Solid-Liquid Reaction Catalyzed by a Liquid Containing Rich Phase-Transfer Catalyst----Synthesis of Hexyl Acetate

Studies in Surface Science and Catalysis, volume 159 Hyun-Ku Rhee, In-Sik Nam and Jong Moon Park (Editors) 9 2006 Elsevier B.V. All rights reserved

181

Solid-Liquid Reaction Catalyzed by a Liquid Containing Rich Phase-Transfer Catalyst .... Synthesis of Hexyi Acetate Hsu-Chin Hsiao*, Chung-Wei Hsu + and Hung-Shan Weng +

*Department of Chemical Engineering, Tung Fung Institute of Technology, Kaohsiung 829, Taiwan, R. O. C. +Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan, R. O. C. Abstract

This work is for the purpose of evaluating the feasibility of synthesizing hexyl acetate (ROAc) from n-hexyl bromide (RBr) and sodium acetate (NaOAc) by a novel phase-transfer catalysis (PTC) technique. In this new technique, the solid-liquid reaction was catalyzed by a catalyst-rich liquid phase. Experimental results reveal that the use of this technique for synthesizing ROAc gives a far higher reaction rate than the solid-liquid-PTC does and a slightly faster rate than tri-liquid PTC. The amount of water added greatly influences the type of the reaction system and the reaction rate. The kind of catalyst affects the conversion of RBr and the fractional yield of ROAc significantly. The catalyst with a longer chain length gives a better performance. However, it will more easily dissolve in the organic phase and hence is more difficult to be recovered and reused after reaction. Tetra-n-butylammonium bromide is the best choice. As the experimental results of reusing catalyst phase show that the conversion decreases due to part of catalyst dissolving into the organic phase, an effort for improving this drawback should be made in the future. 1. INTRODUCTION Although the use of phase-transfer catalysis (PTC) for manufacturing esters has the merits of a mild reaction condition and a relatively low cost [1], PTC has its limitations, such as the low reactivity of carboxylic ion by liquid-liquid PTC [2], a slow reaction rate by solid-liquid PTC, and the difficulty of reusing the catalyst by both techniques. This work was initiated for the purpose of evaluating the feasibility of synthesizing hexyl acetate (ROAc) from n-hexyl bromide (RBr) and sodium acetate (NaOAc) by a novel PTC technique. In this new technique, the solid-liquid reaction was catalyzed by a catalyst-rich liquid phase in a batch reactor. Because there a solid phase and two liquid phases coexist, it is called as a SLL-PTC system [3]. Actually, this liquid phase is the third liquid phase in the tri-liquid PTC system. It might be formed when the phase-transfer catalyst is insoluble or slightly soluble in both aqueous and organic phases. Both aqueous and organic reactants can easily transfer to this phase where the intrinsic reaction occurs [4, 5]. In this study, tetra-n-alkylammonium bromides ((n-Alkyl)4NBr, QBr) were used as the phase-transfer catalysts. The reaction will proceed in the catalyst-rich liquid phase in a way

182 similar to that in the tri-liquid-phase phase transfer catalysis [4, 5]: Organic phase

RBr

Catalyst-rich

RBr + QOAc

= QBr + ROAc

Liquid phase

NaOAc + QBr

= QOAc + NaBr

Solid phase

NaOAc

ROAc

( Scheme I )

NaBr

2. EXPERIMENTAL METHODS The catalyst-rich liquid phase was prepared by forming a third liquid phase in a way similar to that has been described previously [5, 6]. A 125-mL three-neck round-bottom flask was employed as the reactor. The procedure is similar to the previous work [7]. 3. RESULTS AND DISCUSSION 3.1. Formation of the Catalyst-Rich Liquid Phase

The quantity of water added to the phase-transfer catalytic reaction system is a main factor for forming a catalyst-rich liquid phase (the third liquid phase). In the aqueous-organic two phase system, in which the aqueous phase contains phase-transfer catalyst and sodium acetate, when the amount of water is decreased, the volume of aqueous phase will decrease accordingly, even a third liquid phase will appear. The third liquid phase consists of water, sodium acetate, organic solvent and phase-transfer catalyst. If the concentration of sodium acetate is high, most of the phase-transfer catalyst will be salted out to the third liquid phase, hence a catalyst-rich liquid phase is formed. After the appearance of the third liquid phase, the volume of aqueous phase will decline significantly with decreasing the amount of water added, while that of the third liquid phase only shrinks slightly. This fact implies that the catalyst in the third liquid phase attracts a fixed amount of water and will not be affected by the water content in the aqueous phase. Figure 1 reveals the above-mentioned facts. If the amount of water added is continuously decreased, a solid phase (sodium acetate) will appear in the aqueous phase. 3.2. Effect of the Amount of Water Added on the Reaction

Figure 2a reveals that the conversion of RBr increases with increasing the amount of water up to the amount of water being 3 mL, then it declines. Note that the sharp increase of the conversion curves is due to the fast diffusion of RBr from the organic phase to the catalyst-rich liquid phase where only part of RBr reacts with QOAc. Except that RBr can be hydrolyzed to ROH, ROAc (the main product) can be hydrolyzed to form ROH and HOAc (the byproducts) too, so the addition of water will reduce the fractional yield of ROAc. On the contrary, another byproduct, NaBr, can adsorb water to form the hydrated ions. These two contradictory facts result in a peculiar phenomenon (Figure 2b). 3.3. Effect of the Kind of Quaternary Ammonium Salt

183

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Liquid

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Figure 1. Effect of the added amountof water on the volume and Q'=concentrationof each phase. System: 10 mL Octane + 0.014mole NaOAc + 0.006tool Bu4N Br @ 80~ 1000 rpm

()

21)

4(1

Reaction

5

.~-,41--

I()

611

811

time

(m

1 (10

in)

Figure 2. Effect of the added amountof water on the conversion of RBr and the yield of ROAc. System: 25 mL Octane - 0.025 tool RBr + 0.035 moi NaOAc * 0.015 tool Bu.~NBr ~ 80~ 1000 rpm

Figure 3a shows that the use of quaternary ammonium salt with a longer carbon chain gives a higher reaction rate. This is because that a longer carbon chain makes the catalyst more lipophilic. Consequently, the catalyst is easier to react with RBr and more catalyst will dissolve into the organic phase where the catalyst has more chance to react with RBr. However, Figure 3b shows the fractional yield of ROAc decreases obviously with the length of carbon chain in the catalyst. This fact might be attributed to the fact that ROAc produced will stay longer at the catalyst-rich liquid phase which is more lipophilic hence has more chance to be hydrolyzed to ROH because this phase contains water. It should be pointed out that although the catalyst with a longer carbon chain has a higher activity and gives a higher conversion of RBr, it is more difficult to be recovered for reuse due to a larger solubility in the organic phase in addition to a lower fractional yield of ROAc. 3.4. Reuse of Catalyst The feasibility of reusing the phase transfer catalyst via the reuse of the third liquid phase in a tri-liquid catalytic system has been investigated in our laboratory [6,7]. In the present study, the catalyst-rich liquid phase was repeatedly used for four times and the changes in the conversion of RBr and the fractional yield of ROAc were observed. As shown in figure 4, the conversion of RBr decreased significantly when the catalyst phase was reused first time (2nd run), however, no obvious change in the subsequent runs (3rd and 4th runs). The decrease in the conversion might be due to the following reasons: (a) NaBr formed during the reaction would accumulate in the catalyst-rich liquid phase hence the solubility of NaOAc decreased; (b) ROAc produced made the polarity of the organic phase increased hence more catalyst would dissolve into the organic phase; and (c) Part of NaBr produced would precipitate and formed solid particles which would hinder the mass transfer of RBr to the catalyst-rich phase. This drawback should be overcome before the technique of solid-liquid-liquid catalysis to be commercialized.

184

7o

o~

(a)

60

(a)

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.~"

3o ~

20

41)

I0 o~ "

3O

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150

2(K)

R e a c t i o n time ( rain I

t i m c (ill i n )

Figure 3. Effect of the kind of catalyst on the reaction System: 2 mL H20 + 25 mL Octane + 0.025 moi RBr § 0.060 mol NaOAc + 0.025 mole catalyst ~ 80~ 600 rpm

Figure 4. Reuse of the catalyst-rich liquid phase. System: 2 mL H20 + 25 rnL Octane+ 0.025 mol RBr + 0.060 tool NaOAc+ 0.025 tool Bu4NBr @ 80~ 600 rpm

4. CONCLUSIONS Based on the above discussions, we can draw the following conclusions: The use of the SLL-PTC technique for synthesizing ROAc gives a far higher reaction rate than the SL-PTC does and a slightly faster rate than tri-liquid PTC. (2) For forming a SLL reaction system, the amount of water added should be small. (3) For reusing the catalyst, the organic solvent should be nonpolar. (4) Although the catalyst with a longer chain length benefits the reaction rate, it is not easy to be recovered. Tetra-n-butylammonium bromide is the best choice. (5) The amount of catalyst added should not be too high, otherwise the fractional yield of ROAc would decrease. (6) The amount of NaOAc added should not be too high, otherwise the reaction rate of RBr would decrease. (7) A lower concentration of RBr benefits the fractional yield of ROAc. (8) The conversion of RBr will decline when the catalyst-rich liquid phase is reused, mainly due to the accumulation of NaBr and lose of catalyst. This drawback should be overcome before the technique of solid-liquid-liquid catalysis to be commercialized.

(1)

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