MFI zeolites stabilize rhodium in hydroformylation by combining nanoscale confinement with chemical anchoring. Their ten-membered-ring channels and cages restrict Rh motion, orient incoming olefins and limit sintering, while framework oxygen, silanol nests, phosphate or Zn–O motifs coordinate the metal and tune its electron density. The result is not one universal Rh complex but a condition-dependent family of confined sites whose geometry, accessibility and oxidation state determine activity and linear-versus-branched aldehyde selectivity.
What MFI confinement changes at a rhodium site
Hydroformylation requires a rhodium center to bind and rearrange several molecules in sequence: an olefin approaches, inserts into a Rh–hydride bond, carbon monoxide inserts into the resulting Rh–alkyl, and hydrogenolysis releases an aldehyde. A homogeneous Rh complex can change its coordination sphere during that cycle, but an unanchored Rh atom or cluster on a solid surface can also migrate, aggregate and become inactive. MFI zeolites address both problems at once.
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MFI structures such as silicalite-1 and ZSM-5 contain intersecting ten-membered-ring channels and cages. When Rh is placed inside these micropores, the pore walls form a physical boundary around the metal. The available space limits the size and movement of Rh species, making it harder for isolated atoms or subnanometre clusters to meet and sinter. At the same time, the channel gives an approaching olefin a repeatable trajectory. This geometric control can replace part of the ligand control normally provided by phosphines.
The confinement is chemically active rather than an inert container. Framework oxygen atoms, silanol defects and intentionally introduced phosphate or Zn–O groups can bind Rh. Those contacts hold the metal at defined positions, polarize adsorbed CO and olefins, and alter the balance between reduced and oxidized Rh states. Because the Rh–O bonds can lengthen, shorten or exchange ligands during turnover, the working catalyst remains dynamic while the pore prevents uncontrolled aggregation.
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Why confinement favors linear aldehydes
Regioselectivity is set when the olefin inserts into a Rh–hydride or related Rh intermediate. In an open environment, several approaches may be accessible, producing both linear (normal, n) and branched (iso, i) aldehydes. An MFI channel narrows those possibilities. Its sinusoidal passages and cage intersections can favor an orientation in which the terminal carbon remains positioned for formation of the linear aldehyde.
A Nature Catalysis study (2024) reported linear-to-branched aldehyde ratios as high as 400 for C6–C12 alpha-olefins over subnanometre Rh clusters confined in MFI channels. That result demonstrates the potential of pore shape to impose selectivity normally associated with a carefully designed molecular ligand. It does not mean every Rh-MFI catalyst reaches a ratio of 400: the value depends on Rh nuclearity, pore environment, substrate, loading and operating conditions.
Internal and external Rh sites can behave very differently. In an Inorganic Chemistry Frontiers study (2026), Rh@S-1 converted 1-hexene at 98.3%. When bulky 2,5-dimethylbenzenethiol (DMBT) was used to poison accessible external Rh selectively, the reported n/i regioselectivity increased to 59.9 under the study’s low-1-hexene-concentration condition. The experiment separates the contribution of micropore-confined Rh from less selective external species; it is not a universal selectivity value for all Rh@S-1 preparations.
How the framework anchors and electronically tunes Rh
Silanol nests and framework oxygen
Defect sites known as silanol nests contain neighboring Si–OH groups created when framework atoms are missing or hydrolyzed. These hydroxyls can coordinate Rh-containing species and also enrich olefin molecules close to the metal. The local increase in olefin concentration helps compensate for the low reactant concentration inside a micropore, while the defect prevents Rh from freely migrating across the silica surface.
A Nature Communications study (2023) reported an approximately 50,000 h−1 turnover frequency for a silanol-rich MFI-supported Rh system. The figure is a study-specific measurement, not a cross-study benchmark: turnover-frequency definitions, substrate concentration, gas composition, temperature, pressure and Rh loading must be checked before comparing it with another catalyst.
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Even defect-free framework oxygen can provide anchoring contacts. Rh bound to several framework oxygens has less freedom to diffuse than a weakly adsorbed Rh carbonyl. Those oxygen contacts also influence the electron density at Rh, changing how strongly CO, hydrogen and the olefin bind. The optimum is a balance: anchoring must be strong enough to prevent leaching and sintering but labile enough to let the coordination sphere change during insertion and hydrogenolysis.
Phosphate-containing MFI
Phosphorus can be introduced when a phosphonium template is converted to phosphate within the MFI environment. The resulting Rh–(O)–P-MFI motif stabilizes isolated Rh3+ sites and supplies a more ligand-like electronic environment than silica alone. In the cited low-temperature ethylene hydroformylation work, Rh2O3-derived sites were more effective than reduced Rh0-derived sites, indicating that the productive state was not simply metallic Rh.
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Under carbon monoxide and hydrogen, an initially prepared Rh species can change oxidation state and coordination. Rh0 and Rh2O3 precursors were reported to evolve toward species described as Rh(I)(CO)2 or Rh3+, depending on the surrounding environment. Phosphorus-containing sites preferentially stabilized oxidized species associated with productive turnover in that study. The active state therefore has to be identified under reaction gas, not inferred solely from the calcined catalyst.
Zn–O anchoring
Co-encapsulated zinc offers a second route to stabilize isolated Rh. During an induction period, Zn can generate Rhδ+–(O–Zn)x motifs. These mixed metal–oxygen contacts are designed to hold Rh in the pore, reduce leaching and discourage aggregation. They also create an electronically distinct Rh site, so performance reflects both physical confinement and the chemical identity of the anchor.
The active site is a dynamic confined ensemble
It is misleading to assign every MFI catalyst a single formula such as “Rh1+ in a zeolite cage.” Preparation history, defect density, Rh loading and syngas exposure produce different nuclearities and oxidation states. Reported systems include atomically dispersed Rh, subnanometre clusters, and larger particles located partly or wholly outside the micropores.
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Once CO and H2 are introduced, Rh can rearrange. Carbonyl ligands occupy coordination positions, hydrogen can generate Rh–H species, and olefin insertion changes the Rh–carbon framework. A site that is isolated in the fresh solid may therefore become a Rh(I)(CO)2-like species, an oxidized Rh3+ center, or a small cluster under working conditions. The MFI wall and its oxygen-containing defects constrain these transformations without freezing the catalyst into one immutable structure.
Operando spectroscopy and complementary microscopy are consequently essential. Ex situ oxidation-state assignments can miss the species that actually bind olefin and CO. The most defensible description is a family of Rh–O, Rh–P–O or Rh–O–Zn motifs whose nuclearity and coordination evolve during the catalytic cycle.
Access to the site determines whether stabilization helps
Micropore confinement improves control only if reactants can reach the Rh center and products can leave. Conventional MFI crystals can impose long diffusion paths, particularly for C6–C12 olefins and aldehydes. Nanosized crystallites shorten those paths and expose a larger fraction of internal sites. A Journal of the American Chemical Society study (2025) reported about a threefold increase in specific activity for a nanosized Rh-MFI catalyst compared with the corresponding larger-crystal design.
Self-pillared MFI (SPP-MFI) adds interlayer mesopores to the intrinsic micropores. In an Angewandte Chemie International Edition study (2026), Rh1@SPP-MFI contained Rh1.5+–O4 units confined in zeolite cages. Flexible Rh–O bonds supplied the coordination adaptability needed for turnover, while the interlayer mesopores improved access. For 1-octene at 353 K and 2.5 MPa, the reported turnover frequency was 7074 h−1. That number belongs to the stated substrate and pressure-temperature window and should not be compared directly with a gas-phase or solvent-based result using another normalization.
Designing for access involves a trade-off. Opening the structure with mesopores or very small crystals can reduce diffusion resistance, but excessive external surface may create unconfined Rh particles. The useful architecture preserves a high fraction of internal, anchored sites while providing short transport paths.
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Distinct MFI-supported Rh designs
| Design | Stabilizing feature | Reported evidence | What it shows |
|---|---|---|---|
| Silanol-rich MFI | Silanol nests coordinate Rh and enrich olefin near the metal | Approximately 50,000 h−1 TOF (Nature Communications, 2023) | Defects can combine anchoring with local reactant concentration |
| Rh–(O)–P-MFI | Phosphate from a phosphonium template stabilizes isolated Rh3+ | Rh2O3-derived sites outperformed reduced Rh0 sites in low-temperature ethylene hydroformylation | Framework phosphorus can select a productive oxidation-state environment |
| Rh@S-1 internal sites | Atomically dispersed Rh inside silicalite-1 micropores | 98.3% 1-hexene conversion; n/i reached 59.9 after DMBT poisoning of external Rh (Inorganic Chemistry Frontiers, 2026) | Internal and external Rh make different regioselectivity contributions |
| Subnanometre Rh-MFI clusters | Sinusoidal MFI channels restrict cluster geometry | Linear/branched ratio up to 400 for C6–C12 alpha-olefins (Nature Catalysis, 2024) | Pore shape can impose very strong linear selectivity |
| Rh1@SPP-MFI | Rh1.5+–O4 units in cages plus interlayer mesopores | 7074 h−1 for 1-octene at 353 K and 2.5 MPa (Angewandte Chemie International Edition, 2026) | Mesopores can improve access without abandoning cage confinement |
| Rh–Zn MFI | Rhδ+–(O–Zn)x motifs form during induction | Designed to limit Rh leaching and aggregation; a directly comparable activity figure was not stated | A second metal can provide both anchoring and electronic modification |
How a confined Rh cycle proceeds
- Olefin enrichment and approach: The olefin diffuses through an MFI channel. Adsorption at silanol-rich or polar framework regions can raise its local concentration next to Rh, and the channel restricts the approach angle.
- Rh–hydride or hydrogen-derived intermediate: H2 reacts with the working Rh carbonyl environment to generate hydrogen-containing Rh species. The exact oxidation state and coordination number depend on the anchor and syngas conditions.
- Olefin insertion: The confined geometry favors one orientation of insertion. In suitable MFI channels this biases formation of the linear Rh–alkyl pathway over the branched alternative.
- CO insertion: Carbon monoxide coordinates and inserts into the Rh–alkyl bond. Framework oxygen, phosphate or Zn–O contacts help maintain a defined pocket while allowing Rh–O bonds to flex as coordination changes.
- Hydrogenolysis and aldehyde release: Hydrogen converts the acyl intermediate to an aldehyde. Product desorption and diffusion through the pore restore an open coordination site.
- Site regeneration: The Rh center returns to a carbonyl/hydride-capable state. If anchoring is too weak, the center can migrate or aggregate; if the pore is inaccessible, the cycle is transport-limited instead of kinetically controlled.
How to compare two Rh-MFI catalysts fairly
A higher turnover frequency or a larger linear/branched ratio is meaningful only when the measurement context is matched. Use the following checklist before drawing a conclusion:
- Rh nuclearity: Is the material atomically dispersed, a subnanometre cluster or a nanoparticle? These forms have different numbers of accessible Rh atoms and different confinement requirements.
- Anchoring chemistry: Record whether the site is attached through silanol nests, framework oxygen, phosphate, Zn–O or another motif. “Rh on MFI” alone is not a sufficient description.
- Morphology and transport: Note crystal size, self-pillared or mesoporous architecture, and whether the reported rate is affected by diffusion.
- Reaction medium: Compare substrate chain length, concentration, solvent or gas phase, CO/H2 ratio, pressure and temperature.
- Rate normalization: Check whether TOF uses total Rh, exposed Rh, surface Rh or another denominator. A figure reported per exposed atom cannot be treated as equivalent to one reported per total loading.
- Selectivity definition: Confirm whether n/i is measured at the same conversion and substrate concentration; external Rh can change the ratio even when internal sites are identical.
- Durability: Examine evidence for Rh leaching, migration, sintering and coke. Short tests do not establish long-duration stability.
The published activity values summarized here are not directly interchangeable. The studies use different olefins, pressures, temperatures, phases, Rh loadings and normalization methods. A useful comparison therefore reports the full condition set beside every number rather than ranking catalysts by TOF alone.
Common failure modes and what they indicate
High conversion but weak linear selectivity
This pattern can indicate a large population of external Rh or pore openings that do not impose a consistent approach geometry. Selective poisoning experiments, microscopy and probe-molecule adsorption can help distinguish external from internal contributions. Increasing the fraction of confined, anchored Rh is more targeted than simply adding more metal.
Good initial activity followed by decline
Possible causes include Rh migration and aggregation, leaching from weakly bound sites, pore blockage by strongly adsorbed species or carbon deposits, and transport changes as products accumulate. A fresh-catalyst oxidation-state measurement cannot identify which mechanism dominates; post-run Rh distribution and operando observation are needed.
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Over-confinement can make the active pocket inaccessible to bulky olefins or slow aldehyde removal. Large crystals lengthen diffusion paths. Nanosized MFI or self-pillared architectures address transport, but only if the additional external area does not create a new population of unconfined Rh particles.
Different results from Rh0 and Rh2O3 precursors
The precursor can determine the initial dispersion and the oxygen environment, but it is not necessarily the final active state. Under CO/H2, both precursors may transform. The phosphate-containing example shows why precursor choice and framework chemistry must be interpreted together rather than treating metallic Rh as automatically optimal.
What remains unresolved
The strongest evidence supports confinement and anchoring as complementary stabilization mechanisms, but several questions remain material-specific. Operando characterization is needed to establish which Rh coordination state dominates at a given temperature, pressure and syngas composition. The balance between isolated atoms and small clusters is also not universal: a cluster can provide high activity while a single atom can deliver a distinct selectivity, depending on the pore and anchor.
Industrial relevance requires evidence beyond model alpha-olefins. Commercial feeds contain mixtures, impurities and longer-chain components that can compete for pores or promote deactivation. Long-duration operation, regeneration, coke tolerance, Rh recovery and scale-up of nanosized or self-pillared crystals each require separate testing. Reported laboratory TOFs and selectivities establish what MFI confinement can accomplish; they do not by themselves prove durability or economic viability at plant scale.
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The most reliable design principle is to engineer three features together: a pore geometry that selects the desired olefin orientation, an oxygen-containing anchor that keeps Rh isolated yet kinetically flexible, and a crystal morphology that allows rapid reactant and product transport. Silanol nests, phosphate and Zn–O motifs offer different ways to tune the metal, while nanosized and self-pillared MFI structures improve access. Because Rh changes under CO/H2, the decisive test is the identity and stability of the working site—not merely the structure of the fresh catalyst.
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