Periodic Mesoporous Organosilica Materials Research Articles

Periodic mesoporous organosilica materials as sorbents for solid-phase extraction of drugs prior to simultaneous enantiomeric separation by capillary electrophoresis

Two novel periodic mesoporous organosilica materials were synthesized with a neutral phenylene-bridged ligand, 1,4-bis(trimethoxysilylethyl)benzene, one of them using tetraethyl orthosilicate as additional silica source (PMO-TMSEB-1 and PMO-TMSEB-2). A third material was also synthesized with 1,4-bis(triethoxysilyl)benzene ligand (PMO-TESB-1) which use has scarcely been reported. The three materials were evaluated as solid-phase extraction (SPE) sorbents for the off-line extraction of a mixture of seven drugs of different nature (duloxetine, terbutaline, econazole, propranolol, verapamil, metoprolol, and betaxolol) from water samples. Subsequent simultaneous enantiomeric analysis by CE, using sulfated-β-cyclodextrin (2% w/v) dissolved in a 25 mM phosphate buffer (pH 3.0) and a voltage of −20 kV (negative polarity) was carried out. Enantiomeric resolutions ranging from 2.4 to 8.5 were obtained in an analysis time of 16 min. After optimization of SPE parameters, it was shown that using just 100 mg of PMO-TESB-1 as sorbent, a preconcentration factor of 400 with 200 mL solution was achieved, allowing recoveries between 80.5 and 103.1% (except for terbutaline), with good repeatability (% RSD = 2–8 %, n = 5). Analytical characteristics of the method were evaluated in terms of precision, linearity and accuracy with method quantitation limits between 5.6 and 21.9 μg/L. The developed method was applied to the analysis of spiked wastewater samples collected in different treatment plants, with recoveries between 73.9 and 102.9% except for econazole with recovery values ranging between 58.5 and 72.4%.

Cationic amine-bridged periodic mesoporous organosilica materials for off-line solid-phase extraction of phenoxy acid herbicides from water samples prior to their simultaneous enantiomeric determination by capillary electrophoresis

Two novel materials based on periodic mesoporous organosilica (PMO) with cationic amine-bridged ligands, (styrylmethyl)bis(triethoxysilylpropyl)ammonium chloride (PMO-STPA) and bis(3-triethoxysilylpropyl)amine (PMO-TEPA), were synthesized in this work to obtain materials with reverse-phase/strong anionic exchange mixed-mode or strong anionic exchange retention mechanism, respectively. The resulting materials were comprehensively characterized and showed functionalization with cationic amine-bridged ligands, and values of surface areas characteristic of mesoporous materials (higher than 100m2/g). These materials were evaluated for the off-line solid-phase extraction (SPE) of a mixture of six phenoxy acid herbicides (fenoprop, mecoprop, dichlorprop, 2-(4-chlorophenoxy)propionic acid (4-CPPA), 2-(3-chlorophenoxy)propionic acid (3-CPPA), 2-phenoxypropionic acid (2-PPA)) from water samples previous to their analysis by CE with diode-array detection using a dual chiral selector system (20mM of heptakis(2,3,6-tri-O-methyl)-β-CD (TM-β-CD) and 7mM of (2-hydroxypropyl)-β-CD (HP-β-CD) dissolved in 50mM phosphate buffer, pH 7.0) which enabled the simultaneous enantiomeric separation of the six phenoxy acid herbicides in 11min. SPE parameters were optimized and recoveries obtained for PMO-STPA and PMO-TEPA sorbents were compared. Under the optimized conditions, it was demonstrated that using 100mg of PMO-STPA sorbent, a maximum preconcentration factor (PF) of 1500 was achieved with 750mL of standard solution, allowing recoveries between 75.5 and 112.2%, with good repeatability (RSD=1.9–8.7%, n=6). Analytical characteristics of the method were evaluated in terms of precision, linearity and accuracy with method quantitation limits (MQL) between 0.4 and 14.3μg/L. The developed method was applied to the analysis of river samples and effluents from wastewater treatment plants, with recoveries ranging from 78.3 to 107.5%.

Water Transport in Periodic Mesoporous Organosilica Materials

Water transport in periodic mesoporous organosilicas (PMOs) was studied using pulsed field gradient NMR. A series of isogeometric PMO materials with different chemical compositions of the pore walls were investigated and compared to a purely siliceous MCM-41 material with an identical pore size. The long-range water diffusivities measured were found to be largely controlled by the macroscopic textural properties of the materials, namely, by the particle geometry and a degree of the particle agglomeration, and by thermodynamic conditions under which the experiments were performed. It is shown that their combined effect caused water molecules either to propagate predominantly along the capillary-condensed water domains or to frequently alternate their trajectories between these domains and the water phase in the interparticle space. Because the transport rates in these two regimes differ substantially, it is suggested that by a purposeful choice of the PMO composition, both the long-range transport rate and...

Sulfonated periodic-mesoporous-organosilicas column for selective separation of C2H2/CH4 mixtures

Light hydrocarbon mixtures separation is a very important industrial process. Here, two periodic mesoporous organosilicas (PMOs) functionalized with sulfonic acid groups have been successfully synthesized via post-synthetic modification of phenylene- or biphenylene-bridged PMO material, which are used in the separation of C2H2/CH4 mixtures. The biphenylene-bridged PMO material with pendent sulfonate groups (s-PMO-2) shows up highly selective adsorption toward C2H2/CH4 mixture upon the cooperative effect of pore space and sulfonate groups. The efficiency for the separation of C2H2/CH4 mixtures is further demonstrated by column experimental breakthrough.

Synthesis of novel periodic mesoporous organosilicas with large content of lysine‐bridged organosilane skeleton

Novel periodic mesoporous organosilicas (PMOs) as functional nanomaterials based on the skeleton of lysine were synthesised by the co-condensation of lysine-bridged organosilicane (Lys-BOS) and tetraethyl orthosilicate in acidic medium using P123 as template. Furthermore, a large content of Lys-BOS was incorporated into the silica framework. Amino acid organosilicane (Lys-BOS) was first prepared by the multi-step reaction between traditional orgnaosilicane [(3-aminopropyl) triethoxysilane] and carboxyl protected lysine. The small-angle X-ray diffraction and N2 adsorption–desorption isotherms indicate that these PMO materials still retain ordered mesostructure in the high molar concentration of Lys-BOS. Fourier-transform infrared spectroscopy confirms that lysine is incorporated in the framework of the PMO materials.

Periodic mesoporous organosilicas as porous matrix for heterogeneous lyophobic systems

Periodic mesoporous organosilicas (PMO) have been studied for the first time as a porous matrix for heterogeneous lyophobic systems (HLS) for absorption and storage of mechanical energy by high pressure intrusion-extrusion of electrolyte solutions. It has been shown that the intrusion of LiCl aqueous solutions in ethane-bridged PMO material is irreversible that corresponds to a bumper behavior. The intrusion pressure increases strongly with the salt concentration - from 13 MPa for 5 M LiCl aqueous solution to 37 MPa for 20 M one. Such a pressure rise of 2.8 times is the highest observed for HLS based on mesoporous materials. Due to high intruded volume (0.63–0.72 mL/g) specific absorbed energy achieves 27 J/g, which is close to the best values ever obtained for HLS based on zeosils and mesoporous silica. The characterization shows that after the intrusion-extrusion experiments the ordered pore arrangement largely stays intact, but a slight increase of the mesopore volume is observed.

Template-dependent hydrophobicity in mesoporous organosilica films

A template dependence of the degree of self-hydrophobization of methylene-bridged periodic mesoporous organosilica (PMO) films is reported. The film with the smallest pore size of 1.7 nm, templated by CTAC, results in higher hydrophilicity when compared to films with a pore size of 4.1 or 5.3 nm, templated by BrijL4 and BrijS10, respectively. Both the surface and the bulk hydrophilicity were evaluated by water contact angle measurements and water ellipsometric porosimetry and the same trends were observed. Additionally, we provide the first evidence for a steric hindrance of the self-hydrophobization process. We show that a partial template removal results in the methylene-to-methyl transformation being observed at a temperature as low as 200 °C, significantly lower than previously demonstrated. These results should be taken into account when PMO materials are considered for applications such as low-k dielectrics, membranes, catalyst and chromatographic supports, and drug carriers.

Periodic mesoporous organosilica with low thiol density – a safer material to trap Hg(II) from water

This research describes the synthesis and characterization of a periodic mesoporous organosilica (PMO) with low thiol density, and its efficiency to trap Hg(II) from waters under realistic environmental conditions. Despite the low density of SH groups grafted to the silanol groups of the pristine PMO, the functionalized material displays effective (92%) and rapid (5min) extraction of Hg(II) from ultra-pure water. The complete extraction (98%) was achieved in 15min. The kinetic sorption is well fitted by the pseudo second-order kinetic model (R2 ranged from 0.994 to 1.000), suggesting a chemisorption mechanism, under the experimental conditions tested. The maximum sorption capacity is not limited to the number of the sulfur coordination sites, and multi-layer and physical adsorption may occur for high Hg(II) concentrations. In the range of experimental conditions used, the mathematical function that better describes the equilibrium data is given by the Freundlich model (R2=0.969). It is also demonstrated that the PMO with low sulfur density keeps its high efficiency for Hg(II) in spiked natural waters, of different chemical composition and salinity, as a proof-of-concept for its potential application in water treatment systems to remove Hg(II). In comparison with other PMO materials reported in literature, the low sulfur density PMO is a cheap, effective and safe alternative PMO material for application to low Hg(II) contaminated waters, minimizing the hazardous effects that the organic functionalization may cause to the environment and biota, and consequently, to human health.

Synthesis of L-serine modified benzene bridged periodic mesoporous organosilica and its catalytic performance towards aldol condensations

Periodic mesoporous organosilica materials (PMOs) offer unique possibilities towards catalysis due to their high stability and tunable organic bridges. Moreover, the presence of the organic bridges make them, in comparison with the traditional silica materials, much more robust in an aqueous environment. As a result, these materials can be applied as catalysts for the valorization of biomass-based resources which are, typically, performed in the presence of water. In this work, we post-synthetically modified benzene bridged PMO with L-serine, thus obtaining a bifunctional PMO bearing a primary amine with an alcohol group on the β-carbon. The promoting effect of the alcohol, in combination with the amine as a base catalyst, is beneficial in the aldol condensation of acetone and 4-nitrobenzaldehyde. Thorough characterization, including a solid state NMR study, of the L-serine modified PMO was performed as well as a successful catalytic test in an aqueous environment. We obtained a TOF value of 1.61 × 10−4 ± 0.08 × 10−4 s−1 and a high selectivity towards the aldol product.

Novel family of periodic mesoporous organosilicas containing azobenzene within the pore walls

Three new bis-silylated azobenzenes, differently substituted, having terminal triethoxysilyl groups attached directly to the para position of the phenyl rings without linker, namely bis(4-triethoxysilyl)azobenzene, bis(2,6-dimethyl-4-triethoxysilyl)azobenzene and bis(2,6-diisopropyl-4-triethoxysilyl)azobenzene, have been successfully synthesized by Lithium-Iodine exchange from its diiodoazobenzene-derivatives, and used to prepare three novel azobenzene-bridged periodic mesoporous organosilica materials (Azb-PMO-R, R= H, Me and iPr). The synthesis of these materials from 100% of bis-silylated azobenzenes precursors was carried out under basic conditions using octadecyltrimethylammonium chloride (C18TMACl) as structure-directing agent. The successful synthesis was verified by N2 sorption measurements, solid state MAS-NMR spectroscopy, and IR spectroscopy. Materials with pore sizes between 3.7 and 4.6 nm and specific surface areas in the range of 400–700 m2/g were obtained. UV–Visible spectroscopy shows that the prepared PMOs are formed by rigid combination of isomers cis and trans of azobenzene moieties, which do not exhibit a regular photochemical trans-to-cis isomerization of the azo group upon alternating UV–Visible irradiation. However, these materials probably exhibit a dynamic and local bending which could be useful for reversible gas adsorption and release with light.

Carbonization of periodic mesoporous phenylene- and biphenylene-silicas for CO2/CH4 separation

Periodic mesoporous organosilicas (PMO), with phenylene or biphenylene organic linkers, were thermally treated in flowing nitrogen atmosphere upon different conditions aiming the enhancement of their CO2 adsorption/separation properties. As-synthesized and template-extracted phenylene- and biphenylene-PMO were pyrolysed at 800 and 1200 °C. The effects of: i) the type of organic bridge; ii) the presence of nitrogen atoms; iii) the use of an acid catalyst prior to carbonization; and iv) pore size were investigated. It was found that pyrolysis promotes modifications in the physical-chemical and the textural properties of the PMO materials, being the formation of micropores one of the most notable differences. Furthermore, with the exception of biphenylene-PMO, the molecular-scale periodicity of the materials was strongly affected by the pyrolysis treatment probably as a result of SiC bond cleavage. The CO2 adsorption capacity and the selectivity for CO2/CH4 separation of all pyrolysed materials were enhanced. In general, the increase of the microporosity in the pyrolysed PMO is accompanied by an improvement of the CO2 adsorption properties with concomitant reduction of the CH4 adsorption behavior. The most interesting material for CO2/CH4 separation is the biphenylene-PMO pyrolysed at 1200 °C, with a selectivity of 9.5 at 25 °C and 500 kPa.

Architecture of novel periodic mesoporous organosilicas based on the flexible skeleton of aspartic acid-bridged organosilane

Novel periodic mesoporous organosilicas materials (PMOs) based on the flexible skeleton of aspartic acid were synthesized by the co-condensation of aspartic acid-bridged organosilane (Asp-BOSP) and tetraethyl orthosilicate (TEOS) in an acidic medium, using the Pluronic P123 surfactant as a template. Furthermore, a new amino acid organosilane was developed by the simple reaction between traditional organosilicone ((3-aminopropyl) trimethoxysilane) and aspartic acid. The small-angle XRD and N2 adsorption-desorption isotherms demonstrate that these PMOs materials possess ordered 2D hexagonal mesostructures in the region of low molar concentrations of Asp-BOSP (⩽10%). Analysis of FTIR and 29Si MAS solid-state NMR confirm that the aspartic acid is incorporated into the framework of the PMOs materials.

Installing Stable Molecular Chirality within the Walls of Periodic Mesoporous Organosilicas via Self-Assembly

The synthesis of highly ordered chiral periodic mesoporous organosilica (PMO) materials is described using a novel approach. Chiral dopants featuring removable chirality were combined with freely rotating bulk monomers, resulting in a bulk chiral material with handedness related to the chiral dopant. Once incorporated into the PMO, removal of the chiral-linker in the dopant is readily accomplished and occurs with complete preservation of the circular dichroism signal in the PMO material, whereas in the precursor molecule, this transformation would lead to a total loss of chirality. The chirality of the PMO material is retained even after prolonged hydrothermal treatment, indicating stable chirality induction within the walls of solid PMO.

Periodic Mesoporous Organosilica Nanoparticles with Controlled Morphologies and High Drug/Dye Loadings for Multicargo Delivery in Cancer Cells.

Despite the worldwide interest generated by periodic mesoporous organosilica (PMO) bulk materials, the design of PMO nanomaterials with controlled morphology remains largely unexplored and their properties unknown. In this work, we describe the first study of PMO nanoparticles (NPs) based on meta-phenylene bridges, and we conducted a comparative structure-property relationship investigation with para-phenylene-bridged PMO NPs. Our findings indicate that the change of the isomer drastically affects the structure, morphology, size, porosity and thermal stability of PMO materials. We observed a much higher porosity and thermal stability of the para-based PMO which was likely due to a higher molecular periodicity. Additionally, the para isomer could generate multipodal NPs at very low stirring speed and upon this discovery we designed a phenylene-ethylene bridged PMO with a controlled Janus morphology. Unprecedentedly high payloads could be obtained from 40 to 110 wt % regardless of the organic bridge of PMOs. Finally, we demonstrate for the first time the co-delivery of two cargos by PMO NPs. Importantly, the cargo stability in PMOs did not require the capping of the pores, unlike pure silica, and the delivery could be autonomously triggered in cancer cells by acidic pH with nearly 70 % cell killing.

High azobenzene functionalization enhances stability of the cis isomer: Periodic mesoporous organosilica network on the way to new light triggered applicable materials

Azobenzene and its derivatives are in the focus of research for the application as molecular switch for medical uses or industrial processes. An optical switchable azobenzene molecule was covalently anchored on the pore walls of a benzene molecule bridged periodic mesoporous organosilica material (abbreviated bz-PMO). The two step synthesis allowed the preparation of a hybrid material with a very high switch density exhibiting an extraordinary long half-life time of 87 h of the cis-isomer. The material was thoroughly characterized applying UV/Vis- and solid state NMR-spectroscopy demonstrating the successful covalent anchoring of the switch onto the pore walls.

Facile Synthesis of Cooperative Acid–Base Catalysts by Clicking Cysteine and Cysteamine on an Ethylene‐Bridged Periodic Mesoporous Organosilica

AbstractA periodic mesoporous organosilica (PMO) that contains ethylene bridges was functionalized to obtain a series of cooperative acid–base catalysts. A straightforward, single‐step procedure was devised to immobilize cysteine and cysteamine on the support material by means of a photoinitiated thiol‐ene click reaction. Likewise, PMO materials capped with hexamethyldisilazane (HMDS) were used to support both compounds. This resulted in different materials in which the amine site was promoted by carboxylic acid groups, surface silanol groups, or both. The catalysts were tested in the aldol reaction of 4‐nitrobenzaldehyde and acetone. It was found that silanol groups have a stronger promoting effect on the amine than the carboxylic acid group. The highest turnover frequency (TOF) was obtained for an amine‐functionalized material that contained only silanol promoting sites. The loading of the active sites also had a significant effect on the activity of the catalysts, which was rationalized on the basis of a computational study.

Chiral periodic mesoporous organosilica in a smectic-A liquid crystal: source of the electrooptic response

ABSTRACTChiral periodic mesoporous organosilica (PMO) materials have been shown to deracemise a configurationally achiral, but conformationally racemic liquid crystal in which the PMO is embedded. In particular, application of an electric field E in the liquid crystal’s smectic-A phase results in a rotation of the liquid-crystal director by an angle proportional to E, which is detected optically – this is the so-called ‘electroclinic’ effect. Here we present results from electroclinic measurements as a function of frequency and temperature, which allow us to distinguish the component of optical signal that arises from liquid-crystal chirality induced within the PMO’s chiral pores from that induced just outside the silica colloids. Our central result is that the overwhelming source of our electrooptic signal emanates from outside the PMO, and that the contribution from the liquid crystal embedded in the chiral pores is much smaller and below the noise level.

Chiral Copper(II) Bis(oxazoline) Complexes Directly Coordinated to Amine‐Functionalized Phenylene/Biphenylene Periodic Mesoporous Organosilicas as Heterogeneous ­Catalysts

AbstractCopper(II) complexes with commercial chiral bis(oxazoline)s were for the first time directly coordinated onto amine‐modified mesoporous phenylene and biphenylene silicas. All final materials maintained the 2D ordered mesoporous structure. The copper metal coordinates directly to the amine groups within the walls of the materials. The materials were tested as asymmetric heterogeneous catalysts in the kinetic resolution of hydrobenzoin. They were active, selective, and enantioselective in this reaction. The material containing the (S)‐(–)‐2,2′‐isopropylidene‐bis(4‐phenyl‐2‐oxazoline) (Me2PhBox) ligand, the best ligand in the homogeneous phase, presented the highest enantioselectivity of all the materials in the first cycle (73 %). The 2,2′‐methylenebis[(4S)‐4‐phenyl‐2‐oxazoline] (PhBox) ligand rendered the catalyst more stable, independent of the anion or organic moiety in the periodic mesoporous organosilica material; this catalyst was reused over five cycles without any significant loss of catalytic activity or enantioselectivity. This type of ligand plays an important role in the stability of the corresponding copper(II) complex upon immobilization and in the robustness of the heterogeneous catalyst upon reuse.

Electrostatic Grafting of a Palladium N‐Heterocyclic Carbene Catalyst on a Periodic Mesoporous Organosilica and its Application in the Suzuki–Miyaura Reaction

AbstractA periodic mesoporous organosilica material functionalized with a Pd N‐heterocyclic carbene complex was synthesized by applying an electrostatic grafting method. The resulting hybrid material was characterized by solid‐state 29Si and 13C cross‐polarization magic‐angle spinning NMR spectroscopy, XRD, thermogravimetric analysis, and BET measurements. The hybrid was applied as a catalyst for the Suzuki–Miyaura cross‐coupling between aryl chlorides and phenylboronic acid under heterogeneous and aerobic conditions. The supported catalyst exhibited excellent activity and stability and it could be reused at least six times without any loss of activity. Atomic absorption spectroscopy detected no Pd contamination in the products, and leaching tests verified that the reaction was truly heterogeneous.

A Facile Multi-interface Transformation Approach to Monodisperse Multiple-Shelled Periodic Mesoporous Organosilica Hollow Spheres.

The synthesis of well-defined and complex hollow structures via a simple method is still a major challenge. In this work, a facile and controllable "multi-interface transformation" approach for preparation of monodisperse multi-shelled periodic mesoporous organosilica (PMO) hollow spheres has been established by a one-step hydrothermal treatment of successively grown organosilica particles. The multi-shelled PMO hollow spheres have inorganic-organic hybrid frameworks, controllable number (1-4) of shells, high surface area (∼805 m(2)/g), accessible ordered mesochannels (∼3.2 nm), large pore volume (1.0 cm(3)/g), and uniform and tunable diameter (300-550 nm), chamber size (4-54 nm), and shell thickness (10-30 nm). In addition, various organic groups (alkyl, aromatic, and heteroelement fragments) are successfully incorporated into the multi-shelled PMO hollow spheres by successively adding different bridged organosilica precursors. Notably, the distribution of different kinds of organic groups in the multi-shelled PMO hollow spheres can be precisely controlled, showing great potential for future applications. We propose that the formation of the multi-shelled PMO hollow structures is ascribed to the creation of multiple highly cross-linked organosilica interfaces, providing a new and interesting fundamental principle for PMO materials. Due to their unique structure and frameworks, triple-shelled ethane-bridged PMO hollow spheres were successfully loaded with an anti-cancer drug doxorubicin and perfluoropentane gas, which present excellent effects in the killing of cancer cells and ultrasound imaging. It is expected that the multi-interface transformation strategy provides a simple, controllable, versatile, and template-free method for preparation of various multifunctional PMOs for different applications.