Research Areas
The Pacific Northwest National Laboratory Chemical Transformations Initiative (CTI) is developing capabilities needed to transform chemical conversion processes.
By using oxygenated renewable carbon sources with abundant recycled waste as a feedstock, and combining our new processes with renewable energy, we are developing the basis for producing high-density transportation fuels for aviation, shipping, and heavy trucking—with a minimal, and ultimately negative, carbon footprint—and to help secure our Nation’s energy independence.
Constraints with the carbon feedstocks require that the processes operate in highly distributed “mini-refineries.” These mini-refineries will need to operate at low temperatures and pressures, and be able to handle complex reactants.
The essential scientific challenge in these conversion processes is how to develop catalysts that are selective yet still active when operating at lower temperatures, such as those around 200 oC.
Our driving hypothesis is that we can learn from natural systems—enzymes—by abstracting principles that contribute to activity and selectivity, and apply those principles to inorganic systems offering the required site density and robustness.
To test our hypothesis, we are focusing on three research thrusts, each building a new capability in low-temperature, condensed phase catalysis within the Institute for Integrated Catalysis at PNNL:
- Electrocatalytic Hydrogen Addition
- Acid-Base Catalyzed Elimination and Carbon-Carbon Bond Manipulation
- Crosscutting Integration Activities
Electrocatalytic Hydrogen Addition
Understanding Model Systems
This research aims to generate fundamental knowledge of the reductive conversion of oxygenated organic molecules on metal surfaces in the presence of electrical potential in aqueous and biphasic environments, i.e., electrocatalytic hydrogenation. In such conditions, chemical potentials of reactants on the surface are determined not only by their concentration but also by solvation effects, charge accumulation associated with electric potentials, and other components of the solution (e.g., electrolytes). In a word, correlations known in gas phase heterogeneous hydrogenation cannot necessarily be used to predict the hydrogenation activity. Why? Because of differences between interactions in condensed phase and gas phase, as well as the effects of charge on surface chemistry. Thus, we must draw new correlations between applied electric potential and surface coverage/activity in the condensed phase to provide the scientific expertise needed for energy conversion from electrical power to chemical energy.
Complex Substrates and Mixtures
This research will test five main hypotheses:
- Electrocatalytic hydrogenation of the reactive components of biocrude can proceed selectively through a proton-coupled electron transfer (PCET) as well as molecular hydrogen. The hydrogen will be provided by protons generated at the anode. Reduction sites with high over-potentials towards the evolution of molecular hydrogen will facilitate Faradaically efficient PCET.
- Through tailored electrode compositions, structures, and membrane electrode assemblies, electrohydrogenation can efficiently and economically increase the energy content in low value and/or low energy materials, transforming them to higher value and higher energy products.
- Components in the mixtures will compete for active catalyst sites. Different catalyst sites will have different activities for functional group reductions. The electrode chemical composition, architectural structure, and active sites will affect the overall rate and selectivity.
- Electrocatalytic hydrogenation can be engineered to operate at low temperatures and pressures with space-time yields exceeding thermochemical hydrogenation processes.
- The electrodes can be engineered to have a high durability and/or be regenerated in situ.
Acid-Base Catalysis
This research aims to understand fundamental aspects of condensed phase acid-base chemistry in relation to conversion of biocrude electrocatalytic hydrogenation intermediates through oxygen elimination and carbon-chain growth at enhanced rates. Specifically, we will understand the impact of
- Water on the acid-base sites and reactions
- Reactant complexity on the reaction rates
- Nano-environment of acid-base sites on their activity.
Atomic-level structure-activity relationships will be established, and the active site and its microenvironment requirements will be suggested. Our approach combines catalyst synthesis with carefully tailored acid-base sites and their nano-environments, extensive characterization, and kinetic measurements and mechanistic studies. We will use the knowledge to identify catalysts and reaction parameters that have potentials for an integrated and compatible process with electrocatalytic hydrogenation. We believe this research will advance our ability to understand, design, and control acid-base catalysts for chemical transformations of biomass-derived oxygenates to produce fuels and chemicals.
Crosscutting Integration Activities
Theory
This research aims to build predictive theory capabilities and computational tools for modeling the selective electrochemical hydrogenation of surrogate bio-oil, under low-temperature and pressure conditions, at complex electrode interfaces. The approach will deal with the heterogeneities that arise from the interface of multi-component liquids with charged surfaces under the steady-state conditions of in operando electrocatalysis. Models will include explicit atomistic representations of the multi-component liquid phase (water, ions, and organic molecules), along with the electrocatalyst (electrode support, nanoparticle, and membrane) under an applied bias, and the charge transfer between them. We have identified four ingredients that are necessary for the quantitative predictions of kinetics of electrocatalytic systems:
- Charge transfer
- Explicit representation of reactivity at solid-liquid interface
- Inclusion of an external biasing potential
- Accounting for kinetic steady states.
Electrode Synthesis
This research aims to develop a library of robust catalysts for electrochemical hydrogenation of biomass-derived oxygenates—aldehydes, ketones, and phenolic compounds present in pre-processed lignocellulose and waste oils and fats. We will synthesize heterogeneous electrocatalysts to provide hydrogen by manipulating the carbon structure and introducing new inorganic systems in them to achieve high conversion efficiency at low temperatures. This will require synthesis of new solid catalysts in which the active sites are precisely arranged in three-dimensional environments. The team will base the design of active sites and transport pathways on the principles deduced from enzymes and theoretical predictions.
Reactor and Deployment
This research has six goals listed in order of execution and increasing complexity:
- Equip the fundamental and applied subprojects with reactors that can be used to measure intrinsic rates of the two target reactions: electrochemical upgrading of the biocrude feedstock and acid-catalyzed oligomerization of intermediates.
- Devise methods to normalize the electrochemical reaction rates in situ
- Model the performance of the reactor so that larger scale flow reactors can be designed competently.
- Understand non-idealities that attend practicable levels of conversion.
- Identify opportunities to intensify the overall process to enhance yield, minimize fouling, and exploit close coupling among the individual processes to minimize capital and operating costs.
- Integrate the chemical and engineering science into a unit that can be run under conditions that will bear on its long-term behavior and will help populate techno-economic models for the overall process.