Metal-organic frameworks, MOFs are advanced porous coordination polymers that are highly attractive for research and applications involving selective bonding of various polar and non-polar molecules. MOFs contain metal sites and organic linkers with distinct molecular units in them. This is of interest to understand molecular mechanism and kinetics of interactions of small molecules with “host” materials, and MOFs constitute a famous class of the latter. The commonly used methods of molecular spectroscopy often lack the capability of in-situ and time dependent measurements of reactions of intact “host” compounds under ambient or controlled environments. The limitations of existing techniques of vibrational spectroscopy are particularly significant when weak bonding is investigated, such as non-specific Van der Waals interactions of nonpolar molecules, and their evolution in the time domain. Recently, our lab reported a new method of in-situ time-dependent ATR-FTIR spectroscopy for revealing molecular mechanisms of “host-guest” interactions.<br/>We present the further investigation of new capabilities of this powerful high-resolution spectroscopic method to reveal fine detail of interactions of nonpolar “guest” molecule with “host” of complex structure. This is a study of molecular mechanism and kinetics of sorption and desorption of pentane as archetypal nonpolar molecule, to probe specific sites in the MOF (compound 1). This “host” material contains aluminum cations and porphyrin linker of complex structure.<br/>Compound 1 reversibly sorbs and desorbs pentane vapor in ambient air, forming sorption “host-guest” complex compound 2. As revealed by in-situ time-dependent ATR-FTIR spectroscopy, the following functional groups in the “host” interact with “guest” molecules. The moderately polar phenyl groups are directly involved into weak nonspecific interactions that cause distinct spectral shifts. The highly polar hydroxyl groups show spectral shifts due to an indirect effect, the change of crystal lattice upon “host-guest” interaction. Chemical kinetics of this “host-guest” reaction follows the pseudo-first order rate law as found by in-situ time-dependent ATR-FTIR spectroscopy and confirmed by the complementary in-situ time-dependent gravimetry. The novel technique of in-situ time-dependent ATR-FTIR spectroscopy is highly promising in studies of mechanism and kinetics of various "solid-gas" reactions under ambient as well as controlled environments.

Revealing capabilities of novel method of in-situ time-dependent ATR-FTIR spectroscopy: Mechanism and kinetics of reaction involving weak molecular interactions of pentane with porphyrin aluminum MOF

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ACS Fall 2022

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