A Bio-Based Alginate Aerogel as an Ionic Liquid Support for the Efﬁcient Synthesis of Cyclic Carbonates from CO 2 and Epoxides

: In this work, the ionic liquid [Aliquat][Cl] was supported into alginate and silica aerogel matrices and applied as a catalyst in the cycloaddition reaction between CO 2 and a bio-based epoxide (limonene oxide). The efﬁciency of the alginate aerogel system is much higher than that of the silica one. The method of wet impregnation was used for the impregnation of the aerogel with [Aliquat][Cl] and a zinc complex. The procedure originated a well-deﬁned thin solvent ﬁlm on the surface of support materials. Final materials were characterised by Fourier Transform Infrared Spectroscopy, N 2 Adsorption–Desorption Analysis, X-ray diffraction, atomic Field Microscopy. Several catalytic tests were performed in a high-pressure apparatus at 353.2 K and 4 MPa of CO 2 .


Introduction
The utilization of CO 2 as starting material to produce cyclic carbonates is a very active field of research. Cyclic carbonates are particularly attractive as intermediate CO 2 derivatives since they can be formed readily from the catalytic coupling reaction between CO 2 and the corresponding epoxide [1][2][3]. Furthermore, cyclic carbonates are broadly used as polar aprotic solvents, fuel additives, electrolytes for lithium-ion batteries, fine chemicals intermediates as well as monomers in polymerization reactions [4]. Binary catalytic systems composed of homogeneous metal complexes together with ionic liquids have been successfully used [5][6][7]. Several mechanistic studies showed that the ionic liquid plays a crucial role in the reaction kinetics, with the anion being responsible for the nucleophilic attack to the epoxide ring, which is the rate-determining step [8][9][10][11]. Recently, [Aliquat][Cl] was used both as catalyst and solvent in the cycloaddition reaction between CO 2 and propylene oxide at high pressures [12].
The combination of high-pressure CO 2 with ionic liquids originates an efficient biphasic system to carry out liquid phase catalysis. The fact that ionic liquids are practically insoluble in CO 2 , while CO 2 presents high solubility in several types of these organic salts, Table 1. Impregnated silica (SIL) and alginate (ALG) aerogels prepared by the wet impregnation method.

Matrix
Material Reference Substance Impregnated . The y-axis scale is the same for all curves. The corresponding absorption bands of the Zn(II)-AHBD catalyst were not observed clearly in the impregnated samples, which can indicate that only a low amount was impregnated into the aerogels. For the blank alginate aerogel (Figure 1, ALG), it is possible to observe several characteristic absorption bands at 3450 cm −1 (O-H stretching) and the alginic acid bands between 700 and 1800 cm −1 [48]. particular, limonene epoxide has been explored in several studies as a renewable substrate that is also particularly challenging due to intrinsically higher steric demand [43,44].
The biopolymer alginate is naturally derived from the seaweed brown algae and is composed with α-L-guluronic acid and β-D-mannuronic acid blocks, linearly linked by 1,4-glycosidic linkage. Due to its biodegradability, low cost, non-toxicity and stability, it is an attractive candidate for SILP preparation and application in carbonates production [45].

Results and Discussion
The preparation and impregnation methods carried out for silica (SIL) and alginate (ALG) aerogels with the zinc complex (Zn) and/or the ionic liquid [Aliquat][Cl] (IL) are summarized in Table 1. The zinc complex used in this work was synthesized and extensively characterised in a previously work [46,47]. Table 1. Impregnated silica (SIL) and alginate (ALG) aerogels prepared by the wet impregnation method.

Matrix
Material Reference Substance Impregnated . The y-axis scale is the same for all curves. The corresponding absorption bands of the Zn(II)-AHBD catalyst were not observed clearly in the impregnated samples, which can indicate that only a low amount was impregnated into the aerogels. For the blank alginate aerogel (Figure 1, ALG), it is possible to observe several characteristic absorption bands at 3450 cm −1 (O-H stretching) and the alginic acid bands between 700 and 1800 cm −1 [48].   (Figure 1, IL) can be seen at 1480 cm −1 (CH2 blending) and between 2800 and 3000 cm −1 (C-H stretching) [49].  Figure 2 (Silica SIL2).
In this case, it was also possible to observe the [Aliquat][Cl] characteristic peaks in the spectrum. In comparison with alginate aerogel, the silica aerogel SIL2 had weaker absorption bands at 1480 cm −1 and between 2800 and 3000 cm −1 , which indicates a lower [Aliquat] [Cl] loading. For the sample Silica SIL1 (Figure 3), the characteristic peaks of the Zn(II)-AHBD catalyst were not observed, which may indicate that this impregnation was not so successful as the previous ones. Furthermore, silica aerogels presented a highly fragile structure and were not used in high pressure catalytic tests. The corresponding absorption bands of pure [Aliquat][Cl] (Figure 1, IL) can be seen at 1480 cm −1 (CH2 blending) and between 2800 and 3000 cm −1 (C-H stretching) [49]. The obtained spectrum of pure [Aliquat][Cl] was compared with the [Aliquat][Cl]-impregnated alginate aerogels (Figure 1, Zn-IL-ALG and IL-ALG) and it was observed that [Aliquat][Cl] characteristic peaks also appeared in the spectrum, which indicates that [Aliquat][Cl] was successfully impregnated into the alginate aerogels. A similar behaviour of the Zn(II)-AHBD + [Aliquat][Cl]-impregnated silica aerogels was observed, as shown in Figure 2 (Silica SIL2). In this case, it was also possible to observe the [Aliquat][Cl] characteristic peaks in the spectrum. In comparison with alginate aerogel, the silica aerogel SIL2 had weaker absorption bands at 1480 cm −1 and between 2800 and 3000 cm −1 , which indicates a lower [Aliquat] [Cl] loading. For the sample Silica SIL1 (Figure 3), the characteristic peaks of the Zn(II)-AHBD catalyst were not observed, which may indicate that this impregnation was not so successful as the previous ones. Furthermore, silica aerogels presented a highly fragile structure and were not used in high pressure catalytic tests. From the FTIR results presented in Figure 3, the Zn(II)-AHBD impregnation efficiency was difficult to evaluate. In fact, due to the complexity of the catalyst structure (Figure 4), the absorption bands appear overlapping and less defined. This hampers the FTIR analysis of Zn(II)-AHBD content in the aerogels and prompted us to use atomic absorption technique. The corresponding absorption bands of pure [Aliquat][Cl] (Figure 1, IL) can be seen at 1480 cm −1 (CH2 blending) and between 2800 and 3000 cm −1 (C-H stretching) [49]. The obtained spectrum of pure [Aliquat][Cl] was compared with the [Aliquat][Cl]-impregnated alginate aerogels (Figure 1, Zn-IL-ALG and IL-ALG) and it was observed that [Aliquat][Cl] characteristic peaks also appeared in the spectrum, which indicates that [Aliquat][Cl] was successfully impregnated into the alginate aerogels. A similar behaviour of the Zn(II)-AHBD + [Aliquat][Cl]-impregnated silica aerogels was observed, as shown in Figure 2 (Silica SIL2). In this case, it was also possible to observe the [Aliquat][Cl] characteristic peaks in the spectrum. In comparison with alginate aerogel, the silica aerogel SIL2 had weaker absorption bands at 1480 cm −1 and between 2800 and 3000 cm −1 , which indicates a lower [Aliquat] [Cl] loading. For the sample Silica SIL1 (Figure 3), the characteristic peaks of the Zn(II)-AHBD catalyst were not observed, which may indicate that this impregnation was not so successful as the previous ones. Furthermore, silica aerogels presented a highly fragile structure and were not used in high pressure catalytic tests. From the FTIR results presented in Figure 3, the Zn(II)-AHBD impregnation efficiency was difficult to evaluate. In fact, due to the complexity of the catalyst structure (Figure 4), the absorption bands appear overlapping and less defined. This hampers the FTIR analysis of Zn(II)-AHBD content in the aerogels and prompted us to use atomic absorption technique. From the FTIR results presented in Figure 3, the Zn(II)-AHBD impregnation efficiency was difficult to evaluate. In fact, due to the complexity of the catalyst structure (Figure 4), the absorption bands appear overlapping and less defined. This hampers the FTIR analysis of Zn(II)-AHBD content in the aerogels and prompted us to use atomic absorption technique.   Table 2 shows the textural properties of blank and impregnated alginate and silic aerogels. Regarding results obtained for blank aerogels (not impregnated), it was possibl to observe that silica aerogels presented a higher surface area and lower pore diamete than alginate aerogels, which is in accordance with results reported by other authors [50] This difference in pores diameter is regarded as the basis of a lower impregnation effi ciency into silica matrices [35]. When the aerogels were impregnated with Zn(II)-AHBD, a decrease (usually sligh but pronounced in the case of SIL2) of the specific surface area was observed. On the othe hand, when impregnated with the ionic liquid ([Aliquat][Cl]), which is an organic sal liquid at room temperature, the aerogel pores were totally filled, with consequent drasti decrease in the surface areas and pore volumes. This effect is more pronounced for algi nate aerogels, especially the ones that were impregnated with [Aliquat][Cl], for which th BET surface area results were always lower than 1 m 2 /g. Also, the alginate aerogel impreg nated with both [Aliquat] [Cl] and Zn(II)-AHBD presented a BET surface area lower than 1 m 2 /g.

Nitrogen Physisorption Studies
In the case of silica aerogels this effect was not so evident, very likely due to the fac that the impregnation was less efficient. As a complement to the results reported in th Table 2, Figure 5 shows the nitrogen adsorption isotherms of the blank and impregnat alginate and silica aerogels. The isotherms belong to "type IV" which is typical for meso porous materials. When the impregnation was performed with [Aliquat][Cl] (Zn-IL-ALG and IL-ALG, Figure 5) there is no desorption of N2 from the aerogels, which is in accord ance with the BET surface results.  Table 2 shows the textural properties of blank and impregnated alginate and silica aerogels. Regarding results obtained for blank aerogels (not impregnated), it was possible to observe that silica aerogels presented a higher surface area and lower pore diameter than alginate aerogels, which is in accordance with results reported by other authors [50]. This difference in pores diameter is regarded as the basis of a lower impregnation efficiency into silica matrices [35]. When the aerogels were impregnated with Zn(II)-AHBD, a decrease (usually slight but pronounced in the case of SIL2) of the specific surface area was observed. On the other hand, when impregnated with the ionic liquid ([Aliquat][Cl]), which is an organic salt liquid at room temperature, the aerogel pores were totally filled, with consequent drastic decrease in the surface areas and pore volumes. This effect is more pronounced for alginate aerogels, especially the ones that were impregnated with [Aliquat][Cl], for which the BET surface area results were always lower than 1 m 2 /g. Also, the alginate aerogel impregnated with both [Aliquat][Cl] and Zn(II)-AHBD presented a BET surface area lower than 1 m 2 /g.

Nitrogen Physisorption Studies
In the case of silica aerogels this effect was not so evident, very likely due to the fact that the impregnation was less efficient. As a complement to the results reported in the Table 2, Figure 5 shows the nitrogen adsorption isotherms of the blank and impregnate alginate and silica aerogels. The isotherms belong to "type IV" which is typical for mesoporous materials. When the impregnation was performed with [Aliquat][Cl] (Zn-IL-ALG and IL-ALG, Figure 5) there is no desorption of N2 from the aerogels, which is in accordance with the BET surface results.

Powder X-ray Diffraction Studies
X-ray diffraction patterns for silica and alginate aerogels are shown in the Figures 6 and 7, respectively. Since this technique detects the crystallinity of the impregnated Zn(II)-AHBD particles, the aerogels containing only [Aliquat] [Cl] were not analysed by this technique. The X-ray diffraction patterns for blank silica aerogels (SIL1 and SIL2) and Zn(II)-AHBD impregnated silica aerogels (Zn-SIL1 and Zn-SIL2) are represented in the Figure 6. No changes on crystallinity of impregnated silica aerogels were observed. Therefore, the discussions on the effect of impregnation on surface morphology will be focused only in the alginate aerogels The diffractogram of alginate is known to consist of two crystalline peaks around 14 and 23° of 2θ which are related to the lateral packing among molecular chains and the layer spacing along the molecular chain direction, respectively [51,52]. For the blank alginate aerogel (

Powder X-ray Diffraction Studies
X-ray diffraction patterns for silica and alginate aerogels are shown in the Figures 6 and 7, respectively. Since this technique detects the crystallinity of the impregnated Zn(II)-AHBD particles, the aerogels containing only [Aliquat] [Cl] were not analysed by this technique. The X-ray diffraction patterns for blank silica aerogels (SIL1 and SIL2) and Zn(II)-AHBD impregnated silica aerogels (Zn-SIL1 and Zn-SIL2) are represented in the Figure 6. No changes on crystallinity of impregnated silica aerogels were observed. Therefore, the discussions on the effect of impregnation on surface morphology will be focused only in the alginate aerogels.
Catalysts 2021, 11, x FOR PEER REVIEW 6 of 16 Figure 5. Nitrogen adsorption isotherms of the alginate and silica aerogels.

Powder X-ray Diffraction Studies
X-ray diffraction patterns for silica and alginate aerogels are shown in the Figures 6 and 7, respectively. Since this technique detects the crystallinity of the impregnated Zn(II)-AHBD particles, the aerogels containing only [Aliquat] [Cl] were not analysed by this technique. The X-ray diffraction patterns for blank silica aerogels (SIL1 and SIL2) and Zn(II)-AHBD impregnated silica aerogels (Zn-SIL1 and Zn-SIL2) are represented in the Figure 6. No changes on crystallinity of impregnated silica aerogels were observed. Therefore, the discussions on the effect of impregnation on surface morphology will be focused only in the alginate aerogels The diffractogram of alginate is known to consist of two crystalline peaks around 14 and 23° of 2θ which are related to the lateral packing among molecular chains and the layer spacing along the molecular chain direction, respectively [51,52]. For the blank alginate aerogel ( The diffractogram of alginate is known to consist of two crystalline peaks around 14 and 23 • of 2θ which are related to the lateral packing among molecular chains and the layer spacing along the molecular chain direction, respectively [51,52]. For the blank alginate aerogel (Figure 7  Zn(II)-AHBD characteristic peaks appeared. This result indicates that although in low amounts, the Zn(II)-AHBD complex was successfully impregnated into the alginate aerogels.

Aerogels Images
The images of the impregnated alginate and silica aerogels are presented in the In Figure 9a,b are presented images of the silica aerogels SIL1 and SIL2. The first sample is a blank aerogel, followed by a Zn(II)-AHBD impregnated aerogel and in the case of image 11b) also Zn(II)-AHBD+[Aliquat][Cl] impregnated silica aerogel.

Aerogels Images
The images of the impregnated alginate and silica aerogels are presented in the Figures 8 and 9, respectively. AHBD characteristic peaks appeared. This result indicates that although in low amounts, the Zn(II)-AHBD complex was successfully impregnated into the alginate aerogels.

Aerogels Images
The images of the impregnated alginate and silica aerogels are presented in the Figures 8 and 9, respectively. In Figure 9a,b are presented images of the silica aerogels SIL1 and SIL2. The first sample is a blank aerogel, followed by a Zn(II)-AHBD impregnated aerogel and in the case of image 11b) also Zn(II)-AHBD+[Aliquat][Cl] impregnated silica aerogel. In Figure 9a,b are presented images of the silica aerogels SIL1 and SIL2. The first sample is a blank aerogel, followed by a Zn(II)-AHBD impregnated aerogel and in the case of image 11b) also Zn(II)-AHBD+[Aliquat][Cl] impregnated silica aerogel.

Scanning Electron Microscopy
The SEM micrographs of silica and alginate aerogels are presented in the Figures 10  and 11, respectively. Two different magnifications were performed, 5.000 and 30.000, to detect different details of the surface morphology. No changes on the surface morphology of silica aerogels after the impregnation were observed ( Figure 10). Therefore, the discussions on the effect of impregnation on surface morphology will be focused on the alginate aerogels ( Figure 11). In Figure 11a, we observe that the blank alginate aerogel presents a high porous open structure, typical of the surface morphology of alginate aerogels [48].

Scanning Electron Microscopy
The SEM micrographs of silica and alginate aerogels are presented in the Figures 10 and 11, respectively. Two different magnifications were performed, 5.000 and 30.000, to detect different details of the surface morphology.

Scanning Electron Microscopy
The SEM micrographs of silica and alginate aerogels are presented in the Figures 10  and 11, respectively. Two different magnifications were performed, 5.000 and 30.000, to detect different details of the surface morphology. No changes on the surface morphology of silica aerogels after the impregnation were observed ( Figure 10). Therefore, the discussions on the effect of impregnation on surface morphology will be focused on the alginate aerogels ( Figure 11). In Figure 11a, we observe that the blank alginate aerogel presents a high porous open structure, typical of the surface morphology of alginate aerogels [48].
On the other hand, the Zn(II)-AHBD-impregnated alginate aerogels (Figure 11b   No changes on the surface morphology of silica aerogels after the impregnation were observed ( Figure 10). Therefore, the discussions on the effect of impregnation on surface morphology will be focused on the alginate aerogels ( Figure 11). In Figure 11a, we observe that the blank alginate aerogel presents a high porous open structure, typical of the surface morphology of alginate aerogels [48].

Atomic Absorption Studies
The metal loading was determined by atomic absorption. All the samples were analysed (except the ones impregnated only with [Aliquat][Cl]). However, the only sample for which Zn(II)-AHBD was detected was the alginate aerogel impregnated with Zn(II)-AHBD (Zn-ALG). For all the other samples, it was not possible to quantify the Zn(II)-AHBD loading since it was below the detection limit. The concentration obtained for Zn-ALG was only 1.99% (w/w). For Zn-IL-ALG, the presence of Aliquat Cl has significantly reduced the complex solubility in the solution, decreasing the zinc concentration on the materials to bellow the detection limit (which is 8 ppb).
For silica aerogels, textural properties seem to evidence a significant impregnation level, which was not detected by the atomic absorption studies, namely for Zn-SIL2. In this case, the complex should have suffered a process of decomposition in contact with the silica material. Therefore, both textural results and the brown color observed are not due to the presence of zinc, but due to the presence of the ligand AHBD.

Reactions Using Impregnated Aerogels as Catalysts
Impregnated alginate and silica aerogels were evaluated as catalytic systems for the coupling reaction between CO2 and limonene oxide ( Figure 12). The results are summarized in Table 3. Based on our previous studies, all the reactions were performed for 48 h, at 353.2 K and 4 MPa, using 1 mL of limonene oxide [39,53,54]. It should be noted that the aerogel was never used in direct contact with the liquid phase. All reactions were carried out

Atomic Absorption Studies
The metal loading was determined by atomic absorption. All the samples were analysed (except the ones impregnated only with [Aliquat][Cl]). However, the only sample for which Zn(II)-AHBD was detected was the alginate aerogel impregnated with Zn(II)-AHBD (Zn-ALG). For all the other samples, it was not possible to quantify the Zn(II)-AHBD loading since it was below the detection limit. The concentration obtained for Zn-ALG was only 1.99% (w/w). For Zn-IL-ALG, the presence of Aliquat Cl has significantly reduced the complex solubility in the solution, decreasing the zinc concentration on the materials to bellow the detection limit (which is 8 ppb).
For silica aerogels, textural properties seem to evidence a significant impregnation level, which was not detected by the atomic absorption studies, namely for Zn-SIL2. In this case, the complex should have suffered a process of decomposition in contact with the silica material. Therefore, both textural results and the brown color observed are not due to the presence of zinc, but due to the presence of the ligand AHBD.

Reactions Using Impregnated Aerogels as Catalysts
Impregnated alginate and silica aerogels were evaluated as catalytic systems for the coupling reaction between CO 2 and limonene oxide ( Figure 12). The results are summarized in Table 3.

Atomic Absorption Studies
The metal loading was determined by atomic absorption. All the samples were analysed (except the ones impregnated only with [Aliquat][Cl]). However, the only sample for which Zn(II)-AHBD was detected was the alginate aerogel impregnated with Zn(II)-AHBD (Zn-ALG). For all the other samples, it was not possible to quantify the Zn(II)-AHBD loading since it was below the detection limit. The concentration obtained for Zn-ALG was only 1.99% (w/w). For Zn-IL-ALG, the presence of Aliquat Cl has significantly reduced the complex solubility in the solution, decreasing the zinc concentration on the materials to bellow the detection limit (which is 8 ppb).
For silica aerogels, textural properties seem to evidence a significant impregnation level, which was not detected by the atomic absorption studies, namely for Zn-SIL2. In this case, the complex should have suffered a process of decomposition in contact with the silica material. Therefore, both textural results and the brown color observed are not due to the presence of zinc, but due to the presence of the ligand AHBD.

Reactions Using Impregnated Aerogels as Catalysts
Impregnated alginate and silica aerogels were evaluated as catalytic systems for the coupling reaction between CO2 and limonene oxide ( Figure 12). The results are summarized in Table 3. Based on our previous studies, all the reactions were performed for 48 h, at 353.2 K and 4 MPa, using 1 mL of limonene oxide [39,53,54]. It should be noted that the aerogel was never used in direct contact with the liquid phase. All reactions were carried out  Based on our previous studies, all the reactions were performed for 48 h, at 353.2 K and 4 MPa, using 1 mL of limonene oxide [39,53,54]. It should be noted that the aerogel was never used in direct contact with the liquid phase. All reactions were carried out maintaining the aerogel supported in a metallic grid at the top of the reactor in contact with the CO 2 gaseous phase as described in the methodology section. All investigated catalytic systems presented more than 95% selectivity towards limonene carbonate production.
As shown by the results presented in Table 3, all tested silica aerogels showed practically no activity towards the coupling reaction between CO 2 and limonene oxide.
For the materials with references Zn-SIL1 and Zn-SIL2, which are two different silica aerogels impregnated only with the zinc complex, this result can be explained by the absence of the ionic liquid [Aliquat] [Cl].
In fact, results show that even when supported in a silica material, the presence of the chloride anion still plays a crucial effect on the reaction mechanism. Curiously, also for the material with reference Zn-IL-SIL2, which was a silica aerogel impregnated with both zinc complex and [Aliquat] [Cl], only a small amount of carbonate formation was observed under the experimental conditions studied.
Alginate aerogels were used as catalyst support for the cycloaddition between CO 2 and epoxides. A surprisingly high carbonate formation of 35% and 32% was obtained for materials with reference Zn-IL-ALG and IL-ALG, respectively. On the other hand, as for the case of silica aerogels, materials impregnated only with zinc complex (Zn-ALG) gave no carbonate formation. Furthermore, both aerogels Zn-IL-ALG (impregnated with zinc complex and [Aliquat][Cl]) and IL-ALG (impregnated only with [Aliquat][Cl]) presented nearly the same activity, although being slightly higher for the former, which is in line with the conclusion that the zinc complex was impregnated in very low amounts. IL-ALG materials impregnated only with [Aliquat][Cl] were able to efficiently catalyse the reaction due to high [Aliquat][Cl] loadings. The results obtained are in accordance with the general proposed reaction mechanism in which the nucleophile is responsible for the attack to the epoxide ring, which is the rate-determining step [54]. Although IL-ALG materials presented satisfactory catalytic activities, their reutilization was not tested due to the leaching of the ionic liquid during the reactor depressurization at the end of each experiment. This happens mainly due to the high polarity of the substrate used, limonene oxide. Operating in continuous mode under high CO 2 /epoxide ratios will in the future allow to overcome this limitation.

Silica Alcogels Synthesis
The silica alcogels were produced by sol-gel process with two different approaches (SIL1 and SIL2). In the first approach, SIL1, TMOS and methanol were mixed and then a solution of ammonium hydroxide-water was added drop by drop under stirring. A molar ratio of 1 mol TMOS: 3 mol MeOH: 4 mol H 2 O: 5 × 10 −3 mol NH 4 OH was used [55]. In the second approach, SIL2, TMOS was mixed with water, methanol, and hydrochloric acid and a solution of ammonium hydroxide-water was added drop by drop under stirring conditions. A molar ratio 1 mol TMOS: 2.4 mol MeOH: 1.3 mol H 2 O: 1 × 10 −5 mol HCl: 7.7 × 10 −4 mol NH 4 OH was used [56]. For both approaches, the final mixture was stirred at room temperature for 30 min. The resulting mixture was poured into moulds covered with parafilm for gelation. After fifteen min, the gels formed were immersed into methanol for aging process, which lasted at least 7 days. The solvent was changed at least twice during aging to completely remove excess water and traces of reactants from the alcogels.

Alginate Alcogels Synthesis
Alginate hydrogels were prepared via internal gelation based on lowering the pH as described elsewhere [57,58]. In this work, sodium alginate was dissolved in distilled water under stirring conditions of 400 rpm for 2 h. Then, calcium carbonate was added into the solution and stirred for 1 h. After 1 h the addition of GDL was performed, reducing the pH of the solution and allowing it to turn into a gel. The final concentrations of sodium alginate, CaCO 3 and GDL are 3.2%(w/w), 1.6% (w/w) and 1.4% (w/w), respectively. The resulting mixture was placed into moulds, covered with parafilm and stored in refrigerator (4 • C) for 18 h to become solidified. Then, the alginate hydrogels were immersed in an aqueous solution with 30% (v/v) of ethanol and a multistep solvent exchange was followed every 24 h using ethanol-water solutions with increasing ethanol volumetric ratios of 50%, 70%, and 90%. Finally, pure ethanol was exchanged twice in order to totally replace the water content of the hydrogel by ethanol before the supercritical CO 2 drying.

Aerogels Impregnation
After the solvent exchange period, a wet impregnation (WI) method was used in order to impregnate the Zn(II)-AHBD complex into the aerogels matrix [48]. First, the Zn(II)-AHBD complex was dissolved in an organic solvent to produce a saturated solution (15,000 ppm), ethanol was used in the case of alginate aerogels and methanol for silica aerogels. The solution (30 mL) was placed into contact with the final alcogels to allow the Zn(II)-AHBD complex diffusion into the pores of alcogels until equilibration was reached.

Supercritical Drying of Alcogels
The supercritical CO 2 drying of the alcogels (silica and alginate) was performed at 14 ± 0.5 MPa and 45 • C, employing the procedure and equipment described in a previous work ( Figure 13) [59].
Catalysts 2021, 11, x FOR PEER REVIEW 12 of 16 was extracted by supercritical CO2 drying, leading to the precipitation of the compounds on the surface and pores of aerogels due to their insolubility in CO2.

Supercritical Drying of Alcogels
The supercritical CO2 drying of the alcogels (silica and alginate) was performed at 14 ± 0.5 MPa and 45 °C, employing the procedure and equipment described in a previous work ( Figure 13) [59]. About 12 alcogels were placed in the drying reactor and then filled with solvent (ethanol in the case of alginate and methanol in the case of silica) to prevent damage of alcogel structure during the pressurization of the system. The system was slowly pressurized to the desired pressure at a rate of 0.4 MPa/min. After the operating conditions of drying were reached, the solvent in the reactor was slowly removed by opening the lower valve. This strategy allows a continuous circulation of CO2 to extract the solvent from the alcogels. The drying process was conducted in three cycles of 60 min minimum each. At the end of the three cycles, the system was slowly depressurized at a rate of 0.2 MPa/min in order to avoid shrinkage or damage of aerogels.

CO2 and Epoxides Coupling Reactions
The reactions were performed in a high-pressure reactor presented in Figure 14 and described in detail elsewhere [54]. This set-up is built around a stainless-steel cylindrical cell with two sapphire windows and an internal volume of approximately 11 cm 3 . Inside the reactor a metallic net is used to support the aerogel in order to avoid any contact between the aerogel and the liquid phase-see details in Figure 14. A magnetic stirring bar was introduced inside the cell in order to homogenise the solution.
First, the reactor was filled with limonene oxide, catalyst (SILP or bulk) and 1MPa of CO2 pressure. Then, the reactor was immersed in a thermostated silicone bath heated by means of a controller that maintained the temperature within ±0.1 °C. By operating a CO2compressor, the desired pressure was attained into the cell. The pressure in the cell was measured with a pressure transducer 204 Setra (±0.073% full scale accuracy). At the end of the reaction, the high-pressure cell was cooled and then slowly depressurized to atmospheric pressure. Afterward, the cell was opened, and the content was collected. The crude About 12 alcogels were placed in the drying reactor and then filled with solvent (ethanol in the case of alginate and methanol in the case of silica) to prevent damage of alcogel structure during the pressurization of the system. The system was slowly pressurized to the desired pressure at a rate of 0.4 MPa/min. After the operating conditions of drying were reached, the solvent in the reactor was slowly removed by opening the lower valve. This strategy allows a continuous circulation of CO 2 to extract the solvent from the alcogels. The drying process was conducted in three cycles of 60 min minimum each. At the end of the three cycles, the system was slowly depressurized at a rate of 0.2 MPa/min in order to avoid shrinkage or damage of aerogels.

CO 2 and Epoxides Coupling Reactions
The reactions were performed in a high-pressure reactor presented in Figure 14 and described in detail elsewhere [54]. This set-up is built around a stainless-steel cylindrical cell with two sapphire windows and an internal volume of approximately 11 cm 3 . Inside the reactor a metallic net is used to support the aerogel in order to avoid any contact between the aerogel and the liquid phase-see details in Figure 14. A magnetic stirring bar was introduced inside the cell in order to homogenise the solution.
First, the reactor was filled with limonene oxide, catalyst (SILP or bulk) and 1 MPa of CO 2 pressure. Then, the reactor was immersed in a thermostated silicone bath heated by means of a controller that maintained the temperature within ±0.1 • C. By operating a CO 2 -compressor, the desired pressure was attained into the cell. The pressure in the cell was measured with a pressure transducer 204 Setra (±0.073% full scale accuracy). At the end of the reaction, the high-pressure cell was cooled and then slowly depressurized to atmospheric pressure. Afterward, the cell was opened, and the content was collected. The crude reaction mixture was analysed by NMR using deuterated chloroform as a solvent. NMR spectra were recorded on Bruker 500 MHz type (400 MHz) (Birrica, MA, USA). Peak frequencies were compared against solvent, chloroform-d at 7.26 ppm. All liquid solutions were prepared gravimetrically using an analytical balance (Sartorius model R180D) with the precision of ±0.0001 g.  The carbonate formation was determined by integration of the 1H-NMR spectrum of the crude reaction mixture. The calculations were made using the signals from the epoxy groups of limonene oxide (δ = 2.9-3.1 ppm) and from the cyclic carbonate groups (δ = 4.3-4.5 ppm) [60]. Errors in the carbonate formation measurements were estimated by comparing the results of at least 3 integrations of each spectrum. The error in the final results was lower than 10%.

Aerogels Characterization
The aerogel structure was studied by Fourier transform infrared spectroscopy. The spectra were acquired between 4000 and 400 cm −1 with a 4 cm −1 step using an attenuated total reflectance (ATR) sampling accessory (Smart iTR) equipped with a single-bounce diamond crystal on a Thermo Nicolet 6700 Spectrometer.
Textural characteristics of aerogels, mainly specific surface area, mean pore diameter and pore size distribution, were determined by N2 adsorption-desorption analysis at low temperature (NOVA 3000e). The specific pore volume is determined by the single point adsorption method. The shown average pore diameter is based on the desorption isotherm of the Barrett-Joyner-Halenda (BJH) method. Prior to the determination, the alginate and silica aerogels were degassed at 70 and 200 °C, respectively, under vacuum (<1 mPa) for 20 h. The specific area was calculated by the method of Brunauer, Emmett, Teller (BET) and the pore size distribution was calculated from the desorption isotherm.
The crystallinity of the impregnated Zn(II)-AHBD aerogel particles was analyzed by X-ray diffraction (Discover D8-Bruker). A JEOL field emission microscope, model JEM-FS2200 HRP, operating at 200 kV was used for EDX (Energy-Dispersive X-ray spectroscopy).
Particle morphology of the aerogels were analysed by Field Emission Scanning Microscopy (FE-SEM JEOL 7001F). Before analysis the aerogel particles were covered with approximately 300 A° of gold by a sputter-coater in argon atmosphere (Polaron).
The metal loading of the aerogels was determined by atomic absorption (AA) using a VARIAN SPECTRA 220FS analyser. Digestion of the samples was performed with HCl, H2O2 and HF using microwave at 250 °C. The carbonate formation was determined by integration of the 1H-NMR spectrum of the crude reaction mixture. The calculations were made using the signals from the epoxy groups of limonene oxide (δ = 2.9-3.1 ppm) and from the cyclic carbonate groups (δ = 4.3-4.5 ppm) [60]. Errors in the carbonate formation measurements were estimated by comparing the results of at least 3 integrations of each spectrum. The error in the final results was lower than 10%.

Aerogels Characterization
The aerogel structure was studied by Fourier transform infrared spectroscopy. The spectra were acquired between 4000 and 400 cm −1 with a 4 cm −1 step using an attenuated total reflectance (ATR) sampling accessory (Smart iTR) equipped with a single-bounce diamond crystal on a Thermo Nicolet 6700 Spectrometer.
Textural characteristics of aerogels, mainly specific surface area, mean pore diameter and pore size distribution, were determined by N2 adsorption-desorption analysis at low temperature (NOVA 3000e). The specific pore volume is determined by the single point adsorption method. The shown average pore diameter is based on the desorption isotherm of the Barrett-Joyner-Halenda (BJH) method. Prior to the determination, the alginate and silica aerogels were degassed at 70 and 200 • C, respectively, under vacuum (<1 mPa) for 20 h. The specific area was calculated by the method of Brunauer, Emmett, Teller (BET) and the pore size distribution was calculated from the desorption isotherm.
The crystallinity of the impregnated Zn(II)-AHBD aerogel particles was analyzed by Xray diffraction (Discover D8-Bruker). A JEOL field emission microscope, model JEM-FS2200 HRP, operating at 200 kV was used for EDX (Energy-Dispersive X-ray spectroscopy).
Particle morphology of the aerogels were analysed by Field Emission Scanning Microscopy (FE-SEM JEOL 7001F). Before analysis the aerogel particles were covered with approximately 300 A • of gold by a sputter-coater in argon atmosphere (Polaron).
The metal loading of the aerogels was determined by atomic absorption (AA) using a VARIAN SPECTRA 220FS analyser. Digestion of the samples was performed with HCl, H2O2 and HF using microwave at 250 • C.

Conclusions
Alginate and silica aerogels were prepared and explored as supports for the ionic liquid [Aliquat][Cl] and applied as catalysts on the cycloaddition reaction between CO 2 and limonene oxide. Surprisingly, alginate impregnated aerogel materials presented a much higher efficiency as catalysts in comparison with silica aerogels. Physical characterizations showed differences on the impregnation efficiency of [Aliquat][Cl] into alginate and silica aerogels, very likely due to differences in the pore size of the aerogels. This method takes advantage of both homogeneous (high rates and good selectivity) and heterogeneous catalysis (easy separation) together with the utilization of a bio-based support with high catalyst loadings.
Author Contributions: The manuscript was written through contributions of all authors. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.