*3.2. Catalyst Characterization*

The active phase thermal stability of the selected catalyst was evaluated by thermogravimetric analysis (TGA) with the aim of determining whether the Keggin structure would remain at 300 ◦C or not, since it is the calcination temperature of the catalyst that achieved optimal performance. TGA profile shown in Figure 2 revealed that the main weight loss (approx. 7%) occurred until 208 ◦C. The DTG (derivative thermogravimetry) curve revealed four main stages of weight loss. In the first stage, corresponding to peaks of 75 and 120 ◦C, loss of water physically adsorbed in the material is observed. In the second stage, corresponding to a peak of 208 ◦C, crystallization water loss of the solid structure was observed, thus forming an anhydrous acid (Equation (3)). In the third stage, corresponding to a peak of 305 ◦C, acidic proton loss and anhydride structure formation was observed (Equation (4)). Finally, in the fourth stage, corresponding to a peak of 545 ◦C, the Keggin structure starts

decomposing (Equation (5)). According to Kozhevnikov et al. [28] and Alsalme et al. [29], this stage occurs at approximately 600 ◦C. Therefore, it can be concluded that the HPW Keggin structure remains after calcination at 300 ◦C. However, the Keggin structure of the HPW catalyst calcined at 500 ◦C is close to its decomposition temperature, which explains the low yield observed for this catalyst.

$$\text{H}\_3\text{[PW}\_{12}\text{O}\_{40}\text{]} \text{nH}\_2\text{O} \to \text{H}\_3\text{[PW}\_{12}\text{O}\_{40}\text{]} + \text{nH}\_2\text{O},\tag{3}$$

$$\rm H\_3[PW\_{12}O\_{40}] \to \rm [PW\_{12}O\_{38.5}] + 1.5H\_2O,\tag{4}$$

$$\rm I\ [PV\_{12}O\_{38.5}] \rightarrow 0.5P\_2O\_5 + 12\,\text{WO}\_3.\tag{5}$$

**Figure 2.** Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) analysis of HPW samples.

The crystalline structures of the support (Nb2O5), active phase (HPW), and catalyst (HPW/Nb2O5) calcined at 300 ◦C were evaluated by X-ray powder diffraction (XRD) (Figure 3). The XRD pattern of the HPW active phase showed typical diffraction peaks of a HPW Keggin structure at 10.3◦, 20.7◦, 23.1◦, 25.4◦, and 29.5◦ [30,31]. Such result indicates that the Keggin structure has not been decomposed into WO3 after calcination at 300 ◦C, which could occur for the active phase calcined at 500 ◦C. This result is in agreemen<sup>t</sup> with the thermogravimetry analysis results. The XRD pattern of the support (Nb2O5) revealed an amorphous structure, with broad and diffuse diffraction peaks, which is characteristic of Nb2O5 calcined below 500 ◦C [32,33]. Finally, the XRD pattern of the HPW/Nb2O5-300 ◦C catalyst was similar to that of the support with an amorphous characteristic, and exhibited no diffraction peak connected with HPW. This result indicates that HPW was well dispersed on the support surface, with no active phase agglomerations.

The HPW/Nb2O5/300 ◦C catalyst morphology and structure, and HPW dispersion over the niobium pentoxide surface was investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDX). As it can be seen in Figure 4, it is composed of non-uniform crystals with an irregular surface and particles of various sizes. It was not possible to identify a predominant crystalline structure in this catalyst, which was expected, since Nb2O5 calcined at 300 ◦C exhibited an amorphous characteristic. According to the X-ray emission mapping of niobium and tungsten (Figure 4c,d), it is noted that the heteropolyacid was highly dispersed on the support surface, with no active phase agglomeration, which is in agreemen<sup>t</sup> with the XRD pattern obtained for the catalyst.

**Figure 3.** X-ray powder diffraction (XRD) patterns of the HPW active phase, Nb2O5 support, and HPW/Nb2O5 catalyst calcined at 300 ◦C.

**Figure 4.** Scanning electron micrographs of HPW/Nb2O5-300 ◦C catalyst at (**a**) 200× magnification (scale bar: 500 μm), (**b**) 2000× magnification (scale bar: 30 μm), and X-ray emission mapping of niobium (**c**) and tungsten (**d**) obtained at 2000× magnification (scale bar: 40 μm).

Textural and acidity properties of the niobium support and selected catalyst (HPW/Nb2O5-300 ◦C) were also analyzed. As depicted in Table 3, the support showed values of surface area and volume of pores greater than those obtained for the catalyst HPW/Nb2O5/300 ◦C, since HPW impregnation onto the support leads to a massive reduction in these parameters. This occurs because the HPW Keggin structure is dispersed within the support pores (Nb2O5), causing a decrease in average pore volume and surface area. Decreased surface area of the catalyst HPW/Nb2O5/300 ◦C can be understood as an evidence of the chemical interaction between HPW and the support. Surface acidity is an important property which affects catalyst performance in the reaction and may be indicative of an impregnation success. HPW/Nb2O5/300 ◦C showed acidity of 106.98 μmol <sup>H</sup>+/m2, i.e., much higher than that of Nb2O5 used as support.


**Table 3.** Textural and acidity properties of Nb2O5 and HPW/Nb2O5 catalyst.
