Propylsulfonic Acid Functionalized SBA-15 Mesoporous Silica as Efficient Catalysts for the Acetalization of Glycerol

As the main by-product obtained from biomass, glycerol could be converted into valuable chemicals. Tunable propylsulfonic acid functionalized SBA-15 and KIT-6 with different structural parameters have been prepared by different methods while using 3-mercaptopropyltrimethoxysilane (MPTMS) as the source of sulfur. The composition and structure of the synthesized catalysts have been well-characterized by N2 adsorption-desorption (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The catalytic performance of the prepared catalysts have been evaluated and compared in glycerol acetalization with formaldehyde to the mixture of 1,3-dioxane-5-ol and 1,3-dioxolane-4-methanol. Optimum reaction parameters were investigated to enhance the yield of products and control the distribution of glycerol formals. More than 90% yield of cyclic acetals were obtained with the ratio of two isomers 5R to 6R of 42:58.


Introduction
Glycerol (1,2,3-propanetriol or glycerine) is a high value-added by-product of the biodiesel production from the reaction of vegetable oil/triglyceride with alcohols and the amount of glycerol constitutes 10 wt % of total biodiesel production [1,2].Over 68% of glycerol production was achieved through biodiesel in 2015.In addition, biodiesel is the fastest growing biofuel, which may increase to at least 2700 kilo tons by 2022.The biodiesel market size is expected to witness steady gains over the next seven years, which enable it to account for a significant chunk of glycerol market share [3].Glycerol is also the production of ethanol fermentation processes [4].Being a by-product of biodiesel, glycerol supply has ascended rapidly and the already saturated market has been adversely affected [5].The global glycerol market is segmented as follows by application: polyether polyols, pharmaceuticals, and tobacco humectants, etc., and numerous utilization of glycerol have been studied.To stabilize the glycerol market, the main research was focused on hydrogenolysis [6], etherification [7], hydrogenation [8], partial oxidation [9,10], and dehydration [11].Acetalization of glycerol with aldehydes/ketones is also one of valorization pathway to balance the glycerol market and acetals/ketals could be widely used in fine organic synthesis [12].
In terms of catalysts, the acetalization of glycerol is an acid catalyzed reaction and glycerol formals (GF) could be manufactured while using homogeneous catalysts, such as H2SO4 [21], HF, HCl, or PTSA [22], giving a 40:60 ratio of 5R to 6R [23].Acetalization of glycerol with formaldehyde that was performed with PTSA achieved maximum yield (78%) within 2 h and there is no further improvement to prolong reaction times.The preparation of glycerol formal in the presence of different Lewis and Brønsted acid catalysts gave a large spectrum of yield, with 84% being the highest yield reported up to now, with a ratio 5R to 6R of 27:73.However, homogeneous catalysts are toxicant, corrosive, and difficult to separate from the reaction system, which limited the wide application.
In contrast, heterogeneous catalysts have been investigated extensively in recent studies and they present many advantages over homogeneous systems.Acetalization/Ketalization of glycerol using heterogeneous catalysts, like ion exchange resins [24], metal oxides [25,26], and SBA-15 supported Cs2.5H0.5PW12O40[27,28], etc. have been studied.Da Silva et al. [17] investigated the efficiency and the reaction kinetics with solid catalysts, such as Amberlyst-15, H-USY, montmorillonite K-10, and zeolite Beta in comparison with a homogeneous catalyst PTSA in the acetalization of glycerol with acetone and formaldehyde aqueous.Zeolite Bate (Si/Al = 16) leads to a conversion of 95% at 60 min.The conversion of acetone in glycerol acetalization can reach 90% within 40 min.However, the conversion of glycerol acetalization with formaldehyde is only 80%.The yield of glycerol with formaldehyde using Amberlyst-36 can reach 77% and the ratio of 5R to 6R is 24:76 [29].
The ordered mesoporous materials attracted much attention for their high surface areas, controllable pore size, and pore volume.Surface functionalization of molecular sieve is an efficient method to enhance the acid capacity by modifying the surface of mesoporous materials and it has been widely used in the synthesis of Bisphenol-A and biodiesel synthesis [30][31][32].Various groups, such as vinyl, thiol (-SH), and amine were successfully used to functionalize the mesoporous materials [33][34][35].Acid-functionalized mesoporous silica typically containing propylsulfonic acid has been extensively investigated as an efficient catalyst in acid-catalyzed reaction (esterification, selective acylation, dehydration, hydrolysis, and condensation) [36][37][38].SO3H-functionalized mesoporous silica are commonly generated using 3-mercaptopropyltrimethoxysilane (MPTMS) via oxidation of thiol-propyl groups during the co-condensation or post-grafting oxidation methods.Moreover, sulfonation benefits In terms of catalysts, the acetalization of glycerol is an acid catalyzed reaction and glycerol formals (GF) could be manufactured while using homogeneous catalysts, such as H 2 SO 4 [21], HF, HCl, or PTSA [22], giving a 40:60 ratio of 5R to 6R [23].Acetalization of glycerol with formaldehyde that was performed with PTSA achieved maximum yield (78%) within 2 h and there is no further improvement to prolong reaction times.The preparation of glycerol formal in the presence of different Lewis and Brønsted acid catalysts gave a large spectrum of yield, with 84% being the highest yield reported up to now, with a ratio 5R to 6R of 27:73.However, homogeneous catalysts are toxicant, corrosive, and difficult to separate from the reaction system, which limited the wide application.
In contrast, heterogeneous catalysts have been investigated extensively in recent studies and they present many advantages over homogeneous systems.Acetalization/Ketalization of glycerol using heterogeneous catalysts, like ion exchange resins [24], metal oxides [25,26], and SBA-15 supported Cs 2.5 H 0.5 PW 12 O 40 [27,28], etc. have been studied.Da Silva et al. [17] investigated the efficiency and the reaction kinetics with solid catalysts, such as Amberlyst-15, H-USY, montmorillonite K-10, and zeolite Beta in comparison with a homogeneous catalyst PTSA in the acetalization of glycerol with acetone and formaldehyde aqueous.Zeolite Bate (Si/Al = 16) leads to a conversion of 95% at 60 min.The conversion of acetone in glycerol acetalization can reach 90% within 40 min.However, the conversion of glycerol acetalization with formaldehyde is only 80%.The yield of glycerol with formaldehyde using Amberlyst-36 can reach 77% and the ratio of 5R to 6R is 24:76 [29].
The ordered mesoporous materials attracted much attention for their high surface areas, controllable pore size, and pore volume.Surface functionalization of molecular sieve is an efficient method to enhance the acid capacity by modifying the surface of mesoporous materials and it has been widely used in the synthesis of Bisphenol-A and biodiesel synthesis [30][31][32].Various groups, such as vinyl, thiol (-SH), and amine were successfully used to functionalize the mesoporous materials [33][34][35].Acid-functionalized mesoporous silica typically containing propylsulfonic acid has been extensively investigated as an efficient catalyst in acid-catalyzed reaction (esterification, selective acylation, dehydration, hydrolysis, and condensation) [36][37][38].SO 3 H-functionalized mesoporous silica are commonly generated using 3-mercaptopropyltrimethoxysilane (MPTMS) via oxidation of thiol-propyl groups during the co-condensation or post-grafting oxidation methods.Moreover, sulfonation benefits from higher local hydrophobicity of the sulfonic acid sites [39].It was acknowledged that mild hydrothermal treatment could be used to activate the surface of molecular sieve and increase its acid capacity for grafting in the sensitive surface.Inorganic salts, such as KCl, NaCl, and NaF, etc. are usually used to moderate the wall thickness and the degree of polymerization during post-synthesis hydrothermal treatments of SBA-15 [40], and also aid in the dissolution of silica.Thus, we hypothesized that the addition of NaCl under mild hydrothermal conditions could promote the activation of SBA-15 surfaces for grafting [41].
In this paper, an array of propylsulfonic acid functionalized mesoporous silicas SBA-15 and KIT-6 were prepared and catalytic performance of synthesized catalysts in glycerol acetalization with formaldehyde were investigated.We also used NaCl as additive in the process of catalyst preparation [42].The synthesized catalysts were characterized by N 2 adsorption-desorption (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray fluorescence (XRF, bulk), X-ray photoelectron spectroscopy (XPS, surface), and acid capacity based on acid-base titration.Moreover, the effect of various reaction parameters, such as reaction time, reaction temperature, molar ratio of formaldehyde to glycerol, and catalyst loading were studied to enhance the yield of isomeric products.The distribution of the 5R and 6R was also investigated.

Materials Characterization
In this section, PrSO 3 H-SBA-15-x (x = 0, 200, 400, 800) is the propylsulfonic acid functionalized SBA-15 mesoporous silica synthesized by post-grafting with different amount of additive.PrSO 3 H-SBA-15-OP was materials synthesized by one-pot method and PrSO 3 H-SBA-15-Tol means materials that was synthesized by the co-condensation method.PrSO 3 H-KIT-6-T were catalyst synthesized by co-condensation method and aged at different temperature (80 The successful synthesis of parent and grafting ordered mesoporous silica SBA-15 and KIT-6 were first assessed by XRD and BET.The structure and composition of the samples were identified by XRD and XPS.Powder XRD patterns of small-angle range of the series of PrSO 3 H-SBA-15 synthesized by different methods are presented in Figure 1.All of the functionalized materials show similar XRD patterns with common reflections at 0.8 • , which is consistent with the parent SBA-15 and confirms that grafting of propylsulfonic acid group (-PrSO 3 H) and subsequent oxidation does not alter the crystal structure.The structural integrity of precursor is retained after both functionalization by -PrSO 3 H in H 2 O using NaCl as additive and oxidation by H 2 O 2 .
hydrothermal treatment could be used to activate the surface of molecular sieve and increase its acid capacity for grafting in the sensitive surface.Inorganic salts, such as KCl, NaCl, and NaF, etc. are usually used to moderate the wall thickness and the degree of polymerization during post-synthesis hydrothermal treatments of SBA-15 [40], and also aid in the dissolution of silica.Thus, we hypothesized that the addition of NaCl under mild hydrothermal conditions could promote the activation of SBA-15 surfaces for grafting [41].
In this paper, an array of propylsulfonic acid functionalized mesoporous silicas SBA-15 and KIT-6 were prepared and catalytic performance of synthesized catalysts in glycerol acetalization with formaldehyde were investigated.We also used NaCl as additive in the process of catalyst preparation [42].The synthesized catalysts were characterized by N2 adsorption-desorption (BET), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray fluorescence (XRF, bulk), X-ray photoelectron spectroscopy (XPS, surface), and acid capacity based on acid-base titration.Moreover, the effect of various reaction parameters, such as reaction time, reaction temperature, molar ratio of formaldehyde to glycerol, and catalyst loading were studied to enhance the yield of isomeric products.The distribution of the 5R and 6R was also investigated.
The successful synthesis of parent and grafting ordered mesoporous silica SBA-15 and KIT-6 were first assessed by XRD and BET.The structure and composition of the samples were identified by XRD and XPS.Powder XRD patterns of small-angle range of the series of PrSO3H-SBA-15 synthesized by different methods are presented in Figure 1.All of the functionalized materials show similar XRD patterns with common reflections at 0.8°, which is consistent with the parent SBA-15 and confirms that grafting of propylsulfonic acid group (-PrSO3H) and subsequent oxidation does not alter the crystal structure.The structural integrity of precursor is retained after both functionalization by -PrSO3H in H2O using NaCl as additive and oxidation by H2O2.  Figure 2 compares the corresponding N2 adsorption-desorption isotherms and pore size distributions curves of the precursor and sulfonated mesoporous materials, which best represents the characteristics of all the samples.It could be observed that most of the samples isotherms are of Type IV with H1 hysteresis loops, which is the characteristic of mesoporous materials.However, PrSO 3 H-SBA-15-OP and KIT-6-80 are of Type IV with mixed H1-H2-type hysteresis loops.The presence of parallel adsorption and desorption branches suggests the existence of mesoporous structure.Different hysteresis loop occurred at two-dimensional (2D) hexagonal (SBA-15) and three-dimensional (3D) cubic (KIT-6) structures.The calculation from the desorption branch using the Barrett-Joyner-Halenda (BJH) method (Figure 2A) shows that the catalysts present a very narrow pore size distribution.The physical-chemical parameters of various samples are summarized in Table 1.Surface areas were calculated on the basis of Brunauer-Emmett-Teller (BET) equation and pore sizes by the Barrett-Joyner-Halenda (BJH) method.There is an obvious decrease in BET surface area, pore diameter, and pore volume when -PrSO 3 H is grafted on the mesoporous materials.The pore diameter of sulfonated SBA-15 synthesized by post-grafting using NaCl as additive is higher than that of conventional method by one-pot method.The surface area of catalysts synthesized by post-grafting method with different amount of NaCl range from 281.19 m 2 /g to 410.46 m 2 /g and pore volumes range from 0.54 cm 3 /g to 0.87 cm 3 /g.Pore system of molecular sieve SBA-15 consists of hexagonally ordered mesoporous and irregular microporous [43].The decrease of the surface area may be caused by the blockage of the micropore during the grafting.PrSO 3 H-SBA-15 exhibits the smallest surface area and highest bulk sulfur and acid content, which indicates the maximal grafting loading of propylsulfonic acid groups.The broad distribution curves of SBA-15 when compared to that of SBA-15 in previous report may be due to the quality of commercial SBA-15.Figure 2 compares the corresponding N2 adsorption-desorption isotherms and pore size distributions curves of the precursor and sulfonated mesoporous materials, which best represents the characteristics of all the samples.It could be observed that most of the samples isotherms are of Type IV with H1 hysteresis loops, which is the characteristic of mesoporous materials.However, PrSO3H-SBA-15-OP and KIT-6-80 are of Type IV with mixed H1-H2-type hysteresis loops.The presence of parallel adsorption and desorption branches suggests the existence of mesoporous structure.Different hysteresis loop occurred at two-dimensional (2D) hexagonal (SBA-15) and three-dimensional (3D) cubic (KIT-6) structures.The calculation from the desorption branch using the Barrett-Joyner-Halenda (BJH) method (Figure 2A) shows that the catalysts present a very narrow pore size distribution.The physical-chemical parameters of various samples are summarized in Table 1.Surface areas were calculated on the basis of Brunauer-Emmett-Teller (BET) equation and pore sizes by the Barrett-Joyner-Halenda (BJH) method.There is an obvious decrease in BET surface area, pore diameter, and pore volume when -PrSO3H is grafted on the mesoporous materials.The pore diameter of sulfonated SBA-15 synthesized by post-grafting using NaCl as additive is higher than that of conventional method by one-pot method.The surface area of catalysts synthesized by post-grafting method with different amount of NaCl range from 281.19 m 2 /g to 410.46 m 2 /g and pore volumes range from 0.54 cm 3 /g to 0.87 cm 3 /g.Pore system of molecular sieve SBA-15 consists of hexagonally ordered mesoporous and irregular microporous [43].The decrease of the surface area may be caused by the blockage of the micropore during the grafting.PrSO3H-SBA-15 exhibits the smallest surface area and highest bulk sulfur and acid content, which indicates the maximal grafting loading of propylsulfonic acid groups.The broad distribution curves of SBA-15 when compared to that of SBA-15 in previous report may be due to the quality of commercial SBA-15.Complete grafting of thiol-propyl groups on mesoporous materials and the following oxidation to sulfonic acid groups were examined by XRF and XPS.As could be deserved by the bulk sulfur content of the catalysts that were determined by XRF in Table 1, the bulk sulfur content of PrSO3H-SBA-15 synthesized by one pot method and conventional method is 0.06 wt % and 0.14 wt %, respectively.Higher grafting amount of propylsulfonic acid groups was achieved by post-grafting in H2O using NaCl as additive.The relation between bulk sulfur content and acid site also suggests the complete oxidation of thiol-propyl groups, which is consistent with the results of XPS.As shown in S2p XP spectra of both sulfonic acid functionalized materials (Figure 3), a single energy doublet that was caused by propylsulfonic acid groups centered at 168.9 eV, which revealed that a single S chemical environment appeared, confirming the complete oxidization of thiol-propyl groups to propylsulfonic acid groups [44][45][46].It is also notable that surface sulfur amounts range from 3.60 wt % to 5.23 wt % with the amount of NaCl changing from 0 to 800 mg.PrSO3H-SBA-15-400 has the highest bulk S content and smaller pore volume.The concentration of active Si-OH on molecular sieve SBA-15 have an important influence on the grafting efficiency [47].A higher propylsulfonic acid amount is obtained on SBA-15 than the family of KIT-6 by comparing of the bulk and surface sulfur, which suggests the higher density of reactive silicone groups on SBA-15.Complete grafting of thiol-propyl groups on mesoporous materials and the following oxidation to sulfonic acid groups were examined by XRF and XPS.As could be deserved by the bulk sulfur content of the catalysts that were determined by XRF in Table 1, the bulk sulfur content of PrSO 3 H-SBA-15 synthesized by one pot method and conventional method is 0.06 wt % and 0.14 wt %, respectively.Higher grafting amount of propylsulfonic acid groups was achieved by post-grafting in H 2 O using NaCl as additive.The relation between bulk sulfur content and acid site also suggests the complete oxidation of thiol-propyl groups, which is consistent with the results of XPS.As shown in S2p XP spectra of both sulfonic acid functionalized materials (Figure 3), a single energy doublet that was caused by propylsulfonic acid groups centered at 168.9 eV, which revealed that a single S chemical environment appeared, confirming the complete oxidization of thiol-propyl groups to propylsulfonic acid groups [44][45][46].It is also notable that surface sulfur amounts range from 3.60 wt % to 5.23 wt % with the amount of NaCl changing from 0 to 800 mg.PrSO 3 H-SBA-15-400 has the highest bulk S content and smaller pore volume.The concentration of active Si-OH on molecular sieve SBA-15 have an important influence on the grafting efficiency [47].A higher propylsulfonic acid amount is obtained on SBA-15 than the family of KIT-6 by comparing of the bulk and surface sulfur, which suggests the higher density of reactive silicone groups on SBA-15.Complete grafting of thiol-propyl groups on mesoporous materials and the following oxidation to sulfonic acid groups were examined by XRF and XPS.As could be deserved by the bulk sulfur content of the catalysts that were determined by XRF in Table 1, the bulk sulfur content of PrSO3H-SBA-15 synthesized by one pot method and conventional method is 0.06 wt % and 0.14 wt %, respectively.Higher grafting amount of propylsulfonic acid groups was achieved by post-grafting in H2O using NaCl as additive.The relation between bulk sulfur content and acid site also suggests the complete oxidation of thiol-propyl groups, which is consistent with the results of XPS.As shown in S2p XP spectra of both sulfonic acid functionalized materials (Figure 3), a single energy doublet that was caused by propylsulfonic acid groups centered at 168.9 eV, which revealed that a single S chemical environment appeared, confirming the complete oxidization of thiol-propyl groups to propylsulfonic acid groups [44][45][46].It is also notable that surface sulfur amounts range from 3.60 wt % to 5.23 wt % with the amount of NaCl changing from 0 to 800 mg.PrSO3H-SBA-15-400 has the highest bulk S content and smaller pore volume.The concentration of active Si-OH on molecular sieve SBA-15 have an important influence on the grafting efficiency [47].A higher propylsulfonic acid amount is obtained on SBA-15 than the family of KIT-6 by comparing of the bulk and surface sulfur, which suggests the higher density of reactive silicone groups on SBA-15.The synthesis of propylsulfonic acid functionalized mesoporous silica includes the grafting of -SH and successful oxidation of -SH to -SO3H by hydrogen peroxide (H2O2) [48].Oxidation of the -SH to -SO3H is necessary to enhance the acid strength of molecular sieve and to obtain catalysts with good catalytic activity.The ideal oxidation of -SH to -SO3H should be proceeded with the following reaction (R-SH + 2H2O2→R-OSO2H + 3H2O).The acidic properties of the grafted sulfonic acid catalysts were measured by acid-base titration.As shown in Table 1, the acid capacity for NaCl added PrSO3H-SBA-15 was found to be two to three times higher than PrSO3H-SBA-15-Tol.
By SEM and TEM, the shape of the primary crystallites and the order of their packing in catalysts were examined.From SEM images (Figure 4b), we can see that these materials are consisted of some short rods with a relatively uniform length and the short-rod morphology of SBA-15 sample was well preserved after grafting (Figure 4b,c).The TEM image of SBA-15 (Figure 5a) shows well-ordered hexagonal arrays of mesopores and confirms its 2D hexagonal structure.The TEM observations also indicated that ordered mesostructured of SBA-15 and KIT-6 were retained, even after post-grafting.As an exception, the image of PrSO3H-SBA-15-OP is different from the others, which is consistent with the N2 isotherms in Figure 2B.The synthesis of propylsulfonic acid functionalized mesoporous silica includes the grafting of -SH and successful oxidation of -SH to -SO 3 H by hydrogen peroxide (H 2 O 2 ) [48].Oxidation of the -SH to -SO 3 H is necessary to enhance the acid strength of molecular sieve and to obtain catalysts with good catalytic activity.The ideal oxidation of -SH to -SO 3 H should be proceeded with the following reaction (R-SH + 2H 2 O 2 →R-OSO 2 H + 3H 2 O).The acidic properties of the grafted sulfonic acid catalysts were measured by acid-base titration.As shown in Table 1, the acid capacity for NaCl added PrSO 3 H-SBA-15 was found to be two to three times higher than PrSO 3 H-SBA-15-Tol.
By SEM and TEM, the shape of the primary crystallites and the order of their packing in catalysts were examined.From SEM images (Figure 4b), we can see that these materials are consisted of some short rods with a relatively uniform length and the short-rod morphology of SBA-15 sample was well preserved after grafting (Figure 4b,c).The TEM image of SBA-15 (Figure 5a) shows well-ordered hexagonal arrays of mesopores and confirms its 2D hexagonal structure.The TEM observations also indicated that ordered mesostructured of SBA-15 and KIT-6 were retained, even after post-grafting.As an exception, the image of PrSO 3 H-SBA-15-OP is different from the others, which is consistent with the N 2 isotherms in Figure 2B.The synthesis of propylsulfonic acid functionalized mesoporous silica includes the grafting of -SH and successful oxidation of -SH to -SO3H by hydrogen peroxide (H2O2) [48].Oxidation of the -SH to -SO3H is necessary to enhance the acid strength of molecular sieve and to obtain catalysts with good catalytic activity.The ideal oxidation of -SH to -SO3H should be proceeded with the following reaction (R-SH + 2H2O2→R-OSO2H + 3H2O).The acidic properties of the grafted sulfonic acid catalysts were measured by acid-base titration.As shown in Table 1, the acid capacity for NaCl added PrSO3H-SBA-15 was found to be two to three times higher than PrSO3H-SBA-15-Tol.
By SEM and TEM, the shape of the primary crystallites and the order of their packing in catalysts were examined.From SEM images (Figure 4b), we can see that these materials are consisted of some short rods with a relatively uniform length and the short-rod morphology of SBA-15 sample was well preserved after grafting (Figure 4b,c).The TEM image of SBA-15 (Figure 5a) shows well-ordered hexagonal arrays of mesopores and confirms its 2D hexagonal structure.The TEM observations also indicated that ordered mesostructured of SBA-15 and KIT-6 were retained, even after post-grafting.As an exception, the image of PrSO3H-SBA-15-OP is different from the others, which is consistent with the N2 isotherms in Figure 2B.

Catalytic Activity for Glycerol Acetalization
Propylsulfonic acid functionalized mesoporous materials were prepared by different methods and evaluated in the glycerol acetalization with TOX to obtain the mixture of the glycerol formal (Scheme 1, 5R and 6R).It is clear that all sulfonated mesoporous molecular sieves exhibit higher activity than the non-sulfonated parent material.The gap between the performance of sulfonated and non-sulfonated one is owed to the higher accessibility of reactants to the acidic sites of sulfonated catalysts.
Figure 6 shows the yield of glycerol formal and the ratio of 5R to 6R achieved by three different source of formaldehyde (FA): 1,3,5-trioxane (TOX), formaldehyde solution (FA), and paraformaldehyde (PF).When compared with the results that were obtained with the three source of formaldehyde, TOX shows the best yield of glycerol formal and the ratio of 5R to 6R is 42:58.Herein, we used TOX as the source of FA for the further study.

Catalytic Activity for Glycerol Acetalization
Propylsulfonic acid functionalized mesoporous materials were prepared by different methods and evaluated in the glycerol acetalization with TOX to obtain the mixture of the glycerol formal (Scheme 1, 5R and 6R).It is clear that all sulfonated mesoporous molecular sieves exhibit higher activity than the non-sulfonated parent material.The gap between the performance of sulfonated and non-sulfonated one is owed to the higher accessibility of reactants to the acidic sites of sulfonated catalysts.
Figure 6 shows the yield of glycerol formal and the ratio of 5R to 6R achieved by three different source of formaldehyde (FA): 1,3,5-trioxane (TOX), formaldehyde solution (FA), and paraformaldehyde (PF).When compared with the results that were obtained with the three source of formaldehyde, TOX shows the best yield of glycerol formal and the ratio of 5R to 6R is 42:58.Herein, we used TOX as the source of FA for the further study.The effect of the amount of NaCl during sulfonation and the catalytic activity of the synthesized catalysts on glycerol acetalization were studied.Figure 7 showed the catalytic properties of the series of PrSO3H-SBA-15-x (x = 0, 200, 400, 800).Post-grafting synthesized SBA-15 catalysts apparently exhibited higher catalytic performance than the one-pot method and conventional toluene grafted method in the described reaction.When the amount of NaCl is 400 mg, the catalyst has the smallest surface area and highest acid capacity.PrSO3H-SBA-15-400 presented the greatest catalytic performance.The yield of 5R + 6R can reach 91.5% and the ratio of 5R:6R is 42:58.The catalytic activity of the prepared catalysts using different methods increased in the order: PrSO3H-SBA-15-400 > PrSO3H-SBA-15-OP > PrSO3H-SBA-15-Tol.According to acid-base titration, the acid capacity of the series of PrSO3H-SBA-15 range from 0.2 mmol g −1 to 1.2 mmol g −1 , which is in agreement with the catalytic performance.That is to say, the catalysts that were synthesized by post-synthesis method exhibited higher catalytic performance than the conventional methods.The effect of the amount of NaCl during sulfonation and the catalytic activity of the synthesized catalysts on glycerol acetalization were studied.Figure 7 showed the catalytic properties of the series of PrSO3H-SBA-15-x (x = 0, 200, 400, 800).Post-grafting synthesized SBA-15 catalysts apparently exhibited higher catalytic performance than the one-pot method and conventional toluene grafted method in the described reaction.When the amount of NaCl is 400 mg, the catalyst has the smallest surface area and highest acid capacity.PrSO3H-SBA-15-400 presented the greatest catalytic performance.The yield of 5R + 6R can reach 91.5% and the ratio of 5R:6R is 42:58.The catalytic activity of the prepared catalysts using different methods increased in the order: PrSO3H-SBA-15-400 > PrSO3H-SBA-15-OP > PrSO3H-SBA-15-Tol.According to acid-base titration, the acid capacity of the series of PrSO3H-SBA-15 range from 0.2 mmol g −1 to 1.2 mmol g −1 , which is in agreement with the catalytic performance.That is to say, the catalysts that were synthesized by post-synthesis method exhibited higher catalytic performance than the conventional methods.The effect of the amount of NaCl during sulfonation and the catalytic activity of the synthesized catalysts on glycerol acetalization were studied.Figure 7 showed the catalytic properties of the series of PrSO 3 H-SBA-15-x (x = 0, 200, 400, 800).Post-grafting synthesized SBA-15 catalysts apparently exhibited higher catalytic performance than the one-pot method and conventional toluene grafted method in the described reaction.When the amount of NaCl is 400 mg, the catalyst has the smallest surface area and highest acid capacity.PrSO 3 H-SBA-15-400 presented the greatest catalytic performance.The yield of 5R + 6R can reach 91.5% and the ratio of 5R:6R is 42:58.The catalytic activity of the prepared catalysts using different methods increased in the order: PrSO 3 H-SBA-15-400 > PrSO 3 H-SBA-15-OP > PrSO 3 H-SBA-15-Tol.According to acid-base titration, the acid capacity of the series of PrSO 3 H-SBA-15 range from 0.2 mmol g −1 to 1.2 mmol g −1 , which is in agreement with the catalytic performance.That is to say, the catalysts that were synthesized by post-synthesis method exhibited higher catalytic performance than the conventional methods.In previous work, the investigations were aiming at increasing the amount of 1,3-dioxan-5-ols (6R) because 6R could be used as raw materials of 1,3-propanediol derivatives.The product distribution of glycerol acetalization with TOX (Scheme 1, 5R and 6R) is highly dependent on the experimental conditions.Therefore, PrSO3H-SBA-15-400 was used to optimize the reaction condition to reach a high yield of the isomers and ratio of 6R.

Effect of catalyst content:
To optimize the amount of catalyst for glycerol acetalization with TOX, the reaction was conducted over PrSO3H-SBA-15-400 for 8 h at 90 °C using a different amount of catalyst (Figure 8).The yield of glycerol formal was increased from 85.7% to 91.5% by increasing the catalyst amount from 2 wt % to 4 wt %.However, there is no further enhance was achieved and the ratio of 6R decreased gradually when increasing the catalyst amount to 6 wt %.Effect of reaction temperature: Figure 9 showed the effect of temperature variation on the acetalization of glycerol with formaldehyde.Several tests were conducted at different temperature from 40 °C to 120 °C to acquire the optimal parameter.The yield of GF changed from 18.5% at 60 °C to 91.5% at 90 °C.The amount of 6R increased at first with the temperature ranging from 40 °C to 90 °C.However, 6R isomer decreased quickly after 90 °C, which may be caused by the isomerization of the 5R to 6R.In previous work, the investigations were aiming at increasing the amount of 1,3-dioxan-5-ols (6R) because 6R could be used as raw materials of 1,3-propanediol derivatives.The product distribution of glycerol acetalization with TOX (Scheme 1, 5R and 6R) is highly dependent on the experimental conditions.Therefore, PrSO 3 H-SBA-15-400 was used to optimize the reaction condition to reach a high yield of the isomers and ratio of 6R.

Effect of catalyst content:
To optimize the amount of catalyst for glycerol acetalization with TOX, the reaction was conducted over PrSO 3 H-SBA-15-400 for 8 h at 90 • C using a different amount of catalyst (Figure 8).The yield of glycerol formal was increased from 85.7% to 91.5% by increasing the catalyst amount from 2 wt % to 4 wt %.However, there is no further enhance was achieved and the ratio of 6R decreased gradually when increasing the catalyst amount to 6 wt %.In previous work, the investigations were aiming at increasing the amount of 1,3-dioxan-5-ols (6R) because 6R could be used as raw materials of 1,3-propanediol derivatives.The product distribution of glycerol acetalization with TOX (Scheme 1, 5R and 6R) is highly dependent on the experimental conditions.Therefore, PrSO3H-SBA-15-400 was used to optimize the reaction condition to reach a high yield of the isomers and ratio of 6R.

Effect of catalyst content:
To optimize the amount of catalyst for glycerol acetalization with TOX, the reaction was conducted over PrSO3H-SBA-15-400 for 8 h at 90 °C using a different amount of catalyst (Figure 8).The yield of glycerol formal was increased from 85.7% to 91.5% by increasing the catalyst amount from 2 wt % to 4 wt %.However, there is no further enhance was achieved and the ratio of 6R decreased gradually when increasing the catalyst amount to 6 wt %.Effect of reaction temperature: Figure 9 showed the effect of temperature variation on the acetalization of glycerol with formaldehyde.Several tests were conducted at different temperature from 40 °C to 120 °C to acquire the optimal parameter.The yield of GF changed from 18.5% at 60 °C to 91.5% at 90 °C.The amount of 6R increased at first with the temperature ranging from 40 °C to 90 °C.However, 6R isomer decreased quickly after 90 °C, which may be caused by the isomerization of the 5R to 6R.Effect of reaction temperature: Figure 9 showed the effect of temperature variation on the acetalization of glycerol with formaldehyde.Several tests were conducted at different temperature from 40 • C to 120 • C to acquire the optimal parameter.The yield of GF changed from 18.5% at 60 • C to 91.5% at 90 • C. The amount of 6R increased at first with the temperature ranging from 40 • C to 90 • C.However, 6R isomer decreased quickly after 90 • C, which may be caused by the isomerization of the 5R to 6R.

Effect of mole ratio of formaldehyde to glycerol:
The catalytic experiments were carried at 90 °C, 8 h with a catalyst amount of 4 wt %, formaldehyde/glycerol = 1:1, 1.5:1, 2:1, 3:1. Figure 10 indicates the yield of GF and the distribution of the glycerol formal that is influenced by the change of molar ratio of formaldehyde to glycerol.The yield of GF increases from 83.7% to 91.5% when the mole ratio of formaldehyde to glycerol increasing from 1.2:1 to 1.5:1.Further increasing molar ratio from 1.5:1 to 3:1, the yield of GF decreased to 61.0% and the ratio of 6R decreased slightly.

Effect of reaction time:
The experiments were carried out from 1 h to 16 h over PrSO3H-SBA-15-400 while using 0.2 g catalysts at 90 °C (Figure 11).The yield increased rapidly by increasing the reaction time to 8 h and reached 91.3%.There is no further increase in the GF yield while extending the reaction time and 6R is decreased gradually.

Effect of mole ratio of formaldehyde to glycerol:
The catalytic experiments were carried at 90 • C, 8 h with a catalyst amount of 4 wt %, formaldehyde/glycerol = 1:1, 1.5:1, 2:1, 3:1. Figure 10 indicates the yield of GF and the distribution of the glycerol formal that is influenced by the change of molar ratio of formaldehyde to glycerol.The yield of GF increases from 83.7% to 91.5% when the mole ratio of formaldehyde to glycerol increasing from 1.2:1 to 1.5:1.Further increasing molar ratio from 1.5:1 to 3:1, the yield of GF decreased to 61.0% and the ratio of 6R decreased slightly.

Effect of mole ratio of formaldehyde to glycerol:
The catalytic experiments were carried at 90 °C, 8 h with a catalyst amount of 4 wt %, formaldehyde/glycerol = 1:1, 1.5:1, 2:1, 3:1. Figure 10 indicates the yield of GF and the distribution of the glycerol formal that is influenced by the change of molar ratio of formaldehyde to glycerol.The yield of GF increases from 83.7% to 91.5% when the mole ratio of formaldehyde to glycerol increasing from 1.2:1 to 1.5:1.Further increasing molar ratio from 1.5:1 to 3:1, the yield of GF decreased to 61.0% and the ratio of 6R decreased slightly.

Effect of reaction time:
The experiments were carried out from 1 h to 16 h over PrSO3H-SBA-15-400 while using 0.2 g catalysts at 90 °C (Figure 11).The yield increased rapidly by increasing the reaction time to 8 h and reached 91.3%.There is no further increase in the GF yield while extending the reaction time and 6R is decreased gradually.

Effect of reaction time:
The experiments were carried out from 1 h to 16 h over PrSO 3 H-SBA-15-400 while using 0.2 g catalysts at 90 • C (Figure 11).The yield increased rapidly by increasing the reaction time to 8 h and reached 91.3%.There is no further increase in the GF yield while extending the reaction time and 6R is decreased gradually.Hence, the glycerol acetalization was conducted in solvent free condition while using TOX as the source of formaldehyde.The optimum condition is: 4 wt % catalysts amount, formaldehyde: glycerol 1.5:1 at 90 °C for 8 h, and optimal catalyst activity is 91.5% yield of GF and the ratio of 5R to 6R is 42:58.
We then investigated KIT-6 catalyst family in glycerol acetalization under the optimal reaction parameters and compared the PrSO3H-SBA-15 family with PrSO3H-KIT-6.PrSO3H-KIT-6 was synthesized under different aging temperature for 24 h and the aging temperature showed a dramatic effect on the physical properties and the grafting amount of -SO3H.Different aging temperature expands the surface area from 362 m 2 g −1 to 443 m 2 g −1 and it leads to the decrease of wall thickness, which is in accordance with the literature.
As summarized in Table 2, PrSO3H-KIT-6-80 showed optimal performance for the acetalization of glycerol with formaldehyde (Table 2, Entry 2).The yield of 5R + 6R can reach 89.6%, and the ratio of 5R:6R is 34:66.The SBA-15 supported propylsulfonic acid functionalized catalysts showed a slightly higher yield of GF than KIT-6 supported catalysts (Table 2, Entries 2, 5).However, the ratio of isomer 6R catalyzed by the series of KIT-6 is obviously higher than the series of SBA-15.SBA-15 possesses parallel mesopore channels organized in hexagonal arrangement which may provide better transportation channels to get more access to acid sites.As shown in Table 1, the pore volume and pore size of the series of KIT-6 is smaller than propylsulfonic acid functionalized SBA-15.Maybe much microporosity in KIT-6 hinder the diffusion of bulky molecules, resulting in a low activity of the catalysts.

Reaction Mechanism
The formation of GF isomers depend on which two -OH groups joined because of one glycerol molecule has two different hydroxyl groups (-OH).According to da Sliva Ferreira and Sharma S.P. [49,50], the reaction mechanism for the acetalization of glycerol with formaldehyde mainly contains two reversible steps.As shown in Scheme 3, firstly, formaldehyde reacts with the two different -OH groups of glycerol to form two kinds of hemiacetals: 1,2-propanediol, 3- Hence, the glycerol acetalization was conducted in solvent free condition while using TOX as the source of formaldehyde.The optimum condition is: 4 wt % catalysts amount, formaldehyde: glycerol 1.5:1 at 90 • C for 8 h, and optimal catalyst activity is 91.5% yield of GF and the ratio of 5R to 6R is 42:58.
We then investigated KIT-6 catalyst family in glycerol acetalization under the optimal reaction parameters and compared the PrSO 3 H-SBA-15 family with PrSO 3 H-KIT-6.PrSO 3 H-KIT-6 was synthesized under different aging temperature for 24 h and the aging temperature showed a dramatic effect on the physical properties and the grafting amount of -SO 3 H.Different aging temperature expands the surface area from 362 m 2 g −1 to 443 m 2 g −1 and it leads to the decrease of wall thickness, which is in accordance with the literature.
As summarized in Table 2, PrSO 3 H-KIT-6-80 showed optimal performance for the acetalization of glycerol with formaldehyde (Table 2, Entry 2).The yield of 5R + 6R can reach 89.6%, and the ratio of 5R:6R is 34:66.The SBA-15 supported propylsulfonic acid functionalized catalysts showed a slightly higher yield of GF than KIT-6 supported catalysts (Table 2, Entries 2, 5).However, the ratio of isomer 6R catalyzed by the series of KIT-6 is obviously higher than the series of SBA-15.SBA-15 possesses parallel mesopore channels organized in hexagonal arrangement which may provide better transportation channels to get more access to acid sites.As shown in Table 1, the pore volume and pore size of the series of KIT-6 is smaller than propylsulfonic acid functionalized SBA-15.Maybe much microporosity in KIT-6 hinder the diffusion of bulky molecules, resulting in a low activity of the catalysts.

Reaction Mechanism
The formation of GF isomers depend on which two -OH groups joined because of one glycerol molecule has two different hydroxyl groups (-OH).According to da Sliva Ferreira and Sharma S.P. [49,50], the reaction mechanism for the acetalization of glycerol with formaldehyde mainly contains two reversible steps.As shown in Scheme 3, firstly, formaldehyde reacts with the two different -OH groups of glycerol to form two kinds of hemiacetals: 1,2-propanediol, 3-(hydroxymethoxy), and 1,3-propanediol, 2-(hydroxymethoxy).Then, two hydroxyl groups of the hemiacetal condense and form the two glycerol formal isomers 5R and 6R.From the hemiacetal, 1,2-propanediol, 3-(hydroxymethoxy), two kinds of acetal 5R and 6R can be formed, while for 1,3-propanediol, 2-(hydroxymethoxy), only the 5R acetal is formed.Therefore, the fraction of 5R isomer is always higher than 6R.The first step of converting glycerol to hemiacetal is a fast reaction, while the second step that transforms hemiacetal to the product glycerol formal isomers is slow.However, the hemiacetal cannot be detected by GC, owing to the high injector temperature requested for analysis [51].
Catalysts 2018, 8, x FOR PEER REVIEW 12 of 17 (hydroxymethoxy), and 1,3-propanediol, 2-(hydroxymethoxy).Then, two hydroxyl groups of the hemiacetal condense and form the two glycerol formal isomers 5R and 6R.From the hemiacetal, 1,2-propanediol, 3-(hydroxymethoxy), two kinds of acetal 5R and 6R can be formed, while for 1,3-propanediol, 2-(hydroxymethoxy), only the 5R acetal is formed.Therefore, the fraction of 5R isomer is always higher than 6R.The first step of converting glycerol to hemiacetal is a fast reaction, while the second step that transforms hemiacetal to the product glycerol formal isomers is slow.However, the hemiacetal cannot be detected by GC, owing to the high injector temperature requested for analysis [51].
Scheme 3. Possible reaction mechanism for glycerol acetalization with TOX.

Catalyst Reusability
Finally, we evaluated the recyclability of the PrSO3H-SBA-15-400 in the yield of glycerol acetalization and the distribution of isomers under the optimized conditions (Figure 12).In each cycle, the catalyst was easily separated and recovered from the reaction mixture, washed with dichloromethane, and subsequently dried in vacuum before another reaction was performed.The yield of glycerol formal can reach 79.4% in the fifth runs.

Catalyst Reusability
Finally, we evaluated the recyclability of the PrSO 3 H-SBA-15-400 in the yield of glycerol acetalization and the distribution of isomers under the optimized conditions (Figure 12).In each cycle, the catalyst was easily separated and recovered from the reaction mixture, washed with dichloromethane, and subsequently dried in vacuum before another reaction was performed.The yield of glycerol formal can reach 79.4% in the fifth runs.(hydroxymethoxy), and 1,3-propanediol, 2-(hydroxymethoxy).Then, two hydroxyl groups of the hemiacetal condense and form the two glycerol formal isomers 5R and 6R.From the hemiacetal, 1,2-propanediol, 3-(hydroxymethoxy), two kinds of acetal 5R and 6R can be formed, while for 1,3-propanediol, 2-(hydroxymethoxy), only the 5R acetal is formed.Therefore, the fraction of 5R isomer is always higher than 6R.The first step of converting glycerol to hemiacetal is a fast reaction, while the second step that transforms hemiacetal to the product glycerol formal isomers is slow.However, the hemiacetal cannot be detected by GC, owing to the high injector temperature requested for analysis [51].
Scheme 3. Possible reaction mechanism for glycerol acetalization with TOX.

Catalyst Reusability
Finally, we evaluated the recyclability of the PrSO3H-SBA-15-400 in the yield of glycerol acetalization and the distribution of isomers under the optimized conditions (Figure 12).In each cycle, the catalyst was easily separated and recovered from the reaction mixture, washed with dichloromethane, and subsequently dried in vacuum before another reaction was performed.The yield of glycerol formal can reach 79.4% in the fifth runs.

Catalyst Characterization
Structural properties and composition of prepared mesoporous materials were measured by a combination of N 2 adsorption-desorption (BET), and X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), with sulfur content determined by X-Ray (XRF, bulk) and X-ray photoelectron spectroscopy (XPS, surface), and acid capacity based on acid-base titration.
N 2 adsorption-desorption isotherms were recorded while using a TristarII 3020 instrument (Norcross, GA, USA) and pore size distribution curves were calculated from the analysis of desorption branch of the isotherm by the BJH (Barrett-Joyner-Halenda) method.Surface areas were calculated using the Brumauer-Emmett-Teller (BET) method over the relative pressure range at P/P 0 = 0.06-0.28.Before adsorption, the samples were degassed at 423 K for 4 h.
Transmission electron microscopy (TEM) images were recorded on a FEI microscope (Hillsboro, OR, USA) operated at 200 kV equipped with a Gatan K2 Summit camera.Sample were prepared by dispersing in ethanol and dropcasting onto a micro grid.Scanning electron microscopy (SEM) photos were taken with a Model SU8020 (Hitachi, Japan) field emission scanning electron microscopes.
X Ray Fluorescence (XRF) was performed on a PANalytical Magix PW2403 (Almelo, Netherlands).XPS was performed using an ESCALAB 250Xi instrument (Loughborough, UK) fitted with a charge neutralizer and neutraliser and magnetic focusing lens employing Al Kα monochromated radiation (1486.7 eV).Spectral fitting was performed using CasaXPS version 2.3.16.
The acid capacities of the synthesized catalysts were determined by acid-base titration while using NaCl solution as an ion-exchange agent.50 mg sample in powder form was ion-exchanged with 50 mL 2 mol/L NaCl solution at RT for more than 24 h.The filtrates were then titrated with a 0.01 mol/L NaOH solution.

Catalytic Activity Tests
Catalytic activity were evaluated in the glycerol acetalization with 1,3,5-trioxane (TOX).In a typical experiment, glycerol, TOX, and a certain amount of catalyst were added quantificationally into the reactor.The mixture was heated to a desired temperature and stirred at a speed of 600 rpm/min.Upon reaction completion, the qualitative analysis of products were performed by chromatography/mass spectrometry (GC/MS) (Agilent 7980A/5975C, Santa Clara, CA, USA).The products of the reactions were quantified on an Aglient 7890GC (Santa Clara, CA, USA) using a flame ionization detector (FID) with a HP-5 30 m × 0.32 mm × 0.2 µm capillary column.

Conclusions
An array of propylsulfonic acid functionalized SBA-15 were prepared while using different methods with different structure parameter and varying amount of propylsulfonic acid group.The propylsufonic acid functionalized KIT-6 catalysts have been prepared by co-condensation under different aging temperature and compared with the series of SBA-15.PrSO 3 H-SBA-15-400 is the most active catalyst in glycerol acetalization with formaldehyde.The yield of GF and distribution of the GF isomers could be adjusted by varying reaction condition.The glycerol acetalization with TOX was optimized and conducted out under 90 • C, 8 h, with 4 wt % catalyst loading, and the ratio of formaldehyde to glycerol is 1.5:1.PrSO 3 H-SBA-15-400 showed optimal catalyst activity with 91.5% yield and the ratio of 5R to 6R is 42:58.
Author Contributions: J.C. and R.L. conceived and designed the experiments; R.L. and H.S. performed the experiments and analyzed the data; R.L. and H.S. wrote the paper and proofread the manuscript.All authors discussed the results and commented on the manuscript.

Scheme 2 .
Scheme 2. Possible role of NaCl in activating siloxane bridges for grafting.

Figure 10 .
Figure 10.Effect of mole ratio (formaldehyde to glycerol) on the yield of glycerol formal over 0.2 g PrSO3H-SBA-15-400 at 90 °C for 8 h.

Figure 10 .
Figure 10.Effect of mole ratio (formaldehyde to glycerol) on the yield of glycerol formal over 0.2 g PrSO3H-SBA-15-400 at 90 °C for 8 h.

Figure 10 .
Figure 10.Effect of mole ratio (formaldehyde to glycerol) on the yield of glycerol formal over 0.2 g PrSO 3 H-SBA-15-400 at 90 • C for 8 h.