Synthesis and application of porous materials as supports for nanoparticle, single-site, and biomolecule heterogeneous catalysts

Abstract

The application of porous materials as supports for catalysts has been a focus of academia and industry for many years. Mesoporous silicas (MSN) and ordered mesoporous carbons have been used due to their unique properties such as high surface area, tunable pore sizes, and chemical and thermal stability. These support materials often lend further stability to the catalysts they host, enabling their use in a wider variety of reaction environments. One type of porous support that has seen extensive use on an industrial scale is zeolites, and these can be paired with metal carbides formed in situ during methane dehydroaromatization, Fischer-Tropsch synthesis, or biomass conversion. Improvements to the synthesis of these carbides are made by altering the carburization gas mixture or adding promoter metals. Ordered mesoporous carbons templated from MSN are supports that not only have high surface area and good stability, but are also graphitic. However, their use at a large scale is limited by the time-consuming synthesis techniques, a lack of repeatability in batches, and poor thermal management when prepared in bulk. With simple kitchen tools, the process can reliably be scaled up while avoiding these problems. Further, these can be functionalized through a lithiation process, making them excellent hosts for single-site catalysts. MSN materials are effective supports for a variety of catalysts, including nanoparticle, molecular/single-site, and biomolecule. With their large pore size, more than one catalyst can be incorporated onto the support; a tandem Pd/Au nanoparticle system works cooperatively to facilitate the direct oxidative esterification of allyl alcohol at mild reaciton conditions. MSN materials can also be functionalized with organoalkoxysilanes via co-condensation resulting in a homogeneous distribution of functional groups. By adjusting the pre-hydrolysis time and hydrothermal treatment temperature, original particle morpholgy can be recovered after functionalization. These organic handles can further be reacted with a linker molecule to immobilize biocatalysts for unique reactions. Examining both the fundamental synthetic techniques and the applied side of these catalytic supports lends insight into how they can be used at a small or large scale.