**Use of Co**/**Fe-Mixed Oxides as Heterogeneous Catalysts in Obtaining Biodiesel**

#### **Norma Gutiérrez-Ortega 1,\* , Esthela Ramos-Ramírez 2,\*, Alma Serafín-Muñoz 1, Adrián Zamorategui-Molina <sup>1</sup> and Jesús Monjaraz-Vallejo <sup>2</sup>**


Received: 1 February 2019; Accepted: 26 April 2019; Published: 29 April 2019

**Abstract:** Catalyst-type mixed metal oxides with different compositions and Co/Fe ratios were obtained from layered double hydroxides to be used as heterogeneous catalysts in the production of biodiesel. The effect of the Co/Fe ratio on the precursors of the catalysts was analyzed, considering their thermal, textural and structural properties. The physicochemical properties of the catalysts were determined by thermogravimetric analysis (differential scanning calorimetry and thermogravimetric), X-ray diffraction, Fourier-transform infrared spectroscopy, Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy and N2-physisorption. The conversion to biodiesel using the different catalysts obtained was determined by diffuse reflectance infrared Fourier-transform spectroscopy and 1H-Nuclear magnetic resonance spectroscopy, allowing us to correlate the effect of the catalyst composition with the catalytic capacity. The conditions for obtaining biodiesel were optimized by selecting the catalyst and varying the percentage of catalyst, the methanol/oil ratio and the reaction time. The catalysts reached yields of conversion to biodiesel of up to 96% in 20 min of reaction using only 2% catalyst. The catalyst that showed the best catalytic activity contains a mixture of predominant crystalline and amorphous phases of CoFe2O4 and NaxCoO2. The results suggest that cobalt is a determinant in the activity of the catalyst when forming active sites in the crystalline network of mixed oxides for the transesterification of triglycerides, with high conversion capacity and selectivity to biodiesel.

**Keywords:** Cobalt ferrite; layered double hydroxides; ethylesters; biofuels; hydrotalcite; transesterification

#### **1. Introduction**

Currently, one of the main challenges facing humanity is to reduce the use of petroleum fuels and increase the production of fuels from low carbon sources [1]. A viable alternative is biofuels, which have the advantages of being easily extractable from biomass, biodegradable, non-toxic and environmentally friendly [2,3]. Biodiesel is a liquid fuel derived from triglycerides of animal, vegetable and even microbial origin [4,5]. Biodiesel can be obtained by a great variety of techniques such as direct mixtures, microemulsions, pyrolysis and transesterification [6]. The transesterification technique is the most used because it is easy to carry out; however, the type of catalyst used is important to ensure that the reaction has direct effects on the efficiency of the process and the quality of the products. The catalysts used can be homogeneous, heterogeneous or enzymatic (Figure 1) [7–9].

**Figure 1.** Transesterification reaction for biodiesel production.

Each of the catalytic processes used to obtain biodiesel presents advantages and disadvantages. For example, enzymatic transesterification is considered the most effective and environmentally sustainable method to produce biodiesel; however, its reaction speed is very low, costs are extremely high, and scaling is complicated [10]. Basic homogenous transesterification is easy to operate on an industrial level, has a high catalytic activity and the reaction times are short [11], but has disadvantages, such as that the presence of free fatty acids and water produces soap affecting the entire production process, and that this method generates large volumes of wastewater and therefore has a high environmental impact. On the other hand, heterogeneous catalysts have gained interest in the production of biodiesel, because they are neither consumed nor dissolved in the reaction mixture, which facilitates the separation of the product and the reuse of the catalyst. The main disadvantage of this approach is that it is still in the pilot escape phase and problems of diffusion, immiscibility and mass transfer efficiency are still not resolved at the operational level, which affects the speed of the reaction [12–14].

Recently, the heterogeneous catalysts that have presented greater effectiveness and potential to be used in the production of biodiesel are simple and mixed metal oxides, zeolites, anion exchange resins and carbon-based catalysts, among others [15–20]. Layered double hydroxides (LDH) are a family of natural and synthetic compounds that are characterized by a laminar structure-type hydrotalcite (HT). The sheets are octahedral networks containing metals with divalent and trivalent positive charges that generate a positive residual charge which is neutralized in the presence of anions and water molecules in the interlaminar space. Their molecular formula is [M2<sup>+</sup>(1−x)M3<sup>+</sup>x(OH)2][A<sup>n</sup>−]x/n·yH2O [21,22]. Layered double hydroxides can be prepared by different methods such as coprecipitation, hydrolysis with urea, sol–gel, hydrothermal treatment, combustion and mechanical synthesis, among others, with the most common and economic method being coprecipitation [23–25]. Studies have reported that the synthesis method, the conditions of synthesis, the type of cations and structural anions, the metal precursors and the metal ratio M<sup>2</sup>+/M3<sup>+</sup> are determining factors on the properties and applications of LDH [26–29]. An important element to improve the catalytic activity of LDH is the thermal treatment to form mixed spinel-type oxides, specifically in the production of biodiesel by heterogeneous catalysis [30,31]. The most relevant studies have shown good results of conversion to biodiesel, but these reactions were found to require a long synthesis time, high temperatures or critical conditions, high percentages of catalyst and large amounts of methanol, among other factors. Table 1 shows examples of investigations in which catalysts of the LDH type have been used, which mainly consist of metals such as Mg, Al, Zn and Fe.


**Table 1.** Obtaining biodiesel using different layered double hydroxides (LDH)-type catalysts.

<sup>1</sup> HT: hydrotalcite or layered double hydroxide

The above shows a panorama highlighting the need to continue looking for new materials that achieve better efficiencies in the conversion to biodiesel in the shortest possible time and using the smoothest and most controllable reaction conditions.

The modification that has been given to the LDH containing Fe, both in the laminar structure and impregnated, have shown efficient conversion results, but with long reaction times or with systems that require pressure [31,38]. On the other hand, cobalt is a catalytically active element that has been used in many catalytic processes, including transesterification. The use of catalysts with Co content to obtain biodiesel has given interesting results, with the case of the use of materials of type Co(II)-organic compounds in microwave reactors that have reached 80% conversion in 12 h [40]. Other studies have used cobalt supported, for example, in zeolite ZSM5 with in reaction times of 8 h to 95 ◦C and 80% conversion [41]. Additionally, a study reports the direct esterification of fatty acids to form biodiesel by using a mixture of oxides with Co, Sn and Al with 1:32 oil/methanol ratio, 2% catalyst and 60 ◦C, reaching 65% conversion in4h[42].

The present work focuses on the obtaining of a new catalysts obtained by thermal activation of layered double hydroxides Co/Fe, to be used in the esterification and transesterification reactions to produce biodiesel. The use of Co and Fe as active metals in crystalline networks of the simple and mixed oxides type is proposed, considering mainly that the cobalt is a transition element that can contribute to the formation of active sites for the simultaneous esterification and transesterification of the cooking oil to biodiesel. The effect of the variation of the Co/Fe ratio of the LDH precursor on the physicochemical and catalytic properties of its calcination products at 700 ◦C for catalytic activation is studied. Complementarily, the best operating conditions for obtaining biodiesel by heterogeneous catalysis were determined, the foregoing to generate a viable alternative for the industrial production of biodiesel under simple operating conditions.

#### **2. Results**

#### *2.1. Catalyst Characterization*

This section shows the results of the characterization of the catalytic precursors Layered Double Hydroxides (LDH) identified as LDH1, LDH2, LDH3 and LDH4 corresponding to the ratio Co/Fe = 1, 2, 3 and 4 respectively, as well as the products of calcination of LDH identified as MO-LDH1, MO-LDH2, MO-LDH3 and MO-LDH4.

#### 2.1.1. Thermal Evolution of Layered Double Hydroxides (LDH) to Co/Fe-Mixed Oxides (MO-LDH)

Figure 2a shows the thermogravimetric analysis (TGA) of the LDH, corresponding to the different precursors of the mixed Co/Fe oxides. As can be seen, the LDH2 precursor presents a first major weight loss associated with the elimination of water around 200 ◦C corresponding to 12% of its original weight, while the LDH3 and LDH4 precursors present a weight loss of only 3.5% in that same temperature range. The second weight loss occurs between 600 and 700 ◦C which for the solid LDH1 amounts to a loss of 22%, LDH2 of 33%, LDH3 of 34.5% and LDH4 of 46.5%, the latter being the highest total loss reached before losing 50% of its mass at 1100 ◦C. Thermograms by differential scanning calorimetry (DSC) are shown in Figure 2b, where it can be seen that the different Co/Fe precursors synthesized present similar thermal evolutions. The main signals correspond to the dehydration of the molecules housed in the interlaminar at 300 ◦C and the decarbonation and dehydroxylation at 680 ◦C associated with the weight loss described in the TGA, which forms the crystalline and amorphous phases of the simple and mixed oxides. In the case of the LDH3 and LDH4 solids, an additional reaction can be seen at 998 ◦C associated with the sintering of the spinel of the cobalt ferrite (CoFe2O4).

**Figure 2.** Thermal analysis thermograms by differential scanning and thermogravimetric calorimetry of synthesized catalytic precursors: (**a**) thermogravimetric analysis (TGA) and (**b**) differential scanning calorimetry (DSC).

The thermal stability of the LDH synthesized associated with the evolution of the crystalline phases allows us to determine the temperature at which the materials must be activated in order to be used as catalysts in the transesterification of triglycerides to biodiesel. This temperature was found to be 700 ◦C for the transition of the phases.

#### 2.1.2. Modification of Functional Groups by Thermal Activation of the Catalyst

Figure 3 shows the FTIR spectra of the LDHs prior to activation (Figure 3a) compared to the spectra of the catalysts obtained by calcination at 700 ◦C (Figure 3b). As can be seen, the thermal decomposition profiles are correlated to the activation temperature of the catalysts. A broad band of low intensity close to 3300 cm−<sup>1</sup> can be observed in all spectra of Figure 3b, which is associated with hydroxyl groups, where in the case of the catalysts MO-LDH1 and MO-LDH2 it is a double band associated with the interaction of hydroxyl and carbonates that could be reabsorbed on the surface given the precursor reconstruction effect. This signal attributed to the hydroxyl groups is of low intensity for the catalysts MO-LDH3 and MO-LDH4. With respect to the presence of carbonate groups, these signals are observed between 1200 and 1600 cm<sup>−</sup>1. In the case of MO-LDH1 the band belonging to carbonates is not observed, for MO-LDH2 most of the carbonates are interlaminar, in the MO-LDH3 the carbonates are free, that is, they are adsorbed on the surface and the MO-LDH4 carbonates are not observed. For the case of the modifications in the interlaminar bonds of the metals when evolving

from hydroxides to oxides, these signals can be identified in the region between 400 and 1000 cm−1. For the catalyst MO-LDH3 and MO-LDH4, more intense bands can be observed that belong to the Fe–O, Co–O and Fe–O–Co bonds, these signals are characteristic of an infrared spectrum of the cobalt ferrite. which are less intense for the catalysts MO-LDH1 and MO-LDH2 given the over positioning some signals of simple and mixed oxides.

**Figure 3.** Spectra of infrared spectroscopy of the catalysts: (**a**) LDH synthesized and (**b**) LDH calcined at 700 ◦C.

2.1.3. Identification of Crystalline Phases Present in the Catalysts

Figure 4 shows the X-ray diffraction patterns of the catalysts obtained by calcination at 700 ◦C. The solid MO-LDH1 does not present the crystalline phase of CoFe2O4 (cobalt ferrite) but many noise signals associated with amorphous phases. This confirms our observations of the infrared spectra, showing that when cobalt ferrite is present it is still amorphous, in addition to the presence of small crystalline phases secreted by Fe3O4, CoO and CoO2. The solid MO-LDH2 can be observed in the presence of the crystalline phase of CoFe2O4 accompanied by segregated phases of Fe3O4, CoO2 and CoO. In the case of solid MO-LDH3, a better crystallinity of CoFe2O4 is observed, as evidenced in the FTIR spectra, in addition to the presence of a segregated phase of CoO2 and CoNaxO2. In the case of MO-LDH4, the CoFe2O4 phase is observed, combined with CoNaxO2, CoO2 and CoO [43,44].

For the solids MO-LDH3 and MO-LDH4, the presence of sodium in the crystalline networks is due to the fact that the amount of iron is less than cobalt, which allows cobalt to interact with sodium at the moment of synthesis in basic medium, favoring the integration of sodium in the crystalline network and modifying the properties of the catalysts.

**Figure 4.** X-ray diffraction patterns of the catalysts obtained at 700 ◦ C: (**a**) MO-LDH1, (**b**) MO-LDH2, (**c**) MO-LDH3 and (**d**) MO-LDH4.

Given the variation in the composition and segregation of the crystalline phases in the different catalysts obtained from the calcination products at 700 ◦C of the layered double hydroxides with different Co/Fe molar ratio, the effect of the presence may be analyzed of the different simple and mixed oxides of Co and Fe with respect to the catalytic activity.

#### 2.1.4. Determination of Textural Properties of the Catalysts

Figure 5 shows the physisorption isotherms of N2 for the catalysts obtained by calcination at 700 ◦C. The adsorption isotherms for all solids belong to type III, corresponding to physical adsorption in multilayers by a free surface, which is characteristic of macroporous solids (pore size greater than 50 nm). However, a variation in the curl of hysteresis was observed, especially for the solid MO-LDH1. The reason why the solid MO-LDH1 presents a broad curl is associated with an incomplete thermal evolution so that it does not yet collapse the structure of the oxides. The rest of the catalysts show narrow curls, which reflect a collapse of the interlaminar spaces associated with a decrease in the specific area.

**Figure 5.** Isotherms of the physisorption of N2 of the catalysts obtained at 700 ◦C: (**a**) MO-LDH1, (**b**) MO-LDH2, (**c**) MO-LDH3 and (**d**) MO-LDH4.

Table 2 shows the results of the textural characterization of mixed oxides of different molar ratios: Brunauer, Emmett and Teller (BET)-area, diameter and pore volume.


**Table 2.** Textural properties of the catalysts obtained at 700 ◦C.

As can be seen, the catalysts show a trend of the value of the specific area in correlation with the metal molar ratio Co/Fe and with the thermal decomposition profile of the catalysts. For all cases the areas are less than 10 m2/g, which suggests the degree of collapse of the laminar structure associated with thermal evolution to simple and mixed metal oxides. In relation to the pore diameter, only the MO-LDH2 catalyst has mesoporous size, and the rest exhibit a larger size corresponding to macropores.

#### 2.1.5. Surface Analysis of the Particles of the Catalysts

Figure 6 shows the photographs obtained by scanning electron microscopy of the catalysts. As can be observed, the solid MO-LDH1 and MO-LDH2 show an amorphous texture with the presence of few hexagonal crystalline structures, which is because the crystallization by thermal evolution has not concluded for all the crystalline phases of the mixed and simple oxides. For the solids MO-LDH3 and MO-LDH4, the presence of hexagonal particles in cumulus and in the form of thin, defined sheets increases markedly. The hexagonal sheets form due to the large amount of mixed oxide Co/Fe present in the different catalysts, mainly in the MO-LDH3 catalyst. In the case of the MO-LDH4 catalyst, very large hexagonal prisms resulting from the ordering of the catalyst sheets can be clearly observed.

**Figure 6.** Scanning microscopy images of the catalysts obtained at 700 ◦C: (**a**) MO-LDH1, (**b**) MO-LDH2, (**c**) MO-LDH3 and (**d**) MO-LDH4.

#### 2.1.6. Elemental Composition on the Surface of the Particles

Finally, in Figure 7 is shown an elemental analysis performed on the catalysts obtained by calcination at 700 ◦C. As can be seen, the general composition of the mixed oxides is similar for all the catalysts, which was to be expected because they were synthesized with the same methodology. The variation between the catalysts lies in the variation in the metallic Co/Fe ratio in the synthesis of the catalytic precursors, as well as in the proportion of the simple and mixed metal oxides present in the catalyst. The elements present in the catalysts are: cobalt, iron, sodium, oxygen and carbon, where by correlation with the diffractograms are in the form of metal oxides for the case of cobalt, iron and sodium; whereas the carbon by confirmation of the FTIR spectra corresponds to the CO2 physisorbed on the surface of the catalysts.

**Figure 7.** Elemental analysis of the catalysts obtained at 700 ◦C: (**a**) MO-LDH1, (**b**) MO-LDH2, (**c**) MO-LDH3 and (**d**) MO-LDH4.

Table 3 shows the results of the determination of the Co/Fe molar ratio in the catalytic precursors compared with that of the catalysts.

**Table 3.** Comparison of the variation of the Co/Fe molar ratio on the surface of the precursor particles and their corresponding mixed oxides.


As can be seen, for the case of catalytic precursors, the theoretical molar ratio of Co/Fe is very close to the values of the molar ratio calculated from the average quantification on the surface of the

particles. After the heat treatment, it can be seen that for the case of the MO-LDH1 and MO-LDH2 solids the Co/Fe ratio varies very little, which can be attributed to the homogeneity of the dispersion of metals in the particles. These solids were also homogeneous after thermal evolution. In the case of the MO-LDH3 and MO-LDH4 catalysts, a decrease in the Co/Fe molar ratio is observed, which can be attributed to the fact that the particles are more crystalline with defined edges, which concentrate the iron in the crystalline network, thus decreasing the amount of iron on the surface.

#### *2.2. Evaluation of Biodiesel Quality*

The physical and rapid qualitative parameter that was used to determine whether transesterification was performed for the conversion of triglycerides to biodiesel was the kinematic viscosity, starting from the fact that the oil prior to transesterification had a viscosity of 32 cp, while biodiesel with a high percentage of conversion presents values below 6 cp according to ASTMD6751, implying that a decrease in viscosity is associated with the transesterification of the fatty acids present in the oil [45].

Once it was confirmed that the viscosity had been reduced to values close to the norm, we proceeded to the quantification and confirmation of the conversion by diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) and 1H-Nuclear magnetic resonance (NMR) spectroscopy [46], as well as the verification of the international parameters of density, corrosion of the copper foil and turbidity. These parameters can be used to verify the feasibility of the products' application as biofuels in internal combustion engines.

Table 4 shows the best conversion percentages obtained in the shortest reaction times tested for obtaining biodiesel using a 3% catalyst, a molar oil/methanol ratio of 1:12, a reaction temperature of 65 ◦C and constant agitation at 1400 rpm.


**Table 4.** Quality parameters of biodiesel obtained using the different catalysts.

It can be seen that the four catalysts achieved a conversion of triglycerides to biodiesel by lowering the viscosity of the 32 cp oil to values lower than 7 cp. In the case of the MO-LDH1 and MO-LDH2 catalysts, at a reaction time of 30 min, the viscosities were 14.24 and 15.27 cp, respectively, which indicated that the conversion had not yet finished. When the reaction time was extended to 60 min, a decrease in viscosity values close to 7 cp was observed, associated with conversion values greater than 70%. On the other hand, for the case of the MO-LDH3 and MO-LDH4 catalysts, at 30 min they had already reached viscosities below 4 cp and achieved conversions greater than 90%.

Figure 8 shows the 1H-NMR spectra which were used to calculate the % conversion to biodiesel from the M signal corresponding to the methyl group of the biodiesel methylesters that is formed as the reaction is carried out. Associated with the M signal is the G zone corresponding to the monoglycerides, diglycerides and triglycerides of the oil, which disappear in the conversion to methylesters.

**Figure 8.** Nuclear magnetic resonance spectra of 1H in the transesterification reaction for biodiesel production as a function of reaction time.

With this analysis it can be verified that at 60 min the MO-LDH1 and MO-LDH2 catalysts have not yet been able to complete the transesterification reaction of all triglycerides in the oil, but there is a high percentage of conversion greater than 60%. It can be observed that in the case of the MO-LDH1 catalyst, the activity reached at 60 min was 63.3%, which can be attributed to the fact that it consists of particles of amorphous material with a small amount of simple CoO, CoO2 and Fe3O4, which provide a greater specific area than the rest of the catalysts, but with fewer active sites for catalytic conversion. The MO-LDH2 catalyst has a better conversion capacity, reaching 82% in 60 min, because the catalyst contains the mixed oxide CoFe2O4 in addition to the segregated phases of Fe3O4, CoO2 and CoO. For the case of the MO-LDH3 and MO-LDH4 catalysts, a high conversion to biodiesel with values of 92.7% and 96%, respectively, can be observed. In the NMR spectra it can be observed that there are no signals in the G zone, which implies that they have already completed the transesterification reaction for the formation of the methyl esters when a high intensity of the corresponding M signal is observed. This is associated with better conversion capacity of the MO-LDH3 and MO-LDH4 catalysts, attributed to the presence of crystalline particles of the mixed oxides CoFe2O4 and CoNaxO2.

With respect to the complementary quality parameters shown in Table 4, related to the acidity index, corrosion of the sheet and turbidity of the biodiesels obtained with the MO-LDH3 and MO-LDH4 catalysts, it can be seen that these parameters are within the standard ranges. Thus, it is confirmed that the biodiesel obtained with these catalysts can be used in an internal combustion engine.

As the MO-LDH4 catalyst exhibits a better conversion capacity, reaching 96% and presenting a viscosity decrease to 3.66 cp, this catalyst was used to correlate and improve the reaction conditions for obtaining biodiesel with respect to the decrease of the reaction time, the amount of catalyst and the aomount of methanol used in the synthesis.

In relation to the reaction time, a study of the conversion capacity of the catalyst as a function of time was carried out, from 5 to 30 min, while maintaining the reaction conditions of 3% catalyst, a molar ratio of oil/methanol of 1:12, a reaction temperature of 65 ◦C and constant stirring at 1400 rpm. Table 5 shows the results of the biodiesel conversion values quantified by 1H-NMR obtained with the

catalyst MO-LDH4 at times below 30 min, also indicating the viscosity values as a quality parameter of the biodiesel.


**Table 5.** Percentage of conversion of the biodiesel obtained with the MO-LDH4 catalyst at different reaction times.

It can be seen that the MO-LDH4 catalyst has the capacity to convert the oil to biodiesel in only 5 min with viscosity values that comply with the norm, reaching a conversion of 81%. These results exceed the capacity found for the MO-LDH2 catalyst, which required 60 min to reach the same conversion capacity, thus confirming the fast selectivity and conversion capacity of the catalyst as well as the important role of mixed oxides in the process. After 5 min, the biodiesel complies with the viscosity quality parameter. With respect to the variation of the conversion% as a function of time, it can be seen that from 5 to 30 min the values of the conversion increase gradually from 81.3% to 96%. The activity of the MO-LDH4 catalyst, attributed to the presence of particles of the mixed oxides CoFe2O4 and CoNaxO2 and of the simple oxides CoO2 and CoO, is favored by the crystalline structure of the catalyst particles, as well as by the macroporosity that allows access to active sites. An advantage of the catalyst is that the formation of catalytically active sites is favored by the formation of mixed crystalline structures.

The combination of the different particles containing Co species with different oxidation states in both the simple oxides and in the mixed with Fe and Na, as a catalytic composition, allows the formation of different acidic and basic sites for the esterification of the free fatty acids present in the oil, as well as for the transesterification of the fatty acids from triglycerides to biodiesel. The process that could be developing in the catalyst is proposed based on the recent kinetic model of transesterification in two sites of the Eley–Rideal type, where the mechanism has three important steps, the first step is the adsorption of both methanol and oil on the surface of the catalyst, in the second step a tetrahedral intermediate is produced from the alkoxide group which attacks the positively polarized carbon of the triacylglyceride in both the liquid phase and the catalyst surface, and the third step involves the production of a fatty acid ester and the release of a diacylglycerides. this procedure is repeated with the following two fatty acids bound to the glycerol [47].

Table 6 shows the viscosity values obtained by decreasing the amount of methanol used in the synthesis while preserving the synthesis conditions of 3% catalyst, a reaction temperature of 65 ◦C, constant agitation at 1400 rpm and a reaction time of 30 min. It can be seen that the efficiency of the catalyst is not significantly affected by decreasing the oil/methanol ratio from 1:12 to 1:9 and to 1:6. This represents an operational advantage since an excess of methanol as a reagent is not required to displace the reaction to products, as in most procedures reported by heterogeneous catalysis [8,22]. In this case, the conversion capacity is even slightly improved by decreasing the volume of methanol in the reaction.


**Table 6.** Percentage of conversion of the biodiesel obtained with the MO-LDH4 catalyst at different oil/methanol ratios.

The effect of the decrease of the amount of the catalyst used in the transesterification reaction was tested, while maintaining the conditions of a reaction temperature of 65 ◦C, an oil/ethanol molar ratio of 1:12, constant agitation at 1400 rpm and a reaction time of 30 min. Table 7 shows that the efficiency of the process is not significantly affected by decreasing the amount of the catalyst from 3 to 2%, allowing a reduction of up to one third of the catalyst.

**Table 7.** Percentage of conversion of the biodiesel obtained with the MO-LDH4 catalyst at different oil/methanol ratios.


Finally, to verify the best synthesis conditions considering the simultaneous decrease of the catalyst, methanol and reaction time, biodiesel was obtained with 2% catalyst, a oil/methanol molar ratio of 1:6 and a reaction time of 20 min. Thus, we achieved a biodiesel with quality parameters that comply with the ASTM standards, as reported in Table 8.

**Table 8.** Quality parameters of the biodiesel obtained using the MO-LDH4 catalyst under optimal synthesis conditions.


It can be seen that the biodiesel obtained by Gas Chromatography-Flame Ionization Detector (CG-FID) is mainly composed of the methyl esters of oleic acid (C18: 1 cis-9), linoleic acid (C18: 2 cis-9,12) and linolenic acid (C18: 3 cis-9,12,15), and in a lower proportion of palmitic acid (C16: 0) and stearic acid (C18: 0), which were identified according to the chromatograms shown in Figure 9. These results confirm the high selectivity of MO-LDH4 towards unsaturated fatty acids.

**Figure 9.** Chromatograms of biodiesel obtained with 2% MO-LDH4 catalyst compared to the Supelco standard for Fatty Acid Methyl Esters (FAME).

The results of the analysis of the biodiesel composition obtained confirm the formation of the methyl esters of the respective fatty acids that formed the oil used as raw material in the transesterification reaction. The biodiesel obtained complies with the quality standards set by the ASTM international standards. The quality of the biodiesel was improved by the selectivity and high conversion capacity of the catalyst.

#### **3. Materials and Methods**

#### *3.1. Synthesis of the Catalyst*

The catalysts were synthesized by the procedure registered in the patent process MX2015009362 of the Mexican Institute of Industrial Property [48], as described below. A series of Co/Fe layered double hydroxides samples was prepared by the coprecipitation method with a molar ratio Co<sup>2</sup>+/Fe3<sup>+</sup> = 1, 2, 3 and 4 using a stoichiometric amount of cobalt nitrate hexahydrate and iron nitrate nonahydrate. The aqueous solutions containing the metal salts were coprecipitated in a basic medium with an alkaline solution of sodium hydroxide (2M) and sodium bicarbonate (1M) at a constant pH of 11.5. After complete precipitation of the mixture, the precipitates were washed with water at room temperature until obtaining a pH of 9. Subsequently the precipitates were dried in an oven at 70 ◦C for 24 h to obtain the LDH as catalytic precursors. The materials were identified as LDH1, LDH2, LDH3 and LDH4 corresponding to the Co/Fe ratio = 1, 2, 3 and 4 respectively. The catalytically active phases were obtained by calcination of the precursors at 700 ◦C for 4 h. The calcination products of the layered doble hydroxides were identified as corresponding MO-LDH1, MO-LDH2, MO-LDH3 and MO-LDH4.

#### *3.2. Physicochemical Characterization of the Catalyst*

The catalysts were characterized by (a) differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) with a Universal (Irapuato, Mexico) V4.5A TA Instruments: SDT Q600 V20.5 Build 15; (b) X-ray powder diffraction (XRD) in Inel Equinox (Guanajuato, Mexico) with an X-ray tube coupled with a copper anode using monochromatic CuKα radiation; (c) Fourier transform infrared spectroscopy (FTIR) in a Bruker Tensor (Guanajuato, Mexico) 27 with OPUS software version 6.5; (d) textural properties in an ASAP (Guanajuato, Mexico) 2010 and (e) scanning electron microscopy (SEM) using an X Carl Zeiss Evo (Guanajuato, Mexico) HD-15 microscope with an integrated X-ray dispersive energy microanalysis system.

#### *3.3. Production of Biodiesel*

The production of biodiesel was developed by the procedure registered in the patent process MX2015009362 of the Mexican Institute of Industrial Property [48], as described below. To obtain biodiesel, a batch reactor was used with the operating conditions in a reflux system at 65 ◦C, constant agitation at 1400 rpm and a pressure atmosphere. The alcohol used in the synthesis was anhydrous methanol (Golden Bell, Guadalajara, Mexico) and the source of triglycerides was commercial cooking oil. The variables used in the synthesis of biodiesel were the oil/methanol molar ratio (1:6, 1:9 and 1:12), the percentage of oil/catalyst (1, 2 and 3% w/w) and the reaction time (30 and 60 min). Initially, 3 g of calcined LDH was added to 50 mL methanol in a 500 mL three-necked glass flask equipped with a reflux condenser. The mixture was vigorously mixed under atmospheric and heated until the temperature reached 50 ◦C. Then, 100 mL of cooking oil, was added to the reactor and the temperature was increased to 65 ◦C. The mixture was stirred at 1400 rpm during the reaction time (30 min). At the end of the reaction, methanol was recovered by evaporation, catalyst was recovered by vacuum filtration, and subsequently, glycerol, and biodiesel were separated by decantation.

#### *3.4. Evaluation of the Quality of Biodiesel*

The quality parameters of the biodiesel were evaluated in accordance with the ASTM international standards to verify that the quality indicated in the ASTM D6751 standard was met. The viscosity was determined in accordance with ASTM D445. The density was determined according to ASTM D121718. The ASTM D66419 standard was used for the acidity index. The corrosion of copper foil was evaluated according to ASTM D13020. Turbidity was measured according to ASTM 250021. The quantification of the conversion of biodiesel by FTIR with diffuse reflectivity (DRIFTS) was conducted as established in ASTM D7371, as well as by proton nuclear magnetic resonance (1H-MNR). The determination of the methyl esters contained in the biodiesel was carried out in accordance with the ASTM D6584 standard.

#### **4. Conclusions**

Catalytic precursors of the layered double hydroxides type with different Co/Fe ratios were synthesized in a basic medium, which allowed us to obtain catalysts that showed a correlation between their physicochemical properties and their catalytic capacity in the conversion of triglycerides to biodiesel. The best conversion capacity was presented by the MO-LDH4 catalyst with a 96% conversion to biodiesel by transesterification using a molar alcohol/oil ratio of 6:1, with a reaction time of 20 min and the catalyst percentage of 2%, mainly formed by a mixture of crystalline particles in the form of stacked sheets. The high capacity and selectivity for the conversion of triglycerides to biodiesel shown by the MO-LDH4 catalyst is attributed to the fact that it is a composite catalyst that contains a combination of the simple oxides CoO2 and CoO and the mixed oxides CoFe2O4 and CoNaxO2 where the Co with different oxidation states in combination with O, Fe and Na, favors the presence of acidic and basic sites for the transesterification esterification of the oil to biodiesel.

#### **5. Patents**

MX2015009362 Esthela Ramos Ramírez, Norma Leticia Gutiérrez-Ortega, José de Jesús Monjaraz Vallejo, "Obtaining Cobalt Ferrite for its application as a catalyst in the optimization of esterification and transesterification reactions of fatty acids for the biodiesel production process" ("Obtención de Ferrita de Cobalto para su aplicación como catalizador en la optimización de las reacciones de esterificación y transesterificación de ácidos grasos para el proceso de producción de biodiesel" titule in spanish), request patent filed with the Mexican Institute of Industrial Property (IMPI Spanish acronym) Publication date: January 20, 2017.

**Author Contributions:** Conceptualization, G.-O.N. and R.-R.E.; methodology, G.-O.N. and M.-V.J.; validation, G.-O.N. and R.-R.E.; formal analysis, S.-M.A.; investigation, Z.-M.A.; resources, S.-M.A.; data curation, Z.-M.A.; writing—original draft preparation, G.-O.N.; writing—review and editing, R.-R.E.; project administration, R.-R.E.; funding acquisition, G.-O.N.

**Funding:** This research received no external funding. Funding was received from the resources of the university of Guanajuato.

**Acknowledgments:** This work was supported by the Professors Improvement Program (PROMEP Spanish acronym) of the Public Education Secretary (SEP Spanish acronym), and the Direction of Research and Postgraduate Course Support (DAIP Spanish acronym) from the University of Guanajuato. The authors thank the Directorate of Research Support and the Postgraduate Program of the University of Guanajuato for funding and CONACYT for the scholarship received for the completion of postgraduate studies. In addition, Dr. Ramón Zarraga and Dr. Ricardo Navarro from the University of Guanajuato are thanked for the support with the NMR and SEM-EDX techniques, respectively.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Improved Etherification of Glycerol with** *Tert***-Butyl Alcohol by the Addition of Dibutyl Ether as Solvent**

#### **Carmen M. Dominguez \* , Arturo Romero and Aurora Santos**

Chemical Engineering and Materials, Faculty of Chemical Sciences, Universidad Complutense de Madrid, Ciudad Universitaria S/N, 28040 Madrid, Spain; aromeros@ucm.es (A.R.); aursan@ucm.es (A.S.)

**\*** Correspondence: carmdomi@ucm.es; Tel.: +34-913-944-171

Received: 26 March 2019; Accepted: 17 April 2019; Published: 23 April 2019

**Abstract:** The etherification of glycerol with *tert*-butyl alcohol in the presence of acid catalysts gives rise to the production of ethers (monoethers, diethers and triethers) of high added-value, which can be used as oxygenated additives in fuels. This reaction is limited by the thermodynamic equilibrium, which can be modified by the addition of solvents that selectively solubilize the products of interest along with *tert*-butyl alcohol, leading to the progress of the reaction. In this work, it has been demonstrated that the addition of dibutyl ether allows shifting the reaction equilibrium, increasing the production of diethers. From the study of the main operating conditions, it was determined that an increase in the concentration of the solvent has a positive effect on the selectivity towards the production of diethers, the concentration of the catalyst (a commercial ion exchange resin, Amberlyst 15, named A-15) and the reaction temperature were also determining variables. Working with concentrations of *tert*-butyl alcohol above the stoichiometric one did not report great advantages. The optimal operating conditions to maximize the conversion of glycerol and the selectivity towards diethers were: 70 ◦C, 20% catalyst (referred to the total starting mass of the system), the stoichiometric ratio of glycerol:*tert*-butyl alcohol (G:TB = 1:3) and 1:2 molar ratio of dibutyl ether:*tert*-butyl alcohol. A study of three consecutive reaction cycles showed the high stability of the catalyst, obtaining identical results.

**Keywords:** etherification; glycerol; *tert*-butyl alcohol; dibutyl ether; A-15; catalyst stability

#### **1. Introduction**

The renewable and biodegradable character of biodiesel has made it an interesting alternative to the use of conventional fuels [1–3]. Its use as combustible for diesel engines has greatly grown in recent decades [4] and is expected to grow further in the next years [5,6]. Biodiesel is a mixture of methyl esters of fatty acids [7], synthesized from animal or vegetable fats, which are non-toxic, biodegradable and renewable resources [8], obtaining glycerol (G) (about 1 kg of glycerol is generated for every 9 kg of biodiesel) as the main by-product of the process [4,9]. As a direct consequence of the increase in biodiesel production, the price of glycerol has considerably dropped [6]. Therefore, finding solutions for the valorization of this by-product to added value chemicals could improve the economy of the process [10]. Thus, the production of biodiesel on a large scale will be aided if adequate technologies capable of converting glycerol into added value chemicals are developed [11]. There are several industrial processes that use glycerol as raw material (oligomerization/polymerization, pyrolysis and gasification, selective oxidation, steam reforming, selective transesterification, etherification to fuel-oxygenates, etc.) [12,13]. Another alternative is the production of glycerol carbonate (an important glycerol derivative commonly used as a solvent in cosmetics, personal care items and medicine) from the catalytic oxidative carbonylation of the parent compound [14]. Among the existing options, an interesting alternative for the excess of glycerol generated is its transformation into oxygenated additives for liquid fuels [15], which can enhance the combustion efficiency in internal combustion engines and reduce the emission of pollutants, particularly, *tert*-butyl ethers of glycerol [7]. One possibility for obtaining these oxygenated compounds is the conversion of glycerol into ethers from its etherification with isobutene (IB) [16–22]. The use of gaseous isobutene involves the typical drawbacks of a complex three-phase system (mass transfer phenomena, security issues, etc.) [7] and other alternatives have been studied. Thus, it has been shown that the use of *tert*-butyl alcohol (TB), which is a by-product of polypropylene production, instead of isobutene, could be preferable since (i) The reagents are in the same phase, (ii) prevents the oligomerization of isobutene [9,23] and (iii) allows to overcome the technological problems arising from the need to use solvents (i.e., dioxane, dimethyl sulfoxide) able to dissolve glycerol [7,24].

The etherification of glycerol with *tert*-butyl alcohol proceeds according to a consecutive path giving rise to a mixture of five *tert*-butyl ethers, namely monoethers (M1, M2), diethers (D1, D2) and, eventually, triether (T) (Figure 1). These compounds, mainly diethers and triethers, are considered excellent additives for diesel fuels (diesel, biodiesel and their mixtures) [9,19,22]. The solubility of monoethers in diesel fuel is quite low. Therefore, in order to avoid an additional separation step, the etherification of glycerol should address the generation of higher glycerol ethers (di- and triethers) [16]. On the other hand, it has been demonstrated that the production of triether is usually highly limited due to steric hindrance [7,9]. Thus, the selectivity of glycerol towards the production of di-substituted ethers should be maximized. Otherwise, it should to be noted the undesired (and inevitable) reaction of dehydration of TB to isobutene (IB) (secondary reaction) and the generation of water in each stage of the reaction, which negatively affects de etherification equilibrium (Figure 1) and competes with the reagents (glycerol and *tert*-butyl alcohol) for the active adsorption sites of the catalyst [7,25].

**Figure 1.** Reaction scheme of glycerol (G) etherification with *tert*-butyl alcohol (TB) and secondary reaction to isobutene (IB) from TB dehydration. Adapted from Pico et al., 2012 [9].

Interesting results were obtained when water was removed from the reaction medium, leading to an improvement in ethers production [7,25]. The dehydration of the reaction system (etherification of glycerol with *tert*-butyl alcohol with a commercial ion exchange resin as catalyst) by using zeolites as water adsorbents after 6 h reaction time resulted in a net increase in the yield of diethers from 28.5% to 41.5% [7]. Ozbay et al. (2012) [25] also found a significant enhancement of glycerol conversion and diether yield by the in situ removal of water generated during the sequential etherification reactions (the selectivity towards diether increased from 20 to 33% when a zeolite:catalyst ratio of 4:1 was used). Another possibility (very little studied so far) could be adding a solvent (not miscible with water or with low water solubility) to the reaction medium able to selectively dissolve TB along with the generated ethers, allowing in this way the progress of the reaction. Roze et al. (2013) found that performing the reaction in toluene, which works as a water removing agent, influenced the etherification reaction significantly increasing the conversion of glycerol and the yield of glycerol ethers [26]. Two other solvents, 1,4-dioxane and ethanol, were tested, but lead to worse results [26].

This reaction requires the use of acid catalysts, such as sulfuric acid, ion exchange resins, zeolites, mesostructured silicas, etc. [9,26]. Amongst the heterogeneous catalysts (more advantageous from an environmental point of view due to their easy separation at the end of the process and its possible reuse), the most convenient ones have turned out to be ion exchange resins [27]. Among them, the commercial Amberlite 15 resin (A-15) has given interesting results in the etherification of glycerol with *tert*-butyl alcohol [7,9,23] due to its high acidity and good stability. This catalyst exhibited higher activity than other acid systems such as Nafion® on amorphous silica and a home-made mesoestructurated silica supported acid catalyst because of the wider pore diameter of the A-15, which allowed easier accessibility of the reagent molecules [7]. Pico et al. (2013) [23] found that this catalyst also showed better performance in the etherification of glycerol with *tert*-butyl alcohol (higher glycerol conversion and selectivity towards diethers) than other commercial ion-exchange resins (Ambertite 200 and Amberlite IRC-50) because of its higher acidity and better textural properties. However, the maximum selectivity of glycerol towards the products of interest (diethers) in the presence of A-15 catalyst was relatively low (28.5% [7] and 21% [23]). The explanation to this fact was that the thermodynamic equilibrium was reached. Accordingly, further research to modify this equilibrium and increase the yield to di- and triethers is required.

The objective of the present work is to shift the equilibrium of the glycerol etherification reaction with TB towards the production of diethers. To do this, dibutyl ether (DBE), non-miscible with water (DBE solubility in water = 0.113 g L−1), has been introduced as a solvent into the reaction system to selectively solubilize the ethers, partially solubilize TB and minimize the presence of water in this phase. Dibutyl ether was selected among other organic solvents due to its interesting properties: Non-polar nature, low solubility in water, low reactivity, low toxicity, high flash point, etc.

In this way, working with a biphasic system, the progress of the reaction from monoethers to diethers could be favored. A-15 heterogeneous catalyst, a commercial ion-exchange resin, accepted as the most suitable catalyst for this reaction, has been used in this study. The influence of the main operating variables (DBE concentration, catalyst concentration, reaction temperature and TB concentration) has been evaluated with the aim of finding the optimal conditions to maximize the yield of the reaction and the selectivity towards the products of interest. Finally, the stability of the catalyst in three consecutive reaction cycles has been demonstrated.

#### **2. Results and Discussion**

The results obtained have been discussed and evaluated according to the conversion of reagents, glycerol (G) and *tert*-butyl alcohol (TB) (Equations (1) and (2)) and the selectivity towards the generated ethers (M, D and T) at different reaction times, calculated from the following expressions:

$$\chi\_{\mathbf{G}}(\%) = \left( 1 - \frac{[\mathbf{G}]\_{t=\mathbf{x}}}{[\mathbf{G}]\_{t=0}} \right) \cdot 100 \tag{1}$$

$$\chi\_{\rm TB}(\%) = \left(1 - \frac{[\rm TB]\_{t=x}}{[\rm TB]\_{t=0}}\right) \cdot 100 \tag{2}$$

$$\mathbf{S\_M(\%)} = \frac{[\mathbf{M}]\_{\mathbf{t} = \mathbf{x}}}{[\mathbf{M}]\_{\mathbf{t} = \mathbf{x}} + [\mathbf{D}]\_{\mathbf{t} = \mathbf{x}} + [\mathbf{T}]\_{\mathbf{t} = \mathbf{x}}} \cdot 100 \tag{3}$$

$$\mathrm{S\_D(\%)} = \frac{[\mathrm{D}]\_{\mathrm{t=x}}}{[\mathrm{M}]\_{\mathrm{t=x}} + [\mathrm{D}]\_{\mathrm{t=x}} + [\mathrm{T}]\_{\mathrm{t=x}}} \cdot 100 \tag{4}$$

$$\mathbf{S\_{T}(\%)}=\frac{[\text{T}]\_{\text{t}=\text{x}}}{[\text{M}]\_{\text{t}=\text{x}}+[\text{D}]\_{\text{t}=\text{x}}+[\text{T}]\_{\text{t}=\text{x}}} \cdot 100\tag{5}$$

The terms SM and SD include the selectivity towards the two monoethers (M1 + M2) and diethers (D1 + D2), respectively (see Figure 1). The sum of selectivities (SM + SD + ST) is equal to 100%. The selectivity towards the triether (T) was in all the experiments below 1% (probably due to steric hindrance [7,9]) so it has not been considered in the discussion of results.

In addition to the parameters discussed above, the selectivity towards the production isobutene (IB), an undesired product of the reaction (Figure 1), and calculated from Equation (6) at 360 min of reaction time, has also been taken into account:

$$\mathrm{S\_{IB}(\%)} = \frac{[\mathrm{IB}]\_{\mathrm{t=360}}}{[\mathrm{M}]\_{\mathrm{t=360}} + 2 \cdot [\mathrm{D}]\_{\mathrm{t=360}} + 3 \cdot [\mathrm{T}]\_{\mathrm{t=360}} + [\mathrm{IB}]\_{\mathrm{t=360}}} \cdot 100 \tag{6}$$

#### *2.1. Preliminary Results: Solubility Study*

The aim of introducing DBE as a solvent in the etherification reaction between TB and G is to improve the production of diethers (compounds with higher added value than monoethers). For that purpose, it is necessary that the first products of the reaction (M1 and M2), as well as a fraction of TB (to continue the etherification reaction), are selectively solubilized in the organic phase (DBE), keeping most of the water generated in the other phase. In this way, the reaction equilibrium could be modified.

The distribution of the different compounds selected to carry out this essay (see Section 4.3.1. Solubility test) in the two phases (named as hydrophilic and hydrophobic phases), determined by gas-chromatographic and Karl Fischer analyses, is collected in Table 1. As can be seen, 100% of G remained in the hydrophilic phase, while most of TB (77%) and M1 (61%) were solubilized in the hydrophobic phase. Thus, the presence of DBE led to a different compounds-distribution, obtaining M1 (reaction product) and TB (reagent necessary for the progress of the reaction) preferentially in the hydrophobic phase. In view of these results, most of the water (84%) was removed from the reaction phase, an improvement in the selectivity towards the products of interest can be expected.


**Table 1.** Distribution of the compounds involved in the etherification of G in the hydrophilic and hydrophobic phases.

TB (*tert*-butanol), DBE (dibutyl ether), M1 (monoether, 1-terbutoxipropano-2,3-diol), G (Glycerol).

#### *2.2. Role of DBE as Solvent in the Etherification of Glycerol*

To verify this hypothesis (modification of the reaction equilibrium and positive effect of DBE on the etherification reaction of G with TB), the selectivity of G towards the reaction products (M and D) obtained in the presence of DBE (experiment R5, Table 3, Materials and Methods section) was compared, for the same glycerol conversion value (XG = 82%), with that obtained in the absence of DBE (data obtained elsewhere at similar operating conditions [9]). The results are depicted in Figure 2. As can be seen, the introduction of DBE into the reaction system led to a substantial increase in the selectivity towards the diethers (SD increased from 21% to 40% when DBE was added to the system), confirming the positive role of this solvent.

**Figure 2.** Effect of DBE on the selectivity towards the reaction products (M and D).

On the other hand, the equilibrium constants of reactions giving rise to the generation of M and D (Keq,M and Keq,D, respectively) in the absence and the presence of DBE have been also compared. The expression of the equilibrium constants without were obtained from Pico et al. (2012) [9] (Equations (7) and (8)), whereas the corresponding equilibrium constants in the presence of DBE were experimentally obtained by using the Equations (9) and (10) (R1, t = 360 min):

$$\mathcal{K}\_{\text{eq. M.}} = \exp\left(26.37 - \frac{8639.46}{\mathcal{T}}\right) \tag{7}$$

$$\mathcal{K}\_{\text{eq},\text{D}} = \exp\left(21.51 - \frac{8083.91}{\text{T}}\right) \tag{8}$$

$$\mathbb{K}\_{\text{eq}\mathcal{M}(\text{DBE})} = \frac{[\text{M}] \cdot [\text{H2O}]}{[\text{TB}] \cdot [\text{G}]} \tag{9}$$

$$\mathbf{K}\_{\text{eq}, \text{D}(\text{DBE})} = \frac{[\text{D}] \cdot [\text{H2O}]}{[\text{TB}] \cdot [\text{M}]} \tag{10}$$

In Equations (9) and (10), the concentration of reactants and products have been calculated considering a pseudo-homogeneous phase, being expressed all of them in mmol kg−1. The values obtained for the equilibrium constants are summarized in Table 2.

**Table 2.** Equilibrium constants in the absence and presence of DBE as solvent.


Keq,M was slightly modified by the presence of DBE, while Keq,D greatly increased. Thus, it has been confirmed that the addition of DBE as a solvent to the reaction system is beneficial since it shifts the reaction equilibrium towards the formation of diethers.

#### *2.3. Operation Condition Study*

Once the positive effect of DBE on the etherification of glycerol with TB was confirmed, a study of the main operating variables was carried out, including the concentration of the solvent and the catalyst, the operating temperature and the concentration of TB.

#### 2.3.1. Effect of DBE Concentration

The effect of DBE concentration was evaluated by working at 70 ◦C, 4 bar, the stoichiometric reagents molar ratio (G:TB = 1:3) and a catalyst concentration of 20% (referred to the initial mass of the system) using two molar ratios of DBE:TB, 1:2 and 1:5. The results of reagents conversion (XG, XTB) and products selectivity (SM, SD) with reaction time are depicted in Figure 3a,b, respectively.

**Figure 3.** Influence of DBE concentration on G and TB conversion (**a**) and M and D selectivity (**b**) with reaction time. Operating conditions: 70 ◦C, 4 bar, G:TB = 1:3, Ccat = 20% (runs R1 and R2 of Table 3).

The reagents conversion (G and TB) was very fast in the early stages of the reaction (t < 60 min) (Figure 3a). From this reaction time, the conversion of glycerol hardly changed until the end of the experiment (XG ≈ 85%, t = 360 min) and that of TB progressively increased. G conversion was always higher than TB conversion since the stoichiometric dose of these reagents were used and G only participates in the first step of the reaction (Figure 1).

As the etherification of G with TB is a series reaction, it is considered that the selectivity towards M is maximum at zero reaction time. Once produced, M1 and M2 react with TB giving rise to the generation of D1 and D2 (Figure 1). Thus, as the reaction proceeds, the selectivity towards M decreases, while the selectivity towards D increases (Figure 3b).

An increase in DBE concentration led to higher conversions of both reagents (Figure 3a) and therefore, greater production of the interest compounds. Thus, the conversion of G and TB at 360 min increased from 74 and 48% to 84 and 62%, respectively, when using a DBE:TB ratio = 1:5 and 1:2. As can be seen in Figure 3b, the selectivity towards the diethers was also favored when increasing the solvent concentration (from 35 to 44%, at 360 min reaction time). Therefore, a higher proportion of DBE in the reaction medium (with respect to the reagents) increased the production of diethers (greater conversion of reagents and selectivity towards these compounds).

The selectivity towards the production of IB at 360 min reaction time slightly increased with the concentration of DBE (from 35 to 39 when using a ratio DBE:TB of 1:5 and 1:2, respectively), as can be seen in Figure 4a.

Finally, it must be pointed out that the addition of DBE to the reaction system is not a major problem because TB and DBE can be easily recycled from the hydrophobic phase (by distillation) and being reused in a new cycle. Besides, this leads to the recovery of purified ethers from the reaction media. Thus, it was decided to work with the highest concentration tested of this solvent in the following experiments (DBE:TB = 1:2).

**Figure 4.** Effect of different operating variables on IB selectivity. Operating conditions: 70 ◦C, G:TB = 1:3, Ccat = 20% (**a**), 70 ◦C, G:TB = 1:3, DBE:TB = 1:2 (**b**), G:TB = 1:3, DBE:TB = 1:2, Ccat = 20% (**c**) 70 ◦C, DBE:TB = 1:2, Ccat = 20% (**d**).

#### 2.3.2. Effect of Catalyst Concentration

In previous works, it was found that the optimal concentration of A-15 in the etherification of G with TB was 7.5% [9]. However, considering that the reaction equilibrium has been modified due to the addition of DBE, this parameter may have been affected and it must be studied again.

Thus, the influence of A-15 concentration working with three catalyst loads (10, 20 and 30%) and maintaining constant the other operating variables (70 ◦C, 4 bar, G:TB = 1:3, DBE:TB = 1:2) has been evaluated. A control experiment without catalyst was also performed, obtaining negligible conversions of the reagents during 360 min (data not shown), which is in agreement with the results previously published in the absence of co-solvent [7]. As can be seen in Figure 5a, the conversion of G greatly increased by increasing the concentration of A-15 from 10 to 20%, but this increase was negligible for higher concentrations. Surprisingly, the conversion of TB when using a catalyst concentration of 20% (R1) was higher than the corresponding to a catalyst load of 30% (R4). A catalyst content in the reaction medium of 30% could be excessive, resulting probably in the formation of aggregates, which reduces the active area of the catalyst and therefore, part of the catalytic surface could remain inaccessible to the reagents.

Furthermore, the selectivity towards the diethers greatly improved (from 26 to 44% at 360 min reaction time) by increasing the concentration of the catalyst from 10 to 20%, but no improvement was obtained when operating with higher catalyst concentrations (Figure 5b).

On the other hand, the concentration of catalyst seems to have no significant effect on the selectivity towards isobutene (Figure 4b). Consequently, it has been estimated that the optimal catalyst concentration in this system is 20%.

**Figure 5.** Influence of catalyst concentration on G and TB conversion (**a**) and M and D selectivity (**b**) with reaction time. Operating conditions: 70 ◦C, 4 bar, G:TB = 1:3, DBE:TB = 1:2 (R3, R1 and R4).

#### 2.3.3. Effect of Operating Temperature

The equilibrium of the reaction is determined by the operating temperature, so this variable plays a key role. Thus, knowing the effect of this parameter on the etherification of G with TB in the presence of DBE is necessary. The etherification reaction was carried out at three different temperatures (60, 70 and 80 ◦C), always below the maximum temperature at which the catalyst can be subjected to (120 ◦C, property provided by the supplier) and maintaining invariable the other operating conditions (4 bar, G:TB = 1:3, DBE:TB = 1:2, Ccat = 20%). Moreover, it has been previously observed that when working at higher temperatures (≥ 90 ◦C), glycerol was not totally converted due to the occurrence of de-etherification reactions [7,19].

As expected, an increase in the operating temperature led to higher reaction rates and consequently, higher conversions of G and TB at the equilibrium, due to the endothermic nature of the reactions (this effect was especially notable in the case of TB, Figure 6a). These results are in agreement with those published when no solvent was added to the reaction medium. Similarly, as the reaction temperature increased, the selectivity towards the formation of diethers also increased (Figure 6b). Once the reaction equilibrium was reached (360 min) at 80 ◦C (R6), SD was higher than SM (Figure 6b). The introduction of DBE in the reaction system led to a new reaction equilibrium that allowed operating at higher temperatures. It should be noted that at this temperature (80 ◦C), in the absence of DBE, the selectivity towards the diethers was only 23% [9].

Otherwise, the secondary reaction of IB production was also favored by an increase in temperature (Figure 4c), and the yield of the reaction towards the products of interest decreased. For this reason, and in order to reach a compromise between the conversion of reagents, the selectivity towards the products of interest and the selectivity towards IB, 70 ◦C has been selected as the most convenient reaction temperature.

**Figure 6.** Influence of reaction temperature on G and TB conversion (**a**) and M and D selectivity (**b**) with reaction time. Operating conditions: 4 bar, G:TB = 1:3, DBE:TB = 1:2, Ccat = 20% (R5, R1 and R6).

#### 2.3.4. Effect of *Tert*-Butyl Alcohol (TB) Concentration

Three G:TB molar ratios have been tested, one corresponding to the stoichiometric ratio between both reagents for a total conversion to triether (G:TB = 1:3, R1) and two others in which TB was in excess (G:TB = 1:4 and 1:5, R7 and R8, respectively), maintaining constant the other operating conditions (70 ◦C, 4 bar, DBE:TB = 1:2, Ccat = 20%). The results have been depicted in Figure 7a,b. The conversion of G was hardly affected by the concentration of TB (≈ 85% at 360 min, regardless of the G:TB ratio, Figure 7a). Consequently, the extension of the reaction did not change when working with an excess of TB. As expected, the conversion of TB decreased when increasing the concentration of this reagent.

**Figure 7.** Influence of reaction temperature on G and TB conversion (**a**) and M and D selectivity (**b**) with reaction time. Operating conditions: 70 ◦C, 4 bar, DBE:TB = 1:2, Ccat = 20% (R1, R7 and R8).

Regarding the selectivity towards mono and diethers, it has been shown that a modification in the G:TB ratio did not affect the reaction in terms of products distribution and therefore, no significant changes were observed in the selectivity towards M and D (Figure 7b). These results are in agreement with those obtained by Frustreri et al. (2009) when working with a similar reaction system in the absence of DBE [7]. Finally, the generation of non-desired reaction products, IB (Figure 4d) and water (data do not shown) increased when increasing TB concentration. The selectivity towards IB increased from 39 to 48% when the ratio G:TB was 1:3 and 1:5, respectively (Figure 4d). Thus, it was determined that working with an excess of TB is not necessary in this system, thus reducing the costs of the process.

#### *2.4. Catalyst Stability*

The stability of the catalyst is a key factor for the economy of the process, especially taking into account the high concentration of catalyst used in the present study (20%, referred to the total starting mass of the system).

Once the reaction was stopped (360 min), the catalyst was filtered (by using a vacuum pump), washed with distilled water and stored in methanol [22,23]. Before undergoing a new catalytic cycle, the catalyst was separated from methanol and dried for 12 h at 60 ◦C. The stability study was carried out under the experimental conditions of R1 (70 ◦C, 4 bar, DBE:TB = 1:3, DBE:TB = 1:2 and Ccat = 20%), considered the most appropriate conditions from the previous results (2.3. Operation Condition Study). As can be seen in Figure 8a,b, the evolution of the conversion of both reagents (G and TB) and the selectivity towards the products of interest (M and D) with reaction time were constant during the three consecutive reaction cycles. Moreover, the selectivity towards secondary reactions (IB production) was not modified (data not shown). Thus, A-15 showed excellent stability under the selected operating conditions.

**Figure 8.** Influence of reaction temperature on G and TB conversion (**a**) and M and D selectivity (**b**) with reaction time. Operating conditions: 70 ◦C, 4 bar, G:TB = 1:3, DBE:TB = 1:2, Ccat=20% (R1, R9 and R10).

#### **3. Conclusions**

It has been demonstrated that the addition of dibutyl ether (DBE) as solvent in the etherification of glycerol (G) with *tert*-butyl alcohol (TB) in the presence of an acid solid catalyst (ion exchange commercial resin Amberlyst, A-15) had a positive effect since this compound allowed shifting the reaction equilibrium towards the formation of diethers (D).

The study of the main operating conditions revealed that an increase in the concentration of DBE improved the conversion of reagents (G and TB) and the selectivity towards the products of interest (D) without modifying the production of isobutene (IB), an undesired by-product. The concentration of catalyst played an important role, increasing the production and selectivity to diethers. It has been determined that the optimal concentration of A-15 was 20%. On the other hand, an increase in the operating temperature (from 60 to 80 ◦C) greatly increased the selectivity towards diethers. However, it is necessary to reach a compromise value (Ta = 70 ◦C) to minimize the undesired production of IB. Finally, the molar ratio G:TB had negligible influence on the performance of the reaction. Therefore, it has been concluded that working with the stoichiometric ratio (G:TB = 1:3) is the most convenient option.

The reuse of the catalyst in three consecutive reaction cycles showed its high stability, achieving identical results in terms of reagents conversion and selectivity towards the products of interest.

#### **4. Materials and Methods**

#### *4.1. Reagents*

Anhydrous glycerol (G, C3H8O3, purity ≥ 99.8%), supplied by Fluka (Bucharest, Romania), and anhydrous *tert*-butyl alcohol (TB, C4H10O, purity ≥ 99.5%), provided by Sigma-Aldrich (Darmstadt, Germany), were employed as reagents. Dibutyl ether (DBE, C8H18O, purity ≥ 99.3%), purchased by Sigma-Aldrich was used as solvent to improve the performance of the etherification reaction. Pyridine reagent grade (C5H5N, purity ≥ 99.5%), supplied by Scharlau (Madrid, Spain), was used as solvent for the preparation of the samples before analysis. 1-terbutoxipropano-2,3-diol (M1) was used for the calibration of the gas chromatograph (GC). 1-Pentanol (C5H12O, purity ≥ 99%) supplied by Sigma-Aldrich (Darmstadt, Germany), was employed as internal standard compound (ISTD) in the GC analysis. Hydranal-Composite 5 and dry Hydranal-Methanol, both from Sigma-Aldrich (Darmstadt, Germany), were used as reactive compound and solvent, respectively, for the water analysis. Methanol (CH4O, purity ≥ 99.8%, Sigma-Aldrich, -Darmstadt, Germany) was used to wash the catalyst between successive reactions.

#### *4.2. Catalyst*

Commercial acid ion-exchange resin Amberlyst 15 (A-15) was supplied by Fluka (Bucharest, Romania) and used as received, without further treatments. This catalyst was selected by Pico et al. (2012) as the most interesting material among three types of commercial ion exchange resins (i.e., Amberlyst 15, Amberlite 200 and Amberlite IRC-50) tested in the etherification of glycerol [9]. The A-15 resin showed the highest activity (glycerol conversion) and selectivity towards the desired product (diethers, D).

The BET surface area, average pore volume, external surface area, and average pore diameter are SBET <sup>=</sup> 44 m2 <sup>g</sup><sup>−</sup>1, Vp <sup>=</sup> 0.338 cm3 <sup>g</sup><sup>−</sup>1, At <sup>=</sup> 44 m2 <sup>g</sup><sup>−</sup>1, and dp <sup>=</sup> <sup>50</sup> <sup>×</sup> <sup>10</sup>−<sup>8</sup> m, respectively. The exchange capacity of the catalysts studied (NaCl 0.5 mol L−<sup>1</sup> and NaOH 0.1 mol L−1) is 4.05 eq Kg−1. Further information and details about A-15 characterization can be found elsewhere [9].

#### *4.3. Etherification Experiments*

#### 4.3.1. Solubility Test

Firstly, a solubility test of the compounds involved in the reaction (G, TB), as well as the only ether commercially available (M1), was carried out in a water:DBE system (1:1). The aim of this preliminary experiment is to study the distribution of reagents and products between both phases aqueous (hydrophilic) and organic (hydrophobic). This test was carried out at atmospheric pressure and ambient temperature, so the quantitative results could not be directly extrapolated to those

obtained under the reaction conditions, but they will be representative to know the distribution of these compounds between both phases.

The reagents were added to a glass reactor and stirred vigorously with a vortex-type stirrer during 10 min. After that, the aqueous and organic phases were separated with the aid of a decanter and immediately analyzed.

#### 4.3.2. Etherification Reactions

Etherification reactions were carried out batch-wise in an autoclave reactor (BüchiGlasUster. Uster Switzerland, 50 mL) with magnetic stirring at constant temperature and pressure (N2, 4 bar). The stirring speed selected (1000 rpm) guarantee the absence of external mass-transfer resistances [9]. At these conditions, internal mass-transfer limitations were discarded in a previous work [9]. The reactor was held at constant temperature by immersion in a hot water bath. The reactor temperature was continuously measured and controlled with a thermocouple immersed in the liquid reaction medium.

In a typical run, the catalyst was charged into the vessel of the reactor with the appropriate volume of the solvent (DBE). Then, the reactor was stoppered, heated and pressurized under nitrogen atmosphere to the desired conditions. When the selected temperature was reached, the reagents (G and TB) were injected and the stirring started (the total mass of the system was fixed at 40 g). This time was considered the starting point of the reaction. During reaction time, liquid samples (emulsified) were periodically withdrawn from the reactor and immediately injected in a vial (submerged in crushed ice) containing a known volume of cold pyridine. The diluted samples were subsequently analyzed. Gaseous isobutylene (IB), continuously generated from TB dehydration (Figure 1), was recovered with a cold trap filled with pyridine (located just after the reactor to liquefy the sample) during the whole reaction time. The concentration of IB was measured at the end of the run to check the mass balance and calculate the selectivity of TB towards this non-desired compound. After 360 min, the reaction was finally stopped by cooling the reactor.

Table 3 summarizes the operating conditions selected for each experiment. Etherification runs were performed with two solvent concentrations (DBE:TB molar ratios used were 1:2 and 1:5), three different catalyst loading (from 10 to 30 %, referred to the total starting mass, G + TB + DBE), three temperatures within the range 60–80 ◦C, and three different concentrations of TB (the G:TB molar ratios used were 1:3, 1:4 and 1:5). The molar ratio G:TB = 1:3 corresponds to the stoichiometric ratio. Once the optimum reaction conditions have been selected, the stability of the catalyst was studied during three consecutive reaction cycles.


**Table 3.** Operating conditions of the etherification runs.

#### 4.3.3. Analytical Methods

The progress of the reaction was followed by analyzing liquid samples at different reaction times. The reactor composition (TB, G, IB and the different ethers generated, M1, M2, D1, D2 and T) was analyzed by means of an Agilent 6850 gas chromatograph (Santa Clara, CA, USA) fitted with a flame ionization detector (GC-FID). An HP Innowax chromatography column (30 m length

× 0.32 mm internal diameter) was used as stationary phase. The chromatographic conditions selected were the following: Initial oven temperature 40 ◦C, final oven temperature 220 ◦C, program rate 20 ◦C min−1. 1-Pentanol was used as internal standard, ISTD, to minimize the error in the quantification of the different compounds. Commercial 1-*tert*-butoxypropane-2,3-diol (M1) was employed to obtain the corresponding response factor for this compound, which was extrapolated to the non-commercial products such as 2-*tert*-butoxypropane-1,3-diol (M2), 1,3-di-*tert*-butoxypropan-2-ol (D1), 1,2-di-*tert*-butoxypropan-3-ol (D2), and 1,2,3- tri-*tert*-butoxypropane (T).

Water concentration (generated in the different steps of the reaction) was measured at the end of the experiment (360 min) with a quantitative Karl Fischer analyzer (Titromatic 1S Crison, Barcelona, Spain).

**Author Contributions:** A.S. and A.R. made substantive intellectual contributions to this study, especially with the conceptualization and the methodology used. C.M.D. conceived and performed the experiments and wrote the paper. All the authors have been also involved in drafting and revising the manuscript, so that everyone has given final approval of the current version to be published in Catalysts Journal.

**Funding:** This research received no external funding.

**Acknowledgments:** Carmen M. Domínguez acknowledges the Spanish MICINN for the "Juan de la Cierva" post-doctoral grant (FJCI-2016-28462).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Article*
