N2

Subscriber access provided by Olson Library | Northern Michigan University

Article

Influence of Cross-Linking, Temperature and Humidity on CO2/N2 Separation Performance of PDMS Coated Zeolite Membranes Grown within a Porous Polyethersulfone Polymer Bo Wang, and Prabir K Dutta Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Industrial & Engineering Chemistry Research is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 52

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

Industrial & Engineering Chemistry Research

Influence of Cross-Linking, Temperature and Humidity on CO2/N2 Separation Performance of PDMS Coated Zeolite Membranes Grown within a Porous Polyethersulfone Polymer

Bo Wang* and Prabir K. Dutta*

Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210

*Author to whom correspondence should be addressed ([email protected] and [email protected])

1

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

Abstract Studies of zeolite membranes for gas separations, including natural gas purification, biogas purifications and CO2/N2 separation are well documented. In this paper, using CO2/N2 separation as a model system, we examine a series of zeolite (faujasite)/polymer composite membranes. These zeolite membranes were prepared by growing a continuous zeolite layer within a porous polyethersulfone (PES) support followed by a polydimethylsiloxane (PDMS) coating layer to fix defects in the zeolite/PES layer. The effect of PDMS crosslinking, humidity in the feed gas (46 ppm) and temperature on the transport properties were studied. The separation performance was significantly improved with membranes that used cross-linked PDMS versus uncross-linked PDMS. At room temperature, the introduction of humidity completely suppressed CO2/N2 separation. With higher temperatures, the effect of humidity on CO2/N2 separation performance was alleviated to some extent. Exposure to humidity at high temperature led to poor recovery of transport properties upon reexamination of the same membrane with dry gases at room temperature. An improvement in recovery was observed if the zeolite membrane surface was made hydrophobic via ion-exchange with hexadecylamine. This suggests that water can be trapped at the hydrophobic-hydrophilic PDMS-zeolite interface; making the surface hydrophobic assists in removal of the trapped interfacial water. Practical applications of such membranes are limited to dry gas separations. Keywords Zeolite Membrane; Faujasite; Carbon Dioxide; Separation; Humidity 2


Page 2 of 52

Page 3 of 52

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


1. Introduction Membranes are an important technology for gas separations including CO2/N2 separation, natural gas purification, biogas purifications and other industrial separation processes.1,2 Polymer membranes, in particular, find extensive commercial applications. Several strategies are being investigated for CO2/N2 separation using polymers.3 One of the issues with polymer membranes is the observation that higher permeance results in lower selectivity (Robeson upper bound).4 An alternate membrane technology based on inorganic aluminosilicate zeolites does not have the permeance/selectivity relation limitation of polymers. Instead, molecular simulation studies suggest that very high selectivity and permeance is possible with zeolites.5 However, there are other issues with zeolite membranes. They are typically grown on supports, such as alumina, steel, with alumina being the most common. Widespread use of zeolite membranes are limited because of their high cost, stemming from the low reproducibility in manufacture of defect-free membranes.1 Zeolite A on alumina support used for ethanol water separation is the only commercial application.6 Most studies of CO2/N2 separation have focused on faujasite (zeolite X/Y) membranes, which exhibit permeance of 300-3000 GPU, with CO2/N2 selectivity from 2 to 100.7–13 With ultrathin membranes, selectivity of 550 has been reported, but with very low permeance of 8 GPU.11 The variation in the transport characteristics arise from defects in the zeolite membrane. Level of humidity has been reported to have significant impact on separation performance of zeolite.13 The influence of water is manifested via competitive adsorption 3



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

with the gas being separated, and is controlled by the water quantity, the hydrophilicity of the zeolite framework, and the temperature. For example, CO2/N2 separation through zeolite membrane is realized by preferred adsorption of CO2 on zeolite surface due to its higher quadrupole moment and subsequent diffusion through zeolite pores.8 N2 loading within the zeolite is insignificant as compared to CO2 loading. This is manifested in considerable increase in CO2/N2 selectivity for mixed CO2/N2 gases as compared to pure gas transport, with the permeance of N2 exhibiting the primary decrease.8 Therefore, in zeolite membranes, for mixed gas transport, CO2 permeance is through the zeolite pores while N2 permeance is mainly through defects. Thus, if water is adsorbed within the pores of zeolite, CO2 permeance will be low.14 In AlPO4 membranes, CO2 permeance dropped by 55% at 37 °C in humid condition.15 For low Si/Al zeolite A membrane, H2O in feed gas is condensed in both zeolitic micropores and intracrystalline mesopores. CO2 permeance has also been reported to improve under humid conditions, because of the CO2-H2O affinity.16 With humidity, the drop in CO2 transport through zeolite Y membrane (Si/Al 1.71.8) has been noted.13 Under dry conditions at room temperature, CO2 permeance was 59 GPU and CO2/N2 selectivity was 31.2. With increasing temperature, the CO2/N2 selectivity decreases,8,13 eventually approaching that predicted by Knudsen diffusion at 200oC.12 In presence of water vapor with absolute humidity of 0.031 at 25 °C, both CO2 and N2 permeance is significantly lower and CO2/N2 selectivity approached 1.13 As temperature increased, the CO2 permeance difference between dry and wet systems became smaller, mainly attributed to the reduced water adsorption within the zeolite. 4


Page 4 of 52

Page 5 of 52

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


With temperatures higher than 100 °C, CO2/N2 selectivity with humid feed gas actually becomes higher than dry condition.13 There is lower impact of H2O on hydrophobic zeolite membranes.17 For high Si/Al zeolite membranes like MFI, low hydrophilicity and high hydrothermal stability make it potentially competitive for moisture containing gas separations.18 For pure silica DDR zeolites, moisture only has negligible impact on CO2 separation properties at 100 °C.19,20 Water adsorption in zeolite is reversed by heating to high temperatures (400oC). Zeolite-containing polymer membranes, such as mixed matrix membranes (MMM), in which zeolite powder is dispersed within a polymer are an active area of research.21–27 A polymer-zeolite interface is common in mixed matrix membranes (MMM). Moore et al. reported that presence of humidity results in reduction of gas permeance due to water condensation in zeolite pores for a zeolite A-PVAc MMM for O2 separation.28 We have recently reported on a continuous zeolite membrane (ZM) formed within a porous polyether sulfone (PES) support. These ZM/PES membranes have some flexibility, and the faujasite-based membranes were made rapidly, and even in a semicontinuous roll to roll platform, with reproducible transport properties.29–31 For the as-synthesized zeolite membranes grown within the PES, no CO2/N2 gas separation is observed, presumably due to intercrystalline defects. Defect elimination is critical for obtaining gas separation. PDMS coating has been reported to be an effective strategy of sealing defects, and worked satisfactorily on the ZM/PES membranes.29,32,33 Single and mixed gas studies with CO2/N2 were also reported, and there was a factor of two decrease 5



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

in N2 permeance on going from pure N2 gas to 40 : 60 CO2 : N2, while the CO2 permeance changed by 14%.29 These observations are comparable to zeolite/alumina membranes, where the permeance of N2 decreased by a factor of ~3 with the mixed gas (equimolar), while the CO2 permeance changed by ~5%.8 We have also recently reported on the mechanical properties of these PDMS/ZM/PES membranes.34 The elastic modulus of the zeolite membranes grown within the PES (the membranes examined in the present study) was 5.0 GPa, whereas if the zeolite was grown on top of the PES, the elastic modulus was 21.9 GPa, indicating higher flexibility for the ZM/PES membranes. These previous studies dealt with dry CO2/N2 gas, mostly carried out at room temperature. The PDMS was made according to the vendor specifications. In this study, using CO2/N2 separation as a model system, we examine the impact of three issues on transport properties, 1) cross-linking of PDMS, 2) influence of temperature and 3) humidity in the feed gas using the PDMS/ZM/PES membranes. Eight different membrane samples of PDMS/PES, PDMS/nanozeolite/PES, and the ZM/PES with PDMS with different extents of cross-linking were studied. As expected from previous studies with hydrophilic zeolites, there is a negative influence of humidity on the separation,13 a feature that is temperature dependent. Presence of water also alters the membrane such that the original transport properties of the as-synthesized sample is not completely recovered after exposure to humidity. The best samples from the recovery aspect were in which hydrophobicity was imparted to the hydrophilic zeolite by introducing long-chain alkyl surface groups.

6


Page 6 of 52

Page 7 of 52

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. Experimental Procedure 2.1. Chemicals Ludox SM-30 colloidal silica (SiO2, 30%), Ludox HS-30 colloidal silica (SiO2, 30%), tetramethylammonium bromide ((CH3)4NBr, 98%) and aluminum isopropoxide (Al(O-CH(CH3)2)3, 98%) were purchased from Sigma-Aldrich (Milwaukee, WI, USA). Tetramethylammonium hydroxide ((TMAOH), 25% aqueous solution) was purchased from SACHEM Inc. Aluminum hydroxide (Al(OH)3, 76.5%) was purchased from Alfa Aesar. Sodium hydroxide (NaOH, 99.0%) was purchased from Fisher Scientific. Dehesive 944 Polydimethylsiloxane (PDMS) was provided by Wacker Silicones, Inc. Helium (4.5 grade), carbon dioxide (4.0 grade) and nitrogen (4.5 grade) were purchased from Praxair. Polyethersulfone (PES) 300kDa membrane was purchased from MILLIPORE Biomax. H2O used in this study was purified by a Millipore ultrapure water system. All chemicals were used as received without further purification.

2.2. Zeolite Membrane Synthesis 2.2.1. Nanocrystalline Zeolite Y (NZ) Seeds Nanozeolite Y particles (size ~ 30 nm) were synthesized according to our previous work 31,35 with composition of 0.048 Na2O: 2.40 (TMA)2O(2OH): 1.2 (TMA)2O(2Br): 4.35 SiO2: 1.0 Al2O3: 249 H2O, where TMA+ is tetramethylammonium cations. Briefly, 26.2 g Ludox HS-30 and 10.46 g TMAOH were mixed in a sealed bottle and stirred at room temperature for 30 min. 12.5 g aluminum isopropoxide was dissolved in mixture of 76.5 g H2O and 52.3 g TMAOH solution, and heated in a water bath at 70 7



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

ºC until complete dissolution. After cooling to room temperature, 13.1 g TMABr was added to alumina solution followed by mixing with the silicon source. The clear sol was aged at room temperature with stirring for 3 days. Then, aged clear sol was heated in a dehydration-rehydration hydrothermal (DRHT) setup for a 29 hours process, involving water removal and Na+ addition (NaOH pellet dissolved in water). In DRHT setup, water vapor from the refluxing gel was collected in a constant-pressure funnel, which was then added back to reactor (as NaOH) by adjusting the switch on funnel. By adjusting water amount collected in funnel, synthesis solution composition was controlled. After synthesis, nanozeolite particles were captured by ultracentrifugation and washed until pH of supernatant was 7. After washing to pH 7, nanozeolite seed particles were stored in aqueous dispersion with concentration ~ 1 wt%. Detailed experimental procedure can be found in our previous work.31

2.2.2. Nanocrystalline Zeolite Y Seed Deposition As reported in our previous work, nanozeolite seeds (30 nm) were deposited on PES supports by vacuum assisted dip-coating.29,30,36 Before coating, PES supports were soaked in distilled water overnight and then in isopropanol for 1 hour before washing with water again. Nanozeolite dispersion was ultrasonicated for 1 hour and diluted with distilled water to the required concentration, 10 or 900 µg/mL. About 20 mL of nanozeolite suspension was placed in petri dish for deposition. The PES support was dipped in the petri dish for 3 seconds. A ~25 psi vacuum was applied on the back of the

8


Page 8 of 52

Page 9 of 52

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


PES support to pull the seeds onto the support. After coating, the support was dried at room temperature overnight and stored in plastic sample bags.

2.2.3. Zeolite Y Membrane (ZM) Synthesis ZM/PES were synthesized by secondary growth method.29,30,34,36,37 Gel for zeolite membrane growth has composition of 8.3 Na2O: 1 Al2O3: 6.4 SiO2: 483.9 H2O. After dissolving 4.416 g of Al(OH)3 and 14.58 g NaOH in 170.48 g H2O, 27.7 g Ludox SM-30 was added to the gel. Mixed gel was sealed in polypropylene bottle and aged at room temperature for 4 hours. Aged gel was then moved to a dehydration rehydration hydrothermal (DRHT) setup. In 1 hour of heating, 40 mL of water was evaporated, condensed and stored in constant pressure funnel. Then, nanozeolite seeds coated PES support was immersed in the concentrated gel in the reactor, with 40 mL condensed water dropped back to gel with constant rate in the second one hour while heating. Zeolite membranes were washed with flowing water to remove excess ions and molecules.

2.2.4. PDMS coating on Zeolite Y/PES Membranes ZM/PES was spin coated with PDMS (Dehesive 944 kit) before gas separation test.29,30,36 Commercial PDMS comes in 3 separate bottles, PDMS stock (vinyl terminated PDMS polymer, average molecular weight of 25,000); cross-linker (Si-H PDMS, average molecular weight of 2,100) and catalyst (the exact nature of these chemicals is proprietary, and the procedure followed was supplied by the vendor). First, vinyl terminated PDMS polymer was diluted to desired concentration, 2%, with heptane. After 9



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

complete dispersion, cross linker and catalyst were added with the ratio of 100:1:0.5 (PDMS: cross linker: catalyst) to get a precursor solution.38,39 Amount of catalyst was varied in subsequent experiments to change the degree of cross-linking, as specified below. In spin coating process, the PDMS precursor solution was dropped to cover entire membrane surface, and left for 3 seconds before spinning. Samples were spun at 2,000 rpm for 5 seconds followed by 4,000 rpm for 1 min. After coating, PDMS was allowed to form crosslinks by storing at room temperature for 12 hours. The size of the membrane was ~ 2 x2 inches and mounted in a cell. Figure S1 (Supporting Information) is a photograph of the membrane and the cell assembly.

2.2.5. HDA coating on Zeolite Y/PES Membranes Aqueous hexadecylamine (HDA) solution with pH 4 (protonated form) was prepared with 0.1M HCl (protonated form for solubility and ion-exchange) and spin coated on zeolite membranes prior to coating with PDMS.

2.3. Characterization A Bruker D8 X-Ray Diffractometer using CuKα (λ=1.5405 Å) radiation was used to characterize the crystallinity of zeolite membrane samples. To obtain SEM images, samples were taped on a SEM holder with conductive carbon paste and gold coated at 40 mA for 30 s. Electron beams with 30kV voltage and 41pA current were typically used to collect SEM images. A FEI Helios Nanolab 600 Dual Beam Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) was employed to capture both top view and side view 10


Page 10 of 52

Page 11 of 52

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


SEM images. Before N2 adsorption/desorption isotherm collection, zeolite samples were outgassed under vacuum at 400 °C for 24 hours. BET surface area and external surface area were obtained from the isotherm. BET surface area (calculation in p/p0 range of 00.05, which is linear range for zeolite samples). External surface area of zeolite materials were calculated with t-plot method, in the range of p/p0 0.25-0.6. Detailed calculations of different methods employed in this study can be found in the manual of Nova 2200e BET Surface Area Analyzer from Quantachrome. Fourier transform infrared spectrum (FTIR) was obtained from a PerkinElmer Spectrum 400 coupled with a 20-reflection PIKE HATR accessory. X-ray photoelectron spectrum (XPS) was collected with Axis Kratos X-Ray Photoelectron Spectrometer with monochromatized Al Kα source (12 kV, 10 mA) as the X-ray source. Contact angle results of zeolite pellets were captured with Drop Shape Analysis System from Kruss EasyDrop.

2.4. Measurements of Zeolite Degradation with Humidity Nanozeolite seed particles were used to examine the effect of humidity and CO2 at 100 °C on zeolite framework. Before measurements, nanozeolite samples were calcined at 550 °C for 24 hours to remove TMA+. Nanozeolite samples (~30 mg) was loaded in a quartz reactor, sealed by glass fiber on both ends and heated to 100 °C with 60 mL/min gas flow, (N2 + 46 ppm H2O) or (80% N2 + 20% CO2 + 46 ppm H2O) for 3 hours. Collected samples were characterized by N2 adsorption with Brunauer–Emmett– Teller (BET) theory calculation of surface area.

11



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

2.5. CO2/N2 Transport Measurement A gas separation setup was employed to characterize CO2/N2 separation performance of zeolite membrane samples. Gas flow compositions were controlled with a flow box and mass flow controllers from SIERRA Instruments Inc. Feed gas (1 atm pressure) and sweep gas have flow rates of 60 mL/min and 30 mL/min, respectively. Helium was chosen as the sweep gas based on previous studies on zeolite Y membranes, counter diffusion effects with He were not substantial to alter the single gas CO2 permeability in the zeolite, primarily due to the large pore size.7 Argon on the other hand decreased the CO2/N2 permeance, but the selectivity was unaltered. In this study, feed gas composition could be either “dry gas”, (80% N2 + 20% CO2), or “wet gas”, (80% N2 + 20% CO2 +46 ppm H2O). There were two N2 tanks connected on the setup: one was dry and the other was with water vapor concentration of 108 ppm. Water vapor concentration in feed gas was quantitatively controlled by changing the portion of wet N2 flow (108 ppm H2O tank) and dry N2 flow. Compositions of permeate and retentate gas were investigated with a SRI 310C gas chromatograph equipped with a fixed volume sampler, Hysep D column and TCD detector.

2.6 Sample definitions and characteristics 2.6.1

PDMS

The commercially obtained PDMS came as three independent chemicals: PDMS polymer (MW 25000), cross-linker (CL, MW 2100) and a proprietary catalyst. The vendor advises mixing the three reagents in the volume ratios of PDMS: CL: catalyst ≡ 12


Page 12 of 52

Page 13 of 52

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


100: 1: 0.5 to obtain the cross-linked PDMS polymer (which we abbreviate as CLPDMSA). The chemistry of the cross-linking is well-studied and shown in Figure 1.40 We have studied a series of PDMS polymers, as shown in Scheme 1. These include a) PDMS as supplied b) PDMS: CL ≡ 100: 1 (CL-PDMS) c) PDMS: CL: Catalyst ≡ 100: 1: 0.5 (CL-PDMSA) d) PDMS: CL: Catalyst ≡ 100: 1: 1.5 (CL-PDMSB) e) PDMS: CL: Catalyst ≡ 100: 1: 2.5 (CL-PDMSC). By altering the catalyst amount, we intended to alter the amount of cross-linking. 2.6.2

PES as support

Scheme 2 shows a summary of the samples using PES as support (asterisk shows those samples on which transport measurements were made). Figure 2a shows the top view SEM of the PES support which provides the mechanical support. PES has pores of < 100 nm dimensions, with a reported average diameter of ~ 70 nm, and total porosity of ~ 70% (information obtained from vendor). 2.6.3

PDMS coated PES

A layer of CL-PDMSA was deposited on PES. Figure 2b shows the side view SEM image of a CL-PDMSA/PES membrane, and we estimate a thickness of PDMS of ~1 µm. The PDMS is also expected to penetrate into the PES pores, though the depth of this penetration is unclear. 2.6.4

Nanozeolite coated PES

With a nanozeolite dispersion of 900 µg/mL, a ~ 1 µm layer of zeolite particles is formed on the PES, and coated with CL-PDMSA prior to transport measurement. The side view SEM is shown in Figure 2c. 13



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

2.6.5

Zeolite membrane within PES

Use of low NZ concentration for coating the PES (10 µg/mL), followed by hydrothermal growth results in a zeolite membrane formed within the PES (ZM/PES) as described in our earlier papers.29 Figure S2 shows the XRD of such a membrane, indicating the formation of a faujasite-like material (peaks from the PES support are marked with an asterisk). Upon dissolution of the PES by N-methylpyrrolidone, a continuous zeolite layer is observed in the low magnification SEM of Figure 2d. A ~3 µm zeolite stand-alone layer is observed (Figure 2e). The ZM/PES sample was coated with PDMS, PDMS + CL and CL-PDMSA,B,C prior to transport measurements. The side view SEM of CL-PDMSA/ZM/PES is shown in Figure 2f, with a PDMS layer thickness of ~700 nm.

3. Results and Discussion 3.1. Transport Measurements 3.1.1. Control Samples The control samples were cross-linked PDMS on PES and cross-linked PDMS on the 1 µm NZ coating on the PES. The experimental protocol for the CO2/N2 transport measurements involved examination of dry feed gas, followed by wet feed gas (46 ppm humidity) and switching back to dry feed gas. This process was repeated at different temperatures. Zeolites can tolerate high temperatures, and the two polymers PDMS and PES are stable in air up to temperatures of 180-200oC and beyond 300oC,

14


Page 14 of 52

Page 15 of 52

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


respectively.41,42 However, we did not observe any improvement in transport properties beyond 100oC.

3.1.2. CL-PDMSA/PES Transport data with CL-PDMSA on bare PES support is shown in Figure 3. CO2 permeance stays relatively unchanged at 1245- 1346 GPU (changes within 10% of each other are considered to be within error) in temperature range of 25 - 125 °C, while N2 permeance shows an increase (121, 152, 243 and 302 GPU at 25, 50, 100 and 125 °C, respectively). The CO2/N2 selectivity is 11.1, 8.6, 5.2 and 4.2 at 25, 50, 100 and 125 °C, respectively. CO2 permeance remains constant at temperatures from 25 to 125 °C (Figure 3a), due to decreasing CO2 solubility offsetting increased CO2 diffusivity in PDMS.43 The transport properties recover on lowering the temperature. The experiment was repeated with 46 ppm water in the gas stream. As shown in Figure 3b, at 25 °C, CO2 and N2 permeance are unchanged with humidity, with CO2/N2 selectivity of 11.3. With increased temperature, the data are similar to the dry gas conditions, and the membrane recovers completely on lowering the temperature to 25oC. Water vapor in feed gas has no influence on the CO2/N2 transport through CL-PDMSA/ PES.

3.1.3. CL-PDMSA/NZ/PES A 1 µm coating of nanozeolite was applied on the PES, followed by a coating of CL-PDMSA. Figure 4 shows the CO2/N2 separation data at 25 °C with the dry, wet (46 15



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

ppm H2O) and dry gas routine. Steady state CO2 and N2 permeance are in the ranges of 1187 - 1240 GPU and 87 - 94 GPU, respectively, with CO2/N2 selectivity between 13.2 and 13.7 (Figure 4a). With increase in temperature from 25 to 75 °C, CO2 permeance decreases from 1187 to 1021 GPU, N2 permeance increases from 90 to 116 GPU and CO2/N2 selectivity decreases from 13.2 to 8.8 (Figure 4b). The wet feed gas decreases the CO2 permeance slightly. After cooling the system to 25 °C, CO2 and N2 permeance are 1076 and 83 GPU, respectively, showing slight decrease, with CO2/N2 selectivity of 13.0, and we consider this as complete recovery. CO2/N2 transport results with CL-PDMSA/NZ/PES are comparable with CLPDMSA/PES. This indicates that gas permeation in CL-PDMSA/NZ/PES is taking place primarily through the PDMS, and the nanozeolite layer is having minimal effect.

3.1.4. ZM/PES The CO2/N2 transport data of an as-synthesized ZM/PES is shown in Figure 5. The CO2 and N2 permeance are in ranges of 2518 - 2610 GPU and 1998 - 2118 GPU with CO2/N2 selectivity of ~1.3, which remain unchanged with 46 ppm humidity at 25 °C or 75 °C. Low CO2/N2 selectivity indicates the existence of intercrystalline defects in ZM/PES that allows unimpeded flow of CO2 and N2.

3.1.5. CL-PDMSA/ZM/PES With CL-PDMSA/ZM/PES, CO2/N2 transport measurements were tracked with time. As shown in Figure 6, the initial CO2 and N2 permeance is ~ 300 and 18 GPU, 16


Page 16 of 52

Page 17 of 52

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


respectively, with a CO2/N2 selectivity of 12. With 2 hours of dry gas flow at 25 °C, CO2 and N2 permeance and CO2/N2 selectivity increase and reach a steady state. The CO2/N2 selectivity approaches ~ 37 and CO2 and N2 permeance reach 2093 and 57 GPU, respectively. Our explanation is that initially, zeolite pores are filled with water that is present from zeolite membrane growth. Increase of CO2/N2 selectivity with dry gases passing over time indicates drying out of the zeolite pores, similar to observations with zeolite/alumina membranes.8 Coating of a cross-linked PDMS layer on the NZ/PES and ZM/PES leads to very different transport properties. With CL-PDMSA/NZ/PES, the CO2/N2 transport property (Figure 4) is very similar to CL-PDMSA/PES, but with the ZM/PES, there is a significant increase in the CO2/N2 selectively with dry gas, e.g. at 25 °C the CO2/N2 selectivity of CL-PDMSA/ZM/PES is 37.1, considerably higher than CL-PDMSA/NZ/PES at 13. Figure 7a shows a top-view high resolution SEM of the NZ coating on the PES. The surface is made up of closely packed nanozeolite particles with empty spacing between the particles of the order of a few to tens of nanometers. PDMS coating is applied on this nanozeolite layer. The eventual structure is similar to reports of MMM where sedimentation of particles occurs at the bottom of the polymers.26,27 These MMM are reported to have poor gas separation properties since the polymer does not penetrate into the voids between the sedimented particles. A similar explanation is proposed for CL-PDMSA/NZ/PES. PDMS layer does not cover the interzeolite voids. The zeolite layer thus acts as a mere support, and thus the separation characteristics of CO2/N2 represent that of the PDMS layer. 17



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

With the ZM/PES, the observations are strikingly different. Figure 7b shows a high resolution top view electron micrograph of an untreated PES support and Figure 7c the continuous ZM layer formed within the PES support (the PES support has been dissolved with N-methylpryrolidone). Two structural features are obvious. The PES has a network of pores, and the zeolite membrane is formed in a continuous structure within these pores. The big gaps between the interconnected zeolite structure (Figure 7c) was occupied by the PES prior to its dissolution. The ZM/PES exhibited no selectivity. We propose that there are voids between the hydrophobic PES and the zeolite through which the gas transport is occurring. Such voids are noted in MMM.22,44 Upon coating with PDMS, the rubbery silicone fills the defects, and results in the dramatic improvement in selectivity, as CO2 molecules pass through the zeolite. Considering the arrangement of the PDMS, PES and zeolite layer, a resistance inseries-model shown in Figure 8a was examined. The dimension of the PDMS (700 nm) was estimated from the electron micrograph in Figure 2f. With the zeolite layer, this is more difficult, because we have shown in an earlier study29 that the zeolite grows within the PES, and towards the bottom of the zeolite layer, it is not as dense. The actual thickness contributing to the separation is smaller than the thickness seen in Figure 2f. We estimate that about 1 µm of the zeolite layer is active. The manufacturer specifies 70% of the area in PES is open and should be occupied by the zeolite. The permeability of CO2 and N2 for the PDMS, zeolite Y and PES was taken from the literature and listed in Table 1.8,45,46 With the zeolite, there is again a wide variation in permeability values reported in the literature for CO2 and N2. We chose the membrane with the highest 18


Page 18 of 52

Page 19 of 52

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/N2 selectivity of 100.8 The PDMS is in series with the zeolite. In the zeolite polymer composite, there are parallel contributions from PDMS filling the voids (estimated as 5 % of the volume), PES (25 % of the volume) and the zeolite (70 % of the volume, both dry and water blocked zeolite is being considered). The standard equations for a series/parallel model was used based on the literature (explanations are provided in the supporting information).47 Using this model, the calculated CO2/N2 selectivity is 45, with CO2 permeance of 2438 GPU, and N2 permeance of 53, the permeance are higher than what we observe, and the selectivity is within the range of measurements. What is more interesting is how the pore blockage of the zeolite by water (Figure 8a) influences the transport properties. This data is plotted in Figure 8b, and shows that the permeance change is more gradual than the selectivity, which drops off rapidly between 75 to 100% blockage by water. The zeolite sample we use are just air-dried , and so there is water in the zeolite cages at the start of the experiment, and explains the poor transport performance initially as shown in Figure 6. What Figure 8b is suggesting is that even 25% removal of water by room temperature drying with dry gases can result in a functioning membrane.

Impact of Humidity on PDMS/ZM/PES Upon exposure of the CL-PDMSA/ZM/PES membrane to 46 ppm H2O, there is a precipitous drop in the gas permeance and selectivity, as shown in Figure 6. CO2 and N2 permeance drop to 357 and 30 GPU, respectively with CO2/N2 selectivity of 12,

19



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

comparable to CL-PDMSA/PES (Figure 3b) indicating that the zeolite pores are no longer participating in gas separation. By passing dry gases, the permeance and CO2/N2 selectivity gradually increase, but this recovery process is much slower than the original drying step. After 12 hours, CO2 and N2 permeance reach 943 and 36 GPU, respectively, with CO2/N2 selectivity of 26. The CO2 and N2 permeance are recovered by about 50% in 12 hours, with CO2/N2 selectivity recovered by about 70%, compared to the as-synthesized sample. The heat of water adsorption in zeolite X powder is reported as - 51.7 kJ/mol with adsorbed H2O molecules primarily bonded to Na+ ion at site III.14 In CO2 and H2O mixture, CO2 is rapidly adsorbed in zeolite crystals first and H2O displaces CO2 molecules because of its higher affinity toward zeolite framework.48 According to adsorption isotherms obtained both experimentally and with molecular simulations, equilibrium CO2 adsorption in presence of H2O (110 molecules/unit cell) is only 1% of dry conditions.49,50 In faujasite framework, water molecules interact with residual hydroxyl groups with dipole-field interaction and hydrogen bonds.51 Based on adsorption isotherms, we estimate that at 46 ppm H2O, the number of H2O molecules/unit cell of the zeolite to be 110 at 25°C, and effectively blocks CO2 transport.50 The CO2 permeance is low (357 GPU, Figure 6) since only the CL-PDMSA in the defects of the ZM provide a pathway for gas transport through the membrane and is a small fraction of the membrane.

High temperatures 20


Page 20 of 52

Page 21 of 52

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


In zeolites, H2O adsorption can be reversed with temperature. Typically, heating at higher temperatures (>400o C) result in recovery of the adsorption characteristics of zeolite powders and separation characteristics of zeolite membranes.52 This was not possible with the CL-PDMSA/ZM/PES because of the PDMS and PES support. Thus, recovery after exposure to humidity was attempted by passing dry gas at 100oC, and then at 25oC, this data is shown in Figure 9 (we reproduce the 25 °C data in Figure 9a, but only show the steady-state values of permeance and selectivity). In Figure 9b, we examine the membrane at 50 °C. With dry gas, CO2 and N2 permeance increase from 1060 to 1834 GPU and from 25 to 102 GPU upon raising the temperature form 25 to 50oC, respectively, along with a drop in CO2/N2 selectivity from 43 to 18. Upon introduction of 46 ppm H2O at 50 °C, there is significant drop in CO2 and N2 permeance to 272 and 14 GPU, respectively, but the CO2/N2 selectivity is unaltered around 18, indicating that zeolite pores are still acting as a separation medium, unlike the observation at room temperature (Figure 9a). Upon passing dry gases at 50 °C, the CO2 and N2 permeance increase to 858 and 34 GPU, respectively and CO2/N2 selectivity increases 25.2. By lowering the temperature to 25 °C, the original CO2/N2 selectivity of ~ 40 is recovered, but the CO2 and N2 permeance are 645 and 17 GPU, lower than the initial property. This experiment was repeated at 100 °C (Figure 9c). A significant increase in CO2 and N2 permeance from 1691 to 2990 GPU and from 35 to 161 GPU, respectively is observed, with a lowering of selectivity from 48 to 19. With 46 ppm humidity, CO2 and N2 permeance drop to 1053 and 82 GPU, respectively with a drop in CO2/N2 selectivity 21



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 22 of 52

to 12.9, which is still higher than CL-PDMSA/PES at 100 °C (~6, Figure 3), indicating that the zeolite is active in the separation process. Upon switching to a dry stream of gas at 100 °C, both CO2 and N2 permeance and CO2/N2 selectivity remain unchanged. When the temperature is then lowered to 25 °C, CO2/N2 selectivity increases to 21 and CO2 and N2 permeance decrease to 673 and 32 GPU respectively. Compared to the initial steady state measured with dry gas at 25 °C, both the CO2/N2 selectivity and CO2 and N2 permeance exhibit significant decrease. Studies with zeolite/alumina membranes as a function of increasing temperature exhibit a decrease in selectivity and increase in permeance, similar to the present observations.8 With higher temperatures, adsorption of CO2 decreases and N2 transport through the zeolite is facilitated. The three membranes reported in Figure 9 were prepared under similar conditions, but exhibit considerable variation in permeance (note data at 1h), while the selectivity appear to be less variant. In the course of this research, we have prepared about ~50 membranes, and their permeance and selectivity at room temperature under dry gas conditions are shown in Table S1 of the Supporting Information. The average CO2 permeability is 1610 ± 335 GPU, N2 permeance is 43 ± 12 GPU and CO2/N2 selectivity is 39 ± 6. Variations in selectivity were less marked than in permeance. There can be several reasons for the variability in performance: PDMS layer thickness, ability of PDMS to access all of the void sites, differences in the void structure between the zeolite and PES, and the possible intercrystalline defects within the zeolite membrane.

22


Page 23 of 52

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


Thus, there is considerable variability in the transport properties of these membranes. Since our focus in this study was to examine the trends with temperature, exposure to water and subsequent drying, we have not generated error bars for each experiment. Instead of error bars for each sample, we have repeated the experiment with CL-PDMSA/ZM/PES with a new membrane, and show the same trend as Figure 9c. This data is shown in Figure S3.

3.1.6. Influence of PDMS Cross-Linking With reference to Scheme 2, there are 5 PDMS/ZM/PES samples. We have already discussed CL-PDMSA/ZM/PES, and detail below the transport measurements for the other four samples. Figure 10a indicates that at 25 °C and dry gas conditions, un-cross linked PDMS/ZM/PES membrane has CO2 and N2 permeance of 364 and 15 GPU, respectively and CO2/N2 selectivity of 25, significantly lower than the cross-linked (CLPDMSA/ZM/PES, Figure 9, all greater than 1000 GPU). With the membrane at 100 °C, CO2 and N2 permeance increases to 1156 and 71 GPU and CO2/N2 selectivity decreased to 16. With 46 ppm water vapor in the feed gas, CO2 and N2 permeance decrease to 502 and 52 GPU, respectively and CO2/N2 selectivity decrease to ~10. At 100 °C, CO2 permeance and CO2/N2 selectivity was unchanged after switching from wet to dry feed gas. After cooling to 25 °C, CO2 and N2 permeance slightly decreased from 525 to 370 GPU and from 57 to 52 GPU, respectively, while CO2/N2 selectivity dropped from 9 to 7. Similar observations were made with PDMS + cross-linker (PDMS+CL) coated on ZM/PES, as shown in Figure 10b. With both the PDMS and PDMS+CL, the defects in 23



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

the ZM are getting fixed, as evident from high selectivity with dry gases at 25 and 100oC, as compared to CL-PDMSA/PES. However, in the absence of cross-linking of the PDMS, permeability and recovery of the membrane after exposure to humidity is compromised. Comparison between the uncross-linked and cross-linked PDMS coated on ZM/PES clearly show that cross-linking improves gas performance. We speculate that with the uncross-linked polymers, the zeolite pores get blocked similar to the pore blocking effects observed in MMM.23 Several studies on zeolite Y-PDMS composites indicate that the PDMS chains can penetrate into the zeolite pores, and this has been referred to as physical cross-linking of the PDMS by the zeolite.53,54 With cross-linking of the PDMS, the polymer gets more rigid, and interpenetration of zeolite and PDMS will be reduced. In order to test the hypothesis about increase cross-linking, higher amounts of catalyst were used to increase crosslinking as compared to CL-PDMSA (catalyst used as suggested by vendor). In order to verify if increased cross-linking is taking place the FTIR spectra of all the samples were recorded. The goal was to follow the fate of the Si-H bending mode with addition of catalyst.55,56 The IR data are shown in Figure 11 for PDMS, PDMS+CL, CL-PDMSA, CL-PDMSB and CL-PDMSC. The Si-H bending mode appears at 890 cm-1 in the CL, and shifts once cross-linking occurs to 902 cm-1, and has been noted in the literature.54,55 For the PDMS+CL, the band is the strongest since the Si-H groups are all unreacted. With increased amount of catalyst, the band disappears and for CL-PDMSC, it is not observed, indicting increased cross-linking.

24


Page 24 of 52

Page 25 of 52

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


The transport data with CL-PDMSB and CL-PDMSC is shown in Figure 10c and 10d. For CL-PDMSB/ZM/PES, steady state CO2 and N2 permeance is 1356 and 27 GPU, respectively with CO2/N2 selectivity of 51 at 25 °C with dry feed gas. Upon heating to 100 °C, CO2 and N2 permeance increase to 2691 and 137 GPU, respectively and CO2/N2 selectivity decreases to 20. With 46 ppm H2O in feed gas, CO2 and N2 permeance decrease to 1212 and 68 GPU, respectively and CO2/N2 selectivity decreases to 18. After switching back to dry feed gas, CO2 and N2 permeance increase to 1393 and 18 GPU, respectively and CO2/N2 selectivity stays constant. After cooling the system back to 25 °C, CO2 and N2 permeance decrease to 970 and 24 GPU, respectively and CO2/N2 selectivity increases 40. With CL-PDMSC/ZM/PES membrane (Figure 10d), the transport properties are similar to CL-PDMSB/ZM/PES. Upon heating from 25 to 100 °C, CO2 and N2 permeance increase from 1229 to 2646 GPU and from 22 to 129 GPU, respectively and CO2/N2 selectivity decreased from 55 to 20. With 46 ppm water vapor in the feed gas, CO2 and N2 permeance decrease to 1319 and 69 GPU, respectively and CO2/N2 selectivity decrease to 19. By switching back to dry feed gas, CO2 and N2 permeance increased to 1686 and 85 GPU, respectively and CO2/N2 selectivity was unchanged. After cooling to 25 °C, CO2 and N2 permeance decreased to 863 and 26 GPU, respectively, with CO2/N2 selectivity increased to 33. We are defining recovery as comparison of the initial data at 25oC with that of the sample that has been exposed to humidity at 100oC and back to 25oC with dry gases, and the data are shown in Table 2. The recovery in CO2/N2 selectivity (comparison of 25oC 25



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

data before and after high temperature and humidity) for PDMS/ZM/PES, PDMS+CL/ZM/PES, CL-PDMSA/ZM/PES, CL-PDMSB/ZM/PES and CLPDMSC/ZM/PES are 28, 25, 43, 80 and 60%, respectively. Cross-linking of PDMS as compared to the uncross-linked samples is definitely helping to some extent in the recovery, indicating that the H2O molecules at the zeolite-PDMS interface are removed more readily in the cross-linked polymers. PDMS is considered a hydrophobic polymer and the zeolite is hydrophilic. Upon exposure to humid feed gases, water collects at the PDMS-zeolite interface. It has been noted that water is condensed at the interface by capillary condensation between PDMS and sapphire.57 Our hypothesis is that this tightly bound interfacial water is not removed by dry gas even at 100oC, and is the reason for incomplete recovery with the PDMS, PDMS+CL, and CL-PDMSA,B,C samples. In case of the samples with unreacted Si-H groups, these groups can hydrolyze and the Si-OH groups will be attracted to the polar zeolite interface, and can act as water traps.58

3.2. Surface Modification Even though there is improvement in recovery of the transport properties after high temperature humidity treatment by using the CL-PDMSA,B,C (Table 2), none of the samples shows quantitative recovery. With the as-prepared ZM samples, dry gas even at 25oC will remove water from inside the zeolite pores. It is only when water gathers at the PDMS-zeolite interface that water removal becomes difficult, presumably because of the strong capillary forces with which the water is held.59

26


Page 26 of 52

Page 27 of 52

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


The surface of the zeolite membrane within the PES was modified by treatment with hexadecylamine (HDA) prior to CL-PDMSB coating (CL-PDMSB/HDA/ZM/PES). The charged amino groups of the HDA can ion-exchange with the negatively charged zeolite surface increasing the hydrophobicity (the long molecule is not expected to penetrate into the zeolite) (Figure 12a). In order to verify that the HDA is indeed bound to the zeolite surface, XPS studies were carried out, and the data showing the expected N1s peak at 400.5 eV60 is shown in Figure S4. To prove that zeolite surface becomes hydrophobic upon HDA exchange, contact angle measurements were carried out. The contact angle changed from 11.6o to 74.3o upon HDA exchange. Photograph of the zeolite with water drop and the contact angle data are shown in Figure S5. The separation results with the HDA-zeolite are shown in Figure 12b. Compared with CL-PDMSB/ZM/PES, CO2/N2 selectivity is higher (59.5) at room temperature under steady state. By increasing temperature to 100 °C, CO2 and N2 permeance increase to 2719 and 126 GPU, respectively and CO2/N2 selectivity decrease to 21.6. By applying humid feed gas, CO2/N2 selectivity keeps constant around 20 and CO2 permeance drops to 1569 GPU. Upon switching back to dry feed gas, CO2 permeance increases to 2094 GPU and CO2/N2 selectivity increases to 28.3. By cooling to 25 °C, CO2 permeance decreases to 1032 GPU (86% recovery) and CO2/N2 selectivity increases to 53.1 (90% recovery). This sample has the best recovery, and supports our hypothesis that water entrapment at the PDMS-zeolite interface is responsible for the poor recovery.

3.3. Nanozeolite decomposition 27



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

Another possibility for membrane performance degradation could be damage to the zeolite by flowing humid gas at high temperature, as noted for SAPO-34 membranes due to breakage of Al-O bonds.61 To evaluate structural damage by H2O at 100 °C, a sample of the nanozeolite that was used to make the membrane was treated with 46 ppm H2O at 100 °C for 3 hours (time chosen to be typical of the membrane experiments). There was no change in the diffraction pattern or surface area of the zeolite with the higher temperature treatment (data shown in Table S2).

4. Conclusion In this study, we investigated the effect of several parameters, including humidity (46 ppm), temperature and PDMS cross-linking on the CO2/N2 separation properties of zeolite membrane grown within the PES. The as-synthesized zeolite membrane formed within the pores is defective and does not separate CO2 and N2. To achieve separation, it was necessary to coat the membrane with a PDMS layer. Cross-linking of PDMS increases gas permeance through the membrane assembly, as compared to uncross-linked samples. Water that is present within the zeolite during sample preparation can be partially removed by passing dry gas to get good separation properties. However, 46 ppm H2O in the feed gas is enough to block zeolite transport by adsorption into the zeolite pores. A simple series model of transport using the PDMS and zeolite as layers is series did explain the humidity dependence. As operation temperature increases to 100oC (the highest temperature studied), higher tolerance to humidity is observed, because less water is adsorbed within the zeolite. CO2 permeance and CO2/N2 selectivity after humid gas 28


Page 28 of 52

Page 29 of 52

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


exposure at high temperatures, followed by exposure to dry gases and re-examination at room temperature indicated that complete recovery was not attained. Capillary water condensation at the PDMS/zeolite interface was considered to be the cause. The recovery in the transport properties could be significantly improved (reaching 90% recovery) by introducing hydrophobic long-chain alkyl groups at the PDMS-zeolite interface, thus interfering with the interfacial water adsorption. These faujasitic zeolite membranes grown within PES can only be useful for dry gas separations because of the enhanced and competitive water adsorption within the zeolite pores. After exposure to low levels of humidity, the gas transport properties can be recovered by flowing dry gas, especially if the zeolite surface is made hydrophobic.

Supporting Information. Nanozeolite surface area results and XRD patterns of PES and zeolite membrane are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Bo Wang, [email protected]; * Prabir K. Dutta, [email protected]

Funding Sources The Ohio State University Graduate Fellowship program supported this work. 29



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

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This contribution was identified by Dr. William Koros (Georgia Institute of Technology) and Dr. Zhong He (Primus Green Energy) as the Best Presentation in the session “Novel Materials for Gas Separation, Storage & Utilization” of the 2016 ACS Fall National Meeting in Philadelphia, PA. We thank chemistry department machine shop, electric shop and glass shop at The Ohio State University for their help.

30


Page 30 of 52

Page 31 of 52

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


Reference (1)

(2)

(3)

(4) (5)

(6)

(7)

(8)

(9)

(10)

(11)

(12)

(13)

Gascon, J.; Kapteijn, F.; Zornoza, B.; Sebastián, V.; Casado, C.; Coronas, J. Practical Approach to Zeolitic Membranes and Coatings: State of the Art, Opportunities, Barriers, and Future Perspectives. Chem. Mater. 2012, 24 (15), 2829–2844 DOI: 10.1021/cm301435j. Khulbe, K. C.; Matsuura, T.; Feng, C. Y.; Ismail, A. F. Recent Development on the Effect of Water/Moisture on the Performance of Zeolite Membrane and MMMs Containing Zeolite for Gas Separation; Review. RSC Adv. 2016, 6 (49), 42943– 42961 DOI: 10.1039/C6RA03007F. Merkel, T. C.; Zhou, M.; Baker, R. W. Carbon Dioxide Capture with Membranes at an IGCC Power Plant. J. Membr. Sci. 2012, 389, 441–450 DOI: 10.1016/j.memsci.2011.11.012. Robeson, L. M. The Upper Bound Revisited. J. Membr. Sci. 2008, 320 (1–2), 390– 400 DOI: 10.1016/j.memsci.2008.04.030. Krishna, R.; van Baten, J. M. In Silico Screening of Zeolite Membranes for CO2 Capture. J. Membr. Sci. 2010, 360 (1–2), 323–333 DOI: 10.1016/j.memsci.2010.05.032. Ge, Q.; Wang, Z.; Yan, Y. High-Performance Zeolite NaA Membranes on Polymer−Zeolite Composite Hollow Fiber Supports. J. Am. Chem. Soc. 2009, 131 (47), 17056–17057 DOI: 10.1021/ja9082057. Kusakabe, K.; Kuroda, T.; Morooka, S. Separation of Carbon Dioxide from Nitrogen Using Ion-Exchanged Faujasite-Type Zeolite Membranes Formed on Porous Support Tubes. J. Membr. Sci. 1998, 148 (1), 13–23 DOI: 10.1016/S03767388(98)00164-1. Kusakabe, K.; Kuroda, T.; Murata, A.; Morooka, S. Formation of a Y-Type Zeolite Membrane on a Porous α-Alumina Tube for Gas Separation. Ind. Eng. Chem. Res. 1997, 36 (3), 649–655 DOI: 10.1021/ie960519x. Hasegawa, Y.; Watanabe, K.; Kusakabe, K.; Morooka, S. The Separation of CO2 Using Y-Type Zeolite Membranes Ion-Exchanged with Alkali Metal Cations. Sep. Purif. Technol. 2001, 22–23, 319–325 DOI: 10.1016/S1383-5866(00)00154-4. Hasegawa, Y.; Tanaka, T.; Watanabe, K.; Jeong, B.-H.; Kusakabe, K.; Morooka, S. Separation of co2-ch4 and co2-n2 Systems Using Ion-Exchanged Fau-Type Zeolite Membranes with Different Si/Al Ratios. Korean J. Chem. Eng. 2002, 19 (2), 309–313 DOI: 10.1007/BF02698420. White, J. C.; Dutta, P. K.; Shqau, K.; Verweij, H. Synthesis of Ultrathin Zeolite Y Membranes and Their Application for Separation of Carbon Dioxide and Nitrogen Gases. Langmuir 2010, 26 (12), 10287–10293 DOI: 10.1021/la100463j. Anderson, M.; Wang, H.; Lin, Y. S. Inorganic Membranes for Carbon Dioxide and Nitrogen Separation. Rev Chem Eng 2012, 28 (2–3), 101–121 DOI: 10.1515/revce2012-0001. Gu, X.; Dong, J.; Nenoff, T. M. Synthesis of Defect-Free FAU-Type Zeolite Membranes and Separation for Dry and Moist CO2/N2 Mixtures. Ind. Eng. Chem. Res. 2005, 44 (4), 937–944 DOI: 10.1021/ie049263i. 31



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

(14) Li, G.; Xiao, P.; Webley, P. A.; Zhang, J.; Singh, R. Competition of CO2/H2O in Adsorption Based CO2 Capture. Energy Procedia 2009, 1 (1), 1123–1130 DOI: 10.1016/j.egypro.2009.01.148. (15) Guan, G.; Tanaka, T.; Kusakabe, K.; Sotowa, K.-I.; Morooka, S. Characterization of AlPO4-Type Molecular Sieving Membranes Formed on a Porous α-Alumina Tube. J. Membr. Sci. 2003, 214 (2), 191–198 DOI: 10.1016/S03767388(02)00544-6. (16) Aoki, K.; Kusakabe, K.; Morooka, S. Separation of Gases with an A-Type Zeolite Membrane. Ind. Eng. Chem. Res. 2000, 39 (7), 2245–2251 DOI: 10.1021/ie990902c. (17) Calero, S.; Gómez-Álvarez, P. Hydrogen Bonding of Water Confined in Zeolites and Their Zeolitic Imidazolate Framework Counterparts. RSC Adv. 2014, 4 (56), 29571–29580 DOI: 10.1039/C4RA01508H. (18) Sublet, J.; Pera-Titus, M.; Guilhaume, N.; Farrusseng, D.; Schrive, L.; Chanaud, P.; Siret, B.; Durécu, S. Technico-Economical Assessment of MFI-Type Zeolite Membranes for CO2 Capture from Postcombustion Flue Gases. AIChE J. 2012, 58 (10), 3183–3194 DOI: 10.1002/aic.12805. (19) Caro, J.; Noack, M. Zeolite Membranes – Recent Developments and Progress. Microporous Mesoporous Mater. 2008, 115 (3), 215–233 DOI: 10.1016/j.micromeso.2008.03.008. (20) Tomita, T.; Nakayama, K.; Sakai, H. Gas Separation Characteristics of DDR Type Zeolite Membrane. Microporous Mesoporous Mater. 2004, 68 (1–3), 71–75 DOI: 10.1016/j.micromeso.2003.11.016. (21) Bastani, D.; Esmaeili, N.; Asadollahi, M. Polymeric Mixed Matrix Membranes Containing Zeolites as a Filler for Gas Separation Applications: A Review. J. Ind. Eng. Chem. 2013, 19 (2), 375–393 DOI: 10.1016/j.jiec.2012.09.019. (22) Vinh-Thang, H.; Kaliaguine, S. Predictive Models for Mixed-Matrix Membrane Performance: A Review. Chem. Rev. 2013, 113 (7), 4980–5028 DOI: 10.1021/cr3003888. (23) Rezakazemi, M.; Ebadi Amooghin, A.; Montazer-Rahmati, M. M.; Ismail, A. F.; Matsuura, T. State-of-the-Art Membrane Based CO2 Separation Using Mixed Matrix Membranes (MMMs): An Overview on Current Status and Future Directions. Prog. Polym. Sci. 2014, 39 (5), 817–861 DOI: 10.1016/j.progpolymsci.2014.01.003. (24) Moore, T. T.; Vo, T.; Mahajan, R.; Kulkarni, S.; Hasse, D.; Koros, W. J. Effect of Humidified Feeds on Oxygen Permeability of Mixed Matrix Membranes. J. Appl. Polym. Sci. 2003, 90 (6), 1574–1580 DOI: 10.1002/app.2380. (25) Husain, S.; Koros, W. J. Mixed Matrix Hollow Fiber Membranes Made with Modified HSSZ-13 Zeolite in Polyetherimide Polymer Matrix for Gas Separation. J. Membr. Sci. 2007, 288 (1–2), 195–207 DOI: 10.1016/j.memsci.2006.11.016. (26) Chung, T.-S.; Jiang, L. Y.; Li, Y.; Kulprathipanja, S. Mixed Matrix Membranes (MMMs) Comprising Organic Polymers with Dispersed Inorganic Fillers for Gas Separation. Prog. Polym. Sci. 2007, 32 (4), 483–507 DOI: 10.1016/j.progpolymsci.2007.01.008. 32


Page 32 of 52

Page 33 of 52

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


(27) Vu, D. Q.; Koros, W. J.; Miller, S. J. Mixed Matrix Membranes Using Carbon Molecular Sieves: I. Preparation and Experimental Results. J. Membr. Sci. 2003, 211 (2), 311–334 DOI: 10.1016/S0376-7388(02)00429-5. (28) Moore, T. T.; Vo, T.; Mahajan, R.; Kulkarni, S.; Hasse, D.; Koros, W. J. Effect of Humidified Feeds on Oxygen Permeability of Mixed Matrix Membranes. J. Appl. Polym. Sci. 2003, 90 (6), 1574–1580 DOI: 10.1002/app.2380. (29) Wang, B.; Ho, W. S. W.; Figueroa, J. D.; Dutta, P. K. Bendable Zeolite Membranes: Synthesis and Improved Gas Separation Performance. Langmuir 2015, 31 (24), 6894–6901 DOI: 10.1021/acs.langmuir.5b01306. (30) Wang, B.; Jackson, E. A.; Hoff, J. W.; Dutta, P. K. Fabrication of Zeolite/Polymer Composite Membranes in a Roller Assembly. Microporous Mesoporous Mater. 2016, 223, 247–253 DOI: 10.1016/j.micromeso.2015.11.003. (31) Wang, B.; Li, Y.; Shao, C.; Cui, M.; Dutta, P. K. Rapid and High Yield Synthesis Method of Colloidal Nano Faujasite. Microporous Mesoporous Mater. 2016, 230, 89–99 DOI: 10.1016/j.micromeso.2016.05.001. (32) Banihashemi, F.; Pakizeh, M.; Ahmadpour, A. CO2 Separation Using PDMS/ZSM-5 Zeolite Composite Membrane. Sep. Purif. Technol. 2011, 79 (3), 293–302 DOI: 10.1016/j.seppur.2011.02.033. (33) Chiu, W. V.; Park, I.-S.; Shqau, K.; White, J. C.; Schillo, M. C.; Ho, W. S. W.; Dutta, P. K.; Verweij, H. Post-Synthesis Defect Abatement of Inorganic Membranes for Gas Separation. J. Membr. Sci. 2011, 377 (1–2), 182–190 DOI: 10.1016/j.memsci.2011.04.047. (34) Chakraborty, S.; Wang, B.; Dutta, P. K. Tolerance of Polymer-Zeolite Composite Membranes to Mechanical Strain. J. Membr. Sci. 2016, 518, 192–202 DOI: 10.1016/j.memsci.2016.06.040. (35) Holmberg, B. A.; Wang, H.; Norbeck, J. M.; Yan, Y. Controlling Size and Yield of Zeolite Y Nanocrystals Using Tetramethylammonium Bromide. Microporous Mesoporous Mater. 2003, 59 (1), 13–28 DOI: 10.1016/S1387-1811(03)00271-3. (36) Wang, B.; Sun, C.; Li, Y.; Zhao, L.; Ho, W. S. W.; Dutta, P. K. Rapid Synthesis of Faujasite/Polyethersulfone Composite Membrane and Application for CO2/N2 Separation. Microporous Mesoporous Mater. 2015, 208, 72–82 DOI: 10.1016/j.micromeso.2015.01.020. (37) Severance, M.; Wang, B.; Ramasubramanian, K.; Zhao, L.; Ho, W. S. W.; Dutta, P. K. Rapid Crystallization of Faujasitic Zeolites: Mechanism and Application to Zeolite Membrane Growth on Polymer Supports. Langmuir 2014, 30 (23), 6929– 6937 DOI: 10.1021/la5004512. (38) Rezakazemi, M.; Vatani, A.; Mohammadi, T. Synergistic Interactions between POSS and Fumed Silica and Their Effect on the Properties of Crosslinked PDMS Nanocomposite Membranes. RSC Adv. 2015, 5 (100), 82460–82470 DOI: 10.1039/C5RA13609A. (39) Shirazi, Y.; Ghadimi, A.; Mohammadi, T. Recovery of Alcohols from Water Using Polydimethylsiloxane–silica Nanocomposite Membranes: Characterization and Pervaporation Performance. J. Appl. Polym. Sci. 2012, 124 (4), 2871–2882 DOI: 10.1002/app.35313. 33



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

(40) Bosq, N.; Guigo, N.; Persello, J.; Sbirrazzuoli, N. Melt and Glass Crystallization of PDMS and PDMS Silica Nanocomposites. Phys. Chem. Chem. Phys. 2014, 16 (17), 7830–7840 DOI: 10.1039/C4CP00164H. (41) Jovanovic, J. D.; Govedarica, M. N.; Dvornic, P. R.; Popovic, I. G. The Thermogravimetric Analysis of Some Polysiloxanes. Polym. Degrad. Stab. 1998, 61 (1), 87–93 DOI: 10.1016/S0141-3910(97)00135-3. (42) Kopitzke, R. W.; Linkous, C. A.; Nelson, G. L. Thermal Stability of High Temperature Polymers and Their Sulfonated Derivatives under Inert and Saturated Vapor Conditions. Polym. Degrad. Stab. 2000, 67 (2), 335–344 DOI: 10.1016/S0141-3910(99)00135-4. (43) Merkel, T. C.; Gupta, R. P.; Turk, B. S.; Freeman, B. D. Mixed-Gas Permeation of Syngas Components in Poly(dimethylsiloxane) and poly(1-Trimethylsilyl-1Propyne) at Elevated Temperatures. J. Membr. Sci. 2001, 191 (1–2), 85–94 DOI: 10.1016/S0376-7388(01)00452-5. (44) Gonzo, E. E.; Parentis, M. L.; Gottifredi, J. C. Estimating Models for Predicting Effective Permeability of Mixed Matrix Membranes. J. Membr. Sci. 2006, 277 (1– 2), 46–54 DOI: 10.1016/j.memsci.2005.10.007. (45) Hussain, M.; König, A. Mixed‐Matrix Membrane for Gas Separation: Polydimethylsiloxane Filled with Zeolite. Chem. Eng. Technol. 2012, 35 (3), 561– 569 DOI: 10.1002/ceat.201100419. (46) Li, Y.; Guan, H.-M.; Chung, T.-S.; Kulprathipanja, S. Effects of Novel Silane Modification of Zeolite Surface on Polymer Chain Rigidification and Partial Pore Blockage in Polyethersulfone (PES)–zeolite A Mixed Matrix Membranes. J. Membr. Sci. 2006, 275 (1–2), 17–28 DOI: 10.1016/j.memsci.2005.08.015. (47) Funk, C. V.; Lloyd, D. R. Zeolite-Filled Microporous Mixed Matrix (ZeoTIPS) Membranes: Prediction of Gas Separation Performance. J. Membr. Sci. 2008, 313 (1–2), 224–231 DOI: 10.1016/j.memsci.2008.01.002. (48) Shen, C. M.; Worek, W. M. Cosorption Characteristics of Solid Adsorbents. Int. J. Heat Mass Transf. 1994, 37 (14), 2123–2129 DOI: 10.1016/0017-9310(94)903131. (49) Shirono, K.; Endo, A.; Daiguji, H. Molecular Dynamics Study of Hydrated Faujasite-Type Zeolites. J. Phys. Chem. B 2005, 109 (8), 3446–3453 DOI: 10.1021/jp047293t. (50) Joos, L.; Swisher, J. A.; Smit, B. Molecular Simulation Study of the Competitive Adsorption of H2O and CO2 in Zeolite 13X. Langmuir 2013, 29 (51), 15936– 15942 DOI: 10.1021/la403824g. (51) Kawai, T.; Tsutsumi, K. Evaluation of Hydrophilic-Hydrophobic Character of Zeolites by Measurements of Their Immersional Heats in Water. Colloid Polym. Sci. 1992, 270 (7), 711–715 DOI: 10.1007/BF00654048. (52) Funke, H. H.; Frender, K. R.; Green, K. M.; Wilwerding, J. L.; Sweitzer, B. A.; Falconer, J. L.; Noble, R. D. Influence of Adsorbed Molecules on the Permeation Properties of Silicalite Membranes. J. Membr. Sci. 1997, 129 (1), 77–82 DOI: 10.1016/S0376-7388(96)00333-X. 34


Page 34 of 52

Page 35 of 52

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


(53) Vankelecom, I. F. J.; Scheppers, E.; Heus, R.; Uytterhoeven, J. B. Parameters Influencing Zeolite Incorporation in PDMS Membranes. J. Phys. Chem. 1994, 98 (47), 12390–12396 DOI: 10.1021/j100098a038. (54) Vankelecom, I. F. J.; Depre, D.; De Beukelaer, S.; Uytterhoeven, J. B. Influence of Zeolites in PDMS Membranes: Pervaporation of Water/Alcohol Mixtures. J. Phys. Chem. 1995, 99 (35), 13193–13197 DOI: 10.1021/j100035a024. (55) Berean, K.; Ou, J. Z.; Nour, M.; Latham, K.; McSweeney, C.; Paull, D.; Halim, A.; Kentish, S.; Doherty, C. M.; Hill, A. J.; Kalantar-zadeh, K. The Effect of Crosslinking Temperature on the Permeability of PDMS Membranes: Evidence of Extraordinary CO2 and CH4 Gas Permeation. Sep. Purif. Technol. 2014, 122, 96– 104 DOI: 10.1016/j.seppur.2013.11.006. (56) Esteves, A. C. C.; Brokken-Zijp, J.; Laven, J.; Huinink, H. P.; Reuvers, N. J. W.; Van, M. P.; de With, G. Influence of Cross-Linker Concentration on the CrossLinking of PDMS and the Network Structures Formed. Polymer 2009, 50 (16), 3955–3966 DOI: 10.1016/j.polymer.2009.06.022. (57) Buhl, S.; Greiner, C.; Campo, A. del; Arzt, E. Humidity Influence on the Adhesion of Biomimetic Fibrillar Surfaces. Int. J. Mater. Res. 2009, 100 (8), 1119–1126 DOI: 10.3139/146.110146. (58) Quan, X. Properties of Post‐cured Siloxane Networks. Polym. Eng. Sci. 1989, 29 (20), 1419–1425 DOI: 10.1002/pen.760292003. (59) Ahmad, N. N. R.; Mukhtar, H.; Mohshim, D. F.; Nasir, R.; Man, Z. Surface Modification in Inorganic Filler of Mixed Matrix Membrane for Enhancing the Gas Separation Performance. Rev. Chem. Eng. 2016, 32 (2), 181–200 DOI: 10.1515/revce-2015-0031. (60) Zorn, G.; Liu, L.-H.; Árnadóttir, L.; Wang, H.; Gamble, L. J.; Castner, D. G.; Yan, M. X-Ray Photoelectron Spectroscopy Investigation of the Nitrogen Species in Photoactive Perfluorophenylazide-Modified Surfaces. J. Phys. Chem. C 2014, 118 (1), 376–383 DOI: 10.1021/jp409338y. (61) Poshusta, J. C.; Noble, R. D.; Falconer, J. L. Characterization of SAPO-34 Membranes by Water Adsorption. J. Membr. Sci. 2001, 186 (1), 25–40 DOI: 10.1016/S0376-7388(00)00666-9.

35



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 52

Table 1 The permeability of CO2 and N2 for the PDMS, zeolite Y and PES as collected from literature* Material PDMS Zeolite Y PES

CO2 Permeability/Barrer 3499 2980 2.6

N2 Permeability/Barrer 314 30 0.08

*Transport data taken from References 8, 45 and 46

36


CO2/N2 Selectivity 11.1 99.3 32.5

Page 37 of 52

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


Table 2 Comparison of recovery of gas separation properties (measured at 25oC) before and after treatment with 46 ppm humid gas at 100 °C. Sample PDMS/ZM/PES CL-PDMS/ZM/PES CL-PDMSA/ZM/PES CL-PDMSB/ZM/PES CL-PDMSB/HDA/ZM/PES CL-PDMSC/ZM/PES

CO2 Permeance/GPU Before After 364 370 496 264 1691 673 1356 970 1195 1032 1229 863

37


CO2/N2 Selectivity Before After % Recovery 24.9 7.1 28 26.7 6.7 25 48.3 20.9 43 50.7 40.5 80 59.5 53.1 90 55.0 32.7 60


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

Scheme 1. The different PDMS samples used for coating (CL is the cross-linker supplied by the vendor).

38


Page 38 of 52

Page 39 of 52

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


Scheme 2. The different membrane samples examined (asterisk indicates the samples used for transport measurements).

39



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 1 PDMS polymerization mechanism (adapted from reference 40)

40


Page 40 of 52

Page 41 of 52

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 2 (a) Top view SEM image of PES support. Side view SEM images of CLPDMSA coated (b) PES support and (c) nanozeolite layer on PES support. Top view (d) and (e) side view SEM image of ZM after PES dissolution by NMP. Side view (f) SEM image of CL-PDMSA/ZM/PES.

41



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 3 (a) Dry CO2/N2 (80/20, v/v) separation performance of CL-PDMSA/PES support and (b) CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at 25oC (red dots are the CO2/N2 selectivity)

42


Page 42 of 52

Page 43 of 52

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 4 CO2/N2 separation performance of CL-PDMSA/NZ/PES (a) under dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at 25oC and (b) dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at 75oC (yellow shaded region, data in unshaded region is at 25oC). Red dots are the CO2/N2 selectivity.

43



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 5 CO2/N2 (80/20, v/v) separation performance of ZM/PES (a) (a) under dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at 25oC and (b) dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at 75oC (yellow shaded region, data in unshaded region is at 25oC). Red dots are the CO2/N2 selectivity.

44


Page 44 of 52

Page 45 of 52

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 6 Time evolution of transport property of CO2/N2 of CL-PDMSA/ZM/PES with dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at 25oC. Red dots are the CO2/N2 selectivity.

45



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 7 SEM images of (a) NZ close packing layer on PES support; (b) untreated PES support and (c) continuous ZM layer formed within the PES support with the PES support dissolved.

46


Page 46 of 52

Page 47 of 52

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 8 (a) Modeling diagram of PDMS/ZM/PES composite membrane consisting of two layers in series, a PDMS layer and a zeolite layer. Within the zeolite layer, there are parallel pathways through the zeolite (with and without water blockage ‘x’, which was varied in the calculation), PDMS and PES and (b) trend of CO2 permeability and CO2/N2 selectivity with water blockage in the zeolite calculated from the modeling equation.

47



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 9 CO2/N2 separation performance of CL-PDMSA/ZM/PES with dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at (a) 25 °C (b) 50 °C and (c) 100 °C (yellow shaded region is at high temperature, unshaded region at 25oC). Red dots are the CO2/N2 selectivity.

48


Page 48 of 52

Page 49 of 52

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 10 CO2/N2 separation performance with dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas (blue shaded region) at 100 °C of (a) PDMS/ZM/PES (b) CL-PDMS/ZM/PES (c) CL-PDMSB/ZM/PES (d) CLPDMSC/ZM/PES (blue shaded region with humid gas, yellow shaded region is at high temperature 100oC, unshaded region 25oC). Red dots are the CO2/N2 selectivity.

49



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 11 ATR-FTIR spectrum in (a) 1300-860 cm-1 region and (b) 960-860 cm-1 range of (i) PDMS stock; (ii) PDMS with cross-linker; (iii) CL-PDMSA; (iv) CL-PDMSB and (v) CL-PDMSC (all spectra normalized to the band at 1260 cm-1).

50


Page 50 of 52

Page 51 of 52

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 12 (a) Scheme of zeolite surface modification by HDA and (b) CO2/N2 separation performance with dry CO2/N2 (80/20, v/v) and CO2/N2 (80/20, v/v) + 46 ppm H2O in feed gas at 100 °C of CL-PDMSB/HDA/ZM/PES (blue shaded region with humid gas, yellow shaded region is at high temperature 100oC, unshaded region 25oC). Red dots are the CO2/N2 selectivity.

51



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

TOC Graphic

52


Page 52 of 52

N2

Recommend Documents