**3.4 Spinning-nuclear magnetic resonance equipment (MAS-NMR)**

**Figure 3** shows the 27Al MAS-NMR spectrum for the C3 catalyst before calcination. One can observe a broad band with a maximum at around 0 ppm, that can be ascribed to the formation of the Al(OH)3 (gibbsite) compound. This species has low electronegativity and, therefore, it can interact more easily with more electronegative elements [31], such as molybdenum species, rather than with cobalt species. This is relevant for the preparation method, considering that the latter has a tendency to form cobalt aluminate with the support when using impregnation methods [3, 32, 33].

**Figure 4** displays the 27Al MAS-NMR spectra for the calcined catalysts. The CoMoind sample exhibited a broad band at 0 ppm that can be assigned to octahedral species of the support. Besides, formation of tetrahedral species was determined as a weak band at around 60 ppm. These two bands are characteristic of γ-Al2O3 [34, 35]. It is likely that during calcination up to 250°C, the gibbsite formed boehmite (α-AlO(OH)). This compound was transformed to γ-Al2O3, when the temperature reached 500°C [36].

Additionally, **Figure 4** shows that pentahedral species between 40 and 50 ppm appeared for the C5 and CoMoind catalysts. These species have been identified as defects in the support structure, as originated by the replacement of oxygen in the network of octahedral symmetry by hydroxyl groups [37]. For C5 sample, a band around −14 ppm was detected. This band has been related to the presence of the Al2(MoO4)3 species which distorts the octahedral network of the support [35].

Some authors have reported that the Al2(MoO4)3 species can be due to the dissolution of the Al3+ species, which subsequently react with heptamolybdate complexes during the impregnation step, forming the Anderson-type heteropolymolybdate [Al(OH)6Mo6O18] <sup>3</sup><sup>−</sup> [35]. In this study, the appearance of the molybdate species (around −15 ppm) could be due to the formation of an Anderson-type heteropolymolybdate obtained in the synthesis mixture.
