Sol-Gel Synthesis of Ordered β-Cyclodextrin-Containing Silicas
© Trofymchuk et al. 2016
Received: 10 December 2015
Accepted: 17 March 2016
Published: 31 March 2016
New approaches for β-cyclodextrin-containing silicas synthesis were demonstrated. Materials with hexagonally ordered mesoporous structure were prepared by postsynthesis grafting and by co-condensation methods. β-Cyclodextrin activated by a N,N′-carbonyldiimidazole was employed for postsynthesis treatment of 3-aminopropyl-modified MCM-41 support as well as for sol-gel synthesis with β-cyclodextrin-containing organosilane and tetraethyl orthosilicate participation in the presence of cetyltrimethylammonium bromide. The successful incorporation of cyclic oligosaccharide moieties in silica surface layer was verified by means of FT-IR spectroscopy and chemical analysis. Obtained β-cyclodextrin-containing materials were characterized by X-ray diffraction, transmission electron microscopy, and low-temperature adsorption-desorption of nitrogen. In spite of commensurable loading of β-cyclodextrin groups attained by both proposed approaches (up to 0.028 μmol · m–2), it was found that co-condensation procedure provides uniform distribution of β-cyclodextrin functionalities in silica framework, whereas postsynthesis grafting results in modification of external surface of silica surface. Adsorption of benzene from aqueous solutions onto the surface of β-cyclodextrin-containing materials prepared by co-condensation method was studied as the function of time and equilibrium concentration. Langmuir and Freundlich models were used to evaluate adsorption processes and parameters. Adsorption experiments showed that β-cyclodextrin-containing silicas could be promising for the trace amount removal of aromatics from water.
M41S are ordered mesoporous materials with well-defined uniform pores, high surface area and pore volume, which have attracted great interest since 1992. MCM-41, one of the most widely studied M41S materials, consists of an amorphous (alumina, metallo) silicate framework forming hexagonal pores with diameter more than 1.5 nm . The high potential applications of MCM-41 materials as adsorbents [2–4], catalysts [5–7], membranes [8, 9], and drug delivery systems [10–18] have been possible by means of their functionalization with organic compounds. Here, mesoporous silicas are the common basement for obtaining products with unique characteristics. The presence of highly reactive silanol groups in sufficiently large and tunable uniform pores opens up the possibility for introduction of various organic functional moieties, such as methyl and/or trimethylsil [11, 19], chloropropyl , aminoalkyl and triaminoalkyl [7, 9, 12, 21, 22], phenyl , mercaptoalkyl [24, 25], and sulfo [25, 26] groups into the surface layer of MCM-41. In general, silicas can be functionalized in two ways: postsynthesis modification or direct co-condensation . Among a large number of organic compounds for silica functionalization, cyclodextrin macromolecules are very promising because of their ability to form inclusion complexes with chemicals of suitable geometry and functionality .
Postsynthesis modification of silica surfaces was successfully realized by attachment of β-cyclodextrin or its derivatives to the silica support preliminary functionalized with N-(2-aminoethyl)-3-aminopropyl , carboxylated cuccinyl , 3-aminopropyl [31–35], 3-glycidoxypropyl [36, 37], hydrosilyl , ester , and 3-mercaptopropyl  groups, whereas the taken attempts to introduce β-cyclodextrin moieties into the silica framework by sol-gel methods involve the condensation of silica alkoxides with β-cyclodextrin  or β-cyclodextrin-containing silanes [42–44]. The predominant majority of these works relates to the synthesis of functionalized silica materials with disordered porous structure. However, usage of aggressive solvents and activating agents in multistep procedures of organic reactions at postsynthesis treatment as well as functional silanes and pore-expanding agents at sol-gel condensation process may affect the structure of final MCM-41-type materials substantially causing damage of their hexagonally ordered pore structure. At the same time, the chemical immobilization of β-cyclodextrin under mild conditions—through amide bond formation on macroporous silica—was carried out . The activation properties of N,N′-carbonyldiimidazole in the reaction with β-cyclodextrin for the following immobilization of oligosaccharide derivative onto aminopropyl silica surface was used.
The idea of the present research was to use the possibility of β-cyclodextrin activation under mild conditions for preparing functionalized MCM-41-type silica materials with hexagonally ordered mesoporous structure. Here, two principal methods were exploited for β-cyclodextrin-containing MCM-41 silicas producing: postsynthesis attachment to the support by covalent bond formation or sol-gel synthesis using β-cyclodextrin-containing silane. First, aminopropyl-containing mesoporous ordered functionalized organosilica was prepared by co-condensation of tetraethyl orthosilicate and (3-aminopropyl)triethoxysilane, followed by postsynthesis grafting of β-cyclodextrin through the activating agent (N,N′-carbonyldiimidazole) usage. Another method of synthesis supposed the exploration of activated β-cyclodextrin for organosilane obtaining and its subsequent co-condensation with tetraethyl orthosilicate in the presence of cetyltrimethylammonium bromide to yield β-cyclodextrin-MCM-41 silica. Adsorption experiments were carried out to study the role of functionalization for effective uptake of benzene from water. It was expected that proposed synthetic approaches can be useful in preparing of MCM-41-type silica materials with high surface area, pore volume, and narrow pore size distribution as well as sufficient functional groups concentration.
β-cyclodextrin hydrate (β-CD) (99 %, Acros Organics), tetraethyl orthosilicate (TEOS) (≥99 %, Merck), (3-aminopropyl)triethoxysilane (APTES) (≥99 %, Merck), N,N′-carbonyldiimidazole (CDI) (≥98 %, Merck), and cetyltrimethylammonium bromide (CTMABr) (≥97 %, Merck) were used as purchased, and no further purification was performed. Aqueous ammonia (25 %), ethanol (96 %), and hydrochloric acid (37 %) were purchased from Reakhim and used without additional purification. Acetone (extra pure, Merck) and N,N′-dimethylformamide (DMF) (pure analytical, Reakhim) were dried for 48 h before utilization with activated molecular sieves (0.3 nm, Merck). Benzene (pure analytical, Reakhim) was used to prepare benzene solutions in water. Distilled water was used in all experiments.
Postsynthesis Modification of MCM-41 Silica with β-CD Using Activation Agent
Hexagonally ordered NH2-MCM-41 silica support was prepared by hydrothermal sol-gel synthesis in the presence of ionic surfactant compound, CTMABr, by the procedure described in . TEOS and APTES were used as silica sources. The final molar composition of the reaction mixture for NH2-MCM-41 silica preparing by template method was as follows: 0.09 TEOS:0.006 APTES:0.02 CTMABr:0.55 NH4OH:0.56 C2H5OH:14.4 H2O. Obtained amino-functionalized silica support was washed by water and dried at ambient temperature. Then, the template was removed by extraction in acid-ethanol solution. NH2-MCM-41 silica was dried in the air at 423 K for 4 h, cooled, and kept in a desiccator before use.
Then, reaction mixture with activated oligosaccharide (I) was slowly dropped into DMF suspension of NH2-MCM-41 silica with stirring. The grafting step lasted for 24 h at ambient temperature. Next, β-CD-grafted MCM-41 silica (CD-MCM-41ps) was filtered and washed sequentially with DMF, acetone, and distilled water. CD-MCM-41ps was dried in the air at 293 K.
Synthesis of CD-MCM-41 Silica Using β-CD Silane
The co-condensation method was also employed to incorporate β-CD in silica matrix. At the beginning, β-CD-containing organosilane (II) was prepared by modification of APTES with aforementioned activated oligosaccharide (I). Then, the obtained product was used for templated sol-gel synthesis of two types of CD-MCM-41sg silicas. One of them, CD-MCM-41-1sg, was prepared by co-condensation of TEOS and II in the presence of activation reaction by-products. Meanwhile, the second silica (CD-MCM-41-2sg) was synthesized by use of TEOS and purified β-CD-organosilane. Purification of β-CD-organosilane was realized in accordance with the following procedure. Dry acetone was added to the reaction mixture of β-CD-containing organosilane (II) to fall out substituted oligosaccharide [44, 46], and the resultant precipitate was collected by filtration. After drying under vacuum, the yellow solid product was obtained. Finally, purified β-CD-organosilane was dissolved in dry DMF and co-condensed with TEOS in the presence of CTMABr. Ionic template was previously dissolved in water with stirring at room temperature, and NH4OH was added to provide the alkaline medium of the reaction.
The reaction mixtures for both CD-MCM-41-1sg and CD-MCM-41-2sg silicas were agitated on magnetic stirrer for 2 h. In order to complete the condensation process, the hydrothermal treatment in autoclave at 373 K for 24 h was carried out. The final molar composition of the reaction mixture for CD-MCM-41sg silicas preparing was as follows: 0.05 TEOS:0.001 β-CD-organosilane:0.007 CTMABr:0.27 NH4OH:7.2 H2O. Both CD-MCM-41sg materials were washed by small quantities of water and dried at ambient temperature. Then, the template was removed by triple solvent extraction in HCl/C2H5OH solution at room temperature for 24 h. After extraction, silicas were washed with distilled water until the negative test for halogenide anions with AgNO3. Obtained materials were dried in the air at 293 K.
The ordered mesoporosity of the aminopropyl- and β-CD-containing silicas was confirmed by diffraction analysis at low angles (2θ = 1–10 grad) and transmission electron microscopy (TEM).
Powder X-ray diffraction patterns (XRD) were measured on a DRON-4-02 diffractometer using CuKα radiation (λ = 0.154178 nm) and a nickel filter.
TEM experiments were carried out on a JEM JEOL 1230 electron microscope operated at 100 kV. The samples (0.05 g) for TEM measurements were suspended in ethanol (4 ml) and processed with ultrasonic treatment for 3 min (ultrasound power 60 W). Obtained suspensions (50 μL) were supported onto formvar film on a Cu grid, followed by drying at ambient conditions.
The transmission spectra were registered on a Thermo Nicollet NEXUS Fourier transform infrared (FT-IR) spectrophotometer in the range from 4000 to 400 cm–1 for solid pellets of MCM-41-type silicas.
Porosity measurements were obtained with a Kelvin-1042 Sorptometer using low-temperature adsorption-desorption of nitrogen. Prior to measurements, all samples were outgases at 413 K for 20 h.
Specific surface area of CD-MCM-41 silicas was determined using the BET method in the relative pressure range (P/P 0) up to 0.30. The pore size distributions were calculated by applying the non-localized density functional theory (NLDFT) (equilibrium model). The total pore volume (V total) was obtained from the amount of nitrogen adsorbed at P/P 0 = 0.99.
The amount of surface aminopropyl groups was calculated by the difference in pH values (ionometer I-160) of starting and equilibrium acid solutions with CD-MCM-41ps (or CD-MCM-41sg) silica batch after 24-h contact .
The content of β-CD groups chemically immobilized on the surface of CD-MCM-41 silicas was defined by acid hydrolysis of cyclodextrin up to glucose. The concentration of glucose after the reaction with potassium ferrocyanide was defined by spectrophotometry using Specord M-40 equipment (Germany, Carl Zeiss, Jena) at λ = 420 nm [45, 48].
Results and Discussion
Chemical bonding of β-CD with NH2-MCM-41 silica support was realized with coupling agent participation. Since hydroxyl groups of β-CD exhibit great reactivity, the formation of C(O)–N bonds under the influence of CDI activator occurred even at ambient temperature (Scheme 1). The presence of imidazole by-product in the reaction mixture during the grafting on silica support does not lead to drastical changes in pH, and NH2-MCM-41 silica structure did not alter. Imidazole was easily removed from the surface of CD-MCM-41ps silica by washing with organic solvents and distilled water.
Activated β-CD was also used for obtaining the proper organosilane for subsequent co-condensation with TEOS in attendance of CTMABr template. To confirm that by-products of β-CD activation reaction could not affect the structure of the final silica, two types of CD-MCM-41 silicas were prepared by sol-gel synthesis. The nature of surface layer and mesoporous structure of CD-MCM-41-1sg and CD-MCM-41-2sg silicas obtained from β-CD-alkoxide with by-products and purified one, correspondingly, were compared.
Structural properties of ordered β-cyclodextrin-containing MCM-41 silicas
d 100 (nm)
S BET (m2 · g−1)
V total (cm3 · g−1)
D DFT (nm)
(mmol · g−1)
(μmol · m−2)
(mmol · g−1)
(μmol · m−2)
The evidence of MCM-41 silicas functionalization by two proposed synthesis methods was also demonstrated due to chemical analysis of surface compounds. The estimated content of aminopropyl and β-cyclodextrin groups on the surface of synthesized materials is summarized in Table 1. It can be seen that only a small part of aminopropyl groups for CD-MCM-41ps has been reacted with activated β-CD. So many unreacted amino groups for CD-MCM-41ps may indicate that β-CD is mainly grafted at the entrance of the pores, preventing further penetration of oligosaccharide moieties to the inner pore surface with other anchoring sites . The appearance of aminopropyl groups in CD-MCM-41sg (Table 1) points to the partial hydrolysis of amide bonds under hydrothermal treatment of β-CD-containing silicas in the medium of ammonium. It is evident that postsynthesis grafting as well as sol-gel synthesis leads to bifunctional MCM-41 silica obtaining.
The existence of functional groups within the framework of MCM-41 materials causes the change in the surface chemistry and porosity of the solid, which in turn affects the sorption behavior of MCM-41-type silicas. Earlier , we used UV spectroscopy to prove that β-CD and benzene could form 1:1 “host–guest” inclusion complex in aqueous solution. It was shown that formation of “β-CD-benzene” complex is spontaneous and thermodynamically profitable exothermal process. Incorporation of cyclic oligosaccharide in solid supports like MCM-41 silicas makes possible an efficient removing of aromatic pollutants from aqueous solutions by means of supramolecular structure formation.
Adsorption properties of ordered MCM-41 silicas in dilute benzene solution (0.38 g · L−1)
S BET (m2 · g−1)
a (mmol · g−1)
a (μmol · m−2)
0.361 ± 0.043
0.363 ± 0.117
0.508 ± 0.061
0.972 ± 0.117
0.555 ± 0.061
0.956 ± 0.115
0.739 ± 0.089
0.910 ± 0.109
0.696 ± 0.084
0.870 ± 0.104
Parameters of benzene adsorption calculated by Langmuir and Freundlich equations for ordered β-cyclodextrin-containing MCM-41 silicas
K L (L · mg−1)
a m (mg · g−1)
K F (L · mg−1)
It was shown that typical adsorption capacities for activated carbon and silica adsorbent in liquid phase under different conditions are in the range of 12–230 mg · g−1 for benzene [53, 54]. Therefore, prepared cyclodextrin-containing MCM-41 silicas demonstrate adsorption level performance of known samples and could be very promising for the treatment of aqueous solutions with low benzene concentration.
In this research, we realized two principal methods of β-cyclodextrin-functionalized MCM-41-type silicas producing: postsynthesis attachment to the support by covalent bond formation or sol-gel synthesis using β-cyclodextrin-containing silane in the presence of ionic template. β-Cyclodextrin activated by N,N′-carbonyldiimidazole was employed for both synthetic approaches. Obtained functional materials were characterized by XRD, TEM, and chemical analyses, FT-IR spectroscopy, and low-temperature adsorption-desorption of nitrogen. The results of this study indicate that co-condensation method leads to the formation of MCM-41 silicas with higher arrangement of mesoporous channels compared with one obtained by postsynthesis grafting. Moreover, it was proved that by-products of β-CD activation reaction could not affect the structure of the final silica. The proposed synthesis approaches may be applicable for obtaining of ordered β-cyclodextrin-containing functional materials with high affinity to chemicals of suitable geometry. Adsorption study of benzene uptake from aqueous solutions confirms the probability of β-cyclodextrin-functionalized MCM-41-type silica use in water treatment processes.
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