Tailoring Ultramicroporosity To Maximize CO2 Transport within

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Functional Inorganic Materials and Devices

Tailoring Ultramicroporosity to Maximize CO2 Transport within Pyrimidine-Bridged Organosilica Membranes Liang Yu, Masakoto Kanezashi, Hiroki Nagasawa, Meng Guo, Norihiro Moriyama, Kenji Ito, and Toshinori Tsuru ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01462 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 3, 2019

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ACS Applied Materials & Interfaces

Tailoring Ultramicroporosity to Maximize CO2 Transport within Pyrimidine-Bridged Organosilica Membranes

Liang Yu,a Masakoto Kanezashi,a Hiroki Nagasawa,a Meng Guo,a Norihiro Moriyama,a Kenji Ito,b and Toshinori Tsurua,* a

Department of Chemical Engineering, Hiroshima University, 1-4-1 Kagamiyama

Higashihiroshima, 739-8527, Japan b

National Metrology Institute of Japan, National Institute of Advanced Industrial

Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, Japan Corresponding author Tel: +81 824247714; E-mail: [email protected] KEYWORDS: organosilica; CO2 separation; high flux; ultramicroporosity

1

ACS Paragon Plus Environment

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ABSTRACT： Amine-functionalized organosilica membranes are attracting an increasing amount of attention due to significant potential for the capture of post-combustion CO2. The appealing separation performance of these membranes, however, is generally obtained via compromises to gas permeance. In the present study, a novel, ultramicroporositytailored composite (organo)silica membrane with high flux was synthesized via sol-gel co-condensation of a pyrimidine-bridged organoalkoxysilane precursor (BTPP) with a second intrinsically rigid network precursor (BTESE or TEOS). The surface chemistry, ultramicroporosity, and chain-packing state of the initial BTPP-derived membranes can be carefully tuned, which has been verified via Fourier transform infrared (FTIR) spectroscopy, water-contact angle measurement, X-ray diffractions (XRD), and positron annihilation lifetime spectroscopy (PALS). The composite (organo)silica xerogel specimens presented a slightly improved ultramicroporosity with noticeable increases in gas adsorption (CO2 and N2). However, a surprising increase in CO2 permeance (>2000 GPU), with moderate CO2/N2 selectivity (~20), was observed in the resultant composite (organo)silica membranes. Furthermore, gas permeance of the composite membranes far surpassed the values based on Maxwell predictions, indicating a possible molecular-scale dispersion of the composite networks. This novel, porosity-tailored, high-flux membrane holds great potential for use in industrial postcombustion CO2 capture.

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1. INTRODUCTION Excessive CO2 emissions into the atmosphere induced by human activities have led to environmental issues such as global warming and climate change. The inherent advantages of low-energy consumption and a negligible footprint make membrane

a

promising alternative to address these issues.1-3 Numerous membranes designed for CO2 capture and separation from gas mixtures (e.g. CO2/H2, CO2/N2, CO2/CH4) have been therefore reported over the past few decades.4-5 Generally, the membrane performance is decided by the following 3 factors: (i) flux, the amount of penetrate passing through a membrane per unit of time per unit of area; (ii) selectivity, the separation efficiency for a mixture; and, (iii) duration stability, the operation possibilities under a specific condition. The excellent processability makes polymer membranes very attractive for gas separation. Most of these membranes, however, demonstrate a “nonporous” microstructure and often suffer from the so-called Robeson’s upper bound, a trade-off between permeability (flux normalized by operation pressure and membrane thickness) and selectivity.6 In general, high selectivity of a polymer-based membrane can only be obtained by sacrificing permeability, and vice versa. Furthermore, for practical process designs, advancing membrane permeance is more attractive than raising selectivity to enhance the competitiveness of membrane for flue-gas CO2 capture.7 Membranes with a lower permeance requires a larger membrane area to achieve a same separation target, which raises cost. PolarisTM is a well-known commercial membrane manufactured by Membrane Technology and Research (MTR), Inc. This membrane presents only a 3



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moderate selectivity (CO2/N2, ~50) while its CO2 permeance (>1000 GPU) is much higher than most conventional polymer-based membranes.7 Membrane thinning is one of the most promising approaches to achieve high flux. Thus far, many of the developed thin-film composite (TFC) membranes provide high permeance (> 1,000 GPU) but only moderate selectivity (20-50).7-14 However, very thin membranes are generally not sufficiently robust for practical use under harsh conditions that include high temperature and/or high pressure. Alternatively, amorphous silicabased membranes have therefore attracted a considerable amount of attention due to their operation stabilities under harsh conditions. These membranes are generally processed via a facile sol-gel method and demonstrate ultramicropores (< 7 Å).15-16 The ultramicroporous structure permits amorphous silica membranes to separate penetrating gas molecules via molecular sieving and/or surface diffusion mechanisms.17 However, the close kinetic diameters of CO2 (3.3 Å) and N2 (3.64 Å) make tuning of the membrane pore sizes for high-performance separation a challenge. To overcome this limitation, various CO2-philic moieties (e.g. amine and carboxyl groups) that can significantly improve CO2 transport via surface diffusion (and/or solution-diffusion) have been therefore introduced to the amorphous silica microporous structure.18-25 Indeed, with the introduction of flexible CO2-philic moieties, the formation of a dual flexible-rigid membrane matrix has resulted in attractive increases in CO2/N2 permselectivity. However, in our previous studies, we found that the amine type played an important role in CO2 transport behaviors.26-29 The membranes functionalized with sterically hindered amines demonstrated the greater potential for CO2 permeation due 4


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to the reduced CO2 binding energy (or mild-affinity). Pyrimidine-bridged organosilica membranes

fabricated

from

4,6-bis(3-(triethoxysilyl)-1-propoxy)-1,3-pyrimidine

(BTPP) feature a mild-affinity to CO2 and have shown greater potential for CO2/N2 separation in comparison with strong-affinity membranes in our previous studies.26-27 The drive to develop membranes that possess an even higher level of permeance have persisted, however, via improvements to ultramicroporosity and amine accessibility. In the present study, we proposed a new method to further improve the ultramicroporosity of amine-functionalized organosilica membranes via the introduction of intrinsically rigid network precursors, as illustrated in Scheme 1. The typical dual flexible-rigid network precursor, BTPP, was selected as the starting material, and a second intrinsically rigid network precursor (TEOS or BTESE) was used to tailor the porosity of the resultant network via the co-condensation of BTPP/TEOS or BTPP/BTESE systems. The ultramicroporosity-tailored networks can be formed after thermal treatment at a moderate temperature (300 oC). The introduction of rigid moieties has not only improved ultramicroporosity but also largely destroyed the short-range ordered packing of the rich organic moieties of BTPP. Consequently, the ultramicroporosity-tailored membranes demonstrated surprising increases in CO2 permeance while maintaining moderate CO2/N2 selectivity, which may find important applications in practical post-combustion CO2 capture.

5



Dual flexible-rigid network precursor

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Intrinsically rigid network precursors OEt EtO Si OEt OEt

BTESE

TEOS

BTPP

O O Si

O

O

O

Si O Si

Py

Py

Si O O

O

O

O

Si O Si

Py

Si O Si O O

O O Si

Si O Si

Py

O

O

Py

Si O O

O

Dual flexible-rigid network (BTPP) O O Si

(1) H2O/H+ (2) N2, 300 oC O

O

O Si

Py

Si O O

O

O O Si O O

BTPP

O

O Py

O

O

O

Si O Si O Si O

O

O Si

Py

Py

O

Si O O

Si O Si O Si O O

O

O

Si O Si O Si O

O

O

Porosity-tailored network I (BTPP/TEOS) O O Si

O Py

O Si

Si O Si

O

O

O Si

Py

O

O

Si O Si

O Si O

O

O

Py

Si O O

Si O Si

Si O

O

O

O

Porosity-tailored network II (BTPP/BTESE)

Scheme 1. Illustration of the formation of a porosity-tailored network via the cocondensation of a dual flexible-rigid network precursor (BTPP) and intrinsically rigid network precursors (TEOS or BTESE). Step 1: pre-hydrolysis of (organo)silica precursors under acidic conditions; step 2: thermal treatment at 300 oC under a N2 atmosphere.

2. EXPERIMENTAL SECTION 6


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2.1. Preparation of composite (organo)silica sols, xerogel powders/films, and membranes Composite (organo)silica sols were synthesized via hydrolysis and polycondensation reactions of (organo)silica precursors, as previously reported.25-27, 30 BTPP mixed with equivalent moles of either TEOS or BTESE precursors were prehydrolyzed under an organosilica/H2O/HCl/EtOH system with a molar ratio of 1/120/0.3. (Detailed compositions can be found in Table S1, ESI-1). The hydrolysis and polycondensation reactions were conducted at ambient temperature for 12 h, and the resultant sols were stored in a refrigerator at 4 oC for future use. Individual TEOS, BTESE and BTPP sols were also fabricated analogously as the control. All the sols were named after their precursors, as were the analogous xerogel powders/films. The xerogel powders used for characterizations were prepared via the evaporation of solvent at 50 oC in petri dishes. The xerogel films used for the characterization of surface morphology and chemistry, as well as for optical and structural properties, were fabricated via spin coating the specimen sols onto silicon wafers or quartz substrates, followed by calcination at 300 oC under a N2 atmosphere. Porous α-alumina tubes (porosity: 50%, average pore size: 1 µm, length: 100 mm, outside diameter: 10 mm), kindly supplied by the Nikkato Corporation (Japan), were used as supports for the membranes. A preformed SiO2-ZrO2 composite colloidal sol (2 wt.%) was used as the binder for the coating of a layer of α-alumina particles and for the formation of a SiO2-ZrO2 sublayer.31 A 10 wt.% concentration of α-alumina particles with average diameters of either 2 or 0.2 µm was evenly dispersed into the as7



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prepared SiO2-ZrO2 colloidal sol via ultrasonic processing. To minimize the pore size, these α-alumina particle dispersions were sequentially coated onto the outer surface of a porous support, followed by calcination at 550 oC in air for 15 min. The procedures for each type of the α-alumina particles were repeated 3 times to form a uniform layer. The individual SiO2-ZrO2 sol (~0.5 wt. %) was then coated onto the α-alumina particle layer to further minimize the average pore size to approximately 1 nm, followed by calcination at 550 oC in air for 15 min (Pore size evaluation of the intermediate layer before top layer coating can be found in Figure S6, ESI-5). The topmost selective layer was finished with a coating of BTPP-derived sols (~0.25 wt.%), followed by calcination at 300 oC under N2 for 1 h. The resultant membranes were also named after their precursors. 2.2. Characterization of composite (organo)silica sols, xerogel powders/films, and membranes The sizes of the sols were analyzed via dynamic light scattering (DLS) using a Malvern Zetasizer Nano ZS (Malvern Instruments, Ltd.) at 25 °C. The optical properties of the diluted sols and xerogel films coated on quartz substrates were analyzed using an ultraviolet-visible (UV–vis) spectrophotometer (V-570, JASCO, Japan). The thermal stabilities of the initial xerogel powders were established via thermogravimetric mass spectrometer (TG-MS, TGA-DTA-PIMS 410/S, Rigaku, Japan). Details of the microstructures of the xerogel powders fired at 300 oC under a N2 atmosphere were evaluated via N2 adsorption/desorption isotherms at -196 oC using BELMAX equipment (BEL JAPAN INC., Japan). Prior to the measurement, xerogel samples were 8


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evacuated at 180 oC for 12 h to remove the adsorbed water molecules. True densities of the same xerogel powders were also measured via gas displacement at 40 oC using the same equipment, with He as a replacement gas. The surface properties of xerogel films coated on silicon wafers were assessed using Fourier transform infrared (FTIR) spectroscopy (FTIR-JASCO, Japan), a 3D laser scanning confocal microscope (LSCM, VK-X200 series, KEYENCE, Japan), and a contact angle meter (DM-300, Kyowa Interface Science, Japan). The crystal structure information of xerogel films coated on silicon wafers was analyzed using a D2 PHASER X-ray diffractometer (XRD, Bruker, Germany) employing Cu K-α as the X-ray radiation source (1.54 Å). The crosssectional morphologies of the resultant membranes were determined using a JCM 5700 scanning electron microscope (SEM, JEOL, Japan). Positron annihilation lifetime spectroscopy (PALS) for the xerogel films on silicon wafers was conducted at a positron incident energy of 3 keV and at room temperature under vacuum using a

22Na-based

pulsed positron beam at the National Institute of

Advanced Industrial Science and Technology (AIST) as described elsewhere.30,

32

Average lifetimes longer than 1 ns, due to the annihilation of o-Ps, were determined as τ3 with the respective relative intensities, I3, along with the shorter average lifetime components of τ1 and τ2 due to the annihilations of p-Ps and free positrons. The pore radius, r, for the films was estimated from τ3 based on a semi-empirical quantum mechanical model developed by Tao and Eldrup,33-34 as shown in equation (1). 𝑟

1

2𝜋𝑟

𝜏3 = 0.5[1 ― 𝑟 + 0.166 + 2𝜋𝑠𝑖𝑛 (𝑟 + 0.166)] ―1

(1)

2.3. Gas adsorption evaluation of composite (organo)silica xerogel powders 9



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N2 and CO2 adsorption/desorption properties of xerogel powders at near-ambient temperatures were also performed using BELMAX equipment (BEL JAPAN INC., Japan). Henry’s law was used to describe the N2 adsorption behaviors while a dualmode sorption model that has been widely adopted to assess the gas sorption properties in glassy polymers was used for modeling CO2 adsorption isotherms.35 Generally, the dual-mode model can be regarded as a sum of two modes of sorption sites that obey Henry’s law of dissolution and a Langmuir-type sorption. The dual-mode sorption model is expressed in equation (2). 𝐶𝐻∗ 𝑏𝑝

(2)

𝐶 = 𝐶𝐷 + 𝐶𝐻 = 𝑘𝐷𝑝 + 1 + 𝑏𝑝

where C is the total gas concentration in the xerogel powders, CD and CH are the CO2 concentrations contributed from Henry’s law and Langmuir-type sorption, respectively, kD is the sorption coefficient based on Henry’s law, and 𝐶𝐻∗ and b are the Langmuir parameters in terms of maximum capacity and affinity constant, respectively. The isosteric heat (Qst) for adsorption was calculated using an integrated form of the Clausius-Clapyron equation (equation (3)) based on the adsorption information at 5 and 25 oC. 𝑝1

𝑇1𝑇2

(3)

𝑄𝑠𝑡 = 𝑅𝐼𝑛𝑝2(𝑇2 ― 𝑇1)

where p1 and p2 are the equilibrium pressures at the temperatures of T1 and T2, respectively, for the same amount of gas adsorbed, and R is the universal gas constant. 2.4. Membrane performance evaluation Single-gas (He, 2.6 Å; H2, 2.89 Å; CO2, 3.3 Å; N2, 3.64 Å; CH4, 3.8 Å; CF4, 4.7 Å; SF6, 5.5 Å) permeation tests were conducted based on a constant pressure technique at 10


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temperatures ranging from 35 to 200 oC with the experimental setup schematically shown in Figure S2, ESI-3. Values for gas permeance and ideal selectivity were calculated using equations (4) and (5), respectively. 𝐹𝑖

𝑃𝑖 = 𝐴∆𝑝𝑖

(4)

𝛼𝑖𝑗 = Pi

(5)

Pj

where Pi and Pj [mol/(m2 s Pa)] are the permeance values for components i and j, respectively, Fi [mol/s] is the molar flow rate of component i, A [m2] is the membrane effective area, ∆pi [Pa] is the partial pressure drop of component i between the shell and the inner side of the tubular membrane, and αij is the ideal permselectivity (permeance ratio) of component i over j.

3. RESULTS AND DISCUSSION 3.1. Appearance and chemistry of composite (organo)silica sols and xerogel films Figure 1a shows the digital photos of as-synthesized individual and composite (organo)silica sols. Both types of sols demonstrated a clear and transparent appearance with an obvious Tyndall effect, which suggested a smaller sol size and good compatibility of the composite sols. The particle sizes of these sols measured via DLS ranged from 2 to 20 nm, as shown in Figure 1b. Figure 1c and d show the UV-vis absorption spectra for (organo)silica sols and xerogel films after firing at 300 oC under a N2 atmosphere. Both individual BTPP and composite sols exhibited impressive absorption peaks centered at 208, 235, and 276 nm due to the electron transitions from π to π* (208 and 235 nm) and n to π* (276 nm), respectively, as a result of the existence 11



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of the aromatic pyrimidine linking groups in BTPP. However, these absorption peaks turned to broad and flat types that ranged from 200 to 280 nm for corresponding xerogel films after firing at 300 oC under N2, which probably was a result of the formation of complex, amorphous (organo)silica networks that led to multi-mode, complex conjugations and vibrations of the pyrimidine groups. 2D LSCM images of (organo)silica xerogel films coated on silicon wafers are shown in Figure 1e. Both individual and composite (organo)silica pre-hydrolyzed sols formed continuous and uniform films with smooth or rough surfaces after firing at 300 oC, but aggregated particles were observed in partial regions. The continuous and uniform structure of composite xerogel films (BTPP/TEOS and BTPP/BTESE) can be further verified based on the 3D LSCM images shown in Figure 1f and g. In addition, the composite xerogel films (58 o for BTPP/TEOS and 52 o for BTPP/BTESE) demonstrated a compromised surface hydrophilicity that resided between either hydrophilic TEOS (38 o) or BTESE (42 o) and less hydrophilic BTPP (66 o) xerogel films based on the water contact angles, as illustrated in the inset of Figure 1e.

12


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Figure 1.

Appearance and chemistry of individual and composite (organo)silica sols

and xerogel films. (a) Digital photos of as-synthesized (organo)silica sols with the Tyndall effect. (b) Particle size of as-synthesized (organo)silica sols at 25 oC. UV-vis absorption spectra of (c) diluted (organo)silica sols (in a 1x1 cm quartz cell) and (d) xerogel films fired at 300 oC under a N2 atmosphere (coated on quartz sheets with a size of 1.5x1.5 cm). (e) 2D LSCM images of (organo)silica xerogel films coated on silicon wafers fired at 300 oC under a N2 atmosphere (inset: water contact angle on the 13



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corresponding film). 3D LSCM images for (f) BTPP/TEOS and (g) BTPP/BTESE composite xerogel films coated on silicon wafers fired at 300 oC under a N2 atmosphere.

3.2. Thermal stabilities and structural characterizations of composite (organo)silica xerogel powders/films The thermal stabilities of the initial composite xerogel powders/films were verified using TG (powders) and FTIR (films) techniques, and the results are depicted in Figure 2a and 2b, respectively. Individual TEOS and BTESE specimens with intrinsically rigid structures (Si-O-Si) displayed an extremely lower weight loss throughout the entire temperature range. By contrast, both BTPP and composite (organo)silica specimens demonstrated impressive weight losses that were about 30 wt.% higher than individual TEOS and BTESE because of the pyrolysis of the rich pyrimidine linking units between rigid Si-O-Si domains. It is worth noting that the composite (organo)silica specimens showed slightly lower weight losses by comparison with BTPP, as a result of the increased proportion of more thermally stable units (see Scheme 1). Based on the results of our recent studies,26-27 BTPP xerogel specimens can be thermally stable up to 300 oC under an inert atmosphere. Therefore, in the present study, the same temperature was employed for the calcination of composite (organo)silica xerogel specimens (powders, films, or membranes), and weight losses lower than 10 wt.% were observed. These weight losses are generally a result of the evaporation of further condensation-generated water or alcohol molecules (-Si-OH + HO-Si- = -Si-O-Si- + H2O, or -Si-OH + EtO-Si= -Si-O-Si- + EtOH).26-27 The composite structures of BTPP/TEOS and BTPP/BTESE 14


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contained both flexible pyrimidine linking units and rigid Si-O-Si domains, which was further established by the FTIR spectra (Figure 1b). After calcination at 300 oC under a N2 atmosphere, the pyrimidine units and Si-O-Si domains remained in the composite structures (BTPP/TEOS and BTPP/BTESE), which was confirmed by the characteristic vibration absorption of C=N in pyrimidine (~1658 cm-1) and Si-O-Si (~1041 cm-1), respectively. This result is consistent with the UV-vis absorption spectra of composite (organo)silica specimens, as shown in Figure 1d. The microstructures of the composite (organo)silica specimens were assessed via N2 adsorption isotherms at -196 oC, X-ray diffraction, and PALS techniques. Figure 2c and d show the N2 adsorption isotherms at -196 oC under relative pressure that ranged from 10-4 to 1.0. Individual TEOS and BTESE specimens formed intrinsically rigid networks and demonstrated impressively high levels of N2 adsorption that ranged from 100 to 150 cm3(STP)/g at P/P0=0.01 relative to a pore size of ~1 nm based on the Kelvin equation, which suggested a typical microporous structure. BTPP featuring a dual flexible-rigid structure has previously shown negligible levels of N2 adsorption, which indicates a nonporous network.26-27 In the present study, the introduction of a second intrinsically rigid network (TEOS or BTESE) into a BTPP-derived network, however, failed to evidently improve the rigid micropore volume for composite (organo)silica networks, which was indicated by a negligible amount of N2 adsorption. Nevertheless, a careful comparison of N2 adsorption between individual BTPP and composite (organo)silica specimens (Figure 2d) revealed a very small increase in N2 adsorption for the latter type. Indeed, it is generally difficult to use N2 adsorption/desorption 15



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isotherms at -196 oC to probe the microstructure on an ultramicro scale (< 0.7 nm) due to diffusion problems for the molecules inside pores.36 Nevertheless, a comparison of the specific surface area (SBET) and the total pore volume (Vp) calculated from the N2 adsorption isotherms among individual and composite (organo)silica specimens, listed in Table 1, suggested that the composite (organo)silica specimens presented slightly improved microporosity that featured small increases in both SBET and Vp. In addition, comparison of the true densities probed by He at 40 oC (also listed in Table 1) demonstrated a similar trend with both SBET and Vp. Further verification was obtained for the sub-nano-scale microstructure via X-ray diffractions (Figure 2e and Figure S1) and showed that the individual BTPP specimen demonstrated two broad peaks centered at 6.6 o (d-spacing, 1.3 nm) and 21 o (d-spacing, 0.41 nm), which can be ascribed to a short-range ordered lamellar structure contributed by the pyrimidine linking units and silicate lattice (Si-O-Si), respectively. Generally, such sub-nano- and nano-scale lamellar structures can be observed in aromatic groupbridged organosilica materials due to hydrophobic-hydrophilic and/or π-π interactions that can direct the formation of either short-range or long-range ordered structures, as reported by Inagaki et al.37 On the other hand, TEOS or BTESE intrinsically rigid network precursors showed quite broad, or almost flat, diffraction peaks due to an almost purely amorphous structure. Interestingly, however, after the incorporation of either TEOS or BTESE, the composite (organo)silica specimens also demonstrated a highly amorphous microstructure without any impressive diffraction peaks that can be observed in individual BTPP. This suggested that the short-range ordered lamellar 16


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structure was destroyed by the incorporation of intrinsically rigid network precursors. Careful observation of the XRD patterns shown in Figure S1 indicated that the diffraction peaks from both the lamellar structure and the silicate lattice of the individual BTPP were shifted to higher 2 theta angles, suggesting a denser packing of silicate lattice (as revealed by true densities in Table 1) and an improved distribution of pyrimidine linking units. Figure 2f shows the PALS data for the composite (organo)silica xerogel films fabricated on silicon wafers. The data for either individual or composite (organo)silica specimens clearly displayed a long-lived component (>1 ns) due to the annihilation of o-Ps in either rigid or free-volume pores. The average lifetimes of the o-Ps components were longer on the order of BTPP < BTPP/BTESE < BTPP/TEOS 3,000) of BTPP/BTESE membranes, these phenomena probably indicate the formation of a novel composite (organo)silica network induced by the molecular-scale distribution of two different (organo)silica precursors. Similar situations were also observed for the BTPP/TEOS membranes, as shown in Figure S9, ESI-10. These observations are consistent with the conclusions mentioned above suggesting that the composite (organo)silica networks were mixed in a molecular scale, where the formation of a single-type network from each individual precursor was restricted. Generally, predictions of the gas permeation behavior of mixed-matrix membranes are accurate when using the Maxwell model, where the dispersed phase disperses in the continuous phase at a micro-to-macro level and each phase retains its individual gas permeation performance. In the present study, the synthesized composite (organo)silica membranes far surpassed the Maxwell prediction with a very high flux and comparable selectvity, which indicates the formation of a new network that can overcome the limitations of each original network. TEOS, with no organic linking units, usually can generate a highly microporous silica network but with a smaller pore size. BTPP, however, possesses a bit longer pyrimidine linking unit, which leads to the formation of a dual flexible-rigid network with dense packing of chains and a partially ordered structure that limits high-flux CO2 permeation. Mixing the composite (organo)silica on a molecular scale, however, could result in a good balance between a uniform distribtion of pyrimidine linking units and an ultramicroporous structure, which would amount to high-flux CO2 permeation while maintaining an attractive level of CO2/N2 selectivity. 28


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In Figure 7, the CO2/N2 separation performance of the composite (organo)silica membranes synthesized in the present study are compared with the reported state-ofthe-art thin film composite (TFC) and zeolite-based membranes.7-12, 55-60 The optimum region for post-combustion capture based on the economics, proposed by Merkel et al., is also included for comparison. Typically, membranes with a high permeance, > 1,000 GPU (1 GPU =3.348 x 10-10 mol/(m2 s Pa)), and moderate selectivity, > 20, can reach the optimum region. Obviously, membranes with high permeance and moderate selectivity are more useful and cost-effective for industrial applications than lowpermeance membranes, which has resulted in many reports of high-flux membranes, such as TFC membranes.7-12,

55, 56

By comparison with these reported high-flux

membranes, our composite (organo)silica membranes with an ultrahigh permeance of ~2,000 GPU and a moderate selectivity of ~20, demonstrated greater potential to reach the optimum region. In addition, the as-synthesized composite (organo)silica membranes demonstrated sufficiently high thermal stability for practical CO2 capture processes and a satisfactory binary separation performance as shown in Figure S11, ESI-10 and Figure S12, ESI-11. It is worth noting that individual or composite (organo)silica membranes can be effortlessly fabricated on either ceramic (this work) or polymer supports via either sol-gel or Plasma Enhanced Chemical Vapor Deposition (PECVD) methods under mild conditions.61-64 Therefore, this novel, high-flux membrane shows great promise for use in practical applications. On the other hand, the composite (organo)silica membranes that can withstand operation temperatures as high as >200

oC

also demonstrate potential applications in membrane reactor for 29



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transformation of CO2, such as, synthesis of dimethyl carbonate from methanol and CO2.65

Figure 5. (a) Kinetic diameter dependence of gas permeance at 200 oC for composite (organo)silica membranes and a comparison with reported TEOS and BTESE membranes. (b) Comparison of CO2 separation performance of individual BTPP and composite (organo)silica membranes based on a single-gas permeation test at 35 oC. (c) Illustration of the possible difference between a BTPP-derived network via monocondensation and a porosity-tailored network via co-condensation of BTPP and TEOS or BTESE.

30


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Figure 6. Trade-off between CO2 permeance and CO2/N2 selectivity for individual BTESE and BTPP and composite BTPP/BTESE membranes, and a comparison with the Maxwell prediction. The Maxwell predictions are shown in solid lines. The BTESE data were adapted from ref. 46. 2

CO2 permeance [mol/(m s Pa)] 10

CO2/N2 selectivity [-]

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


-8

10

-7

10

Polymer Zeolite (Organo)silica This work

10

Optim um region P7

2

Z1

P4 Z3 BTPP

10

BTESE

P1 Z6

P5

P3

BTPP/TEOS

P8 BTPP/BTESE P6 P7 TEOS @200 oC Z4 P2

1

10

-6

1

2

3

10 10 CO2 permeance [GPU]

10

4

Entry

Membrane material

Ref.

P1

PolarisTM

7

P2

MCM-41/PDMS

55

P3

PEG-based

9

P4

Pebax1657/PEG

56

P5

SFSNPS/PEG

8

P6

PDMA-ODA

10

P7

Pebax2533

11

P8

BPVE-PFCB

12

Z1

Silicalite-1

57

Z2

Na-ZSM-5

58

Z3

Zeolite-like

59

Z4

SAPO-34

60

TEOS

TEOS

48

BTESE

BTESE

46

Figure 7. Comparison of CO2/N2 separation performance at 20-50 oC of composite (organo)silica membranes with that reported for high-flux polymer- and zeolite-based membranes.

31



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4. CONCLUSIONS A novel, porosity-tailored composite (organo)silica membrane with high flux was successfully synthesized via the hybridization of a dual flexible-rigid network precursor (BTPP) and a second intrinsically rigid network precursor (TEOS or BTESE) that serves as a network modifier. Improvements in ultramicroporosity were primarily confirmed by XRD and PALS. The composite (organo)silica xerogel specimens presented noticeable increases in both adsorption amount and isosteric adsorption heat for CO2, indicating the increases in ultramicroporosity and consequent increases in amine accessibility in BTPP network. Compared with individual BTPP membranes, the composite (organo)silica membranes presented a significant increase in CO2 permeance (>2000 GPU) while remained a moderate CO2/N2 selectivity (~20). This surprising increase is attributed to the fine-tuning of the microstructure, where the short-range ordered lamellar structure is destroyed by a second more amorphous and rigid network. The created ultramicroporosity improves the amine accessibility that facilitates CO2 transport within the membrane. Considering that this novel, porosity-tailored, high-flux membrane can be easily developed on various types of membrane supports, independent of materials (polymer or ceramic) and geometric shapes (flat or tubular), it holds great potential for use in industrial post-combustion CO2 capture or CO2 transformation under relatively high temperatures.

ASSOCIATED CONTENT Supporting Information 32


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The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthesis of individual and composite (organo)silica sols, XRD patterns for individual and composite (organo)silica xerogel films, Gas permeation/separation set-up used in this study, Gas adsorption properties of individual and composite (organo)silica xerogel powders, Pore size evaluation of the membrane supports, Temperature dependence of gas permeance for individual and composite (organo)silica membranes, Trade-off comparison for individual and composite (organo)silica membranes, Comparison of CO2/N2 permeation/separation potential based on activation energies for individual and composite (organo)silica membranes, Comparing membrane performance with the Maxwell prediction for BTPP/TEOS composite membranes, Thermal stability of BTPP/TEOS composite membranes, Binary gas separation of BTPP/TEOS composite membranes AUTHOR INFORMATION Corresponding Author Tel: +81 824247714; E-mail: [email protected] Notes The authors declare no competing ﬁnancial interest. REFERENCES (1) Gin, D. L.; Noble, R. D. Designing the Next Generation of Chemical Separation Membranes. Science 2011, 332 (6030), 674-676. (2) Car, A.; Stropnik, C.; Yave, W.; Peinemann, K. V. Tailor‐Made Polymeric 33



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Membranes based on Segmented Block Copolymers for CO2 Separation. Adv. Funct. Mater. 2008, 18 (18), 2815-2823. (3) 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. (4) Pera-Titus, M. Porous Inorganic Membranes for CO2 Capture: Present and Prospects. Chem. Rev. 2013, 114 (2), 1413-1492. (5) Chen, Q.; Luo, M.; Hammershøj, P.; Zhou, D.; Han, Y.; Laursen, B. W.; Yan, C.G.; Han, B.-H. Microporous Polycarbazole with High Specific Surface Area for Gas Storage and Separation. J. Am. Chem. Soc. 2012, 134 (14), 6084-6087. (6) Robeson, L. M. The Upper Bound Revisited. J. Membr. Sci. 2008, 320 (1-2), 390400. (7) Merkel, T. C.; Lin, H.; Wei, X.; Baker, R. Power Plant Post-Combustion Carbon Dioxide Capture: An Opportunity for Membranes. J. Membr. Sci. 2010, 359 (1-2), 126139. (8) Kim, J.; Fu, Q.; Xie, K.; Scofield, J. M. P.; Kentish, S. E.; Qiao, G. G. CO2 Separation Using Surface-Functionalized SiO2 Nanoparticles Incorporated Ultra-Thin Film Composite Mixed Matrix Membranes for Post-Combustion Carbon Capture. J. Membr. Sci. 2016, 515, 54-62. (9) Fu, Q.; Kim, J.; Gurr, P. A.; Scofield, J. M. P.; Kentish, S. E.; Qiao, G. G. A Novel Cross-Linked Nano-Coating for Carbon Dioxide Capture. Energy Environ. Sci. 2016, 9 (2), 434-440. 34


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43



Scheme 1. Illustration of the formation of a porosity-tailored network via the co-condensation of a dual flexible-rigid network precursor (BTPP) and intrinsically rigid network precursors (TEOS or BTESE). Step 1: pre-hydrolysis of (organo)silica precursors under acidic conditions; step 2: thermal treatment at 300 oC under a N2 atmosphere.


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Figure 1. Appearance and chemistry of individual and composite (organo)silica sols and xerogel films. (a) Digital photos of as-synthesized (organo)silica sols with the Tyndall effect. (b) Particle size of as-synthesized (organo)silica sols at 25 oC. UV-vis absorption spectra of (c) diluted (organo)silica sols (in a 1x1 cm quartz cell) and (d) xerogel films fired at 300 oC under a N2 atmosphere (coated on quartz sheets with a size of 1.5x1.5 cm). (e) 2D LSCM images of (organo)silica xerogel films coated on silicon wafers fired at 300 oC under a N2 atmosphere (inset: water contact angle on the corresponding film). 3D LSCM images for (f)

BTPP/TEOS and (g) BTPP/BTESE composite xerogel films coated on silicon wafers fired at 300 oC under a N2 atmosphere.



Figure 2. Thermal stabilities and structural characterizations of individual and composite (organo)silica xerogel powders/films. (a) TG curves of initial xerogel powders under a He atmosphere (ramping rate, 10 oC/min; He, 300 cm3/min). (b) FTIR spectra of (organo)silica xerogel films coated on silicon wafers (The absorbance was normalized based the absorption peak of Si-O-Si). (c and d) N2 adsorption isotherms at -

196 oC of (organo)silica xerogel powders fired at 300 oC under a N2 atmosphere. (e) XRD patterns and (f) PALS data (recorded at a positron incident energy of 3 keV) for (organo)silica xerogel films coated on silicon wafers.


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Figure 3. Gas adsorption properties of individual and composite (organo)silica xerogel powders fired at 300 oC under a N atmosphere. (a) N adsorption isotherms at 25 oC (TEOS is overlapped with BTESE. Solid 2 2 lines: fitting curves according to Henry’s law). (b) CO2 adsorption isotherms at 25 oC (Solid lines: fitting

curves according to dual-mode adsorption). Adsorption heat of (c) N2 and (d) CO2 for temperatures ranging from 5-25 oC. CO2 and N2 adsorption isotherms at other temperatures are shown in Figure S3 and S4, ESI4.



Figure 4. (a) Digital photo of the tubular ceramic membrane used in the present study. (b) Cross-sectional SEM image of an individual BTPP membrane.


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Figure 5. (a) Kinetic diameter dependence of gas permeance at 200 oC for composite (organo)silica membranes and a comparison with reported TEOS and BTESE membranes. (b) Comparison of CO2 separation performance of individual BTPP and composite (organo)silica membranes based on a single-gas permeation test at 35 oC. (c) Illustration of the possible difference between a BTPP-derived network via mono-condensation and a porosity-tailored network via co-condensation of BTPP and TEOS or BTESE.



Figure 6. Trade-off between CO2 permeance and CO2/N2 selectivity for individual BTESE and BTPP and composite BTPP/BTESE membranes, and a comparison with the Maxwell prediction. The Maxwell predictions are shown in solid lines. The BTESE data were adapted from ref. 46.


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Figure 7. Comparison of CO2/N2 separation performance at 20-50 oC of composite (organo)silica membranes with that reported for high-flux polymer- and zeolite-based membranes.



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Tailoring Ultramicroporosity To Maximize CO2 Transport within

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