Article pubs.acs.org/jced
Characterization and Density Functional Theory Optimization of a Simultaneous Binder (FSG-XO) of Two Different Species Exploiting HOMO−LUMO Levels: Photoelectronic and Analytical Applications Milan K. Barman, Mousumi Chatterjee, Bhavya Srivastava, and Bhabatosh Mandal* Analytical Laboratory, Department of Chemistry, Visva-Bharati, Santiniketan 731235, India S Supporting Information *
ABSTRACT: A cost-time effective mesoporous ion-exchange material (FSG-XO) has been synthesized by immobilizing xylenol orange on functionalized silica gel. Its spatially separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) have been used for the simultaneous sorption of two different metal centers at their respective high and low oxidation states. The characterization of its corresponding nanomaterial, {Si[OSi]p=2−4[OH]m=2−0 xH2O}n[−Si(CH3)2−NH−C6H4−NN−XO]4 has been assessed by a set of sophisticated analysis. FSG-XO possesses high SABET (346.22 m2/g), PV (0.431549 cm3/g), uniform pore size (width, 47.1; and diameter, 50.3 nm), high chemical (4 M HNO3) and thermal stability (140 °C), high level of reusability ( 98 %) of Zn(II) with flow rates up to 3.0 mL min−1. Beyond this there was a decrease in the percentage of adsorption due to an insufficient time of equilibration of the analyte with the adsorbent. The sorption kinetics was found to be faster than that of most of the traditional ion-imprinted adsorbents.21−23 This indicates that the resin has high retention ability for Zn(II), which adsorbed easily and diffused quickly to the binding sites. 3.10. Breakthrough Capacity. The breakthrough capacity represents the exhaustion point in terms of feed (μM), after which the adsorbate leaks through into the effluent in gradually increasing amounts that can exceed the preset or desired value.13 The present studies reveal that the effluent content was found to be close to 2 % (3 μM) to that of the influent content (240 μM of Zn(II)). Afterward, a significant decrease in the binding sites for the analyte was observed, which attained the breakthrough capacity value at 240 μM g−1 of dry exchanger. The experimental values are comparable to the TGA/DTA (232.84 μM g−1) when the analyte is present as a monomer. 3.11. Effect of Desorption Condition and Repeated Use. Since the adsorbed amount of Zn(II) on the resin is very low below pH 1.5, one can expect that elution will be favored in acidic solution. Thereby, elution of zinc(II) was studied following the dynamic procedure in which various concen-
Figure 9. UV−visible spectra of extractor and metal.
[Zn4(H2O)4(OH)4]4+ is a soft acid (η[Zn4(OH)4(H2O)4]4+ = 6.72 eV) and has more comparable η values (ηFSG‑XO = 10.44 eV) (Figure 8a) in comparison to [Zn(H2O)6]+2 ion itself (η[Zn(H2O)6]+2 = 0.63 eV). 3.7. Analysis of the Oxidation State of the Sorbed Zinc. To assess the possible oxidation state of the analyte as it binds with the extractor, the time−voltage−current relationship was recorded on VersaStaeTM II (Figure 8b). The significant peaks are observed at an applied potential of −0.8 V and +0.42 V (i.e., −1.04 V and +0.64 on H+/H2 as reference electrode) and strongly approves the presence of hydroxo species of Zn(II)46 which has been well accepted in TGA/DTA analysis. These peak values become −3.46 eV and −5.14 eV respectively in vacuum scale and represent the LUMO and HOMO energy level of the exchanger metal complex. 3.8. Effect of Foreign Ions on Extraction. Systematic studies of foreign ion stress (200 μg to 1000 μg: Cl− and SO42−
Figure 10. DFT-optimized structure of (a) [Zn4(H2O)4(OH)4]4+, (b) HOMO of [Zn4(H2O)4(OH)4]4+, (c) LUMO of [Zn4(H2O)4(OH)4]4+ and (d) [Zn(H2O)6]+2, (e) HOMO of [Zn(H2O)6]+2 (f) LUMO of [Zn(H2O)6]+2. I
DOI: 10.1021/je501013b J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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trations (10·10−3 to 2000·10−3 mol L−1) and volumes (20 mL to 35 mL) of 100·10−3 (M) HClO4 solutions were used for the desorption of retained zinc(II). These results indicate that 100· 10−3 (M) HClO4 (20 mL) is sufficient for quantitative recovery for zinc(II). Moreover, when the flow rate for desorption of zinc(II) was varied from 1.0 to 4.0 mL min−1, the zinc(II) ions can be eluted quantitatively at a flow rate below 2.5 mL min−1. Therefore, the flow rate of 2 mL min−1 was selected for subsequent experiments. To investigate the reusability of the resin, the sorption and desorption cycles were repeated 160 and 200 times by using the same imprinted adsorbent, and the breakthrough capacity was decreased by only 0.2 % and 0.3 %, respectively. The results indicate that the resin can be used many times without significantly decreasing its breakthrough capacity. 3.12. Simultaneous Sorption of Both the High and Low Oxidation State. HOMO and LUMO of FSG-XO are well separated and are present at two different edges of the tetrahedra at any particular time interval. TGA analysis generates the total numbers of both HOMO and LUMO levels as 58.21 g−1. The full saturation of HOMO (electron-rich center) is attained by capturing 240 μM g−1 (TGA, 232.84 μM g−1) of Zn(II). Therefore, zinc as a tetramer ([Zn4(H2O)8]8+) should bind the HOMO of the exchange material by capturing the electron density into its LUMO core. The appearance of a new peak at 640 cm−1 in the Raman spectra of the loaded extractor indicates the presence of the Zn(OH)Zn unit and confirms the tetramer, [Zn4(H2O)8]8+, as the extracted species.47 Having no other alternative, molecular iodine (species of low oxidation state) is compelled to move toward the LUMO (electron deficient center) and the exchanger attains its saturation level (break-through value) by capturing 225 μM g−1 (97 % of sorption) of molecular iodine as I4spcies.48 Zn(II) is a common and stable species (E0 (Zn2+/Zn): −0.76 V),46 remaining present at the HOMO and becoming indifferent to molecular iodine. The extractor remarkably adsorbs molecular iodine at the LUMO and enhances its breakthrough capacity by almost the same amount as attained during the full saturation of the HOMO by Zn(II) (HOMO, 240 μM g−1; and LUMO, 225 μM g−1). The extractor, through its HOMO, binds Zn(II) by donating the electron to the LUMO of the Zn(II) moiety, while molecular iodine utilizes its HOMO during its attachment to the extractor. It should also be noted that in the reverse sequence of adsorption (i.e., molecular I2 followed by Zn(II)), the extraction of I2 was found to be very poor. At the recommended pH (7 to 7.2) for extraction of molecular I2, the exchanger is in its deprotonated state. Consequently, extraction of molecular I2 by virtue of its attachment at the LUMO of less electronegativity was found to be negligible in the reverse sequence of sorption.
BET surface area (346.22 m2/g), high chemical stability in acids (stable up to 4 M HNO3), very high breakthrough capacity (BTC, 240 μM g−1), and spatially well demarcated revolving HOMO−LUMO having no or little chance of charge recombination. The present enquiry further reveals that the HOMO and the LUMO are simultaneously present wide apart from one another at two different spatial locations. Thus, they become able to bind respectively two different metal centers at their high (Zn(II)) and low (molecular I2) oxidation states (Figure 11a). This observation prominently differs in the mode
Figure 11. (a) Synergistic attachment through donation and backdonation operative on two different metal centers; (b) synergistic attachment through donation and back-donation operative on same metal.
of metal binding from what has been reported so far (Figure 11b). Here, in addition to the classical breakthrough value (saturation of HOMO), the material becomes also able to achieve the second breakthrough value (saturation of LUMO) because of the presence of the spatially well separated HOMO−LUMO within the molecular network.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis procedure, the proposed mechanism, DFT optimized structure of the resin, Raman, NMR, mass spectra, effect of foreign ions on extraction, comparable table. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/je501013b.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Tel.: +91- 947-473-8517. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.K.B. gratefully acknowledges the facilities provided by the Department of Chemistry, VisvaBharati, Santiniketan. The authors thankfully acknowledge experimental assistance from the Laboratories CRISMAT, UMR 6508, CNRS/ENSICAEN, Caen Cedex, France, and also are grateful to the Department of Physics, Visva-Bharati, for providing the instrumental supports (XRD).
4. CONCLUSION The synthesis presented here turns out to be more facile in comparison to that of other matrices (given in Supporting Information, Table S3). It needs no refluxing. Silica gel was efficiently end-capped during the formation of FSG, and particle size lies within the size of its precursor, SG. The composition of the material, {Si[OSi]p=2−4[OH]m=2−0·xH2O}n[−Si(CH3)2−NH−C6H4−NN−XO]4 has been assessed by a set of sophisticated experimental analyses, and has been well supported by density functional calculations. Therefore TGA analysis is sufficient to determine the composition of this kind of material. The extractor is mesoporous and it possesses high
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