Vapor phase catalyst design (NREL, ANL)

Overview

The mission of the VPU catalyst design team is to enable ChemCatBio to achieve more rapid advancements in catalyst formulations and design that result in experimentally observed improvements in yield, selectivity, durability, lifetime, or cost. The outcomes of collaborative computational-experimental projects lead to:

Technical Approach

All of our projects are highly integrated with core experimental activities being conducted within the ChemCatBio consortium. We utilized computational tools to collaborate on long-term (1-2 year) catalyst design efforts or to address well-defined specific challenges facing experimental teams (months). We employ first principles-based calculations (density functional theory and other accurate methods) to understand the kinetics and thermodynamics of catalytic reactions. The computational modeling include: Cluster calculations, surfaces, and large scale dynamic simulations. We utilize both in-house capabilities across the DOE national laboratory system as well as academic partners at Colorado School of Mines and Northwestern University.

The following highlights some of our recent activities regarding:

  • Surface chemistry and thermodynamics
  • Toward designing more stable catalysts
  • Zeolite chemistry and kinetics

Surface chemistry and thermodynamics

Biomass reaction kinetics and thermodynamics are influenced not only by the material choice but also by reaction conditions. It can therefore be difficult to interpret experimental observations when dealing with multiple or multifunctional reactants. Our recent work address this question with the development of Surface Phase Explorer (SPE) – an open-access online software used to visualize ab initio results for competitive adsorption under experimentally-relevant conditions. This tool is applicable to metal surfaces as well as other crystalline materials of interest to biomass catalysis, including bimetallics, oxides and carbides. Our current investigations involve using DFT to identify the relationships between multicomponent adsorption, reaction kinetics and the nature of the active site for rational design of catalytic materials in biomass processing.

Towards designing stable catalysts

We have developed molecular scale models of molybdenum carbide (Mo2C) and nickel (Ni) doped Mo2C catalyst surfaces using computations based on periodic density functional theory. Gas phase binding energies of catalyst surface sites with deoxygenation reaction intermediates (O, OH, H, and H2O) are computed. Kinetic parameters related to the formation of OH and H2O species at the clean and Ni*/Mo2C are also estimated.

Zeolite chemistry and kinetics

Zeolite-catalyzed processes play one of most promising approaches in many of biomass conversion routes including gasification and pyrolysis.

Our group is currently investigating the impact of the structure of ZSM-5 using Brönsted and Lewis (using extra-framework metals) acidic sites on the mechanisms and rates of various reactions that are important to vapor phase upgrading of pyrolysis vapors and syngas and the effects of zeolite pore size and shape on diffusivity using density functional theory (DFT) and molecular dynamics (MD) simulations.

Our key research activities include:

  • Diffusivities in both microporous and mesoporous zeolites

Due to the complex nature and speciation of pyrolysis vapors, it is imperative that a correlation be developed to relate molecular properties to diffusivity within zeolite pores. In a recent paper, we have shown that such a correlation needs to take into account both molecular size and weight. Additionally, this work also demonstrates the important effect of molecular orientation on the diffusivity at the configuration regime and both the critical diameter and the length of the longest principal axis should be considered simultaneously, however, the latter factor has usually obtained less attention in the previous literatures. In the future, the diffusivity in mesoporous H-ZSM-5 nanosheets and particle models of H-ZSM-5 catalyst will be investigated.

Diffusion of biomass pyrolysis products in H-ZSM-5 by molecular dynamics simulations, Lintao Bu, Mark R. Nimlos, David J. Robichaud, and Seonah Kim, J. Phys. Chem. C, 121, 500-510 (2017).

  • Various reaction classes (dehydration, dehydrogenation, decarbonylation, and decarboxylation) in gasification and catalytic pyrolysis

DFT calculations have been performed to study reaction mechanisms and kinetics in the micropores of zeolites for various reactions.

The dehydration of ethanol to ethylene is an important model reaction for studying dehydration occurring during the vapor phase upgrading of biomass pyrolysis vapors. This reaction is also an important first step in the production of hydrocarbons or gasoline from bioethanol. We investigated the mechanisms, thermochemistry, and kinetics of ethanol dehydration at Brønsted acid sites in HZSM-5. A concerted mechanism (~50 kcal/mol) is favored over a stepwise mechanism (54 kcal/mol) by ~ 4-5 kcal/mol lower activation energy barrier.

Ethanol Dehydration in HZSM-5 studied by Density Functional Theory: Evidence for Concerted Process, Seonah Kim, David J. Robichaud, Gregg T. Beckham, Robert S. Paton, and Mark R. Nimlos, J. Phys. Chem. A 119 (15), 3604-3614 (2015).

Converting C1 species (methanol or dimethyl ether (DME)) from syngas using zeolite catalyst into high valued fuels and chemicals have been developed from the methanol-to-hydrocarbons (MTH) process. NREL recently developed that Cu-modified BEA zeolites enables to upgrade the non-gasoline-range C4 alkane fraction via C-H bond activation and reincorporation into the chain-growth cycle. Isobutane dehydrogenation as a simple reaction using Cu-BEA zeolite catalyst was studied using the hybrid quantum mechanics/molecular mechanics (QM/MM) calculations to show the capability of activation C-H bonds in light alkanes. This process will be eventually the part of DME homologation to increase carbon efficiency from biomass to fuels.

Exploring low-temperature dehydrogenation at ionic Cu sites in beta zeolite to enable alkane recycle in dimethyl ether homologation, Carrie A. Farberow, Singfoong Cheah, Seonah Kim, Jeffrey T. Miller, James R. Gallagher, Jesse E. Hensley, Joshua A. Schaidle, and Daniel A. Ruddy, ACS Catal. 2017 (under review).

Traditional microporous zeolite suffers the problems of slow diffusivity and long diffusion length of aromatic products, thus resulting in coke formation, which in turn impairs zeolite lifetime. Incorporating mesopores into the microporous H-ZSM-5 by introducing additional diffusion pathways is a very promising approach for developing more efficient zeolites with optimized mass transfer ability.

 

Molecular dynamics (MD) simulation is power approach to investigate the dynamics of molecules in zeolites at various temperatures. The movies show the diffusion of coke precursors (top: benzene, middle: naphthalene, and bottom: anthracene) in mesoporous H-ZSM-5 nano sheet with 20 A for both the pore size and wall thickness. The simulation demonstrates that benzene molecule (top) is able to diffuse into and out of the micropore during 10 ns MD simulation. Naphthalene (middle) can also diffuse into the micropore with its longest principle axis parallel to the straight channels.

 

Due to its large size and high boiling point (613 K), anthracene (bottom) is not observed to diffuse into the micropore at 300 K. Instead, in the 3rd movie, anthracene was placed inside the micropores initially and the simulation shows anthracene can diffuse into the mesopore once it is formed in the micropore. To accelerate the diffusion process, this simulation was conducted at a higher temperature (700 K).

 

Please contact any of the following to discuss opportunities for using computational tools for researching vapor phase catalysts: