Periodic Mesoporous Organosilica Materials Research Articles

Multifunctional Periodic Mesoporous Organosilicas for Biomolecule Recognition, Biomedical Applications in Cancer Therapy, and Metal Adsorption

AbstractThis paper reports a new approach towards the construction of a multifunctional periodic mesoporous organosilica (PMO), which integrates a range of advantages, such as mesoporous structural order, selective nucleobase‐recognition properties, stimuli‐responsive site‐specific delivery of anticancer agents to cancer tissues, and Cu2+ adsorption, into a single entity. First, the appropriate organic‐functional‐receptor precursor was synthesized by a chemical process and used to fabricate a multifunctional pyridine‐containing PMO material (DMPy‐PMO) by a hydrolysis and condensation route. The designed organic–inorganic hybrid mesoporous silica chemosensor showed an intrinsic selective recognition of nucleobase, specifically thymidine, through multipoint hydrogen‐bonding interactions with suitably arrayed receptor sites loaded into the rigid silica framework. An in vitro cytotoxicity test showed that the designed chemosensor materials have good biocompatibility and, therefore, could be promising candidates for the delivery of a range of therapeutic agents. Confocal laser scanning microscopy (CLSM) confirmed that the material can be internalized effectively by cancer cells (MCF‐7 cells). In addition, the DMPy‐PMOs showed efficient Cu2+ ion removal capacity at pH 5.0 with significantly high levels of adsorption (0.95 mmol g–1). These results suggest that the prepared multifunctional PMO hybrid has potential use as a smart material for a range of applications, such as biomolecule recognition, biomedical applications, and as an efficient adsorbent for the removal of metal ions.

Structural and Optical Properties of Titanium Functionalized Periodic Mesostructured Organosilica Framework via Evaporation-Induced Self-Assembly Method

We describe an evaporation-induced self-assembly method for incorporating Ti into framework of ethane-bridged periodic mesoporous organosilica (PMO). The X-ray diffraction (XRD), transmission electron microscopy (TEM), solid-state 29Si and 13C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and UV-vis spectroscopy were used to elucidate the optical and structural properties of Ti-incorporated PMO materials. The degree of mesopore ordering and the structural properties were found to be dependent on the loading of Ti into the framework of PMO materials, as confirmed by XRD, and surface structural properties. UV-vis spectra also support the incorporation of Ti into framework of PMO. TEM analyses suggested the formation of mesopores for the pure PMO and Ti-incorporated PMO samples. The solid-state NMR and FT-IR spectra have confirmed the presence of organic fragment in the final PMO framework.

Novel and Mild Synthetic Strategy for the Sulfonic Acid Functionalization in Periodic Mesoporous Ethenylene-Silica

A new postsynthetic method has been developed for sulfonic acid functionalization of hybrid periodic mesoporous organosilica (PMO) materials containing carbon-carbon double bonds (-C═C-) located in mesoporous wall structures. Hexagonal mesoporous ethenylene-silicas (HME) with different pore sizes were synthesized by using P123, Brij76, and Brij56 surfactants and investigated for postsynthetic functionalization. The present functionalization strategy involves epoxidation of double bonds at -5 °C followed by conversion of the resulting epoxide with bisulfite ions at 65 °C and involves neither the use of well-known mercaptol/H2O2 nor harsh concentrated H2SO4 reagents during the course of -C═C- functionalization. The epoxidation step plays a crucial role in determining the amount of -SO3H groups functionalized onto the silica support which is optimized with respect to different synthesis parameters. The ethenylene-silicas both before and after chemical modification were thoroughly characterized by powder XRD, TEM, N2 adsorption, Raman spectroscopy, 13C and 29Si MAS NMR, and catalytic test reactions. X-ray powder diffraction measurements and sorption data indicated that the mesostructure was intact during the postsynthetic chemical modification. Raman spectra exhibited two strong bands at 1567 and 1290 cm(-1) for ethenylene-silica attributed to -C═C and -C-H stretching vibrations, respectively; whereas after epoxidation and sulfonation, new bands were observed at 1215 and 1035 cm(-1) corresponding to the epoxide and -SO3 stretching vibrations, respectively. 13C CP MAS NMR of surfactant extracted ethenylene-silica exhibits a signal at 146 ppm along with signals at 16.4 and 17.4 ppm. The appearance of new signals at 47.7 and 46.5 ppm is attributed to carbon atom with ≡C-OH and ≡C-SO3H groups, respectively. 29Si MAS NMR spectra exclusively showed T2 and T3 species at -73 and -82 ppm, respectively both before and after chemical modification and negligible amount of Q3 or Q4 species confirms the stability of Si-C bonds during the functionalization. The sulfonic acid-functionalized mesoporous ethenylene-silicas show high catalytic activity in esterification of acetic acid with ethanol under liquid-phase reaction conditions.

ChemInform Abstract: Pd‐Grafted Periodic Mesoporous Organosilica: An Efficient Heterogeneous Catalyst for Hiyama and Sonogashira Couplings, and Cyanation Reactions.

The high surface area of 2D-hexagonal periodic mesoporous organosilica (PMO) containing a phloroglucinol-diimine moiety inside the pore wall has been utilized for grafting Pd(II) at the surface of the mesopores. This Pd-containing PMO material (Pd-LHMS-3) shows excellent catalytic activity in fluoride-free Hiyama cross-coupling reactions in water at alkaline pH conditions. Sonogashira cross-couplings between terminal alkynes and aryl halides take place in the presence of water and hexamine as base in the absence of any Cu co-catalyst. Cyanation of aryl halides is equally promoted with K4[Fe(CN)6] as the cyanide source (in the absence of poisonous KCN, NaCN or Zn(CN)2) over Pd-LHMS-3. Excellent yield of the products, reusability and the facile work-up could make this Pd-grafted PMO material a unique catalyst for the synthesis of substituted benzonitriles, unsymmetrical biphenyls and di-substituted alkynes under environmentally benign reaction conditions. Further good yield of products and no evidence of leached Pd from the catalyst surface during the reaction and its smooth recovery confirm the true heterogeneity in these catalytic reactions.

Periodic mesoporous organosilicas functionalized with a wide variety of amines for CO2 adsorption

The adsorption of CO2 on amine modified periodic mesoporous organosilicas (PMOs) has been investigated. An ethenylene-bridged PMO is modified with a wide range of diamines and polyamines. A variety of dangling N-containing functionalities, i.e., diaminobutane, diaminohexane, diaminododecane, diethylenetriamine and tetraethylenepentamine, as nucleophiles in a substitution reaction is used. The CO2 adsorption capacity of these materials is probed and compared with amine functionalized SBA-15 material, in an effort to reach the maximal CO2/N ratio of 0.5 when using dry conditions in a chemisorption process. The materials showed good CO2 adsorption behaviour and this maximum amine efficiency value has been approximated by the PMO material modified with diaminododecane.

Enantioselective Addition of Malonates and β‐Keto Esters to Nitroalkenes over an Organonickel‐Functionalized Periodic Mesoporous Organosilica

AbstractA periodic mesoporous organosilica (PMO) with chiral cyclohexyldiamine‐based nickel(II) complexes incorporated within the silica framework was prepared through a co‐condensation of (1R,2R)‐cyclohexyldiamine‐derived silane and Ph‐bridged silane followed by complexation of nickel(II) bromide in the presence of (1R,2R)‐N,N′‐dibenzylcyclohexyldiamine. Structural analyses by X‐ray powder diffraction, nitrogen sorption and transmission electron microscopy disclosed its orderly mesostructure while characterization by solid‐state NMR and X‐ray photoelectron spectroscopy demonstrated the well‐defined single‐site chiral bis(cyclohexyldiamine)‐based nickel(II) active centers incorporated within the PMO material. In particular, as a heterogeneous chiral catalyst, this periodic mesoporous organosilica showed high catalytic activity and excellent enantioselectivity in asymmetric Michael addition of 1,3‐dicarbonyl compounds to nitroalkenes (more than 92% conversions and up to 99% ee values). More importantly, this heterogeneous catalyst could be recovered easily and reused repeatedly nine times without obviously affecting its ee value, showing good potential for industrial applications.

Manipulation of the phase structure of vinyl-functionalized phenylene bridging periodic mesoporous organosilica

A series of vinyl-functionalized periodic mesoporous organosilicas (PMOs) were prepared by co-condensation of 1,4-bis (triethoxysilyl) benzene (BTEB) and triethoxyvinylsilane (TEVS) using the triblock copolymer Pluronic P123 as a template under acid conditions. It is found that the mesophases of resultant PMOs can be controlled via altering the fraction of organosilanes in the synthesis mixture and catalyst HCl concentration. With increasing fraction of TEVS, mesophase of the PMOs materials changed from p6mm to Ia3d, and then becomes a disordered material. For PMOs with 15 molar percentage of TEVS, the increase of HCl concentration can induce a transformation of mesophases from hexagonal p6mm to cubic Ia3d, whereas, a mixture of p6mm/disordered structure forms at lower acid concentration for the PMOs containing 5 % TEVS. The mechanisms of mesophase transformation were discussed based on the adsorption of TEVS into the micelles, influence of acid concentration on the hydrolysis and condensation rate, and the relative reactivities of the organosilane precursors.

Thermal behavior, structure, and dynamics of low-temperature water confined in mesoporous organosilica by differential scanning calorimetry, X-ray diffraction, and quasi-elastic neutron scattering

Differential scanning calorimetry (DSC), X-ray diffraction (XRD), and quasi-elastic neutron scattering (QENS) measurements have been made at 200~330 K for capillary-condensed water confined in periodic mesoporous organosilica (PMO) materials with the phenyl groups embedded in silica matrix (Ph-PMO; pore diameter 30 Å). The DSC data showed that the capillary-condensed water in Ph-PMO freezes at 228 K. X-ray radial distribution functions (RDFs) showed that the tetrahedral-like hydrogen-bonded structure of water is distorted in Ph-PMO pores, compared with bulk water; however, with lowering temperature the tetrahedral moiety of water is gradually recovered in the pores. Below the freezing point, cubic ice I c was formed in the Ph-PMO pores. The QENS data showed that the translational diffusion constant and the residence time and the rotational relaxation time of water molecule in Ph-PMO are comparable with those in bulk. The corresponding activation energies suggested that the more hydrophobic the nature of the wall is, the smaller the activation energy of diffusion and rotation of a water molecule; this implies that water molecules confined in the hydrophobic pores are present in the core of the pores, whereas those in the hydrophilic pores strongly interact with the silanol groups.

Distribution of functional groups in periodic mesoporous organosilica materials studied by small-angle neutron scattering with in situ adsorption of nitrogen

Periodic mesoporous materials of the type (R′O)3Si-R-Si(OR′)3 with benzene as an organic bridge and a crystal-like periodicity within the pore walls were functionalized with SO3H or SO3− groups and investigated by small-angle neutron scattering (SANS) with in situ nitrogen adsorption at 77 K. If N2 is adsorbed in the pores the SANS measurements show a complete matching of all of the diffraction signals that are caused by the long-range ordering of the mesopores in the benzene-PMO, due to the fact that the benzene-PMO walls possess a neutron scattering length density (SLD) similar to that of nitrogen in the condensed state. However, signals at higher q-values (>1 1/Å) are not affected with respect to their SANS intensity, even after complete pore filling, confirming the assumption of a crystal-like periodicity within the PMO material walls due to π–π interactions between the organic bridges. The SLD of pristine benzene-PMO was altered by functionalizing the surface with different amounts of SO3H-groups, using the grafting method. For a low degree of functionalization (0.81 mmol SO3H·g−1) and/or an inhomogeneous distribution of the SO3H-groups, the SLD changes only negligibly, and thus, complete contrast matching is still found. However, for higher amounts of SO3H-groups (1.65 mmol SO3H·g−1) being present in the mesopores, complete matching of the neutron diffraction signals is no longer observed proving that homogeneously distributed SO3H-groups on the inner pore walls of the benzene-PMO alter the SLD in a way that it no longer fits to the SLD of the condensed N2.

Assembling Photoluminescent Silicon Nanocrystals into Periodic Mesoporous Organosilica

A contemporary question in the intensely active field of periodic mesoporous organosilica (PMO) materials is how large a silsesquioxane precursor can be self-assembled under template direction into the pore walls of an ordered mesostructure. An answer to this question is beginning to emerge with the ability to synthesize dendrimer, buckyball, and polyhedral oligomeric silsesquioxane PMOs. In this paper, we further expand the library of large-scale silsesquioxane precursors by demonstrating that photoluminescent nanocrystalline silicon that has been surface-capped with oligo(triethoxysilylethylene), denoted as ncSi:(CH(2)CH(2)Si(OEt)(3))(n)H, can be self-assembled into a photoluminescent nanocrystalline silicon periodic mesoporous organosilica (ncSi-PMO). A comprehensive multianalytical characterization of the structural and optical properties of ncSi-PMO demonstrates that the material gainfully combines the photoluminescent properties of nanocrystalline silicon with the porous structure of the PMO. This integration of two functional components makes ncSi-PMO a promising multifunctional material for optoelectronic and biomedical applications.

Functionalized periodic mesoporous organosilicas for selective adsorption of proteins

The periodic mesoporous organosilicas (PMO) with an organobridged (CH2) was synthesized and functionalized with amino or carboxylic groups by post-synthesis methods. The functionalized PMO by changing the hydrophilic/hydrophobic property and the net charge could be used to selectively adsorb and purify proteins with different shapes and different isoelectric points (pI). The experimental result showed that Bovine serum albumin (BSA) was adsorbed quicker than hemoglobin (Hb) on the materials, and lysozyme (Lys) could not be adsorbed on these PMO materials at all. The adsorption capacity of amino groups modified PMO (PMO-(NH2)2) for BSA was 44.67mg/g and 300.0mg/gfor Hb on carboxylic groups modified PMO (PMO-(COOH)2). The adsorption behavior of proteins was affected strongly by the interaction among different constituents in the mixture of proteins. In addition, it is found that the adsorption rate of (PMO-(NH2)2 for adsorption of proteins was much slower than PMO-(COOH)2.

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.

Near-infrared luminescence of periodic mesoporous organosilicas grafted with lanthanide complexes based on visible-light sensitization

A new periodic mesoporous organosilica material (named as bpd-PMO) was synthesized through co-condensation of bis(triethoxysilyl)ethane (BTEE) and a synthesized silsesquioxane precursor 4,4′-bis[Si(OEt)3(CH2)4]-2,2′-bipyridine (bpd-Si) in the presence of cetyltrimethylammonium bromide (CTAB) surfactant as template. Subsequently, three near-infrared (NIR) luminescent PMO hybrid materials (named as Ln(dbm)3bpd-PMO, Ln = Er, Nd, Yb) covalently linked with lanthanide complexes were successfully prepared via a ligand exchange reaction. The materials were characterized by XRD, N2 adsorption–desorption isotherms, SEM, and fluorescence spectra, and the corresponding luminescent decay analyses were recorded. Of importance here is that the excitation spectra of the Ln(dbm)3bpd-PMO extend to the visible-light range. Following the visible-light excitation, the Ln(dbm)3bpd-PMO materials show the characteristic NIR-luminescence of the corresponding Ln3+ ions. The good NIR-luminescence properties of the Ln(dbm)3bpd-PMO with the visible-light excitation would extend the potential applications of the PMO matrix to the NIR optical field.

Probing the pore structure of a chiral periodic mesoporous organosilica using liquid crystals

Periodic mesoporous organosilicas (PMO) are prepared by the surfactant-templated condensation of bridged organosilsesquioxane monomers. By controlling the nature of the organic segment, the type of surfactant and the condensation conditions, one can control the physical and chemical properties of the resulting PMO and produce highly ordered porous structures with a periodicity on the nanometer scale. The development of chiral PMO materials has been of significant interest given their potential in heterogeneous asymmetric catalysis, chiral chromatography and non-linear optics. Characterization of the chirality of pore structures in these materials thus far has been achieved by indirect methods including polarimetry and solid-state circular dichroism. We report herein a general and convenient approach to probe directly the pore structure of chiral PMO materials based on their interactions with inexpensive liquid crystalline solvents, which result in the induction of measurable chiral properties in the nematic (N) and smectic A (SmA) phases of the liquid crystals. The templated co-condensation of a biphenylene organosilsesquioxane monomer and a chiral binaphthyl organosilsesquioxane monomer produced a new chiral PMO material that was investigated as dopant in two different liquid crystal hosts. Measurements of induced circular dichroism and helical pitch in the nematic phase of the cyanobiphenyl liquid crystal 5CB, and the measurement of an induced electroclinic effect in the SmA phase of the phenyl benzoate liquid crystal 9OO4 were carried out. The induced chiral properties measured in these experiments are consistent with chirality transfer taking place inside the pores, and suggest that the inner structure of the pores in the PMO material is indeed chiral.

Pd-grafted periodic mesoporous organosilica: an efficient heterogeneous catalyst for Hiyama and Sonogashira couplings, and cyanation reactions

The high surface area of 2D-hexagonal periodic mesoporous organosilica (PMO) containing a phloroglucinol-diimine moiety inside the pore wall has been utilized for grafting Pd(II) at the surface of the mesopores. This Pd-containing PMO material (Pd-LHMS-3) shows excellent catalytic activity in fluoride-free Hiyama cross-coupling reactions in water at alkaline pH conditions. Sonogashira cross-couplings between terminal alkynes and aryl halides take place in the presence of water and hexamine as base in the absence of any Cu co-catalyst. Cyanation of aryl halides is equally promoted with K4[Fe(CN)6] as the cyanide source (in the absence of poisonous KCN, NaCN or Zn(CN)2) over Pd-LHMS-3. Excellent yield of the products, reusability and the facile work-up could make this Pd-grafted PMO material a unique catalyst for the synthesis of substituted benzonitriles, unsymmetrical biphenyls and di-substituted alkynes under environmentally benign reaction conditions. Further good yield of products and no evidence of leached Pd from the catalyst surface during the reaction and its smooth recovery confirm the true heterogeneity in these catalytic reactions.

Mesostructure-controlled synthesis of chiral norbornane-bridged periodic mesoporous organosilicas

In this contribution, chiral periodic mesoporous organosilica (PMO) materials with three-dimensional body-centered (Im3m) and face-centered (Fm3m) cubic mesostructures were synthesized for the first time. Under acidic conditions and with triblock copolymer F127 as the template, an Im3m-type chiral PMO was obtained with spherical morphology from the self-condensation of chiral norbornane-bridged precursor 1. In contrast, using triblock copolymer P123 as the template and with the aid of a co-surfactant, Gemini surfactant C14-6-14, an Fm3m-type chiral PMO was obtained with an unprecedented pumpkin-like morphology from the self-condensation of chiral precursor 1. The synthetic conditions such as the stirring, standing, and aging time are crucial for the mesostructure and morphology of the synthesized chiral PMO materials.

Impact of curing condition on chemical stability of ultralow-k PMO material.

ABSTRACTIn this work a new generation of periodic mesoporous organosilica (PMO) low-k dielectrics with targeted k-values 2.0 and 1.8 is evaluated. In addition, impact of two different curing processes on properties of the mesoporous material is analyzed. It is shown that removal of templating organics with thermal annealing leads to formation of mechanically robust and chemically very stable material, while application of UV-assisted curing with broadband lamp (λ > 200 nm) causes pronounced decrease of film ability to sustain in diluted HF solution. The explanation of that phenomenon is given in terms of silica-ring structures formed within organosilica skeleton.

Enantioselective catalysis with a chiral, phosphane-containing PMO material

A novel bistriethoxysilyl-BINAP monomer was prepared and co-condensed with a biphenylene-bridged siloxane precursor in the presence of surfactant templates to give periodic mesoporous organosilicas (PMOs) functionalized with BINAP. Complexation of ruthenium followed by asymmetric catalytic hydrogenation and asymmetric transfer hydrogenation were carried out, and demonstrated that high levels of activity and selectivity are achievable with the chiral material.

Smart heating profiles for the synthesis of benzene bridged periodic mesoporous organosilicas

In this study the effects of the heating rate and heating time on the formation of crystal-like benzene bridged periodic mesoporous organosilicas (PMOs) are investigated. The time needed to heat up an autoclave during the hydrothermal treatment has shown to be crucial in the synthesis of PMOs, while the total duration of heating gave rise to only minor differences. By choosing a smart heating profile, superior PMO materials can be obtained in a short time. Different heating profiles in a range from one minute to one hour are adopted by microwave equipment and compared with conventional heating methods. The heating rate has a large influence on the porosity characteristics and the uniformity of the obtained particles. Moreover, two new alternative synthetic strategies to adopt the smart heating profile are presented, in order to give some possible solutions for the expensive microwave equipment.

Performance evaluation of mesoporous host materials in olefin epoxidation using Mo(II) and Mo(VI) active species—Inorganic vs. hybrid matrix

The [Mo VIO 2Cl 2(thf) 2] and [Mo III 2(CO) 3(MeCN) 2] precursor complexes, were immobilized in a hybrid periodic mesoporous organosilicas (PMO)-type material. These new hybrid matrix materials were prepared in one pot synthesis, containing the bidentate N, N′-ligand ( glypy) in the material. In a second approach, the ordered mesoporous silica MCM- glypy was prepared using a tethered approach by a stepwise procedure. The metal complexes were grafted in both types of materials to obtain heterogeneous pre-catalysts. The modified materials have been characterized by powder X-ray diffraction, N 2 adsorption, and solid-state CP MAS NMR ( 13C, 29Si). All metal containing materials were tested as catalysts in epoxidation of 1-octene, cyclooctene and styrene with tert-butylhydroperoxide (TBHP) at 328 K. Selectivity to octene and cyclooctene epoxides was complete. The new PMO materials described in this work are outstanding catalysts for any of the transformations described. In fact, cyclooctene selectivity based on TBHP consumption is found to be almost independent of the mol ratio attesting the efficiency of the catalysts. They are also effective under stoichiometric olefin/oxidant ratio. In addition, a mechanism for acid catalyzed benzaldehyde formation is proposed as well as the conversion of Mo(II) to Mo(VI) under catalysis.