(606a) Structure-Function Relationships for Non-Precious Bimetallic MOF-Derived Catalysts in Vapor-Phase Furfural Hydrogenation
MOF-derived carbon supported materials are a new class of supported catalysts of interest due to their hypothesized potential to create good metal dispersity, high surface area, tunable porosity, and an abundance of metal/metal oxide species. During pyrolysis of MOFs in an inert atmosphere, the organic linkers carbonize to create a carbon matrix, and metal nodes agglomerate into nanoparticles. These nanoparticles are partially encapsulated within the carbon matrix in some cases, which may result in more stable particles. This work specifically investigates different metal types and compositions of non-precious mono- and bi-metallic MOF-precursors and how the different precatalyst structures formed after pyrolysis affect the function of these materials as catalysts in vapor phase hydrogenation of furfural. A typical reaction pathway of the hydrogenation of furfural, which is an aldehyde bound to a furan ring, first begins with hydrogenation to form furfuryl alcohol or decarbonylation to form furan. Furfuryl alcohol can then be further hydrogenated to form 2-methylfuran, tetrahydrofurfuryl alcohol, and other various products. This talk will mostly focus on the hydrogenation of furfural to furfuryl alcohol and 2-methylfuran.
MOF-74 was used as the precursor platform because of the wide array of metals that can be incorporated into the structure. Numerous factors are thought to affect the final catalyst structure and properties, including the MOF type (linker, metal) and pyrolysis conditions, including the environment (inert, oxidizing), temperature, hold time, and ramp rates. When pyrolyzing MOF-74, the carbon-based MOF structure decomposes to create a carbon support with varying porosity, depending primarily on the pyrolysis temperatures and environment. As temperature increases, MOF-74 loses a majority of its surface area (~80%), as much of the microporosity collapses and in its place forms a more disordered, meso- and micro-porous carbon matrix is created. As temperature further increases above 600 °C, the carbon matrix becomes ordered into a graphitic carbon phase, which further decreases porosity.
The metal nanoparticle size in MOF-74 derived catalyst is affected by metal type and amount, support type, pyrolysis temperature, length of time at this temperature, and heating and cooling rates. In general, the higher the pyrolysis temperature, the larger the nanoparticle size and lower the catalytic rates. Sintering is influenced by several variables including the surface-free energies and Huttig and Tammann temperatures for each metal, which are the temperatures at which metal species have enough energy for surface and bulk diffusion, respectively1. For a CuCo MOF-74-derived catalyst, a range of pyrolysis temperatures was studied (300 â 900 Â°C). Compared to these MOFs pryolyzed at higher temperatures, MOFs pyrolyzed at temperatures less than 600 Â°C produced smaller nanoparticles (5-15 nm) which increased catalytic rates. At higher temperatures, compared to cobalt, copper can more easily diffuse and sinter creating larger agglomerates. Metal nanoparticles on the outside of the MOF particles are larger than the nanoparticles seen on the inside of the particles.
The metal-support interaction and metal choice are important in achieving sufficient alloying, which are critical for opening up selective furfural hydrogenation and hydrogenolysis pathways. Although the CuCo MOF-74-derived catalyst shows little to no alloying perhaps due to limited solubility of these metals in each other2 and also weak metal-support interactions, other bimetallic precursors showed more potential to make alloys, such as FeNi. The presence of Fe in alloyed FeNi bimetallic catalysts decreased the selectivity for furan relative to the monometallic Ni catalyst. Overall, combining Fe in a bimetallic catalyst with either Cu, Co, or Ni increased the selectivity to 2-methylfuran, most likely due to iron modifying the surface adsorption of furfural, although the monometallic Fe MOF-74-derived catalyst had very low activity and produced primarily furfuryl alcohol. The extent to which the MOF derived synthesis produces catalysts that offer different catalytic performance compared to the traditional methods reported in the literature will be discussed using previously investigated metal combinations.