Belén Albela

Facile Large-Scale Synthesis of Monodisperse Mesoporous Silica Nanospheres with Tunable Pore Structure

Mesoporous silica nanoparticles (MSNs) are experiencing rapid development in the biomedical field for imaging and for use in heterogeneous catalysis. Although the synthesis of MSNs with various morphologies and particle sizes has been reported, synthesis of a pore network with monodispersion control below 200 nm is still challenging. We achieved this goal using mild conditions. The reaction occurred at atmospheric pressure with a templating sol-gel technique using cetyltrimethylammonium (CTA(+)) as the templating surfactant and small organic amines (SOAs) as the mineralizing agent. Production of small pore sizes was performed for the first time, using pure and redispersible monodispersed porous nanophases with either stellate (ST) or raspberry-like (RB) channel morphologies. Tosylate (Tos(-)) counterions favored ST and bromide (Br(-)) RB morphologies at ultralow SOA concentrations. Both anions yielded a worm-like (WO) morphology at high SOA concentrations. A three-step formation mechanism based on self-assembly and ion competition at the electrical palisade of micelles is proposed. Facile recovery and redispersion using specific SOAs allowed a high yield production at the kilogram scale. This novel technique has practical applications in industry.

Tunable Catalysts for Solvent-Free Biphasic Systems: Pickering Interfacial Catalysts over Amphiphilic Silica Nanoparticles

Stabilization of oil/oil Pickering emulsions using robust and recyclable catalytic amphiphilic silica nanoparticles bearing alkyl and propylsulfonic acid groups allows fast and efficient solvent-free acetalization of immiscible long-chain fatty aldehydes with ethylene glycol.

Polyethylenimine covalently grafted on mesostructured porous silica for CO2 capture

Polyethylenimine (PEI) has been grafted on a 2D hexagonal mesostructured porous silica of MCM-41 type (LUS silica) using 3-glycidoxypropyltrimethoxysilane (GTMS) as a grafting agent to develop sorbents for CO2 capture. The advantage of this tether is to create ethanolamine units upon reaction of the epoxy group with the amine functions of PEI. Two synthetic routes have been explored: (1) reaction of GTMS and PEI and then grafting on a calcined MCM-41 silica (M-1), and (2) grafting of GTMS on the silica and then reaction with PEI (M-2). In both cases, the grafted solids are well structured according to the XRD patterns. The amounts of glycidoxypropylsilane (GS) and PEI are 14 and 9 wt%, and 21 and 16 wt%, respectively, for samples M-1 and M-2. The CO2 adsorption capacity of both materials has been tested at 303 K and 101 kPa and compared to a bare LUS silica sample impregnated with 25 wt% PEI (M-3-25). Samples M-1 and M-2 containing ethanolamine groups show higher CO2 adsorption capacities, with loading of about 150 and 134 mgCO2 / gPEI (36 and 43 mgCO2 / g-sorbent), respectively, while the CO2 adsorption capacity was about 55 mgCO2 / gPEI (14 mgCO2 / g-sorbent) for the impregnated solid M-3-25.

Estimation of spacing between 3-bromopropyl functions grafted on mesoporous silica surfaces by a substitution reaction using diamine probe molecules

We present here the results of the substitution of 3-bromopropyltriethoxysilane-grafted mesoporous SBA15 (pore size 7.6 nm) with a series of diamines, CH3NH(CH2)nNHCH3 (C2DA, C3DA and C6DA for n = 2, 3 and 6, respectively) or NH2(CH2)nNH2 (C4DA and C5DA for n = 4 and 5, respectively). The outcome of the reactions was closely related to the spacing of the surface bromopropyl groups. Amount of bromine that remained after the reaction decreased linearly with the amount of C2DA, and it disappeared completely when C2DA : Br = 1 : 1 in the initial reactant mixture. This result indicates the complete conversion of C2DA and Br by a one-to-one substitution (i.e. formation of a linear species). By contrast, the rate of decrease of Br was twice as fast during substitutions with amines other than C2DA when diamine/Br < 0.5 (in the initial mixture), suggesting a one-to-two substitution and the formation of a bridge species. We show that the C/N ratios in the elemental analysis after the complete substitution of Br are a simple and reliable indicator of the distributions of the pair spacings of the bromopropyl groups. The results are compared by a geometric calculation in which the configurations and the minimum limits of neighbouring distances are ignored. The formation of linear and bridge forms were confirmed by 13C-NMR analysis. Reactions using MCM41 (pore size 2.4 nm) and fumed silica were also explored for comparison. We found that the order of the pair spacings of the bromopropyl groups grafted onto these silicas was MCM41 < SBA15 ≪ fumed silica. This result disagrees with the outcome that was expected from the number of bromopropyl groups per unit nm2, 0.8, 1.1 and 1.2 for MCM41, SBA15 and fumed silica, respectively, which was derived from the BET specific surface area and bulk elemental analyses. All of these results are consistent with a mode of grafting where the reaction with silanols competes with diffusion into the pore channels.

Multistep anchoring of a catalytically active ruthenium complex in porous mesostructured silica

A supported [Ru(dmp)2(Py@LUS)Cl]Cl complex (dmp = 2,9-dimethyl-1,10-phenantroline) was synthesised in LUS, a 2D hexagonal porous mesostructured silica, via a step-by-step approach for the sake of site isolation, unicity and localisation in the confined space of the nanopores of the silica matrix. A pyridine terminated tether, Py@LUS, was homogeneously distributed on the surface using a molecular stencil patterning technique, followed by reaction of [Ru(dmp)2Cl2] at 78 °C. All intermediate materials were thoroughly characterised with a panel of techniques, including X-ray diffraction, elemental analysis, 29Si and 13C solid state NMR, diffuse reflectance UV-vis and FT-IR spectroscopies, and cyclovoltamperometry. Site isolation, unicity and localisation are achieved in the confined space of the nanopores of the silica matrix. The final material is selectively active in the catalytic oxidation of methylphenylsulfide into sulfoxide.

Framework and grafted nickel ethylenediamine complexes in 2D hexagonal mesostructured templated silica

Well-ordered periodic mesoporous organosilicas (PMOs) of the MCM-41 type of structure containing framework Ni(II) complexes (Ni@PMOs) were synthesized via a one-pot synthesis route in mild conditions, using cheap and environmental friendly reactants. These materials were obtained using the N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTMS) ligand in three different molar ratios of Ni, X = 1, 2, 3 and co-condensed with sodium silicate in the presence of cetyltrimethylammonium tosylate as a templating agent. The latter named OP-Ni-1, OP-Ni-2 and OP-Ni-3, respectively, were submitted to a treatment using a mixture of chlorotrimethylsilane (CTMS) and hexamethyldisilazane (HMDSA) that simultaneously removed the surfactant and capped the silanol groups leading to materials OP-Ni-1TA, OP-Ni-2TA and OP-Ni-3TA (∼2 wt% Ni content). For comparison, a series of mesoporous materials were prepared via a post-synthesis route to locate the Ni(II) complexes in the channel (∼2.4 wt% Ni). The latter were prepared using the “molecular stencil patterning” (MSP) technique based on sequential grafting of trimethylsilyl (TMS) groups and the same ligand cited above using AAPTMS/Ni molar ratios of 2 and 3. These steps lead to intermediate materials LUS to LUS-PSE and final materials LUS-MSP-Ni-2 and LUS-MSP-Ni-3. Both series of materials were thoroughly investigated in terms of (i) coordination state compared to molecular analogues, (ii) Ni(II) to Cu(II) exchange ability and (iii) coordination accessibility using ethylenediamine (en) or thiocyanate (SCN−) as probe ligands. All materials were investigated using XRD, TEM, N2 sorption isotherms, UV-visible, FT-IR and EPR spectroscopies. In particular, FT-IR and EPR data were treated quantitatively to monitor both the TMS loading and the concentration of isolated Cu(II) in conjunction with elemental analysis. Pore size decrease, pore volume reduction, nickel to copper exchangeability (> 90%) and ligand accessibility were observed for materials LUS-MSP-Ni-2 and LUS-MSP-Ni-3 as expected with grafted Ni(II) complexes located in the channel. By contrast, a clear opposite trend, observed for Ni@PMOs, OP-Ni-1, OP-Ni-2 and OP-Ni-3, was fully consistent with a complete insertion and captation of the Ni complexes in the siliceous pore walls despite the very small thickness (ca. 1.4 nm) of the wall. This is explained by the state of Ni(II) both in terms of coordination and cavity effect. Indeed, both absence of counterion coming from the solution (nitrate) and bathochromic shift of the d-d electronic transitions, are consistent with the presence of silanolate groups counterbalancing the electrical charges and mainly leading to neutral framework species of formula, [Ni(AAPS)2(SiO)2], [Ni(AAPS)(OL)2(SiO)2], with neutral ligand OL = H2O, SiOH, SiOSi and AAPS = N-(2-aminoethyl)-3-aminopropylsilyl moities.

Mononuclear–Dinuclear Equilibrium of Grafted Copper Complexes Confined in the Nanochannels of MCM‐41 Silica

Following the structural concept of copper-containing proteins in which dinuclear copper centers are connected by hydroxide bridging ligands, a bidentate copper(II) complex has been incorporated into nano-confined MCM-41 silica by a multistep sequential grafting technique. Characterization by a combination of EPR spectroscopy, X-ray photoelectron spectroscopy (XPS), UV/Vis spectroscopy, IR spectroscopy , and solid-state (13)C and (29)Si cross-polarization magic-angle spinning (CP-MAS) NMR suggests that dinuclear Cu complexes are bridged by hydroxide and other counterions (chloride or perchlorate ions), similar to the situation for EPR-undetectable [Cu(II)···Cu(II)] dimer analogues in biological systems. More importantly, a dynamic mononuclear-dinuclear equilibrium between different coordination modes of copper is observed, which strongly depends on the nature of the counterions (Cl(-) or ClO(4)(-)) in the copper precursor and the pore size of the silica matrix (the so-called confinement effect). A proton-transfer mechanism within the hydrogen-bonding network is suggested to explain the dynamic nature of the dinuclear copper complex supported on the MCM-41 silica.

Confinement of a bioinspired nonheme Fe(II) complex in 2D hexagonal mesoporous silica with metal site isolation

A mixed amine pyridine polydentate Fe(II) complex was covalently tethered in hexagonal mesoporous silica of the MCM-41 type. Metal site isolation was generated using adsorbed tetramethylammonium cations acting as a patterned silanol protecting mask and trimethylsilylazane as a capping agent. Then, the amine/pyridine ligand bearing a tethering triethoxysilane group was either grafted to such a pretreated silica surface prior to or after complexation to Fe(II). These two synthetic routes, denoted as two-step and one-step, respectively, were also applied to fumed silica for comparison, except that the silanol groups were capped after tethering the metal unit. The coordination of the targeted complex was monitored using UV-visible spectrophotometry and, according to XPS, the best control was achieved inside the channels of the mesoporous silica for the two-step route. For the solid prepared according to the one-step route, tethering of the complex occurred mainly at the entrance of the channel.

Minute-made and low carbon fingerprint microwave synthesis of high quality templated mesoporous silica

Hexagonal mesostructured templated silicas were produced in less than 10 minutes using an ultra-fast microwave assisted hydrothermal synthesis. Typically, 10 g can be prepared at once in a commercial microwave device usually devoted to analytical digestion. Undesired alcohol side-products were avoided using inexpensive water colloidal silica instead of silicon alkoxides as the silicon source. In comparison with classical heating activation, the absence of pore expansion and pore wall thickening even for synthesis temperatures as high as 190 °C evidenced that heat transfer and diffusion of matter had no time to take place. Comparison between the chemically extracted and calcined samples shows that the structure was better stabilized for autoclaving above 150 °C. However, a fast temperature ramping and final temperatures above 180 °C were required to sear structures of the highest quality comparable to that of the best conventional methods. This is rationalized by assuming a sequential flake-by-flake assembly of the pore-wall at the micelle palisade. Notably, tosylate counterions yielded better structural characteristics than bromide counterions and allowed better opportunities for surfactant recycling.

Surface molecular engineering in the confined space of templated porous silica

Advanced materials for molecular sensing, selective adsorption and heterogeneous catalysis require fine control at the surface–fluid interface. To achieve this objective several aspects of the material have to be considered: (i) the molecular structure of the active sites, (ii) their vicinity at the nanometer scale of length, (iii) their distribution in the solid and (iv) the confinement controlled by the size and shape of pores. The approaches developed to synthesise such materials are reviewed here considering mainly mesoporous templated silicas such as MCM-41, SBA-15 and related materials. In addition, a new nomenclature (InGASE) is proposed to classify hybrid or non-hybrid materials depending on the location of the function in the porous solid support and the nature of the linker. Special attention is devoted to the different strategies described in the literature to control the grafting or anchoring of organic functions and metal complexes. The challenge is to obtain isolated and well-defined unique sites in a confined nanometric space with an appropriate environment. Applications of these materials in the fields of catalysis, adsorption, sensing and drug delivery are briefly surveyed.