Amino-decorated organosilica membranes for highly permeable CO2 capture
Graphical abstract
Introduction
Excessive emissions of CO2 have raised a series of environmental issues that are attracting intense levels of public scrutiny [[1], [2], [3]]. Compared with conventional separation processes, membrane-based separation techniques provide a promising strategy to achieve the goal of effective and energy-efficient CO2 capture [[3], [4], [5]]. Hence, a variety of membranes that include organic [6], inorganic [7,8], and organic-inorganic hybrid varieties [9] have been used extensively for CO2 separation. Among these, organosilica membranes with high hydrothermal stability and impressive molecular sieving properties have attracted a great deal of interest, and have shown great promise for use in CO2 separation [[10], [11], [12], [13]]. However, it is intrinsically challenging to adjust the organosilica membrane structure with a pore size appropriate for high-performance CO2 separation considering the close kinetic diameters of CO2 (0.33 nm), N2 (0.364 nm), and CH4 (0.38 nm). In general, reinforcing the CO2-philic performance and the molecular sieving properties of membranes has been deemed beneficial in promoting highly efficient CO2 capture. The preferentially adsorbed CO2 molecules on the membrane surface or network structures may hinder the permeation of other gases, as a result, the CO2 separation performance are expected to be effectively enhanced. Therefore, we expected the combination of CO2-philic and molecular-sieving properties to accelerate the exploitation of membranes for CO2 separation.
A specific interaction can be generated between CO2 molecules and amino groups [14], and (3-aminopropyl) triethoxysilane (APTES), which consists of aminopropyl groups, has been used both in the fabrication of sensors and as a modifying agent of adsorbents [15,16]. Consequently, with regard to membrane separation, the existence of amino groups in APTES is expected to accelerate the CO2 separation properties of APTES-derived membranes. The resultant APTES membranes, however, have always been beset by the densified structures and the low utilization efficiency of amino groups, which seriously inhibits the application of highly permeable CO2 membranes [11,14]. An APTES membrane reported by Xomeritakis et al. produced a low level of CO2 permeance at 2.6 ✕ 10−8 mol m−2 s−1 Pa−1 (77.7 GPU, 1 GPU equals to 3.348 ✕ 10−10 mol m−2 s−1 Pa−1), albeit with a satisfyingly high CO2/N2 selectivity of 70 [14]. Consequently, APTES has been preferred as a modifier for possible advancements in CO2 separation performance. Suzuki et al. fabricated a hybrid silica membrane via a chemical vapor deposition strategy that used tetraethylorthosilicate (TEOS) and APTES as precursors, and the resultant membrane returned a CO2 permeance of 2.3 ✕ 10−7 mol m−2 s−1 Pa−1 (687 GPU) with a CO2/CH4 selectivity of 40 [17]. In fact, the modification of membranes using organosilica has been a common strategy for elevating the gas separation performance for other types of inorganic membranes [18,19]. The APTES-functionalized ZIF-90 membrane exhibited improved H2/X selectivities (X represents CO2, CH4, C2H6, C3H8) with a relatively high H2 permeance of 2.9 × 10−7 mol m−2 s−1 Pa−1 [18]. The incorporated APTES narrowed the membrane pore size and eliminated the formation of intercrystalline defects. A similar strategy was also applied to the SAPO-34-derived zeolite membrane [19]. The bis(triethoxysilyl)ethane (BTESE) precursor was used to functionalize the SAPO-34 membrane via a strategy of vacuum-assisted deposition to selectively patch the defects, and the as-synthesized composite membrane displayed improved CO2/CH4 selectivity.
In our previous study, an ultramicroporosity-tailored composite organosilica membrane was fabricated by the co-condensation of 4,6-bis(3-triethoxysilyl-1-propoxy)-1,3-pyrimidine (BTPP), which is CO2-philic, and BTESE (or TEOS). This membrane produced an increase in CO2 permeance (~2000 GPU) and a CO2/N2 selectivity that reached ~20 [12]. Generally, high permeance is preferred over high selectivity since the increased permeance through a membrane without a loss of selectivity has been recognized as more important in industrial scenarios. As we know, improving the selectivity to a level of more than 30 will not significantly decrease the operational costs, but enhancing CO2 permeance would [1]. Hence, a higher possible level for CO2 permeance with moderate CO2/N2 selectivity (20–30) would be the most desirable model for energy-efficient CO2 capture [20,21]. The intrinsically dense structure of a TEOS network and the flexible ethane-bridges of a BTESE structure [22,23], however, still restrict the further advance of CO2 separation performance for the resultant composite-membranes, particularly in the case of ultrahigh CO2 permeance.
In the most recent work by our group, the network structure and permeation properties of organosilica membranes were effectively tailored via the control of bond angles [23]. By comparison with BTESE and bis(triethoxysilyl)ethylene (BTESEthy) membranes, bis(triethoxysilyl)acetylene (BTESA) membranes, feature rigid acetylene bridges and a more open and accessible network structure that attributed to the increased bond angles, which has been confirmed via FT-IR spectra and molecular simulation. BTESA membranes show great potential in C3H6/C3H8 separation due to a relatively larger and uniform pore size (around 0.52 nm) [24,25]. Nevertheless, this membrane has been difficult to use in either CO2/N2 or CO2/CH4 separations because of the small molecular sizes of CO2 (0.33 nm), N2 (0.364 nm), and CH4 (0.38 nm). For instance, the selectivity of CO2/N2 (around 7 to 13) was much lower than the target separation performance (CO2 permeance > 1000 GPU, CO2/N2 > 20) [1], even though the CO2 permeance (~6500 GPU) was sufficiently high. Consequently, a design that could effectively increase the CO2/N2 selectivity while ensuring there is no significant sacrifice in the level of CO2 permeance would be a worthwhile development for BTESA membranes.
Herein, APTES was introduced into a BTESA membrane matrix to tailor the microstructure and advance the CO2 capture performance, as illustrated in Fig. 1. A plausible schematic illustration of the BTESA, BTESA-A, and APTES membrane structures appears in Fig. S1 of the supporting information. A co-condensation strategy, which is authoritative and advantageous in fine-tuning the physico-chemical structures of organosilica-based materials, was used to realize the target. The decoration of APTES in a BTESA matrix (referred to BTESA-A) enhanced the affinity of composite membranes to CO2 molecules and simultaneously controlled the membrane pore size, which was confirmed by the measurements of N2 sorption, CO2 isosteric heat of adsorption, and gas permeation properties. In contrast to the pure BTESA and APTES membranes, the composite BTESA-A membranes achieved a high CO2 separation performance that is unprecedented and superior to most of the membranes reported in the literature. The fabricated membranes and the composite strategy reported in the present study provide novel opportunities for high-performance membranes that can be used in critical separation processes.
Section snippets
Sol preparation and membrane fabrication
The (composite) organosilica sols were prepared via hydrolysis and condensation reactions using homemade BTESA [26] and APTES precursors, water, and HNO3 in ethanol. The mixtures were stirred continuously for 2 h in a closed bottle at 50 °C. The molar composition of the reagents was as follows: (composite) organosilica precursors/H2O = 1/240. For the preparation of the individual BTESA sol, the BTESA/HNO3 ratio was controlled at 1/0.01. To suppress the gelation of APTES and control the reaction
Structural characterization of (composite) organosilica materials
Fig. 2 presents the FT-IR spectra of individual BTESA, APTES, and composite BTESA-A films. The functional groups of C≡C (2060 cm−1) and N–H (1630 cm−1) for BTESA and APTES films appear in Fig. 2a, respectively [26,27]. After BTESA and APTES co-polymerization reactions, the composite structure of BTESA-A was endowed with the functional groups from both of the individual precursors, which indicated a successful decoration of APTES onto the BTESA network. The Si–O–Si groups located around
Conclusions
In summary, we have described a facile and effective co-polymerization strategy that yields high-performance membranes for the capture of CO2. This approach produces composite membranes with a controlled pore size and enhanced CO2-philic properties, which was verified by the measurement of N2 sorption, CO2 heat of adsorption, and gas permeation properties. In the separation of CO2/N2 mixtures, the composite organosilica membranes exhibited high CO2 capture capacities that are unprecedented, and
Credit authorship contribution statement
Meng Guo: Writing - original draft. Masakoto Kanezashi: Writing - review & editing, Supervision. Hiroki Nagasawa: Data curation. Liang Yu: Data curation. Joji Ohshita: Resources. Toshinori Tsuru: Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Meng Guo appreciates the support of the China Scholarship Council (CSC) (No. 201708320384).
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