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The Catalysis Program
Alexis T. Bell
Robert G. Bergman
Christopher J. Chang
Michelle C. Chang
Kenneth N. Raymond
T. Don Tilley
F. Dean Toste
The goal of the LBNL Catalysis Program is to gain the fundamental knowledge about the synthesis, reactions, and mechanisms of catalysts and catalytic reactions to convert the field of catalysis from empiricism to design. The research program is built around the three pillars of catalysis:
- the design and synthesis of new catalyst structures,
- the design and deciphering of new mechanisms and methods to study catalytic mechanisms,
- the application of these new catalysts to transformations that will enable efficient energy utilization and chemical synthesis, and reduce the waste produced and energy used by chemical processes.
NEW CATALYST STRUCTURES Our research on new catalysts focuses on
- multimetallic, tailor-made nanoparticles
- methods to create catalysts within confined environments
- systems that combine homogeneous and heterogeneous catalysis
1) Multimetallic, tailor-made nanoparticles create the ability to control reactivity and selectivity.
The activity and selectivity of nanoparticulate catalysts can be tuned by altering the average particle size and the shape of the particles and by alloying a second metal with the first. Additional increases in catalytic performance can be achieved by modifying the support on which metal nanoparticles are dispersed and by decorating the surface of the nanoparticles with metal oxide moieties. We are developing methods to prepare nanoparticle catalysts with uniform composition, size and shape and to control and exploit the interface between metals and metal oxides.
2) Confined catalytic environments provide an ability to select reactants based on size and shape, to mimic the acceleration of reaction rates seen in enzyme active sites by positioning the reactants of interest appropriately in three-dimensional space, and to devise multistep reactions occurring with incompatible catalysts and reagents. Our program exploits several different environments.
Figure I.2.9. Development of an enantiopure supramolecular
M4L6 assembly (2) by incorporation of a terephthalamide ligand
containing chiral point groups.
a. We are exploring the use of supramolecular M4L6 assemblies for encapsulating and manipulating the behavior of reactants. Having demonstrated their effectiveness in controlling reaction outcomes, we will extend our studies to look at control of stereochemistry and catalysis by metal centers within the confined environment.
b. We are inserting novel organometallic centers into enzyme scaffolds, thereby taking advantage of the reaction scope of the metal sites and the ability of the protein to tune and control reactivity of both complex molecules and small hydrocarbons.
|Figure I.2.12. Metal swap for the generation of new porphyrin based artificial metalloenzymes for selective C-H halogenation.|
|Figure XX. Carbon dioxide reduction catalysts |
were incorporated into the sponge-like crystals
of covalent organic frameworks (COFs)
c. We are creating catalytic metal-organic frameworks that marry the catalytic function of a transition-metal coordination sphere and the substrate capture achieved by these frameworks. A particular focus is being placed on reactions of small molecules, such as carbon dioxide.
3) Systems that combine homogeneous and heterogeneous catalysts create an opportunity to draw from the benefits of both classes of catalyst, and utilize immobilization strategies in the pursuit of new concepts, mechanisms and efficiency in catalysis. We are incorporating molecular catalytic centers into a support, specifically into various inorganic supramolecular cages or vessels, which provides a well-defined catalytic center in a protected environment for electro- and photocatalytic reduction of protons and carbon dioxide.
NEW CATALYTIC MECHANISMS. Our research on new reaction mechanisms focuses on
- the activation of typically unreactive bonds
- interactions beyond the first coordination sphere
- methods to study catalytic reactions in operando
1) We are studying new elementary reactions involving the activation of typically unreactive bonds.
- We investigate, with molecular systems, the mechanisms of elementary reactions that occur in catalytic reactions. These studies focus on key reactions that occur in the functionalizations of saturated and unsaturated hydrocarbons, including the mechanisms of the cleavage of typically unreactive X–H bonds and methods to enhance the reactivity and selectivity of molecular catalysts for the cleavage of X-H bonds.
- We are also investigating new insertion reactions of alkenes to form carbon-oxygen and carbon-nitrogen bonds in processes that will convert alkenes derived from petroleum or renewable sources into value-added products. This work features new iridium-catalyzed additions of N-H and O-H bonds across alkenes via migratory insertion of olefins into Rh-X and Ir–X bonds (X=N, O).
- New metal-oxo complexes that cleave C-H bonds, and new systems activated by Lewis acid binding to a remote site on a ligand are under investigation.
|Figure II.1.8. Lewis acid activation of a platinum aryl-alkene complex.|
2) Several projects examine the mechanistic effects of interactions beyond the first coordination sphere to expand the factors that can be used to control catalyst activity and to draw lessons from enzymatic catalysis for the design of small-molecule and hybrid catalysts that react with high selectivities. These multi-faceted studies include molecular, heterogeneous, and enzymatic catalysis.
3) New methods to study catalytic reactions in operando allow the principles of surface science, spectroscopy, and imaging to be applied to true catalytic systems.
- New instrumental methods are being developed, many based on the soft X-ray emission techniques available at the Advanced Light Source at LBNL. In situ XAS, NEXAFS, EXAFS (K-edges) and soft XES (L and M edges) are being utilized with pressure and temperature controlled in situ cells for studies of gas-solid interfaces, gas-solid and liquid-solid catalytic systems, and constrained organic or mesoporous oxide environments. Providing particular insight are combined data sets from 2 or 3 X-ray methods applied to the same system.
- In situ techniques are being extended to provide high spatial resolution of flow reactions and molecular level mapping of catalytic reactions by 1 nm nanoparticles, to understand their catalytic selectivity and to identify the active sites and reaction intermediates. In situ vibrational methods are applied to systems in solution and on surfaces, by FT-IR and Raman. Sum Frequency Generation vibrational spectroscopy based on femtosecond laser sources provides broad infrared monitoring at gas-solid and liquid-solid interfaces.
NEW CATALYTIC TRANSFORMATIONS. The Catalysis Program targets transformations catalyzed by novel structures by novel mechanisms resulting from the investigations just described, as well as catalytic transformations that raise mechanistic questions and lead to the need for new structures. Our research applying these catalysts and mechanisms to catalytic transformations relevant to energy and synthetic efficiency will focus on
- New reactions by molecular catalysts that occur by at typically unreactive strong bonds
- catalytic reactions occurring in confined spaces
- multifunctional systems that catalyze reaction cascades.
1) The discovery of catalytic reactions that cleave strong bonds will facilitate the development of processes such as alkane coupling, biomass conversion and utilization of basic chemical feedstocks containing carbon-carbon multiple bonds. Novel, selective intermolecular N-H and O-H additions to alkenes are under investigation and the highly selective anti-Markovnikov hydroarylation of both terminal and internal alkenes is under further, more extensive development.
|Figure III.1.6. Anti-Markovnikov hydroarylations by a novel C-H bond cleavage |
and functionalization mechansim.
2) Catalytic reactions that occur in confined spaces bridge homogeneous and heterogeneous catalysis. The development of new reactions occurring in confined spaces will allow small molecule reactivity to achieve the selectivity of enzymatic systems and could lead to new control over reaction rates and increased catalyst compatibility.
This research examines reactions of organic molecules within nanovessels (a homogeneous confined environment) ranging from supramolecular host-promoted reactions, assembly-catalyzed photochemical transformations and cyclization reactions catalyzed by encapsulated transition metal catalysts. Supramolecular assemblies that allow catalytic reactions to occur in water and stabilize the guest species will be exploited to develop tandem reactions with biological catalysts.
|Figure III.2.9. Gold catalyzed rearrangements that may be achieved in larger hosts 3 and 4.|
3) Multifunctional catalytic systems will, as in nature, allow catalytic reactions to be linked together in cascades. Such systems can accommodate unstable intermediates and avoid the energy and waste created by separations at each step of a synthetic sequence, while allowing incompatible catalysts or highly reactive catalysts to function together. Our research aims to place active sites within environments that restrict or encourage the transfer of intermediates or inhibitors to and from multiple functions and that stabilize specific transition states.
We are designing catalysts to conduct tandem, incompatible reactions. A current project entails heterogeneous catalysts in which hydrophobic or hydrophilic barriers allow the placement of catalytic sites at locations accessible only to polar or non-polar reactants, products, or intermediates, to prevent deleterious contact of molecules in cascade reactions. Catalysts for systems including alcohol dehydration/alkene oligomerization, alkene metathesis/alkene epoxidation, alkene hydroformylation/aldol condensation are targeted.
|Figure III.3.4. Two types of nanostructure catalysts. |
Structure II represents an inorganic, mesostructured
silica nanoparticle (MSN) coated by a thin polysiloxane
film, a strong barrier to water. Structure III is a highly
condensed silicone nanosphere possessing a covalently
embedded Ru metathesis catalyst.
In each of these projects, the design of the components of a catalytic system, the design of experimental methods, and the design of new ways to deduce catalytic mechanisms is central. By focusing on the principles of catalyst design, our research efforts will help tip the balance in catalysis science away from empiricism, toward design.