Porous Organic Microenvironment Unlocks p-d Orbital Hybridization to Enable Precise Semi-Hydrogenation of Alkynol:...

Achieving high selectivity in alkyne semi-hydrogenation without relying on toxic modifiers remains a major challenge in heterogeneous catalysis. In this work, we have explored the design of a series of heteroatom-functionalized porous organic polymer (POP)-supported Pd catalysts (S-Pd-POP, O-Pd-POP, and N-Pd-POP) to systematically tune catalytic behavior for semi-hydrogenation of 2-methyl-3-butyn-2-ol (MBY) through p-d orbital hybridization. These materials were synthesized via a scalable free-radical polymerization approach followed by Pd incorporation, yielding an amorphous porous network with a distinct electronic microenvironment. Notably, S-Pd-POP contains ultrasmall and well-dispersed Pd nanoparticles (∼2.5 nm), significantly smaller than those in O-Pd-POP (∼6.5 nm) and N-Pd-POP (∼8.5 nm), leading to greater exposure of active sites. Synchrotron X-ray absorption fine structure (XAFS) analysis confirms Pd exists predominantly in a metallic state, while EXAFS results reveal a clear decrease in Pd-Pd coordination numbers from O-Pd-POP to N-Pd-POP to S-Pd-POP, consistent with progressively smaller Pd clusters. We have achieved a good catalytic activity with S-Pd-POP with 95.3% conversion and 97.2% selectivity under mild conditions (60 °C, 5 bar H2), along with a lower activation energy (24.6 kcal mol-1) than its counterparts. Comprehensive density functional theory calculations validate experimental findings, which show strong overlap between S(p) and Pd(d) orbitals near the Fermi level, indicating pronounced p-d orbital hybridization, which is further supported by significant sulfur-to-Pd electron interaction donation from natural bond orbital analysis. In contrast, nitrogen- and oxygen-based systems show much weaker electronic interactions, corroborating their hydrogenation activity data. In situ ATR-IR spectroscopy studies reveal that the reaction proceeds via weakly bound π-intermediates, which promote rapid product desorption and effectively suppress over-hydrogenation. Overall, this work highlights p-d orbital hybridization as a key factor governing catalytic selectivity and provides a practical strategy for designing efficient, modifier-free hydrogenation catalysts.