Controllable Adsorption of CO2 on Smart Adsorbents: An Interplay

Article pubs.acs.org/cm

Cite This: Chem. Mater. 2018, 30, 3429−3437

Controllable Adsorption of CO2 on Smart Adsorbents: An Interplay between Amines and Photoresponsive Molecules Lei Cheng,† Yao Jiang,† Shi-Chao Qi, Wei Liu, Shu-Feng Shan, Peng Tan, Xiao-Qin Liu, and Lin-Bing Sun* State Key Laboratory of Materials-Oriented Chemical Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China

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S Supporting Information *

ABSTRACT: Photoresponsive adsorbents for CO2 capture attract significant attention, because the variation of adsorption capacity can be tailored conveniently by light irradiation. However, all photoresponsive adsorbents reported until now capture CO2 via physical interaction (weak sites). Thus, the selectivity of CO2 over other gases like CH4 is low, and the variation of CO2 adsorption capacity originates from pure structural change via isomerization of photoresponsive units. Despite great challenges, photoresponsive adsorbents for CO2 capture based on chemical interaction (strong sites) are extremely desirable. Here, we report for the first time the construction of photoresponsive adsorbents with chemical adsorption sites for CO2. The control of adsorption capacity is based on the interplay between active amine sites {A, [3-(methylamine)propyl]trimethoxysilane} and photoresponsive azobenzene molecules [P, 4-(3-triethoxysilylpropyl-ureido)azobenzene]. Density functional theory (DFT) calculations reveal that the negative surface potential of amines correlate with the adsorption capacity on CO2. Upon visible light (450 nm) irradiation, transisomers of azobenzene are formed and interact with amines, which leads to decreased surface electrostatic potential of amines, and CO2 can thus be adsorbed freely on exposed active sites. In contrast, ultraviolet light (365 nm) irradiation results in the isomerization of azobenzene from trans to cis conformation. The surface electrostatic potential of amines increases obviously, and the active sites are thus blocked. A maximum variation amount is obtained on the adsorbent with comparable density of A and P (0.34 group nm−2), which confirms the interplay between amine sites and photoresponsive molecules. Because amines are specific active sites for CO2, high selectivity of CO2 over CH4 is obtained, and such selectivity is tunable upon irradiation with UV−vis light. We thus demonstrate the successful control of CO2 captured by chemical sites through the interplay between amines and azobenzene molecules, which is impossible or difficult to realize via conventional structural change caused by isomerization of photoresponsive units.

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INTRODUCTION

Much attention has been given to the functional materials responsive to external stimuli such as temperature, redox potential, pH, and light.10−23 Among diverse stimulation forms, light is considered the most attractive one because it is a rapid and remote stimulus with high precision.24−36 Moreover, irradiation with light does not generate any undesired byproducts.37−40 As a result, the photoresponsive system has been explored for potential applications in CO2 adsorption.41,42 One interesting example was reported by Park et al.;41 they synthesized a metal−organic framework (MOF) by using a photoresponsive ligand 2-(phenyldiazenyl)terephthalate. The obtained MOF realized reversible alteration of CO2 capture upon photochemical treatment. Another attractive example from Lyndon et al. reported the conversion of azobenzene to 4,4′-dicarboxylate, which was utilized to construct an MOF.42 The MOF decorated with photoresponsive motifs could undergo dynamic photoinduced structural versatility, leading

Natural gas, a cleaner energy for industrial and residential heating, has become increasingly important because of the evergrowing global demand.1,2 The active component of natural gas is methane (CH4) while one of the main impurities is carbon dioxide (CO2).3 The existence of CO2 reduces the reactivity and heating capacity; it also causes corrosion of the relevant pipeline and equipment.4 Indeed, the purification of natural gas is very substantial in separation engineering in industrial processes, aiming to meet equipment specifications and relevant standards on caloric value.5,6 Adsorption is considered a highly promising technique for the removal of CO2 from natural gas.7 It is noticeable that conventional adsorbents possess fixed active sites, and the difference in adsorption capacity comes from the variation of pressure and/or temperature.8 The difference in adsorption capacity under different conditions is then used for the design of cyclic adsorption processes. However, the variation of pressure/ temperature is usually energy-intensive, and new methods to control adsorption capacity are expected.9 © 2018 American Chemical Society

Received: March 7, 2018 Revised: April 23, 2018 Published: April 24, 2018 3429

DOI: 10.1021/acs.chemmater.8b01005 Chem. Mater. 2018, 30, 3429−3437

Article

Chemistry of Materials

Scheme 1. Control of CO2 Adsorption Behavior on the Smart Adsorbents through an Interplaya between Amines (A) and Photoresponsive Molecules (P)

a

(A) VIS light irradiation leads to trans isomerization of azobenzene molecule, decreased surface electrostatic potential, and exposure of active sites; (B) UV light irradiation leads to cis isomerization of azobenzene molecule, increased surface electrostatic potential, and blockage of active sites.

time and reversibility between trans/cis conformation.44 The trans and cis isomerization of the azo bond is induced by corresponding irradiation with visible (VIS) and ultraviolet (UV) light. Our calculations from density functional theory (DFT) indicate that the negative surface potential of amines correlates with the adsorption capacity on CO2. Upon VIS light (450 nm) irradiation, azobenzene molecules isomerize from cis to trans conformation, which leads to decreased surface electrostatic potential of amines, and CO2 can be adsorbed freely on exposed active sites. Consequent UV light (365 nm) irradiation converts azobenzene molecules from trans to cis conformation. The surface electrostatic potential increases obviously, and the active sites are thus blocked. Apparently, the interplay between active sites and photoresponsive molecules causes the change of surface electrostatic potential of amines, and subsequently tailors the adsorption behavior of CO2. Because the amines are specified active sites for CO2 rather than CH4, high selectivity is thus obtained on the present photoresponsive adsorbents. This is the first report of photoresponsive adsorbents that can realize the control of CO2 adsorption on amines through chemical interaction as well as high selectivity of CO2/CH4.

to an obvious change for the adsorption of CO2. It is worth noting that the adsorption isotherms of CO2 are almost linear in both cases.41,42 This indicates that the adsorbate−adsorbent interaction is quite weak, and physical adsorption is predominant, which is not beneficial to selective capture of CO2. Actually, in all of the literature the change in CO2 adsorption capacity is derived from pure structural variation via isomerization of photoresponsive groups, because of the lack of suitable active sites such as amines. From the viewpoint of practical applications, photoresponsive adsorbents for CO2 capture based on strong adsorbate−adsorbent interaction (e.g., chemical interaction) are extremely desirable. This may result in controllable adsorption of CO2 with high selectivity from gas mixtures. It is noteworthy that it is difficult to adjust the adsorption of CO2 that interacted strongly with active sites through traditional structural variation induced by photoresponsive molecules. It is interesting to note that Lee et al. cofunctionalized amine with azobenzene groups onto the gold nanoparticles.43 This renders the nanomaterials responsive to light irradiation and CO2 and the reversible assembly/ disassembly behaviors, while the adsorption of CO2 was not mentioned. Apparently, the development of photoresponsive adsorbents for CO2 capture based on chemical adsorption is a great challenge. Here, we report the fabrication of photoresponsive adsorbents with chemical adsorption sites for CO2, for the first time. The adjustment of adsorption capacity originates from the interplay between active sites (amine) and photoresponsive molecules (azobenzene). This is obviously different from pure structural variation via isomerization of photoresponsive molecules for the adsorbents reported in the literature.41,42 The adsorbents were constructed by incorporating amines {A, [3-(methylamine)propyl]trimethoxysilane} and photoresponsive molecules [P, 4-(3-triethoxysilylpropylureido)azobenzene, Figure S1] simultaneously into mesoporous silica (MS) as shown in Scheme 1. It is known that azobenzene is a photoresponsive molecule with short response

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EXPERIMENTAL SECTION

Chemicals. Toluene (≥99.5%) and tetrahydrofuran (99%) were purchased from Sinopharm and Adamas-beta, respectively, and were dehydrated before use. Methanol (≥99.9%) and hexane (Aladdin, 97%) were purchased from Aladdin, and were used as received. 4Aminoazobenzene (98%) was purchased from TCI Co. Ltd. Cetyltrimethylammonium bromide (CTAB; 99%), 3-(triethoxysilyl)propyl isocyanate (95%), [3-(methylamine)propyl]trimethoxysilane (95%), and tetraethylorthosilicate (98%) were purchased from SigmaAldrich Reagent Co. Ltd. Deionized water was used for all of the experiments. Materials Synthesis. The photoresponsive molecule (P) precursor 4-(3-triethoxysilylpropyl-ureido)azobenzene was synthesized according to the literature procedure.45 In a typical process, 3(triethoxysilyl)propyl isocyanate (2.05 g, 8.12 mmol) and 4-amino3430


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Chemistry of Materials Table 1. Textual Properties and Composition of Different Samples elemental composition (wt %) sample

SBET (m2 g−1)

Vp (cm3 g−1)

C

H

N

density of A (group nm−2)

density of P (group nm−2)

MS A1P2@MS A2P2@MS A3P2@MS A2P1@MS A2P3@MS

1365 837 797 749 815 768

1.46 0.86 0.78 0.74 0.88 0.76

2.01 10.47 10.83 11.78 10.28 12.63

1.68 1.95 2.01 2.18 1.91 2.21

0.04 3.07 3.18 3.42 3.01 3.69

0 0.26 0.34 0.42 0.34 0.35

0 0.33 0.34 0.33 0.25 0.67

Adsorption Tests. Adsorption of CO2 and CH4 was conducted on an ASAP 2020 analyzer. CO2 (99.999%) and CH4 (99.999%) gases were used for the adsorption measurements. The free space was determined using He (99.999%), assuming that He could not be adsorbed at the temperatures investigated. The gas adsorption experiments were measured using an ASAP 2020 analyzer. After degassing, the samples (in the quartz tube) were irradiated with UV light (wavelength 365 nm) or VIS light (wavelength 450 nm) for about 3 h by using a xenon lamp (CEL-HXUV300) equipped with a filter, and then used for the adsorption measurement directly. The adsorption isotherms of CO2 and CH4 at 0 and 25 °C were measured in a water bath with circulating water equipment. The isosteric heats of CO2 adsorption (Qst) were calculated from the CO2 adsorption isotherms at temperatures of 0 and 25 °C; the data were simulated with a Virial expression composed of parameters ai and bi that are independent of temperature in terms of eq 1. In general, a nonlinear curve was obtained displaying the connection between ln P and adsorption quantity (N); from the fitting parameter results of ai, the Qst was calculated in terms of eq 2.47 In the equations, P is pressure, N is amount adsorbed, T is temperature, and m and n represent the number of parameters a and b (where m ≤ 5, and n ≤ 2).

azobenzene (1.58 g, 7.85 mmol) were dissolved in tetrahydrofuran (12 mL) and reflux under N2 for 24 h. Then, hexane was added to the solution at −20 °C. After filtration, and being washed with hexane several times, the precipitate was dried at 60 °C for 24 h under vacuum. The MS was prepared according to a previously described method.46 A mixture of CTAB (1.0 g, 2.74 mmol), NaOH (2 M, 3.5 mL), and 480 mL of deionized water was heated to 80 °C, and tetraethylorthosilicate (5.0 mL, 23.0 mmol) was added dropwise to the solution. The mixture was then stirred at 80 °C for 2 h. After filtering, and washing with deionized water and methanol, the as-synthesized MS was obtained. For removal of the surfactant template (CTAB) from as-synthesized MS, the sample was suspended in a mixture of methanol (100 mL) and concentrated HCl (1.0 mL, 2M), and the solution was heated under reflux for 24 h. The precipitate was then filtered and washed extensively with H2O and methanol and placed under vacuum to remove the remaining solvents to yield MS. Smart adsorbents containing methylamine group and azobenzene units were prepared as follows. The MS (0.5 g) was dispersed in 80 mL of toluene with ultrasonic for 5 min. Then, the A precursor [3(methylamine)propyl]trimethoxysilane (70, 85, or 100 mg) and the P precursor 4-(3-triethoxysilylpropyl-ureido)azobenzene (150, 210, or 370 mg) were added to the mixture. Before the reaction, the obtained solution was soaked for 24 h for its components to make adequate contact with each other. After the mixture was stirred at 80 °C under N2 for 12 h, the solid was filtrated, washed with methanol several times, and dried under vacuum for 24 h. The resultant samples were denoted as AmPn@MS samples, where m varies from 1 to 3 and corresponds to the A content ranging from 0.26 to 0.42 group nm−2, and n varies from 1 to 3 and corresponds to the P content ranging from 0.25 to 0.67 group nm−2. The grafted amount of A and P was calculated according to elemental content of nitrogen and TG analysis. Afterward, the density of A or P can be obtained by combining the grafted amount of A or P and the surface areas of AmPn@MS. Materials Characterization. 1H NMR spectra of samples were recorded on a 400 MHz nuclear magnetic resonance instrument (Bruker Avance). UV−vis diffuser reflectance spectra (UV−vis DRS) were collected on the Shimadzu UV devices. X-ray diffraction patterns of samples were recorded on a Rigaku D/MAX-γA instrument. Transmission electron microscopy and EDX analyses were performed using an FEI Tecnai G2 F20 electron microscope operated at 200 kV. Scanning transmission electron microscopy images were collected on Hitachi SU8010 equipment. Nitrogen sorption isotherms were obtained from an ASAP 2020 analyzer at 77 K. The samples were degassed at 80 °C for 24 h prior to analysis. The Brunauer−Emmett− Teller (BET) surface area was calculated using adsorption data in a relative pressure ranging from 0.04 to 0.20. The total pore volume was determined from the amount adsorbed at a relative pressure of 0.99. The pore size distributions were calculated from the adsorption isotherms using the Barrett−Joyner−Halenda (BJH) methods. Liquid UV−vis spectrophotometer spectra were collected on the PerkinElmer Lambda 35 series devices. Fourier transform infrared spectra (IR) were carried out on a Nicolet Nexus 470 spectrometer. The C, H, and N element analyses were performed by a Vario Micro Cube elemental analyzer. Thermogravimetric analysis was performed in a flow of N2 (20 mL min−1) from room temperature to 1000 °C on a NETZSCH thermobalance (STA-499C F3).

m

ln P = ln N +

n

1 ∑ N i+∑ biN i T i=0 i=0

(1)

m

Q st = − R∑ aiN i i=0

(2)

For an investigation of the adsorption selectivity of CO2 over CH4 on adsorbents, the selectivity is defined as S = (x1/y1)/(x2/y2), where x and y represent the composition of the adsorbed phase and the gas phase, respectively. The ideal adsorption solution theory (IAST) of Myers has been reported to predict binary gas mixture adsorption in porous materials accurately,48 and the dual-site Langmuir (DL) model was chosen to fit the adsorption isotherms; then, DL-IAST was utilized to estimate CO2/CH4 selectivity of adsorbents.

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RESULTS Synthesis and Characterization. The photoresponsive unit (P) precursor was first synthesized, and the structure was confirmed by nuclear magnetic resonance (NMR, Figure S2). The precursors of P and A were then incorporated via silanol groups in MS. Corresponding density of A and P is also calculated on the basis of their content and surface area of the support (Table 1). Structural characterization of A- and P-functionalized adsorbents was performed using various methods. The lowangle X-ray diffraction (XRD) patterns of samples are presented in Figure 1. It can be found that the hexagonal pore structure of MS (p6mm) is well preserved after modification. With increasing amount of organic units, the intensity of the reflections becomes low gradually. In the wideangle XRD patterns (Figure S3), a single broad diffraction peak centered at 23° is detected because of the amorphous silica walls. The incorporation of P and A does not bring new 3431


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Figure 1. Low-angle XRD patterns of the samples before and after loading different amounts of A and P.

Figure 3. IR spectra of the samples before and after loading different amounts of A and P.

diffraction peaks, suggesting the lack of long-range order. In N2 adsorption−desorption isotherms, all samples display typical type-IV isotherms, corresponding to mesoporous structure (Figures S4−S6). Figures S5 and S6 exhibit that the pore size of AmPn@MS decreases because of the introduction of organic units. As listed in Table 1, the surface areas and pore volumes decline progressively with the increase of A and P loading, while open channels and high surface areas are still maintained. Electron microscopy was utilized to reveal the morphology and pore structure of the adsorbents. As shown in scanning electron microscopy (SEM) images (Figure S7), the samples give spherical morphologies before and after modification. In transmission electron microscopy (TEM) images, the pristine MS exhibits the regular pore periodicity and straight pore channels (Figure 2A). After modification, the periodic ordering

addition to the typical bands of silica. The vibrational bands at 1480 cm−1 are ascribed to −NH− stretching modes of the A moieties. The bands at 1530 and 690 cm−1 are derived from the −NH−CO−NH− stretching vibration and the skeletal vibrations of the P moieties, respectively.49 The intensity of such peaks increases gradually with increasing content of A (from A1P2@MS to A2P2@MS and A3P2@MS) and P (from A2P1@MS to A2P2@MS and A2P3@MS). TG analysis of the adsorbents is depicted in Figure S9. The weight loss at temperatures up to 100 °C belongs to the removal of physically adsorbed water in the samples. From 125 to 300 °C, the weight loss attributed to the dehydration of silanol condensation is detected. Subsequently, the A and P decompose gradually from 300 to 700 °C. Elemental analysis indicates that the content of C, H, and N elements originated from A and P increases with the growth of their loading (Table 1). On the basis of the above-mentioned results, it is apparent that both the active sites (A) and photoresponsive molecules (P) are successfully incorporated into MS. Moreover, the morphology and ordered mesostructure are well-maintained regardless of the loading amounts of P and A in the adsorbents. Photoresponsive Properties. The photoresponsive property of the azobenzene derivative (P) was first evaluated by using UV−vis spectrometer. Figure S10 shows the absorption spectra of azobenzene derivative exposed to UV or VIS light for different times. The absorption bands of azobenzene derivative appear at around 450 and 350 nm, which belongs to the n−π* transition of the cis-isomer and π−π* transition of the transisomer, respectively.50 With the prolonging time for UV light irradiation, the band at 350 nm decreases apparently, and the band at 450 nm increases, which indicates a progressive transformation from trans- to cis-isomers.51 A photostationary state is reached after 2 min, and no further change appears upon irradiation for longer time. Upon VIS light irradiation, the absorption band at 350 nm restores the initial state (Figure S11). Remarkably, the band intensity even surpasses that of the initial sample slightly, since there are a fraction of cis-isomers in the initial sample. Moreover, the photoisomerization change of the azobenzene derivative is reversible through alternating UV and VIS light irradiation. Afterward, the photoresponsive properties of A- and Pfunctionalized adsorbents were evaluated. For the P units in trans conformation, a signature adsorption band at 370 nm is observed in the UV−vis spectra (Figure 4A and Figure S12). After 3 min of UV light irradiation, the absorption band at 370

Figure 2. TEM images of the samples (A) MS and (B) A2P2@MS. (C) EDX mapping images of C, N, O, and Si elements.

of the mesostructure is preserved to some extent (Figure 2B). The presence of C, N, O, and Si was examined in the energydispersive X-ray (EDX) spectrum for A2P2@MS (Figure S8). Moreover, a homogeneous distribution of C, N, O, and Si is made evident in the elemental mapping image (Figure 2C). According to these results, it is certain that the prepared composites are comparable to pristine MS in the matter of morphology and mesostructure. To confirm the successful introduction of A and P molecules, infrared (IR) spectra of different samples were collected (Figure 3). Several new vibrational bands are detected in 3432


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S16). The adsorption of CO2 on pristine MS, A2@MS, and P2@MS were first examined (Figures S17−S19). Because of the absence of active sites for chemical adsorption, pristine MS gives an almost linear adsorption isotherm. Additionally, the change of adsorption amount upon UV and VIS light irradiation is negligible. The loading of amines presents chemical adsorption sites, and thus obvious adsorption of CO2 at low pressure is observed in A2@MS. However, the sample A2@MS exhibits an unchanged adsorption amount of CO2 during UV and VIS light irradiation, since it does not have any photoresponsive groups. There are photoresponsive units but no amines in the sample P2@MS; thus UV and VIS light irradiation cannot cause any change in CO2 adsorption as well. Interestingly, A- and P-functionalized adsorbents exhibit quite different adsorption behavior upon light irradiation. Taking A2P2@MS as an example, the adsorption amount of CO2 is 72.4 mg g−1 upon UV light irradiation while it increases to 112.3 mg g−1 upon VIS light irradiation (Figure 5). This

Figure 4. (A) Changes in the UV−vis spectra of A2P2@MS upon irradiation with UV and VIS light. (B) Reversible changes in absorbance at 370 nm as a function of cycles for A2P2@MS.

nm decreases, and the sample reaches a photostationary state (cis conformation) after about 5 min of UV light irradiation. Inversely, through VIS light irradiation, the trans conformation is completely restored (Figure S13). In addition, the efficient trans/cis isomerization of photoresponsive units in the A- and P-functionalized adsorbents is totally reversible upon UV and VIS light irradiation (Figure 4B). The XRD and IR data upon UV irradiation and visible irradiation were collected. The XRD patterns of the sample upon UV/vis irradiation are almost the same as shown in Figure S14, indicating that the mesoporous structure of MS is not affected by isomerization of azobenzene groups. Interestingly, the trans/cis isomerization of the composites can be reflected by IR spectra (Figure S15). The IR spectrum of A2P2@MS shows a band at 690 cm−1, which is attributable to the trans-isomer of azobenzene. After UV light irradiation, a new band emerges at 699 cm−1 because of the cis isomerization.14,52 Afterward, the trans-isomer can be restored after VIS light irradiation, accompanied by the disappearance of the band at 699 cm−1. The aforesaid results demonstrate that the photoresponsive properties of azobenzene molecules are well-retained after incorporating into the adsorbents together with amines. The Aand P-functionalized adsorbents exhibit excellent reversibility in trans/cis isomerization upon UV and VIS light irradiation. Adsorption Behavior. The adsorption behavior of CO2 and CH4 on different adsorbents upon UV (365 nm) and VIS (450 nm) light irradiation was systematically studied. For comparison, two adsorbents functionalized separately with A and P were prepared. The content of A or P in the two adsorbents is identical to that in A2P2@MS, and thus, the adsorbents are denoted as A2@MS and P2@MS, respectively (corresponding characterization results are shown in Figure

Figure 5. Adsorption isotherms of CO2 and CH4 on A2P2@MS at 0 °C upon VIS and UV light irradiation.

corresponds to 35.6% of change for the adsorption amount of CO2. In addition, the adsorption of CH4 on the A2P2@MS was examined. In general, the adsorption amount of CH4 is low, because of the functionalization of specific active sites (amines) for CO2 and the absence of active sites for CH4. Moreover, there is almost no detectable change of CH4 adsorption upon UV and VIS light irradiation, suggesting that the isomerization of photoresponsive units has no effect on the adsorption of CH4. The adsorption of CO2 on a series of A- and Pfunctionalized adsorbents with different densities was studied (Figure 6 and Figures S20−S24). Among the adsorbents studied, the change amount of CO2 adsorption reaches a maximum when the density of A and P is comparable (namely, the density is 0.34 group nm−2 for both units). The reversibility of the CO2 adsorption by alternating irradiation with UV and VIS light was evaluated as shown in Figure 7. After the fresh adsorbent was saturated with CO2, regeneration was conducted. The regenerated adsorbent was irradiated with UV/vis light, and then reused for adsorption. The adsorption amount of CO2 in four cycles was recorded. It is noticeable that the adsorption amount as well as the change between UV and VIS light irradiation almost stay at the same level. This demonstrates the high reversibility of the present photoresponsive adsorbents during UV and VIS light irradiation. The IAST model was employed to estimate the selectivity of CO2/CH4 as displayed in Figure 8A. Upon VIS light 3433


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Figure 8. (A) IAST selectivity of CO2/CH4 on A2P2@MS. (B) CO2 isosteric heat of adsorption of the A2P2@MS upon UV and VIS light irradiation. Figure 6. Changes of CO2 adsorption amount upon VIS and UV light irradiation on adsorbents with different loadings of (A) P and (B) A. The numerals above the bars represent the density of P (in part A) and A (in part B) in the adsorbents with a unit of group nm−2.

59.4 kJ mol−1, which decreases to 48.9 kJ mol−1 upon UV irradiation. The foregoing results indicate that the adsorption of CO2 is tunable on A- and P-functionalized adsorbents upon UV/vis irradiation, which is impossible to realize for the adsorbents functionalized with individual A or P. The interplay between A and P is confirmed by the fact that a maximum change of CO2 adsorption is obtained at comparable density of A and P. Because the active sites (A) incorporated are specific for CO2, CH4 is seldom adsorbed. This leads to high selectivity of CO2/ CH4, which can be tailored by UV/vis irradiation as well. Furthermore, the different adsorption behavior on A- and Pfunctionalized adsorbents is totally reversible upon irradiation with corresponding UV or VIS light.

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DISCUSSION Albeit important, the development of photoresponsive adsorbents for CO2 capture based on strong adsorbate− adsorbent interaction has remained a great challenge. In this work, we report the fabrication of photoresponsive adsorbents with chemical adsorption sites for CO2 by incorporating chemical adsorption sites (amines) and azobenzene molecules simultaneously. The adsorption of CO2 on A- and Pfunctionalized adsorbents is controllable upon UV/vis light irradiation. The mechanism for the adjustment of CO2 adsorption is explored. The steric effect caused by isomerization of azobenzene molecules is first considered.41 Upon VIS light irradiation, trans-azobenzene is formed and seldom shows steric hindrance of amines. The exposed active sites can thus capture CO2. Upon UV light irradiation, the trans-azobenzene molecules are converted to their cis-isomers. It is possible for the enhanced steric hindrance to block some active sites. As a

Figure 7. Adsorption cycles of CO2 over A2P2@MS upon UV and VIS light irradiation.

irradiation, the CO2/CH4 selectivity on A2P2@MS at 0 °C and 0.1 bar is 11 293. Upon UV light irradiation, the CO2/CH4 selectivity declines to 260 under the same conditions. The results suggest high selectivity of CO2 over CH4. Furthermore, such selectivity is tunable upon irradiation with UV and VIS light irradiation. In addition, as shown in Table S1, A2P2@MS exhibited excellent selectivity toward CO2 with CH4, in comparison to other amino-functionalized adsorbents. To further understand the affinity of adsorbents with CO2, the isosteric heat of adsorption was calculated (Figure 8B). UV/vis light irradiation causes the difference in heat of adsorption obviously. Upon VIS light irradiation, the heat of adsorption is 3434


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Figure 9. Simulated geometries and surface electrostatic potentials of (A) the methylamine group, (B) trans-azobenzene interacted with the methylamine group, and (C) cis-azobenzene interacted with the methylamine group.

strong chemical shielding effect on methylamine groups after UV irradiation, and the electrostatic potential near the methylamine group suddenly increases to −0.01 eV and even to electrically neutral, which leads to the substantial loss of active sites for CO2 adsorption. Obviously, the controllable CO2 adsorption on A- and Pfunctionalized adsorbents can be attributed to the interplay between amines and azobenzene molecules. Upon VIS irradiation, trans-isomers of azobenzene are formed and interacted with amines, while the negative electrostatic potential of amines is still comparable to free amines. Hence, a high adsorption amount of CO2 is obtained. Irradiation with UV light results in the formation of cis-azobenzene, and the surface electrostatic potential of amines increases sharply because of the interaction with cis-azobenzene. Thus, the adsorption amount of CO2 declines. It is noteworthy that the reference adsorbents functionalized with individual A or P do not show any change on CO2 adsorption upon UV/vis irradiation. This indicates the significance of both A and P in the control of CO2 adsorption. The effect of density of A and P on the control of adsorption behavior was examined as well. It is demonstrated that the A2P2@MS with comparable density of A and P gives the best performance. This thus confirms the interplay between amines and azobenzene molecules. Because amines are specific active sites for CO2, the adsorption amount of CH4 on the same adsorbents is quite low. Moreover, the change amount of CH4 upon irradiation with UV and VIS is neglectable. This leads to high CO2/CH4 selectivity, and the selectivity over the adsorbents upon VIS irradiation is evidently higher than that upon UV irradiation. Similarly, the enhanced surface electrostatic potential of amines leads to decreased interaction between amines and CO2, and thus the heat of adsorption upon UV irradiation is lower than that upon VIS irradiation. In addition, another possible mechanism can be discussed. There is a hydrogen-bond interaction between the azobenzene groups and the amines.53 The cis-azobenzene groups were shown to exhibit strong affinity toward hydrogenbond donors. Methylamine, a secondary amine, can act as a hydrogen-bond donor. Hence, a low adsorption amount of CO2 is obtained because of no affinity to amines. In contrast, the trans-azobenzene groups had no appreciable affinity to

result, the adsorption amount of CO2 diminishes. Nevertheless, the pore size of the MS support is about 3 nm, and the size of the benzene ring in azobenzene is 0.6 nm. Perhaps the azobenzene derivatives block the pores of MS partially, but the CO2 molecule has a kinetic diameter of 0.33 nm and can also enter the pores freely. In addition, it is impossible for the benzene ring with planar structure to fully shield amine sites and subsequently obstruct the adsorption of CO2. Therefore, the steric effect caused by isomerization of azobenzene should play a minor (if any) role on the control of CO2 adsorption. On the basis of the above analysis, an interplay between amines and azobenzene molecules is believed to exist in the present system. DFT calculations were carried out to disclose the mechanism. The fully relaxed molecular geometries and the surface electrostatic potentials are calculated. With Grimme’s dispersion correction of D3 version, Becke’88 exchange and Lee−Yang−Parr correlation functional at Def2-SVP basis sets level are employed. The convergence criteria of maximum force and root-mean-square force are set as 1.5 × 10−5 and 1.0 × 10−5 a.u., respectively. The simulated geometries and surface electrostatic potentials of azobenzene and methylamine groups are shown in Figure 9. The calculated free energies of formation of trans-azobenzene-methylamine and cis-azobenzene-methylamine are −8.6 and −6.3 kcal mol−1, respectively, suggesting that both trans- and cis-azobenzene can interact well with methylamine groups. From the perspective of thermodynamics, both trans- and cis-azobenzene groups are inclined to interact with methylamine groups by van der Waals force. For transazobenzene-methylamine, the azo atoms and the hydrogen atoms of secondary amino are the main sites for the van der Waals interaction (Figure 9B). However, for cis-azobenzenemethylamine, the main sites are the hydrogen atom of benzene ring 1 in azobenzene group and the hydrogen atom of methylamine groups (Figure 9C). Normally, the surface electrostatic potentials can be used to analyze the active sites of CO2 adsorption. As shown in Figure 9A, the negative surface potential of methylamine groups reaches −0.042 eV, which can be regarded as the main region for CO2 adsorption. After VIS light irradiation, the diminished active sites are negligible because the electrostatic potential near the methylamine groups is still around −0.04 eV. Inversely, the benzene ring 2 has a 3435



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amine groups, and the isolated amine groups can be combined with the CO2 molecules freely, resulting in an enhanced adsorption amount of CO2. In short, the controllable adsorption of CO2 on A- and Pfunctionalized adsorbents is ascribed to the interplay between A and P. Upon irradiation with UV and VIS light, the mode for the interaction between A and P changes, which can be reflected quantitatively by the surface electrostatic potential through DFT calculations. The steric effect of azobenzene molecules on the adsorption of CO2 should be tiny, if there is any.

CONCLUSION We have demonstrated the successful fabrication of photoresponsive adsorbents with chemical adsorption sites for CO2 for the first time. Photoresponsive azobenzene molecules (P) and active amine sites (A) are incorporated into mesoporous silica simultaneously. Density functional theory calculations indicate that the interplay between active sites and photoresponsive molecules causes the variation of surface electrostatic potential of amines, and subsequently tailors the adsorption behavior of CO2. This is also confirmed by experimental results that the best performance is obtained on the adsorbent with comparable density of A and P. Because the chemical active sites are specific for amines, high selectivity of CO2 over CH4 is obtained, and such selectivity is tunable upon irradiation with UV/vis light. The present strategy may open up a new way for the construction of smart adsorbents by taking advantage of the interplay between active sites and stimuli motifs, which is impossible or hard to realize by conventional approaches. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01005. 1 H NMR spectrum, IR spectra, TGA curves, SEM images, UV−vis spectra, gas adsorption isotherms, and further information (PDF)

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yao Jiang: 0000-0002-2316-8274 Shi-Chao Qi: 0000-0002-9609-7710 Lin-Bing Sun: 0000-0002-6395-312X Author Contributions †

L.C. and Y.J. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21722606, 21676138, and 21576137), the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0964). 3436


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Controllable Adsorption of CO2 on Smart Adsorbents: An Interplay

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