Acetalization of Glycerol with Citral over Heteropolyacids Immobilized on KIT-6

: Glycerol acetalization with citral was studied using a heteropolyacid (tungstophosphoric acid) supported on KIT-6, as a catalyst, at 100 ◦ C. Different catalysts were synthesized. Catalysts were characterized by scanning electron microscopy (SEM), inductively coupled plasma (ICP), X-ray diffraction (XRD), attenuated total reﬂetion-Fourier transform infrared spectroscopy (ATR-FTIR), and potentiometric titrations. At a ﬁxed time, the glycerol conversion increased with the H 3 PW 12 O 40 (PW) on KIT-6. PW4-KIT-6 material had a higher conversion than other catalysts. The optimization of glycerol’s acetalization with citral was studied under the PW4-KIT-6 catalyst. After 5 h, it was found that, at T = 100 ◦ C, with m = 0.3 g of solid, molar glycerol:citral = 1:2.25, the conversion of glycerol was 89%. Moreover, the PW4-KTI-6 catalyst showed good catalytic stability.

Citral is a molecule derived from biomass that is present in bio-oils and available from terpenes and terpenoids. This terpene is an aldehyde, which can be raw material to the fine chemistry industry [24][25][26]. Glycerol acetalization with citral is a process of valorization of the glycerol and citral, in a circular economy perspective. However, the compounds obtained from the acetalization of glycerol (acetals) can also be used in the fine chemical industry. Therefore, glycerol and citral are molecules with low commercial value that can be transformed into acetals with high commercial value.
Traditionally, the reaction of glycerol with ketones and/or aldehydes was carried out with homogeneous catalysts. However, homogeneous catalysts have some disadvantages, such as difficulty removing from the reaction mixture and reusing. To make this process friendlier to the environment, sulfuric acid was substituted by solid catalysts, such as zeolites, mesostructured silica, and activated carbons [27][28][29]. Glycerol acetalization with citral was carried out over a mesoporous zirconia catalyst. This material was also utilized in the glycerol acetalization with furfural and cinnamaldehyde [30].
Heteropolyacids (HPAs) are very strong acids and less corrosive than HCl or H 2 SO 4 . These materials have been used in different reactions as catalysts [31][32][33].
Heteropolyacids (HPAs) are very strong acids and less corrosive than HCl or H2SO4. These materials have been used in different reactions as catalysts [31][32][33].
In this work, the reaction of glycerol with citral over H3PW12O40 (PW) immobilized on mesostructured silica (KIT-6) is presented. Tungstophosphoric acid is immobilized on KIT-6 during silica synthesis.

Catalyst Characterization
Nitrogen isotherms of the materials are shown in Figure 1. It can be observed that all the materials show isotherms characteristic of KIT-6. The textural characterization of KIT-6 material and HPW immobilized on KIT-6 are shown in Table 1. It can be observed that surface area (ABET) and porous volume (VP) decreased when the amount of heteropolyacid immobilized on KIT-6 increased. This behavior could be due to the heteropolyacid immobilized in the framework of KIT-6, which can reduce the surface area to the adsorption sites of the N2 molecules. Guo et al. [34] and Gagea [35] observed that when the heteropolyacids were immobilized on SBA-15, the ABET and Vp decreased.   Figure 2 shows ATR-FTIR spectra of mesostructured silica (KIT-6), tungstophosphoric acid (HPW), and PW4-K-6. The heteropolyacid (HPW) displays principal IR bands, which are situated at 1080, 985, 890, and 839 cm −1 [36]. The main heteropolyacid bands in the ATR-FTIR spectra of PW4-K-6 are indicated in Figure 2. However, some bands characteristic of Keggin units are overlapped with the bands of the KIT-6. In a previous work [37], when HPW was supported on SBA-15, some major bands were also not observed. Moreover, Pizzio et al. [38] observed similar behavior. This material was also utilized in the glycerol acetalization with furfural and cinnamaldehyde [30]. Heteropolyacids (HPAs) are very strong acids and less corrosive than HCl or H2SO4. These materials have been used in different reactions as catalysts [31,32,33].
In this work, the reaction of glycerol with citral over H3PW12O40 (PW) immobilized on mesostructured silica (KIT-6) is presented. Tungstophosphoric acid is immobilized on KIT-6 during silica synthesis.

Catalyst Characterization
Nitrogen isotherms of the materials are shown in Figure 1. It can be observed that all the materials show isotherms characteristic of KIT-6. The textural characterization of KIT-6 material and HPW immobilized on KIT-6 are shown in Table 1. It can be observed that surface area (ABET) and porous volume (VP) decreased when the amount of heteropolyacid immobilized on KIT-6 increased. This behavior could be due to the heteropolyacid immobilized in the framework of KIT-6, which can reduce the surface area to the adsorption sites of the N2 molecules. Guo et al. [34] and Gagea [35] observed that when the heteropolyacids were immobilized on SBA-15, the ABET and Vp decreased.   Figure 2 shows ATR-FTIR spectra of mesostructured silica (KIT-6), tungstophosphoric acid (HPW), and PW4-K-6. The heteropolyacid (HPW) displays principal IR bands, which are situated at 1080, 985, 890, and 839 cm −1 [36]. The main heteropolyacid bands in the ATR-FTIR spectra of PW4-K-6 are indicated in Figure 2. However, some bands characteristic of Keggin units are overlapped with the bands of the KIT-6. In a previous work [37], when HPW was supported on SBA-15, some major bands were also not observed. Moreover, Pizzio et al. [38] observed similar behavior.   Figure 2 shows ATR-FTIR spectra of mesostructured silica (KIT-6), tungstophosphoric acid (HPW), and PW4-K-6. The heteropolyacid (HPW) displays principal IR bands, which are situated at 1080, 985, 890, and 839 cm −1 [36]. The main heteropolyacid bands in the ATR-FTIR spectra of PW4-K-6 are indicated in Figure 2. However, some bands characteristic of Keggin units are overlapped with the bands of the KIT-6. In a previous work [37], when HPW was supported on SBA-15, some major bands were also not observed. Moreover, Pizzio et al. [38] observed similar behavior. Figure 3I displays the XRD of the materials. KIT-6 shows one peak (110) at the 2θ region of 1 • to 1.2 • . All materials with HPW immobilized on KIT-6 show a peak at the 2θ region. This result may indicate that the mesoporous structure of KIT-6 is well maintained after the incorporation of HPW into silica. According to Ding et al. [39], the incorporation of molybdophosphoric acid in KIT-6 does not seem to impact the silica structure.  Figure 3I displays the XRD of the materials. KIT-6 shows one peak (110) at the 2θ region of 1 0 to 1.2 0 . All materials with HPW immobilized on KIT-6 show a peak at the 2θ region. This result may indicate that the mesoporous structure of KIT-6 is well maintained after the incorporation of HPW into silica. According to Ding et al. [39], the incorporation of molybdophosphoric acid in KIT-6 does not seem to impact the silica structure. Figure 3II shows the XRD of the materials at the 2θ region of 5 0 to 55 0 . It can be observed that peaks characteristic of heteropolyacid ( Figure 3II-line G) do not appear on the XRD of KIT-6 materials. This could be an indication that the particles of PW are extremely well dispersed [36].   Figure 4I) and PW4-K-6 material ( Figure 4II). From the TEM images, the KIT-6 with heteropolyacid (PW4-K-6) apparently shows some variability on the porous system, which is indicated by a white circle. A possible explanation is the interaction between HPW and KIT-6 during their formation process. In previous work, similar results were observed when tungstophosphoric acid was immobilized on SBA-15 [40].
Intensity (a.u.)  Figure 3II shows the XRD of the materials at the 2θ region of 5 • to 55 • . It can be observed that peaks characteristic of heteropolyacid ( Figure 3II-line G) do not appear on the XRD of KIT-6 materials. This could be an indication that the particles of PW are extremely well dispersed [36]. Figure 4 displays the TEM images of KIT-6 material ( Figure 4I) and PW4-K-6 material ( Figure 4II). From the TEM images, the KIT-6 with heteropolyacid (PW4-K-6) apparently shows some variability on the porous system, which is indicated by a white circle. A possible explanation is the interaction between HPW and KIT-6 during their formation process. In previous work, similar results were observed when tungstophosphoric acid was immobilized on SBA-15 [40].
The acidity measurements of KIT-6 and the materials (PW-K-6) were established by potentiometric titrations with n-butylamine ( Figure 5). It determined the Ei (initial electrode potential). The Ei of PW-KIT-6 materials increases with the PW amount supported on mesostructured silica (Table 1), according to the following: E KIT-6 = 50 mV < E PW1-K-6 = 234 mV < E PW2-K-6 = 418 mV < E PW3-K-6 = 516 mV < E PW4-K-6 = 577 mV < E PW5-K-6 = 657 mV. This behavior may be due to the increase in the amount of H + with the amount of PW on KIT-6. According to Pizzio et al. [38], when the Ei is greater than 100 mV, very strong sites are present on the surface.  The acidity measurements of KIT-6 and the materials (PW-K-6) were established by potentiometric titrations with n-butylamine ( Figure 5). It determined the Ei (initial electrode potential). The Ei of PW-KIT-6 materials increases with the PW amount supported on mesostructured silica (Table 1), according to the following: EKIT-6 = 50 mV < EPW1-K-6 = 234 mV < EPW2-K-6 = 418 mV < EPW3-K-6 = 516 mV < EPW4-K-6 = 577 mV < EPW5-K-6 = 657 mV. This behavior may be due to the increase in the amount of H + with the amount of PW on KIT-6. According to Pizzio et al. [38], when the Ei is greater than 100 mV, very strong sites are present on the surface.

Catalytic Experiments
Glycerol acetalization with citral was performed using heteropolyacids (PW) immobilized on KIT-6. Figure 6 exhibits the scheme of glycerol acetalization with citral. The products are (2-(2,6-dimethyl-1,5-heptenyl)-1,3-dioxolan-4-yl)methanol (5R acetal) and 2-(2,6-dimethyl-1,5-heptenyl)-1,3-dioxan-5-ol (6R acetal).  Figure 7 compares the initial activity of KIT-6, PW1-K-6, PW2-K-6, PW3-K-6, PW4-K-6, and PW5-K-6. When the amount of HPW supported on mesostructured silica (KIT-6) increased, the catalytic activity also increased. These results may be due to the increase in the active sites number on KIT-6 ( Table 1). When the amount of heteropolyacid increased from 10.5% (material PW4-K-6) to 15.6% (material PW-5-K-6), the catalytic activity of the PW-5-K6 material did not increase. This result may be due to the existence of internal diffusion limitations. In fact, the SBET and total porous volume decreased with the amount of heteropolyacid supported on KIT-6 (   Figure 7 compares the initial activity of KIT-6, PW1-K-6, PW2-K-6, PW3-K-6, PW4-K-6, and PW5-K-6. When the amount of HPW supported on mesostructured silica (KIT-6) increased, the catalytic activity also increased. These results may be due to the increase in the active sites number on KIT-6 ( Table 1). When the amount of heteropolyacid increased from 10.5% (material PW4-K-6) to 15.6% (material PW-5-K-6), the catalytic activity of the PW-5-K6 material did not increase. This result may be due to the existence of internal diffusion limitations. In fact, the SBET and total porous volume decreased with the amount of heteropolyacid supported on KIT-6 ( Table 1). It is likely that some active sites present on PW5-K-6 catalysts are not accessible to the reactants. Patel et al. [41] observed the same trend in the glycerol's acetalization with benzaldehyde and furfural over tungstosilicic acid anchored on MCM-48.  Selectivity to five-and six-member ring acetal is shown in Table 2. After 5 h of reaction, the PW4-K-6 catalyst exhibited 75% selectivity to the 5R acetal and 25% to the 6R-acetal at 89% glycerol conversion. All materials have great selectivity to 5R acetal, which may be due to the kinetic effects [16,17,42].
Glycerol acetalization with citral was optimized over PW4-K-6 material. The effects of catalyst loading (PW4-K-6), glycerol:citral molar ratio, and temperature of reaction on conversion and selectivity to acetals were studied. the same trend in the glycerol's acetalization with benzaldehyde and furfural over tungstosilicic acid anchored on MCM-48.   Selectivity to five-and six-member ring acetal is shown in Table 2. After 5 h of reaction, the PW4-K-6 catalyst exhibited 75% selectivity to the 5R acetal and 25% to the 6Racetal at 89% glycerol conversion. All materials have great selectivity to 5R acetal, which may be due to the kinetic effects [16,17,42]. Table 2. Glycerol conversion and selectivity to five-member ring acetal and six-member ring acetal.  Table 2 shows the selectivity to 5R acetal and 6R acetal. After 5 h of reaction, the PW4-K-6 catalyst exhibited 75% selectivity to the 5R acetal and 25% to the 6R-acetal at 89% glycerol conversion. All materials have great selectivity to 5R acetal, which may be due to the kinetic effects [16,17,42]. This material was also utilized in the glycerol acetalization with cinnamaldehyde [30]. Heteropolyacids (HPAs) are very strong acids and less corrosive than These materials have been used in different reactions as catalysts [31,32,33] In this work, the reaction of glycerol with citral over H3PW12O40 (PW on mesostructured silica (KIT-6) is presented. Tungstophosphoric acid is im KIT-6 during silica synthesis.

Catalyst Characterization
Nitrogen isotherms of the materials are shown in Figure 1. It can be ob the materials show isotherms characteristic of KIT-6. The textural character 6 material and HPW immobilized on KIT-6 are shown in Table 1. It can be surface area (ABET) and porous volume (VP) decreased when the amount of h immobilized on KIT-6 increased. This behavior could be due to the h immobilized in the framework of KIT-6, which can reduce the surfac adsorption sites of the N2 molecules. Guo et al. [34] and Gagea [35] observ the heteropolyacids were immobilized on SBA-15, the ABET and Vp decreas  Figure 2 shows ATR-FTIR spectra of mesostructured si tungstophosphoric acid (HPW), and PW4-K-6. The heteropolyacid (H principal IR bands, which are situated at 1080, 985, 890, and 839 cm −1 [ heteropolyacid bands in the ATR-FTIR spectra of PW4-K-6 are indicate However, some bands characteristic of Keggin units are overlapped with th KIT-6. In a previous work [37], when HPW was supported on SBA-15, som were also not observed. Moreover, Pizzio et al. [38] observed similar behav

Catalyst Amount
The temperature (T = 100 • C) and the glycerol:citral molar ratio (1:2.25) remained constant. The catalyst loading was modified from 0.1 g to 0.4 g. Figure 9 displays the effects of PW4-K-6 loading on the conversion of glycerol and selectivity to acetal compounds. The conversion of glycerol improved with PW4-K-6 loading. This behavior may be justified by the rise in the number of sites in the reactor. The selectivity to 5R acetal and 6R acetal were also analyzed. The selectivity to 5R acetal was 87% (at 67% conversion), 80% (at 77% conversion), 75% (at 89% conversion), and 72% (at 96% conversion) using 0.10 g, 0.20 g, 0.30 g, and 0.40 g of PW4-K-6, respectively, after 5 h. It appears that the selectivity to the 5R acetal did not change much with the material loading [43,44], but a slight decrease in selectivity (5R acetal) was observed.

Catalyst Amount
The temperature (T = 100 °C) and the glycerol:citral molar ratio (1:2.25) remained constant. The catalyst loading was modified from 0.1 g to 0.4 g. Figure 9 displays the effects of PW4-K-6 loading on the conversion of glycerol and selectivity to acetal compounds. The conversion of glycerol improved with PW4-K-6 loading. This behavior may be justified by the rise in the number of sites in the reactor. The selectivity to 5R acetal and 6R acetal were also analyzed. The selectivity to 5R acetal was 87% (at 67% conversion), 80% (at 77% conversion), 75% (at 89% conversion), and 72% (at 96% conversion) using 0.10 g, 0.20 g, 0.30 g, and 0.40 g of PW4-K-6, respectively, after 5 h. It appears that the selectivity to the 5R acetal did not change much with the material loading [43,44], but a slight decrease in selectivity (5R acetal) was observed. Figure 9. Acetalization of glycerol with citral over PW4-K-6. Effect of catalyst amount after 5 h of reaction.

Glycerol: Citral Molar Ratio
The study of the glycerol:citral molar ratio in the glycerol conversion was performed using PW4-K-6 material. The molar ratios of 1:1, 1:2.25, and 1:5.1 were studied, while the

Glycerol: Citral Molar Ratio
The study of the glycerol:citral molar ratio in the glycerol conversion was performed using PW4-K-6 material. The molar ratios of 1:1, 1:2.25, and 1:5.1 were studied, while the temperature (T = 100 • C) and catalyst loading (m = 0.30 g) were maintained. The conversion of glycerol increased with the increase in the molar ratio (from 1:1 to 1:2.25). However, the glycerol conversion did not improve when increasing the molar ratio from 1:2.25 to 1:5.1 ( Figure 11). This behavior may be due to the occupation of citral molecules over the active positions of material. A large amount of aldehyde impedes the reaction [43].
PW4-K-6 was reused. Different experiments were performed using PW4-K-6 material. Figure 12 indicates the activity of PW4-K-6 achieved. The activity of PW4-K-6 material is great after the fifth utilization. The HPW amount present on the KIT-6 solid was obtained by ICP. The catalyst lost 3% of HPW present on KIT-6. Figure 11. Acetalization of glycerol with citral over PW4-K-6. Effect of glycerol:citral molar ratio, after 5 h of reaction.
PW4-K-6 was reused. Different experiments were performed using PW4-K-6 material. Figure 12 indicates the activity of PW4-K-6 achieved. The activity of PW4-K-6 material is great after the fifth utilization. The HPW amount present on the KIT-6 solid was obtained by ICP. The catalyst lost 3% of HPW present on KIT-6. Great selectivity to 5R acetal was obtained after the fifth utilization of PW4-K-6 ( Table  3). Table 3. Conversion of glycerol and selectivity to 5R acetal and 6R acetal. Great selectivity to 5R acetal was obtained after the fifth utilization of PW4-K-6 ( Table 3). The glycerol acetalization with citral may be triggered by interactions between the oxygen atom of the carbonyl group of citral (aldehyde) and Brönsted acid sites, according to the mechanistic proposal given in Figure 13 [18,41]. Specifically, the formation of a hemiacetal in the reaction of the citral with glycerol may be followed by the elimination of an H 2 O molecule and formation of a carbocation [41], which, in turn, suffers an attack involving the inner or terminal hydroxyl group of the glycerol molecule, finally giving the cyclic acetals 1,3-dioxolane and 1,3-dioxane, respectively. The PW4-K-6 activity for the glycerol acetalization with citral was evaluated with the activity of other materials described in the literature. Table 4 indicates the activity of the materials. The activity (expressed as h −1 ) of the PW4-K-6, ZrO2-350, and PW4-SBA-15 materials was found to increase as follows: 5.  Figure 13. Mechanistic proposal exemplified for the glycerol acetalization with citral over PW4-K-6, leading to the 1,3-dioxolane or 1,3-dioxane acetal products (where R = -CH=C(CH 3 ) CH 2 CH 2 CH=C(CH 3 )(CH 3 )).

Preparation of Catalysts
The KIT-6 material was synthesized according to Pirez et al. [46]. Briefly, 4 g of Pluronic P123 were dispersed in 144 mL of distilled H 2 O, and 7.9 g of 35% HCl was added to the mixture, under stirring at 35 • C. After complete dissolution, 4 g of 1-butanol was added. The mixture was stirred for 1 h. After this period, 8.6 g of tetraethylorthosilicate (TEOS) was added. The solution then remained under stirring at 35 • C for 24 h. At the end of this period, the solution was placed in a closed autoclave and heated at 100 • C for 24 h. The precipitate was filtered and dried in an oven at 100 • C for 24 h. Finally, the fine powder obtained was washed (ethanol and HCl mixture) and calcined in air at 550 • C.
Heteropolyacid (H 3 PW 12 O 40 ) immobilization was prepared by direct synthesis. The first steps of the synthesis were like the steps described by Pirez et al. [45]. The difference in the synthesis material was in the step where TEOS was added. Thus, 8.6 g of TEOS and the required amounts of H 3 PW 12 O 40 in CH 3 CH 2 OH solution were added to the mixture. This mixture was stirred for 24 h. After this period, the mixture was put in an autoclave. This reactor was heated at 100 • C for 24 h. After this period, the solid obtained was treated according to previous work [6].

Materials Characterization
A BET and total volume porous (Vp) of materials were calculated from N 2 isotherms at 77 K using a Micromeritics ASAP 2010.
The amount of HPW supported in KIT-6 was analyzed by ICP. The ATR-FTIR spectra were obtained using a Perkin Elmer Spectrum 100 FTIR spectrophotometer.
TEM photos were executed on a Hitachi S-2400 instrument. The potentiometric titration was determined according to Pizzio et al. [38].

Catalytic Experiments
The reactions were performed using a stirred batch reactor at 100 • C. In a typical reaction, 0.09 mol of citral, 0.04 mol of glycerol, and 0.3 g catalyst were added to the reactor.