Phenylene-bridged Periodic Mesoporous Organosilica Research Articles

Exploring periodic mesoporous organosilicas for ethane–ethylene adsorption–separation

Ethylene is a core building block in the chemical industry and its separation from ethane is very challenging due to high energy requirements. Adsorption-based processes can be an alternative to current processes based on cryogenic distillation. In this work, we explore for the first time the application of periodic mesoporous organosilica (PMO) materials, whose surface properties can be tuned with different functional groups. With the aim of correlating the PMO structure with ethane and ethylene adsorption, eight different PMO materials have been prepared, viz. the phenylene-bridged PMO, PMO aminated at the organic bridges, and PMO functionalized by silylation of free silanol (inorganic) moieties. High pressure adsorption isotherms were measured, and the separation selectivity and phase diagrams of a binary mixture of ethane and ethylene were estimated. Results have shown that, overall, the PMO studied tend to be more selective towards ethylene than ethane probably due to the interactions between the quadrupole moment of ethylene and the free silanols in the samples. After silylation, the novel materials presented surfaces with higher affinity towards ethane than those of the pristine material. For the aminated samples, functionalization with primary amines originated materials displaying better selectivity towards ethylene than those functionalized with secondary or tertiary amines.

Highly Active Ruthenium Catalyst Supported on Magnetically Separable Mesoporous Organosilica Nanoparticles

A facile and direct method for synthesizing magnetic periodic mesoporous organosilica nanoparticles from pure organosilane precursors is described. Magnetic ethylene- and phenylene-bridged periodic mesoporous organosilica nanoparticles (PMO NPs) were prepared by nanoemulsification techniques. For fabricating magnetic ethylene- or phenylene-bridged PMO NPs, hydrophobic magnetic nanoparticles in an oil-in-water (o/w) emulsion were prepared, followed by a sol–gel condensation of the incorporated bridged organosilane precursor (1,2 bis(triethoxysilyl)ethane or 1,4 bis(triethoxysilyl)benzene), respectively. The resulting materials were characterized using high-resolution scanning electron microscopy (HR-SEM), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray (EDX) spectroscopy, powder X-ray diffraction (XRD), solid-state NMR analysis, and nitrogen sorption analysis (N2-BET). The magnetic ethylene-bridged PMO NPs were successfully loaded using a ruthenium oxide catalyst by means of sonication and evaporation under mild conditions. The obtained catalytic system, termed Ru@M-Ethylene-PMO NPS, was applied in a reduction reaction of aromatic compounds. It exhibited very high catalytic behavior with easy separation from the reaction medium by applying an external magnetic field.

Stabilization of nanosized MgFe2O4 nanoparticles in phenylene-bridged KIT-6-type ordered mesoporous organosilica (PMO)

Abstract The unique combination of two functional materials, namely the earth-abundant spinel magnesioferrite (MgFe2O4) and mesoporous phenylene-bridged KIT-6-type organosilica, was developed. The mesoporous organosilica acts as host matrix for the nanosized MgFe2O4 particles which leads to better dispersibility and increased stability towards acids. Additionally, the mesoporous host is a very good material to generate a toolbox towards applicable materials due to flexible functionalization. Nanosized, monodisperse MgFe2O4 crystallites were synthesized via a facile microwave assisted non-aqueous reaction path. Afterwards, the particles were embedded in phenylene-bridged periodic mesoporous organosilica with 3D cubic pore arrangement (KIT-6-type PMO) generating a new kind of mesoporous inorganic-organic hybrid material (MgFe2O4@phe-PMO). The MgFe2O4@phe-PMO exhibits the characteristics of both components: A high specific surface area of 1164 m2g-1 with clearly defined and highly ordered micro- and mesopores (1.5 and 6.8 nm), and the broad absorption of visible and UV light due to the phenylene bridging units in the PMO and the MgFe2O4 particles. The presence of MgFe2O4 nanoparticles in the PMO matrix is proven by UV/Vis spectroscopy, powder X-ray diffraction (PXRD) and transmission electron microscopy (TEM). Selected area electron diffraction (SAED) and scanning TEM in atomic resolution was chosen to demonstrate the crystallinity and phase purity of MgFe2O4 particles in the hybrid material. An additional focus was laid on calcination of the MgFe2O4/PMO hybrids to remove template molecules, while preventing rearrangement or shrinkage of the pore system and to promote further crystallization of the MgFe2O4 nanoparticles.

Synthesis of lamellar mesostructured phenylene-bridged periodic mesoporous organosilicas (PMO) templated by polyion complex (PIC) micelles

Periodic mesoporous organosilicas (PMOs), obtained by the surfactant-mediated hydrolysis-condensation of bridged organosilanes, combine versatile organic functionalities with advantages of a stable inorganic framework. Here, we introduce a novel synthesis of lamellar mesostructured phenylene-bridged PMOs templated by polyion complex (PIC) micelles (PICPMOs). The micelles assemble by electrostatic interactions between oppositely charged polyelectrolytes, with one being part of a double-hydrophilic block copolymer (DHBC), and the other being a polybase oligochitosan (OC). The PICPMO material was characterized by a range of techniques, including TEM, IR spectroscopy, SAXS, TGA and elemental analysis, which indicates that the material exhibits long-range ordering with an inter-lamellae distance of around 15 nm. Advantages of the synthetic approach developed, together with potential applications of the PICPOs, are discussed.

Fine-tuning the release of molecular guests from mesoporous silicas by controlling the orientation and mobility of surface phenyl substituents

Phenyl-functionalized mesoporous silica materials were used to explore the effect of non-covalent interactions on the release of Ibuprofen into simulated body fluid. Variations in orientation and conformational mobility of the surface phenyl groups were introduced by selecting different structural precursors: 1) a rigid upright orientation was obtained using phenyl groups directly bound to surface Si atoms (Ph-MSN), 2) mobile groups were produced by using ethylene linkers to connect phenyl groups to the surface (PhEt-MSN), and 3) groups co-planar to the surface were obtained by synthesizing a phenylene-bridged periodic mesoporous organosilica (Ph-PMO). The Ibuprofen release profiles from these materials and non-functionalized mesoporous silica nanoparticles (MSN) were analyzed using an adsorption-diffusion model. The model provided kinetic and thermodynamic parameters that evidenced fundamental differences in drug-surface interactions between the materials. All phenyl-bearing materials show lower Ibuprofen initial release rates than bare MSN. The conformationally locked Ph-MSN and Ph-PMO have stronger interactions with the drug (negative ΔG of adsorption) than the flexible PhEt-MSN and bare MSN (positive ΔG of adsorption). These differences in strength of adsorption are consistent with differences between interaction geometries obtained from DFT calculations. B3LYP-D3-optimized models show that π-π interactions contribute more to drug adsorption than H-bonding with silanol groups. The results suggest that the type and geometry of interactions control the kinetics and extent of drug release, and should therefore serve as a guide to design new drug delivery systems with precise release behaviors customized to any desired target.

Excited-State Dynamics of Phenylene Moieties in a Framework of the Organosilica Nanotube

To understand the excited-state dynamics of Ph moieties in a framework of the phenylene–silica nanotube (Ph-NT), temperature and time dependence of the fluorescence were investigated. The time dependence of the Ph exciton fluorescence and excimer fluorescence was successfully analyzed by the model including the diffusion of the Ph excitons and their subsequent trapping at the defect center and the excimer sites. The energy migration of the Ph exciton in the Ph-NT is suggested similarly to phenylene-bridged periodic mesoporous organosilica (Ph-PMO) powder with crystal-like wall structure, although X-ray diffraction of Ph-NT indicates less molecular-level periodicity of Ph moieties in the framework of the nanotube. The excited-state dynamics of Ph moieties in Ph-NT will be discussed by comparing that of Ph-PMO.

Simple, mono and bifunctional periodic mesoporous organosilicas for removal of priority hazardous substances from water: The case of mercury(II)

Pristine, amino monofunctionalized and amino and thiol bifunctionalized phenylene-bridged periodic mesoporous organosilicas (Ph-PMO) were synthetized by a simple methodology without any protection/deprotection steps. The Ph-PMO materials were characterized and their potential as new sorbents for Hg(II) was investigated for low-contamination scenarios. The materials have well-ordered mesostructures as confirmed by powder X-ray diffraction and transmission electron microscopy analyses. 13C and 29Si solid-state NMR confirm the amination of the phenylene bridge of the simple Ph-PMO to form the amine-functionalized PMO (NH2-Ph-PMO) and the grafting of the thiol silylated ligand to the free silanols of the NH2-Ph-PMO to obtain the bifunctional (NH2-Ph-PMO-SH). For the initial conditions tested, the bifunctional material showed excellent Hg(II) removal efficiency (98%) and Hg(II) uptake time (30min). The sorption kinetics were also investigated by three of the most studied kinetic models, and the pseudo-second-order kinetic model was the one that better described the kinetic sorption process of Hg(II) ions onto all PMO.

Cellular Internalization and Biocompatibility of Periodic Mesoporous Organosilica Nanoparticles with Tunable Morphologies: From Nanospheres to Nanowires.

This work describes the sol-gel syntheses of para-substituted phenylene-bridged periodic mesoporous organosilica (PMO) nanoparticles (NPs) with tunable morphologies ranging from nanowires to nanospheres. The findings show the key role of the addition of organic co-solvents in the aqueous templates on the final morphologies of PMO NPs. Other factors such as the temperature, the stirring speed, and the amount of organic solvents also influence the shape of PMO NPs. The tuning of the shape of the PMO nanomaterials made it possible to study the influence of the particle morphology on the cellular internalization and biocompatibility.

Evaluation of phenylene-bridged periodic mesoporous organosilica as a stationary phase for solid phase extraction

A periodic mesoporous organosilica, in particular, a phenylene-bridged material (Ph-PMO), was evaluated for the first time as a sorbent for retaining and eluting fenuron, simazine, atrazine, carbaryl and terbutryn in grape must by solid phase extraction (SPE) prior to their determination with capillary electrophoresis coupled with ultraviolet detection (CE-UV). The analytes were used as model compounds to demonstrate the potential of Ph-PMO for increasing the sensitivity of CE. Under optimal conditions, the limits of detection for the analytes ranged from 0.6 to 4μg/L, and their limits of quantitation from 2 to 10μg/L. These values were comparable and, in some cases, even better than those obtained with C18 and HLB materials. Ph-PMO was characterized physicochemically by X-ray diffraction analysis, N2 adsorption–desorption measurements and laser diffraction particle sizing. The sorbent afforded the extraction of atrazine, carbaryl and terbutryn from grape must with mean recoveries ranging from 86 to 105%. Therefore, periodic mesoporous organosilicas possess a high potential as SPE materials.

Preparation of bifunctionalized phenylene-bridged periodic mesoporous organosilica for solid-phase microextraction

Well-ordered bifunctionalized periodic mesoporous organosilicas (BPMOs), with various mesostructures (like hexagonal p6mm, cubic Iad symmetry), relatively large mesopores, a phenylene-bridged skeleton and vinyl functional groups, were first prepared by co-condensation of 1,4-bis(triethoxysilyl)benzene (BTEB) and triethoxyvinylsilane (TEVS) with a triblock copolymer (P123) as a template under acid conditions. It was found that the mesostructure of the BPMOs can be facilely tuned by altering the fraction of organosilanes in the synthesis mixture. With an increase of TEVS content, the mesostructure changed from p6mm to Iad, until disordered. The mechanisms of the mesophase transformation were discussed based on the adsorption of vinyl groups into the micelles and the relative reactivities of the organosilane precursors. In addition, the potential application of the BPMOs as an effective fibre-coating absorbent is tested in solid-phase microextraction (SPME) analyses.

Face-Centered-Cubic Large-Pore Periodic Mesoporous Organosilicas with Unsaturated and Aromatic Bridging Groups

Large-pore ethenylene-bridged (-CH═CH-) and phenylene-bridged (-C(6)H(4)-) periodic mesoporous organosilicas (PMOs) with face-centered-cubic structure (Fm3m symmetry) of spherical mesopores were synthesized at 7 °C at low acid concentration (0.1 M HCl) using Pluronic F127 triblock copolymer surfactant in the presence of aromatic swelling agents (1,3,5-trimethylbenzene, xylenes-isomer mixture, and toluene). In particular, this work reports an unprecedented block-copolymer-templated well-ordered ethenylene-bridged PMO with cubic structure of spherical mesopores and an unprecedented block-copolymer-templated face-centered cubic phenylene-bridged PMO, which also has an exceptionally large unit-cell size and pore diameter. The unit-cell parameters of 30 and 25 nm and the mesopore diameters of 14 and 11 nm (nominal BJH-KJS pore diameters of 12-13 and 9 nm) were obtained for ethenylene-bridged and phenylene-bridged PMOs, respectively. Under the considered reaction conditions, the unit-cell parameters and pore diameters were found to be similar when the three different methyl-substituted benzene swelling agents were employed, although the degree of structural ordering appeared to improve for phenylene-bridged PMOs in the sequence of decreased number of methyl groups on the benzene ring.

Non-ionic block copolymers leading to a highly ordered organosilica material

Phenylene-bridged periodic mesoporous organosilica (PMO) materials with hexaethylene glycol mono hexadecylether (C16(EO)6) as structure directing agent (SDA) in acidic solution were synthesised and the reaction was studied in different conditions, including in situ small angle X-ray scattering at room temperature and at 60 °C. The in situ SAXS investigations at 60 °C show the formation of a 2D hexagonal mesostructure after 3 hours reaction time. In contrast to these results the same reaction performed in the laboratory produced a powder that, after drying at room temperature, shows a large number of diffraction peaks, allowing identifying a lamellar structure, possibly in coexistence of a micellar cubic structure (space group Pm3n). Aposteriori removal of the surfactant from the powder synthesised in the laboratory led to the collapse of the lamellar structure.

The effect of solvent in evaporation-inducedself-assembly: A case study of benzeneperiodic mesoporous organosilica

Volatile organic solvents were considered to have little influence on the synthesis of mesostructured materials through evaporation-induced self-assembly (EISA), because upon evaporation they leave the sol and hence do not interfere with the self-assembly process. We show here that the choice of solvent is crucial in the synthesis of thin films of phenylene-bridged periodic mesoporous organosilica (benzene PMO). Methanol is found to be a better solvent for the synthesis of thin films, whereas ethanol favors the formation of (HO) 3 Si-C 6 H 4 -Si(OH) 3 crystals, the identity and structure of which is established by X-ray diffraction. A ternary reactant composition diagram is designed to visualize the relationship among multiple synthesis experiments and to guide the interpretation of experimental results and optimization of the quality of the periodic mesoporous organosilica film. Our study highlights the importance of solvent choice, a factor often neglected in EISA. We expect it to inspire researchers to explore the effect of solvent in designing the synthesis of mesoporous materials.

Surfactant-Templated Synthesis of Ordered Silicas with Closed Cylindrical Mesopores

Ordered mesoporous silicas with 2-dimensional hexagonal arrays of closed cylindrical pores were synthesized via templating with block copolymer surfactant followed by calcination at appropriately high temperatures. Precursors to closed-pore silicas, including SBA-15 silicas and organosilicas, were selected based on the existence of narrow passages to the mesopores. The increase in calcination temperature to 800–950 °C led to a dramatic decrease in nitrogen uptake by the materials, indicating the loss of accessible mesopores, whereas small-angle X-ray scattering (SAXS) indicated no major structural changes other than the framework shrinkage. Since SAXS patterns for ordered mesoporous materials are related to periodic arrays of mesopores, the existence of closed mesopores was evident, as additionally confirmed by TEM. The formation of closed-pore silicas was demonstrated for ultralarge-pore SBA-15 and large-pore phenylene-bridged periodic mesoporous organosilicas. The increase in the amount of tetraethyl orthosilicate in standard SBA-15 synthesis also allowed us to observe the thermally induced pore closing. It is hypothesized that the presence of porous plugs in the cylindrical mesopores and/or caps at their ends was responsible for the propensity to the pore closing at sufficiently high temperatures. The observed behavior is likely to be relevant to a variety of silicas and organosilicas with cylindrical mesopores.

The effect of solvent in evaporation-induced self-assembly: A case study of benzene periodic mesoporous organosilica

Volatile organic solvents were considered to have little influence on the synthesis of mesostructured materials through evaporation-induced self-assembly (EISA), because upon evaporation they leave the sol and hence do not interfere with the self-assembly process. We show here that the choice of solvent is crucial in the synthesis of thin films of phenylene-bridged periodic mesoporous organosilica (benzene PMO). Methanol is found to be a better solvent for the synthesis of thin films, whereas ethanol favors the formation of (HO)3Si-C6H4-Si(OH)3 crystals, the identity and structure of which is established by X-ray diffraction. A ternary reactant composition diagram is designed to visualize the relationship among multiple synthesis experiments and to guide the interpretation of experimental results and optimization of the quality of the periodic mesoporous organosilica film. Our study highlights the importance of solvent choice, a factor often neglected in EISA. We expect it to inspire researchers to explore the effect of solvent in designing the synthesis of mesoporous materials.

A Mesoporous Organosilica-Supported Organocatalyst

A chiral imidazolidinone catalyst (MacMillan catalyst) was supported on hollow-structured phenylene-bridged periodic mesoporous organosilica (H-PhPMO) spheres. Thus, the supported catalyst (H-PhPMO-Mac) was prepared from 1,4-bis(triethoxysilyl)benzene and trimethoxysilylpropyl azide with hematite (α-Fe2O3) nanoparticles as hard templates through condensation, etching (removal of α-Fe2O3), and Huisgen 1,3-dipolar cycloaddition. The Diels-Alder cyclo­addition of trans-cinnamaldehyde (0.25 mmol) and 1,3-cyclopentadiene (1.25 mmol) was carried out with H-PhPMO-Mac (0.05 mmol) to give 3-phenylbicyclo-[2,2,1]hept-5-ene-2-carbaldehyde in 98% yield [endo/exo = 1:1.1; 81% ee (endo) and 81% ee (exo)].

Heterogeneous Organocatalysis at Work: Functionalization of Hollow Periodic Mesoporous Organosilica Spheres with MacMillan Catalyst

We report a new method for the synthesis of hollow-structured phenylene-bridged periodic mesoporous organosilica (PMO) spheres with a uniform particle size of 100-200 nm using α-Fe(2)O(3) as a hard template. Based on this method, the hollow-structured phenylene PMO could be easily functionalized with MacMillan catalyst (H-PhPMO-Mac) by a co-condensation process and a "click chemistry" post-modification. The synthesized H-PhPMO-Mac catalyst has been found to exhibit high catalytic activity (98% yield, 81% enantiomeric excess (ee) for endo and 81% ee for exo) in asymmetric Diels-Alder reactions with water as solvent. The catalyst could be reused for at least seven runs without a significant loss of catalytic activity. Our results have also indicated that hollow-structured PMO spheres exhibit higher catalytic efficiency than solid (non-hollow) PMO spheres, and that catalysts prepared by the co-condensation process and "click chemistry" post-modification exhibit higher catalytic efficiency than those prepared by a grafting method.

Thermal behaviour, sulfonation and catalytic activity of phenylene-bridged periodic mesoporous organosilicas

The structural and surface changes in a phenylene-bridged periodic mesoporous organosilica after thermal treatment under different conditions are reported. Organic moieties in the pore walls are stable at temperatures close to 500 °C but calcination in the presence of oxygen leads to C–Si bond cleavage as well as the formation of oxidized groups of phenolic and carbonyl type. The oxidized materials are functionalized by reaction with chlorosulfonic acid in a higher extension than those calcined under nitrogen but all of them preserve their structure and surface properties. All sulfonated organosilicas were active as catalysts for the esterification of acetic acid with ethanol, particularly those calcined in the air. Remarkably, some of them can be even more active than Amberlyst-15 when water is used as reaction medium which is of a high interest for the design of sustainable acid-catalyzed chemical processes.

Thermodynamic properties of novel phenylene-bridged periodic mesoporous organosilica as high performance liquid chromatographic stationary phase

The thermodynamic properties of spherical phenylene-bridged periodic mesoporous organosilica (PMOs) as high performance liquid chromatographic stationary phase were investigated and compared with commercial grafted C18 and phenyl stationary phases. The retention behavior of several compounds of polycyclic aromatic hydrocarbons (PAHs) was investigated. The thermodynamic parameters of transfer from mobile phase to stationary phase were discussed. The results showed that the enthalpy-entropy compensation effect does not exist for homemade phenylene-bridged PMOs. The retention mechanism of solutes on the prepared stationary phase involves synergistic interaction of hydrophobic, pi-pi and inclusion interactions, which is different with that on the commercial stationary phases.

Formation Mechanism Studies of Phenylene-Bridged Periodic Mesoporous Organosilicas (PMOs)

The formation of phenylene-bridged periodic mesoporous organosilicas (PMOs) in the presence of three diblock copolymers differing in hydrophobic hydrocarbon chain lengths was investigated. Hexaethylene glycol octadecylether (C(18)(EO)(6)), hexaethylene glycol hexadecylether (C(16)(EO)(6)), and hexaethylene glycol dodecylether (C(12)(EO)(6)) were chosen in order to obtain insight on the influence of the hydrocarbon chain length on the mesostructure. 1,4-Bis(triethoxysilyl)benzene (BTEB) was used as organosilica precursor under mild acidic conditions. The reactions were followed by in situ small-angle X-ray diffraction (SAXD) on-time in a capillary flow setup. It was found that during the reaction the formation of different structures was observed, which is ascribed to the hydrolysis and condensation of the organosilica precursor. In all cases, different structures evolve with time and phase transitions are observed during the measurements independent of the hydrocarbon chain length.