Hydration of Camphene over PW-SBA-15-SO3H

The hydration of camphene was carried out over SBA-15 with sulfonic acid groups and tungstophosphoric acid at 50 °C. The main product of camphene hydration was isoborneol, with camphene hydrate and borneol as byproducts. The catalytic activity increased with the amount of tungstophosforic acid (PW) immobilized on the silica support until a maximum, which was obtained with the PW4-SBA-15-SO3H material (16.4 wt.%). When the amount of PW immobilized on SBA-15 increased (PW5-SBA-15-SO3H, 21.2 wt.%), the catalytic activity decreased. The catalytic activity of PW4-SBA-15-SO3H increased with the water content of the solvent, until a maximum was reached with 50% water. With higher water concentrations, a decrease in the catalytic activity was observed. The selectivity to isoborneol was 90% at 99% camphene conversion in the presence of the PW4-SBA-15-SO3H catalyst. The catalytic stability of the PW4-SBA-15-SO3H material during camphene hydration was studied by performing consecutive batch runs with the same catalyst sample. After the third run, a trend towards stabilized catalytic activity was observed. A kinetic model is also proposed.


Introduction
The hydration of terpenes is an important synthesis route to obtain valuable compounds with many applications in the perfumery and pharmaceutical industries [1][2][3][4][5]. Isoborneol and borneol obtained by camphene hydration have applications in the formulation of soaps, cosmetic perfumes, and medicines [1,2]. Figure 1 shows the scheme of camphene hydration.

Introduction
The hydration of terpenes is an important synthesis route to obtain valuable compounds with many applications in the perfumery and pharmaceutical industries [1][2][3][4][5]. Isoborneol and borneol obtained by camphene hydration have applications in the formulation of soaps, cosmetic perfumes, and medicines [1,2]. Figure 1 shows the scheme of camphene hydration.  Isoborneol is also an intermediate in the synthesis of camphor [6]. Camphene hydration is carried out using homogeneous catalysts, such as HClO 4 [7], H 4 SiW 12 O 40 [8], and H 3 PW 12 O 40 [8]. The homogenous catalysts have some environmental problems and economic inconveniences. For example, it is difficult to remove them from reaction mixtures  . The PW material exhibited principal IR bands at 1080, 985, 890, and 839 cm −1 , which were attributed to the νas P-Oa, W = Ot, W-Oc-W, and W-Oe-W of the Keggin structure of PW, respectively [16,18]. Additionally, the peaks from the -SO3H group were at 530, 620, 1068, and 1190 cm −1 [19]. The main sites present in the reaction mixture [21]. The isoborneol selectivity in amount of catalyst. However, when the amount of catalyst used incre 0.48 g, the selectivity to isoborneol did not increase, which may be due a mass transfer limitation when excess catalyst was used under the sam tions [21]. Figure 10B shows camphene conversion (%) represented by lectivity (%) to isoborneol represented by light bars after 70 h of reactio to isoborneol decreased when the camphene conversion also decreas may be explained by isoborneol being a product obtained from camphe hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The tempe and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A s the initial concentration of camphene on the conversion of this terpene tion rate increased slightly when the initial camphene concentration may be explained by the low quantity of camphene molecules for the   Table 1 shows the acidity of the materials. When the amount of PW on SBA-15 increased, the acidity of the materials also increased, which may be due to the increased amount of H + with PW loading of the SBA-15 material [15,16].  [16,18]. Additionally, the peaks from the -SO 3 H group were at 530, 620, 1068, and 1190 cm −1 [19]. The main bands of PW were present in the ATR-FTIR spectrum of PW-SBA-15-SO 3 H ( Figure 3). However, some bands characteristic of Keggin units were overlapped with the bands of the SBA-15. In a previous work [15][16][17], when PW was supported on SBA-15, some major bands were also not observed. The peak from the -SO 3 H group at 1190 cm −1 was present in the ATR-FTIR spectrum of the PW-SBA-15-SO 3 H material ( Figure 3). In addition, some peaks of the -SO 3 H group were overlapped with the bands of mesostructured silica SBA-15 [19].  . The PW material exhibited principal IR bands at 1080, 985, 890, and 839 cm −1 , which were attributed to the νas P-Oa, W = Ot, W-Oc-W, and W-Oe-W of the Keggin structure of PW, respectively [16,18]. Additionally, the peaks from the -SO3H group were at 530, 620, 1068, and 1190 cm −1 [19]. The main bands of PW were present in the ATR-FTIR spectrum of PW-SBA-15-SO3H ( Figure 3). However, some bands characteristic of Keggin units were overlapped with the bands of the SBA-15. In a previous work [15][16][17], when PW was supported on SBA-15, some major bands were also not observed. The peak from the -SO3H group at 1190 cm −1 was present in the ATR-FTIR spectrum of the PW-SBA-15-SO3H material ( Figure 3). In addition, some peaks of the -SO3H group were overlapped with the bands of mesostructured silica SBA-15 [19].   Figure 4A shows the XRD spectra of the catalysts. SBA-15 shows three diffraction peaks (100), (110), and (200), which corresponded to the two-dimension hexagonal mesostructure. All catalysts with PW immobilized on SBA-15-SO 3 H displayed the diffraction peak at the 2θ region, which indicated that the structure of SBA-15 was preserved after the immobilization of PW and the sulfonic acid groups onto silica [15,16]. mesostructure. All catalysts with PW immobilized on SBA-15-SO3H displayed the diffraction peak at the 2θ region, which indicated that the structure of SBA-15 was preserved after the immobilization of PW and the sulfonic acid groups onto silica [15,16]. Figure 4B displays the XRD spectra of the catalysts at the 2θ region of 5° to 55°. The peaks characteristic of PW ( Figure 3B-(viii)) did not appear on the XRD spectrum of the SBA-15-SO3H material, which suggested that the PW units were very well dispersed [16,17].   Figure 5A) and PW4-SBA-15-SO3H materials ( Figure 5B). The immobilization of PW and the introduction of the sulfonic acid groups did not seem to affect the porous system of the SBA-15 [15][16][17].  Figure 4B displays the XRD spectra of the catalysts at the 2θ region of 5 • to 55 • . The peaks characteristic of PW ( Figure 3B-(viii)) did not appear on the XRD spectrum of the SBA-15-SO 3 H material, which suggested that the PW units were very well dispersed [16,17]. Figure 5 shows the TEM images of the SBA-15 ( Figure 5A) and PW4-SBA-15-SO 3 H materials ( Figure 5B). The immobilization of PW and the introduction of the sulfonic acid groups did not seem to affect the porous system of the SBA-15 [15][16][17].  Figure 4A shows the XRD spectra of the catalysts. SBA-15 shows three diffraction peaks (100), (110), and (200), which corresponded to the two-dimension hexagonal mesostructure. All catalysts with PW immobilized on SBA-15-SO3H displayed the diffraction peak at the 2θ region, which indicated that the structure of SBA-15 was preserved after the immobilization of PW and the sulfonic acid groups onto silica [15,16]. Figure 4B displays the XRD spectra of the catalysts at the 2θ region of 5° to 55°. The peaks characteristic of PW ( Figure 3B-(viii)) did not appear on the XRD spectrum of the SBA-15-SO3H material, which suggested that the PW units were very well dispersed [16,17].  Figure 5 shows the TEM images of the SBA-15 ( Figure 5A) and PW4-SBA-15-SO3H materials ( Figure 5B). The immobilization of PW and the introduction of the sulfonic acid groups did not seem to affect the porous system of the SBA-15 [15][16][17].  The catalytic activity of the materials increased with the amount of PW immobilized on SBA-15-SO3H until a maximum. This behavior can be explained by the increased acidity of the mesostructured silica and the amount of W species [17,20]. The number of active   Figure 6 shows the initial activity of SBA-15, SBA-15-SO 3 H, PW1-SBA-15-SO 3 H, PW2-SBA-15-SO 3 H, PW3-SBA-15-SO 3 H, PW4-SBA-15-SO 3 H, and PW5-SBA-15-SO 3 H. The catalytic activity of the materials increased with the amount of PW immobilized on SBA-15-SO 3 H until a maximum. This behavior can be explained by the increased acidity of the mesostructured silica and the amount of W species [17,20]. The number of active sites on SBA-15 may have been increased (Table 1). However, at a high amount of PW immobilized on SBA-15 (sample PW5-SBA-15-SO 3 H catalyst), the catalytic activity decreased. This behavior may be due to the existence of some internal diffusion limitations inside the SBA-15 material [16,17]. The total porous volume and surface area (A BET ) decreased with the amount of PW on SBA-15 (Table 1). It is important to note that products were not observed on the surface of catalyst. Additionally, no oligomerization of camphene occurred under these reaction conditions.  Figure 6 shows the initial activity of SBA-15, SBA-15-SO3H, PW1-SBA-15-SO3H, PW2-SBA-15-SO3H, PW3-SBA-15-SO3H, PW4-SBA-15-SO3H, and PW5-SBA-15-SO3H. The catalytic activity of the materials increased with the amount of PW immobilized on SBA-15-SO3H until a maximum. This behavior can be explained by the increased acidity of the mesostructured silica and the amount of W species [17,20]. The number of active sites on SBA-15 may have been increased (Table 1). However, at a high amount of PW immobilized on SBA-15 (sample PW5-SBA-15-SO3H catalyst), the catalytic activity decreased. This behavior may be due to the existence of some internal diffusion limitations inside the SBA-15 material [16,17]. The total porous volume and surface area (ABET) decreased with the amount of PW on SBA-15 (Table 1). It is important to note that products were not observed on the surface of catalyst. Additionally, no oligomerization of camphene occurred under these reaction conditions.  Figure 7 shows the selectivity to isoborneol. All catalysts showed high selectivity to the isoborneol compound. Apparently, the selectivity to isoborneol was not affected by the change in the acidity of the materials. According to Valente et al. [13], the selectivity to isoborneol increased slightly with the amount of acid sites. However, in this work, a relationship was not observed between the selectivity to isoborneol and the acidity of the materials. Figure 7 shows the selectivity to isoborneol. All catalysts showed high selectivity to the isoborneol compound. Apparently, the selectivity to isoborneol was not affected by the change in the acidity of the materials. According to Valente et al. [13], the selectivity to isoborneol increased slightly with the amount of acid sites. However, in this work, a relationship was not observed between the selectivity to isoborneol and the acidity of the materials. The effect of the solvent (aqueous acetone) on camphene hydration using the PW4-SBA-15-SO3H catalyst was studied. Figure 8 shows the catalytic activity of the PW4-SBA-15-SO3H material versus acetone (%). The results can be explained as follows:

Catalytic Experiments
at low water content (high amount of acetone), the catalytic activity increased with increasing water content. This behavior may be due to low amount of water inside the PW4-SBA-15-SO3H surface. When the amount of water increased, the catalytic activity increased as well, until a maximum was reached at 50% of water. -at high water content (low amount of acetone), it is expected that the solvent inside the PW4-SBA-15-SO3H pore system was richer in water content than the bulk solution. The layer of water molecules surrounding the active sites form a barrier hindering the diffusion of camphene. Consequently, the camphene sorption coefficient as well as the activity, decreased. sites present in the reaction mixture [21]. The isoborneol selectivity i amount of catalyst. However, when the amount of catalyst used incre 0.48 g, the selectivity to isoborneol did not increase, which may be due a mass transfer limitation when excess catalyst was used under the sam tions [21]. Figure 10B shows camphene conversion (%) represented by lectivity (%) to isoborneol represented by light bars after 70 h of reacti to isoborneol decreased when the camphene conversion also decreas may be explained by isoborneol being a product obtained from camphe hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temp and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A s the initial concentration of camphene on the conversion of this terpen tion rate increased slightly when the initial camphene concentration may be explained by the low quantity of camphene molecules for the alyst. Figure 11B shows camphene conversion (%) represented by da tivity (%) to isoborneol represented by light bars after 70 h of reaction. tion, the camphene conversion was quite similar ( Figure 11B). The sel neol decreased slightly. The effect of the solvent (aqueous acetone) on camphene hydration using the PW4-SBA-15-SO 3 H catalyst was studied. Figure 8 shows the catalytic activity of the PW4-SBA-15-SO3H material versus acetone (%). The results can be explained as follows: at low water content (high amount of acetone), the catalytic activity increased with increasing water content. This behavior may be due to low amount of water inside the PW4-SBA-15-SO 3 H surface. When the amount of water increased, the catalytic activity increased as well, until a maximum was reached at 50% of water. -at high water content (low amount of acetone), it is expected that the solvent inside the PW4-SBA-15-SO 3 H pore system was richer in water content than the bulk solution.
The layer of water molecules surrounding the active sites form a barrier hindering the diffusion of camphene. Consequently, the camphene sorption coefficient as well as the activity, decreased.  Figure 9A shows the effect of the solvent on the camphene profile. The selectivity of the PW4-SBA-15-SO3H catalyst to isoborneol increased with the amount of water in the reaction mixture ( Figure 9B). This behavior may be explained by the increased amount of water molecules inside the pores of the catalyst. The maximum selectivity to isoborneol  Figure 9A shows the effect of the solvent on the camphene profile. The selectivity of the PW4-SBA-15-SO 3 H catalyst to isoborneol increased with the amount of water in the reaction mixture ( Figure 9B). This behavior may be explained by the increased amount of water molecules inside the pores of the catalyst. The maximum selectivity to isoborneol (90%) was obtained with a 50:50 (V/V) mixture of acetone:water. When the amount of water increased above the 50%, the selectivity to isoborneol decreased. This behavior may be due to a higher amount of water inside the pores of the catalyst and, consequently, the concentration of camphene near the active site was low.  Figure 9A shows the effect of the solvent on the camphene profile. The selectivity of the PW4-SBA-15-SO3H catalyst to isoborneol increased with the amount of water in the reaction mixture ( Figure 9B). This behavior may be explained by the increased amount of water molecules inside the pores of the catalyst. The maximum selectivity to isoborneol (90%) was obtained with a 50:50 (V/V) mixture of acetone:water. When the amount of water increased above the 50%, the selectivity to isoborneol decreased. This behavior may be due to a higher amount of water inside the pores of the catalyst and, consequently, the concentration of camphene near the active site was low.

Effect of the catalyst amount
The effect of the amount of PW4-SBA-15-SO3H on camphene conversion was studied. The initial concentration of camphene (C = 0.065 mol.dm −3 ) and the reaction temperature (T = 50 °C) were kept constant. Figure 10A shows the effect of the amount of PW4-SBA-15-SO3H on camphene conversion and isoborneol selectivity. The camphene conversion increased with the amount of catalyst due to the increased number of active ites present in the reaction mixture [21]. The isoborneol selectivity increased with the mount of catalyst. However, when the amount of catalyst used increased from 0.30 to .48 g, the selectivity to isoborneol did not increase, which may be due to the existence of mass transfer limitation when excess catalyst was used under the same reaction condiions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and seectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity o isoborneol decreased when the camphene conversion also decreased. This behavior ay be explained by isoborneol being a product obtained from camphene and camphene ydrate compounds (kinetic model proposed). The initial concentration of camphene was also studied. The temperature (T = 50 °C) nd the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of he initial concentration of camphene on the conversion of this terpene. The initial reacion rate increased slightly when the initial camphene concentration decreased, which ay be explained by the low quantity of camphene molecules for the same amount catlyst. Figure 11B shows camphene conversion (%) represented by dark bars and selecivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reacion, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoboreol decreased slightly.  Figure 9A shows the effect of the solvent on the camphene profile. The selectivity of the PW4-SBA-15-SO3H catalyst to isoborneol increased with the amount of water in the reaction mixture ( Figure 9B). This behavior may be explained by the increased amount of water molecules inside the pores of the catalyst. The maximum selectivity to isoborneol (90%) was obtained with a 50:50 (V/V) mixture of acetone:water. When the amount of water increased above the 50%, the selectivity to isoborneol decreased. This behavior may be due to a higher amount of water inside the pores of the catalyst and, consequently, the concentration of camphene near the active site was low.  Figure 10A shows the effect of the amount of PW4-SBA-15-SO 3 H on camphene conversion and isoborneol selectivity. The camphene conversion increased with the amount of catalyst due to the increased number of active sites present in the reaction mixture [21]. The isoborneol selectivity increased with the amount of catalyst. However, when the amount of catalyst used increased from 0.30 to 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed). 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly. 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly. 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly. 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly. 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly. 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly. 0.48 g, the selectivity to isoborneol did not increase, which may be due to the existence of a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly.

Effect of the Initial Concentration of Camphene
The initial concentration of camphene was also studied. The temperature (T = 50 • C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly. a mass transfer limitation when excess catalyst was used under the same reaction conditions [21]. Figure 10B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. The selectivity to isoborneol decreased when the camphene conversion also decreased. This behavior may be explained by isoborneol being a product obtained from camphene and camphene hydrate compounds (kinetic model proposed).

Effect of the initial concentration of camphene
The initial concentration of camphene was also studied. The temperature (T = 50 °C) and the amount of catalyst (m = 0.48 g) were kept constant. Figure 11A shows the effect of the initial concentration of camphene on the conversion of this terpene. The initial reaction rate increased slightly when the initial camphene concentration decreased, which may be explained by the low quantity of camphene molecules for the same amount catalyst. Figure 11B shows camphene conversion (%) represented by dark bars and selectivity (%) to isoborneol represented by light bars after 70 h of reaction. After 70 h of reaction, the camphene conversion was quite similar ( Figure 11B). The selectivity to isoborneol decreased slightly.  Figure 12 displays the catalytic activity of the PW4-SBA-15-SO 3 H material. The catalyst showed good activity after five uses. After the reaction, the PW4-SBA-15-SO 3 H material was characterized by ICP. A 3% loss of PW occurred. Products were not observed on the surface of the catalyst. Additionally, no oligomerization of camphene occurred under these reaction conditions. The lost PW may be due to some species being adsorbed on the SBA-15 surface. PW existed in the pore wall of SBA-15 [15]. Selectivity to isoborneol was similar (about 90%) after five utilizations of the PW4-SBA-15-SO 3 H catalyst. material was characterized by ICP. A 3% loss of PW occurred. Products were not observed on the surface of the catalyst. Additionally, no oligomerization of camphene occurred under these reaction conditions. The lost PW may be due to some species being adsorbed on the SBA-15 surface. PW existed in the pore wall of SBA-15 [15]. Selectivity to isoborneol was similar (about 90%) after five utilizations of the PW4-SBA-15-SO3H catalyst.

Kinetic Modeling
The Langmuir-Hinshelwood (LH) mechanism has been widely used in the kinetic study of heterogeneous catalytic systems. Based on the surface reaction between two adsorbed species, the LH mechanism forecasted the kinetic data very well in the hydration of cyclohexene over ion-exchange resin and H-ZSM-5 [21], the hydrolysis of ethyl benzoate [22], the liquid-phase dimerization of isoamylenes [23], the liquid-phase hydrogenation of cinnamaldehyde [24], the esterification of lactic acid with ethanol [25], the esterification of propanol with ethanoic acid [26], the synthesis of tert-amyl methyl ether [27], and the catalytic hydrogenation of d-lactose to lactitol [28]. A kinetic model was proposed assuming a Langmuir-Hinshelwood mechanism, where the reaction is controlled by the surface reaction step. Additionally, it was assumed that the camphene and camphene hydrate adsorbed on the active sites, while the water, isoborneol, and other products did not adsorb on the active sites. The proposed reaction scheme for camphene hydration is shown in Figure 13

Kinetic Modeling
The Langmuir-Hinshelwood (LH) mechanism has been widely used in the kinetic study of heterogeneous catalytic systems. Based on the surface reaction between two adsorbed species, the LH mechanism forecasted the kinetic data very well in the hydration of cyclohexene over ion-exchange resin and H-ZSM-5 [21], the hydrolysis of ethyl benzoate [22], the liquid-phase dimerization of isoamylenes [23], the liquid-phase hydrogenation of cinnamaldehyde [24], the esterification of lactic acid with ethanol [25], the esterification of propanol with ethanoic acid [26], the synthesis of tert-amyl methyl ether [27], and the catalytic hydrogenation of d-lactose to lactitol [28]. A kinetic model was proposed assuming a Langmuir-Hinshelwood mechanism, where the reaction is controlled by the surface reaction step. Additionally, it was assumed that the camphene and camphene hydrate adsorbed on the active sites, while the water, isoborneol, and other products did not adsorb on the active sites. The proposed reaction scheme for camphene hydration is shown in Figure 13 The reaction rates are given by: The batch reactor was operated under isothermal and isobaric conditions. The reaction rates are given by: (1) (2) where k 1 , k 2 , k 3 and k 4 are kinetic reaction constants, C represents camphene, and HC represents the camphene hydrate. The molar balance in the batch reactor is given by: where the C represents camphene, HC represents the camphene hydrate, I represents isoborneol, and O represents "other molecules". The optimization was performed using the SOLVER routine in a Microsoft Excel spreadsheet.
The model was fitted to the experimental results (Figures 14-19). The solid lines represented the kinetic model fitted to the experimental data. The kinetic model fit the experimental concentration data quite well.         Table 2 shows the model parameters obtained by application of the kinetic model to the experimental data. The kinetic constants increased with the acidity and amount of PW immobilized on SBA-15-SO3H (Table 1). The adsorption constant of camphene and the camphene hydrate tended to decrease with the amount of PW on the material. There were some changes in the hydrophobic/hydrophilic balance on the catalyst's surface.    Table 2 shows the model parameters obtained by application of the kinetic model to the experimental data. The kinetic constants increased with the acidity and amount of PW immobilized on SBA-15-SO3H (Table 1). The adsorption constant of camphene and the camphene hydrate tended to decrease with the amount of PW on the material. There were some changes in the hydrophobic/hydrophilic balance on the catalyst's surface.   Table 2 shows the model parameters obtained by application of the kinetic model to the experimental data. The kinetic constants increased with the acidity and amount of PW immobilized on SBA-15-SO 3 H ( Table 1). The adsorption constant of camphene and the camphene hydrate tended to decrease with the amount of PW on the material. There were some changes in the hydrophobic/hydrophilic balance on the catalyst's surface.  Table 3 shows the activity and selectivity to isoborneol of different materials used for camphene hydration. The catalytic activity of PW4-SBA-15-SO 3 H for camphene hydration was found to be higher than the catalytic activity of USY zeolite.

Preparation of Materials
The preparation of PW-SBA-15-SO 3 H catalysts was carried out using a similar procedure as described by Yu et al. [29]. Pluronic P-123 (4 g) were dispersed in 144 mL of distilled H 2 O, and different amounts of tungstophosphoric acid and 7.9 g of 35% HCl were added to the mixture under stirring at 40 • C for 1 h. After complete dissolution, 4 g of 1butanol was added. The mixture was stirred for 1 h. After this period, a mixture of 8.6 g of tetraethylorthosilicate (TEOS) with 0.4 g of (3-mercaptopropyl)triethoxysilane was added. The solution was maintained under stirring at 40 • C for 24 h. After this period, 1.6 g of hydrogen peroxide (30%) was added to the mixture. The mixture was placed in a closed autoclave and heated at 100 • C for 24 h. The white solid was filtered and dried at 100 • C for 24 h. Finally, the fine powder obtained was washed (ethanol and HCl mixture) to remove the template.

Materials Characterization
A Micromeritics ASAP 2010 instrument was used to obtain the N 2 isotherms at 77 K. The amount of PW on SBA-15 was evaluated 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.
Acid-base titration was used to obtain the total acid density (mmol H + .g −1 ) of the materials. The acid-base titrations were carried out according to previous work [29].

Catalytic Experiments
The camphene hydration reactions were carried out in a jacketed batch reactor (200 mL) at 50 • C. In a typical hydration experiment, the reactor was loaded with 114 mL of aqueous acetone (1:1, V/V) and 0.482 g of the catalyst. The reactions were initiated by adding 7.5 mmol of camphene.
The material PW4-SBA-15-SO 3 H was reused several times. Nonane was used as an internal standard. The samples were removed from the reactor periodically. The samples were analyzed by GC using a Hewlett Packard instrument equipped with a 30 m × 0.25 mm DB-1 column. The injector temperature was 180 • C and the detector temperature was 300 • C. The oven temperature program was as follows: started at 80 • C (4 min), ramp at 6 • C min −1 to 126 • C, and ramp at 10 • C min −1 to 300 • C. The products were identified by GC-MS using a FISONS MD800 (Leicestershire, UK) with the same column and temperature conditions.
The initial activity was calculated by the expression: where V is the volume, W is the amount of catalyst, and ( dC camphene dt ) 0 is the scope of the line obtained by the linear regression using the camphene concentration during the first 4 h of the reaction.

Conclusions
Camphene hydration was performed using SBA-15 with sulfonic acid groups and PW as a catalyst. Different catalysts with the same amount of sulfonic acid groups but different PW amounts (1.7 to 22.1 wt.%) in SBA-15 were produced. The PW4-SBA-15-SO 3 H material (16.4 wt.%) exhibited higher catalytic activity than other catalysts.
All the catalysts showed great selectivity to isoborneol. The stability of the PW4-SBA-15-SO 3 H catalyst was studied. After the second use, the catalyst presented high activity. The selectivity to isoborneol was not affected.
A kinetic model was developed, which fit the experimental data relatively well. Informed Consent Statement: Not applicable.

Data Availability Statement:
The compound used for the catalysis and raw characterization data studies herein reported are available from the author upon request.