Development of Heterogeneous Metathesis Catalysts with Nanopores

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Olefin metathesis reactions have been performed by transition metal complexes or oxides of molybdenum and tungsten. Among heterogeneous catalysts for metathesis, it is well known that rhenium-based catalysts are powerful ones. For example, supported Re_2O_7 shows higher activity for liquid-phase metathesis reactions of simple olefins than MoO_3 and WO_3 catalysts. Although rhenium-based catalysts have high potential for olefin metathesis, they have serious drawbacks of little activity toward functionalized olefins without help of additives such as R4Sn and R4Pb. In 1991, organometallic methyltrioxorhenium (MTO) was reported to smoothly induce the metathesis of functionalized olefin only on the surface of a solid acid, SiO_2-Al_2O_3. However, the nature of the rhenium species and the role of the catalyst carrier, SiO_2-Al_2O_3, have not been fully illuminated. In this study, we found that a zinc chloride-modified mesoporous alumina, ZnCl_2//meso-Al_2O_3, with a high surface area, relatively uniform mesopores, and Lewis acidic character was developed, and that MTO doped on the alumina, MTO/ZnCl_2//meso-Al_2O_3, demonstrated higher catalytic performance for the metathesis of functionalized olefins than Herrmann's system of MTO on SiO_2-Al_2O_3 (MTO/SiO_2-Al_2O_3). In our catalytic system, ZnCl_2//meso-Al_2O_3 not only immobilizes MTO on its solid surface, but also activates MTO enough to accelerate the metathesis of functionalized olefins.