Fabrication and Microstructure Tuning of a Pyrimidine-Bridged

Jan 11, 2017 - To this end, a modest number of amine-functionalized silica-based membranes for CO2 separation have been synthesized via co-condensatio...
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Fabrication and Microstructure Tuning of a Pyrimidine-Bridged Organoalkoxysilane Membrane for CO2 Separation Liang Yu,† Masakoto Kanezashi,† Hiroki Nagasawa,† Joji Ohshita,‡ Akinobu Naka,§ and Toshinori Tsuru*,† †

Department of Chemical Engineering and ‡Department of Applied Chemistry, Hiroshima University, 1-4-1 Kagamiyama, Higashihiroshima, Hiroshima 739-8527, Japan § Department of Life Science, Kurashiki University of Science and the Arts, 2640 Nishinoura, Tsurajima, Kurashiki, Okayama 712-8505, Japan S Supporting Information *

ABSTRACT: A novel pyrimidine-bridged organoalkoxysilane membrane was developed from 4,6-bis(3-(triethoxysilyl)-1-propoxy)-1,3-pyrimidine (BTPP) via a sol−gel process. Self-catalyzed and HCl-catalyzed BTPP sols with different water molar ratios were prepared for membrane formation to tailor the microstructure of the BTPP membranes. A higher water molar ratio for the HCl-catalyzed sols led to the formation of a silica network with improved porosity and a well-connected structure. Gas adsorption measurements indicated that BTPP xerogels tended to show a dense silica network due to an organic-rich hybrid structure, and these also showed a higher level of CO2/N2 selectivity due to the presence of pyrimidine groups that could conduct special interactions with CO2. Single-gas permeation testing was performed at different permeation temperatures using gases with different kinetic diameters: He (2.6 Å), H2 (2.89 Å), CO2 (3.3 Å), N2 (3.64 Å), CH4 (3.8 Å), and SF6 (5.5 Å). The BTPP membranes showed a sharp kinetic diameter dependence of gas permeance with a higher level of H2/SF6 selectivity (>500). In addition, the relatively dense silica network and organic-rich properties of BTPP membranes resulted in activated diffusion for all gases considered, with the exception of SF6 that could have permeated the BTPP membranes via larger pores or pinholes. CO2 transport behaviors through BTPP membranes were compared according to activation energies for the permeation (Ep) of CO2 and by the differences in Ep between CO2 and N2 (or CH4). The BTPP-HCl-240 membrane that demonstrated the most-improved porosity and the best-connected silica network showed a lower Ep for CO2 and a greater difference in Ep between CO2 and N2 (or CH4). As a result, the BTPP-HCl-240 membrane exhibited great potential in CO2 separation performance for both CO2 permeance and CO2/gas permselectivity. Compared with most of the reported amine-functionalized silica-based membranes, BTPP membranes showed great potential in CO2 separation performance, which could lead to applications in CO2 separation processes.

1. INTRODUCTION Increases in CO2 emissions from the combustion of fossil fuels has led to global climate concerns. To address these global challenges, efficient technologies have been developed for CO2 capture: aqueous solution absorption, solid materials adsorption, cryogenic separation, and membrane separation.1 Among the alternatives, membrane technology shows the most potential and is attracting an increasing amount of attention due to inherent advantages such as high-energy efficiency, ease of operation and scale-up, excellent reliability, and a small footprint.2−6 Thus far, a variety of membrane materials with high CO2 permselectivity have been designed and synthesized for CO2 separations.3,7,8 Amorphous silica-based membranes make up one of the most important categories for fabricating membranes with excellent thermal, chemical, and high-pressure stabilities. Furthermore, amorphous silica-based membranes are always prepared via a facile sol−gel process that offers easy access to the generation of an ultrathin separation layer ( BTPP-HCl-40 > BTPPHCl-240. This suggested that the water molar ratio had the greater influence on improving the hydrolysis level of BTPP in the following order: BTPP-HCl-240 > BTPP-HCl-40 > BTPPself-12. In addition, the peak at around 1170 cm−1 of BTPPself-12 could be attributed to the vibration of Si−O−CH2CH3, and also indicated the relatively lower hydrolysis degree of BTPP-self-12 compared with that of acid-catalyzed xerogel films (BTPP-HCl-40 and BTPP-HCl-240). To further probe the possible differences in the hydrolysis and condensation levels of BTPP-derived xerogels, TG profiles were obtained, as shown in Figure 2d. All three samples presented similar thermal degradation steps, but there were differences in weight losses. For the final residual weight, the increasing order of weight loss was BTPP-HCl-240 < BTPPHCl-40 < BTPP-self-12, which suggested an opposite order for the hydrolysis and condensation degrees. When the water molar ratio is increased, more −OCH2CH3 groups could be hydrolyzed in the xerogel powders, which represented a lower weight loss in the TG process. In addition, as confirmed by Figure 2b, at temperatures below 300 °C, further hydrolysis and condensation could occur without degradation of the organic chains, which also was proved by the TG-MS results (Supporting Information). Hence, both panels c and d of Figure 2 showed that a higher water molar ratio of the BTPP sols led to a higher level of hydrolysis and condensation degree. Moreover, a highly branched silica network of BTPP-self-12 with a large sol size could have resulted in an enrichment of the Si−O−Si structure, which could have improved the thermal stabilities. Therefore, some of the unhydrolyzed ethoxy groups that existed inside the network of BTPP-self-12 xerogels, could have remained at temperatures below 300 °C, as confirmed in Figure 2c,d. 3.1.2. Reactivation of BTPP Xerogels. The activity of pyrimidine groups incorporated in a BTPP silica matrix is an important factor, because this allows the amine−CO 2 interactions that facilitate CO2 transport.18 During the sol preparation of amine-containing organoalkoxysilane, however, the use of excess acid to achieve a temporary inactivation of amine groups is required to avoid gelation of the sol. Therefore, the presence/absence of acid anions, which can bond with a

3. RESULTS AND DISCUSSION 3.1. Characterization of BTPP Sols/Gels. 3.1.1. Hydrolysis and Polycondensation Properties of BTPP Sols, Films, and Xerogel Powders. The sol−gel method permits synthesis at lower temperatures and involves hydrolysis and condensation reactions. Generally, the size and microstructure of sols could be influenced by the water molar ratio and type of catalyst (acid or base).29 An acid-catalyzed approach led to the formation of weakly branched products with smaller sol sizes of usually N2 > SF6

With the exception of CH4, this order was basically consistent with the increasing order of the kinetic diameter of gas molecules, which suggests that the gas transport behavior of BTPP membranes was decided by the diffusion process. The reverse selectivity of N2 (3.64 Å) over CH4 (3.8 Å), however, could have been due to the relatively higher solubility of CH4 in the BTPP membrane matrix, which was the result of both the higher critical temperature (−82.59 °C) of CH4 and the high affinity of pyrimidine with CH4. This phenomenon has always been the case with aromatic polyimide polymers or some nitrogen-bearing heterocyclic compounds.32,41 Solubility is a decreasing function of temperature, whereas diffusivity is an increasing function of temperature, and therefore, improved separation performance of smaller or sorptive gases (H2, CO2, etc.) over larger gases (N2, CH4 ,etc.) is always expected at lower temperatures. Figure 8 shows the temperature depend-

Figure 9. Kinetic diameter dependence of apparent activation energies for the permeation of gases considered for BTPP membranes.

It should be noted that the Ep values of BTPP membranes for He, N2, etc. were larger than those of the general organosilica membranes we previously reported due to the organic-rich structure of BTPP.35 For condensable gases with a high critical temperature of CO2 (31.0 °C) and CH4 (−82.59 °C), which also could interact with BTPP membranes, however, the sorption performance is considerable and, in turn, the negative values of enthalpies of sorption (ΔHs) may contribute significantly to the activation energy for permeation (Ep). Hence, in the present study, the apparent activation energy for the permeation of CO2 displayed the lowest level due to the special CO2−pyrimidine interactions. In addition, the effective sorption enthalpy of CO2 is not only decided by the surface chemistry of a membrane but also could be affected by the microstructure. Interestingly, the apparent activation energies for permeation could also reflect the microstructures of BTPP membranes fabricated with different approaches. Catalyst type and water molar ratio are considered to be two of the most important factors that can affect the microstructure of sol−gelderived silica membranes. BTPP-HCl-240 membranes with higher levels of hydrolysis and condensation could offer greater volume, or voids, in an Si−O−Si network for the permeation of He and H2. This would also increase the possibilities for CO2 access to pyrimidine groups, which would present lower values of Ep for He, H2, and CO2. On the contrary, the higher Ep for N2 and CH4 that could permeate through sufficient free volume and larger pores of membrane BTPP-HCl-240 could be due to the well-connected Si−O−Si network, which is confirmed in Figures 6 and 7. As a result, the BTPP-HCl-240 membrane demonstrated a greater difference in Ep between smaller or sorptive gases (He, H2, CO2, etc.) and larger gases (N2, CH4, etc.). Hence, the BTPP-HCl-240 membrane offers greater potential for the separation of small molecules from larger molecules, as well as for CO2 separations at lower temperatures. However, the larger pore size of membrane BTPP-self-12 due to the heterogeneous microstructure and the organic-rich microstructure of membrane BTPP-HCl-40 due to the overmuch unhydrolyzed ethoxy groups reduced the difference in Ep between smaller or sorptive gases (He, H2, CO2, etc.) and larger gases (N2, CH4, etc.). 3.2.4. CO2 Separation Properties. Figure 10 compares the gas permeance and permselectivity of BTPP membranes at 40 °C as a function of the water molar ratio in BTPP sols. The BTPP-self-12 membrane prepared using the lowest water molar ratio showed a somewhat higher gas permeance, possibly due to the larger pore size, as confirmed by NKP plots. However, this membrane was not readily permselective to CO2/N2. When the

Figure 8. Temperature dependence of the gas permselectivities of BTPP-derived membranes.

ence of the permselectivities of the H2/N2 and CO2/N2 of BTPP-derived membranes. The selectivities of both H2/N2 and CO2/N2 increased as the permeation temperature decreased. Because H2 and N2 have no special interactions, the permselectivity of H2/N2 remained approximately constant, or slightly increased, because the size selectivity of BTPP membranes could increase with decreases in temperature. On the contrary, CO2 has special interactions with BTPP membranes, and the solubility selectivity could contribute much to the total permselectivity of CO2/N2. Activation energies for the permeation of the gases considered are reported in Figure 9. Generally, the activation energy for the diffusion (Ed) of a penetrant is positive and increases following an increasing order of kinetic diameter. On the contrary, the apparent sorption enthalpy (ΔHs) of a penetrant is negative and depends on the effective sorption performance. Hence, the activation energy for permeation (Ep) that is represented as the sum of Ed and ΔHs (Ep = Ed + ΔHs) could reflect the total energy barriers of a membrane for gas permeation. The adsorption enthalpies of gases like He, H2, and N2 are always negligible because of low critical temperatures (He, −268.0 °C; H2, −240.2 °C, N2, −146.9 °C).42 Therefore, the apparent activation energies for the permeation of noncondensable gases follow an increasing order of kinetic diameter: Ep(N2) > Ep(He) ≈ Ep(H 2) H

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with dense and larger-sized particulate colloidal regions. The resultant interparticle voids produced larger pores or pinholes that improved the permeance, but lowered the selectivity, for CO2/N2. A similar phenomenon was reported in our previous work, where a NH3 vapor was used for the post-treatment of polymer-supported organosilica membranes.43 By contrast, HCl-catalyzed sols with a smaller size tended to form uniform membranes that generated fewer pinholes or larger pores. Hence, the hydrolysis and condensation degree became an important factor that decided the membrane performance. A BTPP-HCl-40 membrane that demonstrated a lower degree of hydrolysis presented more unhydrolyzed ethoxy groups that could have reduced the effective pore size and free volume of the silica network. This would also reduce the permeance even for He, the smallest gas, and would also lower the possibility that CO2 could access pyrimidine groups. As a result, both the effective sorption and diffusion performance of the BTPP-HCl40 membrane was decreased and, in turn, so was the apparent CO2 separation performance. On the contrary, the BTPP-HCl240 membrane demonstrated higher levels of hydrolysis and condensation, and the silica network led to improved porosity and was better connected, which could improve the effective CO2 sorption and diffusion selectivity over N2. This tendency toward the water molar ratio effect was in good agreement with our previous work based on organosilica membranes.31,33 Therefore, the microstructure of BTPP membranes was tuned using a higher water molar ratio for the sols to improve the CO2/N2 separation performance. Table 3 compares the CO2 separation performance of some amine-silica hybrid membranes recently reported, and includes primary-, secondary-, and aziridine-amine species. Compared with other amine-silica membranes, BTPP membranes (pyrimidine) showed fair CO2 permeance and relatively higher CO2/N2 permselectivity at a lower temperature under a dry state. The HCl-catalyzed BTPP membrane with a high water ratio (BTPP-HCl-240) demonstrated superior CO2 separation performance with a CO2 permeance of 3.37 × 10−8 mol/(m2 s Pa) and a CO2/N2 selectivity that was as high as 25 at 40 °C. Therefore, the BTPP membrane should be considered a viable candidate for CO2 separation applications.

Figure 10. (a) Gas permeance and (b) permselectivities of BTPP membranes at 40 °C as a function of the water molar ratio in the BTPP sols (water molar ratio: BTPP-self-12, 12; BTPP-HCl-40, 40; BTPP-HCl-240, 240).

water molar ratio of HCl-catalyzed membranes is increased, both CO2 permeance and permselectivity for H2/N2 and CO2/ N2 were significantly increased. By combination of eqs 2 and 3, gas permselectivity of a membrane, αij, can be expressed by an Arrhenius-type equation, similar to that of permeance and diffusivity: αij =

⎛ Epi − Epj ⎞ ⎛ Epi − Epj ⎞ Pi0 exp⎜ − ⎟ ⎟ ∝ exp⎜ − Pj0 RT ⎠ RT ⎠ ⎝ ⎝

(6)

where Pi0 and Pj0 are the pre-exponential factors for components i and j. Considering the possible difference in pre-exponential factors that may depend on the membrane and the penetrant, the difference in activation energies for the permeation of components i (smaller/sorptive) and j (larger), Epi − Epj, could to some extent reflect the potential in permselectivity (αij). For a given system, higher negative values of Epi − Epj possibly could offer higher values of αij, and this trend may increase with a decrease in temperature. Table 2 Table 2. Data Display of Values for the Epi − Epj and αij for the Gas Pairs of CO2/N2 and CO2/CH4 at 40 °C membrane BTPPSelf-12 BTPPHCl-40 BTPPHCl240

Ep(CO2) − Ep(N2) [kJ/ mol]

CO2/N2 selectivity at 40 °C

Ep(CO2) − Ep(CH4) [kJ/ mol]

CO2/CH4 selectivity at 40 °C

−6.1

12.7

−4.7

8.7

−4.8

11

−4.5

8.4

−13.3

24.8

−11.4

25.4

4. CONCLUSIONS Pyrimidine-bridged organoalkoxysilane (BTPP) membranes with mild CO2 affinity were developed via sol−gel processing. By conditioning the water molar ratio and the type of catalyst used for the sols, we tuned the microstructure of BTPP membranes for high-efficiency CO2 transport. A high water molar ratio demonstrated the highest degree of cross-linking from among the resultant BTPP-derived silica networks, as confirmed by FTIR. The N2 and CO2 adsorption measurements indicated that BTPP showed a relatively dense structure, but a higher level of CO2/N2 sorption selectivity. Single-gas permeation tests using a series of permeation temperatures were performed using gases with different kinetic diameters: He (2.6 Å), H2 (2.89 Å), CO2 (3.3 Å), N2 (3.64 Å), CH4 (3.8 Å), and SF6 (5.5 Å). BTPP membranes exhibited sharp kinetic diameter dependence and activated diffusion, possibly due to a dense silica network. The CO2 transport performances of different BTPP membranes were compared according to activation energy generated for the permeation (Ep) of CO2, as well as to the differences in Ep between CO2 and N2 (or CH4). Both the BTPP-self-12 and BTPP-HCl-240 membranes recorded a relatively lower level of Ep for CO2, which suggested

summarizes the values for Epi − Epj and αij for the gas pairs of CO2/N2 and CO2/CH4, which agreed well with eq 6. Both gas pairs CO2/N2 and CO2/CH4 exhibited direct proportional relationships between CO2/gas permselectivity and the associated difference in activation energies for permeation. The BTPP-HCl-240 membrane showed the highest negative values for Ep(CO2) − Ep(N2) and Ep(CO2) − Ep(CH4), and in turn, higher levels in permselectivity for CO2/N2 and CO2/ CH4. This superior CO2 separation performance may be attributed to the higher levels of hydrolysis and condensation, as previously confirmed. Figure 11 illustrates the possible microstructures and gaseous permeation behaviors of BTPP membranes. A BTPP-self-12 membrane prepared from a largersized sol led to the formation of a heterogeneous membrane I

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Figure 11. Illustration of the possible microstructures and gaseous permeation behaviors of (a) BTPP-self-12, (b) BTPP-HCl-40, and (c) BTPPHCl-240 membranes.

Table 3. Comparison of CO2 Separation Performance of Reported Amine−Silica Membranes under Dry Conditions precursor species

amine species

preparation method

CO2 permeance [10−10 mol/(m2 s Pa)]

CO2/N2 (CH4) permselectivity

temp (°C)

ref

APTES APTES TEOS TRIES BTPP BTPP BTPP

primary primary aziridine primary pyrimidine pyrimidine pyrimidine

surface grafting surface grafting surface grafting sol−gel sol−gel sol−gel sol−gel

2.72 10.0 6.03 2000 388 337 337

10 800 0.15 2 13 11 25

120 100 35 200 40 40 40

22 23 24 25 this work this work this work

lower energy barriers for CO2 transport. However, only the BTPP-HCl-240 membrane showed a greater difference in Ep between CO2 and N2 (or CH4), which reflected a greater potential in CO2/gas (gas = N2, CH4) permselectivity due to an improved porosity and a well-connected silica network. By comparison with the most commonly used amine-functionalized silica-based membranes, the BTPP membranes demonstrated a mild affinity for CO2 and showed greater potential in CO2 separation performance. Hence, BTPP membranes with high thermal stability could become one of the most important membranes for CO2 separation.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04460. Synthesis of 4,6-bis(3-(triethoxysilyl)-1-propoxy)-1,3pyrimidine (BTPP), TG-MS results of BTPP-HCl-240 xerogel powders, description of the normalized Knudsenbased permeance (NKP) model for evaluation of membrane pore size, molar composition and appearance of BTPP-derived silica sols employed (Table S1), element composition of BTPP-HCl-240 xerogel powders as a function of post-heat-treatment temperature (Table S2), and textural properties of BTPP-derived xerogel powders (Table S3) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*T. Tsuru. Tel: +81 824247714. E-mail: [email protected]. jp ORCID

Hiroki Nagasawa: 0000-0001-7972-2425 Toshinori Tsuru: 0000-0002-8561-4962 Notes

The authors declare no competing financial interest. J

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DOI: 10.1021/acs.iecr.6b04460 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX