(300d) Microkinetic Modeling of Catalytic Reforming of Oxygenated Hydrocarbons

There has been increasing interest in the utilization of biomass for renewable energy and chemical production. Biomass, which is made up of oxygenated hydrocarbon building blocks, can be more efficiently utilized for chemical and energy production through the use of small-scale catalytic reforming technologies. Small-scale reactors can increase localization of synthesis gas production and increase reaction rates over current large-scale and enzymatic technologies. Distributed synthesis gas will allow for local production of methanol, ammonia, liquid fuels (Fischer-Tropsch process) and hydrogen for fuel cell applications. Recent technological successes in catalytic reforming of biomass derived oxygenates have shown high reaction rates and high product selectivity towards hydrogen for smaller oxygenates such as methanol, ethylene glycol and glycerol. Future improvements and extensions of this technology to larger oxygenates will require rational design for reactor and catalyst optimization.

Microkinetic models provide necessary insights into surface chemistry that are useful for reactor design and catalyst optimization. Current methodologies for model building often center on density functional theory calculations which can yield species thermochesmistry, and, activation barriers and pre-exponential factors of elementary reactions. The use of such calculations is appropriate for smaller chemistry sets (methanol and ethanol reforming) where the total number of elementary reactions in a mechanism is manageable. As the chemistry set becomes larger (glycerol, sorbitol, etc. reforming), not only does the number of elementary reactions increase, so does the expense per DFT calculation, as the number of molecules in the system becomes larger. To overcome this computational hurdle, we have implemented the use of several semi-empirical techniques which allow for initial screening of model parameters which can later be refined via DFT.

Through the use of group additivity methods we are able to extend current models of catalytic reforming of small oxygenates to larger oxygenates. From this extension to C3 oxygenates (glycerol derivatives) we gain important understandings of the reaction pathways involved in glycerol reforming, as well as important reactions in selective conversion of glycerol to valuable chemicals.

Furthermore, previous experimental research has shown that Ni/Pt bimetallic catalysts have increased activity toward oxygenate reforming compared to that of the parent metals. For ethylene glycol, we have found that the cause of this activity is a difference in reaction path which can be traced to increased binding through oxygen for the bimetallic surface. Using this information, this work identifies atomic binding energies as descriptors for activity, and uses linear scaling relationships to identify potentially active metals.

This presentation will focus on the semi-empirical methods themselves, as well as their use in kinetic modeling of C2 and C3 oxygenate reforming. Results shown will focus on the identification of key elementary reactions through the use of reaction path analysis and sensitivity analysis.