Structural Variety and Adsorptive Properties of Mesoporous Silicas with Immobilized Oligosaccharide Groups

Abstract

In this research, we report on the synthesis of mesoporous silicas with various quantities of immobilized oligosaccharide groups and different pore ordering degree. The hydrothermal co-condensation of tetraethyl orthosilicate and β-cyclodextrin-containing organosilane in the presence of cetyltrimethylammonium bromide template was employed. The purpose of this investigation was to show the opportunity of increasing β-cyclodextrin content in silica matrix by changing the molar ratio of initial reagents during organosilane synthesis and to determine whether the enhancing of immobilized groups on the surface influences on model aromatic compound adsorption from water. It was prepared several β-cyclodextrin-organosilanes by modification of (3-aminopropyl)triethoxysilane with oligosaccharide (the molar composition of reaction mixtures were 1:1, 3:1, and 5:1) with using N,N′-carbonyldiimidazole as linking agent. Three types of MCM-41 materials were obtained with 0.018, 0.072, and 0.095 mmol g−1 β-cyclodextrin-group loading according to chemical analysis of silicas. The IR spectroscopy and potentiometric titration were also performed to confirm the presence of functional groups in the silica matrix. Nitrogen sorptometry experiments exhibited the decrease of high surface area (from 812 to 457 m2 g−1) and the average pore diameter (from 1.06 to 0.60 cm3 g−1) of synthesized silicas with increasing of immobilized oligosaccharide groups. The influence of β-cyclodextrin-organosilane presence on the forming of hexagonally arranged porous structure of silicas was evaluated by X-ray diffraction and TEM analyses. As the loading of oligosaccharide groups increases in obtained silicas, the (100) reflex in diffraction patterns is even less intense and broader, denoting the decrease of long-range pore ordering. Adsorption experiments were carried out to study the effect of β-cyclodextrin groups’ attendance in silica matrix on benzene uptakes from aqueous solutions. Experimental kinetic curves of benzene adsorption on synthesized silicas were compared with theoretical models of Lagergren and Ho-McKay for pseudo-first and pseudo-second-order processes. Langmuir and Freundlich isotherm models were used to evaluate adsorption processes and parameters. Obtained β-cyclodextrin-containing MCM-41 silicas demonstrate adsorption level performance of known samples and could be very promising for benzene uptakes from aqueous solutions in water treatment processes.

An atomistic view on liquid phase adsorption

“…For immobilizing cyclodextrin in a silica-pore, two linker concepts used by Huq et al [16] and Trofymchuk et al [19] were utilized for generating the molecule structure for simulation. Since the native cyclodextrin molecules were described by the Amber-compatible q4md-CD force field [38], the linker molecules were parametrized via AmberTools20 [39].…”

“…The internal surface was functionalized with 0.07 and 0.14 µmol m −2 β-cyclodextrin, respectively. For the simulations representing the system by Trofymchuk et al [19], additional 0.11 and 0.22 µmol m −2 of the propylamine group was added to the internal surface, which was experimentally used to immobilize cyclodextrin. This resulted into a total hydroxylation density (OH-groups on the surface) of 8.56 µmol m −2 on the internal surface and 8.82 µmol m −2 on the external surface.…”

“…Table 1 Properties of the cylindrical mesopore model before and after functionalization (func), generated as described by Trofymchuk et al [19], with surface densities (µmol m −2 ) and pore dimensions (nm).…”

“…This is further backed up by the quality of the GAFF-compatible force field q4md-CD for cyclodextrin simulations [38,51]. In order to verify the quality of the force-fields for the solute molecules p-nitrophenol and benzene, which were studied in the work of Huq et al [16] and Trofymchuk et al [19], validating simulations were carried out based on topologies provided by Mobley et al [52]. Water was described by the TIP3P model [53].…”

“…A third type of material is based on a mesoporous silica support. Mesoporous silicas with chemically attached macrocyclic moieties were successfully prepared by sol-gel condensation of tetraethyl orthosilicate and β-cyclodextrin-silane in the presence of a structure directing agent [16][17][18][19], resulting in silica-based materials that possess a uniform framework mesoporosity with defined nanoscaled cavities. The ability of removing aromatic compounds from an aqueous phase was investigated, and it was concluded that the synthesized materials is promising for this purpose [16,17,19].…”