Recent Advances in Catalysis Over Mesoporous Molecular Sieves

Formylation of amines and alcohols using aminopropylated mesoporous SBA-15 silica (APMS) as an efficient and recyclable catalyst

Aminopropylated mesoporous SBA-15 silica (APMS) is introduced as a new, recyclable and efficient catalyst for the formylation of a variety of amines and alcohols by using readily available formic acid under solvent-free conditions.

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.

Mutable Lewis and Brønsted acidity of aluminated SBA-15 as revealed by NMR of adsorbed pyridine-15N

(1)H and variable-temperature (15)N NMR techniques have been used to study the effect of the gradual alumination of SBA-15 on the structure and adsorption properties of this mesoporous material. The interpretation of experimental spectra suggests that aluminum chlorhydrol most effectively reacts with silica surfaces in the confinement of the cavities of rough mesopore walls, instead of forming a homogeneous aluminum film. This first leads to a gradual filling of the cavities and finally results in aluminum islands on the inner surfaces of mesopores. In the sample with a Si/Al atomic ratio of 4.1, up to half of the inner surface area of the mesopores is covered with aluminum. The alumination produces Brønsted acid sites attributed to silanol groups interacting with aluminum but does not affect the proton-donating ability of isolated silanol groups. At high Si/Al ratios, the surface contains only one type of Lewis site attributed to tetracoordinated aluminum. At lower Si/Al ratios, Lewis acid sites with a lower electron-accepting ability appear, as attributed to pentacoordinated aluminum. The numerical values of the surface densities of all chemically active sites have been estimated after annealing at 420 and 700 K. We were surprised to observe that gaseous nitrogen can occupy Lewis acid sites and hinder the interaction of the aluminum with any other electron donor. As a result, aluminated surfaces saturated with nitrogen do not exhibit any Brønsted or Lewis acidity. At room temperature, it takes days before pyridine replaces nitrogen at the Lewis acid sites.

Hoveyda-Grubbs type metathesis catalyst immobilized on mesoporous molecular sieves MCM-41 and SBA-15

A commercially available Hoveyda–Grubbs type catalyst (RC303 Zhannan Pharma) was immobilized on mesoporous molecular sieves MCM-41 and on SBA-15 by direct interaction with the sieve wall surface. The immobilized catalysts exhibited high activity and nearly 100% selectivity in several types of alkene metathesis reactions. Ru leaching was found to depend on the substrate and solvent used (the lowest leaching was found for ring-closing metathesis of 1,7-octadiene in cyclohexane – 0.04% of catalyst Ru content). Results of XPS, UV–vis and NMR spectroscopy showed that at least 76% of the Ru content was bound to the support surface non-covalently and could be removed from the catalyst by washing with THF.

Synthesis, characterization, and in vitro pH-controllable drug release from mesoporous silica spheres with switchable gates

To accomplish pH-controllable drug release on mesoporous carrier, one of the best ways is to graft stimuli-responsive organic molecules around mesopore outlets. In this work, the pH-responsive propyldiethylenetriamine groups (abbreviative phrase: multiamine chains) were grafted around mesopore outlets of mesoporous silica spheres (MSS) and expected to act as pH-responsive gates. To this end, three multiamine-grafted MSS (i.e., NM1, NM2, and NM3) were synthesized under different reaction temperatures and reaction times. The reaction temperature and time for multiamine grafting were 25 °C and 12 h for NM1, 100 °C and 1 h for NM2, and 100 °C and 12 h for NM3, respectively. Through systematic investigations of TEM, SEM, N(2) adsorption/desorption, TG, and (29)Si MAS NMR, it was found that NM3 had the highest grafting amount of multiamine chains. It was further confirmed that the multiamine chains around the pore outlets of NM3 played the role of "molecular switch" that could well control the transport of guest drug molecules. In contrast, the multiamine chains around the pore outlets of NM2 and NM3 did not show gate effect. The difference should be decided by the fact whether the grafting amount of multiamine chains around mesopore outlets were sufficient under determined reaction temperature and time. In the tests of in vitro drug release, multiamine-gated MSS (i.e., NM3) showed highly sensitive response to the solution pH. At high pH (pH 7.5), ibuprofen (IBU) in this carrier released rapidly and completely within 2 h; at low pH (pH 4.0 or 5.0), only a small part of the IBU (13 wt %) was slowly released from this carrier and the most of IBU was effectively confined in mesopores.