Introduction

Macroporous adsorption resins (MARs) are synthetic porous crosslinked polymer beads, have been developed into a kind of novel functional materials since 1950. They have been shown to be potentially powerful separation materials and have been widely applied in many fields, such as chromatographic analysis [1], treatment [2], pharmaceutical [3], food industries, and wastewater disposal [4]. Except for the general advantage of that of the common adsorbent, MARs possess many special characteristics such as high mechanical strength, good acid and alkali resistance, porous availability, high surface area, and a long lifetime that make them more promising [5–7]. To gain a greater adsorption capacity and a higher adsorption selectivity for some specific organic compounds, chemical modification of MARs is often undertaken by introducing special functional groups into the matrix of the polymeric adsorbents [8, 9]. Thus, investigation on the adsorption features of organic compounds on MARs is of great importance [10, 11]. However, theoretical research on the adsorption features of MARs lags far behind. Although there are several reports about the adsorption feature of MARs, and many of adsorption systems have been observed to exhibit multiphasic behavior [12–14]. But little has been done to determine the impacts of the pore size distribution and the functional groups of polymer resins. To the best of our knowledge, there are no detailed theoretical studies available on hydrogen-bonding adsorption mechanism of rutin onto MARs by density functional method.

Recently, we have experimentally synthesized a series of adsorption MARs with hydrogen-bonding groups, which were amino, hydroxyl, and carboxyl groups, respectively (Scheme 1). Adsorption feature of the synthetic resins with respect to the purification effect was investigated systemically by employing rutin as the adsorbate. The experimental results showed that MARs with amino group have the higher adsorption capacity and the adsorption selectivity. In this article, we present a hybrid DFT(B3LYP) study for the adsorption mechanism of rutin onto MARs functionalized with amino, hydroxyl, and carboxyl groups. The MARs are the molecule of p-(CH3NH)PhL (L = NH2, OH, COOH) as model system. With this theoretical study, we hope to obtain a detailed understanding of adsorption mechanism and predict the trends of adsorption capacity for MARs with the different groups. Meanwhile, we expect to find out more about the factors that control the interaction energies of this important reaction and further investigate the effects of solvent of the reaction systems.

Scheme 1
scheme 1

The preparation method for macroporous adsorption resins modified with amino benzene compounds

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Computational methods

All of the geometries of the model complexes were fully optimized using the density functional theory (DFT) method based on the hybrid of Becke’s three-parameter exchange functional and the Lee, Yang, and Parr correlation functional (B3LYP) [15, 16]. The 6-31G basis set [17, 18] with polarization (d) and (p) was used for all of the atoms. Frequency calculations at the same level of theory have been achieved to identify each optimized stationary point as either minima (the number of imaginary frequencies [NIMAG = 0]) and to evaluate the zero-point energy (ZPE) corrections, which are included in all relative energies. The thermodynamic functions, including free energies, were calculated at 298.15 K and 1 atm. In consideration of the solvent effects on the reactions of interest, we applied a continuum medium to do the full geometry solvation energy calculations for all of the species, using UAHF radii on the polarizable continuum model (PCM). The PCM calculations [19–21] were done at the B3LYP/PCM/6-31G(d,p) level of theory. H2O was used as the solvent, corresponding to the experimental conditions, and the dielectric constant was assumed to be 78.39 for bulk aqueous solution. All calculations reported here have been carried out with Gaussian 03 program package [22].

Results and discussion

In the first part of this section, we present results from our calculations on the adsorption process of rutin to the different MARs p-(CH3NH)PhL (L = NH2, OH, COOH). The influence of the number of rutin upon the adsorption energies and structures is investigated in the second part. The optimized stationary structures of the reactants and the adsorption complexes are depicted schematically in Figs. 1, 2, and Table 1 with selected key geometry parameters (bond lengths and bond angles). The total energies with ZPE corrections and the relative energies to the starting materials are listed in Table 2.

Fig. 1
figure 1

The structures of p-(CH3NH)PhL (L = NH2, OH, COOH) and the optimized minimum energy geometry of rutin

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Fig. 2
figure 2

The full optimized minimum energy geometries for rutin with A, B, and C, with selected structural parameters (bond lengths in Å)

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Table 1 The partial bond length (Å) and bond angle (°) of rutin, A, B, C, and the adsorption complexes at B3LYP/PCM/6-31G(d,p) level
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Table 2 Interaction energies for the macroporous adsorption resin with rutin
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Adsorption mechanism of rutin onto p-(CH3NH)PhL (L = NH2, OH, COOH)

Figure 1 shows the optimized geometry of rutin and the structures of p-(CH3NH)PhL (L = NH2, OH, COOH). The main geometry parameters are shown in Table 1. As one can see from Fig. 1 and Table 1, the C1–O1, O1–H1, and O3–H1 bond lengths in rutin are 1.218, 1.804, and 0.953 Å, respectively. The C2–O7–C3, C3–O8–C4, and C4–O12–C5 bond angles are 121.2°, 120.6°, and 117.4°, respectively. The MESP painted on the van der Waals surface is presented in Fig. 3 for the MAR and rutin, which shows strong charge separation in the systems, the oxygen and nitrogen atom are negatively charged while the rest of the molecule (hydrogen and carbon) is positively charged. These pictures also indicate that the H is donated to the O and N. The stronger electronegativity of A–C structures was attributed to the largest micropore surface, where rutin can be intensively adsorbed by a micropore filling mechanism. The structures of the adsorption complexes with the most relevant geometry parameters are shown in Fig. 2 and Table 1.

Fig. 3
figure 3

The electrostatic potential mapped on to the van der Waals surface of a representative set of rutin and the macroporous adsorption resin

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Inspection of Fig. 2 and Table 1 shows that the adsorption process forms the new hydrogen-bonding in the structures of A-rutin, B-rutin, and C-rutin, respectively. The O1–H11 and O8–H12 bond lengths are 2.549 and 2.287 Å, respectively, in A-rutin, and the O1–H1 and C4–O8 bond lengths elongated by 0.007 Å (1.804 Å in rutin) and 0.011 Å (1.384 Å in rutin), respectively, upon going from the reactants to the adsorption complexes. The similar features of the geometry parameters are also found for B-rutin and C-rutin, and the new hydrogen-bonding of O1–H11 in B-rutin and the O1–H11 and O17–H1 bond distances in C-rutin are 2.077, 1.890, and 2.227 Å, respectively. These changes in the bond lengths and angles are attributed to form the more stable adsorption complexes. Meanwhile, Fig. 4 shows the IR spectrum for the stable structures of rutin and A-rutin, B-rutin, and C-rutin at B3LYP/6-31G(d,p) level.

Fig. 4
figure 4

The IR spectrum for the stable rutin, A-rutin, B-rutin, and C-rutin at B3LYP/6-31G(d,p) level and experiment

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The interaction energies for the MARs with rutin are shown in Table 2. The A-rutin, B-rutin, and C-rutin are 8.92, 9.75, and 11.21 kcal mol−1 stable than the corresponding preliminary reactants from the B3LYP/PCM/6-31G(d,p) calculations in water. The trend of the interaction energies for the MAR is A-rutin (amino group) < B-rutin (hydroxyl group) < C-rutin (carboxyl group). That is to say, forming hydrogen-bonding for one molecule MAR with carboxyl group is more stable than the amino and hydroxyl groups. We plotted the frontier molecular orbitals (HOMO and LUMO) for rutin and the adsorption complexes in Fig. 5. The frontier orbital energy level (ΔE g = E LUMO − E HOMO) of rutin is 0.10552 eV, while the ΔE g of A-rutin, B-rutin, and C-rutin is 0.10031, 0.10334, and 0.10560 eV, respectively. Especially, the ΔE g of A-rutin and B-rutin is lower than rutin because of the MAR. The adsorption reaction occurs more easily for amino and hydroxyl groups.

Fig. 5
figure 5

Spatial plots of the frontier molecular orbitals calculated for rutin, A-rutin, B-rutin, and C-rutin. The orbital energies are given in eV

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Adsorption mechanism of rutin onto (p-(CH3NH)PhNH2) n (n = 1–3)

The adsorption experimental results show that the greater adsorption capacity has been found for the MAR with amino group. This MAR contains numerous amino groups, which could form many of hydrogen-bonding with a rutin molecule. Here, we present a detailed computational investigation of the rutin by explicitly coordinating one, two, and three the resin with amino group to the O and H atoms. Figure 2 depicts the optimized geometries found for rutin with (p-(CH3NH)PhNH2) n (n = 1–3, A-rutin to A3-rutin), and the key geometrical parameters are given in Table 1. In A2-rutin and A3-rutin, the N atom of resin attack to the H atom of rutin and the H atom of amino group add to O atom of rutin through forming hydrogen-bonding. The new N–H and O–H bond lengths are 2.017–3.008 Å in A2-rutin and A3-rutin. Examination of Table 2 shows the two p-(CH3NH)PhNH2 molecules adsorption for rutin has a interaction energy of 9.95 kcal mol−1 relative to the starting materials, which is decreased significantly by 1.03 kcal mol−1 compared to the corresponding one p-(CH3NH)PhNH2 adsorption. While the interaction energy for three p-(CH3NH)PhNH2 becomes 10.69 kcal mol−1 and this is decreased by 0.74 kcal mol−1 compared to the analogous energy two p-(CH3NH)PhNH2 (9.95 kcal mol−1). The adsorption complexes becomes more stable as the number of p-(CH3NH)PhNH2 goes from n = 1 to n = 3. Upon saturation of the rutin by coordination of p-(CH3NH)PhNH2 molecules one by one, the interaction energy of species would become relatively constant.

Systematic comparisons of experiment for the adsorption behavior

Figure 4 shows the IR spectra of the resins A (PPDA) and C (PANB). In the spectrum of A, there is an absorption band in the vicinity of 3374.3 cm−1 that belongs to the stretching vibrations of the N–H bond. In addition, the C–N stretching of the benzene ring and NH group are shown to be at 1,024 cm−1. Two other characteristic peaks related to the C=C stretching of phenyl group at 1605 and 1495 cm−1 are much stronger than before reacting, and another two bands at 812.3 and 694 cm−1, which involve C–H out of bending of CH group for mono-substitution of benzene. For the IR spectrum of p-aminobenzoic acid modified resin. A moderate C=O stretching band involved in formaldehyde carbonyl groups comes forth at 1698.4 cm−1. Finally, a weak absorption band appears at 3394.5 cm−1, and this band is related to O–H stretching of hydroxyl hydrogen .

Comparing the adsorption capacity of flavonoids (rutin) on the three adsorbents by experiment method, we can understand that they follow the order A (–NH2) > B (–OH) > C (–COOH), and the MARs with amino group has the highest equilibrium adsorption capacity at the equilibrium concentration. From Table 2, we observed that the computed interaction energies for the adsorption is given in the following order, p-(CH3NH)PhCOOH > p-(CH3NH)PhOH > p-(CH3NH)PhNH2. The result shows that the hydrogen-bonding of p-(CH3NH)PhCOOH with rutin is the more stable compared to p-(CH3NH)PhOH and p-(CH3NH)PhNH2. While the adsorption capacity has been investigated including of many of the factors controlling except for functional group, such as pore diameter, BET surface area, pore volume, particle size, group amount, temperature, etc. In this article, we mainly present a detailed DFT computational investigation of the hydrogen-bonding adsorption mechanism for rutin onto p-(CH3NH)PhL (L = NH2, OH, COOH).

Conclusion

In this article, we have carried out a theoretical investigation at the DFT (B3LYP) level for the adsorption mechanism of rutin onto a series of MARs functionalized with amino, hydroxyl, and carboxyl groups. According to our calculations, the adsorption mechanism of rutin onto the resin is based on hydrogen-bonding. The interaction energies for adsorption complexes are 8.92–11.21 kcal mol−1, and C-rutin is the more stable than A-rutin and B-rutin. The adsorption experiments show that adsorption capacity of A-rutin is the highest. We present a detailed computational investigation of the rutin by explicitly coordinating one, two, and three A-rutin to the O and H atoms of rutin. Upon saturation of the rutin by coordination of p-(CH3NH)PhNH2 molecules one by one, the adsorption complexes becomes more stable. Our computational results are in good explanation for the experimental observations, which has comprehensive ramifications for the adsorbent synthesize, adsorption experimental design, and practical application.