Reac Kinet Mech Cat DOI 10.1007/s11144-014-0727-x
Catalytic performances of potassium and sodium hydroxides/carbonates and calcium and magnesium oxides on hydrolysis of a-chloropropionic acid to lactic acid Changhua Zhang • Lingqin Shen • Hengbo Yin Aili Wang • Xiaobo Yan
•
Received: 26 January 2014 / Accepted: 11 May 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014
Abstract Catalytic hydrolysis of a-chloropropionic acid to lactic acid in liquid phase over different alkaline catalysts, such as alkali metal hydroxides, alkali metal carbonates, and alkaline-earth metal oxides, was investigated. Lactic acid was mainly produced by the hydrolysis of a-chloropropionic acid over the alkaline catalysts. The catalytic activities of the alkaline catalysts were in an order of KOH [ NaOH [ K2CO3 [ Na2CO3 [ CaO [ MgO. The catalytic activities of the alkali metal hydroxides and carbonates were related to both concentrations of their hydroxide anions and hydrated ionic radiuses of K? and Na?. For the alkaline-earth metal oxide catalysts, their catalytic activities in the hydrolysis of a-chloropropionic acid were mainly related to the solubilities of the resultant hydroxides. A power-function type kinetic equation was used for the evaluation of the effect of catalyst loading and reaction temperature on the hydrolysis of a-chloropropionic acid to lactic acid. The activation energies over the alkaline catalysts were in an order of Ea,KOH \ Ea,NaOH \ Ea;K2 CO3 \ Ea;Na2 CO3 \ Ea,CaO \ Ea,MgO. The alkaline catalyst with low activation energy had high catalytic activity for the hydrolysis of a-chloropropionic acid to lactic acid. Keywords
Lactic acid a-Chloropropionic acid Hydrolysis Alkaline catalysts
Introduction a-Chloropropionic acid (a-CPA) as an important fine chemical can be used in the synthesis of medicines, pesticides, dyestuffs, and agricultural chemicals [1, 2]. Electronic supplementary material The online version of this article (doi:10.1007/s11144-014-0727-x) contains supplementary material, which is available to authorized users. C. Zhang L. Shen H. Yin (&) A. Wang X. Yan Faculty of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China e-mail:
[email protected];
[email protected]
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Furthermore, catalytic hydrolysis of a-CPA over alkaline catalysts can produce lactic acid (LA) [3], which is an important feedstock for the synthesis of a variety of valuable chemicals, such as polylactic acid, lactate ester, 1,2-propanediol, pyruvic acid, and acrylic acid [4–6]. It is worth noting that LA has been widely used for the commercial production of biodegradable and biocompatible polylactic acid in a large scale. The global demand for LA was estimated to be 482.7 kilotons in 2010 and is expected to reach 1,076.9 kilotons in 2016, growing at a compound average growth rate of 14.2 % from 2011 to 2016 [7]. Therefore, considerable increase in the worldwide demand for LA is definitely expected in the coming years. Currently, 95 % of LA is produced through bacterial fermentation and the high price of LA limits its application [8]. In addition to the high price, the bacterial fermentation process unavoidably produces bacteria-containing sludge, causing environmental pollution. The commercial chemical method for the production of LA is based on the hydrolysis of lactonitrile, which is produced from acetaldehyde and hydrogen cyanide. However, hydrogen cyanide is a very harmful chemical for humans. Therefore, a new environmentally friendly methodology for the production of LA at low cost is worthy of investigation. It has been reported that LA can be synthesized by the hydrolysis of a-CPA over alkaline catalyst [3]. a-CPA can be obtained by using glycerol as a starting material through dehydration, oxidation, hydrogenation, and chlorination processes. Glycerol produced as a by-product for ca. 10 % of manufactured biodiesel fuel becomes cheaper and more readily available with the escalating production and use of biodiesel [9–13]. Therefore, the production of LA by using renewable glycerol as the starting material is an economic and green process. The conversion of glycerol to acrolein and acrylic acid has been studied by several groups [11, 12, 14–16]. Acrylic acid can be hydrogenated to form propanoic acid over noble metal catalysts and propanoic acid can be chlorinated to form a-CPA. However, to the best of our knowledge, the hydrolysis of a-CPA to LA is seldom investigated [3]. In our present work, the catalytic hydrolysis of a-CPA into LA in liquid phase over different alkaline catalysts, such as alkali metal hydroxides, alkali metal carbonate, and alkali earth metal oxides, was investigated in detail. The effect of the reaction parameters, such as reaction temperature, alkaline type, and catalyst loading on the hydrolysis of a-CPA to LA was investigated and discussed. The reaction routes were briefly illustrated. A power-function type kinetic equation was used for the evaluation of the effect of catalyst loading and reaction temperature on the hydrolysis of a-chloropropionic acid.
Experimental Materials All chemicals were commercially available and used without further purification. aCPA (98 %) was of reagent grade and was purchased from Yuantong Fine Chemical Co. Ltd. NaOH (96 %), KOH (85 %), Na2CO3 (97 %), K2CO3 (97 %), CaO (96 %),
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and MgO (96 %) were of reagent grade and were purchased from Sinopharm Chemical Reagent Co. Distilled water was used through all of the experiments. Catalytic test All experiments were carried out in a 250 mL three-necked Pyrex flask at different reaction temperatures under atmospheric pressure. Given amounts of alkaline catalysts, such as KOH, NaOH, K2CO3, Na2CO3, CaO, and MgO, were first dissolved or dispersed in 30 mL of deionized water. Before adding a-CPA, the solution or suspension of alkaline catalyst was heated in an silicon-oil bath to given reaction temperatures of 80, 90, 100, and 105 °C. Then, 0.1 mol of a-CPA was added into dropwise the Pyrex flask reactor in 20 min. The reaction mixture was taken at different reaction time and analyzed on an HPLC. Product analysis The liquid samples were acidified with hydrochloric acid (37 %) to convert lactate salts into LA and then filtered through a 0.45 mm filter. HPLC analysis was performed with Agilent HPLC system equipped with a tunable absorbance UV detector and C18 column. The mobile phase was methyl cyanide aqueous solution (20:80, V/V) with a flow rate of 1.0 mL/min. The pH value of mobile phase was 2.0, which was adjusted with phosphate buffer. The detection wavelength was 210 nm [17]. The conversion of a-CPA and the yield of LA were used as the parameters to evaluate the catalytic activities of the alkaline catalysts. They are calculated according to the following equations: Conversion ¼
Mole of aCPA fed mole of aCPA remained 100 % Mole of a-CPA fed Yield ¼
Mole of product 100 % Mole of a-CPA fed
ð1Þ ð2Þ
Results and discussion Hydrolysis of a-CPA catalyzed by alkali metal hydroxides Figs. 1 and 2 show the hydrolysis of a-CPA to LA catalyzed by KOH and NaOH catalysts with different catalyst loadings at 105 °C. The conversions of a-CPA over KOH and NaOH catalysts at 80–105 °C are shown in Figs. S1 and S2 (see supplementary information). During the hydrolysis of a-CPA over the alkaline catalysts, LA was detected as the main product and its selectivity was 100 %. When the molar ratio of KOH to a-CPA was 2:1, 200 min was necessary for the 98 % conversion at 80 °C. At the same time, the conversion was 100 % only after 120 and 40 min at 90 and 100 °C. At the 1.5:1 ratio, however, 160 min was
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Reac Kinet Mech Cat
CPA conversion (%)
100
80
60 KOH:CPA 0.8:1 1:1 1.5:1 2:1
40
20
0 40
80
120
160
200
Reaction time (min) Fig. 1 Conversion of a-CPA catalyzed by KOH catalyst with different molar ratios of KOH to a-CPA of 0.8:1–2:1 at 105 °C
CPA conversion (%)
100
80
60
40
NaOH:CPA 0.8:1 1:1 1.5:1 2:1
20
0 40
80
120
160
200
Reaction time (min) Fig. 2 Conversion of a-CPA catalyzed by NaOH catalyst with different molar ratios of NaOH to a-CPA of 0.8:1–2:1 at 105 °C
necessary for the 100 % conversion at 100 °C, and this time decreased to 80 min at 105 °C. When the molar ratio of NaOH to a-CPA was 2:1, 200 min was necessary for the 86.2 % and 97.0 % conversion at 80 and 90 °C. At the same time, the conversion was 100 % only after 80 and 40 min at 100 and 105 °C. At the 1.5:1 ratio, however, 200 min was necessary for the 100 % conversion at 100 °C, and this time decreased to 120 min at 105 °C. It is interesting to find that although KOH and NaOH are both strong alkalies, KOH showed higher catalytic activity (2.2 % conversion/min at 80 °C) than NaOH (1.6 % conversion/min at 80 °C) in the hydrolysis reaction. It is well known that the ionic radius of Na? is smaller than that of K?. However, the ionic radius of hydrated
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Reac Kinet Mech Cat Table 1 Hydration status of K? and Na? [18] K?
Na?
Ionic radius/pm
133
95
Hydrated ionic radius/pm
232
276
Average water molecules of hydration
10.5
16.6
Na? is bigger than that of hydrated K? and the average number of hydrated H2O molecules on Na? is also higher than that for K? (Table 1) [18]. Therefore, the OH- anions around hydrated Na? cations in aqueous solution are more difficult to attack the a-C atoms of a-CPA molecules than those around hydrated K? cations due to the steric hindrance effect. The difference between the hydration states of K? and Na? of KOH and NaOH in aqueous solution affected their catalytic activities in the hydrolysis of a-CPA to LA. Hydrolysis of a-CPA catalyzed by alkali metal carbonates Figs. 3 and 4 show the hydrolysis of a-CPA catalyzed by K2CO3 and Na2CO3 catalysts with different catalyst loadings at 105 °C. The conversions of a-CPA over K2CO3 and Na2CO3 catalysts at 80–105 °C are shown in Figs. S3 and S4 (see supplementary information). During the hydrolysis of a-CPA over the alkaline catalysts, LA was detected as the main product and its selectivity was 100 %. When the molar ratio of K2CO3 to a-CPA was 1:1, 200 min was necessary for the 73.0 % and 87.7 % conversion at 80 and 90 °C. At the same time, the conversion was 100 % only after 120 and 80 min at 100 and 105 °C. At the 0.75:1 ratio, however, 120 min was necessary for the 100 % conversion at 105 °C.
CPA conversion (%)
100
80
60
40
K 2CO3:CPA 0.4:1 0.5:1 0.75:1 1:1
20
0 40
80
120
160
200
Reaction time (min) Fig. 3 Conversion of a-CPA catalyzed by K2CO3 catalyst with different molar ratios of K2CO3 to aCPA of 0.4:1–1:1 at of 105 °C
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Reac Kinet Mech Cat
CPA conversion (%)
100
80
60
40
Na2CO3:CPA 0.4:1 0.5:1 0.75:1 1:1
20
0
40
80
120
160
200
Reaction time (min) Fig. 4 Conversion of a-CPA catalyzed by Na2CO3 catalyst with different molar ratios of Na2CO3 to aCPA of 0.4:1–1:1 at 105 °C
When the molar ratio of Na2CO3 to a-CPA was 1:1, 200 min was necessary for the 56.4 and 84.0 % conversion at 80 and 90 °C. At the same time, the conversion was 100 % only after 160 and 80 min at 100 and 105 °C. At the 0.75:1 ratio, however, 160 min was necessary for the 100 % conversion at 105 °C. The results showed that the catalytic activities of K2CO3 and Na2CO3 were lower than those of KOH and NaOH, respectively. It can be explained as follows. When K2CO3 and Na2CO3 were used as the catalysts, they underwent the following hydrolysis reactions: K2 CO3 þ H2 O ¼ 2Kþ þ HCO 3 þ OH
ð3Þ
Na2 CO3 þ H2 O ¼ 2Naþ þ HCO 3 þ OH
ð4Þ
K2CO3 (or Na2CO3) as weak alkali produces less OH- anions than KOH (or NaOH) does when they are hydrolyzed with the same concentration of alkali metal cations. Therefore, the catalytic activities of K2CO3 and Na2CO3 were lower than those of KOH and NaOH in the alkaline-catalyzed hydrolysis of a-CPA to LA. Furthermore, it was found that the catalytic activity of K2CO3 was higher than that of Na2CO3 in the hydrolysis reaction. The reason can be explained as those mentioned in the catalytic hydrolysis of a-CPA over KOH and NaOH catalysts. The hydration extent of both K? and Na? may affect the alkaline catalytic activity. Hydrolysis of a-CPA catalyzed by alkaline-earth metal oxides When CaO and MgO were used as the catalysts in the hydrolysis of a-CPA to LA with different catalyst loadings at 105 °C, the conversions of a-CPA are shown in Figs. 5 and 6. The conversions of a-CPA over CaO and MgO catalysts at 80–105 °C are shown in Figs. S5 and S6 (see supplementary information). LA was found as the main product in the hydrolysis reaction.
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CPA conversion (%)
100
80
60
40
CaO:CPA 0.4:1 0.5:1 0.75:1 1:1
20
0 40
80
120
160
200
Reaction time (min) Fig. 5 Conversion of a-CPA catalyzed by CaO catalyst with different molar ratios of CaO to a-CPA of 0.4:1–1:1 at 105 °C
CPA conversion (%)
100 80 60 40
MgO:CPA 0.4:1 0.5:1 0.75:1 1:1
20 0 40
80
120
160
200
Reaction time (min) Fig. 6 Conversion of a-CPA catalyzed by MgO catalyst with different molar ratios of MgO to a-CPA of 0.4:1–1:1 at 105 °C
When the molar ratio of CaO to a-CPA was 1:1, the conversions of a-CPA were 41.5, 73.6, 91.8, and 100 % after 200 min at 80, 90, 100, and 105 °C. When the molar ratio of MgO to a-CPA was 1:1, the conversions were 33.2, 61.3, 85.0, and 90.5 % after 200 min at 80, 90, 100, and 105 °C. The results showed that CaO catalyst had higher catalytic activity than MgO catalyst in the hydrolysis reaction. For CaO and MgO catalysts, their catalytic activities should be related to the concentration of their OH- anions formed by hydrolysis reactions. CaO þ H2 O ¼ CaðOHÞ2 ¼ Ca2þ þ 2OH
ð5Þ
MgO þ H2 O ¼ MgðOHÞ2 ¼ Mg2þ þ 2OH
ð6Þ
The solubilities of resultant Ca(OH)2 and Mg(OH)2 in aqueous solution at 20 and 100 °C are 0.16, 0.072 and 0.00082, 0.004 g/L [19]. The solubility of Ca(OH)2 is
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higher than that of Mg(OH)2, meaning that CaO catalyst can produce more OHanions and have higher catalytic activity than MgO catalyst in the hydrolysis of aCPA to LA. The solubilities of alkaline earth metal oxides are crucial to their catalytic activities in the hydrolysis reaction. The catalytic activities of the alkaline catalysts were in an order of KOH [ NaOH [ K2CO3 [ Na2CO3 [ CaO [ MgO in the hydrolysis of a-CPA to LA. The properties of the alkaline catalysts significantly affected their catalytic activities. Reaction mechanism When alkali metal hydroxides, alkali metal carbonates, and alkaline earth metal oxides were used as the catalysts in the hydrolysis of a-CPA, the alkaline catalysts were hydrolyzed to produce OH- anions. The hydrolysis of a-CPA is a substitution reaction by the SN2 mechanism [20]. a-CPA molecules reacted with OH- anions to form a-chloropropionate anions and water. OH- anions attacked the a-carbon atoms of a-chloropropionate anions to form intermediate anions. Then, electrons were transferred from OH- groups to chlorine atoms, causing the break of C–Cl bonds and subsequently giving the formation of lactate anions and Cl- anions. Kinetic analysis A power-function type kinetic equation was used to evaluate the effect of alkaline loading and reaction temperature on the hydrolysis of a-CPA to LA. Generally, a power-function type kinetic equation over the alkaline catalysts can be written as follows: dCA Ea a b rA ¼ ¼ Aexp ð7Þ CA CB dt RT Here, CA and CB are the concentrations of a-CPA and alkaline catalyst, A is the preexponential factor, Ea is the activation energy, R is the ideal gas constant, T is the reaction temperature, and a and b are the reaction orders for a-CPA and alkaline catalyst. The unit of concentration is mol L-1, the unit of reaction time is min, the unit of T is K, and the unit of -rA is mol L-1 min-1. Taking into account of the initial concentration of a-CPA was kept at 3.33 mol L-1 in all the experiments, Eq. 7 can be rearranged as follows. Ea b ð8Þ rA0 ¼ k0 exp CB0 RT where -rA0 is the initial reaction rate of a-CPA, CB0 is the initial concentration of alkaline catalyst, and k0 is the pseudo pre-exponential factor. Linear Eq. 9 is obtained by taking the natural logarithm of both sides of the Eq. 8. InðrA0 Þ ¼ Inðk0 Þ
123
Ea þbln (CB0 Þ RT
ð9Þ
Reac Kinet Mech Cat
To calculate the reaction order (b) and activation energy (Ea) according to Eq. 9, the initial rates were calculated according to the data shown in Figs. S1-S6. The initial reaction rates of a-CPA under different reaction conditions were calculated at the first 40 min and the reaction temperatures of 80, 90, and 100 °C. The data obtained at 105 °C were not used for fitting the reaction kinetics because the conversions of a-CPA were ca. 100 % for both KOH and NaOH catalysts with high concentrations of 6.67 mol L-1. The values of pre-exponential factors (k0), activation energies (Ea), and reaction orders (b) for different alkaline catalysts are listed in Table 2. All experimental data gave good correlation coefficients for the power-function type kinetic in Eq. 9 in the range of 0.9317–0.9746, indicating that the powerfunction type kinetic model appropriated for the evaluation of the effect of catalyst loading and reaction temperature on the hydrolysis of a-CPA to LA. The results showed that the activation energies over the alkaline catalysts were in an order of Ea,KOH \ Ea,NaOH \ Ea;K2 CO3 \ Ea;Na2 CO3 \ Ea,CaO \ Ea,MgO. The low activation energies indicated that the alkaline catalysts had high catalytic activities for the hydrolysis of a-CPA to LA, being in consistent with the conclusion obtained from catalytic test. The reaction orders for all the alkaline catalysts ranged from 0.6 to 0.8.
Conclusions The catalytic activities of different alkaline catalysts, such as KOH, NaOH, K2CO3, Na2CO3, CaO, and MgO in the hydrolysis of a-CPA were investigated at the reaction temperatures ranging from 80 to 105 °C. All alkaline catalysts effectively catalyzed the hydrolysis of a-CPA under the mild reaction conditions. The selectivity of LA was 100 % for all the alkaline catalysts. Alkali metal hydroxides were more active for the hydrolysis of a-CPA than alkali metal carbonates as well as alkaline-earth metal oxides. The catalytic activities of the alkaline catalysts were in an order of KOH [ NaOH [ K2CO3 [ Na2CO3 [ CaO [ MgO. The catalytic activities of the alkali metal hydroxides and carbonates were related to both concentrations of their resultant hydroxide anions and hydrated ionic radiuses of K?
Table 2 Pre-exponential factors (k0), activation energies (Ea), and reaction orders (b) for different alkaline catalysts Catalysts
ln(k0)
Standard error
k0
-Ea/R (K)
Standard error
Ea (kJ/mol)
b
Standard error
R2
KOH
3.5
1.4
33.8
-2764.9
494.4
23.0
0.8
0.1
0.9317
NaOH
5.0
0.8
148.4
-3327.3
292.7
27.7
0.8
0.1
0.9746
K2CO3
16.2
1.7
1.0 9 107
-7254.1
623.6
60.3
0.7
0.1
0.9502
Na2CO3
17.8
2.0
5.6 9 107
-7977.1
726.7
66.3
0.8
0.1
0.9477
CaO
19.6
2.0
3.4 9 108
-8726.7
714.9
72.6
0.8
0.1
0.9537
MgO
21.1
1.6
1.5 9 109
-9309.7
574.4
77.4
0.6
0.1
0.9711
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and Na?. However, for the alkaline earth metal oxide catalysts, their catalytic activities in the hydrolysis of a-CPA were related to the solubilities of the resultant hydroxides. The effect of alkaline catalyst loading and reaction temperature on a-CPA hydrolysis reaction was well evaluated by the power-function type kinetic equation. The activation energies over the alkaline catalysts were in an order of Ea,KOH \ Ea,NaOH \ Ea;K2 CO3 \ Ea;Na2 CO3 \ Ea,CaO \ Ea,MgO. The alkaline catalyst with low activation energy had high catalytic activity for the hydrolysis of a-CPA to LA. Acknowledgments The authors sincerely thank Mrs. Rongxian Zhang for her kindly supporting the HPLC analysis. The authors also sincerely thank the financial support from Jiangsu Province Education Bureau (CXZZ12-0683, 11KJB530002 and 1102120C).
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.
Ogata Y, Watanabe S (1979) J Org Chem 44:2768–2770 Aam BB, Fonnum F (2006) Toxicology 228:124–134 Zeng HL, Guo GS, Han DM, Liu Y (2003) J Beijing Univ Chem Technol 30:108–112 Corma A, Iborra S, Velty A (2007) Chem Rev 107:2411–2502 Gao C, Ma CQ, Xu P (2011) Biotechnol Adv 29:930–939 Vijayakumar J, Aravindan R, Viruthagiri T (2008) Chem Biochem Eng Q 22:245–264 http://www.marketsandmarkets.com/Market-Reports/polylacticacid-387.html. Accessed Oct 2011 Ramı´rez-Lo´pez CA, Ochoa-Go´mez JR, Ferna´ndez-Santos M, Go´mez-Jime´nez-Aberasturi O, AlonsoVicario A, Torrecilla-Soria J (2010) Ind Eng Chem Res 49:6270–6278 Akiyama M, Sato S, Takahashi R, Inui K, Yokota M (2009) Appl Catal A Gen 371:60–66 Kurosaka T, Maruyama H, Naribayashi I, Sasaki Y (2008) Catal Commun 9:1360–1363 Shen LQ, Yin HB, Wang AL, Feng YH, Shen YT, Wu ZA, Jiang TS (2012) Chem Eng J 180:277–283 Shen LQ, Feng YH, Yin HB, Wang AL, Yu LB, Jiang TS, Shen YT, Wu ZA (2011) J Ind Eng Chem 17:484–492 Feng YH, Yin HB, Wang AL, Shen LQ, Yu LB, Jiang TS (2011) Chem Eng J 168:403–412 Witsuthammakul A, Sooknoi T (2012) Appl Catal A Gen 413–414:109–116 Pestana CFM, Guerra ACO, Ferreira GB, Turci CC, Mota CJA (2013) J Braz Chem Soc 24:100–105 Dolores Soriano M, Concepcio´n P, Lo´pez Nieto JM, Cavani F, Guidetti S, Trevisanut C (2011) Green Chem 13:2954–2962 Shen Z, Jin FM, Zhang YL, Kishita BWA, Tohji K, Kishida H (2009) Ind Eng Chem Res 48:8920–8925 Chen RS (2006) In: Huang MJ, Xue DP, Chen RS, Yuan WQ, Du DC (eds) Inorganic and analytical chemistry (Chinese). High Education Press, Beijing, p 197 Liu GQ (2002) In: Liu GQ, Ma LX, Liu J (eds) Chemical property data sheet (Chinese). Chemical Industry Press, Beijing, p 329 Grossman RB (ed) (2003) The art of writing reasonable organic reaction mechanisms. SpringerVerlag, Inc., New York, pp 50–53
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