**2.3. Magnetically recoverable superparamagnetic γ-Fe2O3 nanocatalyst**

Efficient recovery of the catalysts in PET glycolysis is an important aspect that several researchers have attempted to address recently. Among the recoverable catalysts studied are various ionic liquids, which were shown to provide molar yields up to 80% [36–39]. Aside from yield, there are several issues that should be considered in using ionic liquids as industrial glycolysis catalysts such as cost, stability, and robustness with respect to process variables such as moisture content, to which ionic liquids are very sensitive [10,11,40]. Urea was also reported to be a reusable glycolysis catalyst at mild temperatures [41]. Vacuum distillation was used to recover the catalyst, however, whose high energy requirements can be counterproductive.

We have studied magnetic nanoparticles as a recoverable glycolysis catalyst, among which γ-Fe2O3 was chosen as excellent candidate being known to have good performance in a number of reactions [42–44]. This was the first attempt to utilize magnetic nanomaterials in PET depolymerisation. Nanosized γ-Fe2O3 was selected due to its stability, high catalytic activity, and superparamagnetic property [42]. The superparamagnetic behavior allows recovery by application of a magnetic field yet allows good dispersion in the reaction medium, as it has

**Figure 9.** Low-magnification TEM images of (a) GO, (b) GO-Mn3O4, and (c) high-resolution TEM image of GO-Mn3O4 showing the d-spacing of the (101) crystal plane of Mn3O4 and its diffraction rings [11].

**Figure 10.** BHET yields for bare Mn3O4 and GO-supported Mn3O4 catalysts.

The formation of GO-Mn3O4 nanocomposite was verified by various characterization methods such as XRD, XPS, and Raman spectroscopy. TEM images of pristine GO and the obtained composite are shown in Figures 9a and 9b, indicating the coverage of the GO surface. The highresolution TEM image in Figure 9c shows the lattice fringes and diffraction pattern of the Mn3O4 crystal structure. Compared to the silica-supported composites and conventional metal salt catalysts [10,35], the monomer yield using the GO-Mn3O4 nanocomposite was comparable or higher, reaching more than 90% (Figure 10). The yields for the composite were all above 90%, showing improvement from that of bare Mn3O4 at 83%. However, the Mn3O4 without the support aggregated into micron scale. The GO support could prevent the aggregation of

**Figure 8.** Schematic illustration of ultrasound-assisted synthesis of the GO-Mn3O4 composites. MnO4 is first reduced to MnO2 and precipitated onto the GO support by oxidizing carbon. The reduction of MnO2 to Mn3O4 then takes place in

Efficient recovery of the catalysts in PET glycolysis is an important aspect that several researchers have attempted to address recently. Among the recoverable catalysts studied are various ionic liquids, which were shown to provide molar yields up to 80% [36–39]. Aside from yield, there are several issues that should be considered in using ionic liquids as industrial glycolysis catalysts such as cost, stability, and robustness with respect to process variables such as moisture content, to which ionic liquids are very sensitive [10,11,40]. Urea was also reported to be a reusable glycolysis catalyst at mild temperatures [41]. Vacuum distillation was used to recover the catalyst, however, whose high energy requirements can be counterproductive.

We have studied magnetic nanoparticles as a recoverable glycolysis catalyst, among which γ-Fe2O3 was chosen as excellent candidate being known to have good performance in a number of reactions [42–44]. This was the first attempt to utilize magnetic nanomaterials in PET depolymerisation. Nanosized γ-Fe2O3 was selected due to its stability, high catalytic activity, and superparamagnetic property [42]. The superparamagnetic behavior allows recovery by application of a magnetic field yet allows good dispersion in the reaction medium, as it has

Mn3O4 and provide an enlarged and stable active sites [11].

150 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

the following steps.

**2.3. Magnetically recoverable superparamagnetic γ-Fe2O3 nanocatalyst**

zero remanent magnetization. Iron oxides have further advantages being cheap, nontoxic, and abundant [12].
