The invention of an exceptional catalytic material for effective heterogeneous oxo-synthesis – rhodium-zinc (RhZn) intermetallic nanoparticles (iNPs) – was described in a paper published in the Journal of the American Chemical Society.
Study: Intermetallic Nanocatalyst for Highly Active Heterogenized oxo-synthesis. Image Credit: Billion Photos/Shutterstock.com
It is difficult to develop a heterogeneous catalyst exhibiting significant selectivity and activity in oxo-synthesis (hydroformylation), yet it is vital to guarantee the easy recovery and recyclability of valuable catalysts.
RhZn surfaces exhibit low oxo-synthesis initiation energy thresholds and weaker binding affinity of reaction intermediates than pristine rhodium, resulting in far more favorable reaction thermodynamics on RhZn. The researchers also ran simulations to forecast alternative catalyst design techniques for achieving high regioselectivity.
What is Oxo-synthesis?
The creation of carbon-carbon bonding during the oxo-synthesis of alkenes to yield C+1 aldehydes is a critical chemical reaction.
Ever since its discovery, oxo-synthesis has grown to be one of the substantial catalysis processes in the chemical sector, producing over ten million tons of product each year.
During the oxo-synthesis process, the formyl group is typically added on one of the two sides of the alkene double bond, resulting in a combination of linear and branched aldehydes, with the linear being favored. Several attempts have been made to develop catalysts with increased regioselectivity and activity levels for oxo-synthesis.
Organometallic rhodium structures with different triphenylphosphine ligands, in particular, have shown good potency and linear aldehyde selectivity.
Despite being extensively researched and very effective, these homogeneous catalysts have low recyclability, which limits their applicability given the expensive cost of Rh. As a result, heterogenized catalysts are seen as a possible option for the oxo-synthesis process.
Single-Atom Catalysts
Metallic rhodium nanoparticles (NPs), both supported as well as unsupported, have indeed been investigated as options for heterogeneous oxo-synthesis and have been shown to be functional on a wide range of alkene targets. Unfortunately, in comparison with homogeneous catalysts, their catalytic performance and linear aldehyde regioselectivity are poor.
With the fresh discovery of single-atom catalysts (SACs), individual rhodium atoms anchored on ZnO and CoO have been discovered to be incredibly active for oxo-synthesis.
Previous research has shown that SACs may effectively bridge heterogenized and homogenized catalysis. But manufacturing SACs with significant metallic loading and ensuring stability at elevated heat levels continue to be difficult.
Intermetallic Nanoparticles & the Importance of Site Isolation
Intermetallic nanoparticles (iNPs) are one-of-a-kind catalytic substrates noted for their remarkable catalytic activity in a variety of chemical reactions. Inert metals modify the structural and electrical properties of active metallic locations (e.g., Pt/Pd/Rh) such that molecule adsorption behavior differs dramatically from that of comparable monometallic substrates.
This action allows for good environment regulation of the catalytic active region and is similar to SACs as far as site isolation is concerned.
Site isolation also increases the diffusion threshold of hydrogen atoms on intermetallic surfaces, which improves specificity in the pairwise hydrogenation of alkynes and alkenes. Furthermore, the intermetallic complexes are thermodynamically stable, making them resistant to being deactivated by catalysis due to high-temperature sintering, carbon monoxide poisoning, or coking.
These properties may make Rh-based iNPs ideal for heterogenized oxo-synthesis.
While extensive effort has been made in the research of formation mechanism and characteristics of iNPs, Rh-based intermetallic phases have gotten much less focus than platinum or palladium-based intermetallic phases.
While this may be due to the costly nature of rhodium and its less prevalent use than Pt- or Pd-based catalysis, basic research on Rh catalysts is absolutely essential so that they can be applied to particular chemical reactions such as oxo-synthesis.
Key Findings of the Research
RhZn iNPs supported on mesoporous silica (SBA-15) were described as a very active heterogenized catalyst in the oxo-synthesis process in this study.
RhZn iNPs outperform monometallic rhodium NPs and the standard homogeneous Wilkinson's catalyst, RhCl(PPh3)3, employing styrene as a model substrate.
A substrate scope investigation reveals that RhZn iNPs have outstanding activity in the oxo-synthesis of a wide range of alkenes. The RhZn/SBA-15 catalyst can be recycled and is typically functional for a wide variety of alkene targets, indicating that the heterogenization of oxo-synthesis process was effective.
Density functional theory (DFT) analyses of the thermodynamics and kinetics of oxo-synthesis were undertaken to clarify the thorough reactivity of a number of low-energy RhZn intermetallic surfaces, revealing that the intermetallic RhZn framework weakens surface intermediates' binding affinity while lowering initiation energy thresholds, favoring a fast reaction.
Temperature-regulated CO-desorption research confirmed RhZn iNPs' decreased surface CO adsorption. The findings of this study not only set the tone for the use of heterogenized oxo-synthesis to commercial chemical fabrication, but also bring to light the logical design of well-defined heterogenized catalysts to bridge homogeneous and heterogeneous catalysis.
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Reference
Chen, M., Gupta, G., et al. (2021) Intermetallic Nanocatalyst for Highly Active Heterogeneous Hydroformylation. Journal of the American Chemical Society. Available at: https://pubs.acs.org/doi/10.1021/jacs.1c09665
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