This overview covers the recent achievements in the application of mesoporous molecular sieves as catalysts and catalyst supports. Main structural and textural features are outlined in relation to the novel synthesis approaches of mesoporous molecular sieves and understanding of their properties. For application of mesoporous molecular sieves as catalysts different ways of introduction of active sites on their surfaces are provided. The main part describes selected catalytic reactions catalyzed by mesoporous molecular sieves focused on very recent papers. The examples include metathesis, acylation reactions, C–C coupling reactions of Heck and Suzuki types, base-catalyzed reactions (Knoevenagel and Michael additions, Claisen-Schmidt condensation, Tischenko reaction) and last but not least immobilization of organometallic complexes for different catalytic applications including chiral catalysts.

Recent Advances in Catalysis Over Mesoporous Molecular Sieves

Oxides and mixed oxides have a tremendous importance in the field of heterogeneous catalysis, serving either as catalysts or as supports for active species. The performance of a catalyst depends directly on its composition, texture, structure and surface properties, which have to be precisely controlled and adapted to each application. In this context, the sol–gel process is a unique tool for the preparation and understanding of catalytic materials, owing to its exceptional versatility. In the last 10 years, the non-hydrolytic sol–gel (NHSG) or non-aqueous sol–gel process based on nonhydrolytic condensations in nonaqueous media has established itself as a simple and powerful method for the design of a wide range of oxide, mixed oxide and hybrid materials with controlled composition, morphology, texture and structure. NHSG proved particularly interesting for the preparation of catalytic materials, notably mesoporous xerogels, single site catalysts and highly crystalline nanoparticles. This critical review addresses the application of NHSG to the preparation of heterogeneous catalysts, emphasizing the specificities of this process, and giving a comprehensive overview of the literature (251 references).

Non-hydrolytic sol–gel routes to heterogeneous catalysts

Aerosol processing coupled with surfactant-templated sol–gel synthesis is used to produce MoO3-SiO2-Al2O3 catalysts. By quenching the sol–gel kinetics by fast drying of the aerosol, molecular-scale dispersion of each component is achieved. The structuring agent generates an organized porosity at the nanoscale. The catalysts have a high specific surface area and outstanding olefin metathesis activity.

One-pot aerosol route to MoO3-SiO2-Al2O3 catalysts with ordered super microporosity and high olefin metathesis activity

The structure of molybdenum oxide supported by silica SBA-15 has been studied by visible Raman spectroscopy, diffuse reflectance UV–Vis spectroscopy and X-ray absorption spectroscopy in the dehydrated state obtained after thermal treatment at elevated temperatures (≥350 °C). No dependence of the molybdenum oxide structure on preparation procedure or loading has been observed within the range of loadings studied in detail (0.2–0.8 Mo/nm2). X-ray absorption spectroscopy (XAS) reveals that the dehydrated state consists of a mixture of monomeric and connected molybdenum oxide centres. While the presence of crystalline MoO3 can be excluded by Raman spectroscopy, tetrahedrally and octahedrally coordinated MoO4 and MoO6 units are identified by XAS. The MoO6 units possess connectivity similar to that of MoO3 building blocks, whereas the MoO4 units are isolated or connected to other MoxOy units. These results are supported by UV–Vis spectra showing intensity at wavelengths (>300 nm) typical for dimeric and/or oligomeric species.

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Structure of molybdenum oxide supported on silica SBA-15 studied by Raman, UV-Vis and X-ray absorption spectroscopy

Flame-made MoO3/SiO2–Al2O3 metathesis catalysts with highly dispersed and highly active molybdate species

New heterogeneous catalysts for alkene metathesis were prepared by immobilization of molybdenum dioxide bis(acetylacetonate) and molybdenum dioxide bis(glycolate) on mesoporous molecular sieve SBA-15. Thermal spreading (TS) method and wet impregnation (WI) followed by calcinations were employed as synthesis approaches. These new catalysts represent inexpensive, easy-to-prepare and highly active and selective heterogeneous catalysts for metathesis of higher linear 1-alkenes applicable under mild reaction conditions without the necessity of solvent or co-catalyst in the reaction mixture. In metathesis of 1-octene, these catalysts showed up to four times higher activity compared with MoO 3 supported on conventional silica as well as on SBA-15. This is probably due to the better dispersion of the molybdenum oxide phase on the mesoporous support surface. In metathesis of 1-dodecene and 1-tetradecene these catalysts based on SBA-15 exhibited a significantly higher activity than corresponding catalysts on MCM-41, probably due to the larger pores.

Molybdenum oxide catalysts for metathesis of higher 1-alkenes via supporting MoO2(acetylacetonate)2 and MoO2(glycolate)2 on SBA-15 mesoporous molecular sieves

Diwa Mishra

Papers

Molybdenum oxide catalysts for metathesis of higher 1-alkenes via supporting MoO2(acetylacetonate)2 and MoO2(glycolate)2 on SBA-15 mesoporous molecular sieves