Experimental studies on the reforming and hydrotreating of small oxygenates.

摘要

Biomass derivatives are composed of C, H, and O and the products of selective bond breaking reactions can range from synthesis gas (reforming products) to hydrocarbons (deoxygenation products). In applied experimental catalytic processes, many factors can influence the catalyst structure and selectivity for biomass conversion: thermal stability of the reactant, reaction environment (vapor or aqueous including solvent effects), catalyst structure and site type (shape or bimetallic configuration), synthesis methods, and the stability of the support to name a few. To reduce the complexity, guidance and knowledge from fundamental studies are used to direct the research. Extending the understanding from fundamental studies to supported catalysts for biomass conversion gives insight into the working catalyst. In this thesis, with the use of catalyst evaluation and characterization, experimental studies on supported transition metal catalysts and transition metal carbides were utilized to investigate the role of support, reaction environment (vapor and aqueous), structure of bimetallic catalysts, and effect of functional group on biomass conversion. Typically, catalyst characterization is done using ex-situ techniques. Here in-situ characterization experiments were performed on the working catalyst to probe the catalyst structure. Additionally, a computational approach to resolve the bimetallic nanoparticle structure was developed and included a tool to estimate the surface composition of the nanoparticle. In the first half of this thesis, the production of hydrogen from reforming of ethylene glycol is studied. The hydrogen generated from such processes can be used to upgrade biomass through hydrotreating, which is the focus of the second half of this thesis. Using the fundamental understandings from computational and UHV work along with knowledge from synthesis studies, the NiPt system is the catalyst of choice for the reforming experiments. The previous studies found that NiPt has increased activity over Pt due to its stronger oxygen binding energy (a descriptor), which lowers the barriers for early dehydrogenation reactions that were found to be rate limiting. In this thesis, vapor phase and aqueous phase reactor systems were designed and constructed. Steam reforming experiments can act to link information from fundamental surface studies with applied aqueous systems. Exploring the influence of support and active metal, steam reforming experiments were performed. NiPt/C was found to be an active with stable conversion and was used to probe the bimetallic catalyst structure under aqueous reaction conditions. The structural changes in supported NiPt/C were investigated using in-situ extended X-ray absorption fine structure (EXAFS) under aqueous phase reforming (APR) of ethylene glycol conditions. The enhanced activity of NiPt over Pt from reactor studies was correlated to changes in the catalyst structure. Under APR conditions, Ni segregated to the surface of the catalysts, resembling Ni-terminated bimetallic surfaces that were predicted to be more active than Pt from theoretical and experimental studies on model surfaces. A computational framework for nanoparticle structural modeling to resolve the bimetallic structure was developed, which included a surface composition estimation tool. The second half of the thesis focused on hydrotreating of biomass derivatives as a method to upgrade biomass. Computational and ultra-high vacuum experiments predicted molybdenum carbide (Mo2C) to be active for hydrocarbon production. Literature also suggests Mo2C to be a unique material with acid, base, and metal characteristics. Hydrodeoxygenation of ethylene glycol over Mo2C was performed and Mo2C was found to be selective to ethylene production with acetaldehyde as the second major product. This demonstrated that dehydrogenation and C-O cleavage of ethylene glycol is achievable with Mo2C. With interest to see if selectivity could be shifted to reforming products, Ni modified Mo 2C experiments were performed. Although activity was increased, Mo 2C behavior for dehydrogenation and C-O cleavage was dominant over reforming pathways (dehydrogenation and C-C cleavage). Exploring other functional groups, the hydrotreating of isopropanol and acetone on Mo2C was investigated. Isopropanol was found to dehydrogenate to acetone, and acetone was found to be nearly unreactive. From these results, Mo2C catalysts seem to possess base sites with very few (or weak) acid-like sites that would favor C-O cleavage to produce alkenes. These results suggest that the identity of the oxygenate functional group as well as catalytic sites, a possible result from synthesis methods, can influence the observed selectivity.