Mesoporous Silica Framework Research Articles

Development of thioflavin-modified mesoporous silica framework for amyloid fishing

Amyloid beta peptide accumulation in the brain poses a serious threat during Alzheimer’s disease. Various strategies to remove or disrupt these plaques with nanoparticles is an emerging area for the treatment for neurodegenerative diseases. The present work attempts to develop a novel strategy to remove the plaques using magnetic field by employing magnetic nanoparticles conjugated with a plaque-targeting ligand. Thioflavin-S (TF-S) was covalently linked to magnetic iron oxide incorporated mesoporous silica, SBA-15 and the material was characterized using microscopic and spectroscopic techniques. This magnetic conjugate (IO-SBA-TF-S) was found to display the ability to remove in vitro, KLVFF peptide, a recognition motif in β-amyloid that has been implicated in plaque formation. This system represents a potential smart system that could herald in the next generation therapeutic strategy for Alzheimer’s disease and related disorders.

Can surface energy measurements predict the impact of catalyst hydrophobicity upon fatty acid esterification over sulfonic acid functionalised periodic mesoporous organosilicas?

Sulfonic acid functionalised periodic mesoporous organosilicas (PrSO3H-PMOs) with tunable hydrophobicity were synthesised via a surfactant-templating route, and characterised by porosimetry, TEM, XRD, XPS, inverse gas chromatography (IGC) and ammonia pulse chemisorption. IGC reveals that incorporation of ethyl or benzyl moieties into a mesoporous SBA-15 silica framework significantly increases the non-specific dispersive surface energy of adsorption for alkane adsorption, while decreasing the free energy of adsorption of methanol, reflecting increased surface hydrophobicity. The non-specific dispersive surface energy of adsorption of PMO-SO3H materials is strongly correlated with their activity towards palmitic acid esterification with methanol, demonstrating the power of IGC as an analytical tool for identifying promising solid acid catalysts for the esterification of free fatty acids. A new parameter −ΔGNP−PC, defined as the per carbon difference in Gibbs free energy of adsorption between alkane and polar probe molecules, provides a simple predictor of surface hydrophobicity and corresponding catalyst activity in fatty acid esterification.

Alumina-grafted SBA-15 as a high performance support for Pd-catalysed cinnamyl alcohol selective oxidation

Ultrathin alumina monolayers grafted onto an ordered mesoporous SBA-15 silica framework afford a composite catalyst support with unique structural properties and surface chemistry. Palladium nanoparticles deposited onto Al-SBA-15 via wet impregnation exhibit the high dispersion and surface oxidation characteristic of pure aluminas, in conjunction with the high active site densities characteristic of thermally stable, high-area mesoporous silicas. This combination confers significant rate enhancements in the aerobic selective oxidation (selox) of cinnamyl alcohol over Pd/Al-SBA-15 compared to mesoporous alumina or silica supports. Operando, liquid-phase XAS highlights the interplay between dissolved oxygen and the oxidation state of palladium nanoparticles dispersed over Al-SBA-15 towards on-stream reduction: ambient pressures of flowing oxygen are sufficient to hinder palladium oxide reduction to metal, enabling a high selox activity to be maintained, whereas rapid PdO reduction and concomitant catalyst deactivation occurs under static oxygen. Selectivity to the desired cinnamaldehyde product mirrors these trends in activity, with flowing oxygen minimising CO cleavage of the cinnamyl alcohol reactant to trans-β-methylstyrene, and of cinnamaldehyde decarbonylation to styrene.

Functionalized mesoporous silica material and anionic dye adsorption: MCM-41 incorporated with amine groups for competitive adsorption of Acid Fuchsine and Acid Orange II

Adsorption of two representative anionic dyes (Acid Fuchsine (AF) and Acid Orange II (AO)) using MCM-41 functionalized with amine groups in a mesoporous silica framework (NH2–MCM-41) as the adsorbent was investigated.

Ionic conductivity of mesoporous electrolytes with a high density of pyridinium groups within their framework

Introduction of a high density of pyridinium groups within the framework of mesoporous silica enhanced its electrochemical properties such as ionic conductivity (1.4 mS cm−1 at 95 °C under 100% relative humidity).

Mesoporous silica: a highly promising and compatible candidate for optical and biomedical applications

A simple post-co-precipitation strategy is adopted for the construction of a mesoporous silica framework in which the rare earth gadolinium oxide (Gd2O3:Eu3+& Gd2O3:Sm3+, Eu3+) is incorporated as an emissive probe for optical and bio-medical applications. The samples show good emission along with the required theranostic properties. In the blue region, a better red emissive behavior is observed in the case of Eu3+ doped sample while the Sm3+ and Eu3+ co-doped sample exhibit the same in the near ultra-violet region. Moreover, the structural and textural properties of mesoporous silica are preserved even in the presence of rare earth doped Gd2O3 and this can very well be utilized for theranostic applications. A clear confocal microscopy image shows a red spot, and thus its potential towards trackability is confirmed. In addition to this, other admirable properties are also associated with it namely, an excellent drug loading ability, bio-compatibility, easy surface modification by the targeting moiety and excellent longitudinal relaxivity (T1) under magnetic resonance imaging (MRI), which stands par for it to be utilized in the field of theranostics too. This material shows good luminescence properties along with the desired internal–external morphology, and a narrowly ordered porosity can validate the application of these materials in multi-faceted fields such as phosphors for LEDs, bio-imaging, theranostics etc.

Synthesis and application of tin triflate-containing MCM-41 as heterogeneous Lewis acid catalysts for the Mukaiyama aldol reaction at room temperature

Sn-containing MCM-41 (Sn-MCM-41) with different tin content was prepared by hydrothermal synthesis then treated with triflic acid to form tin triflate within the silica framework (SnOTf-MCM-41). XRD and UV–visible measurements revealed that SnOTf-MCM-41 have highly ordered mesoporous structures with tetrahedrally-coordinated Sn species. Moreover, results from FT-IR analysis revealed that triflate ligands are selectively coordinated with Sn4+ species in the mesoporous silica frameworks. SnOTf-MCM-41, as Lewis acid catalysts, promoted the Mukaiyama aldol reaction of benzaldehyde with 1-trimethylsiloxycyclohexene at room temperature and had greater catalytic activity than untreated Sn-MCM-41. Taking into account results from in situ FT-IR experiments using pyridine as probe molecule, the enhanced catalytic performance after triflic acid treatment was attributed to an increase in the number of acid sites, because of appearance of water tolerance by the formation of metal triflate species. In addition, the SnOTf-MCM-41 catalyst was reusable at least three times in the Mukaiyama aldol reaction.

In-situ generation of large microporous skeleton in mesoporous silica framework using different dicarboxylic acids

Mesoporous Santa Barbara Amorphous-15 is known to possess a small fraction of micropores, in the walls of mesopores. The ratio of micropore to mesopore area can have a profound influence on the application of this highly ordered material in various fields. The present work aims to investigate the influence of dicarboxylic acids as organic structure interrupting agents on the micropore to mesopore ratio. The physiochemical characterization of the synthesized samples including electron microscopy, nitrogen adsorption–desorption isotherms, MAS 29Si-NMR, FT-IR demonstrate a distinct change in the morphology and micropore area due to the different dicarboxylic acids used during synthesis. Our results show that the decrease in chain length of the dicarboxylic acid has a direct relation to the increase in micropore area which has a significant role in the adsorption properties of mesoporous silica. Thus the dicarboxylic acid mediated tuning of the micropores area of mesoporous silica can be used for various applications.

Phenol hydroxylation over Fe-incorporated mesoporous materials prepared by coprecipitation

Mesoporous iron-incorporated materials were prepared by coprecipitation method and used as catalyst for the hydroxylation of phenol. Several techniques, including ICP-AES, XRD, FT-IR, UV–vis, BET, TPR and XPS, were used to characterize the structural properties of these materials. The results indicated that iron atoms incorporated into the mesoporous silica framework of tetrahedral coordination environment forming a new type of active sites. With the increase of iron content, part of iron atoms deposited on extraframework. Framework iron atoms were found to be beneficial to phenol hydroxylation, while extraframework small oligometric iron species accelerated the deep oxidation of dihydroxybenzenes producing tar. Under the optimized conditions, a phenol conversion of 47.3% with the yield of 45.7% and selectivity of 96.6% to dihydroxybenzenes was obtained. The leaching of extraframework iron atoms and the blocking of active sites by tar formed led to the gradual attenuation of activity. Tar adsorbed on the catalyst could be burnt up by calcinations making the catalyst regenerated. A stable yield of about 30% to dihydroxybenzenes could be obtained.

Grafting of Precoordinated Cu2+–N-(2-Aminoethyl)aminopropylsilane Complexes onto Mesoporous Silicas and the Adsorption of Aqueous Selenate on Them

N-(2-Aminoethyl)aminopropylsilane (AeAptes) was coordinated with Cu(2+) with Cu:AeAptes ratios of 1:1 and 1:2. These were then grafted onto MCM-41 [the products being denoted as Cu(en)-MCM-41 and Cu(en)2-MCM-41, respectively] and SBA-15 [the products being denoted as Cu(en)-SBA-15 and Cu(en)2-SBA-15, respectively]. The periodic structures of these mesoporous silica frameworks were well-retained after grafting. UV-visible and EPR spectroscopies were used to analyze the coordination structure of Cu(2+), revealing the structure expected from the ratio of Cu(2+) to silane. We also prepared Cu(en)- and Cu(en)2-M7D by the same procedure using Cab-O-sil M7D silica. In contrast to the complexes grafted onto the mesoporous silicas, no significant differences were found in the d-d transition band positions, the g-tensor, and hyperfine coupling constants between Cu(en)-M7D and Cu(en)2-M7D. The structural parameters of these were almost the same as those of Cu(en)-MCM-41 and Cu(en)-SBA-15, suggesting that the grafting onto M7D resulted in the transformation of coordination structure of Cu(2+)(AeAptes)2. The adsorption of selenate was analyzed using the Langmuir equation. The adsorption capacities for Cu(en)- and Cu(en)2-mesoporous silicas were Se/Cu = 0.5 and 1.0, respectively, while the Langmuir coefficients for Cu(en)2-mesoporous silicas were more than twice those for Cu(en)-mesoporous silicas. However, no significant differences in these two parameters were found between Cu(en)- and Cu(en)2-M7D; Se/Cu = 0.53-0.56 at saturation and the Langmuir coefficients were the same. The decomposition of the coordinated Cu complex after grafting onto M7D is discussed with respect to the structure of the silica surface. The electronic state of Cu in Cu(en)- and Cu(en)2-mesoporous silicas was analyzed by XANES spectroscopy before and after the adsorption of selenate.

Metathesis of 2-butene to propylene over W-mesoporous molecular sieves: A comparative study between tungsten containing MCM-41 and SBA-15

Mesoporous silica (MCM-41 and SBA-15) having tungsten oxide in the framework was synthesized by hydrothermal crystallization process using CTAB and P123 as structure directing agents, respectively. Tungsten oxide was supported on the MCM-41 and SBA-15 by wet impregnation method for a comparative study. The synthesized materials were characterized by XRD, N2 adsorption–desorption, NH3-TPD, pyridine-FT-IR, UV–vis DRS and Raman spectroscopy techniques. The catalysts were used to perform metathesis reaction of 2-butene in a fixed-bed reactor at different temperatures and under atmospheric pressure. It was observed that catalysts having tungsten oxide in the framework of mesoporous silica exhibited higher activity as compared to tungsten oxide impregnated catalysts. The results revealed that WO3-MCM-41(30) catalyst has much higher activity at a low temperature (450°C) as compared to WO3-SBA-15(30). At 550°C, WO3-MCM-41(30) and WO3-SBA-15(30) showed comparable activity with highest propylene yield of 39mol% and 37mol%, respectively. The higher metathesis activity of WO3-MCM-41 catalysts can be related to the well-dispersed active tetrahedral tungsten oxide species as evidenced from XRD, UV–vis DRS and Raman spectroscopy results.

Near‐Infrared‐Controlled, Targeted Hydrophobic Drug‐Delivery System for Synergistic Cancer Therapy

Hydrophobicity has been an obstacle that hinders the use of many anticancer drugs. A critical challenge for cancer therapy concerns the limited availability of effective biocompatible delivery systems for most hydrophobic therapeutic anticancer drugs. In this study, we have developed a targeted near-infrared (NIR)-regulated hydrophobic drug-delivery platform based on gold nanorods incorporated within a mesoporous silica framework (AuMPs). Upon application of NIR light, the photothermal effect of the gold nanorods leads to a rapid rise in the local temperature, thus resulting in the release of the entrapped drug molecules. By integrating chemotherapy and photothermotherapy into one system, we have studied the therapeutic effects of camptothecin-loaded AuMP-polyethylene glycol-folic acid nanocarrier. Results revealed a synergistic effect in vitro and in vivo, which would make it possible to enhance the therapeutic effect of hydrophobic drugs and decrease drug side effects. Studies have shown the feasibility of using this nanocarrier as a targeted and noninvasive remote-controlled hydrophobic drug-delivery system with high spatial/temperal resolution. Owing to these advantages, we envision that this NIR-controlled, targeted drug-delivery method would promote the development of high-performance hydrophobic anticancer drug-delivery system in future clinical applications.

Mesoporous silica supported cobalt catalysts for hydrogen generation in hydrolysis of ammonia borane

Mesoporous silica supported cobalt boride (Co–B) catalysts are rationally designed for hydrogen generation in ammonia–borane hydrolysis reactions under ambient conditions. Cobalt boride catalysts are supported on three different mesoporous silica, including beta-zeolite seeded MCM-41 (Co@M41S) and traditional MCM-41 (Co@M41T) via chemical adsorption onto functionalized surface with 3-trihydroxysilylpropylmethylphosphonate (THPMP), and one-step co-precipitation into mesoporous silica framework (Co@M41C). Our preparation strategies provide two insights to the reactions: first, cobalt oxide species are intrinsically deposited as ultra-small nanoparticles (<2 nm) on mesoporous silica supports; subsequently the nanoparticles are converted to active Co–B catalysts by reduction with sodium borohydride (SB). Three catalysts exhibit significant differences in catalytic reactivities with hydrogen production rates ranked in an order of Co@M41S > Co@M41T > Co@M41C. Detailed analysis of the coordination environments from in situ X-ray absorption spectroscopy (XAS) results confirm reducibility in SB. Amorphous nature of Co–B catalysts are responsible for efficient catalytic activity in Co@M41S and Co@M41T. Ammonia temperature programmed desorption (NH3-TPD) demonstrates support acidity that correlates to the degree of high dispersity and effective reducibility to Co–B. Effects from catalyst sizes, reducibility in SB treatment and surface acidity are studied in detail to compare catalytic reactivities among three types of supports.

Preparation of aluminum-containing mesoporous silica with hierarchical macroporous architecture and its enhanced catalytic activities

Aluminum-containing mesoporous silica with hierarchical macroporous architecture (Al-MMS) was successfully prepared using a solvent evaporation method through the combination of precursor solution for synthesis of Al-containing mesoporous silica (Al-MS) and poly(methyl methacrylate) (PMMA) colloidal crystals as a hard template. The porous structure and the state of aluminum were investigated using various characterization techniques. The construction of combined structure of Al-MMS, i.e., hierarchical macroporous architecture consisting of thin mesoporous silica frameworks, led to the formation of many mesopore entrances and the shortening of the mesoporous channels. In the tetrahydropyranylation of linear alcohols with dihydropyran (DHP), Al-MMS exhibited higher catalytic activities for the formation of corresponding tetrahydropyranyl ethers as compared to Al-MS. The advantageous structure of Al-MMS enables the efficient transport of reactants to the catalytically active sites, which realizes the significant enhancement of catalytic performances in the reaction of DHP with alcohols having longer alkyl chains.

An extremely stable and highly active periodic mesoporous Lewis acid catalyst in water-medium Mukaiyama-aldol reaction

A periodic mesoporous Lewis acid catalyst ((OTf)2Sc-SO3-Ph-PMO) was synthesized through chelating scandium triflate (Sc(OTf)3) with sodium benzenesulfonate-functionalized periodic mesoporous silica (PhSO3Na-Ph-PMO). Compared with homogeneous catalyst Sc(OTf)3 and mesoporous SBA-15-supported scandium triflate catalyst ((OTf)2Sc-SO3-Ph-SBA-15), it exhibited superior catalytic activity and selectivity in water-medium Mukaiyama-aldol reaction. Hydrophobicity tests and substrate adsorption experiments demonstrated that its unique catalytic performance was related to the combined advantage of mesoporosity and hydrophobic microenvironment, which effectively stabilized and concentrated the substances as well as decreased intrinsic mass transfer resistance. Noted that the periodically arranged Lewis acids in the mesoporous silica framework inhibited the active sites leaching, leading to its high catalytic recyclability with almost unchanged catalytic efficiency for more than 10 times in water media.

Metal-complex ionosilicas: Cationic mesoporus silica with Ni(II) and Cu(II) complexes in their framework

Metal-complex ionosilicas with cationic complexes into the mesoporous silica framework were prepared using anionic surfactants. The electrostatic interaction between the anionic surfactant and the cationic metal complexes incorporated in the silica framework allows for the fine tuning of the mesoporous structure. The gentle procedure of synthesis developed and mild ion-exchange extraction of the surfactant, allowed a cleaner route for the immobilization of homogeneous cationic catalysts in mesoporous silica, while protecting the structural and chemical integrity of the metal complexes.

Multifluorescently Traceable Nanoparticle by a Single-Wavelength Excitation with Color-Related Drug Release Performance

Monodisperse and nanometer-sized periodic mesoporous organosilicas co-doped with fluorescence resonance energy transfer cascades composed of triple fluorophores at various ratios were prepared. These nanoparticles exhibit multifluorescent emissions by a single-wavelength excitation and were designed for the application as multichannelly traceable drug carriers. Different from the hydrophilic framework of inorganic mesoporous silica and hydrophobic framework of mesoporous carbon, these multifluorescent nanoparticles have intrinsically different and finely tunable pore surface polarities governed by the type and amount of fluorophore inside the framework. When applied as drug carriers, they can achieve synchronous or asynchronous release of different drugs by simply choosing different colored nanoparticles. These colorful mesoporous composites with finely tunable color-related drug release performance provide a strong barcoding system for the potential applications of fluorescent nanoparticles in effective screening of drugs and therapeutic protocols for diseases.

In situ synthesis, characterization, and catalytic performance of tungstophosphoric acid encapsulated into the framework of mesoporous silica pillared clay

Mesoporous silica pillared clay (SPC) incorporated with tungstophosphoric acid (HPW) has been synthesized via in situ introducing P and W source in the acidic suspension of the clay interlayer template during the formation of the silica pillared clay. The samples were characterized by XRD, XRF, FT-IR, TG-DTA, N2 adsorption–desorption, and SEM techniques. The results showed that the HPW formed by in situ method has been effectively introduced into the framework of mesoporous silica pillared clay and its Keggin structure remained perfectly after formation of the materials. In addition, samples with similar HPW loadings were also prepared by impregnation method using SPC as the support. HPW in the incorporated samples was better dispersed into the silica pillared clay than in the impregnated samples. The results of catalytic tests indicated that the encapsulated materials demonstrated better catalytic performance than the impregnated samples in oxidative desulfurization (ODS) of dibenzothiophene (DBT).

Highly Oriented Mesoporous Silica Channels Synthesized in Microgrooves and Visualized with Single-Molecule Diffusion

A novel synthesis method for large-pore, well-aligned 2D hexagonal mesoporous silica thin films is reported. The alignment was achieved by confinement in poly(dimethylsiloxane) (PDMS) microgrooves without the necessity of additional forces (such as electric fields). We describe the influence of various experimental conditions including the way the grooves are filled, surface modification at the solid/liquid interfaces, and the height-to-width ratio of the microgrooves on mesopore alignment. With this technique, highly oriented mesoporous silica channels can be obtained at a length scale of several millimeters. For a nondestructive, detailed, and wide-ranging structural and dynamic characterization of the as-synthesized mesochannel silica network, dye molecules were incorporated into the channels at concentrations suitable for single-molecule microscopy. A "maximum projection" of individual frames recorded with a fluorescence microscope immediately gives a global overview ("map") of the pore structure, thus providing direct feedback for tuning synthesis conditions. In addition, deeper insights into the real nanoscale structure of the mesoporous silica framework were obtained through high-accuracy single-molecule tracking experiments. The high spatial accuracy of the experiments allowed for the direct observation of jumps of single dye molecules between individual channels in the mesoporous silica host. Nevertheless, due to the low concentration of defects, the diffusion could be described as a 1D random walk where the molecules diffuse along the highly oriented, parallel channels and sometimes switch from channel to channel through small defects in the pore walls. Furthermore, it could be shown with single-molecule microscopy that template removal and calcination of the aligned films results in an increased defect concentration; however, the overall order of the structures remained intact.

Simultaneous pore enlargement and introduction of highly dispersed Fe active sites in MSNs for enhanced catalytic activity

An effective post-hydrothermal treatment strategy has been developed to dope highly dispersed iron catalytical centers into the framework of mesoporous silica, to keep the particle size in nanometric scale, and in the meanwhile, to expand the pore size of the synthesized mesoporous silica nanoparticles (MSNs). Characterization techniques such as XRD, BET, SEM and TEM support that the synthesized samples are long period ordered with particles size about 100nm and a relatively large pore size of ca. 3.5nm. UV–vis, XPS and EPR measurements demonstrate that the introduced iron active centers are highly dispersed in a coordinatively unsaturated status. NH3-TPD verifies that the acid amount of iron-doped MSNs is quite high. The synthesized nanocatalysts show an excellent catalytic performance for benzylation of benzene by benzyl chloride, and they present relatively higher yield and selectivity to diphenylmethane with a lower iron content and much shorter reaction time.