Unmasking true confinement effects: ultrahigh linear selectivity and chain-length oscillatory behavior in zeolite-encapsulated rhodium hydroformylation

Hydroformylation of higher α-olefins with high linear selectivity is hampered by rapid isomerization and lack of steric control in conventional heterogeneous catalysts. Here, we selectively passivate external rhodium (Rh) sites on a zeolite-encapsulated Rh@MEL catalyst using bulky 2,4-dimethylbenzenethiol (DMBT), producing Rh@MEL-DMBT that exposes only micropore-confined single Rh sites within 5.3 × 5.4 Å MEL channels. This eliminates interference from unrestricted surface Rh, enabling unambiguous probing of true shape-selective hydroformylation. Rh@MEL-DMBT delivers exceptional performance for C5–C12 α-olefins, achieving linear-to-branched (l/b) ratios up to 600, >95% aldehyde chemoselectivity, and significant suppression of isomerization (<5% internal olefins) observed for most substrates within this range. Strikingly, activity and aldehyde selectivity exhibit sinusoidal oscillation with increasing chain length, directly linked to resonant diffusion of olefins in toluene-filled pores revealed by molecular dynamics simulations. In situ Fourier-transform infrared spectroscopy (FTIR) confirms encaged [HRh(CO)2] and [HRh(CO)] intermediates, while kinetic isotope effect (KIE = 1.2) and pressure-dependent kinetics rigorously identify CO insertion as the rate-determining step. These results establish the first experimentally validated molecular mechanism of zeolite-confined single-site Rh hydroformylation and unveil diffusion-controlled chain-length-dependent regioselectivity in confined catalysis.