Introduction

Heteropolyacids (HPAs) with Keggin structure are friendly acid catalysts for various organic reactions1,2,3,4. In this context, Lewis acidic HPAs have been synthesized through partial exchange of protons with metal cations5,6,7. Sn-exchanged Sn0.75H0.25PW12O40 has been studied in different reactions including the selective hydrolysis of cellulose to glucose8,9, cyanosilylation of ketones and aldehydes10, esterification or transesterification11,12 and Friedel-Crafts alkylation reactions13. Zhu’s group developed a high active Ag-exchanged HPA (Ag1H2PW12O40) for the glycerol esterification with acetic acid14. N. Lingaiah et al. reported the ZnPW12O40 catalyst for the synthesis of glycerol carbonate from glycerol and urea, the high efficiency was attributed to the incorporation of Zn2+ into the secondary structure of heterpolytungstate15. Up to now, only one report had been reported concerning the acidic properties of HnPW11MO40 (M = Ti, Zr and Th)16. Other transition metal mono-substituted PW11O397− (Lewis metals include TiIV, CuII, AlIII, SnIV, FeIII, CrIII, ZrIV and ZnII) have rarely been studied systematically. The focus of our paper is to determine how substituted metals affect the acidic property and catalytic activity of HnPW11MO39. Meanwhile, as one of the most studied processes for evaluation of catalytic activity, esterification between alcohols and organic acids is of great interest in academical and industrial fields. In this case, we selected the esterification of glycerol with acetic acid, which also has an environmental and industrial importance in biomass conversion, as a model reaction to investigate the influence of Lewis center on the catalytic activity. In addition, we also studied the effects of Lewis metals on transesterification. Among all, H5PW11TiO40 was the most active, water-tolerant and acid-tolerant HPAs. Calcination treatment to H5PW11TiO40 made it insoluble in water which confirmed its heterogeneous performance in both esterification and transesterification.

Success of this work might clarify the different effects of Lewis metals on total acidity of HPAs and provide more information on how to select proper HPAs according to different requirements.

Methods

Material and reagent

All the chemicals were of AR grade, which were obtained commercially and used without further purification. Na7PW11O39 was synthesized according to the ref 17.

Instrument

Elemental analysis was carried out using a Leeman Plasma Spec (I) ICP-ES and a PE 2400 CHN elemental analyzer. IR spectra (4000–500 cm−1) was recorded in KBr discs on a Nicolet Magna 560 IR spectrometer. The IR spectra of adsorbed pyridine (Py-IR) were depicted by subtracting the spectra before and after exposure to pyridine. X-ray diffraction (XRD) patterns of the sample were collected on a Japan Rigaku Dmax 2000 X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). SEM micrographs were recorded on a scan electron microscope (XL30 ESEM FEG 25 kV). The concentrations of esters were determined periodically on Shimazu GC-14C fitted with a HP-INNO Wax capillary column (30 m × 0.25 mm) and flame ionization detector18. Each of the catalytic reaction was repeated for three times.

Catalyst preparation

K5PW11TiO40 was synthesized according to the procedure described previously19. Firstly, Ti (SO4)2 solution (6 mmol in 2 M H2SO4) was added to the aqueous solution of Na7PW11O39 (6 mmol). Then, adjusting the pH of the mixture to 5.6 by NaHCO3, followed by adding solid KCl (2.24 g) until white precipitate, K5PW11TiO40, was formed. After that, the precipitate was filtrated and recrystallized with water for three times. Other catalysts were synthesized as following: firstly, 29.4 g (100 mmol) Na2WO4 and 1.29 g (9.1 mmol) Na2HPO4 were dissolved in 80 mL deionized water at room temperature under vigorous stirring. Secondly, 20 mL metal salts aqueous solution (12 mmol) (chlorides for Sn; nitrates for Fe, Cr and Zn; sulfate for Al, Cu and ZrO2·xH2O) was added dropwise to the former solution with continuous stirring. After that, 100 mL deionized water was added additionally. The pH was adjusted to 5.6 by HNO3, then KCl (3.39 g) was added until the precipitate formed. This precipitate was filtrated and recrystallized with water for three times giving potassium salt of KPW11M (M = TiIV, CuII, AlIII, SnIV, FeIII, CrIII, ZrIV and ZnII).

2 g potassium salts of KPW11M (M = TiIV, CuII, AlIII, SnIV, FeIII, CrIII, ZrIV and ZnII) were dissolved respectively in 1000 mL deionized water and the potassium cations were replaced by H+ using strong-acid cation exchange resins (Type 732, 20 g) for several times to give HPW11M, until no K+ can be detected by ICP analysis. Then the HPW11M solution was rotary evaporated at 50 °C to remove water and give powders. These powders were calcinated for 3 h at 200 °C to obtain insoluble products. The formation of HPW11M undergos the following equations17:

Determination of the acidic properties

Titration was used to evaluate the total acid content of the solids20. 0.05 g HPW11M suspended in 45 mL acetonitrile and then the mixture was stirred for 3 h. The density of acid sites in the catalysts was measured by titration with a solution of n-butylamine in acetonitrile (0.05 M) using the indicator anthraquinone (pKa = −8.2). The IR spectra of adsorbed pyridine (Py-IR) helped to measure the acid content and distinguish the properties of acid sites (Lewis or Brønsted). The samples were exposed to the pyridine vapor for 12 h under vacuum (10−3 Pa) at 60 °C. The quantification of acidity was calculated by Lambert–Beer equation:

where A is the absorbance (area in cm−1), ε is the extinction coefficient (m2/mol), W is the sample weight (kg), c is the concentration of acid (mol/kg or mmol/g) and S is the sample disk area (m2), respectively. The amount of Brønsted and Lewis acid sites was estimated from the integrated area of the adsorption bands at ca. 1540 and 1450 cm−1, respectively, using the extinction coefficient values based on the previous report21.

Esterification reaction

A 25 mL three-necked glass flask equipped with a water-cooled condenser was charged with glycerin (2.3 g, 25 mmol), different volume of acetic acid and certain amount of catalyst. Each mixture was vigorously stirred and reacted at desired temperature for the required reaction time. After the reaction, the mixture was rotary evaporated at 55 °C to remove the excess acetic acid. The products, the unreacted glycerol and the solid catalyst were left in the reactor. Then the catalyst was separated by centrifuging, washed with water and calcinated at 100 °C for reuse. The liquid mixture was tested by GC. In addition, we have checked the mixture before and after rotary evaporator by GC which confirmed that there was no additional reaction because the time (1 min) was too short to allow any additional reactions occur. The conversion of glycerol and selectivity of glycerides were calculated by the following equations:

Transesterification reaction

1.88 mL glycerol triacetin was added to a 25 mL three-necked glass flask with ethanol-cooled condenser (the temperature is −3 °C) under vigorous stirring. After being preheated to 65 °C, the specified amount of methanol (molar ratio of oil/methanol was 1: 6) and 4 wt% of catalyst was added under stirring at 300 rpm to keep the system uniform in temperature and suspension. The reaction was maintained for 4 h at 65 °C and atmospheric pressure. After reaction, the mixture was rotary evaporated at 45 °C to remove the excess methanol, while the products, unreacted triacetin and the solid catalyst were left in the reactor. Then the catalyst was separated by centrifuging, washed with water and calcinated at 100 °C for reuse. The liquid mixture was measured by GC. In addition, we have checked the mixture before and after rotary evaporator by GC which confirmed that there was no additional reaction because the time (1 min) was too short to allow any additional reactions occur.

Results and Discussion

Structural characterization of the catalysts

The elemental analyses of HPW11M were given in Table 1. These results confirmed the molar ratio of P: W: M = 1:11:1, which showed the formation of mono-metal substituted undeca-tungstophosphates.

Table 1 The characterization of HnPW11M.
Full size table

Four bands were shown at 1072(8νas P-O), 977 (νas W=O), 891 (νas W−O−W inter-octahedral) and 796 cm−1 (νas W−O−W intra-octahedra) in the FTIR spectra of HPW11M (Fig. 1A), which were attributed to the stretching vibrational peaks of HPA Keggin anions22. Compared with PW11O397− (1082, 957, 873 and 763 cm−1), some shifts (νas W=O and νas W−O−W) occurred due to the substitution of W by M ions to form saturated HPAs (Scheme S1). The FTIR spectra of HPW11M were similar to that of H3PW12O40 (1080, 982, 890 and 797 cm−1), which showed the replacement of tungsten by metal cations formed a dodecatungstophosphoric Keggin structure scussesfully. The IR spectrum gave a band at 580 cm−1 which indicated the existence of an M-O bond23. The XRD patterns of as-prepared catalysts also supported the results of FTIR (Fig. 1B). It was found that the characteristic peaks were similar to those of PW11O397− (10.35, 14.60, 17.85, 20.68, 23.11, 25.44, 31.22 and 34.67°) and only a few changes appeared because of the partial replacement of the W atom by the Ti ion. It demonstrated the successful incorporation of Mn+ into PW11O397− clusters and the formation of good crystals with the Keggin structure.

Figure 1
figure 1

FTIR spectra (A) and XRD patterns (B) of HnPW11MO39 catalysts.

This figure is to confirm the Keggin structure of HnPW11MO39 catalysts.

Full size image

The morphology of H5PW11Ti was measured by SEM and EDAX (Fig. S1). It showed that as-prepared material displayed well-shaped crystalline particles with molar ratio of W: P: Ti = 11.07: 1.14: 1.13.

The acidic properties of the catalysts

The FT-IR spectra of pyridine absorption are a powerful tool for identifying the nature of acid sites21. As shown in Fig. 2, HPW11M presented typical bands at around 1540 and 1639 cm−1 24 corresponding to strong Brønsted sites. Compared with HPW11M, new bands at 1450 and 1610 cm−1 were assigned to the coordinated pyridine adsorption on the Lewis acid sites, while the band at 1489 cm−1 was originated from the combination of pyridine on both Brønsted and Lewis acid sites25. The results indicated that the Lewis acid sites were successfully introduced to HPW11M molecules; Therefore, HPW11M exhibited double acidic sites including Brønsted sites and Lewis ones.

Figure 2
figure 2

FTIR spectra of pyridine adsorption of HnPW11MO39 catalysts.

This figure is a powerful tool for identifying the nature of acid sites, especially to distinguish the lewis acid and Brønsted acid.

Full size image

The total contents of Brønsted acid and Lewis acid were obtained from titration with n-butylamine20 and the separated density was calculated from the strength ratio of Brønsted acid and Lewis acid in FT-IR spectra of pyridine absorption14 (Table 1). It was found that mono-substituted HPW11M gave decreasing trend compared with their parent H3PW12O40 for single Brønsted acid content. In addition, different Lewis metals gave the HPAs with different proton numbers, i.e. the proton numbers were four for Al, Fe and Cr, five for Ti, Cu, Zn and Zr, three for Sn, respectively. For H5PW11MO40, the length of M-O bond decreased as the order of Z/r which is Ti4+(7.55) < Zr4+(5.55) < Zn2+(3.33) < Cu2+(3.23). The oxygen attraction of different metals increased in the same order, which might lead to the decrease of proton attraction. Therefore, protons could easily dissociate from HPAs for H5PW11TiO40 and give low value of dissociation constants (the pK1). For H4PW11MO40 (Al, Fe and Cr), the Z/r values were 7.69, 6.12 and 4.84, respectively, which decided the Brønsted acidity in the following sequence: H4PW11AlO40 > H4PW11FeO39 > H4PW11CrO39. Therefore, the metals with higher Lewis acidity such as Ti and Al could give higher Brønsted acidity. Timofeeva15 demonstrated some dissociation constants (the pK1) in AcOH as H5PW11ThO40 > H5PW11ZrO40 > H5PW11TiO40 > H3PW12O40 and our results also confirmed these results. At the same time, the Brønsted acidity of H3PW11SnO39 was lower than that of H3PW12O40. By the comparison between H4PW11MO40 (Al, Fe and Cr) and H5PW11MO40 (Ti, Zr, Cu and Zn), the Brønsted acidity of five protons HPAs was higher than that of four protons. The number of protons played an important role on the Brønsted acidity. For HPAs with the same electron charges in substituent position, the metals with higher Lewis acidity could enhance the Brønsted acidity. Moreover, compared with their parent H3PW12O40, mono-substituted HPAs performed the downward trend for Brønsted acidity no matter how many protons they had. So H3PW12O40 was still the strongest acid among the tested HPAs.

The total acid contents were in the range of H5PW11TiO40 > H5PW11CuO39 > H3PW12O40 > H3PW11SnO39 > H5PW11ZrO40 ~ H5PW11ZnO39 > H4PW11AlO40 > H4PW11FeO39 > H4PW11CrO39. H5PW11TiO40 and H5PW11CuO39 had a higher acid strength than H3PW12O40 which was attributed to the strong combination of Brønsted acid and Lewis acid.

We have reported a series of TixH3−x/4PW12O4011 with higher acidity which was prepared by exchanging the protons of H3PW12O40 with different numbers of Ti4+ ions (Scheme S1). The difference of Brønsted acidity between H5PW11TiO40 and Ti0.25H2PW12O40 was attributed to their different negative charges of anions, which was −5 for H5PW11TiO40 and −3 for Ti0.25H2PW12O40, respectively. Because of the higher number of negative charges, the protons dissociation of H5PW11TiO40 was more difficult compared with Ti0.25H2PW12O40. The order of acidic capacity for TixH3−x/4PW12O40 was Ti0.6H0.6PW12O40 (2.886 mol·kg−1) > Ti0.5H1PW12O40 (2.705 mol·kg−1) > Ti0.25H2PW12O40 (2.671 mol·kg−1) > Ti0.2H2.2PW12O40 (2.663 mol·kg−1) > Ti0.1H2.6PW12O40 (2.614 mol·kg−1) > Ti0.75PW12O40 (2.496 mol·kg−1) > Ti0.3H1.8PW12O40 (2.469 mol·kg−1). For Tix/4H3−xPW12O40 series, the strength of acidic property was not incoherent with the number of protons, but was influenced significantly by the number of metals. Ti0.25H2PW12O40 gave a higher Brønsted acidity than H3PW12O40, which might be attributed to that Ti shared the attraction force from PW12O403− with proton. In this case, proton could easily dissociate from polyanions which would give a higher dissociation constant than H3PW12O40. Therefore, the combination of Ti and H resulted in the higher Brønsted acidity.

Catalytic activity of HPW11M catalysts in esterification

Commonly, Brønsted acid is active mainly in esterification, whereas Lewis acid is more active in transesterification26. However, some research showed that Lewis acid is also active in esterification of glycerol. Zhu’s group recently reported a highly-active silver-exchanged phosphotungstic acid catalyst Ag1H2PW12O40 which was applied in glycerol esterification with acetic acid, with 96.8% conversion and 48.4, 46.4 and 5.2% selectivity to MAG, DAG and TAG, respectively and the reaction conditions were 120 °C, 15 min with the molar ratio of 10:1. The high efficiency came from the contribution of Lewis metal Ag14. N. Lingaiah’s group also reported the incorporation of Zn into the secondary structure of heteropolytungstate which could promote the conversion of glycerol to carbonate with urea16. In order to evaluate the acidic performance of these HPAs, the esterification of glycerol with acetic acid was conducted in the presence of the HPW11M catalysts (Table 2). The conversion of glycerol changed as following: H5PW11TiO40 > H5PW11CuO39 > H3PW12O40 > H3PW11SnO39 > H5PW11ZrO40 > H4PW11FeO39 > H5PW11ZnO39 > H4PW11AlO40 > H4PW11CrO39. This order was in accordance with their total acidic density, which suggested that the total acidic strength played an important role on glycerol esterification. The conversions of glycerol for H5PW11TiO40 and H5PW11CuO39 were higher than that of H3PW12O40, which was mainly due to their higher acidic contents.

Table 2 The comparison between different catalysts on esterification reactiona.
Full size table

The different catalysts gave different selectivities to glycerol esters. For HPAs with low acidic contents including H4PW11FeO39, H5PW11ZnO39, H4PW11AlO39, H3PW11SnO39 and H4PW11CrO39, the main product was monoacetin whose selectivities were higher than 75%. While for H3PW12O40, H5PW11TiO40 and H5PW11CuO39, the total selectivities to diacetin and triacetin increased. It is known that glycerol esterification with acetic acid is a consecutive reaction including three continuous steps: glycerol + HOAc → MAG + H2O; MAG + HOAc → DAG + H2O; DAG + HOAc → TAG + H2O14. It is highly desirable to achieve the maximum production of the valuable DAG and TAG. By now, the selectivities of DAG (58.2%) and TAG (31.9%) reached the highest value within 4 h under the catalysis of Ag1H2PW12O40, which was superior or at least comparable to the best catalysts14. In our work, the selectivities to DAG and TAG were 55.4 and 6.7% by H5PW11TiO40 and 43.8 and 2.4% by H5PW11CuO39, respectively, which were obtained with the mild conditions : molar ratio of glycerol to acetic acid = 1:5, 75 °C, 3 wt% of catalyst and 1 h. Extending the reaction time could enhance the selectivities to DAG and TAG (Fig. 3a). It was found that from 0.25 h to 4 h, the selectivity toward MAG obviously decreased to 3.3% while the total selectivity to DAG and TAG increased to 96.7%. For H5PW11ZrO40 and H5PW11ZnO39 with almost the same Lewis acidity, but the former tended to produce more DAG while the later was fond of MAG, respectively. From the point of ionic radii for the two ions, the steric crowding around Lewis acid centers decreased with the increase of ionic radii from Zn2+ (0.60 Å) to Zr4+ (0.72 Å); therefore, in the same reaction conditions, H5PW11ZrO40 gave more yield of DAG than H5PW11ZnO39.

Figure 3
figure 3

Glycerol conversion and selectivity as a function of reaction time over H5PW11TiO40 and H3PW12O40.

Reaction conditions: 75 °C, glycerol/acetic acid = 1:5 (molar ratio), 3 wt% catalyst loading relative to glycerol. This figure is to confirm that extending the reaction time could enhance the selectivities to DAG and TAG which are our target products.

Full size image

For H5PW11TiO40 and Ti0.25H2PW12O40, the turnover frequencies (TOFs = conversion of glycerol/amount of catalyst (mmol) × time) were 1007.86 and 1055.10 mmol/mmol·h, respectively. The conversion of glycerol was followed the equation obtained by Kozhevnikov et al.27: log k = 1.04H0 − 3.46, where k is expressed as the rate constant and H0 is the acidity function of the catalyst solution. Therefore, the catalytic activity of Ti0.25H2PW12O40 was higher than that of H5PW11TiO40, which meant that higher Brønsted acidity gave higher glycerol conversion. This was also suitable for the other HPAs substituted by metals. The different selectivities to different esters might be attributed to the pore size distributions of these two catalysts (The pore sizes were 5.62 and 3.83 nm corresponding to H5PW11TiO40 and Ti0.25H2PW12O40, respectively). The large size of H5PW11TiO40 permitted higher selectivities to DAG and TAG.

For H3PW12O40, the combined selectivities to DAG and TAG reached a maximum value of 81.2% for 4 h. The difference between H5PW11TiO40 and H3PW12O40 was because of their tolerance to water. From the above equations, we could know that water was by-product of the glycerol esterification reaction. Hence, the solid acid catalyst was easily destroyed by water in the system. So a water-tolerant solid acid catalyst was desirable. When adding extra water to the reaction system, catalytic activity of the two catalysts could be influenced (Fig. 4). In view of H5PW11TiO40, the water content ranging from 0 to 0.6 wt% did not play a significant role in the glycerol conversion, which meant that H5PW11TiO40 catalyst exhibited a certain water-tolerant property. On the contrary, the catalytic activity of H3PW12O40 decreased dramatically as the growth of water from 0.1 to 0.6 wt%, which suggested that H3PW12O40 was not water-tolerant. It was reported that if H2O molecules were added, they would link with O–H to form O–H····H2O28,29. In the IR spectra of HPAs adsorbing H2O (Fig. S2), the peak at 1670 cm−1 (corresponding to hydrated OH stretching vibrational peak) belonging to H5PW11TiO40 was bigger than that of H3PW12O40, which indicated that more water molecules could be adsorbed by H5PW11TiO40 than by H3PW12O40. Therefore, the water-tolerance of H5PW11TiO40 was better than that of H3PW12O40. But further increasing the amount of water, the ability of water-tolerance could be destroyed. The different water-tolerance determined that H3PW12O40 could not give the selectivities to DAG and TAG as high as H5PW11TiO40.

Figure 4
figure 4

The water-tolerant tests for H5PW11TiO40 and H3PW12O40.

Reaction conditions: 75 °C, glycerol/acetic acid = 1: 5 (molar ratio), 3 wt% catalyst loading relative to glycerol, 20 min. This figure is to confirm that when adding extra water to the reaction system, catalytic activity of the two catalysts could be influenced. which meant that H5PW11TiO40 catalyst exhibited a certain water-tolerant property. While H3PW12O40 was not water-tolerant.

Full size image

Main parameters of the esterification reaction catalyzed by H5PW11TiO40 including temperature, reaction time, catalyst dosage and molar ratio of glycerol to acid were investigated (Fig. S3). It suggested that in order to obtain high yields of DAG and TAG, high molar ratio of glycerol to acid (1:5), long reaction time (4 h) and 75 °C were needed.

Catalytic activity of HPW11M catalysts in transesterification

Based on the suggestion by Santacesaria30, Brønsted acid catalysts were active mainly in esterification while Lewis acid catalysts were more active in transesterification. In order to evaluate the efficiency of the HPW11M on transesterification, glycerol triacetin was selected to investigate the influence of their Lewis and Brønsted acidity (Table 3). It is known that triglyceride transesterification with methanol is a consecutive reaction including three continuous steps31:

Table 3 The comparison between different catalysts on transesterificationa.
Full size table

Table 3 showed that conversion of glycerol triacetin changed in the order of Ti0.25H2PW12O40 ~ H3PW12O40 > H5PW11TiO40 > H5PW11CuO39 > H3PW11SnO39 > H5PW11ZrO40 ~ H5PW11ZnO39 > H4PW11AlO40 > H4PW11FeO39 > H4PW11CrO39. For TOF, the order was as following: Ti0.25H2PW12O40 ~ H3PW12O40 > H5PW11TiO40 > H5PW11CuO39 > H3PW11SnO39 > H5PW11ZrO40 ~ H5PW11ZnO39 > H4PW11AlO40 > H4PW11FeO39 > H4PW11CrO39. The results suggested that HPA catalysts with higher acidity were in favor of high TOF for transesterification of triacetin. The selectivity to glycerol was in the range of Ti0.25H2PW12O40 ~ H3PW12O40 > H5PW11TiO40 > H5PW11CuO39 ~ H4PW11CrO39 > H4PW11FeO39 > H4PW11AlO40 > H5PW11ZnO39 > H5PW11ZrO40 > H3PW11SnO39. HPAs with higher Lewis acidity such as Cr, Fe gave higher selectivity to glycerol and tended to promot the conversion of diacetin and monoacetin to glycerol and methyl acetate.

Main parameters of the transesterification reaction catalyzed by H5PW11TiO40 including temperature, reaction time, catalyst dosage and molar ratio of TAG to methanol were investigated (Fig. S4). In order to obtain high yield of glycerol, high molar ratio of triacetin to methanol (1:6), long reaction time (4 h) and 65 °C were needed, with which we have got a 92.2% conversion and a 83.3% selectivity of glycerol, respectively.

It might be more important to convert low quality feedstocks to biodiesel. However, the presence of high FFAs might have adverse effects on the catalyst activity. H5PW11TiO40 might catalyze the esterification of FFA and transeterification of triacetin simultaneously. Therefore, the conversion of triacetin with some FFA contents was catalyzed by H5PW11TiO40 with Lewis acidity and Brønsted one (Fig. S5).

Reuse of catalysts

It is necessary to determine the nature of H5PW11TiO40 in esterification or transeterification. The solubility of H5PW11TiO40 in acetic acid and methanol was measured by Uv-Vis spectroscopy (Fig. S6). We could see that H5PW11TiO40 was insoluble either in acetic acid or in methanol, which suggested that it performed as a heterogeneous catalyst in both esterification and transesterification reactions. The behavior was attributed to the calcinations treatment to H5PW11TiO40 at 200 °C for about 3 h to form insoluble powders which were also determined by the SEM image and EDAX (Fig. S1). In other word, the catalyst could be easily separated from the production mixture. The catalyst was separated by centrifuging and decanted from the bottom of the reactor and then washed with ethanol and dried at 60 °C overnight for further reaction cycles. As shown in Fig. 5, there was no considerable change in the catalytic activity after five reaction cycles either in esterification or transesterification. The IR spectrum of the products in transesterification (Fig. S7) indicate that no characteristic peaks corresponding to H5PW11TiO40 were observed in range of 790 to 1000 cm−1, which demonstrated that the leaching of H5PW11TiO40 was negligible. The leaching of H5PW11TiO40 was about 0.10 wt% and 0.12 wt% during esterification and transesterification for one cycle, respectively. To further determine the leaching of H5PW11TiO40, the catalyst was seperated after reacting for 20 min (91.0% of glycerol conversion) and was allowed to react further for over 1 h at the same conditions. The result showed that the conversion of glycerol was only 92.4%, which meant that H5PW11TiO40 acted as a heterogeneous catalyst.

Figure 5
figure 5

Reusability test preformed for H5PW11TiO40 in esterification and transesterification.

This figure is to confirm that H5PW11TiO40 still performed high activity after five reaction cycles either in esterification or transesterification.

Full size image

The stability of HPAs during the reaction was determined by IR spectroscopy (Fig. S8). After the reaction, the catalysts still kept its Keggin structure. The peaks at 1072, 977, 891 and 796 cm−1 attributing to the characteristic bands of Keggin structure could also be observed. The SEM image of the catalyst after reaction (Fig. S9) showed no-change of the morphology during the reaction. Therefore, HPAs was stable and could be reused at least for five cycles.

Conclusion

This work demonstrated a variety of HPAs HnPW11M (M = TiIV, CuII, AlIII, SnIV, FeIII, CrIII, ZrIV and ZnII) with both Lewis and Brønsted acidity. The influence of these Lewis metals on the acidic strength of dodecatungstates showed that (1) moderated Lewis metals gave more influence on total acidity including Ti and Cu; (2) compared to TixH3−4xPW12O40, Ti in substituted place played a less important role on total acidity. Their catalytic activities in esterification of glycerol and transeterification of triglycerides were generally followed their acidic properties. Among all these HPAs, H5PW11TiO40 displayed properities like Lewis acid sites, insolubility in polar solvents, water-tolerance, acid-tolerance and stability which were benefit to its excellent performance. To the best of our knowledge, H5PW11TiO40 gave the highest selectivity to desired DAG and TGA (96.7%) in the esterification of glycerol which was higher than the reported Ag1H2PW12O40. Moreover, in transeterification, high conversion of triacetin and high yield of glycerol were obtained by H5PW11TiO40. Furthermore, H5PW11TiO40 did not suffer from leaching and deactivation in five reaction cycles either in esterification or transesterification reactions.

This study provides useful information on Lewis metal substituted HPAs. It could be a promising candidate for the catalytic esterification of glycerol and synthesis of valuable biofuel additives, meanwhile, it could also act as a potential catalyst for production of biodiesel using low quality feedstocks.

Additional Information

How to cite this article: Tao, M. et al. Tailoring the Synergistic Bronsted-Lewis acidic effects in Heteropolyacid catalysts: Applied in Esterification and Transesterification Reactions. Sci. Rep. 5, 13764; doi: 10.1038/srep13764 (2015).