Heterogeneous Catalysis Beyond the Active Site

Metal-Organic Frameworks — Non-noble metal phosphides — Smart Supports

Accessing food, fuels, and feedstocks to supply a growing population while severing our reliance on non-renewable resources places stringent demands on the chemistry of the future. Sustainable synthesis not only places restrictions on raw materials, but it also makes high demands on reaction conditions to minimize energy consumption, formation of problematic side-products, and cumbersome separation steps.

Enzymes perform even highly complex transformations at low temperatures, without toxic reagents and in an aqueous environment, which make them an aspirational model for sustainable catalysis. While transition metals are often key to enzyme active sites, they are embedded in complex protein scaffolds that precisely regulate their environment. Our group aims to increase the precision with which both the spacing and the environment of active sites can be controlled in heterogeneous catalysts to enable new reactivity or permit the use of abundant first row transition metals.

How big is an active site?

Heterogeneous catalysis can offer substantial practical advantages, especially for large-scale reactions, but rational optimization of catalytic performance remains challenging. While homogeneous catalysis involves identical transition metal centers with a well-defined ligand set, traditional heterogeneous catalysts feature a plethora of distinct sites with distinct activity and selectivity. Since metal centers can interact with one another over extended distances, not only the primary coordination environment of the metal needs to be carefully controlled, but also the arrangement of metal centers throughout the catalyst. Synthetic strategies that permit selective synthesis of multiple distinct metal arrangements with precision present substantial opportunities to increase catalyst performance. We found that structural differences between materials that are removed from the active site by 8 chemical bonds or more have drastic effects on active site activity(1).

(1) Qiming Jin, Paolo Cleto Bruzzese, Alessandro Vetere, Claudia Weidenthaler, Eko Budiyanto, Mir Henglin, Nils Nöthling, Alexander Schnegg*, Constanze N. Neumann*

ChemRxiv 2024, doi: 10.26434/chemrxiv-2024-h09tl

Can metal organization unlock new pathways?

Radical intermediates can give rise to fascinating reactivity, but their generation commonly requires substantial energy input. We are interested in enabling energy-efficient radical transfer reactions that do not require the formation of high energy free radical intermediates. We found that Rh(II) porphyrin centers that are site-isolated within a MOF matrix make it possible to assemble a tri-component transition state that comprises Rh(II), a silane and an olefin and leads to the formation of a C-Si bond. The MOF catalyst was thus able to catalyze olefin hydrosilylation at room temperature, while a molecular analogue proved ineffective even at 140 °C (1,2). Metal-organic frameworks (MOFs) provide a pore environment that can be engineered with atomic-level precision which can be leveraged to stabilize new intermediates and transition states.

(1) Zihang Qiu+, Hao Deng+, Constanze N. Neumann* Angew. Chem. Int. Ed. 2024, 63, e202401375 doi.org/10.1002/anie.202401375

(2) Zihang Qiu+, Paolo Cleto Bruzzese+, Zikuan Wang+, Hao Deng+, Markus Leutzsch, Christophe Farès, Sonia Chabbra, Frank Neese, Alexander Schnegg*, Constanze N. Neumann* ChemRxiv 2024 10.26434/chemrxiv-2024-dpqvv-v2

Can Ni match Pd in catalysis?

Catalysts based on noble metals such as palladium still constitute to performance benchmark for many catalytic transformations. Especially for the formation and use of hydrogen - the central pillars of a future hydrogen economy - efficient catalysts are needed that are based on more sustainable metals. We are interested in the extent to which metal phosphides containing non-noble transition metals such as nickel can match the performance and operational convenience of noble metal based catalysts. We found that a simple, safe and economical synthesis of supported metal phosphides optimizes metal phosphide dispersion over a wide range of supports and simultaneously furnishes the metal phosphide with ligands that protect their surface from air oxidation. The air-stable metal phosphide Ni2P/Al2O3 delivers activity on par with the Pd-based benchmark catalyst in alkyne semi-hydrogenation (1).

(1) Leila Karam, Christophe Farès, Claudia Weidenthaler, Constanze N. Neumann Angew. Chem. Int. Ed. 2024, 63, e202404292 doi.org/10.1002/anie.202404292

Smart Supports for Selective Reactions

Extensive research efforts are devoted to honing catalytically active materials to deliver the best possible performance. Whatever shape, size and composition of the active catalyst is selected, however, the majority of heterogeneous transition metal catalysts require a support material, such as carbon, alumina or silica. Despite the fact that the support material makes up a large fraction of the total weight of the active catalyst, there are comparatively few good options from which to select. Common materials such as carbon, alumina, silica or ceria can offer high stability to temperature, high surface areas and some variation in the polarity and basicity of the support surface. Pairing an active catalyst with a suitable support material is crucial for maximizing the stability and activity of catalyst.

We are interested in the design of support materials that not only lead to long-lived highly active catalysts, but take an active role in controlling the outcome of a catalytic transformation. Given the ubiquitous nature of catalyst support materials in heterogeneous catalysis, the development of novel materials that enhance the intrinsic selectivity of the catalytically active material can impact a wide range of catalytic processes.

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