*2.2.1. Ultrasound-assisted catalyst synthesis*

Ultrasound has been used in metal deposition, dish cleaning, chemical reactions, and particle dispersion in a solvent [21]. The cavitation phenomenon includes formation, growth, and collapse of bubbles generated in the aqueous solution under ultrasonic irradiation. The phenomenon causes both high temperature (~5000 K) and pressure (~1000 atm) during the sequence of bubble collapse. In a short span of time for bubble formation, it provides very high heating and cooling rate. This unique condition generates intense energy that converts water into H and OH radicals, promoting the formation of metal nanoparticles [22].

Our group applied the ultrasound irradiation method for the fabrication of metal-doped silica nanocomposites [8,11,23,24]. The acoustic cavitation phenomenon facilitates interparticle collision between metal and support material, inducing the binding between metal and support [25,26]. As a result, metal doped on supporting material can be prepared without surfactants or surface modification of support material in a short reaction time and mild reaction conditions.

## *2.2.2. Ultrasonic deposition of metal oxide catalyst on silica particles*

Silica microparticles (SMPs) and silica nanoparticles (SNPs) were used as support for the catalysts. Silica microparticles with size 1–20 μm were purchased from Junsei Chemicals and used without any treatment. Nanosized silica particles were synthesized using the Stöber method, with some modifications. In a sealed round-bottomed flask, 8.0 ml of ammonium hydroxide (28 wt%) and 6.0 ml of deionized water were added to 100 ml of ethanol and stirred for 15 min. Then, 4.7 ml of the silica precursor tetraethyl orthosilicate (TEOS) were added to this solution and stirred at room temperature for 3 h. The resulting precipitate was centrifuged and washed with water and ethanol several times. The product was dried in an oven at 70°C for 8 h followed by calcination at 500°C for 12 h.

To synthesize the silica-supported manganese oxide or zinc oxide catalysts, a predetermined amount of silica support was added to a 1.0 M solution of the precursor [Mn(NO3)2 xH2O or

Zn(NO3)2 6H2O]. The metal oxide loading was set to be 15 wt%. Using a horn-type sonifier, the solution was sonicated for 45 min. A 0.1 M ammonia solution was added at the start of the sonication step in order to keep the pH at around 9.5. The ultrasound-assisted deposition process is illustrated in Figure 5. After separating the particles by centrifugation, they were washed with water and ethanol. The catalyst samples were then dried at 100°C for 8 h and calcined at 350°C for 3 h.

**Figure 5.** Overall synthesis procedure of metal-doped silica nanoparticle.

**2.2. Supported metal oxide composites via ultrasound-assisted synthesis**

146 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

reaction time, simplify reaction steps, or perform synthesis in milder conditions.

into H and OH radicals, promoting the formation of metal nanoparticles [22].

*2.2.2. Ultrasonic deposition of metal oxide catalyst on silica particles*

for 8 h followed by calcination at 500°C for 12 h.

Ultrasound has been used in metal deposition, dish cleaning, chemical reactions, and particle dispersion in a solvent [21]. The cavitation phenomenon includes formation, growth, and collapse of bubbles generated in the aqueous solution under ultrasonic irradiation. The phenomenon causes both high temperature (~5000 K) and pressure (~1000 atm) during the sequence of bubble collapse. In a short span of time for bubble formation, it provides very high heating and cooling rate. This unique condition generates intense energy that converts water

Our group applied the ultrasound irradiation method for the fabrication of metal-doped silica nanocomposites [8,11,23,24]. The acoustic cavitation phenomenon facilitates interparticle collision between metal and support material, inducing the binding between metal and support [25,26]. As a result, metal doped on supporting material can be prepared without surfactants or surface modification of support material in a short reaction time and mild

Silica microparticles (SMPs) and silica nanoparticles (SNPs) were used as support for the catalysts. Silica microparticles with size 1–20 μm were purchased from Junsei Chemicals and used without any treatment. Nanosized silica particles were synthesized using the Stöber method, with some modifications. In a sealed round-bottomed flask, 8.0 ml of ammonium hydroxide (28 wt%) and 6.0 ml of deionized water were added to 100 ml of ethanol and stirred for 15 min. Then, 4.7 ml of the silica precursor tetraethyl orthosilicate (TEOS) were added to this solution and stirred at room temperature for 3 h. The resulting precipitate was centrifuged and washed with water and ethanol several times. The product was dried in an oven at 70°C

To synthesize the silica-supported manganese oxide or zinc oxide catalysts, a predetermined amount of silica support was added to a 1.0 M solution of the precursor [Mn(NO3)2 xH2O or

*2.2.1. Ultrasound-assisted catalyst synthesis*

reaction conditions.

Metal oxides find important application as catalysts, but agglomeration during their synthesis reduces the active surface area beneficial for catalysis. The combination of solid support and metal has been proposed, since composite materials can isolate the nanoparticle on the supporting material and effectively reduce its size. As a result, such nanocomposites can increase the surface area of metal oxide catalyst [14–16]. Silica, one of the most widely used catalyst supports, is synthesized in this study in the nanoscale and used as a metal oxide support material. Another material considered as support for nanocomposites is graphene oxide (GO). It is compatible with various organic/inorganic nanomaterials, taking advantage of possible chemical modification utilizing the oxygen-containing functional group on its sheet. In addition, it has high chemical stability and specific surface area [17–20]. The synthesis of silica and graphene oxide-based catalysts is presented, where ultrasound was used to reduce

> The properties of the synthesized silica-supported catalysts are given in Table 2. At 1.0 wt% catalyst-to-PET loading, glycolysis was performed at 300°C for 80 min, after which the product monomer was recovered. Shown in Figure 6 are the FT-IR spectra of the recovered BHET crystals and standard BHET sample. The matching spectra confirm that the product obtained using the silica-supported catalyst was indeed the monomer. The spectrum showed the presence of peaks corresponding to the functional groups in BHET: an -OH band at 3,424 and 1,128 cm−1, an aromatic C-H at 1,456–1,502 cm−1, C-O at 1,712 cm−1, and alkyl C-H at 2,879 and 2,964 cm−1 [10].

> From the monomer yield versus reaction time data in Figure 7, the order of catalytic activity can be determined as Mn3O4/SNPs>ZnO/SNPs>Mn3O4 SMPs>ZnO/SMPs. This trend fol‐ lows the same order as the catalyst surface area and the pore volume given in Table 1. Among the four catalyst samples with different size of support and metal oxide doping, the large amount of active surfaces in the nanoparticle support and the activity of the Mn3O4 cata‐ lyst could be responsible for the fastest reaction rate and the maximum monomer yield [10]. Although it is evident that using a silica nanoparticle support could improve the catalytic performance, it also has its drawbacks in the practical and industrial perspective. Using a metal oxide catalyst is desired for easier purification of the glycolysis products, but effi‐ cient separation of a nanosized catalyst can be challenging. A trade-off between catalytic performance and practical applicability will be inevitable, unless an effective method to separate the catalysts is provided.

**Figure 6.** Comparison of FT-IR spectra of standard BHET and BHET recovered from catalyzed glycolysis. Figure from [10].


**Table 2.** BET surface area, pore volume, and average pore diameter of silica-supported metal oxide catalysts [10]

## *2.2.3. Sonochemical synthesis of the GO-Mn3O4 composites*

Mn3O4 and its nanocomposites have been utilized as highly effective catalysts for various applications, including PET glycolysis as demonstrated in our previous studies. Some of the known advantages of manganese oxides include excellent catalytic activity, low cost, abun‐ dance, and being environmental benign [27–30]. Depending on the oxidation states of man‐ ganese, there are several forms of MnOx (e.g. MnO2, MnO, Mn2O3, and Mn3O4), each of which has different applications. Hausmannite (Mn3O4), with both Mn2+ and Mn3+ ions in its crystal structure, has been widely used in catalytic applications. When loaded onto a support to provide a large surface area and prevent aggregation, it could significantly enhance the depolymerization of PET [10].

**Figure 7.** Molar yield of BHET at 300°C and 1.1 MPa using silica-supported catalysts. Figure from [10].

**Catalyst BET surface area (m2**

148 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

[10].

*2.2.3. Sonochemical synthesis of the GO-Mn3O4 composites*

depolymerization of PET [10].

**/g) Pore volume ( cm3**

ZnO/SMPs 2.49 0.014 21.4 Mn3O4/SMPs 3.38 0.020 24.4 ZnO/SNPs 22.44 0.154 30.2 Mn3O4/SNPs 45.09 0.214 18.9

**Figure 6.** Comparison of FT-IR spectra of standard BHET and BHET recovered from catalyzed glycolysis. Figure from

**Table 2.** BET surface area, pore volume, and average pore diameter of silica-supported metal oxide catalysts [10]

Mn3O4 and its nanocomposites have been utilized as highly effective catalysts for various applications, including PET glycolysis as demonstrated in our previous studies. Some of the known advantages of manganese oxides include excellent catalytic activity, low cost, abun‐ dance, and being environmental benign [27–30]. Depending on the oxidation states of man‐ ganese, there are several forms of MnOx (e.g. MnO2, MnO, Mn2O3, and Mn3O4), each of which has different applications. Hausmannite (Mn3O4), with both Mn2+ and Mn3+ ions in its crystal structure, has been widely used in catalytic applications. When loaded onto a support to provide a large surface area and prevent aggregation, it could significantly enhance the

**/g) Average pore diameter (nm)**

Simultaneous formation and direct deposition of MnOx nanostructures have been reported by using redox reaction and electrodeposition [21,29–31]. One of the most favorable methods to do this is the reduction of permanganate ions into the insoluble manganese dioxide induced by carbon such as that in a graphene structure. The procedure is simple and the reaction has a self-limiting character [27,28,32]. This method can also be adapted to deposit Mn3O4 onto graphene through thermal reduction of MnO2 over 1000°C [16]. This is an energy-intensive process, over which alternative methods of synthesis using milder conditions would be preferable [21,33]. In our work, sonochemical methods were used to facilitate mild conditions for synthesis and reduce reaction time involved in Mn3O4 deposition as illustrated in Figure 8.

Using the modified Hummers method, graphene oxide (GO) was prepared from graphite flakes [34]. One gram of graphite was added to 50 mL of concentrated H2SO4 in an ice bath. Then, 3.5 g of KMnO4 were added and stirred for 2 h at 35°C. The suspension was then kept at 98°C to which deionized (DI) water was added dropwise. Then, 25 mL of 3% H2O2 aqueous solution were poured into the mixture and filtered with a 0.1 mm pore diameter Anodisc™ membrane. The product was washed with 10% HCl aqueous solution and DI water. By applying ultrasound to the filtered graphite oxide cake suspended in DI water, exfoliated GO was obtained and subsequently dried. Dispersion of GO in DI water at 0.5g/mL concentration was prepared. Mixtures of 10 mg/mL KMnO4 and the GO dispersion at varied volume ratios of 0.01, 0.03, and 0.05 were used for the synthesis of GO-Mn3O4 composite samples A, B, and C, respectively. These are then subjected to ultrasonication at a power of 80 W/cm for 30 min using a horn-type sonicator. The resulting suspension was then filtered and washed with DI water and then ethanol [11].

**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 the following steps.

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