Metal Phosphomolybdate-Catalyzed Condensation of Furfural with Glycerol

Abstract

In this work, metal salts of phosphomolybdic acid were prepared and evaluated as catalysts in acetalization reactions of glycerol with furfural. These substrates have a renewable origin and play a crucial role in synthesizing bioadditives, which can enhance the physicochemical properties of fossil fuels and mitigate greenhouse gas emissions. Moreover, the biodiesel industry has generated a surplus of glycerol, and its use as a reactant is welcome from both economic and environmental viewpoints. Keggin heteropolyacid salts are less corrosive than traditional Brønsted acid catalysts and are easier to handle. Herein, metal phosphomolybdates were easily obtained from the acid precursor and metal chloride metathesis. A series of metal phosphomolybdates with the general formulae M₃[PMo₁₂O₄₀]_x n H₂O (M^x+ = Al³⁺, Fe³⁺, Co²⁺, Cu²⁺, Ni²⁺) was prepared and tested as catalysts in furfural glycerol acetalization reactions.

1. Introduction

The increasing search for renewable energy sources has become a central issue in science. This demand arises from the need to reduce greenhouse gas emissions and find alternatives to fossil fuels. In this context, exploring energy sources derived from plant biomass is gaining significant importance [1]. Industrial crops that produce cellulose and paper generate large amounts of vegetable residues [2]. These wastes are often burned as fuel for boilers. However, an attractive option is to extract valuable platform molecules from this biomass, which can be transformed into products with a higher added value, such as fine chemicals, solvents, and biofuels. Biomass waste mainly consists of cellulose, hemicellulose, and lignin, which is composed of aromatic polymers [3,4,5]. The acid hydrolysis of fractions rich in saccharides, such as cellulose and hemicellulose, can generate biorefinery platform molecules, including 5-hydroxymethylfurfural, furfuryl alcohol, levulinic acid, and furfural. Their derived products are attractive for the chemical industry [6,7,8].

Furfural is a raw material that produces solvents, bioadditives, and biofuels. Indeed, it is among the biorefinery platform molecules classified by the US Energy Department as “top 10” [9]. The technology for its production is widely established, and annually, around 30,000 tons of furfural have been produced [10,11,12]. On the other hand, glycerol is a highly functionalized compound generated in a proportion of 10% v/v during the biodiesel production process [13]. However, biodiesel-derived crude glycerol has a low purity grade, and before being used as a food ingredient, in cosmetics, or in pharmaceuticals, some purification steps must be performed [14]. An alternative is converting the glycerol to ethers, esters, ketals, or acetals through etherification, esterification, or condensation reactions. These compounds are recognized as efficient fuel additives because they improve the physicochemical properties of fuels [15].

Sustainability is a key aspect of the condensation of glycerol with furfural since they are renewable and abundant raw materials. Moreover, cyclic acetals or ketals produced in such reactions enhance the octane number, antiknock, and anti-freezing properties of the gasoline or can be blended with diesel/biodiesel [16,17,18,19]. Several heterogeneous and homogeneous Brønsted or Lewis acid catalysts have been exploited for the acetalization of furfural with glycerol [20,21,22,23,24,25,26]. Nonetheless, most of these systems operate at temperatures greater than 353 K. Conversely, Keggin heteropolyacids (HPAs) have been active catalysts in condensation reactions of alcohols and aldehydes at room temperature [27,28,29,30,31]. Keggin HPAs are metal-oxygen clusters where a central heteroatom (P, Si) is tetrahedrally coordinated to Mo or W atoms, octahedrally surrounded by oxygen atoms [32,33,34]. Keggin HPAs are Brønsted acids stronger than sulfuric acid. Moreover, their catalytic properties can be enhanced with structural modifications [35]. They can be converted to metal salts by exchanging their protons with metal cations, lowering their corrosive power without diminishing their catalytic activity.

In this work, a series of metal phosphomolybdates with the general formulae M₃[PMo₁₂O₄₀]_x n H₂O (M^x+ = Al³⁺, Fe³⁺, Co²⁺, Cu²⁺, Ni²⁺) were prepared and evaluated as catalysts in furfural glycerol acetalization reactions. The influence of main reaction parameters such as temperature, concentration, and molar ratio of reactants was assessed.

2. Materials and Methods

2.1. Chemicals

All reagents were obtained from commercial sources and used without prior treatment: hydrated H₃PMo₁₂O₄₀ (99 wt.%), AlCl₃·6H₂O (99 wt.%), FeCl₃·9H₂O (97 wt.%), CoCl₂·6H₂O (98 wt.%), CuCl₂·2H₂O (≥99.0 wt.%), NiCl₂·6H₂O (99.9 wt.%), glycerol (≥99.5 wt.%), furfural (99 wt.%), and toluene (99.5 wt.%) were purchased from Sigma-Aldrich (St. Louis, MO, USA, EUA)

2.2. Synthesis of Phosphomolybdate Salts

The metal heteropoly salts were synthesized according to the procedure adapted from the literature [21]. To synthesize it, 2 mmol of the H₃[PMo₁₂O₄₀] was dissolved in 50 mL of distilled water and heated at a temperature of 343 K. A second solution was prepared by dissolving a stoichiometric amount of metal chloride in 30 mL of distilled water. This solution was added dropwise to the first. After mixing, this solution was vigorously stirred at 343 K for 3 h. Afterward, the solution was partially evaporated. When it was almost dry, the solid was transferred to a porcelain crucible and heated in a muffle furnace at 403 K for 12 h. At the end of this stage, a solid salt was obtained with a characteristic color of the metallic cation.

2.3. Catalytic Runs

The catalytic runs were performed in three-way glass flasks (25 mL), coupled to a reflux condenser connected to a thermostatic bath. Typically, 147 µL of glycerol (2 mmol) was dissolved in 9.6 mL of acetonitrile. Then, 167 µL (2 mmol) of furfural and 100 µL of toluene (used as an internal standard) were added. The addition of the appropriate amount of catalyst salt started the reaction. The reaction progress was monitored by GC-FID analysis (Shimadzu, GC-2010 plus), equipped with a Carbowax column 20M (30 m, 0.25 µm film thickness). The temperature profile was as follows: 80 °C/min, 10 °C/min up to 210 °C, and a hold time of 13 min. The injector and detector remained at 250 °C.

Calibration curves obtained with glycerol provided the conversion Equation (1). Dodecane was the internal standard. Under reaction conditions, it is possible that furfural molecules can be condensed with themselves, and consequently, due to their high molecular weight, they cannot be detected. For this reason, to check the mass balance of the reaction, we used the following equation, where Ap is the corrected area of the GC peak of products (dioxolane or dioxane) relative to the furfural GC peak area Equation (1).

$$\% \mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{u}\mathrm{r}\mathrm{a}\mathrm{l}=\left({\mathrm{A}}_0-{\mathrm{A}}_{\mathrm{i}}\right)-\sum ({\mathrm{A}}_{\mathrm{p}})/({\mathrm{A}}_0-{\mathrm{A}}_{\mathrm{i}})\times 100$$

(1)

where the variables are as follows:

• A₀ = GC peak initial area of furfural (time = t₀);
• A_i = GC peak instantaneous area of furfural (time = t);
• ∑A_p = total sum of GC peak corrected areas of the products.

Initially, the pure products were injected to determine the response factor concerning the furfural. The difference between the sum of the corrected GC peak areas of the products and the consumed GC peak area of the furfural gave the condensation products.

2.4. Characterization of Metal Phosphomolybdate Salt Catalysts

The strength of the acid sites present in the metal phosphomolybdate salts was determined by potentiometric titration (BEL potentiometer, model W3B, with glass electrode) with n-butylamine solution [36]. To achieve this, the metal phosphomolybdate salt (ca. 50 mg) was suspended in acetonitrile (30 mL) and magnetically stirred for 3 h. Posteriorly, the sample was titrated with a slow addition of portions of the titrant until the electrode potential remained constant.

Powder XRD patterns were recorded in a Bruker D8 Discovery, with Cu radiation and Ni filter, 40 kV and 40 mA. The 2θ angle was varied from 5 to 80 degrees (1°/min). FT-IR spectra were obtained in a Varian-660 with an ATR accessory.

2.5. Identification of Main Reaction Products

Mass spectroscopy analyses (Shimadzu GC-2010 gas chromatograph coupled with an MS-QP 2010 mass spectrometer) operating at 70 eV identified the main reaction products.

3. Results and Discussion

3.1. Characterization of Catalysts

Infrared spectra of phosphomolybdic acid and its metal salts are displayed in Figure 1. The typical absorption bands of the Keggin ion were seen with minimal modifications in all the spectra. The bands at 1059, 959, 884, and 756 cm⁻¹ wavenumbers were assigned to stretching of P–O, Mo=O_d, Mo–O_b–Mo, and Mo–O_c–Mo bonds, respectively [37].

Since all the absorption bands were preserved, it is possible to conclude that after the proton exchange, the Keggin structure (primary structure) of the heteropolyacid was kept intact.

While FT-IR spectroscopy provides information about the primary structure of Keggin heteropolyacids, the XRD patterns can reflect whether some significant changes have occurred after the replacement of protons with metal cations (Figure 2). XRD patterns are impacted by the number of water molecules present in the structure of the heteropolyanion, as well as by the metal cation radius that replaced the hydronium or di-hydronium cations present in the heteropolyacid. However, the main diffraction lines characteristic of phosphomolybdic acid were at low angles (5 and 10°) (2θ), although with more intense lines, and shifted to lower angles. Between 25 and 30° (2θ) angles, intense diffraction lines were observed in the DRX patterns of metal salts [36].

The strength of acid sites was evaluated through n-butylamine potentiometric titration; if the initial electrode potential (Ei) is greater than 100 mV, the acid sites are classified as very strong [38]. Figure 3 shows that the titration curves of iron, aluminum, copper, and cobalt phosphomolybdate salts had Ei between 550 and 600 mV. Only the nickel phosphomolybdate solution displayed Ei = 400 mV.

Notwithstanding, the curves were considerably different; titration curves of aluminum or iron phosphomolybdates were similar to the pristine heteropolyacid, presenting only one pronounced fall in potential. Conversely, the titration curves of copper, cobalt, nickel, and manganese phosphomolybdates presented two plateaus.

This suggests that in the first group of catalysts, the predominant acid sites had the same nature (pure Brønsted or mixed Brønsted and Lewis sites). On the other hand, the second group also had pure Lewis acid sites, in addition to those mentioned. The nature of acidic sites can be examined by ex situ pyridine adsorbed IR spectroscopy (1600–1400 cm⁻¹). Pedada et al. reported that the phosphomolybdic acid infrared spectrum presents mainly three bands, which those authors assigned to Brønsted (B) (1539 cm⁻¹), Brønsted and Lewis (B + L) (1487 cm⁻¹), and Lewis (L) acidic sites (1442 cm⁻¹), respectively [39].

Essayen et al. studied the acidity of the H₃PMo₁₂O₄₀ at special conditions. They recorded the infrared spectrum of acid samples after a pretreatment under vacuum at 200 °C for 2 h, followed by saturation with Py vapor at room temperature, and posteriorly evacuated for 1 h at 150 °C. Infrared spectra showed vibration bands at 1545 cm⁻¹ resulting from the pyridine protonation, PyH⁺, and at 1490 cm⁻¹ attributed as well to pyridinium species (Brønsted acidity) and pyridine coordinated to Lewis sites [40,41]. The typical adsorption bands of Lewis acid sites, normally seen at 1440–1450 cm⁻¹ wavenumbers, were absent. As verified for the pristine phosphomolybdic acid, the salts of iron and aluminum had pure Lewis sites or Brønsted sites. The other salts presented pure Lewis sites, with absorption bands at 1450 cm⁻¹.

3.2. Catalyst Tests

3.2.1. Effect of Catalyst Nature

The reaction conditions were based on the literature [36,37]. All the catalysts were used at the same concentration. Kinetic curves are presented in Figure 4.

The efficiency of metal phosphomolybdate obeyed the following trend: AlPMo₁₂O₄₀·10H₂O > FePMo₁₂O₄₀·7H₂O > Cu₃[PMo₁₂O₄₀]₂·7H₂O > Co₃[PMo₁₂O₄₀]₂·6H₂O > Ni₃[PMo₁₂O₄₀]₂·6H₂O. The greater efficiency of the AlPMo₁₂O₄₀·10H₂O catalyst was attributed to its higher acidity strength, as evidenced by the potentiometric titration curves previously obtained [36]. In that work, FT-IR and XRD analyses ensured that phosphomolybdic acid was efficiently converted to metal salts. The highest Lewis acidity of the Al³⁺ cation contributes to the hydrolysis of this cation, resulting in a higher release of hydroxonium cations (Equations (2)–(4)).

The nature of metal cations is a key aspect of these reactions. The hydrolysis of metal cations provides the protons, which will be the true catalysts in these processes (Equations (2)–(4)). However, this hydrolysis depends on the Lewis acidity of metal cations. To evaluate the impact of metal cations on the acidity of the solution, pH measurements were performed (

Table 1. Measurements of pH in acetonitrile using 0.05 mol% catalyst at room temperature.

Catalyst	pH
AlPMo12O40 10 H2O	−3.0
FePMo12O40 7 H2O	−4.2
Co3(PMo12O40)2 6 H2O	−2.9
Cu3(PMo12O40)2 7 H2O	−2.8
Ni3(PMo12O40)2 6 H2O	−0.6
Acetonitrile	8.4

). It is important to note that pH is adequate when the acidity measurements are performed in water. However, even performing them in acetonitrile, we will refer to them as “pH”.

The greatest activity of aluminum phosphomolybdate can be assigned to the higher Lewis acidity of the Al³⁺ cation and its higher character of hardness (Pearson Theory). Since the oxygen atom is small and electronegative, it acts as a hard base and strongly interacts with hard acids such as the Al³⁺ cation, which is also small and has a high positive charge. Consequently, the reactions described by Equations (2)–(4) are favored, resulting in a higher release of H⁺ cations that, besides diminishing the pH value, can catalyze the reaction. Although the conversions of iron- or aluminum-catalyzed reactions have been similar, the reaction in the presence of aluminum was quicker; therefore, it was selected to continue the study of other reaction parameters.

$${\mathrm{M}}_{3/\mathrm{X}}\left[\mathrm{P}\mathrm{M}{\mathrm{o}}_{12}{\mathrm{O}}_{40}\right]\to 3 {\mathrm{M}}^{\mathrm{X}\mathrm{+}}+{\left[\mathrm{P}\mathrm{M}{\mathrm{o}}_{12}{\mathrm{O}}_{40}\right]}^{3-}$$

(2)

$${\mathrm{M}}^{\mathrm{X}+}+\mathrm{n} {\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{M}{{\left[{\mathrm{H}}_2\mathrm{O}\right]}_{\mathrm{n}}}^{\mathrm{x}+}$$

(3)

$$\mathrm{M}{{\left[{\mathrm{H}}_2\mathrm{O}\right]}_{\mathrm{n}}}^{\mathrm{x}+}\rightleftharpoons \mathrm{M}{\left[{\mathrm{H}}_2\mathrm{O}\right]}_{\mathrm{n}-1}{\left(\mathrm{O}\mathrm{H}\right)}^{+\mathrm{x}-1}+{\mathrm{H}}_3{\mathrm{O}}^{+}$$

(4)

The Lewis acidity of a cation is related to its ability to receive electron pairs from a Lewis base. To achieve this, it should have empty orbitals; it is the case of the Al³⁺ cation, whose sp² hybridization results in a pure “p” orbital that can easily receive an electron pair of electrons of oxygen. These interactions result in H⁺ cations that can protonate the carbonyl group of furfural, making its carbon atom more susceptible to nucleophilic attack by glycerol, favoring the reaction.

The hydrolysis process depends on the strength of interaction between the metal cation and free water. Cations with high Pearson hardness strongly interact with water, leading to the formation of an aqua-complex. This intermediate should interact with another water molecule and provide an H⁺ cation to it. In this step, the Lewis acidity of the metal strengthens its interaction with coordinated water but weakens the O-H bond, making it more acidic and facilitating its exit. Therefore, more H⁺ cations are produced.

The hydroxonium cations can activate the furfural carbonylic carbon, making it more electrophilic. Consequently, its nucleophilic attack by the hydroxyl groups of glycerol becomes more favorable. In the initial step of the condensation reaction, the protonated furfural has a highly electron-deficient carbonyl carbon. This intermediate can undergo a nucleophilic attack by the terminal hydroxyl group of glycerol. This intermediate loses water, resulting in a carbocation, which other hydroxyl groups of glycerol attack. This intramolecular nucleophilic attack can potentially generate two cyclic intermediates. After each nucleophilic attack, one water molecule will be lost [13]. The resulting cyclic intermediates can have five or six members. If the nucleophilic attack occurs via the secondary hydroxyl group of glycerol, a five-member ring known as dioxolane is formed (Scheme 1). Conversely, if the other terminal hydroxyl group attacks the carbocation formed after water loss, a six-member ring (dioxane) will be produced (Scheme 1).

The impact of different reaction times on selectivity was assessed. However, it was verified that regardless of reaction time, the proportion between the dioxane and dioxolane isomers remained constant. It means that this reaction was not under kinetic or thermodynamic control. Therefore, only the data selectivity obtained after 120 min of reaction is shown in Figure 5. The distinction between the two isomers was easily made by GC-MS analyses and a comparison with data of the previously isolated products.

The reaction regioselectivity deserves to be commented on. Entropically, a six-membered ring has lower angular internal tension and should be more stable. On the other hand, the formation of a five-membered ring is more favorable because the hydroxyl groups involved are neighbors. Therefore, under the conditions studied, neither of the two isomers was predominantly favored. Both five- and six-membered ring products were simultaneously formed, suggesting that no thermodynamic or kinetic control is acting in this reaction.

While the nature of the metal cation strongly impacted the conversion, the selectivities achieved in these reactions were the same: regardless of the catalyst, dioxolane and dioxane were always the main products and obtained at an equimolar proportion. The only exception is the Ni₃[PMo₁₂O₄₀]₂-catalyzed reaction (Figure 5).

After the end of the reaction time, it was stopped, and the products were extracted with ethyl acetate. The remaining catalyst in the reaction solution was extracted with water, which was water-washed and evaporated, giving the solid salt.

3.2.2. Study of the Influence of the Furfural: Glycerol Molar Ratio on Aluminum Phosphomolybdate-Catalyzed Condensation Reactions

Regardless of isomer (dioxane or dioxolane), glycerol–furfuryl acetal is formed by the reaction of one mole of each reactant. In this work, we selected glycerol as a stoichiometric reactant, and thus, we worked with a furfural excess. Acetalization is a reversible reaction, and an increase in furfural load should shift the equilibrium toward products, leading to a higher glycerol conversion. The curves displayed in Figure 6 evidenced this effect.

Conversely, if a higher glycerol load is used (greater than 2 mmol) and the furfural remains constant, a lowering in conversion is verified. Figure 7 clearly shows this effect.

Different from the conversions that were impacted by the variation in glycerol: furfural molar ratio, the reaction selectivities remained almost the same (Figure 7).

3.2.3. Influence of Catalyst Loading

The catalyst concentration does not affect the conversion of reactions that have reached the equilibrium. Nonetheless, herein, the goal was to find the minimum catalyst load that achieves the highest conversion (Figure 8).

The higher concentrations achieved the maximum conversion within the first five reaction minutes (10 or 7.5 mol%). The large amount of water generated as a by-product of the reaction’s beginning may have stopped the reaction’s progress. On the other hand, we suppose that the quick consumption of the substrate may also compromise the reaction rate. Nonetheless, almost all reactions were decelerated after 5 min, regardless of the catalyst load, except those with 2.5 or 5.0 mol% of catalyst, which had a gradual increase in conversion until 20 reaction minutes. Moreover, the reactions with higher catalyst loads were the fastest.

Once more, the variation of a reaction parameter had little or no impact on the reaction selectivity (Figure 9). Regardless of the catalyst load, dioxolane and dioxane were always formed in the same proportion.

3.2.4. Effect of Reaction Temperature

A variation in reaction temperature can shift the equilibrium of reversible, independent to be endo- or exothermic reactions, impacting their conversion. However, sometimes the effect is not easily observed. The initial reaction conditions were kept: glycerol to furfural 1:1 proportion, 0.05 mol% catalyst load. Kinetic curves are displayed in Figure 10.

The reactions carried out at 308 and 318 K were faster than those at room temperature; consequently, greater conversions were also achieved. It was expected that, independent of the conversion, reactions at higher temperatures should present higher initial rates. However, this was not true for all the reactions. The rates of runs performed at 328 or 338 K were lower. Thus, aiming to clarify the effect of reaction temperature, we selected two reactions with great loads of catalyst and performed the catalytic runs at the lowest and highest temperatures (Figure 11).

Analyzing the reactions with the highest catalyst load, it was possible to conclude that the temperature had a minimum impact on the reaction’s initial rate and conversion after 30 min. A similar effect happened with a lower catalyst load. Therefore, it is possible to conclude that the glycerol condensation with furfural at an equimolar proportion in acetonitrile has a minimum dependence on the temperature. The analysis of reaction selectivity showed that the same occurred when the reactions were performed at different temperatures. Figure 12 shows this effect.

3.2.5. Reusability of the Catalyst

The recovery and reuse of the Al[PMo₁₂O₄₀] catalyst were evaluated.

Table 2. Recovery and reuse of Al[PMo₁₂O₄₀] catalyst.

Run	Conversion/%	Recovery Rate/%
1	90	90
2	87	88
3	88	88
4	87	87

Reaction conditions: Al[PMo₁₂O₄₀] catalyst (0.05 mol%), furfural (2 mmol), glycerol (2 mmol), CH₃CN (9.6 mL), time (30 min).

shows the main results. The solvent was evaporated, giving the solid catalyst and liquid products. Then the reaction mixture was extracted with ethyl acetate to separate the products and afterward washed with water. The catalysts were dried in an oven, weighed, and reused.

The catalyst was reused and recovered without loss of activity. The reaction selectivity was kept constant after the successive reuses.

4. Conclusions

The catalytic activity of metal phosphomolybdate salts was evaluated in acetalization reactions of glycerol with furfural. A series of metal phosphomolybdates with the general formulae M₃[PMo₁₂O₄₀]_x (M^x+ = Al³⁺, Fe³⁺, Co²⁺, Cu²⁺, Ni²⁺) was prepared and evaluated at different reaction conditions. Among the catalysts tested, the Al[PMo₁₂O₄₀] 10 H₂O salt was the most active and selective toward the furfural glycerol acetal. This superior activity was assigned to the highest strength of acidity. Despite the Al³⁺ metal cation being a Lewis acid, in the reaction medium, it undergoes hydrolysis by reacting with residual water molecules or those generated in the reaction. This hydrolysis results in hydroxonium cations, which can activate the furfural via protonation of their carbonyl group, making it more electron-deficient. Depending on which hydroxyl group attacks the furfural, cyclic acetals with five- or six-membered rings are formed. Regardless of the reaction conditions, the acetals were always obtained at almost equal proportions. The impacts of the main parameters of the reaction were assessed. An increase in catalyst load improves both the conversion and initial rate of the reactions. Similarly, a higher furfural load increased the reaction conversions. Under the conditions studied, the variation in reaction temperature had a minimum impact on the selectivity. Most studies were carried out at room temperature, and it was verified that using 0.05 mol% of catalyst and a 1:3 glycerol–furfural molar ratio, a high conversion (80%) and a high combined selectivity to acetals (>90%) were achieved. These catalysts are easy to handle and non-corrosive. Moreover, they did not require neutralization at the end of the reactions, reducing the effluent generation.