Abstract
SBA-15 materials were functionalized through a post-synthetic methodology with molybdenum-Schiff bases to provide catalytic activity in epoxidation reactions. Thus, glycidoxypropyl functionalities were first attached to the surface of the mesostructured silica, followed by the reaction of the immobilized oxirane groups with 2-amino propyl pyridine. This reaction allowed the obtaining of (hydroxypropyl)-2-aminomethyl pyridine ligands, directly tethered to the surface of the mesoporous silica-based SBA-15, which resulted in excellent chelating ligands to immobilize dioxo molydenum species by a reaction with MoO2(acac)2. This investigation comprises a thorough characterization of the process for building the immobilized molybdenum-Schiff base complexes, as well as the use of the obtained materials in 1-octene oxidation in the presence of organic hydroperoxides. These materials displayed high intrinsic catalytic activity in the epoxidation of 1-octene with organic hydroperoxides under a wide variety of conditions, although both the reaction solvent as well as the nature of the organic hydroperoxide, exerted a dramatic influence on the catalytic activity of these heterogeneous oxidation catalysts. Thus, whereas nonpolar solvents provided good epoxide yields with high efficiency in the use of the oxidant, polar solvents depressed the catalytic activity of the supported Mo-Schiff bases. These results have been ascribed to the competition with the solvent, when polar, for binding to the metal sites, thus avoiding the formation of the hydroperoxo-metal cycle and the epoxidation of the olefin. The catalysts presented here show good reusability with low catalytic activity decay after the first reuse.
Highlights
Alkene epoxidation is a field of both academic and industrial importance due to the versatility of the epoxides, which are key building blocks for the synthesis of numerous chemicals [1,2]
We proposed a possible solution to these disadvantages by functionalizing mesostructured silicas (SBA-15) with polymer chains containing numerous molybdenum grafting points [15]
More stable coordination of the environment of active sites and less metal leaching during the epoxidation reaction are the main consequences. Taking these facts into account and our previous experience on the modification of polyglycidylmethacrylate with 2-aminomethyl pyridine to generate stable binding sites for metal coordination, we present the facile synthesis, characterization, and olefin epoxidation behavior of an efficient and highly porous Mo(VI) catalyst supported onto mesostructured SBA-15 silica
Summary
Alkene epoxidation is a field of both academic and industrial importance due to the versatility of the epoxides, which are key building blocks for the synthesis of numerous chemicals [1,2]. The heterogenization of homogeneous catalytic systems by immobilization onto solid supports makes easier catalysts recovery and recycling, improving the economic and environmental performance of the process Numerous materials such as zeolites, polymers, amorphous and mesostructured silica, among many others, have been tested as carriers for the synthesis of molybdenum-containing heterogeneous alkene epoxidation catalysts [5,6]. Some remarkable examples are works of Corma et al [7] who described the synthesis of a chiral molybdenum complex anchored onto modified Y zeolite for the selective epoxidation of allylic alcohols This Y zeolite was treated for developing, besides its typical micropores, a Catalysts 2017, 7, 215; doi:10.3390/catal7070215 www.mdpi.com/journal/catalysts
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