Etherification of Glycerol by tert-Butyl Alcohol - American Chemical

Article pubs.acs.org/IECR

Etherification of Glycerol by tert-Butyl Alcohol: Kinetic Model M. Pilar Pico,* Arturo Romero, Sergio Rodríguez, and Aurora Santos Departamento de Ingenieria Quimica, Facultad de Ciencias Químicas, Universidad Complutense Madrid, Ciudad Universitaria S/N, 28040 Madrid, Spain S Supporting Information *

ABSTRACT: The catalytic etherification of glycerol with tert-butyl alcohol over a strong acid ion-exchange commercial resin (Amberlyst 15) has been studied. The etherification reactions were carried out in a glass laboratory autoclave reactor with magnetic stirring without solvent at autogenous pressure. Experimental results were obtained at different temperatures (50−80 °C) and using an 8.5 wt % catalyst loading referred to the starting amount of glycerol. Four ethers (two monoethers and two diethers) and isobutylene were identified as the main products of glycerol etherification and tert-butyl alcohol dehydration (secondary reaction), respectively. Two simplified heterogeneus kinetic models are proposed to describe the process performance. The first model lumps the monoethers and diethers into the species M and D, respectively, and includes three reactions and the corresponding kinetic equations. The second model includes all of the species detected and six stoichiometric equations. Kinetic parameters for each kinetic model were estimated by data fitting. Irreversible dehydration of tert-butyl alcohol can be assumed at the operating conditions employed in the present study. Both kinetic models proposed describe the evolution of the system properly, in terms of both the reactant and product distributions with reaction time in the temperature range studied.

■

INTRODUCTION The use of renewable energy is gaining increasing attention all over the world. One of the most frequently employed energy sources is biodiesel.1−4 The main byproduct of this process is glycerol (10 wt % of the total biodiesel obtained). Moreover, conventional applications of glycerol require a costly purity of crude glycerine (98% purity).5,6 Therefore, valorization of glycerol obtained from biodiesel as an initial raw material for products such as glycerol tertiary butyl ethers, propylene glycol, and glycerol carbonate would be desirable.7−9 One of the most interesting ways to revalorize glycerol is the preparation of alkyl ethers of glycerol by etherification with alcohols or short-chain olefins to be employed as oxygenated additives for diesel fuels (diesel, biodiesel, and their mixtures).10,11 This strategy allows the valorization of a byproduct and the improvement of biodiesel performance as biofuel as these tert-butyl ethers from glycerol offer an alternative to oxygenated compounds such as methyl tertbutyl ether (MTBE) and ethyl tert-butyl ether (ETBE) that are currently added to fuels.12 This reaction produces a mixture of mono-tert-butyl ethers, di-tert-butyl ethers, and tri-tert-butyl ether. The good solubilities of higher ethers (di-tert-butyl glycerols and tri-tert-butyl glycerol) in diesel and gasoline make them excellent additives with a high potential for diesel and biodiesel formulations.11,17 The etherification of glycerol can be carried out using homogeneous acid catalysts (e.g., p-toluenesulfonic acid11 or ionic liquids with Brønsted acidity13), but from an environmental point of view, heterogeneous catalysts are preferred. Different solid catalysts have been employed in the literature, mainly acid ion-exchange resins (e.g., Amberlyst 1514,15 or Amberlyst 3516), sulfonic acid-modified mesostructured silicas,17 large-pore zeolites (e.g., H−Y and H-β16), and others (e.g., sulfonated peanut shell18). Other biobased oxygenated © 2012 American Chemical Society

compounds can be produced using similar heterogeneous catalysts (e.g. ethyl lactate), which is one of the main pillars of a sustainable economy.19,20 On the other hand, these glycerol etherification reactions can be performed with different olefins and alcohols, mainly isobutylene7,13,16−23 and tert-butyl alcohol.10,14,24−27 Etherification reactions of glycerol by tert-butyl alcohol can be carried out in the liquid phase without a solvent, and tert-butyl ethers of glycerol can be dissolved in the reaction mixture. Therefore, the reactive phase could be considered as a single phase if tert-butyl alcohol (instead of isobutylene, a gaseous compound) is selected for glycerol etherification.14,24 However, if tert-butyl alcohol is used as the reagent instead of isobutylene, a lower conversion of glycerol is usually achieved, because of the equilibrium that must be considered for the former. Few works have developed kinetic models that properly describe the etherification of glycerol performed with either isobutylene or tert-butyl alcohol. Klepácǒ vá et al.16 developed a powder kinetic model considering a detailed scheme of reactions for glycerol and ethylene glycol etherification with isobutylene in the presence of Amberlyst 35 (7.5 wt %). In the case of glycerol etherification, they took into account the distributions of products and intermediates of 11 equilibrium reactions. They also tried to use Langmuir−Hinshelwood-type kinetic models, but the additional adsorption parameters caused numerical difficulties. Etherification reactions were carried out in a solvent (dioxane, dimethyl sulfoxide, or sulfolane) at autogenous Received: Revised: Accepted: Published: 9500

February 23, 2012 May 29, 2012 June 19, 2012 June 19, 2012 dx.doi.org/10.1021/ie300481d | Ind. Eng. Chem. Res. 2012, 51, 9500−9509

Industrial & Engineering Chemistry Research

Article

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

pressure and in the temperature range from 50 to 80 °C. A stirring frequency of 1200 rpm was used. Behr et al.21 suggested a simplified kinetic model with three equations in which the two monoethers, the two diethers, and the triether generated from glycerol etherification with isobutylene were lumped in two pseudocomponents: monoethers and high ethers. A power-law kinetic approach on the basis of the overall molar concentrations was used to model the observed reaction rates. The temperature dependence of the reaction rates was described by means of the Arrhenius equation and the nonrandom two-liquid (NRTL) model was used to describe the liquid−liquid equilibrium. All experiments were carried out at 2 MPa, 2 wt % catalyst (p-toluenesulfonic acid), and 1000 rpm. Temperatures of 70 and 90 °C were investigated at isobutylene/glycerol ratios of 1:1 and 1:2. Lee et al.13 proposed a consecutive scheme of three equilibrium reactions of glycerol etherification with isobutylene using different catalysts (i.e., p-toluenesulfonic acid, Amberlyst 15, ionic liquids). Products were lumped as monoethers and

diethers. Furthermore, two side reactions involving the formation of tert-butyl alcohol and oligomers of isobutylene were suggested. Reactions were carried out at 20 bar and 7.5 wt % catalyst. Di Serio et al.23 studied the glycerol etherification reaction with isobutylene and Amberlyst 15 as the catalyst. The reaction conditions were an isobutylene/glycerol ratio of 2:1, a temperature of 90 °C, and a pressure of 15 bar. They estimated the equilibrium constant (NRTL method) for three reversible reactions of glycerol etherification, where monoethers and diethers were lumped into two substances, and the kinetic constant for the secondary reactions of isobutylene oligomerization considering all second-order reactions. Frusteri et al.24 suggested a simplified potential kinetic model to describe the main reaction between glycerol and tert-butyl alcohol, based on empirical reaction orders of the reactants estimated by data fitting. Reaction orders with respect to glycerol and tert-butyl alcohol were determined considering initial glycerol conversion values under the excess method as 9501

dx.doi.org/10.1021/ie300481d | Ind. Eng. Chem. Res. 2012, 51, 9500−9509


Article

Table 1. Kinetic Model with the Lumping Approach for the Etherification of Glycerol with tert-Butyl Alcohol over A15 Catalyst stoichiometric equation k1/ k−1

G + TB ←⎯⎯⎯→ M + H 2O k 2 / k−2

M + TB ←⎯⎯⎯⎯→ D + H 2O k3/ k−3

D + TB ←⎯⎯⎯→ T + H 2O K4

TB → IB + H 2O

reaction rate

(R1)

⎛ CMC H2O ⎞ r1 = k1⎜CGC TB − ⎟ K1 ⎠ ⎝

(2)

(R2)

⎛ C DC H2O ⎞ r2 = k 2⎜CMC TB − ⎟ K2 ⎠ ⎝

(3)

⎛ C TC H2O ⎞ r3 = k 3⎜C DC TB − ⎟ K3 ⎠ ⎝

(4)

(R3)

r4 = k4C TB

(R4)

(5)

Table 2. Kinetic Model Stated for the Etherification of Glycerol with tert-Butyl Alcohol over A15 Catalyst stoichiometric equation k1A / k−1A

G + TB ←⎯⎯⎯⎯⎯⎯⎯→ M1 + H 2O k1B/ k−1B

G + TB ←⎯⎯⎯⎯⎯⎯→ M2 + H 2O k 2A / k−2A

M1 + TB ←⎯⎯⎯⎯⎯⎯⎯→ D1 + H 2O k 2B/ k−2B

M1 + TB ←⎯⎯⎯⎯⎯⎯→ D2 + H 2O k3A / k−3A

M2 + TB ←⎯⎯⎯⎯⎯⎯⎯→ D2 + H 2O k4A / k−4A

D1 + TB ←⎯⎯⎯⎯⎯⎯⎯→ T + H 2O k5/ k−5

D2 + TB ←⎯⎯⎯→ T + H 2O k6

TB → IB + H 2O

reaction rate

(R1A)

⎛ CM1C H2O ⎞ r1A = k1A ⎜CGC TB − ⎟ K1A ⎠ ⎝

(6)

(R1B)

⎛ CM2C H2O ⎞ r1B = k1B⎜CGC TB − ⎟ K1B ⎠ ⎝

(7)

(R2A)

r2A

⎛ C D1C H2O ⎞ = k 2A ⎜CM1C TB − ⎟ K 2A ⎠ ⎝

(8)

(R2B)

⎛ C D2C H2O ⎞ r2B = k 2B⎜CM1C TB − ⎟ K 2B ⎠ ⎝

(9)

(R3A)

⎛ C D2C H2O ⎞ r3A = k 3A ⎜CM2C TB − ⎟ K3A ⎠ ⎝

(10)

⎛ C TC H2O ⎞ r4A = k4A ⎜C D1C TB − ⎟ K4A ⎠ ⎝

(R4A)

⎛ C TC H2O ⎞ r5 = k5⎜C D2C TB − ⎟ K5 ⎠ ⎝

(R5)

(R6)

■

r6 = k6C TB

(11) (12)

(13)

THEORETICAL BASIS tert-Butylation of glycerol generates a set of consecutive equilibrium reactions catalyzed by acids with reaction orders given by the molecularity of elementary reaction steps proposed by Melero et al.,17 Klepacova et al.,16,28 Behr and Obendorf,21 and Karinen and Krause22 using isobutylene as the reagent. Products obtained after glycerol (G) etherification with isobutylene (IB) have been identified in the literature,29 and they are the following: 1-tert-butoxypropane-2,3-diol (M1), 2tert-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-tertbutoxypropane (T). Considering these products as a starting point, the reaction pathway shown in Figure 1 can be proposed for tert-butyl alcohol (TB) as the etherification agent. In this case, the dehydration of tert-butyl alcohol is an independent side reaction that influences the main reactions of tertbutylation through the undesired consumption of tert-butyl alcohol. The kinetic models proposed in this work were developed according to the reaction scheme shown in Figure 1 and previous studies of similar reactions.13,16,23,26,30 They are based on heterogeneous reactions that occur over the catalyst pellets, and 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

reaction conditions. Product distribution was not included in this model. Kiatkittipong et al.26 proposed two lumped kinetic models for glycerol etherification with tert-butyl alcohol: a power law based on activities and mole fractions and a Langmuir− Hinshelwood (LH-A) model in which only the strongest adsorption components were taken into account. The LH-A model gave the best fit of the experimental results. Kinetic parameters were calculated with an Arrhenius equation, and equilibrium constants were obtained with Gani’s group contribution method. The scope of this work was to study the modeling of glycerol etherification with tert-butyl alcohol at different reaction temperatures using a strong acid ion-exchange commercial resin (Amberlyst 15, denoted A15) as the catalyst. Two kinetic models of different complexity were developed to describe both the evolution of reactants and the distribution of products and byproducts in the range of temperatures studied (50−80 °C). The first is a simplified model that gave rise to a more complex model that considers all of the species involved. To the best of our knowledge, such a detailed kinetic study has not been reported in the literature for this reaction. 9502



Article

the heterogeneous catalyst during the reactions is considered as a constant implicit into the rate coefficient. The effect of reaction temperature is included in the kinetic parameters (ki), which are considered as temperature-dependent according to the Arrhenius equation ⎛ E ⎞ ki = ki0 exp⎜ − ai ⎟ ⎝ RT ⎠

In a typical run, 1.26 g (0.014 mol) of glycerol (G) and 4.07 g (0.055 mol) of tert-butyl alcohol (TB) were used as reactants. The reactor was flashed twice with nitrogen to displace the air inside and generate an inert atmosphere. The temperature range studied was from 50 to 80 °C. Catalyst loading was considered as 8.5 wt %, referred to the starting amount of glycerol. All experiments were carried out twice to check the reproducibility of the results. The tert-butyl alcohol/glycerol molar ratios used were 3:1, 4:1 and 5:1. 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 runs to guarantee the absence of external mass-transfer resistances. The absence of internal mass-transfer limitations was checked theoretically, using the isothermal internal effectiveness factor, η, which is a function of the Thiele modulus, φ. A value of 0.8 for the Thiele modulus and a value of 0.96 for the effectiveness factor at 50 °C were obtained. Therefore, internal mass-diffusion effects were also negligible under the experimental conditions studied in this work. The method we applied to calculate the Thiele modulus and effectiveness factor is described in Appendix B (Supporting Information). The reaction was stopped by removing the reactor from the glycerine bath at reaction times from 0 to 480 min. The contents of the reactor were then analyzed. Gaseous isobutylene generated from tert-butyl alcohol dehydration was recovered with a cold trap filled with pyridine located just after the reactor to liquefy the sample; this product was subsequently analyzed. Analysis. The reactor composition (ethers, tert-butyl alcohol, and isobutylene) was analyzed by means of an Agilent 6850 gas chromatograph fitted with a flame ionization detector. An HP Innowax chromatography column (30 m length × 0.32 mm i.d.) was used. The chromatographic conditions were as follows: initial oven temperature 40 °C, final oven temperature 220 °C, program rate 20 °C min−1. n-Heptane was used as the internal standard. Commercial 1-tert-butoxypropane-2,3-diol (M1) was employed to obtain the corresponding response factor, which was extrapolated to noncommercial 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 1,2,3tri-tert-butoxypropane (T). Quantification of glycerol was performed through a mass balance based on the chromatographic analysis of glycerol-derived compounds. An example of a chromatogram is included in Appendix E (Supporting Information). Water content was measured with a quantitative Karl Fischer analyzer (Titromatic 1S Crison). The porous structure of the catalyst was characterized by N2 adsorption−desorption at −196 °C, performed in an SA 3100 surface area analyzer (Beckman Coulter). Amberlyst 15, before and after adsorption, was previously outgassed for at least 4 h at 80 °C. From the N2 isotherm, the apparent surface area was determined by applying the BET equation. The pore volume (Vp) and mean pore diameter (dp) were obtained also from the N2 isotherm data.

(1)

Two kinetic strategies are proposed in this work; a lumping kinetic approach and an extended kinetic model based on the lumped model. Both are expressed in terms of concentrations instead of activities31,32 because an ideal mixture was assumed. These models are described in the next two subsections. Lumping Approach. The simplified approach (based on Figure 1) considers that all monoether and diether species are lumped into species denoted M and D, respectively, yielding the reaction scheme and reaction rates listed in Table 1. Production rates for each compound were calculated in the batch reactor according to the reaction rates in Table 1 and are summarized in Appendix A, Table A1 (Supporting Information). Extended Model. The extended kinetic model considers all reactions shown in Figure 1 and gives rise to the reaction scheme and corresponding reaction rates listed in Table 2. Production rates for each substance were calculated as indicated in Table A2 (Appendix A, Supporting Information), as functions of the reaction rates in Table 2. The acronyms used are those indicated in Figure 1.

■

EXPERIMENTAL SECTION

Catalysts and Chemicals. The commercial strong acid ion-exchange resin Amberlyst 15 (A15, 20−50 mesh), supplied by Fluka, was used as the heterogeneous etherification catalyst, without additional drying. [The catalyst Brunauer−Emmett− Teller (BET) surface area, average pore volume, external surface area, and average pore diameter were SBET = 44 m2/g, Vp = 0.338 cm3/g, At = 44 m2/g, and dp = 50 × 10−8 m, respectively.] Anhydrous glycerol (purity ≥ 99.5%), supplied by Fluka, and anhydrous tert-butyl alcohol (purity ≥ 99.5%), provided by Sigma-Aldrich, were employed as reactants. Reagent-grade pyridine, supplied by Scharlau, was used as the solvent for sample analysis. n-Heptane (95% purity) solvent (Romil Pure Chemistry) was employed as the internal standard compound in the chromatographic analysis. 3-tert-Butoxy-1,2propanediol (purum ≥ 97%, Aldrich) was 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. Apparatus and Procedure. Etherification reactions were carried out in a watertight glass laboratory autoclave reactor (50 mL) with magnetic stirring, without solvent, and at autogenous pressure. The reactor was held at constant temperature by immersion in a glycerol bath, whose temperature was 2 °C higher than that of the reactor. The set-point temperature inside the reactor was reached 10 min after the reactor was introduced into the glycerol bath, and this time was considered as the starting point of the reaction. Reactor temperature was measured continuously by means of a thermocouple immersed in the liquid reaction medium.

■

RESULTS AND DISCUSSION Six runs were carried out under different experimental conditions; they are listed in Table 3. The batch reactor was 9503



Article

catalysts.18 Therefore, runs 4−6 were performed at this ratio of 4:1. The influence of temperature (runs 4−6) on the glycerol and tert-butyl alcohol conversions is shown in Figures 3 and 4, respectively. The product distributions are shown in Figures 5−8, where triether is not included because it was obtained in a negligible amount.

sacrificed at each experimental time listed in Table 3; that is, an individual reaction was completed for each experimental time. Table 3. Experimental Conditions for the Etherification of 0.014 mol of Glycerol with 0.055 mol of tert-Butyl Alcohol run

TB/G ratio

T (°C)

t (min)

1 2 3 4 5 6

3:1 4:1 5:1 4:1 4:1 4:1

60 60 60 50 60 80

360 360 360 0, 90, 120, 240, 360, 480 0, 90, 120, 240, 360, 480 0, 90, 120, 240, 360, 480

Four ethers were identified: M1, M2, D1, and D2. M1 was the most abundant product, followed by M2. Diethers were formed to a lesser extent than monoethers, and the D1 concentration was higher than that of D2. Only traces of T could be quantified in the range of temperatures investigated because of steric hindrance, as also reported in previous works.24 This product distribution can be explained because the etherification of glycerol takes place more easily on primary hydroxyl groups (M1 and D1).13 In addition, isobutylene (IB) was generated as an undesired byproduct. The formation of isobutylene was considered in this work as a nonequilibrium reaction because most of the IB was in the gas phase. Etherification reactions with isobutylene were not considered either because of the small amount of IB produced and its limited solubility in glycerol at the low pressure used.30,33 Experimental values obtained for reactant conversions and product distributions versus time were fitted to kinetic equations depending on the kinetic model used. Nonlinear regression (Marquardt algorithm) was coupled with the Runge−Kutta method, which minimizes the square sum of residuals between the experimental and predicted values. The influence of the TB/G ratio (runs 1−3) can be seen in Figure 2. As this figure shows, an increment in glycerol conversion was observed when this ratio was increased from 3:1 to 4:1. However, a further increase of this ratio to 5:1 produced a lower effect on the conversion. These results match those obtained in previous studies of the etherification with tert-butyl alcohol and Amberlyst 1528 and with isobutylene and other

Figure 3. Influence of the temperature on glycerol conversion. Symbols: experimental data. Dotted lines: values estimated using the lumped model (eq S1, Table A1, Supporting Information). Dashed lines: values estimated using the extended model (eq S8, Table A2, Supporting Information). Catalyst concentration = 8.5 wt %.

Figure 4. Influence of the temperature on tert-butyl alcohol conversion. Symbols: experimental data. Dotted lines: values estimated using the lumped model (eq S2, Table A-1, Supporting Information). Dashed lines: values estimated using the extended model (eq S9, Table A2, Supporting Information). Catalyst concentration = 8.5 wt %.

As shown in Figures 3 and 4, a considerable enhancement in glycerol and tert-butyl alcohol conversions occurred as the temperature increased. Glycerol conversion increased from 0.7 at 50 °C to 0.88 at 80 °C. tert-Butyl alcohol increased from 0.25 to 0.55 at same temperatures. At higher temperatures, kinetic constants increase; therefore, reaction rates rise. Equilibrium conversions of reactants also increase with temperature because of the endothermic nature of the reactions. The results obtained are within the ranges observed in previous works.14,15,24,26,28 In Figures 5 and 6, the evolutions of monoethers and diethers, respectively, are shown. As can be seen in these profiles, mono- and diethers exhibited the behavior of intermediate products according to the reaction pathway shown in Figure 1.

Figure 2. Influence of tert-butyl alcohol/glycerol molar ratio on glycerol conversion (runs 1−3 in Table 5). 9504



Article

Figure 7. Evolution of water distribution over time at different temperatures. Symbols: experimental data. Dotted lines: values estimated using the lumped model (eq S7, Table A1, Supporting Information). Dashed lines: values estimated using the extended model (eq S16, Table A2, Supporting Information). Catalyst concentration = 8.5 wt %.

Figure 5. Evolution of M1 and M2 distributions over time at different temperatures. Symbols: experimental data. Dashed lines: values estimated using the extended model (eqs S10 and S11, Table A2, Supporting Information). Catalyst concentration = 8.5 wt %.

An increase in the M1 concentration with temperature can be seen in Figure 5, because the M1 production rate was higher than the production rates of M2 and the diethers. Furthermore, the trend of the M2 concentration with time exhibited a maximum at all temperatures studied; this is a typical performance of serial step reaction kinetics. In Figure 6, an increase in diether concentration can be observed at increased time and temperature. In this study, the diethers are considered end products because triether was not formed as a result of steric hindrance, as previously stated. At higher reaction times, the diether concentrations indicated that equilibrium was achieved mainly as a result of water formation. The M1 concentration could not decrease because of the achievement of this equilibrium. Both figures show that M1 and D1 are the main products because of the easier etherification of glycerol on primary hydroxyl groups. As can be seen in Figure 7, the water content increased as the TB and glycerol contents decreased (Figure 4), with a positive influence of temperature on both the water production rate and the final water concentration obtained. According to Figure 1, 1 mol of water is generated by each the tert-butyl alcohol dehydration and etherification reaction. An increase from 4.5 to 9 wt % at 480 min was noticed when the temperature was increased from 50 to 80 °C, in agreement with the endothermic character of these reactions.15,28 On the other hand, Figure 8 shows the increase in isobutylene with increasing temperature

Figure 8. Evolution of isobutylene distribution over time at different temperatures. Symbols: experimental data. Dotted lines: values estimated using the lumped model (eq S6, Table A-1, Supporting Information). Dashed lines: values estimated using the extended model (eq S15, Table A2, Supporting Information). Catalyst concentration = 8.5 wt %.

and decreasing TB content (Figure 4). This growth was favored by the absence of isobutylene dimers and the glycerol etherification reaction because of the small amount of isobutylene present in the liquid phase, as explained before.

Figure 6. Evolution of D1 and D2 distributions over time at different temperatures. Symbols: experimental data. Dashed lines: values estimated using the extended model (eqs S12 and S13, Table A2, Supporting Information). Catalyst concentration = 8.5 wt %. 9505



Article

Kinetic Model with Lumping Approach. To apply the kinetic model summarized in Tables 1 and A1 (Appendix A, Supporting Information), the sum of monoethers was lumped as component M, and the sum of diethers was lumped as component D. Triether formation was not considered (therefore, reaction R3 was avoided) because this compound was found at very low concentrations in the product mixture. The evolutions of the weight percentages of these lumped species at different temperatures can be seen in Figures 9 and 10. G, TB, H2O, and IB are shown in Figures 3, 4, 7, and 8, respectively.

temperature was increased, as expected from an endothermic reaction such as etherification. From the data obtained at 60 and 80 °C and 480 min, experimental equilibrium constants were calculated, and the values are reported in Table 4. These Table 4. Experimental and Calculated Equilibrium Constants for Glycerol Etherification Using the Lumped Approach Table 1 reaction

experimental equilibrium constant

calculated equilibrium constant

R1

⎛ 6671.3 ⎞⎟ K1 = exp⎜20.53 − ⎝ T ⎠

⎛ 8639.46 ⎞⎟ K1 = exp⎜26.37 − ⎝ T ⎠

R2

⎛ 9053.5 ⎞⎟ K 2 = exp⎜24.35 − ⎝ T ⎠

⎛ 8083.91 ⎞⎟ K 2 = exp⎜21.51 − ⎝ T ⎠

experimental values were used to estimate the kinetic parameters of the model as fixed variables. On the other hand, to check the reliability of the proposed model, equilibrium constants were also introduced into the model as fitting parameters, and the values estimated by fitting are also included in Table 4. As can be seen, the experimental and estimated equilibrium constants are very similar, although the values obtained are slightly higher than those reported by Kiatkittipong et al.26 Kinetic parameters of glycerol etherification according to the scheme in Tables 1 and A1 (Appendix A, Supporting Information) were calculated by experimental data fitting (compositions of TB, M, D, IB, and H2O as functions of time). Estimated values are reported in Table C1 (Appendix C, Supporting Information) with their standard deviations. In Appendix D of the Supporting Information (Table D1), the percentage of the variation explained for each measured variable is also reported. Therefore, the following kinetic equations for the rates of reactions R1, R2, and R4 (eqs 2, 3, and 5, respectively) listed in Table 1 were obtained (eqs 14−16, respectively). They include the estimated kinetic constants and experimental equilibrium constants for lumped model. These expressions suggest that the fastest reaction is the formation of monoether, which is the majority product. Reaction R4 has been studied in the literature by Abella et al.30 and Honkela et al.,34 among others. The activation energies obtained for R4A with a low standard deviation are related to those obtained in previous studies (1830 and 14234 kJ/mol).

Figure 9. Evolution of lumped species M with time at different temperatures. Symbols: experimental data. Dotted lines: values estimated using the lumped model (eq S3, Table A1, Supporting Information). Catalyst concentration = 8.5 wt %.

⎡ CMC H2O ⎛ 9137.3 ⎞⎟⎢ r1 = exp⎜23.8 − CGC TB − 6671.3 ⎝ T ⎠⎢⎢ exp 20.53 − T ⎣

(

Figure 10. Evolution of lumped species D with time at different temperatures. Symbols: experimental data. Dotted lines: values estimated using the lumped model (eq S4, Table A1, Supporting Information). Catalyst concentration = 8.5 wt %.

)

⎤ ⎥ ⎥ ⎥⎦ (14)

⎡ C DC H2O ⎛ 3714.2 ⎞⎟⎢ ⎜ CMC TB − r2 = exp 4.9 − ⎢ 9053.5 ⎝ ⎠ T exp 24.35 − T ⎣

(

It was observed that the amounts of both M and D increased with increasing temperature. This increase was more significant for D than for M, because the monoethers are formed in a serial step. Regarding the diethers (desired products), concentrations of 2 and 11 wt % at 480 min were obtained at 50 and 80 °C, respectively. The trends in the results are similar to those obtained by other authors.14,15,28 At a long enough time (480 min) and temperatures 60 and 80 °C, values near equilibrium were obtained for G, TB, H2O, M, and D (Figures 3, 4, 7, 9, and 10, respectively). Equilibrium parameters obtained for the products were higher when the

)

⎤ ⎥ ⎥ ⎦ (15)

⎛ 6220.1 ⎞⎟ r4 = exp⎜10.5 − C TB ⎝ T ⎠

(16)

Using eqs 14−16, predicted values for G, TB, H2O, IB, M, and D were calculated according to the scheme in Tables 1 and A1 (Appendix A, Supporting Information). The predicted values are shown as dotted lines in Figures 3, 4, and 7−10, respectively. As can be seen a suitable fit between predicted and experimental values has been obtained. 9506



Article

Extended Kinetic Model. In this case the kinetic model shown in Tables 2 and A2 (Appendix A, Supporting Information) was used. In Figure 5, the evolution of M1 and M2 was shown. The amount of M1 increases when the temperature rises but small differences have been obtained between 60 and 80 °C. This can be due to the fact that diethers formation is faster when temperature is higher and the monoether is involved as reactant and product in the serial step reactions. M2 content decreases as temperature increases indicating a higher activation energy for the disappearing rate of M2 (r3A in Table 2) than for its production (r1B). Diethers evolution is shown in Figure 6. D1 increases when temperature rises as well as D2. Equilibrium constants have been experimentally calculated based on data obtained at high enough times (480 min) and temperatures 60 and 80 °C. These experimental values were fixed to obtain the kinetic parameters as well as in lumped model procedure. These data are shown in Table 5. Equilibrium constants obtained are higher if temperature increases as well as in the lumped approach.

experimental equilibrium constant

⎛ 6690.5 ⎞⎟ = exp⎜20.63 − ⎝ T ⎠

K1A

R1B

⎛ 6114.3 ⎞⎟ K1B = exp⎜15.62 − ⎝ T ⎠

⎛ 6914.77 ⎞⎟ K1B = exp⎜17.91 − ⎝ T ⎠

R2A

⎛ 10128 ⎞⎟ K 2A = exp⎜27.20 − ⎝ T ⎠

⎛ 9908.57 ⎞⎟ K 2A = exp⎜28.20 − ⎝ T ⎠

R2B

⎛ 5481.4 ⎞⎟ K 2B = exp⎜12.13 − ⎝ T ⎠

⎛ 4449.16 ⎞⎟ K 2B = exp⎜12.13 − ⎝ T ⎠

R3A

⎛ 6376.3 ⎞⎟ K3A = exp⎜20.88 − ⎝ T ⎠

⎛ 6369.5 ⎞⎟ K3A = exp⎜18.56 − ⎝ T ⎠

⎤ ⎥ ⎥ ⎥⎦

)

⎤ ⎥ ⎥ ⎥⎦ (17)

(18)

r2A

⎡ C D1C H2O ⎛ 4226.6 ⎞⎟⎢ ⎜ CM1C TB − = exp 6.23 − 10128 ⎝ T ⎠⎢ exp 27.20 − T ⎣

(

⎤ ⎥ ⎥ ⎦

)

(19)

⎡ C D2C H2O ⎛ 7139.0 ⎞⎟⎢ CM1C TB − r2B = exp⎜14.15 − ⎢ 5481.4 ⎝ ⎠ T exp 12.13 − T ⎣

(

)

⎡ C D2C H2O ⎛ 5267.7 ⎞⎟⎢ CM2C TB − r3A = exp⎜10.33 − ⎢ 6376.3 ⎝ T ⎠⎢ exp 20.88 − T ⎣

(

⎛ 6178.5 ⎞⎟ C TB r6 = exp⎜10.40 − ⎝ T ⎠

⎛ 8193.0 ⎞⎟ = exp⎜24.97 − ⎝ T ⎠

K1A

⎡ CM2C H2O ⎛ 4054.6 ⎞⎟⎢ CGC TB − r1B = exp⎜5.75 − ⎢ 6114.3 ⎝ ⎠ T exp 15.62 − T ⎣⎢

(

calculated equilibrium constant

R1A

)

(

Table 5. Equilibrium Constants for Glycerol Etherification with the Extended Model Table 2 reaction

⎡ C M1C H2O ⎛ 7155.6 ⎞⎟⎢ ⎜ CGC TB − r1A = exp 17.62 − 6690.5 ⎝ T ⎠⎢⎢ exp 20.63 − T ⎣

⎤ ⎥ ⎥ ⎦ (20)

⎤ ⎥ ⎥ ⎥⎦ (21)

)

(22)

Equations 17−22 were used in the extended model (replacing eqs 6−10 and 13, respectively, in Table 2), and with these kinetic equations, predicted values of G, TB, M1 and M2, D1 and D2, H2O, and IB were calculated and are shown as dashed lines in Figures 3−8, respectively. As can be seen in these figures, the evaluation of the experimental data showed that the use of kinetic equations based on an extended kinetic model is a reasonable way to describe the changes in reactant concentrations during glycerol etherification with tert-butyl alcohol, as this model fits the results for all of the species. However, the predictions for G, TB, IB, and H2O obtained using the lumped and extended models were quite similar. As M1 was much higher than M2, the two species could be lumped as M. The two types of diethers formed were properly described by the extended model.

Kinetic parameters of glycerol etherification according to scheme in Tables 2 and A2 (Appendix A, Supporting Information) were calculated by fitting the experimental data of all components. Furthermore, equilibrium constants have also been introduced in the model as fitting parameters to check the reliability of the proposed model. The calculated values are also shown in Table 5. As can be seen, both values are similar. The kinetic parameters calculated are reported in Appendix C of the Supporting Information (Table C2) with their standard deviations. The percentage of the variation explained of the parameters calculated using extended model is displayed in Appendix D of the Supporting Information (Table D2). Lumped results have been employed as starting point of the extended model estimations. Therefore, the following kinetic equations for the rates of reactions R1A, R1B, R2A, R2B, R3A, and R6 in Table 2 are summarized in eqs 17−22, respectively. In these expressions, estimated kinetic constants and experimental equilibrium constants for the extended model are included. M1 formation is the fastest reaction. D2 formation from reaction R2B is the slowest, so its contribution could be considered negligible.

■

CONCLUSIONS In the etherification of glycerol using tert-butyl alcohol, it is possible to work in just one phase, in contrast to the reaction using isobutene. Nevertheless, water is generated in a greater quantity than in the isobutene reaction, which limits the maximum conversion by influencing the thermodynamic equilibrium. The triether was not obtained under the employed conditions. The lumped model can predict in a reasonable way how the reactants and products evolve, although it cannot allow distinctions to be made between the two monoethers and between the two diethers. The results obtained with the lumped mathematical analysis were employed as the starting point of the extended simulation. The extended model was found to be capable of predicting the evolutions of all of the products individually. The activation energy of monoether formation (lump M or component M1 as the majority product) was higher than that of diether formation. Therefore, the maximum yield of monoether was found to increase with increasing temperature. 9507



■

Article

formulation: Influence on EN 14214 quality parameters. Fuel 2010, 88 (8), 2011−2018. (8) Balaraju, M.; Rekha, V.; Prasad, S. P. S.; Prabhavathi Devi, B. L. A.; Prasad, R. B. N.; Lingaiah, N. Influence of solid acids as co-catalysts on glycerol hydrogenolysis to propylene glycol over Ru/C catalysts. Appl. Catal. A 2009, 354, 82−87. (9) Climent, M. J.; Corma, A.; Frutos, P. D.; Iborra, S.; Noy, M.; Velty, A.; Concepción, P. Chemicals from biomass: Synthesis of glycerol carbonate by transesterification and carbonylation with urea with hydrotalcite catalysts. The role of acid−base pairs. J. Catal. 2010, 269, 140−149. (10) Chang, J. S.; Chen, D. H. Optimization on the etherification of glycerol with tert-butyl alcohol. J. Taiwan Inst. Chem. Eng. 2011, 42, 760−767. (11) Jong Lee, H.; Seung, D.; Filimonov, I. N.; Kim, H. Etherification of glycerol by isobutylene. Effects of the density of acidic sites in ionexchange resin on the distribution of products. Korean J. Chem. Eng. 2011, 28, 756−762. (12) Ancillotti, F.; Fattore, V. Oxygenate fuels: Market expansion and catalytic aspect of synthesis. Fuel Process. Technol. 1998, 57, 163. (13) Lee, H. J.; Seung, D.; Jung, K. S.; Kim, H.; Filimonov, I. N. Etherification of glycerol by isobutylene: Tuning the product composition. Appl. Catal. A 2010, 390, 235−244. (14) Klepácǒ vá, K.; Mravec, D.; Hájeková, E.; Bajus, M. Etherification of glycerol. Pet. Coal 2003, 45 (1−2), 54−57. (15) Klepácǒ vá, K.; Mravec, D.; Bajus, M. tert-Butalytion of glycerol catalysed by ion-exchange resins. Appl. Catal. A 2005, 294, 141−147. (16) Klepácǒ vá, K.; Mravec, D.; Kaszonyi, A.; Bajus, M. Etherification of glycerol and ethylene glycol by isobutylene. Appl. Catal. A 2007, 328, 1−3. (17) Melero, J. A.; Vicente, G.; Morales, G.; Paniagua, M.; Moreno, J. M.; Roldán, R.; Ezquerro, A.; Pérez, C. Acid-catalyzed etherification of bio-glycerol and isobutylene over sulfonic mesostructured silicas. Appl. Catal. A 2008, 346, 44−51. (18) Zhao, W.; Yang, B.; Yi, C.; Lei, Z.; Xu, J. Etherification of Glycerol with Isobutylene to Produce Oxygenate Additive Using Sulfonated Peanut Shell Catalyst. Ind. Eng. Chem. Res. 2010, 49, 12399−12404. (19) Silva, V. T. M.; Pereira, C.; Rodrigues, A. E. PermSMBRA New Hybrid Technology: Application on Green Solvents and Biofuel Production. AIChE J. 2011, 57, 1841−1851. (20) Pereira, C.; Silva, V. T. M.; Rodrigues, A. E. Ethyl lactate as a solvent: Properties, applications and production processesA review. Green Chem. 2011, 13, 2658−2671. (21) Behr, A.; Obendorf, L. Development of a process for the acidcatalyzed etherification of glycerine and isobutene forming glycerine tertiary butyl ethers. Eng. Life Sci. 2003, 2 (7), 185−189. (22) Karinen, R. S.; Krause, A. O. I. New biocomponents from glycerol. Appl. Catal. A 2006, 306, 128−133. (23) Di Serio, M.; Casale, L.; Tesser, R.; Santacesaria, E. New Process for the Production of Glycerol tert-Butyl Ethers. Energy Fuels 2010, 24, 4668−4672. (24) Frusteri, F.; Arena, F.; Bonura, G.; Cannilla, C.; Spadaro, L.; Di Blasi, O. Catalytic Etherification of Glycerol by tert-Butyl Alcohol to Produce Oxygenated Additives for Diesel Fuels. Appl. Catal. A 2009, 367, 77−83. (25) Ozbay, N.; Oktar, N.; Dogu, G.; Dogu, T. Conversion of Biodiesel By-Product Glycerol to Fuel Ethers over Different Solid Acid Catalysts. Int. J. Chem. Reactor Eng. 2010, 8, A18. (26) Kiatkittipong, W.; Intaracharoen, P.; Laosiripojana, N.; Chaisuk, C.; Praserthdam, P.; Assabumrungrat, S. Glycerol ethers synthesis from glycerol etherification with tert-butyl alcohol in reactive distillation. Comput. Chem. Eng. 2011, 35, 2034−2043. (27) Janaun, J.; Ellis, N. Glycerol Etherification by tert-Butanol Catalyzed by Sulfonated Carbon Catalyst. J. Appl. Sci. 2010, 10 (21), 2633−2637. (28) Klepácǒ vá, K.; Mravec, D.; Bajus, M. Etherification of glycerol with tert-butyl alcohol catalysed by ion-exchange resins. Chem. Pap. 2006, 60 (3), 224−230.

ASSOCIATED CONTENT

S Supporting Information *

Production rates of reactants and products in a batch reactor according to the lumped and extended models, Thiele modulus and effectiveness factor calculations, kinetic parameters of glycerol etherification with the lumped and extended models, residual values obtained with lumped and extended models, and chromatographic separation of products. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

■

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 Comunidad Autónoma de Madrid (Spain) for her research contract.

■

NOMENCLATURE At = external surface area (m2/g) dp = average pore diameter (nm) Eai = activation energy of kinetic constant of reaction i (kJ/ mol) ki = kinetic constant of product formation of reaction i (kg/ mol·min) Ki = equilibrium constant of reaction i R = ideal gas constant (J/mol·K) ri = formation rate of products of reaction i (mol/min) Ri = rate of concentration change for reactants and products of reaction i (mol/min) SBET = BET surface area (m2/g) Vp = average pore volume (cm3/g) W = catalyst loading referred to initial glycerol loading (wt %) η = effectiveness factor φ = Thiele modulus

■

REFERENCES

(1) Paniri, G.; Ghaziaskar, H. S.; Rezayat, M. Ternary solubility of mono- and di-tert-butyl ethers of glycerol in supercritical carbon dioxide. J. Supercrit. Fluids 2010, 55, 43−48. (2) Demirbas, A. Progress and recent trends in biofuels. Prog. Energy Combust. Sci. 2007, 33, 1−18. (3) Lotero, E.; Liu, Y.; Lopez, D. E.; Suwannakarn, K.; Bruce, D. A.; Goodwin, J. G., Jr. Synthesis of Biodiesel via Acid Catalysis. Ind. Eng. Chem. Res. 2005, 44, 5353−5363. (4) Bozbas, K. Biodiesel as an alternative motor fuel: Production and policies in the European Union. Renewable Sustainable Energy Rev. 2008, 12, 542−552. (5) 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, 987−1000. (6) Zhou, C. C.; Beltramini, J. N.; Fan, Y.; Lu, G. Q. Chemoselective catalytic conversion of glycerol as a biorenewable source to valuable commodity chemicals. Chem. Soc. Rev. 2008, 37, 527−549. (7) Melero, J. A.; Vicente, G.; Morales, G.; Paniagua, M.; Bustamante, J. Oxygenated compounds derived from glycerol for biodiesel 9508



Article

(29) Jamróz, M. E.; Jarosz, M.; Witowska-Jarosz, J.; Bednarek, E.; Tecza, W.; Jamróz, M. H.; Dobrowolski, J. C.; Kijénski. Mono- di- and tri-tert-butyl ethers of glycerol. A molecular Study. Spectrochim. Acta A 2007, 67 (3−4), 980−988. (30) Abella, L. C.; Gaspillo, P. A. D.; Maeda, M.; Goto, S. Kinetic Study on the Dehydration of tert-Butyl Alcohol Catalyzed by Ion Exchange Resins. Int. J. Chem. Kinet. 1999, 31 (12), 854−859. (31) Silva, V. T. M.; Rodrigues, A. E. Kinetic studies in a batch reactor using ion exchange resin catalysts for oxygenated production: The role of mass transfer mechanisms. Chem. Eng. Sci. 2006, 61, 316− 331. (32) Graça, N.; Pais, L. S.; Silva, V. T. M.; Rodrigues, A. E. Oxygenated biofuels from butanol for diesel blends: Synthesis of the acetal 1,1-dibutoxyethane catalyzed by Amberlyst-15 ion exchange resin. Ind. Eng. Chem. Res. 2010, 49, 6763−6771. (33) Van Gerpen, J. Biodiesel processing and production. Fuel Process. Technol. 2005, 86, 1097−1107. (34) Honkela, M. L.; Ouni, T.; Krause, A. O. I. Thermodynamics and Kinetics of the Dehydration of tert-Butyl Alcohol. Ind. Eng. Chem. Res. 2004, 43, 4060−4065.

9509


Etherification of Glycerol by tert-Butyl Alcohol - American Chemical

Recommend Documents