Date of Award
Open Access Dissertation
College of Engineering and Computing
Due to the rapidly declining fossil fuel reserves and the onset of global climate change, the development of active, selective, and stable catalytic materials for the efficient production of biomass derived platform chemicals, i.e., levulinic acid (LA), succinic acid (SA), γ-valerolactone (GVL) etc., is receiving considerable attention in order to produce second generation biofuels and commodity chemicals. Though solid lignocellulosic biomass is significantly cheaper than petroleum; however, the available technology is the significant barrier for large scale utilization of biomass for the production of biofuels. The development of biomass conversion technology requires, 1) identification of potential biomass chemicals that can selectively be transformed into targeted molecules, 2) understanding fundamental bond breaking/formation mechanism (i.e. C-H, O-H, C-C bond cleavage and formation) at solid/gas and solid/liquid interface, and 3) identification of active site.
Among the ‘top 10’ platform biomass chemicals identified by the Department of Energy (DOE), GVL is of particular interest because of its widespread application as a gasoline blender and in the production of bio based polymers. Understanding the reaction kinetics governing the aqueous phase hydrodeoxygenation (HDO) of levulinic acid (LA) to γ-Valerolactone (GVL) over Ru surfaces will expedite the design of better catalysts for this conversion process considering that Ru/C catalyst is the most used catalysts for the HDO of LA. In this dissertation, we report a computational investigation of the reaction mechanism of LA to GVL using DFT calculations and mean-field microkinetic modeling in both vapor and liquid phase reaction conditions.
In vapor phase calculations, our model predicts a dominant reaction route that propagates through the alkoxy formation step leading to the formation of a five member ring structure which is subsequently followed by a C-OH cleavage to form GVL. This pathway deviates from the previously proposed mechanism that involves formation of 4-hydroxypentanoic acid (HPA). At low temperature region (T<373 K), our model identifies that at a vapor phase condition Ru(0001) is not the experimentally observed active sites, while at high reaction temperatures (T > 423 K), Ru(0001) constitutes the majority of the active site. At high temperatures (T> 473 K), our model also confirms the experimental observation that α-angelica lactone (AGL) formation pathway is responsible for mild reversible catalysts deactivation. Next, our liquid phase results indicate that polar solvents (i.e. water) have a beneficial effect on the reaction kinetics of the hydrodeoxygenation of LA. Specifically, in an aqueous phase condition and 323 K reaction temperature, reaction rate is 4-5 order of magnitude higher in comparison to the rate at gaseous phase condition which explains the low temperature activity found in experimental studies. In contrast, non-polar solvents (1,4-dioxane) have a detrimental effect on the reaction kinetics, as also confirmed by several experimental studies, due to high solvent coverage on the Ru surface. In addition, our results also show that Ru (0001) is highly active for the hydrogenation of LA to its corresponding alcohol product, 4-Hydroxypentanoic acid (HPA), at a high reaction temperature above 373 K. However, at a low reaction temperature (T < 373 K), the hydrogenation rate is significantly slower than the measured experimental kinetics, even in the presence of an aqueous environment. Considering furthermore that the hydrogenation of various short chain ketones (acetone, butanone-2, and pentanone-2) over Ru (0001) also lead to reaction rates much smaller than the rate predicted by experiments, we conclude that Ru (0001) is not the active site at low temperatures (T<373K) for HPA production, owing to the high activation barrier for the second C-H bond formation, i.e., alkoxy hydrogenation step. To identify the active Ru site for the experimentally observed low temperature activity for HPA formation, we performed a constrained thermodynamics study and identified surface oxygen species as a possible active site for the hydrogenation of LA. A computational investigation of the OH-assisted reaction pathway confirms the presence of a low energy pathway for the hydrogenation of LA to HPA in an aqueous environments.
Mamun, O.(2017). Theoretical Investigation of the Catalytic Hydrodeoxygenation of Levulinic Acid Over Ru (0001) Catalyst Surface. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/4269