Pore-Aperture-Governed Biguanylation of NH ₂ -MIL-101(Cr) to Enhance Basicity for CO ₂ Capture and Catalysis

Pore-aperture engineering is a decisive yet underexplored parameter governing the postsynthetic modification efficiency of amine-functionalized metal–organic frameworks (MOFs). Here, a systematic isoreticular comparison between NH 2 -MIL-125(Ti) and NH 2 -MIL-101(Cr) demonstrates that enlarging the pore apertures improves guanylation. Using a mild aqueous cyanamide protocol, NH 2 -MIL-101(Cr) attains ∼33% guanylation, which is 2-fold higher than the ∼16% achievable in NH 2 -MIL-125(Ti), owing to the substantially reduced diffusional barriers across its 12–15 Å windows. This study confirms the formation of bridging biguanidyl linkages at the pentagonal windows of NH 2 -MIL-101 (Cr), which increases the number of basic sites. The improved functionalization enhances both the base catalytic activity for the Claisen–Schmidt condensation reaction and CO 2 adsorption, arising from a denser population of strong basic sites (bridging biguanidyl groups). Overall, this study establishes pore aperture-governed enhanced functionalization of amine-functionalized MOFs to increase and strengthen the basic sites, providing a mechanistic foundation for designing next-generation sorbents and solid base catalysts.