Polymer-supported Schiff base complexes in oxidation reactions

The design and synthesis of hollow/yolk-shell mesoporous structures with catalytically active ordered mesoporous shells can infuse new vitality into the applications of these attractive structures. In this study, we report that hollow/yolk-shell structures with catalytically active ordered mesoporous aluminosilica shells can be easily prepared by using silica spheres as the silica precursors. By simply treating with a hot alkaline solution in the presence of sodium aluminate (NaAlO(2)) and cetyltrimethylammonium bromide (CTAB), solid silica spheres can be directly converted into high-quality hollow mesoporous aluminosilica spheres with perpendicular pore channels. On the basis of the proposed formation mechanism of etching followed by co-assembly, the synthesis strategy developed in this work can be extended as a general strategy to prepare ordered mesoporous yolk-shell structures with diverse compositions and morphologies simply by replacing solid silica spheres with silica-coated nanocomposites. The reduction of 4-nitrophenol with yolk-shell structured Au@ordered mesoporous aluminosilica as the catalyst has clearly demonstrated that the highly permeable perpendicular pore channels of mesoporous aluminosilica can effectively prevent the catalytically active yolk from aggregating. Furthermore, with accessible acidity, the yolk-shell structured ordered mesoporous aluminosilica spheres containing Pd yolk exhibit high catalytic activity and recyclability in a one-pot two-step synthesis involving an acid catalysis and subsequent catalytic hydrogenation for desired benzimidazole derivative, which makes the proposed hollow ordered aluminosilica spheres a versatile and practicable scaffold for advanced catalytic nanoreactor systems.

Hollow Mesoporous Aluminosilica Spheres with Perpendicular Pore Channels as Catalytic Nanoreactors

Hollow mesoporous silica materials have been intensively pursued because of their unique properties for various applications. Yolk/shell structured hollow mesoporous silica with functional cores inside their hollow interior can further broaden the applications of hollow mesoporous silica. The self-templating strategy has been developed as one of the most important strategies to effectively fabricate hollow mesoporous silicas and their yolk/shell counterparts. In this feature article, we provide an overview of advances in the self-templating synthesis of hollow mesoporous silica based on the following three strategies: surface-protected etching, structural difference-based selective etching, and cationic surfactant assisted self-templating. We then discuss some important applications of these self-templating strategy-derived hollow mesoporous silicas, such as nanoreactors for confined catalysis and multifunctional platforms for combined therapy. Finally, some perspectives for the future development of this active research field are provided.

Self-templating synthesis of hollow mesoporous silica and their applications in catalysis and drug delivery

Solvent-free allylic oxidation of alkenes with O2 mediated by Fe- and Cr-MIL-101

Removal of uranium(VI) ions from aqueous solutions using Schiff base functionalized SBA-15 mesoporous silica materials

The influence of aluminium sources on the acidic behaviour as well as on the catalytic activity of mesoporous H-AlMCM-41 molecular sieves

Allylic oxidation of cyclohexene was carried out over mesoporous (Cr)MCM-41 and (Cr)MCM-48 molecular sieve catalysts. In both the cases, 2-cyclohexen-1-one was obtained as the major product with small amounts of cyclohexene oxide and 1,2-cyclohexandiol. (Cr)MCM-48 showed higher activity than (Cr)MCM-41 owing to the high chromium content in the former. The use of polar solvents such as acetonitrile and methanol decrease the 2-cyclohexen-1-one selectivity; however, such a procedure produces double bond oxidized product, viz. cyclohexene oxide. Furthermore, unlike many other chromium-based solid catalysts, the activity over recycled as well as washed (Cr)MCM-41 and (Cr)MCM-48 remains nearly the same, indicating that the mesoporous chromosilicate materials behave truly as heterogeneous catalysts. In other words, after the initial loss of non-framework chromium ions for the first time, no leaching was noticed for the chromium-based systems.

Allylic oxidation of cyclohexene over chromium containing mesoporous molecular sieves

Liquid phase oxidation of cyclohexane was carried out under milder reaction conditions over mesoporous VMCM-41 molecular sieve catalysts using aqueous hydrogen peroxide as oxidant, acetic acid as solvent, and methyl ethyl ketone as initiator. The catalysts showed high substrate conversion and excellent product (cyclohexanol) selectivity. Although the activity of the catalyst slightly decreased after first recycle, owing to leaching of small amount of non-framework vanadium ions, it, however, remained nearly same thereafter. This observation was further confirmed by washing experiments where the non-framework vanadium ions were removed upon ammonium acetate treatment. Further, the washed catalyst also showed an activity similar to that of the recycled catalyst. Thus, the recycled or washed VMCM-41 behaves truly as heterogeneous catalyst. This observation was complemented and confirmed by both filtrate and quenching studies. The effects of reaction time, temperature, Si/V molar ratio, and catalyst concentration on the catalyst performance were examined in order to optimize the conversion of cyclohexane and selectivity of cyclohexanol. However, the use of strong oxidizing agent, e.g., tertiary butyl hydroperoxide, resulted in the formation of cyclohexanone as the major product. In addition, the use of solvents like methanol, dioxan and acetone showed lower activity.

Mesoporous VMCM-41: highly efficient and remarkable catalyst for selective oxidation of cyclohexane to cyclohexanol

The effect of vanadium sources on the synthesis and catalytic activity of VMCM-41

Mesoporous VMCM-48 molecular sieves with Si/V (molar) ratios of 25–200 were synthesized hydrothermally and systematically characterized by various analytical and spectroscopic techniques, viz, XRD, N 2 sorption, TG-DTA, TEM, EPR, 51 V MAS-NMR, FT-IR, and DRUV-VIS XRD studies suggest that the substitution of vanadium occurs in the silicate framework structure of MCM-48 TEM and ED investigations confirm the highly ordered cubic structure of VMCM-48 EPR and 51 V NMR studies indicate the presence of pentavalent vanadium ions in tetrahedral framework positions, while DRUV-VIS spectra show their existence in two different environments The catalytic activity of these well-characterized (both in the calcined and the washed forms) materials was evaluated for cyclohexane oxidation under mild reaction conditions All these catalysts gave high substrate conversion and excellent product (cyclohexanol) selectivity Furthermore, unlike many other vanadium-based heterogeneous catalysts reported, the mesoporous VMCM-48 catalysts show minimal leaching of the active vanadium species This was confirmed by washing, recycling and quenching experiments where only a small amount of vanadium ions leaches out in the case of calcined samples, while vanadium was not detected for the washed ones Finally, the catalytic activity of VMCM-48 was also compared with mesoporous VMCM-41 as well as microporous VS-1 catalyst The results indicate that the former showed superior activity to those of the latter two, such superiority could, however, be directly related to the amount of vanadium incorporated in MCM-48 being larger than the amounts in MCM-41 and MFI structures

S.E. Dapurkar

Papers

The influence of aluminium sources on the acidic behaviour as well as on the catalytic activity of mesoporous H-AlMCM-41 molecular sieves

Allylic oxidation of cyclohexene over chromium containing mesoporous molecular sieves

Mesoporous VMCM-41: highly efficient and remarkable catalyst for selective oxidation of cyclohexane to cyclohexanol

The effect of vanadium sources on the synthesis and catalytic activity of VMCM-41

Catalytic activity of highly ordered mesoporous VMCM-48