Reaction Kinetics of Transesterification with Titanium Alkoxide-Based

Article pubs.acs.org/IECR

Reaction Kinetics of Transesterification with Titanium Alkoxide-Based Phase-Transforming Catalyst Gayan Nawaratna* and Sandun D. Fernando Department of Biological and Agricultural Engineering, Texas A&M University, 201 Scoates Hall, 2117 TAMU, College Station, Texas 77843, United States ABSTRACT: A catalytic process was developed that uses a phase-transforming catalyst to perform a transesterification reaction. In this process, titanium isopropoxide catalyst is added to the reaction medium as a liquid, and as a result of condensation polymerization, the spent catalyst can be removed as a heterogeneous solid. The kinetics of this system was studied using monoolein as the model monoglyceride and isopropanol as the alcohol. A yield optimization study was also conducted using soybean oil as the triglyceride. The titanium isopropoxide catalyst was able to transesterify monoolein completely, giving >99% ester yields. The kinetic study revealed that the transesterification of monoolein with titanium isopropoxide is secondorder with respect to isopropyl alcohol. The rate constant was evaluated to be 0.0002 L mol−1 min−1. In studies with soybean oil, a maximum ester yield of 71% was observed using 16% (w/w) catalyst under the conditions tested. This catalyst concept, once fully developed, can be quite useful in a multitude of processes because the initial homogeneous catalyst phase alleviates masstransport issues whereas the latter heterogeneity eases separation of the catalyst from the medium.

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INTRODUCTION Homogeneous catalysts that are in the same phase as the substrates generally do not give rise to significant transport barriers in a reaction as compared to heterogeneous catalysts. Nevertheless, heterogeneous catalysts, when available, are favored in industry because of the ease of separation of the spent catalyst from the reaction medium. The primary disadvantage of heterogeneous catalysts is the associated transport barriers (substrate diffusion into and product removal from the catalyst active site), which adversely affect the reaction kinetics. The reaction kinetics of transesterification with heterogeneous catalysts has been studied for a variety of reaction systems.1−9 For example, studies of methyl stearate with n-butanol over Amberlyst-15 catalyst showed second-order kinetics with 3.21 × 109 a rate constant of kgsol/kgcat min.4 However, because the products of this reaction are complex, the kinetic model was reported to be complicated. The study also reported low correlation coefficient (R2) values for the kinetic parameters.4 In another study10 involving the transesterification of oleic acid with methanol in the presence of Amberlyst-15/-16 catalysts, conversions above 95% within 120 min were reported. A study of metal-oxide-catalyzed transesterification reported a rate constant of 0.0085 g2 mol−2 min−1 and different reaction orders for different metal oxides.8 Although many studies have reported the kinetics of common homogeneous and heterogeneous catalysts, this is the first time that the reaction kinetics of a phasetransforming catalyst has been analyzed. With the intention of alleviating transport barriers at the onset of the reaction but also affording the ability to separate the spent catalyst upon completion of the reaction, we developed a process that uses a phase-transforming catalyst. Titanium isopropoxide was used as the catalyst in this process. According to this catalyst concept, the catalyst is added to the reaction medium as a homogeneous liquid, and as a result of concurrent condensation polymerization,11−14 the titanium isopropoxide undergoes a phase transformation that allows removal of the spent catalyst © 2013 American Chemical Society

from the medium as a solid. Transesterification was chosen to test this concept. The specific objective of this work was to elucidate the kinetic aspects of this catalytic process. It should be emphasized here (to avoid any ambiguity in terminology) that the phase-transforming catalysts described here are distinctly different from “phase-transfer catalysts”. Phase-transfer catalysts refer to the type of catalysis in which the catalyst migrates from one liquid phase to another immiscible liquid across a phase boundary.15,16 Transesterification is an industrially relevant reaction that converts triglycerides to fatty acid alkyl esters, commonly known as biodiesel.17−23 This reaction can be carried out with both homogeneous and heterogeneous catalysts24−28 by acidic or basic routes.17,29−31 We have shown that titanium isopropoxide catalyzes this reaction, according to the basic route through solvolysis of an alkoxide nucleophile and subsequent esterification of the triglyceride.26,32 This work focuses on elucidating the kinetic aspects of this process.

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MATERIALS AND METHODS The kinetics of the transesterification reaction with titanium isopropoxide catalyst was elucidated using isopropyl alcohol (because the isopropyl ligand is complementary to both alcohol and the titanumum isopropoxide catalyst). Because the transesterification of soybean oil with isopropanol is a complicated reaction to resolve kinetically, glycerol monooleate was selected as the model glyceride. Oleate was used because it is a common fatty acid in soybean oil and because of the ease of deciphering the kinetic data.33 As monooleate has only one fatty acid chain, the reaction stoichiometry is 1:1 with isopropanol. Received: Revised: Accepted: Published: 8392

March 4, 2013 May 27, 2013 June 3, 2013 June 3, 2013 dx.doi.org/10.1021/ie4006782 | Ind. Eng. Chem. Res. 2013, 52, 8392−8398

Industrial & Engineering Chemistry Research

Article

Samples were withdrawn from the reaction mixture at 10-min intervals initially and at 20-min intervals after 120 min of reaction time. Samples were weighed and analyzed by gas chromatography (GC-6850 Agilent Technologies, Santa Clara, CA) to estimate of ester composition. The gas chromatograph was calibrated with the respective alkyl ester standard (oleate isopropyl ester, Nu-Chek Prep Inc., Elysian, MN) before quantitative yields were obtained. The GC method utilized for oleate ester detection is reported in Table 1.

A prerequisite for the development of an accurate kinetic model is that the external and internal mass-transfer resistances be minimized in the reaction. To ensure that there were no external mass-transfer resistances, the transesterification reaction was carried out with lipid/alcohol/catalytic surfactant ratios such that the reaction mixture remained in a single phase. A lipid/alcohol/surfactant ratio of 3:2:1 (volume basis) was found to satisfy this condition (Figure 1). Transesterification

Table 1. Parameters of the GC Method Utilized for Oleate Ester Detection parameter

value

inlet temperature split ratio injection volum column flow (helium) FIDa temperature H2 flow air flow makeup gas (nitrogen) oven program

250 °C 50:1 1 μL 1.6 mL/min (constant flow) 280 °C 40 mL/min 450 mL/min 30 mL/min 75 °C hold for 1 min, to 200 °C at 50 °C/min, hold for 3 min, to 230 °C at 20 °C/min, hold 10 min 30 m × 0.25 mm × 0.25 μm (DB-wax column)

column a

Figure 1. Phase distribution of the soybean oil, isopropanol, and titanium isopropoxide ternary system at room temperature.

Flame ionization detector.

The concentration of fatty acid alkyl esters was calculated by Chemstation software (Agilent Technologies). The area under the peak from the flame ionization detector (FID) chromatogram corresponded to the concentration of that component. These concentrations were determined using calibrations with pure ester standards along with an internal standard (C-12 ester). Three replicates were carried out, and the results were analyzed using the statistical software Design Expert (Stat-Ease, Minneapolis, MN). Kinetic calculations were performed assuming homogeneous phase conditions in a batch reactor. Another set of experiments was conducted with the intention of optimizing the ester yields from the transesterification of soybean oil. In this study, titanium isopropoxide concentrations of 1%, 2%, 3%, 4%, 6%, and 16% (w/w) were tested as

was carried out in high-pressure reaction tubes (10 mL, glass) with continuous stirring using magnetic stirrers. Figure 2 shows the experimental setup. Monoolein (rac-glycerol 1-monooleate) was purchased from Sigma-Aldrich (St. Louis, MO). Isopropanol was purchased from EMD Chemicals. Titanium isopropoxide catalytic surfactant was purchased from Sigma-Aldrich. The monoglyceride was heated to 70 °C on a standard hot plate under continuous stirring with the reaction vial sealed with a high-pressure septum. Alcohol and the catalyst (titanium isopropoxide) were added as soon as the monoglyceride reached the reaction temperature. A temperature of 70 °C was chosen so that the reaction occurred below the boiling point of isopropanol.

Figure 2. Experimental setup in which monoolein transesterification was carried out. 8393

dx.doi.org/10.1021/ie4006782 | Ind. Eng. Chem. Res. 2013, 52, 8392−8398


Article

Figure 3. Actual setup of the reactor in which high-temperature soybean oil tranesterification reactions were carried out.

Table 2. Kinetic Equations for Each of the Cases case

α

β

1

0

0

2

1

0

3

0

1

4

1

1

(θ − X ) 1 = kCA0t ln (θ − 1) (1 − X )θ

5

2

0

X = kCA0t (1 − X )

6

0

2

X = kCA0t (1 − X )

7

2

1

(1 − X ) ⎤ 1 ⎡ 1 1 2 − ln ⎢ ⎥ = kCA0 t (θ − 1) ⎣ (1 − X ) (θ − 1) (θ − X ) ⎦

8

1

2

θ(1 − X ) ⎤ 1 ⎡ X 1 2 + ln ⎢ ⎥ = kCA0 t (θ − X ) ⎦ (θ − 1) ⎣ (θ − X )θ (θ − 1)

9

2

2

⎡ ⎤ ⎡ (1 − X )θ ⎤ 1 X 3 X(1 − θ)⎢ + ⎥ + 2 ln⎢ ⎥ = kCA0 t ⎣ (X − 1) ⎣ (θ − X ) ⎦ (X − θ)θ ⎦

equation

CA0X = kt

the transesterification catalyst, and soybean oil and isopropanol were used as the substrates. Degummed soybean oil was purchased in bulk from STE Oil Company, San Marcos, TX. Experiments were carried out as described previously.26,32 The yield optimization studies were carried out in a highpressure reactor that had a magnetic drive stirrer with a maximum speed of 2000 rpm and a tachometer module with an accuracy of ±10 rpm. The stainless steel reactor vessel was capable of handling 500 mL of reactants. The reactor was used in batch mode. Figure 3 shows an actual setup of the reactor system. To initiate the transesterification reaction, the catalyst was infused into the reaction chamber containing triglyceride immediately after the contents reached the designated temperature (200 °C) through a high-pressure liquid pump (Eldex 5790, Eldex Laboratories, Napa, CA). An alcohol-to-oil ratio of 3:1 (molar basis) was used for the transesterification reaction.

ln

1 = kt (1 − X )

ln

(θ − X ) = kt θ

Figure 4. Conversion of glycerol monooleate with time.

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RESULTS AND DISCUSSION Figure 4 depicts the glyceride conversion with time. It was noticed that it took up to 1 h for any initial (transesterification) products to be detected. Nevertheless, once initiated, the 8394



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Figure 5. Kinetic equation plots for the nine cases studied.

reaction went to completion within approximately 180 min. These data were used to determine the kinetic parameters of the reaction system. Kinetic aspects of triglyceride transesterification have been widely studied.2,3,7,8,34 However, almost all previous studies concentrated on common basic catalysts (sodium and potassium hydroxides or alkoxides) and inorganic acids (primarily sulfuric acid).2−4,35−38 Although a handful of studies have investigated the kinetic aspects of heterogeneous catalysts,4,8,38 no studies have yet been published on the titanium isopropoxide system. Because the catalyst initiates the reaction in its homogeneous form, the system was analyzed as such. The analytical procedure is discussed in more detail below. The transesterification reaction is reversible. Therefore, excess isopropanol was used to drive the reaction forward. Equation 1 shows the generalized reaction, where A is the monoglyceride, B is isopropanol, C is isopropyl oleate ester, and D is glycerol. The equation also shows the stoichiometric relationship between the reactants and the products. heat

A+B⇐ ⇒C+D

−

dCA = kCA αC B β dt

(2)

where −CA/t is the consumption of reactant A per unit time; k is the rate constant; CA and CB are the concentrations of reactants A and B, respectively, at time t; and α and β are the orders of the reaction with respect to components A and B, respectively. One can write the following equations for species A and B

CA = CA0(1 − X )

(3)

C B = C B0(1 − X )

(4)

θ=

C B0 CA0

(5)

where CA0 and CB0 are initial concentrations of species A and B, respectively; θ is the ratio of the initial concentrations of species B and A; and X is the fractional conversion. Finally, one can write a generalized equation for the conversion as

(1)

dX = kCA0(α + β − 1)(1 − X )α (θ − X )β dt

One can write the general rate equation for this reaction as 8395

(6)



Article

Figure 6. Ester yield (wt %) versus time for different catalyst concentrations.

condensation reaction. The results show that 1% (w/w) catalyst has significantly lower ester yields than any of the other catalyst concentrations. An interesting observation of this catalytic process is the concurrent phase transformation of the titanium isopropoxide catalyst. It was observed that, although the titanium isopropoxide was added to the reaction medium as a liquid, once the reaction completed, the catalyst deposited as a solid (Figure 7). A likely

In this study, nine different cases for reaction order were considered as discussed below. Because most reports in the literature state that transesterification has orders of 0, 1, or 2, these nine different cases were considered. For each case, eq 6 was integrated from t = 0 to t = t with conversion from X = 0 to X = X. The final equations are listed in Table 2. All of these equations are of the (linear) form y = mx, and because the concentration is known, when the conversion (X) is plotted against time in each case, the resulting plot should be a straight line going through the origin. The gradient of the straight line gives the reaction rate constant (k). The correlation coefficients (R2) of the plots were compared to determine the best fit. Figure 5 presents the corresponding plots of all nine scenarios along with the R2 values. It is evident that three cases have correlation coefficients near 0.8. Because cases 3 and 8 have negative gradients, which, in a reaction engineering context, are invalid, those plots were excluded from further analysis (and are not included in Figure 5). In several instances, the distribution(s) are identical (cases 2 and 4, 5 and 9, and 1 and 6). However, in all cases (except cases 1 and 6), the regression coefficients are low. Comparing cases 1 and 6, both have >0.8 correlation, with case 6 being closest to the measured results with an R2 value indicating 88% probability that the model is correct. Thus, it appears that this system behaving according to the case 6 scenario. Accordingly, it can be surmised that the present system has an order of 2 with respect to isopropanol and an order of 0 with respect to triglycerides. This is in agreement with earlier literature (although the order of the transesterification reaction has been reported to vary depending on the situation and type of starting materials2,3,7,17) From the aforementioned information, the rate constant was calculated to be 0.0002 L mol−1 min−1. Figure 6 depicts the results from the yield optimization study. The maximum possible yield was observed for different catalyst percentages. The highest ester yield was 71% with 16% catalyst by weight after 2 h of reaction time. At 16% catalyst (w/w), a reduced ester yield was observed after 3 h of reaction (as compared to shorter reaction durations). This can be attributed to the simultaneous occurrence of the sol−gel reaction in the medium. These observations suggest that longer reaction times are not favored because of the dominating alcohol

Figure 7. Phase transformation of titanium isopropoxide catalyst subsequent to the transesterification reaction.

reason for this behavior is the high-temperature-induced alcohol condensation−polymerization of isopropoxides.11−14 It is noteworthy that, although we were able to obtain complete esterification (>99%) using the monooleate (monolglyceride), it was not possible to reach such yields using the triglyceride. Based on the results, it is likely that the optimal catalyst concentration is somewhere between 6% and 16%. [It should be noted that, because titanium isopropoxide was added to the liquid phase and because of the induced phase transformation (into a gel), it was not possible to add more than a 16% loading of the catalyst to the system.] The spirit of this catalyst technology is not to reuse the spent catalyst in its transformed (solid) form, but to regenerate it by 8396



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reverting back to the liquid isopropoxide form. The oligomerized/ polymerized form of titanium isopropoxide can be transformed to the liquid titanium isopropoxide form using several methods including alcoholysis.39,40

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CONCLUSIONS The kinetic study reported herein revealed that transesterification of monoolein with titanium isopropoxide is second-order with respect to isopropyl alcohol. However, a zeroth-order reaction is also plausible. The rate constant was evaluated to be 0.0002 L mol−1 min−1. Optimization studies revealed that this catalytic system can provide ester yields of up to 71% using 16% (w/w) catalyst. However, the results suggest that better yields might be plausible with a catalyst composition between 6% and 16%. An interesting phenomenon observed during these studies was the phase transformation of the titanium isopropoxide catalyst from the original liquid form to a gel, likely due to condensation polymerization. The fortuitous result is the ability to remove the spent (heterogeneous) catalyst relatively easily for regeneration.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This article is based on work supported by the National Science Foundation under Grant CBET 0924900.

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REFERENCES

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Reaction Kinetics of Transesterification with Titanium Alkoxide-Based

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