Pore size effect of zirconium-based metal-organic frameworks for encapsulation and release of volatile monoterpenes

In this work the sorption properties of three isoreticular Zr-based MOFs, viz., UiO-66, UiO-67, and UiO-68 as adsorbents of active organics were examined and especially the pore size effects are discussed using d-limonene, myrcene, α-terpinene, and α-pinene as probe molecules. Results indicated that the terpene uptake and release are highly dependent on both the MOF pore size and the geometric feature of guest molecules themselves. The shape selective property of UiO-66 holds for the tested terpenes, where the adsorption capacity of d-limonene, myrcene, and α-terpinene in equilibrium with the liquid phase (0.43–0.46 g g−1) was approximately 5-times larger than that of α-pinene with 0.56 nm kinetic diameter (0.09 g g−1). The narrow triangular apertures of UiO-66 act as the permeation-limiting barrier for guest molecules with the dimension close to the pore size. Except for α-pinene, the loading fraction for other terpenes decreased with an increase in the MOF pore diameter. For example, the uptake of UiO-66, UiO-67, and UiO-68 for myrcene reached up to 90 %, 81 %, and 76 % of their theoretical maximum loading, respectively. Differential scanning calorimetry revealed that the melting point of both d-limonene and α-terpinene confined in the 0.71 nm pores of UiO-67 shifts to a higher temperature relative to that in the bulk phase. In contrast, no obvious phase transition was observed for the terpene composites with either UiO-66 or UiO-68 (0.94 nm pore size) under identical conditions. Consistent with the thermal analysis results, the monoterpenes entrapped in the UiO-67 showed a much lower release rate than their counterparts in the other two frameworks.