Etherification of Glycerol with Benzyl Alcohol - Industrial & Engineering

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Etherification of Glycerol with Benzyl Alcohol María Pilar Pico,* Sergio Rodríguez, Aurora Santos, and Arturo Romero Departamento Ingeniería Química, Facultad de Ciencias Químicas, Universidad Complutense Madrid, Ciudad Universitaria S/N, 28040 Madrid, Spain ABSTRACT: A study of the influence of different catalysts on glycerol etherification with benzyl alcohol was performed. The best catalyst was selected based on activity and selectivity values. Experimental results were obtained at different reactant molar ratios (benzyl alcohol/glycerol = 3:1−1:3), temperatures (80−100 °C), and catalyst concentrations (3.45−14.4 wt %) referred to the reaction mass. Four ethers (two monoethers and two diethers) and dibenzyl ether were identified as the main products of glycerol etherification and dimerization of benzyl alcohol (secondary reaction), respectively. Two kinetic models are proposed to describe the process performance: a potential model and an Eley−Rideal model. Kinetic parameters for each model were estimated by data fitting. Both proposed kinetic models describe the evolution of the system properly, in terms of not only the reactants but also the product distribution with reaction time under the conditions studied. To carry out the reaction with nonpurified glycerol, the reaction system must first be neutralized.

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INTRODUCTION Biodiesel is a biomass-derived liquid fuel that has been produced on a large scale during the past few years, and its production is expected to increase in the coming years. Glycerol is the main byproduct in biodiesel production through the transesterification of triglycerides with methanol (about 1 kg of crude glycerol is formed as a byproduct for every 9 kg of biodiesel).1,2 Therefore, finding new outlets for the effective use of glycerol in useful chemical processes could reduce both environmental pollution and economic losses. Glycerol is an abundant renewable resource for the production of biomaterials, as well as a source for a great variety of chemical intermediates.3,4 Glycerol is also an efficient platform for the synthesis of oxygenated compounds such as polyglycerols and glycerol ethers by means of etherification.5−10 New simple and environmentally friendly catalytic routes for the transformation of glycerol to higher-value-added chemicals have been developed.11 Some of these routes are based on monoalkyl glycerol ethers (MAGEs), which exhibit a wide spectrum of biological activities such as anti-inflammatory, antibacterial, antifungal, immunological simulation, and antitumor properties.12−14 Glycerol ethers also have many potential uses, for example, as solvents15 and cryogenic agents.16 They are also important precursors for the preparation of 1,3dioxolan-2-ones17,18 and bis(sodium sulfonate ester)-type cleavage surfactants.19 Dioxolanes are chemicals that can be used as additives to fuel20 and diesel, which can reduce the emission of particles and the production of NOx in diesel fuel combustion.21 Usually, the etherification of glycerol has been studied with olefins such as isobutene22−24 and alcohols such as tert-butyl alcohol.25−28 However, many studies have recently been published using different raw materials. For example, Gu et al.11 studied the etherification reactions between glycerol and different reactants including benzyl, propargyl, and allyl alcohols; olefins; and dibenzyl ethers. MAGEs were obtained in moderate to excellent yields. On the other hand, Shi et al.29 reported the catalytic reductive alkylation for the synthesis of © 2013 American Chemical Society

linear 1-O-alkyl mono- and diglycerol ethers in one step with direct aldehydes using a palladium catalyst and a Brønsted acid as a cocatalyst. da Silva et al.30 investigated the etherification of glycerol with benzyl alcohol using different catalysts with good results in most cases. Meanwhile, Suriyaprapadilok et al.,31 studied the same reaction but with a preceding step to transform glycerol into solketal. The authors obtained some byproducts and water that must be removed to avoid the reversibility of the main reaction. Yadav et al.32 studied the etherification of bioglycerol with 1phenylethanol over supported heteropolyacid. Different types of heteropolyacids (HPAs) supported on hexagonal mesoporous silica (HMS) and K-10 clay were tested. An overall second-order kinetic equation was used to fit the experimental data, under the assumption that all of the species are weakly adsorbed on the catalytic sites. Finally, Gaudin et al.33 studied the direct etherification of glycerol with long-chain alcohols, which tipically suffers from poor contact between the glycerol and alcohol phases. In this case, a dodecylbenzene sulfonic acid catalyst that can establish a suitable contact between reaction phases and provide a direct route for the synthesis of surfactants from MAGEs was used. The scope of this article was to study the influence of different catalysts, namely, Amberlyst 15, Amberlite 200, a propylsulfonic-acid-functionalized mesostructured silica, and a ZSM5 zeolite impregnated with sulfonic groups, on glycerol etherification with benzyl alcohol. The best catalyst was selected to extend this study with the best reactant molar ratio, as well as the effects of temperature and catalyst concentration on the product distribution. Two kinetic models were developed including the effects studied in this work (reactant molar ratio, temperature, and catalyst concentration). Received: Revised: Accepted: Published: 14545

June 27, 2013 September 11, 2013 September 16, 2013 September 16, 2013 dx.doi.org/10.1021/ie402026t | Ind. Eng. Chem. Res. 2013, 52, 14545−14555

Industrial & Engineering Chemistry Research

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To the best of our knowledge, such a detailed study of this process has not been performed. Furthermore, the quantification of the effects of catalyst concentration and temperature on the kinetic rate performed in this work has not yet been reported in the literature for this reaction. Theoretical Basis. The etherification of glycerol with benzyl alcohol involves a set of consecutive equilibrium reactions catalyzed by acids where the reaction orders are given by the molecularity of the elementary reaction steps as proposed in the literature for other reactions such as the tertbutylation of glycerol23,34−37 using isobutylene and tert-butyl alcohol25,26 as reagents. If water is continuously removed, this system can be turned into an irreversible process. The products obtained after glycerol (G) etherification with benzyl alcohol (Bz) are the following: 3-benzyloxy-1,2-propanediol (M1), 2benzyloxy-1,3-propanediol (M2), 1,3-dibenzyloxy-2-propanol (D1), 1,2-dibenzyloxy-3-propanol (D2), and 1,2,3-tribenzyloxypropane (T). Considering these products as a starting point, the reaction pathway shown in Figure 1 can be proposed for

Figure 2. Undesired byproducts from the etherification of glycerol with benzyl alcohol over A15 catalyst.

the proposed rate equations correspond to a heterogeneous model in which the internal and external mass-transfer resistances are negligible. The activity (H+ concentration) of the heterogeneous catalyst during the reactions was considered as a constant implicit in the rate coefficient. The effect of reaction temperature is included in the kinetic parameters (kj), considered as temperature-dependent according to the Arrhenius equation ⎛ Eaj ⎞ kj = k 0j exp⎜ − ⎟ ⎝ RT ⎠

(1)

Two kinetic strategies are proposed in this work: a potential kinetic model and an Eley−Rideal kinetic model. Both are expressed in terms of concentration instead of activities because an ideal mixture is assumed. They are stated in the next two subsections. Potential (PL) Kinetic Model. The potential model (based on Figure 1) considers the reaction scheme and reaction rates shown in Table 1. The production rates for each compound in a batch reactor, according to the reaction rates in Table 1, are summarized in Table 2. Table 1. Potential Kinetic Model for the Etherification of Glycerol with Benzyl Alcohol over A-15 Catalyst reaction 1

Figure 1. Reaction pathway proposed for the etherification of glycerol with benzyl alcohol over A15 catalyst.

2 3

benzyl alcohol (Bz) as an etherification agent. In this case, the dimerization of benzyl alcohol is an independent side reaction that influences the main reactions of etherification through the undesired consumption of benzyl alcohol. However, this product could react with glycerol to produce monoethers.11 These results are in accordance with those reported previously in the literature.11,30 Furthermore, some undesired byproducts (toluene, benzaldehyde, and stilbene) have been detected but in low concentrations. The maximum selectivity corresponds to BZO, and it is around 0.4%. For this reason, the mentioned byproducts were not considered in this study (Figure 2).38 The kinetic models proposed in this work were developed according to the reaction scheme shown in Figure 1. The water content was considered to be negligible because the average amount of water corresponds to less than 5% of the total mass of the reaction system. These kinetic models are based on heterogeneous reactions that occur over the catalyst pellets, and

4 5

stoichiometric equation

reaction rate

equation

n1

r1 = k1W CGC BZ

(2)

k2

r2 = k 2W n2CMC BZ

(3)

k3

r3 = k 3W n3C DC BZ

(4)

r4 = k4W n4C BZ 2

(5)

r5 = k5W n5C DBZCG 2

(6)

k1

G + Bz → M + H 2O

M + Bz → D + H 2O D + Bz → T + H 2O k4

2Bz → DBZ + H 2O k5

DBZ + 2G → 2M + H 2O

Eley−Rideal (ER) Kinetic Model. In the noncompetitive adsorption model, an ER-type model, a molecule adsorbed on the catalyst reacts with a molecule from the bulk phase. Here, it is supposed that only glycerol is adsorbed on the catalyst and benzyl alcohol reacts from the bulk phase because only the strongest adsorption component is taken into account.39 Polar molecules are preferably adsorbed within the sulfonic acid network. As a result, the acidity and accessibility and, therefore, the reactivity of the resin catalyst are strongly influenced by the polarity of the reaction medium.40−42 It is assumed that all of the active surface sites (S) are equal and that each component occupies one surface site when it is adsorbed 14546

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Therefore, substituting eq 19 into eqs 15 and 16 gives eqs 20 and 21. The ER model is given in Table 3. The production rates for each compound were calculated based on a batch reactor equation model according to the reaction rates in Table 3 and are the same as those shown in Table 2.

Table 2. Production Rates of Reactants and Products in a Batch Reactor (Potential Model) compound glycerol (G)

rate

equation (7)

dC G RG = = − r1 − 2r5 dt

benzyl alcohol (Bz) monoether of glycerol and benzyl alcohol (M) diether of glycerol and benzyl alcohol (D) triether of glycerol and benzyl alcohol (T) dibenzyl ether (DBZ) H2O

RBZ =

dC BZ = − r1 − r2 − r3 − 2r4 dt

dC M RM = = r1 − r2 + 2r5 dt

(9)

RD =

dC D = r2 − r3 dt

(10)

RT =

dC T = r3 dt

(11)

RDBZ =

R H2O =

dC DBZ = r4 − r5 dt

(12)

dC H2O

(13)

dt

= r1 + r2 + r3 + r4 + r5

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(8)

EXPERIMENTAL SECTION Catalysts and Chemicals. Two commercial acid ionexchange resins were used as heterogeneous etherification catalysts: Amberlyst 15 (A15, 20−50 mesh, sulfonic acid) supplied by Fluka and Amberlite 200 (A200, 16−50 mesh, sulfonic acid) supplied by Sigma. In addition, a solid acid supported catalyst was prepared by a grafting procedure using a ZSM5 carrier and a solution of (3mercaptopropyl)trimethoxysilane (2 mL) (MPTMS, ≥95% purity, supplied by Aldrich) in anhydrous toluene (4 mL) (≥99%, supplied by Merck). Then, oxidation was carried out by contacting the sample with a solution of hydrogen peroxide (30% w/v, Fischer).43 Furthermore, a propylsulfonic-acid-functionalized mesostructured silica (SBA15) was synthesized following a previously reported procedure.44 The molar composition of the synthesis mixture for 8 g of templating block copolymer (Pluronic 123, Aldrich) was 0.0369 mol of tetraethylorthosilicate (TEOS, ≥98%, Aldrich), 0.0041 mol of MPTMS, 0.0369 mol of H2O2, 0.24 mol of HCl (37%, Panreac), and 6.67 mol of H2O. Glycerol (100% purity), supplied by Fischer Scientific, and anhydrous benzyl alcohol (≥99.5% purity), provided by SigmaAldrich, were employed as reactants. Methanol (≥99% purity), supplied by Scharlau, was used as the solvent for sample analysis. 1-Pentanol solvent (≥99% purity, Sigma-Aldrich) was employed as the internal standard compound in the chromatographic analysis (±)-3-Benzyloxy-1,2-propanediol (≥97% purity), 1,3-dibenzyloxy-2-propanol (≥97% purity), and dibenzyl ether purum (≥98% purity) were utilized to calibrate the gas chromatograph. HYDRANAL-Composite 5 and dry HYDRANAL-methanol were used as the solvent and reactive compound, respectively, for the analysis of water. Catalyst Characterization. The characterization of the catalysts was carried out by different techniques. The porous structure of the catalysts was determined by N2 adsorption−desorption at −196 °C, performed in an SA 3100 surface area analyzer (Beckman Coulter). Catalysts were previously outgassed for at least 4 h at 80 °C, before and after adsorption. From the N2 isotherm, the apparent surface

(14)

G + S ↔ GS

If it is assumed that the surface reaction is the ratedetermining step, the reaction rates for the etherification of G are obtained from eqs 2−6, and the combination of eqs 2 and 6 with eq 14. The results of this union are as follows r1 = k1C BzCGSØG

(15)

r5 = k5C DBZCGS2ØG 2

(16)

where Øi represents the surface concentration of species i on the catalyst (∑Øi = 1) and is given in the Langmuir isotherm by Øi =

K iCi 1 + ∑j KjCj

(17)

where Ki is the adsorption equilibrium constant and Ci is the concentration of species i. The adsorption equilibrium constant is defined by the van’t Hoff equation as

⎛ ΔHi ⎞ ⎟ K i = k 0i exp⎜ − ⎝ RT ⎠

(18)

In this case, only G adsorption is considered, and eq 17 can be rewritten as ØG =

K GCG 1 + K GCG

(19)

Table 3. Eley−Rideal Kinetic Model for the Etherification of Glycerol with Benzyl Alcohol over A-15 Catalyst reaction 1 2 3 4 5

stoichiometric equation

reaction rate

equation

k1

k W n1C BzK GCG r1 = 1 1 + K GCG

(20)

k2

r2 = k 2W n2CMC Bz

(3)

k3

r3 = k 3W n3C DC Bz

(4)

r4 = k4W n4C Bz 2

(5)

G + Bz → M + H 2O

M + Bz → D + H 2O D + Bz → T + H 2O k4

2Bz → DBZ + H 2O k5

DBZ + 2G → 2M + H 2O

r5 =

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k5W n5C DBZK G 2CG 2

(21)

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through a mass balance based on the chromatographic analysis of compounds derived from glycerol. The water content was measured with a quantitative Karl Fischer analyzer, Titromatic 1S Crison.

area was determined by applying the Brunauer−Emmett− Teller (BET) equation. The pore volume (Vp, referred to the volume of pore filled with N2 at a relative pressure of P/P0 = 0.98) and mean pore diameter (Dp) were also obtained from the N2 isotherm data. The acid-exchange capacity of the catalyst was determined by potentiometric titration. In a typical procedure, 0.05 g of solid was suspended in 15 g of 0.5 M NaCl aqueous solution. The resulting suspension was stirred at room temperature until equilibrium was reached and subsequently titrated with 0.1 M NaOH by means of a Metrohm pH electrode. Table 4 lists the main physicochemical properties of the catalysts used.

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RESULTS AND DISCUSSION Eighteen runs were carried out under different experimental conditions (catalyst, molar ratio of reactants, temperature, and catalyst concentration), as detailed in Table 5. Runs 1−4 were Table 5. Experimental Conditions of Glycerol Etherification with Benzyl Alcohol

Table 4. Textural Properties and Acidities of the Catalysts Studied catalyst

Dp (nm)

Vp (cm3/g)

SBET (m2/g)

acidity (equiv/kg)

A-15 A-200 SBA15 ZSM5

30 50 7.2 3.7

0.34 0.31 1.35 0.25

44 48 773 268

4.05 2.42 1.4 1.00

Experimental Procedure. Etherification reactions were carried out in a glass reactor (250 mL) with magnetic stirring, without solvent, and at vacuum pressure with continuous removal of formed water. The reactor was thermostated by immersion in a glycerol bath, and the reactor temperature was measured continuously by a thermocouple immersed in the liquid reaction medium. The set-point temperature inside the reactor was reached 10 min after the reactor had been placed in the glycerol bath, and this time was considered as the starting point of the reaction. In a typical run, 50 g of glycerol (G) and 58.7 g of benzyl alcohol (Bz) were used as reactants. The temperature range studied was 80−100 °C. The catalyst concentration varied from 3.45 to 14.4 wt %, referred to the starting amount of reactants. The benzyl alcohol/glycerol molar ratios used were 3:1, 2:1, 1:1, 1:2, and 1:3. The stirring speed was modified from 600 to 1200 rpm to check the influence of external mass transfer; this resistance was negligible at stirring frequencies higher than 900 rpm at the highest temperature studied. A value of 1200 rpm was chosen for all of the runs to guarantee the absence of external mass-transfer resistances. The absence of internal mass-transport resistances was checked, and the Thiele modulus and effectiveness factor were calculated. The value obtained for the effectiveness factor at the lowest temperature was 0.961. Analysis. The composition of the reaction mixture (ethers, benzyl alcohol, and dibenzyl ether) was analyzed by means of a gas chromatograph, Agilent 6850, integrated with a flame ionization detector. An HP Innowax chromatographic column (30 m length × 0.32 mm i.d.) was utilized. The chromatographic conditions were as follows: initial oven temperature of 40 °C, final oven temperature of 250 °C, and three programmed rates of 10, 0.5, and 10 °C min−1. 1-Pentanol was used as the internal standard. Commercial (±)-3benzyloxy-1,2-propanediol (M1) and 1,3-dibenzyloxy-2-propanol were employed to obtain the corresponding response factor which was extrapolated to noncommercial products as 2benzyloxy-1,3-propanediol (M2), 1,2-dibenzyloxy-3-propanol (D2), and 1,2,3-tribenzyloxypropane (T). Dibenzyl ether was calibrated as well. Quantification of glycerol was performed

run

catalyst

Bz/G

T (°C)

catalyst concentration (wt %)

t (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

A15 A200 SBA15 ZSM5 A15 A15 A15 A15 A15 A15 A15 A15 A15 A15 A15 A15 A15 A15

1:1 1:1 1:1 1:1 3:1 2:1 1:1 1:2 1:3 1:1 1:1 1:1 1:2 1:2 1:2 1:3 1:3 1:3

90 90 90 90 80 80 80 80 80 80 90 100 80 90 100 80 90 100

4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 4.6 3.45, 4.6, 5.5, 9.2 3.45, 4.6, 5.5, 9.2 3.45, 4.6, 5.5, 9.2 4.6, 6.3, 7.6, 12.6 4.6, 6.3, 7.6, 12.6 4.6, 6.3, 7.6, 12.6 4.6, 7.2, 8.6, 14.4 4.6, 7.2, 8.6, 14.4 4.6, 7.2, 8.6, 14.4

0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480 0−480

mainly focused on choosing the most suitable catalyst for this reaction, and different reactant molar ratios were studied in runs 5−9. In addition, the effects of temperature and catalyst concentration on glycerol conversion and product distribution were studied with the catalyst selected in runs 10−18. Finally, the reaction was carried out with crude glycerol from soya bean oil. Influence of Catalyst. Glycerol etherification with benzyl alcohol was studied over four different heterogeneous catalysts: two commercial ion-exchange resins (A15 and A200) and two catalysts specifically made for this work, one synthesized (SBA15) and the other impregnated (ZSM5). Zeolite ZSM5 was previously calcinated at 400 °C for 2 h to remove any water content that could be present in the catalyst matrix. The experimental conditions employed for this study were an initial reactant molar ratio of 1:1, a temperature of 90 °C, and a catalyst concentration of 4.6 wt % referred to the mass of reaction (Table 5, runs 1−4). In Figure 3, the evolution of reactant conversion with the four different catalysts studied is shown. As can be observed, A15 presents the best results concerning reactant conversion for both glycerol and benzyl alcohol. In this case, the glycerol conversion achieved is around 0.55, whereas the conversion is slightly higher for benzyl alcohol (around 0.7), because DBZ formation depends only on the benzyl alcohol concentration. However, A200 shows a lower conversion for reactants. As can be seen in Table 4, the textural properties are very similar for A15 and A200. Hence, the accessibility to acid sites is almost the same; nevertheless, 14548



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Figure 3. Reactant conversions with different catalysts (A15, A200, SBA15, and ZSM5) at 90 °C, 4.6 wt %, and Bz/G = 1:1: (a) glycerol (G), (b) benzyl alcohol (Bz).

Figure 4. (a) Reactant conversions and product distributions at a reaction time of 480 min. (b) Monoether selectivities over different catalysts (A15, A200, SBA15, and ZSM5) at different glycerol conversions and at 90 °C, 4.6 wt %, and Bz/G = 1:1.

conversion, with monoether as the only reaction product observed. The pore size distribution and pore volume of A15 are very similar to those of A200, which explains the similar product distributions obtained with these catalysts. SBA15 and ZSM5 show a smaller pore size and pore volume than the ionexchange resins, despite their greater apparent surface areas. However, the lower acidities of A200, SBA15, and ZSM5 produce slower reactions than those obtained using A15. Actually, a glycerol conversion of 0.23, for instance, is achieved at 106 min with A15, at 460 min with A200, and at 480 min with SBA15. This conversion cannot be achieved with ZSM5 during the reaction time studied. In Figure 4b, the monoether selectivities obtained with all of the catalysts studied are plotted at different glycerol conversions. It can be observed that the values do not change in a significant way for all of the catalysts employed and they are very similar for A15, SBA15, and A200 cases. The monoether selectivity decreases slightly in all cases to produce diethers, except for ZSM5, which leads to only monoether. da Silva et al.30 also obtained similar results in their work. The most acidic catalysts showed a superior performance compared to the other catalysts, producing glycerol benzyl ethers in higher yield than dibenzyl ether. Based on these results, A15 was selected to continue with the study of the glycerol etherification process by benzyl alcohol. Influence of Reactant Molar Ratio. The influence of the initial molar ratio of reactants was studied in this work in runs 5−9 (Table 5). In Figure 5, the product evolution can be seen under different initial conditions. It can be observed that, when

the difference in reactant conversion could be due to the fact that A200 exhibits a lower acidity than A15. On the other hand, SBA15 presents the highest total apparent surface area and pore volume, but its acidity and pore size are lower than those for ion-exchange resins. The products generated by means of glycerol etherification with benzyl alcohol are quite numerous, and SBA15 exhibits a very low value of pore size, which makes it more difficult to access acid sites and hinders diffusion of molecules during the reaction. A similar situation can be observed with ZSM5, whose textural properties are high but its acidity is very low. A poor development of the porous structure and a low acidity value are responsible for the low activity of these catalysts.35,45 Figure 4 shows the reactant conversions and product selectivities obtained with the different catalysts studied. The product selectivities are defined as S=

product concentration × 100 Bz reacted

(22)

In Figure 4a, the glycerol conversions and product distributions obtained with the four different catalysts at the same reaction time (480 min) can be compared. As can be observed, the highest glycerol conversion is achieved with A15, as is the highest diether selectivity. The selectivities to monoether of A15 and A200 are very similar; however, as can be noticed, the selectivity to diether is lower for A200. A glycerol conversion around one-half as high as that for A15 can be seen. On the other hand, SBA15 shows a lower glycerol conversion than A15, but the product distribution is comparable. Finally, ZSM5 shows the lowest glycerol 14549



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Figure 5. Product distribution as a function of Bz/G initial molar ratio (T = 80 °C, 4.6 wt %, and 480 min).

Bz is in excess, M and DBZ are obtained as main products. However, when G is the most abundant reactant, M formation is favored, whereas DBZ production decreases. A 1:1 initial molar ratio of reactants was selected to extend this study, because from this point on, the glycerol conversion decreases slightly and the undesired product DBZ is low enough. These results are in accordance with those of Gu et al.,11 who used a molar ratio of 1:1 and observed a very low production of DBZ. On the other hand, da Silva et al.30 used an initial ratio of 3:1 and obtained M and DBZ as the main products for most of the catalysts they tested. In Figure 6, the product distribution can be seen in front of glycerol conversion. As can be observed in Figure 6a, the lowest monoether selectivity is achieved with a molar ratio of Bz/G = 3:1; furthermore, at higher molar ratios, the monoether selectivity goes down when the glycerol conversion rises because this product reacts to produce diether (Figure 6b). The diether selectivity obtained is very similar in all cases; however, dibenzyl ether is higher when Bz is the majority reactant (Figure 6c). DBZ decreases with glycerol conversion at molar ratios of less than 1:1; that is, when G is in excess or in a stoichiometric amount, DBZ reacts with G to produce monoether. Influence of Temperature. The effects of temperature (runs 10−18, Table 5) on the glycerol and benzyl alcohol conversions are shown in Figure 7. The product distributions are shown in Figure 8, where triether is not included because it was obtained in a negligible amount. As shown in Figure 7, a considerable enhancement in the conversion of glycerol and benzyl alcohol is observed as the temperature increases. The glycerol conversion varies between 0.4 and 0.7 at 80 and 100 °C respectively. Benzyl alcohol ranges between 0.5 and 0.9 at the same temperatures. Similar results were obtained in previous studies with other alcohols such as tert-butanol,25 where reactant conversion increases with rising temperature. Figure 8 shows that M has the behavior of an intermediate product according to the reaction pathway shown in Figure 1. The M selectivity decreases with time at all temperatures; however, the D selectivity increases when temperature rises. In this study, D is considered as an end product because triether is not formed as a result of steric hindrance, as was previously stated. As can be seen in Figure 8c, the DBZ selectivity rises with temperature, but it presents a profile that corresponds to an intermediate product. It decreases with time, especially at higher temperatures, because it reacts with glycerol to produce monoether.

Figure 6. Product distribution as a function of Bz/G initial molar ratio at different glycerol conversions: (a) SM, (b) SD, and (c) SDBZ. Symbols: experimental data. Dotted lines: values estimated using the ER model (T = 80 °C, 4.6 wt %).

Influence of Catalyst Concentration. The study of the effects of the catalyst concentration on the reactant conversion and product distribution was carried out using the selected catalyst A15. The reaction conditions are listed in Table 5, runs 10−18. The catalyst concentration ranged from 3.45 to 14.4 wt % referred to the starting amount of reactants. The effects of the catalyst concentration on the reactant conversion and product distribution can be observed in Figures 9 and 10, respectively. As can be seen in Figure 9, glycerol conversion rises with increasing catalyst amount. As a consequence, the glycerol conversion increases from around 0.55 at 3.45 wt % to 0.75 at 9.2 wt % considering 8 h of reaction time. The evolutions of monoethers, diethers, and dibenzyl ether are shown in panels a−c, respectively, of Figure 10. In Figure 10a, a decrease in M selectivity can be observed when catalyst concentration rises because, as was said before, M is an intermediate product and an increase in catalyst concentration 14550



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Figure 7. Effects of temperature on the conversions of (a) glycerol (G) and (b) benzyl alcohol (Bz). Symbols: experimental data. Dotted lines: values estimated using the ER model (Bz/G = 1:1, 4.6 wt %).

favors D formation. Consequently, the selectivity to final products (diether) is higher when the catalyst concentration increases (Figure 10b). For instance, at 3.45 wt %, the final value corresponds to a selectivity of 3.7%, whereas at 9.2 wt %, a selectivity of 6.2% is achieved. Figure 10c shows the selectivity evolution of DBZ at different catalyst concentrations. As can be seen, its tendency is the same as that obtained at different temperatures. SDBZ decreases with time, and its performance corresponds to an intermediate product. This is because the catalyst concentration increases at this reaction temperature; the reaction of monoether production is favored; and, as a consequence, the DBZ selectivity decreases. The performance of these reactants and glycerol ethers can be observed in previous works carried out with other alcohols such as tert-butanol.26 They show that, when the catalyst concentration rises, increments in the glycerol conversion and diether selectivity can be observed. Figures 11 and 12 represent the glycerol conversion evolution at different catalyst concentrations and temperatures under two different initial molar ratios. In both cases, as well as the 1:1 case, when the catalyst concentration increases, the glycerol conversion rises. At the highest temperatures and catalyst concentrations, glycerol conversion achieves a constant value of around 0.45 for Bz/G = 1:2 and 0.3 for Bz/G = 1:3. The other compounds present a behavior similar to that shown at a 1:1 initial molar ratio. Kinetic Model. The results obtained at different initial molar ratios of reactants, different temperatures, and different catalyst concentration (runs 5−18, Table 5) were used to develop both kinetic models, the potential model and the Eley−Rideal model. Potential Kinetic Model (PL). The kinetic parameters of glycerol etherification according to scheme in Tables 1 and 2 were calculated by experimental data fitting (concentrations of Bz, M, D, and DBZ as functions of time). Nonlinear regression (Marquadt algorithm) was coupled with a Runge−Kutta integration step and therefore employed in the fitting procedure. The values obtained for reaction orders in catalyst concentration were close to 1. Thus, these orders, ni, were fitted to 1, and the kinetic parameters obtained after fitting of the experimental data are summarized in Table 6 with their respective standard deviation values. In the same table, the percentage of the variation explained for each measured variables is also displayed.

Figure 8. Effects of temperature on the selectivities of (a) monoether of glycerol and benzyl alcohol (M), (b) diether of glycerol and benzyl alcohol (D), and (c) dibenzyl ether (DBZ). Symbols: experimental data. Dotted lines: values estimated using the ER model (Bz/G = 1:1, 4.6 wt %).

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Figure 9. Effects of catalyst concentration on the conversions of (a) glycerol (G) and (b) benzyl alcohol (Bz). Symbols: experimental data. Dotted lines: values estimated using the ER model (T = 90 °C, Bz/G = 1:1).

Figure 10. Effects of catalyst concentration on the selectivities of (a) monoether of glycerol and benzyl alcohol (M), (b) diether of glycerol and benzyl alcohol (D), and (c) dibenzyl ether (DBZ). Symbols: experimental data. Dotted lines: values estimated using the ER model (T = 90 °C, Bz/G = 1:1).

Figure 11. Effects of temperature and catalyst concentration on XG at (a) 80, (b) 90, and (c) 100 °C. Symbols: experimental data. Dotted lines: values estimated using the ER model (T = 90 °C, Bz/G = 1:2).

Using equations obtained with these kinetic parameters, predicted values of G, Bz, M, D, and DIB were calculated according to the scheme in Tables 1 and 2. Eley−Rideal Kinetic Model (ER). The kinetic parameters of glycerol etherification from the scheme in Tables 2 and 3 were

calculated by experimental data fitting (concentrations of Bz, M, D, and DBZ as functions of time). As in the first model, nonlinear regression (Marquadt algorithm) was coupled with a Runge−Kutta integration step and therefore employed in the fitting procedure. The orders in catalyst concentration were 14552



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Table 7. Kinetic Parameters and Residual Values Obtained for Glycerol Etherification Reaction Using the ER Model parameter

value

STD

Ea1 (kJ/mol) Ea2 (kJ/mol) Ea4 (kJ/mol) Ea5 (kJ/mol) k01 (kg2/gcat·mol·min) k02 (kg2/gcat·mol·min) k04 (kg2/gcat·mol·min) k05 (kg3/gcat·mol2·min) ΔHG (kJ/mol) k0G (kg/mol) sum of squares

96.18 130.85 159.68 197.63 1.33 × 1011 1.08 × 1013 1.19 × 1017 8.06 × 1018 3.25 2.3 × 10−02 7.92

1.28 10.49 9.58 10.28 5.63 × 103 3.68 × 104 3.67 × 108 5.87 × 109 9 × 10−1 8.23 × 10−04

If the sum of squares obtained with both kinetic models, PL and ER, are compared, it can be observed that the ER model leads to better fitting of the experimental data. Predicted values obtained with this kinetic model are shown on Figures 6−10. As can be observed, they fit the experimental values in an appropriate way. Experiments with Nonpurified Glycerol. Some experiments were carried out with nonpurified glycerol obtained from biodiesel production with soya oil and potassium methoxide as the catalyst. The glycerol characteristics are reported in Table 8. The content of volatile components is high, as well as the amount of alkali metals present in the media (K and Na). Table 8. Characterization of Nonpurified Glycerol

a

Figure 12. Effects of temperature and catalyst concentration on XG at (a) 80, (b) 90, and (c) 100 °C. Symbols: experimental data. Dotted lines: values estimated using the ER model (T = 90 °C, Bz/G = 1:3).

parameter

value 110.90 130.63 159.21 185.93 1.33 × 1.08 × 1.19 × 8.06 × 13.1

1011 1013 1017 1018

STD 1.31 13.74 8.12 22.60 5.78 × 4.81 × 3.11 × 5.88 ×

content

K Na water MeOH othera

23319.5 ppm 123 ppm 525.5 ppm 34600 ppm 2.8%

Esters; mono-, di-, and triglycerides.

Different catalyst concentrations referred to the starting amount of reactants were tested. In Figure 13a, a comparison between the results obtained with pure and nonpurified glycerol can be observed. Glycerol conversion decreases dramatically when nonpurified glycerol is employed because of both the deactivation of the catalyst with OH groups and the proton exchange with metallic cations. To reduce this negative influence, the OH groups and metallic cations were neutralized. For this purpose, sulfuric acid and the ion-exchange resin (A15) were added to nonpurified glycerol before carrying out the reaction. After this pretreatment, a reaction was carried out with a catalyst concentration of 4.6 wt %. The results derived from this experiment are shown in Figure 13b, and as can be observed, the values obtained were very similar to those achieved with pure glycerol. Consequently, nonpurified glycerol with metallic cations and the remaining OH compounds can behave as pure glycerol if they are previously neutralized.

Table 6. Kinetic Parameters and Residual Values Obtained for the Glycerol Etherification Reaction by Using the PL Model Ea1 (kJ/mol) Ea2 (kJ/mol) Ea4 (kJ/mol) Ea5 (kJ/mol) k01 (kg2/gcat·mol·min) k02 (kg2/gcat·mol·min) k04 (kg2/gcat·mol·min) k05 (kg3/gcat·mol2·min) sum of squares

component

105 107 108 109

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also close to 1, so they were fitted to this value. The kinetic parameters obtained after fitting of experimental data are summarized in Table 7 with their respective standard deviation values. Variations explained for each measured variable are included in the same table.

CONCLUSIONS Based on the results obtained, A15 was selected as the best catalyst of the four evaluated for the etherification of glycerol with benzyl alcohol. Its higher activity is due to its high acidity 14553



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Figure 13. Comparison between pure glycerol and (a) nonpurified glycerol, (b) nonpurified glycerol and neutralized glycerol (T = 90 °C, initial Bz/ G molar ratio = 1:1).

■

and better textural properties. This catalyst leads to the highest glycerol conversion and the highest ether selectivities. The most suitable reactants ratio is 1:1, based on the glycerol conversion and byproduct selectivity. Higher temperatures lead to greater reactant selectivities, as well as greater diether production. The monoether shows an intermediate product performance and decreases when temperature rises because its activation energy is the lowest among the products obtained. Byproduct formation is higher when temperature increases; however, at the highest temperatures it decreases to form M. The increase in catalyst concentration yields a higher conversion of reactants and formation of desired products. Catalyst concentration has the same influence as temperature, because M and DBZ behave as intermediate products. Continuous water removal allows the occurrence of irreversible kinetic equations. Fitting of the kinetic models proposed with the experimental data for reactant conversions and product selectivities as functions of time shows a linear dependence of the reaction rate on the catalyst concentration, as expected from an heterogeneous reaction. The ER kinetic model shows better results than the PL kinetic model based on the sum of squares obtained, although both of the models fit the experimental results properly. This reaction can be carried out with nonpurified glycerol if it is neutralized before. The results obtained are equivalent to those presented for pure glycerol reactions.

■

Abbreviations

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.:+34 91 394 41 06. Fax: +34 91 394 41 71.

■

Notes

Bz = benzyl alcohol BZO = benzaldehyde D = diether of glycerol and benzyl alcohol DBZ = dibenzyl ether EST = stilbene G = glycerol M = monoether of glycerol and benzyl alcohol TOL = toluene

REFERENCES

(1) Chornet, E.; Czernik, S. Renewable fuels: Harnessing hydrogen. Nature 2002, 418, 928. (2) Zhou, C. H.; Beltramini, J. N.; Fana, Y. X.; Lu, G. Q. Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals. Chem. Soc. Rev. 2008, 37, 527. (3) Rahmat, N.; Abdullah, A. Z.; Mohamed, A. R. Recent progress on innovative and potential technologies for glycerol transformation into fuel additives: A critical review. Renewable Sustainable Energy Rev. 2010, 14, 987.

The authors declare no competing financial interest.

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NOMENCLATURE Dp = pore diameter (nm) Eai = activation energy of reaction i (kJ/mol) ΔHG = adsorption energy of glycerol (kJ/mol) k0j = pre-exponential factor of the kinetic constant of product formation by reaction j (kg2/gcat·mol·min) KG = equilibrium constant of glycerol adsorption (kg/mol) kj = kinetic constant of product formation by reaction j (kg2/ gcat·mol·min) ni = kinetic reaction order of species i ØG = surface concentration of glycerol on the catalyst Ri = production rate of reactant or product i in a batch reactor (mol/kg·min) rj = rate of reaction j (mol/kg·min) SBET = apparent surface area (m2/g) Si = product selectivity of species i referred to benzyl alcohol reacted (%) t = reaction time (min) Vp = average pore volume (cm3/g) W = catalyst concentration referred to the total reaction mass (gcat/kg) Xi = conversion of reactant i

ACKNOWLEDGMENTS

The authors acknowledge financial support for this research from the Spanish Ministry of Science and Innovation under Projects PRI-PIBAR-2011-1375 and AGI Santander-ECL 2011. M.P.P. thanks to Comunidad Autónoma de Madrid (Spain) for her research contract. 14554



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Etherification of Glycerol with Benzyl Alcohol - Industrial & Engineering

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