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 Å × 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 which is directly linked to resonant diffusion of olefins in toluene-filled pores revealed by molecular dynamics simulations. In situ Fourier-transform infrared spectroscopy 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.