AlPO4-5 channels (either open or filled with SWCNTs), a brief description is provided on the dependence of the potential energy inside micropores on the relation between pore width and dimensions of the diffusing molecules. The potential energy of gas molecules inside micropores may increase or decline depending on the variation of the δ ratio (= dp/dk), thus leading to a respective reduction or enhancement of the mobility of diffusing molecules. Table 3 presents the calculated δ ratios for all gases permeating through the AlPO4-5/PS and CoAPO-5/PS/CNTs composite membranes with respect to two characteristic limits of δ ratio. The first characteristic δ value is 1.24 (i.e. dp = 1.24×dk), which is related to the onset of the potential fields unification 49. The second δ ratio limit is 1.086×dk, which corresponds to the maximum of the potential well depth


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that provides the maximum qst limit and thus, the lowest molecular mobility. As δ ratio decreases over the range of 1.24×dk – 1.086×dk, the deepening of the potential well results in continuous enhancement of qst. Pore diameters smaller than 1.086×dk (or even dp ≤ dk) are associated with stronger and dominating repulsive interactions between gas molecules and pore walls, thus reducing the intra-pore potential energy and decreasing qst. As a result, an energy barrier for diffusion appears and the diffusivity increases rapidly with rising temperature (activated gas transport). On the other hand, when δ > 1.24, the potential energy curve shows a minimum near the pore walls and gas transport can take place by the activated Knudsen diffusion mechanism. The surface diffusion component should also contribute to the total gas transport and in accordance, Jt = JS + JKa, where Jt, JS and JKa denote the total gas phase flux, surface flux and activated Knudsen flux respectively

50

. At low and moderate temperatures (RT ≤ ∆Ha),

activated Knudsen diffusion of heavier gases can be pressure dependent according to Eq.(S7). Since JS is also enhanced with increasing pore surface coverage, the total permeance, Qt becomes higher with increasing pressure gradient. As shown in Fig. 8, the permeances of all probe gases through the CoAPO-5/PS/CNTs membrane exhibit a decreasing tendency with increasing temperature, which is common for all pressure gradients applied. Therefore, adsorption effects on the channels have obviously a considerable contribution on the gas transport through the CoAPO-5/PS/CNTs composite membrane, as in the case of the AlPO4-5/PS membrane. However, it is mentioned that considering that the internal (open core) diameter of the intra-pore grown SWCNTs (~4 Å) is comparable to the kinetic diameter of the employed probe gases (ranging between 2.6 Å (He) and 3.8 Å (CH4)), the notion of surface diffusion inside the CNTs-accommodating channels is not strictly valid, since there is no more clear distinction between “adsorbed phase” and “gaseous” phase.



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As observed in Table 3, the calculated values of dp/dk in the AlPO4-5/PS membrane suggest that combined surface and activated Knudsen diffusion mechanisms should account for the permeation behaviour of the studied gases, which for the applied experimental conditions is described by Eqs. (S3) and (S7). On the other hand, according to the calculated δ ratios, the permeation behaviour of H2, CO2, O2, N2 and CH4 gases through the CoAPO5/PS/CNTs membrane should be most suitably described by Eqs.(S9) and (S10), which refer to a merging of surface and activated transport processes. However, the observed results for these gases are rather indicative of the surface and activated Knudsen diffusion mechanism since their total permeance is depended on the pressure gradient and increases with decreasing temperature and not inversely to the kinetic diameter of the probe gases. This behavior is in opposition to the behavior of molecular sieving membranes, which is the anticipated one for a membrane having pore size of ∼4 Å. Fig. 9 depicts the single gas permeance results of the employed probe gases with relation to their kinetic diameter for both studied membranes at all temperatures and for a pressure gradient of 2 bar. For both composite membranes, CO2 is the least permeable gas and the increasing order of CO2, O2, N2 and CH4 permeances follows the increasing order of their kinetic diameter, in contrast to the tendency expected for molecular sieves. The aforementioned discordance between the experimentally observed permeation behaviour of the CoAPO-5/PS/CNTs membrane and the expected molecular sieving character can be attributed to partial volatilization of the embedded amine template during the pyrolysis of the as-produced hybrid inorganic/organic material, thus leading to formation of a fraction of open channels. The fraction of open zeolitic channels would result in the divergence mentioned above, as these channels have a diameter of 7.3 Å that is 1.9 – 2.8 times the kinetic diameter of the probe gases and for those pores, surface or activated Knudsen diffusion rather than activated diffusion are the predominant gas transport mechanisms.


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As observed in Fig. 9, the permeation rates through the CoAPO-5/PS/CNTs membrane were substantially higher compared to the respective ones through the AlPO4-5 membrane for all probe gases at the temperatures of 35, 50 and 80oC, despite the fact that the existence of the CNTs reduces the pore diameter. The increase in permeance for all studied gases is presented in Fig. 10. In particular, a remarkable enhancement (up to 14 times) was determined for CO2 permeance. This increase cannot be correlated with parameters such as molar mass, kinetic diameter and Lennard-Jones force constant (ε/k). In essence, it is likely to be indicative of fast diffusion through the SWCNTs-bearing zeolitic channels. The contribution of activated diffusion arising from repulsive interactions inside the narrow SWCNTs, which reduce qst and enhance molecular mobility, in addition to the smooth surface of the SWCNTs could impose a significant increase of total permeation rates, especially at higher temperatures.

3.3.4. CO2 separation efficiency of the developed composite membranes Regarding the CO2 separation efficiency of the AlPO4-5/PS composite membrane, H2/CO2, N2/CO2 and CH4/CO2 ideal selectivity reached to 9.91, 2.45 and 3.31 respectively at 80oC. The ideal selectivities for these gas pairs in the CoAPO-5/PS/CNTs membrane were 3.88, 1.45 and 1.97 respectively at 80oC and ∆P = 2 bar. However, with decreasing transmembrane pressure, the aforementioned ideal selectivities increased (up to 7.47, 2.28 and 2.87 respectively at ∆P = 0.5 bar and the same temperature) since H2 and CH4 permeances exhibited negligible reduction whereas for CO2 this reduction was larger. The H2 and CO2 permeances as well as the calculated H2/CO2 ideal selectivities for all temperatures and applied pressure gradients are listed in Table S5 in Supporting Information. Fig. 11 depicts the H2/CO2 ideal selectivity vs. H2 permeance results at 80oC, for the two studied membranes, with regard to respective results for a variety of zeolitic membrane types, reported by Guan et al. 41, Weh et al. 42, R. Roque‐Malherbe et al. 51, Gu et al. 52, Li et al. 53, Yin et al. 54, Tomita et al. 55, Hong



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et al.

56

, Lai et al.

Kanezashi et al.

57

, Coronas et al.

58

, Bakker et al.

59

, Yan et al.

Page 20 of 46

60

, Huang et al.

61

, and

62

. As shown in Fig. 11, the developed CoAPO-5/PS/CNTs membrane

achieved competitive H2/CO2 separation performance in comparison with the majority of the zeolitic membrane types mentioned therein. In specific, the membrane exhibited adequate H2/CO2 separation efficiency (up to 7.47 at 80oC and ∆P = 0.5 bar) combined with relatively high H2 permeation rates.

4. Conclusions

In this work, composite zeolitic membranes incorporating for the first time CNTs of sub-nanometer thickness were fabricated using c-oriented AlPO films as templates. AlPO4-5 and CoAPO-5 membranes were grown via the seeding technique. The zeolite seeds were deposited onto a-alumina supports, functionalized using a silane coupling agent, and attached on the support through an attachment-by-sonication procedure. In order to complete film formation, the seeded supports were subjected to a hydrothermal secondary growth using a proper precursor solution. Transformation of the occluded amine into SWCNTs inside the AFI CoAPO-5 pores was accomplished by pyrolysis at 750°C for 1 hour. Finally, both AlPO4-5 and CoAPO-5 membranes were coated with a polystyrene film to plug the intercrystalline gaps. For both composite membranes, SEM and single-gas permeance results indicated that the inter-crystalline gaps had been completely occluded by PS dense plugs. It was also inferred that gas transport was controlled by the zeolite (or CNTs in the case of the hybrid membrane) channels. Therefore, while polymer managed to fill inter-crystal gaps of the zeolite film due to its high adhesion with the zeolite material, it didn’t control diffusion of the


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probe gases, which was found to take place through the uni-dimensional pore system of the membranes. At all applied temperatures and pressure gradients, the two membranes exhibited the same increasing order of permeation rates, i.e. H2 > He > CH4 > N2 > O2 > CO2. For the CoAPO-5/PS/CNTs membrane the permeance of the employed gases was found to be pressure dependent and did not decline with increasing gas kinetic diameter. The integrity of the membranes was confirmed by permeability experiments using TIPB vapour, which exhibits a diameter larger than the AFI pores, during which no evidence of TIPB vapour was found in the permeate side. For all employed gases, the permeances through the CoAPO-5/PS/CNTs membrane were substantially increased compared to the respective ones determined for the AlPO4-5/PS membrane (up to 14 times for CO2) at the same temperatures, despite the decrease in pore diameter due to CNT growth. The observed increase in permeance might evidence fast transport of gases through the SWCNT-filled zeolitic channels. The contribution of repulsive potentials, which reduce adsorption strength and enhance diffusivity, in addition to the smooth surface of the SWCNTs, possibly imposed the significant increase in the gas permeation rates. Hydrogen was the most permeable gas over the entire temperature range. It was proven that templated CNT growth in appropriately oriented zeolite films can produce composite membranes accommodating SWCNTs of sub-nanometer diameter that are uniform in size, following the morphology, monodispersity, and density of the mother zeolitic channels. The nanotube size can be controlled by varying the AlPO framework, considering the available variety of AlPO frameworks having one-dimensional straight pores with nominal sizes ranging from 0.5 to 1.2 nm. Enhancing the CNT yield (e.g. by increasing metal content of the framework and experimenting with different SDAs with varying carbon content), and



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optimizing thickness, orientation, and intergrowth of the zeolite film (ongoing work in our lab) can lead to membranes exhibiting high selectivity and high transport rates, thus paving the way towards the next breakthrough in membrane technology with respect to CO2, hydrocarbon, hydrogen and other gas separations.

Supporting Information Available: Theoretical background on gas transport through membranes, determination of gas permeance using the cross-flow and Wicke-Kallenbach techniques, experimental procedure for conduction of mixed-gas permeance measurements, monitoring of CO2 permeance at 80oC and several pressure gradients, liquid N2 porosimetry at 77 K, investigation of conformity of the permeation data to the Knudsen diffusion type, ideal selectivity values for the AlPO4-5/PS/CNTs membrane calculated for the gas pairs of SF6 with He, H2, CO2, O2, N2 and CH4 gases, H2/CO2 ideal selectivity and respective permeation rates, and SWCNT growth without external carbon precursor gas. This information is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgments This work was financially supported by: (1) the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement of the IOLICAP (n° 283077) project, (2) by the Greek Ministry of Education, the Greek Secretariat of Research and Technology and the European Social Fund (ESF) through the project “Desalination by Solar Powered Membrane Distillation: Material and Process Optimization”- (SolMed) funded under the Activity “ARISTEIA II” of the Operational Programme (OP) “Education and Lifelong Learning”, and (3) by the Petroleum Institute RIF grant #15314.


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(52)

Gu, X.; Tang, Z.; Dong, J. On-stream modification of MFI zeolite membranes

for enhancing hydrogen separation at high temperature. Microporous Mesoporous Mater. 2008, 111, 441. (53)

Li, Y.; Liang, F.; Bux, H.; Yang, W.; Caro, J. r. Zeolitic imidazolate

framework ZIF-7 based molecular sieve membrane for hydrogen separation. J. Membr. Sci. 2010, 354, 48. (54)

Yin, X.; Zhu, G.; Wang, Z.; Yue, N.; Qiu, S. Zeolite P/NaX composite

membrane for gas separation. Microporous Mesoporous Mater. 2007, 105, 156. (55)

Tomita, T.; Nakayama, K.; Sakai, H. Gas separation characteristics of DDR

type zeolite membrane. Microporous Mesoporous Mater. 2004, 68, 71. (56)

Hong, M.; Falconer, J. L.; Noble, R. D. Modification of Zeolite Membranes

for H2 Separation by Catalytic Cracking of Methyldiethoxysilane. Ind. Eng. Chem. Res. 2005, 44, 4035. (57)

Lai, R.; Gavalas, G. R. ZSM-5 membrane synthesis with organic-free

mixtures. Microporous Mesoporous Mater. 2000, 38, 239. (58)

Coronas, J.; Falconer, J. L.; Noble, R. D. Characterization and permeation

properties of ZSM-5 tubular membranes. AlChE J. 1997, 43, 1797. (59)

Bakker, W. J. W.; Kapteijn, F.; Poppe, J.; Moulijn, J. A. Permeation

characteristics of a metal-supported silicalite-1 zeolite membrane. J. Membr. Sci. 1996, 117, 57. (60)

Yan, Y.; Davis, M. E.; Gavalas, G. R. Preparation of Zeolite ZSM-5

Membranes by In-Situ Crystallization on Porous a-Al2O3. Ind. Eng. Chem. Res. 1995, 34, 1652.



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Huang, A.; Liang, F.; Steinbach, F.; Gesing, T. M.; Caro, J. Neutral and

Cation-Free LTA-Type Aluminophosphate (AlPO4) Molecular Sieve Membrane with High Hydrogen Permselectivity. J. Am. Chem. Soc. 2010, 132, 2140. (62)

Kanezashi, M.; O'Brien-Abraham, J.; Lin, Y. S.; Suzuki, K. Gas permeation

through DDR-type zeolite membranes at high temperatures. AlChE J. 2008, 54, 1478. (63)

Hirschfelder, J. O.; Curtiss, C. F.; Bird, R. B., Molecular theory of gases and

liquids. Wiley: New York, 1954. (64)

Breck, D. W., Zeolite Molecular Sieves; Structure, Chemistry and Use. Wiley:

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Shelekhin, A. B.; Dixon, A. G.; Ma, Y. H. Theory of gas diffusion and

permeation in inorganic molecular-sieve membranes. AlChE J. 1995, 41, 58. (66)

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Buckingham, A. D.; Disch, R. L.; Dunmur, D. A. Quadrupole moments of

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Flygare, W. H.; Benson, R. C. The molecular Zeeman effect in diamagnetic

molecules and the determination of molecular magnetic moments (g values), magnetic susceptibilities, and molecular quadrupole moments. Mol. Phys. 1971, 20, 225. (69)

Stogryn, D. E.; Stogryn, A. P. Molecular multipole moments. Mol. Phys.

1966, 11, 371.


Page 31 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tables:

Table 1. Physical properties of the gases used for single gas permeance measurements. M: molar mass, dk: kinetic diameter, σm and ε/k : parameters of the Lennard-Jones potential, µe: dipole moment (0 Debye) and Qm: quadrupole moment.

Μ

σm63

dk64

ε/k65-66

[Qm × 10 2 6 ] 6 7 -6 9

[g⋅mol-1]

[Å]

[Å]

[K]

[esu⋅cm2]

He

4

2.551

2.60

10.22

0

H2

2

2.827

2.89

59.7

0.637

44.01

3.941

3.30

195.2

4.3

O2

32

3.467

3.46

118.0

0.4

N2

28.01

3.798

3.64

95.05

1.4

CH4

16.04

3.758

3.80

148.2

0

Gas

CO2

Table 2. SF6 single gas permeance (Q) through the CoAPO-5/PS/CNTs membrane with respect to the temperature (T) and trans-membrane pressure (∆P).

T

∆P x 105

Q x 10-8

(oC)

(Pa)

(mol⋅m-2⋅s-1⋅Pa-1)

35

2.903

2.07

50

3.019

1.86

60

3.121

1.74

80

3.249

1.72



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 46

Table 3. Calculated values of δ ratio for all gases in the zeolitic layer of the AlPO4-5/PS and CoAPO-5/PS/CNTs hybrid membranes. The diameter of the zeolitic channels is taken as 7.3 Å and 4.2 Å respectively for the two membranes.

Probe gas He

H2

CO2

O2

N2

CH4

d k (Å)

2.60

2.89

3.30

3.46

3.64

3.80

dp/d k (1) (-)

2.808

2.526

2.212

2.110

2.005

1.921

dp/d k (2) (-)

1.615

1.453

1.273

1.214

1.154

1.105

(1) AlPO4-5/PS (2) AlPO4-5/PS/CNTs


Page 33 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions: Fig. 1: (a) SEM image of the as grown AlPO4-5 film on a-alumina substrate. (b) XRD pattern of the columnar AFI membrane upon calcination - * indicates peaks from the a-alumina substrate. (c) SEM image of AlPO4-5 film after PS coating. (d) SEM image of the CoAPO5/SWCNT film. (e) SEM image the CoAPO-5/SWCNT membrane after PS coating. Fig. 2: (a) XRD pattern, and (b) unpolarized Raman spectra of the CoAPO-5/SWCNT hybrid crystals. The inset is a high–resolution TEM image of the parallel channels of the AlPO framework with diameter of 7.3 Å, and a simulated representation of a nanotube formed inside a channel. Fig. 3: Temperature dependence of permeance of the employed probe gases through the AlPO4-5/PS membrane. Fig. 4: Supply of mixtures of N2 with TIPB or n-Heptane vapours: (a) N2 permeance evolution during the vapours supply; the secondary axis shows N2 permeance when the N2/TIPB mixture is supplied, (b) N2 normalized permeance as a function of the elapsed time of vapours supply. Fig. 5: n-Heptane permeance as a function of the paraffin vapours supply duration. Fig. 6: Dependence of permeance on trans-membrane pressure for (a) He, (b) H2, (c) CO2, (d) O2, (e) N2 and (f) CH4 gases in CoAPO-5/PS/CNTs membrane at the temperatures of 35, 50, 60 and 80oC. Fig. 7: Permeance of He, H2, CO2, O2, N2 and CH4 gases through the CoAPO-5/PS/CNTs membrane vs. temperature at a trans-membrane pressure of (a) 0.5 bar, (b) 1 bar, (c) 1.5 bar and (d) 2 bar. Fig. 8: Long-term repeatability and average permeance values of He, H2, CO2 and N2 gases through the CoAPO-5/PS/CNTs membrane vs. trans-membrane pressure at 80oC. Fig. 9: Correlation between permeance and kinetic diameter of the probe molecules for the membranes (a) AlPO4-5/PS and (b) CoAPO-5/PS/CNTs at ∆P = 2 bar. 33 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 46

Fig. 10: Enhancement of permeance of the studied probe gases through the CoAPO5/PS/CNTs membrane with respect to the AlPO4-5/PS membrane at ∆P = 2 bar. Fig. 11: H2/CO2 ideal selectivity vs. H2 permeance results (at 80oC) for the two studied membranes, with regard to respective results reported in literature for several zeolitic membranes.


Page 35 of 46

Figures:

Fig. 1 (a)

1 μm

7000

(b)

6000

5000

Intensity, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000

* * *

3000

2000

1000

0 10

20

30

40

50

2-theta degrees

(c)

10 μm

(d)

10 μm

(e)

10 μm



Fig. 2

8000

(a)

SWCNT

AFI channels

6000

Intensity a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 46

4000

2000

0 10

20

30

40

50

60

2-theta degrees

(b) G 1598

D 1278

G-

RBM 514

542

1377 1424

598 (4,2) SWNT phonon modes


Page 37 of 46

Fig. 3

5.50E-08 He H2 CO2 O2 N2 CH4

4.90E-08

Permeance (molm-2s-1Pa-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4.30E-08 3.70E-08 3.10E-08 2.50E-08 1.90E-08 1.30E-08 7.00E-09 1.00E-09 20

30

40

50

60

70

80

90

Temperature (oC)



Fig. 4

2,4E-08 N2/n-C7H16

2,2E-08

N2/TIPB

1,35E-08

2E-08 1,1E-08

1,8E-08 1,6E-08

8,5E-09 1,4E-08 6E-09

1,2E-08 1E-08

3,5E-09 8E-09 1E-09

N2 Permeance (mol·m-2·s-1·Pa-1)

1,6E-08

N2 Permeance (mol·m-2·s-1·Pa-1)

6E-09 0

400

800

1200

1600

2000

2400

Time (min)

(a)

1,8 1,6

N2 Normalized permeance [-]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 46

1,4 1,2 1 0,8 0,6

N2/TIPB

0,4

N2/n-C7H16

0,2 0 0

400

800

1200

1600

2000

2400

Time (min)

(b)


Page 39 of 46

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 5



Fig. 6

3.0E-07

35oC 50oC 60oC 80oC

1.94E-07 1.90E-07

Permeance (mol⋅m-2⋅s-1⋅Pa-1)


1.98E-07

1.86E-07 1.82E-07 1.78E-07 1.74E-07 1.70E-07 500

1000

1500

2000

2.9E-07 2.8E-07 2.7E-07 2.6E-07 2.5E-07

2500

Pressure (mbar)

7.8E-08 7.4E-08 7.0E-08 6.6E-08 6.2E-08 5.8E-08 5.4E-08 5.0E-08 4.6E-08 4.2E-08 3.8E-08 3.4E-08 3.0E-08

0

500

(a) 9.2E-08

35oC 50oC 60oC 80oC

1000

1500

2000

2500

Pressure (mbar)

(b)

35oC 50oC 60oC 80oC

8.8E-08



35oC 50oC 60oC 80oC

2.4E-07

0

8.4E-08 8.0E-08 7.6E-08 7.2E-08 6.8E-08 6.4E-08 6.0E-08

0

500

1000

1500

2000

1.04E-07

0

2500

Pressure (mbar)

9.60E-08

1000

1500

2000

2500

Pressure (mbar)

1.35E-07

(d)

35oC 50oC 60oC 80oC

1.30E-07


1.00E-07

500

(c)

35oC 50oC 60oC 80oC


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 46

1.25E-07 1.20E-07

9.20E-08

1.15E-07

8.80E-08

1.10E-07

8.40E-08

1.05E-07

8.00E-08

1.00E-07

7.60E-08

9.50E-08 9.00E-08

7.20E-08 0

500

1000

1500

Pressure (mbar)

2000

2500

0

(e)

500

1000

1500

2000

Pressure (mbar)

(f)


2500

Page 41 of 46

Fig. 7

3,2E-07 He average

2,8E-07

Permeance (mol·m-2·s-1·Pa-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2,4E-07 H2 average

2,0E-07 1,6E-07

CO2 average

1,2E-07 8,2E-08

N2 average

4,2E-08 2,0E-09 0

500

1000

1500

2000

2500

Pressure (mbar)



Fig. 8

3.6E-07

3.6E-07 He

He

1 bar

3.1E-07

H2

2.6E-07

CO2



0.5 bar

CH4 2.1E-07 1.6E-07 1.1E-07

H2

3.1E-07

CO2 2.6E-07

O2 N2

2.1E-07

CH4 1.6E-07 1.1E-07 6.0E-08

6.0E-08

1.0E-08

1.0E-08 20

30

40

50

60

70

80

90

100

20

30

40

Temperature (oC)

50

60

70

80

90

100

Temperature (oC)

(a)

(b)

3.6E-07

3.6E-07 He

1.5 bar 3.1E-07

H2 CO2

2.6E-07 O2 2.1E-07

N2 CH4

1.6E-07

He

2 bar Permeance (mol⋅m-2⋅s-1⋅Pa-1)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 46

1.1E-07

H2

3.1E-07

CO2 2.6E-07

O2 N2

2.1E-07

CH4 1.6E-07 1.1E-07 6.0E-08

6.0E-08

1.0E-08

1.0E-08 20

30

40

50

60

70

80

90

100

20

30

Temperature (oC) (c)

40

50

60

70

80

Temperature (oC)

100

(d)


90

Page 43 of 46

Fig. 9 (a)

He

CO2 O2

H2

5,5E-08

N2 CH4

Permeance (molm-2s-1Pa-1)

4,9E-08 35oC 50oC 80oC

4,3E-08 3,7E-08 3,1E-08 2,5E-08 1,9E-08 1,3E-08 7,0E-09 1,0E-09 2

2,2

2,4

2,6

2,8

3

3,2

3,4

3,6

3,8

4

4,2

4,4

Kinetic diameter (Å)

(b)

He 3,0E-07

Permance (mol⋅⋅ m-2⋅ s-1⋅ Pa-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CO2 O2

H2

N2 CH4

35oC

2,5E-07

50oC 60oC

2,0E-07

80oC

1,5E-07

1,0E-07

5,1E-08

1,0E-09 2

2,2

2,4

2,6

2,8

3

3,2

3,4

3,6

3,8

4

4,2

4,4

Kinetic diameter (Å)



Fig. 10 (a)

Permeance ratio (-)

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

He H2 CO2

CO2 H2 35

He

50 80

(b)

Permeance ratio (-)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 46

14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

O2 N2 CH4

CH4 N2 35

O2 50 80


Page 45 of 46

Fig. 11

100

Roque-Malherbe et al. Gu et al. Li et al. Guan et al.

Ideal selectivity H2/CO 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Yin et al.

10

Tomita et al. Hong et al. Lai et al. Bakker et al. Yan et al.

1

Huang et al. Kanezashi et al. Weh et al. AlPO4-5/PS CoAPO-5/PS/CNTs

0,1

1,0E-08 5,1E-07 1,0E-06 1,5E-06 2,0E-06 2,5E-06 3,0E-06 3,5E-06

Permeance H2 (mol⋅⋅ m-2⋅ s-1⋅ Pa-1)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 46

Table of Contents Image:

SWCNT AFI channels

Microporous CNT membrane


Carbon Nanotube Selective Membranes with Subnanometer

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