Porous and Stable Zn-Series Metal–Organic Frameworks as Efficient Catalysts for Grafting Wood Nanofibers with Polycaprolactone via a Copolymerization Approach

Abstract

A hydrothermal method was used to synthesize two highly stable Zn(II) metal-organic frameworks (MOFs), namely, [Zn2(L)2(HIPA)]n (1) and [Zn9(L)6(BTEC)3(H2O)4·6H2O]n (2) (HL = 3-amino-1H-1,2,4-triazole, H2HIPA = 5-hydroxyisophthalic acid, H4BTEC = benzene-1,2,4,5-tetracarboxylic acid). The physicochemical properties of 1 and 2 were characterized using a range of analytical techniques. The scanning electron microscopy images confirmed the stability of the MOFs under heating at 120 °C for 12 h. Following their preparation, the two MOFs were used as catalysts in the grafting of poly(ε-caprolactone) on wood nanofibers (WNFs) by means of a homogeneous ring-opening polymerization protocol in an ionic liquid. The grafting ratio achieved using catalyst 1 was higher than that achieved for catalyst 2, wherein a maximum of 92.43% was obtained using the former. Under comparable reaction conditions, the grafting ratio of 1 was found to be significantly higher than those achieved using 4-dimethylamino pyridine, Sn(Oct)2, and UiO-67 catalysts. In addition, fluorescence emission was detected from the residual catalysts present in the products. The calculated electrostatic potentials and average local ionization energies indicated that the grafting of ε-caprolactone on the WNFs follows a "coordination-insertion" mechanism. Overall, these two new and efficient MOF catalysts have the potential to replace highly toxic traditional catalysts in polymerization reactions. The grafted cellulose material with fluorescence emission may also be suitable for use in biomedical applications.