Adsorption performance of VOCs in ordered mesoporous silicas with different pore structures and surface chemistry

Ordered mesoporous silicas with different pore structures, including SBA-15, MCM-41, MCM-48 and KIT-6, were functionalized with phenyltriethoxysilane by a post-synthesis grafting approach. It was found that phenyl groups were covalently anchored onto the surface of mesoporous silicas, and the long-range ordering of the mesoporous channels was well retained after the surface functionalization. The static adsorption of benzene and the dynamic adsorption of single component (benzene) and bicomponent (benzene and cyclohexane) on the original and functionalized materials were investigated. As indicated by the adsorption study, the functionalized silicas exhibit improvement in the surface hydrophobicity and affinity for aromatic compounds as compared with the original silicas. Furthermore, the pore structure and the surface chemistry of materials can significantly influence adsorption performance. A larger pore diameter and cubic pore structure are favorable to surface functionalization and adsorption performance. In particular, the best adsorption performance observed with phenyl-grafted KIT-6 is probably related to the highest degree of surface functionalization, arising from the relatively large mesopores and bi-continuous cubic pore structure which allow great accessibility for the functional groups. In contrast, functionalized MCM-41 exhibits the lowest adsorption efficiency, probably owing to the small size of mesopores and 1D mesoporous channels.     

Direct synthesis of polyoxometalates-functionalized mesoporous hybrid silicas

Novel polyoxometalates (POMs)-functionalized mesoporous hybrid silicas (SiW11/MHS) were synthesized for the first time by the co-condensation of tetraethoxysilane (TEOS) with Keggin-type monovacant SiW11 in the presence of block copolymer EO20PO70EO20 (P123). The as-obtained SiW11/MHS samples were characterized by FT-IR, UV/DRS, XRD, ICP-AES, and N2 adsorption-desorption measurements. It is shown that not only a hexagonal packing of channels with homogeneous pore diameter is obtained, but also Keggin units remain perfectly in the hybrid materials, and Si species insert into the vacancies of lacunary SiW11 during the synthesis to form SiW11Si2 structure, which condenses with the framework of mesoporous silica thus grafting POM molecules onto the pore wall of mesoporous silica by covalent linkage. This linkage results in a firmer immobilization of POMs on the mesoporous material. The water-leaching percent of POMs in the hybrid material is much less than those in the impregnated samples. Therefore, the synthesis strategy for the hybrid material is significant for acquiring supported POM catalysts of high efficiency and practicability.     

Synthesis and characterization of Ti-containing mesoporous silicas

Ti-containing mesoporous silicas have been synthesized at ambient temperature using amines with alkyl chains of 10–16 carbon atoms as templating surfactant. Following this synthesis route, it was possible to prepare materials with more than 2 wt% Ti without forming extrawall TiO₂. Physicochemical characterization of the samples showed that Ti atoms were probably tetrahedrally coordinated, in a configuration similar to that observed on TiSi mixed oxides. In contrast to data reported previously, these materials could be obtained within less than 15 min and were thermally stable at 650°C in air. The pore dimensions of Ti-HMS could be varied by changing the amine chain length. Materials with very high surface areas were obtained using relatively low surfactant/SiO₂ ratios. Increasing this ratio in the synthesis gel did not modify the symmetry of the mesoporous silica as reported for M41S materials but drastically decreased its surface area.     

Grafting fluorescence molecules into the pore surface of bimodal mesoporous silicas with different routes

Novel luminescent hybrid silicas have been synthesized through one-step and two-step methods and their different luminescent properties have been investigated. The results showed that the samples synthesized with two-step procedure exhibited a more evident blue shift of photoluminescence performance resulting from the high efficiency of surface amine than that by one-step route. The structure characteristic and texture properties of resultant hybrid silicas were characterized by XRD, TEM, FT-IR and N₂ adsorption-desorption techniques, indicating that inorganic/organic nanocomposites could maintain the bimodal mesoporous structure. Finally, a possible involved mechanism has been put forward.     

Sorption of actinide ions onto mesoporous phosphorus-containing silicas

Equilibrium and kinetic characteristics of template mesoporous silicas containing phosphonic acid residues in sorption of various actinide ions were studied. The sorption equilibrium involving these sorbents is attained within 20 min after introducing the sorbent into the solution. The calculated values of the internal diffusion coefficient \((\bar D)\) and half-exchange time (τ_0.5) in sorption of uranium were ～3.5 × 10^?16 m² s^?1 and ～390 s, respectively. Mesoporous phosphorus-containing silicas efficiently sorb from acid solutions uranyl ions, Th(IV), and Pu(IV). In sorption of uranium from sulfuric acid solutions, the capacity of the sorbents is 125–132 mg g^?1, and in sorption from nitric acid solutions (0.5–3.0 M HNO₃), 276–299 mg g^?1. In sorption of Th(IV) from nitric acid solutions, the capacity of the sorbents is 60–66 mg g^?1. In sorption of microamounts of ²³⁹Pu(IV), the distribution coefficient reaches 4500 cm³ g^?1. Phosphorus-containing silicas in nitric acid solutions do not noticeably sorb ²⁴¹Am, which allows using them for efficient separation of the Pu/Am pair with the separation factor of no less than 2 × 10₃.     

Improving bioavailability of silybin by inclusion into SBA-15 mesoporous silica materials

Mesoporous silica SBA-15 (SC) and template occluded SBA-15 (SP) were used to increase the bioavailability of silybin, an extremely poorly water soluble drug. Silybin was introduced into SC and SP by assembling or self-assembling methods. The results of X-ray powder diffraction, differential scanning calorimetry (DSC) and N2 adsorption-desorption experiment indicated that amorphous silybin was successfully introduced into the mesopores in both SC and SP samples. The transmission electron micrograph (TEM) images revealed that the structure of the mesoporous materials was not destroyed after the drug loading. Results of dissolution rate tests and animal experiments showed that both SC and SP can significantly improve the bioavailability of silybin, SP was thus preferred because the template needs not to be removed by calcination or extraction.     

Synthesis of new silicas with high stable and large mesopores and macropores for biocatalysis applications

Monomodal or bimodal porous silicas with large mesopores, constituted by particles or having a monolithic (block type) morphology, respectively, are synthesized using sodium silicate as siliceous species source, cetyltrimethylammonium bromide (CTAB) as pore template and ethyl acetate (EtAc) as pH modifier. The monomodal porosity is represented by 20–30 nm pores and the bimodal one by these pores and also macropores. These characteristics are modulated in function of the CTAB and EtAc concentrations as well as the pH and hydrothermal treatment. The role of these reagents upon the porosity is rationalized. The presence of high CTAB concentration and a rather low pH decreasing rate (function of EtAc concentration and hydrothermal treatment) are essential for having the already known bimodal mesoporous silicas (BMS). On the contrary a rather high pH decreasing rate promotes the formation of the new bimodal mesoporous–macroporous silicas (BMMS) synthesized in this work, where the EtAc also plays the role of emulsion forming agent. The hydrolytic stability of the synthesized silica under aqueous conditions, at different pH values, makes these silicas good candidates for application in different areas of catalysis, especially in the enzymatic one.     

Morphological control on SBA-15 mesoporous silicas via a slow self-assembling rate

Pluronic 123-templated mesoporous SBA-15 silica rods with a length of ca. 3.0–4.0 μm were easily synthesized in a dilute silicate solution with a pH value of 2.0 at room temperature. Through a good control on the synthetic condition and the chemical components, a high homogeneity (>95%) of the hexagonal SBA-15 silica rods can be achieved. In addition, the effect of the synthetic conditions including acid source, weight ratio of the P123/sodium silicate, temperature, water content, pH value, and applying shearing flow were explored in detail to tailor the morphologies of the SBA-15 mesoporous silicas. In this paper, we also focused on the counterion effect on the synthesis of the SBA-15 mesoporous silicas. It was found that the SO₄ ²⁻ counterion from H₂SO₄ has higher affinity to induce the formation of P123 rod-like micelles than that of Cl⁻, NO₃ ⁻. Meanwhile, we postulated that the self-assembly pathway of the silica species and the neutral tri-block copolymer micelles in a dilute solution with a pH value of 2.0 would occur through an S⁰···I⁰ rather than the S⁰X^−…I⁺ one as previously discussed. We further employed the SAB-15 mesoporous silica rods as the templates to synthesize high-quality CMK-3 mesoporous carbon rods by using commercially available phenol–formaldehyde (PF) resin as the carbon source.     

Adsorption of methanol on mesoporous SBA-15

The adsorption of methanol on mesoporous SBA-15 has been studied by using Brunauer-Emmett-Teller (BET) surface area analysis, scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The BET surface area analysis shows decreases of the surface area from 387 to 383 m²/g, pore volume from 0.88 to 0.81 cm³/g and pore diameter from 9.07 to 8.4 nm after methanol adsorption. The appearance of strong IR bands at 2862 and 2964 cm^− 1 due to methyl (-CH₃) symmetric and asymmetric stretching demonstrate the presence of methanol and evidence of successful methanol adsorption. XPS results show increase of carbon and oxygen content on the surface of SBA-15. Thermogravimetric analysis shows that the methanol adsorbed on SBA-15 is stable up to a temperature of 265 °C and that the methanol adlayers decompose between 265 and 588 °C.     

Engineering nanospaces: ordered mesoporous silicas as model substrates for building complex hybrid materials

This contribution summarizes investigations of organic-inorganic hybrid materials wherein the inorganic phase is ordered mesoporous silica such as MCM-41 and SBA-15. The review, which covers work performed in the last three years, emphasizes studies of: (1) covalently attached functional groups, (2) new approaches to functionalization, (3) approaches for achieving high densities of uniform functional groups, (4) periodic mesoporous organosilicas (PMOs) with hierarchical ordering, (5) new functional chemistries, and (6) the application of new materials to enantioselective catalysis and emerging areas. The review concludes with the authors outlining some outstanding problems in the field.     

Adsorption of vitamin E on mesoporous titania nanocrystals

Tri-block nonionic surfactant and titanium chloride were used as starting materials for the synthesis of mesoporous titania nanocrystallite powders. The main objective of the present study was to examine the synthesis of mesoporous titania nanocrystals and the adsorption of vitamin E on those nanocrystals using X-ray diffraction (XRD), transmission electron microscopy, and nitrogen adsorption and desorption isotherms. When the calcination temperature was increased to 300 °C, the reflection peaks in the XRD pattern indicated the presence of an anatase phase. The crystallinity of the nanocrystallites increased from 80% to 98.6% with increasing calcination temperature from 465 °C to 500 °C. The N₂ adsorption data and XRD data taken after vitamin E adsorption revealed that the vitamin E molecules were adsorbed in the mesopores of the titania nanocrystals.     

Adsorption of lead from water by thiol-functionalized SBA-15 silicas

Thiol-functionalized mesoporous silicas have been synthesized by co-condensation of tetraethoxysilane and varying contents of 3-mercaptopropyltrimethoxysilane in acidic medium with the block copolymer Pluronic 123 as a structure directing agent. Adsorption of lead in aqueous solution on the synthesized materials has been investigated. The adsorption data are fitted to Langmuir isotherms and a maximum adsorption capacity calculated from the Langmuir equation can reach 0.19 mmol of Pb/g. The stoichiometric ratio of S: Pb being 1:1 has been obtained for every adsorbent. The effect of the pH on lead adsorption, and desorption of lead on the lead-loaded materials have been studied.     

Template-directed synthesis of mesoporous silicas containing phosphonic acid derivatives in the surface layer

A template-directed process, using 1-dodecylamine as a template, is developed for the synthesis of mesoporous silicas containing the phosphonic acid derivatives ≡Si(CH₂)₂P(O)(OC₂H₅)₂ and ≡Si(CH₂)₃P(O)(CH₃)(ONa) in the surface layer. The porous materials obtained by removing the template with boiling methanol have specific surfaces of 854 and 505 m²/g, accessible pore volumes of 0.42 and 0.37 cm³/g, and pore diameters of 2.2 and 2.5 nm, respectively. As shown by scanning electron microscopy and x-ray diffraction, the mesoporous silicas are nonuniform in particle shape and size, and their structure is less ordered than that of classic mesoporous silicas, such as MCM-41. IR and ¹³C, ³¹P, and ²⁹Si CP/MAS NMR spectroscopy data indicate that, in the surface layer of the mesoporous silica prepared with the use of ≡Si(CH₂)₂P(O)(OC₂H₅)₂, the functional groups are present in the form of T ² and T ³ structural units. In addition, the surface layer contains alkoxy groups and water, which participates in hydrogen bonding.     

Surface-modified mesoporous silicas as recyclable adsorbents of an endocrine disrupter, bisphenol A

Surface-modified mesoporous silicas (MSs) were investigated for recyclable adsorption of an endocrine disrupter, bisphenol A (BPA). Surface-modified MSs were prepared by (i) post-synthesis surface modification of MSs using surface hydroxyl groups and organosilanes (m-MS) and by (ii) co-condensation of tetraethoxysilane and the corresponding organosilanes (d-MS). Infrared measurements indicated that organic groups mainly existed on the surface of m-MS, which resulted in a surface characterized by high hydrophobicity. Both organic groups and isolated hydroxyl groups existed on the surface of d-MS, resulting in both hydrophobicity and hydrophilicity on the surface. The amount of BPA adsorbed on surface-modified MSs per organic group was similar for m-MS and d-MS, however, the d-MS established equilibrium for BPA adsorption faster than m-MS, as measured by UV-vis spectra. A larger amount of BPA per surface area could be adsorbed on carbon materials than on the surface-modified MSs, however, the regeneration of carbon materials by washing could not be done easily. The surface-modified MSs retain adsorption capacity for BPA after several regeneration cycles, demonstrating that the surface-modified MSs are effective recyclable adsorbents of the endocrine disrupter, bisphenol A.     

Thermal decomposition behavior of amino groups modified bimodal mesoporous silicas as aspirin carrier

Two kinds of amino groups were employed to functionalize bimodal mesoporous silicas and related drug carriers were prepared. The characterization results of XRD, N2 adsorption and desorption, FT-IR and TG all confirmed the structural integrity of the bimodal mesopore architecture after introduction treatment of functional groups and the successful adsorption of aspirin. In order to investigate the interaction among the mesoporous structure, the functional groups grafted onto the mesoporous surface and the existential microenvironment of the drug molecules inside the mesoporous channels, the thermal decomposition behaviors of amino groups modified and aspirin loaded carriers were studied based on the thermogravimetric analysis in details. According to the thermogravimetry and derivative thermogravimetry results, the apparent activation energies E(a) of thermal decomposition for all related samples have been evaluated by Kissinger and Flynn-Wall-Ozawa methods. Meanwhile, their thermal decomposition mechanisms have been suggested by using Coats and Redfern methods. All these featured consequence could provide a deeper understanding for large loading capacity and controlled release of drug-carriers in the pharmaceutical application.     

Elucidation of interactive effects of synthesis conditions on the characteristics of mesoporous silicas templated using polyoxide surfactant

A series of mesoporous silicas (MS-1–MS-9) were synthesized at different gel compositions using a triblock copolymer (TCP), poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), as the surfactant. The interactive effects of acidity, the contents of tetraethyl orthosilicate (TEOS) and the surfactant, and the gelling temperature on the characteristics of the final material were simultaneously characterized. Increasing acidity favored mesopore formation. A material with a surface area of 760 m²/g, mostly in the mesoporous range, was obtained at 1.0(TEOS):0.017(TCP):7.3HCl:115.7H₂O. Mesopore formation was predominantly determined by the TEOS:TCP ratio and was promoted with its increase from 1.56:1 to 2.09:1. A further increase to 2.61:1 was detrimental. Whereas increasing the TCP content to 3.5% w/w improved micellization, a further increase to 4.6% should be avoided. Mesoporous silicas showed low crystallinity but a high degree of hexagonal mesoscopic organization. The weak surface acidity was attributed to surface silanols, the number of which was proportional to mesoporosity.     

Elucidation of interactive effects of synthesis conditions on the characteristics of mesoporous silicas templated using polyoxide surfactant

A series of mesoporous silicas (MS-1-MS-9) were synthesized at different gel compositions using a triblock copolymer (TCP), poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), as the surfactant. The interactive effects of acidity, the contents of tetraethyl orthosilicate (TEOS) and the surfactant, and the gelling temperature on the characteristics of the final material were simultaneously characterized. Increasing acidity favored mesopore formation. A material with a surface area of 760 m²/g, mostly in the mesoporous range, was obtained at 1.0(TEOS):0.017(TCP):7.3HCl:115.7H₂O. Mesopore formation was predominantly determined by the TEOS:TCP ratio and was promoted with its increase from 1.56:1 to 2.09:1. A further increase to 2.61:1 was detrimental. Whereas increasing the TCP content to 3.5% w/w improved micellization, a further increase to 4.6% should be avoided. Mesoporous silicas showed low crystallinity but a high degree of hexagonal mesoscopic organization. The weak surface acidity was attributed to surface silanols, the number of which was proportional to mesoporosity.     

Preparation of polyelectrolyte-functionalized mesoporous silicas for the selective adsorption of anionic dye in an aqueous solution

Polyelectrolytes (PDDA, poly (diallydimethylammonium chloride)) functionalized mesoporous silica adsorbent (PDDA/MS) was prepared and characterized by N₂ adsorption, transmission electron microscopy (TEM) and zeta potential. The PDDA/MS showed ordered pore structure and the surface charge was successfully converted from negative to positive by PDDA functionalization. The adsorption capability of the prepared adsorbents was evaluated using nine different dye solutions with positive, neutral and negative charges. For the comparison purpose, commercial silica (CS, Davisil) and granular activated carbon (GAC), which are conventionally used as adsorbents, were also treated by PDDA impregnation. The batch adsorption experiments showed that the PDDA/MS exhibited the highest selective adsorption capacity of negative acid dye dissolved in an aqueous solution. Taken together, the results of this work indicate that polyelectrolyte functionalization of the inorganic surface could be a simple and suitable method for the surface modified adsorbent and the PDDA/MS suggested in this study could be used for the effective removal of acid dye from aqueous solutions.     

Structural characterization and surface heterogeneity of bimodal mesoporous silicas functionalized with aminopropyl groups and loaded aspirin

Bimodal mesoporous materials were modified with different amount of 3-aminopropyltriethoxysilane and employed as aspirin carriers. The modified and drug loaded bimodal mesoporous materials were characterized with XRD, FT-IR, TEM and elemental analysis methods to explore the influence of amino groups and drug molecules on the mesoporous surface. Meanwhile, the mesoporous surface energy states was calculated by the density functional theory based on the N₂ sorption analysis. According to the surface energy distributions of samples including before and after modification and drug loading, it can be deduced that through superficial modification with amino groups, the surface energy moves to high state, implying the introduction of amino moieties could provide the mesoporous surface with much active sites. Besides, the interaction between the amino groups and drug molecules is weak, and hence the controlled drug delivery would be possible.     

Encapsulation of Coumarin 151 into the mesopores of modified rodlike SBA-15

Dye Coumarin 151 was postgrafted into the rodlike SBA-15 mesoporous materials, which were synthesized by a direct hydrothermal synthesis method and further modified by an organic silane with a terminal amino group. Characterization by powder X-ray diffraction, N₂ adsorption-desorption, transmission electron microscopy, photoluminescence and scanning electron microscopy were carried out. Small-angle X-ray diffraction and N₂ adsorption-desorption characterizations showed that these dye containing materials remained as ordered mesostructures and the pore size was from 6 nm for blank sample to 3.6 nm for postgrafting sample. PL characterization of composite samples exhibited optical properties with different dye concentrations. The characterization showed the existence of Coumarin 151 in the channels of SBA-15 and the composite materials with novel optical properties enabled possible applications in optical sensing and electron acceptors.