Chiral 2D Framework Turns Rhodium Catalyst Into a Recycling Champion

Chemists have long faced an awkward trade-off at the heart of asymmetric catalysis. Homogeneous catalysts, in which every active metal complex floats freely in solution, deliver exquisite control over the handedness of the products they form, but recovering those precious metal complexes from the reaction mixture is often a messy, lossy business. Heterogeneous catalysts, by contrast, can be filtered off and reused indefinitely, yet their rigid solid surfaces frequently scramble the delicate three-dimensional interactions that make a chiral catalyst selective in the first place. A study published in Catalysis Letters by Xiao Feng, Chengzhong Huang, Baogang Yang and Yanling Han now reports a design that appears to sidestep this dilemma almost entirely, by anchoring one of the most celebrated chiral ligands in chemistry onto a framework only a few atoms thick.

The material at the center of the work is a two-dimensional metal-organic framework, or 2D MOF, built from zirconium nodes and the tritopic linker 1,3,5-tris(4-carboxyphenyl)benzene, known as H3BTB. These ultrathin crystalline sheets have attracted intense interest in recent years because of three properties that heterogeneous catalysis has historically lacked: atomic-level thickness, ultrahigh specific surface area and a population of active sites that are, by construction, almost fully exposed. Instead of burying catalytic centers deep inside a bulk solid where reactants struggle to reach them, a 2D MOF displays them across its flat surfaces, closer in spirit to the environment a soluble complex enjoys than to that of a conventional solid pellet.

What the team did was to functionalize this scaffold with derivatives of BINAP, the chiral bisphosphine ligand whose discovery helped launch modern asymmetric catalysis and earned a Nobel Prize in 2001. BINAP is what chemists call a privileged ligand: its rigid, twisted binaphthyl backbone imposes a defined chiral pocket on any metal it coordinates, steering reactions toward one mirror-image product over the other. In the new work, BINAP-derived ligands were immobilized within the 2D MOF structure, and the framework was then metallized with rhodium, the same metal that powers some of the most important industrial and pharmaceutical asymmetric transformations. The resulting material, dubbed Zr-BTB-L-Rh, constitutes a well-defined heterogeneous asymmetric catalyst in which the rhodium active species are atomically dispersed rather than clustered into nanoparticles.

That atomically dispersed character proved to be more than a structural nicety. Powder X-ray diffraction, scanning and transmission electron microscopy, atomic force microscopy, inductively coupled plasma-mass spectrometry and high-angle annular dark-field scanning transmission electron microscopy were among the techniques used to confirm that the hierarchical porous architecture survived the functionalization and metallization steps intact, with individual rhodium sites distributed throughout. In other words, the catalytic centers inside the framework sit in an environment that resembles a solution-phase complex far more closely than a metal surface, which is why the authors describe the setting as quasi-homogeneous.

The benchmark chosen to test the concept was the asymmetric Hayashi-Miyaura reaction, in which an arylboronic acid adds across 2-cyclohexen-1-one to form a carbon-carbon bond with defined chirality. This reaction is a mainstay of asymmetric synthesis, and a rhodium-BINAP combination is its classical catalyst system, so the comparison with the molecular version was direct and demanding. The heterogeneous Zr-BTB-L-Rh delivered yields above 99 percent and enantioselectivities above 99 percent, matching the selectivity thresholds that chemists usually regard as the gold standard for a practical asymmetric process. Producing essentially one mirror image of the product from a recoverable solid is precisely the outcome that decades of heterogenization efforts have been chasing.

The turnover number, a measure of how many product molecules a single catalytic site can produce, tells an even more striking story. At low catalyst loading, the framework-bound rhodium achieved a turnover number up to 99 times that of its homogeneous counterpart. This is not merely a modest improvement in efficiency; it means each active site in the MOF does nearly two orders of magnitude more chemical work before being retired. The likely explanation lies in the same combination of features the framework provides: every rhodium site is exposed, reactants can diffuse efficiently through the hierarchical pores, and the solid support stabilizes the metal species against the aggregation and decomposition pathways that often cap the lifetime of soluble complexes.

Recyclability, the original motivation for heterogenizing catalysts in the first place, held up as well. The team recovered the catalyst after each run and reused it across five consecutive cycles, during which both activity and enantioselectivity remained stable. For a chiral catalyst, maintaining enantioselectivity through repeated use is the harder of the two demands, because even small structural changes to the chiral pocket, such as ligand oxidation or metal leaching, typically show up first as erosion of product handedness. The stable performance across five runs suggests that the coordination environment inside the MOF remains chemically intact, with the rhodium staying bound to its BINAP-derived ligands rather than drifting out of the framework.

The broader significance of the study lies in its position within a growing effort to build catalytic materials that erase the boundary between homogeneous and heterogeneous regimes. Previous work has explored phosphine-functionalized MOFs, chiral porous hybrid solids for asymmetric hydrogenation, and BINAP-based frameworks for enantioselective cyclization, all pursuing the same idea of a solid material that behaves like a precisely engineered molecule. The new contribution shows that the two-dimensional geometry in particular, with its maximal site exposure and quasi-homogeneous microenvironment, can push that idea to quantitative performance levels that rival, and in turnover terms dramatically exceed, the molecular catalysts that inspired it.

Practically, the design points toward asymmetric synthesis routes in which the valuable rhodium and the even more valuable chiral ligand are capital equipment rather than consumables. In pharmaceutical manufacturing, where enantioselective steps are ubiquitous and metal residues in products are tightly regulated, a catalyst that combines near-perfect selectivity with easy separation and reuse could meaningfully change the economics of a process. The authors report no competing interests, and the structural characterization for the study was supported by the Analysis and Testing Center of Xuzhou University of Technology.

Challenges remain before such frameworks become routine tools. Scaling the synthesis of atomically thin MOF nanosheets, verifying long-term stability across far more than five cycles under real process conditions, and extending the platform beyond the Hayashi-Miyaura benchmark to the full breadth of rhodium-catalyzed asymmetric reactions are all open questions. But the demonstration that a 2D MOF can host BINAP-rhodium sites at 99 percent enantioselectivity with a hundredfold boost in turnover is a clear signal that the next generation of chiral catalysts may be less like dissolved molecules and more like engineered crystal sheets.

Subject of Research: Heterogeneous asymmetric catalysis using BINAP-functionalized 2D metal-organic frameworks with atomically dispersed rhodium active sites

Article Title: BINAP-Functionalized 2D Metal-Organic Frameworks for Heterogeneous Rh Catalysts

Article References: BINAP-Functionalized 2D Metal-Organic Frameworks for Heterogeneous Rh Catalysts. (n.d.). https://doi.org/10.1007/s10562-026-05528-9

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05528-9

Keywords: asymmetric catalysis, heterogeneous catalyst, 2D metal-organic frameworks, BINAP, rhodium, Hayashi-Miyaura reaction, enantioselectivity, turnover number, zirconium MOF, quasi-homogeneous catalysis, chiral ligand immobilization, catalyst recycling