Abstract
Macrocyclic calixarene compounds exhibit versatile properties, which have been employed for the complexation, extraction, and transport of various metal ions. However, the potential applications of calixarenes are not straightforward. This review includes the “structural effects” of calixarene derivatives and the concept of adsorbent design incorporating calixarene derivatives after a brief introduction on the molecular design of calixarene derivatives used as extraction reagents. The adsorbents incorporating calixarene moieties are classified into four types and examples of metal adsorption using a variety of adsorbents are also introduced.
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Introduction
Critical metals are indispensable for the production of advanced materials. The securing of valuable metals and removal of hazardous elements have become increasingly important research areas for a sustainable society [1,2,3,4]. Among the various chemical separation processes reported to date [5], solvent extraction and ion exchange have been extensively employed for metal separation as hydrometallurgical methods. Solvent extraction has the advantages of high efficiency, simple operation, versatility, easy scale-up, and mass disposal, whereas it has the disadvantages of utilizing hazardous and flammable organic diluents, as well as the formation of emulsions and third phase, slow phase separation, and low efficiency based on degraded reagents. Ion exchange performed without the need of an organic diluent can solve some of these disadvantages. Various factors in the operation of the solvent extraction process have been investigated. The most important factor is the selection of a suitable solvent extraction reagent. The metal separation and extraction efficiency are obviously affected by the nature of the reagent used. A variety of new reagents have been synthesized to investigate metal extraction [6,7,8]. Solvent extraction is applied to ion exchange, and various factors in the operation of ion exchange, adsorption, and chromatographical processes have also been investigated [9,10,11]. The metal separation and adsorption efficiency are also affected by the nature of the adsorbent used. In this sense, functional groups for metal uptake in ion exchange resins and adsorbents are also important, and a variety of new adsorbents have also been synthesized to investigate metal adsorption [12, 13].
A strategy for adsorbent design incorporating calixarene derivatives chosen from fundamental information via the molecular design of calixarene derivatives as extraction reagents is shown in Fig. 1. The fundamental information shown in Fig. 1a includes the objectives (separation, concentration, analysis, etc.), target metals (size, valency, coordination number, hard and soft acids and bases (HSAB) nature, etc.), and applied methods (solvent extraction, ion exchange, membrane transport, coagulation, precipitation, detection, spectroscopic analysis, etc.). In general, the molecular design of separating reagents, such as extraction reagents and adsorbents, is conducted after gathering relevant information. This is considered from the viewpoint of the functional groups based on HSAB theory [14] and the metal uptake ability and separation efficiency from the side chains related to their steric effect and lipophilicity, together with the denticity related to the number of coordinating atoms. This approach of molecular design of conventional reagents is similar to that of calixarene derivatives used as extraction reagents and adsorbents. As the metal uptake efficiency is highly dependent on the type of functional groups introduced to the adsorbent, it is not easy to drastically change the metal uptake ability or selectivity by substituting the side branches. Therefore, the properties of calixarene derivatives with specific structures are required.
Concept on (c) adsorbent design incorporating calixarene derivatives from (a) fundamental information via (b) molecular design of calixarene derivatives as extraction reagents
Calixarenes synthesized via the condensation reaction of p-alkylphenol and formaldehyde provide a three-dimensionally arranged cavity for guest molecules and a platform for guest ions. Calixarenes have attracted considerable attention as a new type of host compound owing to their discriminative properties originating from the cavity and platform [15, 16]. The selection of the macrocyclic framework such as calixarenes, which have a certain cavity size and shape different from conventional reagents, is an additional conceivable factor for molecular design, as shown in Fig. 1b. A variety of derivatives have been synthesized as extraction reagents [17,18,19]. However, one of the unignorable disadvantages when utilizing them as extraction reagents in an organic diluent is their poor lipophilicity. Although it is not easy to drastically change the solubility of calixarene derivatives in organic solvents [20], their application as ion exchange resins and adsorbents can overcome this drawback.
Although there are some review articles related to the adsorbents incorporating calixarenes, some of them do not specialize in metal adsorption with the introduction of a broad range of target molecules [21, 22], are adversely focused on a certain target molecule [23, 24], and even treat polynucleated metal complexes as gas adsorbents [25]. Metal adsorption using adsorbents incorporating calixarene derivatives have also been reported in my mini review article; however, this only summarized the results obtained from our research group [26]. In this review, the concept of the design of adsorbents incorporating calixarene derivatives from extraction reagents, together with their classification into four types of adsorbents and some results using various adsorbents, have been introduced.
The concept of the molecular design of calixarene derivatives used as extraction reagents has already been reported in detail [27]. Herein, I briefly describe this as an introduction to the next section. As described beforehand, the points to consider for the guest metal ion are as follows: charge, coordination number, coordination geometry related to the coordination number, hardness and softness based on HSAB theory, ion radius, dehydration energy, dehydration rate based on the Eigen mechanism [28], stability constants with anions, solubility products with anions such as hydroxyl ions, etc. Functional groups for metal uptake are common, even general extraction reagents. Calixarenes possess multiple phenolic hydroxyl groups. Functional groups can be introduced on the lower rim via substitution reaction and also on the upper rim via aromatic substitution reactions after dealkylation at the para–position. The resulting derivatives also possess specific structural properties as follows: different sized cavities, two frames with different sizes at the upper and lower rims, different conformational isomers, multiple functionalities, symmetric properties, etc. Therefore, calixarene derivatives exhibit not only the effect of the functional groups but also unique “structural effects”, which are different from those of general reagents owing to their specific cyclic structures. These “structural effects” contain many effects such as size discrimination, conformation, the convergence of multiple functionalities, complementarity, and allosteric nature, as shown in Fig. 2 [27, 29, 30]. Four conformational isomers are known to exist for calix[4]arene derivatives. When functional groups longer than propyl groups are introduced on calix[4]arene, the conformation can be fixed. Calix[4]arene was originally prepared using sodium ion as a template ion. Consequently, most of the cone conformational calix[4]arene derivatives reported to date exhibit selectivity toward Na(I) ions among the alkali metal ions. By contrast, 1,3-alternate conformational derivatives provide a larger coordination site and exhibit higher selectivity toward Cs(I) [31]. To confirm whether these structural effects appear (positively or negatively [32]), a comparison of calixarene derivatives with their corresponding monomeric forms in terms of their extraction and separation efficiency is required. When the structural effects appear to be negative or less positive, a new molecule is designed and synthesized to improve the extraction and separation efficiency, and consequently, the optimal reagent is obtained, which is ready to apply in the next step, such as in ion exchange.
Various structural effects of calixarene derivatives
Concept of the molecular design of calixarene derivatives used as adsorbents
Calixarene is an oligomer originally prepared during the preparation of phenolic resins, but it is considered a monomer from a macromolecular point of view. The molecular design of adsorbents incorporating calixarenes is even more difficult because the design of macromolecules is more complicated when compared to the molecular design of low molecular weight compounds. Therefore, the complexation or extraction of metal ions using ligands or extraction reagents should be preliminarily investigated. The most important approach is to adequately incorporate a well-designed and optimized monomer molecule into the macromolecule. For the molecular design of calixarene derivatives used as adsorbents, careful attentions should be paid to the following aspects:
Synthesis
Most macromolecules, and not only resin-containing calixarene derivatives, have heterogeneous and complicated compositions. Therefore, their preparation should not be long, but simple with fewer steps. As a result, it becomes desirably effortless. Apart from the amount of calixarene introduced to the resin, the preparation of”pie in the sky”-like polymers should be avoided. With the additional synthetic laboriousness of general adsorbents, adsorbents incorporating calixarene derivatives should be synthesized considering many factors, such as the metal selectivity, metal separation efficiency, loading capacity, durability, and regeneration of the adsorbents. For example, the conformation should be considered when calix[4]arene derivatives are incorporated into macromolecules and the binding of calixarenes to macromolecules matrices should be considered relative to the rim and metal uptake sites, etc.
Insolubility into aqueous solutions
Some of the aims of applying calixarenes as adsorbents are to improve their poor solubility in organic diluents and their distribution into the aqueous phase. After their application, these aims should be achieved. The molecular weight of the resulting adsorbents and the degree of polymerization should also be considered.
Selectivity and separation efficiency
The selectivity and separation efficiency should be considered during the molecular design of calixarene derivatives used as extraction reagents and should be maintained after their application. In general, depending on the target ions, the incorporation of amine-nitrogen and ether- or carbonyl-oxygen atoms with strong coordinating power is not preferred because they can participate in the metal uptake process and consequently, reduce the separation efficiency.
Capacity
The loading capacity is one of the most important properties of an adsorbent, particularly from a practical point of view. Needless to say, the functional groups for metal uptake in general adsorbents are of great importance. Calixarene components are also the crucial component in adsorbents. The relationship between the loading capacity of the target ion on the adsorbent and the amount of the incorporated calixarene moiety roughly or almost exactly expresses the stoichiometry between the ions and the calixarene. The loading capacities for most adsorbents, and not only those containing calixarene derivatives, are expressed as mg/adsorbent–g, which is meaningful practically, but not scientifically. Although this expression using mmol/adsorbent–g is also not sufficient, the expression using mmol/ adsorbent–g allows an understanding of the stoichiometry. The loading capacity increases upon increasing the amount of active sites per unit weight of the adsorbent, depending on the stoichiometry. The large molecular weight of calixarene itself causes a decrease in the calixarene content in these adsorbents. In this sense, inert components with high molecular weight in the adsorbent should be removed.
Related to the affinity between the target ion and active site, since many general adsorbents contain ion-exchangeable groups with strong affinity for metals, the adsorption reaction should not be driven by physical adsorption, but simple chemical adsorption (Langmuir model), which describes that a single adsorption site combines with a single adsorbate. The adsorption mechanism for some adsorbents containing more than one coordinating atom with weaker affinity can be complicated. Even so, the adsorbates are not gas molecules, but metal ions, and their affinity is not the weakest London dispersion force or even a weaker dipole–dipole interaction, but less weak dipole–ion interactions. The electrostatic interaction of the latter is sufficiently stronger than the former when discussing the mechanism of metal uptake. However, many articles on adsorbents, and not only those incorporating calixarenes, have discussed the adsorption mechanism using not only the Langmuir model but also the Freundlich (which considers the homogeneous surfaces of the adsorbent with no phenomena explained), Dubinin–Astakhov (which considers the adsorbates occupying micropores), and Temkin (which considers the indirect adsorbate–adsorbate interactions) models to fit the data without mentioning their corresponding adsorption equations or even the active functional groups, together with the pore size, its distribution, and specific surface area. The Donnan equilibrium should be considered for the real-time analysis of the adsorbents; however, this is another topic for discussion.
Repeated use and durability
From the viewpoints of the regeneration of the adsorbents and the recovery of the loaded metal, the metal ions should be repeatedly adsorbed and eluted. For this purpose, it is preferable that the binding site is formed via strong bonds, such as C–C bonds. However, the incorporation of calixarenes into macromolecular matrices using C–C bonds makes no realistic sense.
The concept of the molecular design of adsorbents containing calixarene derivatives is shown in Fig. 1c.
Adsorbents containing calixarenes are not classified based on the macromolecules used. However, they are classified into four types depending on how the calixarene is incorporated into the macromolecules, as shown in Fig. 1c. Each adsorbent has the following advantages and disadvantages:
Polymer-supported adsorbents
The advantages of polymer-supported adsorbents are the stability of their connecting moieties and the wide availability of commercially available polymers used as the supporting matrices. The concerns are as follows: relatively laborious synthesis, that is, the introduction of functional groups at the upper rim may lengthen the synthesis owing to debutylation, while functionalization at the lower rim may require a fixed conformation. Not only do the calixarene monomers relate to the metal uptake, but the coordinating atoms contained in the supporting matrices may also be involved, which lowers the separation efficiency. The success and failure of their synthesis is not clear, that is, the density or amount of the functional groups introduced in the macromolecules is not exact. When the ratio of the supporting matrix to the calixarene monomer is large, the content of calixarene component decreases and the loading capacity may also decrease.
Impregnated adsorbents and polymer inclusion membranes
The favorable advantage of impregnated adsorbents and polymer inclusion membranes is their simple preparation, which involves dissolving the calixarene derivative in an organic solvent, mixing with the microporous material, and drying. There is no chemical bond formed between the supporting matrices and calixarene monomers. If an adequate microporous polymers or membrane matrix is selected, the resulting materials will maintain the metal selectivity of the calixarene monomer used. When the ratio of the macroporous matrix to calixarene monomers is large, the content of the calixarene component decreases and the loading capacity may also decrease. The binding is not a chemical mode, but physical adsorption via hydrophobic interactions, and the leakage of the calixarene monomers is a concern.
Crosslinked adsorbents
The advantages of cross-linked adsorbents include their relatively easy synthesis, the high stability of their connecting moieties, and high capacity due to the relatively small ratio of the supporting matrix to calixarene monomer. On the other hand, the amount of functional groups introduced in the macromolecules may not be exact.
Polymerized adsorbents (vinyl polymerization and copolymerization without vinyl polymerization)
The advantages of polymerized adsorbents are their high capacity due to the relatively small ratio of the supporting matrix to calixarene monomer, the stability of the connecting moieties, particle size control during emulsion, and application to template polymerization for high separation efficiency, whereas their disadvantages are long synthesis and unclear polymerization.
The potential characteristics of the four types of adsorbents containing calixarene derivatives are listed in Table 1.
The results of metal adsorption using each adsorbent containing calixarene derivatives are introduced one by one in the next sections of this review.
Researches on metal adsorption using polymer-supported adsorbents
To the best of my knowledge, the earliest example of metal adsorption using an adsorbent incorporating calixarene was reported by Shinkai et al. in 1988 [33]. They focused on the geometric matching between uranyl ion (U(VI) as UO22+), which exhibit a pseudoplanar hexacoordinate structure, and co-planar calix[6]arene as a superuranophile. They found that the water-soluble hexaacetic acid derivative of calix[6]arene exhibited an extremely high stability constant for uranyl ion when compared to its corresponding tetramer [34], which was then applied as an adsorbent upon supporting the sulfonated calix[6]arene hexaacetic acid derivative on poly(ethyleneimine), as shown in Fig. 3a. The obtained gel also efficiently and selectively adsorbed uranyl ion. A silica-supported adsorbent incorporating calixarene was first reported by Glennon et al. in 1993 [35]. They prepared ethyl ester derivatives of calix[4]- and [6]arenes bearing triethoxy silane, which were suitable to attach to silica. The obtained gel, as shown in Fig. 3b, was used as the stationary phase and applied to the chromatographical separation of alkali metal ions. The calix[4]arene-type gel exhibited selectivity toward Na(I) with a long retention time due to its size-discrimination properties. Glennon et al. prepared an adsorbent using a hydroxamic acid derivative of calix[4]arene supported on octadecylsilica (ODS) to investigate the metal uptake of base metal ions using a chromatography cartridge [36]. They also prepared an adsorbent using the hydroxamic acid derivative of calix[4]arene and a styrene–divinylbenzene microporous polymer (XAD–4). Although it will be described later, it was notable that they referred to the calixarene moieties chemically bound to ODS, in addition to those physically bound to XAD–4. The ODS-supported hydroxamic acid calix[4]arene derivative adsorbent exhibited high selectivity toward Fe(III) over Pb(II), Cu(II), Mn(II), Ni(II), Co(II), Cd(II), and Zn(II), because the hydroxamic acid group is well known to exhibit a high affinity toward Fe(III).
a–f Conceptional structures of supported adsorbents incorporating calixarene derivatives
One of the most practical applications of calixarene adsorbents is the removal of Cs(I). Various kinds of calixcrown compounds have been prepared for Cs(I) removal and it was found that 1,3-alternate conformational calix[4]crown-6 exhibited selectivity toward Cs(I), as reported by Ungaro et al. [31, 40]. Arena et al. applied to an adsorbent incorporating 1,3-alternate conformational calix[4]crown-6 supported on silica toward the removal of Cs(I), as shown in Fig. 3c [41]. Subsequently, Dozol et al. reported the transport of Cs(I) and Na(I) through a solid membrane incorporating calix[4]biscrown-6, although the Cs(I)/Na(I) selectivity was < 350 due to the low hydrophobic environment of the supported membrane [42]. Brindle et al. reported solid-state 29Si- and 13C-NMR spectroscopy of silica-bonded calixarenes without any metal adsorption [43], even though it would contribute to the subsequent studies.
There have been other reports on adsorbents using silica as the supporting matrix in addition to those described above. Chen et al. prepared an adsorbent attaching calix[6]crown-4 on silica using an amidation reaction [44]. The adsorbent exhibited not only hard ion, but also Ag(I) and Hg(II) selectivity. This indicated that the introduced amido groups were hydrolyzed during the binding reaction to form amino groups due to the inconsistency in their HSAB theory. Tabakci et al. prepared an adsorbent incorporating tert-butylcalix[6]arene hexaacetic acid on silica using amidation to investigate the adsorption of Cr(VI) ions [45]. Oddly enough, hexaacetate ligands were selected for Cr(VI) anions, but they only exhibited Cr(VI) adsorption at low pH. This was attributed to the unreacted amino groups on the aminated silica support. In the case of cations as guest ions, the affinities were used for not only anions, but also coordinating atoms with δ− nature and a series of homologous elements with the same charge and different ion radii could be easily compared with each other in terms of their affinity with the host compounds. On the other hand, there are some difficulties such as the low availability of coordinating atoms with δ+ nature, various charges and sizes of anionic species, selectivity toward the Hofmeister series of ions, and easily changeable anionic species with pH, as well as the easily changeable ligand species by protonation upon changing pH. In this sense, the discrimination of anionic species is generally more difficult and careful attention has been being given to this. Tabakci et al. also prepared an adsorbent via the amidation of aminated silica with an acid-chlorinated dinitliro derivative of tert-butylcalix[4]arene to investigate the adsorption of Cu(II) [46]. Although the effective groups for Cu(II) uptake were not mentioned, this can also attributed to the unreacted amino groups on the aminated silica. Chang et al. prepared an adsorbent incorporating calix[4]arene with N,N-dimethylaminomethyl groups on the upper rim into silica upon the reaction of a diepoxidized calix[4]arene derivative with aminated silica to investigate the removal of toxic metals such as Cr(III), Cu(II), Ni(II), Co(II), and Zn(II) [47]. No remarkable selectivity was observed. Yilmaz et al. prepared an adsorbent incorporating bis(3-pyridylamide)calix[4]arene into silica via a silane coupling reaction between the unmodified hydroxyl groups of calix[4]arene and epoxidized silica to investigate the adsorption of anionic species, dichromate anions, and arsenate anion [48]. They suggested that protonated pyridyl groups effectively uptake anionic species assisted by the secondary amide protons. Gubbuk et al. prepared an adsorbent incorporating p-tert-butylcalix[4]-aza-crown into chlorosilicated sporopollenin via a dehydrochloric acid reaction to investigate its adsorption behavior toward Pb(II), Cu(II), and Zn(II), including thermodynamic studies, together with morphological and thermogravimetric studies [49]. Liu and Zhang et al. and Zhao and Zhang et al. prepared an adsorbent incorporating ionic liquid pseudcalixarene compounds, tetraazacalix[2]arene[2]triazine [50] and bis(tetraoxacalix[2]arene[2]triazine) [51], into silica using aminated silica to investigate the chromatographical separation of many analytes, including anionic species such as bromide, bromate, iodate, chlorate, nitrate, iodide, and thiocyanate. They successfully achieved different selectivities for such ions due to the formation of various anion–π complexes between the specific-shape cage host and the diversity of the anions studied. Zhao and Zhang et al. also prepared an adsorbent incorporating bis(1-allyl-limidazolyl)-tert-butylcalix[4]arene into silica to investigate the chromatographical separation of many analytes, including anionic species [52]. The obtained adsorbent also exhibited different selectivity for anions when compared with the adsorbents described beforehand. Su et al. prepared an adsorbent prepared upon the reaction of tri(O-propyl)-mono(O-trismethoxypropylthiobutyl)-p-tert-butylcalix[4]arene or di(O-propyl)-di(O-trismethoxypropylthiobutyl)-p-tert-butylcalix[4]arene with mesoporous silica (SBA–15) to investigate the adsorption of various divalent metal ions [53]. They did not report the effect of the incorporated calixarene on metal adsorption. However, it appeared that four phenoxy oxygen atoms and sulfide-sulfur atoms contribute to metal adsorption. Furthermore, the former may be rather more effective than the latter due to the steric hindrance of the two substituents because the adsorption efficiency of the calix[4]arene adsorbent with di-substituents was less than that with a mono-substituent. Memon prepared an adsorbent incorporating p-morpholinomethylcalix[4]arene on silica gel to discuss the Cu(II) adsorption mechanism assisted by analysis of variance (ANOVA) studies [54]. Other adsorbents prepared using this silane coupling reaction have also been reported. Tabakci et al. prepared an adsorbent incorporating di(O-nitrilo)-tert-butylcalix[4]arene into cellulose using a silane coupling reaction between epoxidized or aminated cellulose and carboxylated di(O-nitrilo)- tert-butylcalix[4]arene to investigate its adsorption behavior (Ni(II) > Cu(II) > Cd(II) > Pb(II) > Hg(II) > Co(II)) [55]. However, the adsorbent did not exhibit any significant selectivity except for trivalent chromium at low pH. Yilmaz et al. prepared an adsorbent incorporating bis(N-methylglucamino)calix[4]arene at the upper rim into magnetic sporopollenin using a ring–opening reaction with epoxidized magnetic sporopollenin to investigate the removal of boron [56]. They used magnetite to collect the prepared magnetic spropollenin adsorbent. However, the adsorbent after the boron removal can also be reused. Boronic acid has multiple pKa values and species, and forms covalent bonds via esterification with polyols. In particular, N-methyl glucamine-type resins are effective upon neutralization by a protonated tertiary amine [57, 58].
Rao et al. modified inert XAD–4 and bonded it to p-tert-calix[8]arene via an esterification reaction to investigate its metal adsorption properties [59]. The p-tert-calix[8]arene-supported XAD–4 exhibited selectivity toward Th(IV) and U(VI) over Cu(II), Fe(II), Zn(II), Ni(II), Co(II), Cd(II), and Pb(II), although they did not refer to its size-discrimination or the hydrolysis of metal ions under high pH condition. Our group also reported a supported adsorbent, as shown in Fig. 3d, in which a chloromethylated calix[4]arene tetraacetic acid derivative with a cone conformation was carefully reacted with polyallylamine at 50 °C in the presence of sodium carbonate in 2-propanol so that quaternary ammonium formation or esterification did not take place [60, 61]. Since it was found that p-tert-octylcalix[4]arene tetraacetic acid in its cone conformation exhibited high Pb(II) selectivity over Cu(II), Zn(II), Ni(II), and Co(II) due to its size-discriminating effect and HSAB theory in our preliminary work [62, 63], the as-obtained adsorbent also exhibited high Pb(II) selectivity when compared with bead-type of polyallylamine, even though the unreacted and remaining amino groups may lower the separation efficiency. The relationship between the amount of Pb(II) and the amount of calix[4]arene component used was discussed and showed a 1:1 stoichiometry (although the stoichiometry in the extraction data was 2:1 (Pb(II):calix[4]arene). Chromatographic separation of Pb(II) and Zn(II) was also carried out and successfully achieved.
Alexandratos et al. prepared supported adsorbents attaching unmodified and diphosphorylated calix[4]arene on polystyrene using an etherification reaction [64]. Although the diphosphorylated calix[4]arene adsorbent exhibited the quantitative adsorption of Fe(III) and Pb(II), the unmodified calix[4]arene and dephosphorylated styrene adsorbents showed poor adsorption properties. Jain et al. prepared an adsorbent containing bis(semicarbazono)calix[4]arene and chloromethylated polystyrene (Merrifield resin) to investigate its adsorption behavior toward La(III), Ce(III), Th(IV), and U(VI) [65]. One or two remaining hydroxyl groups were used for the binding reaction, although they did not mention the conformation of the introduced calix[4]arene moiety. The obtained adsorbent was stable, even in 0.25 M (M = mol dm–3) hydrochloric acid, and was applied toward the chromatographic separation of a ternary metal system. Merrifield resin appears to be effective for the preparation of the supported adsorbent. However, the reaction of the resin with the reactants may be a solid–liquid process, which leads to the content of active groups or calixarene components being unclear. Agrawal et al. prepared an adsorbent incorporating hydroxamate calix[6]arene on polystyrene upon reacting β-hydroxylaminopolystylene with acid chloridated calix[6]arene to investigate its adsorption properties toward Th(IV), U(VI), and Ce(IV) [66]. They did not mention the conformation of the calix[6]arene moiety, but it was expected to be a cone conformation because of the strong hydrogen bonds formed by the six unmodified hydroxyl groups. Although they did also not mention the metal species, Th(IV), U(VI), and Ce(IV) were successfully eluted using a buffer solution at pH 6.5 using 0.1 M and 2 M HCl in sequence. Jain et al. also prepared an adsorbent incorporating di(O-methyl)calix[4]arene with o-vanillinthiosemicarbazone at the upper rim on Merrifield resin to investigate its adsorption behavior toward Cu(II), Cd(II), and Pb(II) [67], Th(IV), U(VI), and other ions [68], Cr(VI), As(III), and Tl(I) [69], and La(III), Ce(III), and other metals [70]. Although no mention of the conformation of the incorporated calixarene moiety and the adsorbed metal species was given, the successful separation of all the ternary metal systems studied was achieved using gradient elution with different concentrations of acid solutions. Merdivan et al. prepared an adsorbent consisting of p-tert-butylcalix[4]arene bearing a 1,2-crown-4 moiety anchored on Merrifield resin by reacting two carboxylic acid groups with the resin to investigate the adsorption properties toward Cu(II), Cd(II), Co(II), Ni(II), and Zn(II) [71]. They did not mention any effects of the calix[4]-1,2-crown-4 moiety. Memon prepared an adsorbent incorporating calix[4]arene bearing thiourea groups on Merrifield resin via the reaction of p-nitrocalix[4]arene with Merrifield resin, which was then modified to thiourea resin. The adsorbent was employed to investigate the adsorptive removal of fluoride ions [72]. No other coexisting anions, except for other halogenide ions, suppressed the removal of fluoride. They employed the same adsorbent for Pb(II) removal [73]. Yilmaz et al. prepared a variety of polymers incorporating calixarene derivatives on Merrifield resin. However, most of them were dissolved in dichloromethane and employed to metal extraction [74,75,76,77,78,79,80].
Chitosan bearing primary amino groups is another polymer matrix. Chen et al. prepared an adsorbent using bis(2-ethoxy)ethoxy-tert-butylcalix[4]arene on chitosan via the reaction of bis(2-chloro)ethoxy-tert-butylcalix[4]arene with suspended and swelled chitosan to investigate its adsorptive behavior toward Ni(II), Cd(II), Cu(II), Pd(II), Ag(I), and Hg(II) [81]. The prepared adsorbent showed a lower adsorption efficiency than the original chitosan material. They attributed this to the decreased nitrogen content and the increased steric hindrance of the calixarene component. Tabakci et al. prepared an adsorbent containing p-tert-butylcalix[4]arene bearing dinitrile and mono carboxylate groups on chitosan using a N,N’-diisopropylcarbodiimide (DIC) coupling reaction to investigate its adsorptive behavior (Hg(II) > Pb(II) > Cd(II) > Cu(II) > Ni(II) > Co(II)) [82]. The prepared adsorbent exhibited a higher adsorption efficiency than the original chitosan material, although they did not mention the pH conditions. They described the preferable adsorption of Pb(II), Hg(II), Cd(II), and Cu(II) due to the NHC = O group based on HSAB theory, but it can be attributed to the unreacted amino groups in chitosan. They also reported the effective adsorption of dichromate anions on the prepared adsorbent at low pH, in which the original chitosan was dissolved. Jumina et al. prepared an adsorbent containing C-4-methoxycarbonylmethoxy-3-methoxycalix[4]resorcinarene on chitosan using an amide exchange reaction [83] and Siswanta et al. [84] investigated its adsorptive behavior toward Pb(II).
Calix[4]resorcinarene not only incorporates components into the polymer matrix. Kałędkowski et al. prepared an adsorbent upon reacting dehydroxylated calix[4]pyrrole[2]thiophene with Merrifield resin to investigate the effect of the modification time on the calixpyrrole content, as shown in Fig. 3e [85]. They also reported the adsorption of “soft” metal ions except for those with mismatched sizes for the ring size of calix[4]pyrrole[2]thiophene. Jain et al. prepared a series of adsorbents using the reaction of two calix[4]pyrrole derivatives with modified styrene–divinylbenzene resin (XAD–2) via a diazo coupling reaction to investigate their adsorption behavior toward Cu(II), Zn(II), and Cd(II) [86]. They did not mention the reason for the observed selectivity. Memon et al. prepared an adsorbent consisting of calix[4]arene tetraester on diazotized XAD–4 to investigate its Pb(II) adsorption properties [87].
Yilmaz et al. prepared two adsorbents upon the reaction of N-methylglucaminocalix[4]arene [88] and benzylpiperidinocalix[4]arene [89] with epoxidized modified magnetite particles. The prepared adsorbents exhibited adsorption abilities for arsenate and dichromate ions. The adsorption of dichromate ions on the N-methylglucamine-type adsorbent was proposed, as shown in Fig. 3f. The protonated N-methylglucamine group contributes to the uptake of HCr2O7–, which was probably assisted by the anion–hydrogen bond interactions formed by multiple hydroxyl groups. Ibrahim et al. prepared an adsorbent consisting of diethanolamino-tert-butylcalix[4]arene on silica-coated magnetite to investigate its Pb(II) adsorption properties [90]. They analyzed many spectroscopic data. Although the incorporated calix[4]arene possessed neutral ester groups, the adsorbent exhibited high Pb(II) selectivity.
Regnouf-de-Vains et al. prepared an adsorbent formed upon the reaction of bis(2,2′-bipyridyl)-tert-butylcalix[4]arene and a benzaldehyde resin to investigate the adsorptive separation of Cu(II) and Zn(II) [91, 92] and Ag(I), Zn(II), and Pb(II) [92] using UV spectroscopy. Bartsch et al. prepared a variety of adsorbents consisting of N-(X)sulfonyl carboxamido-tert-butylcalix[4]arene by changing X-substituent linked silica gel via the reaction of the allyl group with ω-triethoxysilylalkyl mercaptan and cumene hydroperoxide (CHP) to determine the optimal reaction conditions for the adsorbent [93]. They also investigated the effects of X-substituent and spacer length on the Pb(II) adsorption efficiency and found that the adsorbents bearing electron–withdrawing substituents and shorter spacers exhibited high adsorption efficiencies [93, 94]. They also investigated the Pb(II) capacity, Pb(II) concentration factor using eluents at different pH, and competitive Pb(II) adsorption in the presence of Cd(II) and Na(I).
Due to the many advantages and availability of a variety of polymer matrices, a number of studies have been reported.
Researches on metal adsorption using extractant-impregnated adsorbents
The first part of this section summarizes the extractant-impregnated adsorbents reported to date. The earliest metal adsorption process using an adsorbent impregnated with calixarene was reported by Glennon et al. in 1994 [35]. As described beforehand, they also prepared an adsorbent consisting of p-tert-butylcalix[4]arene tetraethyl ester immobilised on octadecylsilica particles, although successful results on the chromatographic retention behaviour toward Na(I) ions were not observed, which was attributed to the less accessibility of calixarene components behind the octadecyl silica-bound side chains. Glennon et al. prepared an adsorbent incorporating p-tert-butylcalix[4]arene tetrahydroxamic acid into a styrene–divinylbenzene type microporous polymer (XAD–4) to investigate the adsorption of Fe(III), Co(II), Pb(II), Cd(II), Mn(II), Ni(II), Zn(II), and Cu(II) [36]. The physically bonded calixarene tetrahydroxamate adsorbent exhibited a similar pattern for metal uptake to that observed with the calixarene tetrahydroxamate adsorbent chemically bonded on ODS, which indicated the contribution of the impregnated calixarene component. Our group have also prepared a series of impregnated adsorbents [95]. When calix[4]arene-supported polyallylamine was used for Pb(II) loading in a previous work [60, 61], the uncontrolled content of the calixarene component in the adsorbent was seen as a problem, which led to the unclear stoichiometry observed between the calixarene component and Pb(II). The most convenient advantage of the impregnated resin was its easy preparation, which involved directly incorporating a fixed weight of the extraction reagent into a fixed weight of the microporous polymer. A series of acetic acid derivatives of calix[6]arene, calix[4]arene, linear trimer, and linear monomer with tert-octyl branches were prepared [96] and a series of the adsorbents were easily prepared by incorporating these acetic acid derivatives into a microporous acrylic ester type polymer (Amberlite XAD–7), which was suitable for impregnating even large calix[6]arene derivatives due to its appropriate polarity, large pore size (80 Å), high porosity (55%), and relatively high specific surface area (450 m2 g–1). It is well known that the impregnation ratio of conventional extractants to XAD–7 is 1:1 because of its high porosity, whereas that of the adsorbent incorporating calixarene derivatives to XAD–7 was 1:3. This was attributed to the occupation of the cavitary structure by the calixarenes. The leakages of the impregnated reagents, linear trimer, and cyclic hexamer and tetramer was < 0.1% at pH 2–5, whereas that of the monomer increased upon increasing the pH (1.7% at pH 5). The adsorbent incorporating the calix[4]arene derivative exhibited high Pb(II) selectivity over Cu(II), Zn(II), and Ni(II) when compared with the other impregnated adsorbents and XAD–7 itself. The most fruitful result was to obtain the amount of the incorporated reagent and the loading capacity of Pb(II) in relation to the stoichiometry. The relationship between the amount of impregnated extraction reagents and Pb(II) loading capacity and stoichiometry is listed in Table 2. The stoichiometry of the calix[4]arene component and Pb(II) was 1:1, whereas a 1:2 stoichiometry was reported in the extraction study [62, 63], as described in the previous section. Another finding was that the content of the active component in the unit weight of the adsorbent was significant. Our group also reported a series of adsorbents consisting of the acetic acid derivatives of calix[6]arene and calix[4]arene with or without a tert-butyl branch to investigate the leakage of the reagents into the aqueous phase, pH dependency of divalent metal ions, loading capacity of Pb(II), elution of the loaded Pb(II) ions, and chromatographic separation of Pb(II) and Zn(II) [97]. Innegligible amount of debutylated calix[4]arene leaked into the aqueous phase at pH 5. The adsorbent incorporating the tert-butylcalix[4]arene derivative exhibited high Pb(II) selectivity and the successful chromatographic separation of Pb(II) and Zn(II) was achieved. Matsumiya and Iki et al. prepared an adsorbent by impregnating p-tert-butylsulfinylcalix[4]arene into XAD–7 to investigate its adsorptive separation of Nb(V) and Ta(V) [98]. Our group also prepared a series of adsorbents incorporating p-tert-octylcalix[4]arene n-pyridyl derivatives into XAD–7 to investigate their Ag(I) adsorption properties [99]. This work was also followed by an extraction study [100]. The 2-pyridino-type adsorbent exhibited efficient Ag adsorption even in the presence of an excess of Na(I) ions. Yilmaz et al. prepared an adsorbent incorporating 2-(piprazinoethylamino)carbonylmethoxycalix[8]arene into XAD–4 to investigate its Cr(VI) adsorption properties [101]. Two piprazyl amino groups appeared to be partially protonated for interaction with anionic species such as Cr(VI) and they concluded a weak physisorption process occurred based on the very low adsorption energies obtained using the Temkin isotherm model. Memon et al. also used the same adsorbent for As(V) adsorption using many models [102]. Yamada et al. prepared an adsorbent incorporating p-diethylphosphonomethylthiacalix[6]arene into XAD–7 used for Pd(II) sorption [103]. They successfully reported the selective sorption of Pd(II) over Pt(IV), Rh(III), Ba(II), Al(III), La(III), and Ce(III), and the sufficient desorption of the loaded Pd(II) ions.
Silica gel is also effective for the impregnation of extraction reagents due to its microporous structure. Zhang et al. prepared an adsorbent impregnated with 1,3-[(2,4-diethyl-heptylethoxy)oxy]-2,4-crown-6-calix[4]arene (Calix[4]arene–R14) together with N,N,N’,N’-tetraoctyl-3-oxapentane-1,5-diamide (TODGA) and octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) in macroporous silica modified with polystyrene to investigate the removal of Sr(II), Pd(II), Zr(IV), Mo(VI), and Cs(I) ions [104]. They prepared three columns packed with TODGA for Sr(II), Pd(II), and Zr(IV), CMPO for Sr(II) and Mo(VI), and Calix[4]arene–R14 for Cs(I). The stepwise and effective elimination of fission product elements was achieved using three columns. They also prepared the above-mentioned adsorbents impregnated with Calix[4]arene–R14 using octanol [105, 106], methyloctyl-2-dimethylbutanemide [107], and tributylphosphate [108] as modifiers to improve the hydrophilicity of Calix[4]arene–R14. Kim prepared an adsorbent by impregnating Calix[4]arene–R14 using dodecanol as a modifier to improve the Cs(I) adsorption efficiency [109]. Wei et al. also prepared an adsorbent impregnated with Calix[4]arene–R14 together with dodecanol and dodecyl benzenesulfonic acid as modifiers to improve the Cs(I) adsorption efficiency [110]. Zhang et al. also prepared adsorbents impregnated with 1,3-calix[4]arene-bis(naph-crown-6) [111] and 25,27-bis(n-octyloxy)calix[4]arene-crown-6 and 25,27-bis(i-octyloxy)calix[4]arene-crown-6 [112] using macroporous silica modified with polystyrene to investigate the adsorption of Na(I), K(I), Rb(I), Cs(I), Sr(II), Ba(II), Fe(III), and Pd(II). The former adsorbent showed selectivity toward Cs(I) and Rb(I), whereas the latter two adsorbents exhibited relatively high selectivity toward Cs(I) over Rb(I). Zhang et al. also prepared an adsorbent impregnated with 1,3-di(1-hexyloxy)-2,4-crown-6-calix[4]arene for Cs(I) [113] and a 1,3-alternate calix[4]crown-5 derivative for Rb(I) [114]. Borai et al. prepared a series of adsorbents impregnated with calix[4]arene-bis(2,3-naphtho-crown-6) using tributylphosphate as a modifier and four zeolites to investigate their Cs(I) adsorption properties [115].
The second part of this section summarizes the polymer inclusion membranes (PIMs) reported to date. Arena et al. prepared a variety of PIMs in 1998. They incorporated a 25,27-bis(N, N-diethylaminocarbonylmethoxy)calix[4larene and three 25,27-dialkylestercalix[4]arene derivatives for Sr(II) removal and a 5,17-bis-(l-ally1)-25,27-bis-(1-propyloxy)calix[4larene-crown-6 in the 1,3 alternate conformation for Cs(I) removal into a mixture of cellulose triacetate (CTA), 2-nitrophenyl octyl ether (NPOE), and dinonylnaphtalenesulfonic acid (DNNS) in dichloromethane, and the resulting mixture was cast and dried to obtain the PIMs. The as-obtained PIMs were used to investigate the transport of Sr(II) and Cs(I) [116]. The effects of the incorporated calix[4]arene carriers and DNNS for facilitated permeability were discussed in detail. Lamb et al. prepared a series of membranes incorporating 1,3-bis(dodecyloxy)calix[4]arene-crown-6 and calix[4]arene-biscrown-6 into CTA and NPOE in methylene chloride for casting and drying [117]. They also investigated the transport of a series of alkali metals and found that the diffusion of the metal-carrier complex in the PIMs was the rate-limiting step in the permeation process. Kim et al. prepared various membranes incorporating 1,3-alternate calix[4]arene-crown-6 and a series of 1,3-dialkoxycalix[4]arene dibenzocrown-6 derivatives to obtain PIMs and to compared their permeation properties for alkali metals, particularly Cs(I) with supported liquid membranes [118]. They reported the faster transport of metal ions using PIMs and excellent durability over 20 d of stirring. Yilmaz et al. prepared membranes incorporating calix[4]arene bearing 4-ethoxycarbonyl-N-piperidinomethyl groups on the upper rim into CTA and NPOE to investigate their Cr(VI) transport properties [119]. They reported 98.6% removal of Cr(VI) after 10 h of transport. Alpoguz et al. prepared a series of membranes incorporating bis(O-benzylaminoethyl)-tert-butylcalix[4]arene [120] and calix[4]arene with bis(1,4-dioxa-8-azaspiro[4,5]decanyl)methyl groups at the upper rim [121] into CTA and three plasticizers for Cr(VI) transport.
Impregnation is the simplest method to obtain the adsorbents and PIMs. Both materials physically contain calixarene derivatives as they are. Most of the obtained materials exhibit approximately the same metal uptake and transport properties as the incorporated calixarene derivatives themselves.
Researches on metal adsorption using calixarene moiety-crosslinked adsorbents
Next, I provide a summary of the adsorbents bearing cross-linked calixarene moieties reported in the literrature. Cross-linked adsorbents incorporating crown ether moieties were prepared in advance of this synthetic method, as well as other types of adsorbents incorporating calixarenes [122,123,124]. A cross-linked adsorbent was first reported by Blanda et al. in 1998, although it did not include adsorption data. They prepared debutylated O-tetraethoxyethoxycalix[4]arene with a fixed cone conformation for the efficient uptake of Na(I) ions [125]. A series of cross-linked copolymers were prepared, as shown in Fig. 4a, via selective formylation, reduction, dichlorination, reaction with p-bromo- or p-cyanophenol, alcoholization or reduction, and coupling with terephthaloyl chloride. Our group first reported the adsorption of Pb(II) using an adsorbent constructed from methylene-cross-linked calixarene, as shown in Fig. 4b [126]. The idea to obtain this adsorbent was very simple: The calixarene bearing phenol units should react with formaldehyde derivatives, similar to the preparation of phenol resin. The acetic acid derivative of debutylated calix[4]arene in its cone conformation was cross-linked with s-trioxane under acidic conditions. The as-obtained adsorbent exhibited high Pb(II) selectivity due to the calix[4]arene tetraacetic acid component and was applied to the chromatographic separation of Pb(II) and Zn(II) with 30 times the concentration of Pb(II) without Zn(II) contamination. A further study prepared a new adsorbent changing the reactant ratio (calix[4]arene tetraacetic acid:s-trioxane) from 1:5 to 1:1 [127]. It was expected that the loading capacity of Pb(II) on the adsorbent would not change because the calixarene contents in the adsorbents were not significantly different upon changing the reactant ratio (i.e. decreasing the number of cross-linked methylene groups). Surprisingly, the Pb(II) capacity was changed and reached 2.10 mol kg–1 using the adsorbent prepared with a 1:1 reactant ratio, as shown in Fig. 5a, which was higher than a commercially available carboxylic acid-type adsorbent. It was concluded that the stoichiometry of calix[4]arene tetraacetic acid and Pb(II) changed from 1:1 to 1:2 upon changing the reactant ratio from 1:5 to 1:1 due to the increased flexibility of the calixarene components. As described in the previous two sections, the stoichiometry of calix[4]arene tetraacetic acid and Pb(II) was determined to be 1:2 from the extraction data [62]. This was supported by a comparison of the amount of calixarene component incorporated in the adsorbent (1.30 vs 1.50 mol kg–1) and Pb(II) loading capacity (1.32 vs 2.10 mol kg–1), together with the reduced loading capacity of Zn(II) (1.20 vs 1.39 mol kg–1), as shown in Fig. 5b [128]. The effects of both metal concentrations on the adsorbed amounts on methylene-crosslinked calix[4]arene adsorbents with tetraacetic acid upon changing the reactant ratio (tetraacetic acid:s-trioxane) from 1:5 to 1:1 are shown in Figs. 5a and b. Yilmaz et al. prepared a number of polymers upon reacting diketones, diesters, and dinitrile derivatives of p-tert-butylcalix[4]arene with triethylene glycol ditosylate [129]. The resulting polymers were telomers, which were used for extraction and exhibited selectivity toward Cu(II), Pb(II), and Hg(II). They also prepared polymers incorporating calix[4]arene-crown-4 and calix[4]arene-crown-5 upon their reaction with 1,8-diaminooctane [130]. The resulting polymers were telomers, which were used for extraction and exhibited selectivity toward Cu(II), Pb(II), and Hg(II). The conformation of the incorporated calix[4]arene component was not fixed, although both adsorbents exhibited high extraction ability for all metal ions studied. They prepared polymers using p-tert-butylcalix[4]arene-thiacrown-4 or calix[4]arene-crown with 1,5-dibromopentane [131]. They found that only three or four monomer units were cross-linked and the resulting adsorbents could be used for the extraction of Cu(II), Hg(II), and Pb(II). They also prepared a polymers using 1,3-di(4-aminobenzyl)-2,4-crown-6-calix[4]arene with tert-butyl branches and a cone conformation with terephthaloyl chloride [132]. However, the obtained polymer also had five monomer units and can be applied to alkali metal extraction. Scoponi et al. prepared a variety of polymers by reacting various calixarenes bearing diols at the lower and upper rims with 2,4-tolylendiisocianate in the presence of dibutyltin dilaurate [133]. They obtained information on the conformation of the incorporated calix[4]arene, the molecular weight after cross-linking, and solubility in organic solvents by changing the length of the spacer, conformation, and position of the diols used. They also investigated the extraction of a series of alkali metals and silver ions using these polymers. The polymers bearing crown moieties exhibited a high extraction efficiency for K(I), Rb(I), Cs(I), and Ag(I) when compared with those without crown moieties. Gong et al. successfully prepared the range of polymers shown in Figs. 4c and d by reacting calix[4]crown-6–2,4-bis(4-aminobutyl ether) with adipoyl chloride or 1,2,4,5-benzenetetracarboxylic dianhydride [134]. They also obtained the information on the molecular weights after cross-linking and solubility in organic solvents, such as ethyl acetate, acetone, methylene chloride, THF, ether, toluene, n-hexane, and benzene upon comparison with calixarene monomer, the last four of which hardly dissolved the as-obtained adsorbents. They did not apply the polymers in an adsorption process, but rather extraction. Both polymers exhibited similar extraction behavior with selectivity toward Cs(I) among the alkali and alkali earth metals studied. Yang et al. polyamidated tert-butylcalix[6]arene hexaester or calix[6]-1,4-crown-4 tetraester with three poly(ethyleneimine), ethylenediamine, diethylenetriamine, and triethylenetetramine to investigate the adsorption of Na(I), K(I), Ag(I), Hg(II), Cu(II), Co(II), and Ni(II) [135]. The adsorbent incorporating the crown unit exhibited a higher adsorption capacity, except for soft ions such as Ag(I) and Hg(II) when compared to that without the crown unit, which meant that the crown unit enhanced hard metal adsorption but suppressed the adsorption of soft ions. Hajipour et al. prepared the adsorbents shown in Fig. 4e by reacting bis[(4-aminobenzyloxy)-tert-butylcalix[4]arene with pyromellitic acid diamide bearing chiral amino acid units using polycondensation or microwave irradiation methods to investigate the adsorption of Na(I), K(I), Cs(I), Ag(I), Co(II), Hg(II), Cd(II), and Cu(II) [136]. All the adsorbents exhibited comparable or lower adsorption efficiencies for all ions studied when compared to the starting diamine compound with the exception of Na(I) ions on the polycondensed adsorbents. Our group also prepared a series of adsorbents cross-linking 2-pyridyl calix[4]arene [137] for Ag(I), hexaacetic acid of calix[6]arene [138] for Pb(II), and the tetraacetic acid derivative of calix[4]arene and hexaacetic acid derivative of calix[6]arene [139] for In(III). All the adsorbents exhibited sufficiently high loading capacities for the target metals. Yoga Priastomo et al. also prepared adsorbents cross-linking calix[4]arenes bearing an O-methyl moiety and their unmodified derivatives via a three step synthesis to investigate the adsorption of Pb(II) and Pt(IV) [140]. Only limited results have been reported on this type of adsorbent, which can be attributed to the low reactivity of the cross-linking reagents caused by steric hindrance. Consequently, the adsorbents were obtained with low molecular weight. Lakouraj et al. prepared two types of adsorbents cross-linking di(3-carboxy-1,2,3-triazolium)-p-tert-butylcalix[4]arene with various diamine compounds in the presence of triphenylphosphine as a condensing agent or diacetylene-tert-butylcalix[4]arene with various diazide compounds, as shown in Figs. 4f and g, respectively [141]. They obtained information on their solubility in organic solvents and physical properties as well as and the XRD spectra of nanocrystalline polytriazolecalixamide adsorbents and amorphous polytriazolecalixarene adsorbents. They also investigated the sorption of divalent metals using these adsorbents. All the adsorbents exhibited little clear selectivity for Pb(II). They proposed the metal forms chelate complexes with the carbonyl oxygen and triazole 2-nitrogen atoms in polytriazolecalixamide and the phenol oxygen and triazole 3-nitrogen atoms in polytriazolecalixarene. Trabolsi et al. prepared the adsorbent shown in Figs. 4h upon reacting 1,3-dihydroxy-calix[4]arenepara-dibromo-monothioether-crown-5 with 1,3,6,8-tetraethynylpyrene in the presence of bis(triphenylphosphine)-palladium(II) chloride and copper(I) iodide [142]. The obtained adsorbent was characterized using FTIR, solid-state 13C NMR spectroscopy, nitrogen adsorption–desorption experiments, SEM, TEM, EDS, and XRD. The obtained adsorbent was also employed for metal adsorption and exhibited high Hg(II) selectivity over Ca(II), Mg(II), Cu(II), and Zn(II) due to the soft sulfur atoms in the crown unit. The Hg(II) uptake was also confirmed using XPS.
a–h Conceptional structures of crosslinked adsorbents incorporating calixarene derivatives
Effects of concentrations of Pb(II) and Zn(II) on adsorbed amount on methylene-crosslinked calix[4]arene adsorbents with tetraacetic acid by change of reactants ratio (tetraacetic acid:s-trioxane) from 1:5 to 1:1. (a) Pb(II), (b) Zn(II)
Researches on metal adsorption using vinyl-polymerized and copolymerized adsorbents
The vinyl-polymerized and copolymerized adsorbents reported to date will be summarized in the next section. Harris and McKervey et al. first prepared a vinyl-polymerized adsorbent in 1991, although they only confirmed its Na(I) uptake in deuterated chloroform and the Na(I) selectivity in an extraction system [143]. They polymerised the triethylacetate monomethacrylate-type tert-butylcalix[4]arene using azobisisobutyronitrile (AIBN) upon heating. The obtained adsorbent shown in Fig. 6a contains six calixarene units due to the steric congestion of the calixarenes. They also tried to polymerise the Na(I)-complexed monomer, but failed because of the inhibition of thiocyanate. Yilmaz et al. prepared a series of adsorbents via the polymerization of trinitrile monomethacrylate-type tert-butylcalix[4]arene itself and its copolymerizing it with styrene (1:5 ratio) using AIBN and heating [144]. The homopolymer had a partial cone conformation, as confirmed by 1H-NMR spectroscopy. The copolymer had a molecular weight of 25,000. However, it was applied to an extraction process. Menon et al. reacted the hydroxylamine derivative of calix[4]arene with an acid chloride derivative of calix[6]arene to prepare a copolymerized adsorbent [145]. Some questions remain though because the nitro groups, and even the hydroxamic acid groups, should be reduced to amino groups using Raney nickel, and derivatives with different cavity sizes and unmodified hydroxyl groups remain after the copolymerization reaction. However, the adsorbent exhibited adsorptive selectivity toward Ga(III) > In(III) > Tl(III) over Cu(II), Zn(II), Pb(II), Co(II), Ni(II), and Al(III). Yilmaz et al. prepared adsorbents upon the self-polymerizing of tert-butyl-25-[2-(acryloxy)ethoxy]-27-methoxycalix[4]arene-crown-4 and copolymerization with styrene using AIBN and heating [146]. The obtained homopolymers and copolymers prepared with different reactant ratio (ligand to styrene = 1:5 and 1:10) had molecular weights of 4400, 42,000, and 60,000, respectively. However, they were again applied to an extraction. Our group prepared a vinyl-polymerized adsorbent containing calix[4]arene tetraacetic acid (Fig. 6b) using a multi-step synthesis involving cyclization, debutylation, O-acetylation, Fries rearrangement, reduction, dehydration with polymerization, esterification, and hydrolysis [147]. The polymerization reaction should be conducted using the tetraacetic acid derivative of calix[4]arene monomer after hydrolysis because the obtained solid-state polymer will suppress the subsequent reactions. However, it occurred during the dehydration using 1-hydroxyethyl derivative and p-toluenesulfonic acid. Nevertheless, the adsorbent exhibited high Pb(II) selectivity over Cu(II), Zn(II), Co(II), and Ni(II), and was applied to the successful chromatographic separation of Pb(II) and Zn(II). A further study involved the preparation of an adsorbent via copolymerization of 2,6-dimethyl-4-vinylphenoxyacetic acid as the monomeric derivative of the calix[4]arene and divinylbenzene. Its physical properties, a comparison of its adsorption behavior under different pH conditions, and the loading capacity of Pb(II) were reported [148]. The adsorbent prepared with the calix[4]arene component exhibited extremely high Pb(II) selectivity and a high loading capacity of Pb(II) (1.82 mol kg–1) when compare with the monomer component due to the size discrimination and multi-functional converging effects of the calixarene as well as the incorporation of the polydivinylbenzene moiety in the monomer adsorbent. The loading capacity of Pb(II) also suggested that the stoichiometry of the calixarene component and Pb(II) was > 1:1, which was similar to that obtained using a methylene cross-linked adsorbent [127]. Prabawati et al. have prepared a variety of adsorbents upon the polymerization of O-di- [149] and mono-allylated [150] tert-butylcalix[6]arene in the presence of concentrated sulfuric acid to investigate the adsorption of Cr(III), Cu(II), and Cd(II). Bayrakcı et al. prepared a fiber-type adsorbent upon the copolymerization of bis(3-aminomethyl-pyridineamido) and bis(2-aminomethyl-pyridineamido)calix[4]arene with polyacrylonitrile by electrospinning at 15 kV [151]. The adsorbent can be classified as a polymer-supported adsorbent. However, it was prepared by radical polymerization, even though the binding of the calixarene component on polyacrylonitrile is not clear. Both fibers exhibit Cr(III) adsorption properties, particularly under low pH conditions.
a and b Conceptional structures of polymerized adsorbents incorporating calixarene derivatives
Conclusions
This review summarizes the structural effects of calixarene derivatives, provides a brief introduction to the molecular design of calixarene derivatives used as extraction reagents, and the concept of the molecular design of adsorbents incorporating calixarene derivatives. The adsorbents were classified into four types based on how the calixarenes are incorporated into the macromolecules. During a survey of the studies on metal adsorption using adsorbents incorporating calixarene derivatives, research on the synthetic methods used to prepare adsorbents incorporating calixarene derivatives without the results of any metal adsorption studies were also obtained. For instance, Prata et al. prepared a variety of polymers, such as the copolymer of monostyrenocalix[4]arene and styrene [152], the copolymer of monostylenocalix[4]arene, stylene and divinylbenzene [153], self-polymerized polymers prepared from 4-ethynylbenzyloxy)calix[4]arenes [154], a radical cyclopolymerized polymer obtained from bis(4-vinyl-benzyloxy)-p-tert-butylcalix[4]arene [155], the copolymers of mono- or divinylbenzylcalix[4]arene and styrene [156], and the copolymers obtained via the cross-coupling reaction of 1,4-bis-(4-tert-butyl-phenoxymethyl)-2,5-diiodo-benzene and 1,4-diethynylbenzene [157]. All these polymers were well-designed so that the incorporated calix[4]arenes would maintain their cone conformation, even though they contained no adsorption data. Other recommendations include ring-opening metathesis polymerization of alkene-bridged calix[4]arene monomers in their three possible conformations with cyclooctene and norbornene [158]. The homo-conformer was not selectively incorporated into the polymer, however, the conformation was in their main concerns. Taylor et al. conducted the free radical polymerization of mono(vinylbenzyl)calix[4]arene in its cone conformation, followed by sulfonation, to obtain a water-soluble polymer [159]. They did not apply this adsorbent to metal complexation, but to viologen. These synthetic methods provide clues to the synthesis new adsorbents for metal ions. Collaborative work by host–guest chemists and hydrometallurgical chemists will synergize the field of metal separation, which will also be applicable to other targets due to their well-designed structures.
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Acknowledgement
I am grateful to Professor Katsutoshi Inoue, Professor Kazuharu Yoshizuka, Professor Takashi Hayashita, Dr. Hidetaka Kawakita, Dr. Hiroki Kodama, and Dr. Bruce. A. Moyer for valuable suggestions and comments. I would like to thank Editage (www.editage.com) for English language editing.
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Ohto, K. Review of adsorbents incorporating calixarene derivatives used for metals recovery and hazardous ions removal: the concept of adsorbent design and classification of adsorbents. J Incl Phenom Macrocycl Chem 101, 175–194 (2021). https://doi.org/10.1007/s10847-021-01053-x
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DOI: https://doi.org/10.1007/s10847-021-01053-x